Guanidinium-Formamidinium Lead Iodide: A Layered Perovskite-Related Compound with Red Luminescence at Room Temperature

Dual-Template-Directed Zero-Dimensional Bismuth Chlorides: Structures, Luminescence, Photoinduced Chromism, and Enhanced Proton Conductivity

Zero-dimensional (0D) hybrid organic-inorganic bismuth halides have attracted immense scientific interest as promising candidates for lead-free materials. Here, by using a typical solvothermal method, two mixed-cation-phase 0D hybrid bismuth chlorides of [HPDA][H2PDA]BiCl6 (1) and [Hbzim][H2PA]BiCl6 (2) (PDA = bis(4-pyridyl)amine, bzim = benzimidazole, PA = 2-picolylamine) have been assembled based on a series of organic amine guests. Both compounds exhibit interesting photoluminescence phenomena, in which compound 1 exhibits a double emission property of blue fluorescence and yellow-green phosphorescence simultaneously, while compound 2 exhibits wide-band yellow-green emission under visible light excitation. The luminescence mechanism is explained by experiments and theoretical calculations. In view of the fact that halometallate units and the conjugated nitrogen heterocyclic systems can act as electron donors and electron acceptors, respectively, both compounds exhibit free radical-driven photochromism induced by electron transfer under xenon lamp irradiation at room temperature. In addition, benefiting from abundant hydrogen bond networks in structures, the two compounds show significant temperature-dependent proton conduction behavior in the range of 298-343 K, and the proton conductivity of both compounds is significantly improved after light irradiation. Our study demonstrates two novel hybrid mixed-cation-phase 0D hybrid bismuth halides with photoluminescence, photochromism, and photomodulated proton conduction properties, which enriches the dual-template-directed metal halide system and provides a feasible scheme for the synthesis of photoresponsive smart materials.

Crystal structure of hexa-glycinium dodeca-iodo-triplumbate

The crystal structure of hexa-glycinium tetra-μ-iodido-octa-iodido-triplumbate, (C2H6NO2)6[Pb3I12] or (GlyH)6[Pb3I12], is reported. The compound crystallizes in the triclinic space group P. The [Pb3I12]6- anion is discrete and located around a special position: the central Pb ion located on the inversion center is holodirected, while the other two are hemidirected. The supra-molecular nature is mainly based on C-H⋯I, N-H⋯I, O-H⋯I and N-H⋯O hydrogen bonds. Dimeric cations of type (A +⋯A +) for the amino acid glycine are observed for the first time.

What Matters for the Charge Transport of 2D Perovskites?

Compared to 3D perovskites, 2D perovskites exhibit excellent stability, structural diversity, and tunable bandgaps, making them highly promising for applications in solar cells, light-emitting diodes, and photodetectors. However, the trade-off for worse charge transport is a critical issue that needs to be addressed. This comprehensive review first discusses the structure of 3D and 2D metal halide perovskites, then summarizes the significant factors influencing charge transport in detail and provides a brief overview of the testing methods. Subsequently, various strategies to improve the charge transport are presented, including tuning A'-site organic spacer cations, A-site cations, B-site metal cations, and X-site halide ions. Finally, an outlook on the future development of improving the 2D perovskites' charge transport is discussed.

2D Hybrid Perovskites: From Static and Dynamic Structures to Potential Applications

2D perovskites have received great attention recently due to their structural tunability and environmental stability, making them highly promising candidates for various applications by breaking property bottlenecks that affect established materials. However, in 2D perovskites, the complicated interplay between organic spacers and inorganic slabs makes structural analysis challenging to interpret. A deeper understanding of the structure-property relationship in these systems is urgently needed to enable high-performance tunable optoelectronic devices. Herein, this study examines how structural changes, from constant lattice distortion and variable structural evolution, modeled with both static and dynamic structural descriptors, affect macroscopic properties and ultimately device performance. The effect of chemical composition, crystallographic inhomogeneity, and mechanical-stress-induced static structural changes and corresponding electronic band variations is reported. In addition, the structure dynamics are described from the viewpoint of anharmonic vibrations, which impact electron-phonon coupling and the carriers' dynamic processes. Correlated carrier-matter interactions, known as polarons and acting on fine electronic structures, are then discussed. Finally, reliable guidelines to facilitate design to exploit structural features and rationally achieve breakthroughs in 2D perovskite applications are proposed. This review provides a global structural landscape of 2D perovskites, expected to promote the prosperity of these materials in emerging device applications.

Unique Perovskitizer N─Pb Bond Switching Induced Polar Photovoltaic Effect in Trilayered Hybrid Perovskite

Polar photovoltaic effect (PPE) has attracted great attention in regulating desired optoelectronic properties, which can be driven by order-disorder and displacive phase transitions. Bond-switching is also a feasible method to induce PPE, but such investigation is very rare. Lead-halide hybrid perovskite (LHHP) is an outstanding photodetection material; lead atoms possess rich coordination modes to provide possibilities to construct switchable bonds. Here, a unique perovskitizer N─Pb bond-switching is disclosed to induce polar photovoltage in the emerging LHHP, PA2MHy2Pb3Br10 (1, PA = n-propylamine, MHy = methylhydrazine). Interestingly, the perovskitizer MHy+ provides 2s2 lone pair while the Pb atom affords empty d orbitals, which coordinate with each other to generate a flexible N─Pb bond. Further, the introduction of N─Pb bonds results in a high distortion of the PbBr6 octahedron to form local polarity and further orientation to induce spontaneous polarization. More importantly, such a flexible N─Pb bond switching mechanism drives a notable PPE and controllable polarized photo-response, a polarization ratio up to 9.7 at the polar phase in striking contrast with the non-polar phase (1.03). The work provides the first demonstration of bond-switching to induce polar phase transition and polar photovoltage in the photoconductive hybrid perovskites for photoelectric applications.

Broadband red emission from one-dimensional hexamethonium lead bromide perovskitoid

Broadband emissions from low-dimensional hybrid perovskites have aroused intense interest. However, the achievement of broadband red emission in lead halide perovskites remains challenging. Herein, we report a one-dimensional (1D) hybrid lead bromide perovskitoid, (HM)Pb2Br6 (HM = hexamethonium), featuring a corrugated "3 × 3" [Pb2Br6]2- chain. The unique structure results in intriguingly red emission peaking at 692 nm, with a PLQY of around 6.24%. Our spectroscopic and computational studies reveal that the red emission derives from self-localized Pb23+, Pb3+ and Br2- species confined within the inorganic lead bromide lattice that function as radiative centres. This finding will benefit the design of perovskite systems for efficient red emission.

