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NANOSTRUCTURED METAL-OXIDE MATERIALS FOR DYE-SENSITIZED SOLAR CELLS SHWETA AGARWALA NATIONAL UNIVERSITY OF SINGAPORE 2011
NANOSTRUCTURED METAL-OXIDE
MATERIALS FOR DYE-SENSITIZED SOLAR
CELLS
SHWETA AGARWALA
NATIONAL UNIVERSITY OF SINGAPORE
2011
NANOSTRUCTURED METAL-OXIDE MATERIALS
FOR DYE-SENSITIZED SOLAR CELLS
Shweta Agarwala
(M.Sc, Nanyang Technological University, Singapore)
A THESIS SUBMITTED
FOR THE DEGREE OF DOCTOR OF PHILOSPHY
DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
NATIONAL UNIVERSITY OF SINGAPORE
2011
i
Acknowledgements
The work presented here would not have been possible without the help of many talented
and supportive people. First and foremost, I would like to thank my advisor Dr. Ho Ghim Wei,
for the intellectual guidance, strong support and trust she offered me throughout my time here. I
had the privilege to learn from her not just lessons of science and technology but of life.
I thank all the members of Dr. Ho’s research group, especially Mr. Ong Weili, Mr. Kevin
Moe and Dr. Lim Zhihan, who have helped me over the years and made my time here both
productive and enjoyable. I would also like to thank all past and present final year students for
assisting me in experiments.
I greatly appreciate Dr. A. S. W Wong’s help and guidance during my Ph.D studies. It
has benefitted me a lot.
Special thanks to Mr. Thomas Ang and Mr. Tay Peng Yeow, whose experience and skills
kept laboratory equipments in smooth running condition.
Big thanks to my friends and co-workers from different departments and laboratories,
who have helped and guided me in one way or the other.
Lastly, I would like to thank my parents and in-laws for their love, patience and support.
This thesis, however, is dedicated to my husband (Mr. Shailendra Agarwala) who believed in me
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and challenged me to be a better person. My journey here and in life would not have been
possible without his encouragement, tolerance and love.
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Table of Contents
Acknowledgements …………………………………………………………………………..… i
Table of Contents ……………………………………………………………………………….. ii
Summary ………………………………………………………………………………….....… viii
List of Tables …………………………………………………………………………………..... x
List of Figures …………………………………………………………………..………...…..... xii
List of Symbols ……………………………………………………………….…………....… xviii
Chapter 1 Introduction ……………………………………………………………………...…… 1
1.1 Photovoltaic effect ……………………………………………………………….....…… 1
1.2 Brief history of photovoltaic …...……………………………………………….……….. 2
1.3 Silicon, III-V and organic solar cell ………...………………………………..………….. 3
1.4 Dye-sensitized solar cell (DSSC) …………....……………………………………….…. 7
1.4.1 Working principle ……………………………………………..…….……...…… 8
1.4.2 Solar cell performance parameters ………………..……..………..……………. 11
1.5 Components of dye-sensitized solar cell ……………… …………………………....…. 14
1.5.1 Metal-oxide semiconductor …………………………………………...…..…… 14
1.5.2 Sensitizer …………………………………………………………...…..………. 24
1.5.3 Electrolyte ……………………………………………………………………… 27
1.5.4 Electrode ……………………………………………………………..............… 31
1.6 Stability of dye-sensitized solar cell ……………………………….…...……………… 32
1.7 Solid-state dye-sensitized solar cell ……………………………...……………....…….. 34
iv
1.8 Dye-sensitized solar cell efficiency ……….......................................………………….. 37
1.9 Synthesis methods for nanomaterials ………………………………..............….……… 38
1.9.1 Vapor-phase synthesis ……………………………………………………...……. 38
1.9.2 Vapor-liquid-solid (VLS) mechanism ……………………………….....……...… 39
1.9.3 Hydrothermal and sol-gel synthesis …………………..………………………….. 39
1.9.4 Microwave and ultrasonic synthesis ……………………..………………….…… 40
References ……………………………………………………………………………………… 42
Chapter 2 Experimental …………………………………………………………...…………… 57
2.1 Materials …………..................................................................................……………… 57
2.2 Sample preparation and DSSC assembly ……….....................................................…… 59
2.4 Characterization techniques …………...............................................................……….. 62
2.4.1 Material morphology ………...............................................................………… 62
2.4.1.1 Scanning electron microscope (SEM) …….......................….…. 67
2.4.1.2 Energy dispersive X-ray (EDX) …………......................…….… 63
2.4.1.3 Tunneling electron microscope (TEM) …….......................….… 63
2.4.2 Material composition ………………………....................................................... 70
2.4.2.1 X-ray diffraction (XRD) ……...................…………………….. 70
2.4.2.2 X-ray photoelectron spectroscopy (XPS) ……..................…….. 71
2.4.2.3 Fourier transform infrared spectroscopy (FTIR) ……................. 73
2.4.2.4 Brunauer–Emmett–Teller (BET) measurement ……..............…. 74
2.4.3 Optical property …………...............................................................................… 76
2.4.3.1 UV-Vis spectroscopy ……...............................................……… 76
v
2.4.3.2 Photoluminescence (Pl) ……………...................................…… 78
2.4.4 Electrical property ……………………….…...........................................……… 79
2.4.4.1 Probe station …………….……………..........................................…….. 79
2.4.5 Photovoltaic device characterization …….........................................………………….. 80
2.4.5.1 Solar simulator ……………….......................................……………….. 81
2.4.5.2 Incident photon-to-current conversion efficiency (IPCE) spectra …....... 83
2.4.5.3 Electrochemical impedance spectroscopy (EIS) ……….……............…. 83
References ………………………………………………………………...........................……. 86
Chapter 3 Mesoporous titanium dioxide (TiO2) film for liquid dye-sensitized solar cell ........... 88
3.1 Introduction …………………………………..............................................…………… 88
3.2 Experimental …………………………………............................................…………… 90
3.2.1 Synthesis of mesoporous TiO2 ……………….......................................……….. 90
3.3 Results and discussion ……………………………..........................................….…….. 91
3.3.1 Effect of polymer concentration …………..........................................….…….. 92
3.3.2 Effect of calcination temperature ……............................................…………… 95
3.3.3 Electrical and optical characterization ……………......................................... 105
3.3.4 Effect of scattering centers on DSSC performance ….................................… 107
3.4 Conclusions ………………………………………………............................................ 118
References ……………………………………………………….....................................……. 120
Chapter 4 Titanium Dioxide (TiO2) nanotubes for liquid dye-sensitized solar cell
(DSSC)….…………………………………………………….................................………….. 123
vi
4.1 Introduction ………………………………………...................................................…. 123
4.2 Experimental …………………………………......................................................…… 125
4.2.1 Anodization of Ti foil ……………………….........................................……... 125
4.3 Results and discussion ……………………………..................................................…. 126
4.3.1 Effect of anodization voltage …………..............................................……..…. 126
4.3.2 Growth mechanism of TiO2 nanotubes …………................................……..… 134
4.3.3 Effect of anodization voltage on DSSC performance …..............................….. 139
4.3.4 Effect of anodization duration on DSSC performance .................................…. 141
4.4 Conclusions………………………................................................................................. 143
References ……………………………….............................................................................…. 144
Chapter 5 Iron Oxide (Fe2O3) nanoflowers for liquid dye-sensitized solar cell (DSSC)........... 146
5.1 Introduction ………………...................................................................................……. 147
5.2 Experimental …………….........................................................................................…. 149
5.2.1 Synthesis of α-Fe2O3 nanoflowers………......................................................… 149
5.3 Results and discussion ……………....................................................………………... 150
5.3.1 Characterization of α-Fe2O3 ………...............................................…………… 150
5.3.2 Effect of varying FeCl3 concentration …………...........................................… 159
5.3.3 Photovoltaic performance of α-Fe2O3 nanoflowers ………..........………….… 165
5.4 Conclusions ………………………………….......................................................…..... 175
References …………………………….............................................................................……. 176
Chapter 6 Quasi-solid-state dye-sensitized solar cell (DSSC) ……….............……………….. 180
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6.1 Introduction …………………………...................................…………………………. 180
6.2 Experimental …………………………………......................................……………… 183
6.2.1 Synthesis of PEO/I2KI/LiI electrolyte …........................................................... 183
6.3 Results and discussion …………….………………………………................……….. 184
6.3.1 Effect of KI concentration ………..............................……….......................… 184
6.3.2 Effect of DPA concentration ……....................................................………….. 186
6.3.3 Effect of KI concentration on DSSC performance ............................................ 188
6.4.4 Effect of DPA concentration on DSSC performance ........................................ 194
6.4.5 Stability of DSSC .................................................................................................. 205
6.4 Conclusions …….......………………………........................................................……. 207
References ………………………………............................................................…………….. 209
Chapter 7 Conclusions …………..……..............................…………………...……………… 212
Chapter 8 Future work ……………………......................................…………………………. 215
Appendix A- Silver (Ag) Nanoparticles .................................................................................... 217
Appendix B- Scattering titanium dioxide (TiO2) particles ........................................................ 227
Appendix C- List of publications related to this thesis …………….....……….......………….. 235
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SUMMARY
Global warming and depletion of fossil fuels have sparked extensive research for clean
energy sources such as solar cells around the world. Dye-sensitized solar cells (DSSC) are
currently subject of intense research as a low-cost photovoltaic device. Among the versatile
group of semiconductors nanostructures used for this device, metal-oxides stand out as one of the
most common and most diverse classes of materials with extensive structural, physical and
chemical properties and functions. The structure, properties and function of small oxide domains,
however, depend sensitively on their sizes and shapes. The nanostructured metal oxides offer
many new opportunities to study fundamental surface processes in a controlled manner and thus
lead to fabrication of new devices.
The main objective of this work is to synthesize nanostructures of titanium dioxide and
iron oxide and understand their growth mechanisms; so that theoretical and practical foundations
for future large-scale production of these nanostructures can be laid. In this work the possibility
of using the synthesized morphologies as new electrode material for DSSC is explored.
Functionality of various nanostructures is investigated through chracterization of microstrcture,
electronic properties and optical properties. The metal-oxide nanostructures are promising
materials to improve the efficiency of DSSC due to enhancement of surface area, light trapping
and efficient electron transport. Solution based synthesis mechanism is adopted to grow these
nanostructures. We understand the role of morphologies on electron transport and thus deduce
the main reason for electron and efficiency loss. Another part of the thesis deals with addition of
different additives and counter ions in iodide based quasi-solid electrolyte system. The work
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involves optimizing the quasi-solid electrolyte in order to be effective in DSSC. We explain and
understand the practical advantages of the new electrolyte systems in comparison to the classic
quasi-solid state electrolytes.
The thesis is organized into eight chapters. Chapter 1 presents an extensive literature
review, necessary to understand the materials and device functioning. Chapter 2 introduces the
materials and methods applied in the present study. This includes synthesis of materials,
fabrication of solar cell device and characterization techniques to investigate the working of the
nanostructures and device performance. Chapter 3 describes the use of high surface area
mesoporous TiO2 film in DSSC. This chapter also discusses the influence of incorporating
scattering centers in the TiO2 film and its effects on the solar conversion efficiency of DSSC.
Chapter 4 discusses the process of anodization of titanium foil to synthesize TiO2 nanotubes.
Effect of TiO2 nanotube parameters and influence of silver (Ag) nanoparticles is studied on the
efficiency of DSSC. Chapter 5 demonstrates a material shift for the photo-electrode by
discussing Fe2O3 nanoflowers and their effects on DSSC. This chapter discusses the possible
growth mechanism of such unique nanostructures and finds a relation between the structure and
its function in DSSC. The main aim of chapter 6 is to develop a stable and filler-free quasi-solid
state electrolyte system. Results and discussion section of this chapter throws light on the
intricate working of this novel LiI and KI based iodide electrolyte. Chapter 7 concludes the
thesis and finally, chapter 8 gives some directions for future work.
x
List of Tables
Table 2.1 List of materials used in this work.............................................................................57
Table 3.1 Physical properties of nanoparticles, mesoporous and mesoporous/P25 films ...... 113
Table 3.2 Photovoltaic characteristics for different films measured under illumination with
AM 1.5 simulated sunlight …………………......................................................... 118
Table 4.1 Photovoltaic characteristic of TiO2 nanotubes grown at different anodization
voltage .................................................................................................................... 141
Table 4.2 Photovoltaic characteristic of TiO2 nanotubes grown for different anodization
time ........................................................................................................................ 143
Table 5.1 BET results for different samples of α-Fe2O3 ……............................................... 164
Table 5.2 Quantity of dye absorbed and photovoltaic characteristics of 0.04 and 1 mM
sample …………………………………………………………………………… 170
Table 6.1 DSSC performance at various KI loadings in the quasi-solid electrolyte ……..... 190
Table 6.2 Computed ionic conductivities for various electrolyte systems …....................... 191
xi
Table 6.3 Fitted impedance parameters for the fresh and aged DSSC with 14.5 wt % KI in the
electrolyte ………………………………….......................................................… 194
Table 6.4 Photovoltaic characteristics of DSSC at various DPA loadings in the quasi-solid-
state electrolyte …………….......................................................................……… 196
Table 6.5 Fit results of impedance spectra for DSSC for DPA free and 0.004 g DPA
incorporated electrolyte system......………………………………………….…… 203
xii
List of Figures
Figure 1.1 (a) Schematic diagram depicting various components (b) Energy level diagram
outlining various processes of dye-sensitized solar cell .…………................…. 11
Fig 1.2 I-V graph of a DSSC showing various parameters of performance...................... 13
Fig 1.3 Schematic diagram showing the Ru dye anchored to TiO2 nanoparticle ............. 16
Fig 2.1 Procedure for DSSC assembly: (a) setup for ‘doctor blading’ the semiconductor
film, (b) coated electrode after annealing, (c) dye loaded semiconductor film and
(d) the sandwich assembly of the coated FTO and Pt-coated electrode .............. 62
Fig 2.2 (a) Picture of SEM (courtesy: Multidisciplinary laboratory, Engineering Science
program, National University of Singapore). (b) Schematic showing the layout of
components inside SEM ………………………………….....................………. 64
Fig 2.3 Schematic diagram showing interaction between electron beam and specimen.. 66
Fig 2.4 (a) Picture (courtesy Institute of Materials Research and Engineering IMRE,
A*Star, Singapore) and (b) Schematic showing the layout of components inside
TEM …................................................................................................................. 69
Fig 2.5 Schematic diagram depicting the technique of x-ray
diffraction…………................................................………………………….…. 71
Fig 2.6 Picture of XPS (image courtesy Surface science laboratory, physics department,
National University of Singapore) ……..................……………………………. 72
Fig 2.7 Schematic depicting the working of FTIR ………………………..………..…... 74
xiii
Fig 2.8 Picture of BET equipment used to obtain surface area and pore distribution
information (image courtesy Multidisciplinary laboratory, Engineering Science
program, National University of Singapore) ……………………….....……….. 75
Fig 2.9 UV-Vis spectrophotometer (a) the equipment and (b) the sample stage for both
solid and powder samples (image courtesy Multidisciplinary laboratory,
Engineering Science program, National University of Singapore) ..................... 77
Fig 2.10 Image of 4-probe station (courtesy: Multidisciplinary laboratory, Engineering
Science program, National University of Singapore) .......................................... 80
Fig 2.11 Solar simulator (a) the equipment and (b) the sample stage covered with black
box (image courtesy Engineering design studio, Engineering Science program,
National University of Singapore) ……………………….…………………….. 82
Fig 2.12 Model used for EIS measurements for DSSC ………….............………………. 85
Fig 3.1 Schematic diagram showing the pore formation in TiO2 ……….....…………… 93
Fig 3.2 SEM images of different pore morphologies by varying copolymer concentration
(a and b) quasi-hexagonal and (c) lamellar pores ……………….......…………. 95
Fig 3.3 HRTEM images and corresponding SAED patterns of mesoporous TiO2 after
annealing at (a) 300 °, (b) 550 ° and (c) 430 °C for 15 min ................................ 97
Fig 3.4 SEM images of mesoporous TiO2 film obtained after calcination at (a) 300 ° (b)
430 ° and (c) 500 °C for 15 min ……..........................................………………. 98
Fig 3.5 TEM images of mesoporous TiO2 annealed at 430 C (a and b) low and (c) high
resolution and (d) TEM image of film annealed at 550 C. Inset shows the
selected electron diffraction pattern of the film annealed at 430 C …...........… 99
xiv
Fig 3.6 Wide angle XRD pattern obtained from mesoporous TiO2 film after calcination at
430 °C for 15 min ……….................………...............…………………..…… 100
Fig 3.7 XPS spectra of Ti2p for (a) as-synthesized film, (b) film calcined at 430 °C and
of O1s for (c) as-synthesized film and (d) film calcined at 430 °C …............... 102
Fig 3.8 (a) FTIR spectra of (i) as synthesized and (ii) after calcination at 430 °C and (b)
TGA plot for the mesoporous TiO2 film containing P123 amphiphilic triblock
copolymer .......................................................................................................... 104
Fig 3.9 (a) IV curves and (b) CV hysteresis for mesoporous TiO2 …………........…… 106
Fig 3.10 UV-Vis transmittance spectrum for mesoporous TiO2 film ……....…..…...…. 107
Fig 3.11 SEM images of (a) low and (b) high magnification mesoporous TiO2 film. SEM
images of (c) top and (d) cross sectional views of mesoporous/P25 film ......... 109
Fig 3.12 XRD patterns of nanoparticles, mesoporous and mesoporous/P25 mixture TiO2
films. ‘A’ represents the anatase and ‘R’ the rutile phase of TiO2 ……...…… 110
Fig 3.13 Nitrogen adsorption-desorption spectra of nanoparticles, mesoporous and
mesoporous/P25 films ………………………………..........................……….. 113
Fig 3.14 Optical properties of nanoparticles, mesoporous and mesoporous/P25 films: (a)
UV-Vis absorption of dye loaded samples and (b) diffused reflectance spectra 115
Fig 3.15 Performance of DSSC made from nanoparticles, mesoporous and
mesoporous/P25 films: (a) Current density–voltage and (b) IPCE curves ........ 117
Fig 4.1 SEM images of TiO2 nantubes grown for 2 h at different voltages of (a) 20, (b)
30, (c) 40 and (d) 50 V …………............................................………………. 127
Fig 4.2 SEM images of TiO2 nanotubes grown at 60 V for 2 h (a) top, (b) tilted and (c)
cross-sectional view…………………….......................................……………. 129
xv
Fig 4.3 (a) Graph showing variation of TiO2 nanotube diameter and length with
anodization voltage. (b) and (c) TEM images of an individual nanotube ......... 130
Fig 4.4 XRD spectrum of TiO2 nanotubes showing the anatase composition ……….. 131
Fig 4.5 SEM images of (a) bottom surface, (b) top surface with nanograss and (c) top
view of TiO2 nanotubes grown on Ti foil ………………….....................……. 133
Fig 4.6 Anodization setup for the growth process of TiO2 nanotubes ........................... 135
Fig 4.7 Schematic for the growth process of TiO2 nanotubes …….........…….......…… 137
Fig 4.8 Current-time graph for anodization process of TiO2 nanotubes …………….... 138
Fig 4.9 J-V curve for the DSSC made with TiO2 nanotubes grown at different anodization
voltages ……………………..........................................……………………… 140
Fig 4.10. (a) Variation of tube length and (b) J-V curve for TiO2 nanotubes grown at
different anodization time ……......…………………………………………… 142
Fig 5.1 FESEM images at (a), (b) low magnification and (c), (d) high magnification of
nanoflower of -FeOOH. (e) and (f) -Fe2O3 nano-flowers after calcination at
400 C ................................................................................................................ 151
Fig 5.2 TEM images of (a), (b) nanoflowers and (c), (d) petal like structures originating
from nanoflowers ............................................................................................... 152
Fig 5.3 (a) TEM, (b) HRTEM and (c) SAED pattern of as-obtained -FeOOH
nanoflowers. (d) TEM, (e) HRTEM and (f) SAED pattern of - Fe2O3
nanoflowers after calcination at 400 C for 2 h ……................………………. 153
Fig 5.4 XRD pattern of (a) as-obtained -FeOOH and (b) -Fe2O3 nanoflowers …..... 154
xvi
Fig 5.5 (a) Absorption and (b) Transmission spectrum of 0.04 mM sample, (c) Plot of
(αh)1/2
versus photon energy for indirect transition and (d) Plot of (αh)2 versus
photon energy for indirect transition ……………........……………………….. 156
Fig 5.6 FESEM images of the products obtained after growth at 120 C for (a) 0.5 h, (b) 1
h, (c) 2 h, (d) 3 h, (e) 4 h and (f) 6 h.………………………………………….. 158
Fig 5.7 TEM images for samples prepared at 120 C for different reaction time: (a) 0.5 h,
(b) 1 h, (c) 4 h and (d) 6 h.………............................................……………...... 159
Fig 5.8 SEM images of samples with various FeCl3 concentrations: (a, b) 0.04 mM, (c, d)
0.12 mM, (e, f) 0.4 mM and (g, h) 1 mM………………….................……….. 161
Fig 5.9 Nitrogen adsorption-desorption isotherms at 77 K for nano-flowers produced at
different FeCl3 concentrations …...................................................................… 165
Fig 5.10 (a) J-V characteristics, (b) IPCE curves of DSSC and (c) UV-vis absorption
spectra of dye loaded samples made with 0.04 and 1mM FeCl3 concentration..169
Fig 5.11 Variation of (a) Z’, (b) Z” as a function of frequency and (c) Nyqusit plots of the
doctor bladed films with 0.04 and 1 mM FeCl3 concentration ...........................172
Fig 6.1 XRD spectra of PEO/LiI based electrolyte with various KI loading ..................186
Fig 6.2 (a) Absorption spectrum for the 0.004 g DPA loaded electrolyte and (b) XRD
spectra of PEO/KI/LiI based electrolyte with and without DPA loading .......... 188
Fig 6.3 I-V curves for DSSC using quasi-solid electrolyte at various KI loading. Inset
shows the calculated cell efficiencies at different wt % of KI ………...........… 189
Fig 6.4 (a) Nyquist plots and (b) the Bode plots of the fresh and aged quasi-solid state
DSSC. Inset shows the equivalent circuit model of the DSSC …...................... 193
xvii
Fig 6.5 J-V characteristics of DSSC employing quasi-solid electrolyte with different
amount of DPA. Inset shows the variation of conversion efficiency with DPA
loading ................................................................................................................ 197
Fig 6.6 IPCE curves of DSSC employing quasi-solid electrolyte with 0.0 and 0.0004 g
DPA .…..........…………………………………………………………….…... 198
Fig 6.7 (a) The equivalent circuit model of the DSSC, (b) Nyquist plots and (c) zoomed
spectra in the high frequency region for 0.0 and 0.004 g DPA added quasi-solid
electrolyte DSSC ................................................................................................ 201
Fig 6.8 Forward bias dependence of (a) transport resistance (Rt), (b) charge transfer
(recombination) resistance (Rct) and (c) chemical capacitance (Cμ) of DSSC with
no and 0.004 g DPA ……………………...…...............................................…. 204
Fig 6.9 (a) DSSC performance of five devices tested for reproducibility. (b) Stability for
the quasi-solid state DSSC containing 0.004 g DPA ......................................... 206
xviii
List of Symbols and Abbreviations
hν Photon energy
h Planck’s Constant (6.62 ×10-34
W.s)
c The speed of light (3 ×108 ms
-1)
q Charge constant of an electron 1.6021 ×10-19
C
Voc Open-circuit photovoltage
Isc Short-circuit current
Jsc Short-circuit current density
FF Fill factor
Ef Fermi level
Pin Intensity of incident light
Pmax Maximum power
σ Cross-section
ε Extinction coefficient
SBET BET specific surface area
xix
ρ Density
Å Angstrom (10-10
m)
α Absorption coefficient
η Overall solar-to-electrical-energy conversion efficiency
Eg Band gap of semiconductor
d Electron transport or collection time
n Electron lifetime
Rt Charge transport resistance
Rct Charge transfer (recombination) resistance
Rb Bulk resistance
Cμ Chemical capacitance
Ln The effective diffusion length
Dn Diffusion coefficient
eV Electron volt
DSSC Dye-sensitized solar cell
SEM Scanning electron microscope
xx
TEM Tunneling electron microscope
IPCE Incident photon-to-current conversion efficiency
EIS Electrochemical impedance spectroscopy
BET Brunauer-Emmett-Teller measurement
BJH Barret-Joyner-Halenda
HOMO Highest occupied molecular orbital
LUMO Lowest unoccupied molecular orbital
MLCT Metal-to-ligand charge transfer
SAED Selected area electron diffraction
FTIR Fourier Transform Infrared
XPS X-ray photoelectron spectroscopy
XRD X-ray Diffraction
1D One dimensional
3D Three dimensional
1
Chapter 1: Introduction
Our reliance on fossil fuels has cost us dearly. Not just the fossils are depleting at
an alarming rate, but have been blamed for the climate change. It has become inevitable
to search for alternative sources of energy to meet world‟s ever-increasing demands.
Amongst the many novel technologies available, solar energy is the cynosure of all eyes
and this thesis will focus on this clean energy. This chapter discusses the contruction,
working, performance of dye-sensitized solar cell (DSSC) along with detailed discussion
on each component of DSSC like the semiconductor, dye, electrolyte and electrode.
Various synthesis methods available for production of semiconductor materials for DSSC
have also been reviewed.
1.1 Photovoltaic effect
The conversion of sunlight into electricity is called the photovoltaic effect. During
the process, a photon excites an electron from valence band of the semiconductor to the
conduction band, leaving a hole behind. After the generation of the electron-hole pair, the
separation of the electron from the hole takes place, eventually transporting it through an
external circuit. This constitutes the production of the electricity. Among all these
processes, electron-hole separation is the most crucial because the excited electron-hole
pair recombines spontaneously as the system wants to be electrically neutral. In a
conventional silicon solar cell, the separation of the charges takes place at the p-n
junction [1]. The depletion of majority carriers on both sides of the junction causes a
2
potential barrier to develop. This built-in potential causes a barrier for majority carrier
motion, while providing a low resistance path to minority carriers. Minority carriers are
photogenerated in p and n layers and reach the junction by diffusion. However, since
electron-hole separation and electron transport take place in a single semiconductor,
electrons can still be captured by defects before being transported to an external circuit.
This leads to low light-to-electricity conversion efficiency. To prevent the recombination
of electrons at defects, a silicon solar cell relies on high quality crystal wafers, which in
turn dramatically increases the manufacturing cost.
1.2 Brief history of photovoltaic
The French physicist Alexandre-Edmond Becquerel first discovered the
phenomenon of the conversion of light to electricity in 1839, known as the photovoltaic
effect [2]. Willoughby Smith later discovered the photovoltaic effect in selenium in 1873
[3]. In 1876 William G. Adams found out that illuminating a junction between selenium
and platinum also gives photovoltaic effect [4]. These two discoveries made the
foundation of the first selenium solar cell in 1877. In 1904, Albert Einstein theoretically
explained the photovoltaic effect [5] and was later awarded with Nobel Prize for his work
in 1921. In 1918, a Polish scientist Czochralski discovered a method for single crystalline
silicon production, which enabled the production of monocrystalline solar cells in 1941
[6]. D. M. Chapin and C. S. Fuller developed the modern solar cells in 1954 at Bell Labs
using a solid-state semiconductor junction [7]. In 1974, the Japanese Sunshine project
commenced, which was followed by establishment of Solec International and Solar
3
Technology International in 1975. Many important events in the field of photovoltaics
appeared in 1980. Researchers of the University of New South Wales in Australia
constructed a solar cell with more than 20 % efficiency in 1985. In 1990 Siemens bought
ARCO Solar and established Siemens Solar Industries, which now is one of the biggest
photovoltaic companies in the world. Solar Energy Research Institute (SERI) founded in
1977 was later renamed National Renewable Energy Laboratory (NREL). During 2000
and 2001 production of Japanese producers increased significantly. Companies like
Sharp, Sanyo and Kyocera produce solar modules with peak power, which is equivalent
to the annual consumption of Germany.
1.3 Silicon, III-V and organic solar cell
Today the photovoltaic market is dominated by silicon solar cells that utilize in
multicrystalline and monocrystalline forms. Silicon solar cell research is mainly
concentrated on using thin-film crystalline silicon, which avoids the costly crystal
growing and sawing processes. Silicon solar cells have some limitations as they require
good crystal quality, have low light absorption and are not compatible with flexible
substrates. Another althernative for polycrystalline silicon is amorphous silicon.
Amorphous solids, like common glass, are materials in which the atoms are not arranged
in any particular order. Such materials contain large numbers of structural and bonding
defects. It was not until 1974 that researchers began to realize that amorphous silicon
could be used in photovoltaic devices by properly controlling the conditions under which
it was deposited. Efficiency for amorphous silicon cells has now reached the order of 10
4
%. Amorphous silicon absorbs solar radiation 40 times more efficiently than single-
crystal silicon, so a film only about 1 micron thick can absorb 90 % of the usable solar
energy. This is one of the key features of this cell. Although the efficiency figure for
amorphous silicon solar cells is lower than crystalline silicon, they are favoured due to
low cost. Today, amorphous silicon is commonly used for solar-powered consumer
devices that have low power requirements (e.g., wrist watches and calculators). Other
principal economic advantages are that amorphous silicon can be produced at lower
temperature and can be deposited on low cost substrates. These characteristics make
amorphous silicon the leading thin-film photovoltaic material. From a solid-state physics
point of view, silicon is not an ideal material for photovoltaic conversion for two reasons:
There is a small spectral mismatch between absorption and the semiconductor and the
Sunlight spectrum, approximated by a black body of 5900 K. A much more serious point
is that silicon is an indirect semiconductor, meaning that valence band maximum and
conduction band minimum are not opposite to each other in k-space. Light absorption is
much weaker in an indirect gap semiconductor than in a direct semiconductor. This has
serious consequences from the materials point of view: for a 90 % light absorption it
takes only 1 μm of GaAs (a direct band gap semiconductor) versus 100 μm of Si. The
photogenerated carriers have to reach the pn-junction, which is near the front surface.
The diffusion length of minority carriers has to be 200 μm or at least twice the silicon
thickness. Thus, the material has to be of very high purity and of high crystalline
perfection. In view of these physical limitations, a lot of effort has been invested into the
search for new materials.
5
The ideal solar cell materials should have a direct band gap in the range of 1.1-1.7
eV. Furthermore, the material should be readily available, non-toxic and easily
processible. The synthesis and deposition techniques should be suitable for large area
production. Alternatively to the search of new inorganic semiconductor materials, other
device geometries have been developed for the purpose of light to electron conversion,
such as various concentrating systems including III/V-tandem cells. Other inorganic
materials that have been used for photovoltaic devices are copper indium diselenide (CIS)
and cadmium telluride. The interest now has expanded towards CuInSe2, CuGaSe2,
CuInS2 and their multinary alloys Cu(In,Ga)(S,Se)2. These films either require two
separate deposition steps followed by annealing or co-evaporation techniques. Laboratory
efficiencies for a small area of these devices are approaching 19 % and large area
modules have reached 12 %. Cadmium telluride solar cells, which show only slightly
lower efficiency, also offer great promise.
In the last few years, the search for new materials has been extended into the field
of organic molecules and polymers, which offer several advantages compared to
inorganic materials. Organic materials are chemically tunable to adjust physical
properties, such as band gap, valance and conduction energies, charge transport,
solubility and morphological properties. Moreover, their processing is easy and well
established. They can be processed through wet-processing (spin coating, cast coating,
ink-jet printing and roll-to-roll processing) as well as dry processing techniques (thermal
evaporation). Organic materials are required in small quantites and can be mass-
produced. This makes organic materials advantageous from economical point of view.
6
Organic photovoltaic solar cells therefore bear an important potential of development in
the search for low-cost modules for the production of domestic electricity. Organic
charge transport materials have either molecular or polymer structure. While charge
transport in molecular systems occurs intermolecularly from molecule to molecule,
polymer charge transport proceeds intramolecularly, along the polymer chain. The field
of organic photovoltaics started with single layers of an organic semiconductor, deposited
between electrodes made up of two different metals, to produce a rectifying device.
