Ingot Height Research Articles

High efficiency solar-grade silicon producing by directional solidification through controlling diffusion of impurity

The precise distribution of impurity of the whole silicon ingot is investigated through a theoretical model and the experiments. This model solves the problem of inaccurate prediction of impurity distribution in the final solidification region. The model was used to calculate the distribution of iron impurity in polycrystalline silicon ingots after directional solidification and compared with experiments. The calculation accuracy was significantly improved compared to the traditional Scheil equation. This paper investigates the influence of cooling rate, silicon ingot height and solidification rate on the yield of purified products after directional solidification. For the experimental equipment and raw materials in this paper, the optimal cooling rate of silicon ingots after directional solidification is 0.33 °C/s, the optimal height of silicon ingots is 0.34 m, and the yield of silicon purification increases with the decrease of directional solidification rate. Based on the research results, a standard for removing the top skin of silicon ingots after directional solidification and a method for improving the purification yield by suppressing impurity diffusion during the cooling stage were obtained. This model can provide theoretical guidance for the development of low-cost and high-efficiency methods for removing impurities from polycrystalline silicon, and is of great significance for the development of the photovoltaic industry. In addition, the theoretical model proposed in this paper can also be applied to the directional solidification purification process of other materials, such as the preparation of high-purity aluminum and high-purity titanium, and can predict the impurity migration behavior and distribution pattern throughout the purification process.

Production of upgraded metallurgical-grade silicon for a low-cost, high-efficiency, and reliable PV technology

Upgraded metallurgical-grade silicon (UMG-Si) has the potential to reduce the cost of photovoltaic (PV) technology and improve its environmental profile. In this contribution, we summarize the extensive work made in the research and development of UMG technology for PV, which has led to the demonstration of UMG-Si as a competitive alternative to polysilicon for the production of high-efficiency multicrystalline solar cells and modules. The tailoring of the processing steps along the complete Ferrosolar’s UMG-Si manufacturing value chain is addressed, commencing with the purification stage that results in a moderately compensated material due to the presence of phosphorous and boron. Gallium is added as a dopant at the crystallization stage to obtain a uniform resistivity profile of ∼1 Ω cm along the ingot height. Defect engineering techniques based on phosphorus diffusion gettering are optimized to improve the bulk electronic quality of UMG-Si wafers. Black silicon texturing, compatible with subsequent gettering and surface passivation, is successfully implemented. Industrial-type aluminum back surface field (Al-BSF) and passivated emitter and rear cell (PERC) solar cells are fabricated, achieving cell efficiencies in the range of those obtained with conventional polysilicon substrates. TOPCon solar cell processing key steps are also tested to further evaluate the potential of the material in advanced device architectures beyond the PERC. Degradation mechanisms related to light exposure and operation temperature are shown to be insignificant in UMG PERC solar cells when a regeneration step is implemented, and PV modules with several years of outdoor operation demonstrated similar performance to reference ones based on poly-Si. Life cycle analysis (LCA) is carried out to evaluate the environmental impact of UMG-based PV technology when compared to poly-Si-based technology, considering different scenarios for both the manufacturing sites and the PV installations.

Numerical Simulation of Electromagnetic Field in Slab Electroslag Remelting Process with Double Electrode Series

In this paper, a mathematical model of an electromagnetic field during an electroslag remelting process of industrial-scale slab was established using a Maxwell 3D module in Ansys Electromagnetics Suite. The distribution characteristics of the magnetic field intensity, current density and Joule heat density during the electroslag remelting process were analyzed in detail. On this basis, the influence of the current frequency on the electroslag remelting process of an industrial scale is studied, and the influence of process parameters such as the slag pool depth, electrode insertion depth and ingot height on the electromagnetic field is also considered. The results show that the Joule heat generated in the slag pool is much greater than that of the electrode and the ingot. The maximum Joule heat is located at the contact between the electrode corner and the slag pool, and the Joule heat near the middle of the two electrodes is greater than the rest. With the increase in the current frequency, the current density distribution in the slag cell is basically unchanged. The current density inside the two electrodes increases obviously with the current frequency. When the current frequency increases from 10 Hz to 50 Hz, the maximum current density at the inner surface of the electrode increases by 14.7%. The current distribution at the lower side of the electrode in the slag pool is relatively uniform, and the current density in this region decreases with the increase in the height of the slag pool and the increase in the depth of the electrode inserted into the slag pool.

