In the first part of the thesis we discuss the applicability of x-ray methods to determine the crystallinity of Poly (3-Hexylthiophen) (P3HT) asa model system for side chain substituted polymers. By means of wide-angle x-ray scattering we obtain crystallinities for bulk P3HT samples at room temperature ranging from 70 to 80%. These values are used to calibrate the reference melting enthalpy of a 100% crystalline P3HT material needed to determine the crystallinity with differential scanning calorimetry (DSC). Based on grazing incidence x-ray diffraction experiments we propose a model for the vertically layered morphology of melt-crystallized thin P3HT films on amorphous substrates. The films show stronglyoriented crystalline structures at the film surface. Additionally, for sufficiently thick films alayer containing unoriented crystals is present below the surface layer. Using fast scanning calorimetry we determined the crystallinity of the P3HT films. Starting from the bulk value for about 350 nm thick films the crystallinity continuously decreases as a function of film thickness.