Effect of Surface Ligands in Perovskite Nanocrystals: Extending in and Reaching out

Abstract

ConspectusThe rediscovery of the halide perovskite class of compounds and, in particular, the organic and inorganic lead halide perovskite (LHP) materials and lead-free derivatives has reached remarkable landmarks in numerous applications. First among these is the field of photovoltaics, which is at the core of today's environmental sustainability efforts. Indeed, these efforts have born fruit, reaching to date a remarkable power conversion efficiency of 25.2% for a double-cation Cs, FA lead halide thin film device. Other applications include light and particle detectors as well as lighting. However, chemical and thermal degradation issues prevent perovskite-based devices and particularly photovoltaic modules from reaching the market. The soft ionic nature of LHPs makes these materials susceptible to delicate changes in the chemical environment. Therefore, control over their interface properties plays a critical role in maintaining their stability. Here we focus on LHP nanocrystals, where surface termination by ligands determines not only the stability of the material but also the crystallographic phase and crystal habit. A surface analysis of nanocrystal interfaces revealed the involvement of Brønsted type acid-base equilibrium in the modification of the ligand moieties present, which in turn can invoke dissolution and recrystallization into the more favorable phase in terms of minimization of the surface energy. A large library of surface ligands has already been developed showing both good chemical stability and good electronic surface passivation, resulting in near-unity emission quantum yields for some materials, particularly CsPbBr₃. However, most of those ligands have a large organic tail hampering charge carrier transport and extraction in nanocrystal-based solid films.The unique perovskite structure that allows ligand substitution in the surface A (cation) sites and the soft ionic nature is expected to allow the accommodation of large dipoles across the perovskite crystal. This was shown to facilitate electron transfer across a molecular linked single-particle junction, creating a large built-in field across the junction nanodomains. This strategy could be useful for implementing LHP NCs in a p-n junction photovoltaic configuration as well as for a variety of electronic devices. A better understanding of the surface propeties of LHP nanocrystals will also enable better control of their growth on surfaces and in confined volumes, such as those afforded by metal-organic frameworks, zeolites, or chemically patterened surfaces such as anodic alumina, which have already been shown to significantly alter the properties of in-situ-grown LHP materials.

Figure 1

Effect of seed aging time (i.e., step 1) on the morphology of intermediate CsPbI₃ (i.e., step 2). As the aging time is increased, the reaction product of stage 1 change from cubes for 1 h of aging time to mixed cubes and thin wires for 7 h of aging and long wires and tubes for 16 h of aging time.

Figure 2

TEM image of the initial Pb⁰ NPs (a) serving as nucleation seeds for the synthesis of 5 nm CsPbBr₃ NPLs (b). (c) Self-assembly of NPLs in the reaction mixture into larger cubes in the case of CsPbBr₃ and wires for CsPbI₃ (d). (e) Schematic representation of the conversion of nanocubes to bulk-type crystals through orientated attachment self-assembly.

Figure 3

Atomic-resolution images of (a) CsPbI₃ and (b) Cs₄PbI₆. CsPbI₃ crystallizes in a perovskite crystal structure with orthorhombic distortion in which PbX₆ octahedra are corner-sharing. The cubic crystals are bound by facets on (001) and (100) planes. The Cs₄PbI₆ structure is rhombohedral with space group R3c. The typical 2-fold symmetry of the high-resolution images of Cs₄PbX₆ is produced by the projection of chains of PbX₆ octahedra in the [122] viewing direction. The habit is such that the rhombohedral crystals are formed by a layering of densely packed PbX₆ with interlayers of cations.

Figure 4

Selective transformation of Cs₄PbBr₆ NCs to CsPbBr₃ NCs of different thicknesses. (a–c) TEM images of the CsPbBr₃ samples emitting at 410, 432, and 490 nm, respectively. (d) Absorption and emission spectra of CsPbBr₃ NCs. Five different absorption and emission peaks correspond to five different thicknesses (1–10 unit cells). Emission peaks from left to right: 410 nm (1 ML), 432 nm (2 MLs), 460 nm (5 MLs), 479 nm (8 MLs), and 488 nm (10 MLs).

Figure 5

(A) Scheme depicting the CsPbBr₃ NC linked by PABA to CdSe NPL. (B) Band diagrams of the hybrid systems (red lines) with indicated ligands, drawn relative to the system’s Fermi level. VB and CB of the pure systems are drawn as well (black lines), corresponding respectively to the top of the valence band and the bottom of the conduction band before complexation. Arrows indicate the electrostatic changes upon hybridization. The local vacuum level at the two constituents of the hybrid structure is indicated by the upper red lines. Technically, the vacuum level in the perovskite domains (indicted by the top red line) was extracted directly from the work-function measurements, whereas the one in the CdSe domains (top dashed red line) could not be resolved from secondary onsets and hence was deduced indirectly (with no compromise in accuracy) from the electrostatic information provided by the elemental core line shifts (an average over corresponding elements).