Enhanced Luminous Efficiency of WLEDs Using a Dual-Layer Structure of the Remote Phosphor Package

Enhanced Luminous Efficiency of WLEDs

Using a Dual-Layer Structure of the

Remote Phosphor Package

Kuo-Ju Chen, Hsin-Chu Chen, Min-Hsiung Shih, Member, IEEE, Chao-Hsun Wang, Hsin-Han Tsai,

Shih-Hsuan Chien, Chien Chung Lin, Member, IEEE, and Hao-Chung Kuo, Senior Member, IEEE

Abstract—This study demonstrates how the insertion of a

thin silicone layer into a dual-layer remote phosphor structure

enhances light extraction in white light-emitting diodes (WLEDs).

In the experiment, a dual-layer phosphor structure yielded a

higher intensity of blue and yellow components than a

conven-tional structure. Moreover, the lumen flux was 5% higher than

a conventional remote phosphor package at the same correlated

color temperature (CCT). Using a TFCalc32 simulation, the

electric field intensity was calculated for different thicknesses of

the dual-layer remote phosphor structures, and the enhanced use

of blue rays was verified. Additionally, the dual-layer structure

reduces chromaticity deviations as the driving current increases.

Index Terms—Coatings, GaN, light-emitting diodes,

optoelec-tronic devices, packaging, phosphor.

I. I

P

HOSPHOR-CONVERTED white light-emitting diodes

(WLEDs) are a promising light source because of their

small size, high energy efficiency, low cost, and color stability

[1]–[4]. WLEDs apply the principle of complementary colors:

Blue light from a blue chip is combined with yellow light

from phosphor [5]. WLEDs have the potential to be used

in solid-state lighting, but their luminous efficiency must be

enhanced [6]. Generally, freely dispersed coating is the most

common method used to fabricate white light. In this process,

transparent encapsulated resin is combined with phosphor

powder and is dispersed on the phosphor package. Although

this approach allows the thickness of the phosphor layer to

Manuscript received December 02, 2012; revised February 07, 2013; accepted February 28, 2013. Date of publication May 03, 2013; date of current version May 27, 2013. This work was supported in part by the National Science Council in Taiwan under Grant NSC100-3113-E-009-001-CC2 and Grant NSC-99-2120-M-009-007.

K.-J. Chen, C.-H. Wang, S.-H. Chien, and H.-C. Kuo are with the Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan (e-mail: [email protected]).

H.-C. Chen is with the Department of Photonics and Institute of Electro-Op-tical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan. He is also with the Electronics and Optoelectronics Research Laboratories, Indus-trial Technology Research Institute, Hsinchu 30010, Taiwan.

M.-H. Shih is with the Department of Photonics and Institute of Electro-Op-tical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan. He is also with the Research Center for Applied Sciences, Academia Sinica, Taipei 115 Taiwan (e-mail: [email protected]).

C. C. Lin is with the Institute of Photonics System, National Chiao Tung University, Tainan 711, Taiwan.

Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JLT.2013.2260322

be controlled easily and reduces much of the cost, it does not

produce high-quality WLEDs [7]. Therefore, the conformal

coating method can be used as an alternative. This method

distributes colors uniformly, resulting in angular homogeneity

of correlated color temperature (CCT) [8]. However, the

disad-vantage of a conformal phosphor structure is the backscattering

effect, which reduces luminous efficiency.

Previous studies have demonstrated the concept of separating

the chip and the phosphor layer of remote phosphor structures

[9], [10]. The enhanced light extraction internal reflection

(ELiXIR) structure, which uses a polymer hemispherical shell

lens with interior phosphor coating, is known to increase

extraction efficiency [11]. Furthermore, an air-gap embedded

structure can enhance luminous efficiency by reflecting

down-ward light [12].

In addition to the structure of the package, the concentration

of phosphor plays an important role on luminous efficiency. The

re-absorption loss in the phosphor layer raises when the

phos-phor concentration increases. Therefore device luminous

effi-ciency would be degraded, especially at lower CCTs [13].

Sim-ilarly, Narendran et al. demonstrated that high occurrence of

scattering and reflecting also reduces luminous efficiency [14].

Therefore, it is essential to enhance the emission of blue and

yellow rays and reduce the amount of light lost from

backscat-tering and reflection.

In this study, a dual-layer phosphor structure was employed

in a remote phosphor package to increase light output

com-pared to a conventional remote phosphor structure at the same

CCT. The experimental results indicate that a dual-layer

phos-phor structure yielded higher light transmission than a

conven-tional phosphor structure. Furthermore, a TFCalc32 simulation

demonstrated that the power intensity between the silicone layer

and the phosphor layers was enhanced. The chromaticity

coor-dinate deviations were also improved by increasing the driving

current of the LED.

