Defect Clusters Research Articles

Synergistic Effect of Ag, Sb Dual‐Cation Substitution on Cu2ZnSn (S, Se)4 High‐Efficiency Solar Cells

ABSTRACTThe poor crystal quality inside an absorber layer and the presence of various harmful defects are the main obstacles restricting the properties of Cu2ZnSn (S, Se)4 (CZTSSe) thin‐film solar cells. Cation doping has attracted considerable research attention as a viable strategy to overcome this challenge. In this paper, based on Sb‐substituted CZTSSe system, we prove that Ag partially substituting Cu may be a feasible strategy. After a series of characterization of the films, it was discovered that the crystal quality and crystallinity of the films were further improved by introducing Ag into Cu2Zn(Sb, Sn) (S, Se)4 (CZTSSSe), and the concentrations of CuZn accepter defects and 2[CuZn + SnZn] defect clusters were effectively inhibited. At the same time, the carrier concentration is increased. The results show that when the Ag doping ratio is 15%, the photovoltaic conversion efficiency (PCE) reaches 8.34%, compared with the single‐doped Sb element, the efficiency is increased by 24%. For the first time, this study investigates the collaborative effect of Sb, Ag dual‐cation substitution in CZTSSe. The solar cell performance enhancement mechanism offers new potential for the advancement of CZTSSe thin‐film solar cell technology in the future.

Ag,Cd double gradient doping at front interface for high efficiency CZTSSe solar cells

Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells have attracted considerable attention in recent years, and considered to be potential alternative solutions to existing commercialized solar cells for future energy demands. However, poor-quality p-n junction causes severe carrier recombination at the front interface, resulting high open-circuit voltage deficit and unsatisfactory efficiency of CZTSSe solar cells. In this work, Ag,Cd double gradient doping was introduced to front interface for interface carrier recombination suppression. Benefited from the formed Ag,Cd double gradient, defect and/or defect cluster at front interface significantly reduced, thus evidently suppressing the carrier recombination at front interface. As a result, the power conversion efficiency (PCE) of champion device increased from 10.04 % (Reference) to 12.66 % (With Ag,Cd). This Ag,Cd double gradient architecture offers new ideas for improvement of high efficiency CZTSSe as well as other solar cells.

Atomistic simulation of chemical short-range order on the irradiation resistance of HfNbTaTiZr high entropy alloy

High entropy alloys (HEAs) have been considered as one of the potential structural material candidates for fourth-generation nuclear reactors and fusion reactors due to their excellent irradiation resistance. Current studies have shown that the chemical short-range order (CSRO) usually exists in HEAs, which has a significant effect on the mechanical properties and irradiation resistance of HEAs. Refractory high entropy alloys (RHEAs), as a new class of HEAs have better mechanical properties at high temperatures than face-centered cubic (FCC) HEAs, and therefore have better prospects of application in the nuclear field. In this study, CSRO and its effect on the irradiation resistance of HfNbTaTiZr are analyzed via molecular dynamics (MD) and Monte Carlo (MC). The primary cascade simulations, multi-cascade simulations and surface bombardment simulations are carried out to simulate the generation and accumulation of irradiation damage. The results of the primary cascade simulations and surface bombardment simulations of CSRO models show that the presence of CSRO induces cascade splitting into subcascades. The presence of subcascades reduces the thermal peak enhancement effect and thus lowers the recombination rate of Frenkel pairs (FPs) in the damage zone when FPs concentrations are low. However, the creation of subcascades increases the size of the damage zone caused by the cascade. Thus, when the concentrations of FPs are high, the larger area of damage zone allows more of the already existing FPs to be included, thus promoting their recombination, i.e., impedes their accumulation when concentrations are high. These subcascades lower the recombination of FPs at low FPs concentrations but inhibit their accumulation at high FPs concentrations. The presence of CSRO is also beneficial in inhibiting the growth of point defect clusters, which further improves the resistance of HfNbTaTiZr to dislocation generation. Furthermore, the presence of CSRO facilitates the irradiation-induced phase transition. But it is found that HfNbTaTiZr shows suppression of HCP cluster growth. And the tendency to break down large HCP clusters into smaller ones is demonstrated in the CSRO model. From our calculations we also find that the irradiation-induced HCP atoms have a higher potential energy relative to the matrix. The potential energy difference between those energetic HCP atoms and the matrix can lead to generating a great number of insurmountable barriers pervading the matrix and largely suppressing the long-term mobility of FPs, thus limiting their aggregation and growth into clusters.

