mc-Si Ingot Research Articles

Effect of High Temperature Annealing on Crystal Structure and Electrical Properties of Multicrystalline Silicon by the Metallurgical Method

The defects of dislocations and grain boundaries (GBs) could significantly reduce the electrical properties of multicrystalline silicon (mc-Si) solar cells. In this manuscript, the influence of crystal defects on the electrical properties of mc-Si was investigated, aims to carry out the research about the influence of crystal defects on the electrical properties of mc-Si. Purified metallurgical grade N type mc-Si ingot was obtained with the method of vacuum directional solidification technique by employing industrial production ingot furnace. The variations in grain boundaries and dislocations were characterized by electron backscatter diffraction (EBSD), and their effects on the electrical properties of mc-Si before and after annealing at different heat insulating time were investigated. Experimental results showed that high temperature annealing process could significantly improve the electrical properties of mc-Si, the minority carrier lifetime was improved from 0.599 μs to 0.770 μs after 10 h insulation, with a significant increase of 28.61%. Experimental results indicated that high temperature annealing process could significantly eliminate the crystal defects of multicrystalline silicon. The crystal defects was eliminated more obviously with prolonged heat preservation time. When treated by high temperature annealing, the proportion of small angle grain boundaries in the same region can be increased by as high as 13.45%. Moreover, the whole grain sizes of partly region of silicon wafer could be increased by 20000μm2, increased by 15.89% compared with the original grain sizes. Based on the experiment phenomena, we attributed the improvement in the electrical properties of mc-Si to the elimination of dislocations, increased grain sizes and migration of GBs.

Defect control based on constitutional supercooling effect in cast multicrystalline silicon: Boron-indium co-doping

The impact of constitutional supercooling (CS) on the grain structure of cast multicrystalline silicon (mc-Si) materials was investigated. Indium impurity was chosen as the CS dopant because of its low equilibrium segregation coefficient so that CS effect could be obtained at relatively low doping concentration. Industrial indium (In) doped G2 and boron-indium (B–In) co-doped G5 seed-assisted multicrystalline silicon (mc-Si) ingots were fabricated and characterized. Self-nucleated grains and epitaxial grains could be obtained, respectively, as the result of tuning the doping amount of indium. Constitutional-supercooling-related defects (CSD) were found in the co-doped ingot and could suppress the growth and propagation of dislocation-type defects. As a result, the overall deleterious defect density of B–In co-doped mc-Si wafers was well reduced compared to that of referential B-doped wafers, and the average solar cell performance of B–In co-doped ingots was obviously enhanced.

Effect of Titanium Carbide Heat Exchanger Block and Retort on Oxygen Impurities in Mc-Silicon: Numerical Modelling

Primarily, mc-Silicon growth is undertaken by directional solidification (DS) process. Reduction of non-metallic impurities such as oxygen, carbon and nitrogen in DS produced mc-Si ingots is a challenging task and strong contaminations come from inner parts of the DS system. In conventional DS system heat exchanger block and retort are made of graphite. In our present work, we have used a heat exchanger block and retort made by titanium carbide (TiC). The simulation has been done for using both graphite and TiC as Heat exchanger block and retort. The simulation results were compared and analysed. The numerical simulation of oxygen impurity distributions in melt and crystal has been investigated. When we used TiC as Heat exchanger block and retort, it gives lower melt convection in the molten stage, more uniform temperature distribution in x- axis and lower temperature gradient in y-axis at the end of solidification. The lower melt convection can reduce the oxygen impurities and results in uniform oxygen distribution. So, TiC based DS system gives better results compared with the graphite-based system.

Oxygen and Carbon Distribution in 80Kg Multicrystalline Silicon Ingot

Multicrystalline silicon (mc-Si) wafers produced by directional solidification still dominate the world market, due to the factor quality/price. The performance of solar cell depends directly to the quality of wafer and impurities distribution in mc-Si ingot. In our study we investigate the distribution of the interstitial oxygen (Oi) and substitutional carbon (Cs), from the bottom to top of the silicon ingot. During the solidification process the solid-liquid interface moves upward with an average growth velocity of 1.2 cm/h, with a slightly convex form. The determination of (Oi) and (Cs) concentrations were performed thanks to the Fourier Transform Infrared Spectrometry (FTIR) technique. The results show that oxygen concentration increases near the crucible wall to the maximum value of 6.3 × 1017 atoms/cm3, and the carbon concentration decrease from maximum value of 9.59 × 1017 atoms/cm3 in the top to the minimal value of 7.84 × 1017 atoms/cm3 in the bottom of ingot. The concentration of global carbon and oxygen in the centre and corner bricks was investigated using the Secondary Ion Mass Spectroscopy (SIMS) technique. The concentration of oxygen and carbon in the center bricks were 1.8 1018 and 2 1018 atoms/cm3, and in the corner bricks 4.6 × 1019 and 9 × 1019 atoms/cm3, respectively. These results provide quantitative information on the concentration of the light impurities in the as-grown mc-Si and allow an overview of their spatial distribution within the final ingot.

