Effect of π-bridge lengths in oligomerized fused-ring electron acceptors on the photovoltaic performance of organic solar cells

Abstract

Oligomer acceptors combine the advantages of both small molecule non-fullerene acceptors and polymer acceptors, offering significant potential for achieving high-efficiency and stable organic solar cells (OSCs). Developing novel central units and incorporating novel linking sites are crucial for controlling molecular aggregation and packing behavior, thereby optimizing film morphology. In this study, two conjugated backbones with different π-bridge unit lengths were utilized to link the central fused-cores of non-fullerene acceptors (NFAs) to construct two novel oligomer acceptors (TBT and CH-TRZ). The designed acceptors with a specific A–DA′D–A structure exhibit a fibrillar network morphology in the film, enhancing charge transport and stability. Notably, introducing the Y5-analogue acceptor conjugate chain (Y5-BO) as a π-bridge unit yielded excellent optoelectronic properties, including a red-shifted absorption spectrum, higher absorption coefficient, improved crystallinity and molecular aggregation, and enhanced electron mobility with moderate molecular packing. Therefore, after subtle tradeoff between open-circuit voltage (V_OC, 0.948 V) and short current density (J_SC, 23.79 mA cm⁻²), an exciting efficiency of 17.07% was achieved in CH-TRZ-based binary bulk-heterojunction OSCs, along with excellent long-term stability (T_80% > 3000 h). This work establishes a novel and feasible strategy for designing specific A–DA′D–A oligomer acceptors and highlights the great potential of π-bridge unit engineering and novel linking sites in achieving efficient and stable OSCs.