Exploring the Potential of ZnMgO Thin Films as a Sustainable Alternative to CdS Buffer Layer in CZTSSe Solar Cells

Recently, Copper Zinc Tin Sulfide Selenide [Cu₂ZnSn(S,Se)₄ or CZTSSe] absorbers based solar cells have exhibited efficiencies exceeding 14%, ending a period of stagnation lasting over a decade. Although a notable V_oc deficit still exists, it is possible to enhance the overall efficiency by optimizing the optical properties of the CZTSSe device. This study aims to investigate the potential of Zn_1-xMg_xO (ZMO) as a buffer layer material for CZTSSe solar cells, with the goal of replacing the toxic CdS buffer layer. The use of ZMO buffer layers offers advantages such as band alignment, high electron mobility, good transparency, and chemical stability. ZMO thin films were deposited using magnetron co-sputtering with different Mg/(Mg+Zn) ratios. The sputtering conditions and film properties were optimized and analyzed. CZTSSe solar cells were fabricated using the ZMO buffer layer and their performance was also evaluated. The ZMO thin films exhibited a tunable bandgap ranging from 3.21 eV to 4.88 eV, depending on the Mg/(Mg+Zn) ratio. The grain size of the films increased with increasing Mg ratio and then decreased at higher Mg concentrations. The highest power conversion efficiency (PCE) of 3.3% with V_oc= 343 mV, J_sc=27.3 mA/cm² and FF=35.5 % was achieved with a ZMO buffer layer composition of Zn_0.84Mg_0.16O having a bandgap of 3.76 eV. This performance compared favorably with that of reference cells utilizing the CdS buffer. This study demonstrates the potential of ZMO as a substitute for the toxic CdS buffer layer in CZTSSe solar cells. The findings contribute to the development of more environmentally friendly and efficient thin-film solar cell technologies.