Quantum dot display
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A quantum dot display is a type of display technology used in flat panel displays 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 tubes (CRTs), liquid crystal displays (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 and allows for 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 cameras, mobile phones, 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.
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.
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, CdSe 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 levels 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 (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.
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 is a fabrication technique suitable for forming large area of ordered monolayers of QDs. A single layer of QDs is formed by spin casting a mixed solution of QD and TPD. 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 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 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 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:
- 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.
- After the solvent evaporates, the formed QD monolayer is transformed onto 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 µm 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 and cons
- 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.
- Brightness: 50~100 times brighter than CRT and LCD displays ~40,000 cd/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.
- 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 luminous than green; have 5 times more power).
Commercialization of quantum dot display has just started and the Kindle Fire HDX 7 is the first product using the technology. 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.
Many expect that quantum dot display technology can compete with or even replace liquid crystal displays (LCDs) in the near future, including the desktop and notebook computer spaces and televisions. Other than display applications, several companies are manufacturing QD-LED light bulbs; these promise greater energy efficiency and longer lifetime.
- Quantum-dot displays could outshine their rivals, New Scientist, 10 December 2007
- Quantum Dot Electroluminescence
- Nanocrystal Displays
- The future of cadmium free QD display technology (QD TV)
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