Luminous efficiency enhancement of
white light-emitting diodes by using a
hybrid phosphor structure
Bing-Cheng Lin
Jhih-Kai Huang
Kuo-Ju Chen
Sheng-Huan Chiu
Zong-Yi Tu
Chien-Chung Lin
Po-Tsung Lee
Min-Hsiung Shih
Mei-Tan Wang
Jung-Min Hwang
Hao-Chung Kuo
Luminous efficiency enhancement of white light-emitting
diodes by using a hybrid phosphor structure
Bing-Cheng Lin,
aJhih-Kai Huang,
aKuo-Ju Chen,
aSheng-Huan Chiu,
aZong-Yi Tu,
aChien-Chung Lin,
bPo-Tsung Lee,
aMin-Hsiung Shih,
c,d,*
Mei-Tan Wang,
eJung-Min Hwang,
eand Hao-Chung Kuo
a,*
aNational Chiao Tung University, Institute of Electro-Optical Engineering,Department of Photonics, No. 1001, University Road, Hsinchu 30010, Taiwan
bNational Chiao Tung University, Institute of Photonics System, No. 301,
Gaofa 3rd Road, Guiren District, Tainan 71150, Taiwan
cAcademia Sinica, Research Center for Applied Sciences, No. 128 Academia Road,
Sec. 2, Nankang, Taipei 115, Taiwan
dIndustrial Technology Research Institute, Green Energy and Environment Research
Laboratories, 195, Sec. 4, Chung Hsing Road, Chutung, Hsinchu 31040, Taiwan
eNational Sun Yat-sen University, Department of Photonics, 70 Lienhai Road,
Kaohsiung 80424, Taiwan
Abstract. This paper presents a “hybrid” structure for the coating of yellow YAG∶Ce3þ phosphor on blue GaN-based light-emitting diodes (LEDs). The luminous efficiency of the hybrid phosphor structure improved by 5.9% and 11.7%, compared with the conventional remote and conformal phosphor structures, respectively, because of the increased intensity of the yellow component. The hybrid structure also has an advantage in the phosphor usage reduction for the LEDs. Furthermore, the electric intensity of the hybrid phosphor structure was calculated for various thicknesses by conducting TFCalc32 simulation, and the enhanced utilization of blue rays was verified. Finally, the experimental results were consistent with the simulation results performed using the Monte-Carlo method. © 2015 Society of Photo-Optical Instrumentation Engineers (SPIE) [DOI:10.1117/1.JPE.5.057603]
Keywords:light-emitting diodes; GaN; phosphor.
Paper 14085SS received Oct. 28, 2014; accepted for publication Dec. 4, 2014; published online Feb. 19, 2015.
1 Introduction
Recently, the development of white light-emitting diodes (WLEDs) has become the latest step toward a revolution in lighting technology.1–3 The advantage of WLEDs is a small size, high
energy efficiency, low cost, and color stability, which improve lighting applications including liquid crystal displays backlighting in mobile phones and other handheld devices, dashboard lighting, and the headlights used in automobiles.4,5 Currently, phosphor-converted white
light-emitting diodes (pcWLEDs), in which a blue chip and yellow-emitting phosphors are combined, are the most popular packaging structure used to produce white light.6 Regarding GaN-based LED chips, the progress in white LEDs is strongly driven by the advances in high-efficiency based LEDs that emit light in the visible spectral regime. These GaN-based LEDs are used as high-power pump excitation sources in pcWLEDs. Recent studies have substantially improved GaN-based LEDs by using new types of active regions featuring reduced charge separation,7–9nano/microphotonics structures,10,11growth and substrate
technol-ogies,12–15and barrier engineering.16–18Thus, the availability of high-efficiency nitride LEDs has enabled the practical implementation of pcWLEDs.
