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

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 Cu2Zn1-xCdxSnS4. 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.