The best curve fitting to measured current-voltage curves of a crystalline solar cell is obtained from the mathematical description of an alternate circuit diagram characterised by the parallel connection of two diodes with the diode saturation currents I01 and I02, and the diode factors n1 and n2, the so-called two-diode model. In the alternate circuit diagram a current source produces an irradiance-dependent photocurrent Iph, part of which flows off at the diodes due to charge carrier recombination. The current loss caused by low resistance at the edges of the solar cell is characterised by the shunt resistance Rp, which lies parallel to the diodes and the current source. If the voltage at the contacts of the solar cell, i. e. at the consumer, is called V, then the somewhat higher voltage V + IRs applies to all the parallel components. Rs is the series resistance of the solar cell across which a voltage drop develops proportionally to the current I.
The two-diode model is represented by an implicit equation of the current, which can only be solved iteratively.
nh n2: Diode factors [-]
I01: Saturation current from base (p-silicon) and emitter (n-silicon) [A]
I02: Saturation current in the space-charge zone [A]
Iph: Photocurrent of the solar cell [A]
Rs: Series resistance [W]
Rp: Bypass resistance [W]
k: Boltzmann constant [1.38046 x 10-23 J/K]
q: Elementary charge [1.602 x 10-19 C]
T: Temperature [K]