LED Materials Bring White Light and More

KAREN A. NEWMAN, GROUP PUBLISHER, karen.newman@photonics.com

Efficiency and stability may make LEDs the lighting choice of the future, but commonly used inorganic materials are expensive and creating the right kind of white light remains a challenge. Researchers around the world are looking for alternatives to cerium, yttrium and other rare-earth elements used in the phosphors added to LEDs to generate white light, and working to create light that will appeal to consumers.

Beyond general illumination, LEDs from one end of the spectrum to the other are being used in applications ranging from UV-C water disinfection to near-IR emitters for night vision and surveillance. While most mass-produced products can face pricing pressures from consumers, price is not always the first consideration when performance is critical.

It will come as no surprise that three key trends pushing the LED materials market — as noted earlier this year by Persistence Market Research — are a general increase in demand for LEDs; a critical need for efficient lighting solutions; and favorable regulatory policies that support the ongoing research and development of energy-efficient products.

Among several reports over the past two years regarding the creation of white LEDs, research ranges from experimenting with new phosphor development to replacing phosphors altogether with quantum dots, as well as work out of Europe on luminescent rubber showing that when it’s combined with an LED of a certain wavelength, it creates white light.

Luminescent proteins

Inspired by biomolecules, a team of German and Spanish researchers at the University of Erlangen-Nuremberg in Germany has used luminescent proteins to convert standard blue or UV LED light into pure white light. By introducing the proteins into a polymer matrix to produce luminescent rubber, they have developed a hybrid device called a BioLED (Figure 1).

Figure 1. Red, green and blue luminescent proteins are introduced into rubber to produce BioLEDs. Courtesy of M.D. Weber.

Starting with either a blue LED or a UV LED — necessary for exciting the rubber — the luminescent polymers can then be combined to create white light. For example, a blue LED can be combined with green and red rubber, or a UV LED with blue, green and red rubber, to emit a pure white light with efficiency similar to inorganic LEDs.

According to researcher Rubén Costa, a co-author of the current study, the idea was first proposed by Thompson and Forrest in 2000 in Advanced Materials (Vol. 12, p. 1678). In the earlier research, the chromophors were prepared in the lab and applied to OLEDs as a normal small molecule, but researchers were not able to stabilize proteins directly obtained from the bacteria. Thus, stabilizing the proteins became the key aspect of the research undertaken by the team at Erlangen-Nuremberg, working in collaboration with professor Uwe Sonnewald.

“As a matter of fact, proteins are only partially stable in aqueous solution, and in our rubbers they are pretty stable,” said Costa. This allowed the team to think of applying this protein-based rubber material to optoelectronics.

“Our first aim was to stabilize the proteins in order to be able to implement them into OLEDS, but surprisingly it turned out that the fluorescent rubbers were more suitable for hybrid LEDs like our BioLEDs,” said Costa.

The BioLEDs are simple to manufacture and their materials are low-cost and biodegradable, the researchers say, allowing them to easily be recycled and replaced. Further, the luminescent properties of the proteins remain intact during months of storage under environmental conditions including light, temperature and humidity.

With a wide palette of fluorescent proteins, it is possible to cover the whole visible spectrum. Once the group had a stable rubber, it decided to aim for the white LED.

They use standard procedures to determine the color quality by using integrating spheres calibrated with white light sources, Costa said.

“But importantly, we focus on the stability of the white. Our main interest is to provide stable electroluminescence spectrum over time, not just a single measurement.”

The team is working to optimize the material for greater thermal stability and a longer operating lifetime, improving the chemical composition of the polymer matrix and identifying proteins that can hold up better under device operating conditions. Its ultimate goal is industrial-scale production.

“We published 150 h of stability. Preliminary results that we have recently obtained point out that this can be greatly enhanced via suitable modification of the components,” said Costa. He and his team are confident they can provide much more stable BioLEDs in the near future.

Today, they can produce BioLEDs for mid-power LEDs, but high-power LEDs produce much more heat and the proteins will not be stable for a long time, according to Costa. They are currently working on how to make them more resistant against temperature. The performance with respect to efficiency and brightness is optimum, he says, for single point applications such as for back screens.

“The stability is the current issue that we expect to solve in the near future,” said Costa.

Common materials reduce cost

Motivated by the desire to reduce the cost of white LEDs, researchers at Rutgers University in Piscataway, N.J., have been working to develop phosphors using only earth-abundant elements to make the devices more sustainable and cost-effective, according to Zhichao Hu, a postdoctoral associate in material sciences and engineering at Rutgers.

As a graduate student, Hu was a member of a team working on the project under the direction of professor Jing Li. The team’s first publication of this development was W. Ki and J. Li’s article in the Journal of the American Chemical Society, Vol. 130, pp. 8114-8115, in 2008.

Their chief concerns at the time were the lack of domestic (U.S.) supply of rare-earth elements, which are key ingredients of commercially available phosphors — and their fluctuating price — which Hu said peaked around 2011.

Because LEDs do not emit white light, manufacturers use technologies that typically rely on a yellow-emitting phosphor coating to shift to white the generally blue color of light emitted by a single semiconductor chip. The phosphors used are made from materials composed of rare-earth elements. Not only are these materials expensive, but the light they put out tends to be harsh and cold, according to the researchers (Figure 2).

Figure 2. An LED coated with a yellow phosphor is shown turned off (left) and then turned on (right). This ‘green’ LED is inexpensive and provides warm white light. Courtesy of Zhichao Hu.

