SPECTROSCOPIC ELLIPSOMETRY STUDIES OF THE OPTICAL-PROPERTIES OF DOPEDPOLY(3,4-ETHYLENEDIOXYTHIOPHENE) - AN ANISOTROPIC METAL

Poly(3,4-ethylenedioxythiophene) (PEDOT) is a conjugated polymer with high electrical conductivity in the doped state, good thermal and chem ical stability and fast electrochemical switching. The material is use d as an antistatic layer, and has potential for use as a (transparent) electrode material and for electrochromic applications. Thin films (< 1 mu m) of doped PEDOT have been studied with variable angle spectrosc opic ellipsometry. Reflectance and transmission measurements have also been performed. From ellipsometry and transmission data, the optical properties and thickness of the PEDOT layers were extracted. By simult aneously fitting optical functions for multiple samples using a recent ly discussed 4 x 4 matrix formalism, the analysis reveals that the mat erial is optically anisotropic and of uniaxial character with the opti c axis normal to the film surface. The uniaxial anisotropy of PEDOT fi lms reveals an entirely different appearance for the ordinary and extr aordinary indices of refraction with the former showing metallic chara cter. The metallic state behavior agrees well with the high electrical conductivity measured for this material at different doping levels. F rom the measured optical properties and from supplementary optical and structural analysis, a preferential structural orientation of the pol ymer chains can be deduced. (C) 1998 Elsevier Science S.A.