Two-dimensional Halide Perovskites Research Articles

Effect of heterostructure engineering on electronic structure and transport properties of two-dimensional halide perovskites

Organic-inorganic halide perovskite solar cells have attracted much attention due to their low-cost fabrication, flexibility, and high-power conversion efficiency. More recent efforts show that the reduction from three- to two-dimensions (2D) of organic–inorganic halide perovskites promises an exciting opportunity to tune their electronic properties. Here, we explore the effect of reduced dimensionality and heterostructure engineering on the intrinsic material properties, such as energy stability, bandgap and transport properties of 2D hybrid organic–inorganic halide perovskites using first-principles density functional theory. We show that the energy stability of engineered perovskite heterostructures is significantly enhanced. The heterostructures with improved stability also show excellent transport properties similar to their bulk counterparts. These layered chemistries demonstrate the advantage of a broad range of tunable bandgaps and high-absorption coefficient in the visible spectrum. The proposed 2D heterostructured material holds potential for nano-optoelectronic devices as well as for effective photovoltaics.

2D/3D Halide Perovskites for Optoelectronic Devices.

The traditional three-dimensional (3D) halide perovskites (HPs) have experienced rapid development due to their highly power conversion efficiency (PCE). However, the instability of 3D perovskite on humidity and UV irradiation blocks their commercialization. In the past few years, two-dimensional (2D) halide perovskites attract much attention because they behave better stability due to the water resistance of the aliphatic carbon chains in the 2D perovskite lattice. In this review, we categorize the 2D/3D perovskites based on the applications [i.e., solar cells (SCs), light-emitting diodes (LEDs) and photodetectors (PDs)]. We further discuss the recent efforts in the performance enhancement of the 2D/3D perovskite-based devices. However, there are still some difficulties before 2D/3D HPs is fully commercialized. We will provide some scientific and technical challenges and prospects in the article to point out the future direction.

M-Phenylenediammonium as a New Spacer for Dion-Jacobson Two-Dimensional Perovskites.

Two-dimensional (2D) halide perovskites have several distinct structural classes and exhibit great tunability, stability, and high potential for photovoltaic applications. Here, we report a new series of hybrid 2D perovskites in the Dion-Jacobson (DJ) class based on aromatic m-phenylenediammonium (mPDA) dications. The crystal structures of the DJ perovskite materials (mPDA)MAn-1PbnI3n+1 (n = 1-3) were solved and refined using single-crystal X-ray crystallography. The results indicate a short I···I interlayer distance of 4.00-4.04 Å for the (mPDA)MAn-1PbnI3n+1 (n = 2 and 3) structures, which is the shortest among DJ perovskites. However, Pb-I-Pb angles are as small as 158-160°, reflecting the large distortion of the inorganic framework, which results in larger band gaps for these materials than those in other DJ analogues. Density functional theory calculations suggest appreciable dispersion in the stacking direction, unlike the band structures of the Ruddlesden-Popper phases, which exhibit flat bands along the stacking direction. This is a consequence of the short interlayer I···I distances that can lead to interlayer electronic coupling across the layers. The solution-deposited films (nominal (mPDA)MAn-1PbnI3n+1 compositions of n = 1-6) reveal improved surface coverage with increasing nominal n value with the higher n films being composed of a mixture of n = 1 and bulk three-dimensional MAPbI3 perovskites. The films made from solutions of these materials behave differently from those of other 2D iodide perovskites, and their solar cells have a mixture of n = 1 DJ and MAPbI3 as light-absorbing semiconductors.

Organic semiconductor-incorporated two-dimensional halide perovskites.

Organic semiconductor-incorporated halide perovskites (OSiP) have emerged as a promising new family of 2D hybrid materials with exceptional structural tunability, optical and electronic properties, and environmental stability.

A Microstructural Analysis of 2D Halide Perovskites: Stability and Functionality

Recent observations have demonstrated that the photoelectric conversion properties of perovskite materials are intimately related to the presence of superlattice structures and other unusual nanoscale features in them. The low-dimensional or mixed-dimensional halide perovskite families are found to be more efficient materials for device application than three-dimensional halide perovskites. The emergence of perovskite solar cells has revolutionized the solar cell industry because of their flexible architecture and rapidly increased efficiency. Tuning the dielectric constant and charge separation are the main objectives in designing a photovoltaic device that can be explored using the two-dimensional perovskite family. Thus, revisiting the fundamental properties of perovskite crystals could reveal further possibilities for recognizing these improvements toward device functionality. In this context, this review discusses the material properties of two-dimensional halide perovskites and related optoelectronic devices, aiming particularly for solar cell applications.

