Cation-Engineered Post-Treatment for Blue Perovskite Light-Emitting Diodes

Halide post-treatment has emerged as a promising strategy for achieving pure-blue emission in mixed-halide perovskites by facilitating the incorporation of low-solubility chloride while simultaneously passivating defects to enhance luminescence. Despite the demonstrated success in anion exchange, the specific influence of organic cations in post-treatment solutions has remained insufficiently explored, limiting further optimization of this technique. Here, the role of cations is investigated by comparing two structurally similar organic cations in halide post-treatment agents, i.e., 4-fluorophenylethylammonium (FPEA⁺) and phenylethylammonium (PEA⁺). The findings reveal a stark contrast in their effects on 3D perovskite surfaces. Through enhanced surface binding and fluorine-induced hydrophobicity, FPEA⁺ mitigates lattice damage from the solvent penetration during post-treatment, enabling a mild perovskite modification that better preserves the 3D perovskite structure while effectively passivating surface defects. Conversely, the PEA⁺ treatment causes partial degradation of the 3D perovskite, forming a defective 2D/3D grain surface and introducing a new 2D layer in inter-grain regions. By replacing PEA⁺ with FPEA⁺ in the conventional halide post-treatment, the mixed-halide perovskite light-emitting diode performance is significantly enhanced, achieving peak external quantum efficiencies of 16.2% at 483 nm, 11.1% at 475 nm, and 7.6% at 466 nm, with all devices exhibiting excellent color purity and spectral stability.