Abrikosov clusters in chiral liquid crystal droplets

Self-organizing triangular lattices of topological vortices have been observed in type-II superconductors, Bose-Einstein condensates, and chiral magnets under external forcing. Liquid crystals exhibit vortex self-organization in dissipative media. In this study, we experimentally investigate the formation of vortex clusters, analogous to Abrikosov lattices, in temperature-driven chiral liquid crystal droplets. Based on a Ginzburg-Landau-like equation, we derive the interaction laws underlying the formation of these Abrikosov clusters of chiral domains. The origin of these is elucidated due to the competition between the repulsive interaction and the spatial effect of the confinement within the droplet. Our results advance the theoretical understanding of localized vortex self-organization in liquid crystals and open up possibilities for controlling the clustering of these topological defects.

A spatially-resolved model of neutron-irradiated tungsten coupling stochastic cluster dynamics and finite deformation plasticity

Structural materials used in nuclear reactors face severe degradation in mechanical properties, such as hardening and embrittlement. At the microscopic scale, this occurs due to creation and accumulation of irradiation-induced defects and their interaction with system dislocations. Although techniques exist which can model evolution of irradiation defects, for instance kinetic transport theory-based models, their interaction with mechanical deformation of the bulk material has not been investigated extensively. In this work, we demonstrate a novel spatially-resolved multiscale coupling between microscopic irradiation defect evolution, modeled using Stochastic Cluster Dynamics (SCD) and macroscopic mechanical deformation modeled using a finite-deformation plasticity model. SCD is used to determine the statistically averaged defect cluster spacing, dependent on operating conditions such as irradiation dose and temperature. This acts as an initial condition that governs the critical resolved shear stress of dislocation glide in the macroscopic plasticity model. This framework is used to predict mechanical behavior in post-mortem test of irradiated Tungsten samples, which has found its importance as structural material used in nuclear reactors. The results obtained using the coupled approach are in good agreement with experimental data of uniaxial tension tests. The model is able to capture the effect of temperature and irradiation dose on the material hardening. Two methods are proposed to estimate hardness – using Tabor's Law relating uniaxial yield stress to hardness and from flat-punch simulations. The results are in reasonable agreement with hardness data from micro-indentation experiments of irradiated Tungsten samples. The model is also able to reveal microstructural details such as spatial variation in defect density and local stress.

The Effects of Defect Cluster CaK-SiP on the Structure and Laser Damage for KDP Crystals: A First-Principles Study

The Effects of Defect Cluster CaK-SiP on the Structure and Laser Damage for KDP Crystals: A First-Principles Study

Mitigating Band Tailing in Kesterite Solar Absorbers: Ab Initio Quantum Dynamics.

Open-circuit voltage deficits are limiting factors in kesterite solar cells. Addressing this issue by suppressing band tailing and nonradiative charge recombination is essential for enhancing the performance. We employ ab initio nonadiabatic molecular dynamics to elucidate the origin of band tailing and charge losses and propose a mitigation strategy. The simulations show that Cu-Zn disorder, associated with antisite defect clusters [CuZn+ZnCu], is a significant source of band tailing in kesterites, as evidenced by the much larger Urbach energy in disordered than ordered kesterites. Cu-Zn disorder gives rise to new sulfur-centered coordination polyhedra, increases structural inhomogeneity, changes electrostatic potential at sulfur centers, and shifts the S(3p) orbital energy. Differences in the S(3p)/Cu(3d) and S(3p)/Sn(5s) hybridization strengths and the S(3p) orbital energy shift reduce the band gap by 0.37 eV. Furthermore, Cu-Zn disorder enhances vibrational motion of sulfur anions and surrounding cations, increasing band gap fluctuations by 15 meV. The stronger electron-phonon interactions reduce charge carrier lifetimes and limit the kesterite solar cell efficiency. Partial substitution of Zn with Cd facilitates structural ordering and significantly suppresses band tailing, particularly in disordered systems. The improvement can be attributed to the larger atomic radius and mass of Cd, which weakens bonding around the anion, suppresses S-related vibrations within the covalent tetrahedra, and reduces nonadiabatic coupling, thereby increasing charge carrier lifetimes. The reported results establish the key influence of cation disorder on band tailing and reduced charge carrier lifetimes in kesterites and highlight cation disorder engineering as a strategy to achieve high-efficiency kesterite solar cells.

