Highly Dynamic Ligand Binding and Light Absorption Coefficient of Cesium Lead Bromide Perovskite Nanocrystals

Abstract

Lead halide perovskite materials have attracted significant attention in the context of photovoltaics and other optoelectronic applications, and recently, research efforts have been directed to nanostructured lead halide perovskites. Collodial nanocrystals (NCs) of cesium lead halides (CsPbX3, X = Cl, Br, I) exhibit bright photoluminescence, with emission tunable over the entire visible spectral region. However, previous studies on CsPbX3 NCs did not address key aspects of their chemistry and photophysics such as surface chemistry and quantitative light absorption. Here, we elaborate on the synthesis of CsPbBr3 NCs and their surface chemistry. In addition, the intrinsic absorption coefficient was determined experimentally by combining elemental analysis with accurate optical absorption measurements. (1)H solution nuclear magnetic resonance spectroscopy was used to characterize sample purity, elucidate the surface chemistry, and evaluate the influence of purification methods on the surface composition. We find that ligand binding to the NC surface is highly dynamic, and therefore, ligands are easily lost during the isolation and purification procedures. However, when a small amount of both oleic acid and oleylamine is added, the NCs can be purified, maintaining optical, colloidal, and material integrity. In addition, we find that a high amine content in the ligand shell increases the quantum yield due to the improved binding of the carboxylic acid.

Low‐Dimensional Perovskites: From Synthesis to Stability in Perovskite Solar Cells

“…[68] Our understanding of the dynamic ligand binding and light absorption coefficient of cesium lead bromide perovskite nanocrystals CsPbX 3 was boosted by a study of De Roo et al, in which they observed more dynamic interactions of CsPbX 3 with capping ligands (Figure 3). [69] Although, the ligand binding to the NP network is more ionic and labile, however, it usually disappeared during the isolation and purification processes. Further NP purification is possible without scarifying the optical, electrical, and colloidal properties when small amounts of both OLA and oleylamine are included.…”

Polymer-Enhanced Stability of Inorganic Perovskite Nanocrystals and Their Application in Color Conversion LEDs

“…An excess of halide may further be attributed to its role in the surface capping of the NCs, where it constitutes the negative counterpart of oleylammonium ions that stabilize the NCs in solution. 22 The NC quantum yield (QY) increased slightly but reproducibly with the polymer addition for both the green-emitting CsPbBr 3 NCs, from 49 ± 4 % to 53 ± 4 %, and the red-emitting CsPbBr 1.6 I 1.4 NCs, from 84 ± 3 % to 88 ± 1 %. We believe the reason behind this slight increase to be a better passivation of the trap states by the more stable ligand shell.…”

“…Samples synthesized in reactions with an upscaling factor of two for all substances were purified as reported earlier. 22 After the reaction the content of the flask was split into portions of 2 mL and centrifuged at 7000 rpm/6300 rcf for 15 minutes. After removing the supernatant 300 µL hexane were added to each portion.…”

“…[16][17][18][19][20] However, while CsPbX 3 (X = Br, I) perovskite NCs can be stored in solution, maintaining their luminescence for several months, they suffer from humidity, light and temperature driven degradation in operation conditions and from a low colloidal stability with a related limited processability. 21 In part, this is associated with a highly dynamic ligand binding to the NC surface, 22 which additionally hinders the NCs' post-synthetic purification and further functionalization. When perovskite NCs with highly ionic components and high surface energies come into contact with a polar surface or solvent, they rapidly degrade to their components.…”

Research Update: Challenges for high-efficiency hybrid lead-halide perovskite LEDs and the path towards electrically pumped lasing

“…72 Roo et al reported that ligand binding to the NC surface is highly dynamic, and therefore, ligands are easily lost during the isolation and purification procedures, thus causing nanocrystal aggregation. 80 To form a uniform perovskite nanocrystal film, Kim et al used a centrifuging method for purification 72 and Deng et al used a dip coating method. 39 As another significant problem, these nanocrystals are very dispersible in most common solvents, limiting the solution-processing of the top layer.…”