2D Halide Perovskites Research Articles

Releasing Charge Transport Barriers at Low Bias in Perpendicularly Aligned 2D Halide Perovskite

2D halide perovskites have garnered significant interest due to the great potential in optoelectronic devices. The 2D Perovskites samples are structured as n layers of [MX6]4− octahedral sheets inducing strong quantum confinement effects. Here, we have synthesized a series of 2D (BA)2MAn-1PbnI3n+1 and its 3D counterpart by controlling the particles alignment to the substrate. We have performed a comprehensive study involving electrical transport and photoconductivity effect as a function of the particle's organization: parallel, perpendicular, and disordered orientation to the electrodes. We have observed that for intermediary value of n, the 2D layered particles are aligned perpendicular to the electrode and parallel to the transport electrical current. A distinct and complex S-shape behavior in V-i curves is disclosed, suggesting two electrical resistance regimes. The sharp increase in resistance suggests the presence of electronic barriers for all samples, which is suppressed for columnar arrays of n = 2 and 3 under illumination. We suggest that this linear behavior in the photoconductivity effect is close related to the absence of electrical barriers at the interface of the particles due to perpendicular alignment to the electrodes. The absence of interfaces in a more columnar array facilitates ionic conductivity enhancing photoelectric effect. Our findings reveal distinct differences in charge transport for perpendicularly aligned 2D samples, opening up possibilities for important applications at low bias.

Vertically oriented low-dimensional perovskites for high-efficiency wide band gap perovskite solar cells.

Controlling crystal growth alignment in low-dimensional perovskites (LDPs) for solar cells has been a persistent challenge, especially for low-n LDPs (n < 3, n is the number of octahedral sheets) with wide band gaps (>1.7 eV) impeding charge flow. Here we overcome such transport limits by inducing vertical crystal growth through the addition of chlorine to the precursor solution. In contrast to 3D halide perovskites (APbX3), we find that Cl substitutes I in the equatorial position of the unit cell, inducing a vertical strain in the perovskite octahedra, and is critical for initiating vertical growth. Atomistic modelling demonstrates the thermodynamic stability and miscibility of Cl/I structures indicating the preferential arrangement for Cl-incorporation at I-sites. Vertical alignment persists at the solar cell level, giving rise to a record 9.4% power conversion efficiency with a 1.4 V open circuit voltage, the highest reported for a 2 eV wide band gap device. This study demonstrates an atomic-level understanding of crystal tunability in low-n LDPs and unlocks new device possibilities for smart solar facades and indoor energy generation.

Linearly programmable two-dimensional halide perovskite memristor arrays for neuromorphic computing.

The exotic properties of three-dimensional halide perovskites, such as mixed ionic-electronic conductivity and feasible ion migration, have enabled them to challenge traditional memristive materials. However, the poor moisture stability and difficulty in controlling ion transport due to their polycrystalline nature have hindered their use as a neuromorphic hardware. Recently, two-dimensional (2D) halide perovskites have emerged as promising artificial synapses owing to their phase versatility, microstructural anisotropy in electrical and optoelectronic properties, and excellent moisture resistance. However, their asymmetrical and nonlinear conductance changes still limit the efficiency of training and accuracy of inference. Here we achieve highly linear and symmetrical conductance changes in Dion-Jacobson 2D perovskites. We further build a 7 × 7 crossbar array based on analogue perovskite synapses, achieving a high device yield, low variation with synaptic weight storing capability, multi-level analogue states with long retention, and moisture stability over 7 months. We explore the potential of such devices in large-scale image inference via simulations and show an accuracy within 0.08% of the theoretical limit. The excellent device performance is attributed to the elimination of gaps between inorganic layers, allowing the halide vacancies to migrate homogeneously regardless of grain boundaries. This was confirmed by first-principles calculations and experimental analysis.

