2014 CE05 CrySaLID DocSci.pdf

7/24/2019 2014 CE05 CrySaLID DocSci.pdf

1/29

CrySaLID

Grands dfis socitaux : Energie propre, sre et efficace / Clean, Secure and efficient Energy2014

1

CrySaLIDCrystallisation of seeded Silicon, impact of Light Impurities and Defects

Cristallisation du Silicium partir de germes, effet des Impurets Lgres et desDfauts

Table of content

1. Abstract in English: ....................................................................................................................................... 2

2. Context and positioning of the project....................................................................................................... 2

2.1 Introduction to the subject: society and economical challenge ............................................................... 2

2.2 State of the art and industrial challenge ................................................................................................... 3

2.4 Scientific challenge and context .............................................................................................................. 6

2.6 Positioning of the project: ..................................................................................................................... 10

3. Scientific and technical program and project organization ......................................................................... 11

3.1 Objectives and scientific breakthrough needed ..................................................................................... 11

3.2 Expected results ..................................................................................................................................... 12

3.3 Description of the scientific program and task organization ................................................................. 13

3.4 Detailed program and task description .................................................................................................. 13

3.5 Gantt chart and table of milestones and deliverables ............................................................................ 20

3.6 Consortium ............................................................................................................................................ 22

3.7 Partner competencies ............................................................................................................................. 22

3.8 Complementarity of the partners ........................................................................................................... 25

3.9 Staff involved in the project .................................................................................................................. 25

3.10 Scientific and technical justification of the funding requested for each partner.................................. 27

4. Strategy for valorisation, protection and exploitation of results.................................................................. 28

4.1 Scientific communication ...................................................................................................................... 28

4.2 Valorisation of expected results ............................................................................................................ 28

4.3 Positioning of the project in the industry strategy ................................................................................. 294.4 Contribution to scientific and technical culture ..................................................................................... 29

4.5 Other impact .......................................................................................................................................... 29


2/29

CrySaLID


2

1. Abstract in English:

The development of the photovoltaic (PV) sector requires significant progress in performance andreductions in cost. For crystalline Si solar cells in particular, it is well-known that the grain structure,impurities and defects left after the crystallisation step, have a major impact on the final PV properties.

However, the involved fundamental mechanisms are still not fully understood which hinder an efficient andreproducible control of the crystallisation processes.

In the last few years, research efforts concerning mono-like silicon crystal growth which is an interestingcompromise between single crystalline and multicrystalline silicon both used for PV applications wereresumed. Indeed, single crystals yield high PV efficiency but their fabrication requires costly crystal growthprocesses. Oppositely, multicrystalline silicon results in lower PV efficiency but can be fabricated usingcheaper solidification processes close to metallurgical casting processes. Knowing this, the objective is toproduce mono-like crystals i.e. with as few grains as possible using casting or directional solidificationprocesses being initiated on seeds or by selecting the crystalline orientation by dendrite growth. Forsuccessful mono-like growth, the nucleation of parasitic grains and twins during crystallisation must beminimized and the subsequent growth controlled to favour the desired crystalline orientation. In addition,impurities and defects especially dislocations have major detrimental effects and are closely linked to the

grain growth from seeds.The scientific objectives of the CrySaLID project are to deepen the understanding of the mechanisms of

crystallisation of silicon grown from seed as parasitic grains nucleation, grain competition and twins withpure silicon or silicon containing light impurities. Moreover, the characterisation of defects and lightimpurities linked to the crystallisation and grain structure will be addressed to understand the generation ofstructural defects and the impact of impurities. The correlation with PV property measurement will also beconducted. The first technological objective is to define processing conditions for improved grain structure,impurity segregation and defect control for a growth from a selected crystalline orientation. The second aimsat developing a 3D and predictive Si growth simulation tool at industrial scale. Concerning the companyEMIX which is part of the consortium, one major objective within the project is to fabricate silicon withlower C levels suitable for subsequent mono-like crystal growth. The objectives of the CrySaLID project willbe achieved by: i) complementary investigations of Si crystallisation mechanisms (grain growth andcompetition) by in situX-ray imaging of its growth, Kyropoulos and mono-like crystal growth, bi-crystalgrowth investigation, ii) structural defects, impurity and PV characterisation, iii) multi-scale modelling(thermodynamic environment, impurity segregation, phase field modelling of grain boundary grooves and3D grain structure industrial scale modelling), iv) Feedback to the industrial process.

2. Context and positioning of the project

2.1 Introduction to the subject: society and economical challenge

Global warning and the exhaustion of fossil energy sources are well known major issues that have

become problems of the present and no longer belong only to the future. They have direct geopoliticalimpacts by creating regional instabilities and economic issues. Indeed, the urgency of stopping climatechange was pointed again by the IPCC (Intergovernmental Panel on Climate Change in English and GIEC:

Groupe dExperts Intergouvernemental sur lEvolution du Climat en franais) in the first release of theirreport in 20131and in the most recent report published in the last few weeks 2. Moreover, according to the

France-Europe 2020 agenda for research, technology, transfer and innovation, it is estimated that the cost ofclimate and environmental change will be a major burden for European countries, and in particular forFrance, in the coming years, if no suitable adaptation or remediation policies are instituted in time. To have a

successful energy transition, a major joint-attack investment will be needed including reduction ofconsumption, development of renewable energies and improvements in energy efficiency. Additionally, it is

foreseen that the associated market has the potential to create new and highly qualified jobs within theEuropean Union and in France in particular. These ambitious objectives will impact on all economic andsocial sectors. One of these sectors is the building industry which is the most energy-consuming sector aswell as local by nature. Several renewable energies are proposed and constitute the energetic mix, all of them

1First Release of the 2014 IPCC report in September 2013

2Climate Change 2014: Impacts, Adaptation, and Vulnerability, Summary for policymakers, 31 March 2014.


3/29

CrySaLID


3

showing specific benefits but also drawbacks. Of the alternative energy resources available for exploitation,solar energy is one that stands out. According to the last survey from the American consultant groupSolarbuzz, an important expansion of the solar PV demand is expected in 2014 with a global market of about49 GW compared to 36 GW in 2013. In this context, to be able to sustain competition with fossil fuels 3andother green energies, solar photovoltaic electricity requires significant progress regarding improvement of

performance and, reduction of costs as well as increase in reliability. Efforts are needed on different materialtechnologies and for integration. Among the materials used to produce solar cells, silicon still dominates themarket with a share of about 90%. However, there is still a lot to do to improve the present crystalline silicontechnology in order for it to face the challenges of the coming decades and for the French and European PVIndustry to remain competitive.

Whithin this context, the CrySaLID project fits perfectly into the general alternative energy issue thatconcerns several challenges identified in the strategic France Europe 2020 document. The project isparticularly relevant to the challenge Clean, Secure and Efficient Energy in the 2014 ANR call. As statedin the call, the Clean, Secure and Efficient Energy challenge aims at identifying or improving technologiesthat are part of the energy mix. Solar energy is one of the most reliable sources of energy and the CrySaLIDproject aims at reinforcing and deepening the knowledge of the fabrication of crystalline silicon for PVapplications to accompany and prepare process improvements with the general objective of increasing theefficiency/cost ratio. The targeted axis within the Clean, Secure and Efficient Energy challenge isInnovative concepts for capture and transformation of renewable energies. In this axis, three main modesof exploitation of solar energy have been identified, among which the development of direct production ofelectricity is fundamental. The CrySaLID project is intended to contribute to the competitiveness of the partof the PV sector focusing on crystalline silicon material. More precisely, the project focuses oncrystallisation of silicon with selected orientation of the initial crystals. In addition, a particular focus will beput on light impurities and defects in this material. Indeed, it was shown in the previous section that there is

still a long way to go to understand essential phenomena involved during the crystallisation of silicon andthat upstream research is sorely needed.

The CrySaLID project is a private/public collaborative project that will be profitable for both public andprivate partners. On the one hand, the project will allow public partners to improve the understanding of

fundamental mechanisms involved by exploring research issues, driven by the industry concerns andaddressed from new points of view. On the other hand, the project will provide the private partner withaccess to high level public research to improve innovation. Lastly, 2014 is the International Year ofCrystallography, which would be perfect timing for the beginning of our project on the crystallisation ofsilicon if granted.

2.2 State of the art and industrial challenge

Silicon technology has clear advantages over other technologies; advantages such as the good availabilityof basic material (silica), the relatively high energy conversion efficiency and the good stability of the solarcells resulting in an expected lifetime of about 30 years. Moreover, silicon has a well-established technologyrepresenting about 90% of the world photovoltaic market in 2010

4. Silicon-based technologies are mainly

single-crystal silicon (c-Si) and cast multicrystalline silicon (mc-Si).

