Organic-Inorganic Hybrid Passivation Enables Perovskite QLEDs with an EQE of 16.48

Highly Efficient Blue Light-Emitting Diodes with Low Efficiency Roll-Off Based on Large-Size and Gradient Alloy Quantum Dots

Quantum dot light-emitting diodes (QD-LEDs) exhibit significant advancements in new-generation display and lighting applications that require high efficiency, high brightness, and high resolution, such as automotive heads-up displays (HUD) and augmented reality (AR)/virtual reality (VR). However, state-of-the-art blue QD-LEDs have yet to meet these requirements due to defect-induced nonradiative recombination and unbalanced carrier injection. Herein, a novel blue quantum dots (QDs), which feature a large-size (≈10.5 nm) CdSe@ZnSe gradient alloy core and an ultra-thin ZnS outermost shell, have been demonstrated through reversely adding seed crystal with composition regulating. The as-synthesized QDs possess near-unity quantum yield, shallower hole injection barrier, and excellent photo-chemical stability. By employing CdSe@ZnSe/ZnS QDs as the emitting layers (EML), blue QD-LEDs with electroluminescence (EL) peak at 475 nm exhibit a record-high external quantum efficiency (EQE) of 24.3% and low efficiency roll-off, sustaining over 90% of the maximum EQE within the luminance of 2,220-22,910 cd m-2. Key to success is the suppression of defect-related nonradiative recombination, reduced leakage current, and improves charge injection balance through QD structural engineering. This work indicates a significant potential of newly developed large-size and gradient alloy QDs in promoting the commercialization of QD-LEDs.

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).

Surface Stress Engineering of CsPbI3 Perovskite Quantum Dots for Efficient Solar Cells

Inorganic CsPbI3 perovskite quantum dots (PQDs) demonstrate high potential for new-generation photovoltaics, but the imbalanced surface stress of PQDs induced by ligand deficiency and incompatibility significantly deteriorates their optoelectronic properties and phase stability, restricting their photovoltaic performance. Herein, a surface lattice regularization strategy is proposed for the surface stress engineering of PQDs, in which a series of onium cations with appropriate dimensions and good affinity with the surface lattice of PQDs are introduced into the surface lattice of PQDs, resulting in substantially ameliorated optoelectronic properties and phase stability of PQDs. Meanwhile, with surface stress engineering, the PQD solid with enhanced stacking orientation is constructed, facilitating charge carrier transport. Consequently, the PQD solar cell with an efficiency of up to 17.01% is obtained, which is one of the highest values of inorganic PQD solar cells. Such a strategy provides feasible access to maximize the optoelectronic properties of PQDs for high-performance optoelectronics.

Europium(II) Complex with d-f Transition: New Emitter for Blue Light-Emitting Electrochemical Cells with an External Quantum Efficiency of 19.8

Light-emitting electrochemical cells (LECs) have good prospects in the solid-state lighting field due to their simple single-layer structure and low manufacturing cost. However, the lack of high-efficiency blue LECs limits their development and further application. To solve this problem, many luminescent materials with various mechanisms such as fluorescence, phosphorescence, and thermally-activated delayed fluorescence are studied as emitters in LECs. Besides these materials, herein lanthanide europium(II) (Eu(II)) complex with d-f transition is demonstrated as a new type of emitter for LECs. In detail, a blue emitting Eu(II) complex bis[hydrotris(3-tert-butylpyrazolyl)borate]europium(II) is dispersed into 9-(3-(triphenylsilyl)phenyl)-9H-3,9'-bicarbazole host and tetrahexylammonium tetrafluoroborate electrolyte to make a blend film, which attains a high photoluminescence quantum yield of ≈100%. The LEC using the blend film as an active layer achieves a blue emission with Commission Internationale de L'Eclairage coordinates of (0.12, 0.18) and a record-breaking external quantum efficiency of 19.8%, corresponding to an exciton utilization efficiency of ≈100%. This work reveals that Eu(II) complex with d-f transition is a promising emitter toward high-performance blue LECs, which can inspire further research on Eu(II) complex based LECs.

Buried Interface Passivation of Regular Perovskite Solar Cells Using Trimethoxysilane-Based Small Molecule Materials

Numerous issues influencing the device performance at the buried interface of perovskite solar cells exist. Modifying the interface constitutes a key to enhancing the efficiency and stability of solar cells. In this work, we propose a small molecule material, (3-bromopropyl)trimethoxysilane (BTS), as an efficient modifier for the buried interface of regular perovskite solar cells. It can interact with the -OH groups on SnO2 and the uncoordinated Pb and the formamidinium (FA) vacancies in the perovskite, thereby passivating the defects in the SnO2 and perovskite layers, improving the extraction and transportation of charge carriers at the interface, inhibiting the nonradiative recombination of charge carriers, and thereby increasing the carrier lifetime. The introduction of BTS significantly improves the performance of the solar cells, with the open-circuit voltage (Voc) reaching 1.169 V and an increase in power conversion efficiency (PCE) from 19.39% to 23.60%. Additionally, the devices with interface modification exhibit outstanding stability, retaining 85.7% of the initial PCE value after being aged for 160 days under a relative humidity of 35% at room temperature.

Understanding the Effect of Acid-Base Equilibria in Ligand Exchange on the Overall Quality of Inorganic Perovskite Nanocrystals and Light-Emitting Devices

Photoluminescence quantum yield (PLQY) losses in inorganic perovskite nanocrystals (PeNCs) due to ligand desorption hamper high external quantum efficiencies (EQE) in corresponding perovskite light-emitting devices (PeLEDs). Their low PLQYs derive mainly from ligand desorption during device fabrication. Post-synthesis treatments contribute to inefficiently adsorbed ligands due to their unfavorable chemical environments. Here the acid/base dynamics of treatments are investigated by applying a chemoselective and aprotic-driven ligand exchange strategy that favors neutral environments, in lieu of traditional acid-mediated strategies. Mild ligand-extracting reagents (LERs) are utilized to gently extract native anchoring ligands with their cations, while their anions temporarily passivate the PeNC's surface, ensuring steady colloidal stability. By applying tri-ethyloxonium tetrafluoroborate (TET) as the LER, PeNCs films displayed PLQYs as high as 92.8%. When paired with the widely-employed di-dodecyldimethylammonium bromide (DDAB) ligand, PeLED devices based on TET-treated PeNCs exhibited a maximum EQE of 22.94% for emissions at λ = 512 nm. The work highlights the versatility of ligand exchange processes by assessing their overall governing factors.

Molecule-induced ripening control in perovskite quantum dots for efficient and stable light-emitting diodes

Perovskite quantum dots (QDs) show an excellent application perspective in semiconductor optoelectronic devices. However, problems of ligand loss during the growth, purification, film formation, and storage process always induce the aggregation and ripening of QDs, adversely affecting QDs' and QD-based devices' performance. Here, we use a bidentate molecule to control ripening toward a notable performance boost in CsPbI3 QDs. The strong interaction between QDs and the bidentate molecules maintains stable surface states of QDs, inhibiting QDs' undesirable ripening and generation of defects. We fabricate QD-based light-emitting diodes (LEDs) with a maximum external quantum efficiency (EQE) of 26.0% at 686 nm and an operating half-life of 10,587 hours at an initial radiance of 190 mW sr-1 m-2 (equivalent to a luminance of 100 cd m-2 for green perovskite LEDs). Benefiting from the high storability of the target QDs, the as-fabricated devices based on the QD solution storing for 1 month show a maximum EQE of 21.7% (20.3% for 3 months).

