Efficient Sky-Blue Perovskite Light-Emitting Devices Based on Ethylammonium Bromide Induced Layered Perovskites

Perovskite Solar Cells and Light Emitting Diodes: Materials Chemistry, Device Physics and Relationship

Solution-processed perovskite solar cells (PSCs) and perovskite light emitting diodes (PeLEDs) represent promising next-generation optoelectronic technologies. This Review summarizes recent advancements in the application of metal halide perovskite materials for PSC and PeLED devices to address the efficiency, stability and scalability issues. Emphasis is placed on material chemistry strategies used to control and engineer the composition, deposition process, interface and micro-nanostructure in solution-processed perovskite films, leading to high-quality crystalline thin films for optimal device performance. Furthermore, we retrospectively compare the device physics of PSCs and PeLEDs, their working principles and their energy loss mechanisms, examining the similarities and differences between the two types of devices. The reciprocity relationship suggests that a great PSC should also be a great PeLED, motivating the search for interconverting photoelectric bifunctional devices with maximum radiative recombination and negligible non-radiative recombination. Specific requirements of PSCs and PeLEDs in terms of bandgap, thickness, band alignment and charge transport to achieve this target are discussed in detail. Further challenges and issues are also illustrated, together with prospects for future development. Understanding these fundamentals, embracing recent breakthroughs and exploring future prospects pave the way toward the rational design and development of high-performance PSC and PeLED devices.

The Rise of Tandem Perovskite Light-Emitting Diode

In 2024, tandem perovskite light-emitting diodes (tandem-PLEDs) achieved a breakthrough external quantum efficiency of 43.42%, with an organic electroluminescence (EL) unit stacked atop a perovskite EL unit, surpassing the previous single-junction perovskite LEDs. This innovative design enables a higher brightness at lower currents, enhancing the longevity and efficiency of the tandem-PLEDs. Additionally, the tandem-PLEDs can also be fabricated by combining a perovskite EL unit with a perovskite quantum dot unit. In this perspective, the key advancements in tandem-PLEDs are highlighted, focusing on the development of perovskite-organic materials, perovskite-perovskite quantum dots, and the design principles for obtaining efficient and stable charge generation layers. But more importantly, the challenges and solutions are discussed in fabricating all-perovskite tandem LEDs using strongly polar solvents that have yet to be reported nowadays. This comprehensive guide aims to support researchers in advancing the practical deployment of tandem-PLED technology.

Stable and Efficient Mixed-halide Perovskite LEDs

Tailoring bandgap by mixed-halide strategy in perovskites has attracted extraordinary attention due to the flexibility of halide ion combinations and has emerged as the most direct and effective approach to precisely tune the emission wavelength throughout the entire visible light spectrum. Mixed-halide perovskites, yet, still suffered from several problems, particularly phase segregation under external stimuli because of ions migration. Understanding the essential cause and finding sound strategies, thus, remains a challenge for stable and efficient mixed-halide perovskite light-emitting diodes (PeLEDs). The review herein presents an overview of the diverse application scenarios and the profound significance associated with mixed-halide perovskites. We then summarize the challenges and potential research directions toward developing high stable and efficient mixed-halide PeLEDs. The review thus provides a systematic and timely summary for the community to deepen the understanding of mixed-halide perovskite materials and resulting PeLEDs.

Manipulation of Charge Dynamics for Efficient and Bright Blue Perovskite Light-Emitting Diodes with Chiral Ligands

Perovskite light-emitting diodes (PeLEDs) emerge as a promising class of optoelectronic devices for next-generation displays and lighting technology. However, the performance of blue PeLEDs lags far behind that of their green and red counterparts, including the unachieved trade-off between high efficiency and high luminance, severe efficiency roll-off, and unsatisfactory power efficiency. Here, a multi-functional chiral ligand of L-phenylalanine methyl ester hydrochloride is strategically introduced into quasi-2D perovskites, which can effectively passivate defects, modulate the phase distribution, improve photoluminescence quantum yield, guarantee high-quality film morphology, and enhance charge transport. Furthermore, ladder-like hole transport layers are established, boosting charge injection and balance. The resultant sky-blue PeLEDs (the photoluminescence peak is 493 nm and the electroluminescence peak is 497 nm) exhibit an external quantum efficiency of 12.43% at 1000 cd m-2 and a record power efficiency of 18.42 lm W-1 , rendering that the performance is among the best blue PeLEDs.

High-performance electrochemiluminescence sensors based on ultra-stable perovskite quantum dots@ZIF-8 composites for aflatoxin B1 monitoring in corn samples

Aflatoxin B1 (AFB1) that is prone to contaminate corns brings a serious threat to human health. Therefore, it is of great significance to construct novel detection methods for AFB1 tracing. Here, methylamine perovskite quantum dots (MP QDs) encapsulated by ZIF-8 metal-organic frameworks (MP QDs@ZIF-8) were prepared and then ultra-stable electrochemiluminescence (ECL) sensors were developed. By the confinement of cavities structure, multiple MP QDs were crystallized and embedded inside ZIF-8 to form MP QDs@ZIF-8, achieving stable and robust ECL responds in aqueous environment. Further combined with AFB1-imprinted polymer, the constructed ECL sensor showed good selectivity and ultra-sensitivity (the detection limit was 3.5 fg/mL, S/N = 3) with a wide linear range from 11.55 fg/mL to 20 ng/mL for AFB1 quantification. Satisfactory recoveries in corn samples indicated the reliable practicability of the proposed sensor for AFB1 assay. This work provided a novel pathway in designing high-performance ECL sensing platform for food safety.

