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Submitted on 1 Jan 1982
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To cite this version:
D. Helmreich. MULTICRYSTALLINE SILICON FOR SOLAR CELLS. Journal de Physique Collo- ques, 1982, 43 (C1), pp.C1-289-C1-305. �10.1051/jphyscol:1982140�. �jpa-00221797�
JOURNAL DE PHYSIQUE
CoZZoque C I , supplkment au nOIO, Tome 4 3 , octobre 1982 page C1-289
M U L T I C R Y S T A L L I N E S I L I C O N FOR SOLAR C E L L S
He Ziotronic GmbH, 0-8263 Burghausen, F . R. G.
Rbsumd. - Des plaquettes de silicium mono et polycristallines utilisables pour l'blaboratbon de cellules solaires terrestres, peuvent Stre prbparges soit par les mdthodes en lingots conventionnelles ou non, demandant B poste'riori le dd- coupage en plaquettes, soit par des techniques de croissance en couches minces.
Le pour et le contre des di;erses mgthodes de croissance et des diffdrentes qualit& de silicium polycristallin ainsi blabord sont discutbs en tenant compte : l.des avantages et perspectives des diffdrentes mbthodes de crois-
sance et de leurs possibilitGs 3 long terme, 2. de la possibilitb d'intbgra- tion d'btapes de raffinement dans la technique, 3. de la rbpercussion des mbthodes de croissance sur la qualitd du matEriau et 4. des propribtbs pho- toblectriques. L'utilisation positive d'un matbriau de qualitb solaire d'une moins grande puretd demande non seulement des modifications des techniques de croissance mais aussi des mzthodes dr61aboration des cellules en rapport avec les feuilles de silicium utilisbes.
Abstract. - Single and multicrystalline silicon wafers for terrestrial solar cell application can be prepared either by semi- and unconventional bulk growth methods including post growth wafering or by a variety of sheet growth techniques. Pros and cons of the various growth techniques and the different multicrystalline silicon qualities obtained with them are discussed in the
light of : ].issues and benefits of individual growth techniques and their po- tential in long-range conceptualization, 2. possibility of integrating refi- ning steps, 3. repercussion of growth techniques on material quality, and 4. photoelectric properties. A successful utilization of a solar grade start- ing material with lower purity requires not only modifications of individual growth techniques but also solar cell preparation techniques adjusted to the respective silicon sheet material.
1. Introduction. - The development of terrestrial photovoltoics (PV) into a novel prospering branch of industry cannot take place smoothly. Furthermore, it has to consider the development of process, product, equipment and market potential.
The process by which innovotion-based industries develop hos been studied by o num- ber of scientists, ond rough conceptual outlines of this process have been developed.
Three stages of industrial innovation ore described by Abernoth~ and Utterback I11 by their dynamic model of product and process change in a growing industry. The three stages of innovation, namely start-up, growth, and maturity can be checked by o set of key-words related to product, process, and equipment design.
This paper is intended to classify the different bulk and sheet growth techniques in terms of key-words given in fig. 1 I S / . Many crystal growth methods have been applied to silicon; the most important ones ore enlisted in fig. 2. The various techniques are described in detoil e.g. in the proceedings of IEEE ond EC PV con- ferences; a number of reviews on silicon sheet growth technologies hove been publi- shed recently.
In the some manner the quality of the different types of silicon ingots ond sheets has to be considered for the judgment of processes by which they had been grown.
Their characteristics are shown in fig. 3 / > I . The purpose of this more qualitative comparison is to illustrate orders of magnitude, trends and contrasts rather thon
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982140
Fig. I : Key words for classificotion of ingot and sheet technologies
key word
characterization
practicability
productivity
processing of MG/TSG-Si
cuttins/ wafering
meaning
melting/ crystallization procedure.
container material
preparation and maintenance of equipment.
rigidity of process.
easiness of process control production costs.
throughput.
possibility of extension.
feasibility of coutinuous operation
tolerance to impurities.
integrability of refining steps
compatible techniques.
yield.
