Double-Color Emitting SrSi₂O₂N₂:Eu²⁺,Yb²⁺ Oxynitridosilicate Phosphor for Warm White-Light Emitting Diode with Highly Stable Color Chromaticity

Si₃N₄, SrCO₃, Eu₂O₃ and Yb₂O₃ were used to synthesize SrSi₂O₂N₂:Eu²⁺,Yb²⁺ green to orange emitting phosphor. All ingredients with at least 3N purity were purchased from Aldrich Company and used without any further purification. The stoichiometric amount of the starting materials were mixed thoroughly by an agate mortar using ethanol, dried 2 h in an oven at 120°C. As prepared powder mixtures were packaged in an alumina crucible, and subsequently, fired at 1400°C for 8 h in a horizontal alumina tube furnace in the N₂/H₂ (5%) gas flow. Eu²⁺ concentration was fixed at 2 at% with respect to that of Sr²⁺ ion, the meantime the amount of Yb²⁺ ion was varied from 0 to 8 at%. A slightly sintered sample has been obtained after firing, which later on, was preliminary grinded into a fine powder. As-prepared phosphor powder was applied to generate white light LED. The obtained phosphor powder mixed with silicon resin (The Dow Chemical Company, EG6301A and EG6301A) then the mixture was packed on top of InGaN based blue-emitting chip, and dried at 120°C for 2 h.

Phase composition analyzes were carried out by using X-ray diffractometer (Rigaku D/max IIIC, Rigaku, Japan; with Cu Kα (λ = 1.542 A°) at RT. The microstructure of the synthesized powder was evaluated using scanning electron microscope (S-5000, Hitachi Ltd., Tokyo, Japan). The photoluminescence spectra were recorded at room temperature using a fluorescent spectrophotometer (F-700, Hitachi Ltd., Tokyo, Japan) with a 150 W Xe lamp as an excitation source. Decay time was recorded by streak camera C4334 (Hamamashu, Japan). The quantum efficiency was recorded using integrated sphere method. Thermal quenching was tested using PTE-VUVD2L-100.

Figure 1a shows the X-ray diffraction patterns of the obtained powder singly doped and co-doped by two ions, e.g. Eu²⁺ and Yb²⁺. The peaks of the X-ray diffraction patterns of all samples are matched fairly well to those of the SrSi₂O₂N₂ reference (JCPDS 49-0840). Several unindexed peaks at 27 to 31° (corresponding peaks are marked with "*") of unknown phase are present, which have been also reported in previous studies.^17–19 However, compared to the results reported by J. Ruan et al. obtained product consists of almost a single phase, and does not contain significant amount of impurities, such as Sr₂SiO₄. With introducing of Yb²⁺ the new peak arises at 28° (corresponding peak is marked with "o"), which can be seen in several samples, however, its origin is not clear. The ionic radii of Eu²⁺ (1.17 Å, 6-coordination number)¹⁸ and Yb²⁺ (1.14 Å, octahedral coordination)²⁰ are close to that of Sr²⁺ (1.18 Å, 6-CN.)¹⁸ As showed in the previous studies,^16,17 Eu²⁺ and Yb²⁺ cations preferably occupy the Sr²⁺-sites in the lattice of SrSi₂O₂N₂. In our experiments negligible displacement of position of the XRD peaks suggests that forming of solid–solution process has occurred when SrSi₂O₂N₂ is doped with Eu²⁺ and Yb²⁺. Worth to note, that results obtained are well matched to previously reported researches.^16,17 Figure 1b shows the micrograph of the SrSi₂O₂N₂:Eu²⁺,Yb²⁺ phosphor. The powder consists of well-crystallized grains with average particle size of 5–10 μm. The powder is well dispersed obviously, although some agglomerates can be observed, too.

