Metal halide perovskite light emitters

Perovskite White Light Emitting Diodes: Progress, Challenges, and Opportunities

As global warming, energy shortages, and environment pollution have intensified, low-carbon and energy-saving lighting technology has attracted great attention worldwide. Light emitting diodes (LEDs) have been around for decades and are considered to be the most ideal lighting technology currently due to their high luminescence efficiency (LE) and long lifespan. Besides, along with the development of modern technology, lighting technologies with higher performance and more functions are desired. Perovskite based LEDs (PeLEDs) have recently emerged as ideal candidates for lighting technology owing to the extraordinary photoelectric properties of perovskite, such as high photoluminescence quantum yields (PLQYs), easy wavelength tuning, and low-cost synthesis. Herein, we open this review by introducing the background of white LEDs (WLEDs), including their light-emitting mechanism, typical characteristics, and key indicators in applications. Then, four main approaches to fabricate WLEDs are discussed and compared. After that, in accordance with the four categories, we focus on the recent progress of white PeLEDs (Pe-WLEDs), followed by the challenges and opportunities for Pe-WLEDs in practical application. Meanwhile, some pertinent countermeasures to their challenges are put forward. Finally, the development promise of Pe-WLEDs is explored.

Energy Resonance Transfer between Quantum Defects in Metal Halide Perovskites

Quantum defects have been shown to play an essential role in nonradiative recombination in metal halide perovskites (MHPs). Nonetheless, the processes of charge transfer assisted by defects are still ambiguous. Herein, we theoretically study the nonradiative multiphonon processes among different types of quantum defects in MHPs using Markvart's model for the induced mechanisms of electron-electron and electron-phonon interactions. We find that the charge carrier can transfer between the neighboring levels of the same type of shallow defects by multiphonon processes, but it will be distinctly suppressed with an increase in the defect depth. For the nonradiation multiphonon transitions between donor- and acceptor-like defects, the processes are very fast and not sensitive to the defect depth, which provides a possible explanation for the phenomenon of blinking of photoluminescence spectra. We also discuss the temperature dependence of these multiphonon processes and find that their variational trends depend on the comparison of the Huang-Rhys factor with the emitted phonon number. These theoretical results not only fill some of the gaps in defect-assisted nonradiative processes in the perovskite materials but also provide deeper physical insights into producing higher-performance perovskite-based devices.

Antisolvent-Processed One-Dimensional Ternary Rubidium Copper Bromine Microwires for Sensitive and Flexible Ultraviolet Photodetectors

Recently, newly emerging halide perovskites have aroused intensive attention in photoelectric fields in virtue of their good properties, such as well-balanced carrier transport, large light absorption coefficient, tunable band gap, and low-temperature solution processing technique. Nevertheless, their future commercial development is severely hampered by lead toxicity and instability of such materials. In this work, one-dimensional Rb₂CuBr₃ single-crystal microwires (MWs) were prepared by antisolvent engineering, and they were further employed as absorbers to prepare sensitive ultraviolet (UV) photodetectors. The optical band gap of Rb₂CuBr₃ MWs is measured to be 3.83 eV, exhibiting an excellent UV absorption. The fabricated device demonstrates a remarkable UV light detection ability with a specific detectivity of 1.23 × 10¹¹ Jones, responsivity of 113.64 mA W^-1, and response speed of 69.31/87.55 ms under light illumination of 265 nm. Meanwhile, the proposed photodetector without any encapsulation shows outstanding stability and repeatability. After storing in ambient air for 2 weeks, the light detection ability remains basically unchanged. Further, a flexible photodetector was fabricated with the same structure, which demonstrates a remarkable bending endurance. These results confirm the great potential of Rb₂CuBr₃ for high-performance UV photodetectors, increasing the possibility for assembly of optoelectronic systems.

Phase Tailoring of Ruddlesden-Popper Perovskite at Fixed Large Spacer Cation Ratio

Ruddlesden-Popper (RP) metal halide perovskites are considered as promising optoelectronic materials due to their good environmental stability and desirable optoelectronic properties. However, the phase composition and ordering in the deposited film, with a fixed ratio of large organic spacer cation in the precursor solution, are hard to be further tailored for specific optoelectronic applications. Herein, it is shown that even with a fixed spacer cation ratio, the phase composition and ordering can still be largely regulated by utilizing different crystallization kinetics of various cations with the inorganic octahedral lead halide. By using two different short cations to compete with the large spacer cation, the phase composition can be continuously tailored from thin multiple quantum wells (MQWs) dominated to 3D perovskite dominated. The phase ordering can be reversed from small n phases' prior to large n phases' prior near the substrate. Finally, with the same amount of large spacer cation protection, the perovskite can be tailored for both high-performance electroluminescence and photovoltaics with favorable energetic landscape for the corresponding desired first-order excitonic recombination and second-order free electron-hole recombination, respectively. This exploration substantially contributes to the understanding of precise phase engineering in RP perovskite and may provide a new insight into the design of multiple functional devices.

Achieving high-performance in situ fabricated FAPbBr3 and electroluminescence

Currently, metal halide perovskite films still encounter the issues of inferior film quality and interfacial electrical properties when they were constructed electroluminescence devices. Herein, efficient and pinhole-free perovskite emissive film was obtained on the poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) layer modified by an ultrathin LiF layer. Owing to the synergetic effect of the LiF interlayer, including better regulation of the perovskite film and a more balanced charge injection capability, an efficient green light-emitting diode based on the perovskite film was achieved with a maximum current efficiency of 25.6 cd/A, which is 58% higher than that of the control device with a plasma-treated PEDOT:PSS layer. Our results not only provide a facile strategy for acquiring efficient perovskite films but also circumvent the expensive and time-consuming plasma treatment process commonly used to improve the wetting properties of the underlying films.

Ultrafast, Self-Powered, and Charge-Transport-Layer-Free Ultraviolet Photodetectors Based on Sequentially Vacuum-Evaporated Lead-Free Cs2AgBiBr6 Thin Films

Researchers have focused on perovskite-based ultraviolet photodetectors due to their significance in fundamental scientific and practical applications. However, toxicity and instability hold back their mass production and commercialization. The lead-free Cs₂AgBiBr₆ double perovskite, promised to be an alternative, is fabricated mostly by spin coating, which restricts the practical application in high-resolution image sensors. Herein, we demonstrate a sequential vacuum evaporation method for the fabrication of the Cs₂AgBiBr₆ film. A self-powered ultraviolet photodetector based on the evaporated Cs₂AgBiBr₆ thin film is further constructed without any carrier-transport layers, for the first time. The best-performing device has a high on/off ratio of 6.6 × 10³, and its response time is fast, less than 6.13 μs. Moreover, the as-prepared devices exhibit salient stability under harsh operational conditions (continuous illumination, high temperature, and humidity). In addition, the pixelated image sensor containing a 25 × 25 Cs₂AgBiBr₆ photodetector array achieves a proof-of-concept special pattern recognition. Our work paves the way for new-generation ultraviolet image sensors composed of environmentally friendly and high-performance perovskite photodetector arrays.

