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Zero-dimensional and one-dimensional nanostructured materials for application in photovoltaic cells

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Zero-dimensional and one-dimensional nanostructured materials for application in photovoltaic cells

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ZERO-DIMENSIONAL AND ONE-DIMENSIONAL NANOSTRUCTURED
MATERIALS FOR APPLICATION IN PHOTOVOLTAIC CELLS

by

Akshay Kumar

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

December 2010

Copyright 2010 Akshay Kumar
ii

DEDICATION

Dedicated to my family

iii

ACKNOWLEDGEMENTS

Many people have contributed towards making my stay at USC a comfortable one. I
would like to take this opportunity to thank them and acknowledge their invaluable
contribution towards this work. First of all, I am greatly indebted to my advisor,
Professor Chongwu Zhou, for believing in me and providing me with the freedom to
think, resources to put together the ideas, and his invaluable suggestions during past four
years. I deeply acknowledge his gesture of allowing me to work in his group and provide
financial support even when I was not committed to doing a PhD in his group. His
understanding of my needs for vacations at odd times is deeply appreciated.
I would like to thank Professor Anupam Madhukar for the inspiration and his
teachings on how to do good research. He has certainly added a new dimension to my
ways of looking at things.
I can not thank my family enough for the kind of unconditional love they have
showered upon me. I have always enjoyed the support of my family for whatever I
decided to do. Just talking to them on phone will take away all the stress I used to have. I
owe my entire life to them for whatever I am today is because of them. Friends are no
doubt an important part of one’s life and I am no exception for I have had the pleasure of
having some great friends both here and in India. I would like to thank Arvind, Anuj,
Pankaj, Arun, Nivedita, Aditya for some great times we spent together whether they were
those tea sharing sessions, those political debates, or vacations outside LA. Thank you
iv

folks. My old friends from IIT, Aloke, Ashish, Srijith, Saket, Ranajoy, Nitesh, thank you
for being there. Your friendship is so fresh always.
My PhD work in the lab carries a lot of support and help from my labmates. I
want to thank Dr. Kougmin Ryu, Dr. Fumiaki Ishikawa, Dr. Po-chian Chen, Lewis, Yi,
Alex, Jialu, Chuan, Ted, Maoqing, Yue, Hshiokang for the help they provided whenever I
needed it. We spent some unforgettable times together and I would always cherish those.
I would also like to thank my collaborator Prof. Mark Thompson and his students
Cody Schlenker, Francisco Navarro, Chao Wu, Rui Zhang for the fruitful discussions and
providing me the opportunity to use their facilities and in the process, diversify my
knowledge in the field.
I want to thank Prof. Edward Goo, Prof. Daniel Dapkus, Prof. Priya Vashishta for
serving on my Qualifying and defense committee.
v

Tables of Contents
Acknowledgements ............................................................................................................ iii

List of Tables ................................................................................................................... viii

List of Figures .................................................................................................................... ix

Abstract.................. …………………………………………….……………………...xvix

Chapter 1 Introduction………………………………………………………………...1
1.1 Overview of Zero- and One-Dimentional Materials……………………………….2
1.1.1 Semiconductor Nanocrystal Quantum Dots (NCQDs)………………………...2
1.1.2 Carbon Nanotubes(CNTs)……………………………………………………..5
1.1.3 One-Dimensional Nanowires (1-D NWS)……………………………………..8
1.2 Introduction to Photovoltaic Cells: Materials and Principle ................................... 10
1.2.1 TiO
2
Nanowire Array for Dye-Sensitized Solar Cells………………………...13
1.2.2 TiO
2
Nanowire Array for QD-Sensitized Solar Cells………………………... 17
1.2.3 Solid-State Dye-Sensitized Solar Cells………………………………………..19
1.3 The Race to Replace ITO: Candidate Materials ..................................................... 20
1.4 Outline of This Work .............................................................................................. 25
Chapter 1 References .................................................................................................... 27

Chapter 2 Growth of Aligned Single-Crystalline Rutile TiO
2
Nanowires on
Arbitrary Substrates and Their Application in Dye Sensitized
Solar Cells…………………………………………………………………35
2.1 Introduction ............................................................................................................. 36
2.2 Experimental Details ............................................................................................... 38
2.2.1 Synthesis of TiO
2
Nanowires ........................................................................... 38
2.2.2 Solar Cell Fabrication ...................................................................................... 39
2.2.3 Structural and Electrical Characterization ....................................................... 39
2.3 Results and Discussions .......................................................................................... 40
2.3.1 TiO
2
Nanowire Growth on Arbitrary Substrates ............................................. 40
2.3.2 Characterization of TiO
2
Nanowires ................................................................ 46
2.3.3 Growth Mechanism of TiO
2
Nanowires .......................................................... 49
2.3.4 Solar Cell Fabrication ...................................................................................... 57
2.4 Summary and Conclusion ....................................................................................... 60
Chapter 2 References .................................................................................................... 61

vi

Chapter 3 Sensitization of Hydrothermally Grown Single Crystalline TiO
2

Nanowires array with CdSeS Nanocrystals for Photovoltaic
Application…………………………………………………………………65
3.1 Introduction ............................................................................................................. 66
3.2 Experimental Details ............................................................................................... 68
3.2.1 Synthesis of TiO
2
Nanowires ........................................................................... 68
3.2.2 Synthesis of CdSeS Nanocrystals .................................................................... 69
3.2.3 Ligand Exchange of CdSeS Nanocrystals ....................................................... 70
3.2.4 CdSeS Sensitization Strategy ........................................................................... 71
3.2.5 Solar Cell fabrication ....................................................................................... 71
3.2.6 Characterization and Electrical Measurements ................................................ 72
3.3 Results and Discussion ........................................................................................... 72
3.3.1 Device Structure and Synthesis ....................................................................... 72
3.3.2 Sensitization Strategies .................................................................................... 79
3.3.3 Solar Cell Measurements ................................................................................. 82
3.4 Conclusion .............................................................................................................. 85
Chapter 3 References .................................................................................................... 86

Chapter 4 Photoelectrode-Infiltrating Hole Transport Media by Physical Vapor
Deposition on Hydrothermally Grown TiO
2
Nanowire Array for
Solid State Dye-Sensitized Solar Cells…………………………………….90
4.1 Introduction ............................................................................................................. 91
4.2 Experimental Details ............................................................................................... 94
4.2.1 Synthesis of TiO
2
Nanowires ........................................................................... 94
4.2.2 Solar Cell fabrication ....................................................................................... 95
4.2.3 Characterization of Solar Cell .......................................................................... 96
4.3 Results and Discussion ........................................................................................... 97
4.3.1 Device Structure and Principle ........................................................................ 97
4.3.2 Infiltration of NNP in TiO
2
Structures ........................................................... 101
4.3.3 Device Characterization ................................................................................. 104
4.3.4 Discussion on NNP as HTM .......................................................................... 109
4.4 Conclusion ............................................................................................................ 111
Chapter 4 References .................................................................................................. 112

Chapter 5 Uniform, Highly Conductive, and Patterned Transparent Films of
Percolating Silver Nanowires Network on Rigid and Flexible
Substrates Using Dry Transfer Technique………………………………..115
5.1 Introduction ........................................................................................................... 116
5.2 Experimental Details ............................................................................................. 118
5.3 Results and Discussion ......................................................................................... 119
5.4 Conclusion ............................................................................................................ 134
Chapter 5 References .................................................................................................. 135

vii

Chapter 6 Scalable Light-Induced Metal to Semiconductor Conversion of
Carbon Nanotubes………………………………………………………...139
6.1 Introduction ........................................................................................................... 140
6.2 Experimental Details ............................................................................................. 141
6.2.1 Wafer scale synthesis of aligned nanotubes on quartz and sapphire ............. 141
6.2.2 Transfer of aligned nanotubes and device fabrication ................................... 142
6.3 Results and Discussion ......................................................................................... 143
6.4 Conclusion ............................................................................................................ 156
Chapter 6 References .................................................................................................. 157

Chapter 7 Conclusions and Future Work ................................................................... 161
7.1 Conclusions ........................................................................................................... 161
7.2 Future Work ........................................................................................................ 163
7.2.1 Future Work in TiO
2
Nanowire solar Cell ..................................................... 163
2.2.1 Future Work in QD-Sensitized Solar Cells .................................................... 164
6.2.2 Future Work in Solid-State Dye-Sentized Solar Cells ................................... 166
Chapter 7 References ................................................................................................. 169

Comprehensive Bibliography..........................................................................................170

viii

List of Tables
Table 1.1 Table 1. Summary of TiO
2
nanowire morphology obtained
on various substrates

46
Table 7.1 Different hole transport molecules and their HOMO levels

168
ix

List of Figures
Figure 1.1 Change in the density of states (DOS) for quantum
confined structures

2
Figure 1.2 Absorption spectra of CdSe nanocrystals of various
sizes. As the size of the nanocrystal increases from 1.8
nm to 20 nm, the absorption peak red-shifts
continuously from 450 nm to 700 nm representing a
change of 1.2 eV in the band gap of CdSe. The bottom
panel shows the corresponding photoluminescence
spectra (PL) for the same nanocrystals

4
Figure 1.3 A carbon nanotube can be constructed by cutting a
graphene layer along lines OB and AC, and then rolling
it up into a tube, so that O meets A, and C meets B.
Vector
OA
is called the chiral vector

6
Figure 1.4 Schematic diagrams of zigzag, armchair, and chiral
nanotubes

6
Figure 1.5 SEM images of different nanowires synthesized and
used in this thesis

9
Figure 1.6 Schematic diagram of a typical photovoltaic cell
depicting the individual steps involved in the process of
electricity generation.
10
Figure 1.7

Typical current-voltage characteristics of a photovoltaic
cell. V
mp
and I
mp
represent the point where the cell
generates maximum output. Equations used to calculate
the cell efficiency are given on the right
12

Figure 1.8

(a)Schematic diagram of a TiO
2
nanoparticle based dye
solar cell. (b) Energy level diagram of the dye solar cell
depicting the energy levels of individual components
and associated processes involved in the generation of
electrons from photons

13

Figure 1.9

Schematic representation of the diffusive transport of
electrons through a mesoporous network of TiO
2

nanoparticles

15
x

Figure 1.10 Schematic diagram of the proposed architecture for
dye solar cell which employs an oriented array of
nanowires, subsequently sensitized with absorbing
entities to fabricate the solar cell

16
Figure 1.11

Plot showing the spectral distribution of sun’s
irradiance. Inset shows the absorption of N719 dye
solution prepared in ethanol

17
Figure 1.12

TEM and SEM images of various NCs synthesized and
used in this thesis

18
Figure 1.13 Various candidate materials for transparent and
conductive electrodes a) SEM of a carbon nanotube
(CNT) film, b) SEM image of a silver nanowire network
film with high optical to electrical conductivity ratio, c)
SEM image of Au nanowire grating fabricated using
nano-imprinting technique, and d) AFM image of
solution processed graphene flakes

23
Figure 2.1 SEM images of Hydrothermally grown TiO
2
Nanowire
Array on FTO substrate

41
Figure 2.2 SEM images of TiO
2
nanowires grown on glass
substrate

42
Figure 2.3 SEM images of TiO
2
nanowires grown on glass
substrate

43
Figure 2.4 SEM images of TiO
2
nanowires grown on glass
substrate
44
Figure 2.5 SEM images of TiO
2
nanowires grown on glass
substrate
45
Figure 2.6 Structural Characterization of TiO
2
nanowires grown at
180
o
C for 4 hours with 10 ml of DI water, 10 ml of
HCl, and 1 ml of TiCl
4.
(a) and (b) XRD pattern of
nanowires grown on the glass and FTO substrate,
respectively. (c) TEM image of a single nanowire. Inset
shows a HRTEM image of the same nanowire. (d)
SAED pattern of a single nanowire.
47
xi

Figure 2.7 (a) schematic diagram of growth habit of rutile
nanowires with (110) side faces and (001) top face. (b)
top view SEM image of TiO
2
nanowires grown using 10
ml of DI water, 10 ml of HCl, and 1 ml of TiCl
4
at 120
o
C for 2 hours showing square cross section.

50
Figure 2.8 SEM images showing the evolution of nanowire
morphology grown using 10 ml of DI water, 10 ml of
HCl for 1 hour with different amount of TiCl
4

concentration and different temperature: (a) individual
nanowires obtained using 0.5 ml TiCl
4
at 150
o
C, inset
shows large area image (b) cross-like structure and (c)
three-leg structures obtained using 0.75 ml TiCl
4
at 150
o
C, inset shows large area image (d) dendrite structure
obtained using 1 ml TiCl
4
at 150
o
C, (e) large area image
of dendritic structures, (f) large area image of tree-like
structures, and (g) tree-like branched structure obtained
using1 ml TiCl
4
at 180
o
C

51
Figure 2.9 Effect of titanium precursor on the growth rate of
nanowires. Variation of nanowire length with the growth
time using 1 ml of TiCl
4
, Ti-isopropoxide, and Ti-
butoxide precursors each and with 10 ml of DI water, 10
ml of HCl at 180
o
C on FTO substrates . Inset shows a
photo-image of a nanowire film peeled-off from the
FTO substrate after a growth time of 8 hours.

53
Figure 2.10 Effect of substrate positioning on the alignment of
nanowires grown at 180
o
C for 2 hours using 1 ml of Ti-
isopropoxide. SEM image of TiO
2
nanowires grown on
(a) FTO substrate placed horizontally, (b) FTO substrate
placed vertically, (c) glass substrate placed horizontally,
and (d) glass substrate placed vertically. Main panel
shows the top view SEM image and inset shows
perspective and cross sectional views. (e) High
resolution SEM of nanowire cross-section grown on
glass substrate, and (f) schematic diagram showing the
growth evolution of nanowires

55
Figure 2.11 Photo-image of TiO
2
nanowires grown on (a) glass rod
before and after growth, and (b) glass tube. (c), (d), and
(e) SEM images of nanowires grown on a solid glass
rod. (f) and (g) SEM images of nanowires extracted
from the inner walls of the teflon cup.
56
xii

Figure 2.12 (a) J-V characteristics of a dye-sensitized solar cell
assembled with FTO substrate covered with 3 µm long
rutile TiO
2
nanowire film as anode and 20 nm Pt
deposited FTO substrate as cathode. Red curve shows
the device with as-synthesized nanowires (without
further treatment) immersed in dye solution for 12
hours, and yellow curve represents device with
nanowires treated with TiCl
4
solution and immersed in
dye solution for 24 hours. (b) External quantum
efficiency data for the device treated with TiCl
4

57
Figure 3.1 (a) Schematic diagram showing the device structure of
the QDSSC. Vertical array of TiO
2
nanowires are
grown directly on FTO substrate, followed by
sensitization with CdSeS QDs. Fabrication is completed
by using Pt-coated FTO as the counter electrode and the
space filled with electrolyte solution. (b) Energy level
diagram of the various components of the cell. CdSeS
QDs and TiO
2
form type-II interface resulting in
migration of photogenerated electrons from QD to TiO
2
.

73
Figure 3.2 (a) Perspective (b) cross-section SEM images of the
TiO
2
nanowire array grown on FTO substrate. The
length of nanowires is ~ 1 µm. (c) High resolution TEM
image of a single nanowire showing distinct {110}
crystal planes. (d) Selected area electron diffraction
pattern of a single TiO
2
nanowire. (e) XRD pattern of
TiO
2
nanowires grown on FTO substrate. Enhanced
(002) peak confirm the vertical growth of nanowires.

75
Figure 3.3 (a) TEM image of CdSeS QDs (inset shows the HR-
TEM of individual QD) (b) UV-Vis absorption spectra
(red curve) and PL spectra (blue curve) of as-
synthesized CdSeS QDs . (c) TEM image of single TiO
2

nanowire sensitized with CdSeS QDs. (d) HRTEM
image of QDs attached on to the nanowire. Black circles
are drawn around the individual QDs as a guide to the
eye.

77
xiii

Figure 3.4 a) I-V characteristics (black and red curve) and output
power characteristics (open squares curve) of TiO
2
-
CdSeS cell fabricated using strategy B. (b) I-V
characteristics (black and red curve) and output power
characteristics (open square curve) of the cell fabricated
using strategy A.

82
Figure 4.1 (a) Schematic diagram of the solid state dye sensitized
solar cell configuration. A vertical array of TiO
2

nanowires is grown directly on FTO substrate, sensitized
with N719 dye molecules, infiltrated with vapor
deposited F4TCNQ doped NNP molecules, and finally
completed with thermally evaporated Cu electrodes. (b)
Energy level alignment of different components of the
sDSC. (c) Schematic diagram showing the hopping
assisted transport mechanism of holes in the doped NNP
layer.

98
Figure 4.2 Structural Characterization of TiO
2
nanowires grown at
180
o
C for 4 hours with 10 ml of DI water, 10 ml of
HCl, and 0.4 ml of TiCl
4.
(a) SEM image (b) XRD
pattern of nanowires grown on the glass and FTO
substrate, respectively. (c) TEM image of a single
nanowire. Inset shows a HRTEM image of the same
nanowire. (d) SAED pattern of a single nanowire.

102
Figure 4.3 (a) Top view SEM image of as-grown TiO
2
nanowire
array on FTO substrate prior to NNP infiltration. (b)
Cross section SEM image of the nanowire array
showing individual nanowires growing vertically on
FTO substrate with a length of 200 nm (c) Top view
SEM image of TiO
2
nanowire array after HTM
deposition showing the encapsulated nanowires with
NNP molecules forming a percolating network. (d)
Cross section SEM image of 200 nm long TiO
2

nanowires deposited with doped NNP molecules
showing excellent infiltration of HTM molecules well
inside the nanowire pores.

103
xiv

Figure 4.4 Current-Voltage (J-V) characteristics of a sDSC
prepared using 200 nm long TiO
2
nanowire array. Red
curve represents the behavior in dark while blue curve
represents the device performance under light
illumination. Open square points represent the power
output of the devices plotted on the right axis.

105
Figure 4.5 (a) External quantum efficiency (EQE) measurement of
the sDSC fabricated with 200 nm long TiO
2
nanowire
array. (b) Light harvesting efficiency (LHE)
measurement of the device.

106
Figure 4.6 J-V characteristics for 400 nm long TiO
2
nanowires
sensitized with dye and infiltrated with NNP molecules.

107
Figure 4.7 I-V characteristics for control device (FTO/TiO
2
/NO
DYE/NNP/Cu)

108
Figure 4.8 I-V for NNP:F
4
TCNQ HTM between FTO and Cu (no
TiO
2
, no dye)

109
Figure 5.1 (a) Schematic representation of transfer process flow. i) press
PDMS stamp against the silver nanowire film on AAO
membrane, ii) Peel off the PDMS stamp, iii) press PDMS
stamp with Ag nanowires on the receiving substrate, and iv)
peel off PDMS stamp leaving nanowire film on the receiving
substrate. (b) Photograph of silver nanowire film transferred
on PET. Arrows show the boundary of the nanowire film. (c)
Photograph of nanowire film on glass substrate. (d) Nanowire
film on PET showing the flexibility of film. (e) Photograph
showing the results of adhesion test. Nanowires remain
adhered to the PET substrate when peeled off using a sticky
tape from the area shown by dotted lines. (f)-(i) SEM images
of the left, top, right and bottom region of the film shown in
Fig. 1 (b) demonstrating film’s uniformity across the entire
area.

120
Figure 5.2 Patterned transfer of silver nanowire film. (a) Schematic
diagram of the patterned PDMS stamp in contact with
the nanowire film. (b) Photograph of patterned nanowire
film transferred on the PET substrate. The size of each
pixel is 1 mm × 1 mm. (c) photo-image of a patterned
film (d) SEM image showing the nanowire network
from one pixel.

122
xv

Figure 5.3 (a) Sheet resistance vs. transmittance plot of silver nanowire
films. Black dots represent resistance values without
annealing and red dots represent resistance values with
annealing the samples at 200
o
C for 20 minutes in air. Inset
shows the transmittance spectrum of the nanowire films with
different thicknesses. (b) SEM image of an annealed
nanowire sample showing melting of nanowires at the ends
and subsequent joining with neighboring nanowires. (c)
Variation of sheet resistance with the annealing time for
nanowires films of different densities. Sheet resistance first
decreased with annealing and subsequently increased with
further annealing (d) SEM images of nanowire films prepared
using various amounts of nanowires. Nanowire density
increases with increased concentration.

124
Figure 5.4 Distribution of sheet resistance for various Ag nanowire
samples with the 85% transmittance. The average sheet
resistance is 10 + 1.5 Ω/sq.

125
Figure 5.5 (a) Transmittance versus wavelength spectra of the
nanowire film with different density (b)Transmittance
versus wavelength spectra of the nanowire film at
different light incident angles.

126
Figure 5.6 SEM image of Ag nanowires showing the melting of
nanowire and formation of droplets when annealed at
300
o
C for 20 min on AAO membrane.

127
Figure 5.7 Two probe measurement of a silver nanowire film
showing ohmic behavior of the film.

128
Figure 5.8 Sheet resistance versus volume of Ag nanowire solution.
The onset of conduction across the sample occurs for
V
c
= 0.2 ml. The power fit of the data indicated value of
critical exponent α = 1.42. The inset shows the
logarithmic plot of the data with a linear fit.

130
Figure 5.9

Sheet resistance versus bending angle plot. Nanowire
film remains conductive under severe bending. Inset
shows the photograph of the measurement setup and
definition of bending angle.

132
Figure 6.1 Wafer-scale synthesis of aligned nanotubes on quartz
and a-sapphire wafers. Photographs of the wafers after
nanotube growth are shown on the left. SEM images on
the right show large arrays of highly aligned nanotubes.
141
xvi

Figure 6.2 Schematic diagram showing the aligned-nanotube
transfer process

142
Figure 6.3 Photograph of a Si/SiO
2
wafer with transferred
nanotubes. SEM image shows that, after being
transferred, nanotubes maintain a good degree of
alignment on the receiving substrate. Au electrodes
deposition, followed by etching of nanotubes outside the
device channel area complete the fabrication of CNT-
FETs

143
Figure 6.4 (a) Schematic diagram showing large arrays of field-
effect transistors comprising of horizontally aligned
carbon nanotubes between source and drain electrodes.
(b) Photograph of a Si/SiO
2
wafer with fabricated
aligned nanotube transistors. The SEM image shows a
typical CNTFET in the arrays. (c) Schematic diagram
illustrating the scalable light irradiation process. (d)
Current vs. gate voltage (I
DS
-V
G
) characteristics of a
CNTFET device, obtained with V
DS
=0.5 V before (black
trace) and after (red trace) light irradiation. The I
On
/I
Off

ratio increased from ~64 to ~10
5
in the nanotube
transistor due to the light irradiation.-mediated oxidation
of nanotube sidewalls leads to metal-to-semiconductor
transition.

144
Figure 6.5 (Left) AFM image of single-nanotube CNT-FET that
shows metal-to-semiconductor transition after 5 hours of
light exposure. Zoomed AFM image shows no visible
damage or cutting on the nanotube structure. (Right)
I
On/
I
Off
evolution of the CNT-FET shown on the left
upon timed light irradiation.

145
xvii

Figure 6.6 Light-mediated oxidation of nanotube sidewalls leads to
metal-to-semiconductor transition. (a) Raman D band
(left) and G band (right) of a metallic nanotube in a
single-nanotube device before and after light irradiation.
(b) I
DS
-V
G
characteristics of the single-nanotube device
shown in the inset of (a), before and after light
irradiation. (c) Comparison between the G/D ratios of
nanotubes before and after one hour irradiation with the
full spectrum, ultraviolet, (250 nm - 400 nm), visible
(380 nm – 700 nm) and near infrared (750 nm – 2000
nm). (d) Percentage of nanotubes exhibiting Raman
RBM signal after light irradiation using the same
irradiation conditions as part (c). The inset shows the
decrease of RBM intensity was more significant for the
small-diameter nanotube than for larger nanotubes. (e)
Schematic showing light-induced oxidation of the
nanotube sidewalls. possible chemical groups introduced
on the nanotube sidewalls upon sp
2
–sp
3
rehybridization
by light-induced oxidation. (f) Comparison of typical
I
DS
-V
G
characteristics of two CNTFETs before and after
irradiation in air and in vacuum.

147
Figure 6.7 Influence of the irradiation time and nanotube diameter
on the metal-to-semiconductor conversion observed in
CNTFETs. (a, b and c) I
On
and I
Off
of single- and few-
nanotube CNTFETs showing different evolutions under
timed light irradiation (V
ds
= 100 mV). (d) Histogram of
CNTFETs that lost electrical conduction or survived
after six-hour light irradiation plotted versus the
nanotube diameter. Clear diameter dependence was
observed. (e) Percentage of CNTFETs that survived
(red) and showed depletable behavior (black) for
different light irradiation durations. The best yield was
found after 4-hour exposure, when the percentage of
depletable devices increased from 32% to 88% while
keeping a survival ratio near 81%.

151
xviii

Figure 6.8 (a) Stacked histograms showing the number of
nanotubes exhibiting RBM vs. the RBM frequency,
before and after light irradiation, as measured with three
excitation lines (532 nm, 633 nm, and 785 nm).
Frequency regions characteristic for metal or
semiconductor nanotubes are highlighted based on
Kataura’s plot. Comparison of the histograms obtained
before and after irradiation for each laser shows a
predominant light-induced oxidation of small-diameter
nanotubes (large RBM frequency). (b) Percentage of
metallic nanotubes in as-grown samples before (gray
columns) and after (red columns) light exposure using
xenon (upper panel) and halogen (lower panel) lamps.
Nanotubes were grouped into two categories based on
their diameter: small-diameter (0.7 - 1.3 nm) and large-
diameter (1.4 - 2.0 nm) nanotubes. A substantial
decrease in the percentage of small-diameter metallic
nanotubes found after light irradiation, for both light
sources employed, indicates their preferential oxidation
over semiconducting small-diameter nanotubes.
Contrarily, the percentage of large-diameter metallic
nanotubes was largely unaffected by light, indicating the
preferential oxidation (metal over semiconductor) is
more effective for small-diameter nanotubes.

