(C6H13N)2BiI5: A One-Dimensional Lead-Free Perovskite-Derivative Photoconductive Light Absorber

Structural Phase Transition in 0D (3,5-DMP)2Bi1-xSbxCl5 Metal Halides: Expression of the Lone Pair Effect and Polyhedral Distortion

Low-dimensional Bi/Sb-based organic-inorganic metal halides (OIMHs) have attracted immense attention from the research community because of their structural diversity and efficient luminescence properties. Further understanding of the relationship between the structure and luminescence properties of these materials is of utmost importance for tuning the luminescence properties for various practical applications. Herein, we have synthesized two lead-free Bi/Sb-based novel OIMHs, (3,5-DMP)2BiCl5 and (3,5-DMP)2SbCl5 [(3,5-DMP) = 3,5-dimethylpiperidine], with zero-dimensional (0D) structures and crystallizing in triclinic (P1 ¯ space group) and monoclinic (P21/c space group) crystal systems, respectively. Both the compounds behave as typical semiconductors with indirect optical band gaps of 3.34 and 3.36 eV for pristine Bi and Sb compounds. These compounds exhibit higher environmental and thermal stability at ambient conditions. Gradual substitution of Sb at the Bi site in (3,5-DMP)2Bi1-xSbxCl5 resulted in the introduction of structural strain due to the significant expression of the lone pair effect, thus leading to a structural transition from the triclinic to monoclinic phase. The effect of the structural phase transition on the optical properties is also studied in (3,5-DMP)2Bi1-xSbxCl5. This work may offer new direction and guidance for exploring various 0D hybrid metal halides and tuning the structures for improvement in the luminescence properties.

0-D and 1-D Perovskite-like Hybrid Bismuth(III) Iodides

Low-dimensional hybrid bismuth halide perovskites have recently emerged as a class of non-toxic alternative to lead perovskites with promising optoelectronic properties. Here, we report three hybrid bismuth(III)-iodides: 0-D (H2DAC)2Bi2I10 ⋅ 6H2O (H2DAC_Bi_I), 0-D (H2DAF)4Bi2I10 ⋅ 2I3 ⋅ 2I ⋅ 6H2O (H2DAF_Bi_I), and 1-D (H2DAP)BiI5 (H2DAP_Bi_I) (where H2DAC=trans-1,4-diammoniumcyclohexane; H2DAF=2,7-diammoniumfluorene and H2DAP=1,5-diammoniumpentane). Their synthesis, single-crystal X-ray structures, and photophysical properties are reported. The first two compounds comprise edge-sharing [Bi2I10]4- dimers, while the last compound has cis-corner-sharing 1-D chains of [BiI6]3- octahedra. Intercalation of triiodide (I3 -) and iodide (I-) ions enhance electronic coupling between the [Bi2I10]4- of H2DAF_Bi_I, leading to enhanced optical absorption, compared to H2DAC_Bi_I which lacks such intercalants. Furthermore, calorimetric and variable temperature X-ray diffraction measurements suggest a centrosymmetric to non-centrosymmetric phase transition (monoclinic P212121↔orthorhombic Pnma) of H2DAP_Bi_I at 448 K (in heating step) and at 443 K (in cooling step).

Polymorphism and Red Photoluminescence Emission from 5s2 Electron Pairs of Sb(III) in a New One-Dimensional Organic-Inorganic Hybrid Based on Methylhydrazine: MHy2SbI5

We explore the crystal structure and luminescent properties of a new 1D organic-inorganic hybrid, MHy2SbI5, based on methylhydrazine. The compound reveals the red photoluminescence (PL) originating from the 5s2 electron pairs of Sb(III) as well as complex structural behavior. MHy2SbI5 crystalizes in two polymorphic forms (I and II) with distinct thermal properties and structural characteristics. Polymorph I adopts the acentric P212121 chiral space group confirmed by SHG, and, despite a thermally activated disorder of MHy, does not show any phase transitions, while polymorph II undergoes reversible low-temperature phase transition and high-temperature reconstructive transformation to polymorph I. The crystal structures of both forms consist of 1D perovskite zig-zag chains of corner-sharing SbI6 octahedra. The intriguing phase transition behavior of II is associated with the unstable arrangement of the [SbI5]2-∞ chains in the structure. The energy band gap (Eg) values, estimated based on the UV-Vis absorption spectra, indicate that both polymorphs have band gaps, with Eg values of 2.01 eV for polymorph I and 2.12 eV for polymorph II.

