Electron Transfer Reactions in Quantum Dot Sensitized Solar Cells

In order to meet the clean energy demands of future generations, solar energy conversion must be achieved in a clean, inexpensive, and relatively efficient manner. Among the many candidate technologies currently vying to meet this need, the quantum dot sensitized solar cell shows much promise, however, the best prototypes of today fall short of the efficiency benchmarks necessary for mass implementation. This dissertation addresses the inefficiencies of quantum dot sensitized cells from the standpoint of electron transfer reactions.

The electron transfer rate from CdSe quantum dots to TiO2 nanoparticles was modeled with both a many-state Marcus model as well as a through space ballistic tunneling model. This particular reaction is among the first in a series of reactions which must occur to generate extractable photocurrent from a quantum dot sensitized solar cell. Electron transfer rates in this system were studied experimentally using ultrafast transient absorption spectroscopy through the elucidation of excited state dynamics of CdSe quantum dot sensitizers. Experimentally determined electron transfer rates across this junction were found to generally agree with prominent models. The phenomenon of hot carrier injection was not observed in these systems at room temperature. A comparison of measured electron transfer rates from CdSe quantum dots to various metal oxide nanoparticles with quantum dot sensitized solar cell device performance demonstrated that this particular electron transfer rate is not the rate limiting step of photocurrent production, and therefore does not contribute to device inefficiencies.

Two other electron transfer reactions which are unique to quantum dot sensitized solar cells, namely that from the electrolyte to the quantum dot, as well as that from TiO2 nanoparticles to the electrolyte, were also investigated with transient absorption spectroscopy and chronopotentiometry, respectively. A qualitative comparison between electron transfer and device performances demonstrates that interaction between the electrolyte and the TiO2 electron transport layer play a role in resultant device photocurrent. Given the studies presented here, future research is directed toward alternative electrolytes to improve quantum dot sensitized solar cell performance.