conversion ratio
KDGB-DgpINpD.RUL-
FUAT ZONING ( 1 9 5 3 )
I
FKD mc lm)/
CONTIGUOUS CAP-I R I B B O N - A m - D R O P COATING (CCC. 1 9 7 7 / C A m X G (lQS9) 1 (RAD. 19763 1 S-WEB, 1 9 0 2 )
SHEET GROWTH
( > 100- mm/min)
EORnONTAL IUBBON GROlPFH (ERG. 1 9 7 5 / t r s S . 1 9 8 0 )
BULK GROWTH
( < I 0 mm/min)
SHEET GROWTH
(10-100 mm/min)
Fig. 5: Crystal growth methods applied to silicon for use in terrestrial photovoltaics (after / 1 6 / ) CZOCBR*ISP DSNDRlTIC WBB (9ZB.1963)
P a t w o ( l o a o )
/
mmov ( s o l e n )RIBBON-TO-RIBBON I
PEDESTAL G R O m ZOMMG (BTR, 1076) ' ROUXR QmCHINC ( 1 9 7 2 )
SILICON-ON-CERAMIC DIBBCTIONAL CIPILURY ACTION SE4PMG SOLXDmCATION
(RQ. 1 9 7 9 ) m'IZR?ACZ-coN~o~
C R W U l U U n O N (1078)
i
(ICC. 1 0 8 1 )EmmKD 9 R P A N o v 1
(IS, 1 9 7 7 )
1
FOIL CASTING TECIiNI0"E RAMP ASSISTEDKDGP-SUPPORW (RAFT 1982)
PC-G (ESP. 1980)
Fig. 3: Key words for characterization of ingots and sheets key word
soundness
crystal perfection
impurit Ms
PV performance
to present exact numbers. Quantitative values - especially in a tabular presentation- may be misleading.
meaning
geometric perfection, mechanical strength, voids, gas bubbles crystalline structure.
grain / twin-boundaries.
disl~cations
impurity level.
main impurities, macro segregation.
grain boundary, segregation.
precipitation, striations diffusion length, efficiency.
effectivity of BSF. BSR.
gettering.
passivat ion.
TSG -Si substrates, epi cells
2. Ingottechnologies. - The technical state of the art in bulk growth is represen- ted by only three different major methods: Czochralski process /3/, directional solidification I 4 1 (Bridgmon, HEM, UCP) and casting 1 4 1 . The other methods left either are of less importance [continuous casting) 1 4 1 or are just coming forth in the photovoltaic scene (float zone 1 5 1 , pedestal 161 )(fig. 4 ) .
A considerable amount of solar generators is already fabricated from unconventional silicon. However, the majority of this evolutionary branch of industry is practiced with monocrystalline silicon prepared by (semi-)conventional techniques. Unconven- tional techniques in general have to form the basis for economic mass production towards the end of this century. The (semi-)conventional approaches will help to ease the solar material market situation in the present decade. To be cost-effective, any ingot technology has to be complemented by an efficient shaping and wafering technique (see fig. 5). Up to now, the increasing efforts in bulk growth techniques have not been matched by adequate activities and advances in cutting and slicing.
The industry still has to live with relatively low production rates and rather labor- intensive technologies. Advanced slicing techniques with high-efficiency and low- cost capabilities are in the state of practical testing.
Classification. - Comparison of the different ingot technolo- gies is done in reference to the key words given in fig. 1. For its partial illustra- tion particular experiences at Wacker with processes and materials are used.
(semi-) conventional crystallization
unconventional crystallization process
advanced CZ
advanced FZ
Fig. 4: (Semi-)conventional a n d unconventional bulk c r y s t a l l i z a t i o n t e c h n i q u e s
(semi-) conventional wafering technique characterization
vibrator feed Auger feed incl.
auxiliary crucible liquid transfer solid rod feed
process Bridgman/
Stock barger
HEM UC P casting
p r o c e s s characterization compan y
SILTEC company Varian T I
SILTEC Hamco Siemens
unconventional wafering techniques characterization
moving container or
kovingtemperature heat exchanger gradual cooling mould casting conti- casting
company UCC I BM CGE N EC
Crystal Systems Solarex Wacker Wacker
Fig. 5 : (Semi-)conventional a n d unconventional w a f e r i n g t e c h n i q u e s
-
FAST
MWS
MBS
fixed abrasive
slurry
slurry
Crystal Systems (RTC)
Yasunaga R TC Varian P.R. Hoffmann Wacker Meyer + B u r g e r
2.1 Characterization of growth method. - Improvements of standard crystallization techniques largely concentrated on the Czochrolski method. This method is now per- formed as a continuous process with different concepts of melt replenfshment: batch- recharging with granular or solid rod feed or continuous liquid melt addition which causes minimal thermal and mechanical disturbances and allows an automatic feed-back control. The route of vertical float zoning ( F Z ) seems to offer promising aspects
in terms of tolerance toward chemical impurities 151.