State of the Art and Prospects for Halide Perovskite Nanocrystals

Metal-halide perovskites have rapidly emerged as one of the most promising materials of the 21st century, with many exciting properties and great potential for a broad range of applications, from photovoltaics to optoelectronics and photocatalysis. The ease with which metal-halide perovskites can be synthesized in the form of brightly luminescent colloidal nanocrystals, as well as their tunable and intriguing optical and electronic properties, has attracted researchers from different disciplines of science and technology. In the last few years, there has been a significant progress in the shape-controlled synthesis of perovskite nanocrystals and understanding of their properties and applications. In this comprehensive review, researchers having expertise in different fields (chemistry, physics, and device engineering) of metal-halide perovskite nanocrystals have joined together to provide a state of the art overview and future prospects of metal-halide perovskite nanocrystal research.

Gold-Cage Perovskites: A Three-Dimensional AuIII-X Framework Encasing Isolated MX63- Octahedra (MIII = In, Sb, Bi; X = Cl-, Br-, I-)

The Cs₈Au^III₄M^IIIX₂₃ (M = In³⁺, Sb³⁺, Bi³⁺; X = Cl^-, Br^-, I^-) perovskites are composed of corner-sharing Au-X octahedra that trace the edges of a cube containing an isolated M-X octahedron at its body center. This structure, unique within the halide perovskite family, may be derived from the doubled cubic perovskite unit cell by removing the metals at the cube faces. To our knowledge, these are the only halide perovskites where the octahedral sites do not bear an average 2+ charge. Charge compensation in these materials requires a stoichiometric halide vacancy, which is disordered around the Au atom at the unit-cell corner and orders when the crystallization is slowed. Using X-ray crystallography, X-ray absorption spectroscopy, and pair distribution function analysis, we elucidate the structure of this unusual perovskite. Metal-site alloying produces further intricacies in this structure, which our model explains. Compared to other halide perovskites, this class of materials shows unusually low absorption onset energies ranging between ca. 1.0 and 2.4 eV. Partial reduction of Au³⁺ to Au⁺ affords an intervalence charge-transfer band, which redshifts the absorption onset of Cs₈Au₄InCl₂₃ from 2.4 to 1.5 eV. With connected Au-X octahedra and isolated M-X octahedra, this structure type combines zero- and three-dimensional metal-halide sublattices in a single material and stands out among halide perovskites for its ordering of homovalent metals, ordering of halide vacancies, and incorporation of purely trivalent metals at the octahedral sites.

Co-luminescence in a zero-dimensional organic-inorganic hybrid antimony halide with multiple coordination units

Zero-dimensional (0D) organic-inorganic hybrid metal halides (OIMHs) containing multiple halometallate species (HMSs) have received extensive attention due to their capability to achieve multifunctional photophysical characteristics. Herein we report a lead-free 0D-OIMH compound, namely [Emim]8[SbCl6]2[SbCl5] (1, Emim = 1-ethyl-3-methylimidazolium), which is the first crystal containing two distinct mononuclear [SbXn]3-n units in one single structure. The optical absorption, temperature/excitation-variable photoluminescence (PL) and PL decay were studied. 1 exhibits a broad emission centered at 577 nm, which is analyzed to be a combination of the emissions from [SbCl6]3- and [SbCl5]2-. The structural effects including SbSb distances and polyhedral distortion of [SbXn]3-n on the PL of antimony-based 0D-OIMHs are discussed in detail. This work would provide guidance for constructing Sb-based 0D OIMHs composed of multiple halometallate species.

Strong Excitonic Magneto-Optic Effects in Two-Dimensional Organic-Inorganic Hybrid Perovskites

This work demonstrates the strong excitonic magneto-optic (MO) effects of magnetic circular dichroism (MCD) and Faraday rotation (FR) in nonmagnetic two-dimensional (2D) organic-inorganic hybrid Ruddlesden-Popper perovskites (RPPs) at room temperature. Due to their strong and sharp excitonic absorption as a result of unique quantum well structures of 2D RPPs, sizeable linear excitonic MO effects of MCD and FR can be observed at room temperature under a low magnetic field (<1 T) compared with their three-dimensional counterpart. In addition, since the band gaps of 2D organic-inorganic hybrid perovskites can be manipulated either by changing the number n of inorganic octahedral slabs per unit cell or through halide engineering, linear excitonic MO effects of 2D-RPPs can be observed through the broadband spectral ranges of visible light. Our result may pave the way for the promising research field of MO and magneto-optoelectronic applications based on 2D organic-inorganic hybrid perovskites with facile solution processes.

Chemically Robust Indium Tin Oxide/Graphene Anode for Efficient Perovskite Light-Emitting Diodes

Graphene is an optimal material to be employed as an ionic diffusion barrier because of its outstanding impermeability and chemical robustness. Indium tin oxide (ITO) is often used in perovskite light-emitting diodes (PeLEDs), and it can release indium easily upon exposure to the acidic hole-injection layer so that luminescence can be quenched significantly. Here, we exploit the outstanding impermeability of graphene and use it as a chemical barrier to block the etching that can occur in ITO exposed to an acidic hole-injection layer in PeLEDs. This barrier reduced the luminescence quenching that these metallic species can cause, so the photoluminescence lifetime of perovskite film was substantially higher in devices with ITO and graphene layer (87.9 ns) than in devices that had only an ITO anode (22.1 ns). Luminous current efficiency was also higher in PeLEDs with a graphene barrier (16.4 cd/A) than in those without graphene (9.02 cd/A). Our work demonstrates that graphene can be used as a barrier to reduce the degradation of transparent electrodes by chemical etching in optoelectronic devices.

Organic Molecule Assisted Growth of Perovskite Films Consisting of Square Grains by Surface-Confined Process

Organic–inorganic perovskite single crystals are promising in the field of optoelectronics due to their excellent optoelectronic properties. However, the ion transport of perovskite precursor is poor in confined spaces, which results in difficulty in the preparation of perovskite single-crystal films. Herein, MAPbBr₃ films consisting of square grains were fabricated by the surface-confined process using the organic molecule PEAI (phenethylammonium iodide). Under the effect of oversaturation gradient, PEA⁺ is combined with the surface of perovskite grain from top to side, which constrains the lateral growth of grains and induces a downward growth of perovskite, leading to the formation of square grains. With the improvement of concentration PEAI, the perovskite film exhibits a decreased side length of grains (from 0.98 to 12.96 μm) and increased grain number and coverage, as well as crystallinity. The perovskite single crystalline grain films with PEAI showed double photoluminescence (PL) emission peaks due to the existence of iodine-rich perovskite. This work may provide a practical way to fabricate high-quality perovskite films for perovskite photoelectronic devices.

