Project Report

The application of quantum dots (QDs) in photovoltaic cells to enhance their conversion efficiencies is a promising and increasingly active field of research. Such cells are termed "future generation" or "third-generation" solar cells. There are several approaches to enhance efficiency in QD-based photovoltaic cells in comparison to conventional bulk semiconductor-based cells. Two such examples are hot carrier injection or multiexciton generation. However, before such phenomena can be utilized, a detailed understanding of the charge transfer mechanisms at the QD interface is needed. Through this NSF-EAPSI funded collaboration with Professor Masumoto at the University of Tsukuba, we examined the dynamics, kinetics, and injection yields of electron transfer from photoexcited PbS QDs to TiO2 nanoparticle acceptor states dispersed in organic suspensions. A fundamental understanding of the electron transfer processes at this interface is crucial for implementation of QDs into "third-generation" solar cells. In Dr. Masumoto’s research laboratory we used time-resolved spectroscopic techniques such as time-correlated single photon counting and up-conversion photoluminescence spectroscopy to characterize electron transfer on both a fast (femtosecond and picosecond) and slow (nanosecond) timescale. We found that electron transfer from PbS QDs molecularly linked to TiO2 nanoparticles occurs on the nanosecond-microsecond timescale. On the faster picosecond timescale, we observed the creation of biexcitons via two-photon absorption. The biexciton decay lifetime is consistent with reported values for Auger recombination. Biexciton relaxation through Auger recombination occurs when thermal energy given off from the recombination of one exciton is transferred to a second exciton creating a "hot" or non-thermalized excited state. Preliminary data suggest the possibility of "hot" electron transfer as a result of the Auger mechanism. Our interpretation of the biexciton decay and its application for hot carrier injection is ongoing. In addition to productive research, the NSF-EAPSI fellowship was reciprocally beneficial for myself and the Masumoto Research Group. I believe that the combination of physics and chemistry perspectives makes our ongoing collaboration unique.

National Science Foundation (NSF)
Office of International and Integrative Activities (IIA)
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Carter Kimsey
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Youker Diane
United States
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