Spontaneous anion-exchange synthesis of optically active mixed-valence Cs2Au2I6 perovskites from layered CsAuCl4 perovskites

A theoretical exploration of the structural feature, mechanical, and optoelectronic properties of Au-based halide perovskites A2AuIAuIIIX6 (A = Rb, Cs; X = Cl, Br, I)

As a possible alternative to lead halide perovskites, inorganic mixed-valence Au-based halide perovskites have drawn much attention. In the current research, we have conducted comprehensive theoretical calculations to reveal the structural feature, thermodynamic and dynamic stability, mechanical behavior, optoelectronic properties, and photovoltaic performance of Au-based halide perovskites A2AuIAuIIIX6 (A = Rb, Cs; X = Cl, Br, I). The structural parameters of these compounds are carefully analyzed. Our calculations indicate that the thermodynamic, dynamic, and mechanical stability of monoclinic Rb2AuIAuIIIX6 and tetragonal Cs2AuIAuIIIX6 are ensured, and they are all ductile. The electronic band structure analysis shows that Rb2AuIAuIIII6 illustrates a direct-gap feature, while Rb2AuIAuIIIX6 (X = Cl, Br) and Cs2AuIAuIIIX6 (X = Cl, Br, I) are indirect-gap materials. The effect of A-site cation substitution on the optical band gaps of the Au-based halide perovskites is elucidated. Our results further suggest that Rb2AuIAuIIIX6 (X = Br, I) and Cs2AuIAuIIIX6 (X = Cl, Br, I) are more suitable for single-junction solar cells due to their suitable band gaps within 1.1-1.5 eV. Furthermore, four compounds A2AuIAuIIIX6 (A = Rb, Cs; X = Br, I) not only have high absorption coefficients in the visible region but also show excellent photovoltaic performance, especially for A2AuIAuIIII6 (A = Rb, Cs), whose efficiency can reach over 29% with a film thickness of 0.5 μm. Our study suggests that inorganic Au-based halide perovskites are potential alternatives for optoelectronic devices in solar cells.

Oxygen and moisture-induced healing of halide double perovskite surface defects

In this work, we studied the impact of environmental constituents such as oxygen (O2) and moisture on halide double perovskite (HDP) films. The transport measurements indicate that an increment in O2 concentration enhances the resistivity of a Cs2AgBiBr6 film by two orders of magnitude. The adsorption of O2 on the film's surface helps in passivation of defects (∼50% reduction in defect density on O2 exposure), which inhibits ion migration and results in an increased resistivity of the film. The process of adsorption and desorption of O2 on the film surface is found to be fully reversible. In contrast, the resistivity of double perovskite films decreases by an order of magnitude in the presence of moisture. This is attributed to the generation of free protons as a result of the dissociation of water molecules at the films' surface, hence exhibiting an increase in current under external bias. The HDP films possess high resistivity (for T < 100 °C) due to the desorption of physisorbed water layers from the surface, which gradually decreases with an increase in the operating temperature. This work demonstrates that O2 and moisture are a good combination for defect passivation in any HDPs, in general.

Lead-Free Halide Double Perovskite for High-Performance Photodetectors: Progress and Perspective

Lead halide perovskite has become a promising candidate for high-performance photodetectors (PDs) due to its attractive optical and electrical properties, such as high optical absorption coefficient, high carrier mobility, and long carrier diffusion length. However, the presence of highly toxic lead in these devices has limited their practical applications and even hindered their progress toward commercialization. Therefore, the scientific community has been committed to searching for low-toxic and stable perovskite-type alternative materials. Lead-free double perovskite, which is still in the preliminary stage of exploration, has achieved inspiring results in recent years. In this review, we mainly focus on two types of lead-free double perovskite based on different Pb substitution strategies, including A₂M(I)M(III)X₆ and A₂M(IV)X₆. We review the research progress and prospects of lead-free double perovskite photodetectors in the past three years. More importantly, from the perspective of optimizing the inherent defects in materials and improving device performance, we propose some feasible pathways and make an encouraging perspective for the future development of lead-free double perovskite photodetectors.

