Propagation of Crystal Defects During Directional Solidification of Silicon

Article Propagation of Crystal Defects during Directional Solidiﬁcation of Silicon via Induction of Functional Defects

Patricia Krenckel 1,* , Yusuke Hayama 2, Florian Schindler 1, Theresa Trötschler 1, Stephan Riepe 1 and Noritaka Usami 2

1 Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstr. 2, D-79110 Freiburg, Germany; [email protected] (F.S.); [email protected] (T.T.); [email protected] (S.R.) 2 Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan; [email protected] (Y.H.); [email protected] (N.U.) * Correspondence: [email protected]

Abstract: The introduction of directional solidiﬁed cast mono silicon promised a combination of the cheaper production via a casting process with monocrystalline material quality, but has been struggling with high concentration of structural defects. The SMART approach uses functional defects to maintain the monocrystalline structure with low dislocation densities. In this work, the feasibility of the SMART approach is shown for larger ingots. A G2 sized crystal with SMART and cast mono silicon parts has been analyzed regarding the structural defects via optical analysis, crystal orientation, and etch pit measurements. Photoluminescence measurements on passivated and processed samples were used for characterization of the electrical material quality. The SMART approach has successfully resulted in a crystal with mono area share over 90% and a conﬁnement

of dislocation structures in the functional defect region over the whole ingot height compared to a mono area share of down to 50% and extending dislocation tangles in the standard cast mono Si.

Citation: Krenckel, P.; Hayama, Y.; Cellular structures in photoluminescence measurements could be attributed to cellular dislocation Schindler, F.; Trötschler, T.; Riepe, S.; patterns. The SMART Si material showed very high and most homogeneous lifetime values enabling Usami, N. Propagation of Crystal solar cell efficiencies up to 23.3%. Defects during Directional Solidification of Silicon via Induction Keywords: directional solidification; crystal structure; dislocations; grain boundaries; seeded growth; of Functional Defects. Crystals 2021, semiconducting silicon 11, 90. https://doi.org/10.3390/ cryst11020090

Received: 15 December 2020 1. Introduction Accepted: 20 January 2021 Due to price pressure on monocrystalline silicon for the PV industry in the last Published: 22 January 2021 years [1], the interest in alternative high-efficiency silicon wafers has increased again. One possibility to combine the high efficiency of monocrystalline Si with the high throughput Publisher’s Note: MDPI stays neutral and cost-efficiency of directionally solidified ingots is the crystallization of the so-called with regard to jurisdictional claims in cast mono silicon ingots (cm Si, also known as “quasi-mono Si” or “monolike Si”) [2]. published maps and institutional affil- iations. Monocrystalline silicon plates with the same grain orientations were put onto the crucible bottom. The adoption of this grain orientation to the newly crystallized material is enabled by remaining a part of these plates solid during the melting process and a quasi-monocrystalline material can be grown. Nevertheless, strain built in the material during the growth process and a mismatch between the seed plates can induce dislocations Copyright: © 2021 by the authors. and dislocation tangles in the growing crystal [3–5]. These dislocation tangles multiply and Licensee MDPI, Basel, Switzerland. spread over ingot height creating cones of dislocation clusters, which reduce the efficiency This article is an open access article of the produced solar cells [6]. Besides the dislocation multiplication, the nucleation of distributed under the terms and conditions of the Creative Commons grains at the crucible walls is a big issue for cm Si. These parasitic grains extend into the Attribution (CC BY) license (https:// ingot and reduce the monocrystalline area [7]. A smaller monocrystalline area reduces creativecommons.org/licenses/by/ the final cell efficiency due to a less effective texturing process and recombination active 4.0/). grain boundaries.

Crystals 2021, 11, 90. https://doi.org/10.3390/cryst11020090 https://www.mdpi.com/journal/crystals Crystals 2021, 11, 90 2 of 10

The SMART approach (Seed Manipulation for ARtificially controlled defect Tech- nique [8,9]) copes with both of these problems, dislocation tangles and parasitic grains, via functional defects. The functional defect region is a combination of large-angle tilt boundaries to block the parasitic grains and small-angle grain boundaries in between to generate dislocations locally and absorb the tensions and impurities. The efficacy of this approach for small scale experiments was already reported [8]. However, the feasibility for larger scaled experiments has yet to be shown. The phase boundary during the crystallization of larger ingots is normally less flat and the probability of process fluctuations at the phase boundary is increased. Both factors might harm the stability and thus functionality of the functional defects and increase dislocation generation and multiplication. Additionally, the greater ingot height leads to a higher probability of dislocation multiplication in the mono area and statistically to a larger amount of parasitic grains impacting the large-angle tilt boundary of the functional defect region. In the end, the SMART Si and standard cm Si technique have to be compared on the electrical level, meaning material lifetime, to evaluate the suitability for high efficiency solar cells. In this work, the cast mono and SMART approach were crystallized in the same G2 sized ingot to guarantee perfect comparability. The propagation of defects was studied by grain structure, photoluminescence (PL), electron back scattering diffraction (EBSD) and etch pit measurements on as cut wafers. PL measurements on passivated wafers reveal the material properties and potential for high-efficiency solar cells.

