Thin-Film Preparation and Characterization of Cs<sub>3</sub>Sb<sub>2</sub>I<sub>9</sub>: A Lead-Free Layered Perovskite Semiconductor
Abstract
In this study, computational, thin-film deposition and characterization approaches have been used to examine the ternary halide semiconductor Cs<sub>3</sub>Sb<sub>2</sub>I<sub>9</sub>. Cs<sub>3</sub>Sb<sub>2</sub>I<sub>9</sub> has two known structural modifications, the 0-D dimer form (space group P6<sub>3</sub>/mmc, No. 194) and the 2-D layered form (P$\bar{3}$<i>m</i>1, No. 164), which can be prepared via solution and solid state or gas phase reactions, respectively. Our computational investigations suggest that the layered form, which is a one-third Sb-deficient derivative of the ubiquitous perovskite structure, is a potential candidate for high-band-gap photovoltaic (PV) applications. In this work, we describe details of a two-step deposition approach that enables the preparation of large grain (>1 µm) and continuous thin films of the lead-free layered perovskite derivative Cs<sub>3</sub>Sb<sub>2</sub>I<sub>9</sub>. Depending on the deposition conditions, films that are c-axis oriented or randomly oriented can be obtained. The fabricated thin films show enhanced stability under ambient air, compared to methylammonium lead (II) iodide perovskite films stored under similar conditions, and an optical band gap value of 2.05 eV. Photoelectron spectroscopy study yields an ionization energy of 5.6 eV, with the valence band maximum approximately 0.85 eV below the Fermi level, indicating near-intrinsic, weakly p-type character. Density Functional Theory (DFT) analysis points to a nearly direct band gap for this material (less than 0.02 eV difference between the direct and indirect band gaps) and a similar high-level of absorption compared to CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>. The photoluminescence peak intensity of Cs<sub>3</sub>Sb<sub>2</sub>I<sub>9</sub> is substantially suppressed compared to that of CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>, likely reflecting the presence of deep level defects that result in non-radiative recombination in the film, with computational results pointing to I<sub>i</sub>, IS<sub>b</sub>, and V<sub>I</sub> as being likely candidates. A key further finding from this study is that, despite a distinctly layered structure, the electronic transport anisotropy is less pronounced due to the high ionicity of the I atoms and the strong anti-bonding interactions between the Sb s lone pair states and I p states, which leads to a moderately dispersive valence band.