15.51 % efficiency all-small-molecule organic solar cells achieved by symmetric thiazolyl substitution

Two new small molecule donors of SW1 and SW2 are designed and synthesized by employing thiophene and thiazole substituents as the side groups of benzo[1,2-b:4,5-b′]dithiophene central unit, respectively. Due to the more electron-withdrawing feature and enhanced intermolecular stacking of SW2 with respect to that of SW1, the all-small-molecule (ASM) organic solar cells (OSCs) based on SW2 afford a much better power conversion efficiency up to 15.51 %, along with both enlarged open-circuit voltage and fill factor. Further systematic investigation demonstrates that thiazole could act as an excellent building block to optimize the energy levels, molecular stacking as well as microscopic morphologies in ASM systems, leading to the highly efficient ASM-OSCs.