Chlorination and Position Isomerization to Enhance the Photovoltaic Performance of Polymer Donors

Substitution of chlorine in a conjugated side chain is a simple and effective method with which to improve the photoelectronic properties of polymer donors. The chlorination and chlorine-substituted position affects the photovoltaic performance of organic solar cells. Herein, the polymers PBDQx-β-H, PBDQx-β-Cl, and PBDQx-α-Cl based on a quinoxaline (Qx) unit were synthesized. The effects of the chlorination and its position isomerization of thiophene side chain on material performance were systematically investigated. After the introduction of a Cl atom at the β-position, the PBDQx-β-Cl achieves dramatically enhanced absorption and aggregation abilities compared with PBDQx-β-H. When the Cl atom is in the α position, a better backbone planarity of PBDQx-α-Cl is assumed and the polymer PBDQx-α-Cl exhibits the much stronger absorption and further improved molecular aggregation in comparison with PBDQx-β-Cl. Meanwhile, when blended with an acceptor, Y6, the PBDQx-α-Cl-based blend film shows a more suitable nanoscale phase separation morphology. Consequently, PBDQx-α-Cl:Y6-based devices achieve an excellent power conversion efficiency of 15.24% with a synchronously promoted open-circuit voltage (VOC), short-circuit current density (JSC), and fill factor (FF). These are much higher than those achieved by the PBDQx-β-H-based devices (7.73%). These results show that the dual strategy of chlorination and position isomerization is a promising approach with which to gradually design and synthesize highly efficient polymer donors.