Proceedings of MATSUS Spring 2025 Conference (MATSUSSpring25)
DOI: https://doi.org/10.29363/nanoge.matsusspring.2025.325
Publication date: 16th December 2024
A new power conversion efficiency record of 15.1% was reported just recently for a CZTSSe-based thin film device in which the polycrystalline CZTSSe absorber layer shows an off-stoichiometric composition. Deviations from stoichiometry cause intrinsic point defects which determine the electronic properties of a semiconductor significantly. A special kind of structural disorder, the Cu/Zn disorder, is always present in these compounds and is discussed as a possible reason for band tailing as well.
To minimize or avoid Cu/Zn disorder cation mutation strategies can be applied. In this way the crystal structure of the material can change from kesterite- to stannite-type to completely avoid this disorder. Substituting Zn2+ with Cd2+ in CZTS is one of the options. In the resulting solid solution series, both end members adopt different crystal structures: Cu2ZnSnS4 crystallizes in the kesterite-type structure whereas Cu2CdSnS4 adopts the stannite-type crystal structure.
We studied crystal structure, cation distribution and intrinsic point defect scenario in Cu2(Zn1-xCdx)SnS4 monograins by neutron diffraction. This method enables us to differentiate the isoelectronic cations Cu+ and Zn2+ in the crystal structure analysis. At the same time the presence of Cd2+ in the samples introduces a huge challenge for neutron diffraction, as Cd is absorbing neutrons, in this way increasing the measuring times significantly.
These investigations enabled us to deduce that in the range between x=0 and 0.38 the mixed crystals adopt kesterite type structure with increasing Cu/II disorder with increasing Cd content. Starting with x=0.57 and until x=1.0 the material adopt stannite type structure, with a complete absence of Cu/II disorder. The change in the cation distribution being so abrupt suggests us that in this solid solution the complex cation re-distribution process within the crystal structure is happening in a very narrow compositional range 0.38 < x < 0.57.
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