Semiconductor quantum dots have potential to be an alternative to the high cost and process-intensive materials that are currently used in photovoltaic devices. Colloidal quantum dots are simple to process via solution-based chemistry and have highly tunable absorption properties that make them ideal for use in photovoltaic devices. However, due to their small size and large surface area to volume ratio, quantum dots are highly sensitive to their chemical environment. Thus it is necessary to understand the reactive nature of the quantum dot surface and determine both ways to circumvent this sensitivity as well as ways to use it to our advantage. More specifically, it is important to demonstrate ways in which surface-adsorbed species influence the excited state behavior of these QDs.
This document first describes the sensitivity of the quantum dot surface to adsorbed oxygen. In a long-term storage study, it was demonstrated that the most oxidative environment was under the exposure of light in the presence of atmospheric oxygen. This was shown to oxidize the nanoparticle surface, a process which was heavily dependent on the relative coverage of the quantum dot surface by organic surfactant molecules.
The importance of these molecules in dictating the excited state properties of quantum dots prompted the study of bi-functional organic linker molecules, which are typically employed to anchor quantum dots to TiO2. Specifically, these studies show that the chemical nature of the linking molecule heavily affects the rate constant of electron transfer between CdSe and TiO2. Linker molecules were assessed on their functional groups, lengths and lower unoccupied molecular orbitals (LUMOs) to determine ideal conditions for electron transfer reactions. These results were modeled using rectangular and trapezoidal tunnel barriers that approximate the barrier created by each molecule and determine the effects of barrier shape on the rate constant of electron transfer reaction.
Lastly, the reaction of the CdSe surface with S2- species was studied to better understand the mechanism for hole transfer in quantum dot solar cells. From these studies it was determined that S2- in solution binds to the quantum dot surface and stabilizes the charges as they separate in the quantum dots. As the holes are stabilized on the quantum dot surface there is a greater probability of electron injection into TiO2. The studies described in this dissertation highlight the sensitivity of the quantum dot surface to storage environment while elucidating information about electron and hole transfer processes that are imperative to the function of quantum dot solar cells.