Growth and performance of polycrystalline organic thin films deposited by Supersonic Molecular Beam Deposition

Conjugated small molecules are very interesting both as a model to study the growth model of crystalline organic films and as a very good performance organic material. Vacuum deposition has shown to be the most suitable technique to obtain high purity and order films. Nevertheless permain some problems due to the high anisotropy of organic that make easy the formation of different polymorphs or/and orientations that strongly limit the quality of the films. The innovative supersonic molecular beam deposition (SuMBD) technique, developed at the IFN-Lab, allows a wider control on the growth. The kinetic energy (EK) of the impinging molecules is the key factor that affects the growth modifying the assembling processes of molecules and their surface mobility [Y. Wu et al., Phys. Rev. Lett., 98 (2007) 076601]. Here we report on the growth of different small conjugated molecules (oligothiophenes and pentacene), investigating the influence of several deposition parameters. We will show how, tuning properly the supersonic beam, it is possible to obtain very ordered thin films with large and smooth monocrystalline islands. A higher kinetic energy always improves the quality of the film in terms of structure, size of the grains and thus also of the device performance. The deposition regime obtainable by SuMBD, permits to achieve field effect mobility values that are the state of the art even in absence of any SAM (self-assembled monolayer) and ex-situ. Our organic thin film transistors (OFETs) show indeed a field effect mobility ?? = 1.3 V?cm-1?s-1, while a-sexithiophene devices show value of ?? = 1.5o10-1 V?cm-1?s-1, twice higher than the best values in literature. This work was financially supported by Provincia Autonoma di Trento Project Nanosmart and the Fondazione CARITRO Projects DAFNE and ODINO.