Stable Light-Emitting Diodes Using Phase-Pure Ruddlesden-Popper Layered Perovskites

Time-Resolved Electrical Scanning Probe Microscopy of Layered Perovskites Reveals Spatial Variations in Photoinduced Ionic and Electronic Carrier Motion

We study light-induced dynamics in thin films comprising Ruddlesden-Popper phases of the layered 2D perovskite (C₄H₉NH₃)₂PbI₄. We probe ionic and electronic carrier dynamics using two complementary scanning probe methods, time-resolved G-mode Kelvin probe force microscopy and fast free time-resolved electrostatic force microscopy, as a function of position, time, and illumination. We show that the average surface photovoltage sign is dominated by the band bending at the buried perovskite-substrate interface. However, the film exhibits substantial variations in the spatial and temporal response of the photovoltage. Under illumination, the photovoltage equilibrates over hundreds of microseconds, a time scale associated with ionic motion and trapped electronic carriers. Surprisingly, we observe that the surface photovoltage of the 2D grain centers evolves more rapidly in time than at the grain boundaries. We propose that the slower evolution at grain boundaries is due to a combination of ion migration occurring between PbI₄ planes, as well as electronic carriers traversing grain boundary traps, thereby changing the time-dependent band unbending at grain boundaries. These results provide a model for the photoinduced dynamics in 2D perovskites and are a useful basis for interpreting photovoltage dynamics on hybrid 2D/3D structures.

Uniaxial Expansion of the 2D Ruddlesden-Popper Perovskite Family for Improved Environmental Stability

The unique hybrid nature of 2D Ruddlesden-Popper (R-P) perovskites has bestowed upon them not only tunability of their electronic properties but also high-performance electronic devices with improved environmental stability as compared to their 3D analogs. However, there is limited information about their inherent heat, light, and air stability and how different parameters such as the inorganic layer number and length of organic spacer molecule affect stability. To gain deeper understanding on the matter we have expanded the family of 2D R-P perovskites, by utilizing pentylamine (PA)₂(MA) _n-1Pb _nI_{3 n+1} ( n = 1-5, PA = CH₃(CH₂)₄NH₃⁺, C5) and hexylamine (HA)₂(MA) _n-1Pb _nI_{3 n+1} ( n = 1-4, HA = CH₃(CH₂)₅NH₃⁺, C6) as the organic spacer molecules between the inorganic slabs, creating two new series of layered materials, for up to n = 5 and 4 layers, respectively. The resulting compounds were extensively characterized through a combination of physical and spectroscopic methods, including single crystal X-ray analysis. High resolution powder X-ray diffraction studies using synchrotron radiation shed light for the first time to the phase transitions of the higher layer 2D R-P perovskites. The increase in the length of the organic spacer molecules did not affect their optical properties; however, it has a pronounced effect on the air, heat, and light stability of the fabricated thin films. An extensive study of heat, light, and air stability with and without encapsulation revealed that specific compounds can be air stable (relative humidity (RH) = 20-80% ± 5%) for more than 450 days, while heat and light stability in air can be exponentially increased by encapsulating the corresponding films. Evaluation of the out-of-plane mechanical properties of the corresponding materials showed that their soft and flexible nature can be compared to current commercially available polymer substrates (e.g., PMMA), rendering them suitable for fabricating flexible and wearable electronic devices.

Effects of n-butyl amine incorporation on the performance of perovskite light emitting diodes

The efficiency of perovskite light emitting diodes (PeLEDs) is crucially limited by leakage current and nonradiative recombination. Here we introduce n-butyl amine (BA) to modulate the growth of perovskite films as well as improve the performance of PeLEDs, and investigate in detail the effects of BA incorporation on the structural, optical, and electrical characteristics of perovskite films. The results indicate that BA would terminate the grain surface and inhibit crystal growth, leading to increased radiative recombination. However, BA overload would make the films loose and recreate shunt paths. The electrical detriment of BA overload outweighs its optical benefit. As a result, optimal PeLEDs can be obtained only with moderate BA incorporation.

