Grain Boundary Groove Research Articles

Smoothed grain boundary grooves, passivated defects and released compressive stresses via bulk molecule doping for efficient perovskite solar cells

In preparing efficient and stable perovskite solar cells (PSCs), it is challenging to simultaneously overcome the inherent problems of grain boundary grooves (GBGs), residual stress, defect states in perovskite film. Herein, a small organic molecule additive, pyridinium 1,4-zwitterionic thiolates (PZT), is synthesized to prepare high-quality PZT-doped perovskite active layer through bulk molecule doping method. The PZT promotes the crystallization of perovskite, optimizes the crystal orientation, and smooths the GBGs of perovskite films. Meanwhile, PZT releases compressive stress and passivates the perovskite defects at the surface/interface as well as internal grain boundaries in the perovskite film, which inhibits the charge recombination and promotes the charge separation and transport efficiency in the bulk perovskite films through the atomic interaction at the perovskite/PZT interface. In addition, the PZT also inhibits ion migration in perovskite film and increases film hydrophobicity. Benefiting from the above benefits, the PSCs based on perovskite:PZT layer achieved a maximum power conversion efficiency of 23.45%, much higher than that (21.79%) of control device. They also show significantly improved long-term stability under different aging test conditions. Our work provides a guide for improving the performance of PSCs by designing suitable multifunctional molecules.

Polycrystallinity Enhances Stress Buildup around Ice.

Damage caused by freezing wet, porous materials is a widespread problem but is hard to predict or control. Here, we show that polycrystallinity significantly speeds up the stress buildup process that underpins this damage. Unfrozen water in grain-boundary grooves feeds ice growth at temperatures below the freezing temperature, leading to fast stress buildup. These stresses can build up to levels that can easily break many brittle materials. The dynamics of the process are very variable, which we ascribe to local differences in ice-grain orientation and to the surprising mobility of many grooves-which further accelerates stress buildup. Our Letter will help understand how freezing damage occurs and in developing accurate models and effective damage-mitigation strategies.

Phase-field modelling of electromigration-induced intergranular slit propagation in metal interconnects

We use a phase-field method to elucidate grain-boundary grooving as a mechanism of genesis and subsequent propagation of intergranular slit in metal interconnects under concurrent surface and grain boundary diffusion. Surface diffusion is assumed to be the rate limiting transport mechanism. Accelerated grain-boundary grooving induced by electromigration is shown to ensue a narrow channel-like slit which advances at a steady state along the grain-boundary preserving its shape near the tip region. The slit characteristics namely the width and velocity dependence on electric field derived are shown to be in excellent agreement with the sharp interface calculations which treat the tip region of the slit independently. Furthermore, the simulations reveal that for the same magnitude of electric field, a slit conceived from a smaller grain exhibits a slower kinetics and delay damage dissemination. The apparent discrepancy from the sharp interface description is resolved on the basis of curvature gradient and electromigration-induced surface flux which heal the root during groove to slit transition and is predominant in smaller grains. Finally, drawing analogy from the nucleation and growth models of electromigration voids, failure due to intergranular slit is divided into serial process of initial grooving followed by slit propagation stage. The lifetime analysis suggests the grooving stage to be the rate determining step in the failure process, yielding an exponent of 1.33 in Black’s law.

Tungstate‐mediated In‐situ Passivation of Grain Boundary Grooves in Perovskite Solar Cells

AbstractPossessed with advantageous optoelectronic properties, perovskites have boosted the rapid development of solution‐processed solar cells. The performance of perovskite solar cells (PSCs) is significantly weakened by the carrier loss at grain boundary grooves (GBGs); however, it receives limited attention and there lacks effective approach to solve this issue. Herein, for the first time, we constructed the tungstate/perovskite heterointerface via a “two step” in situ reaction approach that provides effective defect passivation and ensures efficient carrier dynamics at the GBGs. The exposed perovskite at grain boundaries is converted to wide‐band‐gap PbWO4 via an in‐situ reaction between Pb2+ and tungstate ions, which passivate defects due to the strong ionic bonding. Moreover, recombination loss is further suppressed via the heterointerface energetics modification based on an additional transformation from PbWO4 to CaWO4. PSCs based on this groove modification strategy showed good universality in both normal and inverted structure, with an improved efficiency of 23.25 % in the n‐i‐p device and 23.33 % in the p‐i‐n device. Stable power output of the modified device could maintain 91.7 % after around 1100 h, and the device efficiency could retain 92.5 % after aging in air for around 2110 h, and 93.1 % after aging at 85 °C in N2 for 972 h.

