This study introduces a “greener” green monochromatic phosphor-converted light-emitting diode (pc-LED) using a band-pass filter (BPF) combined with a long-pass dichroic filter (LPDF) and a short-pass dichroic filter (SPDF) to improve the color quality of our previously developed LPDF-capped green pc-LED. This can also address the drawbacks of III-V semiconductor-type green LEDs, which show a low luminous efficacy and a poor current dependence of the efficacy and color coordinates compared to blue semiconductor-type LEDs. The optical properties of green monochromatic pc-LEDs using a BPF are compared with those of LPDF-capped green pc-LEDs, which have a broad band spectrum, and III-V semiconductor-type green LEDs by changing the transmittance wavelength range of the BPF and the peak wavelength of the green phosphors. BPF-capped green monochromatic pc-LEDs provide a high luminous efficacy (134 lm/W at 60 mA), and “greener” 1931 Commission Internationale d'Eclairage (CIE; CIEx, CIEy) color coordinates (0.24, 0.66) owing to the narrowed emission spectrum. We also propose a two-dimensional (2D) polystyrene (PS) microbead (2-μm diameter) monolayer as a scattering layer to overcome the poor angular dependence of the color coordinates of the transmitted light through a nano-multilayered dichroic filter such as an LPDF or BPF. The 2D PS scattering layer improves the angular dependence of the green color emitted from a BPF-capped green pc-LED with only 3% loss of luminous efficacy.
©2013 Optical Society of America
Light-emitting diodes (LEDs) have been studied intensively because of certain advantages such as their high brightness, eco-friendliness (mercury-free composition), long lifetime, small size, low power consumption, fast response, and so on. For these reasons, LEDs can be applied in many types of lightings, signals, and displays [1–3]. However, III-V semiconductor-type green LEDs show a low luminous efficacy and poor current dependence of the luminous efficacy and color coordinates compared to semiconductor-type blue LEDs.
Various approaches have been developed to address these drawbacks. A simple strategy combining green-color conversion materials (phosphors) and blue LEDs, called phosphor-converted LEDs (pc-LEDs) (see Fig. 1(b) ), is not an effective way to realize a pure green color because of color mixing between the blue light passed through the phosphor layer and the green light emitted from the phosphor at a low phosphor concentration. A high phosphor concentration is needed to prevent this color-mixing problem. However, pc-LEDs with a high phosphor concentration have the problem of a relatively low luminous efficacy due to the scattering and reflection loss of the phosphor layer [4–10]. Therefore, we previously reported on the fabrication of highly efficient monochromatic pc-LEDs using an LPDF and various green phosphors to address the problem of the low performance of monochromatic LEDs at various wavelengths in the region of the “green gap” (see Fig. 1(c)) [11, 12].
Figure 2(a) shows the PL spectra of the green phosphors and the CIE values (inset). The green phosphors were measured with a 365nm excitation source. Figures 2(b)-2(d) show the EL spectra and the CIE values (inset) of the green pc-LEDs with glass, LPDF515, and LPDF535 at 60mA with different peak wavelength of the green phosphors. Figure 3 shows the full width at half maximum (FWHM) of the green phosphors, the green pc-LEDs with glass, and the green pc-LEDs with a LPDF as a function of the peak wavelength of the green phosphors. The FWHM increased as the wavelength of the green phosphor increased. The green pc-LED with glass has the largest FWHM because the transmitted blue emission and the emission of the green phosphor were mixed. On the other hand, L535-capped green pc-LEDs can realize the CIE values and colors of green phosphors because the CIE values and the FWHM of the L535-capped green pc-LEDs become very similar to those of the green phosphor by reflecting the blue emission from the blue LED. The LPDF can block and recycle the blue light from the blue LED. This LPDF-capped pc-LED is good for applications in solid-state lighting owing to its broad band spectrum, current and temperature stability, and high luminous efficacy. However, the 1931 Commission Internationale d'Eclairage (CIE) color coordinates and color purity of LPDF-capped green pc-LEDs are limited for use in display