1. Introduction

White light-emitting diodes (LED) have been extensively used as a nonconventional light source from the advantages of low electric consumption, high brightness, long lifetimes and environmental friendliness [1,2]. Typical phosphors, such as yttrium aluminum garnet (YAG), and rare earth-doped nitride/silicate compounds, acting as a lighting converter have been utilized for high efficiency white LED [3–6]. As a new type of the remote phosphor for the white LED, the printing technology of the phosphor layer has been recently reported by using YAG:Ce phosphors which are stable at high temperatures [7]. The reported printing process requires firing of the YAG-based phosphor layer at a temperature higher than 700 °C. To utilize inexpensive high efficiency silicate-based phosphors, the processing temperature for densification and adhesion to the substrate needs to be reduced substantially below ~410 °C. Otherwise, silicate phosphors are chemically degraded resulting in the reduced luminescence performance [8–10]. Such low temperature processing also enables to use a regular soda lime silicate (SLS) glass as a substrate. The allowable maximum processing temperature for the SLS glass substrate without warping is around ~530 °C. Accordingly, the choice of glass having a low softening temperature is very critical in this printing approach. We recently reported the yellow silicate-based phosphor layer screen-printed on an SLS glass substrate with a resultant luminescence efficiency of ~32 lm/W at 200 mA [8].

In this work, the commercial red and green silicate phosphors are used with a low softening glass frit for the purpose of forming desirable thick films of <20 μm in thickness on a common SLS glass substrate at a low temperature of 400 °C. The mixed red and green phosphors were chosen to cover a broader range of emission wavelength compared to that of only yellow phosphor. The red/green approach exhibits the better luminous efficacy of ~55.1 lm/W. White luminescence characteristics induced by the excitation of a 443 nm blue chip are investigated mainly in terms of different ratios of red/green phosphors. The thickness of the phosphor layer and the relative content of the phosphors to the glass frit, which can affect the luminous efficacy, were not changed here. Our purpose is primarily to suggest the potential of the remote red/green silicate phosphor layer printed on the glass substrate as another white LED.

2. Experimental

A commercially-available Bi₂O₅-ZnO-B₂O₃ glass-based frit (BSP718, DAION Co., Incheon, Korea) was selected as a base matrix to promote adhesion with a regular soda lime silicate (SLS) glass substrate. The glass frit has a softening point of ~385 °C and an average particle size of ~3 μm. Commercially-available red Sr₂Si₇Al₃ON₁₃:Eu²⁺ phosphor (SDR630, Yantai shield., China) and green CaMgSi₂O₆:Eu²⁺ phosphor (SDS525, Yantai shield., China) were used. Total content of the two phosphors was fixed as 40 wt% relative to the glass frit while the weight ratio of (red: green) phosphors were varied as (10:0), (9:1), (7:3), (5:5), (3:7), (1:9) and (0:10). Sample ID of the films follows the phosphor ratio. For example, R9/G1 indicates the thick film containing 90 wt % red and 10 wt % green phosphors.

Viscous ink paste was prepared by admixing the silicate phosphors and glass frit with organic vehicles at 1400 rpm for ~6 min using a paste mixer (PDM-150, Daewha Tech, Korea). The vehicle was composed of ethyl cellulose (Kanto Chemical Co., Ltd, Tokyo, Japan), α-terpineol (90%, Aldrich, Milwaukee, WI) and lauric acid (98%, Aldrich, Milwaukee, WI). The resultant paste was screen-printed twice on the SLS glass substrate by using a 250 mesh screen. The printed paste was dried at 120 °C and sintered at 400 °C for 30 min.

Figure 1(a) shows a schematic of the printed remote phosphor structure consisting of the mixed red/green phosphors embedded in a glass matrix. The printed phosphor layer is preferred to be located on the bottom of the glass substrate to minimize the loss of emitted light from the chip source as reported earlier [8]. This preliminary work does not intend to change the thickness of films which must be the main parameter to increase the resultant luminescence efficacy. The thickness of the fired phosphor layer here was fixed as 17-20 μm after the double printing procedure.

 

Fig. 1 (a) A schematic showing the printed phosphor thick film on the bottom of glass substrate as a nonconventional remote phosphor structure, (b) a cross-sectional SEM image of the R5/G5 thick film fired at 400 °C for 30 min and (c) the corresponding surface SEM image with the EDS elemental mapping image of Bi, Sr and Ca.

