Trimethylsulfonium Lead Triiodide: An Air-Stable Hybrid Halide Perovskite

We report on the synthesis, characterization, and optoelectronic properties of the novel trimethylsulfonium lead triiodide perovskite, (CH₃)₃SPbI₃. At room temperature, the air-stable compound adopts a hexagonal crystal structure with a 1D network of face-sharing [PbI₆] octahedra along the c axis. UV-vis reflectance spectroscopy on a pressed pellet revealed a band gap of 3.1 eV, in agreement with first-principles calculations, which show a small separation between direct and indirect band gaps. Electrical resistivity measurements on single crystals indicated that the compound behaves as a semiconductor. According to multi-temperature single-crystal X-ray diffraction, synchrotron powder X-ray diffraction, Raman spectroscopy, and differential scanning calorimetry, two fully reversible structural phase transitions occur at -5 and ca. -100 °C with reduction of the unit cell symmetry to monoclinic as temperature decreases. The role of the trimethylsulfonium cation regarding the chemical stability and optoelectronic properties of the new compound is discussed in comparison with APbI₃ (A = Cs, methylammonium, and formamidinium cation), which are most commonly used in perovskite solar cells.