University of Southern California Dissertations and Theses

Energy management in organic photovoltaics

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Energy management in organic photovoltaics

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Content ENERGY MANAGEMENT IN ORGANIC PHOTOVOLTAICS

by
Andrew N. Bartynski
__________________________________________________________________
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
(CHEMICAL ENGINEERING)

May 2016

Copyright 2015 Andrew N. Bartynski

ii

Dedication

For Ross Victor Speck

iii

Acknowledgements
There are numerous people I wish to acknowledge for their guidance and support, without
which this work would not have been possible. First, Professor Mark Thompson for the
tremendous freedom he provided within his lab, allowing me to explore topics I found
interesting while offering exemplary advice and encouragement. While at USC, I was able
to grow substantially both intellectually and scientifically due to the environment he
cultivated.
Additionally, I would like to thank my other committee members, Professors Richard
Brutchey, Andrea Armani, Noah Malmstadt, and Malancha Gupta. Their input and
inquisitive questions strengthened the quality of my work. The members of the Thompson
group provided exceptional camaraderie and inspiration leading to many fruitful
discussions and helpful insights.
Professor Wolfgang Brutting and his students Mark Gruber, Stefan Grob, Theressa Linderl,
and Tobias Schmidt extended their tremendous hospitality hosting me in Augsburg. My
visit to his lab and the subsequent visits of Professor Brutting and his students to USC are
some of my fondest memories from graduate school. Professor Stephen Forrest and his
students Brian Lassiter, Anurag Panda, Kevin Bergemann, and Jeramy Zimmerman also
welcomed me into their lab at Michigan for which I am thankful.
Finally, I would like to acknowledge my father, mother, and brother for their continued
love and support throughout my graduate career. In closing, I am immensely grateful to
my girlfriend Narin, for being the most brilliant, challenging, and understanding partner. I
wouldn’t want to go on this journey with anyone else.
iv

Table of Contents

Dedication ii

Acknowledgements iii

List of Tables vii

List of Figures viii

Abstract xvi

Chapter 1. Introduction 1
1.1 Motivation 1
1.2 Fundamentals of Photovoltaics 5
1.2.1 A Brief History 5
1.2.2 Introduction to Inorganic Photovoltaics 6
1.2.3 Introduction to Organic Photovoltaics 8
1.3 Current Status, Challenges, and Future Outlook 11
1.3.1 Photocurrent 12
1.3.2 Open Circuit Voltage 15
1.3.3 High Efficiency 19
1.4 Summary of Topics 20
1.5 References 21

Chapter 2. Instrumentation and Analysis 27
2.1 General 27
2.1.1 Thin Film Characterization 27
2.1.2 Device Fabrication 27
2.1.3 Device Characterization 27
2.1.4 Instrumentation 28
2.1.5 Optical Modeling 29
2.2 Specific to Chapter 3 29
2.3 Specific to Chapter 5 30
2.4 References 33

v

Chapter 3. Exciton Blocking and Carrier Extraction in Buffers for Organic
Photovoltaics 34
3.1 Abstract 34
3.2 Mixed Buffers Based on C60:BCP 34
3.2.1 Introduction 34
3.2.2 Characterization of BCP:C 60 blends 36
3.2.3 Exciton Blocking Characteristics 39
3.2.4 Monte Carlo Modeling 41
3.2.5 Conductivity Measurements 42
3.2.6 Buffer Layer Characterization 43
3.3 Mixed Buffers based on NPD:DBP 46
3.4 Amorphous vs. Crystalline Buffer Layers 47
3.4.1 Introduction 47
3.4.2 Optical and Electronic Properties 48
3.4.3 Devices with a Crystalline Buffer 50
3.4.4 Devices with an Amorphous Buffer 52
3.4.5 Additional Buffers 53
3.5 Conclusions and Future Outlook 55
3.6 References 58

Chapter 4. Energy Transfer and Exciton Diffusion 62
4.1 Abstract 62
4.2 Multichromophoric Energy Sensitization of C60 62
4.2.1 Introduction 62
4.2.2 Optical and Electronic Properties 64
4.2.3 Luminescence Quenching Experiments 66
4.2.4 Device Studies 67
4.3 Probing Singlet and Triplet Exciton Diffusion Through the use of
Energy Sensitizers 72
4.3.1 Introduction 72
4.3.2 Materials Design and Selection 76
4.3.3 Experimental Design 78
4.3.4 Fitting the Diffusion Length 79
4.3.5 Diffusion Through Neat C60 Films 81
4.3.6 Diffusion Through BCP:C60 Films 83
4.4 Conclusions and Future Outlook 84
4.5 References 86

Chapter 5. Symmetry Breaking Charge Transfer 90
5.1 Abstract 90
5.2 Kinetics of Symmetry Breaking Charge Transfer in Some Zinc
Dipyrrins 90
vi

5.2.1 Introduction 90
5.2.2 Materials Characterization 94
5.2.3 Photoluminescence Quantum Yeild and Lifetime
Measurements 95
5.2.4 Kinetic Model 96
5.2.5 Fitting the Photoluminescence Quantum Yield and Lifetime
Measurements 98
5.2.6 Extracting the Driving Force 100
5.3 Symmetry Breaking Charge Transfer in a Zinc Chlorodipyrrin
Acceptor for High Open Circuit Voltage Organic Photovoltaics 101
5.3.1 Introduction 101
5.3.2 Optical Characterization 105
5.3.3 Characterization of the Excited State in Various
Environments 106
5.3.4 Frontier Energy Level Characterization 108
5.3.5 Device Characterization 110
5.3.6 Charge Transfer Sate Characterization 112
5.3.7 Conclusion 118
5.4 ZCl: a Versatile Acceptor for High Open Circuit Voltage Organic
Photovoltaics 119
5.4.1 Comparison of ZCl and C60 with a Variety of Donors 119
5.4.2 Comparison of ZCl and Cl6BODIPY 123
5.5 Conclusion and Future Outlook 128
5.6 References 131

Chapter 6. Unconventional Materials for High Open Circuit Voltage
Photovoltaics 138
6.1 Abstract 138
6.2 Introduction 138
6.3 Results and Discussion 140
6.3.1 Photovoltaic Performance and Optimization 141
6.3.2 Determination of the Energy of the Charge Transfer State 144
6.3.3 Morphological Studies 147
6.3.4 Optical Electric Field Effects 148
6.4 Conclusion and Future Outlook 151
6.5 References 152

Bibliography 155

vii

List of Tables

Table 3.1: Summary of Device Performance Characteristics. 40
Table 4.1: Summary of Device Performance Characteristics of standard and
sensitized devices. 71
Table 5.1: Experimental and calculated photoluminescence quantum yield and
lifetime of zDIP2 in various solvent environments. 96
Table 5.2: Rate constants for zDIP2 determined from analysis of lifetime and
quantum yield data. 99
Table 5.3: Summary of DBP/ZCl and DBP/C60 Device Performance
Characteristics. 111
Table 5.4: Summary of ZCl and C60 Acceptor Device Performance
Characteristics. 120
Table 5.5: Summary of CL6BODIPY, ZCl, and C60 Acceptor Device Performance
Characteristics 127
Table 6.1: Device performance for 6T/DBP and DBP/C60 devices. 142
Table 6.2: Device performance for 6T/DBP devices with various thicknesses of
DBP, HTL (MoO3), and ETL (BCP). 144

viii

List of Figures

Figure 1.1: World electricity generation by fuel (TWh) (from IEA) 1
Figure 1.2: World CO2 emissions by fuel (Mt) (from IEA) 2
Figure 1.3: Atmospheric CO2 concentration (ppm) (from NASA global climate
change) 3
Figure 1.4: Global energy potential annually and total reserves for a wide variety
of sources. From Ref. 5 4
Figure 1.5: 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. 9
Figure 1.6: Absorber thickness, cell mass per area and specific power for various
photovoltaic technoligies. Specific power is shown for active layers alone
and for cells with a 25μm polyethylene terephthalate (PET) or 3 mm glass
substrate or encapsulation layer. From Ref. 17 10
Figure 1.7: Historical plot of the highest confirmed power conversion efficiencies
for research cells, from 1976 to the present, for a range of photovoltaic
technologies. (Courtesy of NREL) 11
Figure 1.8: Solar photon flux (left axis) with the photopic response function (right
axis) plotted versus wavelength. From Ref. 19 12
Figure 1.9: Schematic representation of D-A chromophores and basic energy level
diagram demonstrating the narrowing of the bandgap due to hybridization
from the D and A subunits. From Ref. 20. Molecular structures of
PCPDTBT, PTB7, and DTCTB. 13
Figure 1.10: Sample optical interference patterns for tandem devices From Refs. 29
and 32. The plots illustrate how positioning the subcells in areas of intense
optical field impact device efficiency 14

ix

Figure 1.11: Experimental VOC for a wide range of single-junction solar cell band
gaps, from 0.67 to 2.1 eV, showing that the band gap–voltage offset, WOC
= (Eg/q) - VOC, is roughly constant over this range experimentally. The band
gap–voltage offset WOC is calculated for a semiconductor layer with
radiative recombination only, and using the detailed balance model, also
showing the approximate constancy of W OC as predicted from theory. The
measured band gap–voltage offset for some solar cell materials approaches
the calculated value for radiative recombination only. From Ref. 48. Solid
red and green lines are added to show typical values for OPVs. 16
Figure 3.1: Absorption characteristics of C60 in film and solution. Absorption
spectra of C60 in solution (red line) and thin film (blue line) plotted along
with the photon flux for AM 1.5 G illumination (black). The solution
extinction is converted to film absorptivity units by multiplying by the
concentration of the C60 film (2.39 M). The appearance of a band between
λ = 400 and 500 nm in the thin film sample, is assigned to an intermolecular
charge transfer (CT) state. The hatched areas below each absorption line
show the number of photons collected by film (blue) and solution (red)
samples at a thickness of 40 nm, illustrating the importance of CT
absorption in C60-based OPVs. 35
Figure 3.2: Absorption and OPVs based on BCP:C 60 blends. (a) Device
architectures with BCP:C60 volume ratios of 0:1 (1), 1:2 (2), 1:1 (3), and
2:1 (4). (b) Absorption coefficient of BCP:C60 blend films with 0:1 (▼), 1:2
(■), 1:1 (●), 2:1 (▲), and 1:0 (►) calculated from optical constants
determined by variable angle spectroscopic ellipsometry. Inset: Extinction
as a function of C60 fraction for wavelengths of =360 nm (●) and 450 nm
(■), corresponding to Frenkel and CT absorption features, respectively.
Linear and power law (y = x
2.7
) fits are shown for the 360 and 450 nm data,
receptively. (c) External quantum efficiency of 1 - 4. 37
Figure 3.3: Effects of varying the ratio of acceptor to blended layer thicknesses. (a)
Acceptor thicknesses correspond to x = 5 nm (5), 15 nm (6), 25 nm (7), and
35nm (8). (b) Current density vs. voltage characteristics of devices under
one sun AM1.5G illumination. (c) External quantum efficiencies of devices
in (b). 39
Figure 3.4: Blocking properties of the blend films. 3-D Monte Carlo simulation of
exciton diffusion from neat C60 into BCP:C60 blend layer. The neat C60
(0:1) blocks 50% of the excitons, the 1:1 ratio blocks approximately 81%,
the 2:1 ratio blocks ~ 95%, and the 4:1 ratio blocks ~98% of the excitons.
The blocking efficiency is defined as the ratio of the exciton population at
the interface to the population expected for an ideal blocking layer, and all
data are normalized to this value. Inset: 3D illustration of the 1:1 mixed
layer used in the simulation. Green denotes BCP, blue is C60. The neat C60
acceptor layer is shown as the semi-transparent region on the front right
edge of the blend. 41
x

Figure 3.5: Effects of varying the BCP:C60 layer thicknesses. (a) conductivity of
BCP:C60 blends as a function of C60 fraction, (b) device architectures, (c)
current density vs. voltage characteristics of devices under one sun
AM1.5G illumination, and (d) external quantum efficiencies. 42
Figure 3.6: Performance of the optimized organic photovoltaic cell. (a) Schematic
of the devices with various buffer layers: 10 nm thick BCP (9),10 nm thick
BCP:C60 (10), 10 nm thick PTCBI (11), 10 nm BCP:C60/5 nm PTCBI (12),
10 nm BCP:C60/5 nm BCP (13). (b) Current density vs. voltage
characteristics of devices under one sun, AM1.5G illumination. (c) Ratio
of the external quantum efficiency (EQE) at -1 V to its value at 0 V for
devices with various buffer layers as in (b). 45
Figure 3.7: Comparison of the organic photovoltaic cells with and without
NPD:DBP mixed buffer. Current density vs. voltage characteristics of
devices under one sun AM1.5G illumination. 46
Figure 3.8: Schematic drawing of molecular formulae and energy levels of α-NPD,
DIP, DBP, and C60. Energy values of organic materials are taken from
literature.10,11 Absorption characteristics of donor, acceptor, and blocking
materials are shown, with DBP featuring the by far most dominant
absorption, while DIP and α-NPD absorb only weakly. 49
Figure 3.9: Effects of varying the DIP layer thicknesses. (a) Current density vs.
voltage characteristics of devices under one sun AM1.5G illumination, (b)
External quantum efficiencies, and (c) summary of device performance
parameters. 51
Figure 3.10: Effects of varying the NPD layer thicknesses. (a) Current density vs.
voltage characteristics of devices under one sun AM1.5G illumination, (b)
External quantum efficiencies, and (c) summary of device performance
parameters. 52
Figure 3.11: Performance of additional buffers. (a) Current density vs. voltage
characteristics of devices with a DTP buffer under one sun AM1.5G
illumination. (b) Current density vs. voltage characteristics of devices with
Tetracene buffer under one sun AM1.5G illumination. 54
Figure 4.1: (a) Molecular structure of C60, DPSQ, ZCl, and Cl6SubPc (b) Thin film
extinction coefficients of C60, ZCl, Cl6SubPc, C60:ZCl:Cl6SubPc, and DPSQ
compared with the AM 1.5G solar spectrum. Extinction calculated from
optical constants obtained by spectroscopic ellipsometry. 65

xi

Figure 4.2: (a) Singlet and triplet energies of ZCl, Cl6SubPc, and C60. Arrows
indicate possible energy transfer pathways. (b) Redox potential
(a)
(vs Fc/Fc
+
) of C60, ZCl, and Cl6SubPc.
(b)
Ref. 11,
(c)
Ref.20,
(d)
Ref.21
(c) Photoluminescence of ZCl and Cl6SubPc with and without C 60. Films
were excited at λ = 500 nm and 550 nm, respectively. 66
Figure 4.3: (a) Summary of device performance as a function of C60:ZCl:Cl6SubPc
layer thickness. (b) Summary of device performance and (c) external
quantum efficiency curves for optimized devices with C 60, C60:ZCl, and
C60:ZCl:Cl6SubPc. The layer thicknesses were 40 nm, 50 nm, and 70 nm
for C60, C60:ZCl, and C60:ZCl:Cl6SubPc, respectively. 69
Figure 4.4: (a) Device architecture for the reference and sensitized devices
containing DPSQ. (b) J–V curves of devices under one sun AM1.5G
illumination. (c) Plot of external quantum efficiency showing the increase
in spectral responsivity between λ = 500 nm and 650 nm due to the
inclusion of ZCl and Cl6SubPc. 70
Figure 4.5: Schematic diagram of the impact of chromophore separation distance
on the rate of energy transfer. The rate of FRET, which has an R
-6

dependence initially decays rapidly but stabilized over longer distances.
The rate of Dexter energy transfer, which has an exp(-2R) dependence,
rapidly decays with distane. 75
Figure 4.6: State energy diagram depicting the energy level requirements for singlet
and triplet energy transfer. For singlet energy transfer, path A, the singlet
energy of the sensitizer must be greater than C60. For triplet energy transfer,
path B, the sensitizer must rapidly intersystem cross to the triplet, which is
higher in energy than C60. 77
Figure 4.7: (a) Thin film extinction spectra of C60, Zcl, and ICl and emission spectra
of ZCl and ICl. (b) Molecular structures of ICl, ZCl, NPD, C60, and BCP. 78
Figure 4.8: Schematic of devices utilized to measure the exciton diffusion length.
The thickness of the acceptor layer (A) was varied between 10 nm and
70 nm 79
Figure 4.9: Representative EQE of the control (a) and devices sensitized with ZCl
(b). ΔEQE (c) between the sensitized and control devices. Sensitizer
absorption within the device calculated via transfer matrix approach (d)
with spacer thicknesses of 10 nm, 15 nm, 20 nm, and 30 nm. 80
Figure 4.10: ΔIQE for devices using ZCl (▲) and ICl (●) as sensitizers through a
neat layer of C60. Solid lines are fits using equation 3 yielding diffusion
lengths of 45 ± 5 nm and 35 ± 5 nm for singlets and triplets, respectively. 82
xii

Figure 4.11: ΔIQE for devices using ZCl (▲) and ICl (●) as sensitizers through a
layer of BCP:C 60. Solid lines are fits using equation 3 yielding diffusion
lengths of 18 ± 2 nm and 25 ± 3 nm for singlets and triplets, respectively. 83
Figure 5.1: Plots of potential energy surfaces outlining the basics of Marcus Theory.
(a) depicts electron transfer with reorganization energy 𝜆 and positive ∆𝐺 .
(b) depicts electron transfer with ∆𝐺 = 0. (c) depicts barierless electron
transfer. (d) depicts the Marcus inverted regime where increasingly negative
∆𝐺 decreases the rate of electron transfer 92
Figure 5.2: Molecular structures and solution phase absorption of zDIP1-4.
Photoluminescence quantum yield as a function of solvent dielectric for
zDIP1-4. 94
Figure 5.3: Photoluminescence quantum yield and lifetime for zDIP2 in solvent
blends of cyclohexane and THF with different dielectric constants.
Photoluminescence lifetime traces for solutions with selected dielectric
constants. 95
Figure 5.4: Simplified Jablonski diagrams outlining the kinetic schemes for simple
emission and emission involving a CT state. The simple scheme involves
radiative and non-radiative decay from the singlet. The second scheme
involves a charge transfer state in equilibrium with the singlet state. The
rate of forward and reverse electron transfer determines the equilibrium
between the states. 97
Figure 5.5: Comparison of experimental and calculated photoluminescence
quantum yield and lifetime for zDIP2 in solvent blends of cyclohexane and
THF with different dielectric constants. Rate constants extracted from fits
to the photoluminescence quantum yield and lifetime data (filled symbols).
Rate constants extracted from transient absorption (open symbols) 98
Figure 5.6: Gibb’s free energy as a function of solvent dielectric calculated from
electron transfer rates. As the solvent dielectric increases, electron transfer
becomes exergonic 100

xiii

Figure 5.7: (a) Schematic representation of the charge generation process in an
OPV with a conventional donor and a Symmetry Breaking Charge transfer
Acceptor. First, an excited state is created through the absorption of a
photon. Second, symmetry breaking charge transfer (SBCT) occurs on the
molecule generating and intramolecular charge transfer (ICT) state.
Finally, charge transfer (CT) to the donor results in an oxidized donor and
reduced acceptor ligand separated by a neutral acceptor ligand. If the
excitation takes place in the bulk of the SBCT material, away from the D/A
interface, the formed exciton must diffuse to the interface to charge
separate. (b) Molecular structures and extinction spectra for DBP, ZCl, and
C60. 104
Figure 5.8: Photoluminescence quantum yield and lifetime for ZCl in solvents with
different dielectric constants. 106
Figure 5.9: Femtosecond transient absorption of ZCl in cyclohexane (CH) (a),
toluene (b), acetonitrile (MeCN) (c) and PMMA (d) at initial delays.
Excitation pump fluence of 15 µJ/cm
2
were used for (a), (b), and (c) and
45 µJ/cm
2
was used for (c). The red arrows highlight the change in the
transient spectrum. 107
Figure 5.10: Left: Valence and conduction band edges of thick molecular films
deposited on a DBP film. The zero of energy is the measured vacuum level
of the initial DBP film. Right: Position of molecular states and resulting
density of states (DOS) calculated for C60 and ZCl. 109
Figure 5.11: Device architecture (a), illuminated I-V (b), dark I-V (c), and EQE
curves (d) for the devices described in the text. Thickness = 20 nm for
DBP, 40 nm for C60, and 20 nm for ZCl. 110
Figure 5.12: Comparison of simulated and measured EQE for DBP/ZCl device
illustrating the contribution from both DBP and ZCl (a). Calculated
absorption and IQE for the DBP/ZCl device. 112
Figure 5.13: (a) FTPS spectra for DBP/ZCl bilayer, DBP:ZCl blend, and DBP/C 60
bilayer devices. The lowest energy transition in the spectra for DBP:ZCl
blend and DBP/C60 bilayer were fit with Eq. 2. (b) EL spectra for neat
DBP, DBP/ZCl bilayer and DBP:ZCl blend. The lowest energy transition
in the spectra for DBP:ZCl blend and DBP/ZCl bilayer were fit with Eq. 3. 114
Figure 5.14: VOC vs. ECT for a variety of small molecule/fullerene and
polymer:fullerene OPVs. Lines at VOC = ECT - 0.6 ± 0.1 are guides to the
eye. Values taken from Vandewal et al.,
44,53-55
Piersimoni et al.,
56
Ko
et al.,
57
Hoke et al.,
15
Wang et al.,
58
Graham et al.,
41
Tietze et al.,
59
and this
work. 116
xiv

Figure 5.15: Molecular structures and extinction coefficients for 6T, ZnPc, DIP,
and NPD. 119
Figure 5.16: Illuminated current-voltage (I-V) plots of the OPV with 6T, ZnPc,
DIP, and NPD as donors and C60 and ZCl as acceptors. The plots are
grouped by donor. 121
Figure 5.17: External quantum efficiency (EQE) plots of the OPV with 6T, ZnPc,
DIP, and NPD as donors and C60 and ZCl as acceptors. The plots are
grouped by donor. 122
Figure 5.18: Molecular structure, absorption, and emission spectra for
Cl6BODIPY and ZCl 124
Figure 5.19: Cyclic voltammetry traces for Cl6BODIPY and ZCl. Cl6BODIPY
exhibits a reversible reduction at -0.71 V and an irreversible oxidation at
1.40 V. ZCl exhibits two reversible reductions at -1.30 V and -1.54 V and
an irreversible oxidation at 1.22 V. 125
Figure 5.20: Illuminated I-V curves, dark I-V curves, and EQE for devices with
DBP as a donor and either Cl6BODIPY, ZCl or C60 as the acceptor. 126
Figure 6.1: Molecular structure and extinction spectra of 6T, C60, and DIP. 140
Figure 6.2: Current density vs. voltage characteristics under one sun AM1.5G
illumination (a) and external quantum efficiency spectra (b) of 6T/C60,
DBP/C60, and 6T/DBP devices. 141
Figure 6.3: (a) Current density vs. voltage characteristics under one sun AM1.5G
illumination for 6T/DBP devices with various thicknesses of DBP. (b)
Current density vs. voltage characteristics under one sun AM1.5G
illumination for 6T/DBP devices with varying ETL and HTL. 143
Figure 6.4: EQE and EL measurements for DBP/C60 bilayer, DBP/C60 PM-HJ,
6T/DBP bilayer, and 6T/DBP PM-HJ devices. The DBP/C60 devices show
clear CT emission while the 6T/DBP devices show no obvious signal of CT
emission 145
Figure 6.5: Temperature dependent open circuit voltage for DBP/C60 and 6T/DBP
devices. Lines represent fits to the linear portion of the curve. Extrapolation
to 0 K yields ECT. 146
Figure 6.6: AFM images and characteristic line profiles of 6T(60 nm) (a,d),
6T(60 nm)/DBP(10 nm) (b,e), and 6T(60 nm)/DBP(20 nm) (c,f). 148

xv

Figure 6.7: (a) Simulated spatial distribution of the absorbed optical power for
6T/DBP devices with various DBP thicknesses at λ = 450 nm and 610 nm.
(b) Simulated spatial distribution of the absorbed optical power for 6T/DBP
devices with various BCP thicknesses at λ = 450 nm and 610 nm. 150
xvi

Abstract

The design of materials and devices for organic photovoltaic applications is dominated by
considerations related to the management of energy. From the synthesis of materials which
intensely absorb light to the fabrication of devices with optimal phase segregation to
promote charge separation, ensuring that energy is harvested efficiently is a tantamount
concern. The energy generation process can be broadly separated into two categories: one
related to the generation of photocurrent, and the other photovoltage. This dissertation
describes strategies to improve the efficiency of both of these processes through the use of
novel materials and device architectures.
In order to improve photocurrent, we have developed an exciton blocking layer based on a
blend of materials. These blends are optically transparent, preventing parasitic absorption,
and highly conductive, conducive to carrier extraction. The buffer layers increase
photocurrent production by reducing exciton-polaron annihilation within devices. In
addition, we probed the importance of crystallinity in buffer layers finding that crystalline
buffers outperform amorphous buffers as the layer thickness increases. Beyond buffer
layers, we extended the use of an energy sensitization scheme in order to harvest a broader
portion of the solar spectrum. This allows devices to harvest more light and generate larger
photocurrents. The sensitization scheme was then applied to the study of exciton diffusion
in devices.
In order to achieve higher open circuit voltage, we investigated two strategies. First, we
studied a phenomena known as symmetry breaking charge transfer and employed materials
which undergo this process in devices. Studies on the rate of electron transfer in these
xvii

materials in solution reveal ultrafast rates with negligible driving force. In devices, the
materials produce substantially increased open circuit voltages and reduce the voltage
losses due to recombination. Next, we investigated devices based on materials with large
interfacial energy gap. These devices produce large open circuit voltages due to the
significant energetic offset at the donor/acceptor interface. The overall theme in this work
is that through precise understanding of the photocurrent generation process, it is possible
to gain insight into methods to increase efficiency.

1

Chapter 1. Introduction
1.1 Motivation
Driven by an ever growing population and a sustained proliferation of industrialized living
conditions, the global demand for electricity continues to increase. Between 1971 and
2012, electricity consumption has grown by more than a factor of five to more than
20,000 TWh.
1
Figure 1.1 outlines the annual world electricity generation by fuel source,
revealing that the supermajority of electricity generation has been, and continues to be,
accomplished through the combustion of carbonaceous fuels (chiefly oil, coal, and natural
gas).
1
Despite the fact that these fuels are economical, abundant, and energy dense, their
continued use results in severe negative externalities which, if left unchecked, will have
grave consequences on the Earth’s biosphere.
Foremost in these concerns is the release of CO2 generated by the combustion process.

Figure 1.1: World electricity generation by fuel (TWh) (from IEA)
2

Figure 1.2 outlines the annual world CO2 emissions by fuel from which it is apparent that
the entirety of the emission originates from combustion.
1
Historically, the atmospheric
concentration of CO2 has been below 300 ppm for the past 650,000 years.
2
Following the
start of the industrial revolution, the increase in anthropomorphic CO2 generation has
correlated with the increase in atmospheric CO2 concentration. Figure 1.3 gives a
historical record of the atmospheric CO2 concentration.
2
Around 1950, the atmospheric
concentration of CO2 first exceeded 300 ppm. As of 2014, the atmospheric CO2
concentration is approximately 400 ppm. This rapid change in CO2 concentration, a
substantial deviation in a system which had been in homeostasis for more than
500,000 years, is cause for concern.
CO2 is a greenhouse gas which recycles energy radiant on the Earth form the Sun, resulting
in an increase in the temperature of the surface above what it would be in the absence of
the gas. Thus, greatly simplifying the issue, an increase in atmospheric CO2

Figure 1.2: World CO2 emissions by fuel (Mt) (from IEA)
3

concentration results in an increase in the temperature of the Earth’s surface. This is
predicted to have disastrous consequences including sea level rise associated with polar ice
cap melt, more frequent and extreme weather events, and significant decreases in crop
yields.
3

Beyond temperature effects related to the greenhouse effect, increased atmospheric CO2
concentrations also have significant impact on the Earth’s oceans. Atmospheric CO2 is in
equilibrium with the sea water in which it dissolves to form carbonic acid.
4
In this way, an
increase in the concentration of CO2 in the atmosphere results in an increase carbonic acid
concentration in sea water and an overall lowering of the ocean’s pH. This is predicted to
have a deleterious impact on a wide variety of marine organisms, such as plankton, corals,
and snails, which use aragonite (a carbonate mineral) to make their shells or skeletons.
4

The increase in ocean acidity greatly reduces the structural integrity of this material,
threatening the survival of these organisms which form the foundation of the ocean’s food
chain.

