Diethynylbenzo[1,2-b:4,5-b′]Dithiophene-Based Small Molecule and Cross-Conjugated Copolymers for Organic Solar Cells

In this manuscript we reported the synthesis of diethynylbenzo[1,2-b:4,5-b']dithiophene(DEBDT)-based small molecule BDTTIPS and three new cross-conjugated copolymers PBDTE-Th, PBDTE-FBT, and PBDTE-DPP that consist of DEBDT unit. The thermal, optophysical, and electronic properties of these materials were systematically investigated. The small molecule BDTTIPS exhibited higher absorption coefficient than the cross-conjugated copolymers. The LUMO/HOMO energy levels of such three cross-conjugated copolymers were 23.74/ 5.51, -3.77/-5.53, and -4.02/-5.58 eV as estimated from cyclic voltammetry measurements, which were lower than that of -3.63/-5.39 eV for the small molecule BDTTIPS. Due to the unique molecular structure and electronic properties of crossconjugated polymers, hole mobilities of PBDTE-Th and PBDTEDPP were higher than that of BDTTIPS. Photovoltaic performances were investigated by fabricating organic solar cells with the configuration of ITO/PEDOT: PSS/donor: PC61BM/Ca/Al. The small molecule chromophore BDTTIPS exhibited a power conversion efficiency of 4.19%, which was much higher than those obtained from devices based on conjugated polymers as the photoactive layer. These observations indicated that the DEBDT can be a promising building block for the construction of organic semiconducting materials for organic solar cell applications, and the rational molecular design was required for the attainment of high photovoltaic performances. (C) 2016 Wiley Periodicals, Inc.