This study introduces multi-package white light-emitting diodes (LEDs) system with the ability to realize high luminous efficacy and an excellent color rendering index (CRI, Ra) using the RB,MAB,MGB,MCB (RB,MAB,MGB,M denoted as a long-pass dichroic filter (LPDF)-capped, monochromatic red, amber and green phosphor converted-LED (pc-LED) pumped by a blue LED chip, and CB denoted as a cyan and blue mixed pc-LED pumped by a blue LED) system. The luminous efficacy and color rendering index (CRI) of multi-package white LED systems are compared while changing the concentration of the cyan phosphor used in the paste of a cyan-blue LED package and the driving current of individual LEDs in multi-package white LEDs at correlated color temperatures (CCTs) ranging from 6,500K (cold white) to 2,700K (warm white) using a set of eight CCTs as specified by the American National Standards Institute (ANSI) standard number C78.377-2008. A RB,MAB,MGB,MCB white LED system provides high luminous efficacy (≥96lm/W) and a color rendering index (≥91) encompassing the complete CCT range. We also compare the optical properties of the RB,MAB,MGB,MCB system with those of the RB,MAB,MGB,MB and RAGB (red, amber, green, and blue semiconductor-type narrow-spectrum-band LEDs) systems. It can be expected that the cyan color added to a blue LED in multi-package white LEDs based on LPDF-capped, phosphor-converted monochromatic LEDs will meet the needs of the high-quality, highly efficient, full-color white LED lighting market in the near future.
© 2012 OSA
Solid-state lighting is an attractive technology compared to conventional lighting methods such as incandescent and fluorescent lamps due to distinct advantages such as the high brightness, eco-friendliness (mercury-free composition), long-life time, small size, low power consumption and fast response times offered by this type of lighting [1–3]. 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 [4–6]. However, III-V monochromatic semiconductor LEDs, with a wavelength range between green and amber, show low luminous efficacy (known as the “green gap”) and poor current dependence (termed the “green droop”) compared to blue semiconductor LEDs. In previous reports by the authors, we proposed highly efficient long-pass dichroic filter (LPDF)-capped phosphor-converted LEDs (pc-LEDs) which operated in the green-gap wavelength to overcome the green gap and the green droop problems. Furthermore, LPDF-capped pc-LEDs have good current dependence compared to green III-V semiconductor LEDs because they uses blue LEDs, which shows good current dependence, as their phosphor excitation source [7–10].
There are many approaches to generate white light using LEDs. The single-package approaches based on pc-LEDs such as blue LEDs with yellow phosphors (or green and red phosphors) as well as ultraviolet (UV) LEDs with blue and yellow phosphors (or blue, green, and red phosphors) are known for their low cost and high luminous efficiency [11–16]. However, these single-package phosphor-based approaches have many disadvantages as well, such as scattering and color mixing loss of the light emitted from the mixed phosphors, a lack of phosphor color uniformity, and a relatively large fabrication tolerance of the mixed-color points. The semiconductor-type red, amber, green, and blue (RAGB) multi-package approach allows for the dynamic control of color points (See Fig. 1(a) ) [17, 18]. However, this multi-package (chip) approach incurs the low efficiency of green LEDs and is known for wide color/efficacy variations of the blue, green, amber, and red LEDs at different temperatures/currents and low color rendering indexes (CRIs) due to the narrow-band spectrum of each LED. Recently, we proposed a multi-package white LED system combined with InGaN blue LEDs and LPDF-capped green, amber and red pc-LEDs to address the problems associated with conventional multi-package (chip) white LED systems (See Fig. 1(b)) [9, 10].
As previously reported, RB,MAB,MGB,MB four-package white LEDs (RB,MAB,MGB,M denoted as a LPDF-capped, monochromatic red, amber and green phosphor converted-LED (pc-LED) pumped by a blue LED chip) showed excellent luminous efficacy though only fair color rendering indexes (CRIs) above ~82 for the eight correlated color temperatures (CCTs) specified by the American National Standards Institute (ANSI) standard C78.377-2008. This limited CRI of the RB,MAB,MGB,MB four-package white LED is due to the cyan void between the blue and green emission in the white spectrum of the RB,MAB,MGB,MB white LED. Therefore, more elaborate study is necessary to increase the CRIs to more than 90 while maintaining the excellent luminous efficacy of the RB,MAB,MGB,MB white LED at the eight specified CCTs. Recent research has attempted to create white light with an addition of cyan phosphors in a single-package pc-LED, as cyan phosphor can fill the space between the blue and green emission in the RGB-mixed white spectrum [19, 20]. For the same reason, we used a cyan and blue-mixed pc-LED instead of a pure blue LED in multi-package white LEDs to improve the CRIs without decreasing the high luminous efficacy. Here, we propose a multi-package white LED system consisting of a cyan and blue color-mixed pc-LED pumped by an InGaN blue LED chip and LPDF-capped, monochromatic red/amber/green pc-LEDs (a white LED system denoted as RB,MAB,MGB,MCB) in an effort to enhance the CRI of the RB,MAB,MGB,MB white LED system and to control the color points dynamically while reducing the variation in the temperature/current dependence of individual pc-LED (See Fig. 1(c)). Moreover, we compare the CRIs of various multi-package (chip) color-mixing LEDs as a function of the CCT to evaluate the feasibility of LEDs as a designed source of white light.
