How to manufacture lighting phosphors for LEDs

Most "white" LEDs used in lighting today employ InGaN semiconductors that emit light in the blue region of the spectrum, combined with cerium-doped yttrium aluminum garnet (YAG) phosphors. A portion of the blue photons emitted by the LED is absorbed by the phosphor and re-emitted in the yellow region of the spectrum. The mixture of the remaining blue photons and the yellow light creates an approximation of white light that is pleasing to the human eye.

While this combination of LED and phosphor is a proven technology with solid performance, it is not without flaws. One notable drawback is that the lack of a red component in the system's output limits the ability to achieve high Correlated Color Temperatures (CCT) and high Color Rendering Indices (CRI). Combining YAG phosphors with other materials can increase red light emission—enabling the production of "warm" white LEDs—but this often comes at the cost of hard-won luminous efficacy. This article examines this issue in detail, explores how phosphor manufacturers are rising to the challenge, and considers the impact that phosphor research will have on the efficacy, CCT, and CRI of solid-state lighting.

Blue LED + YAG Phosphor = White Light

The phosphor used in high-brightness LEDs is a highly specialized material: cerium-doped YAG (scientifically designated as Y3Al5O12:Ce3+). This material evolved from phosphors originally used in cathode-ray tubes (CRTs) and was first synthesized in 1967.

In operation, blue photons from the LED are absorbed by atoms or molecules within the YAG phosphor, exciting electrons to a higher energy level. These electrons then rapidly drop back to a lower energy state, releasing the absorbed energy partly as heat and partly as photons with longer wavelengths—specifically, some red and green light, and a significant amount of yellow light. The combination of the yellow light emitted by the YAG phosphor and the blue radiation "leaking" directly from the LED through the YAG:Ce coating effectively approximates white light. The phenomenon behind converting blue light into "white light" via phosphor absorption and re-emission is known as the Stokes shift, named after the Irish physicist George G. Stokes, who described the effect in an 1852 paper. Figure 1 illustrates how the Stokes shift occurs through the absorption and emission curves of the YAG phosphor.

Figure 1: Absorption and emission spectra of YAG phosphor exposed to blue light.¹

Figure 2 shows the relative spectral emission curve of a modern white LED using YAG phosphor (in this case, an OSRAM OSLON SSL 150 LED producing 136 lm at a forward current of 350 mA and forward voltage of 3.1 V, with an efficacy of 125 lm/W). The dashed line represents the eye's sensitivity function, illustrating how the eye responds to light of different wavelengths. Although the first peak (corresponding to blue photons directly from the LED) is larger, it is less visually prominent because the eye is less sensitive to that wavelength; in contrast, the photons emitted by the phosphor are centered at 560 nm—the wavelength to which the eye is most sensitive.

Figure 2: Modern white LED (OSRAM OSLON SSL ...spectral emission curve of [150].

As a proven technology capable of delivering acceptable results, YAG serves as the benchmark against which other LED phosphors are measured. YAG phosphors not only strongly absorb blue photons but also exhibit a rapid "decay time"—the interval required for an electron that has absorbed a blue photon to release energy as a yellow photon and heat—thereby preventing a phenomenon known as saturation quenching. This process occurs when the photons that would normally be emitted are effectively "swamped," preventing them from escaping the phosphor matrix amidst high photon flux. YAG phosphors offer several advantages for LED applications. First, under blue LED excitation, the quantum efficiency of YAG phosphors (a measure of the total output photon energy from the phosphor relative to the input photon energy from the LED) is a robust 80%. Second, researchers report no signs of degradation in YAG phosphors during long-term blue LED excitation or upon exposure to moisture. Finally, the synthesis of YAG phosphors is relatively straightforward, utilizing the same high-purity precursors (Y₂O₃, Al₂O₃, CeO₂) that have long been employed in traditional CRT phosphors.

Drawbacks of YAG Phosphors:

Despite their proven performance, YAG phosphors are not without flaws. Two practical issues are "thermal quenching" and relatively low chemical stability. The mechanisms behind thermal quenching are complex, but in layman's terms, elevated temperatures cause electrons—which would normally absorb blue photons and emit yellow ones—to "go missing" (in other words, the atoms become ionized). YAG phosphors are susceptible to thermal quenching at temperatures around 200°C, a level not typically reached during normal LED operation. Furthermore, the material's low chemical stability limits the LED's lifespan—defined as the point where luminous output drops to 70% of its initial value. Manufacturers counter this by pointing out that modern white LEDs can operate for 30,000 hours or longer; however, researchers note that such figures are based on "ideal" operating conditions, and chemical instability could impact the performance of devices used in harsh environments. Yet, perhaps the greatest challenge facing YAG phosphors is aesthetic in nature. Solid-state lighting manufacturers are keen to gain consumer acceptance for this technology, yet a common complaint is that the "harsh" light produced by white LEDs lacks the "warmth" associated with traditional incandescent lighting. This perception stems from the fact that YAG phosphors produce a "blue-shifted" white light with very little red content, resulting in light with a high CCT (5000 to 8300 K). The absence of significant red wavelengths introduces another issue for YAG-based devices: poor CRI. CRI measures how accurately a light source renders object colors compared to sunlight (which has a CRI of 100). Despite their obvious drawbacks, incandescent bulbs boast a CRI of around 95, whereas cool-white LEDs typically range from 70 to 80. (For more information on CCT and CRI, please refer to the TechZone articles "Defining White LED Color Characteristics" and "What Is the Color Rendering Index and Why Is It Important?")