155
Figure 7.1 TEM and SEM images of various NCs synthesized and
used in this thesis

164
Figure 7.2 Top view SEM images of TiO
2
nanowires synthesized
using different concentrations of titanium precursors.
Nanowires become thinner as we go from (a) to (d)
167
xix

ABSTRACT

Zero-dimensional materials such as quantum dots and one-dimensional materials such as
nanorods and nanowires have attracted significant attention in the past two decades and
have been demonstrated as important building blocks for numerous electronic and
optoelectronic device applications. Of course, the starting place for the field is the ability
to grow various nanomaterials in different morphologies. In this thesis, we have
demonstrated successful synthesis of both quantum dots and nanowires belonging to a
totally new material class and have subsequently utilized them for photovoltaic cells in
different device architectures. Apart from the photovoltaic cells, we have demonstrated a
scalable way to fabricate carbon nanotube devices with a high on-off ratio on a wafer
scale.
This dissertation describes the above-mentioned aspects in detail and accordingly
consists of seven chapters. Following an overview and an introduction of fundamental
knowledge of zero-dimensional and one-dimensional nanostructured materials in Chapter
1, Chapter 2 discusses synthesis of vertically aligned array of single crystalline TiO
2

nanowires and their use in traditional dye-sensitized solar cells.
Chapter 3 discusses the continued work on TiO
2
nanowire cells where we employ
novel nanoparticles as sensitizers and demonstrate their advantages over traditional dye
molecules.
xx

Chapter 4 address the issue of liquid based electrolytes in the solar cell and proposes a
novel physical vapor deposited hole transport material to replace the liquid electrolyte
and thus paving a way for solid-state dye sensitized solar cell.
Chapter 5 deals with an important issue of ITO replacement for solar cells.
Researchers have been very actively looking for materials that can work as transparent
and conductive electrodes. We demonstrate the viability of silver nanowire based random
network film as a potential replacement. We achieve remarkable performance in terms of
sheet resistance and transparency which rivals that of ITO, supporting a very strong case
for Ag nanowire film.
In chapter 6, we demonstrate a scale way to fabricate carbon nanowtube devices
with high on-off ratio ( a critical requirement for logic devices) by converting metal
nanotubes to semiconducting ones. This conversion is realized using exposure of as-
grown nanotubes to a broad band light source which induces photochemical reactions.
We demonstrate a high device yield of 82%.
Finally in chapter 7, we conclude by discussing the future directions and work
that needs to be done in order to carry forward the advance made in this thesis.

1

Chapter 1. Introduction

It has always been intriguing as to what lies under the skin of a substance and what
mysteries a material hides under its surface that cause it to behave in a certain manner.
Every material is novel in its own way and the desire of mankind to change the behavior
of a material to fit its requirements allows a lot of scope for research and innovation in
this field.
One very crucial tool that has given rise to the development of whole lot of new
materials is the size and dimensionality of the material. In particular, when a material is
synthesized with reduced size and reduced dimensionality, it starts to exhibit many new
properties that are very different from its bulk properties. The effects of reduced
dimensionality spread over its physical, chemical, and electronic properties. The famous
physicist, Richard Feynman had envisioned the importance of reduced size and
dimensionality of materials long ago when in his famous lectures, he said, “There is
plenty of room at the bottom”. In the past two decades, advances in synthetic chemistry
and development of new synthesis protocols have opened up the possibilities of
synthesizing materials at nanoscale with controlled zero-, one-, and two-dimensionality
and as a result, have paved the way for realizing and observing the theoretically predicted
nature of materials at nanoscale through experimentation.
The most important change in the properties of material, when synthesized at
nanoscale, arises from the fact that when a material is synthesized at nanoscale, there is a
drastic change in the electronic density of states and band structure of the material.
2

Importantly, this change is a sharp function of dimensionality with materials electronic
band structure changing from a discrete delta function for zero-dimensional materials to
continuum structure for bulk materials. Figure 1.1 shows how a semiconductor’s
electronic density of states changes with different degree of reduced dimensionality [1].
Figure 1.1 Change in the density of states (DOS) for quantum confined structures

This change from a continuous DOS to discrete level DOS has profound effects on the
optical properties of these materials. For example, zero-dimensional quantum dots (QDs)
exhibit different band gap as a function of its size.
1.1 Overview of the zero- and one-dimensional materials
1.1.1 Semiconductor Nanocrystal Quantum Dots (NCQDs)
Semiconductor nanocrystal quantum dots (NCQDs) represent a class of quasi-zero-
dimensional objects in which carrier motion is restricted in all three directions. The most
important consequence of this reduced dimensionality is that NCQDs have atom-like
3

discrete energy spectra that are strongly size dependent, as opposed to the continuous
bands spectra in their bulk counterpart. Colloidal chemistry techniques allow synthesis of
NCQDs in various sizes and shapes in a controllable manner resulting in immense control
of the band gap of these particles with somewhat controlled energy spectra. Continued
development in understanding of the growth mechanism further led to the ability to
control size dispersion and defect states. A great level of synthetic flexibility and control
together with chemical manipulations lead to surface modification by exchanging the
passivation layer, formation of layered heterostructures and self-assembly into 3-
dimensional (3D) super lattices.
The system is interesting from fundamental physics point of view since the
properties of this system are different from their bulk counterpart in many aspects.
Quantum confinement effect leads to increase in the band gap and exhibits discrete
energy states both in valence band and in conduction band. Also, since the carriers are
confined in a very small volume, there are greatly enhanced coulomb interactions
between carriers leading to significant manifestations in carrier dynamics. The
discreteness of energy states in the conduction and valence band are manifested in their
absorption spectra which consists of features indicative of well-defined discrete
transitions, with the gap between these energy states reaching few hundreds of meV.
Figure 1.2. shows how the band gap of CdSe nanocrystals can be tuned from 1.7
eV to 2.8 eV as the size of the nanocrystals is changed from 20 nm to 1.8 nm [2]. The
peaks in absorption and photoluminescence (PL) spectra red-shift continuously as the
nanocrystal are grown larger and larger. This ability to tune the band gap opens up
4

enormous possibilities for their use in various optoelectronic devices such as lasers [3-5],
light emitting diodes (LEDs) [6-9], biological labeling [10-12], and solar cell [13-25].

Figure 1.2 Absorption spectra of CdSe nanocrystals of various sizes. As the size of the
nanocrystal increases from 1.8 nm to 20 nm, the absorption peak red-shifts continuously
from 450 nm to 700 nm representing a change of 1.2 eV in the band gap of CdSe. The
bottom panel shows the corresponding photoluminescence spectra (PL) for the same
nanocrystals [2]

5

1.1.2 Carbon Nanotubes (CNTs)
Among various nanomaterials studied so far, carbon nanotubes (CNTs) stand out due to
their remarkable electronic, mechanical, optical and chemical properties. Since their
discovery by Sumio Iijima at NEC Corporation in Japan in 1991 [26] , carbon nanotubes
have stimulated enormous interest for both fundamental research and future applications.
Carbon nanotubes can be viewed as long graphene sheets rolled into seamless
cylinders, and a single walled carbon nanotube (SWNT) can be as long as several
centimeters, while the diameter is only one to two nanometers. The excitement and the
opportunity come from the fact that the electronic properties of a nanotube depends
critically on how one rolls the graphene sheet to make that nanotube. As shown in Figure
1.3, we can cut the graphene along the lines of OB and AC and then roll it up into a
nanotube, so that O meets A, and C meets B. The chiral vector OA is defined on the
hexagonal lattice as OA = ma + nb, where a and b are two basic vectors shown in Figure
1.3, and n and m are two integers that can be used to fully define the structure of this
nanotube. The chiral angle, θ, is measured relative to the direction defined by a. The
diagram in Figure 1.3 has been constructed for (m, n) = (4, 2), and the unit cell of this
nanotube is bounded by OACB [27].
Different values of n and m results in different morphology of nanotubes. Figure
1.4 displays the schematic diagram of three kinds of carbon nanotubes: zigzag, armchair,
and chiral nanotubes. Zigzag nanotubes correspond to either m or n equals to 0, and have
a chiral angle of 0°. Armchair nanotubes (named so, because the configuration is similar

to
ca

F
O
O

F
o an armchai
alled chiral n
Figure 1.3 A
OB and AC, a
OA
is called

Figure 1.4 Sc
ir) have n =
nanotubes [2
A carbon nan
and then roll
the chiral ve
chematic dia
m and a chi
27].
notube can be
ling it up int
ector [27]
agrams of zig
ral angle of
e constructe
to a tube, so
gzag, armch
30°, while o
d by cutting
o that O mee
hair, and chir
other nanotu
g a graphene
ets A, and C
ral nanotubes
ubes are gene
layer along
meets B. V
s [27]
6
erally
lines
Vector
7

The electronic band structure of SWNTs can be expressed by considering the covalent
bonding of the carbon atoms arranged in a hexagonal lattice via sp
2
molecular orbitals
[28]. Depending on their chiralities and diameters, SWNTs can be metallic, semi-metallic,
or semiconductor. For metallic nanotubes, the Fermi energy intersects two bands of the
one dimensional band structure and results in ballistic transport of electrons along the
length of the nanotube [29]; thus enable them to carry high current density of 109 A/cm
2
,
which can be an ideal interconnection element in nano-circuits (e.g., the maximum
current densities for normal metal (e.g. Ag) is about 106 A/cm
2
). In the case of
semiconductor nanotubes, the electronic band structure exhibits characteristic E
-1/2
van
Hove type singularities, which is typical in 1-D systems. The bandgap of the nanotube is
given by the expression (0.9/diameter) eV [30]. A more detailed discussion about
electronic band structure can be found in Ref. 31. In addition, with extraordinary field-
effect mobility of 79,000 cm
2
/Vs and intrinsic mobility of > 100,000 cm
2
/Vs [31],
semiconductor nanotubes have been proposed for nanoelectronics applications such as
high frequency FETs (> 1 GHz), single electron memories, and bio-chemical sensors.
Although carbon nanotube has excellent intrinsic electronic properties, it suffers
from many engineering problems that limit their application in devices. Most prominent
among those are the issues of controllable positioning and removing metal nanotubes
from the channel. While a lot of progress has been made in growing aligned nanotubes,
thus solving the problem of controllable positioning to a large extent, the problem of
removing metal nanotubes from the channel has seen limited progress. The most
commonly used method to remove metal tubes from the channel is the so called electrical
8

breakdown method, where a very high voltage is applied under the presence of gate to
turn off the semiconducting nanotubes while flowing high current through metal
nanotubes leading to their burnout. However, this method requires device to device
operation, making it a cumbersome approach. In our work, we demonstrate a scalable
way to fabricate carbon nanotube devices with high on-off ratio (a critical requirement
for logic devices) by converting metal nanotubes to semiconducting ones. This
conversion is realized by exposing as-grown nanotubes to a broad band light source
which induces photochemical reactions, leading to metal-to-semiconductor conversion of
nanotubes. We demonstrate a high device yield of 82%. This technique is discussed in
detail in chapter 6.

1.1.3 One-Dimensional Nanowires (1-D NWs)
Nanowires are one of the most important class of nanomaterials and hence, are most
widely studied category among all one-dimensional (1-D) structures such as nanotubes,
nanobelts, and nanorods. Once the size of the nanowire becomes smaller than the size of
electron wave function, quantum confinement effect [32] starts to play important roles
(similar to nanocrystals, as discussed in section 1.1.1), leading to demonstration of new
physical, optical, and electronic properties as compared to their bulk counterpart [33][34].
In the past two decades, developments in synthetic techniques have resulted in synthesis
of nanowires belonging to a wide range of materials, such as, wide bandgap materials
(SnO
2
, ZnO, TiO
2
, In
2
O
3
), narrow band gap materials (CdSe, CdS, ZnSe, CdTe, GaAs,
InAs), and metals (Ag, Au, Pt) [35-38]. Figure 1.5 shows SEM images of some of the

n
v
st
ex
ch
u
u

In
in
tr
h

anowires sy
arious appli
tructures suc
xtremely su
hemical pro
sed for appl
sed as a tran
Figure 1.
n addition,
ncluding sing
ransistors [52
ave been suc
ynthesized a
ications. Na
ch as high a
itable for ch
ocesses [39-4
lication in dy
nsparent and
5 SEM imag
Several pro
gle nanowire
2], memory
ccessfully fa
as part of m
anowire mor
spect ratio a
hemical or b
42]. Metal o
ye-sensitized
conductive
ges of differe
ototypes of
e laser [47, 4
units [53, 5
abricated and
my dissertat
rphology ha
and high sur
biological s
oxide nanow
d solar cells
electrode [4
ent nanowire
f nanoscale
48], radio fr
54], logic cir
d demonstrat
tion studies
as many adv
rface to volu
ensing or se
wires such a
[43, 44] wh
45, 46].
es synthesiz
electronic
requency dev
rcuits [55], a
ted.
and subseq
vantages co
ume ratio, wh
ensing for a
as ZnO and
hile Ag nano
ed and used
and optoele
vices [49-51
and photo-d
quently used
ompared to
hich makes
any other su
TiO
2
have
owires have
in this thesi
ectronic dev
], single ele
etectors [56
9
d for
other
them
urface
been
been

is
vices,
ectron
, 57],

1
P
o
fr
an
ju
F
d

F
in

.2 Intro
hotovoltaic
f the ever in
rom sunlight
nd generate
unction, to s
igure 1.6 sh
evice.
Figure 1.6 Sc
nvolved in th
oduction to
devices hav
ncreasing dem
t is a semico
electrons a
separate thes
hows a schem
chematic dia
he process of
o Photovo
ve become on
mand of ene
onductor ma
and holes, a
se carriers al
matic repres
agram of a ty
f electricity
oltaic cell :
ne of the mo
rgy [58, 59]
aterial with
and a mech
llowing the
sentation of
ypical photov
generation f
: Material
ost importan
. What is re
appropriate
hanism, such
flow of elec
the processe
voltaic cell d
from photons
ls and Prin
nt research d
equired to ge
band gap to
h as a semi
ctricity in an
es involved
depicting the
s
nciple
directions in
enerate elect
o absorb sun
iconductor d
n external ci
in a photovo
e individual
10
view
tricity
nlight
diode
ircuit.
oltaic
steps
11

Below we discuss each process in detail:
a) Absorption: Sun’s spectrum has photons of energy varying from far infrared (IR)
all the way to ultra-violet (UV) which need to be absorbed by the solar cell.
Depending upon the device structure, this is achieved by employing a
semiconductor (or a series of semiconductors to absorb different regions of sun’s
spectrum, as in multi-junction solar cells) in different architecture. For example,
in Dye-sensitized solar cells (DSSCs), this is accomplished by using Ruthenium-
based organic dye molecules attached onto the photoanode material, which have
high absorption coefficient. There are a number of dyes available and the most
widely used variant is called N719. Once a photon is absorbed by the dye
molecule, it creates a bound pair of electron and hole, called exciton, as shown in
figure 1.6.
b) Exciton Diffusion and Dissociation: Once the exciton is generated in the
absorbing material, it needs to diffuse to an interface (p-n junction or
heterojunction, depending upon the cell design) where electron and holes are
separated because of the thermodynamic energy considerations. Specifically,
because of the offset between the conduction band of absorber and that of the
transport material (as shown in figure 1.6, process b), electrons find it
energetically favorable to migrate to the transport material and thus dissociating
the bound pair of electron and hole and becoming free carriers.
c) Transport of carriers to the electrode: Once the electron is inside the transport
material, it needs to be collected at the electrode. This is achieved by employing

A
fi
cu
d
an
sh
ce

F
re
th

mater
(for di
d) Collec
they a
charge
A typical cur
igure 1.7. U
urrent and
ownward to
nd the curre
hape of the c
ell is govern
Figure 1.7 T
epresent the
he cell effici
ials with hig
iffusive tran
ction of carr
are collecte
ed carriers in
rent density
Under the dar
extremely l
give rise to
ent goes to z
curve is defin
ned by three
Typical curr
point where
ency are giv
gh electron m
nsport), as rep
riers: After
d at the res
n the externa
– voltage (I
rk condition
low reverse
o a photocurr
zero at certai
ned by a fac
important pa
rent-voltage
e the cell gen
ven on the rig
mobility (fo
presented by
transporting
spective ele
al circuit.
I-V) characte
ns, the cell i
bias curren
rent at zero
in applied bi
ctor called fil
arameters na
characterist
nerates maxi
ght
r drift transp
y process c.
g the carrier
ectrodes and
eristics of a
is basically
nt. Upon ill
applied bias
ias (called th
ll factor. The
amely the sh

tics of a ph
imum output
eff
FF 
 
port) or with
rs inside the
d thus allow
photovoltaic
a diode givi
lumination,
s (called shor
he open circ
e efficiency
hort circuit cu
otovoltaic c
t. Equations
OC
mp
SC
V
V
I





h high diffus
e transport l
wing the flo
c cell is show
ing high for
the curve s
rt circuit cur
cuit voltage)
of a photovo
urrent (I
SC
),
cell. V
mp
an
used to calc
SC
mp
in
OC
I
I
P
F V 
12
sivity
layer,
ow of
wn in
rward
shifts
rrent)
. The
oltaic
open
d I
mp

culate
FF

circuit voltage (V
OC
), an
which represent the maximum output of the cell denoted by V
1.2.1 TiO
2
Nanowire Array for Dye
As mentioned in the earlier section, two important steps in the pro
generation are dissociation of exciton at an interface and subsequent transport to the
electrode. In dye-sensitized solar cell, first reported by Gratzel in 1991
steps are accomplished by employing a mesoporous network of TiO
Because of the suitable band alignment, TiO
figure 1.6) with the dye molecules and thus provides a heterojunction for excition
dissociation. Because of the lower conduction band of TiO

Figure 1.8 (a) Schematic diagram of a TiO
(b) Energy level diagram of the dye solar cell depicting the energy levels of individual
components and associated processes involved in the generation of electrons from
photons [61]

), and the fill factor (FF) of the cell. There is a point on the curve
which represent the maximum output of the cell denoted by V
mp
and I
mp
.
Nanowire Array for Dye-Sensitized Solar Cells
As mentioned in the earlier section, two important steps in the process of electricity
generation are dissociation of exciton at an interface and subsequent transport to the
sensitized solar cell, first reported by Gratzel in 1991 [
steps are accomplished by employing a mesoporous network of TiO
Because of the suitable band alignment, TiO
2
forms a type-II band alignment (shown in
figure 1.6) with the dye molecules and thus provides a heterojunction for excition
dissociation. Because of the lower conduction band of TiO
2
, electrons migrate to TiO
(a) Schematic diagram of a TiO
2
nanoparticle based dye-sensitized solar cell.
(b) Energy level diagram of the dye solar cell depicting the energy levels of individual
components and associated processes involved in the generation of electrons from
13
d the fill factor (FF) of the cell. There is a point on the curve
.
cess of electricity
generation are dissociation of exciton at an interface and subsequent transport to the
[60] both of these
steps are accomplished by employing a mesoporous network of TiO
2
nanoparticles.
II band alignment (shown in
figure 1.6) with the dye molecules and thus provides a heterojunction for excition
, electrons migrate to TiO
2
by
sensitized solar cell.
(b) Energy level diagram of the dye solar cell depicting the energy levels of individual
components and associated processes involved in the generation of electrons from
14

overcoming the binding energy of the exciton. Once the electron is injected in the TiO
2

particles, they diffuse through a thick layer (~ 5- 10 µm) of TiO
2
nanoparticles and get
collected at the electrode subsequently. Figure 1.8 shows a schematic diagram of the cell
and the operating principle of a TiO
2
based dye solar cell.
The efficiency of this cell can be calculated in terms of the operational parameters,
as shown in the previous section. The efficiency of the cell can also be calculated in
terms of the efficiency associated with each step involved in the process, namely,
absorption, injection, and collection.
) 3 ........(
) 2 .......(
) 1 .......(
COL INJ
COL INJ ABS IPCE
IQE
IQE LHE EQE
 
   
 
 
  

Where IPCE refers to Incident photon-to-electron conversion efficiency, ABS refers to
absorption of light, INJ refers to the injection of electron, and COL refers to collection of
electrons at the electrode. This equation can also be written in terms of the quantum
efficiencies as given in equation 2. EQE refers to the external quantum efficiency of the
device (same as IPCE), LHE refers to light harvesting efficiency which represents which
part and how much of the incident sunlight is actually absorbed by the device, IQE refers
to internal quantum efficiency and is given by the product of injection efficiency and
collection efficiency. It basically represents for every photons absorbed, how many
electrons are collected at the electrode.
15

Having discussed about the factors that contribute to the total external efficiency of the
cell, we argue in this thesis, that the limitation in this efficiency comes from poor
absorption efficiency (η
ABS
or LHE) and poor collection efficiency ( η
COL
).
Ways to improve the LHE will be discussed in the next section. In this section, we
discuss the reasons behind the poor charge collection efficiency of the traditional DSSC.
As mentioned earlier, in a traditional solar cell, electrons diffuse through a mesoporous
network of TiO
2
nanoparticles. This process is depicted schematically in figure 1.9. In
order for electron to diffuse to the electrode, it has to hop from particle to particle
overcoming the resistance offered by inherent grain boundaries. This process can be very
inefficient leading to loss of many carriers due to scattering at the particle-particle
interface [62].
Figure 1.9 Schematic representation of the diffusive transport of electrons through a
mesoporous network of TiO
2
nanoparticles

Given the problems associated with nanoparticles based photoanodes, researchers have
grown one-dimensional (1-D) TiO
2
nanotube arrays using anodization method, and have
subsequently used them as photoanodes in DSSCs with improved performance compared
to nanoparticle based anodes. [63-67]. However, the crystalline quality of such grown

n
m
on
m
th
ar
n
dy
fa
ce
in

F
em
to

anotubes are
most optimal
nce the elec
mobility chan
he problem o
rchitecture b
anowires. Fi
ye- and QD
abricated usi
ells. The det
n solar cells
Figure 1.10
mploys an o
o fabricate th
e rather poor
architecture
ctron is injec
nnel. This ar
of poor char
because of
igure 1.10 sh
D-sensitized
ing our nano
tails of the sy
are discusse
Schematic
oriented arra
he solar cell
r and requir
e is an orien
cted in the T
rchitecture h
rge collectio
our ability
hows the sch
solar cells
owire array
ynthesis, cha
ed in chapter
diagram of
ay of nanow
re back illum
ted array of
TiO
2
nanow
holds the po
on efficiency
to grow al
hematic diag
s. We demo
as photo ano
aracterizatio
r 2 of the the
the propose
wires, subseq
mination whi
f single cryst
wire, it can b
otential of pr
y. In this the
igned array
gram of our
onstrate that
ode results i
n, and subse
sis.
ed architectu
quently sensi
ich limits th
talline TiO
2

be transporte
roviding the
esis, we pro
s of single
proposed ar
t the efficie
in improved
equent use o
ure for dye
itized with a
he efficiency
nanowires w
ed along the
e best soluti
opose to use
crystalline
rchitecture o
ency of the
efficiency o
f TiO
2
nanow
solar cell w
absorbing en
16
. The
where
high
on to
such
TiO
2

of the
e cell
of the
wires
which
ntities
17

1.2.2 TiO
2
Nanowire Array for QD-sensitized Solar Cells
As discussed in section 1.2, One of the other factors that limit the efficiency of the solar
cell is the poor light harvesting efficiency (LHE). Poor LHE is mainly a consequence of
the fact that the dye molecules employed in the cell have a narrow window of absorption
resulting in wastage of a large part of sun’s spectrum. Figure 1.11 shows the spectral

Figure 1.11 Plot showing the spectral distribution of sun’s irradiance. Inset shows the
absorption of N719 dye solution prepared in ethanol

distribution of sunlight [68] and the absorption spectrum of the most widely used dye
molecule, N719 is shown in the inset. As can be seen in the figure, N719 starts to absorb
from 750 nm onwards and hence photons belonging to the infrared regions are not
500 750 1000
0.0
0.2
0.4
0.6
0.8
1.0

Absorption (a.u)
wavelength (nm)
N719 dye
measured in ethanolic solution
Infrared
region
missing

ab
in
en
m
ab
F

m
N
se
sy
v
an
bsorbed at a
n the infrare
ntity, there i
missing regio
bsorption an
Figure 1.12 T
material clas
NCQDs offe
ensitizers. F
ynthesized a
arious nanoc
nd their char
ll. As can be
ed region an
is a critical
on of the spe
nd suggests u
TEM and SE
s as alterna
er many ad
Figure 1.12
and used in t
crystals such
racterization
e seen in sun
nd hence wh
need to loo
ectrum. One
using nanocr
EM images o
ative sensitiz
dvantages o
shows SEM
this thesis. C
h as CuInSe
n of nanocrys
n’s spectrum
hile N719 ha
ok for alterna
of the chap
rystal quantu
of various NC
zers for sola
over the tra
M and TEM
Colloidal syn
e
2
, CuInS
2
, C
stals are disc
m, more than
as been the
ative materi
pters of this t
um dots (NC
Cs synthesiz
ar cells. As
aditional dy
M images o
nthetic techn
CdSeS, Cu
2
Z
cussed in cha
half of the l
most widely
ials that can
thesis deals
CQDs) belon
zed and used
mentioned
ye molecule
of some of
niques are u
ZnSnS
4
etc.
apter 3 in gr
ight intensity
y used abso
complemen
with the iss
ging to vario
d in this thes
in section 1
es as altern
the nanocry
sed to synth
Synthetic d
reat detail.
18
y lies
rbing
nt the
ue of
ous
is
1.1.1,
native
ystals
hesize
details
19

1.2.3 Solid State Dye-Sensitized Solar Cell
The widespread applicability of dye-sensitized solar cells is hindered by the volatile
solution phase electrolyte systems used in these cells. The volatile solution phase
electrolyte system, typically comprising acetonitrile and iodide/triiodide, deployed for
dye regeneration in traditional dye-sensitized solar cells (DSCs) represents a stability and
packaging limitation to this otherwise potentially economically viable solar technology.
To address the problems of corrosion and potential leakage associated with these
liquid systems [69], there is substantial interest in developing solid-state DSCs, where a
highly conductive, solid-state hole-transport medium (HTM) replaces the solution
electrolyte system. Currently, HTM deposition is carried out via solution-based
techniques, where materials such as poly(3-hexylthiophene) P3HT
[70-72]
and spiro-
OMeTAD(2,20,7,70-tetrakis-(N,N-di-p-methoxyphenylamine)9,90-spirobifluorene [73-
76] are dissolved in a solvent and drop cast or spin cast onto the substrate. However,
these techniques rely strictly on capillary action [77] for the uptake of HTM molecules
and require concomitant solvent removal from deep within the nanostructured device. As
a result, a considerable mass transport limitation exists for solution-based deposition
methods, due to the required transverse motion of solute molecules with respect to the
surrounding solvent medium.
In this regard, solution based deposition techniques are unattractive for achieving
a high quality conformal HTM coating of the photoanode surface that ensures contact of
the HTM domains to the counter electrode. This is because the oxidized dye molecule
must be rapidly reduced, regenerating the neutral species, following photoinduced
20

electron injection from the dye to TiO
2
. Correspondingly, hole transport to the anode
must be facile, in order to avoid performance losses. In a solid-state dye-sensitized solar
cell (sDSC) efficient dye regeneration requires intimate orbital overlap between the dye
and the molecules of the HTM. Consequently, physical infiltration of molecules
employed in the HTM is critical to achieving high performance [77]. Since achieving
intimate infiltration of HTM materials has proven to be quite challenging using solution
based fabrication methods,

it is compelling to examine alternative HTM deposition
techniques.
In this study, we demonstrate for the first time, the feasibility of employing a
conformal physical vapor condensation technique, known as organic vapor phase
deposition (OVPD), to grow TiO
2
-infiltrating domains of the HTM material 1,4-bis(2-
naphthylphenylamino)benzene (NNP) in intimate contact with the dye layer of sDSC
devices prepared on a TiO
2
nanowire array. Chapter 4 discusses the solid state dye
sensitized solar cell in great details.

