Toward Stable Deep-Blue Luminescent Colloidal Lead Halide Perovskite Nanoplatelets: Systematic Photostability Investigation

Colloidal lead halide perovskite nanocrystals and nanoplatelets have emerged as promising semiconductor nanomaterials because of their spectral tunability, facile processability, and bright emission with high color purity. In particular, strong quantum and dielectric confinement make atomically thin colloidal lead bromide perovskite nano platelets a favorable candidate for next-generation deep-blue emitting (lambda(max) = 437 nm) materials. However, poor photostability poses a critical challenge; colloidal nano platelets suffer from photobleaching or transformation into thicker, less-confined nanostructures with red-shifted emission upon UV irradiation. In this study, we synthesize deep-blue emitting organic inorganic hybrid perovskite nanoplatelets (formula: L-2[ABX(3)]BX4, L: butylammonium and octylammonium, A: methylammonium or formamidinium, B: lead, and X: bromide or iodide) with large lateral dimension (similar to 1 mu m) by ligand-assisted reprecipitation and systematically investigate the factors that affect the photostability of those nanoplatelets. We find that freshness of the prepared precursor solutions for ligand-assisted reprecipitation is critical to obtain better stability with high photoluminescence quantum yield of perovskite nanoplatelets. Photobleaching is found to result from intrinsic instability of the perovskite lattice against UV irradiation in nanoplatelets, whereas transformation into thicker nanostructures results from extrinsic factors-moisture, primarily. Furthermore, we observe that substitution of the organic cation from formamidinium to methylammonium and addition of excess alkylammonium bromide ligands significantly enhance both the ambient and photostability. Lastly, we demonstrate that the dropcast film of methylammonium lead bromide nanoplatelets with excess alkylammonium bromide ligands shows dramatically improved stability both under UV irradiation and under ambient conditions. This study expands our understanding of the factors that affect perovskite nanoplatelet photostability and opens up new possibilities for the fabrication of stable perovskite nanoplatelet-based optoelectronic devices.