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ADetailedStudyonPhosphor-ConvertedLight-EmittingDiodesWithMulti-PhosphorConfigurationUsingthe...

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DOI:10.1109/JQE.2015.2472341

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IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 51, NO. 10, OCTOBER 2015 3200310

A Detailed Study on Phosphor-ConvertedLight-Emitting Diodes With Multi-Phosphor

Configuration Using the Finite-DifferenceTime-Domain and Ray-Tracing Methods

Jia-Sheng Li, Jia-Xiao Chen, Li-Wei Lin, Zong-Tao Li, Yong Tang, Bin-Hai Yu, and Xin-Rui Ding

Abstract— Although the optical simulations have beenwidely adopted to study phosphor-converted light-emittingdiodes (pcLEDs), to the best of our knowledge, there is nostudy that investigates the pcLED model considering the effectof phosphor particles with irregular shapes. In order to explorethis topic, we employ the finite-difference time-domain methodto characterize the phosphor-converted element (PCE) model.Furthermore, the PCE-based pcLED with YAG:Ce and nitridephosphors is investigated by the ray-tracing method. The resultsshow that the rod-shaped nitride phosphor can scatter incidentlight to larger angles compared with the spherical YAG:Cephosphor of the same size. In turn, their different scattering andabsorption characterizations cause that the mixing concentrationof the phosphors has a significant effect on the correlative colortemperature and light output power of the pcLED.

Index Terms— Optical simulation, light-emittingdiodes (LEDs), finite-difference time-domain (FDTD), multi-phosphor configuration.

I. INTRODUCTION

L IGHT-EMITTING diodes (LEDs) are considered tohold significant potential in next-generation solid-state

lighting (SSL) applications on account of their manyadvantages [1]. One of the most promising methods to generatewhite light is by combining a blue LED chip with phosphor-converted elements (PCEs); LEDs that adopt this methodare referred to as phosphor-converted LEDs (pcLEDs). ThePCE configuration critically affects the optical performanceof pcLEDs, in particular their angular color uniformity (ACU),

Manuscript received June 3, 2015; revised August 17, 2015; acceptedAugust 20, 2015. Date of publication August 24, 2015; date of current versionSeptember 4, 2015. This work was supported in part by the Post-DoctoralScience Foundation of China under Grant 2014M560659 and in part bythe National Natural Science Foundation of China under Grant 51375177,Grant 51405161, and Grant U1401249. (Corresponding author: Z.-T. Li.)

J.-S. Li, J.-X. Chen, Y. Tang, B.-H. Yu, and X.-R. Ding arewith the Key Laboratory of Surface Functional Structure Manufacturingof Guangdong, Higher Education Institutes, South China University of Tech-nology, Guangzhou 510641, China.

L.-W. Lin is with the Department of Mechanical Engineering,University of California at Berkeley, Berkeley, CA 94720-5800 USA.

Z.-T. Li is with the Key Laboratory of Surface Functional StructureManufacturing, Guangdong, Higher Education Institutes, South ChinaUniversity of Technology, Guangzhou 510641, China, and also with FoshanNationstar Optoelectronics Company, Ltd., Foshan 528000, China (e-mail:meztli@scut.edu.cn).

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

Digital Object Identifier 10.1109/JQE.2015.2472341

Fig. 1. Scanning electron microscope (SEM) images of (a) YAG:Cephosphors and (b) nitride phosphors.

light output efficiency (LE), etc. Consequently, many peoplehave studied the effect of PCEs on pcLEDs numerically, and interms of the particles size [2]–[4], the extinction coefficient [5],the concentration [6]–10], the distance from source [11], andthe geometric morphology [12]–[16].

Most of these studies simply focus on the YAG phosphorbecause of its excellent luminescent characteristics, long life-time, and impressive stability [17], [18]. However, using thesingle YAG phosphor configuration, it is hard to achievea high color rendering index (CRI) and a low correlativecolor temperature (CCT) [19], thereby making it increasinglydifficult to satisfy the increasing demand for pcLEDs. In orderto expand the excitation spectrum of the PCE and obtaina high CRI, a common approach proceeds by combiningnitride phosphors with YAG phosphors [20]. Some studieson this multi-phosphor configuration have also been carriedout experimentally [21]–[24]. However, to the best of ourknowledge, the pcLED with multi-phosphor configuration hasbarely been studied through optical simulations, unlike thesingle YAG phosphor configuration. This is mainly becauseof the fact that the present studies tend to employ the MonteCarlo method [25] in order to simulate light propagationin the PCE, which is regarded as YAG phosphors particlescloud mixed with silicone. In turn, the optical propertiesof the PCE are solved via the Mie scattering model [26].As far as we know, the Mie scattering model is the onlyexact mathematical solution of scattering parameters for thehomogeneous spherical particles. As a result, the PCE modelis more suitable for spherical phosphor particles, such as thenearly spherical YAG phosphor shown in Fig. 1(a).

