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The surface of multicrystalline silicon solar cells are etched by mixtures of HF, HNO3 and H2SiF6 in order to remove saw damage caused by wafer slicing, as well as to create a water surface topography that provides a low reflectance for incident light, otherwise known as the texture. Topographically analyzing wafer surfaces before and after etching has revealed that the saw damage controls the texturized wafer surface’s final topography.The first key factor is the dimension and magnitude of the plastic stress field introduced by indenting SiC grains into the wafer surface during the wafering process. The second key factor is that lattice-stressed silicon is etched at a higher rate than unstressed bulk silicon. At the wire entrance, side sharp and large SiC grains create the deepest indention pits, and therefore the deepest of the water surface stress fields. The lattice-disturbed silicon inside these pits is etched at a higher rate compared to the pit’s side walls, which are uniformly attacked across the wafer area. Consequentially, existing pits deepen, and these areas generate the wafer’s lowest reflectivity. At the wire exit side, a higher number of smaller and rounder SiC particles indent the surface and create more numerous and shallower indention pits compared to the wire entrance side. The resulting stress field is less deep, so less silicon is removed from inside of these pits during etching compared to the wire entrance side. This yields to a wafer surface region consisting of shallowly etched pits and higher reflectance. It is concluded that the saw damage acts like an etch mask in the texturization of multicrystalline silicon wafers.
Recovery of Li(Ni0.33Mn0.33Co0.33)O2 from Lithium-Ion Battery Cathodes: Aspects of Degradation
(2019)
Nickel–manganese–cobalt oxides, with LiNi0.33Mn0.33Co0.33O2 (NMC) as the most prominent compound, are state-of-the-art cathode materials for lithium-ion batteries in electric vehicles. The growing market for electro mobility has led to a growing global demand for Li, Co, Ni, and Mn, making spent lithium-ion batteries a valuable secondary resource. Going forward, energy- and resource-inefficient pyrometallurgical and hydrometallurgical recycling strategies must be avoided. We presented an approach to recover NMC particles from spent lithium-ion battery cathodes while preserving their chemical and morphological properties, with a minimal use of chemicals. The key task was the separation of the cathode coating layer consisting of NMC, an organic binder, and carbon black, from the Al substrate foil. This can be performed in water under strong agitation to support the slow detachment process. However, the contact of the NMC cathode with water leads to a release of Li+ ions and a fast increase in the pH. Unwanted side reactions may occur as the Al substrate foil starts to dissolve and Al(OH)3 precipitates on the NMC. These side reactions are avoided using pH-adjusted solutions with sufficiently high buffer capacities to separate the coating layer from the Al substrate, without precipitations and without degradation of the NMC particles.
The stoichiometry of wet chemical etching of silicon in concentrated HF/HNO₃ mixtures was investigated. The formation of nitrogen species enriched in the etching mixture and their reactivity during the etching process was studied. The main focus of the investigations was the comprehensive quantification of the gaseous reaction products using mass spectrometry. Whereas previously it could only be speculated that nitrogen was a product, its formation was detected for the first time. The formation of hydrogen, N₂, N₂O and NH₄⁺ showed a dependence on the etching bath volume used, which indicates the formation of nitrogen compounds by side reactions. Simultaneously, the ratio of the nitrogen oxides, NO and NO₂, formed decreases with increasing etching bath volume, while nitric acid consumption increases, so that the formation of NO₂ could also be identified as a side reaction. Based on the stoichiometries obtained, a new reaction scheme for the reduction of nitric acid during etching in HF/HNO₃ mixtures and an electron balance for the oxidation of silicon is presented.
The oxidizing effect of nitric acid in aqueous solutions depends on the concentration of undissociated nitric acid. This makes the concentration of undissociated nitric acid an essential parameter to monitor and control the quality of silicon etching in the industrial manufacturing of solar cells. In the present study, a method known already is extended in such a way that the degree of dissociation of nitric acid can be determined by Raman spectroscopy in HF/HNO3/H2SiF6 acid mixtures over a broad concentration range for the first time and without using an internal or external standard to compensate the typical time‐dependent drift of a Raman spectrometer. The method developed requires the calculation of a peak area ratio from the areas of the unimpeded Raman signals assigned to nitrate (νN − O) at 1,048 cm−1 and to undissociated HNO3 (νN − OH) at 957 cm−1. The correlation between the peak ratio and the degree of dissociation of nitric acid revealed can be described by a simple empirical equation. Using this equation, the degree of dissociation of nitric acid can be determined over a broad concentration range in binary and ternary mixtures of HNO3 with HF and H2SiF6. The impact of the acids HF and H2SiF6 and the total water content in the degree of dissociation of nitric acid is discussed.
