Energy Level Alignment at Donor-Acceptor Interfaces and Its
Correlation with Open-circuit Voltage
by
Peicheng Li
A thesis submitted in conformity with the requirements
for the degree of Master of Applied Science
Graduate Department of Materials Science & Engineering
University of Toronto
© Copyright 2017 by Peicheng Li
ii
Abstract
Energy Level Alignment at Donor-Acceptor Interfaces and Its Correlation with Open-circuit
Voltage
Peicheng Li
Master of Applied Science
Graduate Department of Materials Science & Engineering
University of Toronto
2017
Organic photovoltaic (OPV) cell has attracted great interests due to its low cost manufacturing
method and its flexible form factor. In this thesis, we utilize physical vapor deposition to fabricate
different donor-acceptor interfaces and devices. First, we study the effect of substrate work
function on the energy level alignment at these interfaces by ultra-violet photoemission
spectroscopy (UPS). Second, OPV cells are fabricated based on the same sets of materials used
in the UPS study. Third, a theory is proposed in order to explain the observed relationship
between open-circuit voltage VOC and the donor-acceptor energy offset EDA. The main findings
of this thesis are: (a) the substrate work function has negligible impact on energy level alignment
between donor and acceptor; (b) VOC is closely related to EDA; and (c) a new proposed theoretical
model predicts that VOC reaches a maximum if EDA is equal to the difference between singlet
energy and a constant which relates charge transfer state energy to EDA.
iii
Acknowledgements
There are so many people who provided help and support during my Master program. I would
like to express my sincere thanks to all of them.
First of all, I would like to thank my supervisor, Prof. Zheng-Hong Lu, for the great guidance
throughout my entire research. Whenever I encountered with some problems during my research,
Prof. Lu always spent his time to have a discussion with me and provided a clear direction for me
to solve the problem. Not only did I learned the academic knowledge from Prof. Lu but also the
problem-solving skill which will benefit my career in the future. Thank you, Dr. Lu!
Then, I would like to express my thanks to my group members for the generous collaboration
during my graduate study. Especially, I would like to thank Yiying Li for training me to master
the photoemission facility and Weiji Hong for teaching me how to fabricate a device. My
accomplishment would be impossible without your help. Also, I would like to thank other group
members for the time when we had a discussion about our research and the time we hung out
together. Thank you, all my group members!
In the end, I would like to thank my beloved family. Thanks for the encouragement and the
support you provided when I encountered something bad. Thanks for the happiness we shared
when I accomplished something great. I love you!
iv
Table of Contents
Acknowledgements...................................................................................................................... iii
Table of Contents ..........................................................................................................................iv
List of Figures ................................................................................................................................ v
1. Introduction & Motivation ....................................................................................................... 1
2. Theoretical background ........................................................................................................... 6
2.1 Organic Semiconductor & Molecular Orbitals ................................................................................. 6
2.2 OPV working principle ..................................................................................................................... 7
3. Experimental Technique ........................................................................................................ 10
3.1 Physical Vapor Deposition .............................................................................................................. 10
3.2 Ultra-violet Photoemission Spectroscopy ....................................................................................... 12
3.3 Current density-voltage characteristic measurement ...................................................................... 15
4. Energy Level Alignment at Donor-acceptor Interfaces ......................................................... 18
4.1 Introduction ..................................................................................................................................... 18
4.2 Methods ........................................................................................................................................... 19
4.3 Results & Discussion ...................................................................................................................... 22
4.4 Summary ......................................................................................................................................... 30
5. Correlation between VOC and EDA ......................................................................................... 32
5.1 Introduction ..................................................................................................................................... 32
5.2 Methods ........................................................................................................................................... 33
5.3 Results & Discussion ...................................................................................................................... 34
5.3.1 Device Stability ........................................................................................................................ 34
5.3.2 Correlation between VOC and EDA ............................................................................................ 36
5.4 Summary ......................................................................................................................................... 41
6. Conclusion and Future Work ................................................................................................. 43
v
List of Figures
1.1 Power-conversion efficiency of different solar cells according to the statistics from
NREL. The efficiency of OPV has reached over 10%, which makes its commercialization
possible. --------------------------------------------------------------------------------------------2
1.2 Commonly used organic semiconductors in OPV cells: (a) Copper(II) phthalocyanine; (b)
Boron subphthalocyanine chloride; (c) C60; (d) C70. ------------------------------------------2
1.3 Three main types of OPV cells, namely planar-heterojunction cell (left), bulk-cell
(middle) and low-donor-concentration cell (right).---------------------------------------------3
2.1 (a) conjugated π orbitals in benzene. Carbon and hydrogen atoms are shown as gray and
light gray spheres respectively; (b) molecular orbitals in benzene. Highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are
labeled. ----------------------------------------------------------------------------------------------7
2.2 Schematic diagram of carrier photo-generation in an OPV cell. Blue arrows corresponds
to exciton diffusion, green arrow represents CT state formation and red arrow shows the
transport of free charge carriers from organic layer to the electrode. -----------------------8
3.1.1 Schematic of components in the organic deposition chamber utilized in this thesis. Four
Knudsen cells (K-Cell) are inside the chamber with their respective shutters on the top.
Transfer-arm evaporator (TAE) is used for quick replacement of organic materials. Quartz
crystal microbalance (QCM) is used to monitor the deposition rate and film thickness.
The whole chamber is pumped by a cryogenic pump. --------------------------11
3.1.2 Schematic of chambers in Cluster Tool system for OPV cell fabrication. ----------------12
3.2.1 Schematic of photoemission process and kinetic energy measurement by the spectrometer.
--------------------------------------------------------------------------------------13
3.2.2 Schematic of the spectrometer applied in the UPS facility. UV light excites the valence
electrons which are focused by the electron lens system. The electron lens system
accelerate/decelerate emitted electrons to reach the pass energy E0. Only the electron with
E0 energy can successfully pass through the semi-sphere electron analyzer and is then
measured by the multi-channel detector. ------------------------------------------------------14
vi
3.2.3 UPS spectrum of 10 nm C70 deposited on ITO-coated glass substrate. For insets, the left
one corresponds to secondary electron (SE) region, and the right one is valence band (VB)
region. -------------------------------------------------------------- ------------------------15
3.3.1 Schematic of the facility for J-V measurement. ---------------------------------------------16
3.3.2 A typical J-V characteristic curve of an OPV cell. Three key parameters, VOC, JSC and FF
can be determined from this J-V curve. -------------------------------------------------------17
4.2.1 UPS spectrum of bare ITO substrate (solid curve) and MoOX-coated ITO substrate
(dashed curve). The corresponding work function is labeled. ------------------------------20
4.2.2 the photograph of the multi-access chamber (MAC) cluster tool and PHI 5500 surface
analysis system used for in situ UPS measurement of donor-acceptor interfaces. The
major components are labelled. ----------------------------------------------------------------21
4.2.3 Energy levels (HOMO & LUMO) of donor and acceptor materials used in this study.
Structural formula of these molecules are also shown. --------------------------------------22
4.3.1 (a) UPS spectra of DBP-C70 interface measured on ITO substrate and MoOX-coated ITO
substrate, respectively; (b) Energy level diagram of DBP-C70 interface on ITO substrate;
(c) Energy level diagram of DBP-C70 interface on MoOX-coated ITO substrate. --------23
4.3.2 (a) UPS valence band spectra of DBP with different thicknesses deposited on the top of
MoOX-coated ITO substrate; (b) UPS valence band spectra of DBP with different
thicknesses deposited on the top of ITO substrate; (c) Zooming-up of UPS valence band
spectra measured from 1 nm C70 on the top of 10 nm DBP deposited on ITO and MoOX-
coated ITO substrate, respectively. DBP’s HOMO is observable due to the small thickness
of C70 layer. --------------------------------------------------------------------------24
4.3.3 (a) UPS spectra of CuPc-C70 interface measured on ITO substrate and MoOX-coated ITO
substrate, respectively; (b) Energy level diagram of CuPc-C70 interface on ITO substrate;
(c) Energy level diagram of CuPc-C70 interface on MoOX-coated ITO substrate. -------26
4.3.4 (a) UPS spectra of m-MTDATA-C70 interface measured on ITO substrate and MoOX-
coated ITO substrate, respectively; (b) Energy level diagram of m-MTDATA-C70
interface on ITO substrate; (c) Energy level diagram of m-MTDATA-C70 interface on
MoOX-coated ITO substrate. -------------------------------------------------------------------26
4.3.5 (a) UPS spectra of 2T-NATA-C70 interface measured on ITO substrate and MoOX-coated
ITO substrate, respectively; (b) Energy level diagram of 2T-NATA-C70 interface on ITO
vii
substrate; (c) Energy level diagram of 2T-NATA-C70 interface on MoOX-coated ITO
substrate. -----------------------------------------------------------------------------------------27
4.3.6 (a) UPS spectra of TAPC-C70 interface measured on ITO substrate and MoOX-coated
ITO substrate, respectively; (b) Energy level diagram of TAPC-C70 interface on ITO
substrate; (c) Energy level diagram of TAPC-C70 interface on MoOX-coated ITO substrate.
