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Quantum dot display

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Colloidal quantum dots irradiated with a UV light. Different sized quantum dots emit different color light due to quantum confinement.

A quantum dot display is a display device that uses quantum dots (QD), semiconductor nanocrystals which can produce pure monochromatic red, green, and blue light.

Photo-emissive quantum dot particles are used in a QD layer which converts the backlight to emit pure basic colors; this improves display brightness and color gamut by reducing light losses and color crosstalk in RGB color filters. This technology is used in LED-backlit LCDs, though it is applicable to other display technologies which use color filters, such as wOLED.

Electro-emissive quantum dot displays are an experimental type of new display based on quantum-dot light-emitting diodes (QLED or QD-LED). These displays are similar to active-matrix organic light-emitting diode (AMOLED) and MicroLED displays, in that light would be produced directly in each pixel by applying electric current to inorganic nano-particles. QD-LED displays could support large, flexible displays and would not degrade as readily as OLEDs, making them good candidates for flat-panel TV screens, digital cameras, mobile phones and handheld game consoles.[1][2][3]

As of 2017, all commercial products, such as LCD TVs using quantum dots and branded as QLED, use photo-emissive particles. Electro-emissive QD-LED TVs exist in laboratories only.[4][5]

Working principle

The idea of using quantum dots as a light source emerged in the 1990s. Early applications included imaging using QD infrared photodetectors, light emitting diodes and single-color light emitting devices.[6] Starting from early 2000, scientists started to realize the potential of developing quantum dot for light sources and displays.[7]

QDs are both photo-emissive (photoluminescent) and electro-emissive (electroluminescent) allowing them to be readily incorporated into new emissive display architectures.[8] Quantum dots naturally produce monochromatic light, so they are more efficient than white light sources when color filtered and allow more saturated colors that reach nearly 100% of Rec. 2020 color gamut.[9]

Quantum dot enhancement layer

A widespread practical application is using quantum dot enhancement film (QDEF) layer to improve the LED backlighting in LCD TVs. Light from a blue LED backlight is converted by QDs to relatively pure red and green, so that this combination of blue, green and red light incurs less blue-green crosstalk and light absorption in the color filters after the LCD screen, thereby increasing useful light throughput and providing a better color gamut.

The first manufacturer shipping TVs of this kind was Sony in 2013 as Triluminos, Sony's trademark for the technology.[10] At the Consumer Electronics Show 2015, Samsung Electronics, LG Electronics, TCL Corporation and Sony showed QD-enhanced LED-backlighting of LCD TVs.[11][12][13] At the CES 2017, Samsung rebranded their 'SUHD' TVs as 'QLED'; later in April 2017, Samsung formed the QLED Alliance with Hisense and TCL to produce and market QD-enhanced TVs.[14][15]

Quantum dot on glass (QDOG) replaces QD film with a thin QD layer coated on top of the light-guide plate (LGP), reducing costs and improving efficiency.[16][17]

Traditional white LED backlights that use blue LEDs with on-chip or on-rail red-green QD structures are being researched, though high operating temperatures negatively affect their lifespan.[18][19]

Quantum dot color filters

'QDCF' LED-backlit LCDs would use QD film or ink-printed QD layer with red/green sub-pixel patterned quantum dots, aligned to precisely match the red and green subpixels; blue subpixels can be transparent to pass through the pure blue LED backlight, or can be made with blue patterned quantum dots in case of UV-LED backlight. This configuration effectively replaces passive color filters, which incur substantial losses by filtering out 2/3 of passing light, with photo-emissive QD structures, improving power efficiency and/or peak brightness, and enhancing color purity.[18][20][21] Because quantum dots depolarize the light, output polarizer (the analyzer) needs to be moved behind the color filter and embedded in-cell of the LCD glass; this would improve viewing angles as well. In-cell arrangement of the analyzer and/or the polarizer would also reduce depolarization effects in the LC layer, increasing contrast ratio. To reduce self-excitement of QD film and to improve efficiency, the ambient light can be blocked using traditional color filters, and reflective polarizers can direct light from QD filters towards the viewer. As only blue or UV light passes through the liquid crystal layer, it can be made thinner, resulting in faster pixel response times.[20] [22]