Attempt to correlate dislocations network and distribution to macroscale plane rotations in <001> cast-mono ingots

This work aims to contribute to the understanding of the mechanisms behind electrically active crystalline defect formation in cast-mono (cm) ingots grown along the <001> axis. Both the spatial distributions of background dislocations in the cellular dislocation array and the driving force for dislocation multiplication in the sub-grains are not perfectly understood. Macroscale length crystal rotations are here characterized by the (θ/2θ) X-ray diffraction (XRD) method at different ingot heights and along two perpendicular sample axes. XRD measurements show the presence of crystal rotations in the large single grain domain. Two orthogonal misorientations components are found to evolve linearly along each sample axis forming plane orientation gradients up to (0.05 ± 0.02) degree/cm extending over 12 cm inside the cast-mono grain. (001) crystal structure deformation are also analysed by rocking curve imaging (RCI) using synchrotron radiation on wafers of a neighboring brick. First, results confirm the presence of the plane orientation gradient measured by XRD and its order of magnitude. Second, RCI results demonstrate that the plane orientation gradient is associated to the cellular dislocation network. Therefore, these results strongly suggest that background dislocations network of a cellular form can create an organized orientation gradient at large distance within the ingot. The involvement of the characterized orientation gradients in the formation and extension of electrically active SGBs in cast-mono ingots is discussed. • Crystal structure deformation measurements of cm-Si ingots. • Evidence of the presence of {001} planes orientation gradients. • Plane orientation gradients almost constant along the sample axes. • Orientation gradients increase with the ingot height. • Orientation gradients associated to the cellular network of dislocations.

Adjustment of resistivity for phosphorus-doped n-type multicrystalline silicon

In this work we present a novel approach to reduce inhomogeneity of resistivity for n-type phosphorus-doped directionally solidified silicon ingots by using complex arrangements on influencing dopant transport during growth process, which involves adjustments of melt flow and gas transport. In order to obtain more uniform resistivity profile over ingot's height, different process conditions were tested on G1-sized (22 × 22 × 12 cm 3 ) n-type directionally solidified ingots, including altering of ambient gas pressure, enhanced melt mixing with travelling magnetic fields and variations of argon flow above the melt. The quality of ingots was characterised in terms of electrical resistivity and minority carrier lifetime, as well as the presence of impurities and inclusions. The influence of process parameters on material quality will be discussed. It will be shown that a complex approach to melt stirring and gas transport is an effective way to control resistivity distribution along the height of phosphorus-doped multicrystalline ingots, which makes it possible to provide n-type ingots with resistivity profiles comparable to p-type boron-doped ones. • The mechanisms of phosphorus transport during directional solidification of silicon is investigated. • Process parameters influencing phosphorus transport in G1-size system are defined. • An optimized growth recipe, which ensures more uniform resistivity distribution along the ingot length, is developed. • A multicrystalline ingot showing 45% narrower resistivity variation and high material quality is grown.

Evaluating Plasticity of Alloy AD31 on Ingot Height

The authors present the results of evaluating the distribution of plastic properties on the height of a log ingot with the diameter of 145 mm, obtained by means of semi-continuous casting, from the aluminum alloy AD31. The authors study semi-continuous alloys of three manufacturers. One part of ingots was obtained with the use of only primary aluminum, secondary raw materials and secondary raw materials with the addition of primary aluminum. With the purpose of stabilizing the process of presswork the authors define the values of yield stress in the ingot lower and upper parts and study its microstructure. The research was conducted for the ingots without thermal treatment after accelerated homogenization. It is shown that the ingots, produced with the use of primary aluminum only, have less scattered plastic properties in terms of the height. That is why, for the case of press-working the items with thin components, it is reasonable to use the ingots from primary aluminum.

