Silicon Ingots Research Articles

Comparison of LID and Electrical Injection Regeneration of PERC and Al-BSF Solar Cells from a Cz-Si Ingot

In order to study the effect of device structures and silicon wafer positions on light-induced degradation (LID) and regeneration, five groups of industrial PERC and Al-BSF solar cells were fabricated by using silicon wafers from different positions of a B-doped Czochralski silicon (Cz-Si) ingot. Then, the cells were subjected to a dark annealing (200 °C, 30 min), the first LID (45 °C, 1 sun, 12 h), an electrical injection regeneration (175 °C, 18 A, 30 min) and the second LID (45 °C, 1 sun, 12 h) in order, and the variations of performance of the cells with processing time were measured. It was found that after the electrical injection regeneration, the efficiency losses of PERC cells decreased from 1.28–1.76%absolute in the first LID to 0.09–0.16%absolute in the second LID, while those of Al-BSF cells decreased from 0.3–0.66%absolute in the first LID to 0 in the second LID. The efficiency losses of PERC cells during the first LID were caused by the co-action of B-O-defect-induced LID (BO-LID) and dissociation of Fe-B pairs, and the latter contributed 5.81–9.56% of the efficiency loss, while those of Al-BSF cells during the first LID were almost contributed by BO-LID solely. For both kinds of cells, the cells made from the silicon wafers from middle of the ingot had the best performance throughout the experiment. In addition, the LID and regeneration treatments only affected the spectral response of the cells in the wavelength larger than 700 nm.

COMPUTATIONAL STUDY OF RADIATION CHARACTERISTICS IN THE NICHE OF THE EXPERIMENTAL DEVICES OF THE WWR-K REACTOR

A nuclear reactor is a complex engineering and technical installation that generates radiation. Because of this, the experimental measurement of the radiation characteristics of a nuclear reactor is a difficult task, and in some cases technically impossible. The modern development of calculation codes and tools makes it possible to determine the radiation characteristics of a nuclear reactor with sufficient accuracy and reliability. Due to this, computational modeling of physical processes occurring in a nuclear reactor has become one of the main scientific research methods. The WWRK reactor is a multi-purpose research reactor with a large number of vertical and horizontal irradiation positions used for a wide range of scientific and applied problems. The irradiation position with the largest dimensions in the WWR-K reactor is the niche of experimental devices, which makes it possible to irradiate objects up to 1000 mm in diameter. This position is considered as a candidate for neutron transmutation doping of silicon ingots with a diameter of more than 200 mm. The article presents the radiation characteristics for the current and modernized configuration of the niche of the experimental devices of the WWR-K reactor. It is shown that the upgrade of the configuration leads to an improvement in the neutron characteristics in the irradiated position.

Suppression of dislocations and twins by inducing asymmetrical grain boundaries for casting high-quality monocrystalline silicon ingot

Cast monocrystalline silicon (CMC-Si) is a promising photovoltaic substrate owing to the advantage of low cost of directional solidification. However, the high dislocation density and other crystal defects, such as twins, have limited its further application. We proposed a new seed pavement method, namely, the construction of asymmetrical large angle grain boundaries, to suppress the generation of dislocations and other crystal defects. Two CMC-Si ingots, Ingot-A for the traditional method of seed pavement and Ingot-B for the new method of seed pavement, have been casted. The results show that, the crystal quality of Ingot-A is remarkably lower than that of Ingot-B. For Ingot-A, a large number of dislocations are induced, such as small angle grain boundaries, which come from the seed junctions due to size error of the seeds. And some twins are brought out at the symmetric grain boundaries, which reduces the proportion of the (100) crystal plane. For Ingot-B, there are large angle orientation relationships between the adjacent seeds. This asymmetrical large angle grain boundaries have not formed small angle boundaries, and successfully prevented the generation of dislocations and twins. The average conversion efficiency of Ingot-B (21.81%) is significantly higher than that of Ingot-A (20.63%).

Experimental Study on Surface Integrity of Solar Cell Silicon Wafers Sliced by Electrochemical Multi-Wire Saw.

