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In this study, we propose green/red bilayered freestanding phosphor film-capped white light-emitting diodes (W-LEDs) using InGaN blue LEDs and narrowband red and green phosphors to realize a wide color gamut in a liquid crystal display (LCD) backlight system. The narrowband K2SiF6:Mn4+ (KSF) red and SrGa2S4:Eu2+ (SGS) green phosphors are synthesized using a facile etching synthetic process and flux-aided solid state reaction under a H2S atmosphere, respectively, and the freestanding phosphor films are fabricated using a delamination method with water-soluble polymer, polystyrene sulfonic acid, PEDOT/PSS, and interlayered phosphor film. Various phosphor concentrations of green/red bilayered freestanding phosphor film-capped W-LEDs exhibit a correlated color temperature (CCT) and luminous efficacy range of 11,390 K ~6,540 K and 99 lm/W ~124 lm/W, respectively, with an applied current of 60 mA. The W-LED with green (12.5 wt%)/red (40 wt%) bilayered phosphor film, which exhibited luminous efficacy of 105 lm/W at the CCT of 8,330 K, is selected and the color gamut of the bare LED and phosphor RG and the filtered RGB triangle is calculated to be more than ~95% and ~86.4%, respectively, relative to the NTSC in the 1931 CIE color coordinates space.
© 2015 Optical Society of America
1. Introduction
A decade ago, tri-color white light-emitting diode (LED) lamps using a combination of individual red, green, and blue (RGB) LED chips were launched by Sony [1, 2] in order to replace cold cathode fluorescent lamp (CCFL) backlights in liquid crystal display (LCD) TVs, which had a color gamut of ~75% of the National Television Standard Committee (NTSC) standard. Here, the color gamut is defined as the color reproduction area of the triangle between the color coordinates of R, G, and B colors in the backlight. This LED-backlight LCD TV offered a very wide color reproduction range, delivering a color gamut of ~105% of the NTSC standard in the 1931 Commission Internationale d'Eclairage (CIE) xy color space. This RGB tri-chip approach enabled the facile dynamic control of color points and provided high color rendition as well as stabilization of the chromaticity. However, the disadvantages of the RGB tri-chip approach, e.g. the very low efficiency of the green LEDs (the “green gap” problem) [3, 4], the different temperature and current dependences [5, 6], and longevities [7, 8] of each colored LED, inhibited the wide penetration of these RGB tri-chip white LEDs into a wider range of backlight applications.
Compared with the RGB tri-chip LED approach, phosphor-converted (pc) white LEDs are a more reliable, efficient, and simple backlight technology for LCDs, because the color variations of inorganic powder phosphor materials are narrow with different temperatures, currents, and times [9, 10]. Furthermore, their quantum efficiencies (QEs) of photoluminescence (PL) are sufficiently high for efficient color converters. In the first stage, the two-band pc-LED approach of a blue single-chip and a yellow single-phosphor was used to replace CCFLs; however, its color gamut was limited (~75% of NTSC standard) and could not faithfully reproduce natural colors. Currently, the three-band white pc-LEDs using combined green and red phosphors have been widely used as backlights for both LCD TVs and small-sized LCDs such as cellular phones and tablet displays [11–13]. To date, the widest color gamut of the two-phosphor approach can attain approximately 85.6% of the NTSC standard using narrow-band β-sialon:Eu green and CaAlSiN3:Eu red phosphors with the assistance of the typical color filters of LCDs [14]. The most important prerequisite for developing good color converters for pc-LED backlights is the enlargement level of the color gamut that the RG phosphors and B LED chips can reach. In order to achieve a larger color gamut, the shape of the white LED spectrum should match well with the RGB color filters of the LCDs and the full width at half-maximum (FWHM) of the emission spectrum of the RG color converters should be as narrow as possible.
