  We performed temperature dependent transient photovoltage and photocurrent
measurements on poly(3-hexylthiophene):[6,6]-phenyl-C61 butyric acid
methylester bulk heterojuction solar cells. We found a strongly charge carrier
concentration and temperature dependent Langevin recombination prefactor. The
observed recombination mechanism is discussed in terms of bimolecular
recombination. The experimental results were compared with charge carrier
extraction by linearly increasing voltage (photo-CELIV) measurements done on
the same blend system. We explain the charge carrier dynamics, following an
apparent order larger than two, by dynamic trapping of charges in the tail
states of the gaussian density of states.
