Phase segregation due to ion migration in all-inorganic mixed-halide perovskite nanocrystals

Inhibiting the Appearance of Green Emission in Mixed Lead Halide Perovskite Nanocrystals for Pure Red Emission

Mixed halide perovskites exhibit promising optoelectronic properties for next-generation light-emitting diodes due to their tunable emission wavelength that covers the entire visible light spectrum. However, these materials suffer from severe phase segregation under continuous illumination, making long-term stability for pure red emission a significant challenge. In this study, we present a comprehensive analysis of the role of halide oxidation in unbalanced ion migration (I/Br) within CsPbI2Br nanocrystals and thin films. We also introduce a new approach using cyclic olefin copolymer (COC) to encapsulate CsPbI2Br perovskite nanocrystals (PNCs), effectively suppressing ion migration by increasing the corresponding activation energy. Compared with that of unencapsulated samples, we observe a substantial reduction in phase separation under intense illumination in PNCs with a COC coating. Our findings show that COC enhances phase stability by passivating uncoordinated surface defects (Pb2+ and I-), increasing the formation energy of halide vacancies, improving the charge carrier lifetime, and reducing the nonradiative recombination density.

Mapping Spatial Strain Distribution and Its Effects on Optoelectronic Properties in Wrinkled Perovskite Films

Organic-inorganic halide perovskite films, fabricated by using the antisolvent method, have garnered intense attention for their application in high-efficiency and stable solar cells. These films characteristically develop periodic wrinkled microstructures. Previous research has indicated that macroscopic residual strain significantly influences the optoelectronic behaviors of these films. However, the detailed interplay between the wrinkled morphology, strain distribution, and local photophysical properties at the micro- and nanoscale has not been fully elucidated. Here, we explore the microscopic morphology-strain-property relationship within wrinkled perovskite films employing correlative micro-optical and nanoelectrical microscopy techniques. Microphotoluminescence (PL) mapping supplemented by in situ strain PL measurements identifies a heterogeneous spatial strain distribution across the microstructural hills and valleys. Additionally, light-intensity-dependent photoconductive atomic force microscopy reveals that valleys experiencing less compressive strain exhibit a lower conductivity and a higher propensity for ion migration. The findings underscore the potential of targeted strain engineering to optimize the performance and longevity of perovskite solar cells.

Ultrafast Rejuvenation of Aged CsPbI3 Quantum Dots and Efficiency Improvement by Sequential 1-Dodecanethiol Post-Treatment Strategy

Metal halide perovskite CsPbI3 quantum dots (QDs) have sparked widespread research due to their intriguing optoelectronic. However, the CsPbI3 QDs undergo inevitable aging and luminescence quenching caused by the weak binding ability of oleate (OA-)/oleylammonium (OAm+), hindering further practical application. Herein, we have realized ultrafast rejuvenation of the aged CsPbI3 QDs that have lost their photoluminescence performance based on a 1-dodecanethiol (DDT) surface ligand to restore the outstanding red light emission with a high photoluminescence quantum yield (PLQY) from 25 to 90%. Furthermore, CsPbI3 QDs with DDT surface treatment maintain a cubic phase and high PLQY value even after 35 days. The DDT ligands can form a strong bond with Pb2+ and passivate I- ion vacancies, enhancing radiative recombination efficiency and thereby improving the PLQY of the QDs. The stable yet easily accessible surface of the DDT-capped CsPbI3 QDs was successfully employed as white LEDs and exhibited considerable enhanced luminous performance, suggesting promising application in solid-state lighting fields.

Stable and efficient CsPbI3 quantum-dot light-emitting diodes with strong quantum confinement

Even though lead halide perovskite has been demonstrated as a promising optoelectronic material for next-generation display applications, achieving high-efficiency and stable pure-red (620~635 nm) emission to cover the full visible wavelength is still challenging. Here, we report perovskite light-emitting diodes emitting pure-red light at 628 nm achieving high external quantum efficiencies of 26.04%. The performance is attributed to successful synthesizing strongly confined CsPbI3 quantum dots with good stability. The strong binding 2-naphthalene sulfonic acid ligands are introduced after nucleation to suppress Ostwald ripening, meanwhile, ammonium hexafluorophosphate exchanges long chain ligands and avoids regrowth by strong binding during the purification process. Both ligands enhance the charge transport ability of CsPbI3 quantum dots. The state-of-the-art synthesis of pure red CsPbI3 quantum dots achieves 94% high quantum efficiency, which can maintain over 80% after 50 days, providing a method for synthesizing stable strong confined perovskite quantum dots.

Perovskite Quantum Heterostructure Constructed by Halide Mixing between a Single CsPbI3 Nanocrystal and an Individual CsPbBr3 Microplate

Ion migration is significantly enhanced in lead-halide perovskites with a soft crystal lattice, which can promote the formation of a heterogeneous interface between two such materials with different halide-anion compositions. Here we have deposited a single CsPbI3 nanocrystal (NC) on top of an individual CsPbBr3 microplate to create a mixed-halide CsPbBrxI3-x (0 < x < 3) NC by means of the anion exchange process. The formation of a CsPbBrxI3-x/CsPbBr3 heterostructure is confirmed by the much-enlarged geometric volume of the CsPbBrxI3-x NC as compared to the original CsPbI3 one, as well as by its capability of receiving photogenerated excitons from the CsPbBr3 microplate with a larger bandgap energy. The quantum nature of this heterostructure is reflected from single-photon emission of the composing CsPbBrxI3-x NC, which can also be bulk-like during phase segregation to demonstrate a red shift in the photoluminescence peak that is opposite to the common trend observed in smaller-sized mixed-halide NCs.

Probing the cw-Laser-Induced Fluorescence Enhancement in CsPbBr3 Nanocrystal Thin Films: An Interplay between Photo and Thermal Activation

Perovskite nanocrystals hold significant promise for a wide range of applications, including solar cells, LEDs, photocatalysts, humidity and temperature sensors, memory devices, and low-cost photodetectors. Such technological potential stems from their exceptional quantum efficiency and charge carrier conduction capability. Nevertheless, the underlying mechanisms of photoexcitation, such as phase segregation, annealing, and ionic diffusion, remain insufficiently understood. In this context, we harnessed hyperspectral fluorescence microspectroscopy to advance our comprehension of fluorescence enhancement triggered by UV continuous-wave (cw) laser irradiation of CsPbBr3 colloidal nanocrystal thin films. Initially, we explored the kinetics of fluorescence enhancement and observed that its efficiency (φph) correlates with the laser power (P), following the relationship φph = 7.7⟨P⟩0.47±0.02. Subsequently, we estimated the local temperature induced by the laser, utilizing the finite-difference method framework, and calculated the activation energy (Ea) required for fluorescence enhancement to occur. Our findings revealed a very low activation energy, Ea ∼ 9 kJ/mol. Moreover, we mapped the fluorescence photoenhancement by spatial scanning and real-time static mode to determine its microscale length. Below a laser power of 60 μW, the photothermal diffusion length exhibited nearly constant values of approximately (22 ± 5) μm, while a significant increase was observed at higher laser power levels. These results were ascribed to the formation of nanocrystal superclusters within the film, which involves the interparticle spacing reduction, creating the so-called quantum dot solid configuration along with laser-induced annealing for higher laser powers.