Recently, CdSe/ZnS core/shell II-VI quantum dots (QDs) have become excellent candidates for replacing micro-sized powder phosphors in pc-LED backlight lamps because the FWHMs of Cd-based QDs are below 40 nm [14]. For the best results, Cd-based green and red QDs have reached a wider color gamut of more than 115% NTSC in the 1931 CIE xy color space [15]. However, Cd-based QDs are toxic and environmentally harmful. In order to reduce the use of Cd elements, III-V and I-III-VI2 non-toxic QDs have been extensively investigated as color converters in pc-LEDs with the view to fabricating general lighting instead of backlit devices, because the FWHMs of non-Cd QDs available are significantly broader (> 40 nm) than those of Cd-based QDs [16–19]. Nevertheless, the non-Cd QDs and Cd-based QDs may become viable alternatives to the rare-earth activated inorganic phosphors because rare-earth elements are considered critical materials in the market due to their scarcity and cost [19]. Through the significant efforts made by numerous researchers, the widest color gamut that non-toxic InP/ZnS QDs can reach is ~87% relative to the NTSC standard. In addition, the reliability, photo-/thermo-stability, and longevity remain as important issues from the perspective of acceptance of both Cd-based and non-toxic QDs in pc-LED technology [20]. The developments of various thick-shelling technologies have attempted to enhance the material reliability of QDs [21]; however, the discovery of new types of environmentally viable and reliable QDs with narrow FWHMs remains a challenge for “on-chip” pc-LED backlights.
Paradoxically, it is valuable to locate an inorganic powder phosphor with a narrow emission bandwidth and environmentally clean elements. If remote-types of phosphor films are utilized in “on-edge” or “on-film” types of backlight systems [22, 23], it is possible to reuse less reliable phosphor materials through reducing the temperature surrounding the phosphors and the intensity of light excitation around the phosphors. Recently, narrowband K2SiF6:Mn4+ (KSF) phosphors (FWHM < 10 nm) were developed to replace the wideband Eu-activated nitride red phosphors in order to enhance the color qualities, such as color rendering indexes (CRIs) and special CRI for pure red (R9) [24–26]. It was also reported that the color coordinates of the KSF phosphors are located at the deep red point in the 1931 CIE xy color coordinates diagram. It is necessary to determine whether the combination of one of the various possible green phosphors and the KSF red phosphor is more apt to reach high efficiencies and high levels of color gamut for application in remote-type phosphor films integrated in LED backlights.
In this study, we first synthesized narrowband KSF red phosphors (FWHM < 10 nm) and one candidate for the narrowband green phosphor, i.e. SrGa2S4:Eu2+ (SGS) (FWHM < 50 nm) [24–27], using a facile etching synthetic process and flux-aided solid state reaction under H2S atmosphere, respectively. Then, we characterized the crystal, morphological, and optical properties of the KSF red and SGS green phosphors [24–28]. Next, we introduced a novel strategy for fabricating freestanding remote-type films of phosphor/silicone binder hybrids that can be used as stacked GR films for the generation of white color. When the printing method was coupled with an interlayer and a floating/delamination technique, wafer-scale freestanding GR phosphor films were achievable [29–31]. Finally, we demonstrated several remote-type stacked GR films on a blue LED system that consisted of a narrowband red phosphor (KSF), a thiogallate green phosphor (SGS), and a blue LED to enhance the color gamut. In this work, two bright narrowband phosphors of red KSF and green SGS were synthesized and a novel combination of both phosphor down-converters was exploited for the first time in order to fabricate a high-color gamut white light pc-LED for LCD backlights, which is comparable to the color gamut (87%) of non-toxic InP/ZnS QDs.
2. Experimental methods
2.1 Synthesis of the narrowband K2SiF6:Mn4+ red phosphor
In order to synthesize the red-emitting KSF phosphor, pure SiO2 powders were dissolved in HF (25 vol%) at room temperature for 2 hours to form a H2SiF6 solution. A stoichiometric amount of KMnO4 was dissolved in the H2SiF6 solution. After the solution color changed from colorless to a deep purple, H2O2 (30 vol%) was added drop-by-drop. The solution formed a yellow precipitate of KSF powders. After finishing the reaction, the powders were filtered and dried at 80 °C in the oven [27].
