Rapid introduction of renewable energy is essential in order to meet future energy demands without further polluting the environment. Solar energy conversion D plays a very important role in this, but current solar panels based on silicon are h I. M. Dharmadasa expensive because of the high cost of processing crystalline silicon, a technology a that demands high energy consumption. The way forward is to move towards thin- r m film solar cells using alternative materials and low-cost manufacturing methods. a The photovoltaic community is actively researching thin-film solar cells based on d amorphous silicon, cadmium telluride (CdTe), copper indium gallium diselenide a s (CIGS), dye-sensitised materials, and organic semiconductors/polymers. However, a progress has been slow owing to the complications of the physics behind these devices. This book concentrates on the latest developments in and understanding of T device physics underlying thin-film solar cells. The material presented is mainly H experimental and based on CdTe thin-film solar cells. The author extends these new findings to CIGS- and GaAs-based thin-film solar cells and presents a new I N device design based on graded bandgap multilayer solar cells. This design has been experimentally tested using the well-researched GaAs/AlGaAs system, and initial - F devices have shown impressive device parameters (V ≈ 1175 mV, FF ≈ 0.85, and J ≈ 12 mAcm–2). In particular, the V represents the higochest recorded value togethesrc ILA oc d with the highest possible FF values to date for a single PV device, indicating the M right approach for PV solar cell development. This device is capable of absorbing v all radiation (ultraviolet, visible and infrared) within the solar spectrum as well as a heat energy from the surroundings and combines these with “impact ionisation” Sn and “impurity photovoltaic” effects. The conversion efficiency of graded bandgap Oc device has improved to ~20% using only two growth attempts. The improved e L device understanding presented in this book should impact and guide future s Advances in A device design and low-cost thin-film solar panel development and manufacture. i Rn I. M. Dharmadasa is professor of applied physics and head of THIN-FILM the Electronic Materials and Sensors Group at Materials and C Engineering Research Institute at Sheffield Hallam University, E UK. He has worked extensively in semiconductor research since L becoming a PhD student at Durham University as a Commonwealth SOL AR CELLS L Scholar, in 1977. He has over 200 publications and presentations on both research and development and applications of solar energy and 6 patents on S thin-film solar cells and has successfully supervised 14 PhD programmes to date. Winner of several scholarships and awards, including the 2001 Eurosolar UK Prize for inspiring renewable energy applications, Prof. Dharmadasa is also passionate about promoting the use of renewable energy for economic development and poverty reduction and is active in setting up solar-powered energy hubs in developing countries to empower rural communities through education and commerce. He blogs at www.apsl.org.uk. V213 ISBN-13 978-981-4316-07-1 TThhiiss ppaaggee iinntteennttiioonnaallllyy lleefftt bbllaannkk CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2012 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Version Date: 20120829 International Standard Book Number-13: 978-9-81436-412-6 (eBook - PDF) This book contains information obtained from authentic and highly regarded sources. 