Ion-Diffusion Management Enables All-Interface Defect Passivation of Perovskite Solar Cells

Perovskite solar cells (PSCs) have demonstrated over 25% power conversion efficiency (PCE) via efficient surface passivation. Unfortunately, state-of-the-art perovskite post-treatment strategies can solely heal the top interface defects. Herein, an ion-diffusion management strategy is proposed to concurrently modulate the top interfaces, buried interfaces, and bulk interfaces (i.e., grain boundaries) of perovskite film, enabling all-interface defect passivation. Specifically, this method is enabled by applying double interactive salts of octylammonium iodide (OAI) and guanidinium chloride (GACl) onto the 3D perovskite surface. It is revealed that the hydrogen-bonding interaction between OA+ and GA+ decelerates the OA+ diffusion and therefore forms a dimensionally broadened 2D capping layer. Additionally, the diffusion of GA+ and Cl- determines the composition of the bulk and buried interface of PSCs. As a result, n-inter-i-inter-p, i.e., five-layer structured PSCs can be obtained with a champion PCE of 25.43% (certified 24.4%). This approach also enables the substantially improved operational stability of perovskite solar cells.

Luminescence and Dielectric Switchable Properties of a 1D (1,1,1-Trimethylhydrazinium)PbI3 Hybrid Perovskitoid

The synthesis and investigation of the physicochemical properties of a novel one-dimensional (1D) hybrid organic-inorganic perovskitoid templated by the 1,1,1-trimethylhydrazinium (Me₃Hy⁺) cation are reported. (Me₃Hy)[PbI₃] crystallizes in the hexagonal P6₃/m symmetry and undergoes two phase transitions (PTs) during heating (cooling) at 322 (320) and 207 (202) K. X-ray diffraction data and temperature-dependent vibrational studies show that the second-order PT to the high-temperature hexagonal P6₃/mmc phase is associated with a weak change in entropy and is related to weak structural changes and different confinement of cations in the available space. The second PT to the low-temperature orthorhombic Pbca phase that corresponds to the high change in entropy and dielectric switching is associated with an ordering of the trimethylhydrazinium cations, re-arrangement and strengthening of hydrogen bonds, and slightly shifted lead-iodide octahedral chains. The high-pressure Raman data revealed two additional PTs, one between 2.8 and 3.2 GPa, related to the symmetry decrease, ordering of the cations, and inorganic chain distortion, and the other in the 6.4-6.8 GPa range related to the partial and reversible amorphization. Optical studies revealed that (Me₃Hy)[PbI₃] has a wide band gap (3.20 eV) and emits reddish-orange excitonic emission at low temperatures with an activation energy of 65 meV.

Crystallization Pathways of FABr-PbBr2-DMF and FABr-PbBr2-DMSO Systems: The Comprehensive Picture of Formamidinium-Based Low-Dimensional Perovskite-Related Phases and Intermediate Solvates

In this study, we systematically investigated the phase diversity and crystallization pathways of the FABr excessive regions of two ternary systems of FABr-PbBr₂-DMF and FABr-PbBr₂-DMSO (where FA⁺-formamidinium cations, DMF-dimethylformamide and DMSO-dimethyl sulfoxide solvents). In these systems, a new FA₃PbBr₅ phase with a structure containing chains of vertex-connected PbBr₆ octahedra is discovered, and its crystal structure is refined. We experimentally assess fundamental information on differences in the mechanisms of crystallization process in FABr-PbBr₂-DMF and FABr-PbBr₂-DMSO systems and determine possible pathways of crystallization of hybrid perovskites. We show that intermediate solvate phases are not observed in the system with DMF solvent, while a number of crystalline solvates tend to form in the system with DMSO at various amounts of FABr excess.

In Situ Tetraalkylammonium Ligand Engineering of Organic-Inorganic Hybrid Perovskite Nanoparticles for Enhancing Long-Term Stability and Optical Tunability

Organic-inorganic hybrid perovskite nanoparticles (OIHP NPs) have attracted scientific attention owing to their efficient photoluminescence with optical tunability, which is highly advantageous for optoelectronic applications. However, the limited long-term stability of OIHP NPs has significantly hindered their practical application. Despite several synthetic strategies and encapsulation methods to stabilize OIHP NPs, complicated multi-step procedures are often required. In this study, we introduce an in situ ligand engineering method for stabilizing and controlling the optical properties of OIHP NPs using tetraalkylammonium (TAA) halides with various molecular structures at different concentrations. Our one-pot ligand engineering substantially enhanced the stability of the OIHP NPs without post-synthetic processes. Moreover, in certain cases, approximately 90% of the initial photoluminescence (PL) intensity was preserved even after a month under ambient conditions (room temperature, 20-50% relative humidity). To determine the role of ligand engineering in stabilizing the OIHP NPs, the surface binding properties of the TAA ligands were thoroughly analyzed using Raman spectroscopy. Specifically, the permanent positive charge of the TAA cations and consequent effective electrostatic interactions with the surfaces of the OIHP NPs are pivotal for preserving the initial PL intensity. Our investigation is beneficial for developing OIHP nanomaterials with improved stability and controlled photoluminescence for various optoelectronic applications, such as light-emitting devices, photosensitizers, photodetectors, photocatalysis, and solar cells.

