Cd Incorporation as a way to avoid Cu/Zn disorder in kesterites: the kesterite-stannite structural transition in Cu2(Cd,Zn)SnS4

A new power conversion efficiency record of 16.6% was recently reported for thin-film devices based on Cu2ZnSn(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 Zn2+ by Cd2+ 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 Cu-2(Zn1-xCdx)SnS4 monograins were studied by neutron diffraction. For 0 <= x <= 0.38, the mixed crystals adopt the kesterite structure, with increasing Cu/B-II 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 Cu-2(Zn1-xMnx)SnSe4.