Full Spectrum White LEDs of Any Color Temperature with Color Rendering Index Higher Than 90 Using a Single Broad-Band Phosphor

A new phosphor alloy Ca 1 + x Sr 1-x Ga 2 S 3 F 2 : Ce 3 + , Eu 2 + with x = 0.04 exhibits a full spectrum emission in the entire visible wavelength range from 420 to 700 nm when excited by a LED chip with 400 nm emission. By simply depositing varying thicknesses (amount) of this phosphor powder on a 400 nm excitation LED source, white LEDs with correlated color temperatures (CCT) between 1700 K and 20,000 K and (x,y) chromaticity coordinates lying on the Planckian locus of the CIE chromaticity diagram has been demonstrated. To the best of our knowledge, this is the ﬁrst report on the demonstration of a single full spectrum phosphor based white LEDs exhibiting the entire gamut of white light color temperature with CRI exceeding 90. Speciﬁcally CRI greater than 95 is reported for the CCT range of 2200 K – 8500 K. © 2017. ECS. terms paper is part of the JSS Focus Issue on Visible and Infrared Phosphor Research and Applications.

Humans have evolved over centuries using natural sources of lights originating from natural and artificial blackbody sources, namely, sunlight and fire. Thus one can expect that the properties of light that they have been exposed must have played a significant role in the development of the biological system in addition to other environmental factors. Today, however, in the quest for creating energy efficient and environmental friendly artificial light sources such as using LEDs, the basic characteristics of light that we are being exposed have changed drastically. New studies are slowly emerging on the health impacts of artificial lighting on human health. Recreating the blackbody like sources using LEDs is cumbersome and expensive. This paper reports a new approach that provides the pathway to create LED based light sources with blackbody characteristics.
One of the key attributes of LED based solid state lighting is its potential for delivering tailored full spectrum lighting. Tailored and full spectrum white light emitting diodes (WLEDs) of high optical power and high energy efficiency are needed for a wide gamut of emerging applications. 1 Full spectrum artificial lighting mimicking natural sunlight is anticipated to have beneficial impacts on cognitive psychophysiological health of human beings, safety, work place productivity, education, art and architecture, farming, industrial bio-chemical processes, etc. The phosphor converted white LED (pc-WLED) is the most suitable architecture for creating these miniaturized solid state light sources with the desired emission spectrum. Since pc-WLEDs are fabricated using single emission wavelength GaInN blue or near UV LED chips (the excitation source), it requires significantly simpler electrical driver and control circuits compared to white LED packages fabricated using direct emission LED chips of multiple colors. Therefore, pc-WLEDs provide the most economical solution for full spectrum lighting.
Spectrum tuning in a phosphor converted LED is currently done by varying weight ratios of phosphor powders of different emission wavelengths. This is also referred to as "Phosphor Blends" in the industry. The emission spectrum (color) and the color temperature of a pc-WLED can be tailored by mixing phosphor powders of green, yellow, orange and red emission wavelengths in various weight ratios. [2][3][4] Figure 1 depicts the variation in the (x,y) chromaticity coordinate 5 of the light emitting from the LED as a result of mixing different phosphors or by varying the thickness of a single phosphor layer. 3,4 Both z E-mail: duttap@rpi.edu these processes lead to linear shift in the (x,y) coordinates between the end points of the individual chromaticity points as explained below. In this paper, we report a nonlinear shift in the chromaticity coordinate by using a full spectrum phosphor alloy instead of phosphor blends.
Referring to Figure 1, the (x,y) coordinate B corresponds to the peak emission wavelength around 460 nm of a typical GaInN blue LED chip. The (x,y) coordinates G, Y, O and R correspond to peak emission wavelengths of typical green, yellow, orange and red phosphors, respectively. By increasing the thickness (the quantity) of the phosphor powder on the blue LED chip, the emission intensity ratio of the blue versus the phosphor emission peak decreases. Thus the emission color of the LEDs can be continuously tuned with (x,y) coordinates shifting linearly along the direction of the arrows on the straight dashed lines connecting the points BG, BY, BO and BR in Figure 1. By dispensing appropriate amounts of a yellow or an orange phosphor on a blue LED chip, one could create a cool white (CW) LED with CCT around 6000 K or a warm white LED (WW) with CCT around 3000 K (Figure 1). Though it is desirable to have the (x,y) coordinate of the pc-WLEDs on the Planckian blackbody locus (curve shown in Figure 1 with equivalent blackbody temperatures in Kelvin), it is practically difficult to develop and use a multitude of phosphor compounds for fabricating LEDs with different color temperatures. Furthermore, for achieving high color rendering index (CRI) necessary for many applications, the emission spectrum must be tuned using two or more phosphors. Over the last two decades since the advent of candela class blue GaInN LEDs, 6 there has been wide-spread global research and development of phosphor compounds that could meet a host of stringent criteria for pc-WLED packages. 2,3,7-9 Today, CW and WW pc-WLEDs are fabricated by mixing two or three selected phosphors in optimized (but different) weight ratios (different phosphor blends).
