Lead‐Free Halide Double Perovskite Cs2AgBiBr6 with Decreased Band Gap

Environmentally friendly halide double perovskites with improved stability are regarded as a promising alternative to lead halide perovskites. The benchmark double perovskite, Cs₂AgBiBr₆, shows attractive optical and electronic features, making it promising for high‐efficiency optoelectronic devices. However, the large band gap limits its further applications, especially for photovoltaics. Herein, we develop a novel crystal‐engineering strategy to significantly decrease the band gap by approximately 0.26 eV, reaching the smallest reported band gap of 1.72 eV for Cs₂AgBiBr₆ under ambient conditions. The band‐gap narrowing is confirmed by both absorption and photoluminescence measurements. Our first‐principles calculations indicate that enhanced Ag–Bi disorder has a large impact on the band structure and decreases the band gap, providing a possible explanation of the observed band‐gap narrowing effect. This work provides new insights for achieving lead‐free double perovskites with suitable band gaps for optoelectronic applications.     

Lead-Free Halide Double Perovskite Cs2AgBiBr6 with Decreased Band Gap

Environmentally friendly halide double perovskites with improved stability are regarded as a promising alternative to lead halide perovskites. The benchmark double perovskite, Cs₂AgBiBr₆, shows attractive optical and electronic features, making it promising for high-efficiency optoelectronic devices. However, the large band gap limits its further applications, especially for photovoltaics. Herein, we develop a novel crystal-engineering strategy to significantly decrease the band gap by approximately 0.26 eV, reaching the smallest reported band gap of 1.72 eV for Cs₂AgBiBr₆ under ambient conditions. The band-gap narrowing is confirmed by both absorption and photoluminescence measurements. Our first-principles calculations indicate that enhanced Ag–Bi disorder has a large impact on the band structure and decreases the band gap, providing a possible explanation of the observed band-gap narrowing effect. This work provides new insights for achieving lead-free double perovskites with suitable band gaps for optoelectronic applications.     

High‐Pressure Band‐Gap Engineering in Lead‐Free Cs2AgBiBr6 Double Perovskite

Novel inorganic lead‐free double perovskites with improved stability are regarded as alternatives to state‐of‐art hybrid lead halide perovskites in photovoltaic devices. The recently discovered Cs₂AgBiBr₆ double perovskite exhibits attractive optical and electronic features, making it promising for various optoelectronic applications. However, its practical performance is hampered by the large band gap. In this work, remarkable band gap narrowing of Cs₂AgBiBr₆ is, for the first time, achieved on inorganic photovoltaic double perovskites through high pressure treatments. Moreover, the narrowed band gap is partially retainable after releasing pressure, promoting its optoelectronic applications. This work not only provides novel insights into the structure–property relationship in lead‐free double perovskites, but also offers new strategies for further development of advanced perovskite devices.     

Stable and Highly Efficient Photocatalysis with Lead‐Free Double‐Perovskite of Cs2AgBiBr6

Composition engineering of halide perovskite allows the tunability of the band gap over a wide range so that photons can be effectively harvested, an aspect that is of critical importance for increasing the efficiency of photocatalysis under sunlight. However, the poor stability and the low photocatalytic activity of halide perovskites prevent use of these defect‐tolerant materials in wide applications involving photocatalysis. Here, an alcohol‐based photocatalytic system for dye degradation demonstrated high stability through the use of double perovskite of Cs₂AgBiBr₆. The reaction rate on Cs₂AgBiBr₆ is comparable to that on CdS, a model inorganic semiconductor photocatalyst. The fact of fast reaction between free radicals and dye molecules indicates the unique catalytic properties of the Cs₂AgBiBr₆ surface. Deposition of metal clusters onto Cs₂AgBiBr₆ effectively enhances the photocatalytic activity. Although the stability (five consecutive photocatalytic cycles without obvious decrease of efficiency) requires further improvements, the results indicate the significant potential of Cs₂AgBiBr₆‐based photocatalysis.     

