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This study introduces innovative multi-chip white LED systems that combine an InGaN blue LED and green/red or green/amber/red full down-converted, phosphor-conversion LEDs (pc-LEDs). Efficient green, amber, and red full down-converted pc-LEDs were fabricated by simply capping a long-wave pass filter (LWPF) on top of LED packing associated with each corresponding powder phosphor. The principal advantage of this type of color-mixing approach in newly developed multi-chip white LEDs based on colored pc-LEDs is thought to be dynamic control of the chromaticity and better light quality. In addition, the color-mixing approach improves the low efficacy of green/amber LEDs in the “green gap” wavelength; reduces the wide color/efficacy variations of each primary LED with at different temperatures and currents; and improves the low color rendering indexes of the traditional color-mixing approach in red, green, and blue (RGB) multi-chip white LEDs.
©2011 Optical Society of America
1. Introduction
The electrical performance of commercial solid-state lightings (SSLs) based on white light-emitting diodes (LEDs) has improved tremendously over the last decade. The advantages of white-light sources based on LEDs include their high brightness, long durability, good color rendition, low power consumption, mercury-free composition and easy manufacturability. These advantages have led to the introduction of white LEDs into a wide range of traditional lighting applications, including LCD backlighting, outdoor lighting, indoor lighting, and automobile lighting. Due to the considerable impact of LEDs on energy consumption efforts, the environment, and on human health, interest in the use of white light based on LEDs for general illumination has grown rapidly [1–3]. However, the greatest impact of white light based on LEDs will likely be in general illumination applications that demand a more improved high-quality white-light source.
Currently, there are three popular approaches to generate white light using LEDs. As shown in Fig. 1(a) and (b) , these approaches include the single-chip approach of a blue (B) LED with yellow (YB) phosphors [4,5] (or green (GB) and red (RB) phosphors [6–8]; green and red emission pumped by blue LED), another single-chip approach of an ultraviolet (UV) LED with blue (BUV) and yellow (YUV) phosphors (or blue (BUV), green (GUV) and red (RUV) phosphors [9]; blue, green and red emission pumped by UV LED) and a multi-chip approach that combines red (R), green (G), and blue (B) LEDs [10,11]. Each of these three different approaches of producing white light based on LEDs has its pros and cons. The first two single-chip approaches based on phosphor-converted LEDs (pc-LEDs) benefit from their relatively low cost and high efficiency as well as good color stability over a wide range of temperatures. As a blue LED chip is more efficient and stable compared to a UV LED chip, white light based on blue LEDs is commonly used as a simple long-life white-light source. Presently, the first approach toward the creation of a single pc-LED chip based on a yellow (cerium-doped yttrium aluminum garnet) phosphor-coated blue LED has led to its wide use in various outdoor lighting applications at the expense of a reddish warm white color [4]. Quite recently, the developments of new green and red nitride phosphors that are efficiently excited by a blue LED encourage the use of these phosphors in producing high-quality white light with an improved color rendering index (CRI) and a reduced color temperature [8]. However, the tri-color single-chip approach continues to be associated with the scattering and color-mixing loss of emitted light from the mixed green and red phosphors and the relatively large fabrication tolerance of the mixed color points. The RGB multi-chip approach otherwise allows for the facile dynamic control of color points and provides high color rendition and stabilization of the chromaticity. However, disadvantages of the multi-chip approach, such as the low efficiency of green LEDs (the “green gap” problem), the different temperature/current/time dependence of each colored LED, the reduced quality of the light due to the narrow-band spectrum, and the high fabrication cost inhibit the wide penetration of these multi-chip white LEDs into a wider range of lighting applications [12–14]. Moreover, Su et al. proposed a single package comprised of UV/purple/blue multi-chip LEDs, UV-pumped green phosphor and violet-pumped red phosphor with a UV-mirror-visible-pass omni-directional reflector (ODR) filter (Fig. 1(c)) to combine the advantages of both conventional single-chip and multi-chip approaches [15]. However, the color mixing loss of G,R mixed phosphors, the low efficiency issue of UV LED, and the limited control of the emitted color temperatures continue to be issues in their suggested devices.
