Evaluating the use of blue phosphors in white LEDs: the case of Sr0.25Ba0.75Si2O2N2:Eu2+
© Joos et al.; licensee Springer. 2014
Received: 28 February 2014
Accepted: 5 May 2014
Published: 19 May 2014
The luminescence properties of the blue emitting phosphor Sr0.25Ba0.75Si2O2N2:Eu2+ are extensively investigated. This oxonitridosilicate phosphor features strong 4f65d1 - 4f7 luminescence originating from the Eu2+ ion, with a narrow emission band peaking at 467 nm and a full width at half maximum of only 41 nm. Thermal quenching of the blue luminescence only sets in above 450 K, making this material an interesting candidate as LED conversion phosphor. The fast decay of the luminescence prevents the phosphor to be susceptible to saturation effects at high excitation fluxes. Furthermore it is proven to be chemically stable against moisture. The only drawback is the relatively low quantum efficiency of the synthesized powder, provisionally preventing this material to be used in applications. In addition, the phosphor features a weak yellow emission band, originating from small domains featuring a different crystal structure. It is shown that the majority of the powder grains only exhibit blue emission. Finally, the spectrum of a white LED, based on a UV pumping LED and three (oxy)nitride phosphors is simulated in order to assess the usefulness of blue phosphors in LEDs for lighting. Only a marginal improvement in terms of color quality can be achieved with a narrow banded phosphor, at the expense of a decrease in luminous efficacy and overall electrical to optical power efficiency.
PACS 70 – Condensed Matter: Electronic structure, Electrical, Magnetic, and Optical Properties
PACS 42.70.-a Optical materials
Light-emitting diodes or LEDs are steadily consolidating their share in the lighting and display market. Nevertheless, current technology is not yet fully established and improvements are still desirable, for both lighting and display applications.
For lighting applications, there is still a fair margin for improvement in terms of luminous efficacy and color rendering. The first phosphor converted LEDs (pc-LEDs), based on a blue In1-xGa x N pumping diode and a yellow (typically Y3Al5O12:Ce3+[1, 2]) conversion phosphor, suffered from a low color rendering due to the lack of green and red light in the spectrum and of inherently high correlated color temperatures (CCT). This issue could to a great extend be solved by adding a red phosphor (for example Ca1-xSr x S:Eu2+ or Sr2Si5N8:Eu2+[4, 5]). While the low energy sector of the visible spectrum is then sufficiently covered by the phosphor blend, a void remains in the blue-cyan part of the spectrum, in between the narrow banded spectrum of the pumping LED and the emission of the phosphor(s). The first way to solve this consists of filling the gap by using a cyan phosphor with a small Stokes shift. The second option, which will be pursued in this paper, uses a violet or near UV pumping diode in combination with an additional blue phosphor.
To evaluate whether a luminescent material is suitable for use in phosphor converted LEDs, several requirements have to be fulfilled simultaneously. These are described in detail in and comprise thermal stability of the luminescence, the absence of saturation effects at high excitation fluxes and chemical stability of the host material in combination with more obvious requirements such as excitability, a high conversion efficiency and desirable color point.
The MSi2O2N2:Eu2+ (M = Ca, Sr, Ba) system has already been the subject of extensive and detailed studies[7–14]. All three basic compounds (i.e. with only one type of alkaline earth metal) exhibit strong 4f65d1 - 4f7 luminescence upon doping with divalent europium. While BaSi2O2N2:Eu2+ features a greenish-blue narrow banded emission, SrSi2O2N2:Eu2+ and CaSi2O2N2:Eu2+ are characterized by significantly broader yellowish-green and yellow emission bands, respectively.
