Surface ligand modification of cesium lead bromide nanocrystals for improved light-emitting performance

Biexciton Emission in CsPbBr3 Nanocrystals: Polar Facet Matters

The metal halide perovskite nanocrystals exhibit a remarkable tolerance to midgap defect states, resulting in high photoluminescence quantum yields. However, the potential of these nanocrystals for applications in display devices is hindered by the suppression of biexcitonic emission due to various Auger recombination processes. By adopting single-particle photoluminescence spectroscopy, herein, we establish that the biexcitonic quantum efficiency increases with the increase in the number of facets on cesium lead bromide perovskite nanocrystals, progressing from cube to rhombic dodecahedron to rhombicuboctahedron nanostructures. The observed enhancement is attributed mainly to an increase in their surface polarity as the number of facets increases, which reduces the Coulomb interaction of charge carriers, thereby suppressing Auger recombination. Moreover, Auger recombination rate constants obtained from the time-gated photon correlation studies exhibited a discernible decrease as the number of facets increased. These findings underscore the significance of facet engineering in fine-tuning biexciton emission in metal halide perovskite nanocrystals.

Anisotropic electronic coupling in three-dimensional assembly of CsPbBr3 quantum dots

Cesium lead halide (CsPbX3, X = Cl, Br, or I) perovskite quantum dots (PeQDs) show promise for next-generation optoelectronics. In this study, we controlled the electronic coupling between PeQD multilayers using a layer-by-layer method and dithiol linkers of varying structures. The energy shift of the first excitonic peak from monolayer to bilayer decreases exponentially with increasing interlayer spacer distance, indicating the resonant tunnelling effect. X-ray diffraction measurements revealed anisotropic inter-PeQD distances in multiple layers. Photoluminescence (PL) analysis showed lower energy emission in the in-plane direction due to the electronic coupling in the out-of-plane direction, supporting the anisotropic electronic state in the PeQD multilayers. Temperature-dependent PL and PL lifetimes indicated changes in exciton behaviour due to the delocalized electronic state in PeQD multilayers. Particularly, the electron-phonon coupling strength increased, and the exciton recombination rate decreased. This is the first study demonstrating controlled electronic coupling in a three-dimensional ordered structure, emphasizing the importance of the anisotropic electronic state for high-performance PeQDs devices.

Synthesis-Kinetics of Violet- and Blue-Emitting Perovskite Nanocrystals with High Brightness and Superior Stability toward Flexible Conversion Layer

The low photoluminescence (PL) efficiency and unstable features of small blue-emitting CsPbX3 nanocrystals (NCs) greatly limit their applications in optoelectronics field. Herein, the synergistic and post-treatment kinetics are studied to create highly bright and anomalous stable violet (peak position of ≈408 nm) and blue (peak position of ∼ 466 nm) emitting perovskite NCs. Ligand and ion exchange mechanism are systematic studied by the evolution of absorption, PL, and fluorescence lifetime to evaluate ligand bonding, defect engineering, and non-radiative recombination. Didodecyl dimethyl mmonium chloride (DDAC) and CuX2 post-synergistic treatment created DDAC-CsPbCl3-CuCl2 and DDAC-CsPbCl3-CuBr2 NCs that remained the phase composition, morphology, and size of CsPbCl3 NCs. The PL efficiencies are drastically increased to 42 and 85% for violet- and blue-emitting NCs, respectively. The stability test indicated that the NCs enable against various harsh conditions (e.g., ultraviolet light irradiation and heat-treatment). The NCs retained their initial PL efficiency after 2 months under ambient conditions and UV light irradiation. These NCs also exhibited high stability after heat-treatment at 120 °C. The emitting NCs embedded in flexible films still revealed bright PL and high stability, suggesting current results provide a new avenue for the application in the field of optoelectronics.

Ligand-Enhanced Neodymium Doping of Perovskite Quantum Dots for Superior Exciton Confinement

In this study, all-inorganic perovskite quantum dots (QDs) for pure blue emission are explored for full-color displays. We prepared CsPbBr3 and Cs3NdCl6 QDs via hot injection methods and mixed in various ratios at room temperature for color blending. Nd-doped CsPb(Cl/Br)3 QDs showed a blueshift in emission, and the photoluminescence quantum yields (PLQY, ΦPL) were lower in the 460-470 nm range due to surface halogen and Cs vacancies. To address this, we introduced a silane molecule, APTMS, via a ligand exchange process, effectively repairing these vacancies and enhancing Nd doping into the lattice. This modification promotes the PLQY to 94% at 466 nm. Furthermore, combining these QDs with [1]Benzothieno[3,2-b][1]benzothiophene (BTBT), a conjugated small-molecule semiconductor, in a composite film reduced PLQY loss caused by FRET in solid-state QD films. This approach achieved a wide color gamut of 124% National Television System Committee (NTSC), using a UV LED backlight and RGB perovskite QDs in a BTBT-based organic matrix as the color conversion layer. Significantly, the photostability of this composite was enhanced when used as a color conversion layer (CCL) under blue-LED excitation.

Anti-Stokes Photoluminescence in Halide Perovskite Nanocrystals: From Understanding the Mechanism towards Application in Fully Solid-State Optical Cooling

Anti-Stokes photoluminescence (ASPL) is an up-conversion phonon-assisted process of radiative recombination of photoexcited charge carriers when the ASPL photon energy is above the excitation one. This process can be very efficient in nanocrystals (NCs) of metalorganic and inorganic semiconductors with perovskite (Pe) crystal structure. In this review, we present an analysis of the basic mechanisms of ASPL and discuss its efficiency depending on the size distribution and surface passivation of Pe-NCs as well as the optical excitation energy and temperature. When the ASPL process is sufficiently efficient, it can result in an escape of most of the optical excitation together with the phonon energy from the Pe-NCs. It can be used in optical fully solid-state cooling or optical refrigeration.