The continuing development of crystalline silicon keeps the material competitive and can delay or evenmaybe prevent its replacement by so-called second and third generation technologies for mass production ofenergy. In any case, it is expected that silicon-based materials will maintain a strong market share in theforeseeable future, hence justifying projects aiming at improving this technology. However, the growth of theSi-PV market currently faces major limitations and challenges: the availability of a solar grade silicon sourceat a reasonable cost and the demand for low cost solar panels with increasing PV efficiency. In order to solvethe procurement issue, alternatives to the costly processes used by the microelectronics industry for thepurification of silicon are proposed. For each new source material, a number of issues that had been solvedfor microelectronic grade silicon have to be carefully considered again. Moreover, in a classical industrialdirectional solidification furnace, there are sources of contamination, in particular of carbon. Yet, in amaterial with a much higher impurity level, nucleation of parasitic grains, segregation along grain boundaries

3Basic Energy Science Advisory Comittee, Sci. for Energy Tech.: Strengthening the link between Basic Research and Industry,

U.S.D. of Energy, 2010.4World Energy Outlook 2011. International Energy Agency, ISBN 978-92-64-12413-4 (2011) .


4/29

CrySaLID


4

and extended defects (dislocations, twins) can have catastrophic consequences for the cell conversionefficiency. Indeed, the impurities accumulated at the grain boundaries or at defects may be active and maytrap electron-hole pairs in their vicinity. The problem is especially crucial for the process developed by thecompany EMIX, which is based on an original ingot growth technique using continuous pulling from a coldcrucible. Moreover, EMIX recently diversified their activities to provide source material for single crystal

growth as part of the FerroAtlntica group. This evolution raises key issues concerning impuritycontamination and its consequences on mono-like growth.

Several solidification processes exist to make crystalline silicon for PV applications: Czochralski, casting,cold crucibles, ribbons, wafer moulding. The use of directional solidification favours the growth of acolumnar structure with rather large grains. Indeed, directional solidification allows a better control of grainstructure than classical casting (pouring of molten silicon into a mould) and also incidentally results infurther purification so that most crystallisation of mc-Si is currently achieved by directional solidification.The main directional solidification techniques used for mc-Si are Bridgman, HEM (Heat Exchanger Method)and EMC (Electro-Magnetic Casting). The advantage of directional solidification methods is that they arequicker and thus cheaper in energy than the Czochralski method used for the growth of silicon singlecrystals.

In the Bridgman technique, solidification is achieved by the movement of the molten silicon placed in acrucible inside a temperature gradient, the heat being extracted at the walls of the ingot. This technique isused by Deutsche Solar GmbH (Solar World group) for example. A similar technique named gradientfreeze consists in imposing an initial temperature gradient and then cooling the heaters with the samecooling rate to provoke solidification without moving the charge while keeping a constant temperaturegradient. In the HEM technique, heat is extracted by modifying the heat exchange conditions at the bottom ofthe crucible. Using this technique, the isotherms are flatter than with the Bridgman technique so that thethermo-mechanical stresses are minimised. The solidification front is also flatter, which favours the

crystallisation of homogeneous and large columnar grain structures. This technique is used by severalcompanies, e.g. Photowatt (a subsidiary of EDF ENR), BP SOLAR.

Figure 1: EMC (Electro-Magnetic Casting) process developed at EMIX.

In the original EMC process, solidification takes place in an electromagnetic cold crucible (figure 1). Thistechnique was initially developed in a CNRS laboratory and is industrially exploited by EMIX. Themanufacturing of ingots by Continuous Casting in Cold Crucible (4C) presents several advantages comparedto conventional techniques. Firstly, the main feature is that it is a continuous casting process. In practice, thecasting time may last as long as the segregation has not saturated the silicon with impurities. Secondly, the

casting speed is in the range of 1 mm/min to 2 mm/min, meaning that the productivity is five to ten timeshigher than with a conventional Bridgman device. Moreover, due to the electromagnetic strength, there is nocontact between the silicon bath and the crucible, thus avoiding any contamination with the material of the


5/29

CrySaLID


5

crucible. In parallel, other innovative techniques as wafer moulding technique and ribbon growth techniquesare developed in particular to avoid the wafer cutting phase which is costly and time consuming.

Multicrystalline material, either in bulk or in sheets, is less expensive than its single-crystallinecounterpart but thegrain boundaries inherent in multicrystalline material result in somewhat reduced solarcell performances

5. Although the directional solidification process favours the growth of columnar grains

required for PV applications some defective regions of small grains named grits are observed6. Theseregions need to be removed before cell fabrication or, in the worst case; the wafer has to be rejected after ithas been cut which is very costly. The only alternative is to better control the solidification parameters toavoid the formation of small grain regions. Moreover, it is worth noticing that twins are particularly frequentduring silicon solidification as can be seen in Figure 2.a (zebra regions inside grains). In the EMC process,the unavoidable existence of strong radial temperature gradients leads to an inhomogeneous solidificationgrain structure (Figure 2.b) with a high density of extended defects.

Figure 2: Typical grain structure of a wafer a) from classical directional solidification process (15.6 x

15.6 cm2)

7, b) from the EMIX EMC (Electro-Magnetic Casting) process (15 x 15 cm

2).

Moreover, the interplay between impurities and grain boundaries is complex. On the one hand, grain

boundaries can have a positive effect by purifying the grains themselves of impurities thanks to segregationand trapping. On the other hand, the impurities accumulated at the grain boundaries are still active and maytrap electron-hole pairs in their vicinity

8. In any case, it is necessary to optimize the grain size in mc-Si to

optimise its properties as a function of the purity of the feed source.

Figure 3: As-cut scanned wafers for multi, mono-like and single crystalline silicon (A. Jouini et al.9

).

Recently, interest in obtaining single crystalline or mono-like crystalline silicon (few grains) ingots using

classical directional solidification processes or cast-processes9,10,11

has been reactivated, with the advantage

5Crystal Growth of Si Solar Cells, Springer Berlin Heidelberg, Berlin, 2009, Editors: K. Nakajima, N. Usami.

6N. Mangelinck-Nol et al., Trans. Indian, Inst. Met., 60, No 2-3 (2007) 93-97.7C.W. Lan et al., Journal of Crystal Growth, 360 (2012) 68-75.8R. Shimokawa et al., J. Applied Physics, 59 (1985) 2571-2576.9A. Jouini et al.,Progress in Photovoltaics: Research and Applications, 20 (2012) 735-746.

a) b)


6/29

CrySaLID


6

of increasing the PV efficiency compared to mc-Si while reducing the costs compared to the usual singlecrystalline fabrication processes such as the Czochralski growth method. As an example, ECM technology ispresently developing furnaces for mono-like crystal growth from seeds in directional furnaces.

When fabricating mono-like silicon by directional solidification for PV applications, the main challengeis to be able to grow large mono-like ingots, that is, on an industrial scale. The consequence is that multiple

seeds must be used in the industrial configuration as large seeds are unavailable and/or very costly and alsothat the solidification must be perfectly mastered to grow mono-like crystal with as few parasitic grains aspossible (Figure 3). This accompanies the main scientific issues that will be discussed in the followingsection.

In fact, the distinction between single and multicrystalline silicon may disappear in the future, and thedistinction will be better made based on the solidification/growth process used: cast or grown silicon. In anycase, the main scientific issue consisting in controlling the grain structure and defect formation is shared byall crystalline silicon technologies.

Finally, several silicon solidification processes exist and are either industrialised or in the process of beingso: casting, cold crucibles, ribbons, wafer moulding, mono-like processes. They all end with a different grainstructures (size and arrangement of grains) that have a direct impact on the PV properties.

2.4 Scientific challenge and contextSolar cell research addresses physical metallurgy issues such as surfaces, grain boundaries, dislocations,

precipitates, micro-cracks and other defects inherited from the solidification and cooling stages that canseverely affect the solar-cell efficiency. As seen above in the field of crystalline silicon application,multicrystalline (mc) material, either in bulk or in sheets, is less expensive than its single-crystallinecounterpart, but the grain structure inherent to multicrystalline material, and associated features (grainboundaries, twins, texture, defects and segregation) result in somewhat reduced solar cell performancesbecause they are usually recombination centers for the light generated electrons and holes

12. The highest

efficiency reported for commercial c-Si solar cells is 25.0%, whereas the best efficiency reported forcommercial mc-Si solar cells is currently 20.4%

13. It was stressed in a recent US strategic report on science

for energy technology3 that a full description of the material, the trapping mechanism, and the impact of

contamination is still lacking; and that the impact of grain boundaries on performance and reliability is stillnot fully understood. From a fundamental point of view, it implies that a better understanding of the physics,chemistry, and stability of crystalline defects and grain boundaries in crystalline Si is needed. As stated byNakajima et al.

