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This paper reports a simple approach for the design of blue-excitation-light passing and phosphor-yellow-emission-light reflecting dielectric multilayers to recycle the backward emission of Y3Al5O12:Ce3+ (YAG:Ce) yellow phosphors on top of a blue InGaN light-emitting diode (LED) cup. The insertion of modified quarter-wave films of alternate high- and low-refractive index dielectric films (TiO2/SiO2) into the interface between a YAG:Ce phosphor layer and glass substrate resulted in 1.64 and 1.95 fold increase in efficiency and luminous efficacy of the forward white emission compared with that of a conventional phosphor on top of a blue LED cup with a lower correlated color temperature (< 4000K).
©2009 Optical Society of America
1. Introduction
Technologies for achieving high-efficiency white light-emitting diodes (LEDs) have been studied actively in recent years for applications to solid-state lighting. Major developments in wide-band-gap III-V nitride compound semiconductors and color converted phosphors have led to the commercial production of white phosphor-converted light-emitting diodes (pcLEDs) [1,2]. Blue InGaN technology has made the white LED possible, in which white light is obtained by coating a yellow (Y3Al5O12:Ce3+, or “YAG:Ce”) powder phosphor [3,4] or green/red (SrGa2S4:Eu2+/SrS:Eu2+ sulfides or green/red nitrides) mixed powder phosphors [5–9] onto a blue LED chip. The development of phosphors for white pcLEDs has expanded LED applications to light bulbs or lamps with high durability and low energy consumption.
However, in the conventional pcLED structure, it is important to improve the low conversion efficiency of the phosphor layer in order to meet the goal of solid-state lighting, as defined by the Department of Energy (DOE) and Optoelectrical Industry Development Association (OIDA) [10]. As previously reported, the low conversion efficiency of the phosphor layer is due mainly to the low forward efficiency of the coated phosphor layer [11]. For the conventional case of a InGaN/YAG:Ce white LED, half of the phosphor-converted yellow light is emitted directly back into the InGaN chip or submount. This results in considerable loss of the yellow emitted light because the powder phosphor is emitted omni-directionally into space. Therefore, it is important to reduce this phosphor conversion loss. Recently, several optical schemes have been implemented with the aim of improving the amount of light extraction from pcLEDs. Narendran et al. introduced a scattered photon extraction (SPE) pcLED to separate the LED die and phosphor layer [11]. Their geometry of the optic efficiently transfers the light exiting the InGaN die to the phosphor layer, and allows most of the backscattered light from the phosphor layer to escape the optic. Luo et al. also proposed a remote phosphor layer with a hemispherical dome to reduce the deterministic optical modes trapped inside the encapsulant [12,13]. In both cases, backscattered light is still partially trapped between the phosphor layer and reflector. Quite recently, Allen et al. proposed an almost ideal phosphor-converted white LED to maximize the effective phosphor packaging efficiency [14]. Their enhanced light extraction by internal reflection (ELiXIR) pcLED approach involved physically separating the phosphor layer from the LED chip to reduce absorption. In their design, the blue LED was located at the center of a hemispherical polymer shell with an interior semi-transparent phosphor coating. However, these methods involve a complicated optical design for applications to high power LEDs with a large LED chip, such as thin-film flip-chip (TFFC) LED (chip size > 1x1mm2) [15], which increases the cost of fabrication and produces an unfavorably large size of the LED package in some industrial applications. Furthermore, the large portion of backward phosphor emission into the InGaN chip is barely recovered if the aforementioned optical structures are applied to large TFFC LED chips. In order to recycle the backward phosphor emission into the InGaN chip, we propose a short wave pass edge filter (SWPEF), which functions as a window for blue LED light and a mirror for yellow phosphor emission. This paper reports a simple optical design and structure of SWPEFs to enhance the forward efficiency of the phosphor layer. In this design, a SWPEF coated transparent substrate was inserted into the interface between a blue LED cup and yellow phosphor layer (phosphor-on-cup) and the recycling capability from backward phosphor emission was assessed.
