What causes photovoltaic (solar) cells to degrade?
Why do they degrade? What exactly is reducing their efficiency?


This link outlines several modes of solar panel degradation, and this report by the National Renewable Energy lab is a very detailed review of studies on solar panel degradation worldwide.

To summarize:


Internal resistance of the cell can increase due to infiltration of contaminants (usually water vapor) when the encapsulating material cracks due to long-term UV exposure or temperature cycling.
The shunt resistance (resistance of the path to ground) can decrease when metal ions migrate (diffuse) through the cell. This happens naturally, with the rate depending on temperature and the type of cell. In general, higher temperatures lead to faster diffusion, and amorphous silicon has higher diffusion rates.
The anti-reflective coating can deteriorate. Anti-reflective coatings can be made of lots of different types of materials, but in general, heat, UV exposure, and exposure to contaminants are what cause them to degrade.


These are all degradation mechanisms of the cells themselves - any of the other components of the panel can also degrade. In all cases, the mechanisms are related to thermal stress or mechanical damage due to things like wind or hail, or corrosion from humidity.

Given all of the above, the main causes of solar panel degradation that can be (sort of) controlled by you are:


Temperature
UV exposure
Mechanical damage



  How can this process of degrading be slowed down? E.g. if you don’t need your PV panels, is it possible to conserve them by covering them with a black, opaque coverage like tarp? If not, what else?


Anything you can do to reduce these three factors would help. However, covering a solar panel with an opaque tarp would only reduce UV exposure. The temperature (and cyclical thermal stress) would likely increase.

Unfortunately, both 1 and 2 increase with exposure to sunlight - and exposure to sunlight is the whole point of solar panels. Mechanical damage can be reduced by installing panels in places and orientations that make it unlikely that they will be stressed by winds, or have things hitting them very often.

However, it looks like the only effective option would be to completely enclose the panels in a temperature-controlled low humidity environment when they are not in use. Whether or not the benefit of doing so is worth the cost will depend on the degradation rate and cost of the particular panels.