Spectrum tuning by mixing phosphors is a tedious process. Slight variations in the individual phosphor quantity in the mixture (blends) can shift the (x,y) coordinate away from the desired point. To the best of our knowledge, there are no reported fixed weight ratio of phosphor compounds that could be used to fabricate both CW and WW pc-WLEDs. This paper reports a single phosphor alloy that could be used to fabricate a wide range of pc-WLEDs with (x,y) coordinate always lying on the Planckian locus 5 by simply varying the thickness of the phosphor layer on the 400 nm excitation LED source. The shift in the (x,y) coordinates is shown by the arrows along the Planckian locus in Figure 1. At the same time, high CRI (>90) is maintained throughout the color temperature range. The phosphor compound (alloy) used in this study was selected from a general composition space of Ca 1+x Sr 1-x Ga y In 2-y S z Se 3-z F 2 with (0 ≤ x ≤ 1, 0 ≤ y ≤ 2, 0 ≤ z ≤3) activated with Ce 3+ and Eu 2+ . 10 By varying the x, y and z and the activator species, the emission spectrum of this multi-component phosphor could be tuned to provide either single color or broad band full spectrum LEDs. The focus of this paper is on a specific full spectrum phosphor alloy with x = 0.04, y = 2 and z = 3. The process of arriving at this final composition based on series of experiments will be summarized below. The full spectrum phosphor was synthesized by alloying the necessary mole fractions of pre-synthesized powders of SrS:Eu 2+ , SrS:Ce 3+ Na 1+ , CaS:Eu 2+ , CaF 2 :Eu 2+ and GaS. Sodium sulfide (Na 2 S) was used for Na 1+ charge compensation. The concentration of each activator/dopant species (Eu, Ce, Na) in the starting precursor was kept at 2 mole%. The precursor powders were homogenized in a slurry form with acetone using a ZrO 2 jar, ZrO 2 milling spheres and a Retsch PM 100 planetary ball mill for 30-60 minutes at 600 rpm. After drying the slurry in air at room temperature, the dried powder was reacted at a temperature in the range of 800-850 • C in silica crucible under argon-hydrogen gas for a period of 2-3 hours. X-ray diffraction (XRD) measurements were carried out using a Bruker D8 Discover Diffractometer using Cu Kα radiation at 40 kV and 40 mA. Excitation spectrum measurements were carried out using a Flourolog Tau-3 lifetime measurement system. The emission spectrum and chromaticity (x,y) coordinate measurements were measured using a UPRtek MK350 spectrometer and an Ocean Optics Spectrometer (Jaz Spectroscopy Suite with Spectrasuite software) with an integrating sphere. For the excitation and emission spectrum measurements, the powder was mixed with Epoxy 20-3302 from Epoxies, etc. and coated on glass slides. The surface density of phosphor was increased in the range of 0.2 to 1 mg/mm 2 for changing the CCT between 20,000 K and 1700 K. Figure 2 shows a typical XRD pattern of Ca 1.04 Sr 0.96 Ga 2 S 3 F 2 : Ce 3+ , Eu 2+ phosphors synthesized by solid-state reaction. All peaks in the XRD pattern could be accounted for on the basis of ICDD PDF entries with the following index numbers: 00-025-0895, 00-008-0464, 01-071-4402, 03-065-2936, 01-075-0265, 03-065-1075, 00-035-0816, 00-050-0115 corresponding to known phases of strontium gallium sulfide, calcium sulfide, europium strontium sulfide, strontium calcium sulfide, strontium sulfide, calcium fluoride and strontium cerium oxide, respectively. The well defined narrow peaks present in the XRD pattern clearly indicate the high crystalline phase purity and uniform activator incorporation in the synthesized phosphor. Figure 3 shows photographs captured at the aperture of an integrating sphere during the emission of different pc-WLEDs fabricated using this phosphor. The various color shades of white light emitted by the full spectrum pc-WLEDs with different CCTs are distinctively visible. The (x,y) coordinates and the CRIs of various pc-WLEDs fabricated are presented in Figure 4 (filled triangles). Interestingly, the (x,y) coordinate was found to traverse along the Planckian locus as the CCT was changed over a wide range from 20,000 K to 1700 K by simply increasing the thickness of the phosphor layer. Another interesting observation is that the CRI of each color temperature is greater than 90. In particular, the CCT range of 2670 K to 6500 K exhibited a CRI of 97-98. For the sake of comparison, commercially available pc-WLEDs from prominent vendors in the lighting industry exhibit a