Studies on in-situ sulfurization of copper zinc tin (CZT) thin films prepared by reactive DC magnetron sputtering

Cu₂ZnSnS₄ (CZTS) thin film solar cells show a great promise both in terms of efficiency and cost. The present day understanding of the desirable properties of the absorber CZTS layer is: an optimal optical band gap of 1.5 eV and copper poor and zinc rich stoichiometry. CZTS absorber layers are prepared by DC Magnetron sputtering technique. CZTS absorber layers (of thickness ∼1 μm ) at a temperature 570 K using an alloy target of copper, zinc and tin of composition 2:1:1. These CZT thin films are grown in the presence of hydrogen sulphide (H₂S) plasma. The sulfurization depends upon the flux of the metal atoms interacting with sulphur atoms/ions available in the plasma, two approaches have been followed: first is, sputtering carried out at low H₂S (reactive gas) flow rate with high Ar (sputter gas) flow rates and the second approach is, sputtering at high H₂S flow rate with low Ar flow rate. The CZTS thin films are prepared at target powers of 30 W and 40 W. The optical emission spectra (OES) clearly show the available ion species in the plasma. Results show that the CZTS thin films grown are polycrystalline in nature; the stoichiometry of the films show copper poor zinc rich state; optical band gap varies from 1.5 to 1.7 eV. The carrier concentration and the Hall mobility of CZTS thin films are in the range 1 to 13×10²⁰ cm^-3 and 0.1 to 2.7 cm²V^-1s^-1. The surface work function varies from 4.7 to 5.1 eV. The interesting result of the investigation is that the in-situ sulfurization influences the stoichiometry, consequently, enhances the desirable properties of the CZTS absorber layers. (author)