Influence of Ionic Additives in the PEDOT:PSS Hole Transport Layers for Efficient Blue Perovskite Light Emitting Diodes

Ruddlesden-Popper (RP) perovskites have been gaining traction in the development of high-efficiency or blue-emitting perovskite light emitting diodes (PeLEDs) due to the unique energy funneling mechanism, which enhances photoluminescence intensity, and dimensional control, which enables spectral tuning. In a conventional p-i-n device structure, the quality of RP perovskite films, including grain morphology and defects, as well as device performance can be significantly influenced by the underlying hole-transport layer (HTL). Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is commonly used in several PeLEDs as an HTL because of its high electrical conductivity and optical transparency. Nonetheless, the energy level mismatch and exciton quenching caused by PEDOT:PSS often compromises PeLED performance. Herein, we investigate the mitigation of these effects through addition of work-function-tunable PSS Na to the PEDOT:PSS HTL and assess the impact on blue PeLED performance. Surface analysis of the modified PEDOT:PSS HTLs reveals a PSS-rich layer that alleviates exciton quenching at the HTL/perovskite interface. At an optimal concentration of 6% PSS Na addition, an improvement in the external quantum efficiency is observed, with champion blue and sky-blue PeLEDs achieving 4% (480 nm) and 6.36% (496 nm), respectively, while operation stability is prolonged by fourfold.

Insight into Diphenyl Phosphine Oxygen-Based Molecular Additives as Defect Passivators toward Efficient Quasi-2D Perovskite Light-Emitting Diodes

The introduction of additives has become an important method for enhancing the device performance of quasi-two-dimensional perovskite light-emitting diodes. In this work, we systematically studied the electronic and spatial effects of molecular additives on defect passivation abilities using the methyl, hydrogen, and hydroxyl groups substituted three diphenyl phosphine oxygen additives. The electron-donating conjugation effect of the hydroxyl group on diphenylphosphinic acid (OH-DPPO) leads to a more electron-rich region in OH-DPPO, and the hydroxyl group has a moderate steric hindrance. All these factors endow it with best passivation ability than the other two additives. Furthermore, ion migration was suppressed due to hydrogen bonding between the hydroxyl group and Br. Ultimately, the OH-DPPO passivated devices achieved an external quantum efficiency of 22.44% and a 6-fold improvement in lifetime. These findings provide guidance for developing multifunctional additives in the field of perovskite optoelectronics.

Dual-function perovskite light-emitting/sensing devices for optical interactive display

Interactive display devices integrating multiple functions have become a development trend of display technology. The excellent luminescence properties of perovskite quantum dots (PQDs) make it an ideal luminescent material for the next generation of wide-color gamut displays. Here we design and fabricate dual-function light-sensing/displaying light-emitting devices based on PQDs. The devices can display information as an output port, and simultaneously sense outside light signals as an input port and modulate the display information in a non-contact mode. The dual functions were attributed to the device designs: (1) the hole transport layer in the devices also acts as the light-sensing layer to absorb outside light signals; (2) the introduced hole trapping layer interface can trap holes originating from the light-sensing layer, and thus tune the charge transport properties and the light-emitting intensities. The sensing and display behavior of the device can be further modulated by light signals with different time and space information. This fusion of sensing and display functions has broad prospects in non-contact interactive screens and communication ports.

High-Performance Quasi-Two-Dimensional CsPbBr2.1Cl0.9:PEABr Perovskite Sky-Blue LEDs with an Interface Modification Layer

This paper elucidates the increased luminescence efficiency of CsPbBr2.1Cl0.9 sky-blue perovskite light-emitting diodes (PeLEDs) achieved through the interface modification of 3,4 ethylenedioxythiophene (PEDOT):polystyrene sulfonic acid (PSS)/quasi-two-dimensional (QTD) perovskite using CsCl and CsBr materials, respectively. QTD films were fabricated using ratios of CsPbBr2.1Cl0.9 doped with phenethylamine hydrobromide (PEABr) at 60%, 80%, and 100%. The solvent dimethyl sulfide (C2H6OS) was employed under the excitation of ambient and 365-nm laser lights. The PeLED structure was composed of Al/LiF/2,2',2"-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi)/CsPbBr2.1Cl0.9:PEABr/interface modification layer/PEDOT:PSS/ITO glass. The optimized results revealed that the luminance, current efficiency, and external quantum efficiency of the QTD CsPbBr2.1Cl0.9:80% PEABr PeLED with the CsCl interface modification additive was 892 cd/m2, 3.87 cd/A, and 5.56%, respectively.

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.

High-Performance Blue Quasi-2D Perovskite Light-Emitting Diodes via Balanced Carrier Confinement and Transfer

Extensive investigation of the passivating agents has been performed to suppress the perovskite defects. However, very few attentions have been paid to rationally design the passivating agents for the balance of the carrier confinement and transfer in quasi-2D perovskites, which is essential to achieve high-performance perovskite LEDs (PeLEDs). In this work, tributylphosphine oxide (TBPO) with moderate carbon chain length is demonstrated as a decent passivator for the quasi-2D perovskites by strengthening the carrier confinement for massive radiative recombination within the perovskites, and more importantly providing efficient carrier transfer in the quasi-2D perovskites. Benefiting from these interesting optoelectronic properties of TBPO-incorporated perovskites, we achieve high-efficient blue PeLEDs with an external quantum efficiency up to 11.5% and operational stability as long as 41.1 min without any shift of the electroluminescence spectra. Consequently, this work contributes an effective approach to promote the carrier confinement and transfer for high-performance and stable blue PeLEDs.

