Ruddlesden-Popper Research Articles

Electronic structure of the homologous series of Ruddlesden–Popper phases SrO(SrTiO3) n , (n = 0–3, ∞)

Abstract The system SrO(SrTiO3) n contains promising compounds for several applications, whose functionalities all depend in particular on the band structure of the respective crystal. While the electronic structure of SrO and SrTiO3 is sufficiently clarified in literature, there is a lack of information concerning the Ruddlesden–Popper (RP) phases. In this work, density functional theory is used to compute the electronic structure for the homologous series with n = 0–3, ∞. The according band structures are presented and effective masses are given for the complete system. In addition, the calculations are consulted to discuss the thermodynamical stability of the RP phases, confirming the gain of formation energy up to n = 3, as reported in recent literature. A promising possibility for applications has been found, analyzing theses band structures: As the optical gaps at distinct high-symmetry points of the Brillouin zone show different dependencies on the lattice parameters, as it is reported for SrO in literature, a similar behavior could be expected in particular for the RP phase with n = 1.

Insights on a Ruddlesden-Popper phase as an active layer for a solid oxide fuel cell fed with dry biogas

Solid oxide fuel cell (SOFC) is a mature opportunity for producing power energy in remote areas like islands, where access to the electrical grid is not favoured, and gas distribution is the only viable approach. In this context, generally, biogas represents the most convenient fuel resources in these areas. However, the direct use of biogas in SOFCs is still an issue to be solved due to its negative effect on the conventional Ni-YSZ anode. In this study, to overcome this issue, we suggested using a protective layer coated on the anode of a commercial SOFC. A nickel manganite showing mixed ionic and electronic conductivity tailored specifically for this approach was investigated. The preliminary characterisations showed that the formation of a Ruddlesden-Popper (RP) n = 1 structure supporting fine encapsulated particles based on Ni was formed around 800 °C in consequence of the reducing environment. The electrochemical experiments carried out for 270 h demonstrated for the coated cell significant stability in the presence of dry biogas, albeit an ageing effect was noticed in the electrical percolation of both cell electrodes. The post mortem analyses revealed an attractive redox property for the nickel manganite, which partially returned to the RP n = 2 phase. Moreover, the absence of carbon deposits on the anode suggests possible applications for this approach.

Spacer Organic Cation Engineering for Quasi‐2D Metal Halide Perovskites and the Optoelectronic Application

Recently, organic and inorganic halide perovskites are one of the most popular semiconductors owing to their enormous potential for optoelectronic application, such as photovoltaics (PV), light‐emitting diode (LED), photodetector, and so on. The photoelectric conversion efficiency of 3D organic–inorganic hybrid perovskite solar cells has increased rapidly; however, the commercialization of the related devices is largely hampered by the poor stability of perovskite materials under environmental stress. Compared to 3D halide perovskites, quasi‐2D (Q‐2D) perovskites have improved moisture stability and less tendency for ion migration, which offers a new approach to stabilize perovskite‐based optoelectronic devices. Furthermore, Q‐2D hybrid perovskites have diverse structures with different quantum confinement and dielectric confinement characteristics, which enables the fine‐tuning of their optoelectronic properties through structure engineering. Depending on the different structures of spacer organic cations, the Q‐2D perovskite structure mainly includes Ruddlesden−Popper (RP) phase structure, Dion−Jacobson (DJ) phase structure, and alternating cation in the interlayer space (ACI) phase structure. In this review, the state‐of‐the‐art in Q‐2D perovskites is discussed based on their structural engineering, and an overview of PV and LEDs applications is provided. Finally, a brief outlook with respect to the development of Q‐2D perovskite materials, as well as advanced device, is provided.