Most of the companies involved with the development of unconventional ingot techno- logies work on variations of the BridgmonIStockbarger method. The BridgmanlStockbar- ger technique and the Heat Exchanger Method (HEM) had their benefits from the early development of transparent halide and sapphire crystals which allowed - in the re01 sense of the word - a better insight into solidification mechanism.
Directional solidification can be performed in two different ways:
1 . The principle of a moving container within a fixed temperature field has been
utilized by UCC, IBM and CGE.
2. Container and heating system are in o fixed position and crystallization is effected by reducing the heating energy. HEM and a version developed at NEC I71 work with the additional support of a gas stream which cools the bottom of the crucible.
The aforementioned methods have in common that both melting and solidification are carried out in the container, thus causing a series of problems:
1 . Due to a considerable residence time of the melt in the crucible, contamination of the silicon melt by impurities from the container is quite severe.
2. A solidifying melt odheres to the container walls ond the ingot will crack during the cooling process due to differences in thermal expansion coefficients. HEM utilizes thin-wolled quartz containers which will crack during the cooling process The same supposedly hoppens during ubiquitous crystallization process ( U C P ) . In the case of high-density graphite or mullite of special composition it is possib- le to adjust the thermal exponsion coefficient of the container material to that of solid silicon. Multiple-use moulds can be obtained by lining them with powders of Si N SiO,., Si or grophite I81 or by protecting them with a low-melting slag layer3 4 ' / 9 ~ .
Casting had been alreody utilized since the sixties in the preparation of infrared components. The issue was here to modify geometry of the ingots and to adjust their physical properties to the new field of application. Accordingly the development of procedure and material could be based on an already long-stonding experience with process and equipment.
In the case of mould casting (Wackerl an olready melted silicon mass is poured into a preheated container. Up to qow, grophite has proven to be the most qualified mould material. Rotating the mould ensures symmetric distribution of heat resp.
homogeneous heat extractioo and helps, therefore, to develop a homogeneous chill layer before the silicon melt is able to react with the mould walls. This procedure ensures that t!-e eould can be used repeatedly. The subsequent solidification process is carried out in a controlled vertical temperoture gradient.
An extension of traditional mould casting is the continuous costing process. Early experiences with this m e t h ~ d at Wacker showed that it has promising potentialities for terrestrial photovoltaic application.
Characteristics of the various ingot growth techniques ore given in fig. 6.
In addition to the aforementioned techniques which are practised within a recipient in vacuum or inert gas atmosphere further process ond equipment austeritiy studies have been undertaken. Our work in open heorth casting is an attempt to explore the limits of upscoling and the limitations of melting and costing in a truncated equipment /TO/.
Fig. 6: Current characteristics of silicon ingot growth techniques
linear growth rate maximum width throughput technology /
skill crystal structure PV performance comments
2.2 Practicability. - Essential for a low-cost mass production is a well-controll- able procedure and its implementation in o simplified equipment causing minor efforts for preparation and maintenance. If the process is insensitive to disturbances from the environment of the system itself, decisive parameters can be handled more deli- berately and process can be controlled more easily. This not only reduces production costs by decreased expenditures for labor, it also helps to stabilize the process and, therefore, guarantees homogeneity and continuity of the material quality. Com- prehensive knowledge about the procedure allows some subprocesses to become outo- mated.
HEM and SILSO-casting are the most advanced ones ranging in the stage of "growth".
They already include a product design which is stable enough to have a significant production volume.
mm/min mm(mm2)
k g l h r
% AM 1
The equipment developed for HEM is quite simple, easy to operate and not expensive.
Capacity per unit seems to have already reached its limits with charges of about 40 kg. Up to now special attention had been directed to processing, especially to elaborate and establish an appropriote heating and cooling program.
