Acceptor-Sensitizers for Nanostructured Oxide .../67531/metadc699927/m2/1/high... · 2.5 Synthesis...

APPROVED: Justin Youngblood, Major Professor Thomas Cundari, Committee Member Oliver Chyan, Committee Member Robby Petros, Committee Member William E. Acree, Jr., Chair of the Department

of Chemistry Mark Wardell, Dean of the Toulouse Graduate

School

ACCEPTOR-SENSITIZERS FOR NANOSTRUCTURED OXIDE SEMICONDUCTOR IN

EXCITONIC SOLAR CELLS

Seare Ahferom Berhe

Dissertation Prepared for the degree of

DOCTOR OF PHILOSOPHY

UNIVERSITY OF NORTH TEXAS

August 2014

Berhe, Seare Ahferom. Acceptor-Sensitizers for Nanostructured Oxide Semiconductor

in Excitonic Solar Cells. Doctor of Philosophy (Chemistry-Organic Chemistry), August 2014,

115 pp., 5 tables, 4 schemes, 49 figures, references, 192 titles.

Organic dyes are examined in photoelectrochemical systems wherein they engage in

thermal (rather than photoexcited) electron donation into metal oxide semiconductors. These

studies are intended to elucidate fundamental parameters of electron transfer in

photoelectrochemical cells. Development of novel methods for the structure/property tuning of

electroactive dyes and the preparation of nanostructured semiconductors have also been

discovered in the course of the presented work. Acceptor sensitized polymer oxide solar cell

devices were assembled and the impact of the acceptor dyes were studied.

The optoelectronic tuning of boron-chelated azadipyrromethene dyes has been explored

by the substitution of carbon substituents in place of fluoride atoms at boron. Stability of singlet

exited state and level of reduction potential of these series of aza-BODIPY coumpounds were

studied in order to employ them as electron-accepting sensitizers in solid state dye sensitized

solar cells.

Copyright 2014

by

Seare Ahferom Berhe

ii

iii

ACKNOWLEDGEMENTS

My foremost thanks to God and my parents (Ahferom Berhe and Nechi Asfaha), who

have risen, inspired and made me enjoy and appreciate life. I would like to express the deepest

appreciation to my major advisor Professor Justin Youngblood, for his persistent help and

extreme patience in the face of numerous obstacles. He continually and convincingly conveyed a

spirit and excitement in regard to research. Without his guidance and persistent help this

dissertation would not have been possible. I thank my committee members Prof. Thomas

Cundari, Prof. Oliver Chyan and Prof. Robby Petros for their support. I thank all the faculty and

staff of the department of chemistry at UNT. I thank Center for advanced research and

Technology (CART) UNT for giving me the permission to use their Nano SEM and

Profilometer. I thank the National Science Foundation for funding in terms of providing 400

MHz NMR to characterize my compounds (NSF CHE#0840518). I thank Prof. Francis D’Souza

and Habtom Gobeze for assistance in collecting transient absorption data. I thank my friends and

fellow graduate students for their support, feedback, and friendship. I wish to express my sincere

gratitude to my wife, Tirhas Berhe, who supported me from starting my application to graduate

school all the way into finishing my Ph.D. I thank my sister and brothers and special thanks to

my friends Dr. Yoseph Marcos and Dawit Beyene for their encouragement and full support.

This dissertation is dedicated to my beloved father Ahferom Berhe.

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TABLE OF CONTENTS

Page

ACKNOWLEDGEMENTS ........................................................................................................... iii LIST OF TABLES ........................................................................................................................ vii LIST OF FIGURES ..................................................................................................................... viii LIST OF SCHEMES...................................................................................................................... xi LIST OF ABBREVIATIONS ....................................................................................................... xii CHAPTER 1. INTRODUCTION ................................................................................................... 1

1.1 Introduction ............................................................................................................. 1

1.2 Electronic Property of Polymers ............................................................................. 4

1.3 Effect of Polymer Morphology on Hole Mobility .................................................. 5

1.4 Process of Photocurrent Generation ........................................................................ 6

1.5 Electron Quenching Mechanism ............................................................................. 7

1.6 Fabrication of Devices ............................................................................................ 9

1.7 Characterization of Solar Cell Devices ................................................................... 9

1.8 Scope of the Present Work .................................................................................... 10

1.9 References ............................................................................................................. 13 CHAPTER 2. ELECTRON TRANSPORT IN ACCEPTOR-SENSITIZED POLYMER-OXIDE SOLAR CELLS: ACCEPTOR-QUENCHING AND ELECTRON CASCADE EFFECTS ....... 16

2.1 Introduction ........................................................................................................... 16

2.2 Experimental Section ............................................................................................ 19

2.3 Growth of Nanostructured Oxide Materials ......................................................... 22

2.4 Selection of the Acceptor-Sensitizers and other Interface-Modifier Compounds 24

2.5 Synthesis of the Acceptor-Sensitizers ................................................................... 27

2.6 Fluorescence Quenching and Current- Voltage Behavior in Sensitized P3HT – TiO2 Devices ........................................................................................................ 28

2.7 Surface Dipole Effects in Solid-State Sensitized Oxide Materials and Devices .. 31

2.8 Photosensitization by Acceptor – Sensitizers ....................................................... 33

2.9 Effects of the Sensitizers in P3HT/PCBM – TiO2 Devices .................................. 36

2.10 Effects of the Sensitizers on Electron Transport in ZnO Devices ........................ 38

v

2.11 Conclusion ............................................................................................................ 41

2.12 References ............................................................................................................. 41 CHAPTER 3. INFLUENCE OF SEEDING AND BATH CONDITIONS IN HYDROTHERMAL GROWTH OF VERY THIN (∼20 NM) SINGLE-CRYSTALLINE RUTILE TiO2 NANOROD FILMS .............................................................................................. 46

3.1 Introduction ........................................................................................................... 46


3.2.1 Seeding Methods ....................................................................................... 48

3.2.2 Oxide Nanorods Growth ........................................................................... 48

3.3 Influence of Seed Layer to the Growth of TiO2 Nanorod Array .......................... 49

3.4 Influence of Bath Conditions to the Growth of TiO2 Nanorod Array .................. 52

3.5 Variation of Nanorod Film Thickness in Hybrid Inverted Organic Photovoltaics 57

3.6 Conclusion ............................................................................................................ 59

3.7 References ............................................................................................................. 60 CHAPTER 4. OPTOELECTRONIC TUNING OF ORGANOBORYLAZADIPYRROMETHENES VIA EFECTIVE ELECTRONEGATIVITY AT THE METALLOID CENTER ...................................................................................................... 62

4.1 Introduction ........................................................................................................... 62

4.2 Synthesis of azaBODIPY Derivatives .................................................................. 63

4.3 Experimental Procedure ........................................................................................ 64

4.4 Photochemistry of azaBODIPY Derivatives ........................................................ 69

4.5 Electrochemistry of AzaBODIPY Derivatives ..................................................... 70

4.6 Results and Discussion ......................................................................................... 71

4.7 Conclusion ............................................................................................................ 75

4.8 References ............................................................................................................. 75 CHAPTER 5. ELECTRON TRANSFER IN PHOTOGALVANIC DYE-SENSITIZED SOLAR CELLS INCORPORATING ORGANIC DYES .......................................................................... 78

5.1 Introduction ........................................................................................................... 78


5.2.1 Synthesis of Molecular Acceptors ............................................................ 80

5.2.2 Preparation of TiO2 Nanorod Films .......................................................... 81

5.2.3 Preparation of TiO2 Nanoparticle Films ................................................... 82

5.2.4 Preparation of Films for Transient Spectroscopy Studies......................... 82

vi

5.2.5 Transient Spectroscopy Studies ................................................................ 83

5.2.6 Assembly of Liquid and Solid Based Devices and Their Characterization................................................................................................................... 83

5.3 Results and Discussion ......................................................................................... 85

5.3.1 Synthesis of Acceptor Sensitizers ............................................................. 85

5.3.2 Optoelectronic Properties of the Acceptor- Sensitizers ............................ 86

5.4 Device Studies ...................................................................................................... 91

5.4.1 Device Studies on Solid-State Devices Incorporating the Dyes ............... 91

5.4.2 Device Studies on Liquid-state Devices Incorporating the Dyes ............. 93

5.5 Conclusion ............................................................................................................ 95

5.6 References ............................................................................................................. 96 APPENDIX A. CHARACTERIZATION DATA FOR COMPOUNDS AND TiO2 NANORODS IN CHAPTER 2 ..................................................................................................... 98 APPENDIX B. CHARACTERIZATION DATA FOR TiO2 NANOROD FILMS IN CHAPTER 3................................................................................................................................................... 103 APPENDIX C. CHARACTERIZATION DATA FOR COMPOUNDS IN CHAPTER 4 ....... 111 APPENDIX D. DETAILS OF WORK DONE BY AUTHORS ............................................... 114

vii

LIST OF TABLES

Page

Table 2.1. Redox potentials of organic semiconductors and conduction band edges of ZnO and TiO2 ............................................................................................................................................... 26

Table 2.2. Photocurrent, photovoltage, fill factor, and efficiency of devices prepared in this study. ............................................................................................................................................. 31

Table 4.1. Optical and electrochemical data for compounds 1-5. ................................................ 72

Table 5.1. Photocurrent, photovoltage, fill factor, and efficiency of liquid based DSCs devices prepared in this study. ................................................................................................................... 92

Table 5.2. Photocurrent, photovoltage, fill factor, and efficiency of liquid based DSC devices prepared in this study. ................................................................................................................... 94

viii

LIST OF FIGURES

Page

Figure 1.1. Schematics of dye sensitized solar cell depicting the energy loss due to overpotential.......................................................................................................................................................... 3

Figure 1.2. Schematic structure showing the preferred arrangement of adjacent chains of regioregular HT-P3HT. (b) Plane-on orientation of P3HT with respect to an underlying surface. (c) Alternative edge-on arrangement from reference 20. ................................................................ 5

Figure 1.3. Schematic diagram of the photoelectrochemical cell composed of a donor polymer on TiO2 semiconductor (n-type) and counter electrode. ...................................................................... 7

Figure 1.4. Schematics of donor quenching mechanism. ............................................................... 8

Figure 1.5. Schematics of acceptor quenching mechanism. ........................................................... 8

Figure 1.6. Step by step polymer oxide solar cell fabrication. ....................................................... 9

Figure 1.7. Model I-V curve of photoelectrochemical cell. .......................................................... 10

Figure 2.1. Electron transport events in polymer-oxide excitonic solar cells. .............................. 17

Figure 2.2. Qualitative view of energetic considerations of electron transport at polymer-sensitizer-oxide interfaces ............................................................................................................. 18

Figure 2.3. UV-vis absorption spectra for C60-M (left) and C60-T (right). ................................ 21

Figure 2.4. SEM image of ZnO nanorods by hydrothermal growth. ............................................ 23

Figure 2.5. SEM image of TiO2 nanorods by hydrothermal growth. ........................................... 23

Figure 2.6. Fluorescence of P3HT as neat film, and on bare and sensitized TiO2 nanorods. ...... 29

Figure 2.7. Current-voltage behavior of P3HT/TiO2 devices under 1 sun illumination (upper) and in the dark (lower). ........................................................................................................................ 30

Figure 2.8. Vacuum level shifts caused by surface dipoles. ......................................................... 32

Figure 2.9. Upper: Solution-phase UV-vis absorption spectrum for NcQ (EtOH) ..................... 34

Figure 2.10. IPCE spectra for P3HT-C60-T-TiO2 cell (upper) and P3HT-C60-M-TiO2 cell (lower). .......................................................................................................................................... 35

Figure 2.11. Upper: IPCE spectra for P3HT/PCBM-Acceptor-TiO2 devices. ............................. 37

Figure 2.12. Upper: Current-voltage behavior in P3HT-Acceptor-ZnO devices under 1 sun illumination; Lower: IPCE for P3HT-C60-T-ZnO device. .......................................................... 39

ix

Figure 2.13. Current-voltage behavior in P3HT/PCBM-Acceptor-ZnO devices under 1 sun illumination. .................................................................................................................................. 40

Figure 3.1. Rutile TiO2 grown using the method from ref 17 on (A) bare 7 Ohm/square FTO, (B) a seed layer of MnOOH nanoparticles on FTO, or (C) a continuous sheet of TiO2 on FTO ....... 50

Figure 3.2. Transmission electron microscopy (left, bright field image) and selected area ......... 50

Figure 3.3. Transmission electron microscopy (left, bright field image) and selected area electron diffraction (SAED, right) of a bundle of nanorods grown from a seed layer of MnOOH nanoparticles in a 6 h growth at 55 mM Ti(iOPr)4. ...................................................................... 51

Figure 3.4. Rutile TiO2 nanorods grown to just 200 nm height with minimal bundling of the nanorods. ....................................................................................................................................... 52

Figure 3.5. SEM images of TiO2 nanorods by hydrothermal growth from a continuous sheet of TiO2 using 55 mM Ti(iOPr)4 for 5 h. Film height was 7 microns (see Fig. A3.11 ; left)............. 53

Figure 3.6. Films of rutile TiO2 nanorods grown to heights of (A) 200 nm, (B) 2 μm, and (C) 5 μm under identical growth conditions. ......................................................................................... 54

Figure 3.7. Substrates used for TiO2 nanorod growth: (A) bare FTO, (B) a 10 nm-thick coating of TiO2 on FTO, and (C) MnOOH nanoparticles on FTO. .......................................................... 55

Figure 3.8. UV-vis absorbance data for TiO2 nanorod films sensitized with NcQ, an acenequinone dye with absorbance at 410 nm (absorbance spectrum reproduced from reference 6 of the main paper) ......................................................................................................................... 56

Figure 3.9. Growth rate of single crystalline TiO2 nanorods from TiO2 sheet seeding using 28 mM Ti(iOPr)4 ................................................................................................................................ 57

Figure 3.10. Current−voltage behavior of photovoltaic cells using the grown TiO2 nanorod films....................................................................................................................................................... 59

Figure 4.1. 1H NMR for compound 3. .......................................................................................... 65

Figure 4.2. 13C NMR for compound 3. ......................................................................................... 65

Figure 4.3. 1H NMR for compound 4............................................................................................ 66


Figure 4.5. 1H NMR for compound 5............................................................................................ 68


Figure 4.7. Absorption and fluorescence spectra of azaBODIPYS 1-5 in toluene at rt (left and right, respectively). ....................................................................................................................... 69

x

Figure 4.8. Transient fluorescence of compounds 2 and 3, as measured by time-correlated single photon counting. ........................................................................................................................... 70

Figure 4.9. Cyclic voltammogram of azaBODIPYS in CH2Cl2 ................................................... 71

Figure 5.1. Electron injection by Ru-dye from ‘hot’ vs. thexi states ............................................ 79

Figure 5.2. The figure on the left is UV-vis (solid line) and emission of PcQ(dashed line) in CH2Cl2........................................................................................................................................... 86

Figure 5.3. Absorption spectra of pentacenequinone and naphthacenequinone in ethanol at rt. .. 87

Figure 5.4. Femtosecond transient absorbance spectra (left) and decay rate (right) of MNcQ-2/spiro-OMeTAD blend on glass .................................................................................................. 88

Figure 5.5. Femtosecond transient absorbance spectra (left) and decay rate (right) of TiO2 nanorods films with adsorbed NcQ-7 and overlaid spiro-OMeTAD ............................................ 89

Figure 5.6. Fluorescence transient absorbance spectra of C60-T ester/spiro-OMeTAD blend on glass............................................................................................................................................... 90

Figure 5.7. Transient absorbance spectra of TiO2 nanorods films with adsorbed C60-T and overlaid spiro-MeOTAD blend on glass. ...................................................................................... 90

Figure 5.8. Illuminated IV curve of solid state acceptor sensitized DSCs. .................................. 92

Figure 5.9. IPCE of NcQ-7 and N3 based acceptor sensitized solid state DSCs. ......................... 93

Figure 5.10. Light IV curve of liquid based acceptor sensitized cells. ......................................... 94

Figure 5.11. IPCE of liquid based acceptor sensitized cells. ........................................................ 95

xi

LIST OF SCHEMES

Page Scheme 2.1. Acceptor-sensitizers investigated in this study. C60-M = monoadduct [60]fullerene; C60-T = trisadduct[60]fullerene; NcQ = naphthacenequinone. ................................................... 18

Scheme 2.2. Synthesis of C60-T from commercially available starting materials. ..................... 28

Scheme 2.3. Synthesis of NcQ from commercially available starting materials. ......................... 28

Scheme 4.1. Synthesis scheme of azaBODIPY derivatives. ........................................................ 63

Scheme 5.1. Synthesis of NcQ-2, NcQ-7 and PcQ. ..................................................................... 85

xii

LIST OF ABBREVIATIONS

C60-M...………………………………………………………………………... C60 monoadduct C60-T...…………………………………………………………………………… C60 trisadduct DSCs…………………………………………………………….Liquid dye sensitized solar cells ssDSCs...…………………………………………………………...Solid dye sensitized solar cell NcQ-2...……………………………………………...5,12-Naphtacenequinone-2-carboxylic acid NcQ-7...……………………………………………...5,12-Naphtacenequinone-7-carboxylic acid PcQ……………………………………………………5,14-Pentacenequinone-2-carboxylic acid P3HT...………………………………….…………………………….…Poly(3-hexyl thiophene) Spiro-MeOTAD………...2,2’,7,7’-tetrakis. (N,N-di-methoxyphenylamine)-9,9’-spirobifluorene

1

CHAPTER 1

INTRODUCTION

1.1 Introduction

Fossil fuels have been the main source of energy ever since the industrial revolution. The

dark sides of using fossil fuel as the main source of energy are 1) it is not a clean energy source

to our environment, and 2) it is a limited resource. Burning of fossil fuels produces CO2 gas

which is known as a source for the global warming. The warming of the atmosphere is predicted

to be the cause of many problems in the global ecosystem.1 Research has shown that energy

demand will roughly double by midcentury and triple by 2100 due to a dramatic increase of

world population.1 Considering the limited amount of fossil fuels it will be a challenge to meet

the increasing demands. Therefore, a need for an alternative energy source is apparent.

Geopolitical, environmental, and economic security will likely only be realized by meeting the

energy demand of the projected increase of population by supplying a sustainable and carbon-

neutral energy source within the next 10-20 years.2 In the last couple of decades, interest in a

renewable energy source has increased dramatically. Research has been done on different types

of renewable energy sources such as wind energy, biomass, nuclear energy, and solar energy,

and has shown promising results. The amount of energy which hits the earth from the sun is

substantially higher than the projected energy demand. The current annual worldwide electric

generation capacity is 20 terawatts, and it is estimated that 14 -16 terawatts (TW, x 1012 watts) of

the total comes from fossil fuels.3, 4, 5 The amount of solar power that hits the surface of the earth

is 120,000 terawatts, which amounts to about 6000 times the present rate of the world’s energy

consumption.4 So, the sun is a great potential source of energy to meet all the energy demands.

But, the issue is that we need to be able to harvest the sun’s energy. So, we need an invention

2

and development to efficiently harvest and store the sun’s energy. Since my research focus is in

solar energy, my dissertation mainly focuses on photovoltaic devices and the organic and

inorganic materials that compose the photoactive layer in such devices. Today there are different

types of solar cells such as dye sensitized solar cells, polymer solar cells, hybrid solar cells,

quantum dot solar cells, perovskite solar cells, silicon solar etc. Silicon solar cells are the most

commonly used solar cells. Pure crystalline structures of silicon are required to make silicon

solar cell devices. But making crystalline silicon is energy and time consuming hence very

expensive. Therefore, looking for other easy and cheap methods of making devices would be

imperative. Cheap and easily manufacturable “next generation” cells such as dye sensitized

solar cells, organic solar cells (OPV), hybrid solar cells, perovskite solar cells and polymer solar

cells are some of the methods with great potential as an energy source for future. One advantage

of OPV in particular is its 1) low cost synthesis, 2) easy manufacture of thin film devices and

that 3) organic semiconductor thin films may show high absorption coefficients exceeding 105

M-1cm-1, which makes them good chromophores.6 However, a downside of the current organic

photovoltaic cells is that their efficiency is significantly lower than that for single and

multicrystalline silicon as well as CdTe and CuIn(As)Se cells.11 Also, polymer-based organic

photovoltaic cells are very sensitive to water and oxygen and, hence, need to be carefully sealed

to avoid rapid degradation.4 Dye sensitized solar cells have been studied extensively over the last

two decades. Current highest efficient dye sensitized solar cells reported are 12.3% (porphyrin

dyes)7,8, 12.05% (ruthenium dyes)7,8, and 15% (perovskites)9. The major disadvantage of DSCs

is the liquid electrolyte used which is temperature-sensitive. At low temperatures, the electrolyte

can freeze, thus rendering the solar cell completely unusable. At high temperatures, the liquid

electrolyte expands, making sealing the solar panels a major problem. The use of a liquid

3

electrolyte causes some serious additional problems such as potential instability, limitation of

maximum operation temperature, danger of evaporation, and extra cost for forming an electrical

series connection.10 Other limiting factors for dye sensitized solar cells is an excessive loss of

voltage during the dye-regeneration reaction, the use of the iodide/triiodide electrolyte as a redox

shuttle limits the attainable open-circuit potential.2 Similarly, there is a loss of energy during

electron injection to TiO2. Excited dyes shake of their excess energy when injecting to the

conduction band edge of TiO2 and we call this lost energy an overpotential (Fig 1.1). To solve

the overpotential, determining the potential energy of an electron in the TiO2 conduction band at

equilibrium with an adsorbed dye is essential. Determining equilibrium potential would help

researchers to know just what electrochemical potential must be targeted for efficient electron

injection with the least overpotential.

Figure 1.1. Schematics of dye sensitized solar cell depicting the energy loss due to overpotential.

