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Pyrrolic squaraines and energy management in organic photovoltaics

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Pyrrolic squaraines and energy management in organic photovoltaics

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PYRROLIC SQUARAINES AND ENERGY MANAGEMENT IN ORGANIC
PHOTOVOLTAICS

by
Piyumie Wickramasinghe

A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(CHEMISTRY)

May 2018
Copyright 2018 Piyumie Wickramasinghe
ii

For my parents Sarath Wickramasinghe and Swarnamalie Perera
my sister Sathika Wickramasinghe
and my baby Arali Sarath Hewavitharana

iii

ACKNOWLEDGEMENTS
I was incredibly fortunate to be guided by Professor Thompson during my graduate
career, and this opportunity allowed me scientific and intellectual growth. I am deeply
grateful for his guidance, and appreciate the support he rendered throughout the completion
of my program.
My training in organic synthesis came from Dr. Lincoln Hall. We were the early
birds in our lab and our mornings started with tales of squaraines, stories from our countries
while removing solvents on the rotovaps. I am grateful for his scientific direction and
friendship. I am thankful for the many discussions I have had with Prof. Peter Djurovich. I
would like to thank my committee members Professors Barry Thompson, Smaranda
Marinescu, Ralf Haiges, Berenice Benayoun, and Martin Gundersen for strengthening my
work. Professor Haiges often assisted me with the numerous difficulties I encountered in
obtaining crystal structures for squaraines.
I have drawn inspiration from the members of the Thompson group, and I am
thankful for their friendship. From Dr. Slava Dieve I learnt techniques for receystallization,
Dr. Sarah Conron took me under her wing and trained me on device fabrication, and
discussions with Dr. Tyler Fleetham gave me insights to solving device problems. I truly
appreciate Narcisse Ukwitegetse and Moon Chul Jung for the assistance they gave me
during the last months of my experimental work and Dr. Elsa Couderc for train rides home
with hopping mechanisms conversation. During my time at USC, I was fortunate to form
lifelong friendships with Dr. Priscilla Antunez and Dr. Vandana Suresh. Together, we have
explored science and shared laughter. I am tremendously fortunate to have worked with
iv

Women in Chemistry (WIC), to be part of their family, and for the support I have received
from Women in Science and Engineering (WiSE).
I have been blessed to surrounded by a network of people who have supported and
loved me- I am thankful for Eily Strotman-Martin and Dr. Chand Aryasingha who always
showed up for me. I’m thankful for my aunts Ramani and Mala, and my grandparents Arthur
and Kusuma who showered me with love, my cousins Dileepa and Professor Ruchira
Cumaranatunga for inspiring me to become a woman of strength, Mrs. Chandani
Samarasinghe for instilling the love for math and science in me, Mrs. Dora Boteju and Mrs.
Abeysekara for helping me build confidence in myself through the learning process, and all
my teachers who inspired me.
Finally, I would like to acknowledge my family- my PhD would not have been
possible if not for their love and support. I appreciate my parents Swarnamalie and Sarath
for the incredible sacrifices they have made for me. My parents would leave their lives in
our home in Sri Lanka, for months at a time to help me push through my experimental work
and writing. My father sat down to write a game plan for me to reach my thesis writing goal,
and my mother would lift me up and giving me every bit of strength to make it through. My
sister Sathika, has been my greatest strength during this journey and has been the greatest
blessing of my life. My daughter Arali has been a force reminding me to live in the present
moment, and to be the best expression of myself. I hope my work will inspire her to nurture
a passion for learning, and always keep growing.
Thank you.

v

TABLE OF CONTENTS
Dedication ........................................................................................................................... ii
Acknowledgements ............................................................................................................ iii
Table of Contents ................................................................................................................ v
List of Figures and Tables................................................................................................. vii
Abstract ............................................................................................................................. xii
Chapter 1. Introduction ....................................................................................................... 1
1.1 Motivation for Solar Energy Research .......................................................... 1
1.2 Introduction the Photovoltaics ....................................................................... 5
1.3 Introduction to Organic Photovoltaics ........................................................... 8
1.4 Fundamental Organic Photochemical Processes ......................................... 11
1.5 Characterization of Devices ......................................................................... 31
1.6 Illumination Parameters .............................................................................. 33
1.7 Dark Parameters ........................................................................................... 34
1.8 Spectral Mismatch Factor (M) ..................................................................... 40
1.9 Spectral response (SR) & External Quantum Efficiency (EQE) ................. 41
1.10 Summary of topics ..................................................................................... 41
1.11 Endnotes for Chapter 1 .............................................................................. 43

vi

Chapter 2. Symmetric Pyrrolic Squaraines and Their Application to Organic Photovoltaics
........................................................................................................................................... 49
2.1 Abstract ........................................................................................................ 49
2.2 Introduction .................................................................................................. 50
2.3 Experimental section ...................................................................................... 8
2.4 Results and Discussion ................................................................................ 56
2.5 Photophysical and Electrochemical Characterization .................................. 60
2.6 Application to Organic Photovoltaics .......................................................... 65
2.7 Conclusion ................................................................................................... 68
2.8 Endnotes for Chapter 2 ................................................................................ 70
Chapter 3. Improving the performance of a DBP/ ZCl bilayer device through energy
sensitization....................................................................................................................... 73
3.1 Abstract ........................................................................................................ 73
3.2 Introduction .................................................................................................. 73
3.3 The Screening of Sensitizers ........................................................................ 90
3.4 Optical and Electronic Properties of the System ......................................... 92
3.5 Luminescence Quenching Experiments ....................................................... 96
3.6 Device Studies ............................................................................................. 98
3.7 Optimization of Sensitized Devices ............................................................. 68
vii

3.8 Conclusion ................................................................................................... 68
3.9 Endnotes for Chapter 3 .............................................................................. 109
Bibliography ................................................................................................................... 113

viii

LIST OF FIGURES AND TABLES

Figure 1.1 A warming planet: Trends in earth’s surface temperatures since 1880, with
record high years listed on the right. (Adapted from NASA global climate change
1
)…....1

Figure 1.2. The dramatic increase of CO2 in the atmosphere (Adapted from NASA global
climate change)
1
………………………………………………………….……………….2

Figure 1.3. Annual world CO2 emissions in million tons. Adapted from IEA...........……3

Figure 1.4. Comparison of the sources of electricity generation
3
………………...………4

Figure 1.5. Chronological statistics from 1976 to the present of the best performing
research cells and their efficiencies for a variety of photovoltaic technologies. (Courtesy of
NREL)………………………………..................................................................................5

Figure 1.6. General OPV device Architecture, and different architectures of the active
layer (b) planar (lamellar) heterojunction (PHJ) where the donor and the acceptor are
fabricated as two separate layers, (c) Bulk Heterojunction (BHJ) where there is significant
intermixing of the donor and the acceptor
materials.………………………………………………………………………………….9

Figure 1.7. The process of photocurrent generation in an OPV: (1) A photon is absorbed
forming an exciton; (2) exciton diffusion; (3) charge transfer resulting in the formation of
a charge transfer state; (4) charge separation; (5) charge
extraction.………………………………………………………………………………...11

Figure 1.8. Widely used active layer materials………………………………….………14

Figure 1.9. Exciton diffusion mechanisms: Dipolar Mechanism or Förster Energy
Transfer for singlets and electron exchange or Dexter Energy transfer which includes
triplets...……………………………………………………………………….………...18

Figure 1.10. Plots of potential energy surfaces for reactants and products along the
reaction co-ordinate summarize Marcus theory. (a) Electron transfer takes place in the
Marcus normal region where ΔG and the reorganization energy is positive, ΔG < λ, (b)
Maximum electron transfer rate is reached when reorganization energy is not a barrier for
energy transfer -ΔG = λ, (c) Electron transfer takes place in the Marcus inverted region
where ΔG and the -ΔG < λ, electron transfer rate decreases at this
point.……………………………………………………………………………………21

ix

Figure 1.7. Energy level diagram for commonly used electrodes………………………..29

Figure 1.8. Chemical structures of commonly used anode and cathode ……………........30

Figure 1.9. Characteristic I – V curves of a solar cell in the dark (dashed traces) and under
light (solid traces)..……………………………………………………..…………………33

Figure 1.10. Single diode equivalent circuit model is used for estimating solar cell losses
arising from resistive elements. ……………………………………….…………..……..36
Scheme 2.1. Chemical structures of (a) PSQ (b) NSP………………………………..….51

Figure: 2.1 Reaction schemes (a) 2,4-bis(3,5-dimethyl-1H-pyrrol-2-yl)-3-oxocyclobut-
1-en-1-olate (PSQ) (b) 2,4-bis(1-hexyl-5-(thiophen-2-yl)-1H-pyrrol-2-yl)-3-
oxocyclobut-1-en-1-olate (NSP)………………………………………………………..56

Figure 2.2 (a) ORTEP diagram of PSQ (b) ORTEP diagram of NSP…………………58

Figure 2.3. Crystal packing diagrams for PSQ. (a) herringbone structure with π- stacks
pitched in opposite direction (b) stacking arrangement viewed down the long molecular
axis. Crystal packing diagrams for NSP. (c) herringbone structure with vertical
translations of alternating π- stacks (d) stacking arrangement viewed down the long
molecular axes. Hydrogen atoms were removed for clarity……………………….……59

Figure 2.4. (a) Absorbance spectra for PSQ and NSP in toluene solution (solid lines),
and as neat films (open symbols). (b) Excitation (ex) and emission (em) spectra in
toluene…………………………………………………………………………………..61

Figure 2.5. Calculated energy levels and molecular orbitals of PSQ and NSP at
B3LYP/LACVP** level of theory using Maestro Material Science 10.6 software: (a)
LUMO of PSQ, (b) HOMO of PSQ, (c) LUMO of NSP, (d) HOMO of NSP………...63

Figure 2.6. Cyclic Voltammetry (CV) and Differential Pulse Voltammetry (DPV) diagrams
for PSQ (a) and (b), and NSP (c) and (d) in acetonitrile. Scan rate 100 mV/s………...65

Figure 2.7 (a) Current-density characteristics (b) tabulated OPV performance
characteristics of squaraine donor devices (c) external quantum efficiency (EQE) plots,
(d) AFM images of films of vacuum deposited PSQ (rms = 0.60 nm) (e) spun-cast PSQ
x

(rms = 17 nm) and (f) NSP (rms = 2.4 nm) spun-cast from
chloroform……………………………………………………………………………...68

Figure 3.1. The schematic configuration of the (a) conventional binary OSCs, (b) tandem
OSCs, and (c) ternary OSCs with four possible active
layer. ……………………………………………………………………………………75

Figure 3.2. FRET in the P3HT/SQ/PCBM system (a) Extinction coefficient (blue line)
of SQ in 1,2-dichlorobenzene, and absorption (red solid line) and emission (red dotted
line) spectra of P3HT film. (b) EQE versus wavelength of devices with SQ
concentrations ranging from 0 to 5 wt%. Adapted from reference 46……………….....79

Figure 3.3. PL spectra of organic layer stacks deposited on quartz substrates for different
spacer layer thicknesses (x nm BCP). (a) 20 nm SubPc/x nm CBP, (b) 20 nm SubPc/x nm
CBP/10 nm C60, and (c) 20 nm SubPc/x nm CBP/10 nm SubNc. Adapted from
reference ………………………………………………………………………………...82

Figure 3.4. (a) Absorption spectra of the three active materials complementing each other
to effectively harvest solar light. (b) The measured EQE (solid lines) and IQE (dashed
lines) spectra show efficient photocurrent generation by all three absorbing materials.
Adapted from reference ………………………………………………………………….84

Figure 3.5. EQE for the devices described in the text. Adapted from Reference 95..… .88

Figure 3.6. Schematic representation of molecular formulae and energy levels for
Coumarin 30, DBP and ZCl. Energy values for DBP and ZCl are from literature
2,4
,
energies for Coumarin 30 has been calculated from redox potentials…………………92

Figure 3.7: (a) Singlet and triplet energies of Coumarin 30 and DBP. Possible energy
transfer pathways are given by arrows (a) Determined by TDDFT Calculations (b) Ref
22 (b) Redox potentials were determined (vs Fc/Fc+)…………………………………94

Figure 3.8. The absorption spectra of the neat and blended DBP: BCP films……..….95

Figure 3.9. (a) Emission of Coumarin 30, and absorption of DBP in thin films (b)
Photoluminescence of Coumarin 30 with and without DBP. Films were excited at λ =
380 nm…………………………………………………………………………………..97

Figure 3.10 Device architecture for (a) sensitized device, where the effect of sensitization
is evaluated as a function of the concentration of the sensitizer (b) control
device…………………………………………………………………………..………98

xi

Figure 3.11. Characteristics of OPV devices (a) IV curves of devices under one sun AM
1.5G illumination (b) plot of external quantum efficiency (c) performance parameters as a
function of the concentration of the sensitizer……………………………………….…..99

Figure 3.12. Device architecture for (a) sensitized device, where the effect of sensitization
is evaluated as a function of the thickness of the sensitized layer (b) control device….103

Figure 3.13. Characteristics of OPV devices (a) IV curves of devices under one sun AM
1.5G illumination (b) plot of external quantum efficiency (c) performance parameters as a
function of the thickness of the sensitized layer……………………….………………104

Figure 3.14. Device architecture for (a) sensitized device, where the effect of sensitization
is evaluated as a function of the thickness of the sensitized layer (b) control
device………………………………………………………………………………….105

Figure 3.15. Characteristics of OPV devices (a) IV curves of devices under one sun AM
1.5G illumination (b) plot of external quantum efficiency (c) performance parameters as a
function of the thickness of the neat DBP layer……………………………………….106

TABLES
Table 2.1. Photophysical Data in Solution and in Thin ilms…………………………….60

Table 2.3. Electrochemical Redox Potentials
a
and Calculated HOMO/LUMO
b
nergies..64

Table 3.1. Photophysical and electrochemical data for DBP, ZCl, and potential
sensitizers………………………………………………………………………………..90

Table 3.2. Summary of device performance characteristics of control and sensitized
devices for varied concentrations of the sensitizer ......................................................... 100
Table 3.3. Summary of device performance characteristics of control and sensitized
devices for varied thickness of the sensitized layer ........................................................ 103
Table 3.4. Summary of device performance characteristics of control and sensitized
devices for varied thickness of the neat DBP layer ........................................................ 107
xii

ABSTRACT
Organic photovoltaics (OPV) have remained a research area of great interest in the
past two decades as source of renewable energy. Compared to their inorganic counterparts,
they have the advantages of low-cost roll-to-roll production, lightweight, and flexibility
which makes them available for applications on flexible substrates or mobile phones.
However, OPVs suffer from low performance efficiencies compared to Si-based PVs. This
dissertation explores the utilization molecule design, and device optimization through
energy management of the device to address the limitations of OPVs.

In Chapter 2 we explore the design and development of pyrrolic squaraines as donor
materials for applications in OPVs, comparisons are made between the performance of
devices fabricated through the vacuum deposition and the solution processing of the
squaraine material. In Chapter 3, we present research on improving the photocurrent of a
tetraphenyldibenzoperiflanthene (DBP) / zinc chlorodipyrrin (ZCl) bilayer device, using
Coumarin 30 as a sensitizer. Here an energy sensitization scheme has been used to harvest
a broader region of the solar spectrum. The overall theme of this dissertation is to utilize
material and device design to develop insights on improving the performance of OPVs.

1

CHAPTER 1
INTRODUCTION
1.1 Motivation for Solar Energy Research
The year 2016 set a new global record as the warmest year the earth surface temperatures
have reached since 1880, according to independent studies conducted in 6,300 weather
stations by NASA and the National Oceanic and Atmospheric Administration (NOAA).
1
Global temperatures averaged in 2016 were 0.99°C above the mid-20
th
century mean, record
high temperatures were reached January- September, while October-December remained
equal to the record high temperatures reached during those months in 2015.
1
Since the late
19
th
century the average earth surface temperature has risen by 1.1°C; it is noteworthy to
mention that since 2014, each passing year has set a new record for earth’s surface temperature

Figure 1.1 A warming planet: Trends in earth’s surface temperatures since 1880, with
record high years listed on the right. (Adapted from NASA global climate change
1
)
2

(Figure 1.1), and the global warming phenomenon has been largely driven by the increase in
anthromorphic carbon dioxide (CO2) emissions to the atmosphere.
1

The high-accuracy measurements of atmospheric CO2 concentration introduced by
Charles David Keeling in 1958 serves as evidence of the anthropogenic effect on the chemical
composition of the atmosphere.
2
Atmospheric studies reveal that the CO2 concentration
remained steady from 10,000 years ago until the year 1750, at 280± 20 ppm; during the
industrial era, the concentration rose exponentially to 367 ppm in 1999, and to 382.95 ppm
February in 2007 (Figure 1.2).
1,2
The atmospheric CO2 concentration has continued to rise
since and has reached a historic record 405.61 ppm in February 2017.
CO2 is released to the atmosphere through natural processes such as respiration and
volcanic eruptions. However, human activities such as deforestation and combustion of fossil

Figure 1.2. The dramatic increase of CO2 in the atmosphere (Adapted from NASA global
climate change)
1

3

fuels, have been the largest contributors of CO2 to the planet.
1,2
Atmospheric studies on the
effect of the rising concertation of CO2 on the planet can be traced back as far as 150 years:
The greenhouse effect on earth’s atmospheric composition was first suggested by physicist
John Tyndall in 1860; in 1896 Svante Arrhenius predicted the rise in earth’s surface
temperature as a result of the greenhouse effect; In 1938, G. S. Callendar reported that
doubling the atmospheric CO2 concentration would increase earth’s surface temperature by
2°C, with higher temperatures at the poles, and linked fossil fuel combustion to the rising CO2
concentration and greenhouse effect
1
Indeed as shown in Figure 1.3, since 1971 the annual
world CO2 emission by fuel combustion has more than doubled, with more than 30,000
million tons of CO2 released to the atmosphere in 2014. As compelling evidence mounts up,

Figure 1.3. Annual world CO2 emissions in million tons. Adapted from IEA
4

to sustain the earth, it is necessary to adopt alternative energy sources that do not release
massive amounts of CO2 to the atmosphere and destabilize the environment.
The total electricity generation in the world in 2014 was 23,816 TWh.
1
As outlined
in Figure 1.4, the contribution of geothermal, solar, wind and heat in total was only 6.3%.
When abundance of potential energy sources is reviewed, with an average solar energy
density of 1000 W/m
2
reaching the earth surface, 23,000 TW of solar energy irradiates the
earth annually (Figure 1.4.).
1
While the total energy usage in 2015 was only 18.5 TW, the

Figure 1.4. Comparison of the sources of electricity generation
3

5

estimated total energy usage for 2050 is 27 TW.
1
Although, wind, geothermal, and biofuels
are often considered as sustainable energies, by reviewing the future energy demand, and the
availability of potential energy sources it is evident that solar energy surpasses all others by
orders of magnitude. Therefore, a vast amount of alternative energy research has been focused
on a solar-based future, as has been the motivation for this thesis. Solar, is the only
environmentally sustainable, and economically plausible future of energy.

1.2 Introduction the Photovoltaics
The ability of a material to generate voltage or current upon the exposure to light is
known as the photovoltaic effect, and it was first reported by Alexandre Edmond Becquerel

Figure 1.5. Chronological statistics from 1976 to the present of the best performing
research cells and their efficiencies for a variety of photovoltaic technologies. (Courtesy
of NREL)
6

in 1839, when an acidic silver chloride electrolyte cell, with platinum electrodes produced
current upon irradiation.
2
The P-N junction fabricated by Russel Ohl in 1946 at Bell
Telephone Laboratories is recognized as the first modern solar cell.
3
Since then, the use of
these devices in the US space program has driven their development further, with inorganic
photovoltaics (PVs) dominating the current solar cell technology. Figure 1.5 outlines the
different types of PVs and their efficiencies.
According to the reports of National Renewable Energy Laboratory (NREL), which is
the internationally recognized official verifier of efficiency records for research solar cells
currently, the III-V single junction and multi-junction GaAs solar cells are the most efficient
PVs with efficiencies of 46%. However, owing to their high production cost usage is limited
to aerospace applications. Crystalline silicon based PVs closely follow GaAs PVs with 25%
power conversion efficiency. With its long dominant history silicon PV controls 83% share
of the current PV market.
4,5

The working principle in an inorganic semiconductor is that when a photon with energy
larger than the band gap is absorbed, an electron is promoted from the valance band to the
conduction band. The probability of this process is known as the absorptivity of the material
and increases exponentially with thickness according to Beer’s law 𝑎 = 1−𝑒 −𝛼 𝑥 where 𝑎 is
absorption, α is absorptivity, and x is thickness. In silicon, the absorptivity near the band edge
is on the order of 1000 cm
-1
. Due to the high dielectric constants (ε = 10- 15), the binding
energy of the Wannier-Mott exciton is 0.01 eV, and the weakly bound electron-hole pair
dissociates immediately at room temperature. Charge carriers are then transported against the
built-in potential of the cell through a drift process, and a diffusion motion through the
7

concentration gradient. Silicon displays much higher hole motilities (450 cm
2
V
-1
s
-1
) compared
to organic semiconductors (10
-2
to 10
-6
cm
2
V
-1
s
-1
).
Although silicon PVs dominate the PV market and is a mature technology, their
efficiencies are limited by absorption. As a result of the indirect band gap in crystalline silicon,
a significant change in momentum is required to promote an electron to the band edge, which
causes the absorption coefficient to be low. This means that since silicon is a poor light
absorber, in order to capture 90% of the incident photons, a thickness of 100 μm crystalline
silicon is required, in comparison to 1 μm of GaAs.
6
The thick layers of silicon which can be
brittle requires support on rigid heavy glass, which adds to costs and limits their applications.
Additionally, the efficient transport of charges through a large thickness require ultra-pure
materials with 99.999% purity. This ultra-purity requirement is met by an energy intensive
crystal growth processes.
In contrast, emerging PV technologies use a wide range of inexpensive materials such
as small molecule and polymer solar cells which have reached 12 % efficiencies since their
recent advent, and perovskite solar cells which have reached 22 %. Furthermore, organic
semiconductors display many unique advantages over conventional inorganic materials.
Small molecule organic absorbers are 1,000 times more light absorbing than silicon due to
their high absorption coefficients (ε = 100,000 cm
-1
). Their structural, optical, and electrical
properties are simply tunable through minor molecular modifications in their synthesis
process which can be compatible with commercial scale production. With progress made in
computational chemistry to predict the molecular and optical properties of materials, the
future holds the ease of screening materials to match device requirements. Additionally, they
8

can be fabricated on flexible substrates such as plastics using inexpensive solution processing
techniques commonly used in the plastic manufacturing industry such as large-scale roll-to-
roll printing, which makes them suitable for a wide variety of applications. As a result of these
advantages in organic semiconductors, a future of energy based on earth-abundant materials
that sustain the environment, while being economically and commercially viable is possible.
Indeed, the work of this thesis has been the development of novel organic absorbers for small
molecule solar cells, and energy management to improve their efficiency, and further
understanding the science of solar cells.
1.3 Introduction to Organic Photovoltaics
The first organic photovoltaic cell fabricated in 1975 was composed of a single organic
layer sandwiched between two different electrodes with different work functions.
7
The built-
in potential of these cells was derived from a Schottky barrier- which was the difference
between the work function of the two electrodes. The power conversion efficiency of the
Schottky diode was 0.001%, because of the inability to dissociate the exciton at room
temperature.
8
In 1986 C. W. Tang fabricated a bilayer heterojunction device with an energy
offset in the donor and the acceptor that allowed the dissociation of excitons. This device
architecture is an important milestone, and the power conversion efficiency reached 1% under
75 mW/cm
2
simulated AM2 illumination.
9
In 1991, M. Hiramoto proposed a planar-mixed
molecular heterojunction architecture, where a co-deposited mixed layer is sandwiched
between two neat pigment layers. This co-deposited interlayer is often credited as the
prototype bulk heterojunction layer.
10
In 1992, A. J. Heeger reported the photo induced pico-
scale electron transfer from a conducting polymer to C 60; his lab further improved the charge
9

collection efficiency and the power conversion efficiency of polymer photovoltaic cells by
blending MEH-PPV with fullerene in 1995.
11,12
For his seminal work in semiconducting and
metallic polymers, A. J. Heeger was jointly awarded the Nobel Prize in chemistry in 2000.
One of the factors that limit the performance of OPVs is the narrow absorption bands of the
active materials. This issue has been addressed by tandem solar cells which combine several
active layers in one cell with two BHJs stacked on top of each other, while expanding the
overlap with the solar spectrum. The sub-junctions are connected in series or as a three-
terminal cell when connected in parallel.
13

Figure 1.6. General OPV device Architecture, and different architectures of the active layer
(b) planar (lamellar) heterojunction (PHJ) where the donor and the acceptor are fabricated as
two separate layers, (c) Bulk Heterojunction (BHJ) where there is significant intermixing of
the donor and the acceptor materials.
10

Due to the increased donor/acceptor (D/A) interfacial area in BHJ devices, the exciton
dissociation process is more efficient in them, which significantly improves their performance
compared to bilayer OPVs. Nevertheless, controlling the morphology of the interfacial area
in BHJs is not well understood, although continuous phases of active materials is critical for
charge separation. The general device architecture is given in Figure 1.6. In bilayer devices,
since donor and acceptor materials are deposited as independent layers, the lack of
morphological control is mostly eliminated in during their fabrication. Thus, bilayer devices
help elucidate the fundamental science behind the operation of the photovoltaics cell. For
these reasons, the work in this thesis is primarily based on bilayer device architecture.