CRI of 80 for cool white range (6000-6500 K). Only the warm white LEDs exhibit CRI around 95. The spectrum of natural sunlight was also captured and selected (x,y) coordinates are shown in Figure 4 (filled circles). The CRI of 99 was recorded out doors for direct sunlight on a bright day with CCT varying between 4000 K and 6000 K depending on the time of the day. The sunlight CRI was measured to be less than 99 when the CCT was below 4000 K or above 6000 K, possibly due to absorption/scattering of selective wavelengths (in the solar spectrum) by the earth's atmosphere (pollutants or water vapor). On a cloudy day, the diffused sunlight exhibited a CCT of around 9000 K with CRI of 97. For sunlight inside a room penetrating through a clear glass window, the CRI measured was around 93 when the CCT was around 5000 K, possibly due to absorption/reflection of selective wavelengths in the glass. The calibration of CCT and CRI of the reported full spectrum pc-WLED with natural lighting conditions clearly shows that the quality of indoor lighting provided by this full spectrum artificial source can match or exceed the quality of the natural outdoor lighting conditions. A comprehensive analysis of the phosphor spectrum that is necessary to traverse of the (x,y) coordinates along the Planckian locus was carried out. The exercise of full spectrum engineering was systematically conducted by starting from a low CRI (∼70) cool white LED spectrum that is rich in the blue portion of the visible light spectrum and then step-wise filling the other emission ranges by tuning the phosphor composition while monitoring the change in CRI. Enhancement in peak emission for certain wavelength range (by increasing the concentration of the respective element in the multi-component phosphor) beyond a certain level did not result in further enhancement in the CRI. It must also be kept in mind that to avoid excessive scattering losses in the phosphor layer, the amount of phosphor coated on any blue LED chip must be kept as low as possible (minimized). After the conclusion of this optimization exercise, it was found that the CRI of higher than 95 could be maintained with the broad spectrum and the significant phosphor peaks at 450 nm, 490 nm, 550 nm and 630 nm as long as the molar ratios of various elements in the phosphor used (with Eu 2+ and Ce 3+ ) was approximately the following: (Sr/Ga: 1/2); (Sr/S: 1/3); (Ca/Sr: 1/1) and (Ca/F: 1/2). Any variations in the precursor compounds used in the synthesis process such as using SrGa 2 S 3 or CaGa 2 S 3 instead of SrS, CaS and GaS with the above molar ratios resulted in the similar spectrum and CRI higher than 95. One of the most intriguing observations from this study was the realization of the universal nature of the full spectrum phosphor for delivering high CRI in a wide range of color temperature. Once the full spectrum was optimized to provide CRI greater than 95 for cool white LEDs around 6500 K, the same phosphor could be used in different quantities with the same excitation source to create white LEDs across the entire color temperature gamut (1700 K -20,000 K) with CRI greater than 90. Another unique attribute was that all these high CRI LEDs exhibited color coordinates on the Planckian locus. Figure 5 presents the emission spectra of the LEDs covering the entire visible range. The narrow peak at 400 nm is due to the excitation source. The weak peak around 420 nm is attributed to a phase rich in CaF 2 : Eu 2+ . The peaks around 450 nm and 490 nm were found to be due to the Sr-rich phase of the Sr 1-x Ca x Ga 2 S 3 : Ce 3+ alloy. The peak around 550 nm is due to Ca-rich phase of the Ca 1-x Sr x Ga 2 S 3 : Eu 2+ compound. Finally, the peak around 630 nm is due to the Sr-rich phase of the Sr 1-x Ca x S: Eu 2+ compound. It must be pointed again that the different spectrum presented in Figure 5 were not created by mixing different ratios of various phosphors (which is typically done to change the spectrum of phosphor LEDs). Rather the variations in the spectra occurred naturally as the thickness of the phosphor Figure 5. Emission spectra of Ca 1.04 Sr 0.96 Ga 2 S 3 F 2 : Ce 3+ , Eu 2+ illuminated with a 400 nm LED for CCTs (a) below 5000 K and (b) above 5000 K. The (x,y) coordinate for each spectrum was on the Planckian locus (Figure 4).  layer changed indicating inherent interaction between the emission and excitation of the various luminescence centers (phases) present in the alloy.
