Fabrication of Solution Processed Semiconducting Films...

Fabrication of Solution Processed Semiconducting Films

based Devices

By

Muhammad Yasin

2011-NUST-DirPhD-EE-42

Supervisor

Dr. Tauseef Tauqeer

Co-Supervisor

Prof. Dr. Khasan S Karimov

THIS DISSERTATION IS SUBMITTED TO NUST, IN PARTIAL FULFILLMENT OF

DOCTOR OF PHILOSOPHY IN ELECTRICAL ENGINEERING

Department of Electrical Engineering, School of Electrical Engineering and

Computer Science, National University of Sciences and Technology, Islamabad

2016

II

III

Certificate by All GEC Members

It is certified that PhD thesis in respect of PhD student Reg No. 2011-NUST-DirPhD-EE-42

Name: Muhammad Yasin has been vetted and approved by the following GEC:

Names Signatures

Dr. Tauseef Tauqeer (Supervisor)

Prof. Dr. Khasan S. Karimov (Co-Supervisor)

Prof. Dr. Sait Eren SAN (External GEC Member)

Prof. Dr. Syed Muhammad Hassan Zaidi (GEC Member)

Dr. Hamood Ur Rahman (GEC Member)

Dr. Muhammad Umar Khan (GEC Member)

IV

This work is dedicated to my beloved mother, wife and daughters.

V

Acknowledgements

All honors and glories are for Almighty Allah, The Most Beneficent and Merciful Who enabled

me to complete this dissertation within the given time frame. May Almighty Allah shower His

countless blessings on Muhammad (PBUH), The Last Prophet, who is not only the continuous

source of knowledge, wisdom and torch of guidance for entire mankind but also messenger of

peace for all creatures of Almighty Allah.

I am highly thankful to my supervisor Dr. Tauseef Tauqeer, Co-Supervisor Prof. Dr. Khasan

Sanginovich Karimov and GEC members Prof. Dr. Sait Eren, Prof. Dr. S. M. Hassan Zaidi, Dr.

Hamood Ur Rahman and Dr Muhammad Umar Khan, for their valuable support, supervision,

devoted guidance and constructive criticism. I am unable to express my entire heart feelings in

words but I can pray to Almighty Allah (SWTA) to reward them for their kindness. I feel myself

honored to be the student of such renowned scholars.

I am thankful to Turkish Research and Technological Council (TUBITAK) for providing me

funded opportunity to carry out a part of PhD research work at Polymer Electronics Research

Laboratory, GEBZE Technical University (GTU) Turkey. Special thanks to Prof. Dr. Sait Eren

SAN who not only supervised my research but also organized my research visit at GTU. I am

greatly inspired by his personality and professional skills, and sincerely appreciate his

consideration that enabled me to experience fruitful research in a friendly and relaxed

environment.

Besides Prof. SAN, precious assistance, guidance and moral support of the faculty members,

graduate students and staff of the Department of Physics at GTU, Turkey, must also be

acknowledged. In particular, I am greatly thankful to Prof. Dr. Yusuf Yerli, Prof. Dr. Mustafa

Okutan, Arif Kosemen, Zuhal Alpaslan Kosemen, Cigdem Cakarlar, Dr. Ayesha Akpinar, Noor-

ul-Hassan, Betul canimkurbey, Gokhan Casdaman, Dilek Taskin, Hasan Turkan, Dr. Saadullah

Ozturk, Dr. Hakki Acarlar, Abuzer Yaman, Abdullah HAKAN, Ihlas Cicek, Adem Chen and

Ahmet Nazim for helping me during the fabrication & characterization of electronic devices and

interpretation of results.

VI

I am highly grateful to all the faculty members, graduate students, staff and every one of my

parent institution, National University of Sciences and Technology, School of Electrical

Engineering and Computer Science (NUST-SEECS) for their moral support and guidance. I am

also thankful to Prof. Dr S. M. Hassan Zaidi, Principal & Dean NUST-SEECS who not only

managed and administered my research activities but also encouraged me at every foot step of

my PhD study.

I am grateful to my employer, NESCOM for allowing me to pursue PhD studies. I am also

thankful to my close office colleagues who helped and prayed for the success of this work.

Finally, I highly appreciate my family for their love, kindness and support for the materialization

of this work. They have always been a source of motivation and inspiration for me. In particular,

I am indebted to my wife, Asma and beloved daughters Maryam, Ayesha & Hamna for their

understanding, patience, encouragement and continual support.

VII

Declaration

I hereby declare that the dissertation entitled “Fabrication of Solution Processed

Semiconducting Films based Devices” is my own work. It does not contain any previously

published/ submitted material or work accepted for the award of any degree or diploma at

NUST-SEECS or any other institute, except where due acknowledgement is made in the

dissertation. Any contribution made to this research work by any local or foreigner researcher

with whom I worked, is explicitly acknowledged. It is also certified that the intellectual content

of this dissertation is the outcome of my own work. Approval must be taken prior to use the

work presented in or derived from this dissertation.

Author Name: Muhammad Yasin

Signature:

VIII

Journal Publications

1. M. Yasin, T. Tauqeer, K.S. Karimov, S.E. San, A. Kösemen, Y. Yerli, A.V. Tunc,

P3HT:PCBM blend based photo organic field effect transistor, Microelectronic

Engineering, 130 (2014) 13-17.

2. M. Yasin, T. Tauqeer, H. Rahman, K.S. Karimov, S. San, A. Tunc, Polymer–Fullerene

Bulk Heterojunction-Based Strain-Sensitive Flexible Organic Field-Effect Transistor,

Arab J Sci Eng, 40 (2015) 257-262.

3. M. Yasin; T. Tauqeer; S.M.H. Zaidi; S.E. San; A. Mahmood; M.E. Köse; B.

Canimkurbey; M. Okutan; Synthesis and electrical characterization of Graphene Oxide

films, Thin Solid Films, 590 (2015) 118-123.

4. A. Mahmood; A. Naeem; M. Y. Kaya; M. Yasin; E. Mensur-Alkoy; S. Alkoy; Effect of

Co/Mg ratio on the phase, microstructure, dielectric and impedance properties of lead

zirconate titanate; Accepted for publication in Journal of Materials Science: Materials in

Electronics; DOI: 10.1007/s10854-015-3697-5

5. A. Asimov; M. Ahmetoglu; A. Kirsoy; M. Özer; M. Yasin; The electrical properties of

AU/P3HT/N-TYPE SI schottky barrier diode, Accepted for Publication in Journal of

Nanoelectronics and Optoelectronics

6. Tauseef Tauqeer; Muhammad Yasin; Sait E San; Hamood Ur Rahman; Khasan

Karimov, Fabrication and characterization of P3HT:MR:PCBM blend based organic

phototransistor, Journal of Nanoelectronics and Optoelectronics (Accepted for

Publication)

7. Hidayatullah Khan, Muhammad Amin, Muhammad Ali, Muhammad Iqbal, Muhammad

Yasin, Turkish Journal of Electrical Engg and Computer. Sc. (Accepted for Publication)

IX

Conference Publications

1. M. Yasin, T. Tauqeer, S.E. San, H. Ur Rahman, K.S. Karimov, Fabrication and

characterization of organic bulk heterojunction based displacement and bend sensitive field

effect transistors, in: 12th International Bhurban Conference on Applied Sciences and

Technology (IEEE-IBCAST), 2015, pp. 1-5.

DOI: 10.1109/IBCAST.2015.7058470

2. Muhammad Yasin; T. Tauqeer; Sait Eren San; Zuhal A. Kösemen; Kh. S. Karimov; Asad

Mahmood; Investigation of the effect of gate voltage on the performance of organic bulk

hetero-junction based phototransistor; IEEE 17th

International Multi topic Conference (IEEE-

INMIC), 2014, pp. 445-449.

DOI: 10.1109/INMIC.2014.7097381

3. Erdinc Doganci, Cigdem Cakirlar, Sumeyra Bayir, Sait Eren San, Muhammad Yasin,

Mustafa Okutan, Cavit Uyanik, Faruk Yilmaz, 1st International Conference on Organic

Electronic Material Technologies, Turkey, (25-28 March 2015)

4. Muhammad Yasin; T. Tauqeer; Sait Eren San; Abuzer Yaman; Influence of Polymer

Intrinsic Properties on the Performance of Organic Bulk Heterojunction Solar Cells;

(Submitted)

http://dx.doi.org/10.1109/IBCAST.2015.7058470

http://dx.doi.org/10.1109/INMIC.2014.7097381

X

Abstract

In recent years, solution processed semiconducting layers based devices, such as Solar Cells

(SCs), Field Effect Transistors (FETs) and Light Emitting Diodes (LEDs) have attracted much

interest as potential, inexpensive and flexible alternatives to inorganic devices. Despite

considerable understanding of device physics, investigation of well-known solution processed

semiconductors based devices having simpler configurations and optimal performance is crucial

to further developments in this area. This research work mainly focuses on the fabrication and

characterization of solution processed semiconducting films based electronic devices having

simpler architectures.

Firstly, Organic Solar Cells (OSCs) were developed to investigate the effect of polymer intrinsic

properties on the performance of the devices. Secondly, Organic Field Effect Transistors

(OFETs) were investigated with a motivation to develop low cost sensors for photo, strain, bend

and displacement sensing applications. Lastly, parallel plate capacitor type structures were

developed to study the electrical properties of Graphene Oxide (GO), Cobalt & Magnesium

doped Lead Zirconate Titanate (PbTi0.5Zr0.3(Co1-xMgx)0.2O3) Ceramics and Methacrylate-based

Side Chain Liquid Crystalline Polymers (SCLCP) with an aim to explore their potential for

electronic device applications.

Polymer-Fullerene BHJ based Solar Cells (OSCs) were fabricated using different batches of

poly[2-methoxy,5-(30,70-dimethyl-octyloxy)]-p-phenylene vinylene (MDMO-PPV) in order to

investigate the effect of polymer intrinsic properties i.e. molecular weights, polydispersity (PDI)

values, charge carrier densities (NA) and band gap energies (Eg) on the performance of the

devices. Devices were under dark as well as UV-Vis illuminations. Results showed that polymer

intrinsic properties significantly influenced on the structural properties of active layers.

Efficiency of the devices was found highest for polymer having higher PDI and NA. It was

attributed to the increased photon absorption capability and favorable nano morphology of active

layer for MDMO-PPV batch with higher PDI and NA values.

Organic Bulk Hetero-Junction (BHJ) based transistors were fabricated in MESFET configuration

with top gate and bottom drain source contacts on glass and flexible PET substrates for sensing

XI

applications. Semiconducting layers of the devices were based on P3HT:PCBM blends which

were processed using spin coating technique. Gate and drain/source electrodes of the transistors

were made using Aluminum and Silver, respectively, using physical vapor deposition technique.

Electrical performance of the devices was explained with the help of energy band diagrams.

Organic Transistors developed on glass substrates using P3HT:PCBM (1:1 & 1:0.8 wt/wt ratios)

and P3HT:MR:PCBM blends (1:0.2:1 wt/wt ratio) were investigated as photo sensors. Results

showed that MESFET based photo sensitive devices followed the behavior of typical low voltage

phototransistors with weak saturation trend, in particular under illumination. Photo responsivity

of the devices was found to increase with illumination intensity and to decrease with negative

gate voltage. Further, phototransistors developed using the blend of P3HT:MR:PCBM were

found to have higher photo sensitivity values than P3HT:PCBM blends based photo detectors.

Organic MESFET based devices developed on glass and flexible substrates with 1:1 wt/wt ratio

of P3HT and PCBM blends were also investigated for strain and displacement sensing

applications. Increase in displacement and compressive bending results in increase of drain to

source current. Further, drain to source resistance was reduced from 15.40 MΩ to 13.15 MΩ

when displacement was varied from 0 to 250μm.

Electrical and dielectric properties of Graphene Oxide (GO), PbTi0.5Zr0.3(Co1-xMgx)0.2O3

Ceramics and SCLCPs with varying lengths of aliphatic spacer were studied as Metal-Insulator-

Metal (MIM) capacitors using temperature dependent parallel plate impedance spectroscopic

technique. Analyses showed that GO film possses Direct Current (DC) and Correlated Barrier

Hopping (CBH) mechanisms of conductivity at low and high frequency ranges, respectively.

Analysis of PbTi0.5Zr0.3(Co1-xMgx)0.2O3 Ceramics with varying values of composition exhibited

high magnitude of dielectric (ɛr = 4261) over a wide range of temperature at 100 kHz. Dielectric

constants of the SCLCP films were found to increase with the decrease of flexible spacer length.

Detailed analysis revealed that the conductivity of SCLCP films followed Quantum Mechanical

Tunneling (QMT) and CBH conductivity mechanisms at low frequency regime and, Super

Linear Power Law (SLPL) and DC conductivity mechanisms at high frequency region.

Impedance spectroscopic results suggest that, solution processed films being investigated using

impedance spectroscopy can be proposed as model systems for applications in large area flexible

arrays and other advanced microelectronic devices.

XII

Table of contents

Certificate by All GEC Members ................................................................................................. III

Acknowledgements ........................................................................................................................ V

Declaration ................................................................................................................................... VII

Journal Publications ................................................................................................................... VIII

Conference Publications ............................................................................................................... IX

Abstract .......................................................................................................................................... X

Table of contents .......................................................................................................................... XII

List of Figures ............................................................................................................................ XVI

List of Tables ............................................................................................................................. XXI

List of Abbreviations ................................................................................................................ XXII

Chapter-1: Introduction ................................................................................................................... 1

1.1 Preamble ........................................................................................................................... 1

1.2 Aims and objectives ......................................................................................................... 2

1.3 Thesis Organization.......................................................................................................... 3

Chapter-2: Theory and Background................................................................................................ 5

2.1 Organic Semiconductors .................................................................................................. 5

2.1.1 General Aspects ........................................................................................................ 5

2.1.2 Charge Transport in Organic Semiconductors .......................................................... 7

2.1.3 Classification of Organic Semiconductors ................................................................ 7

2.1.4 Organic Semiconductor Sensors ............................................................................. 10

2.2 Organic Solar Cells ........................................................................................................ 11

2.2.1 Introduction ............................................................................................................. 11

XIII

2.2.2 Energy Conversion Steps ........................................................................................ 12

2.2.3 Solar Cell Performance Parameters ........................................................................ 13

2.2.4 Different Architectures of Solar Cells .................................................................... 14

2.3 Organic Thin Film Transistors (OTFTs) ........................................................................ 15

2.3.1 Device Theory ......................................................................................................... 15

2.3.2 OTFTs based Sensors ............................................................................................. 22

2.4 AC Impedance and Dielectric Spectroscopic Studies .................................................... 22

Chapter-3: Materials and Experimental Details ............................................................................ 28

3.1 Materials ......................................................................................................................... 28

3.1.1 Semiconducting Materials for Solar Cells and Transistors..................................... 28

3.1.2 Semiconducting Materials for MIM Capacitors ..................................................... 31

3.2 Device Fabrication ......................................................................................................... 35

3.2.1 Vacuum Evaporation .............................................................................................. 35

3.2.2 Spin Coating............................................................................................................ 36

3.2.3 Drop Casting Process .............................................................................................. 37

3.3 Characterization Setup for Films and Devices ............................................................... 38

Chapter-4: Polymer-Fullerene BHJ Solar Cells and Phototransistors .......................................... 41

4.1 MDMO-PPV:PCBM blend based Organic Solar Cells ................................................. 41

4.1.1 Introduction ............................................................................................................. 41

4.1.2 Experimental Methods ............................................................................................ 42

4.1.3 Results and Discussion ........................................................................................... 44

4.2 P3HT:PCBM blend based Organic Phototransistors ..................................................... 48

4.2.1 Introduction ............................................................................................................. 49

4.2.2 Fabrication and Characterization ............................................................................ 50


XIV

Chapter-5: Strain and Displacement Sensitive OFETs ................................................................. 68

5.1 Organic Bulk Hetero-junction based Strain/ bend Sensitive Flexible Organic Field

effect Transistors ....................................................................................................................... 69

5.1.1 Introduction ............................................................................................................. 69

5.1.2 Experimental Methods ............................................................................................ 71


5.2 Polymer-Fullerene BHJ Based Displacement Sensitive OFET ..................................... 79

5.2.1 Introduction ............................................................................................................. 79

5.2.2 Experimental Details ............................................................................................... 80


Chapter-6: Electrical Characterization of Graphene Oxide, Lead Zirconate Titanate and Liquid

Crystalline Polymer Films ............................................................................................................ 86

6.1 Electrical Characterization of Graphene Oxide (GO) Films .......................................... 88

6.1.1 Introduction ............................................................................................................. 88

6.1.2 Experimental Procedure .......................................................................................... 89


6.2 Effect of Co/Mg ratio on the structural and electrical properties of lead zirconate

titanate films .............................................................................................................................. 96

6.2.1 Introduction ............................................................................................................. 96

6.2.2 Experimental Part.................................................................................................... 97


6.3 Electrical Conductivity Analysis of Methacrylate-based Liquid Crystal Polymers with

Pendant Cholesterol Groups .................................................................................................... 104

6.3.1 Introduction ........................................................................................................... 104

6.3.2 Experimental ......................................................................................................... 105

6.3.3 Results and discussion .......................................................................................... 106

XV

Chapter-7: Conclusions and Future Work .................................................................................. 113

7.1 Conclusions .................................................................................................................. 113

7.2 Future Work ................................................................................................................. 116

References ................................................................................................................................... 118

XVI

List of Figures

Figure 2-1: Schematic description of sp2 hybridization of Carbon atom [8] .................................. 5

Figure 2-2: Diagram of a HOMO and LUMO level of a molecule [10]......................................... 6

Figure 2-3: Commonly used p-type organic semiconductors (a) P3HT (b) Pentacene (c) CuPc (d)

Rubrene (e) MDMO-PPV (f) MEH-PPV (g) F8BT (h) Alpha-Sexithiophene ............................... 9

Figure 2-4: Commonly used n-type organic semiconductors (a) Fullerene-C60 (b) Fullerene-C70

(c) PCBM (d) Perylene (e) TCNQ (f) BBL (g) 4FPEPTC (h) DBP ............................................... 9

Figure 2-5: Current Voltage (I-V) Characteristics of a typical Solar Cell under dark and

illumination conditions ................................................................................................................. 15

Figure 2-6: Typical top gate top contacts FET ............................................................................. 16

Figure 2-7: Idealized energy band diagram of an OTFT with p-channel operation under given

biasing values [64] ........................................................................................................................ 18

Figure 2-8: Idealized energy band diagram of an OTFT with n-channel operation under given

biasing values [64] ........................................................................................................................ 18

Figure 2-9: (a) Output and (b) Transfer I-V characteristics of a typical OFET [65] .................... 20

Figure 2-10: 3D schematics of OFET in different operating regimes [61] (a) VD<< VG - Vth,

Linear regime (b) VD, sat = VG - Vth, Pinch off point formation near drain contact (c) VD > VD, sat,

Beyond Pinch off point ................................................................................................................. 21

Figure 3-1: Molecular Formula of P3HT ...................................................................................... 29

Figure 3-2: Molecular Structure of MDMO-PPV ......................................................................... 30

Figure 3-3: Chemical Formula of PCBM ..................................................................................... 30

Figure 3-4: Molecular Architecture of Graphene Oxide ............................................................... 31

Figure 3-5: Position and Orientation of molecules when matter is in (a) Crystalline (b) Liquid

Crystalline and ( c) Liquid state .................................................................................................... 33

XVII

Figure 3-6: Schematic illustration of (a) Nematic, (b) Cholesteric and (c) Smectic phase of liquid

crystals .......................................................................................................................................... 34

Figure 3-7: Leybold Univex 450 vacuum evaporator ................................................................... 36

Figure 3-8: (a) Semiconductor in powder form as received from Manufacturer (b) Solvent (c)

Semiconductor solution (d) Stirring Station with temperature control and monitoring ............... 37

Figure 3-9: Glove Box System ..................................................................................................... 38

Figure 3-10: DEKTAK-8 Surface Profiler ................................................................................... 38

Figure 3-11: UV-Vis Spectrophotometer interfaced with computer ............................................ 39

Figure 3-12: Impedance Analyzer HP 4194 and Novotherm Heating Station ............................. 40

Figure 3-13: Semiconductor Characterization System: Keithley 4200 SCS ................................ 40

Figure 4-1: (a) Schematic of the device architecture, and molecular structures of (b) PEDOT:PSS

(c) MDMO-PPV (d) PC71BM ...................................................................................................... 44

Figure 4-2: 10×10 µm2 surface and high resolution 3D AFM Profiles of Polymer: Fullerene Bulk

heterojunction based semiconducting layers for three different batches of MDMO-PPV : A[(a,

b)], B[c, d], and C[(e, f)] ............................................................................................................... 45

Figure 4-3: Absorption Spectra of MDMO-PPV:PCBM nanolayers (with 1:4 w/w ratio) prepared

using three different batches of MDMO-PPV Polymer ................................................................ 46

Figure 4-4: Current Density-Voltage (J-V) Curves of BHJ solar cells developed using three

different batches of MDMO-PPV Polymers under an illumination intensity of 100W/cm2 ........ 47

Figure 4-5: (a) Design schematic of organic phototransistors, and molecular structures of (b)

P3HT (c) MR (d) PCBM............................................................................................................... 51

Figure 4-6: (a) Absorption of P3HT:PCBM blend based active layer (with 1:1 wt/wt ratio) as a

function of wavelength, inset: its SEM based side view and (b) its (10µm × 10µm) surface AFM

view ............................................................................................................................................... 52

Figure 4-7: (a) S-D I-V curve (b) G-S I-V curve when VDS = 0 V (c) Energy Band Diagram of

the phototransistor, showing exciton generation, its dissociation, and transportation of holes and

electrons towards electrodes [93].................................................................................................. 53

XVIII

Figure 4-8: Current Voltage (I-V) Curves (output) of the Phototransistor without being

illuminated, inset: gate leakage current vs drain current at VGS = - 3V as a function of drain

voltage ........................................................................................................................................... 54

Figure 4-9: Transfer I-V Characteristics of the photo-OFET under dark ..................................... 54

Figure 4-10: Specific capacitance as a function frequency between gate and source/ drain

electrodes ...................................................................................................................................... 56

Figure 4-11: Output I-V Characteristics of the Photo-OFET (a) under fixed UV-Vis illumination

intensity of 100mW/cm2 and (b) under given UV-Vis illumination intensities compared with

dark ............................................................................................................................................... 58

Figure 4-12: Absorption spectrum of 350 nm thick P3HT:PCBM blend based active layer ....... 59

Figure 4-13: AFM 3D profiles (5µm×5µm) [(a) & (b)] and SEM images of the active layer (c) 60

Figure 4-14: Phototransistor output I-V characteristics under dark.............................................. 61

Figure 4-15: Phototransistor output I-V characteristics under UV-Vis illumination ................... 62

Figure 4-16: (a) Photo sensitivity and (b) responsivity as a function of Gate Voltage ................ 63

Figure 4-17: (a) Surface AFM image (10µm × 10µm) of P3HT:MR:PCBM blend based active

layer, inset: its high resolution 3D AFM profile, (b) Absorption Spectra, (c) Capacitance of

active layer as a function of frequency between Source/Drain and Gate electrodes and (d)

Capacitance of active layer as a function of DC bias voltage between Source/Drain and Gate

electrodes at specified frequency values. ...................................................................................... 65

Figure 4-18 I-V Characteristics of the Phototransistor under dark: Gate leakage current and drain

current as a function of drain to source voltage at VGS = - 4V. ................................................... 66

Figure 4-19 Output Current -Voltage (I-V) Characteristics of the Phototransistor under dark and

given UV-Vis illumination intensities when (a) VGS = - 3 V (b) VGS = - 4 V. ......................... 67

Figure 5-1: Design schematic of the flexible device .................................................................... 71

Figure 5-2: Energy Band Diagram ................................................................................................ 73

Figure 5-3: (a) 5×5 µm AFM image and (b) Cross section SEM view of 300 nm thick active

layer developed on substrate for sample-1 .................................................................................... 73

XIX

Figure 5-4: Section Roughness of active layer developed on flexible substrate for sample-2 ..... 74

Figure 5-5: (a) Out Put and (b) Transfer I-V Characteristics of sample-1 OFET under no bending

state ............................................................................................................................................... 75

Figure 5-6: Out Put I-V Characteristics of sample-2 OFET under no bending state .................... 75

Figure 5-7: Photographs of the flexible devices under bending state ........................................... 77

Figure 5-8: Schematic of the devices when bent at (a) 0º and (b) 90º w.r.t. drain current ........... 77

