X-ray structural and crystallinity studies of low and high molecular weight poly(3-hexylthiophene)

X-ray diffraction studies were performed in order to understand the crystallinity, the molecular orientation and the nanoscale morphology of a low-(LMW) and a high molecular weight (HMW) fractions of Poly(3-hexylthiophene) (P3HT) thin films. The thickness of films was found to be in the range between 10 nm and 200 nm. Further structural information was obtained using X-ray grazing incidence in-plane and out-of-plane diffraction using synchrotron radiation. The X-ray analysis reveals that the films are mainly amorphous and contain randomly oriented nanocrystallites with the best match for a monoclinic unit cell. Additionally, polymorphism was found for similar to 200 nm thick films of LMW fraction. Interestingly, the film morphology varies as a function of the film thickness. For LMW material the random orientation of nanocrystallites changes to preferential orientation of lamellae along the surface normal when the size of crystallites become of the order of film thickness, i.e. below 20 nm. For the HMW fraction we found an increase of lamellar ordering at about 60 K below the melting point of the material. Temperature-resolved measurements reveal a thickness dependence of crystallinity. For the LMW fraction the character of the phase transition changes from continuous drop of intensity (for thick films) to sharp drop of intensity for extremely thin films, which is a hint for the importance of the film to substrate interaction. The figure shows the proposed monoclinic unit cell of the P3HT low molecular weight fraction.