A Novel Wide-Bandgap Small Molecule Donor for High Efficiency All-Small-Molecule Organic Solar Cells with Small Non-Radiative Energy Losses

In this work, a wide-bandgap small molecule donor, BTTzR, that incorporates the electron-donating alkylthienyl-substituted benzo[1,2-b:4,5-b']dithiophene (BDT-T) unit and the electron-deficient thiazolo[5,4-d]thiazole (TTz) unit as the building block units, is designed and synthesized. BTTzR exhibits strong absorption in the wavelength range of 400–650 nm with a wide optical bandgap of 1.88 eV, low-lying highest occupied molecular orbital (HOMO) energy level of -5.58 eV, and ordered molecular orientation and packing. Hence, the optimized all-small-molecule organic solar cells (SM-OSCs) based on BTTzR as a donor and Y6 as an acceptor achieve an outstanding power conversion efficiency (PCE) of 13.9%, which is among the highest values reported in the literature to date. More importantly, the device exhibits very small non-radiative energy losses of 0.18 eV, which is similar to that of inorganic solar cells. These results indicate that BTTzR is a promising small molecule donor material for the SM-OSCs application and provide a new sight for the design of photovoltaic materials to reduce the non-radiative energy losses in the OSCs.