2.2 Synthesis of the narrowband SrGa2S4:Eu2+ green phosphor
In order to synthesize the SGS green phosphor, SrCO3, Ga2O3, Eu2O3, NaBr, and sulfur were weighed stoichiometrically, mixed, ground using a mortar and pestle, and then placed in an alumina crucible that was heated in a box-type furnace at 800 °C for 2 hours in a H2S stream flow. The NaBr and sulfur were added as fluxes in order to decrease the calcination time and to enhance the luminescence intensity. The furnace with the Al crucible was purged using Ar gas in order to prevent the oxidation of the mixture at temperatures below 500 °C during the initial and final heat-treatment stages. The resulting sintered phosphor cake was removed from the furnace and mortared, washed, and sieved prior to evaluation [28].
2.3 Characterization of phosphors
The emission and excitation of all powder phosphors were measured using a Xe-lamp and spectrophotometer (Darsa, PSI Trading Co., Korea). The crystal structure of the KSF red and SGS green phosphors were investigated using X-ray diffraction with CuKα1 radiation (D-max 2500, JEOL, USA) with JCPDS card no. 85-1382 (K2SiF6) and JCPDS card no. 25-0895 (SrGa2S4). The morphology of the phosphors was measured using a field emission scanning electron microscopy (FE-SEM) including energy-dispersive x-ray spectroscopy (EDS; JEM-7610F, JEOL, Japan).
2.4 Fabrication of freestanding phosphor films
Glass was cleaned and treated under UV irradiation and ozone for 5 min, and polystyrene sulfonic acid (PEDOT/PSS; Sigma-Aldrich, 483095) was sonicated for 10 min. The PEDOT/PSS was spin-coated at 4000 rpm for 35 sec on the glass substrate. In order to fabricate freestanding films, the optimum amount of the KSF (20 wt% - 50 wt%) red and SGS (10 wt% - 20 wt%) green phosphors were dispersed in a silicon binder (OE-6636A and OE-6636B, Dow Corning, Korea) separately to form the green and red phosphor pastes. Each paste was printed on a 150 μm spacer and then dried and hardened at 150 °C for 1 hour [29, 30]. Next, the phosphor coated glass substrates were immersed in distilled water in order to separate the film from the substrate through dissolving the PEDOT/PSS. After the phosphor film floated to the water surface, it was scooped out using mesh or picked up with tweezers [31]. Broad band silicate ((Sr, Ba)2SiO4:Eu (Intematix Corporation), 30 wt%) green, (Y2Al5O12:Ce(Merck), YAG, 15 wt%) yellow, and nitride ((Sr, Ca)AlSiN3:Eu(Intematix Corporation), 2.5 wt%) red freestanding phosphor films were also fabricated as described above.
2.5 Characterization of freestanding G/Y/R phosphor film, Y single layered and G/R bilayered phosphor film-capped W-LED
The optical properties of the freestanding G/Y/R phosphor films, Y single layered and G/R bilayered phosphor film-capped W-LEDs were measured using an InGaN blue LED (Dongbu LED, Inc., Korea) as an excitation source with an applied current of 60 mA. The current dependence and temperature dependence of the G/R bilayered phosphor film capped W-LED were measured from 10 mA to 180 mA with 10 mA intervals in an ambient temperature and from 20 °C to 120 °C with 20 °C intervals at 60 mA, respectively. All optical properties of the InGaN blue LED were measured in an integrated sphere using a spectrophotometer (Darsapro-5000, PSI Trading Co. Ltd., Korea). The cross-sectional images of the freestanding phosphor films were measured using a FE-SEM (JEM-7610F, JEOL, Japan).