Electrochemical multi-wire sawing (EMWS) is a hybrid machining method based on a traditional multi-wire sawing (MWS) system. In this new method, a silicon ingot is connected to a positive electrode; the slicing wire is connected to a negative electrode. Material is removed by the interaction of mechanical grinding and an electrochemical reaction. In this paper, contrast experiments of EMWS and MWS were conducted based on industrialized equipment to verify the beneficial effects of the hybrid method. The experimental statistical results show that the composite processing method improved the processing qualification rate by 1.28%, and the Bow of silicon wafers was reduced by about 2.74 microns. Further testing on the surface of the silicon wafer after electrochemical action showed that obvious holes were present on the surface, and the surface hardness of the wafer decreased significantly. Therefore, the scratches on the surface of wafer sliced by EMWS were reduced; in addition, the thickness of the surface damage layer was reduced by about 9 microns. After standard texturing, the average reflectivity of the wafers sliced by EMWS was about 2–10% lower than that of the wafers sliced by MWS in the wavelength of 300–1100 nm. In this paper, the voltage parameter of the composite machining is set to 48 V; the amount of electrolyte added in each experiment is 2 L; and a good machining effect is obtained. In the future, the electric parameters and cutting fluid components will be further studied to improve the electrochemical effect.

In-Situ LID and Regeneration of Al-BSF Solar Cells from Different Positions of a B-Doped Cz-Si Ingot

In this paper, five groups of industrial aluminium back-surface-field (Al-BSF) solar cells were made from silicon wafers from different locations of a B-doped Czochralski silicon ingot. Then, we performed the first LID (45 °C, 1 sun, 12 h), regeneration (100 °C, 1 sun, 24 h), and second LID (45 °C, 1 sun, 12 h) treatments on the cells, and measured the in-situ changes of their I-V characteristic parameters by using an I-V tester during the experiment. The cells were also characterized by Suns-Voc measurement, full-area light beam induced current scanning, and external quantum efficiency measurement at the four breakpoints of treatments (before and after the first LID, after regeneration and the second LID). It was found that the LID and regeneration of the Al-BSF solar cells can be explained by the LID and regeneration reaction of B-O defects and the LID caused by dissociation of Fe-B pairs. After regeneration, the relative decay rate of efficiency decreased from 2.75–3.8% during the first LID to 0.42–1.23% during the second LID, indicating that regeneration treatment (100 °C, 1 sun, 24 h) can improve the anti-LID ability of Al-BSF solar cells.

Hydrometallurgical purification of solar cell silicon scraps through chemical etching with HF-HNO3-H2SO4

The top-cut scraps of silicon ingots are a major source of waste in the production of polysilicon solar cells. In this study, purification of solar cell silicon scraps through chemical etching was carried out. Certain parameters, such as solvent types, etching temperature, ultrasonic power, and ultrasonic frequency were investigated. The optimal conditions were determined to be HF – H2SO4 – HNO3 mixed acid, reaction temperature of 60 °C, ultrasonic frequency of 45 kHz and ultrasonic power of 240 W. Chemical analysis demonstrated that iron was removed. Inclusions were removed under the observation of a microscope. Results indicated that the solar-cell silicon scraps were purified at a low cost.

Recovery and Purification of Silicon From Diamond Wire Saw Wasted Silicon Powder by a Technique of Induction Smelting Followed by Directional Solidification

A technique of induction smelting followed by directional solidification to recover and purify silicon from diamond wire saw waste silicon powder without the use of additives is presented herein. The silicon reaction and the effect of smelting time on the removal of impurities from silicon powder waste were investigated using a simple induction smelting and directional solidification method. Through the detection of byproducts and condensed volatiles, it is inferred that Si is volatilized and removed as silicon monoxide during smelting. Byproducts containing Si, SiC, or SiO2 phases can be generated by the reaction between silicon monoxide and graphite crucible after volatilization. Because the crucible is sealed with byproducts, the flow of volatile materials in the crucible disrupts the mass transfer balance in the silicon melt. The amount of residual impurities in silicon ingot trends downwards with the extension of smelting time. A 99 pct oxygen removal rate was obtained after the material was smelted three times. The silicon ingot achieved the lowest impurity content after smelting for 4 hours. The silicon powder waste can be potentially used as a metallurgical-grade silicon product after a simple one-step induction smelting process. Further purification of the silicon ingot by directional solidification can reduce residual impurities to 74 ppmw.

