Structural Evolution of Layered Hybrid Lead Iodide Perovskites in Colloidal Dispersions

Abstract

Controlling the structure of layered hybrid metal halide perovskites, such as the Ruddlesden-Popper (R-P) phases, is challenging because of their tendency to form mixtures of varying composition. Colloidal growth techniques, such as antisolvent precipitation, forms colloidal dispersions with properties that match bulk layered R-P phases, but controlling the composition of these particles remains challenging. Here, we explore the microstructure of particles of R-P phases of methylammonium lead iodide prepared by antisolvent precipitation from ternary mixtures of alkylammonium cations, where one cation can form perovskite phases (CH 3 NH 3 + ) and the other two promote layered structures as spacers (e.g. C 4 H 9 NH 3 + and C 12 H 25 NH 3 + ). We determine that alkylammonium spacers pack with constant methylene density in the R-P interlayer, and exclude interlayer solvent in dispersed colloids, regardless of length or branching. Using this result, we illustrate how the competition between cations that act as spacers between layers, or as grainterminating ligands, determines the colloidal microstructure of layered R-P crystallites in solution. Optical measurements reveal that quantum well dimensions can be tuned by engineering the ternary cation composition. Transmission synchrotron wide-angle X-ray scattering and small angle neutron scattering reveal changes in the structure of colloids in solvent and after deposition into thin films. In particular, we find that spacers can alloy between R-P layers if they share common steric arrangements, but tend to segregate into polydisperse R-P phases if they do not mix. This study provides a framework to compare the microstructure of colloidal layered perovskites and suggests clear avenues to control phase and colloidal morphology.

Structure and Surface Passivation of Ultrathin Cesium Lead Halide Nanoplatelets Revealed by Multilayer Diffraction

“…Lead halide perovskites can adopt a cubic, tetragonal, or orthorhombic structure depending on whether and how the [PbX 6 ] 4– octahedra are tilted. , While the debate on bulk and large nanocrystals of CsPbBr 3 points in favor of the orthorhombic polymorph under normal temperature and pressure, this aspect is still not entirely clarified for small nanocrystals and thin platelets. − The different Pb–Pb distances along the horizontal and vertical directions easily exclude the cubic symmetry: we tried to discern between the two options left based on the atoms in the PbBr 2 planes being coplanar (tetragonal) or misaligned (orthorhombic) due to the tilting of octahedra (see SI, Section S5, for details). To do so, we allowed the z -coordinate of the Br – ions in the PbBr 2 planes to relax, finding that | z Pb – z Br | = 0.241 ± 0.054 Å.…”

Interfaces in metal halide perovskites probed by solid-state NMR spectroscopy

“…2b). [231][232][233][234] During polycrystalline lm growth, phase segregation between regions with different 2D connement indices (the n value from the formula unit R 2 A nÀ1 B n X 3n+1 ) can occur. Layered MHPs with a nominal stoichiometry of n greater than 2 typically segregate into regions of higher and lower n-values due to solution-processing kinetics.…”

“…MHP NCs can assemble into microparticles and superlattices in dispersions or upon deposition. 42,233,[261][262][263][264][265][266][267][268][269] Perovskite NC lms are promising candidates for optoelectronic devices, but their limited ambient stability and charge transport across ligand-NC interfaces remains a challenge. 15,51,257,259,270,271 The small size of MHP NCs can also induce strain and modify surface energies, altering the energetics of particular phases (e.g.…”

Small-angle scattering to reveal the colloidal nature of halide perovskite precursor solutions

“…6,9,10 Weakly coordinating solvents tend to favour faster crystallization, whereas stronger coordinating solvents like DMSO form an intermediate state, which leads to a slower crystallization process and thus a better film morphology. 11,12 However, though trends are observable and the resulting film properties are characterised in detail, the actual solution chemistry, including the specific structure of the colloidal particles and a mechanism to explain the fast, lowtemperature crystallization, remains unclear. 13 Methods preferably used for solution characterization, like dynamic light scattering (DLS), UV-vis spectroscopy or cryogenic transmission electron microscopy (cryo-TEM) should be used with caution, as often the nature of the regarded system slightly differs, or crucial properties are only indirectly determined.…”