In mould costing the subtechnologies melting/costing and solidificotionlcooling are carried out within the same recipient. Future generations of equipment will be com- posed of subsystems for the different subtechnologies.
F Z
2 - 4 1 0 0 0
4
very high
DF I
16 no crucible
2.3 Productivity. - The add-on costs are influenced by the four factors: 1 . equip- ment costs and depreciation, 2. labor, 3. energy, 4. expendables. Emphasis on add- on costs is different with different crystallization techniques. In some cases mar- ginal improvements are already sufficient to reduce production costs; some processes, however, require a more fundamental manipulation or alteration.
Although the energy costs are the least important ones, it seems worthwhile to men- tion that casting technol~gy ploys a unique role in this framework. In advanced CZ method and the techniques of directional solidification some 100 kwh per kg silicon single crystal are consumed due to the necessity to keep a mass of silicon in the molten state during crystallization. In the case of casting a volue of 8 kWhlkg
C Z
1 - 2 1 5 0 0
4
high
DF
15 recharge
possible
D S
0 , l - 5 3 4 0 0 1 - 6
low
multigra8n
9 - 15 crucible lost/
multiple use cruc~ble
0.5 - 2 2 1 0 0
5
low
multigrain
9 - 13 multiple use
mold
Open Hearth C a s t ~ n g
0.5 - 1 5 0 0 m
10
very low
m u l t ~ g r a ~ n
@ Silicon refining M (s/l)
@ silicon refining M ( 11 I )
@ melt stock for casting kg Si
I
@ ingot
kg Si @ c~ttingfwaferin~
I
begin of testing begin of testing
Fig. 7: Development in charge dimensions (Wacker ~ r o j e c t ) 1. Liquid-liquid extraction ( p y r ~ m e t a l l u r ~ y ) 2. Solid-liquid extraction ( h y d r ~ m e t a l l u r ~ y ) 3. Melt-stock for casting
4. Ingots, as-cast
5. Ingot charging per slicing unit
silicon ingot has already been realized in a relatively simple pilot equipment and it is believed that it can be operated continuously with a value only a factor of three higher than the theoretical melt energy for silicon (0,7 kWh/kg silicon).
The main production costs are equipment costs plus depreciation and expendables.
In the case of expendables the problem is a fundamental one because it will not be sufficient to reduce the costs merely by simplifying the concepts or by introducing cheaper moteriols. The primary task is here to prolong the time of use through modi- fied procedures. In advanced Czochralski technique the time of use of the expensive quartz crucibles has been extended through continuous operation; in casting or Bridg- man/Stockbarger technique moulds can be utilized repeatedly if special preparation techniques are used.
With respect to equipment the production costs can be eased if future generations of equipment are simpler, inexpensive and built for handling bigger charges and higher throughput.
The possibilities of an increased throughput and of continuous operation have to be judged differently for the various crystallization techniques. Due to the limited availability of auxiliary material (like crucibles) in bigger dimensions an improve- ment of throughput is not likely to be feasible through extension of charges in cer- tain phases of development. In order to be effective large-scale operation in one sub-technology should not be seen independently, furthermore, it has to be related to the pecularities and necessities of the other relevant sub-technologies. Fig.7 shows the development of charge dimensions in the subtechnologies moteriol up-gra- ding, crystallization and slicing which has been reached up to now in the Wacker project and which can be predicted for the next years as a result of present ex- periences / 1 1 / .
An increase in solidification rate is not of interest as long as the material quality is spoiled.Most likely, a solution to this problem is to split the crystallization step into further subtechnologies and to try an optimization of each individual subtechnology.
Material utilization is o further factor essential to productivity. Depending on slicing technique conversion ratio (d silicon sheet/kg ingot) it ranges from 0,5
S e G - S i
aluminothermic product
electrolytic product
-
4Bridgman/
Stockbarger
SlLSO casting open hearth
[casting
I D MBS MWS FAST
Fig. 8: Routes of processing various off-grade silicon qualities in ingot growth
to 1 m2/kg. Material waste of wafering operation is one of the reasons that sheet growth has received much attention for PV application.
2.4 Processing of TSG-silicon. - A variety of off-grade silicon qualities has already been processed in the different crystallization techniques. Such quolities range from a directly prepared SoG-silicon to upgraded MG-silicon and dilutions of SeG- silicon with MG-silicon. The chart in fig. 8 shows the different situations possible.