MnPc Films Deposited by Ultrasonic Spray Pyrolysis at Low Temperatures: Optical, Morphological and Structural Properties

In this work, we report how manganese phthalocyanine (MnPc) films obtained using the ultrasonic spray-pyrolysis technique at 40 °C deposited on glass substrate subjected to thermal annealing at 100 °C and 120 °C. The MnPc films were characterized using UV/Vis spectroscopy, Raman spectroscopy, X-Ray Diffraction (XRD), and Scanning Electron Microscopy (SEM). The absorption spectra of the MnPc films were studied in a wavelength range from 200 to 850 nm, where the characteristic bands of a metallic phthalocyanine known as B and Q bands were observed in this range of the spectrum. The optical energy band (Eg) was calculated using the Tauc equation. It was found that, for these MnPc films, the Eg has the values of 4.41, 4.46, and 3.58 eV corresponded to when they were deposited, annealing at 100 °C and 120 °C, respectively. The Raman spectra of the films showed the characteristic vibrational modes of the MnPc films. In the X-Ray diffractograms of these films, the characteristic diffraction peaks of a metallic phthalocyanine are observed, presenting a monoclinic phase. The SEM images of these films were studied in a cross-section obtaining thicknesses of 2 μm for the deposited film and 1.2 μm and 0.3 μm for the annealed films at 100 °C and 120 °C. Additionally, in the SEM images of these films, average particle sizes ranging from 4 to 0.041 µm were obtained. The results agree with those reported in the literature for MnPc films deposited by performing other techniques.

BiOBr Surface-Functionalized Halide Double-Perovskite Films for Slow Ion Migration and Improved Stability

Surface-tailored lead-free halide double-perovskite (Cs₂AgBiX₆) thin films are utilized for ion migration studies. A thin surface layer of BiOBr/Cl is grown via intentional annealing of the halide films in ambient conditions. Herein, we physically stacked the two films, viz., Cs₂AgBiBr₆ and Cs₂AgBiCl₆, to thermally activate the halide ion migration at different temperatures (room temperature (RT)-150 °C). While annealing, the films' color changes from orange to pale yellow and transparent brown to yellow as a result of the migration of Br^- ions from Cs₂AgBiBr₆ to Cs₂AgBiCl₆ and Cl^- ions from Cs₂AgBiCl₆ to Cs₂AgBiBr₆, respectively. Annealing helps in homogenizing the halide ions throughout the films, consequently leading to a mixed phase, i.e., Cs₂AgBiCl_xBr_6-x/Cs₂AgBiBr_xCl_6-x (x = 0 to 6) formation. The movement of ions is understood by absorption studies performed at regular time intervals. These investigations reveal a redshift (from 366 to 386 nm) and a blueshift (from 435 to 386 nm) in absorption spectra, indicating the migration of Br^- and Cl^- toward Cs₂AgBiCl₆ and Cs₂AgBiBr₆, respectively. The films characterized using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) reveal the presence of a peak at 2θ = 10.90° and binding energy of 158.1 eV, respectively, corresponding to the formation of Bi-O bonds at the film surface. Also, XRD studies show a lower 2θ shift of the diffraction peak in the case of Cs₂AgBiCl₆ films and a higher 2θ shift in the case of Cs₂AgBiB₆ films, which further confirms the migration of Cl^- and Br^- from one film to the other. XPS investigations confirm the compositional change with a gradual increment in the concentration of Br^-/Cl^- with an increase in heating time for Cs₂AgBiCl₆/Cs₂AgBiBr₆ films. All these studies confirm thermal diffusion of halide ions in double-perovskite films. Further, from the exponential decay of the absorption spectra, the rate constant for halide (Br) ion diffusion is calculated, which shows an increment from 1.7 × 10^-6 s^-1 at RT to 12.1 × 10^-3 s^-1 at 150 °C. The temperature-dependent rate constant follows Arrhenius behavior and renders an activation energy of 0.42 eV (0.35 eV) for bromide (chloride) ion mobility. A larger estimated value as compared to the reported values for Cs₂AgBiBr₆ wafers (∼0.20 eV) reveals a slow mobility of halide ions in thin films of Cs₂AgBiBr₆/Cl₆. The formation of a BiOBr passivation layer at the surface of Cs₂AgBiBr₆ thin film might be one of the plausible causes of the slow anion diffusion in the present work. Slow ion migration is an indication that the films are stable and of high-quality.

