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 3 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 CH 3 NH 3 + (MA + ). Two NMR studies were conducted: 133 Cs NMR analysis shows that Cs + and MA + compete for coordination with PbI 3 - in DMF. 207 Pb NMR studies of PbI 2 with cationic iodides show that perovskite-forming Cs + (and, somewhat, Rb + ) do not comport with the 207 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 3 - iodoplumbate than does MA + in a polar solvent environment like DMF. The stronger energy preference for PbI 3 - 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.