High-performance quasi-2D perovskite light-emitting diodes: from materials to devices

Narrow-Linewidth Emission and Weak Exciton-Phonon Coupling in 2D Layered Germanium Halide Perovskites

The photophysical properties of low-dimensional metal-halide semiconductors and their tunability make them promising candidates for light-absorbing and emitting applications. Yet, the germanium-based halide perovskites to date lack desirable light-emitting properties, with so far only very broad, weak, and unstructured photoluminescence (PL) reported due to significant octahedral distortion. Here, the photophysical properties of the 2D layered Ruddlesden-Popper semiconductors (4F-PMA)2GeI4 and (4F-PMA)2PbI4 (4F-PMA: 4-F-phenylmethylammonium) are characterized and compared. Using a combination of single-crystal X-ray diffraction, variable temperature time-resolved PL, and density functional theory, structure-property relations are correlated. Specifically, the results indicate that (4F-PMA)2PbI4 features stronger coupling to longitudinal optical (LO) phonons, assisting emission from a broad bound-exciton state due to a soft, deformable lattice. In contrast, (4F-PMA)2GeI4, benefitting from intermolecular bonding to scaffold a rigid octahedral structure, shows weaker LO-phonon coupling, resulting in the longest PL lifetime and most narrow linewidth (≈120 meV linewidth at 2 K) reported for a Ge-halide perovskite yet, without the occurrence of any additional bound-state emission at low temperatures. These results highlight the potential of germanium halide perovskite materials for optoelectronic applications.

Two-Dimensional Perovskites for Photocatalytic CO2 Reduction

The photocatalytic conversion of Carbon dioxide (CO2) into valuable chemicals is one of the most promising approaches to addressing the CO2 emission problem. However, several issues still need to be resolved to increase the efficiency of photocatalytic reactions. Perovskites possess superior light absorption capacity, tunable band gaps, high defect tolerance, and diverse dimensionality. Among them, two-dimensional (2D) perovskites are more stable under photocatalytic conditions and have exciting excitonic characteristics compared to three-dimensional (3D) perovskites. 2D perovskites have unique physical and chemical properties, such as high stability, polaron formation, quantum well structures, and high exciton binding energies, which remain underexplored for photocatalytic CO2 reduction (pCO2RR). Tuning these properties is easier in 2D perovskites than in 3D perovskites by varying the layer thickness and spacer cations. Therefore, 2D perovskite photocatalysts are emerging as promising materials for reducing CO2 into valuable products. This review discusses the classification and synthesis methods of 2D perovskites, the unique properties that make them favorable for photocatalysis, and recent advances in applying 2D perovskites for pCO2RR by monitoring the operational methodology. It also emphasizes the potential for future developments in photocatalysis using 2D perovskites.

Decoding the Role of Interface Engineering in Energy Transfer: Pathways to Enhanced Efficiency and Stability in Quasi-2D Perovskite Light-Emitting Diodes

Quasi-two-dimensional (quasi-2D) perovskites have emerged as a transformative platform for high-efficiency perovskite light-emitting diodes (PeLEDs), benefiting from their tunable quantum confinement, high photoluminescence quantum yields (PLQYs), and self-assembled energy funneling mechanisms. This review systematically explores interfacial energy transfer engineering strategies that underpin advancements in device performance. By tailoring phase composition distributions, passivating defects via additive engineering, and optimizing charge transport layers, researchers have achieved external quantum efficiencies (EQEs) exceeding 20% in green and red PeLEDs. However, challenges persist in blue emission stability, efficiency roll-off at high currents, and long-term operational durability driven by spectral redshift, Auger recombination, and interfacial ion migration. Emerging solutions include dual-cation/halogen alloying for bandgap control, microcavity photon management, and insulator-perovskite-insulator (IPI) architectures to suppress leakage currents. Future progress hinges on interdisciplinary efforts in multifunctional material design, scalable fabrication, and mechanistic studies of carrier-photon interactions. Through these innovations, quasi-2D PeLEDs hold promise for next-generation displays and solid-state lighting, offering a cost-effective and efficient alternative to conventional technologies.

Dimensionality-Controlled Confinement Effects for Tunable Optoelectronic Properties in Quasi-1D Hybrid Perovskites

Hybrid perovskite dimensional engineering enables the creation of one- to three-dimensional (1D to 3D) networks of corner-sharing metal halide octahedra interspersed by organic cations, offering opportunities to tailor semiconducting properties through quantum- and dielectric-confinement effects. Beyond the discrete options, intermediate dimensionality has been introduced in the form of quasi-2D phases with inorganic layers of varying thickness. The current study extends this approach to quasi-1D lead-iodide systems with variable ribbon widths from 2 to 6 octahedra, stabilized by flexible molecular configurations, cation mixing of organic cations, or guest molecule selection. This family of quasi-1D structures adopts characteristic well-like configurations, with intraoctahedral distortion increasing from the core to the edges. First-principles density-functional theory (DFT) calculations and optical characterizations─i.e., temperature-dependent UV-visible absorption, electro-absorption, photoluminescence, and circular dichroism─collectively demonstrate lower bandgap and exciton binding energy with increased ribbon width due to tailorable quantum confinement and structural distortions. Access to two ribbon widths within a single well-ordered structure yields distinguishable bandgaps and excitonic properties, demonstrating a class of dual-quantum confinement materials within the perovskite family. Our study serves as a starting point, showcasing a paradigm to stabilize increased ribbon widths through further tuning of organic templating effects. This continuum between 2D and 1D structures offers promise for fine-tuning the dimensionality and optoelectronic properties of hybrid perovskites.

Neutral inorganic salt additives universally regulate multicolor perovskites for efficient electroluminescence

Quasi-two-dimensional (quasi-2D) perovskites show great potential in light-emitting diodes. The use of additives plays a key role in the passivation of defects and the suppression of low-dimensional phases. However, the effects of additives vary greatly in different perovskite formulations/films with different light-emitting bands. In this paper, the universal utility of additives was achieved through sodium hexafluorophosphate (NaPF6). Benefitting from the neutral environment of the additives and the hydrogen bonds formed by PF6-/PEA+ (F⋯H-N), the low-dimensional phases are effectively reduced, and the distribution of the medium/high-dimensional phases is more balanced. Meanwhile, the PF6- anionic group can passivate the uncoordinated Pb2+, and reduce the defect density of the film. Finally, a maximum EQE of 16.8% was achieved in quasi-2D PEA2Csn-1PbnBr3n+1 green PeLEDs (514 nm), which was significantly higher than that of the pristine device (a maximum EQE of 10%). Correspondingly, the maximum EQE of sky blue PeLEDs (490 nm) based on the mixed halogen [(PEA)0.75(GA)0.25]2CsPb2X7 (X = Br and Cl) component can be increased from 5.6% to 9.6%. The maximum EQE of pure blue PeLEDs (474 nm) can be increased from 3% to 4%. This class of neutral additives provides a solution for high-performance quasi-2D perovskite electroluminescence.