However, the nitride phosphor, which is broadly used inthe multi-phosphor configuration, has a mostly non-spherical

0018-9197 © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

3200310 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 51, NO. 10, OCTOBER 2015

shape, as shown in Fig. 1(b). It is known that the Mie resultslead to significant discrepancies for non-spherical particlesin the scattering phase function (SPF), especially at thelarge forward scattering angles and at backward scatteringangles [27], [28]. If we still adopt the Mie scatteringmodel to characterize a PCE including irregularly shapednitride phosphors, the simulated results of the multi-phosphorconfiguration may cause some non-negligible errors.

Certain methods exist for dealing with the PCE modelfor pcLEDs that are not limited to the actual shapes ofphosphor particles. For instance, Hsiao et al. developed aPCE model using the measured near-field chromatic luminancedata to define the effective phosphor particle radius thatbest describes the scattering effect of a real PCE [29].Furthermore, Hung and Tien [30] and Chen et al. [31]have introduced the bidirectional scattering distributionfunction (BSDF) to describe the optical properties of the PCE,the BSDF data are obtained experimentally. Their simulatedresults exhibit impressive agreement with the experiments.However, once the PCE configuration changes – for instance,in terms of its phosphor particle size distribution, concen-tration, geometry, etc., – it is necessary to measure thechromatic luminance data or BSDF data again. This approachis therefore inconvenient for predicting the effect of thePCE on pcLEDs.

Contrastingly, the finite-difference time-domain (FDTD)method is not limited to particle shapes, and can obtain theoptical properties of the PCE numerically. This method iswidely adopted for analyzing the scattering properties of non-spherical particles [32], and furthermore displays impressiveagreement with the Mie results for spherical particles [27].However, despite our best efforts, we have not identified anystudies that combine the FDTD method and the PCE modelin pcLED simulations.

In what follows, we propose an effective methodology fordefining the optical properties of PCEs in pcLEDs by employ-ing the FDTD method. Finally, the CCT and light output powerPopt of the pcLED with a multi-phosphor configuration areinvestigated.

II. THE PCE CHARACTERIZATION

A. The Scattering and Absorption Parametersof Phosphor Particles

The FDTD method is employed to define the scatteringcross-section Csca, the absorption cross-section Cabs, and theSPF of phosphor particles. In the FDTD calculation region,the electromagnetic fields are solved on discrete time andspace intervals according to the Maxwell’s equations. In thescattering parameters calculation, the FDTD calculation regionis divided into the scattered field region and the total fieldregion. In the total field region, the incident source electromag-netic field and the scattered electromagnetic field are includedin the calculation; on the other hand, in the scattered fieldregion, only the scattered electromagnetic field is calculated.Therefore, the total scattered power Psca(λ) and the total powerabsorbed by the particles Pabs(λ) can be calculated according

to the following equations:

Psca (λ) =n∑

i=1

T scatteredi (λ) Psource (λ), (1)

Pabs (λ) = (1−n∑

i=1

T totali (λ))Psource (λ), (2)

where T scatteredi (λ) and T total

i (λ) are the scattered transmissionand total transmission obtained from a series of monitorsurfaces in the scattered field region and the total field region,respectively; Psource(λ) is the incident source power (W); λ isthe wavelength (nm).

Next, the Csca and the Cabs can be calculated according tothe following equations [33]:

Csca (λ) = Psca(λ)

Isource(λ), (3)

Cabs (λ) = Pabs(λ)

Isource(λ), (4)

where Isource(λ) is the source irradiance (W/m2).Besides the cross-section properties, the SPF p(λ, θ, φ)

is also an important scattering parameter for describing thenormalized scattering energy distribution in the far field. It isdefined as follows [34]:

p (λ, θ, φ) = 4π F(λ, θ, φ)

k20Csca (λ)

, (5)

where F(λ, θ, φ) is the dimensionless scattering function,k0 is the wavenumber (m−1), and θ and φ are the zenithand azimuth angles. Because the SPF is satisfied by thenormalization condition [35], it can be also written in asimplified form, as follows:

p (λ, θ, φ) = Pfarsca(λ, θ, φ)

Psca(λ), (6)

where Pfarsca(λ, θ, φ) is far-field scattering power in a discrete

region {(θs, φs) | θ + �θ> θ s ≥ θ, φ + �φ > φs ≥ φ} .In far-field spherical coordinates, Pfar

sca(λ, θ, φ) can becalculated as follows [36]:

Pfarsca (λ, θ, φ) =

∫ φ+�φ

φ

∫ θ+�θ

θS (λ, θs, φs)R2sinθsdθsdφs,

(7)

where S (λ, θs, φs) is the module of the Poynting vector(W/m2), R is the distance from the observation point to theparticle (m).

