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

Methylammonium tin iodide systems containing earth-alkaline ions (CH 3 NH 3 Sn 1− x (EA) x I 3 , EA = Ca 2+ , Sr 2+ , Mg 2+ , 0 ≤ x ≤ 0.30) were investigated. The X-ray diffraction patterns detected the formation of tetragonal nearly cubic CH 3 NH 3 SnI 3 (space group P4mm ), SnI 2 , and not identified phases. The morphological analysis confirmed the presence of secondary phases with formation of irregularly shaped crystallites. The Sn3 d and I3 d photoemission spectra revealed the typical position and separation of spin–orbit components for Sn 2+ 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 Sn 2+ and Sr 2+ ions which can be easily exchanged in the SnI 6 2− octahedra.