High-Efficiency Solution-Processed Inorganic Metal Halide Perovskite Light-Emitting Diodes

Modulation of Charge Transport Layer for Perovskite Light-Emitting Diodes

Perovskite light-emitting diodes (Pero-LEDs) have garnered significant attention due to their exceptional emission characteristics, including narrow full width at half maximum, high color purity, and tunable emission colors. Recent efficiency and operational stability advancements have positioned Pero-LEDs as a promising next-generation display technology. Extensive research and review articles on the compositional engineering and defect passivation of perovskite layers have substantially contributed to the development of multi-color and high-efficiency Pero-LEDs. However, the crucial aspect of charge transport layer (CTL) modulation in Pero-LEDs remains relatively underexplored. CTL modulation not only impacts the charge carrier transport efficiency and injection balance but also plays a critical role in passivating the perovskite surface, blocking ion migration, enhancing perovskite crystallinity, and improving light extraction efficiency. Therefore, optimizing CTLs is pivotal for further enhancing Pero-LED performance. Herein, this review discusses the roles of CTLs in Pero-LEDs and categorizes both reported and potential CTL materials. Then, various CTL optimization strategies are presented, alongside an analysis of the selection criteria for CTLs in high-performance Pero-LEDs. Finally, a summary and outlook on the potential of CTL modulation to further advance Pero-LED performances are provided.

Supersaturated Antisolvent-Assisted Crystallization for Highly Efficient Inorganic Perovskite Light-Emitting Diodes

We introduced a strategy to form nanocrystalline CsPbBr3 perovskite films with high luminescence and stability, inhibiting crystal growth using a CsBr supersaturated antisolvent during the antisolvent-assisted crystallization process. We devised this strategy because the supersaturated antisolvent has a higher CsBr concentration over its solubility limit in the saturated antisolvent and consequently will form the smaller perovskite nanocrystalline grains due to the quick precipitation of the CsBr. Here, the CsBr is chosen as a model inorganic antisolvent additive for a crystal growth inhibitor and a passivator. Consequently, we have achieved a nanocrystalline CsPbBr3 film with an average grain size of ∼39 nm by the CsBr supersaturated antisolvent-assisted crystallization process, which is about 41% smaller than the average grain size of the control sample. Hence, the perovskite thin film exhibited a much higher photoluminescence quantum yield than the control film. The maximum current efficiency (CEmax) and the maximum external quantum efficiency (EQEmax) of the corresponding CsPbBr3 perovskite light-emitting diodes (PeLEDs) were approximately twice higher (CEmax of 94.64 cd A-1 and EQEmax of 22.93%) than those of the control device. Simultaneously, the inclusion of CsBr additives played a multifunctional role in diminishing the leakage current of PeLEDs and enhancing their operational lifetime.

Bluish-white Light-emitting 2D Sheets of Lead-free Perovskite Cesium Titanium Bromide (CsTiBr3) by a Two-stage Deposition Technique

Bluish-white light-emitting materials are commonly used in LED lighting because they produce natural-looking light. Here we report the photoluminescent emission (PL) of novel, two-dimensional lead-free CsTiBr3 perovskite prepared via a two-stage deposition process. The formation of two-dimensional nanosheets of CsTiBr3 perovskite is confirmed by XRD, EDAX, and FESEM analysis. The height of the cesium bromide thin film substrate from the titanium bromide vapor source plays an important role in the formation of two-dimensional CsTiBr3. The CsTiBr3 perovskite nanosheets exhibit unique exciton- luminescence at 440 nm and self-trapped exciton emission at 595 nm which are the characteristics of two-dimensional halide structure, along with the band-to-band emission at 400 nm at an excitation wavelength of 340 nm. The resulting bluish-white light PL emission makes two-dimensional CsTiBr3 perovskite an alternative material to the traditional lead-based perovskite in LEDs, display technology, solid-state lighting, and various optoelectronic devices, addressing environmental concerns.

Organic Ligand Engineering for Tailoring Electron-Phonon Coupling in 2D Hybrid Perovskites

In the emerging two-dimensional organic-inorganic hybrid perovskites, the electronic structures and carrier behaviors are strongly impacted by intrinsic electron-phonon interactions, which have received inadequate attention. In this study, we report an intriguing phenomenon of negative carrier diffusion induced by electron-phonon coupling in (2T)2PbI4. Theoretical calculations reveal that the electron-phonon coupling drives the band alignment in (2T)2PbI4 to alternate between type I and type II heterostructures. As a consequence, photoexcited holes undergo transitions between the organic ligands and inorganic layers, resulting in abnormal carrier transport behavior compared to other two-dimensional hybrid perovskites. These findings provide valuable insights into the role of electron-phonon coupling in shaping the band alignments and carrier behaviors in two-dimensional hybrid perovskites. They also open up exciting avenues for designing and fabricating functional semiconductor heterostructures with tailored properties.

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.

Eliminating Nanocrystal Surface Light Loss and Ion Migration to Achieve Bright Mixed-Halide Blue Perovskite LEDs

Blue light-emittin g diodes (LEDs) are important components for perovskite electroluminescence applications, which still suffer from insufficient luminescence efficiency and poor stability. In Cl/Br mixed perovskite NCs, surficial defects cause severe light failure and ion migration, the in-depth mechanism of which is also not clear. To gain insights into these issues, we employ the ligand post-addition approach for mixed Cl/Br NCs by using octylammonium hydrobromide (OctBr) ligands, which effectively decrease surficial light loss and block ion migration pathways. The passivated CsPbCl_1.5Br_1.5 NCs exhibit exceptional blue emission with 95% PLQY, and the electroluminescence spectra of LEDs are located at the initial positions at the initial states. The treated NC blue devices show a negligible color shift as the voltage increases, which proves that electric-field-driven ion migration is drastically suppressed. In addition, OctBr-treated CsPbCl_1.5Br_1.5 and CsPbClBr₂ NC LEDs show high external quantum efficiencies of 2.42 and 3.05% for emission peaks at 456 and 480 nm, respectively. Our work identified the nature of NC surface defects and provided a surficial modification approach to develop high-performance and color-stable blue mixed-halide perovskite LEDs.

