Quantum dot light emitting diode

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Quantum dot light-emitting diodes - also briefly QLEDs called - are electro-optic semiconductor components , which by means of quantum dots ( English quantum dots , QDs) convert electrical energy into light with unique optical properties and radiate. In a broader sense, this also includes traditional light-emitting diodes , the light of which is converted by quantum dots. Depending on the structure of the QDs, the emission color can be selected across the entire spectrum of visible light. As a result, QLEDs can produce almost any color on the CIE diagram . This offers more color options and better color rendering than white LEDs, since the emission spectrum is much narrower, which is characteristic of quantum-limited states.

Quantum dots for light conversion

Photo-excitation with a traditional primary light source LED (typically blue or ultraviolet, i.e. light emitting short wavelengths) is used in this case. An example of the photo-excitation principle is a method developed by Michael Bowers at Vanderbilt University in Nashville , in which a blue LED is coated with quantum dots that glow white depending on the blue light of the LED. This process emits a warm, yellowish-white light similar to that produced by incandescent lamps .

In February 2008, the use of nanocrystals resulted in a luminous efficiency of 300 lumens visible light per watt of radiation (not per electrical watt) and warm light emission.

In February 2011, scientists at PlasmaChem GmbH were able to synthesize quantum dots for LED applications and use them to build a light converter that could efficiently convert light from blue to any color over many hundreds of hours. Such QLEDs can be used to emit visible or near infrared light of any wavelength that is excited by light of a shorter wavelength.

Quantum dots are also used for improved white backlighting of liquid crystal displays (so-called LCD televisions ). With the help of nano-semiconductor crystals applied as a layer on a glass plate or foil and irradiated by blue, short-wave LEDs (e.g. made of gallium nitride ) from behind or from the side, the spectrum of the background lighting of LCDs can be optimized in a previously unattainable way (technical term extended color space ). This form of fluorescence technology is being further developed in various laboratories. The US company 3M, a major supplier of backlighting components for LCDs, is working with Nanosys Inc. on appropriately coated foils. The British company Nanoco Group PLC has also been active in the further development of quantum dots for several years and has concluded cooperation agreements with Asian manufacturers of liquid crystal displays. The US company QD Vision also works with Asian companies that optimize the background lighting of LC displays. The manufacturer Samsung Electronics uses - in contrast to the competition - QLEDs instead of OLEDs in television technology. Samsung Electronics wants to offer its high-quality televisions under the name QLED in order to point out the technology with quantum dots. However, these televisions are still backlit liquid crystal displays (LCD screens). This is intended to prepare the transition to the new screens with actual self-illuminating QLEDs that are still being developed at Samsung.

Actual quantum dot light emitting diodes

In the case of these light-emitting diodes, light of a certain wavelength is generated in the component itself by an electro-optical effect of the QDs. That is, the excitation of the QDs is done by externally supplied electrical energy. The structure of QLEDs is similar to the basic construction of OLEDs. A layer of quantum dots is sandwiched between layers of electron transport and hole transport materials. An applied electric field causes electrons and holes to move into the quantum dot layer and recombine, thereby emitting a photon. This principle has been investigated for quantum dot displays.

The ability to tune the emission wavelengths and the narrow bandwidth is also advantageous as excitation sources for fluorescence imaging. The scanning near field optical microscope (NSOM) was shown using an integrated QLED.

Individual evidence

  1. ^ H. Neidhardt, L. Wilhelm, VA Zagrebnov: A new model for quantum dot light emitting-absorbing devices: proofs and supplements . In: Nanosystems: Physics, Chemistry, Mathematics . tape 6 , no. 1 , 2015, p. 6–45 , doi : 10.17586 / 2220-8054-2015-6-1-6-45 ( PDF ).
  2. ↑ Quantum Dot Displays. Infotip, accessed on January 14, 2017
  3. ^ VL Colvin, MC Schlamp, AP Alivisatos: Light-emitting diodes made from cadmium selenide nanocrystals and a semiconducting polymer . In: Nature . tape 370 , no. 6488 , August 4, 1994, p. 354-357 , doi : 10.1038 / 370354a0 .
  4. ^ Accidental Invention Points to End of Light Bulbs , LiveScience.com. October 21, 21. Retrieved January 24, 2007. 
  5. ↑ Mason Inman: Crystal Coat Warms up LED Light . February 1, 2008. Accessed January 30, 2012. 
  6. ↑ Quantum dot light emitting diode . In: Nanotechnology Current . No. 4 , 2011, ISSN  1866-4997 , p. 98-99 .
  7. ↑ Nanoco Signs Agreement with Major Japanese Electronics Company. Nanoco Group, September 23, 2009
  8. ↑ Display backlighting. Nanosys Inc.
  9. ↑ P. Patel: Quantum dots are behind new displays . In: IEEE Spectrum . tape 49 , no. 8 , 2012, p. 14-17 , doi : 10.1109 / MSPEC.2012.6247547 .
  10. ↑ Color IQ Tech
  11. ↑ Mason Inman: OLED or QLED? . 5th January 2017. 
  12. ↑ Geoffrey Morrison: How QLED TV could help Samsung beat LG's OLEDs. CNET News, June 30, 2016, accessed January 13, 2017.
  13. ↑ QLED versus OLED, Samsung is fueling system dispute with TV technology. Die Wirtschaftswoche , January 5, 2017, accessed on January 13, 2017
  14. ↑ Quantum-dot LED may be a screen of choice for future electronics. Massachusetts Institute of Technology News Office, December 18, 2002
  15. ↑ Kazunori Hoshino, Ashwini Gopal, Micah S. Glaz, David A. Vanden Bout, Xiaojing Zhang: Nanoscale fluorescence imaging with quantum dot near-field electroluminescence . In: Applied Physics Letters . tape 101 , no. 4 , 23 July 2012, p. 043118 , doi : 10.1063 / 1.4739235 .