Efficient and Spectrally Stable Blue Perovskite Light-Emitting Diodes Employing a Cationic π-Conjugated Polymer

Chemical Reaction Modulated Low-Dimensional Phase Toward Highly Efficient Sky-Blue Perovskite Light-Emitting Diodes

Blue perovskite light-emitting diodes (PeLEDs) are crucial avenues for achieving full-color displays and lighting based on perovskite materials. However, the relatively low external quantum efficiency (EQE) has hindered their progression towards commercial applications. Quasi-two-dimensional (quasi-2D) perovskites stand out as promising candidates for blue PeLEDs, with optimized control over low-dimensional phases contributing to enhanced radiative properties of excitons. Herein, the impact of organic molecular dopants on the crystallization of various n-phase structures in quasi-2D perovskite films. The results reveal that the highly reactive bis(4-(trifluoromethyl)phenyl)phosphine oxide (BTF-PPO) molecule could effectively restrain the formation of organic spacer cation-ordered layered perovskite phases through chemical reactions, simultaneously passivate those uncoordinated Pb2+ defects. Consequently, the prepared PeLEDs exhibited a maximum EQE of 16.6 % (@ 490 nm). The finding provides a new route to design dopant molecules for phase modulation in quasi-2D PeLEDs.

Near-unity quantum yield and long-term emission stability in halide perovskite nanocrystal glass composite

Metal halide perovskites are promising optoelectronic materials due to their outstanding luminescent properties. However, the instability of perovskites has long been the bottleneck to their practical applications. Here Cs4PbBr6 nanocrystals based glass composite (Cs4PbBr6 NCs@glass) are successfully prepared, which displays green emission color (520 nm), narrow bandwidth (23 nm) and a near-unity photoluminescence quantum yield (PLQY). The H2O molecules permeating in the lattice of Cs4PbBr6 were found to be a crucial role in the subband energy emission. The Cs4PbBr6 NCs@glass has excellent emission stability; maintains 93 % of initial PL intensity after ultraviolet light irradiation for over 5000 h. In addition, by adjusting the halogen content, we have achieved tunable emission color from blue (450 nm) to green (520 nm) and red (670 nm) on Cs4PbX6 NCs@glass (X = Cl, Br, I), which covers up to 127 % of the National Television Systems Board (NTSC) standard system. Our finding indicates the commercial applications of perovskite materials in lighting and display.

Synergistic Steric Effect of Precursor And Antisolvent Enables Strongly Confined Perovskite Films with Efficient and Spectral Stable Blue Emission

Reducing the crystal size of perovskites to the strong quantum confinement regime is an effective way to realize blue luminescence for light-emitting applications. However, challenges remain in directly constraining the crystal growth during film preparation to achieve three-dimensional quantum confinement, and the widely used long-chain ligands may bring difficulties for charge transport and unfavorably affect the device performance. Herein, we report a novel strategy for fabricating strongly confined blue-emitting perovskite nanocrystalline films via synergistic steric effect modulation by precursors and antisolvents. We synthesize cesium pentafluoropropanoate (CsPFPA) as a new type of precursor agent, where the steric effect of the PFPA group can help constrain the growth of perovskite crystals and passivate the defects. Furthermore, different types of antisolvents with varied molecular sizes and steric hindrance are used to regulate the size of perovskite crystals and improve film quality. Consequently, highly emissive blue perovskite films are realized with the emission wavelength effectively tuned in the blue region by varying the concentration of CsPFPA as well as the type of antisolvents. Based on the strongly confined perovskite films, blue light-emitting diodes (LEDs) are constructed, showing good spectral tunability and stability in the electroluminescence. This work demonstrates a novel pathway for developing bright perovskite blue emitters for LEDs, which may potentially advance their future applications in display and lighting.