11

1.4 Fundamental Organic Photochemical Processes
The working principle of an OPV system is governed by six fundamental processes
that converts photonic energy absorbed into electrical charges. These include: (1) The
absorption of a photon which leads to a localized exciton on the donor (D*) or (A*) (2)
Diffusion of the exciton to the D/ A interface (3) Dissociation of the exciton to form a
Coulombically bound charge transfer (CT) state, which is a geminate complex (D
+
A
-
) (4)
Dissociation of the CT state to form polaron pairs D
+
and A
-
(5) Charge transport through a
hopping mechanism and (6) Charge collection by an external circuit. The basic working
principle is depicted in Figure 1.7, and the following section is a detailed description of each
of the steps in the process.

Figure 1.7. The process of photocurrent generation in an OPV: (1) A photon is absorbed
forming an exciton; (2) exciton diffusion; (3) charge transfer resulting in the formation of a
charge transfer state; (4) charge separation; (5) charge extraction.
Filled
Orbitals
Filled
Orbitals
V acant
Orbitals
V acant
Orbitals
Donor Accpetor
HOMO
LUMO
E
exc
E
exc
= Exciton energy
1 2 3 4 5
+
-
+
-
+
-
12

(1) Photon Absorption
The absorption of a photon leads to an electron being promoted from the HOMO to the
LUMO of the organic semiconductor, which results in an excited electron in the LUMO and
a hole in the HOMO or the formation of an exciton. In inorganic semiconductors with
typically high dielectric constants such as Si (ε= 11.9)
14
, Significant electric field screening
reduces the Coulombic interaction between the electron and the hole. This phenomenon leads
to the formation of Wannier-Mott excitons with binding energies in the order of 0.01 eV,
which can readily separate at room temperature (kBT= 25 meV) to form free charges in the
presence of an electric field. Since the dielectric constants for organic materials (ε~3) is
typically ¼ lower than inorganics, they are inefficient at screening electric fields and result in
forming tightly bound Frenkel-type excitons. These excitons can have binding energies as
large as 0.1- 0.5 eV. The Coulombic attraction between the opposite charges of the excited
molecule stabilizes the Frenkel exciton. Since the binding energy in the organic
semiconductor is twenty times larger than thermal energy, optical absorption does not lead to
the formation of free carriers.
Photon absorption promotes an electron from the ground state (S0) to a higher singlet
excited state S1 or S2. Rapid internal conversion (10
-14
to 10
-11
s) leads to the population of the
lowest vibrionic level of S1. The typical singlet lifetime is several nanoseconds and the singlet
excited states may radiatively recombine to produce fluorescence. Population of the triplet
state (T1) is probable if the intersystem crossing rate can compete with the radiative and non-
radiative decay rates from the singlet state. However, phosphorescence and non-radiative
triplet decay is slower as triplet excitons need spin-orbit coupling to couple radiatively and
13

non-radiatively to the ground state. Triplet lifetime ranges between microseconds to
milliseconds.
A large variety of organic small molecules have been reported in literature as donors
and acceptors, and dye-based molecules are often used as donors. Among these are donors
are porphyrins, phthalocyanines (Pc), subphthalocyanines (SubPC), diketopyrrolopyrroles
(DPP), merocyanine, squaraines (SQ), borondipyrromethene (BODIPY), and perylene
diimides (PDI) the structures of which are given in Figure 1.8. In the first bilayer device that
was published by C. W. Tang in 1986
9
the device comprised of an ITO-coated substrate with
vacuum-evaporated 30 nm Copper (II) phthalocyanine (CuPc) and 50 nm PTCBI used as the
donor and acceptor layers respectively, and a Ag electrode. Although Pc compounds are
structurally related to porphyrins, they are often synthesized without substituents, which
enables the close packing of molecules in the crystal structure which in turn yields longer
exciton diffusion lengths (𝐿 𝐷 ).
76
CuPc molecules pack perpendicular to the ITO substrate in
a herringbone array and exhibits anisotropic charge transfer properties, in which the
orientation of π- π stacking allows the perpendicular transport of charge via a hopping
mechanism.
15
The pairing of CuPc with C60 has become the archetype device in OPV with 𝜂 𝑃
of 0.9% in bilayer and 3.6% in BHJ under 150 mW/cm
2
simulated AM 1.5G
illumination.
77,16,17

When SubPC is substituted as the donor in the Pc/C 60 bilayer device, 𝑉 𝑜𝑐
increases
from 0.42 to 0.97 V corresponding to the deeper lying HOMO in SubPC, and 𝜂 𝑃 increases
from 0.9% to 2.1% compared to CuPc.
16
Squaraines are another family of tunable small
molecule donors with high extinction coefficients (>10
5
M
-1
cm
-1
)
18
, which makes them
14

excellent candidates for OPVs. Vapor deposited iso-butyl squaraine devices fabricated in our
group have demonstrated a 𝑉 𝑜𝑐
= 0.82 V, and 𝜂 𝑃 =3.2% for donor layers as thin as 6.5 nm
when paired with C60.
19
Buckministerfullerene or C60 was discovered by Kroto in 1985,
20
and
is a popular acceptor owing to its favorable physical properties. C60 displays motilities of 10
-
2
-10
-1
cm
2
/(Vs)
21
; allows the reversible electron transfer of up to six electrons
22
; its frontier
orbitals are aligned such that facile electron transfer from common donors is possible;
fullerenes have small reorganization as the spherical shape requires little structural change
upon electron transfer.
23
Additionally, the spherical shape provides anisotropy toward
electron transfer, which means that in a three-dimensional arrangement, electron transfer is
possible from any direction.

Figure 1.8. Widely used active layer materials
15

(2) Exciton Diffusion
2.1 Energy Transfer Mechanisms
Once the exciton has been generated, energy transfer can take place in the
conjugated centers of the organic molecule. In organic crystals with ordered packing systems
the exciton is more delocalized. However, most amorphous organic semiconductors have
highly localized charge carriers, and energy transfer takes place through a hopping
mechanism. The three plausible energy transfer mechanisms responsible for exciton migration
are Photon reabsorption, Förster transfer, and Dexter transfer.
Photon reabsorption Photon reabsorption involves the emission and the subsequent
absorption of a photon between two molecules, and occurs when there is significant overlap
between the emission of the donor molecule and absorption of the acceptor molecule.
24
Photon
reabsorption is a long-range interaction, and should be considered as a possible mechanism
for thicknesses greater than 10 nm. The energy transfer rate decreases as the Stokes shift
between the absorption and the emission spectra increases, or the as the thickness of the
organic film decreases.
Förster Energy Transfer Förster energy transfer
25,26
is a non-radiative energy transfer
mechanism through a dipole-dipole interaction between the donor (d) and the acceptor
molecules (a), while conserving spin. Therefore, the energy transfer takes place in singlets.
The energy transfer process can be visualized as the emission of a photon from the donor
molecule, and subsequent absorption by the acceptor. Therefore, there must be significant
overlap between the photoluminescence spectra of the excited state donor and the absorption
16

of the ground state acceptor. The dipole-dipole interaction can occur non-radiatively through
empty space or through molecular-occupied space, since it does not involve the direct transfer
of electrons. The process can be described as the overlap of the dipolar electric field of d*
with a. The interaction between two electric diploes can be defined by the following equation:
Eq 1.1 𝑘 𝐹 ( 𝑑𝑖𝑝𝑜𝑙𝑒 − 𝑑𝑖𝑝𝑜𝑙𝑒 ) 𝛼
𝜇 𝑑 2
𝜇 𝑎 2
𝑅 𝑑𝑎
6

The strength of the dipole-dipole interaction is defined by the distance between the
donor and the acceptor molecules 𝑅 𝑑𝑎
, and strength of the interacting dipoles 𝜇 𝑑 and 𝜇 𝑎 . The
interacting dipoles correspond to the transitions 𝑑 ∗
→ 𝑑 and 𝑎 ∗
→ 𝑎 , and can be further
defined through extinction coefficient (𝜀 ) and radiative decay rate ( 𝑘 𝑟
) as given in equations
1.2 and 1.3.
6,26

Eq 1.2 𝜇 𝑑 2
( 𝑑 ∗
↔ 𝑑 ) → ∫ 𝜀 𝑑
𝑜𝑟 ∫ 𝑘 𝑟 ( 𝑑 )

Eq 1.3 𝜇 𝑎 2
( 𝑎 ∗
↔ 𝑎 ) → ∫ 𝜀 𝑎
𝑜𝑟 ∫ 𝑘 𝑟 ( 𝑎 )

The rate of the energy transfer can be expressed as equation 1.4:
25,26

Eq 1.4 𝑘 𝐸𝑇
( 𝑑𝑖𝑝𝑜𝑙𝑒 − 𝑑𝑖𝑝𝑜𝑙𝑒 ) = 𝛼 (
2
3
𝑘 𝑟 ( 𝑑 )
𝑅 𝑑𝑎
6
) 𝐽 ( 𝜀 𝑎 )
Here, 𝛼 is the proportionality constant related to concentration and solvent polarity
index of refraction, 𝐽 ( 𝜀 𝑎 ) is the spectral density integral, which is the overlap integral between
emission spectrum of d and the absorption of a including the extinction coefficient of a (𝜀 𝑎 ).
The efficiency of the energy transfer is dependent upon the quantum yield of d, the extinction
coefficient of a and spectral overlap between emission of the donor and the absorption of the
17

acceptor. Due to the large oscillator strength observed in singlet- singlet transitions when
compared to singlet- triplet transitions, singlet-singlet energy transfers are likely to occur
through Förster energy transfer. With reducing intermolecular space, the postulation that the
dipoles of the donor and the acceptor are weakly coupled breaks down. As the electron
densities of the two molecules overlap, direct electron exchange needs to be considered as an
energy transfer mechanism.
Dexter Energy Transfer Dexter energy transfer involves the electron exchange
through the orbital overlap between the excited state donor and the ground state acceptor
molecules. The process can be visualized as the coincidental transfer of an electron and a hole
between the two molecules which is similar to a hopping mechanism as given in Figure 1.9.
Since transitions must conserve total spin in Dexter Transfer singlet-singlet as well as triplet-
triplet transitions are allowed. The energy transfer equation for Dexter transfer in terms of
specific orbital interaction (K), Spectral overlap integral normalized for the ground state donor
(J), and the Van der Waals radius of the molecules (L) is given by:
27

Eq 1.5 𝑘 𝐷 ( 𝑑 ) = 𝐾𝐽 𝑒 (
−2𝑑 𝐿 )

18

The Dexter transfer rate does not depend on the extinction coefficient of the acceptor
in comparison to Förster transfer, and decreases exponentially with increasing distance
between the two neighboring molecules. The energy transfer process is a short-range
interaction typically in the range of 0.1- 1 nm.
Exciton Diffusion Length
Donor molecules that that are directly coupled or possess orbital overlap with a
neighboring acceptor molecule can undergo the charge transfer process through the
mechanisms mentioned above upon absorption of light. However, donor molecules that do
not exhibit dipole coupling or orbital overlap with the acceptor molecules may reach the
donor-acceptor interface through a hopping mechanism and undergo exciton dissociation. As
excitons are a neutral species their propagation is not influenced by an electric field and

Figure 1.9. Exciton diffusion mechanisms: Dipolar Mechanism or Förster Energy Transfer for singlets
and electron exchange or Dexter Energy transfer which includes triplets
Dipole Coupling
d* a
Electron Transfer
d* a
Hole Transfer
Förster Energy Transfer
Dexter Energy Transfer
19

diffusion occurs through random hops. The distance that the excitons can propagate before
decaying back to ground state is known as the exciton diffusion length (𝐿 𝐷 ),
28

Eq 1.6. 𝐿 𝐷 = √𝐷 𝜏 0

where 𝐷 is the exciton diffusivity, and 𝜏 0
is the exciton lifetime. In amorphous organic
films, 𝐿 𝐷 is in the order of 10 nm
29,30
, for highly ordered anthracene crystals 𝐿 𝐷 is in the order
of microns.
31,32
The efficiency of exciton diffusion largely depends on the ratio of 𝐿 𝐷 to the
thickness of the absorbing donor layer. The optimization of device performance may require
an increase in the donor layer thickness which comes at the expense of exciton decay due to
the limitations imposed by 𝐿 𝐷 . To address this problem, the bilayer device architecture is
changed into a volume structure by mixing donor and acceptor molecules to form a bulk
heterojunction
33,34
, which in turn reduces the electrically inactive regions and improves the
exciton diffusion efficiency.
Additionally, investigations in SubPc systems have shown that optimizing the
efficiency of Förster energy transfer in terms of balancing radiative and non-radiative decay
rates yields longer 𝐿 𝐷 . Menke et al.
35
have investigated the effects of the dilution of SubPc
in a wide band gap host species and reported that dilution increases the photoluminescence
efficiency as a result of the decrease of non-radiative decay pathways, thereby favoring
radiative decay and leading to an efficient Förster transfer rate. The non-polar host also
stabilizes the excited state of SubPc, leading to smaller Stokes shifts and increased spectral
overlap, and collectively increasing the 𝐿 𝐷 by 50% compared to neat films. However, excitons
in inhomogeneous energy landscapes may result in being trapped and not contribute to
20

photocurrent. In polymeric organic semiconductors, kinks and torsions in the polymer
backbone leads to the formation of traps; if the exciton does not have sufficient energy to hop
to the adjacent molecule it will be trapped and undergo radiate or non-radiate decay to the
ground state.
Crystalline order is not always beneficial for exciton diffusion. Thermal annealing
is often used to increase crystallinity in amorphous materials. In 2,4-bis[4-(N,N-
diisobutylamino)-2,6-dihydroxyphenyl]squaraine a threefold increase is observed in 𝐿 𝐷 upon
thermal annealing; as cast films yield an 𝐿 𝐷 of 1.7 nm and thermal annealing at 130°C
increases the 𝐿 𝐷 to 5 nm.
36
However, thermal annealing can lead to exciton quenching at the
grain boundaries, or to unfavorable molecular packing, causing 𝐿 𝐷 to decrease as observed in
the thermal annealing of some diketopyrrolopyroles.
37

In planar heterojunction architecture, the thickness of the donor is a critical parameter
that defines complete optical absorption if the layer is substantially thick, or limits the fraction
of excitons that reach the D/A heterojunction before recombining if the thickness is greater
than 𝐿 𝐷 . For most organic semiconductors, the 𝐿 𝐷 is 10- 20 nm while C60 shows a higher 𝐿 𝐷
at 40 nm. The exciton diffusion bottleneck has been circumvented in the bulk heterojunction
architecture by generating excitons near the D/A interface through the mixing the organic
semiconductors.
38,39

21

(3) Exciton Dissociation and Charge Separation
Marcus Theory
Once the exciton diffuses to the D/A heterojunction it must separate into free charge
carriers in order to extract photocurrent. The thermodynamics of electron transfer and charge
generation is an important factor in efficient OPVs, and is often described using Marcus and
Onsager theory.
In his Nobel Prize, winning work for Chemistry, Rudolph A. Marcus expressed the
movement of energy in a chemical reaction as two harmonic oscillators on a single reaction
coordinate: the first harmonic oscillator represents the reactant and its surrounding medium

Figure 1.10. Plots of potential energy surfaces for reactants and products along the reaction co-
ordinate summarize Marcus theory. (a) Electron transfer takes place in the Marcus normal region
where ΔG and the reorganization energy is positive, ΔG < λ, (b) Maximum electron transfer rate is
reached when reorganization energy is not a barrier for energy transfer -ΔG = λ, (c) Electron transfer
takes place in the Marcus inverted region where ΔG and the -ΔG < λ, electron transfer rate decreases
at this point.

22

while the second harmonic oscillator represents the product and its surrounding medium.
40
.
The intersection point is where the energy of the two systems are equal and signifies the
energy and nuclear configuration the reactant must reach for electron transfer to occur. The
rate of electron transfer (𝑘 𝐸𝑇
) is given by equation 1.7.
Eq 1.7. 𝑘 𝐸𝑇
=
2𝜋 ℏ
𝐻 𝑎𝑏
2
√4𝜋𝜆𝑘𝑇 𝑒𝑥𝑝 (
( Δ𝐺 𝜊 +𝜆 )
2
4𝜋𝜆𝑘𝑇 )
Where 𝐻 𝑎𝑏
is the electronic coupling matrix between initial and final states, 𝑘 is the
Boltzmann constant, 𝑇 is the temperature of the reaction, Δ𝐺 𝜊 is the Gibbs free energy- the
energy difference between final and initial states, and 𝜆 is the reorganization energy. The
reorganization energy is a key parameter that is the sum of the internal reorganization energy
which is required for rearranging the reactant molecule from the initial to the final state, and
the internal reorganization energy which is required for rearranging the surrounding medium.
In Figure 1.10 (a) the electron transfer rate is slow because of the high reorganization energy
and activation barrier; as Δ𝐺 𝜊 approaches zero Figure 1.10 (b) the energy that is required to
reach the barrier becomes 𝜆 /4 and the electron transfer rate increases with increasing −Δ𝐺 𝜊 .
When Δ𝐺 𝜊 further increases in Figure 1.10 (c) the electron transfer has no activation barrier
and happens on an ultrafast time scale. Marcus theory shows that a small reorganization
energy will enable the electron transfer process, and increasing −Δ𝐺 𝜊 further as shown in
Figure 1.10 (d) will result in the reorganization energy becoming a barrier, leading to a slow
electron transfer rate. This movement along the energy surface is known as moving to the
“Marcus inverted region”.
23

Ultrafast electron transfer when Δ𝐺 𝜊 = 0 has been reported in perylenediimide
dimers which undergo Symmetry Breaking Charge Transfer (SBCT).
41
Zinc dipyrrin systems
which display a similar phenomenon show charge transfer between 1 ps and 14 ps.
42
In such
systems the donor and the acceptor are two equal subunits which leads to Δ𝐺 𝜊 being zero.
When the intramolecular charge transfer takes place the electron transfer rate is dependent on
the reorganization energy. The resulting hole and electron are localized on the two units with
very little coupling between the electron and the hole. SBCT is a favorable process used on
OPVs due to the negligible driving force which in turn lowers the energy loss during electron
transfer, and reduced back electron transfer rates which decreases charge recombination and
favors charge separation.
Since intermolecular electron transfer can occur in thin films, forward electron
transfer rates are much faster and reverse electron transfers were much slower compared to
polar solutions.
Bimolecular recombination is dominant is solid state while geminate recombination is
central in solution. Charge recombination will be discussed in detail in the following section.
Charge Separation
Excitons that reach the D/A interface progresses into a charge-transfer (CT) state D
+
/A
-

after electron transfer, which can either recombine to the ground state or evolve into
Coulombically unbound charges through a manifold of charge-separated (CS) states. The
energy of the polaron pair can be given by the sum of the adiabatic ionization potential (IP)
of the donor and the electron affinity of the acceptor (EA). This energy is often approximated
24

to be the energy difference (Δ𝐸 𝐷𝐴
) between the HOMO of the donor and the LUMO of the
acceptor, and defines the upper limit for the 𝑉 𝑜𝑐
of the solar cell. At this point it is worthwhile
to note that Δ𝐸 𝐷𝐴
is not an accurate estimation of 𝑉 𝑜𝑐
as shown through spectroscopic and
temperature dependent techniques.
43,44
The upper limit for 𝑉 𝑜𝑐
is the energy from the ground
state to the intermolecular CT (𝐸 𝐶𝑇
) at the D/A interface. The Δ𝐸 𝐷𝐴
parameter is often cited
as the upper limit for Voc because in the CT transition an electron is promoted from the HOMO
of the donor to the LUMO of the acceptor. However, 𝐸 𝐶𝑇
correlates linearly with 𝑞 𝑉 𝑜𝑐
. Typical
energy losses are around 0.6 V due to recombination.
45,46

