Halide Perovskite Light-emitting Diodes Research Articles

Additive-Driven Enhancement of Crystallization: Strategies and Prospects for Boosting Photoluminescence Quantum Yields in Halide Perovskite Films for Light-Emitting Diodes.

Halide perovskite light-emitting diodes (PeLEDs) hold great potential for applications in displays and lighting. To enhance the external quantum efficiency (EQE) of PeLEDs, it is crucial to boost the photoluminescence quantum yield (PLQY) of the perovskite films. The use of additives has emerged as a powerful chemical strategy to control the crystallization process in solution-processed perovskite films. The different types of additives that can be used reflect the various types of chemical interactions with the perovskite materials, influencing their crystallization process in various possible ways. Understanding the relationship between these chemical interactions and their impact on the crystallization process is a key step for designing emitters with improved PLQY and devices with superior EQE. Following the logic chain of additive-perovskite interactions, impacts on crystallization, and subsequent enhancement of PLQY and EQE, this review discusses how additives play a pivotal role in influencing the crystallization process to enhance the PLQY of the perovskite films. Furthermore, this assessment addresses the open challenges and outlines future prospects for the development of PeLEDs.

Regulation of nucleation and crystallization for blade-coating large-area CsPbBr3 perovskite light-emitting diodes

Metal halide perovskite light-emitting diodes (PeLEDs) and large-area perovskite color conversion layers for liquid crystal display exhibit great potential in the field of illumination and display. Blade-coating method stands out as a highly suitable technique for fabricating large-scale films, albeit with challenges such as uneven nucleation coverage and non-uniformity crystallization process. In this work, we developed an in-situ characterization measurement system to monitor the perovskite nucleation, and crystallization process. By incorporating formamidine acetate (FAAc) into perovskite precursor solutions, the nucleation rate and nuclei density of perovskite were increased, leading to more uniform nucleation. In addition, we inserted a layer of [2-(9H-carbazol-9-yl)ethyl] phosphonic acid above the poly(9-vinylcarbazole) hole transport layer. This layer acts as an anchor for the perovskite nano-crystal nuclei formed in the precursor, enhancing the steric hindrance of the solute and subsequently slowing down the crystal growth rate, thereby improving crystal quality. Based on these improvements, large-area perovskite nano-polycrystalline films with significantly improved uniformity and enhanced photoluminescence quantum yield were obtained. A small-area PeLED (2 mm × 2 mm) with a maximum external quantum efficiency of 25.91% was realized, marking the highest record of PeLED prepared by blade-coating method to date. An ultra-large-area PeLED (5 cm × 7 cm) was also prepared, which is the largest PeLED prepared by the solution method reported so far.

Advances and Challenges in Large-Area Perovskite Light-Emitting Diodes.

Metal halide perovskite light-emitting diodes (PeLEDs) have shown promise for high-definition displays and flat-panel lighting because of their wide color gamut, narrow emission band, and high brightness. The external quantum efficiency of PeLEDs increased rapidly from ≈1% to more than 25% in the past few years. However, most of these high-performance devices are fabricated using a spin coating method with a small device area of <0.1 cm2, limiting their commercial applications. Recently, large-area PeLEDs have attracted growing attention and significant breakthroughs have been reported. This perspective first introduces the pros and cons of each technique in making large-area PeLEDs. The advances in the fabrication of large-area PeLEDs are then summarized using spin coating and mass-production methods such as inkjet printing, blade coating, and thermal evaporation. Moreover, the challenging issues will be discussed that are urgent to be solved for large-area PeLEDs.

In-Situ Surface Repair of FAPbBr3 Quantum Dots toward High-Performance Pure-Green Perovskite Light-Emitting Diodes.

Metal halide perovskite light-emitting diodes (PeLEDs) are ideal for high-resolution displays due to their tunable emission, narrow spectra, and low-cost processing. Colloidal FAPbBr3 perovskite quantum dots (PeQDs) enhance radiative recombination, making them efficient for pure-green PeLEDs. However, their low stability and surface defects limit their practical application. Here, we address these challenges by proposing an in situ surface repair strategy using benzhydroxamic acid (BHA) as a modifier. We demonstrated that BHA can coordinate with Pb2+ ions and form hydrogen bonds with FA+ and halide ions, effectively reducing nonradiative recombination and maintaining the integrity of the PeQDs. High-quality FAPbBr3 PeQDs with a photoluminescence quantum yield (PLQY) of up to 92.5% were achieved, leading to pure-green PeLEDs with an external quantum efficiency (EQE) of 24.8% and a maximum luminance of 40,231 cd m-2, providing a feasible and promising perspective for advanced solid-state lighting and displays.

