PV cell reverse characteristics

Empirically, the behaviour of the cell's characteristics under reverse polarisation is quadratic with the applied voltage. This results is from our own measurements, and confirmed in ref [15]:

This expression could be valid till the avalanche zone (Zener), but in reality the dissipation in the cell – which varies as the cube of the reverse voltage – reaches a destructive limit far before this elbow.

By the way the reverse characteristics during reverse or even hot-spot conditions is very unstable (highly varies with the cell temperature). On the other hand, its exact behavior in the I/V characteristics of a full array under partial shading conditions is completely negligible. These are reason why the accuracy of this model is really not important.