Pseudohalogen regulation of benzimidazole small molecule acceptor to optimize molecular crystallinity in Q-PHJ organic solar cells

The cyano group is a pseudohalogen with chemical properties similar to halogens. It has the ability to regulate the stacking and energy levels of molecules in organic solar small molecule non-fullerene acceptor materials. In this study, we introduce two novel alkylated derivatives, BMIC-CN-Me and BMIC-CN-ⁱPr, featuring a cyano-modified benzimidazole core. This central nucleus design is reported for the first time, offering a unique approach to molecular engineering in organic photovoltaics. BMIC-CN-Me, in particular, enhances device performance by meticulously managing molecular stacking through steric hindrance. Analysis via single-crystal X-ray diffraction reveals five distinct stacking modes, with an average intermolecular distance of approximately 3.31 Å, optimizing the efficiency of charge transfer. This precise molecular arrangement elevates the photoconversion efficiency (PCE) of the non-fullerene acceptor, based on the benzimidazole core, to an impressive 17.6 %. Moreover, the material demonstrates exceptional stability, retaining over 80 % of its initial efficiency after 1200 h in a glove box and maintaining more than 80 % of its original efficiency after 500 h of continuous simulated solar irradiation. Compared to the 2-methylimidazole-derived molecules acceptor, devices prepared using BMIC-CN-Me demonstrate superior performance due to their better-matched energy levels and UV–Vis absorption spectrum. Furthermore, when contrasted with BMIC-CN-ⁱPr, BMIC-CN-Me exhibits a more tightly packed molecular arrangement. These factors contribute to more effective exciton dissociation, accelerated charge transport, and the suppression of recombination, leading to an enhanced fill factor and overall photoelectric conversion efficiency. The quasi-planar heterojunction architecture further bolsters device stability. The cyano-modified benzimidazole structure provides additional non-covalent interaction sites, which augment the material’s stability. Our findings indicate that the cyano substitution of the benzimidazole core not only enhances material stability and molecular aggregation but also significantly improves the performance of organic solar cells. This innovative approach to molecular design holds promise for the development of high-performing and durable organic photovoltaic materials.