Defect Passivating Hole Transporting Material for Large-Area and Stable Perovskite Quantum-Dot Light-Emitting Diodes

Organic hole-transporting materials (HTMs) with high hole mobility and a defect passivating ability are critical for improving the performance and stability of perovskite optoelectronics, including perovskite quantum dot light-emitting diodes (Pe-QLEDs) and perovskite solar cells. In this study, we designed two small-molecule HTMs, termed X13 and X15, incorporating the methylthio group (SMe) as defect-passivating sites to enhance the interaction between HTMs and the perovskite layer for Pe-QLED applications. Our study highlights that X15, featuring SMe groups at the para-position of the carbazole unit, demonstrates a strong interaction and superior passivation effects with perovskite quantum dots. Consequently, Pe-QLEDs (0.09 cm2) incorporating X15 as the HTM achieve a maximum external quantum efficiency (EQE) of 22.89%. Moreover, employing X15 in large-area Pe-QLEDs (1 cm2) yields an EQE of 21.10% with uniform light emission, surpassing the PTAA-based devices (EQE ∼ 15.03%). Our finding provides crucial insights into the molecular design of defect-passivating small-molecule HTMs for perovskite light-emitting diodes and related optoelectronic devices.

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.

Synchronous Passivation Boosts the NIR-II Luminescence Efficiency of Ag2S Nanocrystals for Effective Deep Tissue Lymphatic Mapping

Silver sulfide nanocrystals (Ag2S NCs) with second near-infrared (NIR-II) emission and excellent biocompatibility hold great attraction for in vivo imaging. However, current Ag2S NCs suffer severely with low photoluminescence quantum yields (PLQYs), limiting the penetration depth. Herein, based on computational prediction, we proposed a facile synchronous passivation strategy employing non-toxic MgCl2 as a synchronous passivator for simultaneous Mg-doping and chlorination of Ag2S NCs (Syn-Ag2S). Spectroscopic studies including femtosecond transient absorption measurement (fs-TAS) and temperature-dependent PL spectra revealed that syn-passivation successfully enlarged nonradiative activation energy and reduced the defect states and exciton-photon interaction, thus resulting in a record high absolute PLQY of 46 ± 2% at 1220 nm emission among toxic-element-free NIR-II probes. The boosted emission unlocks the potential of Ag2S NCs for deep lymph node imaging. The profile of deep lumbar lymph nodes was clearly distinguished by Syn-Ag2S, while clinical indocyanine green (ICG) failed to do so. This strategy also establishes an approach for developing other ultrabright and non-toxic NCs used not only in bioimaging but also in photovoltaics and LEDs.

Surface passivation strategies for CsPbBr3 quantum dots aiming at nonradiative suppression and enhancement of electroluminescent light-emitting diodes

With many fascinating characteristics, such as color-tunability, narrow-band emission, and low-cost solution processability, all-inorganic lead halide perovskite quantum dots (QDs) have attracted keen attention for electroluminescent light-emitting diodes (QLEDs) and display applications. However, the performance of perovskite QLED devices is intrinsically limited by the inefficient electrical carrier transport capacity. Herein, one facile but effective method is proposed to enhance the perovskite QLED performance by incorporating a short carbon chain ligand of 2-phenethylammonium bromide (PEABr) to passivate the CsPbBr3 QD surface. With the PEABr ligand, the Br- vacancies are passivated, which could eliminate nonradiative recombination of perovskite QDs; thus their optical properties are enhanced. Meanwhile, PEABr can interact with perovskite QDs to adjust the perovskite film morphology, resulting in low current leakage and efficient electron injection. After the PEABr treatment, the CsPbBr3 QD film exhibits strong green emission located at 516 nm, with an average photoluminescence lifetime of 45.71 ns and a photoluminescence quantum yield of up to 78.64%. In addition, the surface roughness of the CsPbBr3 QD film is reduced from 3.61 nm to 1.38 nm, which is essential to prepare a QD film with high surface coverage. As a result, the QLED device with PEABr treated CsPbBr3 QDs exhibits a maximum current efficiency of 32.69 cd A-1 corresponding to an external quantum efficiency of 9.67%, 3.88-fold higher than that of the control device (pure QDs as an emission layer). This research provides an effective strategy for the improvement of the perovskite QLED performance and may be helpful for extending their actual applications.

Highly Efficient Blue Light-Emitting Diodes Enabled by Gradient Core/Shell-Structured Perovskite Quantum Dots

Room temperature (RT) synthesized mixed bromine and chlorine CsPbBrxCl3-x perovskite quantum dots (Pe-QDs) offer notable advantages for blue quantum dot light-emitting diodes (QLEDs), such as cost-effective processing and narrow luminescence peaks. However, the efficiency of blue QLEDs using these RT-synthesized QDs has been limited by inferior crystallinity and deep defect presence. In this study, we demonstrate a precise approach to constructing high-quality gradient core-shell (CS) structures of CsPbBrxCl3-x QD through anion exchange. Characterization shows that these CS-QDs exhibit a type-I band alignment with a high bromine concentration in the core and a high chlorine concentration in the shell. This unique configuration results in a larger exciton binding energy and reduced defect density, leading to enhanced exciton radiative recombination. Consequently, QLEDs using CS-QDs achieve an external quantum efficiency (EQE) of 16.28%, a maximum luminance of 8423.35 cd/m2, and improved operational stability, surpassing the 12.80% EQE of reference QLEDs made with homogeneous structured QDs (HS-QDs). These findings present a strategy for developing high-quality RT-synthesized blue CS-QDs, marking a significant advancement in the field of efficient pure-blue QLEDs.

Gradient Hole Injection Inducing Efficient Exciton Recombination in Blue (475 nm) Perovskite QLEDs

Perovskite quantum dots (QDs) are emerging as excellent light sources for light-emitting diodes (LEDs). However, the performance of blue perovskite QD-based LEDs (QLEDs) still lags behind that of red and green counterparts, which is hindered by blue perovskite QDs with broad bandgaps that tend to increase nonradiative recombination. Here, we designed a gradient energy for hole injection utilizing multiple hole injection layers (HTLs) combined with carbazole-based small-molecule modification to reduce the hole injection barrier between HTLs and QD layers and improve the hole injection efficiency, realizing efficient exciton recombination in blue perovskite QLEDs. Moreover, the QD film on the designed HTLs demonstrates a lower surface roughness and improved photoluminescence properties. The optimized blue CsPbCl3-xBrx QLEDs exhibit an impressive external quantum efficiency of 20.7% with an electroluminescence peak at 475 nm and a turn-on voltage of 2.6 V, representing the state-of-the-art for blue perovskite LEDs emitting in the range of 460-480 nm.

Stable and Highly Emissive Infrared Yb-Doped Perovskite Quantum Cutters Engineered by Machine Learning

Quantum cutting (QC) allows the conversion of high-energy photons into lower-energy photons, exhibiting great potential for infrared communications. Yb-doped perovskite nanocrystals can achieve an efficient QC process with extremely high photoluminescence quantum yield (PLQY) thanks to the favorable Yb3+ incorporation in the perovskite structure. However, conventionally used oleic acid-oleylamine-based ligand pairs cause instability issues due to highly dynamic binding to surface states that have curbed their potential applications. Herein, zwitterionic type C3-sulfobetaine 3-(N,N-Dimethylpalmitylammonio)propanesulfonate molecule is utilized to build a strong binding state on the nanocrystals' surface through a new phosphine oxide synthesis route. Leveraging machine learning and Bayesian Optimization workflow to determine optimal synthesis conditions, near-infrared PLQY above 190% is achieved. The high PLQY is well maintained after over three months of aging, under high-flux continuous UV irradiation, and long continuous annealing. This is the first report of highly efficient and stable perovskite quantum cutters, which will drive the study of fundamental physics phenomena and near-infrared quantum communications.

