High Defect Density Research Articles

Tuning Perovskite Crystal Growth Dynamics Using Additives on Textured Silicon Substrates

Double‐sided textured silicon solar cells with micrometer‐sized pyramid structure are used as bottom cells in monolithic tandem structures to decrease reflection losses. As top cell material, frequently perovskites are used. In this work, various additives are investigated to enhance the perovskite absorber quality, as a top cell fabricated using the hybrid route. In the context of the hybrid route, it is found that urea or methylammonium chloride (MACl) can effectively increase the grain size and improve the absorber quality, while formamidinium chloride (FACl) cannot. With urea, the crystallization can be tuned without leaving any voids in the film (unlike MACl). However, when annealed at a high annealing temperature, the excessive crystal growth with urea causes non‐conformal coating and high defect density. By adjusting the annealing conditions and additive concentration, the crystal growth of the perovskite top cell on the micrometer‐sized silicon pyramids can be fine‐tuned, ensuring that the perovskite layer conformally coated the pyramids. The use of additives not only improves crystallization but also enhances the conversion of the inorganics, particularly at the hole transport layer (HTL) interface. Moreover, this work contributes to a better understanding of perovskite crystallization dynamics and how to control it, especially on textured substrates.

Enhancing anti-adhesion properties by designing microstructure - the microscopy and spectroscopy study of the intercellular bacterial response.

This study is the first one that investigates in detail the bacterial intercellular response to the high density of crystallographic defects including vacancies created in Cu by high pressure torsion. To this aim, samples were deformed by high pressure torsion and afterward, their antibacterial properties against Staphylococcus aureus were analyzed in adhesion tests. As a reference an annealed sample was applied. To avoid the influence of surface roughness, specially elaborated conditions for surface preparation were employed, which do not introduce defects and assure comparable surface roughness. The analysis of the chemical composition and thickness of passive layers by X-ray photoelectron spectroscopy showed that they were comparable for nanostructured and micrograined samples, consisting of Cu2O and CuO, and a thickness of 6nm. The interface bacterium-substrate was prepared by a focused ion beam and further analyzed by scanning transmission electron microscopy and energy dispersive spectroscopy. High pressure torsion processed Cu shows enhanced anti-adhesion properties while in contact with S. aureus than micrograined Cu. There is a linear correlation between luminous intensity and grain size-0.5. The bacterial intercellular defence mechanism includes the creation of Cu2O nanoparticles and the increased concentration of sulphur-rich compounds near these nanoparticles.

Towards invertible 2D crystal structure representation for efficient downstream task execution

Abstract In the study of MoS2 lattice defects, we explore the use of Siamese Neural Networks (SNN) to create invariant embeddings, which respect the crystalline symmetry of the lattice. By training our model with contrastive learning, we successfully differentiate configurations with varying defects, achieving perfect accuracy in recognizing equivalent placements. Our method showcases the capability to predict physical properties like formation energy per site and the bandgap with strong performance across both low and high-defect density scenarios, outperforming traditional methods when enhanced with polynomial features. Despite its effectiveness, the model presents limitations at high defect densities, indicating a need for further refinement. Our approach lays the groundwork for reverse-engineering processes. Thus, we open pathways for generative models that can navigate from specified property ranges to optimal defect configurations, fostering an efficient solution-space exploration for bespoke material synthesis.

Performance Enhancement of Hole Transport Layer-Free Carbon-Based CsPbIBr2 Solar Cells through the Application of Perovskite Quantum Dots.

CsPbIBr2, with its suitable bandgap, shows great potential as the top cell in tandem solar cells. Nonetheless, its further development is hindered by a high defect density, severe carrier recombination, and poor stability. In this study, CsPbI1.5Br1.5 quantum dots were utilized as an additive in the ethyl acetate anti-solvent, while a layer of CsPbBr3 QDs was introduced between the ETL and the CsPbIBr2 light-harvester film. The combined effect of these two QDs enhanced the nucleation, crystallization, and growth of CsPbIBr2 perovskite, yielding high-quality films characterized by an enlarged crystal size, reduced grain boundaries, and smooth surfaces. And a wider absorption range and better energy band alignment were achieved owing to the nano-size effect of QDs. These improvements led to a decreased defect density and the suppression of non-radiative recombination. Additionally, the presence of long-chain organic molecules in the QD solution promoted the formation of a hydrophobic surface, significantly enhancing the long-term stability of CsPbIBr2 PSCs. Consequently, the devices achieved a PCE of 9.20% and maintained an initial efficiency of 85% after 60 days of storage in air. Thus, this strategy proves to be an effective approach for the preparation of efficient and stable CsPbIBr2 PSCs.

