Growth Monitoring of Cu-Poor Prepared CuInS 2 Thin Films
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Growth Monitoring of Cu-Poor Prepared CuInS2 Thin Films Ilka V. Luck, Jacobo Alvarez-Garcia*, Lorenzo Calvo-Barrio*, Axel Werner, Alejandro PerezRodriguez*, J. R. Morante* and Dieter Bräunig Hahn-Meitner-Institut, Solar Energy Research, Technology, Glienicker Straße 100, 14109 Berlin, Germany *Universitat de Barcelona, Facultat de Fisica, Departament d'Electronica, EME, Avda. Diagonal 645-647, 08028 Barcelona, Spain ABSTRACT The CuInS2 thin film formation from a Cu/In precursor stack in the presence of elemental sulfur using a rapid thermal process under Cu-poor conditions has been studied. The process has been aborted at appropriate stages and the corresponding samples were investigated by XRD, Raman spectroscopy and SEM. The sulfurisation starts from elemental Cu and CuIn2. Elemental In and the binary phases Cu11In9 and Cu7In3 appear as intermediate phases. At the end of the sulfurisation the sample contains the ternary phases CuInS2 and CuIn5S8. CuS and β-In2S3 are detected by Raman spectroscopy at the sample surface and at distinct stages of the sulfurisation only. A difference in CuInS2 crystal quality is observed between the surface and the bottom of the samples.
INTRODUCTION Chalcopyrite thin films have been thoroughly investigated over the last years due to their extraordinary potential for photovoltaic applications. Highest efficiencies of 18.8 % have been obtained so far by thin film solar cells based on Cu(In,Ga)Se2 prepared under Cu-poor conditions[1]. The performance of solar cells based on CuInS2 - a material that is more desirable from an environmental point of view - is hampered by a photovoltage that is about 250 mV lower than the bandgap of the material suggests. Efficiencies as high as 12.7 % could be achieved using CuInS2 absorbers that were prepared under Cu excess [2]. This difference in performance is suspected to originate, among others, from the different preparation conditions. Recently a first CuInS2 thin film solar cell was reported using an absorber prepared under Cu-poor conditions with an efficiency of 10.6 % and and open circuit voltage of 760 mV [3]. However, Cu-poor CuInS2 film preparation seems to be much more critical compared to Cupoor Cu(In,Ga)Se2 and Cu-rich CuInS2. The purpose of this work is therefore to further elucidate CuInS2 formation under Cu-poor conditions. EXPERIMENTAL DETAILS A set of Cu-poor CuInS2 thin films have been prepared sequentially from a sputtered Cu/In precursor stack by a rapid thermal process [2]. Soda lime glass substrates with and without H1.4.1
molybdenum (Mo) back contact were used. The preparation has been aborted at various stages of the process as depicted in figure 1. The corresponding samples were characterised by XRD, Raman spectroscopy and SEM. Where appropriate Raman depth profiling was done by removing the upper part of the samples by sputtering. The homogeneity was checked by recording spectra at various spots on the samples.
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