Band Edge Control of Quasi-2D Metal Halide Perovskites for Blue Light-Emitting Diodes with Enhanced Performance

Perovskite light-emitting diodes (PeLEDs) have received great attention for their potential as next-generation display technology. While remarkable progress has been achieved in green, red, and near-infrared PeLEDs with external quantum efficiencies (EQEs) exceeding 20%, obtaining high performance blue PeLEDs remains a challenge. Poor charge balance due to large charge injection barriers in blue PeLEDs has been identified as one of the major roadblocks to achieve high efficiency. Here band edge control of perovskite emitting layers for blue PeLEDs with enhanced charge balance and device performance is reported. By using organic spacer cations with different dipole moments, that is, phenethyl ammonium (PEA), methoxy phenethyl ammonium (MePEA), and 4-fluoro phenethyl ammonium (4FPEA), the band edges of quasi-2D perovskites are tuned without affecting their band gaps. Detailed characterization and computational studies have confirmed the effect of dipole moment modification to be mostly electrostatic, resulting in changes in the ionization energies of ≈0.45 eV for MePEA and ≈ −0.65 eV for 4FPEA based thin films relative to PEA-based thin films. With improved charge balance, blue PeLEDs based on MePEA quasi-2D perovskites show twofold increase of the EQE as compared to the control PEA based devices.     

Strategic Doping in Metal Halide Perovskites for Thermoelectrics

Metal halide perovskites (MHPs) have not only shown unique merits of ultralow thermal conductivity compared to traditional inorganic thermoelectric (TE) materials, but also featured superior Seebeck effect to organic semiconductors, thereby affording great prospect in TE field. However, their severely poor electrical conductivity significantly hinders TE applications, which results from the restrained doping efficiency due to the limited accommodation capability of heterogeneous dopants and the heavy compensation from interior defects in MHPs. Realizing high-effectiveness electrical doping in MHPs becomes imperative yet remains extremely challenging. This Minireview is therefore intended to sort out the diversified doping strategies and highlight their underlying impacts on both thermal and electrical transportation in MHPs. These strategies are systematically classified into bulk and surface/interface doping as dictated by where the dopants are implemented while unravelling how they critically impact TE properties in distinctive means. A rational guideline is hence derived to strengthen electrical doping towards desirable perovskite TEs.     

High Q-Factor and Low Threshold Continuous-Wave Near-Infrared Lasing with Quasi-2D Perovskites

Quasi-2D perovskites have shown great potential in achieving solution-processed electrically pumped laser diodes due to their multiple-quantum-well structure, which induces a carrier cascade process that can significantly enhance population inversion. However, continuous-wave (CW) optically pumped lasing has yet to be achieved with near-infrared (NIR) quasi-2D perovskites due to the challenges in obtaining high-quality quasi-2D films with suitable phase distribution and morphology. This study regulates the crystallization of a NIR quasi-2D perovskite ((NMA)₂FA_n−1Pb_nI_3n+1) using an 18-crown-6 additive, resulting in a compact and smooth film with a largely improved carrier cascade efficiency. The amplified spontaneous emission threshold of the film is reduced from 47.2 to 35.9 µJ cm⁻². Furthermore, by combining the film with a high-quality distributed feedback grating, this study successfully realizes a CW NIR laser of 809 nm at 110 K, with a high Q-factor of 4794 and a low threshold of 911.6 W cm⁻². These findings provide an important foundation for achieving electrically pumped laser diodes based on the unique quasi-2D perovskites.     

Shining Light on the Photoluminescence Properties of Metal Halide Perovskites

Lead halide perovskites are a remarkable class of materials that have emerged over the past decade as being suitable for application in a broad range of devices, such as solar cells, light‐emitting diodes, lasers, transistors, and memory devices. While they are often solution‐processed semiconductors deposited at low temperatures, perovskites exhibit properties one would only expect from highly pure inorganic crystals that are grown at high temperatures. This unique phenomenon has resulted in fast‐paced progress toward record device performance. Unfortunately, the basic science behind the remarkable nature of these materials is still not well understood. This review assesses the current understanding of the photoluminescence properties of metal halide perovskite materials and highlights key areas that require further research. Furthermore, the need to standardize the methods for characterization of PL in order to improve comparability, reliability, and reproducibility of results is emphasized.     