Iron related precipitates in multicrystalline silicon by conductive atomic force microscopy

Multicrystalline silicon (mc-Si) is a widely used material for photovoltaic applications. The presence of metallic contaminated grain boundaries strongly affects the crystal electronic properties enhancing electron-hole recombination, thus reducing the solar cell performance. The present study aims to investigate the electrical activity of metallic contaminated grain boundaries in mc-Si. Two sets of mc-Si wafers, contaminated with iron and aluminium, respectively, were analyzed. The wafers presented grain boundaries whose density and character were characterized by Electron Backscatter Diffraction (EBSD), while their electrical activity was analyzed using Conductive Atomic Force Microscopy (c-AFM). The grain boundary density decreases along the ingot height and the most common coherent grain boundaries have the character Σ3n. The grain boundary electrical activity is mostly due to metallic precipitates located at the grain boundaries. In particular, iron precipitates enhance the current contrast at the grain boundaries. Both fixed voltage maps and current-voltage characteristics at the grain boundaries were measured to understand and clarify the transport phenomena at grain boundaries decorated with metallic impurities. The current profiles measured by c-AFM across a grain boundary were modelled by assuming the contribution of a Coulombic potential introduced by the positively charged precipitate. Quantitative parameters regarding the segregated iron-related precipitates are estimated from the model. The results of this study, based on local electrical characterization and appropriate modelling, will contribute to improving the understanding of the recombination at iron precipitates at grain boundaries in mc-Si.

Propagation of Crystal Defects during Directional Solidification of Silicon via Induction of Functional Defects

The introduction of directional solidified cast mono silicon promised a combination of the cheaper production via a casting process with monocrystalline material quality, but has been struggling with high concentration of structural defects. The SMART approach uses functional defects to maintain the monocrystalline structure with low dislocation densities. In this work, the feasibility of the SMART approach is shown for larger ingots. A G2 sized crystal with SMART and cast mono silicon parts has been analyzed regarding the structural defects via optical analysis, crystal orientation, and etch pit measurements. Photoluminescence measurements on passivated and processed samples were used for characterization of the electrical material quality. The SMART approach has successfully resulted in a crystal with mono area share over 90% and a confinement of dislocation structures in the functional defect region over the whole ingot height compared to a mono area share of down to 50% and extending dislocation tangles in the standard cast mono Si. Cellular structures in photoluminescence measurements could be attributed to cellular dislocation patterns. The SMART Si material showed very high and most homogeneous lifetime values enabling solar cell efficiencies up to 23.3%.

Pore structure analysis of directionally solidified porous copper

Directionally solidified porous copper is considered as a potential candidate in the field of microchannel heat sinks. By the Bridgman-type directional solidification method, a porous copper ingot was fabricated. Evolution of the porosity, pore number density, average pore diameter and average interpore spacing at different ingot heights was investigated. The results show that with the increase of ingot height, the porosity firstly increases and then basically remains unchanged from the ignot height of 65 mm; the pore number density rapidly decreases at first, and the decreasing speed becomes slower when the ignot height higher than 85 mm; the average pore diameter increases and then remains unchanged from the ingot height of 85 mm; the average interpore spacing increases, and the increasing speed of average interpore spacing becomes slower with the increase of height to higher than 85 mm. In order to study the evolution of diameter and spatial distribution of pores, the distribution ranges of pore diameter, nearest-neighbor distance and radial cumulative pore number were analyzed. As the ingot height increases, the distribution ranges of pore diameter and nearest-neighbor distance firstly increase and then tend to be stable. There are no pore clusters and for long distance, the spatial distribution of pores is uniform at different ingot heights. Pore structure and 3D pore morphology of porous copper were observed with the help of light illumination and X-ray tomography. Pore nucleation, pore interruption, pore coalescence, diameter change of pores and lateral displacement of pores were found to exist in the pore structure.

Research the Efficiency of Pulse Feeding of Fuel

Significant structural defects of wells, which influence the uniformity of metal heating, specific fuel consumption and operating costs are revealed. It was noted that the actual task is to increase the energy efficiency of heating high-alloy steel grades by introducing pulsed fuel supply technology. Efficiency of introduction of impulse fuel supply for heating boilers with heating from the center of the dowel is investigated. Comparison of the results of control heating and data obtained in similar conditions under the usual heating method has shown that the use of the automatic control system, which implements impulse fuel supply during the period of holding, will reduce the maximum temperature difference over the ingot height to (20...30) ° C. Natural experiments showed that the pulse width, that is, the ratio of the duration of the pulses to the period of the interleaving of the pulses, at the beginning of exposure is greater than at the end, since the process of absorption of heat by the metal and clay is not completed. As warm up, until the end of exposure, the efficiency is significantly reduced. The proposed system of pulsed heating ensures the maintenance of the given ratio of gas-air during exposure. This allows to eliminate ignored and reduces the conditional fuel consumption. It has been proved that the use of a pulsed method of supplying fuel during the period of holding will allow to intensify heat transfer processes, increase the uniformity of heating and reduce the conditional fuel consumption.