Improved phase purity and film quality in quasi-2D perovskite light-emitting diodes by an additive with the trimethacrylate group

Quasi-2D perovskites are potential materials for optoelectronics like light-emitting diodes (LEDs); compared to their 3D counterparts, they are considered more stable against the atmosphere and more efficient in exciton confining. However, the simultaneous formation of different phases in the quasi-2D perovskite film, i.e., the phase impurity issue, lowers the device performance. We propose using a small molecule additive, trimethylolpropane trimethacrylate (TMPTA), to suppress the phase impurity by mixing it into the antisolvent. The phase pure quasi-2D perovskite film was obtained, and meanwhile, the film quality was also improved. Moreover, the ester functional groups in TMPTA also passivate the charged defects in the perovskite film, minimizing the carrier recombination in the device. Correspondingly, with TMPTA modification, the maximum current efficiency is increased by 25%, and the half lifetime of the PeLEDs is prolonged by three times.

Low-Dimensional Metal Halide Perovskite Crystal Materials: Structure Strategies and Luminescence Applications

Replacing methylammonium (MA+ ), formamidine (FA+ ), and/or cesium (Cs+ ) in 3D metal halide perovskites by larger organic cations have built a series of low-dimensional metal halide perovskites (LDMHPs) in which the inorganic metal halide octahedra arranging in the forms of 2D layers, 1D chains, and 0D points. These LDMHPs exhibit significantly different optoelectronic properties from 3D metal halide perovskites (MHPs) due to their unique quantum confinement effects and large exciton binding energies. In particular, LDMHPs often have excellent broadband luminescence from self-trapped excitons. Chemical composition, hydrogen bonding, and external factors (temperature and pressure etc.) determine structures and influence photoelectric properties of LDMHPs greatly, and especially it seems that there is no definite regulation to predict the structure and photoelectric properties when a random cation, metal, and halide is chosen to design a LDMHP. Therefore, this review discusses the construction strategies of the recent reported LDMHPs and their application progress in the luminescence field for a better understanding of these factors and a prospect for LDMHPs' development in the future.

Low-Dimensional-Networked Perovskites with A-Site-Cation Engineering for Optoelectronic Devices

Low-dimensional-networked (LDN) perovskites denote materials in which the molecular structure adopts 2D, 1D, or 0D arrangement. Compared to conventional 3D structured lead halide perovskite (chemical formula: ABX₃ where A: monovalent cations, B: divalent cations, X: halides) that have been studied widely as light absorber and used in current state-or-the-art solar cells, LDN perovskite have unique properties such as more flexible crystal structure, lower ion transport mobility, robust stability against environmental stress such as moisture, thermal, etc., making them attractive for applications in optoelectronic devices. Since 2014, reports on LDN perovskite materials used in perovskite solar cells, light emitting diodes (LEDs), luminescent solar concentrators (LSC), and photodetectors have been reported, aiming to overcome the obstacles of conventional 3DN perovskite materials in these optoelectronic devices. In this review, the variable ligands used to make LDN perovskite materials are summarized, their distinct properties compared to conventional 3D perovskite materials. The research progress of optoelectronic devices including solar cells, LEDs, LSCs, and photodetectors that used different LDNs perovskite, the roles and working mechanisms of the LDN perovskites in the devices are also demonstrated. Finally, key research challenges and outlook of LDN materials for various optoelectronic applications are discussed.

High-performance quasi-2D perovskite light-emitting diodes: from materials to devices

Quasi-two-dimensional (quasi-2D) perovskites have attracted extraordinary attention due to their superior semiconducting properties and have emerged as one of the most promising materials for next-generation light-emitting diodes (LEDs). The outstanding optical properties originate from their structural characteristics. In particular, the inherent quantum-well structure endows them with a large exciton binding energy due to the strong dielectric- and quantum-confinement effects; the corresponding energy transfer among different n-value species thus results in high photoluminescence quantum yields (PLQYs), particularly at low excitation intensities. The review herein presents an overview of the inherent properties of quasi-2D perovskite materials, the corresponding energy transfer and spectral tunability methodologies for thin films, as well as their application in high-performance LEDs. We then summarize the challenges and potential research directions towards developing high-performance and stable quasi-2D PeLEDs. The review thus provides a systematic and timely summary for the community to deepen the understanding of quasi-2D perovskite materials and resulting LED devices.

Energy-Funneling Process in Quasi-2D Perovskite Light-Emitting Diodes

Quasi-two-dimensional (quasi-2D) perovskites, demonstrating excellent radiative efficiency and facile processability, have been considered as next-generation materials for light-emitting applications. Quasi-2D perovskites with a unique energy-funneling process offer an approach to achieve not only high photoluminescence quantum yields at low excitation but also tunable emission induced by dielectric and quantum confinement. In this Perspective, we highlight the mechanism of the energy-funneling process and discuss the salient position of it in quasi-2D perovskite materials for light-emitting applications; we then present the significance of component and molecular engineering strategies for the energy-funneling process to meet the requirements of stable emission and display technologies. Considering present achievements, we also provide promising directions for future advancements of quasi-2D perovskite materials. We hope this Perspective can provide a new viewpoint for researchers to encourage the commercial progress of quasi-2D perovskites for light-emitting applications.

