No abstract

Halide perovskites are promising photovoltaic, solar cell, and semiconductor materials. Density-functional theory (DFT) models address compressive and tensile biaxial strain effects on APbCl3, where A = (K, Rb, and Cs). This research shows how A-cation impacts bandgap energy and band structure. The direct bandgap for KPbCl3, RbPbCl3, and CsPbCl3 is found 1.612, 1.756, and 2.046 eV, respectively; increases from A=K to Cs. When spin-orbital coupling (SOC) is introduced, bandgaps in KPbCl3, RbPbCl3, and CsPbCl3 perovskites are reduced to 0.356, 0.512, and 0.773 eV, respectively. More tensile strain widens the bandgap; compressive strain narrows it. Without SOC, the bandgaps of KPbCl3, RbPbCl3, and CsPbCl3 were tuned from 0.486 to 2.213 eV, 0.778 to 2.289 eV, and 1.168 to 2.432 eV, respectively. When the compressive strain is increased, the dielectric constant of APbCl3 decreases (red shift) and increases (blue shift) as the tensile strain is increased. Strain improves APbCl3 perovskite's optical performance.

We establish that the formally 0D (R-/S-MBA)2CuBr4, containing R-/S-α-methyl benzylamine (R-/S-MBA) connected to highly distorted CuBr4 tetrahedral units in alternating layers, possesses extraordinary chiro-optical properties. The concentration and path length-independent chiral anisotropy factor, g CD, for this compound is the highest in the orange-red part of the visible spectrum reported so far from any hybrid material, arising from a chirality transfer from the organic component to the inorganic layer through the extensive asymmetric hydrogen bonding network and electronic coupling, driving the CuBr4 tetrahedral units to follow the 21-screw axis. This sensitivity in the orange-red part of the visible spectrum is achieved by incorporating bromine in the copper coordination sphere, which significantly red-shifts the band edge absorption to ∼710 nm compared to ∼490 nm reported for the chloride analogue. DFT/TDDFT calculations allow us to understand the underlying electronic structure responsible for its remarkable optical properties. We find that this compound gets a partial 2D character, crucial for its broadband chiro-optical properties, arising from Cu–Br···Br–Cu interactions connecting the otherwise isolated CuBr4 units.

Two-dimensional (2D) layered Ruddlesden−Popper metal halide perovskites (MHPs) show enhanced stability compared to threedimensional (3D) MHPs. The general formula of 2D layered perovskite is L 2 A n−1 M n X 3n+1 , where L is the large organic spacer and n is the number of metal octahedra. However, the syntheses of such 2D layered perovskites yield a mixture of 3D and 2D layered perovskites with different layers of the metal octahedra. In this work, we have synthesized 2D layered (MA) n+1 Pb n I 3n+1 perovskite by the sonochemical method. We have shown that the dimensionality n can be controlled by the sonication time and reaction temperature. Using absorption and photoluminescence spectroscopy, we have probed the reaction and growth mechanisms of the 2D layered perovskites and their transformation to 3D MAPbI 3 (MAPI). At both lower temperature and early stage of the reaction, 2D layered perovskites with lower dimensionality form and eventually covert to higher-dimensional layered perovskite before transforming to 3D perovskites. The dissimilarity in the solubility of the precursors (PbI 2 and MAI) is responsible for such transformations. We show that these mixed (2D layered and 3D MAPI) perovskites can be used to fabricate a white light-emitting diode.