Si Ingots Research Articles

Crystalline silicon solar cell with an efficiency of 20.05 % remanufactured using 30 % silicon scraps recycled from a waste photovoltaic module

Several studies have aimed to develop methods for recycling and recovering valuable materials from waste photovoltaic (PV) modules to keep up with the increasingly stringent waste disposal guidelines. However, silicon (Si) recovered from disposed PV modules is rarely used as a raw material in the solar industry. In this study, 30 % Si recovered from waste PV modules was added to a feedstock to grow a 6inch single-crystalline Si ingot using the Czochralski method. The single-crystalline Si ingot was re-melted and manufactured to produce a higher quality single-crystalline Si ingot with a minority carrier lifetime of 274–1527 μs and purity of 7N7. Thereafter, a Si wafer was manufactured via cropping, squaring, and wafering. Finally, the Si wafer was used to fabricate a solar cell via a conventional process. Solar cell parameters, such as open circuit voltage, short circuit current density, fill factor, and efficiency, were analyzed using a solar simulator. The manufactured solar cell had an efficiency of 20.05 %, which is approximately 0.97 % lower than that of commercial wafer-based solar cells. Moreover, the factors influencing the solar cell efficiency were evaluated.

Importance of zero cross-point (Cp = 0) control between the vacancy and interstitial Si atom concentration curves to obtain a perfect Si ingot using Si melt growth

The cross-point (Cp), which appears in the vacancy (CV) and interstitial Si atom (CI) concentration curves, is a simple controlling factor for ingot growth and provides a final goal to obtain a defect-free Si ingot using general Si melt growth. The importance of Cp is demonstrated using the noncontact crucible method (NOC), which has a relatively small temperature gradient due to an ingot grown inside the melt. The control of temperature gradient G at a constant growth rate v is simple for the actual ingot growth; thus, the present simulated results are expressed by G at a constant v. The concentration at the Cp point becomes smaller as G becomes smaller and finally becomes zero (Cp = 0), in which CV and CI are practically zero (CV=CI=0). The estimated G and temperature T at Cp = 0 for v = 0.000144 cm s−1 are 6.3, 12.5, and 18.5 K cm−1 and 1216 K (943 °C), 1354 K (1081 °C), and 1371 K (1098 °C), corresponding to each combination. At 1216 K, G should be precisely controlled within 6.3 ± 3 % K cm−1 in the region substantially close to Cp = 0 to maintain the remaining concentration of point defects substantially lower than 1.0 × 1013 cm−3. G should be changed during growth from larger G to smaller G, such as 6.3 K cm−1, to efficiently grow the ingot within the limited time. The changing point, x of G, is an important key parameter for the timing variation, where x denotes the normalized distance from the growing interface. The initial G is large; thus, the changing point of G approaches the growing interface. The growth condition necessary to obtain a defect-free Si ingot can be demonstrated using the Cp = 0 point, which is fully compatible with the perfect critical point where CV and CI are practically zero. The present results are mainly considered using the CV and CI concentration curves. However, these results largely help understand the approximate trend of G and v for the growth of defect-free ingot and the relation between Cp and the commonly used critical point between CV and CI.

Effect of total iron and silicon contents and homogenization on the microstructure and performance of 6201 Al–Mg–Si ingots

Effect of total iron and silicon contents and homogenization on the microstructure and performance of 6201 Al–Mg–Si ingots

Analyzing structure loss in Czochralski silicon growth: Root causes investigation through surface examination

A considerable proportion of the grown Czochralski Si ingots are remelted due to the generation of dislocations or the so-called structure loss. The assessment of the failed ingots is a crucial step toward achieving higher yield. Nonetheless, characterizing Si ingots poses a challenge attributed to their inherent high brittleness and the elevated concentration of dislocations, further complicating the cutting process. In this work, we utilize a non-destructive method to categorize the failed ingots and reveal the root causes of this failure. This study is performed on roughly 80 industrial monocrystalline silicon ingots that are grown for solar cell applications. Most of these ingots have experienced structure loss during the body growth. Analyzing the surface features reveals major categories of the structure loss ingots. Approximately, 66% of the examined ingots seem to be linked with temperature fluctuations, specifically a sudden drop in the melt temperature, and contamination by foreign particles. It is found that the primary causes can be categorized into four major groups: melt contamination, thermal fluctuations, melt vibration, and variations in growth rate. The primary root causes are identified within a fishbone diagram.

