Effect of Cd on cation redistribution and order-disorder transition in Cu2(Zn,Cd)SnS4

Abstract

Cation substitution has been extensively used to improve the fundamental optoelectronic properties and the photovoltaic performance of kesterite solar cells, and some of the most promising results have been obtained by substituting zinc with cadmium. Structurally, the positive effects of Cd have been attributed to the expected increase in the formation energy of defects such as Cu_Zn + Zn_Cu due to the larger ionic radius of Cd²⁺ as compared to Zn²⁺. However, ab initio calculations using density functional theory (DFT) showed similar formation energies for Cu_Zn + Zn_Cu in Cu₂ZnSnS₄ and Cu_Cd + Cd_Cu in Cu₂CdSnS₄. Further, in this report, it is shown that Cd does not directly substitute the zinc lattice sites (2d Wyckoff positions) in the Cu₂ZnSnS₄ structure, but rather, a two-way cation restructuring due to the continuous transformation of the structure from kesterite to stannite leads to Cu replacing Zn, and Cd occupying the Cu sites (2a Wyckoff positions) in the partially Cd-substituted Cu₂Zn_1−xCd_xSnS₄. Hence, the structural reasons for the beneficial effects of Cd need to be reinterpreted. Here, using computational model based on cluster expansion (fitted on DFT data), Monte-Carlo simulations, and differential scanning calorimetry, it is shown that Cu₂CdSnS₄ has less structural disorder than Cu₂ZnSnS₄ even if the thermodynamic point defect formation energy calculated using diluted point-defect models for disorder-inducing Cu_Zn + Zn_Cu and Cu_Cd + Cd_Cu defects in these two materials is predicted to be similar. This difference in the structural disorder is due to a sharp order-disorder transformation in Cu₂ZnSnS₄ at about 530 K, and a continuous order-disorder transformation in Cu₂CdSnS₄ throughout the range of processing temperatures.