All-perovskite tandem solar cells: from fundamentals to technological progress

Organic-inorganic perovskite materials have gradually progressed from single-junction solar cells to tandem (double) or even multi-junction (triple-junction) solar cells as all-perovskite tandem solar cells (APTSCs). Perovskites have numerous advantages: (1) tunable optical bandgaps, (2) low-cost, e.g. via solution-processing, inexpensive precursors, and compatibility with many thin-film processing technologies, (3) scalability and lightweight, and (4) eco-friendliness related to low CO2 emission. However, APTSCs face challenges regarding stability caused by Sn2+ oxidation in narrow bandgap perovskites, low performance due to V oc deficit in the wide bandgap range, non-standardisation of charge recombination layers, and challenging thin-film deposition as each layer must be nearly perfectly homogenous. Here, we discuss the fundamentals of APTSCs and technological progress in constructing each layer of the all-perovskite stacks. Furthermore, the theoretical power conversion efficiency (PCE) limitation of APTSCs is discussed using simulations.

Ordered Perovskite Structure with Functional Units for High Performance and Stable Solar Cells

Ion migration is one of the most critical challenges that affects the stability of metal-halide perovskite solar cells (PSCs). However, the current arsenal of available strategies for solving this issue is limited. Here, novel perovskite active layers following the concept of ordered structures with functional units (OSFU) to intrinsically suppress ion migration, in which a three-dimensional (3D) perovskite layer is deposited by vapor deposition for light absorption and a 2D layer is deposited by solution process for ion inhibition, are constructed. As a promising result, the activation energy of ion migration increases from 0.36 eV for the conventional perovskite to 0.54 eV for the OSFU perovskite. These devices exhibit substantially enhanced operational stability in comparison with the conventional ones, retaining >85% of their initial efficiencies after 1200 h under ISOS-L-1. Moreover, the OSFU devices show negligible fatigue behavior with a robust performance under light/dark cycling aging test (ISOS-LC-1 protocol), which demonstrates the promising application of functional motif theory in this field.

Z-Type Ligand Enables Efficient and Stable Deep-Blue Perovskite Light-Emitting Diodes

During the synthesis of deep-blue perovskite quantum dots (PQDs), they generally emerge as a two-dimensional byproduct with poor yield and low photoluminescence quantum yield (PLQY) due to amine ligand enrichment-induced abundant surface defects. Herein, we provide a colloidal synthesis method to prepare deep-blue CsPbBr3 PQDs in a green nontoxic solvent via strategic Z-type ligand engineering. Z-type ligands of zinc octanoate enable the formation of robust coordination bonds with surface bromide ions of PQDs, maintaining acid-base equilibrium and reducing excess amine enrichment on the PQDs surface. Consequently, homogeneous and monodispersed PQDs with improved PLQY of 73% are successfully synthesized, achieving efficient deep-blue LEDs with a peak EQE of 5.46%, a maximum luminance of 847.6 cd/m2, and an operational half-lifetime of 14 min. The devices exhibit color coordinates of (0.137, 0.049), closely approximating the Rec. 2020 blue standard. Our work offers a potentially eco-friendly and viable route for realizing high-performance LEDs in the deep-blue region.

Stable and Efficient Mixed-halide Perovskite LEDs

Tailoring bandgap by mixed-halide strategy in perovskites has attracted extraordinary attention due to the flexibility of halide ion combinations and has emerged as the most direct and effective approach to precisely tune the emission wavelength throughout the entire visible light spectrum. Mixed-halide perovskites, yet, still suffered from several problems, particularly phase segregation under external stimuli because of ions migration. Understanding the essential cause and finding sound strategies, thus, remains a challenge for stable and efficient mixed-halide perovskite light-emitting diodes (PeLEDs). The review herein presents an overview of the diverse application scenarios and the profound significance associated with mixed-halide perovskites. We then summarize the challenges and potential research directions toward developing high stable and efficient mixed-halide PeLEDs. The review thus provides a systematic and timely summary for the community to deepen the understanding of mixed-halide perovskite materials and resulting PeLEDs.

Kinetically Controlled Synthesis of Quasi-Square CsPbI3 Nanoplatelets with Excellent Stability

Nanoplatelets (NPLs) share excellent luminescent properties with their symmetric quantum dots counterparts and entail special characters benefiting from the shape, like the thickness-dependent bandgap and anisotropic luminescence. However, perovskite NPLs, especially those based on iodide, suffer from poor spectral and phase stability. Here, stable CsPbI3 NPLs obtained by accelerating the crystallization process in ambient-condition synthesis are reported. By this kinetic control, the rectangular NPLs into quasi-square NPLs are tuned, where enlarged width endows the NPLs with a lower surface-area-to-volume ratio (S/V ratio), leading to lower surficial energy and thus improved endurance against NPL fusion (cause for spectral shift or phase transformation). The accelerated crystallization, denoting the fast nucleation and short period of growth in this report, is enabled by preparing a precursor with complete transformation of PbI2 into intermediates (PbI3 -), through an additional iodide supplier (e.g., zinc iodide). The excellent color stability of the materials remains in the light-emitting diodes under various bias stresses.

Ligand Engineering for Mitigating Exciton-Phonon Coupling in Mixed Halide Perovskite Nanocrystals

The vulnerability of mixed halide perovskite nanocrystals (NCs) remains challenging because of the weak interaction between commonly employed ligands, oleic acid/oleylamine (OAm/OA) and halide anions, coupled with substantial surface phonon energy. Here, we introduce 3-aminopropyltriethoxysilane (APTES) as a capping ligand to modify CsPbBrI2 NCs to enhance the interactions between them. The optical properties have been significantly enhanced, and halide segregation has been suppressed, both of which can be attributed to the reduced phonon energy and exciton-phonon coupling strength. Moreover, these APTES-CsPbBrI2 NCs exhibit a broad color gamut and sustained color stability during long-term operation, indicating their promising potential in display technologies. This work may offer insights into surface engineering to enhance the properties and band stability of mixed halide perovskite NCs.

