Influence of impurities and structural properties on the device stability of pentacene thin film transistors

The influence of environmental conditions on the electronic transport and the device stability of polycrystalline pentacene transistors were investigated. Electrical in situ and ex situ measurements of pentacene thin film transistors were carried out to study the influence of dry oxygen and moisture on the device operation. The staggered thin film transistors were fabricated by organic molecular beam deposition on thermal oxide dielectrics. Exposing the pentacene films to oxygen leads to the creation of acceptorlike states in the band gap. The acceptorlike states cause a shift of the onset of the drain current towards positive gate voltages. A simple analytical model will be presented which directly correlates the onset voltage of the transistors with the acceptor concentration in the pentacene film. Exposing the pentacene film to moisture causes a drop of the charge carrier mobility, a reduction of the threshold voltage, and a shift of the onset voltage. Besides the creation of acceptorlike states in the pentacene film the interface between the drain and source electrodes and the pentacene film is affected by moisture. The injection of holes in the highest occupied molecular orbital level of the pentacene film is inhibited, which causes an apparent drop of the charge carrier mobility and a reduction of the threshold voltage.