Photoelectrochemical Behaviors of Orientation-Controlled Thin Films of N,N′-Substituted Perylene-3,4:9,10-bis(dicarboximide)

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Abstract
Orientation-controlled thin films of N,N′-dimethylperylene- and N,N′-dibutylperylene-3,4:9,10-bis(dicarboximide) (Me-PTCDI and Bu-PTCDI) were prepared by vapor deposition onto a (001) cleaved surface of KCl. As the substrate temperature increased, the film structure changed from homogeneous-amorphous to inhomogeneous polycrystalline, then to epitaxially grown states. In the epitaxial films the molecular planes of Me-PTCDI are cofacially stacked parallel to the KCl substrate, whereas the Bu-PTCDI molecules oriented standing on the substrate. In a photoelectrochemical cell of indium tin oxide (ITO)/PTCDI/I3 -,I-/Pt, the homogeneous-amorphous films showed anodic photocurrents corresponding to their n-type semiconduction, whereas the polycrystalline and epitaxial films exhibited cathodic photoresponse due to iodine doping from the electrolyte solution. The photo-response of Bu-PTCDI was much lower than that of Me-PTCDI because of its high excited-state relaxation through fluorescence emission. The epitaxial film exhibited higher fluorescence emission than the polycrystalline films; however the epitaxial orientation, especially the parallel molecular stacking in the Me-PTCDI film, improved the photoelectrochemical efficiency.

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