Synergistic Effect of Cation Composition Engineering of Hybrid Cs1-x FAx PbBr3 Nanocrystals for Self-Healing Electronics Application

Mixed-cation hybrid perovskite nanocrystal (HPNC) with high crystallinity, color purity, and tunable optical bandgap offers a practical pathway toward next-generation displays. Herein, a two-step modified hot-injection combined with cation compositional engineering and surface treatment to synthesize high-purity cesium/formamidinium lead bromide HPNCs(Cs_1-x FA_x PbBr₃ ) is presented. The optimized Cs_0.5 FA_0.5 PbBr₃ light-emitting devices (LEDs) exhibit uniform luminescence of 3500 cd m^-2 and a prominent current efficiency of 21.5 cd A^-1 . As a proof of concept, a self-healing polymer (SHP) integrated with white LED backlight and laser prototypes exhibited 4 h autonomous self-healing through the synergistic effect of weak reversible imine bonds and stronger H-bonds. First, the SHP-HPNCs-initial and SHP-HPNCs-cut possess high long-term stability and dramatically suppressed lead leakage as low as 0.6 ppm along with a low leakage rate of 1.11 × 10^-5 cm² and 3.36 × 10^-5 cm² even over 6 months in water. Second, the Cs_0.5 FA_0.5 PbBr₃ HPNCs and SHP-induced shattered-repaired perovskite glass substrate show the lowest lasing threshold values of 1.24 and 8.58 µJ cm^-2 , respectively. This work provides an integrative and in-depth approach to exploiting SHP with intrinsic and entropic self-healing capabilities combined with HPNCs to develop robust and reliable soft-electronic backlight and laser applications.

Control of n-Phase Distribution in Quasi Two-Dimensional Perovskite for Efficient Blue Light-Emitting Diodes

Pure-bromide quasi-2D perovskite (PBQ-2DP) promises high-performance light-emitting diodes (LEDs), while a challenge remains on control over its n-phase distribution for bright true-blue emission. Present work addresses the challenge through exploring the passivation molecule of amino acid with reinforced binding energy, which generates narrow n-phase distribution preferentially at n = 3 with true blue emission at 478 nm. Consequently, a peak external quantum efficiency of 5.52% and a record brightness of 512 cd m-2 are achieved on the PBQ-2DP-based true blue PeLED, these both values located among the top in the records of similar devices. We further reveal that the electron-phonon coupling results in the red-shifted emission in the PBQ-2DP film, suggesting that the view of n-phase distribution dominated true-blue emission in PBQ-2DP needs to be revisited, pointing out a guideline of electron-phonon coupling suppression to relieve the strait of realizing true blue or even deep blue emission in the PBQ-2DP film.

Morphology and Luminescence Regulation for CsPbBr3 Perovskite Light-Emitting Diodes by Controlling Growth of Low-Dimensional Phases

At present, the high defect density and strong nonradiative recombination rate of all-inorganic cesium lead bromide (CsPbBr₃) perovskite light-emitting diodes (PeLEDs) seriously inhibit the improvement of their quantum efficiency. In this paper, the addition of a short-chain additive, diethylammonium bromide (DEABr), aims to control the generation of a quasi-2D large n-phase to optimize the surface morphology and construct two-dimensional/three-dimensional (2D/3D) heterojunction perovskite structures to enhance the EL efficiency of PeLEDs. Through Kelvin probe force microscopy (KPFM) characterization, we confirmed that the 2D phase grains with a low potential are locally formed on the surface of the perovskite film under the action of DEABr. The existence of the 2D phase effectively improved the surface morphology and suppressed surface defects. In addition, the in situ constructed 2D/3D heterojunction perovskite structure further increases the exciton radiative recombination rate and significantly improves the electroluminescent performance. By optimizing its doping concentration, the optimal all-inorganic PeLED displays a current efficiency (CE) of 30.3 cd A^-1, an external quantum efficiency (EQE) of 9.6%, and a maximum brightness of 32,500 cd m^-2. According to our results, the formation of 2D structures on the surface of the CsPbBr₃ film can improve surface morphology issues and optoelectronic properties of the film.

Ion Migration in Perovskite Light-Emitting Diodes: Mechanism, Characterizations, and Material and Device Engineering

In recent years, perovskite light-emitting diodes (PeLEDs) have emerged as a promising new lighting technology with high external quantum efficiency, color purity, and wavelength tunability, as well as, low-temperature processability. However, the operational stability of PeLEDs is still insufficient for their commercialization. The generation and migration of ionic species in metal halide perovskites has been widely acknowledged as the primary factor causing the performance degradation of PeLEDs. Herein, this topic is systematically discussed by considering the fundamental and engineering aspects of ion-related issues in PeLEDs, including the material and processing origins of ion generation, the mechanisms driving ion migration, characterization approaches for probing ion distributions, the effects of ion migration on device performance and stability, and strategies for ion management in PeLEDs. Finally, perspectives on remaining challenges and future opportunities are highlighted.

Hole Transport Layer Free Perovskite Light-Emitting Diodes With High-Brightness and Air-Stability Based on Solution-Processed CsPbBr3-Cs4PbBr6 Composites Films

Recently, perovskite light-emitting diodes (PeLEDs) have drew widespread attention due to their high efficiencies. However, because of the sensitivity to moisture and oxygen, perovskite luminescent layers are usually prepared in high-purity nitrogen environment, which increases the cost and process complexity of device preparation and seriously hindrances its commercialization of PeLED in lighting and display application. Herein, dual-phase all-inorganic composite CsPbBr₃-Cs₄PbBr₆ films are fabricated from CsBr-rich perovskite solutions by a simple one-step spin-coating method in the air with high humidity. Compared with the pure CsPbBr₃ film, the composite CsPbBr₃-Cs₄PbBr₆ film has much stronger photoluminescence emission and longer fluorescence lifetime, accompanied by increased photoluminescence quantum yield (33%). As a result, we obtained green PeLED devices without hole transport layer exhibiting a maximum brightness of 72,082 cd/m² and a maximum external quantum efficiency of about 2.45%, respectively. More importantly, the champion device shows excellent stability with operational half-lifetime exceeding 1,000 min under continuous operation in the air. The dual-phase all-inorganic composite CsPbBr₃-Cs₄PbBr₆ film shows attractive prospect for advanced light emission applications.

