Charge carrier photogeneration and decay dynamics in the poly(2,7-carbazole) copolymer PCDTBT and in bulk heterojunction composites with PC70BM

Charge carrier photogeneration and decay were studied in poly[N-9(')-hepta-decanyl- 2,7-carbazole-alt-5,5-(4('),7(')-di-2-thienyl-2('),1('),3(')-benzothiadiazole)], PCDTBT, and in phase-separated mixtures of PCDTBT with the fullerene derivative [6,6]-phenyl C-70-butyric acid methyl ester (PC70BM). The absorption spectrum of PCDTBT exhibits two distinct absorption bands centered at approximately 400 nm and 600 nm, respectively. In pristine PCDTBT, higher carrier quantum efficiency and lower photoluminescence efficiency are observed following excitation via the higher-energy absorption band. In contrast, in the PCDTBT-fullerene bulk heterojunction material, the carrier generation efficiency is an order of magnitude larger but essentially independent of the excitation wavelength. The photoinduced absorption of PCDTBT:PC70BM is interpreted in terms of ultrafast charge transfer, trap filling, and long-lived mobile carriers.