Electronic structures, spectroscopic properties, and thermodynamic characterization of sodium- or potassium-incorporated CH₃NH₃PbI₃ by first-principles calculation

Influence of alkali metals such as sodium (Na) and potassium (K) in methyl ammonium lead iodide CH₃NH₃PbI₃ perovskite crystal on electronic structures, spectroscopic properties, and thermodynamic properties was investigated by first-principles calculation. Partial substitution of CH₃NH₃ with Na or K in the perovskite crystal generated 3s, 3p, 4s, and 4p orbitals of Na or K above conduction band, which promoted the charge transfer from the alkali metals to the conduction band, accelerating the carrier generation and diffusion related to the photovoltaic performances. The molar absorption coefficient and oscillation strength were increased by a slight deviation of the charge distribution near ligand in the coordination structure. The splitting of chemical shift of ¹²⁷I-NMR in magnetic field was caused by a slight perturbation of the coordination structure with nuclear quadrupole interaction based on electronic field gradient. Decrease of the Gibbs free energy and entropy indicated the thermodynamic stabilization without scattering carrier diffusion as phonon effectiveness. The decrease of the entropy was based on a slight change of stretching vibration mode of Pb–I bond with vending mode of N–H and C–H bonds in the infrared and Raman spectra. The minor addition of 12.5% Na or K into the perovskite crystal will improve the photovoltaic properties, open voltage related to band gap, and short-circuit current density based on the carrier diffusion with phonon effectiveness.