Bandgap Engineering of Lead‐Free Double Perovskite Cs2AgBiBr6 through Trivalent Metal Alloying

The double perovskite family, A₂M^IM^IIIX₆, is a promising route to overcome the lead toxicity issue confronting the current photovoltaic (PV) standout, CH₃NH₃PbI₃. Given the generally large indirect band gap within most known double perovskites, band‐gap engineering provides an important approach for targeting outstanding PV performance within this family. Using Cs₂AgBiBr₆ as host, band‐gap engineering through alloying of In^III/Sb^III has been demonstrated in the current work. Cs₂Ag(Bi_1−x M_x )Br₆ (M=In, Sb) accommodates up to 75 % In^III with increased band gap, and up to 37.5 % Sb^III with reduced band gap; that is, enabling ca. 0.41 eV band gap modulation through introduction of the two metals, with smallest value of 1.86 eV for Cs₂Ag(Bi_0.625Sb_0.375)Br₆. Band structure calculations indicate that opposite band gap shift directions associated with Sb/In substitution arise from different atomic configurations for these atoms. Associated photoluminescence and environmental stability of the three‐metal systems are also assessed.     

Efficient and Selective Electroreduction of CO2 to HCOOH over Bismuth-Based Bromide Perovskites in Acidic Electrolytes

Metal halide perovskites, primarily used as optoelectronic devices, have not been applied for electrochemical conversion due to their insufficient stability in moisture. Herein, two bismuth-based perovskites are introduced as novel electrocatalysts to convert CO₂ into HCOOH in aqueous acidic media (pH 2.5), exhibiting a high Faradaic efficiency for HCOOH of >80 % in a wide potential range from −0.75 to −1.25 V. Their structural evolution against water was dynamically monitored by in situ spectra. Theoretical calculations further reveal that the formation of intermediate OCHO* on bismuth sites of Cs₃Bi₂Br₉(111) play a pivotal role toward HCOOH production, which has a lower energy barrier than that on Cs₂AgBiBr₆(001) surfaces. Significantly, CO₂ reacts with protons instead of water which can enhance CO₂ reduction rate and suppress hydrogen evolution by avoiding carbonate formation in acidic electrolytes. This work paves the way for the extensive investigation of halide perovskites in aqueous systems.     

Bandgap Engineering of Lead-Free Double Perovskite Cs2AgBiBr6 through Trivalent Metal Alloying

The double perovskite family, A₂M^IM^IIIX₆, is a promising route to overcome the lead toxicity issue confronting the current photovoltaic (PV) standout, CH₃NH₃PbI₃. Given the generally large indirect band gap within most known double perovskites, band-gap engineering provides an important approach for targeting outstanding PV performance within this family. Using Cs₂AgBiBr₆ as host, band-gap engineering through alloying of In^III/Sb^III has been demonstrated in the current work. Cs₂Ag(Bi_1−xM_x)Br₆ (M=In, Sb) accommodates up to 75 % In^III with increased band gap, and up to 37.5 % Sb^III with reduced band gap; that is, enabling ca. 0.41 eV band gap modulation through introduction of the two metals, with smallest value of 1.86 eV for Cs₂Ag(Bi_0.625Sb_0.375)Br₆. Band structure calculations indicate that opposite band gap shift directions associated with Sb/In substitution arise from different atomic configurations for these atoms. Associated photoluminescence and environmental stability of the three-metal systems are also assessed.     

Low‐Bandgap Cs4CuSb2Cl12 Layered Double Perovskite: Synthesis,Reversible Thermal Changes,and Magnetic Interaction

