Improved order and transport in C-60 thin films grown on SiO2 via use of transient templates

The performance of C-60 semiconducting films is linked to the degree of crystallinity and ordering, properties that strongly depend on the substrate, and growth conditions. Substrate-molecule interactions can be specifically tailored by employing growth templates to achieve a desired thin film structure. However, the presence of a growth template after the film deposition is usually not desirable as it may change the properties of the layer of interest. The ability to remove a growth template without any disruption to the active layer would be highly beneficial. A simple method of template removal by annealing is presented here. A variety of small organic molecules (perfluoropentacene, [6]phenacene, and alpha-sexithiophene) were used as a growth template to obtain a high-quality well-ordered C-60 thin film. In situ grazing-incidence wide-angle x-ray scattering was employed to study the structural changes of C-60 thin films during template removal. While a slight disturbance of the thin film structure was observed during template removal caused by evaporated molecules from the growth template escaping through the C-60 layer, the disruption is only temporary. When the annealing process is concluded, only the well-ordered C-60 thin film directly on top of SiO2 is left, which is not achievable without the use of a growth template. Improved crystallinity and grain size of such a thin film, when compared to preparation without a growth template, lead to a significant improvement of the charge carrier mobility. Importantly, template removal prevents the formation of undesired ambipolar transistor characteristics.