Phase control of quasi-2D perovskites and improved light-emitting performance by excess organic cations and nanoparticle intercalation

The optoelectronic properties of quasi-two-dimensional organic-inorganic hybrid perovskites can be tuned by controlling the formation of Ruddlesden-Popper type phases, which enables diverse device applications such as photovoltaics and light-emitting diodes (LEDs). Herein, the influence of excess organic cations on the phase formation of (PEA)2MAn-1PbnBr3n+1 is systematically investigated with various mixing ratios to discover the phase distribution beneficial for light-emitting diodes. It is found that PEA cations exceeding Pb ions in molar ratio are required to produce small-n phases in the films with a strong photoluminescence, while excess MA cations enable the formation of more large-n phases. Low electrical conductivity inherent to the properties of quasi-2D perovskites is further lowered by the introduction of excess organic cations. This is overcome by the intercalation of zinc oxide (ZnO) nanoparticles (NPs) into the blocking layers composed of PEA cations. Importantly, these metal oxide NPs also modulate the phase distribution, enabling the realization of bright green quasi-2D perovskites with a better stability and a maximum luminance of nearly 60 000 cd m-2, which is the highest brightness compared to the so far reported quasi-2D perovskite LEDs incorporating organic cations.

Quantum and Dielectric Confinement Effects in Lower-Dimensional Hybrid Perovskite Semiconductors

Hybrid halide perovskites are now superstar materials leading the field of low-cost thin film photovoltaics technologies. Following the surge for more efficient and stable 3D bulk alloys, multilayered halide perovskites and colloidal perovskite nanostructures appeared in 2016 as viable alternative solutions to this challenge, largely exceeding the original proof of concept made in 2009 and 2014, respectively. This triggered renewed interest in lower-dimensional hybrid halide perovskites and at the same time increasingly more numerous and differentiated applications. The present paper is a review of the past and present literature on both colloidal nanostructures and multilayered compounds, emphasizing that availability of accurate structural information is of dramatic importance to reach a fair understanding of quantum and dielectric confinement effects. Layered halide perovskites occupy a special place in the history of halide perovskites, with a large number of seminal papers in the 1980s and 1990s. In recent years, the rationalization of structure-properties relationship has greatly benefited from new theoretical approaches dedicated to their electronic structures and optoelectronic properties, as well as a growing number of contributions based on modern experimental techniques. This is a necessary step to provide in-depth tools to decipher their extensive chemical engineering possibilities which surpass the ones of their 3D bulk counterparts. Comparisons to classical semiconductor nanostructures and 2D van der Waals heterostructures are also stressed. Since 2015, colloidal nanostructures have undergone a quick development for applications based on light emission. Although intensively studied in the last two years by various spectroscopy techniques, the description of quantum and dielectric confinement effects on their optoelectronic properties is still in its infancy.

Designing a new family of oxonium-cation based structurally diverse organic–inorganic hybrid iodoantimonate crystals

We report proton-bound oxonium cation based iodoantimonate hybrid organic–inorganic crystals with diverse structure–property relationships.

Depth-dependent electronic band structure at the Au/CH3NH3PbI3−xClx junction

The electronic properties of the interface between Au and the lead halide perovskite (CH₃NH₃PbI_3−xCl_x) were investigated by ultraviolet photoelectron spectroscopy (UPS) and X-ray photoemission spectroscopy (XPS).