Protonic Conduction in La2NiO4+δ and La2‐xAxNiO4+δ (A = Ca, Sr, Ba) Ruddlesden–Popper Type Oxides

AbstractA significant hydration and protonic conduction in La2NiO4+δ‐based Ruddlesden–Popper (RP) oxides will enable their use as positrode materials in proton ceramic electrochemical cells (PCECs). Unlike perovskite electrolytes and positrode materials, where protons reside and jump on regular oxide ions, RP oxides such as La2NiO4+δ may contain oxygen interstitials and offer the possibility that protons locate on and migrate by help of these, suggesting a mechanism by which positrodes may operate by triple conduction. Here, theoretical calculations that support proton migration between interstitial oxide ions in the rock‐salt layer via unshared oxide ions in the perovskite layer are reported. Furthermore, water contents of pristine La2NiO4+δ and La2‐xAxNiO4+δ (A = Ca, Sr, Ba) are reported, showing that hydration increases with the level and basicity of the dopant. Hebb‐Wagner blocking electrode measurements show significant partial protonic conductivity in all samples. The effects of doping on hydration and of on the p‐type conductivity suggest that protons reside primarily on structural oxide ion, but a role of interstitial oxide ions for dissolution and migration of protons cannot be ruled out in cases with oxygen excess. The results are instructive for further studies and optimization of RP oxides as potential positrode materials for PCECs.

Thick-Layer Lead Iodide Perovskites with Bifunctional Organic Spacers Allylammonium and Iodopropylammonium Exhibiting Trap-State Emission.

The nature of the organic cation in two-dimensional (2D) hybrid lead iodide perovskites tailors the structural and technological features of the resultant material. Herein, we present three new homologous series of (100) lead iodide perovskites with the organic cations allylammonium (AA) containing an unsaturated C═C group and iodopropylammonium (IdPA) containing iodine on the organic chain: (AA)2MAn-1PbnI3n+1 (n = 3-4), [(AA)x(IdPA)1-x]2MAn-1PbnI3n+1 (n = 1-4), and (IdPA)2MAn-1PbnI3n+1 (n = 1-4), as well as their perovskite-related substructures. We report the in situ transformation of AA organic layers into IdPA and the incorporation of these cations simultaneously into the 2D perovskite structure. Single-crystal X-ray diffraction shows that (AA)2MA2Pb3I10 crystallizes in the space group P21/c with a unique inorganic layer offset (0, <1/2), comprising the first example of n = 3 halide perovskite with a monoammonium cation that deviates from the Ruddlesden-Popper (RP) halide structure type. (IdPA)2MA2Pb3I10 and the alloyed [(AA)x(IdPA)1-x]2MA2Pb3I10 crystallize in the RP structure, both in space group P21/c. The adjacent I···I interlayer distance in (AA)2MA2Pb3I10 is ∼5.6 Å, drawing the [Pb3I10]4- layers closer together among all reported n = 3 RP lead iodides. (AA)2MA2Pb3I10 presents band-edge absorption and photoluminescence (PL) emission at around 2.0 eV that is slightly red-shifted in comparison to (IdPA)2MA2Pb3I10. The band structure calculations suggest that both (AA)2MA2Pb3I10 and (IdPA)2MA2Pb3I10 have in-plane effective masses around 0.04m0 and 0.08m0, respectively. IdPA cations have a greater dielectric contribution than AA. The excited-state dynamics investigated by transient absorption (TA) spectroscopy reveal a long-lived (∼100 ps) trap state ensemble with broad-band emission; our evidence suggests that these states appear due to lattice distortions induced by the incorporation of IdPA cations.

Weak magnetic field-dependent photoluminescence properties of lead bromide perovskites

The strong spin–orbit coupling (SOC) in lead halide perovskites, when inversion symmetry is lifted, has provided opportunities for investigating the Rashba effect in these systems. Moreover, the strong orbital moment, which, in turn, impacts the spin-pair in singlet and triplet electronic states, plays a significant role in enhancing the optoelectronic properties in the presence of external magnetic fields in lead halide perovskites. Here, we investigate the effect of weak magnetic fields (&amp;lt;1 T) on the photoluminescence (PL) properties of CsPbBr3 nanocrystals with and without Ruddlesden–Popper (RP) faults and single crystals of CH3NH3PbBr3. Along with an enhancement in the PL intensity as a function of an external magnetic field, which is observed in both lead bromide perovskites, the PL emission red-shifts in CsPbBr3 nanocrystals. Density-functional theory calculations of the electronic band-edge in CsPbBr3 show almost no change in the energy gap as a function of the external magnetic field. The experimental results, thus, suggest the role of mixing of the triplet and singlet excitonic states under weak magnetic fields. This is further deduced from an enhancement in PL lifetimes as a function of the field in CsPbBr3. In CH3NH3PbBr3, an increase in PL intensity is observed under weak magnetic fields; however, no changes in the peak energy or PL lifetimes are observed. The internal magnetic fields due to SOC are characterized for all three samples and found to be the highest for CsPbBr3 nanocrystals with RP faults.