Naphthalenediimide/Iodobismuthate Hybrid Heterostructures: Water Resistance and Long-Lived Charge-Separated States

Organic-inorganic hybrid iodobismuthate perovskites have become promising semiconductive materials for their environmentally friendly and light-harvesting characteristics. However, their low-dimensional bismuth-iodide skeletons result in poor charge-separation efficiency, limiting their application in optoelectronic devices. To address this issue, the donor-acceptor (D-A) heterostructures have been introduced to the iodobismuthate hybrid materials by incorporating an electron-deficient N,N'-bis(4-aminoethyl)-1,4,5,8-naphthalene diimide (NDIEA) as the electron acceptor and organic counterpart. Five naphthalenediimide/iodobismuthate hybrid heterostructures, named (H2NDIEA)1.5·Bi2I9·3DMF (1), H2NDIEA·[Bi2I8(DMF)2]·2DMF (2), (H2NDIEA)2·Bi4I16·2H2O·4MeOH (3), (H2NDIEA)2·Bi4I16·8H2O (4), and [(H2NDIEA)2·Bi6I22]n·4nH2O (5) (DMF = N,N-dimethylformamide), were synthesized. Their crystal structures, water stabilities, charge-separated behaviors, and electrical properties have been studied through experimental and computational investigations. The results revealed that hybrids 3-5 exhibited high water resistance attributed to their tightly packed structures and robust H-bonds between solvent molecules and organic-inorganic supramolecular frameworks. Density functional theory calculations confirmed characteristic type-IIa band alignments of all the five hybrids, facilitating to the photoinduced charge separation. Moreover, the closer contact caused by the strong anion-π interactions between electron donors and acceptors in hybrid 5 leads to the long-lived charge-separated states and improved electrical properties compared to the other hybrids.

Spectroscopic Evidence of Localized Small Polarons in Low-Dimensional Ionic Liquid Lead-Free Hybrid Perovskites

Rational design is an important approach to consider in the development of low-dimensional hybrid organic-inorganic perovskites (HOIPs). In this study, 1-butyl-1-methyl pyrrolidinium (BMP), 1-(3-aminopropyl)imidazole (API), and 1-butyl-3-methyl imidazolium (BMI) serve as prototypical ionic liquid components in bismuth-based HOIPs. Element-sensitive X-ray absorption spectroscopy measurements of BMPBiBr4 and APIBiBr5 reveal distinct resonant excitation profiles across the N K-edges, where contrasting peak shifts are observed. These 1D-HOIPs exhibit a large Stokes shift due to the small polaron contribution, as probed by photoluminescence spectroscopy at room temperature. Interestingly, the incorporation of a small fraction of tin (Sn) into the APIBiBr5 (Sn/Bi mole ratio of 1:3) structure demonstrates a strong spectral weight transfer accompanied by a fast decay lifetime (2.6 ns). These phenomena are the direct result of Sn-substitution in APIBiBr5, decreasing the small polaron effect. By changing the active ionic liquid, the electronic interactions and optical responses can be moderately tuned by alteration of their intermolecular interaction between the semiconducting inorganic layers and organic moieties.

A New Metal-Free Molecular Perovskite-Related Single Crystal with Quantum Wire Structure for High-Performance X-Ray Detection

The family of metal-free molecular perovskites, an emerging novel class of eco-friendly semiconductor, welcomes a new member with a unique 1D hexagonal perovskite structure. Lowering dimensionality at molecular level is a facile strategy for crystal structure conversion, optoelectronic property regulation, and device performance optimization. Herein, the study reports the design, synthesis, packing structure, and photophysical properties of the 1D metal-free molecular perovskite-related single crystal, rac-3APD-NH4 I3 (rac-3APD= racemic-3-Aminopiperidinium), that features a quantum wire structure formed by infinite chains of face-sharing NH4 I6 octahedra, enabling strong quantum confinement with strongly self-trapped excited (STE) states to give efficient warm orange emission with a photoluminescence quantum yield (PLQY) as high as ≈41.6%. The study accordingly unveils its photoexcited carrier dynamics: rac-3APD-NH4 I3 relaxes to STE state with a short lifetime of 10 ps but decays to ground state by emitting photons with a relatively longer lifetime of 560 ps. Additionally, strong quantum confinement effect is conducive to charge transport along the octahedral channels that enables the co-planar single-crystal X-ray detectors to achieve a sensitivity as high as 1556 µC Gyair -1 cm-2 . This work demonstrates the first case of photoluminescence mechanism and photophysical dynamics of 1D metal-free perovskite-related semiconductor, as well as the promise for high-performance X-ray detector.

Lead-free perovskite-inspired semiconductors for indoor light-harvesting - the present and the future

Are lead-free perovskite-inspired materials (PIMs) the wise choice for efficient yet sustainable indoor light harvesting? This feature article outlines how wide-bandgap PIMs can provide a positive answer to this compelling question. The wide band gaps can hinder sunlight absorption, in turn limiting the solar cell performance. However, PIMs based on group VA of the periodic table can theoretically lead to an outstanding indoor power conversion efficiency up to 60% when their band gap is ∼2 eV. Yet, the research on PIM-based indoor photovoltaics (IPVs) is still in an early stage with highest indoor device efficiencies up to 10%. This article reviews the recent advancements on PIMs for IPVs and identifies the main limiting factors of device performance, thus suggesting effective strategies to address them. We emphasize the poor operational stability of the IPV devices of PIMs being the key bottleneck for the vast adoption of this technology. We believe that this report can provide a solid scaffolding for further researching this fascinating class of materials, ultimately supporting our vision that, upon extensive advancement of the stability and efficiency, PIMs with wide bandgap will become a contender for the next-generation absorbers for sustainable indoor light harvesting.

