Quantum dot display
Encyclopedia
A quantum dot display is a type of display technology used in flat panel display
s as an electronic visual display
. Quantum dots (QD) or semiconductor nanocrystals are a form of light emitting technology and consist of nano-scale crystals that can provide an alternative for applications such as display technology. This display technology differs from cathode ray tube
s (CRTs), liquid crystal display
s (LCDs), but it is similar to organic light-emitting diode
(OLED) displays, in that light is supplied on demand, which enables new, more efficient displays, which is enabling mobile devices with longer battery lives.
Unlike inorganic semiconductor based LEDs
, organic electroluminescent devices can be deposited over larger areas and on flexible or non-planar substrates. Large area display or general illumination devices have been fashioned from these molecules and have begun their entry into the market. However, the light emitting organic molecules tend to degrade and are particularly sensitive to humidity and oxidation. Quantum dots incorporate the best aspects of both organic light emitters and inorganic light emitters. With many promising advantages, QD LED or QLED is considered as a next generation display technology. QDs can be incorporated into a new generation of applications such as flat-panel TV screens, digital camera
s, mobile phone
s, personal gaming equipment and PDAs
.
The properties and performance of these unique crystals is determined by the size and/or composition of the QD. Given QDs are both photo-active (photoluminescent) and electro-active (electroluminescent) they can be readily incorporated into new emissive display architectures.
quantum dot light emission can be gradually tuned from the red region of spectrum for a 5 nm diameter dot, to the violet region for a 1.5 nm dot. The physical reason for QD coloration is the quantum confinement effect and is directly related to the energy level
s of quantum dot. 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 which are also more closely spaced, and this allows the QD to absorb photons of smaller 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.
The structure of QD-LED is similar to basic design of OLED. The major difference is that the light emitting centers are cadmium selenide
(CdSe) nanocrystals, or quantum dots. A layer of cadmium-selenium 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 spectrum of photon emission is narrow, characterized by its full width at half the maximum value.
Bringing electrons and holes together in small regions for efficient recombination to emit photons without escaping or dissipating was one of the major challenges. To address this problem, a thin emissive layer sandwiched between a hole-transporter layer (HTL) and an electron-transport layer (ETL). By making an emissive layer in single layer of quantum dots, electrons and holes may be transferred directly from the surfaces of the ETL and HTL., and resulting high recombination efficiency.
Both ETL and HTL consist of organic materials. It is well known that most organic electroluminescent materials are in favor of injection and transport of holes rather than electrons. Thus, the electron-hole recombination generally occurs near the cathode, which could lead to the quenching of the exciton produced. In order to prevent the produced excitons or holes from approaching cathode, a hole-blocking layer plays dual roles in blocking holes moving towards the cathode and transporting the electrons to the emitting layer, QD layer. Tris-Aluminium (Alq3), bathocuproine (BCP), and TAZ are most commonly used hole-blocking materials. These materials can be used as both electron-transporting layer and hole blocking layer.
The array of quantum dots is manufactured by self-assembly in process known as spin-casting; a solution of quantum dots in an organic material is poured into a substrate, which is then set spinning to spread the solution evenly.
a mixed solution of QD and TPD. This process simultaneously yields QD monolayers self-assembled into hexagonally close-packed arrays and places this monolater 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 surface of the film. Resulting QD structure is affected by many parameters: solution concentration, solvent ration, QD size distribution and QD aspect ratio. Also, important is purity of QD solution and organic solvent.
Although, phase separation is relatively simple fabrication process, it is not suitable for display device applications. Since spin-casting does not allow lateral patterning of different sized QDs (RGB), phase separation cannot result multi-color generating QD-LED. Moreover, it is not ideal to have organic under-layer material for QD-LED; organic under-layer must be homogeneous, a constraint which limits the number of applicable device designs.
The overall process of contact printing:
With contact printing method, fabrication of multi-color generation QD-LED becomes possible. A QD-LED was fabricated with an emissive layer consisting of 25 microns wide stripes of red, green and blue QD monolayers. Before the development of contact printing process, saturated color emission was achieved by depositing multiple QD monolayers with spin casting method. However, with the development of contact printing method, amount of QD required to produce QD-LED got minimized, hence reducing the cost of fabrication and material. The demonstrated color gamut from QD-LEDs exceeds the performance of both LCD and OLED display technologies, which shows great promise of QD display.
