Effect of temperature on capacitance of Al/microcrystalline chlorophyll a/Ag sandwich cells

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Abstract
The capacitance measurements of Al/microcrystalline Chl a/Ag sandwich cells have been carried out as a function of temperature. The results show that while the capacitance at 0.10 Hz is voltage dependent at 20 °C, it is totally voltage invariant at −150 °C. This is explained in terms of thermal mobilization of the trapped charges which are responsible for the depletion layer. The temperature supplies the necessary energy and brings the response time of the trapped charges within the modulation frequency of the applied voltage and makes capacitance voltage dependent. Further, depending upon the frequency, f, of the applied voltage, the temperature needed to mobilize the carriers to the same extent is also different. This is due to the fact that when the frequency is high, the trapped carriers are not able to respond as well to the variations in voltage compared to when the frequency is low. As a result, a comparatively high temperature is needed to mobilize the carriers to the same degree at high frequency than at low frequency. The same reasoning, i.e., the response behavior of the trapped charges with respect to the frequency of the probe voltage can also explain the onset of the geometric capacitance at much lower temperatures at lower frequencies than those at high frequencies. From the plot of log f versus reciprocal characteristic temperature, an activation energy of ∼0.2 eV has been calculated, and corresponds to the energy necessary to detrap and mobilize the charge carriers from the traps. The structural imperfections in the electrodeposited Chl a film are most probably the source of traps.