Novel strategy for preparing a magnetic drawing solute for use in forward osmosis to reclaim water from the wastewater of diamond wire sawing of silicon ingots

The speedy growth of the semiconductor industries and global water scarcity issues have caused great demand for energy-saving water reclamation technologies such as forward osmosis (FO). Magnetic nanoparticles functionalized with osmotic polymer were believed to be one of the emerging materials used as the drawing solute in FO because they can be simply recovered with magnet. This study employed a novel strategy to prepare an FO drawing solute that balances the tradeoff between the amount of surface-immobilized ionic polymer for generating osmotic pressure and particle size with strong magnetization for ensuring steady recovery. The drawing solute was designed to be applied to the FO reclamation of water from the wastewater generated during the diamond wire sawing of Si ingots. Coprecipitation and polyol reduction were employed to synthesize poly(sodium acrylate)-functionalized magnetic nanoparticles (PSA-MNPs) as the drawing solute. The polyol route produced particles (PSA-MNPpolyol) sized 150–220 nm and composed of spherical subunits sized 3–5 nm. By contrast, coprecipitation resulted in irregular aggregated clusters and discrete 3–8-nm particles (PSA-MNPcp). Adding the electrostatic stabilizer sodium acetate to the precursor in the coprecipitation route (creating PSA-MNPcp+AC) significantly improved the particles’ dispersion and slightly increased their osmolality. When the optimal concentration of iron precursor was employed, the saturation magnetization and the osmolality of the PSA-MNPpolyol reached 45.89 emu/g and 258 mOsmol/kg, respectively. The average water flux generated when PSA-MNPpolyol was employed as the drawing solute was approximately 3.8 times higher than that generated when PSA-MNPcp+AC was used over a 3-h FO process on waste produced by ingot sawing. The homogeneous particle sizes of PSA-MNPpolyol are conductive to post-FO recovery through magnetic separation.

Improving quality of cast monocrystalline Si ingot with seed crystal strips and graphite soft felt

A combination of the two techniques, one that employs seed crystal strips and the other that makes use of graphite soft felts have been proposed and studied to improve the casting process of monocrystalline Si (mono-Si) ingots. The evolution of the melting process of Si seed crystals and the growth of edge Si muliticrystals during the cast of mono-Si ingot has been investigated in detail. The results of the study indicated that these two changes increased the proportion of full mono-Si wafers substantially and led to a considerable reduction in the average dislocation ratio of edge Si bricks based on cast mono-Si ingot. Solar cell devices fabricated based on cast mono-Si ingot with the above two improvement measures exhibited enhancement in their overall photoelectric conversion efficiency (η) by 0.29% and 0.64% (absolute value), in comparison to the solar cells based on mono-Si ingot that was cast using only the seed crystal strips and the conventional ingot, respectively. Therefore, the study provides sufficient evidence that utilization of seed crystal strips and graphite soft felts in combination with each other as supplementary measures to the casting process of mono-Si ingot can be scaled up to be used in the photovoltaic production line for the development of solar cells with higher performance.

