Effects of series resistance and inductance on solar cell admittance measurements

Abstract

Admittance measurements have been performed on a variety of 0.43 cm 2 area, copper indium gallium diselenide (CIGS) polycrystalline thin film solar cells for frequencies (f) ranging from 1 kHz to 1 MHz and bias voltages ( V) ranging from −2.0 to 0.2 V. The bulk of these measurements are not presented here. To extend the usefulness of these measurements, and in particular, to extract information about traps, it is first necessary to account for frequency-dependent effects associated with the series resistance and inductance. Such is the purpose of this paper. The complex admittance of a four-element circuit is calculated and compared with measurements on two typical CIGS solar cells. The model circuit consists of a capacitance C in parallel with a resistance r, with this combination in series with a resistor R and inductor L. C accounts for the depletion capacitance of the diode, and R and r account for series and shunt resistances typically observed in solar cell current-voltage measurements. The series inductance was introduced to account for observed resonance effects in the admittance measurements. Model calculations are shown to be in good agreement with measurements on devices. For our cells, measurements above 300 kHz are seldom useful, as these frequencies are in the regime where series R and L dominate the behavior of the circuit. We find-measurements in the 10–50 kHz range to be most appropriate for determining charge densities from C–V scans.