For this study, we proposed adsorbing acceptor dyes instead of donor dyes on TiO2

would be more suitable because with acceptor sensitizer thermal equilibration state and slow

electron injection kinetics could be attained. Ruthenium-based dyes, which have been the

4

champion dyes for most of the history of DSC research, have multiphasic electron injection

kinetics and injection from higher singlet excited states is ultrafast (~1012 s-1) whereas the

thermally-equilibrated metal-ligand charge transfer (MLCT) state injects less quickly (~1010 s-

1)11-17. These makes studying the electron transfer kinetics of donor dyes complex. Photoreduced

acceptor-dyes however do not have a wide distribution of available states from which to

thermally inject an electron, as they are already a thermally equilibrated species. This will greatly

simplify their injection kinetics and make it easier to study the driving force dependence of the

injection process. Therefore, the slow electron injection kinetics and a thermally equilibrated

state of acceptor sensitizer would enable us to study a less complex electron transfer kinetics. My

research focus is in photogalvanic dye sensitized solar cells composed of inorganic oxide

semiconductors, electron-accepting dyes and organic polymers or hole conductor. In galvanic

solar cells the photoreduced (radical anion) unit is in solution and it has to move to the positive

electrode in order to give off its extra electron. As the radical anion moves in solution it can

collide with the oxidized species and undergoes a back electron transfer reaction (energy is

lost).18 Whereas, in photogalvanic dye sensitized solar cells the photoreduced entity does not

need to move because it is chemically attached to the positive electrode. Hence, the back reaction

is minimized.

1.2 Electronic Property of Polymers

π-Conjugated polymers create delocalized electron density, and the interaction of these π

electrons dictates the electronic characteristics of the polymer.19 As the conjugation of these π

electrons increases, the energy gap between the π to π*states decreases resulting in a band

structure somewhat similar to that observed in inorganic solid state semiconductors.19 The

regioregularity and stacking pattern of polymers moves the gap between the π to π*states which

5

affects the absorption profile. Highly regioregular polymers increase the π conjugation by

adopting a planar structure, which will make their absorbance shift towards the red region.20

1.3 Effect of Polymer Morphology on Hole Mobility

Morphology of a polymer layer plays a role in the performance of polymer devices

because it significantly affects the hole mobility.21 The morphology of P3HT polymer films

depends mainly in the regioregularity of the polymer, technique of deposition and nature of the

solvent.20,22-26 Highly regioregular polymers adopt a planar conformation with higher

crystallinity (better packing) which helps the hole mobility. Whereas the regiorandom P3HT

adopts a twisted conformation that makes it lose its conjugation and reduce the hole mobility.24

“Edge-on” and “plane on” have been the two types of morphology observed when making a

P3HT polymer film.21

Figure 1.2. Schematic structure showing the preferred arrangement of adjacent chains of regioregular HT-P3HT. (b) Plane-on orientation of P3HT with respect to an underlying surface. (c) Alternative edge-on arrangement from reference 20.

Charge transport and absorption can be affected by the regioregularity of poly-3-

hexylthiphene (P3HT). Kim et al. made devices with better efficiency using high regioregularity

(95.4%).28 Molecular weight of a polymer has a significant effect on the film morphology. An

edge-on orientation was obtained with a high molecular- weight polymer (Mw = 175 kg/mol, RR

6

= 81%) by drop casting, whereas a plane-on arrangement was observed by spin coating.29 In

contrast, only an edge-on orientation was obtained with low-molecular-weight P3HT (Mw = 28

kg/mol, RR = 96%), regardless of the casting method used.29 Research has shown that drop

casting favors edge-on film morphology whereas spin coating resulted in plane-on

arrangement.20,25 In spin coating self-reorganization is inhibited by fast drying of solvent. So,

spin speed and solvent type plays a big role in the morphology of the film. When time is allowed

for P3HT to crystallize, either by using casting techniques with slow evaporation of solvent or by

using solvents with high boiling points, charge mobility in the resulting films is typically

improved. Surin et al. measured high charge-carrier mobility in films of P3HT cast.27 Annealing

also helps in reorganization and packing of the polymer by removing the last bits of solvent.27,30

1.4 Process of Photocurrent Generation

There are about four steps in the processes of photocurrent generation in a polymer solar

cell. 1) absorption and generation of excitons, 2) diffusion of the excitons until they reach a

donor/acceptor interface, 3) dissociation of each exciton at the interface and 4) collection of

charge carriers. Free electrons and holes are not created directly within an organic semiconductor

upon absorption of a photon, but a bound electron/hole state (exciton) is created due to the large

binding energy (columbic attraction) of the charge carrier.4 The exciton then must be dissociated

to liberate the electron and hole carriers. A potential concern in this charge separation process is

that the electron and hole must overcome their mutual Coulomb attraction.19 The columbic

attraction between an electron and a hole is given by:

V = e2

4πεrε0r

where e is the charge of an electron,

7

εr is the dielectric constant of the surrounding medium,

ε0 is the permittivity of vacuum, and

r is the electron-hole separation distance

As shown in the above equation a medium dielectric constant is important. The higher the

dielectric constant, the lower the columbic attraction is between the electron and the hole. TiO2

has a high dielectric constant of ~80 which makes the Coulomb attraction of electrons and holes

in dye-sensitized photoelectrochemical cells effectively screened. A donor/ acceptor interface

enhances electron dissociation when they have an appropriate difference in electron affinity and

ionization potential.31,32

The aforementioned processes of photocurrent generation can be summarized in the

schematics drawn below.

Figure 1.3. Schematic diagram of the photoelectrochemical cell composed of a donor polymer on TiO2 semiconductor (n-type) and counter electrode.

1.5 Electron Quenching Mechanism

There are two types of electron quenching mechanisms when photoinduced electron

transfer is undertaken in a photoelectrochemical cell – donor (oxidative) and acceptor (reductive)

quenching. When a photon is absorbed by a chromophore in its ground state (So) it gets excited

-5.0

-4.0

-4.5

-3.0

-5.5 D

D* TiO2 Polymer

Conducting glass

Cathode

Voc

Evac

(ev)

hѵ

8

to a singlet electronic excited state (S1). Two different electron-transfer scenarios can emerge

from this: 1) if the excited chromophore has an acceptor molecule around (attached to) it, it can

give off an electron to the acceptor molecule forming a reduced acceptor (radical anion) and an

oxidized chromophore (radical cation). This process is known as donor (oxidative) quenching

and is illustrated in Fig 1.3. 2) Whereas if the excited chromophore has a donor molecule around

(attached to) it, it can accept an electron from the donor forming a reduced chromophore (radical

anion) and oxidized donor (radical cation). This process is known as acceptor (reductive)

quenching and is illustrated in fig.1.4.

Figure 1.4. Schematics of donor quenching mechanism.

Figure 1.5. Schematics of acceptor quenching mechanism.

Fullerenes are good electron acceptors due to their low lying empty orbitals. So it makes

a good electron energy match with the P3HT. Therefore, bulk heterojunction of P3HT: fullerene

(PCBM) solar cells have been made with a high efficiency. Scientists have made inverted solar

cells by replacing organic acceptors with oxide semiconductor as the n-type component. The

So

S1

So

S1

So

S1

hν

Radical anion Radical cation

h

Chromophore

Donor

So

S1

So

S1

So

S1

hν

Radical anion Radical cation Chromophore

Acceptor

9

appeal of substituting an oxide semiconductor for the organic acceptor is the ease and reliability

of nanostructuring the donor/acceptor interface, as well as the superior conductivity of the oxide

semiconductor relative to an organic semiconductor. My research is trying to incorporate

molecular acceptors such as fullerene and acenequinones in between the oxide semiconductor

and the organic donor to study and enhance device performance.

1.6 Fabrication of Devices

Figure 1.6 is showing step by step process of polymer oxide solar cell device fabrication.

Fabrication of ssDSCs were fabricated the same way as the polymer oxide solar cell but using

spiro-MeOTAD as a hole conductor in place of polymer P3HT.

Figure 1.6. Step by step polymer oxide solar cell fabrication.

1.7 Characterization of Solar Cell Devices

Voltage, current and fill factor are three important parameters used in evaluating the

Performance of a photoelectrochemical cell. These values are extracted from an IV curve, a plot

of applied voltage against measured current.

10

Figure 1.7. Model I-V curve of photoelectrochemical cell.

Voc = Open circuit voltage. Jsc = Short circuit current density Vmax = Maximum photovoltage Imax = Maximum photocurrent FF = Fill Factor η = Efficiency Fill factor is a ratio of theoretical maximum power to the practical maximum power. It

has values of between 0 and 1. The efficiency of a solar cell is the ratio of output electrical

power to the optical power incident to the cell and is calculated based on the equation below.

As illustrated in the above equation, the efficiency of a solar cell device is directly

proportional to the current density, open circuit voltage and the fill factor. Recombination of an

electron with its counterpart hole due to columbic attraction is a key factor in lowering current

density of devices.

1.8 Scope of the Present Work

In chapter 2, I explain the synthesis and characterization of molecular acceptors such

as naphthacenequinone and fullerene derivatives. I also discuss the hydrothermal growth and

η = Pmax

Pin

Jsc . Voc . FF Pin

=

11

characterization of oxide semiconductors such as ZnO and TiO2 nanorods. I explain the seeding

and growth bath concentration dependence of these oxide semiconductors. This chapter also

describes the solar cell device assembly procedure and characterization techniques. The device

performance dependence on the different types of oxide semiconductors is also discussed.

Fullerene and acenequinone compounds have been examined as electron mediators between a p-

type semiconductive polymer and two different n-type oxide semiconductors. Composite

interlayer materials were prepared by growth of oxide nanorods, chemisorption of the acceptor-

sensitizers, and infiltration of the polymer phase. Photovoltaic test cells were assembled

between silver and fluorine-doped tin oxide electrodes. These samples were studied for their

fluorescence quenching, current-voltage, and quantum efficiency behavior to characterize the

efficacy of the acceptor-sensitizers as electron-selective interlayers. We find that the sensitizers

are generally more effective with titanium dioxide than with zinc oxide, and that photovoltage

and fill factor increase in a trend that matches the increase in the 1st reduction potential of the

acceptor-sensitizers. We also observe photosensitization of the oxide semiconductor by the

acceptor-sensitizers either in parallel with the polymer as an alternate photosensitizer or in series

with the polymer in a two-photon process, depending according to the acceptor-sensitizers’ 1st

reduction potential. These results have implications for designing electron-selective interlayers

and donor-acceptor sensitizing compounds and materials for photoactive materials and devices.

Surface area of the oxide semiconductor is important in the device performance. The

higher the surface area the more molecular acceptor and donor can be loaded, which increases

the current density. Therefore, chapter 3 is about optimizing the seeding and growth conditions

of TiO2 nanorods to maximize their surface area. Therefore, new seeding conditions have been

examined for the hydrothermal growth of single-crystalline rutile TiO2 nanorods. Rutile

12

nanorods of ~20 nm diameter are grown from seed layers consisting of either (A) TiO2 or

MnOOH nanocrystals deposited from suspension, or (B) a continuous sheet of TiO2. These seed

layers are more effective for seeding the growth of rutile nanorods compared to the use of bare

F-SnO2 substrates. The TiO2 sheet seeding allows lower concentration of titanium alkoxide

precursor relative to previously reported procedures, but fusion of the resulting TiO2 nanorods

into bundles occurs at higher precursor concentration and/or longer growth duration.

Performance of polymer-oxide solar cells prepared using these nanorods shows a dependence on

the extent of bundling as well as rod height.

The absorption extinction coefficient of the acceptor molecules is also an important factor

in the performance of solar cell devices. In chapter 2 the acceptor molecules used do not have

high absorption extinction coefficient – Fullerene [4.9 x 103 M-1cm-1 (400 nm) and < 1000

M-1cm-1 650 nm)]22, acenequinones (1 x 104 M-1cm-1). It is expected that higher absorption

extinction coefficient of an acceptor would lead to a higher current density.23 Therefore, chapter

4 is about the synthesis of new azaBODIPY compounds with very high extinction coefficients-

(75,000 – 85,000 M-1cm-1)24. However, before their application in photovoltaic devices,

reduction potential matching of azaBODIPY with the conduction band of TiO2 oxide

semiconductor is necessary. Therefore in chapter 4 the optoelectronic tuning of boron-chelated

azadipyrromethene dyes has been explored by the substitution of carbon substituents in place of

fluoride atoms at boron. The resulting organoboron-azaBODIPYs have been characterized by

steady-state UV-vis and fluorescence spectroscopies, transient fluorescence spectroscopy, and

cyclic voltammetry. Trends in the absorbance, fluorescence, and redox behavior appear

dependent on the effective electronegativity at the boron atom as tuned by its substituents, with

13

stronger electronegativity correlating to red-shifted absorbance, enhanced fluorescence lifetime

and yield, and positively-shifted redox potentials.

In chapter 1 a P3HT polymer was spin coated on top of the acceptors. The high

extinction coefficient of P3HT dwarfs the absorbance of the acceptor compounds. Consequently,

the effect of acceptors was not prominent when the IPCE of the devices was recorded.

Employing a hole conductor that has a zero or negligible absorbance in the visible region would

make the effect of acceptors more pronounced. Therefore, in chapter 5 2,2’,7,7’-tetrakis (N,N-

di-methoxyphenylamine)-9,9’-spirobifluorene (spiro-OMeTAD) was used as a hole conductor to

study the effect of acceptors. Electron transfer between the acceptors and the spiro-OMeTAD

system was studied using a femtosecond transient absorption system, with help of Habtom

Gobeze from the group of Prof. Francis D’Souza. Devices were made and studied for their

current-voltage and quantum efficiency behavior.

1.9 References

1. Nocera, D. G. Energy Environ. Sci., 2010, 3, 993–995.

2. Cook, T. R.; Dogutan, D. K.; Reece, S. Y.; Surendranath, Y.; Teets, T. S.; Nocera, D. G., Chem. Rev., 2010. 110. 6474.

3. Yang, Z.; Zhang, J.; Kintner-Meyer, M. C. W.; Lu, X.; Choi, D.; Lemmon, J. P.; and Liu, J. Chem. Rev. 2011, 111, 3577.

4. Gratzel, M. Acc.Chem. Res. 2009, 42, 1788.

5. Yarris, L. Berkeley Lab Home Page. http//http://www.lbl.gov/publicinfo/newscenter/features/2008/FS-chilean.html (accessed Jan 16, 2014).

6. Gunes S.; Neugebauer H.; Sariciftci N. S., Chem. Rev. 2007. 107. 1324.

7. Yella, A.; Lee, H.; Tsao, H. N.; Yi, C.; Chandiran, A. K.; Nazeeruddin, M.; Diau, E. G.; Yeh, C. Y.; Zakeeruddin, S. M.; Grätzel, M., Science. 2011, 334, 629-634.

14

8. Clifford, J. N.; Planells, M.; Palomares, E., J. Mater. Chem., 2012, 22, 24195–24201

9. Burschka, U.; Pellet, N.; Moon, S. J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Gratzel, M., Nature. 2013, 499, 316.

10. Shah, A.; Torres, P.; Tscharner, R.; Wyrsch, N.; Keppner, H., Science 1999, 285, 692-698.

11. Kuciauskas, D.; Monat, J. E.; Villahermosa, R.; Gray, H. B.; Lewis, N. S.; McCusker, J. K., J. Phys. Chem. B 2002, 106, 93479358.

12. She, C.; Guo, J.; Lian, T., J. Phys. Chem. B 2007, 111, 69036912.

13. Kelly, C. A.; Farzad, F.; Thompson, D. W.; Stipkala, J. M.; Meyer, G. J., Langmuir 1999, 15, 70477054.

14. Benkö, G.; Kallioinen, J.; Korppi-Tommola, J. E. I.; Yartsev, A. P.; Sundström, V., J. Am. Chem. Soc. 2002, 124, 489493.

15. Kallioinen, J.; Benkö, G.; Sundström, V.; Korppi-Tommola, J. E. I.; Yartsev, A. P., J. Phys. Chem. B 2002, 106, 43964404.

16. Haque, S. A.; Palomares, E.; Cho, B. M.; Green, A. N. M.; Hirata, N.; Klug, D. R.; Durrant, J. R., J. Am. Chem. Soc. 2005, 127, 34563462.

17. 15. Koops, S.; Durrant, J. R., Inorg. Chim. Acta 2008, 361, 663670.

18. Alberry, W. J.; Archer, M. D. Nature 1977, 270, 399

19. Clarke, T. M.; Durrant, J. R., Chem. Rev. 2010, 110, 6736.

20. Dang, M. T.; Hirsch, L.; Wantz, G.; Wuest, J. D., Chem. Rev. 2013, 113, 3734.

21. Salleo, A. Mater, Today 2007, 10, 38.

22. Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Nature 1999, 401, 685.

23. DeLongchamp, D. M.; Vogel, B. M.; Jung, Y.; Gurau, M. C.; Richter, C. A.; Kirillov, O. A.; Obrzut, J.; Fischer, D. A.; Sambasivan, S.; Richter, L. J.; Lin, E. K. Chem. Mater. 2005, 17, 5610.

24. Wang, G.; Swensen, J.; Moses, D.; Heeger, A. J. J. Appl. Phys. 2003, 93, 6137.

25. Wang, G.; Hirasa, T.; Moses, D.; Heeger, A. J. Synth. Met. 2004, 146, 127.

15

26. Heil, H.; Finnberg, T.; von Malm, N.; Schmechel, R.; von Seggern, H. J. Appl. Phys. 2003, 93, 1636

27. Cho, S.; Lee, K.; Yuen, J.; Wang, G.; Moses, D.; Heeger, A. J.; Surin, M.; Lazzaroni, R. J. Appl. Phys. 2006, 100, 114503.

28. Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Giles, M.; McCulloch, I.; Ha, C.S.; Ree, M. Nat. Mater. 2006, 5, 197.

29. Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Nature 1999, 401, 685.

30. Zen, A.; Pflaum, J.; Hirschmann, S.; Zhuang, W.; Jaiser, F.; Asawapirom, U.; Rabe, J. P.; Scherf, U.; Neher, D. Adv. Funct. Mater. 2004, 14, 757

31. Tang, C. W. Appl. Phys. Lett. 1986, 48, 183.

32. Arkhipov, V. I.; Heremans, P.; Bässler, H. Appl. Phys. Lett. 2003, 82, 4605.

33. Sokolyuk, N. T.; Romanov, V. V.; Pisulina, L. P., Russ. Chem. Rev. 1993, 11, 1005 -1024.

34. Jang, S. R; Yum, J. H.; Klein, C.; Kim K. J.; Wagner P.; Officer, D.; Gratzel, M.; Nazeeruddin M. K., J. Phys. Chem. 2008. 113. 1998.

35. Loudet, A.; Burgess, K. Chem. Rev. 2007, 107, 11

16

CHAPTER 2

ELECTRON TRANSPORT IN ACCEPTOR-SENSITIZED POLYMER-OXIDE SOLAR

CELLS: ACCEPTOR-QUENCHING AND ELECTRON CASCADE EFFECTS*

2.1 Introduction

Hybrid inverted solar cells that employ metal oxide semiconductors such as titanium

dioxide (TiO2) and zinc oxide (ZnO) together with organic semiconductive polymers have been

investigated for several years now as an alternative to all-organic bulk heterojunction composites

(Figure 1).1-9 The oxide semiconductors assert the role of the n-type semiconductor component

paired with p-type semiconductive polymers such as poly(3-hexylthiophene) (P3HT) and various

poly(phenylenevinylene) and polyfluorene derivatives. The appeal of substituting the oxide

semiconductor for an organic acceptor is in the ease and/or reliability of nanostructuring the

donor/acceptor interface, as well as in the superior conductivity of the oxide semiconductor

relative to an organic semiconductor. Despite these advantages, hybrid inverted solar cells have

yet to equal, let alone surpass, the performance of all-organic bulk heterojunction devices. The

shortcoming of these hybrid composites is the inferior efficiency of interfacial electron transport

at the donor/acceptor heterojunction when the acceptor is an oxide semiconductor. The yield of

electron transfer at the P3HT/TiO2 interface, for example, has been separately examined by

photoluminescence quenching and time-resolved microwaved conductivity (TRMC),10,11 and

found to be poorly efficient. In P3HT/Zn1-xMgxO composites, TRMC analysis has shown that

the bulk of photogenerated charge carriers form within the P3HT bulk rather than at the

interface, and must transfer as free carriers into the oxide acceptor.12 The yield of free

* This entire chapter is reproduced from Berhe, S. A.; Zhou, J. Y.; Haynes, K. M.; Rodriguez, M. T., Youngblood, W. J. ACS Appl. Mater Interfaces, 2012, 4, 2955-2963 with permission from the American Chemical Society.

17

photogenerated charge carriers within P3HT and other p-type semiconductive polymers upon

illumination is not high, however, and as a compensation for this problem, fullerene acceptors

including C60 and [60]PCBM are commonly blended into the polymer component to increase

the photocurrent yield, creating a donor-acceptor1-acceptor2 electron cascade with at least three

possible electron transfer interfaces: polymer-oxide, polymer-fullerene, and fullerene-oxide.6,13-17

Figure 2.1. Electron transport events in polymer-oxide excitonic solar cells.

More recent studies have substituted or complemented the bulk fullerene acceptor by

placing fullerene acceptors directly at the polymer/oxide interface.18-23 Other interfacial

modifiers, including ruthenium-polypyridyl, zinc-porphyrin and other dyes,18,24-28, as well as

other aromatic and aliphatic molecular species have been explored, to varying degrees of

electron transfer enhancement.20,25,26,29 None of these interfacial modifiers match the

photocurrent boost from blending fullerenes into the polymer bulk, and not all are intended to

create an electron cascade. Other factors cited for the benefits of molecular interfacial modifiers

are: (1) providing a low-dielectric interface environment that improves the wettability of the

18

oxide component by the polymer,24,26 or alters the polymer chain packing near the oxide

surface;29 (2) lowering the conduction band edge by interface dipoles of carboxylate binding

groups and/or the overall molecular dipole of the molecular modifier;30-32 (3) photosensitizing

the oxide with the molecular monolayer and transferring holes into the polymer;27,28 and (4)

inhibiting back electron transfer (recombination) from the oxide conduction band to the

polymer.24,26

Scheme 2.1. Acceptor-sensitizers investigated in this study. C60-M = monoadduct [60]fullerene; C60-T = trisadduct[60]fullerene; NcQ = naphthacenequinone.

