Perovskite Substrates Research Articles

Layered perovskites with exsolved Co-Fe nanoalloy as highly active and stable anodes for direct carbon solid oxide fuel cells

The performance and stability of direct carbon solid oxide fuel cells (DCSOFCs) are severely affected by the electrocatalytic activity of their anode materials. In this work, a cobalt doped layered perovskite, (PrBa)0.95Fe1.8−xCoxNb0.2O5+δ (PBFCoxN, x = 0, 0.1, 0.2, 0.3), is investigated as the anode material for a DCSOFC. Co3Fe7 alloy particles are exsolved on the perovskite substrate under the reduction of carbon, which improves CO chemisorption and electrochemical oxidation. This enhances the electrochemical performance of the anode. The electrolyte-supported cell with a (PrBa)0.95Fe1.6Co0.2Nb0.2O5+δ (PBFCo0.2N) anode achieves a peak power density of 525.6 mW cm−2 at 800 °C, fueled by activated carbon. The performance of this anode material is comparable to or exceeds that of previously reported anodes, indicating that the PBFCo0.2N anode is a promising option for DCSOFCs.

Diethanolamine Modified Perovskite-Substrate Interface for Realizing Efficient ESL-Free PSCs.

Simplifying device layout, particularly avoiding the complex fabrication steps and multiple high-temperature treatment requirements for electron-selective layers (ESLs) have made ESL-free perovskite solar cells (PSCs) attractive. However, the poor perovskite/substrate interface and inadequate quality of solution-processed perovskite thin films induce inefficient interfacial-charge extraction, limiting the power conversion efficiency (PCEs) of ESL-free PSCs. A highly compact and homogenous perovskite thin film with large grains was formed here by inserting an interfacial monolayer of diethanolamine (DEA) molecules between the perovskite and ITO substrate. In addition, the DEA created a favorable dipole layer at the interface of perovskite and ITO substrate by molecular adsorption, which suppressed charge recombination. Comparatively, PSCs based on DEA-treated ITO substrates delivered PCEs of up to 20.77%, one of the highest among ESL-free PSCs. Additionally, this technique successfully elongates the lifespan of ESL-free PSCs as 80% of the initial PCE was maintained after 550 h under AM 1.5 G irradiation at ambient temperature.

Constructing robust heterointerfaces for carrier viaduct via interfacial molecular bridges enables efficient and stable inverted perovskite solar cells

To ensure carrier selectivity, we propose an interfacial molecular bridge comprised of Ph-CH2N+H3−n(CH3)n ammonium cation at the perovskite–substrate interface and reveal its underlying impact on PSCs performance.

Termination Control of (001) and (110) NdGaO3 Single-Crystal Substrates by Selective Chemical Etching

Controlling the surface morphology and composition of the perovskite substrates is a critical aspect in tuning the final properties of the deposited films and of their interfaces. The paper reports on a chemical etching method developed for (110) and (001) NdGaO3 single crystal substrates in order to obtain a well-defined GaO2−x-terminated surface. The etching process is based on a HF + NH4OH solution and includes an annealing step performed in air or under O2 flow at temperatures of 800–1000 °C. In order to obtain the desired composition and surface morphology, the etching procedure was optimized for the vicinal step density at the surface and substrate crystal orientation. Growth nucleation studies of one-unit-cell MeO films (Me = Ti, Sr, Ba) on chemically etched and on only annealed substrates were performed in order to determine the composition of the substrate topmost layer. The results indicate that the chemically etched NdGaO3 substrate surface has a predominantly GaO2−x termination, with a lower free surface energy compared to the NdO1+x termination.

Anisotropic Strain‐Mediated Symmetry Engineering and Enhancement of Ferroelectricity in Hf0.5Zr0.5O2/La0.67Sr0.33MnO3 Heterostructures