CsPbI3-Based Phase-Stable 2D Ruddlesden-Popper Perovskites for Efficient Solar Cells.

Inorganic CsPbI3 perovskite has shown great promise in highly stable perovskite solar cells due to the lack of volatile organic components. However, the inferior phase stability in ambient conditions resulted from the very small Cs+, limiting their practical applications. Here, CsPbI3-based 2D Ruddlesden-Popper (RP) perovskites were developed using two thiophene-based aromatic spacers, namely, 2-thiophenemethylamine hydroiodide (ThMA) and 2-thiopheneformamidine hydroiodide (ThFA), which significantly improved the phase stability by releasing the large inner stress of black-phase CsPbI3. The optimized ThFA-based 2D RP perovskite (n = 5, ThFA-Cs) device achieves a record efficiency of 16.00%. Importantly, the ThFA-Cs devices could maintain an average of 98% of their initial efficiencies after being stored in N2 at room temperature for 3000 h and 92% of their initial value at 80 °C for 960 h. This work provides a new perspective for exploration of the phase-stable CsPbI3-based perovskite with reduced dimensions for high-performance solar cells.

Reversible Pressure-Dependent Mechanochromism of Dion-Jacobson and Ruddlesden-Popper Layered Hybrid Perovskites.

Layered Dion-Jacobson (DJ) and Ruddlesden-Popper (RP) hybrid perovskites are promising materials for optoelectronic applications due to their modular structure. To fully exploit their functionality, mechanical stimuli can be used to control their properties without changing the composition. However, the responsiveness of these systems to pressure compatible with practical applications (<1GPa) remains unexploited. Hydrostatic pressure is used to investigate the structure-property relationships in representative iodide and bromide DJ and RP 2D perovskites based on 1,4-phenylenedimethylammonium (PDMA) and benzylammonium (BzA) spacers in the 0-0.35GPa pressure range. Pressure-dependent X-ray scattering measurements reveal that lattices of these compositions monotonically shrink and density functional theory calculations provide insights into the structural changes within the organic spacer layer. These structural changes affect the optical properties; the most significant shift in the optical absorption is observed in (BzA)2 PbBr4 under 0.35GPa pressure, which is attributed to an isostructural phase transition. Surprisingly, the RP and DJ perovskites behave similarly under pressure, despite the different binding modes of the spacer molecules. This study provides important insights into how the manipulation of the crystal structure affects the optoelectronic properties of such materials, whereas the reversibility of their response expands the perspectives for future applications.

Imaging the Moisture-Induced Degradation Process of 2D Organolead Halide Perovskites.

Two-dimensional (2D) and quasi-2D Ruddlesden–Popper (RP) phase organolead halide perovskites are promising materials for both photovoltaic and optoelectronic devices. Although they are known to be more stable when exposed to moisture than their 3D counterpart, chemical degradation of these materials under moisture, which not only leads to a significant drop in device performance but also leads to lead leakage, yet remains one of the most serious hurdles for their practical applications. To gain insight into the degradation mechanism of 2D/quasi-2D perovskites under moisture conditions, the degradation pathway of 2D/quasi-2D (PEA)2(MA)n−1PbnI3n+1 (PEA = C6H5C2H4NH3+, MA = CH3NH3+, and n is the number of perovskite layers between adjacent organic spacer layers) perovskite single crystals (SCs) and thin film are explored. We observe the degradation process by mapping the photoluminescence of the 2D perovskites and demonstrate that the larger-n phases all directly degrade into the relative stable n = 1 phase and MAI and PbI2, which is a mechanism different from that in previous reports and further confirmed in the 2D perovskite thin film. This degradation process is also found to be independent of the boundary and morphology of the SCs. This discovery provides a new perspective for understanding the chemical degradation of the 2D perovskite materials and may inspire new solutions for improving their moisture stability.