A review of main characterization methods for identifying two-dimensional organic–inorganic halide perovskites

Two-dimensional (2D) halide perovskite has attracted widespread attention because of their structural diversity and adjustable material properties. Their material properties are closely related to structural features, especially the inorganic layers (n) in each structural unit of 2D halide perovskite. The optical properties of 2D perovskite that vary with n can be used to manufacture various optoelectronic devices. This review introduces the basic structure and properties of 2D halide perovskite and then summarizes various experimental methods that can be used to directly or indirectly identify the n value, such as X-ray diffraction (XRD), grazing incidence wide-angle X-ray scattering (GIWAXS), photoluminescence spectra (PL), ultraviolet–visible spectra (UV–vis), atomic force microscopy (AFM), second harmonic generation (SHG) spectroscopy or third harmonic generation (THG) spectroscopy, etc. Moreover, the existence of different n-value phases in 2D halide perovskites has been discussed in terms of the energy transfer characteristics. Finally, we summarize advantages and disadvantages of characterization methods and some issues that still need to be explored in the 2D halide perovskite itself.

The photophysics of Ruddlesden-Popper perovskites: A tale of energy, charges, and spins

Quasi two-dimensional halide perovskites (also known as Ruddlesden-Popper or RPs) are the most recent and exciting evolution in the perovskite field. Possessing a unique combination of enhanced moisture and material stability, whilst retaining the excellent optoelectronic properties, RPs are poised to be a game changer in the perovskite field. Spurred by their recent achievements in solar cells, light-emitting diodes and spintronic devices, these materials have garnered a mounting interest. Herein, we critically review the photophysics of RPs and distill the science behind their structure–property relations. We first focus on their structure and morphology by highlighting the crucial role of large cations: dictating the RPs' layered structure and the statistical distribution of thicknesses (i.e., n-phases). Next, we discuss how optoelectronic properties of RPs differ from conventional halide perovskites. Structural disorder, stronger excitonic, and polaronic interaction shape the nature of photo-excitations and their fate. For example, faster recombinations and hindered transport are expected for charge carriers in thinner n-phases. However, the complex energetic landscape of RPs, which originates from the coexistence of different n-phases, allows for funneling of energy and charges. Presently, the photophysics of RPs is still nascent, with many recent exciting discoveries from coherence effects in the above-mentioned funneling cascade to spin effects. Giant Rashba spin–orbit coupling, also observed in RPs, dictates their spin dynamics and provides exciting spintronics opportunities. To leverage these propitious RPs, future research must entail a cross-disciplinary approach. While materials engineering will unlock new chiral RPs and Dion-Jacobson variants, novel characterization techniques such as in situ synchrotron-based x-ray diffraction, ultrafast electron microscopy, and multidimensional electronic spectroscopy, etc., are essential in unraveling their secrets and unleashing their full potential.

Two-dimensional halide perovskite single crystals: principles and promises

In the last few years, two-dimensional (2D) metal halide perovskites (MHPs) are gaining tremendous attention and are replacing their three-dimensional (3D) congeners rapidly. These next-generation halide perovskites can circumvent the limitations of the 3D MHPs such as stability and structural diversity. The incorporation of bulky organic cation can not only prevent the moisture penetration into the crystal lattice and thus providing greater stability but also expands the field of hybrid semiconducting materials by offering structural diversity. These unique features render even higher tuneability and improved photophysical properties and as a result intensive investigations have been made for 2D MHP–based polycrystalline thin films. However, single crystals based on two-dimensional halide perovskites are still emerging and have great potential for future device applications. Along with the hybrid halide perovskites, the all inorganic halide perovskites are also blossoming. In this review, we have discussed exclusively the development of hybrid as well as all inorganic two-dimensional halide perovskites. First, we have discussed the crystal structure of 2D perovskites. In the next section, different growth procedures reported for preparation of single crystals are discussed. We then highlight the effect of doping on single crystals and their optoelectronic properties. Finally, we discuss the current challenges and future perspectives to further develop 2D single crystals for their efficient use in various devices.

Layer-by-layer anionic diffusion in two-dimensional halide perovskite vertical heterostructures.

