Additive Molecules Adsorbed on Monolayer PbI₂: Atomic Mechanism of Solvent Engineering for Perovskite Solar Cells

Solventengineering is highly essential for the upscaling synthesisof high-quality metal halide perovskite materials for solar cells.The complexity in the colloidal containing various residual speciesposes great difficulty in the design of the formula of the solvent.Knowledge of the energetics of the solvent-lead iodide (PbI2) adduct allows a quantitative evaluation of the coordination abilityof the solvent. Herein, first-principles calculations are performedto explore the interaction of various organic solvents (Fa, AC, DMSO,DMF, GBL, THTO, NMP, and DPSO) with PbI2. Our study establishesthe energetics hierarchy with an order of interaction as DPSO >THTO> NMP > DMSO > DMF > GBL. Different from the common notionof formingintimate solvent-Pb bonds, our calculations reveal that DMFand GBL cannot form direct solvent-Pb2+ bonding.Other solvent bases, such as DMSO, THTO, NMP, and DPSO, form directsolvent-Pb bonds, which penetrate through the top iodine planeand possess much stronger adsorption than DMF and GBL. A strong solvent-PbI2 adhesion (i.e., DPSO, NMP, and DMSO), associated with a highcoordinating ability, explains low volatility, retarded precipitationof the perovskite solute, and tendency of a large grain size in theexperiment. In contrast, weakly coupled solvent-PbI2 adducts (i.e., DMF) induces a fast evaporation of the solvent, accordinglya high nucleation density and small grains of perovskites are observed.For the first time, we reveal the promoted absorption above the iodinevacancy, which implies the need for pre-treatment of PbI2 like vacuum annealing to stabilize solvent-PbI2 adducts. Our work establishes a quantitative evaluation of the strengthof the solvent-PbI2 adducts from the atomic scaleperspective, which allows the selective engineering of the solventfor high-quality perovskite films.