As only blue or UV light passes through the liquid crystal layer, it can be made thinner, resulting in faster pixel response times. Nanosys made presentations of their photo-emissive color filter technology during ; commercial products are expected by , though in-cell polarizer remains a major challenge. Rather than requiring a separate LED backlight for illumination and TFT LCD to control the brightness of color primaries, these QLED displays would natively control the light emitted by individual color subpixels,  greatly reducing pixel response times by eliminating the liquid crystal layer.
The major difference is that the light emitting devices are quantum dots, such as cadmium selenide CdSe nanocrystals. A layer of quantum dots is sandwiched between layers of electron-transporting and hole-transporting organic materials.
An applied electric field causes electrons and holes to move into the quantum dot layer, where they are captured in the quantum dot and recombine, emitting photons. Mass production of active-matrix QLED displays using ink-jet printing is expected to begin in Unlike simple atomic structures, a quantum dot structure has the unusual property that energy levels are strongly dependent on the structure's size.
The physical reason for QD coloration is the quantum confinement effect and is directly related to their energy levels. The bandgap energy that determines the energy and hence color of the fluorescent light is inversely proportional to the square of the size of quantum dot.
Larger QDs have more energy levels that are more closely spaced, allowing the QD to emit or absorb photons of lower energy redder color. In other words, the emitted photon energy increases as the dot size decreases, because greater energy is required to confine the semiconductor excitation to a smaller volume. Newer quantum dot structures employ indium instead of cadmium , as the latter is not exempted for use in lighting by the European Commission RoHS directive.
Moreover, QD-LED offer high color purity and durability combined with the efficiency, flexibility, and low processing cost of comparable organic light-emitting devices. The emission wavelengths have been continuously extended to UV and NIR range by tailoring the chemical composition of the QDs and device structure. Quantum dots are solution processable and suitable for wet processing techniques. Phase separation is suitable for forming large-area ordered QD monolayers. This process simultaneously yields QD monolayers self-assembled into hexagonally close-packed arrays and places this monolayer on top of a co-deposited contact.
During solvent drying, the QDs phase separate from the organic under-layer material TPD and rise towards the film's surface. The resulting QD structure is affected by many parameters: solution concentration, solvent ration, QD size distribution and QD aspect ratio.
Also important is QD solution and organic solvent purity.
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Although phase separation is relatively simple, it is not suitable for display device applications. Moreover, it is not ideal to have an organic under-layer material for a QD-LED; an organic under-layer must be homogeneous, a constraint which limits the number of applicable device designs. The contact printing process for forming QD thin films is a solvent-free water-based suspension method, which is simple and cost efficient with high throughput.
During the process, the device structure is not exposed to solvents. Since charge transport layers in QD-LED structures are solvent-sensitive organic thin films, avoiding solvent during the process is a major benefit. This method can produce RGB patterned electroluminescent structures with ppi pixels-per-inch resolution. The array of quantum dots is manufactured by self-assembly in a process known as spin casting : a solution of quantum dots in an organic material is poured onto a substrate, which is then set spinning to spread the solution evenly.
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Contact printing methods also minimize the amount of QD required, reducing costs. QDs are dispersable in both aqueous and non-aqueous solvents, which provides for printable and flexible displays of all sizes, including large area TVs. QDs can be inorganic, offering the potential for improved lifetimes compared to OLED however, since many parts of QD-LED are often made of organic materials, further development is required to improve the functional lifetime.
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Other advantages include better saturated green colors, manufacturability on polymers, thinner display and the use of the same material to generate different colors. One disadvantage is that blue quantum dots require highly precise timing control during the reaction, because blue quantum dots are just slightly above the minimum size. Since sunlight contains roughly equal luminosities of red, green and blue across the entire spectrum, a display also needs to produce roughly equal luminosities of red, green and blue to achieve pure white as defined by CIE Standard Illuminant D From Wikipedia, the free encyclopedia.
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