Time delayed collection field experiments on polymer: Fullerene bulk-heterojunction solar cells

The recombination of photogenerated charge carriers in poly[2-methoxy-5-(3('),7(')-dimethyloctyloxy)-1,4-phenylene vinylene]:1-(3-methoxycarbonyl)-propyl-1-phenyl-[6,6]C-61 bulk-heterojunction solar cells is investigated using the time delayed collection field technique. Here the lifetime of photogenerated electrons and holes that have escaped charge recombination can be determined from current measurements using a pulsed collection voltage that is delayed with respect to the excitation pulse. At 80 K, the number of long lived charge carriers decays in time according to t(-alpha) with alpha=0.2, practically independent of laser fluence in the range of 1-1000 mu J/cm(2). For excitation density < 4 mu J/cm(2) the number of long lived carriers (n(L)) depends linearly on the fluence. At higher fluence, n(L) is limited by a process that occurs in the time span between generation and carrier extraction under a constant bias (-4 V). Continuous background illumination reduces the lifetime of long lived carriers, probably by filling the low energy trap sites, for which a number density of 10(17) cm(-3) is estimated. Recombination of long lived carriers at 80 K in the 1-1000 mu s range is not limited by the (bimolecular) rate of nongeminate electron and hole encounter. Instead, the rate is limited by a monomolecular process, which may be the detrapping of one type of carrier or the recombination within a Coulombically bound electron-hole pair. (c) 2006 American Institute of Physics.