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
• 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
• 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
• 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
What an array looks like
Silicon the Element
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 jackhammers 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
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
8) Photomask on the collection grid
9) Mount and make electrical connections
10) Encapsulate with glass as an array
1)
2)
3)
4)
5)
6)
7)
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
• 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
•
•
•
•
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
•
•
•
•
Fracture Physics & Materials Science
Interphase Physics ( solids + gas = liquid)
Engineering process modeling
Statistical analysis
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
•
•
•
•
•
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
•
•
•
•
Chemistry
Engineering process modeling
Better thermodynamic data analysis
Chemical kinetics
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
ZSM-5
5-8 A°
3-5 A°
8-13 A°
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
• 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
• 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
CVD Reactor
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
•
•
•
•
Physics and material science
Thermodynamics
Chemistry at high temperatures
Computerized Flow Dynamic (CFD)
modeling
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
Crystal Pulling
Silicon
More about CZ pullers
Parts of a silicon boule
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
• 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
• 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
Kayex Video:
http://www.kayex.com/page.asp?tid=129&na
me=Kayex-Silicon-Crystal-Growing-ProcessDemo
From inside the pull chamber, looking into the
crucible:
http://www.youtube.com/watch?v=cYj_vqcyI78