AbstractHafnium‐based binary oxides have attracted considerable attention due to their robust ferroelectricity at the nanoscale and compatibility with silicon‐based electronic technologies. To further promote the potential of Hafnium oxides for practical device applications, it is essential to effectively harness the interplay between structural symmetry, domain configuration, and ferroelectricity. Here, using Hf0.5Zr0.5O2/La0.67Sr0.33MnO3 (HZO/LSMO) heterostructures as a model system, the anisotropic strain‐mediated symmetry engineering and ferroelectricity enhancement are systematically investigated. By growing the heterostructures on (110)‐oriented perovskite substrates, considerable anisotropic strain is imposed on the LSMO bottom electrodes. Such an anisotropically‐strained LSMO layer acts as a structural template and effectively tune the structural symmetry, polar/non‐polar phase ratio, and ferroelectricity of the HZO top layer. Specifically, the anisotropic tensile strain stabilizes the ferroelectric rhombohedral and orthorhombic phases, thus enhancing the remnant polarization (Pr) up to 22 µC cm−2. In contrast, the anisotropic compressive strain facilitates the formation of non‐ferroelectric tetragonal phases, leading to a suppressed Pr down to 8 µC cm−2. These findings provide a guideline for understanding and modulating the intrinsic structure‐ferroelectricity relationship of HZO through anisotropic strain‐mediated symmetry engineering, which may shed light on the development of hafnium‐oxide‐based electronic devices.

Detour-phased perovskite ultrathin planar lens using direct femtosecond laser writing

Perovskite-enabled optical devices have drawn intensive interest and have been considered promising candidates for integrated optoelectronic systems. As one of the important photonic functions, optical phase modulation previously was demonstrated with perovskite substrate and complex refractive index engineering with laser scribing. Here we report on the new scheme of achieving efficient phase modulation by combining detour phase design with 40 nm ultrathin perovskite films composed of nanosized crystalline particles. Phase modulation was realized by binary amplitude patterning, which significantly simplifies the fabrication process. Perovskite nanocrystal films exhibit significantly weak ion migration effects under femtosecond laser writing, resulting in smooth edges along the laser ablated area and high diffractive optical quality. Fabrication of a detour-phased perovskite ultrathin planar lens with a diameter of 150 μm using femtosecond laser scribing was experimentally demonstrated. A high-performance 3D focus was observed, and the fabrication showed a high tolerance with different laser writing powers. Furthermore, the high-quality imaging capability of perovskite ultrathin planar lenses with a suppressed background was also demonstrated.

High‐Performance Inverted Perovskite Solar Cells Enhanced via Partial Replacement of Dimethyl Sulfoxide with N‐Methyl‐2‐Pyrrolidinone

Herein, a very simple but efficient method is developed to concurrently improve the quality of perovskite film and enhance the interfaces of perovskite solar cells (PSCs) concurrently via partially replacing dimethyl sulfoxide (DMSO) with N‐methyl‐2‐pyrrolidinone (NMP) in inverted planar PSCs. With the systematic investigations, it is found that the perovskite films fabricated with this simple method demonstrate higher crystallinity, larger grain size with less grain boundaries, and better perovskite–substrate interface with very rare voids. All of these lead to efficient all‐round defects suppression and high performance of the corresponding PSCs. As a result, the tri‐mixed solvent (DMF:DMSO:NMP = 4:0.75:0.25) processing renders a power conversion efficiency (PCE) of 21.53%, which is much higher than that of PSCs fabricated with traditional bi‐mixed solvent (DMF:DMSO = 4:1) (only 17.05%). Notably, the simple‐method‐fabricated PSCs demonstrate remarkable humidity and thermal stabilities even without any encapsulation (90.6% and 75.2% of initial PCE are retained after more than 511 h in air with relative humidity (RH) ≈ 50% at room temperature and 216 h in air with RH 70% at continuous heating of 70 °C, respectively). All these results demonstrate that the simple method by partially replacing DMSO with NMP is very efficient to improve the quality of perovskite layer and yield high‐performance PSCs.

Evaluation of La2Ce2O7 (LCO) Protection Layer for Proton Conducting Electrolyte

Due to the susceptible reaction BaCeO3-based proton-conductors toward steam and carbon dioxide, La2Ce2O7(LCO) is proposed as a protect layer on top of solid electrolyte since LCO has shown high proton conductivity with adequate stability against steam. In the study, La(NO3)3·6H₂O and Ce(NO3)3·6H₂O were used as precursors to obtain La2Ce2O7(LCO). Particle size analyzer and XRD are applied to analysis this result. From the XRD analysis, it can be seen at the temperature range above 900°C, the reflections representing fluorite-structured LCO appeared with other reflections from BaCe0.5Zr0.3Y0.2O3-δ(BCZY) perovskite substrate. The result can also demonstrate that there is no reaction between LCO layer and BCZY substrate. It is suggested that LCO exhibits good chemical compatibility with BCZY. This synergic bilayer electrolyte design may be a feasible design to overcome the instability of Zr- or Ce-rich Ba(CeZr)O3−δ electrolysis cells against steam and CO2 for better long-term performance. The stability and protection of La2Ce2O7(LCO) on BaCe0.5Zr0.3Y0.2O3-δ(BCZY) will be evaluated based on structural and electrical properties measurement under humid and CO2 containing environment as function of time and temperatures.

