The Artificial Mixed Fused Quartz Particles and Silicon Particles

Article The Artiﬁcial Mixed Fused Quartz Particles and Silicon Particles-Assisted High-Performance Multicrystalline Silicon

Yanhuan Cai 1, Changcheng Mi 1 and Xinming Huang 1,2,*

1 College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, Jiangsu, China; [email protected] (Y.C.); [email protected] (C.M.) 2 JA Solar Holdings Co., Ltd., Feng Tai District, Beijing 100160, China * Correspondence: [email protected]; Tel.: +86-1338-295-6166

Received: 6 May 2019; Accepted: 28 May 2019; Published: 1 June 2019

Abstract: Mixed seeds of fused quartz particles and silicon particles were laid at the bottom of the crucible to assist the growth of multicrystalline silicon crystals. The full melting process was used, and then we found that the grown crystals had higher quality. The effect of mixed seeds on the growth of multicrystalline silicon was studied. The results showed that fine and uniform initial grains could be obtained by mixed seeds assisting the growth of crystals. Increasing the number of grain boundaries can better release thermal stress and inhibit the proliferation and diffusion of dislocations. The defect density of multicrystalline silicon decreased and the minority carrier lifetime increased, thus improving the conversion efficiency of multicrystalline silicon cells.

Keywords: fused quartz particles; silicon particles; mixed seeds; conversion eﬃciency

1. Introduction At present, crystalline silicon occupies more than 90% of the photovoltaic market [1]. Among crystalline silicon materials, monocrystalline silicon (CZ-Si) and multicrystalline silicon (mc-Si) are widely used. The latter is more popular because of its low cost, high output and large-scale advantages. However, compared with CZ-Si, mc-Si crystals contain more defects, such as dislocations (dislocation clusters), grain boundaries (GBs) and precipitations [2], so the corresponding solar cell efficiency is lower. The traditional multicrystalline silicon is generated with seedless growth. After melting, the silicon raw materials nucleate randomly at the bottom of the crucible, and the initial grains are disorderly. Later, seed-assisted multicrystalline silicon, also known as high performance multicrystalline silicon (HPMC-Si) technology, was developed [3,4]. At present, HPMC-Si has gradually replaced conventional multicrystalline silicon as the most important photovoltaic material by virtue of its excellent product performance. The so-called HPMC-Si can reduce dislocation and improve crystal quality by using seed-assistance to control nucleation and grain growth [4–6]. Full melting and semi-melting are two main processes in HPMC-Si. Fused quartz particles are used as seeds in the full melting process and silicon particles are used as seeds in the semi-melting process. Quartz seeded growth of mc-Si ingots has recently attracted much attention because of the very high yield and high crystal quality [6,7]. Because fused quartz particles are heterogeneous and have no crystal structure and poor infiltration of silicon crystal, the nucleation rate of crystal silicon on its surface is relatively low [8]. The effect of crystallization and the quality of cast silicon ingots are slightly inferior to that of the semi-melting process, while the yield of the semi-melting process is not high due to partial melting of seeds. The full melting process not only has the advantage of yield, but also it does not need seed protection (semi-melting needs to

Crystals 2019, 9, 286; doi:10.3390/cryst9060286 www.mdpi.com/journal/crystals Crystals 2019, 9, 286 2 of 10 controlrequirements the thickness for thermal of seeds, field, high et requirementsc.). The operation for thermal process field, is more etc.). convenient, The operation and process it hasis become more convenient,the mainstream and it hasprocess become in theproduction. mainstream Yin process Changh inao production. et al. [9] Yinused Changhao SiC materials et al.[ 9with] used better SiC materialswettability with to bettercast ingots, wettability but the to actual cast ingots, effect butof the the quality actual eofffect silicon of the ingots quality did of not silicon be improved ingots did as notimagined, be improved it became as imagined, worse. This it became is thought worse. to This be because is thought the to introduction be because theof SiC introduction will increase of SiC the willcarbon increase content the carbonin the ingot, content reduce in the the ingot, minority reduce carrier the minority lifetime carrierand widen lifetime the andred region widen the(thered low regionminority (the carrier low minority lifetime carrier region) lifetime at the bottom region) of at the ingot. bottom of the ingot. InspiredInspired by by Wong Wong et et al. al. [ 10[10],], in in this this paper, paper, we we artificially artificially designed designed the the mixed mixed seeds seeds of of silicon silicon particlesparticles and and fused fused quartz quartz particles particles as as the the nucleation nucleation layer. layer. The The full full melting melting process process was was used used and and thenthen we we found found that that the the crystals crystals grown grown had had higher higher quality quality than than ingots ingots using using fused fused quartz quartz particles. particles. WeWe studied studied the the e ffeffectsects of of the the special special nucleation nucleation layer layer on on the the quality quality of of multicrystalline multicrystalline silicon silicon ingots ingots andand tried tried to to understand understand the the mechanism mechanism of of defect defect reduction reduction after after using using this this design. design. Due Due to to the the special special nucleationnucleation layer layer at theat the bottom bottom of the of crucible,the crucible, the quality the quality of crystal of wascrystal improved. was improved. The results The showed results that,showed with that, the help with of the mixed help seeds, of mixed uniform seeds, and uniform fine initial and fine grains initial could grains be easily could obtained be easily at obtained the initial at stagethe initial of crystallization. stage of crystallization. The seed-assisted The seed-assisted multicrystalline multicrystalline silicon based silicon on this based method on this had method fewer dislocationshad fewer dislocations and higher conversionand higher econversionfficiency. efficiency.

