Synthesis and characterization of low-band-gap poly(thienylenevinylene) derivatives for polymer solar cells

Abstract

A series of conjugated polymers containing thienylenevinylene moieties as the electron donor for polymer solar cells were synthesized through Yamamoto and Stille coupling. The structure and device properties of the homopolymer (E)-poly[2,2′-(1,2-ethenediyl)bisthiophene] (PEBT), and two donor–acceptor-type copolymers (E)-poly[2,2′-(1,2-ethenediyl)bisthiophene-alt-4,7-(2,1,3-benzothiadiazole)] (PEBTBT) and (E)-poly[2,2′-(1,2-ethenediyl)bisthiophene-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PEBTTBT) were investigated. The vinylene group inserted between two thiophene rings has an important role in lowering the band gap of thienylenevinylene derivatives. In addition, the donor–acceptor-type copolymers including a benzothiadiazole unit showed even lower band gap than their own homopolymer due to intramolecular charge transfer (ICT) through their conjugated backbones. As a result, PEBT, PEBTBT and PEBTTBT showed band gaps of 1.77, 1.37 and 1.57 eV, respectively. Bulk heterojunction photovoltaic devices were fabricated using blends of the polymers with [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₀BM) and PEBTBT gave the best power conversion efficiency among them, at 1.07% under AM 1.5, 100 mW cm⁻².