B. The Optical Properties of the PCE

To combine the scattering and absorption characterizationof YAG and nitride phosphors with the pcLED model, theMonte Carlo ray-tracing method is widely applied. Whenstudying light propagation through a PCE consisting of particleclouds, there are three parameters used in the Monte Carlolight scattering model [25]: the absorption coefficient μabs, thescattering coefficient μsca, and the scattering phase functionpPCE. It is assumed that there are negligible particle interactioneffects and multiple scattering events inside particle clouds

LI et al.: DETAILED STUDY ON pcLEDs WITH MULTI-PHOSPHOR CONFIGURATION 3200310

at a low particle concentration. Therefore, the Psca(λ) andthe Pabs (λ) of every phosphor particle can be added directlyto obtain the total scattering and absorption power of thePCE [33]. With the equations (3) to (6), the three parameters ofthe PCE including one type of phosphor with different particlesize take on the following forms:

μabs,i (λ) = NiP̄abs,i(λ)

Isource(λ)= ci

m̄iC̄abs,i (λ), (8)

μsca,i (λ) = NiP̄sca,i(λ)

Isource(λ)= ci

m̄iC̄sca,i (λ), (9)

pCCE,i(λ, θ, φ) =∫

pi,D (λ, θ, φ) Csca,i,D (λ) ni(D)d D∫Csca,i,D (λ) ni(D)d D

, (10)

where Ni is the total number of type i phosphor particlesper unit volume (1/m3), P̄abs,i(λ) and P̄sca,i(λ) is the averageabsorption and scattering power of the type i phosphor withdifferent particle size D respectively, ci is the type i phosphorparticle concentration (g/m3), ni (D) is the type i phosphorparticle size distribution function, pi,D (λ, θ, φ) and Csca,i,D isthe scattering phase function and the scattering cross-sectionof the type i phosphor with particle size D respectively;C̄abs,i (λ) , C̄sca,i (λ) , and m̄i are the absorption, scatteringcross-section, and the particle mass of the type i phosphorintegrated over ni(D), respectively. These quantities, in turn,can be calculated with the following equations:

C̄abs,i (λ) =∫

Cabs,i,D (λ) ni(D)d D∫ni(D)d D

, (11)

C̄sca,i (λ) =∫

Csca,i,D (λ) ni(D)d D∫ni(D)d D

, (12)

m̄i =∫

mi (D) ni (D) d D∫ni (D)d D

, (13)

where Cabs,i,D (λ) and mi (D) is the absorption cross-sectionand particle mass of the type i phosphor with particlesize D.

Based on the same assumption above, the parameters of thePCE with k types of phosphor can be calculated as follows:

μabs (λ) =k∑

i=1

μabs,i (λ), (14)

μsca (λ) =k∑

i=1

μsca,i (λ), (15)

pCCE(λ, θ, φ) =∑k

i=1 NiC̄sca,i (λ) pCCE,i(λ, θ, φ)∫∫ ∑k

i=1 NiC̄sca,i (λ) pCCE,i(λ, θ, φ)dθdφ.

(16)

III. EXPERIMENT AND SIMULATION SETUP

The common YAG:Ce and nitride phosphors that wereadopted in our previous study [23] are further investigated inthis paper. The density and refractive index of the YAG:Cephosphors are 4.5 g/cm3 and 1.78, respectively; for thenitride phosphors, these quantities are 4.5 g/cm3 and 2.17,respectively. In order to define the particles sizes andsimplified shapes, we measured the particles size

Fig. 2. The particle size distributions for (a) YAG:Ce and (b) nitridephosphors.

distributions with scanning electron microscopy (SEM),as shown above in Fig. 1. From these images, we can seethat the YAG:Ce phosphor particles are nearly spherical,whereas the nitride phosphor particles are nearly rod shaped.Therefore, the YAG:Ce phosphor is assumed to be spherical,which is the same with most of the studies on pcLEDssimulation, whereas the other phosphor is assumed to berod shaped, with different lengths and widths. In addition,the particles size distributions and some typical shapes areshown in Fig. 2. We regard the YAG:Ce and nitride phosphorparticle sizes as the diameter and the diagonal coveringthe entire phosphor particle, respectively. On the basis ofthese SEM measurements, the YAG:Ce phosphor and nitridephosphor have an average particle size of 13.7 μm and5.2 μm, respectively, and a standard deviation of 5.0 μm and4.0 μm, respectively.

