Perovskite Slabs Research Articles

Super-high Curie temperature ferroelectrics and superior TC tunability: lattice instability and ferroelectric phase transitions

Ferroelectric materials with high Curie temperatures, TC, and tuning of the TC over a large temperature range are very important for the basic sciences and engineering. This paper reviews the various types of high-temperature ferroelectrics with perovskite, bismuth layered, tungsten bronze, and perovskite slab structures, especially Sr2Nb2O7 with TC = 1342 °C and its family. The lattice instability of the high-temperature ferroelectric phase transitions is also discussed. The superior tunability of the TC is achieved by the substitution of cations at the A- and B-sites in the perovskite-like structures. The variation of the soft optic and relaxation modes and elastic anomalies related to structural phase transitions were discussed in relation to the A- and B-site substitutions of oxygen octahedral ferroelectrics, especially Ba2NaNb5(1-x)Ta5xO15 with a tungsten bronze structure and (BaxSr1-x)2Nb2O7, Sr2(Nb1-xTax)2O7 with the perovskite slab structure.

Nonradiative Relaxation Dynamics of a Cesium Lead Halide Perovskite Photovoltaic Architecture: Effect of External Electric Fields

Lead halide perovskites have attracted much attention as an active material in solar cells. In this first-principles study, we consider a cesium lead halide perovskite slab interfacing with electron transport and hole transport layers, relevant to the practical photovoltaic architecture. We apply external electric fields normal to the surface of the perovskite slab and explore the induced changes onto optoelectronic properties. It is found that the bandgap increases linearly and the conductivity diminishes exponentially with decreasing electric field strengths. Furthermore, we study the influence of electric fields onto nonradiative relaxation of photoexcited electrons and holes using the reduced density matrix in the formalism of Redfield theory. Our calculations provide relaxation rates and relaxation pathways, illustrating the mechanisms of modulations of electric field strengths onto charge carrier dynamics. Our results show that holes have longer lifetimes than electrons at various external electric fields. It is also found that the patterns of charge carrier dynamics depend on the direction of external electric fields. Specifically, in comparison with the system under zero field, our findings show that (i) the positive electric field facilitates the relaxation of electrons and holes and (ii) the negative electric field facilitates the relaxation of electrons but inhibits the relaxation of holes.

Methodological Issues in First-Principle Calculations of CH3NH3PbI3 Perovskite Surfaces: Quantum Confinement and Thermal Motion.

Characterization and control of surfaces and interfaces are critical for photovoltaic and photocatalytic applications. In this work, we propose CH3NH3PbI3 (MAPI) perovskite slab models whose energy levels, free of quantum confinement, explicitly consider the spin–orbit coupling and thermal motion. We detail methodological tools based on the density functional theory that allow achieving these models at an affordable computational cost, and analytical corrections are proposed to correct these effects in other systems. The electronic state energies with respect to the vacuum of the static MAPI surface models, terminated in PbI2 and MAI atomic layers, are in agreement with the experimental data. The PbI2-terminated slab has in-gap surface states, which are independent of the thickness of the slab and also of the orientation of the cation on the surface. The surface states are not useful for alignments in photovoltaic devices, while they could be useful for photocatalytic reactions. The energy levels calculated for the MAI-terminated surface coincide with the widely used values to estimate the MAPI alignment with the charge transport materials, i.e., −5.4 and −3.9 eV for valence band maximum and conduction band minimum, respectively. Our study offers these slab models to provide guidelines for optimal interface engineering.

Perovskite Termination-Dependent Charge Transport Behaviors of the CsPbI3/Black Phosphorus van der Waals Heterostructure*Supported by the National Natural Science Foundation of China (Grant Nos. 51972266, 51672214, 11304248, and 11247230), theNatural Science Basic Research Plan in Shaanxi Province of China (Grant No. 2014JM1014), the Scientific Research Program Funded by Shaanxi Provincial Education Department (Grant No. 2013JK0624), the Fund Program for the Scientific