2. Materials and Methods 2.1. Material Crystallization and Preparation For the direct comparison of the growth development and material properties for the SMART approach and the standard cast mono approach, a seed configuration consisting of monocrystalline seed plates and silicon slices was chosen such that one half of the ingot was grown with intentionally introduced functional defects (SMART approach), the other half without these defects (standard cm approach). With the following seed configuration, a G2-sized ingot with a total mass of 75 kg Si was crystallized in an industrially coated crucible via directional solidification. Highly phosphorous-doped silicon was used to produce n-type doped material with a resistivity around 1 Ωcm. All named seed pieces were etched before being put into the crucible. Quadratic, monocrystalline Si plates with 156 mm side length and 30 mm height were placed on the crucible bottom and used as seed. The crystal orientation for all of these seed plates was <100> in growth direction and at the side faces. For the cm Si, both accordant Si plates were placed with direct contact of the side faces. For the SMART approach, the two Si plates were surrounded by three Si slices with <100> orientation in growth direction and <−110> at the side faces and additionally two of these Si plates between Si plates and crucible (see the seed configuration in Figure1 ). The orientation difference between the Si plates and slices was used to induce a random tilt grain boundary, while the contact between the Si slices with the same orientation could induce a high defect density, as frequently reported for cm Si material [3–5]. A crystallization process featuring a remained seed height of 23 mm and a flat till slightly convex phase boundary was applied. The total process time was around 4000 min including a melting time of 460 min followed by a crystallization time of 1700 min resulting in a growth velocity of around 7 mm/h. The crystallized ingot was cut into three bricks with 125 mm side length covering different parts of the material like functional defect or edge region. The bricks were slurry cut into 200 µm thick wafers. Crystals 2021, 11, 90 3 of 10 Crystals 2021, 11, x FOR PEER REVIEW 3 of 11

FigureFigure 1. Sketch 1. Sketch of the of seedthe seed conﬁguration configuration with with SMART SMART andand cm Si Si approach approach in inone one crystallization crystallization (left () leftand) view and viewinto the into the cruciblecrucible (right ().right).

2.2. CharacterizationA crystallization process featuring a remained seed height of 23 mm and a flat till slightly convex phase boundary was applied. The total process time was around 4000 min includingPrior to a themelting wafering, time of all 460 bricks min followed were analyzed by a crystallization by mapping time of of the 1700 resistivity min result- distribu- tioning via in thea growth Eddy velocity current of technique around 7 mm/h. [10] using a Semilab Wt-2000 tool. AnThe inline crystallized tool was ingot used was to cut take into PL three and br opticalicks with measurements 125 mm side length of all covering as cut wafers. Virtualdifferent cuts parts through of the material these wafer like functional stacks were defect performed or edge region. using The mathematical bricks were slurry methods likecut alignment into 200 µm of thick the imageswafers. and calculation of the optical flow between images of two wafers [11]. For every tenth wafer, images under white light with different illumination angles2.2. Characterization were taken. Due to the different reflectivities of the grains, the grain structure of these wafersPrior to was the calculatedwafering, all via bricks an algorithm. were analyzed A mapping by mapping of theof the grains resistivity and thedistribu- calculation of thetion parametersvia the Eddy suchcurrent as technique grain boundary [10] using length a Semilab and Wt-2000 area weighted tool. median grain size were performedAn inline tool [12]. was As aused first to estimation, take PL and the optical monocrystalline measurements area of all was as calculatedcut wafers. by the areaVirtual percentage cuts through of the these largest wafer identified stacks were coherent performed grain. using mathematical methods like alignment of the images and calculation of the optical flow between images of two wafers Some specific wafer areas (30 × 40 mm2) at particular ingot heights were analyzed [11]. For every tenth wafer, images under white light with different illumination angles in more detail. For this microscopic characterization, EBSD was used to determine the were taken. Due to the different reflectivities of the grains, the grain structure of these grainwafers orientations. was calculated The othervia an structuralalgorithm. defectA mapping class, of dislocations, the grains and was the analyzed calculation by of etch pit densitythe parameters measurements. such as Forgrain these, boundary the samples length and were area prepared weighted by median polishing grain and sizesubsequent were etchingperformed with [12]. a chromium-free As a first estimation, etch (Cr-less the mono etch)crystalline [13] instead area was of calculated the often by used the Secco-etcharea forpercentage preferential of the defect largest etching. identified The coherent existence grain. of these structural defects was compared with theSome recombination specific wafer activity areas (30 detected × 40 mm2 with) at particular micro photoluminescence ingot heights were analyzed (µPL) onin with quinhydrone/Methanolmore detail. For this microscopic passivated characterizat wafers [14ion,]. EBSD was used to determine the grain orientations.For further The analysis, other structural particular defect wafers class, weredislocations, cut down wasto analyzed 100 × 100by etch mm pit2 and den- under- wentsity a measurements. process sequence For these, used inthe the samples tunnel-oxide were prepared passivating by polishing contact and (TOPCon) subsequent solar cell etching with a chromium-free etch (Cr-less etch) [13] instead◦ of the often used Secco-etch◦ concept including a boron emitter diffusion at 890 C and POCl3 diffusion at 800 C[15]. Asfor a texture, preferential random defect pyramids etching. The were existenc chosene of for these the structural cm Si and defects SMART was Si compared material. The with the recombination activity detected with micro photoluminescence (µPL) on with passivation was done with an aluminum oxide layer. After this process chain, spatially quinhydrone/Methanol passivated wafers [14]. resolved lifetime images were obtained via harmonically modulated PL imaging [16].