Interfacial engineering of halide perovskites and two-dimensional materials

Recently, halide perovskites (HPs) and layered two-dimensional (2D) materials have received significant attention from industry and academia alike. HPs are emerging materials that have exciting photoelectric properties, such as a high absorption coefficient, rapid carrier mobility and high photoluminescence quantum yields, making them excellent candidates for various optoelectronic applications. 2D materials possess confined carrier mobility in 2D planes and are widely employed in nanostructures to achieve interfacial modification. HP/2D material interfaces could potentially reveal unprecedented interfacial properties, including light absorbance with desired spectral overlap, tunable carrier dynamics and modified stability, which may lead to several practical applications. In this review, we attempt to provide a comprehensive perspective on the development of interfacial engineering of HP/2D material interfaces. Specifically, we highlight the recent progress in HP/2D material interfaces considering their architectures, electronic energetics tuning and interfacial properties, discuss the potential applications of these interfaces and analyze the challenges and future research directions of interfacial engineering of HP/2D material interfaces. This review links the fields of HPs and 2D materials through interfacial engineering to provide insights into future innovations and their great potential applications in optoelectronic devices.

New Low-Dimensional Lead-Free Perovskite (2-AMP)2BiX7·H2O (X = Cl, Br) Crystals: Synthesis, Stability, and Nonlinear Optical Properties

Organic-inorganic hybrid Pb-halide perovskites have attracted tremendous interest due to their potential application in photovoltaic and optoelectronic fields. However, the toxicity and poor stability of Pb-halide perovskites have become key bottlenecks toward their future commercialization and industrialization. Therefore, in this work, two novel hybrid lead-free perovskite nonlinear optical (NLO) crystals (2-AMP)₂BiX₇·H₂O (X = Cl, Br) with high stability were successfully synthesized. Both the crystals belong to the orthorhombic P2₁2₁2₁ space group, displaying a zero-dimensional perovskite structure. The thermal, environmental, and solvent atmospheric stabilities were comprehensively investigated, with high thermal stability up to 170 °C and high environmental and high solvent atmospheric stability observed for (2-AMP)₂BiBr₇·H₂O. First-principles calculations were carried out to study the relationship of the structure and properties. A large birefringence of 0.1368@1064 nm was determined for (2-AMP)₂BiBr₇·H₂O, which was derived from the strong aeolotropic conjugated π-electron distribution of planar 2-aminomethylpyridium. A second-harmonic generation (SHG) effect that was 0.25 and 0.32 times that of KH₂PO₄ (KDP) was measured for (2-AMP)₂BiCl₇·H₂O and (2-AMP)₂BiBr₇·H₂O, respectively, and the stronger SHG response of bromide was attributed to the larger distortions of {BiBr₆} octahedrons. This work may offer new guidance for exploration of low toxicity and high stability of perovskite NLO materials.

2-D Heterometallic Pb-Iodoargentate Framework [PbAg2I6]n with a Diskoid [Pb(18-crown-6)]2+ Linker and Cocatalyst for Synergistically Enhanced Photocatalytic Properties via g-C3N4 Doping

The heterometallic Pb-iodoargentate hybrid [Pb(18-crown-6)(PbAg₂I₆)]_n (1; 18-crown-6 = 1,4,7,10,13,16-hexaoxacyclooctadecane) was prepared via self-assembly of the tetrahedal AgI₄ and octahedral PbI₆ primary units using crown ether 18-crown-6 as an organic ligand in DMF solvent. The hybrid heterocomposite cocatalyst based on 1 and graphitic carbon nitride (g-C₃N₄) was prepared by a facile solvothermal method. In 1, the dimeric units Ag₂I₆ and Pb₂I₁₀ are joined via face sharing, leading to a ternary heterometallic 1-D [PbAg₂I₆^2-]_n chain containing novel Ag₂Pb₂I₄ cubes. The 1-D chains are joined by the discoid [Pb(18-crown-6)]²⁺ complex cations, forming the 2-D [Pb(18-crown-6)(PbAg₂I₆)]_n hybrid with the skeleton of [Pb₂Ag₂I₆]_n. Compound 1 shows photocatalytic activity in the degradation of MB at room temperature under visible-light irradiation. The photoelectric response measurement showed that the photocurrent of 1 increased from 1.41 to 2.43 μA/cm^-2 when g-C₃N₄ was loaded, indicating that the introduction of a certain amount of g-C₃N₄ on the surface of 1 improves the separation and migration rate of photoinduced electrons and holes. The 1/g-C₃N₄ composite showed much higher photocatalytic efficiency in comparison to pristine 1 and g-C₃N₄ for MB degradation, which suggests the synergistic effect between 1 and g-C₃N₄ toward visible-light-driven photocatalytic performance. Meanwhile, the 1/g-C₃N₄ composite exhibited good reusability and stability in the photocatalytic reaction. Free radical quenching experiments showed that the ^•O₂^- radical is the main reactive substance over catalyst 1, while h⁺, ^•OH, and ^•O₂^- species have synergistic effects over the 1/g-C₃N₄ composite catalyst in the process of photodegradation.

