Fundamental Properties of Low-Dimensional Perovskite-Related Light Absorbers: [(CH₂)₃NH₂]₃Sb₂X₉ (X = I, Br, Cl),...

Over the past decade, many Sb-based organohalides have been synthesized and analyzed due to their intriguing optical and electronic properties. They form low-dimensional (0D, 1D, and 2D) metal halide frameworks due to the 3+ oxidation state of antimony (Sb³⁺). The low-dimensional perovskite derivatives commonly have good moisture stability which makes them attractive for replacing the lead-containing halide perovskites. We focus on the A₃Sb₂X₉ compositions since the band gap tunability of the compounds is noteworthy. Cs₃Sb₂I₉ is known to form both 0D and 2D octahedral networks at room temperature. The A-site in A₃Sb₂X₉ is crucial in determining the configuration of the metal halide framework. Within the hybrid A₃Sb₂X₉ systems, the azetidinium cation (Az⁺, [(CH₂)₃NH₂]⁺) has rarely been studied as a potential A-site candidate. We confirm that the polycrystalline powders and thin films of (Az)₃Sb₂I₉ exhibit a 0D dimer structure, while (Az)₃Sb₂Br₉ and (Az)₃Sb₂Cl₉ adopt a 2D corrugated layer structure at the molecular level. (Az)₃Sb₂I₉ and (Az)₃Sb₂Cl₉ exhibit phase transitions at low temperatures, as shown by differential scanning calorimetry. Thin films grow with a strongly preferred (00l) orientation perpendicular to glass substrates. The Tauc plot analysis from diffuse reflectance spectra exhibits a band edge at 2.11 eV in (Az)₃Sb₂I₉, at 2.63 eV in (Az)₃Sb₂Br₉, and at 3.26 eV in (Az)₃Sb₂Cl₉. A weak excitonic band is identified in the absorption spectrum of the (Az)₃Sb₂I₉ thin film. Interestingly, there is a severe morphological difference in the thin films, depending on the solvent used, resulting in a band edge shift. The temperature dependence of the (Az)₃Sb₂I₉ thin-film XRD data indicates the existence of an intermediate phase.