  Understanding the formation of lead halide (LH) perovskite solution
precursors is crucial to gain insight into the evolution of these materials to
thin films for solar cells. Using density-functional theory in conjunction with
the polarizable continuum model, we investigate 18 complexes with chemical
formula PbX$_2$M$_4$, where X = Cl, Br, I and M are common solvent molecules.
Through the analysis of structural properties, binding energies, and charge
distributions, we clarify the role of halogen species and solvent molecules in
the formation of LH perovskite precursors. We find that interatomic distances
are critically affected by the halogen species, while the energetic stability
is driven by the solvent coordination to the backbones. Regardless of the
solvent, lead iodide complexes are more strongly bound than the others. Based
on the charge distribution analysis, we find that all solvent molecules bind
covalently with the LH backbones and that Pb-I and Pb-Br bonds lose ionicity in
solution. Our results contribute to clarify the physical properties of LH
perovskite solution precursors and offer a valuable starting point for further
investigations on their crystalline intermediates.