Uncovering synergistic effect of chloride additives for efficient quasi-2D perovskite solar cells

• MACl and PbCl 2 binary additives form MAPbCl 3 intermediate phase. • Perovskite crystallization is retarded and volatilization of MACl is prevented. • Such a synergistic effect leads to high-quality Q-2D perovskite films. • Charge recombination is suppressed and charge extraction efficiency is enhanced. • The champion PCE of 18.67% is one of the highest PCE for Q-2D RP PSCs with n ≤ 4. Methylammonium chloride (MACl) and lead chloride (PbCl 2 ) are commonly used for improving perovskite film quality. However, the rapid release of volatile MACl would lead to low-quality perovskite films with pinholes and defects. Here, we report a strategy of employing MACl&PbCl 2 binary additives together to prepare high-quality quasi-two-dimensional (Q-2D) Ruddlesden–Popper (RP) perovskite films. We found that MAPbCl 3 intermediate phase is formed via a reaction between MACl and PbCl 2 , which suppresses the fast volatilization of MACl, resulting in a compact and pinhole-free perovskite film with large grains. Moreover, the trap states are passivated via the incorporation of Cl – into the Q-2D perovskite crystals along with the addition of PbCl 2 , leading to a reduced charge recombination loss and an improved charge extraction. A remarkable power conversion efficiency (PCE) of 18.67% is achieved in such Q-2D RP PSCs, one of the highest PCE values among the low-dimensional RP PSCs with n ≤ 4. The environmental stability of the devices is also significantly enhanced. Our results provide an effective strategy by using chloride based additives for fabricating high-quality low-dimensional RP perovskite films for efficient solar cells.

Ruddlesden–Popper Faults in NdNiO3 Thin Films

The NdNiO3 (NNO) system has attracted a considerable amount of attention owing to the discovery of superconductivity in Nd0.8Sr0.2NiO2. In rare-earth nickelates, Ruddlesden–Popper (RP) faults play a significant role in functional properties, motivating our exploration of its microstructural characteristics and the electronic structure. Here, we employed aberration-corrected scanning transmission electron microscopy and spectroscopy to study a NdNiO3 film grown by layer-by-layer molecular beam epitaxy (MBE). We found RP faults with multiple configurations in high-angle annular dark-field images. Elemental intermixing occurs at the SrTiO3–NdNiO3 interface and in the RP fault regions. Quantitative analysis of the variation in lattice constants indicates that large strains exist around the substrate–film interface. We demonstrate that the Ni valence change around RP faults is related to a strain and structure variation. This work provides insights into the microstructure and electronic-structure modifications around RP faults in nickelates.

Direct Observation of Fast Carriers Transport along Out-of-Plane Direction in a Dion–Jacobson Layered Perovskite

Layered perovskites have been broadly applied in perovskite solar cells and light-emitting diodes to enhance device stability, but the poor carrier transport property along the out-of-plane direction for all known Ruddlesden–Popper (RP) layered perovskites imposes constraints to their application. Herein, using transient photoluminescence mapping, we visualized that the transport of carriers or excitons along the out-of-plane direction is comparable to along the in-plane direction in a Dion–Jacobson (DJ) layered perovskite, which has the reduced ligand length and stronger I···I electronic interaction between inorganic layers.

High Efficiency Perovskite Solar Cells Employing Quasi‐2D Ruddlesden‐Popper/Dion‐Jacobson Heterojunctions

AbstractWhile 2D Ruddlesden‐Popper (RP) perovskites exhibit attractive opto‐electronic properties and stability for use in perovskite solar cells (PSCs), their complicated film‐forming processes often induce a non‐negligible level of defects that significantly undermine the power conversion efficiency (PCE) and stability of PSCs. Here, the use of two organic ammonium salts with the same chain length, namely monoammonium (butylammonium iodide, BAI) and diammonium (1,4‐butanediamine dihydroiodide, BDAI2) for surface defect passivation of RP‐2D perovskite films of (AA)2MA4Pb5I16 (n = 5) are reported. It is found that the diammonium BDAI2 not only effectively reduces the defect density (similarly to using monoammonium BAI) but forms a Dion‐Jacobson (DJ) 2D structure to enhance interfacial charge extraction and suppress surface charge recombination. As a result, a boosted PCE of 18.34% has been obtained with a high open‐circuit voltage of 1.24 V. Owing to the enhanced structural integrity of the DJ phase, the RP‐2D/DJ‐2D perovskite heterojunction films exhibit supreme material robustness, which translates to the impressive environmental stability of devices, showing nearly zero‐degradation of the efficiency after 800 h of continuous thermal aging (60 °C) for 800 h. This work enriches the fundamental understanding of the impacts of the DJ‐2D structure on the surface properties of 2D perovskites.

