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.

共轭侧链的氯取代是改善聚合物给体光电性能的简便高效策略。氯代反应及氯取代位点会显著影响有机太阳能电池的光伏性能。本文合成了基于喹喔啉（quinoxaline, Qx）单元的三类聚合物：PBDQx-β-H、PBDQx-β-Cl与PBDQx-α-Cl。系统研究了噻吩侧链的氯代反应及其位点异构化对材料性能的影响。相较于未氯代的PBDQx-β-H，在β位引入氯原子后的PBDQx-β-Cl的光吸收能力与分子聚集能力均得到显著提升。当氯原子位于α位时，PBDQx-α-Cl的主链平面性更佳，相较于PBDQx-β-Cl，其光吸收强度更高，分子聚集程度进一步优化。同时，当与受体材料Y6共混时，PBDQx-α-Cl基共混薄膜展现出更适宜的纳米级相分离形貌。基于PBDQx-α-Cl:Y6的器件实现了15.24%的优异光电转换效率，其开路电压（open-circuit voltage, VOC）、短路电流密度（short-circuit current density, JSC）与填充因子（fill factor, FF）均同步得到提升。该性能远优于基于PBDQx-β-H的器件（7.73%）。上述研究结果表明，氯代与位点异构化的双重调控策略，是高效聚合物给体材料定向设计与合成的极具潜力的途径。