Figure 1.3: Atmospheric CO2 concentration (ppm) (from NASA global climate change)
4

For the reasons discussed above, it is therefore necessary to adopt the use of alternative
sources of energy which do not release such vast quantities of CO2.
Logically, the first course of action in looking for an alternative is to review availability
and abundance of various potential energy sources. Figure 1.4 outlines the total energy
available via renewable sources annually as well as the total reserves of non-renewable
energy sources and the annual world energy consumption. With an average terrestrial
intensity of approximately 1000 W/m
2
, the sun irradiates the earth with more than
23,000 TW annually.
5
This outstrips current world energy consumption by more than three
orders of magnitude, and when compared to all other energy sources solar is without equal
in its availability. This superabundance motivates the great deal of work in solar energy

Figure 1.4:Global energy potential annually and total reserves for a wide variety of
sources. From Ref. 5
5

conversion which is indeed the focus of this thesis. In order to sustain the future energy
needs well into the next century, solar is truly the only answer.
1.2 Fundamentals of Photovoltaics
1.2.1 A Brief History
The photovoltaic effect is the creation of voltage or electric current in a material upon
exposure to light. The effect was first reported by Edmond Becquerel in 1839 where
current was produced when silver chloride was placed in an acidic solution containing
platinum electrodes and illuminated.
6
This fundamental process of converting radiation
into electrical energy is the underlying principle upon which all photovoltaic devices
function. Following this discovery, the first solid state photovoltaic cell is credited to
Charles Fritts in 1883 who fabricated the device by coating selenium with gold junctions.
7

However this device was quite crude and it was not until significant advances were made
in the understanding of the physics and materials science of semiconductors that the
modern solar cell was developed. Later, in 1906 A. Pochettino reported the first
observations of the photovoltaic effect in organic materials while studying the
photoconductivity of anthracene crystals.
8
The “light-sensitive electronic device” patented
in 1946 by Russel Ohl while at Bell Telephone Laboratories is recognized as the first
modern solar cell as it includes the P-N junction.
9
Following this design, the first practical
silicon solar cell was demonstrated in April of 1954.
7
Since this time, the design,
fabrication, and study of solar cells has been an active area of research and commercial
development.
6

Along with the continued development of inorganic photovoltaics, organic photovoltaics
have also been actively investigated. Early work involved the study of common dyes
sandwiched between electrodes, however these devices produced negligible efficiencies.
In 1964 G. M. Delacote reported a rectifying photodiode formed by sandwiching copper
phthalocyanine (CuPc) between asymmetrical contacts.
10
The modern donor/acceptor
organic photovoltaic device was first developed by Ching Tang in 1986 providing an order
of magnitude improvement in device efficiency.
11
In 1995 the groups of Heeger and Friend
reported the first bulk-heterojunction, where the donor and acceptor are intermixed in order
to increase the surface area of the junction.
12, 13

1.2.2 Introduction to Inorganic Photovoltaics
Owing to their historical priority and absolute dominance of the commercial market, we
will undertake a brief discussion on the operation of inorganic semiconductor
photovoltaics, endeavoring to underline their mechanism of action and highlight strengths
and weaknesses which will be compared to organic photovoltaics. In a semiconductor
device, photons with energy greater than the band gap, the energy difference between the
valance and conduction bands, are absorbed to promote an electron from the valance band
to the conduction band. The probability with which this process occurs is known as the
absorptivity of a material and scales exponentially with thickness by Beer’s law
𝑎 = 1 − 𝑒 −𝛼𝑥
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
. After photon absorption, the
electron rapidly relaxes to the conduction band edge releasing any excess energy in the
form of heat. Owing to the high dielectric constant of inorganic solids, significant
dielectric screening occurs. This reduces the Coulomb interaction between the electron
7

and hole, resulting in the formation of a Wannier-Mott exciton with a binding energy on
the order of 0.01 eV. Following the creation of this weakly bound electron-hole pair, the
charges rapidly separate and are transported out of the device. This occurs through a
combination of processes, drift, due to the action of the electric field within the device on
the charges, and diffusion, the random motion of the carriers along the concentration
gradient. The description of carrier transport is governed by
𝐽 𝑛 ,𝑝 = 𝑞 𝜇 𝑛 ,𝑝 ( 𝑛 , 𝑝 ) 𝐸 + 𝑞 𝐷 𝑛 ,𝑝 𝑑 ( 𝑛 ,𝑝 )
𝑑𝑥
(1)
where J is current, q is the fundamental charge, 𝜇 𝑛 ,𝑝 is the electron or hole mobility,
respectively, E is the electric field, Dn,p is the electron or hole diffusion constant,
respectively, and n and p are the electron and hole concentration, respectively. In summary,
in the ideal inorganic solar cell, photons are absorbed, rapidly generating free electrons and
holes, which are then extracted from the device.
From the description of inorganic semiconductor photovoltaics it is clear that they are
governed by two principal processes which limit their efficiency: absorption and transport.
Correspondingly, in order for a solar cell to be efficient, it must possess a thickness such
that it can absorb all of the incident light. Diametrically opposed to absorption is carrier
transport. The carriers which are produced upon photoexcitation must possess sufficient
lifetime that they can be extracted from the device. This means that recombination
processes must be minimized. Recombination can occur through radiative recombination,
Shockley-Read-Hall recombination, and Auger recombination. In radiative
recombination, an electron and hole meet and recombine to produce a photon equal to the
band gap of the semiconductor. This generally occurs in direct bandgap semiconductors
8

where overlap between the electron and hole wave functions are largest.
Shockley-Read-Hall recombination occurs in devices with defect states where a carrier
becomes trapped in a defect until a reciprocal carrier encounters it and recombines.
14, 15

Shockley-Read-Hall recombination can be mitigated by controlling the purity and doping
density of the material. In Auger recombination an electron and hole recombine but instead
of emitting a photon they radiationlessly transfer the energy to another carrier which is
promoted to a higher energy level.
16
Auger generally occurs at high carrier concentrations.
To combat recombination issues, high efficiency photovoltaic devices utilize a variety of
techniques to limit recombination. Foremost is the use of the P-N junction which maintains
a substantial imbalance in the carrier concentrations in each of the layers.
1.2.3 Introduction to Organic Photovoltaics
Compared to inorganic devices, there are several key differences in the operation of organic
photovoltaics. Figure 1.5 depicts photocurrent generation in OPVs outlining the steps
necessary for current production. The initial process of absorption is similar in an organic
device, however, the extinction coefficient of organic materials are significantly higher (on
the order of 100,000 cm
-1
) and the transition is from state to state instead of band to band.
This results in a narrow, intense absorption window for organics which has interesting
consequences that will be discussed below. Upon photoexcitation, the low dielectric of
organics (εr ~ 3) results in the formation of a strongly bound Frenkel exciton. The exciton
must then diffuse to the donor/acceptor interface where an interfacial offset in molecular
orbital energies drives the transfer of an electron resulting in the formation of a charge
9

transfer state. Next, this charge transfer state separates resulting in the formation of free
carriers. The carriers are then swept out of the device.
Although the efficiency of inorganic and organic devices are governed by similar
processes, the recombination mechanisms which limit their efficiency differ. First, the
excitonic nature of organic photovoltaics ad an additional recombination pathway through
the radiative or non-radiative decay of excitons. This can be mitigated by the fabrication
of a bulk-heterojucntion which reduces the distance an exciton must travel before it can
separate. Additionally, as only electrons or holes exist within a given layer of the device,
polaron-polaron recombination is limited to the donor/acceptor interface. Instead, exciton-
polaron recombination can occur throughout the device.
Despite the limitations imposed by the transport requirements in OPV, there are several
areas in which they have advantages over conventional photovoltaic technologies. Due to
their high extinction coefficient, OPVs are generally quite thin and thus are compatible

Figure 1.5: 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.
10

with extremely light weight applications. Figure 1.6 compares the active layer thickness,
mass per area, and specific power across a variety of photovoltaic technologies.
17
Because
OPVs possess especially thin active layers, their specific power compares favorably to
other technologies even with a lower overall efficiency. Additionally, OPVs are
compatible with flexible substrates allowing them to be deployed in locations and use cases
where heavy, rigid substrates are untenable.
Another unique property of OPVs is that the materials undergo state to state transitions
upon photoexcitation. This means that the absorption has a prescribed bandwidth and only
absorbs in a specific portion of the spectrum. This allows for the design of visibly

Figure 1.6: Absorber thickness, cell mass per area and specific power for various
photovoltaic technoligies. Specific power is shown for active layers alone and for cells
with a 25μm polyethylene terephthalate (PET) or 3 mm glass substrate or encapsulation
layer. From Ref. 17
11

transparent solar cells, which absorb no portion of the visible spectrum, while still
producing power.
18
Despite the limitations this places on the device, it is still theoretically
possible to achieve efficiencies in excess of 20 % without absorbing any visible photons.
19

In these unique niches it is possible that OPVs will see commercial success.
1.3 Current Status, Challenges and Future Outlook
The historical progress in photovoltaic device development is nicely summarized in
Figure 1.7 where the highest reported device efficiencies are reported as a function of time.
Organic photovoltaics are a third generation photovoltaic technology and were first
included on the chart in 2001. Since that time there has been rapid progress in terms of
reported device efficiency due to significant improvements in materials development,
processing, and device design. This section highlights recent advances in OPVs and

Figure 1.7: Historical plot of the highest confirmed conversion efficiencies for research
cells, from 1976 to the present, for a range of photovoltaic technologies. (Courtesy of
NREL)
12

discusses prospects for the future related to the aspects of performance parameters seen
through the lens of materials and device development.
1.3.1 Photocurrent
Photocurrent generation in OPVs is generally limited by two factors: absorption and
exciton diffusion. In order to address these issues, many approaches have been taken in
materials and device design to maximize the photocurrent. Many of the common materials
utilized in early OPVs absorbed in the ultra-violet and visible portion of the solar spectrum.
Figure 1.8 shows the solar spectrum as a function of wavelength revealing that a significant
amount of photon flux exists outside of the visible portion of the spectrum.
Therefore, recent work has focused on shifting absorption into the near-infra red (NIR) in
order to collect additional solar flux. Primarily, a red shift in absorption has been achieved
through the use of push-pull chromophore systems where one subunit possess electron
donating moieties and another possess electron withdrawing moieties.
20
Figure 1.9

Figure 1.8: Solar photon flux (left axis) with the photopic response function (right
axis) plotted versus wavelength. From Ref. 19
13

outlines the design of these chromophores and an extremely simplified picture of how the
resultant bandgap is narrowed in addition to some molecular structures. The most widely
used of these materials are poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b’]-
dithiophene)-alt-4,7-(2,1,3-benzothiadiazole) (PCPDTBT)
21
and (PTB7).
22
Recently, a
small molecule 2-[(7-{4-[N,N-bis(4-methylphenyl)amino]thiophenyl}-2,1,3-benzothia-
diazol-4-yl)methylene]propanedinitrile (DTDCTB) following the same design rule has
shown great promise.
23, 24

In addition to the development of new materials to extend light collection into the near-
infra red, alternative device architectures have been developed to ensure broad spectral
coverage. Ternary blend solar cells comprised of three photoactive compounds have drawn

Figure 1.9: Schematic representation of D-A chromophores and basic energy level
diagram demonstrating the narrowing of the bandgap due to hybridization from the D
and A subunits. From Ref. 20. Molecular structures of PCPDTBT, PTB7, and DTCTB.
14

a great deal of interest.
25
In these devices, the addition of a third component allows for a
wider portion of the solar spectrum to be harvested. In addition to ternary blends, tandem
devices have been fabricated with complimentary absorbing sub cells.
26-29
To ensure broad
spectral coverage, one cell absorbs primarily in the blue and the other absorbs primarily in
the red/NIR. Beyond hetero-tandem devices, homo-tandem devices, comprised of two
identical subcells, allow increased absorption through the use of a thicker overall active
layer which would be resistive if placed in a single cell.
30-32
The optical interference pattern
within the device is also important to consider in the design process. Modeling utilizing
the transfer matrix formalism has proven to be a useful tool to ensure the active layer is
positioned in a location of maximized optical field intensity.
33, 34
Figure 1.10 illustrates
the ways in which optical modeling informs on device design in tandem devices. Through
these means, a broad spectrum of light can be collected while also ensuring intense
absorption.

Figure 1.10: Sample optical interference patterns for tandem devices From Refs. 29 and
32. The plots illustrate how positioning the subcells in areas of intense optical field
impact device efficiency.
15

Further increases in photocurrent have been achieved through precise control of the
microstructure and morphology of the materials within devices. Annealing steps involving
exposing the device to elevated temperatures or solvent vapor have been shown to alter the
morphology of the active layer, leading to an increase in photocurrent.
35-39
Similarly, in
solution processed devices, the addition of a small fraction of a secondary solvent serves
to increase efficiency.
40, 41
In vapor deposited devices, elevated substrate temperatures
during deposition have been shown to have a similar effect.
42, 43
Detailed study of the
morphology in these devices has shown that phase segregation on the order of the exciton
diffusion gives the best performance. It has also been proposed that a three phase system,
one comprised of neat donor, one of neat acceptor, and one of intermixed donor and
acceptor is crucial for efficient devices.
44
This structure provides high carrier mobilities in
the pure domains and constructs an energetic driving force for charge separation within the
device.
45

At present, the internal quantum efficiencies for the best performing devices approach
100 %.
46, 47
This means that continued increases in photocurrent are only going to be
realized by extending absorption to longer wavelengths and ensuring total absorption of all
incident photons. Therefore, an important path to increased photocurrent is the
development of new near-infra red absorbing materials. Along with these new materials
there will need to be device architectures compatible with them in order to ensure that the
quantum efficiency remains high.
1.3.2 Open Circuit Voltage
Compared to the energy of the photons they absorb, the open circuit voltage of OPVs are
generally low. Figure 1.10 shows the open circuit voltage for a variety of photovoltaic
16

technologies as a function of bandgap.
48
OPVs exhibit a substantially larger offset between
their bandgap and VOC relative to other photovoltaics. To attempt to combat this fact, a
number of strategies have been developed to increase the VOC of devices. These methods
take advantage of material properties in order to decrease the losses which are responsible
for the low VOCs observed.
A number of theoretical formulations for VOC have been developed and each gives a
direction in which to steer materials selection to increase VOC. Broadly speaking, these
equations arise from modified versions of the generalized Shockley equation:
49

Figure 1.11: Experimental VOC for a wide range of single-junction solar cell band gaps,
from 0.67 to 2.1 eV, showing that the band gap–voltage offset, WOC = (Eg/q) - VOC, is
roughly constant over this range experimentally. The band gap–voltage offset Woc is
calculated for a semiconductor layer with radiative recombination only, and using the
detailed balance model, also showing the approximate constancy of Woc as predicted
from theory. The measured band gap–voltage offset for some solar cell materials
approaches the calculated value for radiative recombination only. From Ref. 48. Solid
red and green lines are added to show typical values for OPVs.
17

𝑉 𝑂𝐶
=
1
𝑞 (∆𝐸 + 𝑛𝑘𝑇 ln (
𝐽 𝑆𝐶
𝐽 00
)) (2)
where ∆𝐸 is the energetic barrier at the donor/acceptor interface, 𝑛 is the ideality factor, 𝑘
is Boltzmann’s constant T is temperature, 𝐽 00
and is the dark current activation energy.
From Equation # there are two ways in which to increase VOC: increase ∆𝐸 or increase
𝐽 𝑆𝐶
𝐽 00
.
As the increase with ∆𝐸 is linear while
𝐽 𝑆𝐶
𝐽 00
is logarithmic, many material combinations have
been selected to increase ∆𝐸 . An early example of this strategy was utilized by
Mutolo et al. where they replaced CuPc with SubPc resulting in an increase in VOC of
400 meV.
50
Since then, a variety of subpthalocyanines and related compounds have been
utilized in high VOC devices as both donors and acceptors.
51-56
More generally, increasing
∆𝐸 has been the most broadly applied and most successful strategy for increasing VOC.
Another interpretation of the Shockley equation, proposed by Giebink et al.,
57
is given
below:
𝑉 𝑂𝐶
=
1
𝑞 (( 𝐸 𝐻𝐿
− 𝐸 𝐵 )+ 𝑘𝑇 ln (
𝑘 𝑃𝑃𝑟
𝑁 𝐻𝑂𝑀𝑂 𝑁 𝐿𝑈𝑀𝑂 𝜁 𝑚𝑎𝑥 𝐽 𝑥 /𝑟 )) (3)
where 𝐸 𝐻𝐿
is the HOMO-LUMO offset, 𝐸 𝐵 is the exciton binding energy, 𝑘 𝑃𝑃𝑟
is the
polaron-pair recombination rate, 𝑁 𝐻𝑂𝑀𝑂 ,𝐿𝑈𝑀𝑂 are the density of states in the HOMO and
LUMO, 𝜁 𝑚𝑎𝑥 is the maximum density of polaron pairs supported at the interface, 𝐽 𝑥 is the
photocurrent, and r is the spatial extent of the polaron pair. This formulation also gives
several avenues to increase VOC through tuning material properties. Here, VOC can be
enhanced by decreasing the exciton binding energy and altering the spatial extent of the
charge transfer (polaron pair) state. The effect of exciton binding energy can be understood
18

through Onsager-Braun theory where the rate of free carrier recombination (𝑘 𝑟𝑒𝑐
) is
given by
𝑘 𝑟𝑒𝑐
=
𝑞𝜇
𝜀 0
𝜀 𝑟 (4)
where 𝜇 is carrier mobility, and 𝜀 0
is the permittivity of free space, and 𝜀 𝑟 is the relative
dielectric. Similarly the binding energy is given by
𝐸 𝑏 =
𝑒 2
4𝜋 𝜀 0
𝜀 𝑟 𝑟 0
(5)
where e is the elementary charge and 𝑟 0
is the charge separation distance. Following these
design rules, several groups have successfully increased VOC by increasing the dielectric
and spatial separation distance within devices. Tuning the dielectric of the active layer
materials has been accomplished by altering the substituents to include polar groups.
58-60

Alternatively, steric bulk has been introduced through the use of a variety of substituents
and has also been found to increase VOC.
61-63
Thus, controlling the kinetics of charge
separation by altering the driving force has been an effective strategy for realizing high
VOCs in devices.
A third expression for VOC, developed by Vandewal et al.,
64
given below:
𝑉 𝑂𝐶
=
1
𝑞 (𝐸 𝐶𝑇
+ 𝑘𝑇 ln (
𝐽 𝑆𝐶
ℎ
3
𝑐 2
𝑓𝑞 2𝜋 ( 𝐸 𝐶𝑇
−𝜆 )
) + 𝑘𝑇 ln( 𝐸𝑄𝐸 𝐸𝐿
) ) (6)
where 𝐸 𝐶𝑇
is the energy of the charge transfer (CT) state, h is Plank’s constant, c is the
speed of light, f is proportional the CT state absorption, λ is the CT state reorganization
energy and EQEEL is the electroluminescence quantum efficiency of the CT state provides
other insights. Based on this interpretation, decreases in the CT state absorption,
19

donor/acceptor electronic coupling, and CT state emission quantum yield can all increase
VOC. Experimentally, decreasing CT state absorption by altering the concentration of CT
states in a device has been shown to increase VOC.
65
Also, changes in electronic coupling
achieved by altering donor/acceptor interaction have been demonstrated to increase V OC
experimentally.
66, 67

Looking towards the future, a few areas which necessitate improvement stand out. The
offset between the bandgap of the OPV and its VOC must be decreased. Extensions of the
strategies discussed above have already begun to address mitigating the losses observed.
Particularly promising are the design of non-fullerene acceptors which have energy levels
better matched to the donors which they are paired with leading to enhanced VOC. In
addition, the development of organic materials with higher dielectric constants or
incorporating high dielectric materials into the active layer could be a pathway to lower the
exciton binding energy. This in turn would reduce the driving force required for charge
separation which would increase VOC. .
1.3.3 High Efficiency
Ultimately, the devices with the highest reported efficiencies have utilized materials which
perform optimally in terms of both their JSC and VOC. This has been achieved through
judicious selection of active layer materials, precisely controlling their morphology, and
ensuring intelligent device design. Currently, the highest efficiency devices exceed 10 %,
have been fabricated from both polymers and small molecules, and arise from single
junction and tandem designs.
26, 29-32
Therefore it is likely that continued progress will stem
from a variety of different avenues. As outlined in the previous sections, there is still room
for improvement in OPV performance through the development of novel materials with
20

specific properties desirable for high efficiency. Namely, increased absorption in the NIR
to enhance JSC and optimized energetic alignment at the D/A interface to maximize VOC.
Overall, the continuous progress in the realization of devices with ever increasing
efficiency has been and continues to be driven by materials development.
1.4 Summary of Topics
This thesis covers several topics related to the management of energy in organic
photovoltaic devices. There are two broad themes within this work. Chapters 3 and 4 deal
with understanding and control of the processes which generate photocurrent while
Chapters 4 and 5 deal predominately with open circuit voltage. Chapter 2 elaborates the
standard instrumentation and experimental techniques utilized throughout this work.
Following, Chapter 3 details work on buffer layers which selectively extract charge carriers
while reflecting excitons back towards the donor/acceptor interface. Chapter 4 describes
the extension of an energy sensitization scheme which broadens the absorption within
devices and the utilization of sensitization to study excited state diffusion. In Chapter 5,
symmetry breaking charge transfer and its application in organic photovoltaics is
discussed. Finally, Chapter 6 describes devices with large open circuit voltage achieved
through intelligent materials selection.

21

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23

29. Chen, C.-C.; Chang, W.-H.; Yoshimura, K.; Ohya, K.; You, J.; Gao, J.; Hong, Z.;
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Efficiency Exceeding 11%. Advanced Materials 2014, 26, (32), 5670-5677.
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Consisting of Two Identical Sub-Cells. Advanced Materials 2013, 25, (29), 3973-3978.
31. Liu, Y.; Chen, C.-C.; Hong, Z.; Gao, J.; Yang, Y.; Zhou, H.; Dou, L.; Li, G.; Yang,
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efficiency. Sci. Rep. 2013, 3.
32. Che, X.; Xiao, X.; Zimmerman, J. D.; Fan, D.; Forrest, S. R., High-Efficiency,
Vacuum-Deposited, Small-Molecule Organic Tandem and Triple-Junction Photovoltaic
Cells. Advanced Energy Materials 2014, 4, (18), n/a-n/a.
33. Peumans, P.; Yakimov, A.; Forrest, S. R., Small molecular weight organic thin-
film photodetectors and solar cells. Journal of Applied Physics 2003, 93, (7), 3693-3723.
34. Pettersson, L. A. A.; Roman, L. S.; InganÃ¤s, O., Modeling photocurrent action
spectra of photovoltaic devices based on organic thin films. Journal of Applied Physics
1999, 86, (1), 487-496.
35. Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y., High-
efficiency solution processable polymer photovoltaic cells by self-organization of polymer
blends. Nat Mater 2005, 4, (11), 864-868.
36. Wei, G.; Wang, S.; Sun, K.; Thompson, M. E.; Forrest, S. R., Solvent-Annealed
Crystalline Squaraine: PC70BM (1:6) Solar Cells. Advanced Energy Materials 2011, 1,
(2), 184-187.
37. Zimmerman, J. D.; Xiao, X.; Renshaw, C. K.; Wang, S.; Diev, V. V.; Thompson,
M. E.; Forrest, S. R., Independent Control of Bulk and Interfacial Morphologies of Small
Molecular Weight Organic Heterojunction Solar Cells. Nano Letters 2012, 12, (8), 4366-
4371.
38. Wei, G.; Lunt, R. R.; Sun, K.; Wang, S.; Thompson, M. E.; Forrest, S. R., Efficient,
Ordered Bulk Heterojunction Nanocrystalline Solar Cells by Annealing of Ultrathin
Squaraine Thin Films. Nano Letters 2010, 10, (9), 3555-3559.
39. Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J., Thermally Stable, Efficient
Polymer Solar Cells with Nanoscale Control of the Interpenetrating Network Morphology.
Advanced Functional Materials 2005, 15, (10), 1617-1622.
40. Peet, J.; Kim, J. Y.; Coates, N. E.; Ma, W. L.; Moses, D.; Heeger, A. J.; Bazan, G.
C., Efficiency enhancement in low-bandgap polymer solar cells by processing with alkane
dithiols. Nat Mater 2007, 6, (7), 497-500.
24

41. Lee, J. K.; Ma, W. L.; Brabec, C. J.; Yuen, J.; Moon, J. S.; Kim, J. Y.; Lee, K.;
Bazan, G. C.; Heeger, A. J., Processing Additives for Improved Efficiency from Bulk
Heterojunction Solar Cells. Journal of the American Chemical Society 2008, 130, (11),
3619-3623.
42. Yang, F.; Shtein, M.; Forrest, S. R., Morphology control and material mixing by
high-temperature organic vapor-phase deposition and its application to thin-film solar
cells. Journal of Applied Physics 2005, 98, (1), 014906.
43. Yang, F.; Shtein, M.; Forrest, S. R., Controlled growth of a molecular bulk
heterojunction photovoltaic cell. Nat Mater 2005, 4, (1), 37-41.
44. Bartelt, J. A.; Beiley, Z. M.; Hoke, E. T.; Mateker, W. R.; Douglas, J. D.; Collins,
B. A.; Tumbleston, J. R.; Graham, K. R.; Amassian, A.; Ade, H.; Fréchet, J. M. J.; Toney,
M. F.; McGehee, M. D., The Importance of Fullerene Percolation in the Mixed Regions of
Polymer–Fullerene Bulk Heterojunction Solar Cells. Advanced Energy Materials 2012, 3,
(3), 364-374.
45. Burke, T. M.; McGehee, M. D., How High Local Charge Carrier Mobility and an
Energy Cascade in a Three-Phase Bulk Heterojunction Enable >90% Quantum Efficiency.
Advanced Materials 2014, 26, (12), 1923-1928.
46. Park, S. H.; Roy, A.; Beaupre, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.;
Leclerc, M.; Lee, K.; Heeger, A. J., Bulk heterojunction solar cells with internal quantum
efficiency approaching 100%. Nat Photon 2009, 3, (5), 297-302.
47. Xiao, X.; Bergemann, K. J.; Zimmerman, J. D.; Lee, K.; Forrest, S. R., Small-
Molecule Planar-Mixed Heterojunction Photovoltaic Cells with Fullerene-Based Electron
Filtering Buffers. Advanced Energy Materials 2013, 4, (7), n/a-n/a.
48. King, R. R.; Bhusari, D.; Boca, A.; Larrabee, D.; Liu, X. Q.; Hong, W.; Fetzer, C.
M.; Law, D. C.; Karam, N. H., Band gap-voltage offset and energy production in next-
generation multijunction solar cells. Progress in Photovoltaics: Research and Applications
2011, 19, (7), 797-812.
49. Hormann, U.; Kraus, J.; Gruber, M.; Schuhmair, C.; Linderl, T.; Grob, S.;
Kapfinger, S.; Klein, K.; Stutzman, M.; Krenner, H. J.; Brutting, W., Quantification of
energy losses in organic solar cells from temperature-dependent device characteristics.
Physical Review B 2013, 88, (23), 235307.
50. Mutolo, K. L.; Mayo, E. I.; Rand, B. P.; Forrest, S. R.; Thompson, M. E., Enhanced
Open-Circuit Voltage in Subphthalocyanine/C60 Organic Photovoltaic Cells. Journal of
the American Chemical Society 2006, 128, (25), 8108-8109.
51. Cnops, K.; Rand, B. P.; Cheyns, D.; Verreet, B.; Empl, M. A.; Heremans, P., 8.4%
efficient fullerene-free organic solar cells exploiting long-range exciton energy transfer.
Nat Commun 2014, 5, 3406.
25

52. Cnops, K.; Zango, G.; Genoe, J.; Heremans, P.; Martinez-Diaz, M. V.; Torres, T.;
Cheyns, D., Energy Level Tuning of Non-Fullerene Acceptors in Organic Solar Cells.
Journal of the American Chemical Society 2015, ASAP.
53. Gommans, H.; Aernouts, T.; Verreet, B.; Heremans, P.; Medina, A.; Claessens, C.
G.; Torres, T., Perfluorinated Subphthalocyanine as a New Acceptor Material in a Small-
Molecule Bilayer Organic Solar Cell. Advanced Functional Materials 2009, 19, (21), 3435-
3439.
54. Sullivan, P.; Duraud, A.; Hancox, l.; Beaumont, N.; Mirri, G.; Tucker, J. H. R.;
Hatton, R. A.; Shipman, M.; Jones, T. S., Halogenated Boron Subphthalocyanines as Light
Harvesting Electron Acceptors in Organic Photovoltaics. Advanced Energy Materials
2011, 1, (3), 352-355.
55. Verreet, B.; Cnops, K.; Cheyns, D.; Heremans, P.; Stesmans, A.; Zango, G.;
Claessens, C. G.; Torres, T.; Rand, B. P., Decreased Recombination Through the Use of a
Non-Fullerene Acceptor in a 6.4% Efficient Organic Planar Heterojunction Solar Cell.
Advanced Energy Materials 2014, 4, (8), n/a-n/a.
56. Verreet, B.; Rand, B. P.; Cheyns, D.; Hadipour, A.; Aernouts, T.; Heremans, P.;
Medina, A.; Claessens, C. G.; Torres, T., A 4% Efficient Organic Solar Cell Using a
Fluorinated Fused Subphthalocyanine Dimer as an Electron Acceptor. Advanced Energy
Materials 2011, 1, (4), 565-568.
57. Giebink, N. C.; Wiederrecht, G. P.; Wasielewski, M. R.; Forrest, S. R., Ideal diode
equation for organic heterojunctions. I. Derivation and application. Physical Review B
2010, 82, (15), 155305.
58. Yang, P.; Yuan, M.; Zeigler, D. F.; Watkins, S. E.; Lee, J. A.; Luscombe, C. K.,
Influence of fluorine substituents on the film dielectric constant and open-circuit voltage
in organic photovoltaics. Journal of Materials Chemistry C 2014, 2, (17), 3278-3284.
59. Cho, N.; Schlenker, C. W.; Knesting, K. M.; Koelsch, P.; Yip, H.-L.; Ginger, D. S.;
Jen, A. K. Y., High-Dielectric Constant Side-Chain Polymers Show Reduced Non-
Geminate Recombination in Heterojunction Solar Cells. Advanced Energy Materials 2014,
4, (10), n/a-n/a.
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Photovoltage Loss in Organic Photovoltaic Cells. Advanced Materials 2014, 26, (35),
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61. Holcombe, T. W.; Norton, J. E.; Rivnay, J.; Woo, C. H.; Goris, L.; Piliego, C.;
Griffini, G.; Sellinger, A.; Bredas, J.-L.; Salleo, A.; Frechet, J. M. J., Steric Control of the
Donor/Acceptor Interface: Implications in Organic Photovoltaic Charge Generation.
Journal of the American Chemical Society 2011, 133, (31), 12106-12114.
26

62. Perez, M. D.; Borek, C.; Forrest, S. R.; Thompson, M. E., Molecular and
Morphological Influences on the Open Circuit Voltages of Organic Photovoltaic Devices.
Journal of the American Chemical Society 2009, 131, (26), 9281-9286.
63. Erwin, P.; Thompson, M. E., Elucidating the interplay between dark current
coupling and open circuit voltage in organic photovoltaics. Applied Physics Letters 2011,
98, (22), 223305.
64. Vandewal, K.; Tvingstedt, K.; Gadisa, A.; Inganas, O.; Manca, J. V., Relating the
open-circuit voltage to interface molecular properties of donor:acceptor bulk
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65. Vandewal, K.; Widmer, J.; Heumueller, T.; Brabec, C. J.; McGehee, M. D.; Leo,
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Reduced Donor-Acceptor Interface Area. Advanced Materials 2014, 26, (23), 3839-3843.
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Orientation on the Photovoltaic Properties of a Phthalocyanine/Fullerene Heterojunction.
Advanced Functional Materials 2012, 22, (14), 2987-2995.
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Voltage of Organic Solar Cells. Advanced Materials 2013, 25, (42), 6076-6082.

