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

Advances and challenges in molecular engineering of 2D/3D perovskite heterostructures

Organic-inorganic hybrid perovskites have been intensively studied in past decades due to their outstanding performance in solar cells and other optoelectronic devices. Recently, the emergence of two-dimensional/three-dimensional (2D/3D) heterojunctions have enabled many solar cell devices with >25% power conversion efficiency, driven by advances in our understanding of the structural and photophysical properties of the heterojunctions and our ability to control these properties through organic cation configuration in 2D perovskites. In this feature article, we discuss a fundamental understanding of structural characteristics and the carrier dynamics in the 2D/3D heterojunctions and their impact factors. We further elaborate the design strategies for the molecular configuration of organic cations to achieve thorough management of these properties. Finally, recent advances in 2D/3D heterostructures in solar cells, light-emitting devices and photodetectors are highlighted, which translate fundamental understandings to device applications and also reveal the remaining challenges in ligand design for the next generation of stable devices. Future development prospects and related challenges are also provided, with wide perspectives and insightful thoughts.

Improving the Operational Lifetime of Metal-Halide Perovskite Light-Emitting Diodes with Dimension Control and Ligand Engineering

Perovskite light-emitting diodes (LEDs) have emerged as one of the most propitious candidates for next-generation lighting and displays, with the highest external quantum efficiency (EQE) of perovskite LEDs already surpassing the 20% milestone. However, the further development of perovskite LEDs primarily relies on addressing operational instability issues. This Perspective examines some of the key factors that impact the lifetime of perovskite LED devices and some representative reports on recent advancements aimed at improving the lifetime. Our analysis underscores the significance of "nano" strategies in achieving long-term stable perovskite LEDs. Significant efforts must be directed toward proper device encapsulation, perovskite material passivation, interfacial treatment to address environment-induced material instability, bias-induced phase separation, and ion migration issues.

Functionalizing nanophotonic structures with 2D van der Waals materials

The integration of two-dimensional (2D) van der Waals materials with nanostructures has triggered a wide spectrum of optical and optoelectronic applications. Photonic structures of conventional materials typically lack efficient reconfigurability or multifunctionality. Atomically thin 2D materials can thus generate new functionality and reconfigurability for a well-established library of photonic structures such as integrated waveguides, optical fibers, photonic crystals, and metasurfaces, to name a few. Meanwhile, the interaction between light and van der Waals materials can be drastically enhanced as well by leveraging micro-cavities or resonators with high optical confinement. The unique van der Waals surfaces of the 2D materials enable handiness in transfer and mixing with various prefabricated photonic templates with high degrees of freedom, functionalizing as the optical gain, modulation, sensing, or plasmonic media for diverse applications. Here, we review recent advances in synergizing 2D materials to nanophotonic structures for prototyping novel functionality or performance enhancements. Challenges in scalable 2D materials preparations and transfer, as well as emerging opportunities in integrating van der Waals building blocks beyond 2D materials are also discussed.

Stability of Perovskite Light-Emitting Diodes: Existing Issues and Mitigation Strategies Related to Both Material and Device Aspects

Metal halide perovskites combine excellent electronic and optical properties, such as defect tolerance and high photoluminescence efficiency, with the benefits of low-cost, large-area, solution-based processing. Composition- and dimension-tunable properties of perovskites have already been utilized in bright and efficient light-emitting diodes (LEDs). At the same time, there are still great challenges ahead to achieving operational and spectral stability of these devices. In this review, the origins of instability of perovskite materials, and reasons for their degradation in LEDs are considered. Then, strategies for improving the stability of perovskite materials are reviewed, such as compositional engineering, dimensionality control, defect passivation, suitable encapsulation matrices, and fabrication of core/shell perovskite nanocrystals. For improvement of the operational stability of perovskite LEDs, the use of inorganic charge-transport layers, optimization of charge balance, and proper thermal management are considered. The review is concluded with a detailed account of the current challenges and a perspective on the key approaches and opportunities on how to reach the goal of stable, bright, and efficient perovskite LEDs.