In principle, four different refining techniques may be combined with ingot growth:
1. segregation, 2. recrystallization (from alloys), 3. liquid-liquid extraction (by slogging), 4. liquid-gas extraction (through gas blowing or vacuum treatment). Part of them can be integrated in a simple way. Some of these combinations will have their limitations with respect to economy and material quality. For future concepts it is essential to couple at least the final step in purification tightly with the crystallization process.
Impurities can be put into different categories as concerned to their particular influence on material quality, viz. photovoltaic performance.
- Iron, copper and many other transition metals mostly show a morked groin boundary segregation due to their high diffusion coefficients and very low bulk solubilities.
- "111-V" elements, group I1 elements, carbon, and oxygen segregate macroscopically as one is used to observe in single crystal growth. At very high concentrations these impurities mostly appear as inclusions, agglomerates or precipitotes within individual grains. As it is not possible to reduce the concentrations of B, P, and 0 by customary methods, it is presently necessary to provide a TSG-silicon with sufficiently low concentrations of these elements.
Various combinations of sheet qualities and device techniques have been combined with only limited success hitherto.
2.5 Cutting/Wafering. - Likewise the individual crystallization process has to ob- serve the quality of starting material, the cutting ond wafering technology has also to consider the quality of the ingot. Besides ingots grown by advanced Czochralski
fixed abrasive
Fig. 9: Comparative evaluation of operational conditions of alternative silicon slicing methods
140
6 - 20
P 0
B I D MBS FAST MWS
Fig. 10: Verified and projected ranges of add-on costs for alternative silicon slicing methods (after l l i l )
technique all other ingots require a contouring and sectioning into sliceoble pieces.
This can be done either with OD or band saws.
The technical state of the art in silicon slicing is represented by only four diffe- rent methods. A qualitative evaluation of the operational feasibility of these alter- native slicing methods is presented in fig. 9. A comprehensive review on advanced slicing techniques is given by P.G. Werner / 1 2 / .
The characteristics in fig. 9 show that ID sawing in its advanced version is irnrnedio- tely applicable to solar silicon technology and superior to the other advanced slicing methods. Multiblade slurry (MBS) processes are assigned with the lowest ratings of all procticolly applied conventional slicing methods. Cutting of silicon with fixed abrasive rnultiwire packs (FAST) is hampered by the high flexibility of the wire tools and the related low engagement forces between tool and work piece.
low high very high crystallization velocity
Fig. 1 1 : Matrix of ribbon technologies with parameters meniscus height and shaper/melt interaction
Besides ID-sawing MBS is the second slicing technique, which in near future will have a definite chance to play a major role in the production of solar silicon wafers. From all slicing techniques it provides the highest production rates and the lowest add-on costs (fig. 10).
3. Ribbon technologies. - All the ingot technologies suffer from very limited growth rates, sawing costs and material waste, factors which do not enter if silicon sheets can be prepared directly from the melt. Alleviation of problems is expected from ribbon technologies. This fact is reflected in the large body of ribbon technologies studied in various parts of the world.
As there exists already a well-organized literature on the subject of comparative studies of ribbon growth methods /13, 14, 15/ it is not intended to emphasize the characterization of the different techniques but to qualitatively compare them in the light of key-words given in fig. 1 and to try predictions concerning their state of the art and future potential. In this context see also /16/.
3.1 Characterization of growth method. - The individual growing techniques can be distinguished by the degree of interaction between solidifying melt and a guiding system /4/. In addition to shaper/melt interaction T . Ciszek introduced a further criterion - the meniscus height /16/. Techniques with a strong interaction and a high meniscus require less sophisticated thermal and mechanical control than those where solidification takes place from a sub-mm meniscus.
High meniscus methods show a biased (wedge type) crystallization front with a much more efficient heat extraction. Very high growth rates are normally achieved with
these extended meniscus methods. A compilation of silicon ribbon growth methods is done in a matrix similar to that proposed by T. Ciszek in fig. 1 1 .
Most of the 18 ribbon techniques listed in fig. 1 1 are still in an early stage of development and some of them are even already terminated (S, IS, SOC, SCIM, HRG).