Fundamental understanding of interfacial charge behaviors is of great significance for the optoelectronic and photovoltaic applications. However, the crucial roles of perovskite terminations in charge transport processes have not been completely clear. We investigate the charge transfer behaviors of the CsPbI3/black phosphorus (BP) van der Waals heterostructure by using the density functional theory calculations with a self-energy correction. The calculations at the atomic level demonstrate the type-II band alignments of the CsPbI3/BP heterostructure, which make electrons transfer from the perovskite side to monolayer BP. Moreover, the stronger interaction and narrower physical separation of the interfaces can lead to higher charge tunneling probabilities in the CsPbI3/BP heterostructure. Due to different electron affinities, the PbI2-terminated perovskite slab tends to collect electrons from the adjacent materials, whereas the CsI-termination prefers to inject electrons into transport materials. In addition, the interface coupling effect enhances the visible-light-region absorption of the CsPbI3/BP heterostructure. This study highlights the importance of the perovskite termination in the charge transport processes and provides theoretical guidelines to develop high-performance photovoltaic and optoelectronic devices.

Engineered Layer-Stacked Interfaces Inside Aurivillius-Type Layered Oxides Enables Superior Ferroelectric Property

Layer engineering with different layer numbers inside Aurivillius-type layered structure, similar to interface engineering in heterojunctions or superlattices, can give rise to excellent physical properties due to the correlated layer-stacked interfaces of two different layer phases with different strain states. In this work, using the solid-state reactions from Aurivillius-type Bi3TiNbO9 (2-layer) and Bi4Ti3O12 (3-layer) ferroelectric powder mixtures, single-phase compound of Bi7Ti4NbO21 with an intergrowth structure of 2-layer and 3-layer perovskite slabs sandwiched between the Bi-O layers was synthesized and the effects of this layer-engineered strategy on the structure, Raman-vibration and ferroelectric properties were systematically investigated. The mostly-ordered intergrowth phase was observed clearly by utilizing X-ray diffraction and advanced electron micro-techniques. Uniformly dispersions and collaborative vibrations of Ti and Nb ions in the layer-engineered Bi7Ti4NbO21 were demonstrated. Remarkably, dielectric and ferroelectric properties were also recorded and an enhanced ferroelectric response was found in the layer-engineered mixed-layer sample with high ferroelectric Curie temperature, compared with the homogeneous 2-layer and 3-layer samples. Analyses of the Raman spectra and atomic structures confirmed that the performance improvement of the layer-engineered sample is intrinsic to the correlated layer-stacked interfaces inside the Aurivillius-type layered oxides, arising from strain-induced lattice distortions at the interfaces.

Unraveling the Crystallization Kinetics of 2D Perovskites with Sandwich-Type Structure for High-Performance Photovoltaics.

2D perovskite solar cells with high stability and high efficiency have attracted significant attention. A systematical static and dynamic structure investigation is carried out to show the details of 2D morphology evolution. A dual additive approach is used, where the synergy between an alkali metal cation and a polar solvent leads to high-quality 2D perovskite films with sandwich-type structures and vertical phase segregation. Such novel structure can induce high-quality 2D slab growth and reduce internal and surface defects, resulting in a high device efficiency of 16.48% with enhanced continuous illumination stability and improved moisture (55-60%) and thermal (85°C) tolerances. Transient absorption spectra reveal the carrier migration from low n to high n species with different kinetics. An [PbI6 ]4- octagon coalescence transformation mechanism coupled with metal and organic cations wrapped is proposed. By solvent vapor annealing, a recrystallization and reorientation of the 2D perovskite slabs occurs to form an ideal structure with improved device performance and stability.

Formamidinium‐Based Dion‐Jacobson Layered Hybrid Perovskites: Structural Complexity and Optoelectronic Properties

AbstractLayered hybrid perovskites have emerged as a promising alternative to stabilizing hybrid organic–inorganic perovskite materials, which are predominantly based on Ruddlesden‐Popper structures. Formamidinium (FA)‐based Dion‐Jacobson perovskite analogs are developed that feature bifunctional organic spacers separating the hybrid perovskite slabs by introducing 1,4‐phenylenedimethanammonium (PDMA) organic moieties. While these materials demonstrate competitive performances as compared to other FA‐based low‐dimensional perovskite solar cells, the underlying mechanisms for this behavior remain elusive. Here, the structural complexity and optoelectronic properties of materials featuring (PDMA)FAn–1PbnI3n+1 (n = 1–3) formulations are unraveled using a combination of techniques, including X‐ray scattering measurements in conjunction with molecular dynamics simulations and density functional theory calculations. While theoretical calculations suggest that layered Dion‐Jacobson perovskite structures are more prominent with the increasing number of inorganic layers (n), this is accompanied with an increase in formation energies that render n > 2 compositions difficult to obtain, in accordance with the experimental evidence. Moreover, the underlying intermolecular interactions and their templating effects on the Dion‐Jacobson structure are elucidated, defining the optoelectronic properties. Consequently, despite the challenge to obtain phase‐pure n > 1 compositions, time‐resolved microwave conductivity measurements reveal high photoconductivities and long charge carrier lifetimes. This comprehensive analysis thereby reveals critical features for advancing layered hybrid perovskite optoelectronics.