Influence of additional insulation block on multi-crystalline silicon ingot growth process for PV applications

Directional solidification process is the main process for producing the multi-crystalline silicon ingots. It is economically beneficial to grow the good quality mc-Si ingots with lower power consumption. Here we have carried out numerical simualtion for analysing the impact of addition of an insulation block on the DS furnace. The addition of the insulation block has been implemented experimentally and the quality of the grown ingot was evaluated by the minority carrier lifetime measurement. Though the temperature gradient and hence the growth rate are higher in the insulation block added DS furnace, the generated dislocation density has not increased beyond the acceptable limit 108 1/m2. The power used by the DS furnace has been reduced considerably by the addition of the insulation block. The good quality mc-Si ingot with lower power consumption has been obtained as a result of insulation block addition.

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.

Numerical investigation of Directional Solidification process for improving multi-crystalline silicon ingot quality for photovoltaic applications

Numerical simulation is used as a comprehensive tool for analysis and optimization of the multi-crystalline silicon (mc-Si) growth by the Directional Solidification (DS) method. We have used a transient global heat transfer model to optimize the temperature profile of the DS process to produce high-quality multi-crystalline silicon ingots for solar cell application. The melt-crystal interface shape, impurities and von Mises stress were studied by the numerical simulation with different temperature profiles. A slightly convex interface shape, lower thermal stress and lower impurities in the mc-Si were obtained due to the lower temperature gradient by the optimized temperature profile. Simulation results show that the optimized temperature profile is particularly suitable for high quality mc-Si ingot.

The influence of travelling magnetic field on phosphorus distribution in n-type multi-crystalline silicon

The influence of different melt streams on the distribution of phosphorus in multi-crystalline silicon ingot was studied. Phosphorus-doped multi-crystalline silicon (mc-Si) ingots were directionally solidified using travelling magnetic fields (TMF) to alter axial phosphorus profiles. Resistivity distributions in the crystallized n-type ingots were measured along the ingot length. Different Lorentz forces were applied in order to enhance melt stirring and with that to transport phosphorus more rapid towards the melt surface. A new rectangular setup was developed, which enables simultaneous directional solidification of 4 G0-sized mc-Si ingots (80x80x60 mm3) under the influence of TMF. 900 g ingots with different initial level of phosphorus doping were grown, and dopant concentrations in ingots were related to stirring intensities. The phosphorus evaporation rate significantly affects axial dopant profile of mc-Si material, thus this approach can be used as a powerful tool to control and tailor resistivity distribution along phosphorus-doped mc-Si ingots.

Collaborative R&D between multicrystalline silicon ingots and battery efficiency improvement—effect of shadow area in multicrystalline silicon ingots on cell efficiency* * Project supported by the National Natural Science Foundation of China (No. 51532007) and the Fundamental Research Funds for the Central Universities.

We characterized strip-like shadows in cast multicrystalline silicon (mc-Si) ingots. Blocks and wafers were analyzed using scanning infrared microscopy, photoluminescence spectroscopy, laser scanning confocal microscopy, field-emission scanning electron microscopy, X-ray energy-dispersive spectrometry, and microwave photoconductivity decay technique. The effect on solar cell performance is discussed. The results show that the non-microcrystalline shadow region in Si ingots consists of precipitates of Fe, O, and C. The size of these Fe–O–C precipitates found at the shadow region is ~25 μm. Fe–O–C impurities can slightly reduce the minority carrier lifetime of the wafers while severely decrease in shunt resistance, leading to the increase in reverse current of the solar cells and degradation in cell efficiency.

3D visualization and analysis of dislocation clusters in multicrystalline silicon ingot by approach of data science

We report on our attempt to perform the three-dimensional (3D) visualization of dislocation clusters in multicrystalline silicon (mc-Si) ingot by processing photoluminescence (PL) images and analysis of dislocation clusters in mc-Si. As-sliced wafers prepared using a high-performance (HP) mc-Si ingot were sequentially measured by PL imaging with intentional superposition of reflection. Then, various image processing techniques were applied to all the PL images to extract dark regions, which most likely correspond to dislocation clusters, as well as microstructures. By 3D reconstruction using a large quantity of 2D images, we could successfully visualize the generation, propagation and annihilation of dislocation clusters in HP mc-Si ingot. In addition, relationship between source region of dislocation clusters and crystal orientation were investigated by combining data scientific and experimental approaches. As a result, it was suggested that small angle grain boundaries with angular deviation of less than 10 degrees cause the generation of dislocation clusters.

