Depletion layer characteristics of Al/microcrystalline chlorophyll a/Ag sandwich cells

The capacitance measurements of Al/microcrystalline chlorophyll a/Ag sandwich cells have been carried out at various frequencies in the dark and under illumination. The voltage-dependent capacitance, C, and the linear Schottky plots obtained at only low frequencies, ∼ 0.1 Hz, and under illumination suggest that the depletion layer mainly consists of trapped charges that are not able to follow the variations in applied voltage at high frequencies ∼ 100 Hz. From the linear Schottky plots, the depletion parameters, e.g., built-in potential, width at zero bias, and space charge density have been determined in the dark and under illumination (11 μW cm^-2), which are, respectively, 620 mV, 300 Å and 2 × 10²³ m^-3, and 700 mV, 205 Å and 4.78 × 10²³ m^-3. The ageing of the cells in ambient air has been found to have dramatic effect on the capacitance characteristics of the cells. The diffusion of oxygen and presence of water vapour are most probably responsible for an inhomogeneous space charge developing within the depletion layer and the decrease of space charge density with time, respectively.