Multifunctional Luminescent Sensor Based on the Pb2+ Complex Containing a Tetrazolato Ligand

A series of luminescent Pb²⁺ complexes, [Pb(L¹)₂]_n (1), [Pb(L²)₂]_n (2), [Pb(L³)(NO₃)(H₂O)₂]_n (3), [Pb(L³)(Br)(H₂O)]_n (4), [Pb(L³)(Cl)(H₂O)]_n (5), and [Pb(L⁴)(H₂O)₂] (6) have been synthesized by treatment of polydentate tetrazolato ligands with various hydrated Pb²⁺ salts (HL¹ = 2-(1H-tetrazol-5-yl)pyridine, HL² = 3-(1H-tetrazol-5-yl)isoquinoline, HL³ = 6-(1H-tetrazol-5-yl)-2,2'-bipyridine, and H₂L⁴ = 6,6'-bis(1H-tetrazol-5-yl)-2,2'-bipyridine). These complexes have been characterized by IR, TGA, and elemental analysis. Their crystal structures have been determined by X-ray crystallography, and the phase purity of bulk samples were further confirmed by PXRD. Their luminescence properties have been investigated in detail, and their emission origin may involve ligand-centered π-π* transition, metal-centered s-p transition and charge-transfer character. It is interesting to note that 5 exhibits obviously enhanced red-shifted emission, whose photoluminescence quantum yield (PLQY = 16.5%) is much higher than the other compounds (≤2%). Most importantly, the emission property of 5 was strongly affected by temperature. When the temperature rises from 295 to 493 K, the emission maximum gradually shifts to high energy due to the loss of the aqua ligand. In contrast, when the temperature is lowered from 295 to 13 K, two emission bands were observed. The low-energy emission band exhibits a slight blue shift, while a new high-energy emission band appears at around 520 nm, which is assigned to ligand-centered phosphorescence. After removal of the coordinated aqua ligand, the emission of 5-H₂O is very sensitive to the vapors of volatile primary amines and acids, although they have different response mechanisms. This result indicates that 5-H₂O may be a potential multifunctional sensor for temperature, volatile amines, and acids. To decipher the emission origin, DFT calculations have also been carried out based on the structure units of these compounds.

Engineering van der Waals Materials for Advanced Metaphotonics

The outstanding chemical and physical properties of 2D materials, together with their atomically thin nature, make them ideal candidates for metaphotonic device integration and construction, which requires deep subwavelength light-matter interaction to achieve optical functionalities beyond conventional optical phenomena observed in naturally available materials. In addition to their intrinsic properties, the possibility to further manipulate the properties of 2D materials via chemical or physical engineering dramatically enhances their capability, evoking new science on light-matter interaction, leading to leaped performance of existing functional devices and giving birth to new metaphotonic devices that were unattainable previously. Comprehensive understanding of the intrinsic properties of 2D materials, approaches and capabilities for chemical and physical engineering methods, the resulting property modifications and novel functionalities, and applications of metaphotonic devices are provided in this review. Through reviewing the detailed progress in each aspect and the state-of-the-art achievement, insightful analyses of the outstanding challenges and future directions are elucidated in this cross-disciplinary comprehensive review with the aim to provide an overall development picture in the field of 2D material metaphotonics and promote rapid progress in this fast emerging and prosperous field.

Progress and challenges in layered two-dimensional hybrid perovskites

Dimensionality is the game-changer property of a material. The optical and electronic properties of a compound get dramatically influenced by confining dimensions from 3D to 2D. The bulk 3D perovskite materials have shown remarkable up-gradation in the power conversion efficiency, hence grabbing worldwide attention. But instability against moisture, temperature, and ion migration are the factors constantly back-stabbing and hindering from full-scale commercialization. 2D perovskite material has emerged as an excellent bridging entity between structural-chemical stability, and viable commercialization. Organic-inorganic 2D perovskite materials come with a layered structure in which a large organic cation layer as a spacer is sandwiched between two inorganic metal halide octahedra layers. Moreover, hydrophobic spacer cations are employed which isolate inorganic octahedral layers from water molecules. Hydrophobic spacer cations protect the authentic structure from being degraded. These layered structures occur in two phases namely the Ruddlesden-Popper phase and Dion-Jacobson phase, depending on the spacer cation types. Alternating inorganic and organic layers form multiple quantum wells naturally, along with spin-orbit-coupling gives Rashba splitting. 2D perovskite materials are coming up with interesting chemical, physical properties like exciton dynamics, charge carrier transport, and electron-phonon coupling as a result of the quantum confinement effect. Despite appreciable stability, limited charge transport and large bandgap are limiting the application of 2D perovskite materials in solar cells. These limitations can be overcome by using the concept of 2D/3D multidimensional hybrid perovskites, which includes the long-term stability of 2D perovskite and the high performance of 3D perovskite at the same time. Here in this perspective, we have given brief insight on structural versatility, synthesis techniques, some of the unique photophysical properties, potential device fabrication, and recent advancements in the 2D structure to stand against degradation. Certain shortcomings and future outlooks are also discussed to make the perspective more informative.

Quadruple-Cation Wide-Bandgap Perovskite Solar Cells with Enhanced Thermal Stability Enabled by Vacuum Deposition

Vacuum processing of multicomponent perovskites is not straightforward, because the number of precursors is in principle limited by the number of available thermal sources. Herein, we present a process which allows increasing the complexity of the formulation of vacuum-deposited lead halide perovskite films by multisource deposition and premixing both inorganic and organic components. We apply it to the preparation of wide-bandgap CsMAFA triple-cation perovskite solar cells, which are found to be efficient but not thermally stable. With the aim of stabilizing the perovskite phase, we add guanidinium (GA⁺) to the material formulation and obtained CsMAFAGA quadruple-cation perovskite films with enhanced thermal stability, as observed by X-ray diffraction and rationalized by microstructural analysis. The corresponding solar cells showed similar performance with improved thermal stability. This work paves the way toward the vacuum processing of complex perovskite formulations, with important implications not only for photovoltaics but also for other fields of application.

Polarity and Ferromagnetism in Two-Dimensional Hybrid Copper Perovskites with Chlorinated Aromatic Spacers

Two-dimensional (2D) organic-inorganic hybrid copper halide perovskites have drawn tremendous attention as promising multifunctional materials. Herein, by incorporating ortho-, meta-, and para-chlorine substitutions in the benzylamine structure, we first report the influence of positional isomerism on the crystal structures of chlorobenzylammonium copper(II) chloride perovskites A₂CuCl₄. 2D polar ferromagnets (3-ClbaH)₂CuCl₄ and (4-ClbaH)₂CuCl₄ (ClbaH⁺ = chlorobenzylammonium) are successfully obtained. They both adopt a polar monoclinic space group Cc at room temperature, displaying significant differences in crystal structures. In contrast, (2-ClbaH)₂CuCl₄ adopts a centrosymmetric space group P 2₁/ c at room temperature. This associated structural evolution successfully enhances the physical properties of the two polar compounds with high thermal stability, discernible second harmonic generation (SHG) signals, ferromagnetism, and narrow optical band gaps. These findings demonstrate that the introduction of chlorine atoms into the interlayer organic species is a powerful tool to tune crystal symmetries and physical properties, and this inspires further exploration of designing high-performance multifunctional copper-based materials.

