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.

Automated summary

Fluorinated noncovalently fused-ring electron acceptors (NFREAs) are commonly used in high-performing organic solar cells (OSCs) due to their better performance compared to the chemically cheaper chlorinated ones. By modulating central side chains, three novel chlorinated NFREAs were synthesized and one of them, DBT-HD, exhibited better performance in terms of efficient exciton dissociation and charge transport when blended with PBDB-T. This resulted in achieving a champion power conversion efficiency (PCE) and fill factor (FF) for PBDB-T:DBT-HD based devices among the highest values reported in literature for NFREAs based binary OSCs.