CsPbBr3 perovskites: A dual fluorescence sensor to distinguish ethanol from methanol

In recent years, lead halide perovskites have emerged as a promising material with defect tolerance, thermally stable, and optoelectronic properties. However, the instability is the major factor which hinder their potential applications in various fields. This work demonstrates the chemical stability of Cesium Lead Bromide (CsPbBr₃) under different passivation condition with an objective to develop alcohol sensor. Cetyltrimethyl ammonium bromide (CTAB) passivated CsPbBr₃ demonstrated as a turn off fluorescent probe for alcohols and more significantly turn on fluorescent probe for ethanol. Herein, it is shown that CTAB passivated CsPbBr₃ can effectively discriminate ethanol from methanol owing to its different mode of interaction with ethanol and methanol. The outstanding optical properties of halide perovskites with an ultra-low detection limit of 7.3 ppb was obtained for ethanol detection. The sensing performance of the material is also validated with petrol and cough syrup samples showing excellent performance for future implementation with practical applications.

Near-Infrared LEDs Based on Quantum Cutting-Activated Electroluminescence of Ytterbium Ions

Cesium lead halide perovskite nanocrystals (PNCs) exhibit promising prospects for application in optoelectronic devices. However, electroactivated near-infrared (NIR) PNC light-emitting diodes (LEDs) with emission peaks over 800 nm have not been achieved. Herein, we demonstrate the electroactivated NIR PNC LEDs based on Yb³⁺-doped CsPb(Cl_1-xBr_x)₃ PNCs with extraordinary high NIR photoluminescence quantum yields over 170%. The fabricated NIR LEDs possess an irradiance of 584.7 μW cm^-2, an EQE of 1.2%, and a turn-on voltage of 3.1 V. The ultrafast quantum cutting process from the PNC host to Yb³⁺ has been revealed as the main mechanism of electroluminescence (EL)-activated Yb³⁺ for the first time via exploring how the trend between the EL intensity of PNC and Yb³⁺ varies with different voltages along with the tendency of temperature- and doping-concentration-dependent PL and EL spectra. This work will extend the application of PNCs to optical communication, night-vision devices, and biomedical imaging.

Cesium Lead Iodide Perovskites: Optically Active Crystal Phase Stability to Surface Engineering

Among perovskites, the research on cesium lead iodides (CsPbI₃) has attracted a large research community, owing to their all-inorganic nature and promising solar cell performance. Typically, the CsPbI₃ solar cell devices are prepared at various heterojunctions, and working at fluctuating temperatures raises questions on the material stability-related performance of such devices. The fundamental studies reveal that their poor stability is due to a lower side deviation from Goldschmidt's tolerance factor, causing weak chemical interactions within the crystal lattice. In the case of organic-inorganic hybrid perovskites, where their stability is related to the inherent chemical nature of the organic cations, which cannot be manipulated to improve the stability drastically whereas the stability of CsPbI₃ is related to surface and lattice engineering. Thus, the challenges posed by CsPbI₃ could be overcome by engineering the surface and inside the CsPbI₃ crystal lattice. A few solutions have been proposed, including controlled crystal sizes, surface modifications, and lattice engineering. Various research groups have been working on these aspects and had accumulated a rich understanding of these materials. In this review, at first, we survey the fundamental aspects of CsPbI₃ polymorphs structure, highlighting the superiority of CsPbI₃ over other halide systems, stability, the factors (temperature, polarity, and size influence) leading to their phase transformations, and electronic band structure along with the important property of the defect tolerance nature. Fortunately, the factors stabilizing the most effective phases are achieved through a size reduction and the efficient surface passivation on the delicate CsPbI₃ nanocrystal surfaces. In the following section, we have provided the up-to-date surface passivating methods to suppress the non-radiative process for near-unity photoluminescence quantum yield, while maintaining their optically active phases, especially through molecular links (ligands, polymers, zwitterions, polymers) and inorganic halides. We have also provided recent advances to the efficient synthetic protocols for optically active CsPbI₃ NC phases to use readily for solar cell applications. The nanocrystal purification techniques are challenging and had a significant effect on the device performances. In part, we summarized the CsPbI₃-related solar cell device performances with respect to the device fabrication methods. At the end, we provide a brief outlook on the view of surface and lattice engineering in CsPbI₃ NCs for advancing the enhanced stability which is crucial for superior optical and light applications.

Gateway towards recent developments in quantum dot-based light-emitting diodes

Quantum dots (QDs), with their excellent photoluminescence, narrow emission linewidth, and wide color coverage, provide unrivaled advantages for advanced display technologies, enabling full-color micro-LED displays. It is indeed critical to have a fundamental understanding of how QD properties affect micro-LED display performance in order to develop the most energy-efficient display device in the near future. However, to take a more detailed look at the stability issues and passivation ways of QDs is essential for accelerating the commercialization of QD-based LED technologies. Knowing about the most recent breakthroughs in QD-based LEDs can give a good indication of how they might be used in shaping the future of displays. In this review, we discuss the characteristics of QD-based LEDs for the applications of display and lighting technologies. Various approaches for synthesis and the stability improvement of QDs are addressed in detail, along with recent advancements towards QD-based LED breakthroughs. Moreover, we summarize our latest research findings in QD-based LEDs, providing valuable information about the potential of QD-based LEDs for future display technologies.