QD color filters can also be back-lit with OLED or micro-LED panels, improving their efficiency and color gamut.[16]

Nanosys made presentations of their photo-emissive technology during 2017; commercial products are expected by 2019, though in-cell polarizer remains a major challenge. [23][14][24][25][26][27][28] [29]

Active-matrix light-emitting diodes

See also: AMOLED, MicroLED, and LED display

AMQLED displays will use electroluminescent QD nanoparticles functioning as Quantum-dot-based LEDs (QD-LEDs or QLEDs) arranged in an active matrix array. 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,[30] greatly reducing pixel response times by eliminating the liquid crystal layer.

The structure of a QD-LED is similar to the basic design of an OLED. 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.[7][31] The demonstrated color gamut from QD-LEDs exceeds the performance of both LCD and OLED display technologies.[32]

Mass production of active-matrix QLED displays using ink-jet printing is expected to begin in 2020-2021.[33][34][35][29] InP ink-jet solutions are being researched by Nanosys, Nanoco, Nanophotonica, Fraunhofer IAP, and Seoul National University, among others. [28][36]

Optical properties of quantum dots

Performance of QDs is determined by the size and/or composition of the QD structures. Unlike simple atomic structures, a quantum dot structure has the unusual property that energy levels are strongly dependent on the structure's size. For example, CdSe quantum dot light emission can be tuned from red (5 nm diameter) to the violet region (1.5 nm dot). 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.[37]

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.[18][38]

QD-LEDs are characterized by pure and saturated emission colors with narrow bandwidth, characterized by full width at half the maximum value in the range of 20-40 nm.[7][20] Their emission wavelength is easily tuned by changing the size of the quantum dots. Moreover, QD-LED offer high color purity and durability combined with the efficiency, flexibility, and low processing cost of comparable organic light-emitting devices. QD-LED structure can be tuned over the entire visible wavelength range from 460 nm (blue) to 650 nm (red) (the human eye can detect light from 380 to 750 nm). The emission wavelengths have been continuously extended to UV and NIR range by tailoring the chemical composition of the QDs and device structure.[39][40]

Fabrication process

Quantum dots are solution processable and suitable for wet processing techniques. The two major fabrication techniques for QD-LED are called phase separation and contact-printing.[41]

Phase separation

Phase separation is suitable for forming large-area ordered QD monolayers. A single QD layer is formed by spin casting a mixed solution of QD and anb organic semiconductor such as TPD (N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine). 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.[42]

Although phase separation is relatively simple, it is not suitable for display device applications. Since spin-casting does not allow lateral patterning of different sized QDs (RGB), phase separation cannot create a multi-color QD-LED. 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.

Contact printing

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 1000 ppi (pixels-per-inch) resolution.[32]

The overall process of contact printing:

  • Polydimethylsiloxane (PDMS) is molded using a silicon master.
  • Top side of resulting PDMS stamp is coated with a thin film of parylene-c, a chemical-vapor deposited (CVD) aromatic organic polymer.
  • Parylene-c coated stamp is inked via spin-casting of a solution of colloidal QDs suspended in an organic solvent.[contradictory]
  • After the solvent evaporates, the formed QD monolayer is transferred to the substrate by contact printing.

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.

Contact printing allows fabrication of multi-color QD-LEDs. A QD-LED was fabricated with an emissive layer consisting of 25-µm wide stripes of red, green and blue QD monolayers. Contact printing methods also minimize the amount of QD required, reducing costs.[32]

Comparison

Nanocrystal displays would render as much as a 30% increase in the visible spectrum, while using 30 to 50% less power than LCDs, in large part because nanocrystal displays wouldn't need backlighting. QD LEDs are 50-100 times brighter than CRT and LC displays, emitting 40,000 cd/m2. QDs are soluble 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.) In addition to OLED displays, pick-and-place microLED displays are emerging as competing technologies to nanocrystal displays.

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 D65. However the blue component in the display can have relatively lower color purity and/or precision (dynamic range) in comparison to green and red, because the human eye is three to five times less sensitive to blue in daylight conditions according to CIE luminosity function.

See also

References

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