2D and 3D Phases of Chalcogenide Perovskites

Chalcogenide perovskites have been recently under the researchers’ spotlight as novel absorber materials for photovoltaic applications. BaZrS₃, the most investigated compound of this family, shows high absorption coefficient, a bandgap of around 1.8 eV, and excellent environmental and thermal stability.[1- 3] Despite the high temperature synthesis that has been traditionally employed to prepare this material, milder conditions have been successfully applied recently, opening the possibility for its deposition in thin film and device integration.[4] In addition to the 3D perovskite BaZrS₃ the Ba-Zr-S compositional space contains 2-D Ruddlesden-Popper phases Ba_x+1Zr_xS_3x+1 (with x= 1, 2, 3) which have recently been reported. [5, 6] It is important to note that, differently from hybrid lead halide perovskites in which RP phases are achieved through the addition of a bigger cation, these lower dimensional phases can be obtained only through stoichiometric variation of the 3D elements. Consequently, 3D-2D mixtures, with different optoelectronic properties compared to the pure phases, can be easily created, requiring a deep understanding and control of the mechanisms to avoid the formation of secondary phases.<br/>In this paper we report solid state synthesis and characterization of Ba-Zr-S phases. In order to avoid the use of the easily oxidised metals the starting materials used were BaS and ZrS₂. We report the protocols for synthesis at 900°C in quartz capsules, changing the precursor ratio BaS: ZrS₂ and revealing the formation of a mixture of 3D and 2D chalcogenide perovskite phases. To carefully resolve the composition, we complemented the XRD, SEM and EDS characterization of the synthetic products with the analysis of the Raman-IR spectra. For this purpose, we calculated the phonon mode, deriving the Raman and IR spectra, for the 3D and 2D Ba-Zr-S chalcogenide perovskites and for the binary precursors. This thorough characterization will show how complex can be to distinguish between different phases in the Ba-Zr-S compositional space, stressing the importance of the use of multiple techniques to soundly resolve the composition of these materials.<br/><br/>1. Nishigaki Y. <i>et al</i>., <i>Solar RRL</i>, <b>2020</b>, 4, 1-8<br/>2. Perera, S. <i>et al</i>., <i>Nano Energy,</i> <b>2016</b>, 22, 129-135<br/>3. Tiwari, D. <i>et al</i>., <i>Journal of Physics: Energy, </i><b>2021</b>, 3, 034010<br/>4. Comparotto, C. <i>et al.</i>, <i>ACS Applied Energy Materials</i>, <b>2022</b>, 5, 6335-6343<br/>5. Li, W. <i>et al.</i>, <i>Physical Review Materials</i>, <b>2019</b>, 3<br/>6. Niu, S. <i>et al</i>., <i>Journal of Materials Research</i>, <b>2019</b>, 34, 3819-3826