27

Chapter 2. Instrumentation and Analysis
Chapter2.1 General
2.1.1 Thin Film Fabrication
Thin films were deposited on substrates pre-cleaned with tergitol and organic solvents. All
layers (when possible) were deposited by vacuum thermal evaporation (system base
pressure of 1-3x10
-6
torr) at rates between 0.02 and 0.2 nm s
-1
. Solution processed films
were spun-cast from solution concentrations in solvents and rates described in the text.
2.1.2 Device Fabrication
All organic materials (when possible) were purified by gradient sublimation before use.
The devices were deposited on ITO pre-cleaned with tergitol and organic solvents. All
layers (when possible) were deposited by vacuum thermal evaporation (system base
pressure of 1-3x10
-6
torr) at rates between 0.02 and 0.2 nm s
-1
. Blended layers were grown
by vacuum thermal evaporation, simultaneously depositing from two sublimation sources
in the desired ratio (e.g. 0.1 nm s
-1
:0.1 nm s
-1
for a 1:1 blend). Solution processed layers
were spun-cast from solution concentrations in solvents at rates described in the text.
Solvent annealing was performed as described previously.
1
Cathode layers were deposited
through a shadow mask defining a device with an area of 2 mm unless otherwise specified.
The devices were fabricated with the layer thicknesses given in the text.
2.1.3 Device Characterization
I-V measurements were performed in air at 25 °C unless otherwise specified using a
Keithley 2420 Sourcemeter (sensitivity = 100 pA) in the dark and under ASTM G173-03
28

spectral mismatch corrected 1000 W/m
2
white light illumination from an AM1.5G filtered
300 W xenon arc lamp (Asahi Spectra HAL-320W). Routine 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) 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.
2

Performance parameters are reported for the best repeatable efficiencies for the given
device architecture. Errors in VOC and FF arise from variations between devices, and the
error in JSC is primarily from uncertainty in measuring the intensity and spectrum of the
lamp, which also dominates the error in the power conversion efficiency.
2.1.4 Instrumentation
UV-Vis: UV-vis spectra were recorded on a Hewlett-Packard 4853 diode array
spectrometer.
Extinction coefficients: Extinction coefficients for the blended layers were calculated from
optical constants measured by variable angle spectroscopic ellipsometry.
Steady-state emission: Steady-state emission measurements on thin films were performed
using a Photon Technology International QuantaMaster Model C-60SE spectrofluorimeter.
Photoluminescence quantum yield: 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.
29

Photoluminescence lifetime: Fluorescence lifetimes were determined through time-
correlated single-photon counting methods. Fluorescence lifetime measurements were
performed using an IBH Fluorocube instrument equipped with a 405 nm LED excitation
source with the IRF value of 0.4 ns.
Electrochemistry: Cyclic voltammetry (CV) were performed using an EG&G
Potentiostat/Galvanostat model 283. Freshly distilled THF (VWR) was used as the solvent
under inert atmosphere with 0.1 M tetra(n-butyl)ammonium hexafluorophosphate
(Aldrich) 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. Electro-chemical reversibility was established using CV and are reported
relative to a ferrocenium/ferrocene (Fc
+
/Fc) redox couple used as an internal standard. A
scan rate of 100 mV/s was used for all measurements.
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.
2.1.5 Optical Modeling
Optical modeling was performed utilizing the transfer matrix formalism.
2.2 Specific to Chapter 3
Blocking layer modeling: To simulate the conduction and blocking behaviors of the
BCP:C60 blends, we consider a random distribution of available (C60) and unavailable
(BCP) sites for exciton transport, where each molecule is positioned on a cubic lattice. The
excitons then move via hopping to nearest-neighbor sites in a random walk consisting of
30

2000 steps, and their final position is recorded. For a three dimensional random walk, the
expected distance moved in any one dimension is sqrt(steps/3). 2000 steps then
corresponds to an exciton diffusion length of approximately 25 nm. Any exciton that
attempts to move onto an unavailable (BCP) site remains stationary for that step. The
BCP:C60 interface reflects excitons if no available site exists in the blocking layer, while
the edges of the region have cyclic boundary conditions.
Conductivity: The conductivities of the blend films were measured by sandwiching a 50
nm thick layer between ohmic (Al) contacts. The symmetric J-V characteristics are swept
between ± 2 V, and the conductivity is measured at 4 mA/cm
2
, characteristic of the
operating current of an OPV.
2.3 Specific to Chapter 5
Femtosecond transient absorption: Femtosecond pump and probe pulses were derived
from the output of a Ti:Sapphire regenerative amplifier (Coherent Legend, 1kHz, 4 mJ, 35
fs). Approx. 10% of the amplifier 800 nm output was used to pump a type-II OPA (Spectra
Physics OPA-800C) to generate a signal at ~1540 nm and this OPA signal output was
mixed with the residual 800 nm pump in a type-II BBO crystal to generate the 520 nm
(FWHM) using a CaF2 lens. White light supercontinuum probe pulses between 320-950
nm were obtained by focusing a small amount of the amplifier output on a rotating CaF2
disk. The supercontinuum probe was collimated and focused with a pair of off-axis
parabolic mirrors into sample. To suppress the scattering from the excitation pulse, a
perpendicularly oriented pump and probe were used to collect the data by passing the probe
through an analyzing polarizer after the sample. The cross correlation between pump and
31

probe in a thin 1mm quartz substrate gave a fwhm of 150 fs for 520 nm excitation. The
supercontinuum probe was dispersed using a spectrograph (Oriel MS127I) onto a 256-pixel
silicon diode array (Hamamatsu) for multiplexed detection of the probe.
The solutions of ZCl in cyclohexane, toluene, and acetonitrile were placed in a screw-
capped 1 mm quartz cuvette. The concentration of ZCl in cyclohexane and acetonitrle was
adjusted to give an optical density between 0.21 and 0.16 at 520 nm. The solutions were
deaerated by bubbling with N2 prior to analysis. The solid film of ZCl in PMMA was
prepared by spin coating on a quartz substrate to reach an optical density of 0.1 at 520 nm.
The film also had an additional quartz window on top surface and the outer edges were
sealed with epoxy under N2 atmosphere. During data collection, the samples were slowly
oscillated perpendicular to the pump and probe to reduce photodamage to the sample by
the pump. Transient absorption measurements were performed with pump fluences varying
between 5.7 and 40

µJ/cm
2
. Over this range, the signal was found to scale linearly with the
pump energy.
Electron Spectroscopies: Valence and conduction band states where obtained from UV-
photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPS),
respectively. The valence band spectra were measured using an He IIα line, whereas
secondary electron cutoffs were obtained using an He I line, with a 5 V bias applied to the
sample. Conduction band spectra were measured using a primary electron energy of 20.3
eV. The Fermi level of a gold surface in contact with the samples was used as a common
energy reference for all measurements. The instrumental broadening is estimated to 0.1 eV
in UPS and 0.6 eV in IPS.
32

Electronic structure: Electronic structure calculations were performed with the
GAMESS(US) software package using Becke3-Lee-Yang-Parr (B3LYP) three parameter
DFT theory. Geometries of local minima on the potential energy surface were calculated
with a 6-31G basis set. The density of states was obtained as a sum of the individual
electronic states convoluted with a 0.47 eV full width at half maximum Gaussian function.
FTPS-EQE: FTPS-EQE was measured using a Nicolet iS50r FTIR with the external
detector option and QTH light source, as described previously.
3
The photocurrent of the
OPV of interest was amplified by a Stanford Research Systems low-noise current
preamplifier. For electroluminescence measurements, solar cell devices were biased
between 1.2-1.4 V in order to minimize the emission of pure material. An Acton Research
Corporation CCD cooled to -30°C and SpectraPro 500i spectrograph were used as the
emission detection system.

33

2.4 References
1. Zimmerman, J. D.; Xiao, X.; Renshaw, C. K.; Wang, S.; Diev, V. V.; Thompson,
M. E.; Forrest, S. R., Independent Control of Bulk and Interfacial Morphologies of Small
Molecular Weight Organic Heterojunction Solar Cells. Nano Letters 2012, 12, (8), 4366-
4371.
2. Seaman, C. H., Calibration of solar cells by the reference cell method-The
spectral mismatch problem. Solar Energy 1982, 29, (4), 291-298.
3. Vandewal, K.; Goris, L.; Haeldermans, I.; Nesladek, M.; Haenen, K.; Wagner, P.;
Manca, J. V., Fourier-Transform Photocurrent Spectroscopy for a fast and highly
sensitive spectral characterization of organic and hybrid solar cells. Thin Solid Films
2008, 516, (20), 7135-7138.

34

Chapter 3. Exciton Blocking and Carrier Extraction in Buffers for Organic
Photovoltaics
3.1 Abstract
In order to achieve high efficiencies in organic photovoltaic devices, it is necessary to use
buffer layers. These materials serve to ensure that excitons are not quenched at the
metal/organic or oxide/organic interface and to guarantee that there is no reciprocal carrier
collection at the electrodes. In this chapter, we discuss the development of a new class of
buffer layers based on a mixture of a wide bandgap material and the active layer material.
Specifically, we describe the characterization and performance of BCP:C60
1
and NPD:DBP
based buffers, studying their optical and electronic properties and comparing their behavior
in devices to conventional buffers. Next, we compare the performance of amorphous and
crystalline buffer layers, highlighting the importance of transport through the buffer.
2
The
use of buffer layers is integral to the continued development of high efficiency OPVs.
3.2 Mixed Buffers Based on C60:BCP
3.2.1 Introduction
Since their introduction into organic photovoltaic (OPV) cells,
3, 4
fullerenes have become
the most ubiquitously employed acceptor molecules due to their high electron conductivity
and ability to promote efficient charge separation at donor/acceptor (D/A) interfaces.
4-6

The absorption spectrum of C60, for example, is particularly rich, and this has led to some
uncertainty in the importance of its various absorption features in the photocurrent
generation process. For example, the C60 absorption spectrum in solution is dominated by
two features with peaks at wavelengths of  = 260 nm and 340 nm that are attributed to
35

allowed electronic transitions resulting in Frenkel-type (i.e. monomolecular) excited states,
while the absorption at longer wavelengths is low due to a symmetry-forbidden transition.
7

On transition from solution to the solid state, we observe a significant increase in
absorption between  = 400 and 550 nm (Figure 3.1) due to the emergence of an
intermolecular charge transfer (CT) state
7
resulting from the excitation of an electron from
the highest occupied molecular orbital (HOMO) of one fullerene into the lowest
unoccupied molecular orbital (LUMO) of its nearest neighbors. Here, we examine the
relative roles played by direct absorption into the Frenkel or the CT states by blending C60
with the large energy gap (and hence transparent) material, bathocuproine (BCP). We find
that the CT state absorption declines more rapidly than that of the Frenkel-type feature,

Figure 3.1: Absorption characteristics of C60 in film and solution. Absorption spectra
of C60 in solution (red line) and thin film (blue line) plotted along with the photon flux
for AM 1.5 G illumination (black). The solution extinction is converted to film
absorptivity units by multiplying by the concentration of the C 60 film (2.39 M). The
appearance of a band between λ = 400 and 500 nm in the thin film sample, is assigned
to an intermolecular charge transfer (CT) state. The hatched areas below each
absorption line show the number of photons collected by film (blue) and solution (red)
samples at a thickness of 40 nm, illustrating the importance of CT absorption in C 60-
based OPVs.
36

allowing for their separate contributions to be determined as a function of BCP:C 60 blend
ratio. Moreover, when such blends are layered onto the surface of a neat C 60 acceptor in
an OPV, we find by both analytical and experimental methods that the layer serves to
effectively block excitons while allowing for efficient electron transport along percolating
pathways formed by C60 within the BCP matrix. This results in a new architecture for
electron conducting exciton blockers that can be used to enhance the efficiency of both
small molecule and polymer OPVs.
In Figure 3.1, we show the relative the importance of the C60 CT and Frenkel absorption
features, with the CT being the principal source of absorption of solar irradiation. Recently,
it was proposed that CT excitons in C 60 can ionize in the bulk, generating free electrons
and holes;
8
however, this only constitutes 15% of the carriers generated by the C60 acceptor
in an OPV. A similar behavior has also been observed in bulk heterojunctions (BHJs),
where free carriers are produced at photon energies > 2.35 eV (  < 530 nm).
9
The role that
the C60 CT band plays in photocurrent generation is explored here by utilizing blended
BCP:C60 acceptor layers in OPVs. BCP has singlet and triplet energies of 3.17 eV
10
and
2.62 eV,
10
respectively, that are larger than for C60 (with corresponding energies of 1.86
eV
7
and 1.55 eV
11
). Additionally, the HOMO and LUMO energies of BCP are at -6.5 eV
12

and -1.6 eV
10
relative to vacuum, respectively, while for C60 they are -6.4 eV
13

and -4.0 eV.
13
Thus, C60 does not engage in either energy or electron transfer to BCP.
3.2.2 Characterization of BCP:C60 Blends
The absorption spectra of the neat and blended BCP:C60 films are shown in Figure 3.2.
The absorption coefficient, , of the allowed Frenkel transition at  = 340 nm is
37

proportional to the C60 fraction as predicted by Beer’s law, reflecting the monomolecular
nature of this transition. In contrast, absorption of the CT absorption at  = 450 nm exhibits
a power-law dependence, viz.: 𝛼 = 𝑥 𝑚 , where x is the C60 volume fraction and
m = 2.7 ± 0.1 (see inset, Figure 3.2). This implies that the formation of CT excitons
involves two to three molecules, in agreement with previous studies,
7, 14
and that even
modest C60 dilution significantly reduces CT absorption.
The behavior of a BCP:C60 acceptor layer was investigated using a bilayer OPV cell
employing the transparent, wide energy gap donor, N,N’-di-[(1-naphthyl)-N,N′-diphenyl]-
1,1′-biphenyl)-4,4′-diamine (NPD),
15
localizing exciton generation to only the acceptor

Figure 3.2: Absorption and OPVs based on BCP:C 60 blends. (a) Device architectures
with BCP:C60 volume ratios of 0:1 (1), 1:2 (2), 1:1 (3), and 2:1 (4). (b) Absorption
coefficient of BCP:C60 blend films with 0:1 (▼), 1:2 (■), 1:1 (●), 2:1 (▲), and 1:0 (►)
calculated from optical constants determined by variable angle spectroscopic
ellipsometry. Inset: Extinction as a function of C60 fraction for wavelengths of =360
nm (●) and 450 nm (■), corresponding to Frenkel and CT absorption features,
respectively. Linear and power law (y = x
2.7
) fits are shown for the 360 and 450 nm
data, receptively. (c) External quantum efficiency of 1 - 4.

(a)

ITO
MoO
3
15 nm
NPD 11 nm
C
60
5 nm
BCP:C
60
(A:B) 40 nm
BCP 7 nm
Al
V

400 500 600 700
0
10
20
30
40
0
1x10
5
2x10
5
3x10
5
(c)
EQE (%)
Wavelength (nm)
(b)
0.0 0.5 1.0
0.0
0.5
1.0

(au)
C
60
fraction
340 nm
450 nm

 (cm
-1
)
C
60
1:2
1:1
2:1
BCP
(a)

ITO
MoO
3
15 nm
NPD 11 nm
C
60
5 nm
BCP:C
60
(A:B) 40 nm
BCP 7 nm
Al
V

400 500 600 700
0
10
20
30
40
0
1x10
5
2x10
5
3x10
5
(c)
EQE (%)
Wavelength (nm)
(b)
0.0 0.5 1.0
0.0
0.5
1.0

(au)
C
60
fraction
340 nm
450 nm

 (cm
-1
)
C
60
1:2
1:1
2:1
BCP
38

layer. The neat layer of C 60 at the donor/acceptor (D/A) interface ensures that differences
in photocurrent between devices are related to the blends instead of due to interface
dissociation of excitons. .
The external quantum efficiencies (EQE) of devices 1-4 are shown in Figure 3.2, with
additional performance parameters given in Table 3.1. As the ratio of the BCP:C 60 blend
varies from 0:1 to 2:1 by volume, the short circuit current density (JSC) decreases from
JSC = 3.0 ± 0.1 mA/cm
2
to 1.3 ± 0.1 mA/cm
2
due to the corresponding drop in C60
photoresponse, as seen in EQE measurements. The open circuit voltage (VOC) remains
unchanged at VOC = 0.87 ± 0.01 V, and the fill factor increases (FF) from
FF = 0.45 ± 0.01 to 0.49 ± 0.01 as the C60 concentration decreases. The drop in EQE is
consistent with the decrease in CT absorption of the doped C60 films (c.f. the absorption
and EQE spectra in Figure 3.2). That is, for a 1:2 BCP:C 60 film, the EQE at  = 350 nm
remains unchanged, while it decreases by ~30% at  = 450 nm. Further decreases in C60
concentration continue to decrease the response at   450 nm faster than that at
  350 nm.
The decrease in photoresponse with increasing BCP concentration contrasts with the results
of Menke, et al.
16
where the dilution of boron subphthalocyanine chloride (SubPc) with the
wide energy gap p-bis(triphenylsilyl)benzene (UGH2) results in a significant increase in
photocurrent. This was attributed to an increased diffusion length due to an increase in the
photoluminescence efficiency and excited state lifetime of SubPc coupled with a decrease
in the nonradiative exciton decay rate in the doped film. The difference between the
properties of SubPc:UGH2 and BCP:C 60 blends results from the differing nature of the
excitons participating in the photocurrent generation process. In SubPc, absorption is
39

primarily due to the generation of Frenkel states which increases linearly with
concentration, while the increase in exciton diffusion length is more rapid due to its
dependence on Förster transfer which increases roughly as the square of the concentration.
This is in contrast to C60, where CT excitons are generated in addition to Frenkel states.
On dilution, power-law dependence of decay in CT exciton (Figure 3.2 inset) absorption
outweighs concomitant increases in diffusion length. While the blended devices have a
lower photocurrent, the FF does not depend strongly on C60 concentration, and indeed does
not significantly decrease up to C60 dilutions as high as 70%.
3.2.3 Exciton Blocking Characterization
To understand the exciton blocking properties of the blends, OPVs containing a 10 nm
thick BCP:C60 layer sandwiched between two C60 layers (one is x ≤ 35nm thick and the
other is [40 nm – x] thick) were fabricated with the red absorbing donor (2,4-bis[4-(N,N-
diphenylamino)-2,6-dihydroxyphenyl] squaraine) (DPSQ)
17, 18
(see Figure 3.3). The total
thicknesses of neat C60 and BCP:C60 films are 50 nm. The J –V and EQE characteristics of
devices 5-8 with x = 5 nm to 35 nm are shown in Figure 3.3, with other performance

Figure 3.3: Effects of varying the ratio of acceptor to blended layer thicknesses. (a)
Acceptor thicknesses correspond to x = 5 nm (5), 15 nm (6), 25 nm (7), and 35nm (8).
(b) Current density vs. voltage characteristics of devices under one sun AM1.5G
illumination. (c) External quantum efficiencies of devices in (b).

(a)

0.0 0.5 1.0
-6
-4
-2
0
2
5
6
7
8
Current density (mA/cm
2
)
Voltage (V)
(b)

400 500 600 700 800
0
10
20
30
40
EQE (%)
Wavelength (nm)
(c)

0.0 0.5 1.0
-6
-4
-2
0
2
5
6
7
8
Current density (mA/cm
2
)
Voltage (V)
(b)
400 500 600 700 800
0
10
20
30
40
EQE (%)
Wavelength (nm)
(c)
40

parameters given in Table 3.1. The JSC decreases from 6.2 ± 0.3 mA/cm
2
to
4.1 ± 0.2 mA/cm
2
as the BCP:C60 layer is moved toward the D/A interface (i.e. as x is
decreased). This trend is also apparent in the EQE spectra where the response from C60
decreases as the thickness of the neat C60 layer adjacent to the D/A interface decreases
(devices 5 to 8). These data suggest that BCP:C60 prevents excitons generated in the C60
film adjacent to the metal electrode from diffusing to the D/A interface where dissociation
into free charges can occur. In contrast, the doped layer does not impede charge transport,
as inferred from the constant and high FF = 0.72 ± 0.01 and Voc = 0.94 ± 0.01 V.
Increasing the thickness of the C60 layer adjacent to the D/A interface from x = 5 nm to
Table 3.1 Summary of Device Performance Characteristics.
Device JSC (mA/cm
2
) VOC (V) FF PCE (%)
1 3.0 0.87 0.44 1.14
2 2.2 0.87 0.43 0.84
3 1.7 0.86 0.45 0.64
4 1.3 0.86 0.49 0.56
5 4.1 0.92 0.72 2.7
6 4.8 0.94 0.73 3.3
7 5.6 0.94 0.73 3.8
8 6.2 0.94 0.71 4.2
9 7.5 0.95 0.65 4.8
10 7.6 0.95 0.66 4.8
11 7.1 0.95 0.71 4.8
12 8.1 0.95 0.68 5.3
13 8.3 0.95 0.64 5.0

41

35 nm increases the power conversion efficiency under 1 sun, AM 1.5G illumination from
2.7 ± 0.1 % to 4.1 ± 0.1 %.
3.2.4 Monte Carlo Modeling
The blocking mechanism of the BCP:C 60 blends has been explored using a 3-dimensional
Monte Carlo simulation where excitons are randomly generated in a neat C60 acceptor
located adjacent to the BCP:C60 blocking layer (see Methods). For a non-blocking junction
between two materials, the final exciton population at the “interface” corresponds to a
situation where 50% of the excitons enter the “blocking “region, and the other 50% remain
in the acceptor, as shown in Figure 3.4. In the case of a 1:1 blend of BCP:C 60, the mixed

Figure 3.4: Blocking properties of the blend films. 3-D Monte Carlo simulation of
exciton diffusion from neat C60 into BCP:C60 blend layer. The neat C60 (0:1) blocks
50% of the excitons, the 1:1 ratio blocks approximately 81%, the 2:1 ratio blocks ~
95%, and the 4:1 ratio blocks ~98% of the excitons. The blocking efficiency is defined
as the ratio of the exciton population at the interface to the population expected for an
ideal blocking layer, and all data are normalized to this value. Inset: 3D illustration of
the 1:1 mixed layer used in the simulation. Green denotes BCP, blue is C60. The neat
C60 acceptor layer is shown as the semi-transparent region on the front right edge of the
blend.

42

layer is 81% efficient at blocking excitons, whereas for a 1:4 blend, the interface is ~98%
blocking, showing that a seemingly porous interconnected mixed layer can, indeed, act as
an efficient exciton blocking layer.
3.2.5 Conductivity Measurements
Further, we note that the conductivity of the blends is not significantly affected by C 60
concentration. That is, the conductivity of neat C60 at a current of 5 mA/cm
2

(corresponding approximately to the operating current of an OPV) and 1:1, and 1:2
BCP:C60 blends is 6 ± 2 x 10
-6
S/cm, only slightly decreasing to 2 x 10
-6
S/cm for the 2:1
blend. This is similar to previous measurements of mixed, amorphous C60 layers.
19
In

Figure 3.5: Effects of varying the BCP:C60 layer thicknesses. (a) conductivity of
BCP:C60 blends as a function of C60 fraction, (b) device architectures, (c) current density
vs. voltage characteristics of devices under one sun AM1.5G illumination, and (d)
external quantum efficiencies.

NPD (11nm)
C
60
:BCP (1:1),
X nm,
C
60
(20 nm)
ITO
BCP (7nm)
Al
43

contrast, the conductivity is only 4 x 10
-10
S/cm at 0.1V for neat, and insulating BCP. This
behavior suggests the existence of percolating conduction paths for electrons formed at
concentrations of ≥ 30%, at which point the CT absorption feature is very weak, and the
blend is an efficient exciton blocker.
Additional investigation of the conductivity of the buffer layer was conducted by
fabricating devices with increasing buffer layer thicknesses. The J –V and EQE
characteristics of devices containing buffer thicknesses from 20 nm to 80 nm are shown in
Figure 3.5 with performance parameters given in Table 3.1. The JSC increases slightly
from 3.0 ± 0.1 mA/cm
2
to 3.4 ± 0.1 mA/cm
2
when the buffer thickness is increased from
20 nm to 40 nm platuaus and then decreases back to 3.0 ± 0.1 mA/cm
2
for a thickness of
80 nm. However, there is no change in both VOC and FF with buffer thickness indicative
that there is no issue with charge extraction even at buffer thiknesses of 80 nm.
3.2.6 Buffer Layer Characterization
Given their relatively high electron conductivity and transparency, we employed BCP:C60
blends as an exciton blocking/electron transporting buffer layer adjacent to the cathode
contact in an otherwise conventional DPSQ/C60 OPV (see Methods), and its performance
was compared to analogous devices using conventional blockers of pure BCP
20
or
PTCBI,
21
or compound buffers
18
where the BCP:C60 is capped with either BCP or PTCBI.
In these devices, the active layer is comprised of DPSQ/C60 junctions that are solvent vapor
annealed after the deposition of C60 but before the buffer-layer deposition.
21
The J –V
characteristics for devices with various buffer layers are shown in Figure 3.6, with
additional performance parameters provided in Table 3.1. For all devices, Voc =
0.95 ± 0.01 V independent of the buffer. The device capped with a 10 nm thick PTCBI
44

buffer (Device 11), exhibits the smallest JSC = 7.1 ± 0.4 mA/cm
2
.
18
Unlike PTCBI, the
other buffers, BCP (9) and BCP:C60 (10) are transparent, resulting in an increase to
JSC = 7.5 ± 0.4 mA/cm
2
in both cases. The compound buffer layers with a thicknesses of
15 nm, i.e. BCP:C60/PTCBI (12) and BCP:C60/BCP (13), have JSC = 8.3 ± 0.4 mA/cm
2
and
8.5 ± 0.4 mA/cm
2
, respectively. Devices 9 and 13 capped with BCP exhibit
FF = 0.64 ± 0.01 and 0.65 ± 0.01, respectively. The buffer comprised of only BCP:C 60
(10) had a slightly increased FF = 0.66 ± 0.01. The devices capped with PTCBI 11 and
BCP:C60/PTCBI 12, exhibit the largest FF = 0.71 ± 0.01 and 0.68 ± 0.01, respectively.
Due to the increase in photocurrent and FF, the power conversion efficiency of the device
with the BCP:C60/PTCBI buffer is PCE = 5.3 ± 0.3 % compared to 5.0 ± 0.3% for
BCP:C60/BCP, 4.8 ± 0.2 % for BCP:C60, 4.8 ± 0.2 % for PTCBI,
21
and 4.8 ± 0.2 % for
BCP.
The differences in FF for the various buffer layers can be understood by examining the
ratio of the EQE at -1 V, to the EQE at 0 V, in Figure 3.6. The photoresponse from C60
between  = 400 nm and 550 nm is a function of external bias, while the DPSQ response
between  = 600 nm and 825 nm, remains relatively unaffected. The voltage dependence
at short wavelengths for the device with a BCP buffer is consistent with quenching
22
of
excitons by a build-up of electron-polarons at the C60/BCP interface. This quenching
reduces current at forward bias (e.g. near the OPV maximum power point), and reduces
FF. Under reverse bias, the interface electrons are depleted, reducing quenching and
increasing current. Excitons generated at the shorter wavelengths closer to the reflective
cathode, are more strongly influenced by the application of an external field, supporting
the assertion that the quenching occurs near the C60/BCP interface. Device 13, capped with
45

10 nm BCP:C60/5 nm BCP, experiences a smaller voltage dependence than a neat
10 nm BCP (9). This is due to two factors. First, the BCP layer in the compound buffer is
thinner than the neat BCP buffer allowing the metal-induced defect states
23
formed on
cathode deposition to penetrate throughout the BCP layer. Second, the BCP:C 60 layer
blocks excitons while allowing for electron transport, thereby spatially separating these
two species, thereby reducing the probability for exciton-polaron quenching. The devices
capped with PTCBI, 11 and 12, also exhibit a small voltage dependence due to a lack of
build-up of electrons at the organic/PTCBI interface.