Crystals 2021, 11, x FOR PEER REVIEW 4 of 11

For further analysis, particular wafers were cut down to 100 × 100 mm2 and under- went a process sequence used in the tunnel-oxide passivating contact (TOPCon) solar cell concept including a boron emitter diffusion at 890 °C and POCl3 diffusion at 800 °C [15]. As a texture, random pyramids were chosen for the cm Si and SMART Si material. The Crystals 2021, 11, 90 4 of 10 passivation was done with an aluminum oxide layer. After this process chain, spatially resolved lifetime images were obtained via harmonically modulated PL imaging [16].

3. Propagation of of Crystal Crystal Defects Defects 3.1. Blocking Blocking of of Parasitic Parasitic Grains Grains Parasitic grains grains nucleated nucleated on on the the coating coating at at the the crucible crucible wall wall can can be be observed observed to to grow grow towards thethe intendedintended monocrystallinemonocrystalline area.area. ForFor thethe classical classical cm cm Si Si approach, approach, these these parasitic para- siticgrains grains overgrow overgrow the <100>-grain the <100>-grain induced induced by the by seed the seed plate plate (see Figure(see Figure2). This 2). overgrowthThis over- growthreduces reduces the monocrystalline the monocrystalline area and area therefore and therefore the efﬁciency the efficiency in the in the resulting resulting solar solar cell. cell.In case In case of the of SMARTthe SMART Si approach, Si approach, the the functional functional defects defects were were suitable suitable for for the the blocking block- ingof these of these parasitic parasitic grains grains (see (see left left side side of of Figure Figure2 ).2). This Thiswas was alreadyalready reported for for smaller smaller sized ingots with with an an ingot ingot height height of of 60 60 mm mm [17]. [17]. Grains Grains nucleated nucleated at at the the coating coating of of the the crucible wall tend tend to to grow grow into into the the crystal. crystal. The The propagation propagation was was sto stoppedpped by by the the functional functional defects at the outer edge edge of of the the monocrystallin monocrystallinee area. area. Hence, Hence, it it was was possible possible to to maintain maintain the monocrystalline area area nearly nearly constant constant over over the the whole whole ingot ingot height. height.

Figure 2. Virtual cut of grain structure measured via optical images through the bricks with SMART Figure 2. Virtual cut of grain structure measured via optical images through the bricks with SMARTSi (left) andSi (left) cm Siand structure cm Si structure (right). The(right). border The betweenborder between mono- andmono- multicrystalline and multicrystalline area is indicated area is indicatedby the red by lines. the red lines.

