To extend the time between charges, Nathan’s group built a prototype device that converts ambient light into electricity using an array of solar cells made of thin-film hydrogenated amorphous silicon that’s designed to sit within the phone’s screen. The photovoltaic (PV) cell takes advantage of the smartphone displayâ€™s large footprint. In a typical organic light-emitting diode (OLED) display, only about 36 percent of the light generated is projected out of the front of the screen, says Ahnood. Much of it escapes at the edges of the OLED, where it is useless. So Nathan and his collaborators at his Canadian firm IGNIS Innovation set out to harness this wasted light by putting thin-film PV cells around the displayâ€™s edges as well.