Suppression of geminate charge recombination in organic photovoltaic devices with a cascaded energy heterojunction

The effect of cascaded energy heterojunctions on geminate charge recombination in organic photovoltaic devices is examined using a kinetic Monte Carlo model. The structure of the cascaded heterojunction, which encourages spatial separation of the geminate charge pair, is varied to recreate that found in ternary blends and tri-layers, as well as that formed by self-organization in binary blends in which one component crystallizes. It is shown that substantial reductions in charge recombination can indeed be achieved with parameters similar that reported for P3HT:PCBM solar cells. However, the efficacy of cascaded energy heterojunctions is shown to be limited for thick cascade layers (>10 nm). This provides guidance as how to design ternary organic photovoltaics, whilst also offering a possible explanation of low recombination efficiency in some semi-crystalline OPVs.