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, a low-lying highest occupied molecular orbital (HOMO) energy level of -5.58 eV, and an ordered molecular orientation and packing. Hence, 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 devices exhibit a very small non-radiative energy loss of 0.18 eV, which is similar to those of inorganic solar cells. These results indicate that BTTzR is a promising small molecule donor material for SM-OSC application and provide new insight into the design of photovoltaic materials to reduce the non-radiative energy loss in the OSCs.