Structure and surface properties of size-tuneable CsPbBr3 nanocrystals.

This investigation has characterised the structure and surface chemistry of CsPbBr3 nanocrystals with controlled diameters between 6.4 to 12.8 nm. The nanocrystals were investigated via a thorough 133Cs solid state NMR and nuclear relaxation study, identifying and mapping radially-increasing nanoscale disorder. This work has formalised 133Cs NMR as a highly sensitive probe of nanocrystal size, which can conveniently analyse nanocrystals in solid forms, as they would be utilised in optoelectronic devices. A combined multinuclear solid state NMR and XPS approach, including 133Cs-1H heteronuclear correlation 2D (HETCOR) NMR, was utilised to study the nanocrystal surface and ligands, demonstrating that the surface is Cs-Br rich with vacancies passivated by didodecyldimethylammonium bromide (DDAB) ligands. Furthermore, it is shown that a negligible amount of phosphonate ligands remain on the powder nanocrystal surface, despite the key role of octylphosphonic acid (OPA) in controlling the colloidal nanocrystal growth. The CsPbBr3 NCs were shown to be structurally stable under ambient conditions for up to 6 months, albeit with some particle agglomeration.

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

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 P[double bond, length as m-dash]O 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.