Silicon wafer for solar application are produced by multi-wire sawing from 12x12 cm2 silicon bricks. After slicing the wafer surface consists of a several micrometer surface layer of very heterogeneous constitution, the so called saw damage. The topmost layer of the saw damage consists of debris, amorphous silicon and high-pressure silicon phases followed by a very defect-rich and lattice-strained
region of fractures, cracks, and rifts caused by the rupture of the silicon lattice during the slicing process [1,2]. Such a damaged surface exhibits very poor semiconductor properties; therefore the saw damage is removed by chemical etching using mixtures of HF, HNO3 and H2SiF6 in order to produce solar cells.
Recent investigations showed that the etching of the saw damage is a very heterogeneous process [3-5]. The surface consists of spots at which the etching forms very rapidly deep grooves while other spots remain unetched over a considerably long time. Some of the rapidly formed grooves
seem to remain their shape while others grow because of an attack of the side walls, however, without making these grooves significantly deeper. So far there is no explanation for this behavior. The present work is the first study about the locally resolved etching behavior of lattice-strained
silicon. The surface of polished single-crystal wafers were scratched with a diamond tip under defined conditions. By means of confocal Raman microscopy the local state of the silicon lattice in and nearby the scratches were characterized in terms of compression and tensile stress with a lateral resolution of 2 μm. Then, the scratches were etched stepwise using HF-HNO3-H2SiF6 mixtures and measured by confocal microscopy to quantify the local removal of silicon and measured by confocal Raman microscopy to monitor the lattice state. For the first time this study reveals and quantifies the impact of tensile and compressive lattice stress on the etch rate of silicon with the major outcome, that stress leads to a significantly anisotropic etching behavior. From the time dependent development of the
scratch profiles and the topography of the surrounding wafer areas a detailed picture about the formation of highly reactive species and etching behavior against unstrained silicon is deduced.
Multi-wire sawing using an abrasive SiC slurry or diamond wires constitutes the main slicing techniques for multi- and monocrystalline silicon crystals in photovoltaics. The massive mechanical load during the sawing process creates a wafer surface layer characterized by lattice defects, pits, fractures, rifts, cracks, amorphous Si and even some high-pressure Si modifications, otherwise known as saw damage.[1] This highly defect-rich surface causes the rapid recombination of electron-hole pairs, requiring that it be removed by etching in order to manufacture solar cells and to generate a surface morphology having a low reflectivity which directly affects the solar cell’s efficiency.
However, etching of the saw damage features of a heterogeneous and laterally unevenly distributed etch attack and a significantly higher etch rate compared to the underlying bulk silicon.[2,3] The present study is focused on the question of how mechanically introduced lattice strain in single-crystalline silicon alters the chemical reactivity of the silicon atoms affected by the strain field on a microscopic length scale. The magnitude and local distribution of lattice strain were extracted from confocal Raman microscopy measurements according to Ref. 4. One of the parameters used to describe the reactivity of silicon is the local etch rate, which was derived from the local removal before and after etching by confocal microscopy. Wet-chemical etching was performed with HF-HNO3-H2SiF6 acid mixtures of different concentrations. It was found, that the reactivity of silicon increased linearly with the magnitude of lattice strain. In particular, an increase in tensile strain led to a higher increase in reactivity compared to the increase observed with growing compressive strain. The second decisive parameter is the reactivity of the etch mixture. Diluted acid mixtures with a low reactivity attack only the highest strained Si, whereas more concentrated and therefore more reactive acid mixtures are able to attack even slightly strained Si. Side effects, such as the behavior of amorphous or nanocrystalline Si and the generation of highly reactive intermediary species while etching, are discussed.
