A quantum dot display is an experimental type of display technology. Quantum dots (QD) or semiconductor nanocrystals could provide an alternative for commercail applications such as display techno logy. This display technology would be similar to organic light-emitting diode (OLED) displays, which would enable more efficient displays.
Quantum dots could support large, flexible displays but would not degrade as readily as OLEDs, theoretically making them good candidates for flat-panel TV screens, digital cameras, mobile phones and personal gaming equipment. At present, they are used only to filter light from LEDs to backlight LCDs, rather than as actual displays.
properties and perfromance are detemined 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.
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 lght 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 spaces, 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 wavelenght 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 structuer 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 the 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 eletron-transporting and hole-transporting organic materials. An applied eletric 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 challenger 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 eletron-transport layer(ETL). By making an emissive layer layer in a single layer of quantum dots, eletrons 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 materoals. Most organic electroluminescent materials favor injection and transport of holes rather than eletrons. 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 hole from approaching thr cathode, a hole-blocking layer plays dual roles in blocking holes moving towards the cathode and transporting the eletrons to the emitting QD layer. Tris-aluminium (Alq3), bathocuproinr(BCP), and TAZ are the most commonly used hole-blocking materials. These materials can be used as both eletron-transporting layer and hole blocking layer.
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