Figure 5-9: Drain to source current (IDS) as a function of drain to source voltage (VDS) under

suspended gate and the given bending conditions for strains applied at (a) 0o

and (b) 90o

with

respect to the direction of current for sample-1 ............................................................................ 78


suspended gate and the given bending conditions for strains applied at (a) 0o and (b) 90o with

respect to the direction of current for sample-2. ........................................................................... 78

Figure 5-11: Design schematic of Displacement Sensor based on Organic Field Effect Transistor

(OFET) .......................................................................................................................................... 81

Figure 5-12: Section roughness of P3HT PCBM blend based semiconducting layer .................. 83

Figure 5-13: Output I-V Characteristics of the OFET .................................................................. 83


given displacements when gate was suspended, inset1: Schematic of the displacement

measurement setup, inset2: Variation of the drain to source resistance (RDS) with displacement at

VDS = -8V. ..................................................................................................................................... 84

Figure 6-1: (a) Schematic View of parallel plate structures and (b) Chemical Formula of GO ... 90

Figure 6-2: Experimental set up for temperature dependent electrical analysis of GO Films. ..... 91

Figure 6-3: (a) Surface and (b) 3D AFM profiles, (c) Absorption and (d) Transmittance Spectra

of GO Films .................................................................................................................................. 91

Figure 6-4: (a) Real and (b) Imaginary Dielectric Constants, and (c) Loss Tangent Plots of GO

film as a function of frequency at given temperatures. ................................................................. 93

Figure 6-5: AC Conductivity of GO film as a function of frequency at given temperatures ....... 94

XX

Figure 6-6: Complex impedance spectrum (Nyquist Plot) for the GO at various temperatures,

inset: Equivalent RC network ....................................................................................................... 95

Figure 6-7: SEM micrographs of the thermally itched surface of PbTi0.5Zr0.3(Co1-xMgx)0.2O3

ceramics. ....................................................................................................................................... 98

Figure 6-8: Variation of dielectric properties of the PbTi0.5Zr0.3(Co1-xMgx)0.2O3 ceramics with

frequency....................................................................................................................................... 98


temperature ................................................................................................................................... 99

Figure 6-10: Variation of AC conductivity of the PbTi0.5Zr0.3(Co1-xMgx)0.2O3 ceramics with

temperature. ................................................................................................................................ 100


frequency at various temperatures. ............................................................................................. 102

Figure 6-12: Complex impedance plots for PbTi0.5Zr0.3(Co1-xMgx)0.2O3 ceramics at 300 oC .... 103

Figure 6-13: Design Schematic of Parallel Plate Structure used for Impedance Spectroscopic

Analysis of Co-polymers and Homo-polymers .......................................................................... 106

Figure 6-14: Angular frequency evolution of (a) the real (εʹ), and (b) imaginary (εʺ) dielectric

constants of the polymers............................................................................................................ 107

Figure 6-15: Cole–Cole plots of the dielectric constant (ε"– ε') ................................................. 110

Figure 6-16: Specific conductivity of Poly(Chol-n-MMA) and poly(Chol-n-MMA-co-MMA) (n=

3, 7, and 10); inset: High resolution plots of specific conductivity of samples in high frequency

regime ......................................................................................................................................... 111

Figure 6-17: Specific conductivity versus frequency plot of Poly(Chol-n-MMA) and poly(Chol-

n-MMA-co-MMA) (n= 3, 7, and 10). ......................................................................................... 112

XXI

List of Tables

Table 2-1: Pressure Sensors based on OTFTs .............................................................................. 23

Table 2-2: Strain Sensors based on OTFTs .................................................................................. 24

Table 2-3: Environment Monitoring based on OTFT ................................................................... 24

Table 4-1: Various Parameters such as Fill Factor (FF), Short Circuit Current Density (Jsc), Open

Circuit Voltage (Voc) and efficiency (η) of polymer fullerene BHJ based solar cells realized using

three different batches of MDMO-PPV Polymers ........................................................................ 48

Table 6-1: Absorption coefficient relaxation time (o), dielectric parameters (εs, ε∞, ε''max,

and Δε) and critical frequencies (fc) of Poly(Chol-n-MMA) and poly(Chol-n-MMA-co-MMA)

(n= 3, 7, and 10). ......................................................................................................................... 108

XXII

List of Abbreviations

P3HT Poly(3-hexylthiophene)

PCBM [6,6]-phenyl C61-butyric acid methylester

MDMO-PPV Poly[2-methoxy-5-(3‘,7‘-dimethyloctyloxy)-1,4-phenylenevinylene]

CuPc Copper Phthalocyanine

DC Direct current

EA Electron affinity

Eg Band Gap Energy

GaAs Gallium arsenide

HOMO Highest occupied molecular orbital

LUMO Lowest unoccupied molecular orbital

LVDT Linear variable differential transformer

OFETs Organic field effect transistors

OLEDs Organic light emitting diodes

VDS Drain to Source Voltage

VGS Gate to Source Voltage

Vth Threshold Voltage

Ci Capacitance of Active Layer per unit Area

OSCs Organic Solar Cells

Voc Open Circuit Voltage

FF Fill Factor

XXIII

Isc Short Circuit Current

𝜂 Efficiency

QE Quantum Efficiency

OTFTs Organic Thin Film Transistors

Pc Phthalocyanine

PMMA Poly (methyl methacrylate)

SEM Scanning electron microscopy

UV-Vis Ultraviolet Visible

rpm Rotation Per Minute

GUI Graphical User Interface

AMOLED Active Matrix Organic Light Emitting Diode

Co Cobalt

Mg Magnesium

SCLCPs Side Chain Liquid Crystal Polymers

LCs Liquid Crystals

AC Alternating Current

1

Chapter-1: Introduction 1 1

1.1 Preamble

During the last few decades, extensive research has been done on the organic semiconducting

materials for their applications in electronic devices due to their low temperature processing, low

cost and promising features. Further ease of their fabrication on flexible substrates makes them

potential candidates for future electronic applications. Furthermore, properties of organic

semiconductors can be easily tuned for specific applications by modifying their molecular

architecture using doping, a simple chemical synthesis technique [1]. Liu et al. [2] have recently

shown the improvement in the dielectric properties of organic Donor-Acceptor blend by doping

it with salt.

Inorganic semiconductors such as silicon and gallium arsenide have been the backbone of

conventional microelectronic area for the last five decades. History of organic microelectronics

dates back in 1950s, when drift mobility and photoconductivity analysis of Anthracene, a low

molecular weight organic material, was carried out for the first time to investigate its potential as

semiconducting material for electric device applications [3, 4]. Around three decades later,

doped polyacetylene [5], was demonstrated as a promising polymeric organic semiconductor for

electronic industry for the first time in 1977. Discovery of semiconducting properties in

anthracene and polyacetylene triggered an active research for the investigation of

semiconductive properties in a variety of low molecular weight and conjugated polymeric

organic materials. Initially performance, reliability, stability and reproducibility of the devices

developed using organic semiconductors were very poor. However with gradual advancements in

synthesis process and understanding of the architecture of these conjugated materials along with

processing of new materials such as polythiophenes and fullerenes during last two decades, the

demands for utilization of organic semiconductors in Organic Light Emitting Diodes (OLEDs),

Organic Field Effect Transistors (OFETs) and Organic Solar Cells (OSCs) are now greater than

ever due to promising commercial applications [6, 7]. Corporations such as Philips, Kodak,

Samsung and Sony have introduced commercial devices based on organic semiconductors due to

simplicity in processing and low cost. Active research is going on to process new organic

2

semiconductors and to optimize existing device architectures for drastic improvements in organic

microelectronic area.

1.2 Aims and objectives

The main objective of this dissertation is to fabricate and investigate solution processed

semiconducting films based Organic Solar Cells, Organic Field Effect Transistors and parallel

plate type capacitors. Fabrication of the devices was carried out using low cost materials

(commercially available & locally synthesized) having promising features and simpler

technology to introduce low cost devices for various industrial, environmental and medical

applications. By considering the demand of industry for sensors and new materials with adequate

performance, solution processing, low cost, flexibility and simplicity of structure, the objectives

of the current research work were as follows:

I. Realization of MDMO-PPV and [6,6]-phenyl C71-butyric acid methylester (PC71BM)

blend based Organic Solar Cells (OSCs) on Indium Tin Oxide (ITO) coated glass

substrates in order to investigate the effect of polymer intrinsic properties on the

performance of the devices

II. Fabrication and characterization of Organic Bulk Hetero-junction based MESFETs

for the first time, having one of the simplest transistor architecture

III. Investigation of thin film transistors based low cost photo sensors with adequate

sensitivity using commercially available solution processed organic materials

IV. Investigation of Organic BHJ based transistors for strain, bend and displacement

sensing applications

V. Development of capacitors type micro structures having solution processed

active/semiconducting layers sandwiched between two conductive electrodes, in

order to investigate and explore structural, electrical and dielectric properties of

locally synthesized materials with promising features and applications

3

1.3 Thesis Organization

The thesis consists of four major sections, which includes Theory and Literature survey (chapter-

2) and brief overview of Materials and Experimental Details (chapter-3) along with Results &

Discussion (chapter-4 to chapter-6) followed by Conclusion & Future Work (chapter-7). The

chapter wise detail of the thesis is as follows:

Chapter-2: This chapter comprises of three major sections. In the first part (section 2.1), basic

theory and literature related to organic semiconductors is presented which includes description

regarding general aspects, transport properties, types and utilization for the sensing applications.

Section 2.2 briefly describes theory of Organic Solar Cells, whereas section 2.3 is dedicated to

explain basic operation, parameters and potential of OTFTs for sensing applications. The

introduction and literature survey of parallel plate impedance spectroscopic technique, utilized to

investigate electrical properties of locally synthesized solution processed semiconducting

materials, is given in the last part (section 2.4) of the chapter.

Chapter-3: Detailed information about the semiconducting materials used in this work for

different investigations is given in this chapter. Furthermore, fabrication techniques and the

experimental setups used for the fabrication & characterization of various electronic devices

have also been described.

Chapter-4: Fabrication and characterization of Organic bulk hetero-junction based solar cells

and photo-MESFETs are presented in this chapter. The Chapter is divided into two main parts.

First part (section 4.1) presents experimental results of an investigation carried out to study the

effect of polymer intrinsic properties such as molecular weights, polydispersity (PDI) values,

charge carrier densities (NA) and band gap energies (Eg) on the performance of MDMO-

PPV:PCBM blend based Organic Solar Cells. Second part (section 4.2) presents the fabrication

and investigation of organic BHJ based phototransistors. Three different types of

phototransistors, having solution processed semiconducting layers made of P3HT:PCBM (with

1:1 and 1:0.8 wt/wt ratios) and P3HT:MR:PCBM (with 1:0.2:1 wt/wt ratio) blends were

developed and investigated. Electrical behavior of the phototransistors was discussed with the

help of energy band diagram and related literature.

4

Chapter-5: This chapter comprises of two main parts. First part describes fabrication &

characterization of organic bulk hetero-junction based strain/ bend sensitive OFETs, developed

on flexible PET substrates, whereas second part presents displacement sensing analysis of

OFETs realized on glass substrates. Semiconducting layers of the investigated devices were

polymer-fullerene blends having 1:1 wt/wt ratio.

Chapter-6: Development and electrical characterization of electronic microstructures, having

solution processed semiconducting films sandwiched between two parallel conductive plates, is

presented in this chapter. Characterization was carried using temperature dependent impedance

spectroscopic technique, in order to investigate structural, electrical conduction and dielectric

properties along with relaxation behaviors of locally synthesized solution processed materials i.e.

Graphene Oxide, Cobalt & Magnesium doped Lead Zirconate Titanate (PbTi0.5Zr0.3(Co1-

xMgx)0.2O3) ceramics and Methacrylate based side chain liquid crystal polymers as a function of

frequency and temperature.

Chapter-7: This chapter highlights conclusion and few directions for future investigation of the

current work.

5

Chapter-2: Theory and Background 2 1

2.1 Organic Semiconductors

2.1.1 General Aspects

Organic semiconductors are fascinating kind of materials having electrons conducting properties

in addition to other promising features. The interest in organic semiconductors is motivated by

increasing demands of materials that offer easier and inexpensive processing methods. Similar to

all organic compounds, organic semiconductors are based on carbon atoms which in case of

polymers form the main chain. Other elements or functional groups which are attached to the

backbone significantly influence its chemical and electrical properties. Each carbon atom in an

organic semiconductor makes covalent bond with other carbon atoms and partially ionic

interactions to the atoms of other elements. Each carbon atom has four valence electrons.

Carbon atoms form sp2- hybridization with other atoms as shown in Figure 2-1 [8]. Among four

valence electrons of each carbon atom, three are located in three different sp2 orbitals (lie on the

same plane) whereas fourth electron is located in pZ orbital (which lies on a plane perpendicular

to the plane of sp2 orbitals). Bonds formed due to the overlap of sp

2-sp

2 orbitals of two carbon

atoms are called as covalent/ strong bonds or ζ-bond and overlapping of pZ and pZ orbitals

results in weaker bonds called as π bonds. Both these types of bonds (ζ bond and π bond) exist in

a carbon molecule.

Figure 2-1: Schematic description of sp2 hybridization of Carbon atom [8]

6

Further, carbon atoms make single, double or triple bonds. Single and double bonds are regularly

alternated in organic semiconducting material in addition to delocalized out of plane π electrons.

Energy levels in organic semiconductors are discrete which are termed as molecular orbitals. The

two important orbitals are Highest Occupied Molecular Orbital (HOMO) and Lowest

Unoccupied Molecular Orbital (LUMO). These levels in organic materials are equivalent to

valence and conduction bands in inorganic materials, respectively. Compared to energy bands in

inorganic materials, delocalization of electrons across many π-orbitals results in splitting of wide

spread molecular orbital energy levels in organic materials. Energy required for the

delocalization of an electron from HOMO to LUMO level in organic materials is called as band

gap energy (Eg), similar to inorganic material‘s theory. The magnitude of Eg indicates electronic

and optical properties as well as possible applications for effective utilization of materials for

optimal performance of the devices [9]. Figure 2-2 presents diagram of the HOMO and LUMO

of an organic semiconductor molecule. Each circle shows an electron in an orbital; when light of

a high enough frequency and energy is absorbed, electron jumps from HOMO to the LUMO.

Figure 2-2: Diagram of a HOMO and LUMO level of a molecule [10]

7

2.1.2 Charge Transport in Organic Semiconductors

Transportation of charge carriers in crystalline/ inorganic semiconductors occurs in delocalized

states, and can be described by band theory/model. This band model is not applicable to organic

semiconductors having disordered architectures. Further, conduction phenomenon in organic

semiconductors is not entirely uncovered and understood. Weaker intermolecular wander wall

bonds exist in semiconducting polymers as compared to strong covalent bonds in inorganic

semiconductors. Organic materials have lower melting temperature, less hardness and smaller

delocalization due to weaker bonds. Transport properties are strong function of delocalization in

material [11]. Transport in organic materials is due to hopping of charges between localized

states. It is known that in organic materials, mean free path for charge carriers is smaller than the

mean atomic distance [12]. Polymers which are originally insulators are made conductive by the

inclusion of charge carriers in side chains by the process of chemical doping, light exposure or

injection from electrodes. Mobility of charge carriers in organic semiconductors is low compared

to inorganic semiconductors due to disordered transport [13].

It has been shown that mobility of charge carriers in organic materials is the function of

inhomogeneous density of hoping sites, film morphology, molecular weight, molecular

orientation, surface passivation, molecular conformation, packing and environmental

conditions/treatment. In addition, mobility of the charge carriers and other electrical properties

are also sensitive to side chains of the soluble polymers. Charge transport is generally faster

along the polymer chain as compared to bulk. Mobility of charge carriers (holes) along one

dimensional polymer chains of MDMO-PPV is reported to be equal to 0.2-0.5cm2/VS, Whereas

mobility of charge carriers in thin films of MDMO-PPV is found to be five times smaller as

compared to charge carrier‘s mobility in side chains [14]. It is also known that transport

properties of polymers are the function of polymer chain conformation and nano-scale structural

ordering [15]. Thus performance of an organic device can be tuned by varying the properties of

polymer films using different solvents, annealing/thermal curing and doping/ nano-manipulation.

2.1.3 Classification of Organic Semiconductors

Electronic devices based on solution processed thin films of organic semiconductors have

attracted much attention of the researchers in recent years due to their low cost, ease of

processing and variety of promising application. Organic semiconducting materials can be

8

broadly classified as conjugated polymers and low molecular weight materials. Low molecular

weight materials are also called as small molecules. Polymeric materials which constitute

repetition of basic fundamentals units/monomers show solubility in many organic solvents. Low

molecular organic materials are further classified as pigments and dyes. Pigments are in-soluble

and dyes are soluble low molecular weight organic semiconductors. Solution processing

techniques such as spin coating, drop casting, dip coating, spraying, etc, are generally used to

make films of polymeric materials and thermal evaporation/sublimation techniques are utilized

for coating thin films of small molecules. Single crystal films are normally grown using low

molecular weight organic materials. Poly(3-hexlthiophene) and pentacene are the representative

organic semiconductors for polymeric and small molecule categories, respectively.

Based on the electrical conduction, organic semiconductors can be p or n-type. Figure 2-3 and

Figure 2-4 depicts molecular architectures of some commonly known p and n-type organic

semiconductors, respectively. As stated before, alternating single and double bonds are clearly

seen in both types of organic semiconductors, indicating a conjugated π electron system. In the

inorganic electronics area, p-type and n-type semiconductors are developed using the process of

doping. For example addition of small amount of group III and V materials in Silicon (group IV

semiconductor) make it p-type and n-type semiconductor, respectively. However in the organic

electronics area formation of p-type and n-type organic semiconductors does not cover the same

concept, as their realization does not necessarily requires the injection of electron donor and

acceptor species. An organic semiconductor is said to be p-type if mobility of holes is greater

than electrons and n-type if mobility of electrons is higher than holes. Thus organic

semiconductors are classified as p-type and n-type based on the preference transport of holes and

electrons, respectively. Further, injection of electrons from electrodes and interfaces is easy in n-

type organic semiconductors and holes in p-type organic semiconductors. Conductivity of

specific charge carriers (holes and electrons) in organic semiconductors is the function of work

functions of electrodes in comparison to conventional organic semiconductors, where

conductivity of holes and electrons is mainly the function of doping type and its concentration.

9

Figure 2-3: Commonly used p-type organic semiconductors (a) P3HT (b) Pentacene (c) CuPc (d)

Rubrene (e) MDMO-PPV (f) MEH-PPV (g) F8BT (h) Alpha-Sexithiophene

Figure 2-4: Commonly used n-type organic semiconductors (a) Fullerene-C60 (b) Fullerene-C70 (c)

PCBM (d) Perylene (e) TCNQ (f) BBL (g) 4FPEPTC (h) DBP

10

2.1.4 Organic Semiconductor Sensors

A sensor is a device that measures physical, biological, chemical, thermal and environmental

quantities. Sensor converts these quantities into an equivalent electrical signal. Needs for sensors

are ever increasing as these are the vital part of modern measurement and control systems.

Renewable energy harvesting, food safety, environmental monitoring, medical diagnostics, and

security are the possible areas that would be benefitted from the exploitation of sensors. Sensors

are conventionally classified according to the quantity to be measured or specific technique used

for such measurement. For example, some of the organic species based sensors, utilized for

biomedical applications are listed as under [16]:

Displacement Sensors

Flow Sensors

Strain Sensors

Gas Sensors

Temperature Sensors

Speed Sensors

Pressure Sensors

Optical Sensors

Hearing Sensors

Piezoelectric Sensors

Capacity Senors

Oflaction Sensors

Harsanyi Sensors

Humidity Sensors

Enzyme Sensors

Micro-organization Sensors

Immunity Sensors

Tissue Sensors

DNA Sensors

11

Materials having semiconducting properties, also called as semiconductors, are mostly used for

the fabrication of sensors. Materials can be organic, inorganic or composite. Organic materials

based sensors and devices are preferred over inorganic ones due their ease of processing, low

cost, simple architectures and flexibility which make them suitable for large area applications.

Although conventional in-organic semiconductors based sensors are robust but are rigid and

expensive. During the last decade, organic microelectronic researchers have well investigated the

potential of organic semiconductors for low cost sensors development. Recently, pressure [17,

18], strain [19], humidity [20-22] and temperature [23, 24] sensors based on simpler

architectures were realized using organic and organic-in-organic composite materials which

indicates their potential for similar applications.

2.2 Organic Solar Cells

2.2.1 Introduction

Solar cells are used to convert light energy into electrical energy. Initially most of the solar cells

were based on inorganic materials such as crystalline silicon [25]. Maximum efficiency of

inorganic solar cells based on crystalline silicon is 24.5% [26] and 42.3% for various tandem and

multi-junction solar cells [27]. However development of these solar cells involves complex

fabrication processes which makes solar panels costly. Silicon wafer processing technology

limits the development of solar cells in large sizes. Solar cells based on the organic materials are

cheap as these are easily processed in an ordinary clean room. Focused research is required to

enhance the performance of organic solar cells in order to compete with inorganic photoelectric

energy conversion devices and to catch industrial applications.

Organic solar cells are based on the thin films of small molecules or polymer materials. These

can be fabricated utilizing cost effective processing techniques such as thermal evaporation, spin

coating, dip coating, screen printing and spray deposition. There is a huge difference between

organic and conventional solar cells as far as the pattern of carrier generation is concerned, in

both these devices. In conventional/ inorganic solar cells, electrons and holes are generated in the

bulk of the cell and these are not tightly bound to each other [28]. Separation of the carriers is

governed by the built in potential or electric field. After separation these reach at their respective

electrodes and are transported out from the active cell material. Whereas in organic solar cells

12

generated carriers (electron and hole pairs) are tightly bound to each other and, are called as

excitons in organic devices. Further, dissociation of these excitons mainly occurs at the

electrodes or at the interface between donor and acceptor organic materials. Diffusion length of

these excitons is relatively small [29-37]. Current flow in conventional solar cells is due the

diffusion of minority carriers. In contrast to these solar cells, current in organic solar cells is

because of the flow of majority carriers.

Efficiency of the first generation of the cost effective organic solar cells was very low in the

range of 10-3

to 10-2

%. Research and fabrication of these devices gained pace in past years to

achieve the efficiency of 4% for the case of evaporated bilayer devices [38, 39]. More focused

research on the various aspects has further improved their efficiency and current reported

efficiency of ultrathin organic solar cell [40] developed by Stephen et al.‘s cell is 8%. Reduction

in loss during photo excitons generation process to collection of carriers at the electrodes process

would increase efficiency up to 10% [41].

2.2.2 Energy Conversion Steps

Generation of electrical energy from sunlight (solar) energy using organic solar cells consists of

four steps:

2.2.2.1 Exciton Formation

Optical absorption bandwidth of the organic material defines the amount of light absorbed/

excitons formed in any organic solar cell. Only absorbed photons participate in the generation of

excitons. Unfortunately, organic materials normally comprises of short absorption bands and

able to absorb only 40% of the sunlight spectrum. Absorption and exciton formation capability

of the cell can be enhanced by increasing the thickness of the organic layers. But increase in the

thickness of organic layers reduces mobility of the carriers is reduced, which is already low in

the organic materials. Decreasing the band gap of the organic material is one way to increase

light absorption and to improve the efficiency of the device [42, 43]. Donor and acceptors made

up of properly tuned conjugated materials show better harvesting of solar spectrum [44]. Doping

of semiconducting films with dye is another way of increasing the absorption capability of the

device [45]. Kymakis et al. [46] have demonstrated increase in the absorption capability of an

13

organic solar cell due to inclusion of an organic dye at the interface of carbon nanotube and

polymer.

2.2.2.2 Diffusion of Exciton to the Donor and Acceptor (D-A) interface

As there are more excitons near the surface of the cell as compared to the outer surface of the

cell. Therefore excitons start moving towards the D-A interface by the process of diffusion.

Excitons with higher diffusion lengths have more probability to reach at the interface as

compared to those with lower diffusion lengths. Normally triplet type excitons have higher

diffusion length because of their higher lifetime. Thus diffusion length of an exciton is increased

by converting a singlet exciton into triplet exciton [47]. Peumans et al. [29] have developed

efficient double hetero-junction organic solar cell using materials having extended diffusion

lengths. Another way of increasing the chances of exciton diffusion to D-A interface is by

decreasing the distance between donor and acceptor [48-51].

2.2.2.3 Dissociation of excitons

Due to strong bond (Columbic interaction) of electron and hole pair in an exciton, a strong field

or force is normally required at D-A interface for the dissociation of the excitons. Difference in

LUMO and HOMO of donor and acceptor materials mainly provides this field in case of

multilayer and BHJ organic solar cells. However, in single layer organic solar cells dissociation

occurs because of the difference of the work functions of two electrodes.

