Depletion layer studies in organic films: Low frequency capacitance measurements in polycrystalline tetracene

The study of the electrical properties of (metal 1‖ organic ‖metal 2) cells has not led to a consistent model of their origin. On the one hand, conductivity measurements suggest that many organic materials behave as insulators (low carrier concentrations) while photovoltaic properties of some of these same materials may be rationalized using the concepts of semiconductor physics, suggesting that the organic film has a high carrier concentration. In this paper the voltage dependence of the capacitance of a (Al‖tetracene‖Au or Nesatron) cell is investigated in an attempt to better understand this contradiction. The electrical properties of this cell are explained in terms of a model in which depletion layers are made from a high density of immobile trapped charge. The trapped charge can be mobilized by light and then evidence is found for a Schottky depletion region (∼2000 Å thick) in the tetracene film.