The monocrystalline monocrystalline area area share share was was estimated estimated in upper in upper brick brick part partvia the via grain the grainge- ometrygeometry measurements. measurements. The The analyzed analyzed wafer wafer area area has has a size a size of of 125 125 mm mm × ×125125 mm mm and and is is located 20 mmmm awayaway fromfrom thethe ingotingot edge. edge. TheThe phase phase boundary boundary in in this this ingot ingot part part was was ﬂat flat till tillslightly slightly convex. convex. The The segmented segmented grain grain structure structure is is depicted depicted Figure Figure3 for 3 for exemplary exemplary wafers wa- fersat three at three different different ingot ingot heights. heights. The The mono mo areano area is deﬁned is defined via thevia sizethe ofsize the of seeded the seeded grain, grain,the white the grainwhite ingrain the images.in the images. The mono The areamono for area the for cm the Si was cm reducedSi was reduced down to down 50% dueto 50%to parasitic due to grains,parasitic see grains, Table 1see The Table mono 1 areaThe mono share forarea cm share Si shrinks for cm with Si shrinks increasing with brick in- creasingheight as brick expected, height except as expected, for the lastexcept part, for which the last is duepart, to which the determination is due to the determina- of the mono tionshare. of Athe double mono twinshare. boundary A double can twin result bounda in twory can grains result with in two the samegrains grain with orientationthe same grainseparated orientation by a twin separated grain. by If a this twin twin grain. grain If this is twin very grain narrow, is very it will narrow, not be it will detected not be by detectedthe measurement by the measurement and the algorithm and the willalgorithm calculate will bothcalculate grains both with grains same with orientation same ori- as entationone grain as and one thusgrain overestimate and thus overestimate the mono the area. mono The area. mono The area mono for the area SMART for the SMART approach approachfeatures a features nearly constant, a nearly higherconstant, level higher over 90%level as over the blocking90% as the of blocking parasitic grainsof parasitic by the grainsfunctional by the defects functional was successful. defects was successful.

Table 1. Monocrystalline area share at different ingot heights for the cm Si and SMART Si materials.

Ingot Height Mono Area /% % mm cm Si SMART Si 50 110 54.7 98.8 60 130 47.8 97.8 70 155 49.1 96.4 80 175 63.8 94.4 90 200 72.2 92.6 Crystals 2021, 11, 90 5 of 10 Crystals 2021, 11, x FOR PEER REVIEW 5 of 11

Figure 3. Segmented images of wafers from cm Si (left) and SMART Si (right) wafers from different Figure 3. Segmented images of wafers from cm Si (left) and SMART Si (right) wafers from differ- entingot ingot heights. heights. Segmented Segmented grains grains are are colored colored due due to their to their area area size size (white: (white: largest largest grain, grain, logarithmic loga- rithmiccolor scale). color scale). 3.2. Dislocation Conﬁnement in Functional Defects Table 1. Monocrystalline area share at different ingot heights for the cm Si and SMART Si materials. Besides ingrowing grain boundaries causing the reduction of the monocrystalline area, dislocation clusters are a big issue reducing the material quality. The analyzed material is located in theIngot center Height of the ingot with a minimum distance Mono to the Area ingot /% edge of 90 mm. The material% growing from the mm contact between the cm two Si monocrystalline SMART seed plates Si on the cm Si half50 of the ingot shows110 strong recombination 54.7 active structures (see each 98.8 right side of the PL images60 in Figure4). The130 dislocations arising 47.8 from a small angular mismatch 97.8 between Crystals 2021, 11, x FOR PEER REVIEW 6 of 11 the seed70 plates, due to processing155 reasons, align 49.1 to dislocation loops and clusters96.4 elongating in lateral80 direction and thus175 limiting the material 63.8 quality with increasing 94.4 ingot height. 90 200 72.2 92.6

3.2. Dislocation Confinement in Functional Defects Besides ingrowing grain boundaries causing the reduction of the monocrystalline area, dislocation clusters are a big issue reducing the material quality. The analyzed material is located in the center of the ingot with a minimum distance to the ingot edge of 90 mm. The material growing from the contact between the two monocrystalline seed plates on the cm Si half of the ingot shows strong recombination active structures (see each right side of the PL images in Figure 4). The dislocations arising from a small angular mismatch between the seed plates, due to processing reasons, align to dislocation loops and clusters elongating in lateral direction and thus limiting the material quality with increasing ingot height. As expected using the SMART approach, the contact between the Si slices with the same orientation induces dislocation structures in the functional defects, as well. In contrast to the cm Si material, the dislocations are confined between the functional grain boundariesFigureFigure 4.4. PL forming image of themonocrystalline functional defect area area surrounded surrounded region (see by by Figure functional functional 4). defects defects on on as as cut cut wafers wafers at at 2828 mmmm ((leftleft)) andand 7070 mmmm ( (rightright)) ingot ingot height. height.