II. E

In this experiment, a dual-layer structure was fabricated using

the pulse spray coating method, which creates a uniform

phos-phor layer [15]–[17]. The phosphos-phor powder was YAG:

with the particle size of 13

. An InGaN-based blue LED

with a peak emission wavelength of approximately 450 nm was

bonded on silver glue with gold wire in a lead-frame package.

The schematic cross-sectional view of a dual-layer remote

phos-phor structure is shown in Fig. 1. The samples were fabricated

Fig. 1. Schematic cross-sectional view of (a) dual-layer and (b) conventional remote phosphor structures.

by the following steps: First, the silicone encapsulant was filled

in a commercial plastic lead-frame package.

The pulse spray coating method was then employed to spray

phosphor and silicone binder on the surface of each layer of the

remote phosphor structure. Finally, phosphor was sprayed on

top of the remote phosphor structure to obtain the same CCT as

the conventional remote phosphor structure. In the experiment,

the total density of the phosphor in the experiment is set up as

6

for all the samples and the density of phosphor is

approximately 1.0 mg/cm2 from one spray coating step. To

ob-tain the better luminous efficiency of the devices, the thickness

of the top and bottom phosphor layers was adjusted. Thus, the

ratio of the sample A, B, and C represent the different density

of phosphor layer in the dual-layer structure.

III. R

D

The effect of changing the ratios of the first and the second

layers of the phosphor structure is shown in Fig. 2(a), which

represents the transmission of the different samples in relation

to the wavelength. The remote phosphor package composed of

a dual-layer structure yielded the highest transmission at

wave-lengths ranging from 400 to 800 nm. The sample containing

the optimized ratio, Sample C, produced greater transmission

than the other samples in the experiment. Enhanced

transmis-sion means that the LED device emits more photons, reducing

light reflection and increasing light output.

The drop in intensity at a wavelength of 460 nm can be

attributed to the absorption of phosphor. Fig. 2(b) shows a

comparison of the lumen enhancement of the different

sam-ples. Sample C exhibits a 5% lumen enhancement over the

conventional remote phosphor structure. The transmission

measurement verified that this was the result of increased

extraction of rays in the yellow band. Therefore, increasing

light extraction is critical to improving the luminous efficiency

of LED structures.

The emission spectra of Sample C and the conventional

re-mote phosphor structure are shown in Fig. 3(a). These

struc-tures had the same CCT at approximately 5400 K, and both

op-erated at a current of 120 mA. The dual-layer structure produced

a higher intensity in blue and yellow components and yielded

a higher light output than the conventional structure. Fig. 3(b)

shows the luminous flux and the luminous efficiency of both

Sample C and the conventional remote phosphor structure, each

driven at currents from 20 to 420 mA. Since the optical trapping

in the phosphor layer degrades device luminous efficiency due

Fig. 2. (a) The transmission of the different ratios of phosphor layers as wave-length increases (b) The lumen enhancement of the amount of phosphor in the second layer under the regular operation current.

Fig. 3. (a) The emission spectra (b) Luminous flux and luminous efficiency of the dual-layer and the conventional remote phosphor structures driven at cur-rents from 20 to 420 mA.

to the self-absorption of phosphor [18]. Comparing to the

refer-ence structure (the single-layer structure), the dual-layer

struc-ture can reduce the optical trapping of the phosphor layer, and

increase luminous flux.

In general, yellow rays scatter in all directions when pumped

by a blue ray. Consequently, most of the downward rays are lost

Fig. 4. Thickness-dependent of (a) the dual-layer and (b) the conventional LEDs by TFCalc32 simulation.

in the package and light output is reduced. Conversely, in the

dual-layer phosphor structure, a thin silicone layer with a low

refractive index is inserted into the phosphor layer. The

refrac-tive indices of the phosphor and silicone encapsulants used in

this study were 1.8 and 1.4, respectively.

According to Snell’s law, the total reflection angle needed

to be increased because of the phosphor/encapsulant layer.

Re-flecting the blue ray could improve its use because it increases

the probability of phosphor excitation [12]. Therefore, a

TF-Calc32 simulation was employed to observe the actual effect

of blue photons coupling to the phosphor layer.

In the simulation, the lengths of the first and second silicone

layers were approximately 850

and 70

, respectively.

