2D halide perovskite‐like semiconductors are attractive materials for various optoelectronic applications, from photovoltaics to lasing. To date, the most studied families of such low‐dimensional halide perovskite‐like compounds are Ruddlesden–Popper, Dion–Jacobson, and other phases that can be derived from 3D halide perovskites by slicing along different crystallographic directions, which leads to the spatially isotropic corner‐sharing connectivity type of metal‐halide octahedra in the 2D layer plane. In this work, a new family of hybrid organic–inorganic 2D lead halides is introduced, by reporting the first example of the hybrid organic–inorganic post‐perovskite 3‐cyanopyridinium lead tribromide (3cp)PbBr3. The post‐perovskite structure has unique octahedra connectivity type in the layer plane: a typical “perovskite‐like” corner‐sharing connectivity pattern in one direction, and the rare edge‐sharing connectivity pattern in the other. Such connectivity leads to significant anisotropy in the material properties within the inorganic layer plane. Moreover, the dense organic cation packing results in the formation of 1D fully organic bands in the electronic structure, offering the prospects of the involvement of the organic subsystem into material's optoelectronic properties. The (3cp)PbBr3 clearly shows the 2D quantum size effect with a bandgap around 3.2 eV and typical broadband self‐trapped excitonic photoluminescence at temperatures below 200 K.

Halide perovskites are novel photonics materials promising numerous applications in fields such as photovoltaics, LED light sources, microlasers, and radiation detectors. Many halide perovskites are direct-gap semiconductors, and Wannier–Mott excitons play a significant role in their optical properties near the fundamental absorption edge. The high oscillator strength of these states favors applications where efficient interaction with light is required. In this work, to study excitonic states in CsPbBr3 halide perovskite single crystals, the reflection spectroscopy at temperatures from 4 K was used. A reflection coefficient up to 70% was observed for the n=1 exciton state, followed by weak excited states of excitons with n=2 and n=3. It should be noted that the Sommerfeld enhancement factor should be considered for a correct description of the behavior of the dielectric constant, taking into account excitonic effects.

Halide perovskites are a promising optical gain media for lasers. In this work, the phenomenon of random lasing is studied in the halide perovskite MAPbI3 single crystal at a temperature of 4 K and pico- or femtosecond pulsed optical pumping. Spectral, temporal, polarimetric, and threshold measurements facilitate isolating random lasing from the photoluminescence background. Spectral localization of the lasing lines points to defect-related states emission as the lasing origin. Time-resolved photoluminescence study shows that the random lasing is emitted as an ultrashort pulse with a duration below 10 ps, and the photoluminescence background as a long 100 ps afterglow. The mechanism of lasing in MAPbI3 halide perovskite is determined, which is ultrafast radiation from states associated with defects. The obtained information will make it possible to increase the efficiency of lasing in halide perovskites by controlled introduction of radiating states in the material transparency region.

Halide perovskites and their low-dimensional analogs are promising semiconductor materials for solar cells, LEDs, lasers, detectors and other applications in the area of photonics. The most informative optical property of semiconductor photonics materials is the absorption spectrum enabling observation of the fundamental absorption edge, exciton structure, defect-related bands, etc. Traditionally, in the study of halide perovskites, this spectrum is obtained by absorption spectroscopy of thin films or diffuse reflectance spectroscopy of powders. The first method is applicable only to compounds with the developed thin film deposition technology, and in the second case, a large absorption coefficient narrows the observations down to the sample transparency region. In this paper, we suggest the diffuse reflectance spectroscopy with dilution as a method for obtaining the full-range absorption spectrum from halide perovskite powders, and demonstrate its application to practically important cases.

In this work, we apply polarized Raman spectroscopy for study of internal vibrations of the 3-cyanopyridinium cation in the halide post-perovskite (3cp)­PbBr3 (3cp = 3-CN-C5H5NH+). For a single cation, the vibrational frequencies and intensities of the Raman signal were calculated using the density functional theory. Selection rules were established for vibrations of cations in the crystal. These rules together with modeling results were used to identify the internal vibrations of the cation in the Raman spectrum of the crystal. Narrow and isolated internal vibrations of cations could be used as spectators of the crystalline environment.