*Address all correspondence to: Min-Hsiung Shih, E-mail: [email protected]; Hao-Chung Kuo, E-mail: hckuo@ faculty.nctu.edu.tw
To fabricate the pcWLEDs configurations, the yellow-emitting phosphor powders are mixed with transparent encapsulated resin and then dispersed in a cup reflector or directly coated on the LED chip surface. Although this direct dispensing method enables the thickness of the phosphor layer to be easily controlled and reduces much of the cost, it does not produce high-quality WLEDs.19Thus, the conformal phosphor structure can be used as an alternative. This method is used to uniformly distribute colors, causing angular homogeneity of the correlated color tem-perature (CCT).20However, the disadvantage of a conformal phosphor structure is the backscat-tering effect, in which more than half of the light is backscattered to the absorptive LED chip.21 To alleviate this backscattering problem, the phosphor layer can be separated from the chip to suppress the reabsorption. Thus, various phosphor structures, such as the remote phosphor struc-ture, have been designed to address this problem.22,23In this structure, the concave encapsulant surface causes the phosphor thickness to become nonuniform during the fabrication of the remote phosphor structure. A nonuniform phosphor is suggested to cause yellow rings after activating the device. Therefore, the patterned remote phosphor structure and ZrO2-type package
have been employed to improve the uniformity of the angular-dependent CCT.24,25
In this study, a hybrid phosphor structure was employed in an array of blue LED chips on board (COB) to increase light output compared with the conventional remote and conformal phosphor structures. Compared with the conventional package leadframe, the advantages of a COB package are its heat dissipation and thinness, which make the COB package suitable for use in high-power lighting sources.26,27 The experimental results indicated that a hybrid phosphor structure yields a high intensity of yellow components because the trap of the incident pumping blue rays inside the structure increases the absorption probability of the phosphor layer and delivers additional yellow rays. The hybrid structure also has an advantage in the phosphor usage reduction for the white LEDs. The hybrid structure saves more than 17% and 37% in phosphor usage, compare to the remote and conformal structures at color temperature 5300 K. Moreover, a TFCalc32 simulation revealed that the power intensity between the phosphor layers is enhanced when using a hybrid structure.
2 Experiment
In this study, hybrid phosphor structures were fabricated using the pulsed spray coating (PSC) method,18which is used to spray the phosphor slurry in uniform layers and easily control the CCT. Moreover, this method is convenient, does not cause chemical pollution, and is suitable for fabricating planar illumination systems with large areas.28 The phosphor powder used in this
experiment was YAG∶Ce3þ with a particle size of 13μm. The LED leadframe size was 22 mm× 25 mm, and the chip size and thickness were 45 mil2 and 150μm, respectively. Four GaN-based blue LEDs with a peak emission wavelength of approximately 450 nm were bonded on silver glue with gold wire to the bond pad. A SEM cross-sectional view of a hybrid phosphor structure is shown in Fig.1. The samples, which featured hybrid phosphor structures, were fabricated using the following steps: (1) the PSC method was employed in spraying phosphor on the chip to form the bottom phosphor layer; (2) the transparent silicone binder was dispensed into the lead frame and cured in an oven at a temperature of 150°C for 1.5 h; and (3) the phosphor was sprayed on the top of the silicone binder layer to form the hybrid phosphor structure at the same total amount of phosphor. In this experiment, the total density of the phosphor was set as 6 mg∕cm2 for all of the samples, and the density of the phosphor was
approximately 1 mg∕cm2 from one spray coating step. To understand the performance of the
devices and optimize the luminous efficiency, the top and bottom phosphor layers of the hybrid phosphor structure were fabricated in various thicknesses but with the same total amount of phosphor. For convenience, we use a simple weight ratio such as 1 mg∕5 mg (bottom/top) to replace the 1 mg∕cm2 in the bottom phosphor layer and 5 mg∕cm2 in the top phosphor
layer. To compare the hybrid phosphor structure with the conventional phosphor structure, the remote and conformal phosphor structures were fabricated with the same amount of phosphor, and the weight ratio of 0 mg∕6 mg (bottom/top) and 6 mg∕0 mg (bottom/top) rep-resented the conventional remote and conformal phosphor structures, respectively. In this study, every data point is the average value of five LEDs for the proposed structure. The LED devices were measured under continuous-wave conditions at room temperature and the light output was collected by an integration sphere. This integration sphere was 30 cm in diameter. The influence of external impact on the measurement is much smaller in a closed integration sphere system.