Bulk Incorporation of Molecular Dopants into Ruddlesden–Popper Organic Metal–Halide Perovskites for Charge Transfer Doping

Organic metal-halide perovskites (OHPs) have recently attracted much attention as next-generation semiconducting materials due to their outstanding opto-electrical properties. However, OHPs currently suffer from the lack of efficient doping methods, while the traditional method of atomistic doping having clear limitations in the achievable doping range. While doping with molecular dopants, has been suggested as a solution to this problem, the action of these dopants is typically restricted to perovskite surfaces, therefore significantly reducing their doping potential. In this study, successful bulk inclusion of “magic blue”, a molecular dopant, into 2D Ruddlesden–Popper perovskites is reported. This doping strategy of immersing the perovskite film in dopant solution increases the electrical current up to ≈60 times while maintaining clean film surface. A full mechanistic picture of such immersion doping is provided, in which the solvent molecule facilitates bulk diffusion of dopant molecule inside the organic spacer layer. Physical criteria for judicious choice of solvents in immersion doping are developed based on readily available solvent properties. The immersion doping method developed in this study that enables bulk molecular doping in OHPs will provide a strategic doping methodology for controlling electrical properties of OHPs for electronic and optoelectronic devices.     

Improving the Stability of Halide Perovskites for Photo-, Electro-, Photoelectro-Chemical Applications

Owing to their remarkable and adjustable optoelectronic properties, halide perovskites (HPs) have been regarded as a class of promising materials for various optoelectronic applications based on different energy conversion reactions, including photovoltaic cell, photocatalysis, electrocatalysis, and photoelectrochemical (PEC) systems. However, the low stability of HPs upon exposure to ambient conditions (e.g., water, heat, light, electricity) greatly hinders the practical applications of HPs. In the past few years, significant efforts have been devoted to enhancing the eventual stability of the perovskite-based optoelectronic systems, mainly focusing on delivering improvements in the stabilities of halide perovskite materials and the relevant operation conditions of optoelectronic systems, which deserve in-depth and systematic summaries. In this comprehensive review, the in-depth environment-induced decomposition mechanisms of typical HPs are elucidated. Simultaneously, the strategies for addressing the instability issues of halide perovskite materials are critically reviewed, including dimension control, compositional engineering, ligand passivation, and encapsulation engineering. Furthermore, the photoelectric applications based on the modified HPs and operation conditions are discussed systematically. In the last part of this review, future perspectives and outlooks toward the stability of HPs and their photoelectric applications are envisaged respectively.     

Inkjet Printing Epitaxial Metal Halide Perovskites on Various Substrates?

In their Progress Report, Sytnyk et al. claimed the epitaxial deposition of metal halide perovskites by inkjet printing on various substrates. The claims of epitaxy are doubtful due to a lack of convincing experimental evidence. Both optical microscopy and scanning electron microscopy images alone are insufficient to prove epitaxy. Even if the images suggest that epitaxial growth might have been achieved, direct evidences from X-ray diffraction (XRD) or electron diffraction are still needed. Among the six perovskite-substrate systems, only two CH₃NH₃PbI₃ deposits on (111)PbTe/BaF₂ are characterized with XRD. According to their XRD patterns, actually, it is estimated that each of their MAPbI₃ deposits contains only <20% epitaxial components and the component of nearly random orientation is predominant besides considerable impurity PbI₂. None of the MAPbI₃ deposits can be considered epitaxial. It is critical to point out the errors in the methodology for verifying epitaxy, so that researchers would not be misled by Sytnyk's paper. Importantly, a new methodology to quantify the volume fraction of each epitaxial component in a thin film from XRD measurements is proposed. It is reliable to estimate the degree of texture according to the proposed equations, quite different from using Lotgering degree of orientation.     

Domain Distribution Management of Quasi-2D Perovskites toward High-Performance Blue Light-Emitting Diodes

Quasi-2D perovskites, as one of the promising materials applied in perovskite light-emitting diodes (PeLEDs), have attracted great attention for their superior semiconductor properties. The inherent multiquantum well structure can induce a strong confinement effect, which is especially suitable for blue emission. However, compared to their green counterparts, blue emitters constructed from quasi-2D perovskites are more sensitive to n domain distribution (where n represents the number of PbX₆ inorganic layers). Suffering from inefficient domain distribution management, blue PeLEDs now face a variety of negative issues, including color instability, multipeak emission, and poor fluorescence yield. In this review, the development of blue PeLEDs and the optical properties of quasi-2D perovskites are overviewed. Then, a classification and summary of strategies for domain distribution management are proposed. Finally, the challenges and potential directions of domain distribution management in quasi-2D perovskites are summarized. This review is expected to provide a comprehensive perspective and reference on domain distribution management toward efficient blue quasi-2D PeLEDs.     