Temperature dependent phase transitions and their relation to isosbestic point formation. Case study of C(NH2)3PbI3

Besides the vast research regarding the hybrid organic-inorganic perovskite (HOIP) materials used in the solar cell production, their properties are still not fully uncovered. In this paper, detailed investigation on the phase transitions in guanidinium lead iodide (GAPbI₃) using vibrational spectroscopy techniques (IR and Raman) are presented. In addition to the well-known three phases of GAPbI₃ (denoted as I, II and III) another one existing in the temperature range from 48 °C to 160 °C is characterized. The thorough inspection of the vibrational spectra revealed some interesting changes occurring in the low temperature region (from -90 to -62 °C) that suggest presence of a new phase. Finally, a redefinition of the phase nomenclature according to the recommendations given by the IUCr is proposed.

Guanidinium-Pseudohalide Perovskite Interfaces Enable Surface Reconstruction of Colloidal Quantum Dots for Efficient and Stable Photovoltaics

Complete surface passivation of colloidal quantum dots (CQDs) and their strong electronic coupling are key factors toward high-performance CQD-based photovoltaics (CQDPVs). Also, the CQD matrices must be protected from oxidative environments, such as ambient air and moisture, to guarantee air-stable operation of the CQDPVs. Herein, we devise a complementary and effective approach to reconstruct the oxidized CQD surface using guanidinium and pseudohalide. Unlike conventional halides, thiocyanate anions provide better surface passivation with effective replacement of surface oxygen species and additional filling of defective sites, whereas guanidinium cations promote the construction of epitaxial perovskite bridges within the CQD matrix and augment electronic coupling. Additionally, we replace a defective 1,2-ethanedithiol-treated CQD hole transport layer (HTL) with robust polymeric HTLs, based on a judicious consideration of the energy level alignment established at the CQD/HTL interface. These efforts collectively result in high-performance and stable CQDPVs with photocurrents over 30 mA cm^-2, ∼80% quantum efficiency at excitonic peaks and stable operation under humid and ambient conditions. Elucidation of carrier dynamics further reveals that interfacial recombination associated with band alignment governs both the CQDPV performance and stability.

Highly Concentrated, Zwitterionic Ligand-Capped Mn2+:CsPb(Br x Cl1-x )3 Nanocrystals as Bright Scintillators for Fast Neutron Imaging

Fast neutron imaging is a nondestructive technique for large-scale objects such as nuclear fuel rods. However, present detectors are based on conventional phosphors (typically microcrystalline ZnS:Cu) that have intrinsic drawbacks, including light scattering, γ-ray sensitivity, and afterglow. Fast neutron imaging with colloidal nanocrystals (NCs) was demonstrated to eliminate light scattering. While lead halide perovskite (LHP) FAPbBr₃ NCs emitting brightly showed poor spatial resolution due to reabsorption, the Mn²⁺-doped CsPb(BrCl)₃ NCs with oleyl ligands had higher resolution because of large apparent Stokes shift but insufficient concentration for high light yield. In this work, we demonstrate a NC scintillator that features simultaneously high quantum yields, high concentrations, and a large apparent Stokes shift. In particular, we use long-chain zwitterionic ligand capping in the synthesis of Mn²⁺-doped CsPb(BrCl)₃ NCs that allows for attaining very high concentrations (>100 mg/mL) of colloids. The emissive behavior of these ASC18-capped NCs was carefully controlled by compositional tuning that permitted us to select for high quantum yields (>50%) coinciding with Mn-dominated emission for minimal self-absorption. These tailored Mn²⁺:CsPb(BrCl)₃ NCs demonstrated over 8 times brighter light yield than their oleyl-capped variants under fast neutron irradiation, which is competitive with that of near-unity FAPbBr₃ NCs, while essentially eliminating self-absorption. Because of their rare combination of concentrations above 100 mg/mL and high quantum yields, along with minimal self-absorption for good spatial resolution, Mn²⁺:CsPb(BrCl)₃ NCs have the potential to displace ZnS:Cu as the leading scintillator for fast neutron imaging.

Additive Engineering for Efficient and Stable MAPbI3-Perovskite Solar Cells with an Efficiency of over 21

The high density of defects in MAPbI₃ perovskite films brings about severe carrier nonradiative recombination loss, which lowers the performance of MAPbI₃-based perovskite solar cells (PSCs). Here, methylamine cyanate (MAOCN) molecules were introduced into MAPbI₃ solutions to manipulate the crystallizatsion of the MAPbI₃ films. MAOCN molecules can slow down the volatilization rate of the solvent and delay the crystallization process of the MAPbI₃ film. The crystal quality of the MAPbI₃ films is effectively optimized without an additive residue. Perovskite films treated by MAOCN have lower defect density and longer carrier lifetime, which lowers the carrier recombination loss. Meanwhile, the MAPbI₃ film based on MAOCN has a more hydrophobic surface. The final MAPbI₃-based device efficiency reached 21.28% (V_OC = 1.126 V, J_SC = 23.29 mA/cm², and FF = 81.13). After 30 days of storage under atmospheric conditions, the efficiency of unencapsulated MAOCN-based PSCs only dropped by about 5%.

Investigation of Opto-Electronic Properties and Stability of Mixed-Cation Mixed-Halide Perovskite Materials with Machine-Learning Implementation

The feasibility of mixed-cation mixed-halogen perovskites of formula AxA’1−xPbXyX’zX”3−y−z is analyzed from the perspective of structural stability, opto-electronic properties and possible degradation mechanisms. Using density functional theory (DFT) calculations aided by machine-learning (ML) methods, the structurally stable compositions are further evaluated for the highest absorption and optimal stability. Here, the role of the halogen mixtures is demonstrated in tuning the contrasting trends of optical absorption and stability. Similarly, binary organic cation mixtures are found to significantly influence the degradation, while they have a lesser, but still visible effect on the opto-electronic properties. The combined framework of high-throughput calculations and ML techniques such as the linear regression methods, random forests and artificial neural networks offers the necessary grounds for an efficient exploration of multi-dimensional compositional spaces.