2.2. Experimental Experimental

2.1. Preparation of Seeds and Coatings 2.1. Preparation of Seeds and Coatings The experimental ingots seeded by ordinary fused quartz particles and mixed seeds (fused quartz The experimental ingots seeded by ordinary fused quartz particles and mixed seeds (fused particles: silicon particles = 1:1) were respectively cast in two directional solidification furnaces of quartz particles: silicon particles = 1:1) were respectively cast in two directional solidification Jingyuntong (JZ460/660). G6 high purity and high efficiency quartz crucible with a size of 1040 mm x furnaces of Jingyuntong (JZ460/660). G6 high purity and high efficiency quartz crucible with a size of 1040 mm x 540 mm was used in the experiment. The purity of the two kinds of particles was 99.9999% 1040 mm x 1040 mm x 540 mm was used in the experiment. The purity of the two kinds of particles and the particle size was 50–70 meshes (200–300 µm). The proper size of the particles was used to was 99.9999% and the particle size was 50–70 meshes (200–300 μm). The proper size of the particles prevent the gap between the particles being too large to produce tape casting phenomena [11] or too was used to prevent the gap between the particles being too large to produce tape casting small to increase the risk of sticking pot. phenomena [11] or too small to increase the risk of sticking pot. Seeds were evenly fixed to the bottom of the crucible using an automatic particle planter, as shown Seeds were evenly fixed to the bottom of the crucible using an automatic particle planter, as in Figure1. After the seeds were fixed, Si 3N4 coatings needed to be covered. We sprayed two layers of shown in Figure 1. After the seeds were fixed, Si3N4 coatings needed to be covered. We sprayed two Si3N4 coatings on the seed layer and the side wall of the crucible using automatic spraying apparatus layers of Si3N4 coatings on the seed layer and the side wall of the crucible using automatic spraying to make the coatings more uniform. The purpose of Si3N4 coating is to slow down the erosion rate of apparatus to make the coatings more uniform. The purpose of Si3N4 coating is to slow down the moltenerosion silicon rate of on molten the crucible silicon and on the prevent crucible adhesion and prevent between adhesion ingot and between crucible. ingot and crucible.

(a) (b)

FigureFigure 1. 1.Morphology Morphology of of the the crucible crucible with with (a ()a fused) fused quartz quartz seeds seeds and and (b ()b the) the mixed mixed seeds. seeds.

2.2.2.2. Casting Casting Process Process and and Analysis Analysis AsAs a a comparison, comparison, the the technological technological parameters parameters of of the the two two ingots ingots were were consistent. consistent. In In order order to to avoidavoid dendrite dendrite nucleation nucleation at at the the bottom, bottom, the the undercooling undercooling at at the the bottom bottom was was controlled controlled below below 10 10 k, k, soso the the cooling cooling rate rate at at the the bottom bottom of of crucible crucible was was adjusted adjusted to to about about 3 3 k k/min,/min, and and the the growth growth rate rate of of the the siliconsilicon ingot ingot was was 12 12 mm mm/h./h. After casting, the ingot was cut into 6 × 6, totalling 36 silicon blocks, as shown in Figure 2. The initial grain morphology at the bottom of the squares was observed using an infrared detector and a digital camera. The minority carrier lifetime of six blocks B13–B18 was scanned using the minority Crystals 2019, 9, 286 3 of 10

