Cascade Organic Solar Cells

We demonstrate planar organic solar cells consisting of a series of complementary donor materials with cascading exciton energies, incorporated in the following structure: glass/indium-tin-oxide/donor cascade/C 60 /bathocuproine/Al. Using a tetracene layer grown in a descending energy cascade on 5,6-diphenyl-tetracene and capped with 5,6,11,12-tetraphenyl-tetracene, where the accessibility of the π-system in each material is expected to influence the rate of parasitic carrier leakage and charge recombination at the donor/acceptor interface, we observe an increase in open circuit voltage ( V oc ) of approximately 40% (corresponding to a change of +200 mV) compared to that of a single tetracene donor. Little change is observed in other parameters such as fill factor and short circuit current density ( FF = 0.50 ± 0.02 and J sc = 2.55 ± 0.23 mA/cm 2 ) compared to those of the control tetracene–C 60 solar cells ( FF = 0.54 ± 0.02 and J sc = 2.86 ± 0.23 mA/cm 2 ). We demonstrate that this cascade architecture is effective in reducing losses due to polaron pair recombination at donor–acceptor interfaces, while enhancing spectral coverage, resulting in a substantial increase in the power conversion efficiency for cascade organic photovoltaic cells compared to tetracene and pentacene based devices with a single donor layer.