Fabrication Strategies for 2D Halide Perovskite Towards Next-Generation Optoelectronic Applications

AbstractHalide perovskites have emerged as promising materials in high-performance optoelectronics due to their exceptional optoelectrical properties, such as long carrier lifetime and tunable bandgap. Despite the promising capabilities of three-dimensional (3D) halide perovskites in applications like solar cells and light-emitting diodes, their operational stability remains a critical challenge. This review focuses on quasi-two-dimensional (2D) halide perovskites, which offer enhanced stability through their reduced dimensionality. We discuss the unique properties of these materials, including the ability to modify optical and electronic characteristics by altering the organic cations and the layer number in the perovskite structure. Additionally, we review various fabrication techniques, highlighting the shift from traditional low-temperature solution processes to more advanced solid, liquid, and vapor-phase methods, which address the limitations of conventional fabrication and enhance material quality. This comprehensive review aims to provide insights into the development of stable and efficient 2D halide perovskite-based optoelectronic devices, paving the way for their integration into next-generation optoelectronic applications.

Deterministic Synthesis of a Two-Dimensional MAPbI3 Nanosheet and Twisted Structure with Moiré Superlattice.

The synthesis of extremely thin 2D halide perovskites and the exploration of their interlayer interactions have garnered significant attention in current research. A recent advancement we have made involves the development of a successful technique for generating ultrathin MAPbI3 nanosheets with controlled thickness and an exposed intrinsic surface. This innovative method relies on utilizing the Ruddlesden-Popper (RP) phase perovskite (BA2MAn-1PbnI3n+1) as a template. However, the precise reaction mechanism remains incompletely understood. In this work, we systematically examined the dynamic evolution of the phase conversion process, with a specific focus on the influence of inorganic slab (composed of [PbI6]4- octahedrons) numbers on regulating the thickness and quality of the resulting MAPbI3 nanosheets. Additionally, the atomic structure is directly visualized using the transmission electron microscopy (TEM) method, confirming its exceptional quality. To illustrate interfacial interactions in ultrathin structures, artificial moiré superlattices are constructed through a physical transfer approach, revealing multiple localized high-symmetry stacks within a distinctive square moiré pattern. These findings establish a novel framework for investigating the physics of interfacial interactions in ionic semiconducting crystals.

2D and Quasi-2D Halide Perovskite-Based Resistive Switching Memory Systems

Resistive switching (RS) memory devices are gaining recognition as data storage devices due to the significant interest in their switching material, Halide perovskite (HP). The electrical characteristics include hysteresis in its current–voltage (I–V) relationship. It can be attributed to the production and migration of defects. This property allows HPs to be used as RS materials in memory devices. However, 3D HPs are vulnerable to moisture and the surrounding environment, making their devices more susceptible to deterioration. The potential of two-dimensional (2D)/quasi-2D HPs for optoelectronic applications has been recognized, making them a viable alternative to address current restrictions. Two-dimensional/quasi-2D HPs are created by including extended organic cations into the ABX3 frameworks. By adjusting the number of HP layers, it is possible to control the optoelectronic properties to achieve specific features for certain applications. This article presents an overview of 2D/quasi-2D HPs, including their structures, binding energies, and charge transport, compared to 3D HPs. Next, we discuss the operational principles, RS modes (bipolar and unipolar switching), in RS memory devices. Finally, there have been notable and recent breakthroughs in developing RS memory systems using 2D/quasi-2D HPs.

3D Lead-Organoselenide-Halide Perovskites and their Mixed-Chalcogenide and Mixed-Halide Alloys.

