Polymers Stretch Efficiency of Displays

If you have a laptop computer, chances are that it has a liquid crystal display (LCD). And chances are that you have bemoaned the short battery life that results from these power-hungry displays. Now researchers report in tomorrow's issue of Nature that they have come up with a new type of LCD that incorporates specialized polymers to emit the same amount of colored light with far less energy.

LCDs create an image by sending light from a fluorescent panel through a series of polarization filters and liquid crystal layers. In a monochrome display, the first filter allows light with only one type of polarization to pass through. A second filter is oriented so that it transmits light only if it has a different polarization--with the result that the two filters ordinarily block all light. But a liquid crystalline material between the two filters responds to an electric voltage by rotating the polarization of the light, allowing it to get through and create a bright spot on the screen. Multicolor displays use basically the same technology but require a sandwich of polarizing filters and liquid crystal for the three primary colors, red, green, and blue, along with filters for selecting these colors. Only about 5% of the original light makes it through this thick stack of filters.

Now Christoph Weder and his colleagues at the Swiss Federal Institute of Technology in Zürich have found a way to eliminate all the color filters and the first polarizer for each color, thereby greatly reducing the light losses. In February, the group reported in Science that they had developed a film containing a light-emitting polymer that directly produces green polarized light when illuminated by an ultraviolet backlight. But this film also only absorbed a fraction of the backlight; the rest was lost. In their new work, however, the researchers report that they've added special sensitizer molecules to the films. These molecules absorb the light that is missed by the polymer and transfer the "harvested" energy to the film, enhancing its luminescent emission.

The new work is an "important step forward" in the development of LCD displays, says Jos van Haaren of the Philips Research Laboratories in Eindhoven, the Netherlands. If it can be made to work in real world devices that shine in all three primary colors, it could boost the efficiency of LCD displays by 25% or more, allowing manufacturers either to brighten their displays or reduce their power consumption.

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