Layered Lead Halide Research Articles

Mixed-Layered Lead Halide Frameworks with High Stability and Efficient Room-Temperature Phosphorescence.

Room-temperature phosphorescent (RTP) materials play a crucial role in optical anticounterfeiting science and information security technologies. Ionically bonded organic metal halides have emerged as promising RTP material systems due to their excellent self-assembly and unique photophysical property, but their intrinsic instability largely hinders their advanced practical applications. Herein, we employ a coordination-driven synthetic strategy utilizing organocarboxylates for the synthesis of two isostructural layered lead halide frameworks. The frameworks adopt a new mixed-layered topology, consisting of alternating [Pb10X9]11+ (X = Cl-/Br-) layers and [Pb6XO3]11+ (X = Cl-/Br-) layers that are coordinatively sandwiched by organocarboxylate layers. The frameworks exhibit long-lived green afterglow emission with the long lifetime of up to 45.89 ms and the photoluminescence quantum yield (PLQY) of up to 43.13%. The Pb2+-carboxylate coordination accelerates the metal-to-ligand charge transfer from the light-harvesting lead halide layers to the phosphorescent organic component, promoting efficient spin-orbit coupling and intersystem crossing. Moreover, the coordination networks exhibit good structural robustness under ambient conditions for at least 12 months, as well as stability in boiling water, acidic and basic aqueous environments. The highly efficient afterglow and high structural integrity enable multiple anticounterfeiting applications across diverse chemical environments.

Rashba Spin Splitting Limiting the Application of 2D Halide Perovskites for UV-Emitting Devices.

Layered lead halide perovskites have attracted much attention as promising materials for a new generation of optoelectronic devices. To make progress in applications, a full understanding of the basic properties is essential. Here, we study 2D-layered (BA)2PbX4 by using different halide anions (X = I, Br, and Cl) along with quantum confinement. The obtained cell parameter evolution, supported by experimental measurements and theoretical calculations, indicates strong lattice distortions of the metal halide octahedra, breaking the local inversion symmetry in (BA)2PbCl4, which strongly correlates with a pronounced Rashba spin-splitting effect. Optical measurements reveal strong photoluminescence quenching and a drastic reduction in the PL quantum yield in this larger band gap compound. We suggest that these optical results are closely related to the appearance of the Rashba effect due to the existence of a local electric dipole. The results obtained in ab initio calculations showed that the (BA)2PbCl4 possesses electrical polarization of 0.13 μC/cm2 and spin-splitting energy of about 40 meV. Our work establishes that local octahedra distortions induce Rashba spin splitting, which explains why obtaining UV-emitting materials with high PLQY is a big challenge.

Impacts of Exciton Binding Energy and Dielectric Confinement of Layered Lead Halide Perovskites on Carrier Relaxation and Exciton Phonon Interactions.

This work highlights the significance of dielectric confinements and exciton binding energy of hybrid layered perovskites (LPs) in controlling the carrier relaxation dynamics of LPs for designing efficient optoelectronic devices. The polarizability of organic spacer cations in LPs modulates the carrier-phonon and carrier-carrier interactions, which eventually control the carrier relaxation dynamics. Here, we have varied the alkyl-ammonium chain length in the LPs to change the dielectric confinement, and the first-principles calculations reveal that the long-chain organic spacer experiences stronger dielectric confinement in comparison to short-chain organic spacer cation-based LPs. Transient absorption spectroscopic analysis suggests that the larger dielectric confinement and higher exciton binding energy exhibit faster carrier relaxation dynamics. The enhanced exciton-phonon interaction leads to faster carrier relaxation dynamics. The much softer phonon modes are responsible for the higher up-conversion of acoustic modes to optical modes, which leads to slower carrier relaxation dynamics in n-butylamine (BA) based LPs.

Lead Halide Hybrids Templated by Two Coordinating Ligands for Enhanced and Stable Self-Trapped Emission.

