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In the context of solar cells, some films such as aluminium oxide reduce recombination by "field effect passivation". I find this mentioned often, but can't find a definition or clear explanation of the mechanism anywhere.

It seems to be something to do with the concentration of negative charge at the surface, which leads to a layer just beneath the film with a high concentration of holes, but it's not clear to me how this leads to passivation.

Bridget
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  • I thought this had something to do with combining with the free oxygen in the air over time as apposed to putting the full oxide layer down deliberately during manufacturing, but I could be quite wrong about this. Passivation is usually a coating of something inert, like a metal oxide (glass being silicon oxide). – Olin Lathrop Feb 25 '12 at 13:36

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I assume you're talking about thin-film crystalline silicon solar cells.

Surface passivation can be achieved in several ways:

  1. reducing the recombination at the interface (chemical passivation), and
  2. electrostatically shielding the charge carriers from the interface by an internal electric field (field-effect passivation), or
  3. a combination of both.

There has been good success in using plasma-assisted ALD to passivate c-Si using ultrathin films of \$Al_2 O_3\$. Surface passivation has become more important as c-Si wafer solar cells move towards lower substrate thicknesses and the surface-to-volume ratio increases.

The effect of field effect passivation is to decrease the surface recombination velocity. The fixed charges at the surface of the c-Si interact with the charge carriers in the c-Si bulk and induce a depletion or accumulation layer close to the c-Si surface. If the charge density is sufficiently large it can even create an inversion layer at the c-Si surface. A decreased surface recombination velocity means higher carrier lifetimes and thus higher efficiency cells.

slurms
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For recombination to happen, the presence of electron, hole and sometimes medium to support it (defects, phonons,...) is required. Sometimes the surface will be either accumulated or inverted (due to i) fixed charges in the passivation layer ii) work function difference ...), where one type of charge carriers are comparatively very large. In such cases, the probability of finding the other carrier to recombine is very low and hence, recombination is very low - called as field-effect passivation. For eg.: In case of Al2O3 on p-type crystalline silicon [cSi(p)] creates lot of holes on the cSi(p) surface (accumulation) - as it is negatively charged dielectric over cSi(p). Since there are lot of holes on the cSi(p) surface, the electrons will be very low there and hence the recombination. Suppose if the number of electrons and holes are equal, then the recombination rate will be the maximum. In addition, when the medium which assist the recombination is reduced (say surface defects are reduced), recombination reduces - called as chemical passivation. Hope it helps.

ramuamt
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This is only in Thin Film Solar Panels I suppose. I don't know the exact reason for the formation of the film, but i do know how one gets rid of this. 90% of the manufacturers recommend to negatively ground the thin film panels which allows the negative charge to move to the ground thus not allowing the film to be formed.

Dharav
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