Current State of Solar Energy

Identification of Defects and Secondary Phases in Reactively Sputtered Cu2ZnSnS4 Thin FilmsVardaan Chawla, Stacey Bent, Bruce ClemensApril 7th, 2010

Center on Nanostructuring for Efficient Energy ConversionMaterials Science and EngineeringStanford University

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OutlineMotivationProblems with characterization of thin filmsExperimental approachResultsX-ray diffractionRaman SpectroscopyTransmission Electron MicroscopyScanning Auger MicroscopySummary & Acknowledgements2

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Motivation Phase EquilibriumCZTS is a line compound between Cu2SnS3 and ZnSTheoretically even a 2-3% compositional variation could lead to phase separationTernary Phase DiagramBinary Phase Diagram

Cu2SZnSSnS2Cu2SnS3Olekseyuk, I.D. "Phase Equilibria in the Cu2S-ZnS-SnS2 System."Journal of Alloys and Compounds. 368. (2004): 135-143. Print.CZTS3

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Motivation Crystal StructureCrystal structures of secondary phases similar to CZTSAll primary peaks overlap and hard to separateLow intensity peaks cannot be seen easily in thin filmsTheoretical XRD Patterns of CZTS, Cu2SnS3, and ZnS

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Experimental ApproachSubstrateCuZnSnH2SH2SH2SH2SH2SH2S4H2S + 2Cu + Zn + SnCu2ZnSnS4 + 4H2CZTSIntroduce H2S into chamber during sputter depositionSulfur is incorporated into the film in one step (no anneal)Expect to see higher densities and improved film qualityReactive Sputtering5

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Characterization - XRD

Cu2SZnSSnS2Cu2SnS3Varying Zn/(Cu+Sn) RatioZn/(Cu+Sn) ratio is varied while holding Cu/Sn ratio constantImpossible to determine difference between CZTS, CTS, and ZnS from XRD pattern

(101)(200)(220)(312)(112)Olekseyuk, I.D. "Phase Equilibria in the Cu2S-ZnS-SnS2 System."Journal of Alloys and Compounds. 368. (2004): 135-143. Print.6

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Characterization - XRD

Cu2SZnSSnS2Cu2SnS3Varying Cu/(Zn+Sn) RatioCu/(Zn+Sn) ratio is varied while holding Zn/Sn ratio constantNeed to get very far off 2:1:1 stoichiometry before any CuxS phases can be seenCuxS can be removed with KCN etch

CuSCu2S(101)(200)(220)(312)(112)Olekseyuk, I.D. "Phase Equilibria in the Cu2S-ZnS-SnS2 System."Journal of Alloys and Compounds. 368. (2004): 135-143. Print.7

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Characterization RamanVarying Zn/(Cu+Sn) RatioRaman spectra show only minor changes even though composition is varied dramaticallyNo evidence of the CuxS phase shown by other groups at growth temperatures higher than 500C

Varying Cu/(Zn+Sn) Ratio

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500nm

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Device FabricationGlass Substrate3000 mMolybdenum Layer1 m1.75 mCdS (n-type)55 nmZnO:Al (n-type)340 nmAluminum GridCZTS Absorber (p-type)CZTS Device StackZn-rich films incorporated into standard CIGS device stack for testingZnO85 nmZnOZnO:AlCdSCZTSMoSEM Image9

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Device CharacterizationI-V MeasurementEQE MeasurementFirst CZTS devices grown by a reactive sputtering processEfficiency = 1.35%Degraded EQE clearly points to undetected defects in the absorber

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Characterization - TEM

500nm

200nmCZTSSecondphase

5nmCZTSSecondphaseDetrimental secondary phase interspersed in CZTS matrix

Stacking faults in the secondary phase point to a transition between cubic and hexagonal crystal structures

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Characterization - AugerRaster beam over sputtered surface of sample and scan for Cu, Zn, SnOverlay Cu, Zn, Sn signalComposition variation points to CZTS / ZnS (Zn-rich)

2 umCuZnSnCZTSZnS12

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Characterization - CdZnSCd penetration into ZnS lowers the cubic-hexagonal transition temperatureStacking faults in TEM images are created during CBD of CdS layerCdS ZnS Phase Diagram

(101)H-ZnS(220)(312)(112)XRD before and after CBD

CC + HHChen et al.Solid State Phase Equilibria in the ZnS-CdS System."Materials Research Bulletin. 23. (1988): 1667-1673. Print.13

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Characterization - Auger

SnCdOverlay Sn and Cd signalCd ion exchanges with Zn during bath deposition and penetrates the ZnS phaseCZTSCdZnS2 um14

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SummaryCZTS thin films were grown using Reactive SputteringFilms were characterized using X-ray Diffraction and Raman SpectroscopyFull devices have been grown and tested but are limited due to secondary phases in the filmsTransmission Electron Microscopy and Scanning Auger Microscopy can be used to identify these secondary phases15

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AcknowledgementsUS Department of Energy, Office of Basic Energy Sciences as part of an Energy Frontier Research Centerhttp://www.er.doe.gov/bes/EFRC/index.html

Applied Quantum TechnologiesLocal thin film solar startuphttp://www.aqtsolar.com16

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QuestionsQuestions? 17