During exciton dissociation one of two processes can occur. If the internal conversion
rate (k IC) of the CT state within the CT manifold is greater than the rate of charge separation
(kcs), then the exciton will relax fast to the lowest CT state within the manifold (CT1). In CT1
the hole and electron are still strongly bound Coulombically as they sit on the HOMO of the
donor and LUMO of acceptor respectively, with insufficient thermal energy to overcome the
Coulombic barrier.
If kIC < kcs, exciton dissociation can take place via the ‘hot’ CT states within the CT
manifold. In copper phthalocyanine (CuPc)/ C60 systems the generation of free-carriers is
reported to be in the hundred femtosecond regime for direct excitation of CuPc, suggesting
that the dissociation of CT excitons to be ultrafast.
47,48
Hot excitons are 0.3 eV higher in
energy than thermally relaxed excitons, and the internal relaxation process take place in the
pico second regime.
47,48
In these systems it has been proposed that the excitons in the upper
states are loosely bound with greater distance between the hole and the electron and can
25

dissociate readily. These delocalized upper CT states are proposed as the major source of free
charge carriers while CT1 is a trap state and will contribute little to the yield of free charge
carriers.
However, in a recent study by Vandewal et al.
49
which investigates the quantum yield
and the electric field dependence of charge generation via the excitation of the CT manifold
using a wide range of polymer: fullerene, small-molecule: fullerene, and polymer: polymer
blends suggest that the internal quantum efficiency of the OPV systems investigated are
independent of higher energy states in the CT manifold being excited. They report that
providing excess energy to D*, A*, or excitation to ‘hot’ CT states is not required for a high
yield of free carriers, since in state-of-the art bulk heterojunctions for the best performing
materials produce 90% IQE when CT1 is excited, and that free charge carriers are generated
through the efficient dissociation of CT1. Excitation into the higher lying CT states leads to
ultra-fast relaxation into CT1, and in systems such as MEH-PPV: PC61BM where CT1 is
tightly bound, IQE at all photon energies will be less than unity; in systems such as PCDTBT:
PC71BM where CT1 is weakly bound IQE approaches unity for all photon energies.
Additionally, there are more complicated processes that can occur at the D/A interface.
The exciton on a donor near the interface may not transfer electrons to the acceptor, and
instead may energy transfer to the acceptor leading to the formation of A* as observed in
oligophenylene-fullerene dyads.
50
In certain thiophene-oligomers exciton transfer to C60
results in intersystem crossing and the formation of triplet excitons which can hop back to the
donor and not contribute to charge separation.
51
Furthermore, electronic polarization effects
at the D/ A interface and leads to an interface-dipole, which in turn may increase the energy
26

gap between the HOMO of the donor and the LUMO of the acceptor. This increase leads to
the reduction of charge carriers generated at the interface which would reduce the reverse
saturation current, and negatively impacting the V oc.
52
The electronic coupling between the
CT and CS states is an important parameter that affects charge separation and charge
recombination. The deactivation of the CT state due to the electronic coupling between the
triply degenerate C60 LUMO and pentacene has been demonstrated using a computational
approach. Electron transfer to C60 causes the lifting of orbital degeneracy due to Jahn-Teller
effects and the three closely lying C60 anions can contribute to charge separation. Since the
coupling between the lowest lying CT and pentacene S1, and the coupling between the lowest
lying CT and pentacene T1 are now nearly equal, the proximity of the triplet states to the CT
can act as a pathway for deactivating the CT and not contribute to charge separation.
53

Role of morphology of materials
In BHJ devices the morphology of the D/ A interfacial area and domain sizes is a key
parameter that influences the properties of the CT state and charge generation. Phase
segregation that improves charge generation by minimizing geminate recombination, and
improves charge transport is key for device performance. When domain sizes are too large
exciton dissociation becomes limited by 𝐿 𝐷 and some excitons may not reach the interface;
if the domain size is smaller than the Coulomb capture radius because of very fine phase
separation, charges cannot escape geminate recombination. In this case the charges are
confined by coulomb attraction but by the size of the domain.
54
For example, in a comparison
27

of MeLPPP: PCBM and MDMO-PPV: PCBM polymer blends by Müller et al. CT states are
detected only for the former; the negligible presence of the CT is attributed to the aggregation
of MDMO-PPV resulting in large domain sizes.
55
. When the PCBM concentration is
increased in PF10TBT: PCBM films, photoluminescence spectra reveal the weakening of the
CT-state emission and the red-shifting of the CT emission leads to the lowering of 𝑉 𝑜𝑐
. The
reduction in photoluminescence intensity with increasing PCBM concentration is attributed
to the increased dissociation of excitons, which is further proven by the increased presence
of polymer polarons leading to an improvement in photocurrent.
56
The thermal annealing of
PFB:F8BT blend increased the EQE up to 140° C, and decreased upon further increase of
temperature.
56
Additionally, the application of Onsager- Braun theory to this system shows
that the, higher PCBM concentrations which leads to higher photocurrent requires larger
electron-hole separations and higher carrier motilities. High charge carrier motilities allow
charges to escape the Coulomb capture radius. Similarly, the morphology of thin films can
be adjusted by thermal and solvent annealing, and the choice of solvent for spin-coated
devices. Once the charges are separated are percolated pathway is needed to reach the
electrodes while avoiding bimolecular recombination.
(4) Charge Transport
Charge transport in the material is determined by the interaction energy between
the neighboring molecules in the bulk and the polarization energy of the molecule. In organic
films, weak intermolecular electronic interactions, charge trapping defects lead to the
localization of charges. When the polarization energy is large, charges are localized and are
transported through a hopping mechanism. Localized charges can polarize the neighboring
28

molecule, changing their geometry by affecting the spatial distribution of electrons and bond
lengths. The energy change required for this process is the reorganization organization energy
and the rate of hopping can be expressed by Marcus theory. Low overlap in less densely
packed amorphous films exhibit carrier motilities less than in ordered crystalline films.
Mobilities are typically 10
−7
cm
2
V
−1
s
−1
up to 10
−3
cm
2
V
−1
s
−1
for hole transporters and smaller
for electron transporting materials, and are dependent on temperature.
57
Low charge carrier
motilities reduce photocurrent as a result of allowing bimolecular recombination to compete
with charge collection.
(4) Charge Collection
During the fabrication process OPVs are sandwiched between a transparent electrode
such as Indium Tin Oxide (ITO) or Fluorine Tin Oxide (FTO), and a metal counter electrode
that is vacuum deposited. The nature of contacts between the thin films and the electrodes
affect device performance. If ohmic contacts form at the organic/electrode interface, the
barrier for charge collection is minimum with efficiencies close to unity. Low charge
collection rates can lead to the pile-up of carriers at the electrodes which in turn will lower
the exciton dissociation and charge transport efficiencies.
Commonly used materials for electrodes are given in Figure 1.7. ITO offers the
advantages of >85% transmittance in the visible region of the solar spectrum, high
conductance on glass (3000- 6000 S/ cm) and plastic (1500 S/ cm),
58
and wide bandgap (Eg=
3.2- 4.0 eV)
59,60
The work function of ITO is dependent upon the surface treatment provided,
and also on the In/ O and In/ Sn atomic ratios on the surface; the ITO work function is often
29

reported Φ
𝑓 = 4.8- 5.0 eV.
61,62
However, the high cost of ITO substrates has driven the need
for low cost alternatives. Current research in carbon nano-tubes,
63
,, 64
65
) graphene
66,67
fluorine-
doped Indium Tin Oxide (FTO) demonstrates good potential as substitutes.
The back electrode is deposited on the active layer and a lower work function material
is chosen, so that it may provide an electric field that promotes hole extraction from ITO and
electron extraction from the back electrode. Al is a commonly used back electrode (Φ = 4.1);
Ca creates a large electric field (Φ = 2.9); Ag creates a smaller field than Al (Φ = 4.5); and
Au reverses the field (Φ = 5.1) leading to charge flow in the opposite direction, while making
the Au electrode a good candidate for inverse device architecture.
68
6970
-71

The organic/electrode interface is desirably an ohmic contact which does not introduce
an extra resistive component and aids charge injection.
72,73
Additionally, the insertion of a

Figure 1.7. Energy level diagram for commonly used electrodes
3.1
5.0
6.0
4.0
C
60
PSQ
4.7
ITO
5.1
Au
4.5
Ag
4.2
Al
2.9
Ca
e
e
h
30

buffer layer between the active layer and the metal electrode has improved device
performance.
74
Often, wide band gap organic materials are used as buffers. The buffer layer
provides the advantages of minimizing exciton quenching at the interface, preventing the
damage to the active layer when hot metal diffuses, enhancing charge collection without
affecting optical properties, reducing dark current leakage to increase open circuit voltage
(𝑉 𝑜𝑐
),
75
and facilitating hole injection. Additionally, buffers on the back-electrode side can act
as an optical spacer. Here, the cathode can reflect unabsorbed light back to the device and
allows the maximum absorption of light. Devices can be designed to take advantage of this
feature by eliminating negative interference in the active region where incident and reflected
light waves may overlap.
76

Widely used buffer materials in OPVs are given in Figure 1.8.
PEDOT: PSS (poly(3,4ethylenedioxythiophen)poly(styrenesulfonate)) which is a
conducting polymer blend, is one of the common anode buffer layers used in OPV. PEDOT:

Figure 1.8. Chemical structures of commonly used anode and
cathode buffers
31

PSS is known to 4727 ITO, eliminating surface spikes that would cause the device to short
77
;
it increases the anode work function to 5.1 eV while forming ohmic contacts with donors
78
,
and it eliminates the nonconductive regions on the ITO surface.
79
However, PEDOT:PSS is
not an efficient electron blocker
80
, and its high acidity (pH ~ 1) causes the corrosion of ITO,
and at high temperatures it enables the diffusion of indium ions to the active layer.
81,82

Additionally, MoO3 is also used as an anode buffer, or hole transport layer; it is efficient at
locking misdirected electrons, and reducing dark current which restores (𝑉 𝑜𝑐
).
75

On the back-electrode side tris(8-hydoxyquninoline) aluminum or Alq3 has been used
for its good hole blocking and electron transporting; it is also known to prevent the permeation
of water and oxygen and thereby increase the lifetime of unencapsulated devices by 150 as
compared to bathocuproine (BCP) which is also a widely-used buffer.
83
BCP prevents exciton
quenching at the organic/metal interface, and acts as an efficient exciton blocking layer due
to its wide band gap. Since BCP as a high LUMO energy of 3.5 eV
77
, it would appear to act
as an energy barrier for both holes and electrons (e.g. LUMO of C60 is 4.0 eV), however, it
has been proposed that the low lying defect states that form during deposition of the metal
cathode allow the transport of electrons through the buffer onto the cathode.
77,84,85

Additionally, doping BCP with 10 wt% of 3,4,9,10-perylenetetracarobxylic bis-
benzimidazole (PTCBI) prevents the crystallization of BCP at room temperature and
improves device performance. Furthermore, 1,10-phenanthroline (BPhen) can also be an
alternative for BCP since it has similar frontier orbital energies but provides the advantage of
electron mobility that is two orders of magnitude high.
86,87

32

1.5 Characterization of Devices
The next section follows a brief discussion of electrical measurements in OPVs. The
basic electrical measurement process involves connecting the device to an external power
source, sweeping a DC voltage (V) and measuring of the electrical current density (JV). Each
device measurement is made under dark, followed by white light illumination with 1 sun
intensity. The characteristic Current-Voltage (JV) curves of OPV devices are shown in Figure
1.9. JV measurements are made both in the dark and under 1 sun illumination with a simulated
solar spectrum at a range of intensities. A silicon photodetector calibrated by NREL with a
known responsivity is used for intensity calibration. A correction of the mismatch between
the simulated solar spectrum and the AM1.5G standard can then be made with the calibration.
Generally, the JV measurement is performed by sweeping the voltage from negative (reverse
bias) to positive (forward) bias. The electrochemical implication of this process is that under
reverse bias, the organic material is reduced at the anode and oxidized at the cathode.
88
A
discussion of the key parameters extracted measurement: short circuit current (𝐽 𝑠𝑐
), open
circuit voltage (𝑉 𝑜𝑐
), fill factor (𝐹𝐹 ), and power conversion efficiency (ηP (%)) is the given in
the following section.
1.6 Illumination Parameters
𝑱 𝒔𝒄
is a measure of the amount of light generated carriers when no voltage is applied.
The magnitude of 𝐽 𝑠𝑐
depends on the overlap between solar spectrum and the thin film
absorption of the material, the absorptivity of the materials, charge transfer rates, and charge
carrier mobility. One strategy that can be employed to enhance 𝐽 𝑠𝑐
is to utilize the entire solar
33

spectrum. This strategy is used in Chapter 4 where squaraine molecules are used as donors
to utilize the blue region of the solar spectrum. A broad range of squaraine have been
developed for the green- near IR region while the high-energy photons are often overlooked.
𝑽 𝒐𝒄
is the required applied voltage to shut off the current in the device under white light
illumination. At both operating points of 𝐽 𝑆𝐶
and 𝑉 𝑂𝐶
the operating power of the cell is zero.
The 𝑭𝑭 is a measure of the maximum output power relative to the photocurrent and
photovoltage produced by the device and can be given by grey rectangle with peaks at 𝐽 𝑆𝐶
and
𝑉 𝑜𝑐
(the available power at these points is zero) and can be visualized as the “squareness” of
the device charateristic. The common range for OPV 𝐹𝐹 is 0.3 to 0.7. 𝐹𝐹 lowers as a result
7of parasitic resistive losses.

Figure 1.9. Characteristic I – V curves of a solar cell in the dark (dashed traces) and
under light (solid traces).
-1 -0.5 0 0.5 1 Voltage (V)
Current Density (mA/ cm
2
)
P
max
V
oc
J
sc
Light
Dark
FF =
34

Eq. 1.5 𝐹𝐹 =
𝐽 𝑀 .𝑉 𝑀 𝐽 𝑆𝐶
.𝑉 𝑂𝐶

According to Eq. 1.5 for an OPV under illumination the maximum output power
density is given by 𝑷 𝒎𝒂𝒙 in Figure 1.13 corresponds to the area of the filled gray rectangle
in quadrant four.

Eq. 1.6 𝑃 𝑚𝑎𝑥
= 𝐽 𝑆𝐶
. 𝑉 𝑂𝐶
. 𝐹𝐹
Finally, ηP is calculated the ratio of
𝑃 𝑚𝑎𝑥 𝐸 𝑇𝑜𝑡𝑎𝑙 where 𝐸 𝑇𝑜𝑡𝑎𝑙 is the integrated spectral
irradiance (1000 W/ m
2
for AM 1.5). The maximum theoretical efficiency for a p-n junction
has been calculated by William Shockley and Hans J. Queisser in their seminal work using
the detailed balance principle.
89
In effect, for an ideal p-n single junction PV under ideal sunlit
conditions (1000 W/m
2
), with only radiative recombination losses, the maximum theoretical
efficiency is 33.7%.
1.7 Dark Parameters
Under dark conditions the OPV shows characteristic diode behavior with saturated
current density (𝐽 𝑠 ). In an ideal OPV device, under reverse bias, the current flows independent
of the applied voltage. When the applied voltage is further increased (into the forward bias),
an exponential rectification is observed in the current density. In a perfect device with no
charge recombination, where there are no losses from resistive elements, the voltage
dependence of the current density in the forward bias (upon rectification), and the magnitude
of the net current flowing through the device (𝐽 ) can be given by the ideal-diode Eq. 1.7 where
35

𝑞 is the elementary charge; 𝑛 is the diode ideality factor (𝑛 = 2 for organic semiconductors);
𝑘 is the Boltzman constant; 𝑇 is the temperature respectively. The equation suggests that 𝐽 𝑠
characterizes the dependence on voltage during both reverse and forward biases.
Eq. 1.7 𝐽 = 𝐽 𝑠 𝑒𝑥𝑝 [(
𝑞𝑉
𝑛𝑘𝑇 ) − 1]

The magnitude of 𝐽 𝑠 will depend on carrier injection from the electrodes, charge
mobility of the materials, charge recombination at the D/A interface, and temperature. If
temperature is increased, a lower forward-bias voltage is sufficient to obtain the same diode.
90

𝐽 𝑠 is a useful measurement for recombination in the OPV: increased recombination leads to
an increase in 𝐽 𝑠 . A change in the 𝐽 𝑠 results in a change in the turn on voltage for the diode.
When photon absorption occurs (under illumination), the J-V response is characterized
by a raise in the generated current as a reverse-bias is applied. This is a result of the conversion
of photogenerated D
+
and A
-
ions back to the neutral when a reverse bias is applied at the
external contacts. For an ideal diode illumination, the voltage dependence of the current is
given by Eq. 1.8, where 𝐽 𝑝 ℎ
is the photogenerated current density.

Eq. 1.8 𝐽 = 𝐽 𝑠 𝑒𝑥𝑝 [(
𝑞𝑉
𝑛𝑘𝑇 ) − 1] − 𝐽 𝑝 ℎ

36

When the applied bias is 𝑉 = 0 (under short circuit conditions), then 𝐽 = −𝐽 𝑝 ℎ
which
also corresponds to 𝐽 𝑠𝑐
. The dependence of 𝑉 𝑜𝑐
on dark saturation current can be given by Eq.
1.9. When = 0 , 𝑉 𝑜𝑐
can be expressed as:

Eq. 1.9 𝑉 𝑜𝑐
=
𝑛𝑘𝑇 𝑞 ln (
𝐽 𝑠𝑐
𝐽 𝑠 + 1)
In general, it can be assumed that the photocurrent for reverse bias is always much
higher than the dark current (𝐽 𝑠 𝑐 ≫ 𝐽 𝑠 ) which means that
𝐽 𝑠𝑐
𝐽 𝑠 ≫ 1. Eq. 1.9 can then be further
simplified to give Eq. 1.10. Thus, for a given 𝐽 𝑠𝑐
a low dark current (𝐽 𝑠 ) will be reflected with
a high 𝑉 𝑜𝑐
.

Eq. 1.10 𝑉 𝑜𝑐
=
𝑛𝑘𝑇 𝑞 ln (
𝐽 𝑠𝑐
𝐽 𝑠 )

Figure 1.10. Single diode equivalent circuit model is used for estimating solar cell losses
arising from resistive elements.
J
L
J
0
R
P
R
s
+
-
V
37

For the ideal organic semiconductor (n =2) the photovoltage is dependent on 𝐽 𝑠𝑐
, which
represents the frequency of the photogenerated charge collection, and 𝐽 𝑠 which represents the
frequency of the photogenerated charge recombination.
88
However, in practice the presence
of resistive elements in semiconductor causes a shift from ideal-diode behavior as shown by
the single diode equivalent circuit model in Figure 1.10.
For a high-performance device, series resistance (𝑅 𝑠 ) which takes the conductivity of
the organic semiconductors, contact resistance between the semiconductors and adjacent
electrodes, and the resistance of the electrodes and the interconnections into account, needs
to be minimized. Shunt or parallel resistance (𝑅 𝑃 ), which causes the loss of charge carriers
due to possible leakage paths need to be maximized. 𝑅 𝑃 includes the structural defects such
as pin-holes that are commonly seen in organic films, and recombination sites caused by
impurities.
When the non-idealities in the OPV are taken into account, the generalized Shockley
equation can be given by
91,92

Eq. 1.11 𝐽 =
𝑅 𝑃 𝑅 𝑆 +𝑅 𝑃 {𝐽 𝑆 [exp (
𝑞 ( 𝑉 −𝐽 𝑅 𝑠 𝑛𝑘𝑇 ) − 1] +
𝑉 𝑅 𝑃 } − 𝐽 𝑝 ℎ
( 𝑉 )
For solar cells with minimal leakage current 𝑅 𝑃 >> 𝑅 𝑆 , and Eq. 1.11 can be simplified
to
92
:

38

Eq. 1.12 𝐽 = {𝐽 𝑆 [exp (
𝑞 ( 𝑉 −𝐽 𝑅 𝑠 𝑛𝑘𝑇 ) − 1] +
𝑉 𝑅 𝑃 } − 𝐽 𝑝 ℎ
( 𝑉 )
Dark parameters 𝐽 𝑠 and 𝑛 can be determined by fitting the dark JV characteristic using
Eq. 1.12. The origin of the 𝐽 𝑠 is from some thermally generated carriers from the bulk but
most are from thermally generated carriers at the D/A. The 𝐽 𝑠 from the interface generated
carriers can be given by Eq. 1.13 where 𝐸 𝑎 is the activation energy for thermally generated
carriers at the interface.
91,92

Eq. 1.13 𝐽 = 𝐽 𝑠 𝑒𝑥𝑝 (
𝐸 𝑎 𝑛𝑘𝑇 )
Since there are two carriers- a hole and an electron that is generated at the interface,
𝐸 𝑎 = (
∆𝐸 𝐷𝐴
2
) The magnitude of 𝐽 𝑠 depends on the D/ A interface are, density transfer states
at the HOMO and LUMO of the D and A, reorganization energy for the electron process
D*→A, and the conductivity of electrons and holes in the bulk. 𝐽 00
is the pre-exponential
factor which is a measure of the charge recombination to charge generation ratio. Eq. 1.12
and Eq. 1.13 can be combined to express 𝑉 𝑜𝑐
as
92
:
Eq. 1.14 𝑉 𝑜𝑐
=
𝑛𝑘𝑇 𝑞 ln (
𝐽 𝑠𝑐
𝐽 𝑠 ) +
∆𝐸 𝐷𝐴
2𝑞

The ratio
𝐽 𝑠 𝑐 𝐽 𝑠 is logarithmically related to 𝑉 𝑜𝑐
, while both 𝐽 𝑠𝑐
and ∆𝐸 𝐷𝐴
show linear
relationships with 𝑉 𝑜𝑐
. The thermal activation energy required for charge separation at the
39

interface is given by
∆𝐸 𝐷𝐴
2
, and this magnitude approaches 𝑉 𝑜𝑐
at low temperatures.
93
To
achieve the maximum 𝑉 𝑜𝑐
, 𝐽 𝑠 needs to be minimized and 𝐽 𝑠𝑐
must be maximized. Perez et
al.
92
report the molecular properties can be tuned to minimize 𝐽 𝑠 and thereby maximize ∆𝐸 𝐷𝐴
.
In a comparison of tetracene and rubrene donors in OPVs, they report that pendant phenyl
groups in rubrene leads to weal inter molecular interactions between rubrene-rubrene and
rubrene-C60 compared to tetracene which shows broad and red shifted absorbance in film.
Weak molecular interactions lead to a significantly low 𝐽 𝑠 of 2.7  10
-3
μA/cm
2
in rubrene (𝐽 𝑠
= 7.7  10
-2
μA/cm
2
), which leads to a 𝑉 𝑜𝑐
of 0.9 V compared to the 0.55 V in tetracene.
Additionally, based on Marcus theory for electron transfer in the inverted region
Schlenker et al.
6
have proposed that 𝐽 𝑆 depends on the sum of radiative and nonradiative
recombination 𝑘 𝑟𝑒𝑐
as given by for the reaction (𝐷 +
𝐴 −
) → 𝐷 0
+ 𝐴 0
where 𝐷 +
𝐴 −
is the CT
concentration at the D/ A interface. They report that in the dark, this reaction is driven
electrically, when a forward bias is applied to the anode. Under the forward bias, D+ charges
injected from the anode, and A- injected from the cathode may take part in the recombination
process at a rate 𝑘 𝑟𝑒𝑐
.
Eq. 1.15 𝐽 𝑆 = 𝑞 𝑘 𝑟𝑒𝑐
[( 𝐷 +
𝐴 −
) ]
When 𝑅 𝑃 = ∞, 𝑅 𝑠 = 0, and 𝐽 𝑝 ℎ
≫ 𝐽 𝑆 photovoltage can be expressed as
Eq. 1.16 𝑉 𝑜𝑐
=
𝑛𝑘𝑇 𝑞 ln (
𝐽 𝑠𝑐
𝑞 𝑘 𝑟𝑒𝑐 [𝐷 +
][𝐴 −
]
)
This interpretation suggests that recombination of charges is a major loss mechanism
in OPVs. The loss mechanism can be addressed by suppressing charge injection on either side
40

of the heterojunction, reducing charge leakage in to the D/A region by controlling motilities,
and by controlling the injection of holes and electrons from the anode and cathode. However,
since maximizing the charge collection efficiency is equally important, controlling parasitic
charge recombination may lower the FF and 𝐽 𝑠𝑐
in bilayer devices.
1.8 Spectral Mismatch Factor (M)
The efficiency of the OPV depends on the reference solar spectrum, light intensity,
and the OPV temperature, and standard reporting conditions (SRC) are used to maintain the
reproducibility of devices. The established SRC requires a AM1.5G spectrum (Air Mass for
37° south-facing tilt, global, 1000 W/ m
2
light intensity, and 25°C temperature. A mismatch
in spectral response originates from two sources:
1) Even for the best solar simulators the match between the reference AM 1.5 G
spectrum 𝐸 𝑅 ( 𝜆 ) , and the solar simulator spectrum 𝐸 𝑆 ( 𝜆 ) is far from perfect.
2) The mismatch in the responsivity of the reference cell 𝑆 𝑅 ( 𝜆 ) , and the test cell 𝑆 𝑇 ( 𝜆 )
M is then calculated as
94
:
Eq. 1.17 𝑀 =
∫ 𝐸 𝑅 ( 𝜆 ) 𝑆 𝑅 ( 𝜆 ) 𝑑 ( 𝜆 )
∫ 𝐸 𝑆 ( 𝜆 ) 𝑆 𝑅 ( 𝜆 ) 𝑑 ( 𝜆 )

∫ 𝐸 𝑆 ( 𝜆 ) 𝑆 𝑇 ( 𝜆 ) 𝑑 ( 𝜆 )
∫ 𝐸 𝑅 ( 𝜆 ) 𝑆 𝑇 ( 𝜆 ) 𝑑 ( 𝜆 )

Where 𝑀 = 1, the test cell should have the same spectral response as the reference cell
in order to maintain accurate measurements. Correction for the 𝐽 𝑠𝑐
under the reference
spectrum is made dividing the measured 𝐽 𝑠𝑐
(under the simulator spectrum) by M.
Eq. 1.17 𝐽 𝑠𝑐
𝑅𝑇
=
𝐽 𝑠𝑐
𝑆𝑇
𝑀
41

1.9 Spectral response (SR) & External Quantum Efficiency (EQE)
The spectral response of an OPV can be measured under a monochromatic light
source by recording the photocurrent generated as a function of wavelength.
The EQE measures the responsivity from the OPV at each wavelength under short
circuit conditions, and can be expressed in units of electrons generated per absorbed photon.
The OPV under test is illuminated by a Xe arc lamp, chopped and passed through double
grating monochromator, and measured with a lock-in amplifier to perform spectral mismatch
and spectral responsivity measurements. The quantum efficiency can be calculated from the
measured spectral response of the OPV.
Eq. 1.18 𝑆𝑅 =
𝑞𝜆
ℎ𝑐 𝑄𝐸
The measured spectral response can also be used for calculating the theoretical 𝐽 𝑠𝑐
𝑄𝐸
:
Eq. 1.19 𝐽 𝑠𝑐
𝑄𝐸
= ∫ 𝐸 𝑅 ( 𝜆 ) 𝑆 𝑡 ( 𝜆 ) 𝑑𝜆 ∞
0

A percentage difference of 10% between 𝐽 𝑠𝑐
𝑅𝑇
and 𝐽 𝑠𝑐
𝑄𝐸
indicates the agreement between
broadband and monochromatic measurement techniques.

