Novel Cation-substituted Quarternary Chalcogenide Nanocrystals Cu2XSnS4 (X = Zn, Ni, Fe, Co): Properties and Potential Applications

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Cu2ZnSn(S,Se)4 (CZTSSe) has been intensively studied over the last decade as a promising alternative material for photovoltaics. However, bringing it to real-life applications is challenged by its high tolerance to non-stoichiometry, defects, and secondary phases. Substitution of cations in the CZTSSe structure showed great promise in overcoming these challenges and enabling new properties. Ag and Ba are the substituents mostly studied so far, but they cause only a partial amendment of the photovoltaic and other properties. The very interesting iron group of elements (Fe, Ni, Co) are rather unexplored in this respect yet. Systematic investigations are especially lacking for colloidal nanocrystals (NCs) of the Cu2XSnS4 (X = Zn, Ni, Fe, Co) compounds, although such NCs are a promising option for forming energy conversion and storage devices on deliberate substrates and areas by printing. The successful synthesis of CZTSSe NCs, and to a lesser extent of other I2-II-IV-VI4 NCs, has been reported. Advancing these NCs to solar cells, thermoelectrics, catalysis, and other application at a competitive level demands filling the gap in the knowledge about their structure and electronic properties. The aim of the project is to establish the relation between the lattice structure (including defects), distribution of electronic states, and optical spectra of Cu2XSnS4 (X = Zn, Ni, Fe, Co) NCs. We expect that the cation substitutions proposed in this project can (i) lead to an improvement of the properties of the host material due to the passivation of certain defects, (ii) help to establish the origin of certain native defects or electronic states in CZTS itself, and (iii) induce new properties, not existing in CZTS crystals and NCs, in particular magnetic properties. Computational screening will be performed to facilitate the synthetic part, enabling us to focus on the most promising CXTS compositions and to understand the changes in the electronic, optical, and phonon experimental spectra caused by cationic substitution. Furthermore, we will investigate the evolution of the material properties from the ensembles of as-synthesized individual NCs to polycrystalline films formed by annealing. Based on the results obtained, we will perform a preliminary investigation of the obtained material in simple photovoltaic or thermoelectric devices. In 16 years of successful collaboration between the applicants, as confirmed by more than 100 joint publications and most of them related to colloidal chalcogenide NCs, we have acquired sufficient experience and instrumental tools for the successful fulfillment of the current project. Besides established characterisation techniques, selected for the project, rather exclusive methods of deep level spectroscopies and electron paramagnetic resonance, which are available for the Ukrainian partners, are indispensable for building the complete picture of electronic states in such a complex materials as Cu2XSnS4 NCs.