After casting, the ingot was cut into 6 6, totalling 36 silicon blocks, as shown in Figure2. × The initial grain morphology at the bottom of the squares was observed using an infrared detector and a digital camera. The minority carrier lifetime of six blocks B13–B18 was scanned using the carrierminoritycarrier lifetime lifetime carrier instrument instrument lifetime instrument (Semilab, (Semilab, WT-2000). (Semilab, WT-2000). WT-2000). Then Then, the, the squares Then, squares the were were squares cut cut in wereintoto wafers, wafers, cut into and and wafers, the the grain grain and morphologythemorphology grain morphology and and dislocation dislocation and dislocationdistribution distribution distribution were were char characterizedacterized were characterized by by photoluminescence photoluminescence by photoluminescence (PL) (PL) equipment equipment (PL) (BTequipment(BT imaging, imaging, (BT LIS-R2). imaging,LIS-R2). Fabrication Fabrication LIS-R2). Fabrication and and performance performance and performance testing testing of testing of the the solarof solar the cells solarcells werecells were wereperformed performed performed by by YangzhoubyYangzhou Yangzhou JA JA Solar JA Solar Solar Technology Technology Technology Co., Co., Co., Ltd. Ltd. Ltd.

FigureFigureFigure 2. 2. 2. SquaringSquaring Squaring picturepicture picture of of of an an an ingot. ingot. ingot.

3.3.3. Results ResultsResults and andand Discussion DiscussionDiscussion 3.1. Effect of the Mixed Seeds on the Nucleation Process 3.1.3.1. Effect Effect of of the the Mixed Mixed Seeds Seeds on on the the Nucleation Nucleation Process Process The nucleation process is very important. The quality of nucleation directly affects the growth TheThe nucleation nucleation process process is is very very important. important. The The qu qualityality of of nucleation nucleation directly directly affects affects the the growth growth of initial grains. Therefore, it can be said that nucleation is the key factor to improve the quality of ofof initial initial grains. grains. Therefore, Therefore, it it can can be be said said that that nucleation nucleation is is the the key key factor factor to to improve improve the the quality quality of of silicon ingots. In Ding et al.’s paper [12], two nucleation modes were mentioned. One was the contact siliconsilicon ingots. ingots. In In Ding Ding et et al.'s al.'s paper paper [12], [12], two two nu nucleationcleation modes modes were were mentioned. mentioned. One One was was the the contact contact between fused silicon and fused quartz particles, which formed a large number of nucleation cores. betweenbetween fused fused silicon silicon and and fused fused quartz quartz particles, particles, wh whichich formed formed a alarge large number number of of nucleation nucleation cores. cores. This nucleation mode was called "contact nucleation". The other was the gap area between fused ThisThis nucleation nucleation mode mode was was called called "contact "contact nuclea nucleation".tion". The The other other was was the the gap gap area area between between fused fused quartz particles, which was called "gap nucleation". In this experiment, there were also two kinds quartzquartz particles, particles, which which was was called called "gap "gap nucleation". nucleation". In In this this experiment, experiment, there there were were also also two two kinds kinds of of of nucleation modes. When the silicon particles in the nucleation layer melt at high temperature, nucleationnucleation modes. modes. When When the the silicon silicon particles particles in in the the nucleation nucleation layer layer melt melt at at high high temperature, temperature, there there there would be some interspace, which allowed liquid silicon to flow into the interspace and solidify wouldwould be be some some interspace, interspace, which which allowed allowed liquid liquid silicon silicon to to flow flow into into the the interspace interspace and and solidify solidify and and and form bead-like homogeneous seeds to co-crystallize with fused quartz particles. Figure3 is the formform bead-like bead-like homogeneous homogeneous seeds seeds to to co-crystallize co-crystallize with with fused fused quartz quartz particles. particles. Figure Figure 3 3is is the the bottom morphology after demoulding and Figure4 is the nucleation process. bottombottom morphology morphology after after demoulding demoulding and and Fi Figuregure 4 4is is the the nucleation nucleation process. process.

FigureFigureFigure 3. 3. 3. BottomBottom Bottom Morphology MorphologyMorphology after afterafter Demoulding. Demoulding.Demoulding.

