Researchers Create Brighter Flexible Electroluminescent Film

A research team headed by Dr. Byeongdae Choi at DGIST’s Intelligent Devices and Systems Research Group has created an electroluminescent film that is four times brighter than the present one.

They have developed a technology which can increase the luminesence of electroluminescent devices by 422% compared to current ones by applying retro-reflection electrodes that use the principle of nocturnal animal eyes.

This is a structure and characteristics of electroluminescent device using recursive reflection structure. Structures (a~e) and viewing angle characteristics (f) of an electroluminescent device using a retroreflective structure. It exhibits a wide light reflection characteristic (Reflected EL) of -60 ° and 60 ° when viewing angle characteristics are evaluate. Credit: Daegu Gyeongbuk Institute of Science and Technology (DGIST)

Electroluminescent (EL) refers to an optical and electrical occurrence in which a material discharges light in reaction to the passage of an electric current. EL films using the phosphor powder have benefits such as exceptional durability in a deformed state because of elasticity and flexibility, and high efficiency regardless of low cost. However, it was hard to put into feasible use because of their low brightness.

So as to raise the brightness of EL devices, the research team studied the eyes of nocturnal animals with high utilization efficiency of light. The researchers used the retro-reflection attributes that the light returns to the light source without being spread in the retroreflective structure of the nocturnal animal eye while it is distributed in the normal reflection structure.

The researchers made an EL film coated with a light-transmitting luminescent film on a retroreflective electrode and have found that a light source made up of phosphor particles of several micrometers (μm) in size can have a broad viewing angle of reflected light on the prismatic retroreflective electrode surface.

Furthermore, the team has found out that when the light source has a transmittance higher than a specific value, there is no loss of reflected light and it enables it to make a high luminance electroluminescent light source by manipulating the concentration of luminescent particles. The team also created a film that enhanced brightness by 442% (1017 cd/㎡, 6.67 V/μm at 10 kHz) compared to conventional technology by mixing luminescent particle-polymer binder complexes at specific ratios.

In contrast to conventional EL lights, that are made of plastic or glass and used for lighting or light source for advertisement, very bright EL film is bendable and flexible. Furthermore, it is anticipated to replace the current EL lighting as it is able to be created at room temperature.

This study is significant as it has applied the light reflection principle of nocturnal animal eyes, which have high light utilization efficiency, to light emitting devices. Since this technology can also be applied to self-luminous displays, it is expected to contribute to strengthening Korea's competitiveness in the global lighting market, which is estimated to reach more than 120 trillion won per year in the future, as well as the next generation display market.

Dr. Byeongdae Choi, DGIST’s Intelligent Devices and Systems Research Group

The research paper was published as the cover paper of the September edition of Advanced Materials Technologies, a sister journal of Advanced Materials, a well-known global journal in the field of materials engineering.

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