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Heterojunction Quantum Dot Solar Cells Heterojunction Quantum Dot Solar Cells by Navid Mohammad Sadeghi Jahed A thesis presented to the University of Waterloo in fulfillment of the thesis requirement for the degree of Doctor of Philosophy in Electrical and Computer Engineering Waterloo, Ontario, Canada, 2016 © Navid Mohammad Sadeghi Jahed 2016 I hereby declare that I am the sole author of this thesis. This is a true copy of the thesis, including any required final revisions, as accepted by my examiners. I understand that my thesis may be made electronically available to the public. ii Abstract The advent of new materials and application of nanotechnology has opened an alternative avenue for fabrication of advanced solar cell devices. Before application of nanotechnology can become a reality in the photovoltaic industry, a number of advances must be accomplished in terms of reducing material and process cost. This thesis explores the development and fabrication of new materials and processes, and employs them in the fabrication of heterojunction quantum dot (QD) solar cells in a cost effective approach. In this research work, an air stable, highly conductive (ρ = 2.94 × 10-4 Ω.cm) and transparent (≥85% in visible range) aluminum doped zinc oxide (AZO) thin film was developed using radio frequency (RF) sputtering technique at low deposition temperature of 250 °C. The developed AZO film possesses one of the lowest reported resistivity AZO films using this technique. The effect of deposition parameters on electrical, optical and structural properties of the film was investigated. Wide band gap semiconductor zinc oxide (ZnO) films were also developed using the same technique to be used as photo electrode in the device structure. Sputtered ZnO thin films with high electron mobility (30 cm2V-1s-1) is achieved at a deposition temperature of 250 °C to eliminate charge collection losses in the solar cell device. The developed AZO and ZnO thin films are a suitable substitute for the expensive indium tin oxide (ITO) and low carrier mobility titanium oxide (TiO2) films respectively. To increase the junction area in the final device, a novel and scalable method was developed for fabrication of upright ordered arrays of ZnO nanowires (NWs) combining top-down etching of ZnO film and silica sphere nanomasking in fluorine-based plasma. The electrical properties of the resulted NWs can be controlled using this approach. The effect of plasma etching parameters and surface treatments on the structural and surface properties of the ZnO NWs were investigated. A high throughput drop-casting process was next developed using colloidal quantum dots (CQDs) for the formation of light absorbing quantum dot film (QD-film) to replace the widely used input-intensive spin- coating technique. The QD-film provides controllability over thin film properties via tuning the properties of the CQDs building blocks. Crack-free QD-films were formed with single-step drop casting of CQDs from solution (solution-phase ligand-exchange) and Layer-by-layer (L-B-L) replacement of long ligands with short charge transport-compatible ones (solid-phase ligand-exchange). Charge carrier dynamics were also studied at the CQDs and ZnO interface using photoluminescence decay analyses in order to optimize the selection of CQDs for efficient electron transfer which leads to a better working device. For the first time, the electron transfer in a ZnO/CQD/MoO3 structure was studied to iii eliminate the effect of other non-radiative pathways and determine the true electron transfer at the CQD/ZnO interface. Effect of CQD size and ligands on the transfer efficiency was discussed. The device concepts and theoretical treatment of QD-film/ZnO were also highlighted to gain insight on the CQD and ZnO optical and electrical parameters affecting the performance of the QD solar cell device. The developed materials and processes were employed for the fabrication of planar and nanowire heterojunction QD solar cell devices. For planar structure power conversion efficiency of η=2.3% at 100 mW/cm2 solar irradiance were achieved. iv Acknowledgements First and foremost, I would like to thank my parents, Mohtaram and Safar. My parents have sacrificed a lot to raise me up and to tolerate the hardship of me being away when they were in need. I would like to thank my siblings, Neda and Nima, for their moral support and encouragement. This thesis belongs to my family at least as much as it does to me. I would like to thank my graduate advisor, Professor Siva Sivoththaman. He has been a continual source of ideas and support throughout my graduate studies, while giving me the freedom to explore my own ideas. His patience, enthusiasm, endless pursuit of excellence and dedication to research have given me the motivation to work hard and seek perfection in life and my doctoral research. I am truly grateful to all my PhD defense committee members, Professor Hany Aziz, Professor Chris Backhouse and Professor Zhongchao Tan, for their time and effort to improve the quality of this thesis and providing valuable comments. I would like to thank my external examiner, Professor Karin Hinzer, for reviewing my thesis and providing valuable comments and suggestions. I would like to thank Mr. Joseph Street, CAPDS lab manager, for his valuable technical assistance and dedication to CAPDS lab. I would like to thank Dr. Bahareh Sadeghimakki, research associate in CAPDS lab, for her support and advises during my PhD research. I would like to thank all CAPDS lab members and fellow students that I had the privilege of working with, Dr. Roohen S. Tarighat, Dr. Behzad Esfandiarpour, Maziar Moradi, Zhen Gao, Yaxin Zheng, Dr. Christopher Baldus-Jeursen Nasim Bakhshi-Zadeh, Mohsen Mahmoudysepehr and Dr. Bita Janfeshan. Many people influenced my graduate experience, I am grateful to all my friends in Waterloo who made my PhD enjoyable experience and full of unforgettable memories. v Dedication I wish to dedicate this thesis to my parents. vi Table of Contents List of Figures ......................................................................................................................................... xi List of Tables .......................................................................................................................................... xx Acronyms .............................................................................................................................................. xxi Chapter 1 Introduction ............................................................................................................................. 1 1.1 Colloidal quantum dots as “semiconductor ink” ............................................................................ 1 1.2 Third generation photovoltaics based on nanotechnology and new materials ............................... 2 1.3 Motivation and research objectives ................................................................................................ 5 1.4 Organization of this Thesis ............................................................................................................ 7 Chapter 2 Literature review.................................................................................................................... 10 2.1 From colloidal quantum dots (CQDs) to quantum dot film ......................................................... 10 2.1.1 Underlying mechanisms for nucleation and growth of CQDs .............................................. 10 2.1.2 Hot-injection and non-injection synthesis methods .............................................................. 11 2.1.3 Methods for controlling properties of CQDs ........................................................................ 12 2.1.4 Methods for quantum dot film formation from CQDs .......................................................... 21 2.1.5 Minibands in artificial crystals and quantum dot films ......................................................... 23 2.2 Transparent conductive oxides (TCOs) and wide band gap semiconductors ............................... 24 2.2.1 Intrinsic limit for conductivity of transparent conductive oxides ......................................... 24 2.2.2 Transparent conductive oxide for 3rd generation photovoltaic cells ...................................... 25 2.2.3 Wide band gap semiconductor for 3rd generation photovoltaic cells .................................... 26 2.3 Top-down approaches for zinc oxide nanowires fabrication ....................................................... 27 2.3.1 Basics of reactive ion etching (RIE) ...................................................................................... 28 vii 2.3.2 Etchant gases for reactive ion etching of ZnO ...................................................................... 30 2.4 Devices based on CQDs ............................................................................................................... 31 2.4.1 Quantum dot light emitting devices (QD-LEDs) .................................................................. 31 2.4.2 Quantum dot solar cells (QD-SCs) ........................................................................................ 32 2.4.3 Emerging advanced third generation solar cells based on QDs and CQDs ........................... 38 2.5 Summary ...................................................................................................................................... 45 Chapter 3 Development of ZnO-based highly transparent and conductive metal oxide .......................
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