applications because of the broad spectra of inorganic phosphors such as silicates. The LPDF-capped green pc-LEDs can only reproduce the color purity and CIE color coordinates of the green phosphors (See Fig. 2 inset). For the application of these pc-LEDs in the backlight systems of liquid-crystal displays (LCDs), the color gamut, which is defined as the area of the triangle between the RGB color coordinates, should be increased, because this is one of the key factors for the realization of vivid color pictures. The relative color gamut is calculated from the area ratio of the color gamut between the RGB LED backlight and the National Television System Committee (NTSC) RGB colors. The NTSC CIE color coordinates (CIEx, CIEy) are defined as blue (0.14, 0.08), green (0.21, 0.71), and red (0.67, 0.33). In this study, we propose a newly developed green monochromatic pc-LED capped with band-pass filters (BPFs) combined with LPDFs and SPDFs to fabricate efficient and “greener” green-color monochromatic pc-LEDs and increase the color gamut (See Fig. 1(d)). Here, the BPF can narrow the emission band spectrum from the pc-LED by reflecting both the bluish and reddish spectrum edges at far shorter and longer wavelength regions, respectively, than the wavelength at the maximum peak. In addition, we introduce a two-dimensional (2D) polystyrene (PS) microbead (2-μm diameter) monolayer as a scattering layer to overcome the poor angular dependence of the emission color coordinates of dichroic filters such as LPDFs and BPFs [13, 14].
2. Experimental methods
Fabrication of LPDFs and SPDFs: Dielectric LPDFs and SPDFs were fabricated on glass substrates of thickness 0.15 mm. For the fabrication of the stacks, terminal eighth-wave-thick TiO2 and quarter-wave-thick SiO2 nano-multilayered films ((0.5TiO2/SiO2/0.5TiO2)9, LPDF) and terminal eighth-wave-thick SiO2 and quarter-wave-thick TiO2 nano-multilayered films ((0.5SiO2/TiO2/0.5SiO2)9, SPDF) were coated onto glass substrates by e-beam evaporation at 250°C. For the design of the LPDF and SPDF multilayer films, the characteristic matrix method was used to simulate the reflectance (R), transmittance (T), and absorption (A). In this study, two types of LPDFs with nine periods of 0.5TiO2/SiO2/0.5TiO2 multilayers (L515 and L535; 515 and 535 nm at the half-band-edge wavelength, respectively) and two types of SPDFs with nine periods of 0.5SiO2/TiO2/0.5SiO2 multilayers (S550 and S580; 550 and 580 nm at the half-band-edge wavelength, respectively) were fabricated as capping filters for the production of green monochromatic pc-LEDs. Table 1 shows the thickness of each dielectric layer of the LPDFs and SPDFs, and Fig. 4 shows the transmittance spectra and photographs of the two types of LPDFs and SPDFs [11, 12, 15, 16].
Fabrication of BPFs: The BPFs were designed by combining both LPDFs and SPDFs . For the fabrication of the BPFs, the LPDF (L515 or L535) was coated on a glass substrate, and the SPDF (S550 or S580) was subsequently coated on the LPDF-coated glass substrate by e-beam evaporation. Table 2 shows the names of the BPFs resulting from the combination of two different LPDFs and SPDFs. The BPFs are named with the half-band-edge wavelength of the LPDF and SPDF: LPDF (first)-SPDF (last). Figure 5 shows the transmittance spectra and photographs of the four types of BPFs studied in this experiment. Figure 6 shows the transmittance spectra as a function of the viewing angle in the normal direction.
Fabrication of LPDF-capped or BPF-capped green pc-LEDs: For the fabrication of the LPDF-capped or BPF-capped green monochromatic pc-LEDs, a blue chip (λmax = 445 nm, 16 lm/W at 60mA) was used as a blue light source and an excitation source for various green phosphors in the pc-LEDs. The blue LED chips were purchased from Dongbu LED, Inc. In this experiment, a series of orthosilicate green phosphors ((Sr,Ba)2SiO4:Eu) G515, G521, G530, G540, and G550, where the number is the maximum peak wavelength of the emission spectrum) were used for the fabrication of the various-color green pc-LEDs . The powder phosphors were obtained from phosphor companies (Merck Co. Ltd.). Optimum amounts of the green phosphors were dispersed in a silicone binder, and identical amounts of the resulting phosphor pastes were dropped onto each cup-type blue LED. On top of each green pc-LED, an LPDF- or BPF-coated glass substrate was attached with an air gap [11, 12].
