Limits of open circuit voltage in organic photovoltaic devices

Open circuit voltage (Voc) of organic photovoltaic devices has been interpreted with either the metal-insulator-metal (MIM) model or the energy offset between highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor (HOMOD -LUMOA). To elucidate the relation between Voc and the two models, we have used electrodes of a wide range of work functions to connect the CuPc/C60 organic photovoltaic devices. We found that when the work function difference (Δ φelectrodes) between ITO and Al electrode is in the range -3 and 0 eV, Voc increases linearly with Δ φelectrodes as prescribed by the MIM model. Outside this range, Voc saturates with values close to that given by the HOMOD -LUMOA less the exciton binding energy.