Bandgap Funneling in Bismuth-Based Hybrid Perovskite Photocatalyst with Efficient Visible-Light-Driven Hydrogen Evolution

The photocatalytic system using hydrohalic acid (HX) for hydrogen production is a promising strategy to generate clean and renewable fuels as well as value-added chemicals (such as X₂ /X₃ ^- ). However, it is still challenging to develop a visible-light active and strong-acid resistive photocatalyst. Hybrid perovskites have been recognized as a potential photocatalyst for photovoltaic HX splitting. Herein, a novel environmentally friendly mixed halide perovskite MA₃ Bi₂ Cl_9-x I_x with a bandgap funnel structure is developed, i.e., confirmed by energy dispersive X-ray analysis and density functional theory calculations. Due to gradient neutral formation energy within iodine-doped MA₃ Bi₂ Cl₉ , the concentration of iodide element decreases from the surface to the interior across the MA₃ Bi₂ Cl_9-x I_x perovskite. Because of the aligned energy levels of iodide/chloride-mixed MA₃ Bi₂ Cl_9-x I_x , a graded bandgap funnel structure is therefore formed, leading to the promotion of photoinduced charge transfer from the interior to the surface for efficient photocatalytic redox reaction. As a result, the hydrogen generation rate of the optimized MA₃ Bi₂ Cl_9-x I_x is enhanced up to ≈341 ± 61.7 µmol h^-1 with a Pt co-catalyst under visible light irradiation.

Photoinduced Transformation of Cs2Au2Br6 into CsPbBr3 Nanocrystals

Lead-free halide double perovskites offer an environmentally friendly alternative to lead halide perovskites for designing optoelectronic solar cell devices. One simple approach to synthesize such double halide perovskites is through metal ion exchange. CsPbBr₃ nanocrystals undergo exchange of Pb²⁺ with Au(I)/Au(III) to form double perovskite Cs₂Au₂Br₆. When excited, a majority of the charge carriers undergo quick recombination in contrast to long-lived charge carries of excited CsPbBr₃ nanocrystals. This metal ion exchange process is reversible as one can regenerate CsPbBr₃ by adding excess PbBr₂ to the suspension. Interestingly, when subjected to visible light irradiation, Cs₂Au₂Br₆ nanocrystals eject reduced Au from the lattice as evidenced from the formation of larger gold nanoparticles. The presence of residual Pb²⁺ ions in the suspension restores the original CsPbBr₃ composition. The results presented here provide insight into the dynamic nature of Au within the perovskite lattice under both chemical and light stimuli.

Enhanced visible light absorption in layered Cs3Bi2Br9 through mixed-valence Sn(ii)/Sn(iv) doping

Lead-free halides with perovskite-related structures, such as the vacancy-ordered perovskite Cs₃Bi₂Br₉, are of interest for photovoltaic and optoelectronic applications. We find that addition of SnBr₂ to the solution-phase synthesis of Cs₃Bi₂Br₉ leads to substitution of up to 7% of the Bi(iii) ions by equal quantities of Sn(ii) and Sn(iv). The nature of the substitutional defects was studied by X-ray diffraction, ¹³³Cs and ¹¹⁹Sn solid state NMR, X-ray photoelectron spectroscopy and density functional theory calculations. The resulting mixed-valence compounds show intense visible and near infrared absorption due to intervalence charge transfer, as well as electronic transitions to and from localised Sn-based states within the band gap. Sn(ii) and Sn(iv) defects preferentially occupy neighbouring B-cation sites, forming a double-substitution complex. Unusually for a Sn(ii) compound, the material shows minimal changes in optical and structural properties after 12 months storage in air. Our calculations suggest the stabilisation of Sn(ii) within the double substitution complex contributes to this unusual stability. These results expand upon research on inorganic mixed-valent halides to a new, layered structure, and offer insights into the tuning, doping mechanisms, and structure-property relationships of lead-free vacancy-ordered perovskite structures.

Photo generated charge transport studies of defects-induced shuttlecock-shaped ZnO/Ag hybrid nanostructures

We report the stimulating effects of interfacial charge transfer process between spherical Ag nanoparticles and shuttlecock-shaped ZnO nanostructures observed by UV-visible spectroscopy and x-ray absorption spectroscopy. In specific, ZnO nanorods and shuttlecock-shaped ZnO/Ag nanostructures were developed using a simple chemical colloidal method and characterized for structural variations using XRD. The observed red shift in plasmonic peak and the increase in Urbach energy signify interfacial interactions and increased randomness in the hybrid ZnO/Ag nanostructures. Simultaneously, the enhanced intensity of deep-level emission in the ZnO/Ag hybrid suggests the increased recombination rate of electron-hole pairs. The red and blue emissions evolving with temperature subsequently suggests the presence of oxygen vacancies or zinc interstitials in the system. The decrease in intensities and emerging features in O K-edge and Zn L-edge indicates the charge transfer from Ag to ZnO at the interface of ZnO/Ag hybrids. Moreover, the differences in absorption edges with alternating light on/off conditions were analyzed for the exploitation of this ZnO-based system in various applications.