Fabrication of PS microbead scattering layer: A monolayer of polystyrene (PS) microbeads as a 2D scattering layer (2-μm diameter, 50 vol% in ethanol) was prepared in a solution and scooped by the LPDF or BPF substrate using a scooping transfer technique based on a water-air self-assembly process . Figure 7 shows the top-view and side-view images, obtained by field-emission scanning electron microscopy (FE-SEM, JSM 7401F, JEOL) at 10 kV, of the PS microbead 2D scattering layer on the BPF substrate.
Characterization of phosphor powders and green pc-LEDs: The photoluminescence spectra from orthosilicate green phosphors were measured with a 365 nm excitation source. The emission spectra of the forward emissions from green semiconductor-type LEDs and blue-excited LPDF-capped or BPF-capped green monochromatic pc-LEDs were measured in an integrated sphere using a spectrophotometer (PSI Co. Ltd., Darsapro-5000) with an applied current of 60 mA. The angular dependences of the green semiconductor-type LEDs and blue-excited LPDF-capped or BPF-capped green monochromatic pc-LEDs were measured in the normal direction as a function of viewing angle between 0° and 70°. The luminous flux, luminous efficacy, and 1931 CIE color coordinates were calculated using the Darsapro-5000 program. The transmittance of each dichroic filter was measured in the normal direction using a UV-Vis spectrophotometer (SCINCO CO., LTD. S-3100).
3. Results and discussion
We selected an InGaN blue semiconductor LED, of which the dominant wavelength is 445 nm, as the excitation source for the BPF-capped green monochromatic pc-LEDs. We used five orthosilicate green phosphors ((Ba,Sr)2SiO4:Eu) in the wavelength region between bluish green (515 nm) and yellowish green (550 nm), and four types of BPFs, which were prepared by combining two LPDFs and two SPDFs, in order to find the best conditions for the development of a “greener” green monochromatic pc-LED. Figure 8 shows the EL spectra of the BPF-capped green monochromatic pc-LEDs as a function of the peak wavelengths of the green phosphors. The band width of the BPF-capped green monochromatic pc-LEDs decreases compared with those of the LPDF-capped green pc-LEDs (see Figs. 2(c)-2(d), and Fig. 3) because the reddish green color of the long-wavelength region of the green emission from silicate phosphors excited by the blue LED cannot be transmitted by the high reflectivity of the BPF. As shown in Figs. 8(a) and 8(c), when we select the BPFs fabricated by combination with SPDF 550 (BPF515-550 and BPF535-550), the emission spectra of the BPF-capped green pc-LEDs show a significant reddish emission shoulder due to the somewhat large transmission of the reddish wavelength region (see Figs. 5(a) and 5(c)). It is seen that the BPFs obtained from the combination with SPDF 550 (BPF515-550 and BPF535-550) cannot produce the pure green color from these BPF-capped pc-LEDs, although the band widths of the transmitted spectra passed through them are somewhat narrowed. Otherwise, Figs. 8(b) and 8(d) also indicate that the BPFs fabricated from SPDF580 create a greener color from the transmitted light of the pc-LEDs implemented with them, because SPDF580 blocks the shoulder of the emission peaks in the longer-wavelength region. As shown in Fig. 6 the transmittance spectra move to a shorter wavelength as the viewing angle increases in the normal direction. Therefore, the integrated emission spectrum has emission below 520nm, although the transmittance spectrum of BPF has no transmission below 520nm in the normal direction.
