Improving Efficiency and Stability in Quasi-2D Perovskite Light-Emitting Diodes by a Multifunctional LiF Interlayer

Interfacial Regulation toward Efficient CsPbBr3 Quantum Dot-Based Inverted Perovskite Light-Emitting Diodes

Inverted perovskite light-emitting diodes (PeLEDs) based on quantum dots (QDs) are some of the most promising candidates for next-generation lighting and display applications. Due to the strong fluorescence quenching caused by zinc oxide, high performance in such inverted devices remains challenging. Here, we report an efficient inverted green CsPbBr3 QDs LED using an emitting buffer layer. Ultrathin CsPbBr3 QD emitters act as the buffer layer to reduce the interface luminescence quenching reaction at the ZnO/upper emitting layer interface, increasing the probability of exciton recombination within the emissive layer and regulating the charge transport, leading to effective carrier recombination. The resulting device exhibits an external quantum efficiency of 13.1%, enhanced by about 4.7 times compared with that without a buffer layer device. This work provides a path to fabricating high-performance inverted PeLEDs.

Defects in lead halide perovskite light-emitting diodes under electric field: from behavior to passivation strategies

Lead halide perovskites (LHPs) are emerging semiconductor materials for light-emitting diodes (LEDs) owing to their unique structure and superior optoelectronic properties. However, defects that initiate degradation of LHPs through external stimuli and prompt internal ion migration at the interfaces remain a significant challenge. The electric field (EF), which is a fundamental driving force in LED operation, complicates the role of these defects in the physical and chemical properties of LHPs. A deeper understanding of EF-induced defect behavior is crucial for optimizing the LED performance. In this review, the origins and characterization of defects are explored, indicating the influence of EF-induced defect dynamics on LED performance and stability. A comprehensive overview of recent defect passivation approaches for LHP bulk films and nanocrystals (NCs) is also provided. Given the ubiquity of EF, a summary of the EF-induced defect behavior can enhance the performance of perovskite LEDs and related optoelectronic devices.

On-Shelf and Operational Decay Dynamics of Self-Healing Quasi-Two-Dimensional Perovskite Light-Emitting Devices

Currently, the external quantum efficiency (EQE) performance of perovskite light-emitting diodes (PeLEDs) is approaching its theoretical limit. The main drawback of PeLEDs is their stability. Ion migration in the perovskite layer is one of the main causes of the operational decomposition of PeLEDs. Here, we find that butylammonium-based quasi-two-dimensional (quasi-2D) PeLEDs show self-healing ability, revealing the existence of ion migration in the fabricated perovskite layer. Then, on the basis of the analysis of ∼170 operational decay EQE curves, patterns of on-shelf and operational decay in self-healing quasi-2D PeLEDs have been identified. The uneven distributions of resistance on the perovskite film surface are proposed to cause secondary electric fields. The electroluminescent scintillation in certain regions results in fluctuating electroluminescence of PeLEDs, further proving the existence of microcosmic steric ion movement under secondary electric fields. Our work explores the decay patterns of self-healing PeLEDs and highlights the impact of steric ion movements on the decay processes of PeLEDs.

Flexible and Stretchable Light-Emitting Diodes and Photodetectors for Human-Centric Optoelectronics

Optoelectronic devices with unconventional form factors, such as flexible and stretchable light-emitting or photoresponsive devices, are core elements for the next-generation human-centric optoelectronics. For instance, these deformable devices can be utilized as closely fitted wearable sensors to acquire precise biosignals that are subsequently uploaded to the cloud for immediate examination and diagnosis, and also can be used for vision systems for human-interactive robotics. Their inception was propelled by breakthroughs in novel optoelectronic material technologies and device blueprinting methodologies, endowing flexibility and mechanical resilience to conventional rigid optoelectronic devices. This paper reviews the advancements in such soft optoelectronic device technologies, honing in on various materials, manufacturing techniques, and device design strategies. We will first highlight the general approaches for flexible and stretchable device fabrication, including the appropriate material selection for the substrate, electrodes, and insulation layers. We will then focus on the materials for flexible and stretchable light-emitting diodes, their device integration strategies, and representative application examples. Next, we will move on to the materials for flexible and stretchable photodetectors, highlighting the state-of-the-art materials and device fabrication methods, followed by their representative application examples. At the end, a brief summary will be given, and the potential challenges for further development of functional devices will be discussed as a conclusion.

