Influence of the annealing atmosphere and precursor's thickness on the properties of CZTSSe based solar cells

Cu-Zn-Sn-S sulfide precursors with various thicknesses, ranging from 0.8 µm to 1.7 µm were annealed under two types of annealing atmosphere: a pure (Se) atmosphere and a mix of (S+Se) one. Pure (Se) annealing led to the Cu₂ZnSn(S,Se)₄ (CZTSSe) phase with a Se gradient from front to back surface. Annealing under mixture of (S+Se) improved the homogeneity of the CZTSSe solid solution across the whole layer thickness and prevented the S loss from sulfide precursor. This increased the band gap of the absorber, but enhanced the occurrence of S-rich secondary phases, mainly ZnS thin layers formed at the Mo/CZTSSe interface. The decrease of the absorber thickness can limit the occurrence of secondary phases and affects electrical parameters of the solar cells. While the best cell efficiency of 7.1% was obtained for a 1.7 µm-thick layer annealed under pure Se atmosphere, the best and homogeneous average efficiencies on 2.5 × 2.5 cm² samples were obtained for a 0.8 µm-thick layer annealed under (S+Se) atmosphere. These results showed clearly the trade-off to find between the annealing atmosphere and the thickness of the absorber.