Dual-Accepting-Unit Design of Donor Material for All-Small-Molecule Organic Solar Cells with Efficiency Approaching 11%

Design of high-performance small molecule donors for all-small molecule organic solar cells (ASM-OSCs) requires a combinative effort of optimizing the material design and device fabrication. Herein, a new dual accepting-unit medium-bandgap small molecule donor named SBDT-BDD is developed, which consists of a benzodithiophene (BDT) as the central electron donating unit in combination with two rhodanines (A(1)) and two benzo-[1,2-c:4,5-c'] dithiophene-4,8-diones (BDD) (A(2)) as the electron-accepting units, forming a unique A(1)-A(2)-D-A(2)-A(1), structure. The dual accepting units endow the SBDT-BDD with complementary absorption and appropriate energy level with fused-ring electron acceptor IDIC. Further investigations suggest that SBDT-BDD is morphologically compatible with the two acceptors of PC71BM and IDIC, benefiting the formation of ideal film morphology and efficient exciton dissociation as well as suppressed charge recombination in devices, resulting in outstanding current density and fill factor. The film-depth-dependent optical and electronic properties in such small molecule devices are synergistically optimized, to simultaneously manipulate photon harvesting contours and charge transport. The ASM-OSCs based on SBDT-BDD:IDIC blend produces a power conversion efficiency (PCE) of 9.2%, while the SBDT-BDD:IDIC:PC71BM ternary devices exhibit a significantly increased PCE of 10.9%. Upon molecule design to optimize morphology for precise manipulation of film-depth- and wavelength-dependent optical and electronic properties, this work provides a new strategy for a small molecule donor toward high-performance ASM-OSCs.