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

We illustrate the critical importance of the energetics of cation-solvent versus cation-iodoplumbate interactions in determining the stability of ABX 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 CHNH (MA). Two NMR studies were conducted: Cs NMR analysis shows that Cs and MA compete for coordination with PbI in DMF. Pb NMR studies of PbI with cationic iodides show that perovskite-forming Cs (and, somewhat, Rb) do not comport with the Pb 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 PbI iodoplumbate than does MA in a polar solvent environment like DMF. The stronger energy preference for PbI 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.