Showing papers on "Grain boundary published in 2017"

Recombination in Perovskite Solar Cells: Significance of Grain Boundaries, Interface Traps, and Defect Ions

Abstract: Trap-assisted recombination, despite being lower as compared with traditional inorganic solar cells, is still the dominant recombination mechanism in perovskite solar cells (PSCs) and limits their efficiency. We investigate the attributes of the primary trap-assisted recombination channels (grain boundaries and interfaces) and their correlation to defect ions in PSCs. We achieve this by using a validated device model to fit the simulations to the experimental data of efficient vacuum-deposited p–i–n and n–i–p CH3NH3PbI3 solar cells, including the light intensity dependence of the open-circuit voltage and fill factor. We find that, despite the presence of traps at interfaces and grain boundaries (GBs), their neutral (when filled with photogenerated charges) disposition along with the long-lived nature of holes leads to the high performance of PSCs. The sign of the traps (when filled) is of little importance in efficient solar cells with compact morphologies (fused GBs, low trap density). On the other hand,...

Scaling behavior of moisture-induced grain degradation in polycrystalline hybrid perovskite thin films

Abstract: The stability of perovskite solar cells has shown a huge variation with respect to the film process and film morphology, while the underlining mechanism for the morphology-dependent degradation of the perovskite film has remained elusive. Herein, we report a scaling behavior of moisture-induced grain degradation in polycrystalline CH3NH3PbI3 films. The degradation rates of CH3NH3PbI3 films in moisture were shown to be sensitive to the grain sizes. The duration that was needed for different films to degrade by the same percent showed a linear relationship with the grain size, despite the fact that the films were formed by five different deposition methods. This scaling behavior can be explained by the degradation along the in-plane direction, which is initiated at the grain boundary (GB). The GBs of CH3NH3PbI3 films consist of an amorphous intergranular layer, which allows quick diffusion of moisture into the perovskite films. It was found that thermal annealing induced surface self-passivation plays a critical role in stabilizing the surfaces of thin films and single crystals by reducing the moisture-sensitive methylammonium ions at the surface. The determination of the scaling behavior of grain degradation highlights the importance of stabilizing the GBs to improve the stability of perovskite solar cells.

Intragranular cracking as a critical barrier for high-voltage usage of layer-structured cathode for lithium-ion batteries.

Abstract: LiNi1/3Mn1/3Co1/3O2-layered cathode is often fabricated in the form of secondary particles, consisting of densely packed primary particles. This offers advantages for high energy density and alleviation of cathode side reactions/corrosions, but introduces drawbacks such as intergranular cracking. Here, we report unexpected observations on the nucleation and growth of intragranular cracks in a commercial LiNi1/3Mn1/3Co1/3O2 cathode by using advanced scanning transmission electron microscopy. We find the formation of the intragranular cracks is directly associated with high-voltage cycling, an electrochemically driven and diffusion-controlled process. The intragranular cracks are noticed to be characteristically initiated from the grain interior, a consequence of a dislocation-based crack incubation mechanism. This observation is in sharp contrast with general theoretical models, predicting the initiation of intragranular cracks from grain boundaries or particle surfaces. Our study emphasizes that maintaining structural stability is the key step towards high-voltage operation of layered-cathode materials.

Selective increase in CO2 electroreduction activity at grain-boundary surface terminations

Abstract: Altering a material’s catalytic properties requires identifying structural features that give rise to active surfaces. Grain boundaries create strained regions in polycrystalline materials by stabilizing dislocations and may provide a way to create high-energy surfaces for catalysis that are kinetically trapped. Although grain-boundary density has previously been correlated with catalytic activity for some reactions, direct evidence that grain boundaries create surfaces with enhanced activity is lacking. We used a combination of bulk electrochemical measurements and scanning electrochemical cell microscopy with submicrometer resolution to show that grain-boundary surface terminations in gold electrodes are more active than grain surfaces for electrochemical carbon dioxide (CO 2 ) reduction to carbon monoxide (CO) but not for the competing hydrogen (H 2 ) evolution reaction. The catalytic footprint of the grain boundary is commensurate with its dislocation-induced strain field, providing a strategy for broader exploitation of grain-boundary effects in heterogeneous catalysis.