Fig. 1 Schematic diagrams of the color-mixing approaches to generate white light from single-chip and multi-chip LEDs. (a) white-by-blue (YBB or RBGBB) or white-by-violet (RUVGUVBUV) phosphor-converted single chip white LED. (b) AlGaInP red, InGaN green and blue (RGB) multi-chip white LED. (c) white LED package with UV, blue, violet multi-chip LEDs, green/red phosphors, and an ODR filter, (d) full conversion pc-LED red, green, and InGaN blue (RB,MGB,MB) multi-chip white LED. (e) full conversion pc-LED red, amber, green, and InGaN blue (RB,MAB,MGB,MB) multi-chip white LED. (f) Schematic diagram of the mechanism of the blocking and recycling of the forward unabsorbed emission of a blue LED by capped-LWPF into the phosphor-coated LED die.
Very recently, we succeeded in fabricating highly efficient green/yellow/amber monochromatic pc-LEDs using a long-wave pass filter (LWPF) to address the problem of the low performance and wide temperature variation of monochromatic LEDs at wavelengths in the region of the “green gap” [16,17]. A simple modification of a quarter-wave type of LWPF consisting of TiO2/SiO2 nano-multilayered film was introduced on top of various phosphor-coated InGaN-based blue LED dies to block and recycle unabsorbed transmitted blue emission to create various highly efficient full-conversion monochromatic LEDs (Fig. 1(f)). In the present study, we propose the new paradigm of a multi-chip RB,MGB,MB or RB,MAB,MGB,MB (RB,MAB,MGB,M denoted as a LWPF-capped, full down-converted, monochromatic red, amber and green pc-LED pumped by a blue LED chip) white-light approach which combines blue LED and green/red or green/amber/red full-conversion monochromatic pc-LEDs to enhance the performance of green/amber LEDs and mixed white light, reduce the variation in the temperature/current/time dependence of RB,M, AB,M, GB,M, and B LEDs, enhance the color rendering of white light, and dynamically control the color points (Fig. 1(d), (e)). Therefore, this multi-chip approach with one or more than two monochromatic pc-LEDs (RB,MGB,MB or RB,MAB,MGB,MB white light) can serve as an alternative that combines the advantages of the pc-LED single-chip approach and the RGB multi-chip approach while avoiding the disadvantages of previously demonstrated approaches. This study represents the initial fabrication of white LED lighting consisting of a blue LED and full-conversion monochromatic pc-LEDs. We briefly analyze the optical properties and performance levels of various types of multi-chip white light created by diverse combinations of blue LEDs and full-conversion pc-LEDs and compare the results to those of traditional RGB LED multi-chip white light. This straightforward and facile concept of multi-chip white LEDs using a blue LED chip and full-conversion pc-LEDs can lead to further research and developments related to the new application of white LED lightings in the positive-sum application market of SSLs. The approach toward the creation of white light as demonstrated in this work has greater potential for allowing dynamic control of the light’s spectral distribution and color temperature, providing “smart” and efficient lighting capabilities beyond traditional multi-chip RGB LED white lamp systems [18,19].
2. Experimental methods
Fabrication of LWPFs: Two types (L1 and L2) of dielectric LWPFs were fabricated on glass substrates with a thickness of 0.2 mm. For the fabrication of the LWPF stacks, terminal eighth-wave thick TiO2 (L1: 25 nm, L2: 26 nm) and quarter-wave thick SiO2 (L1: 73, L2: 73 nm) nano-multilayered films ((0.5TiO2/SiO2/0.5TiO2)9) were coated onto a glass substrate by e-beam evaporation at 250°C [16]. 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). These measured n and k values were used to simulate the reflectance (R), transmittance (T) and absorption (A) in the design of the various types of LWPFs. For the design of the LWPF multilayer films for the blue-excited pc-LEDs, the characteristic matrix method was used to simulate the reflectance (R), transmittance (T) and absorption (A) of the optical structure of LRF stacks [16]. In the simulation, the thicknesses of the high-index (TiO2) and low-index (SiO2) films were varied to tune the spectral position of the reflectance band. In this publication, two different types of LWPFs with nine periods of 0.5TiO2/SiO2/0.5TiO2 multi-layers (L1 = 510 for green with L2 = 530 nm for amber/red at the band-edge of the long-wavelength) were fabricated as capping filters to fabricate green, amber and red full down-converted pc-LEDs.