Graphene induced structure and doping level tuning of evaporated CsPbBr3 on different substrates

Tuning the structural and electronic properties of halide perovskite material is an effective method to promote its device performance. Increasing the grain size can decrease grain boundaries and further reduce defects. Regulating the band structure will improve the separation and extraction efficiency of carriers. In this work, high-quality CsPbBr3 films with increased crystallinity, better preferential orientation and improved carrier dynamics are achieved, using partial wetting transparent monolayer graphene as an interlayer. It is demonstrated that graphene can partially shield the surface potential field of the substrates, resulting in a changed preferred orientation of the CsPbBr3. The reduced interaction between the substrates and the perovskite effectively decreases the diffusion barrier of adatoms, increasing the grain size and crystallinity. Besides, the presence of graphene also tunes the band structure of the perovskite with increased p-type doping, which changes the band offset at perovskite substrate interface. An improved charge extraction can be achieved on perovskite/G/ITO interface due to increased band offset while a reduced charge extraction is observed on perovskite/G/Au interface. As a result, the corresponding photodetector with graphene demonstrates better performance than that without graphene. Our study will provide more ideas for tuning the perovskite structure and energy bands to achieve better device performance.

Reversibly Controlled Ternary Polar States and Ferroelectric Bias Promoted by Boosting Square-Tensile-Strain.

Interaction between dipoles often emerges intriguing physical phenomena, such as exchange bias in the magnetic heterostructures and magnetoelectric effect in multiferroics, which lead to advances in multifunctional heterostructures. However, the defect-dipole tends to be considered the undesired to deteriorate the electronic functionality. Here, deterministic switching between the ferroelectric and the pinched states by exploiting a new substrate of cubic perovskite, BaZrO3 is reported, which boosts the square-tensile-strain to BaTiO3 and promotes four-variants in-plane spontaneous polarization with oxygen vacancy creation. First-principles calculations propose a complex of an oxygen vacancy and two Ti3+ ions coins a charge-neutral defect-dipole. Cooperative control of the defect-dipole and the spontaneous polarization reveals ternary in-plane polar states characterized by biased/pinched hysteresis loops. Furthermore, it is experimentally demonstrated that three electrically controlled polar-ordering states lead to switchable and nonvolatile dielectric states for application of nondestructive electro-dielectric memory. This discovery opens a new route to develop functional materials via manipulating defect-dipoles and offers a novel platform to advance heteroepitaxy beyond the prevalent perovskite substrates.

Space‐Resolved Photoresponse in Quasi‐Two‐Dimensional Ruddlesden–Popper Perovskites

AbstractQuasi‐two‐dimensional (quasi‐2D) organic–inorganic hybrid perovskites have shown excellent ambient stability with decent photovoltaic performance. Further improvement of device level properties requires a comprehensive understanding of the performance‐limiting mechanisms such as phase segregation, ion/electron coupling conduction, and their effects on charge transport at both the micro‐ and macro‐scale. Here, space resolved investigations of Ruddlesden–Popper (RP) phases of the commonquasi‐2D perovskite are probed using two complementary methods, conducting atomic force microscopy (c‐AFM) and Kelvin probe force microscopy (KPFM). Bias‐driven photocurrent mappings obtained by c‐AFM measurements disclose local inhomogeneous conduction and hysteresis currents in quasi‐2D RP perovskites, while relatively uniform conductivity is observed on individual grains. Bias‐driven KPFM reveals that the surface average photovoltage sign is dominated by the band bending at the buried perovskite–substrate interface. The quasi‐2D RP film exhibits substantial variations in the spatial response of the photovoltage across grains and grain boundaries, which is direct evidence of the inherently benign nature of microstructures, and the final device performance. This research elucidates underlying space‐resolved photoresponse mechanisms behind the lower efficiency of quasi‐2D RP perovskites compared with the 3D perovskites, which is necessary for further development of efficient and stable 2D perovskite‐based devices.

