Multicrystalline solar silicon production for development of photovoltaic industry

Multicrystalline solar silicon production for development of photovoltaic industry A.I. Nepomnyaschikh Institute of Geochemistry, Siberian Branch, Russian Academy of Sciences, E-mail: ainep@igc.irk.ru

CONTENTS Introduction Key issues Solar silicon requirements Experiment Silicon purification by silicon melt refining Production of high purity multicrystalline SG silicon Growth of multi-crystalline silicon for solar cells Conclusion

Introduction Institute of Geochemistry For the past 10 years the annual rate of the world production of solar cells was over 30 %.% In 2005 the production output was 1318 MWt, and it will reach 4 GWt in 2010,, as predicted. [Solar Generation III.. EPIA, September 2006. http://www.epia.org/].

By the end of 2006 the cumulative installed capacity of solar photovoltaic systems around the world had reached more than 6.5 GWt, and by 2020 the forecasted capacity should reach 205 GWt. It is supposed that in 2030 the solar stations will produce about 10%, whereas in 2040 the production will make up from 20 to 28% of the cumulative world volume of electric power production. [Solar Generation IV.. EPIA, September 2007. http://www.epia.org/].

To ensure the existing tendency of the rate growth of the solar module production, the SG silicon production has to be raised to at least 40 000 tons per year by 2010.

Key issues The silicon production volume for solar energy Drastic depreciation of silicon How to approach them?

Operating scheme of silicon production for solar cells Polysilicon Carbothermic reduction SiO 2 + 2C 2 = Si + 2CO Trichlorsylane production Si + 3HCl3 HCl SiHCl 3 + H 2 Trichlorsylane rectification Reduction of trichlorsylane by hydrogen in the Siemens reactors. The silicon output is 15 %. Reduction of granulated solar grade polysilicon Scrap of single crystals

Requirements to SG silicon Over 30 % of electronic grade polysilicon is used for solar Permissible levels of impurity contents for different types of silicon Impurity, ppm EG-silicon energy 10 4 10 3 10 2 10 10 0 10-1 10-2 10-3 * 10-4 * SG-silicon * * * RMG-silicon MG-silicon * Targets: : technology installation for a direct production of multisilicon via recrystallization of refined metallurgical silicon produced from super pure quartzites. * * *

WHY multisilicon? In the past years the multicrystalline silicon became the mostly preferred material for PV production, with a total share of 58% of the shipped modules in 2004. Evolution of PV module shipments by technology between 1996 and 2004

WHY multisilicon? Lowest cost Close efficiency 14.5% Мulti and 16% Мono (commercial) 22.7% Мulti and 24% Мono (laboratory) Lowest waste volume

Solar silicon requirements (cont) Major electro-physical properties of Solar Silicon Properties Conductivity type Resistivity, Ω*cm Lifetime, µs Diffusion length of free run, µm Block size of multicrystalline silicon, mm Value P type 0.4-3 >10 >100 2

SG silicon requirements Impurity contents SG silicon (no more than, ppm) Carbon Oxygen Boron Phosphorous Sodium Magnesium Potassium Aluminium Titanium 3 10 0.3 0.2 0.2 0.2 0.5 0.1 0.01 Copper Nickel Chromium Manganese Iron Cobalt Zinc Barium Calcium How to meet them? 0.1 0.05 0.01 0.01 0.03 0.01 0.1 0.1 0.1

Institute of Geochemistry Growth of multicrystalline silicon for solar cells The Stockbarger method was applied to produce the multicrystalline silicon. It is commonly used in the monocrystal growing process. This process combines refining of metallurgical silicon from impurities and development of columnar Vienna, 13-19 Octoberof2007 structure of silicon, with the size of cross -section blocks not less than 2 mm.