----------------------------------------------------------------------------------------- 27
4.3.7 (a) UPS spectra of mCP-C70 interface measured on ITO substrate and MoOX-coated ITO
substrate, respectively; (b) Energy level diagram of mCP-C70 interface on ITO substrate;
(c) Energy level diagram of mCP-C70 interface on MoOX-coated ITO substrate. -------28
4.3.8 (a) UPS spectra of CBP-C70 interface measured on ITO substrate and MoOX-coated ITO
substrate, respectively; (b) Energy level diagram of CBP-C70 interface on ITO substrate;
(c) Energy level diagram of CBP-C70 interface on MoOX-coated ITO substrate. --------28
4.3.9 (a) UPS spectra of CzSi-C70 interface measured on ITO substrate and MoOX-coated ITO
substrate, respectively; (b) Energy level diagram of CzSi-C70 interface on ITO substrate;
(c) Energy level diagram of CzSi-C70 interface on MoOX-coated ITO substrate. --------29
4.3.10 EDA measured on ITO substrate versus EDA measured on MoOX-coated ITO substrate.
Substrate work function makes little difference to the energy level alignment at donor-
acceptor interfaces. -------------------------------------------------------------------------------30
5.2.1 The photograph of the Cluster Tool Vacuum Deposition System for OPV cells fabrication.
----------------------------------------------------------------------------------------33
5.2.2 The device structure of the OPV cells used in this study. ----------------------------------34
5.3.1.1 (a) J-V characteristics of DBP/C70 OPV cell with and without MoOX anode buffer layer;
(b) Measured open-circuit voltage as a function of the time, recorded once the device is
taken out of the vacuum deposition chamber. ------------------------------------------------35
5.3.2.1 J-V characteristics of various planar-heterojunction OPV cells used in this study. ------36
5.3.2.2 Experimentally measured VOC against the EDA measured by UPS. The blue and red lines
are calculated according to the proposed model. The theory shows that the maximum VOC
can be achieved at a transition EDA point (dash line). The data of SubPc/C70 device is
taken from the literature. ---------------------------------------------------------------------37
5.3.2.3 The Universal plot of VOC versus EDA. The blue and red solid line is plotted according to
Eq.21 and Eq.22. The data point of C60 planar-heterojunction (PHJ, empty circles) cells
viii
and PCBM bulk-heterojunction (BHJ, empty squares) cells are collected from various
literature papers. The value of 𝛽 is obtained by the linear regression fit in the increase
regime. Our measured data including the SubPc literature data are shown as the black
star. ----------------------------------------------------------------------------------------------41
B Quasi Fermi level splitting under illumination for planar-heterojunction (PHJ) and bulk-
heterojunction (BHJ) OPV cells. -------------------------------------------------------------46
C Linear regression fit for (a) PCBM bulk-heterojunction cells and (b) C60 planar
heterojunction cells. The fitting window is 0.2~1.3 eV and 0.5~1.7 eV for PCBM and C60
cells, respectively. ------------------------------------------------------------------------------48
1
Chapter 1
Introduction & Motivation
Solar energy is a clean renewable energy, which people deem to be a promising energy source
to replace traditional fuel and nuclear energy in the future. The silicon-based inorganic
photovoltaic cell is already on the market and its power-conversion efficiency has reached over
25% today [1]. However, the single crystal structure of silicon is often required to fabricate the
high-performance cells [2], which makes its cost of fabrication high. In addition, the high power
consumption to reduce quartz to silicon make the silicon-based solar cell less meaningful, even
though its efficiency is relatively high. Therefore, it is desirable for people to discover new
photovoltaic technologies in order to replace the traditional inorganic solar cell.
One of the emerging photovoltaic (PV) technologies is organic photovoltaics (OPV). The cost
of OPV cell fabrication can be much cheaper than traditional silicon-based cell since an OPV cell
mainly consists of carbon-based organic materials. Furthermore, the OPV cell can be made very
thin and it can be fabricated on flexible substrates which broadens the application of PV. All these
features make OPV as a promising technology to replace the tradition inorganic PV. However,
the power-conversion efficiency of OPV is lower than traditional inorganic PV. According to
NREL’s latest efficiency chart shown in figure 1.1, the efficiency of organic solar cells reaches
11.5% in 2015 [3]. However, its efficiency increases from ~3% to over 10% within just 10 years,
which is better than the development trend of their inorganic counterpart.
2
Figure 1.1: Power-conversion efficiency of different solar cells according to data from NREL [3].
The efficiency of OPV has reached over 10%, which makes its commercialization possible.
Tremendous efforts have been made in order to improve the power-conversion efficiency of
OPV cells. Basically, the strategies can be divide into two directions. The first direction is the
design of organic materials applied in the OPV cell. Copper(II) phthalocyanine (CuPc) is one of
the most commonly used donor materials in the early stage of OPV development [4-6]. There is
a literature paper [7] showing the efficiency of a CuPc/C60 planar OPV device is ~1%. However,
the efficiency can reach ~2% when CuPc is replaced with a novel donor material,
Subphthalocyanine (SubPc). Fullerene is usually used as the acceptor material in OPV cells [8-
10]. The device performance of an OPV cell with C70 is better than that with C60 since C70 has a
better absorption at visible spectrum [11]. Figure 1.2 shows the structure of molecules mentioned
above.
(a) (b)
3
(c) (d)
Figure 1.2: Commonly used organic semiconductors in OPV cells: (a) Copper(II) phthalocyanine;
(b) Boron subphthalocyanine chloride; (c) C60; (d) C70.
And the second direction to improve the OPV performance is the engineering of device
structure. There are three main types of OPV cells, classified by the structure of donor-acceptor
active layer, shown in figure 1.3. The first type is the planar-heterojunction cell [4,7,9] which
means that the active layer is a layer-by-layer donor-acceptor heterojunction. The second type is
the bulk-heterojunction cell [8,12,13], of which the active layer is a mixture of donor and acceptor
materials. The third type of OPV cell is the low-donor-concentration cell (Schottky-junction cell)
[14-16], which means that only a small amount of donor material is mixed in the matrix of
acceptor material. Generally speaking, the device performance of a low-donor-concentration cell
and the bulk-heterojunction one is better than the planar-heterojunction one. Furthermore, the
optimization of the interface between the active layer and the electrode is an effective approach
to maximize the device performance. For instance, the efficiency of OPV cells can be improved
a lot by inserting a proper interlayer between the anode and the active
Figure 1.3: Three main types of OPV cells, namely planar-heterojunction cell (left), bulk-
heterojunction cell (middle) and low-donor-concentration cell (right).
4
layer [12,17,18]; an exciton-blocking layer [19,20] can also be applied in order to improve the
light harvesting in the active layer and, therefore, improves the power-conversion efficiency.
The power-conversion efficiency of an OPV cell is determined by three key parameters. First
is the open-circuit voltage (VOC), second is the short-circuit current (JSC) and the third is the fill
factor (FF). The optimization of efficiency is actually the maximization of these three parameters.
VOC is closely related to the donor-acceptor energy levels and the effective approach to improve
VOC is to optimize the energy level alignment at donor-acceptor interface [21-27]; JSC is not only
related to donor-acceptor energy levels [28] but also depends on the charge carrier transport
properties [29] in the organic materials and the efficiency of charge collection at the electrode-
active layer interfaces [30]; FF is an even more complicated parameter which is related to the
series resistance and shunt resistance of the OPV cell [31]. Therefore, VOC is the easiest parameter
to understand and control. There has already been a bunch of theoretical and experimental studies
[21-27] on the correlation between VOC and donor-acceptor interface energetics, which, however,
is not fully understood yet.
The aim of this thesis is to shed some light on the basic physical properties and processes at
donor-acceptor interfaces in an OPV cell. The energy level alignment at donor/acceptor interfaces
and its correlation with VOC are explored in this thesis, and a mathematical model is proposed to
explain the relationship between VOC and donor-acceptor interface energetics. The model
establishes a guideline for designing OPV cells with maximum VOC.
The structure of this thesis is as follows:
Chapter 2 introduces the basic concepts and working principle in organic photovoltaics.
