Toshiba Corp exhibited a technology that can significantly enhance light extraction efficiency of OLED TVs and lighting equipment at the International Nanotechnology Exhibition & Conference (nano tech 2009).
The new technology is designed to form patterns called "nano-grating," which have a pitch of several hundred nanometers, on a SiO2 substrate.
OLED lighting has a problem of low light extraction efficiency, which is about 25%, unless special measures are taken. This time, Toshiba used the nano-grating for OLED devices and succeeded in improving the light extraction efficiency by 1.6 times compared with a case where no measures are taken.
It has been known that a diffraction grating has an effect to extract light with a wavelength equivalent to the grating space. However, there have been few techniques to accurately produce a diffraction grating with a wavelength spacing corresponding to visible light.
Toshiba has been making efforts to enhance the light extraction efficiency of OLED devices by producing such a diffraction grating with the use of a technology called "self-assembly nanoengineering."
Specifically, a resist is applied onto a SiO2 substrate, and a thin thermoplastic resin is placed on it. Then, fine particles of SiO2, which have a small diameter of several hundred nanometers, are dissolved in a dispersion liquid and deposited on the resin. In this instance, the particles are aligned in an orderly manner at a high density by interaction among themselves, without being controlled. Toshiba calls this phenomenon "self-assembly."
After removing the dispersion liquid, the resin is heated and softened so that the bottom layer of the particles sinks into the resin and is fixed there. The fixed particles act as a kind of mask in the subsequent etching process.
As a result, a grating aligned in an orderly fashion is formed on the SiO2 substrate with a pitch almost equal to the size of the particles. This time, Toshiba formed a diffraction grating with a grating space of 700nm and a grating groove depth of 440nm.
Toshiba created the diffraction grating on a 4-inch SiO2 wafer and used it for a 2mm-square monochrome OLED device. As a result, the total flux was increased by 60% compared to when no grating is used. As far as the emission peak wavelength is concerned, the light emission intensity was virtually doubled, according to the company.
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