Screen-Overprinted Perovskite RGB Microdisk Arrays Based on Wet-Solute-Chemical Dynamics for Full-Color Laser Displays

Owing to outstanding optoelectronic properties, halide perovskites are great candidates for novel laser display applications. However, the realization of their practical flat-panel display applications is challenging because of the incapacity to controllably assemble different halide perovskite microlaser arrays onto an identical substrate as pixelated full-color panels due to intrinsic fragile crystal lattices. Here, perovskite red-green-blue (RGB) microdisk arrays are reported, acting as flat-panels for full-color laser displays. A universal screen-overprinting technology is developed to integrate full-color perovskite microdisk arrays on a prepatterned template, which is on the basis of wet-solute-chemical dynamics involving a combination of surface tailoring and solvent selection. Via such an overprinting method, perovskite RGB microlaser matrices with precise localizations and well-defined dimensions were fabricated on an identical substrate, and each set of RGB microlaser served as a pixel for full-color display panels. On this basis, static and dynamic laser displays have been demonstrated with as-prepared full-color panels. These results will provide novel design concepts and device structures for future full-color laser display applications.

Efficient Inorganic Perovskite Light-Emitting Diodes by Inducing Grain Arrangement via a Multifunctional Interface

Accumulating evidence shows that metal halide perovskite light-emitting diodes (PeLEDs) are well-described to show broad application prospects in lighting and display due to the wide color gamut and high color purity. However, it is still a great challenge to prepare high-quality all-inorganic PeLEDs by a solution method. For example, it is difficult to obtain all-inorganic perovskite films with good crystallinity and high grain orientation because of too fast and uncontrollable crystallization of all-inorganic perovskite films. Here, we demonstrated a multifunctional interface of formamide (FA)-doped PEDOT:PSS, which improved the crystallinity of all-inorganic perovskite films by inducing grain arrangement. As a result, a highly crystalline, ordered, and defect-passivated CsPbBr₃ film was obtained by the multiple roles of FA, and the CsPbBr₃-based PeLED treated with FA achieved both high brightness and high efficiency: the peak external quantum efficiency (EQE) reaches 9.61%, and the maximum brightness is 185,000 cd/m². In addition, Tween 80, used as a passivator of perovskite films, reduced the defect states and suppressed ion migration. Under the synergistic effect of FA interface treatment and Tween 80 passivation treatment, efficient CsPbBr₃-based PeLEDs were obtained with an EQE of 15.02% and an operation lifetime of 182.5 min at an initial brightness of 1000 cd/m², which is among the best reported lifetimes under high brightness. Our study provides a simple and effective strategy for the realization of all-inorganic PeLEDs with high efficiency, high brightness, and ultralong operation lifetime.

High Triplet Energy Level Molecule Enables Highly Efficient Sky-Blue Perovskite Light-Emitting Diodes

The role of triplet states in the interfacial energy transfer in perovskite light-emitting diodes (PeLEDs) has so far not been clarified because of the complex exciton recombination and decay dynamics. This work aims to study this issue and accordingly proposes a novel interfacial-engineering strategy for efficient sky-blue PeLEDs. To this end, bis[2-(diphenylphosphino)phenyl]ether oxide with a high triplet energy level is introduced into sky-blue PeLEDs. It effectively reduces undesirable exciton transfer from the perovskite emission layer to the electron-transport layer, largely suppresses exciton quenching at the interface, and simultaneously passivates defects at the perovskite surfaces. As a result of the multichannel energy-loss reduction, sky-blue PeLED that emits at 488 nm is achieved with a peak external quantum efficiency of 10.17% and a maximum brightness of 6728.41 cd m^-2. This work thus provides indirect evidence for the triplet mechanism of blue emission of mixed-halide perovskites and sheds new light on a promising way of boosting the performance of blue PeLEDs.

Temperature-Dependent Photoluminescence of Manganese Halide with Tetrahedron Structure in Anti-Perovskites

The temperature-dependent photoluminescence (PL) properties of an anti-perovskite [MnBr₄]BrCs₃ sample in the temperature range of 78–500 K are studied in the present work. This material exhibits unique performance which is different from a typical perovskite. Experiments showed that from room temperature to 78 K, the luminous intensity increased as the temperature decreased. From room temperature to 500 K, the photoluminescence intensity gradually decreased with increasing temperature. Experiments with varying temperatures repeatedly showed that the emission wavelength was very stable. Based on the above-mentioned phenomenon of the changing photoluminescence under different temperatures, the mechanism is deduced from the temperature-dependent characteristics of excitons, and the experimental results are explained on the basis of the types of excitons with different energy levels and different recombination rates involved in the steady-state PL process. The results show that in the measured temperature range of 78–500 K, the steady-state PL of [MnBr₄]BrCs₃ had three excitons with different energy levels and recombination rates participating. The involved excitons with the highest energy level not only had a high radiative recombination rate, but a high non-radiative recombination rate as well. The excitons at the second-highest energy level had a similar radiative recombination rate to the lowest energy level excitons and a had high non-radiative recombination rate. These excitons made the photoluminescence gradually decrease with increasing temperature. This may be the reason for this material’s high photoluminescence efficiency and low electroluminescence efficiency.