Advances and Challenges in Two-Dimensional Organic-Inorganic Hybrid Perovskites Toward High-Performance Light-Emitting Diodes

Two-dimensional (2D) perovskites are known as one of the most promising luminescent materials due to their structural diversity and outstanding optoelectronic properties. Compared with 3D perovskites, 2D perovskites have natural quantum well structures, large exciton binding energy (Eb) and outstanding thermal stability, which shows great potential in the next-generation displays and solid-state lighting. In this review, the fundamental structure, photophysical and electrical properties of 2D perovskite films were illustrated systematically. Based on the advantages of 2D perovskites, such as special energy funnel process, ultra-fast energy transfer, dense film and low efficiency roll-off, the remarkable achievements of 2D perovskite light-emitting diodes (PeLEDs) are summarized, and exciting challenges of 2D perovskite are also discussed. An outlook on further improving the efficiency of pure-blue PeLEDs, enhancing the operational stability of PeLEDs and reducing the toxicity to push this field forward was also provided. This review provides an overview of the recent developments of 2D perovskite materials and LED applications, and outlining challenges for achieving the high-performance devices.

Layered Hybrid Formamidinium Lead Iodide Perovskites: Challenges and Opportunities

ConspectusHybrid halide perovskite materials have become one of the leading candidates for various optoelectronic applications. They are based on organic-inorganic structures defined by the AMX₃ composition, were A is the central cation that can be either organic (e.g., methylammonium, formamidinium (FA)) or inorganic (e.g., Cs⁺), M is a divalent metal ion (e.g., Pb²⁺ or Sn²⁺), and X is a halide anion (I^-, Br^-, or Cl^-). In particular, FAPbI₃ perovskites have shown remarkable optoelectronic properties and thermal stabilities. However, the photoactive α-FAPbI₃ (black) perovskite phase is not thermodynamically stable at ambient temperature and forms the δ-FAPbI₃ (yellow) phase that is not suitable for optoelectronic applications. This has stimulated intense research efforts to stabilize and realize the potential of the α-FAPbI₃ perovskite phase. In addition, hybrid perovskites were proven to be unstable against the external environmental conditions (air and moisture) and under device operating conditions (voltage and light), which is related to various degradation mechanisms. One of the strategies to overcome these instabilities has been based on low-dimensional hybrid perovskite materials, in particular layered two-dimensional (2D) perovskite phases composed of organic layers separating hybrid perovskite slabs, which were found to be more stable toward ambient conditions and ion migration. These materials are mostly based on S_xA_n-1Pb_nX_3n+1 composition with various mono- (x = 1) or bifunctional (x = 2) organic spacer cations that template hybrid perovskite slabs and commonly form either Ruddlesden-Popper (RP) or Dion-Jacobson (DJ) phases. These materials behave as natural quantum wells since charge carriers are confined to the inorganic slabs, featuring a gradual decrease in the band gap as the number of inorganic layers (n) increases from n = 1 (2D) to n = ∞ (3D). While various layered 2D perovskites have been developed, their FA-based analogues remain under-represented to date. Over the past few years, several research advances enabled the realization of FA-based layered perovskites, which have also demonstrated a unique templating effect in stabilizing the α-FAPbI₃ phase. This, for instance, involved the archetypical n-butylammonium and 2-phenylethylammonium organic spacers as well as guanidinium, 5-ammonium valeric acid, iso-butylammonium, benzylammonium, n-pentylammonium, 2-thiophenemethylammonium, 2-(perfluorophenyl)ethylammonium, 1-adamantylmethanammonium, and 1,4-phenylenedimethanammonium. FAPbBr₃-based layered perovskites have also demonstrated potential in various optoelectronic applications, yet the opportunities associated with FAPbI₃-based perovskites have attracted particular attention in photovoltaics, stimulating further developments. This Account provides an overview of some of these recent developments, with a particular focus on FAPbI₃-based layered perovskites and their utility in photovoltaics, while outlining challenges and opportunities for these hybrid materials in the future.

Recent Advances on the Applications of Luminescent Pb2+-Containing Metal-Organic Frameworks in White-Light Emission and Sensing

Luminescent Pb2+-based metal-organic frameworks (MOFs) belong to a new class of multifunctional molecular materials with interesting luminescence properties and potential applications within a single crystalline phase. In this mini review, we present the recent advances that have been achieved in their applications as single-phase white-light emitting materials and chemosensors in the last decade. We focus on the trends in the modification of their structures and luminescence by various bridging ligands, and subsequently their multifunctional applications, which may affect the future development of the field.

The 2D Halide Perovskite Rulebook: How the Spacer Influences Everything from the Structure to Optoelectronic Device Efficiency

Two-dimensional (2D) halide perovskites have emerged as outstanding semiconducting materials thanks to their superior stability and structural diversity. However, the ever-growing field of optoelectronic device research using 2D perovskites requires systematic understanding of the effects of the spacer on the structure, properties, and device performance. So far, many studies are based on trial-and-error tests of random spacers with limited ability to predict the resulting structure of these synthetic experiments, hindering the discovery of novel 2D materials to be incorporated into high-performance devices. In this review, we provide guidelines on successfully choosing spacers and incorporating them into crystalline materials and optoelectronic devices. We first provide a summary of various synthetic methods to act as a tutorial for groups interested in pursuing synthesis of novel 2D perovskites. Second, we provide our insights on what kind of spacer cations can stabilize 2D perovskites followed by an extensive review of the spacer cations, which have been shown to stabilize 2D perovskites with an emphasis on the effects of the spacer on the structure and optical properties. Next, we provide a similar explanation for the methods used to fabricate films and their desired properties. Like the synthesis section, we will then focus on various spacers that have been used in devices and how they influence the film structure and device performance. With a comprehensive understanding of these effects, a rational selection of novel spacers can be made, accelerating this already exciting field.