Efficient and Stable 3D/2D Perovskite Solar Cells through Vertical Heterostructures with (BA)4 AgBiBr8 Nanosheets

Perovskite solar cells (PVSCs) have drawn great attention due to their high processability and superior photovoltaic properties. However, their further development is often hindered by severe nonradiative recombination at interfaces that decreases power conversion efficiency (PCE). To this end, a facile strategy to construct a 3D/2D vertical heterostructure to reduce the energy loss in PVSCs is developed. The heterostructure is contrived through the van der Waals integration of 2D perovskite ((BA)₄ AgBiBr₈ ) nanosheets onto the surface of 3D-FAPbI₃ -based perovskites. The large bandgap of (BA)₄ AgBiBr₈ enables the formation of type-I heterojunction with 3D-FAPbI₃ -based perovskites, which serves as a barrier to suppress the trap-assisted recombination at the interface. As a result, a satisfying PCE of 24.48% is achieved with an improved open-circuit voltage (V_OC ) from 1.13 to 1.17 V. Moreover, the 2D perovskite nanosheets can effectively mitigate the iodide ion diffusion from perovskite to the metal electrode, hence enhancing the device stability. 3D/2D architectured devices retain ≈90% of their initial PCE under continuous illumination or heating after 1000 h, which are superior to 3D-based devices. This work provides an effective and controllable strategy to construct 3D/2D vertical heterostructure to simultaneously boost the efficiency and stability of PVSCs.

Pattern-Selective Molecular Epitaxial Growth of Single-Crystalline Perovskite Arrays toward Ultrasensitive and Ultrafast Photodetector

The emergence of organic-inorganic perovskite has provided great flexibility for creating optoelectronic devices with unprecedented performance or unique functionality. However, the perovskite films explored so far have been difficult to be patterned to arrays owing to their poor solvent and moisture stability, which usually lead to serious structural damage of perovskites. The successful preparation of perovskite microarrays with uniform shape and size is more challenging. Here we report a straightforward approach to realize single-crystalline perovskite arrays through a relatively simple pattern-selective molecular epitaxial growth. This approach is applied to create diverse shaped perovskite arrays, such as hexagon, triangle, circle, square, and rectangle. A vertically aligned perovskite photodetector displays both an ultrasensitive and ultrafast photoresponse arising from the reduction in carrier diffusion paths and the high optical absorption. This work demonstrates a general approach to creating perovskite arrays with uniform shape, size, and morphology and provides a rich platform for producing high-performance photodetectors and photovoltage devices.

Wide-Gap Perovskite via Synergetic Surface Passivation and Its Application toward Efficient Stacked Tandem Photovoltaics

Superior bandgap tunability enables solution-processed halide perovskite a promising candidate for multi-junction photovoltaics (PVs). Particularly, optically coupling wide-gap perovskite by stacking with commercially available PVs such as silicon and CIGS (also known as 4-terminal tandem) simplifies the technology transfer process, and further advances the commercialization potential of perovskite technology. However, compared with matured PV materials and the phase-pure FAPbI₃ , wide-gap perovskite still suffers from huge voltage deficits. Here, the authors take advantage of the synergetic effect behind a sequential fluoride and organic ammonium salt surface passivation strategy to control non-radiative energy losses, and obtained a 17.7% efficiency in infrared-transparent wide-gap perovskite solar cells (21.1% for opaque device), and achieved efficiencies of over 25% when stacked with commercial Si and CIGS products with original PCEs of 18-20% under a 4-terminal working condition.

ZnO-Based Electron-Transporting Layers for Perovskite Light-Emitting Diodes: Controlling the Interfacial Reactions

Perovskite light-emitting diodes (PeLEDs) provide new opportunities for cost-effective and large-area electroluminescent devices. It is of interest to use ZnO-based electron-transport layers (ETLs), which demonstrate superior performance in other solution-processed LEDs, in PeLEDs. However, the notorious deprotonation reaction between ZnO and perovskite casts doubt on the long-term stability of PeLEDs with ZnO-based ETLs. This Perspective presents an overview of the chemical reactions that may occur at the interfaces between perovskite and ZnO-based ETLs. We highlight that other interfacial reactions during the fabrication of PeLEDs, including the reactions between ZnO and the intermediate phase during perovskite crystallization and the amidation reactions catalyzed by ZnO, demonstrate critical utilities in the fabrication of high-efficiency and stable PeLEDs. Considering these recent advances, we propose future directions and prospects to design and control the interfacial reactions, aiming to fully exploit the potential of ZnO-based ETLs for realizing high-performance PeLEDs.