Constructing Soft Perovskite-Substrate Interfaces for Dynamic Modulation of Perovskite Film in Inverted Solar Cells with Over 6200 Hours Photostability.

High‐performance perovskite solar cells (PSCs) depend heavily on the quality of perovskite films, which is closely related to the lattice distortion, perovskite crystallization, and interfacial defects when being spin‐coated and annealed on the substrate surface. Here, a dynamic strategy to modulate the perovskite film formation by using a soft perovskite–substrate interface constructed by employing amphiphilic soft molecules (ASMs) with long alkyl chains and Lewis base groups is proposed. The hydrophobic alkyl chains of ASMs interacted with poly(triarylamine) (PTAA) greatly improve the wettability of PTAA to facilitate the nucleation and growth of perovskite crystals, while the Lewis base groups bound to perovskite lattices significantly passivate the defects in situ. More importantly, this soft perovskite–substrate interface with ASMs between PTAA and perovskite film can dynamically match the lattice distortion with reduced interfacial residual strain upon perovskite crystallization and thermal annealing owing to the soft self‐adaptive long‐chains, leading to high‐quality perovskite films. Thus, the inverted PSCs show a power conversion efficiency approaching 20% with good reproducibility and negligible hysteresis. More impressively, the unencapsulated device exhibits state‐of‐the‐art photostability, retaining 84% of its initial efficiency under continuous simulated 1‐sun illumination for more than 6200 h at elevated temperature (≈65 °C).

Top-Layer Engineering Reshapes Charge Transfer at Polar Oxide Interfaces.

Charge-transfer phenomena at heterointerfaces are a promising pathway to engineer functionalities absent in bulk materials but can also lead to degraded properties in ultrathin films. Mitigating such undesired effects with an interlayer reshapes the interface architecture, restricting its operability. Therefore, developing less-invasive methods to control charge transfer will be beneficial. Here, an appropriate top-interface design allows for remote manipulation of the charge configuration of the buried interface and concurrent restoration of the ferromagnetic trait of the whole film. Double-perovskite insulating ferromagnetic La2 NiMnO6 (LNMO) thin films grown on perovskite oxide substrates are investigated as a model system. An oxygen-vacancy-assisted electronic reconstruction takes place initially at the LNMO polar interfaces. As a result, the magnetic properties of 2-5 unit cell LNMO films are affected beyond dimensionality effects. The introduction of a top electron-acceptor layer redistributes the electron excess and restores the ferromagnetic properties of the ultrathin LNMO films. Such a strategy can be extended to other interfaces and provides an advanced approach to fine-tune the electronic features of complex multilayered heterostructures.

Exsolution of phase-separated nanoparticles via trigger effect toward reversible solid oxide cell

The electrocatalytic characteristics of heterostructure nanoparticles have attracted attention for use in devices addressing energy and environmental issues. Among the various features, exsolved alloy or core–shell nanoparticles display high reactive surface area and strong interaction between the metal and substrate oxide. Herein, we report multifunctional heterostructure nanoparticles, namely Fe/Cu Janus nanoparticles (JN) and verify an exsolution trigger effect using Cu as a seed in the La0.43Sr0.37Fe0.09Cu0.03Ti0.88O3-δ (Fe75) perovskite substrate. During the exsolution process, the exsolved Cu particles played a key role in triggering the Fe exsolution by contributing additional surface energy. These Fe/Cu JNs exhibited outstanding catalytic activity in a reversible solid oxide cell fed with H2O/H2 and CO/CO2 fuels. A single cell with Fe75 showed an impressive current density of − 1.11 A cm−2 at 1.3 V and 900 °C. Our study experimentally elucidated the mechanism of the triggering of co-exsolution and demonstrated a multifunctional catalyst using Fe/Cu JNs.

