Quantum dot (QD) display technology

Quantum dot (QD) display technology

1. Introduction

Quantum Dot (QD) is composed of semiconductor materials, which look like a nanocrystal sphere. These nanocrystals emit light with absorbing light due to flow of electric current. The lightning wavelength differs depending on the dot size. Liquid crystal displays (LCD) alone is not capable of emitting its own light. There should be a light source or backlight. A light emitting diodes LED backlight used the three basic colors-blue, green, and red in order to come up with a white color light. Recently, one of the popular LED LCD colors is blue coated with a yellowish phosphor to achieve a white light color and QD LED TV is found as a more advanced version of this. The manufacturer no longer used the blue LED with yellowish phospor, but instead they use QD particles for achieving white color. The QDLED absorbs the emitted blue light then transfer it to red and green to creating the desired white color. This process is also known for achieving an accurate LED colors. OLED is composed of organic molecules made of thin films that create light with the use of electricity. This type of lightning technology is capable of creating crisper and brighter displays on electronic devices than the traditional (LCD) and (LED). Meanwhile, the Organic Light-emitting Diode (OLED) display is made of organic compound, which releases light with the presence of an electric current. The pixel is capable of emitting light on its own. It only means that this type of LED display have no issues regarding backlight leaking. Thus, Organic Light-emitting Diode is different from Quantum Dot Light-emitting Diode in terms of in expressing colors. QD LED uses LCD-based technology, which is slightly difficult to consider as the next generation display. However, OLED is considered as the next generation display, which is transparent, flexible and roll-able. It has relatively low processing temperature suited for a plastic substrate when you wish to make a flexible display and because it does not require a backlight as compared to QD, it can be optimized in creating a transparent generation display [1].

Quantum dot display

QLED means Quantum dot light emitting diodes and are a form of light emitting technology and consist of nano-scale crystals that can provide an alternative for applications such as display technology [2].Quantum dot (QD) is another promising material having unique features because they have narrow band emission, high photoluminescent efficiency, easy color control, and solution processability [3][4].

Quantum dots (QDs) are nanometer size semiconductor particles, in which the band gap is correlated to the size, shape, and composition of the particles. Quantum dots are crystals about 10 nanometres in diameter, made from a semiconduc­tor material, commonly cadmium selenide. They are so tiny that their shape and size affect the quantum properties of their electrons, in particular their energy gap the energy needed to kick electrons into a higher-energy band which determines the colour of light that the material can emit. Whereas a bulk semiconductor is limited to emitting a single colour of light, researchers can tune the precise colour a quantum dot will absorb and re-emit by tailoring its size.

Figure shows a schematic illustration of the relationship between the band gap and the particle size. The band gap decreases with increasing particle size. This is called the “quantum size effect”. Since the band gap is correlated to emission wavelength, the emission color can be controlled by the particle size [3] [4]. Therefore, by preparing QDs with small fluctuations of particle size, pure color emission with sharp emission spectra can be obtained. Typical particle sizes of quantum dots giving visible emissions are 2–8 nm. For example, a 3 nm QD emits saturated green light (λmax of ~535 nm FWHM of ~30 nm) and a 7 nm QD emits saturated red light (λmax of ~630 nm FWHM of ~35 nm) [3]. In a sense, QDs are highly efficient phosphor crystals. While there are several types of QDs, a typical material is core‐shell type QDs such as CdSe/ZnS. In addition, for wet process compatibility, colloidal quantum dots have been developed. Colloidal quantum dots usually have substituents combined on the shell surface. Due to these features of QDs, they have been studied in their application to displays and lighting. In their first generation, QDs have been down‐conversion materials. For example, quantum dots absorb relatively short wavelength light and emit a narrow spectral distribution with a peak wavelength at longer wavelength, based on the size of the quantum dots. Therefore, by pumping with blue light, green and red QDs emit photons in a narrow spectral distribution with a peak wavelength. By using the down‐converting property of QDs, the spectra of backlights for LCDs become sharper, giving high color purity with a combination of color filter of LCDs [3]. In other word, QDs can make sharp the spectra of LEDs that are used as backlights. In the second generation, QDs have been applied to light emitting devices in which the device structure is similar to OLEDs but the emission occurs from QDs instead of organic molecules. The device is called a quantum dot light emitting diode (QLED). In addition, due to the wet process ability of QDs, QLEDs can be applied to wet and/or printable fabrication process [3].