Figure 2.2. Qualitative view of energetic considerations of electron transport at polymer-sensitizer-oxide interfaces. Solid lines indicate photoinduced electron transport; dotted lines indicate dark (thermal) electron transport. CS = charge separation; CR = charge recombination; CB = conduction band; VB = valence band; SC = semiconductor.

We sought to investigate the importance of an electron transport cascade at interfacially-

modified P3HT/oxide semiconductor junctions by examining a set of acceptor-sensitizers (Chart

19

1) having increasingly negative 1st-reduction potentials at interfaces of P3HT with ZnO and TiO2

nanorod films. For interfaces of pristine polymer with oxide semiconductors, we propose that an

increase in reduction potential of the intermediate acceptor-sensitizer should provide greater

overpotential for electron transfer into the oxide (Figure 2), possibly boosting photovoltaic

performance. For interfaces of P3HT/PCBM with an oxide semiconductor, acceptor-sensitizers

with 1st-reduction potentials more negative than that of the PCBM radical anion should provide a

thermodynamic barrier for an electron cascade, inhibiting photocurrent. We find that the use of

these interfacial monolayers is helpful with TiO2, regardless of whether PCBM is included or

omitted in a given device. With ZnO, however, the acceptor-sensitizers were only helpful in the

absence of PCBM. When P3HT:PCBM blends were used with ZnO, the acceptor-sensitizers

reduced the photocurrent and photovoltage. In the absence of PCBM we found that

improvements in photovoltage were consistent with the trend of rising 1st reduction potentials of

the acceptor compounds. Photosensitization by some of the acceptor-sensitizers is also apparent

in at P3HT/TiO2 interfaces, despite insufficient energetics for electron transfer from neutral

photoexcited acceptor molecules to the oxide. We describe how this photocurrent generation

could arise from hole transfer from photoexcited acceptors to P3HT, leading to dark (thermal)

electron injection into the oxide semiconductors by the resulting radical anion acceptor-

sensitizers. When PCBM is included in TiO2-based cells, our results support the previous

assertions that effects other than electron cascade can contribute significantly to the modulation

of electron transport at the organic-oxide interface.

2.2 Experimental Section

Anhydrous toluene and dichloromethane were prepared by distillation over CaH2. NaH

was used as a 60% dispersion in mineral oil. Chemicals were purchased as reagent grade and

20

used as received. Compounds 1 and 4 (Schemes 2.1 and 2.2, pp 26 and 27, respectively) were

prepared according to previously reported procedures.33,44 Analytical data were consistent with

the previous results. Deionized water was obtained from a Millipore water purifier (10 MΩ

resistivity) for oxide growth solutions. UV-vis absorption data were collected with a Varian

Cary spectrophotometer.

Propanedioicacid,1,1’,1”-[1,3,5-benzenetriyltris(oxy-2,1-ethanediyl)]-3,3’,3”-triethylester

(2): A mixture of 1,3,5-tris(2-hydroxyethoxy)benzene (0.83 g, 3.2 mmol) and dry pyridine (1.12

mL, 13.9 mmol) in 48 mL of dry CH2Cl2 kept at 0 °C under Argon. A solution of ethyl 3-chloro-

3-oxopropionate (1.35 ml, 10.5 mmol) in 10 ml of dry CH2Cl2 was added dropwise over a course

of 30 minutes. The reaction was stirred at room temperature for 65 h. The reaction mixture was

purified using gravity column chromatography [SiO2, (1st column CH2Cl2: MeOH (97:3); 2nd

column toluene: EtOAc (1:1)]. The fractions containing the product were concentrated to give a

light yellowish oil (1.24 g, 75%): δH(500MHz, CDCl3): 1.14 (t, 9H), 3.32 (s, 6H), 4.02 (t, 6H),

4.07 (q, 6H), 4.36 (t,6H), 6.00 (s,3Ar-H); δC (125MHz, CDCl3): 13.64, 40.98, 61.16, 63.16,

65.38, 94.16, 159.89,165.96,166.20; ESI-MS obsd 600.67, calcd 600.20 (C27H36O15).

Trisadduct Ester (3): C60 (160 mg, 0.22 mmol) was sonicated in 240 mL of anhydrous

toluene. Compound 2 (128 mg, 0.22 mmol) and I2 (176 mg, 1.39 mmol) were added. 1,8-

diazabicyclo[5.4.0]undec-7-ene(0.248 mL, 1.66 mmol) was diluted in 150 mL of anhydrous

toluene and added dropwise over a course of 3 h. The mixture was stirred at room temperature

under argon for another 20 h. It was purified by gravity column chromatography [SiO2, toluene:

EtOAc (9.5:0.5)]. The fractions containing the product were concentrated to give a red solid. (94

mg, 32%): δH(500MHz, CDCl3): 5.83( s, 3H), 4.82 (d, 3H) 4.46 (q, 6H), 4.39 (m, 3H) 4.25 (t,

3H) 4.07 (d, 3H) 1.38 (t, 9H); δC (125MHz, CDCl3): 163.22, 162.67, 160.09, 147.01, 146.84,

21

146.81, 146.79, 146.70, 146.45, 146.16, 145.91, 145.69, 144.98, 144.19, 143.64, 143.18, 142.74,

142.11, 141.93, 141.15, 140.98, 94.27, 70.88, 69.59, 66.27, 65.93, 63.44, 52.79, 14.36; LD-MS

obsd 1314.08, calcd 1314.16; UV-vis (CHCl3) λmax(nm): 280, 483, 563.

Trisadduct Acid (C60-T): 3 (20 mg, 0.015 mmol) was sonicated in 60 mL of anhydrous

toluene, NaH (36.3 mg, 1.51 mmol) was added and stirred at 60 °C under argon atmosphere. 1

mL of MeOH was added after 3 h. A reddish brown precipitate was formed and collected by

centrifugation. It was washed with toluene, 2 M H2SO4 and then with water, and dried under

vacuum (14 mg, 90%). Analytical data were consistent with a previous report.35

Figure 2.3. UV-vis absorption spectra for C60-M (left) and C60-T (right).

2-Methyl-5,12-naphthacenequinone (5): A mixture of 7-methyl-1,4-Naphthoquinone

(0.17 g, 1.00 mmol), NaI (1.45 g, 9.64 mmol) and α,α,α’,α’-tetrabromo-o-xylene(0.61 g, 1.45

mmol) in 4 mL of DMF was heated to 80°C overnight. The mixture was then cooled to room

temperature and 25 mL of water and 7 mL NaHSO3 were added until color faded from the

solution. The solution was extracted between ethyl acetate and water (3x) and the organic layer

was dried over anhydrous MgSO4, filtered, and concentrated to dryness, giving a brown solid.

The brown solid was dissolved in a mixture of hexanes: EtOAc (4:1), from which the product

precipitated as a yellow solid (70 mg, 50%): mp 242-246oC. δH(400MHz, CDCl3): 8.85 (s,2H)

22

8.30(d, 1H) 8.19 (s, 1H) 8.03 (m, 2H) 7.70 (m, 2H) 7.63 (d, 1H) 2.56 (s,3H); δC(125MHz,

CDCl3): 183.51, 183.08,145,56, 135.44, 135.48, 135.24, 134.54, 132.45, 130.28, 130.05, 129.80,

129.60, 129.60, 129.56, 22.12; LD-MS obsd ; UV-vis (CHCl3) λmax(nm): 303, 392, 409.

5,12-Naphthacenequinone-2-carboxylic acid (NcQ): A teflon autoclave pressure vessel

was charged with 5 (44 mg, 0.16 mmol), sodium dichromate ( 0.105 g, 0.4 mmol) and water

(1.33 mL) and heated in an oven at 225°C for 18 h. The pressure vessel was taken out and cooled

by running tap water over it. The green solid within was sonicated and filtered with warm water.

HCl (0.794 mL) was added and allowed to cool to room temperature overnight to give a yellow

suspension. Filtration gave a yellow solid (28 mg, 56%). δH(400MHz, (CD3)2SO): 8.87 (d,2H)

8.73(s, 1H) 8.39 (d, 1H) 8.36 (s, 1H) 8.32 (m, 2H) 7.79 (m, 2H); δC (125MHz, (CD3)2SO):

182.13, 182.05, 136.49, 134.87,130.38, 130.32, 130.10, 129.68, 129.65, 129.52, 129.38, 129.34,

127.8; LD-MS obsd 302.03, calcd 302.06 (C19H10O4); UV-vis (CHCl3) λmax(nm): 303, 393.

2.3 Growth of Nanostructured Oxide Materials

We prepared ZnO nanorod layers following the seeding method by Ohyama and

coworkers,33 and using a modification of the growth conditions reported by Hodes and

coworkers.65 Films were grown to 300 nm height (Figure S1). This height is sufficient to

achieve near-opacity in the region of the visible spectrum where the polymer absorbs (400-600

nm), and thickness is otherwise minimized to compensate for the weak charge carrier mobility in

the polymer. TiO2 (rutile) nanorods were grown using the hydrothermal method reported by Liu

and Aydil (Figure S2).45 We observed a ~90 minute induction period for nanorod growth using

the TiO2 underlayer, after which the growth is rapid. Achieving short nanorod films of

reproducible height requires careful timing of the growth period. A film height of 700 nm,

which would normally be thicker than desired, was found to give the best photocurrent/voltage

23

behavior for unsensitized TiO2. Shorter films (200 nm) exhibited much lower photovoltages,

perhaps due to partial short circuiting caused by etching of the TiO2 underlayer in the acidic

growth conditions. Longer growth times may allow for some regrowth of TiO2 at the FTO

surface, or may not allow sufficient polymer infiltration for direct P3HT/FTO contact. Shorter

films may yield better performance if the FTO is passivated by chemical or electrochemical

means. Films of P3HT that are thicker than ~300 nm experience greater series resistance due to

the conductivity of charge carriers being inferior in organic semiconductors compared to

inorganic semiconductors. Further work in this direction is in progress to control the growth at

short film heights and narrow the nanorod diameter, but was not crucial to this initial study.

Figure 2.4. SEM image of ZnO nanorods by hydrothermal growth.

Figure 2.5. SEM image of TiO2 nanorods by hydrothermal growth.

24

Assembly and testing of solar cell prototypes: Sensitization of oxide substrates was

performed by soaking the TiO2 nanorod films overnight, while ZnO nanorod films were soaked

for 20 min in solutions of the sensitizers in THF (C60-T) or DMF (C60-M, NcQ). P3HT or a

P3HT/PCBM blend was spincoated onto oxide nanorod films from chlorobenzene solution (10

mg/ml or 10 and 8 mg/ml, respectively). PEDOT:PSS was spincoated from aqueous dispersion.

Films were transferred into a metal evaporator and silver electrodes of ~70 nm thickness were

deposited in island-type geometry with a central dot of 1.1 cm2 area. Each device was capped

with a cover slide and annealed on a hotplate at 125 °C for 5 min. Silver paste was applied to

improve contact to the FTO, and devices were warmed to 50 °C for 3 hours and allowed to cool

to room temperature. Devices were tested on the day after they were assembled.

Photoluminescence studies were done by placing the devices within a fluorescence spectrometer

(Cary Eclipse) with the substrate (glass side of transparent electrode) at a 51° angle with respect

to the incident light, such that the reflection of incident light was seen to avoid the exit slit

leading to the monochromator and detector. Current-voltage measurements were taken with a

sourcemeter (Keithley 2400) and a Class A solar simulator (Solarlight, Inc.). IPCE spectra

(incident photons converted to electrons) were measured against a calibrated silicon photodiode

using monochromatic light from a xenon lamp (PV measurements QEX7).

2.4 Selection of the Acceptor-Sensitizers and other Interface-Modifier Compounds

We chose the easily prepared C60-M (Chart 2.1) as the benchmark acceptor-sensitizer

because monoadduct fullerenes have already been reported to enhance charge transfer at

interfaces between P3HT and either ZnO or TiO2.18-21 These precedents may seem surprising,

given that electron transfer in the reverse direction, from photoexcited ZnO or TiO2 to C60 or

monoadduct fullerenes, has also been reported.35-37 The apparent discrepancy is resolved by the

25

consideration that the conduction band edges of these oxide semiconductors are significantly

lower in the absence of a liquid electrolyte. Electron transfer from the oxides to fullerenes is to

be expected in a solution-phase experiment, whereas the reverse direction is possible when the

interface is dry. The flat band potentials of ZnO and TiO2 are -0.76 and -0.81 vs. SCE in

acetonitrile, respectively,38,39 whereas Kelvin probe measurements of the dry oxides in air

provide band edge potentials corresponding to values of -0.6 V vs. SCE for ZnO (4.1 eV below

vacuum)6 and either -0.5 or -0.25 V vs. SCE for TiO2 (4.2 or 4.45 eV below vacuum – separate

measurements).32,40 The difference in electrochemical potentials under wet/dry conditions is

attributed to the dipoles formed by alignment of electrolyte molecules at the oxide surface that

shift the band edges upwards. Additionally, the ‘dry’ levels reported under air for oxide

semiconductors are influenced by the coordination of oxygen molecules to the oxide surfaces

that shift the band edges further downward, and this has implications for dry interfaces operating

under inert atmosphere.41,42 In a similar manner, the conduction band of TiO2 has been shown to

shift due to the adsorption of molecular sensitizers in a manner related to the direction and

magnitude of the molecular dipoles of the sensitizers (vide infra).25,30,43,44

Additional molecular acceptor compounds with increasing 1st reduction potentials include

an e,e,e-trisadduct fullerene (C60-T), and a naphthacenequinone (NcQ). The equatorial

placement of malonic acids on the trisadduct C60-T was chosen to enable simultaneous binding

of one carboxylic acid from each adduct in an attempt to minimize the diversity of possible

binding modes by enabling a most-stable configuration at the oxide surface. Table 1 shows the

redox potentials, in descending order, for the neutral excited and radical anion states for P3HT

and all the acceptor species examined in this study, along with the band edge potentials for ZnO

26

and TiO2 as measured in air. The excited state oxidation potential (ESOP) is determined via the

ground state oxidation potential (Eox) and the excited state energy (E0−0) by eq 1:

ESOP = Eox – E0-0 (1)

An important caveat in the table is that the Kelvin probe data are reported for anatase

TiO2, whereas the TiO2 nanorods we employ in this study are rutile.45 Although the conduction

band edge of rutile TiO2 is reported to be 200 mV positive of the band edge of anatase TiO2 in

the presence of electrolyte,46 no corresponding offset has been reported in dry conditions.

Although we could find no literature report for a Kelvin probe measurement of undoped rutile

TiO2 at the time of preparing this manuscript, we note that a reported measurement is available

for rutile TiO2 doped with 0.05 wt% Nb, and places the band edge at 4.2 eV below vacuum for

the [110] crystal face,47 which is the operative surface of the TiO2 nanorods as grown for this

study.

Table 2.1. Redox potentials of organic semiconductors and conduction band edges of ZnO and TiO2

Organic/oxide ESOP(*/+) RP(-)/CBE Ref. P3HT (dry) -1.95b 48 P3HT -1.10 49,50 NcQ -1.08S/-0.57T -1.13d,e 51 C60-T -0.44S/-0.17T -0.86e 52 PCBM -0.29S/-0.01T -0.67d 53,54 PCBM (dry) -0.61S/-0.4T b -0.9w/-0.4h 48,54-56 C60-M -0.2S/0.1T -0.64e 52,54,57 ZnO (dry) -0.60h 6 TiO2 (dry) -0.5h/-0.25h 32,40

All potentials determined for solvated species vs. SCE or converted to SCE from other electrodes58 or vacuum level,59 where indicated. ESOP = Excited State Oxidation Potential; RP = Reduction Potential; CBE = Conduction Band Edge; b Originally reported vs. vacuum (UPS). c Reported for 5,12-naphthacenequinone. d Originally reported vs. Fc/Fc+. e Measured from ester derivative. f Originally reported vs. NHE. g AcN = acetonitrile electrolyte. h Originally reported vs. vacuum (Kelvin probe). S Singlet excited state. T = Triplet excited state. w Originally reported vs. vacuum (IPES).

27

There is a need for a clearer understanding of whether and how the redox potentials of

polymeric and molecular semiconductors may shift from a solvated to solventless environment.

Voltammetrically-determined redox potentials for dissolved polymeric and molecular

semiconductors are commonly converted to the vacuum scale and assumed to be relevant to the

solventless environment of solid state material composites such as bulk heterojunctions and

hybrid excitonic solar cells. There are reported measurements for some photoelectroactive

compounds by ‘dry’ methods, such as Kelvin probe contact potentials, ultraviolet photoelectron

spectroscopy (UPS), and inverse photoemission spectroscopy (IPES), but these methods

unfortunately do not consistently agree with one another, let alone with the values obtained for

liquid phase voltammetry. There is not even any clear trend for the disagreement, as for

example, Kelvin probe measurement of the energy level for photoreduced PCBM (-0.4 V vs.

SCE) places the species at lower energy than gauged by cyclic voltammetry in solution (-0.67 V

vs. SCE), which is lower still than that determined from IPES (-0.9 V vs. SCE). In the current

absence of a clear understanding on this issue, we rely on the assumption that the acceptor-

sensitizers must be all affected in a uniform manner, such that the hierarchy of relative energetics

for the 1st reduction potentials of these compounds in solution will be maintained in solid state

films.

2.5 Synthesis of the Acceptor-Sensitizers

We developed synthetic procedures for the tris(malonic acid)-fullerene (C60-T, Scheme

1), and the naphthacenequinone-carboxylic acid (NcQ, Scheme 2). Preparation of the C3-

symmetric C60-T was adapted from a reported synthesis of such equatorial (e,e,e)-trisadduct

fullerenes by Hirsch and coworkers.60 The earlier-reported tris-tethers were designed to allow

selective deprotection of either the interior ester groups (attached to benzene core) or the exterior

28

ester groups. We have simplified the synthesis by using an ethyl group at the exterior ester of 2,

in anticipation of complete hydrolysis of the tris(malonate ester)-adduct 3 to form the

corresponding acid product C60-T.61 Compound NcQ has been previously reported starting

from 1,4-anthraquinone,62 but that synthesis requires a laborious preparation of the unstable

precursor 2-hydroxymethyl-1,3-butadiene,63 so we devised a shorter, easier route through the

fusion of methylnaphthoquinone and α,α, α2,α2 -tetrabromoxylene. The resulting

methylnaphthoquinone 5 is oxidized to the corresponding carboxylic acid NcQ, without

damaging the quinoidal keto groups, by the use of Na2Cr2O7 under forcing conditions.64

Scheme 2.2. Synthesis of C60-T from commercially available starting materials.

Scheme 2.3. Synthesis of NcQ from commercially available starting materials.

2.6 Fluorescence Quenching and Current- Voltage Behavior in Sensitized P3HT – TiO2 Devices

Steady-state fluorescence quenching studies (Figure 3) show that P3HT fluorescence

increases when embedded within TiO2 nanorods, most likely due to the loss of polymer chain

29

crystallization. Qualitatively, this indicates that fluorescence quenching due to photoinduced

electron transport at the P3HT/TiO2 interface must be weak due to an inherently low quantum

yield for the process. Morphology will also play a role in the overall extent of quenching: when

significant amounts of P3HT more than ~10-20 nm from the TiO2 interface, the excited states

will not reach the interface to engage in electron transfer. Scanning electron microscopy images

of the TiO2 nanorods film (Figure S2) indicate that the channels between nanorods are quite

narrow, so no appreciable quantity of P3HT should be far from the polymer/oxide interface.

Some quenching is observed between P3HT and acceptor-sensitizer monolayers as the 1st

reduction potential of the sensitizer trends more positive, reflecting the increasing driving force

for electron transfer. Subtle differences in the shape of the luminescence peaks may indicate

conformational differences in the polymer chain packing induced by the change in wettability at

sensitized oxide surfaces relative to bare TiO2.

Figure 2.6. Fluorescence of P3HT as neat film, and on bare and sensitized TiO2 nanorods.

The most dramatic evidence for altered electron transport behavior is shown in the

current-voltage and quantum efficiency (IPCE: incident photons converted to electrons) behavior

of test devices having P3HT-Acceptor-TiO2 interfaces. Figure 4 shows that a reduction in dark

30

current for sensitized TiO2-based devices compared to bare TiO2. A reduction in dark current is

usually a good sign that photovoltaic performance will be improved, and the acceptor-sensitized

cells do show improved the photocurrent and photovoltage. The photovoltage and fill factor

improvements correlate to the rise in the reduction potentials of the acceptor-sensitizers. Table 2

details the photovoltaic performance of these and other cells tested in this study.

Figure 2.7. Current-voltage behavior of P3HT/TiO2 devices under 1 sun illumination (upper) and in the dark (lower).

31

Table 2.2. Photocurrent, photovoltage, fill factor, and efficiency of devices prepared in this study.

P3HT-Acceptor-Oxide JSC (mA)

VOC (mV) FF

PCE (%)

P3HT—bare—TiO2 1.34 216 0.36 0.11 P3HT—C60-M—TiO2 1.56 241 0.36 0.13 P3HT—C60-T—TiO2 1.92 280 0.37 0.20 P3HT—NcQ—TiO2 1.72 609 0.56 0.59

JSC = short circuit current; mA = milliamps; VOC = open circuit voltage; mV = millivolts; FF = fill factor; PCE = power conversion efficiency.

The fluorescence quenching and photocurrent-enhancing effects of the acceptor-

sensitizers suggest that photoinduced electron transfer at P3HT-Acceptor-TiO2 interfaces is more

efficient than at the binary P3HT-TiO2 interface. The trend in the inhibition of dark current

matches the photovoltage improvement, suggesting that the acceptor-sensitizer monolayer is

behaving as an electron-selective contact, presumably as a tunneling barrier to transport from the

TiO2 conduction band electrons to P3HT. When the reduction potential of the acceptor-

sensitizer is well above the conduction band edge (CBE) of the TiO2, the density of states in the

sensitizer monolayer at the energy level of the TiO2 CBE may be too low to support tunneling of

the oxide-to-P3HT electron transport.

