Panels are almost exclusively sand by weight. When panels are at the end of their life we either recycle them into new panels or simply store them somewhere. They're not particularly difficult to store safely. Right now there is not much recycling going on because there simply isn't the volume of old panels available to make this interesting from an economic perspective.

To cover nights in the Sahara we use some of the absurd excess energy we get from covering the whole desert to make some Hydrogen or Methane and convert that back to electricity. If we don't want PV and gas synthesis for some reason to we can also just store heat in molten salt in solar-thermal installations and drive steam turbines. That's just a bit more expensive than PV today.