Photocurrent enhancement in polymer:fullerene bulk heterojunction solar cells doped with a phosphorescent molecule

We demonstrate photocurrent enhancement of up to 20% in polymer:fullerene bulk heterojunction photovoltaic cells via the incorporation of a phosphorescent dopant, without degradation in the open-circuit voltage or fill factor of the device. The enhancement is shown to originate from multiple sources. First, the phosphor is able to populate the long-lived triplet state of the polymer, leading to longer diffusion length and a larger polymer contribution. Also, there is direct absorption on the dopant leading to enhanced spectral coverage. Finally, the dopant acts as a donor site and therefore increases the fullerene signal. (C) 2009 American Institute of Physics. [doi:10.1063/1.3257383]