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 (BF 4 – ) salts on the surface passivation and photophysical properties of CsPbBr 3 NCs was systematically investigated. A series of BF 4 – salts with inorganic-, aromatic-, and phosphorus-based cations were examined to correlate countercation chemistry with surface reactivity. Structural analyses revealed that most BF 4 – salts efficiently removed metallic Pb 0 defects while maintaining the phase integrity. NH 4 BF 4 promoted the oriented attachment of nanocubes into nanowires, whereas tritylium BF 4 induced the partial decomposition of BF 4 – into BF 3 and F –, forming Pb–F bonds that stabilized the surface and reduced trap densities. In contrast, 2,4,6-triphenylpyrylium BF 4 triggered a Katritzky reaction with oleylamine, leading to aggregation and Cs 4 PbBr 6 formation. Photophysical measurements showed enhanced photoluminescence and increased trap activation energies for most BF 4 – -treated NCs due to suppressed nonradiative recombination. Light-emitting diodes incorporating sodium and tritylium BF 4 -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.