Anionic diffusion in a soft crystal lattice of hybrid halide perovskites affects their stability, optoelectronic properties and the resulting device performance. The use of two-dimensional (2D) halide perovskites improves the chemical stability of perovskites and suppresses the intrinsic anionic diffusion in solid-state devices. Based on this strategy, devices with an enhanced stability and reduced hysteresis have been achieved. However, a fundamental understanding of the role of organic cations in inhibiting anionic diffusion across the perovskite-ligand interface is missing. Here we demonstrate the first quantitative investigation of the anionic interdiffusion across atomically flat 2D vertical heterojunctions. Interestingly, the halide diffusion does not follow the classical diffusion process. Instead, a 'quantized' layer-by-layer diffusion model is proposed to describe the behaviour of the anionic migration in 2D halide perovskites. Our results provide important insights into the mechanism of anionic diffusion in 2D perovskites and provide a new materials platform with an enhanced stability for heterostructure integration.

The 2D Halide Perovskite Rulebook: How the Spacer Influences Everything from the Structure to Optoelectronic Device Efficiency.

Two-dimensional (2D) halide perovskites have emerged as outstanding semiconducting materials thanks to their superior stability and structural diversity. However, the ever-growing field of optoelectronic device research using 2D perovskites requires systematic understanding of the effects of the spacer on the structure, properties, and device performance. So far, many studies are based on trial-and-error tests of random spacers with limited ability to predict the resulting structure of these synthetic experiments, hindering the discovery of novel 2D materials to be incorporated into high-performance devices. In this review, we provide guidelines on successfully choosing spacers and incorporating them into crystalline materials and optoelectronic devices. We first provide a summary of various synthetic methods to act as a tutorial for groups interested in pursuing synthesis of novel 2D perovskites. Second, we provide our insights on what kind of spacer cations can stabilize 2D perovskites followed by an extensive review of the spacer cations, which have been shown to stabilize 2D perovskites with an emphasis on the effects of the spacer on the structure and optical properties. Next, we provide a similar explanation for the methods used to fabricate films and their desired properties. Like the synthesis section, we will then focus on various spacers that have been used in devices and how they influence the film structure and device performance. With a comprehensive understanding of these effects, a rational selection of novel spacers can be made, accelerating this already exciting field.

Enhanced Photocurrent of All-Inorganic Two-Dimensional Perovskite Cs2PbI2Cl2 via Pressure-Regulated Excitonic Features.

Pressure processing is efficient to regulate the structural and physical properties of two-dimensional (2D) halide perovskites which have been emerging for advanced photovoltaic and light-emitting applications. Increasing numbers of studies have reported pressure-induced and/or enhanced emission properties in the 2D halide perovskites. However, no research has focused on their photoresponse properties under pressure tuning. It is also unclear how structural change affects their excitonic features, which govern the optoelectronic properties of the halide perovskites. Herein, we report significantly enhanced photocurrents in the all-inorganic 2D perovskite Cs2PbI2Cl2, achieving over 3 orders of magnitude increase at the industrially achievable level of 2 GPa in comparison with its initial photocurrent. Lattice compression effectively regulates the excitonic features of Cs2PbI2Cl2, reducing the exciton binding energy considerably from 133 meV at ambient conditions to 78 meV at 2.1 GPa. Impressively, such a reduced exciton binding energy of 2D Cs2PbI2Cl2 is comparable to the values of typical 3D perovskites (MAPbBr3 and MAPbI3), facilitating the dissociating of excitons into free carriers and enhancing the photocurrent. Further pressurization leads to a layer-sliding-induced phase transition and an anomalous negative linear compression, which has not been observed so far in other halide perovskites. Our findings reveal the dramatically enhanced photocurrents in the 2D halide perovskite by regulating its excitonic features and, more broadly, provide new insights into materials design toward extraordinary properties.

Two-dimensional halide perovskites: synthesis, optoelectronic properties, stability, and applications.

Halide perovskites are promising materials for light-emitting and light-harvesting applications. In this context, two-dimensional perovskites such as nanoplatelets or Ruddlesden-Popper and Dion-Jacobson layered structures are important because of their structural flexibility, electronic confinement, and better stability. This review article brings forth an extensive overview of the recent developments of two-dimensional halide perovskites both in the colloidal and non-colloidal forms. We outline the strategy to synthesize and control the shape and discuss different crystalline phases and optoelectronic properties. We review the applications of two-dimensional perovskites in solar cells, light-emitting diodes, lasers, photodetectors, and photocatalysis. Besides, we also emphasize the moisture, thermal, and photostability of these materials in comparison to their three-dimensional analogs.

