Effect of the highest occupied molecular orbital energy level offset on organic heterojunction photovoltaic cells

The effect of the highest occupied molecular orbital energy level offset, Delta E(HOMO), on organic photovoltaic cells has been investigated using a set of donor/C(60) heterojunction structures with Delta E(HOMO) up to 1.1 eV provided by various donors. The open-circuit voltage decreases with Delta E(HOMO) whereas the short-circuit current and fill factor increase with Delta E(HOMO) but roll off with Delta E(HOMO) >0.7 eV. These characteristics can be simulated using a Braun-Onsager model and assuming an interfacial charge-transfer state with an initial charge separation distance proportional to Delta E(HOMO) and an increased decay rate at high Delta E(HOMO). (C) 2010 American Institute of Physics. [doi:10.1063/1.3491214]