Chemical Aspects of Halide Perovskite Nanocrystals

Halide perovskite nanocrystals (HPNCs) are currently among the most intensely investigated group of materials. Structurally related to the bulk halide perovskites (HPs), HPNCs are nanostructures with distinct chemical, optical, and electronic properties and significant practical potential. One of the keys to the effective exploitation of the HPNCs in advanced technologies is the development of controllable, reproducible, and scalable methods for preparation of materials with desired compositions, phases, and shapes and low defect content. Another important condition is a quantitative understanding of factors affecting the chemical stability and the optical and electronic properties of HPNCs. Here we review important recent developments in these areas. Following a brief historical prospective, we provide an overview of known chemical methods for preparation of HPNCs and approaches used to control their composition, phase, size, and shape. We then review studies of the relationship between the chemical composition and optical properties of HPNCs, degradation mechanisms, and effects of charge injection. Finally, we provide a short summary and an outlook. The aim of this review is not to provide a comprehensive summary of all relevant literature but rather a selection of highlights, which, in the subjective view of the authors, provide the most significant recent observations and relevant analyses.

Complete Suppression of Phase Segregation in Mixed-Halide Perovskite Nanocrystals under Periodic Heating

Under continuous light illumination, it is known that localized domains with segregated halide compositions form in semiconducting mixed-halide perovskites, thus severely limiting their optoelectronic applications due to the negative changes in bandgap energies and charge-carrier characteristics. Here mixed-halide perovskite CsPbBr1.2 I1.8 nanocrystals are deposited onto an indium tin oxide substrate, whose temperature can be rapidly changed by ≈10 °C in a few seconds by applying or removing an external voltage. Such a sudden temperature change induces a temporary transition of CsPbBr1.2 I1.8 nanocrystals from the segregated phase to the mixed phase, the latter of which can be permanently maintained when the light illumination is coupled with periodic heating cycles. These findings mark the emergence of a practical solution to the detrimental phase-segregation problem, given that a small temperature modulation is readily available in various fundamental studies and practical devices of mixed-halide perovskites.

Ion Migration in Lead-Halide Perovskites: Cation Matters

Metal halide perovskites exhibit remarkable properties for optoelectronic applications, yet their susceptibility to ion migration poses challenges for device stability. Previous research has predominantly focused on the migration of the halide ions. However, the migration of cations, which also has a significant influence on the device performance, is largely overlooked. In this Perspective, we review the migration of cations and their impacts on perovskite materials and devices. Special attention shall be devoted to recent insights into the migration of L-site organic cations in 2D/3D perovskites. We outline inspirations and directions for further research into the cation migration of perovskites, highlighting new possibilities in advancing perovskite optoelectronics.

Monitoring Phase Separation and Dark Recovery in Mixed Halide Perovskite Clusters and Single Crystals Using In Situ Spectromicroscopy

Mixed halide perovskites (MHPs) are a group of semiconducting materials with promising applications in optoelectronics and photovoltaics, whose bandgap can be altered by adjusting the halide composition. However, the current challenge is to stabilize the light-induced halide separation, which undermines the device's performance. Herein we track down the phase separation dynamics of CsPbBr1.2I1.8 MHP single cubic nanocrystals (NCs) and clusters as a function of time by in situ fluorescence spectromicroscopy. The particles were sorted into groups 1 and 2 using initial photoluminescence intensities. The phase separation followed by recovery kinetics under dark and photo blinking analysis suggests that group 1 behaved more like single NCs and group 2 behaved like clusters. Under the 0.64 W/cm2 laser illumination, the phase shifts for single NCs are 3.4 ± 1.9 nm. The phase shifts are linearly correlated with the initial photoluminescence intensities of clusters, suggesting possible interparticle halide transportation.

Mimicking Natural Antioxidant Systems for Improved Photostability in Wide-Band-Gap Perovskite Solar Cells

Fostered by the top power conversion efficiencies (PCEs) of lab-scale devices, industrialization of perovskite solar cells is underway. Nevertheless, the intrinsically poor stability of these materials still represents a major concern. Herein, inspired by Nature, the use of β-carotene in perovskite solar cells is proposed to mimic its role as a protective pigment, as occurs in natural photosynthesis. Laser-mediated photostability (LMPS) assessment, Fourier-transform infrared spectra analysis acquired in attenuate total reflectance (ATR-FTIR), spectroscopy ellipsometry (SE), and time-resolved photoluminescence (TRPL) measurements under stress conditions prove that the inclusion of a thin β-carotene interlayer promotes a high improvement in the photostability of the perovskite films against photooxidation. Importantly, this is accompanied by an improvement of the solar cell PCE that approaches 20% efficiency with no hysteresis, which is among the highest values reported for a mixed halide (I-Br) perovskite with a band gap of 1.74 eV, relevant for coupling with silicon in tandem cells.

How Photogenerated I2 Induces I-Rich Phase Formation in Lead Mixed Halide Perovskites

Bandgap tunability of lead mixed halide perovskites (LMHPs) is a crucial characteristic for versatile optoelectronic applications. Nevertheless, LMHPs show the formation of iodide-rich (I-rich) phase under illumination, which destabilizes the semiconductor bandgap and impedes their exploitation. Here, it is shown that how I2 , photogenerated upon charge carrier trapping at iodine interstitials in LMHPs, can promote the formation of I-rich phase. I2 can react with bromide (Br- ) in the perovskite to form a trihalide ion I2 Br- (Iδ- -Iδ+ -Brδ- ), whose negatively charged iodide (Iδ- ) can further exchange with another lattice Br- to form the I-rich phase. Importantly, it is observed that the effectiveness of the process is dependent on the overall stability of the crystalline perovskite structure. Therefore, the bandgap instability in LMHPs is governed by two factors, i.e., the density of native defects leading to I2 production and the Br- binding strength within the crystalline unit. Eventually, this study provides rules for the design of chemical composition in LMHPs to reach their full potential for optoelectronic devices.

Dendrimer-Encapsulated Halide Perovskite Nanocrystals for Self-Powered White Light-Emitting Glass

Perovskite nanocrystals (PNCs) have attracted substantial attention due to their inspiring intrinsic merits such as low cost, high performance, and solution processability, but when it comes to the usage of blends of different colored PNCs with the purpose of covering the broadband spectrum field, the high degree of instability remains a major bottleneck. Herein, we report a family of dendritic ammonium ligands that act as stiff shell-encapsulating PNCs for improving their stability and suppressing ion permeability in mixed colloidal PNC solutions. The as-synthesized ligand-encapsulated PNCs notably achieve near-unity photoluminescence quantum yields (PLQYs) and strongly resist the unwanted ion exchange reaction under aggressive anion source attack. To fabricate self-powered white-emitting glass, the stabilized mixed colored PNCs were embedded into the laminated glass, which simultaneously acted as absorbers-emitters for luminescent solar concentrators (LSCs) and emitters for white light-emitting glass.