1.10 Summary of topics
This thesis focuses on the energy management of photovoltaic devices using a
sensitizer, and the development of new squaraine donor materials that absorb in different
regions of the solar spectrum. Chapter 2 describes the design and development of pyrrolic
42

squaraines as donor materials in organic photovoltaics. Chapter 3 covers improving the
performance of a solar cell using a sensitizer for additional spectral coverage and energy
transfer.

43

1.11 Endnotes for Chapter 1
(1) NASA, Global Climate Change Report. Report. http://climate.nasa.gov 2011.
(2) IPCC: Intergovernmental panel on climate change- Climate Change 2007: The Physical
Science Basis Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.
Tignor and H.L. Miller (eds.). Cambridge University Press, Cambridge, United
Kingdom and New York, NY, USA, 996 pp.
(3) Perez, R.; Perez, M., A fundamental look at energy reserves for the planet. The IEA
SHC Solar Update 2009, 50, 2-3.
(4) Becquerel, A.-E., Memoire sur les effets electriques produits sous l'influence des
rayons solaires. Comptes Rendus 1839, 9, (567), 1839.

(5) Ohl, R. S., Light-sensitive electric device. In Google Patents: 1946.
(6) Service, R. F., Solar Energy: Can the Upstarts Top Silicon? Science 2008, 319, 718-
720.
(7) Goetzberger, A., Hebling, C., Schock, H. W., Photovoltaic Mater., History, Status and
Outlook. Mater. Sci. Eng., R 2003, 40, (1), 1-46.
(8) Schlenker, C. W., Thompson, M. E., Chem 2012, 312, 175-212.
(9) Tang, C. W., Albrecht, A. C., Photovoltaic Effects of Metal Cholorophyll-a-Metal
Sandwich Cells. J. Chem. Phys. 1975, 62, (6), 2139-2149.
(10) Barth, S., Bassler, H., Intrinsic Photoconduction in Ppv-Type Conjugated Polymers.
Phys. Rev. Lett. 1997, 79, (22), 4445-4448.
(11) Tang, C. W., Two Layer Organic Photovoltaic Cell. Appl. Phys. Lett. 1986, 48, (2),
183-185.

(12) M. Hiramoto, H. Fujiwara, M. Yokohama, Appl. Phys. Lett. 1991, 58,1062.

(13) N. S. Sariciftci, L. Smilowitz, A. J. Heeger and F. Wudl, Science, 1992, 258, 1474.

(14) G. Yu, J. Gao, J. C. Hummenlen, F. Wudl, and A. J. Heeger, Science 1995,270,1789.

(15) S. Tanaka, Appl. Phys. Let, 2009, 94,113506.

(16) Kurt B. and Sajeev J., Phys. Rev. E 1998, 58, 3896.

(17) Sullivan P, Jones T. S., Appl. Phys. Lett. 2007, 91, 233114.
44

(18) Gommans H, Ad., Func. Mater. 2007, 17,2653.

(19) Peumans P, Appl. Phys. Lett. 2002, 80,338.

(20) Law K Y, J. Phys. Chem. 1987, 91, 5184.

(21) Wang S., Appl. Phys. Lett. 2009, 92, 233304.

(22) Kroto H. W, Nature 1985, 318, 162.

(23) Haddock J. N. Org. Elect. 2005, 6, 182.

(24) Fullerene: Principles and applications, Langa F, Royal Doc of Chem Cambridge,
2007.

(25) Imahori H, Adv, Func .Mater. 2004, 14, 525.

(26) R. C. Powell and Z. G. Soos, J. Lumin. 1975, 11, 1.
(27) Förster, T. Annalen Der Physik 1948, 2, 55.

(28) T. Förster, Discuss. Faraday Soc. 1959, 27, 7.

(29) Dexter, D. L. J. of Chem. Phys. 1953, 21, 836.
(30) Kipplen B., Brédas J. L., Energy Environ. Sci., 2009, 2, 251.

(31) P. Peumans, A. Yakimov, and S. R. Forrest. J. of Appl. Phys., 2003, 7, 3693.

(32) Wade A. Luhman and Russell J. Holmes. Adv. Func. Mater., 2011, 4, 764771.

(33) P. Avakian and R. E. Merr, Phys. Rev. Lett.,1964, 18, 13.

(34) D. F. Williams and J. Adolph. J. Chem. Phys., 1967, 11, 46.

(35) M. Hiramoto, H. Fujiwara, and M. Yokoyama. Appl. Phys. Lett., 1991, 10, 1062.

(36) G. Yu, J. Gao, J. C. Hummelen, F. Wudl, and A. J. Heeger. Science, 1995, 5243, 1789.

(37) Menke, S. M., Luhman, W. A. & Holmes, R. J. Nat. Mater. 2013, 12, 152.

(38) G. Wei, R. R. Lunt, K. Sun, S. Wang, M. E. Thompson, and S. R. Forrest, Nano Lett.,
2010, 10, 3555.

45

(39) O. V. Mikhnenko, J. Lin, Y. Shu, J. E. Anthony, P. W. M. Blom, T. Q. Nguyen, and
M. A. Loi, Phys. Chem. Chem. Phys. 2012, 14, 14196.

(40) M. Hiramoto, H. Fujiwara, and M. Yokoyama, Appl. Phys. Lett. 1991, 58, 1062.

(41) G. Yu, J. Gao, J. C. Hummelen, F. Wudl, and A. J. Heeger, Science 1995, 270, 1789.

(42) Marcus, R. A., J. Chem. Phys. 1956, 5, 966.

(43) Marcus, R. A., Electron Transfer Reactions in Chem.: Theory and Experiment
(Nobel Lecture). Ang. Chem. Int. Ed. 1993, 8, 1111.

(44) Giaimo, J. M., Gusev, A. V., Wasielewski, M. R., J. Am. Chem. Soc. 2002, 29,
8530.

(45) Trinh, C., Kirlikovali, K. O., Das, S., Ener, M. E., Gray, H. B., Djurovich, P. I.,
Bradforth, S., Thompson, M. E., J. Chem. Phys. C 2014.

(46) Hormann, U., Kraus, J., Gruber, M., Schuhmair, C., Linderl, T., Grob, S., Kapfinger,
S., Klein, K., Stutzman, M., Krenner, H. J., Brutting, W., Phys. Rev. B 2013, 23, 235307.

(47) Vandewal, K., Tvingstedt, K., Gadisa, A., Inganas, O., Manca, J. V., Phys. Rev. B
2010, 12, 125204.

(48) Gruber, M., Wagner, J., Klein, K., Hörmann, U., Opitz, A., Stutzmann, M., Brütting,
W., Adv. En. Mater. 2012, 9, 1100.

(49) Widmer, J., Tietze, M., Leo, K., Riede, M., Adv. Func. Mater. 2013, 46, 5814.

(50) Jailaubekov, A. E., Willard, A. P., Tritsch, J. R., Chan, W.-L., Sai, N., Gearba, R.,
Kaake, L. G., Williams, K. J., Leung, K., Rossky, P. J., Zhu, X. Y. Nat. Mat. 2013, 12,
66.

(51) Grancini, G., Maiuri, M., Fazzi, D., Petrozza, A., Egelhaaf, H. J., Brida, D., Cerullo,
G., Lanzani, G. Nat. Mat. 2013, 12, 29.

(52) Vandewal, K., Nat. Mat. 2014, 3, 63.

(53) Hukka T. I. Adv Mater. 2006, 18, 1301.

(54) Schueppel R, Phys Rev B: Condens. Matter Mater. Phys 2008, 77, 085311.

(55) Potscavage W. J Appl. Phys. Lett. 2008, 93, 193308.

46

(56) Bredas, J. L. , Norton E, Acc. Chem. Res. 2009,42, 1691.

(57) Ruseckas A, Shaw P. E, Samuel I. D. W, Dalton. Trans.

(58) Müller J. G. Phys Rev. B. 2005, 72,195208.

(59) Veldman D, J. Am. Chem. Soc 2008, 130, 7721.

(60) Yang, Y., Huang, Q. L., Metz, A. W., Ni, J., Jin, S., Marks, T. J., Madsen, M. E.,
DiVenere, A., Ho, S. T. Adv. Mater. 2004, 16, 321-324.

(61) Han, H., Mayer, J. W., Alford, T. L. J. of Appl Phys. 2006, 100.

(62) Antony, A., Nisha, M., Manoj, R., Jayaraj, M. K. Appl. Surf. Sci. 2004, 225, 294-301.

(63) Brumbach, M., Veneman, P. A., Marrikar, F. S., Schulmeyer, T., Simmonds, A., Xia,
W., Lee, P., Armstrong, N. R. Langmuir 2007, 23, 11089-11099.

(64) Paniagua, S. A., Hotchkiss, P. J., Jones, S. C., Marder, S. R., Mudalige, A., Marrikar,
F. S., Pemberton, J. E., Armstrong, N. R. J. of Phys. Chem. C 2008, 112, 7809-7817.

(65) Dabera, G. D. M. R., Jayawardena, K. D. G. I., Prabhath, M. R. R., Yahya, I., Tan,
Y. Y., Nismy, N. A., Shiozawa, H., Sauer, M., Ruiz-Soria, G., Ayala, P., Stolojan, V.,
Adikaari, A. A. D. T., Jarowski, P. D., Pichler, T., Silva, S. R. P. Acs Nano 2013, 7, 556-
565.

(66) Barnes, T. M., Bergeson, J. D., Tenent, R. C., Larsen, B. A., Teeter, G., Jones, K. M.,
Black burn, J. L., van de Lagemaat, J. Appl Phys. Lett. 2010, 96.

(67) Pasquier, A. D., Unalan, H. E., Kanwal, A., Miller, S., Chhowalla, M. Appl Phys. Lett.
2005, 87.
(68) De Arco, L. G., Zhang, Y., Schlenker, C. W., Ryu, K., Thompson, M. E., Zhou, C.
Acs Nano 2010, 4, 2865-2873.

(69) Yang, D., Zhou, L., Chen, L., Zhao, B., Zhang, J., Li, C. Chem. Comm. 2012, 48,
8078-8080.

(70) H. B. Michaelson, J. App. Phys. 1991, 48, 4729.

(71) H. L. Skriver and N. M. Rosengaard, Phys. Rev. B 46, 7157 ,1992.
(72) M. S. White,D. C. Olson, S. E. Shaheen, N. Kopidakis, and D. S. GinleyAppl. Phys.
Lett. 2006, 89, 143517.

47

(73) Christoph J. Brabec, Solar Energy Mater. and Solar Cells, 2004, 83, 273.

(74) Hirose, Y., Kahn, A., Aristov, V., Soukiassian, P., Bulovic, V., Forrest, S. R., Phys.
Rev. B 1996, 54, 13748.

(75) Ishii, H., Sugiyama, K., Ito, E., Seki, K., Adv. Mater. 1999, 11, 605.

(76) Peumans, P., Yakimov, A., Forrest, S. R., J. Appl. Phys. 2003, 93, 3693.

(77) Li, N., Lassiter, B. E., Lunt, R. R., Wei, G., Forrest, S. R., Appl. Phys. Lett. 2009, 2,
023307.

(78) Stubinge T., Brutting W., J. Appl. Phys. 2001, 90, 3632.

(79) Peumans P., Forrest S. R., Appl. Phys. Lett. 2001, 79, 126.

(80) Barbec C. J., Shaheen S.E. Appl. Phys. Lett. 2002,80,1288.

(81) Brumbach M., Veneman P. A, Langmuir 2007, 23, 11089.

(82) Donely C, Dunphy D, Langmuir 2002, 18, 450.

(83) Ni. J, Yan. H, J. Am. Chem. Soc. 2005, 127, 5613.

(84) Wong K. W, Yip H. L., Appl. Phys. Lett. 2002, 80, 2788.

(85) Song Q L, Li. F. Y, Chem. Phys. Lett. 2005, 416, 42.

(86) Rand B.P. Adv. Mater 2005, 17, 2714.

(87) Hirose Y. Kahn A, Phys Rev B 1996 54, 13748.

(88) Ichikawa M., J. Appl. Phys. 2003, 94, 796.

(89) Naka S. Okada H, Appl. Phys. Lett. 2000, 76, 197.

(90) Jenny Nelson, Acc. Chem. Res. 2009, 42, 1768.

(91) Schlenker, C. W., Thompson, M. E. Chem. Comm. 2011, 47, 3702.

(92) W. Shockley, H. J. Queisser., J. Appl. Phys, 1961, 32, 3.

(93) D. A. Neamen, Semiconductor Phys. and Devices, Third Edition edn., McGraw-Hill,
New York, 2003.
48

(94) Dongaonkar, S., Servaites, J. D., Ford, G. M., Loser, S., Moore, J., Gelfand, R. M.,
Mohseni, H., Hillhouse, H. W., Agrawal, R., Ratner, M. A., Marks, T. J., Lundstrom, M.
S., Alam, M. A. J. App. Phys. 2010, 108.

(95) Perez, M.D., Borek, C., Forrest, S.R., and Thompson, M. E., J. Am. Chem.
Soc.,2009, 131,9281.

(96) Rand, B.P., D.P. Burk, and S. Forrest, R., Phys. Rev. B, 2007, 11, 115327.

49

CHAPTER 2
Symmetric Pyrrolic Squaraines and Their Application to Organic Photovoltaics
2.1 Abstract
We report the material characterization and single crystal X-ray analysis of bis(3,5-
dimethyl-1H-pyrrol-2-yl)-3-oxocyclobut-1-en-1-olate and a derivative of this symmetric
squaraine dye with a thiophene substituent that has high solubility and high absorptivity (ε
= 5.2  10
5
M
-1
cm
-1
) absorption in the 532- 690 nm region of the solar spectrum making
it a promising candidate for application in organic photovoltaics (OPVs). The squaraine
derivative shows a markedly broader absorption band in solution than parent derivative,
while both show low quantum yields (ΦPL = 0.047 and 0.014 respectively). OPVs were
fabricated using the following architecture: ITO/ MoO3 (120 Å)/ Squaraine (80-85 Å)/
C60(400 Å)/ BCP (100 Å)/ Al (1000 Å), BCP = bathocuproine, with the vacuum deposition
of the parent and the solution processing of the new derivatives. Devices fabricated with
thin films of the parent squaraine forms smooth films that lead to higher fill factors (FF),
photocurrent and better performance under 1 sun, AM1.5G simulated illumination: open-
circuit voltage Voc = 0.43 V, short-circuit current Jsc = 5.17 mA/cm
2
, FF = 0.52, and power
conversion efficiency η = 1.15 %, compared with the thiophene based squaraine: V oc =
0.56 V, short-circuit current Jsc = 4.34 mA/cm
2
, FF = 0.42, and power conversion efficiency
η = 1.02 %. KEYWORDS: squaraine dyes, organic photovoltaic, solar cell
50

2.2 Introduction
Squaraine dyes are characterized by an intense absorption band in the visible and near
IR spectral regions.
1,2
They possess an electron accepting core that is attached to electron
donating groups and form donor-acceptor-donor (D-A-D) types of systems.
3
The electron
donating groups can be electron rich aromatics or heterocycles, which can be used to tune
the absorption making squaraines a versatile group of chromophores.
4-6
As a result of their
interesting opto-electrical properties squaraines have been used in a wide variety of
commercial applications such as sensitizers in xerography,
7-9
photodynamic therapy,
10-12

nonlinear optics,
13-15
bio-imaging,
16,17
and solar energy conversion.
18-21
The bulk of this
work is based on squaraines that absorb in the 630- 700 nm region, while the analogs that
absorb in the blue region of the solar spectrum are overlooked. Specifically, in the
application to organic photovoltaics utilization of photons from the entire solar spectrum,
including the blue region is important for improving the power conversion efficiencies.
Squaraine dyes made from the condensation of 2,4-dimethylpyrrole with squaric acid
are the among the first squaraines to be reported.
4
The condensation occurs readily over
the highly reactive α-position in the pyrrole ring to produce a deep purple dye. If both
α-positions are available for condensation the formation of insoluble polymeric materials
has been reported.
4
Although the synthesis of 2,4-bis(3,5-dimethyl-1H-pyrrol-2-yl)-3-
oxocyclobut-1-en-1-olate is among the first squaraines reported, they have not been fully
characterized in subsequent literature and the crystal structure has not been reported due to
the difficulty in growing single crystals as a result of poor solubility of the material.
22
We
report the single crystal X-ray analysis of bis(3,5-dimethyl-1H-pyrrol-2-yl)-3-
51

oxocyclobut-1-en-1-olate (PSQ) which show the packing arrangement conducive for
intermolecular charge transport. Correspondingly, introducing a thiophene substituent at
the α-position of the pyrrole ring would induce the intermolecular π–π interaction and
consequently improve charge transport, while allowing the tuning of the absorption profile
and frontier molecular orbitals. In addition, the alkylation of the pyrrolic nitrogen would
improve the solubility of the material. Here we show the facile synthesis of 2,4-bis(1-hexyl-
5-(thiophen-2-yl)-1H-pyrrol-2-yl)-3-oxocyclobut-1-en-1-olate (NSP), the significant
differences in absorption, emission, and electrochemical properties compared to the parent
material and their performance in bilayer organic photovoltaics.

2.3 Experimental section
Instrumentation. General: NMR spectra were recorded on Varian Mercury 500 MHz
spectrometers at room temperature. Mass spectral analysis was performed on a Shimadzu
Prominance-LCMS 2020 equipped with a column oven (T = 40 °C), a PDA photodetector
(200– 800 nm), and an MS spectrometer (LCMS 2020; m/z range: 0– 2000; ionization
(a) (b)
Scheme 1. Chemical structures of (a) PSQ (b) NSP

( a )

52

modes: ESI/APCI). Thermogravimetric analyses were performed using a
thermogravimetric analyzer TG 209 Libra under a high-purity air flow (20 mL min ‑1).
Samples were heated from 30 to 300 C at a linear rate of 15 K min
-1
. UV-visible spectra
were recorded on a Hewlett-Packard 4853 diode array spectrophotometer. Steady-state
emission experiments at room temperature were performed using a Photon Technology
International Quanta Master Model C-60SE spectrofluorimeter. Quantum efficiency
measurements were carried out using a Hamamatsu C9920 system equipped with a xenon
lamp, calibrated integrating sphere, and model C10027 photonic multichannel analyzer.
Electrochemistry: Cyclic voltammetry (CV) and differential pulse voltammetry
(DPV) were performed using an EG&G potentiostat/ galvanostat model 283 under N 2
atmosphere. Anhydrous Acetonitrile was used as solvent with and 0.1 M
tetrabutylammonium hexafluorophosphate (TBAH) was used as the supporting electrolyte.
A glassy carbon rod, a platinum wire, and a silver wire were used as the working electrode,
counter electrode, and pseudoreference electrode, respectively. Electrochemical
reversibility and redox potentials were determined using CV and DPV, respectively. The
redox potentials were calculated relative to an internal ferrocenium/ ferrocene (Fc
+
/ Fc)
reference for the parent squaraine and decamethylferrocenium/ decamethylferrocene
(Me10Fc
+
/ Me10Fc) reference for its derivative respectively.
X-ray Crystallography: A metallic purple needle-like crystal of PSQ obtained from
ethanol/ ether was used for the X-ray crystallographic analysis. The X-ray intensity data
were measured on a Bruker SMART APEX diffractometer equipped with an APEX CCD
system and a Mo sealed-tube fine-focus source (λ = 0.71073 Å). A total of 1216 frames
53

were collected. The total exposure time was 15.20 hours. The frames were integrated with
the Bruker SAINT software package using a SAINT V8.34A (Bruker AXS, 2013)
algorithm. The calculated minimum and maximum transmission coefficients (based on
crystal size) are 0.9730 and 0.9990.
Device Fabrication: Photovoltaic cells were fabricated on ITO-coated glass substrates
that were pre-cleaned with tergitol and organic solvents cleaned and treated in UV-ozone
for 10 min immediately prior to loading into a high vacuum (base pressure 1-3  10
-6
Torr)
chamber. The C60 (MTR limited) and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline
(BCP) (Aldrich) were purified by thermal gradient sublimation in vacuum prior to use. The
hole extraction material MoO3 (Acros) metal cathode material, Al (99.999% pure, Alfa
Aesar), was used as received. The squaraine films were spin-cast in air at 3000 rpm for 45s
from 1.5 mg/mL chloroform solutions. The devices were fabricated with the layer
thicknesses given in the text. The squaraine films were transferred to a high vacuum
chamber for deposition of C60 and BCP by vacuum thermal evaporation at the following
rates: C60 (2 Å/s) and metals: 1000 Å thick Al (3 Å/s). The cathode was evaporated through
a shadow mask defining a device with an area of 2 mm. Current-voltage measurements
were performed in air at 25 °C using a Keithley 2420 Sourcemeter (sensitivity = 100 pA)
in the dark and under ASTM G173-03 spectral mismatch corrected 1 kW/m
2
white light
illumination from a 300WXe arc lamp (Asahi Spectra HAL-320W) equipped with an AM
1.5G filter. Spectral mismatch correction was performed using a silicon photodiode
(Hamamatsu S1787−04, 8RA filter) calibrated at the National Renewable Energy
Laboratory (NREL). Chopped and filtered monochromatic light (250 Hz, 10 nm fwhm)
54

from a Cornerstone 260 1/4 M double grating monochromator (Newport 74125) was used
in conjunction with an EG&G 7220 lock-in amplifier to perform all spectral responsivity
and spectral mismatch correction measurements.
23
Performance parameters are reported
for the best replicable efficiencies in devices. The uncertainty in determining the intensity
and spectrum of the lamp contributes largely to the source of error in J sc, while the errors
in Voc and FF occur primarily from the dissimilarities between devices.
AFM: Atomic force microscopy was taken on a Dimension Icon Scanning Probe
Microscope (Bruker) with PeakForce tapping mode. A Scan Asyst-Air Tip (Bruker) was
used to scan the 5 μm by 5 μm images.
Synthesis: All chemicals, reagents, and solvents were used as received from
commercial sources without further purification. All glassware was oven-dried, and all
reactions were performed under N2. 2,4-bis(3,5-dimethyl-1H-pyrrol-2-yl)-3-oxocyclobut-
1-en-1-olate and 2-(2-thienyl) pyrrole were synthesized using procedures described in the
literature.
24

