Interplay Among Locally Excited, Change-Transfer, and Charge-Separated States in a Donor-Acceptor Copolymer Single Chain

Abstract

Organic photovoltaics (OPVs) have been extensively highlighted as promising photovoltaics for realizing low-cost solar energy conversion with light weight and applicability for flexible devices. With introducing non-fullerene acceptors, the performance of bulk-heterojunction OPV devices has significantly improved, reaching the power-conversion efficiency (PCE) higher than 18%. In this work, we revealed the photoinduced dynamics in a single chain of donor−acceptor (D-A) polymer, PM6, which has been widely used as a donor in the state-of-the-art non-fullerene OPV systems with its top-notch PCE performance[1]. With the femtosecond-resolved absorption and fluorescence spectroscopy spanning sub-ps to μs time domains, we explored the evolution of locally excited (LE), charge-transfer (CT), charge-separated (CS) states in the single chain of PM6[2]. Our observation revealed that an intrachain CT state forms within 200 fs and establishes an ultrafast equilibrium with the parent LE state. We tracked the dependence of the evolution of these equilibrated states on the length of the chain. The equilibrated LE and intrachain CT states persist until they diffuse and reach a chain folding. At the chain folding, the intrachain CT state quickly transforms into an interchain CT state that bifurcates into forming a CS state or annihilation within a picosecond. Our findings highlight the role of chain foldings for maximizing polaron generation in OPVs based on PM6.