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

Abstract

Computational, thin-film deposition and characterization approaches have been used to examine the ternary halide semiconductor Cs 3 Sb 2 I 9 . Cs 3 Sb 2 I 9 has two known structural modifications, the 0-D dimer form (space group P6 3 /mmc, No. 194) and the 2-D layered form (P3m1, 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 highband-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 3 Sb 2 I 9 . 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 3 NH 3 PbI 3 . The photoluminescence peak intensity of Cs 3 Sb 2 I 9 is substantially suppressed compared to that of CH 3 NH 3 PbI 3 , likely reflecting the presence of deep level defects that result in non-radiative recombination in the film, with computational results pointing to I i , I Sb , and V I 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.

Perovskite-Inspired Photovoltaic Materials: Toward Best Practices in Materials Characterization and Calculations

“…Additionally, XRF may be a suitable technique to assist in detecting changes in composition resulting from radiolysis, such as during RBS or XPS measurements. 21,30 Raman spectroscopy has also been put forward as a nondestructive method to measure the ratios of different elements. 127 Other methods to detect beam damage exist, such as analyzing the low-loss region in EELS measurements.…”

“…• H and He not detectable 129 • Damage and decomposition of soft and metastable materials 131 • Preferential sputtering during depth-profiling can remove more volatile components and change composition 30 • Beam-induced changes in oxidation state 129 • Binding energy shift due to charge accumulation, as well as junction and surface photovoltage 129,132 • Gas-phase surface contaminants can occur 129 Auger electron spectroscopy (AES) Sample irradiated with 3-20 keV electrons, ejecting core-level electrons in atoms and creating vacancies. Relaxation of outer electron to vacancy results in emission of Auger electron.…”

“…In particular, inaccuracies may arise from this method if the density of states at the leading edge is low and there is no distinct tangent to take, as occurs for many PIMs reported. 19,27,30,163 A method to visualize gap states is to plot the leading edge on a semilogarithmic scale, since band tails can be described by exponential functions. 172,173 This is illustrated in Figure 6a for organic small molecules.…”

Crystal and Electronic Structures of Complex Bismuth Iodides A3Bi2I9 (A = K, Rb, Cs) Related to Perovskite: Aiding the Rational Design of Photovoltaics

“…Recently, the closely related caesium antimony iodide Cs 3 Sb 2 I 9 has been investigated as solar cell absorber layer and the electronic structure of its polymorphs has been calculated by density functional theory (DFT). 29 In contrast, the investigation of the properties of inorganic bismuth halides has been limited to some work reporting optoelectronic properties 21,[30][31][32][33] and so far as we can discern, apart from a few calculated parameters for Cs 3 Bi 2 I 9 , 34 there are no computational studies of the electronic band structures available. Clearly complex bismuth halides are an interesting family of potentially solution-processable compounds, whose potential as an alternative to toxic lead halides is far from fully explored.…”

A semiconducting molecular ferroelectric with a bandgap much lower than that of BiFeO3

“…Inspired by the recently discovered PV effect in hybrid perovskites A 3 Bi 2 I 9 (A: methylammonium or cesium), 15,16 we have synthesized a hybrid semiconducting ferroelectric (2-(ammoniomethyl)pyridinium)SbI 5 (1). This material shows a narrow bandgap of 2.03 eV, comparable to that for SbSI (E g = 2.0 eV) and much lower than that for BiFeO 3 (E g = 2.7 eV), that may be suitable for PV applications.…”