2,4-bis(3,5-dimethyl-1H-pyrrol-2-yl)-3-oxocyclobut-1-en-1-olate was purified further
with repeated washings using ether, and was recrystallized with ethanol/ ether (2:1) to yield
metallic purple needle-like crystals.
1-hexyl-2-(thiophen-2-yl)-1H-pyrrole. To 2-(thiophen-2-yl)-1H-pyrrole (1 g ,6.70
mmol) in a round bottom flask 10% Tetrabutylammonium hydrogen sulfate catalyst was
added, followed by finely ground KOH (0.752 g, 13.4 mmol). The mixture was sonicated
for 15 min. At the end of the sonication period, 1-bromohexane (1.11 g, 6.70 mmol) of was
55

added to the mixture and stirred at 70°C for 16 hrs. The resulting solution was then washed
with cold water; the product was extracted with CH2Cl2. Solvent was removed under
reduced pressure using a rotary evaporator from the mixture to give a brown oil.
1
HNMR
(CDCl3, 500 MHz) δ 7.42 (dt, J = 5.1, 1.2 Hz, 1H), 7.29 – 7.21 (m, 2H), 6.96 (dd, J = 2.8,
1.8 Hz, 1H), 6.57 (dd, J = 3.6, 1.8 Hz, 1H), 6.48 – 6.40 (m, 1H), 4.20 (dd, J = 8.0, 6.9 Hz,
2H), 2.00 – 1.89 (m, 2H), 1.50 (dp, J = 7.5, 3.9, 3.5 Hz, 5H), 1.18 – 1.08 (m, 3H).
13
C NMR
(CDCl3, 126 MHz) δ 135.40, 127.42, 126.52, 125.44, 124.86, 122.90, 110.58, 108.18,
47.62, 31.78, 31.63, 26.62, 22.81, 14.30.
2,4-bis(1-hexyl-5-(thiophen-2-yl)-1H-pyrrol-2-yl)-3-oxocyclobut-1-en-1-olate. A
mixture of squaric acid (1g, 8.77 mmol) and 1-hexyl-2-(thiophen-2-yl)-1H-pyrrole (4.09
g, 17.54 mmol) were refluxed in a mixture of 1-butanol (30 mL) and toluene (120 mL) at
130 °C for 16 hrs. The reaction mixture was then cooled down; the solvent was removed
under reduced pressure using a rotary evaporator, and the resulting solid was washed with
CHCl3 and filtered to remove unreacted squaric acid. The solvent was removed from the
filtrate under reduced pressure to obtain a crude dark blue solid. The final product was
purified by elution with CH3COOC2H5/hexane on a silica gel column to obtain 0.3 g (6.2
%) of a blue-green solid.
1
HNMR (CDCl3, 600 MHz) δ 7.90 (d, J = 4.5 Hz, 1H), 7.50 (dd,
J = 5.1, 1.2 Hz, 1H), 7.37 (dd, J = 3.7, 1.2 Hz, 1H), 7.17 (dd, J = 5.1, 3.7 Hz, 1H), 6.73 (d,
J = 4.5 Hz, 1H), 4.93 (t, J = 7.7 Hz, 2H), 1.73 (q, J = 7.8 Hz, 2H), 1.59 – 1.54 (m, 2H), 1.36
(p, J = 7.4 Hz, 2H), 1.25 (dh, J = 7.3, 4.4, 3.7 Hz, 4H), 0.87 – 0.80 (m, 3H). Elemental
Analysis: C 70.35, H 6.66, N 5.12, S 11.87.

56

2.4 Results and Discussion
The squaraine materials were synthesized through the condensation of squaric acid
with a pyrrolic precursor as described in Figure 2.1. The composition of PSQ and NSP
were confirmed through
1
H NMR,
13
C NMR, and elemental analyses. Thermal Gravimetric
Analysis (TGA) was performed under nitrogen to assess the thermal stability of the
squaraines. At 266 °C PSQ undergoes 15% weight loss, while NSP undergoes 2% weight
loss. The alkylation of the pyrrolic proton in NSP increases its thermal stability. NSP
decomposes upon sublimation, while PSQ can be sublimed by gradient sublimation under
vacuum at (10
-5
torr) at 190°C-100°C-70°C gradient temperature zones.

(a)

(b)

Figure: 2.1 Reaction schemes (a) 2,4-bis(3,5-dimethyl-1H-pyrrol-2-yl)-3-oxocyclobut-1-en-1-
olate (PSQ) (b) 2,4-bis(1-hexyl-5-(thiophen-2-yl)-1H-pyrrol-2-yl)-3-oxocyclobut-1-en-1-olate
(NSP)
57

The recrystallization of PSQ from ether/ethanol yielded metallic purple needle-like
crystals, while the recrystallization of NSP from chloroform/hexane yielded metallic green
needle-like crystals which we were used for obtaining single crystal X-ray structures. The
crystal structures for PSQ and NSP are given in Figure 2.2 The metrical parameters in the
crystal structure are similar to previously reported squaraines
25
; the pyrrole-squarate bond
distance is relatively short (1.399 Å in PSQ, and 1.391 Å in NSP) as a result of the strong
conjugation between the pyrrole donor group and the squarate core while the C4 – N1 bond
length is longer (1.391 Å in PSQ, and 1.397 Å in NSP) in comparison to pyrrolic bond
lengths, which is typically 1.372 Å.
26
. The crystal packing diagram indicates that both
squaraine molecules are organized in a herringbone-array of dimers separated by 3.98 Å in
PSQ (Figure 2.3 (a)) and 4.41 Å in NSP (Figure 2.3 (c)) between the molecular planes.
The pitch and roll angles as defined by Curtis et al.
27
are 29° and 56° for PSQ and 14° and
55° for NSP respectively. The displacement with respect to the long molecular axis is
defined by the pitch angle, while the displacement relative to the short axis is defined by
the roll angle. The small pitch angle observed in NSP (Figure 2.3(d)) leads to the packing
of the squarate cores one on top of the other in adjacent dimers. As observed in the generic
herringbone arrangement, the moderate pitch angle in PSQ (Figure 2.3(c)) leads to the
slipping of adjacent dimers stacked on top of each other leading to minimal overlap in the
π- interaction between the pyrrolic rings. However,, the view along the short axis of the
dimers in the same stack shows the overlap between the pyrrole ring of one molecule and
the squarate moiety of the molecule slipped under it, leading to donor-acceptor interactions
58

between dimers which in turn enhances the charge and exciton transfer ability in organic
photovoltaics.

Figure 2.2 (a) ORTEP diagram of PSQ (b) ORTEP diagram of NSP

(a)

(b)

59

Figure 2.3. Crystal packing diagrams for PSQ. (a) herringbone structure with π- stacks
pitched in opposite direction (b) stacking arrangement viewed down the long
molecular axis. Crystal packing diagrams for NSP. (c) herringbone structure with
vertical translations of alternating π- stacks (d) stacking arrangement viewed down the
long molecular axes. Hydrogen atoms were removed for clarity.

(a)

(b)

(d)
(c)
60

2.5 Photophysical and Electrochemical Characterization
The absorption properties of PSQ and NSP in toluene solution and in neat thin film
are listed in Table 2.1, and representative spectra are shown in Figure 2.4(a). In solution,
both PSQ and NSP display an intense absorption band with a high extinction coefficient
(10
5
M
-1
cm
-1
) with λmax = 556 nm for PSQ and λmax = 638 nm. The absorption bands of
PSQ and NSP in solution display narrow full-width half maxima (fwhm) of 502 cm
-1
and
1340 cm
-1
, respectively, while the absorption bands in thin film are markedly broader, with
fwhm between 3628 and 3169 cm
-1
. The absorption spectra for both squaraines red-shift
and undergo substantial broadening in thin films (Figure 2.4(a)). The broadening of the
absorption band in films is an important characteristic for photovoltaics as greater overlap
with the solar irradiance spectrum leads to improved efficiencies.

Table 2.1. Photophysical Data in Solution and in Thin Films
Absorption Emission
Solution in Toluene Thin Film in CHCl3 Toluene Solution
ε (10
5
M
-1
cm
-1
) λmax (nm) fwhm (cm
-1
)

λmax nm) fwhm (cm
-1
)

λmax (nm) Φ
PSQ 1.04 556 502 572 3628 576 1.4
NSP 5.22 638 1311 682 3169 672 4.7

61

400 500 600 700 800 900
0.0
0.2
0.4
0.6
0.8
1.0

Noramalized Absorbance
Wavelength (nm)
PSQ Film
PSQ Solution
NSP Film
NSP Solution
554 634
(a)
400 500 600 700 800 900
0.0
0.2
0.4
0.6
0.8
1.0

Normalized Intensity
Wavelength (nm)
em, PSQ
ex, PSQ
em, NSP
ex, NSP
(b)

Figure 2.4. (a) Absorbance spectra for PSQ and NSP in toluene solution (solid lines), and as neat films
(open symbols). (b) Excitation (ex) and emission (em) spectra in toluene
62

PSQ and NSP fluoresce at wavelength of λmax = 576 and 672 nm respectively (Table
2), and the excitation and emission spectra are shown in Figure 2.4(b). The excitation
spectra of both squaraines match the absorption profiles. The photoluminescent quantum
efficiency (Φ) of PSQ in toluene is 1.4 %, whereas NSP is higher at 4.7 %. The Stokes shift
of PSQ is 21 nm and indicates only a minor structural reorganization in the excited state.
On the other hand, NSP displays a larger Stokes shift at 37 nm which suggests that the
excited state displays a greater degree of intramolecular distortion than occurs in PSQ.
Moreover, in both squaraines the Stokes shift decreases with increasing solvent polarity
(negative solvatochromism). For example, the Stokes shift of PSQ is 14 nm in toluene and
12 nm in dichloromethane. In contrast, the Stokes shift for NSP is 24 nm in toluene and 30
nm in dichloromethane. The positive solvatochromic response indicates that the excited
state of both squaraines is more polarizable than the ground state and is stabilized by polar
media.
The solvent-dependent luminescence properties of both squaraines were examined in
further detail. The quantum yield of PSQ decreases sharply in acetonitrile (Φ = 0.002).
NSP follows a similar trend with a high quantum yield in toluene (Φ = 0.047) and a
dramatic decrease in acetonitrile (Φ = 0.029) affected by solvent polarity. The solvent
polarity dependent quantum yield is analogous to previously reported squaraines
18
, where
the decreasing quantum yield with increasing solvent polarity was observed as the result
of a twisted intramolecular charge transfer state which becomes a non-radiative decay
pathway.
28,29

63

Theoretical calculations at B3LYP/LACVP** level of theory using Maestro Material
Science 10.6 software has been performed on PSQ and NSP to gain insight into electronic
structures and the energy levels of the two compounds (Figure 5). The HOMOs of the two
materials are similar while the LUMO of NSP is stabilized by 0.31 V. There is little
contribution to the HOMO and the LUMO from the thiophene unit.

Figure 2.5. Calculated energy levels and molecular orbitals of PSQ and NSP at
B3LYP/LACVP** level of theory using Maestro Material Science 10.6 software: (a) LUMO
of PSQ, (b) HOMO of PSQ, (c) LUMO of NSP, (d) HOMO of NSP.

64

Electrochemical analysis using both cyclic and differential pulse voltammetry were
performed in acetonitrile and referenced to Fc
+
/ Fc (PSQ) and Me10Fc
+
/ Me10Fc (NSP) as
an internal standard, and redox data are listed in Table 2.3. As given in Figure 2.6 PSQ
displays quasireversible oxidation waves in the range 0.17 – 0.92 V and quasireversible
reduction waves between –1.2 V and –1.6 V. NSP displays quasireversible oxidation waves
in the range 0.72 – 0.92V and quasireversible reduction waves between –0.43 V and –1.34
V. Compared to PSQ, both the oxidation and reduction potentials of NSP has shifted by
0.5 V.

Table 2.3. Electrochemical Redox Potentials
a
and Calculated HOMO/LUMO
b
Energies
Squaraines Eox (V) Ered (V) HOMO (eV) LUMO (eV)
PSQ 0.30 –1.42 5.02 3.09
NSP 0.83 –0.92 5.85 3.54
a
Recorded in 0.1 M Bu4N
+
PF6 in CH3CN, PSQ has been referenced to internal Fc+/ Fc; NSP
has been referenced to internal Me10Fc+/ Me10Fc.
b
HOMO and LUMO values have been
calculated from redox data using methods described in the literature.
30,31

65

2.6 Application to Organic Photovoltaics
Organic photovoltaic devices for PSQ and NSP donor materials were fabricated using
C60 as the acceptor. A layer of MoO3 was vacuum deposited on the Indium doped Tin
Oxide (ITO) substrate, and was used as a hole extraction/ electron blocking layer. PSQ
was vacuum deposited at 0.5 Ås
-1
, while NSP was spun cast from chloroform on top of the
MoO3 layer and the solar cells with the structures ITO/ MoO3 (120 Å)/ Squaraine (80-85
Å)/ C60(400 Å)/ BCP (100 Å)/ Al (1000 Å) were fabricated. Current – density versus
voltage characteristics were measured in the dark and under simulated one sun (1 kW/m
2
)
-2 -1 0 1
-20
-10
0
10
20
30
40

I(uA)
Volts Fc/ Fc
+
Fc
Fc
1.47
1.39
0.28
0.36
0.78
(a)
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0
-50
-40
-30
-20
-10
0
10
20
30

I(uA)
Volts Fc/ Fc
+
1.42
Fc
0.30
0.82
(b)

-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
-50
-40
-30
-20
-10
0
10
20
30

I (uA)
Volts (Me
10
Fc
+
/Me
10
Fc)
Me
10
Fc
1.17
-1.01
(c)
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5

I(uA)
Volts ( Me
10
Fc
+
/ Me
10
Fc)
0.83
Me
10
Fc
0.92
(d)

Figure 2.6. Cyclic Voltammetry (CV) and Differential Pulse Voltammetry (DPV) diagrams for
PSQ (a) and (b), and NSP (c) and (d) in acetonitrile. Scan rate 100 mV/s.
66

AM1.5G illumination (Figure 2.7(a)). The power conversion efficiencies for PSQ (ηP =
1.15±0.1) and NSP (ηP = 1.02±0.1) are similar. Figure 2.7(b) shows that the photoresponse
from the squaraines (500- 700 nm) and C60 (400- 500 nm) agrees with the absorption
spectra. The short circuit current for PSQ (Jsc = 5.17±0.1 mA/ cm
2
) is higher than for NSP
(Jsc = 4.34±0.2 mA/ cm
2
), while the increase in the Fill Factor (FF) from 0.42±0.02 in NSP
to 0.52±0.01in PSQ.
The improvement in device performance in PSQ can be explained Atomic Force
Microscopy images (AFM) of amorphous films of the donor materials. Vacuum deposited
PSQ produces smoother films (rms = 0.63 nm) compared to NSP (rms = 2.4 nm) leading
to improved charge carrier transport. As shown in Figure 2.7(c), the spin casting of PSQ
leads to crystalline surfaces (rms = 17 nm) and produces highly rough surfaces that lead to
poor fill factors in devices. Furthermore, solvent annealing and thermal annealing of films
resulted in lowering of the power conversion efficiency.

67

(a)

(b)
Donor Voc (V) J sc (mA/ cm
2
) FF ηP (%)
PSQ (80 Å) 0.43±0.01 5.17±0.1 0.52±0.01 1.15±0.1
NSP (85 Å) 0.56±0.02 4.34±0.2 0.42±0.02 1.02±0.1

(c)

-1.0 -0.5 0.0 0.5 1.0
-6
-5
-4
-3
-2
-1
0
1
2
3
4
5
6

Current Density (mA/ cm
2
)
Voltage (V)
NSP Dark
NSP Light
PSQ Dark
PSQ Light

400 500 600 700 800
0
5
10
15
20
25
30

EQE (%)
Wavelength (nm)
PSQ
NSP
68

2.7 Conclusion
A new derivative of symmetric 2,4-bis(3,5-dimethyl-1H-pyrrol-2-yl)-3-oxocyclobut-
1-en-1-olate squaraine dye has been synthesized using the alkylated 2-(thiophen-2-yl)-1H-
pyrrole as the donor group. The absorption spectra of the new derivative span the 500- 800
(e)

Figure 4. (a) Voltage versus current density (b) tabulated OPV performance characteristics
squaraine donor devices (c) external quantum efficiency (EQE) plots, (d) AFM images of
films of PSQ and NSP (rms = 7.97 nm) and (e) (rms = 17 nm) spun-cast from chloroform.
(d) (e)

(f)

Figure 2.7 (a) Current-density characteristics (b) tabulated OPV performance
characteristics of squaraine donor devices (c) external quantum efficiency (EQE) plots, (d)
AFM images of films of vacuum deposited PSQ (rms = 0.60 nm) (e) spun-cast PSQ (rms
= 17 nm) and (f) NSP (rms = 2.4 nm) spun-cast from chloroform.
69

nm in solid state consequently leading to significant overlap with the solar spectrum. The
incorporation of the thiophene group leads to the lowering of the LUMO energy level. Both
materials display similar power conversion efficiencies, however the parent squaraine
derivative yields higher photocurrent and improved fill factors due to formation of
smoother surfaces from vacuum deposition.
Acknowledgments
Funding for this research was provided by the National Science Foundation (NSF CBET-
1511757)

70

2.8 Endnotes for Chapter 2

(1) Beverina, L.; Salice, P. European Journal of Organic Chemistry 2010, 1207-
1225.

(2) Sreejith, S.; Carol, P.; Chithra, P.; Ajayaghosh, A. Journal of Materials
Chemistry 2008, 18, 264-274.

(3) Beverina, L.; Sassi, M. Synlett 2014, 25, 477-490.

(4) Treibs, A.; Jacob, K. Angewandte Chemie-International Edition 1965, 4, 694.

(5) Treibs, A.; Jacob, K. Annalen Der Chemie-Justus Liebig 1966, 699, 153.

(6) Treibs, A.; Jacob, K. Annalen Der Chemie-Justus Liebig 1968, 712, 123.

(7) Tam, A. C. Applied Physics Letters 1980, 37, 978-981.

(8) Law, K. Y.; Bailey, F. C. Journal of Imaging Science 1987, 31, 172-177.

(9) Law, K. Y. Chemical Reviews 1993, 93, 449-486.

(10) Arunkumar, E.; Sudeep, P. K.; Kamat, P. V.; Noll, B. C.; Smith, B. D. New
Journal of Chemistry 2007, 31, 677-683.

(11) Beverina, L.; Crippa, M.; Landenna, M.; Ruffo, R.; Salice, P.; Silvestri, F.;
Versari, S.; Villa, A.; Ciaffoni, L.; Collini, E.; Ferrante, C.; Bradamante, S.; Mari,
C. M.; Bozio, R.; Pagani, G. A. Journal of the American Chemical Society 2008,
130, 1894-1902.

(12) Quartarolo, A. D.; Sicilia, E.; Russo, N. Journal of Chemical Theory and
Computation 2009, 5, 1849-1857.

(13) Beverina, L.; Ruffo, R.; Patriarca, G.; De Angelis, F.; Roberto, D.; Righetto, S.;
Ugo, R.; Pagani, G. A. Journal of Materials Chemistry 2009, 19, 8190-8197.

(14) Chen, C. T.; Marder, S. R.; Cheng, L. T., Journal of the American Chemical
Society 1994, 116 (7), 3117-3118.

(15) Beverina, L.; Crippa, M.; Salice, P.; Ruffo, R.; Ferrante, C.; Fortunati, I.;
Signorini, R.; Mari, C. M.; Bozio, R.; Facchetti, A.; Pagani, G. A. Chemistry of
Materials 2008, 20, 3242-3244.

(16) Lee, Y. D.; Lim, C. K.; Kim, S.; Kwon, I. C.; Kim, J. Advanced Functional
71

Materials 2010, 20, 2786-2793.

(17) Xiang, Z. M.; Nesterov, E. E.; Skoch, J.; Lin, T.; Hyman, B. T.; Swager, T. M.;
Bacskai, B. J.; Reeves, S. A. Journal of Histochemistry & Cytochemistry 2005, 53,
1511-1516.

(18) Wang, S. Y.; Hall, L.; Diev, V. V.; Haiges, R.; Wei, G. D.; Xiao, X.; Djurovich,
P. I.; Forrest, S. R.; Thompson, M. E. Chemistry of Materials 2011, 23, 4789-4798.

(19) Wang, S. Y.; Mayo, E. I.; Perez, M. D.; Griffe, L.; Wei, G. D.; Djurovich, P. I.;
Forrest, S. R.; Thompson, M. E. Applied Physics Letters 2009, 94, 3.

(20) Binda, M.; Iacchetti, A.; Natali, D.; Beverina, L.; Sassi, M.; Sampietro, M.
Applied Physics Letters 2011, 98, 3.

(21) Zimmerman, J. D.; Lassiter, B. E.; Xiao, X.; Sun, K.; Dolocan, A.; Gearba, R.;
Vanden Bout, D. A.; Stevenson, K. J.; Wickramasinghe, P.; Thompson, M. E.;
Forrest, S. R. Acs Nano 2013, 7, 9268-9275.

(22) Bonnett, R.; Motevalli, M.; Siu, J., Tetrahedron 2004, 60 (40), 8913-8918.

(23) Dale, C. L.; Hill, S. J.; Kellam, B., Medchemcomm 2012, 3 (3), 333-338.

(24) Seaman, C. H., Solar Energy 1982, 29 (4), 291-298.

(25) C. W. Dirk, W. C. Herndon, F. Cervanteslee, H. Selnau, S. Martinez, P.
Kalamegham, A. Tan, G. Campos, M. Velez, J. Zyss, I. Ledoux and L. T. Cheng,
Journal of the American Chemical Society, 1995, 117, 2214-2225.

(26) C. W. N. Cumper, Trans. Faraday Soc., 1958, 54, 1266-1270.

(27) M. D. Curtis, J. Cao and J. W. Kampf, Journal of the American Chemical
Society, 2004, 126, 4318-4328.

(28) C. CornelissenGude, W. Rettig and R. Lapouyade, Journal of Physical
Chemistry A, 1997, 101, 9673-9677.