Modulation of Metal Halide Structural Units for Light Emission

ConspectusWith the development of solid-state lighting technology, efficient light sources that combine high brightness, wide range, and good stability are in high demand for next-generation lighting and displays. Metal halides are emerging as promising luminescent materials due to their versatility for desirable light emission manipulations. This is because the optical activity of the metal halide material depends on the metal halide structural unit and the organic ions or coordinated organic ligands. The different assembly of metal halide units and organic parts can enable versatile light emissions, such as lead halide perovskites (LHPs) and copper halide-organic hybrids. Impressively, the external quantum efficiency of the LHP based light-emitting diodes (LEDs) has improved significantly from 0.1% to over 20% in just five years. With this great progress, the structural lability and toxicity of the LHPs are now the critical issues that need to be addressed for practical applications. These issues are mainly rooted in the intrinsic lead composition and low formation energy crystal structure of the widely adopted LHPs. Thus, the modulation of the structure and composition of the basic metal halide structural units is considered a rational strategy to address these issues.In this Account, we will present a general material design using metal halide structural units as basic building blocks to build up metal halide luminescent materials for solid-state lighting devices. Following this route, we will emphasize the modulation of metal halide structural units to tackle the existing challenges in lead halides, including the instability of crystalline structure, ion migration, and the presence of toxic lead. Considering basic components in structural units, we will highlight ionic engineering in LHPs via ion doping, substitution, and modification to enhance the crystal structural stability and suppress ion migration. To replace toxic lead, we will introduce recent advances in the modulation of lead-free halide structural units by active ion doping and organic ligand coordination to fabricate highly luminescent materials. Finally, we will present future strategies of metal halide structural unit modulation for solid-state light emissions. We hope this Account will provide new insights for designing metal halide materials from the viewpoint of the modulation of the basic building blocks and inspire future studies of advanced metal halide materials for solid-state light emitting applications.

Advances in Perovskite Light-Emitting Diodes Possessing Improved Lifetime

Recently, perovskite light-emitting diodes (PeLEDs) are seeing an increasing academic and industrial interest with a potential for a broad range of technologies including display, lighting, and signaling. The maximum external quantum efficiency of PeLEDs can overtake 20% nowadays, however, the lifetime of PeLEDs is still far from the demand of practical applications. In this review, state-of-the-art concepts to improve the lifetime of PeLEDs are comprehensively summarized from the perspective of the design of perovskite emitting materials, the innovation of device engineering, the manipulation of optical effects, and the introduction of advanced encapsulations. First, the fundamental concepts determining the lifetime of PeLEDs are presented. Then, the strategies to improve the lifetime of both organic-inorganic hybrid and all-inorganic PeLEDs are highlighted. Particularly, the approaches to manage optical effects and encapsulations for the improved lifetime, which are negligibly studied in PeLEDs, are discussed based on the related concepts of organic LEDs and Cd-based quantum-dot LEDs, which is beneficial to insightfully understand the lifetime of PeLEDs. At last, the challenges and opportunities to further enhance the lifetime of PeLEDs are introduced.

Highly emissive halide perovskite nanocrystals: from lead to lead-free

Highly emissive halide perovskite nanocrystals with tunable emission spectra covering the entire visible spectra.

Extremely Stable Luminescent Crosslinked Perovskite Nanoparticles under Harsh Environments over 1.5 Years

Organic-inorganic hybrid perovskite nanoparticles (NPs) are a very strong candidate emitter that can meet the high luminescence efficiency and high color standard of Rec.2020. However, the instability of perovskite NPs is the most critical unsolved problem that limits their practical application. Here, an extremely stable crosslinked perovskite NP (CPN) is reported that maintains high photoluminescence quantum yield for 1.5 years (>600 d) in air and in harsher liquid environments (e.g., in water, acid, or base solutions, and in various polar solvents), and for more than 100 d under 85 °C and 85% relative humidity without additional encapsulation. Unsaturated hydrocarbons in both the acid and base ligands of NPs are chemically crosslinked with a methacrylate-functionalized matrix, which prevents decomposition of the perovskite crystals. Counterintuitively, water vapor permeating through the crosslinked matrix chemically passivates surface defects in the NPs and reduces nonradiative recombination. Green-emitting and white-emitting flexible large-area displays are demonstrated, which are stable for >400 d in air and in water. The high stability of the CPN in water enables biocompatible cell proliferation which is usually impossible when toxic Pb elements are present. The stable materials design strategies provide a breakthrough toward commercialization of perovskite NPs in displays and bio-related applications.

Multifunctional Charge Transporting Materials for Perovskite Light-Emitting Diodes

Despite their low exciton-binding energies, metal halide perovskites are extensively studied as light-emitting materials owing to narrow emission with high color purity, easy/wide color tunability, and high photoluminescence quantum yields. To improve the efficiency of perovskite light-emitting diodes (PeLEDs), much effort has been devoted to controlling the emitting layer morphologies to induce charge confinement and decrease the nonradiative recombination. The interfaces between the emitting layer and charge transporting layer (CTL) are vulnerable to various defects that deteriorate the efficiency and stability of the PeLEDs. Therefore, the establishment of multifunctional CTLs that can improve not only charge transport but also critical factors that influence device performance, such as defect passivation, morphology/phase control, ion migration suppression, and light outcoupling efficiency, are highly required. Herein, the fundamental limitations of perovskites as emitters (i.e., defects, morphological and phase instability, high refractive index with poor outcoupling) and the recent developments with regard to multifunctional CTLs to compensate such limitations are summarized, and their device applications are also reviewed. Finally, based on the importance of multifunctional CTLs, the outlook and research prospects of multifunctional CTLs for the further improvement of PeLEDs are discussed.

Long-Range Exciton Transport in Perovskite-Metal Organic Framework Solid Composites

The encapsulation of perovskite quantum dots (PQDs) in metal organic frameworks (MOFs) is a promising strategy for fabricating stable and functional perovskite solid composites (denoted as PQDs@MOF), which have exhibited great potential for optoelectronics, catalysis, and luminesce applications. However, the exciton diffusion distance, one of the key factors determining the performance of PQDs@MOF in these applications, remains unknown. Herein, by using time-resolved and photoluminescence-scanned imaging microscopy, we report the observation of long-distance exciton transport (278 ± 12.6 nm) and high diffusion coefficient (0.0428 ± 0.0039 cm²/s) in MAPbBr₃ PQDs@MOF microcrystals. We show that the long exciton diffusion length, which is seven times longer than that in colloid MAPbBr₃ PQD solid films, can be attributed to the strong dipole-dipole coupling between adjacent PQDs embedded in the MOF matrix and their long carrier lifetimes. These findings demonstrate the great potential of PQDs@MOF crystals for optoelectronic applications.