In the simulation, the commercial software packages FDTDSolutions and Tracepro were used. Fig. 3(a) shows the detailedmodeling of YAG:Ce phosphors by the FDTD method. Thesimplified model (SM) of the YAG:Ce phosphor was placedin a three-dimensional FDTD simulation region (3D simula-tion mode), which was truncated by the perfectly matchedlayer (PML). In order to divide the simulation region into thetotal filed region and the scattered field region, the total-fieldscattered-field source (TS-source) with a center wavelength

3200310 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 51, NO. 10, OCTOBER 2015

Fig. 3. The detailed finite-difference time-domain (FDTD) model for(a) YAG:Ce and (b)–(e) nitride phosphors. PML: Perfectly matched layer.TS-source: total-field scattered-field source. T-monitor: transmission-monitor.SM: simplified model.

Fig. 4. The ray-tracing model of the phosphor-converted light-emittingdiode (pcLED). PCE: phosphor-converted element.

of 455 nm was applied, which surrounded the SM of theYAG:Ce phosphor. The transmission-monitor (T-monitor) boxconsisted of a series of plane monitors, which were usedfor recording the electromagnetic field information passingthrough the monitors. The background index was depended onthe medium surrounding the phosphor. Similarly, the detailedmodeling of the nitride phosphor similar to that of the YAG:Cephosphor, except for its SM and different angles of orientation.When setting up the optical model of the PCE, which consistedof particle clouds, it was necessary to assume that the phosphorparticles were randomly oriented [33]. This means that theprobability that the incident light propagates into the phosphorparticles should be the same for each angle. Consequently,we defined the average cross-section and SPF of phosphorparticles oriented at different angles in terms of the resultsobtained from phosphor particles that were oriented randomly.

As for the YAG:Ce phosphor particle, which was simplifiedas being spherical, it was easily to account for the randomorientation given its symmetry. However, for the nitridephosphor, it was necessary to consider a variety of angles inorder to obtain properly random results. Therefore, we placedthe SM of the nitride phosphor at 0°, 45°, 90°, and 135°, andconsidered the simulation efficiency and accuracy[see Figs. 3(b)–(e)]. It should be noted that the subsequentdiscussion on the cross-section and SPF is based on therandomly oriented phosphor particles.

The ray-tracing modeling is shown in Fig. 4. The com-mercially available pcLED was composed of a blue LED

chip with a Lambertian distribution, a commercial plastic leadframe package with 90% reflectance, the encapsulant, anda PCE. The encapsulant was made from silicone; its indexof refraction was 1.54, and it was assumed that the siliconewas no absorption for light. The PCE contained the YAG:Cephosphors, nitride phosphors, and silicone; its conversion effi-ciency was 75%, and its index of refraction depended on theconcentrations of these two types of phosphors. The emissionspectra of the blue LED chip was centered at 455 nm, whereasthat of the PCE was centered at 525 nm and 620 nm (becauseof the mixing of the YAG:Ce and nitride phosphors) [23].(The detailed properties of the PCE will be discussed in nextsection.) In order to record the far-field CCT distributionand Popt, a semi-spherical detector with a 1 m radius wasplaced upon the pcLED.

IV. RESULTS AND DISCUSSION

A. The Scattering and Absorption Characterizationsof Phosphor Particles

Firstly, we consider the 8 μm YAG:Ce and nitride phosphorsas an example in order to discuss their scattering character-ization through the electromagnetic field distribution, whichis shown in Fig. 5. It can be observed from Fig. 5(a) thatthe light wave takes on a fan shape that consists of a series ofcontinuously curved waves that gather toward the top region ofthe phosphor. After being scattered by the YAG:Ce phosphor,the propagation of the light wave is still regular, and mostof the light wave is emitted from the top surface. Nevertheless,from Figs. 5(b)–(e) we can see that the propagation of the lightwave in the nitride phosphor is quite different. It is difficultto discern an obvious propagation direction inside the nitridephosphor, it seems that the incident light wave is broken intomany fragments by the scattering effect. However, the lightwave inside the nitride phosphor is more uniform, and does notgather toward the top region, as does the YAG:Ce phosphor.Also, the light wave that is emitted from the surface is moreuniform, though the light wave inside the phosphor is quiteirregular.

