Effects of alkali-metals (X = Li, Na, K) doping on the electronic, optoelectronic, thermodynamic, and X-ray spectroscopic properties of X–SnI₃ halide perovskites

The effect of alkali metals (Li, Na, K) doping at X sites in tin-based X–SnI₃ halide perovskite on structural, electronic, phonon, thermodynamic, and X-ray core-level spectroscopic properties was investigated and correlated with optoelectronic efficiency in the development of a stable and environmentally friendly solar technology to replace toxic lead-based perovskite solar cells (PSCs). These calculations were carried out based on the density functional theory approach using the Quantum Espresso software with the generalized gradient approximation (GGA) exchange correlation Perdew Burke Ernzerhorf functional. In addition, the phonon frequency, thermodynamics, and X-ray spectroscopy calculations were completed in Materials Studio using the CASTEP code. Our results reveal that there is a clear correlation between the alkali metal's ionic radius, changes in the lattice structural parameters, and the electronic band gap, such that the lattice constants are 5.940, 5.868, and 5.897 Å and the band gaps are 1.20, 1.14, and 1.10 eV for LiSnI₃, NaSnI₃, and KSnI₃, respectively. The highest lattice constant obtained for LiSnI₃, indicates that Li atoms occupied interstitial positions in the perovskite crystal, whereas the direct band gap decreases as cationic size increases, with KSnI₃ exhibiting the least band gap. Phonon calculations show that KSnI₃ has the lowest negative vibration region in its phonon dispersion plot, indicating dynamic stability. Interestingly, all the structures are found to portray high optical absorption, high extinction coefficient, high polarizability. Additionally, the free energy computation of the perovskites shows a reduction with rising temperature, but enthalpy increases significantly with increasing temperature. Yet, due to phonon lattice vibrations, the heat capacity increased as the temperature rose. Although the XSnI₃ (X = Li, Na, and K) perovskites under investigation will perform efficiently as semiconductors and can be used for optoelectronic applications, the KSnl₃ perovskite material has the most desirable properties for the fabrication of solar cell devices.