Tungstate-mediated In-situ Passivation of Grain Boundary Grooves in Perovskite Solar Cells.

Possessed with advantageous optoelectronic properties, perovskites have boosted the rapid development of solution-processed solar cells. The performance of perovskite solar cells (PSCs) is significantly weakened by the carrier loss at grain boundary grooves (GBGs); however, it receives limited attention and there lacks effective approach to solve this issue. Herein, for the first time, we constructed the tungstate/perovskite heterointerface via a "two step" in-situ reaction approach that provides effective defect passivation and ensures efficient carrier dynamics at the GBGs. The exposed perovskite at grain boundaries is converted to wide-band-gap PbWO4 via an in-situ reaction between Pb2+ and tungstate ions, which passivate defects due to the strong ionic bonding. Moreover, recombination loss is further suppressed via the heterointerface energetics modification based on an additional transformation from PbWO4 to CaWO4. PSCs based on this groove modification strategy showed good universality in both normal and inverted structure, with an improved efficiency of 23.25% in the n-i-p device and 23.33% in the p-i-n device. Stable power output of the modified device could maintain 91.7% after ~1100 h, and the device efficiency could retain 92.5% after aging in air for ~ 2110 h, and 93.1% after aging at 85 °C in N2 for 972 h.

Effects of a high magnetic field on single-phase interface evolution, additional interfacial energy and nucleation undercooling in Al-based alloy

The effects of a high magnetic field on the evolution of the single-phase interface and the liquid-solid interface energy in Al-Cu alloy were investigated experimentally. It is found that the application of the magnetic field has a significant promotion effect on the migration of liquid droplets, accelerating the formation of the single-phase interface. This should be attributed to the thermoelectric (TE) magnetic convection in the droplets which has enhanced the diffusion and increased the migration speed of liquid droplets. Further, the effect of the high magnetic field on the solid-liquid interface energy is analyzed by an improved grain boundary groove (GBG) method. The average solid-liquid interface energy of the α-Al/Al-Cu and Al2Cu/Al-Cu systems increases and decreases with the increase of the magnetic field, respectively. The above experiment results are well explained based on the formation and interaction of the magnetic dipole at the solid-liquid interface. Moreover, experimental results reveal that the magnetic-field-induced interface energy increases and decreases the nucleation undercooling of the Al-30wt.%Cu alloy and Al-35wt.%Cu alloy, respectively. By studying the effect of the magnetic-field-induced interface energy on the nucleation undercooling, the understanding of the interface energy-induced nucleation undercooling deepens.

Flattening Grain-Boundary Grooves for Perovskite Solar Cells with High Optomechanical Reliability.

Optomechanical reliability has emerged as an important criterion for evaluating the performance and commercialization potential of perovskite solar cells (PSCs) due to the mechanical-property mismatch of metal halide perovskites with other device layer. In this work, grain-boundary grooves, a rarely discussed film microstructural characteristic, are found to impart significant effects on the optomechanical reliability of perovskite-substrate heterointerfaces and thus PSC performance. By pre-burying iso-butylammonium chloride additive in the electron-transport layer (ETL), GB grooves (GBGs) are flattened and an optomechanically reliable perovskite heterointerface that resists photothermal fatigue is created. The improved mechanical integrity of the ETL-perovskite heterointerfaces also benefits the charge transport and chemical stability by facilitating carrier injection and reducing moisture or solvent trapping, respectively. Accordingly, high-performance PSCs which exhibit efficiency retentions of 94.8% under 440 h damp heat test (85% RH and 85 °C), and 93.0% under 2000 h continuous light soaking are achieved.

