To better understand the molecular arrangement within the thin films, specular X-ray diffraction measurements (probing the periodicities normal to the substrate surface) were performed (Fig. 2). It is immediately apparent from the diffraction patterns that the different substrate temperatures have an impact on the film structure which is not reflected in the morphology, as assessed by AFM. In films prepared at 200 K, two broad, weak Bragg peaks are visible at ∼0.5 and 1.9 Å–1 which correspond to d-spacings of 12.45 and 3.29 Å, respectively. The first peak can be interpreted as standing molecules of EPI with a slight tilt of the long molecular axis away from the substrate normal (EPI has a molecular length of ∼12.6 Å).5 The second peak gives a d-spacing similar to the intermolecular distance of π–π interactions, suggesting that molecules lying parallel to the substrate surface are also present. The presence of molecules with two distinct orientations in the same film is not uncommon38–41 and the broad, weak nature of the diffraction peaks observed for films prepared at 200 K shows that the film is not well ordered.42 The fact that films produced at 200 K are mostly disordered can potentially explain, and is consistent with, the observed poor device performance of EPI transistors prepared at substrate temperatures of 200 K,5 as the degree of molecular ordering is strongly correlated with charge transport mobility in thin films.43