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.     

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.     

Exploring Lead‐Free Hybrid Double Perovskite Crystals of (BA)2CsAgBiBr7 with Large Mobility‐Lifetime Product toward X‐Ray Detection

Halide double perovskites have recently bloomed as the green candidates for optoelectronic applications, such as X‐ray detection. Despite great efforts, the exploration of promising organic–inorganic hybrid double perovskites toward X‐ray detection remains unsuccessful. Now, single crystals of the lead‐free hybrid double perovskite, (BA)₂CsAgBiBr₇ (BA⁺ is n‐butylammonium), featuring the unique 2D multilayered quantum‐confined motif, enable quite large μτ (mobility‐lifetime) product up to 1.21×10^?3 cm² V^?1. This figure‐of‐merit realized in 2D hybrid double perovskites is unprecedented and comparable with that of CH₃NH₃PbI₃ wafers. (BA)₂CsAgBiBr₇ crystals also exhibit other intriguing attributes for X‐ray detection, including high bulk resistivity, low density of defects and traps, and large X‐ray attenuation coefficient. Consequently, a vertical‐structure crystal device under X‐ray source yields a superior sensitivity of 4.2 μC Gy_air^?1 cm^?2.     

(CH3NH3)2PdCl4: A Compound with Two‐Dimensional Organic–Inorganic Layered Perovskite Structure

The synthesis of previously unknown perovskite (CH₃NH₃)₂PdCl₄ is reported. Despite using an organic cation with the smallest possible alkyl group, a 2D organic–inorganic layered Pd‐based perovskites was still formed. This demonstrates that Pd‐based 2D perovskites can be obtained even if the size of the organic cation is below the size limit predicted by the Goldschmidt tolerance‐factor formula. The (CH₃NH₃)₂PdCl₄ phase has a bulk resistivity of 1.4 Ω cm, a direct optical gap of 2.22 eV, and an absorption coefficient on the order of 10⁴ cm^?1. XRD measurements suggest that the compound is moderately stable in air, an important advantage over several existing organic–inorganic perovskites that are prone to phase degradation problems when exposed to the atmosphere. Given the recent interest in organic–inorganic perovskites, the synthesis of this new Pd‐based organic–inorganic perovskite may be helpful in the preparation and understanding of other organic–inorganic perovskites.     

Lead‐ and Iodide‐Deficient (CH3NH3)PbI3 (d‐MAPI): The Bridge between 2D and 3D Hybrid Perovskites

3D and 2D hybrid perovskites, which have been known for more than 20 years, have emerged recently as promising materials for optoelectronic applications, particularly the 3D compound (CH₃NH₃)PbI₃ (MAPI). The discovery of a new family of hybrid perovskites called d ‐MAPI is reported: the association of PbI₂ with both methyl ammonium (MA⁺) and hydroxyethyl ammonium (HEA⁺) cations leads to a series of five compounds with general formulation (MA)_1−2.48x(HEA)_3.48x[Pb_1−xI_3−x]. These materials, which are lead‐ and iodide‐deficient compared to MAPI while retaining 3D architecture, can be considered as a bridge between the 2D and 3D materials. Moreover, they can be prepared as crystallized thin films by spin‐coating. These new 3D materials appear very promising for optoelectronic applications, not only because of their reduced lead content, but also in account of the large flexibility of their chemical composition through potential substitutions of MA⁺, HEA⁺, Pb²⁺ and I⁻ ions.     

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.     

Revealing the Mechanism of Pressure-Induced Emission in Layered Silver-Bismuth Double Perovskites

Pressure-induced emission (PIE) associated with self-trapping excitons (STEs) in low-dimensional halide perovskites has attracted great attention for better materials-by-design. Here, using 2D layered double perovskite (C₆H₅CH₂CH₂NH₃⁺)₄AgBiBr₈ as a model system, we advance a fundamental physicochemical mechanism of the PIE from the perspective of carrier dynamics and excited-state behaviors of local lattice distortion. We observed a pressure-driven STE transformation from dark to bright states, corresponding a strong broadband Stokes-shifted emission. Further theoretical analysis demonstrated that the suppressed lattice distortion and enhanced electronic dimensionality in the excited-state play an important role in the formation of stabilized bright STEs, which could manipulate the self-trapping energy and lattice deformation energy to form an energy barrier between the potential energy curves of ground- and excited-state, and enhance the electron-hole orbital overlap, respectively.     

