Designing Displays that Recycle Light

LCD displays embedded with fluorescent dye molecules can convert blocked light into electricity

Kara Manke

A linearly polarized luminescent solar concentrator placed in front of an LCD image. The light is absorbed and waveguided to the edges when the dye molecules are oriented parallel to the polarization of the light (top) and it is transmitted when the dye molecules are oriented perpendicular to the polarization of the light (bottom).

Chances are, you are viewing this article on an electronic device that uses a liquid crystal display, commonly known as an LCD. If so, you might be surprised to hear that only about 8 percent of the light that is being generated by your device is actually reaching your eyes.

In an LCD, images are formed by taking a uniform white backlight and filtering it through a set of polarizers. In some regions, the polarizers absorb light, creating an area that appears dark (such as in the letters of this article) and in other regions, the polarizers allow light to pass, creating an area that appears bright.

The light that is absorbed by these polarizers is normally lost as heat. However, Amador Menendez-Velazquez and his colleagues at the Center for Excitonics Energy Frontier Research Center have developed a way to recycle some of this absorbed light, converting it into electricity for the device. This method would also harvest ambient light from the surroundings and, in essence, convert your iPhone or iPad into a miniature power generator.

“In our strategy, we replace the purely absorptive dyes by fluorescent dyes. In this way, it is possible to re-emit and recycle the filtered light, saving energy and extending the duration of the battery,” said Menendez-Velazquez, who is the lead author on a paper describing this technique. The re-emitted light would be guided to solar cells embedded in the frame of the device, where it would be converted into electricity.

Menendez-Velazquez’ s work on light recycling in displays grew out of his and his colleague Marc Baldo’s interest in a type of solar energy harvester known as a luminescent solar concentrator. A luminescent solar concentrator consists of a panel of a transparent material, such as glass or plastic, that has been embedded with dye molecules. These molecules absorb light from the sun and emit it at longer wavelengths. Because light travels much slower in the glass than it does in air, some of this emitted light becomes trapped within the panel, and is guided to the edges. By installing solar cells along the edges of the panel, this solar energy can be used to generate electricity.

In attempting to incorporate luminescent solar concentrators into electronic displays, Menendez-Velazquez and his colleagues faced one major challenge: not all of the light that is absorbed and emitted by the dye molecules is trapped within the panel. “Fluorescent molecules can emit visible light from the display, potentially corrupting the image,” said Menendez-Velazquez.

They solved this problem by shifting the fluorescence of the molecules outside the detectable range for human eyesight. “To overcome the problem of visible light emission we use a chain of molecules, a cascade. In this scheme, different molecules capture the visible light and sequentially transfer this energy to the terminal dye, which emits it in the near-infrared range,” said Menendez-Velazquez. Designing such a cascade was a difficult challenge, he said, as this requires that each dye molecule efficiently transfer its energy to the next molecule in the chain.

The four-molecule mixture that they devised recycles approximately 20 percent of the light that hits each of the two crossed polarizers in the panel. While they are still exploring ways of increasing the efficiency, this work represents a significant step in the development of light-harvesting displays.