11, the knowledge of the growth mechanism as well as PV properties of various kinds of

grains is essential in order to appropriately design and produce mc-Si to improve the PV conversionefficiency.

In addition, mono-like crystalline growth requires either (a) the formation of a dendrite selecting anorientation at the bottom of the crucible11 or (b) the use of seed crystals. The dendrite casting method14induces the growth of initial dendrites and 3 grain neighbours. However, the control of undercooling is notso easy when applying the method to commercial plants, which limits the application at this stage. Thesimplest way to control crystal structure is thus to use seed pavements with given orientations, and the use ofsingle crystalline seeds has become popular in recent years for the production of the so-called mono-like

ingots10,15

. Nonetheless, in this material, one of the key issues consists in controlling the grain growth inorder to avoid/minimise the nucleation and development of unwanted or parasitic additional grains up to theend of the ingot in order to avoid the destruction of the initial grain structure and to obtain the PV propertiesrequired (Figure 3). Trempa et al.

10have discussed this key issue relying on twin formation mechanisms on

{111} facets (Figure 4).

10M. Trempa et al.,Journal of Crystal Growth, 351 (2012) 131-140.11K. Nakajima et al., Conference Record of the 2006 IEEE 4th World Conference on Photovoltaic Energy Conversion, Vols 1 and 2,(2006) 964-967.12D. Sarti et al.,Solar Energy Materials and Solar Cells, 72 (2002) 27-40.

13M.A. Green et al., Solar cell efficiency tables (version 40). Progress in Photovoltaics: Research and Applications, 2012. 20(5) 606-614.14K. Nakajima et al., Journal of Crystal Growth 344 (2012) 6-11.15A. Jouini et al,.Progress in Photovoltaics: Research and Applications, 20 (2012) 735-746.


7/29

CrySaLID


7

Nevertheless, how grains are nucleated and selected, as well as the development of grain boundaries,during crystal growth remain unclear. Voigt et al.

16showed that the grain orientation could be described by

coincidence orientations showing again the importance of twinning mechanisms in the process of theformation of the grain structure. More recently, Wong et al.

17showed also that even though the percentage of

non-coherent grain boundaries was high in their initial stage, as the growth proceeded, more twin appeared

which was explained by the minimization of interfacial energy, as well as twin nucleation/growth from {111}facets.

a) b) c)

Figure 4: a) Schematic drawing, Scan images revealing different grain orientations of vertical cuts (parallel to the

growth direction) of crystals grown with different axial seed orientations b) , c) (Trempa et al.10

).

Following nucleation, grain growth kinetics plays a major role in the competition between grains that can

lead to very different grain structures. Grain kinetics is dependent on the global process solidificationparameters (temperature gradient, solidification or cooling rate, composition of impurities)and on the localparameters (solute accumulation, local temperature field, undercooling at the growth front18) and the relativecrystalline orientation

19,20. The crystalline orientation of the grains induces different growth kinetics that play

a role in the competition between grains and thus on their final arrangement. As a second step, the relative

arrangement of the grains leads to different kinds of grain boundaries19

and to different PV properties. Indeedgrain boundaries could lead to different mechanical and electrical characteristics according to their

orientations. In particular, the important 3 grain boundaries have a larger mechanical resistance when cutand facilitate ultra-thin wafer slicing. Moreover, they are mainly electrically inactive even in the presence ofimpurities. Additionally, the dislocation density observed is much lower in grains containing twins or withspecific orientations.

In 2012, Lan et al.7released very interesting work on control of the growth front (flat or slightly convex)as an important step to avoid grain growth from the crucible wall and equiaxed grains near the end of thegrowth on an industrial scale. To further control the initial grain growth, both self-seeded and seeded growthswere considered, and the results were promising. However the authors concluded that, although significantprogress has been made in the growth of high-quality mc-Si by grain control, further improvement of thequality and uniformity is required to reduce the performance gap with single crystals. The authors furtherconcluded that the selection of grain orientation and boundaries having better resistance to defect and sub-grain formation could be important and that it is still a challenge for mass production crystal growers.

Although it was shown that twins in themselves have no obvious direct impact on the photovoltaicproperties, successive twinning can modify the final grain structure after solidification and has an effect onthe distribution of the crystalline orientations of the grains in the ingot

21,22. As a consequence, twinning is

frequent and an essential feature of the solidification of Si ingots. Moreover, twinning is a major mechanism

for the formation of parasitic grains in the case of mono-like growth.In addition, the nucleation of grains can happen either on the walls or on precipitates or impurities in the

molten alloy. The occurrence of nucleation is also time-dependent because precipitates can form after some

16A. Voigt et al., 14thEuropean Photovoltaic Solar Energy Conference, Barcelona, Spain, 31june-4july 1997 774-777.17Wong et al., Journal of Crystal Growth 387 (2014) 10-15.18K. Fujiwara et al., J. Crystal Growth 266 (2004) 441-448.

19A. Tandjaoui et al., Journal of Crystal Growth, 377 (2013) 203-211.20T. Duffar et al., C. R. Ac. Sci. Ser. Physique, 14 (2013) 185191.21A. Tandjaoui et al., C.R. Ac. Sci. Ser. Physique, 14 (2013) 141-148.22T. Duffar, Recent Res. Devel. Crystal Growth 5 (2010) 61-111


8/29

CrySaLID


8

solidification by a segregation phenomenon23. It is also necessary todetermine the active precipitates onwhich silicon grains can nucleate and then, their nucleation efficiency

24.

In fact, impurity contamination, segregation and precipitation must be studied in details as impurities notonly interact with the development of the grain structure but interact also with dislocations

25and subgrain

boundaries and thus have a direct impact on the PV properties. Impurities in general, and C and O in

particular, are inherent to directional solidification processes used to fabricate silicon for PV applications.Indeed, the furnace environment (heaters, graphite plate) is often responsible for most of the Ccontamination. In addition, impurities can be found in the starting feedstock used for PV applications. Eventhough they are unavoidable, impurities can by nature drastically reduce the PV efficiency but also modifythe grain structure by favouring grain nucleation and possibly twin nucleation. Moreover, segregation due tothe segregation coefficient of impurities can modify the solid-liquid interface creating additional grainstructure distortion and structural defects. This issue is particularly urgent and important for development ofthe EMIX process as they reoriented their activities in the last few years to provide silicon feedstock formono-like growth.

Another main issue is the formation of structural defects such as grain boundaries, dislocations orstacking faults that are well-known to decrease crystalline solar cell efficiencies

26,27,2829. Indeed, dislocations

can act as recombination-active sites thus reducing PV efficiency. Therefore, a substantial improvementwould be to be able to control and reduce their formation. Grain boundaries in particular were found toseverely impact the minority carrier lifetime when metallic impurities are present at the interface in most

cases30,31

. Moreover, although, defect engineering by temperature annealing and gettering can be used as apowerful tool for performance improvement of crystalline silicon for solar cell, it is not sufficient to avoidcompletely the detrimental effect of dislocations32, and dislocations remain one of the most importantefficiency-limiting defects in silicon for PV applications. All the mechanisms discussed up to now andrelated to grain growth in silicon (impurities, precipitates, grain boundary and grain orientation, sub-

grains between seeds) play an important role in the generation and mobility of dislocations; see forexample

33,34.

Apart from the well-known generation of dislocations from thermo-mechanical stress in the cooling of thesolid, at least two other mechanisms still not perfectly understood have been pointed out in the literature andtake place during solidification35. One mechanism for the formation of dislocations is the stress generated by

the solidification of remaining trapped liquid (the silicon liquid density being higher that its solidcounterpart)36. This phenomenon can happen at triple points in mc-silicon but also at sub-grain boundaries

between seeds during the growth of mono-like silicon. Indeed, multiple seeds are needed in order to growlarge crystals. Silicon grows from the seed with approximately the same crystalline orientation. However, theslight misorientation between seeds is sufficient to create additional undesirable defects (sub-grain

boundaries, impurity segregation and dislocations) that can extend over the whole ingot during growth37

.Indeed, grains and sub-grain boundaries are known to be responsible for the development of dislocations and

later of clusters. In the work of Ryningen et al.38

, it was shown that the majority of the dislocations found insilicon ingots are nucleated at grain boundaries and it was postulated that it may happen, at the solid-liquid

interface during crystal growth, some of the dislocations continuing to grow together with the interface.