Grain boundary stability governs hardening and softening in extremely fine nanograined metals

Abstract: Conventional metals become harder with decreasing grain sizes, following the classical Hall-Petch relationship. However, this relationship fails and softening occurs at some grain sizes in the nanometer regime for some alloys. In this study, we discovered that plastic deformation mechanism of extremely fine nanograined metals and their hardness are adjustable through tailoring grain boundary (GB) stability. The electrodeposited nanograined nickel-molybdenum (Ni–Mo) samples become softened for grain sizes below 10 nanometers because of GB-mediated processes. With GB stabilization through relaxation and Mo segregation, ultrahigh hardness is achieved in the nanograined samples with a plastic deformation mechanism dominated by generation of extended partial dislocations. Grain boundary stability provides an alternative dimension, in addition to grain size, for producing novel nanograined metals with extraordinary properties.

Vacancy-induced dislocations within grains for high-performance PbSe thermoelectrics.

Abstract: To minimize the lattice thermal conductivity in thermoelectrics, strategies typically focus on the scattering of low-frequency phonons by interfaces and high-frequency phonons by point defects. In addition, scattering of mid-frequency phonons by dense dislocations, localized at the grain boundaries, has been shown to reduce the lattice thermal conductivity and improve the thermoelectric performance. Here we propose a vacancy engineering strategy to create dense dislocations in the grains. In Pb1-xSb2x/3Se solid solutions, cation vacancies are intentionally introduced, where after thermal annealing the vacancies can annihilate through a number of mechanisms creating the desired dislocations homogeneously distributed within the grains. This leads to a lattice thermal conductivity as low as 0.4 Wm-1 K-1 and a high thermoelectric figure of merit, which can be explained by a dislocation scattering model. The vacancy engineering strategy used here should be equally applicable for solid solution thermoelectrics and provides a strategy for improving zT.

Top and bottom surfaces limit carrier lifetime in lead iodide perovskite films

Abstract: Carrier recombination at defects is detrimental to the performance of solar energy conversion systems, including solar cells and photoelectrochemical devices. Point defects are localized within the bulk crystal while extended defects occur at surfaces and grain boundaries. If not properly managed, surfaces can be a large source of carrier recombination. Separating surface carrier dynamics from bulk and/or grain-boundary recombination in thin films is challenging. Here, we employ transient reflection spectroscopy to measure the surface carrier dynamics in methylammonium lead iodide perovskite polycrystalline films. We find that surface recombination limits the total carrier lifetime in perovskite polycrystalline thin films, meaning that recombination inside grains and/or at grain boundaries is less important than top and bottom surface recombination. The surface recombination velocity in polycrystalline films is nearly an order of magnitude smaller than that in single crystals, possibly due to unintended surface passivation of the films during synthesis. Understanding surface carrier dynamics enables the design of optimal optoelectronic devices. Yang et al. find that surface recombination limits the total carrier lifetime in polycrystalline lead iodide perovskite films, meaning recombination at surfaces is more important than within and between grains.

Phase segregation enhanced ion movement in efficient inorganic CsPbIBr2 solar cells

Abstract: Organic–inorganic hybrid perovskite solar cells with mixed cations and mixed halides have achieved impressive power conversion efficiency of up to 22.1%. Phase segregation due to the mixed compositions has attracted wide concerns, and their nature and origin are still unclear. Some very useful analytical techniques are controversial in microstructural and chemical analyses due to electron beam-induced damage to the “soft” hybrid perovskite materials. In this study photoluminescence, cathodoluminescence, and transmission electron microscopy are used to study charge carrier recombination and retrieve crystallographic and compositional information for all-inorganic CsPbIBr2 films on the nanoscale. It is found that under light and electron beam illumination, “iodide-rich” CsPbI(1+x)Br(2−x) phases form at grain boundaries as well as segregate as clusters inside the film. Phase segregation generates a high density of mobile ions moving along grain boundaries as ion migration “highways.” Finally, these mobile ions can pile up at the perovskite/TiO2 interface resulting in formation of larger injection barriers, hampering electron extraction and leading to strong current density–voltage hysteresis in the polycrystalline perovskite solar cells. This explains why the planar CsPbIBr2 solar cells exhibit significant hysteresis in efficiency measurements, showing an efficiency of up to 8.02% in the reverse scan and a reduced efficiency of 4.02% in the forward scan, and giving a stabilized efficiency of 6.07%.

