Display Industry Ready for New EU Cadmium Regulations

Abstract

IN LATE MAY, THE EUROPEAN UNION (EU) TIGHTENED ITS restrictions on the use of the toxic metal cadmium in electronic devices, updating a rule that had been in place since 2017. But the new EU regulations should not have a big impact on the display business, say industry insiders, as most manufacturers are already building products on a global basis that comply with the new limits.

Cadmium first made its way into displays over a decade ago in the production of quantum dots (QDs), semiconductor crystals that generate different colors when subjected to light or electricity depending on their size. QDs, which also can be made from other materials such as indium phosphide, are widely used to perform color conversion in LCD displays by “downshifting” white or blue light from an LED backlight to red or green. The result is vivid color reproduction, including high dynamic range (HDR) with wide color gamut.

Cadmium also is one of the elements and compounds regulated by the European Commission (EC, the EU's executive arm) through its Restriction of Hazardous Substances (RoHS) rules, which began in 2006 to limit the use of hazardous chemicals in consumer electronics. The RoHS rules (specifically Directive 2011/65/EU) govern the use of substances such as lead and mercury and generally set limits on a parts-per-million (ppm) basis. The limit for lead and mercury in a “homogenous part” is 1,000 ppm, for example, while cadmium is set at 100 ppm.

Exemptions to the RoHS rules are possible if a strong argument can be made for using a restricted material over a substitute, such as significant performance, efficiency, or related environmental benefit (e.g., less electricity consumption). Such exemption requests usually are evaluated by an outside party, which then makes a recommendation to the EC.

Cadmium has been used in lighting and displays under such an exemption (Exemption 39), which was granted by the EC in 2011 after a lengthy evaluation and recommendation by the Oeko Institute in Germany. At the time, QDs based on cadmium selenide were far more efficient and reliable than any alternative and were being developed for use in encapsulating films by companies such as 3M. The EC granted an exemption allowing “10 micrograms of cadmium per square millimeter of light-emitting area,” or 10 grams per square meter, which is roughly the size of a 65-inch TV.

While the units were different, this worked out to be much more cadmium than the standard 100 ppm limit under RoHS, said Peter Palomaki, owner and chief scientist at Palomaki Consulting. In 2017, the exemption was subsequently modified with Exemption 39(a) to allow only 0.2 micrograms per square millimeter (or 0.2 grams per square meter) of light-emitting area. Palomaki concedes that the differing units of measurement in the EU rules can be confusing.

“The 0.2 grams per unit area, per square meter, is basically saying how much you can use over the whole screen area, which is a very different measurement than a ppm measurement, which is a mass divided by a mass,” explained Palomaki. “And the ppm measurement speaks to a part, a homogenous part, something that can't be physically broken down into small parts.”

According to Palomaki, a QD enhancement film (QDEF) does not count as a homogenous part because it consists of three layers that could be pulled apart: a top layer, QD middle layer, and bottom layer. Palomaki considers the QD middle layer to be the “homogenous part” subject to the 100 ppm rule. That viewpoint also is shared by leading QD supplier Nanosys-Shoei.

“We go by that interlayer, we think that's the right way to do it; that's the spirit of the way it was written,” said Jeff Yurek, vice president of marketing at Nanosys, Shoei Chemical. “But there's been some debate on that.”

While the 0.2 grams threshold under Exemption 39(a) originally was due to expire in October 2019, it has persisted into 2024 while the EC considered several requests to extend it from both lighting and display manufacturers. After another in-depth assessment and recommendation on cadmium usage was delivered from the Oeko Institute in late 2022, the EC solicited public comment on the new recommendation through 2023 before voting on it back in March.

The updated rule, Exemption 39(b), was not officially published until May 21. Manufacturers now have an 18-month grace period until November 21, 2025, to fully comply with the new rule.

“For the last seven years, it's been the 0.2 number that they have to abide by,” said Palomaki. “The 100 ppm is null and void, and it's the 0.2 that's the rule right now. In 18 months, the 0.2 goes away, and it rolls back to the 100 ppm, which is a broad standard for all of the products. The 100 ppm is true for your toaster oven, the microwave, and refrigerator you have. All consumer electronics have that 100 ppm EU RoHS requirement. So TVs and displays will just fall back to that standard 100 ppm limit for cadmium.”

