Applying the heteroatom effect of chalcogen for high-performance small-molecule solar cells

Abstract

Small molecules with defined chemical structures and low quality variation are important for organic solar cells (OSCs). Three thieno[3,4-b]thiophene small-molecule donor materials, STB-C, STB-O and STB-S, with different side-chain substitutions of alkyl, alkoxy and alkylthio were synthesized and applied to investigate the heteroatom effects on the OSC performance. Optimized devices based on STB-C, STB-O and STB-S delivered power conversion efficiencies (PCEs) of 7.84%, 8.68% and 4.05%, respectively, revealing the distinct influence of heteroatoms. Systematic structure–property relationships were further investigated by using incident X-ray diffraction, transmission electron microscopy, atomic force microscopy, and resonant soft X-ray scattering. Compared with those of STB-C and STB-S, the highest efficiency of STB-O can be attributed to the excellent charge transport properties, originating from the stronger π–π stacking, a multi-length scaled phase separation and a slightly elevated LUMO energy level. It is noteworthy that STB-O delivered the highest PCE among small-molecule donors based on alkoxy-substituted BDTs.