Preparation and Characterization of Cu-Ga-Se Films of Ordered Vacancy Compound
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Preparation and Characterization of Cu-Ga-Se Films of Ordered Vacancy Compound S. Nishiwaki, S. Siebentritt, M. Ch. Lux-Steiner Hahn-Meitner-Institute Berlin, Glienicker Str. 100, D-14109 Berlin, Germany ABSTRACT Cu-Ga-Se films with an orderd vacancy compound (OVC) structure were prepared at substrate temperature about 500 °C by thermal co-deposition. With a preparation under extremely Se excess condition, films of the OVC were synthesized within the compositional ratio of 0.73 ≤ [Ga]/([Cu]+[Ga]) ≤ 0.86 along Cu2Se-Ga2Se3 pseudo binary system. The growth on soda-lime glass substrates improves the crystallinity compared to that on alkali-free glass. An increase in the optical bandgaps of OVC films from 1.85 eV to 1.94 eV was observed with an increase in the Ga content of the films. The deposition of Cu and Se onto Ga2Se3 films resulted in a vertically inhomogeneous film: the bottom layer with the OVC structure and the top layer with the chalcopyrite structure. A solar cell using the CuGa5.0Se8.1 film within a ZnO/CdS/CuGa-Se/Mo/soda-lime glass substrate structure showed an open circuit voltage of 947 mV, an efficiency of 2.2 %, a short circuit current density of 4.5 mA/cm2, and a fill factor of 0.52 (Air Mass 1.5, 0.5 cm2, total area). INTRODUCTION Copper chalcopyrite semiconductors have received considerable attention as one of the most promising candidate for thin film solar cells. At present, the record efficiency of solar cells using Cu(In,Ga)Se2 absorber films are approaching 20 % [1]. In the solar cell structure, the surface of the absorber layer plays an important role because of the formation of a junction with the window layer. It has been demonstrated that the absorber surface layer has Cu poor compositions of approximately Cu(In,Ga)3Se5 [2]. Furthermore, it has been observed that the Cu deficient surface layer has a wider bad gap than the bulk part [2]. This layer seems to be crucial for the avoidance of interface recombination [3]. Then it is expected that the surface layer has an ordered vacancy compound (OVC) structure of CuIn3Se5. Different from the chalcopyrite (Ch) structure of the space group of I-42d, the OVC structure has a tetragonal space group of I-42m [4]. However, a surface layer with the different crystallographic structure from the bulk has not been observed so far. Concerning the OVC, structural, optical and electrical properties of single crystals and films of mainly Cu(In,Ga)3Se5 composition were reported [5-13]. In addition, its application for solar cells was also demonstrated [14]. However, to date, the studies are not yet conclusive and results are not always consistent. For instance, in the system of CuIn3Se5-CuGa3Se5, the single crystals showed the OVC structure in its whole range [9], but the films with a Ga rich composition showed a cubic zinc blende type structure instead of the OVC [6]. In this article, Cu-Ga-Se films with compositions in the pseudo-binary system of Cu2SeGa2Se3 were prepared under various conditions, and the crystal structures and optical bandgaps are
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