Buried interface modification and light outcoupling strategy for efficient blue perovskite light-emitting diodes

Perovskite light-emitting diodes (PeLEDs) exhibit remarkable potential in the field of displays and solid-state lighting. However, blue PeLEDs, a key element for practical applications, still lag behind their green and red counterparts, due to a combination of strong nonradiative recombination losses and unoptimized device structures. In this report, we propose a buried interface modification strategy to address these challenges by focusing on the bottom-hole transport layer (HTL) of the PeLEDs. On the one hand, a multifunctional molecule, aminoacetic acid hydrochloride (AACl), is introduced to modify the HTL/perovskite interface to regulate the perovskite crystallization. Experimental investigations and theoretical calculations demonstrate that AACl can effectively reduce the nonradiative recombination losses in bulk perovskites by suppressing the growth of low-n perovskite phases and also the losses at the bottom interface by passivating interfacial defects. On the other hand, a self-assembly nanomesh structure is ingeniously developed within the HTLs. This nanomesh structure is meticulously crafted through the blending of poly-(9,9-dioctyl-fluorene-co-N-(4-butyl phenyl) diphenylamine) and poly (n-vinyl carbazole), significantly enhancing the light outcoupling efficiency in PeLEDs. As a result, our blue PeLEDs achieve remarkable external quantum efficiencies, 20.4% at 487 nm and 12.5% at 470 nm, which are among the highest reported values. Our results offer valuable insights and effective methods for achieving high-performance blue PeLEDs.

Efficient Quasi-2D Perovskite Based Blue Light-Emitting Diodes with Carbon Dots Modified Hole Transport Layer

Quasi-2D perovskites based blue light-emitting diodes (LEDs) suffer from its poor electroluminescence performance, mainly caused by the nonradiative recombination in in defect-rich low-n phases and the unbalanced hole-electron injection in the device. Here, we developed a highly efficient quasi-2D perovskite based sky-blue LEDs behaving recorded external quantum efficiency (EQE) of 21.07% by employing carbon dots (CDs) as additives in the hole transport layer (HTL). We ascribe the high EQE to the effective engineering of CDs: (1) The CDs at the interface of HTLs can suppress the formation of low-efficient n = 1 phase, resulting a high luminescence quantum yield and energy transfer efficiency of the mixed n-phase quasi-2D perovskites. (2) The CDs additives can reduce the conductivity of HTL, partially blocking the hole injection, and thus making more balanced hole-electron injection. The CDs-treated devices have excellent Spectral stability and enhanced operational stability and could be a new alternative additive in the perovskite optoelectronic devices.

High-n Phase Suppression for Efficient and Stable Blue Perovskite Light-Emitting Diodes

Quasi-2D perovskites light-emitting diodes (PeLEDs) have achieved significant progress due to their superior optical and electronic properties. However, the blue PeLEDs still exist inefficient energy transfer and electroluminescence performance caused by mixed multidimensional phase distribution. In this work, transition metal salt (zinc bromide, ZnBr2) is introduced to modulate phase distributions by suppressing the nucleation of high n phase perovskites, which effectively shortens the energy transfer path for blue emission. Moreover, ZnBr2 also facilitates energy level matching and reduces non-radiative recombination, thus improving electroluminescence (EL) efficiency. Benefiting from these combined improvements, an efficient blue PeLEDs is obtained with a maximum external quantum efficiency (EQE) of 16.2% peaking located at 486 nm. This work provides a promising approach to tune phase distribution of quasi-2D perovskites and achieving highly efficient blue PeLEDs.

Monoammonium Modified Dion-Jacobson Quasi-2D Perovskite for High Efficiency Pure-Blue Light Emitting Diodes

Quasi-2D perovskites exhibit impressive optoelectronic properties and hold significant promise for future light-emitting devices. However, the efficiency of perovskite light-emitting diodes (PeLEDs) is seriously limited by defect-induced nonradiative recombination and imbalanced charge injection. Here, the defect states are passivated and charge injection balance is effectively improved by introducing the additive cyclohexanemethylammonium (CHMA) to bromide-based Dion-Jacobson (D-J) structure quasi-2D perovskite emission layer. CHMA participates in the crystallization of perovskite, leading to high quality film composed of compact and well-contacted grains with enhanced hole transportation and less defects. As a result, the corresponding PeLEDs exhibit stable pure blue emission at 466 nm with a maximum external quantum efficiency (EQE) of 9.22%. According to current knowledge, this represents the highest EQE reported for pure-blue PeLEDs based on quasi-2D bromide perovskite thin films. These findings underscore the potential of quasi-2D perovskites for advanced light-emitting devices and pave the way for further advancements in PeLEDs.

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.