Organic Cations Protect Methylammonium Lead Iodide Perovskites against Small Exciton-Polaron Formation.

Working organic-inorganic lead halide perovskite-based devices are notoriously sensitive to surface and interface effects. Using a combination of density functional theory (DFT) and time-dependent DFT methods, we report a comprehensive study of the changes (with respect to the bulk) in geometric and electronic structures going on at the (001) surface of a (tetragonal phase) methylammonium lead iodide perovskite slab, in the dark and upon photoexcitation. The formation of a hydrogen bonding pattern between the -NH3 groups of the organic cations and the iodine atoms of the outer inorganic layout is found to critically contribute to the relative thermodynamic stability of slabs with varying surface compositions and terminations. Most importantly, our results show that the hydrogen bond locking effects induced by the MA groups tend to protect the external two-dimensional lattice against large local structural deformations, i.e., the formation of a small exciton-polaron, at variance with purely inorganic lead halide perovskites.

Diammonium-Cesium Lead Halide Perovskite with Phase-Segregated Interpenetrating Morphology for Photovoltaics.

The insertion of organic spacers into halide perovskite slabs has offered a trade-off between the efficiency and stability of perovskite solar cells (PSCs). The layered structure of diammonium-intercalated cesium lead halide perovskites is virtually unexplored, in contrast to several works on the monoammonium system. In this report, we find that perovskite with 1,4-butanediammonium (BDA) and cesium cations can only form n = 1 and n = 2 layered isologues defined by the chemical formula of (BDA)Csn-1Pbn(I0.7Br0.3)3n+1, while the n = 3-4 ones will self-construct into unique heterostructures comprising separated quantum wells (QWs; n = 1-2) and 3D (n = ∞) perovskites. We highlight that the 2D/3D heterostructures show a structural resemblance to that of bulk heterojunction in organics, thus improving the charge separation and transport more than surface passivation. Solar cells based on the (BDA)Cs3Pb4I9.1Br3.9 (n = 4) absorbing layer delivered a power conversion efficiency (PCE) reaching 9.49% with ideal light and thermal stability.

Computational investigation of Zn-doped and undoped SrEu2Fe2O7 as potential mixed electron and proton conductors.

Understanding the electrode properties at the atomistic level is of great benefit to the evaluation of electrode performance and design of better electrode materials in solid oxide fuel cells. In this work, density functional theory (DFT) calculations are employed to investigate the formation and conducting behaviors of oxygen vacancies and proton defects in Ruddlesden–Popper oxide SrEu2Fe2O7 (SEFO), which has been experimentally characterized as a promising cathode. The calculation results suggest both oxygen vacancies and proton defects can be formed in SEFO, and especially, the formation of these defects is largely dependent on oxygen sites in the special crystal structure with alternative stacking of rock-salt layers and double-layered perovskite slabs. The oxygen vacancies within the perovskite slabs have very low formation energies, but demonstrate high energy barriers for migration and low hydration properties; while in the case of those in the rock salt layers, it’s contrary. Interestingly, protons have similar migration abilities in the perovskite slabs and rock salt layers. And therefore, increasing the vacancy concentration of the rock salt layer is beneficial to increase the concentration of proton defects and to improve the proton conductivity. DFT calculations also indicate that substituting Zn for Fe in SEFO can largely depress the oxygen vacancy formation energy, which helps to increase the concentration of both defects. Importantly, the energy barriers for migration of both oxygen ions and protons are barely enhanced, implying a negligible trapping effect of the Zn dopant.

Competing Polar and Antipolar Structures in the Ruddlesden-Popper Layered Perovskite Li2SrNb2O7.

