Revealing the role of Pb2+ in the stability of organic-inorganic hybrid perovskite CH3NH3Pb1-xCdxI3: an experimental and theoretical study

This paper presents the synthesis of organic-inorganic hybrid perovskite CH3NH3Pb1-xCdxI3. The effect of incorporating Cd2+ or Pb2+ on the stability of the perovskite structure was analysed from a theoretical and experimental viewpoint. The XRD results showed that the tetragonal perovskite structure was formed for x values of up to 0.5, which seems to indicate that the presence of a considerable amount of Pb2+ is necessary to stabilise the structure. In turn, UV-Vis spectroscopy showed how the presence of Cd2+ led to a reduction in the optical band gap of the perovskite structure of up to 9% for CH3NH3Pb0.5Cd0.5I3 with regard to the MAPbI(3) structure. Moreover, periodic-DFT calculations were performed to understand the effect of the increased concentration of Cd on the structural and electronic properties of MAPbI(3) perovskites. The analysis of both the ELF and the non-covalent interaction (NCI) index show the important role played by the Pb2+ ions in stabilizing this kind of hybrid perovskite structures. Finally, the DOS analysis confirmed the experimental results obtained using UV-Vis spectroscopy. The theoretical band gap values decreased as the concentration of Cd increased.