Thursday, October 1, 2009

Does quantum coherence matter in organic solar cells?

At today's COPE science meeting we are discussing a paper, Coherent Intrachain Energy Migration in a Conjugated Polymer at Room Temperature, published in Science early this year by Collini and Scholes. The paper was chosen by Paul Schwenn.
There is also a longer much more detailed description of the work in J. Phys. Chem. A, which helps understand some of the details, particularly because it has some nice pedagogical figures.
Use is made of two-dimensional spectroscopy, for understanding that I find this Figure very helpful.

A key issue in photosynthesis and organic solar cells is the harvesting of light. One wants to channel excitons to "reaction centres" where charge separation will occur, as rapidly as possible and with the maximum efficiency. EET (electronic energy transfer) or exciton transfer usually occurs between molecules via the Forster mechanism (FRET). Often this is incoherent (and irreversible), i.e., there is no phase coherence between the wavefunction of the excitation on the donor and acceptor molecules. If phase coherence is present one can observed quantum interference effects such as Rabi oscillations associated with an excition oscillating back and forward between the donor and acceptor.

An important question is whether coherent transfer will increase the transfer rate and efficiency. Most people seem to claim it does but this is not clear to me. Some of these issues are discussed in an earlier post on similar experiments on a photosynthetic system.

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