Oxide unlocking electron beam melting to separate boron for impurities co-removal of metallurgical grade silicon

Removing stubborn B from Si efficiently and simplifying the purification process are critical challenges that must be addressed in the manufacture of solar grade silicon (SoG-Si) via metallurgical routes. Herein, B is flexibly coupled with other impurities separation procedure, so that metallurgical grade silicon (MG-Si) is purified through simplified electron beam melting (EBM) unlocked by a few oxides (0.1 wt%). EBM fulfills three functions including promotion of oxidation, evaporation of impurities, and induction of directional solidification, giving high-temperature field, high vacuum field, and intense melt convection, which not only ensures the smooth process of B oxidation but also favors the co-removal of other impurities, obtaining high purity Si. As a result, purified Si ingot exhibits a low content of B (from 12.76 ppmw to 2.92 ppmw), and the total content of major impurities is reduced from 1217.24 ppmw to 5.66 ppmw. Significantly, pure SiO2 could more readily access the high-temperature region of Si melt during EBM, offering distinct advantages for the oxidation gas phase separation of B. This work provides an efficient and robust method for expanding ideas of B removal and simplifying metallurgical routes.

Effects of Impurity Barrier Layer on the Red Zone at the Bottom of Cast Monocrystalline Si Ingot for Solar Cells

A technique to prepare low‐cost high‐purity quartz sheets as impurity barrier layers between the crucible and the seed crystal is proposed herein to improve the casting process of monocrystalline Si (mono‐Si) ingots. The results show that the diffusion of metal impurities from the crucible toward a cast mono‐Si ingot can be hindered, increasing the minority carrier lifetime of Si crystals and reducing the height of the red zone at the bottom of the ingot. Solar cell devices assembled from cast mono‐Si ingots exhibit the enhancement in their overall photoelectric conversion efficiency (η) by 0.76% in comparison to those based on conventional mono‐Si ingots. Therefore, high‐purity quartz sheets as impurity barrier layers for the casting of mono‐Si ingots have great application potential in the photovoltaic industry.

Preparation of High-Quality Silicon with Silicon Cutting Waste by a Carbothermal Reduction Method

Silicon cutting waste (SCW) mainly consists of Si (80 ~ 85 wt%), SiO2 (13 ~ 16 wt%) and other impurities (2 ~ 4 wt%). Nowadays, the Si in SCW is commercially recycled to produce Si ingots by a slag refining method, but the SiO2 in SCW is melted into silicon slag and discarded as waste. In this paper, a carbothermal reduction method has been proposed for recycling Si resources from both Si and SiO2 in SCW to prepare high-quality silicon in a submerged arc furnace. Petroleum coke was selected as the carbonaceous reducing agent. Firstly, the effects of carbon content on the equilibrium compositions of the Si-SiO2-C system were simulated. Secondly, SCW was mixed with petroleum coke under the guidance of thermodynamic analysis results. Finally, the mixtures were charged into furnace and smelted. Thermodynamic equilibrium analysis results showed that the value of n(Si):n(C):n(SiO2) should be controlled as 2.62:0.22:0.44 theoretically. Experimental results revealed that the recovery ratio of SCW was 50% and the purity of Si products was 99.40%. This proposed method provides an effective and industrialized applicable approach for recycling SCW.

Efficient recycling of silicon cutting waste for producing high-quality Si-Fe alloys.

A tremendous amount of silicon cutting waste (SCW) is being produced during slicing Si ingots, which leads to a great waste of resources and serious environmental pollution. In this study, a novel method that recycling SCW to produce Si-Fe alloys was proposed, which not only provides a process with low energy consumption, low cost, and short flow for producing high-quality Si-Fe alloys but also achieves a more effective recycling of SCW. The optimal experimental condition is investigated to be a smelting temperature of 1800°C and a holding time of 10min. Under this condition, the yield of Si-Fe alloys and the Si recovery ratio of SCW were 88.63% and 87.81%, respectively. Compared with the present industrial recycling method that uses SCW to prepare metallurgy-grade Si ingot by an induction smelting process, this Si-Fe alloying method can achieve a higher Si recovery ratio of SCW at a shorter smelting time. The promoting mechanism of Si recovery by Si-Fe alloying is mainly expressed as follows: (1) facilitating the separation of Si from SiO2-based slag; (2) reducing the oxidization and carbonization loss of Si by accelerating the heating of raw materials and reducing the exposed area of Si.

