Iodide vs Chloride: The Impact of Different Lead Halides on the Solution Chemistry of Perovskite Precursors

Controlled perovskite growth from solution is crucial for efficient optoelectronic applications and requires a deep understanding of the perovskite precursor chemistry. The so-called "chlorine route" to lead-iodide perovskite, using PbCl2 or MACl additive as a precursor, is frequently employed to form homogeneous perovskite layers by retarding perovskite crystallization. To understand the role of chlorine-containing lead precursors in solution, we analyze the chemical interaction of PbCl2 and PbI2 precursors with commonly employed solvents (gamma-butyrolactone (GBL), N,N-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO)) by combining first-principles simulations and experimental UV-vis spectroscopy in diluted precursor solutions. Ab initio molecular dynamics simulations reveal reduced solvation and an increased free energy barrier of lead-halide bond dissociation of PbCl2 compared to PbI2 with chlorine acting as a stronger ligand, which, in turn, limits the solvent coordination. In contrast to PbI2, PbCl2 absorption spectra lack signatures of high-valent [PbCln](2-n) complexes and show low sensitivity on the employed solvent, as confirmed by combined UV-vis and excited-state time-dependent density functional theory (TD-DFT) analysis. Altogether, our data suggest the presence of residual chlorine coordinated to Pb even in the presence of high iodine excess, which may retard the perovskite growth and could also lead to chlorine incorporation within the lead-iodide perovskite crystal.