Tailoring the optical properties of rubrene films through epitaxy-induced amorphous-to-crystal transition

Crystalline rubrene (RUB) with the orthorhombic structure can be regarded as a workhorse in organic optoelectronics. So far, however, its great potential for device integration has been held back by the struggle to obtain high-quality and photo-oxidation-resistant RUB thin films. Here, we propose an effective strategy to obtain homogeneous, highly crystalline, and oriented RUB thin films, which relies on the spontaneous amorphous-to-crystal transition driven by organic epitaxy occurring at room temperature in vacuum; this crucial process dictates the final morphological, structural, and optoelectronic properties of the film. To probe the kinetics of the transition, we combine ex situ analysis via polarized optical spectroscopy and atomic force microscopy with a photoluminescence investigation carried out in situ, based on monitoring the efficiency of the singlet fission process typical of crystalline RUB. Building on the insights gained, we tune the thin film growth and post-growth parameters to obtain centimeter-scale, highly homogeneous and crystalline RUB thin films, consisting of several mu m-sized and coherently oriented domains, featuring oxidation resistance.We show that the amorphous-to-crystal transition driven by epitaxy is the process behind the growth of rubrene films with single-crystal-like properties. Growth optimisation expedites this transition and greatly improves the homogeneity of the films.