Diamond Wire Sawing Research Articles

A low cost additive-free acid texturing process for large area commercial diamond-wire-sawn multicrystalline silicon solar cells

Diamond-wire-sawing (DWS) technique allows slicing of silicon ingots to produce wafers at cheaper price due to its reduced kerf-loss and increased cutting rate. However, there is no cost-effective and industrially viable method available for texturing DWS wafers, especially for multicrystalline silicon (mc-Si) wafers. Currently additive-based acid texturing process is availed by PV manufacturing units for texturing DWS mc-Si wafers. An additive-free, low cost and energy-efficient acid texturing process is demonstrated for DWS mc-Si wafers in industrial production line. Nearly 10% absolute reduction in weighted average reflectance values (WAR) are noticed for the newly textured wafers from that of as-cut DWS mc-Si wafers and the WAR values are comparable to that of existing additive-based acid textured mc-Si wafers. An absolute reduction in emitter saturation current density by ~17 fA-cm-2 and improved implied open circuit voltage of ~5 mV (absolute) are reported for lifetime sample fabricated using newly textured wafers when compared to the additive-based acid textured lifetime samples. An impressive batch average efficiencies of 18.20% and 18.24% are achieved for the additive-free and existing additive-based acid textured mc-Si cells, respectively. Detailed analysis conclude that ~1.5% (relative) enhancement in short circuit current density can be achieved by further process optimization. Cost analysis indicates that 60% of the chemical cost involved the texturing process can be cut down by replacing the existing additive-based acid process with the new method using the same process equipment. Hence, the texturing process presented has great potential for producing large area high efficiency mc-Si cells favoring cost considerations without compromising performance.

A metal-free additive texturization method used for diamond-wire-sawn multi-crystalline silicon wafers

Diamond-wire-sawn (DWS) technique has been widely applied in PV industry. However, this technique still has a big challenge when it is used for multi-crystalline silicon (mc-Si) because its reflectivity of DWS mc-Si etched by the conventional acid etching solution (HF/HNO3/H2O system) is quite higher. This paper proposes a metal-free additive texturization method, in which polyethylene glycol (PEG) is added to the HF/HNO3/H2O system to reduce the reaction rate of silicon wafers and improve the wettability of the etching liquid and the surface of mc-Si wafers. The etching points will gradually turn into nanopores instead of worm-like structures due to low reaction rates. Those nanopores form a gradually changed refractive index between silicon and air, which immensely reduces the surface reflectivity of mc-Si wafers. The mc-Si wafers processed in the experiment have an average surface reflectivity of about 16.9% in the wavelength range of 300–1100 nm, which is generally lower than the mc-Si wafers etched by the conventional acid etching process. This technical route has been applied to fabricate the 156 mm × 156 mm DWS mc-Si solar cells, and finally, the conversion efficiency of 19.39% has been obtained. Moreover, this approach matches well with the current PV industrial processes, showing a promising prospect.

Surface modification of diamond wire sawn multi-crystalline silicon wafers by grinding pre-treatment for wet acid texturization

Grinding as a pre-treatment method for HF-HNO3-H2O solution etching of diamond wire sawn (DWS) multi-crystalline (multi-Si) silicon wafers was proposed and researched. Significant improvement in morphological uniformity and decrease in light reflectivity of the wet acid etched surface textures were achieved with the pre-treatment. The photoelectric-conversion efficiency gain of solar cells fabricated with the pretreated DWS multi-Si wafers was 0.32%, compared to solar cells fabricated with the same batch of untreated wafers in an identical production line. The grinding pre-treatment was observed to restructure the superficial layer of DWS multi-Si wafers. The surface modification layer was believed to be more easily reacted with the wet acid etching solution, proving responsible for the improvement of photoelectric-conversion efficiency of solar cells. This method could provide a new research idea for the wet acid texturization of DWS multi-Si wafers.

