Synthesis, characterization and photovoltaic properties of thiophene copolymers containing conjugated side-chain

Five side-chain conjugated polythiophene derivatives, P1–P5, were synthesized by Stille coupling reaction. The effects of side-chain structures, bearing Cdouble bond; length as m-dashC double bond, Ctriple bond; length of mdashC triple bond as well as different number of methoxy substituents on the benzene ring of the side-chains, on the optical, electrochemical, and photovoltaic properties of the polymers were investigated. From P1 to P3, the effect of Ctriple bond; length of mdashC triple bond and Cdouble bond; length as m-dashC double bond was compared. The results indicate that the content of the thiophene units with the Ctriple bond; length of mdashC triple bond in their conjugated side-chains not only influences the absorption shape and intensity, but also influences the energy bandgap and the photovoltaic properties of the polymers. From P3 to P5, the effect of methoxy substituents on the benzene ring of the conjugated side-chains was compared. On increasing the number of the methoxy groups on the benzene ring of the conjugated side chains, the visible π–π∗ absorption of the conjugated polymer backbone become stronger both in solution and in film. Electron-donating ability of the methoxy groups decreased the bandgap of the polymers. The best polymer solar cell based on P5 with a structure of ITO/PEDOT:PSS/Polymer:PCBM (1:1 wt/wt)/Mg/Al showed a power conversion efficiency of 1.45% under the illumination of AM1.5, 80 mW/cm2.