Two-dimensional Perovskites Research Articles

Role of Exciton Diffusion in the Efficiency of Mn Dopant Emission in Two-Dimensional Perovskites

Role of Exciton Diffusion in the Efficiency of Mn Dopant Emission in Two-Dimensional Perovskites

Chlorella vulgaris on two-dimensional Ruddlesden–Popper phase can-1mnn-3Nb3O3n+1 (n = 4, 5, 6) perovskite nanosheets for hydrogen generation

Amid the increasing focus on sustainable energy, hydrogen generation through photocatalysis has garnered substantial attention in recent years. Given their distinctive electronic, optical, and structural properties, two-dimensional perovskite nanosheets have surfaced as promising contenders as photocatalysts. Here we present a systematic exploration of Ruddlesden-Popper phase Can-1Mnn-3Nb3O3n+12− (n = 4,5,6) perovskite (CMNO) nanosheets with distinct layers (n = 4, 5, and 6), synthesized via exfoliation approach, for enhanced physicochemical properties and catalytic performances. Rigorous characterizations supported by the Tauc plot and Mott-Schottky analysis reveal the crystal structures, varied bandgaps, and an n-type semiconductor behavior from the CMNO nanosheets. In a novel approach to enhance its photo-absorption, Chlorella vulgaris microalgae, strategically layered onto the CMNO nanosheets, consequently exhibit significant improvements in the onset potential and current density, especially for the CMNO (n = 6) nanosheet. This research lays a foundation for understanding the tailored composition and layer-dependent properties of CMNO nanosheets, propelling their potential application in efficient hydrogen generation through photoelectrochemical water splitting for sustainable energy technologies.

Origin of Persisting Photoresponse of One-Year Aged Two-Dimensional Lead Halide Perovskites Stored in Air under Dark Conditions.

Two-dimensional halide perovskites are promising for advanced photonic, optoelectronic, and photovoltaic applications. However, their long-term stability is still a critical factor limiting their implementation into further commercial applications. Here, we present an environmental stability analysis of BA2(MA)n-1PbnI3n+1 (BA = C4H12N+, MA = CH6N+) two-dimensional perovskites with the lowest quantum well thicknesses of n = 1 and n = 2, after 1 year of aging under ambient humidity, oxygen content, and light conditions. We observed that both crystal phases (n = 1 and 2) degraded similarly, resulting in the removal of organic components and crystal decomposition into PbI2, Pb oxides, and Pb hydroxides. However, we have found a significant difference between their aging under ambient light and dark conditions, affecting their degraded morphology and photoactivity. Both crystal phases exposed to ambient light aged into a morphology characterized by the formation of several pinholes and voids, accompanied by photoluminescence degradation. Samples stored under dark conditions surprisingly preserved their photoluminescence activity, which morphologically aged into microrod structures. We conclude that the observed loss of photoactivity of 2D perovskites aged under ambient light is attributed to photoaccelerated degradation processes causing faster crystal surface photo-oxidation accompanied by a creation of multiple I vacancies and hydration of the inner crystal. The retainment of photoactivity in 2D perovskites aged under dark conditions is attributed to slower surface oxidation processes into Pb salts, as confirmed by X-ray photoemission spectroscopy. The formed surface layer even allows for a layer-by-layer degradation and acts as a protection barrier against further additional loss of I atoms and the consequent hydration of the inner part of samples. We demonstrate that light is the most critical external factor accelerating 2D perovskite degradation processes in ambient air and thus affecting their long-term stability. We conclude in this work that perovskite material structural engineering together with their surface passivation or encapsulation strategical techniques applied is an essential step for their further application into long-term stable commercial devices.

High-Quality Thiophene-Based Two-Dimensional Perovskite Films Prepared with Dual Additives and Their Application in Solar Cells

High-Quality Thiophene-Based Two-Dimensional Perovskite Films Prepared with Dual Additives and Their Application in Solar Cells

Room-temperature moiré superlattices materials based on two-dimensional perovskites

Room-temperature moiré superlattices materials based on two-dimensional perovskites

Optoelectronic and Transport Properties of Layer-dependent Two-Dimensional Perovskite Cs3Bi2I9

