Driving Force Dependence of Electron Transfer Kinetics and Yield in Low-Band-Gap Polymer Donor–Acceptor Organic Photovoltaic Blends

Abstract

The rate of photoinduced electron transfer (PET) (κ_PET), quantum yield of PET (QY_PET), 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 (Δ<i>G</i>_PET) in the range of 0.18–0.57 eV. Femtosecond transient absorption (TA) spectroscopy was used to assess the PET kinetics and QY_PET, while time-resolved charge extraction (TRCE) measurements were employed to assess EQE. Near unity QY_PET was observed in DA blend films with a Δ<i>G</i>_PET of 0.57 and 0.30 eV, whereas no resolvable PET was observed with a Δ<i>G</i>_PET of 0.18 eV. For the DA blends that exhibit PET, both κ_PET and QY_PET appear independent of Δ<i>G</i>_PET, with an average κ_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 Δ<i>G</i>_PET, minimizing associated losses in device open-circuit potential; however, picosecond bimolecular recombination severely limits achievable charge extraction yields in these DA systems