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 −3.74/5.51, −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 cross-conjugated polymers, hole mobilities of PBDTE-Th and PBDTE-DPP 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. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016