Template Pore Size and A‐Site Cation Management Dictate Luminescence Efficiency, Stability, and Wavelength in Confined Perovskite Nanostructures

Abstract A‐site cation composition is a useful lever in optimizing the photophysical properties and stability of metal halide perovskites (MHPs). Independent of this, straightforward preparative routes to MHP nanostructures that employ a single solid‐state template with modest thermal requirements are also in demand. Here both strategies are employed in the fabrication and evaluation of luminescence properties of mixed formamidinium/cesium (Cs x FA 1− x PbBr 3 ) and methylammonium/cesium (Cs x MA 1− x PbBr 3 ) nanostructures formed within confining mesoporous silica of 4 and 7 nm average pore diameters. Use of such small‐pore oxide‐terminated templates produce perovskite nanostructures in the strongly confined regime, with broadly tunable emission from green to sky blue. It is found that the smallest nanostructures that are formamidinium rich exhibit the largest photoluminescence quantum efficiency values, but such values diminish by more than 50% in a 10 day period. In contrast, the same nanostructures formed within a 7 nm porous template retain their efficiency values over the same time window. The likely origins of this size‐dependent behavior are discussed in terms of pore‐size‐dependent capillary forces. Such routes may ultimately lead to improved light‐emitting diode designs composed of controlled quantum‐confined perovskites of greater intrinsic stability than other emitters such as ligand‐based colloidal nanocrystals.