Solar cells have roughly the same behavior as photodiodes. The internal resistance depends on the exposed light. To analyze the internal resistance, we measure the voltage on a known resistor and the opencircuit voltage. Illuminance is also measured to associate it with the internal resistance.
The measurements are fully automated to provide accurate data at any time.
current-voltage curve:
The graph shows clearly a curve like a diode.
open circuit voltage - illuminance
As you can see the open-circuit-voltage of a solar cell changes with the natural logarithm of the irradiance.