Double perovskites (DPs) with a generic formula A₂M′(I)M^IIIX₆ (A and M are metal ions, and X=Cl, Br, I) are now being explored as potential alternatives to Pb‐halide perovskites for solar cells and other optoelectronic applications. However, these DPs typically suffer from wide (≈3 eV) and/or indirect band gaps. In 2017, a new structural variety, namely layered halide DP Cs₄CuSb₂Cl₁₂ (CCSC) with bivalent Cu^II ion in the place of M′(I) was reported, which exhibit a band gap of approximately 1 eV. Here, we report a mechanochemical synthesis of CCSC, its thermal and chemical stability, and magnetic response of Cu^II d⁹ electrons controlling the optoelectronic properties. A simple grinding of precursor salts at ambient conditions provides a stable and scalable product. CCSC is stable in water/acetone solvent mixtures (≈30 % water) and many other polar solvents unlike Pb‐halide perovskites. It decomposes to Cs₃Sb₂Cl₉, Cs₂CuCl₄, and SbCl₃ at 210 °C, but the reaction can be reversed back to produce CCSC at lower temperatures and high humidity. A long‐range magnetic ordering is observed in CCSC even at room temperature. The role of such magnetic ordering in controlling the dispersion of the conduction band, and therefore, controlling the electronic and optoelectronic properties of CCSC has been discussed.     

Small‐Band‐Gap Halide Double Perovskites

Despite their compositional versatility, most halide double perovskites feature large band gaps. Herein, we describe a strategy for achieving small band gaps in this family of materials. The new double perovskites Cs₂AgTlX₆ (X=Cl ( 1 ) and Br ( 2 )) have direct band gaps of 2.0 and 0.95 eV, respectively, which are approximately 1 eV lower than those of analogous perovskites. To our knowledge, compound 2 displays the lowest band gap for any known halide perovskite. Unlike in A^IB^IIX₃ perovskites, the band‐gap transition in A^I₂BB′X₆ double perovskites can show substantial metal‐to‐metal charge‐transfer character. This band‐edge orbital composition is used to achieve small band gaps through the selection of energetically aligned B‐ and B′‐site metal frontier orbitals. Calculations reveal a shallow, symmetry‐forbidden region at the band edges for 1 , which results in long (μs) microwave conductivity lifetimes. We further describe a facile self‐doping reaction in 2 through Br₂ loss at ambient conditions.     

Chemical Origin of the Stability Difference between Copper(I)‐ and Silver(I)‐Based Halide Double Perovskites

Recently, Cu^I‐ and Ag^I‐based halide double perovskites have been proposed as promising candidates for overcoming the toxicity and instability issues inherent within the emerging Pb‐based halide perovskite absorbers. However, up to date, only Ag^I‐based halide double perovskites have been experimentally synthesized; there are no reports on successful synthesis of Cu^I‐based analogues. Here we show that, owing to the much higher energy level for the Cu 3d¹⁰ orbitals than for the Ag 4d¹⁰ orbitals, Cu^I atoms energetically favor 4‐fold coordination, forming [CuX₄] tetrahedra (X=halogen), but not 6‐fold coordination as required for [CuX₆] octahedra. In contrast, Ag^I atoms can have both 6‐ and 4‐fold coordinations. Our density functional theory calculations reveal that the synthesis of Cu^I halide double perovskites may instead lead to non‐perovskites containing [CuX₄] tetrahedra, as confirmed by our material synthesis efforts.     

Directed Electron Delivery from a Pb-Free Halide Perovskite to a Co(II) Molecular Catalyst Boosts CO2 Photoreduction Coupled with Water Oxidation

The development of high-performance photocatalytic systems for CO₂ reduction is appealing to address energy and environmental issues, while it is challenging to avoid using toxic metals and organic sacrificial reagents. We here immobilize a family of cobalt phthalocyanine catalysts on Pb-free halide perovskite Cs₂AgBiBr₆ nanosheets with delicate control on the anchors of the cobalt catalysts. Among them, the molecular hybrid photocatalyst assembled by carboxyl anchors achieves the optimal performance with an electron consumption rate of 300±13 μmol g⁻¹ h⁻¹ for visible-light-driven CO₂-to-CO conversion coupled with water oxidation to O₂, over 8 times of the unmodified Cs₂AgBiBr₆ (36±8 μmol g⁻¹ h⁻¹), also far surpassing the documented systems (<150 μmol g⁻¹ h⁻¹). Besides the improved intrinsic activity, electrochemical, computational, ex-/in situ X-ray photoelectron and X-ray absorption spectroscopic results indicate that the electrons photogenerated at the Bi atoms of Cs₂AgBiBr₆ can be directionally transferred to the cobalt catalyst via the carboxyl anchors which strongly bind to the Bi atoms, substantially facilitating the interfacial electron transfer kinetics and thereby the photocatalysis.     