Fabrication of the full down-converted pc-LED with LWPFs: To fabricate the full down-converted pc-LEDs, a blue chip (λmax = 445 nm) was used simultaneously as a blue light source and an excitation source for the various color phosphors of pc-LEDs. Blue, green and amber monochromatic LED chips were purchased from Alti-semiconductor Co. Ltd. (Sr,Ca)AlSiN3:Eu [20], (Sr,Ba,Ca)3SiO5:Eu [21] and (Sr,Ba)2SiO4:Eu [22] phosphors were also used as the red, amber and green pc-LEDs in this experiment, respectively. The powder phosphors were obtained from several phosphor companies. Optimum amounts of each color phosphor were dispersed in a silicone binder, and the same amounts of the resulting phosphor pastes were dropped onto a cup-type blue LED to create the red, amber, and green pc-LEDs. On top of the variously colored pc-LEDs, a LWPF-coated glass substrate was attached with an air gap.
Characterization of full down-converted pc-LEDs and multi-chip white LEDs: The forward emissions of the emission spectra from III-V blue, green, amber LEDs, blue-excited pc-LEDs and blue-excited LWPF-capped monochromatic pc-LEDs were measured in an integrated sphere using a spectrophotometer (PSI Co. Ltd., Darsar). The luminous efficacy and quantum efficiency were defined as the brightness and the integrated emission spectra of LWPF-assisted pc-LEDs, respectively, at a constant current or power. The external efficiency and color purity of the various full down-converted color pc-LEDs were obtained with the current at optimum phosphor concentrations.
Characterization of multi-chip white LEDs: A set of the primary LEDs with peak wavelengths of 455 nm (blue), 523 nm (green), 594 nm (amber), and 625 nm (red) was selected for RGB three-chip and the RAGB four-chip white LEDs by comparing the optical properties of RB,MGB,MB three-chip and RB,MAB,MGB,MB four-chip white LEDs (Fig. 2 ). The primary LEDs were put into a triangle or square lattice fixture for three-chip or four-chip white LEDs under direct current (DC) operation. Each primary LED was controlled separately by an individual power supply for the particular combination of the primary fixtures required for the selected color points. The efficacies, CRIs and the relative fluxes from each set of white LEDs were measured in an integrated sphere using a spectrophotometer. The fractional applied currents of the primary LEDs in four different multi-chip white LED sets were measured to achieve the set of eight CCTs specified by American National Standards Institute (ANSI) standard (C78.377-2008).
Fig. 2 The overlapped emission spectra of each LED in four different white-light systems; (a) RGB, (b) RAGB, (C) RB,MGB,MB, (d) RB,MAB,MGB,MB multi-chip white LED. The inset shows the color diagram of the CIE (CIE = Commission Internationale d’Eclairage) of each white-light system.
3. Results and discussion
Many combinations of a blue LED and full-conversion monochromatic colored LEDs can be implemented to control particular white points dynamically. The wavelength of the light emitted by the full down-converted pc-LED is dependent on the particular phosphor used. A variety of efficient green, amber, and red monochromatic pc-LEDs can be fabricated by simply capping a long-wave pass filter (LWPF) on top of LED packing associated with each corresponding powder phosphor. There are many red, amber, and green phosphor candidates in the presently commercialized phosphors to fabricate RB,M, AB,M, and GB,M full-conversion pc-LEDs. Among them, we selected (Sr,Ca)AlSiN3:Eu, (Sr,Ba,Ca)3SiO5:Eu and (Sr,Ba)2SiO4:Eu phosphors for the fabrication of red, amber, green full-conversion pc-LEDs in this experiment, as they are widely used in single-chip white pc-LEDs. Figure 3 and Fig. 4 indicate that full down-converted pc-LEDs have reduced or at least similar variations of the efficacy and color coordinates with the current/temperature compared to the wide variation in the levels of current/temperature stability among green/amber/red monochromatic III-V LEDs that do not contain phosphors.