Evaporation crystallization of zero-dimensional guanidinium bismuth iodide perovskite single crystal for X-ray detection

We synthesized zero-dimensional (0D) lead-free guanidinium bismuth iodide ((Gua)3Bi2I9) single crystals via an antisolvent-assisted evaporation crystallization method, and exploited their potential X-ray detection merits.

Four Discrete Silver Iodobismuthates/Bromobismuthates with Metal Complexes: Syntheses, Structures, Photocurrent Responses, and Theoretical Studies

Using in situ formed metal complexes of [Fe(bipy)₃]²⁺ or [Ni(bipy)₃]²⁺ (bipy = 2,2'-bipyridine) as templates, four new Ag-Bi-X (X = I and Br) compounds are first isolated in the metal-complex-decorated heterometallic halobismuthate family, namely [M(bipy)₃]AgBiI₆ (M = Fe (1), Ni (2)), [Fe(bipy)₃]AgBiBr₆ (3), and [Ni(bipy)₃]AgBiBr₆ (4). Compounds 1-4 feature discrete [AgBiX₆]_n^2n- anions, exhibiting three polymorphisms that may be ascribed to the different stackings and the flexible condensations of [BiX₆] octahedrons and [AgX₄] tetrahedra/[AgX₃] triangles. UV-vis diffuse reflectance analyses reveal that they are narrow band gap semiconductor materials (ca. 1.82-2.13 eV). Intriguingly, the title compounds display excellent photoelectrical switching properties, with photocurrent density following the order 3 > 4 > 2 > 1. In addition, the comparative studies of intermolecular interactions, theoretical band structures, density of states, and effective masses of three polymorphisms have also been investigated.

From Zero- to One-Dimensional, Opportunities and Caveats of Hybrid Iodobismuthates for Optoelectronic Applications

The association of the electron acceptor 4,4'-amino-bipyridinium (AmV²⁺) dication and BiI₃ in an acidic solution affords three organic-inorganic hybrid materials, (AmV)₃(BiI₆)₂ (1), (AmV)₂(Bi₄I₁₆) (2), and (AmV)BiI₅ (3), whose structures are based on isolated BiI₆^3- and Bi₄I₁₆^4- anion clusters in 1 and 2, respectively, and on a one-dimensional (1D) chain of trans-connected corner-sharing octahedra in 3. In contrast with known methylviologen-based hybrids, these compounds are more soluble in polar solvents, allowing thin film formation by spin-coating. (AmV)BiI₅ exhibits a broad absorption band in the visible region leading to an optical bandgap of 1.54 eV and shows a PV effect as demonstrated by a significant open-circuit voltage close to 500 mV. The electronic structure of the three compounds has been investigated using first-principles calculations based on density functional theory (DFT). Unexpectedly, despite the trans-connected corner-shared octahedra, for (AmV)BiI₅, the valence state shows no coupling along the wire direction, leading to a high effective mass for holes, while in contrast, the strong coupling between Bi 6p_x orbitals in the same direction at the conduction band minimum suggests excellent electron transport properties. This contributes to the low current output leading to the low efficiency of perovskite solar cells based on (AmV)BiI₅. Further insight is provided for trans- and cis-MI₅ 1D model structures (M = Bi or Pb) based on DFT investigations.

Metal Halide Semiconductors beyond Lead-Based Perovskites for Promising Optoelectronic Applications

In recent decades, metal halide semiconductors represented by lead-based halide perovskites have shown broad potential in optoelectronic applications. This family of semiconductors differs from traditional tetrahedral semiconductors in crystalline structure, chemical bonding, electronic-structure features, optoelectronic properties, as well as material fabrication method. At present, difficulties arising from both intrinsic material properties (including Pb toxicity and long-term stability) and technological aspects hinder their large-scale commercialization. In this Perspective, we focus on up-and-coming lead-free metal halide semiconductors toward high-performance optoelectronic applications. We start by outlining the advantages of metal halide semiconductors and their physical and chemical underpinnings. We then review composition and structure, electronic structure, optoelectronic properties, and device applications according to classification into three material categories, i.e., three-dimensional halide perovskites, low-dimensional perovskites and perovskite-like materials, and materials beyond perovskites. We conclude with an outlook on the challenges and opportunities of metal halide semiconductors and the future development of the field.