3. Results and discussion
KSF red and SGS green phosphors were synthesized using the facile etching method and solid state reaction reported in previous publications. As seen in Figs. 1(a) and 1(b), the powder XRD data demonstrated that all diffraction peaks of the as-synthesized KSF and SGS phosphors could be indexed to the cubic K2SiF6 (JCPDS card no. 85-1382) and cubic SrGa2S4 (JCPDS card no. 25-0895) single phases, respectively, without impurity phases. The SEM pictures of the as-synthesized KSF and SGS phosphors illustrate that micrometer-sized powders with irregular round shapes were observed with slightly different particle sizes. The average particle sizes of the KSF and SGS phosphors were 10.0 and 3.5 μm, respectively. As seen in Figs. 1(a) to 1(d), the XRD and FE-SEM including the EDS data confirmed that the crystal qualities, morphologies, and chemical compositions of the synthesized KSF and SGS phosphors were well matched with those reported in previous publications. Figures 1(e) and 1(f) present the excitation and emission spectra of the as-prepared KSF and SGS phosphors. The PL emission spectrum of the prepared KSF red phosphor had five sharp peaks with a narrow FWHM (< 10 nm) in the main peak (~630 nm), which resulted from the 2E2–4A1 transitions of the Mn4+ ions as reported in previous publications [24–27]. The PL peak wavelength and FWHM of the prepared SGS green phosphors were 530 nm and 55 nm, respectively. The PL emission peaks of the Eu-doped green phosphors originated from the 4f65d1(a1) → 4f7 transitions of the Eu2+ ions in the thiogallate matrix.
Fig. 1 Powder XRDs of the (a) KSF red phosphor (JCPDS card no. 85-1382 (K2SiF6)) and (b) SGS green phosphor (JCPDS card no. 25-0895 (SrGa2S4)). FE-SEM and EDS data of the (c) KSF red phosphor and (d) SGS green phosphor. Emission and excitation spectra of the (e) KSF red phosphor and (f) SGS green phosphor.
Figure 2(a) presents the normalized emissions spectra of a blue LED, a narrowband KSF deep red phosphor, and a SGS green thiogallate phosphor, which are the components of white pc-LEDs. Figure 2(b) also presents the xy color coordinates of the InGaN blue LED, KSF red phosphor, and SGS green phosphor in the 1931 CIE diagram. As expected, the color coordinates of the KSF phosphors were located in the deep red point in the CIE color diagram. Here, we calculated the color gamut of the materials defined using a blue LED, a narrowband KSF deep red phosphor, and a green thiogallate phosphor. As a consequence of the relatively narrow FWHM of the red and green components (FWHM = 10 and 55 nm, respectively), it was possible to obtain the area of the gamut more than 90% compared with the NTSC color gamut standard.
Fig. 2 (a) Normalized emission spectra and (b) 1931 CIE color coordinates of the KSF red phosphor, SGS green phosphor and InGaN blue LED.
Most pc-LEDs are fabricated through the physical blending of red and green phosphors using a silicone resin mixture. This simple method has been widely used for producing commercial white pc-LEDs. However, this method generally has unwanted results such as thermal quenching and phosphor degradation, as well as limiting the material type of phosphors. In order to address these issues, in this work, the freestanding red and green phosphor films were introduced through embedding phosphors in a silicone resin; thus, the formation of remote-type red and green stacked films that can be used in the “on-edge” or “on-film” types of backlight systems is expected. As illustrated in the schematic in Fig. 3(a), the present processing for the fabrication of the phosphor composite film was simple, easily upscaled, and flexible in controlling the phosphor material type, load, and thickness of the film. Figure 3(a) presents two bright silicone composite films embedded with red and green phosphors under UV lamp illumination. Figures 3(b) and 3(c) presents the FE-SEM images of the cross-sectional views of the red and green phosphor films; the figures indicates that both phosphors were uniformly distributed without appreciable agglomeration of phosphor powders in the silicone matrix. The device fabrication schematic in Fig. 3(d) indicates that the individual phosphor films were integrated in a remote fashion with a blue LED.
