applied sciences

Article Fabrication Process and Performance Analysis of CSP LED Filaments with a Stacked Package Design

Nan Jiang 1, Jun Zou 1,2,*, Changran Zheng 3, Mingming Shi 2, Wenbo Li 4, Yiming Liu 1, Bin Guo 3, Jerry Liu 5, Herry Liu 5 and Xavier Yin 5

1 School of Meterial Scince and Engineering, Shanghai Institute of Technoligy, Shanghai 201418, China; [email protected] (N.J.); [email protected] (Y.L.) 2 School of Science, Shanghai Institute of Technology, Shanghai 201418, China; [email protected] 3 School of Material Science and Engineering, Changchun University of Science and Technology, Changchun 130022, China; [email protected] (C.Z.); [email protected] (B.G.) 4 Zhejiang Emitting Optoelectronic Technology Company, Zhejiang 310000, China; [email protected] 5 Tecore Synchem, INC., Tianjin 300451, China; [email protected] (J.L.); [email protected] (H.L.); [email protected] (X.Y.) * Correspondence: [email protected]

 Received: 25 September 2018; Accepted: 8 October 2018; Published: 16 October 2018 

Abstract: This article presents a chip-scale package (CSP) based filament light-emitting diodes (LEDs) with a stacked-type structure and a segmented-type design for higher luminous efficiency and better electrical stability. The innovation strategy improves the luminous efficiency by 7.69% and 18.97% than the traditional dispensing packaged LED filament when the current is at 30 mA and 210 mA, respectively, and reduces the use of red and green phosphors by 66.7% and 26.2%, respectively. With the increase of the current, the luminous efficiency variations of the stacked-packaged filament are lower than that of the traditional dispensing packaged filament. Moreover, the of the filament is no longer determined by the calculation of the ratio of various phosphors. The stacked structure and the segmented design can help us to adjust the specified color temperature quickly by changing the proportion and position of the blue chips and the red CSP LEDs.

Keywords: optical design and fabrication; chip-scale package; light-emitting diode; optical devices

1. Introdution Attributed to their high efficiency, long life, low power consumption, environmental friendliness, tiny size, and directionality, the recently-developed -converted white light-emitting diode (WLED) have become the most promising substitute to conventional light sources [1–4]. However, there are still many problems, including the improvement of phosphor conversion and luminous efficiency [5], and improvements in color purity for their substitution of cheaper incandescent and fluorescent lamps that currently occupy the global market [6]. Presently, light-emitting diode (LED) filaments are a fervent LED product. They have three-dimensional luminescence and high plasticity, and can control light distribution according to the application environment. In contrast, the shortcomings are also very prominent, such as low color purity, low light efficiency, and low heat dissipation, so it is difficult to meet the standards of indoor lighting. The dispensing method of traditional LED filament packaging technology has a large variation in optical performance and the drift of color temperature under different currents [7,8]. Color purity is the key to improving luminous efficiency and light output. The blue InGaN LED chip pumped by blue/green phosphor is a mature technology; however, these lack red components [9].

Appl. Sci. 2018, 8, 1940; doi:10.3390/app8101940 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 1940 2 of 10

