Effect of Molecular Shape on the Properties of Non-Fullerene Acceptors: Contrasting Calamitic Versus 3D Design Principles

The recent development of high-performance nonfullerene acceptors has relied on two main design principles: the calamitic design, where side chains are arranged orthogonally to a planar chromophore, and the 3D design, where the chromophore is forced to adopt a nonplanar structure. Until now, there has been no direct comparison of these designs. In this work, small molecule nonfullerene acceptors with both the calamitic design (FBRCN) and the 3D design (XFBRCN) were synthesized. These molecules bear the same chromophore unit and have similar solubilities. Our results show that the 3D semiconductor XFBRCN has a higher extinction coefficient, broader absorbance bands, and a higher permittivity than the calamitic FBRCN. This is due to the lower crystallinity and better polarizability of the 3D acceptor. In contrast, the calamitic FBRCN had a higher electron mobility, was more miscible with polymeric OPV donors, and was more crystalline than the 3D XFBRCN. This is attributed to the long alkyl chains of FBRCN encouraging efficient solid-state packing. This work highlights the differences between the two material design principles, and helps to elucidate the role of side chains in controlling film morphology and the performance of solid-state electronic devices.