Fig. 3 (a) Schematic of the fabrication process and photograph images of the freestanding phosphor films. Cross-sectional FE-SEM images of the freestanding (b) green and (c) red phosphor film. (d) Schematic diagram of the freestanding phosphor film-capped W-LED.
Figure 4 presents the relative electroluminescence (EL) spectra and CIE color coordinates of the freestanding green and red film phosphors as a function of the phosphor concentration in the silicone resin under a driving current of 60 mA when the phosphor films were attached to blue LEDs. The green and red phosphor emissions in the EL peaked at 537 and 630 nm, respectively, which were well matched with those of the PL emissions. As the green and red phosphor concentrations increased, the intensity of the green and red emissions and the luminances of the freestanding green and red phosphor film-capped LEDs increased due to the enhanced absorption of the green and red phosphor from the blue LED light, as depicted in Figs. 4(a) and 4(b). The CIE color coordinates of the green and red freestanding phosphor films shifted from blue to greenish and reddish due to the increased ratios of green to blue and red to blue in the emission spectrum with increases in the phosphor concentration, as illustrated in the inset of Figs. 4(c) and 4(d). Likewise, the luminous efficacy of the freestanding green and red phosphor films increased from 145 lm/W to 172 lm/W and from 25 lm/W to 35 lm/W, respectively, due to increases in the green and red emissions as the phosphor concentration increased. Due to the human vision functions with much higher sensitivities to green versus red regimes, it is possible to a have higher luminous efficacy from the green-capped LED than red one. These results indicate that the freestanding green and red phosphor films could be used in the layered design of green-on-red-based pc-LEDs in order to realize wide gamut white color; these freestanding bilayered phosphor films could also be applied in the same remote manner as above with the red film side facing an LED chip.
Fig. 4 The optical properties of the green and red remote-type freestanding films on the InGaN blue LED as a function of phosphor concentrations. Emission spectra of the (a) green and (b) red phosphors; 1931 CIE color coordinates of the (c) green and (d) red phosphors.
Figure 5 shows the EL emission spectra of the freestanding green/red phosphor film capped LEDs as a function of green and red phosphor concentrations at an equal current (60 mA). With increasing red phosphor concentration, the red emission in the white spectrum is increased and the green emission in the white spectrum is decreased because the transmitted blue light reaching the green phosphor film is decreased. As the green phosphor concentration increased, the CIE color coordinates of the stacked green/red phosphor film capped LEDs shifted from bluish or reddish white to greenish white due to the increased ratio of green to blue/red in the emission spectrum, as depicted in Fig. 6(a). At a higher red, a more reddish white color could be obtained from the green/red film-capped pc-LED. This indicates that through varying the green and red phosphor concentrations, an optimized white pc-LED with appropriate correlated color temperatures (CCTs), and high color gamut can be fabricated because the optimum white color is tuned with a balance of blue, green, and red lights. Figures 6(b) and 6(c) present the luminous efficacies (lm/W), and CCT of the green/red bilayered film-capped pc-LEDs as a function of the green and red phosphor concentrations at an equal current (60 mA). The luminous efficacies of the green/red bilayered film-capped pc-LEDs increased as the green phosphor concentration increased, while the CCTs of the green/red bilayered phosphor film-capped pc-LEDs decreased from 12,080 K to 5,590 K with increasing in the green phosphor concentration from 10 to 20 wt%. At a higher red phosphor concentration, the CCTs of the green/red bilayered phosphor film-capped pc-LEDs were lowered due to the increased red portion of white emission. These results confirm that white LEDs with three distinct red, green, and blue peaks were fabricated successfully capping the freestanding green/red bilayered SGS G/ KSF R phosphor film on top of the blue LED. As shown in Fig. 6(d), the calculated NTSC of the green/red bilayered film-capped pc-LEDs increased as the red phosphor concentration increased.
Fig. 5 The EL emission spectra of the green/red bilayered freestanding phosphor film-capped WLEDs as a function of the green phosphor concentrations. (a) red 20 wt%, (b) red 30 wt%, (c) red 40 wt%, and (d) red 50 wt%.