These devices produced low efficiencies of 0.01 % [8]. These numbers improved
dramatically when Tang et al. introduced a thin film organic solar cell composed of a
donor–acceptor (DA) heterojunction in1986, yielding efficiency of 0.95 % [9]. The DA
interface allows for efficient dissociation of excitons in comparison to a single organic
layer. The polyacenes [10-12] such as pentacene and tetracene, and the metal
phthalocyanines (MePc), such as CuPc or ZnPc, are among the most studied donor
materials. For acceptors, perylene compounds like perylenetetracarboxylic bis-
benzimidazole (PTCBI) and C60 are commonly used [13-15]. Another promising
combination for organic solar cells is poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl
C61-butyric acid methylester (PCBM) blend. It is the most efficient fullerene derivate-
based donor-acceptor copolymer, which can give solar conversion efficiencies up to 6 %
[16-17].
7
1.4 Dye-sensitized solar cell (DSSC)
Higher costs of silicon solar cells have forced researchers to look for new low cost
materials and devices. The most well known and researched low cost solar cells is dye-
sensitized solar cell developed by O‟Regan and Grätzel in 1991 [18]. Record efficiencies
up to 12 % for small cells and approximately 9 % for mini-modules, and promising
stability data has been accomplished [19]. DSSC may be considered a technology
between the second and third generations of solar cells. It has the potential to become a
third generation technology utilizing the nanoscale properties of the device. Although a
number of issues plague this device, it‟s main advantages of low cost, cheap materials
and ease of fabrication cannot be ignored. The dye, an organic compound, has emerged
as one of the primary alternatives to solid-state semiconductors. Unlike silicon solar cells,
where the semiconductor assumes the role of light absorption, charge separation and
charge transport, these functions are well separated among different materials in a DSSC.
Light is absorbed by a sensitizer, which is anchored to the surface of a wide bandgap
semiconductor. Charge separation takes place in the dye via photo-induced electron
injection from the dye into the conduction band of the solid. Charge is transported in the
conduction band of the semiconductor to the charge collector. DSSC is a unipolar device,
where only electrons flow through the system, and hence the working of this device is
very much different from a silicon solar cell. The main difference between DSSC and
conventional solar cells is that the phenomenon of light adsorption and charge carrier
transport are separated in DSSC, whereas both processes are carried out by the
semiconductor in the conventional cell. Secondly, an electric field is necessary for charge
8
separation in the p-n junction cell. Nanoparticles in DSSC are too small to sustain a built-
in field; causing the charge transport to occur via diffusion rather than drift. Lastly, both
minority and majority charge carriers coexist in a p-n junction. This makes the
conventional solar cells sensitive to any trap impurities that may lead to recombination.
DSSC are majority charge carrier devices in which the electron transport occurs in the
semiconductor film and the regenration of dye occurs through ions in the electrolyte.
Recombination processes can therefore only occur in the form of surface recombination
at the interface.
1.4.1 Working principle
Every solar cell has three main processes: light absorption, charge separation and
charge transport. DSSC is a two-electrode cell, where a layer of wide bandgap
semiconductor anchored with organic dye and electrolyte system is sandwiched between
two transparent conductors. The light or photon absorption takes place in a layer of
organic dye, called the sensitizer. This causes the excited electron to jump from the
highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital
(LUMO), which is then injected into the conduction band of the semiconductor [20].
These electrons travel through the semiconductor film to the fluorine doped tin oxide
(FTO) back electrode. In DSSC, the electricity is generated on the photoelectrode, which
consists of a nanoporous network of metal oxide semiconductor. This film of metal oxide
is sensitized with a monolayer of visible light absorbing dye. The electrons then travel
through the external circuit to the counter electrode. The photoelectrode is in contact with
9
counter electrode by a layer of redox electrolyte. The counter electrode is a platinum
coated FTO, where electrons reduce the
3I back to I and the whole cycle is repeated.
After the injection, the oxidized dye is reduced by found in the electrolyte. The
funcion of the electrolyte‟s redox couple is to transfer the positive hole, generated in the
electronic excitation process of the dye, to the counter electrode. This is achieved via
alternating oxidation states on the photoelectrode. Here the reduced species of the redox
couple gives an electron to the ground state where it is ready to absorb more photons. The
redox species gets oxidized, and diffuses to the counter electrode. The procedure is
illustrated in the schematic of Fig 1.1 and the whole process of sensitization and
electrolyte oxidation/reduction can be summarized by the following equations [21].
*ShS Metal to ligand charge transfer excitation (1.1)
)(* cbeASAS Electron injection (1.2)
IeI 323 Tri-iodide reduction (1.3)
3232 ISIS Dye regeneration (1.4)
The rate of electron injection has been shown to depend on a variety of parameters, such
as the length of the spacer between electron donor and acceptor [22], the density of
acceptor states [23], and the electronic coupling between the dye and the semiconductor.
[24]. Moser et al. has shown that the incident wavelength has an influence on the
quantum yield of the electron injection, implying that electron injection can take place
from hot vibrational states [25]. The competition between the relaxation of the dye
excited state and the injection reaction is contradictionary to Kasha‟s rule [26]. The
electron movement in the nanocrystalline TiO2 to the back contact is significantly slower
in the TiO2 single crystal. This has been demonstrated by photocurrent transient
I
10
measurements after UV illumination of TiO2 particles, which showed a decay time of
milliseconds to seconds [27]. Back reaction of electrons in the TiO2 with the oxidised
ruthenium complex is extremely slow, occurring typically in the microsecond time
domain. This slow recombination rate has been assigned to the weak overlap of the d-
orbital, localised on the Ru metal and the TiO2 conduction band. The dynamics of the
interception of the oxidised dye by the hole conductor have been found to proceed with a
broad range of time constants from 3 ps < τ < 1 ns [28]. Multiple phases of injection
process are assigned to the heterogeneous nature of the heterojunction, incomplete pore
filling and the thereby resulting in lateral hole migration between neighbouring dye
molecules not in contact with the regeneration material [29-30]. The hole injection
quantum yield has been calculated to be 50 % after 900 ps. At longer times, around 10 ns,
the efficiency of hole transfer approaches unity.
11
Figure 1.1 (a) Schematic diagram depicting various components (b) Energy level diagram
outlining various processes of dye-sensitized solar cell.
1.4.2 Solar cell performance parameters
In order to compare and ascertain the performance of various solar cells, a few
parameters are defined. These parameters throw light on various mechanisms and process
12
of the device and also it‟s overall performance. When the solar cell is operated at the
open circuit, I=0, and the voltage across the output terminals is defined as the open
circuit voltage (Voc). When the electrons are injected into the conduction band of the
semiconductor, the electron density increases, raising the Fermi level towards the
conduction band edge. This shift in the Fermi level is responsible for the generation of
the photovoltage in the external circuit. Thus Voc is related to the energy difference
between the Fermi level of the semiconductor and the redox couple in the electrolyte
[20]. The short circuit current (Isc) is the maximum current obtained under the short THE
circuit condition. The overall solar-to-electrical-energy conversion efficiency () for a
solar cell is given by the following equation [20]:
i n
o cs c
P
FFVJ (1.5)
where Jsc is the short-circuit current density, FF the fill factor and Pin the intensity of the
incident light. The FF is defined as the ratio of maximum power (Pmax) of the cell per unit
area divided by the product Voc and Jsc. The FF can assume values between 0 and 1. It is
attributed to the functioning of the metal oxide/electrolyte interface. The higher the
recombination of the conduction band electrons with the acceptor species in the
electrolyte, the lower is the FF [20]:
o cs cVJ
PF F m a x (1.6)
The Pmax is defined as the product of photocurrent and photovoltage, at the voltage where
the power output of the cell is maximal. Apart from the power conversion efficiency,
another fundamental parameter used to evaluate the performance of the solar cell is
„external quantum efficiency‟ (QE). It is also referred to as incident photon-to-current
13
conversion efficiency (IPCE). The IPCE value corresponds to the photocurrent density
produced in the external circuit under monochromatic illumination divided by the photon
flux striking the cell. IPCE provides valuable and practical information about the
monochromatic quantum efficiencies of the solar cell [20]:
)(
)(1240
i n
sc
P
JIPCE (1.7)
Fig 1.2. I-V graph of a DSSC showing various parameters of performance.
By observing the shunt and series resistances of the solar cell, a lot can be
deduced. As the series resistance (Rs) of the device increases, the voltage drop between
the junction voltage and the terminal voltage (V) becomes greater for the same flow of
current. This causes the current-controlled portion of the I-V curve to sag toward the
origin, producing a significant decrease in V and slight reduction in Isc. Very high values
14
of Rs also produce a significant reduction in Isc. When the series resistance dominates the
device performance, the behavior of the solar cell resembles that of a resistor. Series
resistance loss becomes more important at high illumination intensities. As the shunt
resistance (Rsh) decreases, the current diverted through the shunt resistor increases for a
given level of junction voltage. The result is that the voltage-controlled portion of the I-V
curve begins to sag toward the origin, producing a significant decrease in the Isc and
slight reduction in Voc. A badly shunted solar cell takes on operating characteristics
similar to those of a resistor.
1.5 Components of dye-sensitized solar cell
1.5.1 Metal-oxide semiconductor
Semiconductor films are the heart of a DSSC device. The key requirements for a
good semiconductor film are high surface area and high transparency [31]. The electron
collection at the transparent electrode requires the transport of the electrons in the
semiconductor film to be fast, so that the electrons do not undergo recombination. This
process depends on the nature and nano-morphology of the metal oxide semiconductor.
The migration of electrons within the semiconductor framework to the transparent
conducting electrode involves charge-carrier percolation over the porous particle
network. The electrons are involved in random walk via trapping/detrapping mechanism
[32]. This is an important process, which leads to nearly quantitative collection of
injected electrons. Metal oxide films are commonly synthesized through a solution based
15
method. Low cost, good availability and non-toxic nature of metal oxides make them
even more attractive for the applications. The nanoporous structure of the semiconductor
permits the specific surface concentration of the sensitizing dye to be sufficiently high for
total absorption of the incident light. At present, titanium dioxide (TiO2) gives the highest
recorded efficiencies [33], but many other metal oxide systems, such as zinc oxide (ZnO)
[34], tin oxide (SnO2) [35], niobium oxide (Nb2O5) [36] have been tested. Besides these
simple oxides, some mixed oxides like strontium titanate (SrTiO3) [37] and zinc stannate
(Zn2SnO4) [38] have been investigated as well. The low performance of other oxide
semiconductors may be explained by different band structures that influence the electron
density and electron injection. For example, the band structure of Nb2O5 has the same
bandgap energy as TiO2 (3.2 eV) but has its conduction band is approximately 0.2-0.3 eV
more negative. It is, in fact, below the LUMO of the dye. Thus, under visible light,
electron injection from dye occurs on TiO2 but not on Nb2O5 [36]. The effective mobility
of TiO2 is found to increase strongly with carrier concentration. This is coherent with the
trapping/detrapping mechanism of electron transport in DSSC, i.e. movable electron
concentration increases when traps are filled or the barrier height decreases as the Fermi
level is raised. This may be one of the reasons why TiO2 is the best choice for DSSC as
the electron injection by dye-sensitization increases the electron concentration.
16
Fig 1.3 Schematic diagram showing the Ru dye anchored to TiO2 nanoparticle.
Grätzel et al. in 1991 made an impressive breakthrough of DSSC by using
semiconductor films consisting of nanometer-sized TiO2 particles sensitized with a
trimeric ruthenium complex, announcing overall conversion efficiencies of 7 % [18]. The
superiority of nanoparticle films with a high surface area over a 10 μm flat film is
demonstrated. The nanoparticle film showed a porosity of 50-65 % and gave rise to
almost 2000-fold increase in the surface area. In addition, the anatase TiO2 with exposed
(1 0 1) planes played a key role for good connection between the TiO2 and ruthenium-
based dye molecules. This enabled the dye molecules to form high-density monolayer on
the nanoparticle surface, and allowed the electrons in dye molecules to inject efficiently
into semiconductor [18, 32]. TiO2 is a stable, nontoxic oxide, which has a high refractive
index (n=2.4) and is widely used as a white pigment in paint, toothpaste, sunscreen and
self-cleaning materials. Several crystal forms of TiO2 occur naturally, the most common
17
being rutile, anatase, and brookite. Rutile is the thermodynamically most stable form.
Anatase is, however, the preferred structure in DSSC, because it has a larger bandgap
(3.2 eV) and higher conduction band edge energy. This leads to a high Fermi level and
high Voc in DSSC.
Since the start of DSSC, mesoporous TiO2 has been widely studied, due to its high
surface area. Hore et al. compared the base-catalyzed and acid-catalyzed mesoporous
TiO2. He concluded that although the former gave slower recombination and higher Voc,
the latter had better dye adsorption [39]. Changing the amount of polymer in the solution
can control the porosity of the resulting mesoporous film. Higher porosities lead to less
interconnects between the particles and a decrease in charge collection efficiency. Since
1991, large improvement to the TiO2 electrode has been made in terms of light
absorption, light scattering, charge transport, suppression of charge recombination, and
improvement of the interfacial energetic. Well-ordered mesoporous TiO2 structures with
a narrow pore size distribution can be obtained using polymertemplated synthesis.
Zukalova et al. [40] prepared ordered mesoporous TiO2 nanocrystalline films via layer-
by-layer deposition with Pluronic P123 as a template. He found that the mesoporous TiO2
films showed an enhanced solar conversion efficiency resulting from a remarkable
enhancement of Isc, due to the huge surface area accessible to both the dye and the
electrolyte. Kim et al. introduced a long-range ordered mesoporous TiO2 film between
the TiO2 nanoparticle layer and FTO conducting glass substrate during DSSC fabrication
and found Jsc is increased from 12.3 to 14.5 mAcm-1
. The main preparation recipes apply
titanium inorganic salts or titanium alkoxides as titanium sources, which are hydrolyzed
18
and condensed to form Ti-O- networks in the presence of small molecule surfactants or
block copolymers as structure-directing agents [41-42]. When synthesizing various
morphologies for the DSSC, it is difficult to isolate the effect of structure on the device
performance because of the different procedures usually required to create the various
nanostructures. For instance, the crystal size, defect density, and surface potential are
likely to be different between one-dimensional arrays of anodized TiO2 nanotubes as
compared to unoriented films fabricated from sol-gel processed sintered nanoparticles.
One very promising way to produce a range of controlled and highly ordered
morphologies in the same material system and on the same scale is to replicate self-
assembled structures from sacrificial templating systems [43-45]. Diblock copolymers,
consisting of two chemically distinct tethered polymer chains, exhibit self-assembly into
a range of periodically ordered microphases on the 10 nm length scale. This class of
material offers a unique toolbox for exploring nanotechnology applications since, with a
given polymer chemical makeup, we have access to a number of microphase
morphologies by simply tuning the relative volume fraction of each block [46]. Selective
removal of one component of a block copolymer after the phase separation leaves behind
a mesoporous template [47]. For example, a standing array of nanowires can be formed
from cylinder-forming copolymers by first aligning the microphase in an external electric
field [48-49].
Apart from diblock copolymers, the Pluronic family of PEO-PPO-PEO triblock
copolymers has also been widely used as a template [50-54]. However, compared to the
Pluronic copolymers, other kinds of block copolymers have been mainly used to template
19
non-TiO2 systems [55-56] and until recently only a limited number of results about
templating TiO2 nanostructures have been reported [57-59]. For example, Smarsly et al.
used PHB-b-PEO (H(CH2CH2CH2CH(CH3)-CH3)66(OCH2CH2)86H) diblock copolymer
as the structuredirecting agent to prepare crystalline TiO2 mesoporous structures [60].
Wang et al. applied PS-b-PEO as the template and obtained foam like bicontinuous TiO2
nanostructures [59]. Weisner et al. recently reported the results using PS-b-PEO as a
template to synthesize nanocomposite films composed of ordered TiO2 nanoparticles [60-
61]. Toluene and 2-propanol were used as the solvents, in which the block copolymer
could undergo microphase separation to form micelles in the solution, and the titanium
sol-gel precursor is subsequently incorporated into the micelles. With respect to the
structure formation process, Eisenberg and co-workers intensively studied the self-
assembly behavior of amphilic block copolymer PS-b-PAA or PS-b-PEO in solution,
where PAA and PEO are the minority parts of the block copolymer [62]. The block
copolymers are dissolved into 1,4-dioxane, or DMF, which is a good solvent for both
blocks, followed by slow addition of water, a selectively poor solvent for the PS block.
As a result of the slow addition so-called crewcut micelles with large PS cores and thin
PEO or PAA coronas are formed in solution. The morphologies of the crew-cut micelles
are controlled by a force balance between different factors: the stretching degree of the
core-forming block, the interfacial force between the core and surrounding solvents, and
interactions between the coronas. Eisenberg and co-workers adjusted the solution
components to tune the force balance and obtained rich morphologies such as spherical
micelles [63], cylinder micelles [64], lamellae [65] and vesicles [66]. However, the
structures consist of only a pure organic polymer and lack a functional inorganic part.
20
Furthermore, the obtained structures are present in solution, whereas in many
applications the structures are required to be present in the form of dry films.
TiO2 nanotube is another important one-dimensional nanostructure that has
attracted a lot of interest regarding applications in DSSC [67]. Hydrothermal growth has
been reported to be a novel method for synthesis of TiO2 nanotube arrays on FTO glass
substrate. However, issues of high temperature annealing and detachment of the tubes
from substrates have limited their applications. Anodization of Ti foil has been able to
solve these issues. Macák et al. in 2005 reported for the first time that anodic TiO2 might
be used in DSSC [68]. The configuration of this structured solar cell typically adopts
backside illumination geometry, as the titanium foil is opaque to sunlight. TiO2 nanotubes
grown through anodization method are highly aligned and structured and give the
maximum efficiency of 4.24 % for 6 μm long tubes [69]. Instead of DC if AV potential is
used for anodization, so-called bamboo type morphology of the nanotubes is produced
[70]. The advantage of these bamboo-type nanotubes is that they present a more porous
structure and therefore offer larger surface areas for dye adsorption in comparison to
smooth-walled nanotubes created under DC conditions. It is found that the morphology
of the bamboo-type nanotubes is adjustable by changing the AV pulse durations. An
efficiency of ~2.96 % is achieved on the bamboo-type nanotubes that are grown under
AV condition with a sequence of 1 min at 120 V and 5 min at 40 V for 12 h. Besides
vertically aligned TiO2 nanotube array, randomly oriented TiO2 nanowires have also been
studied for DSSC application in view of high porosity of the films [71-72].
21
Oxide aggregate is another important nanostructure reported recently with their
applications in DSSC. The oxide aggregates are typically assembled by nanoparticles or
other nanomaterials to form submicron-sized spheres [73-74]. Since the size of the
aggregates is comparable to the wavelengths of visible light, they have been shown to
generate effective scattering of light. The aggregates of nanomaterials also possess a
highly porous structure and, therefore not only scatter the light but also serve as a
supporting matrix for dye adsorption. In other words, while achieving light scattering, the
aggregates would achieve high internal surface areas for the photoelectrode film. Use of
aggregates of ZnO nanoparticles has more than doubled the conversion efficiency of
DSSC compared to the photoelectrode made of dispersed nanoparticles [73]. Employing
an aggregate structure of TiO2 is anticipated to bring a breakthrough in DSSC solar cell
efficiency in view of better conjunction of the TiO2 with ruthenium-based dyes [73].
Recently, Kim et al. reported a two-step method for the synthesis of TiO2 aggregates.
Firstly TiO2 spheres are synthesized via controlled hydrolysis and then the spheres are
etched under hydrothermal condition. The latter step enables the spheres to be crystalline
with high porosity. The synthesized nanoporous TiO2 spheres are approximately 250 nm
in diameter and consist of nanocrystallites of size 12 nm [75]. These aggregates
demonstrated high surface area of ~117.9 m2g
-1, which is 1.7 times that of nanoparticles.
Applying these aggregates to DSSC yielded a high conversion efficiency of 10.5 %.
In DSSC, the interfacial charge recombination affects the open-circuit voltage by
decreasing the concentration of electrons in the conduction band of semiconductor [76],
and affects the photocurrent by decreasing the forward injection current [77]. Interfacial
22
charge recombination can be especially serious in the case of photoelectrode film
consisting of nanoparticles, because (1) the nanoparticle film gives a large internal
surface and therefore may increase the probability of charge recombination due to an
equally large semiconductor/ electrolyte interface; and (2) the small size of individual
nanoparticles allows limited band bending at the semiconductor surface, thus there is no
electric field that can assist the separation of the electrons in the semiconductor [78-79].
Core-shell structures are developed and applied to DSSC with the idea of suppressing the
interfacial charge recombination. The core-shell nanostructures are a class of
combinatorial systems, which typically comprise of a core made of nanoparticles, or
nanowires/nanotubes, and a shell covering the surface. The coating layer (shell) of these
structures may build up an energy barrier at the semiconductor/electrolyte interface and,
thus retard the reaction between the photogenerated electrons and the redox species in the
electrolyte. This in turn reduces the interfacial charge recombination in the device. Two
approaches have been developed to create the core-shell structures [80]. One involves
synthesis of nanoparticles firstly followed by a shell layer. The energy barrier for these
structures is formed both at the nanoparticle/electrolyte interface and at the individual
cores. In another approach, the photoelectrode film comprised of nanoparticles is
prepared prior to depositing the shell layer.
As the most stable iron oxide phase under ambient conditions, α-Fe2O3 (Eg = 2.2
eV) has been widely used for catalysts, non-linear optics and gas sensors, etc. [81-82].
Quasi-one dimensional nanostructures of α-Fe2O3 have also triggered considerable
interest. In fact, α-Fe2O3 nanostructures can be grown via simple oxidation of pure iron
23
[83]. Wen et al. demonstrated an interesting morphology transition from nanoflakes to
nanowires when heating pure iron at 400, 600, 700 and 800 °C [84]. On the other hand,
Fu et al. reported the large arrays of vertically aligned α-Fe2O3 nanowires grown by
heating pure iron in a gas mixture of CO2, SO2, NO2 and H2O vapor at 540–600 °C [83].
Besides using thermal oxidation of pure iron, α-Fe2O3 nanobelts and nanotubes are also
produced from solution based wet approaches [85]. Wang et al. reported a solution-phase
synthesis method to make nanobelts in FeCl3.6H2O and Na2CO3. After a series of heat
treatment, single crystal α-Fe2O3 nanobelts are obtained. Nanotubes have also been
grown via hydrothermal method, where FeCl3 and NH4H2PO4 are used as precursors. The
formation mechanism of tubular-structured α-Fe2O3 has been proposed as a coordination-
assisted dissolution process. The presence of phosphate ions used in this process is
crucial for the tubular structure formation, which results from the selective adsorption of
phosphate ions on the surfaces of hematite particles and their abilities to coordinate with
ferric ions. Fan et al. investigated the electrical transport properties of α-Fe2O3 nanobelts
[86]. The electrical properties are found similar to ZnO and In2O3. The native oxygen
vacancy renders α-Fe2O3 nanobelts n-type semiconducting in behavior. However, in
contrast to ZnO and In2O3, experiments show that α-Fe2O3 nanobelts can be easily doped
with Zn and converted to p-type at 700 °C. This p-type doping effect is attributed to the
substitution of Fe3+
by Zn2+
ions. The doping effect on the initial n-type behavior
changing to p-type also manifests itself in the modification of the contact property, as
observed in the increasingly non-linear I–V curves. On the other hand, when the doping
process is carried out at lower temperatures, the n-type behavior is enhanced as indicated
at higher conductivities and mobilities.
24
1.5.2 Sensitizer
Sensitizers are normally coordination compounds based on (d)6 transition
metals. Ruthenium (II) polypyridal compounds are most often used in DSSC. Metal-to-
ligand charge transfer (MLCT) mainly dominates their visible absorption spectra. Light
absorption formally promotes an electron from the t2g orbital on the ruthenium metal
centre to a * orbital of a bypyridine ligand [87]. Interfacial electron injection from this
excited state into semiconductor conduction band is reported to be on pico- and femto-
second time scales. Being one of the most crucial parts of DSSC, all sensitizers must
fulfill some basic requirements. Firstly, the absorption spectrum of the sensitizer should
cover the whole visible and some part of near infrared (NIR). Secondly, it should have
some anchoring groups to bind itself to the semiconductor surface. Thirdly, the excited
state level of the dye should be higher in energy than the conduction band of the
semiconductor. Fourthly for dye regeneration, the oxidized state level of the dye must be
more positive than the redox potential of electrolyte. Lastly, the sensitizer should be
electrochemically and thermally stable. Based on these requirements, many different
photosensitizers, including metal complexes, porphyrins, phthalocyanines and metal-free
organic dyes have been designed and applied to DSSC. Dye molecules are attached to the
semiconductor surface through functional groups or ligands on the dye. Problems of poor
electron transfer to the semiconductor arise if dye aggregates, resulting in unsuitable
energetic positioning of the LUMO level [88]. Anchoring of the dye with the
semiconductor takes place through groups like carboxylic acid and phosphoric acid. The
dye complex firstly transfers the electronic charge to the ligand orbital, which is in
25
intimate contact with the conduction band of the semiconductor. An electronic coupling
occurs between the cationic metal and oxygen atom of the carboxylic acid, thus
promoting the electron transfer. The carboxylic acid can either form ester like linkages
(C=O) or caboxylate linkages (C-O-O-) with the semiconductor surface [88]. Most
sensitizers suffer from a fundamental problem of the limited light-capture cross-section
of the dye molecule. The cross-section () is related to the molar extinction coefficient
() by the following formula [89]:
AN
1000. (1.8)
The value of for a dye lies between 104 and 5x10
5 M
-1cm
-1, which computes light
capture cross-section values between 0.0016 and 0.08 nm2. The area the sensitizer
molecule occupies on the surface of the supporting oxide is much larger, e.g., about 1–2
nm2. Hence, at most a few percent of the incident light can be absorbed. A successful
strategy to solve the problem of light absorption through such molecular layers is found
in the application of high surface area films consisting of nanocrystalline oxide particles
with a diameter of 10–20 nm.
Metal complexed dyes are widely popular in DSSC. Among these, Ru complexes
have shown the best photovoltaic properties so far [90-93]. These dyes have a broad
absorption spectrum with suitable excited and ground state energy levels, relatively long
excited-state lifetime, and good chemical stability. Ru complexe used in DSSC has
reached more than 10 % solar cell efficiency under standard measurement conditions.
The first report of sensitization of TiO2 single crystals by Ru complexes was published in
26
1979 [94]. Later O‟Regan and Grätzel achieved 7 % solar cell efficiency using these dyes
[18]. Grätzel and co-workers have worked extensively on mononuclear Ru complexes,
cis-(X)2bis(2,2′-bipyridyl-4,4′-dicarboxylate)ruthenium(II), where X can be Cl, Br, I,
CN, and SCN [95]. The thiocyanato derivative, N3 dye, is found to exhibit outstanding
properties, such as a broad visible light absorption spectrum, long excited state lifetime
(~20 ns) and strong adsorption on the semiconductor surface due to binding with four
carboxyl groups. Efforts have been made by many groups around the world to change the
ligands of Ru complexes and thus optimize the photosensitizers. To extend the spectral
response region of the sensitizer to the near-IR region, Grätzel and co-workers designed
the N749 dye [96], also called the “black dye”. In this dye, the Ru center has three
thiocyanato ligands and one terpyridine ligand substituted with three carboxyl groups.
They found that the red shift in the MLCT band is due to the decrease in the π* level of
the terpyridine ligand and an increase in the energy of the t2g metal orbital. This dye
gave an efficiency of 10.4 % under AM1.5 [97]. Later Grätzel and co-workers
synthesized an amphiphilic heteroleptic ruthenium sensitizer (Z907), which demonstrated
its prominent thermal stability due to the introduction of two hydrophobic alkyl chains on
the bipyridyl ligand [98-100]. Many attempts have also been made to construct
sensitizers with other metal ions, such as Os [101-103], Re [104-105], Fe [106-108], Pt
[109-112] and Cu [113-115]. Osmium (Os) complexes are found to be promising
photosensitizers due to the prominent MLCT absorption band in comparison to the Ru
complex. Arakawa and coworkers developed another class of organic dyes containing
coumarin unit and cyanoacrylic acid unit [116-121]. The coumarin sensitizer exhibits an
effective electron injection process. However, when this dye is used in DSSC, it shows a
27
lower efficiency than Ru complex due to the low response range in the visible region.
Besides organic dyes, other types of organic sensitizers have been used in DSSC. Yang
and co-workers [122] synthesized some anthraquinone dyes with different absorption
spectra for DSSC. However, these dyes showed low power conversion efficiencies. The
best efficiency value obtained by these dyes was only 0.13% under AM1.5 [123].
Recently, polymeric dyes like polyaniline, polypyrrole, polythiophenes and poly(p-
phenylene ethynylene) have been employed as photosensitizers [124].
1.5.3 Electrolyte
The main function of the electrolyte system in DSSC is to reduce the oxidized dye
after charge injection and then subsequently transport the hole to the cathode. Hence, the
electrochemical properties of the redox mediator electrolyte are crucial to the
performance of the DSSC. An efficient and effective electrolyte should be chemically
stable, low in viscosity, a good solvent, should not react with dye or the semiconductor
and should be compatible with the sealing material of the DSSC. Three different types of
electrolyte are usually used in DSSC- liquid, quasi-solid and solid electrolytes. The most
commonly used redox for the electrolytes is iodide/triiodide [18], quinone/hydroquinone
[125], ferrocenium/ferrocene [126], BrBr /3 [127] and Co
III/Co
II [128]. Although other
electrolytes have been implemented in the DSSC, the most successful one so far has been
iodide/triiodide system. Iodide-triiodide redox couple gives the highest power conversion
efficiency among the ruthenium polypyridyl dyes, which is why practically liquid or
quasi-solid DSSC are based on iodine. The source of the iodide ions is typically an alkali
28
metal salt, such as LiI, KI or an imidazolium derivative. Triiodide is generated when
iodide ions react with molecular iodine (I2) in the electrolyte. The species such as 4-tert-
butylpyridine (TBP), which attach on the TiO2 surface sites not covered by the dye, are
also added to the electrolyte to suppress the dark currents in the cell. Several solvents
have been tested for the electrolyte system. The main solvents used for DSSC are nitriles
like acetonitrile, acetomethoxypropio-, valero-, or butyronitrile or mixture of carbonates
like ethylene/propylenecarbonate. The solvent needs to be of low viscosity to dissolve all
the ingredients of the electrolyte. An example of a typical electrolyte composition is 0.5
M LiI, 0.05 M I2 and 0.5 M TBP in 3-methoxypropionitrile.
At present, acetonitrile-based liquid-state electrolytes are commonly used.
However, the demand of perfect sealing of the device in order to avoid leakage and
evaporation of the solvents at a high temperature, as well as the decrease of the tri-iodide
concentration due to sublimation of iodine, which affects the long-term stability, and
performance of the cell, is currently the most challenging issue. Hence much work has
been focused on substituting the liquid electrolyte by polymer electrolytes [129-131], gel
electrolytes [132-133], organic or inorganic hole conductors [134-135] and less volatile
ionic liquids [136-137]. Electrolytes based on both organic solvents and ionic liquids can
be gelated, polymerized, or dispersed with polymeric materials. Inclusion of gelating or
polymeric agents, called fillers, transforms the electrolyte into a quasi-solid electrolyte.
Poly(ethylene oxide) (PEO) based electrolytes are the most commonly used
polymer electrolytes. PEO polymers have over the last 5 years grown to be extremely
29
popular as matrix materials and exist in several forms as electrolyte supports in solid-
state cells. Various pure (of different molecular weight), mixed (often with PVDF), and
composite electrolytes have been used involving nanocomposites with silica, titanium
dioxide, and many other inorganic additives [138-140]. Oligomer size or molecular
weight is shown to influence conductivity and photoelectrochemical properties and
suggest the existence of an optimal chain length [141]. They are especially attractive as
different metal iodides (cations) cause photovoltage to increase with ionic radius in PEO
polymer matrix, thus causing decrease in the recombination reaction [142]. In PEO-based
polymeric systems, the cation size of added iodides is shown to strongly influence
performance and was discussed in terms of intercalation and surface adsorption [143]. De
Paoli and co-workers have thoroughly reviewed both gelated and hole-conducting
electrolytes [143]. Wang et al. published two critical reviews of the field very recently,
which highlighted the possibility of fabricating up-scale flexible solar cells based on
quasi-polymeric electrolytes [144]. Considering the amount of work invested in these
systems, it is remarkable that the maximum conversion efficiency remains fairly constant
around 5 %. Most likely, fundamental insights into interface processes will be required
for significant improvements in performance.
