Large-Scale Room Temperature One-Pot Synthesis of Perovskite Nanoplatelets for Blue Light-Emitting Diodes

Lead halide perovskites (LHPs) have excellent optical properties such as high photoluminescence quantum yield (PLQY), narrow full width at half maximum (FWHM) light emissions, photo-stability, high light absorption, high defect tolerance, and tunable band gap through halogen ions substitution. Hence, LHPs have been extensively applied to various optoelectronic devices, including biological imagers, light-emitting diodes (LEDs), solar cells, photodetectors, and lasers. Imparticular, perovskite nanocrystals (PNCs) are used for LEDs because PLQY and external quantum efficiency (EQE) of the LHPs are significantly enhanced when the LHPs are formed into PNCs.Recently, near-infrared, red, and green solution synthesized PNC-based LEDs (PeLEDs) have been reported to have EQEs of more than 20%, respectively. On the contrary, challenges remain to surpass the significantly lower EQE of blue light-emitting PeLEDs at around 10%. The low solubility in solvents of blue light-emitting PNCs makes them challenging to synthesize. Furthermore, the formation of defect states in the large band gap of blue light-emitting PNCs reduces their intrinsic photoluminescence (PL) efficiency and the electroluminescence (EL) efficiency of the blue light-emitting LEDs.One of the approaches to synthesizing blue-emitting PNCs is to use the mixture of chlorine and bromine to take the X-position in the ABX₃ perovskite structure. However, the blue emission peak of PNCs is often red-shifted or becomes one in the multi-peak emission spectra due to the phase separation-inducing halide ion migration. Hence, others explored reducing at least one of the three dimensions of bromine-only PNCs to form nanostructures like nanoplatelets (NPLs) and induce quantum confinement. NPLs containing a single type of halide ion are not affected by halide ion migration, and their emission peaks are only affected by their thickness. This study demonstrated that CsPbBr₃ NPLs could be synthesized in a single-step one-pot approach at room temperature and ambient air conditions. The synthesis was performed in a homogeneous solution without involving the additional precursor solution preparation steps. A slow reversible ion reaction was induced to reduce the structural defects in synthesizing the CsPbBr₃ NPLs with uniform light emission performance. By adding two ligands (oleic acid and oleylamine) and a small amount of good solvent dimethyl sulfoxide (DMSO) to the bad solvent toluene, the ionization and recrystallization of perovskite crystalline structure repeatedly occurred in cycles during the synthesis process. Furthermore, the ligand concentrations were optimized such that the ligands facilitated the formation of the desired shape and thickness of the PNCs into NPLs. The single-step one-pot approach at ambient conditions in this work allowed the homogenous and steady-state formation of the NPLs, unlike the significantly faster but non-uniform and uncontrollable synthesis of previously reported works. Hence, the single-step one-pot approach at ambient conditions demonstrated is not affected by the size of the synthesis vessel, enabling large-scale and uniform synthesis of NPLs. As a result, we obtained blue NPLs with a 460 nm peak and PLQY of more than 85%.