The effect of the surface patterning by ion beam irradiation on the Ag directional outflow in Ag/AlN nano-multilayers

Surface templating of nano-multilayers is a promising step for the advanced industrial application of these nanomaterials. In the present work, the effect of focused ion beam modification of Ag/AlN nano-multilayer near-surface region on the Ag surface outflow is revealed. Surface patterning by ion beam irradiation leads to the formation of a higher amount of Ag crystals in relation to the intact surface region. Heat treatment initiates grooving of Ag grain boundaries, resulting in the formation of channels in the nano-multilayer volume. These channels act as pathways for mass transport of Ag to the film surface. Ion bombardment creates regions of high concentration of non-equilibrium defects, which locally increase the driving force for grooving and channeling, thereby promoting creation of more pathways for Ag surface migration. The obtained experimental results indicate that surface modification by focused ion beam can be used for production of nano-multilayers with designed templates of surface outflow.

Roughness Suppression in Electrochemical Nanoimprinting of Si for Applications in Silicon Photonics

Metal-assisted electrochemical nanoimprinting (Mac-Imprint) scales the fabrication of micro- and nanoscale 3D freeform geometries in silicon and holds the promise to enable novel chip-scale optics operating at the near-infrared spectrum. However, Mac-Imprint of silicon concomitantly generates mesoscale roughness (e.g., protrusion size≈45nm) creating prohibitive levels of light scattering. This arises from the requirement to coat stamps with nanoporous gold catalyst that, while sustaining etchant diffusion, imprints its pores (e.g., average diameter≈42nm) onto silicon. In this work, roughness is reduced to sub-10nm levels, which is in par with plasma etching, by decreasing pore size of the catalyst via dealloying in far-from equilibrium conditions. At this level, single-digit nanometric details such as grain-boundary grooves of the catalyst are imprinted and attributed to the resolution limit of Mac-Imprint, which is argued to be twice the Debye length (i.e., 1.7nm)-a finding that broadly applies to metal-assisted chemical etching. Last, Mac-Imprint is employed to produce single-mode rib-waveguides on pre-patterned silicon-on-insulator wafers with root-mean-square line-edge roughness less than 10nm while providing depth uniformity (i.e., 42.9±5.5nm), and limited levels of silicon defect formation (e.g., Raman peak shift<0.1cm-1 ) and sidewall scattering.

Twin boundary formation at a grain-boundary groove during the directional solidification of InSb

We investigated twin boundary formation during directional solidification of the compound semiconductor InSb. We directly observed a groove formed at the junction of a solid–liquid interface and a Σ9 grain-boundary by optical microscopy. When the depth of the grain-boundary groove reached about 100 μm, rapid growth occurred to fill the groove. We found that two facet planes of the grain-boundary groove became twin boundary planes. Calculations of the change in free energy associated with nucleation at the grain-boundary groove indicates that the preferential twin nucleation occurs at the bottom of the groove, consistent with the direct observations in this study.

Model of vacancy diffusion-assisted intergranular corrosion in low-alloy steel

A model is presented for intergranular corrosion (IGC) of nonsensitized low-carbon pipeline steels, in which enhanced grain boundary (GB) oxidation is enhanced by vacancies produced by oxidation of reactive Si atoms. Evidence for vacancy injection at corroding GBs had been provided by recent nanoindentation measurements. Model calculations of IGC evolution controlled by lattice vacancy diffusion are compared to experimental measurements. Realistic dihedral angles of corroded GB grooves are predicted based on a vacancy diffusivity within the range of values expected for bcc iron. The calculations rationalize observations of sharpening of GB grooves with time during corrosion exposures, and show that vacancy diffusion quantitatively accounts for enhancement of IGC by reactive solute atoms, without invoking solute segregation or precipitation at GBs.

