Benzothiadiazole-based polymer donors

As the most classic electron-accepting building block, 2,1,3-benzothiadiazole (BT) and the derived structures have achieved great success to construct photovoltaic materials, especially p-type polymers. The first major breakthrough of BT-based polymers in the field of organic photovoltaic (OPV) was made by fullerene derivatives as the electron acceptor, and the power conversion efficiency (PCE) of 11.7% achieved by PffBT4T-C9C13 is still the world record for fullerene-based OPVs. However, the limited tunability of optoelectronic properties of fullerene acceptors hindered the further development. From 2015, a new era of OPVs started with the rapid progress of non-fullerene acceptors (NFAs), which exhibits quite different properties compared to fullerene analogs. Interesting, BT-based polymer donors also set a new landmark for nonfullerene OPVs with remarkable PCEs over 19%. Thus, in this review, we will focus on discussing the structure characteristics, evolution and device development of BT-based p-type polymers. Firstly, the representative BT-based polymers that have achieved milestone efficiency will be introduced. Secondly, how polymer structures affect the optoelectronic properties and photovoltaic performance will be addressed, including the absorption spectra, energy levels, crystallinity, π–π stacking, miscibility with NFAs, bicontinuous network structure and stability of the blend films. Finally, we emphasize the importance of putting more research efforts into designing new BT-derived building blocks and corresponding p-type polymers, and understanding the unrevealed fundamental photoelectron conversion mechanisms.