Li B C N O Na Mg Al 1*10-2 0.8 7*10-2 7*10-6 1.25 2*10-3 2*10-3 2*10-3 V Cr Mn Fe Co Institute of Geochemistry Equilibrium coefficients of impurities in silicon P S Ti 0.35 1*10-5 5*10-6 5*10-6 5*10-6 1*10-5 1*10-5 1*10-5 Ni Cu Zn Ga Ge As Mo W 1*10-4 1*10-4 1*10-5 8*10-3 0.3 0.3 5*10-8 5*10-8 Zr Nb Ta Ag Cd Sb Sn Bi 2*10-8 5*10-7 2*10-8 1*10-4 1*10-6 2*10-2 2*10-2 7*10-4 Effective coefficients of impurity distribution Al Fe Mg Ti Mn Ni B 1,6*10-2 2*10-3 8*10-3 6*10-4 6,5*10-2 2*10-2 1,0

Requirements to high-purity refined silicon, ррm Carbon 3 Oxygen 50 Boron 0.3 Phosphorous 0.3 Sodium 10 Magnesium 10 Potassium 10 Aluminium 30 Titanium 3 Zirconium 3 Total impurities Base element, % Copper Nickel Chromium Manganese Iron Cobalt Zinc Barium Calcium Vanadium 1000 99.9 50 50 50 50 300 50 50 10 10 2

Experiment February 1999: high Institute of Geochemistry high-purity silicon production in one of 25 МVА furnaces of «Kremniy» Enterprise

Chemical impurity contents in raw material, ppm RMS Al Fe Mg Ca Ti Mn Ni V Cu Zr B Р 755 165 800 7.5 20 65 80 30 55 10 40 13 50 873 175 850 11 27 70 35 37 50 8 30 12 29 Average content of impurities in multisilicon samples, ppm Sample Al Fe Mg Ca Ti Mn Ni V Cu Zr B P SC-47 1 3 1 11 1 1 1 5 10 5 9 <10 SC-60 0.1 5 0.4 0.1 1 1 5 7 <10 SC-62 0.1 4 0.2 <10 0.1 1 1 <5 <2 6 7 <10

Silicon refining Institute of Geochemistry Some results of thermodynamic simulation of chemical processes in the silicon melt during refining and tentative experimental data are offered. The thermodynamic model was developed to delineate kinetics from equilibrium effects based on the database Selector software [Karpov I.K.et al., American Journal of Science, 1997. V. 297, No. 8. P. 767-806]

Approach Scheme of Selector program package External text & graphic editors Base of thermodynamics models Assistant programs Selector Generate the modelm Select computational algorithm Compute Statistical manipulation Data base of thermodynamic parameters External scientific & graphic editors

Approach (cont) Gas phase Silicon melt Model of refining silicon. Model of silicon refining was considered like a closed equilibrium system gas-melt. Equilibrium Gibbs minimization was used. Thermodynamic coefficients were corrected by experimental results.

Boron removal 0.0035 0.003 Institute of Geochemistry The thermodynamic calculations were performed initially with the database in the system Si B H 2 O Ar (or air), starting with 1 mol of Si containing 3-60 ppm of B. The calculations were performed to simulate addition of up to 0.5 mol of gas to the system. Temperature range e was 1000-3000 0 C mol 0.0025 0.002 0.0015 0.001 0.0005 0 1000 1250 1500 1750 2000 2250 2500 2750 3000 о С Dependence of boron concentration in silicon melt on temperature.

Boron removal System Si B H 2 O Ar (or air) was involved in calculations 5,0 В moll x10-5 4,5 4,0 3,5 3,0 2,5 2,0 1,5 1,0 0,5 0,0 HBO B в распл. SiO SiO2 1475 1525 1575 1625 1675 1725 1775 Dependence of boron and silicon concentrations on temperature for r system Si 1 mol.; В 0,00005 mol.; H 2 O 0.0101 mol.; Ar 0.2 mol. о С Si moll 0,20 0,18 0,16 0,14 0,12 0,10 0,08 0,06 0,04 0,02 0,00

Experiment Slag Bubbling by humidified gas Water vapour generator H2O Gas In 2003 the experiment was conducted in the 16.5 MWA electro- thermal furnace of Kremny Enterprise, Shelikhov,, Russia. The mass of silicon melt in the laddle was 3000 kg, the water vapour 9 kg and air volume 206 m 3.