Highly Bright and Stable CsPbX3@Cs4PbX6 Hexagonal Nanoarchitectonics Created by Controlling Dissolution-Recrystallization of CsPbX3 Nanomaterials

CsPbBr3@Cs4PbBr6 hexagonal NCs with a bright photoluminescence (PL) peak of 456 nm are created through the dissolution-recrystallization of CsPbBr3 nanoplatelets. Small CsPbBr3 nanocrystals are encapsulated in hexagonal Cs4PbBr6 during recrystallization to form a core-shell structure and keep high brightness and stability. The recrystallization kinetics is systematically investigated to explore the roles of methyl acetate, oleylamine, and n-hexane. Result further indicates that core/shell NCs remained high PL under a variety of harsh conditions (e.g., light irradiation and heat treatment) because of Cs4PbX6 shell and the controlling of recrystallization. Their initial PL intensity is remained after 4 months of storage under ambient conditions and continuous exposure to UV lamp for 180 min. The bright PL is also maintained even treatment at 120 °C. To indicate the universality of this synthesis method, CsPbX3@Cs4PbX6 hexagonal NCs with different emission colors are fabricated by changing temperature, solvent viscosity, and precursors (e,g, oleylamine and halogens). These core-shell samples reveal bright and stable green, orange, and red PL. Because of its high stability, the core/shell NCs are dispersed in flexible films to create diverse patterns. The films also exhibit high brightness and excellent stability. This strategy opens a novel avenue for the application of perovskite nanomaterials in the display field.

Bifunctional Ligand Passivation Enables Stable Blue Mixed-Halide CsPb(Br/Cl)3 Perovskite Quantum Dots toward Light-Emitting Diodes

Mixed-halide CsPb(Br/Cl)3 perovskite quantum dots (PeQDs) have attracted extensive attention in light-emitting diodes (LEDs), but their low photoluminescent efficiency and especially poor stability impede their practical applications. Here, we employ bifunctional didodecyldimethylammonium thiocyanide (DDASCN) with a pseudohalogen SCN- and branched DDA+ to obtain blue-emitting CsPbBr2Cl PeQDs. DDASCN significantly boosts the photoluminescence quantum yield to 92% by inhibiting nonradiative recombination. Importantly, DDASCN PeQDs show excellent stabilities against air, UV light, heat, and polar solvents. These improved performances were explained by density functional theory calculation, which shows that SCN- fills the Cl- vacancy by simultaneously binding with undercoordinated Pb2+ and Cs+, while DDA+ connects undercoordinated Br- and lies parallel to the PeQD core, leading to efficient passivation and a strong binding capacity. Finally, we achieved high-performance white LEDs by integrating our PeQDs, resulting in a color-rendering index of 92.9, a color gamut of 119.61%, and chromaticity coordinates of (0.33, 0.33). This provides an effective method to obtain efficient and stable CsPb(Br/Cl)3 PeQDs for practical applications.

High-performance ultraviolet detector based on self-assembled 3D/2D perovskite heterostructure

Heterogeneous assembly of metal halide perovskites (MHPs) structures offers convenience for promoting the interfacial properties of perovskite heterojunctions, which have been widely used in the new generation of photoelectric devices. In this study, three-dimensional (3D) CsPbBr3 quantum dots (CPB QDs) were epitaxially grown on two-dimensional (2D) (BA)2PbBr4 nanoplates (BPB NPs) via self-assembly in a toluene mixing solution. The morphological, structural, and optical properties of the synthesized structure reveal that a highly-qualified interface and coherence were formed between the two different perovskites. These heterostructures (HSs) facilitate the separation and transportation of electrons and holes in opposite directions. Based on this property, a high-performance ultraviolet light detector was fabricated by depositing a layer of CPB@BPB film on a textured silicon (T-Si) substrate. The prepared CPB@BPB/T-Si detector has shown enhanced properties i.e. quick response time, high responsivity (6.9 A W-1), high detection rate (3.17 × 109 jones), and low detection limit (0.24 μW cm-2). This enhanced performance could be attributed to the large light-absorbing area, effective carrier transport channels in BPB NPs, and improved interfacial properties of the CPB@BPB HS.

Aromatic 2,2-Diphenylethylamine Ligand Exchange of FA0.9Cs0.1PbBr3 Perovskite Nanocrystals for High-Efficiency Pure Green Light-Emitting Diodes

Perovskite nanocrystals (NCs) with long alkyl ligands cannot easily form high-quality composite films owing to their poor dispersibility in π-conjugated small molecules and polymer host materials. In this study, we demonstrated that the aromatic ligand exchange of mixed-cation FA0.9Cs0.1PbBr3 NCs using 2,2-diphenylethylamine (DPEA) can not only enable the fabrication of high-efficiency light-emitting diodes (LEDs) but also allows dispersibility in host materials. The DPEA-NC film exhibited a pure green wavelength of 530 nm and a full width at half-maximum of 20.9 nm with a photoluminescence quantum yield of 90.9%. A DPEA-NC LED achieved a luminance of 39,700 cd/m2 and an external quantum efficiency of 18.6% even in a thick NC film. Interestingly, the DPEA-NCs formed a composite film with small-molecule tris(4-carbazoyl-9-ylphenyl)amine. The operational stability of this composite LED was eight times higher than that of the DPEA-NC LED owing to enhanced hole-electron charge balance and the suppression of perovskite NC degradation. Therefore, the aromatic DPEA ligand exchange of perovskite NCs is an effective way to improve their electrical properties and operational device stabilities.

Surface-binding molecular multipods strengthen the halide perovskite lattice and boost luminescence

Reducing the size of perovskite crystals to confine excitons and passivating surface defects has fueled a significant advance in the luminescence efficiency of perovskite light-emitting diodes (LEDs). However, the persistent gap between the optical limit of electroluminescence efficiency and the photoluminescence efficiency of colloidal perovskite nanocrystals (PeNCs) suggests that defect passivation alone is not sufficient to achieve highly efficient colloidal PeNC-LEDs. Here, we present a materials approach to controlling the dynamic nature of the perovskite surface. Our experimental and theoretical studies reveal that conjugated molecular multipods (CMMs) adsorb onto the perovskite surface by multipodal hydrogen bonding and van der Waals interactions, strengthening the near-surface perovskite lattice and reducing ionic fluctuations which are related to nonradiative recombination. The CMM treatment strengthens the perovskite lattice and suppresses its dynamic disorder, resulting in a near-unity photoluminescence quantum yield of PeNC films and a high external quantum efficiency (26.1%) of PeNC-LED with pure green emission that matches the Rec.2020 color standard for next-generation vivid displays.

