Effect of stoichiometry ratio on the properties of CZTSe thin-film absorber layers for photovoltaic applications

Thin-film solar cells (TFSCs) have received much attention as efficient and economical solutions for solar energy conversion. Cu₂ZnSnSe₄ (CZTSe) has emerged as a promising absorber material because it is made from earth-abundant and environmentally friendly elements, has a high absorption coefficient, and possesses a suitable bandgap for photovoltaic applications. However, achieving high-quality CZTSe films requires careful control of stoichiometry, as minor compositional deviation can significantly impact their structural and optoelectronic properties. In this work, CZTSe thin films were deposited on soda-lime glass (SLG) and Mo-coated SLG substrates. One sample was kept as-deposited, another sample was annealed at 450 °C for 15 min in a selenium atmosphere, and to ensure correct stoichiometry, an additional Cu was sequentially deposited onto the as-deposited film and subsequently annealed at the same temperature to obtain the optimized film. The effect of stoichiometric adjustment on the structural, morphological, optical, and electrical properties of the films was systematically studied. XRD and Raman analyses indicate improved crystallinity and dominant CZTSe phase formation, with no prominent secondary phase peaks observed within the detection limits of the measurements. The optimized films exhibited larger grain size, enhanced electrical conductivity and mobility, and a reduced bandgap. These results demonstrate that maintaining an appropriate stoichiometric ratio significantly enhances CZTSe thin-film quality, improving its suitability for high-performance solar cell applications. In this work, Cu capping was employed as a post-deposition approach to improve the stoichiometry of CZTSe thin films and investigate its influence on material properties.