Displays - Page 7

China-based display maker China Star (CSoT, a subsidiary of TCL) demonstrated a 6.6-inch 384x300 OLED display that uses perovskite quantum dots as a color conversion film.

CSoT is using blue OLED emitter materials, and a perovskite layer to up-convert the color to green (this is a monochrome prototype - evidently a very early prototype). CSoT brands its perovskite-OLEDs as PE-OLED and we believe this is the first time a perovskite-enhanced display has been publicly demonstrated.

Researchers at NTU, lead by Assoc. Prof. Wang Hong, recently demonstrated high light extraction efficiency of perovskite photonic crystals fabricated by delicate electron-beam lithography.

The perovskite photonic crystals exhibit both emission rate inhibition and light energy redistribution simultaneously. They observed 7.9-fold reduction of spontaneous emission rate with a slower decay in perovskite photonic crystals due to photonic bandgap effect (PBG).

What are Quantum Dots?

Quantum dots (QDs) are semiconducting nanocrystals that are very small ' only a few nanometres in size. In display technologies, the most common types of QDs used are composed of a metal chalcogenide core. These QDs have the chemical formula XY ' where X is a metal and Y is sulfur, tellurium or selenium (e.g. CdTe, CdSe, ZnS) ' which is encased with the shell of a second semiconductor (e.g. CdSe/CdS). Their tiny dimensions mean that charge carriers are confined in close proximity, which gives QDs optical and electronic properties that are substantially different from those of large semiconductor crystals.

QLEDs vs OLEDs

In particular, QDs have enhanced light absorption and emission, making them particularly suitable for display technologies. Metal chalcogenide quantum dots (MCQDs) have already made it into commercial products ' most notably, in Samsung's QLED television range. Here, a blue LED backlight excites a layer of quantum dots on an LCD panel, causing them to emit light. The color of light emitted by the quantum dots depends on their size ' with small dots emitting blue light, and progressively larger dots emitting green, yellow, orange, and red light.

Left: Core-shell quantum dot structure. Right: The size of the dot defines the color of light that the dot emits. (Source: Ossila.com)

Quantum Dots are used today in the display industry to enhance the quality and efficiency of LCD-based displays, most notably in TVs (one example is Samsung's premium QLED TV range). While these are still LCD displays enhanced by QDs, quantum dots also have the larger potential to create truly emissive displays (QD-LED) which could compete with OLEDs and even surpass them in quality, efficiency and ease of production.

Several companies (including Samsung, BOE, LG, CSoT and others) are indeed developing QD-LED displays (Samsung, interestingly, is preparing to kick-start hybrid QD-OLED TV pilot production next year). Apple is not left behind, and the company is already known to be looking into QD-LED technologies.

National University of Singapore researchers have developed a perovskite-based color-enhancement film that may enable richer and more natural colors to next-generation flat-panel electronic displays. The research team is currently working with display companies to commercialize the perovskite color-enhancement film, and hopes to see the technology in consumer electronic products within the next two to three years.

Current commercial display technologies such as OLEDs (organic light-emitting diodes) and QLED (quantum dot light-emitting diodes) can only produce slightly more than 50% of the colors visible to the human eye. This limits the color reproduction that these displays can achieve. A research team from the Department of Chemistry and the Solar Energy Research Institute of Singapore (SERIS) at NUS has developed a color-enhancement film that could allow future display technologies to produce more than 75% of all visible colors. This technology is enabled by using perovskites, which can be tuned by changing its chemical composition to emit light strongly and efficiently in a variety of colors. To make the enhancement films, the research team mixed manometer-sized crystals of the perovskite material with a liquid monomer (precursor of plastics), and triggered a polymerization reaction by illuminating the mixture with white light.

Saudi Arabia-based Quantum Solutions demonstrated its perovskite quantum dots (with a focus on its green-colored ones) at SID Displayweek 2018, in addition to its Lead-Sulfide (PbS) QDs.

Quantum Solutions says it uses a flow reactor to create uniform and high-quality QDs at high yields and minimal waste. The company also develops encapsulation technology to protect the perovskite QDs. Their current materials have a lifetime of around 8,000 hours.

China-based QD developer Zhijing Nanotech demonstrated its perovskite-QD film (PQDF) at Displayweek 2018. The company showcased a 55" LCD TV that uses the film to convert the blue LED color to a white background.

Zhijing Nanotech says that its low-cost PQDF can be used to achieve a high 110%+ NTSC color gamut and a high efficiency.

Switzerland-based Avantama demonstrated its perovskite quantum dots at Displayweek 2018. QDs are currently used as color down-conversion films to turn the emission of blue LEDs to white light.

Currently used QDs are either Cadmium-based or Indium-based, and Avantama claims that its pQDs outperform both technologies by a wide margin (3X CdSe, 12x InP), which means that using these will enable much more efficient QD-LCDs. Of course pQDs contain lead, but the amount is very small and it is way below the thresholds required by the EU and other countries.

UK-based Ossila provides components, equipment and materials to enable faster and smarter organic electronics research and discovery. Ossila provides both materials and equipment for perovskite researchers, and the company's lead perovskite scientist, Dr. Jonathan Griffin, was kind enough to answer a few questions we had for him.

Thanks to improved knowledge about salt-solvent interactions, single crystals of perovskites can now be grown. Pictured above are several single-crystal MAPbBr perovskites, alongside the seed crystals used to grow these crystals

Dr. Griffin holds nearly a decade of experience working in organic photovoltaic research and over 5 years of working with perovskites. At Ossila, Jonathan works on technical support for several material ranges, including perovskites, organic photovoltaics, graphene and other 2-D materials. He is also involved in the development of new test equipment and product ranges. Prior to this, he worked in a postdoctoral research position at the University of Sheffield.

Q: Thank you for your time Dr. Griffin. Can you detail for us Ossila's perovskite product range in general?

Fuji Pigment recently reported that it is researching and developing a new type of perovskite quantum dots. Fuji stated that the half width of their emission spectra is substantially narrower than that of InP; this property could very beneficial to the application of the dots in display materials, LED, bio-imaging and more.

emission spectra of perovskite quantum dots under 420 nm of irradiation light

The chemical composition of perovskite quantum dots are either CsPbX3 or CH3NH3PbX3 (X= Cl, Br, I). Their quantum efficiency is 50'80 % and their half width is 15'39 nm. Their base solvent is either hexane or toluene. However, finding alternative solvents is a challenge that is now being addressed.