Crystals 2019, 9, 286 4 of 10

FigureFigure 4. 4.NucleationNucleation process. process. Figure 4. Nucleation process. 3.2. Effect of the Mixed Seeds on the Grain Size 3.2. Effect3.2. of Effect the Mixedof the Mixed Seeds Seeds on the on Grainthe Grain Size Size Figure5 shows the infrared scanning images of B13–B18 silicon blocks in two ingots seeded FigureFigure 5 shows 5 shows the infrared the infrared scanning scanning images images of of B13–B18 B13–B18 siliconsilicon blocks in in two two ingots ingots seeded seeded by by by different seeds. In comparison, the grains on the left side (Figure5a) are relatively loose and differentdifferent seeds. seeds. In comparison, In comparison, the thegrains grains on on th the eleft left sideside (Figure(Figure 5a)5a) areare relatively relatively loose loose and and nonuniform,nonuniform,nonuniform, while while while the the grains the grains grains on onthe on the theright right side side side (Figur (Figur (Figuree e5b) 5b)5 areb) are are more more more vertical vertical and and uniform. and uniform. uniform. In Inorder order In to order to furtherto furtherfurther compare compare compare the initial the initial initial grain grain grainsize, size, two size, two silicon two silicon silicon ingots ingots ingots were were sampled weresampled sampled atat the the bottom atbottom the of bottom of2 mm,2 mm, ofand 2and mm, theand grain thethe morphology grain morphology morphology in the in waferthe in thewafer was wafer was photographed wasphotographed photographed by by digital digital by camera,camera, digital camera,asas shown shown asin in shownFigure Figure 6. in From6. Figure From 6. FigureFrom Figure4 Figureit can 4 itbe4 canit seen can be thatseen be seen thethat initial thatthe initial the grain initialgrain size size grain at atthe the size bottom bottom at the of of bottom fusedfused quartzquartz of fused seeded seeded quartz ingot ingot seededin inFigure Figure ingot 6a 6a in was relatively large, while that at the bottom of mixed seeds seeded ingot in Figure 6b was obviously wasFigure relatively6a was large, relatively while large, that at while the bottom that at of the mi bottomxed seeds of mixed seeded seeds ingot seeded in Figure ingot 6b inwas Figure obviously6b was small. We used Image-Pro analysis software to measure the size of initial grains. According to the small.obviously We used small. Image-Pro We used analysis Image-Pro software analysis to software measure to the measure size of the initial size grains. of initial According grains. According to the to thestatistics statistics on grain on grain size distribution, size distribution, the average the averagegrain size grain of fused size quartz of fused wafers quartz is about wafers 3.36 mm, is about statisticswhile on thatgrain of size mixed distribution, seeds wafers the is aboutaverage 2.79 grain mm, sizethe grain of fused size isquartz reduced wafers about is 17.0%, about and 3.36 the mm, 3.36 mm, while that of mixed seeds wafers is about 2.79 mm, the grain size is reduced about 17.0%, while thatgrain of size mixed is more seeds uniform. wafers Generally is about speaking,2.79 mm, finethe andgrain uniform size is grainsreduced in theabout initial 17.0%, stage and are the and the grain size is more uniform. Generally speaking, fine and uniform grains in the initial stage grain sizebeneficial is more to suppress uniform. the Generally growth of dislocationsspeaking, finein the and later uniform stage, because grains grain in theboundaries initial stagehave a are beneficialare beneficialsignificant to suppress to blocking suppress the effect growth the on growth ofthe dislocations propagation of dislocations inof thedislocations in later thelater stage, [13], stage, because and because can grain better grain boundaries release boundaries thermal have have a significanta significantstress. blocking However, blocking effect the e existenceff ecton onthe theofpropagation these propagation grains ofof di ofdislocationsfferent dislocations sizes in[13], fused [13 and], quar and cantz can particle-seededbetter better release release ingots thermal thermal stress.stress. However,seriously However, reduces the the existence existencethe uniformity of ofthese these grainsof grainsgrain of ofsize,different di ffwhicherent sizes sizesis not in in fusedconducive fused quar quartz tzto particle-seededthe particle-seeded suppression ingots of ingots seriouslyseriouslydislocation reduces reduces in crystals, the uniformityuniformity thus affecting of of grain grain the size, overall size, which qualitywhich is not of is silicon conducive not ingots.conducive to the suppressionto the suppression of dislocation of dislocationin crystals, in thus crystals, affecting thus theaffecting overall the quality overall of quality silicon ingots.of silicon ingots.