Merits and Challenges of Ruddlesden-Popper Soft Halide Perovskites in Electro-Optics and Optoelectronics

Following the rejuvenation of 3D organic-inorganic hybrid perovskites, like CH₃ NH₃ PbI₃ , (quasi)-2D Ruddlesden-Popper soft halide perovskites R₂ A_{n -1} Pb_n X_{3 n +1} have recently become another focus in the optoelectronic and photovoltaic device community. Although quasi-2D perovskites were first introduced to stabilize optoelectronic/photovoltaic devices against moisture, more interesting properties and device applications, such as solar cells, light-emitting diodes, white-light emitters, lasers, and polaritonic emission, have followed. While delicate engineering design has pushed the performance of various devices forward remarkably, understanding of the fundamental properties, especially the charge-transfer process, electron-phonon interactions, and the growth mechanism in (quasi)-2D halide perovskites, remains limited and even controversial. Here, after reviewing the current understanding and the nexus between optoelectronic/photovoltaic properties of 2D and 3D halide perovskites, the growth mechanisms, charge-transfer processes, vibrational properties, and electron-phonon interactions of soft halide perovskites, mainly in quasi-2D systems, are discussed. It is suggested that single-crystal-based studies are needed to deepen the understanding of the aforementioned fundamental properties, and will eventually contribute to device performance.

Oriented Quasi-2D Perovskites for High Performance Optoelectronic Devices

Quasi-2D layered organometal halide perovskites have recently emerged as promising candidates for solar cells, because of their intrinsic stability compared to 3D analogs. However, relatively low power conversion efficiency (PCE) limits the application of 2D layered perovskites in photovoltaics, due to large energy band gap, high exciton binding energy, and poor interlayer charge transport. Here, efficient and water-stable quasi-2D perovskite solar cells with a peak PCE of 18.20% by using 3-bromobenzylammonium iodide are demonstrated. The unencapsulated devices sustain over 82% of their initial efficiency after 2400 h under relative humidity of ≈40%, and show almost unchanged photovoltaic parameters after immersion into water for 60 s. The robust performance of perovskite solar cells results from the quasi-2D perovskite films with hydrophobic nature and a high degree of electronic order and high crystallinity, which consists of both ordered large-bandgap perovskites with the vertical growth in the bottom region and oriented small-bandgap components in the top region. Moreover, due to the suppressed nonradiative recombination, the unencapsulated photovoltaic devices can work well as light-emitting diodes (LEDs), exhibiting an external quantum efficiency of 3.85% and a long operational lifetime of ≈96 h at a high current density of 200 mA cm^-2 in air.

Organic-Inorganic Hybrid Passivation Enables Perovskite QLEDs with an EQE of 16.48

Perovskite quantum dots (QDs) with high photoluminescence quantum yields (PLQYs) and narrow emission peak hold promise for next-generation flexible and high-definition displays. However, perovskite QD films often suffer from low PLQYs due to the dynamic characteristics between the QD's surface and organic ligands and inefficient electrical transportation resulting from long hydrocarbon organic ligands as highly insulating barrier, which impair the ensuing device performance. Here, a general organic-inorganic hybrid ligand (OIHL) strategy is reported on to passivate perovskite QDs for highly efficient electroluminescent devices. Films based on QDs through OIHLs exhibit enhanced radiative recombination and effective electrical transportation properties compared to the primal QDs. After the OIHL passivation, QD-based light-emitting diodes (QLEDs) exhibit a maximum peak external quantum efficiency (EQE) of 16.48%, which is the most efficient electroluminescent device in the field of perovskite-based LEDs up to date. The proposed OIHL passivation strategy positions perovskite QDs as an extremely promising prospect in future applications of high-definition displays, high-quality lightings, as well as solar cells.