Constructing highly active alloy-perovskite interfaces for efficient electrochemical CO2 reduction reaction

The electroreduction of CO2 through solid oxide electrolysis cells (SOEC) is considered as a promising technology for mitigating climate changes related to global warming. A SOEC system can effectively reduce polarization losses and best utilize process heat to electrolyze CO2 to produce CO. However, this technology is hindered by the sluggish cathode kinetics, which is mainly due to the poor catalytic activity for the CO2 reduction reaction. Herein, this work develops an A-site ordered layered perovskite oxide, (PrBa)0.95Fe1.6Ni0.2Nb0.2O5+δ (PBFNN), as an efficient cathode catalyst for CO2-SOEC. FeNi3 nanoparticles are in situ generated on the surface of perovskite substrate after reduction, the resulting active alloy-perovskite interfaces between FeNi3 and PBFNN substrate can effectively promote the CO2 reduction reaction (CO2RR). The nanoparticles and abundant oxygen vacancies of FeNi3-PBFNN significantly enhance CO2 adsorption and activation, which is verified by the CO2-temperature programmed desorption measurement and density functional theory calculations. Distribution of relaxation time analysis and electrochemical performance characterization indicate that this interface engineering at nanoscale greatly improves the catalytic activity of the cathode for CO2RR.

Formamidinium Halide Perovskite and Carbon Nitride Thin Films Enhance Photoreactivity under Visible Light Excitation.

Photochemical and photocatalytic activity of adsorbates on surfaces is strongly dependent on the nature of a given substrate and its resonant absorption of the (visible) light excitation. An observation is reported here of the visible light photochemical response of formamidinium lead bromide (FAPbBr3) halide perovskite and carbon nitride (CN) thin-film materials (deposited on a SiO2/Si(100) substrate), both of which are known for their photovoltaic and photocatalytic properties. The goal of this study was to investigate the role of the substrate in the photochemical reactivity of an identical probe molecule, ethyl chloride (EC), when excited by pulsed 532 nm laser under ultrahigh vacuum (UHV) conditions. Postirradiation temperature-programmed desorption (TPD) measurements have indicated that the C–Cl bond dissociates following the visible light excitation to form surface-bound fragments that react upon surface heating to form primarily ethane and butane. Temperature-dependent photoluminescence (PL) spectra of the FAPbBr3 films were recorded and decay lifetimes were measured, revealing a correlation between length of PL decay and the photoreactivity yield. We conclude that the FAPbBr3 material with its absorption spectrum in resonance with visible light excitation (532 nm) and longer PL lifetime leads to three times faster (larger cross-section) photoproduct formation compared with that on the CN substrate. These results contrast the behavior under ambient conditions where the CN materials are photochemically superior due, primarily, to their stability within humid environments.

Synergistic interaction between in situ exsolved and phosphorized nanoparticles and perovskite oxides for enhanced electrochemical water splitting

Highly active and stable catalysts towards electrochemical water-splitting are critical for the sustainable energy conversion and storage. Perovskite oxides with in situ exsolved metal nanoparticles have emerged as promising candidates, while their activities are still unsatisfying and mechanisms unclear. Here we develop a novel hybrid catalyst with the in situ exsolved and phosphorized Co nanoparticles on Sr2FeMoO6 perovskite oxide surface based on density functional theory predictions, and use it as a model system to investigate the particle-substrate interactions with combined experimental studies and theoretical calculations. The as-prepared electrocatalyst exhibits boosted activity and stability for oxygen evolution reaction, hydrogen evolution reaction and overall water-splitting in alkaline solution, comparable to these of precious-metal catalysts. Further studies reveal the phase transformation during catalysis and the strong interactions between the phosphorized Co nanoparticles and perovskite oxide substrate. The modulated band structure, redistributed charge density and spillover effect lead to the enhanced charge transfer, increased O covalency, optimized intermediate adsorption and reduced energy barrier, enhancing the overall activity synergistically.