Highly luminescent and stable quasi-2D perovskite quantum dots by introducing large organic cations

Herein, ultra-stable quasi-two-dimensional perovskite quantum dots (quasi-2D PQDs) are synthesized by introducing the butylamine cation (BA⁺) into the methylamine lead bromide perovskite (MAPbBr₃). By reducing the dimensionality of the perovskite structure, the quasi-2D perovskite (BA)₂(MA) _x-1Pb _x Br_3x+1 presents higher luminescence efficiency and better environmental stability than traditional 3D perovskites, which is mainly because the dimensionality-reduced perovskite has higher exciton binding energy and formation energy. Under an optimal MA : BA ratio of 1 : 1, the quasi-2D perovskite exhibits about four times higher luminescence efficiency (PLQY = 49.44%) than pristine MAPbBr₃; meanwhile it emits stable luminescence in an environment with 80% humidity for 50 days. Most importantly, carbon quantum dot (CQD) doping has also been applied in this work, which effectively passivates the defects of (BA)₂(MA) _x-1Pb _x Br_3x+1 via H-bond interaction, further improving the stability of the perovskite in water. Inspired by the superior performances of the proposed quasi-2D nanomaterial, a novel colorimetric method based on halide ion exchange has been developed for H₂O₂ detection, which also demonstrates that PQDs show significant potential in the field of environmental monitoring.

Low-Dimensional Metal Halide Perovskite Crystal Materials: Structure Strategies and Luminescence Applications

Replacing methylammonium (MA+ ), formamidine (FA+ ), and/or cesium (Cs+ ) in 3D metal halide perovskites by larger organic cations have built a series of low-dimensional metal halide perovskites (LDMHPs) in which the inorganic metal halide octahedra arranging in the forms of 2D layers, 1D chains, and 0D points. These LDMHPs exhibit significantly different optoelectronic properties from 3D metal halide perovskites (MHPs) due to their unique quantum confinement effects and large exciton binding energies. In particular, LDMHPs often have excellent broadband luminescence from self-trapped excitons. Chemical composition, hydrogen bonding, and external factors (temperature and pressure etc.) determine structures and influence photoelectric properties of LDMHPs greatly, and especially it seems that there is no definite regulation to predict the structure and photoelectric properties when a random cation, metal, and halide is chosen to design a LDMHP. Therefore, this review discusses the construction strategies of the recent reported LDMHPs and their application progress in the luminescence field for a better understanding of these factors and a prospect for LDMHPs' development in the future.

Lead-Free Perovskite Photodetectors: Progress, Challenges, and Opportunities

State-of-the-art photodetectors which apply hybrid perovskite materials have emerged as powerful candidates for next-generation light sensing. Among them, lead-based ones are the most popular beyond doubt on account of their unique and superior optoelectronic properties. Nevertheless, trade-off toward commercialization exists between nontoxicity and high performance, with the poor stability of lead-based perovskites, indicating that it is indispensable to substitute lead with nontoxic element meanwhile bringing about a comparable figure of merit of photodetectors and relatively long-term stability. Herein, recent advances in lead-free perovskite photodetectors are reviewed, analyzing the principle while designing new materials and highlighting some remarkable progress, which are comparable, even superior, to lead-based photodetectors. Furthermore, their potential strategy in optical communication, image sensing, narrowband photodetection, etc., is examined and a perspective on developing new materials and photodetectors with superior properties for more practical applications is provided.

Lead-Free Halide Perovskites for Light Emission: Recent Advances and Perspectives

Lead-based halide perovskites have received great attention in light-emitting applications due to their excellent properties, including high photoluminescence quantum yield (PLQY), tunable emission wavelength, and facile solution preparation. In spite of excellent characteristics, the presence of toxic element lead directly obstructs their further commercial development. Hence, exploiting lead-free halide perovskite materials with superior properties is urgent and necessary. In this review, the deep-seated reasons that benefit light emission for halide perovskites, which help to develop lead-free halide perovskites with excellent performance, are first emphasized. Recent advances in lead-free halide perovskite materials (single crystals, thin films, and nanocrystals with different dimensionalities) from synthesis, crystal structures, optical and optoelectronic properties to applications are then systematically summarized. In particular, phosphor-converted LEDs and electroluminescent LEDs using lead-free halide perovskites are fully examined. Ultimately, based on current development of lead-free halide perovskites, the future directions of lead-free halide perovskites in terms of materials and light-emitting devices are discussed.

Ion-exchange-induced MAPbI3 thin-film 3D–2D and 3D–1D conversions: unveiling structural transformations in films via synergistic and competitive approaches

Interesting gas-induced structural transformations from 3D MAPbI₃ to LD perovskites are investigated, contributing to explore more optoelectronic materials with tunability.

