A Simple Approach to Prepare Chlorinated Polymer Donors with Low-Lying HOMO Level for High Performance Polymer Solar Cells

Both photovoltaic performance and cost of materials are the key factors for commercial application of polymer solar cells (PSCs). In this study, we designed and synthesized two new conjugated polymers with a chlorine substituent at the α position and a flexible substituent on the β position of the thiophene conjugated side chain, J11 (with alkyl flexible substituent), and J12 (with alkoxy flexible substituent), through a relatively simple synthetic method. The two polymers displayed similar UV–vis absorption profiles with deep-lying HOMO (the highest occupied molecular orbital) energy levels. Compared with the alkyl substituted polymer J11, alkoxy substituted polymer J12 displayed slightly weaker aggregation and excessive miscibility with the n-type organic semiconductor (n-OS) m-ITTC (which is an isomeric counterpart of ITTC). As a result, the device based on J11:m-ITTC demonstrated a higher PCE of 12.32% with a high fill factor (FF) of 0.73, while the J12:m-ITTC-based device displayed a lower PCE of 8.74%, with a slightly higher Voc of 0.943 V but a low FF of 0.56, which should be ascribed to the relatively low hole mobility, resulting from its small phase separation. These results indicate that altering the position of the chlorine atom (Cl) from the β- to α-position of thiophene unit is a simple but effective method to prepare efficient chlorinated polymers for further development of PSCs.