Thin-Film Preparation and Characterization of Cs3Sb2I9: A Lead-Free Layered Perovskite Semiconductor

Computational, thin-film deposition, and characterization approaches have been used to examine the ternary halide semiconductor Cs3Sb2I9. Cs3Sb2I9 has two known structural modifications, the 0-D dimer form (space group P63/mmc, no. 194) and the 2-D layered form (P3̅m1, 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 Cs3Sb2I9. 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 CH3NH3PbI3. The photoluminescence peak intensity of Cs3Sb2I9 is substantially suppressed compared to that of CH3NH3PbI3, likely reflecting the presence of deep level defects that result in nonradiative recombination in the film, with computational results pointing to Ii, ISb, and VI 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 antibonding interactions between the Sb s lone pair states and I p states, which leads to a moderately dispersive valence band.