Other advantages include better saturated green, manufacture ability on polymers, thin display, same material used to generate difference colors, and higher resolution.
Commercialization of quantum dot display is yet to come. Compared to LCD and OLED, the manufacturing cost of QD-LED is relatively high and development of novel and more cost-efficient fabrication process is desired in future.
Flat panel display
Flat panel displays encompass a growing number of electronic visual display technologies. They are far lighter and thinner than traditional television sets and video displays that use cathode ray tubes , and are usually less than thick...
s as an electronic visual display
Electronic visual display
An electronic visual display is display technology which incorporates flat panel displays, performs as a video display, output device for presentation of images transmitted electronically, for visual reception, without producing a permanent record....
. Quantum dots (QD) or semiconductor nanocrystals are a form of light emitting technology and consist of nano-scale crystals that can provide an alternative for applications such as display technology. This display technology differs from cathode ray tube
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
s (CRTs), liquid crystal display
Liquid crystal display
A liquid crystal display is a flat panel display, electronic visual display, or video display that uses the light modulating properties of liquid crystals . LCs do not emit light directly....
s (LCDs), but it is similar to organic light-emitting diode
Organic light-emitting diode
An OLED is a light-emitting diode in which the emissive electroluminescent layer is a film of organic compounds which emit light in response to an electric current. This layer of organic semiconductor material is situated between two electrodes...
(OLED) displays, in that light is supplied on demand, which enables new, more efficient displays, which is enabling mobile devices with longer battery lives.
Unlike inorganic semiconductor based LEDs
Light-emitting diode
A light-emitting diode is a semiconductor light source. LEDs are used as indicator lamps in many devices and are increasingly used for other lighting...
, organic electroluminescent devices can be deposited over larger areas and on flexible or non-planar substrates. Large area display or general illumination devices have been fashioned from these molecules and have begun their entry into the market. However, the light emitting organic molecules tend to degrade and are particularly sensitive to humidity and oxidation. Quantum dots incorporate the best aspects of both organic light emitters and inorganic light emitters. With many promising advantages, QD LED or QLED is considered as a next generation display technology. QDs can be incorporated into a new generation of applications such as flat-panel TV screens, digital camera
Digital camera
A digital camera is a camera that takes video or still photographs, or both, digitally by recording images via an electronic image sensor. It is the main device used in the field of digital photography...
s, mobile phone
Mobile phone
A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...
s, personal gaming equipment and PDAs
Personal digital assistant
A personal digital assistant , also known as a palmtop computer, or personal data assistant, is a mobile device that functions as a personal information manager. Current PDAs often have the ability to connect to the Internet...
.
The properties and performance of these unique crystals is determined by the size and/or composition of the QD. Given QDs are both photo-active (photoluminescent) and electro-active (electroluminescent) they can be readily incorporated into new emissive display architectures.
History
The idea of using quantum dot as a light source first developed in 1990s. Early applications included, imaging using QD infrared photodetectors and light emitting diodes and single color light emitting devices. Starting from early 2000, scientists started to realize the potential of developing quantum dot as the next generation light source and display technology.Working principle
Optical properties of quantum dot
Unlike atoms, a quantum dot fabricated from a given material has the unusual property that its energy levels are strongly dependent on its size. For example, CdSeCadmium selenide
Cadmium selenide is a solid, binary compound of cadmium and selenium. Common names for this compound are cadmium selenide, cadmium selenide, and cadmoselite ....
quantum dot light emission can be gradually tuned from the red region of spectrum for a 5 nm diameter dot, to the violet region for a 1.5 nm dot. The physical reason for QD coloration is the quantum confinement effect and is directly related to the energy level
Energy level
A quantum mechanical system or particle that is bound -- that is, confined spatially—can only take on certain discrete values of energy. This contrasts with classical particles, which can have any energy. These discrete values are called energy levels...
s of quantum dot. 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 which are also more closely spaced, and this allows the QD to absorb photons of smaller 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.
Quantum dot light-emitting diodes
Quantum-dot-based LEDs are characterized by pure and saturated emission colors with narrow bandwidth, and their emission wavelength is easily tuned by changing the size of the quantum dots. Moreover, QD-LED combine the color purity and durability of QDs with efficiency, flexibility, and low processing cost of 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 structure of QD-LED is similar to basic design of OLED. The major difference is that the light emitting centers are cadmium selenide
Cadmium selenide
Cadmium selenide is a solid, binary compound of cadmium and selenium. Common names for this compound are cadmium selenide, cadmium selenide, and cadmoselite ....
