Quantum dot display
|This article is outdated. (September 2013)|
A quantum dot display is a type of display technology. Quantum dots (QD) or semiconductor nanocrystals can provide an alternative for applications such as display technology. This display technology is similar to organic light-emitting diode (OLED) displays, in that light is supplied on demand, which enables more efficient displays.
Unlike light-emitting diodes (LEDs), organic electroluminescent devices can be deposited over larger areas and on flexible or non-planar substrates. Large area display or general illumination devices using OLEDs are available commercially. However, the light emitting organic molecules tend to degrade and are sensitive to humidity and oxidation. Quantum dots can support large, flexible displays but do not degrade, making them candidates for flat-panel TV screens, digital cameras, mobile phones and personal gaming equipment.
Properties and performance is determined by the size and/or composition of the QD. QDs are both photo-active (photoluminescent) and electro-active (electroluminescent) allowing them to be readily incorporated into new emissive display architectures.
The idea of using quantum dot 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 from early 2000, scientists started to realize the potential of developing quantum dot for light sources and displays.
Optical properties of quantum dots
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 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.
Quantum dot light-emitting diodes
Quantum-dot-based LEDs are characterized by pure and saturated emission colors with narrow bandwidth. 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 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. 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.
The challenge of bringing electrons and holes together in small regions for efficient recombination to emit photons without escaping or dissipating was addressed by sandwiching a thin emissive layer between a hole-transporter layer (HTL) and an electron-transport layer (ETL). By making an emissive layer in a single layer of quantum dots, electrons and holes may be transferred directly from the surfaces of the ETL and HTL, providing high recombination efficiency.
Both ETL and HTL consist of organic materials. Most organic electroluminescent materials favor 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 the cathode, a hole-blocking layer plays dual roles in blocking holes moving towards the cathode and transporting the electrons to the emitting, QD layer. Tris-Aluminium (Alq3), bathocuproine (BCP), and TAZ are the 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. 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 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 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 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 structure are solvent-sensitive organic thin films, avoiding solvent during 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 (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.
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 minimizes the amount of QD required, reducing costs. The demonstrated color gamut from QD-LEDs exceeds the performance of both LCD and OLED display technologies.
Nanocrystal displays 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 don't need backlighting. QD LEDs are 50~100 times brighter than CRT and LCD 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 are inorganic, offering the potential for improved lifetimes compared to OLED. (However, since many parts of QD-LED are made of organic materials, further development is required to improve the functional lifetime.) Resolution can also be higher.
Other advantages include better saturated green colors, manufactureability on polymers, thinner display and that the same material used to generate difference colors.
However, 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, a display needs to produce approximately equal luminosities of blue, red and green. The human eye requires blue to be about 5 times more luminous than green, requiring 5x more power.
Compared to LCD and OLED, the manufacturing cost of QD-LED for the Kindle Fire HDX was relatively high.
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- Quantum Dot Electroluminescence
- Nanocrystal Displays
- The future of cadmium free QD display technology (QD TV)
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