Fig. 6 The optical properties of the green/red bilayered freestanding phosphor film-capped WLEDs as a function of the green and red phosphor concentrations. (a) The 1931 CIE color coordinates, (b) luminous efficacy, (c) CCTs, and (d) calculated NTSC.
In order to further evaluate the suitability of the green/red bilayered phosphor film for use in white pc-LEDs for LCD backlights, showing a CCT value around 7,000 K ~9,000 K [32] and high luminous efficacy (over 100 lm/W), the white emission of one selected bilayered film-capped pc-LED with 40 wt% red phosphor and 12.5 wt% green phosphor was spectrally resolved into its primary RGB colors by applying the transmission spectra of commercial blue (B), green (G), and red (R) color filters. The transmittance spectra of commercial B, G, and R color filters are shown in Fig. 7(a). The full colors of the displayed objects were realized through control of the transmitted RGB lights from backlights through RGB filters. Therefore, the RGB spectrum and color gamut can be then computed with the white pc-LED spectrum using the transmission spectrum of each color filter. Figure 7(b) shows white and RGB color filtered photographs. Figures 7(c) and 7(d) present the calculated RGB spectra and CIE color coordinates from the green/red bilayered SGS G/KSF R phosphor film-capped white LEDs. The white, B, G, and R emissions corresponded to (0.283, 0.332), (0.153, 0.062), (0.760, 0.674), and (0.648, 0.305), respectively, in the CIE color coordinates. The color gamut of the filtered RGB triangle was calculated to be 86.4% relative to the NTSC color space in the CIE 1931 standard system with luminous efficacy of 105 lm/W.
Fig. 7 (a) Transmittance spectra of the blue, green and red color filters. (b) Photographs of the the green (12.5 wt%)/red (40 wt%) bilayered phosphor film-capped W-LED and RGB color filtered W-LED. (c) The emission spectrum of the green (12.5 wt%)/red (40 wt%) bilayered phosphor film-capped W-LED and the calculated RGB spectra. (d) The 1931 CIE color coordinates of the NTSC standard and the calculated RGB from green (12.5 wt%)/red (40 wt%) bilayered phosphor film-capped W-LED.
Also, we fabricated broad emission band free-standing phosphor films with Y2Al5O12:Ce3+ (YAG) yellow phosphor, (Sr, Ba)2SiO4:Eu silicate green phosphor, and (Sr, Ca)AlSiN3:Eu nitride red phosphor for comparison with a KSF and SGS bilayer system. Figures 8(a) and 8(b) show the emission spectrum and 1931 CIE color coordinates of broad band emitting green, yellow, and red phosphors. To compare the color gamut, NTSC, with a SGS/KSF bilayered remote-type white LED, we measured a free standing film of a YAG Y (15 wt%) single layer system and silicate G (30 wt%)/CASN R (2.5 wt%) bilayer system with a CCT of around 7,000 ~9,000 K. Figures 8(c)-8(e) show the emission spectrum, the calculated R, G, and B color filtered spectrum, and the CIE color coordinates of a remote-type white LED using YAG single layer and silicate G/CASN R bilayered films with a CCT of around 8,000 K and Table 1 shows the optical properties of three types (Blue/SGS G/KSF R, Blue/YAG Y, and Blue/silicate G/CASN R) of white LEDs using freestanding phosphor films. The luminous efficacy of the single layer system, Blue/YAG Y, shows the best value of 111 lm/W and the luminous efficacy of the bilayered film systems (Blue/SGS G/KSF R and Blue/silicate G/CASN R) show similar values of 105 lm/W and 103 lm/W, respectively. The NTSC of the narrow band emitting SGS G/KSF R bilayered system (86.4%) is much higher than that of the broad band emitting YAG single layered (67.9%) and silicate G/CASN R bilayered (74.7%) system.
