Lecithin Capping Ligands Enable Ultrastable Perovskite-Phase CsPbI3 Quantum Dots for Rec. 2020 Bright-Red Light-Emitting Diodes

An Enclosed Encapsulation Strategy toward Stable Perovskite Quantum Dots: Structure, Lead Immobilization, and Backlight Display Applications

Lead halide perovskite quantum dots have drawn great attention in the wide-color gamut display area. However, their poor stability restricts their practical application as well as the leakage of a toxic lead element. In this study, an enclosed encapsulation strategy is adopted to form CsPbBr3 quantum dots embedded in the channels of molecular sieve SBA-15 with SiO2 outer layer as enclosing protection (denoted as CsPbBr3@SBA-15@SiO2), which is achieved through a facile solid-state sintering method with post-treatment. The stability of perovskite quantum dots is enhanced remarkably, of which the emission intently after immersion in water for two months can remain over 95% of the initial value. Efficient lead leakage is achieved simultaneously. The improvement mechanisms were elucidated via the micro- and spectral viewpoints. A wide color gamut of 114% NTSC is demonstrated based on the obtained green emitter. This study could provide new insights into designing stable and environmentally sustainable perovskite quantum dots and promote their applications.

Efficient Rec. 2020 Compliant Pure-Green Mixed-Cation Perovskite Light-Emitting Diodes With Multifunctional Co-Additives

Perovskite light-emitting diodes (PeLEDs) compliant with Rec. 2020 standards have raised increasing attention for next-generation displays. As a class of pure-green emitters, the mixed-cation FAxCs1-xPbBr3 perovskites exhibit compatible band emission, but suffer from inferior luminescence performance. The approach to tackling this issue is hindered by a lack of in-depth understanding of their crystallization manipulating mechanism. This work unveils the crystallization process of mixed-cation FA0.7Cs0.45GA0.1PbBr3 perovskites, demonstrating the fast spontaneous growth readily induces severe crystal defects accompanied by poor charge confinement. This motivates us to introduce additional kinetic barriers to manipulate the perovskite crystallization via the synergistic co-additives of 3-((2-(methacryloyloxy)ethyldimethyl)ammonio)-propane-1-sulfonate (DMAPS) and 1,4,7,10,13,16-hexaoxacyclooctadecane (crown). The multifunctional groups in the co-additives afford robust chemical affinities with the diverse organic and inorganic precursor ions simultaneously, which enable decent nanograin growth with effective crystal defect healing and charge confinement. Ultimately, mixed-cation FA0.7Cs0.45GA0.1PbBr3 perovskites with a high photoluminescence quantum yield of 96% are achieved. The resultant pure-green PeLEDs with the Rec. 2020 compliance exhibit a champion external quantum efficiency (EQE) of 31.89%, average EQE of 29.5%, maximum luminance of 2 × 105 cd m-2 and operational half-lifetime of 3.2 h at an initial luminance of 7000 cd m-2 (extrapolated: ≈3500 h at 100 cd m-2).

TOP-Zn Steric Hindrance Effect Enables Ultra-Uniform CsPbX3 Quantum Dots for Wide-Color Gamut Displays

Perovskite quantum dots (PQDs) are expected to be an ideal candidate for wide-color gamut displays owing to their high color purity. However, their color purity is challenged by remarkable spectral broadening due to non-uniform size distribution and crystal defects. Here, a ligand-ion (TOP-Zn) complex-modulating nucleation strategy is proposed to depress spectral broadening. This is achieved by enhancing the steric hindrance effect during lead-halogen octahedral assembly and reducing the reaction activity/sites of the system. This strategy is universal and has been confirmed to be effective for blue, green, and red PQDs, achieving narrowed spectral full-width-at-half-maximum (FWHM) of 15, 17, and 25 nm, respectively. These FWHMs are record-breaking and contribute to a wide color gamut coverage of ≈130% National Television Standards Committee and ≈100% Rec. 2020 standard. Meanwhile, these PQD-based light-emitting diodes (PeLEDs) exhibit a high external quantum efficiency (EQE) of exceeding 20% at their pure color range. These results provide a feasible path to achieve ultra-uniform and pure-color luminescent PQDs for wide-color gamut displays.

Nanocrystalline Perovskites for Bright and Efficient Light-Emitting Diodes

Nanocrystalline perovskites have driven significant progress in metal halide perovskite light-emitting diodes (PeLEDs) over the past decade by enabling the spatial confinement of excitons. Consequently, three primary categories of nanocrystalline perovskites have emerged: nanoscale polycrystalline perovskites, quasi-2D perovskites, and perovskite nanocrystals. Each type has been developed to address specific challenges and enhance the efficiency and stability of PeLEDs. This review explores the representative material design strategies for these nanocrystalline perovskites, correlating them with exciton recombination dynamics and optical/electrical properties. Additionally, it summarizes the trends in progress over the past decade, outlining four distinct phases of nanocrystalline perovskite development. Lastly, this review addresses the remaining challenges and proposes a potential material design to further advance PeLED technology toward commercialization.

Pure red emission with spectral stability in full iodine-based quasi-2D perovskite films by controlling phase distribution

Quasi-2D perovskites have emerged as a promising candidate material for displays owing to their high photoluminescence quantum yields and low-cost solution synthesis. However, achieving pure red quasi-2D perovskite films with luminescence centered at 630 nm and a narrow emission band presents a critical challenge for high-definition displays. Herein, by incorporating 18-crown-6 as additives that simultaneously passivate defects and regulate phase distribution, full iodine-based quasi-2D perovskite films with a single red emission peak and spectral stability are designed. Additionally, through the introduction of an appropriate amount of chlorobenzene and enhancement of annealing temperature, resulting in a narrower phase distribution, the full width at half maximum (FWHM) of the emission peak is significantly reduced. After optimization of the process, we fabricated quasi-2D perovskite films with pure red emission, which exhibited a PL peak at 627.9 nm and a narrow FWHM of 45.1 nm. Based on these pure red perovskite films, diverse complex patterns such as fluorescent anti-counterfeiting labels are implemented for data storage and information encryption. This study provides an effective approach toward developing quasi-2D perovskites with high color purity for high-definition purposes.

Adaptive In-Sensor Computing for Enhanced Feature Perception and Broadband Image Restoration

Traditional imaging systems struggle in weak or complex lighting environments due to their fixed spectral responses, resulting in spectral mismatches and degraded image quality. To address these challenges, a bioinspired adaptive broadband image sensor is developed. This innovative sensor leverages a meticulously designed type-I heterojunction alignment of 0D perovskite quantum dots (PQDs) and 2D black phosphorus (BP). This configuration enables efficient carrier injection control and advanced computing capabilities within an integrated phototransistor array. The sensor's unique responses to both visible and infrared (IR) light facilitate selective enhancement and precise feature extraction under varying lighting conditions. Furthermore, it supports real-time convolution and image restoration within a convolutional autoencoder (CAE) network, effectively countering image degradation by capturing spectral features. Remarkably, the hardware responsivity weights perform comparably to software-trained weights, achieving an image restoration accuracy of over 85%. This approach offers a robust and versatile solution for machine vision applications that demand precise and adaptive imaging in dynamic lighting environments.