2.7 Surface Dipole Effects in Solid-State Sensitized Oxide Materials and Devices

Another factor in interpreting the data in Figure 4 is that surface dipoles may alter the

effective local vacuum levels of the P3HT and TiO2 domains. The dipole effect has been

observed and described in dye-sensitized solar cells having liquid electrolyte or solid-state hole

conductor as well as for polymer−oxide composites.25,30,55,56 When an intervening monolayer of

molecules can direct a net dipole at the interface, the side at the negative end of the dipole

experiences a higher local vacuum level relative to the side at the positive end of the dipole

(Figure 2.8).

32

Figure 2.8. Vacuum level shifts caused by surface dipoles.

Local dipoles at carboxylate binding groups will push negative toward the oxide, whereas overall

molecular dipoles will vary from one sensitizer to another. The P3HT-acceptor interface may

also generate a charge-transfer dipole which would point the negative end toward the oxide

surface.57 The strength of such a dipole would relate to the degree of charge transfer at the

interface. When a dipole is positive toward the oxide semiconductor, the vacuum level at the

oxide is lowered relative to the polymer phase, with the result that photocurrent increases but

photovoltage suffers (Figure 2.8, left). When a dipole is negative toward the oxide, the vacuum

level at the oxide is raised, and photocurrent suffers but photovoltage is boosted (Figure 2.8,

right). The enhancement of one trait (current or voltage) comes at the cost of the other. Dark

current trends are also consistent for this paradigm, giving reduced dark current when the oxide

semiconductor’s vacuum level is raised, and increased dark current when the oxide’s vacuum

level is lowered. The monolayer is only slightly affected by the shift,55 and the unbound organic

33

phase remains unmodified from its original vacuum level.25 The magnitude of the vacuum level

shift can be determined according to eq 2:

(2) In eq 2, ΔV is the magnitude of the vacuum level shift, μ is the surface dipole moment, Ns is the

density of dipoles in the surface monolayer, cos θ is the angle of the dipoles from the surface

normal, εr is the dielectric constant of the medium, and ε0 is the permittivity of free space. The

inverse relation between the local dielectric constant εr and the magnitude of the vacuum level

shift means that high dielectric oxides like rutile TiO2 (εr = 86)58 will have much weaker dipole-

induced shifts compared to low dielectric oxides like ZnO (εr = 8).59 DFT calculations for our 3

sensitizers indicate that both fullerenes display dipoles that align roughly parallel to the

monolayer (i.e., not directly toward either side of the interface), whereas the NcQ sensitizer has a

dipole that points the negative end toward the P3HT (see Figure S11 in the Supporting

information). The effects seen for the acceptor sensitizers in Figure 4 show increased

photocurrent and photovoltage, and decreased dark current. These traits do not fit to a dipole

effect for a dipole pointing at either side of the interface. We therefore conclude that dipoles are

not the dominating effect in the current−voltage behavior of acceptor sensitized TiO2 interfaces.

The trend of rising first reduction potentials for these sensitizers does fit the pattern of improved

photovoltage and fill factor and reduced dark current. The photocurrent enhancement for NcQ is

less than that for C60- T, which may be attributed to weaker driving force for electron transfer at

the P3HT-NcQ interface, as judged by the lack of fluorescence quenching for the P3HT-NcQ-

TiO2 device (Figure 3).

2.8 Photosensitization by Acceptor – Sensitizers

The external efficiency of a solar cell, also known as the IPCE (incident photons

ΔV = µNscosθ

εrεo

34

converted to electrons), expresses the efficiency of converting the photon flux into photocurrent

at each wavelength according to the equation:

IPCE (%) = 1240 x JSC (3) λ x Pin

In Equation 3, JSC and Pin represent the photocurrent and the power intensity of the

incident light, respectively, at a given wavelength λ. Subtle differences in the ratio of P3HT

photosensitization at 500 vs. 600 nm in the IPCE spectrum for the P3HT-NcQ-TiO2 and P3HT-

bare TiO2 test cells (Figure 6, upper) indicate some conformational difference in the polymer at

the bare vs. coated TiO2 surfaces, in agreement with the fluorescence spectra. For the NcQ-

sensitized cell, the peak at ~400 nm indicates photocurrent contribution from light absorption by

NcQ (Fig. 6, lower).

Figure 2.9. Upper: Solution-phase UV-vis absorption spectrum for NcQ (EtOH). Lower: IPCE spectrum of the P3HT-NcQ-TiO2 cell (solid green) overlaid with the P3HT-TiO2 cell (dotted black).

35

Among the acceptor-sensitizers, only the naphthacenequinone (NcQ) should have

sufficient driving force to donate from its neutral excited states (see Table 1; n.b., S1-T1

crossover: k ≈ 1011s-1).67 Whether the photosensitization by NcQ is by donor-quenching

(injection by neutral NcQ* into TiO2) or by acceptor-quenching (hole transfer from NcQ* to

P3HT, followed by injection of NcQ- into TiO2) is an important question that can only be

definitively answered by transient spectroscopic analysis. Photoexcited naphthacenequinone

exhibits rapid acceptor-quenching in the presence of suitable donors51,68 However, voltammetric

oxidation of naphthacenequinone (1.8 V vs. SCE) is reported to be irreversible,69 so it seems

more likely that a photophysical pathway relying on the formation of NcQ- would lead to a

steady-state photocurrent rather than a mechanism relying on the formation of NcQ+.

Figure 2.10. IPCE spectra for P3HT-C60-T-TiO2 cell (upper) and P3HT-C60-M-TiO2 cell (lower).

36

The IPCE spectra of the fullerene-sensitized cells (Figure 7) are more difficult to

deconvolute. Visible light absorbance features of both C60-M and C60-T show broad

absorbances centered near 500 nm (Figure S4). The IPCE spectrum for the P3HT-C60-T-TiO2

cell shows a shoulder near 600 nm that matches with the P3HT-only device, but has heightened

photocurrent around 400 nm. The IPCE spectrum for the P3HT-C60-M-TiO2 cell has a

significant peak around 400 nm, and only weak shoulder peaks near 500 nm and 600 nm that

indicate photosensitization by P3HT. Whereas the C60-T-device can be interpreted as an

overlay of photosensitization by the polymer and acceptor, the IPCE pattern of the C60-M

device is surprising in that the spectrum is apparently dominated by absorption from the

acceptor, with substantial loss of photosensitization by P3HT. Although the IPCE trace does not

precisely match the UV-vis absorption, we have seen a very similar IPCE spectrum for

photosensitization by C60-M in NiO-based dye-sensitized solar cells.70 The significant

fluorescence quenching in this TiO2-based device and enhanced photocurrent relative to the bare-

TiO2 control device suggest that no deficit of charge generation is occurring, but that perhaps the

C60-M is an inefficient electron mediator, due to its lower-lying 1st reduction potential. We do

not know whether the inadequate driving force for charge injection by C60-M− is because the

TiO2 CBE has raised slightly due to a dipole-induced vacuum level shift or because the first

reduction potential of C60-M in the solventless environment is significantly lower than in

solution, or if both factors are operative at this interface.

2.9 Effects of the Sensitizers in P3HT/PCBM – TiO2 Devices

Loss of P3HT-photosensitization is also experienced in the C60-M sensitized device

using a blend of P3HT/PCBM (Figure 2.11, upper). The scale of quantum efficiency is boosted

between the two C60-M-sensitized devices with/without the PCBM.

37

Figure 2.11. Upper: IPCE spectra for P3HT/PCBM-Acceptor-TiO2 devices. Middle and Lower: Current-voltage behavior in P3HT/PCBM-Acceptor-TiO2 devices under 1 sun illumination and in dark, respectively.

38

This suggests that charges generated in the P3HT/PCBM blend are contributing to photocurrent,

but their passage is gated by photosensitization of the C60-M monolayer, acting as an electron

trap between the blend and the TiO2.In effect, this is Z-scheme photosensitization. Just as a

kinetic analysis of a multistep catalytic cycle would reveal only the slowest process, the quantum

efficiency of such a Z-scheme photosensitization should be dominated by the least efficient step.

Extending the IPCE analysis into the near infra-red (1200 nm) did not reveal the well-known

absorbance peak of the fullerene radical anion at ~1050 nm, but this is due to the dilute

illumination of 1 sun intensity: Methods that generate spectroscopically-observable fullerene

radical anions require either the intense light of a laser pulse (pump-probe spectroscopy) or the

voltammetric reduction of a fullerene sample (spectroelectrochemistry). Energy transfer from

long-lived fullerene triplet-excited states (100 µs)54 in the monolayer could provide the second

photosensitization to radical anion fullerenes scattered within the sensitizer monolayer, thereby

enabling injection into the TiO2 CBE. This apparent trapping of electrons results in the

P3HT/PCBM-C60M-TiO2 cell having reduced photocurrent and increased dark current

compared to the control cell with bare TiO2 (Figure 8, middle and lower). Photosensitization by

the C60-T and NcQ acceptor-sensitizers is not evident in the IPCE of cells using the

P3HT/PCBM blend, most likely because it is drowned out by the significant increase of charge

generation away from the TiO2 surface.

2.10 Effects of the Sensitizers on Electron Transport in ZnO Devices

Electron transport mediation by acceptor-sensitizers on ZnO appears to follow a different

pattern from that on TiO2. Surface coverage of the ZnO is likely to be incomplete because the

sensitization time for coating the ZnO surface must be kept short to avoid etching that occurs at

longer soaking times.71,72

39

Figure 2.12. Upper: Current-voltage behavior in P3HT-Acceptor-ZnO devices under 1 sun illumination; Lower: IPCE for P3HT-C60-T-ZnO device.

ZnO nanorod films were soaked for just 20 minutes in baths of the acceptor-sensitizers,

compared with overnight soaking for the TiO2 nanorods. Photoluminescence quenching

behavior and dark current of ZnO-based devices were similar to that observed for TiO2 devices

(Figures S5 and S6), but more often than not, photocurrent was also reduced. For P3HT-only

cells, the C60-T sensitizer, which is likely to achieve the highest surface coverage due to its

ability to bind with three carboxylic acids to the ZnO, exhibits severely reduced photocurrent

(Figure 9, upper). The IPCE of the P3HT-C60-T-ZnO cell shows no photocurrent contribution

40

from P3HT (Figure 9, lower), having a pattern consistent with acceptor-photosensitization only

at the C60-T monolayer. On ZnO the acceptor-sensitizers are inefficient electron mediators.

C60-M and NcQ show good photocurrent/voltage behavior relative to the control cell with bare

ZnO, but are likely only partially covering the ZnO surface, and may be providing only some

benefit in the wettability of the oxide surface. Even partial coverage appears to be a barrier to

the P3HT/PCBM blend (Figure 10), as cells with P3HT/PCBM-ZnO composition showed greatly

impaired photocurrent for all acceptor-sensitizers. This outcome is a surprising contrast to

results reported by others with fullerene monolayers paired with polymer-ZnO solar cells.21,23 It

may be understood in the context of known issues that have repeatedly shown ZnO to respond

less favorably to injection by donor-sensitizers compared to TiO2:73-75 TiO2 has a greater density

of states at the conduction band edge compared to ZnO; and the t2g orbitals of TiO2 have better

overlap with -conjugated molecular orbitals of the sensitizers compared to the s-type orbitals of

ZnO. The bare ZnO/P3HT composites gave the highest photocurrent of all devices – this is due

to the higher surface area of the ZnO/P3HT interface because the ZnO films have well-separated

nanorods whereas the TiO2 nanorod films are partially fused (Figures S1 & S2).

Figure 2.13. Current-voltage behavior in P3HT/PCBM-Acceptor-ZnO devices under 1 sun illumination.

41

2.11 Conclusion

We have determined that acceptor-sensitizers on TiO2 can provide acceptor-

photosensitized charge separation and can act as a recombination barrier between a remote donor

component (P3HT) and the TiO2 conduction band while ZnO appears to be a less reliable oxide

semiconductor for these purposes. The 1st-reduction potential of the acceptor-sensitizers

determines the extent of donor-quenching and the success for electron mediation of transferred

electrons arising from donor-sensitization. The photoluminescence quenching of the polymer

and apparent trapping of electrons in a monolayer of one sensitizer (C60-M) suggest that

forward dark electron transport by radical anions (i.e., and electron cascade) is an important

pathway for charge generation from photoexcited P3HT. However, this does not constitute proof

that the other sensitizers (C60-T, NcQ) are also engaged in electron cascade transport. Although

energetic considerations suggest that radical anions are the injecting agents in these systems

regardless of whether the polymer or sensitizer is photoexcited, we are currently pursuing more

definitive evidence via transient spectroscopy studies to examine these systems in greater detail.

2.12 References

1. Arango, A. C.; Carter, S. A.; Brock, P. J. Appl. Phys. Lett. 1999, 74, 1698-1700.

2. Coakley, K. M.; McGehee, M. D. Appl. Phys. Lett. 2003, 83, 3380–3382.

3. Beek, W. J. E.; Wienk, M. M.; Janssen, R. A. J. Adv. Mater. 2004, 16, 1009-1013.

4. Beek, W. J. E.; Wienk, M. M.; Janssen, R. A. J. Adv. Funct. Mater. 2006, 16, 1112-1116.

5. Peiro, A. M.; Ravirajan, P.; Govender, K.; Boyle, D. S.; O'Brien, P.; Bradley, D. D. C.; Nelson, J.; Durrant, J. R. J. Mater. Chem. 2006, 16, 2088-2096.

6. White, M. S.; Olson, D. C.; Shaheen, S. E.; Kopidakis, N.; Ginley, D. S. Appl. Phys. Lett. 2006, 89, 143517.

7. Briseno, A. L.; Holcombe, T. W.; Boukai, A. I.; Garnett, E. C.; Shelton, S. W.; Frechet, J. M. J.; Yang, P. Nano Lett. 2010, 10, 334-340.

42

8. Gonzales-Valls, I.; Lira-Cantu, M. Energy Environ. 2008, 2, 19–34.

9. Zhang, F.; Xu, X.; Tang, W.; Zhang, J.; Zhuo, Z.; Wang, J.; Wang, J.; Xu, Z.; Wang, Y. Sol. Energy. Mater. Sol. Cells 2011, 95, 1785-1799.

10. Coakley, K. M.; Liu, Y.; McGehee, M. D.; Frindell, K. L.; Stucky, G. D. Adv. Funct. Mater. 2003, 13, 301–306.

11. Kroeze, J. E.; Savenije, T. J.; Vermeulen, M. J. W.; Warman, J. M. J. Phys. Chem. B 2003, 107, 7696-7705.

12. Piris, J.; Kopidakis, N.; Olson, D. C.; Shaheen, S. E.; Ginley, D. S.; Rumbles, G. Adv. Funct. Mater. 2007, 17, 3849–3857.

13. Ameri, T.; Dennier, G.; Waldauf, C.; Denk, P.; Forberich, K.; Scharber, M. C.; Brabec, C. J.; Hinger, K. J. Appl. Phys. 2008, 103, 084506.

14. Takanezawa, K.; Hirota, K.; Wei, Q.-S.; Tajima, K.; Hashimoto, K. J. Phys. Chem. C 2007, 111, 7218–7223.

15. Umeda, T.; Hashimoto, Y.; Mizukami, H.; Shirakawa, T.; Fujii, A.; Yoshino, K. Synth. Met. 2005, 152, 93–96.

16. Shankar, K.; Mor, G. K.; Prakasam, H. E.; Varghese, O. K.; Grimes, C. A. Langmuir 2007, 23, 12445-12449.

17. Waldauf, C.; Morana, M.; Denk, P.; Schilinsky, P.; Coakley, K. M.; Choulis, S. A.; Brabec, C. J. Appl. Phys. Lett. 2006, 89, 233517.

18. Kudo, N.; Honda, S.; Shimazaki, Y.; Ohkita, H.; Ito, S.; Benten, H. Appl. Phys. Lett. 2007, 90, 183513.

19. Yang, C.; Kim, J. Y.; Cho, S.; Lee, J. K.; Heeger, A. J.; Wudl, F. J. Am. Chem. Soc. 2008, 130, 6444–6450.

20. Hau, S. K.; Yip, H.-L.; Acton, O.; Baek, N. S.; Ma, H.; Jen, A. K.-Y. J. Mater. Chem. 2008, 18, 5113–5119.

21. Hau, S. K.; Cheng, Y.-J.; Yip, H.-L.; Zhang, Y.; Ma, H.; Jen, A. K.-Y. ACS Appl. Mater. Interfaces 2010, 2, 1892–1902.

22. Hsieh, C.-H.; Cheng, Y.-J.; Li, P.-J.; Chen, C.-H.; Dubosc, M.; Liang, R.-M.; Hsu, C.-S. J. Am. Chem. Soc. 2010, 132, 4887-4893.

23. Chen, C.-T.; Hsu, F.-C.; Kuan, S.-W.; Chen, Y.-F. Sol. Energy. Mater. Sol. Cells 2011, 95, 740–744.

24. Ravirajan, P.; Peiro, A. M.; Nazeeruddin, M. K.; Graetzel, M.; Bradley, D. D. C.; Durrant, J. R.; Nelson, J. J. Phys. Chem. B 2006, 110, 7635–7639.

43

25. Goh, C.; Scully, S.; McGehee, M. D. J. Appl. Phys. 2007, 101, 114503.

26. Lin, Y.-Y.; Chu, T.-H.; Li, S.-S.; Chuang, C.-H.; Chang, C.-H.; Su, W.-F.; Chang, C.-P.; Chu, M. W.; Chen, C.-W. J. Am. Chem. Soc. 2009, 131, 3644–3649.

27. Said, A. J.; Poize, G.; Martini, C.; Ferry, D.; Marine, W.; Giorgio, S.; Fages, F.; Hocq, J.; Boucle, J.; Nelson, J.; Durrant, J. R.; Ackermann, J. J. Phys. Chem. B 2010, 114, 11273–11278.

28. Zhu, R.; Jiang, C.-Y.; Liu, B.; Ramakrishna, S. Adv. Mater. 2009, 21, 994–1000.

29. Lloyd, M. T.; Prasankumar, R. P.; Sinclair, M. B.; Mayer, A. C.; Olson, D. C.; Hsu, J. W. P. J. Mater. Chem. 2009, 19, 4609-4614.

30. Kruger, J.; Bach, U.; Gratzel, M. Adv. Mater. 2000, 12, 447-451.

31. Tian, X.; Xu, J.; Xie, W. J. Phys. Chem. C 2010, 114, 3973-3980.

32. Liu, Y.; Scully, S. R.; McGehee, M. D.; Liu, J.; Luscombe, C. K.; Frechet, J. M. J.; Shaheen, S. E.; Ginley, D. S. J. Phys. Chem. B 2006, 110, 3257-3261.

33. Ohyama, M.; Kozuka, H.; Yoko, T. Thin Solid Films 1997, 78-85.

34. Kokotov, M.; Hodes, G. J. Mater. Chem. 2009, 19, 3847-3854.

35. Kamat, P. V. J. Am. Chem. Soc. 1991, 113, 9705-9707.

36. Kamat, P. V.; Bedja, I.; Hotchandani, S. J. Phys. Chem. 1994, 98, 9137-9142.

37. Voigt, M.; Klaumunzer, M.; Ebel, A.; Werner, F.; Yang, G.; Marczak, R.; Spiecker, E.; Guldi, D. M.; Hirsch, A.; Peukert, W. J. Phys. Chem. C 2011, 115, 5561–5565.

38. Kohl, P. A.; Bard, A. J. J. Am. Chem. Soc. 1977, 99, 7531-7539.

39. Redmond, G.; Fitzmaurice, D. J. Phys. Chem. 1993, 97, 1426-1430.

40. Cahen, D.; Hodes, G.; Gratzel, M.; Guillemoles, J. F.; Riess, I. J. Phys. Chem. B 2000, 104, 2053-2059.

41. Lira-Cantu, M.; Norrman, K.; Andreasen, J. W.; Casan-Pastor, N.; Krebs, F. C. J. Electrochem. Soc. 2007, 154, B508-B513.

42. Olson, D. C.; Shaheen, S. E.; Collins, R. T.; Ginley, D. S. J. Phys. Chem. C 2007, 111, 16670-16678.

43. Ruhle, S.; Greenstein, M.; Chen, S.-G.; Merson, A.; Pizem, H.; Sukenik, C. S.; Cahen, D.; Zaban, A. J. Phys. Chem. B 2005, 109, 18907-18913.

44

44. Johansson, E. M. J.; Scholin, R.; Siegbahn, H.; Hagfeldt, A.; Rensmo, H. Chem. Phys. Lett. 2011, 515, 146-150.

45. Liu, B.; Aydil, E. S. J. Am. Chem. Soc. 2009, 131, 3985-3990.

46. Kavan, L.; Gratzel, M.; Gilbert, S. E.; Klemenz, C.; Scheel, H. J. J. Am. Chem. Soc. 1996, 118, 6716-6723.

47. Imanishi, A.; Tsuji, E.; Nakato, Y. J. Phys. Chem. C 2007, 111, 2128-2132.

48. Guan, Z.-L.; Kim, J. K.; Wang, H.; Jayye, C.; Fischer, D. A.; Loo, Y.-L.; Kahn, A. Org. Elec. 2010, 11, 1779-1785.

49. Murphy, A. R.; Liu, J.; Luscombe, C. K.; Kavulak, D.; Frechet, J. M. J.; Kline, R. J.; McGehee, M. D. Chem. Mater. 2005, 17, 4892-4899.

50. Al-Ibrahim, M.; Roth, H.-K.; UZhokhavets, U.; Gobsch, G.; Sensfuss, S. Sol. Energy. Mater. Sol. Cells 2005, 85, 13-20.

51. Roberts, L. W.; Schuster, G. B. Org. Lett. 2004, 6, 3813-3816.

52. Guldi, D. M.; Hungerbuhler, H.; Asmus, K.-D. J. Phys. Chem. 1995, 99, 9380-9385.

53. Ross, R. B.; Cardona, C. M.; Guldi, D. M.; Sankaranarayanan, S. G.; Reese, M. O.; Kopidakis, N.; Peet, J.; Walker, B.; Bazan, G.; Van Keuren, E.; Holloway, B. C.; Drees, M. Nat. Mater. 2009, 8, 208-212.