Figure 3.6: Performance of the optimized organic photovoltaic cell. (a) Schematic of
the devices with various buffer layers: 10 nm thick BCP (9),10 nm thick BCP:C60 (10),
10 nm thick PTCBI (11), 10 nm BCP:C60/5 nm PTCBI (12),
10 nm BCP:C60/5 nm BCP (13). (b) Current density vs. voltage characteristics of
devices under one sun, AM1.5G illumination. (c) Ratio of the external quantum
efficiency (EQE) at -1 V to its value at 0 V for devices with various buffer layers as in
(b).

(a)

0.0 0.5 1.0
-8
-6
-4
-2
0
2
Current density (mA/cm
2
)
Voltage (V)
9
10
11
12
13
(b)

400 500 600 700 800
1.0
1.1
1.2
1.3
1.4
EQE (-1V)/EQE (0V)
Wavelength (nm)
(c)

0.0 0.5 1.0
-8
-6
-4
-2
0
2
Current density (mA/cm
2
)
Voltage (V)
9
10
11
12
13
(b)
400 500 600 700 800
1.0
1.1
1.2
1.3
1.4
EQE (-1V)/EQE (0V)
Wavelength (nm)
(c)
46

The relatively transparent, blended BCP:C60 film acts as an effective cathode buffer by
conducting electrons while blocking the diffusion of excitons to the quenching cathode.
This unique concept of a blended blocker/conduction medium results in a significant
enhancement in power conversion efficiency to 5.3 ± 0.3 % in a bilayer DPSQ/C60 junction
compared to the use of more conventional blocking architectures by reducing exciton-
polaron recombination at the blocker/acceptor interface, and by aiding in electron
extraction at the cathode.
3.3 Mixed Buffers Based on NPD:DBP
Following the development of the BCP:C 60 buffer, we extended this approach to other
material systems. Recently, devices containing DBP have been of great interest to the OPV
community due to the high efficiency and long device lifetimes that have been
demonstrated.
24
This inspired our work on a blended buffer of NPD and DBP. To study
the performance of the blocking layer, the performance of devices where DBP was
deposited directly on MoO3 was compared with those where a 10 nm layer of NPD or

Figure 3.7: Comparison of the organic photovoltaic cells with and without NPD:DBP
mixed buffer. Current density vs. voltage characteristics of devices under one sun
AM1.5G illumination.

47

NPD:DBP was inserted. The J –V characteristics for devices with or without the buffer are
shown in Figure 3.7. Compared to the device without a buffer, the NPD and NPD:DBP
devices exhibit an increase in JSC from 5.5 ± 0.1 mA/cm
2
to 7.9 ± 0.1 mA/cm
2
and
6.1 ± 0.1 mA/cm
2
, respectively. However, the FF decreases from 0.64 ± 0.01 for the
device with no buffer to 0.54 ± 0.01 and 0.57 ± 0.01 for the devices with NPD and
NPD:DBP, respectively. VOC remains largely unaffected across the device set at
0.92 ± 0.01 V.
The increase in JSC in the presence of both buffers indicate that they serve to prevent the
quenching of excitons at the MoO3 interface. The larger increase seen for the pristine NPD
layer is due to the fact that NPD is visibly transparent while the NPD:DBP layer absorbs.
This means the NPD:DBP layer absorbs parasitically, decreasing the amount of excitons
which can reach the donor/acceptor interface compared to NPD. On the other hand, the FF
is increased upon the addition of DBP to the NPD layer indicating again that transport
through the buffer layer is enhanced when a mixture is formed. In the NPD:DBP case, the
FF is reduced in the presence of a buffer, unlike what was observed for BCP:C 60. This
illustrates the importance of the material combination utilized and underscores the need for
specific materials selection in the formation of a mixed buffer.
3.4 Amorphous vs. Crystalline Buffer layers
3.4.1 Introduction
The use of exciton blocking layers in organic donor-acceptor solar cells is well established.
For planar heterojunction (PHJ) organic solar cells, it has already been shown that this
recombination channel can be suppressed by inserting either crystalline
25
or amorphous
48

blocking layers,
26
resulting in higher power conversion efficiencies (PCE). In these
devices the blocking layer increases the short-circuit current density (JSC), while leaving
the open-circuit voltage (VOC) and fill factor (FF) almost unchanged. Beyond the increase
due to the exciton blocking effect, it has been suggested that the use of crystalline blockers
as a nanostructured template may increase the area of the donor/acceptor interface, which
would further enhance JSC.
25

In this work, we seek to clarify the influence of morphology by comparing blocking layers
consisting of either crystalline diindenoperylene (DIP) or amorphous N,N′-bis(naphthalen-
1-yl)-N,N′-bis(phenyl)-2,2′-dimethylbenzidine (α-NPD) in planar devices. The blocking
materials were selected based on the alignment of their energy levels related to highest
occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of
the donor material. To ensure hole transport in addition to efficient exciton blocking, the
HOMO must be smaller (or at least similar), while the band gap must be greater than that
of the donor. To easily detect the blocking effect, a highly absorbing donor material,
Tetraphenyldibenzoperiflanthene (DBP
27
), is used in these studies. DBP has already been
proven to form efficient solar cells in combination with fullerenes providing high PCEs up
to 8.1%.
24
Its ability to absorb efficiently is due to the horizontal orientation of the
molecules, which enables a strong coupling between the incident light and the transition
dipole moment, which is aligned along the long axis of the molecule.
28
As electron
acceptor material, we used the fullerene C60.
3.4.2 Optical and Electronic Properties
Figure 3.8 shows the organic materials used in this study with their energy levels and
absorption spectra. The PEDOT:PSS derivative HIL1.3 was obtained from Clevios
49

(Germany), α-NPD and DBP from Lumtec (Taiwan), DIP from S. Hirschmann (Univ.
Stuttgart, Germany), and BCP from Sigma-Aldrich. HIL1.3 was spin-coated from aqueous
dispersion and subsequently dried at 125 °C for 30 min. All other materials were
evaporated in UHV (<5×10−7 millibars) at 0.5 Å/s. Current-voltage (J-V) characteristics
were recorded using a source measure unit (Keithley 236 SMU) under illumination with a
solar simulator (Oriel 300 W with AM 1.5 G filters) in a glovebox system with nitrogen
atmosphere. The illumination intensity was approved by a calibrated silicon reference cell
(RERA systems, PV Measurement Facility, Radboud University Nijmegen, area 1 × 1
cm
2
). Incident photon-to-current efficiency (IPCE) measurements were carried out using a
monochromatized Xe arc lamp as light source and lock-in detection.

Figure 3.8: Schematic drawing of molecular formulae and energy levels of α-NPD,
DIP, DBP, and C60. Energy values of organic materials are taken from literature.10,11
Absorption characteristics of donor, acceptor, and blocking materials are shown, with
DBP featuring the by far most dominant absorption, while DIP and α-NPD absorb only
weakly.

50

3.4.3 Devices with a Crystalline Buffer
Generally, the architecture of the solar cells is ITO (140 nm)/HIL1.3 (45 nm)/blocking
layer (y nm)/DBP (15 nm)/fullerene (45 nm)/BCP (5 nm)/Al (100 nm), i.e., the only
variables are the blocking layer material and the thickness of that layer. DIP was chosen
to form the crystalline blocking layer because it exhibits exceptionally high structural order
in evaporated thin films.
29
DIP fulfills both requirements for effective exciton blocking:
The energy gap for DIP is wider than DBP, as absorption measurements reveal, while UPS
measurements show identical HOMO-offsets for DIP/C60 and DBP/C60 as required for
efficient hole transport to the anode. Due to its high order in evaporated thin films with
large exciton diffusion lengths of up to 100 nm,
30
DIP is also used as donor material in
organic solar cells, yielding exceptionally high fill factors of nearly 75%.
31
However, the
almost upright standing alignment of the DIP molecules leads to weak absorption and
therefore a much smaller JSC compared to DBP. This weak absorption is also advantageous
in blocking layers, otherwise parasitic absorption can occur and the gain in current could
not solely be ascribed to decreased exciton quenching.
To exclude the impact of DIP absorption, devices varying the thickness of the blocking
layer from 3 nm to 21 nm in 3 nm steps were fabricated. These device produced almost
identical values for JSC (Figure 3.9(a)). This result leads to the assumption that 3 nm of
DIP already forms a (nearly) continuous layer, revealing the tendency of DIP to grow in
Stranski-Krastanov mode on various substrates.
32, 33
Compared to the reference without a
blocking layer, the gain in JSC is between a minimum of 24% (3 nm DIP) and a maximum
of 30% (6 nm DIP). At higher DIP thicknesses, the JSC remains nearly constant. Moreover,
the values for VOC (continuously) and FF (initially) show a small increase (Figure 3.9(c)).
51

The slight but continuous gain in VOC for thicker blocking layers is an additional effect of
the reduced recombination,
34
whereas the fill factor increases from 69% (0 nm) to a
maximum of 72% (6 nm) but then decreases again down to its initial value (21 nm) due to
transport issues.
The PCE increases from 2.8% for the reference up to 3.8% for the best cell in this series
containing a 6 nm DIP blocking layer, an improvement of more than 37%. Furthermore,
the similarity of the J-V-characteristics of the compared solar cells leads to the conclusion
that a possible template effect is not relevant. Due to pronounced island growth for higher
DIP thicknesses, the root mean square roughness for the DIP/DBP interface increases,
however, this effect does not propagate to the DBP/fullerene interface, where it could lead
to enhanced exciton dissociation and thus a higher JSC. Even for DIP grown at elevated
temperatures (TSubstrate = 100 °C), which leads to an enhanced lateral crystallinity of the DIP
layer,
35
no changes in JSC can be observed (open symbols in Figure 3.9(a)). Therefore, the
by far most dominant effect for the gain in JSC is reduced exciton quenching at the
HIL1.3/organic interface, which is also supported by IPCE measurements (Figure 3.9(b)),
revealing that the increment is mainly at wavelengths (λ) between 500 nm and 650 nm,

Figure 3.9: Effects of varying the DIP layer thicknesses. (a) Current density vs. voltage
characteristics of devices under one sun AM1.5G illumination, (b) External quantum
efficiencies, and (c) summary of device performance parameters.

52

where the maximum absorption of DBP occurs (Figure 3.8). In the main absorption region
of C60 (400 nm < λ <500 nm), however, only small differences between the IPCE curves
are visible. This is in accordance with the assumption that less excitons generated within
the DBP layer are quenched at the HIL1.3 buffer layer, but instead dissociate at the
DBP/C60 interface, generating free charge carriers.
3.4.4 Devices with an Amorphous Buffer
N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-2,2′-dimethylbenzidine (α-NPD) also fulfills
the requirements for efficient blocking layers in combination with DBP concerning energy
level alignment and hole transporting ability, while hardly absorbing itself in the visible
range. In contrast to highly crystalline DIP, thermally evaporated α-NPD forms amorphous
thin films.
36
As there is no template effect for DIP, a similar gain in JSC for amorphous
blockers is expected. This assumption can be verified by the J-V-characteristics
(Figure 3.10(a)). For the best cell with a 9 nm thick α-NPD layer, JSC increases by 29%.
Compared to the 30% of the device exhibiting 6 nm DIP, one can state that there is no
difference in JSC between devices with crystalline or amorphous blocking layers within
the range of error. Moreover, the same trends for VOC and FF can be observed compared to

Figure 3.10: Effects of varying the NPD layer thicknesses. (a) Current density vs.
voltage characteristics of devices under one sun AM1.5G illumination, (b) External
quantum efficiencies, and (c) summary of device performance parameters.

53

devices with crystalline blocking layer (Figure 3.10(c)), so that there is again an increase
in PCE of about 37%. This leads to the conclusion that a possible template effect of
crystalline blocking layers as proposed in literature
25
is not occurring or at least its impact
is negligible. However, there is a significant difference in the thickness dependence of
device parameters between these two blocking layers. While there was hardly any
correlation between layer thickness and device performance for the DIP-containing solar
cells, this is not the case for the α-NPD devices. This is connected with two factors. First,
it is assumed that 3 nm and even 6 nm of α-NPD are not sufficient to form a closed layer,
which leads to incomplete blocking and thus less gain in J SC compared to structurally
identical DIP devices. Second, blocking layers exceeding 9 nm show an increasing s-shape
behavior. This feature is ascribed to a growing transport resistance,
37
an effect which is
much more pronounced in amorphous films which have lower charge carrier mobilities.
To confirm, samples with 21 nm α-NPD highly doped with MoOx (9:1 and 4:1) were
prepared. As a result, the s-shape vanishes (open symbols in Figure 3.10(a)). However,
as MoOx also acts as a quencher, JSC decreases again with increasing percentage of MoOx.
The quenching effect is also revealed by the corresponding IPCE characteristics (open
symbols in Figure 3.10(b)). IPCE curves naturally show the same trend as JSC with an
increasing amount of generated charge carriers up to a layer thickness of 9 nm and a
following saturation for thicker blocking layers.
3.4.5 Additional Buffers
In addition to DIP and NPD, we also studied other potential buffer layers chiefly to examine
the impact of frontier orbital energy alignment on device performance. We fabricated
devices with layers comprised of TTP and Tetracene. The HOMO of TTP is -4.8 eV
10
and
54

Tetracene is -5.3 eV.
38
The J –V characteristics for devices with various buffer thicknesses
are shown in Figure 3.11 and performance parameters are summarized in Table II. For the
devices with DTP, JSC increases upon the insertion of the buffer with the greatest value for
a thickness of 10 nm. However, VOC decreases dramatically from 0.90 V to 0.76 V for the
devices without and with the buffer, respectively. FF also decreases from 0.69 to
approximately 0.45 for the devices with the buffer. From the increase in JSC and decrease
in VOC and FF it can be understood that DTP serves as an exciton blocking layer but the
mismatched HOMO results in a significant loss in performance due to changes in charge
transport and extraction.
Similarly, devices with a Tetracene buffer also produce increase in JSC. The device with
3 nm Tetracene produces the largest effect with an increase of JSC from 5.6 mA/cm
2
to
6.7 mA/cm
2
compared to the device with no buffer. However in these devices, the VOC
remains unaffected by the inclusion of the buffer at 0.91 V while the FF decreases from
0.68 to approximately 0.55. In this case the offset in HOMO energies between DBP and

Figure 3.11: Performance of additional buffers. (a) Current density vs. voltage
characteristics of devices with a DTP buffer under one sun AM1.5G illumination.
(b) Current density vs. voltage characteristics of devices with Tetracene buffer under
one sun AM1.5G illumination.

0.0 0.5 1.0 1.5
-8
-6
-4
-2
0
Current density (mA/cm
2
)
Voltage (V)
3nm DTP
5nm DTP
10 nm DTP
REF
0.0 0.5 1.0 1.5
-8
-6
-4
-2
0
Current density (mA/cm
2
)
Voltage (V)
3nm Tet
5nm Tet
10nm Tet
REF
55

Tetracene is smaller and so the deleterious effect of the HOMO offset is not as significant.
These studies highlight the importance of transport level alignment in the selection of a
buffer layer.
3.5 Conclusions and Future Outlook
In conclusion, we have performed detailed studies on a variety of buffers used in organic
photovoltaics. The concept of a mixed buffer layer formed by the combination of an active
layer material and a wide band gap material was developed. The mixed buffer comprised
of BCP:C60 was shown to function as an ideal buffer with high transparency, effective
exciton blocking, and good conductivity. On the other hand, the NPD:DBP buffer was not
as effective. While an increase in JSC was observed, the FF was reduced indicative of
transport issues. Additionally, the increase in JSC was less than that observed for an ideal
buffer indicating parasitic absorption occurs.
The mixed buffer has inspired further study in a variety of applications where it has proven
especially useful. Further study by Bergemann et al. revealed transport though the buffer
layer remains non-dispersive to fullerene concentrations as low as 10 %.
39
Xiao et al.
employed the mixed buffer in a PM-HJ comprised of DBP:C70 resulting in a substantial
increase in FF and an overall efficiency greater than 8 %, among the highest reported at
the time.
24
Later, the mixed buffer was utilized in tandem devices by Che et al. yielding
an efficiency of 11.1 %, the highest value obtained for a published device architecture.
40

The mixed buffer has also been used as an optical spacer where a thicker film allows the
active layer to be positioned in a location with higher optical field intensity. Verreet et al.
took advantage of this effect resulting in a fullerene-free device with an efficiency of 6.4 %,
at the time the highest efficiency for a fullerene-free device.
41

56

As well as the mixed buffer, we also examined the importance of order and energetic
alignment in buffer layers. Both amorphous and crystalline buffers provide the same
exciton blocking effect. However, crystalline buffers possess a higher conductivity and
are thus able to continue to increase efficiency at greater layer thicknesses. Through
utilizing buffer materials with various HOMO-HOMO offsets, we were able to show that
the magnitude of the offset determines the degree to which the device is negatively
impacted. This effect is related to changes in the FF indicating that while exciton blocking
still occurs, but charge extraction is greatly impacted.
The results reported here underscore the importance buffer layers play in the development
of high-efficiency OPVs. Without the ability to selectively extract charges while deflecting
excitons, significant quenching occurs. Additionally, transparent spacer layers allow for
the precise positioning of the active layer in an optimal position within the optical cavity.
High optical transparency ensures all photons are absorbed in the active layer leading to
the generation of photocurrent. The sum result of the properties is increased efficiency
which ensures that the study of buffer layers is and will continue to be an active area of
OPV research.
Looking forward, additional progress in buffer layer development is still required. The
development of a universal buffer, one which is compatible with a wide variety of active
layer materials would greatly simplify the optimization of devices. On the other hand, the
elaboration in the number of optically transparent materials with high conductivity would
broaden the potential compounds which would function as buffers. As the OPV
community looks to move away from fullerenes towards lower cost and more synthetically
tractable alternatives, it is likely that other buffer materials will need to be developed as a
57

consequence. Though they do not generate a photocurrent, buffer layers remain an integral
part of the function of an OPV and are particularly germane to the management of energy
within the device.

58

3.6 References
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J. D.; Forrest, S. R.; Thompson, M. E., A Fullerene-Based Organic Exciton Blocking Layer
with High Electron Conductivity. Nano Letters 2013, 13, (7), 3315-3320.
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Cells: Enhanced Efficiencies via a Network of Internal Donor-Acceptor Heterojunctions.
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4. Peumans, P.; Forrest, S. R., Very-high-efficiency double-heterostructure copper
phthalocyanine/C[sub 60] photovoltaic cells. Applied Physics Letters 2001, 79, (1), 126-
128.
5. Lin, Y.; Li, Y.; Zhan, X., Small molecule semiconductors for high-efficiency
organic photovoltaics. Chem. Soc. Rev. 2012, 41, 4245-4272.
6. Mishra, A.; Bäuerle, P., Small Molecule Organic Semiconductors on the Move:
Promises for Future Solar Energy Technology. Angewandte Chemie International Edition
2012, 51, (9), 2020-2067.
7. Kazaoui, S.; Minami, N.; Tanabe, Y.; Byrne, H. J.; Eilmes, A.; Petelenz, P.,
Comprehensive analysis of intermolecular charge-transfer excited states in C_{60} and
C_{70} films. Physical Review B 1998, 58, (12), 7689-7700.
8. Jeong, W.-I.; Lee, Y. E.; Shim, H.-S.; Kim, T.-M.; Kim, S.-Y.; Kim, J.-J.,
Photoconductivity of C60 as an Origin of Bias-Dependent Photocurrent in Organic
Photovoltaics. Advanced Functional Materials 2012, 22, (14), 3089-3094.
9. Burkhard, G. F.; Hoke, E. T.; Beiley, Z. M.; McGehee, M. D., Free Carrier
Generation in Fullerene Acceptors and Its Effect on Polymer Photovoltaics. The Journal
of Physical Chemistry C 2012, 116, (50), 26674-26678.
10. Wu, C.; Djurovich, P. I.; Thompson, M. E., Study of Energy Transfer and Triplet
Exciton Diffusion in Hole-Transporting Host Materials. Advanced Functional Materials
2009, 19, (19), 3157-3164.
11. van den Heuvel, D. J.; Chan, I. Y.; Groenen, E. J. J.; Schmidt, J.; Meijer, G.,
Phosphorescence of C60 at 1.2 K. Chemical Physics Letters 1994, 231, (1), 111-118.
12. Hill, I. G.; Kahn, A., Organic semiconductor heterointerfaces containing
bathocuproine. Journal of Applied Physics 1999, 86, (8), 4515-4519.
59

13. Wilke, A.; Endres, J.; Hormann, U.; Niederhausen, J.; Schlesinger, R.; Frisch, J.;
Amsalem, P.; Wagner, J.; Gruber, M.; Opitz, A.; Vollmer, A.; Brutting, W.; Kahn, A.;
Koch, N., Correlation between interface energetics and open circuit voltage in organic
photovoltaic cells. Applied Physics Letters 2012, 101, (23), 233301-4.
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Review B 1996, 54, (15), 10970-10977.
15. Rand, B. P.; Burk, D. P.; Forrest, S. R., Offset energies at organic semiconductor
heterojunctions and their influence on the open-circuit voltage of thin-film solar cells.
Physical Review B 2007, 75, (11), 115327.
16. Menke, S. M.; Luhman, W. A.; Holmes, R. J., Tailored exciton diffusion in organic
photovoltaic cells for enhanced power conversion efficiency. Nat Mater 2012, advance
online publication.
17. Wang, S.; Hall, L.; Diev, V. V.; Haiges, R.; Wei, G.; Xiao, X.; Djurovich, P. I.;
Forrest, S. R.; Thompson, M. E., N,N-Diarylanilinosquaraines and Their Application to
Organic Photovoltaics. Chemistry of Materials 2011, 23, (21), 4789-4798.
18. Lassiter, B. E.; Wei, G.; Wang, S.; Zimmerman, J. D.; Diev, V. V.; Thompson, M.
E.; Forrest, S. R., Organic photovoltaics incorporating electron conducting exciton
blocking layers. Applied Physics Letters 2011, 98, (24), 243307-3.
19. Asakawa, T.; Sasaki, M.; Shiraishi, T.; Koinuma, H., Dark and Photoconductivity
Behaviors of Amorphous and Crystalline C60 Films. Japanese Journal of Applied Physics
1995, 34, 1958-1962.
20. Peumans, P.; Bulovic, V.; Forrest, S. R., Efficient photon harvesting at high optical
intensities in ultrathin organic double-heterostructure photovoltaic diodes. Applied Physics
Letters 2000, 76, (19), 2650-2652.
21. Zimmerman, J. D.; Xiao, X.; Renshaw, C. K.; Wang, S.; Diev, V. V.; Thompson,
M. E.; Forrest, S. R., Independent Control of Bulk and Interfacial Morphologies of Small
Molecular Weight Organic Heterojunction Solar Cells. Nano Letters 2012, 12, (8), 4366-
4371.
22. Verreet, B.; Malinowski, P. E.; Niesen, B.; Cheyns, D.; Heremans, P.; Stesmans,
A.; Rand, B. P., Improved cathode buffer layer to decrease exciton recombination in
organic planar heterojunction solar cells. Applied Physics Letters 2013, 102, (4), 043301-
5.
23. Parthasarathy, G.; Burrows, P. E.; Khalfin, V.; Kozlov, V. G.; Forrest, S. R., A
metal-free cathode for organic semiconductor devices. Applied Physics Letters 1998, 72,
(17), 2138-2140.
60

24. Xiao, X.; Bergemann, K. J.; Zimmerman, J. D.; Lee, K.; Forrest, S. R., Small-
Molecule Planar-Mixed Heterojunction Photovoltaic Cells with Fullerene-Based Electron
Filtering Buffers. Advanced Energy Materials 2013, 4, (7), 1301557.
25. Zhou, Y.; Taima, T.; Kuwabara, T.; Takahashi, K., Efficient Small-Molecule
Photovoltaic Cells Using a Crystalline Diindenoperylene Film as a Nanostructured
Template. Advanced Materials 2013, 25, (42), 6069-6075.
26. Hirade, M.; Adachi, C., Small molecular organic photovoltaic cells with exciton
blocking layer at anode interface for improved device performance. Applied Physics
Letters 2011, 99, (15), 153302.
27. Fujishima, D.; Kanno, H.; Kinoshita, T.; Maruyama, E.; Tanaka, M.; Shirakawa,
M.; Shibata, K., Organic thin-film solar cell employing a novel electron-donor material.
Solar Energy Materials and Solar Cells 2009, 93, (6â€“7), 1029-1032.
28. Yokoyama, D.; Qiang Wang, Z.; Pu, Y.-J.; Kobayashi, K.; Kido, J.; Hong, Z., High-
efficiency simple planar heterojunction organic thin-film photovoltaics with horizontally
oriented amorphous donors. Solar Energy Materials and Solar Cells 2011, 98, (0), 472-
475.
29. Durr, A. C.; Schreiber, F.; Munch, M.; Karl, N.; Krause, B.; Kruppa, V.; Dosch,
H., High structural order in thin films of the organic semiconductor diindenoperylene.
Applied Physics Letters 2002, 81, (12), 2276-2278.
30. Kurrle, D.; Pflaum, J., Exciton diffusion length in the organic semiconductor
diindenoperylene. Applied Physics Letters 2008, 92, (13), 133306.
31. Wagner, J.; Gruber, M.; Hinderhofer, A.; Wilke, A.; Bröker, B.; Frisch, J.;
Amsalem, P.; Vollmer, A.; Opitz, A.; Koch, N.; Schreiber, F.; Brütting, W., High Fill
Factor and Open Circuit Voltage in Organic Photovoltaic Cells with Diindenoperylene as
Donor Material. Advanced Functional Materials 2010, 20, (24), 4295-4303.
32. Kowarik, S.; Gerlach, A.; Sellner, S.; Schreiber, F.; Cavalcanti, L.; Konovalov, O.,
Real-Time Observation of Structural and Orientational Transitions during Growth of
Organic Thin Films. Physical Review Letters 2006, 96, (12), 125504.
33. Casu, M. B.; Savu, S. A.; Schuster, B. E.; Biswas, I.; Raisch, C.; Marchetto, H.;
Schmidt, T.; Chasse, T., Island shapes and aggregation steered by the geometry of the
substrate lattice. Chemical Communications 2012, 48, (55), 6957-6959.
34. Cnops, K.; Rand, B. P.; Cheyns, D.; Heremans, P., Enhanced photocurrent and
open-circuit voltage in a 3-layer cascade organic solar cell. Applied Physics Letters 2012,
101, (14), 143301.
35. Gruber, M.; Rawolle, M.; Wagner, J.; Magerl, D.; Hörmann, U.; Perlich, J.; Roth,
S. V.; Opitz, A.; Schreiber, F.; Müller-Buschbaum, P.; Brütting, W., Correlating Structure
and Morphology to Device Performance of Molecular Organic Donor–Acceptor
61

Photovoltaic Cells Based on Diindenoperylene (DIP) and C60. Advanced Energy Materials
2013, 3, (8), 1075-1083.
36. Koch, N.; Elschner, A.; Schwartz, J.; Kahn, A., Organic molecular films on gold
versus conducting polymer: Influence of injection barrier height and morphology on
currentâ€“voltage characteristics. Applied Physics Letters 2003, 82, (14), 2281-2283.
37. Wagner, J.; Gruber, M.; Wilke, A.; Tanaka, Y.; Topczak, K.; Steindamm, A.;
Hormann, U.; Opitz, A.; Nakayama, Y.; Ishii, H.; Pflaum, J.; Koch, N.; Brutting, W.,
Identification of different origins for s-shaped current voltage characteristics in planar
heterojunction organic solar cells. Journal of Applied Physics 2012, 111, (5), 054509.
38. Graham, K. R.; Erwin, P.; Nordlund, D.; Vandewal, K.; Li, R.; Ngongang Ndjawa,
G. O.; Hoke, E. T.; Salleo, A.; Thompson, M. E.; McGehee, M. D.; Amassian, A., Re-
evaluating the Role of Sterics and Electronic Coupling in Determining the Open-Circuit
Voltage of Organic Solar Cells. Advanced Materials 2013, 25, (42), 6076-6082.
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Surprisingly High Conductivity and Efficient Exciton Blocking in Fullerene/Wide-Energy-
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Non-Fullerene Acceptor in a 6.4% Efficient Organic Planar Heterojunction Solar Cell.
Advanced Energy Materials 2014, 4, (8), 1301413.

