In summary, thin films of EPI, with thicknesses of either ∼100 nm or ∼7 nm, deposited at three different substrate temperatures have been structurally characterized. While the film morphologies are relatively similar, different molecular orientations and tilts with respect to the substrate surface are found depending on the substrate temperature used during deposition; molecules are mostly approximately upright-standing in films prepared at 300 and 350 K, while films prepared at 200 K are mostly comprised of lying molecules. In the samples prepared at 300 K, where two approximately upright-standing phases are present, XRR measurements on very thin films (∼7 nm) could not distinguish which phase forms first during early film growth and it is suggested both phases grow at the same time. Films prepared with substrate temperatures of 200 and 350 K do not show a high degree of order, and are mainly comprised of disordered layered structures, with only out-of-plane order and very little in-plane order. An exception is observed for the film deposited at a substrate temperature of 300 K, where several Bragg peaks are observed in the GIXD data which could be indexed to provide a unit cell for EPI in the thin film. From this, the crystal structure in the thin film was solved using MD simulations and DFT optimization, followed by comparison of the structure factors of the most promising simulated structures with the experimental data. As the method does not rely on the evaluation of diffraction peak intensities (the crystal packing is derived from an energetic point of view), it allows for a crystal structure to be determined for systems where only a small number of weak, diffuse Bragg peaks are observed, such as in the case of EPI presented here. A crystal structure similar to the previously determined structure of the fluorinated EPI derivative was found.11 An assessment of the different interactions occurring within the crystal structures of EPI and the halogenated derivatives further emphasizes the similarity of the thin film structure to that of the fluorinated derivative, as opposed to the chlorinated derivative. Comparison of the simulated powder diffraction pattern from the crystal structure solved from the thin film with the diffraction pattern of the bulk EPI powder used for the deposition shows some clear differences, suggesting the structure found in the thin film is in fact a surface-mediated polymorph. It is therefore likely that this polymorph is responsible for the performance of some of the organic devices previously produced using this material,11 while the observation of lying molecules in films produced at 200 K explains the poor performance of OFETs produced from films deposited in such a way.5