Effect of post annealing thermal heating on Cu2ZnSnS4 solar cells processed by sputtering technique

Recently, to improve solar conversion efficiency of Cu₂ZnSnS₄ (CZTS) solar cells it is incorporated dopants like Ge and Cd in the crystal lattice of the absorber layer to reach an energy gap of 1.1 eV. According to Shockley-Queisser photon balance calculations the maximum efficiency is found to be 30% for an energy gap of 1.1 eV. Using Cd one can adjust the energy gap of CZTS from 1.55 to 1.09 eV. Therefore, in this work, a detailed study of the diffusion of Cd contained in CdS window layer towards the CZTS absorber was carried out by means of a thermal treatment (hotplate) of the solar cell.

The thermal treatment of the Mo/CZTS/CdS/i-ZnO-ITO solar cell was carried out at temperatures ranging from 100°C to 400°C and times varying from 3 to 15 minutes in air atmosphere. The CZTS was grown from deposits of metallic precursors (Cu / Sn / Cu / Zn) by sputtering and annealing at two ramps: 200 ° C for 45 min with an argon flow of 1 mbar and 550 °C for 5 min at 1 bar in an atmosphere of S + Sn. For the complete solar cells with a hotplate treatment of 280 °C for 6 minutes, an increase of 2.7% to 5.2%  of  conversion efficiency was observed compared to the solar cell without hotplate treatment. The solar cells with hotplate treatment present an XRD peak shift from 28.5° to 28.24°, with lattice parameters a = b = 5.431 Ǻ and c = 10.929 Ǻ, while 338 cm^-1 Raman peak displacement to the left was observed. The band gap decreased from 1.56 to 1.43 eV and Jsc increased from 12.0 mA/cm² to 15.4 mA/cm².

The final version of this paper has been published in Solar Energy 237 (2022) 196-202