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Cadmium-Free Quantum Dots Offer Vibrant Color for Liquid Crystal Displays

Photonics Spectra
May 2016
Cadmium-free quantum dots are a safer and more sustainable option for manufacturers and consumers, featuring rich and vibrant colors without the risks associated with toxicity.

STEVE REINHARD, NANOCO TECHNOLOGIES LTD.

Display manufacturers are constantly striving to satisfy consumers’ never-ending appetites for better and more true-to-life electronic displays. Accomplishing that has as much to do with color as it does with resolution. To do so, one technology that has been gaining increased momentum in the industry is quantum dots. While cadmium-based quantum dots have been around since the mid-1980s, more recent advancements in their chemical makeup are giving quantum dot technology new potential in the marketplace without performance trade-offs.

These microscopic nanocrystals — so tiny that 10,000 of them span the width of a human hair — emit one specific color when light passes through them. The advantage to the manufacturer is that the quantum dots can be finely tuned to a desired color as determined by their size to meet any given application requirement. For instance, the bigger dots, about 50 atoms thick, glow red, while the smallest — 30 atoms or so — glow green. Just a subtle tweak in the size of the particle can change its color across the entire spectrum.

Nanoco dry Cadmium-Free Quantum Dots in vials.


Nanoco dry Cadmium-Free Quantum Dots in vials. Photo courtesy of Nanoco Technologies Ltd.



Growing numbers of display manufacturers are incorporating these brilliant tiny semiconductors into their backlight units (BLUs), offering new levels of color purity over conventional phosphor-based light-emitting diode (LED) backlit liquid crystal displays (LCDs). Quantum dots offer the unique ability to efficiently absorb and convert light, typically that of a blue LED, into very specific reds and greens — offering more beautiful, vibrant color performance than traditional LCD technology. Traditional red and green have been difficult to mimic on screen, whereas quantum dot displays deliver richer reds and more vibrant greens.

Today, when referencing color gamut — the entire range of colors available on any device — it’s mostly in the range of 70 to 80 percent of the National Television System Committee (NTSC) color triangle. With quantum dots, you get 100 percent of that color range — making a sunset look all the more spectacular or the golfing green during the Masters more lifelike. Because of this bump in color performance, quantum dots allow LCD screens to finally rival OLED products, at just a fraction of the cost.

Quantum dots also present an easy solution for display manufacturers to integrate into their products. While quantum dots’ chemistry is fairly complicated, the capital required to integrate them into a manufacturer’s fabrication process is very minimal. Unlike OLED, which requires a complete and expensive overhaul of a production line, quantum dots leverage the existing LCD infrastructure, enabling OEMs to bring innovative new technologies to consumers without the high expense or complexity of creating new fabrication processes. Additionally, quantum dots require a tiny amount of energy to operate, making them more energy efficient.

Another benefit of quantum dots is their flexible nature. Because particles are easily tunable simply by changing their size, they can be cost-effectively customized for various display manufacturers’ products and specifications.

Three approaches to integrating quantum dots into LCD screens

To achieve the near-perfect color quantum dots can deliver to LCD display screens, there are three different approaches to consider:

On-chip, where the quantum dots are deposited directly into the LED package.

On-edge, where the quantum dots are integrated within a component, such as a thin glass tube that is positioned remotely from, but in close proximity to, the LEDs.

On-surface, where the configuration utilizes a remote quantum dot film that covers the surface area of the display.

Though it has the highest material consumption, the on-surface geometry offers the advantage of operating near room temperature and is more easily and cost-effectively incorporated into the encasing display architecture.

Using the on-surface method, quantum dots, incorporated into a film located in-between the LED light source and the LCD panel, are “excited” by light emitted from blue LEDs, transforming some of it into very pure green and red light. As a result, the LCD panel receives a richer white light and expands the range of color the display can reproduce. And because the quantum dot film is made by printing on a large area web, it can easily be cut down to suit a variety of display sizes from extra large HDTVs to smaller devices like monitors and tablets.

Traditional challenges with quantum dots

The majority of quantum dot products that seeded the display market initially contained II-IV compounds like cadmium selenide. Cadmium is a toxic heavy metal and its use is restricted under European and other environmental law because of its threat to both human health and the environment. For example, with its Restriction of Hazardous Substances (RoHS) Directive, the European Union limits the amount of cadmium, lead and mercury that can be included in electrical and electronic equipment placed in the European market. Cadmium is restricted to 100 ppm in homogeneous material, a figure 10 times less than that for mercury and lead. Anticipating further global restrictions in cadmium use, several major OEMs have stated they would launch products only using cadmium-free technology.

