Alloying and selenization of Cu-In stacked layers evaporated onto large areas
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Alloying and selenization of Cu-In stacked layers evaporated onto large areas Raquel Caballero, Cecilia Guillén and Rocío Bayón. Departamento de Energías Renovables (CIEMAT) Avd. Complutense, 22, 28040, Madrid, SPAIN ABSTRACT In this work, Cu and In thin films, as precursors for CuInSe2 (CIS) formation, have been deposited on glass substrate up to 30 x 30 cm2 area using an electron beam evaporator in sequential processes. In order to obtain a similar global composition, three types of sequential processes of evaporation: A) Cu/In/Cu/In, B) Cu/In/Cu/In/Cu/In and C) In/Cu/In/Cu have been tested. As-grown thin films were studied at room temperature and after 120º C annealing. XRD analysis of these films showed mainly the CuIn2-x (0≤ x ≤ 1) phase at room temperature, and Cu11In9 after annealing at 120º C. After alloying, the films were selenized at temperature between 250º and 400º C in vacuum using elemental selenium vapour. XRD of the selenized thin films corresponding to In/Cu/In/Cu sequence and previously annealing at 120º C, showed the major presence of the polycrystalline chalcopyrite structure CuInSe2 with preferential orientation (112) plane at temperature as low as 250º C. From SEM studies and profilometer measurements a decrease in the mean roughness could be observed after annealing at 120º C. In contrast the resistivity of the films increased. INTRODUCTION Among the polycrystalline materials used as absorber in thin film solar cells, the chalcopyrite based compounds such as CuInSe2, generally alloyed with Ga or S, have achieved the maximum conversion efficiencies for this photovoltaic devices, being of values near 19 % in laboratory cells [1]. CIS has one of the highest optical absorption coefficient, so that it can absorb 90 % of the incident photons with energies higher than 1.0 eV within a 1-3 µm-thickness. This material also has a good radiation tolerance due to its electronic structure [2]. Nevertheless, CIS technology needs improvements in the fabrication system, such as simplification of the processes, increase in deposition rate, better use of the basic materials and a guaranteed control on large areas [3]. High material quality on large areas has to be achieved to combine high efficiencies with low costs. The techniques that have reached the best performance are the coevaporation in vacuum and the selenization/sulphuration of the metal precursors deposited by means of sputtering or evaporation [4]. In this study, we have carried out a two-stage process: sequential evaporation of Cu and In using an electron beam evaporator, and subsequent selenization in a controlled-atmosphere furnace. The interest of the Cu-In-alloyed thin films is that their structural uniformity affects the quality of the semiconductor after the chalcogenization [5]. In this sense the order of the several deposition steps can affect the final sample properties. Selenization procedures have been carried out in vacuum with elemental selenium vapour to avoid the highly toxic H2Se [6-7]. EXPERIMENTAL DETAILS CuInSe2 thin films were o
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