Hybrid Lead Iodide Perovskites with Mixed Cations of Thiourea and Methylamine, From One Dimension to Three Dimensions

Pressure-controlled free exciton and self-trapped exciton emission in quasi-one-dimensional hybrid lead bromides

Hybrid metal halides represent a novel type of semiconductor light emitters with intriguing excitonic emission properties, including free exciton emission and self-trapped exciton emission. Achieving precise control over these two excitonic emissions in hybrid metal halides is highly desired yet remains challenging. Here, the complete transformation from intrinsically broadband self-trapped exciton emission to distinctively sharp free exciton emission in a quasi-one-dimensional hybrid metal halide (C2H10N2)8[Pb4Br18]·6Br with a ribbon width of n = 4, is successfully achieved based on high-pressure method. During compression, pressure-induced phonon hardening continuously reduces exciton-phonon coupling, therefore suppressing excitonic localization and quenching the original self-trapped exciton emission. Notably, further compression triggers excitonic delocalization to induce intense free exciton emission, accompanied with reduced carrier effective masses and improved charge distribution. Controlled high-pressure investigations indicate that the ribbon width of n > 2 is necessary to realize excitonic delocalization and generate free exciton emissions in similar quasi-one-dimensional hybrid metal halides. This work presents an important photophysical process of excitonic transitions from self-trapped exciton emission to free exciton emission in quasi-one-dimensional hybrid metal halides without chemical regulation, promoting the rational synthesis of hybrid metal halides with desired excitonic emissions.

Yellow-Orange Emission in Sb3+-Doped Hexakis(thiocarbamidium) Hexabromoindium(III) Tribromide

A luminescent zero-dimensional organic-inorganic hybrid indium halide (TUH)6[In1-xSbxBr6]Br3 (TU = thiourea, 0 ≤ x ≤ 0.0998) was synthesized via the solvothermal method. In structures, resolved by single-crystal X-ray diffraction, isolated distorted [InBr6]3- and [SbBr6]3- octahedra are linked to organic TUH+ cations by intermolecular N-H···Br and N-H···S hydrogen bonds. The crystals were characterized by elemental analysis, TG-DSC, powder X-ray diffraction, FTIR analysis, and steady-state absorption and photoluminescence spectroscopy. (TUH)6[In1-xSbxBr6]Br3 exhibits a broadband yellow-orange emission centered at 595-602 nm with a half-width of 141-149 nm (0.48-0.52 eV) and a large Stokes shift of 232-238 nm (1.33-1.35 eV). This emission can be attributed to the self-trapped exciton emission of triplet states of the octahedral anion [SbBr6]3- or [InBr6]3-. Two possible emission mechanisms were discussed. Doping with Sb3+ leads to a significant increase in photoluminescence quantum yield from 25.7 at x = 0 to 48.4% at x = 0.0065, when excited at 365 nm, indicating the potential use of (TUH)6[In1-xSbxBr6]Br3 compounds in the field of photonics.

Effect of Dimensionality on Photoluminescence and Dielectric Properties of Imidazolium Lead Bromides

Hybrid organic–inorganic lead halide perovskites have emerged as promising materials for various applications, including solar cells, light-emitting devices, dielectrics, and optical switches. In this work, we report the synthesis, crystal structures, and linear and nonlinear optical as well as dielectric properties of three imidazolium lead bromides, IMPbBr₃, IM₂PbBr₄, and IM₃PbBr₅ (IM⁺ = imidazolium). We show that these compounds exhibit three distinct structure types. IMPbBr₃ crystallizes in the 4H-hexagonal perovskite structure with face- and corner-shared PbBr₆ octahedra (space group P6₃/mmc at 295 K), IM₂PbBr₄ adopts a one-dimensional (1D) double-chain structure with edge-shared octahedra (space group P1̅ at 295 K), while IM₃PbBr₅ crystallizes in the 1D single-chain structure with corner-shared PbBr₆ octahedra (space group P1̅ at 295 K). All compounds exhibit two structural phase transitions, and the lowest temperature phases of IMPbBr₃ and IM₃PbBr₅ are noncentrosymmetric (space groups Pna2₁ at 190 K and P1 at 100 K, respectively), as confirmed by measurements of second-harmonic generation (SHG) activity. X-ray diffraction and thermal and Raman studies demonstrate that the phase transitions feature an order–disorder mechanism. The only exception is the isostructural P1̅ to P1̅ phase transition at 141 K in IM₂PbBr₄, which is of a displacive type. Dielectric studies reveal that IMPbBr₃ is a switchable dielectric material, whereas IM₃PbBr₅ is an improper ferroelectric. All compounds exhibit broadband, highly shifted Stokes emissions. Features of these emissions, i.e., band gap and excitonic absorption, are discussed in relation to the different structures of each composition.

