Two ternary chalcostannates Ba₃Sn₂<i>Q</i>₇ (<i>Q</i> = S and Se): syntheses, crystal structures, and photovoltaic studies

Semiconducting Sn-containing chalcogenides are promising for their photovoltaic properties. Herein, we report the syntheses of two barium tin chalcogenides, Ba₃Sn₂<i>Q</i>₇ (<i>Q</i> = S and Se), by high-temperature reactions of elements. A single-crystal X-ray diffraction study shows that the Ba₃Sn₂Se₇ structure crystallizes in the monoclinic system (space group: <i>P</i>2₁/<i>c</i>) with four formula units (<i>Z</i> = 4). The refined lattice constants of the Ba₃Sn₂Se₇ structure are <i>a</i> = 11.4599(3) Å, <i>b</i> = 6.9268(2) Å, <i>c</i> = 19.6066(6) Å, and <i>β</i> = 101.197(1)°. The Ba₃Sn₂Se₇ structure is pseudo-zero-dimensional, similar to that of the Ba₃Sn₂S₇ structure. However, in contrast to the sulfide counterpart, the Ba₃Sn₂Se₇ structure features one split barium site (Ba1 and Ba2) arising from positional disorder. All atoms of the Ba₃Sn₂<i>Q</i>₇ structures occupy the general positions. The main building blocks of the Ba₃Sn₂<i>Q</i>₇ structures are the isolated anionic [Sn₂<i>Q</i>₇]^6- motifs, which are separated and charge-balanced by the filling of Ba²⁺ cations. Optical bandgap studies of polycrystalline Ba₃Sn₂<i>Q</i>₇ samples confirm their semiconducting nature with the direct bandgap values of 2.3(1) eV (<i>Q</i> = S) and 1.9(1) eV (<i>Q</i> = Se). DFT studies also predict the semiconducting nature of the title phases, and the theoretical band gaps are in good agreement with the optical absorption studies. Photovoltaic characterization of the polycrystalline Ba₃Sn₂S₇ sample reveals a 23.7% increase in power conversion efficiency, attributed to a reduced electron-hole recombination rate. Moreover, the theoretical electronic structures of the title chalcogenides are presented along with the COHPs results, highlighting the relative strengths of chemical bonds and charge transfer between the metals (Ba and Sn) and chalcogen (S/Se) atoms.