Lead halide hybrids templated by coordinating ligands are a class of ultrastable broadband self-trapped emitters that overcome the stability problems of conventional ionically bound halide hybrids. However, enhancing their photoluminescence (PL) performances by crystal engineering remains a huge challenge. Herein, for the first time, we have successfully employed the synthetic strategy of two coordinating ligands to synthesize a series of layered lead halide coordination polymers, [Pb6X10]2+(chdc2-)(2,2'-bpy)2 (X = Cl/Br, chdc = trans-1,4-cyclohexanedicarboxylate), which involves chdc as a pillaring strut and 2,2'-bpy as a chelating ligand. The introduction of a chelating ligand (2,2'-bpy) enables stronger lattice distortion of lead halide layers and enhances UV-light absorption and ligand-to-metal charge transfer (LMCT) process, thereby achieving a substantial improvement of photoluminescence quantum yields (PLQYs) over the control layered materials templated by a single chdc ligand. This class of lead halide hybrids templated by two coordinating ligands exhibit chemical "inertness" after being subjected to various chemical conditions for 48 h, maintaining stable and efficient broadband emission. Density functional theory calculations and femtosecond transient absorption spectra (fs-TA) demonstrate that the broadband emission originates from self-trapped excitons, which are more populated with the LMCT contribution from 2,2'-bpy. This study shows a rational strategy at the molecular level to modulate the photophysical properties of chemically robust lead halide coordination polymers.

Manipulation of the Structure and Optoelectronic Properties through Bromine Inclusion in a Layered Lead Bromide Perovskite.

One of the great advantages of organic-inorganic metal halides is that their structures and properties are highly tuneable and this is important when optimizing materials for photovoltaics or other optoelectronic devices. One of the most common and effective ways of tuning the electronic structure is through anion substitution. Here, we report the inclusion of bromine into the layered perovskite [H3N(CH2)6NH3]PbBr4 to form [H3N(CH2)6NH3]PbBr4·Br2, which contains molecular bromine (Br2) intercalated between the layers of corner-sharing PbBr6 octahedra. Bromine intercalation in [H3N(CH2)6NH3]PbBr4·Br2 results in a decrease in the band gap of 0.85 eV and induces a structural transition from a Ruddlesden-Popper-like to Dion-Jacobson-like phase, while also changing the conformation of the amine. Electronic structure calculations show that Br2 intercalation is accompanied by the formation of a new band in the electronic structure and a significant decrease in the effective masses of around two orders of magnitude. This is backed up by our resistivity measurements that show that [H3N(CH2)6NH3]PbBr4·Br2 has a resistivity value of one order of magnitude lower than [H3N(CH2)6NH3]PbBr4, suggesting that bromine inclusion significantly increases the mobility and/or carrier concentration in the material. This work highlights the possibility of using molecular inclusion as an alternative tool to tune the electronic properties of layered organic-inorganic perovskites, while also being the first example of molecular bromine inclusion in a layered lead halide perovskite. By using a combination of crystallography and computation, we show that the key to this manipulation of the electronic structure is the formation of halogen bonds between the Br2 and Br in the [PbBr4]∞ layers, which is likely to have important effects in a range of organic-inorganic metal halides.

Stable self-trapped broadband emission from an organolead halide coordination polymer with strong layer corrugation and high chemical robustness

Layered lead halide coordination polymers are an emerging class of stable self-trapped emitters, in which the unique corrugated structure are critical to afford out-of-plane distortions for self-trapping.

Photogenerated charge transfer in Dion-Jacobson type layered perovskite based on naphthalene diimide.

