IEEE ELECTRON DEVICE LETTERS, VOL. 35, NO. 6, JUNE 2014 657
Improving Performance and Reducing Amountof Phosphor Required in Packaging of White
LEDs With TiO2-Doped SiliconePin-Chao Wang, Yan-Kuin Su, Fellow, IEEE, Chun-Liang Lin, and Guan-Syun Huang
Abstract— White light-emitting diodes (LEDs) assembled bydoping TiO2 nanoparticles with phosphor silicone encapsulationare investigated. The proposed method can improve lumen outputby 2.7% and correlated color temperature deviation by 39%,and it can reduce the required amount of phosphor by 5% andjunction temperature by 6.5 °C in white LED packaging. Theseimprovements are attributed to the enhanced light scattering abil-ity and refractive index of the encapsulation material and reducedlight loss due to color mixing inside the packages, and theyenhance light performance and reduce thermal accumulation.Therefore, results show that improving light output performanceof white LEDs makes these LEDs advantageous for use insolid-state lighting.
Index Terms— White light-emitting diodes (LEDs), TiO2nanoparticles, light scattering ability, refractive index of encap-sulation material.
I. INTRODUCTION
PHOSPHOR-converted white light-emitting diodes (LEDs)assembled by combining blue LED chips with yel-
low phosphor layers are promising next-generation lightingsources, owing to their many advantages over incandescent andfluorescent light sources, such as their environmentally friend-liness, low power consumption, high efficiency, high chromaticperformance, and durability. When employed as a solid-statelight source, an LED should provide high-quality white lightat low cost. Freely dispersed dispensing technology is theconventional method used to fabricate white LEDs. Althoughthis approach allows the thickness of encapsulation materialsto be controlled easily and reduces much of the manufacturingcost, it does not produce high-quality white LEDs, particularlywith respect to high luminous efficacy and chromatic unifor-mity performance, because of the high refractive index contrast
Manuscript received April 1, 2014; accepted April 9, 2014. Date ofpublication May 12, 2014; date of current version May 20, 2014. Thiswork was supported by the National Science Council and Bureau of Energy,Ministry of Economic Affairs of Taiwan, under Contract NSC 102-2221-E-006-211 and Contract 100-2221-E-006-040-MY2. The review of this letterwas arranged by Editor O. Manasreh.
P.-C. Wang is with the Institute of Microelectronics, Department of Elec-trical Engineering and Advanced Optoelectronic Technology Center, NationalCheng Kung University, Tainan 701, Taiwan.
Y.-K. Su is with the Institute of Microelectronics, Department of Electri-cal Engineering and Advanced Optoelectronic Technology Center, NationalCheng Kung University, Tainan 701, Taiwan, and also with the Departmentof Electrical Engineering, Kun-Shan University, Tainan 710, Taiwan (e-mail:[email protected]).
C.-L. Lin and G.-S. Huang are with the Department of Electro-OpticalEngineering and Nano Technology Research and Development Center,Kun-Shan University, Tainan 710, Taiwan.
Color versions of one or more of the figures in this letter are availableonline at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LED.2014.2318037
at the LED chip/air boundary and disharmony between bluelight and yellow light due to the settling of phosphor powderbefore curing [1], [2]. Light extraction efficiency and colordisharmony are the most important issues in evaluating theperformance of white LEDs. Previous studies have demon-strated new concepts that improve light characteristics ofLEDs, including chip surface designs [3], remote phosphorstructure [4], conformal phosphor structure [5], reflective cupdesign [6], phosphor geometry design [7], scattering effectsimulation [8], and silicone microspheres [9]. Although suchresearch has partially improved the performance of whiteLEDs, it has not led to an overall improvement in thepackaging of LEDs.
In order to comprehensively improve the performanceof white LEDs, TiO2 nanoparticles (NPs) were used toincrease the refractive index of encapsulation material andprovide superior light scattering capability [10]. In this letter,optical-thermal characteristics of white LEDs caused by mix-ing different amounts of TiO2 NPs and phosphor into siliconeencapsulation were studied. The proposed method is simpleand uses existing technology to produce low-cost commercialwhite LEDs without changing package optics and withoutadditional optical elements. The only requirement is control-ling TiO2 and phosphor amounts. Experimental results showedthat TiO2-doped phosphor silicone encapsulation can improvelumen output, uniformity of the correlated color temperature(CCT) distribution, and junction temperature properties, andthat it can reduce the amount of phosphor used in manufac-turing LEDs, compared to a conventional structure withoutTiO2 doping at the same CCT, thereby achieving lower costand higher performance.
II. EXPERIMENTAL PROCESS
Commercial TiO2 NPs that do not absorb light with wave-lengths greater than 400 nm in the visible range were prepared,along with white LED package materials. The LED samplesused were commercial GaN-based blue LED chips with chipsize of 1 mm × 1 mm (rated current of 350 mA) and emissionwavelengths of approximately 450 nm, and they were attachedto the center of a commercial plastic surface mount devicelead-frame package (dimensions : 6.5 mm × 5 mm × 0.9 mm)using silver paste. Electronic connections between the LEDchips and the lead frame were implemented using a wirebonding process involving gold wires. Blue LED chips fea-turing the same characteristics and different amounts of TiO2(particle size: 21 nm) and YAG phosphor (particle size: 8 µm)
658 IEEE ELECTRON DEVICE LETTERS, VOL. 35, NO. 6, JUNE 2014
Fig. 1. Schematic diagrams of (a) conventional phosphor and (b) TiO2-dopedphosphor structures.