Table 1 provides the acronyms of each LED and pc-LED, and Table 2 provides the acronyms of the four-package white LED systems. The subscript “B” means that the semiconductor-type blue LED was used by an excitation source and the subscript “M” means that the pc-LED is a LPDF-capped monochromatic pc-LED.
2. Experimental methods
Fabrication of LPDFs: A dielectric LPDF was fabricated on a glass substrate with a thickness of 0.15 mm. For the fabrication of the LPDF stacks, terminal eighth-wave thick TiO2 (t: 25 nm) and quarter-wave thick SiO2 (t: 73 nm) nano-multilayered films ((0.5TiO2/SiO2/0.5TiO2)9) 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). The measured n and k values were used to simulate the reflectance (R), transmittance (T), and absorption (A) in the design of the LPDFs. For the design of the LPDF multilayer film 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 the LPDF stacks. In the simulation, the thicknesses of the high-index (TiO2) and low-index (SiO2) films were varied in order to tune the spectral position of the reflectance band. In this study, two types of LPDF with nine periods of 0.5TiO2/SiO2/0.5TiO2 multi-layers (535 nm for green and 555 nm for amber/red at the band-edge of the long wavelength) were fabricated as a capping filter to fabricate full down-converted monochromatic pc-LEDs .
Fabrication of a blue color-mixed cyan pc-LED and the LPDF-capped green/amber/red pc-LEDs: To fabricate the blue color-mixed cyan pc-LED (CB), and the LPDF-capped green/amber/red pc-LEDs (GB,MAB,MRB,M), a blue chip (λmax = 445 nm) was used simultaneously as a blue light source and an excitation source for various green, amber and red phosphors of pc-LEDs. The blue LED chips were purchased from Dongbu LED, Inc. In this experiment, orthosilicate cyan and green phosphors ((Sr,Ba)2SiO4:Eu) , a amber phosphor ((Sr,Ba,Ca)3SiO5:Eu) , and a red phosphor ((Sr,Ca)AlSiN3:Eu)  were used as the CB, GB,M, AB,M, and RB,M pc-LEDs, respectively. The powder phosphors were obtained from phosphor companies (Force4, Merck and Intematix). Optimum amounts of cyan, green, amber or red phosphor were dispersed in a silicone binder, and identical amounts of the resulting phosphor pastes were dropped onto each cup-type blue LED to create the CB, GB,M, AB,M, or RB,M. On top of each GB,M, AB,M, or RB,M pc-LED, an LPDF-coated glass substrate was attached with an air gap.
Characterization of CB, GB,M, AB,M, and RB,M pc-LEDs: The emission spectra of forward emissions from blue, green, amber, red semiconductor-type LEDs, blue color-mixed cyan pc-LED, and blue-excited LPDF-capped monochromatic pc-LEDs (GB,M, AB,M, and RB,M) were measured in an integrated sphere using a spectrophotometer (PSI Co. Ltd., Darsapro-5000). The luminous flux, the CRI, and the 1931 CIE (CIE = Commission Internationale d'Eclairage) color coordinates at the set of eight CCTs were calculated using the Darsapro-5000 program.
Characterization of multi-package white LEDs: The multi-package white LED of the RB,MAB,MGB,MCB, RB,MAB,MGB,MB, and RAGB systems were measured while controlling the applied current in each primary LED to evaluate the luminous flux, the luminous efficacy, and the CRIs for the set of eight CCTs (6,500K, 5,700K, 5,000K, 4,500K, 4,000K, 3,500K, 3,000K, and 2,700K) specified by the American National Standards Institute (ANSI) standard C78.377-2008. Figures 1(d)–1(f) show the overlapped electroluminescence spectra and the chemical formula of the semiconductor-type LEDs and phosphors in different white LED systems. Figures 1(g)–1(i) show the 1931 CIE color coordinates of the RAGB, RB,MAB,MGB,MB, and RB,MAB,MGB,MCB systems. The primary LEDs were put into a square lattice fixture for four-package 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 luminous efficacies, CRIs, and the luminous fluxes from each white LED set were measured in an integrated sphere using a spectrophotometer. The fractional applied currents of the primary LEDs in the four-package white LED were measured to achieve the set of eight CCTs.
