Chlorination of Low-Band-Gap Polymers: Toward High-Performance Polymer Solar Cells

Halogenation is an effective way to tune the energy levels of organic semiconducting materials. To date, fluorination of organic semiconducting materials to fabricate polymer solar cells (PSCs) has been used far more than chlorination; however, fluorine exchange reactions suffer from low yields and the resulting fluorinated polymer always comes with a higher price, which will greatly hinder their commercial applications. Herein, we designed and synthesized a series of chlorinated donor–acceptor (D-A) type polymers, in which benzo[1,2-b:4,5-b]dithiophene and chlorinated benzothiadiazole units are connected by thiophene π-bridges with an asymmetric alkyl chain. These chlorinated polymers showed deep highest occupied molecular orbital (HOMO) energy levels, which promoted the efficiency of their corresponding PSCs by increasing the device open circuit voltage. The asymmetric alkyl chain on the thiophene moieties gave the final polymer sufficient solubility for solution processing and strong π–π stacking in fi...