Thermodynamic behaviour of solid–liquid grain boundary grooves

ABSTRACT Grain boundary grooves (GBGs) are local features that develop along polycrystalline solid–liquid interfaces. Interest in GBGs lies in their ability to form interface defects during crystallization that promote, in fact, dominate, morphological instability and affect microstructure formation in cast alloys. Recently, we reported on unobserved subtle aspects of the thermodynamic behaviour of GBG microstructures by combining sharp-interface field theory with diffuse-interface phase-field simulations. A surprising feature revealed about steady-state GBGs is that despite their stationarity they nonetheless support persistent capillary-mediated energy fluxes with divergences that continuously cool their interfaces and increase local curvatures. We now analyse the energetic behaviour of GBGs as ‘open’ thermodynamic systems, and report further details of their formation free energy that show how geometric constraints and capillary-mediated thermodynamic fields self-interact and influence the steady-state shapes of GBGs.

Capillary-mediated solid-liquid energy fields: their detection with phase-field method

Observations of melting crystallites in microgravity showed unusual shape changes as melting proceeded toward extinction. When re-analyzed in 2011, shape evolution data showed needle-like crystallites becoming spheroids as they melted toward extinction, suggesting that some type of capillary phenomenon at solid-liquid interfaces was responsible for an energy release capable of spherodising particles on melting, and stimulating pattern formation during unstable crystal growth. The presence of these previously undetected energy fields was recently uncovered using phase-field simulations that employ an entropy density functional. Simulations allow measurement of interfacial energy distributions on equilibrated solid-liquid interfaces configured as stationary grain boundary grooves (GBGs). Interfacial energy source fields—related to gradients in the Gibbs-Thomson temperature—entail persistent cooling along GBG profiles, a new result that fully confirms earlier predictions based on sharp-interface thermodynamics. This study also provides new insights to improve microstructure control at reduced scales by explaining the thermodynamic fields responsible for pattern formation in castings.

Enhancement of optoelectronic properties in multicrystalline silicon via combination of grooving grain boundaries and porous silicon gettering

In multi-crystalline silicon (mc-Si), the detrimental effect of impurities and grain boundaries (GBs) on charge carrier transport has driven the research focus since many years. In view of curing these limitations, we present an innovative method to enhance the optoelectronic performance of mc-Si wafers via a combination between GBs grooving and porous silicon (PS) gettering. A preferential grooving of GBs was achieved using the HF/HNO3 based solution, the PS layers were formed on both sides of the samples using stain-etching method and the gettering experiment was performed at temperatures ranging from 750 to 900 °C. As a result, it has been shown that the rapid thermal annealing process with chemical grooving gives a positive trend of improvement of the electronic quality and found to be more efficient when used in combination with PS. After removing the PS layer, the minority carrier lifetime increases by a factor of more than 27. In addition, a significant enhancement of majority carrier mobility was obtained, which led to an important decrease of the resistivity.

Determination of solid–liquid interfacial energy of Ni3Sn2 phase by grain boundary groove method in a temperature gradient

The Ni3Sn2 phase plays an important role in numerous fields including solder alloys and lithium storage. However, the precise measurements of many parameters which are essential in describing the physical properties of this phase, such the Gibbs-Thomson coefficient, solid–liquid interfacial energy, grain boundary energy and entropy of fusion per unit volume have not been carried out yet. In this work, the equilibrated grain boundary groove (GBG) shapes for a solid Ni3Sn2 phase in equilibrium with the liquid were observed from Sn-36 at.%Ni peritectic alloy in a linear temperature gradient by using a Bridgman type directional solidification apparatus after experiencing a thermal stabilization time of 8 h. The time required to obtain the GBG shapes in this alloy with a large freezing range was analyzed through the temperature gradient zone melting (TGZM) theory. The Gibbs-Thomson coefficient, solid-liquid interfacial energy and grain boundary energy for the solid Ni3Sn2 phase in equilibrium with liquid were determined in this work, which will help the growth and property control of the fields mentioned above.

