Formation of Cu 2 ZnSnS 4 and Cu 2 ZnSnS 4 -CuInS 2 Thin Films Investigated by In-Situ Energy Dispersive X-Ray Diffracti
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1012-Y03-35
Formation of Cu2ZnSnS4 and Cu2ZnSnS4-CuInS2 Thin Films Investigated by In-Situ Energy Dispersive X-Ray Diffraction Alfons Weber, Immo Kˆtschau, Susan Schorr, and Hans-Werner Schock Solar Energy Division, Hahn-Meitner-Institut, Glienicker Str. 100, Berlin, 14109, Germany ABSTRACT Chalcopyrite CuInS2 and the structurally related kesterite Cu2ZnSnS4 are known as photovoltaic absorber materials. In this study different precursor thin films of the quaternary Cu-Zn-Sn-S system (stacking: Mo/CuS/ZnS-SnS) and of the pentenary Cu-In-Zn-Sn-S system (stacking: Mo/CuIn/ZnS-SnS) were annealed in sulfur atmosphere. The predominant crystalline phases were detected by in-situ energy dispersive X-ray diffraction (EDXRD). Additionally the X-ray fluorescence signals of the film components were recorded to detect diffusion effects. For the quaternary system we found ZnS, CuS, Cu2-xS, Sn2S3 and SnS as main binary phases during annealing. The Sn2S3-SnS phase transition had a significant impact on the later formation of ternary/quaternary phases. High diffusivity of copper can explain the little influence of the precursor stacking on the reaction path and may also be responsible for the poor adhesion of the films. For annealing temperatures above 450∞C Cu2ZnSnS4 can be identified clearly by XRD. The incorporation of indium in the system leads to new diffraction peaks which can be explained by the formation of solid solutions in the system CuInS2-Cu2ZnSnS4. INTRODUCTION Chalcopyrite based photovoltaic absorbers are a promising candidate for the production of cheap, large area thin film solar modules. With the indium price increasing strongly in recent years the share of this metal in chalcopyrite absorbers has become a concern. One option is to replace indium by zinc and tin, thus forming the compound Cu2ZnSnS4. In various studies it was found that the material properties of kesterite Cu2ZnSnS4, with a band gap of about 1.5 eV and an absorption coefficient of 104 cm-1 [1, 2] are well suited for photovoltaic applications. The best cells have been produced in a two stage process with firstly depositing a mixture of binary sulfides and metals and subsequent annealing/sulfurizing of the films, reaching efficiencies of up to 5.7% [3, 4]. For a better performance of the devices a more thorough understanding of the heat treatments under sulfur atmosphere appears to be crucial. In this work we use in-situ Energy Dispersive X-ray Diffraction (EDXRD) to study the phase formation during the annealing step. EXPERIMENT Mo coated soda lime glass was used as substrate for precursor formation. For the In-free precursors Cu was deposited on the substrates using DC magnetron sputtering. In the next step ZnS and SnS were evaporated sequentially on the copper layer via physical vapour deposition (PVD). From earlier studies it is known that SnS [5] and ZnS [6] evaporate congruently. The copper layer sulfurizes during the deposition of SnS and ZnS due to the high sulfur background pressure in the chamber. For the In-containing precursors an additio
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