To understand the interactions between the luminescence centers, we analyzed the excitation spectrum of the phosphor for various emission wavelengths as shown in Figure 6. The intensity of the individual curves has been normalized for the sake of presentation. A preliminary analysis of the intensity of each excitation spectrum and its effect on the absorption and down conversion wavelengths was found to consistently explain the natural evolution and the trend observed in the change of shapes of the emission spectrum (shown in Figure 5). As the thickness of the phosphor layer is increased, the peak intensity of longer wavelength components such as the 630 nm emission increases more rapidly than the other peaks due to absorption of the shorter wavelength emission as evident from the excitation spectra. While the absorption and down conversion processes in a full spectrum phosphor ensemble might appear trivial for the shift in color temperature toward warmer white shades with increasing path length of phosphor, the non-linear traverse of the color coordinates along the Planckian locus requires intricate spectrum engineering as discussed in this paper. Finally, the increase in CRI with spectrum filling was also observed to be a non-linear process. The trade-offs between minimizing the amount of phosphor used on a LED and maximizing the CRI for all color temperatures clearly demonstrate the need for spectrum engineering exercise for each class of LED phosphors. A complete theoretical understanding of the absorption and re-emission processes for the alloy phosphor requires further research. Effect of alloy broadening and competition between various optical transitions in an alloyed system needs to be investigated.
As a part of the current research, a rigorous study was conducted using mixtures (phosphor blends) of a variety of high quality commercially available phosphors to create full spectrum broad-band spectra similar to the ones reported in Figure 5. The following phosphors were used in our studies: blue phosphor (Ba 0.86 Eu 0.14 MgAl 10 O 17 , product number 756512-25G from Sigma Aldrich) with peak emission at 460 nm, cyan phosphor (Lutetium based, product number CLG 500 200 from EMD Performance Materials) with peak emission at 500 nm, green phosphor (Eu doped silicate, product number EG3261 from Intematix) with peak emission at 530 nm, yellow phosphor (Aluminate, product number GAL560-02-13 from Intermatix) with peak emission at 560 nm, orange phosphor (Eu doped silicate, product number O5742 from Intermatix) with peak emission at 600 nm, and red phosphor (Nitride-based, product number XR6436-01-15 from Intermatix) with peak emission at 625 nm. Despite having a full spectrum of the visible wavelength covered by these individual phosphors, it was not possible to recreate the trend and phenomenon shown in Figure 4 with any of the mixtures. Furthermore, CRI of 90 was only achieved for a small range of CCT. This clearly demonstrates that the results reported here are non-trivial and appropriate phosphor spectral (excitation and emission) engineering is key for achieving high CRI for the entire range of CCT as well as achieving chromaticity coordinates lying on the Planckian blackbody locus.
Since 1970's tri-band phosphor blends in commercial applications have been meticulously designed and optimized based on the red, green and blue photoreceptors in the human eye. The tri-phosphors blends enable high efficacy, while preserving good (acceptable) CRI for the human tri-color vision. Since the human eye is not equally sensitive to all wavelengths, any light source built to emit all wavelengths will have a significant portion where the human eye sensitivity is low. Therefore the full spectrum light source is expected to have lower theoretical lumen efficiency than the tri-band phosphor sources. However in recent years, studies on the role of various wavelengths on the human well being have started to emerge. With cost of LED based lamps rapidly decreasing and the energy efficiency of the light sources increasing, the role of lighting beyond illumination is of significant interest. As described earlier, designing full spectrum light sources maintaining high CRI is a non-trivial process. The phosphor alloy approach provides a simple manufacturing process with the degree of freedom for achieving any CCT while maintaining high color quality indexes. For various applications, the full spectrum could help in satisfying emerging color rendering metrics such as color quality scale (CQS), gamut area index (GAI), color fidelity index (CFI), color saturation index (CSI), hue distortion index (HDI), luminance distortion index (LDI), color dulling index (CDI), etc. as well as high individual CRI indexes beyond R9. 11 Therefore, the compromise on lumen efficiency for the full spectrum source can be easily traded with other desirable features of the light quality for various applications.
In summary, a new single phosphor alloy has been reported that provides full spectrum emission in the visible range. Most interestingly, this phosphor has demonstrated white LEDs with color coordinates that naturally falls on the Planckian locus and with high CRI (exceeding 90) for the entire white light color temperature. This is a first of a kind phosphor composition reported in the literature. The general shapes of the spectra (excluding the 400 nm LED peak) were found to match with the sunlight spectra.