Decylammonium sulfate post-treatment for efficient hole-conductor-free printable perovskite solar cells with reduced voltage loss

Hole-conductor-free printable mesoscopic perovskite solar cells (p-MPSCs) have attracted widespread attention for their low cost, up-scalability, and exceptional stability. However, the high defect density of perovskite and the absence of interfacial barrier layer between perovskite and carbon electrode cause profound open-circuit voltage (VOC) loss, which results in uncompetitive power conversion efficiency (PCE). Herein, an anion-cation synergy of decylammonium sulfate (DA2SO4) is utilized for suppressing VOC loss of p-MPSCs via a facile post-treatment method. DA+ cations transform the perovskite adjacent to carbon electrode into wide-bandgap 2D perovskite for blocking electrons, while the SO42− anions interact with undercoordinated lead centers for reducing defect density. As a result, the modified device delivers an enhanced PCE from 17.78% to 19.59%, with an improved VOC from 0.98 V to 1.06 V. Meanwhile, the modified device without any encapsulation exhibits excellent moisture stability with the PCE remained almost 99% of the initial value after 528 h aging in 75% RH air at room temperature.

Efficient and stable perovskite solar cells based on multi-active sites 5-amino-1,3,4-thiadiazole-2-thiol modified interface

The highest certification efficiency of perovskite solar cells (PSCs) has reached 26.7 %. However, the high defect density on the surface of perovskite films prepared by low temperature solution method and the energy mismatch between the carrier transport layers and perovskite layer (PVK) greatly limit the performance improvement of PSCs. The introduction of passivating agent to modify the perovskite interface and grain boundary can reduce the defect density, coordinate the energy level effectively, and improve the efficiency and stability of devices. A Lewis base molecule 5-amino-1,3,4-thiadiazole-2-thiol (AMTD) with multiple active sites is introduced at the interface between PVK and hole transport layer (HTL). The electron-rich groups, such as = S, –S–, –NH2, –N on AMTD, passivate the positive electrical defects on the interface and grain boundary, and increase carrier transport efficiency. The interfacial energy level array is optimized to achieve more efficient charge transportation. In addition, the modified of AMTD has a significant protective effect on the perovskite, which inhibit the moisture erosion of in environment. Consequently, the AMTD-optimized device achieves a power conversion efficiency (PCE) of 24.13 %, compared to the efficiency of 21.62 % for pristine device. The stability of the devices is improved greatly.

Effect of Electrical Current on Sliding Friction and Wear Mechanisms in a-C and ta-C Amorphous Carbon Coatings

This study investigates the sliding wear behaviour of amorphous carbon (a-C) and tetrahedral amorphous carbon (ta-C) coatings, two forms of diamond-like carbon (DLC) coatings, against SAE 52100 steel using a modified ball-on-disk tribometer with applied electrical currents ranging from 100 mA to 1800 mA. It focuses on the variations in sliding friction and wear characteristics of these coatings as electrical currents increase under constant load and speed conditions. The a-C coatings exhibited lower coefficient of friction (COF) values and reduced volumetric wear losses up to 1500 mA, while ta-C coatings studied displayed higher wear, similar to uncoated M2 steel, at 300 mA with degradation occurring at low currents, resulting in failure due to severe oxidational wear. The a-C coatings showed no significant electrical damage at these currents. Raman spectroscopy revealed structural changes on the wear tracks of sp2-rich a-C coatings, specifically the formation of graphene layers. In comparison, the wear tracks of sp3-rich ta-C coatings did not display such transformation under the conditions studied. The graphene coverage on the surfaces of the a-C coatings increased with the increase in the current as revealed by the Raman intensity maps of 2D peaks and this increase was accompanied by a higher defect density in the graphene. The low COF of graphene-covered a-C surfaces was consistent with the proposed mechanisms of moisture adsorption. However, at currents exceeding 900 mA, surface temperatures of a-C coatings exceeded 100 °C, impairing graphene’s ability to maintain low friction, resulting in an increased COF.

