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Domain-4

Quantum Dot Solid-State Lighting

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The white LED market is a key market that is expected to revolutionize the lighting industry through improvements in lamp lifespan and efficiency. Color rendering and efficiency are two important criteria for illuminants for conventional general lighting. The ability of an illuminant to illuminate an object in its true tones is expressed by a color-rendering index. As an example, the sodium vapor lamps used in streetlights have poor color rendering, as evidenced by the difficulty in distinguishing between red and yellow cars under the lighting.

 

Currently, white LED technology adopts method that uses a blue (450nm) LED chip to excite cerium-doped YAG:Ce (yttrium aluminum garnet) phosphors for down-conversion. White light is produced through the mixing of blue light from the LED with the wide-spectrum yellow light generated from the YAG phosphors. Unfortunately, this white light exhibits a degree of bluish cast and is often considered a “cold” or “cool” white. As quantum dots exhibit a wide excitation spectrum with high quantum efficiency, they can be used as phosphors for LED down-conversion. In addition, the emission wavelength can be fully tuned across the visible range simply by changing the dot size and the type of semiconductor material. For this reason, quantum dots hold the potential to create essentially any color, particularly the warm white that is eagerly sought by the lighting industry. Moreover, it is possible in theory to obtain white light with a different color-rendering index by combining three types of dots with emission wavelengths corresponding to green, yellow, and red. These compelling properties are attracting commercial and academic attention to quantum dot LEDs.

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Quantum dot LEDs hold potential beyond white-light applications for general lighting. As an example, green LEDs are not very efficient, but the use of green-emitting QDs with efficient blue LED chips may be able to solve this problem. Similarly, amber LEDs exhibit a temperature dependence that could be improved through the application of quantum dots. In addition, as quantum dot light emission is tunable over a wide range, it is possible to fabricate near-ultraviolet excitation quantum dot LEDs through a combination of quantum dots capable of emitting effectively any color on the chromaticity diagram. This could become important in applications such as the replacement of neon lights in advertising billboards.