High-Efficiency Polycrystalline Perovskite Light-Emitting Diodes Based on Mixed Cations

Highly efficient blue light-emitting diodes based on mixed-halide perovskites with reduced chlorine defects

Perovskite light-emitting diodes (PeLEDs) provide excellent opportunities for low-cost, color-saturated, and large-area displays. However, the performance of blue PeLEDs lags far behind that of their green and red counterparts. Here, we show that the external quantum efficiencies (EQEs) of blue PeLEDs scale linearly with the photoluminescence quantum yields (PL QYs) of CsPb(BrxCl1-x)3 nanocrystals emitting at 460 to 480 nm. The recombination efficiency of carriers is highly sensitive to the chlorine content and the related deep-level defects in nanocrystals, causing notable EQE drops even with minor increases in chlorine defects. Minor adjustments of chlorine content through rubidium compensation on the A-site effectively suppress the formation of nonradiative defects, improving PL QYs while retaining desirable bandgaps for blue-emitting nanocrystals. Our PeLEDs with record-high efficiencies span the blue spectrum, achieving peak EQEs of 12.0% (460 nm), 16.7% (465 nm), 21.3% (470 nm), 24.3% (475 nm), and 26.4% (480 nm). This work exemplifies chlorine-defect control as a key design principle for high-efficiency blue PeLEDs.

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

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

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.

Modulation Phase Distribution of Ruddlesden-Popper Quasi-2D Perovskites with a Similarly Spaced Dion-Jacobson Phase

Quasi-two-dimensional (quasi-2D) perovskites exhibit excellent performance when applied to light-emitting diodes (LEDs). However, quasi-2D perovskite films generally have nonuniform n phases and irregular internal crystal structures, which degrade the device's performance. Here, we propose using a Dion-Jacobson (DJ)-type organic spacer to modulate the phase distribution of the Ruddlesden-Popper (RP) quasi-2D perovskite. A DJ-type organic spacer cation, 1.6-hexamethylenediamine (HDABr2), was introduced into the perovskite as the second spacer cation with propylamine hydrobromide (PABr). As DJ-type and RP-type perovskites have similar spacings, RP-DJ style does not cause a chaotic crystalline structure; instead, it modulates the perovskite crystallization and narrows the phase distribution. In parallel, there is a substantial improvement in the maximum luminance, current efficiency, external quantum efficiency, and device stability of the quasi-2D perovskite LEDs. This work provides a novel concept for combining the organic spacer cations for quasi-2D perovskites.

A generic lanthanum doping strategy enabling efficient lead halide perovskite luminescence for backlights

Metal halide perovskites films have attracted great interest because of their solution-proceed fabrication and potential applications for next-generation displays. However, their non-ideal photoluminescence quantum efficiency (PLQE) and stability still does not meet the requirements of displays. Here, we adopt lanthanum ion (La³⁺) doping strategy to enhance its luminescence performance and the possibility of taking it to commercialization. In addition to the entry of lanthanum ions into the lattice to greatly improve the crystal quality, the excess La³⁺ gives rise to the formation of newly developed formamidinium cesium lanthanum bromine, (FA, Cs)₂LaBr₅, which provides additional energy transport pathways, therefore concentrating more energy onto the perovskite host. Consequently, a near-unity PLQE of 99.5% is realized in standardized green-emission cesium (Cs)/formamidinium (FA) mixed FA_0.7Cs_0.3PbBr₃ perovskite films. The introduction of La³⁺ can also lead to markedly stability with nearly 1000 days' shelf storage half-lifetime and 400 hours' light-irradiation T₉₀ lifetime as well as much-enhanced color purity. Moreover, the films with La³⁺ doping enable full-visible spectral enhancement and high-performance white light emission and trichromatic luminescence with 98.9% coverage of the color gamut area required for the Rec. 2020 standard, which suggest that perovskite films have great application potential for backlights in liquid crystal display technologies.

From Heterostructures to Solid-Solutions: Structural Tunability in Mixed Halide Perovskites

The stability, reliability, and performance of halide-perovskite-based devices depend upon the structure, composition, and particle size of the device-enabling materials. Indeed, the degree of ion mixing in multicomponent perovskite crystals, although challenging to control, is a key factor in determining properties. Herein, an emerging method termed evaporation-crystallization polymer pen lithography is used to synthesize and systematically study the degree of ionic mixing of Cs_0.5 FA_0.5 PbX₃ (FA = formamidinium; X = halide anion, ABX₃ ) crystals, as a function of size, temperature, and composition. These experiments have led to the discovery of a heterostructure morphology where the A-site cations, Cs and FA, are segregated into the core and edge layers, respectively. Simulation and experimental results indicate that the heterostructures form as a consequence of a combination of both differences in solubility of the two ions in solution and the enthalpic preference for Cs-FA ion segregation. This preference for segregation can be overcome to form a solid-solution by decreasing crystal size (<60 nm) or increasing temperature. Finally, these tools are utilized to identify and synthesize solid-solution nanocrystals of Cs_0.5 FA_0.5 Pb(Br/I)₃ that significantly suppress photoinduced anion migration compared to their bulk counterparts, offering a route to deliberately designed photostable optoelectronic materials.

