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 Cs₂Ag_1–xNa_xInCl₆NCs 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 AgCl₆ 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.

中文翻译：

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胶态双掺杂Cs ₂ Ag _{1– x} Na _x InCl ₆纳米晶体：表面配位不足的Cl离子限制了它们的发光效率

仅对于基于铅的钙钛矿，已经系统地完成了对胶体卤化物钙钛矿纳米晶体（NCs）中配体壳组成的理解和调整，而对于无铅钙钛矿体系的表面知之甚少。在这里，我们揭示了Bi掺杂的Cs ₂ Ag _{1– x} Na _x InCl ₆的配体壳结构NC通过核磁共振分析。发现该材料为散装形式，其光致发光量子产率（PLQY）高达86％，是卤化物双钙钛矿的创纪录值。我们的结果表明，胺和羧酸都存在且均匀分布在NC的表面上。值得注意的是，即使对于通过结合十二烷酸和癸胺实现的优化表面配体涂层，最大PLQY值也仅达到37％，而在利用合成后的配体交换程序（涉及Cs-油酸酯，不同的铵盐）时未观察到进一步的改进。卤化物，硫氰酸盐和磺酸）。我们的密度泛函理论计算表明，即使使用最佳的配体组合，也有一小部分未钝化的表面位点，即配位不足的Cl离子，足以产生深陷阱态，这与表现出更高的缺陷容忍度的基于Pb的钙钛矿相反。我们的瞬态吸收测量结果证实了这一点，该测量结果表明，空穴的超快捕获（很可能是由表面Cl陷阱态介导的）与它们在AgCl处的定位竞争₆八面体，从那里，它们可以进行光学活性重组，产生观察到的PL发射。我们的结果表明，应设计出替代的表面钝化策略，以进一步优化双钙钛矿NC的PLQY，其中可能包括将其掺入无机壳中。