Televisions based on organic light emitting diodes (OLED) and quantum dots (QLED) might not be the only or even best ways of reaching a wide color gamut performance for long.

Lumenari, a five-year-old company specializing in narrow-band green phosphor formulation called “Emerald” for LED back lights, may soon help 4K LED LCD TVs cover a wider color gamut with better efficiency.

Lumenari’s CEO Robert Nordsell (pictured at top) told HD Guru that the “Emerald” phosphor currently in development will help televisions achieve close to covering 100% of the DCI-P3 color space recommendations for professional cinemas, and 80% of the aspirational Rec. 2020 color space for next-generation television sets.

One of the last remaining obstacles has been getting Emerald to meet the 30,000 hour to half brightness requirements of TV manufacturers, which was the target for a recently completed successful funding effort.

The majority of wide-color gamut LED LCD TVs that do not use quantum dots today, use phosphors to produce colors. The primary way to get the white back light is using a blue phosphor LED. Red and green phosphor coatings are then typically applied to the LED to generate a gamut of colors.

The green phosphor coating on LEDs used in lower (BT.709) gamut LCD TVs is a single broad-emitting phosphor. New wide color gamut architecture requires a discrete red and green phosphor that are as narrow band as possible.

“The material that we make is a green phosphor that enables an improvement of about 80% coverage of Rec. 2020. Current phosphor material is 68%. That’s close to 100% of the DCI-P3 color space, with greater coverage particularly of the green and cyan spaces,” Nordsell said. “One of the advantages or our green phosphor material is that you expand not only the green gamut point but the red gamut point as well.”

He said Emerald will enable LED LCD TVs that are at least equal to or better than quantum dot displays in gamut coverage, depending on the color filter selection of the television manufacturer.

Compared to current phosphor materials, the green phosphor under development in Emerald “will eventually emit more photons from an LED, so if an equivalent number of LEDs is used in the backlight compared to a current LED backlight, you get a brighter display,” he said.

As for brightness output, Nordsell said Emerald is at about 80% of its nearest competitor, “but so far we haven’t focused as much on improving that aspect. We basically make the material as quickly as we can to work on more of the downstream process R&D. We are doing a faster synthesis which results in lower quantum yields and lower brightness, but the result is faster turns, experimentally.”

Nordsell said the chemical composition of Emerald is proprietary, although it is something that has been known for a while. Luminari made “a few tweaks to it to get the performance of the material to where it is today. We’ve been working on this for a litter over a year now.”

He explained that before a phosphor is ready for a manufacturer to implement in an LED backlight chip it must achieve various performance qualifications. All phosphors must perform to the same minimum levels and complete both performance qualification and a reliability qualification. Reliability qualification is essentially performance lifetime.

“You have to make sure the materials are passivative enough, so that it doesn’t impact performance over a long period of time in the television,” he explained. “So, that’s the problem we have been working on solving and that’s what we have raised this funding for.”

“The longevity of our material is not where it needs to be and the display manufacturers won’t compromise the performance lifetime just for a new material,” Nordsell continued. “We have to meet the current spec, which varies from manufacturer to manufacturer, which is an L50 on the order of 30,000 hours. That means a 50% brightness degradation (half life) after 30,000 hours.”

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Comparatively, Nordsell said quantum dots were very hard materials to make meet the reliability specification, because quantum dots evaporate rapidly in the open air.

“Our materials are nowhere near that unstable,” he said. “Yet, quantum dots have been shown to have been made stable [through use of encapsulation in film], so this is a solvable problem for sure.”

“We have been working on making our materials reliable by using the similar encapsulation methods that quantum dots use, but with quantum dots you are encapsulating them in a film placed some distance above the LEDs themselves. Our end goal, however, is to passivate the phosphor powders themselves so that we can do direct deposition on the LEDs. Just as like the LEDs Sony uses in their televisions today,” Nordsell explained.

He said the advantage over quantum dots is the proximity of the material to the LED chip. With quantum dots a very large area film is required because the closer they get to the LED, the more the quantum dots are susceptible to thermal quenching and blue light quenching, meaning they become less efficient the closer they are positioned to the LED chip.

“Our material doesn’t suffer from that problem, so you can actually get the phosphor film into a much smaller area for a package-level solution rather than a display level solution. This means that rather than making a film that covers the entire area of the display, it would be a much smaller area that only covers the LEDs themselves,” Nordsell said.

Other competitive implementations blend phosphors into a a viscous silicone that is deposited directly onto the LED, and then cured under separate thermal and UV curing processes.

Nordsell said Emerald technology works equally well with full-array and edge-lit LED backlighting approaches, and can be used with 4K Ultra HD and 8K resolution levels.

“In the near term, our advantage using phosphor film is going to be performance based. The current state of the art phosphor materials can not achieve what our material can achieve,” he said. “We are working on having display manufacturers make investments in our approach.”

As for when we will see Emerald in working LED LCD televisions, Nordsell said probably not in 2019 product lines, “but I think we can reach our goals inside of 12 months. We have a good trajectory.”

 

By Greg Tarr

 

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