We believe that these differences are mainly caused by theeffect of the morphology of the phosphor particles on theincident light wave. To achieve a deeper understanding ofthis effect, we further discuss nitride phosphor particles thatare oriented at different angles. When the nitride phosphorparticle is oriented at 0° (Fig. 5(b)), a large number of lightwaves are reflected backward, and only a slight amount oflight is scattered forward from the upper surface. However,when oriented at 45° (Fig. 5(c)), more light is uniformlyscattered from the left surface, and its emission directionseems to be deflected an angle compared to its incidentdirection. As for being oriented at 90° (Fig. 5(d)), more lightis scattered forward from the upper surface as compared withthe 0° orientation because the entrance for the incident lightwave that propagates into the phosphor becomes larger. Whenlight propagates perpendicularly into the surface of the nitridephosphor, we find that the emission direction is generallythe same as the incident direction. Otherwise, the emissiondirection can generally be deflected an angle.

LI et al.: DETAILED STUDY ON pcLEDs WITH MULTI-PHOSPHOR CONFIGURATION 3200310

Fig. 5. The cross-sectional view of the electromagnetic field distribution for 8 μm (a) YAG:Ce phosphor particle and (b)–(e) nitride phosphor particlesoriented at 0°, 45°, 90° and 135° respectively.

As for the spherical YAG:Ce phosphor, its lower surface canbe divided into many sub-surfaces, and the incident angles forevery sub-surface will be different. As a result, the deflectedangles are also different at every sub-surface, which may leadto the gathering of the light wave in the top region of YAG:Cephosphor.

To have a better understanding of the scattering ability forthe randomly oriented YAG:Ce and nitride phosphors, wefurther discuss their far-field scattering power distributionsat three typical particles sizes: 1 μm, 8 μm, and 16μm.Fig. 6 shows their scattering phase distributions (SPDs).Three critical incident wavelengths of blue light (455 nm),yellow-green light (525 nm), and red light (620 nm) areconsidered. At small particles sizes, the SPDs of the YAG:Cephosphor and nitride phosphor are slightly sensitive to theincident wavelength; in particular, the longer wavelength canlead to a slightly stronger scattering effect, which can be seenin Figs. 6(a) and (d); At larger particles sizes, the differencescaused by these three wavelengths can be ignored becausethe changes of the wavelengths are quite smaller than theparticle size (16 μm). But no matter for which wavelength,as the particles size increases, the scattering effect on theincident light of both phosphors becomes weaker. This isprimarily reflected in the changes of the scattering phase atthe 0° direction p(0°). As the particle size increases, the p(0°)becomes larger.

From Figs. 6(a) and (c) and Figs. 6(d) and (f) we can seethat the p(0°) for a particle size of 16 μm is two ordersof magnitude larger than that for a particle size of 1 μm;this means that more incident light is not deflected and stillpropagates along the original incident direction (0° direction)for larger particles sizes. On the other hand, as the particlessize increases, the scattering uniformity of both phosphorsalso decreases. As for the YAG:Ce and nitride phosphors withsizes of 1 μm and under an incident wavelength of 455 nm,the standard deviations of their SPDs pdev are 1.75 and 0.86,respectively; whereas for the phosphors with sizes of 16 μm,the pdev are 27.56 and 15.68, respectively. Therefore, the SPDis more uniform at smaller particle sizes. This demonstratesthat the smaller phosphor particles have stronger abilities toscatter the incident light and obtain a more uniform SPD.

Comparing the nitride phosphor with the YAG:Ce phosphor,we see that the p(0°) for the nitride phosphor is generally oneorder of magnitude smaller than that of YAG:Ce phosphor of

Fig. 6. The scattering phase distributions (SPDs) of (a)–(c) YAG:Cephosphors and (d)–(f) nitride phosphors with particle sizes of 1 μm, 8 μm,and 16 μm, respectively.

the same size. Moreover, the SPD is also more uniform forthe nitride phosphor. These two factors imply that that thescattering ability of the nitride phosphor is more outstanding.Based on the electromagnetic field distribution mentionedabove, one reasonable explanation is that the rod-shapednitride phosphor can scatter the incident light to a large anglewhen being oriented at different angles, which is also helpfulin achieving a more uniform SPD. On the other hand, owingto the symmetry of the YAG:Ce phosphor, most of the incidentlight still emits along the incident direction, regardless of

3200310 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 51, NO. 10, OCTOBER 2015

Fig. 7. The scattering cross-section Csca and absorption cross-section Cabsof (a) the YAG:Ce phosphor and (b) the nitride phosphor.

how the phosphors are oriented. It can be inferred that if thenitride phosphor is also assumed to be spherical or solvedby the Mie method, its scattering ability may become muchweaker. Moreover, we can also see that the nitride phosphorhas a larger backward scattering power because of its higherrefractive index.

