Development of a Liquid Contacting Method for Investigating Photovoltaic Properties of PbS Quantum Dot Solids
Photovoltaic (PV) devices based on PbS quantum dot (QD) solids demonstrate high photon-to-electron conversion yields. However, record power conversion efficiencies remain limited mainly due to bulk and interfacial defects in the light absorbing material (QD solids). Interfacial defects can be formed when a semiconductor, such as QD solid, is contacted by another material and may predetermine the semiconductor/metal or semiconductor/metal-oxide junction properties. The objective of the work
... ve of the work described in this dissertation was set to explore whether electrochemical contacting using liquid electrolytes can provide sufficient means of contacting the QD solids to investigate their PV performance without introducing the unwanted interfacial defects. I have initially focused on optimizing processing conditions for efficient QD solids deposition and studied their photovoltaic properties in a standardized solid-state, depleted heterojunction solar cell configuration. Further, a liquid contacting method was developed to study the relationship between photovoltages of QD solids and the energetics (e.g. reduction potentials) of the liquid contacting media. This electrochemical contacting of PbS QD solids was achieved by using anhydrous liquid electrolytes containing fast, non-coordinating, outer-sphere redox couples. Depending on the energetics of a redox couple, both rectifying and nonrectifying (Ohmic) PbS QD solid/electrolyte junctions were successfully formed with both p-and n-type QD solids. Furthermore, application of the liquid solution contacting method in studies of the PbS QD solids has unprecedentedly demonstrated that an ideal behavior of the photovoltage changes with respect to the changes in the energetics of the contacting media can be achieved. This fact supports the initially proposed hypothesis that such liquid ii contacting method will not introduce surface defects to the studied QD materials, allowing for their intrinsic properties to be better understood. The applicability of this method to both p-and n-type QD solids was demonstrated. Finally, a better understanding of the relationships between the surface and ligand chemistries of both p-and n-type QD solids and their photovoltaic properties was possible via applications of such method in conjunction with XPS and UPS studies. iii Dedication This work is dedicated to my wife, Marina, my parents and my best friends, whose support brings immeasurable values to my life, passions, and education. "To raise new questions, new possibilities, to regard old problems from a new angle, requires creative imagination and marks real advance in science." Albert Einstein iv Acknowledgments I am thoroughly thankful to my research adviser, Dr. Erik Johansson, for guiding and assisting me in my research adventures. His exceptional electrochemistry knowledge, and the accessible way of teaching it, has significantly inspired me to always strive to understand more about the theories and areas of study I was going through. I also want to thank all professors, I had fruitful times working with, at both Portland State and Kyiv State Universities, and in both laboratory and class environments. Their shared experiences do bring unpriced values to my academic and professional growth and teach me how to ask new questions, open new possibilities, solve problems, and how to turn interesting ideas into realizable projects. Rest assured, according to the quote above.