Chlorine substituted 2D-conjugated polymer for high-performance polymer solar cells with 13.1% efficiency via toluene processing

In the past few years, fluorine atom has been widely introduced into organic semiconductor (OS) materials to improve the photovoltaic performance of polymer solar cells (PSCs). In contrast, chlorine atom is rarely concerned, although it is also a halogen element and is more easily introduced into OS materials. Herein, we designed and synthesized a new D-A-type two-dimension (2D)-conjugated polymer, PM7, containing a chlorinated-thienyl benzodithiophene (BDT-2Cl) donor unit and a benzodithiophene-4,8-dione acceptor unit. Compared to the control polymer PBDB-T without chlorine substitution, PM7 shows lower HOMO energy level, higher absorption coefficient, enhanced crystallinity and higher carrier mobility. Moreover, the toluene-processed PSCs based on PM7 as donor and small molecule n-OS IT-4F as acceptor achieved a high power conversion efficiency (PCE) of 13.1% with high open-circuit voltage (V_oc) of 0.88 V, short-circuit current density (J_sc) of 20.9 mA cm⁻² and fill factor (FF) of 71.1%, while the PBDB-T:IT-4F-based PSC only exhibited a low PCE of 5.8% with low V_oc of 0.67 V, J_sc of 15.0 mA cm⁻² and FF of 57.6%. The PCE of 13.1% is among the highest values reported for the PSCs to date. These results indicate that the chlorine substitution is a simple and effective strategy to design high-performance conjugated polymer photovoltaic materials.