Fig. 3 The luminous efficacy of a full down-converted monochromatic pc-LED and a direct-emitting monochromatic semiconductor LED without phosphors as a function of the applied current; (a) green color, (b) amber color, and (c) red color. The normalized luminous efficacy of both pc-LEDs and III-V LEDs as a function of the ambient temperature; (d) green color, (e) amber color, and (f) red color.
Fig. 4 The variations of the CIE color coordinates of a full down-converted monochromatic pc-LED and a direct-emitting monochromatic semiconductor LED without phosphors as a function of the applied current; (a) green color, (b) amber color, and (c) red color and as a function of the ambient temperature; (d) green color, (e) amber color, and (f) red color. Arrows indicate increase of the applied current from 0.5mA to 300mA; (a),(b),(c), and the ambient temperature from 20°C to 120°C; (d), (e), (f).
As is well known, different material systems such as InGaN for blue and green and AlGaInP for red behave very differently with respect to the temperature, applied current and time [23]. The colors of full down-converted pc-LEDs are obtained by phosphor emissions excited by the same blue LED source. The improved temperature and current stability and the reduced variation in the color and efficacy of full-conversion pc-LEDs result from the generation of any color based on more stable blue LED chip used compared to the green, amber, and red semiconductor LEDs used. Figure 3 also shows that the luminous efficacies of the green and amber pc-LED are higher than those of a corresponding green and amber III-V LED over the actual range of current used in practical applications [17]. The efficacies of the green and amber full-conversion pc-LED are 2.0 and 1.5 times higher than that of a direct-emitting green and amber LED without the use of phosphors at 100 mA of applied current. In contrast to green/amber pc-LEDs, the luminous efficacy of a red pc-LED is somewhat lower than that of the AlGaInP red LED due to the relatively low quantum efficiency of (Sr,Ca)AlSiN3:Eu red phosphor and the large spectral loss of the wide-band spectrum into sub-optimal wavelengths, specially into the deep red wavelength. Although the efficacy of the red pc-LED is lower than that of the AlGaInP red LED, the increased temperature/current stability of the red pc-LED enables it to be combined with other colored pc-LEDs to realize multi-chip white LEDs. Therefore, in terms of the enhanced luminous efficacy of amber and green pc-LEDs in the emission of “green gap” wavelengths and the reduced temperature/current/time tolerance of the red, amber and green pc-LEDs combined with the blue LED, our multi-chip approach offers a clear advantage.
Similar to the incident angular dependence of the reflected wavelength [24] and polarization [25] of conventional ODR filters, our modified quarter-wave stacks of LWPFs are also characterized by the incident angle dependence of the reflected color and polarization. This non-uniform color and polarization distribution can lead to another critical issue regarding the use of a LWPF in a single-chip white pc-LED. However, white light is obtained from the spatial mixing and redistribution of the light emitted from each pc-LED and blue LED in our RB,MGB,MB and RB,MAB,MGB,MB multi-chip white LEDs. Some of the leaked blue light from each monochromatic green, amber, and red pc-LED can also participate in producing the white light (Figs. 2(c) and (d)). Therefore, the incidence dependence of the color and polarization in LWPFs and the small amounts of leaked blue light are likely an inconsequential issue given that the white light is obtained from the mixed light from multi-chip LEDs and measured in an integrated sphere. Furthermore, the stop band of our LWPFs was designed to block the blue photons that leak from a LWPF-capped pc-LED completely at any incident angle, as suggested in a previous report [26].