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Crystalline phase of the thick films was identified by using an X-ray diffractometer (XRD: Max-2500, Rigaku, Japan). Surface and cross-sectional microstructures were observed by field emission scanning electron microscopy (FESEM: JSM-5400, JEOL, USA). Photoluminescence (PL) excitation and emission spectra of the fired phosphor thick films containing red/green phosphor were obtained using a fluorescence spectrophotometer (F-7000, Hitachi, Japan) with a Xe lamp at room temperature. Emission spectra and commission Internationale de l’Eclairage (CIE) chromaticity coordinates of the resultant LED device were measured by a spectrum analyzer (DARSA-5200, PSI, Korea) with an integrating sphere system using a 443 nm blue diode as an excitation source under current of 200 mA.

3. Result and discussion

Figure 1(b) shows a selected cross-sectional SEM image of the remote phosphor thick film for the R5/G5 case, which was fired at 400 °C for 30 min. Figure 1(c) demonstrates the corresponding surface image of the sample with the results of EDS elemental mappings with elements of Bi, Sr and Ca, which are the representative ones for glass matrix and red and green phosphors, respectively. Even though the firing temperature of 400 °C is quite low, densification of the thick films seemed to progress very well with the least porosity on surface. The adhesion with the glass substrate was excellent since the adhesion comes from the bonding between the glass frit and glass substrate during firing. The red and green phosphor particles well dispersed through the surface became embedded in the densified glass matrix as evidenced in the microstructures and elemental mappings. As the thick films reaches the almost full densification, some phosphor particles are likely to be protruded from the surface with substantial reduction of the film thickness as seen in the surface microstructure [8,11]. It is critical to observe that such low temperature firing can form a densified thick film layer since the silicate phosphors are known to be damaged easily at high temperatures [8,12].

Figure 2 shows the XRD pattern of the R5/G5 phosphor thick film densified at 400 °C as an example. For comparison, XRD patterns of the red and green phosphors and the glass frit were included. All the reference samples were also fired at 400 °C for 30 min. The glass frit was not crystallized at this temperature. The XRD pattern of the R5/G5 film is simply composed of the mixed peaks of the red and green silicate phases. It implies that the phosphors are not chemically reactive each other or with glass frit used here with regard to significant phase change upon firing.

 

Fig. 2 XRD patterns of the R5/G5 thick film, the red and green phosphors, and only glass thick film, which were fired at 400 °C for 30 min.

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XPS spectra were investigated to confirm the changes in chemical states of activator Eu ion in the red and green phosphor-glass film layers with increasing firing temperature around our firing temperature of 400 °C as shown in Fig. 3. The Eu 3d⁵ core level with identifiable Eu³⁺ and Eu²⁺ peaks are seen in the binding energy range of 1120-1140 eV. In the red phosphor case, the relative peak intensity of Eu²⁺ to Eu³⁺ was kept almost consistently with increasing firing temperature. However, the relative ratio seemed to decrease significantly in the green phosphor layer, which indicates thermal degradation of the phosphors. The transition of Eu²⁺ to Eu³⁺ is known to be associated to the adsorption of oxygen and the transfer of electrons bewteen the adsorbed oxygen and Eu²⁺ [13–15]. Stable existence of Eu²⁺ is preferred to maintain strong luminescence performance that originates from the 5d-4f transition of Eu²⁺ ions because of the strong coupling of the 5s electrons with host lattices [16–18]. The degraded luminous efficacy with increasing firing temperature was reported for yellow silicate phoshors [8]. The selected firing temperature of 400 °C seems to be appropriate since this temperature provides minimal thermal degradation for the phosphors while allowing good densification of the phosphor layer.

 

Fig. 3 XPS spectra at Eu 3d⁵ position of the (a) red and (b) green silicate phosphor layers fired at different temperature of 390 °C, 410 °C and 430 °C for 30 min.

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Figure 4(a) shows the emission spectra of the thick films fired at 400 °C, with different ratios of red/green phosphors. The spectra were obtained by the blue LED chip with an excitation wavelength of 443 nm. Two distinguishable broad emission bands were observed, which correspond to the green emission at ~520 nm and the red emission at ~630 nm. The varying tendency of the emission intensity between the green and red bands is easily discernable. The tendency matches well with the expected results depending on the relative contents of red/green phosphors. The inserted plot in Fig. 4(a) shows the changes of FWHM (full width at half maximum) of two 520 and 630 nm bands with the relative content of green phosphor. The smaller FWHM is evident in the green emission band over the compositional range of 30-90% green phosphors where the two green/red emission bands are apparently observed.

