Effects of π-conjugated side chains on properties and performances of photovoltaic copolymers

Poly[(benzodithiophene-2,6-diyl)(2,5-thienylene)] (P1) and its derivatives with π-conjugated side-chains (P2: thienylethenyl and P3: thienylcyanoethenyl) were designed and synthesized in order to study the effects of the conjugated side-chains on the optical, electrochemical, and photovoltaic properties of the copolymers. It was found that the electronic properties and energy levels of the copolymers can be effectively tuned through changing a conjugated side-chain attached to a polymer backbone. Polymer solar cells based on copolymer/PC61BM exhibited power conversion efficiencies of 2.85%, 3.44%, and 4.49%, respectively, for P1, P2, and P3 under a simulated solar light (AM 1.5 G, 100 mW/cm2), compared with 4.17% for a main-chain type copolymer corresponding to P3. The results reveal that the introduction of the conjugated side-chain with an electron-deficient group to a polymer backbone is an effective approach for improving the performance of the photovoltaic materials.