Achieving over 10 % efficiency in kesterite solar cells via selenium-free annealing

The unsatisfactory crystalline quality of the absorber layer has seriously impeded the development of kesterite photovoltaic devices, including a small grain size, a typical multi-layer structure, and inherent defects and defect clusters. Herein, we simultaneously modify the nature of the absorber bulk and the heterojunction interface by sputtering quaternary alloy targets and subsequent selenium-free annealing to improve the photovoltaic performance of Cu2ZnSnSe4 (CZTSe) solar cells. By adjusting the sputtering power, the elemental content and distribution in the precursor film are effectively controlled, which helps to obtain a single-layer large grain CZTSe absorber layer with better homogenization and compactness. The harmful inherent defects are significantly suppressed by such a modification. Simultaneously, the widened depletion width and enhanced built-in electric field, as well as improved band alignment at the heterojunction, enhance carrier collection. As a consequence, the efficiency of CZTSe solar cells has improved greatly from 6.91 % to 10.12 %. The single-target preparation strategy without selenization provides a simple and novel perspective for simultaneously adjusting the morphology and defects of kesterite photovoltaic materials, paving the way for promoting innovative industrial applications.

Origin of giant dielectric constant in Ta+Gd co-doped TiO2 single crystals by optical traveling floating zone method

The co-doped TiO2 polycrystalline ceramics have dramatic dielectric behavior (>104), but its source is still not completely clarified. Compared to ceramic materials, single crystals can maintain most of the original properties of the material and eliminate the grain boundary and pore interferences, thus facilitating the exploration of the source of the dielectric properties. Here, we chose Gd3+ with a moderate ionic radius as the acceptor ion and Ta5+ as the donor, preparing (Gd0.5Ta0.5)0.01Ti0.99O2 single crystals by the optical traveling floating zone method to investigate giant dielectric properties. It was found that a dielectric constant (ε′ ∼1.5 × 104), and a dielectric loss (tanδ ∼ 0.07) were achieved simultaneously in (Gd0.5Ta0.5)0.01Ti0.99O2 single crystals at 105Hz. Electrochemical impedance spectroscopy and XPS analyses indicate that the high dielectric properties are mainly attributed to electrons pinning defective dipoles clusters. In addition, dielectric relaxation behavior under DC bias suggests that electrode effects also affect the dielectric constant. This study provides insights for the origin of the large dielectric constant and the growth of single crystals in TiO2-based materials.

Fast prediction of irradiation-induced cascade defects using denoising diffusion probabilistic model

Irradiation-induced cascade collisions produce numerous point defects within materials, which can severely deteriorate their thermo-mechanical properties and overall performance. We propose a computational scheme that combines molecular dynamic (MD) simulations with a denoising diffusion probabilistic model (DDPM) to rapidly and accurately predict the spatial coordinates of point defects at any given primary knock atom (PKA) energy, ranging from 0 to 100.0 keV. Importantly, this capability extends to PKA energies that are exclusive from the training data set, demonstrating the robustness and generalizability of the model. The proposed scheme has been thoroughly validated by several designed indicators, including the Fréchet inception distance, the number of point defects, the distance from vacancies and self-interstitial atoms (SIAs) to their respective centroids, the inter-centroid distance between the vacancies and SIAs, the probability density of clustered defect sizes, and the sub-cascade number. Compared to MD simulations, the DDPM can generate point defects at a specific PKA energy at least ten thousand times faster. By offering a rapid and reliable means to model defect distributions across various energy levels, the proposed scheme benefits the comprehension of the cascade process and provides a valuable database for both experimental investigations and large-scale simulations.

