Effect of electron-withdrawing chlorine substituent on photovoltaic properties of all chlorinated benzothiadiazole-based polymers

To clarify the influence of electron-withdrawing chlorine (Cl) substituents, two benzothiadiazole (BT)-based D–A-type polymers, PBCl-BTT and PBCl-BTTCl, were utilized in non-fullerene polymer solar cells (PSCs). PBCl-BTTCl has an extra Cl atom on the BT unit compared to PBCl-BTT. Because of the extra Cl, PBCl-BTTCl exhibits distinct properties, such as low energy levels, blue-shifted absorption, and a larger bandgap. However, despite more chlorine, the inverted-type device based on PBCl-BTTCl displayed a lower efficiency (4.32%) than PBCl-BTT (7.31%). The short-circuit current density dropped significantly with PBCl-BTTCl. These results demonstrate the importance of managing Cl substituents in developing BT-based polymers for high-performance solar cells.