Kesterite-structured Cu 2 ZnSnS 4 (CZTS) is an earth-abundant and non-toxic semiconductor that is being studied for use as the absorber layer in thin-film solar cells. Currently, the power-conversion efficiencies of this technology fall short of the requirements for commercialisation. Disorder in the Cu-Zn sub-lattice has been observed and is proposed as one explanation for the shortcomings of CZTS solar cells. Cation site disorder averaged over a macroscopic sample does not provide insights into the microscopic cation distribution that will interact with photogenerated electrons and holes. To provide atomistic insight into Cu/Zn disorder, we have developed a Monte Carlo (MC) model based on pairwise electrostatic interactions. Substitutional disorder amongst Cu and Zn ions in Cu-Zn (001) planes on the 2c and 2d Wyckoff sites-2D disorder-has been proposed as the dominant form of Cu/Zn disorder in near-stoichiometric crystals. We use our model to study the Cu/Zn order-disorder transition in 2D but also allow Zn to substitute onto the Cu 2a site-3D disorder-including Cu-Sn (001) planes. We find that defects are less concentrated in Cu-Sn (001) planes but that Zn ions readily substitute onto the Cu 2a site and that the critical temperature is lowered for 3D disorder.
Bibliographical noteFunding Information:
We thank Laurie Peter, Mark Weller, David Mitzi, Benjamin Morgan, Ji-Sang Park and Sunghyun Kim for useful discussions. This research has been funded by the EPSRC (Grant No. EP/ L017792/1 and EP/K016288/1), as well as the EU Horizon 2020 Framework (STARCELL, Grant No. 720907). AW is supported by a Royal Society University Research Fellowship. We are grateful to the UK Materials and Molecular Modelling Hub for computational resources, which is partially funded by EPSRC (EP/P020194/1).
© 2019 The Royal Society of Chemistry.