54. Guldi, D. M.; Asmus, K.-D. J. Phys. Chem. A 1997, 101, 1472-1481.

55. Hoppe, H.; Glatzel, T.; Niggemann, M.; Hinsch, A.; Lux-Steiner, M. C.; Sariciftci, N. S. Nano Lett. 2005, 5, 269-274.

56. van Duren, J. K. J.; Yang, X.; Loos, J.; Bulle-Lieuwma, C. W. T.; Sieval, A.; Hummelen, J. C.; Janssen, R. A. J. Adv. Funct. Mater. 2004, 14, 425-434.

57. Boudon, C.; Gisselbrecht, J.-P.; Gross, M.; Isaacs, L.; Anderson, H. L.; Faust, R.; Diederich, F. Helv. Chim. Acta 1995, 78, 1334–1344.

58. Pavlishchuk, V. V.; Addison, A. W. Inorg. Chim. Acta 2000, 298, 97-102.

59. Bredas, J. L.; Silbey, R.; Boudreaux, D. S.; Chance, R. R. J. Am. Chem. Soc. 1983, 105, 6555-6559.

60. Beuerle, F.; Chronakis, N.; Hirsch, A. Chem. Commun. 2005, 3676–3678.

61. Lamparth, I.; Hirsch, A. J. Chem. Soc., Chem. Commun. 1994, 1727–1728.

45

62. Cormier, R. A.; Connolly, J. S.; Pelter, L. S. Synth. Commun. 1992, 22, 2155-2164.

63. Riley, R. G.; Silverstein, R. M.; Katzenellenbogen, J. A.; Lenox, R. S. J. Org. Chem. 1974, 39, 1957-1958.

64. Friedman, L.; Fishel, D. L.; Shechter, H. J. Org. Chem. 1965, 30, 1453–1457.

65. Kotov, M.; Hodes, G. J. Mater. Chem. 2009, 19, 3847–3854.

66. Braun, S.; Salaneck, W. R.; Fahlman, M. Adv. Mater. 2009, 21, 1450-1472.

67. Yamaji, M.; Takehira, K.; Itoh, T.; Shizuka, H.; Tobita, S. Phys. Chem. Chem. Phys. 2001, 3, 5470-5474.

68. Yamaji, M.; Itoh, T.; Tobita, S. Photochem. Photobiol. Sci. 2002, 1, 869–876.

69. Badger, G. M. Quart. Rev. (London) Biol. 1951, 5, 155.

70. Berhe, S. A.; Zhou, J. Y.; Youngblood, W. J., Unpublished results.

71. Jensen, R. A.; Ryswyk, H. V.; She, C.; Szarko, J. M.; Chen, L. X.; Hupp, J. T. Langmuir 2010, 26, 1401–1404.

72. Horiuchi, H.; Katoh, R.; Hara, K.; Yanagida, M.; Murata, S.; Arakawa, H.; Tachiya, M. J. Phys. Chem. B 2003, 107, 2570-2574.

73. Asbury, J. B.; Hao, E.; Wang, Y.; Ghosh, H. N.; Lian, T. J. Phys. Chem. B 2001, 105, 4545-4557.

74. Stockwell, D.; Yang, Y.; Huang, J.; Anfuso, C.; Huang, Z.; Lian, T. J. Phys. Chem. C 2010, 114, 6560–6566.

75. Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. Chem. Rev. 2010, 110, 6595–6693.

46

CHAPTER 3

INFLUENCE OF SEEDING AND BATH CONDITIONS IN HYDROTHERMAL GROWTH

OF VERY THIN (∼20 NM) SINGLE-CRYSTALLINE RUTILE TiO2 NANOROD FILMS*

3.1 Introduction

Nanostructured films of oxide semiconductors such as titanium dioxide (TiO2) with one-

dimensional morphology are useful for solid-state dye solar cells, polymer-oxide solar cells,

photoelectrochemical solar cells, and photocatalytic materials.1−6 Nanorod films are easier to

infiltrate with semiconductive polymers, molecular hole conductors, and supramolecular or

nanoparticulate species compared to films of nanoparticle-based mesoporous oxide

semiconductors.7−10 Additionally, electron transport in a given oxide semiconductor

is faster in one-dimensional assemblies than in to nanoparticle based films.11−13 Films of single-

crystalline nanorods with only one crystal face exposed may be useful for transient spectroscopy

of sensitized oxide semiconductors,14 because they have less diversity of interfaces compared to

nanoparticle based films and have higher surface area than bulk single crystals. The surface area

for nanorod-based films is lower than for nanoparticle-based films of equivalent height, so

photocurrents produced from films of sensitized oxide nanorods are generally lower than for

mesoporous oxide films. To enhance the surface area of single-crystalline rutile TiO2 nanorod

films, nanorods of narrow diameter (<50 nm) and dense packing are needed.

Several methods for the hydrothermal growth of single crystalline rutile TiO2 nanorods

have now been reported. 13,15−20 Their general principle is the use of superheated water or a

mixture of superheated water with an organic cosolvent to achieve solution conditions that favor

the deposition of TiO2 from solvated Ti(IV) species. Superheated water exhibits a reduced

* This entire chapter is reproduced from Berhe, S. A.; Nag, S.; Molinets, Z.; Youngblood, W. J. ACS Appl. Mater. Interfaces, 2013, 5, 1181–1185 with permission from the American Chemical Society.

47

dielectric constant,21 greater self dissociation into hydronium and hydroxide,22 and greater

miscibility with organic solvents.23 Preferential growth along the [001] crystal axis is observed

because of inhibition of growth at the [110] axis by coordination of Cl ions in the growth

bath.16,17,24,25 Grimes and co-workers first reported the hydrothermal/solvothermal synthesis of

rutile TiO2 nanorods of 10−35 nm diameter from a toluene/HCl (10 M) mixture using a sol−gel-

derived TiO2 seed layer on F-SnO2 (FTO).15,18 This method uses an extremely high concentration

of titanium precursors in the growth bath (270 mM Ti(OiPr)4 + 700 mM TiCl4) and presents

difficulty in reproducibly preparing thin films (<1 μm) because the initial growth is rapid.

Mullins and co-workers have reported using n-hexane as the organic phase with lower

concentration of [Ti] (150−300 mM) to produce thin nanorods (5 nm diameter), although

considerable fusion of their nanorod films occurred upon annealing.16 Mallouk and co-workers

have found that using butanone as the organic phase and yet lower [Ti] (100−200 mM Ti(OBu)4)

gave nanorods of 40 nm diameter.13 Liu and Aydil grew rutile TiO2 nanorods directly on FTO

substrates from 5 M HCl (aq) without any seed layer or organic solvent.17 An added benefit of

the Liu/Aydil procedure is the further-reduced [Ti] (55 mM Ti(OiPr)4) relative to the

Grimes/Mullins/Mallouk methods, although the Liu/Aydil nanorods are generally thicker (100

nm diameter). Although Liu and Aydil observed that the TiO2 nanorods did not grow on

unseeded surfaces of Si or SiO2, Zhou and co-workers reported the growth of rutile TiO2

nanorods and “dandelion” multilayers of nanorods on arbitrary substrates, simply by elevating

the [Ti] in the aqueous growth bath to 165 mM.18 Wang and co-workers reported growth on

arbitrary substrates using a seed layer of sol−gel TiO2, obtaining nanorods of ∼100 nm diameter

despite lowering the [Ti] to 33 mM.19 Finally, Dong and co-workers showed TiO2 nanorod

growth from untreated titanium surfaces.20 We report herein methods using transparent seed

48

layers that remove the reliance on FTO as a seeding substrate using low [Ti] (28 mM) and

achieve monolayer films of thin nanorods (∼20 nm).


Deionized water was obtained from a Millipore water purifier (10 MC resistivity) for

oxide growth solutions. Film heights for oxide nanorod films were measured with a Dektak

profilometer. UV-vis absorption data were collected with a Varian Cary spectrophotometer.

3.2.1 Seeding Methods

F-SnO2-coated glass substrates (7Ω/, Pilkington Glass) were sonicated in acetone,

isopropanol and deionized water for 5 minutes respectively, air dried in a laminar flow hood and

treated in an ozone cleaner for 15 minutes. Then one of the following seeding protocols was

followed: (1) Substrates were immersed for 20 minutes in a solution of aq. KMnO4 (5 mM, 20

mL) to which n-butanol (50 μL, 0.6 mmol) had been added, after which the substrates were

rinsed with deionized water and air dried in a laminar flow hood. (2) Substrates were coated with

titanium by thermal evaporation of Ti° metal to 100 Å thickness, then annealed at 450°C for 30

min under ambient atmosphere; (3) An aqueous suspension of nanoparticles of rutile TiO2 (20

nm diameter, NanoAmor Inc.) was spin-coated at 2500 rpm onto each substrate, which was then

wiped clean at the edges with a wet cotton swab and annealed to 450°C for 30 min under

ambient atmosphere.

3.2.2 Oxide Nanorods Growth

Substrates were then placed in a Parr-type Teflon pressure vessel (45 mL capacity) with a

solution of titanium isopropoxide (28 or 55 mM) in aq. HCl (5M). The vessel was closed and

heated in an oven at 150°C (1.5-6 h duration). After the heating period, the pressure vessel was

49

placed under a slow stream of tap water for 15 min to cool before opening. Substrates were

rinsed with deionized water and air dried in a laminar flow hood.

Assembly and testing of solar cell prototypes: FTO substrates were patterned by chemical

etching using Zn dust and 4 M HCl prior to the nanorod growth described above. P3HT was

spin-coated onto oxide nanorod films from chlorobenzene solution (10 mg/ml). PEDOT/PSS was

spin-coated from aqueous dispersion. Silver electrodes of ~70 nm thickness were deposited by

metal evaporation in island-type geometry with a central dot of 1.1 cm2 area. Each device was

capped with a cover slide and annealed on a hotplate at 125°C for 5 min. Silver paste was

applied to improve contact to the FTO. Devices were tested on the day after they were

assembled. Current-voltage measurements were taken with a sourcemeter (Keithley 2400) and a

Class A solar simulator (Solarlight, Inc.). A mask defining an area of 1 cm2 was placed over the

active area of each device.

3.3 Influence of Seed Layer to the Growth of TiO2 Nanorod Array

In reproducing the Liu/Aydil procedure, we found that the method is sensitive to the type

of FTO. Using FTO with 7 Ohm/square resistivity instead of the 15 Ohm/square FTO reported in

the earlier method, we obtained large crystal grains of rutile TiO2 (Figure 3.1A) but no nanorods.

Given the evidence that rutile SnO2 in the FTO acts as a seed layer for the direct growth of rutile

TiO2,17 we attribute this outcome to a difference in the surface roughness in lower resistivity

FTO. Commercial FTO substrates achieve lower resistivity through the layer thickness of the

FTO, and thicker films of FTO need lower surface roughness to minimize haze.26 In a search for

alternative seeding methods, we have discovered that thin single-crystalline (20 nm) rutile TiO2

nanorods can be grown using a seed layer of manganese oxyhydroxide (MnOOH) nanoparticles

50

(Figure 3.1B) or a seed layer of TiO2 comprised of either a thin conformal sheet of TiO2 (Figure

3.1C) or a film of rutile TiO2 nanoparticles (see Figure A3.8 in the Appendix).

Figure 3.1. Rutile TiO2 grown using the method from ref 17 on (A) bare 7 Ohm/square FTO, (B) a seed layer of MnOOH nanoparticles on FTO, or (C) a continuous sheet of TiO2 on FTO. Growth time was 4 h for film A, and 2 h for each of films B and C. Film A is too rough for an average film height, but films B and C were measured at 800 and 750 nm (±50 nm), respectively.

The single-crystallinity of the nanorods was confirmed by small-area electron diffraction

(SAED-TEM; see Figures 3.2 and 3.3).

Figure 3.2. Transmission electron microscopy (left, bright field image) and selected area electron diffraction (SAED, right) of a bundle of nanorods grown from a continuous sheet of TiO2 in a 6 h growth at 28 mM Ti(iOPr)4. The mottled appearance is due to surface pitting, which may be formed during growth or during the sonication/scraping process of removing the nanorods from the substrate.

51

Figure 3.3. Transmission electron microscopy (left, bright field image) and selected area electron diffraction (SAED, right) of a bundle of nanorods grown from a seed layer of MnOOH nanoparticles in a 6 h growth at 55 mM Ti(iOPr)4.

The nanorods are prone to fuse into small or large bundles depending on the seeding and

bath conditions used for their growth. Rutile TiO2 nanoparticles can be spin-coated onto

substrates from aqueous suspension and annealed into a stable seeding layer. Although dense

nanorod films can be grown from such seeding (see Figure S18 in the Supporting Information),

almost tall films prepared from using rutile TiO2 seeding exhibited peeling from the substrate

and/or severe cracking. MnOOH nanoparticles, produced by reduction of KMnO4 with a primary

alcohol, were first reported as a seed layer for the growth of ZnO nanorods by Hodes and co-

workers.27 Whether the nanoparticles are actually MnOOH or MnO2 is not definitively known.

Rutile TiO2 nanowires grown from these seeds appear to form in small bundles of just a few

nanorods, and at varying angles relative to the surface normal (Figure 1B). Films can be grown

as short as 200 nm or as tall as 8 μm, but low growth angles inhibit the growth of many of the

nanorods in taller films. A thin layer of thermally evaporated titanium metal (10−20 nm) can be

annealed to 450 °C under ambient atmosphere to obtain a continuous transparent seed layer of

TiO2. Hydrothermal growth from this thin TiO2 underlayer using the conditions of Liu and Aydil

produces a dense film of TiO2 nanorods (Figure 1C) that we initially estimated at ∼100 nm

52

diameter.28 Closer inspection of our films have led us to realize that the ∼100 nm diameter

‘rods’ were actually bundles of much smaller (∼20 nm) nanorods. Fusion of the nanorods is a

consequence of the [110] crystal face being the only surface at all sides of the nanorods.

3.4 Influence of Bath Conditions to the Growth of TiO2 Nanorod Array

Mullins and co-workers reported that such bundling was dependent on the choice of

titanium precursor and organic cosolvent, and their films of TiO2 nanorods were either

unbundled or severely bundled. In our observation, the bundling shows dependence on the

seeding method, the concentration of the titanium precursor, and the growth height of the film.

Films of just 200 nm thickness grown at 55 mM [Ti] show individual nanorods of 40 nm

diameter (Figure 3.4).

Figure 3.4. Rutile TiO2 nanorods grown to just 200 nm height with minimal bundling of the nanorods.

At film heights above 1 μm, bundling becomes so severe that the films are nearly continuous (see

Figure 3.5).

53

Figure 3.5. SEM images of TiO2 nanorods by hydrothermal growth from a continuous sheet of TiO2 using 55 mM Ti(iOPr)4 for 5 h. Film height was 7 microns (see Fig. A3.11 ; left).

Reducing the concentration of the titanium alkoxide precursor from 55 to 28 mM

provides a less dense seeding, which in turn reduces the extent of nanorod bundling (Figure 3).

In a 200 nm tall film (Figure 3A) grown at 28 mM [Ti], there are individual nanorods of 20 nm

diameter as well as locations where nanorods are coming together to form bundles. In 6 h of

growth, films of ∼2 μm (Figure 3B) are formed with some bundles enclosing a few or several

nanorods. Film growth for durations longer than 6 h provided diminishing returns on nanorod

height, but a given film can be resubmitted to a fresh bath solution for further growth.

Unfortunately, the surface area gained by additional height is more than offset by surface area

loss due to rod fusion. A film grown to 5 μm height by two consecutive 6 h growths has severe

fusion (Figure 3C). Only the TiO2 sheet layer showed growth at 28 mM [Ti], and at

concentrations of the Ti(iOPr)4 below 28 mM, we did not observe nanorod growth. Additional

SEM images of TiO2 nanorod films grown to different heights with all seeding methods at

varying [Ti] can be found in the Supporting Information.

54

Figure 3.6. Films of rutile TiO2 nanorods grown to heights of (A) 200 nm, (B) 2 μm, and (C) 5 μm under identical growth conditions.

The parameters that differentiate these seeding methods from each other as well as from

previously reported methods or the use of bare FTO are the size and spacing of seeding

domains. When the nanorod seeding is dense, rods with low growth angles terminate quickly and

serve as new seeding points, with the overall effect of producing densely packed epitaxially

oriented nanorods that are prone to bundling. The thin conformal coating of TiO2 on FTO cannot

be distinguished from bare FTO by SEM imaging (Figure 4, A vs B). Because TiO2 should be a

better seeding surface than FTO for the growth of TiO2, the conformal TiO2 coating should lead

to the growth of rutile crystals for TiO2/FTO substrates that are at least as large as observed for

bare FTO, but instead we observed nanorod growth.

Lira-Cantu and coworkers have shown that thin sheets of TiO2 on FTO have both anatase

and rutile domains,29 and more favorable seeding at the rutile domains within a mixed-phase

seed layer may explain the small size of nanorods grown in the film. Additionally, the initial

treatment of the TiO2/FTO substrates in 5 M HCl prior to reaching growth temperature (150 °C)

may cause some etching of the TiO2 surface that could contribute to the small seeding domains,

as reported for growth on titanium substrates.20 Growth at 28 mM [Ti] results in a lower density

of nanorods,17 so bundling is less severe at lower growth heights, but progresses as the film is

grown taller.

55

Figure 3.7. Substrates used for TiO2 nanorod growth: (A) bare FTO, (B) a 10 nm-thick coating of TiO2 on FTO, and (C) MnOOH nanoparticles on FTO.

For np-MnOOH seed layers (Figure 4C), Hodes and co-workers reported the deposition of well-

dispersed nanoparticles that are each just a few nanometers in diameter. In our case, although

such small particles may be present, we could not resolve any image of them but we do see large

textured aggregates of MnOOH nanoparticles that do not completely cover the FTO surface. The

lower seeding density and amorphous texture of the np-MnOOH deposits results in nanorod

growth at lower angles to the surface and less bundling compared to the TiO2-sheet seeding. We

observed no delamination of films grown from np-MnOOH seeding or films grown at 28 mM

[Ti] from a TiO2 sheet layer, with growth times as long as 18 h. The previously reported

56

delamination of TiO2 nanorod films17 from the underlying FTO substrate occurs only with films

having denser seeding/growth of nanorods, such as the films grown at 55 mM [Ti] from either

TiO2 sheet or a spin-coated film of rutile TiO2 nanoparticles. Films grown from np-TiO2

seeding delaminated if grown longer than 4 h, whereas films grown from a TiO2 sheet at 55 mM

[Ti] could be grown up to 7 h without delamination. TiO2 films were dipped in NCQ dye and

measured their UV-VIS absorption in order to determine the relative surface area of the TiO2

nanorod films grown using different seeding methods and growth bath conditions. The spectra

(Fig 3.8) indicate that the highest surface area is obtained in films grown at 28 mM Ti(iOPr)4 for

6 h. No films grown at 55 mM Ti(iOPr)4 gave any measurable absorbance, even when some

color of the dyed film could be detected by the human eye.

Figure 3.8. UV-vis absorbance data for TiO2 nanorod films sensitized with NcQ, an acenequinone dye with absorbance at 410 nm (absorbance spectrum reproduced from reference 6 of the main paper). Absorbance of nanorod films was measured against a blank of F-SnO2 conductive glass. Films were sensitized with NcQ dye and then absorbance was remeasured, and the prior absorbance of each undyed nanorod film was subtracted from the absorbance of the dyed film to obtain the above spectra at right.

57

Figure 3.9. Growth rate of single crystalline TiO2 nanorods from TiO2 sheet seeding using 28 mM Ti(iOPr)4. Other growth conditions as described above in Experimental Section. All films grown for this set were seeded with an evaporated layer of Ti° that was annealed in air to form a continuous sheet of TiO2. Because there appears to be an incubation time of at least 1 h, for thermal equilibration of the pressure vessel to oven temperature, a double growth of 6 h each run is multiply represented as either an 11 h or 12 h run.

3.5 Variation of Nanorod Film Thickness in Hybrid Inverted Organic Photovoltaics

One example of the issues that can be addressed using such easily prepared films is the

variation of nanorod film thickness in hybrid inverted organic photovoltaic (HOPV) cells using a

semiconductive polymer with TiO2 nanorods. Bulk heterojunction organic photovoltaic cells

such as those using P3HT and a methanofullerene (PCBM) are kept to 200−350 nm thickness to

accommodate the slower mobility of charge carriers in the organic semiconductors relative to

inorganic semiconductors, despite inadequate light absorption in such thin films. HOPV cells are

generally kept to the same thickness, although it has been proposed that the confinement of an

organic semiconductor into a low-dimensional morphology should allow for operation of devices

58

with thicker active layers.30 Additionally, polymer oxide cells have one domain, the oxide

semiconductor, with faster carrier transport. Rutile TiO2, for example, has an electron mobility of

∼1 cm2/(V s).31 We explored the influence of film thickness on the performance of polymer-

oxide solar cells composed of poly(3-hexylthiophene) (P3HT) and rutile TiO2 nanorod films

grown to heights of 200−3900 nm from baths of either 55 or 28 mM [Ti]. Photovoltaic test

devices with active areas of 1 cm2 were assembled on FTO electrodes with rutile TiO2 nanorod

films. Details of the device assembly methods are provided in the Supporting Information.

Figure 5 shows the current−voltage behavior of the prepared devices. Films grown at 55

mM Ti(iOPr)4 to a height of 750 nm (Figure 1C) gave significantly higher photocurrent and

lower photovoltage than all other films. The stark difference in behavior is most logically

explained by the difference in the proportion of the active layer that is at the polymer/oxide

interface versus away from the interface (i.e., in the ‘bulk’), with greater proportion of material

at the interface leading to more charge separation, and thus more photocurrent, but also more

charge recombination, and therefore lower photovoltage. The bundling of nanorods results in

narrower void channels between TiO2 nanorods. Where gaps between nanorods are thin (<20

nm), excitons generated in the polymer can reach the polymer/oxide interface before decaying,

and free carriers generated within the polymer can reach the oxide surface before recombining.

One pathway to better nanorod spacing could be the use of substrates with greatly reduced

surface roughness.

