Chlorinated unfused acceptor enabling 13.57% efficiency and 73.39% fill factor organic solar cells via fine-tuning alkoxyl chains on benzene core

Fluorinated noncovalently fused-ring electron acceptors (NFREAs) are more commonly exploited in highperforming organic solar cells (OSCs) than chemically cheaper chlorinated ones. Herein, by modulating central side chains on 2-(4-(thiophen-2-yl)phenyl)-dithieno[3,2-b:2 ',3 '-d]pyrrole (DBT) asymmetric core, three novel chlorinated NFREAs are synthesized within 4 steps. All three acceptors exhibit similar energy levels and narrow optical band gap (Egopt) below 1.40 eV. The 2-hexyldecyl substituted DBT-HD blends better with PBDB-T for fibrous phase separation and excellent pi-pi packing, contributing to more efficient exciton dissociation, higher and more balanced charge transport, and lower charge recombination in OSCs. As a result, the champion power conversion efficiency (PCE) of 13.57% and fill factor (FF) of 73.39% are achieved for PBDB-T:DBT-HD based devices, among the highest values for NFREAs based binary OSCs in the literature. The Eloss values of all three devices are found to be as low as 0.52-0.53 eV. Our work manifests that central side chain engineering may be an effective strategy in chlorinated NFREAs for satisfactory device performance, and the DBT core has great potential to explore cost-effective NFREAs.