Chapter 3 reviews the experimental techniques used in this study, namely physical
vapor deposition, photoemission spectroscopy and current density-voltage
characteristics measurement.
Chapter 4 studies the effect of substrate work function on energy level alignment at
donor-acceptor interfaces. It is found that substrate work function has negligible impact
on the energetics at donor-acceptor interfaces.
Chapter 5 deals with the relationship between open-circuit voltage (VOC) and donor-
acceptor energy offsets (EDA). VOC is found to linearly increase with EDA when EDA is
lower than a certain point and to decrease when EDA is too high. A mathematical model
5
is proposed and explains well the experimental data. It is predicted by the model that
there is a transition EDA point where the maximum VOC can be reached.
Chapter 6 presents the concluding remarks and opportunities for future work according
to the work and results in this thesis.
6
Chapter 2
Theoretical background
This chapter introduces the basic concepts of organic semiconductor which is the building block
of organic electronics. The knowledge of molecular orbitals is presented, which is essential to
understand the working principle of organic electronics. Then, a detailed working principle of
OPV is described and some modern theories describing the device parameters are provided as
well.
2.1 Organic Semiconductor & Molecular Orbitals
Organic semiconductor is the building block of organic electronics. The typical structure in an
organic semiconductor is its conjugated π electron system. Figure 2.1 (a) shows the conjugated π
system in benzene ring. Six carbon atoms undergo sp3 hybridization and each carbon atom is
connected to two nearest carbon atoms and one hydrogen atom with σ bonds. There is one
remaining p orbital in each carbon atom. The overlapping of these p orbitals leads to the formation
of conjugated π bond. The molecular orbital is formed once the molecule is formed. Figure 2.1
(b) shows six molecular orbitals formed from six p orbitals in carbon atoms. Six electrons in
carbon’s p orbitals will then occupy these orbitals, following three golden rules, namely energy
minimization rule, Hund’s rule and Pauli exclusion principle [32]. π2 and π3 molecular orbitals
are defined as highest occupied molecular orbitals (HOMO) where hole transport takes place. π4
and π5 molecular orbitals are defined as lowest unoccupied molecular orbitals (LUMO) where
electron transport takes place. The energy difference between HOMO and LUMO is defined as
the bandgap of organic semiconductors.
7
Figure 2.1: (a) conjugated π orbitals in benzene. Carbon and hydrogen atoms are shown as gray
and light gray spheres respectively; (b) molecular orbitals in benzene. Highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are labeled.
2.2 OPV working principle
Donor-acceptor interface is a critical component for efficient OPV cells. Unlike the traditional
inorganic semiconductor with a high dielectric constant, the organic semiconductor has a much
lower dielectric constant and, therefore, the electrostatic attraction between hole and electron is
so strong in an exciton that thermal energy at room temperature is not enough for the exciton
dissociation [33]. Therefore, a donor-acceptor interface is required for efficient charge separation
from the exciton since the energy difference of HOMO and LUMO between donor and acceptor
will facilitate the exciton dissociation at the interface. Figure 2.2 shows a schematic diagram of
charge carrier generation in an OPV cell with a planar device structure.
In general, the carrier photo-generation can be divided into four processes discussed as below:
a. Exciton formation under illuminance: Sunlight goes through the transparent electrode and
then reaches the active layer of the OPV cell. The photon then excites the electron from the
HOMO of either donor or acceptor into their respective LUMO, leaving a positive hole in
HOMO. The hole and electron is attracted by each other due to Coulomb force and form a
Frenkel exciton [34].
b. Exciton diffusion in donor-acceptor interface: Some of the formed excitons diffuse into
donor-acceptor interface within their lifetime.
c. Exciton conversion into charge-transfer state: Donor-acceptor interface is where exciton
dissociation happens. For excitons in the acceptor, there is an energy barrier for electrons
8
to transfer to donor but no barrier for holes. Thus, the hole in the exciton transfers into
HOMO of donor spontaneously. The hole in the donor and the electron in the acceptor form
a charge-transfer exciton or charge transfer state due to the electrostatic interaction [35].
d. Charge-transfer state splitting into free charge carriers: Charge-transfer states eventually
split into pairs of a free hole and an electron which are collected by anode and cathode.
Figure 2.2 Schematic diagram of carrier photo-generation in an OPV cell. Blue arrows
corresponds to exciton diffusion, green arrow represents CT state formation and red arrow shows
the transport of free charge carriers from organic layer to the electrode.
The performance of an OPV cell is evaluated by its current density-voltage (J-V)
characteristics under the illuminance of sunlight. There are three key parameters determined by
the J-V characteristics, namely open-circuit voltage (VOC), short-circuit current (JSC) and fill
factor (FF). The power-conversion efficiency is therefore given by:
𝜂 =𝑉𝑂𝐶𝐽𝑆𝐶𝐹𝐹
𝑃𝑆× 100% (Eq. 1)
where 𝑃𝑆 is the power of input sunlight. Therefore, each parameter needs to be optimized in order
to achieve the best performance of an OPV cell.
10
Chapter 3
Experimental Technique
3.1 Physical Vapor Deposition
Physical vapor deposition (PVD) is one of the commonly used thin-film deposition techniques.
The principle is that the source material is heated above its sublimation temperature in a high-
vacuum environment. Then the sublimated material forms a vapor flux, which reaches the
substrate surface where the material is cooled down and condensed to form a solid thin film.
Figure 3.1.1 shows a schematic diagram of the deposition system employed to deposit organic
thin film for ultra-violet photoemission spectroscopy (UPS) study. The whole chamber is pumped
by a cryogenic pump which keeps the pressure ~10-8 torr. Four different organic materials can be
put into four Knudsen cells (K-Cells) at the same time, which are ready for deposition. There is
also a transfer-arm evaporator (TAE) which allows for quick exchange of organic materials for
deposition and therefore high throughput. A quartz crystal microbalance (QCM) is equipped in
the system in order to accurately monitor the deposition rate and film thickness. The principle of
QCM is that the resonant frequency of a quartz crystal is dependent on the mass of the film.
During the deposition, QCM converts the frequency variation to the mass change per area. Given
a known material density, the thickness of the film can be then calculated. However, the film
thickness on the substrate is usually not equal to the calculated thickness from QCM due to their
different positions. Therefore, a tooling factor must be established in order to calculate the true
thickness on the substrate. The tooling factor is the ratio between the true thickness on the
substrate, independently measured from other instruments
11
such as ellipsometry [36], and the calculated thickness on QCM. In this UPS study, acceptor
material C70 is loaded in one of K-Cells and donor materials are successively loaded in TAE.
Figure 3.1.1 Schematic of components in the organic deposition chamber utilized in this thesis.
Four Knudsen cells (K-Cell) are inside the chamber with their respective shutters on the top.
Transfer-arm evaporator (TAE) is used for quick replacement of organic materials. Quartz crystal
microbalance (QCM) is used to monitor the deposition rate and film thickness. The whole
chamber is pumped by a cryogenic pump.
PVD technique is often used to fabricate OPV devices. Figure 3.1.2 shows the schematic
diagram of the Cluster Tools system for device fabrication. The system consists of different
interconnected chambers. During fabrication, the substrate is firstly put in the load-lock. It is then
transferred through central distribution chamber to different deposition chambers. The solar cell
deposition chamber is used to deposit organic materials and the metal deposition chamber is used
to deposit metal electrode. There is also an OLED (organic light-emitting diode) deposition
chamber which is used to deposit molybdenum oxide interlayer in the OPV cell.
12
Figure 3.1.2 Schematic of chambers in Cluster Tool system for OPV cell fabrication.
3.2 Ultra-violet Photoemission Spectroscopy
Ultra-violet photoemission spectroscopy (UPS) is a powerful technique to study the electronic
structure of the material. Valence electrons feature is well defined in the UPS spectrum since UV
photon has a large photoionization cross section with valence electrons. Figure 3.2.1 shows the
schematic diagram of photoemission process. UV radiation is shined onto the sample surface and
excites valence electrons inside the material. These electrons overcome the binding energy (B.E.)
of their energy level and the work function (Φsample) of the sample, and, therefore, escape from
the sample surface and reach the vacuum with certain kinetic energy (K.E.). The simple
relationship about these parameters is given by:
K. E. = ℎ𝑣 − B. E. −Φsample (Eq. 2)
where ℎ𝑣 is the photon energy of the radiation. In this study, UV radiation is generated by
exciting Helium Iα (ℎ𝑣 = 21.22 eV). Then, the emitted electrons are collected by a spectrometer
which measures the amount of electrons with different kinetic energy. It is worthwhile to note
13
that the kinetic energy measured by the spectrometer is different from that of just emitted
electrons. In a real photoemission setup, sample and electron detector are both grounded, which
means there is a potential difference between sample and detector due to their different work
functions. The measured kinetic energy (K. E.measure) is therefore modified by the work function
difference (Φsample − Φspectrometer). Therefore, the equation is modified as below:
K. E.measure = ℎ𝑣 − B. E. −Φspectrometer (Eq. 3)
Figure 3.2.1 Schematic of photoemission process and kinetic energy measurement by the
spectrometer.