59

Figure 3.10. Current−voltage behavior of photovoltaic cells using the grown TiO2 nanorod films. All devices were prepared from films seeded with a continuous TiO2 sheet seed layer. Legends indicate height of the nanorods films used. Solid line traces indicate films grown at 55 mM [Ti], whereas dotted/dashed traces indicate films grown at 28 mM [Ti].

3.6 Conclusion

We have discovered new seeding methods for the growth of single-crystalline rutile TiO2

nanorod films and shown the dependence of rod fusion upon seeding density. In particular, we

report a facile method for producing films of thin (20 nm) rutile nanorods up to 2 μm in height

without severe bundling. This method consumes less titanium reagent than any other method so

far reported for the growth of such films. The low photocurrents of the solar cells examined in

this study are an indication that further optimization of the nanorod morphology is needed. These

easily grown TiO2 nanorod films will be especially useful to researchers working with polymers,

macromolecules, or chemically synthesized nanoparticles that cannot easily be loaded into

nanoparticulate TiO2 films.

60

3.7 References

1. Wang, M.; Bai, J.; Le Formal, F.; Moon, S.-J.; Cevey-Ha, L.; Humphry-Baker, R.; Gratzel, C.; Zakeeruddin, S. M.; Gratzel, M. J. Phys. Chem. C 2012, 116, 3266-3273.

2. Shankar, K.; Mor, G. K.; Prakasam, H. E.; Varghese, O. K.; Grimes, C. A. Langmuir 2007, 23, 12445-12449.

3. Toyoda, T.; Shen, Q. J. Phys. Chem. Lett. 2012, 3, 1885-1893.

4. Kamat, P. V. J. Phys. Chem. C 2007, 111, 2834-2860.

5. Chen, X.; Mao, S. S. Chem. Rev. 2007, 107, 2891-2959.

6. Hwang, Y. J.; Hahn, C.; Liu, B.; Yang, P. ACS Nano 2012, 6, 5060-5069.

7. Boucle, J.; Ravirajan, P.; Nelson, J. J. Mater. Chem. 2007, 17, 3141–3153.

8. Borchert, H. Energy Environ. Sci. 2010, 3, 1682-1694.

9. Baeten, L.; Conings, B.; Boyen, H.-G.; D'Haen, J.; Hardy, A.; D'Olieslaeger, M.; Manca, J. V.; Van Bael, M. K. Adv. Mater., 23, 2802-2805.

10. Coakley, K. M.; Liu, Y.; McGehee, M. D.; Frindell, K. L.; Stucky, G. D. Adv. Funct. Mater. 2003, 13, 301–306.

11. Kamat, P. V.; Tvrdy, K.; Baker, D. R.; Radich, J. G. Chem. Rev. 2010, 110, 6664-6688.

12. Hochbaum, A. I.; Yang, P. Chem. Rev. 2010, 110, 527-546.

13. Feng, X.; Zhu, K.; Frank, A. J.; Grimes, C. A.; Mallouk, T. E. Angew. Chem. 2012, 124, 2781-2784.

14. Spitler, M. T.; Parkinson, B. A. Acc. Chem. Res. 2009, 42, 2017-2029.

15. Feng, X.; Shankar, K.; Varghese, O. K.; Paulose, M.; Latempa, T. J.; Grimes, C. A. Nano Lett. 2008, 8, 3781-3786.

16. Hoang, S.; Guo, S.; Hahn, N. T.; Bard, A. J.; Mullins, C. B. Nano Lett. 2012, 12, 26-32.

17. Liu, B.; Aydil, E. S. J. Am. Chem. Soc. 2009, 131, 3985-3990.

18. Kumar, A.; Madaria, A. R.; Zhou, C. J. Phys. Chem. C 2010, 114, 7787-7792.

19. Wang, H.-E.; Chen, Z.; Leung, Y. H.; Luan, C.; Liu, C.; Tang, Y.; Yan, C.; Zhang, W.; Zapien, J. A.; Bello, I.; Lee, S.-T. Appl. Phys. Lett. 2010, 96, 263104.

61

20. Dong, L.; Cheng, K.; Weng, W.; Song, C.; Du, P.; Shen, G.; Han, G. Thin Solid Films 2011, 519, 4634-4640.

21. Akerlof, G. C.; Oshry, H. I. J. Am. Chem. Soc. 1974, 72, 2844-2847.

22. Marshall, W. L.; Franck, E. U. J. Phys. Chem. Ref. 1982, 10, 295-304.

23. Siskin, M.; Katritzky, A. R. Chem. Rev. 2001, 101, 825-835.

24. Pottier, A.; Chaneac, C.; Tronc, E.; Mazerolles, L.; Jolivet, J.-P. J. Mater. Chem. 2011, 11, 1116-1121.

25. Allen, P. B. Nano Lett. 2007, 7, 6-10.

26. Szanyi, J. Appl. Surf. Sci. 2002, 185, 161-171.

27. Kokotov, M.; Hodes, G. J. Mater. Chem. 2009, 19, 3847-3854.

28. Berhe, S. A.; Zhou, J. Y.; Haynes, K. M.; Rodriguez, M. T.; Youngblood, W. J. ACS Appl. Mater. Interfaces 2012, 4, 2955-2963.

29. Lira-Cantu, M.; Chafiq, A.; Faissat, J.; Gonzales-Valls, I.; Yu, Y. Sol. Energy. Mater. Sol. Cells 2011, 95, 1362-1374.

30. Coakley, K. M.; Srinivasan, B. S.; Ziebarth, J. M.; Goh, C.; Liu, Y.; McGehee, M. D. Adv. Funct. Mater. 2005, 15, 1927-1932.

31. Kavan, L.; Gratzel, M.; Gilbert, S. E.; Klemenz, C.; Scheel, H. J. J. Am. Chem. Soc. 1996, 118, 6716-6723.

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CHAPTER 4

OPTOELECTRONIC TUNING OF ORGANOBORYLAZADIPYRROMETHENES VIA

EFECTIVE ELECTRONEGATIVITY AT THE METALLOID CENTER*

4.1 Introduction

Azadipyrromethene dye are a class of dipyrrinoid compounds1 that are under increasing

investigation for their coordination chemistry2-6 and their various applications in chemosensing,7-

10 bioimaging,11-14 photodynamic therapy,15,16 electrogenerated chemiluminscence,17

supramolecular charge transfer systems18-21 and excitonic solar cells.22 The optoelectronic

properties of azadipyrromethenes are sensitive to their coordination environment, requiring a

careful choice of chelate for any given application. Difluoroboron-chelates of

azadipyrromethenes (F2-azaBODIPYs) have strong fluorescence that correlates to a stable

excited state with lifetimes in the nanosecond regime, but the free base azadipyrromethene is

almost non-emissive (ΦF <<1%).1,23 Among the known metal chelates [Co(II), Ni(II), Cu(I),

Cu(II), Zn(II), Ag(I), Re(I) Au(I), Hg(II)] only those of monovalent coinage metals are reported

to be emissive, and only weakly (ΦF <1%).3-6 Metal-chelation typically shifts the first reduction

potential of azadipyrromethenes to less positive potential compared to the free base whereas

difluoroboryl chelation shifts the reduction potential to more positive potential.6,17

The utility of azadipyrromethenes as electron-accepting sensitizers19,24-26 in light-

harvesting applications such as photoelectrochemical solar cells would be enhanced by access to

a coordination environment that could simultaneously provide a stable singlet excited state and

an elevated first reduction potential. To that purpose, we have prepared and characterized a series

* This entire chapter is reproduced from Berhe, S. A.; Rodriguez M. T.; Park, E.; Vladimir, N. Nesterov V. N.; Pan, H.; Youngblood, W. J. Inorg. Chem. 2014, 53, 2346-2348 with permission from the American Chemical Society.

63

of organoboron-chelated azadipyrromethenes. These organoboron-azaBODIPYs display a range

of optoelectronic behavior. The trends in singlet emission intensities, fluorescence lifetimes and

first reduction potentials (vide infra) suggest that the effective electronegativity of the boron

atom, as tuned by its substituents, determines the behavior of each chelate, with strong

electronegativity favoring a more stable excited state and a more positive reduction potential.

4.2 Synthesis of azaBODIPY Derivatives

The synthesis of azadipyrromethene 1 followed a route developed by O’Shea et al.27 The

sp3-hybridized carbon substituted boron azaBODIPY 3 was produced by treating

azadipyrromethene with dibutylboron trifluoromethanesulfonate and triethylamine (TEA).

Difluoroboron azaBODIPY 2 was synthesized by reacting boron trifluoride etherate and TEA

and it was used as a starting material for making the sp2- and sp-hybridized carbon-substituted

azaBODIPYS 4 and 5.28 We adapted the work of Ziessel et al. to nucleophilically displace the

fluorine atoms on 2 with alkynyl or alkenyl groups using organometallic reagents.29

Scheme 4.1. Synthesis scheme of azaBODIPY derivatives.

64

4.3 Experimental Procedure

Anhydrous solvents were prepared by storage over molecular sieves. Silica gel (40 mm

average particle size) was used for chromatography. Vinylmagnesium bromide was freshly

prepared from vinyl bromide prior to use. Compounds 1 and 2 were prepared according to

previous reports. Analytical data were consistent with the previous results. All other chemicals

were purchased as reagent grade and were used as received. Fluorescence quantum yields for the

azadipyrrins were measured in toluene by comparison to Rhodamine B (measured in ethanol, Φf

= 0.7) with correction for the differential refractive indices of the solvents used. Cyclic

voltammetry was conducted under inert atmosphere in dry CH2Cl2 with NBu4PF6 (0.1M) using a

gold film working electrode, a platinum wire counterelectrode, and a silver wire reference

electrode. Ferrocene was used as an internal standard. Lifetime measurements were obtained by

time correlated single photon counting (TCSPC) using a nano-LED excitation source. HRMS

data were collected on a MALDI-TOF instrument with POPOP as a matrix (Spectra can be found

in the appendix section).

1,3,5,7-Tetraphenyl-4,4-bis(trimethylsilylethylnyl)-4-bora-3a,4a,8-triaza-s-indacene:(3)

Trimethylsilylacetylene (62 μL, 0.45 mmol) was put in a dry argon-purged round bottom flask

with dry THF (5mL) and was cooled to -78°C. n-Butyllithium (0.314 mL, 0.45mmol) was added

and the mixture was allowed to warm to room temperature. The reaction mixture was cannulated

in to a second flask containing compound 2 (73 mg, 0.15 mmol) in dry THF (10 mL). The color

changed from blue-green to green immediately and all starting material was consumed within 10

min. Purification by column chromatography (SiO2, hexanes/CH2Cl2, 2:1) gave the product as a

metallic brown solid (17 mg, 18%). δH(CDCl3, 400 MHz) -0.13 (s, 18H), 6.96 (s, 2H), 7.37–7.47

(m, 12H) 8.06 (d, 4H), 8.25–8.30 (m, 4H); δC(CDCl3, 100 MHz) 0.0, 119.8, 127.6, 128.7, 129.1,

65

129.4, 130.0, 130.8, 132.8, 132.9, 142.7, 143.5, 159.4; HRMS (MALDI-TOF) m/z M+ Calcd for

C42H40N3B11Si2 653.28538; Found 653.28552; λabs(nm) 286, 506, 647; λem(nm) 669.

Figure 4.1. 1H NMR for compound 3.

Figure 4.2. 13C NMR for compound 3.

66

1,3,5,7-Tetraphenyl-4,4-divinyl-4-bora-3a,4a,8-triaza-s-indacene:(4) Vinylmagnesium

bromide(1.3 M, 0.562 mmol in THF, preheated to 35°C) was added to a flask containing

compound 2 (110 mg, 0.28 mmol) in THF (20 mL). The reaction mixture was refluxed for 3 h.

The color changed from blue-green to blue as starting material was consumed. Purification by

column chromatography (SiO2, hexanes/CH2Cl2, 2:1) gave the product as a brown solid (60 mg,

53%). δH(C6D6, 500 MHz) 5.71 (dd, J = 19.4, 3.5 Hz, 2H), 5.91 (dd, J = 13, 3.5 Hz, 2H), 6.97

(dd, J = 19.4, 13 Hz, 2H), 7.58–7.65 (m, 6H), 6.69 (2H, s), 7.71–7.75 (m, 2H), 7.79–7.84 (m,

4H) 8.59–8.63 (m,4H); δC(C6D6, 125 MHz) 120.2, 123.6, 127.6, 128.8, 129.0, 129.5, 129.7,

130.5, 133.5, 134.2, 142.3, 144.3, 159.7; HRMS (MALDI-TOF) m/z M+ Calcd for C36H28N3B11

513.23763; Found 513.23657; λabs(nm) 292, 618. λem(nm) 665.

Figure 4.3. 1H NMR for compound 4.

67

Figure 4.4. 13C NMR for compound 4.

4,4-Dibutyl-1,3,5,7-tetraphenyl-4-bora-3a,4a,8-triaza-s-indacene (5). Compound 1 (40.0

mg, 0.089 mmol) was dissolved in dry CH2Cl2 (1.0 mL) and treated with triethylamine (50.7 μL,

0.36 mmoL) and dibutylboron trifluoromethanesulfonate (78.2 μL, 0.36 mmoL) and stirred at

room temperature under argon overnight. The reaction mixture was concentrated under reduced

pressure and the residue was purified by column chromatography (SiO2, hexanes/CH2Cl2, 4:1) to

give the product as a metallic blue solid (45 mg, 88%). δH(C6D6, 500 MHz) 0.06-0.13 (m, 4H),

0.63 (t, J = 9.1 Hz, 6H), 0.74–0.87 (m, 4H), 0.89–1.02 (m, 4H), 6.68 (s, 2H), 7.24–7.33 (m,

12H), 7.33–7.39 (m, 4H), 7.98–8.02 (m, 4H); δC(C6D6, 125 MHz) 14.4, 26.5, 28.1, 120.6, 127.8,

128.4, 128.7, 128.8, 129.0, 129.3, 129.7, 133.6, 135.2, 141.5, 144.4, 159.4; HRMS (MALDI-

TOF) m/z M+ Calcd for C40H40N3B11 573.33153; Found 573.32965; λabs(nm) 284, 369, 592.

λem(nm) 663.

68

Figure 4.5. 1H NMR for compound 5. The resonance for the methylene nearest the boron atom is shifted upfield by exposure to the aromatic ring currents of the nearby phenyl rings.

Figure 4.6. 13C NMR for compound 5. One resonance for an sp3 carbon of the butyl chains is visible after zooming in the high field region. Resonance at 28.2 ppm is for protons at carbon alpha to boron. Signal is broadened due to quadrupolar B-C coupling.

69

4.4 Photochemistry of azaBODIPY Derivatives

The UV-vis absorbance and fluorescence spectra of the azaBODIPYs are shown in

Figure 4.7. As fluorine is replaced by the less electronegative sp-carbon alkyne substituents, the

absorption maximum is blue-shifted from 655 nm to 649 nm. Further gradual blue shifting up to

592 nm was observed as the hybridization of the carbon substituent attached to the boron atom

was changed from to sp→sp2→sp3. The strongest fluorescence was observed for the

difluoroboryl chelate, and second strongest fluorescence for the sp-organoboryl chelate. A

successive decrease in fluorescence occurs as the electronegativity of the substituent bonded with

the boron atom decreases. Unlike the absorption maxima, the fluorescence maxima do not show

a hypsochromic shift upon change in the boron substituent, resulting in a greater Stokes shift as

the electronegativity of the boron substituent decreases. Transient fluorescence lifetimes (Table

4.1 and Figure 4.8) are consistent with the steady state fluorescence behavior regarding the

relative stability of the singlet excited state among the aza-BODIPY compounds.

Figure 4.7. Absorption and fluorescence spectra of azaBODIPYS 1-5 in toluene at rt (left and right, respectively).

70

Figure 4.8. Transient fluorescence of compounds 2 and 3, as measured by time-correlated single photon counting. Dashed line represents instrument response from the illuminating LED.

4.5 Electrochemistry of AzaBODIPY Derivatives

Cyclic voltammetric (CV) analysis (Figure 4.9) showed reversible reduction waves for 2,

and quasi-reversible waves for 3-5, although we note that even ferrocene appears quasi-

reversible in those analyses due to the choice of electrolyte (CH2Cl2, 0.1 M TBAPF6). Reduction

potentials for the organoboron-azaBODIPYs were all more negative than for 2 (-0.82 V vs

Fc/Fc+), and the potentials rose along with the increase in p-character of the s/p hybridization of

the organic substituent (Table 1). Although the oxidation potentials are at best quasi-reversible,

the estimated E1/2 values exhibit a shift in oxidation potential to less positive values as the

electronegativity of the boron substituent decreased, but the scale of this change was not as

significant as the raise in the reduction potential to more negative values, resulting in an increase

in the electrochemical potential gap (oxidation reduction) in the order 2<3<4=5. The relative

sizes of the photochemical vs. electrochemical energy gaps for these azaBODIPY compounds

are tuned by the changes in substitution at the boron atom. This, in turn, controls the energetic

ordering of radical anion and neutral (singlet) photoexcited states available to these compounds.

71

The difluoroboryl-azaBODIPY 2, like the free base dipyrromethene, has a more negative

potential for its excited state oxidation potential than for its reduction potential (Table 4.1). The

dialkynylboryl-azaBODIPY 3 has nearly isoenergetic values for these states, and the

divinylboryl and dibutylboryl analogs 4 and 5 have more negative redox potentials for their

reduced forms than for their excited state oxidation potentials.

Figure 4.9. Cyclic voltammogram of azaBODIPYS in CH2Cl2. Fc/Fc+ refers of the ferrocene /ferrocenium couple used as internal reference. Sweep rate = 100 mV/s.

4.6 Results and Discussion

The optoelectronic tuning of azaBODIPY dyes via the organoboron chelation is in some

ways similar to effects seen upon corresponding coordination changes on BODIPY dyes. The

strong shift toward more negative reduction potential (180 mV) in changing from 2 to 3 is

similar to that seen for difluoroboryldipyrrin vs. dialkyldipyrrin compounds.29,31 The

72

accompanying negative shift of the oxidation potential by 60 mV is somewhat smaller than seen

for difluoroboryl/dialkynylboryl dipyrrins, for which the shift is usually ~100 mV.

Table 4.1. Optical and electrochemical data for compounds 1-5.

Compound λabs (nm)a

λem (nm) Ε0,0 (eV) ΦF

b τF (ns) 𝐸𝐸ox𝑖𝑖0 𝐸𝐸red𝑖𝑖0

𝐸𝐸ox𝑖𝑖0− 𝐸𝐸red𝑖𝑖0 𝐸𝐸𝑆𝑆0*/S•+

H-ADP (1) 599 639 1.94 n/ac n/ad 0.55e -1.22e 1.77 -1.39

F2B-ADP (2) 654 678 1.83 0.34 1.94 0.86 -0.82 1.68 -0.97 (TMSCC)2B-ADP (3) 647 671 1.85 0.22 1.48 0.80 -1.01 1.81 -1.05 (C2H3)2B-ADP (4) 618 667 1.86 0.017 n/ad 0.79 -1.13 1.92 -1.07 Bu2B-ADP (5) 592 663 1.87 0.006 n/ad 0.67 -1.25 1.92 -1.20 𝐸𝐸ox𝑖𝑖0 = oxidation potential (E1/2); 𝐸𝐸red𝑖𝑖0 = reduction potential (E1/2); 𝐸𝐸𝑆𝑆0*/S•= excited state oxidation potential (calculated as 𝐸𝐸ox𝑖𝑖0 − 𝐸𝐸0,0). All potentials are V vs. Fc/Fc+. aAbsorbance and fluorescence data measured under ambient conditions (in CHCl3 and toluene, respectively. bQuantum yields calculated with using rhodamine B (ΦF = 0.7 in ethanol, λexc = 490 nm). All ΦF are corrected for changes in refractive index. cThe fluorescence of compound 1 was too weak to quantify accurately. dThe lifetimes of compounds 3, 4 and 5 were not measurable by the instrument (detection limit ~ 300 ps). eData from reference 30. There is a noticeable blue-shift (7 nm) in the absorbance maximum of 3 relative to 2, and a

decrease in the fluorescence quantum yield (0.33→0.22). These changes are extreme in the

comparison of 5 to 2, with a 62 nm blue shift and a nearly 50-fold decrease in fluorescence

(Table 1). Dialkynyl-BODIPYs show maxima either red- or blue-shifted no more than 1-2 nm

compared to difluoroboryl-BODIPYs, and have slightly increased fluorescence quantum

yield.29,31 Kee et al. reported that dialkylboryl-BODIPYs are blue-shifted by only 6-7 nm

compared to difluoroboryl-BODIPYS, and show a 3-fold decrease in ΦF for a dimethylboryl-

BODIPY to difluoroboryl-BODIPY (0.33 vs 0.93).28 Molecular orbitals for these compounds

were generated by density functional theory (see Experimental Section) and indicate no orbital

density in the frontier orbitals at boron. In general, the frontier molecular orbitals are remarkably

similar across all compounds within this study. Based on the lack of a consistent trend in the

73

orbital patterns, we propose that the optoelectronic tuning by boron substituents can be more

easily explained as an effect of the electronegativity of the boron substituent upon the strength of

the dative nitrogen-boron bond: As the electronegativity of the boron substituent increases, the

effective electronegativity of the boron atom is inductively raised, thereby strengthening the

dative N→B bond and enhancing the rigidity of the azaBODIPY core. The azaBODIPYs should

be more sensitive to such N→B bond-weakening effects than standard BODIPY dyes because of

the lower electron density in the azaBODIPY ring system caused by the azomethine nitrogen.