A laser texturing study on multi-crystalline silicon solar cells

A laser texturing process including laser ablation and post-etching has been developed to form a unique microstructure of pits in craters (PIC) on the front surface of the diamond wire sawn multi-crystalline silicon (DWS mc-Si) wafer. A laser ablation is applied to produce craters with size of tens of microns distributed at the front surface of the wafer, followed by a post-etching process to remove silicon slags produced in the laser process and to form the pits of size in few microns embedded in each crater. The structural, optical and electrical characteristics of the PIC structure are investigated and optimized to achieve a good trade-off between light trapping and recombination. Finally, an applicative laser texturing process is demonstrated for the industrial-grade DWS mc-Si solar cells with an increased efficiency of 19.26% compared with the standard acid textured ones (19.10%).

Effects of H2O2, Cu(NO3)2 and HF temperatures on surface texturization of diamond-wire-sawn multicrystalline silicon wafer

Previous studies have considered the same temperature for all components of the silicon etching solution, and the effect of the temperature of the individual components on the etching results has not been explored. In the present study, a diamond-wire-sawn (DWS) multi-crystalline silicon wafer was etched twice for 50 s each time. The initial temperature of HF was selected to be room temperature, 55 and 66 °C, whereas those of H2O2 and Cu(NO3)2 varied between 40 and 70 °C, 40 and 72 °C, respectively. The temperature of the three components of the solution was allowed to change over time. The effects of H2O2, Cu(NO3)2, and HF temperatures on the etch morphology, reflectivity, and energy conversion efficiency of the fabricated solar cells were investigated. The saw marks of a DWS wafer were determined to be the preferred nucleation sites for the formation of the inverted pyramid (IP) structure. The optimum temperatures for etching Si to obtain a large area of IP structure were ~72 °C for Cu2+, ~25 °C for HF, and ~55 °C for H2O2. The strong isotropic etching ability of high-temperature HF caused the silicon crystal surface to deviate from the IP structure and left many recombination centers on the surface. A wafer with an IP structure on the entire surface exhibited the highest energy conversion efficiency of 18.21%. A simple wet-chemical post-processing step is expected to reduce the recombination centers and improve the energy conversion efficiency of the fabricated solar cell.

Experimental and numerical analysis of scratching induced damage during diamond wire sawing of silicon

Diamond wire sawing (DWS) is the dominant manufacturing process for thin silicon substrates. The understanding of micromechanical damaging and material removal mechanisms is essential for improving the reliability of the processes and the quality of the products. Due to the lack of models that are capable to predict subsurface damage during diamond wire sawing of silicon, process analyses are so far only possible on the basis of models originally developed for loose abrasive sawing (LAS). However, the application of LAS models to the DWS process does not seem appropriate, because of the different underlying deformation mechanisms. Within this work a three-dimensional Finite-Element analysis is conducted in order to investigate the cracking mechanisms during the sawing process. Experimental as well as numerical studies concerning median and lateral cracking are carried out using a Vickers indenter scratching on silicon. The results show that the numerically predicted dimensions of damage zones are in good agreement with the experimental results and that the new model is suitable for the analysis of subsurface damage during diamond wire sawing of silicon.

Recovery of silicon kerf waste from diamond wire sawing by two-step sintering and acid leaching method

Silicon kerf waste from diamond-wire sawing of solar grade silicon is regarded as a promising resource for the clean production of high-purity silicon. In this study, silicon kerf waste is collected by a two-step sintering method (TSS) to obtain a ceramic product for the first time and its effect on the leaching behavior of silicon kerf was assessed. The results show that the relative density and grain size of silicon kerf ceramic could reach 91.72% and 0.75 μm under TSS3-10 h condition (1400 °C in the first stage and 1350 °C for 10 h in the second stage). The TSS process is confirmed to be beneficial for decomposing silicon oxide layer and promoting densification. Acid leaching experiment was conducted and the relationship between the relative density and impurity extraction was evaluated. It was found that the removal rate of impurities (except oxygen) from silicon kerf by direct acid leaching was higher than that through TSS3-10 h and acid leaching. However, the combined process can reduce 52.4% oxygen content and the yield of silicon was reached 96.03%. The present study aims to shed light on the feasible application of TSS on the hydrometallurgical recycling of silicon kerf.

