Flexible display cools surfaces, offering energy-efficient alternative to traditional signs

Electronic signs are all around us, giving directions or advertising the latest gadget. In ACS Energy Letters, researchers report that they’ve developed a dynamic display technology that dissipates heat instead of generating it when the color changes, cooling the surface underneath. They also showed the display could be attached to flexible backings and wrapped over skin. The passive cooling mechanism could usher in the next generation of sustainable, flexible outdoor signs and smart devices.

“The technology could be used on building billboards to show ads and also help passively cool a building’s interior,” say Boxiang Wang, Ming Xiao and Shenghao Jin, co-authors of the study.

As climate change boosts temperatures around the globe, the demand for cooling devices—especially those that use little energy—is growing. Researchers have developed materials that stay cool in the sun using passive daytime radiative cooling. But so far, these cooling materials are usually white or silver and can’t switch colors.

A solution could be materials that change color when voltage is applied, producing programmable images. But current versions absorb light uncontrollably when colors switch, which ends up heating the device. So, Wang, Xiao, Jin and colleagues wanted to develop a way to control which wavelengths of light are absorbed.

The team built a proof-of-concept device with a multilayered electrode and an indium tin oxide glass counter electrode with a silver-containing electrolyte solution in between. Depending on the applied voltage, the display appears white or colored. When it’s white, the top layer reflects light and cools the device.

When the voltage is adjusted to display a color, silver from the electrolyte solution deposits onto the bottom glass electrode and only limited wavelengths of light are absorbed. This happens without interfering with the reflected light or generating heat, and it is reversible.

In tests, the display device reduced the temperature of the substrate underneath by 5 to 10 degrees Fahrenheit (3 to 5 degrees Celsius) compared to surrounding air while changing color, a function that is not present in conventional passive cooling materials.

The device outperformed conventional display devices in the summer heat, cooling the surface underneath by 11 to 23 F (6 to 13 C) more than the other devices while displaying vibrant colors like magenta and using less energy.

The researchers also created a pixelated electrode with separate holes for the electrolyte solution. Each independent pixel’s color could be controlled by applying different voltages, forming letters. The letters were visible in indoor lighting and sunlight.

Finally, the device was attached to flexible plastic backings and showed its versatility by wrapping over a person’s arm. The researchers say that the technology could someday be used in smart buildings and vehicles, outdoor displays, and wearable devices.