Boosting External Quantum Efficiency of Blue Perovskite QLEDs Exceeding 23% by Trifluoroacetate Passivation and Mixed Hole Transportation Design

Perovskite quantum dot-based light-emitting diodes (QLEDs) have been considered a promising display technology due to their wide color gamut for authentic color expression. Currently, the external quantum efficiency (EQE) for state-of-the-art blue perovskite QLEDs is about 15%, which still lags behind its green and red counterparts (>25%) and blue film-based LEDs. Here, blue perovskite QLEDs that achieve an EQE of 23.5% at 490 nm is presented, to the best knowledge, which is the highest value reported among blue perovskite-based LED fields. This impressive efficiency is achieved through a combination of quantum dot (QD) passivation and optimal device design. First, blue mixed halide perovskite CsPbCl3- xBrx QDs passivated by trifluoroacetate exhibit excellent exciton recombination behavior with a photoluminescence quantum yield of 84% due to reducing uncoordinated Pb surface defects. Furthermore, the device is designed by introducing a mixed hole-transport layer (M-HTL) to increase hole injection and transportation capacity and improve carrier balance. It is further found that M-HTL can decrease carrier leakage and increase radiative recombination in the device, evidenced by the visual electroluminescence spectrum at 2.0 V. The work breaks through the EQE gap of 20% for blue perovskite-based QLEDs and significantly promotes their commercialization process.

Doping Up the Light: A Review of A/B-Site Doping in Metal Halide Perovskite Nanocrystals for Next-Generation LEDs

All-inorganic metal halide perovskite nanocrystals (PeNCs) show great potential for the next generation of perovskite light-emitting diodes (PeLEDs). However, trap-assisted recombination negatively impacts the optoelectronic properties of PeNCs and prevents their widespread adoption for commercial exploitation. To mitigate trap-assisted recombination and further enhance the external quantum efficiency of PeLEDs, A/B-site doping has been widely investigated to tune the bandgap of PeNCs. The bandgap of PeNCs is adjustable within a small range (no more than 0.1 eV) by A-site cation doping, resulting in changes in the bond length of Pb-X and the angle of [PbX6]4. Nevertheless, B-site doping of PeNCs has a more significant impact on the bandgap level through modification of surface defect states. In this perspective, we delve into the synthesis of PeNCs with A/B-site doping and their impacts on the structural and optoelectronic properties, as well as their impacts on the performance of subsequent PeLEDs. Furthermore, we explore the A-site and B-site doping mechanisms and the impact of device architecture on doped PeNCs to maximize the performance and stability of PeLEDs. This work presents a comprehensive overview of the studies on A-site and B-site doping in PeNCs and approaches to unlock their full potential in the next generation of LEDs.

Significant Impact of Defect Fluctuation on Charge Dynamics in CsPbI3: A Study Combining Machine Learning with Quantum Dynamics

In this study, we developed a machine-learned force field for CsPbI3 using a neural network potential, enabling molecular dynamics simulations (MD) with ab initio accuracy over nanoseconds. This approach, combined with ab initio MD and nonadiabatic MD, was used to study the charge trapping and recombination dynamics in both pristine and defective CsPbI3. Our simulations revealed key transitions affecting carrier lifetimes, especially in systems with iodine vacancy and interstitial iodine defects. An iodine trimer, formed when iodine replaces cesium, exhibits a high-frequency phonon mode. This mode enhances nonadiabatic coupling, accelerating charge recombination in defective systems compared to pristine ones. In the iodine vacancy system, recombination times varied significantly due to differences in NA coupling and energy gaps. The interplay between nonadiabatic coupling and pure dephasing time is crucial in determining recombination times for interstitial iodine defects. Our findings highlight the role of defect evolution in perovskites, offering insights for enhancing perovskite performance.

Polymer-Surface-Mediated Mechanochemical Reaction for Rapid and Scalable Manufacture of Perovskite QD Phosphors

Perovskite quantum dots (QDs) have been considered new-generation emitters for lighting and displays due to their high photoluminescence (PL) efficiency, and pure color. However, their commercialization process is currently hindered by the challenge of mass production in a quick and environmentally friendly manner. In this study, a polymer-surface-mediated mechanochemical reaction (PMR) is proposed to prepare perovskite QDs using a high-speed multifunction grinder for the first time. PMR possesses two distinctive features: i) The ultra-high rotating speed (>15 000 rpm) of the grinder facilitates the rapid conversion of the precursor to perovskite; ii) The surface-rich polymer particulate ensures QDs with high dispersity, avoiding QD aggregation-induced PL quenching. Therefore, PMR can successfully manufacture green perovskite QDs with a high PL quantum yield (PLQY) exceeding 90% in a highly material- (100% yield), time- (1 kg min-1), and effort- (solvent-free) efficient manner. Moreover, the PMR demonstrates remarkable versatility, including synthesizing by various polymers and producing diverse colored and Pb-free phosphors. Importantly, these phosphors featuring a combination of polymer and perovskite, are facilely processed into various solid emitters. The proposed rapid, green, and scalable approach has great potential to accelerate the commercialization of perovskite QDs.

Highly luminescent and stable CsPbBr3 perovskite nanocrystals coated with polyethersulfone for white light-emitting diode applications

Simultaneously improving the stability and photoluminescence quantum yield (PLQY) of all inorganic perovskite nanocrystals (NCs) is crucial for their practical utilization in various optoelectronic devices. Here, CsPbBr3 NCs coated with polyethersulfone (PES) were prepared via an in-situ co-precipitation method. The sulfone groups in PES bind to undercoordinated lead ion (Pb2+) on the CsPbBr3 NCs, resulting in significant reduction of surface defects, thus enhancing the PLQY from 74.2% to 88.3%. Meanwhile, the PES-coated NCs exhibit high water resistance and excellent heat and light stability, maintaining over 85% of the initial PL intensity under thermal aging (70°C, 4 h) and continuous 365 nm ultraviolet (UV) light irradiation (24 W, 8 h) conditions. By contrast, the PL intensity of the control NCs dramatically dropped to less than 40%. Finally, a diode emitting bright white light was fabricated utilizing the PES-coated CsPbBr3 NCs, which exhibits a color gamut of ~110% NTSC standard.

Partial Ligand Stripping from CsPbBr3 Nanocrystals Improves Their Performance in Light-Emitting Diodes

Halide perovskite nanocrystals (NCs), specifically CsPbBr3, have attracted considerable interest due to their remarkable optical properties for optoelectronic devices. To achieve high-efficiency light-emitting diodes (LEDs) based on CsPbBr3 nanocrystals (NCs), it is crucial to optimize both their photoluminescence quantum yield (PLQY) and carrier transport properties when they are deposited to form films on substrates. While the exchange of native ligands with didodecyl dimethylammonium bromide (DDAB) ligand pairs has been successful in boosting their PLQY, dense DDAB coverage on the surface of NCs should impede carrier transport and limit device efficiency. Following our previous work, here, we use oleyl phosphonic acid (OLPA) as a selective stripping agent to remove a fraction of DDAB from the NC surface and demonstrate that such stripping enhances carrier transport while maintaining a high PLQY. Through systematic optimization of OLPA dosage, we significantly improve the performance of CsPbBr3 LEDs, achieving a maximum external quantum efficiency (EQE) of 15.1% at 516 nm and a maximum brightness of 5931 cd m-2. These findings underscore the potential of controlled ligand stripping to enhance the performance of CsPbBr3 NC-based optoelectronic devices.

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%.

