New insights into organic–inorganic hybrid perovskite CH3NH3PbI3 nanoparticles. An experimental and theoretical study of doping in Pb2+ sites with Sn2+, Sr2+, Cd2+ and Ca2+

Abstract

This paper presents the synthesis of the organic–inorganic hybrid perovskite, CH₃NH₃PbI₃, doped in the Pb²⁺ position with Sn²⁺, Sr²⁺, Cd²⁺ and Ca²⁺. The incorporation of the dopants into the crystalline structure was analysed, observing how the characteristics of the dopant affected properties such as the crystalline phase, emission and optical properties. XRD showed how doping with Sn²⁺, Sr²⁺ and Cd²⁺ did not modify the normal tetragonal phase. When doping with Ca²⁺, the cubic phase was obtained. Moreover, DR-UV-Vis spectroscopy showed how the band gap decreased with the dopants, the values following the trend Sr²⁺ < Cd²⁺ < Ca²⁺ < CH₃NH₃PbI₃ ≈ Sn²⁺. The biggest decrease was generated by Sr²⁺, which reduced the CH₃NH₃PbI₃ value by 4.5%. In turn, cathodoluminescence (CL) measurements confirmed the band gap obtained. Periodic-DFT calculations were performed to understand the experimental structures. The DOS analysis confirmed the experimental results obtained using UV-Vis spectroscopy, with the values calculated following the trend Sn²⁺ ≈ Pb²⁺ > Cd²⁺ > Sr²⁺ for the tetragonal structure and Pb²⁺ > Ca²⁺ for the cubic phase. The electron localization function (ELF) analysis showed similar electron localizations for undoped and Sn²⁺-doped tetragonal structures, which were different from those doped with Sr²⁺ and Cd²⁺. Furthermore, when Cd²⁺ was incorporated, the Cd–I interaction was strengthened. For Ca²⁺ doping, the Ca–I interaction had a greater ionic nature than Cd–I. Finally, an analysis based on the non-covalent interaction (NCI) index is presented to determine the weak-type interactions of the CH₃NH₃ groups with the dopant and I atoms. To our knowledge, this kind of analysis with these hybrid systems has not been performed previously.