Incorporating organic semiconducting spacer cations into layered lead halide perovskite structures provides a powerful approach to mitigate the typical strong dielectric and quantum confinement effects by inducing charge-transfer between the organic and inorganic layers. Herein we report the synthesis and characterization of thin films of novel DJ-phase organic-inorganic layered perovskite semiconductors using a naphthalene diimide (NDI) based divalent spacer cation, which is shown to accept photogenerated electrons from the inorganic layer. With alkyl chain lengths of 6 carbons, an NDI-based thin film exhibited electron mobility (based on space charge-limited current for quasi-layered 〈n〉 = 5 material) was found to be as high as 0.03 cm2 V-1 s-1 with no observable trap-filling region suggesting trap passivation by the NDI spacer cation.

Magnetic Doping Induced Strong Circularly Polarized Light Emission and Detection in 2D Layered Halide Perovskite

AbstractThe generation, manipulation, and detection of polarized light are the foundations of spin optoelectronics. Polarized light has a wide range of applications in new‐generation information transmission, storage, and quantum communications. Hybrid organic–inorganic halide perovskites have attracted extensive attention recently because of their strong spin–orbit coupling, Rashba splitting, spin‐dependent optical selection, and have been regarded as promising candidates for application in spin optoelectronic devices. Herein, the authors report the successful synthesis of 2D layered lead halide perovskites (PEA)2PbI4 (PEI) doped with magnetic metal Co2+ and the observation of strong circularly polarized photoluminescence and photoresponse. The introduction of magnetic metals in perovskites can induce an analogous Zeeman effect in the system, which can increase its energy degeneracy, giving rise to an imbalanced population of electronic spin states, thereby enhancing the spin polarization in doped samples. Thus, a maximum circularly polarized PL of 35% is observed at room temperature. Moreover, the achieved Co2+‐doped (PEA)2PbI4 also exhibits a selective photoresponse with circularly polarized light (CPL) emission, and an outstanding anisotropy factor of 0.41 for photocurrent is achieved. The experimental results lay the foundation for the controlled synthesis of magnetically doped perovskites as well as applications for direct CPL detection in spin optoelectronics.

Nonmonotonic Photostability of BA2MAn-1PbnI3n+1 Homologous Layered Perovskites.

Layered lead halide perovskites (2D LHPs) are attracting considerable attention as a promising material for a new generation of solar cell devices. LHPs have been presented as a more stable alternative to the more widespread 3D bulk perovskite materials; however, a critical analysis of their photostability is still lacking. In this work, we perform a comparative study between BA2MAn-1PbnI3n+1 (BA─butylammonium and MA─methylammonium) 2D LHPs with different dimensionalities (n = 1-3) and MAPbI3 3D perovskites. We compare different stability testing protocols including photometrical determination of iodine-containing products in nonpolar solvents, X-ray diffraction, and photoluminescence (PL) spectroscopy. The resulting trends of the photostability in an inert atmosphere based on PL spectroscopy measurements demonstrate a nonmonotonic dependence of the degradation rate on the perovskite layer thickness n with a "stability island" at n ≥ 3, which is caused by a combination of antibate factors of electronic structures and chemical compositions in the family of 2D perovskites. We also identify a critical oxygen concentration in the surrounding environment that affects the mechanism and strongly enhances the rate of layered perovskite photodegradation.

Room-temperature continuous-wave vertical-cavity surface-emitting lasers based on 2D layered organic–inorganic hybrid perovskites

Two-dimensional (2D) layered lead halide perovskites with large exciton binding energies, efficient radiative recombination, and outstanding environmental stability are regarded as supreme candidates for realizing highly compact and ultralow threshold lasers. However, continuous-wave (CW) pumped lasing of 2D lead halide perovskites, as the precondition for the electrically pumped lasing, is still challenging. Here, we tackled this challenge by demonstrating lasing emission in phenylethylammonium lead iodide [(PEA)2PbI4] embedded in a vertical microcavity under continuous pumping at room temperature. The millimeter-sized (PEA)2PbI4 single crystal was obtained from a two-step seed-growth method, showing high crystallization, excellent thermal stability, and outstanding optical properties. We used the exfoliated (PEA)2PbI4 thin flake as the gain medium to construct a vertical-cavity surface-emitting laser (VCSEL), showing robust single-mode CW lasing operation with an ultra-low threshold of 5.7 W cm−2 at room temperature, attributed to strong optical confinement in the high-Q cavity. Our findings provide a strategy to design and fabricate solution-based 2D perovskite VCSELs and mark a significant step toward the next-generation of coherent light sources.