Flexible and Stretchable Light-Emitting Diodes and Photodetectors for Human-Centric Optoelectronics

Optoelectronic devices with unconventional form factors, such as flexible and stretchable light-emitting or photoresponsive devices, are core elements for the next-generation human-centric optoelectronics. For instance, these deformable devices can be utilized as closely fitted wearable sensors to acquire precise biosignals that are subsequently uploaded to the cloud for immediate examination and diagnosis, and also can be used for vision systems for human-interactive robotics. Their inception was propelled by breakthroughs in novel optoelectronic material technologies and device blueprinting methodologies, endowing flexibility and mechanical resilience to conventional rigid optoelectronic devices. This paper reviews the advancements in such soft optoelectronic device technologies, honing in on various materials, manufacturing techniques, and device design strategies. We will first highlight the general approaches for flexible and stretchable device fabrication, including the appropriate material selection for the substrate, electrodes, and insulation layers. We will then focus on the materials for flexible and stretchable light-emitting diodes, their device integration strategies, and representative application examples. Next, we will move on to the materials for flexible and stretchable photodetectors, highlighting the state-of-the-art materials and device fabrication methods, followed by their representative application examples. At the end, a brief summary will be given, and the potential challenges for further development of functional devices will be discussed as a conclusion.

Synergistic Surface Modification for High-Efficiency Perovskite Nanocrystal Light-Emitting Diodes: Divalent Metal Ion Doping and Halide-Based Ligand Passivation

Surface defects of metal halide perovskite nanocrystals (PNCs) substantially compromise the optoelectronic performances of the materials and devices via undesired charge recombination. However, those defects, mainly the vacancies, are structurally entangled with each other in the PNC lattice, necessitating a delicately designed strategy for effective passivation. Here, a synergistic metal ion doping and surface ligand exchange strategy is proposed to passivate the surface defects of CsPbBr3 PNCs with various divalent metal (e.g., Cd2+ , Zn2+, and Hg2+ ) acetate salts and didodecyldimethylammonium (DDA+ ) via one-step post-treatment. The addition of metal acetate salts to PNCs is demonstrated to suppress the defect formation energy effectively via the ab initio calculations. The developed PNCs not only have near-unity photoluminescence quantum yield and excellent stability but also show luminance of 1175 cd m-2 , current efficiency of 65.48 cd A-1 , external quantum efficiency of 20.79%, wavelength of 514 nm in optimized PNC light-emitting diodes with Cd2+ passivator and DDA ligand. The "organic-inorganic" hybrid engineering approach is completely general and can be straightforwardly applied to any combination of quaternary ammonium ligands and source of metal, which will be useful in PNC-based optoelectronic devices such as solar cells, photodetectors, and transistors.

Highly Bright and Stable Lead-Free Double Perovskite White Light-Emitting Diodes

Lead-free double perovskites (DPs) are emerging highly stable emitters with efficient broadband self-trapped exciton (STE) photoluminescence (PL), but their low electroluminescent (EL) efficiency is a critical shortcoming. This work promotes the external quantum efficiency (EQE) and luminance of DP-based white light-emitting diode (wLED) with a normal device structure to 0.76% and 2793 cd m-2 via two modifications: This work prevents the formation of adverse metallic silver, spatially confined STE, and lowers local site symmetry in Cs2 Na0.4 Ag0.6 In0.97 Bi0.03 Cl6 DP by terbium doping; and this work develops a guest-host strategy to improve film morphology, reduce defect density, and increase carrier mobility. These alterations cause substantial increase in STE radiative recombination and charge injection efficiency of perovskite layer. Finally, pure white EL with ideal color coordinates of (0.328, 0.329) and a record-breaking optoelectronic performance is achieved by introducing additional green carbon dots in LED to fill the deficient green component.

In situ and General Multidentate Ligand Passivation Achieves Efficient and Ultra-Stable CsPbX3 Perovskite Quantum Dots for White Light-Emitting Diodes

Inorganic CsPbX3 perovskite quantum dots (PeQDs) show great potential in white light-emitting diodes (WLEDs) due to excellent optoelectronic properties, but their practical application is hampered by low photoluminescence quantum yield (PLQY) and especially poor stability. Herein,  we developed an in-situ and general multidentate ligand passivation strategy that allows for CsPbX3 PeQDs not only near-unit PLQY, but significantly improved stability against storage, heat, and polar solvent. The enhanced optical property arises from high effectiveness of the multidentate ligand, diethylenetriaminepentaacetic acid (DTPA) with five carboxyl groups, in passivating uncoordinated Pb2+ defects and suppressing nonradiative recombination. First-principles calculations reveal that the excellent stability is attributed to tridentate binding mode of DTPA that remarkably boosts the adsorption capacity to PeQD core. Finally, combining the green and red PeQDs with blue chip,  we demonstrated highly luminous WLEDs with distinctly enhanced operation stability, a wide color gamut of 121.3% of national television system committee, standard white light of (0.33,0.33) in CIE 1931, and tunable color temperatures from warm to cold white light readily by emitters' ratio. This study provides an operando yet general approach to achieve efficient and stable PeQDs for WLEDs and accelerates their progress to commercialization.

Recent Advances in Patterning Strategies for Full-Color Perovskite Light-Emitting Diodes

Metal halide perovskites have emerged as promising light-emitting materials for next-generation displays owing to their remarkable material characteristics including broad color tunability, pure color emission with remarkably narrow bandwidths, high quantum yield, and solution processability. Despite recent advances have pushed the luminance efficiency of monochromic perovskite light-emitting diodes (PeLEDs) to their theoretical limits, their current fabrication using the spin-coating process poses limitations for fabrication of full-color displays. To integrate PeLEDs into full-color display panels, it is crucial to pattern red-green-blue (RGB) perovskite pixels, while mitigating issues such as cross-contamination and reductions in luminous efficiency. Herein, we present state-of-the-art patterning technologies for the development of full-color PeLEDs. First, we highlight recent advances in the development of efficient PeLEDs. Second, we discuss various patterning techniques of MPHs (i.e., photolithography, inkjet printing, electron beam lithography and laser-assisted lithography, electrohydrodynamic jet printing, thermal evaporation, and transfer printing) for fabrication of RGB pixelated displays. These patterning techniques can be classified into two distinct approaches: in situ crystallization patterning using perovskite precursors and patterning of colloidal perovskite nanocrystals. This review highlights advancements and limitations in patterning techniques for PeLEDs, paving the way for integrating PeLEDs into full-color panels.

Manipulation of hot-carrier cooling dynamics in CsPbBr3 quantum dots via site-selective ligand engineering

Prolonging the lifetime of photoinduced hot carriers in lead-halide perovskite quantum dots (QDs) is highly desirable because it can help improve the photovoltaic conversion efficiency. Ligand engineering has recently become a promising strategy to achieve this; nevertheless, mechanistic studies in this field remain limited. Herein, we propose a new scenario of ligand engineering featuring Pb2+/Br- site-selective capping on the surface of CsPbBr3 QDs. Through joint observations of temperature-dependent photoluminescence, ultrafast transient absorption, and Raman spectroscopy of the two contrasting model systems of CsPbBr3 QDs (i.e., capping with organic ligand only vs hybrid organic/inorganic ligands), we reveal that the phononic regulation of Pb-Br stretching at the Br-site (relative to Pb-site) leads to a larger suppression of charge-phonon coupling due to a stronger polaronic screening effect, thereby more effectively retarding the hot-carrier cooling process. This work opens a new route for the manipulation of hot-carrier cooling dynamics in perovskite systems via site-selective ligand engineering.

