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.     

Synthese,Kristallstruktur und elektronische Bandstruktur der isotypen Sulfidchloride CuBiSCl2 und AgBiSCl2

Synthesis, Crystal Structure and Electronic Band Structure of the Isostructural Sulfide Chlorides CuBiSCl₂ and AgBiSCl₂ Black, platelet crystals of MBiSCl₂ (M = Cu, Ag) were synthesized by heating stoichiometric amounts of MCl and BiSCl. X‐ray diffraction on powders and single‐crystals revealed that the compounds crystallize isostructural with space group Cmcm (Cu/Ag: a = 396.9(1)/397.1(1); b = 1278.6(3)/1371.2(3); c = 859.7(2)/882.4(2) pm). In the crystal structures the Bi^III cations have a bi‐capped trigonal prismatic coordination of two sulfide and six chloride ions. The polyhedra constitute layers equation/tex2gif-stack-2.gif[BiS_2/2Cl_6/3^—] by sharing common edges and faces. The layers are separated by transition metal cations M^I. Their distorted octahedral coordination comprises two short bonds M—S in trans position and four M—Cl bonds that are about 50 pm longer. From relativistic band structure calculations (DFT) is deduced that AgBiSCl₂ is an indirect semi‐conductor with a band gap of about 1.0 eV.     

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.     

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.     

Natural monoclinic AgPb(Bi2Sb)3S6, an Sb‐rich gustavite

The crystal structure of the Sb‐rich variety of the mineral gustavite, silver lead tris(dibismuth/antimony) hexasulfide, AgPb(Bi₂Sb)₃S₆, consists of blocks of diagonal chains of four octahedra, viz. M1a (Bi), M2a (Sb/Bi), M2b (Bi/Sb) and M1b (Ag), separated by Pb atoms in a trigonal prismatic coordination. Two marginal octahedral sites, M1a and M1b, where the gustavite substitution Ag⁺ + Bi³⁺ = 2Pb²⁺ takes place, are formed by Bi and Ag, respectively. Two central octahedra, M2a and M2b, where the Bi³⁺ = Sb³⁺ substitution takes place, are formed by two mixed Bi/Sb sites with different occupancies of Bi and Sb. The alternating occupation of the M1 site by Bi and Ag atoms (which thus creates two distinct sites M1a and M1b) results in the monoclinic space group P2₁/c. A statistical distribution of Ag/Bi in the M1 position (one mixed Ag/Bi site) was reported for synthetic gustavite, resulting in the orthorhombic space group Cmcm.     

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.     

(H3O)3Sb2Br9: the first member of the M3E2X9 structure family with oxonium cations

(H₃O)₃Sb₂Br₉ [trihydroxonium enneabromidodiantimonate(III)] is the first representative of the M₃E₂X₉ family (M = cation, E = Sb and Bi, and X = Br and I) with oxonium cations. The metastable compound was obtained in trace amounts from a solution of CsBr and SbBr₃ in concentrated aqueous HBr. Single crystals were isolated from the mother liquor and investigated by single‐crystal X‐ray diffraction at 100 K. (H₃O)₃Sb₂Br₉ crystallizes with the Tl₃Bi₂I₉ structure type, which is a distorted defect variant of cubic perovskite. The crystal structure comprises characteristic ²_∞[SbBr₃Br_3/2] double layers of corner‐sharing SbBr₆ octahedra with a [001] stacking direction. Due to the small size of the H₃O⁺ cation and O—H…Br hydrogen bonding, the octahedra are tilted.     

Colloidal Synthesis and Charge‐Carrier Dynamics of Cs2AgSb1−yBiyX6 (X: Br,Cl; 0 ≤y ≤1) Double Perovskite Nanocrystals

A series of lead‐free double perovskite nanocrystals (NCs) Cs₂AgSb_1?yBi_yX₆ (X: Br, Cl; 0≤y≤1) is synthesized. In particular, the Cs₂AgSbBr₆ NCs is a new double perovskite material that has not been reported for the bulk form. Mixed Ag–Sb/Bi NCs exhibit enhanced stability in colloidal solution compared to Ag–Bi or Ag–Sb NCs. Femtosecond transient absorption studies indicate the presence of two prominent fast trapping processes in the charge‐carrier relaxation. The two fast trapping processes are dominated by intrinsic self‐trapping (ca. 1–2 ps) arising from giant exciton–phonon coupling and surface‐defect trapping (ca. 50–100 ps). Slow hot‐carrier relaxation is observed at high pump fluence, and the possible mechanisms for the slow hot‐carrier relaxation are also discussed.     

