The role of polymorphism in organic thin films: oligoacenes investigated from first principles

We investigate the cohesive properties of oligoacenes within the framework of density functional theory including van der Waals interactions. In comparison, we evaluate the local density approximation as well as gradient corrections, but also a widely used semiempirical procedure accounting for the long-range dispersive forces, in terms of their performance for the energetics of such weakly bound systems. Besides the cohesive energies we discuss in detail the surface energies which, in turn, allow for obtaining the crystal shapes based on Wulff's construction for the oligomer series from naphthalene to pentacene. In particular, we focus on comparing two different pentacene polymorphs, i.e. the so-called bulk structure and the thin film phase, the latter being predominately found in thin film growth. We not only study the impact of molecular conformation on the details of these polyhedra, but also the influence of the number of considered index planes and the role of the underlying exchange correlation functional. Based on the relaxed crystal structures for the two polymorphic phases, we compute the electronic band structures as well as the optical spectra. To account for excitonic effects in the latter, we solve the Bethe–Salpeter equation for the electron–hole pairs, thereby considering the coupling between resonant and anti-resonant terms.