Polymorphism and Charge Transport in Vacuum-Evaporated Sexithiophene Films

The influence of deposition temperature on structure, orientation, and morphology of vacuum-evaporated sexithiophene films has been studied by using X-ray diffraction, UV-visible spectroscopy and scanning electron microscopy. We correlate this with the mobility of these films as measured in field-effect transistors. The X-ray study based on meridional 001 reflections shows evidence for various crystalline phases depending on the substrate temperature during vapor deposition. A high degree of orientation can be achieved even in several-micrometer-thick films deposited above 190 °C. It is shown that the field-effect mobility is substantially enhanced for deposition temperatures close to the melting point (290 °C), which is associated with a suitable orientation (002 being the single contact plane) and eventually a favorable crystalline structure and coalescent lamellae morphology.