Structural damage and bubble evolution in SiC-ZrC composite irradiated with 500 keV He-ions at various temperatures

Ceramic composites with high temperature strengths and low neutron cross-sections are promising candidates for core materials in advanced nuclear systems. In present work, SiC-20 vol% ZrC composites were irradiated with 500 keV He-ions at 25, 500 and 800 °C to evaluate the effect of irradiation temperature on the structural damage and bubble evolution in ceramic composites. XRD and Raman spectra analysis give that the irradiation resulted in structural damages of both SiC and ZrC. TEM observations reveal the formation of helium bubbles and defect clusters after irradiation. Moreover, the occurrence of micro-cracks in ZrC grains and amorphization of SiC are observed for the samples irradiated at room temperature. Nanoindentation test showed that there is irradiation induced hardening or softening of the composites which depends on the irradiation temperature or fluence. The correlation between microstructural evolution and mechanical properties response is discussed.

Achieving 11.23 % efficiency in CZTSSe solar cells via defect control and interface contact optimization

The high open-circuit voltage deficit (VOC, def) caused by structural imperfections in the absorber layer and charge loss during carrier transport is a critical barrier affecting the performance of Cu2ZnSn(S,Se)4 (CZTSSe) devices. In this work, Ag was added to the DMF-based Cu+-Sn4+ system, which significantly improved the crystal morphology and electrical properties of the absorber layer. Additionally, optimizing the selenization process not only reduced surface roughness and eliminated voids at the bottom of the absorber layer but also resulted in the formation of a MoSe2 back interface layer with a more suitable thickness. These measures collectively enhanced the overall quality of the absorber layer, reducing the formation of deep-level defect clusters and effectively boosting carrier transport efficiency. Consequently, the concentration of bulk and interfacial defects decreased, and the impact of potential barriers on carrier movement was minimized. With these comprehensive improvements, the power conversion efficiency of CZTSSe solar cells increased from 8.48 % to 11.23 %. Our research demonstrates that optimizing the structure of the absorber layer can effectively enhance the performance of CZTSSe solar cells, providing valuable insights for the fabrication of high-efficiency devices in the future.

Significantly Improved dielectric performance and Vickers-Hardness in High-Entropy TiO2-based Ceramics

Remarkable dielectric properties and enhanced hardness were achieved in high-entropy TiO2-based ceramics in this work by incorporating multiple pairs of donor-acceptor (D-A) cations (D = Nb5+, A = In3+, Al3+, Fe3+, Ga3+, La3+). The introduction of high-entropy engineering may facilitate the creation of favorable and stable microstructural features as lattice distorted and localized defect cluster, thereby enhancing the dielectric performance, thermal stability and mechanic properties. The composition of [Nb0.5(In0.2Al0.2Fe0.2Ga0.2La0.2)0.5]0.01Ti0.99O2 exhibits remarkably colossal permittivity (εr) value of approximately 18488 and low dielectric loss (tanδ) value of around 0.014 at 1 kHz. Furthermore, the thermal stability of the dielectric permittivity for [Nb0.5(In0.2Al0.2Fe0.2Ga0.2La0.2)0.5]0.01Ti0.99O2 ceramic meets the X9D standard, and the dielectric loss stays below ∼ 0.05 in the temperature range from -55 to 150 °C. Besides, the Vickers-hardness of [Nb0.5(In0.2Al0.2Fe0.2Ga0.2La0.2)0.5]0.01Ti0.99O2 gets increased by over 37% as compared to that of (Nb5+, In3+) co-doped ceramic (INTO). We presented in this work the influence of high-entropy on the dielectric and mechanic properties of TiO2-based ceramics, offering valuable insight into future development of colossal dielectric ceramics.

Variations of the residual magnetic fields of X70 pipeline steels with double defects

The aim of this research is to correlate residual magnetic fields (RMFs) with double defects in ferromagnetic steels. Specimens of X70 pipeline steels were machined into smooth plates with two types of defect (single and double defects) for comparison. Tensile tests were carried out to detect the RMF signals on the surface of the defective specimens. The variation of the tangential component of the RMF signals was presented and magnetic parameters were defined to investigate the abnormal magnetic changes of the two types of defect. It was found that the peak value of the tangential component was not influenced by defect types and the valley value was capable of evaluating the interaction effect between defects. Correlation between the RMF signals and the location of double defects was also investigated. This research will promote the investigation of defect clusters in ferromagnetic steels based on the metal magnetic memory (MMM) technique.

