Electrochemical Co-deposition of In-Se and Ga-Se Thin Films for Preparation of CIGS Solar Cells
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1113-F08-03
Electrochemical Co-deposition of In-Se and Ga-Se Thin Films for Preparation of CIGS Solar Cells
Serdar Aksu, Jiaxiong Wang and Bulent M. Basol SoloPower Inc., 5981 Optical Court, San Jose, CA 95035, U.S.A.
ABSTRACT Electrochemical co-depositions of indium (In) with selenium (Se) and gallium (Ga) with selenium were carried out to obtain high-quality In-Se and Ga-Se films. The approach utilized full potential of complexation for the first time by using aqueous electroplating solutions containing complexing agents at alkaline regime. Complexing agents were employed to solubilize In and Ga ions at high pH and to bring their reduction potentials down, closer to that of Se. Since no appreciable complexation occurred between Se and the complexing agents, Se reduction potential could be independently controlled by the amount of dissolved Se. Tartrate and citrate were determined to be suitable complexing agents for In and Ga. By optimizing the concentrations of the metal salts, the complexing agents, the selenium source, pH and the electrodeposition current density, it was possible to obtain adherent and smooth In-Se and Ga-Se films with high repeatability and controllable In/Se and Ga/Se molar ratios.
INTRODUCTION Cu(In,Ga)Se2 (CIGS) is one of the most advanced absorber materials for thin film solar cells due to its direct bandgap, high absorption coefficient and ability to yield good quality devices. CIGS-based solar cells have yielded the highest conversion efficiencies of all thin film solar cells to date, reaching up to about 20%1. One technique used to form CIGS layers is a two-stage approach which involves deposition of a precursor layer on a substrate followed by a high temperature activation step that converts the precursor layer into solar cell grade CIGS. The precursor layers employed in two stage processes may be in the form of stacks containing Cu, In and Ga species. Some examples of such stacks are Cu/In/Se, Cu-Ga/In/Se, Cu/In/Ga, Cu/In-Se, Cu-Se/In-Se/Ga-Se, etc. Although various techniques such as evaporation and sputtering have been employed to prepare precursor layers for CIGS film formation, electrodeposition is especially attractive due to its low cost, efficient utilization of raw materials and scalability to high-volume manufacturing. (In,Ga)Se electrodeposition and its possible application to solar cells have been previously studied. Most of these studies, however, concentrated on acidic electrolyte compositions. Hirano2, for example, electrodeposited In layers out of acidic solutions that contained a suspension of fine Se particles. Since this method forms a composite layer comprising electroplated In and mechanically trapped Se particles without any electrochemical reduction reaction of Se on the cathode surface the technique is not expected to be very repeatable for electronic device fabrication applications. Igasaki and co-workers3 electroplated In-Se material out of an electrolyte containing hydrochloric acid which was used to adjust the pH
value of the solution to a range of 1.0
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