High-Performance Blue Perovskite Light-Emitting Diodes Enabled by Efficient Energy Transfer between Coupled Quasi-2D Perovskite Layers

While there has been extensive investigation into modulating quasi-2D perovskite compositions in light-emitting diodes (LEDs) for promoting their electroluminescence, very few reports have studied approaches involving enhancement of the energy transfer between quasi-2D perovskite layers of the film, which plays very important role for achieving high-performance perovskite LEDs (PeLEDs). In this work, a bifunctional ligand of 4-(2-aminoethyl)benzoic acid (ABA) cation is strategically introduced into the perovskite to diminish the weak van der Waals gap between individual perovskite layers for promoting coupled quasi-2D perovskite layers. In particular, the strengthened interaction between coupled quasi-2D perovskite layers favors an efficient energy transfer in the perovskite films. The introduced ABA can also simultaneously passivate the perovskite defects by reducing metallic Pb for less nonradiative recombination loss. Benefiting from the advanced properties of ABA incorporated perovskites, highly efficient blue PeLEDs with external quantum efficiency of 10.11% and a very long operational stability of 81.3 min, among the best performing blue quasi-2D PeLEDs, are achieved. Consequently, this work contributes an effective approach for high-performance and stable blue PeLEDs toward practical applications.

Phase Control and In Situ Passivation of Quasi-2D Metal Halide Perovskites for Spectrally Stable Blue Light-Emitting Diodes

The fabrication of efficient and spectrally stable pure-blue perovskite light-emitting diodes (LEDs) has been elusive and remains of great interest. Herein, we incorporate diammonium salts into quasi-2D perovskite precursors for phase control of multiple quantum well structures to yield tunable and efficient emission in the blue region. With detailed characterizations and computational studies, we show that in situ passivation by the diammonium salts effectively modifies the surface energies of quasi-2D phases and inhibits the growth of low-band gap quasi-2D and 3D phases. Such phase control and in situ passivation could afford blue light-emitting perovskite thin films with high photoluminescence quantum efficiencies of, for instance, 75% for the emission peak at 471 nm. Using this perovskite thin film as an emitting layer, spectrally stable pure-blue LEDs with an emission peak at 474 nm and a full width at half-maximum of 26 nm could be fabricated to exhibit a brightness of 290 cd m^-2 at 8 V and an external quantum efficiency of 2.17%.

Rearranging Low-Dimensional Phase Distribution of Quasi-2D Perovskites for Efficient Sky-Blue Perovskite Light-Emitting Diodes

Metal halide perovskites have received much attention for their application in light-emitting diodes (LEDs) in the past several years. Rapid progress has been made in efficient green, red, and near-infrared perovskite LEDs. However, the development of blue perovskite LEDs is still lagging far behind. Here, we report efficient sky-blue perovskite LEDs by rearranging low-dimensional phase distribution in quasi-2D perovskites. We incorporated sodium ions into the mixed-Cl/Br quasi-2D perovskites with phenylethylammonium as the organic spacer and cesium lead halide as the inorganic framework. The inclusion of the sodium ion was found to significantly reduce the formation of the n = 1 phase, which was dominated by nonradiative transition, and increase the formation of other small-n phases for efficient exciton energy transfer. By managing the phase distribution, a maximum external quantum efficiency (EQE) of 11.7% was achieved in the sky-blue perovskite LED, with a stable emission peak at 488 nm. Further optimizing the phase distribution and film morphology with Pb content, we demonstrated the sky-blue devices with the average EQE approaching 10%. This strategy of engineering phase distribution of quasi-2D perovskites with a sodium ion could provide a useful way for the fabrication of high-performance blue perovskite LEDs.

Solution-Processed Quasi-Two-Dimensional/Nanoscrystals Perovskite Composite Film Enhances the Efficiency and Stability of Perovskite Light-Emitting Diodes

Solution-processed quasi-two-dimensional (Q-2D)/colloidal perovskite nanocrystals (PNCs) perovskite composite films are first prepared as the emitting layers of perovskite light-emitting diodes (PeLEDs). The subsequent multi-spin-coating of PNCs not only fills the gully-like fluctuations of the nanocrystal pinning-prepared Q-2D perovskite films and decreases their surface roughness but also transforms the bilayer perovskite nanosheets into multilayer ones, thus improving the charge transport and reducing the hole-injection barrier in the composite films. More importantly, the bromide vacancies and Pb defects in the Q-2D perovskites are removed via Br^- supply and Pb-OOC-R interaction, in which the Br ions and COO^- groups (from oleic acid) come from the PNC solution, and the radiation recombination is significantly enhanced. Based on the Q-2D/PNCs perovskite composite emitter, the PeLEDs achieve a maximum luminescence of ∼2.0 × 10⁴ cd/m² and a peak current efficiency of 27.5 cd/A, showing 175 and 337% enhancements compared to the control device with the pristine Q-2D perovskite emitter. The lifetime for the luminance decaying to 50% of the initial intensity increases by a factor of 13.8, demonstrating that the device stability is also improved by the Q-2D/PNCs perovskite composite film.