Two-Dimensional Hybrid Halide Perovskites: Principles and Promises

Hybrid halide perovskites have become the "next big thing" in emerging semiconductor materials, as the past decade witnessed their successful application in high-performance photovoltaics. This resurgence has encompassed enormous and widespread development of the three-dimensional (3D) perovskites, spearheaded by CH₃NH₃PbI₃. The next generation of halide perovskites, however, is characterized by reduced dimensionality perovskites, emphasizing the two-dimensional (2D) perovskite derivatives which expand the field into a more diverse subgroup of semiconducting hybrids that possesses even higher tunability and excellent photophysical properties. In this Perspective, we begin with a historical flashback to early reports before the "perovskite fever", and we follow this original work to its fruition in the present day, where 2D halide perovskites are in the spotlight of current research, offering characteristics desirable in high-performance optoelectronics. We approach the evolution of 2D halide perovskites from a structural perspective, providing a way to classify the diverse structure types of the materials, which largely dictate the unusual physical properties observed. We sort the 2D hybrid halide perovskites on the basis of two key components: the inorganic layers and their modification, and the organic cation diversity. As these two heterogeneous components blend, either by synthetic manipulation (shuffling the organic cations or inorganic elements) or by application of external stimuli (temperature and pressure), the modular perovskite structure evolves to construct crystallographically defined quantum wells (QWs). The complex electronic structure that arises is sensitive to the structural features that could be in turn used as a knob to control the dielectric and optical properties the QWs. We conclude this Perspective with the most notable achievements in optoelectronic devices that have been demonstrated to date, with an eye toward future material discovery and potential technological developments.

Low Power Consumption Red Light-Emitting Diodes Based on Inorganic Perovskite Quantum Dots under an Alternating Current Driving Mode

Inorganic perovskites have emerged as a promising candidate for light-emitting devices due to their high stability and tunable band gap. However, the power consumption and brightness have always been an issue for perovskite light-emitting diodes (PeLEDs). Here, we improved the luminescence intensity and decreased the current density of the PeLEDs based on CsPbI₃ quantum dots (QDs) and p-type Si substrate through an alternating current (AC) driving mode. For the different driving voltage modes (under a sine pulsed bias or square pulsed bias), a frequency-dependent electroluminescent (EL) behavior was observed. The devices under a square pulsed bias present a stronger EL intensity under the same voltage due to less thermal degradation at the interface. The red PeLEDs under a square pulsed bias driving demonstrate that the EL intensity drop-off phenomenon was further improved, and the integrated EL intensity shows the almost linear increase with the increasing driving voltage above 8.5 V. Additionally, compared to the direct current (DC) driving mode, the red PeLEDs under the AC condition exhibit higher operating stability, which is mainly due to the reducing accumulated charges in the devices. Our work provides an effective approach for obtaining strong brightness, low power consumption, and high stability light-emitting devices, which will exert a profound influence on coupling LEDs with household power supplies directly.

Brightness Enhancement in Pulsed-Operated Perovskite Light-Emitting Transistors

Perovskite light-emitting field-effect transistors (PeLEFETs) provide a versatile device architecture to control transport and electroluminescence properties of hybrid perovskites, enabling injection of high charge carrier density and spatial control of the radiative recombination zone. Ionic screening and organic cation polarization effects typical of metal-halide perovskites, however, critically affect PeLEFET efficiency and reliability. In this work, we demonstrate a new device operation mode based on high-frequency modulation of the applied voltages, which allows significant reduction of ionic drift/screening in methylammonium lead iodide light-emitting transistors. In optimized top contact PeLEFETs, AC operation results in brighter and more uniform electroluminescence compared to DC-driven devices, whereas high-frequency modulation enables electroluminescence emission up to room temperature.

All-Perovskite Emission Architecture for White Light-Emitting Diodes

We demonstrate all-perovskite light-emitting diodes (PeLEDs) with white emission on the basis of simultaneously solving a couple of issues including the ion exchanges between different perovskites, solvent incompatibility in the solution process of stacking different perovskites and carrier transport layers, as well as the energy level matching between each layer in the whole device. The PeLEDs are built with a two-dimensional (CH₃CH₂CH₂NH₃)₂CsPb₂I₇ perovskite that emits red light, CsPb(Br,Cl)₃ quantum dots that emit a cyan color, and an interlayer composed of bis(1-phenyl-1H-benzo[ d]imidazole)phenylphosphine oxide (BIPO) and poly(4-butylphenyl-diphenyl-amine) (Poly-TPD). The interlayer is designed to realize desirable white electroluminescence by tuning the electron and hole transportation and distribution in-between multilayers. With this PeLED configuration, we achieve the typical white light with chromaticity coordinates of (0.32, 0.32) in Commission Internationale de L'Eclairage (CIE) 1931 color space diagram and steady CIE coordinates in a wide range of driving current densities (from 2.94 to 59.29 mA/cm²). Consequently, our work, as the starting point for future research of all-perovskite white PeLEDs, will contribute to the future applications of PeLEDs in lighting and display. In addition, we believe that the proposed material and all-perovskite concept will leverage the design and development of more perovskite-based devices.

