Solar Cells based on Cu 2 ZnSnS 4 Thin Films Prepared from Metal Salts and Thioacetamide
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Solar Cells based on Cu2ZnSnS4 Thin Films Prepared from Metal Salts and Thioacetamide Achim Fischereder1,2, Katharina Gruber1, Wernfried Haas2,3, Armin Zankel3, Thomas Rath1,2, Ferdinand Hofer3 and Gregor Trimmel1,2 1
Institute for Chemistry and Technology of Materials, Graz University of Technology, Stremayrgasse 9, 8010 Graz, Austria 2 Christian Doppler Laboratory for Nanocomposite Solar Cells, Graz University of Technology and NanoTecCenter Weiz Forschungsgesellschaft mbH, Austria 3 Institute for Electron Microscopy and Fine Structure Research, Graz University of Technology, Steyrergasse 17, 8010 Graz, Austria
ABSTRACT Copper zinc tin sulfide (Cu2ZnSnS4, CZTS) consists of abundant and cheap elements and is therefore a very promising alternative to semiconductors based on Ga or In as solar absorber material. In addition it displays very beneficial properties like a high optical absorption coefficient and an ideal band gap for photovoltaic applications. In this contribution we present the preparation of CZTS thin films from metal salts (copper(I) iodide, zinc(II) acetate and tin(II) chloride) and thioacetamide as sulfur source by a solution based precursor method. CZTS solar cells based on these films as absorber layer with a simple ITO/CZTS/CdS/Al assembly are fabricated and characterized. Efficiencies up to 0.5% were achieved demonstrating the potential of this precursor method for the preparation of CZTS thin films for photovoltaic applications. INTRODUCTION Inorganic solar cells using chalcopyrite-type semiconductors, like Cu(In,Ga)S2 or Cu(In,Ga)Se2, as absorber layers have obtained remarkable high efficiencies up to nearly 20% [1]. However, gallium and indium have to be used for the preparation of the active layer, which are very rare and expensive elements inhibiting a cost effective large-scale production in the future [2]. To overcome these limitations alternative materials are heavily researched in order to substitute these expensive elements. One of the most promising materials is Cu2ZnSnS4 (CZTS) as it consists of abundant and relatively cheap elements and combines the properties for an ideal absorber layer for photovoltaic applications. A direct band gap of about 1.4 – 1.5 eV [3,4,5] and an optical absorption coefficient higher than 104 cm-1 [6,7] allows to absorb and use a high percentage of the incoming sunlight for charge generation within a very thin active layer. CZTS thin films have been prepared mainly by two general approaches in the recent years: physical vapor methods and chemical deposition methods. Examples for physical vapor methods are thermal evaporation of the elements and binary chalcogenides [8], sulfurization of electron-beam evaporated metallic layers [9,10], as well as Rf sputtered precursor layers [11], Rf magnetron sputtering [5], co-evaporation [12] and hybrid sputtering [13]. Chemical methods involve sulfurization of electrodeposited metal precursors [2,14,15,16,17], single step electrosynthesis [18], photo-chemical deposition [19], sol-gel sulfurization methods
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