Fig. 7 shows the Csca and Cabs of the YAG:Ce phosphor andthe nitride phosphor. We find that their Csca and Cabs valuesincrease as the particle size increases for blue light (455 nm),yellow-green light (525 nm), and red light (620 nm). Thismeans that much more incident power turns into scatteringpower and absorption power. This is mainly the result of thefact that the scattering and absorption properties of particlesprimarily depend on the projected area in the case that theparticle size is larger than the wavelength [28]. Obviously, withthe increasing particle size (i.e., the diameter of the sphericalparticles and the diagonal of the rod-shaped particles), theprojected area of the phosphor particles increases as the squareof that size, while the increasing tendency of the cross-sectionvalues becomes slower. In other words, the scattering andabsorption cross-section of the phosphor particles is moresensitive for smaller particle sizes.

By comparing their cross-sections, it can be seen that thecross-section of the YAG:Ce phosphor is generally one orderof magnitude larger than that of the nitride phosphor ofthe same size. This means that the YAG:Ce phosphor has ahigher scattering power and absorption power. One reasonableexplanation is that the projected area of the spherical YAG:Ce

phosphor is π times of that of the rod-shaped nitride phosphor.Furthermore, the Csca is at least one order of magnitudelarger than the Cabs of both phosphors, which means thatthe phosphor particles can turn more incident power intoscattering power in the light propagating process. Moreover,Csca varies little across different wavelengths owing to thefact that the difference between these three wavelengths ismuch smaller than the particle size. However, Cabs is quitesensitive to the wavelength, which can be also seen from theabsorption spectra of the YAG:Ce and nitride phosphors [23].Both phosphors have a strong absorption effect on blue light(455 nm); moreover, the Cabs (455 nm) is nearly two orders ofmagnitude larger than Cabs (620 nm) for the same particle size.

B. The Optical Properties of the PCE

Based on the scattering and absorption characterizationsof the YAG:Ce and nitride phosphors discussed above, theoptical properties of the PCE with different YAG:Ce andnitride phosphor concentrations are given in Fig. 8. FromFigs. 8(a)–(c), it can be seen that μsca and μabs increasewith the increasing total phosphor concentration cY+N, whichmeans that the probability of scattering and absorption alsoincreases inside the PCE [25]. The reason for this is thatas the total number of phosphors per unit volume becomeslarger; thus the incident light is more easily scattered by thephosphors, which is similar to the results of studies on thesingle YAG phosphor configuration [34]. Similarly, μsca alsoincreases as the nitride phosphor concentration cN increasesfor a particular cY+N, which demonstrates that the nitridephosphor contributes much more strongly to the scatteringability of the PCE.

Two reasons for this observation can be provided. First,the nitride phosphor has a smaller density compared with theYAG:Ce phosphor, and its total volume is so large at a certainconcentration that the incident light more easily propagatesin it. As a result, the probability of scattering also increases.Second, the mean particles size of the nitride phosphor is muchsmaller, which leads to a stronger scattering ability. In otherwords, it has a higher probability of scattering per unit distanceof light propagation. In fact, the total volume of the nitridephosphor is larger than that of the YAG:Ce phosphor within5% at a certain concentration. Therefore, the second of thetwo points just mentioned seems to be the primary factor.The tendency of μabs for different concentrations is generallythe same as μsca except for μabs (455 nm), which showsthat the YAG:Ce phosphor still has stronger absorption abilityper unit volume for blue light, thought it has a larger particlesize.

In addition, the μsca for blue light (455 nm), yellow-greenlight (525 nm), and red light (620 nm) is almost the same,whereas the μabs differs greatly. In particular, μabs (455 nm)is generally one order of magnitude larger than μabs (525 nm)and μabs (620 nm). This demonstrates that the PCE emitsthe converted light mainly depended on the absorption forblue light, and the absorption for its converted light is muchweaker. Therefore, though μabs (525 nm) and μabs (620 nm)increase as cN increases for a particular cY+N, the influenceof the decreasing μabs (455 nm) on the absorption properties

LI et al.: DETAILED STUDY ON pcLEDs WITH MULTI-PHOSPHOR CONFIGURATION 3200310

Fig. 8. The scattering and absorption coefficients, μsca and μabs, of thephosphor-converted element (PCE) under different nitride phosphor concen-tration cN for (a) blue light (455 nm). (b) yellow-green light (520 nm), and(c) red light (620 nm). (d) The refractive index of the PCE.

of the PCE is much stronger, resulting in the total amount ofconverted light decreasing as cN increases. Fig. 8(d) alsoshows that the increasing cN can lead to a higher refractiveindex of the PCE, which causes a stronger total reflectioneffect.