We incorporate Se into the 3D halide perovskite framework using the zwitterionic ligand: SeCYS (+NH3(CH2)2Se-), which occupies both the X- and A+ sites in the prototypical ABX3 perovskite. The new organoselenide-halide perovskites: (SeCYS)PbX2 (X=Cl, Br) expand upon the recently discovered organosulfide-halide perovskites. Single-crystal X-ray diffraction and pair distribution function analysis reveal the average structures of the organoselenide-halide perovskites, whereas the local lead coordination environments and their distributions were probed through solid-state 77Se and 207Pb NMR, complemented by theoretical simulations. Density functional theory calculations illustrate that the band structures of (SeCYS)PbX2 largely resemble those of their S analogs, with similar band dispersion patterns, yet with a considerable band gap decrease. Optical absorbance measurements indeed show band gaps of 2.07 and 1.86 eV for (SeCYS)PbX2 with X=Cl and Br, respectively. We further demonstrate routes to alloying the halides (Cl, Br) and chalcogenides (S, Se) continuously tuning the band gap from 1.86 to 2.31 eV-straddling the ideal range for tandem solar cells or visible-light photocatalysis. The comprehensive description of the average and local structures, and how they can fine-tune the band gap and potential trap states, respectively, establishes the foundation for understanding this new perovskite family, which combines solid-state and organo-main-group chemistry.

3D Lead‐Organoselenide‐Halide Perovskites and their Mixed‐Chalcogenide and Mixed‐Halide Alloys

AbstractWe incorporate Se into the 3D halide perovskite framework using the zwitterionic ligand: SeCYS (+NH3(CH2)2Se−), which occupies both the X− and A+ sites in the prototypical ABX3 perovskite. The new organoselenide‐halide perovskites: (SeCYS)PbX2 (X=Cl, Br) expand upon the recently discovered organosulfide‐halide perovskites. Single‐crystal X‐ray diffraction and pair distribution function analysis reveal the average structures of the organoselenide‐halide perovskites, whereas the local lead coordination environments and their distributions were probed through solid‐state 77Se and 207Pb NMR, complemented by theoretical simulations. Density functional theory calculations illustrate that the band structures of (SeCYS)PbX2 largely resemble those of their S analogs, with similar band dispersion patterns, yet with a considerable band gap decrease. Optical absorbance measurements indeed show band gaps of 2.07 and 1.86 eV for (SeCYS)PbX2 with X=Cl and Br, respectively. We further demonstrate routes to alloying the halides (Cl, Br) and chalcogenides (S, Se) continuously tuning the band gap from 1.86 to 2.31 eV–straddling the ideal range for tandem solar cells or visible‐light photocatalysis. The comprehensive description of the average and local structures, and how they can fine‐tune the band gap and potential trap states, respectively, establishes the foundation for understanding this new perovskite family, which combines solid‐state and organo‐main‐group chemistry.

Rational Strategies to Improve the Efficiency of 2D Perovskite Solar Cells.

In the quest for durable photovoltaic devices, 2D halide perovskites have emerged as a focus of extensive research. However, the reduced dimension in structure is accompanied by inferior optical-electrical properties, such as widened band gap, enhanced exciton binding energy, and obstructed charge transport. As a result, the efficiency of 2D perovskite solar cells (PSCs) lags significantly behind their 3D counterparts. To overcome these constraints, extensive investigations into materials and processing techniques are pursued rigorously to augment the efficiency of 2D PSCs. Herein, The cutting-edge delve into developments in 2D PSCs, with a focus on chemical and material engineering, as well as their structure and photovoltaic properties. The review starts with an introduction of the crystal structure, followed by the key evaluation criteria of 2D PSCs. Then, the strategies around solution chemical engineering, processing technique, and interface optimization, to simultaneously boost efficiency and stability are systematically discussed. Finally, the challenges and perspectives associated with 2D perovskites to provide insights into potential improvements in photovoltaic performance will be outlined.

Mixed cation ordering scaffold polar 2D halide perovskite semiconductor for self-powered polarization-sensitive photodetection

Mixed cation ordering scaffold polar 2D halide perovskite semiconductor for self-powered polarization-sensitive photodetection

Revisiting Sub-Band Gap Emission Mechanism in 2D Halide Perovskites: The Role of Defect States.