Strain Engineering of Metal Halide Perovskites on Coupling Anisotropic Behaviors

The power conversion efficiencies (PCEs) of the solar cells containing metal halide perovskites (MHPs) have rapidly increased and exceeded 25% during the past decade. The photovoltaic properties of these devices are extensively investigated in terms of their microstructures, environmental characteristics, and carrier dynamics, and the MHP structural evolution under high pressure is evaluated. In addition, the energy level structure, electron/hole dynamics, and optical/electronic properties of MHPs with anisotropic crystal structures are examined. However, the correlation between the structural anisotropy and material properties of these perovskites is rarely considered in the literature studies on their high-pressure behavior. In this progress report, the optical/electronic properties of MHPs with anisotropic structures under thermal, mechanically imposed, and in-service strains/stresses that have been previously neglected by researchers are summarized.     

Recent Developments of Optoelectronic Synaptic Devices Based on Metal Halide Perovskites

Massive data processing with high computing efficiency and low operating power is required owing to the rapid development of artificial intelligence and information technology. However, the von Neumann structure computing system with the separated memory and processor can cause large energy consumption and a low running speed during massive data processing. Therefore, the brain-inspired neuromorphic computing system is developed, that can provide hardware support for emulating biological synaptic functions and realizing highly intensive data processing with low power consumption. As a neuromorphic device, the optoelectronic synaptic device (OSD) is regarded as an ideal device to replace the von Neumann-based computer because of its ultrafast signal transmission, large bandwidth, low energy consumption, and wireless communication. Owing to their unique optoelectronic property, metal halide perovskites (MHPs) have received growing attention as effective photosensitive materials in OSDs. Therefore, the review introduces the recent progress on OSDs based on MHPs (MHPs-OSDs) including the structures and properties of MHPs, and the architectures and performance characteristics of MHPs-OSDs. Furthermore, applications of MHPs-OSDs are presented. Finally, the outlook and opportunity of MHPs-OSDs are discussed.     

Photophysics of Two-Dimensional Perovskites—Learning from Metal Halide Substitution

2D perovskites offers a rich playing field to explore exciton physics and they possess a great potential for a variety of opto-electronic applications. Whilst their photophysics shows intricate interactions of excitons with the lattice, most reports have so far relied on single compound studies. With the exception of variations of the organic spacer cations, the effect of constituent substitution on the photophysics and the nature of emitting species, in particular, have remained largely under-explored. Here PEA₂PbBr₄, PEA₂PbI₄, and PEA₂SnI₄ (where PEA stands for phenylethylammonoium) are studied through a variety of optical spectroscopy techniques to reveal a complex set of excitonic transitions at low temperature. Weak high-energy features are attributed to vibronic transitions breaking Kasha's, for which the responsible phonons cannot be accessed through simple Raman spectroscopy. Bright peaks at lower energy are due to two distinct electronic states, of which the upper is a convolution of the free exciton and a localized dark state and the lower is attributed to recombination involving shallow defects. This study offers deeper insights into the photophysics of 2D perovskites through compositional substitution and highlights critical limits to the communities’ current understanding of processes in these compounds.     

Activation of Self-Trapped Emission in Stable Bismuth-Halide Perovskite by Suppressing Strong Exciton–Phonon Coupling

All-inorganic bismuth-halide perovskites are promising alternatives for lead halide perovskites due to their admirable chemical stability and optoelectronic properties; however, these materials deliver inferior photoluminescence (PL) properties, severely hindering their prospects in lighting applications. Here, a novel air-stable but non-emissive perovskite Rb₃BiCl₆ is synthesized, and the material is used as a prototype to uncover origin of the poor optical performance in bismuth-halide perovskite. It is found that the extremely strong exciton–phonon interactions with a large coupling constant up to 693 meV leads to the seriously nonradiative recombination, which, however, can be effectively suppressed to 347 meV by introducing Sb³⁺ ions. As a result, Sb³⁺-doped Rb₃BiCl₆ exhibits a stable yellow emission with unprecedented PL quantum yield up to 33.6% from self-trapped excitons. Systematic spectroscopic characterizations and theoretical calculations are carried out to unveil the intriguing photophysical mechanisms. This work reveals the effect of exciton–phonon interaction, that is often underemphasized, on a material's photophysical properties.     