The dissolution of Si in HF/HNO3 consists of a set of complex reactions and thus a large number of reaction products. The most comprehensive picture of this reaction, the role of the involved reaction products and the reactivity of the HF/HNO3 mixtures depending on their composition is drawn by Steinert et al..[1]-[3] Based on the first systematic investigations on hydrogen formation by Hoffmann et al.[4], Acker et al. succeeded a first mass and electron balance for the reaction of silicon in HF/HNO3.[5] However, there is still a lack in interpretation of the mass end electron balances arising from several nitrous oxides. So far, the identified nitrogen oxides NO, NO2 and N2O were considered in sum[5] and neither separated nor individually studied or quantified.
The aim of this work is to complete the mass and electron balance by the contribution of the individual nitrous oxides and to identify their individual formation pathways. Kinetic measurements of the NO and NO2 formation during the dissolution of Si, NO2 turns out as a result from the oxidation of the primary product NO by the HNO3 in the etching mixtures. Subsequently, NO and NO2 react to N2O3 dissolved in the acid mixture. The kinetics of both reactions were individually studied by bubbling NO in HF/HNO3 mixtures of different composition. The already identified intermediary species N4O62+ turns out to be formed by disproportionation of dissolved NO2 via N2O4 without dissolution of silicon. A detailed kinetic studied showed, that only dissolved N2O3 and not the intermediate N4O62+ contribute to the dissolution rate of silicon in HF/HNO3 acid mixtures. Finally, kinetic measurements revealed that the formed H2 reduces gaseous NO yielding to the final gaseous reaction products N2, N2O as well as to ammonium ions which all are formed with identical reaction rate. This reaction is assumed to proceed via NH2OH as intermediate.
As result of the identification and quantification of intermediary and final reaction products a new reaction scheme needs to be established leading to a new approach to the mass and electron balance for the oxidation of silicon during the dissolution in HF/HNO3 mixtures.
Texturization of multi-crystalline silicon wafers for photovoltaic application comprises the removal of the saw damage and shaping the topography of the bulk surface to create a surface with a low reflectivity, the so-called texture. Etching of multi-crystalline silicon wafers is usually carried out with acid mixtures consisting of hydrofluoric acid (HF), nitric acid (HNO₃) and hexafluorosilicic acid (H₂SiF₆). The present study reveals that such acid mixtures diluted by water or modified by the addition of ammonia solution, NH₃ (added as ammonium hydroxide solution, NH₄OH) can create textures with a significantly increased surface area exceeding that obtained by standard etching mixtures by a factor of 2.5–3. This yields a significantly reduced reflectivity of the etched wafer surface. However, the addition of water or NH₃ causes a very low etching rate, which makes such mixtures inapplicable for industrial application. To overcome this disadvantage, a two-step etching regime was developed to produce surface-enlarged solar wafers within a timespan typical for industrial production lines. This procedure comprises a first step of slow etching with a NH₃-modified etching mixture to pre-shape the ascut wafer surface. The second etching step is performed with a typical HF/HNO₃/H₂SiF₆ etching mixture that finalizes the texturization. Electrical measurements made on solar cells produced from such etched wafer confirm the improved surface quality of the two-step etched wafer compared to the reference wafer.
The present study deals with the phenomenological observation of the corrosion of the positive electrode foil of lithium-ion batteries containing LiNi0.6Co0.2Mn0.2O2 (NMC) as cathode material. Due to the presence of moisture, localized water accumulation is formed on the NMC surface. The water absorbed by the electrolyte reacts with the NMC under Li+/H+ exchange and the resulting pH increase leads to dissolution of the carrier foil and characteristic salt-like blooms on the NMC surface. With the increase in the relative area occupied by the holes in the aluminum foil per time, a sufficiently suitable parameter was found with which to quantitatively determine the extent of corrosion. The degree of degradation depends on time and ambient humidity. It was shown that functional recycling with the water jet method is no longer applicable for degraded foils, since the mechanical stability of the foils decreases as corrosion progresses. Lithium, aluminum, sulfur and oxygen were detected in the blooms using SEM–EDX and Laser-Induced-Breakdown-Spectroscopy (LIBS). The underlying NMC layer was found to contain mainly aluminum and significantly lower lithium content than the non-degraded material. SEM and Raman microscopy analyses also showed that the active material is also locally degraded and therefore no longer suitable for functional recycling.