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Dislocation clusters are the most harmful defect in cast crystalline silicon, and it is very hard to determine the various dislocation origins. In this work, we design an experiment to investigate the impact of silicon melt infiltration on the crystal quality of cast crystalline silicon grown by different types of seeds. The results show that the infiltration of silicon melt at the ingot bottom will introduce more metal impurities, thus increasing the length of low minority carrier lifetime at the ingot bottom, which also proved by the numerical simulation of iron diffusion. Moreover, the solidified silicon melt would introduce local mechanical stress, which significantly increases the generation probability of defects in the mono-Si seeds and seed junctions, but not degrade the seeding quality of the poly-Si seeds. These results give a new insight to reduce dislocation origins for casting high-quality crystalline silicon ingots.

The degradation of inverted metamorphic four-junction (GaInP/GaAs/In0.3Ga0.7As/In0.58Ga0.42As, IMM4J) solar cells irradiated by 1-MeV electrons was investigated via their spectral responses and the characterization of their electrical properties. As in the case of traditional three-junction (TJ) GaInP/GaAs/Ge solar cells, the electrical properties ( I sc, V oc, and P max) decrease with the logarithmic change of the electron fluence. The degradation of open-circuit voltage ( V oc) is slightly more pronounced than that of I sc in IMM4J solar cells because of the sum rule for V oc and the limit rule for I sc. The spectral response analysis showed that an In0.3Ga0.7As subcell was the most damaged subcell in the irradiated IMM4J solar cell, but an In0.58Ga0.42As subcell had the lowest initial short-circuit current density (Jsc), which was maintained throughout the irradiation test. We therefore focused on the In0.58Ga0.42As subcell. Deep-level transient spectroscopy (DLTS) experiments were realized to study emission and capture processes in two special full configurations of In0.58Ga0.42As and In0.3Ga0.7As subcells of the IMM4J solar cell. DLTS measurements reveal a dominant electron trap at 0.52 eV below the conduction band (Ec) of In0.58Ga0.42As, and the electron trap gradually evolved into Ec-0.46eV and Ec-0.58eV after 1-MeV electron irradiation. Based on the first-principles calculation, Ec-0.46 eV and Ec-0.58 eV can be assigned as ${\bf V}_{{\bf Ga}}^{\bf 0}/{\bf V}_{{\bf Ga}}^{{\rm{ - }}1}$ and ${\bf V}_{{\bf In}}^{\bf 0}/{\bf V}_{{\bf In}}^{{\bf - 1}}$ , respectively. However, only one shallow level Ec-0.03eV was observed within the bandgap of In0.3Ga0.7As after irradiation with DLTS detection. We summarize the issues faced for the space application of IMM4J solar cells by comparing the spectral responses of IMM3J, IMM4J, and TJ solar cells. Finally, the optimization of the design and fabrication of IMM solar cells are proposed.

The direct measurement of flexoelectric coefficients in epitaxial thin films is an unresolved problem, due to the clamping effect of substrates which induces a net strain (and hence parasitic piezoelectricity) in addition to strain gradients and flexoelectricity. Herein, we propose and demonstrate the use of van der Waals epitaxy as a successful strategy for measuring the intrinsic (clamping-free = flexoelectric coefficients of epitaxial thin films. We have made, measured, and compared BaTiO3 and SrTiO3 thin film capacitor heterostructures grown both by conventional oxide-on-oxide epitaxy and by van der Waals oxide-on-mica epitaxy, and found that, whereas the former is dominated by parasitic piezoelectricity, the response of the latter is truly flexoelectric. The results are backed by theoretical calculations of the film-substrate mechanical interaction, as well as by direct measurements that confirm the strain-free state of the films. van der Waals epitaxy thus emerges as powerful new tool in the study of flexoelectricity and, in particular, they finally allow exploring flexoelectric phenomena at the nanoscale (where strain gradients are highest) with direct experimental knowledge of the actual flexoelectric coefficients of thin films., This work was supported by the National Natural Science Foundation of China (Grants No. 12174174, No. 51962020, No. 51972157, No. 51702149, No. 11604135, and No. 11964017) and the Natural Science Foundation of Jiangxi Province (Grants No. 20212ACB214011, No. 20202ZDB01006, and No. 20192ACB21017) were also acknowledged. G.C. also acknowledges financial support from Project No. PID2019-108573GB-C21 funded by MCIN/AEI/10.13039/501100011033 and Severo Ochoa Grant No. SEV-2017-0706. L.S. thanks for support from Nanchang University.

The molecular dynamics method was used to simulate the fracture process of monocrystalline silicon with different sizes of point defect under a constant strain rate. The mechanism of the defect size on the mechanical properties of monocrystalline silicon was also investigated. The results suggested that the point defect significantly reduces the yield strength of monocrystalline silicon. The relationships between the yield strength variation and the size of point defect fitted an exponential function. By statistically analyzing the internal stress in monocrystalline silicon, it was found that the stress concentration induced by the point defect led to the decrease in the yield strength. A comparison between the theoretical strength given by the four theories of strength and actual strength proved that the Mises theory was the best theory of strength to describe the yield strength of monocrystalline silicon. The dynamic evolution process of Mises stress and dislocation showed that the fracture was caused by the concentration effect of Mises stress and dislocation slip. Finally, the fractured microstructures were similar to a kind of two-dimensional grid which distributed along the cleavage planes while visualizing the specimens. The results of this article provide a reference for evaluating the size effects of point defects on the mechanical properties of monocrystalline silicon.

