  We determined the dominant polaron recombination loss mechanism in pristine
and annealed polythiophene:fullerene blend solar cells by applying the
photo-induced charge extraction by linearly increasing voltage (photo-CELIV)
method in dependence on temperature. In pristine samples, we find a strongly
temperature dependent bimolecular polaron recombination rate, which is reduced
as compared to the Langevin theory. For the annealed sample, we observe a
polaron decay rate which follows a third order of carrier concentration almost
temperature independently.
