reason for degradation of photovoltaics
Degradation of PV cells has been studied for some time. The dominant factor in such degradation seems to be UV exposure. This seems to occur on two fronts.

1) Encapsulant. The PV cells themselves are contained in a layer of encapsulant, usually EVA (ethyl vinyl acetate), which undergoes slow changes when exposed to UV. First, it can change from transparent to brown, cutting down on light reaching the cells. Second, it can release acetic acid, which corrodes connections. Third, it becomes less elastic and lets water vapor into the cells, which also causes corrosion.

2) Cell structure - PV cells are actually great big diodes, and depend on a fairly precise profile of dopants in the silicon. UV seems to have enough energy to cause slow, small diffusion of the dopants away from their desired concentration profiles, reducing the efficiency of the cells.

To a lesser extent, thermal cycling also causes fatigue in electrical joints, which behaves much like overt corrosion, and also works in tandem with the encapsulant changes listed above.

As to prolonging PV lifetime, protecting them from UV and temperature cycles seem the best measures. Since this means keeping them out of the sunlight, it's clear that this has limited usefulness for working systems.

The encapsulant problem can be reduced by using panels which employ silicone rather than EVA. Unfortunately, silicone is more expensive than EVA.