Recent Progress of Thin Crystal Engineering for Perovskite Solar Cells.

Metal halide perovskite single crystals hold promise for photovoltaics with high efficiency and stability due to their superior optoelectronic properties and weak bulk ion migration. The past several years have witnessed rapid development of single-crystal perovskite solar cells (PSCs) with efficiency rocketed from 6.5% to 24.3%, however, which still lags behind their polycrystalline counterparts. Moreover, the poor device stability under light illumination is contrary to the high ion migration barrier of perovskite single crystals. The key limiting factors should be the low crystalline quality and high surface defect density of solution-grown thin single crystals. Under this circumstance, a review paper summarizing the recent progress and challenges will be instructive for future development of this emerging field. In this manuscript, the crystal engineering used to enhance carrier transport and suppress carrier recombination in vertical single-crystal PSCs will be summarized initially, including crystal growth, component control, surface and interface modification. Subsequently, the application of perovskite single crystals in lateral single-crystal PSCs will be discussed and compared with the conventionally vertical structure. Finally, the challenges and proposed strategies for the development of single-crystal PSCs are provided.

The deformation mechanism of low symmetric Ti–Pt intermetallic compounds containing high density of planar defects

Intermetallic compounds typically exhibit limited plastic deformation capacity due to challenges in activating dislocation slip and deformation twinning, coupled with a lack of alternative deformation mechanisms. Ti–Pt alloys are a prevalent type of intermetallic compound utilized in high-temperature shape memory alloys and as materials for energy applications in electric fields. However, they often exhibit poor deformation capability. Here, we prepared a low-symmetry intermetallic phase, Ti4Pt3, which demonstrates significant plastic deformation capability. This phase features a high density of parallel planar defects, resulting in an exceptionally large lattice periodicity perpendicular to these defects. Through in-situ scanning electron microscope compression tests, we observed substantial plastic deformation in this new phase. Analysis of the deformed Ti4Pt3 phase revealed that the dense planar defects create uniformly distributed sites of internal stress concentration, enabling a rapid increase in back stress within crystals. This phenomenon leads to notable lattice rotation and localized order-disorder transitions, both crucial mechanisms facilitating plastic deformation and enhancing deformation capacity. Our research underscores the potential of leveraging structural asymmetry to enable unconventional deformation mechanisms, thereby enhancing the plasticity of intermetallic materials.

Advancing semantic segmentation of two-dimensional materials using a semantic-adaptive transformer model

Accurate detection and characterization of two-dimensional (2D) materials are essential for their effective utilization in various applications. Traditional techniques, such as chemical vapor deposition, often produce materials with high defect density, while mechanical exfoliation is hindered by its labor-intensive and time-consuming nature. In this Letter, we propose a semantic-adaptive transformer model, termed Semptive, designed specifically for the precise detection of monolayer MoS2. Our approach integrates a semantic adaptation module with a multi-head self-attention mechanism, incorporating deep supervision and leveraging prior knowledge to enhance model performance. The model was trained on a dataset obtained through mechanical exfoliation and validated using photoluminescence spectroscopy. The experimental results reveal that Semptive significantly enhances segmentation performance compared to conventional models, achieving higher Intersection over Union and Dice scores while reducing computational demands. This method represents a notable advancement in the efficient and precise identification of 2D materials, providing substantial improvements for material characterization and device fabrication processes.

Microstructure-induced anisotropic biocorrosion response of laser additive manufactured WE43 alloy

The microstructure and corrosion of laser-processed WE43 was studied. Results suggest that multi-factors like grain structure, crystallographic defect, and precipitate distribution result in an anisotropic corrosion response. XY plane with high crystal defect densities offers more sites for pitting corrosion. Filiform corrosion is detected on both XY plane and XZ plane, where Zr-rich and RE-rich precipitate distribution plays an essential role in corrosion initiation and propagation. The enrichment of Zr around filament corrosion promotes local micro-galvanic corrosion. Notably, the XY-plane, with continuous distribution and extended scanning trajectory edge, is particularly susceptible to filiform corrosion as compared with XZ-plane.