Dimension Stone: Exploration and Exploitation in parts of Lokoja, North-Central Nigeria

The study area Lokoja is the home to the confluence between Rivers Niger and Benue. Dimension stone is cut or quarried stone in accordance with specific dimensions, generally more abundant and are used in greater quantities than ore minerals in pavement and several other constructional activities. The objective of the paper is to present prospects and a lead of rocks in Lokoja area for the production of dimension stone. The rocks in the area are being quarried as aggregates or mined as dimension stone and they met quality parameters of the rocks which includes, must be durable, easy to quarry, work, cut and polish. The particular stones selected depend upon the local weathering conditions which can be determined by amounts of industrial gases present and availability of the rocks locally. The rocks identified are marbles, granite gneisses, migmatites and biotite granites. The granites are being quarried for aggregates, marbles are powdered for cement production while migmatites and granite gneisses were mined for dimension stone.The results of Rock Strength (MPA), Rock Class Factor and Swell Factor are within acceptable standard limits except for migmatites with RS 36%, RCF 7% and SF 1.25%, which is above the limit when compared with previous analysis of AAV 22.0%, ACV23.3%, AIV18%and Absorption 0.54% in Boki area. The dimension stone were separated by more precise and delicate techniques, such as diamond wire saws, diamond belt saws, in the mine the cut dimension stone width are 1m, while many are rectangular in shape. They occur in the same geologic setting of the basement complex of Northcentral Nigeria. In the area investment has been at relatively low stage more investment will significantly increase production of the dimension stone for local usage, export and economic growth.
 Keywords:Dimension stone, durable, available, maintenance, migmatite, Lokoja

Effect of the anisotropy of etching surface morphology on light-trapping and photovoltaic conversion efficiencies of silicon solar cell

A homogeneous diamond wire sawing multicrystalline Si surface with nanoscale oval pits was obtained in an acid solution by adding NaNO2, polyethylene glycol–polyvinyl alcohol, and dodecylbenzene sulfonic acid at 12 °C for 130 s. The textured surface showed orientation dependence. The anisotropy of H/D caused different experimental results. The Rave of incident light originating from the direction parallel to saw marks was 22–27% larger than that from the direction perpendicular to saw marks. The photovoltaic conversion efficiency was 0.6–0.8% higher when the thin grid line of Ag electrode was parallel to the saw marks than when in the perpendicular direction. These results indicated that using saw marks can improve the conversion efficiency of solar cells.

Texturization of diamond wire sawn multi-crystalline silicon wafers by micro-droplet etching

Diamond wire sawn multicrystalline silicon (DWS mc-Si) solar cell wafers have been known to be difficult to texturize by conventional acidic wet etching texturization process. A novel texturization method based on localized micro-droplet etching (MDE) has been developed, in which the micro-droplet is generated by controlled condensation of thermal vapor from a HF–HNO3–H2O bath. Uniformly distributed micron scale etch-pit type surface textures on DWS mc-Si wafers are made by the MDE process. Their light reflectivity is reduced to 18–21%, and their sawmarks are removed. Full size DWS mc-Si wafer samples were MDE textured, and send to a solar cell fabrication line along with the conventionally wet textured DWS mc-Si wafers as reference, without any adjustment of processing parameters for the MDE textured wafers. The average short circuit current of the MDE textured cells has been found to increase as compared to the reference cells, in a relative ratio approximately matching the final light reflection reduction ratio induced by the MDE textures. The average open circuit voltage and fill factor of the MDE textured cells are found to increase slightly, indicating that the MDE texture has not induced any significant increase in surface recombination of carriers, neither poorer contact of printed grid contact on cell surfaces. In average, 0.52% absolute increase of energy conversion efficiency is obtained.

Forming submicron in micron texture on the diamond‐wire‐sawn mc‐Si wafer by introducing artificial defects

AbstractBased on a traditional acid etch system (i.e., HNO3/HF), a complex texture comprising microscale and submicroscale structures was produced on the surface of a diamond‐wire‐sawn (DWS) multicrystalline Si (mc‐Si) wafer, upon whose surface it is typically difficult to form an effective texture for suppressing the reflection of incident light. Immersing the as‐cut wafer into an HF/HNO3/AgNO3 solution introduced a large number of artificial defects onto the wafer surface. A subsequent HNO3/HF etch induced a micron texture expanded from the original DWS‐induced damage as well as a submicron texture converted from the artificial defects. The multiscale textured DWS exhibited a reflectivity of ~19%, which is much lower than the reflectivity after only an HNO3/HF etch (~28%). Therefore, the solar cell performance was improved owing primarily to improved optical antireflection and surface passivation. The method is simple and can be easily scaled up into the in‐line texture process.

