I-V curve of a solar cell is given for a given irradiance and temperature. Given a constant irradiance and temperature, why is the voltage changing in an I-V curve? Is the voltage something that we apply externally?
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why is the voltage changing in an I-V curve?
You apply a variable load to get a current flow. Without a current flow, you can't produce an I-V curve. To make a useful I-V curve, the current flow will be across a wide range of values from the cell being open circuit (maximum output voltage) to the cell being shorted (zero cell voltage).
Hence, it's absolutely inevitable that V must change from a maximum value to some fairly low-value (such as 0 volts).
Is the Voltage something that we apply externally?
No, it's the output voltage produced by the cell under differing loading conditions.
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The effect of illuminating the surface of the n region consists in the generation of electron-hole pairs fired towards the depletion region. This phenomenon is equivalent to an independent current generator with a diode in parallel as in the figure. By applying Kirchhoff's principles we arrive at a Lambert equation of which I report the solution (in addition to the known physical constants there are others that I will not report as there is no need to go into further detail)(This is my work created with MATHCAD 15):