 

Fig. 4 (a) Emission spectra and (b) conversion efficacy of remote phosphor layers containing red/green phosphors which were fired at 400 °C, as a function of relative content of each phosphor. The inset plots correspond to the variations of the FWHM values in the 520 and 630 nm bands with the relative content of green phosphor.

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As expected, a higher content of each phosphor induced a stronger emission at the wavelength of excitation of the phosphor. The emission intensity of the red phosphor was stronger than that of the green phosphor as reported earlier [19]. For example, the stronger emission at 630 nm can be seen in the R5/G5 sample. In addition, the existence of the clear red and green peaks may indicate that there is no significant chemical interaction of the phosphors with the glass matrix over the entire mixture range of the two phosphors, which may deteriorate the intensity of light. Typically, these distinguishable green and red peaks are known to contribute more to the higher level of color rendering index compared to the case of only yellow phosphor [20,21].

Figure 4(b) demonstrates the calculated conversion efficiency of the phosphor-converted LED devices based on the emission spectra of Fig. 4(a). The conversion efficiency η was obtained by a simple relation of η = Q_f /ΔP_blue where Q_f is the intensity of emission and ΔP_blue is the difference between the input intensity (P_i) and the transmitted intensity (P_t) of blue LED. The conversion efficiency tended to increase from 18.4% for pure red phosphor to 24.5% for green phosphor as the relative content of green phosphor increased.

Figure 5 shows the excitation and emission peaks of each red and green phosphors. It confirms the overlapping of the emission wavelength range of green phosphor with the excitation range of red phosphor. The emission intensity of green phosphor is likely reduced relative to that of red phosphor. It is likely that more fraction of green phosphor is needed to compenstae the loss of emitted green emission by the red excitation.

 

Fig. 5 Excitation/emission spectra of individual red and green phosphor.

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Figure 6 shows the variation in luminous efficacy of the thick films with different ratios of red/green phosphors, as a result of the excitation of the 443 nm blue diode. An actual R3/G7 sample with the white light conversion is shown as an inset. The luminous efficacy η of the phosphor layer was obtained by a relation of η = L / P_input where L is the luminous flux (lumens) and P_input is the input power of the blue chip. The luminous efficacy value increased gradually up to the R3/G7 ratio. The maximum value of the R3/G7 sample was ~55.1 lm/W. It is known that the green-rich emission usually results in the stronger luminous efficacy from the afore-mentioned reabsorption of light for red-excitation [22]. It should be mentioned that the luminous efficacy became lower by ~15% when the printed thick film was applied to the top side of the glass substrate. The light intensity may be reduced when the emitted photons from the blue chip pass through the glass substrate [8].

 

Fig. 6 Variation in luminous efficacy of the white LED devices as a function of the relative content of green phosphor, with an actual image of white emission in the R3/G7 sample.

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Figure 7 shows the CIE chromaticity diagram with different red/green ratios for the mixed phosphor thick films under the blue LED excitation. The (0.477, 0.260) coordinate obtained for the red phosphor shifted gradually toward the green region of the (0.247, 0.386) coordinate with increasing the relative content of the green phosphor. The shift is driven by the changed degree of relative intensity of each emission with different red/green ratios [23,24]. Only the R3/G7 sample exhibited the luminance corresponding to the white light region with x = 0.327 and y = 0.319 as clearly seen in the inset of Fig. 7. In addition, the correlated color temperature (CCT) was estimated to see the suitability of the obtained color range. The calculated range over the entire phosphor ratios pertains to the values between 1900 to 9000 K. Particularly, the phosphor of R3/G7 was close enough to the true daylight CCT of ~6500 K [25,26]. The CCT value obtained for the R3/G7 sample was ~6108 K. It may be also valuable to mention that the color rendering index of the R3/G7 sample was 93, which is promising when considered the value for commercial white LEDs [27].

 

Fig. 7 CIE color coordination diagram of different red/green ratio samples, with the highlighted R3/G7 case as an inset.

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4. Conclusion

The screen-printing process for the remote phosphor planar film with a thickness of <20 μm on a regular SLS glass substrate was successfully utilized. The planar thick film consists of commercially-available red and green silicate phosphors dispersed well in a glass matrix. The silicate phosphor layer densified at 400 °C was chemically stable without secondary phases evolved during firing. The relative peak intensity of the red/green emission in the PL spectra depended on the relative content of red/green phosphors. The planar film based on the mixture of 30 wt% red and 70 wt% green phosphors demonstrated the best performance as a white LED by exhibiting the luminous efficacy of ~55.1 lm/W with the promising CIE chromaticity result.