Ultra-bright, efficient and stable perovskite light-emitting diodes

Metal halide perovskites are attracting a lot of attention as next-generation light-emitting materials owing to their excellent emission properties, with narrow band emission^1-4. However, perovskite light-emitting diodes (PeLEDs), irrespective of their material type (polycrystals or nanocrystals), have not realized high luminance, high efficiency and long lifetime simultaneously, as they are influenced by intrinsic limitations related to the trade-off of properties between charge transport and confinement in each type of perovskite material^5-8. Here, we report an ultra-bright, efficient and stable PeLED made of core/shell perovskite nanocrystals with a size of approximately 10 nm, obtained using a simple in situ reaction of benzylphosphonic acid (BPA) additive with three-dimensional (3D) polycrystalline perovskite films, without separate synthesis processes. During the reaction, large 3D crystals are split into nanocrystals and the BPA surrounds the nanocrystals, achieving strong carrier confinement. The BPA shell passivates the undercoordinated lead atoms by forming covalent bonds, and thereby greatly reduces the trap density while maintaining good charge-transport properties for the 3D perovskites. We demonstrate simultaneously efficient, bright and stable PeLEDs that have a maximum brightness of approximately 470,000 cd m^-2, maximum external quantum efficiency of 28.9% (average = 25.2 ± 1.6% over 40 devices), maximum current efficiency of 151 cd A^-1 and half-lifetime of 520 h at 1,000 cd m^-2 (estimated half-lifetime >30,000 h at 100 cd m^-2). Our work sheds light on the possibility that PeLEDs can be commercialized in the future display industry.

Colloidal FAPbBr3 perovskite nanocrystals for light emission: what's going on?

Semiconducting nanomaterials have been widely explored in diverse optoelectronic applications. Colloidal lead halide perovskite nanocrystals (NCs) have recently been an excellent addition to the field of nanomaterials, promising an enticing building block in the field of light emission. In addition to the notable optoelectronic properties of perovskites, the colloidal NCs exhibit unique size-dependent optical properties due to the quantum size effect, which makes them highly attractive for light-emitting diodes (LEDs). In the past few years, perovskite-based LEDs (PeLEDs) have demonstrated a meteoritic rise in their external quantum efficiency (EQE) values, reaching over 20% so far. Among various halide perovskite compositions, FAPbBr3 and its variants remain one of the most interesting and sought-after compounds for green light emission. This review focuses on recent progress in the design and synthesis protocols of colloidal FAPbBr3 NCs and the emerging concepts in tailoring their surface chemistry. The structural and physicochemical features of lead halide perovskites along with a comprehensive discussion on their defect-tolerant properties are briefly outlined. Later, the prevalent synthesis, ligand, and compositional engineering strategies to boost the stability and photoluminescence quantum yield (PLQY) of FAPbBr3 NCs are extensively discussed. Finally, the fundamental concepts and recent progress on FAPbBr3-based LEDs, followed by a discussion of the challenges and prospects that are on the table for this enticing class of perovskites, are reviewed.

Defect Passivation Using a Phosphonic Acid Surface Modifier for Efficient RP Perovskite Blue-Light-Emitting Diodes

Defect management strategies are vital for enhancing the performance of perovskite-based optoelectronic devices, such as perovskite-based light-emitting diodes (PeLEDs). As additives can fucntion both as acrystallization modifier and/or defect passivator, a thorough study on the roles of additives is essential, especially for blue emissive Pe-LEDs, where the emission is strictly controlled by the n-domain distribution of the Ruddlesden-Popper (RP, L₂A_n-1Pb_nX_3n+1, where L refers to a bulky cation, while A and X are monovalent cation, and halide anion, respectively) perovskite films. Of the various additives that are available, octyl phosphonic acid (OPA) is of immense interest because of its ability to bind with uncoordinated Pb²⁺ ( notorious for nonradiative recombination) and therefore passivates them. Here, with the help of various spectroscopic techniques, such as X-ray photon-spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), and photoluminescence quantum yield (PLQY) measurements, we demonstrate the capability of OPA to bind and passivate unpaired Pb²⁺ defect sites. Modification to crystallization promoting higher n-domain formation is also observed from steady-state and transient absorption (TA) measurements. With OPA treatment, both the PLQY and EQE of the corresponding PeLED showed improvements up to 53% and 3.7% at peak emission wavelength of 485 nm, respectively.