Influence of Halide Choice on Formation of Low‐Dimensional Perovskite Interlayer in Efficient Perovskite Solar Cells

Recent advances in heterojunction and interfacial engineering of perovskite solar cells (PSCs) have enabled great progress in developing highly efficient and stable devices. Nevertheless, the effect of halide choice on the formation mechanism, crystallography, and photoelectric properties of the low‐dimensional phase still requires further detailed study. In this work, we present key insights into the significance of halide choice when designing passivation strategies comprising large organic spacer salts, clarifying the effect of anions on the formation of quasi‐2D/3D heterojunctions. To demonstrate the importance of halide influences, we employ novel neo‐pentylammonium halide salts with different halide anions (neoPAX, X=I, Br, or Cl). We find that regardless of halide selection, iodide‐based (neoPA)2(FA)(n‐1)PbnI(3n+1) phases are formed above the perovskite substrate, while the added halide anions diffuse and passivate the perovskite bulk. In addition, we also find the halide choice has an influence on the degree of dimensionality (n). Comparing the three halides, we find that chloride‐based salts exhibit superior crystallographic, enhanced carrier transport, and extraction compared to the iodide and bromide analogs. As a result, we report high power conversion efficiency in quasi‐2D/3D PSCs, which are optimal when using chloride salts, reaching up to 23.35%, and improving long‐term stability.

Oxygen Reduction Electrocatalysis with Epitaxially Grown Spinel MnFe2O4 and Fe3O4

Nanocrystalline MnFe$_{2}$O$_{4}$ has shown promise as a catalyst for the oxygen reduction reaction (ORR) in alkaline solutions, but the material has been lightly studied as highly ordered thin film catalysts. To examine the role of surface termination and Mn and Fe site occupancy, epitaxial MnFe$_{2}$O$_{4}$ and Fe$_{3}$O$_{4}$ spinel oxide films were grown on (001) and (111) oriented Nb:SrTiO$_{3}$ perovskite substrates using molecular beam epitaxy and studied as electrocatalysts for the oxygen reduction reaction (ORR). HRXRD and XPS show synthesis of pure phase materials while STEM and RHEED analysis demonstrate island-like growth of (111) surface terminated pyramids on both (001) and (111) oriented substrates, consistent with the literature and attributed to lattice mismatch between the spinel films and perovskite substrate. Cyclic voltammograms under an N$_{2}$ atmosphere revealed distinct redox features for Mn and Fe surface termination based on comparison of MnFe$_{2}$O$_{4}$ and Fe$_{3}$O$_{4}$. Under O$_{2}$ atmosphere, electrocatalytic reduction of oxygen was observed at both Mn and Fe redox features; however, diffusion limited current was only achieved at potentials consistent with Fe reduction. This result contrasts with that of nanocrystalline MnFe$_{2}$O$_{4}$ reported in the literature where diffusion limited current is achieved with Mn-based catalysis. This difference is attributed to a low density of Mn surface termination, as determined by the integration of current from CVs collected under N$_{2}$, in addition to low conductivity through the MnFe$_{2}$O$_{4}$ film due to the degree of inversion. Such low densities are attributed to the synthetic method and island-like growth pattern and highlight challenges in studying ORR catalysis with single-crystal spinel materials.

Single-atom sites on perovskite chips for record-high sensitivity and quantification in SERS

Surface enhanced Raman scattering (SERS) is a rapid and nondestructive technique that is capable of detecting and identifying chemical or biological compounds. Sensitive SERS quantification is vital for practical applications, particularly for portable detection of biomolecules such as amino acids and nucleotides. However, few approaches can achieve sensitive and quantitative Raman detection of these most fundamental components in biology. Herein, a noble-metal-free single-atom site on a chip strategy was applied to modify single tungsten atom oxide on a lead halide perovskite, which provides sensitive SERS quantification for various analytes, including rhodamine, tyrosine and cytosine. The single-atom site on a chip can enable quantitative linear SERS responses of rhodamine (10−6−1 mmol L−1), tyrosine (0.06–1 mmol L−1) and cytosine (0.2–45 mmol L−1), respectively, which all achieve record-high enhancement factors among plasmonic-free semiconductors. The experimental test and theoretical simulation both reveal that the enhanced mechanism can be ascribed to the controllable single-atom site, which can not only trap photoinduced electrons from the perovskite substrate but also enhance the highly efficient and quantitative charge transfer to analytes. Furthermore, the label-free strategy of single-atom sites on a chip can be applied in a portable Raman platform to obtain a sensitivity similar to that on a benchtop instrument, which can be readily extended to various biomolecules for low-cost, widely demanded and more precise point-of-care testing or in-vitro detection.Electronic Supplementary MaterialSupplementary material is available for this article at 10.1007/s40843-022-1968-5 and is accessible for authorized users.