Temperature Sensitivity of Multicrystalline Silicon Solar Cells

This paper presents an experimental investigation of the temperature coefficients of multicrystalline silicon solar cells. The aim was to determine if some cell parameters can affect positively the temperature sensitivity without detrimental impact on the efficiency. Commercial solar cells with different bulk resistivities, compensation levels, and cell architectures have been studied. We report that the base net doping, the location of the solar cell along the brick and the cell architecture have significant impacts on the temperature coefficients. Moreover, we show how the change in recombination mechanisms along the ingot height affects the temperature coefficients. The compensation level was observed to have no significant effect on the temperature coefficients. We also demonstrate why aluminum back-surface-field and passivated emitter rear contact solar cells have similar temperature sensitivities despite a better passivation and higher open-circuit voltage for the latter cell architecture. Finally, we have found that reducing the bulk resistivity can improve the solar cells' performance in hot climates.

Production of high performance multi-crystalline silicon ingots for PV application by using contamination-free SixNy seed particles

In this work a new type of contamination-free SixNy seed particles for the directional solidification of HPM silicon ingots is introduced and evaluated. Several G1 crystallization experiments have been carried out to examine the influence of the seed particle size, the introduced mass and the influence of the composition of the underlying Si3N4 coating at the crucible bottom. Finally, multi-PERC cells over the ingot height from one of the most promising SixNy seeded ingot have been prepared.The results show that an oxygen contamination is not occurring by the new particles. A minimum seed particle density of around 120/cm2 at the crucible bottom is sufficient to generate a HPM like grain structure with random grain boundary fractions R of 65%±5%, which is very close to typical values of classical silicon seeded HPM. A further increase of the particle density doesn’t lead to a further increase of the R value due to the competition between the very close packed seeds.While lower oxygen levels of the Si3N4-coating promote significant higher R values in crucible setups without seed particles, the Si3N4-coating composition has almost no influence on the resulting grain structure properties when SixNy seed particles are used. This could be shown even for small seed densities of ~30 cm−2 and a remaining coating area fraction of 80%.The cell analysis shows same efficiencies of the ingots seeded on the SixNy particles and on the silicon feedstock layer, but a higher wafer yield can be achieved in the case of the SixNy seeding method since a full melt process can be used.

Technology for Producing a Low-Alloy Martensitic 10Kh3A Steel with an Overequilibrium Nitrogen Content

Dense defect-free 10Kh3A steel ingots with an overequilibrium nitrogen content (0.1–0.17 wt %) are formed by electroslag remelting under nitrogen pressure in a DEShP 0.1 pilot furnace (All-Russia Institute of Aviation Materials). A method is developed for calculating the solubility of nitrogen in multicomponent melts, including model steels, under its atmospheric and working pressure in the furnace chamber. This method determines the nitrogen content in liquid and solid metals and can be used to form 10Kh3A steel ingots with a dense homogeneous structure. Distributions of nitrogen and other alloying elements over the ingot height and cross section are found. The calculated nitrogen solubility in the metal is in agreement with experiment one.

Simulated and measured temperature coefficients in compensated silicon wafers and solar cells