3. Results and discussion
We selected an InGaN blue LED (λmax = 445, rated current = 60mA) as an excitation source for the red, amber, green, and cyan phosphors to compare the optical properties of RB,M, AB,M, GB,M, and CB pc-LEDs. Figure 2 shows the excitation and emission spectra of the cyan, green, amber, and red phosphors as obtained from the phosphor companies. This figure clearly shows that the emitting spectrum of the selected cyan phosphor, with a peak wavelength of 505 nm, can partially fill the void between the blue spectrum of the blue LED and the green spectrum of the green phosphor. Given that the internal quantum efficiency (IQE) of the cyan phosphor was approximately 0.54 after following the same experimental procedures in a previous report, the IQE values for green, amber and red were approximately 0.90, 0.88, and 0.80, respectively [8, 10].
Although the position of the peak wavelength and the IQE of the cyan phosphor do not perfectly meet the requirements of the cyan phosphor, the cyan phosphor used in this experiment is considered to be a good candidate with which to study the effects of an addition of cyan color on the optical properties of a multi-package white LED. During the first step of this study, we compared the luminous efficacy and optical properties of a cyan pc-LED pumped by a blue LED while changing the cyan phosphor concentration in a paste. It is necessary to determine the effects of different concentrations of cyan phosphor in a paste on the optical properties of color-mixed cyan pc-LEDs to find a high-quality white LED system.
Figure 3 shows the electroluminescence spectra, the luminous efficacy (inset), and the 1931 CIE color coordinates of a series of blue color-mixed cyan pc-LEDs as a function of the phosphor concentration (20wt%, 22.5wt%, 25wt%, and 27.5wt%) in the paste. The luminous efficacy of blue color-mixed cyan pc-LEDs showed an increase after increasing the cyan phosphor concentration, as the spectrum shows decreased blue intensity and increased cyan intensity when using a higher cyan phosphor concentration. This shows that the 1931 CIE color coordinates moved toward cyan from blue as the concentration of the cyan phosphor increased.
Next, we compared the effect of the CB pc-LED on the electrical and optical properties of the RB,MAB,MGB,MCB white LED system depending on the concentration of cyan phosphor in the CB pc-LED. As previously reported, the fractional applied currents of the primary LEDs in each multi-package white LEDs system were dynamically tuned to realize white at the eight specified CCTs on a blackbody line [9, 10]. The fractional applied currents of the primary LEDs are slightly different for the RB,MAB,MGB,MCB white LED system when using different cyan phosphor concentrations (CB) with the set of eight CCTs.
Figure 4 shows the fractional applied currents of the primary individual LEDs in different concentrations in the cyan phosphor paste to realize white in the set of eight CCTs for the RB,MAB,MGB,M CB white LED system. The specified colors are produced by combining a different portion of each colored LED with the different luminous efficacy values of the various white sets. As shown in Fig. 4, the fractional applied current of the red pc-LED (RB,M) increased, indicating that the fractional radiant flux increases as the CCT decreases. In contrast, the fractional applied current of blue color-mixed cyan pc-LED (CB) was decreased by decreasing the CCT. The fractional applied current of the CB pc-LED increased as the concentration of the cyan phosphor paste increased from 20wt% to 27.5wt%.
Figure 5 shows the overlapped blue-normalized electroluminescence emission spectra of the RB,MAB,MGB,MCB white LED system with four different cyan phosphor paste concentrations for CB pc-LED at CCTs of 6,500K and 2,700K. The concentration of cyan phosphor paste in the CB pc-LED has a greater effect on the white spectrum at the CCT of 6,500K due to the high fractional applied current of the CB pc-LED but has less of an effect on the CB-pc LED at a CCT of 2,700K due to the low fractional applied current.
