Trapping waste energy

Adding solar cells to liquid-crystal displays could help recover a significant amount of energy that's ordinarily wasted in powering them. For example take LCD in a computer. 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.

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. 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 other 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. They estimate 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!