Benzobis(thiadiazole)-based small molecules as efficient electron transporting materials in perovskite solar cells

Novel small-molecules, B2T and B2F, based on benzobis(thiadiazole) have been designed and synthesized as electron transporting materials for perovskite solar cells (PSCs). Benefiting from the high electron affinity of benzobis(thiadiazole) unit, both B2T and B2F exhibited an efficient electron transporting ability in CH₃NH₃PbI_3-xCl_x planar-heterojunction (PHJ) PSCs. The device based on B2T exhibited an optimal power-conversion efficiency (PCE) of 13.0%, while that of B2F was 15.0% with a relatively low hysteresis. Compared to B2T device, the higher efficiency of B2F device was mainly due to the impressive increase in short-circuit current density (J_SC), from 19.36 mA cm⁻² to 22.13 mA cm⁻². By comprehensive analysis of the energy level and photoluminescence spectra of the devices, we inferred that the higher J_SC was resulted from the lower highest occupied molecular orbital (HOMO) energy level of B2F than B2T, which induced a more efficient hole-blocking and less recombination of charge carriers. Through the molecular design (from thiophene to fluorene), we developed a highly efficient electron transporting material, this strategy may provide a new building block to develop novel efficient interface materials for PSCs.