Plasmonic enhancement for colloidal quantum dot photovoltaics by Daniel Paz-Soldan A thesis submitted in conformity with the requirements for the degree of Master of Applied Science Graduate Department of Electrical and Computer Engineering University of Toronto Toronto, Ontario Canada Copyright © 2013 by Daniel Paz-Soldan Plasmonic enhancement for colloidal quantum dot photovoltaics Abstract Plasmonic enhancement for colloidal quantum dot photovoltaics Daniel Paz-Soldan Master of Applied Science Graduate Department of Engineering University of Toronto 2013 Colloidal quantum dots (CQD) are used in the fabrication of efficient, low-cost solar cells synthesized in and deposited from solution. Breakthroughs in CQD materials have led to a record efficiency of 7.0 %. Looking forward, any path toward increasing efficiency must address the trade-off between short charge extraction lengths and long absorption lengths in the near-infrared spectral region. Here we exploit the localized surface plasmon resonance of metal nanoparticles to enhance absorption in CQD films. Finite-difference time-domain analysis directs our choice of plasmonic nanoparticles with minimal parasitic absorption and broadband response in the infrared. We find that gold nanoshells (NS) enhance absorption by up to 100 % at λ = 820 nm by coupling of the plasmonic near-field to the surrounding CQD film. We engineer this enhancement for PbS CQD solar cells and observe a 13 % improvement in short-circuit current and 11 % enhancement in power conversion efficiency. Daniel Paz-Soldan University of Toronto ii Plasmonic enhancement for colloidal quantum dot photovoltaics Acknowledgments This work was made possible by the collective effort of an exemplary group of individuals. First, I thank my supervisor, Professor Edward H. Sargent, for his constant guidance and support. He sets the bar high and encourages us to think big. I am grateful for having had the opportunity to work in a world-class research group in such an exciting field. I am grateful to Dr. Susanna Thon for her leadership and guidance through thick and thin. I owe many thanks to Dr. Anna Lee, mentor and friend, who taught me the virtues of the scientific process and whose contributions were invaluable. I also thank Dr. Michael Adachi for his objective insight and practical know-how with FDTD simulations. I extend thanks to Dr. Mingjian Yuan, Dr. Pouya Maraghechi, Andre´ Labelle, and Haopeng Dong for their helpful advice and important work on the project. I particularly thank David Zhitomirsky, Dr. Illan Kramer, and Dr. Rui Li for guiding my research focus from the very beginning. I am grateful to Dr. Larissa Levina and Elenita Palmiano for CQD synthesis and tireless preparation of materials for the lab on a daily basis. I also thank Damir Kopilovic and Remigiusz Wolowiec for designing and building anything, on-demand, and in a timely fashion. I feel privileged to have collaborated with a diverse and talented group of individuals outside of the Sargent group while at the Uni- versity of Toronto and I am thankful for their valuable contributions: Dr. Aftab Ahmed, Dr. Kun Liu, Dr. Alexander Melnikov, Dr. Peter M. Brodersen, Anjan Reijnders, and Luke Sandilands. I also thank Dr. Neil Coombs and Dr. Ilya Gourevich for training and assistance with electron microscopy. I thank Dr. Sjoerd Hoogland, Dr. Armin Fischer, and Dr. Oleksandr Voznyy for in- sightful discussions. For scientific and non-scientific discussions alike, I thank: Dr. Yuan Daniel Paz-Soldan University of Toronto iii Plasmonic enhancement for colloidal quantum dot photovoltaics Ren, Dr. Jennifer Flexman, Dr. Shokouh Farvid, Dr. Jin Young Kim, Dr. Ghada Koleilat, Dr. Zhijun Ning, Dr. Silvia Masala, Dr. Philipp Stadler, Alex Ip, Kyle Kemp, Graham Carey, Melissa Furukawa, Lisa Rollny, Xinzheng Lan, Brandon Sutherland, Jixian Xu, Chris Wong, Valerio Adinolfi, and Jeannie Ing. I thank my parents, Carlos and Patricia, and my brothers, Carlos Jr. and Mario. I am privileged to have been born into a family that stands behind me in everything I do. Finally I thank Jessica Nguyen, whose constant encouragement and support through everything made this work possible. Daniel Paz-Soldan University of Toronto iv CONTENTS Plasmonic enhancement for colloidal quantum dot photovoltaics Contents Abstract ii Acknowledgments iii Acronyms vii List of Tables viii List of Figures ix 1 Introduction 1 1.1 Colloidal quantum dots . 3 1.1.1 Recent advances in colloidal quantum dot solar cells . 4 1.1.2 Limitations . 5 1.2 Thesis objectives . 7 2 Background 9 2.1 Solar cell fundamentals . 9 2.1.1 Figures of merit . 10 2.1.2 Quantum efficiency . 12 2.2 Surface plasmons . 12 2.2.1 Localized surface plasmons . 14 2.2.2 Effect of ligand . 14 2.3 State of the art . 15 3 Infrared plasmonic nanoparticles 17 3.1 Optical design considerations . 