Electrochemical Performance of Orthorhombic CsPbI3 Perovskite in Li-Ion Batteries

A facile solution process was employed to prepare CsPbI₃ as an anode material for Li-ion batteries. Rietveld refinement of the X-ray data confirms the orthorhombic phase of CsPbI₃ at room temperature. As obtained from bond valence calculations, strained bonds between Pb and I are identified within PbI₆ octahedral units. Morphological study shows that the as-prepared δ-CsPbI₃ forms a nanorod-like structure. The XPS analysis confirm the presence of Cs (3d, 4d), Pb (4d, 4f, 5d) and I (3p, 3d, 4d). The lithiation process involves both intercalation and conversion reactions, as confirmed by cyclic voltammetry (CV) and first-principles calculations. Impedance spectroscopy coupled with the distribution function of relaxation times identifies charge transfer processes due to Li metal foil and anode/electrolyte interfaces. An initial discharge capacity of 151 mAhg⁻¹ is found to continuously increase to reach a maximum of ~275 mAhg⁻¹ at 65 cycles, while it drops to ~240 mAhg⁻¹ at 75 cycles and then slowly decreases to 235 mAhg⁻¹ at 100 cycles. Considering the performance and structural integrity during electrochemical performance, δ-CsPbI₃ is a promising material for future Li-ion battery (LIB) application.

Defect Passivation in Lead-Halide Perovskite Nanocrystals and Thin Films: Toward Efficient LEDs and Solar Cells

Lead-halide perovskites (LHPs), in the form of both colloidal nanocrystals (NCs) and thin films, have emerged over the past decade as leading candidates for next-generation, efficient light-emitting diodes (LEDs) and solar cells. Owing to their high photoluminescence quantum yields (PLQYs), LHPs efficiently convert injected charge carriers into light and vice versa. However, despite the defect-tolerance of LHPs, defects at the surface of colloidal NCs and grain boundaries in thin films play a critical role in charge-carrier transport and nonradiative recombination, which lowers the PLQYs, device efficiency, and stability. Therefore, understanding the defects that play a key role in limiting performance, and developing effective passivation routes are critical for achieving advances in performance. This Review presents the current understanding of defects in halide perovskites and their influence on the optical and charge-carrier transport properties. Passivation strategies toward improving the efficiencies of perovskite-based LEDs and solar cells are also discussed.

Dimensional structure regulation of organic-inorganic hybrid perovskite and its application in thin film transistors

The effects of dimensional structure on the properties of lead iodide perovskite (C₈H₉NH₃)₂(CH₃NH₃)_n-1Pb_nI_3n+1were investigated. Furthermore, perovskite thin films with different dimensionalities were applied as the channel layer of thin film transistors (TFT). The electrical performance and stability of TFT devices were significantly improved through the regulation of dimensional microstructure of the perovskites. As a result, the quasi-2D (n = 6) perovskite TFTs achieved a field-effect mobility (μ_FE) of 3.90 cm²V^-1s^-1, with 10⁴on-off current ratio and -1.85 V threshold voltage, which can be maintained well after 4 days without degradation at 30% ambient humidity. Moreover, the electrical performance of the TFTs based on Pure-2D and Quasi-2D perovskite also exhibited a good bias stability.

Chiral 2D-Perovskite Nanowires for Stokes Photodetectors

Structural engineering in multiple scales permits the integration of exotic properties into a single material, which boosts the development of ultracompact multifunctional devices. Layered perovskites are capable of cross-linking efficient carrier transport originating from few-layer perovskite frameworks with extended functionalities contributed by designable bulky organic cations and nanostructures, thus providing a platform for multiscale material engineering. Herein, high-performance Stokes-parameter photodetectors for arbitrary polarized light detection are realized on the basis of solution-processed chiral-perovskite nanowire arrays. The chiral ammonium cations intercalated between the perovskite layers are responsive to circularly polarized light with a maximum anisotropy factor of 0.15, while the strictly aligned nanowires with the anisotropic dielectric function result in a large polarized ratio of 1.6 to linearly polarized light. Single crystallinity and pure crystallographic orientation permit efficient in-plane carrier transport along the nanowires, yielding a responsivity of 47.1 A W_^-1 and a detectivity of 1.24 × 10¹³ Jones. By synergy of linear- and circular-polarization response with high optoelectronic performance for providing sufficient photocurrent contrasts, Stokes-parameter photodetection is demonstrated on these nanowires. Our Stokes-parameter photodetectors with a small footprint and high performances present promising applications toward polarization imaging.

