An Investigation of the Optical Analysis in White Light-Emitting Diodes With Conformal and Remote Phosphor Structure

An Investigation of the Optical Analysis in

White Light-Emitting Diodes With Conformal

and Remote Phosphor Structure

Kuo-Ju Chen, Hsin-Chu Chen, Chien-Chung Lin, Member, IEEE, Chao-Hsun Wang, Chia-Chi Yeh,

Hsin-Han Tsai, Shih-Hsuan Chien, Min-Hsiung Shih, Member, IEEE, and Hao-Chung Kuo, Senior Member, IEEE

Abstract—An effective emission model of phosphor film is

proposed by using bidirectional scattering distribution function

system (BSDF), and the model is verified by white light-emitting

diodes (LEDs) with conformal and remote phosphor structure.

The emission model is built to clarify the optical characteristics

by analyzing the angular-dependent distribution of emission

and excitation behaviors in phosphor film. The white LEDs with

conformal and remote phosphor structure are also fabricated for

experimental comparison. The uniformity of angular correlated

color temperature (CCT) in white LEDs can be determined by

the angular distribution of blue and yellow light, which is in turns

decided by the refractive index variation between chip a©nd

phosphor layers. Finally, the experimental results are found to

have good agreement with the simulation results performing by

the Monte Carlo method.

Index Terms—GaN, light-emitting diodes (LEDs), optical

simu-lation, phosphor.

I. I

R

ECENTLY, white light-emitting diodes (LEDs) have

been regarded as the next-generation light source due

to the small size, environmental friendly process as well as

high luminous efficiency [1]–[3]. In general, combining the

blue LED chip with the yellow luminary such as Y Al O

phosphor is the most promising method to generate the white

light [4], [5]. For the significant progress in phosphor-converted

white LEDs had been strongly motivated by the advances in

III-Nitride LEDs [6]–[14] serving as pump excitation sources.

The availability of high performance nitride LEDs enables

the practical implementation of phosphor-based LEDs. The

Manuscript received January 13, 2013; revised March 14, 2013; accepted May 14, 2013. Date of publication July 10, 2013; date of current version November 12, 2013. This work was supported in part by the National Science Council in Taiwan under Grant NSC 102-3113-P-009-007-CC2 and Grant NSC 99-2221-E-009-030-MY3.

K.-J. Chen, C.-H. Wang, C.-C. Yeh, H.-H. Tsai, S.-H. Chien, M.-H. Shih, 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, and also with the Electronics and Optoelectronics Research Laboratories, Industrial Technology Research Institute, Hsinchu 30010, Taiwan.

C.-C. Lin is with the Institute of Photonics System, National Chiao Tung University, Tainan 711, Taiwan (e-mail: [email protected]. edu.tw). M.-H. Shih is with the Department of Photonics and Institute of Electro-Op-tical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan, and also with the Research Center for Applied Sciences, Academia Sinica, Taipei 115 Taiwan.

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

Digital Object Identifier 10.1109/JDT.2013.2272644

advances in III-Nitride LEDs had been attributed to the new

approaches for reducing the charge separation issues in active

region [6]–[8], methods for suppression of efficiency-droop [9],

[10], and growth methods to suppress the dislocation density in

materials [10]–[14]. Furthermore, for the fields of phosphors,

there are also some novel materials developed to use in the

solid-state lighting such as oxyfluoride [15], nitride [16], boride

[17] and phosphide [18] hosts. As the results, in order to meet

the practical need in the solid state lighting, high luminous

efficiency and uniformity of angular-dependent correlated color

temperature (CCT) become two major challenges to overcome

in white LEDs [19].

For the high luminous efficiency, remote phosphor packages

which separate the phosphor layer away from the chip could

effectively reduce the backscattering and exhibit higher

conver-sion efficiency, and many examples were demonstrated before

such as the scattered photon extraction package and the

ring-re-mote structure [20]–[23]. However, the disadvantages of rering-re-mote

phosphor structure such as concave surface and non-uniform

an-gular CCT still exist. Therefore, the patterned structure of

re-mote phosphor structure was proposed to improve the

unifor-mity of CCT [24]. Conversely, for the highly uniform color

dis-tribution, conformal phosphor structure is found to be a more

suitable way to improve the distribution of angular CCT [25],

[26]. In this structure, the scattering and reflection

characteris-tics of the phosphor particles are considered as the key

param-eters because it was shown that nearly 60% re-emitted light are

reflected backward [27]. Therefore, large amount of light is

re-flected back and forth and eventually lost inside the package,

resulting in the lower light output in the conformal phosphor

structure.