1.3 The Race to replace Indium–Tin-Oxide (ITO) : Candidate
Materials
Materials with a remarkable combination of high electrical conductivity and optical
transparency are important components of various optoelectronic devices such as organic
light emitting diodes (OLEDs) and solar cells [78, 79]. In case of solar cells, they work as
21

anodes to extract separated charge carriers from the absorbing region, while in case of
OLEDs, they have to inject charge carriers without affecting the light out-coupling
efficiency. Doped metal oxide films such as tin-doped indium oxide and fluorine-doped
tin oxide have single handedly dominated the field for almost four decades now [80].
Ability to deposit these materials with controlled thickness and controlled doping
concentration has significantly contributed to their widespread applications. However,
next generation of optoelectronic devices require transparent conductive electrode (TCE)
to be light-weight, flexible, cheap, and compatible with large scale manufacturing
methods, in addition to being conductive and transparent. These requirements severely
limit the use of ITO as transparent conductors because ITO films fail under bending,
restricting their use in flexible optoelectronic devices [81]. In addition, limited
availability of indium sources resulting in ever increasing prices of indium creates an
urgent need to look for other materials which can potentially work as transparent
conductors for future optoelectronic devices.
The question arises: What materials can fulfill these requirements? Having
realized the need to replace ITO, research community has made significant advance in
this direction with several potential candidate materials being studied and evaluated
constantly. Among them, the most important ones are carbon nanotube (CNT) films [82-
84], graphene films [85-87], metal gratings [88], and random network of metallic
nanowires [46, 82], Figure 1.13 represents these different materials used for various
optoelectronic applications. A great deal of efforts by many research groups has led to
significant improvement in the performance of CNT films and their subsequent
22

applications in display [89, 90] and photovoltaic devices [91]. Figure 13a shows a typical
SEM image of well interconnected nanotube films, obtained using a vacuum filteration
and PDMS-assisted transfer technique, and subsequently used for an organic light
emitting diode [84].
One of the critical requirement for CNT films is that the density of nanotubes
should be above the threshold for the formation of percolation network [92]. Although
the conductivity of individual nanotube is rather high, it is the high nanotube-nanotube
junction resistance that limits the conductivity of these films [93]. Researchers have
devised many approaches to further enhance the conductivity of the film by manipulating
surfactant molecules, various acid treatments etc [94]. CNT films, in general are
comparatively cheap and can be fabricated over a large area in various thicknesses and
patterns, however, their performance still lags behind that of ITO films. For example, to
achieve a sheet resistance of 10 Ohms/sq, CNT films need to be >100 nm thick, severely
affecting their transparency. New approaches are constantly being explored to enhance
the performance of CNT films.
Along the same lines, a random network of metal nanowires and ordered array of
metallic nanostructures have been put forward recently as leading candidates. Initial
results show that the films of metal nanowires exhibit performance that rival that of ITO,
with sheet resistance values approaching ~ 16 Ohms/sq at a transparency of ~ 86% [46].
23

Figure 1.13 Various candidate materials for transparent and conductive electrodes a)
SEM of a carbon nanotube (CNT) film, b) SEM image of a silver nanowire network film
with high optical to electrical conductivity ratio, c) SEM image of Au nanowire grating
fabricated using nano-imprinting technique, and d) AFM image of solution processed
graphene flakes

Recently, a small molecule photovoltaic cell was fabricated on silver nanowire networks
and was shown to compete well with the cells fabricated on traditional ITO films [46].
Figure 1b shows a SEM of silver nanowire film obtained using a cellulose assisted
transfer method with σ
DC
/ σ
OP
values approaching ~ 500 [82].
Having understood the importance of Ag nanowires and their use as transparent
conductive electrode, we speculated that the applicability of Ag nanowires can be
a
b
c
d
24

increased manifold if one can develop a method to transfer the nanowire films on
arbitrary substrate with many advantages over the previously reported method of
cellulose membrane. In chapter 5, we discuss in detail, a PDMS assisted dry transfer
printing technique to obtain highly uniform and conductive film of Ag nanowires on both
rigid and flexible substrates.

25

1.4 Outline of the Thesis
Having understood the usefulness of both zero-dimensional and one-dimensional
nanostructured materials in many applications, my focus in this thesis is on the synthesis
of quantum dots belonging to different material class for solar cell application, synthesis
of 1-D nanowires for both solar cells and transparent conductive electrodes. Accordingly,
the thesis is organized as follows: chapter 1 introduces the basic concepts of reduced
dimensionality nanostructures and reasons that make them so interesting for various
applications. Following an overview and an introduction of fundamental knowledge of
zero-dimensional and one-dimensional nanostructured materials in Chapter 1, Chapter 2
discusses synthesis of vertically aligned array of single crystalline TiO
2
nanowires and
their use in traditional dye-sensitized solar cells. Chapter 3 discusses the continued work
on TiO
2
nanowire cells where we employ novel nanoparticles as sensitizers and
demonstrate their advantages over traditional dye molecules.
Chapter 4 address the issue of liquid based electrolytes in the solar cell and
proposes a novel physical vapor deposited hole transport material to replace the liquid
electrolyte and thus paving a way for solid-state dye sensitized solar cell.
Chapter 5 deals with an important issue of ITO replacement for solar cells.
Researchers have been very actively looking for materials that can work as transparent
and conductive electrodes. We demonstrate the viability of silver nanowire based random
network film as a potential replacement. We achieve remarkable performance in terms of
sheet resistance and transparency which rivals that of ITO, supporting a very strong case
for Ag nanowire film.
26

In chapter 6, we demonstrate a scale way to fabricate carbon nanowtube devices with
high on-off ratio ( a critical requirement for logic devices) by converting metal nanotubes
to semiconducting ones. This conversion is realized using exposure of as-grown
nanotubes to a broad band light source which induces photochemical reactions. We
demonstrate a high device yield of 82%.
Finally in chapter 7, we conclude by discussing the future directions and work
that needs to be done in order to carry forward the advance made in this thesis.

27

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91. Rowell, M.W., M.A. Topinka, M.D. McGehee, H.J. Prall, G. Dennler, N.S.
Sariciftci, L.B. Hu and G. Gruner, Organic solar cells with carbon nanotube
network electrodes. Applied Physics Letters, 2006. 88(23).
92. Hu, L., D.S. Hecht and G. Gruner, Percolation in transparent and conducting
carbon nanotube networks. Nano Letters, 2004. 4(12): p. 2513-2517.
93. Hecht, D. and G. Grüner, Solution Cast Films of Carbon Nanotubes for
Transparent Conductors and Thin Film Transistors. 2009. p. 297-328.
94. Geng, H.Z., D.S. Lee, K.K. Kim, G.H. Han, H.K. Park and Y.H. Lee, Absorption
spectroscopy of surfactant-dispersed carbon nanotube film: Modulation of
electronic structures. Chemical Physics Letters, 2008. 455(4-6): p. 275-278.

35

Chapter 2. Growth of Aligned Single-Crystalline Rutile TiO
2

Nanowires on Arbitrary Substrates and Their Application in
Dye Sensitized Solar Cells
TiO
2
is a wide band gap semiconductor with important applications in photovoltaic cells
and photo catalysis. As mentioned in the introduction chapter, a nanowire architecture
can potentially improve the charge collection efficiency of the device and thus improving
the total cell efficiency. In this chapter, we discuss the synthesis of single-crystalline
rutile phase TiO
2
nanowires on arbitrary substrates, including fluorine-doped tin oxide
(FTO), glass slides, tin-doped indium oxide (ITO), Si/SiO
2
, Si(100), Si(111), and glass
rods. Nanowires are synthesized using a hydrothermal method, where source precursors
are mixed with water and surfactants to prepare the growth solution. Nanowire growth is
carried out by placing the substrate inside the autoclave and heating it for some time. By
controlling the growth parameters such as growth temperature, precursor concentrations
etc., we demonstrate that anisotropic growth of TiO
2
is possible, leading to various
morphologies of nanowires. Optimization of the growth recipe leads to well-aligned
vertical array of TiO
2
nanowires on both FTO and glass substrates. Effects of various
titanium precursors on the growth kinetics, especially on the growth rate of nanowires are
also studied. Finally, application of vertical array of TiO
2
nanowires on FTO as the
photoanode is demonstrated in dye-sensitized solar cell with an efficiency of 2.9 ± 0.4 %.
The length of the nanowires used for solar cell fabrication is 3 um. In order to achieve
efficiency of ~3% using the nanoparticle based TiO
2
photoanodes, one needs to have
about 5-6 um thick film while in case of nanowire, we achieved this efficiency by
36

employing only 3 um long nanowires, which clealt demonstrates the superiority of TiO2
nanowires over nanoparticles.
2.1 Introduction

Synthesis of metal oxide materials have been an area of extensive research owing to their
wide-spread applications [1]. In the past two decades, there have been great efforts in
synthesizing one-dimensional (1-D) metal oxide nanowires due to their unique shape-
dependent electronic and optical properties [2-11]. Among others, ZnO and TiO
2
are of
particular interests because of their demonstrated applications in a wide-variety of
commercial products including pigment [12], sunscreens [13], paints [14], ointments, and
protective coating etc [15]. Advances in synthetic chemistry techniques have allowed
alteration and manipulation of the magnetic and electrical properties of these materials by
introducing doping impurities into the lattice [16, 17].
TiO
2
has also been one of the most significant materials for photocatalysis and
photovoltaics applications. The extraordinary oxidizing ability of photo-generated holes
in TiO
2
, together with its relative chemical and physical stability, has made TiO
2
a
material of choice for applications related with solar energy such as, dye-sensitized solar
cell (to convert light into electricity) [6, 18, 19], photo-electrolytic hydrogen generation
[20], and UV-mediated photocatalysis [21]. The motivation behind synthesizing 1-D
metal oxide materials has come from the fact that their 1-D shape results in the
appearance of new optical and electrical properties which make them all the more
beneficial for the above-mentioned usage.
37

It is, however, interesting to note that while ZnO has seen much advance in
synthesizing nanowires, synthesis techniques for TiO
2
nanowires are limited. For
example, vertical array of ZnO nanowires have been grown using a simple solution
method on a variety of substrates including tin-doped indium oxide (ITO), fluorine-
doped tin oxide (FTO), silicon, glass, and polyethylene terephthalate (PET) substrates [3].
This partly comes from the fact that ZnO possesses a wurtzite crystal structure with six
nonpolar {1010} faces and rather unstable {0001} polar faces which suggests a growth
habit where c-axis is the fastest growing direction [22].
Unlike ZnO, TiO
2
has witnessed limited progress towards a low temperature
synthesis protocol allowing for its growth on arbitrary substrates. There have been reports
on TiO
2
nanowire growth utilizing heterogeneous nucleation based on vapor-liquid-solid
(VLS) techniques, but they typically require very high growth temperatures [23-25]. In
past, while anodization method has been used to grow TiO
2
nanotubes [26], an oblique
angle deposition method has been demonstrated for TiO
2
nanowire growth [27]. Very
recently, TiO
2
nanowires using hydrothermal [28] and solvothermal [29] methods were
synthesized, but the growth was reported to be restricted to FTO substrate, and attempts
to grow nanowires on glass or silicon substrates were not successful [28]. In this paper,
we report a general synthesis procedure which allows growth of TiO
2
nanowires on
arbitrary substrates including FTO, glass slides, ITO, Si/SiO
2
, and Si(100), Si(111)
substrates. We further demonstrate that a densely packed vertical array of TiO
2
nanowires
can be grown directly on glass slide as well, by controlling the substrate position. In
addition, we have used vertical array of TiO
2
nanowires grown on FTO substrate as
38

photoanode to assemble a dye-sensitized solar cell with an efficiency of 2.9 %. We note
that the growth of TiO
2
nanowires on arbitrary substrates can catalyze many more
applications such as Li-ion battery and water splitting, which may require substrates other
than FTO.

2.2 Experimental Details:
2.2.1 Synthesis of TiO
2
nanowires:
TiO
2
nanowire assemblies were grown on various substrates including FTO, glass slides,
ITO, Si/SiO
2
, and Si(100), Si(111) substrates. In a typical synthesis, the substrate was
ultrasonically cleaned sequentially in acetone, isopropyl alcohol, and de-ionized (DI)
water for 15 minutes each and finally dried under N
2
flow. Separately, 1 ml of a titanium
precursor was added drop-wise to a 1:1 mixture of DI water and concentrated (35%)
hydrochloric acid (HCl) to obtain a clear transparent solution. Titanium precursors used
in this study includes titanium tetrachloride, titanium isopropoxide, and titanium n-
butoxide. The substrate was placed at an angle (see text for discussion on the effect of
substrate placement angle) in a 23 ml Teflon liner and the precursor solution was added
to it. The Teflon liner was loaded in an autoclave and was placed in an oven and the
growth was carried out at different temperatures (120
o
C – 180
o
C) and for different
growth times (2 to 8 hours). Detailed synthesis conditions are discussed at appropriate
places in the text. We note that in our approach, the titanium precursor concentration in
the reaction solution is three times higher than that used in previous work [28], which led
to the significantly different results discussed below.
39

2.2.2 Solar Cell fabrication:
For the fabrication of dye-solar cell, an FTO substrate with vertical array of nanowires
was immersed in 0.3 mM ethanolic solution of cis-bis(isothiocyanato)bis(2,2 ′-bipyrridyl-
4-4 ′-dicarboxylato)-ruthenium(II)bis-tetrabutylammonium dye (N-719 as received from
Solaronix) for 12 to 24 hours, which serves as the light absorbing elements. A 20 nm
platinum coated FTO substrate was used as the counter-electrode. A sheet of parafilm
was used as a spacer and the space between the electrodes was filled with commercially
available electrolytic solution (iodyte 100 from Solaronix) by capillary action to complete
the cell fabrication. For the TiCl
4
treatment, FTO substrate with nanowire array was
immersed in 0.1M solution of TiCl
4
(prepared by adding TiCl
4
dropwise into the
measured amount of ice) at 60
o
C for 30 minutes and subsequently annealed at 400
o
C in
air for 30 minutes.
2.2.3 Characterization:
Scanning Electron Microspcope (SEM) images were obtained on JEOL-7001 SEM
operating at 5.0 kV voltage. Powder X-ray Difraction (XRD) analyses were performed on
a Rigaku Ultima IV X-Ray diffractometer using a Cu K α radiation source ( λ = 1.54 Å).
Transmission electron microscope (TEM) and selected area electron diffraction (SAED)
analyses were carried out on a JEOL JEM-2100 microscope at an operating voltage of
200 kV, equipped with a Gatan CCD camera. To prepare TEM samples, substrate with
as-grown nanowires were sonicated at mild power in IPA solution and a drop of this
solution was put on the 300 mesh Cu grid. Current-voltage characteristics was measured
40

using an Agilent semiconductor parameter analyzer. An Oriel 300 W Xe lamp fitted with
AM 1.5G filter was used to simulate the solar spectrum. The sample with an active area
of 0.3 cm
2
was illuminated by the light from the Xe-lamp and the corresponding
photocurrent and photo-voltage generated were measured.
2.3 Results and Discussions:
2.3.1 TiO
2
Nanowire Growth on Arbitrary Substrates:
We synthesized TiO
2
nanowires on various substrates including FTO, ITO, glass (slide
and rod), Si/SiO
2
, Si(100), and Si(111), and obtained nanowires with slightly different
morphologies depending upon the substrate used. Figure 2.1 shows the field emission
scanning electron microscope (FESEM) images of TiO
2
nanowires on FTO substrate
using TiCl
4
as the titanium precursor. Highly aligned array of TiO
2
nanowire array could
be grown on an FTO substrate with a length of 6.8 µm when the growth was carried out
at 180
o
C for 4 hours. Nanowires grown in this fashion were ~ 100 nm in diameter with a
square cross-section (figure 2.1d). Also, the diameter was larger at the bottom and
seemed to taper toward the tip. Cross sectional image shows the highly ordered and
aligned array with a very close packing (figure 2.1c). The nanowires grew all over the
substrate with uniform morphology.

F
su

In
re
re
su
d
au
su
d
Figure 2.1.
ubstrate
n order to ch
emarkably, n
eport of Liu
ubstrates wa
andelion-mo
utoclave (fi
ubstrate was
iscussed late
SEM image
heck the vers
nanowires g
et. al [28]
as not succe
orphology w
igure 2.2.a),
s loaded ve
er in the chap
es of Hydr
satility of the
grew on the
where they
essful. SEM
when the sub
, while nan
ertically (fig
pter.
othermally
e growth pro
glass substr
reported tha
inspections
bstrate was
nowires form
gure 2.2d).
grown TiO
ocess, we em
rate as well.
at attempted
s demonstrat
kept horizo
med vertica
Effect of su
2
Nanowire
mployed a gl
This is in
d growth on
ted that nan
ontal relativ
al array stru
ubstrate pos
e Array on
lass substrate
contrast wit
glass and si
nowires exhi
ve to the ba
ucture when
sitioning wi
41
FTO
e and
th the
ilicon
ibited
se of
n the
ill be

F

U
h
F
co
w
w
IT
co
Figure 2.2. S
Uniform vert
ave similar
TO [28]. In
onsequently
with ordered
was found to
TO substrat
omparatively
SEM images
tical alignme
crystal struc
contrast, the
the growth
nanowires g
be ~90 nm
es, there w
y lower de
of TiO
2
nan
ent in case
cture,

which
ere is no latti
h on the gla
growing on
m with a leng
as formation
ensity and s
nowires grow
of FTO can
can lead to
ice match be
ass substrate
micro-sized
gth of ~ 4 µ
n of similar
smaller-sized
wn on glass s
n arise from
o epitaxial g
etween TiO
2
e resulted in
d particles. T
m. When th
r dandelion
d dandelion
substrate
the fact tha
rowth of Ti
2
and the gla
n dandelion-
The diameter
he growth w
-type structu
ns (~ 20 µ
at TiO
2
and
O
2
nanowire
ss substrates
-like morpho
r of the nano
as carried o
ures, albeit
m) (figure
42
FTO
es on
s, and
ology
owire
ut on
with
2.3).

In
th
F

W
S
as

ndividual Ti
he growth on
Figure 2.3. S
We further e
i(111) subs
ssemblies.

O
2
nanowire
n glass subst
SEM images
explored the
strates (figu
es grew in ra
trate.
s of TiO
2
nan
e growth of
ure 2.5),
adially outw
nowires grow
f nanowires
and obtaine
ard direction
wn on ITO s
on Si/SiO
2
ed similar
n on ITO sub
substrate
2
(figure 2.4
dandelion-s
bstrate, simi
4), Si (100)
shaped nano
43
lar to
, and
owire

F

W
S
o
In
th
st
d
Figure 2.4. S
We note that
i/SiO
2
subst
f nanowires
nterestingly,
he interface
tart to grow
andelions w
SEM images
it was relat
trates than fr
s at the inte
square shap
of Si/SiO
2
s
from a thin
were larger in
s of TiO
2
nan
tively easier
rom FTO, IT
erface, we c
ped individu
substrate and
film of TiO
n size (~ 40
nowires grow
to peel off
TO and glass
carried out
ual nanowire
d the nanow
2
deposited
µm) in case
wn on Si/SiO
the film of
s substrates.
SEM imagi
es with a size
wire film, wh
on the subs
e of Si(100)
O
2
substrate
TiO
2
nanow
In order to
ing on the p
e of ~ 200 n
hich suggest
strate. The do
with dandel
wires from S
find the stru
peeled-off f
nm were fou
ts that nanow
omain size o
lions merged
44
Si and
ucture
films.
und at
wires
of the
d into

ea
co
F

S
w
ep
ap
th
ach other (
ompared to o
Figure 2.5.
uccessful gr
which do not
pitaxial inte
pparently le
he epitaxial i
(figure 2.5).
other silicon
SEM image
rowth of TiO
t have any ep
erface is no
d to nanowi
interface can
. The densi
n substrates.
s of TiO
2
na
O
2
nanowire
pitaxial inte
ot required
ires of differ
n help the ali
ity of dand
anowires gro
es on substra
erface betwe
to grow T
rent morpho
ignment of n
delions were
own on Si(10
ates such as
en them and
TiO
2
nanow
ologies and d
nanowires, a
e low on S
00) substrate
ITO, glass,
d TiO
2
, sugg
wires, thoug
dimensions.
and therefore
Si(111) sub
e
, Si/SiO
2
, an
gests that suc
h the subst
In case of F
e uniform ve
45
strate
nd Si,
ch an
trates
FTO,
ertical
46

array of nanowires were obtained. In contrast, observation of dandelion-shaped
morphologies on other substrates suggests that, in the
absence of epitaxial interface, TiO
2
first nucleated as islands on these substrates, and
subsequently grew from these islands to form dandelion-like morphologies. The TiO
2

nanostructure topologies from various substrates are summarized in table 1.

Table 1. Summary of TiO
2
nanowire morphology obtained on various substrates

2.3.2 Characterization of TiO
2
Nanowires:
The nanowires were subsequently characterized using X-ray diffraction (XRD),
transmission electron microscope (TEM), and selected-area electron diffraction (SAED).
Nanowires grown in this report, regardless of the substrate used, were found to have
rutile phase. XRD data (figure 2.6a, 2.6b) show an excellent agreement with the standard
rutile structure of TiO
2
(PDF file # 01-086-0147, P4
2
/mnm, a = b = 4.594 Å and c =
2.958 Å).
Substrate Morphology Size of
dandelion
Length Diameter
FTO Vertical array NA 5.6 µm ~ 100 nm
ITO Dandelions ~ 20 µm 4 µm ~ 80 nm
Glass Dandelions ~ 10-20 µm 3 µm ~ 80 nm
Si/SiO
2
Dandelions ~ 10-20 µm 3 µm ~ 90 nm
Si (100) Dandelions ~ 30-40 µm 4 µm ~ 90 nm
Si(111) Dandelions ~ 20 µm 4 µm ~ 90 nm

F
w
n
n
si

F
fi
d
p
in
th
Figure 2.6 S
with 10 ml o
anowires gro
anowire. Ins
ingle nanow
igure 2.6a s
igure 2.6b
ifference in
lanes is vis
ntensity (com
he as-grown
Structural Ch
of DI water,
own on the g
set shows a
ire.
shows the X
shows XRD
the relative
sible in figu
mpared to th
nanowire a
haracterizatio
10 ml of H
glass and FT
HRTEM im
XRD pattern
D data for
e intensities
ure 2.6a and
he powder X
array is high
on of TiO
2
HCl, and 1 m
TO substrate
mage of the
n for nanow
nanowires
of peaks co
d figure 2.6
XRD pattern)
ly oriented w
nanowires g
ml of TiCl
4.
, respectivel
same nanow
ires grown
grown on
orresponding
6b. A sharp
) in case of
with respect
grown at 18
(a) and (b)
ly. (c) TEM
wire. (d) SA
on the glass
an FTO su
g to various
increase in
FTO substr
t to the subs
0
o
C for 4 h
XRD patte
image of a s
AED pattern
s substrate w
ubstrate. A
s crystallogra
n the (002)
rate indicates
strate surface
47
hours
ern of
single
n of a
while
clear
aphic
peak
s that
e. On
48

the other hand, the XRD pattern for the glass substrate exhibits peaks for other reflections
as well. However, the intensity of (002) peak increases from 6.7% of (110) peak intensity
in the standard powder spectrum to 16.3 % of the observed (110) peak intensity
suggesting preferential growth of nanowires in [001] direction. TEM measurements
confirmed the single crystallinity of the nanowires grown in this fashion. Figure 2.6(c)
shows the high-resolution TEM (HRTEM) image of a single nanowire. Lattice planes
corresponding to {110} planes are distinctly visible. Spacing based on 10-planes was
calculated to be 3.23 Å for the [110] planes. Figure 2.6(d) shows a selected-area electron
diffraction (SAED) pattern of a single TiO
2
nanowire with [110] being the zone axis.
Both crystalline and amorphous TiO
2
have been synthesized in the past using
hydrolysis of TiCl
4
[30]. It is well known that when TiCl
4
is mixed with water, it
undergoes hydrolysis (violent reaction releasing HCl) resulting in the formation of
amorphous TiO
2
[30]. In order to obtain crystalline TiO
2
, one important factor is to slow
down hydrolysis of TiCl
4
by providing a highly acidic environment [31]. Addition of HCl
inhibits decomposition of TiCl
4,
which facilitates formation of crystalline TiO
2
[32].
Further addition of HCl slows down the reaction to the extent that thermodynamics
dictates the growth direction rather than the kinetics, as discussed below. Also, rutile
structure has a 4
2
screw axis along the crystallographic c-axis which promotes the growth
along [001] direction, leading to a crystal morphology dominated by the {110} faces [33,
34].

49

2.3.3 Growth Mechanism of TiO
2
Nanowires :
The growth habit of crystals is mainly determined by the relative growth of various
crystal faces bounding the crystal, which is dependent on internal structure factors of a
given crystal and external conditions such as temperature, concentration of precursor, and
pH value of the solution, etc. In principle, one can tune the synthesis conditions to allow
the system to evolve with minimum surface energy, that is, under conditions that are
thermodynamically stable. In case of rutile TiO
2
, every Ti atom is bound by six oxygen
atoms forming a TiO
6
octahedral which shares a pair of opposite edges with the next
octahedral forming a chain like structure [35]. Depending upon the numbers of corners
and edges of the coordination polyhedra available, the growth rate of the different crystal
faces differs and follows the sequence (110) < (100) < (101) < (001) [36].

The growth of
TiO
2
in aqueous solution results from the formation of the growth units and their
incorporation into the crystal lattice. The nature of the growth units, which are Ti(IV)
complex ions, depends critically on the acidity and ligand in solution. In the present
reaction conditions, the formation of TiO
2
might take place according to the following
reactions:
TiCl
4
+ H
2
O ← HCl + Ti(IV) complex
2 Ti(IV) complex TiO
2

Presence of abundant amount of H
+
from the hydrochloric acid significantly restricts the
supply of the growth units, and the morphology is determined by the incorporation
mechanism of the growing crystal. As mentioned above, for rutile TiO
2
, growth rate in
dehydration
50

[001] direction is highest resulting in the growth of stable c-elongated anisotropic crystals
exhibiting (110) faces. In addition, presence of Cl
-
ions are known to restrict growth of
the (110) faces, further enhancing the growth along the (001) direction [28]. Thus
according to the crystal-symmetry and surface-energy considerations, a typical crystal
habit should be acicular and tabular and should exhibit a square cross section, as shown
in figure 2.7.
Figure 2.7 (a) schematic diagram of growth habit of rutile nanowires with (110) side
faces and (001) top face. (b) top view SEM image of TiO
2
nanowires grown using 10 ml
of DI water, 10 ml of HCl, and 1 ml of TiCl
4
at 120
o
C for 2 hours showing square cross
section.

