Influence of phase segregation on recombination dynamics in organic bulk-heterojunction solar cells

The recombination dynamics of charge carriers in organic bulk-heterojunction (BHJ) solar cells made of the blend system poly(2,5-bis(3-dodecylthiophen-2-yl) thieno[2,3-b]thiophene) (pBTCT-C ₁₂ ):[6,6]-phenyl-C ₆₁ -butyric acid methyl ester (PC ₆₁ BM) with a donor-acceptor ratio of 1:1 and 1:4 are studied here. The techniques of charge-carrier extraction by linearly increasing voltage (photo-CELIV) and, as local probe, time-resolved microwave conductivity are used. A difference of one order of magnitude is observed between the two blends in the initially extracted charge-carrier concentration in the photo-CELIV experiment, which can be assigned to an enhanced geminate recombination that arises through a fine interpenetrating network with isolated phase regions in the 1:1 pBTCTC ₁₂ :PC ₆₁ BM BHJ solar cells. In contrast, extensive phase segregation in 1:4 blend devices leads to an efficient polaron generation that results in an increased shortcircuit current density of the solar cells. For both studied ratios a bimolecular recombination of polarons is found using the complementary experiments. The charge-carrier decay order of above two for temperatures below 300 K can be explained on the basis of a release of trapped charges. This mechanism leads to delayed bimolecular recombination processes. The experimental findings can be generalized to all polymer:fullerene blend systems allowing for phase segregation.