Optimizing the resistivity of colloidal SnO2 thin films by ion implantation and annealing

Tin oxide (SnO2) is a critical material for a wide range of applications, such as in perovskite solar cells, gas sensors, as well as for photocatalysis. For these applications the transparency to visible light, high availability, cheap fabrication process and high conductivity of SnO2 benefits its commercial deployment. In this paper, we demonstrate that the resistivity of widely colloidal SnO2 can be reduced by noble gas ion beam modification. After low energy argon implantation with a fluence of 4×1015 at.cm−2 at 25keV and annealing at 200°C in air, the resistivity of as-deposited film was reduced from (178±6)μΩcm to (133±5)μΩcm, a reduction of 25%. Hall effect measurements showed that the primary cause of this is the increase in carrier concentration from (8.1±0.3)×1020 cm−3 to (9.9±0.3)×1020 cm−3. Annealing at 200°C resulted in the removal of defect clusters introduced by implantation, while annealing at 300°C resulted in the oxidation of the films, increasing their resistivity. The concentration of oxygen vacancy defects can be controlled by a combination of low energy noble gas ion implantation and annealing, providing promising performance increases for potential applications of SnO2 where a low resistivity is crucial.

Realization of grain growth and suppressed bulk defects for efficient solution-processed Cu2ZnSn (S, Se)4 solar cells via co-doping strategy

Environmentally benign and earth-abundant constituent’s kieserite Cu2ZnSn (S, Se)4 (Cotises) is considered as a hopeful photovoltaic material. Unfortunately, the severe tail state caused by various harmful deep defects in CZTSSe absorber prevents further development of its device efficiency. Here, we report firstly that the Co was incorporated into CZTSSe films to form the Cu2CoxZn1-xSn (S, Se)4 (CCZTSSe) films using a two-step annealing and solution-processing technique method. Through optimized the Co/(Co+Zn) ratio, the CCZTSSe film has excellent good surface morphology and single-phase composition. Appropriate amount of Co elements replacing Zn elements in CZTSSe films can effectively reduce the degree of Cu-Zn disorder, thus inhibiting [2CuZn+SnZn] defect clusters and CuZn defects. The detrimental deep defects located in the film were effectively passivated to reduce electrostatic potential fluctuations, resulting in relief of the trailing state. Therefore, a champion device having an efficiency of 7.50 % with a fill factor (FF) of 61.00 % was achieved from CCZTSSe with 3 % Co content. This work provides new insights into the impacts of Co doping in CZTSSe by a solution processed method and offers a deeper comprehension of the origin of the efficiency enhancement of CCZTSSe devices.

Quantifying Spatial Distribution of Ventilation Defects in Multiple Pulmonary Diseases With Hyperpolarized 129Xenon MRI.

Hyperpolarized 129Xe MRI assesses lung ventilation, often using the ventilation defect percentage (VDP). Unlike VDP, defect distribution index (DDI) quantifies spatial clustering of defects. To quantify spatial distribution of 129Xe ventilation defects using DDI across pulmonary diseases. Retrospective. Four hundred twenty-one subjects (age = 23.1 ± 17.1, female = 230), comprising healthy controls (N = 60) and subjects with obstructive conditions (asthma [N = 25], bronchiolitis obliterans syndrome [BOS, N = 18], cystic fibrosis [CF, N = 90], lymphangioleiomyomatosis [LAM, N = 50]), restrictive conditions (bleomycin-treated cancer survivors [BLEO, N = 14]; fibrotic lung diseases [FLD, N = 92]), bone marrow transplantation (BMT, N = 53), and bronchopulmonary dysplasia (BPD, N = 19). 3 T, two-dimensional multi-slice gradient echo. Whole-lung mean DDI was extracted from DDI maps; correlated with VDP (percent of pixels <60% of whole-lung mean signal intensity) and pulmonary function tests (PFTs) including FEV1, FVC, and FEV1/FVC. DDI and DDI/VDP, a marker of defect clustering, were compared across diseases. Pearson correlation analysis and Kruskal-Wallis tests. P < 0.0056 for disease groups, P < 0.0125 for categories. DDI was significantly elevated in BMT (8.3 ± 11.5), BOS (30.1 ± 57.5), BPD (16.0 ± 46.8), CF (15.4 ± 27.2), and LAM (12.6 ± 34.2) compared to controls (1.8 ± 3.1). DDI correlated significantly with VDP in all groups (r ≥ 0.56) except BLEO, and with PFTs in CF, FLD, and LAM (r ≥ 0.56). Obstructive groups had significantly higher mean DDI (14.0 ± 32.0) than controls (1.8 ± 3.0) and restrictive groups (4.0 ± 12.0). DDI/VDP was significantly lower in the restrictive group (0.6 ± 0.6) than controls (0.8 ± 0.6) and obstructive group (1.0 ± 1.0). DDI may provide insights into the distribution of ventilation defects across diseases. 3 TECHNICAL EFFICACY: Stage 2.