Temperature-Dependent Optical Properties of Perovskite Quantum Dots with Mixed-A-Cations

In this work, metal halide perovskite quantum dots (QDs) with Formamidinium (FA) and Cs mixed cations were fabricated using a solution-processed method at room temperature. By controlling Cs doping ratios in a precursor, the optical properties of mixed-cation perovskite QDs were systematically studied. With the increase in Cs ion doping, the photoluminescence (PL) spectra of perovskite QDs were blueshifted, which was mainly due to the smaller radius of Cs ions than those of FA. Temperature-dependent PL spectra were conducted on mixed-cation perovskite QDs. As the temperature gradually increased from 4 K to 300 K, PL peaks were blue shifted, and full-width at half maximum (FWHM) was widened, which was directly related to lattice thermal expansion and the carrier-photon coupling effect under temperature variation. At the same time, excess Cs ion doping had a prominent influence on optical properties at low temperatures, which was mainly due to the introduction of detrimental defects in perovskite crystals. Therefore, it is particularly important to control doping concentration in the preparation of high-quality perovskite QDs and efficient photoelectric devices.

Impact of Cesium Concentration on Optoelectronic Properties of Metal Halide Perovskites

Performance of a perovskite solar cell is largely influenced by the optoelectronic properties of metal halide perovskite films. Here we study the influence of cesium concentration on morphology, crystal structure, photoluminescence and optical properties of the triple cation perovskite film. Incorporation of small amount (x = 0.1) of cesium cations into Csx(MA0.17FA0.83)1−x Pb(I0.83Br0.17)3 leads to enhanced power conversion efficiency (PCE) of the solar cell resulting mainly from significant rise of the short-current density and the fill factor value. Further increase of Cs concentration (x > 0.1) decreases the film’s phase purity, carrier lifetime and correspondingly reduces PCE of the solar cell. Higher concentration of Cs (x ≥ 0.2) causes phase segregation of the perovskite alongside with formation of Cs-rich regions impeding light absorption.

Efficient Dion-Jacobson perovskite light-emitting diodes via mixed cation engineering

Quasi-two-dimensional (quasi-2D) perovskites with high exciton binding energy (E_b) can confine carriers to form an energy funnel cascade, accelerate carrier localization to the emitting domain, and decrease nonradiative recombination loss. Herein, it is shown that partially alloying Cs⁺ cations into formamidinium (FA)-based Dion-Jacobson (DJ) perovskites and adjusting the stoichiometric ratio can simultaneously modify the tolerance factor, decrease the phase formation enthalpy, improve the morphology, modulate the phase distribution, and boost the current efficiency. By incorporating CsBr to substitute for some of the FABr, perovskite films with narrower phase distributions and fewer defects are obtained, and the current efficiency is boosted from 18.2 to 25.3 cd/A. A high current efficiency of 42.1 cd/A, a record (as far as we are aware) external quantum efficiency (EQE) of 10.5%, and a maximum luminance of 18600 cd/m² with an emission peak at 529 nm are obtained when the Lewis base passivation agent TPBi is dissolved in the antisolvent. This is the first time, to the best of the authors' knowledge, that efficient 1,4-phenyldimethylammonium dibromide (PHDMADBr)-based green-light DJ perovskite light emitting diodes (PeLEDs) based on mixed Cs⁺ and FA⁺ cations have been fabricated.

Ionic liquid-induced in situ deposition of perovskite quantum dot films with a photoluminescence quantum yield of over 85

Metal halide perovskite quantum dots (QDs) hold great promise as building blocks for next-generation light emitting devices (LEDs). The preparation of perovskite QD films with high photoluminescence quantum yield (PLQY) is the key to realizing efficient LEDs. However, the conventional deposition method of spin-coating of pre-synthesized QD ink solutions results in perovskite QD films with low PLQY (typically <45%) due to non-radiative recombination centers induced in the deposition process. Here, by utilizing the ionic nature and steric hindrance effect of the ionic liquid, we demonstrate an in situ deposition method for perovskite QD films with high PLQY by directly spin-coating precursor solutions containing an ionic liquid. Furthermore, mechanistic study reveals that the ionic liquid not only induces the formation of QDs but also suppresses defect-related recombination through the interaction with uncoordinated metal atoms on the surface of the QDs. As a result, the in situ deposited CsPbBr₃ QD film with a PLQY as high as 85.2% and long-term air stability is achieved. These findings demonstrate that the introduction of an ionic liquid provides an effective strategy to enhance the performance of in situ formed perovskite QD films, which could benefit the development of efficient LEDs and other optoelectronic devices.