Optimizing Excited Charge Dynamics in Layered Halide Perovskites through Compositional Engineering

Dion-Jacobson phase multilayered halide perovskites (MLHPs) improve carrier transport and optoelectronic performance thanks to their shorter interlayer distance, long carrier lifetimes, and minimized nonradiative losses. However, limited atomistic insights into dynamic structure-property relationships hinder rational design efforts to further boost their performance. Here, we employ nonadiabatic molecular dynamics, time-domain density functional theory, and unsupervised machine learning to uncover the impact of A-cation mixing on controlling the excited carrier dynamics and recombination processes in MLHPs. Mixing smaller-sized Cs with methylammonium in MLHP weakens electron-phonon interactions, suppresses the nonradiative losses, and slows down intraband hot electron relaxations. On the contrary, larger-sized guanidinium incorporation accelerates nonradiative relaxations. The mutual information analyses reveal the importance of interlayer distances, intra- and interoctahedral angle dynamics, and A-cation motion in extending the excited carrier lifetime by mitigating nonradiative losses in MLHPs. Our work provides a guideline for strategically choosing A-cations to boost the optoelectronic performance of layered halide perovskites.

Structural rigidity, thermochromism and piezochromism of layered hybrid perovskites containing an interdigitated organic bilayer

Layered hybrid perovskites are intensively researched today as highly tunable materials for efficient light harvesting and emitting devices. In classical layered hybrid perovskites, the structural rigidity mainly stems from the crystalline inorganic sublattice, whereas the organic sublattice has a minor contribution to the rigidity of the material. Here, we report two layered hybrid perovskites, (BTa)2PbI4 and (F2BTa)2PbI4, which possess substantially more rigid organic layers due to hydrogen bonding, π-π stacking, and dipole-dipole interactions. These layered perovskites are phase stable under elevated pressures up to 5 GPa and upon temperature lowering down to 80 K. The organic layers, composed of benzotriazole-derived ammonium cations, are among the most rigid in the field of layered hybrid perovskites. We characterize structural rigidity using in situ single-crystal X-ray diffraction during compression up to 5 GPa. Interestingly, the enhanced rigidity of the organic sublattice does not seem to transfer to the inorganic sublattice, leading to an uncommon material configuration with rigid organic layers and deformable inorganic layers. The deformability of the inorganic sublattice is apparent from differences in optical properties between the crystal bulk and surface. Supported by first-principles calculations, we assign these differences to energy transfer processes from the surface to the bulk. The deformability also leads to reversible piezochromism due to shifting of the photoluminescence emission peak with increasing pressure up to 5 GPa, and thermochromism due to narrowing of the photoluminescence emission linewidth with decreasing temperature down to 80 K. This raises the possibility of applying these phase-stable layered hybrid perovskite materials in temperature and/or pressure sensors.

Low-Threshold Amplified Spontaneous Emission from Quasi-2D Lead-Bromide Perovskites for Lasing Applications

Quasi-two-dimensional (quasi-2D) lead halide perovskite materials have shown great potential as gain media for amplified spontaneous emission (ASE) and lasing. Due to the complexity of the mixed-dimensional perovskite materials, factors influencing their ASE thresholds remain unclear, limiting the pace of development in this emerging area of research. Here, we report exceptionally low ASE thresholds of ∼2.23 μJ cm-2 with high stability in quasi-2D lead-bromide perovskite semiconductors. Improved gain coefficients, suppressed Auger recombination, effective coupling between the optical field and the gain medium, and minimized scattering losses are found to be some of the key contributors to the low-threshold ASE. The optimized materials lead to the demonstration of a low-threshold, single-mode perovskite laser based on a distributed feedback (DFB) optical resonator, yielding a low lasing threshold of 0.69 μJ cm-2. We expect our findings to clarify some of the key design principles of low-threshold ASE in perovskite semiconductors for lasing applications.

Theoretical design of 2D Pca21 SiNOX (X = H, F, and Cl) phases: a new family of flexible wide bandgap semiconductors

By first-principles calculations, a new family of two-dimensional (2D) Pca21 SiNOX (X = H, F, and Cl) phases were rationally designed by theoretical exfoliation of bulk layered α-LiSiON compounds, taking advantage of the in- and out-of-plane bonding anisotropy of the bulk parental compound. It is found that 2D Pca21 SiNOX phases have wide direct and quasi-direct bandgaps of 4.99-6.33 eV using the HSE06 functional with good thermodynamic, mechanical, dynamic, and thermal stabilities. In addition, the flexibility of 2D Pca21 SiNOX structures was evidenced with moderate in-plane Young's moduli of 133.27-141.87 N m-1, ideal strength of 6.06-6.56 N m-1, and out-of-plane bending strength of 1.41-1.57 eV. What is more, the stronger anharmonicity of 2D Pca21 SiNOH leads to lower lattice thermal conductivities, in comparison with 2D Pca21 SiNOF and SiNOCl. Finally, isovalent elemental substitutions are adopted to tune the bandgaps of 2D Pca21 SiNOX phases within the range of 0.54-6.64 eV with the HSE06 functional and ten wide bandgap semiconductors (2D Pca21 CNOH, GeNOH, CNOF, GeNOF, CNOCl, SiNOCl, GeNOCl, SiPOCl, SiNSH, and SiNSeH) were unveiled with bandgaps larger than 3.5 eV. Our findings enrich the family of 2D wide bandgap semiconductors, and also highlight the promising multi-functional electronic applications of 2D Pca21 SiNOX phases.

Efficient Spin-Light-Emitting Diodes With Tunable Red to Near-Infrared Emission at Room Temperature

Spin light-emitting diodes (spin-LEDs) are important for spin-based electronic circuits as they convert the carrier spin information to optical polarization. Recently, chiral-induced spin selectivity (CISS) has emerged as a new paradigm to enable spin-LED as it does not require any magnetic components and operates at room temperature. However, CISS-enabled spin-LED with tunable wavelengths ranging from red to near-infrared (NIR) has yet to be demonstrated. Here, chiral quasi-2D perovskites are developed to fabricate efficient spin-LEDs with tunable wavelengths from red to NIR region by tuning the halide composition. The optimized chiral perovskite films exhibit efficient circularly polarized luminescence from 675 to 788 nm, with a photoluminescence quantum yield (PLQY) exceeding 86% and a dissymmetry factor (glum) ranging from 8.5 × 10-3 to 2.6 × 10-2. More importantly, direct circularly polarized electroluminescence (CPEL) is achieved at room temperature in spin-LEDs. This work demonstrated efficient red and NIR spin-LEDs with the highest external quantum efficiency (EQE) reaching 12.4% and the electroluminescence (EL) dissymmetry factors (gEL) ranging from 3.7 × 10-3 to 1.48 × 10-2 at room temperature. The composition-dependent CPEL performance is further attributed to the prolonged spin lifetime as revealed by ultrafast transient absorption spectroscopy.

Manipulation of Metal Halide Perovskite: Photoelectric Conversion or Light Emission?

Metal halide perovskites (MHPs) display a range of superior photophysical properties, rendering them promising as a candidate for the active medium of high-efficiency photovoltaic and electroluminescence devices. In order to maximize their efficacy in photoelectric conversion or light emission, it is essential to regulate the charge separation efficiency of MHPs in a desired manner. Herein, we demonstrate that the extent of charge separation can be effectively manipulated upon thermal annealing treatment on MHPs. As the annealing time is extended from 10 to 30 min, the accumulation of excess lead halides is observed at the boundaries of MHP grains, resulting in the construction of a quasi-Type II band alignment between the lead halide and the MHP. This facilitates the separation of electron-hole pairs, reducing the exciton binding energy from approximately 102 meV to a level comparable with kBT. Our findings elucidate the transition of MHPs from a light-emission material to a photoelectric-conversion material along with continuous heating treatment, which is anticipated to guide the flexible regulation of MHPs to meet the requirements of specific practical applications.

