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Quantum dot display

 

Quantum dot display

A quantum dot display is a type of organic light-emitting diode (OLED) displays, in that light is supplied on demand, which enables more efficient displays.

Unlike 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.[1][2][3]

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.[4]

Colloidal quantum dots irradiated with a UV light. Different sized quantum dots emit different color light due to quantum confinement.

Contents

  • History 1
  • Working principle 2
    • Optical properties of quantum dots 2.1
    • Quantum dot light-emitting diodes 2.2
  • Fabrication process 3
    • Phase separation 3.1
    • Contact printing 3.2
  • Comparison 4
  • See also 5
  • References 6
  • External links 7

History

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.[5] Starting from early 2000, scientists started to realize the potential of developing quantum dot for light sources and displays.[6]

Working principle

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.[7]

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

  • Quantum Dots: Technical Status and Market Prospects
  • Quantum dots that produce white light could be the light bulb’s successor

External links

  1. ^ Quantum-dot displays could outshine their rivals, New Scientist, 10 December 2007
  2. ^ Quantum Dot Electroluminescence
  3. ^ Nanocrystal Displays
  4. ^ The future of cadmium free QD display technology (QD TV)
  5. ^ R. Victor; K. Irina (2000). "Electron and photon effects in imaging devices utilizing quantum dot infrared photodetectors and light emitting diodes". Proceedings of SPIE 3948: 206–219.  
  6. ^ a b P. Anikeeva; J. Halpert; M. Bawendi; V. Bulovic (2009). "Quantum dot light-emitting deices with electroluminescence tunable over the entire visible spectrum". Nano Letters 9 (7): 2532–2536.  
  7. ^ Saleh, Bahaa E. A.; Teich, Malvin Carl (5 February 2013). Fundamentals of Photonics. Wiley. p. 498.  
  8. ^ Seth Coe;Wing-Keung Woo;Moungi Bawendi; Vladimir Bulovic (2002). "Electroluminescence from single monolayers of nanocrystals in molecular organic devices". Nature 420 (6917): 800–803.  
  9. ^ Tetsuo Tsutsui (2002). "A light-emitting sandwich filling". Nature 420 (6917): 753–755.  
  10. ^ Yue Wang et al. (2009). "Photophysical and charge-transport properties of hole-blocking material-TAZ: A theoretical study". Synthetic Metals 159 (17–18): 1767–1771.  
  11. ^ Coe-Sullivan, Seth; Steckel, Jonathan S.; Kim, LeeAnn; Bawendi, Moungi G.; Bulovic, Vladimir (2005). "Method for fabrication of saturated RGB quantum dot light emitting devices". Progress in Biomedical Optics and Imaging 5739: 108–115.  
  12. ^ Coe-Sullivan, Seth; Steckel, Jonathan S.; Woo, Wing-Keung; Bawendi, Moungi G.; Bulovic, Vladimir (2005). "Large-Area Ordered Quantum Dot Monolayers via Phase Separation During Spin-Casting". Advanced Functional Materials 15 (7): 1117–1124.  
  13. ^ a b Kim, LeeAnn;Anikeeva, Polina O.;Coe-Sullivan, Seth; Steckel, Jonathan S.; Bulovic, Vladimir (2008). "Contact Printing of Quantum Dot Light-Emitting Devices". Nano Letters 8 (12): 4513–4517.  
  14. ^ Quantum Dot Pros and Cons

References

See also

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.[14]

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 often 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.

Comparison

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.[13]

  • 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.

The overall process of contact printing:

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.[13]

Contact printing

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.

[12] 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

Phase separation

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.[11]

Fabrication process

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.

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.[10]

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.[9]

[8][6]

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