Today, Li’s team is developing hybrid phosphor-based technologies that combine common, earth-abundant metals with organic luminescent molecules, producing phosphors that emit a controllable white light from LEDs. By varying both metal and organic components, the colors of the phosphors can be systematically tuned to regions of the visible light spectrum that humans find acceptable, according to the researchers.

The group has two patents pending (WO2015164784 A1 and WO2014201377 A1) on its development of these rare-earth-free phosphors, and work continues on optimizing and simplifying the synthesis of the compounds to make the production process easier to scale up, more environmentally friendly and cost-effective, according to Hu. Regarding future production, he said they use the same commercially available process to produce PC-WLEDs (phosphor-converted white LEDs) with the rare-earth-free phosphors.

The next step for the group will be testing the performance of its phosphors in LED devices.

Red phosphor heightens white LEDs

In 2014, a team from Ludwig Maximilian University of Munich (LMU), revealed details of a red phosphor material based on the nitride Sr[LiAl3N4] (Figure 3).

Figure 3. The new red phosphor material can improve the color rendering for white LEDs. Courtesy of Professor Wolfgang Schnick.

When doped with europium, the material displayed intense luminescence over a narrow range of frequencies in the red band. Peak emission occurs at wavelengths of around 650 nm, the researchers said, and peak width is 50 nm.

In the study, the researchers found that the new red-emitting phosphor material has a significant influence on the color rendering index. Phosphors capable of emitting in the deep-red region can reconcile the conflicting demands of optimal efficiency and color rendition.

“The problem with commercially available white-light LEDs is that there is always a trade-off between optimal energy efficiency and acceptable color rendition,” said Wolfgang Schnick, a professor and chairman of inorganic solid-state chemistry at LMU. Schnick has a long-standing partnership working on LED phosphors with Lumileds Aachen.

Lumileds recently confirmed its plan to release products this year that carry “the very material that was published in Nature in 2014,” according to Niels van der Veen, general manager of LDCA for Lumileds.

“It will indeed be used for warm white LEDs that are characterized by a high color rendering index (CRI) and high red content (R9), as predicted in the publication,” he said.

Figure 4. A small light-emitting diode fabricated from carbon dots. Courtesy of Prashant Sarswat.

Meanwhile, work continues in the effort to develop new phosphors for LEDs. Van der Veen said that the solid-state lighting industry and suppliers around the world are working on systems that will follow the well-known Europium-doped nitride phosphors, pointing to manganese-doped fluorides and nano-materials including quantum dots or quantum wells.

He went on to say that as the various LED markets in which Lumileds is active differentiate in application conditons and requirements, it is likely that rather than a single optimal material, a set of different LED phosphors will allow them to best serve those markets.

With research distributed among LED manufacturers, chemical companies, university researchers and start-ups, van der Veen said it is difficult to estimate the worlwide investment in LED R&D. But the work is far from finished.

“The stakes remain high, however, as there is still room to be gained in terms of efficiency, quality of light, as well as cost,” van der Veen concluded.

LED Materials from UV-C to NIR

Last year, ultraviolet LED maker Crystal IS Inc., of Green Island, N.Y., introduced Optan SMD, an LED based on native aluminum Nitride AlN substrates. The new LED was designed to control biofouling in applications where coatings, traditional UV lamps or mechanical wipers didn’t work.

UV-C LEDs may be an emerging technology, but as with any manufactured goods there is pricing pressure for high volumes, according to the company, and material selection can play a role in optimizing system costs.

“Our customers are looking to optimize their costs beyond the light source unit price to the overall system costs — including operating and maintenance,” said Hari Venugopalan, director of global product management.

Crystal IS customers are generally looking for greater reliability and lifetime across both instrumentation and disinfection, as well as higher power for disinfection applications.

“Crystal IS’ lattice matched native aluminum nitride substrates have fewer defects than lattice mismatched UV-C LEDs based on other materials, like sapphire,” said Venugopalan. “Fewer defects results in a more reliable device capable of higher output powers. Using bulk AlN substrates as the foundation for UV-C LEDs is helping Crystal IS meet the performance requirements for instrumentation and disinfection markets.”

Venugopalan said that Crystal IS is focused on continual improvement of its processes and operations. “Crystal IS has validated our lifetime and reliability data for our Optan products and continues to improve lifetime and reliability by further reducing defects in our already low defect AlN substrate,” he said.

Metal alloy substrate

At the opposite end of the spectrum, Opto Diode Corp., of Camarillo, Calif., last year introduced its OD-110W GaAlAs near-infrared (IR) emitters designed for night vision and surveillance applications.

Designed for harsh industrial environments or deployment in the field, the near-IR LEDs are housed in standard 3-lead, TO-39 hermetically-sealed packages and have gold plating on all surfaces.

“The market spaces we are most successful in — military and medical — are not terribly price-sensitive,” said Russ Dahl, business unit manager. “The OEMs are more interested in performance — which could be efficiency, wavelength control or temperature.”

With high power as a key factor, a metal alloy substrate offers better thermal management, according to Ray Fontayne, product manager, Opto Diode.

Other primary factors are the geometry of the LED die itself and the optical encapsulation type and shape, said Dahl. The height from the die to the window plays a large role in ensuring the wide beam angle. In this particular instance the right die structure had to be chosen to minimize visible light.

Dahl also said that, generally speaking, the package materials are most important. “The part has a lower efficiency due to no index matching epoxy, but it will survive and operate at temperatures beyond the range of a standard LED,” he said.

For more information on LED materials innovations, see the article “Materials Innovations Help LEDs Turn On,” by contributing editor Hank Hogan, https://www.photonics.com/A57443.