What are quantum dots and how are they different from LEDs in the way that they emit light?
In the last column, we took a brief look at OSRAM’s new Osconiq S 3030 QD LED package that incorporates quantum dots into LED package phosphor to achieve good color rendering without sacrificing efficacy. But what are quantum dots? And how are they different from LEDs in the way that they emit light?
Quantum dots are semiconductor particles with typical diameter of 2–10 nm. They are so named because, due to their nanoscale size, quantum effects play a significant part in their light emitting properties. Quantum dots emit light via this mechanism: under external stimulus, some of the electrons of the dot material absorb sufficient energy to escape their atomic orbit. This creates a conductance region in which the electrons can move through the material, effectively conducting electricity. As these electrons drop back into atomic orbit, energy is released in the form of light, the color of which depends on the amount of energy released.
Because of the dots’ small size, the amount of energy released is relatively consistent from electron to electron, yielding emissions of a single color. The color is entirely dependent upon the size of the dot, with larger dots (e.g., 5-6 nm) providing lower energy emissions (i.e., reds and oranges) and smaller dots (e.g., 2-3 nm) providing higher energy emissions (i.e., blues and violets). This property is sometimes referred to as “quantum confinement,” indicating that constraints at the atomic level are predominant.
Because the light emissions are monochromatic, quantum dots have numerous existing and potential applications, including solar cells, medical imaging, and even quantum computing. But they are probably best recognized as the “Q” in QLED TVs. In this application, red, green, and blue dots are arranged in layers and protected against environmental degradation by a film. The quantum dots are stimulated by light from a blue LED backlight to emit monochromatic red, blue, and green, which are combined to achieve the desired color. This method has the advantage of reducing the cross-talk, or overlap, of green and blue, as well as reducing the light absorption by color filters, resulting in an improved color gamut.
In contrast, as current is applied, semiconductor LEDs emit photons in the region between the n-type and p-type layers as electrons and holes recombine. Variation in the initial energy levels of the emitted photons combined with losses incurred as the photons make their way out of the semiconductor material results in a larger spread in the spectrum of the emitted light compared to quantum dots.
Most products for general lighting applications create white light by passing blue light from an LED source through phosphor materials to create a combination of blue and yellow, orange, or red wavelengths that the human eye perceives as white. Cooler white light is created using yellow phosphors, while warmer white is created using orange and/or red phosphors. These redder phosphors have the disadvantage of emitting some energy in longer wavelength far-red which is not visible to the human eye, resulting in diminished efficacy (lumens/watt) of the LED package.
The addition of red quantum dots to yellow phosphor circumvents this shortcoming, allowing for more red spectral content without sacrificing efficacy. Up until recently, however, the use of quantum dots was not viable due to their instability in the local LED package environment. With the introduction of its Osconiq S 3030 QD LED package, OSRAM shows that it has found a solution to this problem, through the encapsulation of the quantum dot material. Each “dot” embedded in the LED package phosphor is actually made up of many smaller particles, forming a red-emitting core surrounded by a protective layer. Using this new encapsulation technology, the Osconiq S 3030 QD package efficacy is roughly 20% better than comparable packages.
Note that there is still some wasted energy in the form of far-red wavelengths in the Osconiq S 3030 QD output spectrum. This could be rectified by changing the “recipe” such that more quantum dots are used. However, because the dots are made with the heavy metal cadmium, which is regulated by restriction of hazardous materials (RoHS) guidelines, the volume of quantum dots that can be added to package phosphor is limited. Research into alternate materials not restricted by RoHS is on-going.