Mixed Ligand Passivation as the Origin of Near-Unity Emission Quantum Yields in CsPbBr3 Nanocrystals

Exogenous Metal Cations in the Synthesis of CsPbBr3 Nanocrystals and Their Interplay with Tertiary Amines

Current syntheses of CsPbBr3 halide perovskite nanocrystals (NCs) rely on overstoichiometric amounts of Pb2+ precursors, resulting in unreacted lead ions at the end of the process. In our synthesis scheme of CsPbBr3 NCs, we replaced excess Pb2+ with different exogenous metal cations (M) and investigated their effect on the synthesis products. These cations can be divided into two groups: group 1 delivers monodisperse CsPbBr3 cubes capped with oleate species (as for the case when Pb2+ is used in excess) and with a photoluminescence quantum yield (PLQY) as high as 90% with some cations (for example with M = In3+); group 2 yields irregularly shaped CsPbBr3 NCs with broad size distributions. In both cases, the addition of a tertiary ammonium cation (didodecylmethylammonium, DDMA+) during the synthesis, after the nucleation of the NCs, reshapes the NCs to monodisperse truncated cubes. Such NCs feature a mixed oleate/DDMA+ surface termination with PLQY values of up to 97%. For group 1 cations this happens only if the ammonium cation is directly added as a salt (DDMA-Br), while for group 2 cations this happens even if the corresponding tertiary amine (DDMA) is added, instead of DDMA-Br. This is attributed to the fact that only group 2 cations can facilitate the protonation of DDMA by the excess oleic acid present in the reaction environment. In all cases studied, the incorporation of M cations is marginal, and the reshaping of the NCs is only transient: if the reactions are run for a long time, the truncated cubes evolve to cubes.

Resonant Multiple-Phonon Absorption Causes Efficient Anti-Stokes Photoluminescence in CsPbBr3 Nanocrystals

Lead halide perovskite nanocrystals, such as CsPbBr3, exhibit efficient photoluminescence (PL) up-conversion, also referred to as anti-Stokes photoluminescence (ASPL). This is a phenomenon where irradiating nanocrystals up to 100 meV below gap results in higher energy band edge emission. Most surprising is that ASPL efficiencies approach unity and involve single-photon interactions with multiple phonons. This is unexpected given the statistically disfavored nature of multiple-phonon absorption. Here, we report and rationalize near-unity anti-Stokes photoluminescence efficiencies in CsPbBr3 nanocrystals and attribute them to resonant multiple-phonon absorption by polarons. The theory explains paradoxically large efficiencies for intrinsically disfavored, multiple-phonon-assisted ASPL in nanocrystals. Moreover, the developed microscopic mechanism has immediate and important implications for applications of ASPL toward condensed phase optical refrigeration.

Synergistic Surface Modification for High-Efficiency Perovskite Nanocrystal Light-Emitting Diodes: Divalent Metal Ion Doping and Halide-Based Ligand Passivation

Surface defects of metal halide perovskite nanocrystals (PNCs) substantially compromise the optoelectronic performances of the materials and devices via undesired charge recombination. However, those defects, mainly the vacancies, are structurally entangled with each other in the PNC lattice, necessitating a delicately designed strategy for effective passivation. Here, a synergistic metal ion doping and surface ligand exchange strategy is proposed to passivate the surface defects of CsPbBr3 PNCs with various divalent metal (e.g., Cd2+ , Zn2+, and Hg2+ ) acetate salts and didodecyldimethylammonium (DDA+ ) via one-step post-treatment. The addition of metal acetate salts to PNCs is demonstrated to suppress the defect formation energy effectively via the ab initio calculations. The developed PNCs not only have near-unity photoluminescence quantum yield and excellent stability but also show luminance of 1175 cd m-2 , current efficiency of 65.48 cd A-1 , external quantum efficiency of 20.79%, wavelength of 514 nm in optimized PNC light-emitting diodes with Cd2+ passivator and DDA ligand. The "organic-inorganic" hybrid engineering approach is completely general and can be straightforwardly applied to any combination of quaternary ammonium ligands and source of metal, which will be useful in PNC-based optoelectronic devices such as solar cells, photodetectors, and transistors.