Hybrid Organic/Inorganic Solar Cells based on Electrodeposited ZnO Nanowire Arrays on ITO and ...

175 Pages·2011·10.63 MB·English

Hybrid Organic/Inorganic Solar Cells based on
Electrodeposited ZnO Nanowire Arrays on ITO and AZO
Cathodes
by
Wei-Te Daniel Wen
A thesis submitted in conformity with the requirements
for the degree of Master of Applied Science
Graduate Department of Materials Science and Engineering
University of Toronto
Copyright (cid:13)c 2011 by Wei-Te Daniel Wen
Abstract
Hybrid Organic/Inorganic Solar Cells based on Electrodeposited ZnO Nanowire Arrays
on ITO and AZO Cathodes
Wei-Te Daniel Wen
Master of Applied Science
Graduate Department of Materials Science and Engineering
University of Toronto
2011
ZnO nanowire arrays (NWAs) and Al-doped ZnO (AZO) cathodes were applied in
hybrid organic / inorganic solar cells for lower-cost solar energy. Parameters for the
electrodeposition of ZnO NWAs and the fabrication of NWA-free baseline devices were
systematically optimized using ITO as the cathodes. High efficiencies of up to 5.4% were
achieved. Incorporation of the ZnO NWAs into the baseline devices significantly reduced
their efficiencies due to possible shorting in the active layer. Devices fabricated using
AZO cathodes were characterized. The AZO-based devices achieved efficiencies of up
to ∼4.8%, showing promising results for the application of AZO as an ITO alternative.
Formation of numerous large nanoplatelets was observed during the electrodeposition
of ZnO NWAs on AZO cathodes. The NWAs grown on AZO cathodes were also non-
uniform. Future studies were proposed to address the issues with incorporation of ZnO
NWAs in hybrid solar cells and their combination with AZO cathodes.
ii
Dedication
This thesis is dedicated to my parents, my family and my girlfriend for their love,
encouragement and support throughout my study. The past few years would have been
very difficult without them.
A special dedication is given to dear Dr. Ping Sun, who had passed away in 2010.
His guidance and creativity was a great inspiration to me and I am forever grateful for
the kindness he offered.
iii
Acknowledgements
First and foremost, I am grateful to my supervisor Professor Harry E. Ruda for his
guidance and support. I am thankful for the great freedom he has offered throughout
the project for me to learn.
I also want to express my thanks to Ms. Dong Gao and Professor Zheng-Hong Lu for
their generous exchange of expertise and sharing of equipments in solar cell fabrication
and characterization. I further thank Dr. Bin Bin Li for sharing her knowledge on
the matter of electrodeposition, Ms. Romane Cassaigne and Ms. Camille Pensec for
performing experiments and obtaining results on a portion of the electrodeposition of
ZnOnanowirearrays. Amajorportionofthethesiswouldnothavebeenpossiblewithout
their contributions.
Thanks to all members of the Centre for Advanced Nanotechnology for the many
advice, assistance, and friendship they have given me throughout the years.
iv
Contents
1 Introduction 1
1.1 Solar Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Organic Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2.1 Photoexcitation of Organic Semiconductors . . . . . . . . . . . . 4
1.2.2 Donor-Acceptor Heterojunction . . . . . . . . . . . . . . . . . . . 7
1.3 Hybrid Organic/Inorganic Solar Cells . . . . . . . . . . . . . . . . . . . . 12
1.3.1 Polymer Donors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.3.2 Metal Oxide Nanostructure . . . . . . . . . . . . . . . . . . . . . 15
1.3.3 Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
1.4 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
1.5 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2 Background for Synthesis of Material 27
2.1 Growth of ZnO Nanowire Array . . . . . . . . . . . . . . . . . . . . . . . 27
2.1.1 Synthesis Techniques . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.1.2 Growth Mechanism in ZnO Electrodeposition . . . . . . . . . . . 30
2.2 Deposition of Al-doped ZnO Transparent Conductive Oxide . . . . . . . 39
2.2.1 Deposition Techniques . . . . . . . . . . . . . . . . . . . . . . . . 39
2.2.2 Al-Doping of ZnO by sol-gel method . . . . . . . . . . . . . . . . 44
v
3 Methodology 49
3.1 Synthesis of ZnO Nanowire Array . . . . . . . . . . . . . . . . . . . . . . 49
3.1.1 Preparation of Substrate/Cathode . . . . . . . . . . . . . . . . . . 49
3.1.2 Electrodeposition of ZnO Nanowire Array . . . . . . . . . . . . . 50
3.2 Synthesis of Al-doped ZnO Transparent Conductive Oxide . . . . . . . . 51
3.3 Completion of Hybrid Organic Solar Cells . . . . . . . . . . . . . . . . . 53
3.4 Characterization Techniques . . . . . . . . . . . . . . . . . . . . . . . . . 54
3.4.1 Scanning Electron Microscopy . . . . . . . . . . . . . . . . . . . . 54
3.4.2 Testing of OSC Devices . . . . . . . . . . . . . . . . . . . . . . . 54
3.4.3 Resistivity Measurement of the TCOs . . . . . . . . . . . . . . . . 58
4 Electrodeposition of ZnO Nanowire Arrays 60
4.1 Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
4.2 ZnO Backing Layer Deposition . . . . . . . . . . . . . . . . . . . . . . . 61
4.2.1 Influence of Different ZnO Backing Layers on Nanowire Morphology 61
4.2.2 Importance of Humidity Control during Backing Layer Preparation 64
4.3 The ZnO Nanowire Array . . . . . . . . . . . . . . . . . . . . . . . . . . 68
4.3.1 Effect of Varying Growth (Solution) Temperature . . . . . . . . . 68
4.3.2 Effect of Varying [ZnCl ] . . . . . . . . . . . . . . . . . . . . . . . 74
2
4.4 Discussion - Selection of Nanowire Array for Application in Hybrid Solar
Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
