Importance of spin‐orbit coupling on photovoltaic properties of Pb‐free vacancy ordered double perovskites halides X 2 TeY 6 ( X = Cs , Rb , and Y = I , Br , Cl ): First‐principles calculations

In this paper, the crystal structures, thermodynamic stability, electronic densities of states, band structures, and optical properties of the nontoxic Pb-free vacancy ordered double perovskites halides X2TeY6 (X = Cs, Rb, and Y = I, Br, Cl) were investigated using first-principles calculations. It was found that at Γ point, the conduction band splits into two bands upon inclusion of spin-orbit coupling (SOC) interaction for X2TeY6 compound. Besides, the highest occupied molecular orbital is unchanged for all studied compounds except that of X2TeI6, which is shifted from W to X point. Our findings appear to be well supported by the experimental data (Annalise E. Maughan et al., J Am Chem Soc. 2016;138:8453-8464). This shows a contradiction with the theoretical work found by Malak Azmat Ali et al., Int. J. Energy Res. 2020;45:8448-8455. Hence, the inclusion of SOC is crucial for the correct characterization of the electronic properties of the X2TeY6 compound. Otherwise, the absorption coefficients shift to the visible region by the substitution of Cl by Br and I ions. Thus, the excellent absorption coefficients percentages of Cs2TeI6 (62%-75%) and Rb2TeI6 (57%-76%) along the visible region, their suitable forbidden bands (1.422 eV for Cs2TeI6, and 1.469 eV for Rb2TeI6) and their high stabilities make both Cs2TeI6 and Rb2TeI6 as new potential candidates' compounds for single-junction solar cells.