[caldwell]

I understand that a passivation layer of aluminum oxide forms very quickly when pure aluminum is exposed to an oxygen atmosphere.  But since aluminum oxide is an insulator, then shouldn't aluminum NOT conduct electricity?  Yet it does. 

So here's why I ask:  aluminum is often used as a counter electrode in thin-film solar cells.  The standard way to deposit the aluminum counter electrode is by physical vapor deposition (e.g. thermal evaporation).  I presume it's done in this manner, instead of building a solar cell up from a sheet of aluminum foil, so that a passivation layer does not form which would otherwise interfere (by acting as an insulator) with the electrostatic potential between semiconductors.  But then I started wondering if this were actually true - especially since the passivation layer doesn't act so much like an insulator as to prevent an electric current. 

Would somebody please clear up my confusion?

Thank you!

[DevaDevil]

I understand that a passivation layer of aluminum oxide forms very quickly when pure aluminum is exposed to an oxygen atmosphere.  But since aluminum oxide is an insulator, then shouldn't aluminum NOT conduct electricity?  Yet it does. 

the oxide film is only on the outside, nit into the bulk. Therefore, if a contact to the aluminium metal is made by pushing a copper lead through the very thin oxide layer -just touching the copper to the aluminium foil would do that- the aluminium will conduct.

It is only the very thin oxide layer that does not conduct, the metal itself conducts just fine (very well in fact).

[caldwell]

Great answer!  Thank you!!