Quasi-single Crystalline Silicon Ingots Research Articles

Application of a new grain boundary technology for quasi-single crystalline silicon ingots

A new functional grain boundary group (FGBG) technology was employed to generate quasi-single crystalline silicon ingots with a high single crystalline area ratio. Fabrication of the FGBG structure was achieved and special grain boundaries were formed between adjacent seed crystal strips with 〈100〉 crystal orientation, with a 30° crystal orientation angle between the special grain boundaries. The influence of FGBG on the evolution of grains, dislocation, and surface morphology was analyzed by photoluminescence and electron back-scattered diffraction. FGBG effectively suppressed the side polycrystalline grains (spontaneous nucleation on the crucible wall) intrusion at the early stage of solidification. Additionally, when the crystal growth height was increased, FGBG acted as a barrier layer for the external polycrystalline grains under the control of an appropriate horizontal temperature gradient and slightly convex solid–liquid interface shape, preventing most of the polycrystalline grains around the ingot from growing into the crystal, and blocking the dislocations generated by polycrystalline grains near the grain boundary. The application of this FGBG technology will greatly improve the quality of quasi-single crystalline silicon ingots with a weight of 1400 kg (G7).

Numerical study on stress control of silicon ingot for photovoltaic applications

When continuous silicon ingot casting is performed using directional solidification—a metallurgical refining method for producing high-purity silicon ingots—the thermal stress and microstructure inside the ingots must be monitored for ensuring the desired yield and purity. This study determined the temperature gradient and thermal stress during silicon ingot casting by applying various controlled parameters to the process simulation. Our results indicate that the temperature gradient influences the thermal stress generated during casting; reducing the temperature gradient can help reduce the thermal stress. The ingot exhibited the lowest thermal stress at an electron beam (E-beam) power of 6 kW, an ingot growth rate of 0.02 mm/min, and a molten silicon level of −10 mm. Quasi-single crystalline silicon ingot was produced via crystal growth simulation under thermal stress-minimizing conditions with single-crystalline seed. The quasi-single crystalline silicon ingot maintained the [001] grain orientation and the purity increased from 2N to 7N6.

Effect of Argon Flow on Oxygen and Carbon Coupled Transport in an Industrial Directional Solidification Furnace for Crystalline Silicon Ingots

Transient global simulations were carried out to investigate the effect of argon flow on oxygen and carbon coupled transport in an industrial directional solidification furnace for quasi-single crystalline silicon ingots. Global calculation of impurity transport in the argon gas and silicon melt was based on a fully coupled calculation of the thermal and flow fields. Numerical results show that the argon flow rate affects the flow intensity along the melt–gas surface, but has no significant effect on the flow patterns of silicon melt and argon gas above the melt–gas surface. It was found that the evaporation flux of SiO along the melt–gas surface decreases with the increasing argon flow rate during the solidification process. However, the net flux of oxygen atoms (SiO evaporation flux minus CO dissolution flux) away from the melt–gas surface increases with the increasing argon flow rate, leading to a decrease in the oxygen concentration in the grown ingot. The carbon concentration in the grown ingot shows an exponential decrease with the increase of the argon flow rate, owing to the fact that the dissolution flux of CO significantly decreases with the increasing argon flow rate. The numerical results agree well with the experimental measurements.

Reduced metal contamination from crucible and coating using a silicon nitride based diffusion barrier for the growth of cast quasi-single crystalline silicon ingots

During the crystallization of directionally solidified silicon such as multicrystalline or cast quasi-single crystalline silicon, the diffusion of metal impurities from crucible and coating into the solid silicon ingot at high temperatures creates a severe contamination issue in the edge region of the ingot, the so called red zone. We present an effective diffusion barrier which greatly reduces this detrimental metal diffusion. Silicon wafers are coated with a layer system of silicon oxide and silicon nitride by chemical vapor deposition and are placed on the bottom of the crucible. Using this technique, the metal contamination in the red zone can be reduced by about one order of magnitude. This leads to better solar cell performance of wafers fabricated from this ingot area. Also, less ingot material has to be discarded leading to higher wafer yield per ingot.

