Correlating Photophysical Properties with Stereochemical Expression of 6s² Lone Pairs in Two‐dimensional Lead Halide Perovskites

Abstract

Two‐dimensional (2D) lead halide perovskites (LHPs) have shown great promises for light‐emitting applications and excitonic devices. Fulfilling these promises demands an in‐depth understanding on the relationships between the structural dynamics and exciton‐phonon interactions that govern the optical properties. Here, we unveil the structural dynamics of 2D lead iodide perovskites with different spacer cations. Loose packing of an undersized spacer cation leads to out‐of‐plane octahedral tilting, whereas compact packing of an oversized spacer cation stretches Pb−I bond length, resulting in Pb2+ off‐center displacement driven by stereochemical expression of the Pb2+ 6s2 lone pair electrons. Density functional theory calculations indicate that the Pb2+ cation is off‐center displaced mainly along the direction where the octahedra are stretched the most by the spacer cation. We find dynamic structural distortions associated with either octahedral tilting or Pb2+ off‐centering lead to a broad Raman central peak background and phonon softening, which increase the non‐radiative recombination loss via exciton‐phonon interactions and quench the photoluminescence intensity. The correlations between the structural, phonon, and optical properties are further confirmed by the pressure tuning of the 2D LHPs. Our results demonstrate that minimizing the dynamic structural distortions via a judicious selection of the spacer cations is essential to realize high luminescence properties in 2D LHPs.

Tuning Film Stresses for Open-Air Processing of Stable Metal Halide Perovskites

“…The control film in Figure 5f showed some nonluminescent grain boundaries, indicating that there were nonradiative recombination centers in these areas. 38,39 Highly tensile films with a low additive concentration showed reduced luminescence at both the boundaries and the bulk of the film (Figure 5g), which could be attributed to an increase in nonradiative recombination across the film. However, for a compressive sample, the grain boundaries were luminescing at a higher intensity (Figure 5h).…”

Manipulating Phase Transition and Ferroelectricity via Enhancing the Stereochemical Activity of Lone-Pair Electrons

“…Huang et al reported that lone-pair activity can be enhanced via increasing the size of the A-site cation of halide perovskites, which promotes Pb off-center displacement. This is specially manifested in two-dimensional (2D) halide perovskites that can accommodate a relatively larger size cation in comparison with their three-dimensional (3D) counterparts, leading to a stronger stereochemical character of LPEs and more possibility to obtain ferroelectricity or antiferroelectricity in 2D halide perovskites. , Although this progress has been made in promoting the stereochemical character of LPEs, there is still a lack of thorough understanding about to what extent the stereochemical activity of LPEs can be enhanced to induce and modulate the physical properties of materials containing LPEs, such as FE properties. Here, by taking bismuth oxychalcogenides Bi 2 O 2 X (X = S, Se and Te) as model systems, we engineer the activity of LPEs systematically to induce the phase transition and ferroelectricity.…”