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 CuZn + ZnCu 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 CuZn + ZnCu in Cu2ZnSnS4 and CuCd + CdCu 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 CuZn + ZnCu and CuCd + CdCu 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. Ministry of Education (MOE) National Research Foundation (NRF) Accepted version S. H., J. M. R. T., and L. W. acknowledge nancial support from National Research Foundation (NRF), Singapore, through the Nanomaterials for Energy and Water Management (SHARE NEW) CREATE programme, MOE Tier 2 (MOE2016-T2-1- 030S). W. C., G.-M. R., and G. H. acknowledge support from the F.R.S.-FNRS. W. C., G.-M. R., and G. H. acknowledge access to various computational resources: the Tier-1 supercomputer of the F´ed´eration Wallonie-Bruxelles funded by the Walloon Region (grant agreement No. 1117545), and all the facilities provided by the Universit´e catholique de Louvain (CISM/UCL) and by the Consortium des ´Equipements de Calcul Intensif en F´ed´eration Wallonie Bruxelles (C´ECI). M. G. and V. I acknowledge support by the H2020 Programme under the project INFINITE-CELL (H2020-MSCA-RISE-2017-777968), by the Spanish Ministry of Science, Innovation and Universities under the IGNITE (ENE2017-87671-C3-1-R), and by the European Regional Development Funds (ERDF, FEDER Programa Competitivitat de Catalunya 2007–2013). Authors from IREC belong to the SEMS (Solar Energy Materials and Systems) Consolidated Research Group of the “Generalitat de Catalunya” (Ref. 2017 SGR 862).