Figure 5 shows the overlapped integrated emission spectra of each LED in four different white-light systems (RGB, RB,MGB,MB, RAGB and RB,MAB,MGB,MB multi-chip white LEDs) along with the Commission Internationale d’Eclairage (CIE) color diagram of each white-light system. The conventional RGB or RAGB multi-chip white systems have a narrow-band (NB) spectrum while the newly developed RB,MGB,MB or RB,MAB,MGB,MB white systems provide a greater wide-band (WB) spectrum for the green, amber and red colors. This widening of the spectrum can provide a high light quality that renders all colors very well. In typical dynamic multi-chip white LEDs, each LED is controlled separately to enable dynamic color changes. Consequently, small changes in the power fraction of each LED allow the realization of almost every color within the triangle and quadrangle CIE diagrams in the insets of Fig. 2 in conventional and in the newly proposed white LED systems. Therefore, all colors within the multi-angle are accessible through the mixing of three or four emissions as defined by the color coordinates of several phosphors and a blue LED. The increased number of chromatic points enables an enlarged color reproduction area of the emitted and reflected white light at the expense of the production cost.
Fig. 5 The emitted spectrum distribution of (a) RGB, (b) RAGB, (C) RB,MGB,MB, (d) RB,MAB,MGB,MB multi-chip white clusters for a set of eight CCTs specified in the ANSI standard.
The emitted spectrum distributions of RGB, RB,MGB,MB, RAGB, and RB,MAB,MGB,MB white light are compared for eight correlated color temperatures (CCTs) specified by the ANSI standard. Previously, Zukaukas et al. suggested that a tetrachromatic lamp with WB blue, green, yellow and red spectra with a reasonable bandwidth (50–70 nm) may offer high efficacy and good color quality [27]. However, such a lamp is difficult to implement owing to the lack of wide-band blue emission in white lamps based on a blue LED. Here, trichromatic and tetrachromatic spectra (RB,MGB,MB and RB,MAB,MGB,MB) containing a NB blue LED and WB full-conversion pc-LEDs are technologically more realistic for realizing smart, high-quality white light. The eight specified CCTs on the blackbody line can be easily obtained by dynamically tuning the fractional applied current of the primary LEDs in each multi-chip white light. The fractional applied currents of the primary LEDs are slightly different for different multi-chip sets at any specified CCT (see Fig. 6 ). The specified colors are produced by combining a different portion of each colored LED with the different luminous efficacy of the various white sets. As shown in Fig. 6, the CCT is decreased by the increase of the fractional radiant flux of the red/amber LED with respect to that of the blue/green LED. Any specified white colors and all colors in the multi-angle area in the CIE diagram are realized by dynamically controlling the fractional applied current in the full-conversion pc-LED multi-chip white LEDs as well as the traditional RGB and RAGB multi-chip LEDs. This dynamic control of the color points is a clear advantage compared to the single-chip white approach with partial-conversion pc-LEDs, as the color points of single-chip pc-LEDs are always fixed when they are fabricated with a specified concentration ratio of green and red phosphors.
Fig. 6 The fractional applied currents of the primary LEDs in four different multi-chip white LED sets as a function of the CCT; (a) RGB, (b) RAGB, (C) RB,MGB,MB, and (d) RB,MAB,MGB,MB multi-chip white LED.
Other important figures of merit in white light applications are the luminous efficacy and color rendering index (CRI, Ra) of the white light at the same color point. As shown in Fig. 7 , the luminous efficacy, relative quantum efficiency, relative brightness, and CRI (Ra) of four different clusters of multi-chip white LEDs were compared as a function of the CCT. They were measured under a constant total applied current (150 mA for trichromatic LEDs and 200 mA for tetrachromatic LEDs) with a specified fractional current of each colored LED. As mentioned above, LWPF-capped full-conversion green and amber LEDs have much higher luminous efficacy than conventional green and amber LEDs. Moreover, the luminous efficacy of full-conversion red pc-LED is lower than that of the AlGaInP red LED (see Fig. 3). At a CCT value lower than 5000K, the red portion of the white color increased and the luminous efficacy of the RB,MGB,MB decreased. In the case of the tetrachromatic set of white LEDs, the amber portion of the white LED becomes more important than the red portion at the eight specified CCTs. Hence, the highest efficacy can be attained in RB,MAB,MGB,MB four-chip white LEDs and the lowest efficacy can be attained in RAGB four-chip LEDs. Figure 7 also shows that the enhancement ratios of both the quantum efficiency and the brightness of the RB,MAB,MGB,MB four-chip and RB,MGB,MB three-chip white LEDs are more prominent than the enhancement ratio of the luminous efficacy. When the newly developed multi-chip color-mixing packages are blended with the light output from discrete colored sources, each colored source is operated under the same applied voltage, as all types of full-conversion pc-LEDs are fabricated using the same type of blue chip. Otherwise, the operational voltages of the discrete colored sources from traditional RGB or RAGB packages are different. Particularly, AlGaInP-based red and amber LEDs show operational voltages lower than those of InGaN-based blue and green LEDs. The discrepancy in the enhancement ratios among the efficacy, efficiency, and brightness result from the different applied operational voltages and different current portion of each colored source for traditional and the newly developed color-mixing packages.
