Crystallization of pentacene thin films on polymeric dielectrics

Abstract

Thin films of pentacene have been deposited on five different organic dielectric layers of three different classes of organic materials: a conventional polymer, a photoreactive polymer and a self assembled monolayer. The morphology and crystallographic properties of the pentacene films were investigated with atomic force microscopy and grazing incidence X-ray diffraction. It is found that the films form well known crystallographic structures but show unusual crystal orientations with respect to the surface. The (001) lattice plane of the Campbell phase is tilted approximately 13° from the substrate surface normal while tilts between 4° and 7° are observed for the thin film phase. The island shape and density are found to be inhomogeneous on all substrates. In films where mainly large dendritic grains are observed the thin-film phase is the prevalent crystallographic phase while films with a high island density and small irregular grains are dominated by the Campbell phase. It is observed that the mean surface roughnesses and mean surface energies of the substrate play a minor role for the crystallization while the substrate's morphology on nanoscopic scale is crucial for the thin film growth.