In order to understand the growth mechanism, we carried out controlled experiments on
ITO substrate. Both the growth temperature and the amount of titanium precursors were
varied, and the obtained nanowire morphology was inspected in high resolution SEM.
Figure 2.8 shows the evolution of the nanowire structure as the growth temperature and
the precursor concentration were increased. 0.5 ml of TiCl
4
and a growth temperature of
(001)
(110)
(110)
50 nm
(001)
(001)
(110)
(110)
50 nm
(001)

1
T
ce
cr

F
m
d
in
u
ob
ar
T

50
o
C for 1 h
The length o
enter and ~
rystal structu
Figure 2.8 SE
ml of DI wate
ifferent tem
nset shows l
sing 0.75 m
btained usin
rea image of
TiCl
4
at 180
o
hour resulte
f the nanow
60 nm at t
ure, epitaxia
EM images
er, 10 ml of
mperature: (a
arge area im
ml TiCl
4
at
ng 1 ml TiCl
f tree-like str
o
C.
d in the grow
wire was 1.5
the ends. Sin
al growth of T
showing the
HCl for 1 ho
) individual
mage (b) cros
150
o
C, in
l
4
at 150
o
C,
ructures, and
wth of indiv
5 ± 0.2 µm
nce the sub
TiO
2
on ITO
e evolution o
our with diff
nanowires
ss-like struct
nset shows l
(e) large are
d (g) tree-lik
vidual nanow
with a diam
strate used
O was not po
of nanowire
ferent amoun
obtained us
ture and (c)
large area i
ea image of
ke branched
wires as show
meter of 100
was ITO w
ossible.
morphology
nt of TiCl
4
c
sing 0.5 ml
three-leg str
image (d) d
dendritic str
structure ob
wn in figure
0 ± 15 nm i
hich has a c
y grown usin
concentration
TiCl
4
at 15
ructures obta
dendrite stru
ructures, (f)
btained using
51
2.8a.
n the
cubic
ng 10
n and
0
o
C,
ained
ucture
large
g1 ml
52

When a higher TiCl
4
amount of 0.75 ml was used, many cross-like structures appeared
(figure 3b). In addition to cross-like structures, we also observed three-leg nanowire
structure (figure 2.8c), although with relatively low proportion. Both individual and
branched nanowires were obtained everywhere on the substrate as observed in large area
SEM images, shown in the insets of figure 2.8a and figure 2.8b. When an even higher
TiCl
4
amount of 1 ml was used, many nanowires started to grow from a central long
nanowire resulting in a dendritic-like structure as shown in figure 2.8d and figure 2.8e.
We believe that the additional nanowires start to grow at certain planes with
incorporation of defects at the interface. When the temperature was increased from 150
o
C to 180
o
C, keeping TiCl
4
amount to be 1 ml, the growth resulted in tree-shaped
structures with many nanowires growing on a central wire (figure 2.8f and 2.8g).
Since the hydrolysis rate of titanium precursor has been established as a key factor which
controls the morphology of nanostructures obtained, it is natural to think that different
titanium precursors, under same reaction conditions, will undergo hydrolysis at different
rates, leading to different growth kinetics. In order to study the effect of different titanium
precursors, we carried out growth using titanium isopropoxide (Ti-iPr) and titanium n-
butoxide (Ti-iBu) in addition to TiCl
4
as titanium source. For growth rate comparisons,
only FTO substrates were used, resulting in nanowires with similar morphology (vertical
array). Depending upon the Ti-precursor used, nanowires of different lengths were
obtained. Figure 2.9 shows the variation of growth rate with three different titanium
precursors. Nanowires were grown on FTO substrates with 10 ml of DI water, 10 ml of
HCl, and 1 ml of each precursor at 180
o
C for different growth time. The results revealed
53

that TiCl
4
led to the fastest growth of nanowires with the length reaching 8 µm for a
growth time of 6 hours (nanowire film could be easily peeled off at this stage, as shown
in the inset of figure 2.9) , whereas it was only 5.6 µm in case of titanium isopropoxide.
This behavior can be understood in terms of the reactivity of these precursors with water.
Titanium tetrachloride is known to have faster hydrolysis rate than the other reagents,
which results in accelerated growth of nanowires because of faster supply of titanium
growth units. Also, the diameter of nanowires grown using TiCl
4
precursor was slightly
larger ( ~100 nm) than those grown using titanium isopropoxide and titanium butoxide
precursors (~ 80 nm).

Figure 2.9 Effect of titanium precursor on the growth rate of nanowires. Variation of
nanowire length with the growth time using 1 ml of TiCl
4
, Ti-isopropoxide, and Ti-
butoxide precursors each and with 10 ml of DI water, 10 ml of HCl at 180
o
C on FTO
substrates .

246
2
4
6
8

Length ( m)
Growth Time (hrs)
Ti-iPr
Ti-iBu
TiCl4
Peels off
54

Having discussed the effects of precursors on the growth kinetics, we have also observed
very interesting effects of the substrate positioning inside the autoclave on the alignment
of nanowires both on FTO and glass substrates. When the FTO substrate was placed
horizontally with respect to the base of autoclave, dandelion-shaped structures were
obtained (figure 2.10a), similar to those obtained on other substrates (e.g. figure 1). On
the other hand, when the substrate was placed vertically (or at a slight angle with FTO
side facing down), a very nice vertically aligned array was obtained (figure 2.10b).
Similar growth behavior was observed in case of glass substrate as well, namely,
dandelion-shaped (figure 2.10c) and aligned array (figure 2.10d) of nanowires were
obtained when the glass substrate was loaded horizontally and vertically respectively.
We believe that when the substrate is placed horizontally, there is deposition of
particles on the surface which act as nucleating centers for further growth of nanowires,
resulting in dandelion-shaped structures. On the other hand, when the substrate is placed
vertically, there is very little chance of reaction by-products depositing on the surface and
thus promoting the growth of vertical nanowires. Closer look at the film cross-section
revealed that nanowires initially emanate radially from the nucleation site (figure 2.10e).
Nanowires that grow inclined away from the substrate normal obstruct nanowires
growing off-normal from the adjacent nucleating site. Therefore, it is only the nanowires
that are normal to the substrate grow unobstructed, resulting in c-axis orientation for the
films. This is illustrated schematically in figure 2.10f. Although the alignment is not as

F
o
C
(a
su
th
re
d
Figure 2.10 E
C for 2 hour
a) FTO sub
ubstrate plac
he top view
esolution SE
iagram show
Effect of sub
rs using 1 m
bstrate place
ced horizont
SEM image
EM of nanow
wing the grow
bstrate posit
ml of Ti-isop
ed horizonta
tally, and (d)
and inset sh
wire cross-s
wth evolutio
tioning on th
propoxide. S
ally, (b) FTO
) glass subst
hows perspe
section grow
on of nanowi
he alignmen
SEM image o
O substrate
trate placed v
ective and cr
wn on glass
ires.
nt of nanowi
of TiO
2
nan
placed ver
vertically. M
ross sectiona
substrate, a
res grown a
nowires grow
rtically, (c)
Main panel s
al views. (e)
and (f) schem
55
at 180
wn on
glass
hows
High
matic

g
p
S
gr
w
S
sh

F
gr
g
te

ood as that
erpendicular
i/SiO
2
subst
We al
rew all over
when we exam
EM images
hown in figu
Figure 2.11
rowth, and (
lass rod. (f)
eflon cup.
t on FTO, n
r direction.
trate as well.
lso employed
r the substra
mined the in
of TiO
2
nan
ure 2.11.
Photo-imag
(b) glass tub
) and (g) SE
nanowires o
Similar effe

d glass rod a
ate with mor
nner wall of
nowires on g
ge of TiO
2
n
be. (c), (d), a
EM images
on glass sh
ects on the
and hollow g
rphologies s
Teflon cup,
glass rod, gla
nanowires g
and (e) SEM
of nanowire
ow decent
nanowire or
glass rod as
similar to th
we found T
ass tube, and
grown on (a
M images of
es extracted
degree of a
rientation w
the substrat
hat on other
TiO
2
nanowir
d inner wall o
a) glass rod
f nanowires g
from the in
alignment in
were observe
te, and nanow
substrates. A
res on the su
of Teflon cu
before and
grown on a
nner walls o
56
n the
ed for
wires
Also,
urface.
up are
after
solid
of the
57

2.3.4 Solar Cell Fabrication:
The as-synthesized TiO
2
nanowires would have significant potential for dye solar cell
application. We employed an FTO substrate covered with 3 µm long nanowires as the
photoanode in assembling a dye-sensitized solar cell (DSSC). The conductivity of the
underlying FTO substrate remains unaffected during the hydrothermal synthesis of TiO
2

nanowires because of mild synthesis conditions used. Figure 2.12a shows the J-V
characteristics from a typical dye-sensitized solar cell.

Figure 2.12 (a) J-V characteristics of a dye-sensitized solar cell assembled with FTO
substrate covered with 3 µm long rutile TiO
2
nanowire film as anode and 20 nm Pt
deposited FTO substrate as cathode. Red curve shows the device with as-synthesized
nanowires (without further treatment) immersed in dye solution for 12 hours, and yellow
curve represents device with nanowires treated with TiCl
4
solution and immersed in dye
solution for 24 hours. (b) External quantum efficiency data for the device treated with
TiCl
4
.

Under an AM 1.5G illumination, the cell exhibits a short-circuit current of 4.8 mA/cm
2

and an open-circuit voltage of 0.54 V with an overall power-conversion efficiency of
0.0 0.2 0.4 0.6 0.8
-10
-5
0
5
10
15
20

Current Density (mA/cm
2
)
Voltage (V)
Dark
Light (without TiCl
4
Treamement)
Light (with TiCl
4
Treamement)
400 500 600 700 800
35
70
Quantum Efficiency (%)
Wavelength (nm)
a
b
0.0 0.2 0.4 0.6 0.8
-10
-5
0
5
10
15
20

Current Density (mA/cm
2
)
Voltage (V)
Dark
Light (without TiCl
4
Treamement)
Light (with TiCl
4
Treamement)
400 500 600 700 800
35
70
Quantum Efficiency (%)
Wavelength (nm)
a
b
58

1.2 %. Remarkably, the performance of the cell increased drastically when a TiCl
4
post-
treatment was carried out on the nanowire anode and the sample was kept immersed in
the dye solution for 24 hours. While the open circuit voltage increased only by 40 mV,
the short circuit current density increased to 8.7 mA/cm,
2
giving an overall power-
conversion efficiency of 2.9 %. In addition, the fill factor of the cell increased from 41%
to 59% upon TiCl
4
treatment.
The effects of TiCl
4
treatment have been previously studied in detail by O’regan
et. al [19]. TiCl
4
treatment not only enhances the fill factor of the cell by reducing shunt
current (because of formation of a thin TiO
2
blocking layer), it also increases the
roughness factor which leads to enhanced dye absorption. In our case, however, we
believe that a two fold increment in the power-conversion efficiency is a result of
combination of two effects, namely, TiCl
4
treatment (resulting in increased roughness
and reduced shunt loss) and greater dye loading due to increased immersion time. The
device exhibits an EQE of ~ 60% at the peak absorption of dye, as shown in figure 2.12b.
Based on SEM analysis, a nanowire on average has 4 sides of
nm nm 80 2000 
and a top
side of
nm nm 80 80 
. The specific surface area of the nanowire array is estimated to be
11±3 m
2
g
-1
by using a weight density of 4.25 g/cm
3
for rutile TiO
2
. Following the dye
desorption analysis, a typical value of 78 nmol/cm
2
for the dye concentration was
obtained for our 3 µm long nanowire sample, which is lower than values reported for
anatase TiO
2
particles, consistent with previous reports of dye adsorption studies on rutile
and anatase TiO
2
particles [37]. We note that we did not observe any shorted devices
after assembling the cell, for samples without TiCl
4
treatment, which suggests that there
59

is a thin layer of TiO
2
layer deposited on the FTO substrate during the nanowire growth,
which works as the barrier layer and prevents the electrolyte solution from touching the
bottom FTO substrate. Although, our efficiency is below the highest reported efficiency
values for TiO
2
nanoparticle film anodes because of much thinner TiO
2
layer and other
device optimizations, it compares well with the recently reported efficiencies of devices
made using rutile nanowire arrays [28, 29].

60

2.4 Summary and Conclusion:

In summary, a general synthesis procedure for the growth of TiO
2
nanowires on virtually
any substrate was demonstrated for the first time. A moderately high concentration of Ti-
precursors under highly acidic environment was used to obtain the growth of TiO
2

nanowire assemblies on arbitrary substrates. Both vertical array and dandelion-shaped
TiO
2
nanowire assemblies can be obtained on various substrates including FTO, glass,
ITO, Si/SiO
2
, Si(100), and Si(111) substrates. Different titanium precursors were found
to influence the growth rate of nanowires, and the behavior was explained in terms of the
reactivity of these precursors towards hydrolysis. Alignment of nanowires was observed
to be critically dependent on the positioning of the substrate inside the autoclave. Finally,
a dye sensitized solar cell with a power conversion efficiency of 2.9 ± 0.4 % was
fabricated using TiO
2
nanowires on FTO as photoanode.

61

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8989-8994.

65

Chapter 3. Sensitization of Hydrothermally grown Single
Crystalline TiO
2
nanowire array with CdSeS nanocrystals for
photovoltaic application

An oriented array of electron transporting nanowires, grown directly on a transparent
conductor constitutes an optimal architecture for efficient photovoltaic applications. In
addition, semiconductor nanocrystals can work as efficient light absorbers because of
their tunable optical properties. In this paper, we use an oriented array of TiO
2
nanowire
grown directly on transparent conductive electrode and subsequently sensitized with
colloidally grown CdSeS nanocrystal quantum dots, using an efficient bi-linker assisted
methodology, to demonstrate photovoltaic cells. Upon excitation with light, exciton
dissociation takes place at the nanowire-nanocrystal interface, after which, electron is
transported to the fluorine-doped tin oxide (FTO) electrode via single-crystalline TiO
2

nanowire channels. We demonstrate that an ex-situ ligand exchange of QDs followed by
sensitization on oxygen-plasma treated TiO
2
nanowires results in enhanced loading of
QDs, as compared to the in-situ ligand exchange approach. An array of 1 µm long TiO
2

nanowire sensitized with CdSeS nanocrystals exhibits photovoltaic effects with short-
circuit current of 2-3 mA/cm
2
, open circuit voltage of 0.6-0.7 V and a fill factor of 40-
60 %.

66

3.1 Introduction:
Photovoltaic devices have become one of the most important research directions in view
of the ever increasing demand of energy [1, 2]. Among various technologies, excitonic
solar cells [3] including organic, hybrid organic-inorganic and dye-sensitized solar cells
(DSSCs) are the prime candidates for low-cost, highly-efficient solar cells. Specifically,
DSSC has seen significant advance in the past decade since its first report by Gratzel [4],
leading to the highest reported efficiency of ~ 11% for fully optimized device structure
[5]. Recently, nanocrystals of different materials have been used as sensitizers to
assemble cells. Although the efficiency of QD-sensitized cell remains much lower than
traditional dye-based solar cells, it continues to draw attention because of the various
potential advantages that QD provides. For example, 1) ability to tune the band gap of
QDs by varying the size of nanocrystals [6-8] to allow for harvesting of different
spectrum of sunlight 2) superior photostability of QDs compared to dye molecules [9],
and 3) ability to generate multiple charge carriers with a single photon [10-12]. Various
QDs, namely CdSe [13, 14], InP [15],

PbS [16],

CdTe [17], and CdS [18] have been used
as sensitizers which harvest different spectrum of sunlight.
In order to increase the efficiency of these QD-sensitized solar cells, researchers
have used one-dimensional nanotube arrays as photoanodes and have demonstrated
improved charge collection efficiencies compared to the nanoparticles based anodes [19-
22]. This improvement mainly comes from several orders of magnitude difference in
electron diffusion coefficient between the mesoporous nanoparticle network and their
single crystal bulk counterpart [23]. This high resistance to electron transport in case of
67

nanoparticle anode arises from the inefficient particle to particle migration of electrons
due to the inherent grain boundaries present at the nanoparticle surface leading to
enhanced scattering of free electrons [24]. Also, since every nanotube is in direct contact
with conducting substrate, it increases the probability of electrons reaching the electrode
by many orders of magnitude. Because of single crystalline defect-free nature of
nanotubes, electrons can travel with relatively higher mobilities making them a fast and
efficient transport channel for injected electrons [25].
Because of the difficulties in growing TiO
2
nanowires, ZnO has attracted a lot of
attention after its growth on TCO using mild solvothermal synthesis was reported [26]. In
order to exploit the vectorial transport of charge carriers in nanowires, researchers have
used ZnO nanowire array for both dye solar cell [27] and QD sensitized solar cell [28].
However, their performance have been rather inferior to TiO
2
because of instability of
ZnO in acidic environments [29]. Very recently, our group [30] and others [31, 32]

reported growth of vertically aligned TiO
2
nanowire array directly on transparent and
conductive glass substrates and utilized them for traditional dye solar cells. The reported
efficiency of ~3 % was achieved using only ~2 µm long nanowires as opposed to a
typical thickness of 5-8 µm required in case of TiO
2
nanoparticles films, supporting the
claim that 1-D anode is better than the nanoparticle-based anode [30-32].
In this chapter, we discuss the use of TiO
2
nanowire array directly grown on FTO
substrate as photoanode in QD sensitized solar cell, for the first time. The central theme
of the work is the combination of two important materials (TiO
2
and QDs) in the most
optimal arrangement (Nanowire anode). Ultrafast charge injection from QDs to TiO
2
and
68

extremely fast charge collection via the nanowire channels form particularly promising
cell architecture. We utilize two strategies to connect QDs to TiO
2
nanowires and show
that the strategy involving ex-situ ligand exchange of QDs with a bilinker molecule is
better than the TiO
2
surface functionalization approach. We demonstrate photovoltaic cell
with efficiency of 0.6% with short circuit current of 2 mA/cm
2
, open circuit voltage of
0.6 V and fill factor of 60% which represent a two-fold increase compared to ZnO
nanowire QD solar cell [28].

3.2 Experimental Details:
3.2.1 Synthesis of TiO
2
Nanowires:
In a typical synthesis, the FTO substrate was ultrasonically cleaned sequentially in
acetone, isopropyl alcohol, and de-ionized (DI) water for 15 minutes each and finally
dried under N
2
flow. Separately, 1 ml of a titanium butoxide was added drop-wise to a
1:1 mixture of DI water and concentrated (35%) hydrochloric acid (HCl) to obtain a clear
transparent solution. The substrate was placed at an angle (see text for discussion on the
effect of substrate placement angle) in a 23 ml Teflon liner and the precursor solution
was added to it. The Teflon liner was loaded in an autoclave and was placed in an oven
and the growth was carried out at different temperatures (120
o
C – 180
o
C) and for
different growth times (2 to 8 hours). Detailed synthesis conditions are discussed at
appropriate places in the text. After the growth, the autoclave was cooled down under
flowing water (takes about 15 minutes) and substrate with the nanowires is rinsed
thorough with DI water and dried with nitrogen. The substrate was then annealed in air at
69

450 0C to remove the residue surfactants. Every substrate used in this study was treated
with TiCl
4
to improve the solar cell performance. A 0.1 M aquous solution of TiCl
4
was
prepared by mixing appropriate amount of TiCl
4
in ice and allowing the ice to melt (note
that we can not prepare this solution by mixing TiCl
4
directly in the water as TiCl
4

undergoes hydrolysis immediately at room temperature and precipitates immediately to
give white amorphous TiO
2
). Nanowires substrates is then immersed in this solution and
heated at 60
o
C for 30 minutes while observing that the TiCl
4
solution remains
transparent. After the treatment, substrate is rinsed thoroughly with DI water and
annealed in air at 450
o
C for 30 minutes.
3.2.2 Synthesis of CdSeS Nanocrystals:
CdSeS QDs were synthesized using injection of precursors into hot non-coordinating
solvents. In a typical synthesis, 0.386 mmol of CdO (Sigma Aldrich, 99.99%) was
transferred to a three neck round bottom flask containing 8 ml of ODE (Sigma-Aldrich,
technical grade) and 1 mmol of Oleic acid (Sigma-Aldrich, technical grade). The mixture
was then heated slowly to a reaction temperature of 270
o
C under argon environment.
During the heating, the mixture turned transparent at ~140
o
C indicating the formation of
cadmium oleate which worked as cadmium monomers. Separately, Se and S powders
were mixed with 1 ml Trioctyl phsophine (TOP) (Sigma-Aldrich, technical grade) in the
ratios of 1:1 and stirred vigorously until all Se and S powder was dissolved in TOP. This
solution was then injected swiftly in the reaction vessel upon which the color changed
continuously indicating the formation of CdSeS QDs. After the reaction, the solution
was cooled to room temperature and precipitated with a mixture of methanol and butanol.
70

The sample was centrifuged and the supernatant was discarded. The process of dissolving
and precipitation was carried out three times to wash away any unreacted precursors. The
final product was vacuum dried overnight to get powder of CdSeS QDs which were
dissolved in solvents such as toluene, hexane and chlorobenzene.
3.2.3 Ligand Exchange Procedure of CdSeS Nanocrystals:
To prepare the CdSeS QDs for attachment onto the TiO2 nanowires, the Oleic acid and
TOP ligands on the surface of the QDs were exchanged with MPA. The exchange
process closely follows the work by Aldana et al [33]. In a typical exchange, 0.2 mmol of
MPA was dissolved in 30 mL of anhydrous methanol and pH-adjusted to 11.4 by
addition of TMAOH with continuous stirring. This solution was placed in a 50 mL three-
neck round bottom flask and heated to 55°C under dry nitrogen gas. A small aliquot (~2
mL) was removed from the reaction vessel and added to a vial containing approximately
40 mg of CdSeS nanocrystals in powder form; the nanocrystals gradually dispersed in the
heated solution. This dispersion was then injected into the 50 mL reaction vessel through
a rubber septum. The vessel was degassed and purged with nitrogen. While under
nitrogen, the contents in the reaction vessel were heated to 65°C and refluxed for
approximately 20 hours in the dark. The nanocrystals were isolated by addition of ethyl
acetate to induce flocculation. The resulting precipitate contained MPA-capped CdSe
QDs that were readily dispersible in protic solvents. Excess ligands were removed by
redispersing the precipitate in methanol and again adding ethyl acetate. This precipitation
cycle was performed 3 times.

71

3.2.4 CdSeS sensitization strategy:
We studied two sensitization strategy of CdSeS nanocrystals on TiO
2
nanowires as
follows:
a) Strategy A: TiCl
4
treated nanowire sample is treated with oxygen plasma for 10
minutes and immediately immersed in a vile containing a methanol solution of
ligand exchanged CdSeS nanocrystals and left for 2 days for QD loading. To
prevent the photooxidation of the MPA ligands, the solution was kept under dark.
After the sensitization, the substrate was taken out and rinsed with methanol to
remove any unattached nanocrystals. By this time, the substrate has red color
showing that QDs are attached on the nanowires.
b) Strategy B: This strategy closely follows the recipe of Kamat’s group [14]. TiCl
4

treated nanowire substrate was immersed in a 1M solution of H
2
SO
4
, mixed with
0.2 M MPA solution in methanol for 12 hours to functionalize the surface of TiO
2

nanowires with MPA molecules. Then this substrate was immersed in the solution
of as-synthesized CdSeS QD solution for sensitization. The nanowire substrate
was again left for 2 days for the attachment and then washed with toluene to
remove any unattached QDs.
3.2.5 Solar Cell fabrication:
For the fabrication of QD-solar cell, a 20 nm platinum coated FTO substrate was used as
the counter-electrode. A sheet of parafilm was used as a spacer and the space between the
electrodes was filled with commercially available electrolytic solution (iodyte 100 from
72

Solaronix) by capillary action to complete the cell fabrication. The cell was fixed using a
paper clip.
3.2.6 Characterization:
Absorption measurements were carried out on a Varian Cary 50 UV-Vis
spectrophotometer and photoluminescence measurements were performed on the Quanta
Master from PTI instruments. The excitation wavelength used for PL measurement was
400 nm for all the experiments. Scanning Electron Microspcope (SEM) images were
obtained on JEOL-7001 SEM operating at 5.0 kV voltage. Powder X-ray Difraction
(XRD) analyses were performed on a Rigaku Ultima IV X-Ray diffractometer using a Cu
K α radiation source ( λ = 1.54 Å). Transmission electron microscope (TEM) and selected
area electron diffraction (SAED) analyses were carried out on a JEOL JEM-2100
microscope at an operating voltage of 200 kV, equipped with a Gatan CCD camera. To
prepare TEM samples, substrate with as-grown nanowires were sonicated at mild power
in IPA solution and a drop of this solution was put on the 300 mesh Cu grid. Current-
voltage characteristics was measured using an Agilent semiconductor parameter analyzer.
An Oriel 300 W Xe lamp fitted with AM 1.5G filter was used to simulate the solar
spectrum. The sample with an active area of 0.3 cm
2
was illuminated by the light from
the Xe-lamp and the corresponding photocurrent and photo-voltage generated were
measured.
3.3 Results and Discussion:
3.3.1 Device Structure and Synthesis
The device configuration, shown in figure 3.1a, utilizes type II band alignment at the
73

TiO
2
/QD interface to separate photogenerated negative and positive charges. Following
absorption of photons by QDs, a bound electron-hole pair (exciton) is generated within
the nanocrystal. In the absence of TiO
2
, these excitons will simply recombine without
producing any charge separation. However, because of heterojuction formed at TiO
2
/QD
interface (figure 3.1b), electrons generated in the nanocrystal find it energetically
favorable to migrate to TiO
2
conduction band. Given that the energy offset between the
nanocrystal conduction band and TiO
2
conduction band is much more than the binding
energy of exciton, the process of electron injection from nanocrystal to TiO
2
is extremely
fast, which typically occurs at picoseconds time scale. Kamat’s group and Durrant’s
group have extensively studied the injection process from QD to TiO
2
using time
resolved spectroscopy [22, 34-36]

.
Figure 3.1. (a) Schematic diagram showing the device structure of the QDSSC. Vertical
array of TiO
2
nanowires are grown directly on FTO substrate, followed by sensitization
with CdSeS QDs. Fabrication is completed by using Pt-coated FTO as the counter
electrode and the space filled with electrolyte solution. (b) Energy level diagram of the
various components of the cell. CdSeS QDs and TiO
2
form type-II interface resulting in
migration of photogenerated electrons from QD to TiO
2
.

