Thin-Film Deposition and Characterization of a Sn-Deficient Perovskite Derivative Cs₂SnI₆

Abstract

In this work, we describe details of a two-step deposition approach that enables the preparation of continuous and well-structured thin films of Cs<sub>2</sub>SnI<sub>6</sub>, which is a one-half Sn-deficient 0-D perovskite derivative (i.e., the compound can also be written as CsSn<sub>0.5</sub>I<sub>3</sub>, with a structure consisting of isolated SnI<sub>6</sub><sup>4-</sup> octahedra). The films were characterized using powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), UV-Vis spectroscopy, photoluminescence (PL), photoelectron spectroscopy (UPS, IPES, XPS) and Hall effect measurements. UV-Vis and PL measurements indicate that the obtained Cs<sub>2</sub>SnI<sub>6</sub> film is a semiconductor with a band gap of 1.6 eV. This band gap was further confirmed by the UPS and IPES spectra, which were well reproduced by the calculated density of states with the HSE hybrid functional. The Cs<sub>2</sub>SnI<sub>6</sub> films exhibited n-type conduction with a carrier density of 6(1) × 10<sup>16</sup> cm<sup>-3</sup> and mobility of 2.9(3) cm<sup>2</sup>/V·s. While the computationally-derived band structure for Cs<sub>2</sub>SnI<sub>6</sub> shows significant dispersion along several directions in the Brillouin zone near the band edges, the valence band is relatively flat along the $Γ$–X direction, indicative of a more limited hole minority carrier mobility compared to analogous values for the electrons. The ionization potential (IP) and electron affinity (EA) were determined to be 6.4 and 4.8 eV, respectively. The Cs<sub>2</sub>SnI<sub>6</sub> films show some enhanced stability under ambient air, compared to methylammonium lead (II) iodide perovskite films stored under similar conditions; however, the films do decompose slowly, yielding a CsI impurity. We discuss these findings in the context of suitability of Cs<sub>2</sub>SnI<sub>6</sub> for photovoltaic and related optoelectronic applications.