The backscattering and reflection of light caused by phosphor

could be minimized by optimizing the size of the phosphor

par-ticles [28]. Furthermore, Yamada et al. defined the transmitted

and reflected flux of the blue and yellow light to build the

phos-phor film model [29] and Zhu et al. used the fiber-guided source

to illuminate the characteristic of the phosphor slide [30].

More-over, some research has simulated the relationship between

par-ticle size of phosphor and efficiency in different packages [31].

In general, the emission distribution of phosphor particle is

usu-ally regarded to be ideusu-ally isotropic to simplify the calculation

in phosphor model. However, the scattering distribution in the

phosphor layer usually disagrees with this assumption.

There-fore, the bidirectional scattering distribution function system

(BSDF) system is employed to measure the scattering

phenom-enon and provide the better understanding, which could be

In this study, the emission model of phosphor film is

inves-tigated using BSDF system. Besides, the emission distribution

of phosphor film and the analysis of emission distribution in

remote and conformal phosphor are demonstrated. Then, the

corresponding white LEDs with remote and conformal

phos-phor structure are both experimentally and numerically

inves-tigated. Finally, the refractive index at the air/phosphor layers

interface is verified as a key factor leading to the intensity

dis-tribution of blue and yellow light in remote and conformal

phos-phor structures.

II. E

The pulsed spray coating (PSC) method is adopted in the

experiment to form a thin, uniform phosphor layer on the

sur-face of the sample [34], [35]. First, the polyethylene

terephtha-late (PET) with transmittance of 90% is used as the substrate.

Phosphor powder, silicone binder, and an alkyl-based solvent

are blended together to form phosphor suspension slurry and

sprayed onto the surface of PET. The thickness and

weight-per-centage of phosphor slurry was about 100 m and 50 wt. %,

respectively. The emission distribution of the PET with

phos-phor sample is measured by BSDF system. The PET films were

applied for BSDF measurement due to the low cost, easy-to-cut

sizes, and high transparency across the visible band. The glass

material, which can provide a higher stability in thermal

treat-ment and wider transparency, can also be considered for the

measurement [36]. Once the BSDF measurement is finished,

the final package is made of phosphor-doped silicone without

any PET films to match the standard process. Furthermore, the

diagrams of conformal and remote phosphor LEDs are shown

in Fig. 1. For the both structures, the blue LED chips have peak

emission wavelength at about 450 nm and are placed in the

com-mercial plastic lead-frame package. For conformal structure,

the phosphor slurry is sprayed in the lead-frame, then filling

the silicone glue and baked at 150 C for two hours. In

re-mote phosphor structure, the silicone encapsulant is filled in the

lead-frame and the PSC method is employed to perform

phos-phor coating on the top of the samples. These samples are driven

at the 120 mA to measure the color temperature. The luminous

efficiency of the conformal and remote phosphor structure was

about 100 lm/W and 105 lm/W, respectively. When the

lumi-nous efficiencies of devices are put together for comparison,

the remote phosphor structure is about 5% higher than the

con-formal phosphor structure at the same CCT.

III. S

In the simulation, OpticsWorks software was used and based

on Monte Carlo method incorporated with Mie scattering, which

is common in the LED simulation [37], [38]. Fig. 3 shows the

TABLE I

PARAMETERS OFSTIMULATEDLEDCHIP

simulated structure of remote and conformal phosphor structure.

The particle size distribution and the extinction coefficients of

the phosphor and were considered as the important condition in

the software. The particle sizes of phosphor are set as average

diameter of 12 m and a standard deviation of 0.5. The blue

emission of LED chip and yellow emission of phosphor are set

as 460 nm and 560 nm.

The simulated LED structures were composed of 4- m-thick

n-type GaN layer,multiple-quantum wells (MQWs) with

2.5-nm-thick wells and 200-nm-thick p-type GaN layer. The

blue LED chips dimension is

and the

re-fractive indexes of n-GaN, p-GaN, and MQW are 2.42, 2.45,

2.54, respectively, as shown in Table I [39]. The reflectance

of the surface in the leadframe was 90% [40]. The emission

spectra of blue LED chip and the phosphor are centered at 455

and 560 nm, which are the same as the experiment. Moreover,

for the phosphor model, the phosphor layer was simulated and

calculated the scattering effect of photons through medium with

particles. Furthermore, the distribution of emission obtained by

experiments could input in the software to verify the results.