Fig. 9. The correlative color temperature (CCT) distribution of the phosphor-converted light-emitting diode (pcLED) with multi-phosphor configuration.

C. The Optical Performance of the pcLED

According to the optical properties of the PCE, the pcLEDwith multi-phosphor configuration is simulated by the ray-tracing method. Fig. 9 shows its CCT distribution withcY+N = 0.6 g/cm3, from which it can be found that theCCT distribution is much higher in its central viewing angles.With the increasing cN, the CCT generally decreases, andits distribution becomes more uniform. As discussed above,with more nitride phosphors, the PCE has stronger scatteringability and a higher refractive index. This means that the totalpropagating distance of blue light within the PCE increasesand excites more converted yellow-green and red light. Fur-thermore, more blue light is absorbed by the lead frameowing to the stronger backward scattering effect of the nitridephosphor and the stronger total reflection effect between thePCE and the encapsulant. Moreover, the nitride phosphor withlong-wavelength red light is also helpful in obtaining a lowerCCT. Thus, the CCT generally decreases with increasing cN.In addition, the lower CCT is less sensitive to the changesof the light mixing proportion in different viewing angles.The nitride phosphor also has a stronger ability to scatter theincident light. In particular, its SPD is more uniform than thatof the YAG:Ce, which significantly contributes to the ACU.Therefore, the CCT distribution becomes more uniform withincreasing cN.

Fig. 10(a) shows the average CCT μCCT and the standarddeviation CCT σCCT of the pcLED under three different cY+Nvalues: 0.6 g/cm3, 0.7 g/cm3, and 0.8 g/cm3. It is obviousthat μCCT and σCCT decrease with the increasing cY+N mainlybecause of the increasing number of absorption and scatteringevents in the PCE. As explained in the discussion on CCTdistribution above, μCCT and σCCT also decrease with theincreasing cN for a particular cY+N. Most interestingly, thePCE with lower cY+N and higher cN can achieve a similarμCCT as that with higher cY+N and lower cN. For instance,the μCCT near 8000 K can be obtained by cY+N = 0.6 g/cm3

(cN = 40%) or cY+N = 0.8 g/cm3 (cN = 30%) at the sametime. However, their respective σCCT values differ greatly. ThePCE with cY+N = 0.8 g/cm3 (cN = 30%) can achieve amore uniform CCT distribution. In particular, σCCT decreasesby 53.23% compared with that obtained by the PCE withcY+N = 0.6 g/cm3 (cN = 40%). This means that the increasing

3200310 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 51, NO. 10, OCTOBER 2015

Fig. 10. (a) The average correlative color temperature (CCT) μCCT, thestandard deviation CCT σCCT, and (b) the normalized light output powerPopt of the phosphor-converted light-emitting diode (pcLED) with differenttotal phosphor concentrations cY+N and nitride phosphor concentration cN.

cY+N is the main factor in improving the ACU of pcLED fora given μCCT.

Fig. 10(b) shows the normalized Popt of the pcLED. ThePopt decreases as cY+N increases owing to the larger amount ofbackward scattering and the color-conversion loss. Moreover,the increasing cN can also lead to a decrease of the Popt onaccount of the stronger backward scattering discussed above.However, it should be noted that the increasing cY+N is stillthe main factor in decreasing the Popt. For instance, the Poptobtained by cY+N = 0.6 g/cm3 (cN = 40%) increases by9.1% compared with that obtained by cY+N = 0.8 g/cm3

(cN = 30%), these two types of configurations can bothachieve the μCCT near 8000 K. This demonstrates that thePCE with lower cY+N and higher cN can achieve a higherPopt compared with the PCE with higher cY+N and lower cN,at a similar μCCT.

V. CONCLUSION

In conclusion, by taking into account the irregular shapesof phosphors, the pcLED was successfully characterized bythe FDTD method and simulated by the ray-tracing method.

Our results show that the nitride phosphor more efficientlyscatters the incident light to larger angles, though it also hasmuch more backward scattering power, which is the resultof its rod shape and high refractive index. We think that thephosphor particle shape has a significant effect on the scat-tering characterization, especially when the phosphor particleis oriented at different angles. Furthermore, the scattering andabsorption parameters of phosphors were used to solve theoptical properties of the PCE. Based on the common YAG:Ceand nitride phosphors with mean particle sizes of 13.7 μmand 5.2 μm, respectively, we have shown that increasing cNcan lead to an increase in μsca, μabs, and the refractive index,except for μabs (455 nm).

