Q:

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. 

A:

I assume you're talking about thin-film crystalline silicon solar cells.
Surface passivation can be achieved in several ways:

reducing the recombination at the interface (chemical passivation), and
electrostatically shielding the charge carriers from the interface by an internal 
electric field (field-effect passivation), or
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.