Highly Stable Perovskite Solar Cells Based on the Efficient Interaction between Pb2+ and Cyano Groups of 4-Aminophthalonitrile

Defects present on the top surface of perovskite films have a pronounced detrimental impact on the photovoltaic performance and stability of perovskite solar cells (PSCs). Consequently, the development of effective defect passivation strategies has become key in enhancing both the power conversion efficiency (PCE) and stability of PSCs. In this study, a small molecule material, 4-Aminophthalonitrile (4-APN), was introduced as a means to mitigate surface defects within perovskite films. Obviously, 4-APN effectively passivates the defects at grain boundaries by combining cyano groups (-C≡N) with Pb2+ , significantly reducing the density of defect states, inhibiting non-radiative recombination at the interface, and promoting the charge transfer efficiency from the perovskite layer to the hole transport layer. The 4-APN modification led to a significant upswing in the PCE, while concurrently bolstering the overall device stability. Importantly, the devices on 4-APN as passivation additive exhibited negligible performance degradation aging for 1200 h.

Broadband Photodetector for Ultraviolet to Visible Wavelengths Based on the BA2PbI4/GaN Heterostructure

Two-dimensional (2D) organic-inorganic hybrid perovskites (OIPs) have exhibited ideal prospects for perovskite photodetectors (PDs) owing to their remarkable environmental stability, tunable band gap, and structural diversity. However, most perovskites face the great challenge of a narrow spectral response. Integrating 2D OIPs with a suitable wide band gap semiconductor gives opportunities to broaden the response spectra. Here, a photodetector based on the BA2PbI4/GaN heterostructure with a broadband photoresponse covering from the ultraviolet (UV) to visible band is designed. We demonstrate that the device is capable of detecting in the UV region by p-GaN being integrated with BA2PbI4. The morphology and material optical properties of BA2PbI4 are characterized by transmission electron microscopy (TEM) and photoluminescence (PL). Additionally, the current-voltage (I-V) characteristics and photoresponses of the BA2PbI4/GaN heterojunction photodetector are investigated. The response spectrum of the photodetector is broadened from the visible to UV region, exhibiting good rectifying behavior in the dark conditions and a broadband photoresponse from the UV to the visible region. Additionally, the energy band is used to analyze the current mechanism of the BA2PbI4/GaN heterojunction PD. This study is expected to provide a new insight of optoelectronic devices by integrating 2D OIPs such as BA2PbI4 and wide-band-gap semiconductors such as GaN to broaden the response spectra.

Potential and perspectives of halide perovskites in light emitting devices

Light emitting diodes (LEDs) have become part of numerous electrical and electronic systems such as lighting, displays, status indicator lamps and wearable electronics. Owing to their excellent optoelectronic properties and deposition via simple solution process, metal halide perovskites possess unique potential for developing halide perovskite-based LEDs (PeLEDs) with superior photoluminescence efficiencies leading to external quantum efficiencies beyond 20% for PeLEDS. However, the limited durability, high operative voltages, and challenges of scale-up are persisting barriers in achieving required technology readiness levels. To build up the existing knowledge and raise the device performance this review provides a state-of-the-art study on the properties, film and device fabrication, efficiency, and stability of PeLEDs. In terms of commercialization, PeLEDs need to overcome materials and device challenges including stability, ion migration, phase segregation, and joule heating, which are discussed in this review. We hope, discussions about the strategies to overcome the stability issues and enhancement the materials intrinsic properties towards development more stable and efficient optoelectronic devices can pave the way for scalability and cost-effective production of PeLEDs.

Rational adjustment to interfacial interaction with carbonized polymer dots enabling efficient large-area perovskite light-emitting diodes

Film uniformity of solution-processed layers is the cornerstone of large-area perovskite light-emitting diodes, which is often determined by the 'coffee-ring effect'. Here we demonstrate a second factor that cannot be ignored is the solid-liquid interface interaction between substrate and precursor and can be optimized to eliminate rings. A perovskite film with rings can be formed when cations dominate the solid-liquid interface interaction; whereas smooth and homogeneous perovskite emitting layers are generated when anions and anion groups dominate the interaction. This is due to the fact that the type of ions anchored to the substrate can determine how the subsequent film grows. This interfacial interaction is adjusted using carbonized polymer dots, who also orient the perovskite crystals and passivate their buried traps, enabling a 225 mm² large-area perovskite light-emitting diode with a high efficiency of 20.2%.

