Effects of energy-level offset between a donor and acceptor on the photovoltaic performance of non-fullerene organic solar cells

Abstract

Minimizing the energy-level offset between a donor and acceptor in order to reduce energy loss (E_loss) is a hot topic in the field of organic solar cells (OSCs). However, for non-fullerene OSCs, the discussion concerning the relationship between energy-level offset and device performance has not yet reached a general conclusion due to their complex molecular structure and wide variety of photoactive materials. This study consists of a systematic investigation of this issue based on a typical polymer donor PBDB-TF and a series of A–D–A-type aromatic hybrid fused-ring electron acceptors with various energy levels. We investigate the charge separation of all devices and build the relationship between E_loss, exciton dissociation efficiency (P_diss) and energy-level offset. We find that in large ΔLUMO cases, E_loss and P_diss are positively correlated with ΔHOMO. Larger ΔHOMO could provide sufficient driving force for more efficient exciton dissociation, and thus, higher power conversion efficiency (PCE) in the corresponding non-fullerene OSCs. When ΔHOMO is below a particular threshold, the exciton dissociation process could be hindered severely, thereby deteriorating device performance. Therefore, sufficient driving force is still necessary for efficient exciton dissociation in PBDB-TF-based non-fullerene OSCs in order to achieve outstanding photovoltaic performance. The particular threshold for efficient exciton dissociation needs to be determined in future research.