1、Guest Lectures12.1 Prof.Zhaosheng Li(Photo Electro-chemical Cells)12.4 Dr.Bin Zhu(Li-ion Battery)2c-Si and mc-Si are made using similar processes Multicrystalline Silicon(mc-Si)Produced via directional solidification.Crystalline Silicon(c-Si)Can be made with CZ or FZ processes.Si crystal formationQu
2、artz Metallurgical Grade Si(mg-Si)Solar Grade Silicon(SoG-Si)mc-Si c-Si (Siemens Process)(Directional Solidification)(CZ growth)(wafer sawing mc-Si and c-Si)wafer processing PV Cell fabrication Module BOS+Installation (wafer sawing mc-Si and c-Si)(wafer doping,surface etching,electrode deposition et
3、c.)PV cell interconnects+bypass diodes,encapsulation,etc.Cell and Module ManufactureMetallurgical Grade Silicon Metallurgical grade Si(mg-Si)is made by heating sand with carbon in a furnace.SiO2+2C Si+2CO 1 million metric tons/yr of mg-Si is made for steel and aluminummg-Si is 98-99%pure.No shortage
4、 of mg-Si!Energy efficient process Lots of(very bad)impurities6Siemens Process for Electronic Semiconductor Grade(eg-Si)and Solar Grade Si(SoG-si)Si+3HCl SiHCl3+H2The boiling point of SiHCl3 is 31.8 C.Multiple fractional distillations are used to make the SiHCl3 99.9999999%pure.The SiHCl3 is exposed
5、 to a heated Si rod.SiHCl3 +H2 Si+3HClThis process consumes a lot of energy.7Making Single Crystal Ingots:Czochralski Growth(CZ)The semiconductor grade Si is melted with the dopant(B).A seed crystal is then slowly pulled from the melt.Oxygen dopes(1018/cm3)Si ingot because the quartz crucible is par
6、tially dissolved.Plummer Silicon VLSI Technology(Chapter 3)8Making Single Crystal Ingots:Float Zone(FZ)Need to use crack free polysilicon rods as the starting material.Higher quality crystals than CZ with few impurities.Plummer Silicon VLSI Technology(Chapter 3)siliconsultant9Comparison Between FZ a
7、nd CZ siliconsultantMulticrystalline Casting Processes11Directional Solidification(DS)crystalsystems18%mc-Si cell.in that square!Most likely 12-14%efficient over a standard module size.13Ribbon SiThis approach has the potential to be cheaper,but produces polycrystalline Si.Takes about 90 minutes to
8、produce two 6-foot long strips(time elapsed video on Evergreen website).evergreensolar/app/en/technology/item/4814Comparison of most popular techniquessiliconsultant15Making Si WafersArcher&Hill,“Clean Electricity from Photovoltaics”,p.167,Fig.4.10.Inside-diameter(ID)saw produces about 25 wafers/hou
9、r High-speed wire saws can produce about 500 wafers/hour 16Polishing the Wafer The wafers are chemically etched to remove the damage from sawing.Wire saws creates shallow surface damage 10 m.ID saws have an average of 30 m in surface damage.17Si Wafer(kerf)Loss 50%of the ingot is lost during the saw
10、ing process!At 20 wafers/cm and a wafer thickness 300 m,breakage is on the order of 15%.When the wafer thickness is decreased to 200 m the breakage is often 40%.18PV Cell FabricationArcher&Hill,“Clean Electricity from Photovoltaics”,p.155,Fig.4.3A19Doping the WaferAn oxide layer w/P forms.T=800-900
11、C.After 20 min.P overrides the B to make the top of the wafer n-type.Green,“Solar Cells”p.109,Fig.6.4Plummer Silicon VLSI Technology(Chapter 7)20Removing the Oxide Accidental doping along the side can reduce Rshunt.The wafers can be stacked like a roll of coins and plasma etched to remove the doped
12、region on the side.Green,“Solar Cells”,p.109,Fig.6.521Depositing Electrodes on a Small ScaleIn research labs,the electrodes are deposited in a thermal evaporator.The entire back side can be coated with Al.The area of metal must be minimized on the front to minimize shadowing.In many cases,a stack of
13、 Ti,Pd and Ag are deposited.A thin layer of titanium is used to promote sticking.Ag is used because of its high conductivity.Pd prevents a reaction between Ti and Ag.The metal is annealed 500-600 to improve the contact with Si.Green,“Solar Cells”,p.110,Fig.6.622Printing Electrodes For low-cost manuf
14、acturing,a paste of silver particles,glass frit and organic medium is squeezed through a patterned screening mesh onto the cell.The paste is dried at low temperature.At high T,the organic is driven off and the silver particles fuse together.The glass promotes adhesion to the Si.For more info,see Arc
15、her&Hill,p.17523Modules Slide Courtesy of Ben Carver(formerly from Sunpower)Module consists of individual photovoltaic cells connected together(72 cells in a 12x6 array is a common size),almost always in series.The most common type of encapsulant is a cross-linking EVA,or ethylene-vinyl acetate.Sola
16、r-grade glass,with excellent optical and mechanical properties at very low cost,is almost universally used as a front layer.A Tedlar-Polyester foil is usually employed as a dielectric on the back of the solar module to prevent electrocution(given that these panels are usually used in 600 or 1000 Volt strings,this layer is pretty important).Built to withstand the elements for 25 years!
