The latest solar powered cell phone was just announced at Mobile World Congress in Spain, the Umeox Apollo. The phone runs on Android, and would take about 2.5 hours of sunlight for refilling the battery after daily use. This is great news if you want to go camping and not be without your smartphone. The phone is headed for production, but there is no word on when it will actually be available in the US.
The phone could also be great for travelling. But for daily use, leaving your phone out in the sun for 2.5 hours at a time probably isn't realistic. More likely, the phone would charge for 10-15 minutes throughout the day in between uses. In the event the phone goes completely dead, it would take up to 17 hours in the sun to charge it back.