Insights into cation disorder and phase transitions in CZTS from a first-principles approach

Abstract

${\mathrm{Cu}}_{2}{\mathrm{ZnSnS}}_{4}$ (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.