Moreover, in the work of Kutsukake et al.38

, the generation of dislocations is associated with a twin boundary

23M. Beaudhuin et al.,Materials & Chemistry and Physics 133 (2012) 284-288.24L. Sylla et al., 19thEuropean PV Solar Energy Conference and Exhibition, Paris, 7-11 Juin 2004, 3775-3778.25I. Prichaud, Solar Energy Materials & Solar Cells 72 (2002) 315-326.26J. Chen et al.,Journal of Applied Physics, 96 (10) (2004) 5490-5495.27S. Pizzini et al., Journal of the Electrochemical Society, 135 (1) (1988) 155-156.28J. Chen et al.,Scripta Materialia, 52 (12) 2005, 1211-1215.29T. Buonassisi et al., Nature Materials, 4(9) (2005) 676-679.30A. Voigt et al.,14thEuropean Photovoltaic Solar Energy Conference, Barcelona, Spain, 31june -4july 1997 774-777.31J. Chen et al.,Journal of Applied Physics, 97 (3) (2005) 033701-1-033701532M. Kivambe et al.,Journal of Applied Physics, 112 (2012) 103528.33N. Usami et al.,Journal of Applied Physics, 109 (2011) 083527.34I. Takahashi et al.,Journal of Crystal Growth, 312 (2010) 897-901.

35C. Haessler et al., 2ndWorld Conference and Exhibition on Photovoltaic Solar Energy Conversion, Vienna 1998.36K. Kutsukake et al.,Journal of Applied Physics, 110 (2011) 083530.37M. Tsoutsouva et al.,Journal of Crystal Growth, (2014) In press.38B. Ryningen et al.,Acta Materialia 59 (2011) 7703-7710.


9/29

CrySaLID


9

formation which is particularly interesting as twins are frequently observed in crystalline silicon. Anotherdislocation nucleation mechanism is related to the existence of impurities and precipitates such as Si3N4andSiC because of their differing thermal expansion coefficient during cooling. This mechanism is related to thesize of the precipitates.

As a conclusion, it is essential to characterise dislocations and to establish the link with grain boundaries

and impurities in order to improve the processes and to avoid or control as much as possible their occurrence.Dislocations are a major issue in mono-like ingots because it is frequent to use several seeds that have slightcrystalline relative misorientations leading to the formation of dislocations at the sub-grain boundaries.

One powerful method for deepening the understanding of the physical mechanisms involved in thecrystallisation of silicon is to carry out and analyse carefully designed benchmark experiments. Thesolidification process is essentially dynamic and post-mortem analysis always provides only incompleteunderstanding. To answer these issues and key points, benchmark experiments have been proposed tocharacterise the growth from silicon melt in situ. Characterisation of the solidification of an undercooledlevitated silicon droplet was performed using an X-ray diffractometer and by recording the droplet surfaceimage using a high speed video camera39. The in situsolidification behaviour of Si droplets on silicon waferswas also characterised using IR thermal imaging

40. A confocal scanning laser microscope was used to carry

out in situ observations of crystal growth behaviour from silicon melt by Fujiwara et al.41,42 providinginformation on the solid-liquid interface features.

Figure 5: Radiograph of the solidification of a metallurgical grade Si (Temperature gradient: G = 8 K/cm, Coolingrate: R = 1K/min). Image processing: division by the last image before cooling.

In the last few years, X-ray synchrotron imagery tools have shown their ability to deepen ourunderstanding of solidification mechanisms43,44 of Al-based alloys for structural applications. Using thecombination of radiography and topography, crystalline orientation, strains and mechanical effects can befollowed in situand in real-time. Recently, Tandjaoui et al.

45developed an original characterisation tool for

Si crystallisation. A furnace devoted to X-ray radiography and topography of the solidification of mc-Siduring its growth was designed. X-ray radiography provides information on the dynamic evolution of thesolid-liquid interface. X-ray topography gives complementary information such as single grain evolution

during growth, crystalline orientation and stresses. Twin formation was studied21

and two mechanisms oftwinning and their interaction with grain competition were described. As an example, figure 5 shows a

radiograph obtained during the solidification silicon. In this experiment, the formation of some twins wasevidenced by the black hatched contrast where they were in the position for Bragg diffraction. Twins form atgrain boundary groove facets at the interface in these experiments. The evolution of grain boundary grooves

during solidification and the subsequent grain competition has also been analysed using this technique19

.It is worth noticing that X-ray topography techniques have been recently put forward in the field of

silicon for PV applications in research groups as well as in private companies as a powerful tool and methodof choice due to their sensitivity far beyond the EBSD technique for revealing structural defects and strains

39K. Nagashio et al., J. Applied Physics 100 (2006) 033524-1-6.40K. Nagashio et al., J. Crystal Growth 275 (2005) e1685-e1690.41K. Fujiwara et al.,J. Crystal Growth 266 (2004) 441-448.

42K. Fujiwara et al.,Acta Materialia, 59 (2011) 4700-4708.43J.A. Spittle, International Materials Reviews 51 No.4 (2006) 247.44G. Reinhart et al., Metallurgical and Materials Transactions A, 39A (2008) 865-874.45A. Tandjaoui et al., Energy Proceedia, 27 (2012) 82-87.

Twinning6 mm


10/29

CrySaLID


10

in this material. In particular, D. Oriwol et al.46 showed that dislocation pile-ups can create sub-grainboundaries and are thus responsible for slight misorientations. Rocking curve imaging was also recentlyapplied to reveal sub-grains and dislocations at the junction between two grains growing from seeds inmono-like growth47. Additionally, a so-called Laue Scanner technique was developed by Lehmann et al.

48to

characterise at a fast rate the orientation and the coincidence site lattice on mc-silicon ingots.

When one wants to improve the solidification process, numerical simulation is complementary toquantitative characterization methods and is absolutely crucial as it can provide predictions of the behaviourwhile modifying parameters. Usually and not only for crystalline silicon, multi-scale modelling is necessaryto tackle all the issues raised during solidification processes. Models are then complementary in the outputthey can provide as they can either be used as a means of improving the understanding of the physical andchemical mechanisms, or to test the process parameter influence and/or process configurations, or to provideinput for large scale modelling. In the case of silicon for PV applications, impurity segregation is an essentialfeature that has been studied by, for example, Y. Delannoy49. 3D modelling of a solidification furnacededicated to mc-Si was also performed to study a real process by modelling the heat and momentumtransport during solidification

50. However, simulation of the grain structure in 3D and in real processes is

rare. In metallurgical materials, CET51

and the grain structure have been modelled together withsegregation

52.The 3D cellular automaton (CA) method is an alternative solution to reach these goals

53,54and

was previously applied to the EMIX process. However, whatever model is used, the result cannot be exactand predictive if the physical mechanisms and growth laws of silicon are not implemented in the model. Toreach the objective of industrial-scale predictive models, benchmark experiments are required as well as

models at different scales and with different but complementary methods. In particular, grain competitionfeatures including twinning must be taken into account. In recent work, Lin et al.

55attempted to simulate

facetted growth in silicon crystals by the phase field method. Progress in the simulation of the facettedsilicon interface was obtained by the authors. However, they mentioned in their conclusion that further workis still needed, in particular to model the kinetic undercooling.

The CrySaLID project aims at answering the challenge of understanding the grain structure formation andits interaction with light impurities and defects. As developed above, the subject remains today of high

importance and addresses key issues for crystalline silicon for PV applications in industrial processes.Indeed, all methods produce, or aim at producing, different grain structures (size and arrangement of grains)that have a direct impact on the PV properties and whose formation must be understood, modelled andsimulated to be wisely controlled. There is still a lot to do to understand the growth mechanisms that controlthe grain orientation and grain size, the impurity and defect interactions and all the more crucial assolidification starts from a seed. As a conclusion, it is clear that a deeper understanding of the basicsolidification phenomena is needed. The CrySaLID project gathers groups with complementary recognisedcompetencies needed to answer to the objectives raised: IM2NP, EMIX, SIMAP, CEMEF, NTNU, SINTEF,KAU.

2.6 Positioning of the project:

As detailed in the review of the state of the art, crystalline silicon is currently the most widely used

material for PV cells and this is expected to be the case for a few more decades, due mainly to the maturityof the industrial processes. However, major challenges must be tackled in a context of strongcompetitiveness.

Part of the team of the CrySaLID project (IM2NP, EMIX, SIMAP, SINTEF, CEMEF) was involved in theSi-X (Caractrisation et comprhension de la cristallisation du SiIicium photovoltaque: imagerie X

46D. Oriwol et al., Acta Materialia 61 (2013) 6903-6910.47M. Tsoutsouva et al.,Journal of Crystal Growth, (2014) In press.48T. Lehmann et al.,Acta Materialia 69 (2014) 1-8.49Y. Delannoy and K. Zadat, EPM conference 2012, Beijing, China, 2012.50Y. Delannoy et al., Journal of Crystal Growth, 303 (2007) 170-174.51M.A. Martorano et al., Metallurgical and Materials Transactions A, 34A (2003) p1657-1674.

