Stabilization of Metastable Halide Perovskite Lattices in the 2D Limit

Abstract

Metal halide perovskites constitute a new class of semiconductors that are structurally tailorable, exhibiting rich structural polymorphs. In this perspective, the polymorphism in lead halide perovskites is described—a material system currently used for high‐performance photovoltaics and optoelectronics. Strategies for stabilizing the metastable perovskite polymorphs based on crystal size reduction and surface functionalization are critically reviewed. Focus is on an unprecedented stabilization of metastable perovskite lattices in the 2D limit (e.g., with a thickness down to a few unit cells) due to the dominance of surface effects. This stabilization allows the incorporation of various A‐cations that deemed oversized for 3D perovskites into the 2D perovskite lattices, which bring new insights on the relationships between the crystal structures and optoelectronic properties and lead to emergent ferroelectricity in halide perovskites. A comprehensive understanding is provided on how the A‐cations influence the structural, optoelectronic, and ferroelectric properties, with an emphasis on the second order Jahn‐Teller distortion caused by the oversized A‐cations. Finally, future perspectives on new structure exploration and studies of fundamental photophysical properties using stabilized perovskite lattices are provided.

Surface Stabilization of Colloidal Perovskite Nanocrystals via Multi-amine Chelating Ligands

“…As a result, in comparison with pure AHDA (see 1 H NMR spectra shown in Figure S3a), the shifts of the 1, 2, and 3 resonance peaks in AHDA-PNCs suggest that AHDA is almost completely in its protonated state when bound to the PNCs surface. 17 At the same time, no signals of phosphoric species were detected in both 31 P NMR and XPS measurements (Figure S3b,c), indicating that BTMPA molecules are virtually absent from the surface of CsPbI 3 PNCs after purification. Thus, we can conclude that AHDA-CsPbI 3 PNCs are passivated by only one type of ligand, namely the protonated AHDA.…”

“…The mentioned structural instability of PNCs stems, to a large extent, from the dynamic and loose binding between their inorganic core and surface organic ligands, , which results in an easy detachment of the latter; water, oxygen, and polar solvents can then be adsorbed by the exposed defect sites, leading to phase transition or degradation of PNCs. ,, At present, various commercially available ligands, such as oleylamine (OLA), oleic acid (OA), dodecylbenzenesulfonic acid, zwitterionic ligands, alkanethiolate, 2,2′-iminodibenzoic acid, trioctylphosphine oxide, , secondary aliphatic amines, l -phenylalanine, and alkylphosphonic acids, have been used to stabilize PNCs. Among them, dodecylbenzenesulfonic acid and zwitterionic ligands have been proven as rather effective ligands to stabilize green-emitting CsPbBr 3 PNCs during post-preparative purification and storage. , However, stabilization of red-emitting CsPbI 3 PNCs still remains a challenging task, due to their intrinsically metastable perovskite structure. − At the same time, CsPbI 3 PNCs possess a narrow band gap and are characterized by a long carrier lifetime, making them promising candidates for applications in red-light-emitting devices and other photonic sources, , solar cells, , and photocatalysis . Thus, offering ligands which would ensure much better protection of CsPbI 3 PNCs is an important step forward toward their practical application .…”

Interplay of Kinetic and Thermodynamic Reaction Control Explains Incorporation of Dimethylammonium Iodide into CsPbI3

“…For example, while large cations, such as DMA and GUA do not form 3D perovskites on their own owing to their large size, they can be incorporated into the cubooctahedral space in 2 D /3D Ruddlesden–Popper phases. 63 − 65 The lack of thermodynamic stability of Cs/DMA iodoplumbate phases can be rationalized by the difference in ionic radii of Cs and DMA. The smaller Cs is replaced by the larger DMA, imposing distortions in the lattice.…”

“…This behavior contrasts with that observed for Cs/FA and Cs/GUA, which yield mixed-cation iodoplumbate phases under thermodynamic conditions. ,,, The importance of kinetic reaction control also has potential bearing on the role and speciation of Rb + and K + in hybrid MHPs doped with these cations, where we have previously observed complete segregation of the inorganic dopants under thermodynamic control. , It is also noteworthy that the overall structure topology affects the incorporation of small organic cations into halide perovskite cages. For example, while large cations, such as DMA and GUA do not form 3D perovskites on their own owing to their large size, they can be incorporated into the cubooctahedral space in 2 D /3D Ruddlesden–Popper phases. − The lack of thermodynamic stability of Cs/DMA iodoplumbate phases can be rationalized by the difference in ionic radii of Cs and DMA. The smaller Cs is replaced by the larger DMA, imposing distortions in the lattice.…”

Phonon-Mediated Slow Hot Carrier Dynamics in Lead-Free Cs3Bi2I9 Perovskite Single Crystal

“…46−48 Additionally, the Jahn− Teller distortion and the Peierls-like structural instability toward lattice distortion of the semimetal Bi can lead to a higher value of Urbach energy. 49,50 Furthermore, we have performed the XPS measurement to investigate the oxidation states of present elements in the crystal. Figure 2a shows a survey scan of a Cs 3 Bi 2 I 9 crystal with prominent peaks corresponding to cesium [Cs (3d)], bismuth [Bi (4f)], and iodine [I (3d)].…”