While meeting the RoHS rule is only technically necessary to sell sets in EU countries, the EU is a big market, and both Palomaki and Yurek expect that manufacturers will make sure their sets comply with the new rule. In general, original equipment manufacturers (OEMs) would rather not make different models for different countries, said Yurek. That extends to Chinese manufacturers that are producing high volumes of LCD sets.

“They want to have a SKU they can ship around the world, and this is the hardest regulation to meet,” said Yurek. “So if we meet that, we're pretty much good anywhere else.”

Given the typical manufacturing cycle for TV sets, with new models being announced in January, hitting store shelves in late spring, and selling at volume during the holiday season, Nanosys is advising all of its clients to ensure their 2025 models are compliant with the new rule.

“And most of them are now, already,” said Yurek.

The changes to the cadmium exemption are significant in that the previous Exemption 39(a), to quote the EU directive, “does not distinguish between different configurations regarding the way cadmium-based material is embedded in the quantum dot.” The new exemption does.

According to an explanatory EC memorandum accompanying the directive, the three main applications of QDs are “on-edge technology,” where the QDs are incorporated into a remote component situated in close proximity to the LED chips; “on-surface technology,” where the QDs are encapsulated in a film that covers the complete display area; and “on-chip technology,” where the QDs are placed directly on the LED surface and encapsulated within its LED package.

Only this third on-chip application is now exempt from cadmium limits and only for use in displays, as per Exemption 39(b), which allows “cadmium in downshifting semiconductor nanocrystal quantum dots directly deposited on LED semiconductor chips for use in display and projection applications.” The limits are up to 5 micrograms of cadmium per square millimeter of LED chip surface and a maximum amount per device (i.e., display) of 1 milligram. The exemption expires for all categories on December 31, 2027.

In explaining its decision, the EC said that on-edge applications, such as Sony's early “Triluminos” edge-lit LCD TVs with QDs enclosed in a glass tube, have become obsolete. Meanwhile, cadmium-free QD alternatives for on-surface applications, which historically have used the most cadmium, now are readily available. There are also cadmium-free alternatives to on-chip technology available for solid-state lighting applications, said the EU, which is why lighting is not included in the updated exemption. But the development of cadmium-free on-chip technology for displays lags behind by comparison and may take another four to five years.

“For some new technologies, like micro displays, no market-ready cadmium-free alternatives or configuration alternatives that are as reliable as cadmium QD on-chip configurations currently exist,” said the EC.

While the changes in Exemption 39(b) would seem to have a big impact on current LCD and QD-OLED sets that use QDs, the reality is that the use of cadmium for QDs has been drastically reduced since the first QD-enabled TVs were introduced in 2013. For starters, some of the world's leading TV brands have never used cadmium in their QDEF sets, but instead have relied on QDs made with indium phosphide. This includes the market share leader Samsung, as well as its top competitors, LG and Sony.

A second flavor of QDEF product is one that contains some cadmium but is RoHS-compliant because the amount of cadmium falls under the 100 ppm limit. Those sets might have 90 ppm cadmium or even up to 99 ppm. Under the EU rules, the manufacturer is obligated to put a “cadmium-free” icon, a stamp of the cross through “Cd,” on the back of the set, just as it would for a set using indium phosphide QDs (Fig. 1).

These RoHS-compliant sets yield some of the benefits of cadmium, such as narrow peak line width (higher-purity color) and perhaps higher efficiency, without running afoul of the RoHS rules, noted Palomaki. But there could be some marketing pushback when consumers find out that the sets contain cadmium, as the “cadmium-free” icon is a bit misleading.

“By law, they are required to put a logo on the back of the device that says Cd with a slash through it on any display with under 100 ppm cadmium,” said Yurek. “So an OLED display has that, an LCD with indium phosphide has that, an LCD with cadmium under 100 ppm has that. We're very surprised by it; it's not how we talk about it, it's not what we say to customers or consumers. But it's how the EU talks about it.”

A third flavor of QDEF product is a display containing cadmium that is over the 100-ppm limit in the film layer part but is still under the 0.2 grams threshold for the entire device. Those TVs are exempt under RoHS for the next 18 months but will have to lower their cadmium concentration to comply after that.