In this paper we perform a thorough investigation of the temperature coefficients of c-Si solar cells and wafers, based on both experimental data and device simulations. Groups of neighboring wafers were selected from different heights of four high performance multicrystalline silicon ingots cast using different dopants concentrations and Si feedstocks; Three different target resistivities of compensated silicon ingots based on Elkem Solar Silicon (ESS®), which are purified through a metallurgical route, and one non-compensated reference ingot. The wafers were processed into Al-BSF and PERCT type solar cells, as well as into lifetime samples subjected to selected solar cell processing steps before being etched down and re-passivated to assess the bulk lifetime of the final cells. Temperature-dependent photoluminescence imaging was used to study the spatial distribution of the carrier lifetime and the temperature coefficient of the lifetime. We observe a beneficial effect of higher doping levels on the temperature coefficients of the short circuit current JSC in PERCT type cells, with the most favorable temperature coefficients are found in the ingot doped to 0.5 Ω-cm. Increasingly beneficial temperature coefficients in JSC is also observed with increasing height in each ingot, which might be attributed to a combination of small variations in the dopant concentrations and to a clear increase in the temperature coefficient of the carrier lifetime with increasing ingot height. The latter point is also in agreement with the changes in the so-called gamma parameter of the open circuit voltage throughout the ingot. To build a more fundamental understanding of the experimental results the temperature dependence of the solar cells was simulated using PC1Dmod6.2. We find that the experimental temperature coefficients of the final cells can be reproduced in device simulations within 8% of and 14% for the VOC and JSC, respectively. The compensation level itself is of minor importance for the cell performance. Instead, the simulations indicate that the observed behavior in the temperature coefficients of the solar cells is mostly explained by differences in the net doping and carrier lifetime of the wafers.

Evaluation of improvement strategies of grain structure properties in high performance multi-crystalline silicon ingots

High performance multi-crystalline silicon (HPM-Si) for the use in photovoltaics is characterized by a very fine grain structure and a high content of random grain boundaries, finally resulting in a low dislocation density and consequently in a high material quality. Typically, the grain size increases and the fraction of random grain boundaries decreases over ingot height due to annihilation mechanisms, especially in the first 150 mm. One approach for further material improvement is to further increase the initial random grain boundary fraction and to maintain it as high as possible over the complete ingot height.In this work, several theoretical approaches to achieve these points were evaluated by experiments in G1 scale. Firstly, the influence of the silicon seeding material on the initial grain structure was investigated regarding the effect of extremely fine Si particles in the µm to nm range and the bulk density of the particle layer. Secondly, the effect of the initial geometrical grain boundary arrangement in the seed layer was evaluated. For that purpose, special seed alignments similar to the Quasimono approach were tested. Finally, the process parameter growth rate was varied in a wide range to investigate its influence on the evolution of the grain boundaries during growth.The results show that the optimum for the initial random grain boundary fraction is already reached by the existing methods/commonly used seed materials. Concerning the decrease of the random grain boundary fraction over ingot height, some technical aspects were identified which are able to keep the amount of random grain boundaries at a high level. However, the practical realization within an industrial setup might be difficult.

Gettering of transition metals in high-performance multicrystalline silicon by silicon nitride films and phosphorus diffusion

High-performance multicrystalline silicon (HP mc-Si) from directional solidification has become the mainstream industrial material for fabricating mc-Si based solar cells for photovoltaic applications. Transition metal impurities are inherently contained in HP mc-Si during ingot growth, and they are one of the major efficiency-limiting drawbacks. In this work, we investigate the gettering of transition metals (Cu, Ni, Fe, and Cr) in HP mc-Si wafers along an industrial-standard p-type HP mc-Si ingot, via examining the metal concentration and distribution in the near-surface gettering layers using secondary ion mass spectrometry. We applied both conventional phosphorus diffusion gettering and the recently developed silicon nitride (from plasma-enhanced chemical vapour deposition) gettering techniques. Both techniques are shown to remove significant quantities of metals from the silicon wafer bulk to the surface gettering layers. Improvements in the bulk minority carrier lifetimes throughout the ingot height are also observed by lifetime measurements and spatially-resolved photoluminescence imaging. The gettered Cu and Ni concentrations, as well as the as-grown dissolved Fe concentrations in the silicon wafer bulk, along the HP mc-Si ingot height are shown to follow a similar concentration profile as the metals in conventional mc-Si ingots.