3 Results and Discussion
As shown in Fig.2(a), the bottom and top phosphor layers of the hybrid structure were fabricated in various ratios to optimize the luminous efficiency at a forward current of 700 mA. The exper-imental results indicated that the optimal weight ratio of the bottom to top phosphor layers is 2 mg∕4 mg (bottom/top), which produces a higher luminous efficiency compared with that of conventional structures. Specifically, the hybrid structure exhibited 5.1% and 14.9% improve-ments in luminous efficiency compared with the remote and the conformal phosphor structures, respectively. Figures 2(b)and 2(c)show the emission spectra and CIE 1931 diagram of the hybrid structure, conventional remote structure, and conformal phosphor structure. The hybrid phosphor structure produced a higher intensity in the yellow component than did the remote and conformal phosphor structures, indicating that this structure can be applied for improving the
0/6 1/5 2/4 3/3 4/2 5/1 6/0 105 110 115 120 Conformal Remote Luminous efficiency (lm/W)
Weight ratio (bottom/top) (a) Hybird 400 500 600 700 800 0 3000 6000 9000 12000 15000 18000 Intensity (a.u.) Wavelength (nm) 0 mg/6 mg (Remote) 2 mg/4 mg (Hybrid) 6 mg/0 mg (Conformal) (b)
Fig. 2 (a) The luminous efficiency of the different ratios of phosphor layers in hybrid phosphor structure. (b) The emission spectra of the remote phosphor structure, the conformal phosphor structure, and the hybrid phosphor structure driven at current of 700 mA. (c) The CIE 1931 chro-matic diagram of remote, conformal, and hybrid phosphor structures.
luminous efficiency of LED structures. Conversely, the hybrid phosphor structure exhibited lower intensity in blue rays than the remote and conformal structures did, because the utilization of the blue light causes an increase in the absorption probability of phosphor and delivers addi-tional yellow rays.
To determine the optical characteristics of the remote, conformal, and hybrid phosphor struc-tures when driven at currents from 100 to 1000 mA, the current-dependent luminous efficiency and luminous flux were analyzed, as shown in Fig.3. The luminous efficiencies of the remote, conformal, and hybrid phosphor structures were 117, 111, and 124 lm∕W, respectively, at a driving current of 700 mA. The output luminous efficiency of the hybrid phosphor structure was observed to increase by 5.9% and 11.7% compared with the conventional remote and con-formal phosphor structures, respectively. When applying a high injection current, the enhance-ment of the luminous efficiency showed a stable value, which could have been attributed to the enhanced use of emissions by both blue and yellow rays in the hybrid phosphor structure.
Figure4shows the measured CCT for various ratios of the bottom to top phosphor layers of the hybrid structure. Regarding the conventional, remote, and conformal phosphor structures, when blue rays excited the phosphor, yellow rays were emitted in all directions. A large pro-portion of downward rays, including blue and yellow rays, were transmitted back to the blue LED chip, causing the utilization of blue rays to be low. The conventional remote and conformal phosphor structures clearly exhibited higher CCTs than the hybrid structure did. The CCT of the hybrid phosphor structure composed of a bottom-to-top phosphor-layer weight ratio of 2 mg∕4 mg was located at 5300 K. Confining the blue rays in the silicone layer can improve its utilization by increasing the probability of phosphor excitation. It is also worth noting that the optimized hybrid phosphor (2 mg∕4 mg) structure saved phosphor usage. In Table1, we listed
0 200 400 600 800 1000 100 110 120 130 140 150 0 mg/6 mg (Remote) 2 mg/4 mg (Hybrid) 6 mg/0 mg (Conformal) Current (mA) Luminous efficiency (lm/W) 0 200 400 600 800 1000 Luminous flux (lm)
Fig. 3 Luminous efficiency and luminous flux of the remote, conformal, and hybrid phosphor structures with a ratio of2∕4 driven at currents from 100 to 1000 mA.
