Effects of chemical treatment of Cu₂ZnSnSe₄ thin films on photoluminescence of solar cells ZnO/CdS/Cu₂ZnSnSe₄/Mo/Glass

Thin films of Cu₂ZnSnSe₄ (CZTSe) were fabricated by the selenisation of metallic precursors sequentially deposited on Mo coated glass substrates. The surface morphology and cross section, studied by a scanning electron microscope (SEM), reveal densely packed grains with sizes in excess of 1 urn and CZTSe film with a thickness of 1.4 urn. The elemental composition of both the precursors and CZTSe films was measured by an energy dispersive X-ray analyser demonstrated a copper deficiency and zinc excess as well as a near stoichiometric content of Se. A set of 4 as deposited CZTSe films was used to produce solar cells with the structure ZnO/CdS/Cu₂ZnSnSe₄/Mo/glass. These films were coated with CdS buffer layer using a standard chemical bath deposition after chemical etching using KCN or/and ammonia: film E was not etched, film B was etched in KCN for 2 minutes, film C was etched in KCN for 5 minutes and film D was etched in KCN for 30s and then in ammonia for 5 minutes. Solar cells were then completed by DC-magnetron deposition of ZnO/ZnO:Al transparent front contacts. Finally solar cells with areas of 3x3 mm² were separated by mechanical scribing. The structural properties of the cells and phase content of the CZTSe layer were examined by x-ray diffraction (XRD). Low intensity reflexes of the secondary phases SnSe and SnSe₂ were also present. The bandgaps E_g of the CZTSe layers in the cells, measured by photoluminescence excitation (PLE) at 4.2 K, were found to be of 1.05 eV in all the four cells. This cell also showed the highest FF of 41.1% whereas the best I_sc of 46.6 mA/cm², demonstrated by the D cell. The highest conversion efficiency of 6.2% was obtained for the E cell, produced without any etch, and for the D one after 30 s etch in KCN plus 5 minutes in ammonia.