Some interesting results have also been obtained using g-butyrolactone (GBL) as
plasticizer for P(EO–EPI) copolymer. For the electrolyte prepared with GBL, P(EO-EPI),
NaI and I2, the maximum ionic conductivity changed from 3x10-5
to 1x10-4
Scm-1
after
addition of 50 wt % of GBL [145]. GBL has been used as plasticizer for electrolytes for
application in batteries, and it is well known to be able to coordinate Li+ ions,
30
contributing to the dissolution of lithium salts in polymer systems [146]. Therefore, GBL
also made possible as the substitution of NaI for LiI in the polymer electrolyte. Recently
Nogueira and coworkers [147] obtained interesting results using the copolymer
poly(ethylene oxide/2-(2-methoxyethoxy)ethyl glycidyl ether) containing 78 % of EO
units P(EO–EM). This material is mixed with LiI, I2 and the plasticizer GBL. The amount
of GBL incorporated into the electrolyte is changed from 30 to 90 % and solar cells are
assembled with this kind of electrolyte, containing different plasticizer/polymer ratios.
The Jsc increases when the amount of GBL is increased from 30 to 70 %, and this effect
is related to the increase of ionic conductivity of the electrolyte.
Adding nanoscale inorganic fillers in the electrolyte is a common practice, to
improve the mechanical, interfacial, and conductivity properties of the (gel) polymer
electrolytes. Since the pioneering work by Scrosati and coworkers [148], addition of TiO2
and other nanoparticles has been extensively employed to improve the ionic conductivity
of polymer electrolytes. It is well known that the presence of such nanoparticles changes
the conduction mechanisms assigned to the ions introduced in the polymer, however, how
these nanoparticles actually act is still unknown. The drawback of adding these materials
is the loss of mechanical properties of gel electrolytes and ionic liquid-based electrolytes.
Their effects on the mechanical stability can result in a loss in electrolyte penetration.
The most used approach is the addition of TiO2 nanoparticles to the polymer matrix [149-
152]. Falaras and coworkers [149] investigated the addition of commercially available
TiO2 nanoparticles (P25, Degussa) to the polymer electrolytes of PEO, LiI and I2. The
filler particles, because of their large surface area, prevented recrystallization, decreasing
31
the crystallinity degree of PEO.
1.5.4 Electrode
Two electrodes are used for fabricating DSSC. Both the electrodes are made of
glass with a thin layer of fluorine-doped tin oxide (FTO) on one surface. Fluorine doped
tin oxide has been recognized as a very promising material because it is relatively stable
under atmospheric conditions, chemically inert, mechanically hard, high-temperature
resistant, has a high tolerance to physical abrasion and is less expensive than indium tin
oxide. The FTO substrates used in this work have sheet resistivity of 15 ohm per square.
The other electrode is the back electrode called the counter electrode. It should ideally
have a high conductivity and exhibit an ohmic contact to the electrode, which requires
that the work function of the counter electrode should match the redox potential of the
electrolyte. Nobel metals with high work functions, such as gold and platinum, are
commonly used. Metal electrodes have the advantage of reflecting the light transmitted
by the photo-electrode, thus enhancing the overall light absorption of the DSSC. They
also serve as an oxygen barrier thus protecting the material underneath. Lastly, they are
easier and simpler to produce via simple sputtering or evaporation. Counter electrodes
can be rather easily prepared by the deposition of a thin catalytic layer of platinum onto a
conducting glass substrate. Without platinum, conducting indium doped tin oxide glass is
a very poor counter electrode and has a very high charge transfer resistance. Pt can be
deposited using a range of methods such as electrodeposition, spray pyrolysis, sputtering,
and vapor deposition. Best performance and long-term stability have been achieved with
32
nanoscale Pt clusters prepared by thermal decomposition of platinum chloride
compounds [153].
New materials like carbon [154], carbon nanotubes [155], cobalt sulphide [156]
and conducting polythiophene polymers [157] are also being investigated. Carbon
materials are suited as catalysts for the reduction of triiodide. Kay and Grätzel developed
a counter electrode from a mixture of graphite and carbon black for use in DSSC with the
monolithic cell geometry [158]. The function of the graphite is electronic conduction as
well as catalytic activity, while the high-surface area carbon black is added for increased
catalytic effect. Pettersson et al. optimized the performance of the counter electrode by
using two carbon layers: one for enhanced adhesion to the substrate and other for the
enhanced catalytic effect (doped with Pt) [159]. Single wall carbon nanotubes showed
good catalytic properties for triiodide reduction as well as good conductivity [160].
Poly(3,4-ethylenedioxythiophene) (PEDOT) doped with toluenesulfonate anions show
good catalytic properties for the reduction of tri-iodide [161-163]. Films are prepared
onto TCO substrates and a charge transfer resistance less than 1 Ωcm2 could be obtained
for films that are more than 1 μm thick. Other conducting polymers like polyaniline and
polypyrrole are much less suited [161]. ITO/PEN, a flexible substrate, outperforms Pt on
the same substrate, with a less charge transfer resistance.
1.6 Stability of dye-sensitized solar cell
The issue of stability of the DSSC is very important, especially for
33
commercializing the device. The issue of stability basically comes from either the dye or
the electrolyte, because both these components tend to decay with time. There is
controversy in the literature concerning the stability of the ruthenium dye. The dye is
believed to be able to sustain 108 redox cycles without noticeable loss of performance,
corresponding to 20 years of operation under sunlight [18]. However, when this dye is
maintained in the oxidized state for long periods, it degrades through the loss of the -NCS
ligand. Therefore, regeneration of the dye in the photovoltaic cell should occur rapidly to
avoid this unwanted side reaction. It is to be noted that the lack of adequate conditions for
regeneration of the dye may lead to dye degradation [164]. This is especially true for
devices comprising polymer electrolytes, due to several factors, such as low ionic
mobility that can lead to slow regeneration or the incomplete filling of the TiO2 sensitized
electrode by the electrolyte. Also, upon exposure for prolonged periods of time at higher
temperatures, such as 80 °C, degradation of DSSC performance has been observed.
Ruthenium complex so far is the standard dye for the application in dye-sensitised solar
cells. This is mainly due to the enhanced stability of the ruthenium complex compared to
other sensitisers. However, some natural dyes, such as chlorophyll [165] and different
porphyrins [166], have been shown to be more stable and may be used in DSSC. Organic
dyes with high extinction coefficients, such as Cyanines and Xanthenes, are known from
their applications in photographic films for the sensitisation of AgCl crystals and are also
used as sensitisers in photoelectrochemical devices to create photocurrent.
Another issue related to the stability of the cell is the electrolyte evaporation.
Liquid electrolytes are volatile and tend to evaporate on exposure to prolonged heat.
34
Stability of quasi-solid and solid state DSSC is better than liquid ones, but they show
lower efficiency. The stability of DSSC with polymer electrolytes is also very important,
although few studies involving this issue can be found. One of the motivations for the
substitution of the liquid electrolyte is that the use of a solid electrolyte can minimize
leakage and solvent evaporation problems and provide long-term stability, thus extending
the life of the devices. At the same time, many questions concerning the stability of such
materials when used in the conditions of device operation have already been raised. De
Paoli and coworkers [167] investigated solar cells assembled with the polymer electrolyte
of P(EO–EPI), NaI and I2, using flexible and rigid glass substrates. These cells were
irradiated under a Xe lamp with UV and IR filters for long periods, alternating with dark
periods. An initial decay in the performance is observed in the first 15 days, followed by
a plateau of stability. Another factor associated with the poor stability of DSSC is the
decrease of the tri-iodide concentration due to sublimation of iodine [168-171]. Kloo‟s
group [169] has shown that stability can be increased through gelation of the ionic liquid
electrolyte, which in turn reduces the sublimation of iodine. In another study, the addition
of phenothiazine served the same purpose [170]. So far, all studies involving the stability
of polymer electrolyte based DSSC show the benefit of the replacement of the liquid
component. The positive effect after long-term operation is more evident in the Jsc
parameter.
1.7 Solid-state dye-sensitised solar cell
The replacement of the liquid charge-transporting medium, i.e. electrolyte with a
35
solid one, has important consequences for the electronic processes in the cell. An entirely
new cell design is required in which the device geometry is adapted to the solid system.
Efforts have been made where liquid electrolyte is replaced with an organic charge
transport material 2,2‟7,7‟-tetrakis(N,N-di-p-methoxyphenyl-amine)-9,9‟-spirobifluorene
(spiro-MeOTAD). Maximum white light conversion efficiencies up to 1.8 % for
irradiation of AM1.5 have been obtained for this DSSC system. Interfacial charge
recombination processes have been identified as the main loss mechanism in this type of
cell. For further development of the solid state DSSC based on spiro-MeOTAD, it is
essential to find strategies to suppress charge recombination and to identify the
differences in charge transport compared to high efficiency liquid electrolyte solar cell. A
better understanding of the electronic processes in this device and the optimization of the
inorganic/organic network promise further improvement. The key problem related to the
replacement of the liquid by solid charge transport material is the less efficient hole
transport in the solid medium as a result of the relatively low hole mobilities in organic
semiconductors. Low conductivities pose high resistance to the charge flow, thus causing
voltage loss, particularly at high current densities. Slow charge transport is expected to
generate concentration gradients in the hole conductor matrix. As a consequence, hole
density might build up inside the pores, accompanied by depletion of holes in the bulk
organic semiconductor. This concentration gradient of hole leads to charging of the
interface, which considerably increases the interfacial recombination. Interface charging
is favoured by imperfect penetration of the hole conductor into the colloidal film. In the
case of the liquid electrolyte cell the penetration is complete and a
semiconductor/electrolyte junction occurs at each nanocrystal. The intimate contact and
36
the high mobility of the electroactive species guarantee efficient screening of any electric
field emerging from electron injection. Assuming that the kinetics of charge transfer to
the electrolyte is much faster for the holes than the recombination processes, the electrons
can create a gradient in the electrochemical potential between the particle and the back
contact. In this gradient, the electrons can be transported through the interconnected
colloidal particles to the back contact (percolation), where they are withdrawn as a
current. It has been established that electron transport in nanocrystalline TiO2 films is
dominated by electron trapping/detrapping mechanism in intraband-gap defect states
[172-175] In a strongly screened environment, the multiple trapping takes place mainly
via thermal activation of trapped electrons [176-177]. As a result of the less efficient
screening of photoinduced electric fields in the solid state DSSC, the electron transport in
the TiO2 may significantly differ, leading to the immobilisation of trapped electrons on
the time scale of the experiment. Assuming that the TiO2 network and hole conductor
film can be treated as a bulk material, a decrease of layer thickness should improve the
charge transport. Thin TiO2 layers in DSSC have smaller active surface areas for dye
adsorption and hence lower light harvesting. It is therefore necessary to find a way to
increase the optical density of the semiconductor film, which allows devices to be
fabricated with thinner semiconductor layers. Another important aspect to be considered
for organic charge transport material is the device stability, which is crucial for
commercialization of the device. The stability of solid state DSSC based on spiro-
MeOTAD has not been addressed in literature so far. Upon storage of the device under
ambient conditions, the short circuit current is reported to decrease, while the open-circuit
voltage and fill factor increase with time. These effects compensate each other and hence
37
no change in overall efficiency is observed.
1.8 Dye-sensitized solar cell efficiency
Since the emergence of DSSC, there have been constant efforts to optimize its
performance and increase the light conversion efficiency above 11 %. The main losses in
the device come from potential drop in the regenerative process and recombination losses
between electrons in semiconductor and acceptor species in the electrolyte. Also, most of
the dyes used are not able to absorb all the wavelengths in the sunlight. Research has
shown that recombination can be suppressed by the use of co-adsorbants, additing
additives in electrolyte and by blocking layer over the transparent electrode. Efforts are
underway to synthesize new dyes, like metal free-dyes, which efficiently absorb the light
available. Akkaya et al. [178] synthesized some boradiazaindacene dyes with different
absorption spectra. Although only 1.7 % solar conversion efficiency was achieved, it
paved a way to develop novel panchromatic sensitizers for DSSC. Co-adsorbants have
also been used to DSSC in an attempt to enhance the efficiency. Zhang et al. [179]
reported the use of 4-guanidinobutyric acid (GBA) as a co-adsorbant to the K-19 dye
resulting in an increase of 50 mV Voc without loss of Isc. Researchers have incorporated
light scattering layers in the semiconductor to increase the light absorption. Many hybrid
semiconductor materials with novel morphologies are being tried on DSSC, in order to
increase the effective surface area of the semiconductor. The high surface area in turn
leads to better dye absorption and higher efficiencies in the device. Tuning and improving
the electrolytes is another area where much research is focused. Hara et al. [180]
38
introduced different imidazolium iodides as iodine source and enhanced the efficiency of
DSSC.
1.9 Synthesis methods for nanomaterials
Nanomaterials have received wide attentions for their novel size- and shape-
dependent properties and have found wide varieties of applications. Hence many
technologies have emerged for the synthesis of these novel nanostructures.
1.9.1 Vapor-phase synthesis
Vapor-phase synthesis is a simple process where the vapor species are firstly
generated by evaporation or chemical gaseous reactions. Subsequently, the species are
transported and condensed onto the surface of a solid substrate. The substrate temperature
is lower than the evaporation zone. The prime advantage of this method is its simplicity
and accessibility, which makes this method widely popular for the synthesis of
nanostructures [181-185]. In principle, this method can produce nanostructures of any
material through proper control of synthesis parameters. It is widely accepted that the
control of supersaturation is a major consideration in obtaining nanostructures. A low
supersaturation is required for whisker growth whereas a medium supersaturation
supports the bulk crystal growth. At high supersaturation, powders are formed by
homogeneous nucleation in the vapor phase [186].
39
1.9.2 Vapor-liquid-solid (VLS) mechanism
A typical process in the VLS growth mechanism involves annealing the metal-
coated substrates above certain temperatures, so that the metal film melts and forms
droplets. Liquid has a higher sticking coefficient compared to solid, which helps the
reactant gas to be adsorbed on the droplet surface. As the droplets supersaturate, nuclei
starts to form at the droplet-substrate interface due to phase segregation. Subsequent
addition of atoms into the nuclei results in the growth of nanostructure, with the droplet
serving as the virtual template by promoting crystal growth at the liquid-solid interface
and restricting growth in other directions. The droplet remains at the tip of the resultant
nanostructure and solidifies in the post-growth cooling phase to form a nanoparticle. The
appearance of such nanoparticles is an indication of VLS growth mechanism. The VLS
mechanism often promotes the formation of one-dimensional nanostructures through an
anisotropic growth process. The size of the metal droplet plays a cruicial role in
determining the diameter of the structure.
1.9.3 Hydrothermal and sol-gel synthesis
The word „hydrothermal‟ comes from earth sciences implying a regime of high
temperature and water pressure. A typical hydrothermal synthesis needs high temperature
and high-pressure apparatus called „auto-claves‟ or „bombs‟. Hydrothermal process
typically involves water both as catalyst and the component of solid phases in synthesis
under elevated temperatures and pressures [187]. The beauty of hydrothermal process lies
40
in its simplicity. The requirements are minimal beginning with known quantities of
precursor materials, autoclaves and ovens. Temperature-difference method is the most
extensively used method in hydrothermal synthesis. The supersaturation of solution is
achieved by reducing the temperature in the growth zone. The raw materials are placed in
the lower part of the autoclave filled with a specific amount of solvent. The autoclave is
then heated in order to create two temperature zones. The raw materials dissolve in the
hotter zone of the autoclaves. The saturated aqueous solution in the lower part is then
transported to the upper part by convective motion of the solution. The cooler and denser
solution in the upper part of the autoclave descends while the counter flow of solution
ascends. The solution becomes supersaturated in the upper part as the result of the
reduction in temperature and eventually crystallizes. A large number of compounds
belonging to practically all class of materials have been synthesized under hydrothermal
conditions: like various oxides of tungstant, molybdenum, silica and germanium.
1.9.4 Microwave and ultrasonic synthesis
Recently a new mode of synthesis using microwaves has emerged. Microwaves
are electromagnetic waves with wavelength ranging from 1-300 mm, or frequency
between 300 MHz-300 GHz. In the last decade, microwave energy has been employed in
many chemical reaction studies, which demonstrated that microwave energy has unique
abilities to influence chemical processes like changing the kinetics and selectivity of the
reaction [188]. Recent results suggest that microwave energy increases the heating rate of
the solution mixture more uniformly. The main advantage of using microwaves for
41
synthesis is the shorter reaction time. Many nanoporous oxides have been attained in
much shorter times, without compromising the structure [189-192]. Sonochemistry is the
area in which molecules undergo a chemical reaction due to the application of powerful
ultrasound radiation (20 kHz–10 MHz) [193]. Sonochemistry involves creation, growth,
and collapse of a bubble that is formed in the liquid. The collapse of the bubble leads to
high temperatures (5000–25000 K) and high cooling rates (1011 Ks-1
), which ensures that
the chemical bonds are broken. As an environmently friendly method for nanomaterial‟s
preparation, sonochemistry has been widely applied in the synthesis of amorphous
products, insertion of nanomaterials into mesoporous materials, deposition of
nanoparticles on ceramic and polymeric surfaces, and formation of micro- and
nanospheres [194-197].
42
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Catal. 201 (2001) 22-36.
[197] N. Perkas, Y. Wang, Y. Koltypin, A. Gedanken and S. Chandrasekaran, Chem.
Commun. 11 (2001) 988-989.
57
Chapter 2: Experimental
This chapter will cover short description of various chracterization techniques used in this
work and their main principle of operation. Sample preparation methods and fabrication
of dye-sensitized solar cel (DSSC) is discussed in detail. The morphology and phase of
the synthesized materials are characterized by scanning electron microscopy (SEM),
transmission electron microscopy (TEM), energy dispersive X-ray (EDX), selected area
diffraction (SAED) and X-ray diffraction (XRD). Optical properties are measured using
UV-visible spectroscopy and photoluminescence. Solar simulator, incident photon-to-
current conversion efficiency (IPCE) and electrochemical impedance spectroscopy (EIS)
are used to test the performance of solar cell.
2.1 Materials
The chemicals used in this work for the synthesis of different materials are listed
below.
Table 2.1. List of materials used in the present work.
Chemical Name Purity Company Purpose
Pluronic P123 - BASF Surfactant for Mesoporous
TiO2
58
Poly(ethylene oxide)
(PEG) (Mw ~20000)
- Fluka Binder for „doctor blading‟
paste
Ethylene glycol (EG) 99.0 % Sigma Aldrich Polymer for anodization of
TiO2 nanotubes
Titanium ethoxide 98.0 % Aldrich Precursor for TiO2
Titanium (IV)
isopropoxide
97.0 % Aldrich Precursor for TiO2
Tetrabutoxytitanium (TBT) 99.0 % Acros
Organics
Precursor for TiO2
Iron chloride 98.0 % Sigma Precursor for Fe2O3
Sodium sulfate 99.0 % Sigma Aldrich Sulphate ion source for
Fe2O3
Lithium Iodide (LiI) 99.9 % Aldrich Iodide source for
electrolyte for DSSC
Potassium iodide (KI) 99.5 % Sigma Iodide source for
electrolyte for DSSC
Iodine (I2) 99.8 % Sigma Aldrich Iodide source for
electrolyte for DSSC
1,2-dimethyl-3-
propylimidazolium iodide
(DMP)
- Solaronix Stabilizer for electrolyte for
DSSC
4-tert-butylpyridine (TBP) 96.0 % Aldrich Stabilizer for electrolyte for
DSSC
Diphenylamine (DPA) 99.0 % Sigma Aldrich
Ammonium fluoride 98.0 % Merck Flouride ion source for
59
(NH4F) anodization of TiO2
nanotubes
N719 Dye - Solaronix Sensitizer for DSSC
Ti Foil (2 mm thick) 99.7 % Aldrich Starting material to grow
TiO2 nanotubes
Hydrochloric Acid (HCl) 37.0 % Sigma Aldrich Acid for synthesis for
mesoporous TiO2
Nitric Acid (HNO3) 99.8 % Sigma Aldrich Acid for synthesis of TiO2
Acetonitrile 99.8 % Sigma Aldrich Organic solvent
Ethanol Absolute Fisher Solvent for various purpose
Silver Nitrate (AgNO3) 99.9 % Sigma Aldrich Precursor for Ag
nanoparticles
2.2 Sample preparation and solar cell assembly
Solution based sol-gel and hydrothermal approach was followed to synthesize
different nanostructures. Solutions were prepared by mixing various chemicals under
vigorous stirring and heating in Teflon containers. The white precipitate was formed and
separated using centrifugation, followed by repeated washing with DI water and ethanol.
This step was necessary to ensure effective removal of all impurities. The nanostructures
were converted into their pure forms by annealing in nitrogen ambience at 300-500 °C at
a heating rate of 1 °Cmin-1
for 30-60 min. The exact recipe for the nanostructures is
discussed individually in subsequent chapters. Dip coating, based on solvent evaporation,
60
was used to prepare films on Si substrate for SEM, TEM, XRD and UV-Vis absorption
analyses. Solution was also dried in air at 60 °C for 24 h for powder XRD analyses.
To prepare the DSSC working electrodes, fluorine doped tin oxide (FTO) glass
(sheet resistivity 15 ohm per square) was used as a current collector. It was cleaned with
detergent, ethanol, isopropyl alcohol and de-ionized (DI) water by using an ultrasonic
bath for 15 min. FTO was then dried at room temperature. The synthesized and dried
powder obtained was converted into fine paste using DI water. It was spread on FTO
glass by “doctor blade” technique (Fig 2.1a) [1]. No binder was used in this preparation.
Only one layer of scotch tape (~40 µm thickness) was used for all the electrodes. The
films were allowed to dry in ambient conditions for 1 h before heating at 450 °C for 45
min in air (Fig 2.1b). Adsorption of the dye to the mesoporous film is achieved by simple
immersion of the coated electrode in an ethanolic solution containing 0.3 mM Ru-dye,
(cis-dithiocyanate-N,N'-bis(4-carboxylate-4 tetrabutyl ammoniumcarboxylate-2,2'-
bipyridine) ruthenium(II) (known as N719, Solaronix), for 24 h at room temperature (Fig
2.1c). This results in the conformal adsorption of the dye monolayer to the film surface.
The films were subsequently rinsed with ethanol and dried. The active area of the
electrodes (0.5 cm x 0.5 cm) was prepared by concealing the extra area by paraffin wax
film. Ionomer Surlyn (Dupont) sheet is used around the active area to later seal the
device. To prepare the counter electrode, a hole was drilled in the FTO glass by sand
blasting. The perforated sheet was washed with ethanol and DI water. Then 20 nm thick
Pt layer was sputtered on the FTO. The dye-covered electrode and Pt-counter electrode
were assembled into a sandwich type cell (Fig 2.1d). A drop of the electrolyte, solution
61
containing 0.1 M LiI, 0.05 M I2, 0.6 M 1,2-dimethyl-3-propylimidazolium iodide, and
0.5M 4-tert-butylpyridine in acetonitrile, was put on the hole in the back of the counter
electrode. The electrolyte was introduced into the cell via vacuum back filling. Finally,
the hole was sealed with a thin glass slide to avoid leakage of electrolyte. The whole
device is then exposed to a hair dryer for 5 min. The heat from the dryer helps the Surlyn
sheet to melt and seal the cell from all sides. In order to have a good electrical contact for
the connections to the measurement setup, the edge of the FTO outside of the cell was
cleaned with ethanol and acetone. The amount of dye loaded in semiconductor film was
obtained by desorbing the samples into 0.02 M sodium hydroxide (NaOH) in 4 mL of
ethanol. All the samples were 2 cm x 2 cm in size at a thickness of approximately 10 μm.
The desorbed dye was then used in a UV-Vis spectrophotometer to calculate the amount
of dye absorbed by the film.
62
Fig 2.1. Procedure for DSSC assembly: (a) setup for „doctor blading‟ the semiconductor film, (b)
coated electrode after annealing, (c) dye loaded semiconductor film and (d) the sandwich
assembly of the coated FTO and Pt-coated electrode.
2.4 Characterization techniques
The working mechanism of different characterization techniques used in this
work, are discussed here.
2.4.1 Material morphology
The structure and morphology of the synthesized materials were studied under
scanning electron microscope, tunneling electron microscope and selected area electron
diffraction.
63
2.4.1.1 Scanning electron microscope (SEM)
SEM is the most heavily used techniques in the characterization of the
nanostructures. As the name suggests, it uses electrons to generate an image (Fig 2.2a). It
provides the users with a highly magnified view with a surface topology down to the
nano-scale. The electron source comes in the form of a metallic filament, which is
typically tungsten. This filament is firstly heated up and acts as a cathode. The anode
plate (called electron gun), which is in close proximity to the filament, exerts an attractive
force on the electrons, accelerating them towards the sample as a beam of electrons (Fig
2.2b). A condenser lens condenses this electron beam before it is focused by the objective
lens to a very fine point of about 5nm on the sample [2]. By varying the voltage produced
by the scan generator, the scan coil generates a magnetic field, which moves the electron
beam in a controlled manner by the means of deflection. The same varying voltage is also
applied to the coils at the Cathode-ray tube (CRT), producing a pattern of light on the
surface of the CRT. The pattern of deflection of the light on the CRT corresponds to that
made by the electron beam. When the electron beam hits and penetrates the surface of the
sample, it results in the emission of electrons and photons from the sample. The emitted
electrons are collected by a detector, and then converted to voltage and amplified. The
resultant amplified voltage is then applied to the grid of the CRT, which causes the
intensity of the spot of light to change. The final image consists of thousands of spots of
varying intensity corresponding to the topography of the sample. It is important to note
that a vacuum environment is needed to operate the SEM due to the number of reasons.
Firstly, if gases are present in the column, they can react with the electron source, thus
64
causing it to burn out and produce random discharges that destabilize the beam.
Secondly, other molecules might hinder the transmission of the electron beam. This
interaction forms compounds, which condense on the sample and compromise the detail
and resolution of the image [3].
Fig 2.2. (a) Picture of SEM (courtesy: Multidisciplinary laboratory, Engineering Science
program, National University of Singapore). (b) Schematic showing the layout of components
inside SEM.
The secondary electrons are ejected from the k-orbitals of the specimen atoms
by inelastic scattering interactions with beam electrons. Due to their low energy, these
electrons originate within a few nanometers from the sample surface (Fig 2.3) [4]. This is
(a) (b)
65
the most common imaging mode in SEM collecting low-energy (<50 eV) secondary
electrons. To detect these electrons, SEM is usually equipped with an Everhart-Thorney
detector. In the detector, when an electron strikes the scintillator material, photons
created are conducted towards the photomultiplier by total internal reflection. These
photons are then converted back as current so that they can be detected.
When the electron beam strikes the sample, the incident electrons (primary
electrons) are scattered. These electrons can be scattered elastically or inelastically.
During the elastic scattering, there is little or no change of energy in the primary electrons
but their momentums are changed. Since the electron mass remains the same, the velocity
vectors of the electrons must change and become scattered. Elastic scattering occurs from
the interaction between negatively charged electrons and the positive nucleus. Electrons
that “rebound” from the sample are known as backscattered electrons. As the name
suggests these electrons are picked up by the Backscatter detector [3].
66
Fig 2.3. Schematic diagram showing interaction between electron beam and specimen.
The morphology and thickness of different nanostructures and films in this study
were directly observed under field emission scanning electron microscope (JEOL FEG
JSM 6700 F) with secondary electron imaging operating at 10 kV. It has a cold cathode
field emission gun, ultra-high vacuum, and sophisticated digital technologies for high-
resolution high quality imaging of microstructures. The JSM-6700F is able to handle
samples up to 8 inches in diameter. It has a unique graphical user interface that controls
condition setup, motor stage drive, imaging, and data filing, which assure stable and
reliable operation.
67
2.4.1.2 Energy dispersive X-ray (EDX)
EDX is usually used in conjunction with SEM. When an incident electron or
photon, such as x-ray or γ-ray, hits an atom at a ground state, an electron from an inner
electron shell is emitted, leaving a “hole” or vacancy site in the shell. A more energetic
valence electron for the outer shell fills this electron vacancy, resulting in the loss of
energy. The excess energy is released in the form of x-ray emission. The energy of the x-
ray emitted is dependent on and unique to the type of elements found in the specimen.
The Lithium drifted Silicon (SiLi) detector used in EDX generates a photoelectron when
an x-ray strikes it. This photoelectron travels through the Si creating electron-hole pairs.
The amplitude of the pulse generated thus depends on the number of electron-hole pairs
created, which is itself dependent on the energy of the x-ray impinging on the detector.
Hence, the x-ray spectrum can be analyzed for information on the composition of the
specimen [5].
2.4.1.3 Tunneling electron microscope (TEM)
Transmission electron microscope (TEM) (Fig 2.4a) uses a microscopy technique
similar to that of a scanning electron microscope. The short wavelength of electrons
makes TEM capable of high-resolution imaging (up to a few order of Armstrong) and
hence a very useful tool in material characterization. TEM is composed of a vacuum
system, which can build a pressure of the order of 10-4
to 10-8
Pa, to allow uninterrupted
passage of electrons. It has an electron emission source, a series of intermediate and
68
projector lenses and electrostatic plates (Fig 2.4b). In TEM, a coherent beam of electrons,
directed at the specimen, interacts and transmits electrons as it penetrates through the
specimen. The transmitted electrons are then focused by the objective lens into an image,
which is then magnified and focused by intermediate and projector lenses onto a
phosphor image screen [6-7]. An image of dark and light contrast is formed due to
different amounts of electrons transmitting through the specimen. Both surface
topography and crystallographic information can be obtained. It is important to note that
for TEM, the specimen needs to be ultra-thin in order for electrons to be able to penetrate
through the sample. Hence, sample preparation is an important aspect in TEM imaging.
For the present work, all the samples were dispersed in ethanol and dropped onto the
carbon grid. The measurements were performed on a JEOL-2100 high-resolution
transmission electron microscope (HRTEM) at an accelerating voltage of 300 kV with a
Lorentz lens. The JEM-2100 features a high-stability goniometer stage specifically tuned
for large angle tilt applications. The JEM-2100 has three independent condenser lenses
and is capable of producing high probe current for any given probe size, which allows for
improved analytical and diffraction capabilities.
69
Fig 2.4. (a) Picture (courtesy Institute of materials research and engineering IMRE, A*Star,
Singapore) and (b) Schematic showing the layout of components inside TEM.
SAED is a useful tool to identify crystal structures and examine crystal defects,
which is used in conjunction with TEM. The atoms act as diffraction gratings to the high-
energy electrons in TEM. This is because their wavelengths are fraction of a nanometer
and the spacing between atoms in a solid is only slightly larger; causing electrons to be
diffracted. Depending upon the crystal structure of the sample to be analyzed, some
fraction of the electrons are scattered at particular angles, while others pass through the
sample without deflection. As a result, the image on the screen is a series of spots called
the selected area diffraction patterns. Each spot corresponds to a diffraction condition of
the sample's crystal structure. If the sample is tilted, the same crystal stays under
illumination, but different diffraction conditions are activated, and different diffraction
spots appear or disappear. In SAED, the users can choose from which part of the
specimen the diffraction pattern is to be obtained. A selected area aperture is located
(a) (b)
70
below the sample holder, which can be inserted into the beam path. This thin strip of
metal contains different size holes and can block the beam. SAED pattern on the small
user defined area is obtained by moving the aperture hole to the section of the sample to
be visualized.
2.4.2 Material composition
The composition of the synthesized materials, phase‟s present, surface area and
crystallinity information was obtained using x-ray diffraction, x-ray photoelectron
spectroscopy, Fourier transform infrared spectroscopy and Brunauer–Emmett–Teller
(BET) Measurements.