Patterning technologies of quantum dots for color-conversion micro-LED display applications

Quantum dot (QD) materials and their patterning technologies play a pivotal role in the full colorization of next-generation Micro-LED display technology. This article reviews the latest development in QD materials, including II-VI group, III-V group, and perovskite QDs, along with the state of the art in optimizing QD performance through techniques such as ligand engineering, surface coating, and core-shell structure construction. Additionally, it comprehensively covers the progress in QD patterning methods, such as inkjet printing, photolithography, electrophoretic deposition, transfer printing, microfluidics, and micropore filling method, and emphasizes their crucial role in achieving high precision, density, and uniformity in QD deposition. This review delineates the impact of these technologies on the luminance of QD color-conversion layers and devices, providing a detailed understanding of their application in enhancing Micro-LED display technology. Finally, it explores future research directions, offering valuable insights and references for the continued innovation of full-color Micro-LED displays, thereby providing a comprehensive overview of the potential and scope of QD materials and patterning technologies in this field.

Flow synthesis and multidimensional parameter screening enables exploration and optimization of copper oxide nanoparticle synthesis

Copper-based nanoparticles (NPs) are highly valued for their wide-ranging applications, with particular significance in CO2 reduction. However current synthesis methods encounter challenges in scalability, batch-to-batch variation, and high energy costs. In this work, we describe a novel continuous flow synthesis approach performed at room temperature to help address these issues, producing spherical, colloidally stable copper(ii) oxide (CuO) NPs. This approach leverages stabilizing ligands like oleic acid, oleylamine, and soy-lecithin, a novel choice for CuO NPs. The automated flow platform facilitates facile, real-time parameter screening of Cu-based nanomaterials using optical spectroscopy, achieving rapid optimization of NP properties including size, size dispersity, and colloidal stability through tuning of reaction parameters. This study highlights the potential of continuous flow synthesis for efficient parameter exploration to accelerate understanding, optimization, and eventually enable scale-up of copper-based NPs. This promises significant benefits for various sectors, including energy, healthcare, and environmental conservation, by enabling reliable production with reduced energy and cost requirements.

Surface modification unleashes light emitting applications of APbX3 perovskite nanocrystals

Engineering the surface of metal halide perovskite nanocrystals (MHPNCs) is crucial for optimizing their optical properties, repairing surface defects, enhancing quantum yield, and ensuring long-term stability. These enhancements make surface-engineered MHPNCs ideal for applications in light-emitting devices (LEDs), displays, lasers, and photodetectors, contributing to energy efficiency. This article delves into an introduction to MHPNCs, their structure and types, particularly the ABX3 type (where A represents monovalent organic/inorganic cations, B represents divalent metal ions mainly Pb metal, and X represents halide ions), synthesis methods, unique optical properties, surface modification techniques using various agents (particularly inorganic molecules/materials, organic molecules, polymers, and biomolecules) to tune optical properties and applications in the aforementioned light-emitting technologies, challenges and opportunities, including advantages and disadvantages of surface-modified APbX3 MHPNCs, and a summary and future outlook. This article explores surface modification strategies to improve the optical performance of MHPNCs and aims to inspire advancements in light emitting applications. Importantly, the challenges and opportunities section of this article will illuminate the path to overcoming obstacles, providing invaluable insights for researchers in this field. This in-depth review explores the surface engineering of MHPNCs for light-emitting applications, highlighting their notable advantages and addressing ongoing challenges. By delving deep into various surface modification strategies, this article aims to revolutionize MHPNC-based light-emitting applications, setting a new benchmark in the field. This paves the way for revolutionary advancements, maximizing the capabilities of surface-engineered MHPNCs and heralding a transformative era in precise light-emitting research.

Bidentate Lewis Base Ligand-Mediated Surface Stabilization and Modulation of the Electronic Structure of CsPbBr3 Perovskite Nanocrystals

The desorption of conventional ligands from the surface of halide perovskite nanocrystals (NCs) often causes their structural instability and deterioration of the optoelectronic properties. To address this challenge, we present an approach of using a bidentate Lewis base ligand, namely, 1,4-bis(diphenylphosphino)butane (DBPP), for the synthesis of CsPbBr3 NCs. The phosphine group of DBPP has a strong interaction with the PbBr2 precursor, forming a highly crystalline intermediate complex during the reaction. In the presence of oleic acid, the uncoordinated phosphine group of DBPP is converted into the phosphonium cation, which strongly binds to the surface bromide of the formed CsPbBr3 NCs through hydrogen bonding. Density functional theory calculations suggest that DBPP can strongly bind to the undercoordinated lead and surface bromide ions of CsPbBr3 NCs through its unprotonated and protonated phosphine groups, respectively. The robust binding of DBPP to the surface of perovskite NCs helps to preserve their structural integrity under various environmental stresses. Moreover, the electron density and energy levels are regulated in DBPP-capped CsPbBr3 NCs by the donation of electrons from the ligands to the NCs, resulting in their improved photocatalytic CO2 reduction performance. Our study highlights the potential of using bidentate ligands to stabilize the surface of perovskite NCs and modulate their optical and electronic properties.

Elucidating the Interplay between Symmetry Distortions in Passivated MAPbI3 and the Rashba Splitting Effect

Hybrid organic-inorganic perovskites play a critical role in modern optoelectronic applications, particularly as single photon sources due to their unusual bright ground state. However, the presence of trap states resulting from surface dangling bonds hinders their widespread commercial application. This work uses density functional theory (DFT) to study the effects of various passivating ligands and their binding sites on Rashba splitting, a phenomenon directly linked to the bright ground state. Our results predict that X2- and X4-type ligands that adsorb at acidic oxygen binding sites and zwitterionic binding sites efficiently eliminate trap states introduced by surface iodine vacancies. Furthermore, our results show that distortions from the nominally symmetric cubic structure of the perovskite predominantly determine the presence and magnitude of the Rashba splitting. Specifically, the loss of more symmetry elements consistently leads to Rashba splitting in both the valence band (VB) and the conduction band (CB) with small Rashba splitting coefficients. Conversely, although inversion symmetry breaking alone fails to guarantee the presence of pure Rashba splitting in both the VB and the CB, it significantly increases the degree of splitting. The adsorption of ligands not only mitigates trap states but also plays a critical role in altering the local symmetry, thus influencing Rashba splitting. DFT predicts a distinct Rashba-Dresselhaus splitting in the CB with X2 ligands, causing the largest splitting. The presence of local electric fields causes consistent Rashba splitting of the VB across all studied systems except for the X4 zwitterionic passivated systems (sulfobetaine and lecithin). Electric fields are predicted to cause significant splitting of the CB, particularly for MAPbI3 and SH passivated MAPbI3 surfaces that possess freely rotating ligand binding sites. This study reveals that the wavelength, tunability of Rashba splitting through an applied electric field, and nature of Rashba-Dresselhaus splitting are influenced by the characteristics of the ligand binding site. On the other hand, pure Rashba splitting is predicted to exhibit a greater susceptibility to symmetry distortion than to specific ligand binding sites. These findings elucidate how surface passivating ligands and symmetry distortions influence Rashba splitting, shaping the optoelectronic properties of perovskite nanocrystals.