Screw-Dislocation-Driven Growth of 2D Perovskite Spiral Microplates

Two-dimensional (2D) organic-inorganic halide perovskites are solution-processable semiconductors that are promising for optoelectronic applications. Understanding crystallization mechanisms to achieve control over nanostructures is important for optimizing desired properties. Here we introduce a versatile strategy to synthesize spiral microplates of diverse 2D perovskites at the air-water interface through screw-dislocation-driven growth. Spirals of 11 2D perovskite compositions (LA)2(A)n-1PbnX3n+1 with different spacer (LA) cations, A-cations, halide (X) anions, and n-number can be grown. They typically consist of single- or few-layer perovskite step heights but exhibit stacking complexity when multiple dislocations interact. The spiral microplates exhibit the characteristic optical properties (photoluminescence and second-harmonic generation) of the underlying 2D perovskites. Fluorescence-detected circular dichroism imaging shows that the chirality of the spiral center does not translate to the observed chiroptical properties of the microplate, consistent with the length scale of the chiral distortion. This solution growth of perovskite spirals diversifies the perovskite microstructures for optoelectronics and other applications.

2D (NH4)BiI3 enables non-volatile optoelectronic memories for machine learning

Machine learning is the core of artificial intelligence. Using optical signals for training and converting them into electrical signals for inference, combines the strengths of both, and thus can greatly improve machine learning efficiency. Optoelectronic memories are the hardware foundation for this strategy. However, the existing optoelectronic memories cannot modulate a large number of non-volatile resistive states using ultra-short and ultra-dim light pulses, leading to low training accuracy, slow computing speed and high energy consumption. Here, we synthesized a van der Waals layered photoconductive material, (NH4)BiI3, with excellent photoconductivity and strong dielectric screening effect. We further employed it as the photosensitive control gate in a floating-gate transistor, replacing the commonly used metal control gate, to construct an optical floating gate transistor which achieves adjustable synaptic weights under ultra-dim light without gate voltage assistance. Moreover, it shows ultra-low training energy consumption to generate a non-volatile state and the largest resistive state numbers among the known non-volatile optoelectronic memories. These exceptional performances enable the construction of one-transistor-one-memory device arrays to achieve ~99% accuracy in Artificial Neural Networks. Moreover, the device arrays can match the performance of GPU in YOLOv8 while greatly reducing energy consumption.

Vapor-Solid Reaction Techniques for the Growth of Organic-Inorganic Hybrid Perovskite Thin Films

Perovskite solar cells are considered next-generation photovoltaic technology due to their remarkable advancements in power conversion efficiency. To transition this technology from the lab to industry, the method for preparing perovskite thin films must support mass production. Currently, the solution-based slot-die technique is the primary method for depositing large-area perovskite thin films. However, solution-based methods are not standard in the semiconductor industry, where vapor-based techniques are favored for their high controllability and reproducibility. The cost of vacuum facilities and the complexity of these processes hinder many researchers, resulting in vapor-based technique development lagging behind solution-based methods in device efficiency and scale. This review focuses on the progress in growing perovskite thin films using vapor-solid reaction techniques, which are believed to offer the most direct path to commercialization. By examining the crystallization and growth mechanisms of perovskite films and discussing specific optimization strategies for vapor-solid reactions, insights into future developments and challenges in fabricating perovskite solar cells using fully vacuum processes are concluded.

Regulation of nucleation and crystallization for blade-coating large-area CsPbBr3 perovskite light-emitting diodes

Metal halide perovskite light-emitting diodes (PeLEDs) and large-area perovskite color conversion layers for liquid crystal display exhibit great potential in the field of illumination and display. Blade-coating method stands out as a highly suitable technique for fabricating large-scale films, albeit with challenges such as uneven nucleation coverage and non-uniformity crystallization process. In this work, we developed an in-situ characterization measurement system to monitor the perovskite nucleation, and crystallization process. By incorporating formamidine acetate (FAAc) into perovskite precursor solutions, the nucleation rate and nuclei density of perovskite were increased, leading to more uniform nucleation. In addition, we inserted a layer of [2-(9H-carbazol-9-yl)ethyl] phosphonic acid above the poly(9-vinylcarbazole) hole transport layer. This layer acts as an anchor for the perovskite nano-crystal nuclei formed in the precursor, enhancing the steric hindrance of the solute and subsequently slowing down the crystal growth rate, thereby improving crystal quality. Based on these improvements, large-area perovskite nano-polycrystalline films with significantly improved uniformity and enhanced photoluminescence quantum yield were obtained. A small-area PeLED (2 mm × 2 mm) with a maximum external quantum efficiency of 25.91% was realized, marking the highest record of PeLED prepared by blade-coating method to date. An ultra-large-area PeLED (5 cm × 7 cm) was also prepared, which is the largest PeLED prepared by the solution method reported so far.

Structurally and Electronically Anisotropic Nature of Bridgman-Grown Cs3Sb2Br9 Perovskite Single Crystal toward Efficient Photodetector

Cs3Sb2Br9, as a sort of novel lead-free perovskite single crystal, has the merits of high carrier mobility and a long diffusion length. However, the large-sized and high-crystallized Cs3Sb2Br9 single crystals are not easily obtained. Herein, we apply the vertical Bridgman method to grow centimeter-sized Cs3Sb2Br9 single crystal. The temperature-dependent crystal structure of Cs3Sb2Br9 is in situ characterized in the temperature range of 100-400 K. A novel crystallographic and electronic structure anisotropy of the as-grown Cs3Sb2Br9 single crystal along the transmission directions of [100] and [001] is experimentally and theoretically proved. Owing to the layered two-dimensional (2D) structure of Cs3Sb2Br9, quantum confinement effects prolong the lifetime of hot carriers, leading to their accumulation within the Sb-Br plane along the [100] direction, thereby resulting in a higher density of electronic states. Accordingly, the [100] device exhibits a carrier mobility higher than that of the [001] device, with the [100] device mobility being 4 orders of magnitude higher than that of the [001] device at 423 K, showing a remarkable anisotropy. The [100] device also shows responsivity ∼10 times higher than that of the [001] device.

Bottom Electrode Modification Enables Efficient and Bright Silicon-Based Top-Emission Perovskite Light-Emitting Diodes

The integration of perovskites with mature silicon platform has emerged as a promising approach in the development of efficient on-chip light sources and high-brightness displays. However, the performance of Si-based green perovskite light-emitting diodes (PeLEDs) still falls significantly short compared to their red and near-infrared counterparts. In this study, it is revealed that the high work function Au, widely employed in Si-based top-emission PeLEDs as the reflective bottom electrode, exhibits considerably lower reflectivity in the green spectrum than in the longer wavelengths. Consequently, Ag electrode is introduced to replace Au to enhance the green light reflectivity, and the ultrathin MoO3 and self-assembled monolayers (SAMs) are sequentially deposited for surface modification. These results indicate that the MoO3 layer removes the energy barrier at Ag/polymer hole transport layer interface, enhancing the hole injection efficiency; while the SAMs firmly anchor onto the MoO3 layer, effectively preventing interfacial defect formation. Benefited from this organic/inorganic dual-layer modification strategy, Si-based green PeLEDs with an impressive peak external quantum efficiency of 18.2% and a maximum brightness of 81931 cd m-2 are successfully fabricated, on par with those of the red and near-infrared counterparts. This achievement marks an advancement in developing high-performance Si-based PeLEDs with full-spectrum output.

