Spectroscopic Ellipsometry of the Optical Functions of Some Widely Used Organic Light Emitting Diodes (OLEDs) Materials
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Spectroscopic ellipsometry of the optical functions of some widely used organic light emitting diodes (OLEDs) materials Z. T. Liu1, C. C. Oey,2 A. B. Djuriši ,1 C. Y. Kwong,2 C. H. Cheung,1 W. K. Chan,3 and P. C. Chui2 1 Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong 2 Dept. of Electrical and Electronic Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong 3 Department of Chemistry. The University of Hong Kong, Pokfulam Road, Hong Kong ABSTRACT In this work, optical functions of some widely used OLEDs materials 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), tris (8-hydroxyquinoline) aluminum (Alq3), (N,N′-di(naphthalene-1-yl)-N,N′- diphenylbenzidine (NPB),poly(3,4,-ethylene dioxythiophene):polystyrene sulfonic acid (PEDOT:PSS) and indium tin oxide (ITO)) were studied using spectroscopic ellipsometry (SE) in the spectral range from 1.55 eV to 4.1 eV (wavelength range of 300 nm to 800 nm). The samples were prepared either by thermal evaporation in high vacuum or spin-coating of thin films onto glass substrates. For determination of the optical functions of ITO, commercial ITO glass was used. Measurements at different incident angles were performed to determine whether the samples can be considered isotropic. The SE data were modeled using an oscillator model (Lorentz for semiconducting and Lorentz-Drude for conducting materials). The absorption spectra were also measured, and the comparison with the data determined by SE is given. INTRODUCTION Organic light emitting diodes (OLEDs) have attracted a great deal of attention in both academic and commercial fields because of their potential applications for the next generation flat panel color displays [1]. In order to further understand the device physics as well as efficiently model and design OLEDs, it is essential to know the optical functions (i.e. the refractive index n and extinction coefficients k as the functions of the wavelength of light) of each layer of the devices [2]. In this work, we used spectroscopic ellipsometry (SE) measurements [3,4] in the spectral range of 1.55–4.1 eV to determine the optical functions of some widely used OLEDs materials: 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) (the hole blocking layer), tris (8-hydroxyquinoline) aluminum (Alq3) (the electron transport and light emitting layer), (N,N′-di(naphthalene-1-yl)-N,N′- diphenylbenzidine (NPB) (the hole transport layer), poly(3,4,-ethylene dioxythiophene):polystyrene sulfonic acid (PEDOT:PSS) (the hole injection layer), and indium tin oxide (ITO) (the anode). The optical functions have been determined from the ellipsometry data by oscillator models. The absorption spectra were also measured. The extinction coefficients obtained from the
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fitting of ellipsometry data were in excellent agreement with absorbance measurement results. EXPERIMENTAL DETAILS The ITO glass substrates were directly used for SE measurements. BCP, Alq3 and NPB were thermally evaporated in high vacuum on silicon substrates and PEDOT:PSS was sp
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