Efficient and bright green InP quantum dot light-emitting diodes enabled by a self-assembled dipole interface monolayer

The interfacial state between the hole transport layer (HTL) and quantum dots (QDs) plays a crucial role in the optoelectronic performance of light-emitting diodes. Herein, we reported an efficient and bright green indium phosphide (InP) QD-based light-emitting diode (LED) by introducing a self-assembled monolayer of 4-bromo-2-fluorothiophenol (SAM-BFTP) molecule to improve interfacial charge transport in LED devices. The molecular dipole layer at the interface of the QD layer and HTL not only reduces the energy barrier of holes injected into QDs through vacuum energy level shift but also inhibits the fluorescence quenching of QDs caused by the HTL. Moreover, copper ions doped into phosphomolybdic acid (Cu:PMA) is selected as the hole injection layer (HIL) into the device system based on the SAM-BFTP molecule, and as a result, a green InP QD LED (QLED) with a maximum external quantum efficiency (EQE) of 8.46% and a luminance of 18 356 cd m^-2 was realized. This work can inform and underpin the future development of InP-based QLEDs with concurrent high efficiency and brightness.

Modulation of the carrier balance of lead-halide perovskite nanocrystals by polyelectrolyte hole transport layers for near-infrared light-emitting diodes

An alternative material, methylamine (MA)-doped poly[3-(4-carboxymethyl)thiophene-2,5-diyl] (P3CT) as hole transport layer (HTL) was investigated for efficient solution-processed near-infrared perovskite light-emitting diodes (NIR PeLEDs). The best NIR PeLEDs performance was achieved with an optimized composition ratio of the MA-doped P3CT (1:1) due to the balance of the electron and hole carrier in the active layer. The charge-balanced NIR PeLEDs exhibit the highest radiance of 858.37 W sr⁻¹ m⁻², a low turn-on voltage of 1.82 V, and an external quantum efficiency of 7.44%. Our findings show that using P3CT as an alternative HTL has the potential to significantly improve PeLED performance, allowing it to play a role in the development of practical applications in high-power NIR LEDs.

Enhanced Amplified Spontaneous Emission in Quasi-2D Perovskite by Facilitating Energy Transfer

Despite the superior optoelectronic properties of quasi-two-dimensional (quasi-2D) Ruddlesden-Popper halide perovskites, the inhomogeneous distribution of mixed phases result in inefficient energy transfer and multiple emission peaks. Herein, the insufficient energy funneling process at the high-energy phase is almost completely suppressed and the excitonic understanding of gain nature is studied in the energy funneling managed quasi-2D perovskite via introducing poly(vinyl pyrrolidone) (PVP) additive. The energy transfer process is facilitated from 0.37 to 0.26 ps after introducing the PVP additive, accelerating the exciton accumulation in the emissive state, and increasing the ratio of the high-dimensional phase for enhancing radiative emission. The gain lifetime is promoted to be as fast as 28 ps to outcompete nonradiative recombination during the build-up of population inversion. Simultaneously, the net gain coefficient is increased by more than twofold that of the pristine perovskite film. Owing to the remarkable gain properties, room-temperature amplified spontaneous emission is realized with a low threshold of 11.3 μJ/cm2, 4 times lower than 43 μJ/cm2 of the pristine film. Our findings suggest that the PVP-treated quasi-2D perovskite shows great promise for high-performance laser devices.

Visualization of 3D to quasi 2D conversion of perovskite thin films via in situ photoluminescence measurement: a facile route to design a graded energy landscape

2D-perovskites are generally more stable than 3D perovskites while charge transport in 2D-perovskites becomes inefficient. On the other hand, the instability of 3D perovskite films under heat, light and environmental conditions makes them inapplicable for practical purposes. Therefore, quasi-2D perovskites could be the optimum solution for stable yet highly efficient devices. Using the post-fabrication treatment method, we have converted methylammonium lead tribromide (MAPbBr₃) 3D perovskite films into a quasi 2D-perovskite interfacial layer. In situ photoluminescence measurement during spin coating indicates a rapid conversion of 3D-perovskite into 2D-perovskites. The kinetics of oxygen and moisture diffusion, ion diffusion and electronic charge transport can be estimated from the time dependent PL measurements in the 3D and 2D/3D perovskite samples. 2D terminated perovskite samples show enhanced photoluminescence and improved stability in moisture and UV-irradiation. We also propose that a relatively wide bandgap of 2D-perovskite can give rise to a graded energy landscape at the interface for favorable charge separation. Simulation results reveal that the power conversion efficiency can be improved from 2.83% to 4.02% due to an increase in open-circuit voltage and fill factor in 2D/3D based MAPbBr₃ solar cells without using any electron transport layer.