Anisotropic Photoconductivity and Long-Lived Charge Carriers in Bismuth-Based One-Dimensional Perovskite with Type-IIa Band Alignment

Bismuth-based perovskites are attracting intense scientific interest due to low toxicity and excellent moisture stability compared to lead-based analogues. However, high exciton binding energy, poor charge carrier separation, and transport efficiencies lower their optoelectronic performances. To address these issues, we have integrated an electronically active organic cation, naphthalimide ethylammonium, between the [BiI₅^2-]_n chains via crystal engineering to form a novel perovskite-like material (naphthalimide ethylammonium)₂BiI₅ (NBI). Single crystal analysis revealed a one-dimensional quantum-well structure for NBI in which inter-inorganic well electronic coupling is screened by organic layers. It exhibited anisotropic photoconductivity and long-lived charge carriers with milliseconds lifetime, which is higher than that of CH₃NH₃PbI₃. Density functional theory calculations confirmed type-IIa band alignment between organic cations and inorganic chains, allowing the former to electronically contribute to the overall charge transport properties of the material.

A Potential Checkmate to Lead: Bismuth in Organometal Halide Perovskites, Structure, Properties, and Applications

The remarkable optoelectronic properties and considerable performance of the organo lead-halide perovskites (PVKs) in various optoelectronic applications grasp tremendous scientific attention. However, the existence of the toxic lead in these compounds is threatening human health and remains a major concern in the way of their commercialization. To address this issue, numerous nontoxic alternatives have been reported. Among these alternatives, bismuth-based PVKs have emerged as a promising substitute because of similar optoelectronic properties and extended environmental stability. This work communicates briefly about the possible lead-alternatives and explores bismuth-based perovskites comprehensively, in terms of their structures, optoelectronic properties, and applications. A brief description of lead-toxification is provided and the possible Pb-alternatives from the periodic table are scrutinized. Then, the classification and crystal structures of various Bi-based perovskites are elaborated on. Detailed optoelectronic properties of Bi-based perovskites are also described and their optoelectronic applications are abridged. The overall photovoltaic applications along with device characteristics (i.e., V OC, J SC, fill factor, FF, and power conversion efficiency, PCE), fabrication method, device architecture, and operational stability are also summarized. Finally, a conclusion is drawn where a brief outlook highlights the challenges that hamper the future progress of Bi-based optoelectronic devices and suggestions for future directions are provided.

Rigid Amine-Induced Pseudo-3 D Lead-Free Bismuth Halide Perovskite with an Improved Band Edge for Visible-Light Absorption

Bismuth organometal halide perovskites have recently been investigated as potential substitutes for lead perovskite solar-cell absorbers because of their lower toxicity. However, the narrowing of the band gap remains a crucial challenge for their practical application. All known Bi-based perovskites have large band gaps, thereby affording weak visible-light absorption. This study concerns a novel, lead-free, pseudo-3 D perovskite optoelectronic material, (MV)BiI₅ (MV²⁺ =methyl viologen). The pseudo-3 D metal-halogen perovskite-like structure is constructed by connecting [BiI₅ ] ²⁺ units via I⋅⋅⋅I contacts. MV, as a rigid organic amine, is located at the center of each parallelepiped to balance the charge and stabilize the structure. (MV)BiI₅ has a narrow band gap of 1.48 eV and a better photoresponse than (MV)BiCl₅ with a 1 D structure. (MV)BiI₅ is the first Bi-based perovskite compound with a band gap energy comparable with (CH₃ NH₃ )PbI₃ , which is encouraging for optoelectronic applications. This research will open a potential pathway for the design of pseudo-3 D Bi-based perovskites with performances comparable with APbX₃ absorbers.

Three-Dimensional Metal-Free Molecular Perovskite with a Thermally Induced Switchable Dielectric Response

Temperature-responsive materials with switching physical properties have been widely researched. Among them, the switchable dielectric perovskite materials show potential applications in the electrical and electronic industries and even the intelligence industries. However, perovskite oxides and hybrid organic-inorganic perovskites, as the most representative switchable dielectric materials, are limited by bad biocompatibility. Herein, we report temperature-dielectric-responsive metal-free perovskite (H₂dabco)(NH₄)[BF₄]₃ constructed by the strategy of substituting the B site in the general formula ABX₃ (doubly protonated 1,4-diazabicyclo[2.2.2]octane = H₂dabco). Meanwhile, structurally similar hybrid material (H₂dabco)Rb[BF₄]₃ was designed as a control. They exhibit similar phase-transition characteristics and dielectric response behaviors around 333 K. More interestingly, the ordered-disordered transformation of their organic "spherical" cations (H₂dabco) was deemed to produce their phase transitions and dielectric response switching. Given its ability to generate a dielectric response, (H₂dabco)(NH₄)[BF₄]₃ will show the potential application of metal-free perovskite in a future thermal sensing device.

