Excess Ion-Induced Efficiency Roll-Off in High-Efficiency Perovskite Light-Emitting Diodes

Elevating Charge Transport Layer for Stable Perovskite Light-Emitting Diodes

Ion migration is a major factor affecting the long term stability of perovskite light-emitting diodes (LEDs), which limits their commercialization potential. The accumulation of excess halide ions at the grain boundaries of perovskite films is a primary cause of ion migration in these devices. Here, it is demonstrated that the channels of ion migrations can be effectively impeded by elevating the hole transport layer between the perovskite grain boundaries, resulting in highly stable perovskite LEDs. The unique structure is achieved by reducing the wettability of the perovskites, which prevents infiltration of the upper hole-transporting layer into the spaces of perovskite grain boundaries. Consequently, nanosized gaps are formed between the excess halide ions and the hole transport layer, effectively suppressing ion migration. With this structure, perovskite LEDs with operational half-lifetimes of 256 and 1774 h under current densities of 100 and 20 mA cm-2 respectively are achieved. These lifetimes surpass those of organic LEDs at high brightness. It is further found that this approach can be extended to various perovskite LEDs, showing great promise for promoting perovskite LEDs toward commercial applications.

Unveiling the Carrier Dynamics of Perovskite Light-Emitting Diodes via Transient Electroluminescence

Perovskite light-emitting diodes (PeLEDs) have garnered significant attention due to their outstanding optoelectronic properties. However, investigating carrier transport and recombination behavior during device operation poses persistent challenges. In this study, we explore the impact of additive and interface engineering on device performance using transient electroluminescence (TREL). Polyethylene glycol (PEG) induces the formation of square-faceted nanocrystals with a homogeneous size distribution and extended fluorescence lifetime. Consequently, these PeLEDs exhibit remarkable stability. Additionally, employing an electron transport layer of 2,4,6-tris[3-(diphenylphosphino)phenyl]-1,3,5-triazine (PO-T2T), which has a better match to the energy bands of the perovskite layer and a higher carrier mobility, allows for lower turn-on voltage and faster response but also suffers from a short decay time and poor stability. Moreover, low-temperature TREL characterization shows that the carrier mobility is also significantly suppressed with decreasing temperature, which reduces the transient response speed.

Eliminating the Adverse Impact of Composition Modulation in Perovskite Light-Emitting Diodes toward Ultra-High Brightness and Stability

Excess ammonium halides as composition additives are widely employed in perovskite light-emitting diodes (PeLEDs), aiming to achieve high performance by controlling crystallinity and passivating defects. However, an in-depth understanding of whether excess organoammonium components affect the film physical/electrical properties and the resultant device instability is still lacking. Here, the trade-off between the performance and stability in high-efficiency formamidinium lead iodide (FAPbI3)-based PeLEDs with excess ammonium halides is pointed, and the underlying mechanism is explored. Systematic experimental and theoretical studies reveal that excess halide salt-induced ion-doping largely alters the PeLEDs properties (e.g., carrier injection, field-dependent ion-drifting, defect physics, and phase stability). A surface clean assisted cross-linking strategy is demonstrated to eliminate the adverse impact of composition modulation and boost the operational stability without sacrificing the efficiency, achieving a high efficiency of 23.6%, a high radiance of 964 W sr-1 m-2 (The highest value for FAPbI3 based PeLEDs), and a prolong lifetime of 106.1 h at large direct current density (100 mA cm-2), concurrently. The findings uncovered an important link between excess halide salts and the device performance, providing a guideline for rational design of stable, bright, and high efficiency PeLEDs.

High-Performance All-Inorganic Architecture Perovskite Light-Emitting Diodes Based on Tens-of-Nanometers-Sized CsPbBr3 Emitters in a Carrier-Confined Heterostructure

Developing green perovskite light-emitting diodes (PeLEDs) with a high external quantum efficiency (EQE) and low efficiency roll-off at high brightness remains a critical challenge. Nanostructured emitter-based devices have shown high efficiency but restricted ascending luminance at high current densities, while devices based on large-sized crystals exhibit low efficiency roll-off but face great challenges to high efficiency. Herein, we develop an all-inorganic device architecture combined with utilizing tens-of-nanometers-sized CsPbBr3 (TNS-CsPbBr3) emitters in a carrier-confined heterostructure to realize green PeLEDs that exhibit high EQEs and low efficiency roll-off. A typical type-I heterojunction containing TNS-CsPbBr3 crystals and wide-bandgap Cs4PbBr6 within a grain is formed by carefully controlling the precursor ratio. These heterostructured TNS-CsPbBr3 emitters simultaneously enhance carrier confinement and retain low Auger recombination under a large injected carrier density. Benefiting from a simple device architecture consisting of an emissive layer and an oxide electron-transporting layer, the PeLEDs exhibit a sub-bandgap turn-on voltage of 2.0 V and steeply rising luminance. In consequence, we achieved green PeLEDs demonstrating a peak EQE of 17.0% at the brightness of 36,000 cd m-2, and the EQE remained at 15.7% and 12.6% at the brightness of 100,000 and 200,000 cd m-2, respectively. In addition, our results underscore the role of interface degradation during device operation as a factor in device failure.