Exploiting a Multiphase Pure Formamidinium Lead Perovskite for Efficient Green-Light-Emitting Diodes

Formamidinium (FA)-based perovskites have demonstrated excellent advances in optoelectronics recently, but in terms of perovskite compounds, the optimal condition of their composition optimization remains controversial. Herein, we boosted the performance of perovskite light-emitting diodes (PeLEDs) in both efficiency and stability through composition and phase engineering. The low-dimensional FA₂PbBr₄ particles emerged and mixed with the FAPbBr₃ composites when the FABr content is excessive. By regulating the phase composition in multiphase FAPbBr₃ perovskites, the maximum external quantum efficiency (EQE_max) of optimal PeLEDs (5.14%) is 2.7-fold higher than that of pure FAPbBr₃-based PeLEDs (1.90%), and device stability is improved. Further optimizing the film quality and emission efficiency with Rb⁺ ions, the EQE_max of PeLEDs can be increased to 6.01%, with a half-lifetime of about 185 s under the high current density. This strategy of phase composition optimization in FA-based perovskites provides an effective way to process high-efficiency and stable PeLEDs.

High-performance large-area quasi-2D perovskite light-emitting diodes

Serious performance decline arose for perovskite light-emitting diodes (PeLEDs) once the active area was enlarged. Here we investigate the failure mechanism of the widespread active film fabrication method; and ascribe severe phase-segregation to be the reason. We thereby introduce L-Norvaline to construct a COO--coordinated intermediate phase with low formation enthalpy. The new intermediate phase changes the crystallization pathway, thereby suppressing the phase-segregation. Accordingly, high-quality large-area quasi-2D films with desirable properties are obtained. Based on this, we further rationally adjusted films' recombination kinetics. We reported a series of highly-efficient green quasi-2D PeLEDs with active areas of 9.0 cm2. The peak EQE of 16.4% is achieved in  = 3, represent the most efficient large-area PeLEDs yet. Meanwhile, high brightness device with luminance up to 9.1 × 104 cd m-2 has achieved in = 10 film.

Self-defect-passivation by Br-enrichment in FA-doped Cs1-xFAxPbBr3 quantum dots: towards high-performance quantum dot light-emitting diodes

Halide vacancy defect is one of the major origins of non-radiative recombination in the lead halide perovskite light emitting devices (LEDs). Hence the defect passivation is highly demanded for the high-performance perovskite LEDs. Here, we demonstrated that FA doping led to the enrichment of Br in Cs_1−xFA_xPbBr₃ QDs. Due to the defect passivation by the enriched Br, the trap density in Cs_1−xFA_xPbBr₃ significantly decreased after FA doping, and which improved the optical properties of Cs_1−xFA_xPbBr₃ QDs and their QD-LEDs. PLQY of Cs_1–xFA_xPbBr₃ QDs increased from 76.8% (x = 0) to 85.1% (x = 0.04), and L_max and CE_max of Cs_1–xFA_xPbBr₃ QD-LEDs were improved from L_max = 2880 cd m⁻² and CE_max = 1.98 cd A⁻¹ (x = 0) to L_max = 5200 cd m⁻² and CE_max = 3.87 cd A⁻¹ (x = 0.04). Cs_1–xFA_xPbBr₃ QD-LED device structure was optimized by using PVK as a HTL and ZnO modified with b-PEI as an ETL. The energy band diagram of Cs_1–xFA_xPbBr₃ QD-LEDs deduced by UPS analyses.

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.

Competitive Nucleation Mechanism for CsPbBr3 Perovskite Nanoplatelet Growth

We analyze nucleation-controlled nanocrystal growth in a solution containing surface-binding molecular ligands, which can also nucleate compact layers on the crystal surfaces. We show that, if the critical nucleus size for ligands is larger and the nucleation barrier is lower than those for crystal atoms, the ligands nucleate faster than the atoms on relatively wide crystal facets but much slower, if at all, on narrow facets. Such competitive nucleation of ligands and atoms results in ligands covering predominantly wider facets, thus excluding them from the growth process, and acts as a selection mechanism for the growth of crystals with narrower facets, the so-called nanoplatelets. The theory is confirmed by Monte Carlo simulations and validated experimentally for CsPbBr₃ nanoplatelets grown from solution. We find that the anisotropic crystal growth is controlled by the growth temperature and the strength of surface bonding for the passivating molecular ligands.