17 3.2 Finite-difference time-domain simulations . 19 3.3 Engineering of optical resonances . 20 3.3.1 Hemisphere-capped nanorods . 20 3.3.2 Arrowhead nanorods . 21 3.3.3 Nanoshells . 23 3.4 Plasmonic particle spacing in CQD films . 25 3.5 Conclusions . 27 4 Integrating MNPs into CQD solar cells 28 4.1 PbS CQD and NS film deposition . 28 4.2 Physical and electrical considerations . 29 4.2.1 Surface modification and purification . 30 4.2.2 Agglomeration . 31 Daniel Paz-Soldan University of Toronto v CONTENTS Plasmonic enhancement for colloidal quantum dot photovoltaics 4.3 Nanoshell film deposition . 31 4.3.1 Solvent controls . 32 4.3.2 MNP film fabrication by spin casting . 32 4.3.3 MNP film fabrication by reservoir drop casting . 33 4.4 Conclusions . 33 5 Characterization of devices 34 5.1 Design of plasmonic CQD solar cells . 34 5.2 Performance of CQD solar cells with NR and ARNR . 35 5.3 Optical properties of plasmonic CQD films . 35 5.4 Device performance . 37 5.5 Quantum efficiency . 38 5.6 Conclusions . 39 6 Conclusion 40 6.1 Contributions to the field . 40 6.2 Future work . 41 References 43 A Materials and experimental procedures 48 A.1 PbS CQD synthesis and solvent exchange . 48 A.2 Photovoltaic device fabrication . 48 A.3 Nanorod synthesis . 49 A.4 Arrowhead nanorod synthesis . 50 A.5 Nanoshell solution preparation . 51 B Measurements and simulations 53 B.1 AM 1.5 photovoltaic performance characterization . 53 B.2 EQE measurements . 53 B.3 Solution absorption . 54 B.4 Film absorption . 54 B.5 Double pass film absorption . 54 B.6 FDTD simulations . 54 Daniel Paz-Soldan University of Toronto vi CONTENTS Plasmonic enhancement for colloidal quantum dot photovoltaics Acronyms AM 1.5 Air Mass 1.5 G solar spectrum MNP Metal Nanoparticle ARNR Arrowhead Nanorod MPA 1-mercaptopropanoic Acid CQD Colloidal quantum dot MW Molecular weight EM Electromagnetic MPP Maximum power point EQE External Quantum Efficiency NR Nanorod NS Nanoshell FF Fill Factor OPV Organic Photovoltaics FTO Fluorine doped Tin Oxide PbS Lead sulfide ITO Indium doped Tin Oxide PCE Power Conversion Efficiency IQE Internal Quantum Efficiency PV Photovoltaics J − V Current-Voltage Characteristic PVP Polyvinylpyrrolidone JSC Short-Circuit Current RS Series Resistance LSPR Localized Surface Plasmon Reso- nance RSH Shunt Resistance MeOH Methanol VOC Open-Circuit Voltage Daniel Paz-Soldan University of Toronto vii LIST OF TABLES Plasmonic enhancement for colloidal quantum dot photovoltaics List of Tables 1 Solar cell performance results with embedded NS with and without pre- sonication . 31 2 Solar cell performance results with embedded NS for spin casting and drop casting . 32 3 Solar cell performance of devices with embedded NRs . 35 4 Solar cell performance of devices with embedded ARNRs . 36 5 Device results using nanoshells . 37 Daniel Paz-Soldan University of Toronto viii LIST OF FIGURES Plasmonic enhancement for colloidal quantum dot photovoltaics List of Figures 1.1 Electricity generation by source in the United States, 2009 . 1 1.2 Spectral tuning of quantum dots across the broad solar spectrum . 4 1.3 The depleted heterojunction architecture for CQD PV . 5 1.4 Measured absorption coefficient spectrum of a PbS CQD film . 6 2.1 Basic operation of a solar cell . 9 2.2 Equivalent circuit of a solar cell . 11 2.3 Dispersion curves (!) for gold and silver . 13 2.4 Simple illustration of a surface plasmon on a metal nanoparticle . 14 2.5 The effect of a ligand shell on the surface plasmon of a silver nanoparticle . 15 3.1 Extinction and schematic of hemisphere-capped gold nanorods . 20 3.2 FDTD simulation of hemisphere-capped gold nanorods . 22 3.3 Extinction and schematic of arrowhead-capped gold nanorods . 23 3.4 FDTD simulation of arrowhead-capped gold nanorods . 24 3.5 Summary of peak scattering and absorption cross sections for NR and ARNR 25 3.6 Experimental and calculated extinction of gold nanoshells . 26 4.1 TEM images of nanoshells before and after purification . 30 5.1 Schematic representation and cross sectional TEM image of plasmonic nanoshell CQD solar cell . 34 5.2 (a) Single pass absorption spectrum of PbS CQD films with and without embedded NS . 36 5.3 External quantum efficiency spectra and J-V characterization of represen- tative samples with and without NS embedded . 38 5.4 Internal quantum efficiency spectra of representative samples with and with- out NS embedded . 39 A-1 TEM image of typical NR . 50 A-2 TEM image of typical ARNR . 51 A-1 Double pass absorption spectrum of PbS CQD films with and without em- bedded NS . 55 Daniel Paz-Soldan University of Toronto ix 1 Introduction Plasmonic enhancement for colloidal quantum dot photovoltaics 1 Introduction Electricity, however produced, is now considered a basic right in the developed world. We demand it and we fight to keep costs low despite the grave implications for the environment.