(CdSe) nanocrystals, or quantum dots. A layer of cadmium-selenium 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 spectrum of photon emission is narrow, characterized by its full width at half the maximum value.
Bringing electrons and holes together in small regions for efficient recombination to emit photons without escaping or dissipating was one of the major challenges. To address this problem, a thin emissive layer sandwiched between a hole-transporter layer (HTL) and an electron-transport layer (ETL). By making an emissive layer in single layer of quantum dots, electrons and holes may be transferred directly from the surfaces of the ETL and HTL., and resulting high recombination efficiency.
Both ETL and HTL consist of organic materials. It is well known that most organic electroluminescent materials are in favor of injection and transport of holes rather than electrons. Thus, the electron-hole recombination generally occurs near the cathode, which could lead to the quenching of the exciton produced. In order to prevent the produced excitons or holes from approaching cathode, a hole-blocking layer plays dual roles in blocking holes moving towards the cathode and transporting the electrons to the emitting layer, QD layer. Tris-Aluminium (Alq3), bathocuproine (BCP), and TAZ are most commonly used hole-blocking materials. These materials can be used as both electron-transporting layer and hole blocking layer.
The array of quantum dots is manufactured by self-assembly in process known as spin-casting; a solution of quantum dots in an organic material is poured into a substrate, which is then set spinning to spread the solution evenly.
Fabrication process
Quantum dots are solution processable and suitable for wet processing techniques. There are two major fabrication techniques for QD-LED, called phase separation and contact-printing.Phase separation
Phase separation is a fabrication technique, which is suitable for forming large area of ordered monolayers of QDs. Single layer of QDs is formed by spin castingSpin casting
Spin casting, also known as centrifugal rubber mold casting , is a method of utilizing centrifugal force to produce castings from a rubber mold. Typically, a disc-shaped mold is spun along its central axis at a set speed. The casting material, usually molten metal or liquid thermoset plastic is...
a mixed solution of QD and TPD. This process simultaneously yields QD monolayers self-assembled into hexagonally close-packed arrays and places this monolater 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 surface of the film. Resulting QD structure is affected by many parameters: solution concentration, solvent ration, QD size distribution and QD aspect ratio. Also, important is purity of QD solution and organic solvent.
Although, phase separation is relatively simple fabrication process, it is not suitable for display device applications. Since spin-casting does not allow lateral patterning of different sized QDs (RGB), phase separation cannot result multi-color generating QD-LED. Moreover, it is not ideal to have organic under-layer material for QD-LED; 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 film is a solvent-free method, which is simple and cost efficient with high throughput of solution-processing methods. During the contact printing process, device structure does not get exposed to solvents. Since charge transport layers in QD-LED structure are solvent-sensitive organic thin films, avoiding solvent during process is one of the major benefits of contact printing method. This method can produce RGB patterned electroluminescent structures with 1000 ppi (pixels-per-inch) print resolution.The overall process of contact printing:
- PolydimethylsiloxanePolydimethylsiloxanePolydimethylsiloxane 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, and is particularly known for its unusual rheological properties. PDMS is optically clear, and, in general, is...
(PDMS) is molded using a silicon master. - Top side of resulting PDMS stamp is coated with a thing film of parylene-c, a chemical-vapor deposited (CVDCVDCVD can refer to:* Cardiovascular disease, the class of diseases that involve the heart or blood vessels* Countervailing duties, a mean to restrict international trade...
) aromatic organic polymer. - ParyleneParyleneParylene is the tradename for a variety of chemical vapor deposited poly polymers used as moisture and dielectric barriers. Among them, Parylene C is the most popular due to its combination of barrier properties, cost, and other processing advantages.Parylene is green polymer chemistry...
-c coated stamp is inked via spin-casting of a solution of colloidal QDs suspended in an organic solvent. - After the solvent evaporates, the formed QD monolayer is transformed on to the substrate by contact printing.
