Two star-shaped small molecule donors based on benzodithiophene unit for organic solar cells

Star-shaped small molecules have attracted great attention for organic solar cells(OSCs) because they have three-dimensional charge-transport characteristics, strong light absorption capacities and easily tunable energy levels. Herein, three-and four-armed star-shaped small molecule donors, namely BDT-3 Th and BDT-4 Th, respectively, have been successfully designed and synthesized, which used benzodithiophene(BDT) as the central unit. The two star-shaped intermediates(2 a and 2 b) could be simultaneously obtained by one-step of Suzuki coupling, and 1,2-dimethoxyethane played a key role in the Suzuki coupling. Both of them have excellent thermal stability, good solubility and broad absorption. Four-armed BDT-4 Th shows a slightly higher extinction coefficient, a deeper HOMO energy level and an obviously better phase separation morphology when blended with Y6 than three-armed BDT-3 Th. As a result, increased power conversion efficiency(PCE) of 5.83% is obtained in the BDT-4 Th:Y6-based OSC devices,which is obviously higher than that of the BDT-3 Th:Y6-based devices(PCE = 3.78%). To the best of our knowledge, this is the highest PCE among the BDT-based star-shaped donors-based OSCs. This result provides an effective strategy to obtain star-shaped small molecule donor materials for high efficient organic solar cells.