Efficient Neutron Irradiation Procedure for Uniform Dopant Distributions in Silicon Doping

Abstract Neutron transmutation doping (NTD) is one of the several techniques used for n-type doping with good homogeneity. NTD performances were investigated in this work within a project funded by the Academy of Scientific Research and Technology (ASRT), Egypt, for the “production of doped Silicon Wafers for Solar Panels and Basic Electronics,” 2015–2018. The Egypt second research reactor (ETRR-2) has two irradiation positions for silicon doping using NTD. The two positions can accommodate silicon ingots with large diameters up to six inches. One crucial parameter that affects the quality of the product is the distribution of the dopant in the irradiated ingot. The irradiation facility provides the option of rotating the sample during irradiation for flattening the radial distribution of the dopant. Since the neutron flux is not flat along the irradiation position due to leakage and the effect of control rods, the generated dopant will be not flat along the axial direction. To overcome this problem, this paper proposed an irradiation procedure to smooth the dopant distribution along the axial direction. Such procedure can be applied for NTD facilities that has not an acceptable axial neutron flux distribution in silicon doping process. The procedure is based on loading the irradiation position by two ingots staked end to end and, at the middle of the irradiation duration, the two ingots swap their places. This strategy was verified by MCNP5 calculations and experimentally.

Quality improvement of multi-crystalline silicon ingot by the Hot-Zone modification

Directional solidification process is the dominant method for the multi-crystalline silicon wafer production due to the lower production cost, simple operating process and mass production. Numerical simulation is the best tool to understand and enhance the multi-crystalline silicon ingot quality by using the directional solidification process for photovoltaic application. We improved the multi-crystalline silicon ingot quality by adding an additional graphite layer on the hot-zone of directional solidification furnace. We have simulated both modified and conventional directional solidification process and compared the temperature distribution, growth rate and stress distribution of both systems. The modified directional solidification system significantly increases the multi-crystalline silicon ingot quality compared to the conventional directional solidification process grown multi-crystalline silicon ingot. With the modified directional solidification system we have prevented the wall growth during the casting process.

Thermal hydraulic characteristics of silicon irradiation in a typical MTR reactor

Abstract In this work, a three-dimensional study is conducted for silicon ingot irradiation facility in a typical MTR reactor. The silicon ingot heat generation is allowed to change around its nominal value in the range of 0.25, 0.5, 0.75, 1.0, and 1.25 W/g. The dimensionless clearance ratio ζ between the ingot and the outer casing is changed for values of 0.01, 0.05, 0.1, 0.15, and 0.2. The computational fluid dynamics code ANSYS FLUENT is employed to carry out these calculations. The temperature profile inside the silicon ingot during irradiation is of significant importance for uniform doping. Therefore, the impact of Rayleigh number and dimensionless clearance ratio ζ on the radial and axial temperature profiles through the ingot is reported. A best estimate correlation for the silicon irradiation facility global Nusselt number in terms of Rayleigh and dimensionless clearance ratio is presented. The dimensionless clearance ratio ζ value of 0.05 gives the best thermal hydraulic performance. Comparison with the available experimental correlations in the literature shows good agreement.

Design and Numerical Study of Argon Gas Diversion System Using Orthogonal Experiment to Reduce Impurities in Large-Sized Casting Silicon

To reduce oxygen and carbon impurities while casting silicon, an argon gas diversion system is proposed. A series of two-dimensional global transient numerical simulations are carried out using Fluent software according to the orthogonal experimental design, including heat transfer, convection of silicon melt and argon gas, and the fully coupling transport of impurities. The numerical results show that when the distance between the outer tube outlet and the cover is 10 mm, the backflow is inhibited by lateral outflow, thus the generation of CO is suppressed and the penetration of impurities into the silicon melt is decreased. The larger the flow rate, the more obvious the effect is. When the outer tube outlet is far from the cover, the effect of removing impurities is no longer significant. In addition, too large or too small an inner tube flow rate is not conducive to impurity reduction. The optimal parameter combination of outer tube flow rate, inner tube flow rate, and the distance between outer tube outlet and the cover are determined by the orthogonal experiment. Compared with the original furnace, the average concentration of oxygen and carbon in casting silicon ingots could be decreased by 7.4% and 59.9%, respectively, by using the optimized argon gas diversion system.

Recycling of silicon scraps by electron beam melting coupled with directional solidification and its mechanism of impurity removal

Silicon scraps, as the by-product of solar cells, have received more and more attention for its recycling with the continuing development of the cleaner production. However, hard particles and metal impurities in the silicon scraps are always difficult to remove deeply in industry. In this paper, electron beam melting coupled with directional solidification was used to purify silicon scraps and a silicon ingot containing low impurity was obtained by simultaneous removal of hard particles and meatal impurities. It is found that the hard particles, such as SiC and Si3N4, occurred to effective collision under the action of melt convection and linked to each other as clusters. With the convection of melt, the clusters would be captured in the lower temperature zone at the bottom of crucible to become a part of the condensate shell, which was greatly beneficial to complete removal of hard particles. In the directional solidification process, the electron beam accelerated the melt convection due to its point source heating mode, and promotes the diffusion of metal impurities. Most impurities were concentrated in the center of the top of the ingot. In other purified areas, the content of Fe, Ti were less than 0.11 ppmw, respectively. Purified silicon scraps were used to manufacture high-performance polycrystalline silicon and its average minority carrier lifetime reached 6.90 μs. The results suggested that this method is feasible to achieve the recycling of silicon scraps.