The emergence of two-dimensional (2D) perovskite provides an ideal platform for designing and fabricating microelectronic and optoelectronic devices. Here, we present detailed ab initio calculations to comprehensively explore the layer-dependent optoelectronic and transport properties of the newly synthesized 2D perovskite Cs3Bi2I9. The calculations reveal that the indirect band gap of monolayer, bilayer, and trilayer Cs3Bi2I9 decreases from 2.41 to 2.35 eV. The charge carriers of few-layered perovskite Cs3Bi2I9 are dominated by electrons, and the highest electron and hole carrier mobilities are 140 and 62 cm²V⁻¹s⁻¹ along the b axis for bilayer Cs3Bi2I9. Exciton binding energies decrease from 2.48 to 1.19 eV with an increment of layer, and the calculated exciton level drops down into the valence band to generate potential exciton insulator. 2D Cs3Bi2I9 exhibits potential in the field of ultraviolet detection and photoluminescent devices due to large exciton energy and ultraviolet absorption.

Regulation of Sensitized Phosphorescence in Two-Dimensional Lead Bromine Perovskites by Tuning Excited-State Interactions.

Excited-state interactions within the organic layer play a critical role in sensitized phosphorescence of two-dimensional (2D) perovskites. Herein, we regulate excited-state interactions utilizing isomeric organic ligands 1-naphthylmethylamine (1-NMA) and 1-(2-naphthyl)-methanamine (2-NMA). Transient absorption and first-principles calculations are employed to elucidate the mechanisms of triplet energy transfer (TET) and triplet excimer formation. The results indicate that wave function hybridization and tunneling effect at the inorganic/organic interface contribute to rapid (∼20 ps) and highly efficient (>98%) TET, with the triplet excimer being generated in (1-NMA)2PbBr4 at picosecond time-scale. However, triplet excimer is barely observed in (2-NMA)2PbBr4 due to varying ligand stacking modes. Despite rapid TET, the efficiency of sensitized phosphorescence is low (<0.5%), which is ascribed to pronounced nonradiative decay. By mixing isomeric ligands and optimizing respective ratio, a maximum phosphorescence enhancement of 7.6 folds is achieved. This work provides a detailed mechanistic understanding of triplet excimer sensitization and regulation of sensitized phosphorescence.

Multi-Noncentrosymmetric Polar Order in 2D Hybrid Lead Chloride with Broadband Emission and High-Temperature Second-Harmonic Generation Switching.

Two-dimensional lead halide perovskites represent a fascinating class of hybrid semiconductors for solar cell, light-emitting, nonlinear optical (NLO), and ferroelectric applications. A notable subset within this category is luminescent ferroelectrics, which have garnered considerable attention for their potential in integrated photoelectronic devices. In this study, we employed an organic amine halogenation strategy (also referred to as halogen engineering), which is renowned for its efficacy in inducing polar order through crystal engineering. Consequently, we synthesized a layered Ruddlesden-Popper (RP) lead chloride comprising 3-chloropropylammonium cations (CPA+), with the chemical formula CPA2PbCl4. This compound features as many as four temperature-dependent crystal phases, with phase transitions observed at T1 = 353.1 K (343.9 K), T2 = 211.7 K (208.6 K), and T3 = 182.0 K (178.2 K) in the heating (cooling) cycles. Employing a multitechnique approach─including thermal analysis, X-ray diffraction, dielectric and pyroelectric current measurements, Raman spectroscopy, and second-harmonic generation (SHG) studies─we determined the mechanisms of the structural phase transitions. Our findings demonstrate polar order of phase II (space group Cmc21), phase III (space group Pna21), and phase IV (space group Pca21), while also confirming the centrosymmetric nature of phase I (space group Cmce). X-ray diffraction data revealed that the I to II PT is of a ferroelectric nature, devoid of ferroelastic strain, a conclusion further supported by pyroelectric measurements. CPA2PbCl4 features negative linear thermal expansion and broadband emission, which transitions to white light above 180 K. Remarkably, CPA2PbCl4 also demonstrates high-temperature SHG on-off switching with a high contrast ratio of 300:1 along with good switching stability, as evidenced by SHG cycling studies at heating/cooling rates ranging from 5 to 50 K/min. This SHG study also sets new standards for the field of SHG switching by providing a method to quantify the thermal responsiveness of SHG-switchable materials using the treq (time requirement) parameter. Overall, our findings show that the halogenation strategy has led to the discovery of a rare example of an RP perovskite exhibiting coexistence of white-light emission, SHG on-off thermal bistability, ferroelectricity, and negative linear thermal expansion.

