Sustainable synthesis of quaternary sulphides: The problem of the uptake of zinc in CZTS

Quaternary semiconducting sulphides are promising and sustainable candidates to replace the silicon-based semiconducting materials currently leading the market of the photovoltaic energy production. Recent efforts were paid to CZTS (Copper Zinc Tin Sulphides) semiconductors, due to their suitable electronic properties, the abundance in nature of their constituents, and the low toxicity. In this study, we synthetized Cu3-xZnxSnS4 samples by two different solvothermal approaches, with the aim of developing a sustainable synthetic route in the cradle-to-grave full life cycle assessment. These two routes are characterized by either a single step, or a two-steps procedure and allow operating in mild conditions limiting the use of additional chemicals. A multi-technique characterization protocol has been adopted to evaluate chemical, structural and functional properties of the final products. The selected analyses demonstrated that, in both cases, members of the kesterite-kuramite solid solution series were obtained. Still, only in the case of the two steps approach pure Cu2ZnSnS4 samples can be synthesized, whereas the single step synthesis yields Zn-poor samples. The optical characterization of the optimized CZTS material obtained with the two-steps approach revealed a band gap value (1.6 eV), in line with literature's values, confirming the suitability of this synthesis for the development of materials for photovoltaic applications.