Polymer non-fullerene solar cells of vastly different efficiencies for minor side-chain modification: impact of charge transfer, carrier lifetime, morphology and mobility

Abstract

The performance of the highly efficient PBDB-T : ITIC system with 11.25% power conversion efficiency is degraded significantly to 4.35% when ITIC is blended with a version of PBDB, dubbed PBDB-O, which has only a minor side-chain modification. We explored the reasons for this difference and investigated the impact of morphology, molecular packing and mobility on carrier lifetime and performance. We employed transient absorption spectroscopy to investigate the hole and electron carrier dynamics in each of the blends and observed significant differences in geminate recombination. In order to understand the influence of morphology and mobility on the carrier dynamics and recombination, we utilized grazing incident wide-angle X-ray scattering (GIWAXS), resonant soft X-ray scattering (RSoXS) and diode mobility measurements. Our results indicate that the difference in performance is difficult to explain with the small differences observed in morphology and packing, and seems to be dominated by the changes in intrinsic mobility that the side-chain modification engenders, which impact both charge creation and extraction. The results highlight the difficulties of predicting the impact of synthetic structural modifications on performance and on a specific device-relevant parameter.