4.5 Fine-tuning of Electrodeposition Parameters . . . . . . . . . . . . . . . . 86
4.6 Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
5 Optimization of Hybrid Organic Solar Cells 92
5.1 Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
5.2 Inverted Bulk-Heterojunction Organic Solar Cells . . . . . . . . . . . . . 93
5.2.1 Initial Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
vi
5.2.2 MoO as Hole Transport / Electron Blocking Anode Buffer Layer 100
3
5.2.3 ZnO as Electron Transport / Hole Blocking Cathode Buffer Layer 104
5.3 Hybrid Organic Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . 108
5.3.1 Effect of Incorporating as-deposited ZnO NWA . . . . . . . . . . 108
5.3.2 Effect of Post-Deposition Annealing of ZnO NWA . . . . . . . . . 113
5.4 Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
6 Application of Al-doped ZnO in Hybrid Solar Cells 120
6.1 Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
6.2 Inverted Organic Solar Cells using Al-doped ZnO as Cathode . . . . . . . 121
6.2.1 Function of ZnO Backing Layer on AZO . . . . . . . . . . . . . . 121
6.2.2 Comparison with ITO-based Devices . . . . . . . . . . . . . . . . 123
6.3 Electrodeposition of ZnO Nanowire Arrays on Al-doped ZnO . . . . . . . 128
6.4 Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
7 Conclusions and Future Works 134
7.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
7.2 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
Bibliography 137
A Control of ZnO Seed Layer Texture and NWA Orientation 153
B Calculation of V , σ , V , and d 156
Dissociated Required P3HT Complete
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List of Tables
1.1 Physical, electrical, and optical properties of rr-P3HT and PCBM [3, 92,
128, 24]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
1.2 Summary of reported results on hybrid solar cell based on ZnO and TiO
2
nanostructure [75, 76, 88, 106, 108, 50, 7]. . . . . . . . . . . . . . . . . . 23
2.1 Theoreticalpredictionofthedependenceofgrowthon[ZnCl ]andT . 40
2 Growth
4.1 Summary of dimensions and densities of NWA grown at various T
Growth
= 50-90◦C. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
4.2 SummaryofdimensionsanddensitiesforNWAgrownusingvarious[ZnCl ]
2
= 0.1-10 mM. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
4.3 Comparison between ideal values estimated and actual values obtained for
NWA grown. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
5.1 Summary of J-V characteristics comparing three pairs of devices with Au
and Ag anode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
5.2 Summary of J-V characteristics comparing devices with 15 nm and 30 nm
ZnO buffer layer on ITO cathode. . . . . . . . . . . . . . . . . . . . . . . 97
5.3 Summary of J-V characteristics comparing devices with ZnO buffer layer
deposited with baking temperature of 130◦C and 150◦C. . . . . . . . . . 98
5.4 Summary of J-V characteristics comparing devices with ZnO buffer layer
with and without treatment in electrodeposition conditions for 1 hr. . . . 99
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5.5 V , J , P , FF, and PCE for devices with 30nm ZnO cathode buffer
OC SC max
and 0 nm, 1 nm, 2 nm, and 4 nm MoO anode buffer. . . . . . . . . . . . 101
3
5.6 V , J , P , PCE, and FF for devices with 1 nm MoO anode buffer
OC SC max 3
and 0-30 nm ZnO cathode buffer/backing layer thicknesses. . . . . . . . . 105
5.7 V , J , P , FF, and PCE for hybrid OSCs with NWA of 0-100 nm
OC SC max
height incorporated. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
5.8 V , J , P , FF, and PCE for hybrid OSCs with 60 nm tall NWA
OC SC max
annealed for 30 min in various surrounding atmosphere. . . . . . . . . . . 115
6.1 Summary of results from measurement of devices on AZO cathode with
and without 30 nm ZnO buffer layer. . . . . . . . . . . . . . . . . . . . . 122
6.2 Summary of differences in dependences of AZO and ITO-based devices on
the inclusion of ZnO buffer layer and the choice of material / deposition
method for the anode buffer layer. . . . . . . . . . . . . . . . . . . . . . . 124
6.3 Summary of results comparing characteristics of optimized AZO and ITO-
based devices. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
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List of Figures
1.1 Summary of progress and best cell efficiencies of various technologies in
research. [43] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 The efficiency-cost relationship of the three generations of solar cells. . . 4
1.3 The chemical structures of some common conjugated polymers . . . . . . 6
1.4 Single layer OSC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.5 Schematics of types of OSC . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.6 Operating mechanism of heterojunction OSC . . . . . . . . . . . . . . . . 9
1.7 Benefits and non-ideality in morphology of BHJ . . . . . . . . . . . . . . 11
1.8 Device structure of a hybrid OSC. 1) LWF TCO cathode. 2) Cathode
buffer layer. 3) Metal oxide nanostructure having feature size and void
size of ∼20 nm. 4) Photoactive conjugated polymer. 5) Anode buffer
layer. 6) HWF metal anode. . . . . . . . . . . . . . . . . . . . . . . . . . 13
1.9 The energy levels of the components in a hybrid OSC plotted using values
summarized from references [88, 51, 107, 108, 36, 59]. . . . . . . . . . . . 13
1.10 a) Wurtzite structure of ZnO. b) ZnO crystal and surfaces. . . . . . . . . 17
2.1 Typical CVD setup showing the source, the substrate, and the carrier gas
flow. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
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