Structural and luminescence properties of europium doped MSi 2 O 2 N 2 phosphors
# cation sites
FWHM (RT) (nm)
QE (RT) int/ext
CaSi 2 O 2 N 2
SrSi 2 O 2 N 2 (triclinic)
SrSi 2 O 2 N 2 (monoclinic)
Sr 0.25 Ba 0.75 Si 2 O 2 N 2 (triclinic)
Sr 0.25 Ba 0.75 Si 2 O 2 N 2 (orthorhombic)
, this work
BaSi 2 O 2 N 2
EuSi 2 O 2 N 2
BaSi2O2N2:Eu2+ is an efficient phosphor, even though it has the lowest quantum efficiency of the basic compounds (Table 1). It features a narrow emission band peaking at 495 nm (full width at half maximum (FWHM) of 32 nm). Depending on the synthesis conditions, an appreciable afterglow and strong mechanoluminescence are observed, making it suitable for use as pressure sensor[10, 20, 23].
SrSi2O2N2:Eu2+ is a very efficient phosphor with a reported internal quantum efficiency of up to 90%. It also exhibits afterglow upon UV excitation. The superior thermal properties make this phosphor a primary candidate as green-yellow conversion phosphor in white phosphor converted LEDs. Up to 50% of Sr2+ can be replace by Ca2+ to gently shift the emission color from yellow-green to yellow while keeping a single crystal phase. Finally, CaSi2O2N2:Eu2+ is characterized by a much worse thermal quenching behavior.
In Bachmann et al., the Sr1-xBa x Si2O2N2:Eu2+ solid solution is investigated for x up to 0.75. By replacing Sr2+ by Ba2+ in the SrSi2O2N2:Eu2+ phosphor, the color is surprisingly red shifted. This was explained by Eu2+ preferentially occupying Sr2+ sites in the lattice, while further substitution of Sr2+ by Ba2+ enlarges the experienced crystal field strength, due to compression of the coordination sphere of the remaining Sr sites. It is impossible to synthesize solid solutions between the yellow (x = 0.75) and blue-green (x = 1) phosphor[7, 12].
In 2012, Seibald et al. reported the remarkably blue luminescence of a Sr0.25Ba0.75Si2O2N2:Eu2+ phosphor and its crystal structure. From single-crystal diffraction, the “averaged” crystal structure was determined to be orthorhombic, while a more detailed investigation with HRTEM allowed to resolve the local cation ordering, resulting in a triclinic structure. This is in contrast to the yellow phosphor with the same stoichiometry, which resembles the triclinic SrSi2O2N2 crystal structure. The host material of this blue emitting material turned out to be composed of a BaSi2O2N2 like structure. However, for this particular composition, the metal layers are corrugated, yielding different luminescent properties than BaSi2O2N2:Eu2+ where the metal layers are parallel.
In another paper, in 2013, Seibald et al. reported the occurrence of an additional monoclinic SrSi2O2N2 phase. A single crystal could be isolated and structurally analyzed. The emission signal is slightly blue shifted compared with the common triclinic SrSi2O2N2 phase. The structural difference between the two phases is the relative orientation of consecutive silicate layers. It was questioned whether it would be possible to synthesize a pure SrSi2O2N2 phase without formation of the other one.
In this paper, a more thorough luminescence study is conducted on the blue oxonitridosilicate phosphor Sr0.25Ba0.75Si2O2N2:Eu2+, which was reported in 2012 but not yet fully characterized until this moment. Possible applications for this phosphor are evaluated, after which the discussion is generalized to evaluate the usefulness of blue phosphors in LED lighting technology.
In one step, with SrCO3 (Alfa Aesar, 99.99%), BaCO3 (Alfa Aesar, 99.95%), Si3N4 (α-phase, Alfa Aesar, 99.9%) and EuF3 (Alfa Aesar, 99.5%) as starting materials. After dry ball mixing, the powder mixture was heat treated at 1400°C for 4 hours (heating rate of 4.7°C/min).
In two steps. First (Sr0.25Ba0.75)2SiO4:Eu is prepared from SrCO3, BaCO3, SiO2 (Alfa Aesar, 99.9%) and EuF3 at 1250°C for 3 hours (heating rate of 4.7°C/min). Second, Si3N4 was dry ball mixed with the obtained orthosilicate and heat treated at 1400°C for 4 hours (heating rate of 4.7°C/min). This is the synthesis recipe described in .
