Driving force dependence of electron transfer kinetics and yield in low-band-gap polymer donor-acceptor organic photovoltaic blends
The rate of photoinduced electron transfer (PET) (κPET), quantum yield of PET (QYPET), and charge extraction yield (EQE) are determined for a series of donor-acceptor (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 (ΔGPET) in the range of 0.18-0.57 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 ΔGPET of 0.57 and 0.30 eV, whereas no resolvable PET was observed with a ΔGPET of 0.18 eV. For the DA blends that exhibit PET, both κPET and QYPET appear independent of ΔGPET, with an average κPET 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 ΔGPET, minimizing associated losses in device open-circuit potential; however, picosecond bimolecular recombination severely limits achievable charge extraction yields in these DA systems.
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