Structural Diversity in White-Light-Emitting Hybrid Lead Bromide Perovskites

Hybrid organic-inorganic halide perovskites are under intense investigations because of their astounding physical properties and promises for optoelectronics. Lead bromide and chloride perovskites exhibit intrinsic white-light emission believed to arise from self-trapped excitons (STEs). Here, we report a series of new structurally diverse hybrid lead bromide perovskites that have broad-band emission at room temperature. They feature Pb/Br structures which vary from 1D face-sharing structures to 3D corner- and edge-sharing structures. Through single-crystal X-ray diffraction and low-frequency Raman spectroscopy, we have identified the local distortion level of the octahedral environments of Pb²⁺ within the structures. The band gaps of these compounds range from 2.92 to 3.50 eV, following the trend of "corner-sharing < edge-sharing < face-sharing". Density functional theory calculations suggest that the electronic structure is highly dependent on the connectivity mode of the PbBr₆ octahedra, where the edge- and corner-sharing 1D structure of (2,6-dmpz)₃Pb₂Br₁₀ exhibits more disperse bands and smaller band gap (2.49 eV) than the face-sharing 1D structure of (hep)PbBr₃ (3.10 eV). Using photoemission spectroscopy, we measured the energies of the valence band of these compounds and found them to remain almost constant, while the energy of conduction bands varies. Temperature-dependent PL measurements reveal that the 2D and 3D compounds have narrower PL emission at low temperature (∼5 K), whereas the 1D compounds have both free exciton emission and STE emission. The 1D compound (2,6-dmpz)₃Pb₂Br₁₀ has the highest photoluminescence quantum yield of 12%, owing to its unique structure that allows efficient charge carrier relaxation and light emission.

Metal Halide Perovskites: Synthesis, Ion Migration, and Application in Field-Effect Transistors

The past several years have witnessed tremendous developments of metal halide perovskite (MHP)-based optoelectronics. Particularly, the intensive research of MHP-based light-emitting diodes, photodetectors, and solar cells could probably reform the optoelectronic semiconductor industry. In comparison, in spite of the large intrinsic charge carrier mobility of MHPs, the development of MHP-based field-effect transistors (MHP-FETs) is relatively slow, which is essentially due to the gate-field screening effect induced by the ion migration and accumulation in MHP-FETs. This work mainly aims to summarize the recent important work on MHP-FETs and propose solutions in terms of the development bottleneck of perovskite-based transistors, in an attempt to boost the research of MHP transistors further. First, the advantages and potential applications of MHP-FETs are briefly introduced, which is followed by a detailed description of the MHP crystalline structure and various material fabrication techniques. Afterward, MHP-FETs are discussed, including transistors based on hybrid organic-inorganic perovskites, all-inorganic perovskites, and lead-free perovskites.

The Role of Metal Halide Perovskites in Next-Generation Lighting Devices

The development of smart illumination sources represents a central challenge for current technology. In this context, the quest for novel materials that enable efficient light generation is essential. Metal halide compounds with perovskite crystalline structure (ABX₃) have gained tremendous interest in the last five years since they come as easy-to-prepare high performance semiconductors. Perovskite absorbers are driving the power-conversion-efficiencies of thin film photovoltaics to unprecedented values. Nowadays, mixed-cation, mixed-halide lead perovskite solar cells reach efficiencies consistently over 20% and promise to get close to 30% in multijunction devices when combined with silicon cells at no surcharge. Nonetheless, perovskites' fame extends further since extensive research on these novel semiconductors has also revealed their brightest side. Soon after their irruption in the photovoltaic scenario, demonstration of efficient color tunable-with high color purity-perovskite emitters has opened new avenues for light generation applications that are timely to discuss herein.

