# How does field effect passivation work?

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.

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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

## 2 Answers

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.

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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.

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