After the heat treatments, the powders were allowed to cool naturally and were lightly ground.
Powder X-ray diffraction (XRD) measurements were performed on a Siemens D5000 diffractometer (40 kV, 40 mA) using CuKα1 radiation.
Photoluminescence emission and excitation spectra were measured with an Edinburgh FS920 fluorescence spectrometer. Measurements as a function of temperature were performed using an Oxford Optistat CF cryostat.
Decay profiles were measured using a pulsed nitrogen laser (wavelength 337 nm) as an excitation source in combination with an Andor intensified CCD.
SEM-EDX-CL measurements were performed with an Hitachi S-3400 N scanning electron microscope (SEM), equipped with a Thermo Scientific Noran 7 energy dispersive X-ray detector (EDX). Furthermore, cathodoluminescence (CL) was collected with an optical fiber and analyzed by a CCD camera (Princeton Instruments ProEM 16002), attached to a spectrograph (Princeton Instruments Acton SP2358). An integrating sphere (LabSphere GPS-SL series) was used to measure the internal and external quantum efficiency (QE) of the phosphor powders upon LED excitation (peak wavelength of 370 nm). Al2O3 was used as a white reflecting standard.
Accelerated aging tests were done inside a Memmert HCP108 humidity chamber, with a constant temperature and relative humidity (RH) of 75°C and 75% RH. The photoluminescence was monitored in situ by excitation with a 370 nm LED.
Results and discussion
Synthesis and XRD
After the heat treatment, visual inspection under UV illumination learns that the bulk of the obtained phosphor powder has the desired blue color. On top of the blue powder, a thin, green emitting layer is formed. For further investigation, this green emitting top layer was removed and kept apart. This green emitting layer was also formed during the dual step synthesis. X-ray diffraction (XRD) and photoluminescence (PL) measurements (see further) were addressed to verify that the result of both syntheses are indeed identical. The powder prepared with the one step synthesis was selected for further investigation.
It was reported that the addition of small amounts of NH4Cl as fluxing agent during the temperature treatment improves the formation and crystallization of the SrSi2O2N2:Eu2+ phosphor. The influence of NH4Cl (2 weight % of the final mass) on the formation of the blue phosphor was investigated. It turns out that the fluxing agent rather stimulates the formation of an undesired yellow emission band next to the blue emission band. Therefore, NH4Cl was omitted from further syntheses.
From the XRD measurement of the green emitting top layer, it can be derived that it is mainly composed of Ba3Si6O9N4:Eu2+ (filled circles, Figure 2) and Ba3Si6O12N2:Eu2+ (open squares, Figure 2). The locations of the experimental reflections are slightly shifted to higher 2θ values, which is explained by the incorporation of some Sr2+ in the crystal structure, decreasing the effective lattice parameters. Ba3Si6O12N2:Eu2+ is known to give green emission, explaining the green color of the top layer under UV excitation[27, 30]. The blue luminescence of Ba3Si6O9N4:Eu2+ is thermally quenched at room temperature. Note that both impurities have the same (Sr,Ba):Si ratio of 1:2, as in the intended stoichiometry. The main difference is the larger oxygen to nitrogen ratio, which is probably due to oxygen traces during the synthesis process.
In the emission spectrum, a second contribution in the yellow range around 560 nm, is clearly visible. The origin of this second emission band might be the occurrence of a Sr1-xBaxSi2O2N2:Eu2+ phase with a different structure than the intended blue phosphor, which has “averaged” an orthorhombic structure. In their paper, Bachmann et al. report a Sr0.25Ba0.75Si2O2N2:Eu2+ phase with the triclinic SrSi2O2N2 structure, emitting a broadband spectrum, peaking at 564 nm[7, 31]. It is not unlikely that this is the phase responsible for the weak yellow emission band. If this impurity phase indeed has the SrSi2O2N2 structure, it is not surprising that the addition of NH4Cl flux during the heat treatment stimulates the formation of it. This yellow emission band also occurred in the spectrum from the original paper reporting the blue phosphor.