Large-Area Exfoliated Lead-Free Perovskite-Derivative Single-Crystalline Membrane for Flexible Low-Defect Photodetectors

Wide applications of personal consumer electronics have tended to cause a huge demand for smart and portable electronics, featuring mechanical flexibility, lightweight, and environmental friendliness. However, most of the recently reported flexible photodetectors based on microcrystalline and amorphous components commonly suffer from severe drawbacks, including plenty of grains, boundaries, and surface defects. Here, we present a new lead-free chiral perovskite-derivative light absorber of (aminoguanidinium)₃Bi₂I₉ (AG₃Bi₂I₉), which displays a narrow direct band gap of about 1.89 eV. High-quality bulk single crystals were successfully grown with dimensions up to 24 × 12 × 5 mm³. Emphatically, as-grown bulk single crystals are easy to be exfoliated for large-area ultrathin wafers with an exfoliated area up to 0.6 cm², showing promise for low-defect flexible optoelectronic applications. The remarkable surface smoothness and crystalline quality of single-crystalline thin layers were further confirmed by TEM, HRTEM, AFM, single-crystalline X-ray diffraction, and space-charge limited current (SCLC) measurements. As expected, the planar photodetectors based on flexible exfoliated wafers are first fabricated and exhibit notable photoelectric performance. This work represents an important step forward as it offers an effective strategy for the fabrication of high-quality large-area flexible exfoliated wafer devices.

Broadband white light emission from one-dimensional zigzag edge-sharing perovskite

We reported 1D (AMP)PbBr₄ and (AMP)PbCl₄ perovskites, which consisted of the 1D zigzag edge-sharing [PbBr₄²⁻ (or PbCl₄²⁻)]_∞ infinite inorganic chains with AMP²⁺ cations, for the white-light emission.

Influence of organic cation planarity on structural templating in hybrid metal-halides

Controlling the connectivity and topology of solids is a versatile way to target desired physical properties. This is especially relevant in the realm of hybrid halide semiconductors, where the long-range connectivity of the inorganic substructural unit can lead to significant changes in optoelectronic properties such as photoluminescence, charge transport, and absorption. We present a new series of hybrid metal-halide semiconductors, (phenH₂)BiI₅·H₂O, (2,2-bpyH₂)BiI₅, (BrbpyH)BiI₄·H₂O, (phenH₂)₂Pb₃I₁₀·2H₂O, and (2,2-bpyH₂)₂Pb₃I₁₀ where (phenH₂)²⁺ = 1,10-phenanthroline-1,10-diium, (2,2-bpyH₂)²⁺ = 2,2'-bipyridine-1,1'-diium and (BrbpyH)⁺ = 6,6'-dibromo-2,2'-bipyridium. These compounds allow us to observe how the planarity of the cation, induced either through structural modification in the case of (phenH₂)²⁺ or through non-covalent interactions in (BrbpyH)⁺, both relative to (2,2-bpyH₂)²⁺, modifies the inorganic substructural unit. While the Pb²⁺ series of compounds show minimal changes in inorganic connectivity, we observe large differences in the Bi³⁺ series, ranging from 0-D dimers to corner- and edge-sharing 1-D chains of octahedra. We find that compounds containing (phenH₂)²⁺ and (BrbpyH)⁺ pack more efficiently than those with (2,2-bpyH₂)²⁺ due to their retention of planarity leading to greater inorganic connectivity. Electronic structure calculations and optical diffuse reflectance reveal that the band gaps of these compounds are influenced by the degree of inorganic connectivity and the inorganic substructural unit distances. These results show that the structure and planarity of organic cations can directly influence both the inorganic connectivity and the optical properties that could be tuned for certain optoelectronic applications.

Structural and Functional Diversity in Lead-Free Halide Perovskite Materials

Lead halide perovskites have emerged as promising semiconducting materials for different applications owing to their superior optoelectronic properties. Although the community holds different views toward the toxic lead in these high-performance perovskites, it is certainly preferred to replace lead with nontoxic, or at least less-toxic, elements while maintaining the superior properties. Here, the design rules for lead-free perovskite materials with structural dimensions from 3D to 0D are presented. Recent progress in lead-free halide perovskites is reviewed, and the relationships between the structures and fundamental properties are summarized, including optical, electric, and magnetic-related properties. 3D perovskites, especially A₂ B⁺ B³⁺ X₆ -type double perovskites, demonstrate very promising optoelectronic prospects, while low-dimensional perovskites show rich structural diversity, resulting in abundant properties for optical, electric, magnetic, and multifunctional applications. Furthermore, based on these structure-property relationships, strategies for multifunctional perovskite design are proposed. The challenges and future directions of lead-free perovskite applications are also highlighted, with emphasis on materials development and device fabrication. The research on lead-free halide perovskites at Linköping University has benefited from inspirational discussions with Prof. Olle Inganäs.

