Effect of Particle Size on the Optical Properties of
Yellow Silicate Phosphor in Light-Emitting Diodes
Min-Suk Jang,Wan-Ho Kim,Young-Rae Kang,Sang-Bin Song,and Jae-Pil Kim*
Lighting Sources&Materials team,Korea Photonics Technology Institute,ChumDan Venturero108-9,
Buk-gu,Gwangju,500-779,Republic of Korea
Jin-Hyeok Kim
Department of Materials Science and Engineering,Chonnam National University,Yongbongro77,
Buk-gu,Gwangju,500-757,Republic of Korea
Understanding the effect of particle size on the optical properties of phosphor is important to increa packaging
efficiency in white light-emitting diodes(LEDs).We have investigated the effect of particle size(10–20l m,20–25l m,
25–32l m)on the optical properties of a yellow silicate phosphor adopted in white LEDs.X-ray diffraction results show neg-ligible modification in crystallinity as the particle size of the yellow silicate phosphor varies,whereas the photoluminescence excitation intensity and quantum yield are enhanced as the particle size incread.LED packages fabricated using phosphors with different mean particle sizes,and their optical properties were analyzed.The radiantflux improved with increasing parti-
cle size,whereas the luminousflux incread with decreasing particle size.The effect of immersion on the optical properties
of the LED light source has been also measured,and the details are discusd.
Introduction
Light-emitting diodes(LEDs)are miconductor devices which emit light from a p-n junction when electric current is applied.1LEDs have attracted much attention as a next-generation light source becau they consume less power than conventional light bulbs and exhibit a much longer lifetime.LEDs can generate light of various wavelengths,for example,red,orange, yellow,green,blue,and ultraviolet,depending on the
*kr
©2013The American Ceramic Society Int.J.Appl.Ceram.Technol.,10[4]617–624(2013)
现身说法的意思DOI:10.1111/ijac.12105
miconducting material ud.2Among their various applications,LEDs as white light sources in general lighting and in liquid crystal display back light unit (LCD BLU)are being conducted various rearches to achieve higher efficiency.3–6结婚说说
支教教师The methods to generate white light with LED can be roughly classified into three types.First,white light can be generated by mixing light from blue, green,and red LEDs.Second,a near-ultraviolet LED combines with phosphors radiating blue,green,and red light,in which high color rendering can be obtained.7,8 Finally,the most widely ud method to produce white LEDs today utilizes the combination of a blue-emitting LED with a yellow phosphor,where part of the light is converted to yellow to realize white light emission.9,10 Among the above methods,the method in which a blue LED chip and yellow or red and green phosphors are ud to produce white light sources offers an eco-nomic advantage over the method that combines red, blue,and green LED chips.Moreover,becau light emitted from a phosphor is ud,changes in the color coordinates of the white light sources manufactured using the former method are smaller over long-term operation.In particular,white LEDs are widely ud as LCD BLUs becau they can solve the problems associ-ated with cold cathodefluorescent lamps,including high power consumption and short-life-cycle Electrolu-minescence(EL),and they can ensure uniform lumi-nance in top-view and si
de-view LCD panels.The white LEDs ud in general lighting are also produced using the former method,becau such LEDs generally exhibit high efficiency and a long luminousflux reten-tion time.
The properties of phosphors in white LED play an important role in the production yield and performance of solid-state lighting.Dutta and Tyagi pointed out the importance and well summarized evolution of phos-phors for white light LED.11Among various phos-phors,Yttrium aluminum garnet(YAG)phosphors and yellow silicate phosphors are typically ud to produce white LEDs.The main properties of phosphors include quantum efficiency,temperature dependency,reliability, and particle size.Among the properties,the particle size distribution is of particular importance as it is a major determining factor in the optical properties of phosphors.In addition to influencing the optical prop-erties,the particle size is also related to the production yield,color coordinate adjustment and light efficiency of phosphors during the manufacture of white LEDs.
