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Development of high-efficiency light-emitting nanocomposite materials composed of block copolymers and quantum dots

The Center for Advanced Materials Discovery towards 3D Display of the KAIST Institute for the Nanocentury (KINC) is dedicated to research into not only existing commercialized display technologies but also new display materials. The center's major research foci are on channel materials, light-emitting materials, and electrode materials, and its research aim is to improve the performance of each of these display materials. In particular, the center members are currently working on light-emitting materials for next-generation displays. Desirable light-emitting materials are those that emit pure light, such that all natural colors can be reproduced by displays.

Meanwhile, a special group of materials under study is called quantum dots, nano-sized semiconductor particles, which exhibit excellent color purity. For that reason, global display companies launched quantum dot-based TVs. However, given that quantum dots are not self-illuminating, requiring backlight to function, the technology is still in an immature state. This is because typical pure quantum dot films do not exhibit high light absorption or extraction efficiency, and the interaction between adjacent quantum dots lowers the luminance efficiency. In the present study, a new light-emitting material was developed by combining a polymer medium filled with air pockets, as in popcorn, with quantum dots. The developed material exhibited enhanced photoluminescent characteristics so that the photoluminescence intensity could be improved throughout the entire wavelength range of visible light. This technology is expected to be applicable not only to quantum dot applications but also to various light-emitting materials.

Development of new light-emitting material composed of quantum dots combined with polymer media

Conventional quantum dot films have disadvantages in that they become less efficient due to mutual interference when they are close to one another, and the absorption efficiency is reduced when these materials are processed into thin films. To address the problem of light absorption and extraction efficiency reduction in a multifaceted manner, the research team attempted to coat solutions contraining block copolymers and quantum dots under humidity-controlled conditions so that a phase separation was achieved between the polymer and water particles with evaporatiom of moisture. This led to the successful development of a porous polymer medium filled with air pockets inside the film, which resembled popcorn.

The developed porous structure increased the scattering of light irradiated on it, thereby resulting in an increase in light path length and thus enhanced light absorption efficiency. Also, increased surface roughness suppressed the occurrence of internal total reflection of light while effectively enhancing light extraction efficiency. As a result, it was confirmed that the photoluminescence performance increased by up to 21 times when compared to that of pure quantum dot films. This phenomenon was attributed to the fact that, with the application of the porous polymer medium, its interaction with light was maximized, and thus the light absorption and light extraction of the quantum dot composite materials were improved by four and five times, respectively.

The corresponding technology was then applied to existing LED devices. As a result, the light-emission intensity increased more than seven times, and the durability improved by 45% or higher when compared to that of pure quantum dots. The significance of the present study lies in the fact that it was possible to increase the photoluminescence intensity while at the same time improving the light extraction intensity with respect to the light absorption efficiency. The developed composite material medium serves to enhance the photoluminescence intensity throughout the entire wavelength range of visible light, and thus will be applied not only to quantum dots but also to various light-emitting materials. Also, the application of this technology will make it possible to achieve excellent photoluminescent performance while using smaller amounts of costly light-emitting materials. Therefore, the technology is expected to contribute to improving the cost competitiveness of next-generation display products going forward.

Prof. Choi, Sung-Yool
Prof. Jang, Min Seok
Prof. Jung, Yeon Sik
2019 KI Annual Report

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