(a) (b)

Figure 5. Infrared scanning images of longitudinal grain distribution of the bricks B13–B18. (a)Fused quartz particles seeded ingot; (b)mixed seeds seeded ingot. (a) (b)

FigureFigure 5. 5.InfraredInfrared scanning scanning images images of oflongitudinal longitudinal grain grain distribution distribution of ofthe the bricks bricks B13–B18. B13–B18. (a ()Fuseda) Fused quartzquartz particles particles seeded seeded ingot; ingot; (b ()mixedb) mixed seeds seeds seeded seeded ingot. ingot. Crystals 2019, 9, 286 5 of 10

(a) (b)

FigureFigure 6. 6.Morphology Morphology of of the the initial initial grains grains on theon bottomthe bottom of the of ingots. the ingots. (a) fused (a) quartzfused particlesquartz particles seeded ingotseeded and ingot (b) mixedand (b seeds) mixed seeded seeds ingot. seeded ingot.

3.3.3.3. EEffectffect ofof thethe MixedMixed SeedsSeeds onon thethe MinorityMinority CarrierCarrier LifetimeLifetime MinorityMinority carriercarrier lifetimelifetime directlydirectly aaffectsffects thethe conversionconversion eefficiencyfficiency ofof solar solar cells. cells. FigureFigure7 7shows shows minorityminority carrier carrier lifetime lifetime maps maps of of B13–B18 B13–B18 squares squares in in ingots ingots seeded seeded by by di ffdifferenterent seeds. seeds. The The red red part part in Figurein Figure7 is the7 is low the minoritylow minority carrier carrier lifetime lifetime zone, commonlyzone, commonly known known as the redas the zone red (minority zone (minority carrier lifetimecarrier lifetime is less than is less 2 µs), than which 2 μ iss), mainly which causedis mainly by impuritycaused by permeation impurity permeation in the crucible, in the coating crucible, and impuritiescoating and (mainly impurities metallic (mainly iron impurities) metallic iron contained impurities) in seeds. contained Because in of seeds. too many Because impurities of too inmany the redimpurities zone, it cannotin the red be usedzone, and it cannot needs tobe beused removed and needs and purifiedto be removed in subsequent and purified processes. in subsequent processes.The average minority carrier lifetime of fused quartz squares in Figure7a is 5.78 µs, while that of mixedThe seeds average squares minority in Figure carrier7b is lifetime 6.20 µs, of with fused a diquartzfference squares of 0.42 in µFigures. In the 7a figureis 5.78 itμs, can while be seen that thatof mixed the minority seeds squares carrier in lifetime Figure of 7b the is mixed6.20 μs, seeds with seededa difference ingot of is higher0.42 μs. and In the more figure uniform, it can whilebe seen a largethat the "waterfall" minority (in carrier red circle) lifetime low of minority the mixed carrier seeds lifetime seeded zoneingot appeared is higher inand the more upper uniform, and middle while parta large of the "waterfall" fused quartz (in seededred circle) ingot. low This minority may be carrier due to thelifetime excessive zone growthappeared of dislocationsin the upper in theand middlemiddle andpart later of stagesthe fused of the quartz crystal, seeded which ingot. resulted This in themay high be distributiondue to the ofexcessive dislocation growth density, of thusdislocations forming in more the minoritymiddle and carrier later recombination stages of the centerscrystal, andwhich reducing resulted the in minority the high carrier distribution lifetime. of dislocationThe color density, of the red thus zone forming at the bottommore minority of the silicon carrier ingot recombination cast with mixed centers seeds wasand darker,reducing which the indicatedminority carrier that the lifetime. minority carrier lifetime was lower and the corresponding impurities were more numerous.The color This of phenomenon the red zone isat inthe line bottom with theof the impurity silicon ingot reflux cast theory with proposed mixed seeds by Gaobing was darker, et al which.