Large cation ethylammonium incorporated perovskite for efficient and spectra stable blue light-emitting diodes

Perovskite light-emitting diodes (PeLEDs) have showed significant progress in recent years; the external quantum efficiency (EQE) of electroluminescence in green and red regions has exceeded 20%, but the efficiency in blue lags far behind. Here, a large cation CH3CH2NH2+ is added in PEA2(CsPbBr3)2PbBr4 perovskite to decrease the Pb-Br orbit coupling and increase the bandgap for blue emission. X-ray diffraction and nuclear magnetic resonance results confirmed that the EA has successfully replaced Cs+ cations to form PEA2(Cs1-xEAxPbBr3)2PbBr4. This method modulates the photoluminescence from the green region (508 nm) into blue (466 nm), and over 70% photoluminescence quantum yield in blue is obtained. In addition, the emission spectra is stable under light and thermal stress. With configuration of PeLEDs with 60% EABr, as high as 12.1% EQE of sky-blue electroluminescence located at 488 nm has been demonstrated, which will pave the way for the full color display for the PeLEDs.

Bright Blue Light Emission of Ni2+ Ion-Doped CsPbClxBr3-x Perovskite Quantum Dots Enabling Efficient Light-Emitting Devices

In recent years, significant advances have been achieved in the red and green perovskite quantum dot (PQD)-based light-emitting diodes (LEDs). However, the performances of the blue perovskite LEDs are still seriously lagging behind that of the green and red counterparts. Herein, we successfully developed Ni²⁺ ion-doped CsPbCl_xBr_3-x PQDs through the room-temperature supersaturated recrystallization synthetic approach. We simultaneously realized the doping of various concentrations of Ni²⁺ cations and modulated the Cl/Br element ratios by introducing different amounts of NiCl₂ solution in the reaction medium. Using the synthetic method, not only the emission wavelength from 508 to 432 nm of Ni²⁺ ion-doped CsPbCl_xBr_3-x QDs was facially adjusted, but also the photoluminescence quantum yield (PLQY) of PQDs was greatly improved due to efficient removal of the defects of the PQDs. Thus, the blue emission at 470 nm with PLQY of 89% was achieved in 2.5% Ni²⁺ ion-doped CsPbCl_0.99Br_2.01 QDs, which increased nearly three times over that of undoped CsPbClBr₂ QDs and was the highest for the CsPbX₃ PQDs with blue emission, fulfilling the National Television System Committee standards. Benefiting from the highly luminous Ni²⁺ ion-doped PQDs, the blue-emitting LED at 470 nm was obtained, exhibiting an external quantum efficiency of 2.4% and a maximum luminance of 612 cd/m², which surpassed the best performance reported previously for the corresponding blue-emitting PQD-based LED.

Bright and Color-Stable Blue-Light-Emitting Diodes based on Three-Dimensional Perovskite Polycrystalline Films via Morphology and Interface Engineering

Substantial progress has been achieved in red and green perovskite light-emitting diodes (PeLEDs). However, blue PeLEDs are still inferior in light-emitting efficiency and luminance compared with their green and red counterparts. Herein efficient blue PeLEDs simultaneously achieving high luminance and high color stability are fabricated based on the polycrystalline perovskites with a 3D Rb-Cs alloyed scaffold. The synergistic manipulation of an isopropanol antisolvent treatment and the PEDOT:PSS/blue perovskite interface modification with RbCl effectively improve the photoluminescence properties of the resultant blue polycrystalline 3D perovskite films and the final electroluminescence performance of the blue PeLEDs. The optimized blue PeLEDs show a maximum external quantum efficiency of 1.66% with an emission peak at 484 nm and a full width at half-maximum of 18 nm as well as CIE coordinates of (0.08, 0.21). Moreover, the optimized blue PeLEDs not only show superior color stability under various luminances but also achieve high luminances. The obtained maximum luminance of 9243 cd m^-2 is one of the highest values among the efficient and color-stable blue PeLEDs.

Vacuum-Deposited Blue Inorganic Perovskite Light-Emitting Diodes

Perovskite light-emitting diodes (PeLEDs) have drawn great research attention because of their outstanding electroluminescence performance by solution processing. PeLEDs made by thermal evaporation are relatively rarely explored but are compatible to existing organic light-emitting diode industrial lines. Blue-emitting PeLEDs are all based on organic-containing perovskites, rather than more stable all-inorganic perovskites because of their poor solubility, too fast crystallization, uneven discrete films, and unattainable pure blue emission. Here, we report all-inorganic, vacuum-processed blue PeLEDs. High-throughput combinatorial approaches are employed to optimize Cs-Pb-Br-Cl composition in our dual-source co-evaporation system to achieve the balance between film photoluminescence and injection efficiency. The as-deposited perovskite films demonstrated excellent intrinsic stability against heat, UV-light, and humidity attack. A series of PeLEDs were obtained covering the standard blue spectral region with a best luminance of 121 cd/m² and an external quantum efficiency of 0.38%. We believe that the vacuum processing strategy demonstrated here provides a very promising alternative way to produce efficient and stable all-inorganic blue-emitting PeLEDs.

A hybrid blue perovskite@metal-organic gel (MOG) nanocomposite: simultaneous improvement of luminescence and stability

Blue light-emitting hybrid perovskite nanocrystals (NCs) are promising candidates for optoelectronic applications. However, these NCs suffer severely from low photoluminescence quantum yield (PLQY) and inferior stability under working conditions. Herein, we report, for the first time, a simultaneous dramatic improvement in both the luminescence and the stability of hybrid perovskite NCs through embedding in a porous metal-organic gel (MOG) matrix. The nanocomposite (EAPbBr3@MOG, EA: ethylammonium) shows sharp emission in the intense blue region (λ max < 440 nm), with a substantial ten-fold enhancement in the PLQY (∼53%) compared with EAPbBr3 NCs (PLQY ∼5%). Incorporation of perovskite NCs into the soft MOG matrix provides the additional benefits of flexibility as well as water stability. As a proof of principle, these nanocomposites were further utilized to fabricate a white light-emitting diode. The combination of high brightness, stability and flexibility of these nanocomposites could render them viable contenders in the development of efficient, blue light-emitting diodes for practical applications.

