Relation of open circuit voltage to charge carrier density in organic   bulk heterojunction solar cells
  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. 
Daniel Rauh, Alexander Wagenpfahl, Carsten Deibel, Vladimir Dyakonov
cond-mat.mtrl-sci