Regulating the Crystallinity and Self‐Aggregation of Fused Ring Electron Acceptors via Branched Side‐Chain Engineering for Efficient Organic Solar Cells

Abstract Fine‐tuning the crystallinity and self‐aggregation features of donors/acceptor materials toward high‐efficiency organic solar cells (OSCs) is of crucial importance. Here, a convenient yet effective way to simultaneously control the crystallinity and self‐aggregation of the fused ring electron acceptor (FREA) is demonstrated by altering the length of the first‐position branched alkyl chain on the cyclic unit. Specifically, three carbazole‐based FREAs, 4TC‐4F‐C6C6, 4TC‐4F‐C8C8, and 4TC‐4F‐C10C10, are synthesized by changing the length of the first‐position branched alkyl chain on the carbazole unit. The crystallinity of the studied acceptors decreases as the branched alkyl chain is lengthened. The ability of the acceptors to undergo self‐aggregation decreases in the order 4TC‐4F‐C10C10, 4TC‐4F‐C6C6, and 4TC‐4F‐C8C8. The medium crystallinity and lower self‐aggregation properties of 4TC‐4F‐C8C8 result in favorable phase separation when blended with poly‐[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl‐3‐fluoro)thiophen‐2‐yl)‐benzo[1,2‐b:4,5‐b′]dithiophene))‐alt‐(5,5‐(1′,3′‐di‐2‐thienyl‐5′,7′‐bis(2‐ethylhexyl)benzo[1′,2′‐c:4′,5′‐c′]dithiophene‐4,8‐dione))] (PM6), which is conducive to effective exciton dissociation and charge transport. Consequently, the OSC device based on PM6:4TC‐4F‐C8C8 delivers the best power conversion efficiency of 14.85%.