2+ Therefore, the mixing of a red phosphor viz. Sr2Si5N8: Eu and green/yellow phosphor, namely, 3+ 2+ yttrium aluminum garnet (YAG): Ce or SrSi2O2N2: Eu , is one method to overcome the problem of impure color. Nevertheless, low color purity can be caused by spectral overlapping, which happens when the partial reabsorption of emitted light by another phosphor, mixed with the design of the two phosphors, occurs. [10–15]. To tackle this problem, stacked phosphor layers were developed by Appl. Sci. 2018, 8, x FOR PEER REVIEW 2 of 10 stacking green and red phosphor layers, which have been found to have higher efficiencies. This design showedovercome that the the reabsorption problem of ofimpure green color. emissions Neverthele by redss, low components color purity is can reduced, be caused when by aspectral red phosphor is usedoverlapping, under the greenwhich phosphorhappens when layer the [ 16partial]. reabsorption of emitted light by another phosphor, However,mixed with thus the far, design most of of the the two research phosphors, on stacked occurs. package[10–15]. To structures tackle this is focusedproblem, onstacked remote type phosphor layers were developed by stacking green and red phosphor layers, which have been found packages,to have such higher as phosphor efficiencies. in This glass design (PiG), showed fluorescent that thefilm, reabsorption and other of green forms emissions [17,18]. by For red example, 3+ Xiang etcomponents al. [19] foundis reduced, that when screen-printing a red phosphor ais redused phosphor under the gree layern phosphor on LuAG:Ce layer [16]. PiG obtained a thermally-stableHowever, warm thus WLED. far, most Lee of et the al. research [20] designed on stacked a new package phosphor structures design is focused for reducing on remote the spectral 3+ 3+ overlappingtype packages, of PiGs, such by cuttingas phosphor and reassemblingin glass (PiG), commercialfluorescent film, green and Luother3Al 5formsO12: Ce[17,18].(LuAG:Ce For ) example, Xiang et2+ al. [19] found +that screen-printing a red phosphor layer on LuAG:Ce3+ PiG and red CaAlSiN3: Eu (CASN:Eu ) PiGs. Inspired by what has been explained above, our team obtained a thermally-stable warm WLED. Lee et al. [20] designed a new phosphor design for planned to solve the performance problem of LED filaments by combining chip scale package (CSP) reducing the spectral overlapping of PiGs, by cutting and reassembling commercial green Lu3Al5O12: with a traditional dispensing method. In recent years, CSP has become a very popular research Ce3+ (LuAG:Ce3+) and red CaAlSiN3: Eu2+ (CASN:Eu+) PiGs. Inspired by what has been explained topic inabove, the LED our team packaging planned field to solve [21 ,the22]. performance The package problem size isof onlyLED filaments slightly largerby combining than an chip LED chip, with anscale area package ratio of(CSP) 1.5:1 with [23 a ].traditional CSP has dispensing the advantages method. In of recent having years, a small CSP has package become sizea very and high assemblypopular density research [21]. topic Thus, in inthe this LED work, packaging we report field [21,22]. a new The phosphor package designsize is only for reducingslightly larger the spectral overlappingthan an of LED LED chip, filaments with an area by usingratio of a 1.5:1 CSP [23]. layer CSP and hasa the layer advantages of dispensing of having phosphor. a small package size and high assembly density [21]. Thus, in this work, we report a new phosphor design for 2. Experimentalreducing the Section spectral overlapping of LED filaments by using a CSP layer and a layer of dispensing phosphor. The fluorescent film used for packaging is made by TECORE SYNCHEM, Inc., Tianjin, China, our partner2. Experimental company, Section using their self-developed preparing technology. A red-emitting phosphor CASN: Eu2+The, withfluorescent a wavelength film used for of packaging 618 nm, is was made used by TECORE to make SYNCHEM, the fluorescent Inc., Tianjin, film China, with a mass fractionour of partner 22.2 wt.%. company, The manufacturingusing their self-developed process ofpreparing CSP LEDs technology. is shown A red-emitting in Figure1 .phosphor Firstly, under a CASN: Eu2+, with a wavelength of 618 nm, was used to make the fluorescent film with a mass certain pressure, rearranged chips were pressed into a softened film under vacuum conditions. Then, fraction of 22.2 wt.%. The manufacturing process of CSP LEDs is shown◦ in Figure 1. Firstly, under a after beingcertain baked pressure, in an rearranged oven for threechips hourswere pressed at a temperature into a softened of 300 film C,under the pressedvacuum conditions. chips coated with film wereThen, cut after into being pieces, baked which in an weoven called for three CSP hours LEDs. at a After temperature that, the of red300 °C, CSP the LEDs pressed were chips welded to a 135-mmcoated long with flexible film were substrate cut into pieces, using which a solid we crystal called CSP machine. LEDs. After Each that, flexible the red filament CSP LEDs substrate were was weldedwelded with 51to CSPa 135-mm chips, long which flexible were substrate connected using ina solid series. crystal The machine. uniform Each mixture flexible of filament green-emitting phosphorsubstrate LuAG:Ce was welded3+ and with package 51 CSP resin chips, was which used were as connected a second in layer series. package The uniform for themixture CSP of filament. green-emitting phosphor LuAG:Ce3+ and package resin was used as a second layer package for the Compared with the traditional dispensing packaging method, this packaging method is more complex CSP filament. Compared with the traditional dispensing packaging method, this packaging method but hasis a more lower complex cost. but has a lower cost.