Screening Fluorination Additives for Efficient Perovskite Light-Emitting Diodes

Fluorinated additives are proposed to address the issue of domain polydispersity in quasi-2D perovskites. However, the lack of established screening criteria for these additives necessitates a laborious and costly trial-and-error process. Herein, this work explores the behind nature for the first time how various fluorination in fluorinated additives affect domain distribution in quasi-2D perovskites. The studies reveal that fully fluorinated additives could suppress undesirable low-dimensional domains, facilitating efficient energy transfer, while partially fluorinated additives adversely cause deteriorated optical properties. The observed trend is ascribed to the molecular dipole moment of the fluorinated additives, offering a reasonable explanation for experimental phenomena that has never been reported before. By employing fully fluorinated additives, the fabricated sky-blue perovskite light-emitting diodes (PeLEDs) deliver a peak external quantum efficiency (EQE) of 20.38% (@488 nm), marking as one of the highest efficiencies reported to date. The finding provides a screening criterion for fluorinated additives to realize efficient PeLEDs.

1.4% External Quantum Efficiency 988 nm Light Emitting Diode Based on Tin-Lead Halide Perovskite

In recent years, metal halide perovskite-based light-emitting diodes (LEDs) have garnered significant attention as they display high quantum efficiency, good spectral tunability, and are expected to have low processing costs. When the peak emission wavelength is beyond 900 nm the interest is even higher because of the critical importance of this wavelength for biomedical imaging, night vision, and sensing. However, many challenges persist in fabricating these high-performance NIR LEDs, particularly for wavelengths above 950 nm, which appear to be limited by low radiance and poor stability. In this study, 3-(aminomethyl) piperidinium (3-AMP) is employed as a bulk additive for a tin-lead halide perovskite. The 3-AMP passivated films exhibit a significantly longer carrier lifetime of over 1 µs compared to neat films (0.43 µs) or to those passivated with a perfluorinated aromatic mono-ammonium molecule (0.41 µs). Our optimized tin-lead halide perovskite-based LEDs show a single emission peak at 988 nm and an external quantum efficiency (EQE) of ≈1.4%.

Electron Correlation in 2D Periodic Systems from Periodic Bootstrap Embedding

Given the growing significance of 2D materials in various optoelectronic applications, it is imperative to have simulation tools that can accurately and efficiently describe electron correlation effects in these systems. Here, we show that the recently developed bootstrap embedding (BE) accurately predicts electron correlation energies and structural properties for 2D systems. Without explicit dependence on the reciprocal space sum (k-points) in the correlation calculation, our proof-of-concept calculations shows that BE can typically recover ∼99.5% of the total minimal basis electron correlation energy in 2D semimetal, insulator, and semiconductors. We demonstrate that BE can predict lattice constants and bulk moduli for 2D systems with high precision. Furthermore, we highlight the capability of BE to treat electron correlation in twisted bilayer graphene superlattices with large unit cells containing hundreds of carbon atoms. We find that as the twist angle decreases toward the magic angle, the correlation energy initially decreases in magnitude, followed by a subsequent increase. We conclude that BE is a promising electronic structure method for future applications to 2D materials.

Optimizing Perovskite Surfaces to Enhance Post-Treatment for Efficient Blue Mixed-Halide Perovskite Light-emitting Diodes

The halide postdeposition treatment technique is a widely used strategy for mitigating defects in perovskite. However, when applied to mixed-halide perovskites, it often leads to surface and internal halide heterogeneity, which compromises luminescence performance and spectral stability. In this work, blue mixed-halide 3D perovskites are engineered with acetate (Ac⁻)-rich surfaces to optimize the post-treatment process and achieve halide homogeneity. The findings demonstrate that the strong interaction between surface Ac⁻ ions and Pb2+ ions significantly reduces the formation of halide vacancy defects caused by the washing effect of isopropanol during post-treatment. This defect reduction slows the infiltration of halide ions into the perovskite lattice, providing more time for surface reconstruction and minimizing the accumulation of introduced halide ions at the surface. As a result, a mild halide redistribution occurs, promoting the formation of a uniform mixed-halide perovskite phase. This approach enabled the development of blue mixed-halide 3D PeLEDs with a record external quantum efficiency of 19.28% (emission peak at 482 nm), comparable to state-of-the-art blue reduced-dimensional perovskite-based PeLEDs. Additionally, the device demonstrated a narrowband and stable electroluminescence spectrum with a full width at half maximum (FWHM) of less than 16 nm.

Bright Structural-Phase-Pure CsPbI3 Core-PbSO4 Shell Nanoplatelets With Ultra-Narrow Emission Bandwidth of 77 meV at 630 nm

Achieving a narrow emission bandwidth is long pursued for display applications. Among all primary colors, obtaining pure red emission with high visual perception is the most challenging. In this work, CsPbI3 halide perovskite nanoplatelets (NPLs) with rigorously controlled 2D  [PbI6]4- octahedron layer number (n) are demonstrated. A perovskite core-PbSO4 shell structure is designed to prevent aggregation and fusion between NPLs, enabling consistent thickness and quantum confinement strength for each NPL. Consequently, exact n = 4 CsPbI3 NPLs are demonstrated, exhibiting emission peaks around 630 nm, with very narrow spectral bandwidths of <24 nm and high absolute photoluminescence quantum yields up to 85%. The emission of n = 4 NPLs falls exactly within the pure-red region, closely aligning with the International Telecommunication Union Recommendation BT.2020  standard. Measurements suggest predominant stability and color homogeneity compared to traditional red-emitting CsPbIxBr3- x nanocrystals. Finally, proof-of-concept pure-red emissive light-emitting diodes (LEDs) are demonstrated by integrating n = 4 CsPbI3 NPLs films with a blue LED chip, showing an excellent external quantum efficiency of 18.3% and high brightness exceeding 3 × 106 nits. Stringent requirements for future display technologies, are satisfied based on the high color purity, stability, and brightness of CsPbI3 NPLs.

Gradient Hole Injection Inducing Efficient Exciton Recombination in Blue (475 nm) Perovskite QLEDs

Perovskite quantum dots (QDs) are emerging as excellent light sources for light-emitting diodes (LEDs). However, the performance of blue perovskite QD-based LEDs (QLEDs) still lags behind that of red and green counterparts, which is hindered by blue perovskite QDs with broad bandgaps that tend to increase nonradiative recombination. Here, we designed a gradient energy for hole injection utilizing multiple hole injection layers (HTLs) combined with carbazole-based small-molecule modification to reduce the hole injection barrier between HTLs and QD layers and improve the hole injection efficiency, realizing efficient exciton recombination in blue perovskite QLEDs. Moreover, the QD film on the designed HTLs demonstrates a lower surface roughness and improved photoluminescence properties. The optimized blue CsPbCl3-xBrx QLEDs exhibit an impressive external quantum efficiency of 20.7% with an electroluminescence peak at 475 nm and a turn-on voltage of 2.6 V, representing the state-of-the-art for blue perovskite LEDs emitting in the range of 460-480 nm.

Exciton Dynamics in Layered Halide Perovskite Light-Emitting Diodes

Layered halide perovskites have garnered significant interest due to their exceptional optoelectronic properties and great promises in light-emitting applications. Achieving high-performance perovskite light-emitting diodes (PeLEDs) requires a deep understanding of exciton dynamics in these materials. This review begins with a fundamental overview of the structural and photophysical properties of layered halide perovskites, then delves into the importance of dimensionality control and cascade energy transfer in quasi-2D PeLEDs. In the second half of the review, more complex exciton dynamics, such as multiexciton processes and triplet exciton dynamics, from the perspective of LEDs are explored. Through this comprehensive review, an in-depth understanding of the critical aspects of exciton dynamics in layered halide perovskites and their impacts on future research and technological advancements for layered halide PeLEDs is provided.