Surface-induced phase engineering and defect passivation of perovskite nanograins for efficient red light-emitting diodes

Organic-inorganic hybrid lead halide perovskites are potential candidates for next-generation light-emitting diodes (LEDs) in terms of tunable emission wavelengths, high electroluminescence efficiency, and excellent color purity. However, the device performance is still limited by severe non-radiative recombination losses and operational instability due to a high degree of defect states on the perovskite surface. Here, an effective surface engineering method is developed via the assistance of guanidinium iodide (GAI), which allows the formation of surface-2D heterophased perovskite nanograins and surface defect passivation due to the bonding with undercoordinated halide ions. Efficient and stable red-emission LEDs are realized with the improved optoelectronic properties of GAI-modified perovskite nanograins by suppressing the trap-mediated non-radiative recombination loss. The champion device with a high color purity at 692 nm achieves an external quantum efficiency of 17.1%, which is 2.3 times that of the control device. Furthermore, the operational stability is highly improved, showing a half-lifetime of 563 min at an initial luminance of 1000 cd m^-2. The proposed GAI-assisted surface engineering is a promising approach for defect passivation and phase engineering in perovskite films to achieve high-performance perovskite LEDs.

Solid-State NMR and NQR Spectroscopy of Lead-Halide Perovskite Materials

Two- and three-dimensional lead-halide perovskite (LHP) materials are novel semiconductors that have generated broad interest owing to their outstanding optical and electronic properties. Characterization and understanding of their atomic structure and structure-property relationships are often nontrivial as a result of the vast structural and compositional tunability of LHPs as well as the enhanced structure dynamics as compared with oxide perovskites or more conventional semiconductors. Nuclear magnetic resonance (NMR) spectroscopy contributes to this thrust through its unique capability of sampling chemical bonding element-specifically (1/2H, 13C, 14/15N, 35/37Cl, 39K, 79/81Br, 87Rb, 127I, 133Cs, and 207Pb nuclei) and locally and shedding light onto the connectivity, geometry, topology, and dynamics of bonding. NMR can therefore readily observe phase transitions, evaluate phase purity and compositional and structural disorder, and probe molecular dynamics and ionic motion in diverse forms of LHPs, in which they can be used practically, ranging from bulk single crystals (e.g., in gamma and X-ray detectors) to polycrystalline films (e.g., in photovoltaics, photodetectors, and light-emitting diodes) and colloidal nanocrystals (e.g., in liquid crystal displays and future quantum light sources). Herein we also outline the immense practical potential of nuclear quadrupolar resonance (NQR) spectroscopy for characterizing LHPs, owing to the strong quadrupole moments, good sensitivity, and high natural abundance of several halide nuclei (79/81Br and 127I) combined with the enhanced electric field gradients around these nuclei existing in LHPs as well as the instrumental simplicity. Strong quadrupole interactions, on one side, make 79/81Br and 127I NMR rather impractical but turn NQR into a high-resolution probe of the local structure around halide ions.

Additive Engineering by Bifunctional Guanidine Sulfamate for Highly Efficient and Stable Perovskites Solar Cells

High efficiency and good stability are the challenges for perovskite solar cells (PSCs) toward commercialization. However, the intrinsic high defect density and internal nonradiative recombination of perovskite (PVK) limit its development. In this work, a facile additive strategy is devised by introducing bifunctional guanidine sulfamate (GuaSM; CH₆ N₃ ⁺ , Gua⁺ ; H₂ N-SO₃ ^- , SM^- ) into PVK. The size of Gua⁺ ion is suitable with Pb(BrI)₂ cavity relatively, so it can participate in the formation of low-dimensional PVK when mixed with Pb(BrI)₂ . The O and N atoms of SM^- can coordinate with Pb²⁺ . The synergistic effect of the anions and cations effectively reduces the trap density and the recombination in PVK, so that it can improve the efficiency and stability of PSCs. At an optimal concentration of GuaSM (2 mol%), the PSC presents a champion power conversion efficiency of 21.66% and a remarkably improved stability and hysteresis. The results provide a novel strategy for highly efficient and stable PSCs by bifunctional additive.

Metal Halide Perovskite@Metal-Organic Framework Hybrids: Synthesis, Design, Properties, and Applications

Metal halide perovskites (MHPs) have excellent optoelectronic and photovoltaic applications because of their cost-effectiveness, tunable emission, high photoluminescence quantum yields, and excellent charge carrier properties. However, the potential applications of the entire MHP family are facing a major challenge arising from its weak resistance to moisture, polar solvents, temperature, and light exposure. A viable strategy to enhance the stability of MHPs could lie in their incorporation into a porous template. Metal-organic frameworks (MOFs) have outstanding properties, with a unique network of ordered/functional pores, which render them promising for functioning as such a template, accommodating a wide range of MHPs to the nanosized region, alongside minimizing particle aggregation and enhancing the stability of the entrapped species. This review highlights recent advances in design strategies, synthesis, characterization, and properties of various hybrids of MOFs with MHPs. Particular attention is paid to a critical review of the emergence of MHP@MOF for comprehensive studies of next-generation materials for various technological applications including sensors, photocatalysis, encryption/decryption, light-emitting diodes, and solar cells. Finally, by summarizing the state-of-the-art, some promising future applications of reported hybrids are proposed. Considering the inherent correlation and synergic functionalities of MHPs and MOFs, further advancement; new functional materials; and applications can be achieved through designing MHP@MOF hybrids.

Incorporated Guanidinium Expands the CH3NH3PbI3 Lattice and Enhances Photovoltaic Performance

Guanidinium (GA) has been widely used as an additive in solar cells for enhanced performance. However, the size of the guanidinium cation is too large to be incorporated in the cage of the perovskite structure. Instead, GA forms a variety of structures with lead iodide, where its role in the perovskite crystal as well as solar cell devices is unclear. In this study, we demonstrate that GA can be incorporated into the structure of MAPbI₃ as (GA)_x(MA)_1-xPbI₃. From single-crystal X-ray crystallographic refinement, we observe lattice expansion and Pb-I bond elongation with GA incorporation similar to exerting "negative pressure", which weakens orbital overlap and widens the band gap from 1.49 to 1.53 eV. We find that the highest percentage of GA that can be incorporated into the 3D MAPbI₃ structure is 5.26%, as confirmed by nuclear magnetic resonance. The alloyed (GA)_x(MA)_1-xPbI₃ exhibits increased PL lifetimes from 154.4 to 266.3 ns after GA incorporation while the V_oc of (GA)_x(MA)_1-xPbI₃ devices enlarges from 1.05 to 1.11 V. High efficiencies in solar cell devices up to 20.38% with a J_sc of 23.55 mA cm^-2, V_oc of 1.11 V, and FF of 0.78 have been achieved, with stable photovoltaic performance for 900 h in air.

