Characterisation of Thin Films CuIn 1-x Al x Se 2 Prepared by Selenisation of Magnetron Sputtered Metallic Precursors
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1012-Y12-02
Characterisation of Thin Films CuIn1-xAlxSe2 Prepared by Selenisation of Magnetron Sputtered Metallic Precursors Guillaume Zoppi, Ian Forbes, Paresh Nasikkar, and Robert W. Miles Northumbria Photovoltaics Applications Centre, Northumbria University, Ellison Building, Newcastle upon Tyne, NE1 8ST, United Kingdom
ABSTRACT Thin films of CuIn1-xAlxSe2 have been produced by the selenisation of magnetron sputtered Cu/In/Al precursor layers using elemental selenium and the chemical and physical properties of the layers have been determined for different conditions of synthesis. For optimum conditions of synthesis it was found possible to produce single phase films with the chalcopyrite structure. These films were pinhole free, had good adhesion and were conformal to the substrate. The films had uniform depth profiles as determined using the MiniSIMS. The layers were highly photoactive, indicating that they have the potential to be used to fabricate thin film photovoltaic solar cell devices. INTRODUCTION Chalcopyrite CuInSe2 (CIS) based solar cells associated with Ga and S have achieved the highest level of performance to date for single junction polycrystalline thin film technology. Current state of the art CuIn1-xGaxSe2 (CIGS) solar devices exhibit efficiencies up to 19.5% [1]. However this material has a relatively low bandgap (Eg = 1.2 eV for x = 0.3) compared to the 1.5 eV required for optimum solar energy conversion. CIGS-based devices fabricated with Eg >1.3 eV (x > 0.5) are found to have substantially reduced efficiencies [2]. This is attributed to fill factor and open circuit voltage losses arising from increased defect density and stronger interfacial recombination when the Ga content is increased. The need for higher bandgap material (Eg > 1.5 eV) is driven by the desire to produce (i) higher efficiency devices and (ii) wider energy bandgap cells, for use as the upper cells in multi-junction devices. Substituting Ga by Al makes it possible to produce CuIn1-xAlxSe2 (CIAS) material with the same energy bandgap as CIGS but using less Al. This is because the variation of Eg with Al content is up to 2.7 eV for CuAlSe2 compared to 1.7 eV for CuGaSe2. Indeed CIAS solar cells with Eg = 1.16 eV and x = 0.13 have been produced with efficiencies of 16.9% [3]. This compares to 16.5% for CIGS deposited under similar conditions but with x = 0.26 (Eg = 1.16 eV) [3]. In this work we have investigated the production of CIAS thin films by a 2 stage process that involves the deposition of Cu/In/Al metallic precursor layers followed by annealing in a selenium environment to synthesis the compound. In principle this method promises improved scalability for commercial production compared to other deposition methods. It could also lead to the production of wider energy bandgap CIAS thin films.
EXPERIMENTAL DETAILS The Cu/In/Al metallic precursor layers were deposited onto soda lime glass or Mo-coated soda lime glass slides using magnetron sputtering. The base pressure in the sputtering chamber was better than 5x10-7 mba
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