Low-Bandgap Small-Molecule Donor Material Containing Thieno[3,4-b]thiophene Moiety for High-Performance Solar Cells

By replacing the central thiophene of STDR, a sepithiophene terminated with two 3-ethylrhodanine moieties, with 2-ethylhexyl 3-fluorothieno[3,4-b]thiophene-2-carboxylate, an A-D-Q-D-A-type small molecule has been developed for high-performance organic solar cells with improved photocurrent. STDR-TbT exhibits a significant bathochromic shift with a low optical bandgap of approximately 1.60 eV in the thin film. Accordingly, STDR-TbT shows broad external quantum efficiency spectral response up to 800 nm. A high short circuit current (Jsc) of 10.90 mA cm(-2) was achieved for STDR-TbT:PC71BM-based devices; this is significantly higher than that of STDR:PC71BM-based devices, Jsc: 5.61 mA cm(-2), with a power-conversion efficiency (PCE) of 5.05%. Compared with STDR-based devices, STDR-TbT-based devices show balanced charge carrier transport, better thin-film morphology, and favorable charge separation/collection.