Fluorination Strategy for Benzimidazole Core Based Electron Acceptors Achieving over 19% Efficiency for Ternary Organic Solar Cells

The expansion of two-dimensional conjugated systems in nonfullerene electron acceptors (NFAs) has significantly advanced the molecular design and efficiency potential of organic solar cells (OSCs). This study introduces a novel class of NFAs featuring a benzimidazole core with varying degrees of peripheral fluorination, designated as YIS-4F, YIS-6F, and YIS-8F, respectively. Through systematic modulation of fluorine content, we observed that OSCs incorporating YIS-6F achieved the highest power conversion efficiency (PCE) of 17.28%, surpassing those with YIS-4F and YIS-8F. Notably, the incorporation of YIS-6F in a ternary blend with D18/N3 yielded a remarkable PCE of 19.43%. The enhanced performance of YIS-6F-based devices is attributed to the optimized energy level alignment and optimized crystallinity, which collectively facilitate efficient exciton dissociation, accelerated charge transport, and minimized charge recombination, culminating in an exceptional fill factor and PCE. Our findings underscore the pivotal role of fluorination of NFAs at the central benzimidazole core in optimizing molecular packing, and consequently enhancing the performance of OSCs.