Long carrier diffusion length in two-dimensional lead halide perovskite single crystals

<h2>Summary</h2> Ruddlesden-Popper (RP) perovskites are two-dimensional semiconductors for high-performance optoelectronic devices. In this work, we report a long in-plane carrier diffusion length in 2D RP perovskite single crystals probed by scanning photocurrent microscopy. Carrier diffusion lengths of 7–14 μm are observed when the number of PbI₆⁻² octahedra between organic spacers increases from 1 to 3. Using detailed light intensity and electric-field-dependent photocurrent measurements, we attribute the observed long diffusion length to the dominating dissociated free carrier transport. This is further validated by time-resolved photoluminescence measurements, where the decay lifetime increases in the presence of an electric field. From our experiments, we conclude that the in-plane transport in RP perovskites is efficient because of the partial free carrier generation, which overcomes strong excitonic effects. Our results suggest that semiconducting devices fabricated from RP perovskite single crystals can be as efficient as their 3D counterparts.

Magnetism in a 2D Hybrid Ruddlesden-Popper Perovskite through Charge Redistribution Driven by an Organic Functional Spacer.

Two-dimensional Ruddlesden-Popper (RP) perovskites are emerging materials offering great synthetic versatility and remarkable features due to the tunability of their crystal structure. We present a novel strategy to provide magnetism in a 2D RP perovskite using histidine molecules as a spacer, which could induce charge rebalancing at the interface of the inorganic layer. We observe that the amide and imidazole groups are close to Pb ions. The interaction with the imidazole indicates that this functional group, possibly assisted by the carboxyl close to the vicinity of the amine terminal, is inducing charge rearrangement from Pb2+ to paramagnetic Pb3+ ions, resulting in a positive magnetic moment. This magnetized 2D hybrid perovskites can be classified as a novel class of promising materials showing a magnetic moment at their interface, which may result in intriguing physical properties due to a delicate balance between magnetism and a quantum well confinement effect in the inorganic layer.

Insight into the Enhanced Charge Transport in Quasi-2D Perovskite via Fluorination of Ammonium Cations for Photovoltaic Applications.

Fluorinated spacer cations in quasi-2D (Q-2D) perovskites have recently been demonstrated to improve the Q-2D perovskite solar cell (PSC) performance. However, the underlying mechanism of fluorination of organic cations on the improvement is still unclear. Here, using fluorinated benzylammonium (named F-BZA) as a spacer cation in Q-2D Ruddlesden-Popper (RP) perovskites, we deeply investigate the effect of fluorination of organic cations on perovskite crystallization and intermolecular interactions for improving the charge transport and device performance. It is found that fluorination of spacer cations can slow down the crystallization rate of perovskites, resulting in vertically aligned large grains. Moreover, the interaction between the adjacent spacer cations is further enhanced, constructing a new faster charge-transport channel with a lifetime of 77 ps. Accordingly, the carrier mobility is improved by an order of magnitude and a power conversion efficiency (PCE) of 16.82% is achieved in much more stable F-BZA-based Q-2D RP PSCs, 35% higher than that of BZA-based devices (12.39%). Our results elucidate the mechanism and its importance of fluorinating spacer cations for high-performance Q-2D PSC development.

Phototransistors Based on Organic Small Molecules–Ruddlesden‐Popper Layered Perovskite Single Crystal Heterojunctions

AbstractIn photodetectors based on heterojunctions of organic semiconductor and hybrid perovskite, the strong light absorbing ability of perovskite and the gate‐tunability of organic semiconductor can be preferably incorporated. However, the charge current transfer at the interface, and the double channel transportation in the heterojunctions are not fully understood. The authors successfully transfer the mechanically exfoliated Ruddlesden–Popper (RP) layered perovskite single crystal to the thermal evaporated organic small molecule semiconductor forming a heterojunction. Based on the heterojunction, two configurations of light detecting devices are prepared in which the perovskite layer acts as either a conducting channel and light absorbing layer or light absorbing layer only. The light detecting performance of such devices is systematically investigated. The charge transfer from the perovskite single crystal to the organic semiconductor layer is observed by Kelvin Probe Force Microscope characterization. They also find that the n values of the RP layered perovskite can significantly influence the property of the heterojunction and charge transfer process. For single channel devices a responsivity of 5.87 × 102 A W−1 and a detectivity of 5.13 × 1012 Jones is realized.

