The morphology of thin films at the polymer-to-insulator interface is of great importance for OFET applications. In order to find a relation between the thickness dependence of structural order and the electrical parameters in low molecular weight (M w ∼ 2.5 kDa) poly(3-hexylthiophene) (P3HT), we have performed grazing-incidence X-ray diffraction and field effect mobility measurements. The samples were prepared from solutions with different concentrations by spin-coating mainly onto HMDS-pretreated Si/SiO 2 substrates, resulting in film thicknesses that vary between 10 and 200 nm. The X-ray diffraction curves display Bragg peaks of nanocrystallites diluted into an amorphous matrix where the orientational distribution of the crystallites changes significantly as a function of film thickness. The orientation of nanocrystals was found to be random for the thickest films. Reducing the film thickness, we found an increase in the alignment of the stacking direction of molecules along the surface normal. At same time the mean crystal size along the film normal decreases less than the decrease of film thickness. This is interpreted by a preferential pinning of nanocrystals at the film-to-insulator interface when the crystal size becomes in the order of the film thickness, i.e., below 25 nm. The model of pinning effect is supported by temperature-resolved X-ray measurements performed between room temperature and melting temperature. For films thicker than 25 nm the phase transition appears rather continuously with temperature, but it becomes sharp for thinner films. In contrast to X-ray measurements the field effect mobility is found to be constant within the whole investigated range. Our findings give evidence that the charge transport in low molecular weight P3HT is dominated by the ultrathin layer stabilized at the film-to-insulator interface. Despite the very uniform orientation of the crystallites within this layer, the field effect mobility remains low for all thicknesses. This is attributed to the presence of amorphous regions between highly crystalline domains, which ultimately limits the charge transport in the layer plane.

We report a temperature dependent crystalline structure of spin-coated thin films of high molecular weight regioregular poly(3-hexylthiophene) (P3HT) (M n ∼ 30000 g/mol) and its correlation with charge carrier mobility. These investigations show a reversible change of the crystalline structure, where the interlayer lattice spacing (100) along the alkyl side chains continuously increases up to a temperature of about 220 °C; in contrast, the in-plane π−π distance reduces with increasing temperature. These changes in structure are reversible and can be repeated several times. The temperature-induced structural properties differ for thick and thin films, pointing to a surface/interface role in stabilization of the layer morphology. In contrast to the structural changes, the carrier mobility is rather constant in the temperature range from room temperature up to 100−120 °C, followed by a continuous decrease. For thick layers this drop is significant and the transistor performance almost vanishes at high temperature, however, it completely recovers upon cooling back to room temperature. The drop of the charge carrier mobility at higher temperatures is in contrast with expectations from the structural studies, considering the increase of crystalline fraction of the polycrystalline layer. Our electrical measurements underscore that the reduction of the macroscopic mobility is mostly caused by a pronounced decrease of the intergrain transport. The thermally induced crystallization along (100) direction and the creation of numerous small crystallites at the film−substrate interface reduce the number of long polymer chains bridging crystalline domains, which ultimately limits the macroscopic charge transport.