The lengths of the first and second phosphor layers were

ap-proximately 100

and 20

, respectively. Regarding the

conventional phosphor structure, the silicone layer was

approx-imately 850

, and the phosphor layer was roughly 120

.

These layers were simultaneously subjected to 450 nm light by a

GaN LED. The results of this simulation are displayed in Fig. 4,

which shows the electric field intensity for the different

thick-nesses of dual-layer and conventional phosphor structures. The

electric field intensity in the second silicone layer was higher

than in the conventional phosphor structure. Therefore, the

ad-vantage of the former structure is that the incident blue ray can

be trapped in the lower refractive index medium, increasing the

absorption ability of the phosphor layer and transferring more

yellow rays than the conventional structure.

The power intensity of dual-layer and conventional remote

phosphor structures in WLEDs can be calculated as shown in

Fig. 5. The chromaticity deviations of (a) CIE coordinate and (b) CIE coordinate with dual-layer and conventional remote phosphor structures at dif-ferent LED injection currents.

(1)–(2) at the bottom of the page, where

and

are the

re-fractive indices silicone and phosphor, and

is the electric

field intensity.

According to (1) and (2), the power intensities of the

dual-layer and conventional remote phosphor structures were 20.2%

and 13.8%, respectively, at a wavelength of 450 nm.

Further-more, the power intensity reached a maximum of 46.4%, an

en-hancement that mainly occurred between the thin silicone layer

and the phosphor layers. The enhanced electric field intensity

allows more blue rays to be trapped and raises the probability

of phosphor excitation. In turn, this recycling of photons allows

for the production of more yellow rays.

Fig. 5 shows the chromaticity deviations of (a) CIE

co-ordinate and (b) CIE

coordinate with dual-layer and

con-ventional remote phosphor structures from 20 to 420 mA, the

light quality of the dual-layer and conventional remote

phos-phor structures can be compared. As the current increased,

inci-dent blue rays were generated from the blue chip and converted

the phosphor layer to yellow photons. Therefore, the manner in

which the phosphor layer is used determines the quality of color

mixing in WLEDs, especially at different currents. The

lowest-(1)

rents. Therefore, the dual-layer remote phosphor structure not

only enhanced the luminous flux of the WLED, but also

pro-vided greater stability of CIE coordinates as the driving current

increased.

IV. C

This study demonstrates that a dual-layer remote phosphor

structure enhances the luminous efficiency of WLEDs. Inserting

a thin silicone layer into the phosphor layer and optimizing the

ratio of the different layers increased transmission, thereby

in-creasing light output. Moreover, TFCalc32 simulation results

verified that more incident blue rays were reflected in the thin

silicone layer of the dual-layer structure, increasing the

proba-bility of phosphor excitation and producing higher yellow

com-ponents in the emission spectra. Accordingly, the luminous flux

of the dual-layer structure was 5% higher than that of the

con-ventional remote phosphor package at a driving current of 120

mA. Finally, the dual-layer structure reduces chromaticity

de-viations as the driving current increases. These results establish

WLEDs as a capable source of solid-state lighting.

A

The authors would like to thank Helio Opto., and

Hermes-Epitek Corp., for their technical support.

R

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Kuo-Ju Chen was born in Taichung, Taiwan, R.O.C. He received a B.S degree

in industry education from National Kaohsiung Normal University (NKNU), Kaohsiung, Taiwan, in 2008, and his M.S. degree from National Taiwan Normal University (NTNU). He is currently pursuing a Ph.D. degree at National Chiao-Tung University, Hsinchu, Taiwan.

His focus is on UV excitable phosphate, which has high luminous efficiency and high stability. He used the combinatorial chemistry to develop UV-excited phosphor for his master thesis. His Ph.D. research includes fabrication, simula-tion, and characterization for high-power light-emitting diodes.

Hsin-Chu Chen was born in Miaoli, Taiwan, R.O.C., in 1981. He received a

Ph.D. degree at Institute of Electro-Optical Engineering, National Chiao Tung University, Taiwan, in 2012. His thesis work focused on quantum dots and the nanostructure with optoelectronics devices, which includes fabrication, simula-tion, and measurement. Since 2013, he has been at Industrial Technology Re-search (ITRI) in Hsinchu, Taiwan, where he holds an engineer. He has been working for the development of high color uniformity and high lumen efficiency of white light LED devices, which includes structure design, fabrication, simu-lation, and measurement.

Min-Hsiung Shih received B.S degree in physics from the National Cheng

Kung University, Tainan, Taiwan, in 1995, the M.S. degree in physics from the National Tsing Hua University (NTHU), Taiwan in 1997, and the Ph.D. degree in electrical engineering/electrophysics from the University of Southern Cali-fornia (USC), Los Angeles, in 2006.