52G. Guillemot et al., ISIJ International 46 No6, (2006) 880-895.53Ch.-A. Gandin et al.,Metall. Trans., 26A (1995) 1543.54T. Carozzani et al.,IOP Conf. Series: Mat. Sci. Eng.33, 012087, 201255H.K. Lin et al.,Journal of Crystal Growth, 385 (2014) 134-139.


11/29

CrySaLID


11

synchrotron) project funded by the ANR in the HABISOL (HABitat SOLaire et intelligent) programme from2008 to 2013. Si-X gathered six research institutes or organisations (IM2NP, SIMAP, CEA-INES, CEMEF,SINTEF, ESRF) and the company EMIX. The Si-X project featured an in-depth study of the link between thesolidification of multicrystalline Si (pure silicon and low-cost but degraded silicon feedstock) and theassociated PV properties. The benchmark data needed to fulfil the objectives of the project issued from

several complementary experiments with different ingot scales and complementary characterizationtechniques. In particular, within the Si-X project, we developed a unique device using X-ray imaging(radiography and topography) that allows in situ and real-time characterisation of the grain structureevolution during solidification. This challenging experiment was not available elsewhere for silicon at thebeginning of the project and remains unique. The project also proposed to relate the X-ray imaging results tolarger scale silicon ingot solidification: wafer moulding and classical directional solidification. The couplingbetween experiments and 3D numerical simulations of the grain structure in mc-Si even for industrial ingotswas an additional originality of the project. The results obtained by the different partners of the projectcontributed to a better understanding and as a consequence, to a better control of the grain structureformation in mc-silicon from a fundamental point of view. Moreover, within the Si-X project, EMIX, whosefocus was on mc-Si at that time, was able to improve its understanding of the link between the grain structureof silicon ingots and the casting parameters of the 4C process through the benchmark experiments.

At the end of the Si-X project, the final report and project was evaluated by the ANR and the conclusionwas that the partners proved that fundamental scientific breakthroughs could have concrete applications inindustrial processes. Moreover, the ANR encouraged the partners to carry on with further research workaiming at deepening fundamental knowledge of silicon crystallisation in order to improve PV efficiency andreinforce the process results at EMIX.

In the CrySaLID project, we will address new and key themes related to the formation of the grain in

mono-like crystalline growth. The essential matter of the impact of light impurities, namely C and O,frequently present in industrial processes and their interaction with structural defects (in particulardislocations) will also be studied. Simulation tools will be used to deepen the understanding of solidificationmechanisms and provide predictive tools to improve solidification processes.

3. Scientific and technical program and project organization

3.1 Objectives and scientific breakthrough needed

As detailed earlier, the underlying physico-chemical mechanisms present during the crystallisationprocess must be first understood to be better controlled in a further step and this is the main scientificobjective of the CrySaLID project. Indeed, the CrySaLID project aims at:

Deepening the understanding of silicon crystallisation mechanisms with or without selected

orientation of the first crystals (grain nucleation, growth and competition, twin formation andevolution during growth);

Characterising the defects and impurities linked to the grain structure (dislocations, impuritycontamination, segregation, precipitation during crystallisation and effect on the grain structure);

Correlating the grain structure, defects and impurities with related PV properties;At the end of the project, benchmark data, quantitative predictive modelling and simulations as well as

knowledge on the fundamental mechanisms and processes will be generated and made available.The fulfilment of these scientific objectives is the necessary condition for the successful achievement of

the technical objectives of the project. The technological objectives of the CrySaLID project will contributeto the development and improvement of PV silicon crystalline technologies in general and of the EMIXprocess in particular. Four major technological outcomes are expected from the CrySaLID project:

Identification of processing conditions for better control of grain structure from selected

orientation and defect control;

Impurity control and in particular, modification of the EMIX industrial device and process tolimit C incorporation and deleterious effects with the objective of producing mono-likecrystalline silicon;

Development of a new method for mono-like silicon growth;


12/29

CrySaLID


12

Development of a 3D predictive simulation tool of grain structure formation during silicongrowth and implementation in a commercially available software.

The final objective is to control the fabrication of crystalline silicon ingots (grain structure, impurities,

defects) for PV applications by providing deeper knowledge of silicon crystallisation and predictivenumerical tools for process control. There is no major modification of the project between the pre-

submission step and the submission step.The objectives of the CrySaLID project will be achieved by investigations with the most advanced

experimental and numerical tools. The methodology will consist in carrying out benchmark solidification

experiments from seeds using complementary experimental configurations, and to study the impact ofcontamination by using reference silicon grades and silicon feedstock produced by the EMIX process. Noveland unique characterisation tools will be used, such as in situand real-time X-ray imaging characterisation ofthe solid-liquid interface during crystallisation to reveal silicon growth kinetics and features. In parallel,modelling and simulations at different scales (phase field, cellular automaton, thermodynamical

environment) will be conducted, addressing several phenomena such as thermal modelling, impuritycontamination, grain structure formation (parasitic grains, twins), grain competition, coupling with thermo-mechanical deformation (dislocation generation), also simulating the EMIX industrial process. Due to thecomplexity of the phenomena, an approach including simulations at different scales and the interactionsbetween these different scales is essential for producing valid modelling and simulations of crystallisation

processes. This is only possible if physico-chemical mechanism models used in the simulations have beenvalidated by benchmark experiments and by industrial-scale tests. One of the strengths of the CrySaLIDproject is to propose experiments able to validate the simulation models and also to propose multi-scalesimulations of the solidification processes. These complementary simulations will enrich the discussion onthe scientific issues and improve feedback to EMIX. In parallel, defects and impurities inside the ingots

grown within the project (from seeds, bi-crystals and mono-like and Kyropoulos) will be characterised andtheir relationships to PV properties evaluated. Indeed, all the experimental and modelling work will becoupled as far as possible to the characterisation of the PV properties in order to link the results ofcrystallisation (grain structure, grain size, texture, impurity segregation) to the properties and to give inputsfor process improvement. As a consequence, further to a deeper understanding of the basic phenomena, this

project will contribute to the improvement and optimisation of the processes by giving a precise knowledgeof the solidification mechanisms and the impact of different process parameters.

The ultimate objective is the optimisation of solar cell efficiency through wise control of the processparameters in all the addressed key issues. Cross-fertilization is expected between experimental, numericalinvestigation and industrial process experiences. The synergy between fundamental knowledge and applied

processes is another major originality and strength of the project.

3.2 Expected results

The CrySaLID project will strengthen the position in the research field of the research groups involved,will indicate potential improvements for the fabrication of crystalline Si solar cells in general and willidentify key elements for improving the process of the industrial partner EMIX in order to yield feedstock ofsuitable grade for single crystal growth. Moreover, the CrySaLID project will accelerate the understanding of

the basic phenomena allowing the development of theoretical models, multi-scale modelling and uniqueexperimental devices, objectives which have been identified as major in the 2014 ANR call. Indeed, ourproject answers to this focus perfectly by providing an experimental validation including experiments atvarious scales with innovative characterisation methods coupled with modelling and simulation work. TheCrySaLID project will reinforce the scientific competencies in the sector of crystalline silicon, deepen ourunderstanding of the phenomena involved and make available knowledge, conditions and tools on a medium-term basis for quantitative modelling and thus grain structure prediction and process improvement. Theultimate objective remains the optimisation of solar cell efficiency through optimisation of growth processparameters, with various and controlled feed materials to provide society with affordable solar cell panelswith improved efficiency. One of the main strengths of the CrySaLID project is to gather Europeanspecialists on silicon for solar cells with complementary competencies related to the crystallisation of silicon,formation of defects, characterisation, modelling and simulation at various scales, and PV properties. Indeed,

the partners involved in the consortium share recognised complementary competencies and expertise neededto obtain breakthroughs in knowledge of the crystallisation of silicon and of how to improve industrialprocesses in general. Moreover, the association of public research partners with a private industrial partner


13/29

CrySaLID


13

will allow richer exchanges regarding the results generated and the exploration of new research issues whilstproviding access to high-level public research to the private partner improve innovation. The tasks and theirorganisation will be presented in the following sections.

3.3 Description of the scientific program and task organization

The duration of the project is 42 months. The project organisation will be driven globally by coordinationtask 0. It falls into four tasks on top of task 0. A schematic drawing of the task organisation includingpartners involved and the partner in charge is given figure 6. In section 3.4, the tasks are detailed includingrisks and alternatives.

Figure 6: Global project structure.