Mixed Cation FAxPEA1–xPbI3 with Enhanced Phase and Ambient Stability toward High-Performance Perovskite Solar Cells

Abstract: In this work, different from the commonly explored strategy of incorporating a smaller cation, MA+ and Cs+ into FAPbI3 lattice to improve efficiency and stability, it is revealed that the introduction of phenylethylammonium iodide (PEAI) into FAPbI3 perovksite to form mixed cation FAxPEA1–xPbI3 can effectively enhance both phase and ambient stability of FAPbI3 as well as the resulting performance of the derived devices. From our experimental and theoretical calculation results, it is proposed that the larger PEA cation is capable of assembling on both the lattice surface and grain boundaries to form quais-3D perovskite structures. The surrounding of PEA+ ions at the crystal grain boundaries not only can serve as molecular locks to tighten FAPbI3 domains but also passivate the surface defects to improve both phase and moisture stablity. Consequently, a high-performance (PCE:17.7%) and ambient stable FAPbI3 solar cell could be developed.

Direct Evidence of Ion Diffusion for the Silver-Electrode-Induced Thermal Degradation of Inverted Perovskite Solar Cells

Abstract: Perovskite solar cells (PSCs) have recently demonstrated high efficiencies of over 22%, but the thermal stability is still a major challenge for commercialization. In this work, the thermal degradation process of the inverted structured PSCs induced by the silver electrode is thoroughly investigated. Elemental depth profiles indicate that iodide and methylammonium ions diffuse through the electron-trasnporting layer and accumulate at the Ag inner surface. The driving force of forming AgI then facilitates the ions extraction. Variations on the morphology and current mapping of the MAPbI3 thin films upon thermal treatment reveal that the loss of ions occurs at the grain boundaries and leads to the reconstruction of grain domains. Consequently, the deteriorated MAPbI3 thin film, the poor electron extraction, and the generation of AgI barrier result in the degradation of efficiencies. These direct evidences provide in-depth understanding of the effect of thermal stress on the devices, offering both experimental support and theoretical guidance for the improvement on the thermal stability of the inverted PSCs.

Deciphering chemical order/disorder and material properties at the single-atom level

Abstract: Perfect crystals are rare in nature. Real materials often contain crystal defects and chemical order/disorder such as grain boundaries, dislocations, interfaces, surface reconstructions and point defects. Such disruption in periodicity strongly affects material properties and functionality. Despite rapid development of quantitative material characterization methods, correlating three-dimensional (3D) atomic arrangements of chemical order/disorder and crystal defects with material properties remains a challenge. On a parallel front, quantum mechanics calculations such as density functional theory (DFT) have progressed from the modelling of ideal bulk systems to modelling 'real' materials with dopants, dislocations, grain boundaries and interfaces; but these calculations rely heavily on average atomic models extracted from crystallography. To improve the predictive power of first-principles calculations, there is a pressing need to use atomic coordinates of real systems beyond average crystallographic measurements. Here we determine the 3D coordinates of 6,569 iron and 16,627 platinum atoms in an iron-platinum nanoparticle, and correlate chemical order/disorder and crystal defects with material properties at the single-atom level. We identify rich structural variety with unprecedented 3D detail including atomic composition, grain boundaries, anti-phase boundaries, anti-site point defects and swap defects. We show that the experimentally measured coordinates and chemical species with 22 picometre precision can be used as direct input for DFT calculations of material properties such as atomic spin and orbital magnetic moments and local magnetocrystalline anisotropy. This work combines 3D atomic structure determination of crystal defects with DFT calculations, which is expected to advance our understanding of structure-property relationships at the fundamental level.

Low-Noise and Large-Linear-Dynamic-Range Photodetectors Based on Hybrid-Perovskite Thin-Single-Crystals.

