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 pi-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 (FP = 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.