Bimolecular recombination losses in polythiophene: Fullerene solar cells

Transient absorption spectroscopy is employed to monitor charge carrier decay dynamics in an annealed poly(3-hexylthiophene): methanofullerene solar cell. Comparisons of device and film data and data under applied bias demonstrate that these dynamics are dominated by bimolecular recombination. These data allow us to quantify the rate constant for bimolecular recombination, found to be strongly carrier density dependent, and thus determine the bimolecular recombination flux. By comparison with the device short-circuit photocurrent we conclude that the open-circuit voltage is primarily limited by bimolecular recombination.