Strategies to Achieve High Circularly Polarized Luminescence from Colloidal Organic-Inorganic Hybrid Perovskite Nanocrystals

Colloidal metal halide perovskite nanocrystals (NCs) with chiral ligands are outstanding candidates as a circularly polarized luminescence (CPL) light source due to many advantages such as high photoluminescence quantum efficiency, large spin-orbit coupling, and extensive tunability via composition and choice of organic ligands. However, achieving pronounced and controllable polarized light emission remains challenging. Here, we develop strategies to achieve high CPL responses from colloidal formamidinium lead bromide (FAPbBr3) NCs at room temperature using chiral surface ligands. First, we show that replacing a portion of typical ligands (oleylamine) with short chiral ligands ((R)-2-octylamine) during FAPbBr3 NC synthesis results in small and monodisperse NCs that yield high CPL with average luminescence dissymmetry g-factor, glum = 6.8 × 10-2. To the best of our knowledge, this is the highest among reported perovskite materials at room temperature to date and represents around 10-fold improvement over the previously reported colloidal CsPbClxBryI3-x-y NCs. In order to incorporate NCs into any optoelectronic or spintronic application, the NCs necessitate purification, which removes a substantial amount of the chiral ligands and extinguishes the CPL signals. To circumvent this issue, we also developed a postsynthetic ligand treatment using a different chiral ligand, (R-/S-)methylbenzylammonium bromide, which also induces a CPL with an average glum = ±1.18 × 10-2. This postsynthetic method is also amenable for long-range charge transport since methylbenzylammonium is quite compact in relation to other surface ligands. Our demonstrations of high CPL and glum from both as-synthesized and purified perovskite NCs at room temperature suggest a route to demonstrate colloidal NC-based spintronics.

High-Brightness Perovskite Light-Emitting Diodes Using a Printable Silver Microflake Contact

Achieving efficient devices while maintaining a high fabrication yield is a key challenge in the fabrication of solution-processed, perovskite-based light-emitting diodes (PeLEDs). In this respect, pinholes in the solution-processed perovskite layers are a major obstacle. These are usually mitigated using organic electron-conducting planarization layers. However, these organic interlayers are unstable under applied bias in air and suffer from limited charge carrier mobility. In this work, we present a high brightness p-i-n PeLED based on a novel blade-coated silver microflake (SMF) rear electrode, which allows for a low-cost nanocrystalline ZnO inorganic electron-transporting layer to be used. This novel SMF contact is crucial for achieving high performance as it prevents the electrical shorting suffered when standard thermally evaporated silver rear contacts are used. The fabricated PeLEDs exhibit an excellent maximum luminance of 98,000 cd/m², a maximum current efficiency of 22.3 cd/A, and a high external quantum efficiency of 4.6% under 5.9 V forward bias. The SMF rear contact can be printed and scaled at low cost to large areas and applied to flexible devices.

Surface-2D/Bulk-3D Heterophased Perovskite Nanograins for Long-Term-Stable Light-Emitting Diodes

Although metal halide perovskite (MHP) light-emitting diodes (LEDs) have demonstrated great potential in terms of electroluminescence efficiency, the operational stability of MHP LEDs currently remains the biggest bottleneck toward their practical usage. Well-confined excitons/charge carriers in a dielectric/quantum well based on conventional spatial or potential confinement approaches substantially enhance radiative recombination in MHPs, but an increased surface-to-volume ratio and multiphase interfaces likely result in a high degree of surface or interface defect states, which brings about a critical environmentally/operationally vulnerable point on LED stability. Here, an effective solution is suggested to mitigate such drawbacks using strategically designed surface-2D/bulk-3D heterophased MHP nanograins for long-term-stable LEDs. The 2D surface-functionalized MHP renders significantly reduced trap density, environmental stability, and an ion-migration-immune surface in addition to a fast radiative recombination owing to its spatially and potentially confined charge carriers, simultaneously. As a result, heterophased MHP LEDs show substantial improvement in operational lifetime (T₅₀ : >200 h) compared to conventional pure 3D or quasi-2D counterparts (T₅₀ : < 0.2 h) as well as electroluminescence efficiency (surface-2D/bulk-3D: ≈7.70 ph per el% and pure 3D: ≈0.46 ph per el%).

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.

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.

