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 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 SiO 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 → HSiCl 3 (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 A°

Tech Challenge #3 - Needs Use molecular size and polarity differences to drive separations to higher 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 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: me=Kayex-Silicon-Crystal-Growing-Process- Demo From inside the pull chamber, looking into the crucible: