The complex admittance of metal/oxide/pentacene thin film junctions is investigated under ambient conditions. At low frequencies, a contribution attributed to proton diffusion through the oxide is seen. This diffusion is shown to be anomalous and is believed to be also at the origin of the bias stress effect observed in organic field effect transistors. At higher frequencies, two dipolar contributions are evidenced, attributed to defects located one at the organic/oxide interface or within the organic, and the other in the bulk of the oxide. These two dipolar responses show different dynamic properties that manifest themselves in the admittance in the form of a Debye contribution for the defects located in the oxide, and of a Cole-Cole contribution for the defects related to the organic.
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