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.

Ion Migration in Lead-Halide Perovskites: Cation Matters

Metal halide perovskites exhibit remarkable properties for optoelectronic applications, yet their susceptibility to ion migration poses challenges for device stability. Previous research has predominantly focused on the migration of the halide ions. However, the migration of cations, which also has a significant influence on the device performance, is largely overlooked. In this Perspective, we review the migration of cations and their impacts on perovskite materials and devices. Special attention shall be devoted to recent insights into the migration of L-site organic cations in 2D/3D perovskites. We outline inspirations and directions for further research into the cation migration of perovskites, highlighting new possibilities in advancing perovskite optoelectronics.

Luminescence and Degeneration Mechanism of Perovskite Light-Emitting Diodes and Strategies for Improving Device Performance

Perovskite light-emitting diodes (PeLEDs) can be a promising technology for next-generation display and lighting applications due to their excellent optoelectronic properties. However, a systematical overview of luminescence and degradation mechanism of perovskite materials and PeLEDs is lacking. Therefore, it is crucial to fully understand these mechanisms and further improve device performances. In this work, the fundamental photophysical processes of perovskite materials, electroluminescence mechanism of PeLEDs including carrier kinetics and efficiency roll-off as well as device degradation mechanism are discussed in detail. In addition, the strategies to improve device performances are summarized, including optimization of photoluminescence quantum yield, charge injection and recombination, and light outcoupling efficiency. It is hoped that this work can provide guidance for future development of PeLEDs and ultimately realize industrial applications.

Realizing High Brightness Quasi-2D Perovskite Light-Emitting Diodes with Reduced Efficiency Roll-Off via Multifunctional Interface Engineering

Quasi-2D perovskites have recently flourished in the field of luminescence due to the quantum-confinement effect and the efficient energy transfer between different n phases resulting in exceptional optical properties. However, owing to the lower conductivity and poor charge injection, quasi-2D perovskite light-emitting diodes (PeLEDs) typically suffer from low brightness and high-efficiency roll-off at high current densities compared to 3D perovskite-based PeLEDs, which is undoubtedly one of the most critical issues in this field. In this work, quasi-2D PeLEDs with high brightness, reduced trap density, and low-efficiency roll-off are successfully demonstrated by introducing a thin layer of conductive phosphine oxide at the perovskite/electron transport layer interface. The results surprisingly show that this additional layer does not improve the energy transfer between multiple quasi-2D phases in the perovskite film, but purely improves the electronic properties of the perovskite interface. On the one hand, it passivates the surface defects of the perovskite film; on the other hand, it promotes electron injection and prevents hole leakage across this interface. As a result, the modified quasi-2D pure Cs-based device shows a maximum brightness of > 70,000 cd m-2 (twice that of the control device), a maximum external quantum efficiency (EQE) of > 10% and a much lower efficiency roll-off at high bias voltages.

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.

High Efficiency Near-Infrared Perovskite Light Emitting Diodes With Reduced Rolling-Off by Surface Post-Treatment

The rolling-off phenomenon of device efficiency at high current density caused by quenching of luminescence in perovskite light-emitting diodes (PeLED) is challenging to be solved. Here, 2-amino-5-iodopyrazine (AIPZ) is dissolved in a mixed solvent of chlorobenzene (CB)/isopropanol (IPA) (7:3 volume ratio) for surface post-treatment of FAPbI₃ perovskite film. The interaction of AIPZ and perovskite surface not only balances the charge injection but also passivates defects to enhance radiative recombination in PeLED. Therefore, the PeLED champion yields peak external quantum efficiency reaching 23.2% at the current density of 45 mA cm^-2 with a radiance brightness of 290 W sr^-1 m^-2 . More importantly, the rolling-off of device efficiency is significantly reduced. The lowest rolling-off devices can maintain 80% of peak EQE (22.1%) at a high current density of 460 mA cm^-2 , whereas the control device only retains 25% of the peak EQE value. This work provides an effective strategy to improve performance and reduce the EQE rolling-off of PeLED for practical application.

Suppressing High-Order Phase for Efficient Pure Red Quasi-2D Perovskite Light-Emitting Diodes

Quasi-two-dimensional (quasi-2D) perovskites are promising for the realization of spectrally stable pure red perovskite light-emitting diodes (PeLEDs) with a single iodide component, because they avoid the halide separation that red three-dimensional perovskites of mixed halides have faced. However, the distribution of high-order phases in solution-processed quasi-2D perovskite films causes the spectral shift away from the pure red region. Here, we introduced a simple approach of adding excessive ligand combinations to redistribute the phase distribution of quasi-2D perovskite and to inhibit the high-order phase. Appropriate excess organic ligands will not affect charge injection but will keep the efficient energy funneling and passivate the defect. The narrowed phase distribution reduced the band tail state and restrained reverse charge transfer, resulting in enhanced radiation recombination. We obtained efficient and spectrally stable pure red PeLEDs at 638 nm (approaching the Rec. 2020 specification) with a peak EQE of 11.8% and maximum luminance of 1688 cd/cm². This study provides guidance for future developments of highly efficient pure red PeLEDs.

