Structural Characterization of a Pentacene Monolayer on an Amorphous SiO_2 Substrate with Grazing Incidence X-ray Diffraction

摘要

Pentacene, a simple linear oligoacene consisting of five fused benzene rings, has emerged as a viable candidate for the semiconducting transport layer in organic thin film transistors (OTFTs). The interest in pentacene films, which exhibit p-type transport, primarily stems from their relatively high hole mobilities and high on-to-off current ratios in OTFTs.5 Carrier transport in the channel between the source and drain electrodes is thought to occur in the first few layers of the semiconductor, or perhaps the first layer, in proximity with the dielectric layer adjacent to the gate electrode. It is also widely recognized that the transport properties of crystalline organic films depend strongly on the intermolecular overlap of electronic wave functions within the semiconductor layer, which is very sensitive to the molecular packing in the crystal. Surprisingly, little is known about the detailed crystal structure of the active transport layers in OTFTs, including pentacene films. We report herein preliminary grazing-angle incidence X-ray diffraction (GIXD) data for a monolayer-thick pentacene film grown on amorphous silicon dioxide (a-SiO_2), a commonly used dielectric layer in OTFTs. The data confirm that the monolayer is crystalline and has a structure that differs from that of bulk pentacene, which has important implications for carrier transport in pentacene-based OTFTs.