Slag refining at various viscosity conditions for SiC inclusion removal during Si scraps recycling

SiC inclusions in cast Si ingots are a key problem for Si quality and in the recycling of top-cut Si scraps. In this paper, SiC separation and Si recovery in the CaO–SiO2–MgO slag refining process were studied by varying temperature, slag/Si–SiC mass ratio, slag composition, and holding time. When the viscosity of the mixed system viscosity was 0.1575 Pa s, it was the best condition for the migration of SiC inclusions from Si to slag, and the removal ratio of SiC is 91.6%. The results indicated that low-viscosity slag and a long holding time are conducive to the migration of SiC. Further Image Pro software analysis showed that increasing the MgO content to 30mol% is beneficial to promote the migration of SiC. It is also found that the system viscosity effected the size of SiC in the slag phase. This work improves the removal of key inclusion SiC, reducing the cost of Si recovery.

Effects of metal impurities at the edges of cast Si ingot on crystal quality and solar cell performance

In this study, the effects of metal impurity diffusion at the edges of cast crystalline Si on crystal quality and solar cell performance were investigated and analyzed. The metal impurity diffusion resulted in a decrease of the minority carrier lifetime within Si crystal, low luminous intensity in photoluminescence (PL) images for wafers, high density leakage points in electroluminescent (EL) images and the decrease of photoelectric conversion efficiency(η) for solar cells. The study reveals that the region of low minority carrier lifetime (i.e., red-zone) in Si wafers can be narrowed down by using annealing treatment, in a temperature range of 500–600 °C. However, the annealing treatment on Si wafers had little effect on the efficiency of solar cells. A novel slicing direction for Si wafers, along to the crystal orientation (ACO), allowed the fabrication of Si wafers with non-red zone or with complete red-zone from the same side Si block. The η of the solar cells based on Si wafers with non-red zone was 1.7 % higher than that of the solar cells based on Si wafers with complete red zone. In addition to that, while being sliced from the same side Si block, the mean η of cells, based on wafers from the ACO slicing method was calculated to be 0.11 % higher than that of cells, which based on wafers from the traditional perpendicular crystal orientation (PCO) slicing method.

Fabrication of Crystalline Si Thin Films for Photovoltaics

Crystalline Si (c‐Si) thin films have been widely studied for their application to solar cells and flexible electronics. However, their application at large scale is limited by their fabrication process. As reviewed in this paper, many approaches have been studied, but only some of them have been made into large‐scale industrial production. The standard wire sawing of Si ingots cannot be scaled down to produce thin c‐Si wafers and films due to the brittle nature of c‐Si material, the resulting significant thickness variations, and the waste of material. Therefore, techniques based on the kerf‐less processes including “top‐down” and “bottom‐up” approaches have been developed in recent decades. In this review, photovoltaic applications of thin c‐Si wafers with thicknesses ranging from 50 μm down to 1 μm are presented first. Then, methods to fabricate c‐Si thin films based on both approaches are detailed, including slim‐cut, “smart‐cut,” epi‐free, as well as various growth processes such as molecular beam epitaxy, liquid phase epitaxy, ion beam, and chemical vapor deposition processes at a wide range of growth temperatures, from 1000 °C down to 150 °C. The advantages and disadvantages of these methods are presented and compared.