Thiourea-Induced Cocrystallization of Mixed-Halide Perovskite Thin Films for High-Performance Pure Blue Perovskite Light Emitting Diodes

Metal halide perovskite light-emitting diodes (PeLEDs) are gaining significant attention towards next-generation display materials owing to their advantageous properties, including color tunability, exceptionally narrow bandwidth, and high luminescence. Recent studies have successfully achieved over 20% external quantum efficiencies (EQE) in green and red PeLEDs. However, the blue PeLEDs lag far behind those of green or red PeLEDs because unstable phase stability in mixed halide perovskite thin films under high turn-on voltage (Von). In specific, the blue emission usually possesses halide ions with smaller ionic radius (Cl- or Br-), which easily migrates under high operative bias generating significant halogen vacancies and phase-transformation. In this context, we suggest thiourea as a promising additive for stabilizing defective perovskite thin films. Thiourea is a well-known hydrogen bond donor to halogens, which allows potential coordination complex of thiourea-halide ions to encompass well-dispersed mixed halides in co-crystallized perovskite lattices. In addition, the strong coordination of the C=S···Pb2+ was also demonstrated the thiourea-incorporated perovskite films, which effectively passivated undercoordinated Pb2+ antisites. Consequently, the thiourea-incorporated perovskite films exhibited stable deep-blue emission within 460 nm, demonstrating excellent luminance and quantum efficiency in pure blue PeLEDs. Moreover, the addition of thiourea achieved decreased Von and excellent spectral stability of pure blue PeLEDs at high applied bias, leading to significant advancements in pure blue perovskite display technology.

Efficient Deep-Blue Light-Emitting Diodes Through Decoupling of Colloidal Perovskite Quantum Dots.

Metal halide perovskite light-emitting diodes (PeLEDs) have exceptional color purity but designs that emit deep-blue color with high efficiency have not been fully achieved and become more difficult in the thin film of confined perovskite colloidal quantum dots (PeQDs) due to particle interaction. Here it is demonstrated that electronic coupling and energy transfer in PeQDs induce redshift in the emission by PeQD film, and consequently hinder deep-blue emission. To achieve deep-blue emission by avoiding electronic coupling and energy transfer, a QD-in-organic solid solution is introduced to physically separate the QDs in the film. This physical separation of QDs reduces the interaction between them yielding a blueshift of ≈7nm in the emission spectrum. Moreover, using a hole-transporting organic molecule with a deep-lying highest occupied molecular orbital (≈6.0eV) as the organic matrix, the formation of exciplex emission is suppressed. As a result, an unprecedently high maximum external quantum efficiency of 6.2% at 462nm from QD-in-organic solid solution film in PeLEDs is achieved, which satisfies the deep-blue color coordinates of CIEy <0.06. This work suggests an important material strategy to deepen blue emission without reducing the particle size to <≈4nm.

Structure and Optoelectronic Properties of Perovskite-like (PEA)2PbBr3Cl on AlN/Sapphire Substrate Heterostructure

This study presents the structure and optoelectronic properties of a perovskite-like (PEA)2PbBr3Cl material on an AlN/sapphire substrate heterostructure prepared using spin coating. The AlN/sapphire substrate comprised a 2 μm thick AlN epilayer on a sapphire wafer deposited via metal–organic chemical vapor deposition (MOCVD). The peak position of (PEA)2PbBr3Cl photoluminescence (PL) on the AlN/sapphire substrate heterostructure was 372 nm. The emission wavelength ranges of traditional lead halide perovskite light-emitting diodes are typically 410 to 780 nm, corresponding to the range of purple to deep red as the ratio of halide in the perovskite material changes. This indicates the potential for application as a UV perovskite light-emitting diode. In this study, we investigated the contact characteristics between Ag metal and the (PEA)2PbBr3Cl layer on an AlN/sapphire substrate heterostructure, which improved after annealing in an air environment due to the tunneling effect of the thermionic-field emission (TFE) mechanism.

