Optical Modeling and Characterization of Layers and Multilayer Structures : Organic Optoelectronic Devices and Spectroscopic Ellipsometry

Sammanfattning: This dissertation is on the subject of modeling and characterization of layers and multilayer structures with respect to their optical properties and characteristics. A number of different systems have been investigated; among them multilayer structures in the form of organic optoelectronic devices, anisotropic layers of conjugated polymers as well as inhomogeneous layers of porous silicon and oxidized silver.Modeling of the optical characteristics of organic optoelectronic devices includes organic photovoltaic devices and organic light-emitting diodes. Experimental short-circuit photocurrent action spectra of poly(3-( 4' -(1",4",7"-trioxaoctyl)phenyl)thiophene) (PEOPT)/fullerene (C60) thin film heterojunction photovoltaic devices were modeled. The modeling was based on the assumption that the photocurrent generation process is the result of creation and diffusion of photogenerated species (excitons), which are dissociated by charge transfer at the PEOPT/C60 interface. At the modeling, exciton diffusion ranges of PEOPT and C60 were obtained and the influence of the geometrical structure with respect to the efficiency of the thin film devices were studied. In addition, an analysis of the internal monochromatic quantum efficiency of organic photovoltaic devices based on poly(3-( 4-octylphenyl)-2,2' -bithiophene) (PTOPT) layers and PTOPT/C60 bilayers were presented. A quantum efficiency of exciton-to-charge generation is defined as the external monochromatic quantum efficiency normalized to the absorption in the active materials of the device. An upper limit of the efficiency can be determined and results show that much of the light is absorbed in photoactive layers of the device whereas only a fraction of the generated excitons is converted to charge carriers and can be collected as photocurrent. Optical characteristics of bilayer organic light-emitting diodes made of poly(3-methyl-4 -octylthiophene) and 2-(biphenylyl)-5-(4-tertbutylphenyl)-1,2,4-oxadiazole layers bet ween electrodes of indium tin oxide and Ca/ Al were also modeled. These model simulations, together with electrochemical measurements, can be interpreted as evidence for an indirect optical transition at the polymer/molecule interface that only occurs in a strong electric field.Anisotropic layers of poly(3,4-ethylenedioxythiophene) doped with toluenesulfonate and poly(4-styrenesulfonate) as well as layers of poly(p-pyridine) were studied. Techniques used in these studies were in the firstplace spectroscopic ellipsometry, but also polarized transmission spectrophotometry, grazing x-ray diffraction, and infrared spectroscopy. Common features of these layers were that they all exhibited uniaxial anisotropy with the optic axis perpendicular to the layer surface. Inhomogeneous layers of porous silicon and oxidized silver were also studied by spectroscopic ellipsometry. In the case of porous silicon, analyses revealed a compositional gradient normal to the surface. A porosity graded layer model was presented and used in the analysis of the material. In the porosity graded layer model, the inhomogeneous layer was built up by a number of thin sublayers with the porosity slightly changing from one sublayer to the next. Complementary studies were performed with crosssectional transmission electron microscopy. Finally, silver oxide and the oxidation process of silver layers were studied with both in situ and ex situ spectroscopic ellipsometry. In the evaluation of experimental optical data of anisotropic and inhomogeneous layers, techniques such as acquiring data at multiple wavelengths and angles of incidence are of great value to enhancethe optical information content of measured data. Furthermore, inclusion of multiple data types and simultaneous analysis of multiple samples are very powerful ways to enhance the optical information content and to eliminate strong parameter correlation from complex models.