Organic solar cells (OSCs) as the promising green energy technology have drawn much attention in the last two decades. In comparison to polymer solar cells, small-molecule organic solar cells (SMOSCs) have the advantages of precise chemical structure and molecular weight, purification feasibility, batch reproducibility, etc. Despite of the recent advances in molecular design, the efficiencies of SMOSCs are still lagging behind those of polymer-based OSCs. In this work, a new small-molecule donor (SMD) with a fused-ring-connected bridge denoted F-MD has been designed and synthesized. When F-MD was applied into SMOSCs, the F-MD:N3 blends exhibited a power conversion efficiency (PCE) of over 13%, which is much higher than that of the linear π-bridged molecule L-MD based devices (8.12%). Further studies revealed that the fused-ring design promoted the planarity of the molecular conformation and facilitated charge transport in OSCs. More importantly, this strategy also lowered the crystallinity and self-aggregation of the films, and hence optimized the microstructure and phase separation in the corresponding blends. Thereby, the F-MD-based blends have been evidenced to have better exciton dissociation and reduced charge recombination in comparison with the L-MD counterparts, explaining the enhanced PCEs. Our work demonstrates that the fused-ring π-bridge strategy in small-molecule-donor design is an effective pathway to promote the efficiency of SMOSCs as well as enhance the diversity of SMD materials.
Keywords: charge transport; fused-ring structure; phase separation; small-molecule donors; small-molecule organic solar cells.