Understanding the sub-band gap luminescence in Ruddlesden-Popper 2D metal halide hybrid perovskites (2D HaPs) is essential for efficient charge injection and collection in optoelectronic devices. Still, its origins are still under debate with respect to the role of self-trapped excitons or radiative recombination via defect states. In this study, we characterized charge separation, recombination, and transport in single crystals, exfoliated layers, and polycrystalline thin films of butylammonium lead iodide (BA2PbI4), one of the most prominent 2D HaPs. We combined complementary defect- and exciton-sensitive methods such as photoluminescence (PL) spectroscopy, modulated and time-resolved surface photovoltage (SPV) spectroscopy, constant final state photoelectron yield spectroscopy (CFSYS), and constant light-induced magneto transport (CLIMAT), to demonstrate striking differences between charge separation induced by dissociation of excitons and by excitation of mobile charge carriers from defect states. Our results suggest that the broad sub-band gap emission in BA2PbI4 and other 2D HaPs is caused by radiative recombination via defect states (shallow as well as midgap states) rather than self-trapped excitons. Density functional theory (DFT) results show that common defects can readily occur and produce an energetic profile that agrees well with the experimental results. The DFT results suggest that the formation of iodine interstitials is the initial process leading to degradation, responsible for the emergence of midgap states, and that defect engineering will play a key role in enhancing the optoelectronic properties of 2D HaPs in the future.

Ultrahigh energy harvesting ability of PVDF incorporated with 2D halide perovskite nanosheets via interface effect

Two-dimensional (2D) perovskites have recently emerged as an new family of ferroelectrics and attracted extensive attention for their specific surface interface characteristics benefitting from the unique structure by inserting hydrophobic organic cations into conventional three-dimensional frameworks. In this study, 2D metal–organic perovskite nanosheets, (C4H9NH3)2CsPb2Br7, ((BA)2CsPb2Br7), were successfully synthesized by facile temperature-cooling method and introduced into poly (vinylidene fluoride) (PVDF) polymer fibers to construct flexible piezoelectric energy harvester (PEH). (BA)2CsPb2Br7/PVDF nanofibers reveal a high piezoelectric coefficient (d33) of 63.3 pm/V, which is 2.9 times of the PVDF polymer (21.7 pm/V). Principally, the PVDF composite with 5 wt% (BA)2CsPb2Br7 nanosheet filler assumes an superior electroactive phase amount of about ∼ 93 % profiting from the interfacial interaction between BA+ cations of (BA)2CsPb2Br7 and the dipoles of PVDF. The PEH based on (BA)2CsPb2Br7 nanosheets embedded in PVDF fibers showed a maximum output voltage, 135 V, which is almost 17 times of the PEH based on neat PVDF fibers. Furthermore, the interfacial interaction between (BA)2CsPb2Br7 and PVDF was analyzed by first principle DFT study which reveals electrons migrate and the interfacial polarization between the interface. Simultaneously, (BA)2CsPb2Br7/PVDF composite fibers based PEH reveals exceptional long-term stabilities and durability. More attractively, the exceptional performance of this fabricated PEH evinces a pressure sensitivity of ∼ 7.3 V/N and bio-mechanical harvesting behavior arising from varied human motions, such as beating, pressing, twisting, walking and running. This cost-effective and high performance of (BA)2CsPb2Br7/PVDF based PEH is favorable for the continuation of 2D halide perovskites for further application in new energy and biomechanical filed.

Resistive random-access memories using quasi-2D halide perovskites for wafer-scale reliable switching behaviors

In this study, we propose the (PEA)2MA3Pb4I13, a quasi-two-dimensional (2D) halide perovskite, in the realm of resistive switching memory devices. A quasi-2D perovskite film (PEA)2MA3Pb4I13, with a thickness measuring 450 nm, has been effectively fabricated on a silicon substrate coated with platinum through an all-solution process conducted at low temperatures. The constructed resistive switching memory devices, incorporating an Ag/(PEA)2MA3Pb4I13/Pt/Ti/SiO2/Si configuration, display dependable and consistent bipolar switching attributes. The device shows an ultra-low operating voltage of less than +0.1 V, an impressive high ON/OFF ratio exceeding 108, reversible resistive switching via pulse voltage operation, and the capability for multilevel data storage. In addition, there is little variation in device characteristics depending on the location on a wafer or device size.

