Development of nanostructured Cu3SnS4 thin films through annealing of the stack of precursors for photonic applications

The stack of copper (Cu), tin (Sn), and sulfur (S) precursor layers was deposited on a Corning 2947 substrate using the thermal evaporation method under a vacuum of approximately 2 × 10^–4 Pa, employing the sequentially evaporated layer deposition (SELD) technique. The as-deposited stack was annealed at 623–723 K under a vacuum of approximately 2 × 10⁻¹ Pa to achieve the Cu₃SnS₄ phase. The stack exhibits amorphous behaviour, while films grown between 623 and 723 K attain nanostructured Cu₃SnS₄ (CTS) form. The influence of T_A on the characteristics of the Cu₃SnS₄ layers was investigated through structural, morphological, compositional, optical, and electrical analyses. The annealed CTS films crystallize in a tetragonal crystal system with the space group I42 m (121). The grown films exhibit granular structures, with particles synthesized at 673 K demonstrating increased size. The bandgap (E_g) of the films decreases from 2.13 eV to 1.78 eV, while the absorption coefficient (α) ranges from 1 × 10⁵ to 3 × 10⁵ cm⁻¹, as the annealing temperature (T_A) increases from 623 to 723 K. At 673 K, the low resistivity of 9.37 × 10⁻³ Ω-cm, high mobility of 56.4 cm²/V-s, and acceptor concentration of 1.19 × 10¹⁹ cm⁻³ result from the increased crystallite size, which reduces grain boundary scattering. Thus, Cu₃SnS₄ is a promising absorber layer for thin-film solar cells due to its tunable bandgap, high optical absorption, low cost, and the use of earth-abundant elements. This study successfully advances photovoltaic technology by developing an economically viable alternative material for solar cell absorber layers, paving the way for large-scale solar cell production.