The influence of carbon content gradient and carbide precipitation on the microstructure evolution during carburizing-quenching-tempering of 20MnCr5 bevel gear

Due to the mechanism of microstructural transition, high-strength low-carbon alloy steel gears exhibit high strength and surface hardness after Carburizing-Quenching-Tempering (CQT) combined heat treatment, while maintaining good toughness in the core. The carbon potential gradient and the precipitation of carbides are the main factors affecting the final microstructure, yet there is a shortage of related research. To address this gap, this study developed a multi-physics coupled model considering the impact of carbon content to simulate microstructural transformation. Using finite element analysis, we simulated the microstructural evolution during the CQT heat treatment of 20MnCr5 bevel gear. By comparing the simulation results with the calculations from the commercial heat treatment software DANTE®, the accuracy of this model in predicting microstructural distribution and hardness has been validated. Furthermore, the study established a carbide precipitation kinetics model that takes into account the influence of grain size, to investigate the precipitation behavior of carbides within the matrix during the tempering stage. The cellular automaton method was applied to demonstrate the evolution process of the quenched microstructure during tempering. The study shows that during tempering, carbides primarily precipitate in areas with higher carbon content, preferentially at the boundaries of fine grains with a high density of lattice defects. As the carbon content increases, the number density of precipitates rises, while their average radius decreases. The reduction in matrix supersaturation due to carbide precipitation is one of the causes for the reduction of tooth surface hardness during tempering. The model constructed in this study provides a new perspective for understanding the laws governing material microstructural evolution and offers a solid theoretical foundation for the study of stress and deformation during the heat treatment process.

Comparison of AlN/GaN heterojunctions grown by molecular beam epitaxy with Al and Ga assistance

Although conventional III-nitride molecular-beam-epitaxy growth theories indicate that high-quality AlN should be prepared in Al-assisted regimes, AlN/GaN heterojunctions grown in Ga-assisted regimes actually demonstrate superior performances. In this letter, we compared Al-assisted and Ga-assisted grown AlN/GaN heterojunctions and explored the underlying mechanisms. Aberration-corrected transmission electron microscope, energy dispersive spectrometer, and atomic force microscopy measurements indicate that the Al-assisted growth results in partly-relaxed AlN of high-density defects, deteriorating AlN/GaN interfaces, and surface morphology consisting of irregular and truncated atomic steps. The Ga-assisted growth produces high-quality pseudomorphic AlN with excellent AlN/GaN interfaces, surface morphology, and electrical properties. It is suggested that the better crystalline quality and higher performances of the Ga-assisted grown samples are attributed to a lower N subsurface diffusion barrier resulting from a higher energy of the fcc adsorption positions in the adlayer-enhanced-lateral-diffusion model and intense thermal motions of Ga adatoms. It helps to understand the kinetics of metal-adlayer-enhanced growth of AlN and AlN/GaN heterojunctions at atomic scale and optimize the growth processes. Meanwhile, it is demonstrated that amorphization and Al diffusion near the AlN surfaces occur due to Pt bombardment on highly tensile-strained AlN during focused-ion-beam fabrications. It implies that large strain in AlN/GaN heterojunctions might be an important issue to be considered for device reliability.

Role of surface roughness in determining surface energy and magnetic properties of Fe80Ce20 films during thermal processing on Si(100) and glass substrates