IV. R

D

In this study, BSDF system is employed to measure the

dis-tribution with different incident angle. First, the emission

distri-butions of the blue chip and blue chip with and without silicone

glue are measured and input into the simulation, as shown in

Fig. 2(a). As a matter of fact, the angular intensity of the blue

light from the LED chip can be directly measured, but the

inten-sity of blue light emitting into the phosphor layer still could not

be measured directly. Therefore, the intensity of blue light in

the phosphor layer is simulated according to the previous

infor-mation in Fig. 2(b). Meanwhile, the angular distribution of the

blue light emitting into phosphor layer is narrow and this could

be attributed to the different refractive index between interfaces.

Fig. 2. Blue light intensity distribution of emitting into (a) glue (b) phosphor layer.

Fig. 3. Simulated structure of (a) remote (b) conformal phosphor structure.

Fig. 4. Distribution of emission at different incident from 15 to 60 .

After identifying the distribution of blue light versus different

interfaces including air, silicone glue and phosphor layer, the

emission distribution of phosphor film is measured by using the

collimated light source whose emission wavelength is about 450

nm. The emission distribution patterns from the phosphor film

at different incident angles from 0 to 60 with interval of 15

are shown in Fig. 4.

Fig. 5. Distribution of emission at all direction with the mixed of blue and yellow light.

The phosphor film could be rotated to measure the emission

distribution at different incident angles and the angle between

incident light and phosphor film is about 90- degree. For the

normal incident light, the emission distribution demonstrates

that the intensity at large angle is higher than that at small angle.

Furthermore, with the larger incident angle, emission

distribu-tion is unsymmetrical due to the incident angle at the different

angle.

We can further combine the data in Fig. 4 to obtain the graph

in Fig. 5, which can be interpreted as possible outcome of a real

chip. The excitation from different angles can simulate the all

direction excitation of phosphors in a blue chip. Therefore, it is

found that there is nearly 50% of light emit backward, which is

similar to the results in [9]. This result could explain the

emis-sion behavior of the phosphor layer which is excited by a blue

LED. Moreover, to verify this model, conformal and remote

phosphor structure is demonstrated both numerically and

exper-imentally as following in Fig. 6. The simulation results show

good agreement with experiment both in the yellow and blue

light.

Our statement on CCT can be also examined in previous

publication that remote phosphor has the higher intensity than

conformal phosphor structure, but the CCT distribution of

con-formal phosphor is much better than remote phosphor [39], [40].

As can be seen in Fig. 7, for conformal phosphor structure, the

intensity of blue light is higher than remote phosphor structure at

the large angle. However, the intensity of yellow light shows

al-most the same phenomenon in both remote and conformal

phos-phor structure. When putting their structure difference into

con-sideration, we could see that the different distribution of

mate-rial leading to different refractive index could be the key.

There-fore, it might be reasonable to cast some calculation to verify it.

Moreover, the calculation of the refractive index (RI) in the

different phosphor concentration, the RI of the phosphor layer

with silicone is given by [43], [44]

where

and

are concentration of the materials. Here, the

RI of silicone glue is 1.4 and the phosphor is 1.8. To verify the

assumption in conformal and remote phosphor structures, the

Fig. 6. Yellow and blue light intensity of (a) remote (b) conformal phosphor structure in simulation and experiment.

Fig. 7. (a) Normalized blue light intensity and (b) normalized yellow light in-tensity of remote and conformal phosphor.

different concentrations of phosphor layer for 20% and 85% in

remote phosphor are fabricated.

The RI of the different phosphor concentration at 20% and

85% is about 1.74 and 1.48, respectively. It is obvious that the

normalized intensity of blue light for the concentration of 20%

is larger than 85% in the large divergent angle, as shown in

Fig. 8(a). However, the yellow light still remain the same at the

Fig. 8. Normalized intensity of different concentration: (a) blue light and (b) yellow.

both concentration in Fig. 8(b). According to Snell’s law, the

blue light, emitted into the air from the package, would cause

a different divergence angle when passing through the different

refractive index. Therefore, the smaller divergence angle could

be attributed to the large discrepancy of the refractive index in

the interface and the refractive index is the main reason to

dom-inate the blue light intensity at large angle.

