Simple-structured small molecule acceptors constructed by a weakly electron-deficient thiazolothiazole core for high-efficiency non-fullerene organic solar cells

Non-fullerene organic solar cells (OSCs) have attracted significant attention recently due to their tremendous advances in power conversion efficiency (PCE). It has been found that most non-fullerenes are small molecule acceptors (SMAs) bearing electron-rich cores of polycyclic arenes. However, these polycyclic arenes have a high synthetic cost. Here, a novel strategy is proposed to construct SMAs using simple and weakly electron-deficient thiazolothiazole (TTz) as the core. Two simple-structured SMAs of TTz1 and TTz2 with an A(-A-A)(2) framework are designed and synthesized for potential applications in OSCs, in which benzothiadiazole and malononitrile derivatives are employed as a dual electron-accepting (A-A) arm. Their optical, electrochemical and photovoltaic performances are systematically investigated. We find that both TTz1 and TTz2 exhibit decreased LUMO and increased HOMO energy levels in comparison to the prototypical small molecules TT0 and TTz0 bearing a single electron-accepting (A) terminal end. Medium optical bandgap of about 1.60 eV and satisfactory energy levels are obtained for both molecules TTz1 and TTz2 with more balanced carrier mobility by introducing a novel TTz core and tuning the A-A arm. The highest PCE value of 8.77% is recorded in TTz1-based OSCs blended with the common electron-donating material J71, which is the highest efficiency among OSCs using simple-structured SMAs as non-fullerenes to date. This study reveals great potential to construct high-efficiency and simple-structured SMAs using a weakly electron-deficient TTz core instead of an electron-donating polycyclic arene core.