Polymerized Hybrid Perovskites with Enhanced Stability, Flexibility, and Lattice Rigidity

The intrinsic soft lattice nature of organometal halide perovskites (OHPs) makes them very tolerant to defects and ideal candidates for solution-processed optoelectronic devices. However, the soft lattice results in low stability towards external stresses such as heating and humidity, high density of phonons and strong electron-phonon coupling (EPC). Here, it is demonstrated that the OHPs with unsaturated 4-vinylbenzylammonium (VBA) as organoammonium cations can be polymerized without damaging the perovskite structure and its tolerance to defects. The polymerized perovskites show enhanced stability and flexibility compared to regular three-dimensional and two-dimensional (2D) perovskites. Furthermore, the polymerized 4-vinylbenzylammonium group improves perovskite lattice rigidity substantially, resulting in a reduced non-radiative recombination rate because of suppressed electron-phonon coupling, and enhanced carrier mobility because of suppressed phonon scattering. 2D polymerized perovskite light-emitting diodes (PeLEDs) with strong electroluminescence at room temperature, and quasi-2D PeLEDs with an external quantum efficiency (EQE) of 23.2% and enhanced operation stability are demonstrated. The work has opened a new way of enhancing the intrinsic stability and optoelectronic properties of OHPs.

Highly Efficient Quasi-2D Perovskite Light-Emitting Diodes Incorporating a TADF Dendrimer as an Exciton-Retrieving Additive

Although small organics or polymer additives have been introduced to enhance film formation and radiative recombination of perovskite light-emitting diodes (PeLEDs), the exciton utilization and quantum efficiency need further optimization. Here, we introduce a thermal-activated delayed fluorescence (TADF) dendrimer as an additive to enhance the surface coverage and reduce the trap state of the grain boundary. More importantly, the TADF nature of such an additive can retrieve the exciton dissociated from perovskite or trapped by the grain boundary and then transfer the energy back to emissive perovskite through the Förster energy transfer process. Since the triplets can be reused by reverse intersystem crossing in such a TADF additive, the theoretical exciton utilization is 100%. As a result, the optimized PeLEDs cooperating with a TADF additive achieved a high current efficiency of 39.0 cd A^-1 and an ultrabright luminescence of 18,000 cd m^-2, which are almost 5 times higher than those of the control device without an additive. Moreover, the device stability monitored by half-lifetime at 1000 cd m^-2 enhanced 2 times after introducing the TADF dendrimer as an additive. The parent dendrimer without a TADF feature was also synthesized as an additive to explore the mechanism action, which found that 54% enhancement of device efficiency can be attributed to defect passivating, while 46% was assigned to retrieved energy. This research first demonstrates that the TADF dendrimer is a promising exciton-retrieving additive for enhancing the performance of PeLEDs by passivating defect, filling up grain boundary, and retrieving leakage exciton.

Recent Progress on Patterning Strategies for Perovskite Light-Emitting Diodes toward a Full-Color Display Prototype

Perovskite light-emitting diodes (PeLEDs) have attracted both academic and industrial interest because of their high efficiency, wide color gamut as well as low material and fabrication costs. However, most state-of-the-art PeLEDs are still fabricated with commonly used spin-coating methods, which are undesirable for large-scale commercial production. Achieving highly emissive perovskite microarrays at high spatial resolution with quick and large-scale growth is one of the critical steps to integrate state-of-the-art PeLEDs into full-color display panels. Here, the fabrication methods and crystallization processes of perovskite materials are first discussed because they are strongly relevant to the patterning process and the quality of the perovskite pixels. Then, the current strategies to realize perovskite patterns that can be likely integrated into display panels, through mask-free or mask-assisted methods, are explored. Self-emitting and down-conversion PeLED devices with a patterned perovskite matrix as the emissive layer are also reviewed. Finally, an outlook is provided on how to further improve the optical and electrical properties of the perovskite patterns and the performance of PeLEDs as well as to develop eco-friendly devices to accelerate the potential commercialization progress of this young technique.

State of the Art and Prospects for Halide Perovskite Nanocrystals

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

Efficient and Stable Blue Perovskite Light-Emitting Devices Based on Inorganic Cs4PbBr6 Spaced Low-Dimensional CsPbBr3 through Synergistic Control of Amino Alcohols and Polymer Additives

Perovskite light-emitting devices (PeLEDs) have drawn a great deal of attention because of their exceptional optical and electrical properties. However, as for the blue PeLEDs based on low-dimensional (LD) CsPbBr₃, the low conductivity of the widely used organic spacers as well as the difficulty of forming pure and uniform LD CsPbBr₃ phase have severely inhibited the device performance such as stability and efficiency. In this work, we report an effective strategy to obtain high-quality LD CsPbBr₃ by using a novel spacer of inorganic Cs₄PbBr₆ instead of the common long-chain ammonium halides. We found that a 3-amino-1-propanol (3AP)-modified PEDOT:PSS was helpful to stimulate the formation of the LD blue emissive CsPbBr₃:Cs₄PbBr₆ composite. We also revealed that an additive of poly(vinylpyrrolidone) (PVP) in the precursor can limit further growth of LD perovskite phase into 3D perovskite phase upon annealing, thus resulting in a uniformly distributed LD perovskite with high color stability. Consequently, efficient blue PeLEDs @ 485 nm with a brightness of 2192 cd/m², current efficiency of 2.68 cd/A, and external quantum efficiency of 2.3% was successfully achieved. More importantly, the device showed much improved working stability compared to those with the spacer of organic ammonium halides. Our results provide some helpful insights into developing efficient and stable blue PeLEDs.

