Electronic Structure and Optical Properties of Tin Iodide Solution Complexes

The emerging interest in tin halide perovskites demandsa robustunderstanding of the fundamental properties of these materials startingfrom the earliest steps of their synthesis. In a first-principleswork based on time dependent density functional theory, we investigatethe structural, energetic, electronic, and optical properties of 14tin iodide solution complexes formed by the SnI2 unit tetracoordinatedwith molecules of common solvents, which we classify according totheir Gutmann's donor number. We find that all considered complexesare energetically stable and their formation energy expectedly increaseswith the donating ability of the solvent. The energies of the frontierstates are affected by the choice of solvent, with their absolutevalues decreasing with the donor number. The occupied orbitals arepredominantly localized on the tin iodide unit, while the unoccupiedones are distributed also on the solvent molecules. Owing to thispartial wave function overlap, the first optical excitation is generallyweak, although the spectral weight is red-shifted by the solvent moleculesbeing coordinated to SnI2 in comparison to the referenceobtained for this molecule alone. Comparisons with results obtainedon the same level of theory on Pb-based counterparts corroborate ouranalysis. The outcomes of this study provide quantum-mechanical insightinto the fundamental properties of tin iodide solution complexes.This knowledge is valuable in the research on lead-free halide perovskitesand their precursors.