  We studied the recombination dynamics of charge carriers in organic bulk
heterojunction solar cells made of the blend system poly(2,5-bis(3-dodecyl
thiophen-2-yl) thieno[2,3-b]thiophene) (pBTCT-C12):[6,6]-phenyl-C61-butyric
acid methyl ester (PC61BM) with a donor--acceptor ratio of 1:1 and 1:4. The
techniques of charge carrier extraction by linearly increasing voltage
(photo-CELIV) and, as local probe, time-resolved microwave conductivity (TRMC)
were used. We observed a difference in the initially extracted charge carrier
concentration in the photo-CELIV experiment by one order of magnitude, which we
assigned to an enhanced geminate recombination due to a fine interpenetrating
network with isolated phase regions in the 1:1 pBTCT-C12:PC61BM bulk
heterojunction solar cells. In contrast, extensive phase segregation in 1:4
blend devices leads to an efficient polaron generation resulting in an
increased short circuit current density of the solar cell. For both studied
ratios a bimolecular recombination of polarons was found using the
complementary experiments. The charge carrier decay order of above two for
temperatures below 300 K can be explained by a release of trapped charges. This
mechanism leads to a delayed bimolecular recombination processes. The
experimental findings can be generalized to all polymer:fullerene blend systems
allowing for phase segregation.
