The Effect of Film Composition on the Texture and Grain Size of CuInS 2 Prepared by Chemical Spray Pyrolysis
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The Effect of Film Composition on the Texture and Grain Size of CuInS2 Prepared by Chemical Spray Pyrolysis Michael H.-C. Jin,1,3 Kulbinder K. Banger,1,3 Jerry D. Harris,2,3 and Aloysius F. Hepp3 1 Ohio Aerospace Institute, 22800 Cedar Point Road, Brookpark, OH 44142, U.S.A. 2 Dept. of Chemistry, Cleveland State University, Cleveland, OH 44115, U.S.A. 3 NASA Glenn Research Center, 21000 Brookpark Road, Cleveland, OH 44135, U.S.A. ABSTRACT Ternary single-source precursors were used to deposit CuInS2 thin films using chemical spray pyrolysis. We investigated the effect of the film composition on texture, secondary phase formation, and grain size. Films with either (112)- or (204/220)-preferred orientation were deposited with most often In-rich composition. The (112)-preferred orientation became more pronounced as the film composition became more In-poor. Films with a (204/220)-preferred orientation were both In-rich and contained a yet unidentified secondary phase. The phase was evaluated as an In-rich compound based on composition analysis and Raman spectroscopy. Further experiments showed that the phase could be removed by depositing a thin Cu layer prior to the growth of CuInS2. Similarly, as-grown Cu-rich (112)-oriented films did not exhibit the Inrich compound. The (204/220) preferred orientation of the film is likely related to the equivalent symmetry between planes of CuInS2 and the In-rich compound. The largest grain size (∼ 0.5 µm) was achieved with Cu-rich (112)-oriented films. INTRODUCTION Thin film polycrystalline materials have been studied extensively for solar cell applications partially because their polycrystalline nature allows their formation on many different types of substrates [1]. Using lightweight polymer substrates for solar cells will particularly benefit space missions by reducing the power requirement for the spacecraft [2]. Previously we have developed new single-source precursors (SSPs) for chalcopyrite thin film deposition [3,4,5], and successfully showed that SSPs’ thermal properties were appropriate for low-temperature processes. CuInS2 is a wide band gap chalcopyrite and is a promising material for thin film solar cells because of its near optimum direct band gap of 1.5 eV and its possible use as a top cell in a tandem structure with Cu(In,Ga)Se2 (CIGS) [6]. Although the best thin film solar cell efficiency reported was set by a CIGS heterojunction cell (18.8 %) made at the National Renewable Energy Laboratory (NREL), CuInS2 solar cells are theoretically expected to show efficiencies superior to those of CIGS cells [7]. A total area efficiency of 11.4 % has been achieved for CuInS2 cells [8]. In order to realize high performance CIGS and CuInS2 solar cells, it is believed that a Curich stage during the film growth is necessary to achieve a large, columnar grain structure because the quasi-liquid Cu-Se (or S) binary phase segregated at the surface, enhances the mobility of Cu and Se (or S) atoms during film growth [9]. It has also observed (for CIGS) that additional indium sho
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