Effects of element ratio on robustness of CZTS films: Variations in sulfurization temperature

In this study, Cu₂ZnSnS₄ (CZTS) thin films are fabricated through radio frequency magnetron sputtering followed with sulfurization process under different annealing temperatures. Scanning electron microscopy, X-ray diffraction, energy dispersive X-rays, Raman spectroscopy, UV–vis–NIR spectroscopy, and Hall effect measurement system are used to characterize the fabricated CZTS films. Elemental ratio variations during the sulfurization process under different temperatures are examined from the perspective of the following decomposition process: Cu₂ZnSnS₄ (s) ⇋ Cu₂S (s) + ZnS (s) + SnS (s) +1/2 S₂ (s). Furthermore, the lattice parameters, grain size, full-width at half-maximum (FWHM), dislocation density, and micro-stress are calculated and analyzed in light of the elemental ratio variations. Meanwhile, the Raman spectra and a joint analysis with the energy band gaps validate the presence of secondary phases including orthorhombic Cu₃SnS₄, cubic Cu₂SnS₃ and tetragonal Cu₂SnS₃. The results indicate that the energy band gap can be adjusted by controlling the amount of S, loss of Sn through annealing temperature, and temperature of 500 °C is a appropriate sulfurization temperature for preventing the decomposition of CZTS during the annealing process. This study is an important reference for experimental works that seek to improve quality of CZTS thin films in solar cells with high conversion efficiency.