Highly Luminescent and Stable Perovskite Nanocrystals with Octylphosphonic Acid as a Ligand for Efficient Light-Emitting Diodes

All inorganic perovskite nanocrystals (NCs) of CsPbX₃ (X = Cl, Br, I, or their mixture) are regarded as promising candidates for high-performance light-emitting diode (LED) owing to their high photoluminescence (PL) quantum yield (QY) and easy synthetic process. However, CsPbX₃ NCs synthesized by the existing methods, where oleic acid (OA) and oleylamine (OLA) are generally used as surface-chelating ligands, suffer from poor stability due to the ligand loss, which drastically deteriorates their PL QY, as well as dispersibility in solvents. Herein, the OA/OLA ligands are replaced with octylphosphonic acid (OPA), which dramatically enhances the CsPbX₃ stability. Owing to a strong interaction between OPA and lead atoms, the OPA-capped CsPbX₃ (OPA-CsPbX₃) NCs not only preserve their high PL QY (>90%) but also achieve a high-quality dispersion in solvents after multiple purification processes. Moreover, the organic residue in purified OPA-CsPbBr₃ is only ∼4.6%, which is much lower than ∼29.7% in OA/OLA-CsPbBr₃. Thereby, a uniform and compact OPA-CsPbBr₃ film is obtained for LED application. A green LED with a current efficiency of 18.13 cd A^-1, corresponding to an external quantum efficiency of 6.5%, is obtained. Our research provides a path to prepare high-quality perovskite NCs for high-performance optoelectronic devices.