Anisotropic In Situ Strain-Engineered Halide Perovskites for High Mechanical Flexibility

Even though halide perovskite materials have been increasingly investigated as flexible devices, mechanical properties under flexible environments have rarely been reported on. Herein, a nonconventional deposition technique that can generate extra compressive or tensile stress in representative inorganic CsPbBr₃ and hybrid MAPbI₃ (methylammonium lead iodide) halide perovskites is proposed for higher mechanical flexibility. As an impressive result of bending fracture evaluation, fracture energy is substantially improved by ≈260% for CsPbBr₃ and ≈161% for MAPbI₃ with the maximum compressive strain of −1.33%. Origin of the flexibility enhancements by the in situ strain is verified with structural simulation where the anisotropic lattice distortion, that is, contraction in the ab plane and elongation along the c-axis, is evident with changes in atomic bond lengths and angles in the halide perovskites. Other mechanical properties such as hardness, film strength, and fracture toughness are also discussed with direct comparisons between the inorganic and hybrid halides. Beyond the successful adjustment of this in situ deposition technique, the strain-dependent mechanical properties are expected to be extensively useful for designing halides-based flexible devices.     

Secondary Ion Mass Spectrometry (SIMS) for Chemical Characterization of Metal Halide Perovskites

Metal halide perovskite (MHP) solar cells have attracted much attention due to the rapidly growing power conversion efficiency that has reached 25.2% in a decade, comparable to established commercial photovoltaic modules. Compositional engineering is one of the most effective methods to boost the performance of MHP solar cells. Further improving the efficiency and the stability of MHP solar cells necessitates good understanding of the chemical–efficiency correlation and the chemical evolution during the degradation of MHP solar cells. In this regard, time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) is a powerful tool to investigate the chemical aspect of MHPs and has played an important role in advancing the development of MHP optoelectronics. However, up to date, a review that can guide future utilization of ToF‐SIMS in the MHP development is missing. Herein, the capabilities of ToF‐SIMS in MHP investigations are summarized and analyzed from simple material synthesis and chemical distribution to more complicated device operation mechanism and stability. The strength of ToF‐SIMS in resolving important issues in this field, such as interface composition, ion migration, and degradation in MHP is highlighted. Finally, an outlook with an emphasis on making the utmost of ToF‐SIMS in developing MHP devices is provided.     

Core/Shell Metal Halide Perovskite Nanocrystals for Optoelectronic Applications

Core/shell structured metal halide perovskite nanocrystals (NCs) are emerging as a type of material with remarkable optical and electronic properties. Research into this field has been developing and expanding rapidly in recent years, with significant advances in the studies of the shell growth mechanism and in understanding of properties of these materials. Significant enhancement of both the stability and the optical performance of core/shell perovskite NCs are of particular importance for their applications in optoelectronic technologies. In this review, the recent advances in core/shell structured perovskite NCs are summarized. The band structures and configurations of core/shell perovskite NCs are elaborated, the shell classification and shell engineering approaches, such as perovskites and their derivative shells, semiconductor shell, oxide shell, polymer shell, etc. are reviewed, and the shell growth mechanisms are discussed. The prospective of these NCs in lighting and displays, solar cells, photodetectors, and other devices is discussed in the light of current knowledge, remaining challenges, and future opportunities.     

Dual Emission of Water‐Stable 2D Organic–Inorganic Halide Perovskites with Mn(II) Dopant

Moisture‐delicate and water‐unstable organic–inorganic halide perovskites (OI‐HPs) create huge challenges for the synthesis of highly efficient water‐stable light‐emitting materials for optoelectronic devices. Herein, a simple acid solution–assisted method to synthesize quantum confined 2D lead perovskites through Mn doping is reported. The efficient energy transfer between host and dopant ions in orange light‐emitting Mn²⁺‐doped OI‐HPs leads to the most efficient integrated luminescence with a photoluminescence quantum yield over 45%. The Mn²⁺ substitution of Pb²⁺ and passivation with low dielectric constant molecules such as phenethylamine, benzylamine, and butylamine enhance water resistivity, leading to water stability. The dual emission process of this water‐stable 2D Mn‐doped perovskite will help in developing highly efficient 2D water‐stable perovskites for practical applications.     