Figure 3.2.2 shows the structure of the spectrometer for UPS measurement. The spectrometer
consists of two major components. One is the electron lens system for focusing and
accelerating/decelerating electrons emitted from the sample. The other one is the hemispherical
electron energy analyzer. There are an inner hemisphere and an outer hemisphere in the analyzer
with their respective radius, R1 and R2. An electrical potential difference (V1-V2) is applied
between these two spheres, which only allows the electron with a certain energy E0 to reach the
multi-channel detector. E0 is called pass energy and is given by:
𝐸0 = 𝑒(𝑉1 − 𝑉2) (𝑅1𝑅2
𝑅22 − 𝑅1
2) (Eq. 4)
14
where 𝑒 is the elemental charge. During UPS measurement, pass energy E0 keeps constant in
order to achieve the same resolution for either low-K.E. or high-K.E. electrons. Then the electron
lens system continuously varies the accelerating/decelerating potential in order to
accelerate/decelerate electrons with different K.E. to reach pass energy. Eventually, the number
of electrons with different K.E. can be measured by the multi-channel detector. A UPS spectrum
is then obtained according to Eq.3. It should be noted that the lower pass energy we select, the
higher spectrum resolution we get. However, the lower pass energy leads to a lower electron
transmission function, which results in a low signal-to-noise ratio in the spectrum. Therefore, it
is important to make a balance between resolution and signal-to-noise ratio in order to obtain a
clear photoemission spectrum.
Figure 3.2.2 Schematic of the spectrometer applied in the UPS facility. UV light excites the
valence electrons which are focused by the electron lens system. The electron lens system
accelerate/decelerate emitted electrons to reach the pass energy E0. Only the electron with E0
energy can successfully pass through the semi-sphere electron analyzer and is then measured by
the multi-channel detector.
A UPS spectrum is composed of two regions. One is the secondary electron (SE) region, and
the other one is valence band (VB) region. Secondary electrons emitted from the sample mainly
contribute to the spectrum in SECO region where the sample work function can be calculated by:
Φsample = ℎ𝑣 − B. E.SECO (Eq. 5)
15
where B. E.SECO is the binding energy of SE cut-off. On the other hand, valence electrons mainly
contributes to the spectrum in the VB region where we can determine the position of valence
band maximum in inorganic semiconductors and HOMO in organic semiconductors, with
reference to the Fermi level, by linear extrapolation of the HOMO leading edge to the noise level.
Figure 3.2.3 shows a UPS spectrum of a 10 nm C70 layer deposited on indium tin oxide (ITO)
coated glass substrate. As shown on the insets, the work function of C70 is calculated to be 4.76
eV in the SE region. And its HOMO is determined to be 1.68 eV below the Fermi level in the VB
region.
Figure 3.2.3 UPS spectrum of 10 nm C70 deposited on ITO-coated glass substrate. For insets, the
left one corresponds to secondary electron (SE) region, and the right one is valence band (VB)
region.
3.3 Current density-voltage characteristic measurement
Current density-voltage (J-V) characteristic under sunlight illumination is commonly used to
evaluate the device performance of an OPV cell. Figure 3.3.1 shows the schematic diagram of
the solar cell testing system used in this thesis. The sunlight simulator, LCS-100, is calibrated
using NREL-traceable silicon reference cell to reach one sun condition. Then the simulated
16
sunlight shines onto the OPV cell which is connected to a power supply, Keithley 2400 unit, to
measure the J-V characteristic.
Figure 3.3.1 Schematic of the facility for J-V measurement.
Three key parameters can be obtained from a J-V characteristic curve, namely open-circuit
voltage (VOC), short-circuit current density (JSC) and fill factor (FF). Figure 3.3.2 shows an
arbitrary J-V curve. VOC is determined by the intercept on voltage axis and JSC is determined by
the intercept on current density axis. One special point can be found on the curve in the first
quadrant, which makes the maximum product of voltage (V) and current density (J). Therefore,
this point corresponds to the maximum output of the solar cell. FF is then given by:
FF =𝐽𝑉
𝐽𝑆𝐶𝑉𝑂𝐶× 100% (Eq. 6)
The power-conversion efficiency (η) of an OPV cell is then calculated according to Eq.1
17
Figure 3.3.2 A typical J-V characteristic curve of an OPV cell. Three key parameters, VOC, JSC
and FF can be determined from this J-V curve.
18
Chapter 4
Energy Level Alignment at Donor-acceptor
Interfaces
4.1 Introduction
As discussed in section 2.2, the free charge carrier generation in an OPV cell involves four
processes: exciton formation, exciton diffusion, exciton conversion to charge-transfer (CT) states
and CT states splitting into free charge carriers. The first two processes are highly dependent on
the properties of donor and acceptor materials. And the last two processes are mainly determined
by the properties of donor-acceptor interfaces. Energy level alignment at donor-acceptor
interfaces is a critical property to determine the performance of an OPV cell since exciton
dissociation into free charge carriers is an energy-driven process which occurs at donor-acceptor
interfaces. For instance, there has been a bunch of literature showing VOC of the OPV cell is
highly dependent on donor-acceptor energy offset EDA (the energy difference between the donor’s
HOMO and the acceptor’s LUMO) [21-27]. In addition, the free charge carrier generation at
donor-acceptor interfaces is also found to be dependent on EDA [37], which is an important factor
determining JSC. Therefore, it is meaningful for people to study and understand the energy level
alignment at donor-acceptor interfaces in order to design and fabricate high-performance OPV
cell.
19
Some work has been previously conducted on the effect of substrate work function on the
energy level alignment at organic-organic interfaces [38-42]. The results often show that the
energy level alignment at interfaces can be tuned by changing different substrate. For example,
Zhou et al. [38] finds that EDA at copper phthalocyanine-C60 interface can vary from 0.66 eV to
1.03 eV by replacing ITO with magnesium substrate. Zou et al. [42] also claims that molybdenum
oxide (MoOX) modified ITO can increase EDA at pentacene-C60 interface from 0.86 eV to 1.54
eV. However, these studies are always restricted to the case of one particular donor-acceptor
interface. Thus, the results may not be applicable to other donor-acceptor interfaces. Obviously,
no universal rule can be discovered from one specific interface and, therefore, no guideline can
be established for designing high-performance OPV cells unless a broader set of donor-acceptor
interfaces is studied.
In this chapter, the effect of substrate work function on energy level alignment at different
donor-acceptor interfaces is well studied. Energy level alignment at a broad range of donor-
acceptor interfaces is studied by using in situ ultra-violet photoemission spectroscopy (UPS).
4.2 Methods
The substrate used for the deposition is indium tin oxide (ITO) coated glass substrate, which
is a well-known anode for organic electronic device. ITO-coated glass substrate is through
standard solution cleaning process before deposition: (1) Water cleaning with soap; (2) Ultra-
sonic cleaning with acetone for 15 mins; (3) Ultra-sonic cleaning with methanol for 10 mins; (4)
UV ozone treatment for 15 mins.
In order to vary substrate work function, a 2.5 nm Molybdenum oxide (MoOX) layer is
deposited on the top of ITO since MoOX has a much larger work function than a bare ITO
substrate. Figure 4.2.1 shows the UPS spectrum of a bare ITO substrate and a MoOX-coated ITO
substrate respectively. The work function of ITO is 4.83 eV while the work function of MoOX-
coated ITO is 6.81 eV. The work function difference of these two anode substrate is 1.98 eV,
which is large enough to study the impact of substrate work function on energy level alignment
at donor-acceptor interfaces.
20
Figure 4.2.1 UPS spectrum of bare ITO substrate (solid curve) and MoOX-coated ITO substrate
(dashed curve). The corresponding work function is labeled.
Figure 4.2.2 shows the picture of the experimental facility for in situ UPS measurement. ITO-
coated glass substrate is immediately put into the load-lock after cleaning. Then the substrate is
transferred into the organic deposition chamber through central distribution chamber to deposit a
10 nm donor layer. After deposition, the sample is in situ transferred into photoemission chamber
for UPS analysis. Now, the position of donor’s energy levels can be extracted from its UPS
spectrum. In order to study the energy level alignment between donor and acceptor, a second
layer of acceptor material must be deposited on the top of the donor layer. Therefore, the sample
after UPS measurement is transferred again into organic deposition chamber to deposit 1 nm
acceptor layer on the top of 10 nm donor layer. After deposition, the sample is transferred into
photoemission chamber for UPS measurement. Thus, the energy level position of the acceptor
material is known. The energy level diagram of this donor-acceptor interface can be drawn by
combining energy level position of the donor and the acceptor due to Fermi-level equilibrium.
