Optical and electronic properties of different thin-film polymorphs of PDIF-CN2 controlled by zone-casting conditions

PDIF-CN2 (N,N′-bis(1H,1H-perfluorobutyl)-dicyanoperylene-3,4:9,10-bis(dicarboximide)) is one of the benchmark molecules for n-type organic semiconductors and part of the large perylene diimide (PDI) family. PDIs are known for their rich polymorphism and strong links between molecular packing order and their electrical and optical properties. Here, meniscus-guided zone-casting is used to selectively deposit aligned crystalline films of two different polymorphs of PDIF-CN2 from solution. The two polymorphs can be identified by their characteristic low-wavenumber Raman modes and distinctive photoluminescence spectra that reflect different intermolecular coupling (J- and H-aggregates). One polymorph corresponds to the common single-crystal structure of PDIF-CN2, while for the other previously unidentified thin film polymorph we propose a likely crystal structure based on polarization-dependent UV-Vis absorption spectra, IR scattering near-field microscopy (IR-SNOM) and grazing incidence wide angle scattering (GIWAXS) data, further supported by quantum mechanical calculations. Despite different crystalline structures, field-effect transistors based on thin films of both polymorphs show similar electron mobilities.