Optical properties of phosphors with various com-positions have been intensively studied as summarized by Dutta and Tyagi.11However,optical properties of phosphors can be further dramatically governed by their size.In the ca of YAG phosphors,size effect on the optical properties have been studied.12,13In the studies,YAG phosphors showed increasing scattering rate with decreasing particle size,and conquently,the amount of phosphor particles in white LED c
ould be reduced,which provides an advantage in cost.How-ever,the effects of particle size on the optical properties of silicate phosphors that are currently on the market have not been reported.Generally,the effect of particle size on the optical properties of a given phosphor is cloly related to the quantum efficiency of the phos-phor.Becau silicate phosphors have a quantum effi-ciency difference from that of YAG phosphors,the relationship between optical properties and particle size for silicate phosphors must be determined.In this study,we determine the optical properties of commer-cialized silicate phosphors(Sr2SiO4)with three different particle size ranges to investigate the potential for improvements in the luminous efficiency of white LEDs.
Experimental
The phosphor studied in this study is a yellow sili-cate phosphor produced by Force4and has ud in white LEDs fabrication.The phosphors are formless with particle sizes of(a)10–20l m,(b)20–25l m, and(c)25–32l m.To study the surface shape and particle size distribution of the phosphor,a scanning electron microscope(SEM)and a S-3500particle size analyzer from Microtra were ud,respectively;the crystallinity as a function of the particle size of the phosphor was investigated using an SPD2000X-ray powder diffractometer(XRD)from SINCO.The photo luminescence excitation(PLE)of the phosphor was measured using an F-4500PL Spectrophotometer from Hitachi t
hat featured a xenonflash lamp emitting light over the wavelength range of300–500nm,and photoluminescence(PL)was measured to determine the emission spectrum of the phosphor at an excitation wavelength of450nm over the range of480–700nm. Then,the quantum yield(QY)of the phosphor was measured using the C9920Absolute PL QY measure-ment system from Hamamatsu.
618International Journal of Applied Ceramic Technology—Jang,et al.Vol.10,No.4,2013
For the LED package,the9218size(width 92mm,length180mm)surface mount device(SMD) package from I-CHIUN was ud,and for the LED chip,a large chip(1mm91mm)from Epistar with an emission peak at450Æ1nm was ud.Then,after mixing3wt%(0.06g),5wt%,7wt%,and10wt% phosphor with an OE-6550encapsulant from Dow–Corning(2g),depending on the particle size,the samples were deaerated for10min at10À1torr and dispend into the package.A convection oven was ud to cure the packages for one hour at70°C and two hours at150°C,quentially.
Phosphor distribution can be controlled by curing condition becau the viscosity of encapsulant is tempera-ture dependent.To enlarge the immersion level of the phosphors according to particle size,silicone encapsulant with10wt%phosphor was preheated for10min in a convection oven at50°C,and then the package was fab-ricated using the same method described above.The optical p
高二数学题roperties and beam spread angle of the com-pleted LED package were measured using a CAS-140CT and LED Goniophotometer-100from Instrument Systems,respectively,at3.6V and350mA.
Results and Discussion
Figure1shows the surface shape and particle size distribution of the phosphors after the phosphors were classified according to particle size.As en in Fig.1, the phosphors were smooth and formless.The particle sizes of the phosphor measured through the particle size analyzer were(a)12.02l m,(b)15.01l m,and(c) 20.16l m at the D50%volume distribution diameter and(a)17.86l m,(b)22.11l m,and(c)29.14l m at D90%,as shown in Table I.
Most phosphors that are currently commercialized are produced by a solid-state reaction,and becau high temperature is applied in thefinal thermal treatment process of phosphor manufacture,phosphor particles
typically aggregate.Therefore,phosphors by a solid-state reaction inevitably undergo crushing and washing pro-cess.However,when phosphors are crushed to permit classification according to size,the particles are physically damaged and their crystallinity is destroyed;moreover, the introduction of impurities can decrea the lumines-cence of the phosphors.Therefore,to study the p
hysical damage and crystallinity of phosphor particles during crushing,the XRD pattern of each phosphor particle size distribution was obtained,as shown in Fig.2.The XRD data show a silicate phosphor pattern with typical Sr2SiO4composition;similar crystalline structure was obrved regardless of particle size.Phosphor particles smaller than10l m are not ud in this study becau impurities are generally mixed in during crushing and washing,and size paration are not well achieved.
