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A quantum dot display is a
display device A display device is an output device for presentation of information in visual or tactile form (the latter used for example in tactile electronic displays for blind people). When the input information that is supplied has an electrical signal the ...
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 LCD backlights and/or display color filters. Quantum dots are excited by the blue light from the display panel to emit pure basic colors, which reduces light losses and color crosstalk in color filters, improving display brightness and color gamut. Light travels through QD layer film and traditional RGB filters made from color pigments, or through QD filters with red/green QD color converters and blue passthrough. Although the QD color filter technology is primarily used in LED-backlit LCDs, it is applicable to other display technologies which use color filters, such as blue/UV
AMOLED AMOLED (active-matrix organic light-emitting diode, ) is a type of OLED display device technology. OLED describes a specific type of thin-film-display technology in which organic compounds form the electroluminescent material, and active matrix ...
/ QNED/ MicroLED display panels. LED-backlit LCDs are the main application of photo-emissive quantum dots, though blue OLED panels with QD color filters are being researched. ''Electro-emissive'' or ''electroluminiscent'' quantum dot displays are an experimental type of display based on quantum-dot light-emitting diodes (QD-LED; also EL-QLED, ELQD, QDEL). 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. Manufacturers asserted that 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 camera A digital camera is a camera that captures photographs in digital memory. Most cameras produced today are digital, largely replacing those that capture images on photographic film. Digital cameras are now widely incorporated into mobile devices ...
s,
mobile phone A mobile phone, cellular phone, cell phone, cellphone, handphone, hand phone or pocket phone, sometimes shortened to simply mobile, cell, or just phone, is a portable telephone that can make and receive calls over a radio frequency link whi ...
s and
handheld game console A handheld game console, or simply handheld console, is a small, portable self-contained video game console with a built-in screen, game controls and speakers. Handheld game consoles are smaller than home video game consoles and contain the co ...
s.Quantum-dot displays could outshine their rivals
New Scientist, 10 December 2007
all commercial products, such as LCD TVs branded as ''QLED'', employ quantum dots as ''photo-emissive'' particles; ''electro-emissive'' QD-LED TVs exist in laboratories only. Quantum dot displays are capable of displaying wider color gamuts, with some devices approaching full coverage of the BT.2020 color gamut. QD-OLED and QD-LED displays can achieve the same contrast as OLED/MicroLED displays with "perfect" black levels in the off state, unlike LED-backlit LCDs.


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 diode A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (co ...
s and single-color light emitting devices. Starting in the early 2000s, scientists started to realize the potential of developing quantum dots for light sources and displays. QDs are either ''photo-emissive'' (
photoluminescent Photoluminescence (abbreviated as PL) is light emission from any form of matter after the absorption of photons (electromagnetic radiation). It is one of many forms of luminescence (light emission) and is initiated by photoexcitation (i.e. photon ...
) or ''electro-emissive'' (
electroluminescent Electroluminescence (EL) is an optical and electrical phenomenon, in which a material emits light in response to the passage of an electric current or to a strong electric field. This is distinct from black body light emission resulting fro ...
) allowing them to be readily incorporated into new emissive display architectures. 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 ITU-R Recommendation BT.2020, more commonly known by the abbreviations Rec. 2020 or BT.2020, defines various aspects of ultra-high-definition television (UHDTV) with standard dynamic range (SDR) and wide color gamut (WCG), including picture ...
color gamut.Ruidong Zhu, Zhenyue Luo, Haiwei Chen, Yajie Dong, and Shin-Tson Wu. Realizing Rec. 2020 color gamut with quantum dot displays
Optics Express, Vol. 23, No. 18 (2015). DOI:10.1364/OE.23.023680


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 , commonly stylized as SONY, is a Japanese multinational conglomerate corporation headquartered in Minato, Tokyo, Japan. As a major technology company, it operates as one of the world's largest manufacturers of consumer and professional ...
in 2013 as ''Triluminos'', Sony's trademark for the technology. At the
Consumer Electronics Show CES (; formerly an initialism for Consumer Electronics Show) is an annual trade show organized by the Consumer Technology Association (CTA). Held in January at the Las Vegas Convention Center in Winchester, Nevada, United States, the event t ...
2015,
Samsung Electronics Samsung Electronics Co., Ltd. (, sometimes shortened to SEC and stylized as SΛMSUNG) is a South Korean multinational electronics corporation headquartered in Yeongtong-gu, Suwon, South Korea. It is the pinnacle of the Samsung chaebol, acc ...
,
LG Electronics LG Electronics Inc. () is a South Korean multinational electronics company headquartered in Yeouido-dong, Seoul, South Korea. LG Electronics is a part of LG Corporation, the fourth largest '' chaebol'' in South Korea, and often considered ...
,
TCL Corporation TCL Technology (originally an abbreviation for Telephone Communication Limited) is a Chinese electronics company headquartered in Huizhou, Guangdong Province. It designs, develops, manufactures, and sells consumer products including televis ...
and Sony showed QD-enhanced LED-backlighting of LCD TVs. 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. 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.https://nccavs-usergroups.avs.org/wp-content/uploads/TFUG2017/TFUG917-1-Hartlove-Rev1.pdf 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.


