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

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.