Pulsed operation of perovskite LEDs: a study on the role of mobile ions

ABSTRACT Metal halide perovskite light-emitting diodes (PeLEDs) are a promising technology for energy-efficient and cost-effective lighting and displays, thanks to their tunable color emission, high brightness, color purity and low-temperature fabrication. However, the mixed ionic-electronic conductivity of perovskite materials presents unique challenges, as ionic defects can redistribute under operation, affecting the energy landscape and charge recombination mechanisms. Our drift-diffusion simulations establish a connection between the transient electroluminescence (TrEL) signals of PeLEDs under pulsed operation and the influence of mobile ions. We find that the TrEL plateau value’s dependence on the duty cycle and end-of-pulse overshoot can be explained by the time-varying distribution of ionic defects. The inclusion of mobile ions is crucial to understand the TrEL response. Moreover, the simulations highlight injection barriers at the perovskite/charge-transport layer interfaces, such as is the case for the hole transport layer in our example, as a significant source of non-radiative charge recombination. These findings contribute to the understanding of transient ionic processes in perovskite-based devices.

Large-Area Flexible Perovskite Light-Emitting Diodes Enabled by Inkjet Printing.

Metal halide perovskite light-emitting diodes (PeLEDs) are attracting increasing attention due to their potential applications in flat panel lighting and displays. The solution process, large-area fabrication, and flexibility are attractive properties of PeLEDs over traditional inorganic LEDs. However, it is still very challenging to deposit uniform perovskite films on flexible substrates using a blade or slot-die coating, as the flexible substrate is not perfectly flat. Here, the inkjet printing technique is adopted, and the key challenges are overcome step-by-step in preparing large-area films on flexible substrates. Double-hole transporting layers are first used and a wetting interfacial layer to improve the surface wettability so that the printed perovskite droplets can form a continuous wet film. The fluidic and evaporation dynamics of the perovskite wet layer is manipulated to suppress the coffee ring effect by solvent engineering. Uniform perovskite films are obtained finally on flexible substrates with different perovskite compositions. The peak external quantum efficiency of the inkjet-printed PeLEDs reaches 14.3%. Large-area flexible PeLEDs (4×7 cm2 ) also show very uniform emission. This work represents a significant step toward real applications of large-area PeLEDs in flexible flat-panel lighting.

Enhancing Efficiency and Stability of Tin Halide Perovskite Light-Emitting Diodes via Engineered Alkali/Multivalent Metal Salts.

Sn-based perovskite light-emitting diodes (PeLEDs) have emerged as promising alternatives to Pb-based PeLEDs with their rapid increase in performance owing to the various research studies on inhibiting Sn oxidation. However, the absence of defect passivation strategies for Sn-based perovskite LEDs necessitates further research in this field. We performed systematic studies to investigate the design rules for defect passivation agents for Sn-based perovskites by incorporating alkali/multivalent metal salts with various cations and anions. From the computational and experimental analyses, sodium trifluoromethanesulfonate (NaTFMS) was found to be the most effective passivation agent for PEA2SnI4 films among the explored candidate agents owing to favorable reaction energetics to passivate iodide Frenkel defects. Consequently, the incorporation of NaTFMS facilitates the formation of uniform films with relatively large crystals and reduced Sn4+. The NaTFMS-containing PEA2SnI4 PeLEDs demonstrate an improved luminance of 138.9 cd/m2 and external quantum efficiency (EQE) of 0.39% with an improved half-lifetime of more than threefold. This work provides important insight into the design of defect passivation agents for Sn-based perovskites.

Enhanced Performance of Inverted Perovskite Quantum Dot Light-Emitting Diode Using Electron Suppression Layer and Surface Morphology Control.

The energy level offset at inorganic layer-organic layer interfaces and the mismatch of hole/electron mobilities of the individual layers greatly limit the establishment of balanced charge carrier injection inside the emissive layer of halide perovskite light-emitting diodes (PeQLEDs). In contrast with other types of light-emitting devices, namely OLEDs and QLEDs, various techniques such as inserting an electron suppression layer between the emissive and electron transport layer have been employed as a means of establishing charge carrier injection into their respective emissive layers. Hence, in this study, we report the use of a thin layer of Poly(4-vinylpyridine) (PVPy) (an electron suppression material) placed between the emissive and electron transport layer of a halide PeQLEDs fabricated with an inverted configuration. With ZnO as the electron transport material, devices fabricated with a thin PVPy interlayer between the ZnO ETL and CsPbBr3 -based green QDs emissive layer yielded a 4.5-fold increase in the maximum observed luminance and about a 10-fold increase in external quantum efficiency (EQE) when compared to ones fabricated without PVPy. Furthermore, the concentration and coating process conditions of CsPbBr3 QDs were altered to produce various thicknesses and film properties which resulted in improved EQE values for devices fabricated with QDs thin films of lower surface root-mean-square (RMS) values. These results show that inhibiting the excessive injection of electrons and adjusting QDs layer thickness in perovskite-inverted QLEDs is an effective way to improve device luminescence and efficiency, thereby improving the carrier injection balance.