Room‐Temperature Ferroelectric Material Composed of a Two‐Dimensional Metal Halide Double Perovskite for X‐ray Detection

Although two‐dimensional (2D) metal–halide double perovskites display versatile physical properties due to their huge structural compatibility, room‐temperature ferroelectric behavior has not yet been reported for this fascinating family. Here, we designed a room‐temperature ferroelectric material composed of 2D halide double perovskites, (chloropropylammonium)₄AgBiBr₈, using an organic asymmetric dipolar ligand. It exhibits concrete ferroelectricity, including a Curie temperature of 305 K and a notable spontaneous polarization of ≈3.2 μC cm^?2, triggered by dynamic ordering of the organic cation and the tilting motion of heterometallic AgBr₆/BiBr₆ octahedra. Besides, the alternating array of inorganic perovskite sheets and organic cations endows large mobility‐lifetime product (μτ=1.0×10^?3 cm² V^?1) for detecting X‐ray photons, which is almost tenfold higher than that of CH₃NH₃PbI₃ wafers. As far as we know, this is the first study on an X‐ray‐sensitive ferroelectric material composed of 2D halide double perovskites. Our findings afford a promising platform for exploring new ferroelectric materials toward further device applications.     

Pressure‐Induced Emission (PIE) and Phase Transition of a Two‐dimensional Halide Double Perovskite (BA)4AgBiBr8 (BA=CH3(CH2)3NH3+)

Two‐dimensional (2D) halide perovskites have attracted significant attention due to their compositional flexibility and electronic diversity. Understanding the structure–property relationships in 2D double perovskites is essential for their development for optoelectronic applications. In this work, we observed the emergence of pressure‐induced emission (PIE) at 2.5 GPa with a broad emission band and large Stokes shift from initially nonfluorescent (BA)₄AgBiBr₈ (BA=CH₃(CH₂)₃NH₃⁺). The emission intensity increased significantly upon further compression up to 8.2 GPa. Moreover, the band gap narrowed from the starting 2.61 eV to 2.19 eV at 25.0 GPa accompanied by a color change from light yellow to dark yellow. Analysis of combined in situ high‐pressure photoluminescence, absorption, and angle‐dispersive X‐ray diffraction data indicates that the observed PIE can be attributed to the emission from self‐trapped excitons. This coincides with [AgBr₆]^5? and [BiBr₆]^3? inter‐octahedral tilting which cause a structural phase transition. High‐pressure study on (BA)₄AgBiBr₈ sheds light on the relationship between the structure and optical properties that may improve the material's potential applications in the fields of pressure sensing, information storage and trademark security.     

Lead‐Free Sodium–Indium Double Perovskite Nanocrystals through Doping Silver Cations for Bright Yellow Emission

Lead‐free halide perovskite nanocrystals (NCs) have drawn wide attention for solving the problem of lead perovskites toxicity and instability. Herein, we synthesize the direct band gap double perovskites undoped and Ag‐doped Cs₂NaInCl₆ NCs by variable temperature hot injection. The Cs₂NaInCl₆ NCs have little photoluminescence because of dark self‐trapped excitons (STEs). The dark STEs can be converted into bright STEs by doping with Ag⁺ to produce a bright yellow emission, with the highest photoluminescence quantum efficiency of 31.1 %. The dark STEs has been directly detected experimentally by ultrafast transient absorption (TA) techniques. The dynamics mechanism is further studied. In addition, the Ag‐doped NCs show better stability than the undoped ones. This result provides a new way to enhance the optical properties of lead‐free perovskites NCs for high‐performance light emitters.     

