Solar panels are obviously not practical in mobile devices, for many reasons. The biggest and most obvious reason is that solar panels must sit in the sun to be of any use. That is the exact opposite of a good idea if that device happens to be a computer (or little computer, AKA mobile device). Computers can overheat. You better believe using your phone in the sun will cause it to overheat. Not to mention it'll give you quite the sun burn.
No, it's not going to work. Normal people wouldn't even go that far, and would probably just keep their phone in their pockets/purses when not using it, negating any benefit from having the solar panel.
It CAN work. Rig a baseball cap or sombrero with mini solar panels, a wire that connects to your iphone
and go for an hour jog in the sun. Voila, phone charged. The phone's form factor stays thin, and so does yours.