62

Chapter 4. Energy Transfer and Exciton Diffusion
4.1 Abstract
Understanding and controlling the flow of excitons through an OPV is integral to guarantee
high performance in devices. In this chapter, we study the ways in which energy can be
captured in an OPV and how the transport of these excitons effects device performance.
First, we expand the energy sensitization scheme developed by Trinh et al.
1
to include
multiple chromophores, extending the absorption of the acceptor layer further into the red.
This results in a significant increase in photocurrent by capturing a broader fraction of the
solar spectrum. Then, we utilize the sensitization concept to study the diffusion of singlet
and triplet excited states through use of singlet and triplet sensitizers. We find that for neat
films of C60 the diffusion length of singlets is greater than that of triplets. However, in
blends of BCP and C60 the diffusion length of the singlet is reduced by a greater amount
than the triplet.
4.2 Multichromophoric Energy Sensitization of C60
4.2.1 Introduction
As absorption of the donor is shifted further into the near-infra red (NIR) to enhance photon
collection from the solar spectrum,
2-9
it is increasingly necessary to utilize additional
absorbing materials to ensure broad spectral coverage.
1, 4, 10-13
This has led to a recent surge
in research related to tandem
14-16
and ternary blended bulk-heterojunction
17, 18
solar cells
where a divide-and-conquer strategy has been employed in which separate regions of the
solar spectrum are absorbed by different materials. However, in ternary blended devices
the processes for photocurrent generation can become quite complex as multiple energetic
63

pathways from incident photon to collected electron are possible
18
and in tandem cells
current matching requires significant effort.
15

To overcome the complexity of ternary blended bulk-heterojunctions and tandem systems
and still achieve an increase in photon collection, Trinh et al.
1
developed a sensitization
strategy to improve the absorption efficiency of a C60-based acceptor layer. In this
sensitization approach, energy absorbed by one material, the sensitizer, is transferred to the
host, in this case C60, which is responsible for exciton transport, charge separation, and
electron conduction. In this scheme, the sensitizers need to be carefully designed to ensure
efficient energy transfer and maintain electron conductivity in the blended host – sensitizer
layer. In the previous work, a chlorinated zinc dipyrrin compound, ZCl, was designed and
established as an excellent sensitizer for the C60 acceptor layer.
In this work, we extend the sensitization approach to include multiple sensitizers absorbing
a broader range of the solar spectrum to further extend the absorption of the acceptor layer.
We demonstrate the improved performance resulting from the inclusion of multiple
sensitizers blended with a single host in donor/sensitized C60 acceptor devices. Two energy
sensitizers, ZCl
1
and hexachloro boron subphthalocyanine (Cl6SubPc),
19
with intense
absorption, are utilized to harvest photons in the visible portion of the solar spectrum and
transfer energy to C60. In devices, the simultaneous energy sensitization of C60 with both
ZCl and Cl6SubPc is demonstrated. Through the inclusion of multiple sensitizers, we are
able to achieve an increase in photocurrent of 30 % from 6.5 mA/cm
2
to 8.6 mA/cm
2
for
the sensitized device. The power conversion efficiency is increased from 3.8 % for the
standard device to 4.7 % for the sensitized device.

64

4.2.2 Optical and Electronic Properties
The molecular structures and thin film extinction spectra of the active layer materials are
presented in Figure 4.1. The C60 film shows strong absorption in the UV due to allowed
transitions and another feature between λ = 400 nm and 550 nm due to an intermolecular
charge transfer transition.
20
ZCl has an extremely intense absorption between λ = 450 nm
and 575 nm with a thin film extinction (α) as high as 4x10
5
cm
-1
. Cl6SubPc absorbs
between λ = 500 nm and 650 nm with α as large as 2.5x10
5
cm
-1
. The absorption of the
sensitizers is almost an order of magnitude more intense than C60 in the regions where they
absorb. The blended C60:ZCl:Cl6SubPc (2:1:1 by volume) film shows contributions from
all three components and has significantly more absorbance than pure C60 between
λ = 500 nm and 670 nm. (2,4-bis[4-(N,N-diphenylamino)-2,6-dihydroxyphenyl]
squaraine) (DPSQ),
4, 10, 21, 22
a typical NIR absorbing donor, absorbs between λ = 600 nm
and 800 nm. The donor absorption in conjunction with that of the codeposited acceptor
film should extend the photoresponse of a sensitized device uniformly from the UV to the
NIR.
In order to efficiently funnel excitons from the sensitizers to C 60 and to ensure electron
transport is also occurring via C60, knowledge of the excited state energies and carrier
transport levels is required. The singlet and triplet state energies and oxidation and
reduction potentials of ZCl,
1
Cl6SubPc,
19
and C60
20
are given in Figure 4.2. The arrows in
Figure 4.2a outline a schematic of possible pathways for energy transfer from ZCl and
Cl6SubPc to C60. Both ZCl and Cl6SubPc should function as sensitizers because their
singlet and triplet energies are greater than or within kBT of the values for C60.
Additionally, if electron transfer were to occur from the sensitizer to C60, it will result in
65

the formation of a CT state with energy equal to the difference between the oxidation
potential of the sensitizer and the reduction potential of C60 less the coulombic stabilization
provided by the oxidized and reduced species. Assuming a coulombic stabilization of
0.3 eV,
1
the ZCl/C60 and Cl6SubPc/C60 CTs will have energies of 1.98 eV and 1.55 eV,
respectively. These CT states will be higher in energy than the triplet of C60 resulting in
recombination to form a triplet on C60 and the net effect of energy transfer from the
sensitizer to C60. These considerations will guarantee that any excitons generated on the
sensitizers will be transferred to C60 and not trapped on the sensitizer. Finally, the reduction
potentials of the sensitizers are more negative than that of C60, ensuring that electrons are
conducted out of the device efficiently via C60. The conductivity of C60 in a blended film
has been demonstrated previously in C60:Bathocuproine (BCP) blends, where the electron
conductivity of the blended film is equivalent to that of neat C60 up to 50 % BCP by
volume.
22

Figure 4.1: (a) Molecular structure of C60, DPSQ, ZCl, and Cl6SubPc (b) Thin film
extinction coefficients of C60, ZCl, Cl6SubPc, C60:ZCl:Cl6SubPc, and DPSQ compared
with the AM 1.5G solar spectrum. Extinction calculated from optical constants obtained
by spectroscopic ellipsometry.
66

4.2.3 Luminescence Quenching Experiments
To investigate energy transfer between the sensitizers (ZCl and Cl 6SubPc) and C60, thin
film photoluminescence quenching experiments were performed utilizing C60 as a
quencher. Measurements were performed on films of the sensitizers blended with either
the wide gap material BCP or C60 at volume concentrations of 50 % to replicate the
compositions used in devices. The ZCl and Cl6SubPc films blended with BCP are emissive
and the spectra can be seen in Figure 4.2c. The substitution of C60 for BCP results in
substantial luminescence quenching due to energy transfer to C60. The luminescence
quenching efficiency (𝜃 ) for C60 can be calculated from the ratio of the integrated emission
spectra through equation 1:

Figure 4.2: (a) Singlet and triplet energies of ZCl, Cl6SubPc, and C60. Arrows indicate
possible energy transfer pathways. (b) Redox potential
(a)
(vs Fc/Fc
+
) of C60, ZCl, and
Cl6SubPc.
(b)
Ref. 11,
(c)
Ref.20,
(d)
Ref.21 (c) Photoluminescence of ZCl and Cl6SubPc
with and without C60. Films were excited at λ = 500 nm and 550 nm, respectively.
-1.30
b
-1.25
c
Redox potential (V)
a
ZCl
-1.06
d
C
60
Cl
6
SubPc
1.22
b
1.26
d
0.85
c
(b)
67

𝜃 = 1 −
𝑃𝐿
𝐶 60
𝑃𝐿
𝐵𝐶𝑃 (1)
where 𝑃𝐿 𝐶 60
is the integrated luminescence spectra for the blend with C60 and 𝑃𝐿 𝐵𝐶𝑃 is the
integrated luminescence spectra for the blend with BCP. C 60 quenches > 95 % of ZCl
emission and > 80 % of Cl6SubPc suggesting that energy transfer to C60 occurs efficiently.
As the excitons generated on the sensitizer are singlets, energy transfer will occur via
Förster resonant energy transfer (FRET),
23
where the rate of energy transfer in three
dimensions, 𝑘 𝐹 , is given by
𝑘 𝐹 =
1
𝜏 (
𝑅 0
𝑅 )
6
(2)
where 𝑅 is the intermolecular separation distance and 𝑅 0
is the Förster radius (the distance
at which the rate of energy transfer equals the rate of radiative decay). Inspection of the
Förster equation reveals the rate of energy transfer is fastest for a donor-acceptor system
separated by a small distance. Based on this fact, the sensitization architecture has been
designed to ensure maximum energy transfer efficiency in the solid state. This is achieved
by codepositing the materials in blends containing 50% C60 by volume in order to obtain
minimum separation between the sensitizer and C60 instead of the lamellar cascade
approach which has been applied previously.
24
The C60:sensitizer ratio was chosen to
maintain the high electron conductivity of C60 in the blend,
22
while simultaneously
maximizing the absorption of the sensitizers.
4.2.4 Device Studies
The performance of the multichromophoric sensitized device was optimized and compared
to both a singly sensitized device and a control with pure C60. For the purpose of probing
68

solely the performance of the acceptor layer, the wide energy gap donor N,N′-di-[(1-
naphthyl)-N,N′-diphenyl]-1,1′-biphenyl)-4,4′-diamine (NPD) was used. The control
device had the structure ITO/MoO3(10 nm)/NPD(11 nm)/C60(50 nm)/BCP(10 nm)/Al, the
singly sensitized device has the structure
ITO/MoO3(10 nm)/NPD(11 nm)/C60(15 nm)/C60:ZCl(1:1 50 nm)/BCP(10 nm)/Al, and the
multichromophoric devices had the structure
ITO/MoO3(10 nm)/NPD(11 nm)/C60(15 nm)/C60:ZCl:Cl6SubPc(2:1:1
x nm)/BCP(10 nm)/Al where x was varied from 30 to 90 nm in 20 nm increments. The
neat layer of C60 is placed between the sensitized layer and the D/A interface in order to
ensure that all excitons that reach the donor/acceptor (D/A) interface are transported
through C60 and to guarantee charge transfer occurs between NPD/C60 eliminating any
changes in the kinetics or thermodynamics of charge separation and recombination due to
the presence of the sensitizers.
25
The device performance was probed as a function of the
thickness of the blended C60:ZCl:Cl6SubPc layer and the results are summarized in
Figure 4.3a. Compared to the control device with 50 nm C60, the devices containing the
C60:ZCl:Cl6SubPc layer all exhibited an increase in photocurrent without substantial
change in VOC or FF. The increase reaches its maximum between a thickness of 50 nm and
70 nm for the C60:ZCl:Cl6SubPc layer where the photocurrent plateaus at
4.2 ± 0.1 mA/cm
2
, 1.2 mA/cm
2
greater than the control with pure C60. Figure 4.3b
summarizes the performance of the optimized devices containing C60, C60:ZCl, and
C60:ZCl:Cl6SubPc and Figure 4.3c compares the external quantum efficiencies (EQEs).
The sensitized devices both show increased J SC compared to the C60 reference with the
C60:ZCl:Cl6SubPc device outperforming the C60:ZCl device. From the EQE it is apparent
69

that the increase in
photocurrent is due to contribution from the sensitizers. For the C 60:ZCl device,
contribution from ZCl is evident between λ = 500 nm and 600 nm. In the
C60:ZCl:Cl6SubPc device, the ZCl enhancement is evident between λ = 500 nm and
575 nm, while the Cl6SubPc signal can be seen between λ = 575 nm and 650 nm. These
data clearly show that the sensitizers contribute to photocurrent production without
negatively impacting other device characteristics and that the inclusion of multiple
sensitizers allows the absorption of the acceptor layer to be shifted further to longer

Figure 4.3: (a) Summary of device performance as a function of C60:ZCl:Cl6SubPc
layer thickness. (b) Summary of device performance and (c) external quantum
efficiency curves for optimized devices with C60, C60:ZCl, and C60:ZCl:Cl6SubPc. The
layer thicknesses were 40 nm, 50 nm, and 70 nm for C60, C60:ZCl, and
C60:ZCl:Cl6SubPc, respectively.
70

wavelengths. However, while the application of the sensitizers with a transparent donor
shows the feasibility of
multichomophoric sensitization, the utility of sensitization can be shown in devices with
an intensely absorbing donor.
The continuing development of NIR absorbing donors has led to a gap in the absorption of
devices containing fullerenes. This is causes an EQE droop which is typified in DPSQ/C 60
devices
4, 10, 21, 22
where the C60 photoresponse ends at λ = 550 nm and the DPSQ
photoresponse is strongest between λ = 700 nm and 800 nm. Devices were fabricated to
further illustrate the impact of sensitization on device performance with a NIR donor (see

Figure 4.4: (a) Device architecture for the reference and sensitized devices containing
DPSQ. (b) J–V curves of devices under one sun AM1.5G illumination. (c) Plot of
external quantum efficiency showing the increase in spectral responsivity between
λ = 500 nm and 650 nm due to the inclusion of ZCl and Cl6SubPc.

Reference Sensitized
(a)
71

device structures in Figure 4.4a). The current – voltage (J–V) and EQE are shown in
Figure 4.4b,c and performance parameters are summarized in Table 4.1. The Voc of the
devices remains unchanged at 0.92 ± 0.01 V for both the reference (DPSQ) and sensitized
(DPSQ(s)) devices. The FF of the reference and sensitized devices are 0.60 ± 0.02 and
0.56 ± 0.02, respectively. Sensitization leads to a marked increase in photocurrent from
Jsc = 6.5 ± 0.2 mA/cm
2
to 8.6 ± 0.2 mA/cm
2
for DPSQ and DPSQ(s), respectively. The
EQE reveals that the increase in response is due to both ZCl and Cl6SubPc sensitization.
Compared to previous work with a single sensitizer, the inclusion of multiple sensitizers
results in a larger increase in photocurrent, 2.1 mA/cm
2
for the multichomophoric device
compared to 1.2 mA/cm
2
for sensitization with a single chromophore.
1
The sensitizers
completely fill the droop in absorption resulting in a dramatically enhanced ηp from
3.6 ± 0.2 % to 4.4 ± 0.3 %. The final device has achieved broadband spectral coverage
with EQE in excess of 20% from λ = 350 nm to 800 nm.
In summary, we have demonstrated that Cl6SubPc can be utilized as a sensitizer in
conjunction with ZCl in blends with C60. Photoluminescence quenching experiments
illustrate that C60 quenches excited states on the sensitizers. The sensitizers function by
absorbing photons and transferring energy to C 60 where C60 then serves to transport
excitons and electrons. In OPVs, it was shown that multiple sensitizers can be employed
within a single acceptor layer to compliment absorption and enhance photocurrent without
Table 4.1 Summary of Device Performance Characteristics of standard and sensitized
devices.
Device
JSC
(mA/cm
2
)
VOC
(V)
FF ηp(%)
DP S Q 6.5 ± 0.2 0.92 ± 0.01 0.60 ± 0.02 3.6 ± 0.2
DP S Q(s) 8.6 ± 0.2 0.92 ± 0.01 0.56 ± 0.02 4.4 ± 0.3

72

deleterious effects to VOC or FF resulting in increased efficiency. The extension of the
acceptor layer response out to λ = 670 nm fully fills the EQE minima exhibited by
C60/DPSQ devices and results in a significantly increased ηp of 4.7 % for the champion
device. These data demonstrate that the sensitization scheme is tolerant to the introduction
of additional sensitizers allowing for facile tuning of the acceptor layer absorption.
Although in this work the sensitization scheme is enacted in the acceptor layer, a similar
strategy could be utilized for donors.
4.3 Probing Singlet and Triplet Diffusion Through the use of Energy Sensitizers
4.3.1 Introduction
Organic electronic devices, such as organic light emitting diodes (OLEDs) or organic
photovoltaics (OPVs), have attracted a great deal of attention due to their potential to
deliver high efficiency devices with low manufacturing costs.
26
In order to extract high
efficiency from these devices is crucial to understand the transport of energy within them
to a high degree of detail. An area of particular difficulty is controlling the excited state
dynamics of an organic system. In OPVs, a great degree of work has been done to study
the properties of systems which primarily singlet excitons, such as conjugated polymers
and small molecules with slow intersystem crossing rates, and systems with primarily
triplet excitons, such as molecules with heavy metal centers which assist in intersystem
crossing, or materials which undergo singlet fission,
27
creating two triplet excitons from
one singlet.
The question as to which spin state is preferred revolves around the exciton diffusion length
(LD), defined as the square root of the product of the excited state diffusivity (D) and
73

lifetime (τ). LD governs OPV performance by providing the characteristic distance over
which excitons can traverse before recombining. Thus, for efficient photovoltaic devices
excitons must travel far enough to reach the D/A interface and charge separate. To
determine whether singlet or triplet excitons are desirable and to characterize their utility
within devices, it is crucial to understand the fundamental physics governing their lifetime
and transport.
Singlet states, being strongly allowed optical transitions, typically have relatively short
excited state lifetimes (on the order of ns) because the excited state and ground state are
strongly coupled and the transition is spin allowed. Triplets, on the other hand, are
extremely weak optical transitions and have orders of magnitude longer excited state
lifetime (on the order of μs to ms), with weak coupling between the excited state and ground
state and the transition is spin forbidden.
Based on lifetime alone, it would appear that triplets are obviously superior to singlets, but
the diffusivity of the species vastly different due to the mechanisms through which they
migrate. Diffusivity is the rate at which excitons flow and can be written as
𝐷 = 𝐴 𝑑 2
𝑘 𝐸𝑇
(3)
where A accounts for disorder, d is the distance over which energy is transferred, and kET
is the rate of energy transfer. There are two predominate mechanisms through which
energy is transferred between identical molecules: Förster resonance energy transfer
(FRET) which involves dipole coupling between nearby molecules and Dexter energy
transfer which occurs via the direct exchange of electrons from one molecule to another.
74

If energy transfer is occurring based on Förster resonance energy transfer (FRET),
then the rate of energy transfer, 𝑘 𝐹 , is given by
𝑘 𝐹 =
1
𝜏 (
𝑅 0
𝑅 )
6
(4)
where 𝑅 is the intermolecular separation distance and 𝑅 0
is the Förster radius (the distance
at which the rate of energy transfer equals the rate of radiative decay) defined by
𝑅 0
6
=
9𝜂 𝑃𝐿
𝜅 2
128𝜋 5
𝑛 4
∗ ∫ 𝜆 4
𝐹 𝐷 ( 𝜆 ) 𝜀 𝐴 ( 𝜆 ) 𝑑 𝜆 (5)
where 𝜂 𝑃𝐿
is the quantum yield of photoluminescence, 𝜅 is the dipole orientation factor,
𝑛 is the index of refraction, 𝜆 is the wavelength, 𝐹 𝐷 is the normalized emission spectra of
the donor, and 𝜀 𝐴 is the extinction spectra of the acceptor.
23
In this case, the rate of energy
transfer has 𝑅 −6
dependence and the magnitude of the extinction and photoluminescence
quantum efficiency have significant impact on the rate of transfer.
On the other hand, if energy transfer is occurring based on Dexter transfer, then the
rate of energy transfer, 𝑘 𝐷 , is given by
𝑘 𝐷 = 𝐾𝑒𝑥𝑝 ( −2𝑅 /𝑅 𝐷𝐴
0
)
∫ 𝜆 4
𝐹 𝐷 ( 𝜆 ) 𝜎 𝐴 ( 𝜆 ) 𝑑 𝜆 (6)
where 𝐾 describes orbital interactions, 𝑅 𝐷𝐴
0
is the Van der Waals contact distance for the
donor an acceptor, and 𝜎 𝐴 is the normalized absorption spectra of the acceptor.
28
In this
case, the rate of energy transfer adopts a 𝑒𝑥𝑝 ( −𝑅 ) dependence and is independent of the
magnitude of the absorption spectra or photoluminescence efficiency.
The impact of chomophore separation on the rate of energy transfer is illustrated in
Figure 4.5. FRET, which has an R
-6
dependence shows a slower energy transfer rate at
extremely short differences. However, at longer distances, the rate remains substantially
75

greater than that achieved through Dexter. Thus, the diffusivity of excitons which can be
transported via a FRET mechanism is substantially larger.
Due to their high extinction coefficient and small red-shift between absorption and
emission, singlets can undergo long range transfer via FRET. On the other hand, triplets
do not undergo FRET and instead diffuse by nearest neighbor hops via Dexter energy
transfer. As FRET can occur over longer distances than Dexter energy transfer, Singlets
have a much higher diffusivity than triplets.
29

Several detailed reviews highlight the theory and study of exciton diffusion in organic
photovoltaics.
30, 31
They survey the broad range of topics which have been developed to
both investigate the diffusion itself as well as methods of increasing the diffusion length of
a particular material. The most common methods for determining exciton diffusion length
have been through the use of fluorescence quenching
29
and electro-optical measurements.
32

Figure 4.5: Schematic diagram of the impact of chromophore separation distance on
the rate of energy transfer. The rate of FRET, which has an R
-6
dependence initially
decays rapidly but stabilized over longer distances. The rate of Dexter energy transfer,
which has an exp(-2R) dependence, rapidly decays with distane.
76

As the fluorescence quantum yield of organic thin films are generally low, quenching
measurements can prove difficult to obtain. Additionally, electro-optical simulations rely
on assumptions, such as perfectly quenching or blocking interfaces and flat, discrete
interfaces, which may not be entirely correct.
33
Thus the range of values obtained for the
exciton diffusion length of a single material can be quite substantial and may reflect
differences in experimental and fabrication conditions. Several schemes have been
developed to enhance the exciton diffusion length of a material. Morphological transition
from an amorphous to crystalline film results in an increase in diffusion length.
34

Optimization of the intermolecular separation has also been shown to increase the diffusion
length by more than 50 %.
35
Finally, interconversion of the exciton from a singlet to a
triplet has resulted in an increase in a doubling diffusion length.
36
Thus further study on
the transport of excitons within organic films is of great interest.
In this section, we compare the excited state diffusion of singlets and triplets in C 60 by
employing two classes of sensitizer, one generating singlets (ZCl) and the other triplets
(ICl), which transfer energy to C60. By monitoring the change in internal quantum
efficiency of the sensitizer as a function of its distance from the D/A interface, we are able
to fit separate diffusion lengths for singlet and triplet excitons. Additionally, we probe the
dependence of singlet and triplet exciton diffusion on concentration, finding the singlet
diffusion length to decrease more rapidly than the triplet.
4.3.2 Materials design and selection
In order to study the singlet and triplet diffusion in C60, it was first necessary to synthesize
molecules which would serve as singlet and triplet sensitizers. The sensitizers function by
absorbing light, producing the desired excited state, and finally undergoing energy transfer
77

to C60. Figure 4.6 outlines the state energy diagram required to ensure energy transfer is
exergonic. In order to function as a singlet sensitizer, the molecule must possess a singlet
energy greater than C60. Additionally, the intersystem crossing rate must be slower than
the rate of energy transfer from the sensitizer to C60 to ensure singlet transfer occurs. For
a triplet sensitizer, the triplet energy must be greater than C60 to guarantee energy transfer
is possible. However, in this case the intersystem crossing rate must be faster than the rate
of singlet energy transfer so that only triplets are transferred to C60.
The design and selection of the sensitizers was based on our previous work utilizing a zinc
dipyrrin to increase the visible photoresponse of OPVs.
1
ZCl is a singlet sensitizer as was
demonstrated by the fact that excitation of ZCl in a blended film results in C 60
fluorescence.
1
Building off of the chlorodipyrrin moiety as a highly absorbing
chromophore, a monodipyrrin Ir(F2PPY)2 derivative (ICl) was synthesized to serve as a
triplet sensitizer.
37

Figure 4.6: State energy diagram depicting the energy level requirements for singlet
and triplet energy transfer. For singlet energy transfer, path A, the singlet energy of the
sensitizer must be greater than C60. For triplet energy transfer, path B, the sensitizer
must rapidly intersystem cross to the triplet, which is higher in energy than C 60.
78

The molecular structure, thin film extinction, and emission of ZCl and ICl are shown in
Figure 4.7. ZCl is an extremely strong absorber with a maximum extinction greater than
4x10
5
cm
-1
at λ = 545 nm. ICl absorbs in the same spectral range as ZCl with a maximum
extinction of approximately 2x10
5
cm
-1
, about half of that of ZCl. The thin film emission
spectra reveal that ZCl is a fluorescent emitter due to the small stokes shift observed
between absorption and emission. On the other hand, ICl exhibits phosphorescent emission
exemplified by the large Stokes observed. Thus ZCl is a singlet sensitizer and ICl is a
triplet sensitizer.
4.3.3 Experimental design
The diffusion length was measured through the construction of a series of devices
containing two nearly identical architectures shown in Figure 4.8. A series of control
devices with the structure
ITO/MoO3(10 nm)/NPD(11 nm)/C60(x nm)/C(20 nm)/BCP(10 nm)/Al(100 nm), where C
is a 1:1 blend of C60:BCP and x was varied between 10 nm and 70 nm, were used to

Figure 4.7: (a) Thin film extinction spectra of C60, Zcl, and ICl and emission spectra
of ZCl and ICl. (b) Molecular structures of ICl, ZCl, NPD, C60, and BCP.
79

measure the baseline response. Then, a corresponding series of sensitized devices with the
structure ITO/MoO3(10 nm)/NPD(11 nm)/C60(x nm)/S(20 nm)/BCP(10 nm)/Al(100 nm),
where S is a 1:1 blend of C60 and a sensitizer, were used to measure the sensitizer response.
Figure 4.9 shows representative EQE spectra from both the control (Figure 4.9a) and
sensitized devices (Figure 4.9b) with various thicknesses of C60 utilizing a sensitized layer
containing 1:1 C60:ZCl. Comparison of the EQE between the two series of devices reveal
there is clear spectroscopic signal attributable to the sensitizer between λ = 500 nm and
600 nm. Subtracting the EQE of the control device from the sensitized device yields the
EQE contribution directly from the sensitizer absorption, referred to here as ΔEQE.
Because the control device and sensitized device have the exact same structure (layer
thicknesses, composition, etc.) except for the sensitized layer, all changes in the EQE are
due to absorption by the sensitizer. Representative ΔEQE can be seen in Figure 4.9c. In
order to account for differences in the absorptivity of the sensitizers, ΔEQE is normalized
by the absorption of the sensitizer in the device, which was calculated via the transfer
matrix method, giving the internal quantum efficiency (IQE) of each sensitizer.

