非晶半导体太阳能电池

MSE-630AmorphousSemiconductorsAndSolarCellsMSE-630AmorphousSemiconductorsareusedinmanyapplications,including:SolarCellsSwitchingdevicesThinFilmDisplaysElectrophotographyMSE-630Thin-FilmTransistor(TFT)LCDDisplaysLiquidCrystalDisplaysDevelopedbyRCALaboratoriesin1968Workbyactingasa“lightvalve”eitherblockinglightorallowingittopassAnelectricfieldisappliedtoalterthepropertiesofeachLiquidCrystalCell(LCC)tochangeeachpixel’slightabsorptionpropertiesColorsareaddedthroughfilteringprocessModernLaptopsproducevirtuallyunlimitedcolorsatveryhighresolutionThin-FilmTransistor(TFT)LCDDisplaysPathoflightthroughaTFTLCDLightcomesfrombehind–eitherLEDorfluorescentsourceBeamoflightispolarized,thengoesthroughTFTmatrix,whichdecideswhichpixelsshouldbe“on”or“off”If“on”,moleculesinLCCwillaligninasingledirection,allowinglighttopassColorfiltersblockallwavelengthsoflightexceptthosewithintherangeofthepixel.Areasbetweenpixelsareprintedblacktoincreasecontrast.ExitinglightpassesthroughanotherpolarizertosharpenimageandeliminateglareMSE-630InanTFTdisplay,eachLCCisstimulatedbyadedicatedthin-filmtransistormatrix,withonetransistorateachpixel.MSE-630LCDAddressingModesThreetypesofaddressinghaveemergedsinceLCDbecamethedisplaymediumin1971:DirectMultiplexActiveMatrixActivematrixallowschargestorage,enablingpixelstorefreshenablingreal-timevideoonlargescreensInDirect,onesignalcontrolsmanysegments.Usefulfornumericdisplays,e.g.,watchesandcalculatorsWiresinMultiplexaresharedthroughamatrixwiringscheme,allowingseparatesignalstobedeliveredtoeachpixelMSE-630ManufacturingandDisplayConfigurationsPhotolithographyusedtolayinsulators,transistorsandconductorsdownonaglasssubstrate–thelowerglassinanLCDTFTdisplaysrequireatransistorandcapacitorforeachpixelForhighestfidelity,RGBisreplacedbyGRGBandRGBDeltaDisplaysMSE-630Threeswitchtechnologies:AmorphousSilicon(a-Si),PolycrystallineSilicon)p-Si)andSingleCrystalSilicon(x-Si)AmorphousSiliconisthestandardforTFTLCDsbecausetheyhave:GoodColorGoodGrayscaleReproductionFastResponseAdvantages:Ana-SiTFTproductionprocessrequiresonly4basiclithographysteps,andproducesgoodqualitylargescreens–lowcostDisadvantage:Becausea-Sihaslowmobility,acapacitormustbeaddedtoeachpixelTFTProcessMobility(cm^2/Vsec)A-Si0.3-0.7conventionalp-Si6eximerp-Si329singecrystalSi(x-Si)600MSE-630PolycrystallineSiliconDisadvantage:Requireshigherprocesstemperaturesthana-Si–600oCsoftensmosttypesofglassAdvantage:Addingonlytwoprocesssteps,NMOSandPMOStransistorscanbeformedMeetrequirementsforHDTVdisplaysp-Sijunctiona-SijunctionMSE-630BreakthroughTechnologyTheeximerlaserannealingprocessiscapableofrecrystallizingp-Sifilm,increasingitsmobility660times.ThisispossiblebecausepolycrystallineSiabsorbsUVlight.Theabsorbedenergyraisesthetemperatureofthep-Sifilm,thusannealingit.Theeximerlaserprocessallowsacheaperandmoreconventionalglasstobeusedasasubstrate,reducingproductioncostsforthemassproductionofp-SiTFTs.MSE-630PhotovoltaicsPhotoelectriceffectdiscoveredbyEdmundBequerelin1830AlbertEinsteinreceivedtheNobelPrizefordescribingthenatureoflightandthephotoelectriceffectin1905BellLaboratoriesmadethefirstphotovoltaicmodulein1954.Thespaceindustryinthe1960sandtheenergycrisisinthe1970sspurredfurtherphotovoltaicdevelopmentMSE-630Photovoltaics–OperatingPrinciplesPhotovoltaics,alsoknownasSolarCellsaresemiconductors,typicallySiliconAsolarcellusesjunctionsofann-typesemiconductor(freelymovingelectrons)withap-typesemiconductor(freelymovingholes)whichcreatesatypeofdiodethatisinelectricequilibriuminthedarkPhotons(electromagneticradiation)fromthesunfreeelectronsandholes,causingaDCcurrenttoflowfromthen-tothep-typematerialSeveralcellsareplacedinseriesinmodulestoachievehighervoltagesandpowerTwoPhotovoltaicCellTypesMSE-630Singlecrystalorpolycrystallinecells–use“doped”crystalsformakingthecells,muchlikecomputerchipsThisisthemostcommontechnologyCrystallinecellsareexpensivebutlastmanyyearswithlittledegradationSiliconisthemostcommonmaterial,althoughothersareunderdevelopment,suchasGalliumArsenideandIndiumSelenideImprovingSolarCellEfficiencyMSE-630TheenergyofaphotonisE=hnElectronsareelevatedtotheconductionbandifthefrequencyofthelightequalsorexceedsthebandgapenergyThismeansthatlightatalowerfrequenciesdonoworkTogetaroundthis,cellswithdifferentbandgapenergiesareassembledintomultijunctioncellsMSE-630Multi-junctionSolarCellsThestackatrightisamultijunctionwithdescendingorderofbandgapenergy,Eg.Junctionmaterialscanbemixed(e.g.,GaAsandSi)providedtheyaredimensionallycompatibletotailorbandgapenergyMultijunctionsolarcellshavereachedefficienciesofupto35%Cellmaterialsofinterestinclude:AmorphousSiliconCopperIndiumDiselenideGalliumAresnideandGalliumIndiumPhosphideMSE-630AmorphousSiliconAmorphousmaterialshavenolong-rangecrystallineorderIn1974,researchersfoundthatphotovoltaicdevicescouldbemadeusingamorphoussiliconbyproperlycontrollingdepositionandcompositionAmorphoussiliconabsorbssolarradiation40timesmoreefficientlythansingle-crystalsilicon–afilm1-micronthickcanabsorb90%oftheusablesolarenergyAmorphoussiliconcanbeprocessedatrelativelylowtemperaturesonlow-costsubstratesmakingitveryeconomicalMSE-630AmorphousSiliconThelackofcrystallineregularityinamorphoussiliconresultsin“danglingbonds”.Here,electronsrecombinewithholes.Whenamorphoussiliconisdopedwithsmallamountsofhydrogen(“hydrogenation”),thehydrogenatomscombinechemicallywiththedanglingbonds,permittingelectronstomovethroughtheamorphoussiliconCellsaredesignedtohaveultra-thin(0.008-micron)p-typetoplayer,athicker(0.5to1-micron)intrinsic(middle)layer,averythin(0.02-micron)n-typebottomlayer.Thetoplayerissothinandtransparentthatmostlightpassesrightthrough.Thep-andn-layerscreateanelectricfieldacrosstheentireintrinsicregionMSE-630SolarCellProcessingStepsMSE-630SolarCellEfficiencyPoweristheproductofvoltageandcurrent:VmaxXImax=PmaxAsolarcellsenergyconversionefficiency,(hor“eta”)isthepercentageofpowerconverted(fromabsorbedlighttoelectricalenergy)andcollected,whenasolarcellisconnectedtoanelectricalcircuit.ItiscalculatedusingtheratioofPmaxdividedbytheinputlightirradianceunder“standard”testconditions(E,inW/m2)andthesurfaceareaofthesolarcell(Acinm2)MSE-630MSE-630MSE-630EconomicsofSolarPowerMSE-630CrystallinePVCellEconomicsTotalcostofconventionalcrystallinePVcellsisabout$500/m2($50/sq.ft)ofcollectorareaTheoutputof1-m2is125Watts,so,atacostof$500/m2,thiscorrespondsto$4/Wattofelectricity,notcountingnecessaryauxiliarycomponentsThelowestreportedcostare$3/Wattforphotovoltaiccellsin2002(IEA).Crystallinesiliconcellsaccountedfor80%ofthetotalworldwidein2002.MSE-630Efficienciesvaryfrom6%foramorphousSicellstoupto35%forexoticGaAsorInSecellsEfficienciesEfficiencyis14-16%incommerciallyavailablemc-SicellsPhotovoltaics-EconomicsPowerdistributionsystemsincludeinverterstoconnecttothegrid–systemefficienciesarebetween5-19%AGaAsorInSecelldelivers4timestheelectricalpower–atover100timesthecost!CostsIn2005,photovoltaicelectricitycost$0.30-$0.60/kWhintheUS.Comparethistothe~$0.10/kWhfromothersourcesThepaybackperiodforsolarcellimplementationcanbefrom1to20years.Atypicalvalueis5yearsMSE-630MSE-630AlternativeenergysourcesMSE-630MSE-630MSE-630MSE-630SolarSteamPlant–FourCorners,CAMSE-630MSE-630OpticalMemoryandDataStorageUseamorphousChalcogen(groupVIelements,e.g.Se,SorTe)Photo-inducedphasetransitionsbetweencrystallineandamorphousphasesPhoto-inducedphasetransitionsbetweencrystallineandamorphousphasesorreversiblephotostructuralchangesintheamorphousphaseLightinducescross-linkingofneighboringchainsinSe.Whenaphotonisabsorbed,anelectronfromoneofthenon-bonding(lone-pair)orbitalsthatformthetopofthevalencebandistransferredintotheconductionband,leavingtheotherelectronunpaired.Thisunpairedelectroncan,throughinteractionwithlone-pairelectronsofaneighboringchain,formanadditiona