Delocalization suppresses nonradiative charge recombination in polymer solar cells

Abstract

Suppressing nonradiative deactivation of charge transfer (CT) states is pivotal to realizing further improvements in the power conversion efficiencies of polymer solar cells (PSCs). According to the energy gap law, the nonradiative decay rate constant knr scales exponentially with decreasing CT state energy ECT; thereby, as long as knr is governed by the energy gap law, a decrease in ECT will inevitably increase nonradiative deactivation of CT states and hence decrease the power conversion efficiency. Here, we report the nonradiative decay dynamics of CT states generated in various nonfullerene-acceptor-based PSCs by using transient absorption spectroscopy. The absence of a strong correlation between knr and ECT indicates that the energy gap law is not valid for these PSCs and that parameters other than ECT contribute significantly to knr. We found that knr decreased with an increase in materials’ crystallinities, indicating that increasing crystallinity leads to CT state delocalization, which in turn mitigates the nonradiative deactivation of CT states.