Figure 4.8: Schematic of devices utilized to measure the exciton diffusion length. The
thickness of the acceptor layer (A) was varied between 10 nm and 70 nm
ITO
MoO
3
10 nm
NPD 11 nm
C
60
x nm
C
60
:S (1:1) 20 nm
BCP 10 nm
Al
V
D A A:S
x 20 nm
80

4.3.4 Fitting the Diffusion Length
Because the D/A interface is quenching for excitons, the IQE serves to inform on the
steady-state population of excitons which reach the D/A interface relative to those which
were generated within the acceptor layer. The transport of excitons through the C 60 layer
can be described by the one-dimensional diffusion equation
𝜕𝑛
𝜕𝑡
= 𝐷 𝜕 2
𝑛 𝜕𝑥
2
−
𝑛 𝜏 (1)
400 500 600 700
0
10
20
30
40
50
EQE (%)
Wavelength (nm)
10 nm
15 nm
20 nm
30 nm
(a)
400 500 600 700
0
10
20
30
40
50
EQE (%)
Wavelength (nm)
10 nm
15 nm
20 nm
30 nm
(b)
500 600 700
0
5
10
15
20
EQE (%)
Wavelength (nm)
10 nm
15 nm
20 nm
30 nm
(c)
500 600 700
0
10
20
30
40
50
Sensitizer Abs. (%)
Wavelength (nm)
10 nm
15 nm
20 nm
30 nm
(d)

Figure 4.9: Representative EQE of the control (a) and devices sensitized with ZCl (b).
ΔEQE (c) between the sensitized and control devices. Sensitizer absorption within the device
calculated via transfer matrix approach (d) with spacer thicknesses of 10 nm, 15 nm, 20 nm,
and 30 nm.
81

where n is the exciton density, D is the exciton diffusivity, and τ is the excited state lifetime.
Solving the steady state diffusion equation
𝜕 2
𝑛 𝜕𝑥
2
=
𝑛 𝐷 𝜏 (2)
with the boundary conditions of
𝜕𝑛
𝜕𝑥
= 0 at x = 0 and n = 0 at x = ∞ yields the exciton density
profile given by
𝑛 = 𝐴 𝑒 −𝑥 /𝐿 𝐷 (3)
where A is a constant equal to the fraction of excitons generated in the sensitized layer
which reach the S/C60 interface. Fitting the decay of the ΔIQE with equation 3 yields the
diffusion length for the excited state in question.
4.3.5 Diffusion Through Neat C60 Films
Figure 4.10 shows the ΔIQE values for the devices with various thicknesses of neat
C60 containing both ZCl and ICl as sensitizers. In both cases the ΔIQE decays with
thickness as would be expected. Fits to the data with Equation 3 allow the extraction of
the exciton diffusion length for both singlets and triplets from the ZCl and ICl devices,
respectively. The singlet diffusion length is 45 ± 5 nm and the triplet diffusion length is
35 ± 5 nm. Additionally, the two sensitizers exhibit distinctly different exciton yields at
the C60/S interface. The ICl devices show that 65 ± 6 % of excitons leave the sensitized
layer while that number is only 34 ± 3 % for the ZCl devices.
These results have several important implications. First, in C60, the diffusion length
of singlets is greater than that of triplets. This means that once in the C 60 layer a larger
fraction of singlets are able to reach the D/A interface. This is in contrast with the exciton
82

yield data which show that more triplets are able to diffuse out of the mixed sensitized layer
into the neat layer. However, this may be due to other confounding factors. Based on these
results, it is advantageous to produce singlet excitons within a neat C60 layer. Additionally,
the long diffusion length observed for C60 is another explanation for its widespread success
seen in devices. The diffusion lengths measured here falls within the range observed by
other methods.
32, 38, 39

The decrease in exciton diffusion length for the triplet sensitizer is in contradiction with
previous reports which have found an increase in diffusion length for triplet states.
36, 40
In
these devcies, the triplet sensitizer was blended throughout the thickness of the donor or
acceptor material. The difference between these results could be due to the differences in
the diffusion lengths of the singlets and triplets of the particular materials used in these
studies. Additionally, it is possible that the charge separation efficiency in the device is

Figure 4.10: ΔIQE for devices using ZCl (▲) and ICl (●) as sensitizers through a neat
layer of C60. Solid lines are fits using equation 3 yielding diffusion lengths of 45 ± 5 nm
and 35 ± 5 nm for singlets and triplets, respectively.
83

different for singlets and triplets which is unaccounted for in the model used to fit the
exciton diffusion length.
4.3.6 Diffusion Through BCP:C60 Films
To understand how the transport of excitons is effected by fullerene concentration,
the same experiment was performed on devices containing a BCP:C 60 layer. Figure 4.11
shows ΔIQE values for the devices with various thicknesses of BCP:C60 containing both
ZCl and ICl as sensitizers. Fitting the data with equation 3 yields diffusion lengths of
18 ± 2 nm and 25 ± 3 nm for singlets and triplets, respectively. Again, the exciton yields
for the two sensitizers can be extracted as well. In this case both the singlets and triplets
demonstrate yields of 38 ± 3 %
Compared to the diffusion lengths measured for the pristine film of C60, both the singlet
and triplet diffusion lengths are shorter in the BCP:C 60 film. The decrease for the singlet,

Figure 4.11: ΔIQE for devices using ZCl (▲) and ICl (●) as sensitizers through a
layer of BCP:C60. Solid lines are fits using equation 3 yielding diffusion lengths of
18 ± 2 nm and 25 ± 3 nm for singlets and triplets, respectively.
84

from 45 ± 5 nm to 18 ± 2 nm, is quite substantial. A decline in the diffusion length by
more than half means that the diffusivity of excitons through a mixed film is substantially
reduced. This is consistent with previous results in which we studied the blocking effect
of mixed C60 films.
22
The triplet diffusion length also decreases from 35 ± 5 nm to
25 ± 3 nm. The difference in the magnitude of the decrease in exciton diffusion length
could be due to the differing mechanisms through which singlet and triplet excitons diffuse.
Singlets, which diffuse via FRET, rely on strong dipole-dipole interactions in order to
transfer energy over large distances. Previous work on the absorption of C 60 has shown
that dilution drastically effects the extinction coefficient, reducing the rate of energy
transfer.
22
On the other hand, triplets diffuse via Dexter energy transfer which is
independent of extinction and primarily occurs between nearest neighbors. It has been
shown that even in dilute films of BCP:C 60 there still exist percolation paths of fullerene
clusters.
41
Thus, triplet diffusion is less effected by dilution than singlet diffusion as
transport through the percolated network remains rapid.
4.4 Conclusions and Future Outlook
In conclusion, we have investigated the role of energy transfer and exciton diffusion in
OPVs. The sensitization scheme developed by Trinh et al. was expanded to include
multiple sensitizers, demonstrating its versatility and robust nature. This strategy enables
broad spectral coverage within a device without impacting VOC or FF. The sensitizers
function by absorbing photons which would not be otherwise absorbed and transferring
that energy to the host which serves to transport excitons to the D/A interface for charge
separation and then carries the charges out of the device. Energy sensitization is a valuable
method for enhancing the photocurrent collection in devices in the future.
85

Utilizing energy sensitization, it was also possible to probe the singlet and triplet diffusion
lengths of C60 in pristine and mixed BCP:C60 films. Through the fabrication of a series of
devices the diffusion lengths for individual excited states could be extracted. We found
that the singlet had a longer diffusion length than the triplet in the pristine C60 film.
However, in the BCP:C60 films the diffusion length of the singlet was significantly reduced
while the triplet was effected to a lesser degree. These changes are due to the
fundamentally different mechanisms through which singlets and triplets diffuse.
In order for there to be continued improvements in OPV efficiency, further studies to
understand the nature of energy transfer and exciton diffusion within devices will be
required. To ensure that devices absorb across the entire solar spectrum, it will be
necessary to utilize sensitization or energy transfer strategies like the one discussed above.
Methods such as this allow for a variety of complimentary absorbers to be employed in a
single device. Additionally, an increased understanding of the transport of excitons will
allow for intelligent device design where active layer thickness can be accurately predicted
based on previously determined diffusion lengths. In conclusion, energy transfer and
diffusion through organic materials underpins the ultimate efficiency attainable in devices.

86

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90

Chapter 5. Symmetry Breaking Charge Transfer
5.1 Abstract
The open circuit voltage in organic photovoltaics (OPVs) are substantially lower than that
observed in traditional inorganic photovoltaics with equivalent band gaps. This
significantly limits the overall power conversion efficiency of OPVs. In this chapter we
investigate a phenomena known as symmetry breaking charge transfer (SBCT) in a family
of zinc dipyrrin compounds and their application within devices. First, we study the
kinetics of symmetry breaking charge transfer through detailed measurements on the
photoluminescence quantum yield and lifetime of the compounds in various solvent
mixtures. The dielectric of the solvent mixture determines the equilibrium between the
charge transfer and singlet excited state by modulating the driving force for electron
transfer. Next we utilize a zinc chlorodipyrrin (ZCl) as an acceptor resulting in a substantial
increase in open circuit voltage compared to a C60 control. We trace the origin of this
voltage increase to the high energy charge transfer state formed in the ZCl devices. Then
the application of ZCl is expanded to include a wide variety of donor materials. Finally, a
comparative study of ZCl, C60, and Cl6BODIPY devices investigate the difference in
performance between the dimer-like ZCl and monomer-like Cl6BODIPY.
5.2 Kinetics of Symmetry Breaking Charge Transfer in Some Zinc Dipyrrins
5.2.1 Introduction
Electron transfer has been and remains an area of intense study in the fields of chemistry
and physics. The process enables a number of useful applications from catalysis to
photovoltaics. A seminal advance in the field was the development of a theoretical model
91

to describe outer sphere electron transfer by Rudolph Marcus for which he was awarded
the Nobel Prize.
1, 2
Marcus Theory allows for the calculation of the rate of electron transfer
(𝑘 𝑒𝑡
) between a donor, A, and acceptor, B, through Equation 1:
𝑘 𝑒𝑡
=
2𝜋 ħ
|𝐻 𝐴𝐵
|
2
1
√4𝜆 𝑘 𝐵 𝑇 𝑒𝑥𝑝 (−
( 𝜆 +∆𝐺 0
)
2
4𝜆 𝑘 𝐵 𝑇 ) (1)
where ħ is Plank’s constant, |𝐻 𝐴𝐵
| is the electronic coupling between the initial and final
states, 𝜆 is the reorganization energy, 𝑘 𝐵 is the Boltzmann constant, 𝑇 is temperature, and
∆𝐺 0
is the Gibbs free energy. The theory correctly describes that the rate of electron
transfer is based on both driving force (free energy) and reorganization energy.
Some canonical examples of energy diagrams for electron transfer based on Marcus Theory
are given in Figure 5.1. Figure 5.1a depicts electron transfer with positive ∆𝐺 0
,
Figure 5.1b depicts electron transfer with ∆𝐺 0
= 0, Figure 5.1c depicts electron transfer
with negative ∆𝐺 0
, and Figure 5.1d depicts electron transfer with negative ∆𝐺 0
in the
Marcus inverted region. Based on the relative positions of the two parabolas the rate of
electron transfer can be intuitively understood. In Figure 5.1a electron transfer requires
overcoming a significant 𝜆 and ∆𝐺 barrier thus the process is slow. In Figure 5.1b, the
electron transfer rate is dominated by the magnitude of 𝜆 with a barrier equal to 𝜆 /4. In
this case electron transfer can be quite fast. For Figure 5.1c, electron transfer is barrierless
and the rate is governed by the magnitude of ∆𝐺 . Under these circumstances, electron
transfer is ultrafast as there is significant driving force with no activation energy. The most
significant aspect of Marcus Theory is depicted in Figure 5.1d. In this case, despite the
fact that the driving force for electron transfer is quite significant, 𝜆 becomes increasingly
92

positive serving as a barrier. This means that even in systems with a large driving force,
the rate of electron transfer can be slow.
An extensive variety of materials with various structural moieties have been fabricated in
order to study electron transfer in covalently linked and self-assembled systems.
3, 4
This
has enabled extensive study on the impact of driving force, spatial separation, and
orientation on the rates of electron transfer. The results of these studies have been useful
in the rational design of materials for artificial photosynthetic and photovoltaic
applications. Of particular interest is developing an understanding of molecular parameters
which allow for efficient electron transfer with a small driving force.

Figure 5.1: Plots of potential energy surfaces outlining the basics of Marcus Theory.
(a) depicts electron transfer with reorganization energy 𝜆 and positive ∆𝐺 . (b) depicts
electron transfer with ∆𝐺 = 0. (c) depicts barierless electron transfer. (d) depicts the
Marcus inverted regime where increasingly negative ∆𝐺 decreases the rate of electron
transfer
93

A special case of electron transfer occurs in symmetric systems where both the donor and
acceptor are the same molecule. This phenomena is known as symmetry breaking charge
transfer (SBCT). As the two subunits involved in the charge transfer process are identical,
meaning ideally ∆𝐺 is equal to zero and the electron transfer rate is completely controlled
by 𝜆 . That makes these systems ideal to study the fundamental aspects of electron transfer
in molecular systems. The most widely explored example of this occurs in bianthryl,
however several other systems have been investigated as well. For example, Holman et al.
synthesized a series of perylenediimide dimers separated by various oligophenylene
bridges and found that electronic coupling was significantly larger for perylenediimide
dimers separated by an even number of phynelenes than those which were separated by an
odd number.
5
In another study, Giaimo et al. synthesized cofacial and linear
perylenediimide dimers.
6
They found that the cofacial dimer underwent SBCT on the
femtosecond timescale in toluene whereas the linear dimer underwent SBCT significantly
slower. These types of work helps to build an understanding of the geometries which are
advantageous for rapid electron transfer with minimal driving force.
In this section, we study SBCT in a zinc dipyrrin in various dielectric environments. We
find that the dielectric of the solvent determines the equilibrium between the charge transfer
state and the singlet. Fitting the excited state lifetime and photoluminescence quantum
yield allows us to determine the rates of electron transfer and recombination. Even at zero
driving force (∆𝐺 = 0), we find an extremely high electron transfer rate of 7 x 10
10
.

94

5.2.2 Materials Characterization
A family of zinc dipyrrin compounds were synthesized by Trinh et al. and found to undergo
symmetry breaking charge transfer in polar complexes.
7
The molecular structures and
solution phase absorption spectra of compounds zDIP1-4 are shown in Figure 5.2. The
compounds absorb strongly from λ = 400 to 550 nm with a maximum absorptivity greater
than 1 x 10
5
M
-1
cm
-1
.
The unique property of symmetry breaking charge transfer means the nature of the
molecular excited state is determined by the dielectric of the media. Figure 5.2 shows the
photoluminescence quantum yield as a function of solvent dielectric for zDIP1-4. All of
the dipyrrins exhibit significant photoluminescence quenching with increasing solvent
dielectric. This is indicative of the equilibrium between the singlet and charge transfer
state shifting towards the charge transfer state which is increasingly stabilized by the high
dielectric. To further study the detailed kinetics of this phenomena we decided to use

Figure 5.2: Molecular structures and solution phase absorption of zDIP1-4.
Photoluminescence quantum yield as a function of solvent dielectric for zDIP1-4.
zDIP4
zDIP3
zDIP2
zDIP1
95

zDIP2 as a probe because of its high quantum yield in cyclohexane allows for a wider
dynamic range to study the luminescence quenching.
5.2.3 Photoluminescence Quantum Yield and Lifetime Measurements
To investigate the impact of solvent dielectric on the photoluminescence quantum yield
and lifetime, a series of solutions with a set concentration of zDIP2 and various fractions
of cyclohexane and THF were prepared. The photoluminescence quantum yield and
lifetime are plotted in Figure 5.3 as a function of the dielectric constant of the resultant
solvent mixture (selected lifetime traces are shown in Figure 5.3b). The measured values
are given in Table 5.1. In cyclohexane the quantum yield of zDIP2 is quite high at
Φf = 0.66. On the other hand, in THF the quantum yield decreases to Φf < 0.01.
Correspondingly, the lifetime in cyclohexane is 4.8 ns decreasing to 2.6 ns in THF. As the
dielectric of the solvent mixture increases, both the quantum yield and lifetime decrease.
However, the decrease in lifetime does not track with the decrease in quantum yield as
would be predicted for simple emission kinetics where there exists a single state which

Figure 5.3: Photoluminescence quantum yield and lifetime for zDIP2 in solvent blends
of cyclohexane and THF with different dielectric constants. Photoluminescence
lifetime traces for solutions with selected dielectric constants.
96

decays either radiatively or nonradiatively. Thus a more complicated kinetic scheme must
be developed to account for the observed behavior.
5.2.4 Kinetic Model
Figure 5.4 shows the state diagram and corresponding rates for simple emission kinetics
and a modified kinetic scheme which includes the formation of a charge transfer state. The
rate equation for simple emission is given in Equation 2:
𝑑 [𝑆 1
]
𝑑𝑡
= −𝑘 𝑓 [𝑆 1
] − 𝑘 𝑛𝑟
[𝑆 1
] (2)
where [𝑆 1
] is the concentration of singlet excited sates, 𝑘 𝑓 is the rate of fluorescence decay,
and 𝑘 𝑛𝑟
is the rate of nonradiative decay. Solving Equation 2 for 𝜏 and 𝛷 gives:
Table 5.1: Experimental and calculated photoluminescence quantum yield and lifetime
of zDIP2 in various solvent environments
Solvent Dielectric QY Exp. LT Exp. (ns) QY Fit LT Fit. (ns)
2.02 0.66 4.8 -- --
2.57 0.54 4.7 0.57 4.712
3.12 0.37 4.2 0.37 4.279
3.67 0.2 4.1 0.20 4.111
4.22 0.12 4 0.12 4.007
4.77 0.063 3.7 0.06 3.682
5.32 0.037 3.3 0.037 3.310
5.87 0.022 3.2 0.022 3.197
6.42 0.015 3.2 0.015 3.180
6.97 0.011 3.1 0.011 3.131
7.52 0.007 2.9 0.007 2.917

97

1
𝜏 = 𝑘 𝑓 + 𝑘 𝑛𝑟
(3)
𝛷 𝑓 =
𝑘 𝑓 𝑘 𝑓 +𝑘 𝑛𝑟
= 𝑘 𝑓 𝜏 (4)
Thus for this scheme 𝜏 and 𝛷 𝑓 are directly proportional. The deviation from this behavior
requires the introduction of a second state which is shown in the modified kinetic scheme.
8

The rate equation for this scheme is give in Equations 5 and 6:
𝑑 [𝑆 1
]
𝑑𝑡
= −𝑘 𝑓 [𝑆 1
] − 𝑘 𝑛𝑟
[𝑆 1
] − 𝑘 𝑒𝑡
[𝑆 1
] + 𝑘 𝑏𝑒𝑡
[𝐶𝑇 ] (5)

𝑑 [𝐶𝑇 ]
𝑑𝑡
= −𝑘 𝑐𝑟
[𝐶𝑇 ] − 𝑘 𝑏𝑒𝑡
[𝐶𝑇 ] + 𝑘 𝑒𝑡
[𝑆 1
] (6)
where [𝐶𝑇 ] is the concentration of the CT state, 𝑘 𝑏𝑒𝑡
is the rate of back electron transfer,
and 𝑘 𝑐𝑟
is the rate of charge recombination from the charge transfer state. Solving
Equations 5 and 6 for 𝜏 and 𝛷 gives

Figure 5.4: Simplified Jablonski diagrams outlining the kinetic schemes for simple
emission and emission involving a CT state. The simple scheme involves radiative and
non-radiative decay from the singlet. The second scheme involves a charge transfer
state in equilibrium with the singlet state. The rate of forward and reverse electron
transfer determines the equilibrium between the states.
S
1
k
F
k
NR
Energy
S
1
k
ET
k
BET
k
F
k
NR
k
CR
Energy
CT
98

1
𝜏 1,2
=
1
2
(𝑘 𝑓 + 𝑘 𝑛𝑟
+ 𝑘 𝑒𝑡
+ 𝑘 𝑏𝑒𝑡
+ 𝑘 𝑐𝑟
∓
√
( 𝑘 𝑏𝑒𝑡
+ 𝑘 𝑐𝑟
− 𝑘 𝑓 − 𝑘 𝑛𝑟
− 𝑘 𝑒𝑡
)
2
− 4𝑘 𝑒𝑡
𝑘 𝑏𝑒𝑡
) (7)
𝛷 𝑓 =
𝑘 𝑓 ( 𝑘 𝑏𝑒𝑡
+𝑘 𝑐𝑟
)
( 𝑘 𝑓 +𝑘 𝑛𝑟
) ( 𝑘 𝑏𝑒𝑡
+𝑘 𝑐𝑟
) +𝑘 𝑒𝑡
𝑘 𝑐𝑟
(8)
Based on these equations it is possible to accurately represent the behavior exhibited by
zDIP2.
5.2.5 Fitting the Photoluminescence Quantum Yield and Lifetime Measurements
Utilizing Equations 7 and 8 the photoluminescence quantum yield and lifetime data can be
fit in order to extract the rates of forward and back electron transfer (ket and kbet).
Figure 5.5 shows the fits to the experimental data and the values extracted for ket and kbet.

Figure 5.5: Comparison of experimental and calculated photoluminescence quantum
yield and lifetime for zDIP2 in solvent blends of cyclohexane and THF with different
dielectric constants. Rate constants extracted from fits to the photoluminescence
quantum yield and lifetime data (filled symbols). Rate constants extacted from transient
absorption (open symbols)
99

Table 5.2
contains the values for kf, knr, ket, kbet, and kcr used to obtain the fits shown in Figure 5.5.
kf and knr were determined to be 0.14 ns
-1
and 0.077 ns
-1
, respectively in cyclohexane and
assumed to be constant across all dielectric environments. kcr is slightly effected by solvent
dielectric increasing from 0.22 ns
-1
in cyclohexane to 0.345 ns
-1
in THF. ket increases
significantly with dielectric constant from 25 ns
-1
in cyclohexane to 420 ns
-1
in THF.
Conversely, kbet decreases with dielectric from 160 ns
-1
in cyclohexane to 7 ns
-1
in THF.
To assess the validity of the fitted data, the extracted rate constants were compared to those
measured previously by transient absorption.
7
At similar dielectric constants, the values
for ket and kbet of 116 ns
-1
and 64 ns
-1
, respectively, determined by transient absorption
agree quite well with the fitted values of 96 ns
-1
and 50 ns
-1
calculated here. The data points
Table 5.2: Rate constants for zDIP2 determined from analysis of lifetime and quantum
yield data.
Solvent
Dielectric kf (ns
-1
) knr (ns
-1
) ket (ns
-1
) kbet (ns
-1
) kcr (ns
-1
) ΔGet (meV)
2.02 0.14 0.071 -- -- -- --
2.57 0.14 0.071 25 160 0.22 47
3.12 0.14 0.071 50 80 0.27 12
3.67 0.14 0.071 96 50 0.26 -16
4.22 0.14 0.071 140 38 0.26 -33
4.77 0.14 0.071 180 25 0.28 -51
5.32 0.14 0.071 230 20 0.31 -63
5.87 0.14 0.071 290 15 0.318 -76
6.42 0.14 0.071 350 12 0.318 -87
6.97 0.14 0.071 400 10 0.322 -95
7.52 0.14 0.071 420 7 0.345 -105

100

from transient absorption are indicated by open symbols in Figure 5.5b and their close
alignment with the fitted values confirms the validity of the fit.
5.2.6 Extracting the Driving Force
Using the values for ket and kbet determined by fitting, it is possible to extract Gibb’s free
energy for electron transfer (∆𝐺 𝑒𝑡
) by
∆𝐺 𝑒𝑡
= −𝑘 𝐵 𝑇𝑙𝑛 (
𝑘 𝑒𝑡
𝑘 𝑏𝑒𝑡
) (9)
∆𝐺 𝑒𝑡
was calculated for each point where there are values for ket and kbet and is plotted in
Figure 5.6 with the values tabulated in Table 5.2. The Gibb’s free energy reflects the
offset in the energy of the singlet and the CT state. In cyclohexane, the CT state is 47 meV
uphill from the singlet state, but as the dielectric increases the CT is stabilized and even
becomes downhill from the singlet. In THF, the CT state is 105 meV below the singlet.
∆𝐺 𝑒𝑡
is equal to zero at a dielectric constant of approximately 3.4. At this point there is no

Figure 5.6: Gibb’s free energy as a function of solvent dielectric calculated from
electron transfer rates. As the solvent dielectric increases, electron transfer becomes
exergonic
101

driving force for electron transfer and the rate of forward and back electron transfer are
equal. Even with no driving force the rate of electron transfer is still ultrafast at
approximately 70 ns
-1
.
The results presented in this section represent a significant increase in the understanding
of the process of SBCT in zinc dipyrrins. Through carefully measuring the photophyscial
properties in a diverse arrange of solvent blends, it is possible to investigate SBCT in
variety of dielectric conditions. By modeling the details of the excited state kinetics of this
system there has been a great deal of information revealed about the nature of electron
transfer. In low dielectric media, electron transfer is retarded due to its endergonic nature.
However, even in a moderate dielectric, equivalent to that measured for organic solids, the
singlet and CT state are equally populated. Ultrafast electron transfer with no driving force
is quite interesting as it allows for the rapid separation of a strongly bound Frenkel exciton
with a minimal amount of energetic loss. Based on these findings, the synthesis of a
material which undergoes SBCT with appropriate properties to function in an OPV would
be of great interest.
5.3 Symmetry Breaking Charge Transfer in a Zinc Chlorodipyrrin Acceptor for High
Open Circuit Voltage Organic Photovoltaics
5.3.1 Introduction
Significant advances have been made in the development of organic photovoltaics (OPVs)
as an emerging source of renewable energy, with reported power conversion efficiencies
in excess of 10 %.
9,10
Nevertheless, the open-circuit voltages (VOC) of organic devices are
generally low and serve as a substantial limit to overall device performance. The vast
102

majority of OPVs reported have VOCs below 1 V,
11
and even the best performing tandem
devices often contain subcells which produce nearly identical voltages.
9, 10, 12
Ultimately,
VOC is restricted by the materials chosen for the donor and acceptor. While substantial
progress has been made in formulating numerous donors which yield high performance,
the corresponding diversity of efficient acceptors is lacking. Fullerenes are without doubt
the most ubiquitous acceptor molecules employed in OPVs and they have many desirable
traits such as their high electron mobility and ability to form efficient heterojunctions with
a wide variety of donor materials.
13
However, despite their widespread use, the reported
VOC for devices with fullerene acceptors are typically between 0.6-0.8 V,
11
with selected
systems producing VOC up to 1.15 V.
14, 15
In an attempt to circumvent these issues, a wide
variety of non-fullerene acceptor systems have been developed.
16
The highest efficiencies
have been achieved utilizing compounds based on perylene diimides (PDIs)
17
and
subphthalocyanines (SubPcs).
18
Large VOCs have been reported for devices with non-
fullerene acceptors. For example, Sullivan et al.
19
reported a SubPc/Cl6SubPc device with
a VOC of 1.33 V and Peng et al.
20
utilized a diketopyrrolopyrrole (DPP) acceptor which
achieved a VOC of 1.19 V. The high VOC values for these devices are the result of a
substantial energy offset between the donor-HOMO and the acceptor-LUMO, limiting
light collection predominantly to the blue part of the solar spectrum.
In a system such as an OPV, where charge separation occurs through the transfer of an
electron from the donor to the acceptor, VOC is thermodynamically limited by the energetic
offset of the HOMO of the donor and the LUMO of the acceptor (EDA). EDA has been
argued to be a rather poor predictor of VOC, and it has been shown through spectroscopic
and temperature dependent techniques that the upper bound for VOC is the energy of the
103

ground-state to intermolecular charge transfer (CT) state transition (ECT) at the
donor/acceptor (D/A) interface.
21-24
ECT is, however, limited by EDA, because in the CT
transition an electron is promoted directly from the HOMO of the donor to LUMO of
acceptor. ECT correlates linearly with qVOC with typical energetic losses around 0.6 eV due
to recombination. Additional losses between the energy of excitons in the strongly
absorbing neat materials and the VOC of a device originate from the offset required to drive
formation of the CT state at the D/A interface.
Currently, the largest VOC values are found for devices with the largest EDA. In order to
further increase VOC without compromising other important device parameters, i.e. short
circuit current, JSC, or fill factor, FF, it would be desirable to minimize the driving force
necessary to form the CT state and to reduce the recombination losses to VOC. Symmetry
breaking charge transfer (SBCT) serves as a potential strategy towards this goal. SBCT
involves closely associated pairs of identical molecules or compounds composed of two or
more identical parts, such as covalently bonded organic dimers or metal complexes with
two or more identical ligands. SBCT occurs when an exciton formed initially on one
molecule or ligand undergoes intramolecular charge transfer (ICT), leading to a state in
which a hole and an electron are localized on different molecules or ligands, with very little
coupling between the hole and electron.
3, 25
SBCT has been observed in molecular dimers
such as 9-9’ bianthryl and in other systems, where excitation of the dimer results in an ICT
state with an electron on one subunit and a hole on the other.
3, 25-27
Thus, SBCT is an
attractive strategy to achieve charge separation with a negligible driving force, directional
specificity, and a greatly retarded back-recombination rate. These properties would be
beneficial in OPVs, where a lower driving force for charge separation ensures a smaller
104

energy loss due to electron transfer, directionality ensures electrons and holes are
positioned towards the appropriate electrode, and retarded back-recombination ensures a
high charge separation yield. A simple schematic view of how charge transfer and charge
separation involving SBCT could occur at the D/A interface of an OPV is illustrated in
Figure 5.7. SBCT can occur on ultrafast timescales and thus can be kinetically competitive
with traditional D/A charge transfer processes, allowing it to participate in the process of
charge generation. For example, subpicosecond charge transfer has been observed in PDI
dimers.
6
Trinh et al.’s recent study of SBCT in a series of zinc dipyrrins revealed charge
transfer between 1 ps and 14 ps.
7
The potential advantages of utilizing SBCT to induce
charge separation, and its unexplored application in OPVs, make it an attractive subject for
further investigation.

