Marcus Hole Transfer Governs Charge Generation and Device Operation in Nonfullerene Organic Solar Cells

Abstract

A critical fundamental question regarding how the energetics governs the hole transfer (HT) rate, efficiency, and device performance in nonfullerene acceptor (NFA) organic solar cells (OSCs) remains unclear. In this study, we thoroughly investigate the HT process in a large group of donor/NFA blends with driving forces varied by ∼0.5 eV. We show that the HT rate increases by more than 2 orders of magnitude with increasing driving force, regardless of materials, which can be well-described by the Marcus electron transfer model with a normal region behavior. Importantly, HT efficiency that depends on the competition between HT and intrinsic relaxation of NFAs remains above 80% as long as the driving force is larger than 50 meV, setting a critical threshold for high HT efficiency. The multivariable correlation between driving force and device performance is also analyzed in connection with the nanoscale morphology. The driving force and morphology affect device performance from different aspects, enabling a new knob for manipulating internal electronic processes separately to obtain optimized device performance.