2.2.2.4 Transportation of electron and hole towards the electrodes

After dissociation of the excitons into electrons and holes, electrons are accepted by the acceptor

material and holes by the donor material where these are transported towards their respective

electrodes. Better efficiency is achieved by the use of materials having good morphology. There

are more chances of the carriers to reach at the electrodes when these are transporting through

the smooth films. Smaller grain size of donor and acceptor favours dissociation and

transportation. Morphology can be adjusted by the annealing of the film [52] and solvent mixing.

2.2.3 Solar Cell Performance Parameters

Basic characterization parameters of an organic solar cells are Open Circuit Voltage (Voc), Short

Circuit Current (Isc), Maximum Power Point (MPP), Fill Factor (FF), Power Conversion

14

Efficiency (η) and Quantum Efficiency (QE). First three of these parameters are highlighted in

current voltage Curve of a typical solar cell shown in Figure 2-5. Fill factor (FF) can be

calculated using formula given below:

𝐹𝑖𝑙𝑙 𝐹𝑎𝑐𝑡𝑜𝑟 = 𝐹𝐹 = 𝐼𝑚𝑉𝑚

𝐼𝑠𝑐𝑉𝑜𝑐 2-1

Where Im and Vm are the values of the current and voltage at the maximum power point

mentioned in the above graph. Power conversion efficiency can be calculated using the following

relation:

𝑃𝑜𝑤𝑒𝑟 𝐶𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 = 𝜂 = 𝐼𝑠𝑐𝑉𝑜𝑐𝐹𝐹

𝑃𝑖𝑛 2-2

Where Pin is the input power to the device i.e. Power of the solar incident radiations. Normally

its value is fixed (100W/cm2) for solar cell characterization purposes. Thus power conversion

efficiency is ratio of the output power to the input power of a solar cell. Output power of device

is also the function of the wavelength or energy of the incident photons. Performance of the

device under varying wavelengths of the incident solar radiations lies under the definition of

Quantum Efficiency (QE). Thus quantum efficiency is a measure of the amount of electrons and

holes collected at the electrodes per incident photon.

2.2.4 Different Architectures of Solar Cells

Basic structure of an organic solar cell consists of one or multiple layers of organic materials

sandwiched between two conductive electrodes. Solar cells with only one organic layer are

called single layer organic solar cells. Tang and Albrecht et al. [53] developed first single layer

organic solar cell in 1975 using chlorophyll-a as the semiconducting material but efficiency of

this cell was very low. Solar Cells in which two organic layers are stacked between the

electrodes, making a planar interface are called bilayer solar cells [54]. Anode is normally made

up of glass coated with a transparent conductive oxide (TCO) such as ITO. Rowel et al. [55]

have shown that a layer of Carbon Nano Tubes (CNTs) can also serve as TCO. Surface of the

active layer which is intact with the anode is degraded due to the diffusion of electrodes‘

particles [56]. PEDOT: PSS layer is used in between active layer and cathode of the cell to get

rid of this issue and to smooth the surface of ITO.

15

Figure 2-5: Current Voltage (I-V) Characteristics of a typical Solar Cell under dark and

illumination conditions

In bilayer device configurations, excitons generated far from the D-A interface have to travel a

lot to reach at the interface, therefore only a few of these excitons can reach at the interface by

the process of diffusion. This limits power conversion efficiency of these devices a lot [57]. In

bulk hetero-junction organic solar cells, this issue is addressed by reducing the distance between

donor and acceptor domains by adopting a specific design pattern [58]. Efficiency of the bulk

hetero-junction devices is much higher as compared to bilayer hetero-junction devices [59].

Tandem organic solar cells consist of two or more than two organic subcells in which one covers

lower range while other covers higher range of wavelengths, separated by an inverter.

2.3 Organic Thin Film Transistors (OTFTs)

2.3.1 Device Theory

Transistors which are processed using organic semiconductors are termed as Organic Thin Film

Transistors (OTFTs). Transistors are the most used electronic devices in integrated electronics

area where these are aimed to perform switching operations [60]. Thin Film Transistors are

widely fabricated as Field Effect Transistors (FETs) with three terminals. These terminals are

16

named as Source, Drain and Gate. Every transistor has a semiconducting layer between its drain

and source terminals. Transistors are classified in various ways depending upon the position of

conductive terminals/electrodes, architectures, semiconductor, substrate and dielectric type. All

basic types of transistors also have a dielectric layer between gate electrodes and semiconducting

layers. A top gate top (drain-source) contacts type transistor is shown in Figure 2-6. L and W

represent channel length and width, respectively. The current between drain source electrodes is

the function of field (voltage) applied at gate terminals in addition to the biasing applied at drain

terminals of the devices. Based on the different biasing conditions different operating regimes of

transistor can be identified. In the enhancement mode FET devices, conductance of the channel

is very low when zero or no gate voltage is applied, and the device is said to be in the off state.

The drain and gate voltages are applied with respect to source and referred as VDS and VGS,

respectively. Due to this reason, sometimes source terminal is called as ground terminal.

Although both organic and inorganic transistors show similar behavior and operation modes,

however electrical and structural properties of organic semiconductors are quite different from

inorganic semiconductors. Like inorganic semiconductor, organic semiconducting materials can

be p-type or n-type.

Figure 2-6: Typical top gate top contacts FET

17

Positions of LUMO and HOMO level in organic semiconductors are the function of applied gate

voltage. Whereas positions of the Fermi levels of electrodes remain fixed until drain voltage is

applied. Figure 2-7 schematically depicts change in the positions of LUMO and HOMO levels of

p-type organic semiconductor of a common source OTFT with change in drain and gate voltage

values of the device. When gate voltage is equal to zero, the current will not flow between drain

source contacts even if a small value of drain to source voltage is applied. This is due to

unavailability of mobile carriers and large difference between the Fermi level (work function) of

drain-source contacts and HOMO level (which carries holes) of organic semiconductor as shown

in Figure 2-7 (a). As the negative gate voltage is applied LUMO and HOMO levels of the

semiconductor shifts up and HOMO level comes in-line with Fermi Level of drain – source

contacts and electrons will come/spill out from semiconductor leaving behind mobile holes as

shown in Figure 2-7 (b). In other words, accumulation positive charges in semiconducting layer;

a few nanometers above dielectric layer is due to polarization in the dielectric layer on

application of negative gate voltage. The number of accumulated charges is proportional to the

applied gate to source voltage [61-63]. The current will flow between drain source contacts in

response to the applied drain to source voltage. This current will be due to the flow of holes as

depicted in Figure 2-7(c), indicating p-type conductivity for the case of p-type organic

semiconductors.

Figure 2-8 schematically depicts idealized energy band diagram of a common source OTFT with

n-channel operation under given biasing values. When gate voltage is equal to zero, the current

will not flow between drain source contacts even if a small value of drain to source voltage is

applied. This is due to unavailability of mobile carriers and large difference between the Fermi

level of drain-source contacts and LUMO level (where electrons reside) of organic

semiconductor as shown in Figure 2-8 (a). As the positive gate voltage is applied, a large electric

field at the semiconductor and dielectric interface is established. Further, LUMO and HOMO

levels of the semiconductor shifts downward and LUMO level becomes resonant with Fermi

Level of drain – source contacts. Now electrons can easily move from the metal contacts to

semiconductor and vice versa as shown in Figure 2-8 (b). The current will flow between drain

source contacts in response to the applied drain to source voltage. This current will be mainly

due to the flow of electrons as depicted in Figure 2-8(c), indicating n-type conductivity as

already known for the case of n-type organic semiconductors. It is pertinent to mention that

18

source is always the charge-injecting electrode irrespective of the polarity of gate voltage, as

shown in Figure 2-7(c), when transistor is operating in hole accumulation mode and Figure

2-8(c), when transistor is operating in electron accumulation mode.

Figure 2-7: Idealized energy band diagram of

an OTFT with p-channel operation under

given biasing values [64]

Figure 2-8: Idealized energy band diagram of

an OTFT with n-channel operation under

given biasing values [64]

19

Current-Voltage Curves of a typical low voltage operatable Organic Thin Film Transistor [65]

are shown in Figure 2-9. Drain current remains almost zero and shows no variation when gate

voltage is less than the threshold voltage (Vth) of the device. However when gate voltage is set

higher than the threshold voltage value then initially drain current increases linearly for lower

values of drain voltage and channel resistance stays constant. The device is said to be in the

Ohmic or linear operation mode. When VDS is further increased, the average cross section of the

current flow between drain source contacts starts reducing and channel resistance increases due

to increase in the width of depletion at higher values of drain voltage. At this state VDS = VGS -

Vth. A depletion region is established near drain contact and device is said to be in the pinched

off state. With further increase in drain voltage, higher electric field is produced and space

charge limited current flows in the channel. In other words, electric field forces charges to move

to drain contact through narrow depletion region near drain contact. The potential difference

between source and depletion region stays constant even if drain voltage is increased further.

Thus drain current saturates at a certain value depending upon the gate voltage and shows a

constant value. The device is said to be in the saturation mode. Thus for every fixed value of gate

voltage (higher than the threshold voltage of the device), variation of the drain current with drain

voltage is linear at lower values of drain voltage, and almost no variation in current at higher

values of drain voltage. Device architecture in different operating regimes can be visualized as

depicted in Figure 2-10. Different electrical parameters of an OTFT which are used for its

characterization are field effect mobility (μ), threshold voltage (Vth), insulator layer capacitance

per unit area (Ci) and switching frequency (fs). Field effect mobility in various regimes of

operation can be calculated from transfer characteristics of the transistor. The mobility value

under linear regime can be calculated using the following equations [61, 66]:

𝐼𝐷𝑆 = 𝜇𝑙𝑖𝑛𝐶𝑖

𝑊

𝐿[ 𝑉𝐺𝑆 − 𝑉𝑡𝑕 𝑉𝐷𝑆 −

1

2(𝑉𝐷𝑆)2]

2-3

Field Effect Mobility in the saturation regime (μsat) can be calculated using the following:

𝜇𝑙𝑖𝑛

= 1

𝑤𝐶𝑖𝑉𝐷𝑆

𝜕𝐼𝐷𝑆

𝜕𝑉𝐺𝑆

2-4

Where W and L are the channel width and length, respectively, Ci is the specific capacitance

(capacitance of the active layer per unit area) and Vth is the threshold voltage.

20

Figure 2-9: (a) Output and (b) Transfer I-V characteristics of a typical OFET [65]

𝐼𝐷𝑆 = 𝜇𝑠𝑎𝑡𝐶𝑖

𝑊

2𝐿[ 𝑉𝐺𝑆 − 𝑉𝑡𝑕

2] 2-5

𝜇𝑠𝑎𝑡

=2𝐿

𝑊𝐶𝑖

(𝜕(𝐼

𝐷𝑆,𝑠𝑎𝑡)

12

𝜕𝑉𝐺𝑆

)2 2-6

Switching speed is also an important parameter of an OFET which can be estimated using the

following relation:

𝑓𝑠

= 𝜇𝑜

𝐿2 1

(𝑉𝐺𝑆 − 𝑉𝑡𝑕) 2-7

Above relation shows that switching speed is not only the function of device mobility but also on

its channel length, switching speed increases when mobility is increased and channel length is

reduced. Thus reducing channel length is a way to enhance the switching speed. However

transistor shows deviation from its ideal saturation behavior when channel length is decreased so

much. Such deviation of the device from its ideal behavior is sometimes called as short channel

behavior [67, 68] of the device.

21

Figure 2-10: 3D schematics of OFET in different operating regimes [61] (a) VD<< VG - Vth, Linear

regime (b) VD, sat = VG - Vth, Pinch off point formation near drain contact (c) VD > VD, sat, Beyond

Pinch off point

22

2.3.2 OTFTs based Sensors

Active research in the area of organic electronics during the last decade has suggested that

OTFTs being developed using solution processing techniques are ideal candidates for the

realization of various kinds of low cost and flexible sensors. OTFTs perform both switching and

sensing functions in these devices. It has been observed, due to physical and environmental

changes, channel current and threshold voltage of these devices varies significantly. These

variations in the electrical parameters of the devices are used to measure the external interacting

world. In this section review of various types of sensors developed using OTFT architecture is

presented. Two recently published articles [69, 70] present review of OTFTs based pressure,

vapour, chemical and bio sensors. Table 2-1 and Table 2-2 describe basic parameters of some

OTFTs realized for pressure and strain sensing applications, respectively. Environment

monitoring includes measuring ambient/environmental parameters such as light, temperature,

humidity, gases, chemicals, bio, etc. OTFTs were also investigated for sensing these

environmental parameters during the last decade.

Table 2-3 lists basic parameters of some OTFTs realized for the purpose of environment

monitoring in recent years.

2.4 AC Impedance and Dielectric Spectroscopic Studies

Impedance or dielectric spectroscopy is a powerful technique used to characterize bulk and

interface electrical properties of materials with conducting electrodes [71]. Dynamics of the

charge carriers of a wide variety of materials such as solid, liquid, semiconducting or insulator

can be studied using this technique. Furthermore, motions and relaxation behavior of the

molecules are also estimated as a function of frequency having a wide frequency range of 10-5

to

1011

Hz. When an electric field is applied across the terminals having a dielectric material in

between, the material polarizes i.e. atoms and molecules of the material are displaced from their

equilibrium positions. Polarization is said to be electronic if only displacement of electrons

within the atoms of a material occurs due to applied field and ionic if atoms of a molecule (which

are bonded with each other due opposite polarities) are displaced from their equilibrium

positions. Polarization in polymeric materials mainly occurs due to charge migration and re-

orientation of permanent dipoles. Impedance spectroscopy determines type as well as strength of

polarization which shows its power and importance in characterizing the material properties.

23

Table 2-1: Pressure Sensors based on OTFTs

Semiconducting

Layer Material

Dielectric

Layer

Material

Substrate

Response

Time/

Sensitivity

Possible

Applications Year References

PDPP3T,

NDI3HU-

DTYM2

PMMA,

CYTOP,

PS

Glass, PI,

PET 192 kPa−1

Acoustic

Wave

Sensing,

Wearable

Electronics,

Robotics

2015 [72]

Pentacene polyimide

precursors PEN 30 kPa−1

Artificial

Skin 2004 [73]

P3HT Rubber PET 22ms Artificial

Skin 2009 [74]

Pentacene

Plastic

Foil

(Mylar)

Plastic

Foil

(Mylar)

Tens to

hundreds of

ms

Physical

sensing 2007 [75]

Ruberene Single

Crystals

Micro

structured

rubber

PET Millisecond

s

Electronic

Skin 2010 [76]

Pentacene PVP Glass 20 s force sensing 2005 [77]

Poly iso indigo

bithiophene-

siloxane

PDMS PET 8.4 kPa

−1,

<10 ms

Electric Skin

and Health

Monitoring

2013 [78]

Pentacene PVP PEN n.a. Hydrophone 2007 [79]

CuPc

No

Dielectric

Layer

Glass n.a. Pressure

Sensing 2010 [80]

CuPc

No

Dielectric

Layer

Glass n.a. Telemetry 2012 [81]

24

Table 2-2: Strain Sensors based on OTFTs

Semiconducting

Layer Material

Dielectric

Layer

Material

Flexible

Substrate

Type

Response

Time/

Sensitivity

Possible


P3HT:PCBM

Blend

No

Dielectric

Layer

PET

0.18 μA/%

and

0.65μA/%

Bend

Sensing 2015 [82]

P3HT,

Pentacene PET PET

Between

100-150 ms

Strain

sensing 2012 [83]

Pentacene PVA PET n.a. Strain

sensing 2013 [84]

Pentacene Polyimide PEN n.a. Bend

Sensing 2005 [85]

Pentacene PVP PEN

1.6 nA/%,

7.2 nA/%,

4.1 nA/%

Bend/Strain

Sensing 2007 [86]

Pentacene

Ta2O5

and PVP

bilayer

PEN 20 s Strain

Sensing 2005 [87]

Pentacene Parylene

C PVDF 0.182 μA/%

Piezoelectric

Conversion 2012 [88]

Table 2-3: Environment Monitoring based on OTFT

Semiconducting

Layer Material

Dielectric

Layer

Material

Flexible

Substrate

Type

Response

Time/

Sensitivity

Possible


Pentacene PVPy Glass n.a. Temperature

Sensing 2012 [89]

P3HT SiO2 Si Wafer See ref. Gas sensing 2015 [90]

CuPc No

Dielectric Glass n.a.

Humidity

Sensing 2010 [91]

Pentacene

SiO2-

PMMA

Bilayer

Si n.a. Amonia

Sensing 2012 [92]

P3HT:PCBM

Blend

No

Dielectric Glass 3

Light

Sensing 2014 [93]

Biotinylated

F8T2 SiO2 Si n.a. Bio Sensor 2012 [94]

isoindigo-based

polymer SiO2 Si n.a

Detection in

the marine

environment

2014 [95]

Gain phase Impedance analyzers and LCR meters are usually used for Impedance Spectroscopic

Analysis. Electrical characterization using this technique involves following measurements:

25

a. Variation of the real part of complex impedance and dielectric constant with frequency,

signal amplitude and temperature

b. Variation of the imaginary part of complex impedance and dielectric constant with

frequency, signal amplitude and temperature

c. Variation of the complex impedance and dielectric constant modulus with frequency,

signal amplitude and temperature

d. Cole-Cole plots ( variation of the imaginary part of dielectric constant as a function of

real part of dielectric constant at certain frequency, signal amplitude and temperature)

e. Nyquist plots ( variation of the imaginary part complex impedance as a function of real

part of complex impedance at certain frequency, signal amplitude and temperature)

f. Dielectric Loss variation as a function of frequency

g. Dielectric Strength

h. Relaxation Type/Phenomenon

i. Relaxation time

j. Absorption Coefficient

k. Critical Frequency

The Complex Dielectric Constant (ɛ*) of the investigated films/materials can be mathematically

described as follows [96]:

휀∗ = 휀 + 𝑖휀ʺ 2-8

Here ɛ′ is real part of dielectric constant and ɛʺ is imaginary part of dielectric constant. Cole-Cole

form of the above relation can be written as shown below [97]:

휀∗ = 휀∞ +

(휀𝑠 − 휀∞)1 + (𝑖𝜔𝜏)1−𝛼 2-9

Here ɛs and ɛ∞ parameters specify dielectric constant values at lowest and highest frequency in

the measured frequency range, respectively. ω is the angular frequency which is equal to 2π

times the frequency, η is the relaxation time and α is the absorption coefficient. Capacitance can

be computed using the following relation:

26

𝑪 = 휀𝒐휀𝐴/𝑑 2-10

C is the capacitance, ɛo is the dielectric constant of free space, A is the surface area and d is the

thickness of the cell. Real dielectric constant (ɛ′) value can also computed from the above

relation if capacitance is known.

Imaginary dielectric constant (ɛʺ) of material is actually the measure of dielectric loss as shown

in the following relation:

εʺ = ε 𝑡𝑎𝑛 δ 2-11

Thus imaginary part of the dielectric constant is dielectric loss (tan δ) times the real part of

dielectric constant. Phase angle (θ) and δ are linked with each other as follows:

𝛿 = 90 − 𝜑 2-12

The complex impedance (Z*) of a material can be written as:

𝑍∗ = 𝑍ˊ + 𝑖𝑍ˊˊ 2-13

Where Zʹ and Zʺ are the real and imaginary parts of complex impedance, respectively. Zʹ can

also be written as follows [98]:

𝑍ˊ = 𝑅𝑠 + 𝑅𝑝/1 + (𝜔

𝜔𝑜)2 2-14

RS, RP and ωo in the above equation represent series resistance, parallel resistance and natural

angular frequency of the system, respectively. The dielectric strength (Δɛ) of the investigated

material can be calculated using the following relation:

∆휀 = 휀𝑠 − 휀∞ 2-15

27

The real part of dielectric constant of a material in cole-cole form can written as follows [99]:

2-16

The AC conductivity dependence of the angular frequency can be expressed by the following

relation, as the empirical Jonscher's universal law [100, 101]:

𝜎𝐴𝐶 𝜔 = 𝜎𝐷𝐶 + 𝐴𝜔𝑠 2-17

Where, A is a constant and s is the frequency exponent parameter which identifies AC

conduction mechanisms. ζDC specify DC conductivity of the material. The magnitude of

exponent s, which represents main body interactions of electrons and impurities, is usually less

than unity. The magnitude of parameter s (angular frequency exponent) is calculated from the

slopes of ln(ζʹ)-ln(ω) plots.

In fact Impedance Spectroscopy (IS) is a powerful tool, utilized to investigate important

parameters of materials in order to study their potential for various devices. Based on the values

of various parameters (stated above), relaxation and conduction mechanisms of the materials are

determined. Relaxation can be Debye, nearly-Debye or non-Debye type. The s parameter values

can be interpreted by conductivity mechanisms for frequency dependent conductivity given by

s≈0 [101] for DC conductivity mechanism, 0<s<0.7 [102] for Correlated Barrier Hoping (CBH)

conductivity mechanism, 0.7≤s<1 [103] for Quantum Mechanical Tunneling (QMT)

conductivity mechanism and 1<s<2 [104] for Super Linear Power Law (SLPL). A wide variety

of materials were investigated using this technique in past. These materials include polymers

[105-110] , Liquid Crystals [96, 99, 111-114], Dielelectrics/ Insulators [115, 116], Ceramics

[117-121] and others [122-128].

)1(2)(

2

1sin1)(21

2

1sin1)(1

)()('

oo

o

s

28

Chapter-3: Materials and Experimental

Details 3 1

In this chapter, the materials, techniques and setup used for the fabrication & characterization of

the devices will be introduced briefly. In section 3.1 general properties and applications of the

active layer‘s materials are presented. These materials are Poly(3-hexylthiophene)(P3HT),

Poly[2-methoxy-5-(3‘,7‘-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV), [6,6]-

phenyl C61-butyric acid methylester (PCBM), Polymer-Fullerene blends, Graphene Oxide (GO),

Side Chain Liquid Crystalline based homo-polymers: Poly (chol- n-MMA) & copolymers: Poly

(chol-n-MMA-co-MMA), and Lead Zirconate Titanate Ceramics. In section 3.2, brief overview

of the techniques and equipment used for the fabrication of various films of the devices is

presented. Contact films of the devices were processed using Vacuum Evaporation. Whereas

active/semiconducting layers of the devices were solution processed using spin coating and drop

casting approaches. At the end, brief introduction of the characterization equipment is presented

in section 3.3. Characterization equipment includes AFM, SEM, DEKTAK-8 profiler,

Impedance Analyzer HP-4194, UV-Vis absorption spectrophotometer (S-3100) and Lamp (Lot

Oriel), Semiconductor Characterization System (Keitheley SCS-4200) and special test fixtures.

3.1 Materials

3.1.1 Semiconducting Materials for Solar Cells and Transistors

3.1.1.1 Poly(3-hexylthiophene)

Poly(3-hexylthiophene) which is commonly known as P3HT, is a well investigated p-type

organic semiconductor in the area of organic electronics, in particular for photo sensing

applications. It shows high solubility in many organic solvents as it contains alkyl-groups.

Further, it belongs to the category of conjugated polymers and is based on sp2 hybridized carbon

atoms which forms thiophene rings. P3HT is synthesized by the addition of 3-hexyl groups as

side chains in the thiophene rings of insoluble polymer Polythiophene following a chemical

process [129]. Based on the arrangement of side chains, P3HT can be divided into two

categories i.e. regiorandom and regioregular. P3HT is called as regiorandom P3HT if side chains

29

are arranged from head-to-head and tail-to-tail, and regioregular P3HT if arrangement of the side

chains is from head-to-tail all along the chain. Regioregular P3HT (rr-P3HT) is used in this work

which is generally preferred over regiorandom P3HT due to its flat and crystalline structure

which helps to achieve high mobility [130]. Baeg et al. [131] have recently reported rr-P3HT

based organic field effect transistor with a relatively high mobility of 0.4cm2/VS. P3HT is

soluble in many solvents such as chloroform, 1,2 dichlorobenzene, toluene etc. Slight variations

in the optical and structural properties of P3HT films are reported due to different solvents [132].

Band gap energy (Eg) of P3HT is around 1.9 eV. Figure 3-1 below shows molecular architecture

of P3HT.

Figure 3-1: Molecular Formula of P3HT

3.1.1.2 MDMO-PPV

Like P3HT Conjugated polymer Poly[2-methoxy-5-(3‘,7‘-dimethyloctyloxy)-1,4-

phenylenevinylene] (MDMO-PPV) is soluble in many organic solvents and has been used in the

realization of OLEDs [133, 134], OSCs [135-138], OTFTs [139] and AMOLEDs [140] . The

band gap of MDMO-PPV is about 2.2 eV. Figure 3-2 is presenting chemical formula of MDMO-

PPV conjugated polymer. MDMO-PPV and other conjugated polymers are potential candidates

for low cost and large area electronics due to solution processing and promising features.