[14]. Theyindicated thought that that the ironminority impurity carrier was lifetime the main was factor lower leading and the to the corresponding red zone. The impurities impurity ironwere in more the meltnumerous. flowed This back phenomenon to the seeds andis in increasedline with the impu impurityrity reflux concentration theory proposed at the bottom, by Gaobing which et resulted al. [14]. inThey an ironthought concentration that iron atimpurity the solid-liquid was the main interface factor higher leading than to the the original red zone. concentration, The impurity and iron then in ditheffused meltto flowed both sidesback toto expandthe seeds the and width increased of the redthe zone,impurity which concentration indirectly reduced at the bottom, the impurity which contentresulted in in the an silicon iron concentration melt. This also at explainedthe solid-liqui the factd interface that the higher minority than carrier the original lifetime concentration, of the yellow lineand inthen the diffused middle of to the both red sides zone to was expand higher the than width that of in the both red sides zone, (as which shown indirectly in Figure reduced7). For the the singleimpurity phase, content fused in silica the sandsilicon was melt. a continuous This also explai phasened with the fewer fact voids,that the causing minority fewer carrier reflux lifetime of fused of silicathe yellow and fewer line in impurities. the middle For of thethe mixedred zone phase, was thehigher melting thanof that silicon in both particles sides (as made shown the voidsin Figure larger 7). andFor thethe refluxsingle more, phase, so thefused corresponding silica sand was impurity a continuous content at phase the bottom with wasfewer higher, voids, which causing made fewer the lengthreflux ofof thefused red silica zone and at the fewer bottom impurities. of the mixed For the phase mixed higher. phase, The the average melting minorityof silicon carrier particles lifetime made ofthe fused voids quartz larger squares and the was reflux 6.35 µmore,s after so removal the co ofrresponding the red zone, impurity while thatcontent of mixed at the seeds bottom squares was washigher, 6.92 whichµs. The made diff erencethe length between of the them red zone was at 0.57 theµ s.bottom This furtherof the mixed shows phase thatthe higher. minority The average carrier lifetimeminority and carrier overall lifetime quality of offused the mixedquartz seedssquares seeded was 6.35 ingot μ weres after significantly removal of the better red than zone, those while of that the fusedof mixed quartz seeds particles squares seeded was ingot.6.92 μs. The difference between them was 0.57 μs. This further shows that the minority carrier lifetime and overall quality of the mixed seeds seeded ingot were significantly better than those of the fused quartz particles seeded ingot. Crystals 2019, 9, 286 6 of 10