Abstract: Organic-inorganic halide perovskites are promising photodetector materials due to their strong absorption, large carrier mobility, and easily tunable bandgap. Up to now, perovskite photodetectors are mainly based on polycrystalline thin films, which have some undesired properties such as large defective grain boundaries hindering the further improvement of the detector performance. Here, perovskite thin-single-crystal (TSC) photodetectors are fabricated with a vertical p-i-n structure. Due to the absence of grain-boundaries, the trap densities of TSCs are 10-100 folds lower than that of polycrystalline thin films. The photodetectors based on CH3 NH3 PbBr3 and CH3 NH3 PbI3 TSCs show low noise of 1-2 fA Hz-1/2 , yielding a high specific detectivity of 1.5 × 1013 cm Hz1/2 W-1 . The absence of grain boundaries reduces charge recombination and enables a linear response under strong light, superior to polycrystalline photodetectors. The CH3 NH3 PbBr3 photodetectors show a linear response to green light from 0.35 pW cm-2 to 2.1 W cm-2 , corresponding to a linear dynamic range of 256 dB.

Anisotropic tensile behavior of in situ precipitation strengthened Inconel 718 fabricated by additive manufacturing

Abstract: This study investigated the microstructure and anisotropic mechanical properties of selective laser melting (SLM) processed Inconel 718 (IN718) component. In as-fabricated alloys, ultrafine columnar grained microstructure with highly dispersed precipitates γ" phases at grains boundary and even-distributed γ' phases inside the grains were observed. It was demonstrated that the as-fabricated longitudinal samples showed lower ultimate tensile strength (UTS) of 1101 MPa but higher elongation of 24.5% compared to the transverse samples which showed UTS of 1167 MPa and elongation of 21.5%. The excellent mechanical properties of both the longitudinal and transverse samples can be ascribed to the refined microstructure of the SLM material resulting from the high cooling rate imposed by laser processing. The anisotropy in strength and ductility was attributed to the {100} fiber texture and columnar grain morphology. The {100} fiber texture of columnar grains leads to high strength in transverse direction, while the columnar grain boundaries also served as a path along which damage can preferentially accumulate, leading to fracture.

Skutterudite with graphene-modified grain-boundary complexion enhances zT enabling high-efficiency thermoelectric device

Abstract: Skutterudite materials are widely considered for thermoelectric waste heat recovery. While the skutterudite structure effectively scatters the high frequency phonons, grain-boundary engineering is needed to further reduce the thermal conductivity beyond simply decreasing grain size. Here, we show that reduced graphene oxide (rGO) increases the grain boundary thermal resistivity by a factor of 3 to 5 compared to grain boundaries without graphene. Wrapping even micron sized grains with graphene leads to such a significant reduction in the thermal conductivity that a high thermoelectric figure of merit zT = 1.5 was realized in n-type YbyCo4Sb12, while a zT of 1.06 was achieved in p-type CeyFe3CoSb12. A 16 leg thermoelectric module was made by using n- and p-type skutterudite–graphene nanocomposites that exhibited conversion efficiency 24% higher than a module made without graphene. Engineering grain boundary complexions with 2-D materials introduces a new strategy for advanced thermoelectric materials.

SLM-processed Sc- and Zr- modified Al-Mg alloy: Mechanical properties and microstructural effects of heat treatment

Abstract: Traditionally 4xxx casting alloys are used for the additive manufacturing of structurally optimised lightweight parts in space, aerospace and automotive. However, for such applications there is a need for hardenable high-strength Al-alloys exceeding the properties of the 4xxx alloys family. The study analyses the hardness response of different heat treatment temperatures and hold durations applied to a Sc- and Zr-modified Al-Mg (5xxx-) alloy (Scalmalloy®) processed by Selective Laser Melting, and compares the mechanical properties and microstructure in the as-processed and annealed condition, and these properties are clearly related to the very fine grained microstructure. The results show that the static mechanical properties are exceptionally good with R m -values exceeding 500 MPa along with almost no build-orientation related anisotropic effects, and a high ductility even in the heat treated condition. These properties are clearly related to the very fine grained material, along with the good hardenability of the alloy. The stress-strain curves show the typical Portevin-Le-Chatelier (PLC) effect as known for other 5xxx alloys. Due to significant grain boundary pinning by different particles the very fine-grained bi-modal microstructure originating from the SLM-process can be maintained even in the heat treated condition, and only a HIP treatment leads to local grain growth only in coarser grained regions.

