Tuning of the conformation of asymmetric nonfullerene acceptors for efficient organic solar cells

In this work, three dithieno[3,2-b:2′,3′-d]pyrrol fused-ring electron acceptors (IPT-2F, IPTT-2F, and IPTTT-2F) have been successfully developed as efficient asymmetric nonfullerene acceptors (NFAs) for organic solar cells (OSCs). The molecular conformation of these NFAs can be subtly tuned by extending the donating cores with thiophene rings. Experimental and theoretical studies indicate the crucial role of the conformation change in asymmetric NFAs played in the aggregation of their blend films with PBDB-T. Indeed, the blend films with S-shaped IPT-2F and IPTTT-2F reveal less trap-assisted recombination and better microphase separation compared with C-shaped IPTT-2F. Decent power conversion efficiency (PCE) values of 14% and 12.3% were achieved for IPT-2F- and IPTTT-2F-based OSCs, respectively. Our results indicate the S-shaped conformation of asymmetric NFAs locked via S⋯O interactions is advantageous to fine-tune the morphology in the active layer for more efficient OSCs.