TiO
2
NWs
QD
e
-
h
+
Pt coated FTO
FTO substrate
CdSeS QD
TiO2 NW
electrolyte
a
b
74

Simultaneously, the remaining hole is transported to the counter electrode via a hole
transporting redox couple comprising of Iodine/Iodide complex. This migration of hole is
again facilitated by favorable energy alignment between the nanocrystal valence band
and the potential of I
3
/I
-
couple. We note that although I
3
/I
-
couple is known to corrode
the nanocrystal surface making the cell unstable for longer period of time, this couple
minimizes the interfacial recombination and thus leading to higher open circuit voltages.
There have been reports on using sulfer/sulfide electrolyte for the QD sensitized solar cell
[17, 37], but these cells typically suffer from extremely low fill factor values. In this
regard, it is important to note that a suitable electrolyte formulation is needed for the QD-
sensitized solar cell which makes the cell stable without compromising on the
performance.
To synthesize TiO
2
nanowire array directly on FTO substrates, we used the
method reported by Lu et. al [32]. with slight modifications. Briefly, ultrasonically
cleaned FTO substrates were loaded in 23 mL Teflon-lined autoclave filled with 10 mL
of DI water, 10 mL of HCl, and 1 mL of titanium isobutoxide. The growth was carried
out at 160
o
C for some time depending upon the length requirements. After cooling the
autoclave under flowing water, substrate was taken out and rinsed with DI water. TiCl
4

treatment was carried out by dipping the substrate in 1M aqueous solution prepared by
mixing TiCl
4
and ice. The sample was kept immersed in this solution for 1 hour at 60
o
C
after which it was rinsed and annealed in air at 450
o
C for 30 minutes. Figure 3.2a shows
the SEM images of thus obtained nanowire array. High resolution SEM imaging reveal
that the nanowires have a typical diameter of ~ 90 nm and length can be controlled by

v
µ
n
F
gr
im
el
gr
arying the g
µm length (f
anowires.
Figure 3.2. (
rown on FT
mage of a s
lectron diffr
rown on FTO
rowth time.
figure 3.2b)
(a) Perspect
TO substrate
single nanow
action patter
O substrate.
For example
) while a 2
tive (b) cros
. The length
wire showin
rn of a single
Enhanced (0
e, 1 hour gro
2.5 hours of
ss-section SE
h of nanowir
ng distinct
e TiO
2
nanow
002) peak co
owth at 160
f growth tim
EM images
res is ~ 1 µ
{110} cryst
wire. (e) XR
onfirm the v
o
C yields na
me results i
of the TiO
2
m. (c) High
tal planes. (
RD pattern o
vertical grow
anowires wit
in ~ 2 µm
2
nanowire
h resolution
(d) Selected
f TiO
2
nanow
wth of nanow
75
th ~ 1
long
array
TEM
area
wires
wires.
76

Nanowire array grows uniformly across the
cm cm 5 . 2 5 . 2 
area substrate. We note that
the use of TiCl
4
as the titanium precursor results in extremely dense array of nanowires
making them unfavorable for QD penetration. These arrays also suffer from peeling-off
problem, consistent with the observation of Lu et. al [32] On the other hand, Titanium
butoxide, due to its slower hydrolysis rate, results in individual nanowire with reasonable
areal density. Effects of titanium precursors and other growth conditions are discussed in
detail in our earlier paper [30].
Nanowires grown using this method were found to have rutile phase and the XRD
data (figure 3.2e) show an excellent agreement with the standard rutile structure of TiO
2

(PDF file # 01-086-0147, P4
2
/mnm, a = b = 4.594 Å and c = 2.958 Å). As evident from
the XRD pattern shown in figure 3.2e, the (002) peak is extremely intense indicating the
vertical alignment of nanowires on FTO substrate. High resolution TEM (HRTEM)
imaging (figure 3.2c) clearly shows the well defined crystallographic planes with [001]
growth direction. Lattice spacing based on 10-planes were calculated to be 3.23 Å which
correspond to {110} planes. Selected Area Electron Diffraction (SAED) pattern (figure
3.2d) shows sharp spots suggesting that the nanowires are highly crystalline. Every
nanowire sample used in this report was subjected to a TiCl
4
treatment step in order to
enhance the fill factor of the cell.

S
W
th
o
g
m

F
(b
C
H
in
eparately, C
We chose all
heir higher P
f the CdSeS
ap tuning i
mixture rathe
Figure 3.3. (
b) UV-Vis a
CdSeS QDs
HRTEM ima
ndividual QD
CdSeS nanoc
loyed CdSeS
PL efficiency
S nanocrytals
s realized b
er than the si
a) TEM ima
absorption sp
. (c) TEM i
age of QDs a
Ds as a guide
crystals wer
S nanocrysta
y. The highe
s which lead
by varying
ze giving an
age of CdSe
pectra (red c
mage of sin
attached on
e to the eye.
re synthesiz
als in stead
er efficiency
ds to extrem
the relative
n additional p
S QDs (inse
curve) and P
ngle TiO
2
na
to the nanow
ed following
of binary C
comes from
mely low def
e concentrati
parameter to
et shows the
PL spectra (b
anowire sens
wire. Black
g the publis
CdSe nanocry
m the graded
fect densities
ion of Se/S
o control the
HR-TEM o
blue curve) o
sitized with
circles are d
shed recipe
ystals becau
internal stru
s. Also, the
S in the rea
band gap. A
of individual
of as-synthe
CdSeS QDs
drawn aroun
77
[38].
use of
ucture
band
action
A
l QD)
esized
s. (d)
nd the
78

ligand exchange procedure was carried out to replace TOPO/TOP surfactants from
nanocrystals with smaller MPA molecules, following the standard recipe [33]. After the
ligands were exchanged, nanocrystals were readily soluble in protic solvents like
methanol affirming that SH- group has successfully removed TOPO molecules.
Figure 3.3a shows TEM image of CdSeS nanocrystals with average size of ~5.3 nm. The
band gap of nanocrystals were obtained from UV-Vis absorption spectra and PL spectra
as shown in figure 3.3b. A narrow FWHM of ~ 35 nm in PL spectra confirms narrow size
distribution of as-syntheszed nanocrystals. A clear peak at ~ 550 nm can be seen in
absorption spectra of nanocrystals corresponding to first excitonic peak.

3
In
d
su
Q
o
Q

S
se
S
su

.3.2 Sensitiz
n order to se
epicted sche
ubjected to
QD solution i
f QDs on to
QDs from the
Scheme 1. (
ensitization
trategy B: S
ubstrate is im
zation Strat
ensitize TiO
2
ematically in
10 minutes
in methanol
o nanowires
e sample. In
Top) Strate
is done by
Surface funct
mmersed in a
tegies:
2
nanowires
n scheme 1.
of oxygen p
. The sampl
after which
approach B,
gy A: ligan
dipping the
tionalization
as-synthesiz
with CdSeS
In approach
plasma and
e was kept i
h, it was wa
, TiO
2
nanow
nd exchange
e nanowire
n of TiO2 na
ed QD solut
S QDs, two a
h A, a TiCl
4
t
immediately
immersed fo
ashed in met
wire sample
e of QDs is
substrate in
anowires is c
tion for sens
approaches w
treated nano
y immersed
or 2 days to a
thanol to rem
was first im
s carried ou
n the QD so
carroed out
itization.
were follow
owire sample
d in MPA-ca
allow for loa
move unatta
mmersed in M
ut first and
olution. (Bot
first and the
79
wed as
e was
apped
ading
ached
MPA
then
ttom)
en the
80

solution for 12 hours and then transferred to a vile containing as-synthesized (without any
ligand exchange) nanocrystal solution in toluene. We observe that strategy A leads to
much higher adsorption of QDs on the nanowire substrate resulting in enhanced
photocurrent and hence enhanced overall efficiency. The effect of two sensitization
strategy is discussed below. Figure 3.3c shows a TEM image of a TiO
2
nanowire
sensitized with QDs using strategy A. Individual nanocrystals can be clearly seen on the
surface of nanowires as marked by black circles, shown in the high resolution TEM
image (figure 3.3d).
The efficiency of charge injection from QD to TiO
2
nanowires depends critically
on the nature of interface between them in terms of the covalent binding between
nanowire and QDs. In principle, for obtaining a high efficiency cell, two criteria are
extremely important, i.e. high loading of QDs on the nanowire surface and defect-free
interface with good electronic coupling between each QD and TiO
2
. Different QD
deposition strategies have been reported in literature including electrodeposition [39],
SILAR deposition [40], and bifunctional linker approach [14]. Out of these, the approach
using a bifunctional linker molecule has yielded the most promising results. Typically, II-
VI nanocrystals synthesized in coordinating or noncordinating solvents have large
surfactants such as TOP, TOPO, oleic acid on the surface, which act as an insulating
barrier for electron injection. A bilinker molecule such as SH-R-COOH is an optimal
linking molecule where SH-end binds with the nanocrystal surface and COOH- group
attaches with the metal oxide surfaces. Charge injection efficiency also depends on the
length of the chain, R and smaller chains result in efficient electron injection.
81

Although it is clear that a sensitization strategy involving SH-R-COOH molecules yields
good results, we argue that the overall cell efficiency depends critically on the way this
sensitization step is carried out. The ligand exchange strategies, and the general surface
modification methods used to bind colloidal QDs to TiO
2
, are illustrated in scheme 1.
Traditionally used approach is shown in scheme 1a where surface modification of freshly
prepared TiO
2
particles is first carried out by exposing the substrate to a concentrated
solution of MPA, and then exposed to a solution of unmodified QDs (i.e. capped with
TOP, oleic acid) [36]. We argue that this approach does not lead to high loading of QDs
because it is difficult for SH- group to replace the bulky TOPO/TOP molecules from the
surface of QDs at room temperature. A more prudent way is to carry out ligand exchange
of reversibly bound, bulky organic ligands from the QD surface first with MPA
molecules (as depicted schematically in scheme 1b) which will ensure that SH- groups
are bound to nanocrystals and COOH- groups are open to binding with nanowire surface
(strategy B). Norris et. al [28] had used similar approach to sensitize CdSe nanocrystals
with ZnO nanowires in conjunction with oxygen plasma treatment of the nanowire
substrate.
Following the above arguments, we fabricated cells with two different
sensitization strategies and observed that ligand exchange of QDs before attaching with
TiO
2
nanowires yields more efficient cells. After sensitization of nanowires with QDs, an
FTO substrate with 10 nm of e-beam evaporated Pt was used as the counter electrode.
The two electrodes were clamped together with a parafilm spacer in between. Finally, the
82

space between the electrodes was filled with the electrolyte (iodyte IN-50, as received
from solaronix) to complete the cell fabrication.

3.3.3 Solar Cell Performance:
The current-voltage (I-V) characteristics of the solar cell were measured using an Agilent
parameter analyzer. A Xe-lamp fitted with an AM 1.5G filter was used as the
illumination source. Under dark conditions, both devices A and B showed typical diode
behavior while upon illumination, significant photovoltaic effect was observed. Figure
3.4a shows the I-V characteristics for the device B in which QDs were used without
external ligand exchange (strategy B). Under AM1.5G illumination, the device exhibited
a short circuit current density of 0.5 mA/cm
2
, an open circuit voltage of 0.67 V, and a fill
factor of 65%, resulting in the power conversion efficiency of 0.2 %. It is important to
note that a fill factor of 65% obtained in our
Figure 3.4. (a) I-V characteristics (black and red curve) and output power characteristics
(open squares curve) of TiO
2
-CdSeS cell fabricated using strategy B. (b) I-V
characteristics (black and red curve) and output power characteristics (open square curve)
of the cell fabricated using strategy A.
-0.2 0.0 0.2 0.4 0.6 0.8
-0.5
0.0
0.5
1.0
1.5
Power Density (mW/cm
2
)

Dark
Light
Current Density (mA/cm
2
)
Voltage (V)
-0.01
0.00
0.01
0.02
0.03
Power Density
-0.2 0.0 0.2 0.4 0.6 0.8 1.0
-2
0
2
4
6
Power Density (mW/cm
2
)

Current Density (mA/cm
2
)
Voltage (V)
Dark
Light
0.0
0.5
1.0
Power Density
ab
83

device is the highest reported for any QD-sensitized solar cell both on TiO
2
nanoparticle
anode and on TiO
2
nanotube anode. We speculate that single crystalline nature of every
nanowire results in significantly reduced recombination centers for photogenerated
charge carrriers, facilitating our remarkably high fill factor values.
On the other hand, device A, in which nanocrystals underwent ligand exchange
step before attaching with the TiO
2
nanowires (strategy A), exhibited a 4-fold increase in
short-circuit current resulting in total power conversion efficiency of 0.6%. We argue that
this increment in short circuit current is a direct consequence of the fact that device A had
much more concentration of CdSeS nanocrystals attached onto the nanowires than in
device B, and a better electronic binding between QD and TiO
2
. Presence of excess bulky
molecules such as TOP, oleic acid, etc. in the sensitization solution inhibit QD binding.
Previous work on ligand dynamics of colloidal QDs best describes the deleterious effects
of excess organic surfactants on the observed low photocurrent yields [41].

In the in-situ
ligand exchange approach, sensitization by as-synthesized QDs on TiO
2
-MPA surface
becomes unfavorable due to inability of MPA molecules to penetrate into the TOP/oleic
acid ligand shell which would be required for good electronic coupling. Also, since the
in-situ exchange is performed in the organic solvent, deprotonation of surface bound SH-
group is not favored which is required to form stable Cd-thiolate bond. This is consistent
with the pH dependent study of MPA-CdSe binding and dissociation by Peng and co-
workers [42].
Based on these arguments, we propose that the in-situ ligand exchange approach
leads to inefficient loading and binding of QDs to TiO
2
, resulting in the observed low
84

photocurrents of devices made using this approach. For the devices made using approach
A, we note that although short-circuit current increased significantly, open circuit voltage
and fill factor of the cell reduced marginally. For instance, open circuit voltage reduced
from 0.67V for device A to 0.58 V for device B while fill factor reduced from 65% for
device A to 45% for device B. This reduction might be because of increased rate of
surface recombination at the surface of TiO
2
nanowires coming from increased number of
nanocrystal-nanowire interfaces. These results suggest that one can not keep on
increasing the length of nanowires to sensitize more numbers of QDs because although
this will enhance the loading of more numbers of light absorbing QDs, it will come at a
cost of increased numbers of recombination centers resulting in an eventual decrease in
the overall efficiency. Similar results of decreasing efficiency for increase in nanowire
length was observed recently for TiO
2
nanowire/dye solar cell [31]. This problem can be
solved, to some extent, by overcoating the nanowire surface with some other material
which will eliminate these recombination centers.
We note that the efficiency of our device is better than the previously reported
ZnO-CdSe solar cell, even when the length of ZnO nanowire in that case was 12 µm,
which is 12 times more than the length of nanowires used in our study. This result
establishes the superiority of TiO
2
nanowires over ZnO, in the present device
configuration using Iodine/Iodide redox couple. We also note that our efficiency is
comparable to that obtained in Kamat’s group using TiO
2
nanotube array [17, 22] but
again those efficiencies were obtained with much longer nanotubes.

85

3.4 Summary and Conclusion:
In summary, we utilized vertical array of TiO
2
nanowires grown directly on FTO
substrate in conjuction with CdSeS nanocrystals to obtain photovoltaic cells.
Nanocrystal-nanowire form a type II interface which drives the dissociation of excitons
created in nanocrystals following light absorption. Subsequently, separated electrons are
transported via single crystalline TiO
2
nanowires and holes are transported through the
liquid electrolyte media. We carried out detailed study on quantum dot attachment
strategies and demonstrate that ligand exchanged QDs together with oxygen plasma
treated nanowires leads to an efficient sensitization of nanowires. This architecture
exploits advantages associated with QDs in terms of harvesting wider spectrum of light
where traditional dye have serious limitations, and with nanowire morphology which
provides direct electrical pathways for efficient electron transport. Future work will
require replacing corrosive liquid electrolytes and exploiting multiple exciton generation
properties of QDs to achieve even higher efficiencies.

86

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nanowires with CdSe quantum dots for photovoltaic devices. Nano Lett., 2007.
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90

Chapter 4. Photoelectrode-Infiltrating Hole Transport Media
by Physical Vapor Deposition on Hydrothermally Grown TiO
2

Nanowire Arrays for Solid State Dye-Sensitized Solar Cells

The widespread applicability of dye-sensitized solar cells (DSCs) is hindered by the
volatile solution phase electrolyte system. Also, an array of vertically aligned nanowires
constitutes optimal anode architecture due to potential increase in charge collection
efficiency. In this paper, we report a solid state dye sensitized solar cell (sDSC)
constituting a novel hole transporting layer deposited by a conformal physical vapor
deposition method to obtain TiO
2
-infiltrating domains of a highly conductive hole
transport medium comprising 1,4-bis(2-naphthylphenylamino)benzene (NNP) doped with
tetrafluorotetracyano-p-quinodimethane (F
4
TCNQ). We demonstrate that complete
infiltration by physical vapor condensation of the HTM from deep within an array of
isolated single-crystal nanowires facilitates regeneration of the dye following
photoinduced electron transfer from the dye to the nanowire array. Using a 200 nm long
TiO
2
nanowire array in conjunction with N719 dye and vapor deposited NNP layer, we
obtain solar cells with 0.2% efficiency. This work constitutes a promising new route to
achieving a highly infiltrating HTM system for stable solid state dye sensitized solar cells.

91

4.1 Introduction

An ever-increasing demand of energy has stimulated significant amount of research
efforts towards developing photovoltaic cells [1, 2].

Among various technologies,
excitonic solar cells [3] including organic, hybrid organic-inorganic, and dye-sensitized
solar cells (DSSCs) are prime candidates for low-cost, highly-efficient solar cells.
Among these, DSSCs have significant promise, resulting from their relatively high power
conversion efficiencies, and continue to improve since first proposed by Gratzel [4]. In a
typical cell, a dye molecule is adsorbed to the surface of a high surface area wide-gap
semiconducting metal oxide.
An incident photon absorbed by the dye results in an electronically excited state
of the molecule. Subsequently, rapid photoinduced electron transfer from the dye into the
conduction band of the metal oxide, commonly a mesoporous network of TiO
2

nanoparticles, can result if the energy required to remove an electron from the
electronically excited dye molecule, the excited state ionization energy (E
i
*), is less
negative than the conduction band edge of the metal oxide. The reversibly oxidized dye
is then regenerated via an iodide electrolyte that reacts with the dye cation to form
triiodide. This I
-
/I
3
-
redox couple facilitates hole collection at a counter electrode. In the
past decade, efforts have been focused on two main areas namely, to replace mesoporous
TiO
2
nanoparticle anode with one-dimensional nanowire/nanotube arrays to improve
charge collection efficiencies [5-9] and to replace volatile organic solvent-based
electrolyte systems traditionally used in these cells [10].

92

Researchers have used TiO
2
nanotube arrays grown on both Ti foil [5, 7, 9]

and on
transparent conducting oxides (TCOs) [11, 12] as photoanodes. More recently, our group
[13] and others [14, 15] have grown vertically aligned arrays of TiO
2
nanowires directly
on TCOs and subsequently used them for traditional solution based dye sensitized solar
cells. The electron diffusion coefficients for mesoporous nanoparticle networks are
several orders of magnitude lower than their single-crystalline counterparts. As a result,
single-crystal nanowire arrays have the potential to significantly enhance cell
performance by increasing charge collection efficiency [16]. Moreover, the possibility to
carefully control enhanced local photon absorption via a tailored array of nanostructured
scattering sites is also attractive [17]. In this regard, it is compelling to investigate an
ordered and highly interconnected nanowire array as the model photoanode architecture.
The volatile solution phase electrolyte system, typically comprising acetonitrile
and iodide/triiodide, deployed for dye regeneration in traditional dye-sensitized solar
cells (DSCs) represents a stability and packaging limitation to this otherwise potentially
economically viable solar technology. To address the problems of corrosion and potential
leakage associated with these liquid systems [18], there is substantial interest in
developing solid-state DSCs, where a highly conductive, solid-state hole-transport
medium (HTM) replaces the solution electrolyte system. Currently, HTM deposition is
carried out via solution-based techniques, where materials such as poly(3-hexylthiophene)
P3HT

[19-21]

and spiro-OMeTAD(2,20,7,70-tetrakis-(N,N-di-p-
methoxyphenylamine)9,90-spirobifluorene [22-25] are dissolved in a solvent and drop
cast or spin cast onto the substrate. However, these techniques rely strictly on capillary
93

action [26] for the uptake of HTM molecules and require concomitant solvent removal
from deep within the nanostructured device. As a result, a considerable mass transport
limitation exists for solution-based deposition methods, due to the required transverse
motion of solute molecules with respect to the surrounding solvent medium.
In this regard, solution based deposition techniques are unattractive for achieving
a high quality conformal HTM coating of the photoanode surface that ensures contact of
the HTM domains to the counter electrode. This is because the oxidized dye molecule
must be rapidly reduced, regenerating the neutral species, following photoinduced
electron injection from the dye to TiO
2
. Correspondingly, hole transport to the anode
must be facile, in order to avoid performance losses. In a solid-state dye-sensitized solar
cell (sDSC) efficient dye regeneration requires intimate orbital overlap between the dye
and the molecules of the HTM. Consequently, physical infiltration of molecules
employed in the HTM is critical to achieving high performance [26]. Since achieving
intimate infiltration of HTM materials has proven to be quite challenging using solution
based fabrication methods,

it is compelling to examine alternative HTM deposition
techniques.
In this study, we demonstrate for the first time, the feasibility of employing a
conformal physical vapor condensation technique, known as organic vapor phase
deposition (OVPD), to grow TiO
2
-infiltrating domains of the HTM material 1,4-bis(2-
naphthylphenylamino)benzene (NNP) in intimate contact with the dye layer of sDSC
devices prepared on a TiO
2
nanowire array. This device fabrication process, which is
expected to be scalable and relatively inexpensive, occurs across a sharp thermal gradient
94

by condensation of the HTM material from deep within the interstitial spaces of the
nanostructured TiO
2
electrode, with the primary advantage of being completely solvent-
free. Thus, immediately upon deposition, we generate the desired molecular orientation,
with intimate contact between the HTM and the adsorbed dye on the surface of the TiO
2
.
Rather than relying on capillary action for the uptake of HTM molecules and subsequent
molecular reorientation during solvent removal, OVPD occurs locally at the solid-vapor
interface, with only an inert carrier gas as the surrounding medium. In principle this is
highly desirable, because it eliminates the mass transport problems associated with
solution deposition methods, for which the dynamics or solvent removal will tend to
create voids and non-uniformities in the HTM. Thus, in addition to facilitating extensive
infiltration, the OVPD technique is also expected to yield a highly desirable HTM
structure, by reducing the tendency to form high resistance grain boundaries in the HTM
layer, which can also result from solution-based deposition methods. Using a model 200
nm long nanowire photoanode and a vapor deposited HTM layer, we demonstrate
photovoltaic cells with power conversion efficiencies up to 0.2%.

4.2 Experimental Details:
4.2.1 Synthesis of TiO
2
Nanowires:
In a typical synthesis, the FTO substrate was ultrasonically cleaned sequentially in
acetone, isopropyl alcohol, and de-ionized (DI) water for 15 minutes each and finally
dried under N
2
flow. Separately, 1 ml of a titanium butoxide was added drop-wise to a
1:1 mixture of DI water and concentrated (35%) hydrochloric acid (HCl) to obtain a clear
95

transparent solution. The substrate was placed at an angle (see text for discussion on the
effect of substrate placement angle) in a 23 ml Teflon liner and the precursor solution
was added to it. The Teflon liner was loaded in an autoclave and was placed in an oven
and the growth was carried out at different temperatures (120
o
C – 180
o
C) and for
different growth times (2 to 8 hours). Detailed synthesis conditions are discussed at
appropriate places in the text. After the growth, the autoclave was cooled down under
flowing water (takes about 15 minutes) and substrate with the nanowires is rinsed
thorough with DI water and dried with nitrogen. The substrate was then annealed in air at
450 0C to remove the residue surfactants. Every substrate used in this study was treated
with TiCl
4
to improve the solar cell performance. A 0.1 M aquous solution of TiCl
4
was
prepared by mixing appropriate amount of TiCl
4
in ice and allowing the ice to melt (note
that we can not prepare this solution by mixing TiCl
4
directly in the water as TiCl
4

undergoes hydrolysis immediately at room temperature and precipitates immediately to
give white amorphous TiO
2
). Nanowires substrates is then immersed in this solution and
heated at 60 oC for 30 minutes while observing that the TiCl
4
solution remains
transparent. After the treatment, substrate is rinsed thoroughly with DI water and
annealed in air at 450 oC for 30 minutes.
4.2.2 Solar Cell fabrication:
For the fabrication of dye-solar cell, an FTO substrate with vertical array of nanowires
was immersed in 0.3 mM ethanolic solution of cis-bis(isothiocyanato)bis(2,2 ′-bipyrridyl-
4-4 ′-dicarboxylato)-ruthenium(II)bis-tetrabutylammonium dye (N-719 as received from
Solaronix) for 12 to 24 hours, which serves as the light absorbing elements. Nanowire
96

substrates were then immersed in a 0.3 mM ethanolic solution of N719 dye for 24 hours.
The dye loaded nanowire samples were transferred to the OPVD chamber and coated
with a highly conductive HTM layer of NNP doped with tetrafluorotetracyano-p-
quinodimethane (F
4
TCNQ). A glass reactor OVPD system was used for HTM growth,
where the organics are sublimed inside a tube furnace, impelled by an inert carrier gas
(e.g., N
2
) and condense on the surface of a cooled substrate. The OVPD method allowed
us to tune the film quality by controlling ambient pressure, substrate temperature,
deposition rate, and inert carrier gas flow rate. The HTM films were grown under a
pressure of 1 Torr, inert gas flows of 40 sccm and organic deposition source temperatures
of 240
o
C - 280
o
C and 140
o
C - 150
o
C for NNP and F
4
TCNQ, respectively. Deposition
rates of 0.1 - 0.5 Å/s and 1 - 30 Å/s were used for F
4
TCNQ and NNP, respectively, with 1
- 5 % F
4
TCNQ doping concentrations. After HTM deposition, the samples were loaded
into a metal evaporator and 100 nm Cu contacts were deposited through a shadow mask
at ~ 1 × 10
-6
Torr as a counter electrode.

4.2.3 Characterization:
Scanning Electron Microspcope (SEM) images were obtained on JEOL-7001 SEM
operating at 5.0 kV voltage. Powder X-ray Difraction (XRD) analyses were performed on
a Rigaku Ultima IV X-Ray diffractometer using a Cu K α radiation source ( λ = 1.54 Å).
Transmission electron microscope (TEM) and selected area electron diffraction (SAED)
analyses were carried out on a JEOL JEM-2100 microscope at an operating voltage of
200 kV, equipped with a Gatan CCD camera. To prepare TEM samples, substrate with
97

as-grown nanowires were sonicated at mild power in IPA solution and a drop of this
solution was put on the 300 mesh Cu grid. Current-voltage characteristics was measured
using an Agilent semiconductor parameter analyzer. An Oriel 300 W Xe lamp fitted with
AM 1.5G filter was used to simulate the solar spectrum. The sample with an active area
of 0.3 cm
2
was illuminated by the light from the Xe-lamp and the corresponding
photocurrent and photo-voltage generated were measured.
4.3 Results and Discussions:
4.3.1 Device Structure and Principle
Our device structure is illustrated schematically in Figure 4.1a. A vertically aligned array
of TiO
2
nanowires was first grown on fluorine-doped tin oxide (FTO) coated glass
substrates using a hydrothermal method. The synthesis conditions are relatively mild and
have virtually no deleterious impact on FTO conductivity. Detailed synthetic procedures
were reported in our earlier work [13]. Briefly, FTO-coated glass substrates were
immersed in sequential ultrasonic solvent baths of acetone, isopropanol, and deionized
(DI) water, dried under nitrogen flow, and placed in a Teflon lined autoclave containing
the reaction mixture of 10 ml DI water, 10 ml HCl, and 1 ml titanium isopropoxide. The
autoclave was heated to 120
o
C and the reaction time varied according to the desired
nanowire length. The autoclave was cooled under flowing water, the sample removed
and thoroughly rinsed with DI water, dried under nitrogen flow, and immersed in a 0.1M
aqueous solution of TiCl
4
for 30 minutes at 60
o
C. Each nanowire array was
subsequently annealed at 400
o
C for 30 minutes in air.