Synergistic passivation and stepped-dimensional perovskite analogs enable high-efficiency near-infrared light-emitting diodes

Formamidinium lead iodide (FAPbI₃) perovskites are promising emitters for near-infrared light-emitting diodes. However, their performance is still limited by defect-assisted nonradiative recombination and band offset-induced carrier aggregation at the interface. Herein, we introduce a couple of cadmium salts with acetate or halide anion into the FAPbI₃ perovskite precursors to synergistically passivate the material defects and optimize the device band structure. Particularly, the perovskite analogs, containing zero-dimensional formamidinium cadmium iodide, one-dimensional δ-FAPbI₃, two-dimensional FA₂FA_n-1Pb_nI_3n+1, and three-dimensional α-FAPbI₃, can be obtained in one pot and play a pivotal and positive role in energy transfer in the formamidinium iodide-rich lead-based perovskite films. As a result, the near-infrared FAPbI₃-based devices deliver a maximum external quantum efficiency of 24.1% together with substantially improved operational stability. Combining our findings on defect passivation and energy transfer, we also achieve near-infrared light communication with device twins of light emitting and unprecedented self-driven detection.

Core–shell carbon-polymer quantum dot passivation for near infrared perovskite light emitting diodes

High-performance perovskite light-emitting diodes (PeLEDs) require a high quality perovskite emitter and appropriate charge transport layers to facilitate charge injection and transport within the device. Solution-processed n-type metal oxides represent a judicious choice for the electron transport layer (ETL); however, they do not always present surface properties and energetics compatible with the perovskite emitter. Moreover, the emitter itself exhibits poor nanomorphology and defect traps that compromise the device performance. Here, we modulate the surface properties and interface energetics between the tin oxide (SnO2) ETL with the perovskite emitter by using an amino functionalized difluoro{2-[1-(3,5-dimethyl-2H-pyrrol-2-ylidene-N)ethyl]-3,5-dimethyl-1H-pyrrolato-N}boron compound and passivate the defects present in the perovskite matrix with carbon-polymer core–shell quantum dots inserted into the perovskite precursor. Both these approaches synergistically improve the perovskite layer nanomorphology and enhance the radiative recombination. These properties resulted in the fabrication of near-infrared PeLEDs based on formamidinium lead iodide (FAPbI3) with a high radiance of 92 W sr−1 m−2, an external quantum efficiency (EQE) of 14%, reduced efficiency roll-off and prolonged lifetime. In particular, the modified device retained 80% of the initial EQE (T80) for 33 h compared to 6 h of the reference cell.

Trade-off between the Performance and Stability of Perovskite Light-Emitting Diodes with Excess Halides

Incorporation of excess bulky organoammonium halides as additives is an efficient way to enhance the performance of perovskite light-emitting diodes (PeLEDs). The excess organoammonium halides can decrease the grain size and minimize the trap density to enhance radiative recombination. In this work, we reveal that the halides in excess additives also play a critical role in the operation stability of PeLEDs. With an increasing excess halide ratio, perovskite films gradually change from being rich in halide vacancies (VI) to being rich in halide interstitials (Ii), both of which can promote halide migration and reduce the operation stability. By using mixed 4-fluorophenylmethylammonium iodide and 4-fluorophenylmethylamine as additives, the excess halide ratio can be controlled and both VI and Ii can be minimized. Therefore, the operation stability of methylammonium lead iodide-based PeLEDs is enhanced significantly from 40 to 520 min. This work emphasizes the importance of controlling excess halide concentrations in terms of device performance and operation stability.

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

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

Enhancing the Luminance Efficiency of Formamidinium-Based Dion-Jacobson Perovskite Light-Emitting Diodes via Compositional Engineering

In this paper, the phase formation mechanism of formamidinium (FA)-based Dion-Jacobson (DJ) perovskites is uncovered for the first time, which includes the formation of the n₁ domain (n = 1 perovskite phase) and a trace amount of the large n domain in the spin-coating process and the growth of the n₂ domain and the large n domain during the thermal annealing stage. The phase formation mechanism clearly reveals the different phase distributions between FA-based Ruddlesden-Popper (RP) and FA-based DJ perovskite films at different stages of film preparation due to the larger formation energy of the DJ perovskites. According to this phase formation mechanism, we put forward an effective strategy of the small A-site cation compositional engineering. Then, excess perovskitizer cations (FA⁺) are introduced to increase the crystallinity, modulate the phase distribution and passivate defects without disturbing the structure of DJ perovskites simultaneously. The final green-light DJ PeLED devices show a maximum luminance of 41 520 cd/m² and a maximum current efficiency of 31.1 cd/A (EQE: 8.5%), which are the record values so far. The final DJ PeLEDs show an improved operational lifetime of ca. 15 min at an initial luminance of ca. 800 cd/m². Our results suggest that DJ perovskites can be promising for light-emitting applications.