Vertically Concentrated Quantum Wells Enabling Highly Efficient Deep-Blue Perovskite Light-Emitting Diodes

Deep-blue perovskite light-emitting diodes (PeLEDs) based on quasi-two-dimensional (quasi-2D) systems exist heightened sensitivity to the domain distribution. The top-down crystallization mode will lead to a vertical gradient distribution of quantum well (QW) structure, which is unfavorable for deep-blue emission. Herein, a thermal gradient annealing treatment is proposed to address the polydispersity issue of vertical QWs in quasi-2D perovskites. The formation of large-n domains at the upper interface of the perovskite film can be effectively inhibited by introducing a low-temperature source in the annealing process. Combined with the utilization of NaBr to inhibit the undesirable n=1 domain, a vertically concentrated QW structure is ultimately attained. As a result, the fabricated device delivers a narrow and stable deep-blue emission at 458 nm with an impressive external quantum efficiency (EQE) of 5.82 %. Green and sky-blue PeLEDs with remarkable EQE of 21.83 % and 17.51 % are also successfully achieved, respectively, by using the same strategy. The findings provide a universal strategy across the entire quasi-2D perovskites, paving the way for future practical application of PeLEDs.

Brightening deep-blue perovskite light-emitting diodes: A path to Rec. 2020

Deep-blue perovskite light-emitting diodes (PeLEDs) of high purity are highly sought after for next-generation displays complying with the Rec. 2020 standard. However, mixed-halide perovskite materials designed for deep-blue emitters are prone to halide vacancies, which readily occur because of the low formation energy of chloride vacancies. This degrades bandgap instability and performance. Here, we propose a chloride vacancy-targeting passivation strategy using sulfonate ligands with different chain lengths. The sulfonate groups have a strong affinity for lead(II) ions, effectively neutralizing vacancies. Our strategy successfully suppressed phase segregation, yielding color-stable deep-blue PeLEDs with an emission peak at 461 nanometers and a maximum luminance (Lmax) of 2707 candela per square meter with external quantum efficiency (EQE) of 3.05%, one of the highest for Rec. 2020 standard-compliant deep-blue PeLEDs. We also observed a notable increase in EQE up to 5.68% at Lmax of 1978 candela per square meter with an emission peak at 461 nanometers by changing the carbon chain length.

Improving the Operational Lifetime of Metal-Halide Perovskite Light-Emitting Diodes with Dimension Control and Ligand Engineering

Perovskite light-emitting diodes (LEDs) have emerged as one of the most propitious candidates for next-generation lighting and displays, with the highest external quantum efficiency (EQE) of perovskite LEDs already surpassing the 20% milestone. However, the further development of perovskite LEDs primarily relies on addressing operational instability issues. This Perspective examines some of the key factors that impact the lifetime of perovskite LED devices and some representative reports on recent advancements aimed at improving the lifetime. Our analysis underscores the significance of "nano" strategies in achieving long-term stable perovskite LEDs. Significant efforts must be directed toward proper device encapsulation, perovskite material passivation, interfacial treatment to address environment-induced material instability, bias-induced phase separation, and ion migration issues.

In Situ Hydrolysis of Phosphate Enabling Sky-Blue Perovskite Light-Emitting Diode with EQE Approaching 16.32

The performance of blue perovskite light-emitting diodes (PeLEDs) lags behind the green and red counterparts owing to high trap density and undesirable red shift of the electroluminescence spectrum under operation conditions. Organic molecular additives were employed as passivators in previous reports. However, most commonly have limited functions, making it challenging to effectively address both efficiency and stability issues simultaneously. Herein, we reported an innovatively dynamic in situ hydrolysis strategy to modulate quasi-2D sky-blue perovskites by the multifunctional passivator phenyl dichlorophosphate that not only passivated the defects but also underwent in situ hydrolysis reaction to stabilize the emission. Moreover, hydrolysis products were beneficial for low-dimensional phase manipulation. Eventually, we obtained high-performance sky-blue PeLEDs with a maximum external quantum efficiency (EQE) of 16.32% and an exceptional luminance of 5740 cd m-2. More importantly, the emission peak of devices located at 485 nm remained stable under different biases. Our work signified the significant advancement toward realizing future applications of PeLEDs.