Suppressing Auger Recombination in Cesium Lead Bromide Perovskite Nanocrystal Film for Bright Light-Emitting Diodes

All-inorganic cesium lead halide perovskite colloidal nanocrystals are attractive for next-generation light-emitting diodes because of their high color purity, but the nonradiative Auger recombination in perovskite nanocrystal film limits the efficiency and brightness of the fabricated devices. Here, we introduce a surface-engineering process to exchange the original long-chain oleic acid/oleylamine ligands by the cerium-tributylphosphine oxide hybrid ligands to suppress nonradiative Auger recombination in CsPbBr₃ NC film for bright and low-efficiency roll-off light-emitting diodes. Using ultrafast transient absorption and time-resolved photoluminescence spectroscopy, we demonstrate that the hybrid ligand passivation can efficiently remove surface trap states to enhance radiative recombination and homogenize the exciton concentration to suppress nonradiative Auger recombination in the CsPbBr₃ nanocrystal thin film. Consequently, we fabricate a light-emitting diode with efficient charge injection into the CsPbBr₃ nanocrystal emitting layer, achieving a pronounced improvement of electroluminescence with an external quantum efficiency from 5.5% to 9.1%. More importantly, the efficiency roll-off characteristics of high-brightness light-emitting diodes is effectively mitigated. Our reported hybrid ligand passivation suppressed Auger recombination strategy shows a great potential for fabricating high-brightness cesium lead halide perovskite light-emitting diodes.

Revealing Electrical-Poling-Induced Polarization Potential in Hybrid Perovskite Photodetectors

Despite recent rapid advances in metal halide perovskites for use in optoelectronics, the fundamental understanding of the electrical-poling-induced ion migration, accounting for many unusual attributes and thus performance in perovskite-based devices, remain comparatively elusive. Herein, the electrical-poling-promoted polarization potential is reported for rendering hybrid organic-inorganic perovskite photodetectors with high photocurrent and fast response time, displaying a tenfold enhancement in the photocurrent and a twofold decrease in the response time after an external electric field poling. First, a robust meniscus-assisted solution-printing strategy is employed to facilitate the oriented perovskite crystals over a large area. Subsequently, the electrical poling invokes the ion migration within perovskite crystals, thus inducing a polarization potential, as substantiated by the surface potential change assessed by Kelvin probe force microscopy. Such electrical-poling-induced polarization potential is responsible for the markedly enhanced photocurrent and largely shortened response time. This work presents new insights into the electrical-poling-triggered ion migration and, in turn, polarization potential as well as into the implication of the latter for optoelectronic devices with greater performance. As such, the utilization of ion-migration-produced polarization potential may represent an important endeavor toward a wide range of high-performance perovskite-based photodetectors, solar cells, transistors, scintillators, etc.

Highly luminescent biocompatible CsPbBr3@SiO2 core-shell nanoprobes for bioimaging and drug delivery

The encapsulation of lead halide perovskite nanocrystals (PNCs) with an inert protective layer against moisture and the environment is a promising approach to overcome hinderances for their practical use in optoelectronic and biomedical applications. Herein, a facile method for synthesizing highly luminescent and biocompatible CsPbBr₃@SiO₂ core-shell PNCs with a controlled SiO₂ thickness, which are suitable for both cell imaging and drug delivery, is reported. The synthesized CsPbBr₃@SiO₂ core-shell PNCs exhibit bright green emission at 518 nm upon excitation of 374 nm. Interestingly, a significant increase in the photoluminescence intensity is observed with an increase in the SiO₂ shell thickness, which varies with the increasing reaction time. Cytotoxicity results indicate that the CsPbBr₃@SiO₂ core-shell PNCs are nontoxic, making them suitable for in vitro cell imaging using HeLa cells. Furthermore, doxorubicin physically adsorbed on the surface of CsPbBr₃@SiO₂ core-shell PNCs is efficiently released in cells when the drug-loaded perovskite nanoprobes are injected in the cells, indicating that these core-shell nanoparticles can be used for drug loading and delivery. The results of this study suggest that the CsPbBr₃@SiO₂ core-shell PNCs can pave the way for new biomedical applications and processes.

Phase Control and In Situ Passivation of Quasi-2D Metal Halide Perovskites for Spectrally Stable Blue Light-Emitting Diodes

The fabrication of efficient and spectrally stable pure-blue perovskite light-emitting diodes (LEDs) has been elusive and remains of great interest. Herein, we incorporate diammonium salts into quasi-2D perovskite precursors for phase control of multiple quantum well structures to yield tunable and efficient emission in the blue region. With detailed characterizations and computational studies, we show that in situ passivation by the diammonium salts effectively modifies the surface energies of quasi-2D phases and inhibits the growth of low-band gap quasi-2D and 3D phases. Such phase control and in situ passivation could afford blue light-emitting perovskite thin films with high photoluminescence quantum efficiencies of, for instance, 75% for the emission peak at 471 nm. Using this perovskite thin film as an emitting layer, spectrally stable pure-blue LEDs with an emission peak at 474 nm and a full width at half-maximum of 26 nm could be fabricated to exhibit a brightness of 290 cd m^-2 at 8 V and an external quantum efficiency of 2.17%.

Improving Efficiency and Stability in Quasi-2D Perovskite Light-Emitting Diodes by a Multifunctional LiF Interlayer

Owing to the enlarged exciton binding energy and the ability to confine charge carriers compared to their three-dimensional (3D) counterparts, research on quasi-two-dimensional (quasi-2D) perovskite materials and the correlative application in light-emitting diodes (LEDs) has attracted considerable attention. However, high density of defects, exciton emission trapping, and unbalanced charge injection are still the main intractable obstacles to their further development and practical application. Herein, we report an efficient multifunctional interlayer, lithium fluoride (LiF), to boost the performance of green-emitting quasi-2D perovskite LEDs (PeLEDs) by simultaneously overcoming the aforementioned issues. The introduced LiF interlayer not only eliminates the defects at perovskite grain boundaries and the surface by reinforcing the chemical bonds with uncoordinated lead ions but also restrains the emission of perovskite from quenching triggered by the electron transport layer and reduces excess electron injections to effectively balance carriers in the device. As a result, the resulting green quasi-2D PeLED shows a maximum external quantum efficiency of 16.35%, which is the best value obtained for quasi-2D perovskite-based LEDs reported so far, with simultaneous improvement in the operating lifetime of the device.