Inhibiting Ion Migration Through Chemical Polymerization and Chemical Chelation Toward Stable Perovskite Solar Cells

The migration of ions is known to be associated with various detrimental phenomena, including current density-voltage hysteresis, phase segregation, etc., which significantly limit the stability and performance of perovskite solar cells, impeding their progress toward commercial applications. To address these challenges, we propose incorporating a polymerizable organic small molecule monomer, N-carbamoyl-2-propan-2-ylpent-4-enamide (Apronal), into the perovskite film to form a crosslinked polymer (P-Apronal) through thermal crosslinking. The carbonyl and amino groups in Apronal effectively interact with shallow defects, such as uncoordinated Pb2+ and iodide vacancies, leading to the formation of high-quality films with enhanced crystallinity and reduced lattice strain. Furthermore, the introduction of P-Apronal improves energy level alignment, and facilitates charge carrier extraction and transport, resulting in a champion efficiency of 25.09 %. Importantly, P-Apronal can effectively suppress the migration of I- ions and improve the long-term stability of the devices. The present strategy sets forth a path to attain long-term stability and enhanced efficiency in perovskite solar cells.

Discrete Elemental Distributions inside a Single Mixed-Halide Perovskite Nanocrystal for the Self-Assembly of Multiple Quantum-Light Sources

An in-depth understanding of the structure-property relationships in semiconductor mixed-halide perovskites is critical for their potential applications in various light-absorbing and light-emitting optoelectronic devices. Here we show that during the crystal growth of mixed-halide CsPbBr1.2I1.8 nanocrystals (NCs), abundant Ruddlesden-Popper (RP) plane stacking faults are formed to release the lattice strain. These RP planes hinder the exchange of halide species across them, resulting in the presence of multiple nanodomains with discrete mixed-halide compositions inside a single CsPbBr1.2I1.8 NC. Photoluminescence peaks from these pre-segregated nanodomains, whose correlated intensity and wavelength variations signify the interactions of coupled quantum dots within a single CsPbBr1.2I1.8 NC, can be simultaneously resolved at cryogenic temperature. Our findings thus point to a fascinating scenario in which a semiconductor nanostructure can be further divided into multiple quantum-light sources, the interaction and manipulation of which will promote novel photophysics to facilitate their potential applications in quantum information technologies.

Ligand-Induced Cation-π Interactions Enable High-Efficiency, Bright, and Spectrally Stable Rec. 2020 Pure-Red Perovskite Light-Emitting Diodes

Achieving high-performance perovskite light-emitting diodes (PeLEDs) with pure-red electroluminescence for practical applications remains a critical challenge because of the problematic luminescence property and spectral instability of existing emitters. Herein, high-efficiency Rec. 2020 pure-red PeLEDs, simultaneously exhibiting exceptional brightness and spectral stability, based on CsPb(Br/I)3 perovskite nanocrystals (NCs) capping with aromatic amino acid ligands featuring cation-π interactions, are reported. It is proven that strong cation-π interactions between the PbI6 -octahedra of perovskite units and the electron-rich indole ring of tryptophan (TRP) molecules not only chemically polish the imperfect surface sites, but also markedly increase the binding affinity of the ligand molecules, leading to high photoluminescence quantum yields and greatly enhanced spectral stability of the CsPb(Br/I)3 NCs. Moreover, the incorporation of small-size aromatic TRP ligands ensures superior charge-transport properties of the assembled emissive layers. The resultant devices emitting at around 635 nm demonstrate a champion external quantum efficiency of 22.8%, a max luminance of 12 910 cd m-2 , and outstanding spectral stability, representing one of the best-performing Rec. 2020 pure-red PeLEDs achieved so far.

In situ synchrotron X-ray total scattering measurements and analysis of colloidal CsPbX3 nanocrystals during flow synthesis

In situ X-ray scattering measurements of CsPbX3 (X = Cl, Br, I) nanocrystal formation and halide exchange at NSLS-II beamlines were performed in an automated flow reactor. Total scattering measurements were performed at the 28-ID-2 (XPD) beamline and small-angle X-ray scattering at the 16-ID (LiX) beamline. Nanocrystal structural parameters of interest, including size, size distribution and atomic structure, were extracted from modeling the total scattering data. The results highlight the potential of these beamlines and the measurement protocols described in this study for studying dynamic processes of colloidal nanocrystal synthesis in solution with timescales on the order of seconds.

Potential and perspectives of halide perovskites in light emitting devices

Light emitting diodes (LEDs) have become part of numerous electrical and electronic systems such as lighting, displays, status indicator lamps and wearable electronics. Owing to their excellent optoelectronic properties and deposition via simple solution process, metal halide perovskites possess unique potential for developing halide perovskite-based LEDs (PeLEDs) with superior photoluminescence efficiencies leading to external quantum efficiencies beyond 20% for PeLEDS. However, the limited durability, high operative voltages, and challenges of scale-up are persisting barriers in achieving required technology readiness levels. To build up the existing knowledge and raise the device performance this review provides a state-of-the-art study on the properties, film and device fabrication, efficiency, and stability of PeLEDs. In terms of commercialization, PeLEDs need to overcome materials and device challenges including stability, ion migration, phase segregation, and joule heating, which are discussed in this review. We hope, discussions about the strategies to overcome the stability issues and enhancement the materials intrinsic properties towards development more stable and efficient optoelectronic devices can pave the way for scalability and cost-effective production of PeLEDs.

Formation of Iodide-Rich Domains During Halide Segregation in Lead-Halide Perovskite Nanocrystals

Mixed iodide-bromide methylammonium lead perovskite (MAPbIxBr3-x) nanocrystals (NCs) hold promise for use in light-emitting applications owing to the size- and composition-tunability of their bandgap. However, the segregation of halides during light exposure causes their band gaps to become unstable and narrow. Here, we use transient absorption spectroscopy to track excited-state dynamics during photoinduced halide segregation. The Auger recombination dynamics are observed to accelerate as the bandgap narrows, suggesting enhanced electron-hole overlap. We simulate the motion of iodide within the NC and estimate the evolving bandgap and electron-hole overlap during two possible mechanisms of halide segregation. Our results support a segregation mechanism in which iodide anions form a domain within the NC, rather than a mechanism in which iodide anions independently segregate toward the NC surface. Such mechanistic insight will contribute to future NC bandgap stabilization strategies.

