From Alloy-Like to Cascade Blended Structure: Designing High-Performance All-Small-Molecule Ternary Solar Cells.

Ternary blending strategy has been used to design and fabricate efficient organic solar cells by enhancing the short-circuit current density and the fill factor. In this manuscript, we report all-small-molecule ternary solar cells consisting of two compatible small molecules DR3TBDTT (M1) and DR3TBDTT-E (M2) as donors and PC71BM as acceptor. A transformation from an alloy-like model to a cascade model are first realized by designing a novel molecule M2. It is observed that after thermal and solvent vapor annealing M2 shifts from the mixed region to donor-acceptor (D-A) interfaces which ameliorates the charge transfer and recombination processes. The optimal ternary solar cells with 10% M2 exhibited a power conversion efficiency of 8.48% in the alloy-like model and 10.26% in the cascade model. The proposed working mechanisms are fully characterized and further supported by the density functional theory and atomistic molecular dynamics simulations. This provides an important strategy to design high-performance ternary solar cells which contains one molecule not only is compatible with the main donor molecule but also performs a preference to appear at the D-A interfaces hence builds cascade energy levels.