Figure3a shows the variation in the PL spectrum of the phosphor with particle size at an excitation
(a)
(b)
(c)
Fig.1.scanning electron microscope(SEM)image and phos-phor particle size distributions:(a)10–20l m,(b)20–25l m, and(c)25–32l m.血色残阳
wavelength of 450nm.The peak wavelength of the luminescence of the phosphor was 546nm,and the results showed that the PL intensity incread as the particle size incread.Moreover,the peak wavelength of the luminescence of the phosphor was not affected by particle size.This variation in PL intensity is most likely caud by the luminous efficiency and scattering loss of the phosphors.Figure 3b shows the PLE inten-sity of the silicate phosphor at 546nm.The intensity gradually incread as the particle size incread,and the highest intensity was obrved at 450nm becau Eu 2+electrons are best excited from the 4f ground state to the 5d excited state at 450nm.14Currently,the most widely ud white LEDs u blue LED chips with light emission in this wavelength band as excitation sources,which are manufactured to have the best exci-tation efficiency around 450nm.
QY was measured to more accurately determine the efficiency of the phosphors as a function of particle size.In QY measurements,the ratio of the number of emitted photons to the number of photons absorbed in the integrating sphere is calculated,and this method allows the measurement of efficiency without requiring a phosphor to be packaged.Becau factors such as
LED package and reflector structure are excluded when measuring phosphor efficiency,this method has the advantage of being able to measure the luminous effi-ciency of a phosphor itlf.QY differs fr
om quantum efficiency (QE)becau QY includes the QE of a phos-phor and reabsorption loss arising particle scattering;therefore,it can measure the actual efficiency of a phos-phor more accurately.15,16Fig.4shows QY as a func-tion of phosphor particle size.QY was obrved to increa with particle size,for the 0.06g sample,large phosphor particles showed a 0.2%higher
efficiency
Table I.Phosphor Particle Size Distribution
No Sample P/N (D10%)(l m)P/N (D50%)(l m)P/N (D90%)(l m)1A (10–20l m)8.3512.0217.862B (20–25l m)10.6115.0122.113
C (25–32l m)
14.86
20.16
29.14
620International Journal of Applied Ceramic Technology—Jang,et al.Vol.10,No.4,2013
than small particles.Note that the deviation of the equipment adopted in this experiment from repeated measurement is within0.1%.This trend most likely occurred becau more scattering is produced when phosphor particles are relatively small and scattered light is reabsorbed by other phosphor particles and lost as shown in Fig.3.Although the difference is not sig-nificant,our argument is consistent with the following. The measured QY is obrved to decrea with increas-ing phosphor mass.When the particle size of the mea-sured phosphor was(b)20–25l m,the QY was88.0% when a phosphor mass of0.01g was measured and 85.7%when a phosphor mass of0.06
g was measured, which constitutes a decrea of approximately2%that most likely resulted from an increa in the scattering reabsorption loss with increasing phosphor mass.
The trend obrved for the efficiency of the silicate phosphor investigated in this study is similar to that obrved by Huang et al.for the efficiency of a YAG phosphor.17Huang ported that,in the ca of a YAG phosphor,as the particle size incread,the effi-ciency of the phosphor incread;similarly,in the pres-ent study,large phosphor particles showed high QY and luminous efficiency.This trend is caud by the inherent QE of the phosphor and the reabsorption from the lf-scattering of phosphor particles.Figure4 compares the QY of a commercial YAG phosphor with that of the phosphor ud in this study.The YAG phosphor also showed a decrea in QY with an increa of phosphor mass,which indicates that the YAG phosphor undergoes a similar reabsorption loss by scattering.That is,when the phosphor mass is incread,relatively more scattering events occur and the reabsorption loss increas.
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Figure5shows the change in the luminous spec-trum of the LED package with phosphor particle size. The results show that when the particles are small,the intensity of the blue emission region is reduced and the luminous intensity of the yellow emission region is enhanced.The reason will be discusd later.
To make the analysis more preci,LED packages with varying phosphor contents were fabricated and their optical properties were studied.Figure6shows the radiantflux as a function of particle size when elec-tric current was applied at350mA after LEDs were fabricated with3wt%,5wt%,7wt%,and10wt% phosphor.At the same phosphor content,smaller parti-cles showed a lower radiantflux than larger particles and an even lowerflux when more phosphor was
added
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