A quantum dot display is a display device that uses quantum dots (QD), semiconductor nanocrystals which can produce pure monochromaticred, green, and blue light.
Photo-emissive quantum dot particles are used in a QD layer which uses the blue light from a backlight to emit pure basic colors which improve display brightness and color gamut by reducing light losses and color crosstalk in RGB LCD color filters, replacing traditional colored photoresists in RGB LCD color filters. This technology is used in LED-backlit LCDs, though it is applicable to other display technologies which use color filters, such as blue/UV OLED or MicroLED.LED-backlit LCDs are the main application of quantum dots, where they are used to offer an alternative to OLED displays.
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. 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.
As of 2019, 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, although Samsung is working to release Electro-emissive QDLED displays "in the near future",while others doubt that such QDLED displays will ever become mainstream.
Emissive quantum dot displays can achieve the same contrast as OLED and MicroLED displays with "perfect" black levels in the off state. Quantum Dot displays are capable of displaying wider color gamuts than OLEDs with some devices approaching full coverage of the BT.2020 color gamut.
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.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) or electro-emissive (electroluminescent) 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 color gamut.
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.At the Consumer Electronics Show 2015, Samsung Electronics, LG Electronics, TCL Corporation 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.
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.
QD color filter/converter (QDCF/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 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.
Nanosys made presentations of their photo-emissive color filter 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 filters appeared on the market.
QD color filters/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 filters 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.
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, 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.InP (indium phosphide) ink-jet solutions are being researched by Nanosys, Nanoco, Nanophotonica, OSRAM OLED, Fraunhofer IAP, and Seoul National University, among others. As of 2019, InP based materials are still not yet ready for commercial production due to limited lifetime.
Quantum dot nanorod emitting diode (QNED) display is a further development of QD-OLED displays, 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 a QD color convertors are placed on top of red/green subpixels.Samsung Display is to begin test production of QNED panels in 2021.
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.
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,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.
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 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 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.
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:
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.
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 (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 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.
A liquid-crystal display (LCD) is a flat-panel display or other electronically modulated optical device that uses the light-modulating properties of liquid crystals combined with polarizers. Liquid crystals do not emit light directly, instead using a backlight or reflector to produce images in color or monochrome. LCDs are available to display arbitrary images or fixed images with low information content, which can be displayed or hidden. For instance: preset words, digits, and seven-segment displays, as in a digital clock, are all good examples of devices with these displays. They use the same basic technology, except that arbitrary images are made from a matrix of small pixels, while other displays have larger elements. LCDs can either be normally on (positive) or off (negative), depending on the polarizer arrangement. For example, a character positive LCD with a backlight will have black lettering on a background that is the color of the backlight, and a character negative LCD will have a black background with the letters being of the same color as the backlight. Optical filters are added to white on blue LCDs to give them their characteristic appearance.
A plasma display panel (PDP) is a type of flat panel display that uses small cells containing plasma: ionized gas that responds to electric fields. Plasma TVs were the first large flat panel displays to be released to the public.
An organic light-emitting diode, also known as organic electroluminescentdiode, is a light-emitting diode (LED) in which the emissive electroluminescent layer is a film of organic compound that emits light in response to an electric current. This organic layer is situated between two electrodes; typically, at least one of these electrodes is transparent. OLEDs are used to create digital displays in devices such as television screens, computer monitors, portable systems such as smartphones and handheld game consoles. A major area of research is the development of white OLED devices for use in solid-state lighting applications.
A flat-panel display (FPD) is an electronic display device used to enable people to see content in a range of entertainment, consumer electronics, personal computer, and mobile devices, and many types of medical, transportation and industrial equipment. They are far lighter and thinner than traditional cathode ray tube (CRT) television sets and are usually less than 10 centimetres (3.9 in) thick. Flat-panel displays can be divided into two display device categories: volatile and static. Volatile displays require that pixels be periodically electronically refreshed to retain their state. A volatile display only shows an image when it has battery or AC mains power. Static flat-panel displays rely on materials whose color states are bistable, and as such, flat-panel displays retain the text or images on the screen even when the power is off. As of 2016, flat-panel displays have almost completely replaced old CRT displays. In many 2010-era applications, specifically small portable devices such as laptops, mobile phones, smartphones, digital cameras, camcorders, point-and-shoot cameras, and pocket video cameras, any display disadvantages of flat-panels are made up for by portability advantages.