Efficient and Stable Red Perovskite Light-Emitting Diodes via Thermodynamic Crystallization Control

Efficient and stable red perovskite light-emitting diodes (PeLEDs) demonstrate promising potential in high-definition displays and biomedical applications. Although significant progress has been made in device performance, meeting commercial demands remains a challenge in the aspects of long-term stability and high external quantum efficiency (EQE). Here, an in situ crystallization regulation strategy is developed for optimizing red perovskite films through ingenious vapor design. Mixed vapor containing dimethyl sulfoxide and carbon disulfide (CS2) is incorporated to conventional annealing, which contributes to thermodynamics dominated perovskite crystallization for well-aligned cascade phase arrangement. Additionally, the perovskite surface defect density is minimized by the CS2 molecule adsorption. Consequently, the target perovskite films exhibit smooth exciton energy transfer, reduced defect density, and blocked ion migration pathways. Leveraging these advantages, spectrally stable red PeLEDs are obtained featuring emission at 668, 656, and 648 nm, which yield record peak EQEs of 30.08%, 32.14%, and 29.04%, along with prolonged half-lifetimes of 47.7, 60.0, and 43.7 h at the initial luminances of 140, 250, and 270 cd m-2, respectively. This work provides a universal strategy for optimizing perovskite crystallization and represents a significant stride toward the commercialization of red PeLEDs.

Monoammonium Modified Dion-Jacobson Quasi-2D Perovskite for High Efficiency Pure-Blue Light Emitting Diodes

Quasi-2D perovskites exhibit impressive optoelectronic properties and hold significant promise for future light-emitting devices. However, the efficiency of perovskite light-emitting diodes (PeLEDs) is seriously limited by defect-induced nonradiative recombination and imbalanced charge injection. Here, the defect states are passivated and charge injection balance is effectively improved by introducing the additive cyclohexanemethylammonium (CHMA) to bromide-based Dion-Jacobson (D-J) structure quasi-2D perovskite emission layer. CHMA participates in the crystallization of perovskite, leading to high quality film composed of compact and well-contacted grains with enhanced hole transportation and less defects. As a result, the corresponding PeLEDs exhibit stable pure blue emission at 466 nm with a maximum external quantum efficiency (EQE) of 9.22%. According to current knowledge, this represents the highest EQE reported for pure-blue PeLEDs based on quasi-2D bromide perovskite thin films. These findings underscore the potential of quasi-2D perovskites for advanced light-emitting devices and pave the way for further advancements in PeLEDs.

CsPb2Br5 Plates/Quasi-2D Perovskite Heterojunction for Efficient Sky-Blue Light-Emitting Diodes

Perovskite light-emitting diodes (PeLEDs) have gained significant attention owing to their remarkable tunability and color stability, and substantial progress has been made with green and red PeLEDs. However, the advancement of blue PeLEDs still lags far behind their red and green counterparts. In this study, we report efficient sky-blue PeLEDs utilizing an in situ fabricated CsPb2Br5 plates/quasi-2D perovskite heterojunction using chelating molecules to modulate the crystallization process of perovskites. The wide bandgap of CsPb2Br5 facilitated the formation of a type-I band alignment at the heterojunction, allowing efficient carrier transfer from CsPb2Br5 to CsPbBr3. This heterojunction leads to a noteworthy enhancement of device efficiency. The PeLEDs exhibit a maximum brightness of 2311 cd m-2, accompanied by a maximum external quantum efficiency of 12.86% at 487 nm. Our tailored design of CsPb2Br5/perovskite heterojunction thin films offers a promising avenue for advancing PeLED performance. This work contributes valuable insights into the burgeoning field of perovskite electroluminescence, paving the way for further optimization of PeLED technologies.

Coherence Programming for Efficient Linearly Polarized Perovskite Light-Emitting Diodes

Although quasi-two-dimensional (quasi-2D) perovskites are ideal material platforms for highly efficient linearly polarized electroluminescence owing to their anisotropic crystal structures, so far, there has been no practical implementation of these materials for the demonstration of linearly polarized perovskite light-emitting diodes (LP-PeLEDs). This scarcity is due to difficulty in orientation and phase distribution control of the quasi-2D perovskites while minimizing the defects, all of which are required to manifest aligned transition dipole moments (TDMs). To achieve this multifaceted goal, herein, we introduce a synergistic strategy to quasi-2D perovskites by incorporating both a trimethylolpropane triacrylate anchoring layer and 18-Crown-6 molecular passivator into the film fabrication process. It is found that the interfacial anchoring layer guides the oriented growth of perovskites along the (110) plane, whereas the molecular passivator reduces the number of defects and homogenizes the crystal phase. As a result, a quasi-2D perovskite film with macroscopically aligned TDM that renders high radiative recombination and the degree of linear polarization (DoLP) is constructed. This "coherence-programmed emission layer" demonstrates highly efficient LP-PeLEDs, not only achieving a maximum external quantum efficiency of ∼23.7%, a brightness of ∼36,142 cd/m2, and a DoLP of ∼38%, but also significantly improving the signal-to-interference-and-noise ratio in a multi-cell visible light communication system.

Reducing Dielectric Confinement Effect Enhances Carrier Separation in Two-Dimensional Hybrid Perovskite Photocatalysts

Two-dimensional organic-inorganic hybrid perovskites (OIHPs) with alternating structure of the organic and inorganic layers have a natural quantum well structure. The difference of dielectric constants between organic and inorganic layers in this structure results in the enhancement of dielectric confinement effect, which exhibits a large exciton binding energy and hinders the separation of electron-hole pairs. Herein, a strategy to reduce the dielectric confinement effect by narrowing the dielectric difference between organic amine molecule and [PbBr6]4- octahedron is put forward. The Ethanolamine (EOA) contains hydroxyl groups, resulting in the positive and negative charge centers of O and H non-overlapping, which generated a larger polarity and dielectric constant. The reduced dielectric constant produces a smaller exciton binding energy (71.03 meV) of (C2H7NO)2PbBr4 ((EOA)2PbBr4) than (C8H11N)2PbBr4 ((PEA)2PbBr4 (156.07 meV), and promotes the dissociation of electrons and holes. The increasing of lifetime of photogenerated carrier in (EOA)2PbBr4 are proved by femtosecond transient absorption spectra. Density functional theory (DFT) calculations have also indicated that the small energy shift of the total density of states (DOS) between the C/H/N and the Pb/Br in (EOA)2PbBr4 favors the separation of electrons and holes. In addition, this work demonstrates the application of (PEA)2PbBr4 and (EOA)2PbBr4 in the field of photocatalytic CO2 reduction.

Van der Waals integrated single-junction light-emitting diodes exceeding 10% quantum efficiency at room temperature

The construction of miniaturized light-emitting diodes (LEDs) with high external quantum efficiency (EQE) at room temperature remains a challenge for on-chip optoelectronics. Here, we demonstrate microsized LEDs fabricated by a dry-transfer van der Waals (vdW) integration method using typical layered Ruddlesden-Popper perovskites (RPPs). A single-crystalline layered RPP nanoflake is used as the active layer and sandwiched between two few-layer graphene contacts, forming van der Waals LEDs (vdWLEDs). Strong electroluminescence (EL) emission with a low turn-on current density of ~20 pA μm-2 and high EQE exceeding 10% is observed at room temperature, which sets the benchmark for the EQE of vdWLEDs ever recorded. Such efficient EL emission is attributed to the inherent multiple quantum well structure and high photoluminescence quantum yield (~35%) of RPPs and a low charge injection barrier of ~0.10 eV facilitated by the Fowler-Nordheim tunneling mechanism. These findings promise a scalable pathway for accessing high-performance miniaturized light sources for on-chip optical optoelectronics.