Attempted Abstraction of the Halogenides in (HNEt3)[Re(CH3CN)2Cl4] and Crystal Structures of cis‐[Re(CH3CN)2Cl4]·CH3CN and cis‐[Re(NHC(OCH3)CH3)2Cl4]

The abstraction of the halogenide ligands in [Re(CH₃CN)₂Cl₄]^? should result in a solvent‐only stabilized Re^III complex. The reactions of salts of [Re(CH₃CN)₂Cl₄]^? with silver(I) and thallium(I) salts were investigated and the solid‐state structures of cis‐[Re(CH₃CN)₂Cl₄]·CH₃CN and cis‐[Re(NHC(OCH₃)CH₃)₂Cl₄] are described.     

A two‐dimensional organic–inorganic hybrid perovskite‐type semiconductor: poly[(2‐azaniumylethyl)trimethylphosphanium [tetra‐μ‐bromido‐plumbate(II)]]

Recently, with the prevalence of `perovskite fever', organic–inorganic hybrid perovskites (OHPs) have attracted intense attention due to their remarkable structural variability and highly tunable properties. In particular, the optical and electrical properties of organic–inorganic hybrid lead halides are typical of the OHP family. Besides, although three‐dimensional hybrid perovskites, such as [CH₃NH₃]PbX₃ (X = Cl, Br or I), have been reported, the development of new organic–inorganic hybrid semiconductors is still an area in urgent need of exploration. Here, an organic–inorganic hybrid lead halide perovskite is reported, namely poly[(2‐azaniumylethyl)trimethylphosphanium [tetra‐μ‐bromido‐plumbate(II)]], {(C₅H₁₆NP)[PbBr₄]}_n, in which an organic cation is embedded in inorganic two‐dimensional (2D) mesh layers to produce a sandwich structure. This unique sandwich 2D hybrid perovskite material shows an indirect band gap of ～2.700 eV. The properties of this compound as a semiconductor are demonstrated by a series of optical characterizations and indicate potential applications for optical devices.     

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‐Suppressed Carrier Trapping Leads to Enhanced Emission in Two‐Dimensional Perovskite (HA)2(GA)Pb2I7

A remarkable PL enhancement by 12 fold is achieved using pressure to modulate the structure of a recently developed 2D perovskite (HA)₂(GA)Pb₂I₇ (HA=n‐hexylammonium, GA=guanidinium). This structure features a previously unattainable, extremely large cage. In situ structural, spectroscopic, and theoretical analyses reveal that lattice compression under a mild pressure within 1.6 GPa considerably suppresses the carrier trapping, leading to significantly enhanced emission. Further pressurization induces a non‐luminescent amorphous yellow phase, which is retained and exhibits a continuously increasing band gap during decompression. When the pressure is released to 1.5 GPa, emission can be triggered by above‐band gap laser irradiation, accompanied by a color change from yellow to orange. The obtained orange phase could be retained at ambient conditions and exhibits two‐fold higher PL emission compared with the pristine (HA)₂(GA)Pb₂I₇.     

The Crystal Structures of Trimethylphosphane supported Nickel‐ and Cobalt‐Methyl Complexes: Octahedral mer‐cis‐[CoIIII(CH3)2(PMe3)3] and Square Planar trans‐[NiIICl(CH3)(PMe3)2]

Single crystals of octahedral mer‐cis‐[Co^IIII(CH₃)₂(PMe₃)₃] ( 1 ) and square planar trans‐[Ni^IICl(CH₃)(PMe₃)₂] ( 2 ), were obtained from solvent mixtures (methylcylohexane / pentane 1:1) and have been analyzed by X‐ray crystallography for the first time.     

Mosaic CuI−CuII−InIII 2D Perovskites: Pressure-Dependence of the Intervalence Charge Transfer and a Mechanochemical Alloying Method

The perovskite (BA)₄[Cu^II(Cu^IIn^III)_0.5]Cl₈ ( 1_BA ; BA⁺=butylammonium) allows us to study the high-pressure structural, optical, and transport properties of a mixed-valence 2D perovskite. Compressing 1_BA reduces the onset energy of Cu^I/II intervalence charge transfer from 1.2 eV at ambient pressure to 0.2 eV at 21 GPa. The electronic conductivity of 1_BA increases by 4 orders of magnitude upon compression to 20 GPa, when the activation energy for conduction decreases to 0.16 eV. In contrast, Cu^II perovskites achieve similar conductivity at ≈50 GPa. The solution-state synthesis of these perovskites is complicated, with more undesirable side products likely from the precursor mixtures containing three different metal ions. To circumvent this problem, we demonstrate an efficient mechanochemical synthesis to expand this family of halide perovskites with complex composition by simply pulverizing together powders of 2D Cu^II single perovskites and Cu^IIn^III double perovskites.     