Velocity gradient is the basis of many vortex recognition methods, such as Q criterion, Δ criterion, λ2 criterion, λci criterion and Ω criterion, etc. Except the λci criterion, all these criterions recognize vortices by designing various invariants, based on the Helmholtz decomposition that decomposes velocity gradient into strain rate and spin. In recent years, the intuition of “no vortex in straight flows” has promoted people to analyze the vortex state directly from the velocity gradient, in which vortex can be distinguished from the situation that the velocity gradient has a couple complex eigenvalues. A specious viewpoint to adopt the simple shear as an independent flow mode was emphasized by many authors, among them, Kolař proposed the triple decomposition of motion by extracting a so-called ‘effective’ pure shearing motion; Li et al. introduced the so-called quaternion decomposition of velocity gradient and proposed the concept of eigen rotation; Liu et al. further mined the characteristic information of velocity gradient and put forward an effective algorithm of Liutex (namely eigen rotation), and then developed the vortex recognition method. However, there is another explanation for the increasingly clear representation of velocity gradient, that is the local streamline pattern based on the critical-point theory. In this paper, the tensorial expressions of the right/left real Schur forms of velocity gradient are clarified from the characteristic problem of ∇v. The relations between the involved parameters are derived and numerically verified. Comparing with the geometrical features of local streamline pattern, we confirm that the parameters in the right eigen-representation based on the right real Schur form of velocity gradient have good meanings to reveal the local streamline pattern. Some illustrative examples from the DNS data are presented.

In this work, we proposed a new method of seed recycling for casting monocrystalline silicon ingots. Different regions at the bottom of the monocrystalline silicon bricks were cut for controlling its size and crystal defects, which were used as recycled seeds and reasonable splicing to induce grain boundaries. The results showed that the recycled seeds with higher concentration of metal impurities will result in a longer red zone and the yield of mono-Si ingot would be slightly decreased. The defect density in the recycled seeds increased significantly, but decreased greatly during the seeding and growth. The defect distribution in PL images at the top of ingots grown with recycled seeds was similar to that of ingots grown with new seeds. Furthermore, the average cell efficiency of the solar cells fabricated by the wafers grown with recycled seeds was only 0.1% different from that of the wafers grown with new seeds. Therefore, this low-cost seed recycling method is of great significance for reducing the cost of cast monocrystalline silicon.

The number of distinct components of a high-order material/physical tensor might be remarkably reduced if it has certain symmetry types due to the crystal structure of materials. An nth-order tensor could be decomposed into a direct sum of deviators where the order is not higher than n, then the symmetry classification of even-type deviators is the basis of the symmetry problem for arbitrary even-order physical tensors. Clearly, an nth-order deviator can be expressed as the traceless symmetric part of tensor product of n unit vectors multiplied by a positive scalar from Maxwell’s multipole representation. The set of these unit vectors shows the multipole structure of the deviator. Based on two steps of exclusion, the symmetry classifications of all even-type deviators are obtained by analyzing the geometric symmetry of the unit vector sets, and the general results are provided. Moreover, corresponding to each symmetry type of the even-type deviators up to sixth-order, the specific multipole structure of the unit vector set is given. This could help to identify the symmetry types of an unknown physical tensor and possible back-calculation of the involved physical coefficients.

This work proposes to utilize Si particles embedded in Si3N4 layer as low-cost seeds to produce high-quality multi-crystalline silicon (mc-Si) ingot by full-melting process. The nucleation mechanism and its effects on minority carrier lifetime distribution, PL defects and oxygen concentration were studied. The results show that the incompletely melted silicon particles embedded in the Si3N4 layer can effectively nucleate fine grains with uniform size and few defects. The yield of experiment ingot can be significantly increased due to the quite short length of bottom red zone. Finally, the prepared solar cells can also achieve high conversion efficiency and show low light-induced degradation (LID) value.

A composite nucleant with Si powder as the main component was introduced to prepare high-performance multicrystalline (HPM) silicon ingot in this work. The effect of different sizes of Si powder on the ingot performance was investigated and compared with the conventional HPM silicon ingot which seeded from poly-Si particles. It is proved that the composite nucleant with Si powder of large particle size (D50 = 355 μm) can induce small and uniform grains, showing uniform minority carrier lifetime distribution and low dislocation cluster area ratio. Meanwhile, the length of red zone at the ingot bottom is significantly shorter than conventional HPM silicon ingot, which significantly increases the yield of the ingot. The ingot grown with suitable composite nucleant can achieve high cell efficiency and the solar cells have low light-induced degradation (LID) value due to low interstitial oxygen concentration which are comparable to the conventional HPM silicon ingot.

A simply and reproducible way is proposed to significantly suppress the nucleation density of graphene on the copper foil during the chemical vapor deposition process. By inserting a copper foil into a tube with one close end, the nucleation density on the copper foils can be reduced by more than five orders of magnitude and an ultra-low nucleation density of ~10 nucleus/cm(2) has been achieved. The structural analyses demonstrate that single crystal monolayer graphene with a lateral size of 1.9 mm can be grown on the copper foils under the optimized growth condition. The electrical transport studies show that the mobility of such single crystal graphene is around 2400 cm(2)/Vs.