Fig. 7 The luminous efficacy, relative quantum efficiency, relative brightness, and CRIs (Ra) of four different (RGB, RAGB, RB,MGB,MB, and RB,MAB,MGB,MB) clusters of multi-chip white LEDs as functions of the CCT. The relative quantum efficiency and relative brightness were compared with the emission spectrum of a blue InGaN LED.
Although CRI is not the only accepted figure of merit for the ranking of the quality of a white lighting source at present, the color rendering index is still widely used to measure the ability to reproduce colors of illuminated objects with high fidelity [28]. Here, we compare the CRIs of various multi-chip color-mixing packages as a function of the CCT to evaluate the feasibility of LEDs as a designing source of white light. As shown in Fig. 7(d), the color rendering of three-chip RGB white LEDs is not an acceptable value for the eight specified CCTs due to the narrow spectral band of the RGB LEDs. The CRI values of the three- or four-chip white LEDs with a blue LED and the full-conversion pc-LEDs show an improvement to the point that they exceed 80. These figures clearly indicate that three-chip white LEDs with green and red full-conversion pc-LEDs offer excellent color rendering (> 88) for indoor lighting as well as good luminous efficacy (32 ~43 lm/W) for the eight specified CCTs. Furthermore, four-chip white LEDs with the green, amber and red full-conversion pc-LED design are shown to have good color rendering (> 82) for indoor lighting as well as excellent luminous efficacy (42 ~47 lm/W) for the eight specified CCTs.
4. Conclusions
Similar to the traditional color-mixing approach of RGB or RAGB LEDs, the principal advantage of the newly developed color-mixing approach consisting of a blue LED and full-conversion pc-LEDs to produce white light is the promise of dynamic control to realize virtually all color temperatures. In addition, this color-mixing approach improves the three problems of the traditional color-mixing approach. First, the luminous efficacy is significantly improved by enhancing the luminous efficacy of green and amber pc-LEDs over green and amber LEDs. Second, the color variations of white light can be reduced with different temperatures, currents and times by reducing the color variations in each green, amber, and red pc-LED, as full-conversion pc-LEDs are fabricated by combining LWPF filters, the corresponding phosphors and the same type of blue LED chip. Third, the color rendering indices of the three-chip as well as the four-chip white LEDs with full conversion monochromatic pc-LEDs are significantly improved through the use of green, amber and red phosphors with a WB spectrum. Moving beyond this performance will require further optimization of the LWPFs and phosphors, particularly the narrowband phosphors in each color region, as well as improvements in the efficiency of the blue LED. Although more elaborate studies involving further improvements in the efficacy and color quality are necessary before these types of lights are used in traditional or newly developed white lighting markets, this type of color-mixing approach can provide alternative possibilities to realize efficient and high-quality “smart” lighting systems that are far beyond traditional discrete color-mixing lamp systems.
Acknowledgements
This research was supported by the Future-based Technology Development Program (Nano R&D Program grant # 2008-03573, ERC program grant # R11-2005-048-00000-0 and MEST program grant number NRF-2009-C1AAA001-0092938) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology.
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