F
co
se
N
E
sh

Figure 4.1.
onfiguration
ensitized wi
NNP molecu
Energy level
howing the h
(a) Schem
n. A vertical
ith N719 dy
ules, and fin
alignment
hopping assi
matic diagra
l array of T
ye molecules
nally compl
of different
isted transpo
am of the
iO
2
nanowir
s, infiltrated
eted with th
component
ort mechanism
solid state
res is grown
d with vapor
hermally ev
s of the sD
m of holes in
e dye sensi
n directly on
r deposited
vaporated Cu
SC. (c) Sc
n the doped
itized solar
n FTO subs
F4TCNQ d
u electrodes
hematic dia
NNP layer.
98
cell
strate,
doped
s. (b)
agram
99

Nanowire substrates were then immersed in a 0.3 mM ethanolic solution of N719 dye for
24 hours. The dye loaded nanowire samples were transferred to the OPVD chamber and
coated with a highly conductive HTM layer of NNP doped with tetrafluorotetracyano-p-
quinodimethane (F
4
TCNQ). A glass reactor OVPD system was used for HTM growth,
where the organics are sublimed inside a tube furnace, impelled by an inert carrier gas
(e.g., N
2
) and condense on the surface of a cooled substrate. The OVPD method allowed
us to tune the film quality by controlling ambient pressure, substrate temperature,
deposition rate, and inert carrier gas flow rate. The HTM films were grown under a
pressure of 1 Torr, inert gas flows of 40 sccm and organic deposition source temperatures
of 240
o
C - 280
o
C and 140
o
C - 150
o
C for NNP and F
4
TCNQ, respectively. Deposition
rates of 0.1 - 0.5 Å/s and 1 - 30 Å/s were used for F
4
TCNQ and NNP, respectively, with 1
- 5 % F
4
TCNQ doping concentrations. After HTM deposition, the samples were loaded
into a metal evaporator and 100 nm Cu contacts were deposited through a shadow mask
at ~ 1 × 10
-6
Torr as a counter electrode.
As shown in figure 4.1b, the excited state ionization energy ( E
i
* ≈ - 3.8 eV) of
the dye relative to the conduction band edge ( E
c
= - 4.2 eV) of TiO
2
drives photoinduced
electron transfer from the dye to the metal oxide. Moreover, the ionization energy (E
i
≈ -
4.8 eV) of NNP [27] lies between that of the dye and the work function of polycrystalline
Cu, favoring the shuttling of holes from the dye through the doped NNP layer to the Cu
electrode. Doping of the NNP layer with F
4
TCNQ, a strong oxidant, results in the
appearance of vacant electronic levels, through which charge carriers can migrate via a
localized hoping mechanism, as illustrated in figure 4.1c. The DC conductivity of this
100

chemically doped HTM is relatively high, with a value on the order of σ ~ 1.9 × 10
-3

S/cm, measured in an FTO/NNP:F
4
TCNQ/Cu planar configuration. The measured σ for
our HTM is comparable to that of archetypical hole transport layers commonly studied
for organic light emitting device applications [28, 29]

where conductivity enhancement
are believed to arise from a dopant induced narrowing of the depletion region that
facilitates tunneling at the electrical contact [28]. The electron affinity of NNP is
estimated to be less negative than E
i
* of N719 by > 2.0 eV, precluding efficient electron
transfer from the excited dye molecule to the HTM, due to the large energy barrier, ΔE ~
100 kT. As previously mentioned, the energy offset between the TiO
2
conduction band
edge and E
i
* of N719, favors electron injection from the photoexcited dye to the TiO
2

nanowire and subsequent collection at the FTO electrode. We expect the thermodynamic
open-circuit voltage limit to be governed by the energy offset between the Fermi level of
TiO
2
and the HOMO level of the NNP layer.
As previously shown, by varying the preparation conditions, we can easily control
the aspect ratio of the incorporated nanowires. The TiO
2
nanowire growth kinetics can
be found in our earlier publication [13]. Nanowires grown in this fashion are highly
crystalline as determined from high resolution transmission electron microscope (HR-
TEM) imaging (figure 4.2c) and selected area electron diffraction (SAED) analysis
(figure 4.2d). With the (001) face growing the most rapidly and the (110) face growing
more slowly, this method typically yields nanowires with a square cross-section and
sidewalls bound by {110} faces.

101

4.3.2 Infiltration of NNP in TiO
2
Structures
To minimize recombination losses, the molecules of the hole transport layer and the
adsorbed dye must be situated in close spatial proximity, within the sum of their van der
Waals radii, to insure direct intermolecular orbital overlap. This requires extensive
physical infiltration of the HTM into the interstices of the TiO
2
network. Achieving this
infiltration by liquid-based solution deposition techniques has proven difficult in practice,
apparently due to mass transport limitations. That is, solvent evaporation and premature
nucleation of the solute increase the solution’s viscosity and the HTM molecules begin to
agglomerate before a high quality conformal coating is achieved. Thus, incomplete
infiltration of the hole transport layer via liquid-based solution deposition is a significant
challenge [26].

F
w
p
o
p

In
d
n
re
Figure 4.2: S
with 10 ml o
attern of nan
f a single n
attern of a si
n contrast, O
eposition o
anostructure
espectively,
Structural C
of DI water,
nowires grow
nanowire. In
ingle nanow
OVPD emplo
of the HTM
e. Figure 4
obtained for
haracterizati
10 ml of H
wn on the gl
nset shows a
wire.
oys thermal-
M layer via
4.3a, b depic
r a typical a
ion of TiO
2
HCl, and 0.4
lass and FTO
a HRTEM im
gradient-ass
a physical
cts the top
array of vert
nanowires g
4 ml of TiCl
O substrate,
mage of the
sisted mass t
vapor con
view and c
ically aligne
grown at 18
l
4.
(a) SEM
respectively
e same nano
transport to i
densation f
cross-section
ed individua
80
o
C for 4 h
image (b)
y. (c) TEM im
owire. (d) S
induce confo
from within
nal SEM im
al nanowires
102
hours
XRD
mage
SAED
ormal
n the
mages,
with
103

diameter ~ 80 nm and a length ~ 200 nm before HTM deposition. Respectively, the top-
view and cross-sectional SEM images in Figures 4.3c and 4.3d illustrate the TiO
2

Figure 4.3. (a) Top view SEM image of as-grown TiO
2
nanowire array on FTO substrate
prior to NNP infiltration. (b) Cross section SEM image of the nanowire array showing
individual nanowires growing vertically on FTO substrate with a length of 200 nm (c)
Top view SEM image of TiO
2
nanowire array after HTM deposition showing the
encapsulated nanowires with NNP molecules forming a percolating network. (d) Cross
section SEM image of 200 nm long TiO
2
nanowires deposited with doped NNP
molecules showing excellent infiltration of HTM molecules well inside the nanowire
pores.

nanowires from Figures 4.3a and 4.3b, following a nominally 300 nm HTM deposition.
From the top-view image, the conformally coated TiO
2
array, which, prior to HTM
200 nm
200 nm 200 nm
1 µm
Before NNP Deposition
After NNP Deposition
ab
cd
FTO
NW/NNP
104

deposition appeared as sharp monolithic features, appears encapsulated with the doped
NNP material that is beginning to form a percolating network. Importantly, in Figure
4.3d we do not observe the presence of an HTM over layer on top of the nanowire array.
That would suggest substantial growth in an undesirable coplanar orientation with the
substrate surface, but little coaxial coating. Instead, we observe in the cross-sectional
image, a fairly homogenous distribution of HTM material throughout the entire height
profile of the nanowire array, obscuring the resolvability of individual nanowires under
the HTM coating. This OVPD method appears to allow the HTM to penetrate deep
inside the nanowire network and, thus, should allow collection of charge carriers from
dye molecules attached along the entire TiO
2
surface.
4.3.3 Device Characterization:
Current-density (J) as function of applied voltage (V) characteristics of the solid state dye
solar cell were measured in air at room temperature, in the dark and under spectral
mismatch corrected 100 mW/cm
2
white light illumination from an AM-1.5G filtered 300
W Xenon arc lamp (Newport Inc.). Routine spectral mismatch correction for ASTM
G173-03 was performed using a filtered silicon photodiode, calibrated by the National
Renewable Energy Laboratory (NREL) to reduce measurement errors. Frequency
modulated monochromatic light (250 Hz, 10 nm FWHM) and lock-in detection were
used to perform all spectral responsivity and spectral-mismatch correction measurement.
Figure 4.4 shows a typical J-V curve and corresponding output power density (P = JV)
curve. Red and blue traces correspond to the current density measured in the dark and
under illumination, respectively.
105

The output power density of the cell is shown as the open circle trace, for which the
maximum point on the curve corresponds to the maximum power output density (P
max
).
Our cell exhibits a typical rectifying behavior under dark and shows significant
photovoltaic effect under light exposure. For an incident power density (P
inc
) of 100
mW/cm
2
, our device exhibits a short circuit current of 0.56 mA/cm
2
, open circuit voltage
of 0.355 V, and a fill factor of 46% , resulting in a power conversion efficiency ( η
P
=
P
max
/P
inc
) of 0.1%.

Figure 4.4. Current-Voltage (J-V) characteristics of a sDSC prepared using 200 nm long
TiO
2
nanowire array. Red curve represents the behavior in dark while blue curve
represents the device performance under light illumination. Open square points represent
the power output of the devices plotted on the right axis.

The efficiency of our cells is lower compared to traditional liquid electrolyte based
devices, mainly due to lower light harvesting efficiency (LHE). Since the length of our
nanowires are only ~ 200 nm, the amount of dye present in the cell is orders of magnitude
-0.8 -0.4 0.0 0.4 0.8
-1.0
-0.5
0.0
0.5
1.0
10
-6
10
-4
10
-2
-0.15
-0.10
-0.05
0.00
0.05
0.10
0.15
(A/cm
2
)
(mA/cm
2
)
Voltage (V)

Current density
Dark (mA/cm2)
Light (mA/cm2)
Power (mW/cm2)
Power Density (mW/cm
2
)
106

lower than those in TiO
2
nanoparticle based devices, where the typical thickness of TiO
2

layer is 5-8 µm. Using dye desorption analysis, surface coverage of 0.586 nmol/cm
2
was
obtained, which is significantly lower than thick nanoaprticle film based devices. Based
on the SEM image analysis, an individual nanowire comprises 5 faces available for dye
adsorption, the area of the top face being 80 nm × 80 nm and the area of side face being
200 nm × 80 nm. The total surface area of one nanowire is 70400 nm
2
and assuming a
packing density of 25%, a weight density of 4.25 g/cm
3
for rutile TiO
2
, a specific surface
area of 12.9 m
2
/g was calculated. Assuming an area of 1.46 nm
2
occupied by one dye
molecule, a geometric roughness factor of ~ 5 was calculated. Figure 5b shows the plot
of light harvesting efficiency as a function of wavelength calculated as

Where Γ represents the no. of moles sensitized per unit area in mol/cm
2
, and σ is
absorption cross section in cm
2
/mol
Figure 4.5. (a) External quantum efficiency (EQE) measurement of the sDSC fabricated
with 200 nm long TiO
2
nanowire array. (b) Light harvesting efficiency (LHE)
measurement of the device.

LHE( ) 1 10
 .  ( )
400 500 600 700 800
0
5
10
15
Quantum Efficiency (%)
Wavelength (nm)
400 500 600 700
0
5
10
15
20

Light Harvesting Efficiency (%)
Wavelength (nm)
ab
107

As shown in Figure 4.5b, the peak LHE is only about 17% suggesting that the efficiency
of our device is mainly limited by the poor light harvesting. In order to increase the
amount of dye loading, we used nanowires of ~ 400 nm length and observed an increase
of 33% in the power conversion efficiency, due largely to an increase in short circuit
current density (J
sc
), from 0.6 mA/cm
2
for the 200 nm TiO
2
devices to 0.9 mA/cm
2
for
the 400 nm TiO
2
devices (figure 4.6). Note that we observe less than 2-fold increment in
J
sc
despite increasing the nanowire length by a factor of two. This can be understood
based on the following. The extended growth time allotted for increasing the nanowire
length also gave rise to larger diameter nanowires in addition to increased length. This
reduces the packing density of nanowires on the substrate and, thus, reduces the total
internal surface area available for dye adsorption. Presently, further attempts to increase
the nanowire length using extended growth time have been met with limited success,
since the titania eventually coalesces and the opening to the interstitial space becomes
constricted.

Figure 4.6 J-V characteristics for 400 nm long TiO2 nanowires sensitized with dye and
infiltrated with NNP molecules.
-0.8 -0.4 0.0 0.4 0.8
-2
-1
0
1
2 10
-8
10
-6
10
-4
10
-2
-0.2
-0.1
0.0
0.1
0.2
(A/cm
2
) (mA/cm
2
)
Voltage (V)

Current density
Dark (mA/cm2)
Light (mA/cm2)
Power (mW/cm2)
Power Density (mW/cm
2
)
108

Growth of longer nanowires with small diameter may significantly enhance the
performance of these devices, which will be the focus of future work.
Figure 4.5a shows the external quantum efficiency measurement performed on a
200 nm long TiO
2
nanowire sample. The device exhibits an efficiency of ~ 5% at the
peak absorption wavelength of dye. In order to ascertain the origin of photocurrent, we
fabricated controlled devices with TiO2 nanowires infiltrated with doped NNP molecules
but without the sensitization with dye molecules. As expected, we observe no
photocurrent from this device in J-V measurement (figure 4.7). Also, the quantum
efficiency measurement results in featureless spectra confirming the fact that all the
photocurrent is mainly contributed by the dye molecules.

Figure 4.7. I-V characteristics for control device (FTO/TiO
2
/NO DYE/NNP/Cu)

-0 .4 0 .0 0 .4 0 .8
-1 0
0
10
20
30
(mA/cm
2
)
Vo lta g e (V )

Current density
Lig h t- D a r k
D a rk (m A /c m 2 )
Lig h t (m A /c m 2)
400 500 600 700 800
0
5
10
15
Quantum Efficiency (%)
QE% test
Wavelength (nm)
109

4.3.4 Discussion on NNP as potential HTM
In principle, a hole transporting electrolyte in the dye-solar cell should have the following
characteristics: (1) The conductivity of electrolyte should be extremely high (2) it should
infiltrate completely inside the TiO
2
network to collect all the photo generated charge
carriers (3) its ionization energy should less negative than that of the dye molecule in
order to facilitate hole transport. We discuss each of these characteristics for our vapor
deposited hole transport layer in the following:
(1) We control the conductivity of our NNP layer by doping the film with F
4
TCNQ.
To estimate the HTM conductivity, we deposited doped NNP layers on pristine
FTO substrates and evaporated 100 nm Cu contact pads. A typical two probe I-V
measurement of a 5% F
4
TCNQ doped NNP layer can be found in figure 4.8. As
previously discussed, the conductivity for this doped HTM materials is on the
order of 2 mS/cm.

Figure 4.8. I-V for NNP:F
4
TCNQ HTM between FTO and Cu (no TiO2, no dye)
-1.2 -0.8 -0.4 0.0 0.4 0.8 1.2
-1000
-500
0
500
1000
(mA/cm
2
)
Voltage (V)

Light-Dark
Dark (mA/cm2)
Light (mA/cm2)
110

(2) As shown in figure 4.2, our vapor deposited NNP layer infiltrates very well inside the
nanowire network. This is achieved via deposition rate optimization. We varied
deposition rate of NNP molecules from 1A/s to 30 A/s and observed that a growth rate
more than 20 A/s resulted in poor devices.
(3) The HOMO – LUMO levels of NNP layers are estimated to be -4.8/-1.3 eV [27],
making this material, when doped with F
4
TCNQ, an attractive candidate for transporting
holes from the photooxidized dye molecules on the TiO
2
surface.
In addition to all these characteristics, our vapor deposited molecules presents volatile
solvent free electrolytes which constitutes a solid state layer. Use of solvent based
electrolytes forces strict encapsulation requirements for the outdoor application of cells.
A vapor deposited solid state hole transport layer is a paradigm shift in this direction and
can significantly increase dye-solar cell’s outdoor application and can push this
technology closer to commercialization.

111

4.4 Conclusion:

A vapor deposited hole transport layer constitutes a paradigm shift in the direction of
dye-sensitized solar cells. It addresses one of the most challenging issue of cell stability
and corrosion caused by the use of volatile solvent based liquid electrolytes in the
traditional solar cells. In addition, it also significantly reduces the packaging and
encapsulation requirements of the traditional solar cell. In this work, we successfully
demonstrate a solid state dye-sensitized solar cell using F
4
TCNQ doped NNP molecules
as the hole transport layer. We chose a vertically aligned array of TiO
2
nanowires grown
directly on a transparent conductive oxide as a model photoanode to demonstrate the
concept of a novel vapor deposited HTM layer in dye-sensitized solar cells, for the first
time. Aligned array of TiO
2
nanowires constitute a perfect model for this type of study
because it provides open spaces to the NNP molecules all the way to the end of
nanowires and thus maximizing the overlap between the dye molecules and HTM. This
study will not be possible with TiO
2
nanoparticle based photo anodes, as used in
traditional solar cells. In addition to being a model system, nanowire array is potentially
an optimal anode architecture. By virtue of the single-crystalline nature of every
nanowires, they are inherently free from grain boundaries, which work as scattering
centers for charge carriers and thus limiting their transport. Using a nanowire array with
200 nm long TiO
2
nanowires, N719 dye molecules, and F
4
TCNQ doped NNP molecules
as hole transport layer, we demonstrate cells with 0.2 % of overall power conversion
efficiency.

112

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115

Chapter 5. Uniform, Highly Conductive, and Patterned
Transparent Films of Percolating Silver Nanowires Network
on Rigid and Flexible Substrates using Dry Transfer
Technique

Silver nanowire films are promising alternatives to tin-doped indium oxide (ITO) films as
transparent conductive electrodes. In this paper, we report the use of vacuum filtration
and Polydimethylsiloxane (PDMS)-assisted transfer printing technique to obtain silver
nanowire films on both rigid and flexible substrates, with advantages such as capability
of patterned transfer, best performance among various ITO alternatives (10 Ω/sq at 85%
transparency), and good adhesion to the underlying substrate, thus eliminating the
previously reported adhesion problem. In addition, our method also allows the
preparation of high quality patterned films of silver nanowires with different line widths
and shapes in a matter of few minutes, making it a scalable process. Furthermore, use of
anodized aluminum oxide (AAO) membrane in the transfer process allows annealing of
nanowire films at moderately high temperature to obtain films with extremely high
conductivity and good transparency. Using this transfer technique, we obtain silver
nanowire films on flexible polyethylene terephthalate (PET) substrate with a transparency
of 85 %, sheet resistance of 10 Ω/sq, with good mechanical flexibility. In-depth analysis
reveals that the Ag nanowire network exhibits 2D percolation behavior with good
agreement between experimental and theoretical values.

116

5.1 Introduction:
Materials with a remarkable combination of high electrical conductivity and optical
transparency are important components of many electronic and optoelectronic devices
such as liquid crystal displays, solar cells, and light emitting diodes [1-3].

Doped-metal
oxides such as tin-doped indium oxide (ITO) and fluorine-doped tin oxide (FTO) have
been leading candidates for these technological applications [4-6]. Although ITO and
related materials have witnessed 50 years of extensive research and consequent
applications, recently, there has been a quest to look for materials that can replace ITO as
the leading transparent conductive electrode (TCE). This need mainly arises because of
the rising cost of ITO (due to low availability of indium sources), its brittleness and hence
limited applicability in flexible electronic devices, and high temperature processing step
required to fabricate it.
Towards this objective, various alternatives have been pursued by different
groups, among which thin metal films [7, 8] and metal grids [9, 10] have shown
performance comparable to ITO. However, these approaches require processing and
fabrication using high vacuum equipments and shows limited bending ability. Carbon
nanotube (CNT) films [11-15] and, recently, graphene films [16-19] have attracted
significant attention and have been successfully used as TCE in organic light emitting
diodes [20-22] and solar cells [23, 24]. However, their performance in terms of sheet
resistance and transparency are still inferior to ITO, creating a need to continue the quest
for new materials which can act as cheap and reliable TCE.
117

Very recently, two pioneering approaches were reported in which metal nanowire
networks were used to fabricate TCE [25, 26] and successfully used metal nanowire TCE
in organic solar cell [26]. Both reports represent significant and exciting advancements in
the effort to replace ITO; however, silver nanowires used by Lee et al. [26] had a
surfactant coating of Polyvinylpyrrolidone (PVP) molecules which require a high
temperature (200
o
C) annealing step to obtain good conductivity. This limits these
nanowires’ application on flexible polyethylene terephthalate (PET) substrate. Transfer
method reported by De et al. [25] enables transfer of silver nanowire films to flexible
substrates, but this method suffers from poor adhesion between nanowire film and PET,
and is inherently a chemical intensive process requiring very careful and exhaustive
transfer technique.
In this work, we demonstrate a facile PDMS-assisted dry transfer printing
technique [15] to fabricate TCE using Ag nanowires, with advantages including easy and
fast transfer to both rigid and flexible substrates, good adhesion between the nanowire
film and the substrate, and patterned transfer of various line widths and geometries to
obtain pixilated electrodes. In past, Rogers’ group has studied stamp-assisted dry printing
technique extensively and has demonstrated printing of various metal and semiconductor
nanostructures [27-31]. Furthermore, use of AAO membrane allows annealing of
nanowires which results in fusion of nanowires. Thus, superior quality silver nanowire
films were obtained with a sheet resistance (R
sh
) of 10 Ω/sq at a optical transmittance (T)
of 85 %. This R
sh
value at 85% transparency is at least an order of magnitude lower than
CNT and graphene films, and is comparable to Ag nanowire electrodes values of R
sh
= 13
118

Ω/sq with T = 85% [25] and R
sh
= 16.1 Ω/sq with T = 86% [26]. In addition, in-depth
study of the conduction behavior of our films using percolation theory [32-34] shows
critical exponent value ( α) of 1.42 which represents 2-dimensional (2D) percolation
behavior, with good agreement between experimentally observed and theoretically
predicted value of critical volume.

5.2 Experimental Details:

Silver nanowires used during the fabrication of film were purchased from Seashell
Technologies supplied with a concentration of 12.5 mg/ml in isopropyl alcohol. The
dispersion was further diluted to a concentration of 0.208 mg/ml in isopropyl alcohol
which was subsequently used in all the transfer process. PDMS stamps were fabricated
using SYLGARD 184 silicone elastomer kit (Dow Corning, Inc.). Patterned PDMS was
made by standard optical lithography using SU-8-50 resist (MicroChem, Inc). Silver
nanowire dispersion was vacuum filtered by using AAO filter (47 mm, 0.2 µm, Whatman,
Inc). The nanowire film was transferred on PDMS by making a conformal contact
between the filter and stamp. Glass substrates were functionalized by 3-
aminopropyltriethoxysilane (APTES) by dipping it in 1 mM solution for 20 min and then
blow dried. No functionalization was performed on PET. The receiving substrate was
placed on the hot plate at 120
o
C, and PDMS was pressed against it for ~ 1 min. The
receiving substrate was removed slowly and the silver nanowire film was obtained.
Optical transmission spectra were obtained using Cary Varian 50 Conc, with bare
PET used as reference. Scanning electron microscopy measurements were made by using
119

JEOL 7001 field emission scanning electron microscope. Sheet resistance was measured
using four probe techniques by depositing silver paint at the corners in square shape with
side of ~3 mm and at least 10 locations across the sample, and values reported is the
average value obtained across the entire film. Agilent 4156B was used as a source meter
for sheet resistance measurement. The nanowires on the AAO membrane were annealed
in air on hotplate at 200
o
C for various durations. Bending test was performed by placing
the film between two platforms and bent by reducing the distance between the platforms.
Sheet resistance measurements were performed at various bending angles by placing four
probes on the films with silver paint at the corners (Figure 5.5 inset).

5.3 Results and Discussions:

Figure 5.1(a) shows the schematic of the transfer process. Briefly, a measured amount of
silver nanowire suspension (as received from Seashell Technologies, without any
intentional surfactant coating) in isopropanol (IPA) is filtered on a commercially
available AAO membrane to obtain uniform film of nanowires. A PDMS stamp is
brought in contact with the AAO membrane and the nanowire film is picked up on the
stamp with a yield of 100 %. The PDMS stamp with nanowires is pressed against the
receiving substrate at a moderate temperature (see experimental section). The pressure is
released after a few minutes and the PDMS stamp is peeled off slowly from the substrate
resulting in transfer of nanowires to the substrate. The entire process takes just about a
few minutes and the size of the transfer is limited only by the size of the starting AAO
120

membrane. We note that the same PDMS stamp and the AAO membrane can be used
multiple times.
We used both glass and PET substrate to transfer large area nanowire films. Figures 5.1(b)
and 5.1(c) show the optical micrographs of the nanowire films on PET and glass,

Figure 5.1. (a) Schematic representation of transfer process flow. i) press PDMS stamp
against the silver nanowire film on AAO membrane, ii) Peel off the PDMS stamp, iii)
press PDMS stamp with Ag nanowires on the receiving substrate, and iv) peel off PDMS
stamp leaving nanowire film on the receiving substrate. (b) Photograph of silver
nanowire film transferred on PET. Arrows show the boundary of the nanowire film. (c)
Photograph of nanowire film on glass substrate. (d) Nanowire film on PET showing the
flexibility of film. (e) Photograph showing the results of adhesion test. Nanowires remain
adhered to the PET substrate when peeled off using a sticky tape from the area shown by
dotted lines. (f)-(i) SEM images of the left, top, right and bottom region of the film
shown in Fig. 1 (b) demonstrating film’s uniformity across the entire area.

d e c
10 µm
f
10 µm
g
10 µm
h
10 µm
i
b
a
Left Top Right Bottom
121

respectively. Figure 5.1(d) illustrates the flexibility of the transferred film on PET. The
films look highly transparent as the letters in the background can be clearly seen through
the film. We note that while nanowire film could be transferred on PET substrate without
the aid of any adhesive layer, the film couldn’t be transferred on the bare glass substrate.
This can be explained in terms of weak adhesion strength of noble metals, e.g
silver, towards highly polar SiO
2
surface [35]. On the other hand, PET surface can be
characterized by a dominant dispersive component in its surface tension promoting
stronger adhesion to noble metals [35]. In order to achieve good transfer on the glass
substrate, we functionalized glass surface by depositing a monolayer of 3-
aminopropyltriethoxysilane (APTES) molecules, which makes the surface slightly
positively charged because of the presence of NH
2
groups, facilitating the transfer of
nanowires [36]. SEM inspections of the transferred film on PET substrate reveal that
transfer process is extremely uniform over the entire area of the film resulting in uniform
density of nanowires everywhere on the substrate (Figure 5.1 f,g,h,i). We note that our
nanowire film adhere strongly to the underlying PET substrate, as opposed to the
previous report where the adhesion between nanowires and PET was very weak, and the
nanowires could be easily peeled-off from the substrate using a sticky tape [25].
To demonstrate this behavior, we tried to peel off the nanowire film from PET
using a sticky tape by firmly attaching it in the region shown by the dotted line in figure
5.1(e), but nanowires remained on the PET without any visible change to the film
exhibiting their strong adhesion with the substrate. We believe this is a result of strong
conformal pressure applied by the PDMS stamp as opposed to the flexible cellulose

m
T
su
in
o
m
P

F
p
n
(c
on
membrane us
The ability to
uch as OLE
nherently a c
f nanowires
method suffe
atterning, in
Figure 5.2.
atterned PD
anowire film
c) photo-ima
ne pixel.
sed in previo
o obtain in-p
ED. Our dry
clean proces
on the subs
ers from the
n our case, is
Patterned t
DMS stamp i
m transferred
age of a patt
ous study [25
plane pixilat
y contact pr
ss; it also all
strate. We n
e limitation
s simply achi
transfer of s
in contact w
d on the PET
terned film (
5].
tion is a crit
rinting meth
ows patterni
note that prev
of obtainin
ieved by usin
silver nanow
with the nan
T substrate.
(d) SEM ima
tical require
hod is not o
ing of the na
viously repo
ng patterned
ng a patterne
wire film. (a
owire film.
The size of
age showing
ement for an
only solvent
anowire film
orted cellulo
d films over
ed PDMS st
a) Schematic
(b) Photogr
each pixel i
g the nanow
ny display d
t-free, maki
m to obtain p
ose-based tra
r the large
amp in place
c diagram o
raph of patte
is 1 mm × 1
ire network
122
device
ing it
pixels
ansfer
area.
e of a
of the
erned
mm.
from
123

plane stamp. The entire process of transfer remains the same giving an unprecedented
ability to obtain both patterned an unpatterned uniform nanowire films on any substrate.
The pattern PDMS stamp illustrated was fabricated using standard optical lithography.
Figure 5.2(a) shows the schematic of the patterned PDMS stamp and figure 5.2(b) is an
optical micrograph of patterned nanowire film on PET substrate. The length of the
printed squares is 1mm with a spacing of 0.5 mm between them. SEM inspection of the
transferred region shows high density nanowires on the substrate, suggesting high fidelity
of the patterned transfer process (Figure 5.2(d)). Several kinds of patterns can be
fabricated with the minimum feature size limited by the resist used in optical lithography
to pattern the stamp.
In addition, use of AAO membrane to deposit the film of silver nanowires also
allows the annealing of the film to get further enhanced electrical conductivity in the
films, unlike cellulose membrane which cannot withstand high temperature. To study the
effects of annealing, we prepared Ag nanowire films of various densities on AAO
membrane, and then each sample was split into two pieces after which one went through
annealing, and other did not. All the films were then transferred to PET using PDMS
stamp, followed by thorough characterization. Figure 5.3(a) shows the sheet resistance vs.
transparency plot of the nanowire films with different densities for samples with and
without annealing.
As expected, samples with higher transparency typically showed higher sheet
resistance. Importantly, there was a significant decrease in the sheet resistance for films
annealed at 200
o
C for 20 minutes. For example, a sample with a transparency of 85% (at
124

550nm wavelength) had a sheet resistance of 30 Ω/sq without annealing while the
annealed sample showed a sheet resistance of 10 Ω/sq. The sheet resistance of 10 Ω/sq at
85% transparency for our nanowire film is comparable to both drop-coated film (16.1
Ω/sq at T=86%) [26] and cellulose membrane assisted film (13 Ω/sq at T=85%) [25].
Figure 5.3. (a) Sheet resistance vs. transmittance plot of silver nanowire films. Black
dots represent resistance values without annealing and red dots represent resistance
values with annealing the samples at 200
o
C for 20 minutes in air. Inset shows the
transmittance spectrum of the nanowire films with different thicknesses. (b) SEM image
of an annealed nanowire sample showing melting of nanowires at the ends and
subsequent joining with neighboring nanowires. (c) Variation of sheet resistance with the
annealing time for nanowires films of different densities. Sheet resistance first decreased
with annealing and subsequently increased with further annealing (d) SEM images of
nanowire films prepared using various amounts of nanowires. Nanowire density increases
with increased concentration.