“Those products are being sold right now in the market; you can find them in TVs in the US even,” said Palomaki. “I haven't measured the concentration; that's a more difficult measurement. But I can tell optically by just measuring them which ones contain cadmium and which ones don't.”

According to Palomaki, by using an optical spectrum analyzer, it is easy for him to tell what QDEF sets contain cadmium.

“I'm looking for how broad or narrow the red and green peaks are,” he said. “And there's a pretty clear threshold—indium phosphide can't get much below around 35 nanometers [the width of the peak]. And cadmium is regularly under 30, sometimes 25, or even sometimes 22 nanometers. So there's a pretty big difference between a cadmium QD and an indium phosphide QD that optically you can pretty much tell the difference right away.”

That being said, the final result in the viewer's eyes when looking at the TV is based on more than just the QDs, said Palomaki, regardless of what material they are made from.

“The color reproduction that we see at the front of the screen goes through a lot of other optical things besides the quantum dots,” he said. “Color filters, for just one example, can make a huge impact on color quality and brightness. And TCL's choice of color filter and Samsung's choice of color filter may be totally different, and it may lead to the final light that's coming out to our eyes being very different even if Samsung uses indium phosphide and TCL uses cadmium.”

Nanosys made a big splash in 2017 with its introduction of Hyperion, a new QD product that provided cadmium-like performance with 90 percent coverage of the BT.2020 color gamut standard. It met the RoHS limits by combining a cadmium-free red emitter with an ultra-low cadmium green emitter. But Yurek said his sense is that there is very little product using Hyperion still in the market, though he is not sure of exact inventory.

“What's happened is the [QDEF] film makers have been able to go with cadmium red and green combined and deliver under 100 ppm cadmium,” said Yurek. “And that's partly because of performance improvements in the QDs, and so they don't need quite as many QDs to get the performance out. So what we see are full cadmium solutions under 100 ppm.”

Nanosys’ lower-cost xQDEF extruded product, which combines diffuser plate technology with air-stable QDs, also uses cadmium, but it is well under the 100 ppm RoHS limit, said Yurek (Fig. 2). That is because the plastic diffuser plate, which is much heavier than a thin film, tilts the mass/mass equation in Nanosys’ favor.

“Those are cadmium, and by virtue of the way the RoHS regulation is written, those are miles below the 100 ppm limit because they are a heavier component,” said Yurek.

Nanosys has been making high volumes of indium-phosphide QDs for years, and it was one of the companies urging the EC to do away with the cadmium exemption for LCD sets. Yurek said that while cadmium QDs still have better performance by a small margin over indium phosphide QDs, the brands using them tend to be more cost-conscious and are not investing in components such as the dual brightness enhancement film (DBEF) that help make the TV brighter. Meanwhile, the brands using indium phosphide are optimizing their TVs to achieve the best overall performance.

“One of the things we showed them was that, in fact, if you go to buy a TV, the highest-performance TVs on the market are cadmium-free,” said Yurek. “That's because of other choices and variables. It's not the quantum dots.”

Both Yurek and Palomaki said it is unclear how the remaining on-chip exemption for cadmium will play out for microLED development, which is still in its nascent stages from a manufacturing point-of-view.

“It's hard because there's no accepted, widely used implementation strategy for QDs on microLED,” said Palomaki. “There's tons of different ideas. You could inkjet print right on top of each individual microLED. You could create a microLED with a reservoir around it and deposit the QDs into that reservoir that's kind of contained. That reservoir may have angled walls or straight walls, and all of this is going to change the amount of QDs that ends up being required to get the color conversion. But the die size stays the same. So the way in which you implement the color conversion could, in theory, change a lot from one company to the next.”

Either way, Palomaki expects that microLED manufacturers will make sure they are meeting the 5 microgram limit, or preferably the 100 ppm limit, “so they never have to worry about it again.”

Nanosys is already supporting cadmium-free color conversion for microLED in partnership with Applied Materials, which is using red, green, and blue cadmium-free QDs from Nanosys on top of ultraviolet microLEDs. Yurek said other customers also are looking at cadmium-free QDs for their microLED efforts, and that Nanosys is not doing any development of cadmium for either microLED color conversion or electroluminescent-quantum dot (EL-QD) display technology.

“We think moving forward, let's put the cadmium behind us and not live through that again,” said Yurek. “We did it already; we don't want to do it again, and this is the right thing to do.”