2. Experimental methods
The basic sequence of the SWPEF stacks used in this study is shown in the right-side inset in Fig. 1 . The SWPEF stack is made up of a number of symmetrical periods. This is simply to add a pair of eighth-wave layers to the quarter wave stack, one at each end. For the fabrication of the SWPEF stacks, terminal eighth-wave thick SiO2 (52 nm) and quarter-wave thick TiO2/SiO2 (68/104 nm) films were coated onto a glass substrate by e-beam evaporation at 250°C. The base pressure in the e-beam chamber was fixed at 4.0 x 10−5 torr. The deposition was performed at an acceleration voltage of 7 kV with an oxygen partial pressure of 1.9 x 10−4 torr. The refractive indices (n) and extinction coefficients (k) of the e-beam evaporated SiO2 and TiO2 films were measured using a spectroscopic ellipsometer (Sentech, SE800). Figure 2 shows the wavelength dispersion of the n and k values of the as-grown SiO2 and TiO2 films. These measured n and k values were used for simulating the reflectance (R), transmittance (T), and absorption (A) in the design of the optimum SWPEF.
Fig. 1 Schematic diagram of the phosphor-on-cup white LED device structure with the embedded blue light passing through and yellow light reflecting from the dielectric multilayer coated glass substrate and PL measurement system. The enlarged side view shows the basic structure of the SWPEF stacks and the mechanism for the enhancement of forward emission from the YAG:Ce yellow phosphor layer.
Fig. 2 Measured refractive indices (n) and extinction coefficients (k) of the as-grown (a) SiO2 and (b) TiO2 films as a function of the wavelength.
YAG:Ce phosphor powders (Nichia Co. Ltd.) were coated onto the SWPEF-assisted and plain glass substrates using a sedimentation method. YAG:Ce phosphor powders were suspended in a 1 wt % potassium silicate solution. The phosphor suspension was then poured into a 0.5 wt % barium nitrate solution with a substrate plate at the bottom of the container. After the phosphor powders had sedimented, water was slowly decanted and the sedimented powders were then dried at room temperature for 1 day. The phosphor density was determined by weighing the substrate plate before and after screening.
The normally directed emissions and angular dependence of the emission spectra from both the YAG:Ce-coated conventional and SWPEF-assisted LEDs were measured using a spectrophotometer (PSI Co. Ltd., Darsar). The thicknesses and structures of the SWPEF were measured with a field-emission type scanning electron microscope (FESEM) (JSM 7401F, JEOL) operated at 10 kV.
3. Results and discussion
Figure 1 shows a schematic diagram of the phosphor-on-cup white LED device structure with the embedded SWPEF multilayer coated glass substrate. In general, a conventional quarter-wave film of alternating high- and low-refractive index dielectric films is considered to be a candidate for SWPEF dielectric stacks. However, a conventional quarter-wave film shows strong interference oscillations of transmission peaks in the shorter wavelength region, as determined by our simulations, which can decrease the transmission of blue light. In this study, a stack of modified quarter-wave films was proposed as a good candidate for a SWPEF substrate. In the proposed SWPEF dielectric multilayer stack, a blue light (LED emission) pass filter is required, i.e., one that freely transmits at wavelengths shorter than the high reflectance band at the green/yellow/red wavelength (yellow or green/red phosphor emission). As previously reported, certain variations from the quarter-wave stack design were employed by also depositing terminal eighth-wave low-index films [16].
Here, the combination 0.5L(H)0.5L (eighth-wave low-index SiO2 (0.5L) and quarter-wave high-index TiO2 (H); 0.5SiO2(TiO2)0.5SiO2) between the glass substrate G and air A is repeated m times. The standard wavelength is 608 nm.
The enlarged view in Fig. 1 shows the mechanism for the enhanced forward reflection from the backward yellow emission of the phosphors into the LED cup. The high transmission of the SWPEF stacks at the blue wavelength excites the over-coated yellow phosphor layer on the SWPEF-assisted substrate as much as a phosphor on a conventional glass substrate. In addition, the high reflectance band of the SWPEF stacks at the yellow wavelength can recycle a large amount of the backward yellow phosphor emission that would otherwise be absorbed by the LED die/cup. Therefore, the enhanced efficiency of forward emission from the phosphor package from the phosphor-on-cup white LED with the embedded SWPEF stack is due to the high reflection of backward emission from the yellow phosphor into the LED cup as well as to the high transmission of forward emission of the blue LED chip into the phosphors.
