Analysis of dark current-voltage characteristics of Al/chlorophyll a/Ag sandwich cells

Abstract

The analysis of dark current-voltage (I-V) characteristics of Al/chlorophyll a/Ag cells at room temperature with respect to the elucidation of conduction mechanisms and evaluation of cell parameters is presented. It is seen that the presence of series and shunt resistances can considerably affect the I-V plots. It is therefore important to remove their effects for correct and meaningful analysis of the I-V curves. The results suggest that for Al/Chl a/Ag cells with microcrystalline Chl a∼3000 Å thick, the conduction mechanism for voltages between 0.53 and 1 V can be described by a modified Shockley equation from which the values of Rs, Rsh, n, and I0 obtained are 3.2×104 Ω, 1.7×109 Ω, 1.74, and 2.4×10−15 A, respectively. Rs is most likely due to the combined effect of bulk Chl a and the electrodes, particularly the insulating layer of Al2O3 that is formed as a result of oxidation of Al in air. For higher forward biases, i.e., between 1 and 2 V, the current transport is due to the space-charge-limited current in presence of exponentially distributed traps. The density of traps obtained is ∼1018 cm−3. Because of the breakdown of Al/Chl a/Ag cells at fields higher than 107 V m−1, the transition voltage to observe the trap-filled-limit situation was not possible. For a low-bias region, the conduction mechanisms seem to be dominated by Schottky emission over an Al/Al2O3 barrier; however, the temperature dependence of I-V characteristics is necessary to confirm this. The barrier Al/Al2O3 calculated is ∼1.17 eV.