Lasers in Solar Cell Manufacturing SolarNovus aspect of the manufacturing process that is critical...

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LasersandSolarCellManufacturing

ByDickRieley,SalesManager,MidAltanticandSoutheastRegions,Ophir‐SpiriconLLC

TheGreenmovementisencouragingtheuseofenergyefficienttechnologiessuchassolarcells.Thistechnologyhasbeenmetwithresistanceduetoitsslowreturnoninvestment.Thisiscoupledwiththe

challengeoftheefficiencyofthematerialused.Reducingthecostofmanufacturingsolarcellsislargelyinfluencedbyproductionefficienciesaswellasthetypeofphotovoltaicmaterialsused.Manufacturingefficiencieshavebeenaddressedthroughhighspeed,nearlyfullyautomatedproductionprocessesfrom

handlingofthepanels,depositionofmaterials,tofinalpackaging.Theuseofcosteffectivephotovoltaicmaterials,however,representsarealdilemma.Thelowercostdepositionmaterialsarelessefficient,whereasthehigherefficiencymaterialscostmoreandcanpossesscarcinogenicelementsthatare

federallycontrolled.

Thesolarcellmanufacturingcommunity,althoughaddressingmoreefficientandenvironmentallysafe

materials,hasconcentratedonreducingcoststhroughautomatedprocesses.Thisrequiresmethodsthatareconsistentandreliable.Theriskoftheseoperations,however,isthattheycanproducealarge

quantityoflowcost,highqualitymaterialor,justasquickly,theycanproducesubstandardpanels.

Oneaspectofthemanufacturingprocessthatiscriticalisthescribingofthephotovoltaicmaterialontheindividualcellsonthelargepanels.Lasershaveproventobehighlyreliable,consistent,and

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predictableintheirproductionresults.Inthishighspeedmanufacturingarena,laserscaneasilykeepupwithproduction,aresilenttouse,andtypicallycontributetoacleanmanufacturingenvironment.

LaserBeamDiagnostics

However,withallthatlasershavetoofferandtheadvantagestheycanprovide,laserbeamdiagnostics

areanessentialpartoftheprocess,apartthatisoftenoverlooked.Therearetwokeydiagnosticmeasurementsthatareneededtoensurelaserconsistencyfrompaneltopanel:1)laserbeampower,and2)laserbeamdiagnostic‐size,shape,andintensity.

LaserPower:Thefirstmeasurement‐monitortheoutputpowerofthelaserusingaNISTcalibrated,

laserpowermeter.Regardlessofthequalityofthelaserbeam,ifthepowerisbelowspecification,thescribingprocesswillberejected.AsmostlaserscribersoperateatKHzrates,measuringaveragepowerissufficient.Thelaserpowerdetectorshouldhaveanactiveareatwicethesizeofthelaserbeam,with

adynamicmeasurementrangefrom100mWto100W,includingtheNISTcalibrationatthespecificoperationalwavelengthoftheprocess–typically1064nm,or532nm.Laserpower,whenmeasuredonaregularbasis,willprovideabenchmarkofprocessstabilityanduniformity.

WhenalaserpowermeteristiedintothemanufacturinglaserconsolusingUSB,automaticandperiodicmeasurementscanbetakenwithpass/failcriteriaestablishedforimmediatenotificationofameasurementoutofspecification.

LaserBeamDiagnostics:Alongwithlaserpower,thespatialinformationonthelaserbeamisequally

importanttomeasureandcontrol.Laserbeamdiagnosticstypicallyinvolvethreemeasurements;thelaserbeamsize,shape,andintensity.Intheproductionofsolarcells,thelaserbeamisusedtoscribe(ablate)thedepositedlayersofphotovoltaicmaterialdowntothebaseglass,therebyestablishingthe

individualelectricalcircuitcellsonthepanel.Withacontrolledbaselineofspatialmeasurements,theprocesscanbecontrolledwithoutallowingdeviationsinthelaserbeamthatcouldallowresidualmaterialinthescribeareapotentiallycausinganelectricalshortintheoverallpanelcircuit,resultingina

defectivepanel.

Themosttypicalmeasurementsthataremostinformativeforprocesscontrolinclude:

1)Thesizeofthelaserbeam.Theindustrystandardforbeammeasurementis13.5%ofPeak(1/e2).Thisisusefulasthecalculationisbasedonthepeakofthebeamwheretheprocessofablationis

performedandlesssoonthewingsofthebeam.

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2)Theelipticity(roundness)ofthelaserbeam.Shouldthebeamelipticitychangefromroundtoout‐of‐roundthentheablationpatternwillchangeandtheuniformityofthescribelineswillnotbeconsistent,

producingofout‐of‐specpanels.

3)Shapeoftheoverallbeam.Thiscanaffectthesizeandeffectivenessoftheablationpatternontheglasspanel.Theabovepictureisa3D/2DimageofatypicalTopHatBeamwheretheenergyisuniform

acrossthetopportionofthebeampattern.Thisisquiteusefulinpreparingauniformscribeofacertainwidthanddepth.Dependingupontheprocess,aGaussianBeampatternmaybecalledforaswell.IneithercaseabeamdiagnosticmeasurementthatcomparestheTopHatorGaussianshapetoanidealfit

isaveryinformativevalue.Trackingthesevaluestomonitorchangeisessentialforcloseandtightprocesscontrolofthesolarcelllaserprocess.

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