Mixed-Halide Double Perovskite Cs2 AgBiX6 (X=Br, I) with Tunable Optical Properties via Anion Exchange

Lead-free double perovskites, A2 M+ M'3+ X6 , are considered as promising alternatives to lead-halide perovskites, in optoelectronics applications. Although iodide (I) and bromide (Br) mixing is a versatile tool for bandgap tuning in lead perovskites, similar mixed I/Br double perovskite films have not been reported in double perovskites, which may be due to the large activation energy for ion migration. In this work, mixed Br/I double perovskites were realized utilizing an anion exchange method starting from Cs2 AgBiBr6 solid thin-films with large grain-size. The optical and structural properties were studied experimentally and theoretically. Importantly, the halide exchange mechanism was investigated. Hydroiodic acid was the key factor to facilitate the halide exchange reaction, through a dissolution-recrystallization process. In addition, the common organic iodide salts could successfully perform halide-exchange while retaining high mixed-halide phase stability and strong light absorption capability.

Polysalt ligands achieve higher quantum yield and improved colloidal stability for CsPbBr3 quantum dots

Colloidal lead halide perovskite quantum dots (PQDs) are relatively new semiconductor nanocrystals with great potential for use in optoelectronic applications. They also present a set of new scientifically challenging fundamental problems to investigate and understand. One of them is to address the rather poor colloidal and structural stability of these materials under solution phase processing and/or transfer between solvents. In this contribution, we detail the synthesis of a new family of multi-coordinating, bromide-based polysalt ligands and test their ability to stabilize CsPbBr₃ nanocrystals in polar solutions. The ligands present multiple salt groups involving quaternary cations, namely ammonium and imidazolium as anchors for coordination onto PQD surfaces, along with several alkyl chains with varying chain length to promote solubilization in various conditions. The ligands provide a few key benefits including the ability to repair damaged surface sites, allow rapid ligand exchange and phase transfer, and preserve the crystalline structure and morphology of the nanocrystals. The polysalt-coated PQDs exhibit near unity PLQY and significantly enhanced colloidal stability in ethanol and methanol.

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.

Recent Advances in Ligand Design and Engineering in Lead Halide Perovskite Nanocrystals

Lead halide perovskite (LHP) nanocrystals (NCs) have recently garnered enhanced development efforts from research disciplines owing to their superior optical and optoelectronic properties. These materials, however, are unlike conventional quantum dots, because they possess strong ionic character, labile ligand coverage, and overall stability issues. As a result, the system as a whole is highly dynamic and can be affected by slight changes of particle surface environment. Specifically, the surface ligand shell of LHP NCs has proven to play imperative roles throughout the lifetime of a LHP NC. Recent advances in engineering and understanding the roles of surface ligand shells from initial synthesis, through postsynthetic processing and device integration, finally to application performances of colloidal LHP NCs are covered here.

Balanced charge transport and enhanced performance of blue quantum dot light-emitting diodes via electron transport layer doping

The unbalanced charge transport is always a key influencing factor on the device performance of quantum dot light-emitting diodes (QLEDs), particularly for the blue QLEDs due to their large optical band gap. Here, a method of electron transport layer (ETL) doping was developed to regulate the energy levels and the carrier mobility of the ETL, which resulted in more balanced charge injection, transport and recombination in the blue emitting CdZnS/ZnS core/shell QLEDs. Consequently, an enhanced performance of blue QLEDs was achieved by modulating the charge balance through ETL doping. The maximum external quantum efficiency and luminance was dramatically increased from 2.2% to 7.3% and from 3786 cd m^-2to 9108 cd m^-2, respectively. The results illustrate that charge transport layer doping is a simple and effective strategy to regulate the charge injection barrier and carrier mobility of QLEDs.

Polymeric Hole Transport Materials for Red CsPbI3 Perovskite Quantum-Dot Light-Emitting Diodes

In this study, the performances of red CsPbI₃-based all-inorganic perovskite quantum-dot light-emitting diodes (IPQLEDs) employing polymeric crystalline Poly(3-hexylthiophene-2,5-diyl) (P3HT), poly(9-vinycarbazole) (PVK), Poly(N,N'-bis-4-butylphenyl-N,N'-bisphenyl)benzidine (Poly-TPD) and 9,9-Bis[4-[(4-ethenylphenyl)methoxy]phenyl]-N2,N7-di-1-naphthalenyl-N2,N7-diphenyl-9H-fluorene-2,7-diamine (VB-FNPD) as the hole transporting layers (HTLs) have been demonstrated. The purpose of this work is an attempt to promote the development of device structures and hole transporting materials for the CsPbI₃-based IPQLEDs via a comparative study of different HTLs. A full-coverage quantum dot (QD) film without the aggregation can be obtained by coating it with VB-FNPD, and thus, the best external quantum efficiency (EQE) of 7.28% was achieved in the VB-FNPD device. We also reported a standing method to further improve the degree of VB-FNPD polymerization, resulting in the improved device performance, with the EQE of 8.64%.

Lead-Free Halide Perovskites for Light Emission: Recent Advances and Perspectives

Lead-based halide perovskites have received great attention in light-emitting applications due to their excellent properties, including high photoluminescence quantum yield (PLQY), tunable emission wavelength, and facile solution preparation. In spite of excellent characteristics, the presence of toxic element lead directly obstructs their further commercial development. Hence, exploiting lead-free halide perovskite materials with superior properties is urgent and necessary. In this review, the deep-seated reasons that benefit light emission for halide perovskites, which help to develop lead-free halide perovskites with excellent performance, are first emphasized. Recent advances in lead-free halide perovskite materials (single crystals, thin films, and nanocrystals with different dimensionalities) from synthesis, crystal structures, optical and optoelectronic properties to applications are then systematically summarized. In particular, phosphor-converted LEDs and electroluminescent LEDs using lead-free halide perovskites are fully examined. Ultimately, based on current development of lead-free halide perovskites, the future directions of lead-free halide perovskites in terms of materials and light-emitting devices are discussed.

