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Sustainable Energy

Energy-Harvesting Displays

Adding solar cells to screens could prolong the battery life of many electronic gadgets.

Adding solar cells to liquid-crystal displays could help recover a significant amount of energy that’s ordinarily wasted in powering them. Two research groups have created light filters that double as photovoltaic cells, a trick that could boost the battery life of phones and laptops.

Polarizing power: This film, which could be used in an LCD, lets through light of one polarization and can convert into electricity some of the rest.

Over 90 percent of the displays sold this year will use liquid-crystal display (LCD) technology. LCDs are, however, tremendously inefficient, converting only about 5 percent of the light produced by a backlight into a viewable image. The LCD in a notebook computer consumes one-third of its power.

This type of screen remains dominant because manufacturers can make LCDs inexpensively on a huge scale. More energy-efficient kinds of displays either are too expensive to manufacture or cannot produce high-quality images. “The LCD is very inefficient, but it works,” says Jennifer Colegrove, an analyst at Display Search, a market-research and consulting firm.

Two independent groups—one at the University of California, Los Angeles, the other at the University of Michigan—are tackling two of the biggest culprits of wasted light in LCDs: polarizers and color filters.

Polarizers filter out light that is incompatible with the liquid-crystal shutters in an LCD pixel, accounting for 75 percent of the total light wasted by LCD screens, and conventional color filters toss out two-thirds of the light that hits them. The two research groups have created plastic photovoltaic versions of these two display components, which convert light into electricity.

“We want to take an energy-wasting component that everybody uses and turn it into an energy-saving one,” says Yang Yang, professor of materials science and engineering at UCLA. Yang’s group created plastic solar cells that can act as polarizers. The researchers simply rub one layer in the solar-cell film with a cloth to align all the molecules in one direction. This alignment turns the cell into a polarizer that converts into electricity some of the light that doesn’t pass through.

Yang’s work is part of a three-year project being funded by Intel; in the coming year, his team plans to integrate the photovoltaic polarizer into a working display. In a paper published online in the journal Advanced Materials, the team reports that its polarizer can convert into electricity 3 or 4 percent of the light that’s normally wasted by a filter. Yang expects to get this up to about 10 percent by tinkering with the materials used.

The photovoltaic polarizers can harvest ambient light too, so they could potentially help charge a phone when it’s not in use. “When the phone sits, it could work in the background, collecting energy and recycling it back to the battery,” says Youssry Botros, program director at the Intel Labs Academic Research Office.

The second group, led by Jay Guo of the University of Michigan, is developing energy-harvesting color filters. Color filters are used in many types of displays, but the ones made by Guo’s team are appropriate for use in reflective “electronic paper” screens. These contain arrays of sub-pixels that absorb ambient light and then reflect red, green, or blue light.

Guo and colleagues combined a common polymer solar-cell material with a kind of color filter that his group invented last year. The photovoltaic color filter converts into electricity about two percent of the light that would otherwise be wasted.

Guo estimates that full displays incorporating this photovoltaic filter could generate tens of milliwatts of power, enough to make a difference to the life of a cell phone battery. The photovoltaic color filter is described in a paper published online in the journal ACS Nano.

“It’s an intriguing idea,” says Gary Gibson, a scientist developing reflective color displays at HP Labs in Palo Alto, California. Low brightness is a recurring problem for color electronic paper. If the color filter proves practical, says Gibson, energy harvested from ambient light could be used to power a backlight and make the display brighter.

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