A pre-solution mixing precursor method for improving the crystallization quality of perovskite films and electroluminescence performance of perovskite light-emitting diodes

Quasi-two-dimensional (Q-2D) perovskites are one kind of efficient luminescent material with fast energy transfer and radiative decay of excitons due to the energy cascade formed by the mixed perovskite phase. However, the existence of monolayer or bilayer nanosheets in the Q-2D perovskite film results in poor charge transport, high trap density and rough film surface because of the high ratio of ligands, which leads to poor performance of Q-2D perovskite light-emitting diodes (PeLEDs). Herein, we proposed a new strategy of a pre-solution mixing (PSM) precursor to inhibit the formation of ultrathin perovskite nanosheets, which significantly enhanced the charge carrier mobility, reduced the concentration of defects and improved the film morphology. The PeLEDs based on the PSM precursor achieved the maximum luminescence of 7832.1 cd m^-2 (∼218% enhancement) and the peak current efficiency of 6.0 cd A^-1 (∼131% enhancement). By introducing mixed cations in the PeLED, the maximum brightness of 14 211.0 cd m^-2 and current efficiency of 14.6 cd A^-1 were realized, demonstrating the generality of our PSM method for the preparation of high performance PeLEDs.

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.

Direct Synthesis of Quaternary Alkylammonium-Capped Perovskite Nanocrystals for Efficient Blue and Green Light-Emitting Diodes

Cesium lead halide nanocrystals (CsPbX3 NCs) are new inorganic light sources covering the entire visible spectral range and exhibiting near-unity efficiencies. While the last years have seen rapid progress in green and red electroluminescence from CsPbX3 NCs, the development of blue counterparts remained rather stagnant. Controlling the surface state of CsPbX3 NCs had proven to be a major factor governing the efficiency of the charge injection and for diminishing the density of traps. Although didodecyldimethylammonium halides (DDAX; X = Br, Cl) had been known to improve the luminescence of CsPbX3 NCs when applied postsynthetically, they had not been used as the sole long-chain ammonium ligand directly in the synthesis of these NCs. Herein we report a facile, direct synthesis of DDAX-stabilized CsPbX3 NCs. We then demonstrate blue and green light-emitting diodes, characterized by the electroluminescence at 463-515 nm and external quantum efficiencies of 9.80% for green, 4.96% for sky-blue, and 1.03% for deep-blue spectral regions.

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

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

CH3NH3Br solution as a novel platform for the selective fluorescence detection of Pb2+ ions

The development of a simple fluorescent sensor for detecting the Pb²⁺ heavy metal is fundamentally important. The CH₃NH₃PbBr₃ perovskite material exhibits excellent photoluminescence properties that are related to Pb²⁺. Based on the effects of Pb²⁺ on the luminescent properties of CH₃NH₃PbBr₃, we design a novel platform for the selective fluorescence detection of Pb²⁺ ions. Herein, we use a CH₃NH₃Br solution at a high concentration as the fluorescent probe. Incorporation of PbBr₂ into the CH₃NH₃Br solution results in a rapid chemical reaction to form CH₃NH₃PbBr₃. Hence, the nonfluorescent CH₃NH₃Br material displays a sensitive and selective luminescent response to Pb²⁺ under UV light illumination. Moreover, the reaction between CH₃NH₃Br and PbBr₂ could transform Pb²⁺ into CH₃NH₃PbBr₃, and therefore, CH₃NH₃Br may also be used to extract Pb²⁺ from liquid waste in recycling applications.

Enhancing the performance of perovskite light-emitting devices through 1,3,5-tris(2-N-phenylbenzimidazolyl)benzene interlayer incorporation

Interface engineering is important for enhancing the luminance efficiency and stability of perovskite light-emitting devices. In this work, we study the effects of spin-coated 1,3,5-tris(2-N-phenylbenzimidazolyl)benzene (TPBi) layer incorporation on the crystal structure, morphology, photo-physics, and charge transport characteristics of the underlying MAPbBr3 layer. Introduction of such a TPBi interlayer effectively reduces defect density and increases radiative recombination in the MAPbBr3 layer. Related perovskite light-emitting devices with a TPBi interlayer show a maximum external quantum efficiency of 9.9% and power efficiency of 22.1 lm W-1, which are 2.0 and 1.6 times those of the devices without a TPBi interlayer, respectively. The study provides a simple and effective method to enhance the performance of perovskite light-emitting devices.

Appraisement of Crystal Expansion in CH3 NH3 PbI3 on Doping: Improved Photovoltaic Properties

Three-dimensional hybrid perovskite materials (CH₃ NH₃ PbI₃ ) suffer from intrinsic instability owing to organic cation evaporation and ion migration. The inclusion of a large organic cation such as guanidinium has been probed to stabilize the structure. This work proposes the inclusion of imidazolium iodide (C₃ N₂ H₅ I) as an organic cation inside the CH₃ NH₃ PbI₃ matrix, as a reservoir to control the spontaneous loss of iodide. The introduction of imidazolium iodide in amounts below 20 % has an impact on the crystallization process but not on the optical properties. It also positively controls non-radiative recombination and improves the open-circuit voltage of the solar cells. The present study paves way for a deeper insight into the limit of multi-dimensional perovskite to further push the performance.

