Cd Incorporation as a way to avoid Cu/Zn disorder in kesterites: the kesterite-stannite structural transition in Cu 2 (Cd,Zn)SnS 4

Abstract A new power conversion efficiency record of 16.6% was recently reported for thin-film devices based on Cu 2 ZnSn(S,Se) 4 (CZTSSe) off-stoichiometric polycrystalline absorber layers. Deviations from stoichiometry introduce intrinsic point defects that strongly affect the electronic properties of the material. In addition, Cu/Zn disorder is always present in these compounds and is discussed as a possible origin of band tailing. Cation mutation strategies can be used to minimize this disorder by inducing a structural change from kesterite—to stannite-type. One option is the substitution of Zn 2+ by Cd 2+ in CZTS. In the resulting solid solution, the end members crystallize in different structures: Cu₂ZnSnS₄ in the kesterite type and Cu₂CdSnS₄ in the stannite type. The crystal structure, cation distribution, and intrinsic point defect scenario of Cu₂(Zn 1−x Cd x )SnS₄ monograins were studied by neutron diffraction. For 0 ⩽ x ⩽ 0.38, the mixed crystals adopt the kesterite structure, with increasing Cu/ B I I disorder as Cd content increases. For 0.57 ⩽ x ⩽ 1.0, the material crystallizes in the stannite-type structure with a complete absence of Cu/ B I I disorder. The abrupt change in cation distribution indicates that the transition occurs within a narrow compositional range (0.38 < x < 0.57). Based on the tetragonal distortion ( c/2a ) and cation distributions at x = 0.38 and 0.57, the transition mechanism is likely similar to that in Cu₂(Zn 1−x Mn x )SnSe₄.