A two-diode model regarding the distributed series resistance

Abstract

The two-diode model is widespread for interpreting dark and illuminated current–voltage characteristics of solar cells, though it does not hold correctly for high current densities due to the distributed character of the series resistance. This is one reason for the fact that fitting the dark and the illuminated characteristic leads to two different sets of two-diode parameters. After locally analyzing a typical multicrystalline solar cell, it is found that here the grid conductivity represents the most significant contribution to the distributed series resistance. A simplified equivalent circuit implying a 1-dimensional current distribution is used for simulating current-dependent effective (lumped) series resistances in the dark and under illumination. It is found that the influence of the distributed series resistance on both characteristics can be described empirically by introducing a series resistance being variable for high currents. Moreover, well-known departures from the superposition principle often cannot be neglected. Therefore the second diode contribution is generally stronger under illumination than in the dark, and also the parallel resistance may be affected. We introduce only one additional series resistance parameter and consider that the second diode parameters and the parallel resistance may be different under illumination and in the dark. In this way, the current dependence of both the dark and the illuminated series resistance can be described with the same consistent set of first diode and series resistance parameters. Based on these findings, a two-diode model with an analytically given variable series resistance is proposed, which may describe both the dark and the illuminated characteristic up to large current densities in good approximation with one and the same physically meaningful parameter set.