Wafer-scale patterning of high-resolution quantum dot films with a thickness over 10 μm for improved color conversion

Quantum dots (QDs) are promising color conversion materials for efficient full-color micro light-emitting diode (micro-LED) displays owing to their high color purity and wide color gamut. However, achieving high-resolution QD patterns with enough thickness for efficient color conversion is challenging. Here, we demonstrate a facile and compatible approach by combining replicate molding, plasma etching and transfer printing to produce QD patterns with a sufficient thickness over ten micrometers in a wide range of resolutions. Our technique can remarkably simplify the preparation of QD inks and minimize optical damage to QD materials. The pixel resolution and thickness of QD patterns can be controlled by well-defining the microstructures of the molding template and the etching process. The transfer printing process allows QD patterns to be assembled sequentially onto a receiving substrate, which will further improve the original pixel resolution and avoid repetitive optical damage to QDs during the patterning process. Consequently, various QD patterns can be fabricated in this work, including perovskite quantum dot (PQD) patterns with a pixel resolution of up to 669 pixels per inch (ppi) and a maximum thickness of up to 19.74 μm, a wafer-scale high-resolution PQD pattern with sufficient thickness on a flexible substrate, and a dual-color pattern comprising green PQDs and red CdSe QDs. Furthermore, these fabricated QD films with a thickness of over 10 μm show improved color conversion when integrated onto a blue micro-LED, revealing the potential of our technique for full-color micro-LED displays.

The role of oxygen defects in the electronic, optical and phonon dispersion of the LAGO perovskite: a density functional theory investigation

The study aims to investigate the electronic, optical and phonon dispersion properties of a pure and 2.5% O-defect induced LAGO perovskite, using density functional theory (DFT) with generalized gradient approximation (GGA) and the PBE functional. The research reveals a significant reduction in the band gap from 3.27 eV in pure LAGO to 2.18 eV in defect-induced LAGO. The defect-induced LAGO exhibits relatively strong light absorption in the visible region compared to pure LAGO. The phonon-dispersion analysis identifies one acoustic and two transverse optical mode branches. The calculated Debye temperatures for pure and defect-induced systems are 469.92 K and 463.69 K, respectively, attributed to weaker bonds in defect-induced LAGO. The findings offer fundamental insights into the impact of oxygen vacancies on the electronic, optical, and phonon properties of the LAGO perovskite that can potentially improve the electronic and optoelectronic devices operating across a wide range of spectral frequencies.

Thermal Management Enables Stable Perovskite Nanocrystal Light-Emitting Diodes with Novel Hole Transport Material

The severely insufficient operational lifetime of perovskite light-emitting diodes (LEDs) is incompatible with the rapidly increasing external quantum efficiency, even as it approaches the theoretical limit, thereby significantly impeding the commercialization of perovskite LEDs. In addition, Joule heating induces ion migration and surface defects, degrades the photoluminescence quantum yield and other optoelectronic properties of perovskite films, and induces the crystallization of charge transport layers with low glass transition temperatures, resulting in LED degradation under continuous operation. Here, a novel thermally crosslinked hole transport material, poly(FCA60 -co-BFCA20 -co-VFCA20 ) (poly-FBV), with temperature-dependent hole mobility is designed, which is advantageous for balancing the charge injection of the LEDs and limiting the generation of Joule heating. The optimised CsPbI3 perovskite nanocrystal LEDs with poly-FBV realise approximately a 2-fold external quantum efficiency increase over the LED with commercial hole transport layer poly(4-butyl-phenyl-diphenyl-amine) (poly-TPD), owing to the balanced carrier injection and suppressed exciton quenching. Moreover, because of the Joule heating control provided by the novel crosslinked hole transport material, the LED utilising crosslinked poly-FBV has a 150-fold longer operating lifetime (490 min) than that utilizing poly-TPD (3.3 min). The study opens a new avenue for the use of PNC LEDs in commercial semiconductor optoelectronic devices.

Bilayer phosphine oxide modification toward efficient and large-area pure-blue perovskite quantum dot light-emitting diodes

Blue emissive halide perovskite light-emitting diodes (LEDs) are gaining increasing attention. Reducing defects in halide perovskites to improve the performance of the resulting LEDs is a main research direction, but there are limited passivation methods for achieving efficient and spectrally-stable pure-blue LEDs based on mixed-halide perovskites. In this work, double modification layers containing phosphine oxides, i.e., diphenyl[4-(triphenylsilyl)phenyl]phosphine oxide (TSPO1) and 2,7-bis(diphenylphosphoryl)-9,9'-spirobifluorene (SPPO13), are developed to passivate mixed-halide perovskite quantum dot (QD) films. The comprehensive spectroscopic and structural characterization results indicate the presence of strong interactions between TSPO1/SPPO13 and the QDs. Besides, the combination of the bilayer exhibits a synergistic hole-blocking effect, improving the charge balance of the LEDs. LEDs based on the QD/TSPO1/SPPO13 films deliver stable electroluminesence at 469 nm and present a maximum external quantum efficiency (EQE) and luminance of 4.87% and 560 cd m-2, respectively. Benefiting from the uniform QD/TSPO1/SPPO13 film over a large area, LEDs with an area of 64 mm2 show a maximum EQE of 3.91%, which represents the first efficient large-area mixed-halide perovskite LED with stable pure-blue emission. This work provides a method to improve the perovskite QDs-based film quality and optoelectronic properties, and is a step toward the fabrication of highly-efficient large-area blue perovskite LEDs.

A ligand strategy retarding monovalent copper oxidation toward achieving Cs3Cu2I5 perovskite emitters with enhanced stability for lighting

0D copper-based perovskites (Cs3Cu2I5) have fascinating optical properties, such as strong exciton binding energy, high photoluminescence quantum yield (PLQY) and large Stokes shifts from self-trapped excitons (STEs), which make them highly considerable candidates in the field of lighting. However, the stability of Cs3Cu2I5 is compromised by the oxidation of Cu+ to Cu2+ during the storage or operation process. Here, we proposed a ligand engineering strategy to improve the stability of Cs3Cu2I5via an organic molecule (ethylenediaminetetraacetic acid, EDTA) with multiple functional groups. The strong interaction between carboxyl groups and Cu+ was evidenced through FTIR and XPS, and it could retard monovalent copper oxidation. After storing for 90 days, the EDTA-engineered Cs3Cu2I5 (EDTA-Cs3Cu2I5) maintained its original crystalline structure, while the control Cs3Cu2I5 exhibited an impurity phase. Through quantitative analysis, the content of Cu2+ in EDTA-Cs3Cu2I5 was found to be 83.9% lower than that in control Cs3Cu2I5. Benefiting from the inhibition of Cu+ oxidation, EDTA-Cs3Cu2I5 exhibited improved light emission stability. For example, the optimized EDTA-Cs3Cu2I5 retained 74.7% of the initial photoluminescence (PL) intensity after 90-day storage under ambient conditions, while the pure Cs3Cu2I5 retained only 41.7%. Furthermore, EDTA could passivate defects and enhance the PL properties of the optimized Cs3Cu2I5, which showed a PLQY of 94.7%, much higher than that of 71.4% for pure Cs3Cu2I5. We further constructed a WLED based on the EDTA-engineered Cs3Cu2I5, which showed CIE at (0.3238, 0.3354), a CRI of 91.7, and a T50 of 361 h. The proposed EDTA ligand strategy provides a new way to regulate the light-emitting properties and stabilities of Cs3Cu2I5 for future industrialization.