Two Heteromorphic Crystals of Antimony-Based Hybrids Showing Tunable Optical Band Gaps and Distinct Photoelectric Responses

Organic-inorganic hybrid perovskites, most markedly CH₃NH₃PbI₃, have attracted extensive interest because of their potential use in optoelectronic and photovoltaic applications. Nevertheless, the toxicity of lead restricts their further application. Here, we successfully synthesized two lead-free heteromorphic hybrids, (C₇H₁₈N₂O)₃Sb₄I₁₈·H₂O (1) and (C₇H₁₈N₂O)Sb₂I₈·H₂O (2, C₇H₁₈N₂O²⁺ is N-aminopropylmorpholinium), both of which belong to the zero-dimensional tetranuclear perovskite-like structure. However, the inorganic [Sb₄I₁₈] cluster of 1 adopts a tetragonal topology, while 2 features the distorted [Sb₄I₁₆] motif; this disparity leads to a significant distinction between their electronic structures as well as an optical band gap ( E_g). Their absorption cutoffs are measured to be 708 nm (for 1, E_g = 1.71 eV) and 578 nm (for 2, E_g = 2.11 eV), respectively. In particular, 1 exhibits a stronger photoelectric response in a wider optical region compared to that of 2, and the "on/off" ratio of conductivity of 1 is estimated to ∼300 under sunlight illumination. Density functional theory calculation discloses that different inorganic motifs make greater contributions to their electronic structure and photoelectric response. It is believed that the heteromorphic method allows a potential pathway for construction of new lead-free hybrid materials as light absorbers for photoelectric application.

Bismuth Halide Perovskite-Like Materials: Current Opportunities and Challenges

Metal halide perovskites have recently emerged as promising photovoltaic materials for application in solar cells with high power conversion efficiencies exceeding 23 %. In the years since such high efficiencies have been attained, investigations have mainly focused on the state-of-the-art 3 D Pb-based halide perovskite materials. However, the high toxicity of Pb and intrinsic instability of the pristine perovskite materials have become great obstacles to their industrial application and commercialization. To address these serious issues, it is imperative to explore low-toxicity metal halide perovskites or their derivatives to substitute Pb-based materials for better future development. Currently, Bi-based halide perovskite-like materials are attracting increased interest as environmentally friendly alternatives for photovoltaic applications. This Concept highlights recent advances of Bi-based halide perovskite-like materials in terms of understanding and modifying their fundamental properties and related device performance, with a focus on current challenges, opportunities for future development, and diversification of device applications.

Tunable optical absorption in lead-free perovskite-like hybrids by iodide management

Tunable optical absorption in lead-free perovskite-like hybrids by iodide management.

[((R)-C8H12N)4][Bi2Br10] and [((S)-C8H12N)4][Bi2Br10]: Chiral Hybrid Bismuth Bromides Templated by Chiral Organic Cations

Single crystals of organically templated chiral bromobismuthates(III), [((R/S)-C₈H₁₂N)₄][Bi₂Br₁₀], have been grown for the first time via a slow evaporation method. Each of the chiral molecular compound consists of (R) or (S)-1-phenylethylammonium ([C₈H₁₂N]⁺) cations and [Bi₂Br₁₀]^4- anions. Both the title compounds reveal thermal and moisture stabilities up to ca. 220 °C and over 1 month, respectively. The newly prepared Bi³⁺-based organic-inorganic hybrid materials show optical band gap of ca. 2.88 eV. The noncentrosymmetric [((R)-C₈H₁₂N)₄][Bi₂Br₁₀] and [((S)-C₈H₁₂N)₄][Bi₂Br₁₀] exhibit second harmonic generation efficiency of ca. 20 times that of α-SiO₂ and are type I nonphase matchable. Uniformly deposited thin films of [((R)-C₈H₁₂N)₄][Bi₂Br₁₀] and [((S)-C₈H₁₂N)₄][Bi₂Br₁₀] have been also successfully obtained by a simple spin-coating method. The circular dichroism spectra for both reported thin films are symmetrical, attributable to the corresponding Cotton effect. The selectively deposited chiral thin films are expected to be used as a useful platform for various surface reactions and interface engineering.

Lead-free hybrid perovskites for photovoltaics

This review covers the state-of-the-art in organo-inorganic lead-free hybrid perovskites (HPs) and applications of these exciting materials as light harvesters in photovoltaic systems. Special emphasis is placed on the influence of the spatial organization of HP materials both on the micro- and nanometer scale on the performance and stability of perovskite-based solar light converters. This review also discusses HP materials produced by isovalent lead(II) substitution with Sn2+ and other metal(II) ions, perovskite materials formed on the basis of M3+ cations (Sb3+, Bi3+) as well as on combinations of M+/M3+ ions aliovalent to 2Pb2+ (Ag+/Bi3+, Ag+/Sb3+, etc.). The survey is concluded with an outlook highlighting the most promising strategies for future progress of photovoltaic systems based on lead-free perovskite compounds.