Self-assembly of a robust hydrogen-bonded octylphosphonate network on cesium lead bromide perovskite nanocrystals for light-emitting diodes

Abstract

We report the self-assembly of an extensive inter-ligand hydrogen-bonding network of octylphosphonates on the surface of cesium lead bromide nanocrystals (CsPbBr₃ NCs). The post-synthetic addition of octylphosphonic acid to oleic acid/oleylamine-capped CsPbBr₃ NCs promoted the attachment of octylphosphonate to the NC surface, while the remaining oleylammonium ligands maintained the high dispersability of the NCs in non-polar solvent. Through powerful 2D solid-state ³¹P–¹H NMR, we demonstrated that an ethyl acetate/acetonitrile purification regime was crucial for initiating the self-assembly of extensive octylphosphonate chains. Octylphosphonate ligands were found to preferentially bind in a monodentate mode through P–O⁻, leaving polar PO and P–OH groups free to form inter-ligand hydrogen bonds. The octylphosphonate ligand network strongly passivated the nanocrystal surface, yielding a fully-purified CsPbBr₃ NC ink with PLQY of 62%, over 3 times higher than untreated NCs. We translated this to LED devices, achieving maximum external quantum efficiency and luminance of 7.74% and 1022 cd m⁻² with OPA treatment, as opposed to 3.59% and 229 cd m⁻² for untreated CsPbBr₃ NCs. This represents one of the highest efficiency LEDs obtained for all-inorganic CsPbBr₃ NCs, accomplished through simple, effective passivation and purification processes. The robust binding of octylphosphonates to the perovskite lattice, and specifically their ability to interlink through hydrogen bonding, offers a promising passivation approach which could potentially be beneficial across a breadth of halide perovskite optoelectronic applications.