A Multifunctional Ionic Liquid Additive Enabling Stable and Efficient Perovskite Light-Emitting Diodes

The electroluminescence performance and long-term stability of perovskite light-emitting diodes (PeLEDs) are greatly affected by the film quality of perovskite emitting layer. Herein, the authors employ an ionic liquid, 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([BMIm]OTf), to manipulate the growth of quasi-2D perovskite films by providing heterogeneous nucleation sites. The [BMIm]OTf molecules simultaneously realize uniform perovskite films by reducing the contact angles of precursor solution on the hole transport layer (HTL), and eliminate defect states through bonding [BMIm]⁺ cations to negatively-charged uncoordinated Br and OTf^- anions to uncoordinated Pb²⁺ defects that effectively suppresses the defect states assisted nonradiative recombination in perovskite films. As a result, the efficiency and the operational lifetime of the resultant PeLED are enhanced by more than twofold and threefold, respectively, achieving a maximum external quantum efficiency of 17.6% and an operational lifetime of over 500 min at an initial brightness of 100 cd m^-2 .

Operational Stability Issues and Challenges in Metal Halide Perovskite Light-Emitting Diodes

Metal halide perovskite light-emitting diodes (Pe-LEDs) have shown promise for high-definition displays because of their wide color gamut (∼140%) and narrow emission band. Although the external quantum efficiency (EQE) of Pe-LEDs increased rapidly from ∼1% to more than 20% in several years, they still suffer from poor operational stability, which has been recognized as the bottleneck for commercial application of Pe-LEDs. Although the environmental sensitivity of perovskites can be avoided by encapsulation approaches, the ion migration of perovskites is easily induced by crystal defects under the action of an electric field in the operating state, thus accelerating irreversible phase transition and physical degradation of the perovskites. Additionally, the unbalanced carrier injection of Pe-LEDs could induce great Auger recombination and Joule heating, which deteriorate the operational stability of devices. Considering these issues, coping strategies, such as surface engineering, ion doping, and quantum confinement control of perovskites and structure design and thermal management of devices, are discussed in this Perspective.

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

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

Ionic Liquid Passivation Eliminates Low-n Quantum Well Domains in Blue Quasi-2D Perovskite Films

In quasi-two-dimensional (quasi-2D) perovskite films, carriers transport in the cascade structural systems involving various quantum wells (QWs) n, but their efficiency is limited by the severe nonradiative recombination within plentiful n = 1, 2, 3 domains induced by traditional ammonium bromide passivation. Here, we fabricate the quasi-2D films with the elimination of n = 1, 2, 3 domains by introducing the ionic liquid n-butylamine acetate (BAAc) instead of n-butylamine hydrobromide (BABr), which increases the photoluminescence quantum yield (PLQY) and lowers the surface roughness of films. Due to the anion exchange between BAAc and methylamine hydrobromide (MABr), BAAc exhibits a sole passivation effect on methylamine-based perovskites. As a result, the ionic liquid-derived perovskite light-emitting diodes (PeLEDs) display blue emission at 479 nm and show significantly improved performance on external quantum efficiency (EQE) and luminance. Our finding provides insights into the passivating effect of ionic liquid on quasi-2D perovskites and will benefit fabricating PeLEDs with enhanced performance.

Advances and Challenges in Two-Dimensional Organic-Inorganic Hybrid Perovskites Toward High-Performance Light-Emitting Diodes

Two-dimensional (2D) perovskites are known as one of the most promising luminescent materials due to their structural diversity and outstanding optoelectronic properties. Compared with 3D perovskites, 2D perovskites have natural quantum well structures, large exciton binding energy (E_b) and outstanding thermal stability, which shows great potential in the next-generation displays and solid-state lighting. In this review, the fundamental structure, photophysical and electrical properties of 2D perovskite films were illustrated systematically. Based on the advantages of 2D perovskites, such as special energy funnel process, ultra-fast energy transfer, dense film and low efficiency roll-off, the remarkable achievements of 2D perovskite light-emitting diodes (PeLEDs) are summarized, and exciting challenges of 2D perovskite are also discussed. An outlook on further improving the efficiency of pure-blue PeLEDs, enhancing the operational stability of PeLEDs and reducing the toxicity to push this field forward was also provided. This review provides an overview of the recent developments of 2D perovskite materials and LED applications, and outlining challenges for achieving the high-performance devices.