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

Abstract

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.