Limits of open circuit voltage in organic photovoltaic devices

Open circuit voltage (V-oc) 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 V-oc 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 (Delta phi(electrodes)) between ITO and Al electrode is in the range -3 and 0 eV, V-oc increases linearly with Delta phi(electrodes) as prescribed by the MIM model. Outside this range, V-oc saturates with values close to that given by the HOMOD-LUMOA less the exciton binding energy.