Lead-Free Hybrid Perovskite: An Efficient Room-Temperature Spin Generator via Large Interfacial Rashba Effect.

Two-dimensional (2D) hybrid organic-inorganic perovskite (HOIP) shows great potential for developing flexible and wearable spintronic devices by serving as spin sources via the bulk Rashba effect (BRE). However, the practical application of BRE in 2D HOIP faces huge challenges, particularly due to the toxicity of lead, which is crucial for achieving large spin-orbit coupling, and the restrictions in 2D HOIP candidates to meet specific symmetry-breaking requirements. To overcome these obstacles, we designed a strategy to exploit the interfacial Rashba effect (IRE) of lead-free 2D HOIP (C6H5CH2CH2NH3)2CuCl4 (PEA-CuCl), manifesting as an efficient spin generator at room temperature. IRE of PEA-CuCl originates from the large orbital hybridization at the interface between PEA-CuCl and adjacent ferromagnetic layers. Spin-torque ferromagnetic resonance measurements further quantify a large Rashba effective field of 14.04 Oe per 1011 A m-2, surpassing those of lead-based HOIP and traditional all-inorganic heterojunctions with noble metals. Our lead-free 2D HOIP PEA-CuCl, which harnesses large IRE for spin generation, is efficient, nontoxic, and economic, offering huge promise for future flexible and wearable spintronic devices.

Reducing Dielectric Confinement Effect Enhances Carrier Separation in Two-Dimensional Hybrid Perovskite Photocatalysts.

Two-dimensional organic-inorganic hybrid perovskites (OIHPs) with alternating structure of the organic and inorganic layers have a natural quantum well structure. The difference of dielectric constants between organic and inorganic layers in this structure results in the enhancement of dielectric confinement effect, which exhibits a large exciton binding energy and hinders the separation of electron-hole pairs. Herein, a strategy to reduce the dielectric confinement effect by narrowing the dielectric difference between organic amine molecule and [PbBr6]4- octahedron is put forward. The Ethanolamine (EOA) contains hydroxyl groups, resulting in the positive and negative charge centers of O and H non-overlapping, which generated a larger polarity and dielectric constant. The reduced dielectric constant produces a smaller exciton binding energy (71.03 meV) of (C2H7NO)2PbBr4 ((EOA)2PbBr4) than (C8H11N)2PbBr4 ((PEA)2PbBr4 (156.07 meV), and promotes the dissociation of electrons and holes. The increasing of lifetime of photogenerated carrier in (EOA)2PbBr4 are proved by femtosecond transient absorption spectra. Density functional theory (DFT) calculations have also indicated that the small energy shift of the total density of states (DOS) between the C/H/N and the Pb/Br in (EOA)2PbBr4 favors the separation of electrons and holes. In addition, this work demonstrates the application of (PEA)2PbBr4 and (EOA)2PbBr4 in the field of photocatalytic CO2 reduction.

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.

Efficient Electroluminescence from Organic Fluorophore-Containing Perovskite Films.

Two-dimensional perovskites containing an organic fluorophore can be a unique emitter for light-emitting diodes (LEDs). However, external quantum efficiencies (EQEs) of fluorophore-containing perovskite LEDs reported thus far are still very low. In this study, these are able to boost the EQE to ≈10% by choosing an organic fluorophore with appropriate energy levels for the perovskite structure organization. In the fluorophore-containing perovskite LEDs, carrier transport and exciton formation take place in the perovskite's metal halide framework, thereby avoiding the direct formation of nonradiative triplet excitons on the organic fluorophores. Subsequently, the bright triplet excitons formed in the metal halide framework are transferred to form the radiative singlet states of the organic fluorophores, leading to efficient electroluminescence (EL) from the organic fluorophores regularly dispersed inside the perovskite structure. Unexpectedly higher light-outcoupling efficiency, which is caused by the light scattering in the polycrystalline perovskite layer, will be another reason for efficient EL. These findings will contribute toward the fabrication of LED-based products with high performance at a low cost.

Two-Dimensional Lead Iodide Perovskites with Extremely Reduced Dielectric Confinement: Embedded Self-Erasing Second-Harmonic Generation Switching, Thermochromism, and Photoluminescence

Two-Dimensional Lead Iodide Perovskites with Extremely Reduced Dielectric Confinement: Embedded Self-Erasing Second-Harmonic Generation Switching, Thermochromism, and Photoluminescence

Homogenized contact in all-perovskite tandems using tailored 2D perovskite.

