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In order to have a full picture of the structural and thermodynamic behavior of organic semiconductors in thin films, we employ a rational combination of fast scanning calorimetry (FSC), grazing incidence X-ray scattering (GIWAXS and GISAXS) and polarized optical microscopy/spectroscopy (POM/S) experiments that allow us to probe both crystalline and glassy phases.
We employ fast scanning calorimetry (FSC) to study thermodynamic changes in organic semiconductors, such as the crystallization, the melting, liquid crystalline transitions and the glass transition. Thanks to the fast scanning rates that can be applied—up to 10,000 ºC s-1, FSC allows probing semiconducting thin films that are processed following identical procedures as for device fabrication. It is thus a powerful method to investigate processing-structure-properties relationships in device-like samples. Information that we obtain from fast scanning calorimetry include (i) the impact of thermal treatments on the solid-state microstructure of polymers (Nature Commun. 2019, 10, 3365), (ii) the nature of glassy phases in organic semiconductors (J. Phys. Chem. Lett. 2018, 9, 990), (iii) the composition of intermixed domains in organic solar cells via the analysis of the Tgs (manuscript submitted), and (iv) the solid-state microstructure of high-performing semiconducting polymers (manuscript submitted).
Grazing incidence X-ray scattering in wide (GIWAXS) and small angles (GISAXS) are used to probe the solid-state microstructure of thin films in the length scale between 0.1 and 100 nm. In situ experiments conducted in synchrotron beamlines allow to study, for example, the thermotropic behavior of semiconducting materials (manuscript submitted), the impact of solvent and temperature treatments on their structure and their crystallization during the solution processing (e.g. blade coating).
In order to assess thermally-induced phase transitions in thin films (e.g. crystallization, melting, mesomorphic transitions, etc.), we record the optical transmittance spectra under crossed polarization while the desired thermal protocol is applied to the sample. Spectra thus obtained are integrated to calculate the total light intensity transmitted, which is related to the relative amount of ordered (birefringent) phase(s) in the thin film at each temperature. Hence, the kinetics of the phase transitions involving measurable birefringence changes can be followed by this technique (Chem. Mater. 2018, 30, 748, Nature Commun. 2019, 10, 3365).