Above pic is an artist's mockup

A researcher at London Centre for Nanotechnology, named Arman Ahnood, is working on a prototype phone that uses an OLED display combined with solar-cells. He teases the possibility of a self-sustaining smartphone in the future, as he works toward that lofty goal. Here's a quote from the Engadget article with some details on the budding tech,

Ahnood's handiwork relies on an array of solar cells that surround and sit beneath the phone's display. Currently, the system averages 11 percent efficiency in its energy capture, with a peak efficiency of 18 percent. Naturally, there are gains to be made. Of the light generated from OLEDs, Ahnood approximates that only 36 percent is actually projected outward. Critically speaking, this also suggests that similar power savings could be achieved with dimmer, more efficient displays, but we'll let that slide. As it stands, Ahnood's system is able to generate approximately five milliwatts of additional power, given a screen size of 3.7-inches.
Right now, the efficiency is far too low to come close to self-sustainability, but it's really cool to see that there are folks actively working to make it a reality. That's one of the great things about technology... it always has an optimistic view of the future as it marches inevitably towards it.

Source: Engadget