Differences:

Quantum dots with narrow emission at controllable wavelengths have great advantages compared to conventional phosphors. However the previous QD-LEDs showed only 7 lm W^-1 efficacy, and the peak emissions of red and green lights were not optimized for displays. The low efficacy of QD-LEDs would result in initial low QEs for the QDs as well as the decrease of QE during processing for device fabrication [5].

It looks like whether the color producing crystals are organic or inorganic is the primary difference. As we know, many of the first LED had blue phosphors that only lasted about 7000 hours and we suspect the red and green phosphors were not much better. Now, those numbers have increased considerably. But LEDs are made from rare earth materials and thus are expensive to produce as those materials become harder to get. Qds are made from inorganic semiconductor crystal substance, which is one of the reason manufacturers are probably excited about the technology. It will likely be much less expensive to produce. Another difference is that LEDs directly emit light whereas Qds conduct pass through light, thus LEDs would theoretically have slightly better viewing characteristics in this regard from the off angle viewing perspective. Supposedly, many of the same companies that develop LED technology are supporting QD. Sony, Samsung, LG and Sharp are rumoured to be the main TV manufacturers assisting with the development of the technology [6].

Conclusion:

Though the structure of QLED is quite similar to OLED, you can easily tell the difference between the two by checking its light emitting centers which is made of cadmium selenide nanocrystals or simply called quantum dots. QLEDS are known for having less manufacturing costs and lower power consumption. QLED’s manufacturers claim that QLED TVs are more power efficient than OLEDS with same color purity. When OLED hit the market, it was the absolute, most perfect TV technology ever. The type of picture quality that it produced was simply not seen with any previous technology. It can produce a wide range of colors, deep blacks, and superb contrasts that render brilliant pictures. QLED, as an improvement over OLED, significantly improves the picture quality. QLED can produce an even wider range of colors than OLED, which says something about this new tech. QLED is also known to produce up to 40% higher luminance efficiency than OLED technology. Further, many tests conclude that QLED is far more efficient in terms of power consumption than its predecessor, OLED that QLED is far more efficient in terms of power consumption than its predecessor, LED [1].

References

[1]        A. M. Bagher, ‘Quantum Dot Display Technology and Comparison with OLED Display Technology’, p. 6.

[2]        A. M. Bagher, ‘Solar cell quantum dots’, Am. J. Renew. Sustain. Energy, vol. 2, no. 1, pp. 1–5, 2016.

[3]        M. Koden, ‘OLED Displays and Lighting’, p. 235.

[4]        M. Bruchez, M. Moronne, P. Gin, S. Weiss, and A. P. Alivisatos, ‘Semiconductor Nanocrystals as Fluorescent Biological Labels’, Science, vol. 281, no. 5385, pp. 2013–2016, Sep. 1998, doi: 10.1126/science.281.5385.2013.

[5]        Hsueh-Shih Chen, Cheng-Kuo Hsu, and Hsin-Yen Hong, ‘InGaN-CdSe-ZnSe quantum dots white LEDs’, IEEE Photonics Technol. Lett., vol. 18, no. 1, pp. 193–195, Jan. 2006, doi: 10.1109/LPT.2005.859540.

[6]        B. S. Mashford et al., ‘High-efficiency quantum-dot light-emitting devices with enhanced charge injection’, Nat. Photonics, vol. 7, no. 5, pp. 407–412, May 2013, doi: 10.1038/nphoton.2013.70.