Colloidal Bi-Doped Cs₂Ag_1–xNa_xInCl₆ Nanocrystals: Undercoordinated Surface Cl Ions Limit their Light Emission Efficiency

Abstract

Understanding and tuning the ligand shell composition in colloidal halide perovskite nanocrystals (NCs) has been done systematically only for Pb-based perovskites, while much less is known on the surface of Pb-free perovskite systems. Here, we reveal the ligand shell architecture of Bi-doped Cs 2 Ag 1– x Na x InCl 6 NCs via nuclear magnetic resonance analysis. This material, in its bulk form, was found to have a photoluminescence quantum yield (PLQY) as high as 86%, a record value for halide double perovskites. Our results show that both amines and carboxylic acids are present and homogeneously distributed over the surface of the NCs. Notably, even for an optimized surface ligand coating, achieved by combining dodecanoic acid and decylamine, a maximum PLQY value of only 37% is reached, with no further improvements observed when exploiting post-synthesis ligand exchange procedures (involving Cs-oleate, different ammonium halides, thiocyanates and sulfonic acids). Our density functional theory calculations indicate that, even with the best ligands combination, a small fraction of unpassivated surface sites, namely undercoordinated Cl ions, is sufficient to create deep trap states, opposite to the case of Pb-based perovskites that exhibit much higher defect tolerance. This was corroborated by our transient absorption measurements, which showed that an ultrafast trapping of holes (most likely mediated by surface Cl-trap states) competes with their localization at the AgCl 6 octahedra, from where, instead, they can undergo an optically active recombination yielding the observed PL emission. Our results highlight that alternative surface passivation strategies should be devised to further optimize the PLQY of double perovskite NCs, which might include their incorporation inside inorganic shells.

Halide Perovskite Nanocrystal Emitters

“…In this respect, more attention should be also paid to fine‐tune the selection of the most appropriate computational method to specifically study the nanocrystal systems. Recently, the classical force‐field method has been adopted for the PNCs modeling to overcome the limitations of DFT related to the high number of atoms of the systems under consideration, [ 229 ] the presence of ligands (the size of ligands is commonly not considered), and the interactions with the solvent that are typically neglected. Moreover, the classical force‐field calculations require considerably less time than the corresponding DFT ones.…”

Guidelines for the characterization of metal halide nanocrystals

“…In this regard, inductively coupled plasma (ICP) in conjunction with optical emission spectroscopy (OES) or mass spectroscopy (MS), which is routinely used to analyze traditional metal chalcogenide NCs, provides only limited information in the case of MH NCs, since halide and organic ions cannot be detected. The use of this technique is therefore limited to those cases in which the metal cation doping level of NC systems has to be measured, especially if the amount of dopant is below 0.1% (the detection limit of other commonly used techniques described later; Table 1) [6,[17][18][19]. Consequently, other techniques, capable of detecting both cations and halide anions, are typically preferred.…”

“…To date, the most versatile compositional analysis technique, in terms of reliability and data acquisition time, is certainly SEM-EDS [5][6][7]9,15,17,[19][20][21]27]. Most of the reports assess the robustness of this approach against side effects such as halide desorption or degradation of MH NCs, most likely thanks to the lower dose rate than TEM-EDS.…”

“…Nuclear magnetic resonance (NMR) spectroscopy is currently the most suitable technique to investigate the ligand-surface interactions in colloidal NCs, including both classical and MH systems (Figure 3) [3,16,[43][44][45]. 1 H NMR (or 13 C and/or 31 P NMR) can be readily exploited to identify bound ligands: their resonances are slightly up-field shifted and broadened with respect to those of the free molecules in solution [3,11,16,17,[43][44][45][46]. Such line broadening has been attributed to the limited degrees of freedom (mainly restricted rotational mobility) experienced by a ligand bound to a NC surface.…”

Lead‐Free Halide Double Perovskite Nanocrystals for Light‐Emitting Applications: Strategies for Boosting Efficiency and Stability

“…For Bi 3+ doped Cs 2 Ag 1− x Na x InCl 6 nanocrystals, the insufficient Cl − ions on nanocrystals surface would lead to deep trap states, which were different from lead halide perovskite with highly defect tolerance. [ 189 ] In addition, to avoid the addition of oleylamine, the ligands with strong anchoring groups can be used to passivate surface of HDP nanocrystals for improving stability and PLQY , which is conducive to facilitate tight binding between ligands and nanocrystals. The functional ligands also have a great probability to further modify properties of HDP nanocrystals by judicious molecular design, which will be beneficial for further functionalization of the lead‐free HDP nanocrystals. iii)Tailoring the HDP nanocrystals compositions precisely via metal doping/alloying strategies.…”