The fabrication of scalable all-perovskite tandem solar cells is considered an attractive route to commercialize perovskite photovoltaic modules1. However, The certified efficiency of 1-cm2 scale all-perovskite tandem solar cells lags behind their small-area (~0.1 cm2) counterparts2,3. This performance deficit originates from inhomogeneity in wide-bandgap (WBG) perovskite solar cells (PSCs) at a large scale. The inhomogeneity is known to be introduced at the bottom interface and within the perovskite bulk itself4,5. Here we uncover another crucial source for the inhomogeneity - the top interface formed during the deposition of the electron transport layer (ETL, C60). Meanwhile, the poor ETL interface is also a significant limitation of device performance. We address this issue by introducing a mixture of 4-fluorophenethylamine (F-PEA) and 4-trifluoromethyl-phenylammonium (CF3-PA) to create a tailored two-dimensional perovskite layer (TTDL), in which F-PEA forms a two-dimensional perovskite at the surface reducing contact losses and inhomogeneity, CF3-PA enhances charge extraction and transport. As a result, we demonstrate a high open-circuit voltage of 1.35 V and an efficiency of 20.5% in 1.77-eV WBG PSCs at a square centimeter scale. By stacking with a narrow-bandgap perovskite sub-cell, we report 1.05 cm2 all-perovskite tandem cells delivering 28.5% (certified 28.2%) efficiency, the highest among all reported so far. Our work showcases the importance of treating the top perovskite/ETL contact for upscaling perovskite solar cells.

Investigation of perovskite materials for solar cells using scanning tunneling microscopy.

The issue of energy scarcity has become more prominent due to the recent scientific and technological advancements. Consequently, there is an urgent need for research on sustainable and renewable resources. Solar energy, in particular, has emerged as a highly promising option because of its pollution-free and environment-friendly characteristics. Among the various solar energy technologies, perovskite solar cells have attracted much attention due to their lower cost and higher photoelectric conversion efficiency (PCE). However, the inherent instability of perovskite materials hinders the commercialization of such devices. The utilization of scanning tunneling microscopy/spectroscopy (STM/STS) can provide valuable insights into the fundamental properties of different perovskite materials at the atomic scale, which is crucial for addressing this challenge. In this review, we present the recent research progress of STM/STS analysis applied to various perovskites for solar cells, including halide perovskites, two-dimensional Ruddlesden-Popper perovskites, and oxide perovskites. This comprehensive overview aims to inspire new ideas and strategies for optimizing solar cells.

Wafer-scale integration of two-dimensional perovskite oxides towards motion recognition

Two-dimensional semiconductors have shown great potential for the development of advanced intelligent optoelectronic systems. Among them, two-dimensional perovskite oxides with compelling optoelectronic performance have been thriving in high-performance photodetection. However, harsh synthesis and defect chemistry severely limit their overall performance and further large-scale heterogeneous integration. Here, we report the wafer-scale integration of highly oriented nanosheets by introducing a charge-assisted oriented assembly film-formation process and confirm its universality and scalability. The shallow-trap dominance induced by structural optimization endows the device with a distinguished performance balance, including high photosensitivity close to that of single nanosheet units and fast response speed. An integrated ultra-flexible 256-pixel device demonstrates the versatility of material-to-substrate integration and conformal imaging functionality. Moreover, the device achieves efficient recognition of multidirectional motion trajectories with an accuracy of over 99.8%. Our work provides prescient insights into the large-area fabrication and utilization of 2D perovskite oxides in advanced optoelectronics.

Anion Exchange and Lateral Heterostructure Formation in Ferromagnetic PEA2Cr(Cl,Br)4 Two-Dimensional Perovskites.