Unraveling Size-Dependent Ion-Migration for Stable Mixed-Halide Perovskite Light-Emitting Diodes

Mixed-halide perovskites show tunable emission wavelength across the visible-light range, with optimum control of the light color. However, color stability remains limited due to the notorious halide segregation under illumination or an electric field. Here, a versatile path toward high-quality mixed-halide perovskites with high emission properties and resistance to halide segregation is presented. Through systematic in and ex situ characterizations, key features for this advancement are proposed: a slowed and controllable crystallization process can promote achievement of halide homogeneity, which in turn ensures thermodynamic stability; meanwhile, downsizing perovskite nanoparticle to nanometer-scale dimensions can enhance their resistance to external stimuli, strengthening the phase stability. Leveraging this strategy, devices are developed based on CsPbCl1.5 Br1.5 perovskite that achieves a champion external quantum efficiency (EQE) of 9.8% at 464 nm, making it one of the most efficient deep-blue mixed-halide perovskite light-emitting diodes (PeLEDs) to date. Particularly, the device demonstrates excellent spectral stability, maintaining a constant emission profile and position for over 60 min of continuous operation. The versatility of this approach with CsPbBr1.5 I1.5 PeLEDs is further showcased, achieving an impressive EQE of 12.7% at 576 nm.

Optical Memory, Switching, and Neuromorphic Functionality in Metal Halide Perovskite Materials and Devices

Metal halide perovskite based materials have emerged over the past few decades as remarkable solution-processable optoelectronic materials with many intriguing properties and potential applications. These emerging materials have recently been considered for their promise in low-energy memory and information processing applications. In particular, their large optical cross-sections, high photoconductance contrast, large carrier-diffusion lengths, and mixed electronic/ionic transport mechanisms are attractive for enabling memory elements and neuromorphic devices that are written and/or read in the optical domain. Here, recent progress toward memory and neuromorphic functionality in metal halide perovskite materials and devices where photons are used as a critical degree of freedom for switching, memory, and neuromorphic functionality is reviewed.

Enhanced Hot-Phonon Bottleneck Effect on Slowing Hot Carrier Cooling in Metal Halide Perovskite Quantum Dots with Alloyed A-Site

A deep understanding of the effect of the A-site cation cross-exchange on the hot-carrier relaxation dynamics in perovskite quantum dots (PQDs) has profound implications on the further development of disruptive photovoltaic technologies. In this study, the hot carrier cooling kinetics of pure FAPbI3 (FA+ , CH(NH2 )2 + ), MAPbI3 (MA+ , CH3 NH3 + + ), CsPbI3 (Cs+ , Cesium) and alloyed FA0.5 MA0.5 PbI3 , FA0.5 Cs0.5 PbI3 , and MA0.5 Cs0.5 PbI3 QDs are investigated using ultrafast transient absorption (TA) spectroscopy. The lifetimes of the initial fast cooling stage (<1 ps) of all the organic cation-containing PQDs are shorter than those of the CsPbI3 QDs, as verified by the electron-phonon coupling strength extracted from the temperature-dependent photoluminescence spectra. The lifetimes of the slow cooling stage of the alloyed PQDs are longer under illumination greater than 1 sun, which is ascribed to the introduction of co-vibrational optical phonon modes in the alloyed PQDs. This facilitated efficient acoustic phonon upconversion and enhanced the hot-phonon bottleneck effect, as demonstrated by first-principles calculations.

Mitigating halide ion migration by resurfacing lead halide perovskite nanocrystals for stable light-emitting diodes

Lead halide perovskite nanocrystals are promising for next-generation high-definition displays, especially in light of their tunable bandgaps, high color purities, and high carrier mobility. Within the past few years, the external quantum efficiency of perovskite nanocrystal-based light-emitting diodes has progressed rapidly, reaching the standard for commercial applications. However, the low operational stability of these perovskite nanocrystal-based light-emitting diodes remains a crucial issue for their industrial development. Recent experimental evidence indicates that the migration of ionic species is the primary factor giving rise to the performance degradation of perovskite nanocrystal-based light-emitting diodes, and ion migration is closely related to the defects on the surface of perovskite nanocrystals and at the grain boundaries of their thin films. In this review, we focus on the central idea of surface reconstruction of perovskite nanocrystals, discuss the influence of surface defects on halide ion migration, and summarize recent advances in resurfacing perovskite nanocrystal strategies toward mitigating halide ion migration to improve the stability of the as-fabricated light-emitting diode devices. From the perspective of perovskite nanocrystal resurfacing, we set out a promising research direction for improving both the spectral and operational stability of perovskite nanocrystal-based light-emitting diodes.

Mixed-Halide Perovskites with Halogen Bond Induced Interlayer Locking Structure for Stable Pure-Red PeLEDs

Mixed-halide perovskites enable precise spectral tuning across the entire spectral range through composition engineering. However, mixed halide perovskites are susceptible to ion migration under continuous illumination or electric field, which significantly impedes the actual application of perovskite light-emitting diodes (PeLEDs). Here, we demonstrate a novel approach to introduce strong and homogeneous halogen bonds within the quasi-two-dimensional perovskite lattices by means of an interlayer locking structure, which effectively suppresses ion migration by increasing the corresponding activation energy. Various characterizations confirmed that intralattice halogen bonds enhance the stability of quasi-2D mixed-halide perovskite films. Here, we report that the PeLEDs exhibit an impressive 18.3% EQE with pure red emission with CIE color coordinate of (0.67, 0.33) matching Rec. 2100 standards and demonstrate an operational half-life of ∼540 min at an initial luminance of 100 cd m^-2, representing one of the most stable mixed-halide pure red PeLEDs reported to date.

Quantifying and Reducing Ion Migration in Metal Halide Perovskites through Control of Mobile Ions

The presence of intrinsic ion migration in metal halide perovskites (MHPs) is one of the main reasons that perovskite solar cells (PSCs) are not stable under operation. In this work, we quantify the ion migration of PSCs and MHP thin films in terms of mobile ion concentration (N_o) and ionic mobility (µ) and demonstrate that N_o has a larger impact on device stability. We study the effect of small alkali metal A-site cation additives (e.g., Na⁺, K⁺, and Rb⁺) on ion migration. We show that the influence of moisture and cation additive on N_o is less significant than the choice of top electrode in PSCs. We also show that N_o in PSCs remains constant with an increase in temperature but μ increases with temperature because the activation energy is lower than that of ion formation. This work gives design principles regarding the importance of passivation and the effects of operational conditions on ion migration.