Strategies To Achieve Long-Term Stability in Lead Halide Perovskite Nanocrystals and Its Optoelectronic Applications

The lead halide perovskite (LHP) nanocrystals (NCs) research area is flourishing due to their exceptional properties and great potential for a wide range of applications in optoelectronics and photovoltaics. Yet, despite the momentum in the field, perovskite devices are not yet ready for commercialization due to degradation caused by intrinsic phase transitions and external factors such as moisture, temperature, and ultraviolet (UV) light. To attain long-term stability, we analyze the origin of instabilities and describe different strategies such as surface modification, encapsulation, and doping for long-term viability. We also assess how these stabilizing strategies have been utilized to obtain optoelectronic devices with long-term stability. This Mini-Review also outlines the future direction of each strategy for producing highly efficient and ultrastable LHP NCs for sustainable applications.

A single-gold-atom addition regulates sharp redshift in the fluorescence of atomically precise nanoclusters

The manipulation of emission peaks at the atomic level and the investigation of the fluorescent origin mechanism are important issues. In this study, a phosphine-mediated modification method was employed on Au36(TBBT)24 nanocluster to produce a new gold nanocluster Au37(TBBT)21(TPP)2. The structural comparison revealed that Au37(TBBT)21(TPP)2 has a structural framework similar to that of Au36(TBBT)24 except for the reconstruction of its surface motifs, the addition of one gold atom into the kernel, and local structural distortion. Interestingly, compared with Au36(TBBT)24, the emission peak of Au37(TBBT)21(TPP)2 is red-shifted into the NIR-II windows (972 nm vs. 1152 nm in CDCl3) with a quantum yield of 1.5%. Furthermore, the origin of the NIR-II fluorescence in Au37(TBBT)21(TPP)2 and the red-shift mechanism of the emission peak were explored by combining the crystal structure and DFT calculations. The results reveal that the insertion of the 37th gold atom into the core can increase the contribution of the gold atoms to the HOMO orbitals and change the origin of their fluorescence from local excitation (LE) to inter fragment charge transfer (IFCT).

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

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

Strongly-confined colloidal lead-halide perovskite quantum dots: from synthesis to applications

Colloidal semiconductor nanocrystals enable the realization and exploitation of quantum phenomena in a controlled manner, and can be scaled up for commercial uses. These materials have become important for a wide range of applications, from ultrahigh definition displays, to solar cells, quantum computing, bioimaging, optical communications, and many more. Over the last decade, lead-halide perovskite nanocrystals have rapidly gained prominence as efficient semiconductors. Although the majority of studies have focused on large nanocrystals in the weak- to intermediate-confinement regime, quantum dots (QDs) in the strongly-confined regime (with sizes smaller than the Bohr diameter, which ranges from 4-12 nm for lead-halide perovskites) offer unique opportunities, including polarized light emission and color-pure, stable luminescence in the region that is unattainable by perovskites with single-halide compositions. In this tutorial review, we bring together the latest insights into this emerging and rapidly growing area, focusing on the synthesis, steady-state optical properties (including exciton fine-structure splitting), and transient kinetics (including hot carrier cooling) of strongly-confined perovskite QDs. We also discuss recent advances in their applications, including single photon emission for quantum technologies, as well as light-emitting diodes. We finish with our perspectives on future challenges and opportunities for strongly-confined QDs, particularly around improving the control over monodispersity and stability, important fundamental questions on the photophysics, and paths forward to improve the performance of perovskite QDs in light-emitting diodes.

Competition among recombination pathways in single FAPbBr3 nanocrystals

Single particle level microscopy of immobilized FAPbBr3 nanocrystals (NCs) has elucidated the involvement of different processes in their photoluminescence (PL) intermittency. Four different blinking patterns are observed in the data from more than 100 NCs. The dependence of PL decays on PL intensities brought out in fluorescence lifetime intensity distribution (FLID) plots is rationalized by the interplay of exciton- and trion-mediated recombinations along with hot carrier (HC) trapping. The high intensity-long lifetime component is attributed to neutral exciton recombination, the low intensity-short lifetime component is attributed to trion assisted recombination, and the low intensity-long lifetime component is attributed to hot carrier recombination. Change-point analysis (CPA) of the PL blinking data reveals the involvement of multiple intermediate states. Truncated power law distribution is found to be more appropriate than power law and lognormal distribution for on and off events. Probability distributions of PL trajectories of single NCs are obtained for two different excitation fluences and wavelengths (λex = 400, 440 nm). Trapping rate (kT) prevails at higher power densities for both excitation wavelengths. From a careful analysis of the FLID and probability distributions, it is concluded that there is competition between the HC and trion assisted blinking pathways and that the contribution of these mechanisms varies with excitation wavelength as well as fluence.

Engineering colloidal semiconductor nanocrystals for quantum information processing

Quantum information processing-which relies on spin defects or single-photon emission-has shown quantum advantage in proof-of-principle experiments including microscopic imaging of electromagnetic fields, strain and temperature in applications ranging from battery research to neuroscience. However, critical gaps remain on the path to wider applications, including a need for improved functionalization, deterministic placement, size homogeneity and greater programmability of multifunctional properties. Colloidal semiconductor nanocrystals can close these gaps in numerous application areas, following years of rapid advances in synthesis and functionalization. In this Review, we specifically focus on three key topics: optical interfaces to long-lived spin states, deterministic placement and delivery for sensing beyond the standard quantum limit, and extensions to multifunctional colloidal quantum circuits.

Ligand Equilibrium Influences Photoluminescence Blinking in CsPbBr3: A Change Point Analysis of Widefield Imaging Data

Photoluminescence intermittency remains one of the biggest challenges in realizing perovskite quantum dots (QDs) as scalable single photon emitters. We compare CsPbBr3 QDs capped with different ligands, lecithin, and a combination of oleic acid and oleylamine, to elucidate the role of surface chemistry on photoluminescence intermittency. We employ widefield photoluminescence microscopy to sample the blinking behavior of hundreds of QDs. Using change point analysis, we achieve the robust classification of blinking trajectories, and we analyze representative distributions from large numbers of QDs (Nlecithin = 1308, Noleic acid/oleylamine = 1317). We find that lecithin suppresses blinking in CsPbBr3 QDs compared with oleic acid/oleylamine. Under common experimental conditions, lecithin-capped QDs are 7.5 times more likely to be nonblinking and spend 2.5 times longer in their most emissive state, despite both QDs having nearly identical solution photoluminescence quantum yields. We measure photoluminescence as a function of dilution and show that the differences between lecithin and oleic acid/oleylamine capping emerge at low concentrations during preparation for single particle experiments. From experiment and first-principles calculations, we attribute the differences in lecithin and oleic acid/oleylamine performance to differences in their ligand binding equilibria. Consistent with our experimental data, density functional theory calculations suggest a stronger binding affinity of lecithin to the QD surface compared to oleic acid/oleylamine, implying a reduced likelihood of ligand desorption during dilution. These results suggest that using more tightly binding ligands is a necessity for surface passivation and, consequently, blinking reduction in perovskite QDs used for single particle and quantum light experiments.

