Properties of CuIn 1-x Ga x Se 2 films prepared by the rapid thermal annealing of spray-deposited CuIn 1-x Ga x S 2 and
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Properties of CuIn1-xGaxSe2 films prepared by the rapid thermal annealing of spraydeposited CuIn1-xGaxS2 and Se Laura E. Slaymaker1, Nathan M. Hoffman1, Matthew A. Ingersoll1, Matthew R. Jensen1, JiĜí Olejníþek1, Christopher L. Exstrom1, Scott A. Darveau1, Rodney J. Soukup2, Natale J. Ianno2, Amitabha Sarkar2, and ŠtČpán Kment2 1
Department of Chemistry, University of Nebraska at Kearney, Kearney, NE 68849, U.S.A.
2
Department of Electrical Engineering, University of Nebraska-Lincoln, Lincoln, NE 685880511, U.S.A. ABSTRACT Many reported CuIn1-xGaxSe2 (CIGS) thin films for high-efficiency solar cells have been prepared via a two-stage process that consists of a high-vacuum film deposition step followed by selenization with excess H2Se gas or Se vapor. Removing toxic gas and high-vacuum requirements from this process would greatly simplify it and make it less hazardous. We report the formation of CuIn1-xGaxSe2 (x = 0, 0.25, 0.50, 0.75, 1.0) thin films achieved by rapid thermal annealing of spray-deposited CuIn1-xGaxS2 and Se in the absence of an additional selenium source. To prepare the Se layer, commercial Se powder was dissolved by refluxing in ethylenediamine/2,2-dimethylimidizolidine. After cooling to room temperature, this mixture was combined with 2-propanol and the resulting colloidal Se suspension was sprayed by airbrush onto a heated glass substrate. The resulting film was coated with nanocrystalline CuIn1-xGaxS2 via spray deposition of a toluene-based “nanoink” suspension. The two-layer sample was annealed at 550 oC in an argon atmosphere for 60 minutes to form the final CIGS product. Scanning electron microscopy images reveal that film grains are 200-300 nm in diameter and comparable to sizes of the reactant CuIn1-xGaxS2 nanoparticles. XRD patterns are consistent with the chalcopyrite unit cell and calculated lattice parameters and A1 phonon frequencies change nearly linearly between those for CuInSe2 and CuGaSe2. INTRODUCTION Chalcopyrite CuIn1-xGaxSe2 (CIGS) thin films for use as absorbers in high-efficiency photovoltaic devices have been prepared via a two-stage process in which a thin film of sputtered or thermally evaporated CuIn1-xGax is reacted with H2Se or Se vapor at high temperature [1-3]. Efforts to lower productions costs by eliminating high-vacuum processing steps have stimulated research in solvothermal methods of CIGS preparation. While chemically pure nanocrystalline CIGS may be prepared in this manner [4-6], the removal of solvent from films cast from nanoparticle suspensions (or “nanoinks”) by heating results in films with voids and small grains that lead to low energy conversion efficiencies [7]. Guo et. al. [7] have demonstrated that a dense thin film of chalcopyrite-phase CuInSe1.8S0.2 may be prepared by reaction of H2Se gas with a film cast from a CuInS2 nanoink suspension. Replacement of the S2- ions with the larger Se2- ions reduces void formation and device efficiencies as high as 5.55% have been reported [7]. In selenization reactions such as this, atmospheres of excess H2Se or
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