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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]

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 Cs2Cu(Cl/Br)4 perovskite crystal. Furthermore, by employing this trick to 2D perovskite MA2Cu(Cl/Br)4 (MA=methylammonium), we achieve a phase‐pure easily exfoliated 2D mixed‐cation (MA/Cs)2Cu(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. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) lead-based hybrid perovskites have shown strong photochemical stability. However, the high toxicity of lead hinders their commercialization. Here, we report the synthesis of 2D Cu-based mixed-halide perovskite (CH3NH3)2CuClxBr4−x thin films by the exfoliation of centimeter-sized single crystals and study their structural, thermal, and optical properties, and thermochromism. A strong orientation along the c-axis direction with preferable (00l) diffraction peaks in the XRD spectrum and no grain boundary in the SEM image of the bulk crystal confirm single crystal formation. As the Br content increases, mixed-halide Cu-perovskites exhibit tunable band gap energy ranging from 2.24 eV to 1.43 eV. Upon temperature variation, the Cl-rich perovskites show a reversible thermochromic behavior with a color change between yellow, orange, red, and brown, but no observable color change is observed for Br-rich perovskites. The in situ temperature-dependent X-ray diffraction reveals that thermochromism originates from structural changes in the 2D crystal. [ABSTRACT FROM AUTHOR]