Molecular Design Engineering Regulates the Ground-State Passivation Ability of Benzophenone Derivatives in Perovskite Solar Cells

Intramolecular hydrogen bonding (H-bonding) involved in the excited-state proton transfer (ESPT) process results in benzophenone derivatives (BPDs) with an excellent ability to passivate defects. However, the BPDs are in a continuing dynamic transition process between the ground state and the excited state under light radiation conditions. The ground-state BPDs may lose their ability to passivate defects, resulting in an increased defect density of the perovskite. Therefore, enhancing the passivation ability of the ground-state BPDs can help to achieve the full passivation ability of their ground state to excited state. Herein, we have researched the various BPDs by density functional theory and found that intramolecular H-bonding can weaken the passivation ability of ground-state BPDs, but intramolecular H-bonding is indispensable in the ESPT process. To address the issue, we investigated the influence of electron-donor properties and dipole moments of hydroxyl (-OH), methoxy (-OCH3), and n-octyloxy (-OC8H17) groups in BPD molecules on their coordination capacity through molecular design engineering. Ultimately, 2-hydroxy-4-n-octyloxy-benzophenone (UV5) with strong electron-donor n-octyloxy (-OC8H17) and elongated carbon-chain structure was selected as an additive, which enhances the passivate defect capability in both the ground and excited states. As a result, the UV5-based champion device achieved a power conversion efficiency (PCE) of 24.46% and remained at 75% of the initial PCE with exposure to UV light. This work focuses on the defect passivation capability of ground-state BPDs for the first time and opens a new concept for applying BPDs in PSCs.

Toward Thin-Film Laser Diodes with Metal Halide Perovskites

Metal halide perovskite semiconductors hold a strong promise for enabling thin-film laser diodes. Perovskites distinguish themselves from other non-epitaxial media primarily through their ability to maintain performance at high current densities, which is a critical requirement for achieving injection lasing. Coming in a wide range of varieties, numerous perovskites delivered low-threshold optical amplified spontaneous emission and optically pumped lasing when combined with a suitable optical cavity. A progression toward electrically pumped lasing requires the development of efficient light-emitting structures with reduced optical losses and high radiative efficiency at lasing-level current densities. This involves a set of important trade-offs in terms of material choice, stack and waveguide design, as well as resonator integration. In this Perspective, the key milestones are highlighted that have been achieved in the study of passive optical waveguides and light-emitting diodes, and these learnings are translated toward more complex laser diode architectures. Finally, a novel resonator integration route is proposed that is capable of relaxing optical and electrical design constraints.

Improvement of the Stability of Quantum-Dot Light Emitting Diodes Using Inorganic HfOx Hole Transport Layer

One of the major challenges in QLED research is improving the stability of the devices. In this study, we fabricated all inorganic quantum-dot light emitting diodes (QLEDs) using hafnium oxide (HfOx) as the hole transport layer (HTL), a material commonly used for insulator. Oxygen vacancies in HfOx create defect states below the Fermi level, providing a pathway for hole injection. The concentration of these oxygen vacancies can be controlled by the annealing temperature. We optimized the all-inorganic QLEDs with HfOx as the HTL by changing the annealing temperature. The optimized QLEDs with HfOx as the HTL showed a maximum luminance and current efficiency of 66,258 cd/m2 and 9.7 cd/A, respectively. The fabricated all-inorganic QLEDs exhibited remarkable stability, particularly when compared to devices using organic materials for the HTL. Under extended storage in ambient conditions, the all-inorganic device demonstrated a significantly enhanced operating lifetime (T50) of 5.5 h, which is 11 times longer than that of QLEDs using an organic HTL. These results indicate that the all-inorganic QLEDs structure, with ITO/MoO3/HfOx/QDs/ZnMgO/Al, exhibits superior stability compared to organic-inorganic hybrid QLEDs.

Recent Advances in Structure Design and Application of Metal Halide Perovskite-Based Gas Sensor

Metal halide perovskites (MHPs) are emerging gas-sensing materials and have attracted considerable attention in gas sensors due to their unique bandgap structure and tunable optoelectronic properties. The past decade has witnessed significant developments in the gas-sensing field; however, their intrinsic structural instability and ambiguous gas-sensing mechanisms hamper their practical applications. Herein, we summarize the recent advances in MHP-based gas sensors. The physicochemical properties of MHPs are discussed at first. The structure design, including dimension design and engineering design, is overviewed as well as their fabrication methods, and we put forward our insights into the gas-sensing mechanism of MHPs. It is believed that enhanced understanding of gas-sensing mechanisms of MHPs are helpful for their application as gas-sensing materials, and structure design can enhance their stability, sensing sensitivity, and selectivity to target gases as gas sensors. Subsequently, the latest developments in MHP-based gas sensors are summarized according to their different application scenarios. Finally, we conclude with the current status and challenges in this field and propose future perspectives.

Enhanced exciton-phonon coupling in pseudohalide 2D perovskite for X-ray to visible light detection

Two-dimensional Cs2Pb(SCN)2I2 perovskite has an ultra-small interlayer spacing, inducing a series of interesting semiconductor properties, such as efficient interlayer carrier transport as well as lower exciton binding energy compared to perovskites with long alkylammonium cations. The prepared Cs2Pb(SCN)2I2 photodetector achieves a high on/off ratio of 2.1 × 104 with a specific detectivity of 6.2 × 1011 jones to 450 nm visible light, and a high sensitivity of 2747 μC Gyair-1 cm-2 to X-rays.

Pure-Phase Perovskite Quantum Well for Green Light-Emitting Diodes

Perovskite multiple quantum wells (MQWs) have shown great potential in the field of light-emitting diodes (LEDs). However, the random formation of QWs with varying well widths (n numbers) often leads to suboptimal interface defects and charge transport issues. Here, we reveal that the crystallization sequence of bromide-based perovskite MQWs is large-n QWs preceding small-n QWs. With this insight, we prevent the crystallization of subsequent small-n QWs by reducing the crystallization rate, ultimately resulting in the crystallization of only n = 5 QWs. This reduction in the crystallization rate is achieved through the chemical interaction of dual additives with perovskite constituents. Additionally, the chemical interaction effectively passivates the uncoordinated lead ions defects. Consequently, pure-phase perovskite QWs with a high photoluminescence quantum efficiency of 75% are achieved. The resulting green LEDs achieve a peak external quantum efficiency of 17.1% and a maximum luminance of 29,480 cd m-2, which is attractive for full-color display applications of perovskites.

Chemical Reaction Modulated Low-Dimensional Phase Toward Highly Efficient Sky-Blue Perovskite Light-Emitting Diodes

Blue perovskite light-emitting diodes (PeLEDs) are crucial avenues for achieving full-color displays and lighting based on perovskite materials. However, the relatively low external quantum efficiency (EQE) has hindered their progression towards commercial applications. Quasi-two-dimensional (quasi-2D) perovskites stand out as promising candidates for blue PeLEDs, with optimized control over low-dimensional phases contributing to enhanced radiative properties of excitons. Herein, the impact of organic molecular dopants on the crystallization of various n-phase structures in quasi-2D perovskite films. The results reveal that the highly reactive bis(4-(trifluoromethyl)phenyl)phosphine oxide (BTF-PPO) molecule could effectively restrain the formation of organic spacer cation-ordered layered perovskite phases through chemical reactions, simultaneously passivate those uncoordinated Pb2+ defects. Consequently, the prepared PeLEDs exhibited a maximum EQE of 16.6 % (@ 490 nm). The finding provides a new route to design dopant molecules for phase modulation in quasi-2D PeLEDs.

