Synthesis and photovoltaic properties of copolymers based on benzo[1,2-b:4,5-b′]dithiophene and thiophene with electron-withdrawing side chains

Six alternating conjugated copolymers (PL1-PL6) of benzo[1,2-b:4,5-b′]dithiophene (BDT) and thiophene, containing electron-withdrawing oxadiazole (OXD), ester, or alkyl as side chains, were synthesized by Stille coupling reaction. The structures of the polymers were confirmed, and their thermal, optical, electrochemical, and photovoltaic properties were investigated. The introduction of conjugated electron-withdrawing OXD or formate ester side chain benefits to decrease the bandgaps of the polymers and improve the photovoltaic performance due to the low steric hindrance of BDT. Bulk heterojunction polymer solar cells (PSCs) were fabricated based on the blend of the as-synthesized polymers and the fullerene derivative [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) in a 1:2 weight ratio. The maximum power conversion efficiency of 2.06% was obtained for PL5-based PSC under the illumination of AM 1.5, 100 mW/cm².