FigureFigure 1. Schematic 1. Schematic stacked stacked type type chip-scale chip-scale package package (CSP) (CSP) filament filament design design and fabrication. and fabrication. Appl. Sci. 2018, 8, 1940 3 of 10 Appl. Sci. 2018, 8, x FOR PEER REVIEW 3 of 10

As mentioned above, stacked packaging structure can improve the optical efficiency of remote As mentioned above, stacked packaging structure can improve the optical efficiency of remote encapsulation of an LED. However, due to the air gap between the interfaces of the two packages, encapsulation of an LED. However, due to the air gap between the interfaces of the two packages, there are many optical phenomena, such as refraction, reflection and scattering at the interfaces [24]. there are many optical phenomena, such as refraction, reflection and scattering at the interfaces [24]. Therefore, LEDs with stacked packages will not reach the ideal light effect, and the light effect will Therefore, LEDs with stacked packages will not reach the ideal light effect, and the light effect will even be reduced due to the excessive absorption and barrier of red phosphor to blue light when the red even be reduced due to the excessive absorption and barrier of red phosphor to blue light when the phosphor layer is under the green phosphor layer. Therefore, a dry–wet separation package method, red phosphor layer is under the green phosphor layer. Therefore, a dry–wet separation package on the basis of a stacked packaging structure, was adopted to eliminate the air gap by the wet layer method, on the basis of a stacked packaging structure, was adopted to eliminate the air gap by the wrapping the dry layer completely. wet layer wrapping the dry layer completely. As we know, red-emitting phosphors CASN:Eu2+ have strong absorption and blocking effects on As we know, red-emitting phosphors CASN:Eu2+ have strong absorption and blocking effects blue emissions [25]. The electro luminescence (EL) spectra obtained under the conditions of 30 mA and on blue emissions [25]. The electro luminescence (EL) spectra obtained under the conditions of 30 140 V, and the corresponding Commission Internationale de L’Eclairage-1931 (CIE-1931) chromaticity mA and 140 V, and the corresponding Commission Internationale de L’Eclairage-1931 (CIE-1931) coordinates tested by the Yuan Fang integral ball under a condition of 30 mA and 140 V, are shown chromaticity coordinates tested by the Yuan Fang integral ball under a condition of 30 mA and 140 in Figure2a,b. The EL emission of the red CSP filament without the green package is composed of V, are shown in Figure 2a,b. The EL emission of the red CSP filament without the green package is a blue emission at 450 nm, and a broad emission that occurs at wavelengths ranging from 550 to composed of a blue emission at 450 nm, and a broad emission that occurs at wavelengths ranging 700 nm; the former arises from the InGaN LED chip and the latter from the red CASN:Eu2+ phosphors. from 550 to 700 nm; the former arises from the InGaN LED chip and the latter from the red However, the EL emission of the red CSP filament with a green package includes little blue emission at CASN:Eu2+ phosphors. However, the EL emission of the red CSP filament with a green package 450 nm, which means that the upper green phosphor almost completely absorbs the blue emissions includes little blue emission at 450 nm, which means that the upper green phosphor almost completelyemitted from absorbs the red the CSPblue LEDs.emissions With emitted an increasing from the forward red CSP driving LEDs. With current, an increasing from 30 to forward 210 mA, drivingthe corresponding current, from CIE-1931 30 to chromaticity210 mA, the coordinatescorresponding are presentedCIE-1931 inchromaticity Figure2b. Acoordinates comparison are of presentedCommission in Figure Internationale 2b. A compar de L’Eclairageison of (CIE)Commission color coordination Internationale reveals de thatL’Eclairage the emitting (CIE) lights color of coordinationthese two samples reveals are that all the in red.emitting lights of these two samples are all in red.