Structural chemistry of layered lead halide perovskites containing single octahedral layers.

We present a comprehensive review of the structural chemistry of hybrid lead halides of stoichiometry APbX 4, A 2PbX4 or A A'PbX 4, where A and A' are organic ammonium cations and X = Cl, Br or I. These compounds may be considered as layered perovskites, containing isolated, infinite layers of corner-sharing PbX 4 octahedra separated by the organic species. First, over 250 crystal structures were extracted from the CCDC and classified in terms of unit-cell metrics and crystal symmetry. Symmetry mode analysis was then used to identify the nature of key structural distortions of the [PbX 4]∞ layers. Two generic types of distortion are prevalent in this family: tilting of the octahedral units and shifts of the inorganic layers relative to each other. Although the octahedral tilting modes are well known in the crystallography of purely inorganic perovskites, the additional layer-shift modes are shown to enormously enrich the structural options available in layered hybrid perovskites. Some examples and trends are discussed in more detail in order to show how the nature of the interlayer organic species can influence the overall structural architecture; although the main aim of the paper is to encourage workers in the field to make use of the systematic crystallographic methods used here to further understand and rationalize their own compounds, and perhaps to be able to design-in particular structural features in future work.

Benzodithiophene-Based Spacers for Layered and Quasi-Layered Lead Halide Perovskite Solar Cells.

Incorporating extended pi‐conjugated organic cations in layered lead halide perovskites is a recent trend promising to merge the fields of organic semiconductors and lead halide perovskites. Herein, we integrate benzodithiophene (BDT) into Ruddlesden–Popper (RP) layered and quasi‐layered lead iodide thin films (with methylammonium, MA) of the form (BDT)2MAn−1PbnI3n+1. The importance of tuning the ligand chemical structure is shown as an alkyl chain length of at least six carbon atoms is required to form a photoactive RP (n=1) phase. With N=20 or 100, as prepared in the precursor solution following the formula (BDT)2MAN−1PbNI3N+1, the performance and stability of devices surpassed those with phenylethylammonium (PEA). For N=100, the BDT cation gave a power conversion efficiency of up to 14.7 % vs. 13.7 % with PEA. Transient photocurrent, UV photoelectron spectroscopy, and Fourier transform infrared spectroscopy point to improved charge transport in the device active layer and additional electronic states close to the valence band, suggesting the formation of a Lewis adduct between the BDT and surface iodide vacancies.

Discerning between Exciton and Free-Carrier Behaviors in Ruddlesden–Popper Perovskite Quantum Wells through Kinetic Modeling of Photoluminescence Dynamics

Two-dimensional (2D) Ruddlesden–Popper (RP)-layered lead halide perovskites have recently emerged as a more stable alternative to their three-dimensional (3D) counterparts while also exhibiting int...

Bright Exciton Fine-Structure in Two-Dimensional Lead Halide Perovskites

The fast-growing field of atomically thin semiconductors urges a new understanding of two-dimensional excitons, which entirely determine their optical responses. Here, taking layered lead halide perovskites as an example of unconventional two-dimensional semiconductors, by means of versatile optical spectroscopy measurements, we resolve fine-structure splitting of bright excitons of up to ∼2 meV, which is among the largest values in two-dimensional semiconducting systems. The large fine-structure splitting is attributed to the strong electron-hole exchange interaction in layered perovskites, which is proven by the optical emission in high magnetic fields of up to 30 T. Furthermore, we determine the g-factors for these bright excitons as ∼+1.8. Our findings suggest layered lead halide perovskites are an ideal platform for studying exciton spin-physics in atomically thin semiconductors that will pave the way toward exciton manipulation for novel device applications.