Reusability of contaminated seed crystal for cast quasi-single crystalline silicon ingots

Reusing seed crystal is beneficial for reducing the production costs for cast quasi-single crystalline (QSC) silicon ingots. We numerically investigate the reusability of seed crystal in the casting processes with quartz crucible and silicon feedstock of different purities. The reused seed crystal is recycled from the standard QSC ingot and has been highly contaminated by iron impurity. Transient simulations of iron transport are carried out and special attention is paid to the diffusion and distribution characteristics of iron impurity at the ingot bottom. The heights of the bottom iron contaminated region are compared for silicon ingots grown from normal and recycled seed crystals. The results show that the purity of quartz crucible can influence the reusability of seed crystal more significantly than that of the feedstock. The recycled seed crystal with high iron concentration can be reused for casting processes with standard crucible, whereas it is not recommended for reusing for processes with pure crucible.

Performance of solar cells fabricated from cast quasi-single crystalline silicon ingots

We comprehensively investigated the performance of solar cells made from quasi-single crystalline silicon (QSC-Si) cast by the seed-assisted directional solidification (DS) method and its relationship with the area ratio of 〈100〉 -oriented single crystal of a QSC-Si wafer. It is found that the cell performance improvement triggered by alkaline texturing is more notable for the QSC-Si wafers with a morphology of higher 〈100〉 -oriented single crystal ratio, and the difference in cell efficiency between alkaline and acidic texturing is up to an absolute value of 1.05%. As the single crystal ratio of the QSC-Si wafer decreases from 100% to 50%, the cell efficiency decreases from 18.15% to 17.04%. We also compared the performance of typical CZ-Si solar cells and the QSC-Si solar cells made from wafers with fully 〈100〉 -oriented single crystal. It is found that the average efficiency of QSC-Si solar cells is about 0.8% absolutely lower than that of the CZ-Si solar cells, and it has a substantially wider range of distribution due to dislocation propagation.

Distribution and propagation of dislocation defects in quasi-single crystalline silicon ingots cast by the directional solidification method

We investigated the distribution and propagation of dislocation defects in quasi-single crystalline (QSC) silicon ingots. The dislocation distributions in both the central single crystalline region and the surrounding polycrystalline region of the ingot are measured and compared. Although the dislocation density in the central single crystalline region is much lower than that in the surrounding polycrystalline region, dislocations in this region propagate and spread with the growth of the silicon crystal, especially at the final stage of solidification. The performances of solar cells made of wafers from different parts of the QSC silicon ingot are also compared. The results show that solar cells made of wafers from the middle part of the ingot have the best performance and those made of wafers from the top part of the ingot have the worst performance.

Iron contamination in cast quasi-single crystalline silicon ingots

Iron is an important metallic impurity in crystalline silicon for solar cells and it can significantly influence the minority carrier lifetime. We numerically investigated the transport and distribution characteristics of iron impurity in the directional solidification process for quasi-single crystalline (QSC) silicon ingots, and attempted to reveal the relationship between the distributions of iron concentration and the minority carrier lifetime map. Additionally, the seed preservation time was varied to investigate its influence on the diffusion and distribution of iron impurity. The results show that iron diffusion from the silicon melt to the seed crystal during the preservation process can lead to a peak of iron concentration near the seed-crystal interface at the early stage of solidification. This peak gradually disappears during crystal growth, and iron diffusion results in a large region enriched with iron at the ingot bottom after complete solidification. Therefore, we conclude that the iron impurity is related to the large low lifetime region at the ingot bottom. We also discussed the effect of iron contamination on the high-low-high lifetime distribution pattern in the axial direction near the seed-crystal interface, which is a phenomena sometimes reported in QSC silicon ingots. The effect of seed preservation time on iron contamination in the QSC silicon ingot was also investigated. We found that the single-crystalline seed is highly contaminated even after a short time of seed preservation.