0/6 1/5 2/4 3/3 4/2 5/1 6/0 5200 5400 5600 5800 6000 6200 Remote Conformal
Correlated color temperature (K)
Weight ratio (bottom/top) Hybird
Fig. 4 The correlated color temperature (CCT) of the different ratios of phosphor layers in hybrid phosphor structure.
the total amount of phosphor in three white LED structures, which have the same CCT at almost 5300 K. The total amount of phosphor for the remote, conformal, and hybrid phosphor structures were 21.9, 25.8, and 18.8 mg, respectively, at the same CCT. In our experiment, to achieve the CCT we need, the amount of yellow-emitting YAG phosphor for remote and conformal phosphor structures was larger than that of the hybrid phosphor structure, which significantly increased the cost of high efficiency white LEDs. As shown in Table1, the experimental results indicated that the phosphor saved in the hybrid phosphor structure amounted to 17% for the remote phosphor structure and 37% for the conformal phosphor structure at 5300 K. In order to clarify the CCT uniformity, the CCT deviations of the hybrid structure, conventional remote structure, and conformal phosphor structure were investigated. The angular CCT deviation curves from −70 to 70 deg are shown in Fig.5. The CCT deviations were 1760, 2200, and 1630 K for the hybrid structure, remote structure, and conformal phosphor structure, respectively. Therefore, a better uniformity of angular-dependent CCT was obtained in the hybrid structure compared to the conventional remote phosphor structure.
To understand the effect of blue photons coupling to the phosphor layer, a TFCalc32 sim-ulation was employed in the experiment.29In the simulation of the hybrid structure, the lengths
of the silicone layer, bottom phosphor layer, and top phosphor layer were approximately 500, 20, and 40μm, respectively. Regarding the remote and conformal phosphor structures, the lengths of the silicone layer and phosphor layer were approximately 500 and 60μm, respectively; 450-nm light was emitted incident to these layers by a GaN LED chip. Figure6shows the electric field intensity for the various thicknesses of remote, conformal, and hybrid phosphor structures. The electric field intensity in the hybrid structure was higher than that in the other two structures. Therefore, the advantage of the hybrid phosphor structure is that incident blue rays can be trapped in the silicone layer, increasing the absorption ability of the phosphor layer and allowing the transfer of more yellow rays compared with the other two structures.
The power intensity in phosphor layers of remote, conformal, and hybrid phosphor structures applied in white LEDs can be calculated as follows:
W ¼ R710μm 650μmn 2 P× jEj 2dT ½R150μm 0 n 2 GaN× jEj 2dT þR650μm 150μm n 2 S× jEj 2dT þR710μm 650μm n 2 P× jEj 2dT; (1)
Table 1 Phosphor usage in three LED structures.
Sample Remote Conformal Hybrid Total phosphor amount at 5300 K 21.9 mg 25.8 mg 18.8 mg Phosphor saved (%) þ16% þ37% — -80 -60 -40 -20 0 20 40 60 80 4000 4500 5000 5500 6000 6500 7000 7500
Correlated color temperature (K)
Angle (deg) 0 mg/6 mg (Remote) 2 mg/4 mg (Hybrid) 6 mg/0 mg (Conformal)
Fig. 5 The CCT deviations of the remote, conformal, and hybrid phosphor structures.
