not a cheap solution...
example:
http://www.allelectronics.com/make-a...R-PANEL/1.html
you will need a minimum of two of these [~2.3amps /17v total -in full direct sunlight] for a small charger, able to charge a 2s lipo at around 4amps...
thus, the cost will be around 400$ [not including any regulating array that might be involved]...