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

Abstract

Fluorinated noncovalently fused-ring electron acceptors (NFREAs) are more commonly exploited in high-performing 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 π-π 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.