Figure 2. (a) Electro luminescence (EL) spectra of pure red CSP filament before and after package; Figure(b) the 2. Commission (a) Electro Internationaleluminescence de(EL) L’Eclairage spectra of (CIE)-1931 pure red CSP chromaticity filament coordinates before and ofafter pure package; red CSP (bfilament) the Commission before and afterInternationale package. de L’Eclairage (CIE)-1931 chromaticity coordinates of pure red CSP filament before and after package. In order to fully stimulate the red-emitting phosphors and let parts of the blue light overflow stimulateIn order the to green fully emitting stimulate phosphors, the red-emitting a small phosphors improvement and was let appliedparts of tothe the blue filament light overflow structure. stimulateAs shown the in Figuregreen emitting3, blue chips phosph wereors, arranged a small regularlyimprovement between was red applied CSP LEDs, to the andfilament the proportions structure. Asof redshown CSP in and Figure blue light3, blue chips chips were were 2:1, 1:1,arranged 1:2 and regularly 1:3, respectively. between This red package CSP LEDs, can beand seen the as proportionsa combination of red of stacked-typeCSP and blue and light segmented-type chips were 2:1, packages.1:1, 1:2 and The 1:3, mass respectively. fraction ofThis red package CASN:Eu can2+ bein seen the film as a was combination determined, of stacked-type and the mass and fraction segmented-type of green LuAG:Ce packages.3+ and The the mass proportion fraction of CSPred CASN:Euand blue2+ chips in the is controllable.film was determined, Thus, luminous and the performance mass fraction can be of designed green LuAG:Ce by controlling3+ and thesethe proportiontwo parameters. of CSP and blue chips is controllable. Thus, luminous performance can be designed by controlling these two parameters. Appl. Sci. 2018, 8, x FOR PEER REVIEW 4 of 10 Appl.Appl. Sci. Sci. 20182018,, 88,, x 1940 FOR PEER REVIEW 44 of of 10 10

Figure 3. Samples with different chip proportions. Figure 3. Samples with different chip proportions. The four kinds of filamentsfilaments mentionedmentioned above were dispensed with silicone resins in which the The four kinds of filaments mentioned3+ above were dispensed with silicone resins in which the mass fractionsfractions ofof green green LuAG:Ce LuAG:Ce3+ phosphorphosphor were were 14.3%, 14.3%, 20%, 20%, 29.4%. 29.4%. The The CIE CIE values values obtained obtained for thefor mass fractions of green LuAG:Ce3+ phosphor were 14.3%, 20%, 29.4%. The CIE values obtained for designedthe designed CSP CSP LED LED filament filament samples, samples, tested tested by theby the Yuan Yuan Fang Fang integral integral ball ball under under the the conditions conditions of the designed CSP LED filament samples, tested by the Yuan Fang integral ball under the conditions 30of 30 mA mA and and 140 140 V, areV, are shown shown in Figure in Figure4, from 4, from which which we canwe seecan thatsee thethat CIE the colorCIE color coordination coordination has of 30 mA and 140 V, are shown in Figure 4, from which we can see that the CIE color coordination ahas wide a wide range range of distribution, of distribution, and and the the correlated correlated color color temperatures temperatures can can be obtainedbe obtained from from 2200 2200 K toK has a wide range of distribution, and the correlated color temperatures can be obtained from 2200 K 4600to 4600 K. K. According According to to the the requirements requirements of of indoor indoor atmosphere atmosphere lamps, lamps, samples samples withwith correlatedcorrelated color to 4600 K. According to the requirements of indoor atmosphere lamps, samples with correlated color temperature (CCT)(CCT) near near 2500 2500 K wereK were used used and comparedand compared with the with traditional the traditional packaged packaged LED filaments. LED temperaturefilaments. (CCT) near 2500 K were used and compared with the traditional packaged LED filaments.

Figure 4. CIE values obtained for designed CSP LED filamentfilament samples. Figure 4. CIE values obtained for designed CSP LED filament samples. Appl. Sci. 2018, 8, x FOR PEER REVIEW 5 of 10 Appl. Sci. 2018, 8, x 1940 FOR PEER REVIEW 5 of 10

3. Results and Discussion 3. Results and Discussion Figure 5 shows the practicality of two samples before and after the second-layer packaging. The 3+ first Figuretwo samples5 shows were the pure practicality red CSP of filame two samplesnts, before before and and after after the thegreen second-layer LuAG:Ce3+ packaging.packaging, 3+ Therespectively. first two samplesThe last weresample pure was red our CSP designed filaments, filament, before and with after a CCT thegreen of 2500 LuAG:Ce K, whichpackaging, emits the warmrespectively. white light, The last and sample the upper was one our is designed the sample filament, before with the adesigned CCT of 25002500 K,K whichfilament emits packaging. the warm It whitecan be light,seen that and the colors upper of one each is the sample’s sample emitti beforeng thelight designed were different. 2500 K filamentAlthough packaging. the last filament It can inbe seenFigure that 5 the is colors a segmented-type of each sample’s luminescence emitting light design, were different. the luminescence Although the appeared last filament very in homogeneous.Figure5 is a segmented-type This phenomenon luminescence was due to design, the second the luminescence layer of green appeared packaging very providing homogeneous. ample Thisspace phenomenon for light reflection, was due diffusion to the second and deflection layer of green in the packaging silicone matrix. providing The ample CSP filament space for of light the laminatedreflection, diffusionpackage is and shown deflection in Figure in the 6, silicone and the matrix. dimensions The CSP of each filament partof can the be laminated seen clearly. package The thicknessis shown inof Figurethe green6, and packaging the dimensions layer was of eachnearly part three can times be seen that clearly. of the The red thickness film thickness, of the which green packagingwas a favorable layer was space nearly for three photon times multiple that of the conv redersions, film thickness, so the whichfilament was could a favorable give spaceuniform for photonillumination. multiple conversions, so the filament could give uniform illumination.

