Edited by Daniel Abou-Ras, Thomas Kirchartz, and Uwe Rau Advanced Characterization Techniques for Thin Film Solar Cells Related Titles Würfel, P. Physics of Solar Cells FromBasicPrinciplestoAdvancedConcepts 2009 ISBN:978-3-527-40857-3 Poortmans,J.,Arkhipov,V. (eds.) Thin Film Solar Cells Fabrication,CharacterizationandApplications 2006 ISBN:978-0-470-09126-5 Luque,A., Hegedus, S.(eds.) Handbook of Photovoltaic Science and Engineering SecondEdition 2010 ISBN:978-0-470-72169-8 Edited by Daniel Abou-Ras, Thomas Kirchartz, and Uwe Rau Advanced Characterization Techniques for Thin Film Solar Cells TheEditors AllbookspublishedbyWiley-VCHarecarefully produced.Nevertheless,authors,editors,and Dr.DanielAbou-Ras publisherdonotwarranttheinformationcontained Helmholtz-ZentrumBerlin inthesebooks,includingthisbook,tobefreeof fürMaterialienundEnergie errors.Readersareadvisedtokeepinmindthat Berlin,Germany statements,data,illustrations,proceduraldetailsor [email protected] otheritemsmayinadvertentlybeinaccurate. Dr.ThomasKirchartz LibraryofCongressCardNo.: appliedfor ImperialCollegeLondon London,UnitedKingdom BritishLibraryCataloguing-in-PublicationData [email protected] Acataloguerecordforthisbookisavailablefromthe BritishLibrary. Prof.Dr.UweRau Bibliographicinformationpublishedby ForschungszentrumJülich theDeutscheNationalbibliothek Jülich,Germany TheDeutscheNationalbibliothekliststhis [email protected] publicationintheDeutscheNationalbibliografie; detailedbibliographicdataareavailableonthe Internetathttp://dnb.d-nb.de. #2011WILEY-VCHVerlagGmbH&Co.KGaA, Boschstr.12,69469Weinheim,Germany Allrightsreserved(includingthoseoftranslationinto otherlanguages).Nopartofthisbookmaybe reproducedinanyform–byphotoprinting, microfilm,oranyothermeans–nortransmittedor translatedintoamachinelanguagewithoutwritten permissionfromthepublishers.Registerednames, trademarks,etc.usedinthisbook,evenwhennot specificallymarkedassuch,arenottobeconsidered unprotectedbylaw. Typesetting ThomsonDigital,Noida,India PrintingandBinding StraussGmbH,Mörlenbach CoverDesign Adam-Design,Weinheim PrintedintheFederalRepublicofGermany Printedonacid-freepaper ISBN:978-3-527-41003-3 V Contents Preface XVII List of Contributors XXI Acknowledgments XXVII Abbreviations XXXI Partone Introduction 1 1 IntroductiontoThin-FilmPhotovoltaics 3 ThomasKirchartzandUweRau 1.1 Introduction 3 1.2 ThePhotovoltaicPrinciple 5 1.2.1 TheShockley–QueisserTheory 5 1.2.2 FromtheIdealSolarCelltoRealSolarCells 9 1.2.3 LightAbsorptionandLightTrapping 10 1.2.4 ChargeExtraction 12 1.2.5 NonradiativeRecombination 16 1.3 FunctionalLayersinThin-FilmSolarCells 18 1.4 ComparisonofVariousThin-FilmSolar-CellTypes 20 1.4.1 Cu(In,Ga)Se 20 2 1.4.1.1 BasicPropertiesandTechnology 20 1.4.1.2 Layer-StackingSequenceandBandDiagramofthe Heterostructure 22 1.4.2 CdTe 23 1.4.2.1 BasicPropertiesandTechnology 23 1.4.2.2 Layer-StackingSequenceandBandDiagramofthe Heterostructure 24 1.4.3 Thin-FilmSiliconSolarCells 25 1.4.3.1 HydrogenatedAmorphousSi(a-Si:H) 25 1.4.3.2 Metastabilityina-Si:H:TheStaebler–WronskiEffect 27 1.4.3.3 HydrogenatedMicrocrystallineSilicon(mc-Si:H) 27 VI Contents 1.4.3.4 MicromorphTandemSolarCells 27 1.5 Conclusions 28 References 28 PartTwo DeviceCharacterization 33 2 FundamentalElectricalCharacterizationofThin-FilmSolarCells 35 ThomasKirchartz,KainingDing,andUweRau 2.1 Introduction 35 2.2 Current/VoltageCurves 36 2.2.1 ShapeofCurrent/VoltageCurvesandtheirDescriptionwith EquivalentCircuitModels 