Figure 5.7: (a) Schematic representation of the charge generation process in an OPV
with a conventional donor and a Symmetry Breaking Charge transfer Acceptor. First,
an excited state is created through the absorption of a photon. Second, symmetry
breaking charge transfer (SBCT) occurs on the molecule generating and intramolecular
charge transfer (ICT) state. Finally, charge transfer (CT) to the donor results in an
oxidized donor and reduced acceptor ligand separated by a neutral acceptor ligand. If
the excitation takes place in the bulk of the SBCT material, away from the D/A
interface, the formed exciton must diffuse to the interface to charge separate. (b)
Molecular structures and extinction spectra for DBP, ZCl, and C60.
105

In this section, we study the photophysical and electronic properties of a zinc
chlorodipyrrin (ZCl) and utilize ZCl as an acceptor in planar heterojunction OPVs.
Transient absorption (TA) studies in a variety of solvents reveal that ZCl undergoes SBCT,
evidenced by changes in the excited state dynamics in high dielectric solvents. We probe
the LUMO energy of ZCl and C60 by inverse photoelectron spectroscopy (IPS), revealing
that ZCl has a similar LUMO energy as C60 (-4.1 eV) indicating its ability to function as
an acceptor. When used in an OPV with tetraphenyldibenzoperyflanthrene (DBP) as
donor, ZCl gives markedly higher VOC than the corresponding OPV with C60.
Measurements of ECT from Fourier-transform photocurrent spectroscopy and OPV
electroluminescence show that C60 forms a CT state of 1.45 ± 0.05 eV, while ZCl forms a
higher energy CT state at 1.70 ± 0.05 eV with the same donor. This results in a large VOC
of 1.33 V for DB/ZCl devices in contrast with the VOC of 0.88 V for DBP/C60. Comparison
of ECT and VOC show that the energetic losses due to recombination are substantially
reduced in ZCl devices. These findings demonstrate exciting possibilities for this class of
metallo-dipyrrins as acceptors, and the use of SBCT in OPVs.
5.3.2 Optical Characterization
The absorption spectra in the solid state and molecular structure of ZCl, C 60, and DBP are
given in Figure 5.7. The absorption of DBP is particularly intense as it orients itself largely
parallel to the substrate allowing for strong coupling between the molecular transition
dipole and incident electric field, resulting in a maximum extinction greater than
2.510
5
cm
-1
at 610 nm.
28
Based on its absorption onset of 650 nm, we estimate an optical
gap (Eg) of 1.9 eV for DBP. ZCl exhibits characteristic dypirrin absorption with a
maximum extinction of 4 10
5
cm
-1
at 545 nm. Compared to C60, ZCl absorbs with almost
106

an order of magnitude greater extinction between 500 nm and 600 nm. Eg for C60 has been
determined previously to be 1.85 eV.
29

5.3.3 Characterization of the Excited State in Various Environments
Solvent dielectric dependent photophysics is a hallmark of SBCT, where higher dielectric
solvents provide electrostatic stabilization of the resultant CT state. Thus the polarity of
the medium determines the nature of the excited state: i.e. a ligand localized excited state
in nonpolar media and an ICT state in polar media. Recently, this behavior has been
explored in a series of homoleptic zinc dipyrrin complexes which undergo SBCT in high
dielectric media.
7
Similar to these complexes, ZCl photoluminesces from a single dipyrrin
ligand in low dielectric solvents and from an ICT state in high dielectric media, albeit
weakly, exemplified by a decrease in quantum yield with increasing solvent dielectric
constant (Figure 5.8).
Our interpretation of the quantum yield measurements were corroborated with transient
absorption (TA) measurements of ZCl in cyclohexane (non-polar), toluene (weakly-polar),

Figure 5.8: Photoluminescence quantum yield and lifetime for ZCl in solvents with
different dielectric constants.
107

and acetonitrile (polar), media. The TA of ZCl in cyclohexane (shown in Figure 5.9a)
shows that the depopulation of the ground state (ground state bleach between 450-550 nm)
excited by 520 nm radiation leads to the appearance of the stimulated emission (SE)
between 530 and 600 nm
7
and excited state absorption (ESA) at 360 nm from a localized
excited state S1. At later times, this localized excited state S1 relaxes to the ground state
with an excited state lifetime of ~2.5 ns.
30
Similar to cyclohexane, in a weakly polar
medium such as toluene (Figure 5.9b), SE and ESA from S1 state appear immediately after
the excitation of ZCl. However, within 6 ps, the SE and ESA bands start to disappear with
concurrent rising bands at 415 nm and 545 nm. These new bands are assigned to the ICT
state based on similar observations on a series of homoleptic zinc dipyrrin complexes in

Figure 5.9: Femtosecond transient absorption of ZCl in cyclohexane (CH) (a), toluene
(b), acetonitrile (MeCN) (c) and PMMA (d) at initial delays. Excitation pump fluence
of 15 µJ/cm
2
were used for (a), (b), and (c) and 45 µJ/cm
2
was used for (c). The red
arrows highlight the change in the transient spectrum.
108

high dielectric solvents.
7
As time increases, this ICT state decays to form a T1 state with a
time constant of ~ 550 ps (long time data with global analysis is shown in supporting
information, Figure S2, S4). In highly polar solvent like acetonitrile, the TA measurements
(Figure 5.9c) show a faster rate (~1.5 ps) of ICT state formation because of higher
stabilization of the ICT state in acetonitrile compared to toluene.
To examine whether ICT occurs in the solid state, TA measurements were performed on a
film of ZCl dispersed in PMMA. Similar to toluene, the TA measurements (Figure 5.9d)
show the evolution of similar ICT states (rising bands at 415 nm and 545 nm) with
simultaneous decay of SE and ESA from S1 state. Interestingly, the generation rate of ICT
state is faster (~ 0.7 ps) but the amplitude of the bands at 415 nm and 545 nm are lower in
PMMA. The lower production is likely due to a smaller stabilization of the ICT state via
solvation due to restricted reorientation in the PMMA matrix. The fast generation rate (0.7-
6 ps) of the ICT state in PMMA matrix ( r ~ 3.5) and toluene ( r = 2.38) show that moderate
polarity, even without solvent reorganization, is sufficient to induce SBCT resulting in the
formation of an ICT state. The presence of this ICT state within a device is expected to
markedly affect charge transfer and separation at the D/A interface and thus VOC.
5.3.4 Frontier Energy Level Characterization
We measured the occupied and unoccupied electronic states of ZCl and compared them to
those of C60 using ultraviolet photoelectron spectroscopy (UPS) and inverse photoelectron
spectroscopy (IPES), respectively. The left side of Figure 5.10 contains the valence band
(VB) and the conduction band (CB) spectra measured on 3nm films of C 60 and ZCl
deposited on an ITO substrate. These experimental spectra are all referenced with respect
to the vacuum level (VL). The experimental electronic structure can be directly interpreted
109

using the density of states calculated for molecular C60 and ZCl, and shown in the right
side of Figure 5.10. From the VB spectra, a linear extrapolation of the HOMO features to
the background indicates HOMO onsets for C60 and ZCl at -6.0 ± 0.1 eV and -6.4 ± 0.1 eV,
respectively. A similar procedure using the CB spectra indicates that the LUMO onsets of
ZCl and C60 are found at -4.1 ± 0.1 eV and -4.2 ± 0.1 eV, respectively. The similarity of
the LUMO values are in conflict with the trend in the frontier molecular orbitals derived
from the reduction potentials of these materials measured previously.
30
This discrepancy
could be due to stabilization in the solid state due to intermolecular interactions.
31

Additionally, the values for the LUMO of C60 calculated by the correlation between
electrochemistry and IPS
32
deviate significantly from what has been measured
independently by IPS
33
which agree with the values measured here. The IPS values are the
most relevant as they measure the electron affinity of in the solid state, the same

Figure 5.10: Left: Valence and conduction band edges of thick molecular films
deposited on a DBP film. The zero of energy is the measured vacuum level of the initial
DBP film. Right: Position of molecular states and resulting density of states (DOS)
calculated for C60 and ZCl.
110

environment seen in devices, while the electrochemistry was performed in solution.
Because the LUMOs of the two acceptors are within 100 meV in energy, we expect ZCl to
function as an acceptor similar to C60.
5.3.5 Device Characterization
To study the performance of ZCl in devices, vapor deposited planar-heterojunction OPVs
were fabricated with the structure
ITO/MoO3(10nm)/Donor/Acceptor/BCP(10nm)/Al(100nm), where the donor is DBP and
the acceptor is either ZCl or C60. The thickness of the donor layer was 20 nm and the
thickness of the acceptor layer was 40 nm and 20 nm for devices containing C60 and ZCl,

Figure 5.11: Device architecture (a), illuminated I-V (b), dark I-V (c), and EQE curves
(d) for the devices described in the text. Thickness = 20 nm for DBP, 40 nm for C60,
and 20 nm for ZCl.
111

respectively. The device architectures, illuminated current-voltage (I-V), dark current, and
external quantum efficiency (EQE) of the OPVs are given in Figure 5.11 and relevant
parameters are given in Table 5.3. The DBP/ZCl device gives a power conversion
efficiency (PCE) of 1.4 % with JSC of 2.4 mA/cm
2
, VOC of 1.33 V, and FF of 0.42. The
DBP/C60 device gives a PCE of 3.6 % with JSC of 6.2 mA/cm
2
, VOC of 0.88 V, and FF of
0.68 in agreement with previous results.
28, 34
The JSC for the ZCl devices are lower than
the C60 analogue due to reduced absorption between 350 nm and 500 nm which is reflected
in the EQE. Optical modeling using the transfer matrix formalism
35
reveal that both ZCl
and DBP contribute to the photocurrent (Figure 5.12) with an internal quantum efficiency
(IQE) of approximately 30 %, consistent with the estimated exciton diffusion length of
7 nm. However, the VOC for the ZCl device is significantly larger. The increase in VOC is
also reflected in a significant reduction in dark current. Nevertheless, the PCE of the ZCl
devices is less than that achieved with C60. It is important to note that the goal of the
present study is to determine the feasibility of using SBCT to enhance VOC. As such,
efficient broadband light collection, needed to achieve high PCE, was not incorporated into
the devices reported here. In order to achieve a high PCE for these SBCT based OPVs, the
lack of light absorption between 350 and 550 nm can be corrected by sensitization
30, 36
and
the EQE values raised by shifting to an optimized bulk heterojunction, planar-mixed
heterojunction, or tandem structure.
37-39

Table 5.3: Summary of DBP/ZCl and DBP/C60 Device Performance Characteristics
Device
J SC
(mA/cm
2
)
VOC
(V)
FF
η
(%)
DBP(20 nm)/ZCl(20 nm) 2.4 1.33 0.42 1.4
DBP(20 nm)/C60(40 nm) 6.2 0.88 0.68 3.6

112

5.3.6 Charge Transfer State Characterization
To understand the difference in VOC seen between the ZCl and C60 devices, it is necessary
to measure the parameters which govern VOC. Kinetic,
40
temperature dependent,
21
and
spectroscopic
22
treatments of VOC exist and each provide an understanding of the physics
governing the generation of photovoltage in organic heterojunction systems. The
temperature dependent and spectroscopic treatments are best suited to experiment as they
contain parameters which can be independently measured. Through a detailed balance
approach, ECT can be related to VOC by Equation 10:
22

𝑉 𝑂𝐶
=
1
𝑞 (𝐸 𝐶𝑇
+ 𝑘𝑇 ln (
𝐽 𝑆𝐶
ℎ
3
𝑐 2
𝑓𝑞 2𝜋 ( 𝐸 𝐶𝑇
−𝜆 )
) + 𝑘𝑇 ln( 𝐸𝑄𝐸 𝐸𝐿
) ) (10)
where q is the elementary charge, k is the Boltzmann constant, T is temperature, h is Plank’s
constant, c is the speed of light, f is proportional the CT state absorption, λ is the
reorganization energy and EQEEL is the electroluminescence quantum efficiency of the CT
state. The second and third terms on the right side of Equation 1 correspond to the radiative

Figure 5.12: Comparison of simulated and measured EQE for DBP/ZCl device
illustrating the contribution from both DBP and ZCl (a). Calculated absorption and IQE
for the DBP/ZCl device.
113

(ΔVrad) and nonradiative (ΔVnonrad) recombination voltage losses, respectively.
22
This
model has been shown to accurately predict the VOC of numerous OPVs containing a
variety of donors and fullerene acceptors.
21-23, 41
Using this framework, we will
characterize the CT state of both ZCl and C60 based devices and use the results to
understand their respective VOCs.
EQE spectra, measured by Fourier-transform photocurrent spectroscopy (FTPS),
42
can be
seen in Figure 5.13a. ECT, f, and λ for the devices with a DBP donor were determined from
fitting the FTPS data to Equation 11:
22

𝐸𝑄𝐸 ∝
𝑓 𝐸 √4𝜋𝜆𝑘𝑇 𝑒 (
−( 𝐸 𝐶𝑇
+𝜆 −𝐸 )
2
4𝜆𝑘𝑇 )
(11)
where geometrical reorganization, parameterized by λ, is assumed to be the origin of the
Gaussian lineshape of the CT absorption band. In the DBP/C60 device ECT is
1.45 ± 0.05 eV, λ is 0.242 eV and f is 2.8410
-4
eV
2
. The offset between the ECT and qVoc
is 0.57 eV, ΔVrad is 0.23 eV, and ΔVnonrad is 0.34 eV. These results are in good agreement
with previous measurements in other systems for both polymer/fullerene and small-
molecule/fullerene devices.
22, 23, 41
In contrast, inspection of the FTPS data for the
DBP/ZCl bilayer device exhibit no clear evidence of CT state absorption. This behavior
could be indicative of extremely weak coupling between the donor and acceptor or,
alternatively, the CT state energy could lie close to the singlet absorption of the donor
which is obscuring its detection. To increase the CT state absorption strength, allowing
detection, we increased the interfacial D/A area by fabricating vacuum deposited BHJs
with the structure ITO/MoO3(10nm)/ DBP:ZCl(40nm)/BCP(10nm)/Al(100nm). There is
114

visible enhancement in the FTPS between 1.6 - 1.8 eV which is attributed to ground state
CT absorption, however the signal is still too weak to perform a reliable lineshape fit.
To support the CT absorption measurements, electroluminescence (EL) measurements
43

were performed on a DBP/ZCl bilayer, DBP:ZCl BHJ, and neat DBP and are shown in
Figure 5.13b. Both the bilayer and BHJ emission display a low energy feature which is
not present in neat DBP. Although this emission is in the same region as the ZCl triplet
( max = 1.74 eV and FWHM = 0.05 eV)
25
, the substantially broader linewidth and
redshifted intensity maximum relative to the ZCl triplet and DBP emission spectra lead us
to attribute it to the CT. Because the bilayer and BHJ emission spectra display features
from multiple luminescent states, the spectra were fitted with multiple Gaussian peaks.
The bilayer EL exhibits contributions from DBP emission and a broad low energy band
attributed to the CT state. The BHJ EL reveals a much larger contribution from the CT

Figure 5.13: (a) FTPS spectra for DBP/ZCl bilayer, DBP:ZCl blend, and DBP/C 60
bilayer devices. The lowest energy transition in the spectra for DBP:ZCl blend and
DBP/C60 bilayer were fit with Eq. 2. (b) EL spectra for neat DBP, DBP/ZCl bilayer
and DBP:ZCl blend. The lowest energy transition in the spectra for DBP:ZCl blend and
DBP/ZCl bilayer were fit with Eq. 3.
115

state with a weaker band attributed to DBP. Fitting the lowest energy emitting state in the
EL to Equation 12, analogous to that of absorption,
22

𝐼 𝐹 𝐸 ∝
𝑓 √4𝜋𝜆𝑘𝑇 𝑒 (
−( 𝐸 𝐶𝑇
−𝜆 −𝐸 )
2
4𝜆𝑘𝑇 )
(12)
where IF is the emission intensity, we extract ECTs for the bilayer and BHJ of
1.70 ± 0.05 eV and 1.77 ± 0.03 eV, respectively. The CT state measurements corroborate
the observed device behavior where ZCl exhibits a larger VOC than C60.
The 250 meV increase in ECT for the DBP/ZCl devices compared to DBP/C60 could be due
to a number of factors. First, morphological differences between the acceptor layers in the
DBP/C60 and DBP/ZCl devices could modify their interface with DBP resulting in different
donor-acceptor interactions. Grazing incidence X-ray diffraction measurements indicate
that vapor deposited C60 is polycrystalline while ZCl is amorphous.
30
It has been shown in
polymer:fullerene BHJs and bilayer small-molecule/fullerene devices that increased
crystallinity of the active layer correlates with a shift in ECT to lower energies, in agreement
with our findings for C60 and ZCl.
41, 44
These observations could be the result of increased
delocalization of the CT state in a crystalline environment, a phenomena which has been
suggested as a crucial requirement for high efficiency OPVs.
45
Differing molecular
orientation will similarly lead to changes in the electronic coupling between the donor and
acceptor as different conformations may exist at the donor/acceptor interface. This effect
has been invoked to explain differences in VOC, and therefore ECT, seen in various
systems.
46-49
Orientation dependence of electronic coupling between the donor and
acceptor has also been observed in computational studies on pentacene/C 60,
50
ZnPc/C60,
48

and Sq/C60.
51
Additionally, it has been suggested that steric effects at the D/A interface
116

can increase the energy of the CT state by increasing separation between the donor and
acceptor, resulting in a higher VOC.
52
However, recent studies suggest that the steric
properties of the donor and/or acceptor may not be a strong contributor to the VOC.
41
We
hypothesize that the increase in ECT is due to weak electronic coupling between donor and
acceptor seen in the ZCl devices caused by incorporation of SBCT into the charge
separation process. We propose that initial charge transfer occurs via SBCT and
subsequent hole transfer results in an oxidized DBP and reduced dipyrrin ligand separated
by a neutral dipyrrin ligand (see Figure 5.7).
In conjunction with the increase in CT state energy, charge separation in the ZCl devices
significantly reduces the energetic losses due to recombination from the CT state.
Figure 5.14 shows VOC as a function of ECT for a variety of material systems which exhibits
a linear relation with a slope of 1 and an intercept of -0.6 eV. These values are taken from

Figure 5.14: VOC vs. ECT for a variety of small molecule/fullerene and polymer:fullerene
OPVs. Lines at VOC = ECT - 0.6 ± 0.1 are guides to the eye. Values taken from
Vandewal et al.,
44,53-55
Piersimoni et al.,
56
Ko et al.,
57
Hoke et al.,
15
Wang et al.,
58

Graham et al.,
41
Tietze et al.,
59
and this work.
117

Vandewal et al.,
44, 53-55
Piersimoni et al.,
56
Ko et al.,
57
Hoke et al.,
15
Wang et al.,
58
Graham
et al.,
41
Tietze et al.,
59
and this work. Lines with an intercept of -0.6 ± 0.1 eV are also
added to show the majority of systems fall within close proximity of this relationship. An
energy difference between ECT and qVOC of 0.37 eV is observed for the DBP/ZCl device,
which falls significantly outside the trend seen in all other systems.
The smaller energetic loss can be attributed to a change in the processes which govern
recombination losses to VOC. These losses are described above in terms two and three of
Equation 10, which are catagorized as radiative and non-radiative, respectively. Due to the
logarithmic dependences of these voltage losses, parameters which can only vary by less
than an order of magnitude before becoming unphysical, such as JSC, ECT, and λ, have little
effect on their size. This means the bulk the bulk of the change in recombination losses
obsrved between material systems are attributable to differences in f and EQEEL. A weakly
coupled CT state would result in a reduction in f and a corresponding decrease in
recombination. Previous measurements of various devices have shown values for f
between 10
-3
to 10
-6
eV
2
,
22, 41
indicative of the wide range of coupling strengths which can
occur in these systems. Correspondingly, an increase in EQEEL will also reduce non-
radiative recombination and values on the order of 10
-6
to 10
-9
have been recorded for other
systems.
22
In order to explain the reduction in recombination losses observed for the
DBP/ZCl device, 0.37 eV as opposed to the typical 0.6 eV, there must be a factor of 10
4

change in f and EQEEL. Due to the combination of weak CT absoption and emission
observed for the bilayer device, we conclude the majority of the change in recombination
loss is due to a reduction in f. Overall, the recombination losses of 0.37 eV measured in
the ZCl devices is equivalent to those seen in high efficiency Si and GaAs based devices.
60

118

The increase in ECT and decrease in CT state extinction also fundamentally effect the
maximum efficiency attainable in an OPV. A thermodynamic description of the efficiency
limit for OPVs accounting for their excitonic nature has been formulated based on a
modified Shockley-Queisser analysis,
24
which describes the impact of E (the energy
difference between the lowest energy singlet and ECT) and the CT state absorption ( αCT) on
the highest possible efficiency for a given junction. The study concludes that at the two
extremes, when ΔE or αCT = 0, the behaviour of OPVs will mimic what is seen for inorganic
single-juncton devcies and traditional Shockley-Queisser analysis
61
will apply. The results
presented here represent a significant step in this direction as both ΔE and αCT have been
reduced. Further modification and optimization of the donor and acceptor energy levels in
conjunction with the use of SBCT to enable efficient charge transfer with ΔE close to zero
and substantially reduced coupling is an attractive strategy to bypass the fundamental
limitations imposed on OPV performance.
5.3.7 Conclusion
In conclusion, we studied the photophysical and electronic properties of a non-fullerene
acceptor, ZCl, and subsequently compared its performance in OPVs to C 60. SBCT was
demonstrated to occur in ZCl through transient absorption studies. IPES measurements
reveal that ZCl has the same LUMO as C60 (-4.1 eV). In OPVs, we observed that ZCl
yields substantially larger VOC than analogous devices with C60, 1.33 V compared to 0.88 V
for devices with a DBP donor. Measurements of the CT state reveal that the increase in
VOC originates from a combination of an increase in ECT and a decrease in energetic losses
due to recombination. It is proposed that these effects are related to SBCT which
substantially modulates the coupling between the donor and acceptor. In the future, we
119

envision that SBCT materials will be particularly useful as interface materials in OPVs,
where a thin layer will be placed at the D/A interface to significantly enhance V OC. The
results presented for ZCl are the first documented application of a material which
undergoes SBCT in an OPV and the resultant VOC illustrates the great potential of this
family of materials, and the utilization of SBCT in general, in OPVs. Additionally, the
recombination losses from ECT to VOC for the DBP/ZCl devices are equivalent to what has
been measured for Si and GaAs devices. Further improvement of the performance of ZCl
devices including the use of energy sensitizers to increase absorption at wavelengths
shorter than 500 nm and the fabrication of optimized bulk-heterojunction and tandem
devices to increase photocurrent are underway.
5.4 ZCl: a Versatile Acceptor for High Open Circuit Voltage Organic Photovoltaics
5.4.1 Comparison of ZCl and C60 with a Variety of Donors
Beyond the initial results reported with DBP, we also conducted a wider screening of the
performance of ZCl as an acceptor with a variety of donor materials and compared them to

Figure 5.15: Molecular structures and extinction coefficients for 6T, ZnPc, DIP, and
NPD
120

devices with a C60 acceptor. The molecular structures and absorption for the donor
materials 6T, ZnPC, NPD, and DIP are given in Figure 5.15. Their HOMO energies
are: -4.7 eV for 6T,
33
-5.3 eV for ZnPc,
62
-5.3 eV for DIP,
33
and -5.4 eV for NPD.
63
These
materials span a wide variety of properties from their absorption, morphology, and frontier
molecular orbitals. They were selected as a diverse set of compounds with which we could
determine whether the increase in VOC for devices with a ZCl acceptor was ubiquitous.
Devices with the general structure ITO/Donor(x nm)/Acceptor (y nm)/BCP (10 nm)/Al
(100 nm) were fabricated. The DIP and NPD devices both had a hole transport layer
comprised of 10 nm MoO3 between the ITO and the donor layer. Each donor was paired
with an acceptor layer comprised of either 40 nm C60 or 20 nm ZCl. The donors utilized
and the thickness of the donor and acceptor layers are tabulated in Table 5.4 along with
the performance characteristics of the devices. The I-V plots of the OPVs grouped by
donor are depicted in Figure 5.16. The EQE plots of the OPVs grouped by donor are
depicted in Figure 5.17.
Table 5.4: Summary of ZCl and C60 Acceptor Device Performance Characteristics
Device
J SC
(mA/cm
2
)
VOC
(V)
FF
η
(%)
6T(20 nm)/ZCl(20 nm) 3.5 0.83 0.57 1.6
6T(20 nm)/C60(40 nm) 3.5 0.38 0.43 0.57
DIP(40 nm)/ZCl(20 nm) 1.3 1.29 0.57 0.95
DIP(40 nm)/C60(40 nm) 4.2 0.88 0.52 1.9
NPD(11 nm)/ZCl(20 nm) 1.2 1.28 0.38 0.57
NPD(11 nm)/C60(40 nm) 3.0 0.88 0.44 1.2
ZnPc(40 nm)/C60(40 nm) 5.9 0.52 0.54 1.7
ZnPc(40 nm)/ZCl(20 nm) 2.9 0.81 0.58 1.4

121

The 6T/C60 device gives a JSC of 3.5 mA/cm
2
, VOC of 0.38 V, FF of 0.43, and η of 0.57 %
in agreement with previous reports.
64
The 6T/ZCl device gives a JSC of 3.5 mA/cm
2
, VOC
of 0.83 V, FF of 0.57, and η of 1.7 %. Similar to what is seen with DBP, ZCl gives a
substantially increased VOC compared to the C60 device. Both devices produce equivalent
JSC however the FF is significantly improved in the ZCl device. The net effect is a nearly
threefold increase in η. The EQE reveals that the increased absorption by ZCl between
λ = 450 nm and 550 nm compensates for the loss in absorption at λ < 450 nm upon the
replacement of C60.

Figure 5.16: Illuminated current-voltage (I-V) plots of the OPV with 6T, ZnPc, DIP,
and NPD as donors and C60 and ZCl as acceptors. The plots are grouped by donor.
122

Next, ZnPc devices were fabricated. The ZnPc/C60 device gives a JSC of 5.9 mA/cm
2
, V OC
of 0.52 V, FF of 0.54, and η of 1.7 % in agreement with previous reports. The 6T/ZCl
device gives a JSC of 2.9 mA/cm
2
, VOC of 0.81 V, FF of 0.58, and η of 1.4 %. For the ZnPc
devcies, there is an increase in VOC and FF when ZCl is the acceptor, however there is a
decrease in JSC. Overall the efficiency of the ZCl device is less than that of the C 60 device,
primarily due to the discrepancy in JSC. The EQE shows a reduction in photorespose at
λ < 450 nm and λ > 600 nm for the ZCl device. This could be due to either optical
interference effects or a change in the internal quantum yield of the ZCl devcies.

Figure 5.17: External quantum efficiency (EQE) plots of the OPV with 6T, ZnPc, DIP,
and NPD as donors and C60 and ZCl as acceptors. The plots are grouped by donor.
123

The DIP/C60 device gives a JSC of 4.2 mA/cm
2
, VOC of 0.88 V, FF of 0.52, and η of 1.9 %
in agreement with previous reports.
65
The DIP/ZCl device gives a JSC of 1.3 mA/cm
2
, VOC
of 1.29 V, FF of 0.57, and η of 0.95 %. Once more, the ZCl device has a much greater VOC
and slightly increased FF at the cost of a diminished JSC. The EQE reveals a reduction in
photorespose across almost the entire spectrum for the ZCl device. This is because DIP is
not a strongly absorbing material due to its transition dipole moment being oriented parallel
to the direction of incident light.
66

The NPD/C60 device gives a JSC of 3.0 mA/cm
2
, VOC of 0.88 V, FF of 0.44, and η of 1.2 %
in agreement with previous reports.
36
The NPD/ZCl device gives a JSC of 1.2 mA/cm
2
, VOC
of 1.28 V, FF of 0.44, and η of 0.57 %. In devices with NPD, the ZCl acceptor gives a
larger VOC, but FF and JSC are reduced. Similar to the other material combinations, the
narrow absorption of ZCl limits the JSC and therefore η.
Summarily, the VOC of the ZCl devices is substantially augmented compared to that found
for devices with C60.
5.4.2 Comparison of ZCl and Cl6Bodipy
In order to assess symmetry breaking charge transfer in zinc dipyrrin complexes, Trinh
et al. synthesized heteroleptic zinc complexes with a single dipyrrin ligand to serve as a
control.
7
Following a similar strategy, we set out to synthesize a dipyrrin compound which
would be unable to undergo SBCT but would possess similar properties to ZCl. The
synthesis of a chlorinated BODIPY, analogous to one half of ZCl, has been reported
previously.
67
Following their procedure we synthesized the compound and compared its
properties to ZCl.
124

The molecular structure, absorption, and emission spectra for Cl 6BODIPY and ZCl are
shown in Figure 5.18. Both are intense absorbers with peak molar absorptivities greater
than 10
5
M
-1
cm
-1
. The absorption maxima of Cl6BODIPY at λ = 536 nm is slightly
redshifted compared to ZCl with a maxima at λ = 517 nm. Both ZCl and Cl6BODIPY
exhibit intense fluorescence with the photoluminescence quantum yields in cyclohexane of
0.69 and 0.26 for Cl6BODIPY and ZCl, respectively. The photoluminescence lifetime in
cylcohexane were 5.4 ns and 2.2 ns for Cl6BODIPY and ZCl, respectively. From the
quantum yeild and lifetime data, radiative and nonradiative rates of 1.3 x 10
8
s
-1
and
5.7 x 10
7
s
-1
, respectively, were determined for Cl 6BODIPY. ZCl exhibited radiative and
nonradiative rates of 1.2 x 10
8
s
-1
and 3.4 x 10
8
s
-1
, respectively. It is encouraging that the
radiative rates for both Cl6BODIPY and ZCl are nearly identical because the chromophores
responsible for emission are extremely similar and posess similar oscillator strength. ZCl
has a larger nonradiatve rate than Cl6BODIPY, however this could be due to competing
nonradiative processes which can occur in the dimer-like ZCl.

