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 elect ron microscopy. We correlate this with the mobility of these films as measured in field-effect transistors. The X-ray study based on meridio nal OOl reflections shows evidence for various crystalline phases depe nding on the substrate temperature during vapor deposition. A high deg ree of orientation can be achieved even in several-micrometer-thick fi lms deposited above 190 degrees C. It is shown that the field-effect m obility is substantially enhanced for deposition temperatures close to the melting point (290 degrees C), which is associated with a suitabl e orientation (002 being the single contact plane) and eventually a fa vorable crystalline structure and coalescent lamellae morphology.