Novel Reaction Media of Na2CO3–CaO for Silicon Extraction and Aluminum Removal from Diamond Wire Saw Silicon Powder by Roasting–Smelting Process

Exploring a sustainable process to dispose diamond wire saw silicon powder (DWSSP) waste produced in the solar crystal silicon wafering process is a research field with significant theoretical rese...

Optical properties of chain inverted pyramids on silicon.

Pyramidal structures, including upright pyramids and inverted pyramids (IPs), are commonly used as light-trapping structures for silicon solar cells and silicon photodetectors. In this paper, the possible ray propagation paths in a pyramidal structure are analyzed by establishing a mathematical model in which up to seven ray paths may exist either in a regular or random pyramidal structure. To reduce the reflectivity, the proportion of the quadruple bounce should be increased because of its lower reflectivity. Therefore, a chain IP structure with a quadruple bounce proportion of 10.33% is proposed, of which the overlap value $\Delta x/w$Δx/w is 0.4. According to theoretical ray-tracing calculations, the weighted average reflectivity is reduced by 0.75% compared to that of a random IP structure. Experimentally, chain IP structures are fabricated from the surface line damage produced by the diamond wire sawing of a silicon wafer as a mask, and the reflectivity of the structures is 0.80% lower than that of a random IP structure. The theoretical analysis and experimental results both show that the chain IP structure has better optical properties than the random IP structure, indicating promising prospects for the abovementioned applications.

Analytical prediction of subsurface microcrack damage depth in diamond wire sawing silicon crystal

Subsurface microcrack damage depth (SSD) in silicon wafer processed by diamond wire sawing has a great influence on the subsequent processing process and fracture strength of silicon wafer. Based on the indentation fracture mechanics, a new numerical model for predicting the SSD of silicon wafer on the basis of half-penny crack system is proposed in this paper. In this model, effect of the random distribution characteristics of size, position and tip half angle of abrasives and different material removal modes to the contour of workpiece cutting groove are all considered. Median cracks and radial cracks are regarded that propagating into one body, that is half-penny crack system. Validity of this model is verified by comparing with experimental results in the literature. From this model, the relationship between SSD and feed rate, wire speed, size of abrasives and density of abrasives on the surface of saw wire is analyzed, the critical speed ratio of feed rate to wire speed that can achieve pure ductile processing is discussed as well. The results show that using the maximum extended length of half-penny crack system as the SSD of silicon wafer is more reasonable than using the maximum extended length of median cracks as the SSD of silicon wafer. As wire speed increases, feed rate decreases, or density of abrasives on the surface of saw wire increases, the SSD will decrease. Keeping other parameters of saw wire unchanged, the size of abrasives has no significant effect on the change of SSD. Keeping the ratio of feed rate to wire speed unchanged, the SSD varies within a small range. When the speed ratio is in the range of 10.43–16, the slice surface tends to be formed by pure ductile removal of material. The results can be used to guide the optimization of diamond wire sawing process.

A fracture mechanics approach to enhance product and process sustainability in diamond wire sawing of silicon wafers for solar cells through improved wire design

By decreasing the subsurface damage in silicon wafers produced by diamond wire sawing, the mechanical strength of wafers can be increased, and the amount of silicon to be etched in subsequent manufacturing steps can be minimised, enhancing both product and process sustainability. Apart from the sawing process parameters, the subsurface damage in as-sawn silicon wafers is influenced by the design of the diamond wire. We present a fracture mechanics approach for the design of fixed abrasive diamond wires used in wire sawing of silicon wafers for solar cells. Starting from an allowable damage (crack) depth, indentation fracture mechanics and contact analysis are used to determine the wire design parameters, namely the grit protrusion and peripheral distribution of diamond abrasives. The improved wire design can reduce subsurface damage and thereby improve the surface integrity (product sustainability), and reduce the processing time and chemicals used in the subsequent saw-damage removal step (process sustainability).

