Better, brighter phosphors

New formulations increase the quality and intensity of LED lighting

BY HERB SCHLEGEL & NICOLE RUTHERFORD FROM INTEMATIX

Throughout the offices, homes and hotels of the world, LED bulbs are rapidly replacing incandescent and fluorescent sources, thanks to their high efficacy and long lifetime. These strengths make them a cost-effective choice in the long run, despite higher up-front costs.

As this transition from a niche to a mainstream product is taking place, demand is growing for LED sources that deliver a higher quality of white light. There are various metrics to measure this, but normally a source is considered to be of high quality if it delivers a wide range of Correlated Colour Temperatures (CCT) and a high Colour Rendering Index (CRI) − and maintains that colour performance over time.

When the LED lighting market was in its infancy, a source with a CCT of 4000K at 70 CRI was deemed acceptable for most applications, but within the last few years retail and hospitality lighting has demanded a CCT of 3000K at 80 CRI. Moreover, with recent advances, these sectors are now asking for CCTs
ranging from 2200K to 4000K and CRIs as high as 98.

To meet this demand, manufacturers of phosphor materials must develop products for the efficient generation of white light that provides a very wide spectral range.

At Intematix of Fremont, CA, we are doing just that, with an ever-expanding portfolio of phosphors that include green and yellow aluminates, green and yellow garnets, red nitrides and silicates.

These phosphors span a wide range of wavelengths, have very high efficiencies "“ they can reach a quantum efficiency of more than 97 percent "“ and can be applied to a wide range of applications from mobile to TV displays and general and automotive illumination.

Generating white light

The LEDs that are used in lighting today are an evolution of the first white-emitting LEDs, which emerged from Japan in the 1990s and employed a blue-emitting chip to pump a yellow phosphor (see Figure 2a). In this form of white-emitting device, not all of the blue emission is absorbed by this phosphor, and white light results from colour mixing of blue and yellow. This approach, still widely used today, has the merit of a very high efficiency. However, the colour rendering index is low, due to the lack of emission in the red and orange regions of the visible spectrum.

To improve the CRI of solid-state sources, manufacturers are increasingly turning to a two- or even three-phosphor strategy. A broader range of CCTs and higher CRIs are possible by supplementing the yellow phosphor with one that emits in the red, while even higher values can be obtained with the combination of a red and green phosphor (see Figures 2b and 2c).

The pioneers of the white LED based on a blue-emitting chip "“ Shuji Nakamura of Nichia and the academic group led by Isamu Akasaki and Hiroshi Amano from Nagoya University "“ were first to discover that the combination of YAG and a blue emitting semiconductor chip delivered white light with high luminance. To protect their discovery, Nichia filed patents on this combination and in 1996 was granted US Patent 5,998,925. This patent, which describes the use of a phosphor that "˜absorbs part of the blue light and emits a yellowish light', places restrictions on the use of a blue chip in combination with any garnet composition containing at least one element from: the group of yttrium, lutetium, selenium, lanthanum and samarium; and one element from the group aluminium, gallium and indium and being activated by cerium.

It is worth noting that the Nichia patent does not restrict the manufacture and sale of YAG phosphor, but rather, its application to create a white LED. Consequently, companies that have a license under this patent may purchase YAG from any manufacturer for use in their LED devices.  Nevertheless, the license imposes an extra cost that most LED manufacturers would like to avoid by using phosphors other than YAG. 

Figure 1: Intematix produces a range of phosphor materials, including those based on YAG, silicates, nitrides and aluminates.

To address this need, in 2006 we began commercial production of phosphors based on silicate materials. Being one of the early producers with unique compositions, our silicates quickly became widely used, thanks to their high brightness, wide range of available colours, and relatively narrow emission bandwidths "“ characteristics highly valued by manufacturers of displays, the dominant market for white LEDs.  Three years later, we expanded our offering with our own brand of garnet phosphor, which we called NYAG.

In the last few years, we have continued to broaden our product line with a focus on meeting the requirements of the emerging lighting market by launching green and yellow aluminate (GAL) and red nitride products. These innovations have increased the number of high-performance phosphor families available to LED manufacturers and we continue to fine-tune and optimise each family for brightness and particle size, shape, finish and consistency. We now offer one of the industry's widest ranges of particle sizes, allowing customers to choose the best size and distribution for a particular application. For example, smaller particles are easier to disperse in silicone and generally give the best angular colour uniformity, while larger particles tend to deliver higher brightness.

Looking to the solid state lighting market, our GAL phosphor offers a broader emission spectrum than silicate phosphors, or even YAG materials, so it is particularly ideal for bulb replacement applications where a broad spectral output similar to incandescent light is required. Meanwhile, our red nitride phosphors have an emission bandwidth that is narrower, similar to those of silicates. When emitting in the red, a narrow emission is desired "“ if there is spectral output at infrared wavelengths, where the eye is insensitive, the energy is wasted on unseen light.