Figure 5. An actual image of the stacked type packaged filament. Figure 5. An actual image of the stac stackedked type packaged filament. filament.

Figure 6. A microscopic image of stacked type packaged filament.filament. Figure 6. A microscopic image of stacked type packaged filament.

The ELEL emissionemission ofof thethe stacked stacked package package and and traditional traditional dispensing dispensing package package filaments, filaments, shown shown in FigureThe7, EL were emission composed of the of stacked a blue emissionpackage and at 450trad nm,itional and dispensing a broad emissionpackage filaments, that occured shown at in Figure 7, were composed of a blue emission at 450 nm, and a broad emission that occured at wavelengths rangingranging fromfrom 500 500 to to 700 700 nm, nm, the the former former from from the the InGaN InGaN LED LED chip chip and and the the latter latter from from the wavelengths ranging3+ from 500 to 7002+ nm, the former from the InGaN LED chip and the latter from green LuAG:Ce and3+ red CASN:Eu phosphors.2+ The green EL emission intensity of the stacked-type the green LuAG:Ce3+ and red CASN:Eu2+ phosphors. The green EL emission intensity of the packagedstacked-type filament packaged was found filament to be was higher found than to the be traditionalhigher than dispensing the traditional packaged dispensing filament. packaged This may stacked-typebe ascribed topackaged the decreased filament absorption was found of to the be LuAG:Ce higher than3+’s the green traditional emission dispensing by the red-emitting packaged filament. This may be ascribed to the decreased absorption of the LuAG:Ce3+’s green emission by filament. 2+This may be ascribed to the decreased absorption of the LuAG:Ce3+’s green emission by CASN:Eu phosphor, thereby2+ increasing green emission intensity. These phenomena clearly indicated the red-emitting CASN:Eu2+ phosphor, thereby increasing green emission intensity. These thethat spectralred-emitting overlapping CASN:Eu was effectivelyphosphor, reduced thereby using increasing a stacked-type green packageemission and intensity. segmented-type These phenomena clearly indicated that spectral overlapping was effectively reduced using a Appl. Sci. 2018, 8, 1940 6 of 10 Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 10 stacked-type package and segmented-type luminescence design. The decrease in spectral luminescence design. The decrease in spectral overlapping observed in the current design of the CSP overlapping observed in the current design of the CSP filament can be attributed to the successful filament can be attributed to the successful organization of the proximity of the luminescent elements organization of the proximity of the luminescent elements in the assembled CSP filaments. The in the assembled CSP filaments. The interaction of these optical centers at an optimum distance interaction of these optical centers at an optimum distance promotes the spatial delocalization of promotes the spatial delocalization of energy, minimizes energy transfer between the luminescent energy, minimizes energy transfer between the luminescent centers and thereby reduces the centers and thereby reduces the spectral overlapping. spectral overlapping.