Over the past few years, several studies have reported the existence of polar phases in Ruddlesden-Popper layer perovskites by trilinear coupling of oxygen octahedral rotations (OOR) and polar distortions, a phenomenon termed as hybrid improper ferroelectricity. This phenomenon has opened an avenue to expand the available compositions of ferroelectric and piezoelectric layered oxides. In this study, we report a new polar Ruddlesden-Popper layered niobate, Li2SrNb2O7, which undergoes a structural transformation to an antipolar phase when cooled to 90 K. This structural transition results from a change in the phase of rotation of the octahedral layers within the perovskite slabs across the interlayers. First-principles calculations predicted that the antipolar Pnam phase would compete with the polar phase and that both would be energetically lower than the previously assigned centrosymmetric Amam phase. This phase transition was experimentally observed by a combination of synchrotron X-ray diffraction, powder neutron diffraction, and electrical and nonlinear optical characterization techniques. The competition between symmetry breaking to yield polar layer perovskites and hybrid improper antiferroelectrics provides new insight into the rational design of antiferroelectric materials that can have applications as electrostatic capacitors for energy storage.

Efficient Ruddlesden–Popper Perovskite Light‐Emitting Diodes with Randomly Oriented Nanocrystals

AbstractRuddlesden–Popper phase (RP‐phase) perovskites that consist of 2D perovskite slabs interleaved with bulky organic ammonium (OA) are favorable for light‐emitting diodes (LEDs). The critical limitation of LED applications is that the insulating OA arranged in a preferred orientation limits charge transport. Therefore, the ideal solution is to achieve a randomly connected structure that can improve charge transport without hampering the confinement of the electron–hole pair. Here, a structurally modulated RP‐phase metal halide perovskite (MHP), (PEA)2(CH3NH3)m−1PbmBr3m+1 is introduced to make the randomly oriented RP‐phase unit and ensure good connection between them by applying modified nanocrystal pinning, which leads to an increase in the efficiency of perovskite LEDs (PeLEDs). The randomly connected RP‐phase MHP forces contact between inorganic layers and thereby yields efficient charge transport and radiative recombination. Combined with an optimal dimensionality, (PEA)2(CH3NH3)2Pb3Br10, the structurally modulated RP‐phase MHP exhibits increased photoluminescence quantum efficiency, from 0.35% to 30.3%, and their PeLEDs show a 2,018 times higher current efficiency (20.18 cd A−1) than in the 2D PeLED (0.01 cd A−1) and 673 times than in the 3D PeLED (0.03 cd A−1) using the same film formation process. This approach provides insight on how to solve the limitation of RP‐phase MHP for efficient PeLEDs.

Crystal structure of LaSr3Fe3O9 and its phase relation with LaSr3Fe3O10

The Ruddlesden-Popper phase LaSr3Fe3O10 and its reduced derivatives with nominal composition LaSr3Fe3O10–x, with 0 < x < 1.21, are described in detail with respect to crystal structure, magnetic order, thermal stability and likely vacancy mechanisms. The crystal structure of the LaSr3Fe3O9 (x = 1) phase is described. The LaSr3Fe3O10-δ phase prevails for x < 0.51, whereas LaSr3Fe3O9±ε prevails for x > 0.85. The phases are separated by a two-phase region 0.51 < x < 0.85. By combined Rietveld refinements of high-resolution powder synchrotron and neutron diffraction, the crystallographic and magnetic structure of LaSr3Fe3O9 is described. LaSr3Fe3O9 takes a layer like RP3 type structure with features of the brownmillerite structure in the triple perovskite type slabs; unit cell dimensions a = 28.7562(13) Å, b = 5.5280(2) Å, c = 5.4583(2) Å, space group Cmcm. LaSr3Fe3O9 is an antiferromagnet with TN > 350 K and with a G-type magnetic structure, magnetic space group PCbcm. Based on unit cell volume considerations, local oxygen vacancy ordering schemes are suggested.