Numerical Study on Effect of Side Insulation Movement in the Multi-crystalline Silicon Growth Process

Numerical simulation is the best tool to understand and optimize the directional solidification (DS) process to grow good quality multi crystalline silicon (mc-Si) ingot for PV application. We have numerically investigated the effect of side insulation lifting distance. Here the mc-Si ingot was grown for four different heights of side insulation movement, such as 50, 100, 150 and 160 mm height from the bottom insulation. During the solidification process, the side insulation is lifted at constant velocity, 0.1 mm/min. which was fixed at four heights for four different processes till the end of solidification. From the investigations lower maximum thermal stress, lower maximum shear stress, lower dislocation rate and convex isothermal lines were obtained at the side insulation lifting distance of 150 mm.

Simulation Analysis of Direction Solidification Process with Fixed Partition Block to Grow Multi Crystalline Silicon Ingot

The simulation of Directional Solidification (DS) process without partition block and with partition block has been carried to grow multi crystalline silicon (mc-Si) ingot and the results were compared and analyzed. The melt-crystal interface shape, power consumption and dislocation densities for both furnaces have been analyzed. Their results revealed that the partition block largely influences the shape of the melt-crystal interface of the mc-Si ingot and consumes lower power than the DS furnace without partition block. The partition block in the DS furnace separates the heater region from the heat dissipation region and prevents radiant heat energy loss about 10 to 12 KWh during the solidification process. However the dislocation density in the mc-Si ingot grown with partition block DS furnace is slightly increased due to the higher radial temperature gradient. Since the dislocation density in both cases has not increased higher than 10− 8 1/m2 it will not affect the solar cell conversion efficiency. Here the effect of partition block on melt-crystal interface shape, dislocation density and power consumption in the DS furnace has been analyzed.

Orientation relationship between β-Si3N4 and Si in multicrystalline silicon ingots for PV applications

β-Si3N4 particles are thought to be nucleation sites for silicon in multicrystalline silicon (mc-Si) ingots for PV applications. The orientation relationship (OR) between Si and β-Si3N4 has been investigated by transmission electron microscopy (TEM). The following OR was found by analyzing diffraction patterns from a site specific sample prepared by focused ion beam (FIB)0001β-Si3N4‖11¯1Si4¯51¯0β-Si3N4‖011SiThe β-Si3N4 particle was identified as a likely nucleation site based on the grain boundaries (GB) extending from it. It was shown that these GBs were Σ3 boundaries. The nucleation process was discussed as a source for formation of twins in mc-Si.

Numerical Simulation on the Suppression of Crucible Wall Constraint in Directional Solidification Furnace

We carried out a global modelling and transient simulation of bulk multi-crystalline silicon (mc-Si) growth in Directional Solidification (DS) furnace. The temperature distribution and the heat flux have been investigated. The stress and dislocation have been reduced by altering the crucible shape. The stress due to the crucible wall constraint has been reduced. The simulation results show that the maximum value of stress and dislocation density can also be decreased by crucible shape and these input parameters can be implemented in real system to grow a better mc-Si ingot for solar cell applications.

Improving Heat Transfer Properties of DS furnace by the Geometrical Modifications for Enhancing the Multi Crystalline Silicon Ingot (mc-Si) Quality Using Transient Simulation

Heat transfer plays a main role on Directional Solidification (DS) process that determines the multi-crystalline silicon (mc-Si) ingot quality. The 2D axi-symmetric model based numerical simulations have been carried out to analyze the heat transfer properties of the DS furnace and to study the thermal effects on mc-Si ingot during the solidification process. For enhancing the heat transfer properties of the DS system and the quality of the mc-Si ingot, the conventional DS furnace was subjected to three types of modifications. The simulation has been made for conventional and modified furnaces and their results were compared and analyzed which confirms that the geometrical modification has influenced the thermal distribution, melt-crystal interface, growth rate, thermal stress and dislocation density of the mc-Si ingot. The following modifications have been taken in conventional DS furnace in sequence: a) For lowering the axial and radial temperature distributions, the bottom insulation was modified (DS-1). b) For melt-crystal interface optimization, the heat exchanger block is grooved in the center part (DS-2). c) Finally, the grooved surface has been subjected to Ar gas flow (DS-3). The evaluations are performed on conventional, DS-1, DS-2 and DS-3 furnaces and their results such as axial-radial temperature distributions, melt-crystal interface, growth rate and thermal stress and dislocation densities are compared and analyzed. The results revealed that the modified furnaces give lower thermal stress and dislocation densities than the conventional furnace. The lower thermal stress with lower dislocation density at the end of the solidification process can prevent the multiplication of dislocations during the cooling process that lead to a good quality mc-Si ingot.