Zoom In Zoom Out Reset image size

Figure 2 shows the excitation and emission spectra of Eu²⁺ and Yb²⁺-doped SrSi₂O₂N₂ phosphor. The emission spectrum of SrSi₂O₂N₂:Eu²⁺ phosphor shows a single emission band peaked at 540 nm, which is assigned to the 4f⁶5d¹→4f⁷5d⁰ transition of Eu²⁺ ion.^15–19 The emission is observed at longer wavelengths compared to well known silicate²¹ or aluminate²² based phosphors due to higher degree of covalency between the activator ion and surrounding ligands.¹⁶ The full-width at half maximum (FWHM) of the emission band is 76 nm. The excitation spectra of Eu²⁺ monitored at 540 nm shows the excitation band in 200–500 nm range. The excitation spectra consist of several unresolved sub-bands attributed to the 4f⁶5d¹ excited states of Eu²⁺ ions.^16,17 The spectra of emission and excitation fairly agree to those of reported earlier.^14–20 The emission of Yb²⁺ doped SrSi₂O₂N₂ phosphor is a single emission band peaked at 612 nm attributed to relaxation to 4f¹³5d⁰ level of Yb²⁺ ions (FWHM = 130 nm). The excitation spectrum of SrSi₂O₂N₂:Yb²⁺ phosphor consists of several bands, which is assigned to f-d transition. The unusual red-shifting of the emission band of Yb²⁺ ion in the SrSi₂O₂N₂:Yb²⁺ phosphor is observed and well described by V. Bachmann et al.¹⁶ Emission was ascribed to an Yb²⁺-trapped exciton luminescence. Notable, as can be seen the excitation spectrum of the SrSi₂O₂N₂:Yb²⁺ is partially overlapped with the emission of SrSi₂O₂N₂:Eu²⁺ phosphor in 470–550 nm range. According to Dexter's theory,²³ the energy transfer rate is proportional to the spectral overlap between the energy donor (e.g. Eu²⁺) emission band and the energy acceptor (Yb²⁺) absorption band.

Zoom In Zoom Out Reset image size

Figure 3 shows the emission spectra of SrSi₂O₂N₂ phosphor co-doped with Eu²⁺ (2 at%) and Yb²⁺ (1–8 at%) ions. The obtained phosphor shows broad band emission, which can be well decomposed into two Gaussian curves peaked at 540 and 612 nm (doted lines in figure 3, showed the decomposed curves for 4% Yb²⁺ ion contents). Those two sub-bands are characteristically for single doped SrSi₂O₂N₂ phosphor as shown above. As content of Yb²⁺ ion increases in the SrSi₂O₂N₂:xEu²⁺,yYb²⁺ phosphor, the green emission (λ_em. = 540 nm) originated from Eu²⁺ decreases and, consequently, orange emission from Yb²⁺-trapped exciton (λ_em. = 612 nm) appears. This phenomena may be explained by the energy transfer between the Eu²⁺ ion emission and Yb²⁺ ion absorption bands. The curve shape, and emission intensity vs. activator ion concentration trend is similar to that of reported by J. Ruan et al.¹⁷ At relatively low concentration range of Yb²⁺ (1–2 at%) the intensity at 612 nm increases, and then, above 2 at% the PL intensity decreases due to concentration quenching ascribed to the cross relaxation process between trapping site of large number of Yb²⁺ ions.¹⁷ Accordingly the (x, y) CIE coordinates are shifted to the orange region (see inset of figure 3).

Zoom In Zoom Out Reset image size

The spectral overlap between the Eu²⁺ emission and Yb²⁺ absorption bands gives rise to the energy transfer between them. The energy transfer among luminescent centers is verified by decreased values of decay time constant. Figure 4 shows the luminescence decay time curves monitored at 540 nm of the obtained phosphor recorded at room temperature. The luminescence decay curves can be well approximated by double-exponential decay curve given by equation 1.²⁴

Here I is the luminescence intensity; t is the time; A₁ and A₂ are constants; and τ₁ and τ₂ are short and long lifetimes for exponential components respectively. Based on the determined values for all aforementioned parameters, the average decay time (τ^*) can be calculated by the equation 2.

Zoom In Zoom Out Reset image size

The average decay time (τ^*) is calculated to be 0.66, 0.56, 0.44, and 0.35 μs for 0, 1, 3, and 6 at% of Yb²⁺ ion content respectively. The obtained decay time τ^* is close to the typical value (1 μs) for the parity and spin allowed 5d–4f emission of Eu²⁺ ion.^{6,8,16,17,23,24} By increasing the content of Yb²⁺ ion the decay time of Eu²⁺ decreases down to 0.35 μs indicating energy migration to trapping site of Yb²⁺ ion.