The Rise of Perovskite Light-Emitting Diodes

Recently, metal halide perovskite materials have attracted great interest in both photovoltaic and electroluminescent devices. The external quantum efficiency of the perovskite light-emitting diodes (pero-LEDs) has grown to over 20% within four years, and the operational lifetime has been improved to tens of hours. These achievements make pero-LEDs very promising technology in solid lighting and displays. In this Perspective, we first give a general introduction of the pero-LEDs; we then discuss various pero-LEDs with different cell configurations and perovskite emitting structures; to conclude, we propose some efficiency improvement strategies and development opportunities of pero-LEDs. Provided that we can continually improve efficiency and operational lifetime of pero-LEDs, we can make them more and more compatible with other mature technologies, like organic LEDs and inorganic LEDs, and then finally realize their practical application in daily life.

Spectra stable blue perovskite light-emitting diodes

Device performance and in particular device stability for blue perovskite light-emitting diodes (PeLEDs) remain considerable challenges for the whole community. In this manuscript, we conceive an approach by tuning the 'A-site' cation composition of perovskites to develop blue-emitters. We herein report a Rubidium-Cesium alloyed, quasi-two-dimensional perovskite and demonstrate its great potential for pure-blue PeLED applications. Composition engineering and in-situ passivation are conducted to further improve the material's emission property and stabilities. Consequently, we get a prominent film photoluminescence quantum yield of around 82% under low excitation density. Encouraged by these findings, we finally achieve a spectra-stable blue PeLED with the peak external quantum efficiency of 1.35% and a half-lifetime of 14.5 min, representing the most efficient and stable pure-blue PeLEDs reported so far. The strategy is also demonstrated to be able to generate efficient perovskite blue emitters and PeLEDs in the whole blue spectral region (from 454 to 492 nm).

Modulation of recombination zone position for quasi-two-dimensional blue perovskite light-emitting diodes with efficiency exceeding 5

In recent years, substantial progress has been made in developing perovskite light-emitting diodes with near-infrared, red and green emissions and over 20% external quantum efficiency. However, the development of perovskite light-emitting diodes with blue emission remains a great challenge, which retards further development of full-color displays and white-light illumination based on perovskite emissive materials. Here, firstly, through composition and dimensional engineering, we prepare quasi-two-dimensional perovskite thin films with improved blue emission, taking advantages of reduced trap density and enhanced photoluminescence quantum yield. Secondly, we find a vertically non-uniform distribution of perovskite crystals in the PEDOT:PSS/perovskite hybrid film. Through modulating the position of the recombination zone, we activate the majority of quasi-two-dimensional perovskite crystals, and thus demonstrate the most efficient blue perovskite light-emitting diode to date with emission peak at 480 nm, record luminance of 3780 cd m-2 and record external quantum efficiency of 5.7%.

Blue perovskite light-emitting diodes: progress, challenges and future directions

Metal halide perovskites have excellent optical and electrical properties and can be easily processed via low-cost solution-based techniques like blade-coating and inkjet printing, promising a bright future for various optoelectronic applications. Recently, encouraging progress has been made in perovskite light-emitting diodes (PeLEDs). Green, red, and near-infrared PeLEDs have achieved high external quantum efficiencies of more than 20%. However, as historically blue electroluminescence remains challenging in all previous LED technologies, we are witnessing a similar case with the development of blue PeLEDs, an essential part of displays and solid-state lighting, which lag far behind those of their counterparts. Herein, we review the recent progress of blue PeLEDs and discuss the main challenges including colour instability, poor photoluminescence efficiency and emission quenching by interlayers. Future directions are provided to facilitate the development of efficient blue PeLEDs.

Incorporation of rubidium cations into blue perovskite quantum dot light-emitting diodes via FABr-modified multi-cation hot-injection method

Solution-processed lead halide perovskite quantum dots (QDs) are emerging as one of the most promising candidates for emissive display application. Although perovskite QDs with a full spectrum of visible light emissions have been realized for years, realizing the efficient electroluminescence of blue perovskites at room temperature still faces severe challenges. Herein, we demonstrate both the efficient photoluminescence and electroluminescence of the blue perovskite QDs via a simple FABr-modified multi-cation hot-injection (FMMHI) method. The FMMHI method is unique in both the addition of FABr into the PbBr2 precursor solution and the incorporation of small rubidium (Rb+) into the blue perovskite QDs light-emitting diodes (QLEDs). The addition of FABr into the precursor solution can realize strong quantum confinement effect, large exciton binding energy and high-quality perovskite QD films. Besides, the bandgap can be enlarged by the Rb+-induced perovskite octahedral distortion and strong quantum confinement effect. Excellent PLQYs of 64.5% and 49.8% were achieved for the developed greenish-blue QDs (Rb0.33Cs0.67)0.42FA0.58PbBr3 and deep-blue QDs (Rb0.33Cs0.67)0.42FA0.58PbCl1.25Br1.75 in solid film state. Moreover, maximum external quantum efficiencies (EQEs) of 3.6% and 0.61% were also achieved with an electroluminescence peak wavelength at 502 and 466 nm, respectively, indicating that the perovskite QDs incorporated with Rb+ possess great potential for the development of high-performance blue perovskite electroluminescence diodes.