With contact printing method, fabrication of multi-color generation QD-LED becomes possible. A QD-LED was fabricated with an emissive layer consisting of 25 microns wide stripes of red, green and blue QD monolayers. Before the development of contact printing process, saturated color emission was achieved by depositing multiple QD monolayers with spin casting method. However, with the development of contact printing method, amount of QD required to produce QD-LED got minimized, hence reducing the cost of fabrication and material. The demonstrated color gamut from QD-LEDs exceeds the performance of both LCD and OLED display technologies, which shows great promise of QD display.
Pros
- Color range: Nanocrystal displays should be able to yield a greater portion of the visible spectrum than current technologies. As shown in the diagram, QD Vision calculates as much as 30% more of the visible spectrum would be available using QDs in a QD-LED vs. a CRT TV.
- Low power consumption: QD Vision estimates its nanocrystal displays could use 30 to 50% less electrical power than an LCD, in large part because nanocrystal displays don't need a backlight.
- Color accuracy: Nanocrystal displays would yield more purity in colors than other types of display technologies. Some display technologies, such as LCDs, can’t create a pure red, green, or blue for the display; instead, they need to add a few other colors to those three to display pure colors. Quantum dots, on the other hand, create pure red, green, and blue to create all other colors.
- Brightness: 50~100 times brighter than CRT and LCD displays ~40,000 cdCandelaThe candela is the SI base unit of luminous intensity; that is, power emitted by a light source in a particular direction, weighted by the luminosity function . A common candle emits light with a luminous intensity of roughly one candela...
/m2 - Added flexibility: QDs are soluble in both aqueous and non-aqueous solvents, which provides for printable and flexible displays of all sizes, including large area TVs
- Improved lifetime: QDs are inorganic, which can give the potential for improved lifetimes when compared to alternative OLED technologies. However, since many parts of QD-LED are made of organic materials, further development is required to improve the functional lifetime.
Other advantages include better saturated green, manufacture ability on polymers, thin display, same material used to generate difference colors, and higher resolution.
Cons
- Less saturated blue: Blue quantum dots are difficult to manufacture due to the timing control during the reaction. A blue quantum dot is just slightly above the minimum size, where red to green can be easily obtained. Sunlight contains roughly equal luminosities of red, green and blue. So in order to display an acceptable range of colors, a display needs to be capable to produce approximately equal luminosities of blue as of red and green (even though in order to achieve the same brightness as perceived by the human eyes, blue needs to be about 5 times more luminuous than green; have 5 times more power).
Commercialization of quantum dot display is yet to come. Compared to LCD and OLED, the manufacturing cost of QD-LED is relatively high and development of novel and more cost-efficient fabrication process is desired in future.
Market
Many expect that quantum dot display technology can compete or even replace liquid crystal displays (LCDs) in near future, including the desktop and notebook computer spaces and televisions. These initial applications alone represent more than a $1-billion addressable market by 2012 for quantum dot-based components. Other than display applications, several companies are manufacturing QD-LED light bulbs; these promise greater energy efficiency and longer lifetime.See also
- Bandwidth (signal processing)
- Electron holeElectron holeAn electron hole is the conceptual and mathematical opposite of an electron, useful in the study of physics, chemistry, and electrical engineering. The concept describes the lack of an electron at a position where one could exist in an atom or atomic lattice...
- Energy levelEnergy levelA quantum mechanical system or particle that is bound -- that is, confined spatially—can only take on certain discrete values of energy. This contrasts with classical particles, which can have any energy. These discrete values are called energy levels...
- NanotechnologyNanotechnologyNanotechnology is the study of manipulating matter on an atomic and molecular scale. Generally, nanotechnology deals with developing materials, devices, or other structures possessing at least one dimension sized from 1 to 100 nanometres...
- Organic light-emitting diodeOrganic light-emitting diodeAn OLED is a light-emitting diode in which the emissive electroluminescent layer is a film of organic compounds which emit light in response to an electric current. This layer of organic semiconductor material is situated between two electrodes...
- Potential wellPotential wellA 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 because it is captured in the local minimum of a potential well...
- Quantum dotQuantum dotA quantum dot is a portion of matter whose excitons are confined in all three spatial dimensions. Consequently, such materials have electronic properties intermediate between those of bulk semiconductors and those of discrete molecules. They were discovered at the beginning of the 1980s by Alexei...
- Spectral linewidthSpectral linewidthThe spectral linewidth characterizes the width of a spectral line, such as in the electromagnetic emission spectrum of an atom, or the frequency spectrum of an acoustic or electronic system...