Coexistence of Grain-Boundaries-Assisted Bipolar and Threshold Resistive Switching in Multilayer Hexagonal Boron Nitride

Abstract: The use of 2D materials to improve the capabilities of electronic devices is a promising strategy that has recently gained much interest in both academia and industry. However, while the research in 2D metallic and semiconducting materials is well established, detailed knowledge and applications of 2D insulators are still scarce. In this paper, the presence of resistive switching (RS) in multilayer hexagonal boron nitride (h-BN) is studied using different electrode materials, and a family of h-BN-based resistive random access memories with tunable capabilities is engineered. The devices show the coexistence of forming free bipolar and threshold-type RS with low operation voltages down to 0.4 V, high current on/off ratio up to 106, and long retention times above 10 h, as well as low variability. The RS is driven by the grain boundaries (GBs) in the polycrystalline h-BN stack, which allow the penetration of metallic ions from adjacent electrodes. This reaction can be boosted by the generation of B vacancies, which are more abundant at the GBs. To the best of our knowledge, h-BN is the first 2D material showing the coexistence of bipolar and threshold RS, which may open the door to additional functionalities and applications.

Grain Structure Control of Additively Manufactured Metallic Materials.

Abstract: Grain structure control is challenging for metal additive manufacturing (AM). Grain structure optimization requires the control of grain morphology with grain size refinement, which can improve the mechanical properties of additive manufactured components. This work summarizes methods to promote fine equiaxed grains in both the additive manufacturing process and subsequent heat treatment. Influences of temperature gradient, solidification velocity and alloy composition on grain morphology are discussed. Equiaxed solidification is greatly promoted by introducing a high density of heterogeneous nucleation sites via powder rate control in the direct energy deposition (DED) technique or powder surface treatment for powder-bed techniques. Grain growth/coarsening during post-processing heat treatment can be restricted by presence of nano-scale oxide particles formed in-situ during AM. Grain refinement of martensitic steels can also be achieved by cyclic austenitizing in post-processing heat treatment. Evidently, new alloy powder design is another sustainable method enhancing the capability of AM for high-performance components with desirable microstructures.

Impact of grain boundaries on efficiency and stability of organic-inorganic trihalide perovskites

Abstract: Organic-inorganic perovskite solar cells have attracted tremendous attention because of their remarkably high power conversion efficiencies. To further improve device performance, it is imperative to obtain fundamental understandings on the photo-response and long-term stability down to the microscopic level. Here, we report the quantitative nanoscale photoconductivity imaging on two methylammonium lead triiodide thin films with different efficiencies by light-stimulated microwave impedance microscopy. The microwave signals are largely uniform across grains and grain boundaries, suggesting that microstructures do not lead to strong spatial variations of the intrinsic photo-response. In contrast, the measured photoconductivity and lifetime are strongly affected by bulk properties such as the sample crystallinity. As visualized by the spatial evolution of local photoconductivity, the degradation process begins with the disintegration of grains rather than nucleation and propagation from visible boundaries between grains. Our findings provide insights to improve the electro-optical properties of perovskite thin films towards large-scale commercialization.

Grain Boundary Engineering for Achieving High Thermoelectric Performance in n-Type Skutterudites

Abstract: Grain or phase boundaries play a critical role in the carrier and phonon transport in bulk thermoelectric materials. Previous investigations about controlling boundaries primarily focused on the reducing grain size or forming nanoinclusions. Herein, liquid phase compaction method is first used to fabricate the Yb-filled CoSb3 with excess Sb content, which shows the typical feature of low-angle grain boundaries with dense dislocation arrays. Seebeck coefficients show a dramatic increase via energy filtering effect through dislocation arrays with little deterioration on the carrier mobility, which significantly enhances the power factor over a broad temperature range with a high room-temperature value around 47 μW cm−2 K−1. Simultaneously, the lattice thermal conductivity could be further suppressed via scattering phonons via dense dislocation scattering. As a result, the highest average figure of merit ZT of ≈1.08 from 300 to 850 K could be realized, comparable to the best reported result of single or triple-filled Skutterudites. This work clearly points out that low-angle grain boundaries fabricated by liquid phase compaction method could concurrently optimize the electrical and thermal transport properties leading to an obvious enhancement of both power factor and ZT.

