Tue, Nov. 11, 2014, 4:30pm
Frick Chemistry Laboratory, Taylor Auditorium
Host: Greg Scholes
Photo-induced electron transfer processes in organic photovoltaic solar cells
Although the rapid growth in conversion efficiencies of organic photovoltaic solar cells is subsiding, the underlying science of how these devices function remains both topical and important. This presentation is motivated by a desire to understand the role that the extensive history of photo-induced electron transfer in molecular systems can play in addressing some of the more fundamental science questions. The mechanism of charge carrier generation and recombination in polymer-based bulk heterojunction solar cells remains a topic of great interest and debate. In addition to the difficulties in deciding where in the complex polymer-fullerene blend carriers are generated, an agreed understanding of the actual exciton-to-carrier conversion process remains equally elusive. At the heart of this misunderstanding is the role played by a charge-transfer (CT) state, which is assumed to reside at the polymer-fullerene interface. Most models locate this species at the interface of a pure polymer phase with a pure fullerene phase, and cite carrier delocalization as a means of explaining how the strong coulombic binding energy is overcome to yield the separated carriers. And yet there is also growing opinion that a well-mixed phase of polymer and fullerene is responsible for the majority of free carriers. So how do we rationalize these two scenarios?
Using flash photolysis, time-resolved microwave conductivity (fp-TRMC) and photoluminesence spectroscopy, the presentation will focus on carrier generation and recombination mechanisms in conjugated polymers that are doped lightly with a range of molecular dopants that includes both fullerenes and a number of macrocycles.