Quantum dot color converter

QD color converter (QDCC) LED-backlit LCDs would use QD film or ink-printed QD layer with red/green sub-pixel patterned (i.e. aligned to precisely match the red and green subpixels) quantum dots to produce pure red/green light; 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. Because quantum dots depolarize the light, output polarizer (the analyzer) needs to be moved behind the color converter 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 the QDCC 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.Haiwei Chen, Juan He, and Shin-Tson Wu. Recent advances on quantum-dot-enhanced liquid crystal displays
IEEE Journal of Selected Topics in Quantum Electronics Vol. 23, No. 5 (2017). DOI 10.1109/JSTQE.2017.2649466
H. Chen, G. Tan, M. C. Li, S. L. Lee, and S. T. Wu. Depolarization effect in liquid crystal displays
Optics Express 25 (10), 11315-11328 (2017). DOI 10.1364/OE.25.011315
Nanosys made presentations of their photo-emissive color converter technology during 2017; commercial products were expected by 2019, though in-cell polarizer remained a major challenge. As of December 2019, issues with in-cell polarizer remain unresolved and no LCDs with QD color converter appeared on the market since then. QD color converters can be used with OLED or micro-LED panels, improving their efficiency and color gamut. QD-OLED panels with blue emitters and red-green color converters are researched by Samsung and TCL; as of May 2019, Samsung intends to start production in 2021. In October 2019, Samsung Display announced an investment of $10.8 billion in both research and production, with the aim to convert all their 8G panel factories to QD-OLED production during 2019–2025. Samsung Display presented 55" and 65" QD-OLED panels at CES 2022, with TVs from Samsung Electronics and Sony to be released later in 2022. QD-OLED displays show better color volume, covering 90% of Rec.2020 color gamut with peak brightness of 1500 nits, while current OLED and LCD TVs cover 70–75% of Rec.2020 (95–100% of DCI-P3). A further development of QD-OLED displays is quantum dot nanorod emitting diode (QNED) display which replaces blue OLED layer with InGaN/ GaN blue nanorod LEDs. Nanorods have a larger emitting surface compared to planar LED, allowing increased efficiency and higher light emission. Nanorod solution is ink-printed on the substrate, then subpixels are aligned in-place by electric current, and QD color convertors are placed on top of red/green subpixels. Samsung Display was expected to begin test production of QNED panels in 2021, with mass production in 2024-2025, but test production has been postponed as of May 2022.


Self-emissive quantum dot diodes

Self-emissive quantum dot displays will use electroluminescent QD nanoparticles functioning as Quantum-dot-based LEDs (QD-LEDs or QLEDs) arranged in either active matrix or passive 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, greatly reducing pixel response times by eliminating the liquid crystal layer. This technology has also been called true QLED display, and Electroluminescent quantum dots (ELQD, QDLE, QDEL, EL-QLED). 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. The demonstrated color gamut from QD-LEDs exceeds the performance of both LCD and OLED display technologies. Mass production of active-matrix QLED displays using ink-jet printing is expected to begin in 2020–2021. As cadmium-based materials cannot be used in lighting applications due to their environmental impact, InP (
indium phosphide Indium phosphide (InP) is a binary semiconductor composed of indium and phosphorus. It has a face-centered cubic (" zincblende") crystal structure, identical to that of GaAs and most of the III-V semiconductors. Manufacturing Indium phosphide ...
) ink-jet solutions are being researched by Nanosys, Nanoco, Nanophotonica, OSRAM OLED, Fraunhofer IAP, Merck, and Seoul National University, among others. As of 2019, InP based materials are still not yet ready for commercial production due to limited lifetime.


