光伏发电技术应用（推荐） 47p

Applications of Photovoltaic Technologies,2,Summery of losses in Solar cell,3,Summery of losses in Solar cell,4,Typical cell parameters,5,Typical cell parameters,6,Design tradeoffs: Efficiency Vs cost,Cost of electricity (Rs / kWh) ,Module cost (Rs / W) ,10%,15%,20%,25%,High cell efficiencies are obtained in the laboratory using State-of-the-art technology in the lab produces,But commercially mass produced cell efficiency lies between 13 16% research techniques used in the laboratory are not suitable for commercial production,With higher efficiency modules, the cost per unit area can be much higher for a given cost target of electricity in kWh. (With high efficiency module additional costs, land, material are less.),7,Solar PV Technologies,Wafer based Si solar cells,Thin-film solar cells,Production of Si,Semiconductor Fundamentals, P-N Junction, Solar cell Physics, Solar cell design,8,Contents- Production of Si,Solar PV Chain Why Si for PV?Demand for Si feedstockSi wafer production process EG poly-Si (Siemens type, FBR)CZ & FZ process of ingot productionwafer dicingSi feedstock from various sourcesMulti-crystalline Si wafers and ribbon Si,9,Si for PV,Solar energy (PV) is a very fast growing market where the basic technology depends on availability of pure Si. This material is today in high demand and a shortage is expected.Most analysts assume that silicon will remain the dominant PV material for at least a decade.,One of Shells energy scenario indicates that solar energy will be the single largest energy source within 2060. Solar PV would play important role in it,10,Why Silicon?,At the time being it is almost the only material used for solar cell mass production Easily found in nature, Silicon oxide forms 1/3 of the Earths crust It is non-poisonous, environment friendly, its waste does not represent any problems It is fairly easy formed into mono-crystalline form Its electrical properties with endurance of 125C Si is produced with 99.9999999% purity in large quantities.,11,Solar PV market,Crossing the GW-level: Last year alone worldwide solar cell production reached 1,256 MW (in 2004), 67 percent increase over the 750 MW output in 2003.,Solar PV industry has recorded a growth of 30% in the last decade,12,Contribution of Si in PV market,Others include CdTe, CIGS, C-Si/a-Si (4.5%) Over 90% of solar cell are made of Si,13,Companies producing Si,Si Wafer Manufacturers Hemlock (USA) SEH, SUMCOWacker Chemie (Germany)Tokuyama Soda (Japan)ASiMi (USA)MEMC Electronic Material Inc., (USA),Dedicated manufacturers for PV (wafers and cells)Kyocera (Japan), BP Solar (USA), Shell Solar (USA), Photowatt (France). RWE Schott (USA/Germany),14,Wafers for solar cells,Crystal typeSingle crystal Si wafersMulti-crystal Si Wafers,Shape Circular Pseudo square Square,15,Si Wafer Production,High temp, Carbon,16,Solar cell Silica to Si wafer,17,Solar PV Chain,There are several steps from raw material to power systems,18,Metallurgical grade (MG) Si,MG-Si is material with 98-99% purity, Produced in about 1 Million tons per yearProduced in countries which cheap electricity and quartz deposits (USA, Europe, Brazil, Australia, Norway),Average price is 2 to 4 $/kgMG-Si is produced by reduction of SiO2 with C in arc furnace at 1800 oC.SiO2 + C Si + CO2,Application in producing chlorosilane for electronic grade Si production, production of Al and SteelTypical impurities are iron (Fe), aluminium (Al), calcium (Ca) and magnesium (Mg),19,Electronic grade (EG-Si),Electronic grade (EG-Si), 1 ppb Impurities (i.e. 99.99999999% purities) MG-Si EG-Si: impurities reduction by five order of magnitude is required convert MG-Si to gaseous chlorosilanes or silane, purified by distillation For instance Trichlorosilane SiHCl3 and silane SiH4,On chlorination of MG-Si Si + 2Cl SiCl4The following reactions result in tri-chlorine-silane gas: SiCl4 + HCl SiHCl3,20,Poly Si- Siemens type reactor, Deposition process is slow 10 days/ton using 12 Siemens reactors,Generate by-products containing chlorineWacker, Hemlock, Mitsubishi, Tokuyama, Sumitomo SiTiX, MEMC Italia,21,Poly-Si -Fluidized bed reactor (BFR),Continuous process considerably higher production rates and lower energy consumption Yielding silicon of the highest purity,Silicon seed particles are held in suspension by a gas mixture (H2 and SiH4) At 600C gas phase decomposition takes place, causing the seed particles to grow up to 2 mm in size Big particles falls due to weight Si is collected from the bottom of the jar,22,Wafer-manufactured process,23,Production of sc-Si, Poly-EGS is melted in a quartz crucible (SiO2) Seed particle introduced to begin crystallization Seed pulled to generate desired wafer diameter Ingot is cooled Crucible is discarded (warping and cracking),Czochralski (CZ) process,24,Czochralski (CZ) process,25,Production of sc-Si,Rod of solid, highly purified but polycrystalline silicon is melted by induction heating Single crystal is pulled from the molten zone.,More expensive than Czochralski (Cz) material,This material is of exceptional purity because no crucible is needed Record efficiency solar cells have been manufactured with float zone,Float Zone (FZ),26,Wafer dicing,Inner diameter (ID) saw where diamond particles are imbedded around a hole in the saw blade Si is hard material Almost 50% of the material is lost with ID sawing,Using wire sawing thinner wafers can be produced and sawing losses are reduced by about 30%,Inner diameter sawing,wire sawing,Diamond particles,Sawing of pseudo square wafer,27,Solar grade Si (SOG-Si),Cost of electronic grade Si is 30-45 $/kg too high for solar cells (area related),Production with process modifications with relaxed specification allowing the silicon materials industry to produce at lower cost while meeting the requirements,Earlier approaches in 1980 did not work did not work Low production volume, insufficient purification,Present efforts to produce solar grade Si by purifying metallurgical-grade (MG) silicon Modifying Seimens reactor process and fluidized bed reactor process REC +ASiMi produced 2000 tons of Solar grade Si in 2003,28,Production of mc-Si,mc-Si ingot,Si melt,Heat exchanger,Direction solidification,Poly-Si,Casting,29,Dicing of mc-Si,Wire sawing,mc-Si Ingot,mc-Si wafer,Dicing,30,Production of mc-Si,Reducing material consumption by:Producing thinner wafersReducing kerf loss,Wafer thickenss 250 m Very low kerf loss Efficiency over 14%,EFG growth,Methods of producing Si ribbonsThe edge defined film fed growth process (EFG)Ribbon growth on substrate (RGS)Silicon sheets from powder (SSP),SSP growth,Si sheet from powder,Si ribbons,31,What is the best material for PV?,According to solid state physics Si in not the best material 90% absorption of spectrum requires 100 m of Si while only 1 m of GaAs Si indirect bandgap material Larger thickness also demand for higher quality material, generated carrier needs to diffuse longer Diffusion length should be double of wafer thickness, at least 200 m Si still is material of choice due to well developed micro-electronics industry,32,Optimum efficiency vs bandgap,Efficiency,33,Ideal solar cell material,Bandgap between 1.1 to 1.7 evDirect band structureConsisting of readily available, non-toxic material Easily reproducible deposition techniques, suitable for large area production Good PV conversion efficiency Long-term stability,34,Early Si solar cells,Grown Jn,Cell reported in 1941, Grown junction, Efficiency much less than one percent,Cell reported in 1952, Implanted junction Efficiency about one percent,Cell reported in 1954, Bell LabsHigh temperature diffused junction Single crystal, CZ method 6% cell efficiency,35,Early Si solar cells,In 1960s solar cell were used only for space craft applications Cell design as shown here cell efficiencies up to 15%,In 1970 cell design was changed (COMSAT labs) Thinner emitter and closed spaced metal fingers (improved blue response) Back surface field so called “violet cell” due to lower wavelength reflection,Further improvement in cell efficiencies have been obtained due to anisotropic texturingThese approached improved the current collection ability of solar cells,36,High efficiency solar cells,In 1980s it was clear that cell surface Passivation is key to obtain high open circuit voltage Passivated emitter solar cell (PESC) exceeded 20% efficiency in 1985 Passivation was obtained by thin thermally grown oxide layer use of photolithography to have small contact area and high aspect ratio,Buried contact solar cells New feature incorporating laser grooving and electroplating of metal to avoid photolithography Oxide layer is also used as a mask for diffusion in groves and metallization High metal aspect ratio,37,High efficiency solar cells,light,Rear point contact solar cell demonstrated 22% efficiency in 1988 Both contacts are made at rear surface no shadowing due to metal contact design is feasible only when high quality of Si is used mostly used under concentrated sunlight,Highest efficiency Si cell structure reported till now (24.7%) PESC with both front and rear side Passivation Local diffusion at rear side to make low resistance contact,Light,38,Features of High Efficiency Solar Cell,Techniques for highest possible efficiencies: lightly phosphorus diffused emitters, to minimize recombination losses and avoid the existence of a dead layer at the cell surface; closely spaced metal lines, to minimize emitter lateral resistive power losses; very fine metal lines, typically less than 20 m wide, to minimize shading losses;,Route to high efficiency solar cells,Low recombination High carrier absorption,Solar cell efficiencies increased with technological development,39,Features of High Efficiency Solar Cell,Techniques for highest possible efficiencies: top metal grid patterning via photolithography; low metal contact areas and heavy doping beneath the metal contact to minimize recombination; use of elaborate metallization, such as titanium/palladium/silver, that give very low contact resistances; good rear surface passivation, to reduce recombination; use of anti-reflection coatings, which can reduce surface reflection from 30% to well below 10%.,40,Generic industrial mc-Si Cell Process,Wafer Cutting,Wet Acidic Isotropic texturing,POCl3 Diffusion,Parasitic Junction Removal,PECVD SiNx:H ARC layer,Co-firing,Screen Printed Metallisation,Standard process,Process simplifications Mono-Si block-cast mc-Si wafers Si ribbons to avoid kerf losses Double layer ARC single layer ARC Photolithographic finger patterns screen printing,Solar cell performance: 12 - 16%,41,Choice of staring wafer,Starting wafer: 400 m thick, area 10 X 10 cm2, or 12.5 X 12.5 cm2. P-type doped with boron concentration of 1016 - 1017 cm-3,high doping to reduce minority carriers concentration, & low doping to increase minority carrier life time,lower the minority concentration lower forward bias diffusion current and higher is Voc,lower the doping higher minority carrier lifetime higher is Voc,Doping 1016 #/cm3,42,Junction formation,43,Etching & texturing,Wet chemical etching & Dry chemical