2.4.2.1 X-ray diffraction (XRD)
XRD is a common technique used to determine the crystallographic structures of
solids, including lattice constants, orientation of single crystals, defects, stresses and the
chemical composition. A beam of x-rays with wavelengths ranging between 0.7 and 2 Å
is projected on the specimen, which is diffracted by the crystalline phases present in the
specimen by Bragg‟s law, given by:
nλ = 2d sinθ (2.1)
Here d is the spacing between atomic planes in the crystalline phase, λ is the wavelength
of the x-ray and θ is the angle between the incident ray and scattering planes.
71
Fig 2.5. Schematic diagram depicting the technique of x-ray diffraction.
A crystal lattice is made up of 3-D spatial distribution of atoms. These atoms are
arranged to form a series of parallel planes separated by a distance of d [8]. The planes
vary in orientation from material to material each with its own unique d-spacing. In XRD,
a monochromatic x-ray beam is projected onto a crystalline material at an angle of θ (Fig
2.5). Diffraction only occurs when the distance traveled by the rays reflected from
successive planes differ by a whole number n of wavelengths. The XRD pattern is
obtained by plotting the angular positions and intensities of the resultant diffracted peaks.
A Philips x-ray diffractometer with Cu Kα radiation was used in this work.
2.4.2.2 X-ray photoelectron spectroscopy (XPS)
X-ray photoelectron spectroscopy (XPS) is a surface chemical analysis technique
that measures the elemental composition of a material. The material to be analyzed is
irradiated by a beam of monochromatioc and low energy x-rays and the resulting energy
72
of the photoelectrons that have escaped from the surface are measured. The analysis and
detection of photoelectrons require that the sample be placed in a high-vacuum chamber.
An electrostatic analyzer analyzes the energy of the photoelectrons, and an electron
multiplier tube or a multichannel detector, such as a microchannel plate, detects the
photoelectrons. A spectrum with a series of photoelectron peaks is thus obtained. As the
binding energy of the peaks is characteristic of each element, these peaks can be used to
identify the elements that exist within the material surface. In addition, XPS also provides
information on the existing chemical bonding in the specimen since the chemical state of
the photoelectron emitted can alter the binding energy. XPS has to be performed under
ultra-high vacuum (UHV) conditions at pressures lower than 10−7
Pa [9-10]. XPS for this
work was performed on a VG ESCALAB MKII spectrometer equipped with a non-
monochromatized Mg-Kα x-ray source (1253.6 eV photons).
Fig 2.6. Picture of XPS (image courtesy Surface science laboratory, physics department, National
University of Singapore).
73
2.4.2.3 Fourier transform infrared spectroscopy (FTIR)
FTIR is another widely used infrared (IR) spectroscope for quantitative analyses,
where IR radiations pass through a sample. Some of the IR radiation is absorbed by the
sample and some of it is transmitted. The resulting IR spectrum represents fingerprint of
the sample with absorption peaks corresponding to the frequency of vibrations between
the bonds of the atoms. Because each material is a unique combination of atoms, no two
compounds produce the exact same infrared spectrum. Therefore, infrared spectroscopy
can result in a positive identification of different kinds of material. In addition, the size of
the peaks in the spectrum is a direct indication of the amount of material present. In
general the instrument consists of a black body source from where the IR is emitted (Fig
2.7). The beam firstly passes through an aperture, and then enters the interferometer
where the “spectral encoding” takes place. Finally the beam enters the sample
compartment where it is transmitted through or reflected off from surface of the sample
[11]. This is where specific frequencies, unique to the sample, are absorbed. The beam
passes through the detector for final measurements. Perkins Elmer spectrum 2000
equipment was used in this study.
74
Fig 2.7. Schematic depicting the working of FTIR
2.4.2.4 Brunauer–Emmett–Teller (BET) measurement
BET measurements on samples were conducted using Quantachrome Nova 1200
with N2 as the adsorbate at liquid nitrogen temperature. BET method is used to measure
the total surface area of a material. In 1938, Stephen Brunauer, Paul Hugh Emmett,
and Edward Teller [12] published an article about the BET theory for the first time;
“BET” consists of the first initials of their family names. BET theory aims to explain the
physical adsorption of gas molecules on a solid surface and serve as the basis for analysis
of the specific surface area of a material. The concept of the theory is an extension of
the Langmuir theory, which is a theory for monolayer molecular adsorption, to multilayer
adsorption. It follows certain hypotheses like (1) gas molecules physically adsorb on a
solid in layers infinitely; (2) there is no interaction between each adsorption layer; and (3)
the Langmuir theory can be applied to each layer. Surface area and pore size distribution
75
are measured by the use of nitrogen adsorption/desorption isotherms at liquid nitrogen
temperature and relative pressures (P/Po) ranging from 0.05-1.0. The large uptake of
nitrogen at low P/Po indicates filling of the micropores (< 20 Å) in the material. The
linear portion of the curve represents multilayer adsorption of nitrogen on the surface,
and the concave upward portion of the curve represents filling of meso- (20-500 Å) and
macropores (> 500 Å). An entire isotherm is needed for one to calculate the pore size
distribution of the material.
Fig 2.8. Picture of BET equipment used to obtain surface area and pore distribution information
(image courtesy Multidisciplinary laboratory, Engineering Science program, National University
of Singapore).
76
2.4.3 Optical property
The optical properties of the synthesized materials were studied using
spectroscopy technique and photoluminescence. The reagent solution was dip coated on
Si substrates and dried in air at 60 °C for 24 hr, before annealing in nitrogen ambience at
300-500 °C with heating rate of 1 °Cmin-1
for 30-60 min.
2.4.3.1 UV-visible spectroscopy
When radiation interacts with matter, a number of processes can occur like
reflection, scattering, absorbance, transmittance and fluorescence/phosphorescence.
When measuring UV-Visible spectra, we want only absorbance to occur. Since light is a
form of energy, absorption of light by matter causes the energy content of the molecules
(or atoms) to increase. In general, the total potential energy of a molecule can be
represented as the sum of its electronic, vibrational and rotational energies. The amount
of energy a molecule possesses in each form is not a continuum but a series of discrete
states. In some molecules, photons of UV and visible light have enough energy to cause
transitions between different electronic energy levels. The wavelength of light absorbed
is equal to the energy required to move an electron from a lower energy level to a higher
energy level.
77
Fig 2.9. UV-Vis spectrophotometer (a) the equipment and (b) the sample stage for both solid and
powder samples ((image courtesy Multidisciplinary laboratory, Engineering Science program,
National University of Singapore).
When light passes through or is reflected from a sample, the amount of light
absorbed is the difference between the incident radiation (Io) and the transmitted radiation
(I). The amount of light absorbed is expressed as either transmittance or absorbance, and
they are defined as follows:
oI
IT (2.2)
and TA log (2.3)
A spectrophotometer is an instrument to measure the absorbance of the sample as a
function of wavelength of the electromagnetic radiation (Fig 2.9). The key components
comprises of a tungsten-halogen or deuterium lamp to generate the radiation, a dispersion
device and a detector, which converts visible light into electrical signal. Experiments
were done on UV-1800 Shimadzu for this work.
(a) (b)
78
2.4.3.2 Photoluminescence (PL)
If a semiconductor material is exposed to light, with photons of energy equal or
greater than the optical energy gap of the solid, the light is absorbed by exciting the
electrons from the valence to conduction extended states. This creates a hole in the
valence extended state. Thus, the absorption of every photon creates a pair of electron
and hole in a semiconductor. The Coulomb interaction existing between electrons and
holes gets activated as soon as they are excited [13]. Eventually, these excitations relax
and the electrons return to the ground state. Photoluminescence (PL) occurs when an
excited pair of charge carriers in a solid recombines radiatively [13]. This light can be
collected and analyzed to yield a wealth of information about the material. The
wavelength of the light provides information on transition energies, and the intensity is a
measure of the relative rate of radiative and non-radiative recombination [14]. The optical
transition energy obtained from PL analysis can be used to determine the key
specifications for optoelectronic devices, the color of LEDs and the wavelength of laser
diodes [14]. In this work, PL properties were measured using Accent Rapid
Photoluminescence Mapping (RPM 2000) with He-Cd laser at 325 nm and 1.8 mW. Four
key parameters define PL peaks: peak wavelength, peak intensity, full width at half
maximum (FWHM) and area of the peaks (integrated PL intensity). The PL peak
parameters are all excitation power dependent [14].
79
2.4.4 Electrical property
Information regarding the electronic behavior of the synthesized materials was
obtained through performing current-voltage (I-V) and capacitance-voltage (C-V)
measurements on a probe station. Hall measurements were also carried out on
synthesized materials coated on a Si substrate.
2.4.4.1 Probe station
The probe station (Fig 2.10) is a setup with four probes to probe two terminal and
three terminal devices. I-V and C-V measurements were carried out on HP 4140B
semiconductor parameter analyzer and Agilent 4284A LCR analyzer, respectively. The
probe station uses MDV-CV system to minimize any possible environmental influences.
The measurements are taken with a 4-point probe measuring system or on a Tektronix
370A programmable curve tracer. The curve tracer is controlled by a computer to get the
portable data. Field effect transistors, diodes and contact resistance measurements are
typically done on this system. Important device characteristics, such as frequency
dispersion, flatband voltage, interface trap density and leakage current, can be extracted
from I-V and C-V curves. The metal-oxide-semiconductor (MOS) structure (area 7.8x10-3
cm2) was fabricated on TiO2 using SiO2 dielectric and Al electrode. The IV characteristic
was obtained using gold contacts on the film. For I-V measurements, the gate voltage is
swept from -3V to +3V and measured at 10 kHz. For C-V measurements, the MOS
80
device on SiO2 substrate was used with aluminum (Al) as back contact. For Hall
measurements gold was evaporated on the edges of TiO2 film.
Fig 2.10. Image of 4-probe station (courtesy: Multidisciplinary laboratory, Engineering Science
program, National University of Singapore). The sample stage is in the center.
2.4.5 Photovoltaic device characterization
The assembled solar cells were tested for solar energy conversion efficiency and
incident photon-to-collected-electron conversion efficiency. The electron transport
properties and recombination effects in solar cells were studied by electrochemical
impedance spectroscopy. The cell performance is evaluated over a number of fixed days
at specific intervals to ascertain the stability of the device. After every evaluation, the
cells were stored in the desiccator cabinet at room temperature.
81
2.4.5.1 Solar simulator
A Solar Simulation system also known as sun simulator reproduces full spectrum
light equal to natural sunlight. The ground level spectrum of natural sunlight is different
for various locations on earth. Sun's radiation must travel through the atmosphere before
reaching the earth and this distance changes as the day progresses, due to the changing
angle of the sun. With the sun is directly overhead, the radiation travels the shortest
distance through earth's atmosphere to reach the earth. The spectrum of this radiation is
referred to as "Air Mass 1 Direct" (AM1D). For standardization purposes sea level is
used as a standard reference site. The global radiation with the sun overhead is referred to
as "Air Mass 1 Global" (AM1G). The spectrum of sun's radiation in space does not pass
through any air mass hence it is referred to as “Air Mass 0” (AM0). Since solar radiation
reaching the earth's surface varies significantly with atmospheric condition, location, time
of the day, earth/sun distance, and solar activity, standard spectra have been developed to
provide a basis as the standardization of theoretical evaluation of the effects of solar
radiation. The most widely used standard spectra are those published by The Committee
Internationale d'Eclaraige (CIE), the world authority on radiometric and photometric
nomenclature and standards. The solar simulator consists of a light source and a power
supply. The light source has an ellipsoidal reflector that surrounds the lamp and collects
most of the lamp output. The radiation from the lamp is focused onto an optical integrator
that helps to produce a uniform diverging beam. The beam is diverted 90° by a mirror
onto a collimating lens. Special filters are placed between the mirror and the collimating
lens to shape the radiation spectra to match various air masses. The output is a uniform
82
beam that closely matches the sun's radiation spectra for a given air mass. The power
supply unit provides constant electrical power to the xenon arc lamp. All of our systems
come with a standard closed loop light intensity controller. This helps in assuring stable
light intensity. In addition, the power supply unit houses control circuitry for several
control features. For this work, the photocurrent-voltage of the samples was measured
with AM1.5 solar simulator (Newport Inst., model 91160A) with an AM filter (81088A).
A 150 W Xe lamp was used as the light source. The photoelectrochemical performance
of the solar cells was measured at 25 °C with a source meter (Keithley 2420) and
Newport IV test station software. The light intensity corresponding to AM 1.5 (100
mWcm-2
) was calibrated using a standard silicon solar cell (Oriel, SRC-1000-TC). The
lamp provides uniform illumination in the area of 10 × 10 cm2.
Fig 2.11. Solar simulator (a) the equipment and (b) the sample stage covered with black box
(image courtesy Engineering design studio, Engineering Science program, National University of
Singapore).
(a) (b)
83
2.4.5.2 Incident photon-to-current conversion efficiency (IPCE)
IPCE is the ratio of number of charge carriers collected by the solar cell to the
number of photons of a given energy shining on the solar cell. It relates to the response of
the solar cell to the various wavelengths in the spectrum. If all the photons of a certain
wavelength are absorbed and the resulting minority carriers are collected, the IPCE of
that particular wavelength has a value of one. IPCE ideally has a square shape, where the
efficiency value is fairly constant across the entire spectrum of wavelengths measured.
The reduced value of efficiency at any wavelength may be due to recombination effects
in the solar cell. In other words, IPCE can be viewed as the collection probability due to
the generation profile of a single wavelength, integrated over the device thickness and
normalized to the number of incident photons [15]. IPCE is also sometimes referred to as
quantum efficiency. IPCE spectra were measured at a spectral resolution of 5 nm using a
300 W xenon lamp and a grating monochromator equipped with order sorting filters
(Newport/Oriel). The incident photon flux was determined using a calibrated silicon
photodiode (Newport/Oriel). Photocurrents were measured using an auto ranging current
amplifier (Newport/Oriel). The control of the monochromator and recording of
photocurrent spectra were performed using the TRACQ Basic software (Newport).
2.4.5.3 Electrochemical impedance spectroscopy (EIS)
EIS is a recent tool in laboratories that is slowly making its way into the service
environment. It is also called AC Impedance Spectroscopy. The usefulness of this
84
technique lies in the ability to distinguish the individual contributions of components
under investigation. For example, if the behavior of a coating on a metal when in salt
water is required, by the appropriate use of impedance spectroscopy, a value of resistance
and capacitance for the coating can be determined through modeling of the
electrochemical data. The EIS technique involves applying small sinusoidal potential
perturbations to an electrochemical system over a wide frequency range and measuring
the magnitude and phase of the resulting current. The modeling procedure uses electrical
circuits built from components, such as resistors and capacitors to represent behaviors of
the material under investigation [16]. Changes in the values for the individual
components indicate their behavior and performance. EIS is a non-destructive technique
and so can provide time dependent information about the ongoing processes. Here a small
sinusoidal voltage is placed on the sample over a wide frequency range 105-10
-3 Hz. The
controlling computer system measures the magnitude of the current induced by the
potential and in addition the phase angle between the potential and current maxima.
EIS for this work was recorded with a potentiostat PGSTAT 302N (Autolab, Eco
Chemie, The Netherlands) under an illumination of 100 mWcm−2
. The frequency range
was varied from 0.05 Hz to 100 KHz and magnitude of the alternating signal was 10 mV.
The conductivity of the electrolyte system was derived from the complex impedance
measurements. Fig 2.12 shows the transmission line model used to model a DSSC. This
model serves for the analysis of thin film electrodes. The model applies either to porous
electrodes or to diffusion in thin films. Here Rs describes the resistance of the quasi-solid
electrolyte, Rct and Cct describe the recombination resistance and the chemical
85
capacitance of the DSSC, Dx1 relates to the interface of the photoelectrode and the
electrolyte and Ws encompass the finite Warburg impedance elements associated with the
diffusion of tri-iodide in the electrolyte. Rdiff is the diffusion resistance of the electrolyte.
Fig 2.12. Model used for EIS measurements for DSSC.
WRs Rct
Cct
Dx1 Ws
86
References
[1] I. K. Ding, J. Melas-Kyriazi, N. L. Cevey-Ha, K. G. Chittibabu, S. M. Zakerruddin,
M. Grätzel and D.McGehee, Organic Elect. 11 (2010) 1217-1222.
[2] G. Z. Cao, Nanostructures & Nanomaterials: Synthesis, Properties & Application,
Imperial College Press (2004).
[3] Welcome to the World of Scanning Electron Microscopy, Iowa State University,
Materials Science & Engineering Dept. (2009). Webpage:
http://mse.iastate.edu/microscopy/college.html
[4] G. I. Goldstein, D. E. Newbury, P. Echlin, D. C. Joy, C. Fiori and E. Lifshin,
Scanning electron microscopy and x-ray microanalysis. New York: Plenum Press (1981)
[5] C. C. Lin, H. P. Chen and S. Y. Chen, Chemical Physics Lett. 404 (2005) 1-3.
[6] L. A. Bendersky and F. W. Gayle, J. Res. Natl. Stand. Technol. 106 (2001) 997-1012.
[7] C. T. K. -H. Stadtländer, Microscopy and Microanalysis 9 (2003) 269-271.
[8] B. E. Warren, X-ray Diffraction, Addison-Wesley Pub. Co. (1990).
[9] J. Chastain (Ed.), Handbook of X-Ray Photoelectron Spectroscopy. Minmesota:
Perkin-Elmer Corporation (1992).
[10] J. C. Vickerman, Surface Analysis-The Principal Techniques, John Wiley & Sons
(1997).
[11] H. Günzler and H. U Gremlich, IR Spectroscopy, Wiley-VC Pub. (2002).
[12] S. Brunauer, P. H. Emmett and E. Teller, J. Am. Chem. Soc. 60 (1938) 30.
[13] J. Singh and K. Shimakawa, Advances in Amorphous Semiconductor, Taylor &
Francis, London (2003) 96-98.
87
[14] S. Wang, High Speed Photoluminescence Mapping a Vital Production Tool (Special
Feature Characterization), CompoundSemiconductor.net (2001).
[15] Source Webpage: http://en.wikipedia.org/wiki/Quantum_efficiency
[16] B. E. Conway, J. Bockris and R. E. White, Modern Aspects of Electrochemistry,
Kluwer Academic/Plenum Pub., New York, 32 (1999) 143-248.
88
Chapter 3: Mesoporous Titanium Dioxide (TiO2) Film for
Liquid Dye-Sensitized Solar Cell (DSSC)
This chapter discusses the mesophase ordering and structuring of titanium dioxide
(TiO2) carried out to attain optimized pore morphology, high crystallinity, stable porous
framework and crack-free films. It is observed that the pore structure (quasi-hexagonal
and lamellar) can be controlled via the concentration of copolymer, resulting in two
different types of micellar packing. The calcination temperature is also controlled to
ensure a well-crystalline and stable porous framework. P25 nanoparticles are added in
mesoporous TiO2 film, which act as scattering centers and also function as active binders
to prevent formation of microcracks. Dye-sensitized solar cell (DSSC) made finally made
from different pastes and tested for efficiency.
3.1 Introduction
Grätzel and co-workers [1-4] reported the solar cells based on nanocrystalline TiO2,
which imitates the process of photosynthesis. Since then, DSSC has been attracting much
attention throughout the world. The metal oxide semiconductor is one of the main
components that strongly influence the light harvesting capability of the solar cell.
Among wide-band gap metal oxide electrodes, TiO2 has been the cynosure of all eyes
owing to its peculiar physical and chemical properties [5], although few studies have
89
explored other oxides like ZnO [6-7], SnO2 [8], Nb2O5 [9] and CeO2 [10]. However, after
the demonstration of the hierarchically ordered uniform mesoporous TiO2 thin films by
D. Grosso [11], a new avenue of titanium dioxide applications is opened. Since then,
ordered porous films of TiO2 have been used in photocatalyst [12], water purification
[13], gas sensing [14] and photovoltaic devices [15].
It has already been shown that ordered mesoporous film of TiO2 could increase the
efficiency of photovoltaic devices. This comes from the benefits of increased surface
roughness, surface area and uniform porosity [16-18]. Well-ordered mesopores with good
degree of crystallinity provide high transparency and sufficient surface area for dye
absorption [19]. The challenge is, however, to achieve high degree of crystallinity and at
the same time preserve the order of the mesoporous structure. Although much work has
been carried out to solve this problem, there is still a scope to better understand the
mechanisms and parameters involved. Normally dip-coating/ spin-coating, and
precalcination procedures are reapeated many times to obtain thick TiO2 films [20-21].
Apart from being time consuming, the repetitive coating and sintering procedures lead to
the degradation of the film‟s mesostructure, eventually leading to cracks in the film [22].
Thus it is difficult to obtain a thick and crack free film. Films made up of smaller size
particles are used to obtain a high surface area. On the other hand, good light scattering
can be achieved either by employing larger nanoparticles or through additional scattering
layers in the TiO2 layer [23-25]. The usual consequence is that films, with high surface
area (small nanoparticles) exhibit poor light scattering while films with good light
scattering (big nanoparticles) end up with smaller surface areas for dye adsorption.
90
This work reports the synthesis of well-organized and highly crystalline mesoporous
TiO2 film. Triblock copolymer (Pluronic P123) is used as the template and the process of
evaporation induced self-assembly (EISA) is carried out. Annealing time and temperature
are varied to optimize the films for solar cell application, and issue of microstructure
collapse is addressed. A good crystallinity film with continuous, ordered pores and
anatase phases is obtained when annealed at 430 °C for 15 min. The synthesized film is
crack free with TiO2 particle size in the range of 10-15 nm, thickness of about 150 nm
and pore diameter in the range of 8-10 nm. The pore structure can be controlled by the
concentration of copolymer, which results in two different types of micellar packing,
forming quasi-hexagonal pores and lamellar pores. The short-circuit photocurrent density
(Jsc) of the cell made from a mesoporous/P25 TiO2 film has a higher efficiency of ~6.5 %
compared to the other homogeneous films. A combination of factors, such as increased
surface area, introduction of light scattering particles and crystalline nature of the
mesoporous/P25 film, are the main factors leading to enhanced cell performance.
3.2 Experimental
3.2.1 Synthesis of mesoporous TiO2
All reagents and chemicals were of analytical grade and were used as received. For
synthesis, 1.2 g triblock copolymer HO(CH2CH2O)20(CH2CH(CH3)O)70(CH2CH2O)20OH
designated EO20PO70EO20 or Pluronic P123 was added slowly into 20 g ethanol. The
amount of Pluronic P123 was varied between 1.0 to 2.0 g to investigate its effect on the
91
pore structure. Another solution was prepared by slowly adding 3.4 g of titanium
ethoxide into 0.3 ml concentrated HCl. The precursor solution was prepared by mixing
the two solutions under vigorous stirring for 2-3 h. The solution was then aged in a sealed
glass container at 13 °C for 2 days. Dip coating, based on solvent evaporation, was used
to prepare thin mesoporous films on Si(100) substrates. For DSSC application, the aged
solution was dried at room temperature to form a gel. The gel was converted into fine
paste using deionized (DI) water, and then spread on fluorine doped tin oxide (FTO)
glass (resistivity 15 Ω per square) by „doctor blade‟ technique. No binder was used in this
preparation. The films were allowed to dry in ambient conditions before heating at
various temperatures ranging between 350-550 °C for 30 min in air. The deposited films
on Si and FTO were also calcined at above temperature ranges with the heating rate of 1
°C min-1
in nitrogen ambience for 15-30 min to obtain mesoporous films. However, the
mesoporous TiO2 film on FTO was found to crack after heat treatment. Hence,
commercial TiO2 nanoparticles Degussa P25 (~30 nm in diameter) were added into the
mesoporous TiO2 gel at 5 % concentration by weight. Three types of thick films namely
the TiO2 nanoparticles, mesoporous and mesporous/P25 mixture were prepared on FTO
for DSSC applications. These films were calcined at 430 °C for 15 min.
3.3 Results and discussion
92
3.3.1 Effect of polymer concentration
Mesoporous TiO2 is obtained through micellization by using polymers as
structure directing agents. The pore size and geometry (hexagonal, cubic or lamellar) are
determined by the structure of the polymer. Micelle formation is a process of force
balance and no strong bonding is involved. The balance of the force is disturbed by any
change in the solution (like pH, polymer concentration etc.), and usually results in the
change of micellar structure. Typical evolution of micelle structure is: spherical →
cylindrical → wormlike → liquid crystal. In general, polymer monomers segregate and
form micelles. Then, oxide precursors undergo hydrolysis and condensation around these
micelles. The initial micelles are spherical in shape and individually dispersed in the
solution. They change to a cylindrical shape due to the shift of force balance inside the
micelle, which happens with increased polymer concentration. Continued increase of
polymer concentration results in an ordered parallel hexagonal packing of cylindrical
micelles. At a higher concentration, lamellar micelles would form. Direct arrangement of
spherical micelles forms cubic structures, while hexagonal and tetragonal structures result
from direct assembly of cylindrical micelles.
It is essential to be able to tune and tailor specific mesoporous structures since
they determine the specific surface area and charge carrier transport path of the material.
Tuning of the pores is firstly carried out via the self-organisation of TiO2 nanoparticles,
which is directly related to the block copolymer P123 template (Fig 3.1). P123
amphiphilic triblock copolymer is a nonionic surfactant and the driving force for the
93
micelle forming is the result of the hydrophilicity between the polyethylene oxide (PEO)
and polypropylene oxide (PPO) chains. Titanium dioxide hydrate has an affinity for
binding to the P123 unit, which then creates the self-assembled mesoporous structures.
The fabrication can be obtained following the evaporation induced self-assembly process
which is based on the evaporation of an organic solvent promoting self-assembly of the
surfactant and the simultaneous condensation of the inorganic oxide precursor. The aging
temperature is controlled to be as low as 13 °C to slow down the mesostructure formation
process. The long aging process promotes full framework condensation to effectively
allow self-assembly of nanostructures into ordered pores.
Fig 3.1. Schematic diagram showing the pore formation in TiO2 film.
94
The pore morphology can be easily tuned with the appropriate amount of
copolymer. SEM images (Fig 3.2) are samples synthesized at different amounts of block
copolymer. In general, when 1.0 and 1.5 g of P123 were used, well-structured pores were
obtained (Fig 3.2a and b respectively). High magnification shows that the film has a high
density of quasi-hexagonal pores of ~10 nm in diameter. With an increase in copolymer
concentration to 2.0 g, stripes of lamellar pores are observed (Fig 3.2c). It is noted that
quasi-hexagonal pores transform to stripes of lamellar pores as the amount of copolymer
increased beyond a critical micelle concentration (CMC) to Ti ratio due to the occurrence
of copolymer micelles fusion or aggregation. It is noteworthy that the tuning of pore
structure with two different types of alignments is due to micellar packing; quasi-
hexagonal pores resulting from the close-packed spherical micelles and stripes of aligned
pores resulting from the close-packed cylindrical micelles [26].
95
Fig 3.2. SEM images of different pore morphologies obtained by varying copolymer
concentration (a and b) quasi-hexagonal and (c) lamellar pores.
3.3.2 Effect of calcination temperature
Apart from good control of the well-structured pores, one is also concerned with
the crystallinity quality and stability of the porous framework. A high degree of
crystallinity is necessary for efficient photovoltaic operation, which requires calcination
at elevated temperatures. These temperatures are much higher than the degradation
100 nm
100 nm
100 nm
(a)
(b)
(c)
96
temperature of polymer P123. During annealing of the film, the oxidation of polymer
P123 takes place, which results in degradation of the hydrophilic chains, and eventually
forms pores. It has been shown that at lower temperatures, cleavage of C-O bond takes
place, while at higher temperatures C-C cleavage and dehydration become favorable
leading to pore formation [27]. However, it is known that upon calcination at elevated
temperatures, the ordering of mesoporous matrix collapses, due to the thermal diffusion
of TiO2 species. Therefore, optimization is required to synthesize ordered film with high
crystallinity.
In order to observe changes in pore ordering and crystallinity, films are synthesized at
various temperatures and then analyzed under TEM. Fig 3.3 shows TEM images of the
mesoporous TiO2 films synthesized at three different temperatures. The film grown at
300 °C for 15 min shows ordered pores (Fig 3.3a) with diffused diffraction rings in
SAED pattern indicating that the film is amorphous in nature (inset Fig 3.3a). However,
upon calcination at 550 °C for 15 min, the ordered matrix of the pores collapsed (Fig
3.3b) but much sharper diffraction rings can be observed (inset Fig 3.3b). The diffraction
ring pattern indicates that the film calcined at 550 °C is polycrystalline in nature. The
first few rings indexed for this film represent (101) and (004) planes. Hence, as the
temperature is increased from 300 to 550 °C, degree of crystallinity is improved while
ordering collapses. Fig 3.3c shows TiO2 film synthesized at 430 °C for 15 min. This film
prepared under optimized conditions shows ordered porous mesostructure with good
crystallinity. The mesoporous film prepared at optimal conditions is thermally stable and
has a well-organized microstructure. The average pore size estimated from TEM for this
97
film is about 9 nm. The film morphologies are also investigated by SEM, as shown in Fig
3.4.
Fig 3.3. HRTEM images and corresponding SAED patterns of mesoporous TiO2 after annealing
at (a) 300 °, (b) 550 ° and (c) 430 °C for 15 min.
(101)
(004)
20 nm 20 nm 20 nm
(b)(a) (c)
98
Fig 3.4. SEM images of mesoporous TiO2 films obtained after calcination at (a) 300 ° (b) 430 °
and (c) 500 °C for 15 min.
The pore size and pore volume are adjustable to a certain extent simply by
varying concentration of the templating compounds. Low-resolution TEM images (Fig
3.5a and b) of films calcined at 430 °C for 15 min are shown with short range ordering of
pores. Lattice fringes of (101) plane of TiO2 anatase phase are revealed (Fig 3.5c), with
an inter-planar spacing, d value of ~0.36 nm. The electron diffraction pattern (inset Fig
3.5c) displays the Debye-Scherrer rings of anatase TiO2. Diffraction rings are indexed as
(101) and (004), which suggest the formation of crystalline anatase TiO2 film with fine
(a)
(b)
(c)
100 nm
100 nm
100 nm
99
grains. However, upon calcination at 500 °C (Fig 3.5d) the ordered matrix of the pores
collapses or rather coalescences. As the temperature increases from 350 to 550 °C, the
degree of crystallinity is observed to improve while the ordering of the mesoporous
structure deteriorates. From the TEM results, the collapse of the quasi-hexagonal pores
and the aggregation of TiO2 nanoparticles are observed for the mesoporous film which
has been calcined at 550 °C. Thus, calcination temperature of 430 °C is optimized to
achieve high crystallinity without compromising the stability of the pores framework. All
the characterization techniques discussed henceforth, are on mesoporous TiO2 films
obtained by calcination at 430 °C for 15 min.
Fig 3.5. TEM images of mesoporous TiO2 films annealed at 430 C (a and b) low and (c) high
resolution and (d) TEM image of film annealed at 500 C. Inset shows the selected electron
diffraction pattern of the film annealed at 430 C.
50 nm
50 nm100 nm
10 nm
(a) (b)
(d)(c)
100
Fig 3.6 shows the wide angle XRD pattern of the mesoporous TiO2 film after
calcination at 430 °C, which confirms the formation of crystalline oxide and well
organized structured film. The XRD pattern obtained clearly shows well-resolved, sharp
peaks of anatase phase with peaks at 2θ = 25.6, 38.1, 48.03 and 54.05° (JCPDS no. 21-
1272). The crystallinity and planes of the film revealed by the XRD pattern are consistent
with the SAED results.
Fig 3.6. Wide angle XRD pattern obtained from mesoporous TiO2 film after calcination at 430 °C
for 15 min.
