Low band gap thienothiophene-thiophene copolymer: synthesis, characterization and application in polymer solar cells

Polymer solar cells (PSCs) have recently attracted considerable attention due to their advantages of low cost, light weight, processability and compatibility with large scale flexible substrates. The most efficient device architecture of polymer solar cells is based on the bulk heterojunction concept in which the active layer, sandwiched between two electrodes with different work functions, consists of a blend of electron donating material, e.g. a p-type conjugated polymer, and an electron accepting material (n-type), such as fullerene derivatives.Polythiophenes are quite interesting materials for this purpose, combining the above described advantages with an enhanced photochemical stability, compared with poly(p-phenylene vinylene) and poly p-phenylene. In this work the synthesis, the spectroscopic and photovoltaic characterization of a thienothiophene-thiophene copolymer are reported. Poly[2-nonanoylthieno[3,4-b]thiophene-4,6-diyl-alt-4,4’-bis(octylsulfanyl)-2,2’-bithiophene]) was obtained by Stille coupling between the dibromo derivative of 2-nonanoylthienothiophene and a distannyl derivative of bis(octylsulfanyl)bithiophene.Bulk heterojunction solar cells using these polymers as donor materials blended with PCBM as acceptor were prepared and investigated. The geometry of the device is: glass/ITO/PEDOT:PSS/Polymer:PCBM/Ca/Al. An efficient charge transfer between the donor and PCBM takes place and the best device obtained has and efficiency of 1% under simulated light (AM1.5G). Further improvement of the efficiency using different acceptors, optimizing layer thickness and lowering polymer impurities can be envisaged.