The photodiode or solar cell open circuit voltage doesn't mean anything.
Irradiance is best monitored by measuring photodiode current at zero bias, using a transimpedance amplifier. However, simpler circuits like this one work OK if you calibrate the response against a commercial instrument. More info on pyranometers here.The photodiode or solar cell open circuit voltage doesn't mean anything.
this question appears to be about characterizing PV cells
Is there a way to connect this system to arduino?
My comment was about measuring solar irradiance, for which purpose the open circuit voltage of a PV cell is irrelevant.Figure 3 of the first link in reply #1 has a simple circuit with a photodiode/phototransistor connected to a resistor. The junction labeled "output" is intended to be connected to an analog input of an Arduino or other recording device.