For the study of donor-acceptor energy level alignment on MoOX-coated ITO substrate, the bare
ITO substrate is firstly transferred into MoOX deposition chamber the deposition of the donor
material.
21
Figure 4.2.2 the photograph of the multi-access chamber (MAC) cluster tool and PHI 5500
surface analysis system used for in situ UPS measurement of donor-acceptor interfaces. The
major components are labelled.
The donor materials used in this study are tetraphenyldibenzoperiflanthene (DBP); copper
phthalocyanine (CuPc); 4,4’,4”-Tris(3-methylphenylphenylamino)triphenylamine (m-
MTDATA); 4,4,4-trisN,-2-naphthyl-N- phenylamino-triphenylamine (2T-NATA); 1,1-bis-(4-
bis(4- methyl-phenyl)-amino-phenyl)-cyclohexane (TAPC); 4,4’-Bis(N-carbazolyl)-1,1’-
biphenyl (CBP); 9-(4-tert-butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole (CzSi) and 1,3-
Bis(N-carbazolyl)benzene (mCP). And the acceptor material is fullerene C70. The reason to
choose C70 as the acceptor is that C70 is a very promising acceptor material due to its good
absorption at visible spectrum of the sunlight, which makes the efficiency of the C70-based OPV
cell higher than traditional one such as the C60-based OPV cell [43]. In addition, even though C70
is a ‘hot’ material used in OPV community, there is still a lack of experimental UPS measurement
on C70-based organic heterojunction, which is an important knowledge for people to understand,
design and optimize the C70-based OPV cell.
Figure 4.2.3 shows the structural formula of the donor and the acceptor materials mentioned
above as well as their respective energy levels reported in the literature [44-51].
22
Figure 4.2.3 Energy levels (HOMO & LUMO) of donor and acceptor materials used in this study.
Structural formula of these molecules are also shown.
4.3 Results & Discussion
Figure 4.3. 1 (a) shows the UPS raw spectra of DBP-C70 interface on bare ITO substrate and
MoOX-coated ITO substrate respectively. The spectrum in the SE region is shifted so that the
intercept of SE cut-off with energy axis is equal to the work function. Therefore, the SE cut-off
represents the vacuum level and the whole spectrum directly shows the energy level positions of
the donor and the acceptor. Figure 4.3. 1 (b) (c) show the deduced energy level diagram of DBP-
C70 interface on ITO substrate and MoOX-coated ITO substrate, respectively. The LUMO
positions of DBP and C70 are calculated according to their bandgaps reported in the literature (2.0
eV for DBP, 2.2 eV for C70) [52]. As shown in the diagram, the HOMO offset (energy difference
between HOMO and Fermi-level) of DBP is 0.84 eV and 0.57 eV on ITO substrate and MoOX-
coated ITO substrate respectively. The HOMO offset is minimized by the substrate
23
Figure 4.3.1 (a) UPS spectra of DBP-C70 interface measured on ITO substrate and MoOX-coated
ITO substrate, respectively; (b) Energy level diagram of DBP-C70 interface on ITO substrate; (c)
Energy level diagram of DBP-C70 interface on MoOX-coated ITO substrate.
with high work function. According to the universal energy alignment rule at metal oxide-organic
interface, if the work function of the metal oxide is larger than the ionization energy (IE)of the
organic semiconductor, chemical equilibrium will drive the electron in organic semiconductor to
flow into the metal oxide, leaving the HOMO offset of the organic molecule to be ~0.3 eV [53].
In this case, the work function of MoOX-coated ITO substrate is 6.81 eV which is way larger than
the IE (work function plus HOMO offset) of DBP, 5.4 eV. Therefore, the HOMO offset is
minimized as compared with that of DBP deposited on ITO (work function is 4.83 eV). However,
the HOMO offset of DBP is not comparable to ~0.3 eV reported in the literature, which is
attributed to the dielectric hole screening effect [54]. Figure 4.3. 2 (a) (b) shows the UPS valence
band spectra of DBP with different thicknesses deposited on MoOX-coated ITO substrate and
bare ITO substrate, respectively. The HOMO offset remains to be 0.22 eV on MoOX and 0.78 eV
on ITO substrate when the thickness is below 3 nm. The spectrum is shifted to lower energy when
the thickness continues increasing.
One may argue that whether the dielectric hole screening will affect the UPS results since the
UPS measurement is conducted on 1 nm C70 on the top of 10 nm donor layer. Figure 4.3.2 (c)
24
Figure 4.3.2 (a) UPS valence band spectra of DBP with different thicknesses deposited on the top
of MoOX-coated ITO substrate; (b) UPS valence band spectra of DBP with different thicknesses
deposited on the top of ITO substrate; (c) Zooming-up of UPS valence band spectra measured
from 1 nm C70 on the top of 10 nm DBP deposited on ITO and MoOX-coated ITO substrate,
respectively. DBP’s HOMO is observable due to the small thickness of C70 layer.
25
shows the blow-up UPS valence spectra measured from 1 nm C70 layer on 10 nm DBP layer
deposited on ITO substrate and MoOX-coated ITO substrate, respectively. The HOMO of DBP
can still be observed thank to the small thickness of the overlayer. The HOMO offset is 0.84 eV
and 0.57 eV on ITO and MoOX-coated ITO substrate, respectively, which is exactly the same as
that measured from a pure 10 nm DBP layer as shown in 4.3.1. This means that a 1 nm C70
overlayer does not aggravate the dielectric hole screening effect, which shifts the spectrum of
both DBP and C70 towards lower energy with the same magnitude. Thus, the relative positions of
HOMO of DBP and C70 remains to be fixed and the energy level diagram deduced from the these
UPS data is reliable.
In order to compare the energy level alignment at donor-acceptor interfaces on two different
substrates, the donor-acceptor energy offset EDA, which is the energy difference between the
donor’s HOMO and the acceptor’s LUMO, is chosen as a criterion since EDA is a very important
parameter for the OPV cell. As shown in Figure 4.3.1 (b) (c), EDA at the DBP-C70 interface is
1.33 eV and 1.25 eV on ITO substrate and MoOX-coated ITO substrate, respectively. The
difference is less than 0.10 eV, which means that substrate work function actually makes
negligible influence on the energy level alignment at the DBP-C70 interface. In addition to EDA,
the interfacial dipole Δ can also be used to evaluate the band alignment. The interfacial dipole Δ
is 0.10 eV and 0.07 eV on ITO and MoOX-coated ITO substrate, which means the extent of the
charge transfer at the interface on different substrates is almost the same. The experimental results
actually disapprove the literature reports.
In order to rule out the possibility that DBP-C70 interface is a special case, UPS measurements
are successively conducted on other selected donor-C70 interfaces. Their UPS raw spectra and
deduced energy level diagrams are shown in Figure 4.3.3~4.3.9. The LUMO positions of all the
donor and the acceptor C70 are calculated by adding their respective bandgaps cited from the
literature. For all cases, the energy level alignment at the donor-C70 interface on ITO substrate is
similar to that on MoOX-coated ITO substrate. (Interested readers are recommended to explore
these figures in details).
26
Figure 4.3.3 (a) UPS spectra of CuPc-C70 interface measured on ITO substrate and MoOX-coated
ITO substrate, respectively; (b) Energy level diagram of CuPc-C70 interface on ITO substrate; (c)
Energy level diagram of CuPc-C70 interface on MoOX-coated ITO substrate.
Figure 4.3.4 (a) UPS spectra of m-MTDATA-C70 interface measured on ITO substrate and
MoOX-coated ITO substrate, respectively; (b) Energy level diagram of m-MTDATA-C70
interface on ITO substrate; (c) Energy level diagram of m-MTDATA-C70 interface on MoOX-
coated ITO substrate.
27
Figure 4.3.5 (a) UPS spectra of 2T-NATA-C70 interface measured on ITO substrate and MoOX-
coated ITO substrate, respectively; (b) Energy level diagram of 2T-NATA-C70 interface on ITO
substrate; (c) Energy level diagram of 2T-NATA-C70 interface on MoOX-coated ITO substrate.