Quasi-two-dimensional perovskite light emitting diodes for bright future

The fundamentals, promise and challenges of metal halide quasi-two-dimensional (quasi-2D) perovskites for a next generation emitter in light emitting diode devices are systematically reviewed.

Phenylalkylammonium passivation enables perovskite light emitting diodes with record high-radiance operational lifetime: the chain length matters

Perovskite light emitting diodes suffer from poor operational stability, exhibiting a rapid decay of external quantum efficiency within minutes to hours after turn-on. To address this issue, we explore surface treatment of perovskite films with phenylalkylammonium iodide molecules of varying alkyl chain lengths. Combining experimental characterization and theoretical modelling, we show that these molecules stabilize the perovskite through suppression of iodide ion migration. The stabilization effect is enhanced with increasing chain length due to the stronger binding of the molecules with the perovskite surface, as well as the increased steric hindrance to reconfiguration for accommodating ion migration. The passivation also reduces the surface defects, resulting in a high radiance and delayed roll-off of external quantum efficiency. Using the optimized passivation molecule, phenylpropylammonium iodide, we achieve devices with an efficiency of 17.5%, a radiance of 1282.8 W sr-1 m-2 and a record T50 half-lifetime of 130 h under 100 mA cm-2.

Solid-phase hetero epitaxial growth of α-phase formamidinium perovskite

Conventional epitaxy of semiconductor films requires a compatible single crystalline substrate and precisely controlled growth conditions, which limit the price competitiveness and versatility of the process. We demonstrate substrate-tolerant nano-heteroepitaxy (NHE) of high-quality formamidinium-lead-tri-iodide (FAPbI₃) perovskite films. The layered perovskite templates the solid-state phase conversion of FAPbI₃ from its hexagonal non-perovskite phase to the cubic perovskite polymorph, where the growth kinetics are controlled by a synergistic effect between strain and entropy. The slow heteroepitaxial crystal growth enlarged the perovskite crystals by 10-fold with a reduced defect density and strong preferred orientation. This NHE is readily applicable to various substrates used for devices. The proof-of-concept solar cell and light-emitting diode devices based on the NHE-FAPbI₃ showed efficiencies and stabilities superior to those of devices fabricated without NHE.

Though literature reports metal halide perovskite epitaxial growth on various substrates, controlling film growth for device applications remains a challenge. Here, the authors report kinetic-controlled growth of halide perovskite thin films on various substrates via layered perovskite templates.

Suppression of Electric Field-Induced Segregation in Sky-Blue Perovskite Light-Emitting Electrochemical Cells

Inexpensive perovskite light-emitting devices fabricated by a simple wet chemical approach have recently demonstrated very prospective characteristics such as narrowband emission, low turn-on bias, high brightness, and high external quantum efficiency of electroluminescence, and have presented a good alternative to well-established technology of epitaxially grown III-V semiconducting alloys. Engineering of highly efficient perovskite light-emitting devices emitting green, red, and near-infrared light has been demonstrated in numerous reports and has faced no major fundamental limitations. On the contrary, the devices emitting blue light, in particular, based on 3D mixed-halide perovskites, suffer from electric field-induced phase separation (segregation). This crystal lattice defect-mediated phenomenon results in an undesirable color change of electroluminescence. Here we report a novel approach towards the suppression of the segregation in single-layer perovskite light-emitting electrochemical cells. Co-crystallization of direct band gap CsPb(Cl,Br)3 and indirect band gap Cs4Pb(Cl,Br)6 phases in the presence of poly(ethylene oxide) during a thin film deposition affords passivation of surface defect states and an increase in the density of photoexcited charge carriers in CsPb(Cl,Br)3 grains. Furthermore, the hexahalide phase prevents the dissociation of the emissive grains in the strong electric field during the device operation. Entirely resistant to 5.7 × 106 V·m−1 electric field-driven segregation light-emitting electrochemical cell exhibits stable emission at wavelength 479 nm with maximum external quantum efficiency 0.7%, maximum brightness 47 cd·m−2, and turn-on bias of 2.5 V.