Upcycling of silicon scrap collected from photovoltaic cell manufacturing process for lithium-ion batteries via transferred arc thermal plasma

As the supply of photovoltaic industry products increases rapidly, measures to solve the upcoming related waste problem are urgently required. In particular, the fabrication process of Si wafers leads to significant Si scrap residue, and approximately 40% of crystalline Si ingots are wasted as Si scrap. Here, a feasibility study was conducted to investigate the recycling of Si scrap waste collected during the solar panel manufacturing process using the transferred arc thermal plasma. Si-NP with a particle size of 100 nm or less applicable as an anode material for lithium-ion batteries (LIBs) was successfully upcycled by the transferable arc thermal plasma method, and the developed LIB showed excellent electrochemical performance. Si-NPs electrodes show a discharge capacity of 2920 mAh g−1 at 0.1 A g−1, and 2167 mAh g−1 at 0.4 A g−1. In addition, silicon-graphite (Si-G) composites were also prepared to improve the electrochemical properties of LIBs. Furthermore, the possible pros and cons of the method proposed in this study were discussed including economic evaluation. These results show that the transferred arc thermal plasma is a simple, eco-friendly, and cost-effective method of upcycling Si-NPs anode material from Si scrap waste for high-performance LIBs.

Two-dimensional distribution of vacancy concentration at cross points for the ingot length and the temperature gradients to largely reduce point defects and the growth of a dislocation-free NOC ingot for evaluation of its quality

The noncontact crucible (NOC) method has a large and deep low-temperature region in Si melt to grow a uniform large Si single ingot. The cross point with the same concentration of vacancy (CV) and interstitial Si atom (CI) is very important to reduce the point defects in NOC ingot. The relation between the temperature gradient, G, the position and concentration of cross point was estimated using equations derived from Voronkov’ explanation. For NOC growth, two stages for temperature gradient are required as G1 at the growing interface and G2 at the melt surface. The linear T profile is useful for the two-stage growth. Concentration curve of vacancy and interstitial Si atom was calculated on the two stages as a function of the distance from the growing interface. A cross point appears above the melt surface even for the two-stage calculation with a long ingot inside the melt. By the two dimensional-distribution of CV at cross point for G2/G1 and d1, CV has a tendency to become higher as G2/G1 becomes larger. On the whole, the higher CV area appears in the larger G2/G1 and shorter d1 region, and the lower CV area appears in the smaller G2/G1 and longer d1 region. The latter condition is suitable for the NOC growth. Such trend of CV at cross points should be utilized to reduce the remained point defects in ingot. Lower CV and CI at the cross point can reduce the remained point defects in a NOC ingot using smaller G2/G1, smaller G1 and longer d1 near the critical point. To confirm the quality of NOC ingot grown using the present method, a dislocation-free Si single ingot was grown utilizing silica crucibles by the NOC growth method. The maximum diameter and total length of the grown ingot are 17.5 and 15.5 cm, respectively. A Si ingot was experimentally grown by the NOC method. The bottom of the NOC ingot had a convex shape in the growth direction which is very advantageous to NOC growth.It was confirmed by habit lines and etch-pit density of dislocation whether the ingot was dislocation-free or not. A dislocation-free Si single ingot was realized by the NOC method for the first time.

High-yield synthesis of ultrathin silicon nanosheets by physical grinding enables robust lithium-ion storage

Ultra-thin two-dimensional (2D) silicon nanosheets (SiNSs) have potential applications in electronic, energy storage, and energy conversion devices owing to their unique properties. However, high-yield and large-scale manufacturing of high-quality ultra-thin 2D SiNSs remains a great challenge. This report describes a simple, high-yield (>98%), and large-scale method for preparing ultra-thin 2D SiNSs. The developed approach improves the yield of SiNSs (thickness < 5 nm) by controlling the interaction force between the Si ingot and the abrasive grains during the diamond wire grinding process. The ultra-thin SiNSs deliver enhanced tap density and a limited variable solid electrolyte interphase growth interface. A dynamic chemical vapor deposition technique is carried out to increase the uniformity of the carbon coating on the ultra-thin SiNSs. Lithium-ion batteries employing SiNS-carbon composite anodes exhibit ultra-high initial Coulombic efficiency (88.1%) at a high Si content (79%). The full battery constructed with the fabricated SiNS-carbon composite anode and a commercial LiFePO4 cathode exhibits strong stability (1C over 600 cycles with a capacity retention rate > 80%). The results presented herein confirm the significant potential applicability of the developed method for synthesizing ultra-thin SiNSs with carbon coatings.