Why choosing the right partner is important: stabilization of ternary CsyGUAxFA(1-y-x)PbI3 perovskites

Lead halide perovskites with mixtures of monovalent cations have attracted wide attention due to the possibility of preferentially stabilizing the perovskite phase with respect to photovoltaically less suitable competing phases. Here, we present a theoretical analysis and interpretation of the phase stability of binary (CH6N3)x[HC(NH2)2](1-x)PbI3 = GUAxFA(1-x)PbI3 and ternary CsyGUAxFA(1-y-x)PbI3 mixtures. We first estimate if such mixtures are stable and if they lead to a stabilization of the perovskite phase based on static Density Functional Theory (DFT) calculations. In order to investigate the finite temperature stability of the phases, we also employ first-principles molecular dynamics (MD) simulations. It turns out that in contrast to the FA+-rich case of FA/Cs mixtures, although mixing of FA/GUA is possible, it is not sufficient to stabilize the perovskite phase at room temperature. In contrast, stable ternary mixtures that contain 17% of Cs+ can be formed that lead to a preferential stabilization of the perovskite phase. In such a way, the enthalpic destabilization due to the introduction of a too large/too small cation that lies outside the Goldschmidt tolerance range can be (partially) compensated through the introduction of a third cation with complementary size. This allows to suggest a new design principle for the preparation of stable perovskite structures at room temperature with cations that lie outside the Goldschmidt range through mixtures with size-complementary cations in such a way that the effective average cation radius of the mixture lies within the stability range.

Structural Diversity in Layered Hybrid Perovskites, A2PbBr4 or AA'PbBr4, Templated by Small Disc-Shaped Amines

We present three new hybrid layered lead(II) bromide perovskites of generic composition A₂PbBr₄ or AA'PbBr₄ that exhibit three distinct structure types. [TzH]₂PbBr₄ ([TzH⁺] = 1,2,4-triazolium) adopts a (001)-oriented layer structure and [AaH]₂PbBr₄, ([AaH⁺] = acetamidinium) adopts a (110)-oriented type, whereas [ImH][TzH]PbBr₄, ([ImH⁺] = imidazolium) adopts a rare (110)-oriented structure with enhanced corrugation (i.e., 3 × 3 type). The crystal structures of each are discussed in terms of the differing nature of the templating molecular species. Photoluminescent spectra for each are reported and the behaviors discussed in relation to the different structure of each composition.

Lead-Halide Scalar Couplings in 207Pb NMR of APbX3 Perovskites (A = Cs, Methylammonium, Formamidinium; X = Cl, Br, I)

Understanding the structure and dynamics of newcomer optoelectronic materials - lead halide perovskites APbX3 [A = Cs, methylammonium (CH3NH3+, MA), formamidinium (CH(NH2)2+, FA); X = Cl, Br, I] - has been a major research thrust. In this work, new insights could be gained by using 207Pb solid-state nuclear magnetic resonance (NMR) spectroscopy at variable temperatures between 100 and 300 K. The existence of scalar couplings 1JPb-Cl of ca. 400 Hz and 1JPb-Br of ca. 2.3 kHz could be confirmed for MAPbX3 and CsPbX3. Diverse and fast structure dynamics, including rotations of A-cations, harmonic and anharmonic vibrations of the lead-halide framework and ionic mobility, affect the resolution of the coupling pattern. 207Pb NMR can therefore be used to detect the structural disorder and phase transitions. Furthermore, by comparing bulk and nanocrystalline CsPbBr3 a greater structural disorder of the PbBr6-octahedra had been confirmed in a nanoscale counterpart, not readily captured by diffraction-based techniques.

Large Optical Anisotropy in Two-Dimensional Perovskite [CH(NH2)2][C(NH2)3]PbI4 with Corrugated Inorganic Layers

Optical anisotropy plays an indispensable role in a variety of optical components. Organic halide perovskites often rely on artificially oriented nanostructures to enhance optical anisotropy due to their in-plane isotropic crystal structure, which results in unnecessary optical losses and fabrication difficulties. Here, we report the large optical anisotropy in two-dimensional perovskite [CH(NH₂)₂][C(NH₂)₃]PbI₄ crystals. Without specially designing their morphology, we achieved a large photoresponse linear dichroic ratio of 2 and a photoluminescence linear dichroic ratio of 4.7. Furthermore, we identified that the polarization orientation is parallel to the corrugated inorganic layers on every crystal plane by density functional theory calculations. The anisotropy of the ab-plane and ac-plane changes in opposite trend with temperature, suggesting that the perovskite can selectively generate polarized light or unpolarized light from different crystal planes by tuning the temperature. Our studies provide a new platform toward two-dimensional perovskite-based optical polarization devices.

Large-area transparent flexible guanidinium incorporated MAPbI3 microstructures for high-performance photodetectors with enhanced stability

Unveiling the transparency and flexibility in perovskite-based photodetectors with superior photoresponse and environmental stability remains an open challenge. Here we report on guanidinium incorporated metal halide perovskite (MA_1-xGua_xPbI₃, x = 0 to 0.65) random percolative microstructure (RPM) fabrication using an ultra-fast spray coating technique. Remarkably, RPMs over a large area of 5 × 5 cm² on flexible substrates with a transparency of ∼50% can be achieved with enriched environmental stability. Transparent photodetectors based on MA_1-xGua_xPbI₃ (x = 0.12) RPMs manifest excellent performance with a responsivity of 187 A W^-1, a detectivity of 2.23 × 10¹² Jones and an external quantum efficiency of 44 115%. Additionally, the photodetectors exhibited superior mechanical flexibility under a wide range of bending angles and large number of binding cycles. Integrating features including transparency, high performance, stability, flexibility and scalability within a photodetector is unmatched and holds potential for novel applications in transparent and wearable optoelectronic devices.

Non-transient thermo-/photochromism of iodobismuthate hybrids directed by solvated metal cations

Two iodobismuthate-based organic-inorganic hybrids, [M(DMSO)₈][Bi₂I₉] (M = La (1), Bi (2)), have been successfully designed and synthesized by using solvated metal cations as structure-directing agents (SDAs). 1 displays transient high-temperature thermochromism, which is similar to that of the characteristic low-temperature thermochromic properties of bulk bismuth iodide and iodobismuthate hybrids. In contrast, 2 exhibits distinguishing non-transient thermochromic properties stimulated by the different temperature ranges of the thermal treatments. More importantly, a comparison of the optical inertness of 1 and 2 also reveals novel photochromic behavior. The completely different thermo-/photo-responsive properties of 1 and 2 are mainly ascribed to the different binding abilities of the central metal cations with DMSO molecules, which cause a distinct transformation of the inorganic moiety and consequent modulation of band gaps.