For the design of SWPEF multilayer films, the characteristic matrix method was used to simulate the reflectance (R), transmittance (T), and absorption (A) of the optical structure of the SWPEF stacks [17]. In the first stage, the reflectance and transmittance of the SWPEF [0.5L(H)0.5L]m stacks consisting of alternating low-index SiO2 and high-index TiO2 were simulated as a function of the number of periods, m. The reflectance at the yellow region increased with increasing periodic number of stacks up to m = 9, becoming saturated above m = 9. For a wavelength within the stop band, the reflectance through the sample was increased to >99% [16]. The simulation results showed that the short-pass form, [0.5L(H)0.5L]m, provides a short-wavelength (blue) window with high transmittance. The transmittance at the blue region is maintained over 90% irrespective of the periodic number of stacks because interference oscillation is minimized in the window due to the positioning of the thinnest stack adjacent to the glass substrate. Figure 3 shows a side-view scanning electron microscopy (SEM) image of a real fabricated SWPEF dielectric multilayer comprised of modified of alternate TiO2 and SiO2 quarter-wave films of the ninth periods. As shown in Figs. 1 and 3, the thicknesses of obtained TiO2/SiO2 films satisfactorily match those of dielectric films in the designed SWPEF. Figure 3 also shows the measured optical transmittance of the SWPEF film. The measured high transmittance at the blue wavelength, perfect reflectance at the yellow wavelength and shape of the stop band in the fabricated SWPEF stacks were similar to those of the calculated SWPEF film. There was a slight difference in interference oscillations in the blue window and width of the stop-band in the yellow mirror between the calculated and fabricated SWPEF film. This is mainly because the parameters used in the simulations, such as the thickness and optical properties (refractive index and extinction coefficient), are not identical to the actual values.
Fig. 3 Calculated and measured transmittance spectra of a SWPEF stack [0.5L(H)0.5L]9 on a glass substrate. The inset shows a side-view scanning electron microscopy (SEM) image of the ninth periods of [0.5SiO2(TiO2)0.5SiO2] films.
Various phosphor densities of sedimented YAG:Ce phosphor-coated SWPEF-assisted substrates were coated directly onto the blue LED cup to demonstrate the maximum possible enhancement of the forward emitting efficiency in a phosphor-on-cup white LED embedded within the SWPEF film coated substrate (Fig. 1). Figure 4 shows the transmitted spectra of conventional YAG:Ce-coated plain glass and YAG:Ce-coated SWPEF-assisted glass-covered LEDs corresponding to one phosphor density and a sedimented phosphor layer of 4mg/cm2. The normally directed output spectra of the conventional and SWPEF-assisted devices are the superposition of nonabsorbed blue light and forward phosphor emission. The SWPEF-assisted phosphor-on-cup LED showed much stronger intensity than the conventional LED at the yellow phosphor emission wavelength and similar intensity to that of a conventional LED at the blue LED emission wavelength.
Fig. 4 The blue-conversion-white spectra of a conventional and YAG:Ce-coated SWPEF-assisted glass covered LED cup corresponding to a sedimented phosphor layer of 4mg/cm2 .
Figure 5(a) shows the relative intensities of the transmitted blue and yellow light for the conventional and SWPEF-inserted phosphor-on-cup white LED as a function of the phosphor density. The phosphor density was set to 4.9 mg/cm2 as an upper limit of the experimental range because blue light is almost converted to yellow light above this density. This indicates that the enhancement of the yellow emission of SWPEF-assisted white LED increases with increasing density of the YAG:Ce phosphor. The change in intensity of the blue peak for both the conventional and SWPEF-assisted LED showed similar trends, and the intensities of both samples decreased with increasing density of the YAG:Ce phosphor. The enhancement ratio achieved with the insertion of SWPEF-assisted substrates into a conventional white pc-LED was measured. Figure 5(b) shows both the measured enhancement values at the relative extraction efficiency and the relative luminous efficacy as a function of the phosphor density. For a systematic comparison of these media, the enhancement ratio of both the relative extraction efficiency and relative luminous efficacy was defined as the ratio of the efficiency and luminous efficacy of a SWPEF-assisted phosphor-on-cup white LED to those of an equivalent conventional LED. As shown in both graphs in Fig. 5(b), the enhancement ratios of the extraction efficiency and luminous efficacy showed similar trends depending on the phosphor density. The increases in extraction efficiency and luminous efficacy at a phosphor density of 4.9 mg/cm2 were 1.64 ± 0.05 and 1.95 ± 0.05, respectively. This enhanced yellow emission of the YAG:Ce phosphor layer is similar to the expected enhancement by the high reflection (>0.99) of a SWPEF-assisted substrate at the yellow wavelength.
