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.