Minority carrier lifetime evaluation of periphery edge region in high-performance multicrystalline ingot produced by seed-assisted directional solidification

A high-performance multicrystalline silicon (mc-Si) ingot was produced by seed-assisted directional solidification, and the minority carrier lifetime of the periphery edge region was evaluated. The defects and impurities in the periphery edge region of the silicon wafers were systematically studied with photoluminescence (PL) imaging, minority carrier lifetime mapping, and Fourier transform infrared (FTIR) spectroscopy. Their relationships with the minority carrier lifetime were investigated. The concentration of substitutional carbon, interstitial oxygen, and dislocation clusters is not directly correlated with the low minority carrier lifetime of the edge zone of the mc-Si ingot. Inhomogeneous grain size distribution and contamination with iron impurities were demonstrated to be the main factors affecting the low minority carrier lifetime. By controlling the impurities and improving the grain size distribution, a modified furnace was designed and a higher-quality mc-Si ingot was manufactured.

Simulation Studies of Annealing Effect on a mc-Si Ingot for Photovoltaic Application

A multi-crystalline silicon (mc-Si) ingot was grown by the directional solidification (DS) process for photovoltaic (PV) application. We have numerically investigated shear stress and thermal stress for different annealing time and temperature of the directionally solidified mc-Si ingot after the solidification and also we discuss the melt-crystal (m-c) interface during the solidification. Initially the planar m-c interface is observed during the solidification process, after that a slightly convex m-c interface is obtained for the rest of the solidification process. Maximum shear stress has least value at the center region of the mc-Si ingot for 900 K annealing temperature. Maximum shear stress has least value at the peripheral region of the mc-Si ingot for 700 K annealing temperature. The whole mc-Si ingot has lower thermal stress at 700 K annealing temperature. 5 h annealing time is enough to decrease the internal stress of the mc-Si ingot. Increase of the annealing time beyond 5 h does not further decrease the stress significantly.

Simulation of directional solidification furnace with bottom opening insulation to grow quality mc-Si ingot for PV applications

Abstract2D axi-symmetric transient simulation based on the thermo-plastic model was carried out for analyzing the thermal field of directional solidification (DS) furnaces during the solidification process. The simulation was carried out for two different furnaces: One conventional DS furnace and the other a modified DS furnace. In the modified DS furnace the bottom insulation was provided with a circular shape opening. The simulation was performed for conventional and modified furnaces which suggest that the lifting velocity of the side insulation in the conventional DS furnace significantly affects the solidification process of mc-Si ingot and the opening speed of the bottom insulation in the modified DS furnace has less influence on solidification and also gives optimal conditions to grow good quality mc-Si ingots. The results for conventional and modified DS grown mc-Si ingots were taken with two different insulation lifting profiles and their influence on thermally induced stress, dislocation density and growth rate were compared and analyzed.

Numerical analysis on effect of annealing mc-Si ingot grown by DS process for PV application

Silicon solar cells play a crucial role in Photo voltaic (PV) application. We have numerically investigated thermal stress and normal stress components (Sigma 11, Sigma 22, Sigma 33 and sigma 12) by using finite volume method. The maximum thermal stress has low value at the centre region for 900K and 700K annealing temperatures comparing all the cases. The maximum thermal stress at peripheral region is low for 700K annealing compared to 900K annealing. The annealing effect of mc-Si ingot normal stress components is discussed. At 700K annealing temperature the normal stress in 11 and 33 direction has lower maximum and at the 900K annealing temperature the normal stress in 22 and 12 direction has lower maximum.

Numerical Simulation of Multi-Crystalline Silicon Crystal Growth Using a Macro–Micro Coupled Method during the Directional Solidification Process

In this work, the crystal growth of multi-crystalline silicon (mc-Si) during the directional solidification process was studied using the cellular automaton method. The boundary heat transfer coefficient was adjusted to get a suitable temperature field and a high-quality mc-Si ingot. Under the conditions of top adiabatic and bottom constant heat flux, the shape of the crystal-melt interface changes from concave to convex with the decrease of the heat transfer coefficient on the side boundaries. In addition, the nuclei form at the bottom boundary while columnar crystals develop into silicon melt with amzigzag-faceted interface. The higher-energy silicon grains were merged into lower energy ones. In the end, the number of silicon grains decreases with the increase of crystal length.