Message board poster "rupert.rawnsley" pointed out that you can get solar batteries, standalone chargers, and solar cells actually integrated into the phone. "I like the integrated battery option because you could have two and leave one sitting in the window to charge," Rawnsley writes.
I think I'd personally prefer a standalone charger like this because you can continue you to use it if you get a new handset.
Either way, none of these options are very affordable. The batteries start around $50 for common handset models, and you can quickly spend more if you want fancy. The solar panels also take four hours or more to charge up a phone. Still, if you use a phone as much as I do, a solar cell that works without power in most parts of the world could be a good investment.
— Dan Jones, Site Editor, Unstrung