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

The open circuit voltage V-oc and the corresponding charge carrier density were measured in dependence of temperature and illumination intensity by current-voltage and charge extraction measurements for poly(3-hexylthiophene-2,5-diyl) (P3HT):[6,]-phenyl-C-61 butyric acid methyl ester (PCBM) and P3HT:bisPCBM solar cells. At lower temperatures a saturation of V-oc was observed which can be explained by energetic barriers at the contacts (metal-insulator-metal model). Such injection barriers can also influence V-oc 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-V-oc is given by the effective band gap. (C) 2011 American Institute of Physics. [doi:10.1063/1.3566979]