Vacuum-Processed Metal Halide Perovskite Light-Emitting Diodes: Prospects and Challenges

In less than a decade, organic-inorganic metal halide perovskites (MHPs) have shown tremendous progress in the field of light-emitting applications. Perovskite light-emitting diodes (PeLEDs) have reached external quantum efficiencies (EQE) exceeding 20 % and they have been recognized as a potential contender of the commercial display technologies. However, perovskite thin films in PeLEDs are generally deposited via a spin-coating process, which is not favourable for large area device fabrication. Despite the great success of solution-processed PeLEDs, very few articles have been reported on vacuum processed PeLEDs and the improvements in their optoelctronic performances are also progressing slowly. On the other hand, vacuum processing techniques are mostly used in organic LED technology as they can guarantee (i) the absence of solvent during thin-film growth, (ii) process scalability over large area substrates, and (iii) precise thin-film thickness control. This thin-film growth process is suitable for application in the advancement of a large variety of display technologies. In this Review, we present an overview of current research advances in the field of perovskite thin films grown via vacuum techniques, a study of their photophysical properties, and integration in PeLEDs for the generation of different colors. We also highlight the current challenges and future prospects for the further development of vacuum processed PeLEDs.

Advances in Perovskite Light-Emitting Diodes Possessing Improved Lifetime

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

The dual-defect passivation role of lithium bromide doping in reducing the nonradiative loss in CsPbX3 (X = Br and I) quantum dots

Lithium bromide as a new duel-defect passivation agent can effectively enhance luminescence properties by reducing the non-radiative loss.

B-Site Co-Alloying with Germanium Improves the Efficiency and Stability of All-Inorganic Tin-Based Perovskite Nanocrystal Solar Cells

Colloidal lead-free perovskite nanocrystals have recently received extensive attention because of their facile synthesis, the outstanding size-tunable optoelectronic properties, and less or no toxicity in their commercial applications. Tin (Sn) has so far led to the most efficient lead-free solar cells, yet showing highly unstable characteristics in ambient conditions. Here, we propose the synthesis of all-inorganic mixture Sn-Ge perovskite nanocrystals, demonstrating the role of Ge2+ in stabilizing Sn2+ cation while enhancing the optical and photophysical properties. The partial replacement of Sn atoms by Ge atoms in the nanostructures effectively fills the high density of Sn vacancies, reducing the surface traps and leading to a longer excitonic lifetime and increased photoluminescence quantum yield. The resultant Sn-Ge nanocrystals-based devices show the highest efficiency of 4.9 %, enhanced by nearly 60 % compared to that of pure Sn nanocrystals-based devices.

Boosting the Efficiency of NiOx-Based Perovskite Light-Emitting Diodes by Interface Engineering

Nickel oxide (NiO_x) is a promising hole-transporting material for perovskite light-emitting diodes (PeLEDs) because of its low cost, excellent stability, and simple fabrication process. However, the electroluminescence efficiencies of NiO_x-based PeLEDs are greatly limited by inefficient hole injection and exciton quenching at the NiO_x-perovskite interfaces. Here, a novel interfacial engineering method with sodium dodecyl sulfate-oxygen plasma (SDS-OP) is demonstrated to simultaneously overcome the aforementioned issues. Experimental results reveal that a short OP treatment on the top of the SDS-coated NiO_x significantly deepens the NiO_x work function (from 4.23 to 4.85 eV) because of the formation of a large surface dipole, allowing for efficient hole injection. Moreover, the SDS-OP layer passivates the electronic surface trap states of perovskite films and suppresses the exciton quenching by NiO_x. These improvements inhibit the nonradiative decays at the NiO_x-perovskite interface. As a result, the external quantum efficiency of CsPbBr₃ LEDs is increased from 0.052 to 2.5%; that of FAPbBr₃ nanocrystals LEDs is increased from 5.6 to 7.6%.

Boosting the Efficiency and Stability of Perovskite Light-Emitting Devices by a 3-Amino-1-propanol-Tailored PEDOT:PSS Hole Transport Layer

Properties of the underlying hole transport layer (HTL) in perovskite light-emitting devices (PeLEDs) play a critical role in determining the optoelectronic performance through influencing both the charge transport and the quality of the active perovskite emission layer (EML). This work focuses on manipulating the carrier transport behavior and obtaining a high-quality EML film by tailoring the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HTL with previously unused amino alcohol 3-amino-1-propanol (3AP). The modified PEDOT:PSS rendered a deeper work function that is more suitable for the hole injection from the HTL to EML. More importantly, the 3AP-modified PEDOT:PSS film can induce a low-dimensional perovskite phase that can passivate the defects in the EML, resulting in a significantly improved light emission. Such ameliorations consequently result in a dramatical enhancement in performance of PeLED with a low turn-on voltage of 2.54 V, a maximum luminance of 23033 cd/m², a highest current efficiency of 29.38 cd/A, a corresponding maximum external quantum efficiency of 9.4%, and a prolonged lifetime of 6.1 h at a proper Cs/Pb ratio.

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

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

Bifacial passivation towards efficient FAPbBr3-based inverted perovskite light-emitting diodes

A unique technique to passivate both bottom and top sides of perovskite has been successfully developed to achieve highly efficient inverted perovskite light-emitting diodes (PeLEDs). For the bottom passivation, an organic/inorganic hybrid electron transporting layer (ETL) replaces the widely adopted inorganic ETL to overcome the disadvantages of the pure inorganic ETL. The ZPM (ZnO-in-polymer matrix) ETL, which consists of ZnO nanoparticles blended into polyvinylpyrrolidone, not only passivates the surface defects of ZnO nanoparticles, but also improves the morphology and stability of FAPbBr₃ film. For the top passivation, smaller grains and a FAPbBr₃/PEA₂PbBr₄ 3D/2D hybrid structure are obtained by applying a small amount of PEABr solution. The synergetic interplay of organic/inorganic hybrid ETL and organic halide salt surface modification substantially shrinks the grain size to facilitate radiative recombination, and suppresses non-radiative recombination both at the interface of ETL/perovskite and HTL/perovskite, and in the perovskite layer. As a result, the highly efficient green PeLED sets a new record of device performance for FAPbBr₃-based inverted PeLEDs, with current efficiency of 39.7 cd A^-1, external quantum efficiency of 9.0%, power efficiency of 46.4 lm W^-1, maximum luminance of 6.03 × 10⁴ cd m^-2, and half-lifetime of 297 minutes at an initial brightness of ∼100 cd m^-2.