Electrochemical study of symmetrical intermediate temperature - solid oxide fuel cells based on La0.6Sr0.4MnO3 / Ce0.9Gd0.1O1.95 for operation in direct methane / air

La0.6Sr0.4MnO3 (LSM), which is considered a state-of-the-art solid oxide fuel cell (SOFC) cathode, is investigated for application as an anode in direct methane intermediate temperature - solid oxide fuel cells (IT-SOFCs). Ce0.9Gd0.1O1.95 (CGO) is used as the electrolyte. The characterized electrode is a composite LSM/CGO, prepared in two different configurations: LSM crushed electrospun nanofibers / GDC powders, and LSM powders / GDC powders. The electrodes are tested in both air and direct methane conditions. At 815 °C, the polarization resistance Rp = 1.6 Ωcm2 in air, and Rp = 0.15 Ωcm2 in methane. Since perovskite-type manganites may show poor stability in reducing atmosphere, electrode stability is investigated. It is found that LSM shows a reversible modification of the crystal structure, assuming a Ruddlesden-Popper (RP) structure when exposed to methane. The RP structure is expected to be more stable compared to the LSM single perovskite. Furthermore, the composite electrode is expected to benefit from the presence of CGO, which is stable in reducing conditions.

Electro-Oxidation Reaction of Methanol over La2–xSrxNiO4+δ Ruddlesden–Popper Oxides

How materials’ crystalline structure influences the underlying electronic configuration, along with redox properties, and plays a pivotal role in electrocatalysis is an intriguing question. Here, solution combustion-synthesized La2–xSrxNiO4+δ (x = 0–0.8) Ruddlesden–Popper (RP) oxides were explored for an electrocatalytic methanol oxidation reaction. Optimal doping of bivalent Sr2+ in the A site enabled the tetragonal distortion and oxidation of Ni2+ to Ni3+ that resulted ultimately in enhanced covalent hybridization of Ni 3d–O 2p with a closer proximity of the O 2p band to the Fermi level. The RP oxide La1.4Sr0.6NiO4+δ exhibited the highest methanol oxidation reactivity vis-à-vis the formation of HCO2H. The proposed mechanism over La1.4Sr0.6NiO4+δ considers a lattice oxygen-mediated methanol oxidation reaction, owing to Fermi-level “pinning” at the top of the O 2p band, which facilitated lattice oxygen atoms prone to oxidation. A high surface concentration of the key active species of Ni–OOH was observed to form during the methanol oxidation reaction with the help of lattice oxygen atoms and oxygen vacancies in La1.4Sr0.6NiO4+δ. The present study offers a uniquely comprehensive exploration of structural and surface properties of RP oxides toward methanol oxidation reactions.

Layered 2D Halide Perovskites beyond the Ruddlesden–Popper Phase: Tailored Interlayer Chemistries for High‐Performance Solar Cells

AbstractLayered halide perovskites (LHPs) with crystallographically 2D structures have gained increasing interest for photovoltaic applications due to their superior chemical stability and intriguing anisotropic properties, which are in contrast to their conventional 3D perovskite counterparts. The most frequently studied LHPs are Ruddlesden–Popper (RP) phases, which suffer from a carrier‐transport bottleneck due to the van der Waals gap associated with their intrinsic organic interlayer structures. To address this issue, Dion–Jacobson (DJ) and alternating‐cation‐interlayer (ACI) LHPs have rapidly emerged, which exhibit unique structural and (opto)electronic characteristics that may resemble those of the 3D counterparts owing to the eliminated or reduced van der Waals gap. Improved photophysical properties have been achieved in DJ and ACI LHPs, leading towards better photovoltaic performance. Here we provide a comprehensive discussion on the merits and promises of DJ and ACI LHPs from a chemistry perspective. Then, we review recent progress on the synthesis and tailoring of DJ and ACI LHP crystals and thin films, as well as their optoelectronic properties and photovoltaic performance. Finally, we discuss possible pathways to overcome critical challenges to realize the full potential of DJ and ACI LHPs for high‐performance solar cells and beyond.