Mere Anarchy is Loosed: Structural Disorder in Cu2Zn1-xCdxSnS4

Abstract

Substitution of Cd into the photovoltaic material Cu2ZnSnS4 has been previously shown to improve power conversion efficiencies, and although various models for the structural disorder have been proposed, full characterization of Cu2Zn1-xCdxSnS4 has been hampered by a lack of understanding about the local structure around individual atoms. Through a combination of powder X-ray diffraction and solid-state NMR spectroscopy (119Sn, 113Cd, 63Cu, and 67Zn), the detailed structures of the entire series Cu2Zn1-xCdxSnS4 (x = 0-1), prepared as bulk samples by reaction of the elements at 800 °C, have been examined. The ordered kesterite structure in space group I4¯ of the end-member Cu2ZnSnS4 (x = 0) was retained up to Cu2Zn0.7Cd0.3SnS4 (x = 0.3) and the stannite structure in space group I4¯ 2m from Cu2Zn0.6Cd0.4SnS4 (x = 0.4) to the end-member Cu2CdSnS4 (x = 1). Although the structural transition at x = 0.4 was confirmed, 113Cd NMR spectroscopy reveals the coexistence of phases with I4¯ and I4¯ 2m structures in the intermediate region (x = 0.3-0.4), evidence for which was previously unattainable from XRD data alone. The 119Sn NMR spectra indicated the presence of Sn sites surrounded within distinct local chemical environments imparted by the distribution of Zn and Cd atoms disordered within the second coordination sphere, which could be resolved by their isotropic chemical shifts and quantified by their peak areas. Ultrahigh-field (21.1 T) 63Cu NMR spectroscopy revealed the two sets of Cu sites in the I4¯ structure expected for the lightly Cd-substituted members (up to Cu2Zn0.8Cd0.2SnS4) and one set of Cu sites in the I4¯ 2m structure for the more heavily substituted members. The balance of evidence from the comparison of experimental and DFT-computed 63Cu and 67Zn NMR spectra suggests that Cu/Zn disorder does not occur. Instead, Cd substitution in Cu2Zn1-xCdxSnS4 proceeds exclusively through disorder of Zn and Cd atoms, at least in the bulk samples examined.