Reduced-Dimensional Perovskites: Quantum Well Thickness Distribution and Optoelectronic Properties

Reduced-dimensional perovskites (RDPs), a large category of metal halide perovskites, have attracted considerable attention and shown high potential in the fields of solid-state displays and lighting. RDPs feature a quantum-well-based structure and energy funneling effects. The multiple quantum well (QW) structure endows RDPs with superior energy transfer and high luminescence efficiency. The effect of QW confinement directly depends on the number of inorganic octahedral layers (QW thickness, i.e., n value), so the distribution of n values determines the optoelectronic properties of RDPs. Here, it is focused on the QW thickness distribution of RDPs, detailing its effect on the structural characteristics, carrier recombination dynamics, optoelectronic properties, and applications in light-emitting diodes. The reported distribution control strategies is also summarized and discuss the current challenges and future trends of RDPs. This review aims to provide deep insight into RDPs, with the hope of advancing their further development and applications.

Blade-coated perovskite nanoplatelet polymer composites for sky-blue light-emitting diodes

Colloidal perovskite nanoplatelets (NPLs) have shown promise in tackling blue light-emitting diode challenges based on their tunable band gap and high photoluminescence efficiencies. However, high quality and large area dense NPL films have been proven to be very hard to prepare because of their chemical and physical fragility during the liquid phase deposition. Herein, we report a perovskite-polymer composite film deposition strategy with fine morphology engineering obtained using the blade coating method. The effects of the polymer type, solution concentration, compounding ratio and film thickness on the film quality are systematically investigated. We found that a relatively high-concentration suspension with an optimized NPL to polymer ratio of 1 : 2 is crucial for the suppression of phase separation and arriving at a uniform film. Finally, sky-blue NPL-based perovskite light-emitting diodes were fabricated by blade coating showing an EQE of 0.12% on a device area of 16 mm2.

Perovskite nanocomposites: synthesis, properties, and applications from renewable energy to optoelectronics

The oxide and halide perovskite materials with a ABX3 structure exhibit a number of excellent properties, including a high dielectric constant, electrochemical properties, a wide band gap, and a large absorption coefficient. These properties have led to a range of applications, including renewable energy and optoelectronics, where high-performance catalysts are needed. However, it is difficult for a single structure of perovskite alone to simultaneously fulfill the diverse needs of multiple applications, such as high performance and good stability at the same time. Consequently, perovskite nanocomposites have been developed to address the current limitations and enhance their functionality by combining perovskite with two or more materials to create complementary materials. This review paper categorizes perovskite nanocomposites according to their structural composition and outlines their synthesis methodologies, as well as their applications in various fields. These include fuel cells, electrochemical water splitting, CO2 mitigation, supercapacitors, and optoelectronic devices. Additionally, the review presents a summary of their research status, practical challenges, and future prospects in the fields of renewable energy and electronics.

Data-Driven Design of Electroactive Spacer Molecules to Tune Charge Carrier Dynamics in Layered Halide Perovskite Heterostructures

Crafting rational heterojunctions with nanostructured materials is instrumental in fostering effective interfacial charge separation and transport for optoelectronics. Layered halide perovskites (LHPs) that form heterojunctions between organic spacer molecules and inorganic metal halide layers exhibit tunable photophysics owing to their customizable band alignment. However, controlling photogenerated carrier dynamics by strategically designing layered perovskite heterojunctions remains largely unexplored. We combine a data-driven approach with time-domain density functional theory (TD-DFT) and non-adiabatic molecular dynamics (NAMD) to screen and select electronically active spacer dications (A') that introduce a type-II heterojunction in the lead iodide-based Dion-Jacobson phase LHPs. The composition-structure-electronic property correlations reveal that the number of nitrogens in aromatic heterocycles is the key factor in designing electron-accepting spacers in these perovskites. The detailed atomistic simulations validate the design strategy further by modeling (A')PbI4 perovskites, which incorporate three different screened electroactive A' spacers. The computed excited charge carrier dynamics illustrate the phonon-mediated ultrafast interfacial electron transfer from the inorganic conduction band edge to the lower-lying unoccupied orbitals of spacers, exhibiting photoluminescence quenching in these (A')PbI4 perovskites. The spatially separated electrons and holes at the type-II heterojunction interface prolong the excited charge carrier lifetime, boosting the carrier transport and exciton dynamics. Our work illustrates a robust in silico approach for designing LHPs with exciting optoelectronic properties originating from their fine-tuned heterojunctions.

Rational Strategies to Improve the Efficiency of 2D Perovskite Solar Cells

In the quest for durable photovoltaic devices, 2D halide perovskites have emerged as a focus of extensive research. However, the reduced dimension in structure is accompanied by inferior optical-electrical properties, such as widened band gap, enhanced exciton binding energy, and obstructed charge transport. As a result, the efficiency of 2D perovskite solar cells (PSCs) lags significantly behind their 3D counterparts. To overcome these constraints, extensive investigations into materials and processing techniques are pursued rigorously to augment the efficiency of 2D PSCs. Herein, The cutting-edge delve into developments in 2D PSCs, with a focus on chemical and material engineering, as well as their structure and photovoltaic properties. The review starts with an introduction of the crystal structure, followed by the key evaluation criteria of 2D PSCs. Then, the strategies around solution chemical engineering, processing technique, and interface optimization, to simultaneously boost efficiency and stability are systematically discussed. Finally, the challenges and perspectives associated with 2D perovskites to provide insights into potential improvements in photovoltaic performance will be outlined.

Chiral 2D and quasi-2D hybrid organic inorganic perovskites: from fundamentals to applications

Chiral 2D and quasi-2D hybrid organic-inorganic perovskites (HOIPs) are emerging as promising materials for a variety of applications principally related to optoelectronics and spintronics, thanks to the combined benefits deriving from both the chiral cation and the perovskite structure. Since its recent birth, this research field is tremendously growing, focalizing on the chemical composition tuning to unveil its influence on the related functional properties as well as on developing devices for practical applications. In this review, we focused on the properties of 2D and quasi-2D chiral HOIPs, firstly providing an overview on their chiroptical behaviour followed by their potential exploitation in devices investigated so far for various applicative fields.

Perovskite Light-Emitting Diodes with Quantum Wires and Nanorods

Perovskite materials, celebrated for their exceptional optoelectronic properties, have seen extensive application in the field of light-emitting diodes (LEDs), where research is as abundant as the proverbial "carloads of books." In this review, the research of perovskite materials is delved into from a dimensional perspective, with a focus on the exemplary performance of low-dimensional perovskite materials in LEDs. This discussion predominantly revolves around perovskite quantum wires and perovskite nanorods. Perovskite quantum wires are versatile in their growth, compatible with both solution-based and vapor-phase growth, and can be deposited over large areas-even on spherical substrates-to achieve commendable electroluminescence (EL). Perovskite nanorods, on the other hand, boast a suite of superior characteristics, such as polarization properties and tunability of the transition dipole moment, endowing them with the great potential to enhance light extraction efficiency. Furthermore, zero-dimensional (0D) perovskite materials like nanocrystals (NCs) are also the subject of widespread research and application. This review reflects on and synthesizes the unique qualities of the aforementioned materials while exploring their vital roles in the development of high-efficiency perovskite LEDs (PeLEDs).