Colloidal Synthesis and Charge‐Carrier Dynamics of Cs2AgSb1−yBiyX6 (X: Br,Cl; 0 ≤y ≤1) Double Perovskite Nanocrystals

A series of lead‐free double perovskite nanocrystals (NCs) Cs₂AgSb_1?yBi_yX₆ (X: Br, Cl; 0≤y≤1) is synthesized. In particular, the Cs₂AgSbBr₆ NCs is a new double perovskite material that has not been reported for the bulk form. Mixed Ag–Sb/Bi NCs exhibit enhanced stability in colloidal solution compared to Ag–Bi or Ag–Sb NCs. Femtosecond transient absorption studies indicate the presence of two prominent fast trapping processes in the charge‐carrier relaxation. The two fast trapping processes are dominated by intrinsic self‐trapping (ca. 1–2 ps) arising from giant exciton–phonon coupling and surface‐defect trapping (ca. 50–100 ps). Slow hot‐carrier relaxation is observed at high pump fluence, and the possible mechanisms for the slow hot‐carrier relaxation are also discussed.     

The luminescence of Cs3Bi2Cl9 and Cs3Sb2Cl9

The luminescence properties of Cs₃Bi₂Cl₉, α-Cs₃Sb₂Cl₉, and β-Cs₃Sb₂Cl₉ are reported and compared with those of Cs₃Bi₂Br₉. The first two compounds have comparable luminescence properties which can be described in terms of a band model. Deep center emission is observed for both compounds, whereas edge emission is observed only for Cs₃Bi₂Cl₉. The optical transitions of β-Cs₃Sb₂Cl₉ are localized on the Sb³⁺ ion. The orientation of the lone-pair orbitals of the ns² ions seems to play an important role in the formation of the cationic valence band. The α-β transformation must therefore have a considerable influence on the spectral properties of Cs₃Sb₂Cl₉.     

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.     

Ag3Bi14Br21: ein Subbromid mit Bi24+‐Hanteln und Bi95+‐Polyedern – Synthese,Kristallstruktur und Chemische Bindung

Ag₃Bi₁₄Br₂₁: a Subbromide with Bi₂⁴⁺ Dumbbells and Bi₉⁵⁺ Polyhedra – Synthesis, Crystal Structure and Chemical Bonding Black crystals of Ag₃Bi₁₄Br₂₁ = (Bi₉⁵⁺)[Ag₃Bi₃Br₁₅^3?](Bi₂Br₆^2?), the first argentiferous bismuth subhalide, were obtained from a stoichiometric melt of Ag, Bi, and BiBr₃. The compound crystallizes in the monoclinic space group P2₁/m with lattice parameters a = 1277.78(5) pm, b = 1466.87(6) pm, c = 1342.62(5) pm, and β = 108.47(1)° at 110(5) K. In contrast to all other bismuth subhalides that contain an electron‐rich transition metal, the silver atoms are not bonded to bismuth atoms. Instead they are integrated into the anionic bromometallate network, which consists of [MBr₆]‐octahedra (M = Ag, Bi) that share edges and vertices. These corrugated sheets alternate with tessellated layers formed by Bi₉⁵⁺ polycations and hitherto unknown (Bi^II₂Br₆)^2? groups. The latter anions contain Bi₂⁴⁺ dumbbells (299 pm) and can be represented by the structured formula [Br₂Bi^II(μ–Br)₂Bi^IIBr₂]^2?. The multi‐center bonding within the Bi₉⁵⁺ cluster and the bent single‐bond in the Bi₂ dumbbell can be visualized using the electron localization indicator (ELI‐D).     

Enhanced Photocatalytic Activity of MIL‐125 by Post‐Synthetic Modification with CrIII and Ag Nanoparticles

NH₂‐MIL‐125, [Ti₈O₈(OH)₄(bdc‐NH₂)₆] (bdc^2?=1,4‐benzene dicarboxylate) is a highly porous metal–organic framework (MOF) that has a band gap lying within the ultraviolet region at about 2.6 eV. The band gap may be reduced by a suitable post‐synthetic modification of the nanochannels using conventional organic chemistry methods. Here, it is shown that the photocatalytic activity of NH₂‐MIL‐125 in the degradation of methylene blue under visible light is remarkably augmented by post‐synthetic modification with acetylacetone followed by Cr^III complexation. The latter metal ion extends the absorption from the ultraviolet to the visible light region (band gap 2.21 eV). The photogenerated holes migrate from the MOF’s valence band to the Cr^III valence band, promoting the separation of holes and electrons and increasing the recombination time. Moreover, it is shown that the MOF’s photocatalytic activity is also much improved by doping with Ag nanoparticles, formed in situ by the reduction of Ag⁺ with the acetylacetonate pendant groups (the resulting MOF band gap is 2.09 eV). Presumably, the Ag nanoparticles are able to accept the MOF’s photogenerated electrons, thus avoiding electron–hole recombination. Both, the Cr‐ and Ag‐bearing materials are stable under photocatalytic conditions. These findings open new avenues for improving the photocatalytic activity of MOFs.     

