Real-time Structural and Electrical Investigation of PDI8-CN2 based OFET

Semiconductor thin-film devices based on organic molecules are of great interest for the development of high performance organic field effect transistors (OFETs) and organic light emitting diodes (OLEDs), as well as to underscore fundamental charge transport effects in molecular solids. Among the n-type organic molecules, perylene derivatives are very promising. In particular PDI-8CN2, N,N’-bis(n-octyl)-dicyanoperylene-3,4:9,10-bis (dicarb-oximide), has been reported to allow the fabrication of OFETs with excellent electrical performance (high-mobility: 0.16-0.6 cm2 V-1s-1) and remarkably high stability in air. In these systems, the charge mobility depends on the overlap between π-π orbitals of vicinal molecules, which is mainly influenced by the structure and morphology of the first layers of organic film at the interface with the dielectric. Since the structure of these first layers may significantly differ from that of the bulk, the determination of the molecular orientation and packing of organic molecules at the substrate interface is a crucial input for modelling the electronic band structure and the associated charge-transport properties. For this reason we have performed Grazing Incidence X-ray Diffraction (GIXD) and X-Ray Reflectivity (XRR) measurements, in situ and real time during the UHV deposition of PDI-8CN2. Moreover, in situ and real time electrical measurements were performed on FET structures during the semiconductor deposition. Thanks to these time resolved measurements we could describe i) the thin-film growth dynamics, ii) the molecular packing and microstructure of the organic thin film, iii) the influence of the substrate temperature and the deposition flux, and iv) the relation between the charge transport properties and the growth mechanism of the thin film.