Enhancing the electroluminescence of perovskite light-emitting diodes by optimizing the morphology of perovskite film to suppress leakage current

Organic-inorganic hybrid perovskites have attracted much attention due to their high color purity, low material cost, ease of tuning of bandgap, and flexible manufacture. They are considered to be a new generation of promising light-emitting materials. In this paper, an appropriate doping of polyethylene oxide (PEO) into the perovskite precursor solution acquires the high quality and low defect density of perovskite film. In addition, the passivation of perovskite grain boundaries by PEO are investigated. The perovskite light-emitting diode (PeLED) is fabricated with a structure of ITO/PEDOT:PSS/MAPbBr₃/TPBi/LiF/Ag. As the doping concentration of PEO is at 10 mg/ml, MAPbBr₃-PEO-based PeLEDs exhibit a maximum luminance of 4665.1cdm^-2, while their external quantum efficiency (EQE) and current efficiency (CE) are also maximized to be 1.355% and 5.556cdA^-1, respectively.

A solution-processed ternary copper halide thin films for air-stable and deep-ultraviolet-sensitive photodetector

Recently, the newly emerging lead-halide perovskites have received tremendous attention in the photodetection field because of their intrinsic large light absorption and high well-balanced carrier transport characteristics. Unfortunately, the issue of instability and the existence of toxic lead cations have greatly restricted their practical applications and future commercialization. Furthermore, the previous studies on perovskite photodetectors mainly operate in visible and near-infrared light region, and there are practically no relevant reports aimed at the deep-ultraviolet (DUV) region. In this study, an air-stable and DUV-sensitive photoconductive detector was demonstrated with a solution-processed ternary copper halides Cs₃Cu₂I₅ thin films as the light absorber. The proposed photodetector is very sensitive to wavelengths of light below 320 nm and unresponsive to the visible light. Because of the high material integrity and large surface coverage of the Cs₃Cu₂I₅ thin films, the detector presents an outstanding photodetection performance with a photoresponsivity of ∼17.8 A W^-1, specific detectivity of 1.12 × 10¹² Jones, and fast response speed of 465/897 μs, superior to previously reported DUV photodetectors based on other material systems. Unlike traditional lead-halide perovskites, the lead-free Cs₃Cu₂I₅ shows remarkable stability against heat, UV light, and environmental oxygen/moisture. Thus, the unsealed photodetector demonstrates good operation stability for 11 h of continuous running in open air. Even after 80-day storage in ambient air, its photodetection capability can nearly be maintained. The results suggest that non-toxic Cs₃Cu₂I₅ could be a potential candidate for stable and environment friendly DUV detectors, enabling an assembly of optoelectronic systems in the future.

Electroplated Silver-Nickel Core-Shell Nanowire Network Electrodes for Highly Efficient Perovskite Nanoparticle Light-Emitting Diodes

The low sheet resistance and high optical transparency of silver nanowires (AgNWs) make them a promising candidate for use as the flexible transparent electrode of light-emitting diodes (LEDs). In a perovskite LED (PeLED), however, the AgNW electrode can react with the overlying perovskite material by redox reactions, which limit the electroluminescence efficiency of the PeLED by causing the degradation of and generating defect states in the perovskite material. In this study, we prepared Ag-Ni core-shell NW electrodes using the solution-electroplating technique to realize highly efficient PeLEDs based on colloidal formamidinium lead bromide (FAPbBr₃) nanoparticles (NPs). Solvated Ni ions from the NiSO₄ source were deposited onto the surface of AgNW networks in three steps: (i) cathodic cleaning, (ii) adsorption of the Ni-ion complex onto the AgNW surface, and (iii) uniform electrodeposition of Ni. An ultrathin (∼3.5 nm) Ni layer was uniformly deposited onto the AgNW surface, which exhibited a sheet resistance of 16.7 Ω/sq and an optical transmittance of 90.2%. The Ag-Ni core-shell NWs not only increased the work function of the AgNW electrode, which facilitated hole injection into the emitting layer, but also suppressed the redox reaction between Ag and FAPbBr₃ NPs, which prevented the degradation of the emitting layer and the generation of defect states in it. The resulting PeLEDs based on FAPbBr₃ NPs with the Ag-Ni core-shell NWs showed high current efficiency of 44.01 cd/A, power efficiency of 35.45 lm/W, and external quantum efficiency of 9.67%.

Water Passivation of Perovskite Nanocrystals Enables Air-Stable Intrinsically Stretchable Color-Conversion Layers for Stretchable Displays

Conventional organic light-emitting devices without an encapsulation layer are susceptible to degradation when exposed to air, so realization of air-stable intrinsically-stretchable display is a great challenge because the protection of the devices against penetration of moisture and oxygen is even more difficult under stretching. An air-stable intrinsically-stretchable display that is composed of an intrinsically-stretchable electroluminescent device (SELD) integrated with a stretchable color-conversion layer (SCCL) that contains perovskite nanocrystals (PeNCs) is proposed. PeNCs normally decay when exposed to air, but they become resistant to this decay when dispersed in a stretchable elastomer matrix; this change is a result of a compatibility between capping ligands and the elastomer matrix. Counterintuitively, the moisture can efficiently passivate surface defects of PeNCs, to yield significant increases in both photoluminescence intensity and lifetime. A display that can be stretched up to 180% is demonstrated; it is composed of an air-stable SCCL that down-converts the SELD's blue emission and reemits it as green. The work elucidates the basis of moisture-assisted surface passivation of PeNCs and provides a promising strategy to improve the quantum efficiency of PeNCs with the aid of moisture, which allows PeNCs to be applied for air-stable stretchable displays.

Recent Progress in Engineering Metal Halide Perovskites for Efficient Visible-Light-Driven Photocatalysis

Artificial photosynthesis has attracted increasing attention due to recent environmental and energy concerns. Metal halide perovskites (MHPs) demonstrating excellent optoelectronic properties have currently emerged as novel and efficient photocatalytic materials. Herein, the structural features of MHPs that are responsible for the photoinduced charge separation and charge migration properties are briefly introduced, and then important and necessary photophysical and photochemical aspects of MHPs related to photoredox catalysis are summarized. Subsequently, the applications of MHPs for solar energy harvesting and photocatalytic conversion, including H₂ evolution, CO₂ reduction, degradation of organic pollutants, and photoredox organic synthesis, are extensively demonstrated, with a focus on strategies for improving the performance (e.g., selectivity, activity, stability, recyclability, and environmental compatibility) of these MHP-based photocatalytic systems. To conclude, existing challenges and prospects on the future development of MHP-based materials towards photoredox catalysis applications are detailed.

