Enhanced charge separation by interchain hole delocalization in nonfullerene acceptor-based bulk heterojunction materials

Abstract

Bulk heterojunction (BHJ) composites show improved power conversion efficiencies when optimized in terms of morphology using various film processing methods. A reduced carrier recombination loss in an optimized BHJ was characterized previously. However, the driving force that leads to this reduction was not clearly understood. In this study, we focus on the decreased carrier recombination loss and its driving force in optimized nonfullerene acceptor-based PTB7-Th:IEICO-4F BHJ composites. We demonstrate that the optimized BHJ shows deactivation in the sub-nanosecond nongeminate carrier recombination process. The driving force for this deactivation was determined to be the improved interchain hole delocalization between the polymers. An enhanced interchain hole delocalization was observed using steady-state photoinduced absorption (PIA) spectroscopy. In particular, increased splitting between the polaron PIA bands was noted. Moreover, improved interchain hole delocalization was observed for other state-of-the-art BHJ materials, including D18:Y6 with optimized morphologies.