Phase Engineering for Highly Efficient Quasi-Two-Dimensional All-Inorganic Perovskite Light-Emitting Diodes via Adjusting the Ratio of Cs Cation

Quasi-two-dimensional (2D) perovskites have received intensive attention as a new class of luminescent materials owing to large exciton binding energy and high photoluminescence efficiency. However, there usually contains a mixture of phases in these materials, and excessive low-dimensional phase perovskite is harmful for luminescence efficiency owing to the strong exciton-phonon quenching at the room temperature. Herein, a simple and effective method is proposed to suppress the growth of low-dimensional phase components in quasi-2D perovskite film via carefully adjusting the molar ratio of cesium bromide (CsBr) and phenylpropylammonium bromide (PPABr). The device based on this optimized film has achieved a peak brightness of 2921 cd m^-2 and peak current efficiency of 1.38 cd A^-1, far away higher than that of the pristine CsPbBr₃ device. This research proves a new way for modulating the phase composition in quasi-2D perovskites to fabricate highly efficient perovskite light-emitting diodes (PeLEDs).

Vivid and Fully Saturated Blue Light-Emitting Diodes Based on Ligand-Modified Halide Perovskite Nanocrystals

CsPbX₃ (X = I, Br, Cl) perovskite nanocrystals (NCs) have recently emerged as emitting materials for optoelectronic and display applications owing to their easily tunable emissions, high photoluminescence quantum yield (PLQY), and vivid color purity (full width at half maximum of approximately 20 nm). However, the lagging quantum yields of blue-emitting perovskite NCs have resulted in low efficiency compared to green or red perovskite light-emitting diodes (PeLEDs); moreover, the long insulating ligands (such as oleylamine and oleic acid) inhibit charge carrier injection. In this study, we demonstrated a facile ligand-mediated post-treatment (LMPT) method for high-quality perovskite NCs with changing optical properties to allow fine-tuning of the target emission wavelength. This method involves the use of a mixed halide ion-pair ligand, di-dodecyl dimethyl ammonium bromide, and chloride, which can induce a reconstruction through a self-anion exchange. Using the LMPT method, the PLQY of the surface-passivated blue-emitting NCs was dramatically enhanced to over 70% within the 485 nm blue emission region and 50% within the 467 nm deep-blue emission region. Through this treatment, we achieved highly efficient blue-PeLED maximum external quantum efficiencies of 0.44 and 0.86% within the 470 and 480 ± 2 nm electroluminescence emission regions, respectively.

Perovskites for Next-Generation Optical Sources

Next-generation displays and lighting technologies require efficient optical sources that combine brightness, color purity, stability, substrate flexibility. Metal halide perovskites have potential use in a wide range of applications, for they possess excellent charge transport, bandgap tunability and, in the most promising recent optical source materials, intense and efficient luminescence. This review links metal halide perovskites' performance as efficient light emitters with their underlying materials electronic and photophysical attributes.

Facile Synthesis of Stable and Highly Luminescent Methylammonium Lead Halide Nanocrystals for Efficient Light Emitting Devices

Metal halide perovskites are promising candidates for use in light emitting diodes (LEDs), due to their potential for color tunable and high luminescence efficiency. While recent advances in perovskite-based light emitting diodes have resulted in external quantum efficiencies exceeding 12.4% for the green emitters, and infrared emitters based on 3 D/2D mixed dimensional perovskites have exceeded 20%, the external quantum efficiencies of the red and blue emitters still lag behind. A critical issue to date is creating highly emissive and stable perovskite emitters with the desirable emission band gap to achieve full-color displays and white LEDs. Herein, we report the preparation and characterization of a highly luminescent and stable suspension of cubic-shaped methylammonium lead triiodide (CH₃NH₃PbI₃) perovskite nanocrystals, where we synthesize the nanocrystals via a ligand-assisted reprecipitation technique, using an acetonitrile/methylamine compound solvent system to solvate the ions and toluene as the antisolvent to induce crystallization. Through tuning the ratio of the ligands, the ligand to toluene ratio, and the temperature of the toluene, we obtain a solution of CH₃NH₃PbI₃ nanocrystals with a photoluminescence quantum yield exceeding 93% and tunable emission between 660 and 705 nm. We also achieved red emission at 635 nm by blending the nanocrystals with bromide salt and obtained perovskite-based light emitting diodes with maximum electroluminescent external quantum efficiency of 2.75%.

