Investigation of earth-alkaline (EA = Mg, Ca, Sr) containing methylammonium tin iodide perovskite systems

Methylammonium tin iodide systems containing earth-alkaline ions (CH3NH3Sn1−x (EA) x I3, EA = Ca2+, Sr2+, Mg2+, 0 ≤ x ≤ 0.30) were investigated. The X-ray diffraction patterns detected the formation of tetragonal nearly cubic CH3NH3SnI3 (space group P4mm), SnI2, and not identified phases. The morphological analysis confirmed the presence of secondary phases with formation of irregularly shaped crystallites. The Sn3d and I3d photoemission spectra revealed the typical position and separation of spin–orbit components for Sn2+ in halides. Static thermogravimetric measurements (T = 85 °C) showed a barely measurable weight loss for EA = Mg, a dramatic decrease of the weight loss rate for EA = Ca, and recorded weight losses till t ≈ 1.5 h only for EA = Sr, respectively. The optical spectra displayed absorption edges which increased at increasing the (EA)-content with maximum values for x = 0.050 (λ on-set = 1754 nm, EA = Mg; λ on-set = 1692 nm, EA = Ca; and λ on-set = 1338 nm, EA = Sr, respectively). The Tauc plots revealed a direct semiconducting behavior with band energy gaps depending on the nature and amount of the (EA)-ions. The photoluminescence (PL) spectra showed, for EA = Mg, an increase of the PL-band intensity at increasing the Mg content with a maximum at x = 1.0 and, for EA = Ca, an increase of band intensity at increasing the Ca-content and for EA = Sr, a band intensity maximum at x = 0.025. This was explained by the similar ionic radius between Sn2+ and Sr2+ ions which can be easily exchanged in the SnI6 2− octahedra.