Insights Into Cation Disorder And Phase Transitions In Czts From A First-Principles Approach

Cu2ZnSnS4 (CZTS) is a promising thin-film photovoltaic absorber material consisting of earth-abundant and nontoxic elements, though the efficiency achieved in actual devices is well below the theoretical one. Cation disorder has been proposed as a possible explanation; it is, however, still badly understood. A recent investigation based on high-resolution neutron diffraction experiments by Bosson et al. [J Mater. Chem. A 5, 16672 (2017)] contradicts previous reports and indicates Cu-Zn disorder, not just in the 2c and 2d Wyckoff sites but also in the 2a site of Cu. To clear up this confusion on the exact nature of the disorder, we employ a ternary cluster expansion in combination with Monte Carlo simulations to analyze the phase characteristics of CZTS while undergoing order-disorder transitions. Our calculations show that, in excellent agreement with experiments, a low-temperature phase transition takes place close to 545 K and that the nature of the corresponding disorder is the same as the one recently observed experimentally by Bosson et al.