文摘
The rate of photoinduced electron transfer (PET) (魏PET), quantum yield of PET (QYPET), and charge extraction yield (EQE) are determined for a series of donor鈥揳cceptor (DA) organic photovoltaic systems, comprising low-band-gap polymer donors and the phenyl-C61-butyric acid methyl ester (PCBM) acceptor. The energetic alignment of these polymer donors relative to PCBM provides driving forces for PET (螖GPET) in the range of 0.18鈥?.57 eV. Femtosecond transient absorption (TA) spectroscopy was used to assess the PET kinetics and QYPET, while time-resolved charge extraction (TRCE) measurements were employed to assess EQE. Near unity QYPET was observed in DA blend films with a 螖GPET of 0.57 and 0.30 eV, whereas no resolvable PET was observed with a 螖GPET of 0.18 eV. For the DA blends that exhibit PET, both 魏PET and QYPET appear independent of 螖GPET, with an average 魏PET of 420 fs for the 70% PCBM blends. An increase in nanosecond charge separation yield (TA) and EQE (TRCE) between DA systems was observed, which appears not to be due to the PET process but rather the subsequent recombination processes. DA systems should be designed to minimize 螖GPET, minimizing associated losses in device open-circuit potential; however, picosecond bimolecular recombination severely limits achievable charge extraction yields in these DA systems.