Impact of excitation energy on the excitonic luminescence of cesium lead bromide perovskite nanosheets

An excitation-energy-dependent luminescence phenomenon is reported in cesium lead bromide (CsPbBr₃) perovskite nanosheets. At 10 K, the relative integrated luminescence intensity between the trapped exciton (TX) emission and the free exciton (FX) emission shows an interesting tendency with increasing the optical excitation energy from 2.431 eV (510 nm) to 3.758 eV (330 nm). To interpret such phenomenon, we develop a quantitative model on the basis of the biological population growth theory. A good agreement between experiment and theory is obtained. It is thus revealed that the lower capture coefficient of the TX level to the excited excitons via multiphonon emission relative to the FX level shall be the major cause of the observed phenomenon. These findings may help to deepen the current understanding of the complex luminescence mechanisms of these emerging light-emitting materials.

Emergence of Impurity-Doped Nanocrystal Light-Emitting Diodes

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

Efficient Flexible Inorganic Perovskite Light-Emitting Diodes Fabricated with CsPbBr3 Emitters Prepared via Low-Temperature in Situ Dynamic Thermal Crystallization

The present study systematically investigates the morphology and crystallization process of inorganic CsPbBr₃ perovskite layer films fabricated by thermal coevaporation in conjunction with continuous low-temperature thermal annealing to promote in situ dynamic thermal crystallization. The results confirm for the first time that both the crystal grain size and the compactness of the CsPbBr₃ films can be tuned during the thermal coevaporation fabrication process via in situ dynamic thermal crystallization. The performance of the PeLEDs employing the CsPbBr₃ films as the emitter layer is investigated in detail with respect to the substrate temperature and deposition rate employed during deposition of the CsPbBr₃ film. This study provides guidelines for developing suitable film production processes and highlights future challenges that must be addressed to facilitate the commercial development of large-area, uniform, and flexible perovskite-based optoelectronic devices.

Rational Interface Engineering for Efficient Flexible Perovskite Light-Emitting Diodes

Although perovskite light-emitting diodes (PeLEDs) are promising for next-generation displays and lighting, their efficiency is still considerably below that of conventional inorganic and organic counterparts. Significant efforts in various aspects of the electroluminescence process are required to achieve high-performance PeLEDs. Here, we present an improved flexible PeLED structure based on the rational interface engineering for energy-efficient photon generation and enhanced light outcoupling. The interface-stimulated crystallization and defect passivation of the perovskite emitter are synergistically realized by tuning the underlying interlayer, leading to the suppression of trap-mediated nonradiative recombination losses. Besides approaching highly emissive perovskite layers, the outcoupling of trapped light is also enhanced by combining the silver nanowires-based electrode with quasi-random nanopatterns on flexible plastic substrate. Upon the collective optimization of the device structure, a record external quantum efficiency of 24.5% is achieved for flexible PeLEDs based on green-emitting CsPbBr₃ perovskite.

All-Inorganic CsPbBr3 Perovskite Films Prepared by Single Source Thermal Ablation

Hybrid organo-lead halide perovskites are becoming the benchmark material for next generation photovoltaics and a very important player for other applications such as photodetectors and light emitting diodes. Nevertheless, the most important issue hindering the large-scale application of these materials remains their intrinsic instability due to the organic cation. Although the substitution with inorganic cesium (Cs) enhances stability, in most cases solution deposition methods of fully inorganic perovskites result in high surface roughness and poor surface coverage. This work reports on the evaporation of the CsPbBr₃ precursor by Single Source Thermal Ablation, showing that just after deposition films consist of a mixture of CsPbBr₃, CsPb₂Br₅, and Cs₄PbBr₆ due to a vertical composition gradient. We point out that mild post deposition treatments lead to the conversion of CsPb₂Br₅ and Cs₄PbBr₆ into CsPbBr₃ due to its higher thermodynamic stability. Conversion results into smooth and pinhole-free CsPbBr₃ films with good light absorption and emission properties. We demonstrate the suitability of obtained films for planar devices by preparing perovskite-based pure-green light emitting diodes, thus promoting Single Source Thermal Ablation as a promising alternative deposition technique for all-inorganic perovskite-based devices.

Photoluminescence signatures of thermal expansion, electron-phonon coupling and phase transitions in cesium lead bromide perovskite nanosheets

In this article, photoluminescence (PL) behaviors of inorganic cesium lead bromide (CsPbBr3) nanosheets are investigated in a broad temperature range from 5 to 500 K. As can be seen from the temperature evolution of the PL peak position, the bandgap blueshift induced by thermal lattice expansion is found to be gradually compensated by the bandgap redshift caused by electron-phonon coupling for the temperature variation from 5 K to 360 K. As the temperature is further increased, the nearly completely compensated PL peak position turns to exhibit a rapid blueshift, again at ∼360 K. Such turning behavior is consistent with an orthorhombic-tetragonal (γ-β) phase transition at this critical temperature. For the PL linewidth, it shows a continuous broadening at temperatures beyond 40 K, suggesting the dominant role of phonon scattering, especially at high temperatures. These findings of temperature dependent photophysical properties of CsPbBr3 nanosheets may provide useful information of their further applications in the emerging nano photo-electronic devices.