Effective removal of the organic template is verified by XPS technique. The XPS
spectra for Ti of as-prepared sample show two peaks for Ti2p3/2 at 457 and 458.7 eV (Fig
3.7a), implying two different chemical environments for Ti ions. The binding energy of
101
457 eV corresponds to the formation of Ti2O3 [28], while 458.8 eV signifies TiO2. Fig
3.7b shows the Ti2p spectra for annealed film with binding energy for Ti2p3/2 at 458.2
eV, while for Ti2p1/2 at 463.6 eV, indicating the formation of anatase TiO2 [29]. The
Ti2O3 at 457 eV is not observed in the annealed film (Fig 3.7b), since it is fully converted
to TiO2 upon annealing. Fig 3.7c shows O1s for the as-prepared sample, fitted with three
peaks. Binding energy of 529.5 eV signifies Ti-O in TiO2 and 531 eV corresponds to
hydroxyl groups (–OH) and physiosorbed water. The peak at 532.7 eV corresponds to C-
O bonds [30]. The large hydroxyl content is attributed to the nature of the mesoporous
surface, which reacts rapidly with moisture to form hydroxylated surface as a means of
producing better neutralization of charge, following Pauling‟s rule of electro-neutrality
[31]. Also, the defective sites on the TiO2 film surfaces are supposed to be good
adsorbents for water molecules [32]. The binding energy corresponding to the C-O
bonds disappears upon the thermal treatment of the film (Fig 3.7d) due to removal of
organic content.
102
Fig 3.7. XPS spectra of Ti2p for (a) as-synthesized film, (b) film calcined at 430 °C and of O1s
for (c) as-synthesized film and (d) film calcined at 430 °C.
The FTIR spectra obtained for both as-synthesized and annealed films are shown
in Fig 3.8a. The almost disappearance of C–O stretching band between 1025-1250 cm-1
is
used to determine the presence of surfactant (P123) before and after ethanol extraction
and calcination at 430 °C. For as-synthesized film before annealing, the symmetrical and
asymmetrical –CH2 stretching occurred at 2877 and 2969 cm-1
along with a sharp peak at
around 1081 cm-1
, due to incomplete condensation. These peaks are reduced after heating
the film. Strong transmittance observed at 1714 and 3300 cm-1
arises from vibrations of -
103
CH3 and C-H bonds. These peaks are also reduced after calcinations, as the organic
species present in the film degrade. To understand the thermal properties of the fabricated
porous TiO2 film and to determine the change in weight loss, thermal analysis is carried
out. TiO2 films showed substantial weight loss during 100-150 °C (Fig 3.8b) when
polymer P123 decomposed. Evaporation of volatile species, like water, ethanol and HCl,
has taken place at temperatures lower than 150 °C. Between 300-450 °C, remaining
residual organic matter is removed. The as-synthesized film revealed total weight loss of
63 %.
104
Fig 3.8. (a) FTIR spectra of (i) as synthesized and (ii) after calcination at 430 °C and (b) TGA
plot for the mesoporous TiO2 film containing P123 amphiphilic triblock copolymer.
105
3.3.3 Electrical and optical characterization
I-V characteristics are obtained using gold contacts on the film (Fig 3.9a).
Synthesized TiO2 film is photosensitive, giving higher current density under illumination
with UV lamp at a wavelength of 254 nm. Photoexcitation leads to a higher reverse
current, due to the generation of charge carriers. The TiO2 film shows a small current
value even at zero bias voltage. It can also be seen that under reverse bias, the magnitude
of the current is lower than that during forward bias, indicating that holes are the minority
carriers. For C-V measurements, the MOS device on SiO2 substrate is used with
aluminum (Al) as back contact. Fig 3.9b shows the hysteresis curve obtained for the film
at 1000 KHz, concurs with the ideal C-V curve for n-type substrate with Al contacts. This
confirms that mesoporous TiO2 film is indeed n-type. The area under the hysteresis
signifies the amount of traps and defects present in the dielectric film. These defects
primarily arise from oxygen vacancies in TiO2 [33].
For Hall measurements, gold is evaporated on the edges of TiO2 film. Hall
coefficient is determined to be approximately -0.03 m2C
-1. The number of conducting
electrons is computed to be 20.8x1019
m-2
with a sheet resistivity of 17.77 ohm per
square. The negative Hall coefficient confirmed that the synthesized TiO2 film is indeed
n-type, which is consistent with I-V and C-V results. Even though these results match
with those published in literature [34], it should be noted that the values obtained should
be treated as an estimation, to illustrate the relative values rather than the absolute values.
106
Fig 3.9. (a) I-V curves and (b) C-V hysteresis for mesoporous TiO2 film.
The TiO2 solution is dip coated on FTO glass and calcined at 430 °C for 15
min for UV-vis measurements. The average transmittance is found to be about 90 % in
the visible region (Fig 3.10). Since visible light constitutes 50 % of the solar energy
reaching the surface, if the film is used as photoanode, 90 % transmittance would allow
the entire incident light to be absorbed by the dye.
0
2
4
6
8
10
12
14
-2.5 -1.5 -0.5 0.5 1.5 2.5
Cu
rre
nt
(mA
)
Voltage (V)
In Dark
Under Light
0
1
2
3
4
5
6
7
8
9
-2 -1 0 1 2
Cap
acit
ance
(F)
Voltage (V)
(a)
(b)
107
Fig 3.10. UV-Vis transmittance spectrum for mesoporous TiO2 film.
3.3.4 Effect of scattering centers on DSSC performance
After the optimization of pores structures and calcination temperatures, thick TiO2
pastes are prepared on FTO glass by doctor blade technique. Steps pertaining to DSSC
fabrication are described in chapter 2 under section 2.2. To have a better dye absorption,
TiO2 films of approximately 10 µm thickness are prepared. However, thick mesoporous
TiO2 films (Fig 3.11a) are observed to crack, leading to a poor film quality. The
mesoporous TiO2 film shows numerous cracks of about tens to hundreds of micrometer in
length across the whole surface. Though the film shows numerous microcracks, ordered
quasi-hexagonal pores can still be observed at high magnification (Fig 3.11b). The
cracking issue is caused by the stress induced during the film shrinkage due to the
decomposition of triblock copolymer and subsequent crystallization of the film,
especially prominent with thicker films [22]. Subsequently, TiO2 nanoparticles of ~30 nm
0
10
20
30
40
50
60
70
80
90
100
250 300 350 400 450 500 550 600 650 700
Tran
smis
sio
n (%
)
Wavelength (nm)
UV Visible
108
diameter are added to mesoporous TiO2, essentially functioning as an active binder
(“glue”) to strongly bind the mesoporous TiO2 matrix to circumvent delamination and
cracking issues. The top and cross-sectional SEM views of mesoporous/P25 mixture film
are shown (Fig 3.11 c and d). The mesoporous/P25 film is uniform as well as crack-free
and shows no delamination from the FTO substrate.
Three types of thick films namely the TiO2 nanoparticles, mesoporous and
mesporous/P25 mixture are prepared. Fig 3.12 shows the XRD patterns of all the calcined
samples. All the samples show formation of crystalline TiO2 phase in the films. XRD
pattern clearly shows well-resolved and sharp peaks of anatase phase for mesoporous
TiO2 film. The crystallinity and planes of this mesoporous film are consistent with SAED
results as discussed earlier. It is noted that high degree of crystallinity is necessary for
efficient photovoltaic operation. Rutile phase of TiO2 has been reported to exhibit lower
efficiency than anatase phase [35]. This is due to the close packed structure of rutile
phase, which leads to lower surface area [36]. Moreover, electron transport is relatively
faster in anatase phase than in rutile phase, as anatase phase has higher conduction-band
edge energy [37]. Hence, anatase is the preferred phase for efficient functioning of
DSSC. Thus it is important to analyze the ratios of anatase and rutile phases present in
each sample by taking the ratio of the area under the two most intense peaks of the
anatase IA (101) and rutile IR (110) diffraction peaks [38].
109
Fig 3.11. SEM images of (a) low and (b) high magnification mesoporous TiO2 films. SEM
images of (c) top and (d) cross sectional views of mesoporous/P25 film.
TiO2 nanoparticle spectra show the composition of both anatase (~80%) and
rutile phase (~20%), while mesoporous TiO2 has pure anatase phase. On the other hand,
the mesoporous/P25 film shows predominantly anatase phase (~87%) and small amount
of rutile phase (~13%). The approximate crystallite sizes of the samples are calculated
using Scherrer‟s formula [39] as follows:
(3.1)
where t is thickness of crystallite, k constant dependent on the shape of the crystallite, λ
x-ray wavelength, B full width at half maxima (FWHM) and θ Bragg angle. The values
(b)
100 nm
(d)
5 µm
50 µm
(a)
(c)
50 µmFTO
TiO2
B B cos
k t
110
are summarized in Table 3.1. The calculated crystallite sizes of the TiO2 nanoparticles,
TiO2 mesoporous and mesoporous/P25 films are 29, 10 and 15 nm, respectively.
Fig 3.12. XRD patterns of nanoparticles, mesoporous and mesoporous/P25 mixture TiO2 films.
„A‟ represents the anatase and „R‟ the rutile phase of TiO2.
Table 3.1 summarizes various parameters obtained from BET characterization for
the three samples. The specific surface area of mesoporous TiO2 is found to be 189 m2g
-1,
which is about 3.4 times higher than the surface area of TiO2 nanoparticles (56 m2g
-1).
The addition of TiO2 nanoparticles to mesoporous matrix does not change the surface
area significantly (180 m2g
-1). A high surface area is desired for a good amount of dye to
be chemisorbed [40]. The particle size D is given by the following equation:
20 30 40 50 60
Inte
nsi
ty (
a.u
.)
2θ (Degree)
P25 Nanoparticles
Mesoporous TiO₂
Mesoporous/P25
A
A
A
A
A
A
AR R RR
R
111
D= 6000/ [SBET . ρ] (3.2)
where SBET is the BET specific surface area and ρ the density of the TiO2 (4.2 gml-1
). The
average particle size is estimated by assuming that particles are of similar density as well
as similar spherical shape and size, thus contributing to some extent towards inaccuracies
especially for the mesoporous/P25 sample. The particle size obtained by the BET method
shows similar trend as the crystallite size calculated from the Scherrer‟s formula, though
large discrepancies of the crystallite size data are expected.
Fig 3.13 reveals Nitrogen (N2) adsorption-desorption isotherms for all the
samples. The mesoporous TiO2 and mesoporous/P25 mixture samples exhibit type IV
nitrogen isotherm while the TiO2 nanoparticles exhibits a type III isotherm. Type III
isotherm has adsorption and desorption branches of isotherm that coincide, since no
adsorption-desorption hysteresis is displayed, indicating a non-porous material. On the
other hand, adsorption on mesoporous and mesoporous/P25 TiO2 samples proceeds via
multilayer adsorption followed by capillary condensation (Type IV isotherm). The
adsorption process is initially similar to those of macroporous solids, but at higher
pressures the amount adsorbed rises steeply due to the capillary condensation in
mesopores. Capillary condensation and evaporation do not take place at the same
pressure, thus leading to the formation of hysteresis loops. The isotherm of mesoporous
TiO2 exhibits hysteresis loop type H1, which represents materials with cylindrical pore
geometry and high degree of pore size uniformity [41]. The isotherm of mesoporous/P25
seems to be an intermediate between Type H1 and H3. H3 hysteresis loop does not level
off at relative pressures close to the saturation vapor pressure. It signifies material
112
comprises of aggregates (loose assemblages) of mesoporous particles [41]. This implies
that after mixing nanoparticles into the TiO2 mesoporous, it does not lose its
mesoporosity. Monolayer adsorption for the mesoporous TiO2 is completed at a relative
pressure of 0.4. However, the monolayer adsorption for mesoporous/P25 is not completed
until it reaches a relative pressure of 0.6. This indicates that mesoporous TiO2 possesses
much smaller pore sizes than the mesoporous/P25 sample. The pores in mesoporous TiO2
originate from the previously occupied block copolymer micelles space. The removal of
the triblock copolymer template leads to the formation of pores with narrow size
distribution (~8-10 nm). For mesoporous/P25 sample, the larger pores ~16 nm (Table
3.1) are presumably formed by the aggregation of small mesoporous TiO2 particles with
larger TiO2 nanoparticles when calcined at 450 °C. Larger pores are expected to have
better electrolyte/dye wetting and filling, which prevents charge from building up in the
dye layer which escalates charge recombination process.
113
Fig 3.13. Nitrogen adsorption-desorption spectra of nanoparticles, mesoporous and
mesoporous/P25 films.
Table 3.1. Physical properties of nanoparticles, mesoporous and mesoporous/P25 TiO2 films
Sample Anatase
%
Rutile
%
Crystallite
size
(nm)
Particle
size
(nm)
Surface
Area
(m2g
-1)
Pore
size
(nm)
Dye
Adsorbed
(molecm-3
)
P25 80 20 29 25.5 56 12 6.1 x 10-7
Mesoporous
TiO2
100 0 10 7.5 189 9 4.7 x 10-7
Mesoporous
/P25
88 12 15 8.0 180 16 1.5 x 10-6
0
20
40
60
80
100
120
140
160
180
200
0 0.2 0.4 0.6 0.8 1
Vo
lum
e S
TP (
cc/g
)
Relative Pressure (P/PO)
P25 Nanoparticles
Mesoporous TiO₂
Mesoporous/P25
114
The adsorption of the N719 dye-molecules in the three TiO2 films (same
thickness) is compared before the cell construction. The amount of dye loaded in each
sample is obtained by desorbing the samples into 0.02 M NaOH in 2 ml ethanol. All the
samples are 2 cm x 2 cm in size with a thickness of approximately 10 µm. Fig 3.14a
shows the UV-Vis absorption spectra of the dye loaded samples. All Ruthenium-based
dyes exhibit ligand-centered charge transfer (LCCT) transitions (π-π*) as well as metal-
to-ligand charge transfer (MLCT) transitions (4d-π*). N719 dye shows the absorption
maximum at wavelengths corresponding to 380 and 518 nm. The distinct peaks seen
around wavelengths 372 and 509 nm for the TiO2 nanoparticles, mesoporous and
mesoporous/P25 films are attributed to the MLCT transfer absorption band of the N719
dye. A small shift (~10 nm) is observed in the MLCT peaks of the dye loaded samples as
compared to the peaks of N719 the dye alone. This is due to the interactions of dye
molecules with TiO2. The dye loading capability of each sample is shown in Table 3.1.
The results show the high dye absorbance of the mesoporous/P25 film as compared to the
other two homogeneous (TiO2 nanoparticles and mesoporous) samples. This may be
attributed to the presence of relatively larger pores, which are expected to have better dye
wetting and loading.
Light scattering properties of the TiO2 films are investigated and shown in Fig
3.14b. The incident light, which is reflected back into the cell results in enhanced
absorption in the cell. It can be observed that the mesoporous/P25 film exhibits an
improved reflectance over the mesoporous TiO2 film, although the nanoparticles film
shows the highest reflectance properties. This suggests that the addition of larger
115
nanoparticles promotes light scattering, which contributes to an increase in the diffuse
reflection component in the spectrum. It can also be seen that the mesoporous/P25 films
possess good reflectance characteristics both in the short and long wavelength region
ranging from 350-800 nm, unlike mesoporous film, which has a slight dip in reflectance
in region from 400 to 600 nm. From the reflectance spectra, it is apparent that there is an
improvement ~30 % in the reflectance (visible light range) due to the contribution of
larger particles and rough surfaces of the mesoporous/P25 film.
Fig 3.14. Optical properties of nanoparticles, mesoporous and mesoporous/P25 films: (a) UV-Vis
absorption of dye loaded samples and (b) diffused reflectance spectra.
(a)
(b)
325 375 425 475 525 575 625
Ab
sorb
ance
Wavelength (nm)
P25 Nanoparticles
Mesoporous TiO₂
Mesoporous/P25
0
10
20
30
40
50
60
300 400 500 600 700 800
Ref
lect
ance
(%)
Wavelength (nm)
P25 Nanoparticles
Mesoporous TiO₂
Mesoporous/P25
116
Fig 3.15a displays the photocurrent density-voltage characteristics of the cells
made from different films under simulated irradiation (global AM 1.5. 100 mWcm2). The
device fabrication procedure is described in detail in chapter 2 under section 2.2. Table
3.2 shows the summary of their photovoltaic properties. The active area of the electrodes
is 0.5 cm x 0.5 cm. The low efficiency of the mesoporous film (<1 %) is due to the
presence of microcracks, which leads to poor adhesion to the photoanode. As can be
seen, Jsc of the mesoporous/P25 film is about 1.6 and 7.2 times higher than homogeneous
TiO2 nanoparticles and mesoporous DSSC respectively. The results suggest better charge
injection and transportation of photoexcited electrons in the DSSC made from
mesoporous/P25 film. The increase in the open circuit voltage (Voc) and the fill factor
(FF) of the DSSC mesoporous/P25 film indicates the suppression of the charge
recombination process at the TiO2/electrolyte interface. Hence, mesoporous/P25 DSSC
gives a higher efficiency of 6.46 % than the other DSSC (TiO2 nanoparticles and
mesoporous) films. It is well known that the IPCE of a DSSC is a combined result of the
light harvesting efficiency, electron-transfer yield and electron collecting efficiency in the
external circuit.
Fig 3.15b shows the IPCE spectra obtained for the three DSSC structures. The
absolute IPCE for mesoporous/P25 DSSC is significantly higher than the other two
DSSC over the entire wavelength region. Adding large particles of Degussa P25
influences the electrical contact between the TiO2 nanoparticles and also increases the
light harvesting capability of the entire cell, thus increasing the electron yield of the
DSSC. This result is in good agreement with the high short-circuit photocurrent density
117
observed in the solar cell. The increase in efficiency may be attributed to four main
factors. Firstly, the mesoporous/P25 TiO2 film is crystalline and predominantly anatase
phase which helps in better electron transport. Secondly, the high surface area of
mesoporous/P25 film helps in better dye adsorption. Thirdly, addition of larger TiO2
nanoparticles of size ~30nm helps in better light scattering and formation of bigger pores.
Lastly, mesoporous structure of hybrid film helps in rapid and efficient interfacial
electron transfer between TiO2 and redox active species of the electrolyte [5].
Fig 3.15. Performance of DSSC made from nanoparticles, mesoporous and mesoporous/P25
films: (a) Current density–voltage and (b) IPCE curves.
0
2
4
6
8
10
12
0 0.2 0.4 0.6 0.8
Ph
oto
curr
ent
De
nsi
ty (
mA
.cm
-2)
Photovoltage (V)
P25 Nanoparticles
Mesoporous TiO₂
Mesoporous/P25
0
10
20
30
40
50
60
70
400 450 500 550 600 650 700
IPC
E (%
)
Wavelength (nm)
P25 Nanoparticles
Mesoporous TiO₂
Mesoporous/P25
(a)
(b)
118
Table 3.2. Photovoltaic characteristics for different films measured under illumination with AM
1.5 simulated sunlight.
3.4 Conclusions
Systematic tuning of pores structures has been demonstrated via the concentration of
copolymer, which results in two different types of micellar packing. Also the calcination
temperature is optimized to be 430 C as it results in well-crystalline and stable porous
framework. The synthesized mesoporous TiO2 film shows poor adhesion to FTO
substrate, leading to microcracks formation. This issue is addressed by adding
nanoparticles into mesoporous film. The nanoparticles essentially function as active
binders for the mesoporous film. The resultant film has dominant anatase phase, high
surface area, big pore diameter and high diffuse reflectance properties. The photocurrent
density of the DSSC made from hybrid TiO2 film is 1.6 and 7.2 times higher. The overall
current conversion efficiency of 6.46 % is achieved for DSSC made from
mesoporous/P25 mixture film. Enhanced photovoltaic performance is a result of uniform
DSSC Voc
(V)
Jsc
(mAcm-2
)
FF
(%)
Efficiency
(%)
Nanoparticle 0.77 7.60 67.9 4.08
Mesoporous 0.77 1.84 63.0 0.90
Mesoporous/P25 0.77 12.02 70.1 6.46
119
crack free film, better dye loading and improved light harvesting capability of the
modified film. The mesoporous/P25 film with integrated nanoparticles scattering centers
and mesoporous structure has resolved conflicting factors of providing large surface areas
as well as strong light scattering for high efficiency DSSC.
120
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123
Chapter 4: Titanium Dioxide (TiO2) Nanotubes for Liquid
Dye-Sensitized Solar Cell (DSSC)
3D nanoparticles in the mesoporous TiO2 film exhibit high surface area but suffer
from low diffusion coefficient, which limits the conversion efficiency of DSSC. 1D
nanostructures provide directional mobility and enhance the diffusion coefficient. This
chapter concentrates on the fabrication of highly ordered titanium dioxide (TiO2)
nanotubes on Ti foil using electrochemical anodization technique. The anodization
voltage and duration have been systematically varied to obtain nanotubes of varying
diameters and lengths, which are then used in DSSC.
4.1 Introduction
Ever since Desilvestro et al. [1] reported the absorption of Ru-based complex on
TiO2 electrode, a new road for solar energy conversion has been opened. Later on Grätzel
and co-workers [2-4] extended this concept to make a DSSC. For a high conversion
efficiency of DSSC, both reaction kinetics and thermodynamics of the cell are crucial.
The rate constant for the electron injection from conduction band of semiconductor has to
be faster than the de-excitation of the dye [5]. Also, the regeneration time for the dye has
to be fast to avoid depletion effects [6]. DSSC made with nanoparticle morphology
suffers from low electron transport time constants, like electron diffusion coefficients [7-
124
8]. The reason is the transport of electron via trapping and de-trapping mechanism in the
semiconductor, leading to electron recombination via defects, surface states and grain
boundaries. To overcome this problem research has now been focused on utilizing one-
dimensional (1D) nanostructures for the DSSC.
1D nanostructures are expected to significantly improve the electron transport due
to directional electron mobility and decreased intercrystalline contacts. Accelerated
electron transport and lower recombination are expected, as electron motion is not limited
by the random walk inside the semiconductor. Among 1D nanostructures, nanotubes are
especially researched. Sol-gel and hydrothermal synthesis are the most common routes to
synthesize them [9-10]. The nanotubes produced, however, lack in 1D orientation and
good crystallinity. This hinders the unidirectional electron flow that is expected along the
length of the tube. Therefore, synthesis of vertically aligned nanotubes by anodization in
an appropriate electrolyte has been significant. Zwilling [11] and co-workers firstly
introduced the process of anodization of Ti and Ti alloys in chromic acid-HF mixture.
Electrochemical anodization also has the versatility to grow TiO2 nanotube arrays on any
Ti substrate independent of its geometry.
In this work, vertically oriented TiO2 nanotube arrays with controllable diameter
and length were synthesized on a Ti foil. Due to the high performance [12-13], ease of
preparation and availability of materials, ethylene glycol (EG) based electrolyte is chosen
for anodization. The nanotubes are investigated for their potential applications in liquid
DSSC. Variation in the length and diameter of the nanotube arrays has been demonstrated
125
to critically influence the solar conversion efficiency of the DSSC. It is demonstrated that
even with low surface area morphology, high DSSC efficiency can be achieved if
directional transport with less grain boundaries is available for electrons to transport.
4.2 Experimental
4.2.1 Anodization of Ti foil
The electrochemical electrolyte was prepared by adding 0.25 wt % of ammonium
fluoride (NH4F) and 1 vol % of distilled water into ethylene glycol (C2H6O2, EG). The
mixture was used in the anodization after aging for 24 h. 2.0 mm thick Ti foil was
sonicated with isopropyl alcohol, methanol, and ethanol and finally dried under nitrogen
stream to serve as anode. The cathode was prepared by sputtering 280 nm thick platinum
layer on a glass slide. Both anode and cathode were separated by approximately 3 cm in a
beaker and supported by retort stand to a programmable DC power supply. Applying
varying voltages for 2 h with 10 V intervals grew the nanotubes. Finally, the grown TiO2
nanotubes were annealed at 450 °C for 30 min at a ramp rate of 5 °C/min in Carbolite
CWF1200 burn-off furnace. Nanotube grown Ti foil is used as the working electrode for
fabricating DSSC, as described in section 2.2 under chapter 2. The Ti foils were reused
after each experiment by polishing with 600-grade silicon carbide paper for first 10 min
and 800-grade silicon carbide paper for another 10 min. This polishing process is to
achieve a smooth surface for nanotubes to grow without the formation of disordered top
layers of nanotubes (“nanograss”).
126
4.3 Results and discussion
4.3.1 Effect of anodization voltage
Anodization of Ti foil is carried under different voltages for 2 h using the EG
electrolyte. Below 20 V, no growth of nanotubes is observed. When anodization is
carried out at 20 V, nanotubes of ~30 nm diameter are formed, but the growth is not
uniform (Fig 4.1a). The nanotubes are not fully formed and broken tops can be observed.
Also the tube length varies across the sample indicating that the anodization is not
consistent throughout the foil. Fig 4.1b, c and d show the FESEM images of the TiO2
nanotube array at 30, 40 and 50 V, respectively. It can be seen that the diameter of
nanotubes increases as the anodization voltage increases. When the anodization voltage
reaches 50 V, the inter-tube spacing broadens and leads to formation of larger diameter
tubes. The increased spacing between the tubes is not good for fabricating DSSC, as the
electrolyte seeps through and leads to short-circuit. Fig 4.2a shows the top view of the
nanotubes grown by anodization at 60 V for 2 h. The TiO2 nanotubes in the array have
similar length (~14 μm), internal diameter (~180 nm) and wall thickness (~10 nm). The
tubes are uniform and compact as revealed by the tilted view of FESEM (Fig 4.2b). Such
a compact packing of the arrays can reduce the contact between Ti foil and electrolyte,
which is considered vital to suppress the dark current in the DSSC [14]. Fig 4.2c reveals
that the TiO2 nanotubes grow vertical to the Ti foil, featuring highly parallel 1D
nanostructures.
127
Fig 4.1. SEM images of TiO2 nantubes grown for 2 h at different voltages of (a) 20, (b) 30, (c) 40
and (d) 50 V.
Despite the substantial changes in the nanotube diameter, it is noteworthy that the
overall nanotube architecture is retained over the entire anodization voltage range. Fig
4.3a displays the trend of nanotube growth with the anodization voltage. Nanotube
diameters seem to obey power law dependence from 30-60 V. TiO2 nanotubes are well
aligned and no change is the alignment is observed when the applied voltage is varied.
Also the nanotubes produced using higher anodization voltages are much straighter and
have rough surface. This may be ascribed to the slower chemical dissolution rate of the
anodic reaction, which leads to formation of straight-line tubes in stable sites. The lower
anodization voltages produce nanotubes by fast chemical dissolution process, which leads
128
to smother tube surface because there is insufficient time for the nanotubes to grow on
stable sites of Ti foil. Fig 4.3a and b show a bunch of closely packed nanotubes annealed
at 450 °C for 30 min. The inset of Fig 4.3a reveals that the exterior walls are relatively
smooth and continuous. The dark areas may result from either varying crystalline density
or due to overlap of nanotubes. The broken edges of the nanotubes arise during the
sample preparation for TEM measurements. The selected area electron diffraction
(SAED) pattern of the calcined nanotubes (inset of Fig 4.3b) shows polycrystalline
structure composed of anatase basic units with phase of (101).
129
Fig 4.2. SEM images of TiO2 nanotubes grown at 60 V for 2 h (a) top, (b) tilted and (c) cross-
sectional view.
130
Fig 4.3. (a) Graph showing variation of TiO2 nanotube diameter and length with anodization
voltage. (b) and (c) TEM images of an nanotubes.
The crystal structure is also confirmed by XRD analysis (Fig 4.4). The as-
prepared TiO2 nanotube arrays are commonly amorphous after anodization. The XRD
pattern shown indicates that calcined nanotubes are crystalline and composed of pure
anatase TiO2 phase (JCPDS file no. 21-1272). Good crystallinity is required for smooth
electron flow in the nanotubes for an efficient DSSC. Annealing TiO2 nanotubes at 450
°C does not lead to single crystalline material but forms very large grains. These grain
131
sizes are considerably larger than those observed for Grätzel nanoparticles and therefore
higher electron mobility can be expected for nanotubes.
Fig 4.4. XRD spectrum of TiO2 nanotubes showing the anatase composition.
The top and bottom morphologies of the nanotubes are investigated to understand
the growth mechanism of the tubes. Tiny pits are observed on the bottom oxide surface,
which are created due to localized dissolution of the Ti metal (Fig 4.5a). This happens
when the growth of the oxide at the Ti/TiO2 interface becomes equal to the rate of
dissolution of the Ti foil. The rate of chemical dissolution is determined by the fluoride
concentration in the electrolyte. Small voids are formed, which cause the tube to space
out, and distinct tube walls to form. The chemical etching during the growth makes the
132
tubes walls thinner at the top than at the bottom. Under severe cases, the upper part of the
nanotubes may become so thin that the tube upper parts may collapse completely.
Formation of „nanograss‟ on tube tops is a common problem encountered when TiO2
nanotubes are grown for extended periods of time (Fig 4.5b and c). This feature is
believed to originate due to inhomogeneous etching of the tube top, which leads to the
thinning of the tube walls. The grass on the tube tops poses a problem for dye adsorption
and hence affects the electron transport through the nanotubes. Ti foil is polished prior to
anodization to get rid of the grass [15]. A protective layer is formed over the top surface
of the Ti foil which delays the chemical attack on the tubes.
133
Fig 4.5. SEM images of (a) bottom surface, (b) top surface with nanograss and (c) top view of
TiO2 nanotubes grown on Ti foil.
Since the Ti Foil is opaque to light and constitutes both the photoanode as well as
electrode, the DSSC has to be illuminated from the counter electrode side. Hence, it is
134
called back illumination. As a result, light has to pass through the Pt-coated FTO, which
cuts down the amount of light reaching the dye molecules. It is, therefore, advisable to
use a thinner layer of Pt in these cells. In this work the counter electrode is coated with 2
nm Pt layer. In addition, the light has to pass through the electrolyte as well, which
further reduces the amount of light that reaches the dye. These two factors can adversely
affect the overall efficiency of the cell.
4.3.2 Growth mechanism of TiO2 nanotubes
The electrochemical anodization is a process in which a layer of oxide is created
over a metal surface (fig 4.6). The metal being anodized (titanium foil) is connected to
the anode of a DC power supply and an inert metal (Platinum sputtered on glass) is
connected to the cathode. The two electrodes are immersed in an electrolyte, which
consists of EG, NH4F and H2O. When the DC power is turned on, electrons flow from the
Pt to the Ti foil. Growth of the nanotubes can be described as selective etching and is a
competition between several electrochemical and chemical reactions listed below.
Anodix oxide layer formation on Ti foil can be described as:
Ti + 2H2O → TiO2 + 4H+ + 4e
- (4.1)
Afterwards, chemical dissolution of TiO2 as soluable fluoride complex takes place.
TiO2 + 6F-+ 4H
+ → [TiF6]
2- + 2H2O (4.2)
Direct complexation of Ti4+
ions migrating through the film also happens.
Ti4+
+ 6F- → [TiF6]
2- (4.3)
135
Fig 4.6. Anodization setup for the growth process of TiO2 nanotubes.
When the DC power supply is switched on, the voltage across the anode and the
cathode induces an electric field. Firstly an oxide layer is formed, leading to decay in
current due to the reduction of electroconductivity of the layer. However, the oxide layer
that has formed resists the current flow. After the oxide layer is formed, the Ti4+
metal
ions migrate from the Ti foil to the electrolyte. The migration happens under the
influence of an electric field, caused by the applied voltage. Next, a field-assisted
dissolution of TiO2 takes place at the Ti/electrolyte interface [16-17]. Due to the
roughness of the barrier layer, different parts of the film have different film thickness L1
and L2 (fig 4.7). Such non-uniformity results in an uneven distribution of electric field
with the film, causing faster ion migration in the thinner areas. This effect can be
particularly pronounced due to the high mobility of the fluoride ions. The difference in
ion transport results in difference in local current density, leading to difference in local
dissolution rates. During this stage, the surface is activated and pores start to grow. This
is accompanied by rise in the current due to an increase in available surface area. Afetr
136
sometime, many pores have been initiated and a tree-like growth takes place. The
individual pores start interfering with each other and competing for the available current.
This leads to re-distribution of local current density, resulting in equal sharing of current
between pores and accompanied by the self-assembly of the pores. The current passing
through the electrode is stabilized and steady-state growth of nanotubes occurs. The
overall rate of the whole process in steady-state phase is limited by the diffision of
fluoride ions inside the channel from the bulk solution to the growing nanotubes and
transport of [TiF6]2-
in the opposite direction. Both field-assisted dissolution and chemical
dissolution of the oxide layer occur during this process, but the field-assisted process
dominates in the initial stages of the anodization [18]. This is due to the stronger electric
field across the initially thin oxide layer. The length of the nanotube stops increasing
when the rate of chemical dissolution at the top of the tube equals to the rate of oxide
formation and inward movement of the Ti/TiO2 interface at the base of the tube.