36 2.2.2 MeasurementofCurrent/VoltageCurves 41 2.2.3 DeterminationofIdealityFactorsandSeriesResistances 42 2.2.4 Temperature-DependentCurrent/VoltageMeasurements 44 2.3 QuantumEfficiencyMeasurements 47 2.3.1 Definition 47 2.3.2 MeasurementPrincipleandCalibration 49 2.3.3 QuantumEfficiencyMeasurementsofTandemSolarCells 51 2.3.4 DifferentialSpectralResponse(DSR)Measurements 52 2.3.5 InterpretationofQuantumEfficiencyMeasurementsinThin-Film SiliconSolarCells 53 References 58 3 ElectroluminescenceAnalysisofSolarCellsandSolarModules 61 ThomasKirchartz,AnkeHelbig,BartE.Pieters,andUweRau 3.1 Introduction 61 3.2 Basics 62 3.3 SpectrallyResolvedElectroluminescence 65 3.4 SpatiallyResolvedElectroluminescenceofc-SiSolarCells 68 3.5 ElectroluminescenceImagingofCu(In,Ga)Se Thin-Film 2 Modules 71 3.6 ModelingofSpatiallyResolvedElectroluminescence 75 References 77 4 CapacitanceSpectroscopyofThin-FilmSolarCells 81 JenniferHeathandPawelZabierowski 4.1 Introduction 81 4.2 AdmittanceBasics 82 4.3 SampleRequirements 83 4.4 Instrumentation 84 4.5 Capacitance–VoltageProfilingandtheDepletionApproximation 85 4.6 AdmittanceResponseofDeepStates 86 4.7 TheInfluenceofDeepStatesonCVProfiles 90 4.8 DLTS 91 Contents VII 4.8.1 DLTSofThin-FilmPVDevices 94 4.9 AdmittanceSpectroscopy 95 4.10 DriveLevelCapacitanceProfiling 97 4.11 Photocapacitance 98 4.12 TheMeyer–NeldelRule 99 4.13 SpatialInhomogeneitiesandInterfaceStates 100 4.14 Metastability 102 References 102 PartThree MaterialsCharacterization 107 5 CharacterizingtheLight-TrappingPropertiesofTexturedSurfaces withScanningNear-FieldOpticalMicroscopy 109 KarstenBittkau 5.1 Introduction 109 5.2 HowDoesaScanningNear-FieldOpticalMicroscopeWork? 110 5.3 LightScatteringintheWavePicture 112 5.4 TheRoleofEvanescentModesforLightTrapping 113 5.5 AnalysisofScanningNear-FieldOpticalMicroscopyImagesbyFast FourierTransformation 116 5.6 HowtoExtractFar-FieldScatteringPropertiesbyScanningNear-Field OpticalMicroscopy? 120 5.7 Conclusion 122 References 122 6 SpectroscopicEllipsometry 125 SylvainMarsillac,MichelleN.Sestak,JianLi,andRobertW.Collins 6.1 Introduction 125 6.2 Theory 127 6.2.1 PolarizedLight 127 6.2.2 ReflectionfromaSingleInterface 128 6.3 EllipsometryInstrumentation 129 6.3.1 RotatingAnalyzerSEforEx-SituApplications 130 6.3.2 RotatingCompensatorSEforReal-TimeApplications 131 6.4 DataAnalysis 133 6.4.1 ExactNumericalInversion 133 6.4.2 Least-SquaresRegression 134 6.4.3 VirtualInterfaceAnalysis 134 6.5 RTSEofThinFilmPhotovoltaics 134 6.5.1 ThinSi:H 135 6.5.2 CdTe 139 6.5.3 CuInSe 141 2 6.6 SummaryandFuture 145 6.7 DefinitionofVariables 145 References 146 VIII Contents 7 PhotoluminescenceAnalysisofThin-FilmSolarCells 151 ThomasUnoldandLeventGütay 7.1 Introduction 151 7.2 ExperimentalIssues 154 7.2.1 DesignoftheOpticalSystem 154 7.2.2 Calibration 156 7.2.3 Cryostat 156 7.3 BasicTransitions 157 7.3.1 Excitons 158 7.3.2 Free-BoundTransitions 159 7.3.3 Donor–AcceptorPairRecombination 160 7.3.4 PotentialFluctuations 162 7.3.5 Band–BandTransitions 163 7.4 CaseStudies 164 7.4.1 Low-TemperaturePhotoluminescenceAnalysis 164 7.4.2 Room-TemperatureMeasurements:EstimationofV from oc PLYield 168 7.4.3 SpatiallyResolvedPhotoluminescence:Absorber Inhomogeneities 170 References 173 8 Steady-StatePhotocarrierGratingMethod 177 RudolfBrüggemann 8.1 Introduction 177 8.2 BasicAnalysisofSSPGandPhotocurrentResponse 178 8.2.1 OpticalModel 178 8.2.2 SemiconductorEquations 180 8.2.3 DiffusionLength:Ritter–Zeldov–WeiserAnalysis 