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

Abstract 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.