The effect of soft-annealing on sputtered Cu₂ZnSnS₄ thin-film solar cells

In this study, we investigate the effect of soft-annealing on the efficiency of Cu₂ZnSnS₄ (CZTS) kesterite solar cells. The absorbers were grown on Mo-coated soda-lime glass by sputter deposition of Cu, SnS, and ZnS targets, and sulfurized at 585^∘C for 15 min under an N₂ atmosphere. Before sulfurization, the films were subjected to a soft-annealing process in a temperature range from 150 to 350^∘C. All absorbers were characterized by Raman spectroscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and current density–voltage (J–V) characteristics. The highest device efficiency of 6.1% was obtained at a soft-annealing temperature of 150^∘C, while the median efficiency was 5.04% (∼ 20% higher than the reference). Further increase in the soft-annealing temperature lowers device efficiency. The Raman spectra of the CZTS absorbers show a strong Raman peak at 337 cm^{- 1}, a less-intense peak at 288 cm^{- 1}, and no secondary phases were detected. Interestingly, we find that soft-annealing affects Cu/Zn disorder in the CZTS absorbers, with a higher ordering observed at 150^∘C, which coincides with the highest device efficiency. Finally, our results reveal that soft-annealing does not significantly affect the composition of the absorbers. Moreover, SEM images show that the impact of the soft-annealing temperature on the average grain size and morphology is insignificant.