Since the XRD measurements suggest the occurrence of small quantities of BaSi6N8O:Eu2+, it is important to verify whether this has an influence on the photoluminescence of the powders. The emission spectrum of BaSi6N8O:Eu2+ features a broad band (FWHM = 102 nm), centered at 503 nm and can be excited between 200 nm and 400 nm, although no values for the QE have been reported. To verify the occurrence of this blue-green phosphor, two emission scans of the prepared powder were compared, one at 310 nm excitation and one at 410 nm excitation, the former chosen in the maximum of the excitation band of BaSi6N8O:Eu2+, the latter chosen outside the excitation band of BaSi6N8O:Eu2+. Since no observable differences between the two scans were noticed, it can be concluded that the light-emitting BaSi6N8O:Eu2+ phase, as described by R.-J. Xie et al., is not observed in the prepared powder. This has therefore no influence on the characterization of the luminescence of the Sr0.25Ba0.75Si2O2N2:Eu2+ powders apart for a possibly negative influence on the overall quantum efficiency of the phosphor.
Herein, Ninc, Nabs and Nem are the number of incident, absorbed and emitted photons respectively. A is the absorbed fraction. For the Sr0.25Ba0.75Si2O2N2:Eu2+ phosphor, values of ηext = 30% and ηint= 41% were obtained. This is significantly lower than the internal quantum efficiencies of benchmark phosphors (typically ηint ≈ 90%[11, 32]). The grey shade in the body color of the powder already hinted towards an insufficient QE. If impurity phases which do not emit light (potentially BaSi6N8O) are present in the powders, the measured QE will be lower than the intrinsic QE of the Sr0.25Ba0.75Si2O2N2:Eu2+ phosphor. However, a sound optimization of the synthesis process (e.g. finding a suitable fluxing agent) might improve the quantum efficiency considerably because there does not seem to be a fundamental reason why the Sr0.25Ba0.75Si2O2N2:Eu2+ phosphor should have a lower QE than comparable phosphors such as BaSi2O2N2:Eu2+ or SrSi2O2N2:Eu2+.
From the photoluminescence measurements, it is clear that not a phase pure material is obtained. Combining cathodoluminescence (CL) and energy dispersive X-ray spectroscopy (EDX) inside a scanning electron microscope (SEM) should help to get a better grasp of the different phases that occur in the phosphor powder and their luminescence.
At a few places, a higher than average silicon content could be found, although it could not be linked to a specific chemical composition, nor was it reflected in the CL spectra. These Si-rich areas are possibly related to the BaSi6N8O phase which might be present in the powder. SEM-CL confirms the result from the PL measurement that there is no light emission originating from this phase. No results from thermal quenching or quantum efficiency measurements of the BaSi6N8O:Eu2+ phosphor are reported in literature. Furthermore, given the peculiar high Stokes shift of this material (1.14 eV or 9200 cm-1) and width of the emission band (estimated about 0.5 eV or 4033 cm-1), one can doubt whether the reported luminescence is originating from a ”normal” Eu2+ activated phosphor[29, 34, 35].
The studied area, indicated by the blue rectangle in Figure 5, was divided into 256 by 192 pixels and in each pixel a full cathodoluminescence (E = 15 keV) emission spectrum was recorded. Then for each spectrum, key luminescence parameters, such as the band width (FWHM) and the peak emission wavelength (λmax) were determined. Figure 5 clearly shows that all studied particles have an emission peaking between 465 and 468 nm, while the FWHM ranges between 37 and 43 nm. This effect is larger than the mere consequence of the use of wavelength units. Averaging out over all pixels, a CL emission spectrum is obtained which is similar to the PL emission spectrum. Although the emission is very homogeneous over the studied area, there appears to be some correlation between λmax and the FWHM, as longer peak emission wavelengths tend to coincide with wider emission bands (Figure 5), which could be due to local variations in composition (e.g. Eu concentration or Sr:Ba ratio).
