Prof. John Asbury, Pennsylvania State University

Pathways to More Efficient Organic Solar Cells: what we can learn by watching electrons move in real time

Physical Chemistry Seminar Series

Organic solar cells are promising candidates for inexpensive photovoltaics for large area applications because they can be processed from solution using roll-to-roll technology. The efficiencies of current organic solar cells are limited by partial overlap with the solar spectrum and sub-optimal open-circuit voltage characteristics. Efforts to extend the absorption spectrum of organic solar cells into the near-infrared have produced many promising low band-gap polymers, but enhancements in device efficiency have been incremental. New understanding of the photophysics of these low band-gap polymers suggests that the efficiency of the corresponding devices is limited by incomplete charge separation at electron donor/acceptor interfaces. To elucidate the origin of this limitation, we undertook a study of the dynamics of charge separation in a photovoltaic polymer blend consisting of the conjugated polymer, CN-MEH-PPV, blended with the electron accepting functionalized fullerene, PCBM, using ultrafast vibrational spectroscopy. We take advantage of a solvatochromic shift of the vibrational frequency of the carbonyl (C=O) stretch of PCBM to directly measure the rate of escape of electrons from their Coulombically bound radical pairs. Our findings demonstrate that the rate of free carrier formation is temperature independent indicating that excess vibrational energy resulting from the electron transfer reaction plays an important role in mediating charge separation. These observations suggest that efforts to develop new low band-gap polymers for organic solar cells should target electron donor and acceptor pairs capable of advantageously redistributing excess vibrational energy to efficiently separate charge with minimal donor-acceptor energy level offsets.

For more information contact Prof. Christine Payne (404-385-3125).