Group electronegativity values for carbon in different hybridizations, along with many other

functional groups, have been calculated and/or correlated to empirical data to yield sets of values

that do not always agree quantitatively, but do always agree on qualitative trends: increasing s-

character in carbon hybridization leads to increased electronegativity, and values for alkyl

(2.2−2.5), alkenyl (2.3−2.8), and alkynyl groups (2.5−3.1) are all far below of that of fluorine

(4.0).32-37 Alkynyl substituent electronegativity are near to the electronegativity of elemental

nitrogen (3.0-3.2) but below that of oxygen (3.5). We note that azaBODIPYs with oxygen

substituents at boron have been reported to exhibit fluorescence yields and redox behavior quite

similar to difluoroboryl-azaBODIPYs.22 While this manuscript was under completion, Jiang and

co-workers reported the synthesis of dialkynyl- and diarylboryl-azaBODIPYs using similar

methods to those of this paper, and they noted the significant differences in absorbance maxima

and fluorescence yields between the two chelates, with the diarylboryl-azaBODIPYs being of

depressed fluorescence yield similar to the divinylboryl-compound 4 reported herein.38 The

slightly higher ΦF for the diarylboryl-azaBODIPY compared to 4 (0.027 vs. 0.017, respectively)

may be due to slightly stronger electronegativity for aryl vs. vinyl groups,32 or may be an artifact

of standard deviation for measurements of such weak emission.

74

At the present time there are no reported examples of azaBODIPYs with nitrogen

substituents. According to the electronegativity trend, groups including azide, isocyanate,

isothiocyanate, nitrosyl and nitro should exhibit fluorescence quantum yields and redox behavior

similar to dioxoboryl- and difluoroborylazaBODIPYs. It may also be possible to probe the

intermediary regime of declining fluorescence yield and elevated reduction potential between

compounds 3 and 4 of this study using carbon-based substituents such as difluoromethyl,

trifluoromethyl, formyl and acyl groups. Recent advances in facile substitution at

dichloroboryldipyrromethenes,39,40 if reproducible with azadipyrromethenes, could enable the

more detailed exploration of the optoelectronic tuning we have observed in this study. Additional

control over the hierarchy of excited states could be explored by extending the π-conjugation of

the azaBODIPY chromophore to reduce the photochemical band gap (E0,0).24,25,41 The

optoelectronic tuning of azaBODIPY dyes via the substituents at the boron atom and other sites

may enhance their applicability to photochemical, electrochemical, and photoelectrochemical

processes. As an example, we note that in the past two decades of research on azaBODIPYs,

these compounds have yet to be reported as sensitizers within dye-sensitized solar cells. Free

base azadipyrromethene is insufficiently stable in the singlet excited state for photochemical

applications, and the excited state oxidation potential and reduction potential of azaBODIPY (2)

are both below the conduction band edge of most n-type oxide semiconductors. Dialkynyl-

azaBODIPYs should have sufficient driving force to sensitize TiO2 in the absence of an

electrolyte from either the neutral photoexcited or radical anion state. Additional enrichment of

electron density in the chromophore at other sites may enable them to function even within

electrolyte-wetted photoelectrochemical cells. Efforts to explore these opportunities are currently

underway.

75

4.7 Conclusion

The optoelectronic tuning of boron-chelated azadipyrromethene dyes has been explored

by the substitution of carbon substituents in place of fluoride atoms at boron. The resulting

organoboron-azaBODIPYs have been characterized by steady-state UV-vis and fluorescence

spectroscopies, transient fluorescence spectroscopy, and cyclic voltammetry. Trends in the

absorbance, fluorescence, and redox behavior appear dependent on the effective electronegativity

at the boron atom as tuned by its substituents, with stronger electronegativity correlating to red-

shifted absorbance, enhanced fluorescence lifetime and yield, and positively-shifted redox

potentials

4.8 References

1. Loudet, A.; Burgess, K. Chem. Rev. 2007, 107, 4891-4932.

2. Palma, A.; Gallagher, J. F.; Muller-Bunz, H.; Wolowska, J.; McInnes, E. J. L.; O'Shea, D. F. Dalton Trans. 2008, 273-279.

3. Teets, T. S.; Partyka, D. V.; Updegraff III, J. B.; Gray, T. G. Inorg. Chem. 2008, 47, 2338-2346.

4. Teets, T. S.; Updegraff III, J. B.; Esswein, A. J.; Gray, T. G. Inorg. Chem. 2009, 48, 8134-8144.

5. Partyka, D. V.; Deligonul, N.; Washington, M. P.; Gray, T. G. Organometallics 2009, 28, 5837-5840.

6. Bessette, A.; Ferreira, J. G.; Giguere, M.; Belanger, F.; Desilets, D.; Hanan, G. S. Inorg. Chem. 2012, 51, 12132-12141.

7. Coskun, A.; Yilmaz, M. D.; Akkaya, E. U. Org. Lett. 2007, 9, 607-609.

8. Gawley, R. E.; Mao, H.; Haque, M. M.; Thorne, J. B.; Pharr, J. S. J. Org. Chem. 2006, 72, 2187-2191.

9. Killoran, J.; McDonnell, S. O.; Gallager, J. F.; O'Shea, D. F. New J. Chem. 2008, 32, 483-489.

10. Palma, A.; Tasior, M.; Frimannsson, D. O.; Vu, T. T.; Meallet-Renault; O'Shea, D. F. Org. Lett. 2009, 11, 3638-3641.

76

11. Chen, J.; Burghart, A.; Derecskei-Kovacs, A.; Burgess, K. J. Org. Chem. 2000, 65, 2900-2906.

12. Murtagh, J.; Frimannsson, D. O.; O'Shea, D. F. Org. Lett. 2009, 11, 5386-5389.

13. Tasior, M.; Murtagh, J.; Frimannsson, D. O.; McDonnell, S. O.; O'Shea, D. F. Org. Biomol. Chem. 2010, 8, 522-525.

14. Tasior, M.; O'Shea, D. F. Bioconjugate Chem. 2010, 21, 1130-1133.

15. Adarsh, N.; Avirah, R. R.; Ramaiah, D. Org. Lett. 2010, 12, 5720-5723.

16. Batat, P.; Cantuel, M.; Jonusauskas, G.; Scarpantonio, L.; Palma, A.; O'Shea, D.; McClenaghan, N. D. J. Phys. Chem. A 2011, 115, 14034-14039.

17. Nepomnyashchii, A. B.; Broring, M.; Ahrens, J.; Bard, A. J. J. Am. Chem. Soc. 2011, 133, 8633-8645.

18. Flavin, K.; Lawrence, K.; Bartelmess, J.; Tasior, M.; Navio, C.; Bittencourt, C.; O'Shea, D. F.; Guldi, D. M.; Giordani, S. ACS Nano 2011, 5, 1198-1206.

19. Amin, A. N.; El-Khouly, M. E.; Subbaiyan, N. K.; Zandler, M. E.; Supur, M.; Fukuzumi, S.; D’Souza, F. J. Phys. Chem. A 2011, 115, 9810-9819.

20. D'Souza, F.; Amin, A. N.; El-Khouly, M. E.; Subbaiyan, N. K.; Zandler, M. E.; Fukuzumi, S. J. Am. Chem. Soc. 2012, 134, 654-664.

21. Bandi, V.; El-Khouly, M. E.; Nesterov, V. N.; Karr, P. A.; Fukuzumi, S.; D'Souza, F. J. Phys. Chem. C 2013, 117, 5638-5649.

22. Leblebici, S. Y.; Catane, L.; Barclay, D. E.; Olson, T.; Chen, T. L.; Ma, B. ACS Appl. Mater. Interfaces 2011, 3, 4469-4474.

23. Flavin, K.; Lawrence, K.; Bartelmess, J.; Tasior, M.; Navio, C.; Bittencourt, C.; O'Shea, D. F.; Guldi, D. M.; Giordani, S. ACS Nano 2011, 5, 1198–1206.

24. Gao, L.; Senevirathna, W.; Sauve, G. Org. Lett. 2011, 13, 5354-5357.

25. Gao, L.; Tang, S.; Zhu, L.; Sauve, G. Macromolecules 2012, 45, 7404-7412.

26. Mueller, T.; Gresser, R.; Leo, K.; Riede, M. Sol. Energy. Mater. Sol. Cells 2012, 99, 176-181.

27. Gorman, A.; Killoran, J.; O'Shea, C.; Kenna, T.; Gallagher, W. M.; O'Shea, D. J. Am. Chem. Soc. 2004, 126, 10619-10631.

28. Kee, H. L.; Kirmaier, C.; Yu, L.; Thamyongkit, P.; Youngblood, W. J.; Calder, M. C.; Ramos, L.; Noll, B. C.; Bocian, D. F.; Scheidt, W. R.; Birge, R. R.; Lindsey, J. S. J. Phys. Chem. B 2005, 109, 20433-20443.

77

29. Goze, C.; Ulrich, G.; Ziessel, R. J. Org. Chem. 2007, 72, 313-322.

30. Gresser, R. PhD Thesis, Technischen Universitat Dresden, 2007.

31. Ziessel, R.; Goze, C.; Ulrich, G. Synthesis 2007, 936-949.

32. Inamoto, N.; Masuda, S. Tetrahedron Lett. 1977, 37, 3287-3290.

33. Inamoto, N.; Masuda, S. Chem. Lett. 1982, 1003-1006.

34. Marriott, S.; Reynolds, W. F.; Taft, R. W.; Topsom, R. D. J. Org. Chem. 1984, 49, 959-965.

35. Mullay, J. J. Am. Chem. Soc. 1984, 1106, 5842-5847.

36. Mullay, J. J. Am. Chem. Soc. 1985, 107, 7271-7275.

37. Datta, D.; Singh, S. N. J. Phys. Chem. 1990, 94, 2187-2190.

38. Jiang, X.-D.; Fu, Y.; Zhang, T.; Zhao, W. Tetrahedron Lett. 2012, 53, 5703-5706.

39. Lundrigan, T.; Crawford, S. M.; Cameron, T. S.; Thompson, A. Chem. Commun. 2012, 48, 1003-1005.

40. Lundrigan, T.; Thompson, A. J. Org. Chem. 2013, 78, 757-761.

41. Bellier, Q.; Pegaz, S.; Aronica, C.; Le Guennic, B.; Andraud, C.; Maury, O. Org. Lett. 2011, 13.

78

CHAPTER 5

ELECTRON TRANSFER IN PHOTOGALVANIC DYE-SENSITIZED SOLAR CELLS

INCORPORATING ORGANIC DYES

5.1 Introduction

Dye-sensitized solar cells (DSCs) based on nanocrystalline semiconductors have been

intensively studied because of their potential low cost, ease of processing, and high

performance.1-4 There is a large volume of ongoing research and publication dedicated to the

synthesis and study of new dye compounds, redox shuttles, and surface treatments in the hope of

finding cell components that overcome various thermodynamic and/or kinetic issues of DSCs.5,6

Photoinduced interfacial electron transfer (ET) between molecular adsorbates and semiconductor

nanoparticles has been a subject of intense recent interest,7,8 because this topic is essential for

improving the efficiency of dye-sensitized solar cells.9, 10

Some photosensitizers have slow injection kinetics on TiO2, and some oxide

semiconductors have slower injection kinetics with various dyes compared to TiO2. Having an

intermediate molecular acceptor between donor and oxide could allow for the injecting species

(radical anion) to have a very long lifetime for the injection process (thermal injection) to occur

in high yield regardless of the donor or choice of oxide semiconductor. Ruthenium-based dyes,

which have been the champion dyes for most of the history of DSCs research, have multiphasic

electron injection kinetics: injection from higher (“hot”) singlet excited states is ultrafast (~ 1012

s-1, figure 1) whereas the thermally-equilibrated metal-ligand charge transfer (MLCT) state

injects less rapidly (~ 1010 s-1).11-17

79

Figure 5.1. Electron injection by Ru-dye from ‘hot’ vs. thexi states. Adapted from figure 9 of reference 15.

Thermal injection occurs from a narrow distribution of potential energy with respect to

the injecting species, so the kinetics of thermal injection can, in theory, be less complicated than

photoexcited injection, which may occur from a variety of upper excited states or lowest excited

state (singlet or triplet). Long-lived injecting species with simple injection kinetics would make a

suitable model system for probing slow injection kinetics, as expected for injection with low

driving force to low-lying acceptor states in an oxide semiconductor where the density of states

may be reduced, as is known to be the case for TiO2.12, 16 Knowledge about this subject could

enable researchers to reduce the overpotential for electron injection into the oxide

semiconductor, which could alternately be exploited to examine longer-wavelength absorbing

dyes or to increase photovoltage in photodiode devices. Therefore, the study of solid state DSCs

is to find systems with confirmed behavior of thermal electron injection.

Enhancement of steady-state photovoltage in polymer-oxide solar cells was observed

using intermediary molecular acceptor species composed of fullerene and acenequinone species,

(see chapter 2) and some contribution to the photocurrent by acenequinone acceptor NcQ was

80

observed despite its weak contribution to the cell absorbance.18 Whether this is due to

photoexcited or thermal electron injection is not known. TiO2/dye/P3HT films are opaque and

not easy to prepare with sufficient transparency for pump/probe experiments. In this chapter, I

describe efforts to better understand the photosensitizing behavior of electron-accepting organic

dyes by examining them as the only photoactive component of excitonic solar cells. Two new

acenequinone dyes, NcQ-7 and PcQ were synthesized, and examined along with dyes NcQ-2

(previously NcQ), C60-M, and C60-T as photosensitizers in liquid and solid-state dye-sensitized

solar cells. The electron transport dynamics at TiO2-acceptor-donor interfaces using spiro-

OMeTAD as the donor species via transient UV-vis spectroscopy were also examined.

Briefly, the findings are that the organic acceptor-sensitizers function well in solid-state

DSCs but not in liquid DSCs. The lifetime of the photoexcited dyes within the adsorbed

monolayer may be much shorter than in dilute solution, and not sufficiently long-lived to engage

in efficient electron transfer with the solution-phase redox shuttle within liquid DSCs. The

transient spectroscopy analysis of solid-state TiO2-dye-donor interfaces strongly supports the

interpretation of the injection mode as being thermal electron injection from radical anion dyes

into the TiO2.


5.2.1 Synthesis of Molecular Acceptors

Tetrabromodimethylnaphthalene: N-bromosuccinimide (2.34 g, 13.1 mmol) and benzoyl

peroxide (0.064 g, 0.27 mmol) were added to 3,4-dimethylnaphthalene (0.50 g, 3.2 mmol)

dissolved in carbon tetrachloride (20 mL). The colorless heterogeneous reaction mixture turned

yellow with a white solid suspended on top after refluxing overnight. The solution was filtered

81

and concentrated. The residue was recrystallized from acetonitrile to give a white solid (0.70 g,

47%). δH(400MHz, (CD3)2CO): 8.09 (m, 2H) 7.70 (m, 2H) 7.69 (s, 2H) 2.98 (s, 2H)

Methylpentacenequinone (MPcQ-7): Tetrabromonaphthalene (0.25 g, 0.53 mmol),

methylnaphthquinone (46 mg, 0.26 mmol) and sodium iodide (0.40 g, 2.7 mmol) were mixed in

dimethylformamide (5 mL). The solution was heated to 80°C for 4 h, during which a color

change to red was observed. Water (4 mL) was added and a precipitated formed. Sodium

bisulfite (5 mL, 10%) was added into the solution until it turned from brown to light yellow. The

filtrate was extracted with ethyl acetate and purified using gravity column chromatography

[SiO2, (CH2Cl2/Hexanes, 1:1)]. A yellow solid (57.8 mg, 67%) was collected after concentration.

δH (500MHz, C6D6): 9.12 (m, 2H) 8.44 (d, 1H) 8.32(m, 1H) 8.07 (d, 2H) 7.62 (m, 3H) 7.00(d,

2H) 1.96(s, 3H); UV-vis (CH2Cl2) λmax(nm): 289, 327, 341, 432, 456.

Pentacenequinone acid (PcQ) A microwave tube was charged with

methylpentacenequinone (20 mg, 0.12 mmol), sodium dichromate (51.7 mg, 0.17 mmol) and

water (2 mL) and placed in a microwave and heated to 205°C and 19 atm pressure for 5 hours.

The reaction mixture was a bright yellow solution with a green precipitate. A precipitate was

formed after the filtrate was acidified with 5 M HCl. UV-vis (ethanol) λmax(nm): 289, 327, 341,

432, 456.

5.2.2 Preparation of TiO2 Nanorod Films

F-SnO2-coated glass substrates (7 Ω/, Pilkington Glass) were sonicated in acetone,

isopropanol and deionized water for 5 minutes respectively, air dried in a laminar flow hood and

treated in an ozone cleaner for 15 minutes. Substrates were coated with titanium by thermal

evaporation of Ti° metal to 100 Å thickness, then annealed at 450°C for 30 min under ambient

atmosphere.

82

Substrates were then placed in a Parr-type Teflon pressure vessel (45 mL capacity) with

a solution of titanium isopropoxide (28 mM) in aq. HCl (5 M). The vessel was closed and heated

for 6 h in an oven at 150°C. After the heating period, the pressure vessel was placed under a

slow stream of tap water for 15 min to cool before opening. Substrates were rinsed with

deionized water and air dried in a laminar flow hood.

5.2.3 Preparation of TiO2 Nanoparticle Films

The first step in the preparation of the TiO2 nanoparticle films is making the paste which

was the synthesis of a polyester-based titanium oxide sol using a precursor with the molar ratio

of 1:6:24[Ti(iOPr)4: citric acid: ethylene glycol]. This is called the Pecchini method.22 The sol

was prepared by heating ethylene glycol to 60°C and during stirring the titanium isopropoxide

was added. Finally, the corresponding amount of citric acid was added and the temperature

increased to 90°C. The solution was stirred at this temperature until it turned clear. Finally the

corresponding TiO2 powder was added and stirred overnight. The paste was transferred to a

plastic Nalgene cup filled with zirconia beads. The cup was closed and rolled for two weeks

using a mineral-polishing tumbler. The paste was applied on FTO glass substrates using a doctor

blading technique and air dried overnight in a laminar flow hood. The films were annealed at

450°C for 30 min (heating 1°C per minute).

5.2.4 Preparation of Films for Transient Spectroscopy Studies

F-SnO2-coated glass substrates were sonicated in acetone, isopropanol and deionized

water for 5 minutes respectively, air dried in a laminar flow hood and treated in an ozone cleaner

for 15 minutes. Two samples were prepared from the clean FTO slides: 1) A blend of NcQ-2

(4.0 mg, 0.026 M) and spiro-OMeTAD (104 mg, 0.170 M) was dissolved in 0.5 mL of

chlorobenzene. The blend was spin coated on a clean FTO substrate for 40 s at 800 RPM. 2)

83

TiO2 nanorod films were prepared and soaked in NcQ-2 (2 mM) and C60-T (2 mM) solution

overnight. 170 mM spiro-OMeTAD, 130 mM 4-tert-butylpyridine, 13 mM LiN(CF3SO2)2, and

0.3mM tris(4-bromophenyl)aminium hexachloroantimonate in chlorobenzene was applied to the

sensitized TiO2 nanorod films by leaving the solution to penetrate into the film for 45 s and then

spin-coating for 40 s with 1200 RPM.

5.2.5 Transient Spectroscopy Studies

Femtosecond transient absorption spectral measurements: Femtosecond transient

absorption spectroscopy experiments were performed using an Ultrafast Femtosecond Laser

Source (Libra™) by Coherent incorporating diode-pumped, mode locked Ti:Sapphire laser

(Vitesse™) and diode-pumped intra cavity doubled Nd:YLF laser (Evolution™) to generate a

compressed laser output of 1.45 W. For optical detection a Helios™ transient absorption

spectrometer coupled with femtosecond harmonics generator both provided by Ultrafast Systems

LLC was used. The source for the pump and probe pulses were derived from the fundamental

output of the laser (compressed output 1.45 W, pulse width 91 fs) at a repetition rate of 1 kHz.

95% of the fundamental output of the laser was introduced into harmonic generator which

produces second and third harmonics of 400 and 267 nm besides the fundamental 800 nm for

excitation, while the rest of the output was used for generation of white light continuum. In the

present study, the second harmonic 400 nm excitation pump was used in all the experiments.

Kinetic traces at appropriate wavelengths were assembled from the time-resolved spectral data.

All measurements were conducted at 298 K.

5.2.6 Assembly of Liquid and Solid Based Devices and Their Characterization

Sensitization of oxide substrates was performed by soaking the TiO2 nanorod films

overnight. A solution of spiro-OMeTAD (170 mM), 4-tert-butylpyridine (130 mM),

84

LiN(CF3SO2)2(13 mM), and tris(4-bromophenyl)aminium hexachloroantimonate (0.3mM) in

chlorobenzene was applied to the sensitized TiO2 nanorod films by leaving the solution to

penetrate into the film for 45 s and then spin-coating for 40 s with 1200 RPM. PEDOT:PSS was

spin-coated from aqueous dispersion. Films were transferred into a metal evaporator and silver

electrodes of ~70 nm thickness were deposited in island-type geometry with a central dot of 1.1

cm2 area. Devices were tested on the day after they were assembled. Current-voltage

measurements were taken with a sourcemeter (Keithley 2400) and a Class A solar simulator

(Solarlight, Inc.). IPCE spectra (incident photons converted to electrons) were measured against

a calibrated silicon photodiode using monochromatic light from a xenon lamp (PV measurements

QEX7).

Sensitization of oxide substrates was performed by soaking the TiO2 nanorod films

overnight in a solution of a given dye in organic solvent (THF or DMF). Counterelectrodes were

prepared by applying a drop of H2PtCl6 on FTO glass, air drying, and annealing at 380°C for 30

min. A piece of surlin™ was sandwiched between the sensitized electrode and the

counterelectrode, and the two electrodes were sealed with the film using a heat press. A drop of

an electrolyte (I-/I3-, Co(di-t-butylbpy)+2/+3) was applied at a gap in by the Surlin films™ and

was allowed to enter and fill the inside volume through capillary action. Devices were tested on

the same day that they were assembled. Current-voltage measurements were taken with a

sourcemeter (Keithley 2400) and a Class A solar simulator (Solarlight, Inc.). IPCE spectra

(incident photons converted to electrons) were measured against a calibrated silicon photodiode

using monochromatic light from a xenon lamp (PV measurements QEX7).

85

5.3 Results and Discussion

5.3.1 Synthesis of Acceptor Sensitizers

Scheme 5.1. Synthesis of NcQ-2, NcQ-7 and PcQ.