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Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com August3,2012 16:23 PSPBook-9inx6in 00-Dharmadasa–prelims Contents Preface xiii ListofSymbolsandAbbreviationsUsedintheBook xvii 1 PhotovoltaicSolarEnergyConversion 1 1.1 Introduction 1 1.2 PhotovoltaicEffect 2 1.3 SolarEnergyMaterials 3 1.4 ElectronicDevicesUsedforSolarEnergyConversion 4 1.4.1 p-nJunctions 5 1.4.2 p-i-nJunctions 6 1.4.3 Hetero-Junctions 7 1.4.4 n-nandp-pJunctions 8 1.4.5 Metal/Semiconductor(orSchottky)Contacts 8 1.4.6 Metal-Insulator–SemiconductorInterfaces 11 1.5 CharacteristicsofaSolarCell 13 1.5.1 I-VCharacteristicsofaSolarCellUnder DarkConditions 13 1.5.2 I-VCharacteristicsofaSolarCellUnder IlluminatedConditions 16 1.5.3 HowtoMaximiseV 19 oc 1.5.4 HowtoMaximise J 20 sc 1.5.5 HowtoMaximiseFF 20 1.6 Next-GenerationSolarCells 21 1.7 Summary 22 2 StatusReportonSolarEnergyTechnologies 25 2.1 Introduction 25 2.2 SiSolarCellTechnology 27 2.3 PV-ManufacturingCostBasedonSiTechnology 30 2.4 PVTechnologyBasedonIII-VCompounds 31 August3,2012 16:23 PSPBook-9inx6in 00-Dharmadasa–prelims vi Contents 2.5 NewTechnologyforPVandNano-Divices 32 2.6 EmergingLow-CostThin-FilmTechnologies 33 2.7 Summary 35 3 ElectrochemicalDepositionofSolarEnergyMaterials 37 3.1 Introduction 37 3.2 ElectrodepositionofSemiconductors 38 3.3 StrengthsandAdvantagesofElectrodeposition 40 3.3.1 Simplicity,Low-Cost,Scalability,and Manufacturability 40 3.3.2 Self-PurificationandBuilt-inHydrogen Passivation 41 3.3.3 ExtrinsicandIntrinsicDoping 42 3.3.4 AbilityinBandgapEngineering 43 3.3.5 OtherAdvantagesofElectrodeposition 43 3.4 ExperimentalEvidence 44 3.4.1 ObservationsinXRD 44 3.4.2 ObservationsinXRF 44 3.4.3 ObservationsinPECCellMeasurements 46 3.4.4 ObservationsinOpticalAbsorption Measurements 49 3.4.5 ObservationsinPhotoluminescence 49 3.4.6 ImpurityControlinSemiconductors 51 3.5 IssuesinElectrodepositionofSemiconductors 51 3.6 CurrentWorkandFutureProspects 53 3.7 Summary 56 4 BackgroundoftheCdTeSolarCelland theNewDeviceConcept 59 4.1 Introduction 59 4.2 TheConventionalModelforaGlass/Conducting Glass/CdS/CdTe/MetalSolarCell 60 4.3 KeyObservationsThatLedtotheFormulationof aNewModel 63 4.3.1 SurfaceModificationofCdTe 63 4.3.2 EffectsofSurfaceModificationonDefectLevels 64 4.3.3 EffectsofDefectLevelsonElectronicDevices 65 August3,2012 16:23 PSPBook-9inx6in 00-Dharmadasa–prelims Contents vii 4.3.4 SimilarObservationsonThin-FilmCdS/CdTe SolarCells 66 4.4 NewConceptforCdS/CdTeSolarCell 68 4.5 DescriptionofExperimentalResultsUsingthe TwoModels 71 4.5.1 Current-Voltage(I-V)Characteristics 72 4.5.2 Capacitance-Voltage(C-V)Characteristics 73 4.5.3 ElectronBeam–InducedCurrent Measurements 73 4.5.4 ObservationofDiscreteBarrierHeightsand V Values 74 oc 4.5.5 AThin-FilmCdTeSolarCellDeviceWithout aCdSLayer 74 4.5.6 ResultsFromElectricalContactingWork 75 4.5.7 DopingofCdSandCdTeLayers 76 4.5.8 FurtherExperimentalEvidencetoConfirm