Gram-Scale Synthesis of Blue-Emitting CH3NH3PbBr3 Quantum Dots Through Phase Transfer Strategy

Reprecipitation synthesis has been demonstrated to be a simple and convenient route to fabricate high quality perovskite quantum dots toward display applications, whereas the limited chemical yields (< 10%) and difficulty of purification limited its further application. In order to overcome this issue, we here report a modified emulsion synthesis by introducing phase transfer strategy, which achieving effective extraction of newly formed perovskite quantum dots into non-polar solvent and avoiding the degradation of perovskite quantum dots to a large extent. Based on this strategy, gram-scale CH₃NH₃PbBr₃ quantum dots were fabricated in 10 mL (~0.02 mol/L) colloidal solution with chemical yields larger than 70%. The as fabricated CH₃NH₃PbBr₃ quantum dots exhibit an emission peak of 453 nm and a full width at half maximum of only 14 nm. Moreover, electroluminescent devices based on blue emitting CH₃NH₃PbBr₃ quantum dots were also explored with a maximum luminance of 32 cd/m², showing potential applications in blue light emitting devices.

Recombination Dynamics Study on Nanostructured Perovskite Light-Emitting Devices

The field of organic-inorganic hybrid perovskite light-emitting diodes (PeLEDs) has developed rapidly in recent years. Although the performance of PeLEDs continues to improve through film quality control and device optimization, little research has been dedicated to understanding the recombination dynamics in perovskite thin films. Likewise, little has been done to investigate the effects of recombination dynamics on the overall light-emitting behavior of PeLEDs. Therefore, this study investigates the recombination dynamics of CH₃ NH₃ PbI₃ thin films with differing crystal sizes by measurement of fluence-dependent transient absorption dynamics and time-resolved photoluminescence. The aim is to find out the link between recombination dynamics and device behavior in PeLEDs. It is found that bimolecular and Auger recombination become more efficient as the crystal size decreases and monomolecular recombination rate is affected by the trap density of perovskite. By defining the radiative efficiency Φ(n), which relates to the monomolecular, bimolecular, and Auger recombination, the fundamental recombination properties of CH₃ NH₃ PbI₃ films are discerned in quantitative terms. These findings help us to understand the light emission behavior of PeLEDs. This study takes an important step toward establishing the relationship between film structure, recombination dynamics, and device behavior for PeLEDs, thereby providing useful insights toward the design of better perovskite devices.

Color-stable highly luminescent sky-blue perovskite light-emitting diodes

Perovskite light-emitting diodes (PeLEDs) have shown excellent performance in the green and near-infrared spectral regions, with high color purity, efficiency, and brightness. In order to shift the emission wavelength to the blue, compositional engineering (anion mixing) and quantum-confinement engineering (reduced-dimensionality) have been employed. Unfortunately, LED emission profiles shift with increasing driving voltages due to either phase separation or the coexistence of multiple crystal domains. Here we report color-stable sky-blue PeLEDs achieved by enhancing the phase monodispersity of quasi-2D perovskite thin films. We selected cation combinations that modulate the crystallization and layer thickness distribution of the domains. The perovskite films show a record photoluminescence quantum yield of 88% at 477 nm. The corresponding PeLEDs exhibit stable sky-blue emission under high operation voltages. A maximum luminance of 2480 cd m⁻² at 490 nm is achieved, fully one order of magnitude higher than the previous record for quasi-2D blue PeLEDs.

Perovskite light-emitting diodes show promising color tunability and device performance but suffer from emission color shift at higher driving voltages. Here Xing et al. report color stable blue light-emitting diodes by drastically increasing the phase purity of the quasi-2D perovskite thin films.

Organic-Inorganic Hybrid Ruddlesden-Popper Perovskites: An Emerging Paradigm for High-Performance Light-Emitting Diodes

Recently, lead halide perovskite materials have attracted extensive interest, in particular, in the research field of solar cells. These materials are fascinating "soft" materials with semiconducting properties comparable to the best inorganic semiconductors like silicon and gallium arsenide. As one of the most promising perovskite family members, organic-inorganic hybrid Ruddlesden-Popper perovskites (HRPPs) offer rich chemical and structural flexibility for exploring excellent properties for optoelectronic devices, such as solar cells and light-emitting diodes (LEDs). In this Perspective, we present an overview of HRPPs on their structural characteristics, synthesis of pure HRPP compounds and thin films, control of their preferential orientations, and investigations of heterogeneous HRPP thin films. Based on these recent advances, future directions and prospects have been proposed. HRPPs are promising to open up a new paradigm for high-performance LEDs.

Modified Conducting Polymer Hole Injection Layer for High-Efficiency Perovskite Light-Emitting Devices: Enhanced Hole Injection and Reduced Luminescence Quenching

Modification of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) with sodium-poly(styrenesulfonate) leads to a ca. 0.3 eV increase in the work function and 15 times enhancement in the photoluminescence intensity of the overlying perovskite layer, which is closely correlated with the formation of a highly PSS-enriched top layer. As a direct result, the hybrid halide perovskite light-emitting devices with a modified PEDOT:PSS layer show the maximum external quantum efficiency of 7.2% and power efficiency of 19.0 lm W^-1, which are 14-20 times those of the analogous devices using a pristine PEDOT:PSS layer and among the best reported values for the light-emitting devices using a neat perovskite emission layer. Our results illustrate that insufficient hole injection and luminescence quenching at the PEDOT:PSS anode are among the most important factors limiting the external quantum efficiencies of inverted perovskite light-emitting devices.