A novel copolymer from benzodithiophene and alkylsulfanyl-bithiophene: Synthesis, characterization and application in polymer solar cells

In this paper we present the synthesis and characterization of a novel copolymer from thienothiophene and bithiophene units carrying nonanoyl and octylsulfanyl side chains, poly[1-(thieno[3,4-b]thien-2-yl)nonan-1-one-alt-4,4’-bis(octylsulfanyl)-2,2’-bithiophene] (PolyH). The structural, electrochemical and photophysical properties of PolyH were investigated by gel permeation chromatography (GPC), thermogravimetric analysis (TGA), NMR, UV-VIS-NIR spectroscopy and cyclic voltammetry (CV). PolyH possesses good solubility in common organic solvents, filmability, a proneness to form π-stacks and a moderate solvatochromism. PolyH was tested as donor material in combination with [70]PCBM (electron acceptor) in bulk-heterojunction polymer solar cells. The geometry of the device is: glass/ITO/PEDOT:PSS/ PolyH:[70]PCBM/Ca/Al. The external quantum efficiency curve of the best device (realized using a blend of PolyH:[70]PCBM, 1:2 weight ratio) shows a broad response from 350 to 1000 nm. The power conversion efficiency under 100 mW/cm2 AM 1.5G illumination is greater than 1%.