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.     

Structural Variety of Defect Perovskite Variants M3E2X9 (M = Rb,Tl, E = Bi,Sb, X = Br,I)

The compounds Rb₃Sb₂Br₉, Rb₃Sb₂I₉, Rb₃Bi₂Br₉, Rb₃Bi₂I₉, and Tl₃Bi₂Br₉ were synthesized and their crystal structures determined from single crystal X‐ray diffraction data. The compounds Rb₃Sb₂Br₉, Rb₃Sb₂I₉, and Rb₃Bi₂I₉ crystallize in the Tl₃Bi₂I₉ type of structure (space group P2₁/n, no. 14). Rb₃Bi₂Br₉ and Tl₃Bi₂Br₉ crystallize in a new but closely related type of structure (space group P2₁/a, no. 14). Both structure types feature characteristic double layers comprising corner‐sharing EX₆ octahedra. The space groups are set in a way that the stacking direction of the layers is the [001] direction. The group‐subgroup relations to cubic perovskite ABO₃ are discussed. Differences between M₃E₂X₉ types are attributed to distortions of the underlying MX₃ close packing. Depending on the atomic size ratio, the distortions are quantified by an order parameter.     

Synthesis,Crystal Structure,and Optical Property of the Quaternary Semiconductor La4Cd4In2S13

The sulfide La₄Cd₄In₂S₁₃ in the quaternary RE/M/N/Q (RE = rare‐earth metal; M = Zn, Cd; N = Ga, In; Q = chalcogenides) system was prepared from stoichiometric mixtures of the elements by solid‐state reactions at 1223 K in an evacuated silica tube. The La₄Cd₄In₂S₁₃, crystallizing in the Pbam space group, is isostructural with Pb₄Bi₄In₂S₁₃. The structure of La₄Cd₄In₂S₁₃ consists of double chains of CdS₆ octahedra extending along [001] direction that are interconnected by single chains of InS₆ octahedra by edge‐sharing into 1D ribbons. These ribbons are further fused by the infinite one‐dimensional chains of InS₄ tetrahedra by corner‐sharing into Z‐shaped [Cd₄In₂S₁₃]^12– layers perpendicular to the b direction separated by the La³⁺ ions. UV/Vis/NIR diffuse reflectance spectroscopy study shows its optical gap of around 2.27 eV.     

Wide-coverage and Efficient NIR Emission from Single-component Nanophosphors through Shaping Multiple Metal-halide Packages

Wide-coverage near infrared (NIR) phosphor-converted LEDs possess promising potential for practical applications, but little is developed towards the efficient and wide-coverage NIR phosphors. Here, we report the single-component lanthanide (Ln³⁺) ions doped Cs₂M(In_0.95Sb_0.05)Cl₆ (M=alkali metal) nanocrystals (NCs), exhibiting emission from 850 to 1650 nm with high photoluminescence quantum yield of 20.3 %, which is accomplished by shaping the multiple metal halide octahedra of double perovskite via the simple alkali metal substitution. From Judd-Ofelt theoretical calculation and spectroscopic investigations, the shaping of metal halide octahedra in Cs₂M(In_1−xSb_x)Cl₆ NCs can break the forbidden of f-f transition of Ln³⁺, thus increasing their radiative transition rates and simultaneously boosting the energy transfer efficiency from host to Ln³⁺. Finally, the wide-coverage NIR LEDs based on Sm³⁺, Nd³⁺, Er³⁺-tridoped Cs₂K_0.5Rb_0.5(In_0.95Sb_0.05)Cl₆ NCs are fabricated and employed in the multiplex gas sensing and night-vision application.     