MDMO-PPV serve as donor in bulk hetero-junction based systems. It is known that degradation

rate of MDMO-PPV is reduced when blended with fullerens [141]. Three different batches of

MDMO-PPV polymers with varying intrinsic properties were used to investigate their effect on

the performance of organic BHJ solar cells.

30

Figure 3-2: Molecular Structure of MDMO-PPV

3.1.1.3 PCBM

PCBM is soluble derivative of n-type low molecular weight organic semiconductor, Buckminster

Fullerene C60. It has been widely investigated as electron acceptor in polymer organic solar cells

[142] and n-type semiconductor for organic thin film transistors [143]. Its good acceptor

characteristics are probably due its spherical shape and suitable electron affinity value. Higher

cost and low absorption in the visible spectrum are the major disadvantages of PCBM. Band Gap

Energy (Eg) of PCBM is around 2.3 eV. Chemical Formula of PCBM is presented in Figure 3-3

below.

Figure 3-3: Chemical Formula of PCBM

31

3.1.1.4 Polymer Fullerene Blends

In recent years, polymer fullerene blends in which polymer serve as electron donor and fullerene

as electron acceptor were well investigated for photonic applications, in particular for Organic

Solar Cells [144]. Under illumination, photons having energy higher than the band gap energy of

polymers are absorbed and excitons (electron and hole pairs) are produced. These excitons move

towards donor and acceptor interface due to diffusion, dissociated into charge carriers and are

collected at the electrodes of the devices due to internally developed or applied electrical field.

P3HT:PCBM blends were used to realize OTFTs and MDMO-PPV:PCBM blends were utilized

to develop solar cells.

3.1.2 Semiconducting Materials for MIM Capacitors

3.1.2.1 Graphene Oxide

Graphene oxide (GO) has sp2 and sp

3 hybridized carbon atoms and can be considered as

insulating material compared to graphene. GO exhibits various carboxyl and hydroxyl functional

groups and is soluble in water. The optical and electronic properties of GO can be tuned by

varying the content of oxygen [145]. Recently, GO was demonstrated as channel and dielectric

material for field effect transistors [146], and efficient hole transport layer for organic solar cells

[147]. Due to the presence of oxygen based functional networks on the basal plane of GO, it can

make bonds with variety of organic and inorganic materials [148-150]. The optical and electrical

properties of GO can be enhanced using composites with conductive polymers. In this study,

Dielectric and Electrical properties of GO were investigated using parallel plate impedance

spectroscopic technique. Molecular structure of GO is shown in Figure 3-4.

Figure 3-4: Molecular Architecture of Graphene Oxide

32

3.1.2.2 Lead Zirconate Titanate (PbZrTiO3) Ceramics

Lead zirconate titanate (PbZrTiO3 or PZT) based materials/ceramics with perovskite ABO3 ((A =

Mono –or divalent; B = Tri- hexavalent ion)) structure has attained much focus of the researchers in

recent years because of the simplicity of its crystal structure which helps to understand the

relation between structural changes and physical properties. Perovskite based piezoelectric

materials show high electromechanical transformation, due to which these are commonly utilized

in electro-ceramic applications in industry. Furthermore PZT, due its excellent piezoelectric

properties is commonly utilized during the development of underwater/ultrasonic transducers,

such as hydrophones, actuators and underwater transducers. In industry, materials showing

thermally stable dielectric as well as piezoelectric properties around transition temperature are

required [151-155]. The dielectric properties of PZT can be tuned by using various isovalent and

aliovalent dopants. Under certain conditions when PZT crystals possess tetragonal or

rhombohedral symmetry, each crystal has a dipole moment.

PZT materials, exhibit a unique range of properties which make them suitable for the

development of sensors and actuators. In a basic sense, if a piezoelectric material is deformed, an

electrical signal proportional to bending/deformation due to piezoelectric effect. Also if electrical

signal is applied deformation or bending is observed in these materials due to inverse

piezoelectric effect. In order to improve the electrical properties of PZT, various attempts have

been reported in recent years. Doped PZT are mostly synthesized by using one of the following

methods:

a. conventional solid state sintering route,

b. peroxohydroxide method,

c. citrate precursor method, precipitation method,

d. hydrothermal method and

e. solgel method.

In order to investigate the effect Cobalt/Magnesium ratio on the electrical and structural

properties of PZT films, Polycrystalline PbTi0.5Zr0.3(Co1-xMgx)0.2O3 (x = 0.00, 0.25, 0.45, 0.65,

0.85) ceramics films were solution processed using sol-gel route and characterized using parallel

plate impedance spectroscopy. Mg doped PZT ceramics possess activation energy (Ea 1.05

33

eV) which is comparable with the Ea values (11.1 eV) associated with doubly ionized oxygen

vacancies (VÖ) common in perovskite oxide piezoelectric materials.

3.1.2.3 Side Chain Liquid Crystalline Polymers

PMMA derivatives of side chain liquid crystalline homo-polymers (Poly (chol- n-MMA)) and

copolymers Poly (chol-n-MMA-co-MMA) were investigated using parallel plate dielectric and

impedance spectroscopic technique as an alternative to conventional solution processed PMMA

dielectric layer for OTFTs. These polymers were having a broken focal-conic fan texture with

smectic phase. Side chain liquid crystals demonstrate both properties of polymer and liquid

crystalline, which are formed by attaching polymer as side chain. Properties of the materials

were found to be the function of their backbone, aliphatic spacer length and mesogen. Liquid

crystals are intermediate state of matter, in between the crystalline (solid) and isotropic (liquid).

It must have some features of a liquid (e. g. fluidity, inability to support shear, formation and

coalescence of droplets) as well as solids (anisotropy in optical, electrical, and magnetic

properties, periodic arrangement of molecules in one spatial direction, etc.). Molecules are highly

order when material is in solid (crystalline) state and show long range of periodicity in three

dimensions. Molecules of a liquid have no specific intrinsic order. Although molecules in liquid

crystal do not exhibit any positional order similar to the case of liquids, but they do possess a

certain degree of orientational ordering of molecules. Above stated difference between solid,

liquid crystal and liquid states of a matter is schematically explained with the help of Figure 3-5.

Figure 3-5: Position and Orientation of molecules when matter is in (a) Crystalline (b) Liquid

Crystalline and ( c) Liquid state

34

Liquid crystals are generally anisotropic materials and their physical properties are dependent on

the average alignment of the molecules. Depending upon the amount of order in the material,

liquid crystals can have Nematic, Smectic and Cholesteric phases.

Nematic liquid crystals

In this type of liquid crystals, molecules can be freely positioned but oriented parallel to each

other. As can be seen in Figure 3-6 (a), the molecules point vertically but with no particular

order.

Cholesteric liquid crystals

This type of liquid crystal is based on a twisted structure or in other words the director rotates

about an axis as you move through the material. Further this phase consists of nematic layers

with different direction vector. Thus Cholesteric phase (shown in Figure 3-6 (b)) can also be

referred as helically twisted nematic phase. Cholesterics has no long-range orientation order and

no long-range order in positions of the centers of mass of molecules.

Smectic liquid crystals

As shown in Figure 3-6 (c), both positional and orientational ordering exist in this phase of liquid

crystals. Further this phase has three different types called as A, B and C.

Figure 3-6: Schematic illustration of (a) Nematic, (b) Cholesteric and (c) Smectic phase of liquid

crystals

.

35

3.2 Device Fabrication

In this part, brief description of the equipment and techniques used for the fabrication of devices,

is presented. Electronic devices are generally based on semiconducting, dielectric and metallic

films. Organic microelectronic devices are preferred over in-organic devices due to their low cost

and ease of processing. Organic semiconductors can be low molecular weight or conjugated

polymers. Low molecular weight organic films are generally processed using vacuum

evaporation whereas polymeric organic films are developed using drop casting, dip coating,

spraying and spin coating. In the conventional/in-organic semiconductors based electronic

devices different techniques are used for the development of dielectric films such as thermal

oxidation or plasma assisted CVD for the deposition of SiO2 dielectric film on Si substrate. In

organic microelectronic and in particular flexible electronics area, dielectric films are solution

processed using spin coating, drop casting and dip casting. Physical Vapor Deposition techniques

such as thermal oxidation and sputtering are used for the development of contacts for the

devices. The film‘s morphology and the properties of devices are dependent upon the fabrication

technology [156, 157]. Spin coating technique was used for the processing of polymer

semiconducting and dielectric films, drop casting for the development of GO films, sol-gel

process for lead zirconate titanate ceramics and Vacuum/Thermal evaporation for making

metallic contact films.

3.2.1 Vacuum Evaporation

Vacuum or thermal evaporation is one of the common and oldest techniques used for depositing thin

films [158, 159] and widely used for the deposition of metals, metal composites and semiconductors.

Film material is evaporated in vacuum which condensed into solid state after reaching at the substrate

surface

The following sequential steps to take place during film formation using vacuum evaporation

process:

i) Cleaning of the chamber, source material and Crucibles/Coils

ii) Fixing of crucibles/boats to electrodes

iii) Placing of suitable amount of source material in crucibles or coils

iv) Placement of sample/substrate on the sample holder station and adjust the distance

between samples holder and source material.

v) Establishment of required vacuum level with the help of vacuum pumps

36

vi) Heating of source boat (Coils or crucibles) to evaporate metal in the form of vapors

In this research work, Leybold Univex 450 Vacuum Evaporator has been used for metallic

contact deposition. The Univex Leybold 450 is multi-station vacuum coater, specifically

designed for the deposition of various metal films under high vacuum. The vacuum pumping

system for the Evaporator comprises a TRIVAC D 40 B two-stage rotary vane pump backed by a

TURBOVAC 1000 turbo molecular pump. Figure 3-7 shows photograph of vacuum evaporator.

3.2.2 Spin Coating

Spin coating is a technique used to develop uniform thin films on flat substrates. Polymer

semiconducting films of the investigated devices were processed using spin coating process after

preparing homogenous solution using magnetic stirring station shown in Figure 3-8. A specified

small amount of semiconducting solution is applied on the samples after placing samples on the

spin coating station. Then spinning of the samples is undertaken in steps, initially with low speed

of around 800 rpm for 20 seconds followed by high speed of around 2000 rpm for 50 seconds.

Films deposited using spin coating processes are generally uniform and reproducible. Further

thicknesses and structural properties of the films are the function of spin speed and time.

Figure 3-7: Leybold Univex 450 vacuum evaporator

http://en.wikipedia.org/wiki/Thin_film

http://en.wikipedia.org/wiki/Substrate_(printing)

37

Figure 3-8: (a) Semiconductor in powder form as received from Manufacturer (b) Solvent (c)

Semiconductor solution (d) Stirring Station with temperature control and monitoring

After formation of semiconducting films, samples were annealed. Annealing process was carried

out by using hot plate system placed in the nitrogen filled glove box. Photograph of the Glove

Box is shown in Figure 3-9. Glove box is generally used to store processed samples in order to

prevent oxidation of films.

3.2.3 Drop Casting Process

Drop casting involves dropping solution on the substrate and evaporation of solvent. This

deposition technique was used for the development of graphene oxide films. In order to

evaporate solvent, thermal curing of the samples was done in industrial oven. This is a simple

process of film formation. Unlike spin coating process, no wastage of material occurs. However

films deposited using this process are less uniform. Further, it is hard to control the thickness of

the films deposited using this method.

38

Figure 3-9: Glove Box System

3.3 Characterization Setup for Films and Devices

Structural properties of the films which involve surface roughness, film morphology, uniformity,

size of the film constituents along with inter-particle distance and overall film thicknesses were

studied using Multimode Atomic Force Microscope and Scanning Electron Microscope (XL30,

FEi Co., Hillsboro). Film thicknesses were measured using DEKTAK-8 profiler shown in Figure

3-10. Optical properties of the films were investigated using UV–Vis Spectrophotometer, S-3100

(Shown in Figure 3-11).

Figure 3-10: DEKTAK-8 Surface Profiler

39

Figure 3-11: UV-Vis Spectrophotometer interfaced with computer

Dielectric and electrical conduction properties of the films were studied using Spectrum

Analyzer HP 4194. Spectrum Analyzer was interfaced with a computer, Novotherm heating

system and a temperature controller. Process parameters can be easily adjusted using Labview

based GUI. Figure 3-12 shows photograph of the spectrum analyzer and Novotherm heating

station used for the electrical characterization of the investigated films. Current Voltage (I-V)

characteristics of the films were studied using Keithely Semiconductor Characterization System

SCS-4200. Precise measurement in the Pico Amperes range is possible using Keithley 4200

measurement system shown in Figure 3-13.

40

Figure 3-12: Impedance Analyzer HP 4194 and Novotherm Heating Station

Figure 3-13: Semiconductor Characterization System: Keithley 4200 SCS

41

Chapter-4: Polymer-Fullerene BHJ Solar

Cells and Phototransistors 4 3

4.1 MDMO-PPV:PCBM blend based Organic Solar Cells

Investigation of organic bulk heterojunction solar cells (OSCs) realized using three different

batches of poly[2-methoxy,5-(30,70-dimethyl-octyloxy)]-p-phenylene vinylene (MDMO-PPV)

with varying intrinsic properties such as molecular weights, polydispersity (PDI) values, charge

carrier densities (NA), band gap energies (Eg), etc. is presented in this section. Blend of MDMO-

PPV and [6,6]-phenyl C71-butyric acid methylester (PC71BM) as photoactive layer of the devices.

Properties of the active layers were initially studied using UV-Vis Absorption Spectroscopic and

Atomic Force Microscopic techniques. Polymer intrinsic properties have significantly influenced

on the structural properties of active layers. Devices were characterized under dark and

illumination conditions, and their results were compared. Efficiency of the device was found

highest for polymer having higher PDI and NA values and vice versa. It was attributed to the

increased photon absorption capability and favourable nano morphology of active layer for

MDMO-PPV batch with higher PDI and NA values. These results showed that intrinsic properties

of polymer play a vital role on the performance parameters of Polymer-Fullerene bulk hetero-

junction based OSCs.

4.1.1 Introduction

During the past decade, investigation of solar cells using organic semiconductors have attracted

much attention of the optoelectronics researchers due to their potential applications for the

realization of flexible, light weight and cost effective energy conversion platforms [160].

Organic bulk heterojunction solar cells, in which photoactive layer is based on the blends of p-

type and n-type organic semiconductors, have consistently been playing a leading role in this

emerging area of photovoltaics. P-type and n-type organic semiconductors which are also called

as Donor and Acceptor, respectively, form an interpenetrating network with a nanoscale phase

separation in bulk volume of the organic layer. Processing of semiconducting layer is mostly

carried out using vapour deposition approach for the case of low molecular weight organic

materials whereas solution processing is adopted for the case of polymer based systems [161].

42

Polymer fullerene blends are among the well-studied combinations for bulk hetero-junction solar

cells due to their ease of processing and promising features [51, 162]. Thompson et al. [163]

have reviewed various polymer fullerene bulk hetero-junction (BHJ) solar cells and discussed the

effect of BHJ film‘s morphology and polymer-fullerene interactions on the performance of these

devices.

MDMO-PPV:PCBM blend is among the well investigated polymer fullerene combination for

BHJ solar cells [164, 165]. Earlier studies on this typical polymer fullerene blend include the

investigation of the effect of solvent type [166, 167], blend ratio [168, 169], temperature-

degradation [170] and electric field variation [171] on the their properties. It has been shown that

these variations significantly influence upon the morphology of these films, and short-circuit

current (Jsc), being the strong function of organic film morphology [172], changes as a result.

Thus performance of a solar cell can be modified by tuning the morphology and transport

properties of its BHJ photoactive layer.

Performance of polymer solar cells is known to be the function of polymer intrinsic properties, in

particular, weights of the molecules and their orientations. Liu et al. [173] have investigated the

effect of PTB7 polymer‘s molecular weight on the performance of polymer fullerene BHJ solar

cells and reported 1.6 times increase in its efficiency with the increase of polymer molecular

weight from 18 to 128 kg/mol. Recently, Tumbleston et al. [174] have shown that the

performance of BHJ solar cell is also function of the orientation of the molecules in the organic

Donor-Acceptor blend based films. In this work (which is continuation of the work undertaken

by Yaman et al. [175]), combined effect of polymer intrinsic properties such as Molecular

Weight, Polydispersity, Charge carrier density and band gap energy have been studied on the

performance of MDMO-PPV:PCBM blend based Solar Cells which is not previously

investigated to our best knowledge. Three different batches of MDMO-PPV polymer with

varying intrinsic properties were used for this study.

4.1.2 Experimental Methods

Effect of polymer intrinsic properties on the performance of BHJ solar cells is investigated in

this study using three different batches of MDMO-PPV polymer denoted as polymer batch A, B

and C. Recently, Tunc et al. [176] have investigated intrinsic properties of these polymer batches

43

using different methods and investigated the effect of polymer intrinsic properties on the I-V

characteristics of Organic Field Effect Transistors (OFETs), in particular on the short channel

effect of the OFETs. Polymer batch-B has the highest polydispersity (PDI) and Charge Carrier

Density (NA) values, whereas batch-C carries the highest Mn and Mw values among all the

polymer batches. Mn and Mw are the number and weight averages of the molecular weights,

respectively, and polydispersity is the ratio of Mn and Mw. Polymer batch-A has the lowest Mn,

Mw and PDI values among all the investigated polymer batches. Varying NA values of different

polymer batches indicates differences in their impurity levels. Among all other properties of

polymers, Eg is of particular importance for photovoltaic applications because photon absorption

is the direct function of it. Band gap energy (Eg) values of polymer batch A, B and C are equal to

2.12, 2.08 and 2.06 eV, respectively. It is pertinent to mention that polymer batch-A which has

the lowest molecular weights, carries the highest energy gap. It is expected that these parameters

will influence on the electrical properties of the organic films by affecting on the inter-chain

ordering of the molecules.

Fabrication of the devices was started with the cleaning of Indium Tin Oxide (ITO) coated

patterned glass substrates in an ultrasonic bath using detergent, de-ionized water, acetone and

isopropyl alcohol. Poly(3,4-ethylenedioxylenethiophene)-polystyrenesulfonate (PEDOT:PSS)

film was coated on ITO substrates using spin coating technique. This hole conductive layer not

only smoothes ITO surface but also used to increase the work of bottom electrode so that it can

accept holes, and to decrease the work function of top electrode so that it can accept electrons

from the active layer [177]. PEDOT:PSS coated samples were annealed at 110 ºC for 30 minutes.

MDMO-PPV:PCBM blends with 1:4 w/w ratio, were prepared in dichlorobenzene by the process

of stirring at 50 ºC for 24 hours, using three different batches of polymer (MDMO-PPV). In

order to attain similar thicknesses of all the films i.e. ≈ 100nm, varying concentrations [12, 6 and

7 mg/mL] of polymer batch A, B and C were used. Optimal performance of MDMO-PPV:PCBM

blends based BHJ solar cells has been demonstrated with 1:4 blends ratio [178], which is the

reason of selecting this specific ratio of these blends for this study. Spin coating technique was

used to coat organic semiconducting layers on the samples using these MDMO-PPV:PCBM

blends. Then samples were thermally cured in the Glove Box at 50 ºC for 10 minutes. Finally,

around 50 nm thick top contact was made on the organic films by the evaporation of Aluminium

in the evaporation chamber. Figure 4-1 schematically presents the structure of devices along with

44

the molecular formulas of PEDOT:PSS, MDMO-PPV and PCBM. Current-voltage (J–V)

characteristics of the devices were studied under dark and UV-Vis illumination of 100mW/cm2,

using Semiconductor Characterization System, SCS-Keithley4200, and 150W Oriel Solar

Simulator with AM1.5 filter. Solar Simulator was calibrated by a reference solar cell during the

measurements. Structural profiles of the semiconducting layers were studied using NanoScope

Multimode Atomic Force Microscope (AFM). Absorption spectra were extracted using UV-Vis

Spectrophotometer (S-3100). Thicknesses of the organic and electrode films were measured

using DEKTAK-8 profiler. All the test and measurements were performed in the ambient

conditions.

Figure 4-1: (a) Schematic of the device architecture, and molecular structures of (b) PEDOT:PSS

(c) MDMO-PPV (d) PC71BM

4.1.3 Results and Discussion

Figure 4-2 shows 10×10 µm2 surface and high resolution 3D AFM Profiles of Polymer:Fullerene

bulk hetero-junction based semiconducting layers developed using three different batches of

MDMO-PPV polymers. Thin film casted from polymer batch-A exhibits the highest height

variations. Root Mean Square (RMS) roughness values of the blend films cast from polymer

45

batch A, B and C were found to be 3.104, 0.909 and 1.647 nm, respectively. Lowest surface

roughness and highest PDI value was found for the polymer batch with moderate molecular

weights i.e. batch-B. Further, phase separation of the composite film developed using polymer

batch-B was found significantly larger as compared to the films prepared using other polymer

batches.

Figure 4-2: 10×10 µm2 surface and high resolution 3D AFM Profiles of Polymer: Fullerene Bulk

heterojunction based semiconducting layers for three different batches of MDMO-PPV : A[(a, b)],

B[c, d], and C[(e, f)]

Figure 4-3 shows UV-Vis absorption spectra of Polymer:Fullerene bulk heterojunction based

nanolayers prepared from three different batches of MDMO-PPV polymers. Thin films, prepared

using polymer batch-B showed highest UV-Vis absorption with maximum absorption peak (λmax)

at 497 nm, and two shoulder peaks at 416 and 598 nm. Thin films prepared from polymer batch-

46

A demonstrated the lowest absorption of photons. Further, no significant peaks were observed

for polymer-fullerene blend based films prepared from batch-A and batch-C.

Figure 4-3: Absorption Spectra of MDMO-PPV:PCBM nanolayers (with 1:4 w/w ratio) prepared

using three different batches of MDMO-PPV Polymer

Formation of excitons, their diffusion towards Donor and Acceptor (D-A) interface and

dissociation into free carriers (electrons and holes) at D-A interface, and transportation of these

carriers towards electrodes are among the basic steps which are essentially followed in BHJ solar

cells during the process of solar energy conversion into electrical power [179]. Therefore, role of

the organic film morphology and D-A interface is of primary importance in these devices. The

ideal film architecture is the one which supports the excessive generation of excitons, minimizes

the chances of exciton and carrier recombination simultaneously enhancing charge carrier

generation and their transportation towards electrodes.

The performance of the devices developed using three different batches of MDMO-PPV were

compared using their Current Density-Voltage (J-V) curves, shown in Figure 4-4. Current

Density (Jsc), Open Circuit Voltage (Voc), Fill Factor (FF) and Efficiency (η) values of devices,

under an illumination intensity of 100 mW/cm2, are summarized in Table 4-1. Results showed

that polymer intrinsic properties have significantly influenced on the Jsc and FF values of the

devices. In earlier investigations, these parameters of BHJ solar cells have been shown to be

47

sensitive to the type of solvents [180, 181], annealing conditions [182, 183] evaporation rate

[184], doping species [185], molecular weights of polymers [173] etc. In most of these reports,

variation in the FF and Jsc parameters of the devices was suggested due to differences in the

morphology of their organic films. BHJ solar cell developed using polymer batch-B with

moderate molecular weights and highest PDI and NA values, among the investigated polymer

batches, has shown highest efficiency which was attributed due to higher photon absorption

capability (Figure 4-3) and favourable morphology of its photoactive layer. Furthermore, proper

separation of Donor and Acceptor phases for the case of polymer batch-B (Figure 4-2), may also

be the reason of its enlarged efficiency. Thus based on the earlier investigations [173, 186] and

current study, it is suggested that the performance of a polymer fullerene BHJ solar cell can be

enhanced by using polymer having higher PDI values. In short, in this work an effort was made

to highligt the importance of polymer intrinsic properties for MDMO-PPV:PC71BM blend based

solar cells.