Luminescence and Degeneration Mechanism of Perovskite Light-Emitting Diodes and Strategies for Improving Device Performance

Perovskite light-emitting diodes (PeLEDs) can be a promising technology for next-generation display and lighting applications due to their excellent optoelectronic properties. However, a systematical overview of luminescence and degradation mechanism of perovskite materials and PeLEDs is lacking. Therefore, it is crucial to fully understand these mechanisms and further improve device performances. In this work, the fundamental photophysical processes of perovskite materials, electroluminescence mechanism of PeLEDs including carrier kinetics and efficiency roll-off as well as device degradation mechanism are discussed in detail. In addition, the strategies to improve device performances are summarized, including optimization of photoluminescence quantum yield, charge injection and recombination, and light outcoupling efficiency. It is hoped that this work can provide guidance for future development of PeLEDs and ultimately realize industrial applications.

CsPbCl3 → CsPbI3 Exchange in Perovskite Nanocrystals Proceeds through a Jump-the-Gap Reaction Mechanism

Halide exchange is a popular strategy to tune the properties of CsPbX3 nanocrystals after synthesis. However, while Cl → Br and Br → I exchanges proceed through the formation of stable mixed-halide nanocrystals, the Cl ⇌ I exchange is more elusive. Indeed, the large size difference between chloride and iodide ions causes a miscibility gap in the CsPbCl3-CsPbI3 system, preventing the isolation of stable CsPb(ClxI1-x)3 nanocrystals. Yet, previous works have claimed that a full CsPbCl3 → CsPbI3 exchange can be achieved. Even more interestingly, interrupting the exchange prematurely yields a mixture of CsPbCl3 and CsPbI3 nanocrystals that coexist without undergoing further transformation. Here, we investigate the reaction mechanism of CsPbCl3 → CsPbI3 exchange in nanocrystals. We show that the reaction proceeds through the early formation of iodide-doped CsPbCl3 nanocrystals covered by a monolayer shell of CsI. These nanocrystals then leap over the miscibility gap between CsPbCl3 and CsPbI3 by briefly transitioning to short-lived and nonrecoverable CsPb(ClxI1-x)3 nanocrystals, which quickly expel the excess chloride and turn into the chloride-doped CsPbI3 nanocrystals found in the final product.

Colloidal semiconductor nanocrystals: from bottom-up nanoarchitectonics to energy harvesting applications

Colloidal semiconductor nanocrystals (NCs) have been extensively investigated owing to their unique properties induced by the quantum confinement effect. The advent of colloidal synthesis routes led to the design of stable colloidal NCs with uniform size, shape, and composition. Metal oxides, phosphides, and chalcogenides (ZnE, CdE, PbE, where E = S, Se, or Te) are few of the most important monocomponent semiconductor NCs, which show excellent optoelectronic properties. The ability to build quantum confined heterostructures comprising two or more semiconductor NCs offer greater customization and tunability of properties compared to their monocomponent counterparts. More recently, the halide perovskite NCs showed exceptional optoelectronic properties for energy generation and harvesting applications. Numerous applications including photovoltaic, photodetectors, light emitting devices, catalysis, photochemical devices, and solar driven fuel cells have demonstrated using these NCs in the recent past. Overall, semiconductor NCs prepared via the colloidal synthesis route offer immense potential to become an alternative to the presently available device applications. This feature article will explore the progress of NCs syntheses with outstanding potential to control the shape and spatial dimensionality required for photovoltaic, light emitting diode, and photocatalytic applications. We also attempt to address the challenges associated with achieving high efficiency devices with the NCs and possible solutions including interface engineering, packing control, encapsulation chemistry, and device architecture engineering.

Multifunctional Short-Chain 2-Thiophenealkylammonium Bromide Ligand-Assisted Perovskite Quantum Dots for Efficient Light-Emitting Diodes

Lead halide perovskite quantum dots (QDs) have attracted great interest for application in light-emitting diodes (LEDs) due to their high photoluminescence quantum yield (PLQY), solution processability, and high color purity, showing great potential for next-generation full-color display and lighting technologies. Conventional long-chain insulating oleic acid (OA)/oleamine (OAm) ligands exhibit dynamic binding to the surface of QDs, resulting in a plethora of extra surface defects and inferior optoelectronic properties. Herein, a sole multifunctional ligand with optimized carbon chain length, that is, 2-thiophenepropylamine bromide (ThPABr), was creatively designed and introduced into CsPbBr3 QDs, which not only replaces OAm and provides a bromine source but also coordinates with the uncoordinated surface Pb2+ of QDs through the thiophene, passivating surface defects and increasing the PLQY of the film to 83%. More importantly, the interaction between the electron donor-thiophene ring and QDs can enhance electron injection and improve carrier balance. The resulting green LED exhibited significant performance improvement, showing ultrahigh spectral stability under high operating voltage, achieving a maximum external quantum efficiency of 10.5%, and extending the operating lifetime to 5-fold that of the reference. Designing a single multifunctional ligand presents a promising and convenient strategy for selecting surface ligands that can enhance the performance of LEDs or other optoelectronic devices.

Interface-Induced Crystallinity Enhancement of Perovskite Quantum Dots for Highly Efficient Light-Emitting Diodes

Perovskite quantum dot light-emitting diodes (QLEDs) with high color purity and wide color gamut have good application prospects in the next generation of display technology. However, colloidal perovskite quantum dots (PQDs) may introduce a large number of defects during the film-forming process, which is not conducive to the luminous efficiency of the device. Meanwhile, the disordered film formation of PQDs will form interfacial defects and reduce the device performance. Here, we report an interface-induced crystallinity enhancement (IICE) strategy to increase the crystallinity of PQDs at the hole transport layer (HTL)/PQD interface. As a result, both the Br- vacancies in the PQD film and the interfacial defects were well passivated and the leakage current was also suppressed. We achieved QLEDs with a maximum external quantum efficiency (EQE) of 16.45% and current efficiency (CE) of 61.77 cd/A, showing improved performance to more than twice that of the control devices. The IICE strategy paves a new way to enhance the crystallinity of PQD films, so as to improve the performance of QLEDs for application in the future display field.

Synergistic Halide- and Ligand-Exchanges of All-Inorganic Perovskite Nanocrystals for Near-Unity and Spectrally Stable Red Emission

All-inorganic perovskite nanocrystals (NCs) of CsPbX3 (X = Cl, Br, I) are promising for displays due to wide color gamut, narrow emission bandwidth, and high photoluminescence quantum yield (PLQY). However, pure red perovskite NCs prepared by mixing halide ions often result in defects and spectral instabilities. We demonstrate a method to prepare stable pure red emission and high-PLQY-mixed-halide perovskite NCs through simultaneous halide-exchange and ligand-exchange. CsPbBr3 NCs with surface organic ligands are first synthesized using the ligand-assisted reprecipitation (LARP) method, and then ZnI2 is introduced for anion exchange to transform CsPbBr3 to CsPbBrxI3-x NCs. ZnI2 not only provides iodine ions but also acts as an inorganic ligand to passivate surface defects and prevent ion migration, suppressing non-radiative losses and halide segregation. The luminescence properties of CsPbBrxI3-x NCs depend on the ZnI2 content. By regulating the ZnI2 exchange process, red CsPbBrxI3-x NCs with organic/inorganic hybrid ligands achieve near-unity PLQY with a stable emission peak at 640 nm. The CsPbBrxI3-x NCs can be combined with green CsPbBr3 NCs to construct white light-emitting diodes with high-color gamut. Our work presents a facile ion exchange strategy for preparing spectrally stable mixed-halide perovskite NCs with high PLQY, approaching the efficiency limit for display or lighting applications.