10 μm
0.3 ml
10 μm
0.75 ml
10 μm
8 ml
10 μm
3 ml
200 nm
0 50 100 150
0.1
1
10
100
1000

Sheet Resistance ( sq)
Anealing Time (Minutes)
0.3 ml
0.75 ml
3 ml
a
b
c
d
fusion
60 70 80 90
0.1
1
10
400 500 600 700 800
40
50
60
70
80
90
100
0.75 ml
0.5 ml

Transmittance
Wavelength (nm)
0.3 ml

Sheet resistance ( /sq)
Transmittance
Without Annealing
With Annealing

W
an
p
m
re
re

F
tr

W
n
h
We note that
nd further d
erformance.
many data p
epresentation
esistance val
Figure 5.4: D
ransmittance
We attribute
anowires be
owever in th
t different p
developmen
We also no
points measu
n of the sam
lues for a film
Distribution
e. The averag
the increase
ecause of an
heir case, the
reparation m
nt of all thr
ote that all
ured at diff
mple’s cond
m with trans
of sheet resi
ge sheet resi
e in conduc
nnealing step
e primary ob
means will l
ree preparat
our reported
ferent part o
ductivity. St
smittance of
istance for v
stance is 10
ctivity to the
p. Similar re
bjective for d
likely be use
tion method
d sheet resis
of the samp
tatistical dat
f 85% is show
various Ag n
+ 1.5 Ω/sq.
e better con
esults were
doing annea
ed for differ
ds may lead
stance value
ple, giving
ta on the v
wn in figure
nanowire sam

ntact and fus
obtained by
ling was to r
rent applicat
d to even b
es are averag
a more acc
ariation of
5.4.
mples with th
sion between
y Lee et al.
remove the P
125
tions,
better
ge of
curate
sheet
he 85%
n the
[26];
PVP-

ca
an
le
co
th
d
tr
w

F
d
d

In
w
apping agen
nnealed nan
eading to su
ontact resista
The v
he full trans
ensities. An
ransmittance
with respect t
Figure 5.5 (
ifferent den
ifferent light
n order to op
with different
nts surround
nowires on A
uperior cont
ance betwee
values of the
smittance sp
ngular depe
e changes by
to the substra
(a) Transmi
nsity (b) Tra
t incident an
ptimize the a
t densities o
ding the na
AAO membr
tact between
en nanowires
e transmittan
ectrum is sh
endence of
y ~ 10% wh
ate normal.
ittance versu
ansmittance
ngles.
annealing te
of nanowires
anowires dur
rane clearly
n them (Fig
s resulting in
nce used for
hown in figu
f transmitta
hen the ligh
us waveleng
versus wave
emperature a
s, correspond
ring the syn
shows melt
gure 5.3 (b)
n highly cond
r the plot co
ure 5.5(a) f
ance is sho
ht incident an
gth spectra
elength spec
and annealin
ding to start
nthesis. SEM
ting and fusi
)). This fur
ductive nano
rrespond to
for films wit
own in fig
ngle is chan
of the nan
ctra of the n
ng time, we p
ting Ag nano
M image o
ion of nanow
rther reduce
owire films.
λ = 550 nm
th three diff
gure 5.5(b)
nged from 0
nowire film
nanowire fil
prepared sam
owire solutio
126
f the
wires
s the

m and
ferent
The
0
o
-60
o

with
lm at
mples
on of
127

0.3 ml, 0.75 ml, 3ml and 8 ml, as shown in Figure 5.3 (d). It is evident from the figure
that the nanowires form a percolated network which becomes denser with increasing
dispersion volume. We then carried out time-dependent annealing study, with
representative results shown in Figure 5.3 (c) for samples corresponding to 0.3 ml, 0.75
ml and 3 ml starting solution. A general trend of initial decrease and subsequent increase
in sheet resistance can be seen for all the samples in figure 5.3(c). 20 minutes of
annealing at 200
o
C results in an immediate decrease of sheet resistance from 30 ohms/sq
to 10 ohms/sq.

Figure 5.6 SEM image of Ag nanowires showing the melting of nanowire and formation
of droplets when annealed at 300
o
C for 20 min on AAO membrane.

With further increase in annealing time, sheet resistance starts to increase because of
droplet formation, consistent with the observation of Lee et al [26]. When the film is
3 µm
128

heated to 300
o
C, nanowires melt completely forming small droplets and losing all the
conductivity (supporting information, Figure 5.6). This systematic study led us to the
optimized temperature of 200
o
C and an optimized time of 20 minutes to obtain highly
conductive films without disrupting the integrity of the nanowire network. A decrease in
the sheet resistance of all the films by a factor of three was observed after annealing. A
two-probe measurement of the nanowire film resulted in linear current-voltage
characteristic demonstrating the ohmic behavior of the nanowire film (Figure 5.7).
Nanowire films showed no appreciable change in sheet resistance after exposing the films
to ambient conditions for 90 days.

Figure 5.7 Two probe measurement of a silver nanowire film showing ohmic behavior of
the film.

0.00 0.02 0.04 0.06 0.08 0.10 0.12
0.00
0.02
0.04
0.06

Current (A)
Voltage (V)
129

Further analysis of Ag nanowire films was carried out by studying the percolation
behavior of the network. For a given volume of the nanowire solution (V), with a
concentration of C (0.208 mg/ml), the number of nanowires can be calculated by the
equation given by
No. of nanowires =
2
4
Ag
CV
DdL 
(1)
where D
Ag
is the density of bulk silver (10.5 gm/cc), d is the average diameter (75 nm)
and L is the average length (12.5 µm) of the Ag nanowire provided by the supplier. After
vacuum filtration, these nanowires were evenly distributed on the AAO membrane of 47
mm diameter (D). Hence, the density of nanowires, N can be calculated by dividing the
number of nanowire by area of the AAO membrane, and is given by,
N =
2
2
4
/4
Ag
CV
D
DdL


(2)
Substituting the corresponding values in the above equation, we get N = 0.20697*V µm
-2

for a given volume (V) of the nanowire solution used. According to the standard
percolation theory, the density dependence of conductivity is given by,

()
c
NN


(3)
where σ is the conductivity in three dimension or sheet conductance in two dimensions,
N
c
is the critical nanowire density required for the onset of conduction in a random
network, and α is a critical exponent which depends on the dimensionality of the
space. Theoretical value of α is 1.33 for 2D percolation network and 1.94 for 3D
percolation network [37]. For 2D network,
130

R
sh
=1/sheet conductance, relation between sheet resistance (R
sh
) and volume (V) of
nanowire solution can be obtained using equation (3) as given below.
()
sh c
RVV
 

(4)
Figure 5.8 shows the variation of sheet resistance with the volume of nanowire solution
used and the solid line represent the fitted curve obtained using equation (4).
Figure 5.8 Sheet resistance versus volume of Ag nanowire solution. The onset of
conduction across the sample occurs for V
c
= 0.2 ml. The power fit of the data indicated
value of critical exponent α = 1.42. The inset shows the logarithmic plot of the data with
a linear fit.

In order to obtain the fitting curve we used the value of V
c
as 0.20 ml as this was the
minimum volume required to achieve a conducting film. No conductive film was
0.00.5 1.01.5 2.02.5 3.0
0
25
50
75
100

Rsh
Power Fit of Rsh
R
sh
( )
Vol. of nanowire solution
 = 1.42
-3 -2 -1 0 1
0
2
4

Log R
sh
Log (V-V
c
)
131

obtained by using 0.15 ml of nanowire solution. Theoretically, for a random distribution
of nanowire model [33], N
c
for nanowire with length l is given by

4.236
c
lN  
(5)
Substituting the length of Ag nanowire (12.5 µm) in the above equation, we can calculate
the critical density for the network to be 0.03657 µm
-2
. Substituting this critical density in
equation (2), the theoretical critical volume V
c
is 0.18 ml which is very close to the
experimentally observed critical volume of 0.20 ml. We note that the value of the critical
volume obtained experimentally is a little higher than theoretical value which might be
due to the formation of multilayer stacking of nanowire in some regions as compared to
the ideal monolayer.
The best fit of the equation (4) in figure 5.8 is obtained by using α = 1.42, which
is very close and marginally higher than the value predicted for the 2D theoretical model.
The discrepancy can arise as the nanowire film is not a perfect 2D film and have an
intermediate characteristic between 2D and 3D hence resulting in a value between α =
1.33 (2D) and α = 1.94 (3D). The inset shows the logarithmic fit of the above equation
resulting in a linear plot with good agreement between the experimental and predicted
values.
Another major disadvantage of ITO films as transparent conductive electrodes lies
in its brittleness restricting its application in flexible electronics [38]. Flexible electronic
and optoelectronic devices have attracted a lot of interest in past few years with many
groups reporting flexible OLED [38],

flexible photovoltaic devices [39], flexible
supercapacitor [40], and flexible logic inverter [41], among other applications. Silver
132

nanowire film can fulfill this requirement by replacing ITO as the flexible transparent
conductive electrodes. The potential of silver nanowire film as TCE becomes even
stronger due to the fact that this added advantage of flexibility does not come at the cost
of its performance in terms of sheet resistance and transparency. As mentioned earlier,
R
sh
of 10 Ω/sq at T = 85 % for nanowire film is very much comparable to the ITO
performance. In order to test the durability of nanowire film under stressed conditions,
we carried out measurement of its sheet resistance under different bending angles. The
results are shown in figure 5.9.
Figure 5.9 Sheet resistance versus bending angle plot. Nanowire film remains conductive
under severe bending. Inset shows the photograph of the measurement setup and
definition of bending angle.

0 40 80 120 160
5
10
15
20
25
30
Forward
reverse

Sheet resistance( /sq)
Bending Angle,  (degrees)
θ
133

The inset shows the setup for the measurement of change in sheet resistance with bending
angles where the bending angle is the angle between the tangents drawn from the bent
substrate. As evident from the plot, the sheet resistance varies from 11 Ω/sq to only 17
Ω/sq when subjected to a bending angle of up to 160 degrees. Using an equivalent
thickness of 100 nm for a film with a transmittance of 80% [25], we estimate that the
average normal strain value [42] varies from 0.012 to 0.007 corresponding to bending
angles from 10
o
to 160
o
. No failure was observed up to bending angle of 160
o
. The
overall integrity and conductivity of nanowire film remains intact when the stress is
released. SEM inspections reveal that there is no appreciable change in nanowire
morphology or the network (not shown). In comparison, the conductivity of ITO film on
PET substrate reduces by three orders of magnitudes even when it was bent to only 60
degrees demonstrating its limitations for applications in flexible devices [43].
We would like to note that complete transfer for samples with higher densities
(corresponding to T% <60) could not be achieved regularly as some of the nanowires
remained adhered to the PDMS membrane. Also, the adhesion of nanowires with the
substrate degraded with the increasing density of nanowires on the substrate. We believe
this is due to lack of interaction between nanowires, which are away from the substrate,
and the substrate which results in poor adhesion of the nanowires. Although this is a
problem, it occurs only in the films with lower transmittance which do not find much
application in optoelectronic devices.

134

5.4 Conclusion
In conclusion, we demonstrated a simple, rapid, chemical-free, dry transfer printing
technique to obtain high quality silver nanowire films on both glass and PET substrate.
The PDMS assisted transfer renders extremely uniform films with strong adhesion to
PET substrate over a large area. Use of patterned PDMS stamp gives the ability to obtain
patterned nanowire films with high fidelity. Films thus obtained have extremely low
sheet resistance of 10 ohms/square at a transparency of 85% which is comparable to ITO
films. The nanowire films forms a 2D percolation network and retains its good
conductivity even under bending and recovers back to the original low resistance once
the film is released.

135

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139

Chapter 6. Scalable light-induced metal to semiconductor
conversion of carbon nanotubes
Coexistence of metallic and semiconducting carbon nanotubes in as-grown samples sets
important limits to their application in high-performance electronics. We present the
metal-to-semiconductor conversion of carbon nanotubes for field-effect transistors based
on both aligned nanotubes and individual nanotube devices. The conversion process is
induced by light irradiation, scalable to wafer-size scales and capable of yielding
improvements in the channel-current on/off ratio up to five orders of magnitude in
nanotube-based field-effect transistors. Inactivation of metallic nanotubes in the channels
was achieved as a consequence of a diameter-dependent photochemical process that led
to a controlled oxidation of the nanotube sidewall, and hence stronger localization of π-
electrons. Optimization of irradiation conditions yields nearly 90% of depletable
nanotube field-effect transistors.

140

6.1 Introduction:
The outstanding properties of single-walled carbon nanotubes (SWNTs) have earned
them numerous applications in different technological areas [1-3]. Carbon nanotube field-
effect transistors (CNTFETs) have acquired great importance due to their potential to
switch on and off much faster than current silicon technologies [4-6] and the foreseen
limits in the downscaling of silicon transistors [7, 8]. In spite of significant progress made
toward integrated nanotube circuits [4, 9-11], the assembly and integration of nanotube
electronics still faces significant challenges due to the coexistence of metallic and
semiconducting nanotubes in as-synthesized samples.
Different approaches have been followed to obtain CNTFETs containing only
semiconducting nanotubes in the channels either by selective synthesis [12-14], post-
synthesis separation methods [15-18], or post-synthesis methods to selectively etch
metallic nanotubes [19, 20]. Pioneering works that include the use of monochromatic
light irradiation [21] and broadband light irradiation [22] to selectively etch metallic
nanotubes have been reported. An alternative approach is to induce a metal-to-
semiconductor transition in carbon nanotubes. Electron beam irradiation and hydrogen
plasma have yielded metal-to-semiconductor conversion of SWNTs [23-25], but the
limited size of the electron beam and instability of the plasma represent limiting hurdles
for scalability.

141

6.2 Experimental Details
6.2.1 Wafer scale synthesis of aligned nanotubes on quartz and sapphire
Random orientation of as-grown carbon nanotubes on silicon substrates constitute one of
the main obstacles for CMOS integration; therefore aligned SWNTs were synthesized on
quartz and sapphire wafers and transferred onto silicon wafers. A thin layer of either
elemental iron or Fe (III) was deposited on sapphire and quartz substrates prior to
nanotube growth. After this, wafers were loaded inside a horizontal quartz tube of 6 inch
in diameter in a heating coil furnace. Carbon nanotube growth was performed under a
gaseous mixture of H
2
, CH
4
, and C
2
H
4
at 900 °C for 30 minutes. As shown in Figure 6.1,
large arrays of horizontally aligned nanotubes with densities between 1 and 10
SWNTs/ μm can be obtained at full wafer scales.

Figure 6.1. Wafer-scale synthesis of aligned nanotubes on quartz and a-sapphire wafers.
Photographs of the wafers after nanotube growth are shown on the left. SEM images on
the right show large arrays of highly aligned nanotubes.

(a)
a-sapphire
quartz
142

6.2.2 Transfer of aligned nanotubes and device fabrication
Wafer-scale transfer of aligned nanotubes to Si/SiO
2
wafers brings the potential to
achieve high-density two-dimensional arrays of nanotubes on silicon by repeated transfer
on the same substrate, which is needed to significantly surpass the performance of current
silicon-based CMOS technologies. Figure 6.2. Shows the schematic diagram of the
transfer process.

Figure 6.2. Schematic diagram showing the aligned-nanotube transfer process.

A 100 nm film of Au was deposited on top of the as-grown nanotubes over the entire
wafers, after which a thermally activated adhesive polymer (Revalpha tape from Nitto
Denko) was placed on the Au film. Peeling-off the tape resulted in picking up the
nanotube/Au film. The thermal tape/Au film/aligned SWNT film was placed onto the
A Au u
P Pe ee el l- -o of ff f
A Au u + + S SW WN NT Ts s
A Al li ig gn ne ed d
S SW WN NT Ts s
T Tr ra an ns sf fe er r
o on nt to o S Si i/ /S Si iO O
2 2

A Au u
d de ep po os si it ti io on n
143

target Si/SiO
2
substrate and the whole structure was heated to detach the thermal tape.
Gold etchant was employed to dissolve the Au film leaving behind the aligned SWNT
arrays on the target substrate. By using this approach, the transfer efficiency obtained was
nearly 100%. After nanotube transfer, photolithography was used to fabricate back-gated
carbon nanotube field-effect transistors (Fig. 6.3).

Figure 6.3. Photograph of a Si/SiO
2
wafer with transferred nanotubes. SEM image shows
that, after being transferred, nanotubes maintain a good degree of alignment on the
receiving substrate. Au electrodes deposition, followed by etching of nanotubes outside
the device channel area complete the fabrication of CNT-FETs.

6.3 Results and Discussion
In this work, we report the use of light irradiation to induce the metal-to-semiconductor
conversion of carbon nanotubes for transistors based on aligned nanotubes and individual
nanotube devices. This conversion process is easy to implement and scalable to complete
wafers. Aligned single-walled nanotubes were synthesized over complete quartz and
sapphire wafers (Fig. 6.1) and then transferred to a Si/SiO
2
substrate for device
fabrication (Fig. 6.2). These aligned nanotubes offer significant potential for nanotube
assembly and integration [26, 27], as nanotube devices can be easily fabricated at wafer
scale, as shown in Fig. 6.4 (a). Fig. 6.4 (b) shows the photograph of an array of devices
based on aligned nanotubes transferred to a 4 inch Si/SiO
2
wafer. The SEM image of a
typical device is shown in Fig. 6.4 (b). Figure 1C illustrates the light irradiation process,
Full wafer
processing
144

where a collimated white light beam from either a xenon or halogen lamp is used to
irradiate the fabricated wafer with nanotube devices for durations from 30 min to several
hours. Fig. 6.4 (d) shows the drain current (I
DS
) vs. gate voltage (V
G
) for a typical device
with 6 aligned nanotubes before and after light irradiation with an accumulated energy of
30 kJ/cm
2
. A remarkable increase in the channel current on/off ratio (I
On
/I
Off
) was
observed from 50 before irradiation to 1.2 x 10
5
after irradiation.

Figure 6.4. (a) Schematic diagram showing large arrays of field-effect transistors
comprising of horizontally aligned carbon nanotubes between source and drain electrodes.
(b) Photograph of a Si/SiO
2
wafer with fabricated aligned nanotube transistors. The SEM
image shows a typical CNTFET in the arrays. (c) Schematic diagram illustrating the
scalable light irradiation process. (d) Current vs. gate voltage (I
DS
-V
G
) characteristics of a
CNTFET device, obtained with V
DS
=0.5 V before (black trace) and after (red trace) light
irradiation. The I
On
/I
Off
ratio increased from ~64 to ~10
5
in the nanotube transistor due to
the light irradiation.-mediated oxidation of nanotube sidewalls leads to metal-to-
semiconductor transition.

c
b
a
d
Source
Drain
5 m
-20 -10 0 10 20
1E-11
1E-10
1E-9
1E-8
1E-7
1E-6
1E-5
I
DS
(A)
V
G
(V)
Before
After
145

This process is highly scalable as compared to the traditional electrical breakdown
approach [19], which has to be carried out device by device. Inspection of the nanotubes
after irradiation revealed no visible cut in the nanotubes, suggesting the increase in
I
On
/I
Off
is due to metal-to-semiconductor conversion of nanotubes in the channel.We
synthesized aligned nanotubes on 3 inch quartz and sapphire wafers after synthesis,
source/drain Ti/Au electrodes were patterned. Fig. 6.5 shows an AFM image from a
metallic CNT-FET, comprising of only one nanotube within the channel, after being
exposed for 5 hours to light.

Figure 6.5. (Left) AFM image of single-nanotube CNT-FET that shows metal-to-
semiconductor transition after 5 hours of light exposure. Zoomed AFM image shows no
visible damage or cutting on the nanotube structure. (Right) I
On/
I
Off
evolution of the CNT-
FET shown on the left upon timed light irradiation.

d = 1.60 nm
300 nm
-10 -5 0 5 10
1E-9
1E-8
1E-7
1E-6
1E-5
Before
3 Hours
5 Hours
V
G
(V)
I
DS
(A)
01 23 45
1E-9
1E-8
1E-7
1E-6
1E-5
I
DS
(A)
Time (h)
I
On
I
Off
146

The nanotube (1.60 nm in diameter) was found to be free from obvious structural damage
after exposure. In general, irradiated nanotubes that showed metal-to-semiconductor
conversion after being irradiated up to 5 hours were found to be continuous even after
being annealed at temperatures in hydrogen atmosphere.
To investigate the effect of light irradiation, we carried out electrical
measurements and micro Raman characterization for nearly 200 devices with one to five
nanotubes in the channel. Fig. 6.6 (a) and 6.6 (b) show the D-band, G-band and I
DS
-V
G

curves for a device with a single metallic nanotube before and after light irradiation.
Analysis of the Raman spectra of this nanotube before and after irradiation reveals an
increase of the Raman band intensity at 1345 cm
-1
(D band) and a decrease of the G band
intensity (1590.4 cm
-1
), accompanied by an upshift of 5.5 cm
-1
for the G band The ratio
between the G and D Raman band intensities (I
G
/I
D
) is regarded as an assessment of the
sp
2
/sp
3
ratio in carbon nanotubes, and thus the five-fold decrease in I
G
/I
D
after irradiation
(Fig. 6.6 (a)) is attributed to an increase in the defect density due to an increase in the sp
3

nature of irradiated nanotubes [24]. It is known that rehybridization defects due to
conversion of sp
2
to sp
3
sites lead to π-electrons localization that can readily open or
increase the bandgap of nanotubes [28], and result in the conversion of metallic to
semiconductor nanotubes. The I
DS
-V
G
curves of Fig. 6.6 (b) shows that the metallic
single-nanotube FET exhibited stronger gate bias dependence after light irradiation, thus
indicating that an increase in the sp
3
nature of metallic nanotubes leads to metal to
semiconductor conversion [4, 28-31].
147

Figure 6.6. Light-mediated oxidation of nanotube sidewalls leads to metal-to-
semiconductor transition. (a) Raman D band (left) and G band (right) of a metallic
nanotube in a single-nanotube device before and after light irradiation. (b) I
DS
-V
G

characteristics of the single-nanotube device shown in the inset of (a), before and after
light irradiation. (c) Comparison between the G/D ratios of nanotubes before and after
one hour irradiation with the full spectrum, ultraviolet, (250 nm - 400 nm), visible (380
nm – 700 nm) and near infrared (750 nm – 2000 nm). (d) Percentage of nanotubes
exhibiting Raman RBM signal after light irradiation using the same irradiation conditions
as part (c). The inset shows the decrease of RBM intensity was more significant for the
small-diameter nanotube than for larger nanotubes. (e) Schematic showing light-induced
oxidation of the nanotube sidewalls. possible chemical groups introduced on the nanotube
sidewalls upon sp
2
–sp
3
rehybridization by light-induced oxidation. (f) Comparison of
typical I
DS
-V
G
characteristics of two CNTFETs before and after irradiation in air and in
vacuum.

f
1300 1400
Raman shift (cm
-1
)
Before
After
D band
c
e
d
a b

1500 1550 1600 1650 1700

Raman shift (cm
-1
)
Before
After
5.5 cm
-1
G-band

-10 -5 0 5 10
1E-8
1E-7
1E-6
I
DS
(A)
V
G
(V)
Before
After

Full UV Vis NIR
0.0
0.2
0.4
0.6
0.8
1.0
1.2
G/D ratio (Normalized)
Before
After

Full UV Vis NIR
0
50
100
150
Percentage %
RBM after
irradiation

100 150 200 250
B efore
After
R am an shift (cm
-1
)

O
2
+ h ν  2 O
-
O
-
+ O
2
 O
3
O
-
+ H
2
O  2 OH
-

-10 -5 0 5 10
1E-8
1E-7
1E-6
I
DS
(A)
V
G
(V)
Device in vacuum before
Device in vacuum after
Device in air before
Device in air after
148

