(775a) Investigating the Role of Lewis Acid-Base Interactions in Halide Perovskite Solutions

When processed in solution, metal halide perovskite solar cells, provide a unique cost advantage over existing solar cell technologies. However, the morphology and solar cell efficiency of the final crystal film is known to be heavily influenced by the perovskite material’s many components of (A, B, and X-site atoms) and solvent selection. As these effects remain poorly understood, studying the nucleation and early-stage growth of these materials in solution would provide much needed insight into the effect of solvents on final film morphology. Molecular simulation approaches have a significant role to play in this endeavour by uncovering the solvent-solute interactions (described by Lewis acid-base chemistry) which influence the formation and aggregation of moieties that will ultimately nucleate and grow into thin films. In this study, we leverage first principles density functional theory (DFT) and molecular dynamics (MD) to elucidate the role of these interactions in the pre-nucleation and early-stage growth of metal halide perovskites. Specifically, we:

• Quantify the binding affinity of Lewis bases (solvents) towards Lewis acids (perovskite salts and cations) – over 600 combinations were explored.
• Identify metrics towards selecting ideal solvent candidates for perovskite processing without the need for additional computation (rules of thumb and simple linear models).
• Uncover their impact on the solution precursors and solid-state intermediate structures preceding the final perovskite film.

A key result from this study is that no single metric can describe solvent interactions towards A-site cations (e.g., Cs⁺) and B-site cation species (PbI₂). For example, the Gutmann donor number (DN), a Lewis affinity scale metric that has gained popularity in recent years, adequately describes solvent interactions with isolated B-cation salt complexes (BX_n). Here B represents B-site cations of group 14 and 15 elements (Pb, Sn, Ge, Bi, and Sb) and X represents the perovskite halide ions (Cl-, Br- and I-). On the other hand, the lithium cation affinity (LCA) scale best describes solvent interactions towards group 1 A-site cations (Cs⁺ and Rb⁺), while the methyl ammonium cation affinity (MACA) approximates the affinity of solvents to the methylammonium cation (MA⁺) in perovskite solutions. Using computation, we elucidate how the affinity of solvents towards these Lewis acids can impact the formation of higher order iodoplumbate complexes and the intercalation of A-site cation species into the perovskite framework – two key reactions occurring in solution that are essential to the nucleation and growth of the final perovskite film. Finally, through leveraging our generated metrics, one could design processing protocols that optimize the quality of the final perovskite film via judicious solvent selection.