Room-Temperature Ferroelectric Material Composed of a Two-Dimensional Metal Halide Double Perovskite for X-ray Detection

Although two-dimensional (2D) metal–halide double perovskites display versatile physical properties due to their huge structural compatibility, room-temperature ferroelectric behavior has not yet been reported for this fascinating family. Here, we designed a room-temperature ferroelectric material composed of 2D halide double perovskites, (chloropropylammonium)₄AgBiBr₈, using an organic asymmetric dipolar ligand. It exhibits concrete ferroelectricity, including a Curie temperature of 305 K and a notable spontaneous polarization of ≈3.2 μC cm⁻², triggered by dynamic ordering of the organic cation and the tilting motion of heterometallic AgBr₆/BiBr₆ octahedra. Besides, the alternating array of inorganic perovskite sheets and organic cations endows large mobility-lifetime product (μτ=1.0×10⁻³ cm² V⁻¹) for detecting X-ray photons, which is almost tenfold higher than that of CH₃NH₃PbI₃ wafers. As far as we know, this is the first study on an X-ray-sensitive ferroelectric material composed of 2D halide double perovskites. Our findings afford a promising platform for exploring new ferroelectric materials toward further device applications.     

The atomic-level structure of bandgap engineered double perovskite alloys Cs2AgIn1−xFexCl6

Although lead-free halide double perovskites are considered as promising alternatives to lead halide perovskites for optoelectronic applications, state-of-the-art double perovskites are limited by their large bandgap. The doping/alloying strategy, key to bandgap engineering in traditional semiconductors, has also been employed to tune the bandgap of halide double perovskites. However, this strategy has yet to generate new double perovskites with suitable bandgaps for practical applications, partially due to the lack of fundamental understanding of how the doping/alloying affects the atomic-level structure. Here, we take the benchmark double perovskite Cs₂AgInCl₆ as an example to reveal the atomic-level structure of double perovskite alloys (DPAs) Cs₂AgIn_1−xFe_xCl₆ (x = 0–1) by employing solid-state nuclear magnetic resonance (ssNMR). The presence of paramagnetic alloying ions (e.g. Fe³⁺ in this case) in double perovskites makes it possible to investigate the nuclear relaxation times, providing a straightforward approach to understand the distribution of paramagnetic alloying ions. Our results indicate that paramagnetic Fe³⁺ replaces diamagnetic In³⁺ in the Cs₂AgInCl₆ lattice with the formation of [FeCl₆]³⁻·[AgCl₆]⁵⁻ domains, which show different sizes and distribution modes in different alloying ratios. This work provides new insights into the atomic-level structure of bandgap engineered DPAs, which is of critical significance in developing efficient optoelectronic/spintronic devices.

Through Fe³⁺-alloying, the bandgap of benchmark double perovskite Cs₂AgInCl₆ can be tuned from 2.8 eV to 1.6 eV. The atomic-level structure of Cs₂AgIn_1−xFe_xCl₆ was revealed by solid-state nuclear magnetic resonance (ssNMR).     

Experimental and Theoretical Investigation of the Structural and Opto-electronic Properties of Fe-Doped Lead-Free Cs2AgBiCl6 Double Perovskite

Lead-free double perovskites have emerged as stable and non-toxic alternatives to Pb-halide perovskites. Herein, the synthesis of Fe-doped Cs₂AgBiCl₆ lead-free double perovskites are reported that display blue emission using an antisolvent method. The crystal structure, morphology, optical properties, band structure, and stability of the Fe-doped double perovskites were investigated systematically. Formation of the Fe-doped Cs₂AgBiCl₆ double perovskite is confirmed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis. XRD and thermo-gravimetric analysis (TGA) shows that the Cs₂AgBiCl₆ double perovskite has high structural and thermal stability, respectively. Field emission scanning electron microscopy (FE-SEM) analysis revealed the formation of dipyramidal shape Cs₂AgBiCl₆ crystals. Furthermore, energy-dispersive X-ray spectroscopy (EDS) mapping shows the overlapping of Cs, Bi, Ag, Fe, and Cl elements and homogenous incorporation of Fe in Cs₂AgBiCl₆ double perovskite. The Fe-doped Cs₂AgBiCl₆ double perovskite shows a strong absorption at 380 nm. It extends up to 700 nm, suggesting that sub-band gap states transition may originate from the surface defect of the doped perovskite material. The radiative kinetics of the crystals was studied using the time-correlated single-photon counting (TCSPC) technique. Lattice parameters and band gap value of the Fe-doped Cs₂AgBiCl₆ double perovskites predicted by the density functional theory (DFT) calculations are confirmed by XRD and UV/Visible spectroscopy analysis. Time-dependent photo-response characteristics of the Fe-doped Cs₂AgBiCl₆ double perovskite show fast response and recovery time of charge carriers. We believe that the successful incorporation of Fe in lead-free, environmentally friendly Cs₂AgBiCl₆ double perovskite can open a new class of doped double perovskites with significant potential optoelectronics devices fabrication and photocatalytic applications.     