Excitation Dynamics in Layered Lead Halide Perovskite Crystal Slabs and Microcavities

Lead halide perovskites are promising materials for various optoelectronic device applications such as solar cells, light-emitting diodes, and lasers. Three-dimensional perovskites, for example, CH...

Structural variations in (001)-oriented layered lead halide perovskites, templated by 1,2,4-triazolium.

A new hybrid lead(ii) halide perovskite, (TzH)2PbCl4, ([TzH+] = 1,2,4-triazolium), adopts a (001)-oriented layered perovskite structure, which can be considered as derived from the n = 1 Ruddlesden-Popper (RP) type. Variable temperature single crystal X-ray diffraction reveals a structural phase transition in the region 125 K < T < 173 K between a high temperature, high symmetry polymorph, space group Cmcm, and a low temperature, low symmetry chiral polymorph, space group P212121, which has a tripled unit cell volume. UV-Vis spectra suggest a band gap of 3.30 eV for (TzH)2PbCl4. A second polymorph of the bromide analogue, (TzH)2PbBr4-II, is also reported, and structural relationships between all three variants are discussed.

Phase-Pure Hybrid Layered Lead Iodide Perovskite Films Based on a Two-Step Melt-Processing Approach

Layered lead halide perovskites have recently been heavily investigated due to their versatile structures, tunable electronic properties, and better stability compared with 3D perovskites and have ...

Robust, Cationic Lead Halide Layered Materials with Efficient Broadband White-Light Emission

Only a selected class of corrugated 2D hybrid lead halide perovskites exhibit broadband white-light emission from self-trapped excitons. Recently, we have discovered ultrastable layered lead halide photoemitters overcoming the stability problems of perovskites, despite in need of deep-UV irradiation to achieve photoluminescence quantum efficiency (PLQE) of over 10%. Herein, we have employed a robust, nonconjugated dicarboxylate ligand to pillar the cationic 1D [PbBr]+ chains. The unique corrugated stacking of [PbBr]+ chains facilitates the structural deformation to form self-trapped excitons, thus enabling an eightfold enhancement of PLQE over our previous reported bilayer bromoplumbate structures. The PLQE of 17.2% is not only among the highest in all of the layered lead halide white-light emitters, but overcoming the problem of our previous photoemitters requires deep-UV LED excitation. In addition, by tuning the stacking mode of the pillaring molecules, the chloride analog successfully incorporates a s...

Mn Doping in Centimeter-Sized Layered 2D Butylammonium Lead Bromide (BA2PbBr4) Single Crystals and Their Optical Properties

Layered lead halide perovskites such as butylammonium lead bromide (BA2PbBr4) possess a multiple-quantum-well structure, which is expected to enhance the interaction between the excitons and Mn dop...

Melting temperature suppression of layered hybrid lead halide perovskites via organic ammonium cation branching.

Hybrid organic-inorganic lead halide perovskites have attracted broad interest because of their unique optical and electronic properties, as well as good processability. Thermal properties of these materials, often overlooked, can provide additional critical information for developing new methods of thin film preparation using, for example, melt processing-i.e., making films of hybrid perovskites by solidification of a thin layer of the melt liquid. We demonstrate that it is possible to tune the melting temperature of layered hybrid lead iodide perovskites over the range of more than 100 degrees by modifying the structures of alkylammonium-derived organic cations. Through the introduction of alkyl chain branching and extending the length of the base alkylammonium cation, melting temperatures of as low as 172 °C can be achieved and high quality thin films of layered hybrid lead iodide perovskites can be made using a solvent-free melt process with no additives and in ambient air. Additionally, we show that a similar concept can be translated to the corresponding layered bromides, with slightly higher observed melting temperatures. The design rules established here can guide the discovery of new melt-processable perovskite materials for low-cost high performance devices.