Purple LEDs to Replace Blue LEDs?

News Source: 
Aladdin Lighting Network

“It is only a matter of time before white LEDs using blue LED chips will disappear from the market,” said Shuji Nakamura at a forum on GaN technology in July organized by Nikkei Asian Review. The comment from the inventor of the blue LED chip and Nobel Laureate shocked the industry. Nakamura shares a Nobel Prize in Physics with two Japanese inventors Isamu Akasaki and Hiroshi Amano for their invention of the blue LED. Yet, Nakamura’s recent remark completely derails his previous accomplishments. Industry insiders in China try to decode Nakamura’s comments to determine whether it reflects embedded flaws in blue LEDs design, or whether his motive is to promote Soraa’s purple LEDs.

A comparison between Soraa three phosphor violet pump LED colors versus competitors using traditional blue LEDs for white lighting. (Photo Courtesy of Soraa)

Is Nakamura just trying to promote Soraa?

LEDs are basically narrowband light sources made from semiconductor components, capable of emitting wavelengths ranging from IR to UV rays. The first LED wavelength ranged from IR wavelengths to green lights. Prerequisites for blue LED research included high quality crystal growth technology, and P-type doping technology control in wide bandgap semiconductors. All these technologies emerged in the late 1980s, only then was GaN based systems realized. Additionally, highly efficient blue LED R&D also requires the manufacturing of different GaN-based compounds, while combining it with different types of substrate materials to create a multi-layer quantum dot well structure.

Excited by blue LED rays, phosphor coatings on the LEDs can emit different color of lights including green and red.  White light is created by combining these colors. Additionally, merging complimentary colored LEDs, such as red, green and blue can also create white light. These two technologies have been applied in highly efficient white light sources, and achieved significant energy saving results.

Nakamura pointed out the wavelength of light emitted from blue LEDs combined with phosphor powder is usually not uniform, and there tends to be overtly high blue light peak wavelength values. This can disrupt people’s sleeping patterns, commonly known as the “blue light issue”. On the other hand, the average white light made from blue LED and phosphor does not contain UV rays. Hence, there are color differences in LEDs, which are sometimes different from natural sunlight, UV rays, and other light sources.

Therefore, Nakamura announced “white LEDs using blue LEDs will eventually disappear from the market”, emphasizing Soraa’s white LED solution is the best.

Nakamura is the co-founder of Soraa, a company that has mostly been selling purple LED chips that combined red, green, and white LEDs that combine blue LEDs with phosphor. Is Nakamura’s remark an attempt to promote and benefit his company?

Most LED industry insiders believe Nakamura is trying to promote his company products.

“It’s quite amusing to see an inventor to belittle his own invention,” said a representative from WhichLEDs.

Other attendees agree. “It’s very natural for people to assume Nakamura intent was to promote his business when he made this comment,” said Haipo Wang, Vice President of China Association of Lighting Industry (CALI) and head of Institute of Optoelectric Material Department at Nanjing Tech University.

Others believe Nakamura’s comments were based on his expertise in the field. “Nakamura founded a company focused on purple LED R&D because he saw its potential,” said Wu Hongjian, Chairman of Shanghai Lighting Association. “It is difficult to say whether his remarks were solely based on the company profits.” Wu also is an expert from the Chinese Society of Rare Earths.

Chao Liang, Deputy General Manager at Jiangsu Bree Optronics, also believed as a scientist and Nobel Prize Winner, Nakamura has reasonable theories and logic reasoning in his negative outlook on blue LEDs.

Lattice Power Executive Vice President Zhengyi Chen agreed too. “I believe he based his judgment on his understanding of LED technology,” said Chen.

The advantages of purple LEDs

It will be difficult to determine the motive of Nakamura’s comment, but an objective comparison can be drawn between purple and blue LEDs.

Purple LED potential and outlook was affirmed by Wu Hong. “White light efficiency is higher when converted from purple LEDs, lighting distribution is more uniform, and has better CRI,” he said. “These can cut energy, reduce carbon emission, and improve lighting quality. All these carry significant meaning. Even though it has not become widespread on the market because of its high pricing, its advantages are quite obvious.”

What specific advantages does white LEDs made from purple LEDs have?

  1. High CRI. When white light is created from exciting phosphor coated on purple LEDs, the peak wavelength of the blue light is not as intense. Moreover, the output encompasses all visible light wavelengths. Hence, CRI is much higher, and closer to ideal white light found in sunlight. It has a high red CRI R9, and can reach Ra 95.
  2. High luminous efficacy:
  •  Purple LEDs are manufactured using GaN substrates, and the chip is produced by placing GaN semiconductors on a GaN substrate. In short, this is a type of GaN-on-GaN LED. GaN semiconductors have better crystal quality, and higher purple LED luminous efficiency. Its luminous efficiency has risen rapidly.
  • Purple LED chips are usually triangular, compared to normal LED chips rectangular shape. The triangular purple LED’s luminescent layer has better light emittance compared to square shaped LEDs. Due to purple LEDs high crystal quality and good light extraction efficiency, its wall-plug efficiency (WPE) can reach 84%. Average blue LEDs WPE in general is between 50% to 60%.
  • Average blue LED chips are manufactured on sapphire substrates. Since GaN semiconductors tend to have a different lattice constant compared to sapphire, mismatches can occur and lead to flaws in the crystal. Mismatches almost never exist in GaN substrates, so the resulting crystals are normally flawless. Compared to sapphire substrate products, crystal mismatch can be lowered to about 1/1000.
  • Another advantage of purple LED chips is its less likely to have droop issues. Droop occurs when the power from the driver is raised to increase the brightness of the chip, the issue of droop presents difficulties in increasing brightness in LED chips. In comparison, GaN-on-GaN’s higher crystal quality compared to sapphire substrates reduces droop. Hence, GaN-based products are able to drive voltages five to 10 times higher than sapphire substrate products.