Figure 4.3.6 (a) UPS spectra of TAPC-C70 interface measured on ITO substrate and MoOX-coated
ITO substrate, respectively; (b) Energy level diagram of TAPC-C70 interface on ITO substrate;
(c) Energy level diagram of TAPC-C70 interface on MoOX-coated ITO substrate.
28
Figure 4.3.7 (a) UPS spectra of mCP-C70 interface measured on ITO substrate and MoOX-coated
ITO substrate, respectively; (b) Energy level diagram of mCP-C70 interface on ITO substrate; (c)
Energy level diagram of mCP-C70 interface on MoOX-coated ITO substrate.
Figure 4.3.8 (a) UPS spectra of CBP-C70 interface measured on ITO substrate and MoOX-coated
ITO substrate, respectively; (b) Energy level diagram of CBP-C70 interface on ITO substrate; (c)
Energy level diagram of CBP-C70 interface on MoOX-coated ITO substrate.
29
Figure 4.3.9 (a) UPS spectra of CzSi-C70 interface measured on ITO substrate and MoOX-coated
ITO substrate, respectively; (b) Energy level diagram of CzSi-C70 interface on ITO substrate; (c)
Energy level diagram of CzSi-C70 interface on MoOX-coated ITO substrate.
Figure 4.3.10 shows EDA measured on ITO substrate versus EDA measured on MoOX-coated
ITO substrate for all studied donor-C70 interfaces. There is no significant difference between EDA
measured on two substrates for all interfaces. This means that changing substrate work function
is not a valid approach to tune the energy level alignment between donor and acceptor. The
discrepancy from the previous literature results originates from the different region we are dealing
with. In those cases, one of the chosen substrates forces the Fermi-level to be strongly pinned to
the energy level of over-layer organic molecule. The significant difference of the work function
between under-layer and over-layer molecule leads to a large interfacial dipole which influences
the energy level alignment at the interface. However, in our cases, both substrates hardly make
the Fermi-level to be pinned to the LUMO of C70. There are also some cases (m-MTDATA/C70
and TAPC/C70 on ITO) which fall into the pinning region. However, the energy level alignment
at the donor-acceptor interfaces still shows no dependence on the substrate work function because,
in these cases, the LUMO of C70 is just pinned to the Fermi-level and the the vacuum level
alignment is still expected at these interfaces. This is why our results give a conclusion
contradictory to others’ results [38-42].
30
Figure 4.3.10 EDA measured on ITO substrate versus EDA measured on MoOX-coated ITO
substrate. Substrate work function makes little difference to the energy level alignment at donor-
acceptor interfaces.
4.4 Summary
In summary, in situ UPS technique has been conducted on a wide set donor-C70 interfaces
deposited on two different substrates, ITO substrate and MoOX–coated ITO substrate. The
substrate work function is found to have negligible impact on energy level alignment at all these
interfaces. The result shows that the energy level alignment at donor-acceptor interfaces cannot
be significantly changed by changing substrate work function. Therefore, switching different
substrates is not an effective approach to tune the device performance of the OPV cells. Our
results involves a broad range of donor-acceptor interfaces and, therefore, is much more
convincing than those literature results which always focus on one donor-acceptor heterojunction.
31
Previous reports about substrate dependence on donor-acceptor interfaces is not contradictory to
our results due to the different regime we are dealing with, but they can be misleading in terms
of designing and optimizing the novel high-performance OPV cell.
32
Chapter 5
Correlation between VOC and EDA
5.1 Introduction
As mentioned in section 2.2, the open-circuit voltage (VOC) is one of the key parameters to
determine the power-conversion efficiency of an OPV cell. Increasing VOC is, therefore, an
effective approach to achieve high-efficiency OPV cells. For instance, Mutolo et al. [7] reported
that the efficiency of SubPc-C60 planar OPV cell is almost doubled as that of CuPc-C60 cell due
to the significant VOC increase. Now, the question comes: what determines the VOC of an OPV
cell?
A bunch of experimental and theoretical studies has been conducted in order to explore the
factors that determines the VOC of OPV cells. The classical metal-insulator-metal (MIM) model
predicts that VOC is determined by the built-in potential generated by the work function difference
of the anode and the cathode [26]. However, it is found that the VOC of an OPV cell is more
related to the electronic structure at donor-acceptor interfaces [21-27]. Scharber et al. [26]
claimed that the VOC of the OPV cell is dependent on the donor-acceptor energy offset EDA, which
is the energy difference between the donor’s HOMO and the acceptor’s LUMO. Wilke et al. [21]
also found that VOC has a linearly dependence on EDA. Blakesley et al. [25] derived a theoretical
VOC formula based on quasi-Fermi level splitting and Boltzmann approximation, which shows
that VOC linearly increase with EDA with an offset determined by the free charge carrier
concentration inside the cell. In contrast, Graham et al. [55] shows that the VOC is more related
to charge transfer (CT) state energy than EDA. And Burke et al. [56] derived a theoretical
33
VOC equation, based on the chemical equilibrium between free charge carriers and charge transfer
states, which shows that VOC is highly dependent on CT state energy. Therefore, there is still a
discrepancy in the OPV community and more experimental and theoretical studies on the
relationship between VOC and donor-acceptor interface energetics are required to solve this
problem.
In this chapter, we use physical vapor deposition (PVD) technique to fabricate different OPV
cells based on the donor and the acceptor material used in the UPS study. VOC of all the cells are
measured through current density-voltage (J-V) measurement under simulated sunlight. The
correlation between VOC and EDA, which is extracted from the UPS measurement, is presented.
Furthermore, a mathematic model is developed in order to fit the observed trend. The proposed
theory sheds light on how to design an OPV cell with a maximum VOC in terms of donor-acceptor
energy levels.
5.2 Methods
The OPV cell fabrication is done through the PVD technique. The substrate is a patterned ITO-
coated glass susbtrate with a sheet resistance of 15 Ω/sq. The substrate is subjected to the standard
solution cleaning and UV ozone treatment as mentioned in section 4.2. After cleaning, the
substrate is immediately loaded into the K.J. Lesker Luminos vacuum thermal evaporation
system for deposition. The photograph of the facility is shown as below:
Figure 5.2.1 The photograph of the Cluster Tool Vacuum Deposition System for OPV cells
fabrication.
34
The donor and the acceptor materials used in this study is consistent with those in the UPS
measurement, which is shown in Figure 4.2.3. The device structure is the planar-heterojunction
structure, which is shown in Figure 5.2.2. A 2.5 nm MoOX buffer layer is first deposited onto the
clean ITO-coated glass substrate, which is necessary to fabricate an environmentally stable device
(More discussion in section 5.3). 10 nm donor layer and 40 nm C70 layer are subsequently
deposited. A 5 nm BCP layer is deposited on the top, which is one of the commonly used exciton-
blocking layers, follow by the final deposition of 100 nm Ag as the cathode.
Figure 5.2.2 The device structure of the OPV cells used in this study.
Current density-voltage (J-V) characteristics are measured under one sun condition. The
defined active area of each pixel is 2 mm×1 mm. The voltage range for the test is from -1.0 V to
1.5 V. Voltage step is set to be 0.02 V.
5.3 Results & Discussion
5.3.1 Device Stability
As discussed in section 4.3, the substrate effect on the energy level alignment at donor-acceptor
interface is negligible. However, it is necessary to modify the anode surface in order to achieve
a stable and high-performance device. Figure 5.3.1.1 (a) shows the J-V characteristics of DBP/C70
35
device with and without MoOX anode buffer layer, respectively. For ITO-only device, VOC is
0.70 V, JSC is 5.95 mA/cm2 and FF is 54.76 %. The power-conversion efficiency of this device is
2.28 %. For MoOX device, VOC is 0.87 V, JSC is 5.55 mA/cm2 and FF is 63.47 %, which gives an
efficiency of 3.07 %. The MoOX interlayer significantly improve the device performance though
increasing VOC and FF. Figure 5.3.1.1 (b) shows the plot of measured VOC against the
measurement time. The measurement time is recorded once the device is taken out of the
fabrication system. It clearly shows that MoOX device yields a much more stable VOC as
compared with the ITO-only device.
Figure 5.3.1.1 (a) J-V characteristics of DBP/C70 OPV cell with and without MoOX anode buffer
layer; (b) Measured open-circuit voltage as a function of the time, recorded once the device is
taken out of the vacuum deposition chamber.
The increase in VOC may originate from two aspects. One is the decrease of the hole injection
barrier at anode/DBP interface (0.22 eV on MoOX and 0.78 eV on ITO, see Figure 4.3.2), which
leads to a decrease of energy loss at this interface. The second possible reason is the measurement
error induced by the VOC degradation. The measured VOC is recorded from a device experiencing
the degradation. The mechanism behind the improved stability by inserting a MoOX buffer layer
is still not fully understood, the hydrophilicity of MoOX has been suggested as one reason for the
improved device performance [57,58]. Therefore, our OPV cells are fabricated on the MoOX-
coated ITO substrates, which yield stable VOC for quantitative analysis.