It is not quite astonishing that the EFG method has received the most attention up
to now and that it is the ribbon growth method that has been studied most widely.
Like the other small meniscus methods this method requires sophisticated equipment, high expenditures for measuring and control ond high skill in operation. The small meniscus methods (except RTR) suffer from o contamination of ribbons as a result of strong meltldie interaction. The choice of die material is limited due to the problem of compatibility with molten silicon and adequate wetting behavior. Various refractory materiols have been studied 1171. EFG and CAST are accomplished with wetting graphite dies. In the meantime it was possible to reduce the problems with Sic precipitation. A better understanding of the basic crystallization process en- abled an improved die design with displaced die plates /18, 19/. This resulted in on increase of meniscus height which helps to improve growth stability, crystallinity and to ease contamination effects.
In RTR-technology another source of contamination has to be considered. Feed ribbons are obtained by CVD or high pressure plasma (HPP) deposition on temporary molybdenum substrates and molybdenum silicides are of concern. Recrystallization of microcry- stalline feedstock is either done with a scanning electron beam or with a laser.
A recent process modification uses "rigid edges" which provide a more stable growth than in the cases where melt zone extends till the edges.
In WEB, ESP and ESR stabilization of freezing meniscus is achieved by stabilization of the ribbon edges through silicon dendrites resp. carbon or quartz filaments. A speciality of ESR process liOl is the inclination of the free surface of silicon melt which is accomplished through an inclined ramp. As the ribbon is pulled verti- cally the angle between free surface of silicon melt and pulling direction is less than 90°. This results in an increase of solid/liquid interface and, therefore, a faster heat extraction. The CCC and S-WEB processes utilize a rigid net for built- in stabilizotion of the meltfilm to be crystallized. In the substrate-bound processes SCIM and RAD stabilization of growth process is achieved by the carbon or ceramic material.
The high meniscus methods have in common that temperature control is relatively easy.
An exception is WEB because dissipation of crystallization heat takes place via rib- bon and undercooled melt; on the other hand a tight control of supercooling is re- quired to enable o controlled propagation of dendrites.
The extended meniscus methods benefit from an inclined solid liquid interface which enlargens drastically the heat loss surface and enables, therefore, very high cry- stallization rates. As the interface is nearly parallel to the surface a crystalliza- tion perpendicular to the surface takes place, thus meeting the conditions for a colurnnor structure. In HRG and LASS process the ribbon is pulled nearly horizontally off the meniscus from a larger pool of molten silicon. Latent heat is either removed by radiation or convection. Both methods use active resp. passive thermal control;
a tight control of temperature profiles and melt level is imperative. Up to now, uniformity of ribbon geometry and crystal structure need further improvement. The
ICC method uses conductive heat loss removal through a liquid slag and a part of the crucible wall. The ribbon geometry is defined by the ramp geometry and assisted by the thin slag film which markedly reduces the surface energy of the silicon melt.
Furtheron this liquid encapsulation prevents random silicon nucleation from the supporting material. ICC can be operated either in a vertical or in a nearly hori- zontal version.
Extreme coo.ling rates are obtained with methods used for glassy metal preparation.
Two of them, chill-block melt spinning and roller quenching hove been applied to pure silicon. The fascinating aspects of high throughput rates are complemented by insufficient crystal structure, foil geometry and problems with high thermally in- duced stress. These detrimental influences on materiai quality forbid harnessing of the procedures with such extreme cooling rates (10 K/sec) being correlated with the material silicon.
3.iProcticability. - The major part of the ribbon growth methods - especially the low meniscus methods - require sophisticated equipment. On account of requirements
capacity per equipment unit
I , PV performance 1 0 % ; 1 0 0 cm2 2 1 Wp
2) k e r f loss etc. considered 3 ) Wacker ingot technology ')
growth methods with limited heat extraction
continuous ingot casting slicing 3,
sheet technology multiple ribbon pulling("standard")
foil casting
Fig. 12: Estimates for future large-scale production conditions capacity ( M w p I y e a r ) ')
3
3 0 0 1 0
3 3 0 0
in thermal control efforts in measuring and control technique is an important task (especially in the case of wide ribbons). In order to free the basic process from subjective control, automation of growth process should be attempted.