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.     

Synthese,NMR‐Spektren und Molekülstruktur von [(CH3)2Ga{μ‐P(H)Si(CH3)3}2Ga(CH3)2{μ‐P(Si(CH3)3)2}Ga(CH3)2]

Synthesis, NMR Spectra and Structure of [(CH₃)₂Ga{μ‐P(H)Si(CH₃)₃}₂Ga(CH₃)₂{μ‐P(Si(CH₃)₃)₂}Ga(CH₃)₂] The title compound has been prepared in good yield by the reaction of [Me₂GaOMe]₃ (Me = CH₃) with HP(SiMe₃)₂ in toluene (ratio 1 : 1,1) and purified by crystallization from pentane or toluene, respectively. This organogallium compound forms (Ga–P)₃ ring skeletons with one Ga–P(SiMe₃)₂–Ga and two Ga–P(H)SiMe₃–Ga bridges and crystallizes in the monoclinic space group C2/c. The known homologous Al‐compound is isotypic, both (M^III–P)₃ heterocycles have twist‐conformations, the ligands of the monophosphane bridges have trans arrangements.     

Tailored Engineering of an Unusual (C4H9NH3)2(CH3NH3)2Pb3Br10 Two‐Dimensional Multilayered Perovskite Ferroelectric for a High‐Performance Photodetector

Two‐dimensional (2D) layered hybrid perovskites have shown great potential in optoelectronics, owing to their unique physical attributes. However, 2D hybrid perovskite ferroelectrics remain rare. The first hybrid ferroelectric with unusual 2D multilayered perovskite framework, (C₄H₉NH₃)₂(CH₃NH₃)₂Pb₃Br₁₀ ( 1 ), has been constructed by tailored alloying of the mixed organic cations into 3D prototype of CH₃NH₃PbBr₃. Ferroelectricity is created through molecular reorientation and synergic ordering of organic moieties, which are unprecedented for the known 2D multilayered hybrid perovskites. Single‐crystal photodetectors of 1 exhibit fascinating performances, including extremely low dark currents (ca. 10⁻¹² A), large on/off current ratios (ca. 2.5×10³), and very fast response rate (ca. 150 μs). These merits are superior to integrated detectors of other 2D perovskites, and compete with the most active CH₃NH₃PbI₃.     

An A‐Site Mixed‐Ammonium Solid Solution Perovskite Series of [(NH2NH3)x(CH3NH3)1−x][Mn(HCOO)3] (x=1.00–0.67)

The A‐site mixed‐ammonium solid solutions of metal–organic perovskites [(NH₂NH₃)_x(CH₃NH₃)_1?x][Mn(HCOO)₃] (x=1.00–0.67) exhibit para‐ to ferroelectric diffuse phase transitions with lowered transition temperatures from x=1.00 to 0.67. These properties are due to the decreased framework distortion and polarization in their low temperature ferroelectric phases caused by the increased CH₃NH₃⁺ concentration.     

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.     

Heterobimetallische Komplexe von Lithium,Aluminium und Gold mit dem N‐[2‐N′,N′‐(Dimethylaminoethyl)‐N‐methyl‐aminoethyl]‐ferrocenyl‐Liganden (η5‐C5H5)Fe{η5‐C5H3[CH(CH3)N(CH3)CH2CH2NMe2]‐2}

Heterobimetallic Complexes of Lithium, Aluminum, and Gold with the N ‐[2‐ N ′, N ′‐(dimethylaminoethyl)‐ N ‐methyl‐aminoethyl]‐ferrocenyl Ligand (η⁵‐C₅H₅)Fe{η⁵‐C₅H₃[CH(CH₃)N(CH₃)CH₂CH₂NMe₂]‐2} N‐[2‐N′,N′‐(dimethylaminoethyl)‐N‐methyl‐aminoethyl]ferrocene FcN,NH ( 1 ) reacts with ⁿBuLi under formation of the lithium organyl (FcN,N)Li ( 2 ). At reactions of 2 with AlBr₃ and AuCl · PPh₃ the heterobimetallic organo derivatives (FcN,N)AlBr₂ ( 3 ), (FcN,N)Au · PPh₃ ( 4 ) are formed. A detailed characterization of 2 – 4 was carried out by single crystal x‐ray analyses as well as by NMR and Mößbauer spectroscopy.