137
Fig 4.7. Schematic for the growth process of TiO2 nanotubes.
In aqueous solution electrolytes at constant potential, most metals give rise to
current-time curves with an exponential decay shape. This shape correspondes to the
passivation of the electrode surface which, results from a barrier layer formation of low
conductivity. However, addition of fluoride ions results in an initial exponential decay of
current followed by an increase in current and later quasi-steady state level. Fig 4.8 plots
the curve of current density, with respect to time in the initial anodization state. The
steady state growth of nanotubes starts from stage “A”, as marked in the Fig 4.8.
Considerable current fluctuations could arise from the suppressed heat transfer around the
electrodes in ethylene glycol solution. As seen from the graph, the anodization starts with
a relatively high current density. This is because there is no barrier layer present to inhibit
138
the current flow. However, the current decreases sharply due to the formation of the
oxide layer. The initial oxide layer persists only for a few seconds at the beginning and
thereafter the formation of oxide bundle occurs. Thus, the drop in the current is both due
to the formation of compact oxide layer and the formation of oxide bundles on the
surface, after which the bundles begin to disperse into the electrolyte. As the anodization
proceeds, the conductivity of the electrolyte increases causing the applied anodization
potential to shift away from the electrolyte towards the oxide layer. With a larger electric
field across the oxide layer, the oxide starts to form more rapidly, thus causing the current
density to rise to a maximum value, which is represented by the peak around 500 s. The
current then begins to drop to a rate of oxide formation, which is determined by the rate
of field-assisted oxide growth, field-assisted dissolution and chemical etching.
Fig 4.8 Current-time graph of the anodization process for the growth of TiO2 nanotubes.
139
4.3.3 Effect of anodization voltage on DSSC performance
The effect of anodization voltage on the physical features of TiO2 nanotubes is
studied. The voltage applied during anodization is varied from 20-60 V, keeping duration
constant to 2 h, to obtain an optimized morphology for the DSSC photoanode. Fig 4.9
shows the J-V curves for the DSSC assembled with different photoanodes. The
parameters of the DSSC are summarized in Table 4.1. It can be observed that as the tube
diameter increases the conversion efficiency of the DSSC increases. As discussed earlier
with a low anodization voltage, the nanotube growth is not uniform, which leads to low
solar conversion efficiency. With the application of higher voltages (50-60 V) the
nanotubes are longer, bigger in diameter and more uniform. Due to their aspect ratio the
nanotubes are expected to have a low surface area compared to nanoparticle morphology.
The length of the nanotubes can be tuned from 2 to 14 μm by simply increasing the
anodization voltage from 20 to 60 V, accompanied with increased internal diameter from
30 to 180 nm. The fill factor (FF) and conversion efficiency first decreases and then
gradually rises with the anodization voltage. The initial decrease in FF may be due to a
thicker barrier layer that forms at higher voltages, thus increasing the series resistance of
the DSSC. The Voc is slightly lower for 20 and 40 V nanotubes. The lower Voc may be
scribed to the higher series resistance and higher number of recombination centers
present for the tubes. Consistent with reported literature, the internal quantum efficiency
of the DSSC increases with the tube length [19-21]. The important point to note is that
even though nanotubes have low surface area, they still show a decent efficiency. This
efficiency comes from directional electron transport in the nanotubes. The nanotubes
140
grown at 60 V for 2 h shows the best solar conversion efficiency and are used to
incorporate Ag nanoparticles.
Fig 4.9. J-V curve for the DSSC made with TiO2 nanotubes grown at different anodization
voltages.
0
2
4
6
8
10
12
0 0.2 0.4 0.6 0.8
Ph
oto
curr
en
t D
en
sity
(m
A.c
m-2
)
Photovoltage (V)
20 V
30 V
40 V
50 V
60 V
141
Table 4.1. Photovoltaic characteristic of TiO2 nanotubes grown at different anodization voltages.
Anodization
Voltage
(V)
Tube
Diameter
(nm)
DSSC
Efficiency
(%)
Fill
factor
(%)
Jsc
(mAcm-2
)
Voc
(V)
20 30.0 0.2 64.4 0.6 0.50
30 40.0 1.0 53.8 3.0 0.62
40 60.0 2.3 58.0 7.0 0.58
50 100.0 3.7 58.6 9.7 0.63
60 180.0 4.5 68.2 10.6 0.62
4.3.4 Effect of anodization duration on DSSC performance
The anodization duration is also varied to ascertain its effect on DSSC
performance. The anodization voltage was kept constant at 60 V for this experiment. As
anodization duration increases, the length of TiO2 nanotubes increases almost linearly
until 2 h. There is a sharper increase in tube length between 1 and 2 h, which corresponds
to the relatively larger current densities during the initial growth process. Overall
conversion efficiency increases with the length of TiO2 nanotubes up to 14 μm. Any
further increase in tube length results in a decrease in conversion efficiency (fig 4.10). As
the length of the nanotubes increases, the electron diffusion length becomes shorter and
more defects are introduced in the tube morphology, thus hindering the electron transport.
142
Also, longer tubes give poorer efficiency as they start to loose stability and break mid-
way. Thus the optimized TiO2 nanotube has an approximatel length of 14 μm. The DSSC
performance parameters for the nanotubes grown at different anodization time intervals
are summarized in Table 4.2.
Fig 4.10. J-V curve for the DSSC made with TiO2 nanotubes grown for different anodization
time.
143
Table 4.2. Photovoltaic characteristic of TiO2 nanotubes grown for different anodization duration.
Anodization
Duration
(h)
Tube
Length
(μm)
DSSC
Efficiency
(%)
Fill
factor
(%)
Jsc
(mAcm-2
)
Voc
(V)
0.5 4.5 0.3 49.0 1.0 0.62
1.0 8.0 1.7 65.2 3.7 0.70
2.0 14.0 4.5 68.2 10.6 0.62
3.0 20.0 3.0 60.0 7.6 0.65
4.0 9.0 1.1 55.0 3.4 0.60
4.4 Conclusions
The aim of this work is to demonstrate the effect of directional mobility in 1D
nanostructures. Such morphology leads to high efficiency DSSC even when the surface
area is low. High efficiency DSSC using TiO2 nanotubes have been demonstrated in this
work. TiO2 nanotubes are optimized by varying electrochemical anodization conditions
(e.g. voltage and duration). It is found that the performance of the solar cells strongly
depends on the morphology and structure of the nanotubes. DSSC with an overall
conversion efficiency of 4.5 % are obtained due to the optimization of the nanotubes.
144
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146
Chapter 5: Iron Oxide (Fe2O3) Nanoflowers for Liquid Dye-
sensitized Solar Cell (DSSC)
Three-dimensional (3D) nanoparticles in mesoporous TiO2 film exhibit good
optical transparency and large surface area but have weakness of small diffusion
coefficient. In order to overcome these limitations, one-dimensional (1D) nanotubes are
introduced. However, nanotubes have the drawback of low surface area. Hybrid
nanostructures are combination of 3D and 1D morphologies and have recently emerged
as a promising candidate for DSSC, as they can combine good dye adsorption with better
electron transport properties. A lot of research on nanomaterials has been carried out in
the recent years. Currently there is lack of research on such nanostructures which have
efficient electron transport and high surface area. This chapter discusses on developing
hybrid α-Fe2O3 flowerlike morphology, which exhibit both superior electron transport
and high surface area. Due to its simplicity, the synthesis process can be easily
reproduced and scaled up. In-depth studies on dye-sensitized solar cell (DSSC) are
carried out so as to gain understanding on the effect of morphological variation on
surface area and transport properties. Furthermore, the hybrid α-Fe2O3 nanostructures are
studied as potential candidates for DSSC for the first time.
147
5.1. Introduction
Metal oxides are topic of intense research due to their potential use in the
development of electronic devices. Many exciting properties emerge when the spatial
dimensions of these materials are reduced to nanoscale. Hence, over the years a large
number of metal oxides with complex structures have been successfully synthesized [1-
5]. Among these much attention has been focused on hematite (-Fe2O3) nanostructures
because of their various applications [6-11]. It is an abundant, stable and environmental
friendly n-type semiconductor with a band gap of 2.2 eV, sufficient to utilize ~ 40 % of
the sunlight. Alpha iron oxide (hematite) is the most frequently occurring polymorph
with a rhombohedral-hexagonal structure. The hematite lattice is composed of alternating
iron bi-layers with oxygen layers in parallel to (001) basal plane. Various shapes and
dimensions of -Fe2O3 nanostructures are available such as nano-wires [12], nanorod
[13], nanotubes [14], hollow fibers
[15], nanorings
[16] and cubes
[17]. The performance
of -Fe2O3 is strictly influenced by its morphology, size and porosity. Jain et al. [18]
reported that the electrochemical properties of nanocrystalline -Fe2O3 are dramatically
superior to those of microscale -Fe2O3. Yu et. al. [19] reported enhanced visible light
photocatalytic activity for hollow spheres of -Fe2O3. Apart from chemical vapor
deposition and template-assisted methods, solution based methods for the preparation of
-Fe2O3 nanostructures is gaining widespread popularity due to ease of fabrication and
economic reasons. However, self-assembly synthesis of different -Fe2O3 morphologies
still remains a great challenge.
148
Three-dimensional (3D) nanoparticles have been extensively employed in DSSC
due to combination of optical transparency and large surface area. However, nanoparticle
based DSSC have the primary weakness of small diffusion coefficient (D). The electron
transport phenomenon in DSSC is a combination of trapping and de-trapping
mechanisms, which is highly dependent on the Fermi level. A typical DSSC made with
TiO2 nanoparticles [20] shows D of the order of 10
-4 cm
2s
-1. In order to overcome the
apparent limitation of nanoparticles, one-dimensional nanorods and nanotubes are
introduced [21-22]. The appreciable increase in diffusion coefficient of nanorods and
nanotubes comes from increased diffusion length (L), thus benefitting the charge
separation and transport. Compared to 3D nanoparticles, nanorod morphology lacks in
surface area, significantly decreasing the photocurrent density and therefore limiting the
conversion efficiency [21]. Hybrid nanostructures have recently emerged as a promising
candidate for DSSC, as they can combine good dye adsorption with better electron
transport properties. Achieving high dye adsorption is possible on a 3D spherical surface,
while presence of 1D nanostructures can direct the electron transport. Hybrid
combination of TiO2 nanowires with P25 nanoparticles in DSSC achieved 6.01 % solar
conversion efficiency, which is 60 % higher than DSSC based on only TiO2 nanowires
[23]. 7.12 % conversion efficiency is recorded when TiO2 nanorods (~20 nm diameter)
are used in conjunction with P25 nanoparticles [24]. The efficiency of ZnO based DSSC
is also improved using hybrid structures [25].
In this work, hybrid α-Fe2O3 flower-like morphology is synthesized to achieve
high surface area with high diffusion coefficient for better electron transport. A simple
149
and reproducible solution route is used to synthesize the nanostructures. Abundance of
Fe2O3 in nature and economical solution based method make this work very attractive for
fabricating cheap devices. The performance of devices is investigated for different
morphologies of α-Fe2O3 nanostructures, through chracterization of microstructure,
electronic and optical properties. The possibility of using -Fe2O3 as a new electrode
material for dye sensitized solar cells is explored for the first time. To the best of our
knowledge the photovoltaic properties of Fe2O3 in DSSC have not been reported so far.
The performance of the devices is evaluated based on the surface area, grain boundaries
and diffusion coefficients characteristics. The findings on probing the effect of hybrid
nanostructures on various properties are of great importance to surface dominated
applications.
5.2. Experimental
5.2.1 Synthesis of α-Fe2O3 nanoflowers
In a typical experiment, 2 mmol iron chloride (FeCl3.6H2O) and 2 mmol sodium
sulfate (Na2SO4) are added into 40 mL deionized water and stirred until totally dissolved.
The solution is heated in autoclave at 120 °C for 6 h and then cooled to room
temperature. The products are collected by centrifugation and washed with de-ionized
(DI) water and absolute ethanol several times and later dried in air at 60 °C for 12 h. The
as-prepared FeOOH powder is annealed in air at 400 °C for 2 h at a ramping rate of 1
°Cmin-1
to convert to Fe2O3. Various samples with different amounts of iron chloride
150
hexahydrate ranging from 0.04 to 1 mM are prepared. The powders are converted to fine
paste using DI water and poly(ethylene glycol) (PEG) for DSSC. The process of
fabricating DSSC is described in detail in section 2.2 under chapter 2.
5.3. Results and discussion
5.3.1 Characterization of α-Fe2O3
Fig 5.1 depicts the top view SEM images of 0.04 mM sample with a flower like
morphology. The film is composed of bundle of nanoflowers (Fig 5.1a). The morphology
is almost spherical in shape and uniform throughout the sample (Fig 5.1b). At high
magnification, it can be seen that the film is an interconnected network of large petal like
structures originating from the core (Fig 5.1c). The average diameter is about 1 m with a
petal thickness of about 50 nm. A closer observation reveals that few of the nanoflowers
are hollow (Fig 5.1d). Other researchers have also reported such hollow structures among
other morphologies [26, 27-28]. Dissolution of the core in the reacting solution is
postulated to be one of the possibilities. However, the exact mechanism of formation was
unclear. The 0.04 mM sample is calcined in air at 400 C for 2 h. Calcination does not
change the overall morphology of the nanoflowers (Fig 5.1e and f). The size and texture
of the nano-flowers remains almost the same, as confirmed by SEM. The TEM images of
nanoflowers (Fig 5.2a and b) clearly depict closely packed petals, which agree with the
SEM images. The petals of the nanoflower are straight and have relatively uniform
151
thickness (Fig 5.2c). However, TEM image of the petals reveal that some of them may
have been disjointed or misaligned during the process of calcination (Fig 5.2d).
Fig 5.1. FESEM images at (a), (b) low magnification and (c), (d) high magnification of
nanoflower of -FeOOH. (e) and (f) -Fe2O3 nano-flowers after calcination at 400 C.
152
Fig 5.2. TEM images of (a), (b) nanoflowers and (c), (d) petal like structures originating
from nanoflowers.
A high resolution TEM (HRTEM) image taken from as prepared -FeOOH
sample is shown in fig 5.3a and b. The lattice fringes are visible with a spacing of 0.25
nm and 0.34 nm corresponding to (211) and (130) planes of -FeOOH respectively. The
corresponding SAED pattern (Fig 5.3c) of as prepared sample is not very sharp,
representing almost amorphous nature of the nanoflowers. Upon calcination, the lattice
fringe spacing being determined is 0.25 nm (Fig 5.3e), corresponding to the (110) lattice
plane of -Fe2O3. The nanoflowers are polycrystalline as seen in SAED data (Fig 5.3f).
153
Fig 5.3. (a) TEM, (b) HRTEM and (c) SAED pattern of as-obtained -FeOOH nanoflowers. (d)
TEM, (e) HRTEM and (f) SAED pattern of - Fe2O3 nanoflowers after calcination at 400 C for
2 h.
The ferric solution processed at 120 C does not yield hematite phase directly.
The XRD analysis (Fig 5.4a) identifies the phase of the product to be goethite (α-
FeOOH) [JCPDS No: 29-0713] and akaganeite (β-FeOOH) [JCPDS No: 34-1266]. Hence
the final phase of the 0.04 mM un-annealed sample is composed of a mixture of - and β-
FeOOH. The nanoflowers seem to be polycrystalline in nature before calcination. The
crystallite size of the sample is calculated using Scherrer‟s formula [29], and is calculated
to be approximately 0.9 m, and close to value estimated by SEM. A phase change
occurs on calcination at 400 C of the goethite structures, as detected by XRD (Fig 5.4b).
All peaks are indexed to rhombohedral phase of hematite [JCPDS No: 89-2810]. The
154
strong and sharp peaks indicate that calcined Fe2O3 powder is highly crystalline. The
crystallite size of the calcined nanoflowers is estimated to be 0.6 m. The results agree
well with the SAED data.
Fig 5.4. XRD pattern of (a) as-obtained -FeOOH and (b) -Fe2O3 nanoflowers.
α-Fe2O3 is an extrinsic semiconductor with an indirect band gap of 2.2 eV, but a
direct band gap has also been reported [30-31]. UV-Vis measurements were employed to
155
deduce the band gap of α-Fe2O3 films. The films prepared of α-Fe2O3 are red-brown and
semi-transparent. The thickness is measured to be approximately 1.5 μm. The onset of
absorption is revealed around 2 eV in the UV-Vis absorption spectrum of Fig 5.5a. The
transmittance spectrum of the film is shown in Fig 5.5b. The average transmittance is
found to be about 55 % in the visible region. However, still Fe2O3 is chosen for DSSC as
it is cheap, easily available and chemically stable. The nature and onset of the electronic
transitions can be estimated using the following equation.
mhAh g0 E (5.1)
where α is the absorption coefficient, hν the photon energy and Eg the band gap energy.
Ao and m are constants and depend on the type of electronic transition, where m takes the
value of ½ for direct and 2 for indirect band gap transitions. To establish the type of
band-to-band transition in the α-Fe2O3 samples, the absorption data is fitted to eq. 1 for
both indirect and direct band gap transitions. Fig 5.5c shows the (αhν)1/2
versus photon
energy plot for an indirect transition and Fig 5.5d shows the (αhν)2 versus photon energy
for a direct transition. The approximate value of the band gap Eg was calculated by
extrapolating photon energy to αhν = 0. The extrapolation yields an Eg value of 2.18 eV
for indirect and Eg = 3.46 eV for direct transition. The estimated values are similar to the
ones reported in literature [31-32]. The indirect band gap allows absorption in UV and
blue part of visible light region, which gives α-Fe2O3 its reddish-brown color. For direct
band gap semiconductors, electronic transition from the valence band to the conduction
band is electrical dipole allowed. The electronic absorption as well as emission are
usually strong for such semiconductors. For indirect band gap semiconductors, the
valence band to the conduction band electronic transition is electrical dipole forbidden
156
and the transition is phonon assisted, i.e., both energy and momentum of the electron–
hole pair are changed in the transition. Both their absorption and emission are weaker
compared to those of direct band gap semiconductors, since they involve a change in
momentum. Hence a direct band gap transition would result in a more efficient
absorption of solar energy and therefore much better photovoltaic devices. This might be
the research direction for use of α-Fe2O3 as an efficient photovoltaic material.
Fig. 5.5 (a) Absorption and (b) Transmission spectra of 0.04 mM sample, (c) Plot of
(αh)1/2
versus photon energy for indirect transition and (d) Plot of (αh)2 versus photon energy
for indirect transition.
157
Time dependent experiments were carried out to understand the growth
mechanism of the -FeOOH nanoflowers. The samples were collected from reacting
solution mixture at different time intervals during the growth process are observed under
SEM. Fig 5.6a shows the growth obtained after 30 min with 0.04 mM of FeCl3.
Aggregates of solids can be clearly observed. With increased reaction time to 1 h, more
growth of particles can be seen (Fig 5.6b). The average particle size as estimated from
SEM is approximately 100 nm. For the growth time of 2 and 3 h, the particles increase in
size (~400 nm) and tend to agglomerate together (Fig 5.6c and d). Assembly of 3D
structures can be observed when the reaction time reaches 4 h (Fig 5.6e). However, at this
time the structures are of varying shapes and sizes. At 6 h reaction time almost spherical
nanoflower morphology can be obtained (Fig 5.6f). Authors have also carried out TEM
analysis on the time dependent experiments to further investigate the growth mechanism
(Fig 5.7). On the basis of the time dependent experiments, the α-Fe2O3 nanoflowers are
grown via aggregation of nanoparticles followed by nucleation and growth of
nanoneedles. In the initial stage, primary α-FeOOH nanoparticles are formed through the
hydrolyzation of Fe3+
, which serve as the nuclei in the growth process. In the work
reported, the presence of Na2SO4 is believed to play a crucial role for the formation of the
unique self-assembled structures. It is well known that the aggregation process involves
the formation of larger nanoparticles by reducing the interfacial energy of the smaller
particles. However, the interaction between unprotected building units is generally not
competent to form stable and uniform microstructures [32]. The presence of Na2SO4
provides the necessary strong surface protection to form flower-like structures [32]. The
sulphate ions serve as ligand to Fe3+
, and may adsorb on the facets parallel to the c-axis
158
of -FeOOH nuclei [33]. The nanoneedle gradually evolved into 3D flower-like
superstructures through oriented attachment to reduce the surface energy. It is speculated
that the flower like structures form because bidentate (Fe-O-SO2-O-Fe) type structures
which are formed between the nanoneedles [33].
Fig 5.6. FESEM images of the products obtained after growth at 120 C for (a) 0.5 h, (b) 1
h, (c) 2 h, (d) 3 h, (e) 4 h and (f) 6 h.
159
Fig 5.7 TEM images for samples prepared at 120 C for different reaction time: (a) 0.5 h, (b) 1 h,
(c) 4 h and (d) 6 h.
5.3.2 Effect of varying FeCl3 concentration
The effect of FeCl3 dosage on the final morphology of the product is investigated.
Amount of Na2SO4, DI water and other experimental conditions are kept constant. The
reaction mixture for growing nanoflowers contains Fe3+
ions from ferric chloride and
SO42-
ions from sodium sulphate. The ratio of these ions in solution determines the
reaction rate and hence the final morphology. Fig 5.8a is the SEM image of the product
obtained with 0.12 mM FeCl3 as the dosage. 0.12 mM dosage yields almost spherical
nanoflower like morphology. However, no hollow structures could be observed as with
0.04 mM concentration. On the contrary, the nanoflowers have a solid core as revealed
by the SEM image of a broken nanoflower (Fig 5.8b). The nanoflower morphology is
conserved until the concentration of FeCl3 reaches 0.4 mM (Fig 5.8c and d). When the
concentration is increased to 0.8 mM (higher concentration of Fe3+
ions), the spherical
160
shape of the nanoflower starts to give way (Fig 5.8e). The nanoflowers are no longer
spherical and nanorods are disoriented (Fig 5.8f). Also the nanorods grow longer and
wider. When the Fe3+
ions are further increased in the reacting solution (1 mM), the
flower like morphology is completely absent (Fig 5.8g). The final product at 1 mM is
composed of needle like palates, which tend to originate from the center (Fig 5.8h). The
needles are sparse and wider in size. The change in morphology with dosage of FeCl3 is
also confirmed by TEM. The spherical nanoflower morphology at 0.12 mM
concentration and disoriented nanorod formation at 1 mM is clearly visible in TEM
images (Fig 5.9). With increasing FeCl3 concentration nanorods seem to grow in size. For
0.12 mM solution the range of length and width of petals is 0.8-1 μm and 50-70 nm
respectively. The petal shaped structures grow 1.5 μm in length and 90-130 nm in width
when the concentration reaches 1 mM. TEM imaging brings out the difference in nanorod
dimension for 0.12 and 1 mM samples (Fig 5.9).
161
Fig 5.8. SEM images of samples with various FeCl3 concentrations: (a, b) 0.04 mM, (c, d) 0.12
mM, (e, f) 0.4 mM and (g, h) 1 mM.
Zeng et al. [34] previously studied the effect of SO42-
ions concentration on the
morphology of the product. They observed that with low SO42-
ion concentration, urchin-
162
like morphology is obtained, but with higher SO42-
concentration, the reaction rate
becomes faster and only nanoparticles are formed. It is well established that a prerequisite
for obtaining controlled 3D growth is due to the separation of nucleation and growth
process. This separation is absent when the amount of SO42-
ions increases in the
solution. Combining the knowledge from the two studies (Zeng et al. and this work), it
can be safely inferred that if the amount of Fe3+
ions is higher, the reaction speed tends to
slow down, thus leading to oriented 3D growth. Hence, spherical nanoflower morphology
is obtained. If the amount of Fe3+
ions is increased (as for 0.8 and 1 mM samples),
needle-like petals are formed. At this concentration, there is no clear boundary separating
the nucleation and growth process. The aggregation of nanoparticles may be absent,
leading to core-less structures with only petals growing in all directions (Fig 5.8h).
Table 5.1 summarizes the various parameters obtained from BET characterization
for the different α-Fe2O3 samples. The specific surface area of the sample made from
0.04 mM FeCl3 is found to be the highest (~109 m2g
-1). As the FeCl3 concentration is
increased in the reacting solution, the surface area tends to decrease. This could be due to
formation of large and compact structures leading to a reduction of surface to volume
ratio. A high surface area is desired for a good amount of dye to be chemisorbed for
DSSC [35]. BJH analysis shows that there is not much change in the pore size. However,
pore volume shows a drastic decrease as the FeCl3 concentration is increased. For 0.04
mM sample with nanoflower morphology the pore volume is the highest (~0.27 ccg-1
),
but it decreases to 0.05 ccg-1
for 1 mM sample, with the disordered array of petal shaped
structures. As discussed earlier, core formation is absent for 1 mM sample. The
163
decreasing pore volume corresponds to decreasing core size of the nanoflowers with
increasing FeCl3 dosage. This tends to indicate that the core of the nanoflowers is porous
in nature. The core of the nanoflowers may be composed of aggregates of smaller
particles contributing towards the pore volume. This also explains the high surface area
for the 0.04 mM sample. Fig 5.9 reveals N2 adsorption-desorption isotherms for all the
samples. The 0.04 and 0.12 mM products exhibit type IV nitrogen isotherm while 0.4 and
1 mM samples exhibit type III isotherm. Type III isotherm has adsorption and desorption
branches of isotherm that coincide, since no adsorption-desorption hysteresis is
displayed, indicating a non-porous material. On the other hand, adsorption on 0.04 and
0.12 mM samples proceeds via multilayer adsorption followed by capillary condensation
(Type IV isotherm). The adsorption process is initially similar for both these samples, but
at higher pressures the amount adsorbed rises for the 0.04 mM morphology due to the
capillary condensation in pores. Capillary condensation and evaporation do not take place
at the same pressure, thus leading to the formation of hysteresis loop in the range of 0.6-
1.0 P/Po. Monolayer adsorption for the sample containing 0.04 mM FeCl3 is completed at
a relative pressure of 0.61. However, the monolayer adsorption for 0.12 mM sample is
not completed until it reaches a relative pressure of 0.65. This indicates that 0.04 mM
possesses slightly smaller pore size than 0.12 mM sample.
164
Table 5.1. BET results for different samples of α-Fe2O3.
Sample Surface Area
(m2g
-1)
Pore size
(nm)
Pore Volume
(ccg-1
)
0.04 mM 109 25 0.27
0.12 mM 75 30 0.15
0.4 mM 59 34 0.08
0.8 mM 55 29 0.07
1 mM 30 25 0.05
165
Fig 5.9. Nitrogen adsorption-desorption isotherms at 77 K for nano-flowers produced at different
FeCl3 concentrations.
3.5 Photovoltaic performance of α-Fe2O3 nanoflowers
Depleting fossil fuels and climate changes have sparked intense interest in solar
energy harvesting. The focus of the research community is to fabricate high efficiency
photovoltaics from low cost and environmental friendly semiconductors. Iron oxide
makes a big portion of the earth‟s crust and that makes it an attractive candidate for low
cost photovoltaic. However, Fe2O3 has not been exploited for photoconversion due to
poor charge carrier mobility. This work demonstrates that answer to this problem may lie
in ordered quasi-3D nanostructures with high surface areas. Fig 5.10a depicts J-V
characteristics of the DSSC made from 0.04 and 1 mM FeCl3 concentration, named D1
166
and D2 respectively. 0.04 mM concentration is chosen to make the photoanode because it
possessed a high surface area (~109 m2g
-1), while 1 mM sample depicts the 1D nanorod
arrangement. Under direct illumination, D1 has a short-circuit current density (Jsc) of
around 4.3 mAcm-2
, accompanied by an open-circuit voltage (Voc) of 0.6 V and fill factor
(FF) of 63.5 %. On the other hand, DSSC made of ball milled 1 mM powder, shows
poorer performance. The reported values for D2 cell are: 2.7 mA.cm-2
Jsc, 0.5 V Voc and
58.5 % FF. The photoconversion efficiency recorded for D1 and D2 cells are 1.8 and 0.8
% respectively. It should be noted that the series resistance of the D1 cell (R at Voc) is
much less compared to D1 device (Table 5.2). The J-V behavior of the DSSC can be
modeled using an equivalent circuit with the following equation:
s h
ssp h
R
JRAV
nkT
AJRVqJJJ
/1exp0 (5.2)
and
s
sh
sphRAJ
RJ
JRAV
J
JJ
q
nkTJAP 22
00
/ln
(5.3)
where Jph is the photocurrent density, Jo the initial current density, J the total current
density, Rs the series resistance, Rsh the shunt resistance, A the effective cell area, n the
diode factor, k the Boltzmann constant, T the temperature and P the output power. As the
series resistance increases the output power, leading to a higher FF. Hence, it is clear that
the higher photovoltaic activity of the hybrid quasi-3D nanoflowers originate from their
unique structures.
The photocurrent action spectra of the cells are shown in Fig 5.10b, which
displays IPCE spectra as a function of wavelength. D1 cell shows a better photo-electrical
167
response, and its absolute IPCE is obviously higher than that of D2 cell over the entire
wavelength region of 400-700 nm. This is also in good agreement with the observed
higher Jsc of D1 cell. The maxima of IPCE contributed by the N719 dye absorption at
approximately 520 nm in the visible range are 23 % for D1 and 8.4 % for D2,
respectively. An ideal photoanode has to satisfy many criteria, which can be illustrated by
the continuity equation, introduced for DSSC by Lindquist [36]. The equation describes
the electron generation, diffusion and interception.
0)(
2
2
n
ax
oi nj
xn
x
nDeI
t
n
(5.4)
with boundary conditions,
(5.5)
(5.6)
where n, ηinj, Io and α represent electron concentration, charge injection efficiency,
incident photon flux and absorption coefficient respectively. It is clear that the
absorption coefficient is directly proportional to the effective molar concentration of the
dye, which is determined by the effective surface area available. Furthermore, the pore
volume affects the DSSC performance by changing both the light absorption coefficient
and the diffusion coefficient (D). It is evident that increased Jsc for D1 cell comes from
high dye loading capability of the film due to high surface areas. The influence of the
surface area can also be observed from the relation of Voc and Jsc for a DSSC.
TK
qV
o Benn )0(
0)(
dx
ddn
168
r
inj
ocI
I
q
kTV ln
(5.7)
where
][ 3
IknI etcbr (5.8)
Here Iinj is the current due to the electron from the excited dye, Ir the recombination
current, ncb the concentration of electrons at the electrode interface and ket the rate
constant for the reduction of the tri-iodide in the electrolyte by the electrons in the
conduction band of Fe2O3. Because Iinj is proportional to the amount of dye on the
electrode, it must increase by using a high surface area film. This is exactly what is
observed for the cells made. The hybrid quasi-3D α-Fe2O3 nanostructures of D1 are
superior to 1D nano-rods of D2, as they have high surface areas and high pore volumes.
An increased Voc also suggests a decreased rate of Ir by lowering the electric resistance
for the electron transport. In order to confirm this theory, UV-vis adsorption spectra of
dye washed samples are recorded (Fig 5.10c). The amount of dye loaded in photoanode is
obtained by desorbing the samples into 0.02 M NaOH (Table 5.2). Distinct peaks can be
seen at ~375 and 515 nm for D1 and D2, which are attributed to the metal-to-ligand
charge transfer (MLCT) absorption band of N719 dye. The results clearly show higher
dye absorbance for D1 sample. This may be attributed to the presence of larger pores and
higher surface area of the film, leading to better dye wetting and loading.
169
Fig 5.10 (a) J-V characteristics, (b) IPCE curves of DSSC and (c) UV-vis absorption spectra of
dye loaded samples made with 0.04 and 1mM FeCl3 concentration.
0
0.1
0.2
0.3
330 430 530
Ab
sorb
an
ce (
a.u
.)