The multiple ribbon growth method had been proposed as one of the most effective approaches to low-cost production />I/. The difficulties in keeping satisfactory stability in steady-state growth and high-purity conditions in the furnace are sig- nificant; therefore, modifications have been proposed to relievq the procedure.
One alternative is to eliminate edge effects and to grow polygonal tubes /Si/.-Another approach />3/ endorses the growing of several ribbons simultaneously by controlling a plurality of individual growth stations.
In high meniscus methods the necessary efforts in technology and skill are not so stringent because of reduced requirements in thermal and geometric control.
The extended meniscus methods except ICC still suffer from inadequate geometric con- trol. Unsolved problems seem still to be left with respect to long-term stability, continuous recharging and handling of grown ribbons. These problems are intensified at increased growth speeds.
3.3 Productivity. - Part of the ribbon technologies still has to be considered as a proposal for on economic production. In some coses the potentiolities of the indi- vidual procedure have been predestined on the basis of hazardous projections. Exten- sive experiences have been obtained up to now only in the EFG process /i4/; since several years a pilot production is running with this process. Primary experiences in a continuous production have been reported for WEB / 2 5 / and RAD / i 6 / . Multiple ribbon growth with RTR has recently been demonstrated.
The data in fig. 1 1 show an increase of growth velocity from low meniscus to exten- ded meniscus method, which is equated by an increase of throughput per equipment unit. Although the data in fig. 1'2 are compiled in a crude manner and should be
judged cautiously, they reveal some striking results.
Two preparation01 methods are far ahead of the others. Whereas continuous casting has to suffer from the bottleneck of slicing, continuous foil casting appears to be the least-limited condidate for the future.
3.4 Processing of MG/TSG-silicon. - Most of the ribbon methods hove to get along with effective segregation coefficients near 1. 1-e., segregation is negligible and, as a purification effect is missing the starting material has to be of higher qua- lity than that being used in ingot techniques. Less stringent are the high meniscus methods which (due to their more favorable segregation coefficient k<keff=l) ccn tolerate growth from impure silicon. However, in the sensitive WEB O
process high impurity levels cause a breakdown of growth interface. ESP is the only process reported which has been sucessfully used to grow silicon sheets from MG-silicon.
PV efficiences of diffused cells are not yet acceptable, only EPI cells gave satis- factory results with efficiencies between 8,3 and 10,5%. Experiences with MG- or SOG-silicon are still missing in extended meniscus methods.
4. Characterization of sheet materiol. - Silicon sheet materials obtained by the various growth techniques reveal most distinct characteristics which can be classi- fied with the key-words given in fig. 3. The method of preparation of multicrystalli- ne specimens can hove a strong influence on the types and distribution of crystal defects and impurities and thereby influence the photovoltaic conversion efficiencies in various ways. Through a complex set of interactions which include the effects of crystal defects, impurities, thermal history and thermodynamics of the system the material reaches a certain state which has to be considered for the following solar cell preparation. This state of the material is neither exactly to be defined nor definitive and stable.
Numerous investigations are under way to clarify the synergistic effects of crystal defects and impurities on photovoltaic parameters 14, 27, 28, 291. A straightforward way to elucidate the ongoing solid state reaction in the silicon matrix is the topo- graphic evaluation and coordination of crystollite pattern, impurity distribution and PV performance.
Depending on growth mechanism and actual state crystal defects show different effects on PV characteristics. Grain boundaries (gb's) act as sinks for metallic impurities and thus neutralize these liketime killers. On the other side gb-enhanced recombina- tion is of main concern in the PV application. Twin boundaries are not always found electrically inactive, decoration with dislocations renders them (locally) electri- cally active. Likewise problematic are dislocations appearing as arrays and clusters as well as point defect clusters. Coarse grained and single crystalline materials seem to be more prone to these defects because of the missing sinks at grain boundories.
Up to now up-grading processes concentrated mainly on the reduction of metallic im- purities and dopants in metallurgical silicon. But more and more carbon and oxygen turned out to be quite distinctive troublemakers 130, 31, 32, 331. These latter im- purities are electrically inactive as long as they are incorporated in substitutio- nal resp. interstitiol lattice places. If they appear in concentrations exceeding saturation precipitation is provoked thus degrading PV performance. It is not
possible to extrapolate the overall influence of the entity of impurities on PV per- formance from single effects of the individual impurities which have been evaluated by 1341.
