Influence of the Intermediate Density-of-States Occupancy on Open-Circuit Voltage of Bulk Heterojunction Solar Cells with Different Fullerene Acceptors

Electron density of states (DOS) and recombination kinetics of bulk heterojunction solar cells consisting of a poly(3-hexylthiophene) (P3HT) donor and two fullerene acceptors, either [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) or 4,4'-dihexyloxydiphenylmethano[60]fullerene (DPM(6)), have been determined by means of impedance spectroscopy. The observed difference of 125 mV in the output open circuit voltage is attributed to significant differences of the occupancy of the DOS in both fullerenes. Whereas DPM(6) exhibits a full occupation of the electronic band, occupancy is restricted to the tail of the DOS in the case of PCBM-based devices, implying a higher rise of the Fermi level in the DPM(6) fullerene. Carrier lifetime describes a negative exponential dependence on the open.. circuit voltage, exhibiting values on the microsecond scale at 1 sun illumination.