Growth Mechanisms of Electrodeposited CuInSe 2 and Cu(In,Ga)Se 2 Determined by Cyclic Voltammetry
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Growth Mechanisms of Electrodeposited CuInSe2 and Cu(In,Ga)Se2 Determined by Cyclic Voltammetry M. Estela Calixto, Kevin D. Dobson, Brian E. McCandless and Robert W. Birkmire Institute of Energy Conversion, University of Delaware, Newark, DE 19716, U.S.A ABSTRACT Electrodeposition (ED) of CuInSe2-based thin films from a buffered single-bath on dcsputtered Mo layers has been investigated. In order to understand the film growth, cyclic voltammetry (CV) was used to identify mechanisms leading to the formation of CuInSe2 and Cu(In,Ga)Se2. Similar CV data were observed for deposition from Cu-Se, Cu-In-Se, Cu-Ga-Se, and Cu-In-Ga-Se baths. A preliminary mechanism for CuInSe2 and Cu(In,Ga)Se2 film growth has been proposed from CV, composition and glancing incidence x-ray diffraction (GIXRD) data. Incorporation of In into the growing films occurs via reaction with H2Se, formed by reduction of the initially deposited Cu3Se2, to form In2Se3, which is in turn rapidly assimilated into the film by reaction with copper selenides to form CuInSe2. The incorporation of Ga may occur via a similar mechanism, however, the precipitation of Ga(OH)3 can not be ruled out as a possible route for Ga uptake. All as-deposited ED CuInSe2–based films have poor crystallinity and require annealing in H2Se prior to device processing. Preliminary device results are presented, reporting a conversion efficiency of 6.5% and 6.2% for electrodeposited CuInSe2 and Cu(In,Ga)Se2, respectively. INTRODUCTION In our previous work [1], we have described the development of a vacuum-free single-step ED of CuInSe2 and Cu(In,Ga)Se2 films, from buffered baths, for PV device processing. Buffering of the baths stabilizes bath chemistry and decreases the uptake of metal oxides during film deposition and allows incorporation of significant Ga levels, up to 8 at%, into deposited Cu(In,Ga)Se2. Good control of as-deposited film composition and morphology was achieved, allowing deposition of films suitable for device processing with no additional vacuum deposition step required to adjust composition. Promising device results were obtained for single-bath ED CuInSe2 and ED Cu(In,Ga)Se2. Despite the successful deposition of CuInSe2-based thin films from a single electrochemical bath described earlier and widely reported by several groups, only a few groups have attempted to describe bath chemistry and mechanisms of film growth [2,3,4]. In this work, a preliminary growth mechanism for CuInSe2 and Cu(In,Ga)Se2 film growth has been proposed from CV data, coupled with composition and GIXRD results. The CV behavior of Cu-Se, Cu-Ga-Se, Cu-In-Se and Cu-In-Ga-Se baths on Mo/glass electrodes was monitored. To our knowledge, this is the first reported CV study of the electrodeposition of Cu(In,Ga)Se2 films. EXPERIMENTAL DETAILS ED of CuInSe2 was carried out using acidic aqueous baths containing 2.6mM CuCl2.2H2O, 9.6mM InCl3 and 5.5mM H2SeO3, with 0.236M LiCl added as the supporting electrolyte to improve bath conductivity. For Cu(In,Ga)Se2 ED, baths containing 2.5mM CuCl2.2H2O
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