Branched alkyl-chain engineering of chlorinated asymmetrical acceptors for improved organic photovoltaic performance

Side chains generally dictate molecular packing and film morphology and critically affect the efficiency of organic solar cells (OSCs), and hence, side-chain engineering plays a substantial role in achieving high-performance OSCs. In this work, a series of non-fullerene acceptor molecules with A-D-A structures, L6-L11, having gradient branched alkyl-chains on dithieno[3,2-b:2 ',3 '-d]pyrrole (DTP)-based asymmetrical chlorinated acceptors were designed and synthesized. The effects of branched alkyl-chain length, ranging from n-butyl to 2-decyldodecyl chains, on their optoelectronic properties, thin film molecular packing, blend film morphology and overall photovoltaic performance were systematically studied. Interestingly, the results indicated that with the increase in alkyl-chain length, the open-circuit-voltage (V-OC) is monotonously increased, while the short-circuit current density (J(SC)), fill factor (FF) and power conversion efficiencies (PCEs) perceive a distinct parabolic trend. The reasons for the variation trend of photoelectric parameters were analyzed. Finally, a 2-butyloctyl chain-containing acceptor L8-based device demonstrated a champion PCE of 15.40% with a V-OC of 0.864 V, a J(SC) of 23.63 mA cm(-2) and an FF of 0.754, which is the highest PCE for non-fullerene binary OSCs based on asymmetric ITIC-type acceptors. Further studies indicate that the proper 2-butyloctyl side chain could induce more favorable face-on molecule orientation, enhance carrier mobility, balance charge transport and suppress recombination loss. Our results will provide valuable guidelines for accelerating the understanding of the acceptor structure-photovoltaic performance relationship of OSC materials.

Automated summary

This study systematically investigated the effects of different side-chain alkyl chain lengths on the performance and photovoltaic efficiency of non-fullerene acceptors. The results showed that increasing alkyl chain length improved the open-circuit voltage but had a parabolic trend on short-circuit current density, fill factor, and power conversion efficiencies. The proper alkyl side chain also optimized the molecular packing and carrier mobility, enhancing the overall photovoltaic performance.