LED manufacturers have partially addressed these CCT and CRI challenges by blending YAG phosphors with a second phosphor that adds red wavelengths, thereby shifting the CCT toward the warmer spectrum and improving CRI. Replacing blue LEDs with UV LEDs can yield further improvements in CCT and CRI; manufacturers offer commercial UV products for this purpose. For instance, Philips Lumileds offers a UV version of its Luxeon LED. This device emits wavelengths between 395 and 400 nm, with a minimum radiant intensity of 525 mW/sr at 500 mA. (Because the human eye cannot perceive ultraviolet light, UV LEDs are rated by radiant intensity—watts per steradian—rather than the more familiar efficacy metric—lumens per watt—used for white LEDs.)

The primary downside of adding "red" phosphors is a reduction in LED efficacy. Compounding this issue, red phosphors have an even lower thermal quenching threshold than YAG phosphors, further diminishing efficacy at typical LED operating temperatures. Cree’s XLamp® XT-E white LED operates at 350 mA and 2.85 V; the cool-white (5000 K) version delivers 135 lm with an efficacy of 135 lm/W, whereas the warm-white (2700 K) version offers 97 lm and 97 lm/W, respectively. Warm-white products from other manufacturers exhibit similarly lower efficiency compared to their cool-white counterparts. Figure 3 illustrates the differences in CCT and CRI between blue LEDs paired with YAG phosphors versus UV LEDs paired with a mixture of YAG and red phosphors.

Figure 3: Combining UV LEDs with yellow/red phosphors improves CCT range and CRI but reduces efficacy.³

Improving Phosphor Efficiency

Over the past fifteen years, the pursuit of higher LED efficiency has been a key driver in the development of solid-state lighting components by major manufacturers. Significant progress has been made in both the initial generation of photons and their extraction from the die. A major concern for manufacturers is the relatively low conversion efficiency of red phosphors, which limits the luminous output of warm-white LEDs. Manufacturers are working to produce new materials that promise commercial phosphors with higher conversion efficiencies, while also supporting a wide CCT range and good color rendering indices. The most promising candidates are nitride and oxynitride materials, which utilize the rare-earth element europium (Eu) as an activator, replacing the cerium (Ce) used in traditional phosphors. Many of these new phosphors can be excited by violet or blue LEDs and match the high-temperature quantum efficiency of YAG:Ce phosphors. Furthermore, nitride phosphors do not degrade under high-temperature or high-humidity conditions, making them suitable for LED lighting in harsh environments.

One class of highly efficient oxynitride phosphors consists of MSi2O2N2:Eu²⁺ compositions (where M = Ca²⁺, Sr²⁺, Ba²⁺); these emit in the 575–675 nm range and exhibit quantum efficiencies exceeding 85% at temperatures above 200°C (Figure 4). The yellow, orange, and red Eu²⁺ emissions from these nitride phosphors can be combined with YAG:Ce to create warm-white LEDs that are more efficient than current commercial products. Mixtures of YAG:Ce and CaAlSiN3:Eu²⁺ phosphors, combined with blue LEDs, have already been used to manufacture high-CRI warm-white lamps.

Figure 4: Nitride and oxynitride materials show promise as high-quantum-efficiency phosphors for warm-white LEDs.2

One drawback is that, despite the immense potential of these new materials, synthesizing them is far more difficult than producing traditional phosphors. However, this has not deterred some enterprising manufacturers from commercializing nitride phosphors. In 2011, Intematix added red nitride materials to its phosphor product line. The company's phosphors are used in high-efficiency warm-white LEDs for general lighting, offering customers the added benefit of immunity from certain patent licensing issues.

The company claims that these new phosphors will enable the lighting market to create warm-white applications with higher efficiency and higher CRI (up to 98) than traditional YAG phosphor solutions. Using these new phosphors reportedly allows customers to (legally) circumvent certain patents associated with YAG phosphor applications that would otherwise incur licensing fees.

As manufacturers strive to convince consumers that solid-state lighting is a practical alternative to traditional lighting, the efficacy of LEDs has improved significantly. However, further gains in photon generation and light extraction are becoming harder to achieve, prompting LED manufacturers to increasingly focus on other aspects of chip characteristics to enhance performance.

One such aspect is phosphors—specifically those used to add red wavelengths to the output of white LEDs, giving the light a warmer appearance. Traditional solutions expanded the CCT and CRI ranges of products, meeting consumer demand for diverse color temperature options and faithful color reproduction; however, they often negated some of the hard-won efficacy gains achieved through other technological advancements. Phosphors based on europium-doped nitrides and oxynitrides offer the promise of new materials that deliver desirable CCT and CRI values ​​while maintaining quantum efficiencies comparable to (or better than the pure YAG phosphors used in modern, high-efficiency cool-white LEDs. With certain materials now reaching the market, engineers can look forward to the upcoming launch of a new generation of more efficient warm-white LEDs.