Figure 5.18: Molecular structure, absorption, and emission spectra for Cl6BODIPY and
ZCl
125

Electrochemical characterization of ZCl and Cl6BODIPY was performed in order to
determin their oxidation and reduction potentials. Figure 5.19 contains cyclic voltammetry
traces for ZCl and Cl6BODIPY with the oxidation and reduction potentials labeled. ZCl
exhibits two reversible reductions at -1.30 V and -1.54 V vs. Fc/Fc
+
along with an
irreversible oxidation at 1.22 V vs. Fc/Fc
+
.
30
Cl6BODIPY has a reversible reduction
at -0.71 V vs. Fc/Fc
+
and an irreversible oxidation at 1.40 V vs. Fc/Fc
+
. It is quite
surprising that ZCl and Cl6BODIPY have such different reduction potentials. The
reduction potential of C60 has been measured previously at -1.06 V vs Fc/Fc+.
It has been shown previously that oxidation and reduction potentials correlate with HOMO
and LUMO energies.
32, 68
However, these correlations are not universal and many
materials do not exactly follow this relationship. This is due to intermolecular interactions

Figure 5.19: Cyclic voltammetry traces for Cl6BODIPY and ZCl. Cl6BODIPY exhibits
a reversible reduction at -0.71 V and an irreversible oxidation at 1.40 V. ZCl exhibits
two reversible reductions at -1.30 V and -1.54 V and an irreversible oxidation at 1.22 V.
126

which are present in the solid state but not in solution. Additionally, conformational and
orientational changes in the solid state can have a significant impact on HOMO and LUMO
energies which cannot be captured in solution phase measurements. Nevertheless, based
on the electrochemical data, we would expect the LUMO of Cl6BODIPY to be greater than
ZCl and thus it should also function as an acceptor.
To compare the acceptor performance of Cl6BODIPY, ZCl, and C60, devices with the
structure ITO/MoO3(10 nm)/DBP(20 nm)/Acceptor (x nm)/BCP(10 nm)/Al(100 nm)
were fabricated. The Cl6BODIPY and ZCl layers were 20 nm thick and the C60 was 40 nm

Figure 5.20: Illuminated I-V curves, dark I-V curves, and EQE for devices with DBP
as a donor and either Cl6BODIPY, ZCl or C60 as the acceptor.
127

thick.
The illuminated I-V curves, dark I-V curves, and EQE are given in Figure 5.20 with
performance parameters given in Table 5.5. The C60 device gives the largest JSC which
can be understood through the EQE where the device has a significantly broader spectral
response. The Cl6BODIPY and C60 devices exhibit similar VOCs of 0.90 V and 0.88 V,
respectively, while the ZCl gives a significantly larger VOC of 1.33 V. The FF of the
Cl6BODIPY and ZCl devices are similar at 0.43 and 0.42, respectively, whereas the C60
device has a FF of 0.68. Overall, the C60 device has the highest efficiency of 3.6 % while
the ZCl device has an efficiency of 1.4 % and the Cl6BODIPY device has an efficiency
of 0.8 %.
The trends observed in VOC for this set of devices is fascinating. As we studied in
excruciating detail above, the VOC of the ZCl device is greater than the C60 device despite
the fact that the materials share the same LUMO as measured by IPES. Now, Cl6BODIPY,
a material with a reduction potential significantly lower than C60, exhibits a similar VOC in
devices. Without IPES measurement of the LUMO, there is not sufficient evidence to
corroborate the trend, however, it seems that these chlorinated dipyrrins produce higher
Table 5.5: Summary of CL6BODIPY, ZCl, and C60 Acceptor Device Performance
Characteristics
Device
J SC
(mA/cm
2
)
VOC
(V)
FF
η
(%)
DBP(20nm)/
Cl
6
Bodipy(20 nm)
2.0 0.90 0.43 0.8
DBP(20 nm)/C
60
(40 nm)
6.2 0.88 0.68 3.6
DBP (20nm)/ZCl(20 nm) 2.4 1.33 0.42 1.4

128

VOC than they should. Several potential scenarios exist for the present set of data. If the
LUMOs of C60, Cl6BODIPY, and ZCl are all similar, then the VOC data suggest that SBCT
leads to a substantially increased VOC. If the LUMO of Cl6BODIPY is less than C60 and
ZCl, then it appears that chlorinated dipyrrins function especially well producing open
circuit voltages much higher than fullerenes for equivalent energetic offsets. The dark I-V
curves corroborate the second scenario because both the Cl6BODIPY and ZCl produce
substantially smaller dark current than what is observed in the C60 device. It is very
interesting that the Cl6BODIPY and C60 devices produce nearly identical VOC while the
dark current is significantly reduced in the Cl6BODIPY device.
5.5 Conclusion and Future Outlook
In conclusion, we have conducted extensive work investigating symmetry breaking charge
transfer as a fundamental phenomena and extending its application of OPVs for the first
time. By examining the photophysical properties of a zinc dipyrrin in a variety of solvent
environments, we were able to study the rates of electron transfer to a high degree of detail.
In low dielectric media, the CT state lies slightly higher in energy than the singlet state and
thus the equilibrium is highly weighted towards the singlet. In a dielectric of approximately
3.4, the singlet and CT state are equal in energy and the rate of electron transfer is
approximately 70 ns
-1
. The ultrafast electron transfer with no driving force is quite
interesting as it allows for the rapid separation of a strongly bound Frenkel exciton with a
minimal amount of energetic loss. As the dielectric continues to increase, the CT state is
further stabilized and the rate of electron transfer increases. This work provides insight
into the electron transfer process in these materials
129

In an attempt to take advantage of this behavior, we fabricated OPVs utilizing a zinc
dipyrrin acceptor, ZCl. The devices exhibited substantially higher VOC than comparable
devices utilizing C60. By studying the properties which govern VOC, the frontier molecular
orbitals and charge transfer state formed at the D/A interface, we were able to ascertain
that the ZCl devices form a higher energy CT state. Moreover, the recombination losses
from the CT state in the ZCl device are smaller than what is typically observed in OPVs,
an unprecedented result. This has consequences which fundamentally effect the ultimate
efficiency possible for OPVs. If this effect can be extended to other material systems it is
possible to greatly increase the maximum efficiency achievable.
Next we expanded the scope of donors which we used in devices with a ZCl acceptor and
systematically compared them to devices utilizing C60. Across all donors, the VOC was
greater in the device which contained ZCl. However, the overall efficiency of the devices
largely suffered from the narrow absorption of ZCl which limits the ability for the cell to
efficiently harvest the solar spectrum. Overall, the omnipresent increase observed for VOC
is encouraging and should promote future exploration of additional material combinations.
Finally, we utilized Cl6BODIPY as a foil for ZCl which cannot undergo SBCT. The optical
properties of the two compounds are quite similar, however their electronic properties
differ. The reduction potential for Cl6BODIPY is significantly smaller than ZCl. When
used in devices compared to a C60 standard. The Cl6BODIPY device produces the same
VOC as C60 whereas the ZCl device has a significantly larger VOC. In the absence of an
accurate measurement of the LUMO of Cl6BODIPY we cannot attribute this effect to
SBCT or another phenomena unique to ZCl.
130

Looking forward, SBCT is a topic ripe for future study both phenomenologically and
applied to OPVs. The ability to probe electron transfer in well-defined systems allows for
increased understanding of structures and orientations which promote desired electron
transfer rates. In OPVs, we have only begun to explore the potential applications of SBCT.
The synthesis of additional materials with various structural changes to alter the
morphological, photophysical, and electronic properties of SBCT compatible compounds
could yield a myriad of results. A diverse library of SBCT compounds would elucidate the
many questions which remain about the effectiveness and utility of SBCT in high
efficiency devices.

131

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HOMO/LUMO Alignment at PTCDA/ZnPc and PTCDA/ClInPc Heterointerfaces
Determined by Combined UPS and XPS Measurements. The Journal of Physical
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heterojunction organic photovoltaic cells. Organic Electronics 2008, 9, (5), 582-590.
65. Wagner, J.; Gruber, M.; Hinderhofer, A.; Wilke, A.; Bröker, B.; Frisch, J.;
Amsalem, P.; Vollmer, A.; Opitz, A.; Koch, N.; Schreiber, F.; Brütting, W., High Fill
Factor and Open Circuit Voltage in Organic Photovoltaic Cells with Diindenoperylene as
Donor Material. Advanced Functional Materials 2010, 20, (24), 4295-4303.
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138

Chapter 6. Unconventional Materials for High Open Circuit Voltage Photovoltaics
6.1 Abstract
In order to increase the open circuit voltage in an OPV, it is necessary to maximize the
energy of the charge transfer state formed at the D/A interface. In this chapter, devices
utilizing an unconventional pairing of donor and acceptor for the purpose of maximizing
VOC are described. Compared to devices with a C60 acceptor, the 6T/DBP devices exhibit
a substantially larger VOC. To study the increase in VOC, we measure the energy of the
charge transfer state confirming that higher CT state energy yields a higher VOC. Then, the
DBP devices are optimized through the use of various active layer and buffer thicknesses.
AFM images reveal that 6T grows extremely textured films and the DBP infills forming a
pseudo-BHJ. Optical modeling shows that although thicker films of DBP absorb more
light, exciton generation is shifted away from the D/A interface and more excitons are lost
to recombination. Alternatively, increasing the buffer layer thickness shifts the DBP layer
into a region of higher field intensity increasing performance until the buffer becomes
resistive.
6.2 Introduction
Organic photovoltaics (OPVs) have been touted for their low material costs and
compatibility with low-cost processing techniques.
1
Nevertheless, their power conversion
efficiencies continue to lag behind traditional inorganic solar technologies. Over the past
several years, the internal quantum efficiency for state of the art organic photovoltaic
devices has approached unity.
2-4
This has been due to the development of precise
understanding and control of the morphology present as well as the phenomena occurring
139

within devices. However, despite substantial gains achieved in efficient photocurrent
production, the open-circuit voltage (VOC) of organic devices is generally low and serves
as a substantial limit to overall device performance.
5
The poor VOC can ultimately be traced
back to limitations imposed by fullerenes which, despite their widespread use, typically
exhibit voltages of less than 1 V in devices.
5
To overcome this problem, alternative
strategies have been developed to obtain high VOC in organic devices through the judicious
selection of active layer materials.
Of particular interest in small-molecule OPVs have been a series of compounds related to
perylene, diindenoperylene (DIP)
6
and tetraphenyldibenzoperiflanthrene (DBP), which
have demonstrated high performance as donor materials. Due to their high hole mobilities
and favorable film morphologies, devices employing these materials have been shown to
exhibit large fill factors (FF) in excess of 0.70 in planar heterojunction devices.
6, 7

Additionally, luminescence quenching measurements have revealed these materials have
relatively long exciton diffusion lengths.
7, 8
Planar-mixed heterojunction devices
comprised of a DBP:C70 active layer have exhibited efficiencies of 8.1 % in single junction
devices
2
and a tandem device with 11.1 % efficiency has been fabricated utilizing two
DBP:C70 subcells.
9
Thus, it would be extremely desirable to develop a device utilizing
DBP with a substantially enhanced VOC in order to improve both single-cell and tandem
device performance.
In conjunction with its high hole mobility, DIP also exhibits high electron mobility
10
and
OPVs utilizing DIP as an acceptor have a reported VOC of 1.24 V.
11
However, these devices
suffer from low photocurrents due to the poor absorption of DIP. In this paper, we examine
140

the performance of planar heterojunction 6T/DBP solar cells in comparison to DBP/C60.
The 6T/DBP devices exhibit a large VOC of 1.27 ± 0.01 V. Variable-temperature device
measurements reveal that the large VOC is due to a higher energy charge transfer state
compared to the value measured for DBP/C60 devices. AFM and optical modeling are used
to understand the active layer performance. Further optimization of the devices was
performed to illustrate the importance of hole and electron transport layers.
6.3 Results and Discussion
The molecular structures and extinction spectra for C60, 6T, and DBP are shown in
Figure 6.1. Both 6T and C60 predominantly absorb in the short wavelength range of the
solar spectrum up to λ = 550 nm, while DBP absorbs intensely between λ = 500 nm
and 650 nm. In conjunction, both DBP/C60 and 6T/DBP absorb complementarily. The
frontier molecular orbital energies measured for these materials show that DBP has a
HOMO of 5.4 eV and LUMO of 3.1 eV,
12
C60 a HOMO 6.4 eV of and LUMO of 4.0 eV,
13

and 6T a HOMO of 4.7 eV.
13

Figure 1: Molecular structure and extinction spectra of 6T, C60, and DIP.
141

6.3.1 Photovoltaic Performance and Optimization
To compare the photovoltaic performance of these materials, OPVs with the structure
ITO/6T(60nm)/DBP(20nm)/BCP(10nm)/Al and
ITO/MoO3(10nm)/DBP(20nm)/C60(40nm)/BCP(10nm)/Al were fabricated through
vacuum deposition. For completeness we should also consider OPVs with a 6T donor and
C60 acceptor. This device has been reported previously
14
and gives very poor performance.
An OPV with the structure ITO/6T(60nm)/C60(40nm)/BCP(10nm)/Al gives a VOC of
0.38 V and JSC and FF of 3.5 mA/cm
2
and 0.43, respectively, at 1 sun AM1.5 illumination.
Due to the poor performance of these devices we will not include them in our comparison
for the present study. Current vs. voltage (J-V) and external quantum efficiency (EQE)
curves are shown in Figure 6.2 for the two DBP based devices. The DBP/C60 and 6T/DBP
devices yield short-circuit currents (JSC) of 6.1 ± 0.1 mA/cm
2
and 3.9 ± 0.1 mA/cm
2
,
respectively. EQE measurements reveal the lower photocurrent in the 6T/DBP devices
derives from a reduced photoresponse between λ = 400 and 500 nm. In contrast to the

Figure 6.2: Current density vs. voltage characteristics under one sun AM1.5G
illumination (a) and external quantum efficiency spectra (b) of 6T/C60, DBP/C60, and
6T/DBP devices.
142

lower photocurrent, the VOC of the 6T/DBP device is 1.27 ± 0.01 V, which is substantially
larger than the 0.86 ± 0.01 V observed for DBP/C60. The FF of the DBP/C60 and 6T/DBP
devices were 0.68 ± 0.01 and 0.55 ± 0.01, respectively. Overall, the power conversion
efficiency (η) was 3.6 ± 0.1 % for the DBP/C60 device and 2.7 ± 0.1 % for 6T/DBP. The
VOC observed for the 6T/DBP device is among the largest reported for a single-junction
OPV.
In order to optimize the 6T/DBP device, studies to examine the impact of the DBP
thickness, 6T thickness, electron transport layer (ETL), and hole transport layer (HTL)
were performed. Investigation of the effect of 6T layer thickness revealed no significant
change in device performance between 60 nm and 120 nm as it has minor impact on the
optical cavity in agreement with previous reports.
14
Devices with thicknesses of DBP
varying between 10 nm and 40 nm were fabricated, with the J-V curves presented in
Figure 6.3 and device performance characteristics summarized in Table II. The VOC
increases slightly with increasing DBP thickness from 1.25 ± 0.01 V for 10nm DBP to
1.30 ± 0.01 for 40 nm DBP. The JSC increases from 2.8 ± 0.1 mA/cm
2
for 10 nm DBP to
4.3 ± 0.1 mA/cm
2
for 30 nm DBP, and then decreases to 3.9 ± 0.1 mA/cm
2
for 40 nm DBP.
The FF decreases monotonically from 0.60 ± 0.01 for the 10 nm device to 0.43 ± 0.01 for
the 40 nm device, indicative of an increased series resistance with increasing DBP
Table 6.1: Device performance for 6T/DBP and DBP/C60 devices
Device
J SC
(mA/cm
2
)
VOC
(V)
FF

η
(%)
6T/DBP 3.9 1.27 0.55 2.7
DBP/C60 6.1 0.86 0.68 3.6

143

thickness. As the DBP thickness increases, η increases from 2.1 ± 0.1 % for 10 nm DBP
to a maximum value of 2.8 ± 0.1 % for 30 nm DBP.
In an attempt to circumvent the loss in FF associated with increasing DBP thickness while
simultaneously optimizing the optical electric field, devices with ETL thicknesses between
0 and 20 nm BCP were fabricated. Representative I-V curves are shown in Figure 3. JSC
increases monotonically with BCP thickness from 2.9 ± 0.1 mA/cm
2
to 4.4 ± 0.1 mA/cm
2
.
The FF of the devices increased from 0.35 ± 0.01 for 0 nm BCP to 0.60 ± 0.01 for 5 nm
BCP before decreasing to 0.45 ± 0.01 for 20 nm BCP. The devices with 0 nm BCP and
20 nm BCP show evidence of an “s”-shaped J-V characteristics indicative of issues with
charge injection and extraction.
15

In addition to the effect of the ETL, the impact of the HTL was investigated. Devices with
10 nm MoO3 were fabricated and compared to those without MoO3. The device with MoO3
exhibited a FF of 0.62 ± 0.01, significantly larger than the 0.55 ± 0.01 observed for the

Figure 6.3: (a) Current density vs. voltage characteristics under one sun AM1.5G
illumination for 6T/DBP devices with various thicknesses of DBP. (b) Current density
vs. voltage characteristics under one sun AM1.5G illumination for 6T/DBP devices with
varying ETL and HTL.
144

device with no HTL. However, the photocurrent decreased from 3.9 ± 0.1 mA/cm
2
to
3.1 ± 0.1 mA/cm
2
upon the addition of MoO3. These findings are in agreement with
previous work which has shown what the MoO3/organic interface quenches excitons.
16

However, this problem can be overcome by using the appropriate exciton blocking layer.
12

The VOC was unaffected by the presence of the MoO3, remaining at 1.27 ± 0.01 V.
6.3.2 Determination of the Energy of the Charge Transfer State
The energy of the intermolecular charge transfer state (ECT) formed between the donor and
acceptor has been shown to correlate linearly with VOC and the two quantities can be related
through a modified Shockley-Queisser analysis yielding Equation 1:
17

𝑉 𝑂𝐶
=
1
𝑞 (𝐸 𝐶𝑇
+ 𝑘𝑇 ln (
𝐽 𝑆𝐶
ℎ
3
𝑐 2
𝑓𝑞 2𝜋 ( 𝐸 𝐶𝑇
−𝜆 )
) + 𝑘𝑇 ln( 𝐸𝑄𝐸 𝐸𝐿
) ) (1)
Table 6.2: Device performance for 6T/DBP devices with various thicknesses of DBP,
HTL (MoO3), and ETL (BCP).
Device
J SC
(mA/cm
2
)
VOC
(V)
FF

η
(%)
DBP (10 nm) 2.8 1.23 0.60 2.6
DBP (20 nm) 3.9 1.27 0.55 2.7
DBP (30 nm) 4.3 1.28 0.50 2.8
DBP (40 nm) 3.9 1.30 0.43 2.1
BCP (0 nm) 2.9 1.13 0.35 1.1
BCP (5 nm) 3.5 1.25 0.60 2.6
BCP (10 nm) 3.9 1.27 0.55 2.7
BCP (20 nm) 4.4 1.26 0.45 2.5
MoO3 (10 nm)/
BCP (10 nm)
3.1 1.26 0.62 2.4

145

where q is the elementary charge, k is the Boltzmann constant, T is temperature, h is
Planck’s constant, c is the speed of light, f is proportional the CT state absorption, λ is the
reorganization energy, and EQEEL is the electroluminescence quantum efficiency of the CT
state. ECT can be experimentally determined through sensitive measurements of device
EQE or electroluminescence (EL). Values for ECT can be extracted by fitting the low
energy portion of the spectra to Equations 2 and 3 for EQE and electroluminescence,
respectively:
𝐸𝑄𝐸 ∝
𝑓 𝐸 √4𝜋𝜆𝑘𝑇 𝑒 (
−( 𝐸 𝐶𝑇
+𝜆 −𝐸 )
2
4𝜆𝑘𝑇 )
(2)

Figure 6.4: EQE and EL measurements for DBP/C60 bilayer, DBP/C60 PM-HJ, 6T/DBP
bilayer, and 6T/DBP PM-HJ devices. The DBP/C60 devices show clear CT emission
while the 6T/DBP devices show no obvious signal of CT emission
146

𝐼 𝐹 𝐸 ∝
𝑓 √4𝜋𝜆𝑘𝑇 𝑒 (
−( 𝐸 𝐶𝑇
−𝜆 −𝐸 )
2
4𝜆𝑘𝑇 )
(3)
where IF is the emission intensity, and E is photon energy. The results of the EQE and EL
measurements for DBP/C60 planar heterojunction and 6T/DBP planar heterojunction
devices are available in Figure 6.4. The DBP/C60 device shows a clear signal of both CT
absorption and emission which can be fit to an ECT of 1.45 eV. On the other hand, the
6T/DBP device shows no distinct CT absorption or emission features and thus a value
could not be determined.
An alternative method to determine ECT is the linear extrapolation of VOC as a function of
temperature to 0K.
11, 17
The results of the temperature-dependent measurements of VOC for
DBP/C60 and 6T/DBP devices are shown in Figure 6.5. Fits to the linear portion of the
data reveal ECTs of 1.35 eV and 1.85 eV for the DBP/C60 and 6T/DBP, respectively. ECT
has been shown to have a slight temperature dependence thus the values determined by
extrapolation to 0K provide a lower limit on the value of ECT at ambient temperature. This

Figure 6.5: Temperature dependent open circuit voltage for DBP/C60 and 6T/DBP
devices. Lines represent fits to the linear portion of the curve. Extrapolation to 0 K yields
ECT.
147

temperature dependence also explains the discrepancy between the value for the DBP/C 60
device determined by temperature-dependent measurement and the value measured at
room temperature by EQE and EL. The observed increase in ECT accounts for the increase
in VOC.
6.3.3 Morphological Studies
The concomitant increase in photocurrent with DBP thickness is due to a variety of factors
related to the film morphology and optical field intensity within the device. To probe the
structure of the donor/acceptor (D/A) interface and final active layer, AFM images were
collected on films of 6T and 6T covered with various thicknesses of DBP. The AFM
images and representative cross sections can be seen in Figure 6.6. The 60 nm 6T film is
extremely textured with surface features on the order of 50 nm and an RMS roughness of
9.2 nm. This large surface roughness leads to an increase in the surface area of the D/A
interface, allowing for increased exciton harvesting. Upon deposition of DBP, the surface
roughness decreases to exhibit features on the order of 30 nm with an RMS roughness of
6 nm. This decrease in roughness is indicative of the DBP infilling the 6T microstructure
rather than simply templating the surface which would propagate the surface roughness to
the next layer. With increasing DBP thickness, the size of the surface features decrease
along with the RMS roughness. These results help rationalize the substantial rise in
photocurrent from 10 to 20 nm DBP followed by a smaller increase past 20 nm. For
devices with only 10 nm DBP, the trenches in the 6T film become filled and the
interdigitated nature of the D/A interface allows for collection of a large fraction of
excitons. For 20 nm, the DBP now coats the surface of the film and excitons must diffuse
further to reach the
148

D/A interface and a fraction are thus lost to recombination. At higher thicknesses, despite
the enhanced absorption, a smaller fraction of excitons can diffuse to the D/A interface.
6.3.4 Optical Electric Field Effects
In conjunction with the surface effects studied via AFM, the optical electric field within
the device is substantially effected by DBP layer thickness. Utilizing the transfer matrix
formalism,
18
we modeled the 6T/DBP devices with various thicknesses of DBP for an
understanding of the optical cavity effects within these devices. As the AFM data reveal
substantial surface roughness while the transfer matrix formalism assumes pristine, flat
interfaces, these simulations are intended to be a qualitative guide for understanding and

Figure 6.6: AFM images and characteristic line profiles of 6T(60 nm) (a,d), 6T(60
nm)/DBP(10 nm) (b,e), and 6T(60 nm)/DBP(20 nm) (c,f).
149

not an absolute predictor of performance. Absorbed optical power as a function of position
are plotted in Figure 6.7a for 10 nm, 20 nm, 30 nm, and 40 nm DBP devices. To clearly
demonstrate the impact on both materials, the absorbed optical power is plotted at
individual wavelengths: λ = 450 nm to illustrate 6T and λ = 610 nm for DBP. Two
significant trends can be observed with increasing DBP thickness. First, increasing DBP
thickness results in increased absorption which can be calculated by integrating the
absorbed power curves with respect to thickness. Normalized to the absorbed optical
power of 10 nm DBP, 20 nm results in an increase in absorption by a factor 1.75, 30 nm
by a factor of 2.14, and 40 nm by a factor of 2.32. Second, increasing DBP thickness
effects the shape and intensity of the absorbed optical power profile within the device. For
10 nm DBP, absorption is most intense at the 6T/DBP interface and decreases sharply with
thickness. 20 nm DBP absorbs with the same intensity as 10 nm DPB at the 6T/DBP
interface, but the slope of the absorbed optical power is decreased. At 30 nm and 40 nm
DBP, the intensity of the absorbed optical power at the 6T/DBP interface decreases and
the slope continues to decrease. These simulations corroborate the observed device
performance where JSC initially increases with thickness due to enhanced absorption but
then plateaus and decreases due to exciton generation shifting away from the D/A interface.
This is due to the exciton diffusion length of 10 nm which has been reported for DBP.
7, 19

The devices with varying thickness of ETL attempt to maximize the absorbed optical power
within a layer thickness that is limited by the exciton diffusion length. Absorbed optical
power as a function of position are plotted in Figure 6.7b for devices with 0 nm, 5 nm,
10 nm, and 20 nm BCP. To clearly demonstrate the impact on both materials, the absorbed
optical power is plotted at individual wavelengths: λ = 450 nm to illustrate 6T and
150

λ = 610 nm for DBP. The simulations show that absorbed optical power increases
monotonically with BCP thickness. The absorption can be calculated by integrating the
absorbed power curves with respect to thickness. Normalized to the absorbed optical
power of 0 nm BCP, 5 nm results in an increase in absorption by a factor 1.16, 10 nm by a
factor of 1.30, and 20 nm by a factor of 1.48. These values are in staggeringly good
agreement with the observed trend in JSC which increases by a factor of 1.2 for 5 nm, 1.3
for 10 nm, and 1.5 for 20 nm.
The 6T/DBP devices are particularly interesting in the context of tandem OPV devices.
Tandem devices have exhibited exceptional performance by relaxing some of the extensive
requirements placed on OPV materials. For example, in tandem devices, it is possible to
specifically target separate portions of the solar spectrum with separate subcells
20, 21
or
utilize multiple, thinner identical subcells resulting in reduced resistivity and increased
absorption.
22, 23
However, despite the high power conversion efficiencies exhibited in these

Figure 6.7: (a) Simulated spatial distribution of the absorbed optical power for 6T/DBP
devices with various DBP thicknesses at λ = 450 nm and 610 nm. (b) Simulated spatial
distribution of the absorbed optical power for 6T/DBP devices with various BCP
thicknesses at λ = 450 nm and 610 nm.
151

devices, the VOC has not been optimized to maximize subcell voltage. For example, the
tandem device with 11.1 % efficiency reported by Xiao et al. contains of two DBP:C 70
subcells which only produce 0.86 V each. These subcells absorb photons with energies
greater than 1.8 eV and more than half of that energy is lost. The replacement of the
DBP:C70 cells of the 11.1 % device with 6T/DBP cells producing equivalent JSC and FF
would result in an overall increase in voltage of 0.7 V and a power conversion efficiency
of 14 %.
6.4 Conclusion
In summary, we fabricated a series of OPVs to examine the functionality of DBP as an
acceptor in devices with 6T as a donor. The 6T/DBP device exhibits VOC of 1.27 ± 0.01 V,
among the highest demonstrated in an OPV, and η of 2.8 ± 0.1 %. Compared to a DBP/C60
device, the 6T/DBP device exhibits substantially larger VOC due to its higher ECT. Studies
on the impact of DBP, HTL, and ETL reveal that selection of the appropriate material and
thickness can significantly affect both the JSC and FF of devices. Specifically, the JSC can
be increased to 4.4 ± 0.1 mA/cm
2
with 20 nm BCP and the FF to 0.62 ± 0.01 with 10 nm
MoO3. AFM reveals that 6T forms a highly textured surface which allows for a pseudo-
idealized bulk-heterojunction upon deposition of DBP. Optical modeling shows that
shifting the active layer away from the cathode increases absorbed power by a factor of
1.5. The results presented here demonstrate that through careful materials selection, it is
possible to substantially increase the open circuit voltage of an OPV without compromising
FF or spectral responsivity.
152

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Abstract (if available)

Abstract The design of materials and devices for organic photovoltaic applications is dominated by considerations related to the management of energy. From the synthesis of materials which intensely absorb light to the fabrication of devices with optimal phase segregation to promote charge separation, ensuring that energy is harvested efficiently is a tantamount concern. The energy generation process can be broadly separated into two categories: one related to the generation of photocurrent, and the other photovoltage. This dissertation describes strategies to improve the efficiency of both of these processes through the use of novel materials and device architectures. ❧ In order to improve photocurrent, we have developed an exciton blocking layer based on a blend of materials. These blends are optically transparent, preventing parasitic absorption, and highly conductive, conducive to carrier extraction. The buffer layers increase photocurrent production by reducing exciton-polaron annihilation within devices. In addition, we probed the importance of crystallinity in buffer layers finding that crystalline buffers outperform amorphous buffers as the layer thickness increases. Beyond buffer layers, we extended the use of an energy sensitization scheme in order to harvest a broader portion of the solar spectrum. This allows devices to harvest more light and generate larger photocurrents. The sensitization scheme was then applied to the study of exciton diffusion in devices. ❧ In order to achieve higher open circuit voltage, we investigated two strategies. First, we studied a phenomena known as symmetry breaking charge transfer and employed materials which undergo this process in devices. Studies on the rate of electron transfer in these materials in solution reveal ultrafast rates with negligible driving force. In devices, the materials produce substantially increased open circuit voltages and reduce the voltage losses due to recombination. Next, we investigated devices based on materials with large interfacial energy gap. These devices produce large open circuit voltages due to the significant energetic offset at the donor/acceptor interface. The overall theme in this work is that through precise understanding of the photocurrent generation process, it is possible to gain insight into methods to increase efficiency.

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Asset Metadata

Creator Bartynski, Andrew N. (author)

Core Title Energy management in organic photovoltaics

School Viterbi School of Engineering

Degree Doctor of Philosophy

Degree Program Chemical Engineering

Publication Date 04/25/2016

Defense Date 07/28/2015

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

Tag OAI-PMH Harvest,organic electronics,organic photovoltaics,solar cells

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Thompson, Mark E. (committee chair), Armani, Andrea (committee member), Brutchey, Richard L. (committee member)

Creator Email anb210@gmail.com,bartynsk@usc.edu

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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...

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