Anisotropic optical properties of thin poly(3-octylthiophene)-films as a function of preparation conditions

We have determined the anisotropic dielectric function of thin spin-coated as well as drop-cast poly(3-octylthiophene)-films from near-normal reflectance and transmittance data and from spectroscopic ellipsometry. The influence of the growth parameters (i.e. spin frequency, concentration of the polymer in solvent, type of solvent) and of the substrate on the anisotropic dielectric function of the films was studied. The determined anisotropic dielectric functions were used to investigate the orientation of the polymer chains within these films. For the spin-coated films we found, that the optical anisotropy increases with increasing spin frequency and decreasing polymer concentration in the solution, indicating that polymer chains become more aligned parallel to the substrate. The drop-cast films were found to be even more anisotropic. In addition, the anisotropy increases considerably if the property of the Si substrate is changed from hydrophilic to hydrophobic (done by HF-treatment). Summing up, all results are discussed in dependence of the film thickness as the decisive parameter for the anisotropy. (C) 2003 Elsevier B.V. All rights reserved.