Ligand Boosting of CsPbBr₃ Nanoplatelets for Highly Stable Efficiency Blue LED

Perovskite light-emitting diodes (PeLEDs) with an external quantum efficiency (EQE) exceeding 20% have been achieved in both green and red emissions. On the other hand, the efficiency of blue perovskite materials has lagged far behind, with EQEs of 12.3% emitting the sky blue (wavelength at 475~490 nm) and 8.8% emitting the blue (wavelength at 460~475 nm). Several strategies have been proposed for the synthesis of blue-emitting perovskite nanocrystals such as (i) mixed halide perovskites and (ii) low-dimensional nanoplatelets (NPLs). Whereas the mixed halide perovskites show a variable energy bandgap, they have critical drawbacks such as a deep trap state in the bandgap owing to the formation of Cl^- vacancies and phase segregation arising from ion migration. In CsPbBr₃ NPLs, the emission wavelength can be controlled depending on the number of [PbBr₆]^4– layers. However, the high density of surface defects in 2D perovskite nanocrystals results in a low photoluminescence quantum yield (PLQY), which imposes challenging issues.
Here, we demonstrate that the PLQYs of CsPbBr₃ NPLs could be significantly enhanced by adopting inorganic ligands, which can effectively passivate the surface defects. To boost the PL and PLQY of NPLs, several organic/inorganic ligands (such as PbBr₂, C₈H₁₂BrN, and N₂H₃Br (HZBr) are used. The PLQY of CsPbBr₃ NPLs is significantly enhanced from 34% to 90% when HZBr is used as the ligand can be effectively coordinated at the surface defects of the CsPbBr₃ NPLs. In addition, we conducted the cryogenic PL spectroscopic measurement. Interestingly, in the HZBr-treated sample, the activation energy for carrier trapping is increased from ~180 to 290meV indicating that the surface vacancies and the associated defect states are well passivated. Furthermore, the exciton-longitudinal optical (LO) phonon coupling coefficient and LO phonon energy are reduced from ~280 to 100meV and from ~30 to 20meV, respectively. It suggests that the contribution of exciton-LO coupling to the broadening of PL became weaker.