AsWith expected increasing using ingot the height, SMART line-like approach, recombination the contact active between structures the Si slices emerge with on the the samecm Si orientation half of the induces ingot with dislocation the origin structures close to in the the functional functional defect defects, region as well. (see In Figure contrast 4, toright). the cm These Si material, line-like the structures dislocations are aremost conﬁned probably between dislocations the functional on glide grain planes. boundaries Further formingmicroscopic the functional investigations defect are region needed (see to Figure specif4y). the type of defect. µPL maps of passivated samples enable a more detailed analysis of the functional defect region. The recombination active functional defects can be identified as dislocations loops surrounded by straight grain boundaries (see Figure 5). The straight grain boundaries were induced by the orientation difference between the Si plate and Si slices. EBSD measurements confirm

a 45° tilt grain boundary. The confined dislocation loops between the functional grain boundaries have no fixed position during the crystallization. Nevertheless, they stay relatively stable and do not generate further dislocations between the functional grain boundaries. Dislocation loops developing towards the functional grain boundaries during the crystal growth were observed to introduce dislocation generation in the neighboring monocrystalline area (see Figure 5). Two of the dislocation loops in the shown area, marked by black arrows, develop towards the right functional grain boundary between ingot height 28 mm and 32 mm. At the ingot height of 36 mm, the dislocation loops are no longer visible, but line-like dislocation structures are detected in the neighboring grain (right side). Hence, the movement and following interaction of the dislocation loops with the functional grain boundary induces dislocation generation in the neighboring grain.

Figure 5. µPL map of dislocation loops between functional grain boundaries.

The generation of line-like dislocation structures was only observed in the cm Si half of the crystallized ingot. The movement and subsequent interaction of the dislocation loops seem to happen only towards the cm Si side of the ingot. Whether this unidirectional

Crystals 2021, 11, x FOR PEER REVIEW 6 of 11

Crystals 2021, 11, 90 6 of 10 Figure 4. PL image of monocrystalline area surrounded by functional defects on as cut wafers at 28 mm (left) and 70 mm (right) ingot height.

WithWith increasing increasing ingot ingot height, height, line-like line-like recombination recombination active active structures structures emerge emerge on on the the cmcm SiSi halfhalf ofof thethe ingotingot withwith thethe originorigin closeclose toto thethe functionalfunctional defectdefect regionregion (see(see FigureFigure4 ,4, right).right). TheseThese line-likeline-like structuresstructures areare mostmost probably probably dislocations dislocations onon glide glide planes. planes. Further Further microscopicmicroscopic investigations investigations are are needed needed to to specify specif they the type type of defect.of defect.µPL µPL maps maps of passivated of passiv- samplesated samples enable enable a more a detailedmore detailed analysis analysis of the functionalof the functional defect region. defect Theregion. recombination The recom- activebination functional active functional defects can def be identifiedects can be as identified dislocations as loopsdislocations surrounded loops by surrounded straight grain by boundariesstraight grain (see boundaries Figure5). The (see straight Figure grain5). The boundaries straight grain were boundaries induced by were the orientation induced by differencethe orientation between difference the Si plate between and the Si slices. Si plat EBSDe and measurementsSi slices. EBSD confirmmeasurements a 45◦ tilt confirm grain boundary.a 45° tilt grain The confined boundary. dislocation The confined loops disl betweenocation the loops functional between grain the boundaries functional havegrain noboundaries fixed position have duringno fixed the position crystallization. during the Nevertheless, crystallization. they Nevertheless, stay relatively they stable stay and rel- doatively not generate stable and further do dislocationsnot generate between further the dislocations functional grainbetween boundaries. the functional Dislocation grain loopsboundaries. developing Dislocation towards loops the functional developing grain toward boundariess the functional during grain the crystal boundaries growth during were observedthe crystal to growth introduce were dislocation observed to generation introduce indislocation the neighboring generation monocrystalline in the neighboring area (seemonocrystalline Figure5). Two area of the(see dislocation Figure 5). loops Two inof thethe shown dislocation area, markedloops in by the black shown arrows, area, developmarked towardsby black thearrows, right develop functional towards grain boundarythe right functional between ingotgrain heightboundary 28 mm between and 32ingot mm. height At the 28 mm ingot and height 32 mm. of 36At mm,the ingot the dislocationheight of 36 loops mm, the are dislocation no longer visible,loops are but no line-likelonger visible, dislocation but line-like structures dislocation are detected stru inctures the neighboringare detected grain in the (right neighboring side). Hence, grain the(right movement side). Hence, and following the movement interaction and following of the dislocation interaction loops of the with dislocation the functional loops grain with boundarythe functional induces grain dislocation boundary generation induces dislocation in the neighboring generation grain. in the neighboring grain.

FigureFigure 5.5.µ µPLPL mapmap ofof dislocationdislocation loopsloops between between functional functional grain grain boundaries. boundaries.