Charge Transfer in 2D Halide Perovskites and 2D/3D Heterostructures

Charge Transfer in 2D Halide Perovskites and 2D/3D Heterostructures

High‐Performance Nanogap Photodetectors Based on 2D Halide Perovskites with a Novel Spacer Cation

High‐Performance Nanogap Photodetectors Based on 2D Halide Perovskites with a Novel Spacer Cation

Intrinsic Limits of Charge Carrier Mobilities in Layered Halide Perovskites

Layered halide perovskites have emerged as potential alternatives to three-dimensional (3D) halide perovskites due to their improved stability and larger material phase space, allowing fine tuning of structural, electronic, and optical properties. However, their charge carrier mobilities are significantly smaller than those of 3D halide perovskites, which has a considerable impact on their application in optoelectronic devices. Here, we employ state-of-the-art approaches to unveil the electron-phonon mechanisms responsible for the diminished transport properties of layered halide perovskites. Starting from a prototypical AMX3 halide perovskite, we model the case of n=1 and n=2 layered structures and compare their electronic and transport properties to the 3D reference. The electronic and phononic properties are investigated within density functional theory (DFT) and density functional perturbation theory (DFPT), while transport properties are obtained via the Boltzmann transport equation. The vibrational modes contributing to charge carrier scattering are investigated and associated with polar-phonon scattering mechanisms arising from the long-range Fröhlich coupling and deformation-potential scattering processes. Our investigation reveals that the lower mobilities in layered systems primarily originate from the increased electronic density of states at the vicinity of the band edges, while the electron-phonon coupling strength remains similar. Such an increase is caused by the dimensionality reduction and the break in octahedra connectivity along the stacking direction. Our findings provide a fundamental understanding of the electron-phonon coupling mechanisms in layered perovskites and highlight the intrinsic limitations of the charge carrier transport in these materials. Published by the American Physical Society 2024

Direct & efficient detection of x-ray radiation using thermally evaporated 2D hybrid lead halide perovskite (BA2PbBr4)(BA=n-Butylammonium=C4H9NH3) film

This report demonstrates fast response and direct detection of x-ray radiation by thermally evaporated 2D hybrid halide perovskite (BA2PbBr4)(n-BA=Butylammonium=C4H9NH3) based film. A highly efficient pin-hole free/compact film (thickness∼1 μm), via a controlled evaporation process, has been grown to fabricate solid-state x-ray radiation detector film (typical dimension ∼1 × 1 cm2) of 2D hybrid halide perovskite BA2PbBr4 ( BAPB) material. Direct detection of x-rays is possible using this detector by simple electrical readout method, where a change in resistance occurs while exposed to x-ray radiation at ambient temperature. The detector current increases when an x-ray impinges on it and exhibits the highest sensitivity ∼1000 μC/Gy/cm2. The detector film shows fast response, exhibiting quick response-recovery time ∼ ms towards x-ray. The detector film exhibits reasonably high dark resistivity ∼ 1010Ω−cm and good mobility-lifetime ( μτ) product 3.308×10−6cm2/s. The 2D hybrid halide perovskite (BA2PbBr4) based detector can sustain an effective high electric field ∼5000 V cm−1 corresponding to 40 V operational bias. It shows very good radiation resistant towards x-ray and retains its crystal structure, as confirmed from structural data under sustained x-ray exposure. The 2D halide perovskite detector (BAPB) shows long-term stability or shelf life (∼4 months) and response is highly repeatable with less than 5% fluctuation, which is an important key performance metric for any radiation detector. This detector has good application potential in several areas like, medical imaging, security checking, etc.

Tailoring a Two-Dimensional Halide Perovskite Composed of the Secondary Amine Cation for Anisotropic X-ray Responses.