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

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

Tautomeric mixture coordination enables efficient lead-free perovskite LEDs.

Lead halide perovskite light-emitting diodes (PeLEDs) have demonstrated remarkable optoelectronic performance1-3. However, there arepotential toxicity issues with lead4,5 and removing lead from the best-performing PeLEDs-without compromising their high external quantum efficiencies-remains a challenge. Here we report a tautomeric-mixture-coordination-induced electron localization strategy to stabilize thelead-free tinperovskiteTEA2SnI4 (TEAI is2-thiopheneethylammonium iodide) by incorporating cyanuric acid. We demonstrate that a crucial function of the coordination is to amplify theelectronic effects, even for those Sn atoms that aren't strongly bonded with cyanuric acidowing to the formation of hydrogen-bonded tautomeric dimer and trimer superstructures on the perovskite surface. This electron localization weakens adverse effects from Anderson localization and improves ordering in the crystal structure of TEA2SnI4. These factors result in a two-orders-of-magnitude reduction in thenon-radiative recombination capture coefficient and an approximately twofold enhancement inthe exciton binding energy. Our lead-free PeLED has an external quantum efficiency of up to 20.29%, representing a performance comparable to that of state-of-the-art lead-containing PeLEDs6-12. We anticipate that these findings will provide insights into the stabilization of Sn(II) perovskites and further the development of lead-free perovskite applications.

High Curvature PEDOT:PSS Transport Layer Toward Enhanced Perovskite Light-Emitting Diodes.

The rapidly developed metal halide perovskite light-emitting diodes (PeLEDs) are considered as a promising candidate for next-generation display and illumination, but the unbalanced charge transport is still a hard-treat case to restrict its efficiency and operational stability. Here, a high curvature PEDOT:PSS transport layer is demonstrated via the self-assembly island-like structures by the incorporation of alkali metal salts. Benefiting from the dielectric confinement effect of the high curvature surface, the modified CsPbBr3 -based PeLEDs present a 2.1 times peak external quantum efficiency (EQE) from 6.75% to 14.23% and a 3.3 times half lifetime (T50 ) from 3.96 to 13.01h. Besides, the PeLEDs show high luminance up to 44834cdm-2 . Evidently, this work may provide a deep insight into the structure-activity relationship between the micro-structures at the PEDOT:PSS/perovskite interface and the performance of PeLEDs, and crack the codes for ameliorating the performance of PeLEDs via interfacial micro-structured regulation.

Effect of Passivating Molecules and Antisolvents on Lifetime of Green Dion-Jacobson Perovskite Light-Emitting Diodes.

We investigated the influence of two passivating molecules containing a P═O group on the performance of quasi-2D Dion-Jacobson halide perovskite light-emitting diodes, namely, triphenylphosphine oxide (TPPO) and diphenyl-4-triphenylsilylphenyl phosphine oxide (TSPO1). We found that both passivating molecules lead to increased efficiency compared to control devices, while they had opposite effects on device lifetime, with a decrease observed for TPPO and an increase observed for TSPO1. The two passivating molecules resulted in differences in energy-level alignment, electron injection, film morphology and crystallinity, and ion migration during operation. While TPPO resulted in improved photoluminescence decay times, overall higher maximum external quantum efficiency (EQE) and device lifetime were obtained for TSPO1 compared to TPPO (14.4% vs 12.4% EQE, 341 min vs 42 min T50).

Toward High-Performances of Halide Light-Emitting Diodes: The Importance of Ligands Engineering

Halide perovskite light-emitting diodes (PeLEDs) have attracted great attention because of their superior optical properties, such as extremely high photoluminescence (quantum yield up to nearly 100%) of active layers with tunable wavelengths over the entire visible spectral range. With a suitable modification of halide perovskites, carrier transport materials, and their interfaces, external quantum efficiencies exceeding 10%, 25%, and 20% have been achieved for blue-colored (465 nm), green-colored (512 nm), and red-colored (640 nm) LEDs, respectively. Many strategies for pursuing high performances of devices have been successfully demonstrated, among which ligand engineering has always played an important role in the active layer. Herein, we present a perspective to illustrate the effects and roles of the ligands in cesium lead bromide light-emitting diodes. This perspective is mainly classified into three parts: (1) ligands for CsPbBr3 LEDs could improve radiative recombination of perovskites and contribute to better efficiency of LEDs; (2) ligands could confine CsPbBr3 growth for blue emission of LEDs; (3) stabilities of materials and devices become better with ligand engineering. Finally, the summary and perspective on PeLEDs are highlighted and possible solutions are provided.