Design Rules for Obtaining Narrow Luminescence from Semiconductors Made in Solution

Solution-processed semiconductors are in demand for present and next-generation optoelectronic technologies ranging from displays to quantum light sources because of their scalability and ease of integration into devices with diverse form factors. One of the central requirements for semiconductors used in these applications is a narrow photoluminescence (PL) line width. Narrow emission line widths are needed to ensure both color and single-photon purity, raising the question of what design rules are needed to obtain narrow emission from semiconductors made in solution. In this review, we first examine the requirements for colloidal emitters for a variety of applications including light-emitting diodes, photodetectors, lasers, and quantum information science. Next, we will delve into the sources of spectral broadening, including "homogeneous" broadening from dynamical broadening mechanisms in single-particle spectra, heterogeneous broadening from static structural differences in ensemble spectra, and spectral diffusion. Then, we compare the current state of the art in terms of emission line width for a variety of colloidal materials including II-VI quantum dots (QDs) and nanoplatelets, III-V QDs, alloyed QDs, metal-halide perovskites including nanocrystals and 2D structures, doped nanocrystals, and, finally, as a point of comparison, organic molecules. We end with some conclusions and connections, including an outline of promising paths forward.

Origins of Photoluminescence Instabilities at Halide Perovskite/Organic Hole Transport Layer Interfaces

Metal halide perovskites are promising for optoelectronic device applications; however, their poor stability under solar illumination remains a primary concern. While the intrinsic photostability of isolated neat perovskite samples has been widely discussed, it is important to explore how charge transport layers─employed in most devices─impact photostability. Herein, we study the effect of organic hole transport layers (HTLs) on light-induced halide segregation and photoluminescence (PL) quenching at perovskite/organic HTL interfaces. By employing a series of organic HTLs, we demonstrate that the HTL's highest occupied molecular orbital energy dictates behavior; furthermore, we reveal the key role of halogen loss from the perovskite and subsequent permeation into organic HTLs, where it acts as a PL quencher at the interface and introduces additional mass transport pathways to facilitate halide phase separation. In doing so, we both reveal the microscopic mechanism of non-radiative recombination at perovskite/organic HTL interfaces and detail the chemical rationale for closely matching the perovskite/organic HTL energetics to maximize solar cell efficiency and stability.

Suppressing Phase Segregation in CsPbIBr2 Films via Anchoring Halide Ions toward Underwater Solar Cells

Inorganic CsPbIBr₂ perovskite solar cells (PSCs) have accomplished many milestones, yet their progress has been constrained by ion migration and phase separation. This study explores the modulation of perovskite crystallization kinetics and halide ion migration through chlorobenzene (CB) antisolvent with bis(pentafluorophenyl)zinc (Zn(C₆F₅)₂) additive. The photoluminescence and absorption spectra reveal the significantly reduced phase segregaton in CsPbIBr₂ film treated by CB with Zn(C₆F₅)₂. Moreover, this research analyzes the CsPbIBr₂ film's free carrier lifetime, diffusion length, and mobility using time-resolved microwave conductivity and transient absorption spectroscopy after Zn(C₆F₅)₂ modification. Consequently, the modified CsPbIBr₂ PSCs offer a 12.57% power conversion efficiency (PCE), the highest value among CsPbIBr₂ PSCs with negligible hysteresis and prolonged stability. Furthermore, under 1-m-deep water, CsPbIBr₂ PSCs display a PCE of 14.18%. These findings provide an understanding of the development of phase-segregation-free CsPbIBr₂ films and showcase the prospective applications of CsPbIBr₂ PSCs in underwater power systems.

Electrochemical Control over the Optical Properties of II-VI Colloidal Nanoplatelets by Tailoring the Station of Extra Charge Carriers

An electrochemical (EC) method has been successfully applied to regulate the optical properties of nanocrystals, such as reducing their gain threshold by EC doping and enhancing their photoluminescence intensity by EC filling of trap states. However, the processes of EC doping and filling are rarely reported simultaneously in a single study, hindering the understanding of their underlying interactions. Here, we report the spectroelectrochemical (SEC) studies of quasi-two-dimensional nanoplatelets (NPLs), intending to clarify the above issues. EC doping is successfully achieved in CdSe/CdZnS core/shell NPLs, with red-shifted photoluminescence and a reversal of the emission intensity trend. The injection of extra electrons (holes) into the conduction (valence) band edges needs high bias voltages, while the passivation/activation process of trap states with the shift of Fermi level starts at lower EC potentials. Then, we explore the role of excitation light conditions in these processes, different from existing SEC research studies. Interestingly, increasing the laser power density can hinder EC electron injection, whereas decreasing the excitation energy evades the passivation process of trap states. Moreover, we demonstrate that EC control strategies can be used to realize color display and anti-counterfeiting applications via simultaneously tailoring the photoluminescence intensity of red- and green-emitting NPLs.

Temperature-Dependent Reversal of Phase Segregation in Mixed-Halide Perovskites

Understanding the mechanism of light-induced halide segregation in mixed-halide perovskites is essential for their application in multijunction solar cells. Here, photoluminescence spectroscopy is used to uncover how both increases in temperature and light intensity can counteract the halide segregation process. It is observed that, with increasing temperature, halide segregation in CH₃ NH₃ Pb(Br_0.4 I_0.6 )₃ first accelerates toward ≈290 K, before slowing down again toward higher temperatures. Such reversal is attributed to the trade-off between the temperature activation of segregation, for example through enhanced ionic migration, and its inhibition by entropic factors. High light intensities meanwhile can also reverse halide segregation; however, this is found to be only a transient process that abates on the time scale of minutes. Overall, these observations pave the way for a more complete model of halide segregation and aid the development of highly efficient and stable perovskite multijunction and concentrator photovoltaics.

Super-Bright Green Perovskite Light-Emitting Diodes Using Ionic Liquid Additives

Halide perovskites have potential for use in next-generation low-cost, high-efficiency, and highly color-pure light-emitting diodes (LED) that can be used in various applications, such as flat and flexible displays and solid-state lighting. However, they still lag behind other mature technologies, such as organic LEDs and inorganic LEDs, in terms of performance, particularly brightness. This lag is partly due to the insulating nature of the long-chain organic ligands used to control the perovskite-film morphology. Herein, a 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid (IL) is incorporated as a potential additive with CsPbBr₃ perovskite precursors, which results in a super-bright green perovskite light emitting diode (PeLED) achieving a peak luminance of 3.28 × 10⁵  cd m^-2 only at a bias voltage of 6 V, with a peak external quantum efficiency of 13.75%. This achievement is the outcome of multirole support from IL that simultaneously enables superior control over the perovskite-film morphology, passivates defects, modifies the band energy levels, and prevents ion migration. Hence, this work demonstrates IL as a novel alternative additive with the potential to outperform conventional long-chain ligands in high-performance PeLED device fabrication.