All-inorganic lead halide perovskite nanocrystals applied in advanced display devices

Advanced display devices are in greater demand due to their large color gamut, high color purity, ultrahigh visual resolution, and small size pixels. All-inorganic lead halide perovskite (AILHP) nanocrystals (NCs) possess inherent advantages such as narrow emission width, saturated color, and flexible integration, and have been developed as functional films, light sources, backlight components, and display panels. However, some drawbacks still restrict the practical application of advanced display devices based on AILHP NCs, including working stability, large-scale synthesis, and cost. In this review, we focus on AILHP NCs, review the recent progress in materials synthesis, stability improvement, patterning techniques, and device application. We also highlight the important role of materials systems in creating advanced display devices, followed by the challenges and opportunities in industrial processes. This review provides beneficial inspiration for the future development of AILHP NCs in colorful and white backlight, as well as high resolution full-color displays.

CsPbBr3 and Cs4PbBr6 perovskite light-emitting diodes using a thermally evaporated host-dopant system

This article shows the results of fabricating a device through vacuum deposition by synthesizing a perovskite thin film in the powder form. Light emitting diodes (LEDs) were fabricated using a single-source and host-dopant system of the perovskite produced in the powder form. Both CsPbBr₃ and Cs₄PbBr₆ used in the host-dopant system were green, and the host was tris(8-quinolinolato) aluminum(III). It is confirmed that the display efficiency and optical characteristics are significantly improved by the dopant ratio. The 3%-doped CsPbBr₃ based LED shows a luminance of 9083 cd m^-2, 3.36% external quantum efficiency (EQE), and 96% photoluminescence quantum yield (PLQY) efficiency (for the undoped CsPbBr₃ LED, luminance: 844 cd m^-2/EQE: 1.93%/PLQY: 85%). The LED based on 5%-doped Cs₄PbBr₆ shows a luminance of 11 440 cd m^-2, an EQE of 6.27%, and 99% PLQY efficiency (for the undoped Cs₄PbBr₆ LED, luminance:1113 cd m^-2/EQE: 1.64%/PLQY: 93%). It is expected that the results of this research will contribute to the perovskite LED research performed by thermal evaporation in the future.

Surface Reconstruction of CsPbBr3 Nanocrystals by the Ligand Engineering Approach for Achieving High Quantum Yield and Improved Stability

Oleylamine/oleic acid (OAm/OA) as the commonly used ligand is indispensable in the synthesis of perovskite nanocrystals (PNCs). Unfortunately, poor colloidal stability and unsatisfactory photoluminescence quantum yield (PLQY) are observed, resulting from a highly dynamic binding nature between ligands. Herein, we adopt a facile hybrid ligand (DDAB/ZnBr₂) passivation strategy to reconstruct the surface chemistry of CsPbBr₃ NCs. The hybrid ligand can detach the native surface ligand, in which the acid-base reactions between ligands are suppressed effectively. Also, they can substitute the loose capping ligand, anchor to the surface firmly, and supply sufficient halogens to passivate the surface trap, realizing an exceptional PLQY of 95% and an enhanced tolerance toward ambient storage, UV irradiation, anti-solvents, and thermal treatment. Besides, the as-fabricated white light-emitting diode (WLED) utilizing the PNCs as the green-emitting phosphor has a luminous efficiency around 73 lm/W; the color gamut covers 125% of the NTSC standard.

Pseudohalogen Resurfaced CsPbBr3 Nanocrystals for Bright, Efficient, and Stable Green-Light-Emitting Diodes

Lead halide perovskite nanocrystals (LHP NCs) are regarded as promising emitters for next-generation ultrahigh-definition displays due to their high color purity and wide color gamut. Recently, the external quantum efficiency (EQE) of LHP NC based light-emitting diodes (PNC LEDs) has been rapidly improved to a level required by practical applications. However, the poor operational stability of the device, caused by halide ion migration at the grain boundary of LHP NC thin films, remains a great challenge. Herein, we report a resurfacing strategy via pseudohalogen ions to mitigate detrimental halide ion migration, aiming to stabilize PNC LEDs. We employ a thiocyanate solution processed post-treatment method to efficiently resurface CsPbBr₃ NCs and demonstrate that the thiocyanate ions can effectively inhibit bromide ion migration in LHP NC thin films. Owing to thiocyanate resurfacing, we fabricated LEDs with a high EQE of 17.3%, a maximum brightness of 48000 cd m^-2, and an excellent operation half-life time.

A and B sites dual substitution by Na+ and Cu2+ co-doping in CsPbBr3 quantum dots to achieve bright and stable blue light emitting diodes

Light-emitting perovskite quantum dots (PeQDs) are extensively investigated owing to their evident merits. However, it is still a challenge to adjust their intrinsic emissions and enhance their thermal stability to achieve full-color highly emissive QD-based light-emitting diodes (QLEDs), especially blue QLEDs. Herein, we demonstrate an effective strategy to fundamentally stabilize the crystal structure of CsPbBr₃ QDs by codoping Na⁺ and Cu²⁺ ions, which are designed to substitute Cs⁺ (A sites) and Pb²⁺ (B sites), respectively. It is found out that the codoping metal ions have significantly improved the thermal stability and the optical properties of the QDs. 40% of the emission intensity can be remained after 8 thermal cycles (20-120 °C) for CsPbBr₃: Na⁺/Cu²⁺ QDs, whilst less than 10% is maintained for undoped CsPbBr₃ QDs. Accordingly, stable blue QLEDs are packed by CsPbBr₃: Na⁺/Cu²⁺ QDs. Strong electroluminescence with the maximum luminance of 7161 cd m^-2 and low turn-on voltage of 2.4 V are realized. The CIE coordinates are tuned from green (0.10, 0.74) to blue (0.17, 0.25) via Na⁺ and Cu²⁺ codoping. The maximum external quantum efficiency (EQE_max) is obtained as 4.52% for PeLEDs based on codoped QDs. The proposed metal ions A and B sites dual substitution strategy guarantees PeQDs as an extremely promising prospect in potential applications as high-resolution displays and high-quality lightings.

Challenges and developments for the blue perovskite nanocrystal light-emitting diodes

Perovskite nanomaterials have been highly thought as next-generation light emitters after recent development owing to their benefits of simple synthesis, low-cost, large-area, and wide color gamut. Encouragingly, the external quantum efficiencies (EQEs) of green, red, and near-infrared perovskite light-emitting diodes (PeLEDs) have exceeded more than 20%. However, the performance of the blue PeLEDs is still lower than other analogs, which severely limits the applications of PeLEDs in future full-color displays. Herein, we have reviewed the advances in blue perovskite NCs and their applications in blue PeLEDs. Promising blue perovskite emitters and strategies for fabricating highly efficient blue PeLEDs based on perovskite NCs are investigated and highlighted. Moreover, we point out the main challenges in blue perovskite NC LEDs including low electroluminescence efficiency (EL), spectral instability, the difficulty of charge injection, and device optimization. The perspectives for the further development of blue PeLEDs are also presented.

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.