Depletion layer studies in organic films: Temperature dependence studies of low frequency capacitance measurements in tetracene and magnesium phthalocyanine

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Abstract
The voltage dependence of capacitance in two photovoltaic sandwich cells, (Al ‖ tetracene ‖ Nesatron) and (Al ‖ magnesium phthalocyanine ‖ Nesatron), is reported as a function of temperature. The tetracene cell shows a voltage independent (geometric) capacitance at room temperature which upon heating to 120 °C changes to a depletion layer type capacitance at 10−1 Hz. Values for depletion width, carrier concentration, and barrier voltage are consistent with those reported in a previous publication where carriers in tetracene were mobilized by light. Magnesium phthalocyanine cells, on the other hand, show a depletion layer capacitance at room temperature and upon cooling revert to a geometric capacitance at some lower temperature depending on the frequency of the measurement. A theoretical treatment is presented which describes the frequency and temperature dependence of the depletion layer capacitance. From this analysis a method for measuring activation energies and rate constants for detrapping is obtained. This method is demonstrated for magnesium phthalocyanine cells.