TABLE I
COMPARISON OF CONVENTIONAL PHOSPHOR AND
TiO2-DOPED PHOSPHOR STRUCTURES
mixed with the silicone encapsulation at the same CCT of∼5600K were used. The amounts of TiO2/phosphor powdersmixed for the conventional phosphor and TiO2-doped phos-phor structures were 0/6 wt% and 0.02/5.7 wt%, respectively.The encapsulation materials were deposited into the lead frameusing a dispensing technique. The LEDs were then cured atan ambient temperature of 150 °C. In this letter, the optical-thermal properties and the amounts of phosphor used in theLEDs were compared and investigated. Fig. 1 shows schematicdiagrams of the two LED structures.
III. RESULTS AND DISCUSSION
Table I shows a comparison between the conventionalphosphor and TiO2-doped phosphor structures. The lumenoutputs of conventional phosphor and TiO2-doped phosphorstructures LEDs were 136.8 and 140.5 lm, respectively; thelumen output of the TiO2-doped phosphor structure was 2.7%greater than that of the conventional structure. This improve-ment was attributed to the fact that the average refractiveindex and light-scattering ability of the encapsulation materialwere increased owing to the doping of TiO2 NPs (n = 2.5)with phosphor (n = 1.8) and silicone (n = 1.5). Using theTiO2-doped phosphor structure, the total reflection lossbetween the LED chips and the air was reduced, and the lumenoutput of the LEDs was thereby improved. Based on the exper-imental results, the junction temperatures of the conventionalphosphor and TiO2-doped phosphor structures were 103.6and 97.1 °C, respectively; the junction temperature of theTiO2-doped phosphor structure was 6.5 °C less than that of theconventional structure. The decrease in junction temperaturefor TiO2-doped phosphor structure can be attributed to reducedlight absorption inside the packages and reduced light losswithin the LED chips. TiO2 doping also indirectly reduced
Fig. 2. Angular-dependent CCT of the conventional phosphor andTiO2-doped phosphor structures.
Fig. 3. Variation in (a) dominant wavelength and (b) CIE chromaticitycoordinates at different currents.
the amount of energy transformed into heat, thereby preventingheat from accumulating in the packaging and further reducingthe junction temperature of the LED chips.
In investigating LED CCT deviation characteristics, angular-dependent CCT uniformity was defined as maximumCCT minus minimum CCT. The angular-dependent CCTdeviations of the conventional phosphor and TiO2-dopedphosphor structures were measured and are presented in Fig. 2.The angular-dependent CCT deviations of the conventionalphosphor and TiO2-doped phosphor structures were 605 and367 K, respectively, in the angle range of −70° to 70°.The TiO2-doped phosphor structure had lower CCT deviations,showing 39% improvement compared to the conventionalphosphor structure. This improvement was attributed to thescattering ability of TiO2. Without TiO2 doping, blue andyellow light were easily trapped and reflected in the package,causing ineffective color mixing of blue and yellow light[11]. The uniform distribution of TiO2 in the phosphor sili-cone encapsulation provided an effective scattering capability,and the blue and yellow light could then be distributeduniformly.
Fig. 3 (a) and (b) shows the variation in the dominantwavelength and the Commission Internationale de l’Eclairage(CIE) chromaticity coordinates at different currents. The TiO2-doped phosphor structure showed smaller wavelength andchromaticity coordinate shifts than the conventional phosphorstructure. The reason was that the proposed structure couldefficiently maintain a stable ratio of blue and yellow lightbecause the yellow phosphor could efficiently convert bluelight to yellow light. Therefore, the reduction in accumulatedheat from the LED chips could effectively reduce the changein the dominant wavelength and CIE chromaticity coordinates[12]–[14]. In addition, a TiO2-doped phosphor structure withother amounts of TiO2/phosphor powders (0.2/5 wt%) was
WANG et al.: IMPROVING PERFORMANCE AND REDUCING AMOUNT OF PHOSPHOR 659
also fabricated. However, significant improvement could notbe easily observed.
According to the experimental results, the TiO2-dopedphosphor structure showed not only improved lumen outputand uniformity of angular-dependent CCT but also reducedjunction temperature in LEDs. Moreover, compared to theconventional structure, the TiO2-doped phosphor structureshowed a 5% reduction in the amount of phosphor used. Thisreduction was attributed to the introduction of TiO2, with itshigh scattering ability into the wavelength conversion layerof the phosphor silicone encapsulation, which enhanced thescattering effect of white LEDs by increasing the likelihood ofblue light encountering phosphor powders and being convertedto long-wavelength light [8]. Results indicate that the amountof relatively expensive phosphor required can be reduced bydoping with low-cost TiO2. Therefore, the proposed methodnot only improves the performance of LEDs but also reducesthe manufacturing cost of white LEDs.
IV. CONCLUSIONS
This letter proposes a simple method for producing whiteLEDs by doping phosphor silicone encapsulation with TiO2NPs. The resulting structure offers appropriate refractive effi-ciency and light scattering ability, which reduces the lightloss due to color mixing inside the packages, thereby improv-ing lumen output and CCT deviation and reducing thermalaccumulation and the required amount of phosphor. Alteringthe refractive index and scattering ability of the encapsulationby mixing phosphor silicone with TiO2 effectively reducesmanufacturing costs and improves light performance, provid-ing a suitable LED light source.
ACKNOWLEDGMENT
The authors would like to thank the National ScienceCouncil and Bureau of Energy, Ministry of Economic Affairsof Taiwan.
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