Abstract
The molecular orientations of conjugated materials on the substrate mainly include edge-on, face-on, and end-on. Edge-on and face-on orientations have been widely observed, while end-on orientation has been rarely reported. Since in organic solar cells (OSCs) charge transport is along the vertical direction, end-on orientation with conjugated backbones perpendicular to the substrate is recognized as the ideal microstructure for OSCs. In this work, we for the first time obtained the preferential end-on orientation in a conjugated molecular dyad that contains a conjugated backbone as donor and perylene bisimide side units as acceptor. This was realized by introducing a fused-ring structure to replace linear terthiophenes with conjugated backbones, yielding F-MDPBI and L-MDPBI respectively. Surprisingly, a shifting trend of the molecular orientation from dominating edge-on in L-MDPBI to preferential end-on in F-MDPBI was observed. As a consequence, vertical charge carrier mobilities in F-MDPBI are one order of magnitude higher than those with preferential edge-on orientation, so single-component OSCs based on this molecular dyad as a single photoactive layer provided a power conversion efficiency of 4.89% compared to 1.70% based on L-MDPBI with preferential edge-on orientation.