A display device is an output device for presentation of information in visual or tactile form. When the input information that is supplied has an electrical signal the display is called an electronic display.
Quantum dots (QDs) are semiconductor particles a few nanometres in size, having optical and electronic properties that differ from larger particles due to quantum mechanics. They are a central topic in nanotechnology. When the quantum dots are illuminated by UV light, an electron in the quantum dot can be excited to a state of higher energy. In the case of a semiconducting quantum dot, this process corresponds to the transition of an electron from the valence band to the conductance band. The excited electron can drop back into the valence band releasing its energy by the emission of light. This light emission (photoluminescence) is illustrated in the figure on the right. The color of that light depends on the energy difference between the conductance band and the valence band.
A television set or television receiver, more commonly called a television, TV, TV set, telly, or tele, is a device that combines a tuner, display, and loudspeakers, for the purpose of viewing and hearing television broadcasting through satellites or cables, or using it as a computer monitor. Introduced in the late 1920s in mechanical form, television sets became a popular consumer product after World War II in electronic form, using cathode ray tube (CRT) technology. The addition of color to broadcast television after 1953 further increased the popularity of television sets in the 1960s, and an outdoor antenna became a common feature of suburban homes. The ubiquitous television set became the display device for the first recorded media in the 1970s, such as Betamax, VHS and later DVD. It has been used as a display device since the first generation of home computers and dedicated video game consoles in the 1980s. By the early 2010s, flat-panel television incorporating liquid-crystal display (LCD) technology, especially LED-backlit LCD technology, largely replaced CRT and other display technologies. Modern flat panel TVs are typically capable of high-definition display and can also play content from a USB device.
Liquid-crystal-display televisions are television sets that use liquid-crystal displays to produce images. They are, by far, the most widely produced and sold television display type. LCD TVs are thin and light, but have some disadvantages compared to other display types such as high power consumption, poorer contrast ratio, and inferior color gamut.
Nanosys is a nanotechnology company located in Milpitas, California and founded in 2001. The company develops and manufactures quantum dot materials for display products.
A field-emission display (FED) is a flat panel display technology that uses large-area field electron emission sources to provide electrons that strike colored phosphor to produce a color image. In a general sense, an FED consists of a matrix of cathode ray tubes, each tube producing a single sub-pixel, grouped in threes to form red-green-blue (RGB) pixels. FEDs combine the advantages of CRTs, namely their high contrast levels and very fast response times, with the packaging advantages of LCD and other flat-panel technologies. They also offer the possibility of requiring less power, about half that of an LCD system.
A backlight is a form of illumination used in liquid crystal displays (LCDs). As LCDs do not produce light by themselves—unlike, for example, cathode ray tube (CRT) displays—they need illumination to produce a visible image. Backlights illuminate the LCD from the side or back of the display panel, unlike frontlights, which are placed in front of the LCD. Backlights are used in small displays to increase readability in low light conditions such as in wristwatches, and are used in smart phones, computer displays and LCD televisions to produce light in a manner similar to a CRT display. A review of some early backlighting schemes for LCDs is given in a report Engineering and Technology History by Peter J. Wild.
A surface-conduction electron-emitter display (SED) is a display technology for flat panel displays developed by a number of companies. SEDs use nanoscopic-scale electron emitters to energize colored phosphors and produce an image. In a general sense, an SED consists of a matrix of tiny cathode ray tubes, each "tube" forming a single sub-pixel on the screen, grouped in threes to form red-green-blue (RGB) pixels. SEDs combine the advantages of CRTs, namely their high contrast ratios, wide viewing angles and very fast response times, with the packaging advantages of LCD and other flat panel displays. They also use much less power than an LCD television of the same size.
