Solar cell physical change under load?

[Circlotron]

Say we have an unloaded solar panel in the bright warm sunshine. All these photons are falling on it but no electrical energy is being extracted. Then we put a load on it. Those photons are now being utilised, so does the panel get cooler? Does it get darker colour? Would there be some physical change?

[RJSV]

   First thing, I wouldn't be expecting the electrons set free by previous incoming light, to behave like a hot gas, when electric wire contact happens. There could be an 'outrush' of current due to potential, a small bit stored in inherent capacitance of panel, and realtime potential from sun.
   I would, in a betting scenario, bet money that a small, normal (panel) heating effect due to electric current that starts flowing, through connected load.  I suppose that it's plausible that normal current could cause a slight infrared signature.
But when unconnected, after 1 second, would not expect any further progression of stored charge.

[fourfathom]

Say we have an unloaded solar panel in the bright warm sunshine. All these photons are falling on it but no electrical energy is being extracted. Then we put a load on it. Those photons are now being utilised, so does the panel get cooler? Does it get darker colour? Would there be some physical change?

An unloaded panel runs hotter than a loaded (or shorted) one.  The equivalent cell model shows this and temperature measurements agree with the model behavior.  I doubt that there is visible color change.

[Geoff-AU]

fourfathom nailed it.  A loaded panel runs cooler because energy is being extracted to do useful work elsewhere.

Maybe I haven't thought about it enough, I didn't realise there was a difference between matched (MPP) load and a short.  I thought the shorted panel would be the same temperature as the unloaded panel, dissipating the energy as resistive losses internally.

[Someone]

I doubt that there is visible color change.

Not visible to the eye, but there is a measurable optical emission within the visible wavelength. However since its not visible, it's only a tiny part of the energy loss

[Kleinstein]

The efficiency of solar panels is not very high, maybe 20% with good ones. So extracting that power would lower the temperature rise above environment by some 20%. With some 50 C of temperature rise this would mean some 10 K cooler for the good panel delivering power.

The color change is in the IR range (around 1 µm) - the same wavelength a silicon diode emitts a little. However silicon is rather low efficency as a LED and the effect is thus more minor.
The indirect temperature effect (temperature dependence of the effectiveness of the anti reflective coating) may be the larger one.

[Seekonk]

In a perfect world all the cells would be identical, but cells are just crappy diodes. Here is a FLIR picture of a heavily loaded panel operating at about 1/3 of its power point voltage.  While some get cooler, weak cells get hotter.

[JohnG]

A solar cell, to first order, looks like a current source in parallel with a diode. The current is approximately proportional to the photon flux intercepted by the panel. If the current doesn't go to a load, it goes through the diode formed by the PV cell PN junction, which means the power is dissipated in the cell.

If you pull some of the current to an external load, less power will be dissipated in the cell.

John