Random lasing in cesium lead iodide (CsPbI3) thin films with no surface passivation

Investigation of the stability of organic-inorganic halide perovskite thin films: Insight from experimental and simulation

Herein, we investigated the stability of lead halide perovskites under ambient conditions after mixing the two cations Formamidinium (FA) and Cesium (Cs). The CsxFA1-xPbI3 perovskites solutions were prepared with different contents of x (0.0, 0.3, 0.5, 0.7 and 1.0) and deposited on substrates by spin-coating technique. The CsxFA1-xPbI3 films were, afterwards, characterized using the X-ray diffraction (XRD), UV-visible spectroscopy, photoluminescence (PL) spectra and scanning electron microscopy (SEM) to figure out their crystallinity, morphology, and optical properties. We noticed a stable perovskite structure for the mixed compounds unalike the pure FA and Cs films. The XRD analysis revealed, even after two weeks, the growth and good stability after two weeks of the desired black cubic α-phase perovskite structure in opposite to FAPbI3 and CsPbI3 which, respectively, showed faster degradation and transition into non-perovskite δ-phase and ɣ-phase no perovskite phases. The mixed perovskites Cs-FA also displayed a high absorbance especially for the ones with 30% of Cs and 70% of FA or 50% of each, with an excellent band gap energy ranging between 1.52 and 1.7 eV where FAPbI3 and CsPbI3 were showing a bandgap between 1.5 and 1.9 eV respectively. Moreover, the performance of the CsxFA1-xPbI3 based solar cells were simulated with SCAPS by using the band gaps obtained from the experimental study and after by varying the band gap, the thickness of the absorber layers and then different types of Electron Transport Layer (ETL). The simulation results revealed that the Cs0.3FA0.7PbI3 based solar cells had the highest higher efficiency around 22.36%.

2D tunable all-solid-state random laser in the visible

A two-dimensional (2D) solid-state random laser emitting in the visible is demonstrated, in which optical feedback is provided by a controlled disordered arrangement of air-holes in a dye-doped polymer film. We find an optimal scatterer density for which threshold is minimum and scattering is the strongest. We show that the laser emission can be red-shifted by either decreasing scatterer density or increasing pump area. We show that spatial coherence is easily controlled by varying pump area. Such a 2D random laser provides with a compact on-chip tunable laser source and a unique platform to explore non-Hermitian photonics in the visible.

Unusual Spectrally Reproducible and High Q-Factor Random Lasing in Polycrystalline Tin Perovskite Films

An unusual spectrally reproducible near-IR random lasing (RL) with no fluctuation of lasing peak wavelength is disclosed in polycrystalline films of formamidinium tin triiodide perovskite, which have been chemically stabilized against Sn²⁺ to Sn⁴⁺ oxidation. Remarkably, a quality Q-factor as high as ≈10⁴ with an amplified spontaneous emission (ASE) threshold as low as 2 µJ cm^-2 (both at 20 K) are achieved. The observed spectral reproducibility is unprecedented for semiconductor thin film RL systems and cannot be explained by the strong spatial localization of lasing modes. Instead, it is suggested that the spectral stability is a result of such an unique property of Sn-based perovskites as a large inhomogeneous broadening of the emitting centers, which is a consequence of an intrinsic structural inhomogeneity of the material. Due to this, lasing can occur simultaneously in modes that are spatially strongly overlapped, as long as the spectral separation between the modes is larger than the homogeneous linewidth of the emitting centers. The discovered mechanism of RL spectral stability in semiconductor materials, possessing inhomogeneous broadening, opens up prospects for their practical use as cheap sources of narrow laser lines.

Role of inorganic cations in the excitonic properties of lead halide perovskites

We theoretically investigate lead iodide perovskites of general formula APbI₃ for a series of metallic cations (namely Cs⁺, Rb⁺, K⁺, Na⁺ and Li⁺) by means of density functional theory, the GW method and the Bethe-Salpeter equation including spin-orbit coupling. We demonstrate that the low-energy edges (up to 1.3 eV) of the absorption spectra are dominated by weakly bound excitons, with binding energies E_b of ∼ 30-80 meV, and the corresponding intensities increase as metallic cations become lighter. The middle parts of the spectra (1.8-2.4 eV), on the other hand, contain optical dipole transitions comprising more confined excitons (E_b ∼ 150-200 meV) located at PbI₃. These parts of the spectra correspond to the optical-gain wavelengths which are experimentally achieved in optically pumped perovskite lasers. Finally, the higher energy parts, from about 2.8 eV (LiPbI₃) to 4.3 eV (CsPbI₃), contain optical transitions with very confined excitons (E_b ∼ 220-290 meV) located at halide atoms and the empty states of the metallic cations.