A donor-acceptor ligand boosting the performance of FA0.8Cs0.2PbBr3 nanocrystal light-emitting diodes

A donor-acceptor ligand, 3-amino-2-bromo-6-methoxypyridine (ABMeoPy), was introduced to passivate FA0.8Cs0.2PbBr3 nanocrystals (NCs) by a post-processing method. The donor-acceptor interaction can greatly enhance the coordination bond of pyridine-Pb2+, and improve FA0.8Cs0.2PbBr3 NC performance with 95.99% photoluminescence quantum yield (PLQY), 6-month stability in solution, and 26% trap density decrease. In the light of ABMeoPy passivation of FA0.8Cs0.2PbBr3 NCs, the maximum luminance, the maximum current efficiency, and EQE of light-emitting diodes (LEDs) increased 69%, 110%, and 111%, respectively. The strategy of using D-A molecules to passivate perovskite NCs is quite simple and effective, which can be widely promoted in perovskite-based LEDs or solar cells.

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 efficient ligand-modified manganese ion doped CsPbCl3 perovskite quantum dots for photon energy conversion in silicon solar cells

Manganese ion doped CsPbX₃ perovskite quantum dots (QDs) demonstrate high absorption of ultraviolet (UV) light and efficient orange emission with a large Stokes shift, and are almost transparent to visible light, which are ideal photon energy converters for solar cells. In this work, Mn²⁺ ion doped CsPbCl₃ QDs were synthesized by incorporating a long-chain ammonium ligand dodecyl dimethylammonium chloride (DDAC), in which the DDAC ligand not only played the role of replacing the surface ligands of QDs, but also enhanced the efficiency and stability of Mn²⁺ ion doped QDs. The as-prepared QD sample displayed a photoluminescence quantum yield (PLQY) as high as 91% and served as a photon energy converter for silicon solar cells (SSCs). The photoelectric conversion efficiency (PCE) of SSCs increased from 19.64% to 20.65% with a relative enhancement of 5.14%. This work displays a method to tune the efficiency of QDs by modifying the surface ligands and an efficient photon energy converter for SSCs, which is of great importance for practical applications.

Self-defect-passivation by Br-enrichment in FA-doped Cs1-xFAxPbBr3 quantum dots: towards high-performance quantum dot light-emitting diodes

Halide vacancy defect is one of the major origins of non-radiative recombination in the lead halide perovskite light emitting devices (LEDs). Hence the defect passivation is highly demanded for the high-performance perovskite LEDs. Here, we demonstrated that FA doping led to the enrichment of Br in Cs_1−xFA_xPbBr₃ QDs. Due to the defect passivation by the enriched Br, the trap density in Cs_1−xFA_xPbBr₃ significantly decreased after FA doping, and which improved the optical properties of Cs_1−xFA_xPbBr₃ QDs and their QD-LEDs. PLQY of Cs_1–xFA_xPbBr₃ QDs increased from 76.8% (x = 0) to 85.1% (x = 0.04), and L_max and CE_max of Cs_1–xFA_xPbBr₃ QD-LEDs were improved from L_max = 2880 cd m⁻² and CE_max = 1.98 cd A⁻¹ (x = 0) to L_max = 5200 cd m⁻² and CE_max = 3.87 cd A⁻¹ (x = 0.04). Cs_1–xFA_xPbBr₃ QD-LED device structure was optimized by using PVK as a HTL and ZnO modified with b-PEI as an ETL. The energy band diagram of Cs_1–xFA_xPbBr₃ QD-LEDs deduced by UPS analyses.

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.

Improving the Quality and Luminescence Performance of All-Inorganic Perovskite Nanomaterials for Light-Emitting Devices by Surface Engineering

Lead halide perovskites and their applications in the optoelectronic field have garnered intensive interest over the years. Inorganic perovskites (IHP), though a novel class of material, are considered as one of the most promising optoelectronic materials. These materials are widely used in detectors, solar cells, and other devices, owing to their excellent charge-transport properties, high defect tolerance, composition- and size-dependent luminescence, narrow emission, and high photoluminescence quantum yield. In recent years, numerous encouraging achievements have been realized, especially in the research of CsPbX₃ (X = Cl, Br, I) nanocrystals (NCs) and surface engineering. Therefore, it is necessary to summarize the principles and effects of these surface engineering optimization methods. It is also important to scientifically guide the applications and promote the development of perovskites more efficiently. Herein, the principles of surface ligands are reviewed, and various surface treatment methods used in CsPbX₃ NCs as well as quantum-dot light-emitting diodes are presented. Finally, a brief outlook on CsPbX₃ NC surface engineering is offered, illustrating the present challenges and the direction in which future investigations are intended to obtain high-quality CsPbX₃ NCs that can be utilized in more applications.

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.

High-efficiency perovskite nanocrystal light-emitting diodes via decorating NiOx on the nanocrystal surface

Nickel oxides exhibit a great potential as hole transport layers for the fabrication of efficient perovskite light-emitting diodes (LEDs) due to their high carrier mobility and good energy band matching with perovskite nanocrystals. In this work, nickel oxides were directly decorated on the CsPbBr3 nanocrystal surface through adsorption and a sequential oxidation treatment. The resulting sample shows a high photoluminescence quantum-yield of 82%. The LED using CsPbBr3 nanocrystals with nickel oxides achieves a high external quantum efficiency (EQE) of up to 16.8% with a low turn-on voltage of 2.8 V, which is much superior to that of the counterpart LED based on pristine CsPbBr3 nanocrystals (EQE = 0.7%, turn-on voltage = 5.6 V). The excellent performance of the nickel oxide decorated CsPbBr3 nanocrystal device could be attributed to the better energy level matching between the decorated nanocrystals and the transport layers of the device and more balanced charge carrier injection. Furthermore, the operational lifetime of the nickel oxide decorated CsPbBr3 nanocrystal device is 40 times longer than that of the pristine CsPbBr3 nanocrystal device.