Hysteretic Piezochromism in a Lead Iodide-Based Two-Dimensional Inorganic-Organic Hybrid Perovskite

Two-dimensional inorganic-organic hybrid perovskites are in the limelight due to their potential applications in photonics and optoelectronics. They are environmentally stable, and their various chemical compositions offer a wide range of bandgap energies. Alternatively, crystal deformation enables in situ control over their optical properties. Here, we investigate (C6H9C2H4NH3)2PbI4, a hybrid perovskite whose organic linkers are 2-(1-cyclohexenyl)ethylammonium. Pressure-dependent optical absorption and emission spectroscopy reveal a hysteretic piezochromism that was not reported for other lead iodide-based 2D perovskites. We combine our optical studies with high-pressure X-ray diffraction experiments and first-principles calculations to demonstrate that the deformation of the inorganic lead iodide layers is the main reason for the observed changes in the optical bandgap.

Single Crystal Ruddlesden-Popper and Dion-Jacobson Metal Halide Perovskites for Visible Light Photodetectors: Present Status and Future Perspectives

2D metal halide perovskites (MHPs), mainly the studied Ruddlesden-Popper (RP) and Dion-Jacobson (DJ) phases, have gained enormous popularity as optoelectronic materials owing to their self-assembled multiple quantum well structures, tunable semiconducting properties, and improved structural stability compared to their bulk 3D counterparts. The performance of polycrystalline thin film devices is limited due to the formation of defects and trap states. However, as studied so far, single crystal-based devices can provide a better platform to improve device performance and investigate their fundamental properties more reliably. This Review provides the first comprehensive report on the emerging field of RP and DJ perovskite single crystals and their use in visible light photodetectors of varied device configurations. This Review structurally summarizes the 2D MHP single crystal growth methods and the parameters that control the crystal growth process. In addition, the characterization techniques used to investigate their crystal properties are discussed. The review further provides detailed insights into the working mechanisms as well as the operational performance of 2D MHP single crystal photodetector devices. In the end, to outline the present status and future directions, this Review provides a forward-looking perspective concerning the technical challenges and bottlenecks associated with the developing field of RP and DJ perovskite single crystals. Therefore, this timely review will provide a detailed overview of the fast-growing field of 2D MHP single crystal-based photodetectors as well as ignite new concepts for a wide range of applications including solar cells, photocatalysts, solar H2 production, neuromorphic bioelectronics, memory devices, etc.

Pressure-Induced Changes in the Phase Distribution and Carrier Dynamics of Quasi-Two-Dimensional Ruddlesden-Popper Perovskites

Quasi-two-dimensional (quasi-2D) perovskites hold significant potential for diverse design strategies due to their tunable structures, exceptional optical properties, and environmental stability. Due to the complexity of the structure and carrier dynamics, characterization methods such as photoluminescence and absorption spectroscopy can observe but cannot precisely distinguish or identify the phase distribution within quasi-2D perovskite films or correlate phases with carrier dynamics. In this study, we used pressure to modulate the intralayer and interlayer structures of (PEA)2Csn-1PbnBr3n+1 quasi-2D perovskite films, investigating charge carrier dynamics. Steady-state spectroscopy revealed phase transitions at 1.62, 3, and 8 GPa, with free excitons transforming into self-trapped excitons after 8 GPa. Transient absorption spectroscopy elucidated the structural evolution, energy transfer, and pressure-induced transition mechanisms. The results demonstrate that combining pressure and spectroscopy enables the precise identification of phase distribution and pressure response ranges and reveals photophysical mechanisms, providing new insights for optimizing optoelectronic materials.

Building High-Density Polar Hybrid Perovskites via Intercalation of Cs+ and Aromatic Diamine for Passive X-ray Detection

2D organic-inorganic hybrid perovskites (OIHPs) have shown great promise in direct X-ray detection. The development of high-performance passive X-ray detectors in 2D OIHPs calls for an increase in material density while maintaining structural polarity, which is becoming quite challenging. Here, a high-density, polar 2D alternating-cation-intercalated (ACI) perovskite, (4-AP)Cs2Pb2I8 (B, 4-AP = 4-amidinopyridinium), capable of addressing this problem is successfully constructed by introducing heavy Cs+ into the interlayer space of an aromatic Dion-Jacobson (DJ) perovskite (4-AP)PbI4 (A). Through such a DJ-to-ACI design, the newly developed 2D OIHP B not only significantly increases its density to 4.23 g cm-3 (even higher than that of 3D MAPbI3) but also crystallizes in a polar space group (Ama2), which further leads to enhanced X-ray attenuation and an obvious polar photovoltage (1.1 V) under X-ray irradiation. As a result, X-ray detectors fabricated by high-quality single crystals of B exhibit excellent and stable detection performance under self-powered mode with a high sensitivity of 107 μC Gy-1 cm-2 and a low detection limit of 289 nGy s-1. This work provides implications for the future exploration and regulation of novel ACI OIHPs for high-performance photoelectronic devices.

Rashba-Type Band Splitting Effect in 2D (PEA)2PbI4 Perovskites and Its Impact on Exciton-Phonon Coupling

Despite a few recent reports on Rashba effects in two-dimensional (2D) Ruddlesden-Popper (RP) hybrid perovskites, the precise role of organic spacer cations in influencing Rashba band splitting remains unclear. Here, using a combination of temperature-dependent two-photon photoluminescence (2PPL) and time-resolved photoluminescence spectroscopy, alongside density functional theory (DFT) calculations, we contribute to significant insights into the Rashba band splitting found for 2D RP hybrid perovskites. The results demonstrate that the polarity of the organic spacer cation is crucial in inducing structural distortions that lead to Rashba-type band splitting. Our investigations show that the intricate details of the Rashba band splitting occur for organic cations with low polarity but not for more polar ones. Furthermore, we have observed stronger exciton-phonon interactions due to the Rashba-type band splitting effect. These findings clarify the importance of selecting appropriate organic spacer cations to manipulate the electronic properties of 2D perovskites.

Organic ammonium salt assisted crystallization and defect passivation of a quasi-two-dimensional pure blue perovskite at the buried interface

Quasi-two-dimensional (quasi-2D) perovskites exhibit excellent performance in light-emitting diodes (LEDs). However, the quality of perovskite films prepared via the solution method is significantly impacted by the enormous number of defects that unavoidably form at the grain boundaries and interfaces during the precursor to the crystal formation process. Here, we propose a strategy to assist perovskite crystallization and defect passivation at the buried interface through interfacial modification. The organic ammonium salt, ethylamine chloride (EACl), is added to the hole transport material and modifies the buried interface of the perovskite film. EACl introduces the nucleation sites for perovskite precursors, and promotes the crystallization process of the perovskite grains, contributing to the formation of high-quality perovskite films. At the same time, the presence of Lewis base (-NH2) groups in EACl and their lone electron pairs effectively inactivate unlocated Pb2+ ions at the buried interface, thereby reducing non-radiative recombination. In addition, chloride ions help to mitigate defects and to improve the morphology of perovskite films. Devices with this modification show a higher performance than control devices on all metrics. This work proposes a facile but efficient way for improving quasi-2D pure blue perovskite crystallization and growth.