Do solar cells age? Why, and how?

Question

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?

Answer 1

The National Renewable Energy Lab (NREL) estimates something like 0.4%-1% degradation per year for solar panels. I don't know what the chemical action is here (the electrical result being an increase in series resistance of the PV cells), but the long-term degradation of the panels is correlated with UV exposure. Although damaging to the PV material, the UV rays do contain energy that we want to collect, so you won't be able to filter it out completely without defeating the purpose of the panel.

Other causes for degradation are mostly weather-related, but in the form of mechanical stresses to the system. Anything from wind to snowfall can damage the panels by bending the panels to produce minor cracks in the solar cells and embedded electrical connections. Other factors in a degraded PV panel include corrosion, or the lifetime of the supporting electrical devices needed to maintain a functioning system (these other devices may have shorter lifespans than the panel, and if they are expensive enough it would cause the system to be unsustainable financially compared to replacing the entire panel system).

Source: http://www.engineering.com/ElectronicsDesign/ElectronicsDesignArticles/ArticleID/7475/What-Is-the-Lifespan-of-a-Solar-Panel.aspx (See the Read More link for the NREL report)

Answer 2

Keep in mind that P-N junctions are created in the first place by diffusing impurities (dopants) into the silicon at elevated temperatures (1000 - 1500 K). The efficiency of the junction is related to how sharply the dopant concentration changes at the junction (its gradient).

At lower temperatures, such as those to which a solar array is exposed (say, 270 - 330 K), diffusion is much, much slower, but it does not completely stop. Given enough time, the dopant gradients become less steep, reducing the effectiveness of the P-N junction. Over the extreme long term (millennia), semiconductor devices will stop working as their junctions disappear altogether.

Answer 3

For a different angle, my experience with solar cells is space-based, rather than terrestrial. The biggest drivers for degradation of space based solar cells are particle radiation and orbital debris impacts. The particle radiation degrades by wearing down the junction, and the orbital debris degrades primarily by increasing the internal series resistance of the cells. Because there is no air to cool the cells, reverse bias heating can also be a serious problem if a solar array gets partially shadowed by other structure.