Recombination in polymer-fullerene bulk heterojunction solar cells

Recombination of photogenerated charge carriers in polymer bulk\nheterojunction (BHJ) solar cells reduces the short circuit current (Jsc) and\nthe fill factor (FF). Identifying the mechanism of recombination is, therefore,\nfundamentally important for increasing the power conversion efficiency. Light\nintensity and temperature dependent current-voltage measurements on polymer BHJ\ncells made from a variety of different semiconducting polymers and fullerenes\nshow that the recombination kinetics are voltage dependent and evolve from\nfirst order recombination at short circuit to bimolecular recombination at open\ncircuit as a result of increasing the voltage-dependent charge carrier density\nin the cell. The "missing 0.3V" inferred from comparison of the band gaps of\nthe bulk heterojunction materials and the measured open circuit voltage at room\ntemperature results from the temperature dependence of the quasi-Fermi-levels\nin the polymer and fullerene domains - a conclusion based upon the fundamental\nstatistics of Fermions.\n