Efficient Large-Area Quantum Cutting Photoconversion Films for Silicon Solar Cells on Photovoltaic Glass Using Knife Coating

Yb3+ doped perovskite nanocrystals (PNCs) serve as efficient photoconverters, exhibiting quantum cutting emission at ∼980 nm, which aligns precisely with the optimal response region of silicon solar cells (SSCs). However, severe nonradiative recombination caused by defects in the crystal lattice and film boundaries, along with limitations in small-scale film preparation, restricts their commercial application. Here, we used Ru3+ to mitigate lattice defects in CsPbCl3 PNCs and adjusted the quantum cutting luminescence, achieving a 175% photoluminescence quantum yield (PLQY). The results show that Ru3+ ions enter the perovskite lattice, fill lead vacancies, and passivate the lattice defects. Furthermore, cysteine effectively eliminates surface defects in PNCs by forming Pb-S bonds, resulting in films with a remarkable 117% PLQY, demonstrating strong photoconversion capabilities. Uniformly knife-coated on 20 × 20 cm2 photovoltaic glass, these films increased SSC efficiency from 21.45% to 23.15%. This study showcases a cost-effective photoconverter and a scalable coating method to boost the photovoltaic efficiency of large-area SSCs.

Ligand Engineering of Inorganic Lead Halide Perovskite Quantum Dots toward High and Stable Photoluminescence

The ligand engineering of inorganic lead halide perovskite quantum dots (PQDs) is an indispensable strategy to boost their photoluminescence stability, which is pivotal for optoelectronics applications. CsPbX3 (X = Cl, Br, I) PQDs exhibit exceptional optical properties, including high color purity and tunable bandgaps. Despite their promising characteristics, environmental sensitivity poses a challenge to their stability. This article reviews the solution-based synthesis methods with ligand engineering. It introduces the impact of factors like humidity, temperature, and light exposure on PQD's instability, as well as in situ and post-synthesis ligand engineering strategies. The use of various ligands, including X- and L-type ligands, is reviewed for their effectiveness in enhancing stability and luminescence performance. Finally, the significant potential of ligand engineering for the broader application of PQDs in optoelectronic devices is also discussed.

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

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

Efficient pure-red perovskite light-emitting diodes with strong passivation via ultrasmall-sized molecules

Perovskite light-emitting diodes (PeLEDs) have attracted great attention in recent years; however, the halogen vacancy defects in perovskite notably hamper the development of high-efficiency devices. Previously, large-sized passivation agents have been usually used, while the effect of defect passivation is limited due to the weak bonding or the large space steric hindrance. Here, we predict that the ultrasmall-sized formate (Fa) and acetate (Ac) have more efficient passivation ability because of the stronger binding with the perovskite, as demonstrated by density functional theory calculation. We introduce ultrasmall-sized cesium salts (CsFa/CsAc) into buried interface, which can also diffuse into the bulk, resulting in both buried interface and bulk passivation. In addition, the improved perovskite growth has been found due to the enhanced hydrophily after introducing CsFa/CsAc as additive. According to these advantages, a pure-red PeLED with 24.2% efficiency at 639 nm has been achieved.

Nanosurface-reconstructed perovskite for highly efficient and stable active-matrix light-emitting diode display

Perovskite quantum dots (QDs) are promising for various photonic applications due to their high colour purity, tunable optoelectronic properties and excellent solution processability. Surface features impact their optoelectronic properties, and surface defects remain a major obstacle to progress. Here we develop a strategy utilizing diisooctylphosphinic acid-mediated synthesis combined with hydriodic acid-etching-driven nanosurface reconstruction to stabilize CsPbI3 QDs. Diisooctylphosphinic acid strongly adsorbs to the QDs and increases the formation energy of halide vacancies, enabling nanosurface reconstruction. The QD film with nanosurface reconstruction shows enhanced phase stability, improved photoluminescence endurance under thermal stress and electric field conditions, and a higher activation energy for ion migration. Consequently, we demonstrate perovskite light-emitting diodes (LEDs) that feature an electroluminescence peak at 644 nm. These LEDs achieve an external quantum efficiency of 28.5% and an operational half-lifetime surpassing 30 h at an initial luminance of 100 cd m-2, marking a tenfold improvement over previously published studies. The integration of these high-performance LEDs with specifically designed thin-film transistor circuits enables the demonstration of solution-processed active-matrix perovskite displays that show a peak external quantum efficiency of 23.6% at a display brightness of 300 cd m-2. This work showcases nanosurface reconstruction as a pivotal pathway towards high-performance QD-based optoelectronic devices.

Ligand-mediated exciton dissociation and interparticle energy transfer on CsPbBr3 perovskite quantum dots for efficient CO2-to-CO photoreduction

Perovskite quantum dots (PQDs) hold immense potential as photocatalysts for CO2 reduction due to their remarkable quantum properties, which facilitates the generation of multiple excitons, providing the necessary high-energy electrons for CO2 photoreduction. However, harnessing multi-excitons in PQDs for superior photocatalysis remains challenging, as achieving the concurrent dissociation of excitons and interparticle energy transfer proves elusive. This study introduces a ligand density-controlled strategy to enhance both exciton dissociation and interparticle energy transfer in CsPbBr3 PQDs. Optimized CsPbBr3 PQDs with the regulated ligand density exhibit efficient photocatalytic conversion of CO2 to CO, achieving a 2.26-fold improvement over unoptimized counterparts while maintaining chemical integrity. Multiple analytical techniques, including Kelvin probe force microscopy, temperature-dependent photoluminescence, femtosecond transient absorption spectroscopy, and density functional theory calculations, collectively affirm that the proper ligand termination promotes the charge separation and the interparticle transfer through ligand-mediated interfacial electron coupling and electronic interactions. This work reveals ligand density-dependent variations in the gas-solid photocatalytic CO2 reduction performance of CsPbBr3 PQDs, underscoring the importance of ligand engineering for enhancing quantum dot photocatalysis.