Data-Driven Optimization and Experimental Validation for the Lab-Scale Mono-Like Silicon Ingot Growth by Directional Solidification.

The casting mono-like silicon (Si) grown by directional solidification (DS) is promising for high-efficiency solar cells. However, high dislocation clusters around the top region are still the practical drawbacks, which limit its competitiveness to the monocrystalline Si. To optimize the DS-Si process, we applied the framework, which integrates the growing experiments, transient global simulations, artificial neuron network (ANN) training, and genetic algorithms (GAs). First, we grew the Si ingot by the original recipe and reproduced it with transient global modeling. Second, predictions of the Si ingot domain from different recipes were used to train the ANN, which acts as the instant predictor of ingot properties from specific recipes. Finally, the GA equipped with the predictor searched for the optimal recipe according to multi-objective combination, such as the lowest residual stress and dislocation density. We also implemented the optimal recipe in our mono-like DS-Si process for verification and comparison. According to the optimal recipe, we could reduce the dislocation density and smooth the growth rate during the Si ingot growing process. Comparisons of the growth interface and grain boundary evolutions showed the decrease of the interface concavity and the multi-crystallization in the top part of the ingot. The well-trained ANN combined with the GA could derive the optimal growth parameter combinations instantly and quantitatively for the multi-objective processes.

Semisolid Casting and Die Casting of Al-4.8%Mg-2%Si Alloy

An aluminum alloy, Al–4.8%Mg–2%Si, was cast by die casting and thixocasting, and the properties of the cast specimens were investigated. When the poured molten metal temperature was lower than 640 °C during die casting, it was lower than the liquidus temperature, and the metal became a semisolid slurry in the sleeve of the die casting machine; this fulfills the conditions for rheocasting. A tension test was conducted to investigate the effects of semisolid casting on the mechanical properties of Al–4.8%Mg–2%Si. The ultimate tensile strength and elongation of the ingots cast by die casting and rheocasting were affected by the size of ingot. When the ingot had a circular base of 4.5 mm diameter, the ultimate tensile strength and elongation were excellent; however, when the cross section of the ingot was a square with a side length of 20 mm, the tensile strength and elongation were inferior. The thixocasting was conducted using square ingots with a side length of 20 mm, and the tensile strength and elongation were poor in this case as well. The results of this study demonstrate that semisolid casting cannot improve the mechanical properties of Al–4.8%Mg–2%Si ingots with a high thickness. Semisolid casting cannot produce fine-grained Mg2Si, and the mechanical properties of the material could not be improved by this casting method.

Features of Impurity Segregation and Microstructure of Si Ingots obtained by Electron-Beam Purification of Metallurgical Grade Silicon

Features of Impurity Segregation and Microstructure of Si Ingots obtained by Electron-Beam Purification of Metallurgical Grade Silicon

Optimal Magnetic Graphite Heater Design for Impurity Control in Single-Crystal Si Grower Using Crystal Growth Simulation

In this paper, we report a successfully modified single-crystal Si growth furnace for impurity control. Four types of arbitrary magnetic heater (AMGH) systems with 3, 4, 5, and poly parts were designed in a coil shape and analyzed using crystal growth simulation. The concentration of oxygen impurities in single-crystal Si ingots was compared among the designed AMGHs and a normal graphite heater (NGH). The designed AMGHs were confirmed to be able to control turbulence and convection in a molten state, which created a vortex that influenced the oxygen direction near the melt–crystal interface. It was confirmed that replacing NGH with AMGHs resulted in a reduction in the average oxygen concentration at the Si melt–crystal interface by approximately 4.8%.