As I heard, the life expectancy of photovoltaic cells is normally some decades, as so. This is very long, regarding their high cost, it has significant (negative) effect on the total costs.
Why do they age? I can see the only possibility which could damage their atomic structure, and this is the very high energetic ultraviolet (or even soft röntgen) photon spectrum of the sun (which is only a small part of its power).
And, I think, these photons could be maybe easily filtered out by a transparent plastic layer over the silicon.
Thus, why do the solar cells age, and what happens in them on the atomic level?