Blue LEDs to exit the market?

High white LED Color Rendering Index (CRI) can be reached by using purple light or RGB phosphor excited by purple LED light. With purple LED chip power efficiency upgraded to a whole new level, more industry experts are preferring purple LEDs. Hence, will blue LEDs eventually exit the market?

Purple and RGB phosphor has reached mass production levels, said a representative from WhichLEDs. Purple LEDs developed to replace traditional fluorescent lamps use high voltage mercury vapor arc to create UV rays, but white light formed from UV LEDs does not have overwhelming advantages. Since blue LED is a primary color, and a necessity in creating white light, it is unlikely to disappear from the market in the near future.

“There are merits and demerits in using blue LEDs to create white light, issues include blue light wavelength peaks, and point light source,” said Wang. “Currently, manufacturers are using phosphor powder technology to lower the intensity of blue light and scale up the light source’s color rendering index (CRI). Light distribution technology can reduce glare,” said Wang. The quality of blue light is also being optimized. “Blue LED researchers are raising energy quality, and believe blue light will become more competitive on the market.”

The outcome of the race between blue and purple LEDs technology is unclear. Relationship between the two is comparable to LCD and plasma display TVs, even though the two serve the same purpose, different technologies were incorporated to achieve the desired results, said Wang. The race between blue and purple LEDs has been “inconclusive”. “The winner on the market can be compared to taking the university entrance exam, it depends on the exam questions and the person’s test results. It also depends on how others fare on the exam, and who has the most advantages.”

Blue LED is still the mainstream technology used to produce white light, said Zhen Chen. Purple LEDs can be used in high end white LED lighting applications. “Technology plays a small role in determining whether a product will be replaced,” said Chen. “Costs, consumer habits, and mainstream manufacturers choices are some of the main factors affecting a product’s market share.”

“It is difficult to judge at the moment whether purple LED can lead future market trends, and force blue LEDs to exit the market,” said Chao Liang. “Related policies, material, equipment, supply chain, and support level will all effect industry development.’

Is White LEDs a short-lived technology?

Whether blue or purple LED light sources resemble natural light is unimportant, since the issue of glare and blue light radiation still exist and is harmful to the human eye, according to some netizens. Additionally, there might be better alternative light sources than LEDs in terms of color rendering, and emitting a gentle and less harsher light. Could white LED technology more similar to natural light vanish or will it congregate into a big development trend.

Glare is not unique to LED light sources, all high brightness light sources can cause glare, analyzed WhichLEDs. Higher energy light beams can cause greater damage to a person’s skin and eye cells, so glare is not LEDs greatest disadvantage. Glare can be controlled using reasonable luminaire design. In the future, controlling blue light intensity in the white light spectrum can effectively control blue light radiation.

Purple LEDs blue light radiation might be even stronger, which makes it difficult to promote purple LEDs. Currently, Philips has added 410 nm wavelength purple LEDs in its Crisp White COB product. It can excite the fluorescent compounds in fibers to emit brilliant white colors. However, such luminaires cannot be used in most mainstream lighting, and can only be used in clothing retail or photo studios. Long term exposure to such lighting environments can greatly damage the skin.

“LEDs are not particularly strong in color rendering, and it is possible for other lighting technologies to achieve very high color rendering index and gentle beam angle. There are corresponding lighting products for every application field. Even though LEDs are not the best replacements, it can replace certain or some application field products. It is impossible for one type of technology to fulfill every market demand, analyzed WhichLEDs.

LEDs will remain the most important replacement light source, said Chen. “By sacrificing brightness and costs, color rendering and gentler light emissions are adjustable parameters,” said Chen. Alternative technologies on the market include OLED and Light Emitting Plasma (LEP), but both are still far from replacing LEDs. The two technologies possess certain advantages in niche markets, but will be used mostly as a supplementary to LED light sources, he added.

Other industry experts including Wang and Wu believe Quantum Dot LEDs, OLED, and LEP are still being developed and will not be the final stop in solid state lighting (SSL) developments.

To conclude, WhichLEDs outlined three white LED trends. The LEDs will be applied to meet general lighting market demands, and it will have to possess high energy efficiency. Lighting expenditure amounted for 12% of global energy consumption. Future white LEDs will have high color rendering index that is close to natural sunlight. Lastly, LEDs will have high color saturation and white LEDs will be able to cover most of the wavelengths on the spectrum. This development will be partly spurred by developments in phosphor technology.

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