Electrophoretic Deposited Quartz Powder-Assisted Growth of Multicrystalline Silicon

Ingot multicrystalline silicon (Mc-Si) needs to be improved in quality and reduced in cost compared with Czochralski monocrystalline silicon. A uniform and dense quartz nucleation layer was obtained by the electrophoretic deposition of quartz powder on the surface of the silicon wafer. The deposited silicon wafer was annealed at 600 °C for 1 h, and one side of the silicon wafer with the quartz layer was glued to the crucible. During the growth of Mc-Si crystal, the dense quartz powder can play a nucleation role. The results show that the average lifetime of the minority carriers a of quartz-assisted silicon ingot is 7.4 μs. The overall dislocation density of an electrophoretic deposition quartz-assisted silica ingot is low, and the defect density in the middle of the silica ingot is 1.5%, which is significantly lower than that of spray quartz (3.1%) and silicon particle (4.2%). Moreover, electrophoretic deposited quartz-assisted mc-Si can obtain oriented grains, which offers a potential to apply alkaline texturing on mc-Si wafers.

Effects of Heat Extraction Methods on the Quality of High Performance Multi-Crystalline Silicon Ingot

Effects of Heat Extraction Methods on the Quality of High Performance Multi-Crystalline Silicon Ingot

Impurity-impurity interaction during the growth of UMG-Si-based mc-Si

This article investigates the relationship between the chemical composition and electrophysical properties of p- and n-type multicrystalline silicon ingots based on metallurgical silicon with a purity of 99.99 at.%. In particular, the role of impurity-impurity interactions in the production of multisilicon by the Bridgman vertical method is evaluated in order to identify approaches to controlling this process effectively. The phase equilibrium calculations in the “silicon–all impurities” and “silicon-impurity-oxygen” systems were carried out based on the Gibbs energy minimization in the Selector software package. The study investigates the rank correlations of the concentrations of various impurities with each other, as well as with the specified electrical resistivity (SER) and the lifetime of nonequilibrium charge carriers (NCC) in the direction of crystal growth. Pair correlations of the element distribution profiles were considered based on the role of the main factor represented by the ratio of individual impurity solubilities in solid or liquid silicon (k0), as well as from the standpoint of direct interaction between two elements. It was found that the k0 value for two individual impurities in silicon does not automatically lead to the pair correlation of their distribution profiles in the ingot. A significant effect on the distribution profiles of impurities in multisilicon with k0→0 has the factor of binding some part of the impurity into such a form that this impurity can be incorporated easily into a growing crystal. Binding may be induced by the interaction of the impurity in the melt with the oxygen background, its segregation at the grain boundaries, and its capture by the crystallization front in the composition of the liquid inclusion. Significant correlations of impurity distribution profiles in the ingot were demonstrated by the pairs whose elements interact without the formation of chemical compounds in the 25–1413 °C temperature range. The conducted phase equilibrium calculations for the “silicon–all impurities” system revealed the possibility of forming the VB2, TiB2, ZrB2, and MgTiO4 solid phases in the melt.

Polycrystalline Silicon Membranes for Solar Cells Fabricated Using Water-soluble Sacrificial Layers

During the fabrication of crystalline silicon solar cells, kerf-loss caused by the wire-sawing of silicon ingots to produce thin wafers inevitably limits the reduction of electricity production cost. To avoid the kerf-loss, direct growth of crystalline silicon wafers of 50-150 μm with a porous separation layer that can be mechanically broken during the exfoliation process, has been widely investigated. However, several issues including flattening of the surface after the exfoliation remain unsolved. In this work an alternative method that utilizes a water-soluble Sr3Al2O6 (SAO) sacrificial layer inserted between the mother substrate and the grown crystalline silicon layers is introduced. Polycrystalline silicon layers were grown on SAO/Si by plasma-enhanced CVD process and silicon membranes could be successfully obtained after the dissolution of SAO in the water. Same process could be applied to obtain flexible amorphous silicon membranes. Further research is being conducted to increase the size of the exfoliated wafer, which expects to reduce the production cost of crystalline silicon solar cells effectively.