Figure 4-4: Current Density-Voltage (J-V) Curves of BHJ solar cells developed using three

different batches of MDMO-PPV Polymers under an illumination intensity of 100W/cm2

48

Table 4-1: Various Parameters such as Fill Factor (FF), Short Circuit Current Density (Jsc), Open

Circuit Voltage (Voc) and efficiency (η) of polymer fullerene BHJ based solar cells realized using

three different batches of MDMO-PPV Polymers

Polymer Batch JSC (mA/cm2) Voc(V) FF %η

A 2.78 0.84 0.3 0.7

B 5.16 0.8 0.51 2.1

C 4.61 0.78 0.49 1.76

In conclusion, Polymer-Fullerene BHJ based organic solar cells were fabricated using three

different batches of MDMO-PPV polymer with a motivation to investigate the effect of polymer

intrinsic properties i.e. Molecular Weights, Polydispersity values, Charge Carrier Densities and

HOMO/LUMO Levels, on the performance of these devices. Polymer batches with significant

varying intrinsic properties were used for this investigation. Structural and optical properties of

MDMO-PPV:PCBM blends based nanolayers developed using three different batches of

polymer, were studied using Atomic Force Microscopic and UV-Vis absorption spectroscopic

techniques, respectively. Current Density-Voltage (J-V) characteristics of the devices were

investigated under an UV-Vis illumination intensity of 100mW/cm2 and their results were

compared. Differences in the nano morphology of organic films, casted using three different

batches of polymers were observed which have significantly influenced on Jsc and FF parameters

of the devices. Efficiency of the solar cell, realized using polymer with moderate Molecular

Weights (Mn= 85 kg/mol and Mw= 1400 kg/mol), and highest Polydispersity (16.8) and Charge

Carrier Density (3.2×10-17

cm-3

) values, was found maximum, which was attributed to higher

photon absorption capability and favorable morphology of its photoactive layer.

4.2 P3HT:PCBM blend based Organic Phototransistors

This section describes fabrication and characterization of BHJ based photo organic Metal

Semiconductor Field Effect Transistors (MESFETs). Drain/ Source and Gate electrodes of the

devices were made using Silver and Aluminum, respectively. Development of the electrodes was

carried out using Physical Vapor Deposition technique, named as Vacuum or Thermal

evaporation. Gate electrode which was made on the semiconducting layer of the devices has

shown Schottky type contact properties whereas Drain/Source contacts which were made on the

49

glass substrates prior deposition of semiconducting layers, have shown Ohmic type contact

behavior with the semiconducting/active layers of the devices. Surface profiles of the active

layers were studied using AFM. Photon absorption capability of the semiconducting layers were

studied a function of wavelength using UV-Vis absorption spectroscopy. Current-Voltage (I-V)

characteristics of the devices were investigated under dark and UV-Vis illumination using SCS,

4200 (made of Keithley intruments) and Lamp (made of LOT Oriel). Devices have shown low

voltage operation and weak saturation trend. Fabrication & characterization of phototransistors,

results and conclusions are presented in sequence to uncover the details of this work.

4.2.1 Introduction

Organic Semiconducting layers based devices have gained much attention of the microelectronic

researchers in recent years due to their various promising features such as low cost, room

temperature processing in ordinary lab environment, flexibility and variety of advanced

applications. Some examples of these devices are Organic Transistors [187], Solar Cells [160],

Light Emitting Devices [188], Photodiodes and Photo-transistors [189].

In particular, optoelectronic researchers have well investigated organic phototransistors with an

aim to replace conventional inorganic photo-detectors with these low cost devices in recent

years. Output current of a phototransistor, not only depends upon the electric field which is

applied at its gate terminal but also on the intensity of the light being illuminated over it. In order

to optimize the performance of OPTs, much effort has been made during the last decade.

Researchers have used films of low molecular weight organic semiconductors (which are

normally processed using thermal sublimation techniques) [190-194], polymers (which are

solution processed) [195-199], and blends of polymers & low molecular weight semiconductors

to investigate the effect of low molecular weight and polymer type semiconductor‘s properties

on the performance of OPTs [200].

Phototransistors can be MOSFET (Metal Oxide Semiconductor Field Effect Transistor) or

MESFET (Metal Semiconductor Field Effect Transistor) type. The investigated devices were

developed in MESFET architecture. MESFETs are preferred over MOSFETs because their low

voltage operation and fewer processing steps [201]. Before this work, MESFET architecture

based phototransistors were known but with only one organic semiconductors [202, 203]. The

50

devices were fabricated using the blends of poly(3-hexylthiophene)(P3HT) and [6,6]-phenyl

C61-butyric acid methylester (PCBM) with 1:1 & 1:0.8 wt/wt ratio and blends of poly(3-

hexylthiophene)(P3HT), [6,6]-phenyl C61-butyric acid methylester (PCBM) and Methyl Red

(MR) with 1:1:0.2 wt/wt ratio.

4.2.2 Fabrication and Characterization

MESFET type OFETs were fabricated on the glass substrate. Figure 4-5 shows schematic

diagram of the devices and molecular structures of active layer‘s materials. Devices fabrication

initiated with the deposition of 50 nm thick Silver Drain/ Source contacts on the cleaned glass

slides. Drain/ source contacts were made by PVD process at 10-6

mbar pressure in the

sublimation chamber (Leybold-Univex-450). Three different types of semiconducting solutions

were prepared using overnight magnetic stirring process/ station. Solution-1 was prepared using

P3HT (Aldrich) and PCBM (Aldrich) with 1:1 wt/wt ratio in 1,2 dichlorobenzene (Alfa Aesar),

solution-2 was prepared using P3HT (Merck) and PCBM (Ossila) with 1:0.8 wt/wt ratio 1,2

dichlorobenzene (Merck) and Solution-3 was prepared using P3HT(Aldrich), Methyl

Red(Aldrich) and PCBM (Aldrich) with 1:0.2:1 wt/wt ratio 1,2 dichlorobenzene (Alfa Aesar).

Semiconducting layers were spin coated on Drain Source coated glass substrates using these

semiconducting. Channel length (L) and width (W) were found equal to 30µm and 2mm,

respectively, for the case of P3HT:PCBM blends with 1:1 wt/wt ratio and P3HT:MR:PCBM

blends. Whereas channel length (L) and width (W) of the devices realized using P3HT:PCBM

with 1:0.8 wt/wt ratio were found to be equal to 50µm and 2mm, respectively. Band gaps of

P3HT and PCBM are known to be equal to 1.9 eV and 2.3 eV, respectively. After formation of

active layers, samples were placed in the glovebox and annealed in the nitrogen environment at

110 ºC for 30 minutes. At the end, gate contacts were made by depositing Aluminium on the BHJ

solution processed films in the vacuum chamber.

Following equipment was used to characterize the devices:

Electrical Characterization System, Keithley-4200

UV-Vis Lamp Source

Photodiode (as reference)

Spectro-photometer, S-3100

51

AFM

SEM

Impedance/ Network Analyzer, HP-4194.

Surface Profiler, Dektak-8.

Figure 4-5: (a) Design schematic of organic phototransistors, and molecular structures of (b) P3HT

(c) MR (d) PCBM


4.2.3.1 Phototransistors based on P3HT:PCBM blends with 1:1 wt/wt ratio

Results of the phototransistor fabricated on glass substrate using P3HT:PCBM blends with 1:1

wt/wt ratio are presented and discussed in this section. The absorption spectrum of the 230 nm

thick active layer of the device is shown in Figure 4-6 (a), inset presents cross section SEM cross

section view of the active layer on glass substrate. The AFM image (10µm × 10µm) of the active

layer is shown in Figure 4-6 (b). Root mean square (RMS) roughness of the BHJ polymer-

fullerene film was observed to be equal to 4.89 nm. It is known that Aluminium (Al) shows

Schottky type rectifying contact whereas Silver (Ag) makes Ohmic type linear contact with

organic semiconducting materials, respectively [204]. Electrical results (I-V curves shown in

Figure 4-7 (a) and (b)) indicated Ohmic type contact of the semiconducting layer with Silver

52

drain-source electrodes and Schottky type (rectifying) contact with Aluminum gate electrode

which is in accordance with earlier investigations [205]. Figure 4-7 (c) presents energy band

diagram of the organic phototransistor. Energy bands are drawn as straight lines as band bending

which happens due to gate field, has not been taken into consideration. HOMO level of P3HT is

-4.9 eV. Work function of Silver is -4.7 eV [206, 207] and LUMO energy level of PCBM is -3.7

eV. In this typical configuration, holes can easily enter into the drain source electrodes from the

semiconducting film or from drain source electrodes to semiconducting film than electrons,

indicating limited injection of electrons. Rectifying type contact of the semiconducting BHJ film

with gate electrode of the device was assumed due to the energy difference of 0.6 eV between

the work function of gate electrode and HOMO level of P3HT & LUMO level of PCBM.

Figure 4-6: (a) Absorption of P3HT:PCBM blend based active layer (with 1:1 wt/wt ratio) as a

function of wavelength, inset: its SEM based side view and (b) its (10µm × 10µm) surface AFM view

53

Figure 4-7: (a) S-D I-V curve (b) G-S I-V curve when VDS = 0 V (c) Energy Band Diagram of the

phototransistor, showing exciton generation, its dissociation, and transportation of holes and

electrons towards electrodes [93]

The output Current-Voltage (I-V) curves of the phototransistor without being illuminated, are

depicted in Figure 4-8, inset compares gate leakage current with drain current at VGS = - 3 V.

Figure 4-9 shows drain current (IDS) as a function of gate voltage (VGS) without illumination at

VDS = -5 V. I-V curves of the device were found similar to p-type mode characteristics of a

typical ambipolar FET [208, 209]. Weak kink in the output characteristics (at 0 gate to source

voltage) confirmed ambipolar nature [210, 211] of the device‘s semiconducting layer. Drain

current was found to increase with negative gate voltage as can be seen in Figure 4-8 which is an

indication of p-type behavior of ambipolar MESFET. It was also observed that this device can

54

operate as transistor 0 to -5 V gate voltages because gate current increases rapidly if gate voltage

is further increased. Further, ON-OFF current ratio of the device was also observed to be low

similar to earlier investigations [212].

Figure 4-8: Current Voltage (I-V) Curves (output) of the Phototransistor without being illuminated,

inset: gate leakage current vs drain current at VGS = - 3V as a function of drain voltage

Figure 4-9: Transfer I-V Characteristics of the photo-OFET under dark

55

Drain/ channel current (IDS) can be found mathematically following the relation as under [213]:

𝐼𝐷𝑆 =𝜇𝑊𝐶𝑖

2𝐿(𝑉𝐺𝑆 − 𝑉𝑡𝑕)2 4-1

Where

μ = field effect mobility,

Ci = Dielectric/ gate capacitance per unit area,

W = channel width,

L = channel length,

VGS = gate to source voltage and

Vth = threshold voltage.

In the saturation regime, μ can be determined using following relation:

𝜇 = 𝑚2 2𝐿

𝑊𝐶𝑖 4-2

Where ‗m‘ in the above relation is the slope of (IDS)1/2

- VGS curve. Figure 4-10 is showing gate

specific capacitance variation as a function of frequency, indicating Ci value to be equal to

0.21µF/cm2 at 100 Hz. Field effect mobility value of the device was determined as 1.6×10

-4

cm2V

-1s

-1 which is much higher as compared to the mobility values of earlier investigated

devices, having similar configurations [91, 214]. However device mobility was observed lower

as compared to MOSFET architecture based organic transistors which may be attributed due to

thicker semiconducting films in MESFET based devices [215].

The output I-V characteristics of the photo-OFET at fixed UV-Vis illumination of 100mW/cm2

are shown in Figure 4-11(a). Pronounced kinks were observed in the output I-V characteristics of

the device at 0 and -1 gate at higher drain voltage (greater than -6 V) indicating increase in the

ambipolarity of active layer under illumination. The Current-Voltage Curves of the device at VGS

= -5 V under dark and varying fixed UV-Vis illuminations are described in Figure 4-11 (b). It

was observed that drain current increases with illumination intensity. It was attributed that

56

Photovoltaic (generation of voltage) and Photoconductive (change in the conductivity of the

semiconducting layer due to illumination) phenomena were responsible for the photosensitive

properties of these devices as discussed in [189]. Photo-responsivity (R) of the phototransistor

can be determined using the following relation [216]:

𝑅 =𝐼𝑝𝑕

𝑃=

(𝐼𝑑𝑙 − 𝐼𝑑𝑑 )

𝐸𝐴 4-3

Where

Iph = Drain/ Channel Current generated due to illumination,

Idl = Drain current under illumination,

Idd = Drain current under dark and

P = Incident Optical Power (which can be estimated by multiplying device channel area with

intensity of the incident light).

Figure 4-10: Specific capacitance as a function frequency between gate and source/ drain electrodes

57

Another parameter which is sometime used to determine the performance of a photo-sensor is

ratio of drain current illumination to drain current under dark (r) as under [21]:

𝑟 = 𝐼𝑑𝑙𝐼𝑑𝑑

4-4

Photo responsivity and ‗r‘ values of the device were calculated at VGS = 0 V and VDS = -8 V, and

were determined to be equal to 0.047 A/W and 3, respectively, under an illumination intensity of

100mW/cm2 UV-Vis. Photo-responsivity value of the device was found higher as compared to

reported single semiconductor based photo organic MESFETs [202, 203], comparable with the

photo-responsivity values of glass substrate based photo organic MOSFETs [217, 218] and lower

than Si/SiO2 substrate based photo-OFETs [200, 219]. Although photo-responsivity values of

Si/SiO2 substrate based phototransistors are superior but these are rigid and comparatively costly

which spurs the interest in organic semiconductor based low cost photo-detectors, having option

of their processing on flexible substrates in addition to other promising features.

4.2.3.2 Phototransistors based on P3HT:PCBM blends with 1:0.8 wt/wt ratio

Figure 4-12 presents absorption spectrum of 350 nm thick active layer based on P3HT:PCBM

blends with 1:0.8 wt/wt ratio. Highest peak was found at 534 nm with two subsequent peaks at

556 nm and 600 nm which were originated from the crystalline domains of P3HT. Further,

absorption in the range of 370 nm to 450 nm is attributable to PCBM. Figure 4-13 (a) and (b)

shows 5µm×5µm AFM 3D profiles of P3HT:PCBM blend based active layer. The surface

topography of the blended 350 nm thick film showed a rough surface morphology, with Root

Mean Square (RMS) and Average roughness values equal to 26.9 nm and 21.3 nm, respectively.

The nano-scale phase separation of P3HT:PCBM blend is observable in the 3D AFM image. No

considerable structural defects were found in ther semiconducting film surface. Figure 4-13 (c)

shows SEM surface image of the active layer. The SEM micrograph of the active layer showed a

crack-free surface at a given resolution of 20 m. The inset of Figure 4-13 (c) which presents

SEM image at high resolution (2 m) showed no considerable porosity. Further, no aggregates

were found in the SEM images of the active layer. Aluminium electrodes make Rectifying type

contacts whereas Silver electrodes show Ohmic contacts with semiconducting layers [80].

58

Figure 4-11: Output I-V Characteristics of the Photo-OFET (a) under fixed UV-Vis illumination

intensity of 100mW/cm2 and (b) under given UV-Vis illumination intensities compared with dark

59

Through I-V measurements, Schottky type contact of semiconducting layer with Aluminium gate

electrode and Ohmic type contact with Silver drain-source electrodes was noted. Rectification

ratio was found equal to 10 at ±2 V gate to source voltage when drain to source voltage was

equal to zero. Previously, contact properties of P3HT:PCBM blend based organic film have been

elaborated with the help of the energy band diagram in this specific device configuration by

Yasin et al.[93]. Figure 4-14 shows output I-V characteristics of device under dark. Drain current

was found to increase with negative gate voltage which evidenced p-type conductivity of the

semiconducting layer. P-type behavior of the semiconducting layer was associated to Silver

based drain-source electrodes as already reported for the case of P3HT:PCBM blends [217].

Further, Current- Voltage (I-V) characteristics of the device were found similar to typical p-type

organic semiconductors based field effect transistors [197, 220]. Gate leakage current (IGS)

increases rapidly and becomes comparable with drain current when positive or higher negative

gate voltage (higher than -5 V) is applied. Therefore, the proposed device can only work for

lower negative gate voltages. As already known for the case of organic MESFETs [221], device

has shown low voltage operation, with threshold voltage Vth equal to 0.1 V.

Figure 4-12: Absorption spectrum of 350 nm thick P3HT:PCBM blend based active layer

60

Drain to source current (IDS) of the device in the saturation regime can be mathematically found

using following relation [222]:

𝐼𝐷𝑆 = 𝐾 (𝑉𝐺𝑆 − 𝑉𝑡𝑕)2 4-5

Where VGS is the voltage between gate and source electrodes, Vth is the threshold voltage and k

is the conduction parameter which can be determined using following relation:

𝐾 = 𝜇ɛ𝑊

2𝑎𝐿 4-6

Here, µ is the field effect mobility, ε and a are the parameters which specify permittivity and

thickness of the semiconducting layer, respectively. L and W represent channel length and

width, respectively.

Figure 4-13: AFM 3D profiles (5µm×5µm) [(a) & (b)] and SEM images of the active layer (c)

61

Figure 4-14: Phototransistor output I-V characteristics under dark

Specific capacitance (at 100 Hz) and permittivity values were found equal to 0.2µF/cm2 and

0.84×10-11

F/cm, respectively. Field effect mobility value of the device was found to be 7.7×10-3

cm2V

-1S

-1 at VGS = -3 V and VDS = -8 V. Figure 4-15 shows output I-V characteristics of the

phototransistor under UV-Vis illumination of 90mW/cm2. Enhancement in the drain to source

current of the device is due to photo generated carriers which were produced due to the

absorption of photons having energy higher than the band gap energy of the polymer i.e.

P3HT. Pronounced kinks in the I-V characteristics of the device at lower gate to source voltages

(0 to -1 V) when drain to source voltage becomes higher than -6 V, specifies ambipolar nature

of the active layer as already known [211, 223]. This nonlinear behavior of the device is due to

injection of both types of carriers from drain source electrodes under these biasing conditions.

Further, Current-Voltage (I-V) characteristics of the device under illumination were found in

good agreement to p-type mode characteristics of typical ambipolar FETs [209].

Performance of a photo detector can be indicated by its photo sensitivity (P) and responsivity (R)

parameter values which can be found using the following relations, respectively [224]:

62

Figure 4-15: Phototransistor output I-V characteristics under UV-Vis illumination

𝑃 = 𝐼𝐿𝑖𝑔𝑕𝑡 − 𝐼𝐷𝑎𝑟𝑘

𝐼𝐷𝑎𝑟𝑘 4-7

𝑅 = 𝐼𝐿𝑖𝑔𝑕𝑡 − 𝐼𝐷𝑎𝑟𝑘

𝑃𝐼𝑛𝑐𝑖𝑑𝑒𝑛𝑡 4-8

Where ILight and IDark are the values of drain to source current under illumination and dark,

respectively, and PIncident is the optical power incident on the channel of the device which was

determined by multiplying channel area of the device with incident light intensity. In order to

investigate the effect of gate voltage on the performance of phototransistor, both its performance

parameters i.e. photo sensitivity and responsivity, were plotted as a function of gate voltage

when drain to source voltage was equal to -8 V, as can be seen in Figure 4-16.

Photo sensitivity and responsivity values were found maximum at zero gate to source voltage.

Further, both these values were found to decrease exponentially with the increase of negative

gate voltage. This is due to reason that at lower gate voltages both types of carriers are injected

in drain and source electrodes. Photo sensitivity of the device was analyzed at two different

illumination intensities i.e. 80 and 90mW/cm2 and its value is found higher for 90mW/cm

2

illumination intensity throughout the investigated gate voltage range as expected. Thus, based on

63

Figure 4-16: (a) Photo sensitivity and (b) responsivity as a function of Gate Voltage

the experimental results, significant effect of gate voltage on the performance of investigated

MESFET type phototransistor is being reported.

4.2.3.3 Phototransistors based on P3HT:MR:PCBM blends

Results of the devices fabricated using P3HT:MR:PCBM blends with 1:0.2:1 ratio are described

in this section. Methyl red which is reported to be rich in π-electrons, is a PH indicator azo dye

64

and is soluble in acidic solutions [225]. Methyl Red, having conjugated structure with molecular

formula NC6H4COOH is not well investigated as compared to P3HT and PCBM. During the

recent years, it has been investigated as dopant in Liquid Crystals by various research groups

around the world [114, 226-228]. Further, solution processed Methyl Red films based organic

devices such as diodes [229], solar cells [230] and humidity sensors [22] with promising features

highlight its potential and future applications. Figure 4-17(a) presents 10µm × 10µm AFM image

of P3HT:MR:PCBM blends based 350 nm thick semiconducting/active layer; inset shows its

high resolution (2µm × 2µm) 3D AFM profile. Nano-scale phase separation of donor-acceptor

blends is observable in high resolution AFM image. Root mean square and average roughness

values were found to be equal to 13.95 nm and 10.96 nm, respectively. As can be seen from the

AFM profiles, organic film presents homogenous distribution of polymer: fullerene and methyl

red particles and is free from any major structural disorders which are generally associated with

solution processed films. UV-Vis absorption spectra of P3HT:PCBM and P3HT:MR:PCBM

blends based semiconducting layers is shown in Figure 4-17(b). Methyl Red is the most

nonlinear dye ever known which experiences Trans-Cis photoisomerization under light [231]

which is a reversible process. As can be seen, active layer with Methyl Red absorbs more

photons when wavelength is in the range of 450 nm to 600 nm which is known to be the main

absorption range of Methyl Red [232]. Aluminum based gate electrode of the device made

Schottky type contact with P3HT:MR:PCBM blends based semiconducting layer. Investigation

in hand is the continuation of our previous work [93], in which P3HT:PCBM blend based

semiconducting layer has also showed Schottky type contact with gate electrode. This behavior

of P3HT:PCBM blends between Aluminum and Silver electrodes were explained with the help

of energy band diagram. However, rectification ratio was found higher for the case of

P3HT:MR:PCBM blends, indicating comparatively better Schottky contact properties. Figure

4-17(c) presents variation in the capacitance of active layer between gate-source/ drain electrodes

as a function of frequency. As can be seen, at low frequency ranges (1 KHz – 100 KHz), the

capacitance of the film was found to be equal to 0.7nF, indicating no significant effect of

frequency variation. However capacitance decreases with increase in frequency above 100 KHz

following the behavior of typical Schottky diodes and dielectric films. The effect of DC voltage

on the capacitance of active layer between gate and source/drain electrodes was also studied as

shown in Figure 4-17(d), at some specified frequency values (1 KHz, 10 KHz, and 100 KHz).

65

The capacitance was found to increase significantly with increase in applied DC voltage when

Schottky diode was reversed biased. This is due to increase in the width of depletion layer with

applied DC voltage.

Figure 4-17: (a) Surface AFM image (10µm × 10µm) of P3HT:MR:PCBM blend based active layer,

inset: its high resolution 3D AFM profile, (b) Absorption Spectra, (c) Capacitance of active layer as

a function of frequency between Source/Drain and Gate electrodes and (d) Capacitance of active

layer as a function of DC bias voltage between Source/Drain and Gate electrodes at specified

frequency values.

Figure 4-18 shows Current - Voltage (I-V) characteristics of device under dark conditions. Drain

to source current (IDS) and gate to source current (IGS) as a function of drain to source voltage

(VDS), were measured for comparison. IDS and IGS values were found to be equal to 22nA and

0.94pA at VDS = -5V and VGS = -4V, respectively. Output Current - Voltage (I-V) characteristics

of the device under given UV-Vis illuminations are shown in Figure 4-19 (a) and (b) when gate

to source voltage values were equal to - 3 V and - 4 V, respectively. As can be seen, illumination

66

Figure 4-18 I-V Characteristics of the Phototransistor under dark: Gate leakage current and

drain current as a function of drain to source voltage at VGS = - 4V.

significantly enhances drain to source (IDS) of the device. Photons absorbed under illumination

move towards donor-acceptor interface due to diffusion where these are dissociated into

electrons and holes. Conversion of excitons into electrons and holes is the function of film

microstructure, diffusion length and life time of excitons etc. Electrons and holes are then

collected at source drain electrodes due to applied biasing, to enhance the channel current. Drain

current was found to increase with illumination intensity and negative gate voltage following the

behavior of p-type organic semiconductors based phototransistors [197, 220], with weak

saturation trend and low voltage operation. Increase in the drain to source current (IDS) with gate

voltage was attributed due to variation in the depletion width of active layer as shown in Figure

4-17 (d). Weak saturation behavior of the device was attributed due to presence of both p-type

and n-type organic species. Gate leakage current increases rapidly and becomes comparable with

drain current when positive gate voltage is applied. Therefore, device can only work for negative

gate voltages. Capacitance of the active layer was found to be equal to 0.7nF at 1KHz. Field

effect mobility value of the device was estimated using equations 4-5 and 4-6 and found to be

equal to 5.8 × 10-3

cm2/VS at VGS = - 4V and VDS = -5V, which is similar to field effect mobility

values of polymer-fullerene BHJ based OFETs, recently investigated for strain and displacement

sensing applications [82, 233]. The values of photo responsivity (R) and light to dark current

67

ratio (r) of the investigated device at VGS = - 4 V and VDS = - 5 V were found to be equal to

0.3mA/W and 27, respectively. Photo responsivity (R) of the investigated device is lower

whereas ratio of drain current under light to drain current under dark (r) is higher as compared to

photo sensitive MESFET recently investigated using the blend of P3HT and PCBM as active

semiconducting layer [93]. Higher value of r for the case of P3HT:MR:PCBM blends may be

due to lower value of gate leakage current and higher absorption of photons in UV-Vis regime.

