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 ach-ieved 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, 7C-7C 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 con-version mechanisms.