3.4 Detailed program and task description

Task 0: Coordination.Leader:Nathalie Mangelinck-Nol (IM2NP)Other partners involved: NA

The project comprises joint approaches with several experiments and simulations and feedback to theprivate partner. It is thus necessary to settle a coordination task in this project. The role of the coordinatorwill be to orient the different experiments and to ensure the constant link between the experimental work andthe software development as well as with the industrial considerations. The first task of the coordinator willbe to settle the consortium agreement. The coordinator will also be responsible for the smooth organisation

of the project with focused meetings as well as regular meetings for preparation of deliverables andmilestones. Meetings will be organised in round-turn at each partner site as far as possible or during

workshops and conferences in the field as well as by videoconference. The coordinator will also beresponsible for the regular and final reports concerning scientific and financial aspects. It is foreseen tocreate a shared and secured website for downloading of relevant documents as publications, reports and

presentations.Milestones: Half yearly reports and partial state of expensesDeliverables: Final report and global state of expenses

Task 1: Grain structure control with seedLeader: K. Zadat (SIMAP)Partners involved:SIMAP, IM2NP, NTNU

Task 1 is devoted to grain structure formation and control in solidification processes from seed. The task 1

objectives will be tackled by three subtasks. Subtask 1.1 will focus on fundamental solidificationexperiments using the in situand X-ray imaging device of IM2NP(MCA). This subtask will provide insightinto the fundamental mechanisms of formation of parasitic grains, grain competition, twinning when starting

Task 0 : Coordination (IM2NP, N. Mangelinck-Nol)

Task 1 : Grain structurecontrol with seed

(SIMAP, K. Zadat)

Partners involved: SIMAP,

IM2NP, NTNU

Task 2 : Impact and control ofthe impurities and defects

(EMIX, E. Pereira)

Partners involved:

EMIX, IM2NP, SIMAP,SINTEF, NTNU, KAU

Task 3 : Multi-scale modelingand simulation of grain

structure(CEMEF, Ch-A. Gandin)

Partners involved:CEMEF, IM2NP

Task 4 : Result Synthesis and industrial scale assessment, feedback to industrial process(IM2NP, N. Mangelinck-Nol)

Partners involved:All


14/29

CrySaLID


14

growth from a seed. Subtask 1.2 will be devoted to complementary experiments on bi-crystals growth tostudy the importance of crystalline orientation on grain competition in the general frame of mono-likegrowth and will be taken in charge by NTNU. In subtask 1.3, experiments will be performed in a furnacedeveloped by SIMAP to produce mono-like ingots with original techniques (Kyropoulos and the seed-assisted method

56). All these contributions will generate the information needed to understand the grain

structure formation and evolution when starting growth from seeds.

Subtask 1.1:In situX-ray imaging of silicon growth from seed (IM2NP(MCA))Regarding crystallisation, the originality of the approach of IM2NP for the study of the solidification

microstructure is the characterisation of the dynamics of crystallisation phenomena in silicon by in situandreal-time X-ray imaging. This challenging experiment is not available elsewhere for silicon. The use of theX-ray synchrotron characterization device unveils physical mechanisms. This device will be used in subtask1.2 to address the issues of grain structure formation from seed as in mono-like crystallisation, themechanisms of growth from selected crystal orientations, the dynamics of grain competition and nucleation,and twinning. In this subtask, high purity silicon will be used to study these mechanisms.

These experiments will be conducted at the ESRF (European Synchrotron Radiation Facility) on beamlineBM05 in a high temperature furnace consisting of two resistive graphite heaters that can reach a temperatureof 1800C in a secondary dynamical vacuum of 10-4 mbar. Solidification can be achieved in two ways: bypulling the sample from the hot zone (top) to the cold zone (bottom) by mean of a vertical translation systemwith a velocity range from 0-200m/s, or by decreasing the temperature of the two heaters. Two imaging

modes can be used alternatively during one solidification experiment: X-ray radiography and X-raytopography which are described in the following.

X-ray radiography mode: This consists in illuminating the sample with white radiation (containing thefull energy spectrum) provided by the synchrotron source. The beam is monochromated after the sample,rather than before, in order to keep a constant heat load on the sample, since white beam is required fortopography mode (see below) during the same solidification experiments. This imaging mode enables us toobserve in situand in real time the dynamic evolution of the Solid/Liquid (S/L) interface and its morphologyduring solidification, and to evaluate the interface velocity.

X-ray topography mode: The sample is also illuminated by the white beam radiation. The transmittedbeam is cut by a beam-stop and diffracted beams are collected on X-ray sensitive films. The diffraction spotsrecorded, called topographs, are then observed and analyzed using an optical microscope. The X-raytopography mode is a complementary tool to the X-ray radiography observations and allows us tocharacterize the grain orientation, twinning, crystal quality and stresses throughout the solidificationexperiment.

Risk and alternatives of subtask 1.1:In situ experiments proposed by IM2NP(MCA) in the CrySaLID project are performed at the ESRF

(European Synchrotron Radiation Facility) BM05 beamline. Following the success of the Si-X project forwhich the utilisation of synchrotron X-ray imaging was a key point, the ESRF expressed its interest incontinuing to support this kind of activity. Due to reorganisation of the ESRF, the BM05 beamline is nowclosed to external beamtime applications and the ESRF team has to act as a sub-contractor and not anymoreas a partner. For beamtime bought within the CrySaLID project, the ESRF will contribute without charge to

the scientific discussions on data analysis and to give advice on measuring equipment.As a consequence, no major risk is expected for this task.

Subtask 1.2: Bi-crystal solidification (NTNU)NTNU currently has two PhD students working on projects that could contribute to the CrySaLID project.

Their main subjects are the study of bi-crystal growth from seeds: nucleation and growth of Si ingots withseed crystals, the effects of process parameters on grain structure development.

Risk and alternatives of Subtask 1.2:

No major risk is expected as NTNU has its own funding for these activities and research work has alreadystarted.

56Miyamura et al.,Journal of Crystal Growth (2014) In press.


15/29

CrySaLID


15

Subtask 1.3: Mono-like crystallisation using Kyropoulos and single seed-assisted techniques (SIMAP)The SIMAP laboratory developed in partnership with the Cyberstar company a furnace dedicated to the

growth of mono-like silicon crystal for solar applications using the Kyropoulos method. This device, namedROCKY, was developed within the project PICKS funded by ANR PROGELEC 2011.

Two techniques will be used to perform mono-like crystallisation:

. The Kyropoulos method: seed on top of the melted silicon, and cooling.

. The single seed-assisted method: seed below the melted silicon as used by Miyamura et al.56

.This latter method allows mono-like crystals to grow from a single germ. To achieve this, a thermal mask

able to guide the thermal flow and thus to favor the growth of a G1 ingot from a single germ will beimplemented. This mask will ensure a concave interface to allow the growth of the single germ. This task isclosely link to task 2 because the use of multiple seeds is often responsible for the formation of dislocationswhich could be avoided by using this technique.

Risks and alternatives of subtask 1.3:Melting of the seed in the Kyropoulos furnace. This risk is limited due to the experience gained by the

SIMAP team during the PICKS project.

Milestones of task 1:X-ray imaging experiments scheduling at the ESRF (t0+12months)Availability of ROCKY furnace (t0+18months)Deliverables of task 1:Report on benchmark experimental X-ray imaging results concerning mono-like growth in pure material

(t0+ 24 months)Results of solidification of bi-crystals (t0+ 24 months)Report on solidified ingot by the Kyropoulos technique (t0+ 30 months)Results of first mono-like growth with single seed-assisted method (t0+ 36 months)

Task 2: Impact and control of the impurities and defectsLeader: Elodie Pereira (EMIX)Partners involved:EMIX, IM2NP, SIMAP, NTNU, SINTEF, KAU

Task 2 addresses the key point of light impurities in particular the carbon and oxygen impurities andstructural defects known to create alone or in combination deleterious effects for the PV efficiency. This taskfalls into four subtasks. Subtask 2.1 is devoted to the simulation of the thermodynamic and chemical

environment in the EMIX furnace in order to develop concrete solutions for the reduction of contamination.In parallel, simulation of the impurity effective segregation coefficient will be conducted. This subtask willbe conducted by SIMAP and EMIX and will include some FTIR (Fourier Transform Infra-Red)measurements achieved by IM2NP(OptoPV) to determine the concentration of light impurities, in particularC and O. Subtask 2.2 is devoted to mono-like crystallisation using EMIX feedstock in two devices: the X-ray

imaging furnace for characterisation of the effect of impurities on grain evolution at IM2NP(MCA) and, themedium scale furnace at SIMAP in order to study these effects at a larger scale. In conjunction, subtask 2.3will focus on the characterisation of structural defects and of the implied mechanisms: in situand ex situX-

ray topography on mono-like samples (IM2NP(MCA)), dislocations in between seeds in bi-crystals (NTNU),dislocations linked to the grain structure (SINTEF). Subtask 2.4 will be a transverse task to characterise thePV properties in relation to the impurities and structural defects as well as the associated grain structure. Thiswill be performed jointly at IM2NP(OptoPV) by lifetime and resistivity measurements and at KarlstadUniversity with an additional important contribution for which LBIC (Light Beam Induced Current)measurements are compared to dislocation maps.

