Two new processes for the atomic layer deposition of copper indium sulfide (CuInS₂) based on the use of two different sets of precursors are reported. Metal chloride precursors (CuCl, InCl₃) in combination with H2S imply relatively high deposition temperature (Tdep = 380 °C), and due to exchange reactions, CuInS₂ stoechiometry was only achieved by depositing In₂S3 layers on a CuxS film. However, the use of acac- metal precursors (Cu(acac)₂, In(acac)₃) allows the direct deposition of CuInS₂ at temperature as low as 150 °C, involving in situ copper-reduction, exchange reaction and diffusion processes. The morphology, crystallographic structure, chemical composition and optical band gap of thin films were investigated using scanning electronic microscope, x-ray diffraction under grazing incidence conditions, x-ray fluorescence, energy dispersive spectrometry, secondary ion mass spectrometry, x-ray photoelectron spectroscopy and UV-vis spectroscopy. Films were implemented as ultra-thin absorbers in a typical CIS-solar cell architecture and allowed conversion efficiencies up to 2.8%.

The authors present the elaboration of zinc indium sulfide (ZnInxSy) thin films in the context of a cadmium-free buffer layer development for copper indium gallium diselenide photovoltaic solar cells. The films were deposited by atomic layer deposition (ALD) from ZnEt2 (DEZ), In(acac)3 (acac = acetylacetonate), and H2S at 200 °C. In situ growth kinetics studies were performed with the quartz crystal microbalance technique to determine the respective mass gain per cycle of ZnS and In2S3 layers, allowing determination of the atomic compositions of the ZnInxSy thin films to be expected if the deposition strictly follows the rule of mixtures. As the experimental atomic compositions of the ZnInxSy films differ significantly from this rule, a comprehensive study of the growth mechanism was performed to determine the nature of the side reactions. First, an exchange reaction between In2S3 and the Zn precursor was identified, though this process is not sufficient to account for the experimental data, and therefore, a second process which corresponds to the diffusion of species within the film was also found to take place. Ultimately, the atomic compositions of the ZnInxSy films can be explained by a rate-limited exchange reaction at the surface between DEZ and the In2S3 layer, combined with diffusion of the species in the whole film. More generally, such side reactions should be considered in ALD of multinary compounds, even at low temperature.

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