Synergy Effect of Symmetry-Breaking and End-Group Engineering Enables 16.06% Efficiency for All-Small-Molecule Organic Solar Cells

Molecular innovation is an urgent necessity to realize efficient all-small-molecule organic solar cells (ASM-OSCs). Asymmetric strategy and end-group engineering have been widely utilized for efficient photovoltaic materials with great success. However, the synergistic effect of the asymmetric strategy combined with end-group engineering on blend film morphology and photovoltaic performance remains insufficiently explored. In this vein, two asymmetric small molecule donors with thiophene/thiazolyl side chains and different end-groups of 3-(2-ethylhexyl)-2-thioxo-4-thiazolidinone (Reh) and cyanoacetic acid esters (CA), W2-CA and W2-Reh, were designed to gain insight into the combined effects of symmetry-breaking and end-group engineering. Compared to W2-Reh, W2-CA exhibits a preferable face-on orientation and good bicontinuous phase-separated morphology, which benefit improving carrier mobility and ensuring a high-efficiency charge transfer pathway in the blended films. 16.06% power conversion efficiency (PCE) is achieved in W2-CA-based ASM-OSCs, one of the highest efficiencies reported up to now for binary ASM-OSCs. A promising avenue for high-efficiency small molecule donor design is provided to achieve efficiency ASM-OSCs.