theTrueStructureoftheDevice 78 4.6 PredictionsforFurtherDevelopmentofCdS/CdTe SolarCellsandLatestObservations 80 4.6.1 DopingofWindowandAbsorberMaterials withn-Dopants 80 4.6.2 ImprovementstoBackContactUsingMIS-Type Structures 86 4.6.3 AMulti-LayerGradedBandgapApproach 88 4.6.4 DealingwithDefects 89 4.7 Summary 91 5 ExtensionoftheNewModeltoCIGSThin-Film SolarCells 95 5.1 Introduction 95 5.2 SummaryofAccumulatedKnowledgeonCIGS-Based Materials 96 5.2.1 DifferentGrowthTechniques 96 5.2.2 Structural,Optical,andElectronicProperties 96 5.2.3 OrderedDefectCompoundLayer 97 5.2.4 LatestDevelopmentsinMaterialsGrowth 97 5.3 SummaryofAccumulatedKnowledgeonCIGS-Based SolarCells 98 August3,2012 16:23 PSPBook-9inx6in 00-Dharmadasa–prelims viii Contents 5.3.1 ConventionalDeviceStructure 98 5.3.2 FrequentlyUsedEnergyBandDiagram 99 5.4 CurrentViewsofthePhysicsBehindCIGSSolarCells 100 5.4.1 p-CIGS/n-CdSHetero-Junction 101 5.4.2 p-CIGS/n-CIGSHomo-Junction 101 5.4.3 p-CIGS/n-ODCHetero-Junction 101 5.5 ReportedDevicePerformance 102 5.6 RecentWorkonMetal/p-CIGSInterfaces 104 5.7 DeeperUnderstandingofMo/CIGS/CdS/i-ZnO/ n-ZnO:Al/Metal-GridSolarCells 106 5.7.1 Type-ICIGS-BasedSolarCell 106 5.7.2 Type-IICIGS-BasedSolarCell 109 5.8 DiscussiononFurtherImprovementsofCIGS SolarCells 112 5.8.1 OptimisationofGrowth,Doping,and BandgapEngineering 112 5.8.2 DefectLevelIdentificationandEngineering 113 5.8.3 GrowthofCIGSwithControlledOrientation 113 5.8.4 ReplacementofMoUsingTCOforTandem andDouble-FacedSolarCells 114 5.8.5 FurtherImprovementsoftheDeviceStructure 114 5.9 Conclusions 115 5.10Summary 117 6 EffectiveHarvestingofPhotons 123 6.1 Introduction 123 6.2 TandemSolarCells 123 6.2.1 ConnectioninSeries 124 6.2.2 ConnectioninParallel 125 6.3 ComparisonoftheTwoConnectingMethods 127 6.3.1 DisadvantagesofSeriesConnections 128 6.3.2 AdvantagesofParallelConnections 129 6.4 Conclusions 131 6.5 Summary 132 7 Multi-LayerGradedBandgapSolarCells 135 7.1 Introduction 135 7.1.1 IncorporationofImpurityPVEffect 136 August3,2012 16:23 PSPBook-9inx6in 00-Dharmadasa–prelims Contents ix 7.1.2 IncorporationofImpactIonisation 137 7.2 SummaryofGrowthandProcessDetailsof theDeviceStructure 137 7.3 ExperimentalResultsofFullyProcessed Devices 138 7.3.1 ElectricalPropertiesUnderDarkConditions 139 7.3.2 ElectricalPropertiesUnderAM1.5Illumination 141 7.3.3 IPCEMeasurements 143 7.3.4 EBICMeasurements 143 7.3.5 SIMSProfiling 144 7.3.6 OptimisationofSiDopingConcentration 147 7.4 Discussions 151 7.5 Summary 153 8 SolarCellsActiveinCompleteDarkness 155 8.1 Introduction 155 8.2 SummaryofExperimentalResults 155 8.3 SearchforExperimentalEvidenceoftheImpurity PVEffect 156 8.4 ResponsivityMeasurements 158 8.5 I-VMeasurementsUnderDarkConditions 158 8.5.1 I-VasaFunctionofLightIntensity 159 8.5.2 I-VMeasurementsUnderCompleteDarkness 160 8.6 Discussion 162 8.7 Conclusions 164 8.8 Summary 164 9 EffectsofDefectsonPhotovoltaicSolarCell Characteristics 167 9.1 Introduction 167 9.2 VariationsofI-VCharacteristicsofMetal/n-CdTe Interfaces 168 9.3 EffectsonthePerformanceofCdS/CdTeSolarCells 171 9.4 VariationsinGaAs/AlGaAsSolarCells 173 9.4.1 DeviceStructuresUsed 173 9.4.2 InstabilityofI-VCharacteristics 175 9.4.3 ApplicationofElectricalStressestotheDevice 175 9.4.4 DiscussionandPossibleExplanations 177