Whereas lower value of R is attributable to higher surface roughness of photo sensitive layer

with more charge carrier trap sites. Investigation of the effect of Methyl Red concentration on the

performance of BHJ based photo detector is left as a future work.

Figure 4-19 Output Current -Voltage (I-V) Characteristics of the Phototransistor under dark and

given UV-Vis illumination intensities when (a) VGS = - 3 V (b) VGS = - 4 V.

68

Chapter-5: Strain and Displacement

Sensitive OFETs 5 4

In this chapter solution processed Polymer (P3HT)-Fullerene (PCBM) BHJ based Organic Field

Effect Transistors (OFETs) were investigated for strain/ bend and displacement sensing

applications, respectively. P3HT is a well-known p-type organic semiconductor, soluble in many

solvents. PCBM is soluble derivative of n-type low molecular weight organic semiconductor.

OFETs were processed on flexible and glass substrates following low cost fabrication procedure.

Metal contacts of the devices were made using Physical Vapor Deposition (PVD) Technique,

Thermal Evaporation whereas semiconducting layers of the devices which were developed using

spin coating technique made Ohmic type contacts with its Silver based Drain & Source contacts

and Schottky type contacts with its Aluminum based Gate electrodes, and demonstrated

ambipolar properties. This particular behavior of the BHJ based semiconducting layers of the

devices was explained with the help of Energy Band Diagram. Surface roughness and structural

properties of the semiconducting layers were analyzed using Atomic Force Microscope (AFM)

and Scanning Electron Microscope (SEM). Devices have shown low voltage operation and

threshold voltages. Current Voltage (I-V) characteristics of the devices were measured using

Semiconductor Characterization System (SCS), Keithley 4200 and found similar to P-type mode

I-V characteristics of typical ambipolar OFETs.

In order to study strain sensing properties, devices developed on flexible PET substrates were

bent around the cylinders of varying radii and corresponding variation in the drain to source

current of the devices was noted. Furthermore, bending was applied at 0º and 90º w.r.t. drain to

source current. It was found that drain current increases with increase in bending from both sides

of the devices. Strain sensitivity of the devices was found much higher than recently investigated

OFETs for strain sensing applications. It was also concluded that strain sensitivity is not only

dependent on the geometrical parameters, applied biasing conditions and substrate sizes of the

devices but also on the bending directions.

Displacement sensing properties were investigated using a special test fixture after placing thin

plastic film and thick rubber in sequence on the gate films of the devices. Upper terminal of the

69

test fixture was moved downward until it touches top surface of rubber. This position of the

upper arm was considered as zero displacement position and then upper arm is moved downward

in 50μm steps and corresponding variation in drain current was measured. Current was found to

increase with increase in the displacement. Drain to source resistance was reduced to 13.15 MΩ

from 15.40 MΩ when displacement was varied from 0 to 250 μm.

Thus significant variation in the drain current of the devices was noticed due to bending and

displacement variation. The changes in the structural and morphological properties of

semiconducting layer due to applied tensile strain were attributed as the reason of the variation in

the electrical properties of these devices.

5.1 Organic Bulk Hetero-junction based Strain/ bend Sensitive Flexible

Organic Field effect Transistors

5.1.1 Introduction

During the past decade, electronic devices based on organic semiconductors such as conjugated

polymers, oligomers and low molecular weight materials have attracted much attention due to

their low cost, flexibility, ease of processing, large area capability and variety of applications.

Prominent examples of organic microelectronic devices are Organic Field Effect Transistors

(OFETs) [187], Organic Solar Cells (OSCs) [160], Organic Light Emitting Diodes (OLEDs)

[188], Organic Photodiodes (OPDs) and Phototransistors (OPTs) [234].

Investigation of OFETs for sensing applications has gained much attention in recent years [235,

236]. OFETs are responsible for performing both switching and sensing functions in these

devices [237]. Due to this, strain sensors based on OFETs were also well investigated using

various types of semiconducting and dielectric materials since the demonstration of first organic

semiconductor based strain sensor by Soyoun Jung and Tom Jackson [238]. Cosseddu P. et al.

[83] have demonstrated P3HT and Penatcene based strain sensors on a flexible substrate and

reported the strong influence of mechanical stress on the electrical behavior of these devices.

Scenev et al. [84], Yang et al. [239] and Sekitani et al. [240] have correlated change in the

electrical properties of pentancene based flexible strain sensors with change in the structural and

morphological properties of the pentacene thin films due to bending. Flexible organic strain

70

sensors based on transistor like Wheatstone bridge configuration were also investigated and

sensitivity of these devices was reported to be dependent on the direction of applied strain [241-

243].

OFETs can be based on MOSFET (Metal Oxide Semiconductor Field Effect Transistor) or

MESFET (MEtal Semiconductor Field Effect Transistor) type configurations. MOSFETs show

higher field effect mobility and ION/IOFF ratio, whereas MESFETs are preferred over

MOSFETs due to their low voltage operation and fewer processing steps [201]. Pressure

sensitive OFETs using organic MESFET architecture in which gate makes Schottky type contact

with the active semiconducting layer, have been reported in the past [80, 244], but with only one

organic semiconducting material and on rigid glass substrates. However surprisingly, organic

bulk hetero-junction based flexible organic MESFETs and their sensing properties are not

investigated until now.

In this work, bend or strain sensing properties of organic bulk hetero-junction based MESFETs is

being reported for the first time. Two devices having different sizes of its flexible PET substrates

were fabricated and investigated using the blend of P3HT (soluble p-type polymer) and PCBM

(soluble derivative of n-type fullerene), making bulk hetero-junction with each other, as active

layer. Active layer made Schottky type (rectifying) contact with gate electrode and, Ohmic

contact with drain and source electrodes. Operation of the devices was found similar to the p-

type mode operation of typical ambipolar field effect transistors. Further, devices have

demonstrated low voltage operation and threshold voltage. In order to study strain sensing

properties of the device, varying bending strain parallel and perpendicular to the direction of

drain to source current (IDS) was applied. Significant influence of the bending strain on the drain

to source current of the device was observed. However variation in the current (IDS) under

applied bending was found function of bending direction. It was also observed that increase in

the drain current is sensitive to force applied by the bending arm in addition to bending direction.

That is why maximum increase in the drain current was noticed for the case of device developed

on small size substrate when bending was applied perpendicular to the drain current direction.

71

5.1.2 Experimental Methods

Figure 5-1 shows design schematic of the flexible devices, being developed for strain sensing

applications. Two different types of OFETs were developed on flexible PET substrates for this

investigation. The device realized on comparatively small substrate was named as sample-1

whereas the one prepared on large substrate was named as sample-2. Fabrication of the devices

was started with the cleaning of PET substrates in an ultrasonic bath by using acetone, isopropyl

alcohol and DI water. Drying of substrates was accomplished using nitrogen blow. Silver (Ag)

drain and source contacts with a thickness of around 50 nm were made on the flexible cleaned

substrates by physical vapor deposition process at 10-6

mbar using Vacuum Evaporator (Leybold

Univex 450). Active layer was prepared by mixing P3HT (Aldrich) and PCBM (Aldrich) with

1:1 (wt/wt ratio) in 1,2 dichlorobenzene (Alfa Aesar) and spin coated on the flexible substrate

with previously deposited drain and source electrodes. P3HT:PCBM blend based films, in which

P3HT serve as donor and PCBM as acceptor are well investigated for polymer bulk hetero-

junction solar cells [142].

Figure 5-1: Design schematic of the flexible device

Band gaps of P3HT and PCBM are around 1.9eV and 2.3eV, respectively. After formation of

active layer, devices were annealed at 100 oC for 20 minutes. At the end, 40 nm thick gate

contact was made by depositing Aluminium directly on the semiconducting layer through the

shadow mask in the vacuum chamber at 10-5

mbar pressure. Hot metal vapors can easily pass

through the organic film, and can make a short contact with the drain source electrodes. This was

prevented by making a thin gate film with a low evaporation speed.

72

Current–Voltage (I-V) characteristics of the devices were measured with Semiconductor

Characterization System (Keithley 4200SCS). In order to investigate the surface properties of the

active layers, Nanoscope Multimode Atomic Force Microscope (AFM) was used. Capacitance

measurements of the samples were carried out using impedance analyzer (HP-4194). A signal of

1V peak to peak was applied to the samples during capacitance measurements. Thicknesses of

active layers and various electrode films were measured using Scanning Electron Microscope

(SEM). All the tests and measurements were performed at ambient air and room temperature.


5.1.3.1 Electrical Contact Properties of Polymer Organic Film

Earlier investigations showed that aluminum (Al) makes Schottky type contact, whereas silver

(Ag) makes Ohmic type contact with organic semiconducting materials, respectively [204].

Through I-V measurements, Schottky type (rectifying) contact between gate electrode (Al) and

active layer, and Ohmic contact between source drain electrodes (Ag) and the active layer was

noted. Such behavior of the active layer was discussed with the help of Figure 5-2(a), which

presents energy band diagram of organic semiconductors (used in this study) with respect to the

work functions of source/drain and gate electrodes. Energy bands are shown as straight lines as

band bending which occurs due to gate voltage was not taken into consideration. HOMO level of

P3HT (-4.9 eV) is quite close to the work function of Ag (-4.7 eV) [207, 245] but there is an

energy difference of around 1eV between the work function of Ag and LUMO level of

PCBM (-3.7 eV). Therefore holes can easily be injected into the active layer from drain source

electrodes as compared to electrons. Due to this close matching of the work functions of drain

source electrodes with the HOMO level of P3HT, holes of active layer are making Ohmic

contact with drain source electrodes. However electrons cannot be easily injected in the active

layer due to the presence of an injection barrier of about 1eV between the work function of drain

source electrodes and LUMO level of PCBM. Therefore injection of the electrons was limited in

our experiments. Schottky type contact of the active layer with gate electrode of the device is due

to the energy difference of 0.6eV between the work function of gate electrode (-4.3 eV) and

HOMO level of P3HT & LUMO level of PCBM.

73

Figure 5-2: Energy Band Diagram

5.1.3.2 Structural Properties of Solution Processed P3HT:PCBM Blend based Organic

Films

Figure 5-3(a) and (b) show 5µm × 5µm AFM image and cross section SEM view of 300 nm

thick active layer, respectively. Root mean square (RMS) roughness value of the active layer was

found to be equal to 4.34 nm. Figure 5-4 presents section roughness of active layer developed on

flexible PET substrate for sample-2. The RMS and average roughness values were found to be

equal to 14.70 nm and 11.25 nm, respectively.

Figure 5-3: (a) 5×5 µm AFM image and (b) Cross section SEM view of 300 nm thick active layer

developed on substrate for sample-1

74

Figure 5-4: Section Roughness of active layer developed on flexible substrate for sample-2

5.1.3.3 Current Voltage (I-V) characteristics of the devices under no bending conditions

Figure 5-5 describes output & transfer I-V characteristics of the sample-1 flexible OFET,

whereas Figure 5-6 shows output I-V characteristics of sample-2 OFET under no bending

condition. I-V characteristics of the devices were found similar to p-type mode characteristics of

typical ambipolar field effect transistors [208, 209]. Weak kink in the output characteristics of

the device (at 0 gate to source voltage) was observed which confirms ambipolar nature of its

active layer [246]. Non linear increase in the drain to source current at higher drain to source

voltage (higher than -4 V) and 0 gate to source voltage, is due to injection of both types of

carriers in the the channel. Meric I. et al. [223] have investigated ambipolar graphene based

field effect transistors and discussed the reasons of kinks in the I-V characteristics (at lower gate

to source voltages) with the help of schematic drawings. Gate voltage in organic MESFETs

controls the distribution of injected charge carriers and bulk conductivity of organic

semiconducting layer [247]. In our case, with the increase of negative gate voltage, holes are

drawn towards gate electrode which increases the bulk conductivity of the active layer, and

current between drain source electrodes increases as a result. As already known for the case of

organic MESFETs [221], devices have shown low voltage operation and threshold voltage (Vth )

values.

75

Figure 5-5: (a) Out Put and (b) Transfer I-V Characteristics of sample-1 OFET under no bending

state

Figure 5-6: Out Put I-V Characteristics of sample-2 OFET under no bending state

In the saturation regime, drain to source current (IDS) can be determined by using following

equation [222]:

𝐼𝐷𝑆 = 𝑘(𝑉𝐺𝑆 − 𝑉𝑡𝑕)2 5-1

76

Where VGS is the voltage between gate and source, Vth is the threshold voltage and k is the

conduction parameter, which can be determined by using a following relation:

𝑘 = 𝜇휀𝑠𝑊 2𝑎𝐿 5-2

Where, L, W and a, are the channel length, width and thickness respectively. ɛs is the permittivity

of the semiconducting material, and µ is the field effect mobility. The effective capacitance (Ci at

100 HZ) and permittivity (ɛs) was found to be 1.2×10-5

F/cm2 and 3.6×10

-10 F/cm, respectively.

Field effect mobility values of the devices were determined at VDS = -5 V and VGS = -3 V using

equation (5.1) and (5.2), and its values were found to be equal to 3.53×10-4

cm2V

-1s

-1 and

2.05×10-4

cm2V

-1S

-1 for sample-1 and Sample-2 respectively. Field effect mobility values of the

devices were higher as compared to the mobility values of MESFET based humidity sensitive

organic field effect transistor investigated by Murtaza et al. [91] and lower than usually reported

for organic MOSFET based devices. Structures of organic MESFETs are thicker as compared to

organic MOSFETs which may be the reason of their lower field effect mobility values [215].

5.1.3.4 Current Voltage (I-V) Characteristics of the devices under bending conditions

In order to investigate strain sensing properties of the devices, flexible devices (shown in Figure

5-7) bent around cylinders with radii (R) of 15, 10 and 5mm with corresponding strain values of

1, 1.6 and 3.2%, respectively. Bending was applied parallel and perpendicular to the drain to

source current (IDS) as indicated with the help of device schematics shown in Figure 5-8 and

corresponding strain was calculated using the following relation [248]:

Ɛ = 𝑍 𝑅 5-3

Where Ɛ is the strain generated in the device, Z is the thickness of the PET substrate and R is the

bending radius. Figure 5-9 and Figure 5-10 presents variations in drain to source current as a

function of drain to source voltage under shown bending conditions for sample-1 and sample-2,

respectively, when gate terminal was suspended. Proportional increase in drain current for both

parallel and perpendicular bending with respect to the current axis was observed. The bend or

strain sensitivity of the devices is significantly higher as compared to those reported for the case

of organic MOSFET based strain sensors [243]. Further, strain sensitivity of the transistors is the

77

function of induced geometric asymmetry as already reported [242, 249]. Although in both

samples drain current was found to increase due to applied bending from both sides however its

sensitivity was found strong function of bending direction, device geometrical parameters and

induced forces strength of the bending arm. Variation of current with applied tensile strain was

assumed due to changes in the transport and structural properties (such as inter molecular

distance) of polymer-fullerene bulk hetero-junction based semiconducting layer. The goal of this

work was to develop and demonstrate the results of an organic bulk hetero-junction based strain

sensitive organic MESFET.

Figure 5-7: Photographs of the flexible devices under bending state

Figure 5-8: Schematic of the devices when bent at (a) 0º and (b) 90º w.r.t. drain current

78


suspended gate and the given bending conditions for strains applied at (a) 0o

and (b) 90o

with

respect to the direction of current for sample-1

Polymer-Fullerene bulk heterojunction based strain sensitive flexible OFETs were successfully

fabricated and demonstrated using MESFET configuration for the first time. Results showed that

active layer which was the blend of p-type and n-type organic semiconducting materials, made

Ohmic contact with drain/source electrodes and Schottky contact with gate electrode, and

adopted the behavior of an ambipolar organic semiconductor. Current-voltage (I-V)

characteristics of the devices were found similar to the p-type operation mode characteristics of a


suspended gate and the given bending conditions for strains applied at (a) 0o and (b) 90o with

respect to the direction of current for sample-2.

79

typical low voltage ambipolar field effect transistor. Strain sensing properties of the devices were

studied by bending it around cylinders of varying radii, at 0o and 90

o with respect to drain to

source current direction. Strain from both sides of the device has significantly influenced drain to

source current of the device. However strain sensitivity of the devices was found function of the

bending direction, device physical parameters and induced forces of bending arms. Realization

of such low voltage devices will provide potential for the future development of low cost

portable large area flexible sensor arrays and other low power electronic devices.

5.2 Polymer-Fullerene BHJ Based Displacement Sensitive OFET

5.2.1 Introduction

In recent decades, conjugated polymers, oligomers and low molecular weight organic

semiconductors based microelectronic devices have attracted much attention of the researchers

due to their low cost, flexibility, ease of processing, large area capability and variety of

applications. Prominent organic microelectronic devices include Organic Field Effect Transistors

(OFETs) [250], Organic Solar Cells (OSCs) [251], Organic Light Emitting Diodes (OLEDs)

[252], Organic Photodiodes (OPDs) [253] and Phototransistors (OPTs) [254].

Investigation of organic field effect transistors (OFETs) for sensing applications has seen

progress in recent years [95, 255]. OFETs, which are responsible for performing both sensing

and switching functions in sensors, were well investigated for various sensing applications

during the last decade which includes bio-sensing [256-259], humidity sensing [91, 260, 261],

pressure sensing [74, 262, 263], strain sensing [84, 85, 264], photo sensing [265, 266] etc.

Transistors in these sensors were realized in various configurations, utilizing different

semiconducting and dielectric materials on flexible and rigid substrates, in order to investigate its

effect on the performance of these devices.

OFETs can be based on MOSFET (Metal Oxide Semiconductor Field Effect Transistor) or

MESFET (MEtal Semiconductor Field Effect Transistor) architectures. Transistors which are

based on MOSFET architecture contain a dielectric layer between its semiconducting layer and

gate contact. However MESFETs in which gate makes Schottky type contact with the

semiconducting layer, do not contain dielectric layer. Further MOSFET based devices show

higher field effect mobility and ION/IOFF ratio, whereas MESFETs are preferred over MOSFETs

80

due to their low voltage operation, fewer processing steps and low cost [267], thus most of the

reported OFET based sensors were developed using MOSFET type configuration. Someya et. al.

[268] have realized pressure sensitive flexible OFETs using pentacene based semiconducting and

polyimide based dielectric layers, respectively. The drain to source current of these sensors was

found to increase from 15nA to 6.7µA under the pressure of 30 kPa. Stefan S. C. B. et. al. [269]

have recently demonstrated flexible organic MOSFETs based pressure sensing array using

rubbery dielectric layers. Displacement sensitive OFETs using organic MESFET architecture

have also been reported, but with only one organic semiconducting material. Karimov et. al. [81,

270] have fabricated Copper Pthalocyanine (CuPc) based displacement sensitive MESFETs and

reported exponential decrease of drain to source resistance with the increase of displacement.

However organic bulk hetero-junction based displacement and pressure sensitive MESFETs are

not investigated until now. Displacement sensors were realized in the past with capacitive,

resistive, linear variable differential transformers and optical instruments [271, 272]. The

displacement range of these sensors was in the orders of 10-6

to 10-3

meters.

In this work, displacement sensing properties of polymer fullerene bulk hetero-junction based

MESFETs were investigated for the first time. The transistor was fabricated on glass substrate

using the blend of P3HT (soluble p-type polymer) and PCBM (soluble derivative of n-type

fullerene), making bulk hetero-junction with each other, as active layer.

5.2.2 Experimental Details

Figure 5-11 shows design schematic of displacement sensitive Organic Field Effect Transistor

(OFET). Transistor was fabricated in MESFET type configuration on glass substrate. Fabrication

of the devices was started with the sequential cleaning of glass substrates in an ultrasonic bath by

using acetone, methanol, ethanol, isopropyl alcohol and DI water. Drying of the substrates was

carried out using nitrogen blow and thermal oven. Silver (Ag) drain and source contacts with a

thickness of around 60 nm were made on the cleaned substrates by physical vapor deposition

(PVD) process at 10-5

mbar using Vacuum Evaporator (Leybold Univex 450). Channel length (L)

and width (W) were found equal to 50µm and 2mm, respectively. P3HT (Sigma Aldrich) and

PCBM (Sigma Adrich) with 1:0.8 (wt/wt ratio) were added in 1,2 dichlorobenzene (Sigma

Aldrich) and solution was prepared by stirring it for 24 hour. The temperature was maintained at

50oC during the process of stirring. Semiconducting layer was coated on drain source electrodes

81

deposited glass substrates using spin coating technique. The thickness of the active layer was

estimated to be equal to 280 nm by SEM measurements. P3HT:PCBM blend based nanolayers,

in which P3HT serve as donor and PCBM as acceptor, are well investigated for polymer bulk

heterojunction solar cells in recent years [142]. Band gaps of P3HT and PCBM are around 1.9

eV and 2.3 eV, respectively. Then samples were annealed in the glove box at 110oC for 30

minutes. At the end, 50 nm thick gate contact was made on the semiconducting layer by

evaporation of Aluminium through the shadow mask in the vacuum chamber at 10-5

mbar

pressure. Penetration of hot metal vapours through the organic film, to make a short contact with

the drain source electrodes, was prevented by making a thin Aluminium gate film with a low

evaporation speed. Five devices with similar aforementioned geometrical dimensions and

various film thicknesses were fabricated.

Figure 5-11: Design schematic of Displacement Sensor based on Organic Field Effect Transistor

(OFET)

Current–Voltage (I-V) characteristics of the devices were measured with Semiconductor

Characterization System (Keithley 4200SCS). Surface properties of the active layer, were

studied using Nanoscope Multimode Atomic Force Microscope (AFM). Capacitance

measurements of the samples were carried out using impedance analyser (HP-4194). A signal of

1V peak to peak was applied to the samples during capacitance measurements. Displacement

sensing properties of the samples were investigated using a special test fixture after placing a

thin plastic sheet and a thick rubber in sequence on the gate films of the devices. Range of the

vertical displacement depends upon the elastic properties and thickness of the rubber film.

Thicknesses of active layers and various electrode films were estimated using Scanning Electron

Microscope (SEM) and DEKTAK profilometer. All the tests and measurements were performed

at ambient air and room temperature.

82


Earlier investigations showed that aluminum (Al) makes Schottky type contact, whereas silver

(Ag) makes Ohmic type contact with organic bulk hetero-junction based semiconducting layers,

respectively [93]. Through I-V measurements, Schottky type (Rectifying) contact between gate

electrode (Al) and active layer, and Ohmic contact between source-drain electrodes (Ag) and the

active layer was observed. Such behavior of P3HT:PCBM blend based active layer has been

previously demonstrated [273] with the help of schematic energy level diagram of organic

semiconductors with respect to the work functions of source/drain and gate electrodes. Figure

5-12 shows section roughness of active layer. The RMS and Average roughness values of the

active layer were found equal to 11.297nm and 8.862 nm, respectively. No considerable

structural defects exist in the AFM profile, which are generally associated with solution

processed films.

Figure 5-13 describes output I-V characteristics of the OFET. I-V characteristics of the device

are similar to p-type mode I-V characteristics of typical ambipolar organic field effect transistor.

Weak kink in the output I-V characteristics of the device at lower gate voltages (0 to -1V) with

the increase of drain to source voltage, confirms ambipolar nature of its active layer [211]. In our

case, with the increase of negative gate voltage, holes are drawn towards gate electrode which

increases the bulk conductivity of the active layer, and current between drain source electrodes

increases as a result. Further, gate leakage current becomes comparable with drain to source

current when positive or higher negative gate voltage (above -5 V) is applied. Thus this device

can work for only lower negative gate voltages. Furthermore, our device, similar to reported

ambipolar field effect transistors [223], has shown weak saturation properties due to ambipolar

nature of its active layer. Field effect mobility of the device was calculated using Equation 5-2

and its value was found to be equal to 1.6×10-3

cm2V

-1S

-1. Displacement was varied from 0 to

250 µm and its effect on the drain to source current (IDS) of the device was studied.

Figure 5-14 presents drain to source current as a function of drain to source voltage under given

displacements when gate was suspended. It was found that drain to source current increases with

the increase of displacement. Behavior of the device under displacement is similar to CuPc based

displacement and pressure sensors [80, 81, 270] which were based on organic MESFET

83

architectures. Significant increase in drain to source current (was observed when displacement

was increased from 0 to 250µm.