TheThe generationgeneration ofof line-likeline-like dislocationdislocation structures structures was was only only observed observed in in the the cm cm Si Si half half ofof thethe crystallizedcrystallized ingot.ingot. TheThe movementmovement andand subsequentsubsequent interactioninteraction ofof thethe dislocationdislocation loopsloops seem seem to to happen happen only only towards towards the the cm cm Si Si side side ofof thethe ingot.ingot. WhetherWhether thisthis unidirectionalunidirectional movement occurs due to differences in the material or due to the crystallization process could not be clariﬁed so far. One conceivable reason regarding the material would be a different stress for the SMART Si and cm Si half caused by the non-optimal stress reduction in the cm Si part. Assuming dislocation clusters to grow perpendicular to the solid-liquid interface, a in this region of the crystal non-symmetric, convex solid-liquid interface, lowered towards the cm Si half of the ingot, would be a possible explanation regarding the crystallization process. As explained in Section 2.1, the solid-liquid interface is planar till slightly convex and symmetric as estimated by Eddy current measurements. However, this method is not suitable to detect small irregularities in the form of the solid-liquid interface, which could be decisive in this case with a more or less ﬂat solid-liquid interface. Further experiments investigating stress and the solid–liquid interface in a microscopic scale and enabling statistics due to a larger variety of samples are necessary.

3.3. Cellular Structures in Monocrystalline Area While the monocrystalline area surrounded by the functional defects shows a relatively uniform PL signal in the as cut condition promising a homogeneous monocrystalline material, recombination active cellular structures in the monocrystalline area of the lower ingot height are visible via µPL of passivated samples (see Figure6, top). These cellular Crystals 2021, 11, x FOR PEER REVIEW 7 of 11

movement occurs due to differences in the material or due to the crystallization process could not be clarified so far. One conceivable reason regarding the material would be a different stress for the SMART Si and cm Si half caused by the non-optimal stress reduction in the cm Si part. Assuming dislocation clusters to grow perpendicular to the solid- liquid interface, a in this region of the crystal non-symmetric, convex solid-liquid interface, lowered towards the cm Si half of the ingot, would be a possible explanation regarding the crystallization process. As explained in Section 2.1, the solid-liquid interface is planar till slightly convex and symmetric as estimated by Eddy current measurements. However, this method is not suitable to detect small irregularities in the form of the solid- liquid interface, which could be decisive in this case with a more or less flat solid-liquid interface. Further experiments investigating stress and the solid–liquid interface in a microscopic scale and enabling statistics due to a larger variety of samples are necessary.

3.3. Cellular Structures in Monocrystalline Area While the monocrystalline area surrounded by the functional defects shows a rela- Crystals 2021, 11, 90 tively uniform PL signal in the as cut condition promising a homogeneous monocrystal-7 of 10 line material, recombination active cellular structures in the monocrystalline area of the lower ingot height are visible via µPL of passivated samples (see Figure 6, top). These cellularstructures structures are less recombination are less recombination active than active the dislocation than the dislocation loops in the loops functional in the defects, func- visibletional defects, at the upper visible edge at the of upper this µPL edge map, of this but spreadµPL map, all overbut spread the monocrystalline all over the mono- area crystallinein the lower area ingot in the part. lower With ingot increasing part. With ingot increasing height, theseingot cellularheight, these patterns cellular increase pat- ternsin diameter, increase but in diameter, structures but are structures less recognizable are less andrecognizable at an ingot and height at an ofingot 100 height mm not of visible100 mm anymore. not visible Whether anymore. the Whether underlying the defect underlying disappears defect or disappears the recombination or the recombi- activity nationis reduced activity is not is yetreduce clariﬁed.d is not yet clarified.

Figure 6. µPLµPL map of functional defects surrounding mono area at 28 mm ingot height ( toptop),), grain grain orientation mapping mapping of of the the direction direction perpendicular perpendicular to tothe the wafer wafer surface surface at 32 at mm 32 mmingot ingot height height (middle) and detected etchetch pitspits (EPD)(EPD) afterafter Cr-lessCr-less etchetch atat 2828 mmmm ingotingot heightheight ((bottombottom).).

