Directional lateral crystallization of vacuum-deposited C8-BTBT thin films via liquid crystal phase by a seeded horizontal temperature gradient cooling technique

We have demonstrated the directional lateral crystallization of thin film C8-BTBT which is pre-vacuum-deposited on a SiO₂/Si substrate, with good controllability of the crystallization rate, by cooling from its liquid crystal state with a constant horizontal temperature gradient using a custom-made substrate holder. From in situ polarized optical microscope observation of the crystallization process in detail, it was revealed that the lateral crystal growth proceeds via liquid crystal phase from the lower-temperature side with seed crystals to the neighboring higher temperature side with small supercooling between them as a driving force. Moreover, the propagation rate of crystallization could be intentionally controlled by changing the cooling rate of and the degree of temperature gradient over the substrate, realizing large single crystal domains under the optimized conditions. The orientation analysis of the obtained C8-BTBT thin films with large single-crystal domains was carried out by means of grazing incidence X-ray diffraction and their field effect transistor performance was discussed as well.