Overcome Charge Transfer Barrier via Electrostatically Stabilized CsPbBr₃ Nanocrystals for Efficient Perovskite Light-Emitting Diodes

Electrically insulating organic ligands with long hydrocarbon chains cause bottlenecks, which hinder efficient charge injection and transportation in CsPbBr₃ nanocrystal (NC)-based light-emitting diodes (LEDs). Shorter ligand exchange with precise control of the surface ligand density through additional purification processes is a common strategy used to improve charge transport in optoelectronics. However, practical applications have been limited owing to poor structural integrity and colloidal stability associated with the low steric hindrance of short surface-capping ligands. Here, we report a post-synthesis treatment on CsPbBr₃ NCs using hydrazine monohydrobromide (N₂H₅Br; HZBr), and tetrafluoroborate derivatives (XBF4, X=Na⁺, NO⁺, NH₄⁺, ...). Despite their short molecular structure, these inorganic ligands provide an excess amount of charged ions to the NC surface, enhancing the colloidal dispersion of the CsPbBr₃ NCs through electrostatic stabilization. Cryogenic photoluminescence spectroscopic analysis identified that the exciton-phonon coupling at the surface of CsPbBr₃ NCs is significantly reduced by the presence of short inorganic ligands, preventing the formation of both permanent and transient exciton traps. Furthermore, enhanced surface stabilization to improve the radiative recombination was accomplished by excess Br^- and F^-anions passivating Br^- ion vacancies at the CsPbBr₃ NC surfaces. LEDs based on the short inorganic ligands-treated CsPbBr₃ NCs showed improved maximum current efficiency of 19.46 cd/A, and the external quantum efficiency of 6.04 % increased by a factor of 2.8. Additional electron and hole-only device measurements affirmed the enhanced hole-electron injection and transport into the CsPbBr₃ NCs.