W ¼ R210μm 150μmn2P× jEj2dT ½R150μm 0 n2GaN× jEj2dT þ R210μm 150μm n2P× jEj2dT þ R710μm 210μm n2S× jEj2dT ; (2) W ¼ R170μm 150μm n 2 P× jEj2dT þ R710μm 670μmn 2 P× jEj2dT ½R150μm 0 n 2 GaN× jEj 2dT þR170μm 150μm n 2 P× jEj2dT þ R670μm 170μmn 2 S× jEj 2dT þR710μm 670μmn 2 P× jEj2dT ; (3) wherenGaN,ns, andnpare the refractive indices of GaN, silicone, and phosphor, andjEj2is the
electric field intensity. According to Eqs. (1), (2), and (3), the power intensities ratioW in the phosphor layers of the remote, conformal, and hybrid phosphor structures were 8.5%, 8.8%, and 10.1%, respectively, at a wavelength of 450 nm. The power intensity can be enhanced by 18%, which causes additional blue rays to be trapped in the silicone layer. In addition, the absorption probability of phosphor can be enhanced, enabling additional yellow rays to be produced through the recycling of photons.
Moreover, the LightTools simulation software based on the Monte-Carlo method was employed to simulate the ray tracing for the three structures.30In the simulation, the parameters of the refractive index in the phosphor, silicone, and LED chip were set to 1.6, 1.53, and 2.4, respectively. The average particle diameter and the concentration of phosphors were set to 13μm and 50%, respectively, which was uniformly distributed in the silicone binder. The phosphor absorbed blue light from the LED chips and emitted isotropic yellow light. The amount of light being absorbed and scattered by phosphor particles was simulated using Mie theory. Figure 7 shows that the capability to use the blue rays in the silicone layer was higher in the hybrid phosphor structure than in the other two structures, indicating that the utilization of blue rays and lumen flux increased. Moreover, the simulated luminous flux of remote, conformal, and hybrid phosphor structures were 120, 117, and 125 lumen, respectively. These results correspond with the aforementioned finding—the hybrid structure provides greater light output than conventional structures do.
-100 0 100 200 300 400 500 600 700 800 0.4 0.8 1.2 1.6 2.0 2.4 2.8 |E | 2 |E | 2 |E | 2 Thickness ( m) (a) -100 0 100 200 300 400 500 600 700 800 0.4 0.8 1.2 1.6 2.0 2.4 2.8 (b) -100 0 100 200 300 400 500 600 700 800 0.4 0.8 1.2 1.6 2.0 2.4 2.8 (c) µ Thickness ( m)µ Thickness ( m)µ
Fig. 6 Thickness-dependentjEj2of the (a) remote, (b) conformal, and (c) hybrid phosphor struc-tures by TFCalc32 simulation.
4 Conclusion
In conclusion, the hybrid phosphor structure enhances the luminous efficiency of WLEDs; this enhancement was caused by the increased utilization of blue rays from the WLEDs. The results indicated that the luminous efficiency of the hybrid phosphor structure improved by 5.9% and 11.7% compared with the conventional remote and conformal phosphor structures, respectively. Moreover, the hybrid structure save more than 17% and 37% in phosphor usage compare to the remote and conformal structures at the same color temperature of around 5300 K. Finally, the TFCalc32 simulation revealed that the utilization of blue rays was higher in the hybrid phosphor structure than in the other structures, increasing the luminous efficiency.
Acknowledgments
The authors would like to thank Helio Opto., for their technical support. This study was sup-ported by the National Science Council of Taiwan under Contract Nos. NSC102-3113-P-009-007-CC2 and NSC-102-2221-E-009-131-MY3.
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Bing-Cheng Lin received a BS degree in electrical engineering from Feng Chia University, Taiwan, in 2002, and an MS degree from the Institute of Photonics from NCUE, Taiwan, in 2011. He is currently working toward the PhD degree in the Department of Photonics at the Institute of Electro-Optical Engineering, National Chiao Tung University, Taiwan. In 2011, he was with the Semiconductor Laser Technology Laboratory, NCTU, where he was engaged in research on III-V semiconductor materials for LEDs.
Jhih-Kai Huang received a BS degree in the Department of Electrical Engineering from National Central University, Taoyuan, Taiwan, in 2007, and an MS degree in electro-optical engineering from National Chiao Tung University, Hsinchu, Taiwan, in 2009. He is currently working toward the PhD degree at the Department of Photonics and the Institute of Electron-Optical Engineering, National Chiao Tung University (NCTU), Hsinchu, Taiwan. His research areas include GaN-based device fabrication and nanoimprint technology.