Novel Lewis Base Cyclam Self-Passivation of Perovskites without an Anti-Solvent Process for Efficient Light-Emitting Diodes

Metal halide perovskites have been focused as a candidate applied as a promising luminescent material for next-generation high-quality lighting and high-definition display. However, as perovskite films formed, high density of defects would be produced in solution processing inevitably, leading to low exciton recombination efficiency in light-emitting diodes (LEDs). Herein, a facile and novel self-passivation strategy to inhibit defect formation in perovskite films for constructing high-performance LEDs is developed. For the first time, we introduce 1,4,8,11-tetraazacyclotetradecane (cyclam) in perovskite precursor solution, and it spontaneously passivates defect states of CsPbBr₃-based perovskites by coaction between amine and uncoordinated lead ions during spin-coating without an anti-solvent process. Furthermore, as a delocalized system, cyclam also possesses chemical properties that facilitate exciton transportation. The proposed passivation strategy boosts the external quantum efficiency from 1.25% (control device) to 16.24% (cyclam-passivated device). Furthermore, defect passivation is also conductive to reduce LED degradation paths and improve device stability as the extrapolated lifetime (T₅₀) of LEDs at an initial brightness of 100 cd/m² is increased from 0.9 to 127 h. These findings indicate that the introduction of cyclam is highly effective to enhance the performance of LEDs, and such a strategy in effectively reducing the defects could be also applied in other perovskite-based devices, such as lasers, solar cells, and photodetectors.

Highly stable Na: CsPb(Br,I)3@Al2O3 nanocomposites prepared by a pre-protection strategy

Among the leading energy materials, metal tri-halide perovskite quantum dots (PQDs) with outstanding optoelectronic properties are at the forefront of current research. However, enormous challenges remain to be addressed, including hazardous components and poor stability, before achieving practical applications of PQDs. Although there are diverse methods to improve the stability of PQDs, it is of central importance to avoid damage during operation. Herein, we develop a pre-protected strategy in which the coating combines the advantages of doping with sodium ions to jointly improve stability. Because the stable Na-rich surface acts as a defence, it protects the PQDs from damage during the coating process; therefore, they retain their initial fluorescence. When employing these Na-rich PQDs as core materials of a coating, the highly fluorescent Na: CsPb(Br,I)3@Al2O3 nanocomposites can maintain good stability even when directly immersed in water or exposed to illumination. Clearly, the combination of these features sheds light on the stabilization and applications of PQDs.

Charge transfer between lead halide perovskite nanocrystals and single-walled carbon nanotubes

A charge transfer study between lead halide-based perovskite nanocrystals and single-walled carbon nanotubes (PNC@CNT nanocomposite) was performed. Solution-processed MAPbX₃ PNCs displayed very bright luminescence, but it quenched in the presence of CNTs. This was attributed to the electron transfer from PNCs to CNTs. The detailed changes in fluorescence lifetime were investigated through time-correlated single-photon counting (TCSPC), which suggested mixed static and dynamic quenching along with a decrease in the lifetime. Morphological changes were investigated via transmission electron microscopy (TEM) and attributed to the incorporation of PNCs on long CNTs. Also, the PNC@CNT nanocomposite was explored for photoinduced current response, which indicated an ∼3 fold increase in photoconductivity under light illumination (with a 1 mV bias). This electron transfer study between PNCs and CNTs contributes to the exploration of charge dynamics.

White light-emitting devices based on ZnCdS/ZnS and perovskite nanocrystal heterojunction

Perovskite white light-emitting devices (WLEDs) without intercalation layers have not been achieved due to the ion exchange. Although the intercalation layers prevent ion exchange between perovskite nanocrystals (NCs), it also creates a new problem of charge imbalance and the structure becomes more complex. In this study, blue emitting ZnCdS/ZnS NCs with high quantum yield and stability are introduced to work with the yellow emission from CsPb(Br/I)₃ perovskite NCs for WLEDs. The WLEDs are constituted of ITO/ZnO/PEI/ZnCdS/ZnS NCs/CsPb(Br/I)₃ NCs/TCTA/MoO₃/Au. This design avoids ion exchange between different perovskites NCs, and realizes white light emission by simple fabrication. As a result, we achieved the white light coordinates of (0.34, 0.34) and a correlated color temperature of 5153 K.

Ruddlesden-Popper Perovskites: Synthesis and Optical Properties for Optoelectronic Applications

Ruddlesden-Popper perovskites with a formula of (A')2(A) n -1B n X3 n +1 have recently gained widespread interest as candidates for the next generation of optoelectronic devices. The variations of organic cation, metal halide, and the number of layers in the structure lead to the change of crystal structures and properties for different optoelectronic applications. Herein, the different synthetic methods for 2D perovskite crystals and thin films are summarized and compared. The optoelectronic properties and the charge transfer process in the devices are also delved, in particular, for light-emitting diodes and solar cells.