Anisotropic magnetic property and exchange bias effect in a homogeneous Sillen-Aurivillius layered oxide

A new Sillen-Aurivillius oxide Bi7Fe2Ti2O17Cl (BFTOCl), in which four perovskite layers are sandwiched by Sillen slabs, is designed and prepared by the solid-state reaction. Compared with pure Aurivilius structure, the Sillen slab requires one less positive charge for charge compensation due to the intercalation of Cl anions between two (Bi2O2)2+, which increases the concentration of magnetic Fe3+ ions in perovskite slab, and may depress the interaction of Fe3+ ions existing in neighboring perovskite slabs. This unique natural superlattice structure and the highly orientated texture of the ceramic result in a complex spin structure and an interesting magnetic anisotropy. The magnetism in in-plane direction (0.00487 emu/g at 100 K) is about 2.5 times larger than that in out-of-plane direction. Remarkably, exchange bias anisotropy has been observed in the BFTOCl ceramic, of which the exchange bias fields in in-plane and out-of-plane directions are 345 and 174 Oe at 100 K, respectively.

Fabrication of highly emissive and highly stable perovskite nanocrystal-polymer slabs for luminescent solar concentrators

The high-performance light-management slabs made of low-cost, highly emissive, and ultra-stable nanocrystal (NC)-polymer composites are desirable for application in large-area luminescent solar concentrators (LSCs).

Synthesis, optical, and magnetic properties of six‐layered Aurivillius bismuth ferrititanate

AbstractThis work reports on the preparation, structure, photochemical, and magnetic properties of six‐layered Aurivillius bismuth ferrititanates, that is, Bi7Ti3Fe3O21, Bi7(Ti2Nb)Fe3O21+δ, and Bi7(Ti2Mg)Fe3O21−δ nanoparticles. The samples were prepared through the modified citrate complexation and precursor film process. The XRD Rietveld refinements were conducted to study the phase formations and crystal structure. The morphological and chemical component characteristics were investigated using SEM, TEM, and EDX analyses. Bi7Ti3Fe3O21, Bi7(Ti2Nb)Fe3O21+δ, and Bi7(Ti2Mg)Fe3O21−δ nanoparticles present an indirect allowed transitions with band energies of 2.04, 2.03, and 2.02 eV, respectively. The hybridized (O2p+Fet2g+Bi6s) formed the valence band (VB) and electronic components of (Ti–3d+Fe–eg) formed the conduction band (CB) of this six‐layered Aurivillius bismuth ferrititanate. The three samples showed efficient photocatalytic degradation of Rhodamine B (RhB) dyes with the excitation wavelength λ &gt; 420 nm. The optical absorption, photodegradation, and magnetic abilities were improved through microstructural modification on “B” site via partial substitution of Mg2+ and Nb5+ for Ti4+. The photocatalytic results were discussed based on the layer structure and multivalent Fe ions. Fe3+/2+ in the perovskite slabs (Bi5Fe3Ti3O19)2− could act as the catalytic mediators in the photocatalysis process. As a photocatalyst, Aurivillius Bi7(Ti2Mg)Fe3O21−δ nanoparticle is advantageous due to its photocatalytic and magnetically recoverable abilities.

Intrinsic Carrier Transport of Phase-Pure Homologous 2D Organolead Halide Hybrid Perovskite Single Crystals.

This work reveals the intrinsic carrier transport behavior of 2D organolead halide perovskites based on phase-pure homologous (n = 1, 2, and 3) Ruddelsden-Popper perovskite (RPP) (BA)2 (MA)n -1 Pbn I3n+1 single crystals. The 2D perovskite field effect transistors with high-quality exfoliated 2D perovskite bulk crystals are fabricated, and characteristic output and transfer curves are measured from individual single-crystal flakes with various n values under different temperatures. Unipolar n-type transport dominated the electrical properties of all these 2D RPP single crystals. The transport behavior of the 2D organolead halide hybrid perovskites exhibits a strong dependence on the n value and the mobility substantially increases as the ratio of the number of inorganic perovskite slabs per organic spacer increases. By extracting the effect of contact resistances, the corrected mobility values for n = 1, 2, and 3 are 2 × 10-3 , 8.3 × 10-2 , and 1.25 cm2 V-1 s-1 at 77 K, respectively. Furthermore, by combining temperature-dependent electrical transport and optical measurements, it is found that the origin of the carrier mobility dependence on the phase transition for 2D organolead halide perovskites is very different from that of their 3D counterparts. Our findings offer insight into fundamental carrier transport behavior of 2D organic-inorganic hybrid perovskites based on phase-pure homologous single crystals.