EBSD measurements were performed in this area to investigate the underlying defect for the cellular structures. The induced 4545°◦ titiltlt grain boundary could be verifiedverified andand thethe material grown grown above above the the monocrystalline monocrystalline seed seed plate plate turned turned out out to tobe bereally really monocrystal- monocrys- linetalline (see (see Figure Figure 6, middle).6, middle). No cellular No cellular struct structuresures could could be detected be detected with the with EBSD the system EBSD ◦ withsystem an withangular an angular resolution resolution of 0.3°. of 0.3 . In microscope images of polished and subsequently Cr-less etched samples at the same position, etch pits with some cellular pattern could be detected (see Figure6, bottom). Therefore, these cellular recombination active clusters are regarded at dislocation clusters with a distinct cellular pattern in conjunction with a lower recombination activity in comparison to the aforementioned dislocation tangles. The dislocation density detected via etch pits in the monocrystalline area, as exemplarily shown in Figure6, bottom, is 7 × 103 cm−2, which is up to two magnitudes lower than in other cast-mono Si reported in literature with values from 1 × 104 cm−2 to 6 × 105 cm−2 [7,18]. The comparison of the PL signal and the detected etch pits in a magnified region of the same area (Figure6 , right) shows that etch pits seem to align and these lines of higher dislocation density are roughly corresponding to the low PL signal regions. This alignment of dislocations to a cellular pattern was already observed by Oliveira et al. [19,20] and Krause et al. [20]. The size of the cellular patterns is comparable to that reported in the literature, albeit the density of etch pits is lower in our case. Oliveira et al. [19] and Krause et al. [20] observed the cellular pattern perpendicular to the growth direction, the patterns shown here were observed parallel to the growth direction. In combination, this means that the three-dimensional geometry of these patterns is a bubble, rather than a column. This explains the observation that the aligned etch pits do not exactly fit the low PL signal regions because the measurement was performed on parallel wafers and thus the specific geometry of the dislocation structure might have changed between those two. However, Crystals 2021, 11, 90 8 of 10

the cellular pattern with low PL signal could be attributed to a cellular dislocation pattern in the monocrystalline area.

3.4. Summary The SMART approach worked well for the blocking of parasitic grains. The monocrystalline area was kept almost constant over 90% for the whole ingot height. This enables a high material quality and potential for solar cells due to nearly no recombination active grain boundaries and the possibility to use the effective texturing process for monocrystalline material. As expected, dislocations structures built between the functional grain boundaries. The confinement of these dislocations was almost successful for most parts. Dislocation structures developing towards functional grain boundaries induced new line-like defects in the mono area next to the functional defect. As a possible further improvement, a more stable phase boundary would decelerate the lateral movement of the dislocation structures and thus inhibit the nucleation of new dislocations in the neighboring grain. Another possibility could be the use of a second functional grain boundary surrounding the functional defect region. This could impede the propagation of the dislocation structures as they are additionally blocked by the second large angle grain boundary. Moreover, parasitic grains nucleated at the crucible wall could be blocked even more effectively. Although dislocations could be confined in the functional defect region, a dislocation density of 7 × 103 cm−2 was detected in the monocrystalline area. The dislocations aligned in a cellular pattern reduce the lifetime and thus are visible as low PL signal regions for low ingot heights. Dislocations multiply and rearrange due to thermo-mechanical stress. Therefore, a modification of the crystallization process in combination with SMART Si approach is needed for a further reduction of the dislocation density and the resulting recombination activity.

4. Evaluation of Material Quality and Solar Cell Efficiency To compare the material quality, high-performance multicrystalline (hp mc) Si material with fine crystal grains crystallized with a similar set-up and thermal recipe was processed in the same high-temperature batch as the cm Si and SMART Si material discussed in Crystals 2021, 11, x FOR PEER REVIEW 9 of 11 this paper. The materials crystallized with cm Si and SMART approach feature more homogeneous and higher average effective lifetimes than the hp mc Si material (see Figure7 ). However, it should be noted that the hp mc Si material features a lower resistivity partlypartly responsibleresponsible forfor increasedincreased recombination.recombination. Nevertheless,Nevertheless, thisthis comparisoncomparison showsshows thethe efficientefficient suppressionsuppression ofof stronglystrongly recombinationrecombination activeactive structuralstructural crystalcrystal defectsdefects byby thethe cmcm SiSi andand especiallyespecially thethe SMARTSMART approach.approach.

FigureFigure 7.7. LifetimeLifetime imagesimages of of bulk bulk lifetime lifetime samples samples after after a processa process chain chain typical typical for for TOPCon TOPCon process pro- of hpcess mc of Si, hp cm mc Si, Si, and cm SMARTSi, and SMART Si wafers Si at wafers an illumination at an illumination of 1/20 suns of 1/20 and suns an ingot and heightan ingot of 130height mm . Theof 130 hp mm. mc SiThe material hp mc wasSi material crystallized was crystallized in a similar in experiment a similar experiment but features but a features lower resistivity a lower of 0.5resistivityΩcm while of 0.5 the Ωcm while Si and the SMART cm Si Siand material SMART feature Si material a resistivity feature of a 0.9 resistivityΩcm. of 0.9 Ωcm.

