Do you own a smartphone? Ever notice how warm they can get? What if you could harness some of that wasted energy and put it back into the phone’s battery? A group of scientists from Massachusetts Institute of Technology in the US have done just that.

Liquid crystal displays (LCD) are used in a large number of electronic devices, including iPads and smartphones. To improve the contrast of the screen and remove reflection from sunlight and room lights, for example, LCDs are designed to absorb rather than reflect light. Typically, this absorbed ambient light, as well as backlight, is wasted as heat. Amador Menéndez-Velázquez and colleagues have designed a device that guides this absorbed light to the frame of the display where it is harvested by solar cells.

‘We noticed that the display surfaces of electronic devices are designed to be as black as possible, like solar cells,’ comments Menéndez-Velázquez. ‘We realised that we could exploit this to harvest energy without any loss in the display’s performance. For indoor applications, we estimate that the energy harvested could noticeably extend the battery life of portable devices. Sunlight is much stronger, and outdoors, devices could be powered only by light absorbed in their displays.’

The linear polarised luminescent solar concentrator (LSC) used in this device harvested the energy and transmitted it to the solar cells in the frame. Unlike other LSCs, the emission occurs at the near infrared, which means there is no loss in image quality. The polariser was made with dye-doped liquid crystals. But instead of purely absorptive dyes found in conventional polarisers, they use a combination of four different fluorescent dichroic molecules. The solar cells could generate an excess of 10µW cm^-2 indoors, and up to 1–10mW cm^-2 outdoors in the sun.

‘Battery life in mobile devices has become a market differentiator, and the team has demonstrated a creative approach to recapturing otherwise wasted light and recycling some of its energy back to the battery,’ comments Seth Darling, who works on solar energy conversion at the Centre for Nanoscale Materials at Argonne National Laboratory, US. ‘This process necessarily comes at the expense of a wavelength shift all the way into the near-IR and IR, but thereby overcomes the challenge of unwanted emission from the display.’

Work on improving the polarisation selectivity of the device to reach industry standards, as well as better packaging to prevent the loss of harvested light is still needed. The next step is to create a prototype with specifically designed materials but we appear to be one step closer to a phone that can power itself.