Thermal Properties of Polymer Hole-Transport Layers Influence the Efficiency Roll-off and Stability of Perovskite Light-Emitting Diodes.

While the performance of metal halide perovskite light-emitting diodes (PeLEDs) has rapidly improved in recent years, their stability remains a bottleneck to commercial realization. Here, we show that the thermal stability of polymer hole-transport layers (HTLs) used in PeLEDs represents an important factor influencing the external quantum efficiency (EQE) roll-off and device lifetime. We demonstrate a reduced EQE roll-off, a higher breakdown current density of approximately 6 A cm-2, a maximum radiance of 760 W sr-1 m-2, and a longer device lifetime for PeLEDs using polymer HTLs with high glass-transition temperatures. Furthermore, for devices driven by nanosecond electrical pulses, a record high radiance of 1.23 MW sr-1 m-2 and an EQE of approximately 1.92% at 14.6 kA cm-2 are achieved. Thermally stable polymer HTLs enable stable operation of PeLEDs that can sustain more than 11.7 million electrical pulses at 1 kA cm-2 before device failure.

Vacuum-deposited perovskite CsPbBr3 thin-films for temperature-stable Si based pure-green all-inorganic light-emitting diodes

Metal halide perovskite light-emitting diodes (PeLEDs) are excellent candidates in the field of lighting and display due to their outstanding optical-electrical properties. However, the solution-processed technology of perovskite films and the organic electron/hole transport layers of PeLEDs make it still challenging to improve the operational stability of devices. Herein, we successfully prepared highly luminescent CsPbBr3 perovskite films via vacuum-deposited method and then fabricated all-inorganic PeLEDs with the heterostructure of p-NiO/CsPbBr3/n-Si. Our device exhibits pure-green emission with a wavelength of 527 nm, a narrow full width at half-maximum of 18 nm, and a maximum luminance of 51933 cd/m2, representing one of the best brightness pure-green PeLEDs. Most importantly, the PeLEDs exhibited great thermal stability with a heat resistance up to 80 °C. The electroluminescence peak position of the PeLEDs remains consistent when the ambient temperature increases from 40 °C to 110 °C. Moreover, the all-inorganic PeLEDs can maintain their good luminescence performance after seven thermal cycling tests (30 °C–100 °C). This work not only demonstrated a facile strategy to prepare high-quality pure-green CsPbBr3 perovskite films, but also provided an important all-inorganic device structure for high thermal stability of PeLED.

P‐11.1: The interface engineering with quaternary ammonium ionic liquid enabling efficient blue perovskite light‐emitting diode

Metal halide perovskite light-emitting diodes (PeLEDs) have become a rising star in recent years. Interface plays an important role in the development of the PeLEDs performance. In this work, we demonstrate that the interface engineering with quaternary ammonium ionic liquid that could be used to improve the efficiency of the blue PeLEDs. We find the interface engineering can balance hole and electron injection. In addition, quaternary ammonium cation promotes the growth and nucleation of the blue perovskite emitter, improving its film morphology and reducing the occurrence of defects. The use of the ionic liquid interfacial layer is also helpful to inhibit the ion migration in the perovskite under forward bias operation. In conclusion, by exploring the quaternary ammonium ionic liquid interfacial layer, the luminous efficiency can be greatly improved.

Reducing Nonradiative Losses in Perovskite LEDs through Atomic Layer Deposition of Al2O3 on the Hole-Injection Contact.

Halide perovskite light-emitting diodes (PeLEDs) exhibit great potential for use in next-generation display technologies. However, scale-up will be challenging due to the requirement of very thin transport layers for high efficiencies, which often present spatial inhomogeneities from improper wetting and drying during solution processing. Here, we show how a thin Al2O3 layer grown by atomic layer deposition can be used to preferentially cover regions of imperfect hole transport layer deposition and form an intermixed composite with the organic transport layer, allowing hole conduction and injection to persist through the organic hole transporter. This has the dual effect of reducing nonradiative recombination at the heterojunction and improving carrier selectivity, which we infer to be due to the inhibition of direct contact between the indium tin oxide and perovskite layers. We observe an immediate improvement in electroluminescent external quantum efficiency in our p-i-n LEDs from an average of 9.8% to 13.5%, with a champion efficiency of 15.0%. The technique uses industrially available equipment and can readily be scaled up to larger areas and incorporated in other applications such as thin-film photovoltaic cells.