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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

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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

  • 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 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 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

  • 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&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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