Structural, morphological and electrical properties of pentacene based thin film transistors

Similar to other organic materials pentacene has a strong tendency to form weakly bound van-der-Waals molecular single crystals and highly ordered polycrystalline thin films. During the growth process the molecules arrange in a herringbone structure within each elementary cell. In the thermodynamic limit the growth morphology is determined by the balance of the interfacial free energies involved. For temperatures far from thermodynamic equilibrium, the thin film morphology will be governed by kinetic effects. If the interlayer mass transport is fast enough to allow atoms to leave the tops of growing 2D islands as soon as they arrive, the growing layer will be completed before second layer nucleation sets in and smooth layerby- layer growth results. If, however, the interlayer mass transport is hindered by a sufficiently large diffusion barrier at the island edge, growth in the next layer starts before the previous one is filled and 3D structures develop (multilayer or 3D growth). As the nucleation density of the first crystalline molecular layer is determined by the ratio of the growth rate to the diffusion constant of pentacene on the substrate, high evaporation rates and poorly controlled surfaces with a lot of reactive bonds result in enhanced layer-by-layer growth and smooth densely packed films with small grains and a high grain-boundary density.