A few developments in the material science literature relating to LEDs caught my attention, and I summarize them here.
A recent paper entitled "Investigation on Combustion Derived BaMgAl10O17: Eu2+ Phosphor Powder and Its Corresponding PVP/BaMgAl10O17:Eu2+ Nanocomposite" (Dalton Trans., 2014, 43, 1072) was of interest to me due to the description of using a UV excitation source to generate emissions in the visible range. The paper, authored by Nathalie Pradal (Clermont Universite, ENSCCF, Institut de Chimie de Clermont-Ferrand) and colleagues, describes a method using microwave enhanced combustion to form nanoparticles of a blue phosphor, and casting it in PVP (poly(N-vinylpyrrolidone)) to form phosphor films. Ms. Pradal graciously provided me a review copy of the paper. The main message of the work is that the method can produce small and controllable particle sizes of the phosphor, which can lead to enhanced performance compared to phosphor powders. The phosphor studied down-converts UV light into blue light, which can be used as part of a system to produce white light acceptable for lighting purposes.
Particle size matters
Pradal et al. state in the paper that "[it] has been proved in several recent studies that white LEDs using smaller particle size, with narrower grain size distribution, required a lower amount of phosphor to get similar efficiency to LEDs using bigger particles." In particular, the paper cites S. C. Huang et al., Int. J. Appl. Ceram. Technol., 2009, 6, 465. In that paper, the authors painstakingly created YAG:Ce phosphors of different sizes and relatively narrow size distributions, then determined the weight percentage in a silicone binding matrix needed to achieve the same conversion percentage of the blue excitation energy. They found that particle size of 2 μm required only 4% w/w powder loading to achieve the same conversion as nearly 6% w/w of ~15 μm particles in a packaged LED. In other words, in that study, having larger particles could require 50% more phosphor. In a real phosphor powder, the result is that larger particles in a distribution of sizes result in diminished phosphor performance.
Pradal et al. also note potential benefits of film phosphors over powder, including higher thermal conductivity, better uniformity, and better adhesion, while using a lower mass of phosphor per unit area. Additional criticisms are levied against conventional phosphor production involving "an energy greedy solid state reaction" that leads to "large and irregularly shaped particles."
Impressively, in the study, the cast films were characterized using X-ray diffraction, infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, confocal fluorescent microscopy, and small-angle X-ray scattering. In addition, accelerated aging of the films were studied by exposing the films to UV-A/B energy from mercury vapor lamp sources.
The conclusions of the detailed material study may be summarized as the process produces platelet-shaped particles of relatively uniform size/shape, which, absorbing UV from a spectrum peaking around 340 nm, results in emission a bit above 450 nm. The particles can be cast in a polymer film at low weight loadings and retain their beneficial properties as a phosphor. Notably, the film did degrade in the aging study, and due to oxidation products of the polymer accumulating in the film the overall performance was degraded. Such aging is certainly an important consideration for some applications of OLEDs, such as automotive, which may be expected to endure long-term exposure to solar UV radiation.
Quantum dots
Some of our readers may have noticed some press about Lumisands earlier this year, announcing a non-rare earth phosphor product using quantum dots. Laser Focus World reported on the development, stating that the phosphor was claimed to achieve 41% to 51% quantum yield and convert UV at 365 and 405 nm to visible red light. The phosphor is purported to be made using a silicon wafer, which is "etched to make it porous and the resulting material scraped off, etched, passivated, and mixed with diphenylsilanediol in ethanol. The result is 1- to 5-μm-diameter particles with 5-nm-diameter QDs on their surfaces." Thus, such a phosphor might be the second of the three desired ingredients of a white LED, adding red to the blue described earlier. (As a side note: Lumisands gets the award for the simplest website in the LED industry.)
After reading the LFW article, I touched base with IHS analyst Stewart Shinkwin, who follows packaged LEDs and Optoelectronic components. He notes that improvements in red emission (leading to a high R9 value) could have a significant role in high-CRI white LEDs, although he feels that high-CRI LED lamps with high R9 are of interest mainly to niches like museums. (Note: R9 is a measure of rendering of red colors, but is not included in the determination of CRI, hence it has importance to color rendering beyond that expressed by CRI.) Of course, there could be many such niches, as evidenced by the editor's reports from Lightfair.
Vacuum UV
If all this hasn't yet piqued your interest, consider a paper published in April by Masahiro Yanagihara and colleagues at the Nagoya Institute of Technology and other cooperating institutions in Japan. Yanagihara et al. demonstrated a solid-state source (specifically, a field emission lamp) that emits in the vacuum UV from 220 down to 140 nm. The feat was enabled by their development of a new phosphor based on KMgF3, which, although requiring a source of fluorine (either F2 or CF4 gas), was noted as not requiring rare earth elements or other dangerous, toxic substances such as beryllium. If you read the paper you may notice that the researchers start with KMgF3, which was prepared using argon and CF4 at 1,220 C. After two days of cooling, the team then used pulsed laser deposition to deposit a thin film of the phosphor onto an MgF2 crystal, which was then used to demonstrate emission at 155 nm and 180 nm. Although clearly not ready to leave the lab, development of UV phosphors may help create better UV LEDs, which could then be used with advanced phosphors to create white light.
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