Finally, the optical performance of the pcLED containingthe PCE above was investigated. We found that increasingcY+N and cN can both lead to the improvement of the ACUand the loss of Popt. The PCE with lower cY+N and higher cNcan achieve a higher Popt, whereas the PCE with higher cY+Nand lower cN can achieve a more uniform CCT distributionfor a similar μCCT. As a result, for applications demandinghigh Popt values, the PCE with low cY+N and high cN isrecommended. However, for applications demanding highACU, the PCE with high cY+N and low cN is recommended.We think that the different scattering and absorption char-acterizations of the phosphors have a significant impact onthe optical performance of the pcLED with multi-phosphorconfiguration. However, more studies on the PCE with multi-phosphor configuration are still necessary. The methodologyintroduced in this paper should provide detailed informationfor understanding the effect of phosphors (especially thosewith irregular shapes), and is important in discussing theoptical performance of pcLEDs.

In the future, we plan to use this PCE characterizationmethod to study the effect of the phosphor shape on the opticalperformance of pcLEDs. We believe that our findings willsignificantly contribute to the development of LED lightingapplications.

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Jia-Sheng Li received the B.E. degree in mechanical engineering andautomation from the South China University of Technology, Guangzhou,China, in 2014.

He is currently pursuing the master’s degree with the Key Laboratory ofSurface Functional Structure Manufacturing of Guangdong, Higher EducationInstitutes, South China University of Technology. His current research focuseson the LED packaging and lighting quality.

Jia-Xiao Chen received the B.E. degree in mechanical engineering andautomation from the South China University of Technology, Guangzhou,China, in 2014.

He is currently pursuing the master’s degree with the Key Laboratory ofSurface Functional Structure Manufacturing of Guangdong, Higher EducationInstitutes, South China University of Technology. His current research focuseson the LED packaging process and reliability.

Li-Wei Lin received the B.S. degree in power mechanical engineering fromthe National Tsing Hua University, Taiwan, in 1986, and the M.S. and Ph.D.degrees in mechanical engineering from the University of California, Berkeley,in 1991 and 1993, respectively.

He currently serves as the Chancellor’s Professor of the Department ofMechanical Engineering with the University of California, Berkeley, and theCo-Director of the Berkeley Sensor and Actuator Center. His research interestsand activities include microelectromechanical systems, nanoelectromechanicalsystems, nanotechnology, and microelectronics.

Zong-Tao Li received the Ph.D. degree in mechanical engineering with aminor in microelectronics manufacture engineering from the South ChinaUniversity of Technology, Guangzhou, China, in 2014.

He is currently with the Key Laboratory of Surface Functional StructureManufacturing of Guangdong, Higher Education Institutes, South ChinaUniversity of Technology, and the Optoelectronics Engineering TechnologyResearch and Development Center, Foshan NationStar Optoelectronics Com-pany, Ltd., Foshan, China. His major research interests include packaging ofhigh-power LEDs, lighting quality, and device reliability.

Yong Tang received the Ph.D. degree in mechanical engineering from theSouth China University of Technology, Guangzhou, China, in 1994. He hasover ten years of experience in surface coating technology and over eightyears in optoelectronic LED packaging.

He is currently a Professor with the School of Mechanical and AutomotiveEngineering, South China University of Technology, and the Director of theKey Laboratory of Surface Functional Structure Manufacturing of Guangdong,Higher Education Institutes, Guangdong. His research interests include surfaceproperties in clean energy and its high efficient usage, especially in energy-saving solid-state lighting.

3200310 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 51, NO. 10, OCTOBER 2015

Bin-Hai Yu received the Ph.D. degree in mechanical and electronic engineer-ing from the Huazhong University of Science and Technology, Wuhan, China,in 1997.

He joined Foshan Nationstar Optoelectronics Company, Ltd., Foshan,China. He was a Post-Doctoral Researcher with Foshan EnterprisePost-Doctoral Work Station from 1999 to 2001. He became the Vice Chairmanfrom 2007 to 2014. He is currently a Professor with the School of Mechan-ical and Automotive Engineering, South China University of Technology,Guangzhou, China, and the Key Laboratory of Surface Functional StructureManufacturing of Guangdong, Higher Education Institutes, Guangdong. Hisresearch includes the LED luminaire, SMD LED, high-power LED packaging,and new packaging process.

Xin-Rui Ding received the Ph.D. degree in mechanical engineering witha minor in microelectronics manufacture engineering from the South ChinaUniversity of Technology, Guangzhou, China, in 2015.

He is currently with the Key Laboratory of Surface Functional StructureManufacturing of Guangdong, Higher Education Institutes, South ChinaUniversity of Technology. His major research interests include the LEDpackaging, lighting quality, and reliability.

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