Focus on performance of perovskite light-emitting diodes

Perovskite-based optoelectronic devices, especially perovskite light-emitting diodes (PeLEDs) and perovskite solar cells, have recently attracted considerable attention. The National Renewable Energy Laboratory (NREL) chart inspires us to develop a counterpart for PeLEDs. In this study, we collect the record performance of PeLEDs including several new entries to address their latest external quantum efficiency (EQE), highest luminance, and stability status. We hope that these performance tables and future updated versions will show the frontiers of PeLEDs, assist researchers in capturing the overview of this field, identify the remaining challenges, and predict the promising research directions.

Bifacial passivation towards efficient FAPbBr3-based inverted perovskite light-emitting diodes

A unique technique to passivate both bottom and top sides of perovskite has been successfully developed to achieve highly efficient inverted perovskite light-emitting diodes (PeLEDs). For the bottom passivation, an organic/inorganic hybrid electron transporting layer (ETL) replaces the widely adopted inorganic ETL to overcome the disadvantages of the pure inorganic ETL. The ZPM (ZnO-in-polymer matrix) ETL, which consists of ZnO nanoparticles blended into polyvinylpyrrolidone, not only passivates the surface defects of ZnO nanoparticles, but also improves the morphology and stability of FAPbBr₃ film. For the top passivation, smaller grains and a FAPbBr₃/PEA₂PbBr₄ 3D/2D hybrid structure are obtained by applying a small amount of PEABr solution. The synergetic interplay of organic/inorganic hybrid ETL and organic halide salt surface modification substantially shrinks the grain size to facilitate radiative recombination, and suppresses non-radiative recombination both at the interface of ETL/perovskite and HTL/perovskite, and in the perovskite layer. As a result, the highly efficient green PeLED sets a new record of device performance for FAPbBr₃-based inverted PeLEDs, with current efficiency of 39.7 cd A^-1, external quantum efficiency of 9.0%, power efficiency of 46.4 lm W^-1, maximum luminance of 6.03 × 10⁴ cd m^-2, and half-lifetime of 297 minutes at an initial brightness of ∼100 cd m^-2.

Structured Perovskite Light Absorbers for Efficient and Stable Photovoltaics

Organic-inorganic hybrid lead-halide perovskite materials (ABX₃ ) have attracted widespread attention in the field of photovoltaics owing to their impressive optical and electrical properties. However, obstacles still exist in the commercialization of perovskite photovoltaics, such as poor stability, hysteresis, and human toxicity. A-site cation engineering is considered to be a powerful tool to tune perovskite structures and the resulting optoelectronic properties. Based on the selection and combination of A-site cations, three types of perovskite structures, i.e., 3D perovskite, reduced-dimensional (2D/quasi-2D) perovskite, and 2D/3D hybrid perovskite can be formed. Herein, the remarkable breakthroughs resulting from these three perovskite structures are summarized, and their corresponding properties and characteristics, as well as their intrinsic disadvantages, are highlighted. By summarizing recent research progress, a new viewpoint for improving the performance and stability of perovskite photovoltaics is provided.

Improved photoemission and stability of 2D organic-inorganic lead iodide perovskite films by polymer passivation

2D organic-inorganic lead iodide perovskites hold great promise for functional optoelectronic devices. However, their performances have been seriously limited by poor long-term stability in ambient environment. Here, we perform a systematic study for the stability improvement of a typical 2D organic-inorganic lead iodide perovskite (PEA)2PbI4. The degradation of the (PEA)2PbI4 films can be attributed to the interaction with the humidity in environment, which leads to decomposition of the perovskite components. Then, we demonstrate that polymer passivation provides an effective approach for improving the crystal quality and stability of the (PEA)2PbI4 films. Correspondingly, the photoemission of the polymer-passivated (PEA)2PbI4 films has been enhanced due to the decreased trap states. More importantly, a hydrophobic polymer (Poly(4-Vinylpyridine), PVP) will protect the (PEA)2PbI4 films from humidity in ambient environment, which can greatly improve the physical and chemical stability of the 2D perovskite films. As a result, the PVP-passivated (PEA)2PbI4 films can produce a bright emission even after long-term (>15 d) exposure to ambient environment (25 °C, 80% RH) and continuous UV illumination. This work provides a convenient and effective approach for improving the long-term stability of 2D organic-inorganic lead iodide perovskites, which shows great promise for fabricating large-area and versatile optoelectronic devices.