Figure 7. EL spectra of traditional dispensing packaged filament and stacked-type packaged filament. Figure 7. EL spectra of traditional dispensing packaged filament and stacked-type packaged filament.With the same correlated color temperature of 2500 K, the CIE color coordinates of the aforementioned filaments are shown in Figure8. The CIE color coordinates of both samples move With the same correlated color temperature of 2500 K, the CIE color coordinates of the to the left, but with two different trajectories, while the current increases from 30 to 210 mA. In the aforementioned filaments are shown in Figure 8. The CIE color coordinates of both samples move to stacked-type filament, red fluorescent film can be considered to be in a state of excitation saturation; the left, but with two different trajectories, while the current increases from 30 to 210 mA. In the thus, with the increase in current, the emission of red light did not change obviously. Instead, stacked-type filament, red fluorescent film can be considered to be in a state of excitation saturation; the green phosphor stimulated more green light with an increasing intensity of blue light, so the color thus, with the increase in current, the emission of red light did not change obviously. Instead, the coordinates moved directly to the blue–green area. On the contrary, in the traditional dispensing green phosphor stimulated more green light with an increasing intensity of blue light, so the color package, phosphors were evenly dispersed in the silicone resins. As seen in Table1, the luminous coordinates moved directly to the blue–green area. On the contrary, in the traditional dispensing efficiency of the stacked-packaged filament was higher than that of the traditional dispensing packaged package, phosphors were evenly dispersed in the silicone resins. As seen in Table 1, the luminous filament. With the increase in current, the luminous efficiency variations of the stacked-packaged efficiency of the stacked-packaged filament was higher than that of the traditional dispensing filament were lower than that of the traditional dispensing packaged filament. This phenomenon packaged filament. With the increase in current, the luminous efficiency variations of the can mainly be attributed to the reduction of photon reabsorption and the simplification of the photon stacked-packaged filament were lower than that of the traditional dispensing packaged filament. transfer path. This phenomenon can mainly be attributed to the reduction of photon reabsorption and the simplificationTable 1. Electroluminescenceof the photon transfer data path. of traditional dispensing packaged filament and stacked-type packaged filament. Table 1. data of traditional dispensing packaged filament and stacked-type packaged filament. Traditional Dispensing Method Stacked Packaging Method CCT at 30 mA (K) 2499 2506 Traditional Stacked CRI at 30 mA (Ra) 89.7 87.6 CIE at 30 mA (0.4797,Dispensing 0.4180) Method Packaging (0.4797, 0.4180) Method Luminous efficiency at 30 mA (Lm/W) 68.62 73.88 Luminous efficiencyCCT at at 30 210 mA mA (Lm/W)(K) 44.95 2499 53.48 2506 The variationsCRI in luminousat 30 mA efficiency (Ra) 23.67 89.7 20.4 87.6 The rate of variationsCIE in at luminous 30 mA efficiency 34.94% (0.4797, 0.4180) (0.4797, 27.61% 0.4180) Luminous efficiency at 30 mA (Lm/W) 68.62 73.88 TheLuminous general efficiency luminescence at 210 mechanism mA (Lm/W) of Pc-White LEDs 44.95 is the mixing of short wavelength 53.48 light emittedThe by variations blue or UV in luminous LED chips efficiency and long-wavelength light 23.67 emitted from phosphors 20.4 pumped by theThe short rate wavelength of variations light in [luminous26]. It is considered efficiency a complicated 34.94% energy-conversion and 27.61% optical tracing process when the input light interacts with a phosphor. With the law of conservation of energy, when a shortThe wavelength general luminescence light excites amechanism phosphor, partof Pc-White of the energy LEDs is is converted the mixing to longof short wavelength wavelength light lightand theemitted other by part blue is dissipatedor UV LED in chips the form and oflong heat.-wavelength As shown light in Equation emitted (F1),from the phosphors input power pumped from by the short wavelength light [26]. It is considered a complicated energy-conversion and optical tracing process when the input light interacts with a phosphor. With the law of conservation of Appl. Sci. 2018, 8, 1940x FOR PEER REVIEW 7 of 10 energy, when a short wavelength light excites a phosphor, part of the energy is converted to long the blue LED chip (E ) can be transferred to the blue photon energy that transmits out of silicone wavelength light andinput the other part is dissipated in the form of heat. As shown in Equation (F1), the (Etransmitted), the absorbed energy by phosphors that is used for the light conversion (Econverted) and input power from the blue LED chip (Einput) can be transferred to the blue photon energy that the heat generated from both Stokes shift (Ess) and non-radiation (EnonRad). transmits out of silicone (Etransmitted), the absorbed energy by phosphors that is used for the light conversion (Econverted) and the heat generated from both Stokes shift (Ess) and non-radiation (EnonRad). Einput = Etransmitted + Econverted + Ess + EnonRad (1) Einput = Etransmitted + Econverted + Ess + EnonRad (1)

Figure 8.8. CIE-1931CIE-1931 chromaticity chromaticity coordinates coordinates of of tradit traditionalional dispensing packaged filament filament and stacked-type packaged filament.filament.

