Spectroscopic ellipsometry of electrochemically prepared thin film polyaniline

We report on UV-visible-near IR spectroscopic ellipsometry of electrochemically prepared polyaniline. Accurate modeling and fitting to experimental data can only be obtained on very thin samples (<30 nm). Samples with thickness greater than 100 nm exhibit very complicated characteristics due to absorption effects as well as complex structural effects such as anisotropy, graded and other ambiguous factors. A Lorentz model, with three oscillators centered at approximately 1.5, 2.8 and 3.6 eV and a free carrier contribution, according to the model of the defect states in polyaniline proposed by McCall et al. [Phys. Rev. B 41 (1990) 5202] give a reasonable fit to the experimental data for the thinnest sample. The central transitions of the three oscillators correspond to the transitions from the valence to hole polaron, the occupied polaron level to the conduction band and valence to conduction band (π→π*), respectively. These oscillators also give best fitting for thick samples despite the large mean square error value because of the large absorption αL>1 and structural complexity. The electrochemically prepared polyaniline films may appear to possess uniaxial anisotropy and multilayer structure due to different growth stages and the rigidity of the polymer chain.