Current-voltage Curve Analysis for Non-linear Silicon Solar Cells and Operation Conditions Beyond Low Injection

Abstract

This paper addresses the limitation of the double-diode model to low injection conditions. Assuming a constant carrier lifetime, we re-formulate the current of the first diode for general injection. The new parameter Y is introduced to quantify the share of the diode saturation current J01 that is due to lightly doped regions that go into high injection at higher voltages. The remaining J01 (1-Y) accounts for the highly doped regions that remain in low injection. When Y equals 0, the extended model is identical to the conventional model. A least square fit of the Isc-Voc characteristics to the general-injection model makes the determination of J01 more reliable and indicates whether the recombination is dominated by the highly doped regions. We also propose to use a double-diode model with a variable J01(V) to evaluate non-linear silicon solar cells. A J01 range between ∼400 mV and the maximum power point can be obtained from the bias-light dependent internal quantum efficiency (IQE). Based on the electrical and optical model that describes the IQE, the excess carrier concentration in the base when the solar cell is in short circuit is calculated for every bias light and used to attribute a voltage. The J01(V) for larger voltages is extracted from the ISC-VOC characteristics after an appropriate extrapolation of the parasitic currents through the second diode and the shunt.