Synchronously Polishing the Lead-Rich Surface and Passivating Surface Defects of CsPb(Br/I)3 Quantum Dots for High-Performance Pure-Red PeLEDs

Mixed-halide CsPb(Br/I)3 perovskite quantum dots (QDs) are regarded as one of the most promising candidates for pure-red perovskite light-emitting diodes (PeLEDs) due to their precise spectral tuning property. However, the lead-rich surface of these QDs usually results in halide ion migration and nonradiative recombination loss, which remains a great challenge for high-performance PeLEDs. To solve the above issues, we employ a chelating agent of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid hydrate (DOTA) to polish the lead-rich surface of the QDs and meanwhile introduce a new ligand of 2,3-dimercaptosuccinic acid (DMSA) to passivate surface defects of the QDs. This synchronous post-treatment strategy results in high-quality CsPb(Br/I)3 QDs with suppressed halide ion migration and an improved photoluminescence quantum yield, which enables us to fabricate spectrally stable pure-red PeLEDs with a peak external quantum efficiency of 23.2%, representing one of the best performance pure-red PeLEDs based on mixed-halide CsPb(Br/I)3 QDs reported to date.

Dual-Ligand Red Perovskite Ink for Electrohydrodynamic Printing Color Conversion Arrays over 2540 dpi in Near-Eye Micro-LED Display

The lack of stability of red perovskite nanocrystals (PeNCs) remains the main problem that restricts their patterning application. In this work, the dual-ligand passivation strategy was introduced to stabilize PeNCs and inhibit their halogen ion migration during high-voltage electrohydrodynamic (EHD) inkjet printing. The as-printed red arrays exhibit the highest emisson intensity and least blue shift compared with samples with other passivation strategies under a high electric field during EHD inkjet printing. Combining with blue and green PeNC inks, single-color and tricolor color conversion layer arrays were successfully printed, with minimum pixel size of 5 μm and the highest spatial resolution of 2540 dpi. The color coordinate of CsPbBrI2 NCs arrays are located close to the red point, with a color gumat of 97.28% of Rec. 2020 standard. All of these show great potential in the application of color conversion layers in a near-eye micro-LED display.

Unraveling the Luminescence Quenching Mechanism in Strong and Weak Quantum-Confined CsPbBr3 Triggered by Triarylamine-Based Hole Transport Layers

Luminescence quenching by hole transport layers (HTLs) is one of the major issues in developing efficient perovskite light-emitting diodes (PeLEDs), which is particularly prominent in blue-emitting devices. While a variety of material systems have been used as interfacial layers, the origin of such quenching and the type of interactions between perovskites and HTLs are still ambiguous. Here, we present a systematic investigation of the luminescence quenching of CsPbBr3 by a commonly employed hole transport polymer, poly[(9,9-dioctylfluorenyl-2,7diyl)-co-(4,4'-(N-(4-sec-butylphenyl) diphenylamine)] (TFB), in LEDs. Strong and weak quantum-confined CsPbBr3 (nanoplatelets (NPLs)/nanocrystals (NCs)) are rationally selected to study the quenching mechanism by considering the differences in their morphology, energy level alignments, and quantum confinement. The steady-state and time-resolved Stern-Volmer plots unravel the dominance of dynamic and static quenching at lower and higher concentrations of TFB, respectively, with a maximum quenching efficiency of 98%. The quenching rate in NCs is faster than that in NPLs owing to their longer PL lifetimes and weak quantum confinement. The ultrafast transient absorption results support these dynamics and rule out the involvement of Forster or Dexter energy transfer. Finally, the 1D 1H and 2D nuclear overhauser effect spectroscopy nuclear magnetic resonance (NOESY NMR) study confirms the exchange of native ligands at the NCs surface with TFB, leading to dark CsPbBr3-TFB ensemble formation accountable for luminescence quenching. This highlights the critical role of the triarylamine functional group on TFB (also the backbone of many HTLs) in the quenching process. These results shed light on the underlying reasons for the luminescence quenching in PeLEDs and will help to rationally choose the interfacial layers for developing efficient LEDs.

Nucleophilic Reaction-Enabled Chloride Modification on CsPbI3 Quantum Dots for Pure Red Light-Emitting Diodes with Efficiency Exceeding 26

High-performance pure red perovskite light-emitting diodes (PeLEDs) with an emission wavelength shorter than 650 nm are ideal for wide-color-gamut displays, yet remain an unprecedented challenge to progress. Mixed-halide CsPb(Br/I)3 emitter-based PeLEDs suffer spectral stability induced by halide phase segregation and CsPbI3 quantum dots (QDs) suffer from a compromise between emission wavelength and electroluminescence efficiency. Here, we demonstrate efficient pure red PeLEDs with an emission centered at 638 nm based on PbClx -modified CsPbI3 QDs. A nucleophilic reaction that releases chloride ions and manipulates the ligand equilibrium of the colloidal system is developed to synthesize the pure red emission QDs. The comprehensive structural and spectroscopic characterizations evidence the formation of PbClx outside the CsPbI3 QDs, which regulates exciton recombination and prevents the exciton from dissociation induced by surface defects. In consequence, PeLEDs based on PbClx -modified CsPbI3 QDs with superior optoelectronic properties demonstrate stable electroluminescence spectra at high driving voltages, a record external quantum efficiency of 26.1 %, optimal efficiency roll-off of 16.0 % at 1000 cd m-2 , and a half lifetime of 7.5 hours at 100 cd m-2 , representing the state-of-the-art pure red PeLEDs. This work provides new insight into constructing the carrier-confined structure on perovskite QDs for high-performance PeLEDs.

Efficient CsPbBr3 Perovskite Light-Emitting Diodes via Novel Multi-Step Ligand Exchange Strategy Based on Zwitterionic Molecules

Perovskite nanocrystals have absorbed increasing interest, especially in the field of optoelectronics, owing to their unique characteristics, including their tunable luminescence range, robust solution processability, facile synthesis, and so on. However, in practice, due to the inherent instability of the traditional long-chain insulating ligands surrounding perovskite quantum dots (PeQDs), the performance of the as-fabricated QLED is relatively disappointing. Herein, the zwitterion 3-(decyldimethylammonio)propanesulfonate (DLPS) with the capability of double passivating perovskite quantum dots could effectively replace the original long-chain ligand simply through a multistep post-treatment strategy to finally inhibit the formation of defects. It was indicated from theexperimental results that the DLPS, as one type of ligand with the bimolecular ion, was very adavntageous in replacing long-chain ligands and further suppressing the formation of defects. Finally, the perovskite quantum dots with greatly enhanced PLQY as high as 98% were effectively achieved. Additionally, the colloidal stability of the corresponding PeQDs has been significantly enhanced, and a transparent colloidal solution was obtained after 45 days under ambient conditions. Finally, the as-fabricated QLEDs based on the ligand-exchanged PeQDs exhibited a maximum brightness of 9464 cd/m2 and an EQE of 12.17%.

Defects in lead halide perovskite light-emitting diodes under electric field: from behavior to passivation strategies

Lead halide perovskites (LHPs) are emerging semiconductor materials for light-emitting diodes (LEDs) owing to their unique structure and superior optoelectronic properties. However, defects that initiate degradation of LHPs through external stimuli and prompt internal ion migration at the interfaces remain a significant challenge. The electric field (EF), which is a fundamental driving force in LED operation, complicates the role of these defects in the physical and chemical properties of LHPs. A deeper understanding of EF-induced defect behavior is crucial for optimizing the LED performance. In this review, the origins and characterization of defects are explored, indicating the influence of EF-induced defect dynamics on LED performance and stability. A comprehensive overview of recent defect passivation approaches for LHP bulk films and nanocrystals (NCs) is also provided. Given the ubiquity of EF, a summary of the EF-induced defect behavior can enhance the performance of perovskite LEDs and related optoelectronic devices.