Figure 8 inset shows the 1931 CIE color coordinates of the four BPF-capped green monochromatic pc-LEDs as a function of the peak wavelengths of the green silicate phosphors. It shows that the selection of the G521 phosphor provides the greenest color among the five phosphors under combination with all four BPFs. The color coordinates of the three pc-LEDs consisting of the G521 phosphor and BFPs (BFP 515-580, BFP 535-550, and BFP 535-580) are located in the upper left region of the 1931 CIE color coordinates box (CIEx ≤ 0.25, CIEy ≥ 0.65). When we select the blue semiconductor-type LED (CIEx 0.16, CIEy 0.03), red pc-LED (CIEx 0.64, CIEy 0.35), and green pc-LED, which are located in the upper left region of the 1931 CIE color coordinates box, the relative color gamut obtained is greater than 85% compared to that of NTSC.
As shown in Fig. 9 , the BPF-capped green pc-LED shows narrow FWHM compared to the green phosphor because the reddish green color of the long-wavelength region of the green emission from silicate phosphors excited by the blue LED cannot be transmitted by the high reflectivity of the BPF.
Figure 10 shows the luminous efficacies of the LPDF-capped and BPF-capped green monochromatic pc-LEDs as a function of the peak wavelengths of the green phosphors. In the case of the LPDF-capped green pc-LEDs incorporating the G530 phosphor, those capped with L515 and L535 show the highest luminous efficacies of 187 and 184 lm/W, respectively. Although these LPDF-capped green pc-LEDs incorporating the G530 phosphor show excellent luminous efficacies, the color coordinates are located in the yellowish green region (see the insets of Figs. 2(c) and 2(d)). The wide bandwidths and yellowish color coordinates of the emitting spectrum bands were reproduced only by the G530 phosphors themselves. As reported previously, these characteristics indicate that the LPDF-capped green pc-LEDs have excellent merits for the realization of a high color-rendering index for applications in solid-state lighting, but also have the drawback of a reduced color gamut for applications in the backlight systems of LCDs. Otherwise, the BPF-capped green pc-LED incorporating phosphor G521 can filter out the reddish region of the emitting green spectrum, owing to the additive effects of both the SPDF and LPDF deposited on the glass substrate. Figure 8 also indicates that the luminous efficacy of the BPF-capped green pc-LED decreases with the addition of the SPDF on top of the LPDF-coated substrate, because of the narrowing of the band width of the emitting spectrum. Therefore, the color purity and luminous efficacy of the BPF-capped pc-LEDs show a trade-off relationship. The luminous efficacies of the BPF535-550- and BPF535-580-capped green pc-LEDs incorporating the G521 phosphor are 1.53 times (98 lm/W) and 2.09 times (134 lm/W) higher than that of the III-V semiconductor-type green LED (64 lm/W).
Figures 8, 9 and 10 indicate that the best conditions for a greener color and good efficacy are produced by the BPF535-580-capped green monochromatic pc-LED incorporating the G521 green phosphor. The CIE color coordinates of this BPF535-580-capped G521 green monochromatic pc-LED are x = 0.24 and y = 0.66. We also compare the angular, current, and temperature dependences of the BPF535-580-capped G521 green monochromatic pc-LED and the III-V semiconductor-type green LED.
Figure 11 shows the relative luminous flux and EL spectrum of each green LED and pc-LED as a function of viewing angle. As shown in Figs. 11(e) and 11(h), the BPF-capped green pc-LED has a poor angular dependence of the emitting spectrum and color coordinates because the band-edge wavelength of the BPF moves significantly to a shorter wavelength upon changing the viewing angle from the 0° normal direction to 70°. The poor angular dependence of the spectrum and color appears when any one of BPF, LPDF, or SPDF is selected and capped on the LED cup, because the 1D photonic crystal (PC) types of dichroic filters have large angular characteristics of transmittance and reflectance light. In order to address this problem, we adopt the 2D polystyrene (PS) monolayer as a scattering layer by scooping a PS monolayer from the water surface to the top of the BPF substrate (see Fig. 11(c)). PS microbeads (2-μm diameter) are considered to be a good light-scattering layer, because this size shows a large light-scattering coefficient when calculated from Mie scattering theory [15, 16]. Thus, the angular dependence of the optical properties is reduced by coating a 2D PS scattering layer on top of the BPF-capped green pc-LEDs. The luminous efficacy of the BPF-capped green pc-LED is only reduced by 3% (130 lm/W) after the addition of a 2D PS scattering layer. It can be speculated that this minimum reduction in luminous efficacy and maximum increase in scattering capability are due to the formation of a highly crystallized, wafer-scale 2D monolayer of PS microspheres, as shown in Fig. 7 and in our previous publication .
