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How quantum dots enhance the color spectrum in LCD displays

LCDs might be one of the oldest display technologies around, but going quantum has helped them paint a whole new picture.

Like most decades-old technologies, liquid-crystal displays (LCDs) had a fairly unglamorous start. The first LCDs emerged around the late 1960s, and were used mainly to represent the numbers on digital watches and pocket calculators. It would take almost 20 years for engineers to transform LCDs from one-dimensional arrays into two-dimensional, active-matrix displays. And it would take another dozen years to make the first LCD manufacturers’ dreams a reality; to create a wall-mountable display that could rival the cathode-ray tube television.

Since then, the LCD has been steadily refined to generate increasingly crisper, brighter, and overall more pleasing images. While different companies have put their own twist on how LCDs look and function, the principles are largely the same across the board.

A panel of white LEDs is used as the light source for image generation. This light contains a broad spectrum of wavelengths, including all colours in the visible and near-infrared ranges (Figure 1(a)). Each pixel within an LCD contains three colour filters, which allow red, blue, or green light to pass through to reach the viewer. When these subpixels allow all of the LED backlight through, the full pixel appears white. When the subpixels block all of the backlight, the pixel appears black. The trick to generating different colours is modulating the intensity of light that each subpixel allows to pass through.

That is where liquid crystals come in.

Liquid crystals are, quite literally, substances made up of many crystals that collectively behave as a liquid. Imagine each crystal as a tiny grain of rice. Spreading these grains into a thin layer will mean that some grains are oriented vertically, some are oriented horizontally, and all others oriented along some angle in between. This, it turns out, makes for a great light filter.

Sandwiched between two polarisers, a liquid crystal film can allow light with intensity values ranging from 0 to 255 to pass through the individual subpixels in an LCD. This is made possible by an electric field, which alters the orientation of the crystals in the film. Because the light intensity penetrating each subpixel can be altered individually (and each distinct combination of light intensity represents a unique colour), a standard LCD can produce around 16 million (256 x 256 x 256) different colours.

That means that LCDs cast a wide net across the full range of colours visible to the human eye, also known as the colour gamut.

But there’s a limit to how far traditional LCDs can stretch that colour net. And that’s due to the LED backlight. Because ultimately, a display’s output is only as vivid as its light source, and despite being broad-spectrum, the white LEDs in an LCD produce rather diffuse light that must pass through several filters. Overall, that makes for sub-optimal picture quality.

So how are modern LCD manufacturers dealing with this problem? By simply adding a quantum-sized tweak to the traditional backlighting system.

Instead of a single layer of white LEDs, the backlighting of next-generation LCDs is deconstructed into two layers: a panel of LEDs that shine blue light and a polymer film of quantum dots (QDs) that convert that light into red and green light (Figure 1(b)). Unlike the amalgam of diffuse blue, red, and green light emitted by an all-white LED, this deconstructed system produces high-intensity light of each wavelength. Due to the highly pure light generated by the QDs, these LCDs can produce around 25 million colours- about 50% more than a conventional LCD (Figure 2). This is a result of higher colour gamut coverage of QD LCD than standard LCD in CIE chromaticity diagram space (Figure 1(c)). While standard LCD systems can cover up to 70-75% of Rec2020 colour standard, QD LCD systems can reach up to 85 % of the colour standard. That translates to wider coverage of the colour gamut, higher contrast, and a brighter image—all while using less energy.

Figure 1. LCD schematic: a) conventional, b) with quantum dots, c) color gamut coverage of LCD
Figure 2. Images of conventional (left) and quantum dot (right) LCD displays (image source)

The quantum dots commonly used in today’s QD LCDs are made of cadmium selenide (CdSe) and indium phosphide (InP). Quantum Solutions is currently developing a new class of QDs for display applications made from perovskite quantum dots.

“Perovskite” refers to a crystal structure with the formula ABX3. The structure is composed of four octahedra containing a halogen (e.g., chlorine, bromine, or iodine) at their vertices and a transition metal (e.g., lead or tin) at their centre. At the center of the four octahedra is a cesium ion or an organic formamidinium [CH(NH2)2+] or methylammonium [CH3NH3+] ion [4].

Perovskites are garnering tremendous interest in the world of photovoltaics, due to their combination of broad absorption, a strong ability to shuttle charge carriers, high defect tolerability, and ease of manufacturing.

Perovskite QDs offer higher efficiency and colour purity than their CdSe and InP counterparts, and could represent the future of LCD technology. The technology is promised to deliver 80 – 95 % of Rec2020 standard colour gamut coverage, more than any other quantum dot technology.

Useful links

https://www.displaydaily.com/article/display-daily/it-s-2020-so-where-is-rec2020

https://www.wired.com/2015/01/primer-quantum-dot/

https://news.samsung.com/global/why-qled-matters-for-4k-hdr-content

https://quantum-solutions.com/blog/what-are-perovskite-quantum-dots/

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