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 Cadmium selenide is an inorganic compound with the formula Cd Se. It is a black to red-black solid that is classified as a II-VI semiconductor of the n-type. Much of the current research on this compound is focused on its nanoparticles. Stru ...
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 level A quantum mechanical system or particle that is bound—that is, confined spatially—can only take on certain discrete values of energy, called energy levels. This contrasts with classical particles, which can have any amount of energy. The t ...
s. 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 Indium is a chemical element with the symbol In and atomic number 49. Indium is the softest metal that is not an alkali metal. It is a silvery-white metal that resembles tin in appearance. It is a post-transition metal that makes up 0.21 parts ...
instead of
cadmium Cadmium is a chemical element with the Symbol (chemistry), symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12 element, group 12, zinc and mercury (element), mercury. Li ...
, as the latter is not exempted for use in lighting by the
European Commission The European Commission (EC) is the executive of the European Union (EU). It operates as a cabinet government, with 27 members of the Commission (informally known as "Commissioners") headed by a President. It includes an administrative body ...
RoHS The Restriction of Hazardous Substances Directive 2002/95/EC (RoHS 1), short for Directive on the restriction of the use of certain hazardous substances in electrical and electronic equipment, was adopted in February 2003 by the European Uni ...
directive, and also because of cadmium's toxicity. QD-LEDs are characterized by pure and saturated emission colors with narrow bandwidth, with FWHM ( full width at half maximum) in the range of 20–40 nm. 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.


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.


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 an 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 A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
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. 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. The overall process of contact printing: *
Polydimethylsiloxane Polydimethylsiloxane (PDMS), also known as dimethylpolysiloxane or dimethicone, belongs to a group of polymeric organosilicon compounds that are commonly referred to as silicones. PDMS is the most widely used silicon-based organic polymer, as its ...
(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 A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
. * 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 The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer ( American spelling), also commonly known as a micron, is a unit of length in the International System of Uni ...
wide stripes of red, green and blue QD monolayers. Contact printing methods also minimize the amount of QD required, reducing costs.


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 
nits The National Institutes of Technology (NITs) are the central government-owned-public technical institutes under the ownership of Ministry of Education, Government of India. They are governed by the National Institutes of Technology, Scienc ...
( cd/m2). 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.) In addition to OLED displays, pick-and-place microLED displays are emerging as competing technologies to nanocrystal displays. Samsung has developed a method for making self-emissive quantum dot diodes with a lifetime of 1 million hours. 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 Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is scattered and filtered through Earth's atmosphere, and is obvious as daylight when ...
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 Dynamic range (abbreviated DR, DNR, or DYR) is the ratio between the largest and smallest values that a certain quantity can assume. It is often used in the context of signals, like sound and light. It is measured either as a ratio or as a base ...
) 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. In contrast to traditional LCD panels, QDs suffer from the same
screen burn-in Screen burn-in, image burn-in, or ghost image, is a permanent discoloration of areas on an electronic display such as a cathode ray tube (CRT) in an old computer monitor or television set. It is caused by cumulative non-uniform use of the s ...
effect as OLED panels.


See also

*
Bandwidth (signal processing) Bandwidth is the difference between the upper and lower frequencies in a continuous band of frequencies. It is typically measured in hertz, and depending on context, may specifically refer to '' passband bandwidth'' or ''baseband bandwidth'' ...
*
Electron hole In physics, chemistry, and electronic engineering, an electron hole (often simply called a hole) is a quasiparticle which is the lack of an electron at a position where one could exist in an atom or atomic lattice. Since in a normal atom or ...
*
Energy level A quantum mechanical system or particle that is bound—that is, confined spatially—can only take on certain discrete values of energy, called energy levels. This contrasts with classical particles, which can have any amount of energy. The t ...
*
Nanotechnology Nanotechnology, also shortened to nanotech, is the use of matter on an atomic, molecular, and supramolecular scale for industrial purposes. The earliest, widespread description of nanotechnology referred to the particular technological goal ...
* Organic light-emitting diode *
Potential well A potential well is the region surrounding a local minimum of potential energy. Energy captured in a potential well is unable to convert to another type of energy ( kinetic energy in the case of a gravitational potential well) because it is ca ...
*
Quantum dot Quantum dots (QDs) are semiconductor particles a few nanometres in size, having optical and electronic properties that differ from those of larger particles as a result of quantum mechanics. They are a central topic in nanotechnology. When the ...
*
Spectral linewidth A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to ident ...


Notes


References


External links


Quantum Dots: Technical Status and Market ProspectsQuantum dots that produce white light could be the light bulb’s successor
{{DEFAULTSORT:Quantum Dot Display Quantum electronics Display technology Emerging technologies Quantum dots