Figure 2: Today three different methods are used to make a white-emitting LED product. These variations are associated with the phosphors, which are directly applied to the LED chip. The pioneers of the white-emitting LED added a yellow phosphor to a blue LED (a). Higher colour quality, which comes at the expense of inferior efficiency, is possible by adding a red phosphor (b), or employing the combination of a red and green phosphor (c).

Both our red and green phosphors have the edge over earlier silicates in terms of performance in mid to high power LEDs. In a high power LED, the phosphor typically reaches a temperature of 100 °C or higher, and in that regime the GAL and red nitride phosphors can deliver an efficiency that is 10-15 percent higher than that of silicates.  This greater thermal stability is highlighted by measurements showing that the emission intensity of GAL phosphors declines by just a few percent between 0 °C and 200 °C, compared to a fall of more than 20 percent for traditional YAG (see Figures 3 and 4).

Figure 3: Figures 3: Green aluminate (GAL) has excellent thermal stability, allowing it to aid the production of efficient LED light bulbs operating at high current densities.

Figure 4: Intematix has improved thermal stability by turning to new materials.

What's more, the combination of our GAL and red phosphors can enable a near-perfect colour rendering of up to 98 CRI.

While GAL phosphor's broad green emission enables high CRI, our red phosphors hold the key to producing higher values of R9, which is a measure of how well an illumination source can capture deep, saturated shades of red.  These phosphors, which are used in white LEDs serving retail and hospitality markets, are setting a new benchmark for colour stability: one that is imperceptible to the human eye, and measured at just 2SDCM (that is, a 2-step MacAdam ellipse, which is a well-known metric for measuring shifts in colour) in aggressive accelerated lifetime tests. In addition, these phosphors are relatively new, so different wavelengths and methods to improve the efficiency of each composition are continually being discovered (see Figure 5). 

Figure 5: Intematix has steadily increased the brightness of its red nitride materials.

A bright future

Shipments of phosphors will rise as the LED-based lighting market grows. McKinsey & Company have reported that the LED-based lighting market should increase in value at an average of 38 percent per year between 2012 and 2016, and we anticipate that phosphor sales should increase even faster, as LED makers must use more red and green phosphor in the production of higher quality white-light sources with lower CCTs and higher CRI characteristics.  Demand for more advanced phosphor materials is also rising as LED makers adopt more complex mixing strategies to compete on optimised brightness, CRI and R9 values.

Another reason we are very positive about the future is that, increasingly, our phosphor portfolio not only has materials with stable chemical structures that will last upwards of 50,000 hours − that is at least the lifetime of the bulb − but it also contains materials with excellent thermal stability. 

Bulb makers want to drive their LEDs very hard, because this reduces the total chip count and thus the bill of materials, and this requires phosphors that are more chemically and thermally stable at high temperatures. Our newest phosphors perform particularly well in this regime.

For example, a chip packaged with GAL can maintain 97-98 percent of its initial brightness at 150 °C, and our red nitride survives the most rigorous lifetime tests with virtually no loss of brightness and perfectly stable colour. Bulb makers are also moving away from phosphor-coated LEDs to a remote-phosphor architecture. Because phosphor particles are small, the light they create is naturally diffuse "“ so in a phosphor-coated LED, as light is created, a portion is emitted back toward the chip surface and is lost. This loss is avoided with remote phosphor, which also has the benefit of operating at a lower temperature, further increasing the overall efficiency.

To support the growth of solid-state lighting based on remote phosphors, we have developed materials specifically for this application, which can be applied to either a two-dimensional sheet or a three-dimensional shape, such as dome or a tube. Thanks to superior geometries, efficacy is typically 15 percent higher. However, a 30 percent hike in efficiency is possible compared to a white LED with a diffuser.

By delivering this improvement, while ensuring excellent colour quality and stability with our robust, highly efficient phosphors, we are helping to drive a revolution in solid-state lighting.

This revolution is not just about trimming our carbon dioxide footprint by using highly efficient LEDs, more and more it is about illuminating homes and offices with a superior white light, making them better, more productive places to be.

Phosphors and China

In mainland China, the vast majority of LED packaging fi rms are hampered by a lack of LED device IP associated with using the combination of blue chips and YAG or silicate phosphor. As a result, most Chinese companies face restrictions that impair their exports of Chinese products. 

Intematix is helping to address the diffi culties faced by these Chinese companies, by delivering superior product performance, leveraging our local China manufacturing base, and offering an IP portfolio that covers both red and green phosphor compositions and their application in LEDs. Intematix phosphors are particularly valuable for Chinese LED makers who want to complete with higher light quality and must offer IP protected products to drive product exports to luminaire manufacturers selling to end customers such as Home Depot and Ikea.