Gamma-Ray Irradiation Stability of Zero-Dimensional Cs3Cu2I5 Metal Halide Scintillator Single Crystals

Zero-dimensional Cs₃Cu₂I₅ is one of the most promising metal halide scintillators due to its large Stokes shift, photoluminescence quantum yields, freedom from toxic elements, and excellent energy spectrum resolution. To unlock the full potential of Cs₃Cu₂I₅ as an effective alternative to traditional scintillators for gamma-ray detection, the irradiation stability of Cs₃Cu₂I₅ single crystals under ⁶⁰Co gamma rays with a maximum accumulated dose of 800 krad was explored. Although the luminescence mechanism remained unchanged after irradiation, the optical properties of Cs₃Cu₂I₅ single crystals demonstrated a dose-dependent change at low accumulated doses (<600 krad). However, a further increase in the accumulated dose did not lead to more severe degradation and even slight performance recovery occurred. Electron paramagnetic resonance and theoretical calculation results revealed that the irradiation-induced Cs⁺-related Frenkel defects contribute to performance degradation. These results shed light on the microscopic mechanism of gamma-ray irradiation damage of Cs₃Cu₂I₅ single crystal and provide guidance to their real application.

A Multifunctional "Halide-Equivalent" Anion Enabling Efficient CsPb(Br/I)3 Nanocrystals Pure-Red Light-Emitting Diodes with External Quantum Efficiency Exceeding 23

Pure-red perovskite LEDs (PeLEDs) based on CsPb(Br/I)₃ nanocrystals (NCs) usually suffer from a compromise in emission efficiency and spectral stability on account of the surface halide vacancies-induced nonradiative recombination loss, halide phase segregation, and self-doping effect. Herein, a "halide-equivalent" anion of benzenesulfonate (BS^- ) is introduced into CsPb(Br/I)₃ NCs as multifunctional additive to simultaneously address the above challenging issues. Joint experiment-theory characterizations reveal that the BS^- can not only passivate the uncoordinated Pb²⁺ -related defects at the surface of NCs, but also increase the formation energy of halide vacancies. Moreover, because of the strong electron-withdrawing property of sulfonate group, electrons are expected to transfer from the CsPb(Br/I)₃ NC to BS^- for reducing the self-doping effect and altering the n-type behavior of CsPb(Br/I)₃ NCs to near ambipolarity. Eventually, synergistic boost in device performance is achieved for pure-red PeLEDs with CIE coordinates of (0.70, 0.30) and a champion external quantum efficiency of 23.5%, which is one of the best value among the ever-reported red PeLEDs approaching to the Rec. 2020 red primary color. Moreover, the BS^- -modified PeLED exhibits negligible wavelength shift under different operating voltages. This strategy paves an efficient way for improving the efficiency and stability of pure-red PeLEDs.

Negligible Ion Migration in Tin-Based and Tin-Doped Perovskites

Ion migration is a notorious phenomenon observed in ionic perovskite materials. It causes several severe issues in perovskite optoelectronic devices such as instability, current hysteresis, and phase segregation. Here, we report that, in contrast to lead halide perovskites (LHPs), no ion migration or phase segregation was observed in tin halide perovskites (THPs) under illumination or an electric field. The origin is attributed to a much stronger Sn-halide bond and higher ion migration activation energy (E_a ) in THPs, which remain nearly constant under illumination. We further figured out the threshold E_a for the absence of ion migration to be around 0.65 eV using the CsSn_y Pb_1-y (I_0.6 Br_0.4 )₃ system whose E_a varies with Sn ratios. Our work shows that ion migration does not necessarily exist in all perovskites and suggests metallic doping to be a promising way of stopping ion migration and improving the intrinsic stability of perovskites.

Inorganic Halide Perovskite Quantum Dots: A Versatile Nanomaterial Platform for Electronic Applications

Metal halide perovskites have generated significant attention in recent years because of their extraordinary physical properties and photovoltaic performance. Among these, inorganic perovskite quantum dots (QDs) stand out for their prominent merits, such as quantum confinement effects, high photoluminescence quantum yield, and defect-tolerant structures. Additionally, ligand engineering and an all-inorganic composition lead to a robust platform for ambient-stable QD devices. This review presents the state-of-the-art research progress on inorganic perovskite QDs, emphasizing their electronic applications. In detail, the physical properties of inorganic perovskite QDs will be introduced first, followed by a discussion of synthesis methods and growth control. Afterwards, the emerging applications of inorganic perovskite QDs in electronics, including transistors and memories, will be presented. Finally, this review will provide an outlook on potential strategies for advancing inorganic perovskite QD technologies.

Excitation Intensity- and Size-Dependent Halide Photosegregation in CsPb(I0.5Br0.5)3 Perovskite Nanocrystals

Although broad consensus exists that photoirradiation of mixed-halide lead perovskites leads to anion segregation, no model today fully rationalizes all aspects of this near ubiquitous phenomenon. Here, we quantitatively compare experimental, CsPb(I_0.5Br_0.5)₃ nanocrystal (NC) terminal anion photosegregation stoichiometries and excitation intensity thresholds to a band gap-based, thermodynamic model of mixed-halide perovskite photosegregation. Mixed-halide NCs offer strict tests of theory given physical sizes, which dictate local photogenerated carrier densities. We observe that mixed-anion perovskite NCs exhibit significant robustness to photosegregation, with photosegregation propensity decreasing with decreasing NC size. Observed size- and excitation intensity-dependent photosegregation data agree with model predicted size- and excitation intensity-dependent terminal halide stoichiometries. Established correspondence between experiment and theory, in turn, suggests that mixed-halide perovskite photostabilities can be predicted a priori using local gradients of (empirical) Vegard's law expressions of composition-dependent band gaps.

Ionic-electronic halide perovskite memdiodes enabling neuromorphic computing with a second-order complexity

With increasing computing demands, serial processing in von Neumann architectures built with zeroth-order complexity digital circuits is saturating in computational capacity and power, entailing research into alternative paradigms. Brain-inspired systems built with memristors are attractive owing to their large parallelism, low energy consumption, and high error tolerance. However, most demonstrations have thus far only mimicked primitive lower-order biological complexities using devices with first-order dynamics. Memristors with higher-order complexities are predicted to solve problems that would otherwise require increasingly elaborate circuits, but no generic design rules exist. Here, we present second-order dynamics in halide perovskite memristive diodes (memdiodes) that enable Bienenstock-Cooper-Munro learning rules capturing both timing- and rate-based plasticity. A triplet spike timing-dependent plasticity scheme exploiting ion migration, back diffusion, and modulable Schottky barriers establishes general design rules for realizing higher-order memristors. This higher order enables complex binocular orientation selectivity in neural networks exploiting the intrinsic physics of the devices, without the need for complicated circuitry.