36
5.3.2 Correlation between VOC and EDA
Figure 5.3.2.1 shows all J-V characteristic curves for studied OPV cells. A normal J-V curve
for a solar cell possesses an exponential shape, e.g. DBP/C70, CuPc/C70. However, other curves
more or less show ‘S shape’ features, which is also frequently reported in the literature [59-61].
This ‘S’ kink is thought to results from the reduced recombination velocity at the anode interface
and the imbalanced charge mobilities in donor and acceptor [62-63]. VOC of all OPV cells is then
extracted to study its relationship with EDA.
Figure 5.3.2.1. J-V characteristics of various planar-heterojunction OPV cells used in this study.
Figure 5.3.2.2 shows the plot of the measured VOC as a function of measured EDA. The VOC of
SubPc/C70 device is taken from the literature where a similar device structure is employed [64].
The EDA of SubPc-C70 heterojunction is estimated according to the energy level position reported
in the literature [7,51]. It can be clear found that VOC linearly increase with EDA up to EDA ~ 1.5
eV. By a linear regression fit, the empirical equation for VOC in this regime is given by:
eVOC = EDA − 𝛽 (Eq. 8)
where β=0.40 eV in our case. This linear increase trend is in a good agreement with the literature
results. Scharber et al. [26] reported that VOC linearly increase with EDA with β=0.3 eV in bulk-
37
heterojunction cells. Wike et al. [21] also showed that β=0.47 eV for planar-heterojunction cells.
Surprisingly, in our result, a reverse trend is observed when EDA is higher than 1.7 eV. This
reverse trend is scarcely reported in the literature and the significant deviation from the linear
increase trend need to be explained theoretically.
Figure 5.3.2.2 Experimentally measured VOC against the EDA measured by UPS. The blue and red
lines are calculated according to the proposed model. The theory shows that the maximum VOC
can be achieved at a transition EDA point (dash line). The data of SubPc/C70 device is taken from
the literature.
In order to explain the experimental results, a simple mathematic model is developed. As
mentioned in section 2.2, the free charge carrier generation mainly involves two processes. The
first process is that the exciton converts into the charge transfer (CT) exciton (or state) at donor-
acceptor interface. The second process is that the CT exciton splits into one pair of free hole and
electron. Below shown are these two key processes with their respective energy difference
between final state and initial state (ΔE):
38
Singlet → CT exciton ∆ES→CT = (EDA + 𝛼) − ES (Eq. 9)
CT exciton → free carriers ∆ECT→fc = EDA − (EDA + 𝛼) (Eq. 10)
where the exciton is assumed to be the singlet of C70 since the selected donor materials usually
have a much wider band gap than that of C70. The most part of visible spectrum is absorbed by
C70. There are some donors such as CuPc, DBP and SubPc which also have a good absorption in
the visible spectrum. Readers will find that these special cases also follow the proposed model
based on the singlet energy of C70. Its reason will be provided later. The energy of CT exciton is
equal to EDA plus a constant 𝛼. In the early days, scientists measured the CT exciton emission in
solution, which yields 𝛼 = 0.15 eV [65]. A recent study on the CT exciton emission of different
donor-acceptor heterojunction shows that 𝛼 = 0.20 eV [66]. In the following calculation, we
choose 𝛼 = 0.20 eV which is consistent with the literature result. The energy of a free hole-
electron pair is equal to EDA.
As clearly shown in Eq.10, there is no energy barrier for a CT exciton to convert into the free
charge carriers, which means that the free carrier generation is not restricted by CT exciton
splitting. However, in Eq.9, the energy difference is dependent on EDA and other parameters are
constants. A transition EDA point (EDA0 ) can be calculated by equating Eq.9 to zero:
EDA0 = ES − 𝛼 (Eq. 11)
There will be an energy barrier for singlet conversion into CT exciton if EDA > EDA0 . The Miller-
Abrahams hopping model [67] is applied to quantify this process. The Miller-Abrahams hopping
theory was developed to describe the charge transport process in disordered system in 1960.
Organic semiconductors are amorphous materials. The charge is highly located inside the organic
molecule. The charge transport between molecule and molecule is through a hopping process
involving the tunneling and thermal activation. A simple Miller-Abrahams equation is given as
below:
𝑘 = 𝑘0 exp (−∆E
𝑘𝐵𝑇) when ∆E ≥ 0 (Eq. 12)
𝑘 = 𝑘0 when ∆E < 0 (Eq. 13)
where 𝑘 is the hopping rate, 𝑘0 is a constant including the tunneling term, 𝑘𝐵 is the Boltzmann
constant and 𝑇 is the temperature. The exponential term refers to the thermal activation. Thus,
39
the hopping rate is controlled by tunneling effect with no energy barrier. However, the thermal
activation kicks in when an energy barrier exists. In our case, singlet conversion into CT exciton
is also a hopping process that the hole of the singlet generated in the acceptor hops into the donor.
Therefore, the Miller-Abrahams hopping theory is applicable to the exciton dissociation at the
donor-acceptor interface. In fact, the Miller-Abrahams hopping theory as well as the Marcus
electron transfer theory are widely used in the literature to describe the charge generation as well
as the charge transport in OPV cells [24, 68-72].
By combining the Miller-Abrahams hopping equation with Eq.9, we arrive to the equation for
singlet conversion rate (𝑘S→CT) as shown below:
𝑘S→CT = 𝑘0 exp (−∆E
𝑘𝐵𝑇) when EDA ≥ EDA
0 (Eq. 14)
𝑘S→CT = 𝑘0 when EDA < EDA0 (Eq. 15)
Assuming the final free charge carrier concentration is proportional to this conversion rate
under open-circuit condition, a similar equation for free charge carrier concentration (n and p) is
obtained:
𝑛 = 𝑝 = 𝐴 exp (−∆E
𝑘𝐵𝑇) when EDA ≥ EDA
0 (Eq. 16)
𝑛 = 𝑝 = 𝐴 when EDA < EDA0 (Eq. 17)
where A refers to the free charge carrier concentration under a certain light intensity when no
energy barrier exists between CT exciton and singlet state. A theoretical formula for VOC has
been already derived by Blakesley et al. [25] based on the quasi-Fermi level splitting and
Boltzmann approximation (See details in the Appendix B):
eVOC = EDA − 𝑘𝐵𝑇ln (𝑁𝑛𝑁𝑝
𝑛𝑝) (Eq. 18)
where 𝑁𝑛 and 𝑁𝑝 are the respective density of states for electrons in acceptor and holes in donor.
By plugging the free carrier concentration in Eq.16 and Eq.17 into Eq.18, we are able to calculate
a new formula for VOC as below:
eVOC = −EDA + 2(ES − 𝛼) − 𝑘𝐵𝑇ln (𝑁𝑛𝑁𝑝
𝐴2) 𝐸𝐷𝐴 > 𝐸𝑆 − 𝛼 (Eq. 19)
40
eVOC = EDA − 𝑘𝐵𝑇ln (𝑁𝑛𝑁𝑝
𝐴2) 𝐸𝐷𝐴 ≤ 𝐸𝑆 − 𝛼 (Eq. 20)
As clearly shown in the new formula, VOC will linearly increase with EDA if EDA is lower than
EDA0 . However, a complete reverse trend is predicted by this model when EDA is higher than EDA
0 .
Here, the term 𝑘𝑇ln (𝑁𝑛𝑁𝑝
𝐴2) can be treated as a constant and is equal to β according to Eq.8. Eq.19
and Eq.20 is calculated using all known parameters (β=0.4 eV, ES=1.9 eV [86] and 𝛼=0.2 eV)
and plotted 5.3.2.1 (Blue and red solid lines). This pair of theoretical lines explained quite well
the experimental data. The transition EDA in this case is 1.7 eV, which corresponds to the
maximum achievable VOC (1.3 V) in C70-based donor-acceptor heterojunction device.
Now, it is time to explain why those special cases mentioned above also follow this theory. In
the case where both donor and acceptor absorb the visible light, the total amount of singlets is
shared by the donor and the acceptor with a certain ratio. The singlet from both sides will convert
into CT exciton. The three special cases, e.g. CuPc, DBP and SubPc, are in the linear increase
regime, which means the singlet generated in these donor materials also experiences no energy
barrier during the conversion into CT exciton. For CuPc, the singlet energy is 1.7 eV [73] which
is high than the CT exciton energy (~0.8+0.2 eV); For DBP and SubPc, the singlet energy is 2.0
eV [74] and 2.1 eV [75] which is even higher than that of C70. Therefore, in these cases, the
singlets from both sides experience no energy barrier, which is exactly why these data points still
follow the theoretical trend.