Fig. 5 (a) The relative intensities and (b) the enhancement ratios of the extraction efficiency and luminous efficacy of transmitted blue and yellow light for a conventional and SWPEF-inserted phosphor-on-cup white LED as a function of the phosphor density.
Simple mixing between the blue emission of an InGaN LED chip and the yellow emission of a YAG:Ce phosphor provides an opportunity to obtain a white point in the 4000-8000 K correlated color temperature (CCT) region [18]. As the density of the YAG:Ce phosphor is controlled in Fig. 6(a) , the correct portion of the blue and yellow light differs for both the conventional and the SWPEF-inserted pcLED to produce the desired white light (< 4000 K), which has a chromaticity typically of the blackbody locus. For a practical application, the phosphor densities for reaching a CCT ~5000 K of conventional and SWPEF-assisted LEDs were 4.0 mg/cm2 and 3.2 mg/cm2, respectively. A comparison of the optical characteristics of both a conventional and SWPEF-assisted LED at the practical white point (CCT ~5000 K) revealed enhancement ratios for the forward efficiency and luminous efficacy of 1.76 ± 0.05 and 1.80 ± 0.05, respectively. In conventional YAG:Ce-coated pcLEDs, the narrow blue and broad yellow emission provide a reasonable CRI in the range of ~70–80. However, the CCT is limited at the low end to approximately 4000K. Figure 6(b) shows the all color rendering indices for conventional and SWPEF-assisted phosphor-on-cup LEDs as a function of the phosphor density. All curves in Fig. 6 indicate that the color rendering index (CRI) of most SWPEF-assisted LEDs is typically < 70, even though the forward efficiency of white LEDs is enhanced significantly and a lower CCT (< 4000 K, warm white) is obtained by inserting a SWPEF stack into the phosphor-on-cup LEDs.
Fig. 6 (a) The correlated color temperature and (b) color rendering indices of transmitted blue and yellow light for a conventional and a SWPEF-inserted phosphor-on-cup white LED as a function of the phosphor density.
Color variation with the viewing angle is undesirable for lighting applications and should be taken into consideration. Figure 7 plots the measured luminous efficacy and CCT as a function of the viewing angle for the conventional and SWPEF-assisted LEDs with a phosphor density of 4.0 mg/cm2 taken under identical excitation conditions. The luminous efficacies of both LEDs show Lambertian behaviors, as shown in the inset of Fig. 7. The shift of CCT (4070 → 3760 K) of the YAG:Ce-coated SWPEF-assisted LED was measured and found to be relatively small compared to that (4930 → 4150 K) of the conventional LED when the viewing angle changes in the range of 0-80°. This improved angular dependence of CCT of SWPEF-assisted LEDs is attributed to the reduced contribution of the transmitted blue light to the total white spectrum and color temperature.
Fig. 7 Angular dependence of CCT of a conventional and a SWPEF-inserted phosphor-on-cup white LED corresponding to a sedimented phosphor layer of 4mg/cm2. The inset shows the measured luminous efficacy profiles of a conventional (blue solid line) and a SWPEF-inserted phosphor-on-cup white LED (red solid line).
4. Conclusions
Inserting blue passing and yellow reflecting dielectric multilayers [0.5SiO2(TiO2)0.5SiO2]9 into a YAG:Ce phosphor-on-cup LED recycles the backward light in the backside architectures of a LED cup towards the forward direction. This enhances the yellow emission from the YAG:Ce phosphor layers. The use of a single-phosphor (YAG:Ce) approach aided by the yellow reflection band of the SWPEF stacks enhanced the forward efficiency and luminous efficacy by approximately 1.64 ± 0.05 and 1.95 ± 0.05 times, respectively, with a lower CCT (< 4000K) and CRI (< 70). Studies examining a SWPEF assisted two-phosphor approach to realize a “warm white” phosphor-on-top LED with a highly enhanced efficiency and reasonable CRI are currently underway.
Acknowledgments
This research was supported by a grant (code# F0004100-2008-31) from Information Display R&D Center, one of the 21st Century Frontier R&D Program funded by the Ministry of Knowledge Economy of Korean government. This study was also supported by grant number 2008-03573 of the Nano R&D Program through the Korea Science and Engineering Foundation grant funded by the Ministry of Education, Science and Technology.
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