The SEM-CL study also allows to probe the origin of the yellow emission band, peaking at 558 nm, when preparing Sr0.25Ba0.75Si2O2N2:Eu2+. Note that this emission band is found both in and in this work. The yellow emission amounts to no more than 20% of the total emission intensity, and certainly no areas with pure yellow emission (i.e. in the absence of the blue emission band) could be found, which would be the case for a separately formed impurity phase. For these areas with a larger fraction of long wavelength emission, no deviation in stoichiometry could be found by means of SEM-EDX. This is no surprise since it was shown by Seibald et al. by a combination of TEM/HRTEM/TEM-EDX that a domain structure is present at a nanometer length scale, impossible to resolve with SEM-EDX. The yellow emission is originating from domains with a Sr-richer content, composed of the SrSi2O2N2 structure[14, 19].
The SEM-CL study shows that yellow emission is always accompanied by the blue emission, within the same phosphor particles, supporting the conclusion by Seibald et al. that the yellow emission is due to intergrowth on the nanoscale. Nevertheless, the main fraction of the studied phosphor particles shows only the blue emission band, which offers the promise to prepare a purely blue emitting phosphor, without the additional yellow emission band from the Sr-rich domains. Regardless the impossibility of a SEM to resolve the nanoscopic domain structure, this clearly illustrates that the majority of the micrometer sized grains exhibit only the blue luminescence. Therefore, the submicron resolution of CL in a SEM offers a fast and elegant way to probe the luminescence behavior at the single particle level, by being complementary to the aforementioned TEM study at the nanoscale.
Features of the emission spectrum of Sr 0.25 Ba 0.75 Si 2 O 2 N 2 :Eu 2+ as a function of temperature
du’v’(T, 300 K)
Decay of luminescence
Both components are in the expected range for lifetimes of the 4f65d1 excited state in the case of blue emission. This is slightly faster than the luminescent lifetime of Eu2+ in BaSi2O2N2 (Table 1). This lies within expectations because the luminescence lifetime shortens when the emission color is blue shifted on condition that the refractive index does not change. The origin of the faster decay component, presumably related to a non-radiative decay path, could not yet be clarified.
Because of the fast decay of the 4f65d1 excited state of Eu2+ in this host lattice, it is expected that high excitation fluxes can be used without sublinear response, as is the case in some applications.
The chemical stability of the oxynitride phosphor was inspected by monitoring the in situ photoluminescence (excitation with a 370 nm LED) during an accelerated aging test inside a humidity chamber (75°C, 75% relative humidity). In a timescale of 200 hours, no significant decrease of the luminescence could be measured for both the blue and yellow components (not shown).
Blue phosphor for increased color rendering?
Almost all current white pc-LEDs are composed of a blue pumping LED (peaking typically at 455 to 460 nm and 20 nm FWHM) and a yellow phosphor or green-red phosphor blend. As it was suggested that the blue phosphor under study can be applied in pc-LEDs with high color rendering index (CRI), a simulation is conducted in order to estimate the increase in color rendering that can be achieved by using an additional blue phosphor.
In the simulations, a white LED with a correlated color temperature (CCT) of 4000 K is pursued. For lower CCT, the blue spectral region will only have a minor influence on the color rendering. To account for the green and red spectral region, standard broadband phosphors were selected for this: SrSi2O2N2:Eu2+ and Sr2Si5N8:Eu2+, a typical combination which is known to yield white light of good color quality and color rendering when combined with a blue pumping diode[6, 40].
