Spectroscopic ellipsometry on thin films of conjugated polymers

Abstract

This work presents results from spectroscopic ellipsometry measurements of thin films of conjugated polymers constructed by spin casting with rapid solvent evaporation. Accurate data for the optical functions of MEH-PPV, PF2/6am5, PPy and PmPy films are given for the first time in a broad spectral range. A uniaxial anisotropic character is present in these functions with the optical axis parallel to the surface normal. The amount of anisotropy is found to be fairly high even in polymer layers with less rigid rod molecule chains (PF2/6am5 and MEH-PPV) and very high for the highly 7r-stiff polymer PPy. The influence of an anisotropic and highly dispersive optical response on the efficiency of light emitting diodes made from PF2/6am4 is investigated. The out-coupling efficiency in such devices is found to be highly dependent on the emission wavelength. The results obtained are compared to the commonly used approximation n =const. = 2. The importance of a proper knowledge of the optical behavior of this material group when utilized in devices is shown by the significant difference between these methods. The ability of ellipsometry to monitor structural changes in conjugated polymers is demonstrated. The perfect rigid rod behavior of PPy and the loss of rigidity when forming a copolymer together with meta mPy is clearly visible in the ellipsometry data. The amount of anisotropy is dependent of the ratio of mPy and an isotropic respond is obtained for pure PmPy. Furthermore, the blue shifted absorption spectrum for higher ratios of PmPy validate breaks in the conjugation by the additional meta linkages in the chain.