环境友好型无铅卤化物钙钛矿太阳能电池研究进展

ABX_3(A为甲胺、甲脒等有机离子或铯离子,B为铅或锡等金属离子,X为溴、碘等卤化物离子)卤化物钙钛矿材料具有优异的光电特性,是当前太阳能电池研究的前沿和热点之一。然而,这类太阳能电池普遍面临含毒性元素铅和稳定性差等问题,极大地阻碍了钙钛矿太阳能电池商业化应用进程。因此,发展新型高效无铅钙钛矿太阳能电池势在必行。本文评述了环境友好型无铅卤化物钙钛矿太阳能电池的最新研究动态和进展,探讨了该类太阳能电池的制备、性能及其稳定性等问题,展望了其未来发展趋势。     

High Compression‐Induced Conductivity in a Layered Cu–Br Perovskite

We show that the onset pressure for appreciable conductivity in layered copper‐halide perovskites can decrease by ca. 50 GPa upon replacement of Cl with Br. Layered Cu–Cl perovskites require pressures >50 GPa to show a conductivity of 10^?4 S cm^?1, whereas here a Cu–Br congener, (EA)₂CuBr₄ (EA=ethylammonium), exhibits conductivity as high as 2×10^?3 S cm^?1 at only 2.6 GPa, and 0.17 S cm^?1 at 59 GPa. Substitution of higher‐energy Br 4p for Cl 3p orbitals lowers the charge‐transfer band gap of the perovskite by 0.9 eV. This 1.7 eV band gap decreases to 0.3 eV at 65 GPa. High‐pressure X‐ray diffraction, optical absorption, and transport measurements, and density functional theory calculations allow us to track compression‐induced structural and electronic changes. The notable enhancement of the Br perovskite's electronic response to pressure may be attributed to more diffuse Br valence orbitals relative to Cl orbitals. This work brings the compression‐induced conductivity of Cu‐halide perovskites to more technologically accessible pressures.     

A Review on Cs-Based Pb-Free Double Halide Perovskites: From Theoretical and Experimental Studies to Doping and Applications

Despite the progressive enhancement in the flexibility of Pb-based perovskites for optoelectronic applications, regrettably, they are facing two main challenges; (1) instability, which originates from using organic components in the perovskite structure, and (2) toxicity due to Pb. Therefore, new, stable non-toxic perovskite materials are demanded to overcome these drawbacks. The research community has been working on a wide variety of Pb-free perovskites with different molecular formulas and dimensionality. A variety of Pb-free halide double perovskites have been widely explored by different research groups in search for stable, non-toxic double perovskite material. Especially, Cs-based Pb-free halide double perovskite has been in focus recently. Herein, we present a review of theoretical and experimental research on Cs-based Pb-free double halide perovskites of structural formulas Cs₂M⁺M³⁺X6 (M⁺ = Ag⁺, Na⁺, In⁺ etc.; M³⁺= Bi³⁺, In³⁺, Sb³⁺; X = Cl⁻, Br⁻, I¯) and Cs₂M⁴⁺X₆ (M⁴⁺ = Ti⁴⁺, Sn⁴⁺, Au⁴⁺ etc.). We also present the challenges faced by these perovskite compounds and their current applications especially in photovoltaics alongside the effect of metal dopants on their performance.