Figure 5-12: Section roughness of P3HT PCBM blend based semiconducting layer

Figure 5-13: Output I-V Characteristics of the OFET

84

Figure 5-14: Drain to source current (IDS) as a function of drain to source voltage (VDS) under given

displacements when gate was suspended, inset1: Schematic of the displacement measurement setup,

inset2: Variation of the drain to source resistance (RDS) with displacement at VDS = -8V.

The drain-source resistance (RDS) of the device can be calculated using following relation [274]:

𝑅𝐷𝑆 =𝜌𝐴

𝑑=

𝑉𝐷𝑆

𝐼𝐷𝑆 5-4

Where ρ is the resistivity, A is the effective cross-sectional area of the device, d is the inter-

electrode distance which is equal to channel length, VDS and IDS are the voltage and current

between drain source electrodes, respectively. The value of RDS at 0 displacement was found

equal to 15.40 MΩ which reduces to 13.15 MΩ when displacement was increased to 250µm.

Variation of the current with displacement was assumed due to changes in the transport and

structural properties (such as inter molecular distance) of polymer-fullerene bulk hetero-junction

based semiconducting layer. As drain to source current is also function of the thickness of

semiconducting layer (Equation 5.2), therefore decrease in the thickness and increase in the

conductivity of semiconducting layer with displacement may also be the reason of the increase of

current due to displacement. Flexible strain sensors based on transistor like Wheatstone bridge

configuration and pentacene nanolayer were recently investigated [86, 87]. Significant influence

of mechanical stress on the electrical properties of these strain sensors was reported. The changes

85

in the structural and morphological properties of semiconducting layer due to applied tensile

strain were suggested as the reason of the variation of electrical properties of these devices. The

goal of this work was to develop and demonstrate the results of an organic bulk heterojunction

based displacement sensitive organic MESFET.

Solution processed Polymer-Fullerene bulk hetero-junction nanolayer based displacement

sensitive OFET was successfully fabricated and demonstrated using MESFET configuration for

the first time. Active layer which was the blend of p-type and n-type organic semiconducting

materials, demonstrated Ohmic contact with drain/source electrodes and Schottky contact with

gate electrode, and adopted the behavior of p-type organic semiconductor. The operation of the

device was found similar to a typical p-type low voltage organic field effect transistor.

Significant influence of the displacement variation on the electrical properties of the device was

observed. It was found that drain current increases with the increase of displacement. Mainly

ease of the processing of these devices motivated us for this work, and our results constitute a

concept basis for possible flexible devices and applications, because the fiction and assembly

would be the same in flexibles. In addition, one can expect that these devices will be ideal

candidates for future large area sensing applications.

86

Chapter-6: Electrical Characterization of

Graphene Oxide, Lead Zirconate Titanate

and Liquid Crystalline Polymer Films 6 6

In this chapter electrical characterization of solution processed Graphene Oxide (GO), Lead

Zirconate Titanate (PbTi0.5Zr0.3(Co1-xMgx)0.2O3) ceramics and methacrylate based Side Chain

Liquid Crystal Polymers (SLCP) films is presented. Parallel plate impedance/ dielectric

spectroscopic technique was used for this investigation. The chapter is divided into three parts.

Electrical characterization of GO, ceramic and SLCP films is described and presented in

sequence.Organic Electronic Devices are preferred over conventional in-organic semiconducting

materials based devices due to their low cost, room temperature solution processing and other

promising features which include flexibility, potential to develop large area sensing arrays, etc.

However, efficiency of organic microelectronic devices is comparatively low. To enhance the

performance of organic semiconducting devices, one way is to synthesize new organic materials

with desired features and to develop the devices using these newly developed materials.

Graphene Oxide (GO) which is reported to be highly sensitive to humidity, has been recently

investigated/ demonstrated as dielectric [275, 276] & semiconducting [277] material for Organic

Transistors and efficient hole transport material for Organic Solar Cells [147]. Variation in the

electrical properties (in particular transport and dielectric properties) of GO films with

temperature is being investigated in detail in this work. GO films were drop casted on Indium

Tin Oxide (ITO) coated glass substrates. AFM was used to study the films‘ structural and surface

profile. Electrical measurements of the films were taken using Impedance Analyzer in the

frequency range of 100 Hz to 10 MHz at different temperatures. It was noted that Analog

Current conductivity (ζac) of the films varies with angular frequency (ω). The electrical

parameters of GO were found to be dependent on both frequency and temperature. Results

indicated that GO film possesses direct current (DC) and Correlated Barrier Hopping (CBH) type

of conduction mechanisms at low and high frequency ranges, respectively. Excellent photon

absorption as well as transmittance capability in the visible range suggests suitability of the films

for the realization of flexible organic solar cells and large area photo sensing arrays.

87

Experimental results of Polycrystalline PbTi0.5Zr0.3(Co1-xMgx)0.2O3 (x = 0.00, 0.25, 0.45, 0.65,

0.85) ceramics, synthesized by sol-gel route at 1150 oC for 2 h in air are presented and discussed

in the subsequent section. The final ceramics were characterized by using SEM and LCR-meter.

The dielectric constant (ɛr) was observed to increase with temperature in the investigated

temperature range of 30 oC to 300

oC). The x = 0.25 composition exhibited high magnitude of ɛr

(4261) and dielectric loss (tan = 0.05) over a wide range of temperature at 100 kHz. The

magnitude of tan of sample x = 0.65 was recorded to be 0.009 around room temperature at 100

kHz. The resistance of the ceramic samples was found to decrease increase in temperature

indicating a non-Debye type relaxation phenomenon. The AC conductivity of PbTi0.5Zr0.3(Co1-

xMgx)0.2O3 samples increased gradually with an increase in the both temperature and frequency.

Electrical characterization of a series of methacrylate based side chain liquid crystalline

copolymers (SCLCPs) containing cholesteryl mesogen (poly(Chol-n-MMA-co-MMA)) with

various lengths of aliphatic spacer (n = 3, 7, 10) by the chemical binding of cholesterol

molecules with side chains of comb like polymers, prepared via free radical polymerization is

presented in the last section of the chapter. The dielectric properties of homopolymers i.e.

poly(Chol-n-MMA) and copolymers i.e. poly(Chol-n-MMA-co-MMA) with various lengths of

aliphatic spacer (n = 3, 7, 10), were analyzed by impedance spectroscopic technique in the

frequency range of 100 Hz to 15 MHz at room temperature. Dielectric constants of the liquid

crystalline films were found to increase with the decrease of flexible spacer length. Cole-Cole

plots investigation indicated non-Debye type relaxation behavior of the samples except for co-

polymer with chain length 7 and homo-polymer with chain length 3 which showed nearly Debye

type relaxation properties. AC conductivity (ζac) of the crystalline polymer films was observed to

vary with angular frequency, ω as ωS with S<2. Detailed conductivity analysis revealed that the

conductivity of polymer films followed QMT (Quantum Mechanical Tunneling) and CBH

(Correlated Barrier Hoping) conductivity mechanisms at low frequency regime and, the SLPL

(Super Linear Power Law) and DC (Direct Current) conductivity mechanisms at high frequency

region. In the light of current study, films based on the polymers being investigated are proposed

as model systems for applications in large area microelectronic devices.

88

6.1 Electrical Characterization of Graphene Oxide (GO) Films

6.1.1 Introduction

During the past decade, materials such as fullerenes, nanotubes and graphene were demonstrated

as promising candidates for potential applications in electronic industry due to superior electrical

and optical properties. Graphene is a two dimensional monolayer of hexagonally arranged sp2

hybridized carbon atoms [278]. Due to excellent physiochemical, electrical, thermal and

mechanical properties, graphene is effectively used in many technological applications such as

hydrogen storage, field effect transistor, ultra-capacitors and organic solar cells [279-281] .

Graphene can be produced from water soluble graphene oxide using reduction process [282-

284].

Graphene oxide (GO) has sp2 and sp

3 hybridized carbon atoms and can be considered as

insulating material compared to graphene. GO exhibits various carboxyl and hydroxyl functional

groups and is soluble in water. The optical and electronic properties of GO can be changed by

varying the content of oxygen and water molecules. Recently, GO was demonstrated as channel

and dielectric material for field effect transistors [146], and efficient hole transport layer for

organic solar cells [147]. Due to the presence of oxygen based functional networks on the basal

plane of GO, it can make bonds with variety of organic and inorganic materials [148-150]. The

optical and electrical properties of GO can be enhanced using composites with conductive

polymers.

Purpose of this work is to synthesize and investigate temperature dependent transport properties

of solution processed Graphene Oxide thin films in order to explore the possibility of their usage

for flexible organic electronic devices. Transport properties of GO were studied using impedance

spectroscopy, a well-known technique used for similar investigations. Temperature dependent

AC conductivity analysis of the films indicated its semi-conductive behavior. Further, GO films

have shown DC and CBH conductivity mechanisms at lower and higher frequency ranges,

respectively.

89

6.1.2 Experimental Procedure

6.1.2.1 Synthesis of Graphene Oxide

Graphene Oxide (GO) was synthesized by researchers at TUBITAK Marmara Research Center,

Turkey adopting the following procedure. 0.5 g of graphite (Aldrich) and 25 mL of sulfuric acid

(98%) was mixed at room temperature for 2 hours. Then, 3 g of KMnO4 (Aldrich) was added

into this solution. The mixture was stirred at 50° C for 24 hours and left to cool down to room

temperature. Distilled water of about 40 mL was gradually added to above mixture. Then, 100

mL of distilled water was added and then 3 mL of H2O2 (35 wt % aqueous solution, Aldrich) was

added drop wise to the above solution until the reaction solution turned yellow. The yellow

graphene oxide dispersion was centrifuged at 7000 rpm for 2 hours to precipitate the GO. The

supernatant was removed after centrifugation. 30 mL of distilled water was further added to the

dispersion. The resulting dispersion was ultrasonically treated for 15 minutes to make a uniform

dispersion and remove any agglomeration. The dispersion was centrifuged again at 5,000 rpm for

30 minutes. This centrifugation process was repeated several times and supernatant was removed

at each cycle until the excess of acid was removed from the GO solution. Acid removal was

monitored by measuring the changes in the pH of supernatant. The neutralized GO solution was

then homogenized ultrasonically for 2 h to prepare 5mg/mL graphene oxide solution.

6.1.2.2 Structure Formation

ITO coated glass substrates were ultrasonically cleaned using methanol, ethanol, isopropyl

alcohol and de-ionized water, and dried using nitrogen blow. 3µm thick GO films were coated on

the substrates using drop casting technique and samples were baked in oven at 55° C for 3 hours.

Finally, around 50 nm thick conductive contacts were made on the GO films by evaporating

silver in the vacuum chamber at a pressure of 10-5

mbar using Univex Leybold Vacuum

Evaporator. The active area of the investigated samples was 0.06 cm2. The schematic view of the

fabricated parallel plate structures and, the chemical formula of graphene oxide are presented in

Figure 6-1.

90

Figure 6-1: (a) Schematic View of parallel plate structures and (b) Chemical Formula of GO

6.1.2.3 Characterization

Surface profiles of the active layers (GO Films) were studied using Nanoscope Multimode AFM.

Absorption and transmittance spectra of the films were taken using Cary5000 UV-Vis-NIR.

Electrical parameters of the samples were measured using the setup presented in Figure 6-2. This

setup consists of an impedance analyzer, HP 4194A and NOVOTHERM Temperature Control

System interfaced with computer. Electrical parameters such as real and imaginary dielectric

constants, dielectric loss and AC conductivity of the sample were measured as a function of

frequency at different temperatures. An AC signal with amplitude of 1V (RMS) was applied to

the samples during these measurements. The temperature was monitored with the help of a PT

100 resistor which was in direct contact with the sample.


AFM images and absorption & transmittance spectrums of GO film having 10µm×10µm

dimension are shown in Figure 6-4. The Root Mean Square and Average roughness values of the

films were found equal to 18.18 nm and 14.38 nm, respectively. It is clear from the AFM profiles

that films are free of any major structural disorder. Furthermore, surface and 3D AFM profiles of

the GO film presents a homogenous distribution of GO particles. The absorption peak found at

around 355 nm, can be associated with the * transition of carbon bonds (C-C) in aromatic

ring. Maximum transmittance of the films was found equal to 74% at 487nm.

91

Figure 6-2: Experimental set up for temperature dependent electrical analysis of GO Films.

Figure 6-3: (a) Surface and (b) 3D AFM profiles, (c) Absorption and (d) Transmittance Spectra of

GO Films

92

The variation in real & imaginary dielectric constants (ε and ε) and dielectric loss of GO film

as a function of frequency at different temperatures is shown in Figure 6-4. The frequency of the

applied AC signal was varied from 100 Hz to 10MHz. Figure 6-4(a) shows the real dielectric

constant - frequency plots of the films in the frequency range of 100 Hz to 10 MHz. At 100 Hz,

the magnitude of εwas observed to be decreased from 2.81 at 30 oC to 1.12 at 100

oC.

Furthermore, magnitude of ε was found to decrease with increase in frequency from 100 Hz to 2

KHz. However, no significant effect of frequency and temperature variation was observed as

frequency becomes higher than 2 KHz. The magnitude of ε' was found to be equal to around 0.5

within the frequency range of 2 KHz to 10 MHz at all the investigated temperature values.

Capacitance (C) of the film can be calculated using the following equation [106]:

𝐶 = 휀𝑜휀𝐴/𝑑 6-1

Where εo is the dielectric constant of free space, A is the surface area and d is the thickness of

active layer. Figure 6-4 presents the ɛ - frequency plots of the films in the frequency range of

100 Hz to 10 MHz at selected temperatures (30, 40, 50, 60, 70, 80, 90 and 100ºC).The behavior

of ɛ as a function of frequency at different temperatures was found similar to εʹ-frequency trend.

The magnitude of εʺ is given as [106]:

휀ʺ = 휀 𝑡𝑎𝑛 𝛿 6-2

Where tan δ, is called as loss tangent or dielectric loss. Thus ɛ is actually measurement of the

tan δ of the investigated films. The variation of the tan δ as a function of frequency and

temperature is shown in Figure 6-4. The magnitude of tan δ was observed to decrease with an

increase in both frequency and temperature.

The graph of lnζAC - lnω of the GO film at different temperatures is shown in Figure 6-5. It

shows strong dependency of AC conductivity (ζAC) on temperature at low frequencies.

Furthermore at low frequencies, a plateau was observed which characterizes the direct current

conductivity (DC), while at high frequency; the conductivity was increased gradually with an

increase in frequency. AC conductivity was observed to decrease with an increase in temperature

at low frequency, however, at high frequency, no significant effect of temperature on AC

93

conductivity of the film was observed. The dissipation of water molecules and impurities, out

from sample, with increase in temperature at low frequency might be the reason of decrease in

conductivity ofthe sample [285]. The sample has demonstrated semi-conductive behavior within

the investigated temperature range (30-100 ºC) as reported by Huang et al. [286] for the case of

GO paper. The AC conductivity dependence of the angular frequency can be expressed by the

following relation, as the empirical Jonscher's universal law [101, 287]:

𝜎𝐴𝐶 𝜔 = 𝜎𝐷𝐶 + 𝐴𝜔𝑠 6-3

Where, A is a constant, ω is the angular frequency and s is the frequency exponent parameter

which identifies AC conduction mechanisms. The magnitude of exponent s, which represents

main body interactions of electrons and impurities, is usually less than unity [112].

Figure 6-4: (a) Real and (b) Imaginary Dielectric Constants, and (c) Loss Tangent Plots of GO

film as a function of frequency at given temperatures.

94

Figure 6-5: AC Conductivity of GO film as a function of frequency at given temperatures

The magnitude of parameter s (angular frequency exponent) was calculated from the slopes of

Figure 6-5. In this work, the value of parameter s was found equal to 0 at low frequency region,

showing DC conductivity behavior [101] of GO film and very close to unity at high frequency

region. Furthermore its magnitude was found to vary slightly with increase in temperature. So

based on the magnitude of parameter s (which is close to unity at high frequency region) and its

temperature dependent behavior, it can be concluded that the results are consistent with the

correlated barrier hopping (CBH) model [288-290]. This model proposes temperature activated

hopping of charge carriers between localized sites [291]. Thus GO film possesses DC and CBH

mechanisms of conduction at low and high frequency regimes, respectively. The transition

region from DC to CBH conductivity shifted to low frequency with increase in temperature.

The Complex Impedance Spectra of Graphene Oxide (GO), obtained by plotting ZS Vs ZS at

different temperatures is shown in Figure 6-6. This pattern not only informs about the electrical

processes occurring within the sample but also presents their correlation with the microstructure

of the sample when modeled in terms of an electrical equivalent circuit [292] as presented in the

inset of the Figure 6-5. R2 in the equivalent circuit represents contact resistance whereas R1 in

parallel with ZC1 (impedance of C1) represents polarization resistance. The impedance spectrum

of the GO is based on semicircular arcs whose pattern is found to change with increase in

95

temperature. As can be seen the Nyquist plots of GO sample, complete semicircles does not form

at higher temperatures. This is due to very high impedance of GO sample at higher temperatures.

Figure 6-6: Complex impedance spectrum (Nyquist Plot) for the GO at various temperatures, inset:

Equivalent RC network

In this work, dielectric and transport properties of GO (synthesized using modified Hummers

Method) as a function of frequency and temperature Impedance spectroscopic technique.

Graphene Oxide films were drop casted on Indium Tin Oxide (ITO) coated glass slides. Atomic

force microscopy (AFM) was used to characterize the films structural and surface profiles.

Electrical measurements were taken using impedance analyzer in frequency regime varying from

100 Hz to 10 MHz) at different specified temperatures. Results indicated that AC conductivity,

ζac, of the films‘ varies with ω, the angular frequency. Temperature and frequency variation has

significantly influenced on the electrical properties of GO film, in particular at low frequency

regions. Based on the results it was attributed that film bears DC and CBH conductivity

mechanisms at low and high frequency ranges, respectively. Realization and understanding the

transport properties of similar solution processed films will provide potential and foundation for

the development of advanced microelectronic devices in future.

96

6.2 Effect of Co/Mg ratio on the structural and electrical properties of lead

zirconate titanate films

6.2.1 Introduction

Lead zirconate titanate (PbZrTiO3 or PZT) based materials/ceramics with perovskite ABO3 ((A =

Mono –or divalent; B = Tri- hexavalent ion)) structure has attained much focus of the researchers in

recent years because of the simplicity of its crystal structure which helps to understand the

relation between structural changes and physical properties. Perovskite based piezoelectric

materials show high electromechanical transformation, due to which these are commonly utilized

in electro-ceramic applications in industry. Furthermore PZT, due its excellent piezoelectric

properties is commonly utilized during the development of underwater/ultrasonic transducers,

such as hydrophones, actuators and underwater transducers. In industry, materials showing

thermally stable dielectric as well as piezoelectric properties around transition temperature are

required [151-155]. The dielectric properties of PZT can be tuned by using various isovalent and

aliovalent dopants. Under certain conditions when PZT crystals possess tetragonal or

rhombohedral symmetry, each crystal has a dipole moment.

PZT materials, exhibit a unique range of properties which make them suitable for the

development of sensors and actuators [293]. In a basic sense, if a piezoelectric material is

deformed, an electrical signal proportional to bending/deformation due to piezoelectric effect.

Also if electrical signal is applied deformation or bending is observed in these materials due to

inverse piezoelectric effect. In order to improve the electrical properties of PZT, various attempts

have been reported in recent years. Doped PZT are mostly synthesized by using one of the

following methods [293]:

f. conventional solid state sintering route,

g. peroxohydroxide method,

h. citrate precursor method, precipitation method,

i. hydrothermal method and

j. solgel method.

97

In order to investigate the effect Cobalt/Magnesium ratio on the electrical and structural

properties of PZT films, Polycrystalline PbTi0.5Zr0.3(Co1-xMgx)0.2O3 (x = 0.00, 0.25, 0.45, 0.65,

0.85) ceramics films were solution processed using sol-gel route and characterized using parallel

plate impedance spectroscopy. Mg doped PZT ceramics possess activation energy (Ea 1.05

eV) which is comparable with the Ea values (11.1 eV) associated with doubly ionized oxygen

vacancies (VÖ) common in perovskite oxide piezoelectric materials.

6.2.2 Experimental Part

Polycrystalline PbTi0.5Zr0.3(Co1-xMgx)0.2O3 (x = 0.00, 0.25, 0.45, 0.65, 0.85) powders,

synthesized by researchers at the Department of Materials Engineering, GEBZE Technical

University, Turkey, were electrically characterized. In order to prepare ceramics, PVB binder

was added in 5 wt% to the powders and mix-milled in agate and mortar for 15 minutes. The

powders were then pressed into pellets and binder was burned out at 600 oC for 30 minutes with

a heating step size of 5 oC/min and subsequently sintered at 1150

oC for 2 h to get dense

ceramics. The ceramic samples were used for further characterization.

Scanning Electron Microscopy (SEM) was used in order to investigate the microstructure of

sintered ceramics using a XL30, FEi Co., Hillsboro. The electrical measurements were

performed on parallel-plate capacitor configuration using an Agilent, HP 4194A LCR meter. In

order to make contacts of the samples, the opposite polished surfaces of the sintered pellets were

coated with Ag-paste and cured at 600 oC for 30 min. The impedance of the samples was

monitored between 30 oC to 300

oC using NOVOTHERM Temperature Control System.

Complete electrical characterization system (shown in Figure 6-2) includes impedance analyzer,

computer station, Novotherm temperature heating and control system.


Figure 6-7 presents the SEM views of PbTi0.5Zr0.3(Co1-xMgx)0.2O3 ceramics surfaces at high

resolution. The SEM micrographs of surface showed well connected grain microstructures with

limited porosity, which is sometime observed for the case of PbO based ceramics. Figure 6-8

shows the variation of dielectric constant (ɛr) and Dielectric Loss (Tan δ) of PbTi0.5Zr0.3(Co1-

xMgx)0.2O3 ceramics as a function of frequency, ranging from 100Hz to 1MHz. It was observed

that the magnitude of ɛr for the x = 0 composition decreases with an increase in frequency, which

98

Figure 6-7: SEM micrographs of the thermally itched surface of PbTi0.5Zr0.3(Co1-xMgx)0.2O3

ceramics.


frequency

99

is attributed due to the typical relaxation phenomena in the ferroelectric Perovskites [294].

Further, the magnitude of ɛr for the x > 0.00 compositions was almost constant with frequency.

This behavior confirmed that the Mg-doped samples exhibited non-relaxor characteristics. The

magnitude of ɛr for the x = 0.25 composition was highest among the investigated samples/

compositions and decreased with an increase in Mg content in the given frequency regime (100

Hz1MHz).

Figure 6-9 (a) and (b) shows the variation of ɛr and tan with temperature measured at 100 kHz,

respectively. The ɛr value was found to increase gradually with increase in the temperature. The

phase transition temperature was not observed in the current experimental setup and investigated

temperature range i.e. 30 oC to 300

oC. The tan value was constant over the wide range of

temperature. The magnitude of tan was observed to decrease with an increase in Mg content.

The conduction mechanism of the PbTi0.5Zr0.3(Co1-xMgx)0.2O3 ceramic samples were studied by

the measurement AC conductivity at 100 kHz in the temperature regime from room temperature

to 300 oC. The results are depicted in the Figure 6-10. The AC conductivity of the x = 0

composition was independent of temperature in the low temperature regime up to 250 oC which

then gradually increased with further increase in temperature. The x = 0.25 composition

exhibited high magnitude of conductivity in the investigated temperature regime.


temperature

100


temperature.

Variation of AC conductivity of PbTi0.5Zr0.3(Co1-xMgx)0.2O3 ceramic samples with frequency at

various temperatures is presented in Figure 6-11. No significant difference in the conductivity of

the x = 0.00 to 0.85 compositions in the low frequency regime (up to 105 Hz) was observed,

which increased instantly/ abruptly with further increase in frequency for all the ceramic

samples.

The frequency dependent properties of a material can be described as complex permittivity (ɛ*),

complex impedance (Z*), complex admittance (Y*), complex electric modulus (M*) and

dielectric loss can be related by the following equations:

ɛ ∗ = ɛ + 𝑗ɛ 6-4

ɛ ∗ = ɛ + 𝑗ɛ 6-5

𝑍∗ = 𝑍 + 𝑗𝑍 = 1/𝑗𝐶𝑜ɛ∗ 6-5

𝑌∗ = 𝑌 + 𝑗𝑌 = 𝑗𝐶𝑜ɛ∗ 6-6

101

𝑀∗ = 𝑀 + 𝑗𝑀 = 1/ɛ∗ = 𝑗𝐶𝑜𝑍∗ 6-7

𝑡𝑎𝑛 = ɛ/ɛ = 𝑀/𝑀 = 𝑌/𝑌 = 𝑍/𝑍 6-8

where, = 2 is the angular frequency, Co is the free geometrical capacitance. The

microstructure of ceramics comprises of grains (bulk) and grain boundaries, exhibiting different

resistivities (ρ) and dielectric permittivities (ɛ). Impedance spectroscopy can be used to interpret

various complex parameters such as the real (ɛ , Z , Y , M) and imaginary (ɛ , Z , Y , M)

components of various parameters, in order to understand the electrical structure of materials.

