Mal

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SILICON DEPOSITION FROM A CHLORIDE BASED MELT
Espen Olsen, Karen Sende Osen SINTEF Materials
Technology Georg Hagen NTNU, Dep. Materials
Technology and Electrochemistry
EUCHEM Conference on Molten Salts, Oxford 2002
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LACK OF SOLAR GRADE SILICON?
Feedstock (scrap) MT
MWp
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From sand to solar cells
MG-Si SoG-Si Wafers Cells Modules
Systems
Elkem
Solar Silicon ?
Scan Wafer
Scan Cell
Scan Module
Sol Energy
Research and education at NTNU and
SINTEF
4
US /kg Si
0.03
1
60
25
MG-Si
SiO2
Primary process
Siemens process
EG-Si
SoG-Si
Feedstock limitations from EG-scrap
99.9999999- 99.99999999
99.5-99.9
Carbon
Current process
SiO2
?
Primary process
New SoG process
SoG-Si
99.99-99.9999?
MG-Si
Carbon
New direct route to SoG-Si?
lt20 /kg?
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Target impurity concentrations (ppmw) in SoG
silicon
SartiEinhaus (2001)
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Influence of impurities in Si on solar cell
efficiency
Davies et al (1980)
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Why an electrochemical process?

• Main contaminants in metallurgical Si (MG-Si)
  originates from the raw materials carbon, quartz
  (SiO2) and processing equipment
• In an electrochemical process, carbon may be
  avoided and other materials can be used for
  construction of the process equipment
• Molten salt electrolysis of Al occurs at lt1000
  ºC.
• Very pure Si can be deposited in fluoride based
  electrolytes from dissolved SiO2 (Stubergh et
  al.)
• Electrochemical refining of MG-Si can be
  conducted as in the so called three layer
  process

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Challenges

• Finding a suitable electrolyte capable of
  dissolving SiO2 (molten salt?)
• Processing of the deposited Si must be simple and
  economical - including a potentially purifying
  step
• The deposited Si must have acceptable purity with
  respect to non- purifyable elements (ie. B and P)
• Environmentally acceptable

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Possible chemical system

• CaCl2 - CaO - SiO2
• Reported to dissolve 5 - 10 w SiO2
• Chloride system - water soluble
• SiO2 (diss) ? Si (s) O2 (g)
• E800rev 1.86V (E800iso 2.22 V)
• No fundamental environmental problems

The CaCl2 - rich corner of the system CaCl2 - CaO
- SiO2
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Electrochemical cell
-

• Anode material Pt, graphite
• Anode product Oxygen (chlorine?)
• Cathode material Si-alloy, other metal or
  electrically conducting refractory (nitride,
  carbide)
• Cathode product Si (s) (Ca?)
• Resembles in many ways the Hall-Heroult process
  for Al production

Si-alloy, TiB2
CaCl2 - CaO - SiO2
50 mm
graphite
52 mm
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Cathodical Si deposition on Si-alloy
Si-alloy electrode before and after electrolysis
Cleaved cell after electrolysis. Note the
Si-alloy cathode with particulate deposits
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Characteristics of the deposited Si
XRD - diffractogram of the deposited Si after
treatment with water and HCl
XRD - diffractogram of the deposited Si before
treatment with water and HCl
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Contents of P and B in deposited Si
1) SINTEF Materials Technology, 2) NGU
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Cell voltage and C. E for different cathode
materials at various C. Ds
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Comments and future work

• Electrolysis of SiO2 can be conducted in the
  molten salt system chosen
• Si (s) is deposited cathodically as particles
• Current efficiency 40 - 70
• Easy removal of excess electrolyte
• Meets purity spec. wrt B (0.4 ppmw) with Si-
  alloy cathode
• Does not meet purity spec. wrt P

• Selection of raw materials - focus on P content
• Fine tuning of the process - bath chemistry,
  current density, temperature
• Electrode materials
• Chemical analysis of the deposited Si
• Scaling up
• New, previously undescribed process

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The authors wish to acknowledge support from

• SINTEF Materials Technology
• The Industrys Innovation Fund for NTNU
• The Research Council of Norway
• Elkem and ScanWafer
• The project is a part of the research program
  From Sand to Solar Cells