Multi-Species Surface Reconstruction for High-Efficiency Perovskite Nanocrystal Light-Emitting Diodes

Although colloidal perovskite nanocrystal (PNC) solution has exhibited near-unity photoluminescence quantum yield (PLQY), the luminance would be severely quenched when the PNC solution is assembled into thin films due to the agglomeration and fusion of NCs caused by the exfoliation of surface ligands and non-radiative Förster resonance energy transfer (FRET) from small to large particle sizes, which seriously affected the performances of light-emitting diodes (LEDs). Here, we used Guanidine thiocyanate (GASCN) and Sodium thiocyanate (NaSCN) to achieve effective CsPbI3 PNC surface reconstruction. Due to the strong coordination ability of these small molecules with the anions and cations on the surface of the PNCs, they can provide strong surface protection against PNC fusion during centrifugal purification process and repair the surface defects of PNCs, so that the original uniform size distribution of PNCs can be maintained and FRET between close-packed PNC films is effectively suppressed, which allows the emission characteristics of the films to be preserved. As a result, highly oriented, smooth and nearly defect-free high-quality PNC thin films are obtained, with PLQY as high as 95.1 %, far exceeding that of the original film, and corresponding LEDs exhibit a maximum external quantum efficiency of 24.5 %.

Progress and Application of Halide Perovskite Materials for Solar Cells and Light Emitting Devices

Halide perovskite materials have attracted worldwide attention in the photovoltaic area due to the rapid improvement in efficiency, from less than 4% in 2009 to 26.1% in 2023 with only a nanometer lever photo-active layer. Meanwhile, this nova star found applications in many other areas, such as light emitting, sensor, etc. This review started with the fundamentals of physics and chemistry behind the excellent performance of halide perovskite materials for photovoltaic/light emitting and the methods for preparing them. Then, it described the basic principles for solar cells and light emitting devices. It summarized the strategies including nanotechnology to improve the performance and the application of halide perovskite materials in these two areas: from structure-property relation to how each component in the devices affects the overall performance. Moreover, this review listed the challenges for the future applications of halide perovskite materials.

Top-Down Exfoliation Process Constructing 2D/3D Heterojunction toward Ultrapure Blue Perovskite Light-Emitting Diodes

3D perovskites with low energy disorder and high ambipolar charge mobility represent a promising solution for efficient and bright light-emitting diodes. However, the challenges of regulating the nanocrystal size to trigger the quantum confinement effect and control the surface trap states to reduce charge loss hinder the applications of 3D perovskites in blue perovskite light-emitting diodes (PeLEDs). In this study, we present a top-down exfoliation method to obtain blue 3D perovskite films with clipped nanocrystals and tunable bandgaps by employing methyl cyanide (MeCN) for post-treatment. In this method, the MeCN solvent exfoliates the surface components of the 3D perovskite grains through a partial dissolution process. Moreover, the dissolved precursor can be further utilized to construct an ingenious 2D/3D heterostructure by incorporating an organic spacer into the MeCN solvent, contributing to efficient defect passivation and improved energy transfer. Consequently, efficient PeLEDs featuring ultrapure blue emission at 478 nm achieve a record external quantum efficiency of 12.3% among their 3D counterparts. This work emphasizes the significance of inducing the quantum confinement effect in 3D perovskites for efficient blue PeLEDs and provides a viable scheme for the in situ regulation of perovskite crystals.

Investigating the Mobility-Compressibility Properties of Conjugated Polymers by the Contact Film Transfer Method with Prestrain

Up to now, researches on the mobility-stretchability of semiconducting polymers are extensively investigated, but little attention was  paid to their morphology and field-effect transistor characteristics under compressive strains, which is equally crucial in wearable electronic applications. In this work, a contact film transfer method is applied to evaluate the mobility-compressibility properties of conjugated polymers. A series of isoindigo-bithiophene conjugated polymers with symmetric carbosilane side chains (P(SiSi)), siloxane-terminated alkyl side chains (P(SiOSiO)), and combined asymmetric side chains (P(SiOSi)) are investigated. Accordingly, a compressed elastomer slab is used to transfer and compress the polymer films by releasing prestrain, and the morphology and mobility evolutions of these polymers are tracked. It is found that P(SiOSi) outperforms the other symmetric polymers including P(Si─Si) and P(SiO─SiO), having the ability to dissipate strain with its shortened lamellar spacing and orthogonal chain alignment. Notably, the mechanical durability of P(SiOSi) is also enhanced after consecutive compress-release cycles. In addition, the contact film transfer technique is demonstrated to be applicable to investigate the compressibility of different semiconducting polymers. These results demonstrate a comprehensive approach to understand the mobility-compressibility properties of semiconducting polymers under tensile and compressive strains.

