The Crucial Influence of Fullerene Phases on Photogeneration in Organic Bulk Heterojunction Solar Cells

The conjugated polymer, poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b] thiophene) (pBTTT-C-16), allows a systematic tuning of the blend morphology by varying the acceptor type and fraction, making it a well-suited structural model for studying the fundamental processes in organic bulk heterojunction solar cells. To analyze the role of intercalated and pure fullerene domains on charge carrier photogeneration, time delayed collection field (TDCF) measurements and Fourier-transform photocurrent spectroscopy (FTPS) are performed on pBTTT-C-16:[6,6]-phenyl-C-61-butyric acid methyl ester (PC61BM) solar cells with various stoichiometries. A weak influence of excess photon energy on photogeneration along with a photogeneration having a weaker field dependence at increasing fullerene loading is found. The findings are assigned to a dissociation via thermalized charge transfer (CT) states supported by an enhanced electron delocalization along spatially extended PC61 BM nanophases that form in addition to a bimolecular crystal (BMC) for PC61 BM rich blends. The highly efficient transfer of charge carriers from the BMC into the pure domains are studied further by TDCF measurements performed on non-intercalated pBTTT-C-16:bisPC(61) BM blends. They reveal a field dependent charge generation similar to the 1:4PC(61) BM blend, demonstrating that the presence of pure acceptor phases is the major driving force for an efficient, field independent CT dissociation.