Postsynthetic vapor-phase anion exchange in the ferromagnetic two-dimensional (2D) hybrid metal-halide perovskite, PEA2CrCl4 (PEA+ = phenethylammonium), is reported. Anion exchange using vaporous trimethylsilyl bromide (TMS-Br) is shown to drive complete conversion of solution-processed PEA2CrCl4 polycrystalline thin films to PEA2CrBr4. Low-temperature magnetic circular dichroism spectroscopy indicates ferromagnetic ordering in these PEA2CrCl4 and PEA2CrBr4 films. Via partial anion exchange of exfoliated flakes of PEA2CrCl4 single crystals, we demonstrate that it is possible to generate abrupt lateral PEA2CrCl4/PEA2CrBr4 magneto-heterointerfaces. Kinetic studies reveal that lateral heterostructure formation is dictated by rapid edge-site halide exchange followed by slower intralayer bromide diffusion, and there is negligible interlayer (3D) bromide or TMS-Br diffusion. The importance of the bulky PEA+ interlayer cation in suppressing 3D diffusion is highlighted by parallel anion-exchange experiments on MA2CrCl4 (MA+ = methylammonium), which instead show 3D exchange. Comparison of anion-exchange reactions in PEA2CrCl4, PEA2MnCl4, and PEA2PbCl4 shows that 2D bromide diffusion is slowest in PEA2CrCl4, attributed to the antiferrodistortive ordering found in this composition. In addition to demonstrating both postsynthetic composition control and heterostructure formation in ferromagnetic Cr-based 2D perovskites for the first time, these results also advance our fundamental understanding of ion-exchange processes in this relatively unexplored family of 2D perovskites, broadening opportunities for investigation and control of novel spin effects in low-dimensional metal-halide perovskites.

Two-dimensional Ruddlesden-Popper perovskite solar cells with an efficiency exceeding 19 % by inserting a self-assembled monolayer

Two-dimensional Ruddlesden-Popper (2DRP) phase perovskites have excellent long-term environmental and structure stability. However, the efficiency of 2DRP perovskite solar cells (PSCs) still lags seriously behind 3D counterparts due to the large exciton binding energy between the large-volume organic spacer and the inorganic plate compared to their 3D analogs. Here, a thin layer of self-assembled monolayer material (2-(3,6-dibromo-9H-carbazol-9-yl)ethyl)phosphonic acid (2PACz-Br) is employed between poly (3,4-ethylene dioxythiophene): poly(styrene sulfonate) (PEDOT: PSS) and β-guanidinopropanoic acid (β-GUA)-based perovskite film (β-GUA)2(FA)4Pb5I16 for efficient and stable 2DRP-based PSCs. The 2PACz-Br-based 2DRP perovskite films show superior crystallinity and quality compared with the control perovskite films. The defect density in 2PACz-Br-based perovskite films is reduced, and the non-radiative recombination is further suppressed. The experimental results show that introducing 2PACz-Br can effectively promote the matched energy levels between the perovskite thin film and the charge-carrier transport layer. Benefiting from the conducive collection and transmission of charge carriers, (β-GUA)2(FA)4Pb5I16 (n = 5)-based PSCs can realize an outstanding power conversion efficiency of 19.13 %, much higher than that of devices without 2PACz-Br modification (17.70 %). Moreover, 2PACz-Br-based 2DRP PSCs show better stability than the control devices. Our work paves the way to efficient and stable 2DRP-based PSCs by inserting a multifunctional self-assembled monolayer.

Photo-ferroelectric perovskite interfaces for boosting VOC in efficient perovskite solar cells

Interface engineering is the core of device optimization, and this is particularly true for perovskite photovoltaics (PVs). The steady improvement in their performance has been largely driven by careful manipulation of interface chemistry to reduce unwanted recombination. Despite that, PVs devices still suffer from unavoidable open circuit voltage (VOC) losses. Here, we propose a different approach by creating a photo-ferroelectric perovskite interface. By engineering an ultrathin ferroelectric two-dimensional perovskite (2D) which sandwiches a perovskite bulk, we exploit the electric field generated by external polarization in the 2D layer to enhance charge separation and minimize interfacial recombination. As a result, we observe a net gain in the device VOC reaching 1.21 V, the highest value reported to date for highly efficient perovskite PVs, leading to a champion efficiency of 24%. Modeling depicts a coherent matching of the crystal and electronic structure at the interface, robust to defect states and molecular reorientation. The interface physics is finely tuned by the photoferroelectric field, representing a new tool for advanced perovskite device design.

Band Gap Variation and Structural Disorder in the Two-Dimensional Mixed-Halide Hybrid Lead Perovskites

Band Gap Variation and Structural Disorder in the Two-Dimensional Mixed-Halide Hybrid Lead Perovskites