Experimental Investigation of the Sawn Surface of Monocrystalline Silicon Cut by Endless Diamond Wire Sawing

Abstract The aim of this study was to investigate the influence of the cutting parameters on monocrystalline silicon cut by diamond wire sawing. The sawn surface was analyzed in terms of surface morphology, surface roughness, material removal mechanism and residual stress (by Raman spectroscopy). The surface morphology exhibited evidence of both material removal mechanisms: the brittle mode and the ductile mode. The surface roughness increased with a high vf, which promoted the formation of craters on the sawn surface. On applying a higher vc, the surface roughness reduced, since this favored the formation of damage-free grooves. The Raman spectrum showed evidence of different residual crystalline phases on the sawn surface, which confirms the material removal mechanisms. An increase in vf, for the same vc, caused at reduction in the compressive stress, since the brittle mode predominated as the material removal mechanism. Maintaining vf constant and increasing vc results in higher compressive stress, caused by plastic deformation of the silicon during chip formation.

A fracture mechanics approach to enhance product and process sustainability in diamond wire sawing of silicon wafers for solar cells through improved wire design

A fracture mechanics approach to enhance product and process sustainability in diamond wire sawing of silicon wafers for solar cells through improved wire design

Influence of Cerchar hardness index of hard rock granite on wear of diamond tools

Wear is an important technical and economic parameter in the assessment of cutting performance of diamond tools. It is influenced by many parameters and, the material hardness of the cut surface is one needs to be examined. The current work presents a study of correlation between the Cerchar hardness index (CHI) of hard granite rocks and wear of diamond wiresaw beads. Further the correlation between CHI and wear of large diameter circular disc cutter segments is investigated. Moreover, the linear least regression models for the dataset is developed to grasp the statistical relationship between rock hardness and diamond tool wear. This work may provide significantly valuable insights to the stone and diamond tool manufacturing industry.

Experiment and theoretical prediction for surface roughness of PV polycrystalline silicon wafer in electroplated diamond wire sawing

The electroplated diamond wire sawing technology is the mainstream processing method of cutting PV polycrystalline silicon ingots. Surface roughness is one of the most significant evaluation indexes for wafers surface quality, and has an important influence on subsequent processes such as surface texturization, screen printing, subsurface damage layer prediction and so on. In this paper, a mathematical model, considering the random distribution characteristic of abrasive parameters and its location on saw wire surface, was developed. The influence of brittle and ductile material removal modes on the wafers surface morphology was taken into account, the model can obtain the cutting depths of abrasives by discretizing the cutting groove and wire profile during the sawing process, and then the material removal modes of abrasives are judged by cutting depth. According to the calculation nodes closest to the sawn surface, the wafer surface profile and roughness along the feed speed direction were calculated. The correctness and reasonability of the model were verified by experiments. Then, the model was used to predict the variation trend of surface roughness under different process parameters. The results showed that the relationships between surface roughness and feed speed Vf, wire speed Vs and their ratio Vf /Vs satisfied power functions. Finally, the calculation result of surface roughness was used to generate a 3D surface morphology of wafer.

High-efficiency multi-crystalline black silicon solar cells achieved by additive assisted Ag-MACE

It is important for uniform texture on crystallites with different orientation of diamond wire sawn multi-crystalline silicon solar cells by Ag metal-assisted chemical etching method (Ag-MACE). In this paper, an additive was adopted to make the nanostructures on crystallites with different orientation of multi-crystalline silicon wafers with roughly the same depth and diameter. This additive contains biological enzymes, which are macromolecular proteins with specific catalytic functions. The uniform texture produced by the additive is demonstrated to be helpful in improving the cell performance in terms of surface morphology, reflectivity, effective minority carrier lifetime, auger recombination rate, internal quantum efficiency and external quantum efficiency measurements as well as electroluminescence spectra characterizations. The chemical mechanism of additive assisted etching reaction during the texturing process was discussed. Finally, mass-produced multi-crystalline silicon solar cells made by Ag-MACE with the additive achieve a maximum efficiency and average efficiency of 19.56%, and 19.24%, respectively, which were 0.65% and 0.64% absolutely higher than that without the additive. Moreover, the effective minority carrier lifetime and auger recombination rate of the former was nearly twice and a third of that of the latter respectively.