Efficient sky-blue cesium lead bromide light-emitting diodes with enhanced stability via synergistic interfacial induction and polymer scaffold inhibition

All inorganic CsPbX3 perovskite has aroused broad interests in building efficient light-emitting devices with wide color gamut and flexible fabrication process. So far, the realization of high-performance blue perovskite light-emitting devices (PeLEDs) is still a critical challenge. Herein, we propose an interfacial induction strategy to generate low-dimensional CsPbBr3 with sky blue emission by employing γ-aminobutyric acid (GABA) modified poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The interaction between GABA and Pb2+ inhibited the formation of bulk CsPbBr3 phase. Further assisted by the polymer networks, the sky-blue CsPbBr3 film exhibited much improved stability under both photoluminescence and electrical excitation. This can be ascribed to the scaffold effect and the passivation function of the polymer. Consequently, the obtained sky-blue PeLEDs exhibited an average external quantum efficiency (EQE) of 5.67% (maximum of 7.21%) with a maximum brightness of 3308 cd/m2 and a working lifespan reaching 0.41 h. The strategy in this work provides a new opportunity for exploitation the full potential of blue PeLEDs towards application in lighting and display devices.

Fluorine Passivation Inhibits "Particle Talking" Behaviors under Thermal and Electrical Conditions of Pure Blue Mixed Halide Perovskite Nanocrystals

Owing to outstanding optoelectronic properties, lead halide perovskite nanocrystals (PNCs) are considered promising emitters for next-generation displays. However, the development of pure blue (460-470 nm) perovskite nanocrystal light-emitting diodes (PNC-LEDs), which correspond to the requirements of Rec. 2020 standard, lag far behind that of their green and red counterparts. Here, pure blue CsPb(Br/Cl)3 nanocrystals with remarkable optical performance are demonstrated by a facile fluorine passivation strategy. Prominently, the fluorine passivation on halide vacancies and strong bonding of Pb-F intensely enhance crystal structure stability and inhibit "particle talking" behaviors under both thermal and electrical conditions. Fluorine-based PNCs with high resistance of luminescence thermal quenching retain 70% of photoluminescent intensity when heated to 343 K, which can be attributed to the elevated activation energy for carrier trapping and unchanged grain size. Fluorine-based PNC-LEDs also exhibit stable pure blue electroluminescence (EL) emission with sevenfold promoted luminance and external quantum efficiencies (EQEs), where the suppression of ion migration is further evidenced by a lateral structure device with applied polarizing potential.

Self-assembled monolayer-based blue perovskite LEDs

Blue perovskite light-emitting diodes (LEDs) have shown external quantum efficiencies (EQEs) of more than 10%; however, devices that emit in the true blue-those that accord with the emission wavelength required for Rec. 2100 primary blue-have so far been limited to EQEs of ~6%. We focused here on true blue emitting CsPbBr3 colloidal nanocrystals (c-NCs), finding in early studies that they suffer from a high charge injection barrier, a problem exacerbated in films containing multiple layers of nanocrystals. We introduce a self-assembled monolayer (SAM) active layer that improves charge injection. We identified a bifunctional capping ligand that simultaneously enables the self-assembly of CsPbBr3 c-NCs while passivating surface traps. We report, as a result, SAM-based LEDs exhibit a champion EQE of ~12% [CIE of (0.132, 0.069) at 4.0 V with a luminance of 11 cd/m2], and 10-fold-enhanced operating stability relative to the best previously reported Rec. 2100-blue perovskite LEDs.

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

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

High Color-Purity and Efficient Pure-Blue Perovskite Light-Emitting Diodes Based on Strongly Confined Monodispersed Quantum Dots

Here, we develop an in situ photoluminescence (PL) system to monitor the nucleation and growth of perovskite nanocrystals and control the monomer supply rate to achieve strongly confined and monodispersed quantum dots (QDs) with average size of 3.4 nm. Pure-blue (460 nm wavelength) CsPbBr₃ QDs with near unity PL quantum yield and narrow size distribution (small size dispersion of 9.6%) were thus produced. Light-emitting diodes (LEDs) based on these QDs were prepared by using an all-solution processing route, which showed narrow electroluminescence with full width at half-maximum of 20 nm and a high color purity of 97.3%. The device also had a high external quantum efficiency of 10.1%, maximum luminance of 11 610 cd m^-2, and continuous operation lifetime of 21 h at the initial luminance of 102 cd m^-2, corresponding to the state-of-art for pure-blue perovskite LEDs.