Within the optical tracing process, the light scattering, absorption and conversion are normally considered asas dominant dominant effects effects in mostin most YAG YAG based based yellow yellow phosphor phosphor models models [27]. As [27]. shown As inshown Figure in9, lightFigure 1 is9, thelight blue 1 is light the blue emitted light by emitted the chip, by whichthe chip, is directly which is emitted directly through emitted the through package. the When package. the blueWhen light the emittedblue light by theemitted chip propagatesby the chip to propagat the greenes phosphor, to the green it is absorbedphosphor, and it transformedis absorbed intoand greentransformed light like into light green 3, and light the like propagation light 3, and direction the propagation is isotropic. direction Light 3shows is isotropic. that the Light green 3 lightshows is notthat absorbed,the green butlight directly is not scatteredabsorbed, through but directly the green scattered phosphor through when the the green light phosphor fired by onewhen green the powderlight fired again by one propagates green powder to another again green propagates phosphor. toThere another are green very fewphosphor. blue lights There absorbed are very by few the greenblue lights phosphor absorbed that canby the propagate green phosphor to the red that phosphor, can propagate as light 4,to thatthe red can phosphor, then reabsorb as light the photon 4, that andcan then turn intoreabsorb red light. the photon The transmission and turn into direction red li ofght. the The emitting transmission light of thedirection red phosphor of the emitting follows thelight same of the isotropy. red phosphor When afollows blue light the emittedsame isotropy. by the chipWhen reaches a blue thelight red emitted phosphor by the in thechip film, reaches it is convertedthe red phosphor into red in light the like film, light it is 5, converted and then directly into red ejected. light like Some light light, 5, likeand lightthen 6,directly does not ejected. reach anySome red light, phosphor like light and 6, goesdoes straightnot reach through any red the phosphor film. This and kind goes of straight blue light through is only the absorbed film. This by thekind green of blue phosphor light is and only emitted absorbed into by green the light.green Light phosphor 7 indicates and emitted that the into emitting green light light. of Light the red 7 phosphorindicates that will the not emitting be absorbed light when of the the red emitting phosphor light will of thenotred be absorbed phosphor when is transmitted the emitting to the light green of phosphor,the red phosphor but instead is transmitted the scattering to phenomenon the green phosphor, occurs. Light but 8instead indicates the that scattering blue light phenomenon is absorbed byoccurs. the red Light phosphor 8 indicates and no that spectral blue transformation light is absorbed occurs. by This the is becausered phosphor the quantum and efficiencyno spectral of thetransformation red phosphor occurs. is much This lower is be thancause that the of quantum the green efficiency phosphor, of and the the red probability phosphor of is non-radiative much lower recombinationthan that of the is green larger. phosphor, and the probability of non-radiative recombination is larger. Appl. Sci. 2018, 8, 1940 8 of 10 Appl. Sci. 2018, 8, x FOR PEER REVIEW 8 of 10

Figure 9. LuminescenceLuminescence mechanism mechanism of stacked type packaged filament. filament.