For the blue component, a gaussian spectral shape was taken, serving as an approximation to the spectrum of a blue phosphor in a UV-pumped pc-LED or a blue LED in a blue-pumped pc-LED. To examine the influence on the color quality scale (CQS) and luminous efficacy of the radiation (LER), the peak wavelength and width (FWHM) of the blue component were varied. It was opted to study the CQS instead of the CRI, since the former was designed to account for the defects inherent to the definition of the latter.
It can be seen that it is possible to improve the color quality (CQS) by replacing the blue pumping LED by a UV pumping LED and a blue phosphor. However, the improvement is rather limited because good color quality can already be achieved in the traditional way with a pumping LED of 455 nm. The increase in CQS (from a value of 87 to 89), is at the expense of a decrease in luminous efficacy (315 lm/W instead of 331 lm/W). Higher color qualities can be achieved if blue phosphors with a broader emission spectrum are used (CQS up to 95 for a phosphor with FWHM ≥ 60 nm).
In this formula, ηLED and ηextr represent the electrical to optical power conversion efficiency of the pumping LED and the extraction efficiency of the LED package respectively. N phosphors are used with relative weights f i , internal quantum efficiencies ηint,i and spectra I i (E). f0 is the fraction of the spectrum of the pumping LED which is not absorbed by the phosphors. For UV pumping LEDs, f0 is ideally 0. The ratios in equation 5 are called the quantum deficits, originating from the Stokes shifts of the phosphors.
Given the similar external quantum efficiencies of In1-xGa x N LEDs (ηLED) in the blue and UV spectral region, there is no possibility to overcome the efficiency difference of at least 20% between blue and UV LED pumped white LEDs.
Because of the decrease in luminous efficacy and wall-plug efficiency while the color quality improves only slightly, it is very unlikely that relatively narrow band blue phosphors will be used in future high color quality LEDs for lighting. For devices with high color quality, whether expressed in CQS or CRI, much broader emission bands are to some extent beneficial.
For certain display or projection applications, based on violet laser excitation, blue phosphors with a narrow emission spectrum such as Sr0.25Ba0.75Si2O2N2:Eu2+ are interesting. In this case, color rendering is irrelevant and saturated colors, corresponding with narrow emission bands are compulsory to achieve a large color gamut for the display.
In this work, a complete characterization of the luminescence of a recently reported phosphor in the oxonitridosilicate family is given. The blue emitting material with stoichiometry Sr0.25Ba0.75Si2O2N2:Eu2+ is characterized by broadband emission peaking at 467 nm at room temperature and a good thermal stability of both the emission intensity and color. An additional weak yellow emission band was observed. The currently obtained internal quantum efficiency of 41% is too low to allow this phosphor to be used in applications. Nevertheless, this might be improved by an optimization of the synthesis procedure. The phosphor was found to be chemically stable. As a conclusion of this feasibility study, this blue oxonitridosilicate can be suitable for the use in applications if the quantum efficiency can be improved.
Additionally, the microscopic structure of this phosphor was studied. It turned out that no grains with a pure yellow emission could be found, the blue emission is everywhere dominant. The majority of the powder particles do however only emit blue light.
Finally, the potential of blue phosphors to improve the color quality of white LEDs for lighting was investigated because this is often quoted as motivation to study blue phosphors. It was found that only a minor improvement of color quality can be achieved by using a saturated blue phosphor such as the oxonitridosilicate which is subject of this paper, or the BaMgAl10O17:Eu2+ phosphor. This increase in color quality is at the expense of a decrease in luminous efficacy and overall electrical-to-optical conversion efficiency of the LED, leading to the conclusion that only the use of blue phosphors with a significant broader emission band (FWHM ≥ 60nm) is justifiable to produce LEDs with very high color quality (CQS > 90). Narrow band phosphors with a saturated blue color are however useful in case of projection or display applications based on conversion of near-UV light, e.g. in the case of laser diode excitation.
This work is financially supported by the agency for Innovation by Science and Technology (IWT) and BOF-UGent. The authors would like to acknowledge Olivier Janssens, Tareq Ahmad and Marlies Decraene for the assistance in the experimental work.
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