Structural Descriptors to Correlate Pb Ion Displacement and Broadband Emission in 2D Halide Perovskites

Abstract

2D hybrid lead halide perovskites exhibit versatile photoluminescent behaviors for narrowband to broadband emissions (BBEs) and have become attractive candidates for potential applications such as solid-state lighting. Establishing the relationship between the perovskite structural distortion and BBE is key but challenging in designing and optimizing the perovskite luminophores. Conventional attention is given to analyzing the intra-octahedron distortion of the [PbX6]4– (X = halide) unit that has not yet provided a clear structure–luminescence relationship. Herein, we introduce a descriptor, Pb displacement, to describe the inter-octahedron distortion to clarify the structure–emission relationship. The displacement of adjacent Pb centers represents the lattice distortion, which determines the broadband/narrowband emission instead of the octahedron distortion itself. We find a kite-type quadrilateral rule in (001) type 2D perovskites, that is, the degree to which the four octahedral central ions deviate from a square relates to the BBE. The kite-type arrangement of the Pb ions usually corresponds to the BBEs due to the large structure distortions. In contrast, the square-type arrangement of the Pb ions corresponds to the narrowband emissions because of the small distortions. The distortion descriptor magnifies the distortion scale, making it larger than the conventional one for the intra-octahedron distortion, which matches the general concept of excitons based on the scale of the crystal lattice. Therefore, the set of structural descriptors is better to correlate the perovskite structures and emission properties.

Spacer-Dependent and Pressure-Tuned Structures and Optoelectronic Properties of 2D Hybrid Halide Perovskites

“…In addition, we quantitatively analyzed the evolutions of the bandwidth of the PL peaks as shown in Figure S1b. We found that the bandwidth of the PL in Phase I is less than 14 nm, which is consistent with the free-exciton model established for narrow bands of PL emission for a wide range of 2D perovskites . Upon compression into Phase II, the bandwidth for both PL peaks increases almost linearly with pressure to several tens of nanometers, together with PL intensity reduction.…”

“…47 Additionally, the asymmetric emission profile has been associated with the structural distortion of PbI 6 octahedra, which is believed to be an important structural factor for emission band shape. 9,48 Upon compression, the intensity of the major PL peak increases until 0.8 GPa, where the peak intensity is about 10 times stronger than that at ambient pressure (Figure 3b), which indicates a remarkable pressure-enhanced PL efficiency. Concurrently, the PL emission peak becomes significantly more symmetric upon compression, which suggests a structural change to more regular (nondistorted) lead iodide octahedra.…”

“…We found that the bandwidth of the PL in Phase I is less than 14 nm, which is consistent with the free-exciton model established for narrow bands of PL emission for a wide range of 2D perovskites. 48 Upon compression into Phase II, the bandwidth for both PL peaks increases almost linearly with pressure to several tens of nanometers, together with PL intensity reduction. This observation is indicative of the pressure-induced structural distortion of PbI 6 octahedra, as discussed below.…”

“…The narrow, symmetric PL emission band of most 2D perovskites originates from free excitons, whereas a broad, asymmetric PL profile could be associated with the emission from self-trapped excitons (STE). , However, in iodide-based 2D perovskites, broad emission may result from defect-related origins rather than STE . Additionally, the asymmetric emission profile has been associated with the structural distortion of PbI 6 octahedra, which is believed to be an important structural factor for emission band shape. , …”

Design, synthesis, and characterization of a new hybrid organic–inorganic perovskite with a high-Tc dielectric transition

“…Phonon changes will lead a lattice deformation and the position change of atoms in the cell. 34 As fluorescence occurs, electrons go back to the conduction band and minor deformations of the lattice are also corrected. When electrons are excited to the conduction band, small and irregular lattice deformations change the position of atoms, which affects the electron cloud density distribution and broadens the energy range of an electronic transition.…”