Spectroscopic ellipsometry studies of the optical properties of doped poly(3,4-ethylenedioxythiophene): an anisotropic metal

• Published in: Thin solid films Vol. 313; no. 1-2; pp. 356 - 361
• Main Authors: Pettersson, Leif A.A., Carlsson, Fredrik, Inganäs, Olle, Arwin, Hans
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.02.1998
• Subjects: Optical anisotropy; Poly(3,4-ethylenedioxythiophene); Polymer; Spectroscopic ellipsometry; Uniaxial; Uniaxial; Polymer; Spectroscopic ellipsometry; Poly(3,4-ethylenedioxythiophene); Optical anisotropy
• ISSN: 0040-6090; 1879-2731
• DOI: 10.1016/S0040-6090(97)00846-8

Poly(3,4-ethylenedioxythiophene) (PEDOT) is a conjugated polymer with high electrical conductivity in the doped state, good thermal and chemical stability and fast electrochemical switching. The material is used as an antistatic layer, and has potential for use as a (transparent) electrode material and for electrochromic applications. Thin films (<1 μm) of doped PEDOT have been studied with variable angle spectroscopic 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 simultaneously fitting optical functions for multiple samples using a recently discussed 4×4 matrix formalism, the analysis reveals that the material is optically anisotropic and of uniaxial character with the optic axis normal to the film surface. The uniaxial anisotropy of PEDOT films reveals an entirely different appearance for the ordinary and extraordinary indices of refraction with the former showing metallic character. The metallic state behavior agrees well with the high electrical conductivity measured for this material at different doping levels. From the measured optical properties and from supplementary optical and structural analysis, a preferential structural orientation of the polymer chains can be deduced.