Design principles for electronic charge transport in solution-processed vertically stacked 2D perovskite quantum wells

State-of-the-art quantum-well-based devices such as photovoltaics, photodetectors, and light-emission devices are enabled by understanding the nature and the exact mechanism of electronic charge transport. Ruddlesden–Popper phase halide perovskites are two-dimensional solution-processed quantum wells and have recently emerged as highly efficient semiconductors for solar cell approaching 14% in power conversion efficiency. However, further improvements will require an understanding of the charge transport mechanisms, which are currently unknown and further complicated by the presence of strongly bound excitons. Here, we unambiguously determine that dominant photocurrent collection is through electric field-assisted electron–hole pair separation and transport across the potential barriers. This is revealed by in-depth device characterization, coupled with comprehensive device modeling, which can self-consistently reproduce our experimental findings. These findings establish the fundamental guidelines for the molecular and device design for layered 2D perovskite-based photovoltaics and optoelectronic devices, and are relevant for other similar quantum-confined systems.

Solution-processed two-dimensional perovskite quantum-well-based optoelectronic devices have attracted great research interest but their electrical transport is poorly understood. Tsai et al. reveal that the potential barriers of the quantum wells dominate the transport properties in solar cell devices.

Organic-Inorganic Hybrid Ruddlesden-Popper Perovskites: An Emerging Paradigm for High-Performance Light-Emitting Diodes

Recently, lead halide perovskite materials have attracted extensive interest, in particular, in the research field of solar cells. These materials are fascinating "soft" materials with semiconducting properties comparable to the best inorganic semiconductors like silicon and gallium arsenide. As one of the most promising perovskite family members, organic-inorganic hybrid Ruddlesden-Popper perovskites (HRPPs) offer rich chemical and structural flexibility for exploring excellent properties for optoelectronic devices, such as solar cells and light-emitting diodes (LEDs). In this Perspective, we present an overview of HRPPs on their structural characteristics, synthesis of pure HRPP compounds and thin films, control of their preferential orientations, and investigations of heterogeneous HRPP thin films. Based on these recent advances, future directions and prospects have been proposed. HRPPs are promising to open up a new paradigm for high-performance LEDs.

Hybrid Dion-Jacobson 2D Lead Iodide Perovskites

The three-dimensional hybrid organic-inorganic perovskites have shown huge potential for use in solar cells and other optoelectronic devices. Although these materials are under intense investigation, derivative materials with lower dimensionality are emerging, offering higher tunability of physical properties and new capabilities. Here, we present two new series of hybrid two-dimensional (2D) perovskites that adopt the Dion-Jacobson (DJ) structure type, which are the first complete homologous series reported in halide perovskite chemistry. Lead iodide DJ perovskites adopt a general formula A'A _n-1Pb _nI_{3 n+1} (A' = 3-(aminomethyl)piperidinium (3AMP) or 4-(aminomethyl)piperidinium (4AMP), A = methylammonium (MA)). These materials have layered structures where the stacking of inorganic layers is unique as they lay exactly on top of another. With a slightly different position of the functional group in the templating cation 3AMP and 4AMP, the as-formed DJ perovskites show different optical properties, with the 3AMP series having smaller band gaps than the 4AMP series. Analysis on the crystal structures and density functional theory (DFT) calculations suggest that the origin of the systematic band gap shift is the strong but indirect influence of the organic cation on the inorganic framework. Fabrication of photovoltaic devices utilizing these materials as light absorbers reveals that (3AMP)(MA)₃Pb₄I₁₃ has the best power conversion efficiency (PCE) of 7.32%, which is much higher than that of the corresponding (4AMP)(MA)₃Pb₄I₁₃.