A LED display is a flat panel display that uses an array of light-emitting diodes as pixels for a video display. Their brightness allows them to be used outdoors where they are visible in the sun for store signs and billboards. In recent years, they have also become commonly used in destination signs on public transport vehicles, as well as variable-message signs on highways. LED displays are capable of providing general illumination in addition to visual display, as when used for stage lighting or other decorative purposes. LED displays can offer higher contrast ratios than a projector and are thus an alternative to traditional projection screens, and they can be used for large, uninterrupted video walls. microLED displays are LED displays with smaller LEDs, which poses significant development challenges.
AMOLED 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 refers to the technology behind the addressing of pixels.
A LED-backlit LCD is a liquid-crystal display that uses LEDs for backlighting instead of traditional cold cathode fluorescent (CCFL) backlighting. LED-backlit displays use the same TFT LCD technologies as CCFL-backlit LCDs, but offer a variety of advantages over them.
Nanoco Technologies Ltd. (Nanoco) is a UK-based nanotechnology company that spun out from the research group of Prof. Paul O’Brien at the University of Manchester in 2001. The company's development has been driven by Dr Nigel Pickett, Nanoco's Chief Technology Officer, whose pioneering work on the patented "molecular seeding" process has formed the basis of Nanoco's unique technology, and Dr Michael Edelman, who joined Nanoco as CEO in 2004, leading the company's growth from a two-man start-up to a publicly traded organisation with more than 120 employees across the globe. Since 2004, Nanoco has focussed its research efforts into the development of quantum dots and other nanoparticles that are entirely free of cadmium and other regulated heavy metals. Nanoco has licensed its technology to Dow, Wah Hong, and Merck.
Universal Display Corporation is a developer and manufacturer of organic light emitting diodes (OLED) technologies and materials as well as provider of services to the display and lighting industries. It is also an OLED research company. Founded in 1994, the company currently owns or has exclusive, co-exclusive or sole license rights with respect to more than 3,000 issued and pending patents worldwide for the commercialization of phosphorescent based OLEDs and also flexible, transparent and stacked OLEDs - for both display and lighting applications. Its phosphorescent OLED technologies and materials are licensed and supplied to companies such as Samsung, LG, AU Optronics CMEL, Pioneer, Panasonic Idemitsu OLED lighting and Konica Minolta.
A see-through display or transparent display is an electronic display that allows the user to see what is shown on the screen while still being able to see through it. The main applications of this type of display are in head-up displays, augmented reality systems, digital signage, and general large-scale spatial light modulation. They should be distinguished from image-combination systems which achieve visually similar effects by optically combining multiple images in the field of view. Transparent displays embed the active matrix of the display in the field of view, which generally allows them to be more compact than combination-based systems.
microLED, also known as micro-LED, mLED or µLED, is an emerging flat-panel display technology. microLED displays consist of arrays of microscopic LEDs forming the individual pixel elements. When compared with widespread LCD technology, microLED displays offer better contrast, response times, and energy efficiency.
In light-emitting diode physics, the recombination of electrons and electron holes in a semiconductor produce light, a process called "electroluminescence". The wavelength of the light produced depends on the energy band gap of the semiconductors used. Since these materials have a high index of refraction, design features of the devices such as special optical coatings and die shape are required to efficiently emit light. An LED is a long-lived light source, but certain mechanisms can cause slow loss of efficiency of the device or sudden failure. The wavelength of the light emitted is a function of the band gap of the semiconductor material used; materials such as gallium arsenide, and others, with various trace doping elements, are used to produce different colors of light. Another type of LED uses a quantum dot which can have its properties and wavelength adjusted by its size. Light-emitting diodes are widely used in indicator and display functions, and white LEDs are displacing other technologies for general illumination purposes.