Suppressing ion migration in metal halide perovskite via interstitial doping with a trace amount of multivalent cations

Cations with suitable sizes to occupy an interstitial site of perovskite crystals have been widely used to inhibit ion migration and promote the performance and stability of perovskite optoelectronics. However, such interstitial doping inevitably leads to lattice microstrain that impairs the long-range ordering and stability of the crystals, causing a sacrificial trade-off. Here, we unravel the evident influence of the valence states of the interstitial cations on their efficacy to suppress the ion migration. Incorporation of a trivalent neodymium cation (Nd³⁺) effectively mitigates the ion migration in the perovskite lattice with a reduced dosage (0.08%) compared to a widely used monovalent cation dopant (Na⁺, 0.45%). The photovoltaic performances and operational stability of the prototypical perovskite solar cells are enhanced with a trace amount of Nd³⁺ doping while minimizing the sacrificial trade-off.

Br Vacancy Defects Healed Perovskite Indoor Photovoltaic Modules with Certified Power Conversion Efficiency Exceeding 36

Indoor photovoltaics (IPVs) are expected to power the Internet of Things ecosystem, which is attracting ever-increasing attention as part of the rapidly developing distributed communications and electronics technology. The power conversion efficiency of IPVs strongly depends on the match between typical indoor light spectra and the band gap of the light absorbing layer. Therefore, band-gap tunable materials, such as metal-halide perovskites, are specifically promising candidates for approaching the indoor illumination efficiency limit of ∼56%. However, perovskite materials with ideal band gap for indoor application generally contain high bromine (Br) contents, causing inferior open-circuit voltage (VOC ). By fabricating a series of wide-bandgap perovskites (Cs0.17 FA0.83 PbI3- x Brx , 0.6 ≤ x ≤ 1.6) with varying Br contents and related band gaps, it is found that, the high Br vacancy (VBr ) defect density is a significant reason that leading to large VOC deficits apart from the well-accepted halide segregation. The introduction of I-rich alkali metal small-molecule compounds is demonstrated to suppress the VBr and increase the VOC of perovskite IPVs up to 1.05 V under 1000 lux light-emitting diode illumination, one of the highest VOC values reported so far. More importantly, the modules are sent for independent certification and have gained a record efficiency of 36.36%.

Enlighten the non-illuminated region by phase segregation of mixed halide perovskites

The well-known ion migration in mixed halide perovskites has been intensely investigated within the area under uniform light illumination. Here, the authors demonstrate that the anion segregation in these materials is a nonlocal effect of which the ion redistribution may occur at a macroscopic or mesoscopic scale beyond.

Investigation on the stability improvement of hybrid halide all-inorganic perovskite quantum dots

Perovskite quantum dots (QDs) with stable luminous properties are crucial to for the construction of corresponding light-emitting diodes (LEDs). Hybrid halide perovskite QDs, especially those contain iodine element emitting red light, usually demonstrate poor emission stability owing to the halide segregation. Moreover, red component is indispensable for the construction of white LEDs (WLEDs). Hence, it is essential to improve the luminous stability of hybrid halide perovskite QDs containing iodine element. Here, magnesium dopant and silica matrix were employed to improve the stability of hybrid halide CsPb(IBr)₃QDs. Red, green and blue are three primary colors for constructing WLEDs. Therefore, silica-coated CsPbBr₃QDs emitting green light were also synthesized. The fabricated silica-coated Mn:CsPb(IBr)₃/PMMA film delivered good emission stability during a 42 d observation period, exhibiting the improved stability compared with the corresponding Mn:CsPb(IBr)₃QDs in solution. WLEDs were fabricated by integrating the mixture of silica-coated Mn:CsPb(IBr)₃QDs, silica-coated CsPbBr₃QDs and silicon sealant with a blue-emission LED chip. The as fabricated device exhibited a longer lifetime to be lit than that of those reported previously. During the 36 d observation period for the as fabricated device, the red emission from the silica-coated Mn:CsPb(IBr)₃QDs experienced a peak-emission shift of 34 nm, which is much less than that in Mn:CsPb(IBr)₃QDs solution. Their overall intensity downtrend combined the peak-emission shift are responsible for the spectrum shape change, so as to the fluctuation of color correlated temperature and color rendering index. Our study provides a good starting point for the further improvement of the stability of the hybrid halide perovskites QDs and the corresponding light-emitting devices. With deep studies on the synthesis method and luminous mechanism for hybrid halide CsPb(IBr)₃QDs, red-emission perovskite QDs with satisfied properties are expected to be obtained.

Assessing the Drawbacks and Benefits of Ion Migration in Lead Halide Perovskites

Since the inception of the unprecedented rise of halide perovskites for photovoltaic research, ion migration has shadowed this material class with undesirable hysteresis and degradation effects, limiting its practical implementations. Unfortunately, the localized doping and electrochemical reactions triggered by ion migration cause many more undesirable effects that are often unreported or misinterpreted because they deviate from classical semiconductor behavior. In this Perspective, we provide a concise overview of such effects in halide perovskites, such as operational instability in photovoltaics, polarization-induced abnormal external quantum efficiency in light-emitting diodes, and energy channel shift and anomalous sensitivities in hard radiation detection. Finally, we highlight a unique use case of exploiting ion migration as a boon to design emerging memory technologies such as memristors for information storage and computing.

Stability of Perovskite Light-Emitting Diodes: Existing Issues and Mitigation Strategies Related to Both Material and Device Aspects

Metal halide perovskites combine excellent electronic and optical properties, such as defect tolerance and high photoluminescence efficiency, with the benefits of low-cost, large-area, solution-based processing. Composition- and dimension-tunable properties of perovskites have already been utilized in bright and efficient light-emitting diodes (LEDs). At the same time, there are still great challenges ahead to achieving operational and spectral stability of these devices. In this review, the origins of instability of perovskite materials, and reasons for their degradation in LEDs are considered. Then, strategies for improving the stability of perovskite materials are reviewed, such as compositional engineering, dimensionality control, defect passivation, suitable encapsulation matrices, and fabrication of core/shell perovskite nanocrystals. For improvement of the operational stability of perovskite LEDs, the use of inorganic charge-transport layers, optimization of charge balance, and proper thermal management are considered. The review is concluded with a detailed account of the current challenges and a perspective on the key approaches and opportunities on how to reach the goal of stable, bright, and efficient perovskite LEDs.