Why thermally excited charge carriers not contribute to electricity generation like photo induced electrons?
They do contribute positively to some extent but on the whole the effect is negative.
If you consider a p-n junction semiconductor typically used in solar cells you will find that room temperature is sufficient to "activate" the junction by making electrons available in the conduction band. In fact room temperature (27c) is usually the temperature at which you obtain the peak power and results described in the data sheet. This is the "lab" temperature.
If the temperature were 0K then there would be no conduction electrons to produce electricity. So appropriate amount of heat is necessary to generate electricity from p-n junction.
As the operating temperature of the cells increase more electrons will be available in the conduction band and hence it will increase the conductivity. Infact semiconductor conductivity increases exponentially with temperature.
https://fog.ccsf.edu/~wkaufmyn/ENGN45/Course%20Handouts/15_ElectricalProps/06_TemperatureConductivitySemiConductor.html
One more effect that heat has is that it reduces the band gap of the p-n junction. This is analogous to applying a forward bias to the cells. In simple words hotter electrons at rest have more "inner" energy and therefore they need less "external" energy to surmount the p-n barrier. Therefore the potential difference decreases. And hence you get less voltage out of the circuit.
The combined result of greater number of collisions between energetic electrons and less potential difference decreases the overall output. This is why thermally excited electrons don't contribute much to electricity, in solar cells.
For mathematics and some approximate numbers check out this page
https://www.pveducation.org/pvcdrom/solar-cell-operation/effect-of-temperature
[In thermoelectric systems precisely opposite happens. Heat generates electricity. More specifically, temperature difference generates electricity. Though it's a separate topic altogether. ]
While the electrons and photons are easy to quantize heat is a very difficult thing to quantize. Heat introduces a degree of randomization. Now we don't just have electrons moving there are vibrations at atomic , molecular , crystal lattice level. The discrete model of one photon of appropriate energy exciting one electron of appropriate band gap does not hold.
By comparison electricity is a very civil (almost tame) form of energy. Heat is wild,primal. And while it does excite electrons it makes the effective motion close to 0. In electrical energy they have net movement. In heat energy they simply collide with each other.
There are efforts in place to somehow utilize this heat in electricity conversion. If you are interested in this topic do checkout hot carrier cells
https://www.spiedigitallibrary.org/conference-proceedings-of-spie/9178/917802/Hot-carrier-solar-cell-absorbers-materials-mechanisms-and-nanostructures/10.1117/12.2067926.full?SSO=1
With the advancement in understanding the question is slowly changing from "why not" to "how".
#energy #materials
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