In this study, Fe80Ce20 films were deposited on Si(100) and glass substrates using sputtering in a high-vacuum environment and subsequently heat-treated in a vacuum annealing furnace. The films, with thicknesses ranging from 10 nm to 50 nm, were annealed at temperatures of 100 °C, 200 °C, and 300 °C. The objective was to investigate the effects of film thickness and annealing temperature on the structural, surface energy, and magnetic properties of the material. X-ray diffraction (XRD) analysis confirmed the crystalline nature of both Si(100)/Fe80Ce20 and Glass/Fe80Ce20 films. Atomic force microscopy (AFM) revealed a smooth film surface that became rougher after annealing. Contact angle measurements indicated hydrophilic properties, with the highest surface energy observed in the as-deposited films due to a higher density of defects and grain boundaries. The hysteresis loop demonstrated exceptional soft magnetic properties and a high saturation magnetization (Ms) of approximately 2000 emu/cm³. Annealing at 100 °C altered the magnetic domain structure from a striped labyrinthine configuration to an aligned pattern. At higher annealing temperatures, the films exhibited grain growth, increased surface roughness, and new grain formation. Following annealing, the grains in the Fe80Ce20 films became larger, surface roughness increased, and the water contact angle increased, rendering the films more hydrophobic. The as-deposited films displayed the highest surface energy due to the high density of defects and grain boundaries. Increased surface roughness led to more grain boundary defects and irregularities, which served as pinning points for magnetic domain walls, thus increasing coercivity (Hc). Rough surfaces induced local stress fields, increased magnetic anisotropy, and decreased saturation magnetization. Thermal perturbations from higher annealing temperatures further disrupted the alignment of the magnetic moments, leading to a reduction in saturation magnetization.

Achieving 1.7 GPa Considerable Ductility High-Strength Low-Alloy Steel Using Hot-Rolling and Tempering Processes.

To achieve a balanced combination of high strength and high plasticity in high-strength low-alloy (HSLA) steel through a hot-rolling process, post-heat treatment is essential. The effects of post-roll air cooling and oil quenching and subsequent tempering treatment on the microstructure and mechanical properties of HSLA steels were investigated, and the relevant strengthening and toughening mechanisms were analyzed. The microstructure after hot rolling consists of fine martensite and/or bainite with a high density of internal dislocations and lattice defects. Grain boundary strengthening and dislocation strengthening are the main strengthening mechanisms. After tempering, the specimens' microstructures are dominated by tempered martensite, with fine carbides precipitated inside. The oil-quenched and tempered specimens exhibit tempering performance, with a yield strength (YS) of 1410.5 MPa, an ultimate tensile strength (UTS) of 1758.6 MPa, and an elongation of 15.02%, which realizes the optimization of the comprehensive performance of HSLA steel.

Enhancing photovoltaic performance and stability of inverted perovskite solar cells via multifunctional molecular additive

The film quality of perovskite absorber plays a fundamental role in determining the efficiency and stability of perovskite solar cells (PSCs), of which high crystallinity and low defect density are consistently persuaded. Here, a strategy using multifunctional additive of N,N′-Diallyl-L-tartardiamide (NDT) to produce high-quality perovskite film is proposed. The rich coordination and hydrogen bonding between NDT and perovskite effectively passivate trap states, improve film crystallinity, stabilize perovskite crystal structure and confine ions migration, resulting in enhancement of charge transport and suppression of nonradiative recombination. Consequently, compared with the control device (19.07 %), the NDT-modified devices achieve a champion efficiency of 21.71 % with negligible hysteresis as well as excellent air and light stability. This work presents a facile and effective approach via multifunctional molecular additive to achieve high-performance inverted (p-i-n) PSCs.

Molecularly Engineered Alicyclic Organic Spacers for 2D/3D Hybrid Tin-based Perovskite Solar Cells.

The high defect density and inferior crystallinity remain great hurdles for developing highly efficient and stable Sn-based perovskite solar cells (PSCs). 2D/3D heterostructures show strong potential to overcome these bottlenecks; however, a limited diversity of organic spacers has hindered further improvement. Herein, a novel alicyclic organic spacer, morpholinium iodide (MPI), is reported for developing structurally stabilized 2D/3D perovskite. Introducing a secondary ammonium and ether group to alicyclic spacers in 2D perovskite enhances its rigidity, which leads to increased hydrogen bonding and intermolecular interaction within 2D perovskite. These strengthened interactions facilitate the formation of highly oriented 2D/3D perovskite with low structural disorder, which leads to effective passivation of Sn and I defects. Consequently, the MP-based PSCs achieved a power conversion efficiency (PCE) of 12.04% with superior operational and oxidative stability. This work presents new insight into the design of organic spacers for highly efficient and stable Sn-based PSCs.

