Crystal structure and cation disorder in bulk Cu2ZnSnS4 using neutron diffraction and X-ray anomalous scattering

Cu₂ZnSnS₄ (CZTS) is a promising material for the absorber layer in sustainable thin film photovoltaic cells, but its best efficiencies are currently limited by low open-circuit voltages, thought to be due to defects such as disorder between the copper and zinc lattice sites. Samples of CZTS with stoichiometric and Cu-poor, Zn-rich (optimally doped for device application) starting compositions were fabricated by solid state reaction and their crystal structures were investigated using high-resolution neutron diffraction and anomalous scattering synchrotron X-ray diffraction. Both samples were found to be exclusively kesterite with significant mixing of Cu and Zn on the 2a, 2c, and 2d Wyckoff sites but negligible defect presence on the Sn 2b or S 8g sites. Importantly, the occupancies of the 2a and 2c sites were found to be identical, meaning the kesterite symmetry is preserved.