A bendable organic LED with a natural mica backing releases a strong, candlelight-like glow. Credit: Andy Chen and Ambrose Chen

Giving off a soft comfortable glow, candles set the ambiance for a special dinner or just a peaceful evening at home. However, some lighting alternatives, such as electronic candles, give off unwanted blue wavelength light that interferes with the body’s circadian rhythm. Now, researchers reporting in the journal ACS Applied Electronic Materials have fabricated an improved flexible organic LED that releases candlelight-like light for bendable lighting and smart displays that people can comfortably use at night.

Previously, Jwo-Huei Jou and other researchers developed organic LEDs that emit warm-white light, similar to that produced by candles. However, the devices still emitted some blue wavelengths, which can interfere with sleep because it reduces the body’s production of melatonin.

These devices were made of rigid materials and weren’t bendable. One option for making them flexible is to use a plastic backing, as has been done for other organic LEDs. But plastics don’t hold up well to repeated bending. Another option for the backing is mica — a natural mineral with extreme temperature tolerance that can be split into bendable, transparent sheets. So, Jou, Ying-Hao Chu, and colleagues wanted to develop an even better organic LED and apply it to a mica backing, creating a bendable candle-like light with a long lifespan.

The researchers deposited a clear indium tin oxide film onto a transparent mica sheet as the LED’s anode, which could bend 50,000 times without breaking. Next, the team mixed the luminescent substance N,N’-dicarbazole-1,1’-biphenyl with red and yellow phosphorescent dyes to produce a light-emitting layer. This layer was then placed between electrically conductive solutions with the anode on one side and an aluminum layer on the other side, creating a flexible organic LED.

When a constant current was applied to the device, it produced a bright, warm light with even less blue wavelength emissions than natural candlelight. Calculations showed that exposure to the LED for 1.5 hours would suppress a person’s melatonin production by about 1.6%, whereas light from a cold-white compact fluorescent lamp would suppress melatonin production by 29%. The researchers say that the flexibility of their candlelight-like organic LED opens up the design opportunities for blue-light-free nighttime devices.

Reference: “Flexible Candlelight Organic LED on Mica” by Tun-Hao Chen, Hao-Jen Chen, Ching-Yu Weng, Feng-Rong Chen, De-Shiang Liou, Sun-Zen Chen, Ying-Hao Chu and Jwo-Huei Jou, 9 May 2022, ACS Applied Electronic Materials.DOI: 10.1021/acsaelm.2c00123

The authors acknowledge funding from the Ministry of Science and Technology (Taiwan, Republic of China).