From the schematic diagram, diagram, Figure Figure 9,, we we can clearly see that allall lightlight propagatespropagates inin thisthis specialspecial package structure. The The excellent excellent lumine luminescencescence performance performance of of the the filament filament is is mainly mainly due due to to the absorption and transformation transformation of of some some blue blue light light directly directly by bythe the green green phosphor, phosphor, which which makes makes up for up thefor defect the defect that the that red the phosphor red phosphor in the infilm the blocks film blocksthe light the path, light which path, leads which to a leads reduction to a reductionin luminous in efficiency.luminous efficiency. We also found that this special packaging struct structureure can save a a large amount of of phosphor phosphor cost, cost, using a simple calculation. The The total total mass mass of of all all packaging packaging materials materials used used for each traditional 2+ dispensing packaged filament filament was 0.278 g, andand thethe massmass fractionfraction ofof redred CASN:Eu CASN:Eu2+ phosphor and 3+ green LuAG:Ce 3+ phosphor werewere 3.0%3.0% and and 20.89%, 20.89%, respectively. respectively. Thus, Thus, the the masses masses of redof red phosphor phosphor and andgreen green phosphor phosphor in each in each filament filame werent were 0.00834 0.00834 g and g 0.05807and 0.05807 g, respectively. g, respectively. In comparison, In comparison, in each in 2+ eachstacked-type stacked-type packaged packaged filament, filament, the total the mass total of mass the red of CASN:Euthe red CASN:Eufilm was2+ film 0.0125 was g, in0.0125 which g, thein whichmass fraction the mass of redfraction phosphor of red was phosphor 22.2%; meanwhile,was 22.2%; themeanwhile, mass of the th compositione mass of the of composition the silicone matrixof the 3+ siliconewas 0.257 matrix g, in which was 0.257 the mass g, in fraction which ofthe green mass LuAG:Ce fraction ofphosphor green LuAG:Ce was 16.67%.3+ phosphor As shown was in 16.67%. Table2, 2+ 3+ Asthe shown mass of in the Table red CASN:Eu 2, the massphosphor of the red and CASN:Eu the green2+ phosphor LuAG:Ce andphosphor the green on eachLuAG:Ce stacked3+ phosphor filament onwas each reduced stacked by 66.7% filament and 26.2%,was reduced respectively, by 66.7% compared and with26.2%, the respectively, traditional packaged compared filament. with the traditional packaged filament. Table 2. Mass of red CASN:Eu2+ phosphors and green LuAG:Ce3+ phosphors in each filament with different packaging methods. Table 2. Mass of red CASN:Eu2+ phosphors and green LuAG:Ce3+ phosphors in each filament with different packaging methods. Mass of Red CASN:Eu2+ Mass of Green LuAG:Ce3+ Phosphor (g) Phosphor (g) Mass of Red CASN:Eu2+ Mass of Green

Stacked-type filamentPhosphor 0.00278 (g) LuAG:Ce3+ Phosphor 0.0428 (g) Traditional dispensing filament 0.00834 0.05807 Stacked-type filament 0.00278 0.0428 Traditional dispensing 0.00834 0.05807 4. Conclusions filament We have designed an innovative CSP filament with a stacked-type structure and a segmented-type 4.luminescence Conclusions design, opening a new area in luminous design and providing a better understanding of theWe tunability have designed of optical an properties. innovative A fasterCSP andfilament more accuratewith a methodstacked-type to control structure the luminous and a segmented-typeparameters of filament luminescence has been developed.design, opening CCT, CRIa new and area CIE canin beluminous set by arranging design and the proportionproviding ofa betterred CSP understanding LEDs and blue of chips, the tunability or by changing of optical the mass properties. fraction ofA greenfaster phosphors.and more accurate This fabrication method also to controlaimed to the solve luminous the spectral parameters overlapping of filament of green has and been red developed. components CCT, for highCRI powerand CIE phosphor-based can be set by arrangingLED filaments. the proportion The proposed of red strategy CSP LEDs improves and blue luminous chips, efficiencyor by changing by 7.69% the andmass 18.97%, fraction more of green than phosphors.the traditional This dispensing fabrication packaged also aimed LED to filament, solve the when spectral the current overlapping is at 30 of mA green and and 210 mA,red componentsrespectively, andfor high this reducespower thephosphor-based use of red phosphors LED filaments. and green The phosphors proposed by strategy 66.7% andimproves 26.2%, luminousrespectively. efficiency The protocol by 7.69% for and the optical18.97%, design more than of stacked-type the traditional structure dispensing filaments, packaged possessing LED filament, when the current is at 30 mA and 210 mA, respectively, and this reduces the use of red phosphors and green phosphors by 66.7% and 26.2%, respectively. The protocol for the optical Appl. Sci. 2018, 8, 1940 9 of 10 excellent tunability and high luminous efficiency, is an effort to make smarter high-powered LED filament devices.

Author Contributions: Conceptualization, J.Z.; Methodology, M.S.; Software, N.J. and C.Z.; Validation, N.J., W.L. and M.S.; Formal Analysis, N.J.; Investigation, N.J.; Resources, J.L., H.L., Y.L. and X.Y.; Data Curation, J.N.; Writing-Original Draft Preparation, N.J.; Writing-Review & Editing, N.J.; Visualization, C.Z.; Supervision, J.Z. and B.G.; Project Administration, W.L.; Funding Acquisition, J.Z. Funding: This work was supported by the Science and Technology Planning Project of Zhejiang Province, China (2018C01046), Enterprise-funded Latitudinal Research Projects (J2016-141), (J2017-171), (J2017-293), (J2017-243). Conflicts of Interest: The authors declare no conflict of interest.

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