A quick sanity check is that there aren't many solar-powered flying creatures.

A 15W solar panel is about three square feet, and probably weighs several pounds. A 15W quadcoptor is about two square feet and has a takeoff weight of less then three pounds.

You might be able to carry a small solar panel as a backup in case the copter runs out of power in a remote location, but it would have to sit for quite a while before it's ready to fly again.