Figure 12 shows the luminous flux, luminous efficacy, and 1931 CIE color coordinates of the III-V semiconductor-type green LED, BPF535-580-capped G521 green pc-LED, and 2D-scattering layer modified BPF535-580-capped G521 green pc-LED as a function of applied current and ambient temperature. The III-V semiconductor-type green LED shows a relatively low luminous flux and luminous efficacy and wide variation of the 1931 CIE color coordinates compared with the BPF-capped green pc-LEDs as a function of applied current. As shown in Fig. 12(d), the blue LED shows good temperature dependence compared to the III-V semiconductor-type green LED and both the BPF-capped and 2D-scattering-layer modified BPF-capped green pc-LEDs. Although both BPF-capped and 2D-scattering-layer modified BPF-capped pc-LEDs used the same blue LED as an excitation source, they show different temperature dependence characteristics compared to blue LED due to phosphor thermal quenching. As previously reported, silicate-based green phosphor, as used for the fabrication of the variously-colored green pc-LEDs in this study, shows poor temperature stability compared to other phosphors. However, the thermal stability of the pc-LED is similar to that of the III-V semiconductor-type green LED. Therefore, the temperature stability of the pc-LED can show a greater improvement compared to that of the III-V semiconductor-type green LED if introducing a phosphor with good thermal stability.
The III-V semiconductor-type green LED shows a slight change in the luminous efficacy as a function of ambient temperature because the peak wavelength of the III-V semiconductor-type green LED moves from 517 to 524 nm and the spectrum band width increases from 36 to 41 nm with increasing temperature. These changes in peak position and bandwidth can increase the luminous flux. Therefore, both the BPF-capped and 2D-scattering-layer modified BPF-capped green pc-LEDs provide an improved luminous flux and color variation with respect to changes in current and temperature compared with the III-V semiconductor-type green LED.
We have developed efficient “greener” green monochromatic pc-LEDs by attaching a BPF modified with a 2D scattering layer on top of the green pc-LEDs. We optimized the optical properties of the BPF-capped green pc-LEDs by combining four different types of BPFs (BPF515-550, BPF515-580, BPF535-550, and BPF535-580) and five different types of green phosphors (G515, G521, G530, G540, and G550) for use in display applications that require high luminous efficacy and a good color gamut. The optimized BPF535-580-capped G521 pc-LED showed a high luminous efficacy (134 lm/W) and good 1931 CIE color coordinates (0.24, 0.66). However, the BPF-capped green pc-LEDs showed a poor angular dependence of the color coordinates. To improve the angular dependence of the BPF-capped green pc-LEDs, we applied a 2D scattering layer on top of the BPF substrate by forming a highly crystallized 2-μm PS-bead monolayer. The BPF-capped green pc-LEDs modified with the PS microbead monolayer still showed a high luminous efficacy (130 lm/W), good optical properties with changes in current and temperature, and decreased color variation with viewing angle. When we selected the blue semiconductor-type LED (CIEx 0.16, CIEy 0.03), red pc-LED (CIEx 0.64, CIEy 0.35), and BPF535-580-capped green pc-LED modified with the 2D PS scattering layer (CIEx 0.24, CIEy 0.66), 87.5% of the NTSC RGB color gamut was achieved. These results demonstrate that the use of the BPF-capped pc-LED modified with a 2D PS scattering layer could facilitate development in the signage and display market.
This work was supported by the National Research Foundation (NRF) grant funded by the Ministry of Education, Science and Technology (MEST) of Korea (no. 2011-0017449, NRF-C1AAA001-2009-0092938, and ERC program, no. R11-2005-048-00000-0).
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