Charging

[girolez]

Why isn't that large glass back on modern phones made up of solar panels which charge up the phone?

 

[mikedt]

One issue, unless you leave them outside in direct sunlight, solar panels just don't work very well, especially with indoor fluorescent or LED lighting.

 

[Fox Mulder]

Mike's right, a built-in solar panel on the phone would be virtually useless. They are inefficient and you need a large surface area to generate a useful amount of current.
The only application I could see for this would be if you were stuck on a desert island with no other way to charge your phone. If you left it in the sun for a few days it would be able to charge the battery. Of course you wouldn't have any service out there so you still couldn't call for help, although you could at least pass the time by playing games.