Figure 5a shows the temperature dependence of the luminescence emission of SrSi₂O₂N₂ phosphor co-doped with 2 at% Eu²⁺ and 6 at% Yb²⁺. With increasing temperature, the orange component decreases significantly compared to the green component. This phenomenon also supports that the orange emission is originated from Yb²⁺-trapping site which is sensitive to the temperature variation. CIE coordinates gradually moves from [0.5095; 0.4832] to [0.455; 0.5344] by increasing temperature, which can be well seen on the picture 5b. We also applied the obtained phosphor with 2 at% Eu²⁺ and 6 at% Yb²⁺ composition as a down-conversion phosphor for white LED application. Commonly, the mixture of individual green and orange-red emitting down-conversion phosphors are being used to generate white light in combination with InGaN-based blue LED chip.²⁵ Thus, the emission color balance can be easily tuned by varying the amount of each phosphor. In the prepared LED, since only single green to orange emitting phosphor was being used, we varied its amount to achieve the highest color rendering (figure 6, spectra are normalized based on blue emission). Here the shape of curve in green to orange range is different from the corresponding one (2 at% Eu²⁺ and 6 at% Yb²⁺) presented in the figure 3, where orange emission is higher compared to green emission. However, this difference is ascribed to the effect of temperature on PL emission. Thus, the curve shape obtained at LED operation (figure 6) is similar to that of in the figure 5a corresponding to 75–120°C range. Notably, this temperature range is matched with the operating condition of the LED chip. At relatively low amount of phosphor compared to encapsulation resin, which indicated by "m1", the blue emission dominates (CIE x = 0.2805; CIE y = 0.2014). By increasing the total amount of light converting phosphor by m1 < m2 < m3 < m4, emission from blue range gradually moves to white area, and then to green-orange area (m4 sample, CIE x = 0.4071; CIE y = 0.3789). Inset of figure 6 graphically indicates the CIE coordinates for corresponding LED chips.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

After the fine tuning of the phosphor emission characteristics, and the amount of powder used in LED preparation, we could achieve up to Ra = 89 for the color rendering index of the as-prepared white LED. The high Ra has been achieved when the ratio of red to green component in LED spectrum is >1.²⁵ Figure 7 shows the emission spectra of our LEDs based on utilization of further-optimized SrSi₂O₂N₂:xEu²⁺,yYb²⁺ (x = 2 and y ≅ 5.5) as a down-conversion phosphor in combination with InGaN blue LED chip (λ = 450 nm). Table I summarizes full set of the general color rendering index Ra and 9 CRI of the prepared LED. As can be seen from the figure 7, by applying the forward-bias current from 100 to 500 mA at 3 V operating voltage the emission coming out from the prepared LED is getting intense. Figure 7 inset shows that the CIE chromaticity coordinate remains stable in wide range of forward-bias current. Table II summarizes the values of the color rendering index (CRI), color correlated temperature (T_cc), and CIE chromaticity coordinates.

Zoom In Zoom Out Reset image size

In conclusion, a double-color emitting SrSi₂O₂N₂:Eu²⁺,Yb²⁺ oxynitridosilicate phosphor has been developed for singly used in a warm white LED for the first time. The obtained SrSi₂O₂N₂:Eu²⁺,Yb²⁺ phosphor shows broad band emission centered at 540 and 612 nm for Eu²⁺ and Yb²⁺ ions respectively. The relative ratio of green and orange emissions was found to be dependent on the relative concentration of activator ions in the phosphor. Short decay time of Eu²⁺ emission confirmed the energy transfer from Eu²⁺ ion to Yb²⁺ trapping sites. The fabricated white LED, integrating 450 nm emitting InGaN chip with a double color emitting SrSi₂O₂N₂:Eu²⁺,Yb²⁺ phosphor, shows warm white light, high color rendering index, and superior color stability against input power.

This work was supported by Components & Materials Technology Development program of Ministry of Knowledge Economy (project - 10036981) and BK21 Program.