Detection of Cu₂Zn₅SnSe₈ and Cu₂Zn₆SnSe₉ phases in co-evaporated Cu₂ZnSnSe₄ thin-films

Cu₂ZnSnSe₄ thin-films for photovoltaic applications are investigated using combined atom probe tomography and ab initio density functional theory. The atom probe studies reveal nano-sized grains of Cu₂Zn₅SnSe₈ and Cu₂Zn₆SnSe₉ composition, which cannot be assigned to any known phase reported in the literature. Both phases are considered to be metastable, as density functional theory calculations yield positive energy differences with respect to the decomposition into Cu₂ZnSnSe₄ and ZnSe. Among the conceivable crystal structures for both phases, a distorted zinc-blende structure shows the lowest energy, which is a few tens of meV below the energy of a wurtzite structure. A band gap of 1.1 eV is calculated for both the Cu₂Zn₅SnSe₈ and Cu₂Zn₆SnSe₉ phases. Possible effects of these phases on solar cell performance are discussed.