Competitive Nucleation Mechanism for CsPbBr3 Perovskite Nanoplatelet Growth

We analyze nucleation-controlled nanocrystal growth in a solution containing surface-binding molecular ligands, which can also nucleate compact layers on the crystal surfaces. We show that, if the critical nucleus size for ligands is larger and the nucleation barrier is lower than those for crystal atoms, the ligands nucleate faster than the atoms on relatively wide crystal facets but much slower, if at all, on narrow facets. Such competitive nucleation of ligands and atoms results in ligands covering predominantly wider facets, thus excluding them from the growth process, and acts as a selection mechanism for the growth of crystals with narrower facets, the so-called nanoplatelets. The theory is confirmed by Monte Carlo simulations and validated experimentally for CsPbBr₃ nanoplatelets grown from solution. We find that the anisotropic crystal growth is controlled by the growth temperature and the strength of surface bonding for the passivating molecular ligands.

Functionalized PFN-X (X = Cl, Br, or I) for Balanced Charge Carriers of Highly Efficient Blue Light-Emitting Diodes

All-inorganic perovskite nanocrystals (PeNCs), CsPbX₃ (X = Cl, Br, or I), have been considered as one of the prospective emissive materials for display applications, which showed superior photoluminescence quantum yield and high color purity with narrow spectral line width. Recently, high-performance green and red perovskite light-emitting diodes (PeLEDs) were introduced; however, the efficiency of blue PeLEDs still lagged owing to PeNCs' deep HOMO energy level (∼6.0 eV), which is in discord with the adjacent organic interlayer. In this work, we demonstrated an interfacial engineering strategy with conjugated polyelectrolytes, functionalized PFN (poly[(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]) with halide anions, between the hole injection layer and PeNCs. By introducing PFN-X (X = Cl, Br, or I), they exhibit well-balanced charge carriers and resultant effective radiative recombination in the PeNC layer with reduced hole injection barrier and electron blocking behavior. Among them, in particular, the PFN-Cl-treated PeLEDs display a maximum external quantum efficiency of 1.34% at 470 nm electroluminescence emission with enhanced spectral operating stability.

Lead-Free Perovskite Variant Solid Solutions Cs2 Sn1- x Tex Cl6 : Bright Luminescence and High Anti-Water Stability

Underwater lighting is important for the exploration of the underwater world in different areas. It is of great significance for developing underwater emitters with high penetrability, high luminous efficiency, good anti-water stability, and environmental friendliness. Stable lead-free perovskite luminescent materials, represented by vacancy-ordered double perovskites, are worthy of research because they can almost meet the above requirements. Here, lead-free perovskite variant solid solutions with the formula of Cs₂ Sn_{1- x} Te_x Cl₆ are reported. Upon the exchange of Sn/Te ions, strong Jahn-Teller distortion of octahedra occurs in the lattice structure. The combination of Te luminescent center and Jahn-Teller-like self-trapped excitons gives this material yellow-green luminescence with a wavelength of 580 nm and a high photoluminescence quantum yield of 95.4%. Moreover, these solid solutions can withstand the extreme conditions of immersion in water probably due to the formation of amorphous alteration phase. Such good anti-water stability is also supported by the molecule dynamics simulation result that no reaction occurs on the water/Cs₂ SnCl₆ interface. The high luminous, suitable wavelength, and good anti-water stability enable the solid solutions suitable for the application for underwater lighting.

Strategies to Achieve High Circularly Polarized Luminescence from Colloidal Organic-Inorganic Hybrid Perovskite Nanocrystals

Colloidal metal halide perovskite nanocrystals (NCs) with chiral ligands are outstanding candidates as a circularly polarized luminescence (CPL) light source due to many advantages such as high photoluminescence quantum efficiency, large spin-orbit coupling, and extensive tunability via composition and choice of organic ligands. However, achieving pronounced and controllable polarized light emission remains challenging. Here, we develop strategies to achieve high CPL responses from colloidal formamidinium lead bromide (FAPbBr3) NCs at room temperature using chiral surface ligands. First, we show that replacing a portion of typical ligands (oleylamine) with short chiral ligands ((R)-2-octylamine) during FAPbBr3 NC synthesis results in small and monodisperse NCs that yield high CPL with average luminescence dissymmetry g-factor, glum = 6.8 × 10-2. To the best of our knowledge, this is the highest among reported perovskite materials at room temperature to date and represents around 10-fold improvement over the previously reported colloidal CsPbClxBryI3-x-y NCs. In order to incorporate NCs into any optoelectronic or spintronic application, the NCs necessitate purification, which removes a substantial amount of the chiral ligands and extinguishes the CPL signals. To circumvent this issue, we also developed a postsynthetic ligand treatment using a different chiral ligand, (R-/S-)methylbenzylammonium bromide, which also induces a CPL with an average glum = ±1.18 × 10-2. This postsynthetic method is also amenable for long-range charge transport since methylbenzylammonium is quite compact in relation to other surface ligands. Our demonstrations of high CPL and glum from both as-synthesized and purified perovskite NCs at room temperature suggest a route to demonstrate colloidal NC-based spintronics.

High-Performance Perovskite Light-Emitting Diodes with Surface Passivation of CsPbBrxI3-x Nanocrystals via Antisolvent-Triggered Ion-Exchange

Inorganic lead halide perovskite nanocrystals (PeNCs) have intensively drawn attention as efficient light-emitting materials for optoelectronic applications due to their fine optoelectronic properties with a high photoluminescence quantum yield and easily tunable saturated emission color. However, the poor stability of the red-emitting PeNCs has become an obstacle because of the uncontrollable iodine substitution from the PeNCs due to weak Pb-I bonding. In this work, we have demonstrated a ligand-mediated post-treatment (LMPT) method using a halide ion-pair ligand, tridodecylmethyl ammonium iodide (TrDAI), for the air stable and high-quality red-emitting PeNCs. Through the LMPT method, the optoelectronic properties of red-emitting PeNCs are dramatically improved resulting in a PLQY of 88.7% at 637 ± 2 nm emission with an increased carrier lifetime from 20.77 to 31.52 ns. We achieve highly efficient red perovskite light-emitting diodes exhibiting a maximum current efficiency of 7.69 cd A^-1 and an external quantum efficiency of 6.36% at 637 ± 2 nm electroluminescence emission with a sharp full-width at half maximum of 31 nm.

Micro- and Nanopatterning of Halide Perovskites Where Crystal Engineering for Emerging Photoelectronics Meets Integrated Device Array Technology

Tremendous efforts have been devoted to developing thin film halide perovskites (HPs) for use in high-performance photoelectronic devices, including solar cells, displays, and photodetectors. Furthermore, structured HPs with periodic micro- or nanopatterns have recently attracted significant interest due to their potential to not only improve the efficiency of an individual device via the controlled arrangement of HP crystals into a confined geometry, but also to technologically pixelate the device into arrays suitable for future commercialization. However, micro- or nanopatterning of HPs is not usually compatible with conventional photolithography, which is detrimental to ionic HPs and requires special techniques. Herein, a comprehensive overview of the state-of-the-art technologies used to develop micro- and nanometer-scale HP patterns, with an emphasis on their controlled microstructures based on top-down and bottom-up approaches, and their potential for future applications, is provided. Top-down approaches include modified conventional lithographic techniques and soft-lithographic methods, while bottom-up approaches include template-assisted patterning of HPs based on lithographically defined prepatterns and self-assembly. HP patterning is shown here to not only improve device performance, but also to reveal the unprecedented functionality of HPs, leading to new research areas that utilize their novel photophysical properties.

