In organic field-effect transistors, conductivity is achieved by electronically injected charges that form high-density accumulation layers. We report self-consistent calculations of Poisson's equation, carrier statistics, and the Drude permittivity of the carrier gas at the interface between semiconductors and insulators. The results show that the injected carriers efficiently screen local potentials. Additionally, the AC permittivity of the carriers reduces electrical fields particularly at frequencies of several THz. This dynamic screening may affect the formation of large polarons and the transient localization of carriers.

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