Growth process control of pentacene thin films and its application in full organic thin film transistors

Abstract

Due to its outstanding carrier transport capabilities pentacene is a prominent candidate for the active semiconducting layer in organic thin film transistors. This compound crystallizes in a layered structure with herringbone arrangement within each layer. Pentacene appears in several polymorphic structures, which differ basically by their c-axis lengths, meaning that the angle at which the molecules adsorb relative to the substrate changes from phase to phase. Obviously the interaction of the π-electron systems between adjacent molecules depends strongly on the stacking nature of the molecules. It has been argued, that a smaller angle between the molecular axis and the surface normal results in a larger orbital overlap which is expected to give better carrier transport properties. Therefore it is of major interest to clarify and control the growth conditions for the different phases. We have synthesized high-quality pentacene epitaxial thin films under different growth conditions and investigated them by atomic force, scanning electron and polarization microscopy. The aim was to identify the critical growth parameters with respect to surface quality, thickness and crystallinity of the thin films and with respect to homogeneity, size and shape of grains, ordering, substrate dependence, morphology and phase formation of the polycrystalline thin films. The polymorphic phase identification and the macrostructure of the films were determined by Xray diffraction, whereas micro-structural differences, small impurity concentrations and the temperature dependence of the structure were