Tiny crystals of semiconducting compounds just a few nanometers across can emit light with high color purity when excited by another source of light. The emission arises through nanoscale quantum effects, and the ‘quantum dot’ (QD) technology has the potential to make a major impact in consumer electronics. Getting the emission color right for specific applications such as televisions, however, remains a challenge. Eunjoo Jang and colleagues at the Samsung Advanced Institute of Technology in Korea1 have now constructed QD-based light-emitting diodes (LEDs) that emit the highly saturated white light needed for applications such as liquid crystal display (LCD) panels.

Conventional LEDs are already used in LCDs, but because they emit light in very narrow wavelength bands, a combination of different-colored LEDs or the addition of phosphors is required to achieve broad-band white emission. For example, red, blue and green LEDs combined into a single structure produce white light. Green LEDs, however, are still quite inefficient, and powering three separate elements makes for an expensive device.

Fig. 1: Saturate white-light emission from a quantum-dot-based LED

Jang and her colleagues took an alternative approach that involved embedding multilayered quantum dots in a single indium gallium nitride (InGaN) LED. This type of LED emits a pure blue light, so the researchers embedded quantum dots that converted some of the blue light into red and green (Fig. 1). After coming up with the right combination of semiconductor layers for the two types of quantum dots, almost every blue photon absorbed by a quantum dot created either a red or green photon. “These quantum dots show higher efficiency, better reliability and better color quality compared with conventional phosphors,” explains Jang.

The multilayer structure of the quantum dots also had a second beneficial effect. Quantum dots generally deteriorate during fabrication and through use, severely limiting their working lifetime. The extra layers in the researchers’ new structures seem to prevent this degradation: the white LEDs were operated for up to 2,200 hours without any change in output color.

Jang and her colleagues proved the usefulness of their white-light source (Fig. 1) by integrating it into a 46-inch LCD television. “The color reproducibility of the QD-LED TV panel was a 100% match to the standards of the National Television System Committee,” says Jang.