Giving off a comfortable glow, candles set the ambiance for a special dinner or just a quiet evening at home. However, some lighting alter­natives, such as electronic candles, give off unwanted blue wavelengths that interfere with the body’s circadian rhythm. Now, researchers from the National Tsing Hua University have fabricated an improved bendable organic LED that releases candle­light-like light for flexible lighting and smart displays that people can comfortably use at night.

Previously, Jwo-Huei Jou and other researchers developed OLEDs that released warm-white light, similar to that produced by candles. However, the devices still emitted some blue wavelength light, which can inter­fere with sleep because it dampens the body’s production of melatonin. These devices were made of solid materials and weren’t flexible. One option for making them bendable is to use a plastic backing, as has been done for other OLEDs. But plastics don’t stand 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 OLED 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 OLED. 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 percent, whereas light from a cold-white compact fluorescent lamp would suppress melatonin pro­duction by 29 percent. The researchers say that the flexi­bility of their candle­light-like OLED opens up the design oppor­tunities for blue-light-free nighttime devices. (Source: ACS)

Reference: T.-H. Chen et al.: Flexible Candlelight Organic LED on Mica, ACS Appl. Electron. Mater. 4, 2298 (2022); DOI: 10.1021/acsaelm.2c00123

Link: Dept. of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan, ROC