Structural diversity of lead halide chain compounds, APbX3, templated by isomeric molecular cations

Three new 1D chain structure type hybrid organic-inorganic lead(ii) halides are presented: IQPbBr3, QPbBr3 and QPbI3, templated by large organic cations, isoquinolinium ([IQ+] = protonated isoquinoline) and its isomer quinolonium ([Q+] = protonated quinoline). All three compounds possess the same generic formula as cubic perovskite, ABX3, but adopt different structures. IQPbBr3 adopts a 1D face-sharing single chain hexagonal perovskite structure type, and the other two, QPbBr3 and QPbI3, adopt a non-perovskite structure which is built from 1D edge-sharing octahedral double chains. Crystal structures and preliminary photophysical properties are discussed. Two of them have lower bandgaps than the other reported materials with the same structure type, indicating the value of further exploratory studies for these types of materials.

Ruddlesden-Popper Perovskites: Synthesis and Optical Properties for Optoelectronic Applications

Ruddlesden-Popper perovskites with a formula of (A')2(A) n -1B n X3 n +1 have recently gained widespread interest as candidates for the next generation of optoelectronic devices. The variations of organic cation, metal halide, and the number of layers in the structure lead to the change of crystal structures and properties for different optoelectronic applications. Herein, the different synthetic methods for 2D perovskite crystals and thin films are summarized and compared. The optoelectronic properties and the charge transfer process in the devices are also delved, in particular, for light-emitting diodes and solar cells.

Intrinsic Self-Trapped Emission in 0D Lead-Free (C4 H14 N2 )2 In2 Br10 Single Crystal

Low-dimensional lead halide perovskite materials recently have drawn much attention owing to the intriguing broadband emissions; however, the toxicity of lead will hinder their future development. Now, a lead-free (C₄ H₁₄ N₂ )₂ In₂ Br₁₀ single crystal with a unique zero-dimensional (0D) structure constituted by [InBr₆ ]^3- octahedral and [InBr₄ ]^- tetrahedral units is described. The single crystal exhibits broadband photoluminescence (PL) that spans almost the whole visible spectrum with a lifetime of 3.2 μs. Computational and experimental studies unveil that an excited-state structural distortion in [InBr₆ ]^3- octahedral units enables the formation of intrinsic self-trapped excitons (STEs) and thus contributing the broad emission. Furthermore, femtosecond transient absorption (fs-TA) measurement reveals that the ultrafast STEs formation together with an efficient intersystem crossing has made a significant contribution to the long-lived and broad STE-based emission behavior.

Incorporating Large A Cations into Lead Iodide Perovskite Cages: Relaxed Goldschmidt Tolerance Factor and Impact on Exciton-Phonon Interaction

The stability and formation of a perovskite structure is dictated by the Goldschmidt tolerance factor as a general geometric guideline. The tolerance factor has limited the choice of cations (A) in 3D lead iodide perovskites (APbI₃), an intriguing class of semiconductors for high-performance photovoltaics and optoelectronics. Here, we show the tolerance factor requirement is relaxed in 2D Ruddlesden-Popper (RP) perovskites, enabling the incorporation of a variety of larger cations beyond the methylammonium (MA), formamidinium, and cesium ions in the lead iodide perovskite cages for the first time. This is unequivocally confirmed with the single-crystal X-ray structure of newly synthesized guanidinium (GA)-based (n-C₆H₁₃NH₃)₂(GA)Pb₂I₇, which exhibits significantly enlarged and distorted perovskite cage containing sterically constrained GA cation. Structural comparison with (n-C₆H₁₃NH₃)₂(MA)Pb₂I₇ reveals that the structural stabilization originates from the mitigation of strain accumulation and self-adjustable strain-balancing in 2D RP structures. Furthermore, spectroscopic studies show a large A cation significantly influences carrier dynamics and exciton-phonon interactions through modulating the inorganic sublattice. These results enrich the diverse families of perovskite materials, provide new insights into the mechanistic role of A-site cations on their physical properties, and have implications to solar device studies using engineered perovskite thin films incorporating such large organic cations.

Solid-state NMR of hybrid halide perovskites

Recent advances in the development of perovskite based solar cells have increased the demand for in-depth characterisation of the perovskite structures and the dynamics of their various constituents in relation to the potential impact on the photovoltaic performance. NMR can play an important role in this respect; NMR has been used to study the incorporation of different ionic species, characterize their internal dynamics and diffusion, and monitor the chemical stability of these technologically relevant materials, including upcoming lower dimensional perovskite materials. Furthermore, the flexibility of NMR allows the study of the materials under relevant conditions e.g. under illumination. Here we present an overview of the recent literature on NMR of (hybrid) halide perovskites, focusing on the insights that NMR can provide.

Organic cation directed one-dimensional cuprous halide compounds: syntheses, crystal structures and photoluminescence properties

In recent years, organic-inorganic hybrid metal halides have emerged as a highly promising class of semiconducting light emitting diodes (LEDs) due to their fascinating photoluminescence properties. Here, by specifically selecting different organic cations as templates, a series of new hybrid cuprous halides have been solvothermally prepared, namely [Me-Py]CuI₂ (1), [(Me)₂-DABCO]Cu₂I₄ (2), [Me-MePy]Cu₂I₃ (3), and [H₂DABCO]Cu₃X₅ (X = I (4) and Br (5)). These hybrid cuprous halides feature one-dimensional (1D) [CuI₂]^-, [Cu₂I₃]^- and [Cu₃X₅]^2- (X = Br, I) chains surrounded and charge-balanced by organic cations. Under UV photoexcitation, these hybrid cuprous halides exhibit strong tunable photoluminescence from cyan (480 nm) to red (675 nm) emissions with large Stokes shifts (345 nm). The intrinsic nature of PL emissions is also investigated based on temperature-dependent PL emission, lifetime, photoluminescence quantum efficiencies, etc.