Spontaneous crystallization of strongly confined CsSnxPb1-xI3 perovskite colloidal quantum dots at room temperature

The scalable and low-cost room temperature (RT) synthesis for pure-iodine all-inorganic perovskite colloidal quantum dots (QDs) is a challenge due to the phase transition induced by thermal unequilibrium. Here, we introduce a direct RT strongly confined spontaneous crystallization strategy in a Cs-deficient reaction system without polar solvents for synthesizing stable pure-iodine all-inorganic tin-lead (Sn-Pb) alloyed perovskite colloidal QDs, which exhibit bright yellow luminescence. By tuning the ratio of Cs/Pb precursors, the size confinement effect and optical band gap of the resultant CsSnxPb1-xI3 perovskite QDs can be well controlled. This strongly confined RT approach is universal for wider bandgap bromine- and chlorine-based all-inorganic and iodine-based hybrid perovskite QDs. The alloyed CsSn0.09Pb0.91I3 QDs show superior yellow emission properties with prolonged carrier lifetime and significantly increased colloidal stability compared to the pristine CsPbI3 QDs, which is enabled by strong size confinement, Sn2+ passivation and enhanced formation energy. These findings provide a RT size-stabilized synthesis pathway to achieve high-performance pure-iodine all-inorganic Sn-Pb mixed perovskite colloidal QDs for optoelectronic applications.

Designer phospholipid capping ligands for soft metal halide nanocrystals

The success of colloidal semiconductor nanocrystals (NCs) in science and optoelectronics is inextricable from their surfaces. The functionalization of lead halide perovskite NCs1-5 poses a formidable challenge because of their structural lability, unlike the well-established covalent ligand capping of conventional semiconductor NCs6,7. We posited that the vast and facile molecular engineering of phospholipids as zwitterionic surfactants can deliver highly customized surface chemistries for metal halide NCs. Molecular dynamics simulations implied that ligand-NC surface affinity is primarily governed by the structure of the zwitterionic head group, particularly by the geometric fitness of the anionic and cationic moieties into the surface lattice sites, as corroborated by the nuclear magnetic resonance and Fourier-transform infrared spectroscopy data. Lattice-matched primary-ammonium phospholipids enhance the structural and colloidal integrity of hybrid organic-inorganic lead halide perovskites (FAPbBr3 and MAPbBr3 (FA, formamidinium; MA, methylammonium)) and lead-free metal halide NCs. The molecular structure of the organic ligand tail governs the long-term colloidal stability and compatibility with solvents of diverse polarity, from hydrocarbons to acetone and alcohols. These NCs exhibit photoluminescence quantum yield of more than 96% in solution and solids and minimal photoluminescence intermittency at the single particle level with an average ON fraction as high as 94%, as well as bright and high-purity (about 95%) single-photon emission.

Entropy-Driven Strongly Confined Low-Toxicity Pure-Red Perovskite Quantum Dots for Spectrally Stable Light-Emitting Diodes

Spectrally stable pure-red perovskite quantum dots (QDs) with low lead content are essential for high-definition displays but are difficult to synthesize due to QD self-purification. Here, we make use of entropy-driven quantum-confined pure-red perovskite QDs to fabricate light-emitting diodes (LEDs) that have low toxicity and are efficient and spectrum-stable. Based on experimental data and first-principles calculations, multiple element alloying results in a 60% reduction in lead content while improving QD entropy to promote crystal stability. Entropy-driven QDs exhibit photoluminescence with 100% quantum yields and single-exponential decay lifetimes without alteration of their morphology or crystal structure. The pure-red LEDs utilizing entropy-driven QDs have spectrally stable electroluminescence, achieving a brightness of 4932 cd/m2, a maximum external quantum efficiency of over 20%, and a 15-fold longer operational lifetime than the CsPbI3 QD-based LEDs. These achievements demonstrate that entropy-driven QDs can mitigate local compositional heterogeneity and ion migration.

Nickel(II)-Doped Lead-Free Halide Crystals Exhibiting Highly Efficient Tunable Blue-Emitting out of Antiferromagnetic Ni-Ni Coupling

Metal halide crystals are widely used in optoelectronic fields due to their excellent optical properties. The hunt for a lead-free halide semiconductor with superior optical performance is a particularly fascinating topic in order to avoid the toxicity of lead. Here, we incorporate Ni2+ into a series of halide nanocrystals (NCs) through solution-phase synthesis. By modifying the A-site and varying the halide compositions, we successfully achieved significant tunability of the blue emission of the Ni2+-doped AX (A = K+, Rb+, NH2CH = NH2+ (FA), CH3NH3+ (MA); X = Br, I) NCs, ranging from 375 to 490 nm, due to the antiferromagnetic polaron (AMP), which is in contrast with the excitonic magnetic polarons (EMP) from those with ferromagnetic (FM) coupling between transition metal ions in similar compounds. This work shows that Ni2+-doped halide crystals could become a typical example providing AMP excitation as the optional emission centers for use in light emitting devices.

Photocatalysis Based on Metal Halide Perovskites for Organic Chemical Transformations

Heterogeneous photocatalysts incorporating metal halide perovskites (MHPs) have garnered significant attention due to their remarkable attributes: strong visible-light absorption, tuneable band energy levels, rapid charge transfer, and defect tolerance. Additionally, the promising optical and electronic properties of MHP nanocrystals can be harnessed for photocatalytic applications through controlled crystal structure engineering, involving composition tuning via metal ion and halide ion variations, dimensional tuning, and surface chemistry modifications. Combination of perovskites with other materials can improve the photoinduced charge separation and charge transfer, building heterostructures with different band alignments, such as type-II, Z-scheme, and Schottky heterojunctions, which can fine-tune redox potentials of the perovskite for photocatalytic organic reactions. This review delves into the activation of organic molecules through charge and energy transfer mechanisms. The review further investigates the impact of crystal engineering on photocatalytic activity, spanning a diverse array of organic transformations, such as C-X bond formation (X = C, N, and O), [2 + 2] and [4 + 2] cycloadditions, substrate isomerization, and asymmetric catalysis. This study provides insights to propel the advancement of metal halide perovskite-based photocatalysts, thereby fostering innovation in organic chemical transformations.