Two-dimensional (2D) metal-halide perovskites have shown broad application prospects in the field of optoelectronic detection. The presence of the natural quantum-well structure results in strong anisotropy of physical properties, while studies on anisotropic X-ray responses remain insufficient. Here, we present an intriguing anisotropy of X-ray-responsive behaviors in a 2D halide perovskite, (t-ACH)2(DMA)Pb2Br7 (1, where t-ACH is trans-4-(aminomethyl)cyclohexanecarboxylate and DMA is dimethylamine), in which the secondary amine DMA+ cation with a large ionic radius locates inside the perovskite cage to form inorganic frameworks. The alternative alignment of inorganic slabs and organic bilayers creates a typical quantum-well architecture, which accounts for the generation of photoelectronic anisotropy. High-quality crystals of 1 exhibit notable semiconducting properties with a large μτ product (1.9 × 10-4 cm2 V-1). Intriguingly, 1 has better X-ray detection sensitivity (∼569.9 μC Gyair-1 cm-2) along the in-plane direction, which is attributed to its excellent charge carrier transport performance in this direction. Conversely, the higher resistance stemming from the organic barrier results in a lower detection limit along the out-of-plane direction (∼78.1 nGyair s-1), much lower than the medical diagnostic criteria (∼5.5 μGyair s-1). This work might open up new possibilities for the creative use of hybrid perovskites in direct X-ray detection.

Structural and photophysical properties of two-dimensional lead bromide hybrid perovskite (C8H17NH3)2PbBr4

The structural versatility of organic compounds imparts manifold properties to hybrid organic−inorganic perovskites, where two-dimensional (2D) halide perovskites are potential future platforms for light-emitting devices because of their unique quantum well configuration and dielectric confinement. Notwithstanding the numerous studies on the organic−inorganic perovskites carried out to date, precisely charactering the structure and photoluminescence mechanism remain considerable challenges. Herein, by applying Rietveld refinement, we have obtained crystal structure and atomic sites of 2D layered OA2PbBr4 perovskite, which undergoes a phase transition at 190 K with activation energies of 29.6 and 3.8 meV for the two phases, respectively. The theoretical analysis based on the crystal structure were performed to understand the nature of vibrations and band structure. Steady-state PL shows a narrow emission and a broad emission located at 409 and 487 nm, respectively. With temperature cooling down to 118 K, narrow emission gradually narrowed and enhanced. Meanwhile, intensity of broad emission first weakens and then increases. The turning point occurs near 198 K. By applying time-resolved PL (TRPL) and transient absorption (TA) spectra experiment, we observed a rapid carrier transfer from the FE to the STE state, and demonstrated the narrow and broad bands originate from free exciton and self-trapped exciton emission, respectively. And the self-trapping depth is proved to be 26.3 meV by low-temperature PL. Our findings indicate the synthesized 2D perovskite has potential applications in optoelectronic and display devices.

A Lead-Free Ferroelectric 2D Dion-Jacobson Tin Iodide Perovskite.

2D hybrid organic-inorganic halide perovskites emerge as a new class of 2D semiconductors with the potential to combine excellent optoelectronic properties with symmetry-enabled properties such as ferroelectricity. Although many lead-based ferroelectric 2D halide perovskites are reported, there is yet to be a conclusive report of ferroelectricity in tin-based 2D perovskites. Here, the structures and properties of a new series of 2D Dion-Jacobson (DJ) Sn perovskites: (4AMP)SnI4, (4AMP)(MA)Sn2I7, and (4AMP)(FA)Sn2I7 (4AMP = 4-(aminomethyl)piperidinium, MA = methylammonium, and FA = formamidinium), are reported. Structural characterization reveals that (4AMP)SnI4 is polar with in-plane spontaneous polarization whereas (4AMP)(MA)Sn2I7 and (4AMP)(FA)Sn2I7 are centrosymmetric. Further, (4AMP)SnI4 displays second harmonic generation (SHG) and polarization-electric field hysteresis measurements confirm it is ferroelectric with a spontaneous polarization of 10.0 µC cm-2 at room temperature. (4AMP)SnI4 transitions into a centrosymmetric structure above 367 K. As the first direct experimental observation of the spontaneous ferroelectric polarization of a Sn-based 2D hybrid perovskite, this work opens up environmentally friendly 2D tin halide perovskites for ferroelectricity and other physical property studies.