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The Top Five Technical Challenges in Poly-crystalline Silicon. Canisius College April 5, 2011. Larry Coleman, Engineering Consultant. What is Polysilicon ?. Industry jargon for polycrystalline silicon Key raw material in manufacture of photovoltaic arrays = solar cells

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the top five technical challenges in poly crystalline silicon

The Top Five Technical Challenges in Poly-crystalline Silicon

Canisius College

April 5, 2011

Larry Coleman, Engineering Consultant

what is polysilicon
What is Polysilicon ?
  • Industry jargon for polycrystalline silicon
  • Key raw material in manufacture of

photovoltaic arrays = solar cells

  • Only abundant element to exhibit photovoltaic response ~ 20% efficiency
  • Other photovoltaic materials with better efficiency are either rare or toxic
  • Can be amorphous, poly- or single crystal
photovoltaic array economics
Photovoltaic Array Economics
  • Average US residential use is 20- 30 KW-hr/day
  • Locally we pay $0.18/KWH, some places double
  • Array cost = $2500/KW, get about 8 KW-hr/day from a 1 KW array, averaged over the year
  • Payouts are about 5 years locally, no subsidy
  • No disconnect from the grid, need battery backup
  • Adding in batteries & inverters doubles the payout time
  • Europe leads the way with subsidies
photovoltaic basics
Photovoltaic Basics
  • Solar radiation is function of latitude and cloud cover
  • Typical silicon efficiency is up to 17% of solar spectrum. Room for improvement.
  • Uses p-n junction to generate DC power
  • Wafer is about 1/100 inch thick
where do you find silicon
Where do you find silicon
  • Silicon is ~25% of the earth’s crust, as silica and silicates
  • You make MG silicon by reducing quartz silica with carbon (coke) at 2600°F = 1430°C in a submerged electric arc furnace

SiO2 + 2 C → Si + 2CO

  • Cool it down to solidify. Break it up with automatic jack-hammers to 4” chunks
  • Grind it to suit in a ball-mill or hammer-mill
  • Metallurgical Grade Silicon is about 98.5% pure
  • Used historically in steel-making and aluminum alloys
mgs to solar silicon in 10 steps
MGS toSolar Silicon in 10 steps
  • React quartz with coke → MGS
  • Fluidize and react with HCl → HSiCl3 (TCS)
  • Purify the TCS with distillation & adsorbents
  • Decompose in CVD reactor → poly rods
  • Break the rods & CZ pull single crystal boules
  • Slice the boules into wafers
  • Epitaxially react with doped gas to make p-n junction solar wafers
  • Photomask on the collection grid
  • Mount and make electrical connections
  • Encapsulate with glass as an array
five biggest technology challenges
Five biggest technology challenges
  • Challenges are where technology is lacking and improvements will make a big difference in production costs
  • Does not / can not include new advances and developments
  • European $$ support is now, especially in Germany. US support is lagging behind.
  • Where science needs more development, development needs more scientists
  • US technology is very prized globally
tech challenge 1 status
Tech Challenge #1 - Status
  • Metallurgical silicon grinding and fluidization
  • MGS is used primarily in steel- and aluminum making for alloying strength, where particle size doesn’t matter
  • Solids fluidization is mature for FCC catalyst and coal combustion – MGS is hardly known to the industry
  • Fluidization is a new and niche science
tech challenge 1 needs
Tech Challenge #1 - Needs
  • How to make a tighter grind distribution
  • How to characterize pneumatic conveying
  • How to model reaction shrinking
  • How to minimize losses, while maximizing reactivity
  • How to track electronic impurities
tech challenge 1 science tie ins
Tech Challenge #1 – Science Tie-ins
  • Fracture Physics & Materials Science
  • Interphase Physics ( solids + gas = liquid)
  • Engineering process modeling
  • Statistical analysis
tech challenge 2 status
Tech Challenge #2 - Status
  • Disproportionation Reactions
  • Used to change trichlorosilane to silane
  • Solid catalyst reactors re-arrange Si-H and Si-Cl bonds
  • Adds purification to the process ( removes Boron and Phosphorus)
  • Mechanisms and kinetics are unknown
  • Diffusion suspected in playing a role
tech challenge 2 needs
Tech Challenge #2 - Needs
  • How to model the reactions
  • How to minimize the reactor sizes
  • How to enhance the kinetics
  • How to maximize impurity retention
  • How to construct processes to recycle waste streams
tech challenge 2 science tie ins
Tech Challenge #2 – Science Tie-ins
  • Chemistry
  • Engineering process modeling
  • Better thermodynamic data analysis
  • Chemical kinetics
tech challenge 3 status
Tech Challenge #3 - Status
  • Molecular Sieve and Membrane Purification
  • Modified zeolites and other structures have potential for removing contaminants from electronic gases - minimum R&D has been done
  • Metallic membranes can remove hydrogen and other light gases, but fouling is still a problem
  • Most purification by distillation is expensive and limited in application by diffusion
molecular sieves
Molecular sieves