InAs Nanorod Colloidal Quantum Dots with Tunable Bandgaps Deep into the Short-Wave Infrared

InAs colloidal quantum dots (CQDs) have emerged as candidate lead- and mercury-free solution-processed semiconductors for infrared technology due to their appropriate bulk bandgap, which can be tuned by quantum confinement, and promising charge-carrier transport properties. However, the lack of suitable arsenic precursors and readily accessible synthesis conditions have limited InAs CQDs to smaller sizes (<7 nm), with bandgaps largely restricted to <1400 nm in the near-infrared spectral window. Conventional InAs CQD synthesis requires highly reactive, hazardous arsenic precursors, which are commercially scarce, making the synthesis hard to control and study. Here, we present a controlled synthesis strategy (using only readily available and less reactive precursors) to overcome the practical wavelength limitation of InAs CQDs, achieving monodisperse InAs nanorod CQDs with bandgaps tunable from ∼1200 to ∼1800 nm, thus crossing deep into the short-wave infrared (SWIR) region. By controlling the reactivity through in situ precursor complexation, we isolate the reaction mechanism, producing InAs nanorod CQDs that display narrow excitonic features and efficient carrier multiplication. Our work enables InAs CQDs for a wider range of SWIR applications.

Effect of ion migration on lead halide perovskite on visible light communication system

Benefiting from the high modulation bandwidth (BW), low energy consumption and excellent optical performance, lead halide perovskite has attracted wide attention in visible light communication (VLC). However, the ion migration which results in mobile point defects in perovskite structures is recognized as a crucial key factor inducing the performance degradation. Here, the influence of ion migration in perovskite devices on the performance of VLC was systematically studied. The ion migration process is realized by mixing CsPbBr3 and CsPbI3 quantum dots, during which, the performance of the VLC system is reduced, but it can return to its initial state after stabilization. The on-off keying (OOK) modulation scheme of the perovskite light-emitting diode (LED) device was carried out, achieving a data rate of 90 Mbps.

Laser-Driven Insulator-Metal Phase Transitions in CsPbI3 Quantum Dots and Influence of Doped Metal Nanowires

All-inorganic CsPbI3 perovskite quantum dots (QDs) have received extensive attention in developing optoelectronic devices due to their outstanding properties. Here, using time-dependent density functional theory (TDDFT), the optical properties of the three distinct phases (α, γ, and δ) of the CsPbI3 QDs are investigated. Surprisingly, the δ phase structured QDs exhibit stronger optical absorption properties than the α and γ phase QDs when exposed to equivalent laser irradiation. Considering the quantum size effect, size regulation is also performed on the three structures, the results reveal a significant improvement in optical properties as the size increases in the direction of laser irradiation. More interestingly, Ag-hybrid QDs show better optical gain and maintain a laser-driven metallic state. Our results demonstrate the great potential of size adjustment and metal nanowire coupling in improving the optoelectronic properties of QDs and developing efficient photovoltaic devices.

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

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

A high-performance metal halide perovskite-based laser-driven display

Laser-driven liquid crystal displays (LCDs) comprising metal halide perovskites (MHPs) as the blue-to-green/red color converters are at the forefront of ongoing intense research on the development and improvement of display devices. However, the inferior high photoluminescence quantum yield (PLQY) of MHPs under the excitation of high-power blue light and photoluminescence deterioration at high temperatures remain major concerns. Herein, we design a kind of octylamine-modified MHP via binding energy engineering, and the synthesized materials show PLQY of 97.6% under the excitation of a blue laser at 450 nm. Meanwhile, this design endows a structural self-healing ability to achieve a high PLQY and luminescence stability under high temperature (90 °C) and high flux excitation (386 mW cm-2). The blue light-excitable MHPs with a near unity PLQY, strong stability, and low PLQY deterioration are further encapsulated into a laser-driven LCD device. This prototype demonstrates excellent color gamut (132% NTSC, 98% Rec. 2020), illuminance intensity (>10 000 lux), and energy consumption (47.5% of commercial consumption), and hence is expected to be beneficial for the reduction of energy consumption in backlight display devices, particularly in large-screen outdoor displays.

Surface polarization-induced emission and stability enhancement of CsPbX3 nanocrystals

Recently, the polarization effect has been receiving tremendous attention, as it can result in improved stability and charge transfer efficiency of metal-halide perovskites (MHPs). However, realizing the polarization effect on CsPbX3 NCs still remains a challenge. Here, metal ions with small radii (such as Mg2+, Li+, Ni2+, etc.) are introduced on the surface of CsPbX3 NCs, which facilitate the arising of electric dipole and surface polarization. The surface polarization effect promotes redistribution of the surface electron density, leading to reinforced surface ligand bonding, reduced surface defects, near unity photoluminescence quantum yields (PLQYs), and enhanced stability. Moreover, further introduction of hydroiodic acid results in the in situ formation of tert-butyl iodide (TBI), which facilitates the successful synthesis of pure iodine-based CsPbI3 NCs with high PLQY (95.3%) and stability under ambient conditions. The results of this work provide sufficient evidence to exhibit the crucial role of the surface polarization effect, which promotes the synthesis of high-quality MHPs and their applications in the fields of optoelectronic devices.

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

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

Light-Induced Wavelength Dependent Self Assembly Process for Targeted Synthesis of Phase Stable 1D Nanobelts and 2D Nanoplatelets of CsPbI3 Perovskites

Despite black cubic phase α-CsPbI₃ nanocrystals having an ideal bandgap of 1.73 eV for optoelectronic applications, the phase transition from α-CsPbI₃ to non-perovskite yellow δ-CsPbI₃ phase at room temperature remains a major obstacle for commercial applications. Since γ-CsPbI₃ is thermodynamically stable with a bandgap of 1.75 eV, which has great potential for photovoltaic applications, herein we report a conceptually new method for the targeted design of phase stable and near unity photoluminescence quantum yield (PLQY) two-dimensional (2D) γ-CsPbI₃ nanoplatelets (NPLs) and one-dimensional (1D) γ-CsPbI₃ nanobelts (NBs) by wavelength dependent light-induced assembly of CsPbI₃ cubic nanocrystals. This article demonstrates for the first time that by varying the excitation wavelengths, one can design air stable desired 2D nanoplatelets or 1D nanobelts selectively. Our experimental finding indicates that 532 nm green light-driven self-assembly produces phase stable and highly luminescent γ-CsPbI₃ NBs from CsPbI₃ nanocrystals. Moreover, we show that a 670 nm red light-driven self-assembly process produces stable and near unity PLQY γ-CsPbI₃ NPLs. Systematic time-dependent microscopy and spectroscopy studies on the morphological evolution indicates that the electromagnetic field of light triggered the desorption of surface ligands from the nanocrystal surface and transformation of crystallographic phase from α to γ. Detached ligands played an important role in determining the morphologies of final structures of NBs and NPLs from nanocrystals via oriented attachment along the [110] direction initially and then the [001] direction. In addition, XRD and fluorescence imaging data indicates that both NBs and NPLs exhibit phase stability for more than 60 days in ambient conditions, whereas the cubic phase α-CsPbI₃ nanocrystals are not stable for even 3 days. The reported light driven synthesis provides a simple and versatile approach to obtain phase pure CsPbI₃ for possible optoelectronic applications.