In-situ growth of low-dimensional perovskite-based insular nanocrystals for highly efficient light emitting diodes

Regulation of perovskite growth plays a critical role in the development of high-performance optoelectronic devices. However, judicious control of the grain growth for perovskite light emitting diodes is elusive due to its multiple requirements in terms of morphology, composition, and defect. Herein, we demonstrate a supramolecular dynamic coordination strategy to regulate perovskite crystallization. The combined use of crown ether and sodium trifluoroacetate can coordinate with A site and B site cations in ABX₃ perovskite, respectively. The formation of supramolecular structure retard perovskite nucleation, while the transformation of supramolecular intermediate structure enables the release of components for slow perovskite growth. This judicious control enables a segmented growth, inducing the growth of insular nanocrystal consist of low-dimensional structure. Light emitting diode based on this perovskite film eventually brings a peak external quantum efficiency up to 23.9%, ranking among the highest efficiency achieved. The homogeneous nano-island structure also enables high-efficiency large area (1 cm²) device up to 21.6%, and a record high value of 13.6% for highly semi-transparent ones.

Stabilized Low-Dimensional Species for Deep-Blue Perovskite Light-Emitting Diodes with EQE Approaching 3.4

Despite recent encouraging developments, achieving efficient blue perovskite light-emitting diodes (PeLEDs) have been widely considered a critical challenge. The efficiency breakthrough only occurred in the sky-blue region, and the device performance of pure-blue and deep-blue PeLEDs lags far behind those of their sky-blue counterparts. To avoid the negative effects associated with dimensionality reduction and excess chloride typically needed to achieve deep-blue emission, here we demonstrate guanidine (GA⁺)-induced deep-blue (∼457 nm) perovskite emitters enabling spectrally stable PeLEDs with a record external quantum efficiency (EQE) over 3.41% through a combination of quasi-2D perovskites and halide engineering. Owing to the presence of GA⁺, even a small inclusion of chloride ions is sufficient for generating deep-blue electroluminescence (EL), in clear contrast to the previously reported deep-blue PeLEDs with significant chloride inclusion that negatively affects spectral stability. Based on the carrier dynamics analysis and theoretical calculation, GA⁺ is found to stabilize the low-dimensional species during annealing, retarding the cascade energy transfer and facilitating the deep-blue EL. Our findings open a potential third route to achieve deep-blue PeLEDs beyond the conventional methods of dimensionality reduction and excessive chloride incorporation.

Lattice strain modulation toward efficient blue perovskite light-emitting diodes

The successful implementation of perovskite light-emitting diodes (PeLEDs) in advanced displays and lighting has proven to be challenging because of the inferior performance of blue devices. Here, we point out that a strained system would lead to the quasi-degenerate energy state to enhance the excited-state transition due to the formation of double-polarized transition channel. The tensile strained structure also brings about a synergetic control of the carrier dynamics in virtue of lattice structure deformation and reduced dimensional phase regulation to promote carrier population in large bandgap domains and to realize near-unit energy transfer from the large bandgap phases to the emitter phases. Accordingly, high external quantum efficiencies of 14.71 and 10.11% are achieved for the 488- and 483-nanometer PeLEDs. This work represents a versatile strategy using a strained system to achieve enhanced radiative emission for the development of efficient PeLEDs.

3D and 2D Metal Halide Perovskites for Blue Light-Emitting Diodes

Metal halide perovskites (MHPs) are emerging next-generation light emitters that have attracted attention in academia and industry owing to their low material cost, simple synthesis, and wide color gamut. Efficient strategies for MHP modification are being actively studied to attain high performance demonstrated by commercial light-emitting diodes (LEDs) based on organic emitters. Active studies have overcome the limitations of the external quantum efficiencies (EQEs) of green and red MHP LEDs (PeLEDs); therefore, the EQEs of PeLEDs (red: 21.3% at 649 nm; green: 23.4% at 530 nm) have nearly reached the theoretical limit for the light outcoupling of single-structured planar LEDs. However, the EQEs of blue PeLEDs (12.1% at 488 nm and 1.12% at 445 nm) are still lower than approximately half of those of green and red PeLEDs. To commercialize PeLEDs for future full-color displays, the EQEs of blue MHP emitters should be improved by approximately 2 times for sky-blue and more than 20 times for deep-blue MHP emitters to attain values comparable to the EQEs of red and green PeLEDs. Therefore, based on the reported effective approaches for the preparation of blue PeLEDs, a synergistic strategy for boosting the EQE of blue PeLEDs can be devised for commercialization in future full-color displays. This review covers efficient strategies for improving blue PeLEDs using fundamental approaches of material engineering, including compositional or dimensional engineering, thereby providing inspiration for researchers.