A selenophene-containing conjugated organic ligand for two-dimensional halide perovskites.

A selenophene-containing conjugated organic ligand, 2-(4'-methyl-5'-(5-(3-methylthiophen-2-yl)selenophen-2-yl)-[2,2'-bithiophen]-5-yl)ethan-1-aminium (STm), was synthesized and incorporated into a Sn(II)-based two-dimensional perovskite, (STm)2SnI4. The band offset between the perovskite and ligand can be fine-tuned by introducing the STm ligand. Both field-effect transistor and light-emitting diode devices based on (STm)2SnI4 films exhibit high performance and enhanced operational stability.

Towards superior X-ray detection performance of two-dimensional halide perovskite crystals by adjusting the anisotropic transport behavior

Replacing the spacer cations from BA to i-BA decreases the interlayer distance and barrier height in 2D perovskite, resulting the decrease of the degree of anisotropy. The (BA)2CsPb2Br7 crystal along the ab plane exhibits superior X-ray sensitivity.

Semiconductor physics of organic-inorganic 2D halide perovskites.

Achieving technologically relevant performance and stability for optoelectronics, energy conversion, photonics, spintronics and quantum devices requires creating atomically precise materials with tailored homo- and hetero-interfaces, which can form functional hierarchical assemblies. Nature employs tunable sequence chemistry to create complex architectures, which efficiently transform matter and energy, however, in contrast, the design of synthetic materials and their integration remains a long-standing challenge. Organic-inorganic two-dimensional halide perovskites (2DPKs) are organic and inorganic two-dimensional layers, which self-assemble in solution to form highly ordered periodic stacks. They exhibit a large compositional and structural phase space, which has led to novel and exciting physical properties. In this Review, we discuss the current understanding in the structure and physical properties of 2DPKs from the monolayers to assemblies, and present a comprehensive comparison with conventional semiconductors, thereby providing a broad understanding of low-dimensional semiconductors that feature complex organic-inorganic hetero-interfaces.

Halide Perovskite Epitaxial Heterostructures

ConspectusThe past decade has witnessed the great success and rapid development of halide perovskite materials in photovoltaics and light-emitting devices, which is due to the inherently high photoluminescence quantum yield, long carrier diffusion length, high light absorption coefficient, great defect tolerance, and remarkable tunability. For example, the record efficiency of perovskite solar cells has ramped up to 25.5%, comparable to single-crystal silicon solar cells. In combination with the facile solution processing, perovskite solar cells have gained tremendous attention and interest from both academia and the photovoltaic market. Moreover, as a newly emerging class of semiconductors with excellent properties and low manufacturing cost, it is also promising and meaningful to explore new applications beyond optoelectronic devices, such as transistors, lasers, and memory devices. To this end, the integration of different halide perovskites into epitaxial heterostructures provides an ideal option. On the other hand, from the perspective of fundamental studies, halide perovskite heterostructures also provide a platform to directly visualize ion migration and to study the charge transfer behavior and anomalous exciton phenomena, etc.The aim of our Account is to summarize the synthetic strategies to achieve halide perovskite epitaxial heterostructures and to highlight the effective routes to stabilize halide perovskites and heterostructures. We focus this Account on two-dimensional (2D) halide perovskites and emphasize the important roles the organic ligands play. Specifically, we discuss our recent findings on the incorporation of conjugated organic ligands into 2D halide perovskites leading to enhanced thermal and environmental stability and suppressed halide interdiffusion in the corresponding heterostructures. Molecular dynamics simulation and low-dose aberration corrected transmission electron microscopy were used as powerful tools to unveil and elucidate the working mechanism of stabilization and anion interdiffusion suppression. Following this strategy, diverse halide perovskite heterostructures between different halides, metals, or organic cations and more complex heterostructures and superlattices can be realized. Due to the reduced ion motion and improved stability, the interface of the heterostructures can be kept quite sharp. In the outlook of this Account, we discuss the challenges and a few promising directions with a special focus on the vertical van der Waals (vdW) heterostructures, which are expected to offer more flexibility in heterostructure integration and more opportunities in studying the charge transfer and carrier interaction along the out-of-plane direction. Moreover, the existence of semiconducting organic ligands provides new insights to tune the carrier and exciton behaviors along both in-plane and out-of-plane directions.