Subtask 2.1: Simulation of the thermodynamic and chemical environment in the EMIX process (EMIX,SIMAP, IM2NP(OptoPV))

In this subtask, the scientific issue studied by EMIX will focus on the silicon purity needed in order togrow a single crystal. In particular, preliminary evaluation has shown that light impurities such as C and O

can prevent the growth of large single crystals.

Many parameters in the EMC process can influence the concentration of impurities in solid silicon andespecially the carbon concentration. To reduce the configurations that have to be tested experimentally on the

EMC pilot we have to implement it first numerically with models that are able to describe the EMIX process


16/29

CrySaLID


16

in the most accurate way. We propose then in this first part to pay attention to the simulation of the velocityfield close to the interface regarding the evaluation of the chemical equilibria involved in EMC conditions.Itwill involve knowledge of electromagnetic forces, thermodynamics, fluid mechanics and chemical reactions.The development of an electromagnetic model will be performed using the Comsol tool as a first step. ThisComsol model based on the equivalent heat capacity assumption will allow the determination of the location

of the liquid-solid interface during the pulling. Experimental measurements of the solidification front shapeon sliced EMC ingots will be compared with simulation results for validation.

Data from the EMC Comsol models such as the interface shape, the induction working frequency or theinduced power will serve as input to a turbulent mass transport Fluent model developed by SIMAP. Thismodel will permit characterisation of the purification ability of the EMC process, kefffor each point on theliquid-solid interface and for different induction input parameters. GDMS (Glow Discharge MassSpectrometry) and SIMS (Secondary Ion Mass Spectrometry) analysis will be performed all along EMCingots to compare experiments and simulations. A particular focus will be put on the description of carboncontamination in the liquid silicon. Light impurity measurements will also be performed at IM2NP(OptoPV)

by FTIR. The keffcalculated will be directly used to adapt induction parameters at EMIX in order to increase

the purification ability of the EMC process. The simulations developed in this work will be translated intoComsol at EMIX.

Risks and alternatives:No significant risk is foreseen in this subtask. The main risk is not succeeding in implementing the

models (FactStage and Fluent) developed at SIMAP in the COMSOL software for modelling the EMC

process. An alternative would be for EMIX to buy the same tools as that used at SIMAP.

Subtask 2.2: Mono-like crystallisation using EMIX feedstock in two devices: the X-ray imaging furnaceand in a medium scale furnace (IM2NP(MCA), SIMAP)

This task aims to grow mono-like crystalline silicon from particular EMIX feedstock at a small scale forin-situX-ray imaging during solidification in the IM2NP device at the ESRF on one hand, and at a largerscale in the mono-like crystallisation furnace at SIMAP on the other hand.

a.In-situ X-ray characterization of EMC feedstock Role of carbon related defects.

In this part, experiments will be conducted on silicon samples presenting different carbon concentrationfrom 2.5 ppmw down to 0.1 ppmw and hence involving different kinds of carbon related defects (interstitialcarbon, micro-sized SiC particles, nano-sized decorated SiC particles...). Objectives of this subtask are tocharacterize these defects (SiC polytypes, particle size, dislocations density) and then to perform in situ

characterization during the solidification into a single or mono-like crystal. From these experiments weexpect a deeper understanding of the mechanisms involved during single crystal growth of carbon-contaminated samples and then a better control of the grain selection and defects formation.

b. Crystal growth experiment with the SIMAP furnace.In order to achieve large mono-like silicon crystals, SIMAP has developed a furnace based on the

Kyropoulos process. Because of its differences compared to the Czochralski process, this technique needs apurified silicon feedstock (carbon content less than about 0.1 ppmw). The silicon purified with the modifiedEMC process that EMIX will have by the end of the task 2 will serve as feedstock for the Kyropoulos

furnace (ROCKY).Risks and alternatives:

The main risk for this subtask is that samples containing 2.5 ppmw of carbon will be too contaminated tolead to the solidification of a single or mono-like crystal. In that case, in order to ensure a successful mono-

like growth, low carbon-containing silicon samples may be required. However, there is a strong scientificinterest the impact of highly contaminated samples on growth.

Subtask 2.3: Characterisation of structural defects and of the implied mechanisms (IM2NP(MCA),SINTEF, NTNU)

a. Ex situ and in situ characterisation of dislocations by X-ray topography

The team will characterise defects such as dislocations, SiOx precipitates (by X-ray topography they

induce a characteristic butterfly or coffee bean contrast), or by simple surface etching mechanically and

chemically) in the grains or at the grain boundaries. The solidification of single crystal Si samplesintentionally contaminated with impurities (e.g. metals which can (Fe, Ni, Cu) or cannot (Au) form silicides

or light impurities) will be carried out as reference experiments to understand the impact of impurities on


17/29

CrySaLID


17

dislocation formation. The impurities can have different lattice parameters and expansion coefficients, ascompared to the values for the Si matrix. Ex situ X-ray topography should first be performed oncontaminated samples prior to fusion/solidification studies to check on the possible formation of precipitatesor dislocations. Then, the samples will be processed inside the X-ray imaging device. The melting of thewhole sample must not be complete: the bottom part of it must remain crystalline. A slow growing rate will

favour the metal segregation in the liquid phase and may evidence the behaviour of impurities as comparedto a pure single crystal Si reference sample. Moreover, we could deform, at low temperature (700C, to avoidfurnace contamination), pure single Si wafers by cantilever bending to introduce initial dislocations. Thepart of the sample having the highest dislocation density will have to be kept solid during fusion, prior tosolidification tests.

b. Dislocation formation

The contribution and interest of SINTEF in relation to the proposed project regards dislocation formationin crystalline silicon. Within its Norwegian funded project, SINTEF intends to develop knowledge aboutimpurity transport processes and impurity-defect interactions. Research covers topics of interest to theCrySaLID project some of which will be shared with the partners. Focus is put on the determination ofconditions for reducing defect density in mc-Si crystals, development of a model of transport processes ofrelevant impurities and better understanding of impurity-defect interactions. NTNU will also contribute tothis task with their work on defect-impurity interactions and formation at the sub-grain boundaries created atthe contact of multiple seeds in mono-like crystal growth.

Risks and alternatives:No major risk has been identified for this subtask.

Subtask 2.4:Electrical and PV characterisation linked to defects and impurities (IM2NP(OptoPV), KAU)

One important goal of this project is to study the influence of oxygen and carbon impurities, dependingon their position in the silicon crystal lattice, on the crystallization, on defect formation and on the electricalactivity of the defect. Oxygen and carbon concentrations will be determined and electrical characterizationswill be carried out. Oxygen and carbon concentrations will be determined by FTIR, MicroFTIR and SIMStechniques. Electrical characterization techniques such as lifetime mapping, Conductive AFM mapping andLBIC (Light Beam Induced Current) will be carried out. The correlations between the electrical and the

chemical techniques will be performed at different scales (several m or several mm) depending on the FTIRsignal obtained with the different samples. These studies will be done on samples submitted to crystallizationafter melting in the ESRF furnace used to investigate the crystallization thanks to X-ray radiography and onsamples coming from SIMAP and EMIX, from different ingots and from different heights in these ingots. AtKarlstad University, it is planned to support the project with high resolution LBIC (Light Beam InducedCurrent) and dislocation density mapping. Indeed, a correlation of the electric performance with the defectdensity (both measured on the same solar cells with a few micrometer spatial resolution) leads to specificmeasures of the recombination activities of the defects. As a consequence, we can precisely measure maps ofthe dislocation density. By a correlation of the LBIC maps with the dislocation density, a specific measure ofthe dislocations recombination activity are obtained. From the LBIC maps alone, the recombination activityof grain boundaries can also be measured.

Risks and alternatives:No major risk has been identified for this subtask.