Methylpentacenequinone (MPcQ) was successfully synthesized following a procedure

for the synthesis of methylnaphtacenequinone (MNcQ-2).18 Conversion of the methyl group to a

carboxylate in a superheated aq Na2Cr2O7 gave a black solution as supposed to green (from

chromium (III) oxide as observed in NcQ-2 preparation). Filtration of the black solution gave a

black precipitate (possibly chromium(II) oxide) and a colorless filtrate. Acidifying the colorless

filtrate didn’t form any precipitate. Increasing the quantity of sodium dichromate gave the same

black mixture. A microwave experiment was conducted at 205°C and 19 atm for 20 minutes and

no reaction was observed. Increasing the time gradually to 5 h gave a green precipitate and a

yellow filtrate. Acidifying the filtrate gave the desired product as a yellow precipitate.

Chapter 1 discussed vacuum energy level shift of TiO2 films depending on the dipole of a

monolayer attached to it. The vacuum level shifts up if the negative end of the monolayer is

towards the TiO2 and shifts down if the positive end is towards TiO2.18 DFT calculations show

MNcQ-2

MPcQ

86

that NcQ-2 sensitizer has a dipole that points the negative end toward the P3HT. So, to study the

dipole effect on photovoltage, we reversed the direction of molecular dipole of NcQ and point

the negative dipole toward the TiO2 by moving the carboxylate group to position 7 of

naphthacenequinone ring (NcQ-7, Scheme 5.1).

5.3.2 Optoelectronic Properties of the Acceptor- Sensitizers

The optical and redox properties of NcQ-7 are assumed to be the same as for NcQ-2

which was described in Chapter 2 along with the optoelectronic properties of C60-T. The redox

properties of 5,12-pentacenequinones are not previously determined, but such a small amount

was available through the attempted synthetic methods that I was unable to perform cyclic

voltammetry on a sample of the compound. However, it is reasonable to assume that the

reduction potential of PcQ is equal or negative of the corresponding reduction potential of NcQ.

Since optical measurements of absorption and emission properties are nondestructive, we were

able to characterize the photochemical behavior of MPcQ before converting it to PcQ.

Figure 5.2. The figure on the left is UV-vis (solid line) and emission of PcQ(dashed line) in CH2Cl2. The figure on the left is UV-vis and emission of 1,4 tetracenequinone (1,4-TQ) CCl4 from reference 19.

Tobita and his group reported that the fluorescence quantum yield and excited state life

time of 1,4-tetracenequinone (1,4-TQ) were 0.03 and 1.4 ns, respectively, at 295 K.19 We

Wavelength (nm) ( )

O

O

87

measured the UV-vis and fluorescence spectrum of PcQ and it is almost identical with 1,4-TQ.

Therefore, we expect the quantum yield and excited state life time to be similar to that of 1,4-

TQ. The emission spectrum has λmax at 492nm.

Figure 5.3. Absorption spectra of pentacenequinone and naphthacenequinone in ethanol at rt. Normalized with respect to absorption maxima of visible light: 432 nm (NcQ-2) to 456 nm (PcQ).

In chapter 2, NcQ-2 showed a significant photovoltage enhancement to the polymer solar

cells even though it has week absorption in the visible region. Adding another ring to the

structure might extend the conjugation which would improve the absorption in the visible region.

Therefore, PcQ was synthesized, which is extra-annulated by one benzene ring compared to

NcQ-2. Figure 5.3 compares the UV-vis of NcQ-2 and PcQ. The additional ring in PcQ indeed

resulted in red shifting of the absorption spectrum. In addition to that, NcQ-2 is not fluorescent at

room temperature but PcQ is. The fluorescence of PcQ means that it has a more stable singlet

excited state than NcQ-2. If a molecule does not have a stable excited state, it will relax back to

ground state very quickly after excitation and electron transfer will be less likely to happen. But

if a compound has a stable excited state it means that the relaxation to ground state is slower,

allowing more time for electron transfer. The UV-vis of PcQ shows a λmax of 432 nm and 456

0

1

2

3

4

5

6

330 380 430 480 530

Inte

nsity

Wavelength (nm)

NcQ-2

PcQ

88

nm. The fluorescence spectra shows a λmax of 492nm when exited at 420nm. There is a stoke

shift of 36 nm.

Figure 5.4. Femtosecond transient absorbance spectra (left) and decay rate (right) of MNcQ-2/spiro-OMeTAD blend on glass. The samples were excited using 400 nm laser pulses of 100 fs.

Figure 5.4 shows the femtosecond transient absorbance and decay rate of MNcQ-2/spiro-

OMeTAD blend on glass. Electron transfer from the spiro-OMeTAD to the MNcQ-2 generates

charged spiro-OMeTAD (radical cation) and charged MNcQ-2 (radical anion). Three distinct

spectral features are observed, a peak around ~610 for the radical anion MNcQ-2 (-.) and two

peaks ~500 nm and ~760 nm for the radical cation spiro-OMeTAD (+.). Both the radical cation

and radical anion coexist beyond the instruments response window (3 ns). The time profile of

the absorption of the radical anion could be fitted to triexponential fit (t1 = 2ps, t2 = 97 ps, t3 =

793 ps). The faster component could be a charge transfer to ambient oxygen and the slower

components are charge recombination.

89

Figure 5.5. Femtosecond transient absorbance spectra (left) and decay rate (right) of TiO2 nanorods films with adsorbed NcQ-7 and overlaid spiro-OMeTAD. The samples were excited using 400 nm laser pulses of 100 fs.

Figure 5.5 shows femtosecond transient absorbance spectra and decay rate of TiO2

nanorod films with adsorbed NcQ-7 and overlaid spiro-OMeTAD. The peak for NcQ-7 (-.)

radical anion is at ~ 590 nm (blue shifted by 10 nm) and spiro-OMeTAD radical cation has two

peaks at ~500 nm and ~750 nm. As shown above, the radical cation persisted for up to 3 ns in the

blend, whereas the peak for the radical anion is nearly completely gone only after 0.1 ns when

adsorbed on TiO2. Therefore the fast decay rate (almost 30 times faster) of the radical anion

when it was adsorbed on TiO2 suggests that an electron is injected into TiO2. The decay rates in

figure 5.5 (right) show the radical anion decaying very quickly while the radical cation is quite

prominent. The time profile of the absorption for NcQ-7 (-.) band could be fitted to a

biexponential fit (t1 = 2 ps and t2 = 17 ps) whereas radical cation has a slower multiphasic decay

(t1 = 4 ps; t2 = 46 ps and t3 = 610 ps). We attribute the faster decay of the radical anion NcQ-7 (-.)

to thermal electron injection into TiO2.

90

Figure 5.6. Fluorescence transient absorbance spectra of C60-T ester/spiro-OMeTAD blend on glass. The samples were excited using 400 nm laser pulses of 100 fs.

Unfortunately, absorbance peaks for the C60-T and spiro-MeOTAD radical ion pair do

overlap closely in the NIR spectral region, so observing their decay is more difficult. Figure 5.6

shows transient absorption spectra of C60-T ester/spiro-OMeTAD blend on glass. In agreement

with the literature results of femtosecond transient spectra, the charged spiro-OMeTAD (radical

cation) absorbs around 1450 nm and the charged C60-T ester (radical anion) absorbs around

1050 nm.20 The peaks for both the charged spiro-OMeTAD and C60-T ester, show up beyond 3

ns.

Figure 5.7. Transient absorbance spectra of TiO2 nanorods films with adsorbed C60-T and overlaid spiro-MeOTAD blend on glass.

C60-T ester

C60-T

C60-T

91

Figure 5.7 shows the absorbance spectra of TiO2 nanorods films on glass with adsorbed

C60-T and overlaid spiro-MeOTAD blend. Absorbance by C60-T radical anion on TiO2 is

around 1100 (red shifted by about 50 nm) and that of charged spiro-OMeTAD (radical cation) is

around 1425 nm (slightly blue shifted by about 15 nm). The red shift of C60-T could be due to

the dipole of TiO2 film. Even though quantitatively analyzing the decay rate was difficult due to

the overlap of the two peaks, it is qualitatively evident that the C60-T radical anion peak is gone

with only 0.3 ns. That is about 10 times faster than the blend on glass. This fast decay of radical

anion shows that there is an electron injection from the radical anion to the TiO2.

5.4 Device Studies

5.4.1 Device Studies on Solid-State Devices Incorporating the Dyes

Figure 5.8 shows IV curves of acceptor sensitized solid state DSCs. Solid state DSCs

were made employing the molecular acceptors (C60-T, NcQ-2, NcQ-7 and PcQ) as a dye and

spiro-OMeTAD as a hole conductor. A standard cell was made using N3 dye, a well know dye

for its high performance in liquid DSCs, in order to compare their performance. Despite the

narrow absorption in the visible and low extinction coefficient of the molecular acceptors, their

performance of is quite promising. Table 5.1 has the open circuit voltage (Voc) and closed circuit

current (Jsc) of the devices studied.

92

Figure 5.8. Illuminated IV curve of solid state acceptor sensitized DSCs.

Table 5.1. Photocurrent, photovoltage, fill factor, and efficiency of liquid based DSCs devices prepared in this study.

spiro-OMeTAD-Acceptor- TiO2 JSC (mA) VOC (mV) IPCE (%)

spiro-OMeTAD —PcQ—TiO2 0.155 555 4.98

spiro-OMeTAD — NcQ-7—TiO2 0.472 438 7.93

spiro-OMeTAD —C60-T—TiO2 0.524 615 -

spiro-OMeTAD —N3—TiO2 0.795 775 3.8, 2.98(410 nm and 490 nm)

-900

-700

-500

-300

-100

0.0 0.2 0.4 0.6 0.8

J SC

(µA/

cm2 )

Voltage (V)

NcQ-7C60-TPcQ-2N3

93

Figure 5.9. IPCE of NcQ-7 and N3 based acceptor sensitized solid state DSCs.

IPCE comparison of solid state DSC using dyes NcQ-7 and N3 is shown in figure 5.9. It

shows that the absorption band of NcQ-7 is narrower than N3 dye. But the photocurrent

generation efficiency in that narrow band is higher for NcQ-7 than for N3. This indicates that the

monochromatic photon to current conversion efficiency is higher for NcQ-7. Therefore an

acceptor-sensitizer molecular acceptor with a broader optical absorption property should result in

a higher efficiency.

5.4.2 Device Studies on Liquid-state Devices Incorporating the Dyes

Figure 5.10 shows current-voltage (IV) curves of liquid based acceptor sensitized DSCs

cells. The NcQ molecular acceptor shows better performance with enhancement in both

photocurrent and photovoltage. Despite having a more stable excited state, PcQ didn’t show

better performance as a sensitizer. Comparing the two NcQ acceptors, the dipole of NcQ-7

points to the TiO2 surface whereas NcQ-2 points away from TiO2 surface. NcQ-7 show a little

more improvement both in photocurrent and photovoltage than NcQ-2. As it is mentioned above,

0.0

2.0

4.0

6.0

8.0

10.0

300 400 500 600 700

Inte

rnal

QE

(%)

Wavelength (nm)

N3

NcQ-7

94

pointing a negative dipole at the surface of TiO2 shifts the vacuum level up. The higher

photovoltage from NcQ-7 is consistent with the shifting of vacuum level due to its dipole.

However, since we did not make multiple devices with each sensitizer, we cannot be certain that

the photovoltage difference is not within a standard deviation.

Figure 5.10. Light IV curve of liquid based acceptor sensitized cells.

Table 5.2. Photocurrent, photovoltage, fill factor, and efficiency of liquid based DSC devices prepared in this study.

I-/I3--Acceptor- TiO2

JSC (mA)

VOC (mV)

IPCE (%)

I-/I3-—PcQ—TiO2 0.211 380 23

I-/I3-—C60-T—TiO2 0.268 440 23

I-/I3- —NcQ-2—TiO2 0.508 463 27

I-/I3-—NcQ-7—TiO2 0.570 512 37

95

Figure 5.11. IPCE of liquid based acceptor sensitized cells.

IPCE of the devices follows the IV-curve trend. NcQ-7 has the highest photocurrent and

photovoltage and similarly it has the highest IPCE. Only NcQ-2 and NcQ-7 show a hump from

400 nm to 450 nm. The major peak at 350 nm is from the TiO2 nanoparticle film. The absorption

features of the acceptor sensitizers were not clearly identifiable in the IPCE curves because the

main photocurrent generation was the TiO2.

5.5 Conclusion

We have shown convincing evidence supporting the assertion that acceptor-sensitizers

operate via thermal electron injection from the radical anion state. We have also identified a

possible difference in operative excited state lifetimes between the dyes present in adsorbed

monolayers and dyes in dilute solution, which might be explained by the process of triplet-triplet

annihilation, wherein two triplet-excited chromophores interact to produce one singlet-excited

chromophore and one ground state chromophore; subsequent intersystem crossing by the singlet-

excited chromophore then returns it to the triplet state, with the overall result of nonradiative

decay of one of the original chromophores. In a densely packed monolayer, lateral energy

0

10

20

30

300 350 400 450 500

IPC

E (%

)

Wavelength (nm)

NcQ-2

PcQ

C60-T

NCQ-7

96

transfer among adsorbed dyes could permit triplet-excited states to migrate until they meet and

engage in this process, through which many excited states could be led to decay to the ground

state.

Future studies in this area need dyes with stronger extinction coefficient and tunable

redox potentials. We identify azadipyrrins as a candidate class of compounds for future acceptor

sensitizers, along with perylenemonoimides and perhaps cyclopenta[hi]aceanthrylenes.

5.6 References

1. Yum, J.; Hardin, B. E.; Moon, S.; Baranoff, E.; Nuesch, F.; McGehee, M. D.; Gratzel, M, Angew. Chem. Int. Ed. 2009, 48, 9277-9280.

2. K. Tennakone, G. R. R. A. Kumara, A. R. Kumarasinghe, K. G. U. Wijayantha, P. M. Sirimanne, Semicond. Sci. Technol.1995, 10, 1689;

3. U. Bach, D. Lupo, P. Comte, J. E. Moser, F. Weissortel, J. Salbeck, H. Spreitzer, M. Gratzel, Nature 1998, 395, 583;

4. O'Regan, B.; Gratzel, M., 1991, 353, 737-740.

5. Peter, L. M., J. Phys. Chem. Lett. 2011, 2, 1861-1867.

6. Mora-Sero, I.; Bisquert, J., Solar Cells". J. Phys. Chem. Lett. 2010, 1, 3046-3052

7. Miller, R. J. D.; McLendon, G. L.; Nozik, A. J.; Schmickler, W.; Willig, F. Surface Electron Transfer Processes; VCH publishers: New York,1995.

8. Coord. Chem. ReV. 2004, 248 (Michael Graetzel Frestschrift).

9. Hagfeldt, A.; Gratzel, M. Chem. ReV. 1995, 95, 49.

10. Moser, J. E.; Bonnote, P.; Gratzel, M. Coord. Chem. ReV. 1998,171, 245.

11. Kuciauskas, D.; Monat., J. E.; Villahermosa, R.; Gray, H. B.; Lewis, N. S.; McCusker, J. K., J. Phys. Chem. B 2002, 106, 9347

12. She, C.; Guo, J.; Lian, T., J. phys. Chem. B 2007, 111, 6903-6912.

13. Kelly, C. A.; Farzad, F.; Thompson, D. W.; Stipkala, J. M.; Meyer, G. J., Langmuir 1999, 15, 7047-7054.

14. Benko, G.; Kallioinen, J.; Korppi-Tommola, J. E. I.; Yartsev, A. P.; Sundstorm, V., J. Am. Chem. Soc. 2002, 124, 489-493.

97

15. Kallioinen, J.; Benko, G.; Sundstorm, V.; Korppi-Tommola, J. E. I.; Yartsev, A. P., J. Phys. Chem. B 2002, 106, 4396-4404.

16. Haque, S. A.; Palomares, E.; Cho, B. M.: Green, A. N. M.; Hirata, N.; Klug, D. R.; Durrant, J. R., J. Am. Chem. Soc. 2005, 127, 3456-3462.

17. Koops, S.; Durrant, J.R., Inorg. Chim. Acta 2008, 361, 663-670.

18. Berhe, S. A.; Zhou, J. Y.; Haynes, K. M.; Rodriguez, M. T., Youngblood, W. J. ACS Appl. Mater Interfaces, 2012, 4, 2955–2963

19. Yamaji, M.; Takehira, K.; Takao, I.; Shizuka, H.; Tobita, S. Phys. Chem. Chem. Phys., 2001, 3, 5470 – 5474

20. Olson, C.; Veldman, D.; Bakker, K.; Lenzmann, F. Intl. J. Photoenergy., 2011 doi:10.1155/2011/513089

21. Hocevar, M.; Krasovec, V.O.; Bergine, M.; Drazic, G.; Hauptman, N.; Topic, M. J Sol-Gel Sci Technol 2008 48,156–162

98

APPENDIX A

CHARACTERIZATION DATA FOR COMPOUNDS AND

TiO2 NANORODS IN CHAPTER 2

99

Figure A.1.1H NMR spectrum for compound 2.

Figure A.2. 13C NMR spectrum for compound 2.

100

Figure A.3.1H NMR spectrum for compound 3.

Figure A.4. 1C NMR spectrum for Compound 3.

101

Figure A.5.1H NMR spectrum for Compound 5.

Figure A.6.13C NMR spectrum for Compound 5.

102

Figure A.7. 1H NMR spectrum for Compound NcQ.

Figure A.8. 13C NMR spectrum for Compound NcQ.

103

APPENDIX B

CHARACTERIZATION DATA FOR TiO2 NANOROD FILMS IN CHAPTER 3

104

Figure B.1. SEM image of TiO2 nanorods by hydrothermal growth from a seed layer of MnOOH nanoparticles using 55 mM Ti(iOPr)4 for 6 h. Film height was 8 microns.

Figure B.2. Cross-sectional SEM image of TiO2 nanorods grown hydrothermally from a seed

layer of MnOOH nanoparticles using 55 mM Ti(iOPr)4 for 2.5 h. Film height was 2.3 microns.

105

Figure B.3. SEM image of TiO2 nanorods from hydrothermal growth at 28 mM Ti(iOPr)4 for 2 h, using a continuous TiO2 sheet as seed layer. Film height was 200 nm (see Fig. A3.11, right).

A cropped version of this image appears in the main text as Figure 3.6A.

Figure B.4. SEM image of TiO2 nanorods from hydrothermal growth at 28 mM Ti(iOPr)4 for

2.67 h, using a continuous TiO2 sheet as seed layer. Film height was 700 nm.

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Figure B.5. SEM image of TiO2 nanorods from hydrothermal growth at 28 mM Ti(iOPr)4 for 6

h, using a continuous TiO2 sheet as seed layer. Film height was 2 microns (see Fig. A3.12, right). A cropped version of this image appears in the main text as Figure 3.6B.

Figure B.6. Cross-sectional SEM image of TiO2 nanorods grown hydrothermally from a TiO2

sheet seed layer at 28 mM Ti(iOPr)4 for 6 h. Film height was 2.2 microns.

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Figure B.7. SEM image of TiO2 nanorods grown twice for 6 h each run at 28 mM Ti(iOPr)4.

Film height was 5 microns (see Fig. A3.13). A cropped version of this image appears in the main text as Figure 3.6C.

Figure B.8. SEM image of TiO2 nanorods by hydrothermal growth from a spin-coated seed layer of rutile TiO2 nanoparticles (NanoAmor, Inc.) using 55 mM Ti(iOPr)4 for 3 h. Film height was 3.3 microns. The apparently faster growth of np-TiO2 seeded films compared to TiO2-sheet

seeding films is due to longer incubation time for the TiO2-sheet seeding substrates.

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Figure B.9. Profilometry of TiO2 nanorods grown for 1.33 h at 55 mM Ti(iOPr)4, with seeding

by continuous TiO2 sheet(left). Profilometry of TiO2 nanorods grown for 2 h at 55 mM Ti(iOPr)4, with seeding by MnOOH nanoparticles(right).

Figure B.10. Profilometry of TiO2 nanorods grown for 3 h at 55 mM Ti(iOPr)4, with seeding by continuousTiO2 sheet(left). Profilometry of TiO2 nanorods grown for 4 h at 55 mM Ti(iOPr)4,

with seeding by continuousTiO2 sheet(right).

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Figure B.11. Profilometry of TiO2 nanorods grown for 5 h at 55 mM Ti(iOPr)4, with seeding by continuousTiO2 sheet (left). Profilometry of TiO2 nanorods grown for 2 h at 28 mM Ti(iOPr)4,


Figure B.12. Profilometry of TiO2 nanorods grown for 3 h at 28 mM Ti(iOPr)4, with seeding by continuousTiO2 sheet (left). Profilometry of TiO2 nanorods grown for 6 h at 28 mM Ti(iOPr)4,


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Figure B.13. Profilometry of TiO2 nanorods grown for 2 runs of 6 h each at 28 mM Ti(iOPr)4,

with seeding by continuousTiO2 sheet.

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APPENDIX C

CHARACTERIZATION DATA FOR COMPOUNDS IN CHAPTER 4

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Figure C.1. UV-vis absorbance of 3 in CHCl3(Left) and fluorescence of 3 in toluene(Right).

Figure C.2. UV-vis absorbance of 4 in CHCl3 (Left), and fluorescence of 4 in toluene (Right).

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Figure C.3. UV-vis Absorbance of 5 in CHCl3(left), and fluorescence of 5 in toluene (right).

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APPENDIX D

DETAILS OF WORK DONE BY AUTHORS

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D.1 Details of Work Done by Authors for Chapter 2

Joy Y. Zhou synthesized first NcQ-2. Keith M. Haynes helped with methodology

development for device assembly and testing and Marco T. Rodriguez did the molecular

modelling for molecular dipoles.


Soumya Nag did the TEM studies and Zachary Molinets made a TiO2 film at 55 mM

Ti(iOPr)4 with rutile nanoparticle seeding.


Marco T. Rodriguez did molecular modelling, Eunsol Park grew crystals of compound 3,

Vladmir N. Nestrov did XRD of solid crystal structures for compounds 3 and 4 and Hongjun Pan

did HMQC experiments to compounds 3, 4 and 5.

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