Interface morphologies and embrittlement behavior of α-Fe in liquid lead-bismuth eutectic at elevated temperature

The grain-boundary penetration behavior and interfacial characteristics of α-Fe in lead-bismuth eutectic (LBE) at 900 °C have been investigated. The results indicate that α-Fe can be completely embrittled by liquid LBE in forms of grain boundary (GB) grooving by liquid Bi and Pb segregation at GBs. Deep GB grooves and long-narrow LBE penetrated film occur at low dihedral angle of α grains along triple junctions; while discontinuous LBE particles with 1 μm in diameter can spread at GBs of high dihedral angle or distribute within α grain interior. The discontinuous LBE particles can further aggregate and transfer into grain-boundary films to cleave α grains from substrate. GB morphology strongly dominates liquid LBE grooving and penetration depth, depending on the original GB dihedral angle. Obvious microcracks and dynamic dihedral angle ahead of GB root determine the LBE penetration direction and subsequent grooving, probably depending on the grain orientation and energies. Meanwhile, the interfacial Fe-atom removal/redeposition and strong segregated Pb/Bi atom multilayer absorption may govern solid/liquid interfacial grooving, diffusion and penetration of LBE film via solid/liquid bridge-joint interface atom transfer mechanism.

Subgrain Special Boundaries in Halide Perovskite Thin Films Restrict Carrier Diffusion

Photoluminescence microscopy reveals subgrain special boundaries in FAPbI3 perosvkite thin films that restrict carrier diffusion and serve as nonradiative recombination sites. These subgrain boundaries, which cannot be observed by conventional AFM or SEM measurements of grain-boundary grooves, may explain why the performance of perovskite solar cells has not scaled with apparent grain size.

Measuring solid\u2013liquid interfacial energy fields: diffusion-limited patterns

The Leibniz–Reynolds transport theorem yields an omnimetric interface energy balance, i.e., one valid over all continuum length scales. The transport theorem, moreover, indicates that solid–liquid interfaces support capillary-mediated redistributions of energy capable of modulating an interface’s motion—a thermodynamic phenomenon not captured by Stefan balances that exclude capillarity. Capillary energy fields affect interfacial dynamics on scales from about 10 nm to several mm. These mesoscopic fields were studied using entropy density multiphase-field simulations. Energy rate distributions were exposed and measured by simulating equilibrated solid–liquid interfaces configured as stationary grain boundary grooves (GBGs). Negative rates of energy distributed over GBGs were measured as residuals, by subtracting the linear potential distribution contributed by applied thermal gradients constraining the GBGs from the nonlinear distributions actually developed along their solid–liquid interface. Rates of interfacial cooling revealed numerically confirm independent predictions based on sharp-interface thermodynamics, variational calculus, and field theory. This study helps answer a long-standing question: What creates patterns for diffusion-limited transformations in nature and in material microstructures?

Phase-field modeling of grain-boundary grooving and migration under electric current and thermal gradient

Grain-boundary migration, void formation as well as associated hillock formation are important mechanisms which lead to the failure of interconnects in the microelectronic packages. An understanding of the underlying physics of each of the phenomena can allow better design of interconnects. In this paper, we formulate a new phase-field model based on a grand-potential formalism for studying the phenomena of grain-boundary grooving under the combined influence of pure diffusion controlled transport, electric current and thermal gradient. We separately investigate the contributions of each of the stimuli towards the process of grain-boundary migration and hillock formation, by performing phase-field simulations as well as comparing with analytical theories. Additionally, we qualitatively reproduce the phenomena observed in experiments on polycrystalline metals, wherein electromigration and thermomigration may act in unison or against each other towards their contributions in grooving, hillocking and void growth.

Electromigration-Induced Surface Drift and Slit Propagation in Polycrystalline Interconnects: Insights from Phase-Field Simulations

Electric current flowing through a metal can permanently move its atoms, and this $e\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}m\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}g\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}n$ (EM) is important in ultrasmall electronics. To prolong the life of a device, deeper insight into the interplay of multiple transport mechanisms during EM-induced failure is essential. Phase-field modeling is employed to unravel the roles of surfaces and grain boundaries during EM-mediated formation of grain-boundary grooves in nanoscale interconnects. Simulations capture the entire sequence of microstructural evolution of the sample. The scaling laws derived here provide useful guidelines for predicting the lifetimes of polycrystalline interconnects, and for efficient design.