Ambient-Processed, Additive-Assisted CsPbBr3 Perovskite Light-Emitting Diodes with Colloidal NiOx Nanoparticles for Efficient Hole Transporting

In this study, the electrically driven perovskite light-emitting diodes (PeLEDs) were investigated by hybridizing the organic polyethylene oxide, 1,3,5-tris (N-phenylbenzimiazole-2-yl) benzene (TPBi), and bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl) iridium III (FIrpic) with CsPbBr3 in the emission layer and adopting the colloidal NiOx nanoparticle (NP) hole transport layer. The synthesized NiOx NPs, having an average size of ~5 nm, can be spin-coated to become a smooth and close-packed film on the indium–tin–oxide anode. The NiOx NP layer possesses an overall transmittance of ~80% at 520 nm, which is about the peak position of electroluminescence (EL) spectra of CsPbBr3 emission layer. The coating procedures of NiOx NP and CsPbBr3 layers were carried out in ambient air. The novel PeLED turned on at 2.4 V and emitted bright EL of 4456 cd/m2 at 7 V, indicating the remarkable nonradiative-related defect elimination by organic additive addition and significant charge balance achieved by the NiOx NP layer.

Controllable Growth of High-Quality Inorganic Perovskite Microplate Arrays for Functional Optoelectronics

Inorganic perovskite single crystals have emerged as promising vapor-phase processable structures for optoelectronic devices. However, because of material lattice mismatch and uncontrolled nucleation, vapor-phase methods have been restricted to random distribution of single crystals that are difficult to perform for integrated device arrays. Herein, an effective strategy to control the vapor-phase growth of high-quality cesium lead bromide perovskite (CsPbBr₃ ) microplate arrays with uniform morphology as well as controlled location and size is reported. By introducing perovskite seeds on substrates, intractable lattice mismatches and random nucleation barriers are surpassed, and the epitaxial growth of perovskite crystals is accurately controlled. It is further demonstrated that CsPbBr₃ microplate arrays can be monolithically integrated on substrates for the fabrication of high-performance lasers and photodetectors. This strategy provides a facile approach to fabricate high-quality CsPbBr₃ microplates with controllable size and location, which offers new opportunities for the scalable production of integrated optoelectronic devices.

High-Performance Quasi-2D Perovskite Light-Emitting Diodes Via Poly(vinylpyrrolidone) Treatment

In this work, we fabricate poly(vinylpyrrolidone) (PVP)-treated Ruddlesden-Popper two-dimensional (quasi-2D) PPA₂(CsPbBr₃)₂PbBr₄ perovskite light-emitting diodes (PeLEDs) and achieved a peak brightness of 10,700 cd m^-2 and peak current efficiency of 11.68 cd A^-1, threefold and tenfold higher than that of the pristine device (without PVP), respectively. It can be attributed that the additive of PVP can suppress the pinholes of perovskite films owing to the excellent film-forming property, inhibiting the leakage current. Besides, PVP treatment facilitates the formation of compact perovskite films with defect reduction. Our work paves a novel way for the morphology modulation of quasi-2D perovskite films.

Printable CsPbBr3 perovskite quantum dot ink for coffee ring-free fluorescent microarrays using inkjet printing

Printable perovskite quantum dot (QD) ink is very important for achieving high quality coffee ring-free fluorescent microarrays for different kinds of emerging perovskite optoelectronic applications using inkjet printing. In this work, we prepared a printable CsPbBr₃ perovskite QD ink by mixing high-boiling point dodecane with low-boiling point toluene as a solvent. The evaporation rate, viscosity and surface tension of the ink were carefully optimized by tuning the volume ratio of these two solvents for forming appropriate Marangoni flow, so as to balance the capillary flow and eliminate the coffee ring effect further. Successfully, CsPbBr₃ perovskite microarrays with uniform surface, low roughness and no coffee rings were achieved by inkjet printing the optimized perovskite QD ink on a PVK (poly-(9-vinylcarbazole)) layer. Furthermore, we patterned the CsPbBr₃ perovskite QD ink, and the printed patterns were only visible under ultraviolet (UV) light, which can be applied in invisible anti-counterfeiting labels and encryption in the future.

Interfacial Mechanism for Efficient Resistive Switching in Ruddlesden-Popper Perovskites for Non-volatile Memories

Ion migration, one origin of current-voltage hysteresis, is the bane of halide perovskite optoelectronics. Herein, we leverage this unwelcome trait to unlock new opportunities for resistive switching using layered Ruddlesdsen-Popper perovskites (RPPs) and explicate the underlying mechanisms. The ON/OFF ratio of RPP-based devices is strongly dependent on the layers and peaks at n̅ = 5, demonstrating the highest ON/OFF ratio of ∼10⁴ and minimal operation voltage in 1.0 mm² devices. Long data retention even in 60% relative humidity and stable write/erase capabilities exemplify their potential for memory applications. Impedance spectroscopy reveals a chemical reaction between migrating ions and the external contacts to modify the charge transfer barrier at the interface to control the resistive states. Our findings explore a new family of facile materials and the necessity of ionic population, migration, and their reactivity with external contacts in devices for switching and memory applications.

Study of Metal-Semiconductor-Metal CH3NH3PbBr3 Perovskite Photodetectors Prepared by Inverse Temperature Crystallization Method

Numerous studies have addressed the use of perovskite materials for fabricating a wide range of optoelectronic devices. This study employs the deposition of an electron transport layer of C₆₀ and an Ag electrode on CH₃NH₃PbBr₃ perovskite crystals to complete a photodetector structure, which exhibits a metal–semiconductor–metal (MSM) type structure. First, CH₃NH₃PbBr₃ perovskite crystals were grown by inverse temperature crystallization (ITC) in a pre-heated circulator oven. This oven was able to supply uniform heat for facilitating the growth of high-quality and large-area crystals. Second, the different growth temperatures for CH₃NH₃PbBr₃ perovskite crystals were investigated. The electrical, optical, and morphological characteristics of the perovskite crystals were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), ultraviolet-visible spectroscopy, and photoluminescence (PL). Finally, the CH₃NH₃PbBr₃ perovskite crystals were observed to form a contact with the Ag/C₆₀ as the photodetector, which revealed a responsivity of 24.5 A/W.

