Following isothermal and non-isothermal crystallization of poly(3- hexylthiophene) thin films by UV-vis spectroscopy

We investigated non-isothermal and isothermal crystallization of spin-coated poly(3-hexylthiophene) (P3HT) thin films from the melt by in-situ ultraviolet-visible (UV-vis) absorption spectroscopy. To screen the developing order in the films, we analyzed the absorption spectra taken during the crystallization process according to the Franck-Condon principle, allowing for a quantitative assessment of the degree of crystallinity and quality of the order within crystalline regions of the films. Measured at crystallization temperature (Tc), the sample crystallized at a lower Tc (170 °C) showed higher crystallinity, but lower crystalline quality than the sample crystallized at a higher Tc (225 °C). This was further supported by higher fusion enthalpy, but lower melting temperature of the sample crystallized at 170 °C deduced from calorimetry. Measured at room temperature, we observed similar degree of crystallinity for all differently crystallized films. The highest quality of order, however, as deduced from the absorption spectra, was found for the P3HT film cooled slowly from the melt and for the films crystallized at high crystallization temperatures, fully consistent with results obtained by X-ray diffraction. Atomic force microscopy images did not show a well-defined structure of ordered domains of preferentially aligned lamellae for the as-cast films and the films rapidly cooled from the melt, in contrast to the other differently crystallized films.