Nanoco Cadmium-Free Quantum Dots LCD TV stack.
Nanoco Cadmium-Free Quantum Dots LCD TV stack. Photo courtesy of Nanoco Technologies Ltd.

Other inherent challenges with quantum dots are the same as with any new material that’s introduced into the market: How do you physically make enough of the material that the industry can successfully “bolt” it into products? And how can those products become affordable?

Let’s address these issues.

Moving beyond cadmium

The presence of cadmium has hindered the broad adoption of quantum dots in devices, keeping display manufacturers from realizing the benefits of the technology and products out of consumers’ hands. However, research into the synthesis and mass manufacture of heavy- metal-free quantum dots is of growing interest.

Cadmium-free quantum dots offer a safer and more sustainable option for manufacturers and consumers, giving them the color benefit associated with the technology without the risks associated with toxicity or potential regulatory limitations. Cadmium-free quantum dots also open up potential not just for displays, but for many more applications including lighting, solar and biomedicine.

Nanoco Cadmium-Free Quantum Dots Orion light.
Nanoco Cadmium-Free Quantum Dots Orion light. Photo courtesy of Nanoco Technologies Ltd.

To address the market need for safer materials, Nanoco has developed a unique quantum dot matrix of semiconductor alloy that is cadmium free. By tailoring the structure of the quantum dot and allowing the strength of the bonding interactions to be manipulated, the quantum confinement effects are reduced. The result: Nanoco has made considerable advances in narrowing the full width half maximum (FWHM) and improving the photoluminescence quantum yield of its cadmium-free quantum dots, making them perform nearly identically in a LCD display as their cadmium counterparts.

To address the issue of making enough material, the company uses a manufacturing technique called the “molecular seeding method.” This process is different from traditional quantum dot synthesis because it is scalable and allows for large-batch production of high-quality cadmium-free quantum dots.

Scientist holding Nanoco Cadmium-Free Quantum Dots film for LCD.
Scientist holding Nanoco Cadmium-Free Quantum Dots film for LCD. Photo courtesy of Nanoco Technologies Ltd.

New applications: LED lighting, solar

It’s rare to see a material that is a true platform technology, meaning the same material can be used across multiple applications. But eliminating toxic materials like cadmium from the makeup of quantum dots has opened up a range of new application areas, proving the value of quantum dots.

One application area is LED lighting, where cadmium-free quantum dots provide outstanding color performance without sacrificing efficiency. Applications where color quality is an absolute must include high-end retail store lighting where consumers can determine precise colors of clothing and high-value goods; surgical lights that enable accurate diagnosis and smooth operations; supermarkets that showcase the freshness and quality of meat and produce; and in agricultural uses, where LEDs provide many benefits to the grow-light and horticultural lighting industry to further enhance chlorophyll absorption peaks and ensure healthier, higher-yielding plants.

Another application is solar. Nanoco has developed a method of fabricating an efficient solar cell using a copper-indium-gallium-diselenide (CIGS) nanoparticle ink that can be deposited on thin films that are inexpensive to produce. This technology advancement results in a dollar-per-watt figure of less than 35 cents a watt on the panel. Because solar panel manufacturers can use a simple low-cost printing technology to get the nanoparticles onto the substrate, the CapEx requirements to make these panels are low.

Further, large amounts of the CIGS material have been produced in an R&D setting, proving that the synthesis method is scalable.

There are also promising new applications in biomedicine, where researchers are testing quantum dots as a mechanism to colorfully illuminate tumors to improve the safety and efficacy of cancer surgery. Doctors could potentially use the quantum dots to illuminate molecules, which then bind themselves to cancer tumors, allowing the surgeon to easily distinguish the healthy from the diseased tissue. The quantum dots are more photostable than the organic dyes that surgeons use today, which means that the illuminated cells are brighter for longer periods, giving the surgeon an extended time window to perform the surgery and improve results.

As legislation restricting the use of heavy metals gets ever tighter, cadmium-free quantum dots are uniquely positioned as the future-proof quantum dot display technology on the market. Thanks to this innovative technology, the future indeed looks more colorful — and safer — for both manufacturers and consumers of LCDs, and for the range of other application areas beginning to leverage heavy-metal-free quantum dots.

Meet the author

Steve Reinhard is vice president of business development for Nanoco Technologies Ltd., with over 14 years of experience in the display and lighting industries. He has an MBA from Binghamton University and earned his B.S. in industrial engineering from Pennsylvania State University; email: sreinhard@nanocotechnologies.com.


GLOSSARY
quantum dots
Also known as QDs. Nanocrystals of semiconductor materials that fluoresce when excited by external light sources, primarily in narrow visible and near-infrared regions; they are commonly used as alternatives to organic dyes.  
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