Inorganic and Layered Perovskites for Optoelectronic Devices

Organic-inorganic halide perovskites are making breakthroughs in a range of optoelectronic devices. Reports of >23% certified power conversion efficiency in photovoltaic devices, external quantum efficiency >21% in light-emitting diodes (LEDs), continuous-wave lasing and ultralow lasing thresholds in optically pumped lasers, and detectivity in photodetectors on a par with commercial GaAs rivals are being witnessed, making them the fastest ever emerging material technology. Still, questions on their toxicity and long-term stability raise concerns toward their market entry. The intrinsic instability in these materials arises due to the organic cation, typically the volatile methylamine (MA), which contributes to hysteresis in the current-voltage characteristics and ion migration. Alternative inorganic substitutes to MA, such as cesium, and large organic cations that lead to a layered structure, enhance structural as well as device operational stability. These perovskites also provide a high exciton binding energy that is a prerequisite to enhance radiative emission yield in LEDs. The incorporation of inorganic and layered perovskites, in the form of polycrystalline films or as single-crystalline nanostructure morphologies, is now leading to the demonstration of stable devices with excellent performance parameters. Herein, key developments made in various optoelectronic devices using these perovskites are summarized and an outlook toward stable yet efficient devices is presented.

Microcarrier-Assisted Inorganic Shelling of Lead Halide Perovskite Nanocrystals

The conventional strategy of synthetic colloidal chemistry for bright and stable quantum dots has been the production of epitaxially matched core/shell heterostructures to mitigate the presence of deep trap states. This mindset has been shown to be incompatible with lead halide perovskite nanocrystals (LHP NCs) due to their dynamic surface and low melting point. Nevertheless, enhancements to their chemical stability are still in great demand for the deployment of LHP NCs in light-emitting devices. Rather than contend with their attributes, we propose a method in which we can utilize their dynamic, ionic lattice and uniquely defect-tolerant band structure to prepare non-epitaxial salt-shelled heterostructures that are able to stabilize these materials against their environment, while maintaining their excellent optical properties and increasing scattering to improve out-coupling efficiency. To do so, anchored LHP NCs are first synthesized through the heterogeneous nucleation of LHPs onto the surface of microcrystalline carriers, such as alkali halides. This first step stabilizes the LHP NCs against further merging, and this allows them to be coated with an additional inorganic shell through the surface-mediated reaction of amphiphilic Na and Br precursors in apolar media. These inorganically shelled NC@carrier composites offer significantly improved chemical stability toward polar organic solvents, such as γ-butyrolactone, acetonitrile, N-methylpyrrolidone, and trimethylamine, demonstrate high thermal stability with photoluminescence intensity reversibly dropping by no more than 40% at temperatures up to 120 °C, and improve compatibility with various UV-curable resins. This mindset for LHP NCs creates opportunities for their successful integration into next-generation light-emitting devices.

Hybrid light emitting diodes based on stable, high brightness all-inorganic CsPbI3 perovskite nanocrystals and InGaN

Despite important advances in the synthesis of inorganic perovskite nanocrystals (NCs), the long-term instability and degradation of their quantum yield (QY) over time need to be addressed to enable the further development and exploitation of these nanomaterials. Here we report stable CsPbI3 perovskite NCs and their use in hybrid light emitting diodes (LEDs), which combine in one system the NCs and a blue GaN-based LED. Nanocrystals with improved morphological and optical properties are obtained by optimizing the post-synthesis replacement of oleic acid ligands with iminodibenzoic acid: the NCs have a long shelf-life (>2 months), stability under different environmental conditions, and a high QY, of up to 90%, in the visible spectral range. Ligand replacement enables the engineering of the morphological and optical properties of the NCs. Furthermore, the NCs can be used to coat the surface of a GaN-LED to realize a stable diode where they are excited by blue light from the LED under low current injection conditions, resulting in emissions at distinct wavelengths in the visible range. The high QY and fluorescence lifetime in the nanosecond range are key parameters for visible light communication, an emerging technology that requires high-performance visible light sources for secure, fast energy-efficient wireless transmission.

Device Engineering for All-Inorganic Perovskite Light-Emitting Diodes

Recently, all-inorganic perovskite light-emitting diodes (PeLEDs) have attracted both academic and industrial interest thanks to their outstanding properties, such as high efficiency, bright luminance, excellent color purity, low cost and potentially good operational stability. Apart from the design and treatment of all-inorganic emitters, the device engineering is another significant factor to guarantee the high performance. In this review, we have summarized the state-of-the-art concepts for device engineering in all-inorganic PeLEDs, where the charge injection, transport, balance and leakage play a critical role in the performance. First, we have described the fundamental concepts of all-inorganic PeLEDs. Then, we have introduced the enhancement of device engineering in all-inorganic PeLEDs. Particularly, we have comprehensively highlighted the emergence of all-inorganic PeLEDs, strategies to improve the hole injection, approaches to enhance the electron injection, schemes to increase the charge balance and methods to decrease the charge leakage. Finally, we have clarified the issues and ways to further enhance the performance of all-inorganic PeLEDs.

Enhancing the efficiency of CsPbX3 (X = Cl, Br, I) nanocrystals via simultaneous surface peeling and surface passivation

Inorganic CsPbX3 (X = Cl, Br, I) perovskite nanocrystals (PNCs) are promising materials for next-generation optoelectronic applications due to their tunable emission and high color purity. However, there is still room to improve their photoluminescence quantum yields (PLQYs) in order to promote their applications. Herein, the PLQY of blue light emitting CsPb(Cl/Br)3 PNCs was increased to 83% with ammonium hexafluorophosphate by choosing an appropriate treatment time. The salt peeled off the outermost surface of PNCs with halide vacancies and then passivated the surface. This method is effective at improving the PLQYs of different CsPbX3 (X = Cl, Br, I) PNCs covering the entire visible spectrum; the PLQYs were improved to 25% for CsPbCl3 at 398 nm, 83% for CsPb(Cl/Br)3 at 448 nm, 96% for CsPbBr3 at 504 nm, 86% for CsPb(Br/I)3 at 568 nm, and 98% for CsPbI3 at 687 nm.

