Polymorph modulation of sexithiophene crystals in sexithiophene and pentacene mixed thin films prepared by codeposition

The polymorphism of α-sexithiophene (6T) crystals in mixed thin films prepared by the codeposition of the 6T and pentacene (PEN) molecules on a SiO₂ substrate was investigated by X-ray diffraction (XRD). We found that the fractions of different types of 6T crystal polymorphs in the mixed thin films can be tuned by changing the 6T:PEN molecular ratio in these films. When the 6T:PEN molecular ratio was adjusted from 5:1 to 2:1, several 6T crystal polymorphs of different types appeared. A detailed analysis of out-of-plane and in-plane XRD patterns, along with two-dimensional grazing incidence XRD images, revealed that the fraction of the β phase in 6T decreases as the proportion of PEN increases in the mixed thin films. Atomic force and optical microscopy images also indicated that 6T and PEN molecules primarily aggregated and crystallized in spatially distinct regions in the mixed thin films.