Figure 6-12 shows the Nyquist plots, obtained by plotting Z vs. Z at various temperatures for

the PbTi0.5Zr0.3(Co1-xMgx)0.2O3 ceramic samples. The plot showed single semicircular arcs

throughout the investigated frequency range (100 Hz to 1 MHz) for all the ceramic samples (at

300 oC). A complete semicircle was not obtained for the x = 0.85 composition at 300

oC. The

semicircles are depressed to certain degrees which showed deviation from ideal behavior. Such

behavior can be associated with the overlapping of two or more semicircles which could not be

resolved in the current experimental setup. Bulk capacitances (Cb) of the samples were calculated

from the peaks of semicircles using relation:

𝜔𝑚𝑎𝑥 𝐶𝑏𝑅𝑏 = 1 6-6

In summary, Polycrystalline PbTi0.5Zr0.3(Co1-xMgx)0.2O3 ceramics were electrically characterized

using parallel plate impedance spectroscopic technique. The microstructure of ceramics was

found to be dependent on Mg doping content. The x = 0.00 composition showed a relaxor

behavior in the frequency regime from 100 Hz to 1 MHz. The magnitude of tan and ɛr of

PbTi0.5Zr0.3(Co1-xMgx)0.2O3 ceramic samples were decreased with an increase in Mg doping

content. The x = 0.25 composition was found to exhibit a high ɛr (4261) and tan (0.05) around

room temperature at 100 kHz frequency.

102


frequency at various temperatures.

103

Figure 6-12: Complex impedance plots for PbTi0.5Zr0.3(Co1-xMgx)0.2O3 ceramics at 300 oC

104

6.3 Electrical Conductivity Analysis of Methacrylate-based Liquid Crystal

Polymers with Pendant Cholesterol Groups

6.3.1 Introduction

Liquid Crystal Polymers (LCPs) are unique materials which combine the features of liquid

crystals and polymers. In order to exhibit liquid crystal characteristics for polymers, mesogenic

groups (rod-like or disk-like units) must be attached to polymer on the main or side chain. If they

are the part of the main chain of polymer, polymer is called main chain liquid crystal polymer

(MCLCP). Otherwise, side chain liquid crystal polymer (SCLCP) is formed when mesogens are

incorporated into polymer as side groups. MCLCPs cannot show mesogenic property in large

temperature ranges, however, mesogenic behavior of SCLCPs are in a large range [295-297].

SCLCPs can be varied by altering their components, which are the polymer backbone, the

mesogenic group, and the flexible spacer between the polymer backbone and the mesogen [298-

300].

The literature suggests that SCLCPs can be synthesized mostly with chiral compounds such as

cholesterol (Chol), isosorbide, and menthol [301-304]. Chol which plays a crucial role in the

field of self-organising organic systems is a rigid component of many natural lipid bilayers

[305]. In Chol containing polymers, the Chol groups produce mesomorphous structures [306]. 80

LCP with pendant Chol side groups have significant utility due to their optical properties [307].

They form chiral nematic and smectic phases and can be usually employed in electro- and

thermooptical applications [308] By changing the polymer backbone (i.e., acrylate, methacrylate,

and siloxane) and flexible spacers (i.e., methylene and siloxane), Chol containing SCLCPs have

been prepared [309, 310]. The synthesis of block copolymers [(Poly(methyl

methacrylatecholesterol-b-styrene) and poly(cholesterylmethacrylate-b-2-hydroxyethyl

methacrylate)] have been also reported [311, 312].

In semiconductor industry, liquid crystals are one of the most promising materials especially for

microelectronic devices [313-315]. In the last decades, organic field effect transistors (OFETs)

attract a great deal of attention as an important type of electronic devices for their use in

lightweight, elastic, and low-cost electronic instruments [316-319]. They can be implemented in

wide range of applications such as flat-panel displays, electronic paper, and chemical sensors

105

[320]. Potentially, SCLCPs could be used as a key component in the field of organic

microelectronics [321].

Poly(methyl methacrylate) (PMMA) is one of the well-known polymers for dielectric materials.

It is a hard, hydrophobic, less-polar, brittle, and easily prepared material. An advance property of

PMMA relies on its high resistance and non-tracking characteristics in high voltage applications.

It has a dielectric constant that ranges from about 3.5 (at 5 kHz) to 2.6 (at 1 MHz) and can be

used as a gate dielectric layer in OFETs [322, 323].

In this work, electrical and dielectric properties of methyl methacrylate-based SCLC

homopolymers (poly (chol-n-MMA)) and copolymers (poly (chol-n-MMA-co-MMA))

containing Chol pendant groups with different lengths of flexible aliphatic spacer (n= 3, 7, and

10 methylene units) were studied using parallel impedance spectroscopic technique in the

frequency range of 100 Hz to 15 MHz at room temperature, to fully exploit their possible

applications for flexible OFETs and organic photo voltaic (OPV) assemblies. The polymers

were synthesized by researchers at Department of Chemistry, Gebze Technical University

(GTU), and used as received. The dielectric constant values of the polymers were found to

increase with decreasing aliphatic methylene spacer length. Investigation of the polymers via

Cole-Cole plots technique demonstrated the fact that most of the polymers obey non- Debye type

relaxation behavior except homopolymer and copolymer with n = 3, which showed nearly Debye

type relaxation. Alternating current (AC) conductivity (ζac) of the liquid crystalline polymeric

films was observed to vary with angular frequency, ω as ωS with S<2. Detailed conductivity

analysis revealed that the conductivity of the polymeric films follow quantum mechanical

tunneling (QMT) and correlated barrier hoping (CBH) conductivity mechanisms at low

frequency regime, whereas it obeys super linear power law (SLPL) and direct current (DC)

conductivity mechanisms at high frequency region. In the light of the current study, the

investigation of the polymeric films helped to propose a model system for their possible potential

applications such as large area flexible arrays or as other advanced microelectronic devices.

6.3.2 Experimental

Electrical properties of the polymers were analyzed using Parallel Plate Impedance

Spectroscopic Technique. Parallel plate type structures were developed on glass substrates as

106

shown in Figure 6-13. Fabrication of these structures were started with the cleaning of Indium

Tin Oxide (ITO) coated glass substrates using Soap, Acetone, Methanol, Ethanol and Isopropyl

Alcohol in an ultrasonic bath. Drying of the substrates was carried out using Argon blow and

Thermal Oven. Solutions of the polymer samples were prepared in Toluene (Merck) by the

process of magnetic stirring for 24 hours at room temperature. Polymer films were coated on

cleaned ITO coated glass substrates by the process of spin coating at 2000 rpm using filtered

polymer solution. The size of the filter was 0.45µm. Then samples were kept in the nitrogen

environment for 12 hours in order to evaporate solvent. Finally, 50 nm thick top contact was

made by evaporating Aluminum on the polymer films through the shadow mask using Leybold

Univex 450, Vacuum Evaporator at 10-5

mbar pressure. Thicknesses of various polymer films

were measured using DEKTAK-8 profiler. Impedance Analyzer, HP-4194 was used to determine

the electrical and dielectric properties of the samples within the frequency range of 100Hz to

15MHz. A signal of 1 V peak to peak was applied to the samples during these measurements. All

the tests and measurements were performed in the ambient conditions.

6.3.3 Results and discussion

Electrical characterization of solution processed methacrylate based co-polymer (chol-n-MMA-

co-MMA) and homo-polymer (poly (chol-n-MMA)) is presented in this section, Where n = 3, 7,

10. The real εʹ(ω) and the imaginary εʺ(ω) parts of the complex dielectric constant are described

as ε*(ω) = εʹ(ω)-iεʺ(ω) [324]. Here, ε

*(ω) is the complex dielectric constant.

Figure 6-13: Design Schematic of Parallel Plate Structure used for Impedance Spectroscopic

Analysis of Co-polymers and Homo-polymers

107

Figure 6-14 shows the frequency dependence of the real εʹ(ω) and the imaginary εʺ(ω) parts of

the complex dielectric constant on a semi-log scale at room temperatures in the parallel plate

device under test, respectively. As can be seen, all the samples have shown almost constant value

of εʹ at low frequencies up to 10 kHz, indicating a good and stable polarization over a wide

frequency band. Among the investigated samples, polymers with chain length 3 i.e. Poly(Chol-

MMA-3) and Poly(Chol-MMA-3-co-MMA) have shown the highest dielectric values. Whereas

polymers with chain length 7 i.e. Poly(Chol-MMA-7) and Poly(Chol-MMA-7-co-MMA) have

demonstrated lowest values of dielectric constant. Real dielectric constant values of polymers

with chain length 10 i.e. Poly(Chol-MMA-10) and Poly(Chol-MMA-10-co-MMA) were found

higher than the polymers with chain length 7 and lower than those with chain length 3

throughout the investigated frequency range.

The real ε'(ω) part of the dielectric constant is described as [99]:

6-7

Figure 6-14: Angular frequency evolution of (a) the real (εʹ), and (b) imaginary (εʺ) dielectric

constants of the polymers

)1(2)(

2

1sin1)(21

2

1sin1)(1

)()('

oo

o

s

108

Here, ε∞ and εs are high and low angular frequency dielectric constant, ω=2π times the

frequency, η0 is a generalized relaxation time, and α is the absorption coefficient. The absorption

coefficient α parameter changes from zero to one (0 < α ≤1). If α =0, it corresponds to standard

Debye type relation, nearly-Debye type when α value is close to zero and non-Debye type for 0 <

α ≤1 regions. η0, relaxation time and α, absorption coefficient was calculated from equation (1)

and fitted by the results given in Figure 6-14 (a). The calculated values of η 0 and α are given in

Table 6-1. The values of parameter α are higher than zero which indicates a non-Debye type

relation [325]. The absorption coefficient values of the samples suggested that the investigated

polymers obeyed non-Debye type and nearly-Debye type relaxation processes. Table 6-1 also

presents the values of parameters εs, ε∞ and Δε which were estimated from Figure 6-14 (a). These

parameters change due to chain length of different samples. Equivalent dielectric constants vary

according to the chain length for both homopolymers and copolymers. Critical frequency of all

polymers is determined from Figure 6-14 (b). This frequency value corresponds to the maximum

peak frequency of imaginary dielectric constant. Significant variation in the critical frequency of

the samples is observed, which is in agreement with the variation of other parameters.

Table 6-1: Absorption coefficient relaxation time (o), dielectric parameters (εs, ε∞, ε''max, and

Δε) and critical frequencies (fc) of Poly(Chol-n-MMA) and poly(Chol-n-MMA-co-MMA) (n= 3, 7,

and 10).

Samples (Adj. R-Square 0.99)

α 0 εs ε∞ ε''max. Δε fc[MHz]

Poly(Chol-3-MMA-co-

MMA) 0.39 7.07×10

-8 3.82 -0.102 1.96 3.902 1.31

Poly(Chol-7-MMA-co-

MMA) 0.04 5.80×10

-8 2.39 -0.135 1.43 2.425 3.26

Poly(Chol-10-MMA-co-

MA) 0.27 6.86×10

-8 3.25 -0.117 1.66 3.367 2.26

Poly(Chol-3-MMA) 0.064 5.22×10-8

4.02 -0.301 2.36 4.321 3.91

Poly(Chol-7-MMA) 0.28 6.64×10-8

2.66 -0.100 1.3 2.76 2.72

Poly(Chol-10-MMA) 0.74 8.01×10-8

2.85 0.029 1.3 2.821 0.53

Cole-Cole plots of the samples are shown in Figure 6-15. These Cole–Cole plots were obtained

from the imaginary versus real values of dielectric constants (ε"–ε'). Equivalent circuit model

109

analysis of the Cole–Cole plots show parallel Resistance-Capacitance (RC) and series Resistance

(R) regimes for the investigated samples.Plots have shown non–Debye and nearly-Debye type

relaxation phenomena. The non-Debye type plot are most suitable for the description of the

interface molecular relaxation mechanisms. εʺ– εʹ curves (Cole-Cole plots) of the samples show

depressed semicircles. Poly(Chol-MMA-7-co-MMA) and Poly(Chol-3-MMA) samples are close

to x-axis as they are exhibited nearly-Debye type properties. Angular frequency evolutions of the

ac conductivity (ζʹ) of samples are shown in Figure 6-16. To understand the conductivity

mechanism of the samples, lnζʹ-lnω graphs of the samples were drawn and investigated. As

indicated in Figure 6-17, some of these samples show frequency linear regions on their plots.

The values of parameter s (angular frequency exponent) were calculated from the slopes of

Figure 6-17. Linear increase in the Figure 6-17 with frequency is fitted and ac conductivity is

acquired from this fit. During the fit slope, linear equation, y=a+b*x, expressing b=ln/ln with

R-Square: 0.99 was used. The variations of lnAC with angular frequency have been given in

Figure 6-17 for Poly(Chol-n-MMA) and Poly(Chol-n-MMA-co-MMA) samples. Since the

angular frequency exponent s is used to determine the electrical conduction mechanism for

Poly(Chol-n-MMA) and Poly(Chol-n-MMA-co-MMA), its values have been calculated from the

slopes of Figure 6-17 for the low and high frequency regions. The s parameter values can be

interpreted by conductivity mechanisms for frequency dependent conductivity given by s≈0

[101] for DC conductivity mechanism, 0<s<0.7 [102] for Correlated Barrier Hoping (CBH)

conductivity mechanism, 0.7≤s<1 [103] for Quantum Mechanical Tunneling (QMT)

conductivity mechanism and 1<s<2 [104] for Super Linear Power Law (SLPL) high frequency

regions are given in inset tables in Figure 6-17.These samples contain four conductivity

mechanisms in the low and high frequency regions. These mechanisms are the QMT and CBH

conductivity mechanisms at low frequency regime and the SLPL and DC conductivity

mechanisms at high frequency region. As can be seen, the values of exponent ―s‖ are varying

from 0.029 to 1.81 (±0.004).

Generally, all samples shows conduction mechanism at high or low frequencies. At low and high

frequencies all the samples demonstrated QMT and SLPL conduction mechanism respectively,

except Poly(Chol-7-MMA-co-MMA) sample which showed CBH mechanism at low

frequencies. Furthermore, angular frequency dependent measurement of Poly(Chol-10-MMA)

110

Figure 6-15: Cole–Cole plots of the dielectric constant (ε"– ε')

sample has shown three different conductivity mechanisms. The significance of this sample is, it

reaches saturation at high frequencies with no slope and parameter s value near to zero, which

indicates DC conductivity behavior of the sample at higher frequencies. This behavior of the

sample suggests its suitability for the development efficient organic photovoltaic devices

operative at higher frequencies.

Performance of an Organic Field Effect Transistor is function of the properties of its gate

dielectric films. Peng et al. [326] have realized all organic field effect transistors using a variety

of polymer dielectrics and change in the field effect mobility values of the devices was correlated

with varying dielectric constants values of dielectric films. Dielectric constants and other

promising dielectric properties of the investigated SCLC polymers suggests their suitability for

the development of high frequency operated flexible Organic Field Effect Transistors (OFETs)

for large area sensing and other advanced applications.

111

Figure 6-16: Specific conductivity of Poly(Chol-n-MMA) and poly(Chol-n-MMA-co-MMA) (n= 3,

7, and 10); inset: High resolution plots of specific conductivity of samples in high frequency regime

Electrical characterization of a series of SCLC type homopolymers (poly(Chol-n-MMA)) and

copolymers (poly(Chol-n-MMA-co-MMA)) with flexible methylene spacer (n = 3, 7, and 10)

between the poly(methyl methacrylate) backbone and cholesteryl mesogen is undertaken in this

work. Real and imaginary dielectric constants, absorption coefficient, relaxation time, dielectric

strength, and critical frequency values of the polymers were determined using parallel plate

impedance spectroscopic technique. The dielectric constant values of the polymers were found to

increase with decreasing aliphatic methylene spacer length. εʺ– εʹ curves (Cole-Cole plots) of the

polymers show the depressed semicircles indicating non-Debye type relaxation behavior of the

polymers except for the copolymer and homopolymer with n=3, which showed nearly Debye

type relaxation properties. Detailed conductivity analysis revealed that the polymeric films

followed QMT and CBH conductivity mechanisms at low frequency regime and the SLPL and

DC conductivity mechanisms at high frequency regions. Furthermore, homopolymer with n= 10

showed the saturation or DC conductivity behavior at higher frequencies which represents its

suitability for the development of high frequency organic photovoltaic devices.

112

Figure 6-17: Specific conductivity versus frequency plot of Poly(Chol-n-MMA) and poly(Chol-n-

MMA-co-MMA) (n= 3, 7, and 10).

113

Chapter-7: Conclusions and Future Work 7 8

7.1 Conclusions

The research work was mainly focused on the fabrication and characterization of solution

processed semiconducting films based electronic devices. The solution processed films based

devices are preferred over conventional microelectronic devices due to their low cost, ease of

processing, flexibility and large area sensing applications. The important results obtained from

the research work presented in this dissertation are summarized as:

1. Polymer-Fullerene BHJ based organic solar cells were fabricated using three different

batches of MDMO-PPV polymer with a motivation to investigate the effect of polymer

intrinsic properties i.e. Molecular Weights, Polydispersity values, Charge Carrier

Densities and HOMO/LUMO Levels, on the performance of these devices. Polymer

batches with significant varying intrinsic properties were used for this investigation.

Structural and optical properties of MDMO-PPV:PCBM blends based nanolayers

developed using three different batches of polymer, were studies using Atomic Force

Microscopic and UV-Vis absorption spectroscopic. Current Density-Voltage (J-V)

characteristics of the devices were investigated under an UV-Vis illumination intensity of

100mW/cm2 and their results were compared. Important findings of the work are as

follows:

a. Differences in the nano morphology of organic films, casted using three different

batches of polymers were observed which have significantly influenced on Jsc and

FF parameters of the devices.

b. Efficiency of the solar cell, realized using polymer with moderate Molecular

Weights (Mn= 85 kg/mol and Mw= 1400 kg/mol), and highest Polydispersity

(16.8) and Charge Carrier Density (3.2×10-17

cm-3

) values, was found maximum,

which was attributed due to higher photon absorption capability and favorable

morphology of its photoactive layer.

2. Organic BHJ based Transistor‘s investigation in metal semiconductor metal architecture

known as MESFET architecture was undertaken for the first time for photo, strain/ bend

and displacement sensing applications. Transistors were fabricated on Glass and flexible

114

substrates. Drain/ Source and Gate electrodes of the devices were made using Silver and

Aluminum, respectively. Important findings of this work are summarized as follows:

a. Gate electrode and Drain/Source electrodes showed Schottky and Ohmic type

contacts with the semiconducting/active layers of the devices, respectively.

b. Semiconducting layer of the devices which was the blend of poly(3-

hexylthiophene)(P3HT) and [6,6]-phenyl C61-butyric acid methylester (PCBM)

demonstrated Schottky and Ohmic type contacts with Gate and Drain/Source

electrodes, respectively, and showed ambipolar properties which was discussed

with the help of energy band diagram.

c. Semiconducting layer of the devices showed ambipolar properties

d. MESFET devices low voltage operation

e. Drain current was found to increase with the illumination intensity

f. Photo responsivity and sensitivity values of BHJ based MESFETs were found

higher as compared to single organic semiconductor based photo-MESFETs

g. Photo responsivity and sensitivity values of the devices were found to decrease

with increase in negative gate voltage

h. Photo sensitivity was found to increase when P3HT:PCBM blend based

semiconducting layer mixed with Red Methyl Dye. However gate leakage current

and photo sensitivity values of the devices were reduced with Red Methyl

blending.

3. Influence of temperature and frequency variation on the dielectric and transport

properties of Graphene Oxide (synthesized using modified Hummers Method) was

investigated using impedance spectroscopic technique. Graphene Oxide films were

prepared using drop casting method on Indium Tin Oxide (ITO) coated glass substrate.

Atomic force microscopy was used to characterize the films microstructure and surface

topography. Electrical characterization was carried out using LCR meter in frequency

regime (100 Hz to 10 MHz) at different temperatures. Important findings of the work are

as follows:

a. AC conductivity ζac of the films‘ varied with angular frequency, ω as ωS, with S

< 1. Temperature and frequency variation has significantly influenced on the

electrical properties of GO film, in particular at low frequency regions.

115

b. GO film contains DC and CBH conductivity mechanisms at low and high

frequency ranges, respectively.

4. Polycrystalline PbTi0.5Zr0.3(Co1-xMgx)0.2O3 ceramics, developed using sol-gel route were

electrically characterized using parallel plate impedance spectroscopic technique.

a. The microstructure of ceramics was found to be dependent on Mg doping content.

The x = 0.00 composition showed a relaxor behavior in the frequency regime

from 100 Hz to 1 MHz.

b. The magnitude of tan and ɛr of PbTi0.5Zr0.3(Co1-xMgx)0.2O3 ceramic samples were

decreased with an increase in Mg doping content.

c. The x = 0.25 composition was found to exhibit a high ɛr (4261) and tan (0.05)

around room temperature at 100 kHz frequency.

d. Complex impedance spectroscopic analysis revealed that PbTi0.5Zr0.3(Co1-

xMgx)0.2O3 (x = 0.000.85) exhibited both grain boundary and bulk contributions.

e. The total resistance was decreased with an increase in temperature showing a

typical ceramic behavior.

f. The AC conductivity was increased with an increase in temperature due to the

production of oxygen vacancies associated with doping of aliovalent dopants

(Co3+

, Mg2+

) on Ti4+

/Zr4+

.

5. Electrical characterization of a series of SCLC type homopolymers (poly(Chol-n-MMA))

and copolymers (poly(Chol-n-MMA-co-MMA)) with flexible methylene spacer (n = 3, 7,

and 10) between the poly(methyl methacrylate) backbone and cholesteryl mesogen is

undertaken in this work. Important findings are as follows are as follows:

a. The dielectric constant values of the polymers were found to increase with

decreasing aliphatic methylene spacer length.

b. εʺ– εʹ curves (Cole-Cole plots) of the polymers show the depressed semicircles

indicating non-Debye type relaxation behavior of the polymers except for the

copolymer and homopolymer with n=3, which showed nearly Debye type

relaxation properties.

c. Detailed conductivity analysis revealed that the polymeric films followed QMT

and CBH conductivity mechanisms at low frequency regime and the SLPL and

DC conductivity mechanisms at high frequency regions.

116

d. Homo-polymer with n= 10 showed the saturation or DC conductivity behavior at

higher frequencies.

It is expected that detailed characterization of solution processed semiconducting films based

devices will provide potential for the development of low cost large area flexible sensor arrays

and other low power electronic devices in future.

7.2 Future Work

In order to fully understand and explore the potential applications of solution processed films

based microelectronic devices for industry; the current work can be further extended into the

following directions:

1. Polymer-Fullerene BHJ based Solar Cells were fabricated to analyze the effect of

polymer intrinsic properties on the photo conversion efficiency of the devices. Since,

performance of the solar cells is function of the structural properties of photo sensitive

semiconducting films. Therefore this work could be further extended to investigate the

effect of polymer intrinsic properties on the surface profiles of the semiconducting films.

2. Organic BHJ based MESFETs, having simpler transistor architectures, were fabricated

for the first time and investigated for photo, strain/bend and displacement sensing

applications. This work can be extended further into the following directions, in order to

further understand physics underlying these structures and more sensing applications for

low cost developments in future:

a. Investigation of the effect of transistor‘s physical parameters such as channel

length, channel width, semiconducting and contact layer thicknesses on the

performance of the devices

b. Investigation of the effect of humidity, temperature, various gases and tensile

forces on the electrical properties of the devices

c. Fabrication of small molecules semiconducting layers based BHJ MESFET and

their comparison with the investigated devices

d. Fabrication and characterization of solution processed BHJ films based dual gate

MESFETs

e. Simulation of the devices using device physics simulator, SILVACO TCAD

117

3. Simulate the fabricated Organic Solar Cells and Transistors using SILVACO TCAD so as

to further understand the physics of the devices.

4. Investigate the effect of elevated temperatures (higher than 100 ºC) and humidity

variations on the electrical conduction and dielectric properties of Graphene Oxide films

5. Study the effect of temperature on the electrical properties of SCLCP films.

118

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