Understanding the origin of disorder in kesterite-type chalcogenides A 2 ZnBQ 4 (A = Cu, Ag; B = Sn, Ge; Q = S, Se): the influence of inter-layer interactions

• Published in: Physical chemistry chemical physics : PCCP 2019-09, Vol.21 (35), p.19311-19317
• Main Authors: Mangelis, Panagiotis, Aziz, Alex, da Silva, Ivan, Grau-Crespo, Ricardo, Vaqueiro, Paz, Powell, Anthony V
• Format: Article
• Language: English
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/C9CP03630J

Semiconducting quaternary chalcogenides with A ZnBQ stoichiometry, where A and B are monovalent and tetravalent metal ions and Q is a chalcogen (e.g. Cu ZnSnS or CZTS) have recently attracted attention as potential solar-cell absorbers made from abundant and non-toxic elements. Unfortunately, they exhibit relatively poor sunlight conversion efficiencies, which has been linked to site disorder within the tetrahedral cation sub-lattice. In order to gain a better understanding of the factors controlling cation disorder in these chalcogenides, we have used powder neutron diffraction, coupled with Density Functional Theory (DFT) simulations, to investigate the detailed structure of A ZnBQ phases, with A = Cu, Ag; B = Sn, Ge; and Q = S, Se. Both DFT calculations and powder neutron diffraction data demonstrate that the kesterite structure (space group: I4[combining macron]) is adopted in preference to the higher-energy stannite structure (space group: I4[combining macron]2m). The contrast between the constituent cations afforded by neutron diffraction reveals that copper and zinc cations are only partially ordered in the kesterites Cu ZnBQ (B = Sn, Ge), whereas the silver-containing phases are fully ordered. The degree of cation order in the copper-containing phases shows a greater sensitivity to the identity of the B-cation than to the chalcogenide anion. DFT indicates that cation ordering minimises inter-planar Zn Zn electrostatic interactions, while there is an additional intra-planar energy contribution associated with size mismatch. The complete Ag/Zn order in Ag ZnBQ (B = Sn, Ge) phases can thus be related to the anisotropic expansion of the unit cell on replacing Cu with Ag.