To elucidate which part of the light spectrum plays the major role in the sp
2
/sp
3

conversion of carbon nanotubes, we irradiated devices with ultraviolet (250 nm - 400 nm),
visible (380 nm – 700 nm) and near-infrared (750 nm – 2000 nm) radiation for one hour
by using different band-pass filters between the light source and the devices. Fig. 6.6 (c)
displays the Raman I
G
/I
D
before (gray bars) and after (red bars) light exposure using the
full spectrum, ultraviolet, visible and near infrared irradiation. It is observed that both full
spectrum and ultraviolet irradiation led to a decreased I
G
/I
D
similar in magnitude, whereas
the effect of visible and near infrared irradiation is minor and within the statistical margin
of error. Further confirmation about the role of ultraviolet light is carried out by
examining the Raman radial breathing mode (RBM), which is observed to be highly
sensitive to light irradiation. Prolonged irradiation led to a decrease in intensity and
eventual disappearance of RBM for many nanotubes (Fig. 6.6 (d) inset), which can be
attributed to the presence of rehybridization defects that perturb the symmetry of this
vibration mode [32].
Fig. 6.6 (d) shows the percentage of nanotubes that remained exhibiting RBM
peaks after light irradiation. Again, while the full spectrum and ultraviolet irradiation
delivered similar effect (40% of examined nanotubes showed disappearance of RBM
bands after irradiation), the visible and near infrared irradiation delivered much less
significant effect. Careful examination of Fig. 2D inset also reveals that the nanotube
with diameter d = 1.42 nm (174 cm
-1
) displayed more significant decrease in RBM
intensity than the nanotube with d = 1.74 nm (144 cm
-1
), indicating that small-diameter
nanotubes are more reactive under ultraviolet irradiation than large-diameter nanotubes
149

[33]. Binding energies for carbon-carbon bonds with sp
2
hybridization (6.37 eV) lie well
above the energy associated with photons in the UV region employed here (3.3~5.0 eV),
for which the defect density increase observed upon irradiation cannot be regarded as a
consequence of direct photolysis.
We attribute the observed metal-to-semiconductor conversion to UV-assisted
photo-oxidation of nanotubes in oxygen-containing environment. Oxygenated groups on
the nanotube samples after light irradiation were found using mid FTIR spectroscopy,
showing increased presence of oxygen functionalities in irradiated nanotubes mainly in
the form of hydroxilic groups (see supplementary information). UV-irradiated oxygen
forms oxygen radicals and ozone, which are strong gas-phase oxidants [34, 35]. Fig. 6.6
(e) shows a schematic of carbon nanotube oxidation due to light irradiation in air. The
radical driven reactions can be triggered by UV radiation and readily contribute to the
surface functionalization of nanotubes with oxygen-containing groups. Oxidation of
nanotube sidewall is also consistent with the upshift seen in the Raman G band (Fig. 6.6
(a)), which can be related to interaction between electron-withdrawing oxygen
functionalities and the -electron system on the nanotube sidewall [36, 37].
Further confirmation of the role of oxygen in the photo-assisted metal-to-
semiconductor conversion was obtained by comparing the effects of light irradiation on
nanotube transistors in air and in vacuum (3x10
-5
Torr). Typical examples are shown in
Fig. 6.6 (f). The device exposed to light irradiation in vacuum (black and red curves)
exhibited little change in the on-state current and the I
On/
I
Off
. In contrast, the device
exposed to irradiation in air (blue and green curves) displayed an increase in I
On
/I
Off
and a
150

drop in the on-state current. This unambiguously confirms the important role of oxygen
for the light-assisted metal-to-semiconductor conversion of nanotubes.
To further elucidate the underlying mechanism and to optimize the yield of
depletable transistors by light irradiation, we have carried out detailed experiments on 38
working devices with single or a few nanotubes with varying irradiation time. A “lost”
device is defined as a device with I
On
< 100 pA at V
ds
= 0.1 V. In contrast, devices with
I
On
≥ 100 pA at V
ds
= 0.1 V are categorized as survived devices, among which depletable
devices are defined as having I
On
/I
Off
≥ 10 and non depletable devices are those with
I
On
/I
Off
< 10. AFM was performed to obtain the diameter of each nanotube. Light-induced
metal-to-semiconductor conversion was found to be stable and irreversible at ambient
conditions, in contrast to the behavior observed by Ajayan [38].
CNTFETs were grouped into three categories according to their transport
characteristics: i) CNTFETs that showed metallic behavior, but were converted into
semiconducting and remained depletable throughout the irradiation process (Fig. 6.7 (a),
non-depletable to non-depletable to depletable [ND ND D]); ii) Non-depletable
CNTFETs that first became depletable, and then lost electrical conduction upon
continued irradiation (Fig. 6.7 (b), non-depletable to depletable to lost [ND D Lost]);
and iii) CNTFETs that were depletable before irradiation, and then lost electrical
conduction after continued light irradiation (Fig. 6.7 (c), depletable to depletable to lost
[D D Lost]). The CNTFET in Fig. 6.7 (a) has 2 nanotubes with d = 1.17 nm and 1.20
nm connecting source and drain electrodes. I
On
/I
Off
for this device changed as
1.53.8203 for 0, 3 and 5 hours of light exposure, respectively.
151

Before 3h 3.5h 4h 4.5h
0
25
50
75
100
Percentage %
Time (h)
D e p l e t a b l e
S u r v i v a l

0
5
10

Number of devices
L o s t
0.60.9 1.21.5 1.8 2.1 2.4
0
5
10

S u r v i v e
D i a m e t e r ( n m )
a
b
c
d
e
01 23 45
1E-9
1E-8
1E-7
1E-6
1E-5 ND D
I
DS
(A)
Time (h)
I
On
I
Off
ND
d = 1.17 nm
1.20 nm
01 23 45
1E-12
1E-11
1E-10
1E-9
1E-8
D Lost
I
DS
(A)
Time (h)
I
On
I
Off
D
d = 1.1 nm
01 23 45
1E-13
1E-11
1E-9
1E-7
1E-5
Lost D
I
DS
(A)
Time (h)
I
On
I
Off
ND
d = 0.92 nm

Figure 6.7. Influence of the irradiation time and nanotube diameter on the metal-to-
semiconductor conversion observed in CNTFETs. (a, b and c) I
On
and I
Off
of single- and
few-nanotube CNTFETs showing different evolutions under timed light irradiation (V
ds
=
100 mV). (d) Histogram of CNTFETs that lost electrical conduction or survived after six-
hour light irradiation plotted versus the nanotube diameter. Clear diameter dependence
was observed. (e) Percentage of CNTFETs that survived (red) and showed depletable
behavior (black) for different light irradiation durations. The best yield was found after 4-
hour exposure, when the percentage of depletable devices increased from 32% to 88%
while keeping a survival ratio near 81%.

On the other hand, the nanotube in the single-nanotube device shown in Fig. 6.7 (b) has a
diameter of 0.92 nm and its I
On
/I
Off
changed as 2.4 191 for 0 and 3 hours, respectively.
Radial tension possessed by small-diameter carbon nanotubes decrease their stability and
152

increase their reactivity compared to those with larger diameters [39], which explains
why light irradiation for the device in Fig. 6.7 (b) would first make it depletable but later
too resistive for charge transport. Furthermore, I
On
/I
Off
for the single nanotube CNTFET
in Fig. 6.7 (c) (d = 1.1 nm) changed as 2135 846 upon light irradiation for 0 and 3 hours,
respectively. Comparison between Fig. 6.7 (a) and Fig. 6.7 (b) confirms that small-
diameter nanotubes are more reactive, as electrical conduction was lost in Fig. 6.7 (b), but
persisted in Fig. 6.7 (a) after 5 hour exposure.
The effect of prolonged light exposure on a chip with 38 working CNTFETs is
shown in Fig. 6.7 (d). Devices with nanotube diameters ranging from 0.6 nm to ~1.3 nm
became nearly open circuits, while those with nanotube diameters larger than 1.4 nm in
general survived light exposure. After nearly 6 hours of irradiation, most (90%) of the
surviving devices (20 CNTFETs) were depletable and exhibited clear semiconducting
behavior. Similar diameter dependence has also been observed for nanotube devices
exposed to H
2
plasma [24, 25] and CH
4
plasma [22] due to the higher curvature of small-
diameter nanotubes that makes them more reactive than their large-diameter counterparts.
As demonstrated above, prolonged light-assisted oxidation of the nanotube sidewall may
lead to highly resistive devices. Thus, it is important to find the exposure time that best
optimize the trade-off between depletable and surviving devices. Fig. 6.7 (e) shows the
change in the percentage of depletable (black) and surviving (red) CNTFETs from a chip
with 31 working devices, as a function of light exposure time with a power density of 2.2
W/cm
2
. The best yield was obtained after 4 hours of light exposure, which offered ~88%
depletable devices and a survival rate of ~81%. This yield is typical for devices with
153

about less than five nanotubes in the channel. However, the effect of the metal to
semiconductor conversion in I
On
/I
Off
is generally less pronounced in devices with larger
number of nanotubes, which can be improved by using CNTFETs with optimum and
narrower diameter distribution.
Now that the diameter dependence has been elucidated, we have carried out micro
Raman measurements to ascertain whether light irradiation provides selectivity between
metal and semiconductor tubes of similar diameters (Fig. 6.8). Covalent functionalization
of SWNT sidewall is accompanied by a decrease in the RBM intensity below the noise
level (disappearance of RBM bands) [40, 41]. For complete characterization, we have
carried out micro Raman spectroscopy with three different excitation lines (532 nm, 633
nm, and 785 nm). Fig. 6.8 (a) shows plots of the number of nanotubes exhibiting RBM
v.s. the RBM frequency before and after irradiation as measured with all three excitation
lines.
The frequencies characteristic for metal and semiconductor nanotubes are
highlighted based on Kataura’s plot. Detailed analysis of the data shown in Fig. 6.8 (a),
for all lasers employed, reveals a diameter-dependent decrease of nanotubes showing
RBM, which correlates to the increased sp
3
character of nanotubes upon light irradiation.
By comparing the histograms in Fig. 6.8 (a) before and after light irradiation, one can
clearly see that predominantly small-diameter nanotubes (with large RBM frequency)
underwent disappearance of RBM bands, which is consistent with the diameter
dependence shown in Fig. 6.7. Interestingly, most metallic nanotubes with M
11
bandgaps
in resonance with visible laser wavelengths 532 nm (2.32 eV) and 633 nm (1.96 eV) have
154

diameters lower than ~1.3 nm, which means they are more likely to be oxidized by light
irradiation. NIR laser energy with wavelength of 785 nm (1.58 eV) is, in contrast, in good
resonance with S
11
and S
22
bandgaps of semiconducting nanotubes with diameters lower
than 1.3-1.4 nm, and metallic nanotubes with larger diameters. Results shown in Fig. 6.8
(a) further confirm, for all lasers employed, a well marked diameter-dependent oxidation
of nanotubes.
We irradiated as-grown nanotubes with the full spectrum of xenon and halogen
light sources. Fig. 6.8 (b) shows the percentage of metallic nanotubes in the samples,
before and after irradiation. Nanotubes were grouped into two categories based on their
diameter (0.7 - 1.3 nm and 1.4 - 2.0 nm). Importantly, we observed for both light sources
a marked preferential oxidation of metallic nanotubes with diameters between 0.7 and 1.3
nm over their semiconducting counterparts, with a decrease in the percentage of metallic
nanotubes from 45 to 7% and 35 to 18% for xenon and halogen irradiation, respectively.
The difference observed in the effect of xenon and halogen light sources over the
oxidation of small-diameter nanotubes can be related to the higher intensity of UV
photons of the former [42]. On the other hand, there was no significant difference
between the oxidation of large-diameter metallic and semiconducting nanotubes in the
diameter range of 1.4 – 2.0 nm. This means that higher radial tension, added to the
presence of free electrons on the conduction band of small-diameter metallic nanotubes
makes them more reactive, upon light-induced oxidation, than semiconducting nanotubes
of similar diameters; for which, appropriate irradiation times as well as narrow diameter
distribution are key to obtain semiconducting CNTFET arrays by light irradiation.
155

Figure 6.8. (a) Stacked histograms showing the number of nanotubes exhibiting RBM vs.
the RBM frequency, before and after light irradiation, as measured with three excitation
lines (532 nm, 633 nm, and 785 nm). Frequency regions characteristic for metal or
semiconductor nanotubes are highlighted based on Kataura’s plot. Comparison of the
histograms obtained before and after irradiation for each laser shows a predominant light-
induced oxidation of small-diameter nanotubes (large RBM frequency). (b) Percentage of
metallic nanotubes in as-grown samples before (gray columns) and after (red columns)
light exposure using xenon (upper panel) and halogen (lower panel) lamps. Nanotubes
were grouped into two categories based on their diameter: small-diameter (0.7 - 1.3 nm)
and large-diameter (1.4 - 2.0 nm) nanotubes. A substantial decrease in the percentage of
small-diameter metallic nanotubes found after light irradiation, for both light sources
employed, indicates their preferential oxidation over semiconducting small-diameter
nanotubes. Contrarily, the percentage of large-diameter metallic nanotubes was largely
unaffected by light, indicating the preferential oxidation (metal over semiconductor) is
more effective for small-diameter nanotubes.

M M
M M M M
100 125 150 175 200 225 250 275 300 325
0
10
RBM Raman shift (cm
-1
)
0
10
785 nm
100 125 150 175 200 225 250 275 300 325
0
30
0
30
633 nm
100 125 150 175 200 225 250 275 300 325
0
20

0
20
532 nm
M M
SC SC
SC SC SC SC
SC SC
SC SC
M M M M
M M M M M M M M M M M M
100 125 150 175 200 225 250 275 300 325
0
10
RBM Raman shift (cm
-1
)
0
10
785 nm
100 125 150 175 200 225 250 275 300 325
0
30
0
30
633 nm
100 125 150 175 200 225 250 275 300 325
0
20

0
20
532 nm
M M M M
SC SC
SC SC SC SC
SC SC
SC SC
a b

0.7-1.3 1.3-2.0
0
10
20
30
40
50
Metallic nanotubes %
Diameter (nm)
Before
After
Xenon

0.7-1.3 1.3-2.0
0
10
20
30
40
50
Metallic nanotubes %
Diameter (nm)
Before
After
Halogen
156

6.4 Summary and Conclusion:
Light irradiation of nanotubes constitutes a breakthrough scalable process for nanotube-
based electronic devices via a defect-assisted metal-to-semiconductor conversion
stimulated by light-induced oxidation. This process was found to be diameter dependent
and faster in small-diameter metallic nanotubes. I
on
/I
off
improvements obtained in
CNTFETs were typically in the range of 10
2
up to 10
5
and can be easily scaled and
integrated as a customizable technology over larger-diameter wafers. The approach
presented in this work offers clear advantages over conventional processes to eliminate
metallic nanotubes from CNTFETs and constitutes a significant advance towards large
scale fabrication of carbon nanotube based electronic devices.

157

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42. Spectral irradiance of xenon and halogen lamps was provided by Newport Co

161

Chapter 7. Conclusion and Future Work

7.1 Conclusion
Materials with reduced dimensionality offer numerous fascinating physical, mechanical,
and electronic properties that render them as potential candidates as building blocks for
many electronic and optoelectronic devices. On the one hand, carbon nanotubes are
emerging as the material of choice for replacing silicon based electronics in order to
continue the Moore’s law. On the other hand, semiconductor nanoparticles and nanowires
are increasingly being studied and utilized in photovoltaic devices covering the broad
range of different components of the cells, such as, absorber materials, transport materials,
and transparent conductive electrodes.
In this thesis, we used nanoparticles and nanowires to propose an optimal
architecture for dye- and QD-sensitized solar cells. We demonstrate that by using an
aligned array of TiO
2
nanowires, it is possible to improve the charge collection efficiency
and thus enhancing the overall efficiency of the device. Chapter 1 introduces the
operation principles of dye-sensitized solar cells and discusses the problems that limit the
efficiency and provides potential solutions based upon the scientific reasoning.
Chapter 2 demonstrates the usefulness of 1-D TiO
2
nanoparticles in the solar cell. We
grow TiO
2
nanowires by a simple hydrothermal method and characterize them with SEM,
TEM, and electrical measurements. A significant enhancement in the cell efficiency is
observed in case of nanowire anode compared to the traditional nanoparticles based
anodes.
162

Chapter 3 incorporates the nanocrystal quantum dots as efficient sensitizers in the same
dye-sensitized solar cell configuration. We synthesize high quality CdSeS nanocrystals
using colloidal chemistry techniques and use a bilinker approach to attach them with
TiO
2
nanowires. We demonstrate that the efficiency of the cell depends critically on the
sensitization strategy.
Chapter 4 addresses the problems of stability and corrosion associated with the
use of liquid based electrolytes. We use a novel physical vapor deposited hole transport
layer to replace the liquid electrolyte and show a proof-of-concept by fabricating solid-
state dye-sensitized solar cells. We find that infiltration of this hole transport molecules
inside the pores of nanowires is a critical requirement for obtaining high efficiency cells.
In chapter 5, we propose a random film of silver nanowires as transparent conductive
electrodes for use in solar cells and other optoelectronic devices. We also demonstrate a
facile dry transfer printing technique to transfer the nanowire film on arbitrary substrates.
This technique offers many advantages compared to other techniques. We obtain
performances that rival that of the traditionally used indium-tin oxide based electrodes.
In chapter 6, we address a very important problem in the carbon nanotube based
logic devices. Specifically, we discuss a scalable way to convert metal nanotubes to
semiconducting ones by exposing them to a broad band white light source. This
conversion results in field effect transistors that have only semiconducting nanotubes in
the channel and hence exhibit high on-off ratios.

163

7.2 Future Work
7.2.1 Future work in TiO
2
nanowires solar cells
We demonstrated dye-sensitized solar cells with 3% power conversion efficiency.
However, this was achieved using only 3 um long TiO
2
nanowires. While, TiO
2

nanowires can potentially improve the charge collection efficiency by virtue of their
being defect-free single-crystalline nature, they reduce the effective surface area available
for dye absorption. 1-dimensional materials inherently have lower surface to volume ratio
compared to the zero-dimensional materials. So using nanowire based architecture is a
trade-off between improved charge collection efficiency and reduced light absorption.
The effects of reduced surface area can be overcome by using longer nanowires.
However, the problem with the growth of longer nanowires is the fact that they grow
laterally also and hence reducing the surface area. Future work will require synthetic
techniques to controllably grow long TiO
2
nanowires with small diameters. Nanowires
with higher aspect rations will have extremely high surface area allowing for increased
absorption of dye molecules.
While increasing the length of nanowires will help in the absorption of more photons, it
will also result in increased numbers of nanowire-dye interface. This can potentially
increase the recombination centers and thus affecting the overall efficiency. In fact, this
effect has been observed recently in which increment in the nanowire length beyond a
certain length resulted in a decrease in efficiency [1]. An overcoating of TiO
2
nanowires
with a wider band gap material such as MgO and Nb
2
O
5
can help suppress these
recombination centers.

7
A
C
o
h
b
co
cl
cr
F

.2.2 Future
As discussed
CdSeS nanop
f nanocrysta
arvest differ
elonging to
oefficient. In
lass. Namely
rystalline qu
Figure 7.1 T
work in QD
in chapter 3
particles to d
als is the ab
rent regions
II-III-VI
2
f
n this thesis
y, we synthe
uality. It will
TEM and SE
D-sensitized
3, QDs offer
demonstrate
bility to tune
of sun’s spe
family as th
s, we have
esized CuInS
l be worthwh
M images of
d solar cells
r many adva
the proof-of
e the band g
ectrum. Tow
hey are kno
synthesized
Se
2
, CuInS
2
,
hile to
f various NC
antages over
f-concept. Th
gap of the p
wards this en
own to have
various nan
and Cu
2
ZnS
Cs synthesize
the dye mo
he most imp
particles and
nd, one can u
e extremely
noparticles b
SnS
4
nanopar
ed and used
lecules. We
portant advan
d hence they
use nanoapr
high absor
belonging to
rticles with h
in this thesi
164
used
ntage
y can
rticles
rption
o this
high
s
165

sensitize the TiO
2
nanowires with these nanoparticles. Figure 7.1 shows some of the
nanocrystals synthesized in our lab with an objective of using them as sensitizers.
Recently, a few reports have appeared which talk about the synthesis of these particles
and utilize them for traditional film based solar cells [2-7]. One of the most important
issue in the QD-sensitized solar cells is the nature of interface between individual
nanocrystal and the TiO
2
particle/nanowire. In our work on CdSeS NCs based solar cell,
we used a bilinker molecule to covalently bind the QD with the TiO
2
nanowires forming
an electronically coupled system. This strategy requires an efficienct ligand exchange
technique to remove the large organic capping with smaller chain entities. While such a
chemistry is well developed for II-VI family of nanocrystals, II-IV-VI
2
class of
nanoparticles have witnessed limited progress in this direction and would require further
research work to be able to exchange the bulky capping agents with more electronically
conducting molecules.
One very important problem of QD-sensitized solar cell is the incompatibility of
Iodine/Iodide based electrolytes. Most of the nanocrystals are known to be prone to
corrosion by the liquid electrolyte, some of them not able to survive even for a few hours.
Although researchers have used sulfur/sulfide based electrolytes towards this objective,
and have observed enhanced stability of nanoparticles in this solution, it comes at a cost
of reduced performance. The efficiency is drastically reduced with sulfur based
electrolytes mainly because of increased recombination rate leading to significant drop in
the open circuit voltage. In this respect, research community certainly needs to look for
alternative materials that can replace the iodine based electrolytes without compromising
166

the performance. One solution to this problem can be to move away from any liquid
based electrolyte and use vapor deposited solid state electrolytes. We have already
demonstrated the usefulness of a solid state electrolyte in the case of dye-sensitized solar
cells, but fundamentally, the same approach can be used for QD-sensitized solar cells as
well.

7.2.3 Future work in Solid-State Dye-Sensitized Solar Cell
As discussed in chapter 4, our novel vapor deposited hole transport layer presents a
paradigm shift in the dye-sensitized solar cells as it holds the potential to solve the
problems of stability in outdoor use. While our cells suffer from rather poor efficiency
but we have demonstrated proof-of-concept for this materials. However, the main factor
that limits the efficiency is the poor light harvesting efficiency of our cells. It is a
consequence of the fact that we used only 200 nm long TiO
2
nanowires and hence
extremely limited surface area available for dye absorption. In order to increase the
efficiency of the cells, we need to grow longer nanowires but with sufficient spacing
between the nanowires to allow the infiltration of hole transport layer. By tuning the
synthetic recipe for TiO
2
nanowire synthesis, we have achieved some good results, as
shown in figure 7.2. As can be seen in the figure, we can grow thinner and thinner
nanowires as we go from figure 7.2 (a) to 7.2 (d). Long and thin nanowires not only have
higher effective surface area, they also have sufficient space between them to allow for
deep infiltration of hole transport layer and hence enhancing the efficiency. Certainly,
this will require optimization of deposition conditions of the hole transport layer and of

d
n

F
co

In
u
pr
by
tr
sh
p
oping conce
ecessitates h
Figure 7.2 T
oncentration
n addition to
sing differen
rinciple, the
y the differ
ransport ma
hallower and
ossible hole
entrations sin
higher dopin
Top view S
ns of titanium
o obtaining n
nt hole tran
e open circui
rence betwee
aterials. Acc
d deeper HO
transport m
nce longer n
g to maintai
SEM image
m precursors
nanowire sam
sport layers
it voltage of
en the Ferm
cordingly w
OMO levels t
materials that
nanowires re
n high condu
es of TiO
2
s. Nanowires
mples with r
to improve
f the solid st
mi level of T
we are work
to verify this
can be poten
equire thicke
uctivity of th
nanowires
s become thi
right dimens
e the open c
tate dye-sen
TiO
2
and th
king on or
s conjecture
ntially used
er hole trans
he organic la
synthesized
inner as we g
ions, we are
circuit voltag
sitized solar
he HOMO l
rganic mole
. Table 7.1 g
in dye-sensi
sport layer w
ayer.
d using diff
go from (a) t
e also workin
ge of the ce
r cell is gove
level of the
ecules with
gives some o
itized solar c
167
which
ferent
to (d)
ng on
ell. In
erned
hole
both
of the
cells.
168

The table also gives the HOMO levels associated with each molecule.

Hole Transport Materials HOMO energy level (in eV)
NNP 4.8
TPP 4.9
TPD 5.1
NPD 5.3
AlQ
3
5.6
AND 5.7
CBP 5.8
mCP 6.0
BCP 6.2
UGH 7.1

Table 7.1 Different hole transport molecules and their HOMO levels

Since every molecule has different intrinsic conductivity, they will require optimization
of doping concentrations to achieve high conductivity of holes in order to obtain higher
internal quantum efficiency of the cells.

169

Chapter 7 References
1. Feng, X.J., K. Shankar, O.K. Varghese, M. Paulose, T.J. Latempa and C.A.
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Nanowire Arrays Grown Directly
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3. Guo, Q.J., H.W. Hillhouse and R. Agrawal, Synthesis of Cu2ZnSnS4 Nanocrystal
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4. Steinhagen, C., M.G. Panthani, V. Akhavan, B. Goodfellow, B. Koo and B.A.
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5. Riha, S.C., B.A. Parkinson and A.L. Prieto, Solution-Based Synthesis and
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6. Panthani, M.G., V. Akhavan, B. Goodfellow, J.P. Schmidtke, L. Dunn, A.
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7. Guo, Q., S.J. Kim, M. Kar, W.N. Shafarman, R.W. Birkmire, E.A. Stach, R.
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Abstract (if available)

Abstract Zero-dimensional materials such as quantum dots and one-dimensional materials such as nanorods and nanowires have attracted significant attention in the past two decades and have been demonstrated as important building blocks for numerous electronic and optoelectronic device applications. Of course, the starting place for the field is the ability to grow various nanomaterials in different morphologies. In this thesis, we have demonstrated successful synthesis of both quantum dots and nanowires belonging to a totally new material class and have subsequently utilized them for photovoltaic cells in different device architectures. Apart from the photovoltaic cells, we have demonstrated a scalable way to fabricate carbon nanotube devices with a high on-off ratio on a wafer scale.

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Asset Metadata

Creator Kumar, Akshay (author)

Core Title Zero-dimensional and one-dimensional nanostructured materials for application in photovoltaic cells

School Viterbi School of Engineering

Degree Doctor of Philosophy

Degree Program Materials Science

Publication Date 09/22/2010

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

Tag nanomaterials,OAI-PMH Harvest,photovoltaics,TiO2 nanowires

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Zhou, Chongwu (committee chair), Dapkus, P. Daniel (committee member), Goo, Edward K. (committee member)

Creator Email akshay.kedia@gmail.com,akshayku@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-m3466

Unique identifier UC1128352

Identifier etd-Kumar-4109 (filename),usctheses-m40 (legacy collection record id),usctheses-c127-408099 (legacy record id),usctheses-m3466 (legacy record id)

Legacy Identifier etd-Kumar-4109.pdf

Dmrecord 408099

Document Type Dissertation

Rights Kumar, Akshay

Type texts

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

Repository Name Libraries, University of Southern California

Repository Location Los Angeles, California

Repository Email cisadmin@lib.usc.edu