Crystal growth and resistivity modulation of n-type phosphorus-doped cast mono-like silicon

Resistivity tailing seriously reduces the production of phosphorus-doped n-type cast mono-like silicon (CM-Si) ingots and increases costs. However, it is believed that a uniform axial dopant profile can be efficiently obtained by adjusting furnace pressure to enhance phosphorus evaporation at the ingot top due to high saturated vapor pressure of phosphorus in molten silicon. To increase the resistivity at the ingot top, we compared the growth of n-type cast mono-like silicon ingots under normal and reduced pressure. Ingots quality was characterized, and resistivity distribution was measured and discussed. The fitting results of resistivity suggest that Scheil’s equation which describes the solute redistribution during the non-equilibrium solidification process of the crystals can be used to calculate theoretical resistivity distribution of lightly phosphorus-doped ingots under normal furnace pressure (600 mbar), but it need to be modified in depressurization process. The modified results shows that the physical growth under reduced pressure is a coupling result of evaporation and segregation, of which a model to calculate the mass transfer coefficients of phosphorus in silicon is established. The obtained mass transfer coefficients illustrate that the transfer of phosphorus through gas phase to external environment is the controlling step, and depressurization can enhance the evaporation of phosphorus to a certain extent by increasing the total mass transfer coefficient. This work is supposed to pave an economy-effective way to fabricating n-type CM-Si ingots with homogenized axial resistivity profile in industry.

Numerical and experimental investigation on mc-Silicon growth process by varying the Si3N4 coating thickness of crucible

In the present work, the bottom-opening directional solidification technique was adopted to grow multi-crystalline silicon ingot. By varying the thickness of Si3N4 coating on the quartz crucible, the variation in carbon and oxygen concentration in the ingot has been analyzed. The axis-symmetric time-dependent 2D modeling has been done to analyze heat and mass transfer within the directional solidification furnace. We have analyzed the concentration of carbon monoxide, silicon oxide, silicon carbide, carbon and oxygen in the grown mc-Si ingots by varying the Si3N4 thickness. The oxygen and carbon impurities concentration are higher for the Si3N4 coating thickness of 50 μm to 150 μm and lower for 200 μm. The minority carrier lifetimes of the top, middle and bottom wafers have been analysied. The results show that the middle wafer has higher minority carrier lifetime compared to top and bottom wafers.

Investigation of orientation, surface morphology, impurity concentration and reflectivity of the multi-crystalline silicon wafers

Multi-crystalline silicon (mc-silicon) ingot was grown using the directional solidification (DS) method. The grown ingot was sliced, and the grain orientations, grain size, surface morphology, and impurities concentration in the wafers were studied. The XRD pattern of as-cut wafers exhibited the formation of a more number of (111) oriented plane in the grown ingot. The scanning electron microscopy images showed the formation of worm-like trenches after the chemical etching of the sliced wafers. FTIR spectra showed the higher carbon concentration on the top wafers and higher oxygen concentration at the bottom wafers. At the central wafer of the middle brick, the calculated carbon and interstitial oxygen concentration result exhibited 4.740 × 1016 and 1.369 × 1017 atoms/cm3, respectively. The impurity concentration of carbon and oxygen is validated with simulation results. The reflectivity spectra revealed low reflectivity for the wafers sliced from the central brick compared to the peripheral brick. The presence of very less impurity concentration and low reflectivity of the middle wafer in the central portion of the ingot enhances the light absorption, which is beneficial for photovoltaic energy generation.

Improve solar cell performance of high-performance multicrystalline silicon seeded with low-cost compact nucleation layer

In order to further improve the quality of high efficiency multicrystalline silicon and the performance of multicrystalline silicon solar cells, we designed a compact nucleation layer on the crucible bottom for casting high performance multicrystalline silicon ingots. The morphology of nucleation grains, the minority carrier lifetime mappings and the defects were analyzed and compared with that of the conventional nucleation layer. The results show that the initial nucleation grains of the compact nucleation layer were finer and more uniform, and there are fewer grains with high defect density at the ingot bottom，which is beneficial to reduce the defect ratio of the whole ingot. Moreover, the solar cell efficiency and electrical performance parameters, such as Rsh was also discussed. It was found that the Rsh has a great impact on the efficiency of multi-busbars solar cells, and the average solar cell conversion efficiency of the wafers grown with the compact nucleation layer is 0.11% higher than that of the wafers grown with the conventional nucleation layer.