(755e) Atomic Layer Deposition of the Quaternary Chalcogenide Cu2ZnSnS4 (CZTS)

Atomic layer deposition (ALD) is a layer-by-layer synthesis method to deposit conformal thin films with thickness and compositional control on sub-nanometer length scales.  While many materials have been synthesized by ALD, the technologically-important metal sulfides are underexplored, and homogenous quaternary metal sulfides are absent from the literature.  We report an ALD process to synthesize Cu₂ZnSnS₄ (CZTS), a potentially low cost semiconductor being explored for photovoltaic applications.  Two strategies are reported: one in which a trilayer stack of binary metal sulfides (i.e. Cu₂S, SnS₂ and ZnS) is deposited and mixed by thermal annealing, as well as a supercycle strategy that is similar to the conventional ALD procedure for forming nanolaminates.  Both routes rely on the facile solid state diffusion of chalcogenides for mixing.  For this ALD route to the CZTS system, the challenges are: nucleation, ion-exchange between the film and the volatile chemical precursors, and phase-stability of binary SnS₂.  The CZTS thin films are made with no sulfurization step and appear phase-pure by x-ray diffraction and Raman spectroscopy.  Photoelectrochemical measurements under simulated AM1.5 illumination using Eu⁺³ as an electron acceptor reveal that they are also photoactive and have an average internal quantum efficiency (IQE) of 12 %.