Obviously, all three materials show a difference in recombination active structural defects reducing the lifetime as expected. While the amount of grain boundaries and dislocations is strongly decreased from hp mc Si to cm Si, the SMART Si material shows even less. A large fraction of the lifetime deterioration in the cm Si and SMART Si is not due to the material but rather process-related artefacts like the area of reduced lifetime at the lower (cm Si), respectively upper edge (SMART Si). Highest effective lifetime values at 1/20 suns are around 1200 µs for all three materials in the inner grain and mono area. The influence of the structural defects is very pronounced at the depicted measurement at 1/20 suns, which illustrates the injection condition of a final solar cell at 1 sun at the maximum power point. This minimized lifetime reduction at working conditions is a further advantage of the SMART Si material. Due to the promising material quality of the SMART Si, this material was used as the base material of a TOPCon solar cell [15]. A sample from the previously shown ingot height of 130 mm and a resistivity of 0.9 Ωcm obtained the certified record efficiency of 23.3% (Voc = 699 mV, Jsc = 40.9 mA/cm², FF = 81.5%) [21]. The sample was textured with random pyramids and was processed with a double layer anti-reflection coating.

5. Conclusions The SMART approach was successfully transferred to a G2-sized silicon ingot enabling better material quality than the standard cm Si. The blocking of parasitic grains was effective in such a way that the monocrystalline area could be held higher than 90% all over the ingot height, which is a big improvement compared to the cm Si with a mono area declining to 50%. In addition, an improved control of the other crystal defect, dislocations, was reported. The confinement of dislocations in the functional defects was relatively stable but has still room for future improvement, where different compositions of the functional defects could be evaluated. A recombination active cellular structure was observed in the monocrystalline area which could be correlated with the dislocation distribution even if the dislocation density is low. It could not be clarified yet if these cellular structures limit the material quality for solar cells. Lifetime measurements after a typical high temperature process chain would be useful for this evaluation. The comparison of the material quality of SMART Si, cm Si, and hp mc Si at 130 mm showed a clear benefit of the SMART Si material due to its high homogeneity and low defect density. This SMART Si material was processed as a TOPCon solar cell and obtained a certified record efficiency of 23.3%.

Crystals 2021, 11, 90 9 of 10

Obviously, all three materials show a difference in recombination active structural defects reducing the lifetime as expected. While the amount of grain boundaries and dislocations is strongly decreased from hp mc Si to cm Si, the SMART Si material shows even less. A large fraction of the lifetime deterioration in the cm Si and SMART Si is not due to the material but rather process-related artefacts like the area of reduced lifetime at the lower (cm Si), respectively upper edge (SMART Si). Highest effective lifetime values at 1/20 suns are around 1200 µs for all three materials in the inner grain and mono area. The influence of the structural defects is very pronounced at the depicted measurement at 1/20 suns, which illustrates the injection condition of a final solar cell at 1 sun at the maximum power point. This minimized lifetime reduction at working conditions is a further advantage of the SMART Si material. Due to the promising material quality of the SMART Si, this material was used as the base material of a TOPCon solar cell [15]. A sample from the previously shown ingot height of 130 mm and a resistivity of 0.9 Ωcm obtained the certified record efficiency of 2 23.3% (Voc = 699 mV, Jsc = 40.9 mA/cm , FF = 81.5%) [21]. The sample was textured with random pyramids and was processed with a double layer anti-reflection coating.

5. Conclusions The SMART approach was successfully transferred to a G2-sized silicon ingot enabling better material quality than the standard cm Si. The blocking of parasitic grains was effective in such a way that the monocrystalline area could be held higher than 90% all over the ingot height, which is a big improvement compared to the cm Si with a mono area declining to 50%. In addition, an improved control of the other crystal defect, dislocations, was reported. The confinement of dislocations in the functional defects was relatively stable but has still room for future improvement, where different compositions of the functional defects could be evaluated. A recombination active cellular structure was observed in the monocrystalline area which could be correlated with the dislocation distribution even if the dislocation density is low. It could not be clarified yet if these cellular structures limit the material quality for solar cells. Lifetime measurements after a typical high temperature process chain would be useful for this evaluation. The comparison of the material quality of SMART Si, cm Si, and hp mc Si at 130 mm showed a clear benefit of the SMART Si material due to its high homogeneity and low defect density. This SMART Si material was processed as a TOPCon solar cell and obtained a certified record efficiency of 23.3%.

Author Contributions: Conceptualization, P.K., Y.H., S.R. and N.U.; software, T.T.; investigation, P.K., T.T. and F.S.; writing—original draft preparation, P.K.; writing—review and editing, F.S., T.T., S.R. and N.U. All authors have read and agreed to the published version of the manuscript. Funding: This work was partially supported by the German Federal Ministry for the Economy and Energy, grant number 0325491 and 0324034, and Japan Science and Technology Agency, grant number JPMJCR17J1. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: The authors would like to thank Wacker Polysilicon for the silicon materials and express their gratitude to the crystallization, cell processing and characterization teams at Fraunhofer ISE, Fraunhofer THM, and Nagoya University for their support and input in many valuable discussions. Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results. Crystals 2021, 11, 90 10 of 10

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