Two, if the wing were made of a flexible substance it could actually be coaxed into changing its curvature over a truly curved surface. Assume that this flexible surface tends to spring back to a flat shape. Then, in order to create a curvature, simply bend it some by pulling it together at the ends with a cord or cable. Seems simple enough to me. Actually, solar robotics of any kind is quite fascinating to me.
Sunlight is absolutely totally free.
In fact, it's going to waste all around us. Why not simply scoop it up since it's hitting our robot anyway and use it for something. Solar power on a ground-based robot is a serious problem because Earth is far enough away from the sun that the amount of energy that hits a given area at Earth's distance is pretty small. As a result, a robot would have to lug a round some pretty ungainly solar panels. However, flying robots already have tons of surface area, their wings.