A systematic study of the anisotropic optical properties of thin poly(3-octylthiophene)-films in dependence on growth parameters

Abstract Poly(3-octylthiophene) (P3OT) is a promising candidate for plastic solar cells. We have determined the anisotropic dielectric function (DF) of thin spin-coated and drop-cast P3OT-films deposited on Si and glass substrates from both near-normal reflectance and transmittance spectroscopy and from variable angle spectroscopic ellipsometry. The influence of the deposition parameters (such as spin frequency, concentration of the polymer in solvent, type of solvent, type of substrate) on the anisotropic DF of the film was investigated. The anisotropy of the DF can be related to the orientation of the polymer chains in the film. The optical anisotropy of spin-coated films increases with increasing spin frequency and decreasing polymer concentration in the solvent, indicating that polymer chains become more aligned parallel to the substrate. The drop-cast films were also found to be anisotropic. In addition, the anisotropy increases considerably if the property of the Si substrate is changed from hydrophilic to hydrophobic (by treatment with hydrofluoric acid). With increasing film thickness the optical anisotropy decreases. Summing up, all results are discussed in dependence of the film thickness as the decisive parameter.