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Quantum dots: a new era for liquid crystal display backlight

Liquid crystal displays (LCDs) have become both ubiquitous and indispensable in our daily lives. They have, however, begun to face strong competition from organic LEDs (OLEDs). Both of these technologies present advantages and disadvantages. LCDs are low cost, feature low power consumption, and have a high resolution density. On the other hand, OLEDs achieve superior response times, vivid colors, flexibility, a high contrast ratio (in dark ambient), and allow a thinner profile. Blue phase LCD is currently being developed with the aim of reducing response times,1 but color saturation must also be improved while maintaining high optical efficiency. Great effort has been made toward improving LCD color performance. Presently, the majority of LCDs use single-chip white LED as backlight, comprising a blue LED with yellow phosphor. Due to the broad emission band of yellow phosphor, its color gamut is 75% of the AdobeRGB (red, green, blue) standard. Narrow-band green/red phosphor materials are under development, but their optical efficiency remains relatively low. Although discrete RGB LEDs can significantly extend the color gamut, they require complicated driving circuits.2 Quantum dot (QD) technology is emerging as a novel backlight source that could help to increase the color performance of LCDs. QDs are composed of nanoparticles with diameters of 5–20nm. When charge carriers (electrons and holes) are confined to such small particles, the larger bandgap of the surrounding material creates a barrier. The discrete energy levels that arise as a result of this barrier consequently dictate the material properties. This effect (size quantization) gives QDs several attractive features, including high quantum efficiency, a broad absorption band, narrow emission linewidth, and a controllable emission peak.3 Figure 1 shows the emission properties of the QD samples that we used, which have a cadmium sulfide/selenide core and a zinc sulfide shell (CdSxSe1 x/ZnS: the ratio of CdS is a Figure 1. Emission spectrum of a quantum dot (QD) with a cadmium sulfide/selenide core and a zinc sulfide shell (CdSxSe1 x/ZnS: the ratio of CdS is x, while the ratio of CdSe is 1–x). Varying the composition or particle size alters the bandgap of the core, enabling a tunable emission wavelength. Inset shows the emission color when QDs are excited by a 459nm blue LED.