Characterization of Organic/Organic' Heterojunctions: Electronic and Optical Measurement of Ordered Interfaces and Ultrathin Film Heterojunctions

Abstract

The frontier orbitals of organic semiconductors at interfaces as they relate to organic electronic device applications, both relative energy and possible relative orientations, are the focus of this work. Heterojunctions between perylenetetracarboxylicdianhydride (PTCDA) or N,N'-di-n-butylperylene bis(dicarboximide) (C4-PTCDI) and metal centered phthalocyanines, including chloroaluminum, chloroindium, zinc, and copper phthalocyanine, have been characterized with ultraviolet photoelectron spectroscopy (UPS).Organic semiconductors heterojunctions clearly demonstrate that they cannot be treated as insulators, that vacuum level shifts occur at many organic semiconductor heterojunctions, and that Fermi level alignment is achieved but the individual nature of the organic Fermi levels must considered. UPS shows that the n-type semiconductors PTCDA and C4-PTCDI have organic Fermi levels pinned at the lower edge of the LUMO. Phthalocyanines have organic Fermi levels approximately midway between the HOMO and LUMO. The same Fermi levels are applicable for organic semiconductors at interfaces with gold as with other organic semiconductors. Further, heterojunctions of the organic semiconductors on gold show that although the alignment farther from the interface is determined by Fermi level alignment, at the immediate interface the interface dipole is determined by different factors as described by the additive model of interface dipole formation which includes factors for metal surface dipole, charge transfer, and molecular dipole moments. This model and the role of the molecular dipolar have been well characterized with alkanethiol and fluorinated alkanethiol self-assembled monolayers on gold, leading to the conclusions that the effective work function of the gold surface could be modified over a range of ca. 1.5eV with the SAM dipole and that the gold-sulfur bond is largely covalent.Fluorescence spectroscopy of phthalocyanine heterojunctions with PTCDA and C4-PTCDI was able to determine favored interfacial exciton dissociation pathways, and that charge transfer dissociation to form mobile charges is favored at PTCDA heterojunctions but energy transfer to create phthalocyanine excitons dominates at C4-PTCDI heterojunctions. The wavelength and progression of fluorescence emission from monomer phthalocyanines and aggregated phthalocyanine structures was also able to characterize thin film growth and the resultant polymorphs created by vacuum deposition of phthalocyanines on KCl (100) surfaces and on PTCDA and C4-PTCDI thin films.