Optical, Structural and Electronic Properties of CuInS 2 Solar Cells Deposited by Reactive Magnetron Sputtering
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Optical, Structural and Electronic Properties of CuInS2 Solar Cells Deposited by Reactive Magnetron Sputtering T. Unold, T. Enzenhofer and K. Ellmer Hahn-Meitner Institut, Dept. Solare Energetik Glienicker Str.100, 14109 Berlin, Germany
CuInS2 thin films have been prepared by reactive magnetron sputtering from metallic targets in an Ar/H2S atmosphere. The Cu/In ratio and substrate temperature have been varied and absorber layers were processed to solar cell devices using the HMI baseline process. Device efficiencies up to 8.8 % have been achieved. The electronic and optical properties of the solar cell devices were analyzed by Scanning Electron Microscopy, Raman, photoluminescence, current-voltage and quantum efficiency measurements. Raman measurements indicate that Cu-Au defect ordering is present in some of the films, which can be eliminated by appropriate choice of the growth parameters. Photoluminescence measurements show spectra very similar to the spectra found for cells from the two-step sequential process indicating a similar defect structure in these films. No principle obstacles to establishing a one-step reactive magnetron sputtering process for CuInS2 solar cell absorbers have been identified. INTRODUCTION Thin-film solar cells based on CuInS2 absorbers are usually prepared by coevaporation [1] or by a sequential process consisting of the sputtering deposition of the metal precursors and a sulfurization step in a rapid thermal anneal process at temperatures around 500 °C [2]. During the last years these processes have been shown to be reliable and yield efficiencies above 10%. However, for a large scale production of CuInS2 solar cells a scalable one-step deposition process would be advantageous. Magnetron sputtering is a well established deposition method, which allows low temperature thin film growth on large areas. The main application of magnetron sputtering at the present is in the architectural glass industry were 3x6m2 glass substrates are coated with several dielectric layers at deposition rates > 2nm/s. In the context of thin film solar cells, magnetron sputtering so far is only used for the preparation of the molybdenum back contact, the transparent ZnO front contact, and for the deposition of metallic precursors layers. The main difference of the sputtering process to an evaporation process is the presence of ions and neutral particles with relatively high kinetic energies between 2 to some hundred eV in the plasma. A considerable fraction of these particles hits the substrate during deposition and can affect the thin film growth. Since the kinetic energies are much larger than typical defect formation energies, it is still an open question whether devices with good majority and minority carrier properties can be produced by sputtering, without resorting to very high substrate temperatures [3]. In this paper we will discuss some of the optoelectronic properties of reactively sputtered CuInS2 absorber layers and compare them in detail to the films obtained in the sequential sputtering
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