Modulating the Photoluminescence of CsPbBr ₃ Nanocrystals via Cation Variation in BF ₄ ^– Salts

Despite metal halide perovskite nanocrystals (NCs) having shown great promise for light-emitting applications, their performance is often limited by surface defects and unstable ligand environments that promote nonradiative recombination. To overcome these challenges, the influence of countercations in tetrafluoroborate (BF4–) salts on the surface passivation and photophysical properties of CsPbBr3 NCs was systematically investigated. A series of BF4– salts with inorganic-, aromatic-, and phosphorus-based cations were examined to correlate countercation chemistry with surface reactivity. Structural analyses revealed that most BF4– salts efficiently removed metallic Pb0 defects while maintaining the phase integrity. NH4BF4 promoted the oriented attachment of nanocubes into nanowires, whereas tritylium BF4 induced the partial decomposition of BF4– into BF3 and F–, forming Pb–F bonds that stabilized the surface and reduced trap densities. In contrast, 2,4,6-triphenylpyrylium BF4 triggered a Katritzky reaction with oleylamine, leading to aggregation and Cs4PbBr6 formation. Photophysical measurements showed enhanced photoluminescence and increased trap activation energies for most BF4–-treated NCs due to suppressed nonradiative recombination. Light-emitting diodes incorporating sodium and tritylium BF4-treated NCs exhibited improved emission stability and electroluminescence. These findings highlight countercation-dependent surface chemistry as a key factor in achieving efficient defect passivation and stable perovskite optoelectronic performance.