A MAPbBr3:poly(ethylene oxide) composite perovskite quantum dot emission layer: enhanced film stability, coverage and device performance

Metal halide perovskite quantum dots (PQDs) have become the most promising optoelectronic material in new generation displays and lighting devices due to their excellent optical properties. However, their stability and surface defects have affected their large-scale applications. We demonstrate enhanced stability of high-surface-area colloidal PQDs encapsulated with long-chain organic polymer poly(ethylene oxide) (PEO) via simply mixing the PQDs with this high molecular-weight polymer. The introduction of the polymer deactivates the surface defects of PQDs and sufficiently protects the organic ligands on the surfaces of PQDs with obviously improved ambient stability and photoluminescence quantum yield. Moreover, the use of PEO efficiently confines electrons within the PQD emission layer and generates extremely stable green electroluminescence spectra in MAPbBr3 light-emitting diodes (LEDs) over a wide voltage operation range of 5-12 V, accompanied by significant improvements in external quantum efficiencies with enhancement factors of 18.3 times. Our work provides a robust platform for the fabrication of stable PQDs, high-quality PQD films and efficient PQD-based LEDs.

Versatile Defect Passivation Methods for Metal Halide Perovskite Materials and their Application to Light-Emitting Devices

Metal halide perovskites (MHPs) have emerged as promising emitters because of their excellent optoelectronic properties, including high photoluminescence quantum yields (PLQYs), wide-range color tunability, and high color purity. However, a fundamental limitation of MHPs is their low exciton binding energy, which results in a low radiative recombination rate and the dependence of PLQY on the excitation intensity. Under the operating conditions of light-emitting diodes (LEDs), the injected current densities are typically lower than the trap density, leading to a low actual PLQY. Moreover, the defects not only initiate the decomposition of MHPs caused by extrinsic factors, but also intrinsically stimulate ion migration across the interface and lead to the corrosion of electrodes due to interaction between those electrodes, even under inert conditions. The passivation of defects has proven to be effective for mitigating the effects of defects in MHPs. Herein, the origins and theoretical calculations of the defect tolerance in MHPs and the impact of defects on both the performance and stability of perovskite LEDs are reviewed. The passivation methods and materials for MHP bulk films and nanocrystals are discussed in detail. Based on the currently reported advances, specific requirements and future research directions for display applications are suggested.

Strong Blue Emission from Sb3+-Doped Super Small CsPbBr3 Nanocrystals

Colloidal lead halide perovskite nanocrystals (NCs) have high tunability in the visible light region and high photoluminescence quantum yields (PL QYs) for green and red emissions, but bright blue emission is still a challenge. Super small CsPbBr₃ perovskite NCs emit blue light around 460 nm with a narrow peak width, and they do not have the problem of phase separation like their Cl-Br counterparts. However, the blue emission from super small CsPbBr₃ NCs easily becomes green over time, and their PL QY is still low. The doping of Sb³⁺ ions successfully reduced the surface energy, improved the lattice energy, passivated the defect states below the band gap, eventually boosted the PL QY of blue emission to 73.8%, and resulted in better spectral stability even at elevated temperatures in solution (40-100 °C). Its CIE coordinates were (0.14, 0.06), which are close to the primary blue color (0.155, 0.070) according to the NTSC TV color standard.

Surface Ligand Engineering for Efficient Perovskite Nanocrystal-Based Light-Emitting Diodes

Lead halide perovskites (LHPs) are emerging as promising materials for light-emitting device applications because of the tunability of the band gap, narrow emission, solution processability, and flexibility. Typically, LHP nanocrystals (NCs) with surface ligands show high photoluminescence quantum yields because of charge-carrier confinement with higher exciton binding energy ( E_b). However, the conventionally used oleylamine (OAm) ligands result in the low electrical conductivity and stability of perovskite NCs (PNCs) because of a long carbon chain without conjugation bonds and weak interaction with the surface of NCs. Here, we report the effect of bulkiness and chain length of ligand materials on the properties and stability of CsPbBr₃ PNCs by replacing OAm with other suitable ligands. The effect of the bulkiness of quaternary ammonium bromide (QAB) ligands was systemically studied. The less bulky QAB ligands surrounded the surface of NCs effectively, and brought better surface passivation and less aggregation compared to bulky QAB ligands, and finally the optical property and stability of CsPbBr₃ PNCs were enhanced. Furthermore, the electrical property of CsPbBr₃ PNCs was optimized by tuning the long-chain length of QAB ligands for balanced charge-carrier transport. Finally, we achieved highly efficient green emissive CsPbBr₃ PNC light-emitting diodes (LEDs) by using PNCs with optimized didecyldimethyl ammonium bromide ligands with a current efficiency of 31.7 cd A^-1 and external quantum efficiency of 9.7%, which were enhanced 16-fold compared to those of CsPbBr₃ LEDs using PNCs with conventional OAm ligands.

Observation and implication of halide exchange beyond CsPbX3 perovskite nanocrystals

Anion exchange between pre-synthesized all-inorganic nanocrystals with a perovskite structure is a promising approach to tune their chemical composition and optical properties. Herein we have reported the first study of internanocrystal anion exchange reactionsin the cesium lead halide family, including CsPbX3, Cs4PbX6 and CsX, and we found that the anion exchange dynamics is highly dependent on their crystalline phase. In stark contrast to the fast rate in CsPbX3, cesium based non-perovskite NCs display much slower halide mobility. The reaction time is increased to several hours in Cs4PbX6 and days in CsX, respectively. Furthermore, we confirm that mixing these NCs with the same halide but different structures will induce halide diffusion from Cs4PbX6 NCs and CsX NCs to CsPbX3 NCs. This feature can be explored to utilize the Cs4PbX6 NCs and CsX NCs as a halide source to improve the photoluminescence efficiency and colloidal stability of CsPbX3 NCs.

