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Two-dimensional (2D) hybrid perovskites have exhibited multiple features owing to their multiple quantum well structure. Here, we show the synthesis and characterization of 2D mixed-halide Cu-based perovskite crystals, R2Cu(Cl/Br)4, with three different alkyl chains, where R is methyl, butyl, or hexyl ammonium cations. Exfoliated perovskite flakes exhibit reversible thermochromism with changes in color darkness with temperature variation. Differential scanning calorimetry and temperature-dependent X-ray diffraction measurements reveal that the thermochromic behavior mechanism of R2Cu(Cl/Br)4 depends on the nature of the alkyl chain. [ABSTRACT FROM AUTHOR]

Lead-free, vacancy-ordered, Cs3Bi2Br9 perovskite is considered promising inorganic, stable, and non-toxic halide perovskite for optoelectronic and photovoltaic applications. However, its wide bandgap limits its state-of-art applications. We notice an observable enhancement of light absorption of Cs3Bi2Br9 perovskite crystals upon CuBr2 addition to the perovskite precursor. X-ray diffraction, 133Cs solid state NMR, Raman spectroscopy, X-ray photoelectron spectroscopy, and density functional theory calculations were studied to find out the nature of the perovskite crystal formation mechanism. With the addition of up to 50% CuBr2, the Cs3Bi2Br9 perovskite retains its matrix structure with homogeneously distributed Cs2CuBr4 large domains. Drop-casting of perovskite/DMSO solutions over TiO2 thin films reveals a reduction of the direct bandgap from 2.56 eV for pristine Cs3Bi2Br9 to 1.77 eV for a 50% Cu2+ alloyed one. First-principles calculations reveal the possibility of Cs2CuBr4 phase formation with higher Cu alloying, which can be the main reason behind bandgap narrowing. For homogeneously Cu alloyed Cs3Bi2Br9, reduction of the effective bandgap can occur due to the formation of compensating defect levels (VBr, CuBr) above the pristine valence band maxima (VBM). These results highlight the importance of alloying for structure modulation for bandgap engineered vacancy-ordered perovskite structures. [ABSTRACT FROM AUTHOR]

Halide perovskites have exhibited great research impact for developing innovative materials with novel properties. Here, we report the synthesis of different caesium lead bromide perovskites using different (Cs/Pb) molar ratios and fabrication of their corresponding perovskite/polyvinylidene fluoride (PVDF) composites, as well as study of their structural and UV-photodetection properties. Spin-coated perovskite/PVDF composite thin films revealed strong oriented XRD diffraction peaks along the c -axis direction (00l) with homogeneously distributed perovskite microcrystals in the polymer matrix. The high-Cs containing perovskite/PVDF composite, with Cs/Pb (3/1) molar ratio, demonstrated the highest green emission under UV light and its corresponding UV-photodetector exhibited the highest UV photo-responsivity. These results highlight the importance of structural modulation and additive manufacturing for tailoring the optoelectronic properties of halide perovskites., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Perovskite based on cesium bismuth bromide offers a compelling, non-toxic alternative to lead-containing counterparts in optoelectronic applications. However, its widespread usage is hindered by its wide bandgap. This study investigates a significant bandgap tunability achieved by introducing Fe doping into the inorganic, lead-free, non-toxic, and stable Cs 3 Bi 2 Br 9 perovskite at varying concentrations. The materials were synthesized using a facile method, with the aim of tuning the optoelectronic properties of the perovskite materials. Characterization through techniques such as X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, energy dispersive spectroscopy (EDS), and UV-vis spectroscopy was conducted to elucidate the transformation mechanism of the doping materials. The substitution process results in a significant change in the bandgap energy, transforming from the pristine Cs 3 Bi 2 Br 9 with a bandgap of 2.54 eV to 1.78 eV upon 70% Fe doping. The addition of 50% Fe in Cs 3 Bi 2 Br 9 leads to the formation of the orthorhombic structure in Cs 2 (Bi,Fe)Br 5 perovskite, while complete Fe alloying at 100% results in the phase formation of CsFeBr 4 perovskite. Our findings on regulation of bandgap energy and crystal structure through B site substitution hold significant promise for applications in optoelectronics., Competing Interests: The authors declare that they have no conflict of interest., (This journal is © The Royal Society of Chemistry.)

Two-dimensional (2D) hybrid perovskites with novel functionalities and structural diversity are a perfect platform for emerging optoelectronic devices such as photodetectors, light-emitting diodes, and solar cells. Here, we demonstrate that excess concentration of Cesium bromide (CsBr) is key to the formation of easily exfoliated 2D Cs 2 Cu(Cl/Br) 4 perovskite crystal. Furthermore, by employing this trick to 2D perovskite MA 2 Cu(Cl/Br) 4 (MA=methylammonium), we achieve a phase-pure easily exfoliated 2D mixed-cation (MA/Cs) 2 Cu(Cl/Br) 4 perovskite crystal, which exhibits reduced bandgap (1.53 eV) with ferromagnetic behavior and photovoltaic property. The resultant mixed-cation structured device reveals enhanced efficiency compared to all MA and all Cs counterparts. These findings demonstrate the importance of cation-engineering in developing innovative materials with novel properties., (© 2022 Wiley-VCH GmbH.)