Three imidazolium lead bromides of various chemical compositions and crystal structures display broadband photoluminescence that can be tuned from bluish-green to orange. All compounds exhibit two structural phase transitions, which lead to interesting optical and electrical properties such SHG activity, ferroelectricity, or dielectric switching.

Modeling bismuth insertion in 1D hybrid lead halide TMSO(PbxBiy)I3pseudo-perovskites

The structures of the disordered 1D (pseudo-)perovskites of general TMSO(Pb_xBi_y)I₃formulation [TMSO = (CH₃)₃SO⁺], obtained by doping the TMSOPbI₃species with Bi³⁺ions, are investigated through the formulation of a statistical model of correlated disorder, which addresses the sequences of differently occupied BI₆face-sharing octahedra (B = Pb, Bi or vacant site) within ideally infinite [(BI₃)^-]_nchains. The x-ray diffraction patterns simulated on the basis of the model are matched to the experimental traces, which show many broad peaks with awkward (nearly trapezoidal) shapes, under the assumption that the charge balance is fully accomplished within each chain. The analysis allowed to establish a definite tendency of the metal species to cluster as pure Pb and Bi sequences. The application of the model is discussed critically, in particular as what concerns the possibility that further B-site neighbors beyond the second may influence the overall B-site occupancies.

(C5H6N)Pb2X5 (X = Cl, Br): Hybrid Lead Halides Based on Seven-Coordinate Pb(II)

Hybrid lead halides are a diverse family of compounds, of interest for their optoelectronic properties, that vary in the dimensionality and connectivity of their inorganic substructures. The great majority of these compounds are based on lead-centered octahedra, with few examples featuring inorganic architectures containing higher coordination numbers. Here, we report the synthesis and characterization of a pyridinium lead bromide phase that is based on seven-coordinate Pb(II) centers. Through edge- and face-sharing, the polyhedra form a corrugated, two-dimensional inorganic substructure. Electronic structure calculations were used to examine the band structure and the role of the stereoactive lone pair in the inherently asymmetric, seven-coordinate Pb(II) geometry. For reference, we have visualized the role of the lone pair in the binary halide PbBr₂, which also has a seven-coordinate inner ligand sphere. A comparison of the new structure with the limited number of existing hybrid lead halides with similar inorganic architectures highlights the templating role of the organic cation for these compounds. We also contribute characterization and discussion of isomorphic pyridinium lead chloride, which had been deposited in the Cambridge Structural Database but never, to our knowledge, addressed in the literature. The compounds were synthesized using solution conditions and structures determined with single-crystal X-ray diffraction. The materials were also characterized via powder X-ray diffraction, combustion elemental analysis, and diffuse reflectance UV-vis spectroscopy. While the structures reported here are centrosymmetric, the seven-coordinate, capped trigonal prismatic geometry that we have identified is a source of local asymmetry that could be used as a component in designing globally noncentrosymmetric structures.

2D Lead Iodide Perovskite with Mercaptan-Containing Amine and Its Exceptional Water Stability

Two dimensional (2D) hybrid perovskites have attracted a great deal of interest because of their appropriate photovoltaic efficiency and environmental stability. Although some 2D hybrid perovskites with sulfur-containing amines have been reported, the cation having the mercaptan group has not been well explored yet. In this work, cysteamine (Cya, HS(CH₂)₂NH₂), a mercaptan-containing amine, was introduced into 2D hybrid perovskite. Two 2D lead iodides with different structures, (HCya)₂PbI₄ (1) and (HCya)₇Pb₄I₁₅ (2), were isolated as a red low-temperature phase and a yellow high-temperature phase, respectively. X-ray single-crystal structural analysis showed that the red phase 1 is a single layered corner-shared perovskite and that the yellow phase 2 is a corner/edge-shared quasi-2D perovskite. A thermo-induced reversible 1 to 2 phase transition was found in this synthetic system. The configuration of HCya cation greatly influences the crystallization equilibrium, generating different structures of the lead halides. The single-crystal structure of 1 is discussed in comparison with that of (HAE)₂PbI₄ (AE = HO(CH₂)₂NH₂), an analogue of 1. The different effects of OH and SH groups on the 2D frameworks are studied based on their hydrogen bonding properties. More remarkably, although the two perovskites have similar structures, the (HCya)₂PbI₄ (1) has an intrinsic water stability that is much more stable than (HAE)₂PbI₄, which should be attributed to the affinity of the SH group with lead on the surface of the lead halide.