He is current an associate research fellow in the Research Center for Ap-plied Sciences (RCAS), Academia Sinica, Nankang, Taiwan. He has authored more than 50 journal and conference publications. His research interests include integrated photonic circuits, photonic crystals, GaN based lasers, surface plas-monics, and cavity quantum electrodynamics.

Chao-Hsun Wang received his B.S. and M.S. degrees from the Department

of Photonics at National Chiao-Tung University, Hsinchu, Taiwan, in 2008 and 2009, respectively. He is currently working toward a Ph.D. degree in the De-partment of Photonics, National Chiao-Tung University. His current research interests include the efficiency droop behavior in GaN-based LEDs and high lumen efficiency white LEDs.

Hsin-Han Tsai is currently a master’s candidate in the Institute of

Electro-Op-tical Engineering, National Chiao Tung University, Taiwan. His focus is on white light LED packaging, which can improve angular-dependent CCT unifor-mity and enhance light output. His master’s research includes white light LED packaging, simulation, and measurement.

Shih-Hsuan Chien was born in Taoyuan, Taiwan, ROC. He received the B.S

degree in National Chiao-Tung University(NCTU), HsinChu, Taiwan, in 2012. He is currently pursuing an M.S. degree at National Chiao-Tung University, Hsinchu, Taiwan. His master’s focus is on high-powered white light-emitting diodes, including fabrication, simulation, measurement, and optical character-istic study. In addition, he researches optical applications of a promising mate-rial quantum dot.

Chien-Chung Lin (S’93–M’02) was born in Taipei, Taiwan, R.O.C., in 1970.

He received a B.S. degree in electrical engineering from National Taiwan Uni-versity in 1993, and his M.S. and Ph.D. degrees in electrical engineering from Stanford University, Stanford, CA, in 1997 and 2002, respectively. His thesis work focused on design, modeling, and fabrication of micromachined tunable optoelectronic devices. Since 2009, he has been at National Chiao-Tung Univer-sity (NCTU) in Tainan, Taiwan, where he holds a position as assistant professor. The major research efforts in his group are in the design and fabrication of semi-conductor optoelectronic devices, including LEDs, solar cells, and lasers. Be-fore joining NCTU, he worked for different start-ups in the United States. After graduating from Stanford in 2002, he joined E2O Communications, Inc, in Cal-abasas, California, as a senior optoelectronic engineer. His main research in-terests then were in optically and electrically pumped long-wavelength vertical cavity surface emitting lasers. In 2004, he joined Santur Corporation in Fre-mont California, where he initially worked as a member of technical staff and then became Manager of Laser Chip Engineering. He had worked on various projects such as monolithic multi-wavelength DFB Laser arrays for data and telecommunications applications, yield and reliability analysis of DFB Laser arrays, and so on. Dr. Lin has more than 30 journal and conference publications and is a member of the IEEE Photonic Society and Electron Devices Society.

Hao-Chung Kuo (S’98–M’99–SM’06), received a B.S. degree in physics from

National Taiwan University, Taiwan, R.O.C., an M.S. degree in electrical and computer engineering from Rutgers University, New Brunswick, NJ, in 1995, and his Ph.D. degree from the Electrical and Computer Engineering Depart-ment, University of Illinois at Urbana Champaign, in 1999. He has an extensive professional career both in research and industrial research institutions that in-cludes being a Research Consultant in Lucent Technologies, Bell Laboratories (1993–1995); and a Member of Technical Staff in Fiber-Optics Division at Ag-ilent Technologies (1999–2001) and LuxNet Corporation (2001–2002). Hong Kong ASTRI (2004). Since October 2002, he has been with National Chiao Tung University as a Faculty Member of the Institute of Electro-Optical Engi-neering. He is now the Chairman of the Department of Photonics and Institute of Electro-Optical Engineering, NCTU. His current research interests include semiconductor lasers, VCSELs, blue and UV LED lasers, quantum-confined op-toelectronic structures, opop-toelectronic materials, and solar cells. He has authored and coauthored 200 SCI journal papers, 200 conference papers, and 8 granted and 14 pending patents. Prof. Kuo is an IEEE Senior member and Assoc. Editor of IEEE/OSA J. of Lightwave Technology and JSTQE-special issue Solid State Lighting. He received the Ta-You Wu Young Scholar Award from the National Science Council and the Young Photonics researcher award in 2007. He was elected as OSA fellow in 2012.