# Irradiance and Isc and Voc of a solar cell

## Main Question or Discussion Point

When irradiance increases what happens to open circuit voltage and short circuit current of a solar cell? Do they both increase linearly? Because temperature affects open circuit voltage, so I'd assume open circuit voltage doesn't increase linearly then...

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Baluncore
2019 Award
Voltage does not increase linearly with radiance. Current does increase with radiance.
But you want power so you must find the point where the product of voltage and current is close to the maximum.
That cannot be with short circuit current because then there is no voltage.
That cannot be with an open circuit voltage because then there is no current.
https://en.wikipedia.org/wiki/Maximum_power_point_tracking#I-V_curve
Voltage drop across the resistance of the cell increases with temperature and dominates voltage reduction due to rising temperature.
PN junction voltage falls slightly with temperature increase.

Ok, so open circuit voltage doesn't increase with irradiance, so then it must decrease due to temperature effects.

I was confused between light intensity and irradiance. As light intensity increases open circuit voltage and short circuit current increase, but light intensity and irradiance are two different things.

Baluncore
2019 Award
Ok, so open circuit voltage doesn't increase with irradiance, so then it must decrease due to temperature effects.
Correct. All other things being equal, the open circuit voltage will have the negative temperature coefficient of the cell material, probably silicon.
I was confused between light intensity and irradiance.
Avoid the confusion by referring to incident power. Only photons with wavelength, energy sufficient to overcome the semiconductor band gap are being considered.
Twice the power, is twice the photons, is twice the electrons, is twice the current.

• says
As irradiance increases, we can assume temperature will increase (more irradiance=later in the day=temperature increasing), this means that Voc decreases due to temperature effects, and short circuit current increases due to more irradiance.

Baluncore
2019 Award
A cell is never operated at open-circuit voltage or short-circuit current. It is operated close to the MPP.
As output current increases the voltage falls since the cell internal resistance drops more voltage.
There is also the smaller drop due to the temperature of PN junction.

The rising panel temperature is is due to heat from;
1. Internal I2R resistive losses.
2. Incident long wavelength radiation with insufficient energy to overcome the bandgap.
3. Thermal environment later in the day as angle to Sun changes and as air warms.

"
A cell is never operated at open-circuit voltage or short-circuit current.
Solar cell is operated close to the MPP.
That cannot be with short circuit current because then there is no voltage.
That cannot be with an open circuit voltage because then there is no current."

thank yall for this simple summary