(29) W. Rettig, Angewandte Chemie-International Edition in English, 1986, 25, 971-
988.

(30) B.W. D'Andrade, S. Datta, S. R. Forrest, P. Djurovich, E. Polikarpov, M. E.
Thompson, Organic Electronics 2005, 6, 11-20.
72

(31) P. Djurovich, E. I. Mayo, S. R. Forrest, and M. E. Thompson, Organic Electronics
2009, 10, 515-520.

73

CHAPTER 3
Improving the performance of a DBP/ ZCl bilayer device through energy
sensitization

3.1 Abstract
As new materials are designed to absorb in specific regions of the solar spectrum
in organic photovoltaics (OPVs), it is also important to utilize a broad region of the solar
spectrum for exciton harvesting. In this study, we investigate the energy sensitization of a
tetraphenyldibenzoperiflanthene (DBP)
1
/ zinc chlorodipyrrin (ZCl)
2
bilayer device, using
Coumarin 30
3
as a sensitizer, where DBP is the donor and ZCl function as the acceptor
respectively. As a result of capturing a broader region of the solar spectrum the
photocurrent for the sensitized device increases by 1.94 mA/ cm
2
compared to the control
device
95
while the power conversion efficiency increases from 1.4% to 2.24%.

3.2 Introduction
Small molecule high absorbing dyes are excellent candidates for organic OPVs, due
to the low amount of material required for processing, and potentially low-cost fabrication.
However, OPVs suffer from low PCE, while PCE > 10% have been reported.
5,6
Attempts
to improve the PCE has involved the development of Near-IR materials to provide greater
coverage with the solar spectrum. Achieving high performance efficiencies is challenging
due to the tradeoffs that must be made in device architecture between exciton harvesting
and charge collection. Exciton harvesting can be improved through controlled molecular
74

orientation and using triplet excitons with longer lifetimes,
7-9
or using energy transfer as a
mechanism for transporting excitons.
10,11
The simplest energy transfer model is for the
direct energy transfer from the donor to the acceptor. However, the efficiency of energy
transfer in this model is limited since the exciton collection is improved in one material.
The inclusion of an additional energetic material is an effective way to enhance the PCE is
to utilize the entire solar spectrum for exciton harvesting.
12-16

This approach eliminates the performance limitations imposed on single-junction
solar cells, such as the Shockley-Queisser efficiency limit. Cascading energy levels is a
strategy that has been employed to circumvent the single-junction Shockley-Queisser limit.
There are several ways to organize a cascading energy structure, however, the principle
remains that the excitons generated in a large band gap material are transferred to a smaller
band gap material via Förster energy transfer. This can be achieved by stacking two or
more sub-cells in a tandem device architecture, where different materials are employed to
absorb different regions of the solar spectrum.
17-21
The wide-band gap absorption in this
architecture allows enhanced current generation. The energy levels of the third material
must be compatible with the energy levels of the donor and the acceptor to produce a
cascading energy model that would separate charges and reduce geminate recombination
by allowing the spatial separation of charges. Each of the sub-cells operate independently
with no charge transfer or energy transfer between each other. The sub cells are connected
in series or parallel. Tandem cells improve efficiency by providing broad spectrum
coverage, and by the reduction of loss from thermalization from above bandgap excitations.
The photocurrent is maximized by optimizing the thickness of the active layer depending
75

on the absorption coefficient and the band gap. The maximum short circuit current is
dependent on the minimum short circuit current in the sub-cells, while the maximum open
circuit voltage is computed by the sum of the open circuit voltage of the sub-cells.
However, there are many complications that arise in the fabrication of tandem cells due to
the number of different parameters that need to be optimized and the number of different
layers that need to processed during fabrication, and maximizing charge collection by
effectively designing the intermediate layers between sub-cells; all of these factors lead to
increased cost in tandem solar cell processing.
22
These difficulties can be overcome by
using small molecule sensitizers to increase the solar spectrum coverage.
23

Figure 3.1. The schematic configuration of the (a) conventional binary OSCs, (b) tandem
OSCs, and (c) ternary OSCs with four possible active layer. Adapted from Reference 46.
Error! Bookmark not defined.
morphologies according to the location of the third component.
76

Ternary solar cells are high performance cells fabricated with multiple materials
observed in tandem cells resulting in increased photon harvesting, and provide simplified
fabrication processes. They provide the combined advantages of binary single junction
and tandem solar cells. They consist of a donor, acceptor and a third component which can
be a polymer, small molecule, or nano particle.
24-28
The third component could be
completely embedded in the donor, embedded in the acceptor, or located at the D/A
interface, or form parallel linkage channels as seen in Figure 3.1. Ternary solar cells can
be composed of two donor materials and an acceptor (D1/D2/A)
29-33
allowing for photon
harvesting in a broad range on the solar spectrum through a simple device architecture.
They can also be fabricated using one donor and two acceptor materials (D/A1/A2)
34-37
or a donor, additive component, and acceptor (D/X/A)
38-41
The third component is
chosen to form a cascading energy architecture so that excitons and charge traps do not
form in the active layer, and is located at the interface of the donor and the acceptor layers,
since it relies on the percolating pathways formed by the D/A system for transporting
charges. In the (D1/D2/A) system, excitons generated at in the donor materials can be
dissociated at the D1/A or D2/A interfaces, while holes are transported to the anode through
a percolating pathway formed by one of the donors, and electrons reach the cathode through
the acceptor layer. Additionally, the positioning of the third component is dependent on
the crystallinity and the surface energy of the D/A system.
42,43
In a ternary blend dominated
by P3HT: PCBM, with 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine
as the third component, the surface energy of the squaraine (41.5 mJ m
-2
) lies between
those of P3HT (27.5 mJ m
-2
) and PCBM (47.9 mJ m
-2
), and the squaraine is expected to be
77

located at the P3HT: PCBM interface due to the surface energies of the third component.
44

The location of the squaraine is further confirmed by X-ray elemental mapping where the
nitrogen signal from the squaraine lies next to the Sulphur signal from P3HT.
Charger transfer or energy transfer between the donors with two different bandgaps
can be explored through Photoluminescence (PL), where decreased emission intensity
would be observed for the higher bandgap donor material, while increased emission
intensity would be observed for the lower bandgap material. If charge transfer occurred
between the two donor materials, the emission from one of the materials would be
quenched with no observed increase in intensity for the second material. In a study that
explores if energy transfer or charge transfer occurs in P3HT: SMPV1 polymer blends, PL
quenching in the films were measured for different doping ratios of SMPV1 in the blended
films. As the SMPV1 doping increase in the blended film, the emission intensity of both
P3HT and SMPV1 decreases, confirming that charge transfer occurs in the blended film.
43

The charge transfer between the two donors is further confirmed through the current
density plots for solar cells fabricated with no acceptor but with neat donors of P3HT and
SMPV1 and a blended donor layer. The short circuit current measured for the P3HT:
SMPV1 (1:1) blended layer is much larger than for the devices with neat layers of P3HT
or SMPV1, which is attributed to the charge carrier transfer between the two donor
molecules.
Energy transfer between the two materials can be viewed as a process that competes
with charge transfer. The mechanism for energy transfer can occur through Förster Energy
Transfer (FRET) which is Coulombic interaction that occurs through a dipole-induced
78

interaction in materials or through Dexter energy transfer which involves the electron
exchange of materials through orbital overlap. A fundamental requirement for energy
transfer to occur is that the emission spectra of one material has significant overlap with
the absorption profile of the second material. FRET is a strategy that is often used in system
to promote long range exciton migration. In ternary blend solar cells with the D1/D2/A
system, excitons generated in D1 can transfer energy to D2 via FRET or Dexter
mechanisms, and excitons generated in the D2 can dissociate at the D2/A interface. Electron
transport to the cathode occurs through the percolating pathways formed by the acceptor,
while holes generated in D2 can directly reach the anode or transfer to D1 and then reach
the anode through the pathways formed by D1.
45

Taylor et al. demonstrate how photon absorption and exciton harvesting can be
improved through FRET in a P3HT: PCBM system by incorporating 2,4-bis[4-(N,N-
diisobutylamino)-2,6-dihydroxyphenyl] squaraine (SQ) (Figure 3.2).
46
due to the large
spectral overlap between P3HT and SQ, transient absorption spectroscopy displays energy
transfer efficiency of 96% within the first few pico seconds. Due to the high absorption
coefficient of the SQ (10
5
M
-1
cm
-1
), SQ can serve as a long-range absorber, and when
combined with FRET leads to significant improvement in the overall efficiency. The
cumulative absorption profile of P3HT and SQ overlaps with a significant range of the
solar spectrum, with limited overlap between the two materials, which ensures that SQ will
not impede the absorption coming from P3HT. Additionally, the emission spectrum of
P3HT shows strong overlap with the absorption spectrum of SQ, which is an important
requirement for energy transfer. The P3HT/SQ/PCBM forms a cascading energy level
79

alignment which makes photoinduced charge transfer possible between P3HT and PCBM
and SQ and PCBM.

Figure 3.2. FRET in the P3HT/SQ/PCBM system (a) Extinction coefficient (blue line) of SQ
in 1,2-dichlorobenzene, and absorption (red solid line) and emission (red dotted line) spectra
of P3HT film. (b) EQE versus wavelength of devices with SQ concentrations ranging from 0
to 5 wt%. Adapted from reference.
46

80

The possibility of the occurrence of FRET in the system was investigated using PL
measurements, and it was observed that with increasing concentration of SQ, the emission
intensity from P3HT decreased while the emission intensity from SQ continued to increase.
Additionally, since FRET provides a non-radiative decay path for the excited state donor,
with increasing concentration of the acceptor, the excited state life time of the donor is
expected to decrease. When the fluorescence decay of P3HT was probed a s a function of
SQ through femtosecond fluorescence up conversion, in the absence of SQ, P3HT was
found to have an average lifetime of 223 ps. Upon the addition of 1 wt% of SQ, the P3HT
lifetime reduced to 52.4 ps, and to 9.9 ps with the increase of SQ to 5 wt%. These
observations clearly indicate that excitation energy can be transferred from P3HT to SQ
successfully.
Furthermore, the control devices fabricated with the P3HT: PCBM blend display
current densities of 10.3 mA/ cm
2
, FF 53 %, and PCE of 3.27 %. The incorporation of 1
wt% of SQ to the ternary blend, the current density increases by 1.3 mA/cm
2
, FF by 11%,
without significant change to the Voc, which increases the PCE by 1.1 %. The absorption
profiles of P3HT and SQ show the absorption maximum at 550 nm, and 670 nm
respectively, which compares well with the response in the EQE for the two materials. For
the purpose of analyzing the enhancement of the photocurrent, the EQE is divided into two
regions, 330-620 nm where P3HT absorbs, and 620-900 nm region where the SQ absorbs.
With increasing SQ wt % in the blended layer, the photocurrent in the 330-620 nm region
increases by 0.52 mA/cm
2
at 1 wt %, and by 0.64 mA/ cm
2
at 5 wt%. In the control
experiment, exciton dissociation takes place at the P3HT: PCBM interface, therefore the
81

enhanced photocurrent in this region is attributed to the additional excitons that are
harvested through the energy transfer to SQ using the FRET mechanism. With 5 wt% SQ
in the blended layer the photocurrent in the 620-900 nm region doubles, and this
observation is attributed to the absorbance of the SQ.
In addition to the energy transfer observed between two donor materials, energy
transfer can occur between two acceptor materials in a given system. This phenomenon has
been investigated in solar cells with α-sexithiophene (α-6T) as the donor component and
boron subnaphthalocyanine chloride (SubNc) and boron subphthalocyanine chloride
(SubPc) as acceptor materials where the cell configuration for the active layer is α-6T/
SubNc/ SubPc.
96
Excitons generated in the SubPc layer can transfer energy to the SubNc
layer that is sandwiched between α-6T and SubPc. In this device architecture, SubNc is
responsible for being the energy acceptor for excitons formed in the SubPc layer, and for
being the charge acceptor at the donor interface. These processes occur as exciton
dissociation take place at the α-6T/SubNc interface, once charge separation occurs,
electrons transfer through the SubNc layer to the SubPc and are collected at the cathode
while holes transfer from the SubNc through percolated pathways formed by α-6T, to the
anode.
Cnops et al.
96
further investigated the energy transfer process from SubPc to SubNc
using the high bandgap material 4,40-bis(N-carbazolyl)- 1,10-biphenyl (CBP) as an optical
spacer between the SubPc and the SubNc layers (Figure 3.3). Although in such a system,
long-range energy transfer through the CBP layer may still occur, energy transfer occurring
directly at the interface would be blocked by the optical spacer. SubPc produces two
82

emission peaks at 620 nm and 710 nm, and PL was measured for different thicknesses of
the CBP layer. As a control experiment, PL quenching in the SubPc/ CBP/C 60 was
examined with the C60 serving as the exciton quenching layer. In the control experiment,
energy transfer from SubPc to C60 is limited since there is no overlap between the emission
spectrum of SubPc and the absorption spectrum of C 60. Results indicate that the emission
intensity of the SubPc layer increases with a CBP layer as small as 5 nm, compared to the
SubPc/ SubNc films, and emission can be substantially quenched by SubNc in the presence

Figure 3.3. PL spectra of organic layer stacks deposited on quartz substrates for different
spacer layer thicknesses (x nm BCP). (a) 20 nm SubPc/x nm CBP, (b) 20 nm SubPc/x nm
CBP/10 nm C60, and (c) 20 nm SubPc/x nm CBP/10 nm SubNc. Adapted from reference.
33

83

of the optical spacer layer that is 35 nm in thickness. Additionally, a significant amount of
long range energy transfer occurs from SubPc to SubNc.

Furthermore, α-6T, SubPc, and SubNc are materials with high absorption
coefficients (10
5
cm
-1
) and the combined system shows significant overlap with the solar
spectrum, absorbing in the 300- 800 nm wavelength range. Both SubPc and SubNc
contribute to the photocurrent, and this observation is confirmed by the EQE, which
matches with the absorption profile for all three materials, but displays a higher
contribution from the SubNc layer in the 450 nm to 650 nm region (Figure 3.4). This
increase can be attributed to the fact that while excitons generated in the SubNc layer
dissociate at the α-6T/ SubNc interface, excitons generated in the SubPc layer energy
transfer to the SubNc layer followed by dissociation at the α-6T/ SubNc interface. The
photocurrent for the α-6T/ SubNc/ SubPc system is 14.55 mA/cm
2
which is 2.50 mA/cm
2
higher than in the α-6T/ SubPc system, and 7 mA/cm
2
higher than in the α-6T/ SubNc
system respectively. As a result, the PCE for the α-6T/ SubNc/ SubPc system is 8.4% which
is 3.7% and 2.4% higher than the α-6T/ SubPc and α-6T/ SubNc systems respectively.
Although Ternary blended devices with their cascading energy level alignment
leads to enhanced photocurrents, the introduction of the third component leads to the
formation of additional heterojunctions in the device. Morphological control of these
interfaces is essential for optimal charge generation and for minimizing recombination
losses, which can be a complicated problem to address in each interface. On the other hand,
the dye-sensitization based approach is a less complicated strategy since the dye molecule
84

stays electrically neutral as energy is transferred from the sensitizer to the acceptor.
Therefore, the sensitization process has no significant influence on charge transport or
recombination.
In addition to ternary blends, Hesse et al.
29
demonstrated the concept of fullerene
sensitization through increased photon harvesting and enhanced photocurrent generation,
using perylene-diimid (PDI) is a sensitizer in a PC 60BM device. PDI dye molecules are

Figure 3.4. (a) Absorption spectra of the three active materials complementing each other
to effectively harvest solar light. (b) The measured EQE (solid lines) and IQE (dashed
lines) spectra show efficient photocurrent generation by all three absorbing materials.
Adapted from reference.
33

85

high absorbers (ε = 80000 M
-1
cm
-1
) and produce fluorescence quantum yields close to
unity, making them excellent sensitization candidates. The exceptional charge extraction
properties of the fullerene, and the strong absorption of the dye molecule are key factors
for the performance of this device. Sensitization in the device occurs when the photoexcited
PDI molecules transfers excitation energy through FRET to PC 60BM. Charge separation
takes place at the D/ A interface, and excitons generated in the PDI can also charge separate
by hole transfer to the donor, followed by charge transfer to PC 60BM and regenerating the
dye. The energy transfer process is verified through PL measurements, where the addition
of PDI to PC60BM increased the emission of PC60BM. A loading ratio of (9:1) PDI:
PC60BM lead to an 18-fold increase in the emission for PC60BM. This increase in emission
was attributed to the energy transfer from the high absorbing PDI to PC 60BM. A bilayer
solar cell was fabricated using Hexa-peri-hexabenzocoronene (HBC) as the donor, and the
HBC/ (PDI:PC60BM) device displays enhancements in photocurrent and Voc compared to
the HBC/ PC60BM control device, with a 50% increase in photocurrent at the optimal dye
loading ratio. The origin of the additional charge carries can be clearly seen in the EQE
measurement, there is a drop in the region where HBC and PC60BM absorbs, followed by
a significant increase in the 450- 600 nm region where PDI absorbs. With increased PDI
doping, more photons are absorbed by the acceptor blend, energy is transferred to PC60BM,
and separated at the HBC: PC60BM interface. The increase in the Voc is attributed to the
increased charge carrier density and the reduced biomolecular recombination in the device.
A significant part of OPV research has been focused on developing novel donor
materials, while fullerenes such as PC60BM and C60 have continued to remain the preferred
86

acceptor materials due to their ability to facilitate efficient charge separation at the D/A
interface and excellent electron conducting properties, and the ability to be well paired with
a wide range of donor materials. C60 is the commonly used acceptor and its intermolecular
charge transfer leads to absorption in the 400 -500 nm region, while the lowest energy
transition at 670 nm is symmetry forbidden leading to an overall lower absorption in the
visible region of the solar spectrum. This poor overlap is a drawback in fullerenes, since
the donor materials must be designed to harvest a large region of the spectrum. There has
been a large amount of research done on developing Near-IR absorbing donors, however
when paired with fullerene acceptors, this leaves the high-energy region of the solar
spectrum unabsorbed. Therefore, it is necessary to develop and utilize non-fullerene
acceptors that has good overlap with the solar spectrum while retaining the charge transport
properties of fullerenes. This study utilizes ZCl as an acceptor.
2

Trinh et al.
2
have demonstrated the Symmetry Breaking Charge Transfer (SBCT)
phenomena in ZCl which leads to high Voc values in OPVs. In OPVs charge separation
occurs through the transfer of an electron from the donor to the acceptor. V oc is
thermodynamically limited by the energy offset between the HOMO of the donor and the
LUMO of the acceptor (ΔEDA), although ΔEDA is not an accurate predictor of Voc since the
upper limit for the Voc is the energy of the ground state to intermolecular CT transition
(ECT) at the D/A interface as demonstrated through spectroscopic and temperature
dependent studies.
(97-
9899
100)
However, since an electron is directly promoted from the HOMO
of the donor to the LUMO of the acceptor during the CT transition, ECT is dependent on
ΔEDA. ECT can be linearly correlated to qVoc, with an energy loss of 0.6 eV due to charge
87

recombination, while further losses occur from the driving force needed to form the CT
state at the D/A interface. SBCT is a strategy that minimizes the recombination loss and
the energy required to drive the formation of the CT state. When SBCT occurs, an exciton
is formed on a molecule or ligand which undergoes ICT, which leaves a hole and an
electron localized on different molecules or ligands with weak coupling between the hole
and the electron. Since SBCT can drive charge separation with negligible driving force, it
minimizes energy loss from electron transfer, provides directional specificity to ensure that
electrons and holes are directed to the specific electrodes, and the reduced back
recombination increases the separation yield. Trinh et al.
2
report that SBCT occurs in
ultrafast timescales: in zinc dipyrrins charge transfer occurs between 1 ps and 14 ps.
Bartynski et al.
95
report that ZCl undergoes SBCT, and that excited state dynamics
change in high dielectric solvents. The high dielectric solvents provide electrostatic
stabilization for the CT state. When ZCl is used as an acceptor with DBP as a donor in an
OPV, the Voc achieved is significantly higher than the corresponding DBP/C 60 device.
When the ECT was measured through Fourier-transform photocurrent spectroscopy and OPV
electroluminescence, it was observed that with DBP as the donor, C 60 forms a lower energy CT
state of 1.45 eV, while ZCl forms a higher energy CT state at 1.70 eV. In the resulting devices,
DBP/C60 gives a Voc of 0.88 V, while DBP/ZCl gives a V oc of 1.33 V. The enhancement
of the Voc for the ZCl device is due to the formation of the high-energy CT state.
88

Furthermore, photocurrents of 6.2 mA/ cm
2
and PCE of 3.6% has been reported for
the DBP/C60 device.
95
Although there is a substantial increase in the Voc for the DBP/ZCl
device, the photocurrent of this device is low at 2.4 mA/ cm
2
, and thus the PCE decreases
to 1.4%.
95
The low photocurrent is a result of the reduced absorbance of the device in the
350- 500 nm region compared to the C60 device as given in Figure 3.5. This dip in the
absorbance can be corrected by energy sensitization. The goal of the present study is to fill
the gap in the 350- 500 nm region. The selected sensitizer needs to absorb in the 350 – 500
nm region in order to achieve this goal.

Here we demonstrate the use of a small molecule sensitizer for increasing the PCE
of the donor material DBP, which is widely-used due to its high performance in
combination with fullerenes.
101
Due to the horizontal orientation of DBP molecules, there

Figure 3.5. EQE for the devices described in the text. Adapted from Reference 95

89

is strong coupling between the transition dipole moment of the molecule and incident light,
which leads to efficient absorbance of light.
102

Energy sensitization has been demonstrated as a technique to improve the
absorption efficiency in the C60 acceptor layer by Trinh et al.
2
In this technique, a sensitizer
absorbed energy in the region where C60 does not absorb, and will then transfer the energy
through a FRET
103
mechanism to the C60 host. The C60 acceptor layer is then responsible
for exciton diffusion, charge separation, and electron conduction. For this system, ZCl was
carefully designed as the sensitizer.

In the current study, the energy sensitization approach has been applied to the DBP/
ZCl system to extend the absorption of the parent device and cover the 350- 500 nm region.
We demonstrate that by using Coumarin 30 which is a strong absorber in the 400- 500 nm
region, exciton harvesting can be enhanced and the transfer of energy to DBP can be
achieved. Through the utilization of the sensitizer the PCE increases for the sensitized
device from 1.4 % to 2.24 % compared to the parent device, while the photocurrent
increases from 2.4 to 4.34 mA/ cm
2
without significant change to Voc or FF.

90

3.3 The Screening of Sensitizers
The goal of the current study is to fill the gap in the 350- 500 nm region of the DBP/
ZCl device using a sensitizer which absorbs in the same region. Two high absorbing dyes
were identified as potential candidates for sensitizing the DBP/ZCl system: Coumarin 30
and Coumarin 334, since these dyes absorb in the 350- 500 nm region.