Wavelength (nm)
D1
D2
0
1
2
3
4
5
0 0.2 0.4 0.6
Ph
oto
curr
ent
Den
sity
(mA
/cm
2)
Voltage (V)
D1
D2
0
5
10
15
20
25
400 500 600 700
IPC
E (
%)
Wavelength (nm)
D1
D2
(c)
(b)
(a)
170
Table 5.2. Quantity of dye absorbed and photovoltaic characteristics of 0.04 and 1 mM samples.
DSSC Voc
(V)
Jsc
(mAcm-
2)
FF
(%)
R at
Voc
R at
Isc
η
(%)
Dye
Adsorbed
(molescm-3
)
D
(cm2s
-1)
D1 0.6 4.3 63.5 77.7 8697.2 1.8 4.7 x 10-7
1.0 x 10-2
D2 0.5 2.7 58.5 119.6 8521.3 0.8 3.0 x 10-7
2.0 x 10-3
It is already shown that the hybrid quasi-3D nanostructures are similar to 3D
nanostructures but superior to 1D nanorods as far the surface area is concerned. To
understand the role of morphology on the device performance and evaluate diffusion
coefficient, electrochemical impedance spectroscopy is carried on photoanode of D1 and
D2. The complex impedance analysis helps in determining the inter-particle interactions
like grain boundary effects. Due to the polycrystalline nature of hybrid α-Fe2O3 samples,
the barrier is modeled as a double layer. It can be observed that Z‟ decreases frequency,
indicating an increase in AC conductivity of the films. All the curves merge at high
frequencies (>103 Hz) (Fig 5.11a). The variation of Z” with frequency reveals peaks in
the low-frequency region. The peak intensity for the 0.04 mM sample is lower and the
peak position shifts towards lower frequency. This indicates that the losses are lower in
the film. The curves show that Z” exhibits maximum value and then decreases
continuously with further increase in frequency. Such behavior indicates the presence of
171
relaxation in the system (Fig 5.11b). The Nyquist plots of D1 and D2 are shown in Fig
5.11c. Both the samples show one arc, indicating that the relaxation time of the bulk and
grain boundaries are close to each other and overlap. This response is attributed to
FeIII
/FeII redox species in the α-Fe2O3 matrix
[37]. Diameter of the D1 sample is smaller,
indicating a smaller transport resistance. The intercept of the semicircle with the real axis
at a low frequency represents the sum of the resistances of the grains and grain
boundaries. It is evident from Fig 5.11b that the resistance is lower for D1 samples with
hybrid morphology, thus paving the path for better electrical transport.
172
Fig 5.11. Variation of (a) Z‟, (b) Z” as a function of frequency and (c) Nyqusit plots of the doctor
bladed films with 0.04 and 1 mM FeCl3 concentration.
The electron transport in DSSC is through diffusion since there is negligible band
bending at the nanostructured semiconductor surface and the cationic species. The
0E+0
4E+7
8E+7
1E+8
2E+8
20 400
-Z"
(Ω)
Frequency (Hz)
0E+0
4E+7
8E+7
1E+8
2E+8
0E+0 1E+8 2E+8 3E+8
-Z"
(Ω)
Z' (Ω)
0.04 mM
1 mM
(c)
(b)
0E+0
1E+8
2E+8
3E+8
4E+8
50 500
Z' (
Ω)
Frequency (Hz)
(a)
173
electrolyte solution surrounds the nanostructured electrode and screens the injected
electrons effectively. In order for the injected electrons to reach the FTO, electron
diffusion length (L) should be longer than the thickness of the film. It is important to
study the electron diffusion coefficient, L = (D)1/2, where is the electron lifetime also
because D depends on the electrode morphology. The value of D is computed for both
D1 and D2 samples for a 10 m thick film (Table 5.2). Diffusion coefficient of hybrid
quasi-3D nanostructures is higher than 1D nanorods. The effect of increased D can be
many folds. The following relation relates D to the mobility of electrons (μe):
(5.9)
As the diffusion coefficient increases, the mobility of the semiconductor increases.
However, in DSSC the mobility is trap controlled. This indicates that for D1 sample of
hybrid nanostructures, the mobility is high and traps are smaller in number. Charge
transport in metal oxides is expected to be governed by a multiple-trapping process,
where the electrons thermally detrap from sub-bandgap states to the conduction band, and
are transported a short distance before being retrapped [38]. However, the location of the
traps and trap densities are still debatable. Diffusion coefficient relates inverse
proportionality to the number of traps [39]. Assuming that the traps are located at the
surface of the nanostructures, diffusion coefficient should scale with r2/3
, where r is the
radius of nanoparticles.
With all other parameters equal, and assuming that electron motion is a random
walk between uniformly distributed trap sites, idealized 1D diffusion should be faster
than a fully 3D case [40]. However, hybrid quasi-3D nanoflowers synthesized in this
e
kTD e
174
work lie in between 1D and 3D nanostructures. With identical structural sizes and
fabrication method, it is reasonable to expect comparable trap densities in both 1D nano-
rods and quasi-3D nanoflowers. However, the transport performance of the nanorods is
lower quasi-3D nanoflowers. This could be due to the random orientation of the
nanorods, some contacts exist among them. The other explanation could be that hybrid
quasi-3D nanoflowers have more number of grain boundaries than 1D nanorods. Thus the
number of possible necks is enhanced providing multiple paths for electrons to
transverse. Kopidakis et al. [41] has demonstrated a strong dependence of electron
diffusion with crystal size and the nano-flowers used here are much larger than
nanoparticles used for fabricating standard DSSC. Therefore, quantitative comparison
between the 3D nanoparticles and quasi-3D nanoflowers is not justified.
This work demonstrates a beginning stage for Fe2O3 as the potential candidate for
photovoltaics. As already discussed the synthesized α-Fe2O3 is an indirect band gap
semiconductor. We feel any step taken for synthesizing direct band gap morphology
would lead towards a more favorable material for the solar cell applications. This can be
achieved by incorporating the structures with relevant dopants. It is also theoretically
possible to enhance the conversion efficiency by changing the shape, size, porosity and
thickness of the synthesized morphology. Detailed experimental investigations at these
factors are underway in our laboratory.
175
5.4. Conclusions
In summary, quasi-3D nanoflowers of α-Fe2O3 are synthesized using solution
process with FeCl3 and Na2SO4 as the precursors. Tunability of the material is
demonstrated through the change in precursor concentration. Formation of such
structures requires suitable Fe3+
:SO42-
ion ratio in the reaction mixture. The proposed
growth mechanism indicates that an intermediate product of flower like goethite is
formed first, which changes to hematite phase on calcination at 400 °C. An in-depth
understanding is gained on of how surface area and transport properties are affected by
variation in morphology. The high surface area leads to better dye wetting in DSSC,
which contributes to 1.8 % power conversion efficiency. A systematic understanding of
the devices performance is derived from the investigation of few important parameters,
which include surface area, grain boundaries and diffusion coefficients of the photoanode
film. The understanding of structural (morphology) directed properties will have huge
impact on the future utilization of such hybrid nanostructures for myriad surface
dominated applications such as photocatalyst, solar cell, sensor etc.
176
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180
Chapter 6: Quasi-Solid-State Dye-sensitized Solar Cell (DSSC)
This chapter explores the potential of replacing liquid electrolyte with quasi-solid-
state electrolyte made from poly (ethylene oxide)/LiI/KI system. It is observed that the
introduction of KI in a conventional PEO/I2/LiI electrolyte system prevents the
crystallization of the polymer matrix and enhances the ionic conductivity. The electrolyte
is optimized for various potassium iodide (KI) concentrations. Furthermore,
diphenylamine (DPA) is added to reducethe sublimation of iodine leading to even high
ionic conductivity, without compromising on the electrochemical and mechanical
stability. The most promising feature of the electrolyte is increased device stability.
6.1 Introduction
Depleting fossil fuels and climate changes have sparkled intense interest in solar
energy harvesting. The focus of the research community is to fabricate high efficiency
photovoltaic from low cost and environmental friendly semiconductors. One of the
promising concepts for photovoltaic is DSSC. In the DSSC, the key ingredient is
electrolyte, which provides the internal electric conductivity. At present, acetonitrile-
based liquid-state electrolyte is commonly used. The use of liquid electrolyte, however,
demands perfect sealing of the device to avoid leakage and evaporation of the solvents.
Also, current liquid electrolytes suffer from poor stability due to the decrease of tri-iodide
concentration through sublimation of iodine. Both these issues affect the long-term
181
stability and performance of the liquid electrolyte. Hence much work has been focused on
substituting the liquid electrolyte by polymer electrolytes [1-3], gel electrolytes [4-5],
organic or inorganic hole conductors [6-8] and less volatile ionic liquids [9-10].
Among the many alternatives, PEO based electrolytes have sparked widespread
interest after Ren et al. [11] used them in quasi-solid-state DSSC and reported an
efficiency of 3.6 %. The repeating units (-CH2-CH2-O-) in PEO present a favorable
arrangement for the effective interaction of the electron pair on the oxygen with the metal
cation. This happens because PEO chains are arranged in a helical conformation with a
cavity, which creates an ideal distance for oxygen-cation interaction. Pure PEO
electrolytes, however, suffer from low ionic conductivity (~8.4x10-5
Scm-1
) [12]. The
problems associated with the use of pure PEO polymer electrolytes arise from low ionic
diffusion, which resulted in low penetration of the polymer inside a nanostructured
photoanode. This increases the interfacial charge-transfer resistance between the
electrode and the electrolyte. The ionic conductivity of these electrolytes is based on
segmental motion combined with strong Lewis-type acid-base interactions between the
cation and the donor atoms. Higher ionic mobility is prevalent in the amorphous
polymermatrix rather than in the crystalline state.
In order to enhance the overall conversion efficiency and the transport properties
of the DSSC, the nature or composition of the electrolytes must be improved. One way is
to add organic/inorganic molecules (plasticizers) in the electrolyte. Inorganic
nanoparticles are now commonly used after Scrosati [13] successfully incorporated TiO2
182
nanoparticles to the polymer electrolyte. Inorganic nanoparticles act as fillers and
decrease the degree of crystallization of the polymer matrix. Although, plasticized
polymer electrolytes exhibit high efficiencies [14-16], they tend to compromise on the
mechanical properties of the electrolyte [17]. Selecting a proper metal salt for the
electrolyte is also important. This is because a small size metal cation easily steers
through the dye and tends to intercalate in the pores of the working electrode [18]. Park et
al. [19] has shown that a small size cation (like Li+) leads to a smaller solvated ion cloud,
faster diffusion and higher efficiency. However, a larger cation (like K+ or Na
+) helps to
better separate the polymer chains and decreases the crystallinity, thus leading to higher
ionic conductivity [12]. Work from several groups have shown that electrolyte stability
can be increased either through gelation of the ionic liquid electrolyte or by addition of
phenothiazine, which in turn reduces the sublimation of iodine [20-23].
The aim of this work is to systematically investigate the enhancement of ionic
conductivity and long-term stability of PEO based electrolyte with diphenylamine (DPA)
in a TiO2 nanoparticle photoanode, without compromising the mechanical properties.
Thus, a filler free polymer electrolyte (with metal cation) is synthesized here. Keeping
the importance of cation size in mind, both Li+ and K
+ ions are used in the present
electrolyte. Using KI in conjunction with LiI enhances the ionic conduction of the PEO
based polymer electrolyte by decreasing the crystallization of the PEO matrix and
separating the polymer chains. In addition, diphenylamine (DPA), which forms an
interactive bond with I2, is added to the polymer electrolyte to reduce the sublimation of
183
iodine. The effect of different dosages of DPA on DSSC performance is evaluated here
and its effect systematically studied.
6.2 Experimental
6.2.1 Synthesis of PEO/I2KI/LiI electrolyte
Polymer electrolyte was prepared by adding 0.264 g of PEO (Mw ~200,000) to 50
ml of acetonitrile. 0.1 g of lithium iodide (LiI) and 0.019 g of iodine were then added to
this mixture under continuous stirring. Finally potassium iodide (KI) was added. A total
of six electrolyte solutions were made, with KI weight percentage varying from 10.5 % to
20.5 %. Finally, diphenylamine (DPA) was added. A total of five electrolyte solutions
were made, with the DPA content varying from 0.002 to 0.01 g. The electrolytes were
stirred continuously overnight. All the electrolytes were heated (~70 °C) to evaporate the
solvent, up to the point that the final product had a gel-like character. DSSC is fabricated
using commercially available TiO2 powder Degussa P25, N719 dye and Pt counter
electrode. The process of making DSSC is described in detail in section 2.2 under chapter
2.
184
6.3 Results and discussion
6.3.1 Effect of KI concentration
It is well known that the conductivity of PEO-blended electrolytes containing
metal salts increases with the size of the cation [24]. However, ionic conductivity of the
electrolyte also depends on many other factors. The total conductivity of an electrolyte
can be expressed by the following equation:
(6.1)
where n is the total number of dissociated charge carriers, q the total charge carried and μ
the mobility of the carriers. Dissociated carriers (n) are in turn related to the dissociation
energy (U) and dielectric constant (ε) as follows [25]:
(6.2)
Hence, ionic conductivity can be enhanced by incorporating metal salts, having bigger
ionic radius, higher mobility, higher dielectric constant and lower dissociation energy.
The ionic radius of the K+ ions (1.38 Å) is larger than that of Li
+ ions (0.76 Å) [29].
Hence, it is expected that K+ ions would have higher ionic conductivity. However, K
+
ions have lower ε (596.34) than Li+ ions (722.95), which will tend to lower the ionic
conductivity. Hence, we chose to incorporate both the metal ions in the polymer chain to
balance out the negative effects of both. Li+ cations get adsorbed onto the TiO2 layer [26]
and help decrease the Fermi energy level of the TiO2 layer for better charge collection.
K+ ions get attracted to the ether oxygen [21] and help to make the polymer more
185
amorphous. Adding bigger K+ ions in the electrolyte helps to decrease the crystallization
of the PEO polymer, thus acting as filler.
Figure 6.1 shows the XRD pattern of the quasi-solid state electrolyte with varying
amount of KI. The electrolyte without KI has sharp and well-defined peaks at 26.8°,
34.0°, 38.2°, 51.9°, 60.9° and 62.1° indicating semi-crystalline nature. The crystalline
peak intensities are progressively decreased with the increase in the KI concentration, and
nearly vanish at 14.5 wt%. This suggests that KI does prevent PEO from crystallizing.
The reduced crystallinity of PEO is due to interaction of KI with ether oxygens of the
polymer chain. This increased amorphous phase of PEO with 14.5 wt % KI will favor
inter and intra-chain ion movements and improve the electrical conduction. Further
addition of KI causes agglomeration and phase separation, thus increasing the
crystallinity and the diffusion resistance of the electrolyte.
186
Fig 6.1. XRD spectra of PEO/LiI based electrolyte with various KI loading.
6.3.2 Effect of DPA concentration
One of the conditions for the electrolyte to be suitable for use in DSSC is that the
light absorption by the electrolyte in the visible region should be as low as possible. This
is to ensure that maximum light is absorbed by the N719 dye. N719 dye has an
absorption maximum in the range of 350-550 nm with two prominent peaks around 380
and 530 nm. Thus it is clear that the absorption coefficient of the electrolyte should be
187
minimum around the above wavelengths. The UV-vis absorption measurements of quasi-
solid electrolyte (fig 6.2a) indicate that the absorption maximum occurs at the range of
350-400 nm. The UV-vis measurements were made on “doctor bladed” film of the
electrolyte on a glass slide. The broad maxima around 365 nm is due to [I ] and [I-]
[27].
The absorption spectrum of the DPA incorporated electrolyte overlaps with the N719 dye
in the range of 380-400 nm, while the rest of the visible region has insignificant
absorption. This weak absorption for wavelengths above 450 nm allows the use of highly
concentrated electrolyte in DSSC.
The XRD patterns of the KI incorporated PEO/LiI/I2 quasi-solid electrolytes with
and without DPA are shown (Fig 6.2b). Both the electrolytes are mainly amorphous in
nature, which is a clear indication that the KI ions effectively prevent the polymer PEO
from crystallization. Since the intensity of the broad peak at ~25 is diminished (19 %),
we can infer that the amorphous nature of the electrolyte is slightly increased on addition
of DPA. This also confirms the effective interaction between DPA and the polymer
chain. The increased amorphous phase of PEO with DPA will favor inter- and intra-chain
ion movements and thus improve the electrical conduction. The route of reducing the
crystallinity of electrolyte to enhance the ionic conductivity (σ) in PEO polymer
electrolytes is well known in literature.
3
188
Fig 6.2. (a) Absorption spectrum for the 0.004 g DPA loaded electrolyte and (b) XRD spectra of
PEO/KI/LiI based electrolyte with and without DPA loading.
6.3.3 Effect of KI concentration on DSSC performance
Figure 6.3 displays the photocurrent density-voltage characteristics of the cells
under simulated irradiation (global AM 1.5), made from different KI concentrations in
189
the electrolyte. It was found that the short-circuit current density (Jsc) increased with KI
weight percentage. For the PEO based electrolytes, the ionic conductivity is governed by
the transport of ions [28]. The increased Jsc is attributed to decreased crystallinity in
the polymer chain, leading to enhanced charge transfer. The highest Jsc (9.10 mAcm-2
)
was reported for 14.5 wt % KI. The inset of Fig 6.3 shows the trend of the conversion
efficiency with KI concentrations. The Jsc decreases when the KI concentration increases
beyond 14.5 wt %. This may be due to formation of ion pairs and crosslinking sites that
hinder the motion of the ions in the polymer chain and reduce the ionic mobility [29].
Table 6.1 summarizes the solar cell performance parameters obtained for different KI
concentrations.
Fig 6.3. I-V curves for DSSC using quasi-solid electrolyte at various KI loading. Inset shows the
calculated cell efficiencies at different wt % of KI.
0
1
2
3
4
5
6
7
8
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Cu
rren
t d
en
sity
/
mA
cm
-2
Photovoltage / V
10.5 wt %
12.5 wt %
14.5 wt %
16.5 wt %
18.5% wt %
012345
10 12 14 16 18 20
Eff
icie
ncy
/ %
KI concentration / wt %
190
Table 6.1. DSSC performance at various KI loadings in the quasi-solid electrolyte.
KI
Concentration
(wt %)
Voc
(V)
Jsc
(mAcm-2
)
FF
(%)
Efficiency
(%)
R at Voc
(Ω)
10.5 0.6 5.5 67.5 2.5 67.0
12.5 0.7 6.7 73.0 3.6 65.8
14.5 0.7 9.1 66.2 4.5 59.7
16.5 0.6 6.8 62.3 2.8 63.7
18.5 0.6 6.2 59.4 2.4 73.0
The room temperature ionic conductivity (σ) of the PEO/I2/LiI/KI (14.5 wt % KI)
system is compared with pure polymer PEO/I2/LiI, PEO/I2/KI and plasticized
PEO/I2/LiI/TiO2 systems. All the electrolytes (film thickness ~100 µm) are deposited on
the Pt coated FTO glasses, using a spacer (~0.05 mm). Table 6.2 lists the values of the
ionic conductivities obtained. The ionic conductivity values of the pure polymer and
plasticized electrolyte systems are close to that reported in literature [12, 30-31]. The
optimized electrolyte with 14.5 % KI in PEO/I2/LiI matrix gives a conductivity value of
3.0 x 10-3
Scm-1
, which is much higher than polymer and metal salt matrix electrolytes.
The ionic conductivity of 14.5 wt % KI is almost comparable to that achieved by adding
TiO2 filler (Table 6.2). Hence, this work demonstrates the enhancement of ionic
conductivity of the polymer electrolyte without incorporating any inorganic fillers.
191
Table 6.2. Computed ionic conductivity values for various electrolyte systems.
Electrolyte σ (Scm-1
)
PEO/I2/LiI 1.9 x 10-5
PEO/I2/KI 5.6 x 10-5
PEO/I2/LiI/TiO2 2.0 x 10-3
PEO/I2/LiI/KI 3.0 x 10-3
In order to determine the factors that influence the stability of the quasi-solid state
electrolyte DSSC, EIS is carried out. The nyquist plot for the fresh and 5 days aged
sample is shown in Fig 6.4a. As expected, the aged device showed higher diffuse related
resistance and electron transport resistance compared to the fresh device. Also, in the
kinetic region, the fresh device showed a blue shift in the peak related to the electrolyte
diffusion suggesting more rapid charge transfer than the aged device. With time, the
solvent tends to evaporate and hence the polymer chains start to crystallize leading to
poor ionic conductivity in the device. The inset of Fig 6.4a shows the equivalent circuit
model of the impedance of the quasi-solid electrolyte DSSC. The transmission line model
is used where Rs describes the resistance of the quasi-solid electrolyte, Rct and Cct
describe the recombination resistance and the chemical capacitance of the DSSC, Dx1
relates to the interface of the photoelectrode and the electrolyte and Ws encompass the
192
finite Warburg impedance elements associated with the diffusion of tri-iodide in the
electrolyte. Rdiff is the diffusion resistance of the
3/ II in the quasi-solid electrolyte.
Aging of the DSSC mainly affects Rct and Rdiff, whose values change by twice the
initial value. Hence, the loss in the DSSC efficiency can be attributed to the degradation
of the electrolyte over time, which tends to increase the recombination in the device. The
bode phase plots of EIS spectra (Fig 6.4b) display the frequency peaks of the charge
transfer process at different interfaces for the two devices. The electron lifetime for the
recombination (τe) in the devices is determined by the equation, τe = 1/ωmin = 1/2πfmax.
Table 6.3 summarizes the quantitatively fitted results using the equivalent circuit model.
It can be observed that the fresh device exhibits lower values for Rs, Rct and Rdiff,
implying a more efficient charge transfer process at the TiO2/electrolyte interface and the
Pt counter electrode/redox electrolyte interface. The circuit model uses constant phase
elements of the capacitance (CPE). This comes from the non-ideal frequency dependent
capacitance arising from the non-uniform distribution of the current by the material
heterogeneity [32]. CPE is defined by two values, CPE-T and CPE-P. CPE-P is a
constant ranging from 0 to 1. Our results show a CPE-P value of 1, which is caused by
the appearance of a double layer capacitance due to rough and porous surface of the
photoelectrode [32]. From the above discussion, it can be concluded that with aging the
overall resistance of the device increases (due to crystallization of the polymer matrix),
which leads to shorter electron lifetime and higher electron recombination in the device.
The stability of the device with quasi-solid electrolyte is much better than what is
observed with liquid electrolytes.
193
Fig 6.4. (a) Nyquist plots and (b) the Bode plots of the fresh and aged quasi-solid state DSSC.
Inset shows the equivalent circuit model of the DSSC.
194
Table 6.3. Fitted impedance parameters for the fresh and aged DSSC with 14.5 wt % KI in the
electrolyte.
D2 Film
Thickness
(μm)
Rs
(Ω)
Rct
(Ω)
Rdiff
(Ω)
Cct
(µF)
τe
(ms)
D
(cm2s
-1)
Fresh
Device
9.0 23.7 16.7 23.8 21.0 23.0 3.2 x 10-5
Aged
device
9.0 24.5 6.3 42.5 24.4 11.0 1.8 x 10-5
6.3.4 Effect of DPA concentration on DSSC performance
Table 6.4 shows the photovoltaic behavior of DSSC containing different amounts
of DPA. All quasi-solid-state DSSC are similar in assembly and measurement procedure.
It can be seen that the cell efficiency and Jsc increases even on addition of a small
quantity of DPA (Fig 6.5). When the DPA is introduced into the KI filler polymer
electrolyte, the cell efficiency increases from 4.5 to 4.7 %. Jsc initially increases with the
concentration of DPA up to 0.004 g, where a power conversion efficiency of 5.8 % is
achieved. Jsc drops beyond 0.004 g DPA concentration as a result of increased
recombination in the device. Another reason could be a higher concentration of DPA
absorbs some of the visible light in the range of 350-420 nm, thus lowering the visible
195
light absorption of the N719 dye. It is thus evident that there exists an optimum
concentration of DPA below or above which the quasi-solid electrolyte loses its optimum
performance. For iodine-based electrolyte, the decrease in [I
3 ] ion concentration is
accompanied with increase in [I-] concentration [33]. The increase in the photocurrent
values as more DPA is added may be a direct result of increased conductivity of the
electrolyte. This could be a consequence of an increased mobility of the redox species in
the polymer electrolyte, which intensifies the mass transfer in such medium. When 0.002
g DPA is added in the electrolyte, Voc decreases from 0.75 to 0.68 V. The relatively high
values of fill factor reflect low series resistance of the 0.004 g DPA electrolyte. For all
other DSSC, the high series may result from resistance losses arising from low ionic
mobility of the redox species. The cell efficiencies at different DPA loadings in the KI
polymer electrolyte are also displayed (inset Fig 6.5). The enhanced device performance
with DPA incorporated electrolytes can be explained by the interaction of highest
occupied molecular orbital (HOMO) of donor DPA with the lowest unoccupied
molecular orbital (LUMO) of acceptor iodine. Higher stabilization energy can be
achieved by reducing the energy difference between HUMO of DPA and LUMO of
iodine, resulting in improved charge-transfer from donors to acceptors. The observed
power conversion efficiencies of these cells are much higher than those reported using
pure polymer of PEO/KI/I2 system (=8.4x10-5
Scm-1
) [12] and filler incorporated
polyethylene glycol systems using TiO2 nanotubes ( =2.4x10-3
Scm-1
) [34].
196
Table 6.4. Photovoltaic characteristics of DSSC at various DPA loadings in the quasi-solid-state
electrolyte.
DPA
Concentration
(wt % )
Voc
(V)
Jsc
(mA cm-2
)
FF
(%)
Efficiency
(%)
R at Voc
(Ω)
0.0 0.75 9.1 66.2 4.5 60.0
0.002 0.68 12.9 60.0 5.1 46.0
0.004 0.60 13.9 66.6 5.6 40.0
0.006 0.68 12.8 58.7 5.1 57.0
0.008 0.64 12.6 60.0 4.8 50.3
0.01 0.55 12.2 65.4 4.4 32.8
197
Fig 6.5. J-V characteristics of DSSC employing quasi-solid electrolyte with different amount of
DPA. Inset shows the variation of conversion efficiency with DPA loading.
IPCE is a powerful method to evaluate the light absorption, charge separation and
transport, and charge collection in the DSSC device. Photocurrent action spectra of the
DSSC employing the polymer electrolyte with and without DPA are shown (Fig 6.6).
Because of the UV cut-off effect caused by the glass substrate, the spectra under 400 nm
are deteriorated. IPCE obtained for electrolyte with DPA is higher than that without
DPA in the wavelength region of 490-560 nm. An increased absorption in visible light
region for the range of 575-700 nm is observed for DPA added electrolyte. The evidence
of the improvement in IPCE further confirms the higher Jsc achieved for the DPA
incorporated electrolyte. The initial number of photo-generated carriers, electron
198
injection efficiency and the rate of recombination determine Jsc for the DSSC. Assuming
the same injection efficiency and dye loading capacity of the TiO2 nanoparticle films, it is
reasonable to say that the increment in Jsc is due to reduced recombination rate in the
device.
Fig 6.6. IPCE curves of DSSC employing quasi-solid electrolyte with 0.0 and 0.0004 g DPA.
Inset shows the variation of conversion efficiency with DPA loading.
To further elucidate the photovoltaic performance of DSSC, made using 30 ml
titanium isopropoxide solution, the EIS is performed. The best devices using
PEO/KI/LiI/I2 and PEO/KI/DPA/LiI/I2 electrolyte system are used to study the effect of
DPA addition in the electrolyte. The role of the electrolyte in DSSC is to facilitate the
reduction of the photo-oxidized dyes. An effective electrolyte should be capable of
suppressing the recombination at the semiconductor-electrolyte interface, while
promoting effective charge transport. The equivalent circuit that is used to model the
199
impedance of the DSSC is shown (Fig 6.7a). The Nyquist plots for the DSSC device with
and without DPA incorporated PEO/KI/LiI/I2 electrolyte systems are also shown (Fig
6.7b). Typically there are three arcs in the impedance spectrum of a DSSC. The response
at the high-frequency range (500000-1000 Hz) is attributed to charge transfer process at
the electrolyte/Pt interface. The mid-frequency range between 1000-1 Hz is governed by
charge transfer process at TiO2/dye/electrolyte interface. It is generally assumed that
these frequencies represent the recombination process between electrons in TiO2 and
electrolyte. In the low-frequency region of 1-0.0002 Hz, the arc represents the mass
transport resistance of ions in the electrolyte. A large semi-circle at low frequencies for
DPA added electrolyte indicates an improved ion motion in the electrolyte. In the fit, the
mid-frequency arc is not distinguishable and sometimes missing in the spectra, indicating
a small charge-transfer resistance at the TiO2/dye/electrolyte interface. It may also
happen that the charge-transfer at the TiO2/dye/electrolyte interface is strongly dominated
by that at Pt/electrolyte interface. As seen from Table 6.6, the charge transport resistance
(Rt) is lower for the DPA electrolyte DSSC, indicating that introduction of DPA reduces
the charge transport resistance at the interface of Pt counter electrode and quasi-solid
state electrolyte. At the same potential, the DPA incorporated electrolyte DSSC has a
slightly larger charge transfer (recombination) resistance (Rct), which confirms the earlier
claim of lower recombination rate in DPA electrolyte. The difference in Rct for both
electrolytes at the electrolyte/Pt counter electrode interface is clearly visible in the
zoomed spectra in high frequency region (Fig 6.7c). Rt is observed to be much smaller
than Rct, which is typically required for a high efficiency device [35].
200
The electron transport or collection time (d) associated with the electron transport
from the injection sites to the FTO and the electron lifetime (n) can be calculated as:
d = Rt . C (6.3)
n = Rct . C (6.4)
Comparing the electrolytes, it can be observed that electrolyte with DPA has the longest
lifetime but collection time is shorter for electrolyte without DPA. Since the efficiency of
the DSSC employing DPA incorporated electrolyte is higher, it can be inferred that
although electron collection time is better for electrolyte without DPA, its effect is easily
compensated by the increased electron lifetime in DPA added electrolyte. The effective
diffusion length (Ln) of electrons in TiO2 film is yet another critical parameter that affects
the charge collection efficiency. It can be calculated as:
d
nnnn LDL
(6.5)
201
Fig 6.7. (a) The equivalent circuit model of the DSSC, (b) Nyquist plots and (c) zoomed spectra
in the high frequency region for 0.0 and 0.004 g DPA added quasi-solid electrolyte DSSC.
As can been seen from Table 6.5 that Ln increases as the DPA is added into the
PEO/KI/LiI/I2 electrolyte. Ln is much larger than the TiO2 thickness for both the
electrolytes, which indicates good collection efficiency. The increase in Ln may be caused
by suppression of the recombination rate in DPA electrolyte, as already seen. The
202
conductivity of the electrolytes (σ) is derived from the complex impedance
measurements. It is calculated using the following equation:
bAR
L
(6.6)
where Rb is the bulk resistance determined from the intersection of the high-frequency
semicircle with the real axis, L is the thickness of the film and A is the area of the sample.
An increment of ~30 % is achieved when DPA is added in the filler free PEO/KI/LiI/I2
electrolyte system. It is important to note that this increase in ionic conductivity is
achieved without compromising the mechanical strength of the electrolyte, as no
inorganic filler is added. The fitted values of Rt, Rct and Cμ at bias of 0.65 and 0.7 V are
shown (Fig 6.8). The dependence of Rct on the applied forward bias [36] can be expressed
as:
)exp(TK
qVRR
B
oct
(6.7)
where Ro is a constant, β the transfer coefficient, q the elemental charge, KB the
Boltzmann constant and T the temperature. The plot demonstrates a much stronger that
forward bias dependence for electrolyte without DPA. It is known that once the electrons
transport along the TiO2 surface, the surface traps enhance Rt and reduce σ. When DPA is
added to the electrolyte, the trap related recombination might be suppressed, which
reduces Rt and increases σ. The reduction in Rt is consistent with increased Rct at the
TiO2/electrolyte interface. The chemical capacitance (Cμ) increases with the applied
potential in both the electrolytes, indicating a high charge accumulation and effective
electrical communication between the Fermi level in TiO2 and FTO. The Cμ can be
estimated as
203
]exp[TK
qVCC
B
o
(6.8)
where Co is a constant and α = T/To; To being the characteristic temperature indicating
the depth of the distribution. From this discussion, it is clear that the only downside of
adding DPA in the electrolyte seems to be lower Voc for the device. Voc is mainly
influenced by the rate of recombination in DSSC and the shift in the conduction band
edge (Ec) of metal oxide semiconductor. Li+ cations are known to shift Ec of TiO2
downwards. It is suspected that DPA incorporated electrolyte shifts the Ec by a greater
quantity, which in turn leads to lower Voc. However, the suppressed interfacial charge
recombination and better electron lifetime compensates the adverse effect of band edge
movement on Voc. Hence the net effect is increased overall conversion efficiency of the
DSSC employing DPA added electrolyte.