(a)

(b)

Figure 7. Longitudinal minority carrier lifetime mapping of the bricks B13–B18. (a) fused quartz Figure 7. Longitudinal minority carrier lifetime mapping of the bricks B13–B18. (a) fused quartz particles seeded ingot and (b) mixed seeds seeded ingot. (red colour indicates the low lifetime regions particles seeded ingot and (b) mixed seeds seeded ingot. (red colour indicates the low lifetime and blueregions indicates and blue the highindicates lifetime the high regions). lifetime regions). 3.4. Effect of the Mixed Seeds on the Dislocations 3.4. Effect of the Mixed Seeds on the Dislocations Due toDue thermal to thermal stress stress and and diff erentdifferent orientations, orientations, dislocations dislocations occuroccur during the the crystallization crystallization process,process, and the and initial the initial dislocations dislocations are theare the sources sources of of other other dislocations. dislocations. During the the cooling cooling process process after solidification,after solidification, dislocation dislocation density alsodensity increases also rapidly.increases Dislocationsrapidly. Dislocations induce deep induce concentration deep centersconcentration in the conduction centers and in the valence conduction bands and of valence silicon bands to become of silicon composite to become centers composite of electrons centers of and holes, whichelectrons seriously and holes, affect thewhich electrical seriously and affect photoelectric the electrical properties and ofphotoelectric multicrystalline properties silicon solarof cells. Therefore,multicrystalline we need silicon to reducesolar cells. the Therefore, formation we of need dislocations to reduce duringthe formation crystal of growth. dislocations during Figurecrystal8 shows growth. PL diagrams of wafers from the C15 square at the bottom, middle and top of the Figure 8 shows PL diagrams of wafers from the C15 square at the bottom, middle and top of the two ingots. It can well characterize the grain morphology and dislocation distribution in the cut wafers. two ingots. It can well characterize the grain morphology and dislocation distribution in the cut The dislocation density in the wafer is expressed by the percentage of dislocation area to the total area wafers. The dislocation density in the wafer is expressed by the percentage of dislocation area to the of wafer,total Rd. area The of Rdwafer, of the Rd. fused The Rd quartz-seeded of the fused qu ingotartz-seeded was 3.74%, ingot and was that 3.74%, of mixedand that seeds-seeded of mixed ingot wasseeds-seeded 2.96%. By ingot contrast, was the2.96%. average By contrast, dislocation the densityaverage of dislocation the mixed density seeds-seeded of the ingotmixed was reducedseeds-seeded by about 20.9%. ingot was reduced by about 20.9%. For theFor fused the quartzfused quartz seeded seeded ingot, thereingot, werethere somewere dislocationsome dislocation clusters clusters in the in PL the diagram PL diagram (shown in the red(shown circle), in the while red thecircle), grain while size the of grain the mixed size of seeds-seededthe mixed seeds-seeded ingot was ingot smaller was smaller and more and uniform, more and theuniform, dislocation and the clusters dislocation were clusters almost were invisible. almost Although invisible. Although there were there contact were nucleationcontact nucleation and gap nucleationand ingap both nucleation nucleation in both processes, nucleation the latterprocesse hads, largerthe latter and had more larger uniform and gapmore nucleation uniform gap space nucleation space due to the melting of silicon particles, which had a much smaller impact on the due to the melting of silicon particles, which had a much smaller impact on the overall nucleation. So, there are two more reasonable nucleation modes and their proportion. On the one hand, the generated gap can better release thermal stress. On the other hand, more fine and uniform initial grains were overall nucleation. So, there are two more reasonable nucleation modes and their proportion. On the Crystalsone 2019hand,, 9, 286 the generated gap can better release thermal stress. On the other hand, more fine7 of 10and uniform initial grains were formed, which provided a large number of grain boundaries to inhibit the proliferation and diffusion of dislocations [15], thus reducing the dislocation density. formed, which provided a large number of grain boundaries to inhibit the proliferation and diffusion With the growth of the ingots, dislocations (black region) in the ingots proliferated and diffused of dislocations [15], thus reducing the dislocation density. along the direction of growth. In the mid-growth stage, the dislocation density in the ingot seeded by With the growth of the ingots, dislocations (black region) in the ingots proliferated and diffused mixed seeds increased (shown in Figure 8e), but the uniformity of grain size remained at a high along the direction of growth. In the mid-growth stage, the dislocation density in the ingot seeded level. However, the dislocation density of the fused quartz-seeded ingot in Figure 8b was by mixed seeds increased (shown in Figure8e), but the uniformity of grain size remained at a high significantly higher, and most dislocations were concentrated in areas with fine grains. This is level. However, the dislocation density of the fused quartz-seeded ingot in Figure8b was significantly because the growth rates of grains with different sizes were different; large grains are continuously higher, and most dislocations were concentrated in areas with fine grains. This is because the growth extruded and eroded by surface energy, resulting in additional extrusion stress, which lead to more rates of grains with different sizes were different; large grains are continuously extruded and eroded dislocations [15]. by surface energy, resulting in additional extrusion stress, which lead to more dislocations [15]. At the later stage of crystallization (Figure 8c,f), the size gap between grains gradually enlarged, At the later stage of crystallization (Figure8c,f), the size gap between grains gradually enlarged, and and the growth competition among grains became more and more fierce, resulting in more extrusion the growth competition among grains became more and more fierce, resulting in more extrusion stress. stress. The cumulative release of stress resulted in the rapid growth of dislocations in fused The cumulative release of stress resulted in the rapid growth of dislocations in fused quartz-seeded ingots. quartz-seeded ingots. The PL diagram and Rd values show that mixed seeds crystallization can better inhibit the The PL diagram and Rd values show that mixed seeds crystallization can better inhibit the generation and multiplication of dislocations and dislocation clusters. The results also correspond well generation and multiplication of dislocations and dislocation clusters. The results also correspond to the minority carrier lifetime distribution in Figure7. well to the minority carrier lifetime distribution in Figure 7.

