Design Principles for Trap-Free CsPbX₃ Nanocrystals: Enumerating and Eliminating Surface Halide Vacancies with Softer Lewis Bases

Abstract

We introduce a general surface passivation mechanism for cesium lead halide perovskite materials (CsPbX 3 , X = Cl, Br, I) that is supported by a combined experimental and theoretical study of the nanocrystal surface chemistry. A variety of spectroscopic methods are employed together with ab initio calculations to identify surface halide vacancies as the predominant source of charge trapping. The number of surface traps per nanocrystal is quantified by 1 H NMR spectroscopy, and that number is consistent with a simple trapping model in which surface halide vacancies create deleterious under-coordinated lead atoms. These halide vacancies exhibit trapping behavior that differs between CsPbCl 3 , CsPbBr 3 , and CsPbI 3. Ab initio calculations suggest that introduction of anionic X-type ligands can produce trap-free bandgaps by altering the energetics of lead-based defect levels. General rules for selecting effective passivating ligand pairs are introduced by considering established principles of coordination chemistry. Introducing softer, anionic, X-type Lewis bases that target under-coordinated lead atoms results in absolute quantum yields approaching unity and monoexponential luminescence decay kinetics, thereby indicating full trap passivation. This work provides a systematic framework for preparing highly luminescent CsPbX 3 nanocrystals with variable compositions and dimensionalities, thereby improving fundamental understanding of these materials and informing future synthetic and post-synthetic efforts towards trap-free CsPbX 3 nanocrystals.

Reducing Detrimental Defects for High‐Performance Metal Halide Perovskite Solar Cells

“…This means that the presence of O atoms (that weakens the hydrogen‐bonding strength) is preferred for enhancing the passivation effect on perovskite surfaces with V I . This is in agreement with the previous discussion that the Lewis base strength of the passivating molecules plays a role in the coordination with under‐coordinated Pb 2+ defects; a soft Lewis base molecules is preferred in passivating under‐coordinated Pb 2+ defects . Further analyses suggest that the signal of Δ E ad = E ad,V − E ad,P provides the preferential coordination with surface sites: the negative Δ E ad indicates preferred interaction with V I defects, while the positive Δ E ad indicates preferred interaction with FA + cations on defect‐free surfaces (Figure a).…”

“…Similar to HMDA, DDDA is not an effective passivating molecule because its N and O atoms are almost isolated resulting in strong hydrogen bonding ability of amino groups. On the contrary, ODEA showed even a better passivation efficiency (EQE≈19 %) compared with EDEA because of its optimal hydrogen‐bonding strength when interacting with under‐coordinated Pb 2+ defects (Figure a) . These studies highlight a central message that not only the passivating functional groups are important, but also the molecular structure itself is key for the passivation effect and efficacy.…”

“…proposed that because the under‐coordinated Pb 2+ shows relatively a soft Lewis acid nature, the hardness or softness of the Lewis base of passivating molecules plays a role in efficacy of passivation (Figure a–c). Passivating molecules with harder species (e.g., alkylcarboxylates, carbonates, and nitrates) are ineffective passivating ligands, while softer species (e.g., alkylphosphonates, fluorinated carboxylates, and sulfonates) were found to be effective in passivating under‐coordinated Pb 2+ . Although the reported passivating molecules are effective in suppressing the surface dominated charge traps, the chemical stability of these molecules is an important consideration.…”

“…Two approaches of post‐passivation treatment and additives added in the perovskite precursor solution were proposed as defect engineering approaches . On the basis of Table S2 and Figure b, a central discussion of the types of defects and their passivation strategies are focused on the 1) halide vacancies (e.g., Cl − , Br − , I − ) leading to exposure of under‐coordinated positively charged Pb 2+ atoms, 2) negatively charged Pb‐I anti‐sites (PbI 3 − ) or halide‐excess, 3) cation vacancies (e.g., Cs + , MA + ), 4) metallic lead (Pb 0 ) surface terminated, 5) mobile or volatile I − anion and MA + cations, and 6) I 0 (I 2 ) defects …”

“…Nenon et al . proposed that because the under‐coordinated Pb 2+ shows relatively a soft Lewis acid nature, the hardness or softness of the Lewis base of passivating molecules plays a role in efficacy of passivation (Figure a–c).…”