A prototype LED as thin as wallpaper — that glows like the sun

Light bulbs come in many shapes and styles: globes, twists, flame-like candle tips and long tubes. But there aren’t many thin options. Now, researchers report in ACS Applied Materials & Interfaces that they have created a paper-thin LED that gives off a warm, sun-like glow. The LEDs could light up the next generation of phone and computer screens and other light sources while helping users avoid disruptions to their sleep patterns.

“This work demonstrates the feasibility of ultra-thin, large-area quantum dot LEDs that closely match the solar spectrum,” says Xianghua Wang, a corresponding author of the study. “These devices could enable next-generation eye-friendly displays, adaptive indoor lighting, and even wavelength-tunable sources for horticulture or well-being applications.”

People want indoor lighting that looks natural and creates a comfortable atmosphere. Previous researchers accomplished this with flexible LEDs containing red and yellow phosphorescent dyes that produced a candle-like glow. Alternatives to light-emitting dyes are quantum dots that convert electric energy into colored light. Other teams have used quantum dots to create white LEDs, but they have struggled to match the full spectrum of colors that comprise the sun’s white light, especially in the yellow and green wavelengths where it shines most strongly. So, Lei Chen and colleagues wanted to develop quantum dots that would mimic the desired natural glow when incorporated in a thin, white quantum dot LED (QLED). And in collaboration, a research group led by Wang suggested a strategy for slim electrically conductive materials that operates at modestly low voltage.

The researchers first synthesized red, yellow-green and blue quantum dots wrapped in zinc-sulfur shells and found the ratio of the three colors that produced an emission spectrum closest to sunlight. Then they built a QLED on top of an indium tin oxide glass substrate, depositing layers of electrically conductive polymers, the quantum dot mixture, metal oxide particles, and finally a layer of aluminum or silver. The quantum-dot layer was only tens of nanometers thick — significantly thinner than conventional color conversion layers — giving the final white QLED a thickness similar to that of wallpaper.

In initial tests, the thin QLED performed best under a 11.5-volt (V) power supply, giving off the maxmium bright, warm white light. The emitted light had more intensity in red wavelengths and less intensity in blue wavelengths, which is better for sleep and eye health, according to the researchers. Objects illuminated by the QLED should appear close to their true colors, scoring over 92% on the color rendering index.

In further experiments, the researchers made 26 white QLED devices, using the same quantum dots but different electrically conductive materials to optimize the operating voltage. These light sources required only 8 V to reach maximum light output, and about 80% exceeded the target brightness for computer monitors.

The authors acknowledge funding from the National Natural Science Foundation of China, the Natural Science Foundation of Anhui Province, and the Major Science and Technology Special Project of Zhongshan City.

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