The properties of these dyes were measured in order to screen potential candidates
(Table 3.1). The photo physical and electrochemical criteria for screening sensitizers have
been defined by Trinh et al.
2

1. Both singlet and triplet energies of the sensitizer need to be higher than the
host to guarantee efficient energy transfer to the host.
2. The oxidation potential of the sensitizer must be higher to ensure that the
energy of the CT state is greater than the triplet of the host.
Es ET Eox Ered HOMO LUMO
Coumarin 30 3.04 2.18 0.76 -1.53 5.56 2.95
Coumarin 334 2.74 2.24 0.50 -1.35 5.30 3.17
ZCl 2.37 1.75 1.22 -1.3 6.31 3.23
DBP 2.38 1.34 0.68 -1.08 5.4 2.4

Table 3.1. Photophysical and electrochemical data for DBP, ZCl, and potential sensitizers.
(a) Coumarin 30, and Coumarin 334 absorption was measured in toluene, redox
potentials vs. Fc
+
/Fc, Triplet energies were calculated from TDDFT calculations
(b) ZCl data
2

(c) DBP Data
1

91

3. To maintain good electron conductivity by the host, the reduction potential
of the sensitizer must be more negative than the host.

The comparison of the energy levels suggests that neither candidate is suitable for
sensitizing ZCl since both will form a hole trapping CT state with ZCl. However, Coumarin
30 can energy transfer efficiently to DBP through the singlet and triplet energy levels
without forming traps. Additionally, Coumarin 30 and Coumarin 334 provide excellent
spectral coverage in the 350- 500 nm region. However, Coumarin 334, is not a suitable
candidate for sensitizing DBP since it has an oxidation potential too low so it will form a
hole trapping CT state with DBP. The basic evaluation of the potential candidates reveal
that Coumarin 30 meets the criteria as a possible sensitizer, and these characteristics are
further illustrated in the next section.

92

3.4 Optical and Electronic Properties of the System
Figure 3.6 depicts the molecular structures and thin film extinction spectra of the
active layer materials in the device architecture. The Coumarin 30 film shows strong
absorption at λ = 410 nm with a thin film extinction (α) of 4  10
5
cm
-1
. DBP absorbs
intensely between λ = 500 nm and 650 nm with α as large as 4.84  10
5
cm
-1
while ZCl has
an intense absorption between λ = 450 nm and 575 nm α as high as 4  10
5
cm
-1
. The

400 500 600 700 800 900
0
2
4
6
8
10

 ( x 10
5
cm
-1
)
Wavelength (nm)
DBP
ZCl
Coumarin 30

Figure 3.6. Schematic representation of molecular formulae and energy levels for Coumarin 30,
DBP and ZCl. Energy values for DBP and ZCl are from literature
2,4
, energies for Coumarin 30
has been calculated from redox potentials.
2.9
5.5
Coumarin 30
3.1
5.4
DBP
3.2
6.3
ZCl
HO M O
L UM O
93

absorption of the sensitizer is in the same order of magnitude DBP and ZCl. The
comparison of the codeposited DBP: Coumarin 30 (2:1: by volume) film/ ZCl and DBP:
ZCl film shows contributions from all three components and fills the λ = 370 – 500 nm
region where the standard does not absorb.
To utilize successful energy transfer in the system a blended donor layer has been
designed using DBP as the host material which conducts both excitons and holes, for the
Coumarin 30 sensitizers which plays the role of the light absorbing guest. When the
sensitizer is excited, energy is transferred to the DBP host, followed by exciton transport
to the D/A interface. Upon charge separation, holes are conducted by the DBP host to the
anode. As outlined in Figure 3.7 sensitization of DBP can be achieved through (A) singlet
energy transfer, (B) triplet energy transfer, or (C) electron transfer from the sensitizer to
the host via a CT state where the energy of the CT is equal to the difference between the
oxidation potential of the sensitizer and the reduction potential of DBP minus the
coulombic stabilization provided by the oxidized and reduced species. It is necessary that
all three pathways are available for the sensitization of DBP, since a non-viable pathway
can trap excitons or charges in the blended DBP: Coumarin 30 film. To make all three
pathways viable and ensure efficient energy transfer, both singlet and triplet energies of
the sensitizer need to be greater than DBP. The satisfaction of these conditions will ensure
that excitons generated on the sensitizer will be transferred to DBP instead of being
trapped. Additionally, the reduction potential of the sensitizer needs to be more negative,
and the oxidation potential more positive than that of DBP for efficient conduction of
charges out of the devices.
94

Figure 3.7: (a) Singlet and triplet energies of Coumarin 30 and DBP. Possible energy
transfer pathways are given by arrows
(a)
Determined by TDDFT Calculations
(b)
Ref 22
(b) Redox potentials were determined (vs Fc/Fc
+
)

Figure
2.18
T
1
2.25
S
1
1.34
3.04
Coumarin30
DBP
Energy (eV)
(a)
(a)
(b)
Coumarin30
DBP
Reduction Potential (V)
(b)
-1.53
0.76
0.68
-1.08
95

To understand how blending guest molecules into DBP affects its absorption
profile, DBP was blended with transparent, large band gap material bathocuproine (BCP).
DBP is unable to transfer energy BCP due to the high singlet (3.17 eV)
8
and triplet (2.62
eV)
8
energies of BCP or electron transfer since the HOMO and LUMO energies of BCP
are at -6.5 eV
57
and -1.6 eV
8
relative to vacuum, respectively. Figure 3.8 shows the
absorption spectra of the neat and blended DBP: BCP films and even the modest dilutions
of DBP significantly lowers absorption. Since the excitons generated on the sensitizer are
singlets, the energy transfer will take place via FRET and the efficient energy transfer will
take place when the host-guest system is separated by a small distance, while maximizing
the absorption of the sensitizer.
400 500 600 700 800
0
1
2
3
4
5

Wavelength (nm)
 (10
5
cm
-1
)
BCP
DBP
DBP: BCP (1:1)
DBP: BCP (1:2)
DBP: BCP (2:1)

Figure 3.8. The absorption spectra of the neat and blended DBP: BCP films
96

3.5 Luminescence Quenching Experiments
The combined absorption spectra of Coumarin 30 and DBP covers a significant region of
the solar spectrum with little overlap. This suggests that Coumarin 30 will not inhibit the
absorption of DBP. Additionally, as seen in Figure 3.9 (a) the emission of Coumarin 30
strongly overlaps with the absorbance of DBP which constitutes for FRET. To investigate
the energy transfer between Coumarin 30 and DBP thin film photoluminescence quenching
experiments were performed using DBP as the quencher. Coumarin 30 was blended with
BCP or DBP at a ratio of 2: 1 (DBP: Coumarin 30) to replicate the composition of the best
performing devices. The Coumarin 30: BCP films are emissive and the spectra can be seen
in Figure 3.9 (b). The substitution of DBP instead of BCP leads to luminescence quenching
due to the energy transfer to DBP. The luminescence quenching efficiency (θ) has been
calculated for sensitizers previously.
4
For the DBP: Coumarin 30 blended film (𝜃 ) can be
calculated as 95 %, which means that DBP quenches 95% of the Coumarin 30 emission
and efficient energy transfer occurs from the sensitizer to DBP.
4

97

400 500 600 700
0.0
0.2
0.4
0.6
0.8
1.0

Normalized Intensity
Wavelength (nm)
Coumarin 30 Emission
DBP Absorption
(a)

400 500 600 700 800
0.0
0.2
0.4
0.6
0.8
1.0

Emission Intensity (AU)
Wavelength (nm)
Coumarin 30: DBP
Coumarin 30: BCP
(b)

Figure 3.9. (a) Emission of Coumarin 30, and absorption of DBP in thin films (b)
Photoluminescence of Coumarin 30 with and without DBP. Films were excited at λ = 380 nm
98

3.6 Device Studies
The performance of the sensitized device was optimized and compared with a
control DBP/ZCl device. The control device had the ITO/ MoO3 (11 nm)/ DBP (20 nm)/
ZCl (20 nm)/ BCP (10 nm)/ Al, while the sensitized device has the structure ITO/ MoO3
(11 nm)/ DBP: Coumarin 30 (x nm, z %)/ DBP (y nm)/ ZCl (20 nm)/ BCP (10 nm)/ Al,
where z was varied to change the concentration of DBP in the blended layer, and the
thickness of this layer was optimized by varying x.
Sensitized devices can be fabricated by depositing a blended layer of DBP:
Coumarin 30 of varied concentrations directly in contact with the ZCl acceptor layer.
However, the photoresponse received from this approach cannot be elucidated to
photosensitization alone, since the charge separation between DBP and Coumarin 30 at the
D/A interface may also contribute to photoresponse. To eliminate the ambiguity in
photoresponse, sensitized devices were fabricated by inserting a neat layer of DBP between
the blended and acceptor layers and the thickness of the neat DBP layer was varied to
optimize device performance. The device architecture is given in Figure 3.10. The
performance of the sensitized devices was evaluated as a function of concentration of the
sensitizer in the blended layer, and the results are summarized in Figure 3.11 and Table
3.2.

(a) (b)

Figure 3.10 Device architecture for (a) sensitized device, where the effect of
sensitization is evaluated as a function of the concentration of the sensitizer (b)
control device

V

99

(

-1.0 -0.5 0.0 0.5 1.0 1.5
-4
-2
0
2
4

Current Density (mA/ cm
2
)
Voltage (V)
3:2 Dark
3:2 Light
1:1 Dark
1:1 Light
2:1 Dark
2: 1 Light
Control Dark
Control Light
(a)

400 500 600 700
0
5
10
15
20
25
30
35

EQE (%)
Wavelength (nm)
(1: 1) 16 nm
(2: 1) 16 nm
(3: 2) 16 nm
Control Device

(b)

Control (1:1) (3:2) (2:1)
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
 (%), V
oc
, FF
DBP: Coumarin 30 (x:y)
(c)
2.0
2.5
3.0
3.5
4.0
4.5

J
sc
(mA/ cm
2
)

Figure 3.11. Characteristics of OPV devices (a) IV curves of devices under one sun
AM 1.5G illumination (b) plot of external quantum efficiency (c) performance
parameters as a function of the concentration of the sensitizer
100

The current
The current density characteristics (I-V) under one sun illumination (AM 1.5G at
100 mW/cm
2
) are given in Figure 3.11a. The control devices fabricated with DBP/ ZCl as
the active layer show typical performances of 1.4 % with 𝐽 𝑠𝑐
of 2.40 mA/cm
2
, 𝑉 𝑜𝑐
of 1.33
V and FF of 42%. Incorporating Coumarin 30 in (1:1), (3:2), and (2:1) ratios to the DBP
films led to significant enhancements in the photocurrent without a substantial change in
FF or Voc. The active layer with 2:1 (DBP: Coumarin 30) produced the highest efficiency
of 2.24% and the highest 𝐽 𝑠𝑐
of 4.34 mA/ cm
2
, leading to an 80% improvement in
photocurrent. Even in the device with the lowest DBP concentration of 1:1 the photocurrent
and the PCE is higher than the control device.

Table

Device
(DBP: Coumarin 30)
Jsc (mA/ cm
2
)
±0.1
Voc (V)
±0.01
FF
±0.02
ηp (%)
±0.1
(1:1) 3.64 1.31 0.37 1.76
(3:2) 3.78 1.32 0.37 1.85
(2:1) 4.34 1.29 0.40 2.24
Control Device 2.40 1.33 0.42 1.40

Table 3.2. Summary of device performance characteristics of control and sensitized
devices for varied concentrations of the sensitizer
101

The enhancements observed in 𝐽 𝑠𝑐
corresponds well with the EQE measurements
(Figure 3.11b) which clearly indicates the origin of additional charge carriers that
contribute to the increase of the photocurrent in the sensitized device. The photoresponse
from the different regions of the EQE can be correlated with the absorption of the three
materials in the active layer. The photoresponse between the λ = 370 to 460 nm region is
the contribution from Coumarin 30 while the combined photoresponse in the λ = 500 to
580 nm region is from DBP and ZCl and the photoresponse in the λ = 580 to 650 nm region
is from DBP. The control device shows limited photoresponse in the 300 to 450 nm region,
with 22% and 13% quantum efficiency for the two peaks observed at 550 nm and 620 nm
respectively. While all the sensitized devices show enhanced photoresponse compared to
the control device, the optimal performing sensitized device shows a quantum efficiency
of 17% at 410 nm for Coumarin 30. The photoresponse in the 370 to 460 nm region leads
to a better overlap with the solar spectrum and leads to an enhancement in the photocurrent
for all the sensitized devices. Additionally, a 10% increase in the quantum yield is observed
for both the DBP peak at 620 nm, and the DBP and ZCl combined peak at 550 nm in the
optimal performing device. In the control device, the photocurrent is generated by the
excitons formed in the DBP layer, and followed by dissociation at the DBP/ZCl interface.
The increased photoresponse in the 460 to 680 nm region can be attributed to the additional
excitons transferred to DBP via FRET followed by dissociation to free charge carriers.
These results indicate that the sensitizer contributes to photocurrent production
without negatively impacting other device characteristics as shown in Figure 3.11c. 𝑉 𝑜𝑐

remains consistently high in both controlled and sensitized devices, which suggests that
102

increasing Coumarin 30 loading in the active layer does not lead to bimolecular
recombination. Additionally, the FF of the (2:1) sensitized device is comparable to the
control device at 40% which suggests that the doped film does not lead to parasitic losses.
Since 𝑉 𝑜𝑐
and FF remains consistent with the control device, the overall 60% improvement
in PCE in the 2:1 sensitized device is directly due increased exciton harvesting through
sensitization which leads to the enhancement of the photocurrent.

3.7 Optimization of Sensitized Devices
The thickness of the sensitized layer was optimized by fabricating a series of
devices with a constant DBP: Coumarin 30 ratio of 2:1 while varying the thickness. Figure
1.12 represents the sensitized and control device architectures. The performance of the
sensitized devices was evaluated as a function of concentration of the sensitizer in the
blended layer, and the results are summarized in Figure 3.13.
The blended layer of 14 nm produces a 𝐽 𝑠𝑐
of 3.78 mA/ cm
2
, 𝑉 𝑜𝑐
of 1.31 V and FF
of 38% and a PCE of 1.88%. In this lowest performing sensitized device, the photocurrent
shows a 58% improvement and a 40% PCE improvement compared to the control device,
while 𝑉 𝑜𝑐
and FF remain consistent.
Figure 3.12a and b compares the IV and EQE and Figure 3.12c summarizes the
performance of the optimized devices. As seen in the previously sensitized devices 𝑉 𝑜𝑐
and
FF remain largely unchanged, while the photocurrent shows a substantial increase from
3.78 to 4.34 mA/ cm
2
at 14 nm and 16 nm respectively, and decrease slightly at 18 nm. The
EQE shows that at 14 nm the photoresponse from the sensitizer is only 10%, which leads
103

to a small increase of 2.7 % in the DBP response in the λ = 600 to 640 nm region compared
to the control device. With increasing thickness, the response from the sensitizer increases
up to 17.4% and the DBP response in the λ = 600 to 640 nm region increases by 10%
compared to the control device. The performance parameters of the devices are
summarized in Table 3.3.

Figure 3.12. Device architecture for (a) sensitized device, where the effect of sensitization
is evaluated as a function of the thickness of the sensitized layer (b) control device

(a)

V

V

Device Jsc (mA/ cm
2
)
±0.1
Voc (V)
±0.01
FF
±0.02
ηp (%)
±0.1
(2:1) 18 nm 4.08 1.23 0.37 1.86
(2:1) 16 nm 4.34 1.29 0.40 2.24
(2:1) 14 nm 3.78 1.31 0.38 1.88
Standard Device 2.40 1.33 0.42 1.34

Table 3.3. Summary of device performance characteristics of control and sensitized devices
for varied thickness of the sensitized layer

(b)
104

-1.0 -0.5 0.0 0.5 1.0 1.5
-4
-2
0
2
4

Current Density (mA / cm
2
)
Voltage (V)
14 nm Dark
14 nm Light
18 nm Dark
18 nm Light
16 nm Dark
16 nm Light
Control Dark
Control Light
(a)

400 500 600 700
0
5
10
15
20
25
30
35

EQE (%)
Wavelength (nm)
(2: 1) 18 nm
(2: 1) 16 nm
(2: 1) 14 nm
Control Device
(b)

14 15 16 17 18
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2

(%), V
oc
, FF
Thickness (nm)
(c)
3.7
3.8
3.9
4.0
4.1
4.2
4.3
4.4
J
sc
(mA/ cm
2
)

Figure 3.13. Characteristics of OPV devices (a) IV curves of devices under one sun AM
1.5G illumination (b) plot of external quantum efficiency (c) performance parameters as a
function of the thickness of the sensitized layer
105

The device performance was further optimized by varying the thickness of the neat
DBP layer that is in contact with the ZCl acceptor and the device architecture is represented
in Figure 3.14. Device performance indicates that Jsc, Voc, and FF remain largely
unchanged with varying thickness of the DBP layer as seen in Figure 3.15. The highest
photocurrent achieved is 4.34 mA/ cm
2
with an efficiency of 2.24% for a neat DBP layer
of 5 nm. Decreasing the thickness of the neat layer to 3 nm or increasing the thickness to 7
nm results in slight reductions of the current, and overall performance.
The EQE indicates that the photoresponse from sensitizer remains unaffected,
while increasing the DBP neat layer thickness up to 7 nm results in slight increases in the
DBP/ZCl peak and the DBP standalone peak due to the increased photoresponse coming
from DBP. For optimal device performance, the thickness of the DBP neat layer was
maintained at 5 nm.

Figure 3.14. Device architecture for (a) sensitized device, where the effect of sensitization is
evaluated as a function of the thickness of the sensitized layer (b) control device

(a)

V
(b)

106

-1.0 -0.5 0.0 0.5 1.0 1.5
-6
-4
-2
0
2
4
6
8

Current Density (mA/ cm
2
)
Voltage (V)
(a)
(2:1) 16 nm + DBP 7 nm Dark
(2:1) 16 nm + DBP 7 nm Light
(2:1) 16 nm + DBP 5 nm Dark
(2:1) 16 nm + DBP 5 nm Light
(2:1) 16 nm + DBP 3 nm Dark
(2:1) 16 nm + DBP 3 nm Light
Control Dark
Control Light

400 500 600 700
0
5
10
15
20
25
30
35

EQE (%)
Wavelength (nm)
(2:1) 16 nm + DBP 7 nm
(2:1) 16 nm + DBP 3 nm
(2:1) 16 nm + DBP 5 nm
Control Device
(b)

3 4 5 6 7
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2

(%), V
oc
, FF
Thickness of neat DBP layer (nm)
3.0
3.2
3.4
3.6
3.8
4.0
4.2
4.4
J
sc
(mA/ cm
2
)

Figure 3.15. Characteristics of OPV devices (a) IV curves of devices under one sun AM
1.5G illumination (b) plot of external quantum efficiency (c) performance parameters as a
function of the thickness of the neat DBP layer
107

3.8 Conclusions and Future Outlook
In summary, we have demonstrated that Coumarin 30 can be used as a sensitizer to
increase the efficiency of the DBP/ZCl system by increasing the photocurrent while largely
unaffecting Voc or FF. Photoluminescence quenching experiments demonstrate that DBP
quenches excited states on the sensitizer. Coumarin30 functions by absorbing photons and
transferring energy to DBP, while DBP is responsible for transporting excitons and holes.
The extension of the sensitized layer fully fills the EQE minima observed in DBP/ZCl
devices in the λ= 400- 500 nm region. In the champion, sensitized device, the photocurrent
has been increased by 1.9 mA/ cm
2
and in turn the efficiency has been increased by 0.9%
compared to the control device. Energy sensitization is a valuable method filling gaps in
the solar spectrum unabsorbed by donors and for enhancing the photocurrent collection in
devices in the future.

Device Jsc (mA/ cm
2
)
±0.1
Voc (V)
±0.01
FF
±0.02
ηp (%)
±0.1
3 nm 4.26 1.30 0.38 2.12
5 nm 4.34 1.29 0.40 2.24
7 nm 4.29 1.30 0.39 2.18
Standard Device 2.40 1.33 0.42 1.40

Table 3.4. Summary of device performance characteristics of control and sensitized devices
for varied thickness of the neat DBP layer

108

Further improvement to these devices can be made by understanding the singlet
and triplet exciton diffusion lengths of Coumarin 30 in neat and DBP: Coumarin 30 films.
This information will provide valuable insights on the different mechanisms for the
diffusion of singlet and triplets, and exciton diffusion within the device. Understanding the
mechanisms for exciton transport will be important to optimize device performance, where
the exciton diffusion lengths can be used to predict the optimal thickness of the active layer.
Additionally, to utilize absorbance across the entire solar spectrum a range of
complimentary absorbers can be used employing sensitization or energy transfer methods
as discussed above.

109

3.9 Endnotes for Chapter 3
(1) D. Fujishima, H. Kanno, T. Kinoshita, E. Maruyama, M. Tanaka, M. Shirakawa and
K. Shibata, Solar Energy Materials and Solar Cells, 2009, 93, 1029.

(2) C.Trinh, K. O. Kirlikovali, A. N. Bartynski, C. J. Tassone, M. F. Toney, G. F.
Burkhard, M. D. McGehee, P. I. Djurovich and M. E. Thompson, Journal of the
American Chemical Society, 2013, 135, 11920-11928.

(3) S. Senthilikumar, S. Nath and H. Pal, Photochemistry and Photobiology, 2004, 80, 104.

(4) A. N. Bartynski, M. Gruber, S. Das, S. Rangan, S. Mollinger, C. Trinh, S. E. Bradforth,
K. Vandewal, A. Salleo, R. A. Bartynski, W. Bruetting and M. E. Thompson, Journal of
the American Chemical Society, 2015, 137, 5397.

(5) M. M. Li, K. Gao, X. J. Wan, Q. Zhang, B. Kan, R. X. Xia, F. Liu, X. Yang, H. R.
Feng, W. Ni, Y. C. Wang, J. J. Peng, H. T. Zhang, Z. Q. Liang, H. L. Yip, X. B. Peng, Y.
Cao and Y. S. Chen, Nature Photonics, 2017, 11, 85.

(6) M. Hosoya, H. Oooka, H. Nakao, T. Gotanda, S. Mori, N. Shida, R. Hayase, Y. Nakano
, M. Saito. Organic thin film photovoltaic modules. Proceedings of the 93rd Annual
Meeting of the Chemical Society of Japan2013; 21.

(7) W. A. Luhman, R. J. Holmes, Appl. Phys. Lett. 2009, 94, 153304.

(8) S. M. Menke, R. J. Holmes, Energy Environ. Sci. 2014, 7, 499−512.

(9) S. M. Menke, W. A. Luhman, R. J. Holmes, Nat. Mater. 2013, 12, 152−157.

(10) R. Koeppe, O. Bossart, G. Calzaferri, N. Sariciftci, Sol. Energy Mater. Sol. Cells
2007, 91, 986.

(11) K. Feron, W. J. Belcher, C. J. Fell, P. C.Dastoor, Transfer. Int. J. Mol. Sci. 2012, 13,
17019.

(12) J. D. Zimmerman, B. E. Lassiter, X. Xiao, K. Sun, A. Dolocan, R. Gearba, D. A.
Vanden Bout, K. J. Stevenson, P. Wickramasinghe, M. E. Thompson and S. R. Forrest,
Acs Nano, 2013, 7, 9268.

(13) Y. Z. Lin, Y. F. Li and X. W. Zhan, Chemical Society Reviews, 2012, 41, 4245.

(14) J. Peet, A. B. Tamayo, X. D. Dang, J. H. Seo and T. Q. Nguyena, Applied Physics
Letters, 2008, 93.
110

(15) J. S. Huang, T. Goh, X. K. Li, M. Y. Sfeir, E. A. Bielinski, S. Tomasulo, M. L. Lee,
N. Hazari and A. D. Taylor, San Diego, CA, 2013.

