Fast A‐Site Cation Cross‐Exchange at Room Temperature: Single‐to Double‐ and Triple‐Cation Halide Perovskite Nanocrystals

We report here fast A‐site cation cross‐exchange between APbX3 perovskite nanocrystals (NCs) made of different A‐cations (Cs (cesium), FA (formamidinium), and MA (methylammonium)) at room temperature. Surprisingly, the A‐cation cross‐exchange proceeds as fast as the halide (X=Cl, Br, or I) exchange with the help of free A‐oleate complexes present in the freshly prepared colloidal perovskite NC solutions. This enabled the preparation of double (MACs, MAFA, CsFA)‐ and triple (MACsFA)‐cation perovskite NCs with an optical band gap that is finely tunable by their A‐site composition. The optical spectroscopy together with structural analysis using XRD and atomically resolved high‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) and integrated differential phase contrast (iDPC) STEM indicates the homogeneous distribution of different cations in the mixed perovskite NC lattice. Unlike halide ions, the A‐cations do not phase‐segregate under light illumination. The A‐cation cross‐exchange between perovskite nanocrystals (NCs) is found to be as fast as the halide exchange in the presence of excess A‐oleate complexes in the colloidal solution. The synthesis of a wide range of mixed (double and triple)‐cation and mixed halide perovskite NCs is demonstrated. The mixed A‐cations were found to be uniformly distributed across the crystal lattice and they do not segregate under intense light illumination.