Relation of open circuit voltage to charge carrier density in organic bulk heterojunction solar cells

Abstract: The open circuit voltage Voc and the corresponding charge carrier density were measured in dependence of temperature and illumination intensity by current-voltage and charge extraction measurements for P3HT:PCBM and P3HT:bisPCBM solar cells. At lower temperatures a saturation of Voc was observed which can be explained by energetic barriers at the contacts (metal-insulator-metal model). Such injection barriers can also influence Voc at room temperature and limit the performance of the working solar cell, as was assured by macroscopic device simulations on temperature-dependent IV characteristics. However, under most conditions - room temperature and low barriers - Voc is given by the effective bandgap.