Milestones of task 2:- Determination of the solidification front shape with COMSOL (t0 + 6 months)

- X-ray imaging experiments scheduling at the ESRF (t0+ 18months)- Combination of models developed at SIMAP and EMIX (t0 + 36 months)

- Structural defect characterisation by X-ray topography (t0 + 24 months)Deliverables of task 2:- Report on comparison between solidification front shape modelling and experiments from EMC process

(t0 + 12 months)- Effective distribution coefficients for carbon in the EMC process (t0 + 18 months)

- Thermodynamic study of interaction between carbon and oxygen with molten silicon (t0 + 24 months)- Availability of EMC feedstock (t0+ 20months)

- Report on segregation of carbon and oxygen in the EMC process (t0 + 24 months)

- Report on defect characterisation in relation to impurities (t0 + 36 months)- Report on electrical and PV characterisation (t0 + 36 months)- Report on X-ray imaging experiments performed with EMC feedstock (t0 + 36 months)


18/29

CrySaLID


18

- Report on Kyropoulos experiments performed at SIMAP with EMC feedstock (t0 + 42 months)

Task 3: Multi-scale modeling and simulation of grain structureLeader:Charles-Andr Gandin (CEMEF)Partners involved:CEMEF, IM2NP

Task 3 is devoted to the development and validation of multi-scale modelling and simulation of grainformation and evolution. As stated in the scientific background, modelling and simulation are essential fordeepening our understanding of physical mechanisms involved but also to predict grain structure which, atan industrial level, saves costly test procedures. Two complementary models are proposed in task 3. Insubtask 3.1, IM2NP(TMS) will develop a phase field model of a grain boundary with a facetted interface,allowing the simulation of the evolution of a grain boundary and of the solid-liquid interface in silicon. Insubtask 3.2, CEMEF will develop an industrial scale grain structure simulation in 3D. One of the mainobjectives of task 3 is to unify the models and simulations at multiple scales together with a focus ondifferent physico-chemical mechanisms in order to propose effective simulation tools to industry. Integrationof the grain structure prediction with process simulation in the commercial 3D finite-element FE softwareTherCast is foreseen.

Subtask 3.1: Phase field modelling of silicon growth: effect of grain boundaries (IM2NP(TMS))In a recent theoretical study of directional solidification, the dynamics of a grain boundary rough/rough

groove was investigated at low velocities by using a linear stability analysis57

. The lateral drift of the groovewas shown to decrease as the growth velocity increases and to stop when the cellular instability threshold isapproached. In addition, a phase-field model corresponding to the case of three coexisting phases wasintroduced and shown to simulate tri-junctions between solid 1, solid 2 and liquid

58.

In this subtask, we propose to study grain boundary groove dynamics during silicon solidification bycombining these two approaches. The difficult part of this task will be to adapt the two methods to the caseof facetted materials. We expect that the results obtained this way will provide valuable input to higher-scalenumerical growth models used in this project, such as CAFE (see subtask 3.2). Our long-standing experience

with phase-field modelling, especially for facetted materials will help up to fulfil this task.Risks and alternatives:No major risk has been identified for this subtask.

Subtask 3.2: 3D grain structure modelling (CEMEF)

Simulation of the grain structure in 3D and in real processes remains rare. However, it is essential tomodel the grain structure in 3D, in particular for industrial ingots. To reach this objective, simulation codesin general must be fed by model experiments revealing both physical and chemical mechanisms. As aconsequence, the 3D grain structure model will be fed by all the results produced by the project:experimental and modelling at different scales and in particular by the smaller-scale phase field model.

Within the project, the CEMEF partner proposes to further develop its grain structure model forquantitative modelling of 3D ingots including particular features of silicon growth, and, more precisely,adequate growth kinetics for silicon, macro-segregation relevant to impurity distribution, and coupling with

thermo-mechanical deformation. Modelling of the 3D grain structure in EMIX process is also foreseen andwill be performed thanks to the inputs obtained in task 2 and concerning the thermal environment and

impurity segregation coefficients. A major deliverable will be the integration of grain structure predictionwith process simulation in the commercial 3D software TherCast.

Risks and alternatives: The only risk for this subtask would be the absence of information onthemodynamic parameters and on growth mechanism that are core results which will be produced by theCrySaLID project. As a consequence, no major risk has been identified for this subtask.

Milestones of task 3:- Write a triple phase-field code and adapt it to run on Graphics Processor Units (GPU) (t0+12 months)

- Insert the necessary anisotropies for the different interfaces (t0+12 months)- Implement of a silicon growth algorithm in CAFE (t0+12 months)

- Simulate macrosegration for light impurities in CAFE (t0+12 months)

57G. Faivre et al., Comptes Rendus Physique 14 (2-3) (2013) 149-155.58R. Folch et al., Physical Review E 72 (2005) 011602


19/29

CrySaLID


19

- Perform simulations and collect data for the different grain boundary groove cases (t0+24 months)- Compare with experiments (t0+24 months)

Deliverables of task 3:- Synthetize results of phase field modelling and produce input for the larger-scale growth models

(CAFE) (t0+30 months)

- 3D simulation of EMIX process with CAFE model (t0+36 months)- Integration in the commercial software TherCast (t0+42 months).

Task 4: Result synthesis and industrial scale assessment and feedback to industrial processLeader:Nathalie Mangelinck-Nol (IM2NP)Partners involved:all

Task 4 is devoted to the synthesis of results generated in the project, industrial scale assessment andfeedback to the industrial process. Task 4 will fall into two subtasks. Subtask 4.1 will focus on synthesis ofresults of the experiments and analysis performed in tasks 1 and 2 and to the comparison with grain structuremodels developed in task 3. Subtask 4.2 will be devoted to the development of simulation tools for and atEMIX and to feedback from EMIX on the device modifications proposed in the CrySaLID project and onmono-like growth with EMIX feedstock.

Subtask 4.1: Synthesis of the experiments and analysis

The first objective of this subtask is to unify the experimental results obtained in task 1 on the grainstructure obtained with growth on seeds with the results obtained in task 2 dealing with the impact of lightimpurities and with the associated structural defects. A clear link between these features and the PVproperties, including characterisation of industrial ingots provided by EMIX, will be established. Moreover,the second objective is to conduct comparisons between experiments and simulations performed in task 3.Finally, it will reinforce the predictive character of the simulation model by cross comparison. The thirdobjective is to obtain a better understanding of the phenomena involved in the solidification of silicon.

Risks and alternatives: No risk was identified for this subtask.

Subtask 4.2: Feedback to EMIX process

Control of the impurities in the EMIX process and effect of the furnace atmosphere and of the

graphite plateIt is well known today that carbon can react with liquid silicon from the gas phase. Natural gas convection

is often responsible for the transport of carbon from graphite afterheaters to the melt through CO/CO2reactions. One way to get rid of this source of contamination is to properly isolate the silicon bath from thefurnace atmosphere. The challenge will be then to find non-contaminating equipment ensuring a reliable gasisolation wherein convection and composition of the gas is controlled. During these experiments, CO andCO2concentrations in the argon atmosphere will be monitored just above the melt with a CO/CO2sensor.

Moreover, melt ignition of the EMC process is achieved thanks to a graphite plate that transmits heat tothe silicon charge by thermal conduction until the temperature at which silicon can conduct electrical current

is reached. From this point, silicon starts to melt in contact with the solid graphite plate. During this phase, acertain amount of carbon reaches the silicon melt and saturates it until SiC particles can precipitate. Thisprecipitation phenomenon locally purifies the melt of carbon, then allowing further dissolution of other

carbon atoms from the plate to the melt. It can be found in the literature that the saturation concentration ofcarbon in solid silicon strongly depends on the process conditions but can be assumed on average to staybetween 3 and 5 ppmw. Hence, the equilibrium conditions allowing SiC particles to precipitate are reachedvery fast.

We propose here to carry out castings with new non-contact melt ignition conditions. These tests will

require adjustments of the process parameters in terms of thermal distribution in the plate following Comsolsimulations. Depending on the results obtained from the themodynamic and chemical simulations in task 2,several thermal and atmosphere conditions will be tested so as to avoid oxidation of graphite parts of theafter heaters. Secondly, if the previous conditions are not efficient enough to reduce the carbon content in

EMC ingots, it is planned here to replace the standard graphite after heaters by KANTHAL after heaterscontaining no graphite parts. The validation of the modifications will be sustained by the impurity analysesperformed in Task 2.


20/29

CrySaLID


20

Feedback on mono-like crystal growths with EMIX purified silicon.In this subtask, the experiments carried out by IM2NP and SIMAP in task 2 will be further analysed in

order to understand the mechanisms inducing defects responsible for unsuccessful mono-like crystal growth.The critical defects will be spotted and EMIX will be able to focus on the improvements needed.

Risks and alternatives: The main risk of the device modifications is that the confining set-up above the

melt could introduce metallic impurities due to the mechanical shocks undergone by the feedstock grainswhile flowing t

Documents

2014 CE05 CrySaLID DocSci.pdf