Untying the Cesium "Not": Cesium-Iodoplumbate Complexation in Perovskite Solution-Processing Inks Has Implications...

We illustrate the critical importance of the energetics of cation-solvent versus cation-iodoplumbate interactions in determining the stability of ABX₃ perovskite precursors in a dimethylformamide (DMF) solvent medium. We have shown, through a complementary suite of nuclear magnetic resonance (NMR) and computational studies, that Cs⁺ exhibits significantly different solvent vs iodoplumbate interactions compared to organic A⁺-site cations such as CH₃NH₃⁺ (MA⁺). Two NMR studies were conducted: ¹³³Cs NMR analysis shows that Cs⁺ and MA⁺ compete for coordination with PbI₃^- in DMF. ²⁰⁷Pb NMR studies of PbI₂ with cationic iodides show that perovskite-forming Cs⁺ (and, somewhat, Rb⁺) do not comport with the ²⁰⁷Pb chemical shift trend found for Li⁺, Na⁺, and K⁺. Three independent computational approaches (density functional theory (DFT), ab initio Molecular Dynamics (AIMD), and a polarizable force field within Molecular Dynamics) yielded strikingly similar results: Cs⁺ interacts more strongly with the PbI₃^- iodoplumbate than does MA⁺ in a polar solvent environment like DMF. The stronger energy preference for PbI₃^- coordination of Cs⁺ vs MA⁺ in DMF demonstrates that Cs⁺ is not simply a postcrystallization cation "fit" for the perovskite A⁺-site. Instead, it may facilitate preorganization of the framework precursor that eventually transforms into the crystalline perovskite structure.