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 to Solar Silicon in 10 steps

1) React quartz with coke → MGS

2) Fluidize and react with HCl → HSiCl3 (TCS)

3) Purify the TCS with distillation & adsorbents

4) Decompose in CVD reactor → poly rods

5) Break the rods & CZ pull single crystal boules

6) Slice the boules into wafers

7) 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

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

3-5 A° 8-13 A°

ZSM-5

5-8 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 PullingSilicon

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