Responsibility of small defects for the low radiation tolerance of coated conductors

Abstract Rare-earth-barium-copper-oxide based coated conductors exhibit a relatively low radiation robustness compared to e.g. Nb3Sn due to the d-wave symmetry of the order parameter, rendering impurity scattering pair breaking. The type and size of the introduced defects influence the degrading effects on the superconducting properties; thus the disorder cannot be quantified by the number of displaced atoms alone. In order to develop degradation mitigation strategies for radiation intense environments, it is relevant to distinguish between detrimental and beneficial defect structures. Gadolinium-barium-copper-oxide based samples irradiated with the full TRIGA Mark II fission reactor spectrum accumulate a high density of point-like defects and small clusters due to n - γ capture reactions of gadolinium. This leads to a 14–15 times stronger degradation of the critical temperature compared to samples shielded from slow neutrons. At the same time both irradiation techniques lead to the same degradation behavior of the critical current density as function of the transition temperature J c ( T c ) . Furthermore, annealing the degraded samples displayed the same T c recovery rates, indicating the universality of the defects responsible for the degradation. Since the primary knock on atom of the n - γ reaction as well as the recoil energy is known, we used molecular dynamics simulations to calculate which defects are formed in the neutron capture process and density functional theory to assess their influence on the local density of states. The defects found in the simulation were mainly single defects as well as clusters consisting of Oxygen Frenkel pairs, however, more complex defects such as Gd C u antisites occurred as well.

Facilely tuning the triazine-heptazine ratios in crystalline g-C3N4 for efficient photo-degradation of tylosin

Intrinsically, g-C3N4 has high-density defects, insufficient efficiencies of photogenerated charge carrier separation and transfer, and inadequate photocatalytic ability against antibiotics. A facile crystallinity engineering on g-C3N4 assisted by molten-salts and simultaneously fine-tuning the triazine-heptazine ratio led to forming type-II homojunctions, without introducing foreign atoms and being metal-free. It was successfully achieved through modulating the calcination temperatures and using KCl-LiCl. The salts accelerated deamination reaction kinetics, tuned the unit ratio, and improved the degree of carbon nitride polymerization. At a triazine-heptazine ratio of 74:26 obtained at 450 °C, 10 mg/L of tylosin was completely mitigated within 9 min by the catalyst. Active radicals including O2− and OH were boosted from charge carriers under visible light, and the recombination of photogenerated excitons was relieved on the interfaces of the homojunctions. Breaking glycosidic bonds and dropping off deoxyglycosyl moieties are considered the main degradation mechanism. The cytotoxicity and acute toxicity of the degradation product are classified as harmless. In addition, 3D-molecular docking between the transformation products and target proteins (L4 and L22) in ribosome confirms toxicity reduction. This work paves a way for efficient and harmless photocatalytic degradation of tylosin by facilely modulating the unit ratio and enhancing the crystallinity of carbon nitride without introducing foreign atoms.

Effect of ultrasonic micro-forging on the microstructure and properties of GH4169 superalloy deposited by laser direct energy deposition

Nickel-based superalloys prepared by laser direct energy deposition (DED) still suffer from microstructure, elemental segregation, and unstable strength, which cannot meet the service requirements and severely limit the application and development of DED-prepared superalloy parts. In this paper, GH4169 specimens with high density and free of defects were prepared by ultrasonic micro-forging assisted laser direct energy deposition (UMT-DED) method. The effects of UMT on the dendritic structure, elemental segregation, and precipitation phases of the alloys prepared by UMT-DED, as well as the densities, hardness, and tensile properties at room and high temperatures were investigated. The contribution of the strengthening mechanism to the yield strength was estimated. The grain refinement mechanism and the deformation mechanism of the Laves phase in GH4169 were discussed. The results show that UMT achieves grain refinement by forming fine crystal strips between the layers. The proportion of fine grain areas increased from 5.9 % to 33.3 %. The plastic deformation introduced by UMT provides external work for dislocation movement to break and fracture part of the Laves phase during plastic deformation, with dislocation cutting of the Laves phase being the main deformation mechanism. The residual stresses was transformed into residual compressive stress. The tensile strength, yield strength, and elongation of the specimens at 25 °C were increased by 4.23 %, 6.42 %, and 23.67 % respectively.