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

Abstract

Computational,\nthin-film deposition, and characterization approaches\nhave been used to examine the ternary halide semiconductor Cs₃Sb₂I₉. Cs₃Sb₂I₉ has two known structural modifications, the 0-D dimer form\n(space group <i>P</i>6₃/<i>mmc</i>,\nno. 194) and the 2-D layered form (<i>P</i>3̅<i>m</i>1, no. 164), which can be prepared via solution and solid-state\nor gas-phase reactions, respectively. Our computational investigations\nsuggest that the layered form, which is a one-third Sb-deficient derivative\nof the ubiquitous perovskite structure, is a potential candidate for\nhigh-band gap photovoltaic (PV) applications. In this work, we describe\ndetails of a two-step deposition approach that enables the preparation\nof large grain (>1 μm) and continuous thin films of the lead-free\nlayered perovskite derivative Cs₃Sb₂I₉. Depending on the deposition conditions, films that are <i>c</i>-axis oriented or randomly oriented can be obtained. The\nfabricated thin films show enhanced stability under ambient air, compared\nto methylammonium lead­(II) iodide perovskite films stored under similar\nconditions, and an optical band gap value of 2.05 eV. Photoelectron\nspectroscopy study yields an ionization energy of 5.6 eV, with the\nvalence band maximum approximately 0.85 eV below the Fermi level,\nindicating near-intrinsic, weakly p-type character. Density functional\ntheory (DFT) analysis points to a nearly direct band gap for this\nmaterial (less than 0.02 eV difference between the direct and indirect\nband gaps) and a similar high-level of absorption compared to CH₃NH₃PbI₃. The photoluminescence peak\nintensity of Cs₃Sb₂I₉ is substantially\nsuppressed compared to that of CH₃NH₃PbI₃, likely reflecting the presence of deep level defects that\nresult in nonradiative recombination in the film, with computational\nresults pointing to I_i, I_Sb, and V_I as being likely candidates. A key further finding from this study\nis that, despite a distinctly layered structure, the electronic transport\nanisotropy is less pronounced due to the high ionicity of the I atoms\nand the strong antibonding interactions between the Sb <i>s</i> lone pair states and I <i>p</i> states, which leads to\na moderately dispersive valence band.