ZSM-5

5-8 A°

3-5 A°

8-13 A°

tech challenge 3 needs
Tech Challenge #3 - Needs
  • Use molecular size and polarity differences to drive separations to higher degree @ low price
  • Follow the example of air separation, but to greater purification on smaller quantities
  • Develop new types of sieves, rather than just “A” and “X” types. Example = ZSM-5 for gasoline
  • Better membrane coatings that can stand up to corrosive environments. Ties to a hydrogen economy.
tech challenge 3 science tie ins
Tech Challenge #3 – Science Tie-ins
  • Nano-technology / Nano-engineering
  • Molecular structure of zeolites and molecules - physics
  • Chemical modification of zeolites and other mole sieve structures
  • Physics of membrane processes – molecular diffusion
tech challenge 4 status
Tech Challenge #4 - Status
  • Chemical Vapor Deposition (CVD)
  • Some crude models available for CVD onto silicon rods and silicon wafers
  • Anecdotal evidence is that deposition rates can be boosted by 50% = large power reduction
  • Good thermophysical data is scarce
  • On-going work with CVD deposit of carbon onto graphite for better high temperature reactors
tech challenge 4 needs
Tech Challenge #4 - Needs
  • Accurate thermophysical values of components
  • Decomposition model driven by hard production data, including diffusion effects near the rod
  • Development of SiC CVD technology to retain purity and lower energy costs
  • Development of decomposition models with higher purity silane
  • Develop CVD model for carbon-graphite
tech challenge 4 science tie ins
Tech Challenge #4 – Science Tie-ins
  • Physics and material science
  • Thermodynamics
  • Chemistry at high temperatures
  • Computerized Flow Dynamic (CFD) modeling
tech challenge 5 status
Tech Challenge #5 - Status
  • Crystal and Ribbon Pulling of molten silicon at 1410°C = 2570°F
  • Continuous melt replentishment promises lower costs but hampered by crucible materials
  • Existing quartzware only lasts 100 hours
  • New ceramics and coatings are only partial solution
  • Ribbon pulling has the best long term potential
  • 30% of the silicon is lost in wafering
tech challenge 5 needs
Tech Challenge #5 - Needs
  • Improved materials of construction for crucibles that will last more than 2 runs. Quartz dissolves
  • Some work has started on silicon nitride coatings to prevent wetting.
  • Best option is improved pyroltyic graphite made by CVD deposit of carbon vapor @ 1800°C
  • Melt replentishment is a target technology to make crystal pulling more continuous. Problems with hydrogen content being too high.
  • Ultimate goal is being able to pull single crystal ribbon
tech challenge 5 science tie ins
Tech Challenge #5 – Science Tie-ins
  • Materials Science
  • Physics of sub-cooled liquids
  • High temperature chemistry (reactions) with quartzware and ceramics
  • Measurement of impurities at electronic levels: FTIR, mass-spec GC
education levels needed
Education levels needed
  • Physics – likely PhD, with specialization in high temperature silicon processing, crystallography, ceramics, and zeolites
  • Chemistry – likely MS with specialization in analytical sciences ( FTIR, GC-MS, epitaxy)
  • Chem. Engineering – BS or MS, with concentration in thermodynamics, silicon chemistry, fluidization, and chemical kinetics
  • Engineering Simulation Science - MS
  • Nano-engineering – likely PhD, specialized in membranes and zeolite modification
cz puller videos
CZ Puller Videos

Kayex Video:

http://www.kayex.com/page.asp?tid=129&name=Kayex-Silicon-Crystal-Growing-Process-Demo

From inside the pull chamber, looking into the crucible:

http://www.youtube.com/watch?v=cYj_vqcyI78

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