Emergence of Impurity-Doped Nanocrystal Light-Emitting Diodes

In recent years, impurity-doped nanocrystal light-emitting diodes (LEDs) have aroused both academic and industrial interest since they are highly promising to satisfy the increasing demand of display, lighting, and signaling technologies. Compared with undoped counterparts, impurity-doped nanocrystal LEDs have been demonstrated to possess many extraordinary characteristics including enhanced efficiency, increased luminance, reduced voltage, and prolonged stability. In this review, recent state-of-the-art concepts to achieve high-performance impurity-doped nanocrystal LEDs are summarized. Firstly, the fundamental concepts of impurity-doped nanocrystal LEDs are presented. Then, the strategies to enhance the performance of impurity-doped nanocrystal LEDs via both material design and device engineering are introduced. In particular, the emergence of three types of impurity-doped nanocrystal LEDs is comprehensively highlighted, namely impurity-doped colloidal quantum dot LEDs, impurity-doped perovskite LEDs, and impurity-doped colloidal quantum well LEDs. At last, the challenges and the opportunities to further improve the performance of impurity-doped nanocrystal LEDs are described.

Two-dimensional perovskites with alternating cations in the interlayer space for stable light-emitting diodes

Lead halide perovskites have attracted tremendous attention in photovoltaics due to their impressive optoelectronic properties. However, the poor stability of perovskite-based devices remains a bottleneck for further commercial development. Two-dimensional perovskites have great potential in optoelectronic devices, as they are much more stable than their three-dimensional counterparts and rapidly catching up in performance. Herein, we demonstrate high-quality two-dimensional novel perovskite thin films with alternating cations in the interlayer space. This innovative perovskite provides highly stable semiconductor thin films for efficient near-infrared light-emitting diodes (LEDs). Highly efficient LEDs with tunable emission wavelengths from 680 to 770 nm along with excellent operational stability are demonstrated by varying the thickness of the interlayer spacer cation. Furthermore, the best-performing device exhibits an external quantum efficiency of 3.4% at a high current density (J) of 249 mA/cm² and remains above 2.5% for a J up to 720 mA cm^-2, leading to a high radiance of 77.5 W/Sr m² when driven at 6 V. The same device also shows impressive operational stability, retaining almost 80% of its initial performance after operating at 20 mA/cm² for 350 min. This work provides fundamental evidence that this novel alternating interlayer cation 2D perovskite can be a promising and stable photonic emitter.

Phase Selection of Cesium Lead Triiodides through Surface Ligand Engineering

The cesium lead triiodide (CsPbI₃) perovskite is a promising candidate for stable light absorbers and red-light-emitting sources due to its outstanding stability. Phase engineering is the most important approach for the commercialization of CsPbI₃ because the optically inactive nonperovskite structure is more stable than three-dimensional (3-D) perovskite lattices at ambient temperature. This study presents an in-depth evaluation to find the optimum surface ligand and to reveal the mechanism of phase stabilization by surface ligands. Thermodynamic evaluations combined with density functional theory calculations indicate the criteria for forming stable 3-D CsPbI₃ perovskites under surface and volume free energy competition between perovskite and nonperovskite phases. Comparative calculations for ammonium, alcohol, and thiol groups show that ammonium groups enhance the phase stability of 3-D perovskites the most. In addition, ammonium-passivated CsPbI₃ is relatively robust against defect formation and H₂O adsorption.

Colloidal Synthesis of Ternary Copper Halide Nanocrystals for High-Efficiency Deep-Blue Light-Emitting Diodes with a Half-Lifetime above 100 h

Currently, the blue perovskite light-emitting diodes (PeLEDs) suffer from a compromise in lead toxicity and poor operation stability, and most previous studies have struggled to meet the crucial blue NTSC standard. In this study, electrically driven deep-blue LEDs (∼445 nm) based on zero-dimensional (0D) Cs₃Cu₂I₅ nanocrystals (NCs) were demonstrated with the color coordinates of (0.16, 0.07) and a high external quantum efficiency of ∼1.12%, comparable with the best-performing blue LEDs based on lead-halide perovskites. Encouraged by the remarkable stability of Cs₃Cu₂I₅ NCs against heat and environmental oxygen/moisture, the proposed device was operated in a continuous current mode for 170 h, producing a record half-lifetime of ∼108 h. The device stability was further verified by an aggressive thermal cycling test (300-360-300 K) and a 35-day storage test. Together with the eco-friendly features and facile colloidal synthesis technique, the 0D Cs₃Cu₂I₅ NCs can be therefore regarded as a promising candidate for deep-blue LEDs applications.

Reduction of Optical Gain Threshold in CsPbI3 Nanocrystals Achieved by Generation of Asymmetric Hot-Biexcitons

Lead halide perovskite nanocrystals (NCs) are a class of promising light-emitting materials and have been considered as gain media in lasers. Strong exciton-exciton interactions in NCs cause an energy shift of the lowest optical transition and affect the optical gain threshold. Here, we clarify the dynamics of exciton-exciton interactions in highly photoexcited CsPbI₃ NCs by double-pump transient absorption spectroscopy. This method provides control over the population of each excited state by varying the time interval between the two pump pulses. We find that the band-edge energy shift induced by the formation of asymmetric hot-biexcitons (comprising one ground-state exciton and one hot exciton) is smaller than that induced by hot excitons and hot biexcitons in the ensemble. We demonstrate that the generation of asymmetric hot-biexcitons reduces the optical gain threshold in the CsPbI₃ NC ensemble.

In Situ Inkjet Printing of the Perovskite Single-Crystal Array-Embedded Polydimethylsiloxane Film for Wearable Light-Emitting Devices

Metal halide perovskites are promising light-emitting materials for applications such as wearable lighting devices and flat panel displays because of their high photoluminescence efficiency, high color purity, and facile solution processability. However, the intrinsic ambient instability and crystal friability issues have fundamentally hindered the practical applications of perovskites. Here, we solve this problem through a liquid to liquid self-encapsulation inkjet-printing technique. Perovskite inks are directly inkjet-printed into the liquid polydimethylsiloxane (PDMS) precursor to in situ form the self-encapsulation perovskite single-crystal-embedded PDMS structure. We show that the space-confined effect of the liquid PDMS precursor can significantly retard the perovskite crystallization process and promote the embedded growth of perovskite single crystals in PDMS. Benefiting from the sealing function of PDMS, the printed perovskite single crystals show excellent ambient stability and flexibility. Furthermore, we demonstrate that wafer-scale air-stable and flexible perovskite fluorescent patterns can be produced in PDMS by direct inkjet printing, which is cost-effective and free of complex microfabrication processes. This method provides a facile approach to scalable fabrication of air-stable and wearable perovskite fluorescent patterns, which will be of great significance for potential application in perovskite light-emitting diode display.