Structure, Texture and Phases in 3D Printed IN718 Alloy Subjected to Homogenization and HIP Treatments

Abstract: 3D printing results in anisotropy in the microstructure and mechanical properties The focus of this study is to investigate the structure, texture and phase evolution of the as-printed and heat treated IN718 superalloy Cylindrical specimens, printed by powder-bed additive manufacturing technique, were subjected to two post-treatments: homogenization (1100 °C, 1 h, furnace cooling) and hot isostatic pressing (HIP) (1160 °C, 100 MPa, 4 h, furnace cooling) The Selective laser melting (SLM) printed microstructure exhibited a columnar architecture, parallel to the building direction, due to the heat flow towards negative z-direction Whereas, a unique structural morphology was observed in the x-y plane due to different cooling rates resulting from laser beam overlapping Post-processing treatments reorganized the columnar structure of a strong {002} texture into fine columnar and/or equiaxed grains of random orientations Equiaxed structure of about 150 µm average grain size, was achieved after homogenization and HIP treatments Both δ-phase and MC-type brittle carbides, having rough morphologies, were formed at the grain boundaries Delta-phase formed due to γ″-phase dissolution in the γ matrix, while MC-type carbides nucleates grew by diffusion of solute atoms The presence of (Nb078Ti022)C carbide phase, with an fcc structure having a lattice parameter a = 443 A, was revealed using Energy dispersive spectrometer (EDS) and X-ray diffractometer (XRD) analysis The solidification behavior of IN718 alloy was described to elucidate the evolution of different phases during selective laser melting and post-processing heat treatments of IN718

Dielectric relaxation and Maxwell-Wagner interface polarization in Nb2O5 doped 0.65BiFeO3–0.35BaTiO3 ceramics

Abstract: Electrical characterizations of Nb2O5 doped 0.65BiFeO3–0.35BaTiO3 (0.65BF–0.35BT) ceramic were carried out over broad temperature and frequency ranges through dielectric spectroscopy, impedance spectroscopy, and ac conductivity measurements. The dielectric constant and loss tangent are drastically reduced with introducing Nb2O5 into the 0.65BF–0.35BT system. Two dielectric anomalies are detected in the temperature regions of 100 °C ≤ T ≤ 280 °C and 350 °C ≤ T ≤ 480 °C, and the Curie temperature (TC) was confirmed in higher temperature region. A dielectric relaxation with large dielectric constants was detected near the TC. This dielectric relaxation becomes even stronger with the gradual increase in the Nb2O5 content. Impedance spectroscopy results clearly show the contributions of grains and grain boundaries in the frequency range of 100 Hz ≤ f ≤ 1 MHz, and the relaxation processes for grains and grain boundaries are non-Debye-type. The grain boundaries are more resistive than that of the grains, revealing the inhomogeneity in samples. The experimental results are well fitted based on a Maxwell-Wagner (MW) interfacial polarization model below 100 kHz, and the MW interfacial polarization effect becomes more and more obvious with the increase in the Nb2O5 content. The increase in dielectric constant is possibly related to space charge polarization, which is caused by charges accumulated at the interface between the grain and grain boundaries. Frequency dependence of the ac conductivity confirms the MW interfacial polarization effect below 100 kHz.

Tailoring the thermal and electrical transport properties of graphene films by grain size engineering

Abstract: Understanding the influence of grain boundaries (GBs) on the electrical and thermal transport properties of graphene films is essentially important for electronic, optoelectronic and thermoelectric applications. Here we report a segregation-adsorption chemical vapour deposition method to grow well-stitched high-quality monolayer graphene films with a tunable uniform grain size from ∼200 nm to ∼1 μm, by using a Pt substrate with medium carbon solubility, which enables the determination of the scaling laws of thermal and electrical conductivities as a function of grain size. We found that the thermal conductivity of graphene films dramatically decreases with decreasing grain size by a small thermal boundary conductance of ∼3.8 × 109 W m-2 K-1, while the electrical conductivity slowly decreases with an extraordinarily small GB transport gap of ∼0.01 eV and resistivity of ∼0.3 kΩ μm. Moreover, the changes in both the thermal and electrical conductivities with grain size change are greater than those of typical semiconducting thermoelectric materials.