Influence of the copper deficiency and anionic composition on band-energy diagram of bulk kesterite CZTSSe

The influence of the chemical composition on band edges and Fermi level of the Cu2ZnSn(S x Se1-x)(4) stoichiometric compounds was investigated. The Cu-poor compounds, Cu1.70Zn1.15SnSSe3 and Cu1.70Zn1.15SnS2Se2, were also studied according to the same approach. As already reported, the absorption threshold increases linearly from 1.05 to 1.48 eV with increasing sulfur content. Simultaneously, the conduction band moves from -4.4 to -3.8 eV respectively, while the position of the uppermost level of the valence bands and the Fermi levels are almost not influenced by the S/Se ratio. In contrast, for Cu-poor compounds, a significant change in the chemical surface composition is witnessed compared to that of the bulk. Moreover, the absorption capability is enhanced without modification of the optical gap. The Cu off-stoichiometry influences also notably the positioning of Fermi level into the band gap. These features could be at the origin for the singular performances of the Cu poor CZTSSe-based solar cell.