In order to validate our proposed theory, a bunch of literature data is collected regarding VOC
and EDA [21,26,27,76-85]. These data includes the planar-heterojunction cells based on the
acceptor C60 as well as the bulk-heterojunction cells based on [6,6]-phenyl-C61-butyric acid
methyl ester (PCBM) as the acceptor. To apply our theory to different OPV systems, Eq.19 and
Eq.20 are mathematically modified as below:
eVOC + 𝛽 − ES = −(EDA − ES) − 2𝛼 (EDA − ES) > −𝛼 (Eq. 21)
eVOC + 𝛽 − ES = (EDA − ES) (EDA − ES) ≤ −𝛼 (Eq. 22)
According to Eq.21 and Eq.22, one is able to plot a universal plot where eVOC + 𝛽 − ES is the
ordinate and EDA − ES is the abscissa. The value of 𝛽 is obtained by the linear regression fit in
the increase regime for different acceptor systems. 𝛽 is 0.58 eV for C60-based planar-
heterojunction cells and 0.35 eV for PCBM-based bulk-heterojunction cells (See Appendix C).
41
The singlet energy of C60 and PCBM is 2.0 eV [87] and 1.7 eV [88], respectively. By plugging
these parameters, a universal plot is obtained as below:
Figure 5.3.2.3 The Universal plot of VOC versus EDA. The blue and red solid line is plotted
according to Eq.21 and Eq.22. The data point of C60 planar-heterojunction (PHJ, empty circles)
cells and PCBM bulk-heterojunction (BHJ, empty squares) cells are collected from various
literature papers. The value of 𝛽 is obtained by the linear regression fit in the increase regime.
Our measured data including the SubPc literature data are shown as the black star.
In Figure 5.3.2.3, our theoretical lines are in an excellent agreement with the literature data as
well as our own data. Therefore, the proposed model is applicable to not only different donor-
acceptor systems, but also the bulk-heterojunction OPV cells.
5.4 Summary
In summary, different donor materials as well as the acceptor C70 are used to fabricate planar-
heterojunction OPV cells. J-V measurement is conducted on each cell to record its VOC. VOC is
42
found to linearly increase with EDA with EDA up to 1.5 eV, while decrease when EDA is larger
than 1.7 eV. The linear increase regime agrees well with the literature results. However, the
reverse trend is hardly reported in the literature. Therefore, a mathematic model based on a two-
step process for a singlet exciton to dissociate into free charge carriers is proposed. The model
shows that the free charge carrier generation is dominated by the conversion process from a
singlet exciton to CT exciton. By applying the Miller-Abrahams hopping theory, the conversion
rate is then quantified, which leads to a new formula for VOC. This model predicts that there is a
transition EDA point (EDA0 = ES − 𝛼), which determines the maximum achievable VOC (1.3 V for
C70-based donor-acceptor OPV cells). VOC linearly increases or decreases with EDA when EDA is
lower or higher than EDA0 . The proposed model is validated by collecting the literature data for
C60-based planar-heterojunction cells and PCBM-based bulk-heterojunction cells. Hence, the
model is applicable to different donor-acceptor systems as well as different device structures
(planar and bulk). The proposed model establishes a guideline for designing novel organic
materials with proper energy levels to achieve the OPV cells with maximum VOC.
43
Chapter 6
Conclusion and Future Work
In this thesis, two major issues regarding OPV cells are addressed. The first one is the energy
level alignment at donor-acceptor interface, which is an important property to determine the
performance of an OPV cell. By using two substrates with different work functions, the impact
of substrate work function on the energy level alignment at donor-acceptor interfaces is
systematically studied based on different donor-acceptor heterojunctions. It is found that the
substrate work function has negligible influence on the energy levels, which makes a
contradiction to the literature results. This discrepancy is attributed to the different alignment
regime. The second issue is the correlation between VOC and EDA, which is still a hot topic in the
OPV community. The experimental results show that VOC linearly increase with EDA when EDA
is below a certain point, which is in a good agreement with literature results. However, a reverse
trend at a high-EDA regime is observed, which is hardly reported in the literature. A mathematic
model based on a two-step process for a singlet exciton to dissociate into free charge carrier is
developed in order to quantify the relationship between VOC and EDA. The theoretical trend line
describes quite well of the experimentally observed trend. A bunch of literature data regarding
different donor-acceptor systems as well as different device structures is collected to validate the
proposed theory. Our theory is proven to be valid for both planar-heterojunction and bulk-
heterojunction OPV cells, which establishes a guideline for designing organic materials with
proper energy levels in order to achieve the maximum VOC.
Even though the performed experiments and the proposed theory have proven successful, there
remains some unanswered questions and related opportunities in the future research:
44
There is still a discrepancy between theory and experimental results regarding the energy
of charge-transfer exciton. From the point of physics, the energy of CT state is equal to
EDA minus the binding energy, while the CT exciton emission measured in both solution
and vacuum deposited donor-acceptor heterojunction shows that the CT state energy is
actually larger than EDA [65,66]. Therefore, more experimental data are required to
resolve this problem.
Short-circuit current is also an important parameter in OPV cells and could be a potential
indicator of donor-acceptor interface properties, but it also involves the charge transport
property in donor and acceptor materials as well as the charge extraction process at the
electrode/active layer interface, which complicates the whole picture. Therefore, a theory
about JSC considering all these properties is required to achieve a high-performance
organic solar cell.
In conclusion, the organic photovoltaic cell has become a potential candidate for the next-
generation commercial solar cells due to its low-cost, flexibility and eco-friendly nature. There
is still a large space for improving the power-conversion efficiency, which requires people to
fully understand the working mechanism behind. Hence, a large number of both experimental
and theoretical studies is imperative to guide the design of high-end organic solar cells.
45
Appendix A
Photoemission Spectroscopy Calibration
Gold sample is used to calibrate the binding energy of the photoemission spectroscopy. The
work function of the spectrometer in the software is first adjusted to make sure that the middle
point of the Fermi edge is at 0±0.05 eV binding energy and -15±0.05 eV binding energy for XPS
and UPS (-15 V bias is applied) respectively. Then the spectrometer is adjusted so that the Au
4f7/2 core-level binding energy is at 84.00±0.05 eV.
46
Appendix B
Quasi-Fermi level and VOC
Figure B Quasi Fermi level splitting under illumination for planar-heterojunction (PHJ) and bulk-
heterojunction (BHJ) OPV cells.
Under solar illumination, the photo-generated free holes and electrons occupy their respective
energy levels, HOMO in donor and LUMO in acceptor. Such condition is called quasi-
equilibrium. Holes and electrons establish the quasi-equilibrium with their own quasi-Fermi level,
which represents the electrochemical potential of the free charge carriers. As shown in Figure B,
in a planar heterojunction, the quasi-Fermi level of holes is in the donor layer and that
47
of electrons is in the acceptor layer. For Bulk-heterojunction, these two quasi-Fermi levels are in
the same mixing layer. The open-circuit voltage is determined by the difference between the
quasi-Fermi level for holes and that for electrons:
eVOC = EF,n − EF,p (Eq. B. 1)
The quasi-equilibrium follows Boltzmann function:
𝑛 = 𝑁𝐴 exp (𝐸𝐹,𝑛 − 𝐿𝑈𝑀𝑂𝐴
𝑘𝐵𝑇) (Eq. B. 2)
𝑝 = 𝑁𝐷 exp (𝐻𝑂𝑀𝑂𝐷 − 𝐸𝐹,𝑝
𝑘𝐵𝑇) (Eq. B. 3)
By plugging Eq.B.2 and Eq.B.3 into Eq.B.1, one is able to get a formula for VOC:
eVOC = EDA − 𝑘𝐵𝑇ln (𝑁𝑛𝑁𝑝
𝑛𝑝) (Eq. B. 4)
48
Appendix C
Linear regression fits of the literature data for
PCBM bulk-heterojunction and C60 planar-
heterojunction cells
Figure C Linear regression fit for (a) PCBM bulk-heterojunction cells and (b) C60 planar
heterojunction cells. The fitting window is 0.2~1.3 eV and 0.5~1.7 eV for PCBM and C60 cells,
respectively.
Figure C shows the linear regression for the literature data, the slope is fixed at 1 according to
Eq.20. β=0.35±0.01 eV and 0.58±0.03 eV for PCBM and C60 cells, respectively.
49
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