High-performance and stable CsPbBr3 light-emitting diodes based on polymer additive treatment

Because of their high efficiency and sharp emission, perovskite light-emitting diodes are a promising candidate for next-generation lighting techniques. However, the relatively poor stability of perovskite light-emitting diodes lowers their utility. Therefore, a highly stable perovskite light-emitting diode has to be developed to meet the commercial demand. Herein, we report a highly stable CsPbBr₃ light-emitting diode via simple polymer treatment. The addition of 2-methyl-2-oxazoline in perovskite film assists the formation of CsPbBr₃ nanocrystals, improving the quality and photoluminescence property of perovskite film. Based on such CsPbBr₃ nanocrystals and polymer hybrid film, our device presents a high external quantum efficiency and luminance of around 3.0% and 16 648 cd m⁻², respectively. Moreover, an excellent device half-lifetime of more than 2.4 hours has been achieved, under continuous operation at a relatively high initial luminance of 1000 cd m⁻², representing one of the most stable PeLEDs operated at such high initial luminance.

The 2-methyl-2-oxazoline additive induced the formation of high-quality CsPbBr₃ nanocrystals and a stable PeLED with a half-lifetime of 2.4 hours at high initial luminance of 1000 cd m⁻² was demonstrated, representing one of the most stable PeLEDs.

Mixed Lead-Tin Halide Perovskites for Efficient and Wavelength-Tunable Near-Infrared Light-Emitting Diodes

Near-infrared (NIR) light-emitting diodes (LEDs), with emission wavelengths between 800 and 950 nm, are useful for various applications, e.g., night-vision devices, optical communication, and medical treatments. Yet, devices using thin film materials like organic semiconductors and lead based colloidal quantum dots face certain fundamental challenges that limit the improvement of external quantum efficiency (EQE), making the search of alternative NIR emitters important for the community. In this work, efficient NIR LEDs with tunable emission from 850 to 950 nm, using lead-tin (Pb-Sn) halide perovskite as emitters are demonstrated. The best performing device exhibits an EQE of 5.0% with a peak emission wavelength of 917 nm, a turn-on voltage of 1.65 V, and a radiance of 2.7 W Sr^-1 m^-2 when driven at 4.5 V. The emission spectra of mixed Pb-Sn perovskites are tuned either by changing the Pb:Sn ratio or by incorporating bromide, and notably exhibit no phase separation during device operation. The work demonstrates that mixed Pb-Sn perovskites are promising next generation NIR emitters.

Retracted Article: Facile preparation of bithiazole-based material for inkjet printed light-emitting electrochemical cell

Light-emitting electrochemical cell of bithiazole-based material was fabricated by solution processing rendered high external quantum efficiency over 12.8% and luminance of 1.8 10⁴ cd m⁻².

Light-emitting electrochemical cell of bithiazole-based material was fabricated by solution processing rendered high external quantum efficiency over 12.8% and luminance of 1.8 10⁴ cd m^−2.

Excitonic phenomena in perovskite quantum-dot supercrystals

Quantum confinement and collective excitations in perovskite quantum-dot (QD) supercrystals offer multiple benefits to the light emitting and solar energy harvesting devices of modern photovoltaics. Recent advances in the fabrication technology of low dimensional perovskites has made the production of such supercrystals a reality and created a high demand for the modelling of excitonic phenomena inside them. Here we present a rigorous theory of Frenkel excitons in lead halide perovskite QD supercrystals with a square Bravais lattice. The theory shows that such supercrystals support three bright exciton modes whose dispersion and polarization properties are controlled by the symmetry of the perovskite lattice and the orientations of QDs. The effective masses of excitons are found to scale with the ratio of the superlattice period and the number of QDs along the supercrystal edge, allowing one to fine-tune the electro-optical response of the supercrystals as desired for applications. We also calculate the conductivity of perovskite QD supercrystals and analyze how it is affected by the optical generation of the three types of excitons. This paper provides a solid theoretical basis for the modelling of two- and three-dimensional supercrystals made of perovskite QDs and the engineering of photovoltaic devices with superior optoelectronic properties.

Enhanced Electroluminescence from Organic Light-Emitting Diodes with an Organic-Inorganic Perovskite Host Layer

The development of host materials with high performance is essential for fabrication of efficient and stable organic light-emitting diodes (OLEDs). Although host materials used in OLEDs are typically organics, in this study, it is shown that the organic-inorganic perovskite CH₃ NH₃ PbCl₃ (MAPbCl₃ ) can be used as a host layer for OLEDs. Vacuum-evaporated MAPbCl₃ films have a wide band gap of about 3 eV and very high and relatively balanced hole and electron mobilities, which are suitable for the host material. Photoluminescence and electroluminescence take place through energy transfer from MAPbCl₃ to an organic emitter in films. Incorporation of an MAPbCl₃ host layer into OLEDs leads to a reduction of driving voltage and enhancement of external quantum efficiency as compared to devices with a conventional organic host layer. Additionally, OLEDs with an MAPbCl₃ host layer demonstrate very good operational stability under continuous current operation. These results can be extensively applied to organic- and perovskite-based optoelectronics.