Table 6.5. Fit results of impedance spectra for DSSC for DPA free and 0.004 g DPA incorporated
electrolyte system.
Device Rt
(Ω)
Rct
(Ω)
Cct
(µF)
τd
(ms)
τn
(ms)
Dn
(cm2s
-1)
Ln
(cm)
ηcc
(%)
σ
(Scm-1
)
No
DPA
3.0 18.4 470.0 1.4 8.6 2.9x 10-3
5.0 84.0 2.7x10-3
With
DPA
2.4 21.5 360.0 0.8 8.0 4.5x 10-3
6.0 89.5 3.3x10-3
204
Fig 6.8. Forward bias dependence of (a) transport resistance (Rt), (b) charge transfer
(recombination) resistance (Rct) and (c) chemical capacitance (Cμ) of DSSC with no and 0.004 g
DPA.
205
6.4.5 Stability of DSSC
In order to ascertain the reproducibility, five different devices from different
batches of materials were made and tested. The trend of η % and Jsc are recorded for the
tested devices (Fig 6.9a). The study of the stability of DSSC with the DPA incorporated
quasi-solid state electrolyte is important. One of the motivations for the substitution of
liquid electrolyte with the quasi-solid electrolyte is to minimize leakage and solvent
evaporation; thus ensuring longer stability and longer life for the DSSC. An evaluation of
the efficiency during the ageing period for DSSC assembled with PEO/KI/DPA/LiI/I2
electrolyte system is given (Fig 6.9b). An initial decay in efficiency is reported which is
followed by a slight increase. This could be attributed to better wetting of the TiO2
nanoparticle film by the electrolyte through improved seepage of the electrolyte. The
efficiency, however, continues to drop after a week but eventually a plateau of stability is
achieved after 25 days.
206
Fig 6.9. (a) DSSC performance of five devices tested for reproducibility. (b) Stability for the
quasi-solid state DSSC containing 0.004 g DPA.
It has been suggested that the initial decay in the performance followed by a
plateau of stability might be an intrinsic property of DSSC assembled with polymer
electrolytes. This trend is independent of the cell size, substrate type and exposure time
(a)
(b)
5.6
5.7
5.8
1 2 3 4 5η
%
13.7
13.8
13.9
1 2 3 4 5
J sc
(mA
.cm
-2)
No. of Devices Tested
2
2.5
3
3.5
4
4.5
5
5.5
6
1 4 7 10 13 16 19 22 25 28 31 34 37 40
η %
Time (days)
0.004 g DPA
No DPA
207
[24]. The deposition of the quasi-solid electrolyte takes place under heating; hence no
residue solvent is left behind. Therefore, the loss of performance for the DSSC cannot be
attributed to the evaporation of the solvent. The loss of power conversion efficiency over
time may come from degradation of the N719 dye. It is well understood that the
regeneration of the dye in DSSC is a fast process. However, under lack of favorable
conditions the dye may be in oxidized state for long periods, leading to its degradation
through the loss of –NCS ligand [37]. This is especially true for the quasi-solid state
DSSC because the ion mobility is low in the electrolyte. This can lead to slow
regeneration or incomplete filling of TiO2 nanoparticles by the electrolyte. Hence, the
degradation of the DSSC device may be due to a number of reasons, which may not be
independent of each other. Desorption of the dye, degradation of counter electrode and
changes in electron transport in the semiconductor are some of them. The DPA added
quasi-solid DSSC loses only 11 % of the initial performance. The stability figures are
much impressive compared to liquid electrolyte DSSC, which losses almost half of the
conversion efficiency after 5 days. It is also note worthy that this stability is achieved
without adding any plasticizer or inorganic filler, which might compromise the
mechanical strength of the electrolyte. The stability results of the DPA incorporated
quasi-solid electrolyte ascertain the benefits of replacing the liquid component.
6.4 Conclusions
The work focuses on fabricating quasi-solid-state DSSC using DPA and KI in the
electrolyte. The addition of KI to PEO/LiI/I2 increases the ionic conductivity of the
208
electrolyte. The concentration of KI is optimized to obtain a high efficiency DSSC. 14.5
wt % of KI yields a homogeneous polymer electrolyte with the conductivity of the order
3x10-3
Scm-1
and energy conversion efficiency of 4.5 %. Further increase in the KI
concentration caused a drop in the DSSC efficiency due to the formation of cross-linking
networks in the electrolyte, as confirmed by XRD analysis. An energy conversion
efficiency of 5.8 % was obtained by employing a light scattering layer with the KI
incorporated polymer electrolyte. Furthermore, DPA is added to stabilize the electrolyte
yielding conversion efficiency of 5.8 %, which is attributed to the enhanced ionic
conductivity (~3.5x10-3
Scm-1
). The device is stable and retains 89 % of the initial
efficiency after 40 days. Results indicate that PEO/KI/DPA/LiI/I2 system would form a
promising polymer electrolyte for the DSSC.
209
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212
Chapter 7: Conclusions
Metal oxides are topic of intense research due to their potential uses in the
development of electronic devices. Many exciting properties emerge when the spatial
dimensions of these materials are reduced to nanoscale. The present research has
concentrated on both the fundamental and the engineering aspects regarding next
generation metal-oxide nanomaterials for solar cells. While this thesis did not find the
"holy grail" solution to the energy problem, but several important strides are made in this
direction. This thesis represents thorough investigation of metal-oxide nanostrctures in
DSSC application, an alternative technology for silicon solar cells. The focus of the thesis
is on synthesizing large surface area, high crystallinity and defect free structures with the
aim of better dye loading and better electron transport in the solar cells. Investigations are
carried out on structural, electrical, optical and transport properties of the synthesized
materials. The discussion begins with a review on the materials, their applications and
common practices adopted in the research community. To investigate the metal-oxide
nanostructures, the first task is to synthesize the nanomaterials. Solution based processes
like hydrothermal and sol-gel are adopted in this work, as these are easy to perform, cost
effective and can be easily extended to different substrates.
The first chapter emphasizes the importance of obtaining optimum thickness, high
crystallinity, high surface area and controlled porosity of 3D mesoporous TiO2 film for
213
fabricating high solar conversion efficiency DSSC. The study also helped create TiO2
film with high surface area (using smaller size particles) and good light scattering
properties (larger nanoparticles) without employing the conventional repetitive coating
and sintering procedures. The solar conversion efficiency is improved to 5.46 % by
incorporating light scattering centers to better harvest the incoming light.
1D nanostructures of TiO2 are synthesized through the anodization route to help
improve the electron transport due to directional electron mobility and decreased inter-
crystalline contacts. J-V curves reveal accelerated electron transport and lower
recombination in the DSSC devices made of TiO2 nanotubes. This is because the electron
motion is not limited by the random walk inside the semiconductor, as is the case for 3D
nanoparticles.
After investigating the performance of 3D nanoparticles and 1D nanotubes,
hybrid nanostructures are synthesized. 3D nanoparticle based morphologies have the
primary weakness of small diffusion coefficient (D), whereas 1D nanostructures lack
good surface areas. Hybrid nanoflowers of Fe2O3 are used in DSSC for the first time, as
they combine good dye adsorption with better electron transport properties. The hybrid
-Fe2O3 nanostructures comprises of 1D nanorods, to direct the electron transport,
nucleating radially from 3D porous core required for high dye adsorption. We tried to
understand how hybrid morphology affects the device performance and compared it with
1D nanorods.
214
The DSSC with the above morphologies are assembled with liquid iodide
electrolyte. The last part of the thesis deals with fabricating DSSC using quasi-solid
electrolyte. LiI and KI are incorporated in the PEO polymer chain to enhance ionic
conductivity and avoid issues of leakage and evaporation of the solvents. The important
finding of this research is incorporating LiI and KI together, without any inorganic fillers,
which helps to maintain the mechanical strength of the electrolyte. In addition,
diphenylamine (DPA), which forms an interactive bond with I2, is added to the polymer
electrolyte to reduce sublimation of iodine. The effect of different dosage of KI and DPA
on DSSC performance is evaluated and its effect systematically studied.
215
Chapter 8: Future Work
Charge transport in mesoporous systems is a puzzling phenomenon and remains
the most researched topic even today. Several interpretations have been given like the
diffusion theory [1] and the Montrol Scher model [2], but the exact mechanism is far
from understood. Simulating the results for different trap densities, particle sizes, porosity
and ionic motion in the electrolyte can help us better understand the electron transport
mechanism in the mesoporous TiO2 system.
For the work carried out on mesoporous TiO2, future research can be directed
towards synthesizing long range ordered systems. A desirable morphology of the film
would have mesoporous channels aligned in parallel to each other and vertically to the
FTO glass electrode. Such morphology will give better control towards of pores and lead
to better diffusion of the dye and electron transport.
In order to fabricate a device for energy conversion based on the as-synthesized
1D nanostructures, it is very important to have a controlled growth of these
nanostructures. In other words, it is essential to be able to pattern the growth of these
nanostructures. Future work can be directed towards fabricating ordered TiO2 nanotubes
using templating methods.
216
The working of DSSC with different morphology metal oxides and electrolyte
systems can be better understood through the use of time and frequency domain
measurements techniques such as intensity modulated photocurrent spectroscopy (IMPS)
and photovoltage spectroscopy (IMVS). These techniques will help throw light on the
behavior of recombination kinetics and carrier mobility in different metal oxide
nanostructures. The rate constants of charge transfer and recombination in the
photoanode films can be obtained and quantitatively related to the electron transport
resistance across the film.
There is also need to synthesize alternative redox couples for the DSSC
employing plasmonic effects. This is required because iodine in the redox electrolyte
reacts with metal nanoparticles and etches them away. Hence the stability of plasmonic
DSSC with Ag nanoparticles is an issue. Cobalt based complexes using derivatives of
[Co(bip)2]3+/2+
may be attractive alternative to triiodide/iodide couple redox for Ag
nanoparticle system.
References
[1] H. Rensmo, S. Sodergren and L. Patthey Chem. Phys. Lett. 274 (1997) 51-57.
[2] J. Nelsom Phys. Rev. B 59 (1999) 15374-15380.
217
Appendix A- Silver (Ag) Nanoparticles
A.1 Introduction
Apart from manipulating the semiconductor morphologies for enhancing the solar
conversion efficiencies, other routes have also been employed, like adding scattering
centers and using reflective surfaces. Another promising concept to improve the solar
conversion efficiency is to use localized surface plasmon. In the present study, silver
(Ag) nanoparticles are used with TiO2 nanotubes to scatter the light and generate
localized surface plasmonic effect. Ag is chosen for the work due to its higher absorption
and lower cost. Surface plasmon resonance when applied to solar cells increases the
photocurrent response over a wide range of wavelengths [1]. The resonant frequencies of
the surface plasmon depend greatly on the particle‟s material, size, shape and spatial
orientation [2]. The challenge in using surface plasmon resonance in solar cells rely on
optimizing the geometric properties of the metallic nanoparticles to obtain a large
electromagnetic field while having a coverage low enough such that it does not block off
too much of the incident light. Fortunately, the light interaction cross-section is larger
than the geometric cross section of the metallic particle as the polarizability increases at
its resonant frequency. Being a new field especially in photovoltaic applications, the
excitation of surface plasmon to increase the efficiency of DSSC is not well studied.
Furthermore plasmon enhancements for Ag nanoparticles have not been as thoroughly
218
investigated as Au nanoparticles. The higher absorbance helps it to serve as a better
material for use in solar cells.
A.2 Synthesis of silver (Ag) nanoparticles
1 mL of 38.8 mM tri-sodium citrate (C6H5O7Na3.2H2O) was added into 49 mL of
boiling aqueous solution containing 9 mg of silver nitrate (AgNO3) under vigorous
stirring. The mixture was boiled for 1 h and cooled to room temperature. The as-prepared
silver colloid was centrifuged at 8500 rpm for 10 min to remove larger nanoparticles.
Half of the collected silver nanoparticles were dried in Memmert oven to obtain SEM
images and the other half were suspended in distilled water to be implemented into
DSSC. The suspension of Ag nanoparticles was spin coated on the TiO2 nanotube film at
1500 rpm for 40 sec. The concentration of sodium citrate was varied between 20-80 mM
to study the effect of sodium citrate concentration on the silver nanoparticles size. Other
parameters, such as reaction temperature and reaction time were also varied.
A.3 Characterization of Ag Nanoparticles
In metal nanoparticles, collective electron oscillations known as surface plasmons
can be excited by light. When light is impinged on the localized surface plasmon on the
metal nanoparticle surface, the irradiated light is scattered and absorbed on the surface.
This results in an evanescent wave with a strong electromagnetic field to be generated on
the metal nanoparticle surface. This wave remains localized on the surface at a distance
219
less than the diameter of the metal nanoparticle. Localized surface plasmon enhances the
optical phenomenon such as Raman scattering and light absorption, and the level of
enhancement strongly depends on the material and the surface states [3]. Stuart and Hall
[4] pioneered the work on Plasmon enhamced light sensitive devices. When a particle is
under the action of an electromagnetic (EM) field, electron start to oscillate, transforming
energy from the incident EM wave into another form of energy. The electrons can also be
accelerated, and then they radiate energy in a scattering process. Two mechanisms are
proposed to explain the photocurrent enhancement by metal particles in solar cells: light
scattering and near-field concentration of light. The contribution of each mechanism and
resonance frequency of the oscillations is determined by the dielectric properties, shape
and size of the metal nanoparticles. The research done here is to obtain spherical silver
nanoparticles with different dimensions. The SEM images of the synthesized Ag
nanoparticles are shown in fig A.1. Concentration of tri-sodium citrate is varied to obtain
nanoparticles of different sizes. Citrate acts as stabilizing agent after silver nanoparticles
are formed [5]. For 20 mM tri-sodium citrate the average diameter of 105 nm is obtained
with almost spherical nanoparticles (fig A.1a). The diameter increases to 115 nm for 40
mM tri-sodium citrate (fig A.1b). For higher concentration of tri-sodium citrate, the
diameter of nanoparticles tends to decrease. For 60 and 80 mM concentrations the
nanoparticle diameter is 87 and 85 nm respectively (fig A.1c and d). For 80 mM the
nanoparticles are elongated in shape and agglomeration is observed.
220
Fig A.1. SEM images of Ag nanoparticles grown at different sodium citrate concentrations of (a)
20, (b) 40, (c) 60 and (d) 80 mM.
The variation of diameter with the concentration of sodium citrate is plotted in
graph A.2a. The reaction time of the synthesis process for Ag nanoparticles is also varied
(Fig A.2b). The time is varied from 0.5-2 h keeping the temperature and tri-sodium
citrate concentration fixed at 90 °C and 40 mM respectively. The Ag nanoparticles are
relatively larger for reaction time of 0.5 and 2 h. it is to be noted that when size of the
metal particle becomes large (>50 nm), the radiation effects becomes important [6]. The
displacement of the electronic cloud is no longer homogenous even for spherical
particles. Thus high multipolar charge distributions are induced [6].
221
Fig A.2. Size variation of Ag nanoparticles with (a) concentration of sodium citrate and (b)
reaction time.
UV-Vis absorbance is measured for synthesized Ag nanoparticles (Fig A.3). Ag
nanoparticles exhibit intense absorption peaks due to the surface Plasmon excitation [7].
The absorption band in visible region (400-550) is typical for Ag nanoparticles. It can be
observed that as the particle size increases (for 40 and 80 mM samples) the absorption
peak shifts towards the red wavelength [8]. Larger particles exhibit more scattering and
re-radiation, leading to radiative damping correction. The overall effect is broadened
plasmon resonance [9]. For larger particles the accelerated electrons produce an
222
additional polarization field that depends on the ratio between the size of the particles and
the wavelength of the incident light [9]. This secondary radiation causes electrons to lose
energy and they experience a damping effect, which makes the surface plasmon
resonance wider, as observed for the Ag nanoparticles. The peak for 80 mM sample is
much broader than others. This broadening can be attributed to the clusters of
nanoparticles.
Fig A.3. UV-Vis absorbance spectra of Ag nanopartcles synthesized using different
concentrations of sodium citrate.
223
A.4 Effect of Ag nanoparticles on DSSC performance
Fig A.4 and Table A.1 illustrates the relation between the size of Ag nanoaprticles
and their DSSC performance. The measurements were made under AM 1.5 simulated
sunlight. Particles grown using 40 mM tri-sodium citrate concentration (diameter 115
nm) show low Jsc. This may be because the particle size is big enough to show surface
plasmonic effect. Ag nanoparticles with 20 mM concentration (105 nm) also show low
Jsc, as the particles are agglomeration. 60 mM Ag nanoparticles when used for DSSC
showed the best efficiency due to better dispersion and surface Plasmon effect. The
DSSC efficiency decreased again as the concentration of sodium citrate is increased to 80
mM, due to the formation of some rod like structures along with nanoparticles. These
rods may lower the effective surface of the Ag nanoparticles leading to lower dye
absorption and lesser scattering effect. The DSSC using 60 mM Ag nanoparticles appears
to have the largest improvements in J-V characteristics compared to the other cells. This
can be explained by the higher coverage of Ag nanoparticles on the DSSC surface,
resulting in higher plasmon resonance effect that enhances the electric field around the
particles, increasing the effective molecular absorbance of the dye. The Voc, in general,
shows a negative shift as the Ag nanoparticles decreases in size. For small Ag
nanoparticles, the Voc adopts more negative potential; whereas bigger Ag particles take up
more positive potentials. The magnitude of Voc generally decreases with increasing Ag
nanoparticle size, except for 80 mM nanoparticles. The larger phototovoltage (0.7 V)
for DSSC with 60 mM Ag nanoparticles suggest more negative Fermi-levels in the
photoanode film. This is consistent with the results obtained by Kamat et al. [10], whose
224
previous studies showed that noble metal or metal ion doped semiconductor
nanocomposites exhibit negative shifts in their Fermi levels compared to the pure
semiconductor. By shifting the Fermi level closer to the conduction band, the
semiconductor film facilitates charge rectification and improves the conversion
efficiency. It is to be noted that the performance of DSSC without Ag nanopartcicles and
with 40 mM is almost the same. This may point out to the fact that large nanoparticles
(>100 nm) may not show any surface plasmon resonance. DSSC made with only TiO2
nanotube anode yields a solar conversion efficiency of 4.5 % with Jsc of 10.6 mAcm-2
.
When 60 mM Ag nanoparticles are added on top of TiO2 nanotubes, the Jsc increases
(11.6 mAcm-2
) leading to conversion efficiency of 5.5 %.
Fig A.4. (a) J-V curve for DSSC decorated with different concentration Ag nanoparticles and (b)
energy-level diagram of DSSC.
225
Table A.1. Photovoltaic characteristic of TiO2 nanotubes grown at 60 V for 2 h and loaded with
different Ag nanopartilces.
Sodium Citrate
Concentration
(mM)
Ag Nanoparticle
Diameter
(nm)
DSSC
Efficiency
(%)
Fill
factor
(%)
Jsc
(mAcm-2
)
Voc
(V)
0 - 4.5 68.2 10.6 0.62
20 90-100 3.6 64.2 8.6 0.65
40 100-120 4.5 68.3 10.5 0.62
60 70-80 5.5 68.8 11.6 0.69
80 90-140 3.1 64.2 7.2 0.65
It can be concluded that the absorption coefficient of the DSSC increases by
localized surface plasmon of Ag nanoparticles, which in turn increases the number of
photoelectrons generated. The scattering of light and the evanescent wave with a strong
electromagnetic field are considered localized surface plasmon effects in DSSC. When
exposed to light, the surface of each Ag nanoparticle produces a strong local magnetic
field due to a large absorption coefficient of the Ag surface plasmon. The large size of Ag
nanoparticles (>50 nm) may also lead to the scattering of light, which may increase the
absorption of sunlight by the DSSC. Enhanced scattering increases the Isc due to increase
in optical path lengths, thus leading to higher efficiency.
226
A. 5 Conclusions
Further increase in cell performance up to 1 % has been achieved using surface
plasmon resonance effect by Ag nanoparticles. This significant improvement in
efficiency is due to plasmonic effect and scattering mechanism, leading to 10 % increase
in photocurrent.
Reference
[1] M. Ihara, K. Tanaka, K. Sakaki, I. Honma and K. Yamada, J. Phys. Chem. B 101
(1997) 5153-5157.
[2] Y. Shinkai, H. Tsuchiya and S. Fujimoto, ECS Transactions 16 (2008) 261-266.
[3] K. Arya, Z. B. Su and J. L. Birman, Phys. Rev. Lett. 54 (1985) 1559-1562.
[4] H. R. Stuart and D. G. Hall, Appl. Phys. Lett. 73 (1998) 3815-3817.
[5] A. Van Hoonacker and P. Englebienne, Current Nanoscience 2 (2006) 359-371.
[6] C. Noguez, J. Phys. Chem. C 111 (2007) 3806-3819.
[7] X. Gao, G. Gu, Z. Hu, Y. Guo, X. Fu and J. Song, Colloids and Surfaces A 254
(2005) 57-61.
[8] T. Huang and X. H. N. Xu, J. Mater. Chem. 20 (2010) 9867-9876.
[9] M. Meier and A. Wokaun, Opt. Lett. 8 (1983) 581-583.
[10] P. V. Kamat, Pure Appl. Chem. 74 (2002) 1693-1706.
227
Appendix B- Scattering Titanium dioxide (TiO2) Particles
B.1 Introduction
Exceptionally high refractive index and whiteness of TiO2 makes it attractive for
wide variety of products including coatings, paints, plastics, paper, rubber printing inks,
synthetic fibers, ceramics, cosmetics, and even toothpaste. Improvement of light harvest
efficiency in the dye-sensitized solar cell with titanium dioxide (TiO2) electrode by light
scattering has been reported [1-5] in literature. The scattering is determined by the
composition of the incident light, optical properties of the particles, medium, size, shape,
concentration, surface roughness, and spatial arrangement of the particles. Light-
scattering effect can be achieved by additional layers on top of TiO2 films. Addition of
the scattering layers with the large particles ensures adequate light trapping in the device
[6], due to the increase of absorption path length of photons and optical confinement.
Characterization of the scattering of light by titanium dioxide particles has been the
subject of significant research for many decades. Ferber and Luther [2] and Rothenberger
et al. [3] confirmed the light-scattering effect with the transport theory and the many-flux
model, respectively.
In this work, light scattering particles of TiO2 were synthesized. The morphology
was optimized by varying the concentration of tetrabutoxytitanium (TBT). Pure anatase
228
phase TiO2 nanoparticles with diameter in the range of 90-130 nm are then used with
quasi-solid state electrolyte to fabricate DSSC.
B.2 Synthesis of TiO2 scattering particles (TiO2-SP)
For the synthesis of scattering layer of TiO2, 0.5 ml tetrabutoxytitanium (TBT)
was added to 10 ml of ethylene glycol under nitrogen purging. The solution was
magnetically stirred overnight at room temperature. The resulting solution was poured
into an acetone bath (~120 ml) containing 0.3 ml of DI water, under vigorous stirring.
The white precipitate was separated using centrifugation, followed by repeated washing
with DI water and ethanol. This step was necessary to ensure removal of ethylene glycol
from the surface of titanium dioxide glycolate particles. The spherical particles of
titanium gylcolate were converted to pure anatase TiO2 by annealing in air at 450 °C for
30 min.
B.3 Characterization of TiO2-SP
TiO2-SP nanoparticles are prepared from four different concentrations of TBT in
acetone. The diameter and density of the nanoparticles can be easily tuned by changing
the concentration of TBT. When 0.05, 0.1 and 0.5 ml TBT is used well dispersed uniform
and spherical nanoparticles are obtained (Fig B.1a, b,c). This uniformity can be attributed
to ethylene glycol which reduces the hydrolysis rate of TBT. As observed, the diameter
of these particles increases with the amount of TBT (50-130 nm). This happens due to
229
increased number of nuclei present to form titanium glycolate. With further increase in
TBT (Fig B.1d), the particles tend to fuse together to form larger agglomerates. The TBT
concentration of 0.5 ml in 120 ml acetone gives polydispersed particles with largest
diameter in the range of 90-130 nm. Hence, this concentration of TBT is used to prepare
light scattering layer for the DSSC. The spherical nature of the TiO2-SP is also confirmed
by TEM. Fig B.1 e and f show the TEM images of the particles after annealing at 450 °C
for 30 min. The image clearly shows that the spherical morphology of the particles is
essentially preserved during the annealing process.
Fig B.1. SEM images at (a) 0.05 ml, (b) 0.1 ml, (c) 0.5 ml, (d) 1 ml TBT concentration, (e) and (f)
TEM images of TiO2-SP.
230
Fig B.2 shows XRD pattern of TiO2-NP films after annealing at 450 ºC for 30
min. For TiO2-SP, the sample containing 0.5 ml TBT was chosen. XRD pattern displays
well-resolved and sharp peaks. It can be observed that the particles are polycrystalline in
nature and have anatase as the predominant phase. The peaks are indexed corresponding
to (101), (004), (200) and (105) anatase phase of TiO2 (JCPDS file No. 21-1272).
However, there is a small amount of (110) rutile phase also present in TiO2-SP (JCPDS
file No. 21-1276).
Fig B.2. XRD spectrum of TiO2-SP film. „R‟ represents the rutile phase of TiO2.
B.4 DSSC Performance of TiO2-SP with quasi-solid electrolyte
To enhance the conversion efficiency of DSSC, a 300 nm thick scattering layer
(TiO2-SP) is coated on TiO2 electrode. The photovoltaic performance of the device D1
231
(without scattering layer) and D2 (with scattering layer) are shown in Fig B.3a. An
increase of 29 % was recorded in the conversion efficiency for D2 compared to D1. For
the same quasi-solid electrolyte, energy conversion efficiency was increased to 5.8 % for
D2. The improved photovoltaic performance is mainly due to increased Jsc and fill factor.
Enhanced Jsc is attributed to better dye loading and increased light harvesting capability
of the film. TiO2-SP particles (~100 nm) also promote better penetration of the dye,
which reduces the series resistance of device. The enhanced fill factor is linked to the
rapid diffusion of the polymer electrolyte in the TiO2 film. However, the photovoltage
decreases due to poor connectivity between the FTO and the electrolyte. The summary of
the various solar cell parameters is given in Table B.1. The inset (Fig B.3a) shows the
incident photo-to-current conversion efficiency (IPCE) obtained for D1 and D2. It is
well-known that light of a shorter wavelength is relatively more scattered on a rough
surface than longer wavelength. This is because the scattering efficiency of light is
proportional to λ-4
, where λ is the wavelength of incident light. The IPCE spectra indicate
that the improvement of quantum efficiency for D2 is relatively higher in shorter
wavelength region (400-600 nm) than in longer wavelengths.
The light scattering properties of the synthesized TiO2 films are investigated with
diffuse reflectance spectra as shown in Fig B.3b. Considerable increase in the reflectance
can be observed after the addition of 100 nm TiO2-SP particles in D2 electrode. TiO2-SP
exhibits high reflectance in the whole visible region (400-800 nm). The reflection for D1
and D2 reduces dramatically under 400 nm because of the light absorption caused by the
band transition of TiO2 (band gap 3.0 eV).
232
Fig B.3. (a) IV characteristics and (b) reflectance spectra of DSSC with and without TiO2-SP
layer. Inset shows IPCE of the DSSC.
233
Table B.1 Photovoltaic characteristics of DSSC with 14.5 wt % KI electrolyte measured under
illumination with AM 1.5 simulated sunlight.
DSSC Voc
(V)
Jsc
(mAcm-2
)
FF
(%)
Efficiency
(%)
D1 0.7 9.1 66.2 4.5
D2 0.6 12.6 71.0 5.8
B.5 Conclusions
DSSC device is modified by adding light scattering particles to a TiO2 electrode
in quasi-solid state electrolyte. The TiO2 films with light scattering incorporated showed
enhanced performance (29 %), compared with nanocrystalline TiO2 films, which were
used as the controls. In particular, the photocurrent density (Jsc) reached ~12.6 mAcm-2
under a one-sun condition. This was attributed to the light scattering effect and decrease
in internal resistance through the porous structure with a minor loss in electron transport.
234
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[2] J. Ferber and J. Luther, Sol. Energy Mater. Sol. Cells 54 (1998) 265-275.
[3] G. Rothenberger, P. Comte and M. Grätzel, Sol. Energy Mater. Sol. Cells 58 (1999)
321-336.
[4] A. Usami, Sol. Energy Mater. Sol. Cells 59 (1999) 163-166.
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(2006)
1176-1188.
235
Appendix C- List of Publications Related to this Thesis
Journals
1) S. Agarwala, G. W. Ho, “Self-Ordering Anodized Nanotubes: Enhancing the
Performance by Surface Plasmon for Dye-Sensitized Solar Cell”, J. Solid State
Chem. (in press).
2) S. Agarwala, L. Zhihan, E. Nicholson, G. W. Ho, “Probing Morphology-Device
Relation of Fe2O3 Nanostructures towards Photovoltaic and Sensing
Applications”, Nanoscale 4 (2012) 194-205.
3) B. Zhang, L. Yu, S. Agarwala, M. L. Yeh, H. E. Katz, “ Structure, sodium ion
role, and practical issues for β-alumina as a high-k solution processed gate layer
for transparent and low-voltage electronics”, ACS Appl. Mater. Interfaces 3
(2011) 4254-4261.
4) S. Agarwala, C. K. N. Peh, G. W. Ho, “Investigation of Ionic Conductivity and
Long-term Stability of LiI and KI coupled Diphenylamine Quasi-Solid-State Dye-
Sensitized Solar Cell”, ACS Appl. Mater. Interface 3 (2011) 2383-2391.
5) S. Agarwala, L. N. S. A. Thummalakunta, C. K. N. Peh, A S W Wong, G W Ho,
“Co-existence of LiI and KI in filler-free, quasi-solid-state electrolyte for efficient
and stable dye-sensitized solar cell”, J. Power Sources 196 (2011) 1651.
236
6) S. Agarwala, K. Moe, A. S. W. Wong, V. Thavasi, G. W. Ho, “Mesophase
ordering of TiO2 with high surface area and strong light harvesting for dye-
sensitized solar cell”, ACS Appl. Mater. Interfaces 2 (2010) 1844.
7) S. Agarwala, G. W. Ho. “Synthesis and tuning of ordering and crystallinity of
mesoporous titanium dioxide film”, Mater. Lett. 63 (2009) 1624.
Conference Proceedings
1) S. Agarwala, G. W. Ho, “Electronically functional metal-oxide for solar cells
applications”, International conference on materials for advanced technologies
(ICMAT), Singapore (Jun 2011).
2) G. W. Ho, W. L. Ong, Z. Lim, M. Kevin, S. Agarwala, Z. Lee, G. H. Lee,
“Synthesis, Alignment, and fabrication of metal-oxide nanostructures on non
conventional substrates for multifunctional room temperature sensors”,
International conference on materials for advanced technologies (ICMAT),
Singapore (Jun 2011).
3) G. W. Ho, S. Agarwala, M. Kevin, “Mesophase Ordering and Structuring of
Porous Titanium Dioxide and other Oxide Nanomaterials with High Surface Area
and Strong Light Harvesting Matrix for Dye-sensitized Solar Cell”, Materials
Research Society (MRS) Spring Meeting, San Francisco, USA (Apr 2011).
4) G. W. Ho, S. Agarwala, W. L. Ong, “Titanium Dioxide Nanobelts and
Mesoporous Film for Dye-sensitized Solar Cell and Gas Sensing Applications”,
Materials Research Society (MRS) Spring Meeting, San Francisco, USA (Apr
237
2011).
5) S. Agarwala, G. W. Ho, “Mesophase ordering and structuring of porous TiO2 for
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