Real-Time Structural and Optical Study of Growth and Packing Behavior of Perylene Diimide Derivative Thin Films: Influence of Side-Chain Modification

We study the growth of two n-type small-molecule organic semiconductors from the perylene diimide family: N,N'-bis-(2-ethylhexyl)dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDIR-CN2) and N,N'-1H,1H-perfluorobutyl-dicjyanoperylene-3,4: 9,10-bis (dicarboximide) (PDIF-CN2) whose chemical structures differ only in the imide substituents, branched alkyl chains -C8H16 and linear fluoroalkyl chains -C4F7H2, respectively. Both types of substituents introduce some degree of steric hindrance for intermolecular interactions, affecting solid-state packing during thin film formation, and thus induce specific structure-dependent optoelectronic properties in thin films. The transition from an amorphous structure to crystalline domains with strong intermolecular coupling was followed in situ and in real time during growth. We investigated the structural and morphological properties by X-ray diffraction and atomic force microscopy as a function of the substrate temperature and chemical structure. We examined the relationship between the structural properties and thin film optical signatures probed via differential reflectance spectroscopy, ellipsometry, and temperature-dependent photoluminescence. A new crystalline PDIR-CN2 polymorph at high temperatures emerges. In addition, we observed in PDIF-CN2 that the fluorinated chains contribute to crystallization inhibition because of the higher overall steric hindrance compared to the alkyl chains.