Horizontal‐, Vertical‐, and Cross‐Conjugated Small Molecules: Conjugated Pathway‐Performance Correlations along Operation Mechanisms in Ternary Non‐Fullerene Organic Solar Cells

A family of the SM‐axis series based on benzo[1,​2‐​b:4,​5‐​b′]​dithiophene and 3‐ethylrhodanine (RD) units with structurally different π‐conjugation systems are synthesized as a means to understand the structure–property relationship of conjugated pathways in ternary non‐fullerene organic solar cells (NF‐OSCs) as a third component. The optical and electrochemical properties of the SM‐axis are highly sensitive both to the functionalized direction and to the number of RD groups. Enhanced power conversion efficiencies (PCEs) of over 11% in ternary devices are obtained by incorporating optimal SM‐X and SM‐Y contents from PBDB‐T:ITIC binary NF‐OSCs, while a slightly lower PCE is observed with the addition of SM‐XY. The results of in‐depth studies using various characterization techniques demonstrate that working mechanisms of SM‐axis‐based ternary NF‐OSCs are distinctly different from one another: an energy‐transfer mechanism with an alloy‐like model for SM‐X, a charge transfer with the same model for SM‐Y, and an energy transfer without such a structure for SM‐XY. As extension of the scope, a SM‐X‐based ternary NF‐OSC in the PM6:IT4F system also shows a greatly enhanced PCE of over 13%. The findings provide insights into the effects of conjugated pathways of organic semiconductors on mechanisms of ternary NF‐OSCs, advancing the understanding for synthetic chemists, materials engineers, and device physicists.