Figure 6 shows the luminous efficacies and the CRIs of four RB,MAB,MGB,MCB white LED systems with different cyan phosphor paste concentrations in CB pc-LEDs as a function of the set of eight CCTs. The luminous efficacies and color rendering properties are excellent at all CCTs in the RB,MAB,MGB,MCB white LED systems. This RB,MAB,MGB,MCB system provides a luminous efficacy of 92lm/W and above and CRIs of 91 and above. As mentioned earlier, the luminous efficacy of the CB pc-LED was increased by increasing the concentration of the cyan phosphor, whereas the luminous efficacy in the RB,MAB,MGB,MCB white LED system decreased as the concentration of the cyan phosphor was increased in the paste. Thus, as the concentration of cyan phosphor increases, the fractional applied current of the CB pc-LED increases at cold white CCTs. The luminous efficacy values of the RB,MAB,MGB,MCB white LED system reached their maximum value in the range of 3,500K-4,500K because the fractional applied current of each primary pc-LED was distributed evenly. The luminous efficacy and CRIs at a CCT of 6,500K (cold white) were higher than at a CCT of 2,700K (warm white) owing to the better distributed fractional applied current of each pc-LED. The RB,M pc-LED dramatically increased and the CB pc-LED decreased with a decrease in the CCT. As a result, these RB,MAB,MGB,MCB white LEDs have excellent luminous efficacy and CRIs at a CCT of 6,500K as compared to those at a CCT of 2,700K. The RB,MAB,MGB,M CB white LED system with a cyan phosphor concentration of 20wt% has the best luminous efficacy levels of 96lm/W ~106lm/W, and the CRIs are 92 ~94 for the set of eight CCTs. If we select the RB,MAB,MGB,MCB white LED system with a cyan phosphor concentration of 27.5wt%, the luminous efficacy values are 92lm/W ~103lm/W and the CRIs are 92 ~97 for the set of eight CCTs. The luminous efficacies and CRIs are in a trade-off relationship in these systems (20wt% and 27.5wt%). For additional comparison studies of multi-package white LEDs, we selected the RB,MAB,MGB,MCB white LED system with a cyan phosphor concentration of 25wt%, which provides both excellent luminous efficacy values of 96lm/W ~103lm/W and CRIs of 91 ~95.
Finally, we compared the luminous efficacy values, CRIs, and optical properties of the RB,MAB,MGB,MCB (cyan phosphor paste concentration of 25wt%) system and the RB,MAB,MGB,MB and RAGB systems for the set of eight CCTs. Figure 7 shows the blue-normalized emission spectra of the three different sets of white systems of RB,MAB,MGB,MCB, RB,MAB,MGB,MB, and RAGB at CCTs of 6,500K and 2,700K. The RAGB white LED system shows a narrow spectrum of each colored LED, while the other systems, combined with pc-LEDs, show a broad spectrum at CCTs of 6,500K and 2,700K. The luminous efficacy ratings of the RB,MAB,MGB,MCB, RB,MAB,MGB,MB, and RAGB white LED systems were all measured with the set of eight CCTs shown in Fig. 8(a) , i.e., 96lm/W ~104lm/W, 94l/W ~102lm/W, and 44lm/W ~58lm/W, at all CCTs. The RB,MAB,MGB,MCB white system with the spectrum in the sky blue wavelength region is broader than the RB,MAB,MGB,MB and RAGB white LED systems. For this reason, the CRIs of the RB,MAB,MGB,MCB, RB,MAB,MGB,MB, and RAGB white LED systems resulted in CCTs of 91 ~95, 86 ~90, and 66 ~83, respectively (see Fig. 8(b)).
In summary, a multi-package white LED system combined with red/amber/green LPDF-capped pc-LEDs and blue/cyan color mixed pc-LED pumped by a blue LED provides excellent luminous efficacy (≥96lm/W) and CRIs (≥91) at various CCTs from 6,500K to 2,700K as specified by the ANSI standard. We selected the RB,MAB,MGB,MCB white LED system with a cyan phosphor concentration of 25wt% in a paste, as this combination provides both excellent luminous efficacy in the range of 96lm/W ~103lm/W and CRIs ranging from 91 to 95 for a set of eight CCTs. The CRI values are significantly enhanced in the multi-package white LED system when introducing a cyan phosphor-based LED in a multi-package white LED system (RB,MAB,MGB,MCB) as compared to those without the cyan phosphor (RB,MAB,MGB,MB). The luminous efficacy ratings of the RB,MAB,MGB,MCB white LED system are 2.2 times and 1.7 times higher than the RAGB white LED system at CCTs of 6,500K and 2,700K, respectively. More elaborate experiments are required to realize a white lighting system of a higher quality in which the quantum efficiency of the InGaN blue LED chip and phosphors are enhanced, the wavelength of the cyan phosphor is tuning, the edge wavelength of the LPDF is modified, and the combination of each colored LED is properly selected in a multi-package white LED system. This approach, which utilizes a multi-package white LED system with LPDF-capped pc-LEDs and a blue color-mixed cyan pc-LED, can lead to the creation of high-quality smart lighting systems.
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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