First-principles investigation on the structural, optical, and laser-induced damage characteristics of boron impurity and boron-oxygen defect cluster in potassium dihydrogen phosphate crystals

The DFT+U method combined with HSE06 hybrid functional and finite-size correction has been employed to investigate the defect formation energies, lattice modifications and optical spectra of Potassium Dihydrogen Phosphate (KDP) crystals with Boron. The B defects contain the B interstitials (Bi), the B substituted P sites (BP), and the boron-oxygen defect clusters ([BP+VO]). The results demonstrate that these B defects can trigger lattice distortions. The BP and [BP+VO] have a minor lattice change rate and greater stability, while Bi has a relatively large local distortion inducing crystal damage. We have also utilized conformational coordinate diagrams to explore the optical absorption/emission processes arising from B defects. The BP and [BP+VO] were found to generate absorption peaks at 330 nm and 255 nm, while the Bi leads to absorption peaks near the absorption edge. The absorption peaks within the near-ultraviolet range might have an energy buildup at the defects, which would decrease laser utilization and harm the crystal.

Passivation of Deep‐Level Defects through Chemical Environment Regulation for Efficient CZTSSe Solar Cells Based on Active Selenium Adsorption–Desorption Process

AbstractInsufficient selenization and uneven distribution of elements caused by the poor diffusion and reaction activity of selenium clusters is one of the main issues limiting the efficiency of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. Here, this work designs a simple and feasible strategy to improve the activity of selenium (Se) by implementing high‐temperature treatment on graphite boxes loaded with Se pellets. The rapid adsorption/desorption characteristics of graphite on active gaseous small‐molecule selenium have successfully introduced hyperactive Se4, Se3, and Se2 into the selenization process. The results indicate that the adsorbed non‐toxic gaseous active Se3 and Se4 can quickly and uniformly diffuse into the precursor film at low temperatures, thereby inducing nucleation and grain growth at both surface and back interface simultaneously, which inhibits the upward migration and aggregation of cations, especially Cu, and promotes the homogenization of elements. The overall relatively Cu‐poor chemical environment suppresses the formation of CuZn defects and [2CuZn+SnZn] defect clusters, and also promotes the generation of favorable VCu. The band tail states and non‐radiative recombination are then optimized. Finally, the CZTSSe solar cells achieve a power conversion efficiency (PCE) of 14.5%, with VOC/VOCSQ of 67% being one of the highest in the literature.

On the capabilities of k-ART over MD for the study of the kinetics of small point defect clusters in α-Fe

Molecular Dynamics simulations, while contributing to the understanding of the mechanisms of diffusion and interactions of point defects and their clusters, are inherently limited in their temporal scope (few nanoseconds). This constraint becomes particularly evident when studying the dynamics of vacancies at low temperatures, where their jump frequency is exceedingly low, posing challenges for accurate reproduction. Additionally, the size of the simulation box imposes constraints, influencing the representation of the system and potentially affecting the accuracy of results. A relatively new kinetic activation-relaxation technique (k-ART) efficiently resolves the limitations of MD simulations, such as computation time and system dimensionality, without the need for a priori knowledge of the simulated system. This technique enables simulations lasting up to several seconds and encompassing systems with higher dimensions. In this paper we check the validity of k-ART to reproduce accurately the migration mechanisms and energies of point defects and small clusters, previously obtained by MD and validated experimentally. We point out the advantages and difficulties of using AKMC with k-ART.