Solar powered flight is not easy/cheap. The first through the night solar flight was only pulled off this previous summer by Solar Impulse (it was manned). It's a -big- plane (width wise anyways). Its wings are as wide as a jumbo-jet's. You're not going to save much weight making it a UAV either, since I suspect most of the load is actually batteries.

On the side, it's -friggin- cool. Here's the Solar Impulse site: http://www.solarimpulse.com/index.php