Considering the still rather obscure conditions in the various sheet materials it seems impossible to recommend an appropriate solar cell preparation technique. High efficiency cell technology (MLAR, BSF, BSR, shallow junction), as they are used in space technology, are of only gradual help if applied to multicrystalline silicon /29/. A good quality material con be improved with these techniques, but a material of minor quality cannot be transformed to a high-quality one. Gettering and passi- vation are only effective under certain circumstances /35/. A distinct improvement through gettering is observed if there existed a short circuit problem. Highly con- sidered techniques are recently low temperature processes to keep the thermal stress of the bulk materiol low and to obviate further deleterious solid state reactions during cell preparation. The processes investigated (e.g. Schottky barrier devices 1361, ion irnplantotion /37/, laser assisted diffusion 138, 39l)hove not yet reached a stage of development which qualifies them as candidates for o low-cost mass pro-
continuous casting 0 0
+ 4- excellent + good 0 fair - insufficient
Fig. 13: Characterization of ingots
mould castin
state of start up @ growth mrturity ( not yct t r @ ~
development
Fig. 14: Classification of ingot technologies
duction
5. Conclusion. - Comparative surveys for classification of crystallization techno- logy and characterization of silicon sheet are given in figs. 13 - 15. Presently none of the crystallization techniques discussed has reached the stage of maturity according to the goals of the photovoltaic program. Only two of the sheet growth techniques (EFG, WEB) have advanced to the state of "growth". No ideal material seems to have yet been attained which can be handled easily in solar cell processes.
Near term solutions can be expected through empirical procedures for fine tuning technology and material. But most important seems a basic understanding of the material's problems as a prerequisite of optimization process technology.
+ + excellent + good 0 fair - insufficient R A F T
R Q C C C S - W E B R AD SOC
F i g . 15: C h a r a c t e r i z a t i o n of s h e e t m a t e r i a l s
6. Acknowledgment. - T h i s s t u d y has been s u p p o r t e d b y t h e B u n d e s m i n i s t e r i u m f u r Forschung und T e c h n o l o g i e under c o n t r a c t N r . NT 084510846.
+ -
0
+ +
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DISCUSSION
M. ROD0T.- You mentioned C c o n t e n t s of o r d e r 200 ppm. Is it n o t t o o high t o avoid S i c p r e c i p i t a t e s ?
D. HELMRE1CH.- UMG S i h a s a b o u t 500 ppm carbon. I n g o t s grown from g r a p h i t e moulds keep a high C c o n c e n t r a t i o n , c o n t r a r y t o q u a r t z moulds. The f i g u r e I gave should n o t b e confused w i t h t h a t r e l a t i v e t o c a s t i n g o t s i n vacuum o r i n e r t gas atmosphere.
I f t h e C-content i n s i l i c o n i s above s o l u b i l i t y l i m i t ( 2 - 3 . 1 0 ~ ~ / c m ~ i n t h e s o l i d a t m e l t i n g p o i n t ) you w i l l always f i n d S i c p r e c i p i t a t e s e i t h e r i n t h e g r a i n boundaries or/and i n g r a i n s .
G. HARBEKE.- Could you d e f i n e t h e term t e r r e s t r i a l - g r a d e s i l i c o n ?
D. HELMRE1CH.- T e r r e s t r i a l - ( o r S o l a r ) grade s i l i c o n i s d e f i n e d i n terms of t h e s t a r t i n g m a t e r i a l i n combination w i t h t h e c r y s t a l l i z a t i o n p r o c e s s and t h e c e l l technology. The s t e p s have t o be f i t t e d t o g e t h e r i n o r d e r t o g e t e f f i c i e n c i e s i n t h e module t o l e r a b l e i n t e r r e s t r i a l p h o t o v o l t a i c s (10 % AM1).
D e f i n i t i o n s l i k e i m p u r i t y c o n t e n t and spectrum a r e n o t worthwhile i n t h i s c o n t e x t because the same s t a r t i n g m a t e r i a l behaves d i f f e r e n t l y w i t h d i f f e r e n t c r y s t a l l i z a t i o n methods.