Problem of attaining constant impurity concentration over ingot height

The possibility of growing crystals with homogeneous impurity distribution over crystal height has been demonstrated in a study of segregation during silicon and germanium growth from thin melt layers using the submerged heater method. Numeric simulation of 200 mm diam. antimony-doped germanium crystallization has shown that, beginning from a 40 mm melt layer thickness, the exact problem solution with convection allowance is identical to the unidimensional heat exchange problem solution in the central ingot part. The conditions under which convection can be ignored in mass transport calculation are more rigorous: the melt layer height must be within 20 mm. In this case Tiller’s ratio can be used for calculating the longitudinal impurity distribution for predominantly diffusion-controlled mass transport pattern. Analysis of the existing attempts to describe the experimental crystal growth results using the simplified formulae shows that they only yield acceptable results if the actual growth rate or change in melt layer thickness during crystallization are taken into account, e.g. as in the formula suggested by Marchenko et al. One can therefore analytically describe the longitudinal impurity distribution in the ingot, e.g. B and P distribution in silicon, and recommend the degree of additional doping of the melt zone under the heater so that to provide a constant impurity concentration over the ingot height. Homogeneous material can be obtained after residual layer solidification in the end portion of the ingot if the growth rate is controlled through varying the cooling rate.

Monte Carlo simulations of criticality safety assessments of transuranic element storage in a pyroprocess facility

Abstract In this study, criticality safety assessments of the potential for storing transuranic element (TRU) ingots via a pyroprocess were evaluated to determine the appropriate TRU storage design parameters, in this case the ratio of the TRU ingot height to the radius and the number of TRU ingot canisters stacked within a container. Various accident situations were modeled over a modeling period of 5 years for a cumulative inventory of TRU ingots with various water densities in submerged containers and with various pitches between the containers in the facility. Under these combinations, we calculated the threshold of TRU height and radius ratio depending on the number of canisters in a container to keep the stored TRU in a subcritical state. The ratio of the TRU ingot height to radius should not exceed 4.5, 1.1, 0.5, 0.3, and 0.2 for two, three, four, five, and six levels of stacked canisters in a container, respectively.

Defect related radiative recombination in mono-like crystalline silicon wafers

We report on studies of sub-bandgap defect related photoluminescence (DRL) signals originating from radiative recombination through traps in the bandgap of cooled mono-like silicon wafers. Spectrally resolved photoluminescence (SPL) and multivariate curve resolution (MCR) have been used in combination, to study the behaviour of sub-bandgap photoluminescence (PL) emissions in wafers cut from different heights in a pilot-scale mono-like silicon ingot. No DRL signals were found in the main mono-like body. Strong defect related sub-bandgap emissions correlating with heavily dislocated areas, are found directly above some of the seed junctions. The DRL signal exhibits a correlation with the number of axis with small angle misalignment in the junctions of the seeds. The signal conventionally labelled D1 (0.80 eV) decreases with ingot height. A mechanism relating this signal to oxygen is proposed. The signals D3 (0.94 eV) and D4 (1.00 eV) are found to co-occur, supporting previous studies, and similarly to the D2 (0.87 eV) signal, their strength is found to increase with ingot height. As the content of the transition metal impurities in the ingot is supposed to increase with height, this supports a reported link between the D3 and D4 signals with Fe, as well as a link between D2 and other impurities. An emission previously found in multicrystalline material and labelled D07 (0.70 eV), is found to solely exist as the only DRL signal recorded by us in parasitic crystals, growing into the main mono-like ingot from the crucible walls. This contradicts the common notion that the D1–D4 signals are strongly related to, and always follow dislocations. Total photoluminescence spectrum (right) and distribution (left) of the PL signal with centre energy 0.70 eV emanating from the parasitic crystals growing into the bulk mono-like Si crystal from the crucible walls.

Preparation of silicon target material by adding Al-B master alloy in directional solidification

The silicon target material was prepared by adding Al-6B master alloy in directional solidification. The microstructure was characterized and the resistivity was studied in this work. The results showed that the purity of the silicon target material was more than 99.999% (5N). The resistivity was ranges from 0.002 to 0.030 Ω·cm along the ingot height. It was revealed that the particles of AlB2 in Al-6B master alloy would react spontaneously and generate clusters of [B] and [Al] in molten silicon at 1723 K. After directional solidification, the content of B and Al were increasing gradually with the increase of solidified fraction. The measured values of B were in good agreement with the curve of the Scheil equation below 80% of the ingot height. The mean concentration of B was about 17.20 ppmw and the mean concentration of Al was about 8.07 ppmw after directional solidification. The measured values of Al were fitting well with the curve of values which the effective segregation coefficient was 0.00378. It was observed that B co-doped Al in directional solidification polysilicon could regulate resistivity mutually. This work provides the theoretical basis and technical support for industrial production of the silicon target material.