Efficient Light-Emitting Diodes Based on in Situ Fabricated FAPbBr3 Nanocrystals: The Enhancing Role of the Ligand-Assisted Reprecipitation Process

In this paper, we reported the in situ fabrication of highly luminescent formamidinium lead bromide (FAPbBr₃) nanocrystal thin films by dropping toluene as an anti-solvent during the spin-coating with a perovskite precursor solution using 3,3-diphenylpropylamine bromide (DPPA-Br) as a ligand. The resulting films are uniform and composed of 5-20 nm FAPbBr₃ perovskite nanocrystals. By monitoring the solvent mixing of anti-solvent and precursor solution on the substrates, we illustrated the difference between the ligand-assisted reprecipitation (LARP) process and the nanocrystal-pinning (NCP) process. This understanding provides a guideline for film optimization, and the optimized films obtained through the in situ LARP process exhibit strong photoluminescence emission at 528 nm, with quantum yields up to 78% and an average photoluminescence lifetime of 12.7 ns. In addition, an exciton binding energy of 57.5 meV was derived from the temperature-dependent photoluminescence measurement. More importantly, we achieved highly efficient pure green perovskite based light-emitting diode (PeLEDs) devices with an average external quantum efficiency (EQE) of 7.3% (maximum EQE is 16.3%) and an average current efficiency (CE) of 29.5 cd A^-1 (maximum CE is 66.3 cd A^-1) by adapting a conventional device structure of ITO/PEDOT:PSS/TFB/perovskite film/TPBi/LiF/Al. It is expected that the in situ LARP process provides an effective methodology for the improvement of the performance of PeLEDs.

Less-Lead Control toward Highly Efficient Formamidinium-Based Perovskite Light-Emitting Diodes

A formamidinium (FA)-based perovskite is an ideal option for the potential efficient light-emitting diode (LED) in view of its high tolerance factor closer to 1. In this work, FA cation-based perovskite nanocrystals FA_0.8Cs_0.2Pb _xBr₃ ( x = 1.0, 0.8, 0.7, and 0.6) are fabricated with stoichiometric modification. The adoption of less-lead precursor is confirmed to be a feasible and effective approach in inhibiting nonradiative recombination by diminishing the presence of uncoordinated metallic Pb atoms. Note that the subsequent devices require the optimized lead ratio for an optimum behavior, a clear influence of Pb ratio on a perovskite LED has been established. No surprisingly, the less-lead perovskites exert positive roles on the perovskite LED performance, not only in terms of efficiency but also in stability. With an optimized composition FA_0.8Cs_0.2Pb_0.7Br₃, the perovskite LED displays the prominent performance with a current efficiency of 28.61 cd A^-1, about 11-fold improvement than the previous best record of pure FA-based perovskite. Additionally, the perovskite device degradation can be mitigated under operating conditions by properly altering precursor stoichiometry, which can be attributed to the hydrogen reaction under moisture-induced ambient. The stoichiometric optimization of the metal Pb in the perovskite is an important strategy on the road to the further development of perovskite LEDs.

Phonon Interaction and Phase Transition in Single Formamidinium Lead Bromide Quantum Dots

Formamidinium lead bromide (FAPbBr₃) quantum dots (QDs) are promising materials for light emitting applications in the visible spectral region because of their high photoluminescence (PL) quantum yield (QY) and the enhanced chemical stability as compared to, for instance, methylammonium based analogues. Toward practical harnessing of their compelling optical characteristics, the exciton recombination process, and in particular the exciton-phonon interaction and the impact of crystal phase transition, has to be understood in detail. This is addressed in this contribution by PL studies on single colloidal FAPbBr₃ QDs. Polarization-resolved PL measurements reveal a fine structure splitting of excitonic transitions due to the Rashba effect. Distinct phonon replica have been observed within energetic distances of 4.3 ± 0.5, 8.6 ± 0.9, and 13.2 ± 1.1 meV from the zero phonon line, which we attribute to vibrational modes of the lead bromide lattice. Additional vibrational modes of 18.6 ± 0.3 and 38.8 ± 1.1 meV are found and related to liberation modes of the formamidinium (FA) cation. Temperature-dependent PL spectra reveal a line broadening of the emission caused by exciton phonon interaction as well an unusual energy shift which is attributed to a crystal phase transition within the single QD.