CAPACITANCE CHARACTERISTICS OF MIXED MONOLAYERS OF CHLOROPHYLL-A AND CAROTENOID CANTHAXANTHIN

Capacitance measurements of the cells, obtained by sandwiching mixed monolayers of chlorophyll a and canthaxanthin between Al and Ag electrodes, have been carried out at various frequencies in the dark and under illumination. The results show that at frequencies of approximately 100 Hz, one observes the voltage-independent geometric capacitance; however, as the frequency of applied bias decreases, a voltage-dependent capacitance, characteristic of a Schottky barrier, appears; this is evidenced by the linear plots of reciprocal capacitance square (1/C2) versus applied voltage, V(a). This is explained as due to the depletion layer consisting mainly of trapped charges that are not able to follow the rapid variations in applied voltage at higher frequencies of approximately 100 Hz. Similar observations have also been made with illuminated cells. Light is believed to detrap and mobilize the trapped charges. From linear plot of 1/C2 versus V(a), the depletion layer parameters, e.g., built-in potential, width at zero bias and space charge density have been determined, and are, respectively, approximately 0.7 V, 290 angstrom and 2.5 x 10(23)/m3 in the dark.