Synthesis of Thermally Stable and Highly Luminescent Cs5 Cu3 Cl6 I2 Nanocrystals with Nonlinear Optical Response

Low-dimensional Cu(I)-based metal halide materials are gaining attention due to their low toxicity, high stability and unique luminescence mechanism, which is mediated by self-trapped excitons (STEs). Among them, Cs₅ Cu₃ Cl₆ I₂ , which emits blue light, is a promising candidate for applications as a next-generation blue-emitting material. In this article, an optimized colloidal process to synthesize uniform Cs₅ Cu₃ Cl₆ I₂ nanocrystals (NCs) with a superior quantum yield (QY) is proposed. In addition, precise control of the synthesis parameters, enabling anisotropic growth and emission wavelength shifting is demonstrated. The synthesized Cs₅ Cu₃ Cl₆ I₂ NCs have an excellent photoluminescence (PL) retention rate, even at high temperature, and exhibit high stability over multiple heating-cooling cycles under ambient conditions. Moreover, under 850-nm femtosecond laser irradiation, the NCs exhibit three-photon absorption (3PA)-induced PL, highlighting the possibility of utilizing their nonlinear optical properties. Such thermally stable and highly luminescent Cs₅ Cu₃ Cl₆ I₂ NCs with nonlinear optical properties overcome the limitations of conventional blue-emitting nanomaterials. These findings provide insights into the mechanism of the colloidal synthesis of Cs₅ Cu₃ Cl₆ I₂ NCs and a foundation for further research.

3D/2D Core/Shell Perovskite Nanocrystals for High-Performance Solar Cells

All-inorganic lead halide perovskite nanocrystals (NCs) emerge as a rising star in photovoltaic fields on account of their excellent optoelectronic properties. However, it still remains challenging to further promote photovoltaic efficiency due to the susceptible surface and inevitable vacancies. Here, this work reports a 3D/2D core/shell perovskite heterojunction based on CsPbI₃ NCs and its performance in solar cells. The guanidinium (GA⁺ ) rich 2D nanoshells can significantly passivate surface trap states and lower the capping ligand density, resulting in improved photoelectric properties and carrier transport and diminished nonradiative recombination centers via the hydrogen bonds from amino groups in GA⁺ ions. Consequently, an outstanding power conversion efficiency (PCE) of up to 15.53% is realized, substantially higher than the control device (13.77%). This work highlights the importance of surface chemistry and offers a feasible avenue to achieve high-performance perovskite NCs-based optoelectronic devices.

Ligands in Lead Halide Perovskite Nanocrystals: From Synthesis to Optoelectronic Applications

Ligands are indispensable for perovskite nanocrystals (NCs) throughout the whole lifetime, as they not only play key roles in the controllable synthesis of NCs with different sizes and shapes, but also act as capping shell that affects optical properties and electrical coupling of NCs. Establishing a systematic understanding of the relationship between ligands and perovskite NCs is significant to enable many potential applications of NCs. This review mainly focuses on the influence of ligands on perovskite NCs. First of all, the ligands-dominated size and shape control of NCs is discussed. Whereafter, the surface defects of NCs and the bonding between ligands and perovskite NCs are classified, and corresponding post-treatment of surface defects via ligands is also summarized. Furthermore, advances in engineering the ligands towards the high performance of optoelectronic devices based on perovskite NCs, including photodetector, solar cell, light emitting diode (LED), and laser, and finally to potential challenges are also discussed.

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

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

Ultra-small α-CsPbI3 perovskite quantum dots with stable, bright and pure red emission for Rec. 2020 display backlights

The synthesis of α-CsPbI₃ perovskite quantum dots (QDs) with pure red emission around 630 nm is in high demand for display backlight application. However, the phase transition of α-CsPbI₃ to yellow non-emitting δ-CsPbI₃ has been proven to be a great challenge for the classic colloidal synthesis route for perovskite QDs in octadecene (ODE). Herein, we report a novel colloidal synthesis route by replacing ODE with lauryl methacrylate (LMA) as the reaction solvent to improve the solubility of precursors, resulting in small sized α-CsPbI₃ QDs with a diameter of only 4.2 nm, which are the smallest red PQDs reported so far. The corresponding CsPbI₃ QD films exhibit a tunable photoluminescence (PL) emission peak in the bright pure red region of 627 to 638 nm. The CsPbI₃ QD polymer composite films with PL emission at 630 nm exhibit a superior photoluminescence quantum yield (PLQY) and photostability to mixed halide CsPbBrI₂ films under intense illumination. Perovskite light emitting diodes (LED) with the color gamut reaching 96% of the Rec. 2020 standard are achieved using these films. This study provides a high-performance pure red fluorescent material with a robust, low-cost, and reproducible colloidal chemistry that will pave the way for the adoption of perovskite QDs in display backlight application.

Halide-Driven Halogen-Hydrogen Bonding versus Chelation in Perovskite Nanocrystals: A Concept of Charge Transfer Bridging

The choice of surface functionalized ligands to encapsulate semiconductor nanocrystals (NCs) is important for tailoring their optoelectronic properties. We use a small bidentate 8-hydroxyquinoline (HQ) molecule to surface functionalize CsPbX₃ perovskite NCs (X = Cl, Br, I), along with traditional long-chain monodentate ligands. Our experimental results using optical and ultrafast spectroscopy depict a halogen-hydrogen bonding formation in the HQ functionalized CsPbCl₃ and CsPbBr₃ NCs, which act as a charge transfer (CT) bridging for the interfacial hole transfer from the NCs to the HQ molecule as fast as 540 fs. In contrast, weak chelation is observed for HQ-coupled CsPbI₃ NCs without an active CT process. We explain two distinct surface coupling mechanisms via the polarizability of halides and larger PbI₆^4- octahedral cage size. Control of two contrasting halide-dependent surface coupling phenomena of a small molecule that further regulate the CT process may have significant implications in their development in optoelectronics.

Recent progress of single-halide perovskite nanocrystals for advanced displays

Light-emitting diodes based on lead halide perovskite nanocrystals (LHP NCs) have shown an astonishing increase in efficiency in just several years of academic research, reaching high external quantum efficiencies exceeding 20%. The extensive color-tunability and narrow emission bandwidth of LHP NCs, in particular, are of great importance in the creation of the next generation of ultra-high-definition displays, as defined by the Rec. 2020 standard recommendation. In fact, whereas the colour of LHP NCs can be easily tuned by the compositions of halogens, the ion migration in mixed-halide perovskites under the electric field will seriously affect the spectral stability and operational lifetimes of perovskite light-emitting diodes (PeLEDs). Therefore, it is essential to realize efficient colour-saturated PeLEDs based on single-halide perovskite NCs. In this review, we focus on the recent progress in LHP NC-based PeLEDs and highlight the strategy of tuning the spectral emission based on quantum confinement or cation alloying/doping in single-halide perovskite NCs. Finally, we will give an outlook on future research avenues for preparing high-efficiency pure green, red and blue PeLEDs based on single-halide perovskite NCs.