Site Selectivity of Hydride in Early-Transition-Metal Ruddlesden-Popper Oxyhydrides.

Layered perovskite titanium oxyhydrides have been prepared by low-temperature topochemical CaH2 reduction from Ruddlesden-Popper Sr n+1Ti nO3 n+1 phases ( n = 1, 2) and structurally characterized by combined synchrotron X-ray and neutron diffraction data refinements. In the single-layered Sr2TiO3.91(2)D0.14(1) material, hydride anions are statistically disordered with oxides on the apical site only, as opposed to known transition-metal oxyhydrides exhibiting a preferred occupation of the equatorial site. This unprecedented site selectivity of H- has been reproduced by periodic DFT+ U calculations, emphasizing for the hydride defect a difference in formation energy of 0.24 eV between equatorial and apical sites. In terms of electronic structure, the model system Sr2TiO3.875H0.125 is found to be slightly metallic and the released electron remains mostly delocalized over several Ti atoms. On the other hand, hydride anions in the double-layered Sr3Ti2O6.20H0.12 material show a clear preference for the bridging apical site within the perovskite slabs, as confirmed by DFT calculations on the Sr3Ti2O6.875H0.125 model system. Finally, the influence of the B-site chemical nature on the hydride site selectivity for early 3d transition metals is theoretically explored in the single-layered system by substituting vanadium for titanium. The V3+ electronic polaron is suggested to play a role in stabilizing H- on the equatorial site in Sr2VO4- xH x for x = 0.125.

Electrically controlled transformation of memristive titanates into mesoporous titanium oxides via incongruent sublimation

Perovskites such as SrTiO3, BaTiO3, and CaTiO3 have become key materials for future energy-efficient memristive data storage and logic applications due to their ability to switch their resistance reversibly upon application of an external voltage. This resistance switching effect is based on the evolution of nanoscale conducting filaments with different stoichiometry and structure than the original oxide. In order to design and optimize memristive devices, a fundamental understanding of the interaction between electrochemical stress, stoichiometry changes and phase transformations is needed. Here, we follow the approach of investigating these effects in a macroscopic model system. We show that by applying a DC voltage under reducing conditions on a perovskite slab it is possible to induce stoichiometry polarization allowing for a controlled decomposition related to incongruent sublimation of the alkaline earth metal starting in the surface region. This way, self-formed mesoporous layers can be generated which are fully depleted by Sr (or Ba, Ca) but consist of titanium oxides including TiO and Ti3O with tens of micrometre thickness. This illustrates that phase transformations can be induced easily by electrochemical driving forces.

New, Efficient, and Reliable Air Electrode Material for Proton-Conducting Reversible Solid Oxide Cells.

Driven by the demand to minimize fluctuation in common renewable energies, reversible solid oxide cells (RSOCs) have drawn increasing attention for they can operate either as fuel cells to produce electricity or as electrolysis cells to store electricity. Unfortunately, development of proton-conducting RSOCs (P-RSOCs) faces a major challenge of poor reliability because of the high content of steam involved in air electrode reactions, which could seriously decay the lifetime of air electrode materials. In this work, a very stable and efficient air electrode, SrEu2Fe1.8Co0.2O7-δ (SEFC) with layer structure, is designed and deployed in P-RSOCs. X-ray diffraction analysis and High-angle annular dark-filed scanning transmission electron microscopy images of SEFC reveal that Sr atoms occupy the center of perovskite slabs, whereas Eu atoms arrange orderly in the rock-salt layer. Such a special structure of SEFC largely depresses its Lewis basicity and therefore its reactivity with steam. Applying the SEFC air electrode, our button switches smoothly between both fuel cell and electrolysis cell (EC) modes with no obvious degradation over a 135 h long-term test under wet H2 (∼3% H2O) and 10% H2O-air atmospheres. A record of over 230 h is achieved in the long-term stability test in the EC mode, doubling the longest test that had been previously reported. Besides good stability, SEFC demonstrates great catalytic activity toward air electrode reactions when compared with traditional La0.6Sr0.4Co0.2Fe0.8O3-δ air electrodes. This research highlights the potential of stable and efficient P-RSOCs as an important part in a sustainable new energy power system.