It takes a considerably higher level of technology to harness photovoltaic energy than it does to produce electricity. The only reason solar energy is around today (and the reason it isn't more popular) is because we have the luxury of expending a lot of energy toward a lesser source of energy in the hopes that it will eventually be worthwhile. For a fantasy society, photovoltaic technology would represent an effort even modern Earth is not equipped to undertake, with minimal returns and a delicate technology which must be exposed to the harsh conditions of a D&D world to function.
I can't really see it happening. It's a neat idea though.