Improving the Stability of Colloidal CsPbBr₃ Nanocrystals with an Alkylphosphonium Bromide as Surface Ligand Pair

High Resolution Image Download MS PowerPoint Slide In this study, we synthesized a phosphonium-based ligand, trimethyl(tetradecyl)phosphonium bromide (TTP-Br), and employed it in the postsynthesis surface treatment of Cs-oleate-capped CsPbBr 3 nanocrystals (NCs). The photoluminescence quantum yield (PLQY) of the NCs increased from ∼60% to more than 90% as a consequence of replacing Cs-oleate with TTP-Br ligand pairs. Density functional theory calculations revealed that TTP + ions bind to the NC surface by occupying Cs + surface sites and orienting one of their P–CH 3 bonds perpendicular to the surface, akin to quaternary ammonium passivation. Importantly, TTP-Br-capped NCs exhibited higher stability in air compared to didodecyldimethylammonium bromide-capped CsPbBr 3 NCs (which are considered a benchmark system), retaining ∼90% of their PLQY after 6 weeks of air exposure. Light-emitting diodes fabricated with TTP-Br-capped NCs achieved a maximum external quantum efficiency of 17.2%, demonstrating the potential of phosphonium-based molecules as surface ligands for CsPbBr 3 NCs in optoelectronic applications.