Halide Perovskite Materials for Energy Storage Applications

Halide perovskites, traditionally a solar‐cell material that exhibits superior energy conversion properties, have recently been deployed in energy storage systems such as lithium‐ion batteries and photorechargeable batteries. Here, recent progress in halide perovskite‐based energy storage systems is presented, focusing on halide perovskite lithium‐ion batteries and halide perovskite photorechargeable batteries. Halide‐perovskite‐based supercapacitors and photosupercapacitors are also discussed. The photorechargeable batteries and photorechargeable supercapacitors employ solar energy to photocharge the battery; this saves energy and improves device portability. These lightweight, integrated halide perovskite‐based systems, which are pertinent to electric vehicles and portable electronic devices, are reviewed in detail. Suggestions on future research into the design of halide‐perovskite‐based energy storage materials are also given. This review provides a foundation for the development of integrated lightweight energy conversion and storage materials.     

Impact of Processing on Structural and Compositional Evolution in Mixed Metal Halide Perovskites during Film Formation

Understanding crystallization processes and their pathways in metal‐halide perovskites is of crucial importance as this strongly affects the film microstructure, its stability, and device performance. While many approaches are developed to control perovskite formation, the mechanisms of film formation are still poorly known. Using time‐resolved in situ grazing incidence wide‐angle X‐ray scattering, the film formation of perovskites is investigated with average stoichiometry Cs_0.15FA_0.85PbI₃, where FA is formamidinium, using the popular antisolvent dropping and gas jet treatments and this is contrasted with untreated films. i) The crystallization pathways during spin coating, ii) the subsequent postdeposition thermal annealing, and iii) crystallization during blade coating are studied. The findings reveal that the formation of a nonperovskite FAPbI₃ phase during spin coating is initially dominant regardless of the processing and that the processing treatment (e.g., antisolvent dropping, gas jet) has a significant impact on the as‐cast film structure and affects the phase evolution during subsequent thermal treatment. It is shown that blade coating can be used to overcome the nonperovskite phase formation via solvothermal direct crystallization of perovskite phase. This work shows how real‐time investigation of perovskite formation can help to establish processing–microstructure–functionality relationships.     

Compositional and Dimensional Control of 2D and Quasi‐2D Lead Halide Perovskites in Water

Blue light emitting two dimensional (2D) and quasi‐2D layered halide perovskites (LHPs) are gaining attention in solid‐state lighting applications but their fragile stability in humid condition is one of the most pressing issues for their practical applications. Though water is much greener and cost effective, organic solvents must be used during synthesis as well as the device fabrication process for these LHPs due to their water‐sensitivity/instability and consequently, water‐stable blue‐light emitting 2D and quasi‐2D LHPs have not been documented yet. Here, water‐mediated facile and cost‐effective syntheses, characterizations, and optical properties of 16 organic–inorganic hybrid compounds are reported including 2D (A′)₂PbX₄ (A′ = butylammonium, X = Cl/Br/I) (8 compounds), 3D perovskites (4), and quasi‐2D (A′)_pA_x?1B_xX_3x+1 LHPs (A = methylammonium) (4) in water. Here, both composition and dimension of LHPs are tuned in water, which has never been explored yet. Furthermore, the dual emissive nature is observed in quasi‐2D perovskites, where the intensity of two photoluminescence (PL) peaks are governed by 2D and 3D inorganic layers. The Pb(OH)₂‐coated 2D and quasi‐2D perovskites are highly stable in water even after several months. In addition, single particle imaging is performed to correlate structural–optical property of these LHPs.     

Amplified Spontaneous Emission Based on 2D Ruddlesden–Popper Perovskites

2D Ruddlesden–Popper perovskites (RPPs) are a class of quantum‐well (QW) materials, composed of layered perovskite QWs encapsulated between two hydrophobic organic layers. Different from widely investigated 3D‐perovskites with free carriers at room temperature, 2D‐RPPs exhibit strongly bound electron–hole pairs (excitons) for high‐performance solar cells and light emitting diodes (LEDs). Herein, it is reported that self‐organized multiple QWs in 2D‐RPP thin films naturally form an energy cascade, which enables an ultrafast energy transfer process from higher energy‐bandgap QWs to lower energy‐bandgap QWs. Therefore, photoexcitations are concentrated on lower‐bandgap QWs, facilitating the build‐up of population inversion. Room‐temperature amplified spontaneous emission (ASE) from 2D‐RPP thin films is achieved at dramatically low thresholds, with gain coefficients as high as >300 cm^?1, and stoichiometrically tunable ASE wavelengths from visible to near‐infrared spectral range (530–810 nm). In view of the high efficiency reported for LEDs, these solution‐processed 2D‐RPP thin films may hold the key to realize electrically driven lasers.