Experiment Institute of Geochemistry T, 0 C arb. M Si (mol) arb. M water (mol) arb. M air (mol) Boron content, ppm Fe content, % C 0 C r C 0 C r 1760 1 0.005 0.08 53 35 0.34 0.31 C 0 impurity concentration in non-refined silicon C r impurity concentration in refined silicon

Technology to produce SG multisilicon Location of pure quartz deposits: (i) deposit of quartzite Bural- Sardag in Eastern Sayan Mts in the Republic of Buriatia, Russia and (ii) deposit of vein quartz Sarykul near Ushtobe town, Kazakhstan. Geologists of the Institute of Geochemistry SB RAS discovered deposit of super pure quartzites Bural-Sarjdag in Buriatia, which may be used for solar silicon production Super quartzites appear to be a variety of extremely pure quartzites, which represent a new type of commercial raw materials for producing super pure quartz materials and metallurgical silicon in Russia. Such kinds of deposits are known only in the USA (North Carolina) and Norway. They represent up to 80 % of the global output of super-pure quartz material.

Technology to produce SG multisilicon New deposit of particularly pure quartzite «Bural- Sardag»,, (impurity amount never more than 50 ppm) was discovered by geologists of the Institute of Geochemistry in the Eastern Sayan Mts.

Technology to produce SG multisilicon Quartzite, Charcoal Electrodes Bural-Sardag Ash: : 1.1% 1% Fe 0.0005 0.0015 0.01 Al 0.008 0.008 0.01 Ca 0.0005 0.4 0.003 P 0.0001 0.04 0.0025 B 0.00001 0.0007 0.0008 Ti 0.0005 0.0002 0.001 Ni 0.0001 0.00006 0.0002 Cr 0.0001 0.00004 0.0003 Содержание примесей в исходных материалах,, %.

Technology to produce SG multisilicon Fe Al Ca P B Ti Ni Cr Non-refined 300 350 460 20 3 26 11 30 Refined 100 30 10 0.3 0.3 3 11 30 Impurity contents in metallurgical silicon, ppm

The new technology specifically developed for multisilicon production to generate solar energy includes: Carbothermal reduction of silicon with highly-pure quartzites and specially treated charcoal applied; New technology of silicon melt refining in a ladle; Purification of Silicon from most impurities in directed crystallization of silicon. Carbothermal reduction MG Melt refining Multisilicon growth Gases Technology to direct the SG multisilicon production from highly pure refined MG silicon.

Project-oriented oriented products Multisilicon Carbon 3 Copper 0,1 Oxygen 10 Nickel 0,05 Boron 0,3 Chromium 0,01 Phosphorus 0,2 Manganese 0,01 Sodium 0,2 Iron 0,01 Magnesium 0,2 Cobalt 0,01 Potassium 0,5 Zinc 0,1 Conductivity type Parameter Value Р type Aluminium Titanium 0,1 0,01 Barium Calcium 0,1 0,1 Specific resistance, Оm*cm 0,4-3 Life time ннз, мкс >10 Diffusion length of free run ннз, мкм >80 Dimensions of blocks in multisilicon, mm 2 >2

Conclusion The Joint Stock Company Solar silicon was set up in Russia to implement the industry-based project. Agreements have been concluded with a number of the Russian and international foundations both for direct and venture funding. The investment program is targeted to industrial testing the technology of direct solar silicon production and subsequent organization and management of full-scale production of multi-crystalline silicon at the test-and and-industrial sites of Russia and Kazakhstan. The group of companies referred to TSC Group on behalf of KazSilicon Enterprise pioneered to launch the high-purity refined silicon production in Central Asia.

Conclusion Our joint plan is: (i) to produce in 2007 the first experimental- industrial lot of highly-pure refined silicon estimated as 50-100 tons; (ii) to commence on multisilicon production.

Tanks for your attention!