Preventing Anion Exchange between Perovskite Nanocrystals by Confinement in Porous SiO2 Nanobeads

All-inorganic CsPbX₃ (X = Cl, Br, I) perovskite nanocrystals (NCs) are highly attractive due to their outstanding optical and electrical properties. However, poor stability and easy anion exchanges between CsPbX₃ nanocrystals with different halides limit their applications in light-emitting diodes (LEDs). To solve the problems, we developed an approach to in situ synthesize CsPbX₃ NCs into porous silica colloidal spheres, which can effectively prevent anion exchange and increase photo stability. Based on our results, we first proved that the anion exchange between CsPbX₃ nanocrystals is mainly driven by physical collision of the nanocrystals, not requiring a bridge such as a solvent. We subsequently used an optimized ratio of green, red, and blue SiO₂/CsPbX₃ composites as solid-state luminescent materials to fabricate single-layer white light-emitting diodes (WLEDs). No anion exchanges have been observed in the LED fabrication and lighting process.

Methylammonium-Mediated Crystallization of Cesium-Based 2D/3D Perovskites toward High-Efficiency Light-Emitting Diodes

Two-dimensional (2D)/three-dimensional (3D) perovskites have been successfully applied in high-efficiency light-emitting diodes (LEDs) because of their large exciton binding energy (E_b) caused by the quantum and dielectric confinements. Thermal annealing and antisolvent treatments are usually executed in order to promote the crystallization and film quality of perovskites, which add complexity to the device fabrication process. Here, the cesium-based 2D/3D perovskite was prepared by introducing ammonium halide benzamidine hydrochloride (BMCl) as the additive. By further introducing an appropriate amount of MABr and PbBr₂, BM₂(Cs_1-xMA_xPbBr₃)_n-1PbBr₄ crystals can be formed rapidly without any additional treatments, while inhibiting the formation of the unfavorable Cs₄PbBr₆ phase. The optimized 2D/3D perovskite-based LEDs achieved a maximum luminance of 12 367 Cd/m², a current efficiency of 17.4 Cd/A, and an external quantum efficiency of 5.2%. Our results suggest that appropriate perovskite crystallization can be achieved at room temperature by the regulation of precursor solution, making the perovskite crystallization process easier to control with reduced processing complexity.

Interface and Defect Engineering for Metal Halide Perovskite Optoelectronic Devices

Metal halide perovskites have been in the limelight in recent years due to their enormous potential for use in optoelectronic devices, owing to their unique combination of properties, such as high absorption coefficient, long charge-carrier diffusion lengths, and high defect tolerance. Perovskite-based solar cells and light-emitting diodes (LEDs) have achieved remarkable breakthroughs in a comparatively short amount of time. As of writing, a certified power conversion efficiency of 22.7% and an external quantum efficiency of over 10% have been achieved for perovskite solar cells and LEDs, respectively. Interfaces and defects have a critical influence on the properties and operational stability of metal halide perovskite optoelectronic devices. Therefore, interface and defect engineering are crucial to control the behavior of the charge carriers and to grow high quality, defect-free perovskite crystals. Herein, a comprehensive review of various strategies that attempt to modify the interfacial characteristics, control the crystal growth, and understand the defect physics in metal halide perovskites, for both solar cell and LED applications, is presented. Lastly, based on the latest advances and breakthroughs, perspectives and possible directions forward in a bid to transcend what has already been achieved in this vast field of metal halide perovskite optoelectronic devices are discussed.

Strategies to Improve Luminescence Efficiency of Metal-Halide Perovskites and Light-Emitting Diodes

Metal-halide perovskites (MHPs) are well suited to be vivid natural color emitters due to their superior optical and electrical properties, such as narrow emission linewidths, easily and widely tunable emission wavelengths, low material cost, and high charge carrier mobility. Since the first development of MHP light-emitting diodes (PeLEDs) in 2014, many researchers have tried to understand the properties of MHP emitters and the limitations to luminescence efficiency (LE) of PeLEDs, and have devoted efforts to increase the LE of MHP emitters and PeLEDs. Within three and half years, PeLEDs have shown rapidly increased LE from external quantum efficiency ≈0.1% to ≈14.36%. Herein, the factors that limit the LE of PeLEDs are reviewed; the factors are characterized into the following groups: i) photophysical properties of MHP crystals, ii) morphological factors of MHP layers, and iii) problems caused by device architectures. Then, the strategies to overcome those luminescence-limiting factors in MHP emitters and PeLEDs are critically evaluated. Finally, research directions to further increase the LE of MHP emitters and the potential of MHPs as a core component in next-generation displays and solid-state lightings are suggested.

Single-phase alkylammonium cesium lead iodide quasi-2D perovskites for color-tunable and spectrum-stable red LEDs

While red is one of the primary colors for display applications, the investigation of visible red emitting perovskites, particularly 2D perovskites, is relatively limited. In this work, we demonstrate a single-phase Ruddlesden-Popper quasi-2D (C3H7NH3)2CsPb2I7 perovskite for red color LEDs. Through increasing the annealing temperature of (C3H7NH3)2CsPb2I7 perovskite thin films, we have successfully achieved tunable emission wavelengths from 654 to 691 nm. Equally important, for all the quasi-2D perovskite LEDs, once the annealing temperature is fixed, the emission spectrum is independent of bias voltages, which is very important for their use in lighting and displays. With the analysis of the crystallinity, morphology, and thermodynamic stability of the quasi-2D perovskite, we find that the obtained (C3H7NH3)2CsPb2I7 perovskite is a single-phase quasi-2D perovskite with only n = 2 phase. Besides, we found that the red shifting of emission wavelength is caused by the increase of perovskite crystal size while increasing the annealing temperature. Our results also show that the temperature-induced color tunability can be applied to a series of quasi-2D perovskites with different alkylammonium cations. Importantly, we find that short alkylammonium spacers offer better electrical properties for efficient current transport and high performance in LED applications. This work contributes to controlling the optoelectronic properties of quasi-2D perovskites via controlling their crystal growth as well as paves the way to realize practical lighting and display applications of perovskite LEDs.