Multifunctional π-Conjugated Additives for Halide Perovskite

Additive is a conventional way to enhance halide perovskite active layer performance in multiaspects. Among them, π-conjugated molecules have significantly special influence on halide perovskite due to the superior electrical conductivity, rigidity property, and good planarity of π-electrons. In particular, π-conjugated additives usually have stronger interaction with halide perovskites. Therefore, they help with higher charge mobility and longer device lifetime compared with alkyl-based molecules. In this review, the detailed effect of conjugated molecules is discussed in the following parts: defect passivation, lattice orientation guidance, crystallization assistance, energy level rearrangement, and stability improvement. Meanwhile, the roles of conjugated ligands played in low-dimensional perovskite devices are summarized. This review gives an in-depth discussion about how conjugated molecules interact with halide perovskites, which may help understand the improved performance mechanism of perovskite device with π-conjugated additives. It is expected that π-conjugated organic additives for halide perovskites can provide unprecedented opportunities for the future improvement of perovskite devices.

Toward Continuous-Wave Pumped Metal Halide Perovskite Lasers: Strategies and Challenges

Reliable and efficient continuous-wave (CW) lasers have been intensively pursued in the field of optoelectronic integrated circuits. Metal perovskites have emerged as promising gain materials for solution-processed laser diodes. Recently, the performance of CW perovskite lasers has been improved with the optimization of material and device levels. Nevertheless, the realization of CW pumped perovskite lasers is still hampered by thermal runaway, unwanted parasitic species, and poor long-term stability. This review starts with the charge carrier recombination dynamics and fundamentals of CW lasing in perovskites. We examine the potential strategies that can be used to improve the performance of perovskite CW lasers from the materials to device levels. We also propose the open challenges and future opportunities in developing high-performance and stable CW pumped perovskite lasers.

Cesium Lead Halide Perovskite Nanocrystals Assembled in Metal-Organic Frameworks for Stable Blue Light Emitting Diodes

All inorganic cesium lead trihalide nanocrystals are promising light emitters for bright light emitting diodes (LEDs). Here, CsPb(BrCl)1.5 nanocrystals in metal-organic frameworks (MOF) thin films are demonstrated to achieve bright and stable blue LEDs. The lead metal nodes in the MOF thin film react with Cs-halide salts, resulting in 10-20 nm nanocrystals. This is revealed by X-ray scattering and transmission electron microscopy. Employing the CsPbX3 -MOF thin films as emission layers, bright deep blue and sky-blue LEDs are demonstrated that emit at 452 and 476 nm respectively. The maximum external quantum efficiencies of these devices are 0.72% for deep blue LEDs and 5.6% for sky blue LEDs. More importantly, the device can maintain 50% of its original electroluminescence (T50 ) for 2.23 h when driving at 4.2 V. Detailed optical spectroscopy and time-of-flight secondary ion mass spectroscopy suggest that the ion migration can be suppressed that maintains the emission brightness and spectra. The study provides a new route for fabricating stable blue light emitting diodes with all-inorganic perovskite nanocrystals.

Phosphonate/Phosphine Oxide Dyad Additive for Efficient Perovskite Light-Emitting Diodes

Additives play a critical role for efficient perovskite light-emitting diodes (PeLEDs). Here, we report a novel phosphonate/phosphine oxide dyad molecular additive (PE-TPPO), with unique dual roles of passivating defects and enhancing carrier radiative recombination, to boost the device efficiency of metal-halide perovskites. In addition to the defect passivation effect of the phosphine oxide group to enhance the photoluminescence intensity and homogeneity of perovskite film, the phosphonate group with strong electron affinity can capture the injected electrons to increase local carrier concentration and accelerate the carrier radiative recombination in the electroluminescence process. Owing to their synergistic enhancement on device efficiency, quasi-two-dimensional green PeLEDs modified by this dyad additive exhibit a maximum external quantum efficiency, current efficiency, and power efficiency of 25.1 %, 100.5 cd A^-1 , and 98.7 lm W^-1 , respectively, which are among the reported state-of-the-art efficiencies.