(16) Y. J. Cho, J. Y. Lee, B. D. Chin and S. R. Forrest, Organic Electronics, 2013, 14,
1081.

(17) V. S. Gevaerts, A. Furlan, M. M. Wienk, M. Turbiez and R. A. J. Janssen, Advanced
Materials, 2012, 24, 2130.

(1) S. Sista, M. H. Park, Z. R. Hong, Y. Wu, J. H. Hou, W. L. Kwan, G. Li and Y. Yang,
Advanced Materials, 2010, 22, 380.

(18) J. Y. Kim, K. Lee, N. E. Coates, D. Moses, T. Q. Nguyen, M. Dante and A. J.
Heeger, Science, 2007, 317, 222-225.

(1) C. H. Chou, W. L. Kwan, Z. R. Hong, L. M. Chen and Y. Yang, Advanced Materials,
2011, 23, 1282.

(19) A. Hadipour, B. de Boer and P. W. M. Blom, Advanced Functional Materials, 2008,
18, 161.

(20) C. J. Brabec, S. Gowrisanker, J. J. M. Halls, D. Laird, S. J. Jia and S. P. Williams,
Advanced Materials, 2010, 22, 3839.

(21) A. N. Bartynski, C. Trinh, K. O. Kirlikovali and M. E. Thompson, Applied Physics
Letters, 2014, 105.

(22) A. B. Yusoff, D. Kim, H. P. Kim, F. K. Shneider, W. J. da Silva and J. Jang, Energy
Environ. Sci., 2015, 8, 303.

(23) Z. Hu, S. Tang, A. Ahlvers, S. I. Khondaker and A. J. Gesquiere, Appl. Phys. Lett.,
2012, 101, 053308.

(24) M. Zhang, F. Zhang, Q. An, Q. Sun, J. Wang, L. Li, W. Wang andJ.Zhang,
Sol.EnergyMater.Sol.Cells,2015, 141,154.

(25) B. H. Lessard, J. D. Dang, T. M. Grant, D. Gao, D. S. Seferos and T. P. Bender,
ACS Appl. Mater. Interfaces, 2014, 6, 15040.

(26) M. M. Mandoc, F. B. Kooistra, J. C. Hummelen, B. de Boer and P. W. M. Blom,
Appl. Phys. Lett., 2007, 91, 263505.

111

(27) H. C. Hesse, J. Weickert, C. Hundschell, X. Feng, K. Mu ¨llen, B. Nickel, A. J.
Mozer and L. Schmidt-Mende, Adv. Energy Mater., 2011, 1, 861.

(28) S. Honda, T. Nogami, H. Ohkita, H. Benten and S. Ito, ACS Appl. Mater. Interfaces,
2009, 1, 804.

(29) Q. An, F. Zhang, L. Li, Z. Zhuo, J. Zhang, W. Tang and F. Teng, Phys. Chem.
Chem. Phys., 2014, 16, 16103.

(30) Q. Sun, F. Zhang, J. Hai, J. Yu, H. Huang, F. Teng and W. Tang, Electron. Mater.
Lett., 2015, 11, 236.

(31) Q. An, F. Zhang, J. Zhang, W. Tang, Z. Deng, and B. Hu Energy Environ. Sci.,
2016, 9, 281.

(32) S.-J. Ko, W. Lee, H. Choi, B. Walker, S. Yum, S. Kim, T. J. Shin, H. Y. Woo and J.
Y. Kim, Adv. Energy Mater., 2015, 5, 1401687.

(33) Y. Santo, I. Jeon, K. Sheng Yeo, T. Nakagawa and Y. Matsuo, Appl. Phys. Lett.,
2013, 103, 073306.

(34) T.-Y. Huang, D. Patra, Y.-S. Hsiao, S. H. Chang, C.-G. Wu, K.-C. Ho and C.-W.
Chu, J. Mater. Chem. A, 2015, 3, 10512.

(35) W. L. Leong, S. R. Cowan and A. J. Heeger, Adv. Energy Mater., 2011, 1, 517.

(36) Y.-J. Cheng, C.-H. Hsieh, P.-J. Li and C.-S. Hsu, Adv. Funct. Mater., 2011, 21,
1723.

(37) Q. Wei, T. Nishizawa, K. Tajima and K. Hashimoto, Adv. Mater., 2008, 20, 2211.

(38) F. Bonaccorso, N. Balis, M. M. Stylianakis, M. Savarese, C. Adamo, M. Gemmi, V.
Pellegrini, E. Stratakis and E. Kymakis, Adv. Funct. Mater., 2015, 25, 3870.

(39) Z. Zhuo, F. Zhang, J. Wang, J. Wang, X. Xu, Z. Xu, Y. Wang and W. Tang, Solid-
State Electron., 2011, 68, 83.

(40) S. Honda, H. Ohkita, H. Benten and S. Ito, Adv. Energy Mater., 2011, 1, 588.

(41) S. Honda, S. Yokoya, H. Ohkita, H. Benten and S. Ito, J. Phys. Chem. C, 2011, 115,
11306.

(42) J. S. Huang, T. Goh, X. K. Li, M. Y. Sfeir, E. A. Bielinski, S. Tomasulo, M. L. Lee,
N. Hazari and A. D. Taylor, Nat. Photonics, 2013, 7, 486.
112

(43) Q.i An, F. Zhang, J. Zhang, W. Tang, Z Denga and B. Hu, Energy Environ. Sci.,
2016, 9, 281

(44) J-S Huang, T. Goh, X. Li, M. Y. Sfeir, E. A. Bielinski, S. Tomasulo, M. L. Lee, N.
Hazari, and A. D. Taylor, Nature Photonics, 2013, 7, 479.

(45) K. Cnops, B. P. Rand, D. Cheyns, B. Verreet, M. A. Empl, and P. Heremans, Nat.
Commun., 2014, 5, 3406.

(46) U. Hormann, J. Kraus, M. Gruber, C. Schuhmair, T. Linderl, S. Grob, S. Kapfinger,
K. Klein, M. Stutzman, H. J. Krenner, W. Brutting, Phys. Rev. B 2013, 88, 235307.

(47) K. Vandewal, K. Tvingstedt, A. Gadisa, O. Inganas, J. V. Manca, Phys. Rev. B
2010, 81, 125204.

(48) J. Widmer, M. Tietze, K. Leo, M. Riede, Adv. Func. Mat. 2013, 23, (46), 5814-
5821.

(49) M. Gruber, J. Wagner, K. Klein, U. Hörmann, A. Opitz, M. Stutzmann, W. Brütting,
Adv. En. Mat. 2012, 2, 1100.

(50) X. Xiao, K. J. Bergemann, J. D. Zimmerman, K. Lee and S. R. Forrest, Advanced
Energy Materials, 2014, 4.

(51) D. Yokoyama, Z. Q. Wang, Y. J. Pu, K. Kobayashi, J. Kido and Z. R. Hong, Solar
Energy Materials and Solar Cells, 2012, 98, 472.

(52) T. Forster, Discussions of the Faraday Society, 1959, 7-17.

(53) M. Wu, D. N. Congreve, M. W. B. Wilson, J. Jean, N. Geva, M.Welborn, T. V.
Voorhis, V. Bulović, M. G. Bawendi, and M A. Baldo, Nature Photonics, 2016, 10, 31.

(54) J. B. You, L. T. Dou, K. Yoshimura, T. Kato, K. Ohya, T. Moriarty, K. Emery, C. C.
Chen, J. Gao, G. Li and Y. Yang, Nature Communications, 2013, 4.

113

Bibliography
(1) Perez, R.; Perez, M., A fundamental look at energy reserves for the planet. The IEA
SHC Solar Update 2009, 50, 2-3.

(2) Becquerel, A.-E., Memoire sur les effets electriques produits sous l'influence des
rayons solaires. Comptes Rendus 1839, 9, (567), 1839.

(3) R. S.Ohl, Light-sensitive electric device. In Google Patents: 1946.

(4) R. F.,ervice, Solar Energy: Can the Upstarts Top Silicon? Science 2008, 319, 718-720.

(5) A. Goetzberger., Hebling, C., Schock, H. W., Photovoltaic Mater., History, Status and
Outlook. Mater. Sci. Eng., R 2003, 40, (1), 1-46.

(6 ) C. W. Schlenker, M. E. Thompson, Chem 2012, 312, 175-212.

(7 ) C. W. Tang, Albrecht, A. C., Photovoltaic Effects of Metal Cholorophyll-a-Metal
Sandwich Cells. J. Chem. Phys. 1975, 62, (6), 2139-2149.

(8) S. Barth, Bassler, H., Intrinsic Photoconduction in Ppv-Type Conjugated Polymers.
Phys. Rev. Lett. 1997, 79, (22), 4445-4448.

(9 ) C. W. Tang, Two Layer Organic Photovoltaic Cell. Appl. Phys. Lett. 1986, 48, (2),
183-185.

(10) M. Hiramoto, H. Fujiwara, M. Yokohama, Appl. Phys. Lett. 1991, 58,1062.

(11) N. S. Sariciftci, L. Smilowitz, A. J. Heeger and F. Wudl, Science, 1992, 258, 1474.

(12) G. Yu, J. Gao, J. C. Hummenlen, F. Wudl, and A. J. Heeger, Science 1995,270,1789.

(13) S. Tanaka, Appl. Phys. Let, 2009, 94,113506.

(14) Kurt B. and Sajeev J., Phys. Rev. E 1998, 58, 3896.

(15) P. Sullivan, Jones T. S., Appl. Phys. Lett. 2007, 91, 233114.

(16) H. Gommans, Ad., Func. Mater. 2007, 17,2653.

(17) P. Peumans, Appl. Phys. Lett. 2002, 80,338.

(18) K. Y. Law, J. Phys. Chem. 1987, 91, 5184.

114

(19) S. Wang., Appl. Phys. Lett. 2009, 92, 233304.

(20) H. W Kroto, Nature 1985, 318, 162.

(21) J. N. Haddock, Org. Elect. 2005, 6, 182.

(22) Fullerene: Principles and applications, Langa F, Royal Doc of Chem Cambridge,
2007.

(23) H. Imahori, Adv, Func .Mater. 2004, 14, 525.

(24) R. C. Powell and Z. G. Soos, J. Lumin. 1975, 11, 1.

(25 ) T. Förster,. Annalen Der Physik 1948, 2, 55.

(26) T. Förster, Discuss. Faraday Soc. 1959, 27, 7.

(27) D. L. Dexter, J. of Chem. Phys. 1953, 21, 836.

(28) B. Kipplen., J. L Brédas., Energy Environ. Sci., 2009, 2, 251.

(29) P. Peumans, A. Yakimov, and S. R. Forrest. J. of Appl. Phys., 2003, 7, 3693.

(30) A. Wade, Luhman and Russell J. Holmes. Adv. Func. Mater., 2011, 4, 764771.

(31) P. Avakian and R. E. Merr, Phys. Rev. Lett.,1964, 18, 13.

(32) D. F. Williams and J. Adolph. J. Chem. Phys., 1967, 11, 46.

(33) M. Hiramoto, H. Fujiwara, and M. Yokoyama. Appl. Phys. Lett., 1991, 10, 1062.

(34) G. Yu, J. Gao, J. C. Hummelen, F. Wudl, and A. J. Heeger. Science, 1995, 5243, 1789.

(35) S. M.Menke, Luhman, W. A. & Holmes, R. J. Nat. Mater. 2013, 12, 152.

(36) G. Wei, R. R. Lunt, K. Sun, S. Wang, M. E. Thompson, and S. R. Forrest, Nano Lett.,
2010, 10, 3555.

(37) O. V. Mikhnenko, J. Lin, Y. Shu, J. E. Anthony, P. W. M. Blom, T. Q. Nguyen, and
M. A. Loi, Phys. Chem. Chem. Phys. 2012, 14, 14196.

(38) M. Hiramoto, H. Fujiwara, and M. Yokoyama, Appl. Phys. Lett. 1991, 58, 1062.

(39) G. Yu, J. Gao, J. C. Hummelen, F. Wudl, and A. J. Heeger, Science 1995, 270, 1789.
115

(40) Marcus, R. A., J. Chem. Phys. 1956, 5, 966.

(41) Marcus, R. A., Electron Transfer Reactions in Chem.: Theory and Experiment
(Nobel Lecture). Ang. Chem. Int. Ed. 1993, 8, 1111.

(42) Giaimo, J. M., Gusev, A. V., Wasielewski, M. R., J. Am. Chem. Soc. 2002, 29,
8530.

(43) Trinh, C., Kirlikovali, K. O., Das, S., Ener, M. E., Gray, H. B., Djurovich, P. I.,
Bradforth, S., Thompson, M. E., J. Chem. Phys. C 2014.

(44) Hormann, U., Kraus, J., Gruber, M., Schuhmair, C., Linderl, T., Grob, S., Kapfinger,
S., Klein, K., Stutzman, M., Krenner, H. J., Brutting, W., Phys. Rev. B 2013, 23, 235307.

(45) Vandewal, K., Tvingstedt, K., Gadisa, A., Inganas, O., Manca, J. V., Phys. Rev. B
2010, 12, 125204.

(46) Gruber, M., Wagner, J., Klein, K., Hörmann, U., Opitz, A., Stutzmann, M., Brütting,
W., Adv. En. Mater. 2012, 9, 1100.

(47) Widmer, J., Tietze, M., Leo, K., Riede, M., Adv. Func. Mater. 2013, 46, 5814.

(48) Jailaubekov, A. E., Willard, A. P., Tritsch, J. R., Chan, W.-L., Sai, N., Gearba, R.,
Kaake, L. G., Williams, K. J., Leung, K., Rossky, P. J., Zhu, X. Y. Nat. Mat. 2013, 12,
66.

(49) Grancini, G., Maiuri, M., Fazzi, D., Petrozza, A., Egelhaaf, H. J., Brida, D., Cerullo,
G., Lanzani, G. Nat. Mat. 2013, 12, 29.

(50) Vandewal, K., Nat. Mat. 2014, 3, 63.

(51) Hukka T. I. Adv Mater. 2006, 18, 1301.

(52) Schueppel R, Phys Rev B: Condens. Matter Mater. Phys 2008, 77, 085311.

(53) Potscavage W. J Appl. Phys. Lett. 2008, 93, 193308.

(54) Bredas, J. L. , Norton E, Acc. Chem. Res. 2009,42, 1691.

(55) Ruseckas A, Shaw P. E, Samuel I. D. W, Dalton. Trans.

(56) Müller J. G. Phys Rev. B. 2005, 72,195208.

116

(57) Veldman D, J. Am. Chem. Soc 2008, 130, 7721.

(58) Yang, Y., Huang, Q. L., Metz, A. W., Ni, J., Jin, S., Marks, T. J., Madsen, M. E.,
DiVenere, A., Ho, S. T. Adv. Mater. 2004, 16, 321-324.

(59) Han, H., Mayer, J. W., Alford, T. L. J. of Appl Phys. 2006, 100.

(60) Antony, A., Nisha, M., Manoj, R., Jayaraj, M. K. Appl. Surf. Sci. 2004, 225, 294-301.

(61) Brumbach, M., Veneman, P. A., Marrikar, F. S., Schulmeyer, T., Simmonds, A., Xia,
W., Lee, P., Armstrong, N. R. Langmuir 2007, 23, 11089-11099.

(62) Paniagua, S. A., Hotchkiss, P. J., Jones, S. C., Marder, S. R., Mudalige, A., Marrikar,
F. S., Pemberton, J. E., Armstrong, N. R. J. of Phys. Chem. C 2008, 112, 7809-7817.

(63) Dabera, G. D. M. R., Jayawardena, K. D. G. I., Prabhath, M. R. R., Yahya, I., Tan,
Y. Y., Nismy, N. A., Shiozawa, H., Sauer, M., Ruiz-Soria, G., Ayala, P., Stolojan, V.,
Adikaari, A. A. D. T., Jarowski, P. D., Pichler, T., Silva, S. R. P. Acs Nano 2013, 7, 556-
565.

(64) Barnes, T. M., Bergeson, J. D., Tenent, R. C., Larsen, B. A., Teeter, G., Jones, K. M.,
Black burn, J. L., van de Lagemaat, J. Appl Phys. Lett. 2010, 96.

(65) Pasquier, A. D., Unalan, H. E., Kanwal, A., Miller, S., Chhowalla, M. Appl Phys. Lett.
2005, 87.
(66) De Arco, L. G., Zhang, Y., Schlenker, C. W., Ryu, K., Thompson, M. E., Zhou, C.
Acs Nano 2010, 4, 2865-2873.

(67) Yang, D., Zhou, L., Chen, L., Zhao, B., Zhang, J., Li, C. Chem. Comm. 2012, 48,
8078-8080.

(68) H. B. Michaelson, J. App. Phys. 1991, 48, 4729.

(69) H. L. Skriver and N. M. Rosengaard, Phys. Rev. B 46, 7157 ,1992.
(70) M. S. White,D. C. Olson, S. E. Shaheen, N. Kopidakis, and D. S. GinleyAppl. Phys.
Lett. 2006, 89, 143517.

(71) Christoph J. Brabec, Solar Energy Mater. and Solar Cells, 2004, 83, 273.

(72) Hirose, Y., Kahn, A., Aristov, V., Soukiassian, P., Bulovic, V., Forrest, S. R., Phys.
Rev. B 1996, 54, 13748.

(73) Ishii, H., Sugiyama, K., Ito, E., Seki, K., Adv. Mater. 1999, 11, 605.

117

(74) Peumans, P., Yakimov, A., Forrest, S. R., J. Appl. Phys. 2003, 93, 3693.

(75) Li, N., Lassiter, B. E., Lunt, R. R., Wei, G., Forrest, S. R., Appl. Phys. Lett. 2009, 2,
023307.

(76) Stubinge T., Brutting W., J. Appl. Phys. 2001, 90, 3632.

(77) Peumans P., Forrest S. R., Appl. Phys. Lett. 2001, 79, 126.

(78) Barbec C. J., Shaheen S.E. Appl. Phys. Lett. 2002,80,1288.

(79) Brumbach M., Veneman P. A, Langmuir 2007, 23, 11089.

(80) Donely C, Dunphy D, Langmuir 2002, 18, 450.

(81) Ni. J, Yan. H, J. Am. Chem. Soc. 2005, 127, 5613.

(82) Wong K. W, Yip H. L., Appl. Phys. Lett. 2002, 80, 2788.

(83) Song Q L, Li. F. Y, Chem. Phys. Lett. 2005, 416, 42.

(84) Rand B.P. Adv. Mater 2005, 17, 2714.

(85) Hirose Y. Kahn A, Phys Rev B 1996 54, 13748.

(86) Ichikawa M., J. Appl. Phys. 2003, 94, 796.

(87) Naka S. Okada H, Appl. Phys. Lett. 2000, 76, 197.

(88) Jenny Nelson, Acc. Chem. Res. 2009, 42, 1768.

(89) Schlenker, C. W., Thompson, M. E. Chem. Comm. 2011, 47, 3702.

(90) W. Shockley, H. J. Queisser., J. Appl. Phys, 1961, 32, 3.

(91) D. A. Neamen, Semiconductor Phys. and Devices, Third Edition edn., McGraw-Hill,
New York, 2003.

(92) Dongaonkar, S., Servaites, J. D., Ford, G. M., Loser, S., Moore, J., Gelfand, R. M.,
Mohseni, H., Hillhouse, H. W., Agrawal, R., Ratner, M. A., Marks, T. J., Lundstrom, M.
S., Alam, M. A. J. App. Phys. 2010, 108.

(93) Perez, M.D., Borek, C., Forrest, S.R., and Thompson, M. E., J. Am. Chem.
Soc.,2009, 131,9281.
118

(94) Rand, B.P., D.P. Burk, and S. Forrest, R., Phys. Rev. B, 2007, 11, 115327.

(95) A. N. Bartynski, M. Gruber, S. Das, S. Rangan, S. Mollinger, C. Trinh, S. E. Bradforth,
K. Vandewal, A. Salleo, R. A. Bartynski, W. Bruetting and M. E. Thompson, Journal of
the American Chemical Society, 2015, 137, 5397.
(96) K. Cnops, B. P. Rand, D. Cheyns, B. Verreet, M. A. Empl, and P. Heremans, Nat.
Commun., 2014, 5, 3406.

(97) U. Hormann, J. Kraus, M. Gruber, C. Schuhmair, T. Linderl, S. Grob, S. Kapfinger,
K. Klein, M. Stutzman, H. J. Krenner, W. Brutting, Phys. Rev. B 2013, 88, 235307.

(98) K. Vandewal, K. Tvingstedt, A. Gadisa, O. Inganas, J. V. Manca, Phys. Rev. B
2010, 81, 125204.

(99) J. Widmer, M. Tietze, K. Leo, M. Riede, Adv. Func. Mat. 2013, 23, (46), 5814-
5821.

(100) M. Gruber, J. Wagner, K. Klein, U. Hörmann, A. Opitz, M. Stutzmann, W.
Brütting, Adv. En. Mat. 2012, 2, 1100.

(101) X. Xiao, K. J. Bergemann, J. D. Zimmerman, K. Lee and S. R. Forrest, Advanced
Energy Materials, 2014, 4.

(102) D. Yokoyama, Z. Q. Wang, Y. J. Pu, K. Kobayashi, J. Kido and Z. R. Hong, Solar
Energy Materials and Solar Cells, 2012, 98, 472.

(103) T. Forster, Discussions of the Faraday Society, 1959, 7-17.

(104) M. Wu, D. N. Congreve, M. W. B. Wilson, J. Jean, N. Geva, M.Welborn, T. V.
Voorhis, V. Bulović, M. G. Bawendi, and M A. Baldo, Nature Photonics, 2016, 10, 31.

Abstract (if available)

Abstract Organic photovoltaics (OPV) have remained a research area of great interest in the past two decades as source of renewable energy. Compared to their inorganic counterparts, they have the advantages of low‐cost roll‐to‐roll production, lightweight, and flexibility which makes them available for applications on flexible substrates or mobile phones. However, OPVs suffer from low performance efficiencies compared to Si‐based PVs. This dissertation explores the utilization molecule design, and device optimization through energy management of the device to address the limitations of OPVs. ❧ In Chapter 2 we explore the design and development of pyrrolic squaraines as donor materials for applications in OPVs, comparisons are made between the performance of devices fabricated through the vacuum deposition and the solution processing of the squaraine material. In Chapter 3, we present research on improving the photocurrent of a tetraphenyldibenzoperiflanthene (DBP) / zinc chlorodipyrrin (ZCl) bilayer device, using Coumarin 30 as a sensitizer. Here an energy sensitization scheme has been used to harvest a broader region of the solar spectrum. The overall theme of this dissertation is to utilize material and device design to develop insights on improving the performance of OPVs.

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Asset Metadata

Creator Wickramasinghe, Piyumie (author)

Core Title Pyrrolic squaraines and energy management in organic photovoltaics

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Chemistry

Publication Date 04/11/2018

Defense Date 04/10/2018

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag OAI-PMH Harvest,organic photovoltaics,squaraines

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Thompson, Mark (committee chair)

Creator Email pwickram@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c89-11732

Unique identifier UC11671550

Identifier etd-Wickramasi-6210.pdf (filename),usctheses-c89-11732 (legacy record id)

Legacy Identifier etd-Wickramasi-6210.pdf

Dmrecord 11732

Document Type Dissertation

Rights Wickramasinghe, Piyumie

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus MC 2810, 3434 South Grand Avenue, 2nd Floor, Los Angeles, California 90089-2810, USA