Theoretical Studies of Atomic Transport in Ternary Semiconductor Quantum Dots and Charge Transport in Organic Photovoltaic Active Layers

DEDICATION To my Mother, Father, and Sister v ACKNOWLEDGMENTS I would like to thank my advisor Prof. Dimitrios Maroudas for his guidance throughout my graduate research. I could not resolve all the challenging problems without his patience, encouragement, and insightful advice. I would also like to thank my committee members Prof. Mountziaris, Prof. Ramasubramaniam, and Prof. Venkataraman. Their suggestions during my proposal defense were very useful for the improvement of my final thesis. I also thank all my friends in Amherst. They are always there to share my happiness and difficulties. Finally, I would like to thank my family. My mother, father, and sister are always there to support me. Ternary semiconductor quantum dots with thermodynamically stable structures are particularly important for achieving optimal performance in optoelectronic and photovoltaic applications. Ternary quantum dots (TQDs) are typically synthesized in the form of core/shell structures. However, misfit strain induced by the abrupt core/shell interface can change the nature of the TQDs dramatically, leading to unstable optoelectronic function. In this thesis, a transient species transport model is developed to predict species distributions in TQDs during their thermal annealing. Specifically, the interdiffusion kinetics is analyzed of group-VI species in ZnSe 1-x S x and ZnSe 1-x Te x TQDs and of group-III species in In x Ga 1-x As TQDs. The modeling results are used to interpret the evolution of near-surface species concentration during thermal annealing and predict the equilibrium species distribution as a function of TQD size and composition. A database of constituent species transport properties is generated for further design of post-growth processes that enables the development of thermodynamically stable TQD structures with optimal optoelectronic function grown through simple one-step colloidal synthesis techniques. vii Nanoparticle assemblies of organic semiconducting materials are particularly appealing for next-generation organic photovoltaic (OPV) devices because their low-cost aqueous synthesis reduces the usage of chlorinated solvents. Another class of novel semiconducting materials, organometallic halide perovskites, have emerged as promising materials for solar cells because of their high photo-absorption coefficient and high power conversion efficiency (PCE). Based on deterministic charge carrier transport models, this thesis presents a computational analysis of charge transport in photovoltaic devices with active layers of the above two types of materials and develops design protocols for improving photovoltaic device efficiency. Our results demonstrate that charge transport efficiencies in centrifuged organic nanoparticle assemblies are comparable with those in drop cast thin films. The effects on charge …