Research Advances on Benzotriazole-based Organic Photovoltaic Materials

Over the past two decades, organic solar cells (OSCs) have been developed rapidly with the power conversion efficiency rapidly rising from less than 5% to over 18%, which is mainly promoted by the development of various new donor and acceptor materials. As a typical electron-deficient penta-heterocycle, benzotriazoles (BTAs) derivates a variety of high-performance photovoltaic materials, including polymer donor, small-molecule donor materials, as well as non-fullerenes acceptor and polymer acceptor. Among them, the J series of polymer donors and Y series of non-fullerenes acceptors are typical examples, and thus are specially highlighted in this review. Meanwhile, molecular design strategies of those BTA-based photovoltaic materials have also been discussed. It shows that donor-acceptor (D-A) conjugated strategy is still the most efficient thus far, where A units is the BTA unit or its derivatives, and D units commonly used in BTA-based photovoltaic materials are benzodithiophene, benzodifuran, dithienosilole, indacenodithiophene, thiophene, etc. The D-A strategy is both applied for donor molecules (with the molecular structure of D-A, D-pi-A-pi, D-A-D-A-D, etc.), and for acceptor molecules (with the molecular structure of A-D-A, A-pi-D-pi-A, A-DAD-A, etc.). By adjusting their molecular structures and/or pairing of differential D and A units, various properties such as absorption band and energy levels of molecules, as well as the morphology and charge carrier mobilities in OSCs can be well controlled. Furthermore, through side-chain engineering, such as flexible side-chains (alkyl, alkoxy, alkylthiol, alkylsilyl, etc.), conjugated side-chains (substituted-thiophene or benzene, etc.), electron-withdrawing groups (F atoms, Cl atoms, dicyanomethylene, etc.), their photovoltaic properties can be further regulated. Here, this review focuses on the research progress on BTA-based photovoltaic materials and related molecular design strategies developed in recent years, and also presents perspective on its future development.