Syntheses, structures, optical properties, and theoretical calculations of Cs2Bi2ZnS5, Cs2Bi2CdS5, and Cs2Bi2MnS5

Three new compounds, Cs₂Bi₂ZnS₅, Cs₂Bi₂CdS₅, and Cs₂Bi₂MnS₅, have been synthesized from the respective elements and a reactive flux Cs₂S₃ at 973 K. The compounds are isostructural and crystallize in a new structure type in space group Pnma of the orthorhombic system with four formula units in cells of dimensions at 153 K of a=15.763(3), b=4.0965(9), c=18.197(4) Å, V=1175.0(4) Å³ for Cs₂Bi₂ZnS₅; a=15.817(2), b=4.1782(6), c=18.473(3)  Å, V=1220.8(3)  ų for Cs₂Bi₂CdS₅; and a=15.830(2), b=4.1515(5), c=18.372(2) Å, V=1207.4(2) Å³ for Cs₂Bi₂MnS₅. The structure is composed of two-dimensional _∞²[Bi₂MS₅²⁻] (M=Zn, Cd, Mn) layers that stack perpendicular to the [100] axis and are separated by Cs⁺ cations. The layers consist of edge-sharing _∞¹[Bi₂S₆⁶⁻] and _∞¹[MS₃⁴⁻] chains built from BiS₆ octahedral and MS₄ tetrahedral units. Two crystallographically unique Cs atoms are coordinated to S atoms in octahedral and monocapped trigonal prismatic environments. The structure of Cs₂Bi₂MS₅, is related to that of Na₂ZrCu₂S₄ and those of the AMM′Q₃ materials (A=alkali metal, M=rare-earth or Group 4 element, M′= Group 11 or 12 element, Q=chalcogen). First-principles theoretical calculations indicate that Cs₂Bi₂ZnS₅ and Cs₂Bi₂CdS₅ are semiconductors with indirect band gaps of 1.85 and 1.75 eV, respectively. The experimental band gap for Cs₂Bi₂CdS₅ is ≈1.7 eV, as derived from its optical absorption spectrum.     

Homologous Silver Bismuth Chalcogenide Halides (N,x)P. III. The (4, x)P, (5, x)P,and (7, x)P Structure Families of Modular Compounds with Tunable Composition and Structure

The crystal structures of six members of the homologous series with general formula [BiQX]₂[Ag_xBi_1?xQ_2?2xX_2x?1]_N+1 (Q = S, Se; X = Cl, Br; 1/2 ≤ x ≤ 1) and N = 4, 5, or 7 were determined by single‐crystal X‐ray diffraction. The series are characterized by the parameters N and x and are denoted ^{(N, x)}P. Ag₃Bi₄S₆Cl₃ (x = 0.60) (I) , Ag_3.5Bi_3.5S₅Br₄ (x = 0.70) (II) and Ag_3.65Bi_3.35Se_4.70Br_4.30 (x = 0.73) (III) belong to ^{(4, x})P series Ag_5xBi_7?5xQ_12?10xX_10x?3 and adopt the AgBi₆S₉ structure type. The ^{(5, x)}P compound Ag_3.66Bi_4.34S_6.68Br_3.32 (IV) , which corresponds to x = 0.61 in Ag_6xBi_8?6xS_14?12xBr_12x?4, crystallizes isostructurally to AgBi₃S₅. The compounds Ag_4.56Bi_5.44Se_8.88Br_3.12 (x = 0.57) (V) and Ag_5.14Bi_4.86S_7.76Br_4.24 (x = 0.64) (VI) , which are members of ^{(7, x)}P series Ag_8xBi_10?8xQ_18?16xBr_16x?6, adopt the Ag₃Bi₇S₁₂ structure type. In the monoclinic crystal structures (space group C2/m) two kinds of layered modules alternate along [001]. Modules of type A uniformly consist of paired rods of face‐sharing monocapped trigonal prisms around Bi atoms with octahedra around mixed occupied metal positions (M = Ag/Bi) between them. Modules of type B are composed of [MZ₆] octahedra, which are arranged in NaCl‐type fragments of thickness N. All structures exhibit Ag/Bi disorder in octahedrally coordinated metal positions as well as Q/X mixed occupation of some anion positions. Corresponding to their black color, all compounds are narrow‐gap semiconductors (E_g = 0.35 eV for (II) ). General characteristics of the entire class of ^{(N, x)}P compounds are gathered in a catalogue.     

Antimony(V) Bromide and Polybromide Complexes with N‐alkylated Quinolinium or Isoquinolinium Cations: Substituent‐dependent Assembly of Polymeric Frameworks

Reactions of [Sb^IIIBr₆]^3– with Br₂ in HBr in the presence of N‐substituted quinolinium or isoquinolinium cations result in new complexes of hexabromidoantimonates of Sb^V and their polybromide adducts: (N‐MeQuin)₂{[SbBr₆](Br₃)} ( 1 ), (N‐MeIsoquin)₂{[SbBr₆](Br₃)} ( 2 ), and (N‐EtQuin)[SbBr₆] ( 3 ). Thermal stability was studied; estimated energies of supramolecular Br ··· Br interactions were calculated.