Stable Yellow Light-Emitting Devices Based on Ternary Copper Halides with Broadband Emissive Self-Trapped Excitons

Great successes have been achieved in developing perovskite light-emitting devices (LEDs) with blue, green, red, and near-infrared emissions. However, as key optoelectronic devices, yellow-colored perovskite LEDs remain challenging, mainly due to the inevitable halide separation in mixed halide perovskites under high bias, causing undesired color change of devices. In addition to this color-missing problem, the intrinsic toxicity and poor stability of conventional lead-halide perovskites also restrict their practical applications. We herein report the fabrication of stable yellow LEDs based on a ternary copper halide CsCu₂I₃, addressing the color instability and toxicity issues facing current perovskite yellow LED's compromise. Joint experiment-theory characterizations indicate that the yellow electroluminescence originates from the broadband emission of self-trapped excitons centered at 550 nm as well as the comparable and reasonably low carrier effective masses favorable for charge transport. With a maximum luminance of 47.5 cd/m² and an external quantum efficiency of 0.17%, the fabricated yellow LEDs exhibit a long half-lifetime of 5.2 h at 25 °C and still function properly at 60 °C with a half-lifetime of 2.2 h, which benefits from the superior resistance of CsCu₂I₃ to heat, moisture, and oxidation in ambient environmental conditions. The results obtained promise the copper halides with broadband light emission as an environment-friendly and stable yellow emitter for the LEDs compatible with practical applications.

Frenkel-Holstein Hamiltonian applied to absorption spectra of quaterthiophene-based 2D hybrid organic-inorganic perovskites

For the prototypical two-dimensional hybrid organic-inorganic perovskites (2D HOIPs) (AE4T)PbX₄ (X = Cl, Br, and I), we demonstrate that the Frenkel-Holstein Hamiltonian (FHH) can be applied to describe the absorption spectrum arising from the organic component. We first model the spectra using only the four nearest neighbor couplings between translationally inequivalent molecules in the organic herringbone lattice as fitting parameters in the FHH. We next use linear-response time-dependent density functional theory (LR-TDDFT) to calculate molecular transition densities, from which extended excitonic couplings are evaluated based on the atomic positions within the 2D HOIPs. We find that both approaches reproduce the experimentally observed spectra, including changes in their shape and peak positions. The spectral changes are correlated with a decrease in excitonic coupling from X = Cl to X = I. Importantly, the LR-TDDFT-based approach with extended excitonic couplings not only gives better agreement with the experimental absorption line shape than the approach using a restricted set of fitted parameters but also allows us to relate the changes in excitonic coupling to the underlying geometry. We accordingly find that the decrease in excitonic coupling from X = Cl to Br to I is due to an increase in molecular separation, which in turn can be related to the increasing Pb-X bond length from Cl to I. Our research opens up a potential pathway to predicting optoelectronic properties of new 2D HOIPs from ab initio calculations and to gain insight into structural relations from 2D HOIP absorption spectra.

Critical Role of Organic Spacers for Bright 2D Layered Perovskites Light-Emitting Diodes

Light-emitting diodes (LEDs) made with quasi-2D/3D and layered perovskites have undergone an unprecedented surge as their external quantum efficiency (EQE) is rapidly approaching other lighting technologies. Manipulating the charge recombination pathway in semiconductors is highly desirable for improving the device performance. This study reports high-performance layered perovskites LEDs with benzyl ring as spacer where radiative recombination lifetime is longer, compared with much shorter alkyl chain spacer yields. Based on detailed optical and X-ray absorption spectroscopy measurements, direct signature of charges localization is observed near the band edge in exchange with the shallow traps in benzyl organics containing layered perovskites. As a result, it boosts the photoluminescence intensity by 7.4 times compared to that made with the alkyl organics. As a demonstration, a bright LED made with the benzyl organics with current efficiency of 23.46 ± 1.52 cd A-1 is shown when the device emits at a high brightness of 6.6 ± 0.93 × 104 cd m-2. The average EQE is 9.2% ± 1.43%, two orders of magnitude higher than the device made with alkyl organics. The study suggests that the choices of organic spacers provide a path toward the manipulation of charge recombination, essential for efficient optoelectronic device fabrications.

High-Brightness Perovskite Light-Emitting Diodes Using a Printable Silver Microflake Contact

Achieving efficient devices while maintaining a high fabrication yield is a key challenge in the fabrication of solution-processed, perovskite-based light-emitting diodes (PeLEDs). In this respect, pinholes in the solution-processed perovskite layers are a major obstacle. These are usually mitigated using organic electron-conducting planarization layers. However, these organic interlayers are unstable under applied bias in air and suffer from limited charge carrier mobility. In this work, we present a high brightness p-i-n PeLED based on a novel blade-coated silver microflake (SMF) rear electrode, which allows for a low-cost nanocrystalline ZnO inorganic electron-transporting layer to be used. This novel SMF contact is crucial for achieving high performance as it prevents the electrical shorting suffered when standard thermally evaporated silver rear contacts are used. The fabricated PeLEDs exhibit an excellent maximum luminance of 98,000 cd/m², a maximum current efficiency of 22.3 cd/A, and a high external quantum efficiency of 4.6% under 5.9 V forward bias. The SMF rear contact can be printed and scaled at low cost to large areas and applied to flexible devices.

Structural and spectroscopic studies of a nanostructured silicon-perovskite interface

While extensively investigated in thin film form for energy materials applications, this work investigates the formation of APbBr3 structures (A = CH3NH3+ (MA), Cs+) in silicon and oxidized silicon nanotubes (SiNTs) with varying inner diameter. We carefully control the extent of oxidation of the nanotube host and correlate the relative Si/Si oxide content in a given nanotube host with the photoluminescence quantum efficiency (PLQE) of the perovskite. Complementing these measurements is an evaluation of average PL lifetimes of a given APbBr3 nanostructure, as evaluated by time-resolved confocal photoluminescence measurements. Increasing Si (decreasing oxide) content in the nanotube host results in a sensitive reduction of MAPbBr3 PLQE, with a concomitant decrease in average lifetime (τave). We interpret these observations in terms of decreased defect passivation by a lower concentration of oxide species surrounding the perovskite. In addition, we show that the use of selected nanotube templates leads to more stable perovskite PL in air over time (weeks). Taken in concert, such fundamental observations have implications for interfacial carrier interactions in tandem Si/perovskite photovoltaics.