a d

b e

Figure 8. Cont. Crystals 2019, 9, 286 8 of 10

c f c f Figure 8. PL mappings of the wafers from bottom, middle and top parts of brick C15 from the ingots FiguregrownFigure 8. with PL8. PL mappings(a –mappingsc) fused of quartz theof the wafers seeds wafersfrom and from ( bottom,d– bottom,f) mixed middle middl seeds. ande and top top parts parts of brickof brick C15 C15 from from the the ingots ingots growngrown with with (a– c(a)– fusedc) fused quartz quartz seeds seeds and and (d– (fd)– mixedf) mixed seeds. seeds. 3.5.3.5. EEffectffect of of the the Mixed Mixed Seeds Seeds on on the the Cell Cell E Efficiencyfficiency 3.5. Effect of the Mixed Seeds on the Cell Efficiency FigureFigure9 shows9 shows the the e ffi efficiencyciency distribution distribution of solar of so cells.lar cells. The averageThe average efficiency efficiency of solar of cells solar made cells Figure 9 shows the efficiency distribution of solar cells. The average efficiency of solar cells ofmade silicon of waferssilicon with wafers diff erentwith seedsdifferent are 18.60%seeds are and 18.60% 18.69%, and respectively. 18.69%, respectively. Compared with Compared fused quartz with made of silicon wafers with different seeds are 18.60% and 18.69%, respectively. Compared with particles,fused quartz the solar particles, cells made the solar from cells mixed made seeds-seeded from mixed ingots seeds-seeded not only have ingots higher not average only have effi ciency,higher fused quartz particles, the solar cells made from mixed seeds-seeded ingots not only have higher butaverage also have efficiency, narrow but distribution. also have narrow The preliminary distribution. experimental The preliminary results showedexperimental that using results the showed mixed average efficiency, but also have narrow distribution. The preliminary experimental results showed seedsthat using as the the seed mixed layer seeds could as improve the seed the layer cell ecouldfficiency improve of high the performance cell efficiency multicrystalline of high performance silicon that using the mixed seeds as the seed layer could improve the cell efficiency of high performance undermulticrystalline the same casting silicon conditions.under the same Cell casting efficiency conditions. can also reflectCell efficiency the quality can ofalso silicon reflect ingots, the quality such multicrystalline silicon under the same casting conditions. Cell efficiency can also reflect the quality asof minoritysilicon ingots, carrier such lifetime, as minority dislocations carrier and lifetime, impurities. dislocations This resultand impurities. is in good This agreement result is with in good the of silicon ingots, such as minority carrier lifetime, dislocations and impurities. This result is in good minorityagreement carrier with lifetimethe minority distribution carrier shown lifetime in dist Figureribution7 and shown the dislocation in Figure distribution 7 and the showndislocation in agreement with the minority carrier lifetime distribution shown in Figure 7 and the dislocation Figuredistribution8. shown in Figure 8. distribution shown in Figure 8.

FigureFigure 9. 9.Conversion Conversion e efficiencyﬃciency distribution distribution of of the the solar solar cells. cells. Figure 9. Conversion efficiency distribution of the solar cells. 4. Conclusions 4. Conclusions Crystals 2019, 9, 286 9 of 10

4. Conclusions Multicrystalline silicon ingots were grown using fused quartz particles/silicon particles as mixed seeds. We found that: (1) Mixed seeds exhibit a great crystallization effect. The average initial grain size was reduced by 17.0% compared with fused quartz crystals, and the uniformity of grain size was improved. Ingot growth using mixed seeds could effectively reduce the dislocation density and improve the quality of ingots. (2) The average dislocation density of mixed seeds ingots was about 20.9% lower than that of fused quartz seeded ingots. In addition, the minority carrier lifetime was higher, the distribution was more uniform and the average cell conversion efficiency was increased by 0.1%. We conclude that using fused quartz particles and silicon particles as mixed seeds to assist the growth technology of multicrystalline silicon is beneficial to the preparation of high-efficiency multicrystalline silicon solar cells.

Author Contributions: For research articles with several authors, a short paragraph specifying their individual contributions must be provided. The following statements should be used “conceptualization, Y.C. and X.H.; methodology, Y.C.; software, Y.C.; validation, Y.C. and C.M.; formal analysis, Y.C.; investigation, Y.C. and C.M.; resources, Y.C. and C.M.; data curation, Y.C.; writing—original draft preparation, Y.C.; writing—review and editing, X.H.; visualization, Y.C.; supervision, X.H.; project administration, X.H.; funding acquisition, X.H.”. Acknowledgments: This work was partly supported by the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT) [Grant number IRT1146]; a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD); a project funded by the Natural Science Foundation of Jiangsu Higher Education Institutions [Grant number 13KJB430016]; and a project funded by the Natural Science Foundation of the Jiangsu Province [Grant number BK20141460]. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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