Critical role of metal ions in surface engineering toward brightly luminescent and stable cesium lead bromide perovskite quantum dots

The highly dynamic binding ligands on the surface of all-inorganic cesium lead halide perovskite quantum dots (PQDs), which can be easily lost or detached leading to a deterioration in the optical properties and stability, are one of the greatest challenges for the practical storage and application of PQDs. Herein, we report a facile metal ion-assisted ligand surface engineering strategy to synchronously boost the photoluminescence quantum yield and stability of CsPbBr3 PQDs by a sequential short-chain ligand (didodecyl dimethylammonium sulfide, DDA+-S2-) exchange and subsequent metal salt (In(Ac)3) treatment. From detailed characterization of the critical role of the metal ions, these enhancements were found to originate from the promoted ligand capping induced by the metal ions attached on the surface of the PQDs. Considering the shortened ligands and robust surface passivation, the modified CsPbBr3 PQDs exhibit drastically enhanced performance in an electroluminescent device. Our results have provided an insightful understanding of surface ligand engineering for high-quality and stable perovskite QDs and their effective optoelectronic applications.

Rationalizing and Controlling the Surface Structure and Electronic Passivation of Cesium Lead Halide Nanocrystals

Colloidal lead halide perovskite nanocrystals (NCs) have recently emerged as versatile photonic sources. Their processing and luminescent properties are challenged by the lability of their surfaces, i.e., the interface of the NC core and the ligand shell. On the example of CsPbBr₃ NCs, we model the nanocrystal surface structure and its effect on the emergence of trap states using density functional theory. We rationalize the typical observation of a degraded luminescence upon aging or the luminescence recovery upon postsynthesis surface treatments. The conclusions are corroborated by the elemental analysis. We then propose a strategy for healing the surface trap states and for improving the colloidal stability by the combined treatment with didodecyldimethylammonium bromide and lead bromide and validate this approach experimentally. This simple procedure results in robust colloids, which are highly pure and exhibit high photoluminescence quantum yields of up to 95-98%, retained even after three to four rounds of washing.

An overview on enhancing the stability of lead halide perovskite quantum dots and their applications in phosphor-converted LEDs

Beyond the unprecedented success achieved in photovoltaics (PVs), lead halide perovskites (LHPs) have shown great potential in other optoelectronic devices. Among them, nanometer-scale perovskite quantum dots (PQDs) with fascinating optical properties including high brightness, tunable emission wavelength, high color purity, and high defect tolerance have been regarded as promising alternative down-conversion materials in phosphor-converted light-emitting diodes (pc-LEDs) for lighting and next-generation of display technology. Despite the promising applications of perovskite materials in various fields, they have received strong criticism for the lack of stability. The poor stability has also attracted much attention. Within a few years, numerous strategies towards enhancing the stability have been developed. This review summarizes the mechanisms of intrinsic- and extrinsic-environment-induced decomposition of PQDs. Simultaneously, the strategies for improving the stability of PQDs are reviewed in detail, which can be classified into four types: (1) compositional engineering; (2) surface engineering; (3) matrix encapsulation; (4) device encapsulation. Finally, the challenges for applying PQDs in pc-LEDs are highlighted, and some possible solutions to improve the stability of PQDs together with suggestions for further improving the performance of pc-LEDs as well as the device lifetime are provided.

Simultaneous Strontium Doping and Chlorine Surface Passivation Improve Luminescence Intensity and Stability of CsPbI3 Nanocrystals Enabling Efficient Light-Emitting Devices

A method is proposed to improve the photo/electroluminescence efficiency and stability of CsPbI₃ perovskite nanocrystals (NCs) by using SrCl₂ as a co-precursor. The SrCl₂ is chosen as the dopant to synthesize the CsPbI₃ NCs. Because the ion radius of Sr²⁺ (1.18 Å) is slightly smaller than that of Pb²⁺ (1.19 Å) ions, divalent Sr²⁺ cations can partly replace the Pb²⁺ ions in the lattice structure of perovskite NCs and cause a slight lattice contraction. At the same time, Cl^- anions from SrCl₂ are able to efficiently passivate surface defect states of CsPbI₃ nanocrystals, thus converting nonradiative trap states to radiative states. The simultaneous Sr²⁺ ion doping and surface Cl^- ion passivation result in the enhanced photoluminescence quantum yield (up to 84%), elongated emission lifetime, and improved stability. Sr²⁺ -doped CsPbI₃ NCs are employed to produce light-emitting devices with a high external quantum yield of 13.5%.

Yb3+ and Yb3+/Er3+ Doping for Near-Infrared Emission and Improved Stability of CsPbCl3 Nanocrystals

Lead halide perovskite nanocrystals (NCs) exhibit excellent tunable emissions covering the entire visible spectral region, but they do not emit near-infrared (NIR) light. We synthesized rare earth element doped CsPbCl3 NCs for NIR emission. The Yb3+ doped CsPbCl3 NCs emitted strong 986 nm NIR light; the Yb3+/Er3+ co-doped CsPbCl3 NCs emitted at 1533 nm. The total photoluminescence quantum yield (PL QY) of the CsPbCl3 NCs changed from 5.0% to 127.8% upon incorporating 2.0% Yb3+, a factor of 25.6 times enhancement. The material's stability was tested under continuous ultraviolet (365 nm) irradiation. The doped CsPbCl3 NCs exhibited a better stability than the undoped one. The PL intensity of the undoped CsPbCl3 NCs dropped to 20% of the initial value in 27 h, while the doped one took 85 h.