Theoretical investigation on interactions between lithium ions and two-dimensional halide perovskite for solar-rechargeable batteries

Two-dimensional (2D) halide perovskite materials have recently been proposed for solar energy conversion and energy storage systems, namely, photo-rechargeable batteries. However, the theoretical understanding of the 2D halide perovskite materials for the photo-rechargeable batteries lags behind the experimental advancement. In this manuscript, we investigate the adsorption and transport properties of lithium ions on an experimentally verified 2D halide perovskite material, (C6H9C2H4NH3)2PbI4, via first-principles calculations. Our study reveals the efficient adsorption and transport properties of the lithium ion at the 2D lead halide perovskite surface, and demonstrates the prospects of the metal halide perovskite system as an effective anode material for lithium-ion batteries as well as the photo-charging medium for photo-rechargeable batteries. In addition, the interlayer diffusion of the lithium ions is calculated, which demonstrates the efficient transport of the lithium ions between the 2D halide perovskite layers that contribute to the most of capacity. Most importantly, the displacement of the lithium ion is demonstrated to be available upon the hole polaron formation, which is suggested to signify the photo-charging behaviour initiating the unidirectional lithium ion movement on the electrode surface. The present study for the first time rationalizes the photo-charging behaviour of the photo-rechargeable battery, and facilitates the fundamental understanding on the halide perovskites for the energy storage applications including the lithium-ion batteries and the photo-rechargeable batteries.

Broadband and sensitive two-dimensional halide perovskite photodetector for full-spectrum underwater optical communication

Full-spectrum underwater optical communication (UOC) is of great significance for major strategic needs including resource development, scientific exploration, and homeland security. As the core of the full-spectrum UOC system, photodetectors (PDs) are plagued by stringent requirements including a broadband response, intrinsic water resistance, and a high detectivity. In this work, two-dimensional (2D) halide perovskites (HPs) and corresponding PDs are constructed by stearamine (SA), representing the rarely explored long-chain aliphatic amine series, to own waterproofness, ultralow noise, and superior optoelectronic performance, which consequently enable a high suitability for UOC. By dimensionality and composition modulations to extend the absorption onset down to 1.5 eV, a broadband response covering the entire transmission window of water (> 1.55 eV) for full- spectrum UOC can be obtained. Besides, featuring a high responsivity of 3.27 A·W−1, a peak external quantum efficiency (EQE) of 630%, fast rise/decay times of 0.35 ms/0.54 ms, a superior detectivity up to 1.35 × 1012 Jones and the capability to distinguish various waveforms and light intensities, the PDs present sensitive and persistent photoresponse underwater. As a result, proof-of-concept wireless transmission of ASCII codes in water is demonstrated.

Broad Tunability of Carrier Effective Masses in Two-Dimensional Halide Perovskites

The effective mass of charge carriers is a crucial parameter for the design of any optoelectronic device. The estimated values of the effective mass of 2D halide perovskites currently span a broad range, providing an unwelcome source of confusion in this promising material system. Here we highlight how the distortion imposed by the organic spacers, and orbital hybridization effects by the metal cation, govern the effective mass. As a result, the effective mass in 2D halide perovskites can be easily tailored over a wide range. To demonstrate this, we have directly measured the reduced effective mass of charge carriers in phenethylamine (PEA)-based 2D halide perovskites. Combining the experimental results with electronic band-structure calculations, we propose a scaling diagram for the effective mass value versus the distortion of the octahedra imposed by the organic cations.

Interlayer Polarization Explains Slow Charge Recombination in Two-Dimensional Halide Perovskites by Nonadiabatic Molecular Dynamics Simulation.

Two-dimensional (2D) perovskites for applications in photovoltaics and optoelectronics are attracting a great deal of research interest. The nonradiative electron-hole (e-h) recombination is the major efficiency loss channel. Herein, we report a study of the thickness dependence of the e-h recombination dynamics in diamine-based 2D perovskite via ab initio NAMD. For multilayer structures, due to the emergence of spontaneous interlayer electric polarization, which is induced by the collective and correlated reorientation of methylammonium molecules, the electron and hole at the band edges are localized in different inorganic layers, suppressing the e-h recombination. Furthermore, a broad range of phonon excitation also inspired rapid pure dephasing related to the microscopic origin for longer recombination times. The combination of the two effects leads to the observation of a prolonged carrier lifetime in multilayer 2D perovskites, which is essential to understanding the nonradiative e-h recombination mechanism in such materials.