Systematic studies on structural and optical properties of organic thin films on inorganic substrates

摘要

Molecular organic compounds are employed as active materials in a variety of applications,including organic light emitting diodes (OLEDs) and organic thin film transistor (OTFTs). These devices consisting of sequentially deposited layers of organic materials have been demonstrated with active device thicknesses of only a few hundred nanometers or less. For a better understanding and design of such devices, it is necessary to understand the structural and optical properties of the organic thin films employed. Polymers based on the family of five-membered thiophene conjugated ring and its derivates, due to their self-assembling nature, have been considered as one of the most common conducting organic polymers used in OTFT devices. We have studied the adsorption of 3-nitrothiophene/selenophen/tellurophen on Cu(110) surface, utilising ab initio calculations. These calculations provide a basic understanding and forecasting of intra- and inter-molecular interactions present in the observed structures. Monolayers of 3-nitrothiophen/selenophen/tellurophen on a Cu(110) surface are well ordered with the aromatic ring perpendicular to the surface, in a close packed structure. Due to the strong lateral interaction between the hydrogen atoms of the neighboring conjugated rings, they rotate relative to the nitro (-NO2) group. Tris-(8-hydroxyquinoline) aluminum (Alq3) and N,N’-diphenyl-N,N’-bis(1-naphthyl)-1-1’biphenyl-4,4”iamine (alpha-NPD) are among the most commonly used electron-transport and hole-transport materials suitable for OLED applications. The optical and structural properties of the thin films of these materials deposited by organic vapor phase deposition (OVPD) have been studied by spectroscopic ellipsometry (SE). Employing this technique enables the precise determination of the dielectric function as well as thickness of the organic thin films of each material. This result can be explained by the characteristic features of electronic states in organic molecules. In order to tailor and modify thin film properties to be suitable for desired applications, e.g. organic light emitting devices (OLED) in this case, it is necessary to study and understand the influence of deposition parameters on thin film growth. Our study is focused on the influence of deposition rate and substrate temperature on (alpha-NPD) film morphology. A remarkable dependence of the film morphology on deposition rate and substrate temperature is observed. A detailed quantitative morphology analysis provides an excellent description of the growth mechanism of OLED films.