Kuo-Ju Chenreceived a BS degree in industry education from National Kaohsiung Normal University (NKNU), Kaohsiung, Taiwan, in 2008, and an MS degree from National Taiwan Normal University (NTNU). He is currently pursuing the PhD degree at the National Chiao-Tung University, Hsinchu, Taiwan. His PhD research includes fabrication, simulation, and characterization for high-power light-emitting diodes.
Sheng-Huan Chiureceived a BS degree in National Chiao-Tung University (NCTU), Hsinchu, Taiwan, in 2012, and is currently pursuing an MS degree at the National Chiao-Tung University, Hsinchu, Taiwan. His master’s degree focused on high-powered white light-emitting diodes, including fabrication, simulation, measurement, and optical characteristic study. In addition, he researches include optical applications of a promising material quantum dot.
Zong-Yi Tureceived a BS degree in the Department of Electronic and Computer Engineering from National Taiwan University of Science and Technology (NTUST), Taipei, Taiwan, in 2013. He is currently pursuing an MS degree at National Chiao-Tung University (NCTU), Hsinchu, Taiwan. His master’s degree focused on high luminous efficiency and high CRI white light-emitting diodes, including fabrication, simulation, measurement, and optical characteristic study.
Chien-Chung Linreceived a BS degree in electrical engineering from the National Taiwan University in 1993, and MS and PhD degrees in electrical engineering from Stanford University, Stanford, California, USA, in 1997 and 2002, respectively. Since 2009, he has been with National Chiao Tung University (NCTU) in Tainan, Taiwan, where he holds a position as an assistant professor. The major research efforts in his group are in design and fabrication of semiconductor optoelectronic devices.
Po-Tsung Leereceived a BS degree in physics from National Taiwan University in 1997. She received MS and PhD degrees both in electrical engineering/electrophysics from University of Southern California, USA, in 1998 and 2003, respectively. She joined the Institute of Electro-Optical Engineering, National Chiao Tung University as an assistant professor in August 2003, and has been an associate professor since 2007.
Min-Hsiung Shihreceived a BS degree in physics from the National Cheng Kung University, Tainan, Taiwan, in 1995, an MS degree in physics from the National Tsing Hua University (NTHU), Taiwan, in 1997, and a PhD degree in electrical engineering/electrophysics from the University of Southern California (USC), Los Angeles, USA, in 2006. He is currently an associate research fellow in the Research Center for Applied Sciences (RCAS), Academia Sinica, Nankang, Taiwan.
Mei-Tan Wang received a PhD degree from the Graduate Institute of Photonics and Optoelectronics from the National Taiwan University, Taipei, Taiwan, in 2013. She has over eight years of experience in optoelectronic/LED epitaxy and packing. She is currently a researcher for Industrial Technology Research, Hsinchu, Taiwan. Her main research includes microstructure package design and its high efficient usage in solid-state lighting.
Jung-Min Hwangreceived MS and PhD degrees in electrical engineering from the National Tsing Hua University, Hsinchu, Taiwan, in 1999 and 2005, respectively. Now his research focuses on novel integrated LED light engine and multicolor smart lighting systems at the Industrial Technology Research Institute, Taiwan. He has published more than 30 journals and 50 conference papers and has been granted more than 28 patents in the field of LED and LED lighting application.
Hao-Chung Kuoreceived a BS degree in physics from National Taiwan University, Taiwan, an MS degree in electrical and computer engineering from Rutgers University, New Brunswick, New Jersey, USA, in 1995, and a PhD degree from Electrical and Computer Engineering Department, University of Illinois at Urbana Champaign, in 1999. Since October 2002, he has been with the National Chiao Tung University as a Faculty Member of the Institute of Electro-Optical Engineering.