Optimization via simulation of a seeded directional solidification process for quasi-single crystalline silicon ingots by insulation partition design

In this study, transient global simulations are used to investigate the effects of the insulation partition design upon the heat transfer and thermal stress in an industrial seeded directional solidification (DS) furnace for quasi-single crystalline (QSC) silicon ingots. To optimize the DS process and improve the thermal field, a motionless insulation partition block with two position placements and a dynamic block are designed for a seeded DS furnace. Simulation results show that the dynamic partition block moving along the side insulation layer with a constant velocity can significantly reduce the total heating power consumption and can improve the crystal growth rate. Moreover, the melt–crystal interface shape in the dynamic case is the most favorable for growth of QSC silicon ingots. In addition, the thermal stress in the silicon ingot grown using the dynamic partition block is less than that in other two cases using motionless blocks.

Preservation of Seed Crystals in Feedstock Melting for Cast Quasi-Single Crystalline Silicon Ingots

The preservation of seed crystals is important for the casting of quasi-single crystalline (QSC) silicon ingots. We carried out transient global simulations of the feedstock melting process in an industrial-sized directional solidification (DS) furnace to investigate key factors influencing seed preservation. The power distribution between the top and side heaters is adjusted in the conventional furnace for multicrystalline silicon ingots and in the evolved furnace with a partition block for QSC silicon ingots. The evolution of the solid-liquid interface for melting and the temperature distribution in the furnace core area are analyzed. The power distribution can influence the temperature gradient in the silicon domain significantly. However, its effect on seed preservation is limited in both furnaces. Seed crystals can be preserved in the evolved furnace, as the partition block reduces the radiant heat flux from the insulation walls to the heat exchange block and prevents the heat flowing upwards under the crucible. Therefore, the key to seed preservation is to control radiant heat transfer in the DS furnace and guarantee downward heat flux under the crucible.

Local design of the hot-zone in an industrial seeded directional solidification furnace for quasi-single crystalline silicon ingots

To preserve the seed crystal in the melting process and improve the thermal field in the hot-zone during the solidification process aiding the formation of a quasi-single crystalline silicon ingot, an insulation partition block was designed for use in the hot-zone of an industrial seeded directional solidification furnace. A global model taking into account thermal conduction, thermal radiation, melt convection and argon flow was established to investigate the effects of the insulation configuration design on the thermal field, solidification interface shape, melt convection, argon recirculation and power consumption. In addition to comparing insulation configuration designs with and without the partition block, we carried out a comprehensive parameter study of the local design of the hot-zone, including the position, the width and the thickness of the insulation partition block. The results show that a suitable temperature gradient and a flat or slightly convex interface during seed preservation and bulk crystal growth can be achieved to maintain the quasi-single crystal structure all the way to the top of the silicon ingot. Furthermore, the argon recirculation and energy consumption can be reduced and the melt flow motion can be controlled by good insulation partition block design.

Influence of an insulation partition on a seeded directional solidification process for quasi-single crystalline silicon ingot for high-efficiency solar cells

An insulation partition was designed in a seeded directional solidification (DS) furnace of industrial-size to produce quasi-single crystalline silicon ingot for solar cells of high efficiency. We used a transient global model to study the effects of the insulation partition block on the temperature and thermal stress fields in the solidified silicon ingot during the solidification process. We validated the transient global model by comparing the calculations with the experimental measurements. Simulation results show that an insulation partition block can significantly reduce the total heating power consumption and influence the temperature and velocity fields in the silicon melt. We also found that the melt-crystal interface (m–c interface) shape changes from concave to convex to the melt with a larger crystal growth rate with an insulation partition, while it always remains flat with a smaller crystal growth rate without a partition block. The axial temperature gradient and thermal stress in the grown silicon crystal increased with a partition block. The solar cells from the grown quasi-single crystalline silicon ingot exhibited higher short-circuit current (Isc) and open-circuit voltage (Voc). The average conversion efficiency of solar cells is increased to 17.8% and about 1.2% more than that based on mc-Si ingot from the conventional furnace.