V. C

In conclusion, the emission model of phosphor film with

BSDF system is investigated and verified both in conformal

and remote phosphor structure. Accordingly, the simulation

results agree well to experimental results in conformal and

remote phosphor structure. Furthermore, the blue and yellow

light are treated separately to discuss the optical characteristic

in simulation and experiment. Finally, we think the refractive

index between air and phosphor layers is the main reason for

the different distribution of the intensity in the blue and yellow

light, which could influence the uniformity of angular CCT in

white LEDs. Such phosphor model could provide the

informa-tion to understand the influence of phosphor, and is important

in discussing about the optical characteristic in white LEDs.

A

The authors would like to thank HELIO Optoelectronics

Cor-poration, Kismart CorCor-poration, and Wellypower Optronics for

their technical support.

R

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at the National Chiao-Tung University, Hsinchu, Taiwan.

His study is focus on UV excitable phosphate which has high luminous ef-ficiency and high stability. He used the combinatorial chemistry to develop UV-excited phosphor for his master thesis. His Ph.D. research includes fab-rication, simulation, and characterization for high power light-emitted diodes.

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

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 Research (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, simulation, and measurement.

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

He received the B.S. degree in electrical engineering from the National Taiwan University in 1993, and the 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 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, including LEDs, solar cells, and lasers. Before joining NCTU, he worked for different start-ups in the United States. After graduating from Stanford in 2002, he joined E2O Communications, Inc in Calabasas, Cali-fornia as a senior optoelectronic engineer. His main research interests then were in optically and electrically pumped long-wavelength vertical cavity surface emitting lasers. In 2004, he joined Santur Corporation in Fremont California, where he initially worked as a member of technical staff then became Manager of Laser Chip Engineering later. He had worked on various projects such as monolithic multi-wavelength DFB Laser arrays for data and telecommunica-tions applicatelecommunica-tions, yield and reliability analysis of DFB Laser arrays, etc.

Dr. Lin has more than 30 journal and conference publications and is a member of the IEEE Photonics Society and IEEE Electron Devices Society.

Chao-Hsun Wang received the B.S. and M.S. degrees in Department of

Pho-tonics from the National Chiao-Tung University, Hsinchu, Taiwan, in 2008 and 2009, respectively, and is currently working toward the 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.

cludes 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, and is currently pursuing the M.S. degree at National Chiao-Tung University, Hsinchu, Taiwan. His master’s degree focus is on high-powered white light-emitting diodes, including fabrication, simulation, measurement, and optical characteristic study. In addition, he researches optical applications of a promising material quantum dot.

Min-Hsiung Shih received the 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 currently 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.

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

from National Taiwan University, Taiwan, the M.S. degree in electrical and computer engineering from Rutgers University—State University of New Jersey, New Brunswick, NJ, in 1995, and the Ph.D. degree from Electrical and Computer Engineering Department, University of Illinois at Urbana Champaign, in 1999.

He has an extensive professional career both in research and industrial research institutions that includes: Research Assistant in Lucent Technologies, Bell Laboratories (1993–1995); and a Senior R&D Engineer in Fiber-Optics Division at Agilent Technologies (1999–2001) and LuxNet Corporation (2001–2002). Since October 2002, he has been with the National Chiao Tung University as a Faculty Member of the Institute of Electro-Optical Engineering. He is now the Associate Dean, Office of International Affair, NCTU. His current research interests include semiconductor lasers, VCSELs, blue and UV LED lasers, quantum-confined optoelectronic structures, optoelectronic materials, and Solar cell. He has authored and coauthored 300 internal journal papers, 2 invited book chapter, 6 granted and 12 pending patents.

Prof. Kuo is an Associate Editor of IEEE/OSA JOURNAL OFLIGHTWAVE

TECHNOLOGY and IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM

ELECTRONICS Special Issue on Solid State Lighting (2009). He received Ta-You Wu Young Scholar Award from National Science Council Taiwan in 2007 and Young Photonics researcher award from OSA/SPIE Taipei chapter in 2007. He was elected as OSA fellow and SPIE fellow in 2012.