181 8.2.3.1 EvaluationSchemes 183 8.2.4 MoreDetailedAnalyses 184 8.2.4.1 InfluenceoftheDarkConductivity 184 8.2.4.2 InfluenceofTraps 184 8.2.4.3 Minority-CarrierandMajority-CarrierMobility-Lifetime Products 186 8.3 ExperimentalSetup 187 8.4 DataAnalysis 189 8.5 Results 192 8.5.1 HydrogenatedAmorphousSilicon 192 8.5.1.1 TemperatureandGenerationRateDependence 192 8.5.1.2 SurfaceRecombination 193 8.5.1.3 Electric-FieldInfluence 193 8.5.1.4 Fermi-LevelPosition 194 8.5.1.5 DefectsandLight-InducedDegradation 194 8.5.1.6 Thin-FilmCharacterizationandDepositionMethods 195 8.5.2 HydrogenatedAmorphousSiliconAlloys 196 Contents IX 8.5.3 HydrogenatedMicrocrystallineSilicon 196 8.5.4 HydrogenatedMicrocrystallineGermanium 197 8.5.5 OtherThin-FilmSemiconductors 197 8.6 Density-of-StatesDetermination 198 8.7 Summary 198 References 198 9 Time-of-FlightAnalysis 203 TorstenBronger 9.1 Introduction 203 9.2 FundamentalsofTOFMeasurements 204 9.2.1 AnomalousDispersion 205 9.2.2 BasicElectronicPropertiesofThin-FilmSemiconductors 207 9.3 ExperimentalDetails 208 9.3.1 AccompanyingMeasurements 210 9.3.1.1 Capacitance 210 9.3.1.2 Collection 212 9.3.1.3 Built-inField 212 9.3.2 CurrentDecay 212 9.3.3 ChargeTransient 215 9.3.4 PossibleProblems 217 9.3.4.1 DielectricRelaxation 217 9.3.5 InhomogeneousField 218 9.4 AnalysisofTOFResults 219 9.4.1 MultipleTrapping 219 9.4.1.1 OverviewoftheProcesses 219 9.4.1.2 EnergeticDistributionofCarriers 220 9.4.1.3 TimeDependenceofElectricalCurrent 223 9.4.2 SpatialChargeDistribution 223 9.4.2.1 TemperatureDependence 223 9.4.3 DensityofStates 225 9.4.3.1 WidthsofBandTails 225 9.4.3.2 ProbingofDeepStates 226 References 228 10 Electron-SpinResonance(ESR)inHydrogenatedAmorphous Silicon(a-Si:H) 231 KlausLips,MatthiasFehr,andJanBehrends 10.1 Introduction 231 10.2 BasicsofESR 232 10.3 HowtoMeasureESR 235 10.3.1 ESRSetupandMeasurementProcedure 235 10.3.2 PulseESR 238 10.3.3 SamplePreparation 239 10.4 ThegTensorandHyperfineInteractioninDisorderedSolids 240 X Contents 10.4.1 ZeemanEnergyandgTensor 240 10.4.2 HyperfineInteraction 243 10.4.3 Line-BroadeningMechanisms 245 10.5 DiscussionofSelectedResults 248 10.5.1 ESRonUndopeda-Si:H 248 10.5.2 LESRonUndopeda-Si:H 252 10.5.3 ESRonDopeda-Si:H 253 10.5.4 Light-InducedDegradationina-Si:H 257 10.5.4.1 ExcessCharge-CarrierRecombinationandWeakSi–SiBond Breaking 258 10.5.4.2 Si–HBondDissociationandHydrogenCollisionModel 260 10.5.4.3 TransformationofExistingNonparamagneticCharged Dangling-BondDefects 260 10.6 AlternativeESRDetection 263 10.6.1 HistoryofEDMR 264 10.6.2 EDMRona-Si:HSolarCells 265 10.7 ConcludingRemarks 268 References 269 11 ScanningProbeMicroscopyonInorganicThinFilmsforSolarCells 275 SaschaSadewasserandIrisVisoly-Fisher 11.1 Introduction 275 11.2 ExperimentalBackground 276 11.2.1 AtomicForceMicroscopy 276 11.2.1.1 ContactMode 277 11.2.1.2 NoncontactMode 278 11.2.2 ConductiveAtomicForceMicroscopy 279 11.2.3 ScanningCapacitanceMicroscopy 280 11.2.4 KelvinProbeForceMicroscopy 282 11.2.5 ScanningTunnelingMicroscopy 284 11.2.6 IssuesofSamplePreparation 285 11.3 SelectedApplications 286 11.3.1 SurfaceHomogeneity 286 11.3.2 GrainBoundaries 288 11.3.3 Cross-SectionalStudies 291 11.4 Summary 294 References 294 12 ElectronMicroscopyonThinFilmsforSolarCells 299 DanielAbou-Ras,MelanieNichterwitz,ManuelJ.Romero, andSebastianS.Schmidt 12.1 Introduction 299 12.2 ScanningElectronMicroscopy 299 12.2.1 ImagingTechniques 301 12.2.2 ElectronBackscatterDiffraction 302