Partial ligand exchange on CsPbI3 nanocrystals with alkyl phosphonium iodide improves phase stability and emission efficiency in light-emitting diodes.

All-inorganic colloidal CsPbI3 nanocrystals are promising materials for light-emitting applications owing to their excellent optoelectronic properties. However, their practical implementation remains limited by surface defects, ligand instability, and phase degradation under ambient conditions. Here, we employ trimethyl(dodecyl)phosphonium iodide (TDP-I) as a post-synthetic ligand for partial ligand exchange on CsPbI3 nanocrystals initially coated with a ligand shell that is primarily composed of oleylammonium ions. X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, and nuclear magnetic resonance analyses confirm that TDP-I is only partially replacing the native ligand shell. These findings are in line with classical molecular dynamics calculations, which indicate that a mixed oleylammonium/TDP+ ligand shell is more stable than a full TDP+ one. The TDP-I treatment improved photophysical properties including an increase in the photoluminescence quantum yield (from ∼77% to ∼90%), accompanied by prolonged photoluminescence lifetime and reduced nonradiative recombination. The TDP-I-treated nanocrystals also exhibit improved stability, with delayed transformation from the orthorhombic phase to the non-perovskite δ-phase under ambient conditions. When incorporated into light-emitting diodes, the TDP-I-treated CsPbI3 nanocrystals deliver a maximum external quantum efficiency of 12.4%. These results demonstrate that partial phosphonium ligand exchange provides an effective strategy for optimizing the surface chemistry, optical properties, and device performance of CsPbI3 nanocrystals, highlighting the importance of mixed-ligand surface engineering for perovskite optoelectronics.