Fig. 8 (a) PL emission spectrum and (b) 1931 CIE color coordinates of silicate G, YAG Y, CASN R phosphors. (c) EL emission spectrum and calculated color filtered spectrum and (b) 1931 CIE color coordinates of YAG Y (15 wt%) single layered phosphor film-capped W-LED. (e) EL emission spectrum and calculated color filtered spectrum and (f) 1931 CIE color coordinates of silicate G (30 wt%)/CASN R (2.5 wt%) bilayered phosphor film-capped W-LED. (insets: schematic diagram of phosphor film-capped W-LED).
Table 1. Optical properties of remote-type white LEDs using freestanding phosphor films.
It indicates that the freestanding bilayered green/red phosphor film-capped white pc-LED, which consisted of narrow band red KSF and green SGS phosphors, is superior to the previously reported phosphor combinations in pc-LED backlights. This enables the reproduction of colors previously impossible with nitride phosphor-based pc-LEDs and it creates more vibrant and richer images in LCD displays.
Figure 9 presents the evolution of the EL luminous flux (lm) and luminous efficacy (lm/W), and the changes of the color coordinates of the bilayered green/red phosphor film-capped white pc-LED as functions of both the applied current of 10-180 mA and the ambient temperature of 20-120 °C. As a result, the CIE color coordinates were varied only in (0.282-0.279, 0.329-0.332) within 10-180 mA. The luminous efficacy of the bilayered green/red phosphor film-capped white pc-LED exhibited 124 lm/W at 10 mA and 129 lm/W at 20 mA, but steadily decreased to 71 lm/W at 180 mA. This decreasing trend primarily resulted from the degradation of the blue LED chip itself that arises from the increased efficiency drop in the enhanced applied current because the stacked green and red phosphor films do not come into contact with the blue LED in phosphor film-capped LED. For the same reason, Fig. 9(b) shows the CIE color coordinates of the bilayered green/red phosphor film-capped white pc-LED changed only slightly regardless of the applied current. In addition, Fig. 9 demonstrates that variations of the luminous flux, luminous efficacy, and CIE color coordinates for bilayered green/red phosphor film-capped white pc-LEDs are very small or almost constant regardless of the ambient temperature. These results confirm that the freestanding bilayered green/red phosphor film has an acceptable current dependence and good thermal stability, which renders it applicable for use in white pc-LEDs for LCD backlights.
Fig. 9 Current dependence of the green (12.5 wt%)/red (40 wt%) bilayered phosphor film-capped W-LED: (a) luminous flux and luminous efficacy and (b) 1931 CIE color coordinates. Temperature dependence of the green (12.5 wt%)/red (40 wt%) bilayered phosphor film-capped W-LEDs: (c) luminous flux and luminous efficacy and (d) 1931 CIE color coordinates.
4. Conclusions
We successfully synthesized narrowband emitting KSF red phosphor and SGS green phosphor using a facile etching synthetic process and a flux-aided solid state reaction under a H2S atmosphere, respectively. The optical and crystal properties of the red and green phosphors were also characterized in order to enhance the color gamut. Furthermore, we fabricated various phosphor concentrations of green/red bilayered freestanding phosphor films and green/red bilayered phosphor film capped white pc-LEDs in order to realize high-color gamut white light pc-LED for LCD backlights. The W-LED with green (12.5 wt%)/red (40 wt%) bilayered phosphor film exhibited a luminous efficacy of 105 lm/W at a CCT of 8,330 K with an applied current of 60 mA. The 1931 CIE color coordinates of white, B, G, and R emissions corresponded to (0.283, 0.332), (0.153, 0.062), (0.76, 0.674), and (0.648, 0.305), respectively; the color gamut of the filtered RGB triangle was calculated to be 86.4% relative to the NTSC with small variations of the optical properties as a function of the current and temperature. The narrowband green/red bilayered phosphor film capped W-LEDs are good candidates for LCD backlights with a high color gamut and flexibility of use.
Acknowledgments
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP (Ministry of Science, ICT&Future Planning)) (No. 2011-0017449).
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