Quasi-single Crystalline Silicon Research Articles

Reduction of the initial defects generated during casting of quasi-single crystalline silicon by reserving gaps between seed crystals

The presence of dislocation is a significant impediment to enhancing the quality of quasi-single crystalline silicon cast by seed-assisted directional solidification. A novel approach was proposed to reduce dislocations in G7 166-type silicon ingots, especially in the region of seed crystal seams, by reserving seed crystal gaps during loading. We calculated that the seed crystal's linear thermal expansion after heating from 298.15 to 1683.15 K was 0.9 mm. However, in consideration of the actual deviation in seed processing and measurement, we increased the gap between seeds to 1.2 mm to prevent compressive stress on all 49 seed crystals. The actual residual seed gap measured after ingot casting was approximately 0.325 mm, which is relatively close to the calculated value. It was discovered that the average defect ratio in the silicon blocks decreased from 26.95% (without gaps) to 6.79% (with 1.2 mm seed gaps) for the proposed method. This indicates that a portion of the dislocation source was reduced by reducing compressive stress caused by linear expansion between adjacent unmelted seeds. The average conversion efficiency of the solar cells prepared using the gapless seed method for p-type polycrystalline cells is 22.03%, whereas the novel approach resulted in an average cell efficiency of 22.22%, featuring a higher proportion of high-efficiency sections.

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).

A new form of impurity cluster in casting quasi-single crystalline silicon

The casting quasi-single crystalline silicon technology is very promising in the development of high quality and low cost crystals for manufacturing photovoltaic devices. In this study, we find a new type of impurity cluster named as “black spot” in the casting quasi-single crystalline, which is different from the “shadows” caused by SiC/Si3N4 particles discovered in previous research. The “black spot” is systematically studied and characterized using inductively coupled plasma mass spectrometry (ICP-MS), photoluminescence (PL), and infrared (IR) detectors. The test results show that the “black spot” is composed of metal impurities. According to our analysis, the formation mechanism of the “black spot” may be caused by the accumulation of the metal impurities in certain region of the solid/liquid interface. These clustering impurities are not absorbed by enough grain boundaries in the casting quasi-single crystalline after being solidified into the crystal and thus exhibit as “black spot”. An optimization method is proposed in the direction of numerical simulation and verified by experiments to avoid the formation of the “black spot”.

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.

Effect of oxygen concentration on minority carrier lifetime at the bottom of quasi-single crystalline silicon

The formation mechanism of red zone in casting silicon ingots was investigated. Oxygen and metal distribution in ingots of crystalline Si were determined and analyzed. It was found that metal impurities is not the only factor to form bottom low minority carrier lifetime region. Interstitial oxygen with relative high concentration also decreases carrier lifetime at ingot bottom.

Optimization of the controlling recipe in quasi-single crystalline silicon growth using artificial neural network and genetic algorithm

The thermal field in the seeded directional solidification (DS) of quasi-single crystalline (QSC) silicon needs precise control to preserve the seed and maintain a flat solidification interface. Compared with adjusting the furnace configuration and materials properties, optimizing controlling recipe parameters of the DS process is more convenient and efficient in reality for improving the thermal field. However, controlling elements in the furnace interact with each other and the cooperation effect cannot be estimated by qualitatively deducing, which adds difficulties to the design and improvement of controlling recipe. In this study, an optimization system was built to optimize the controlling recipe during crystal growth and an industrial G6 DS furnace was taken as an application example. Flattening the solidification interface, reducing the thermal stress in the growing crystal and shortening the casting time were chosen as three optimization targets. Two independent heaters and a heat gate were the three controlling elements whose recipe parameters were the optimization variables. A 2D heat transfer model along with a thermal stress model was developed and an Artificial Neural Network (ANN) was trained to study the mapping relationship between optimization variables and targets in order to save computing resource and time. Genetic Algorithm (GA) was employed to yield the optimal recipes of the three controlling elements. The results showed that the optimal recipe achieved better crystal quality compared with the original recipe. The effectiveness and efficiency of this optimization system indicates that it can achieve the optimal result that theoretic analysis may hardly access.

Designing functional Σ13 grain boundaries at seed junctions for high-quality cast quasi-single crystalline silicon

Dislocation clusters and sub-grain boundaries (Sub-GBs) induced by the misorientation between adjacent seeds are destructive to the quasi-single crystal (QSC) Si ingot quality and the solar cell efficiency. Here, we have designed an artificial Σ13 GB between two adjacent <100>-oriented seeds at the crucible bottom. It is found that the generation of dislocation clusters and sub-GBs from the seed junctions is significantly suppressed owing to the low energy barrier potential of the Σ13 GB. Although some twins could generate from the vertical Σ13 GB, they will not give a bad influence on the ingot quality. The efficiency of solar cells is absolutely high in industrial circles, with an average value of 20.1%. These results pave a new way to the fabrication of high quality QSC-Si ingots for photovoltaic application.

Recombination activity of sub-grain boundaries and dislocation arrays in quasi-single crystalline silicon

Crystallographic defects in quasi-single crystalline (QSC) silicon are seriously detrimental to the performances of solar cells. In this work, we have identified these crystallographic defects as sub-grain boundaries (sub-GBs) and dislocation arrays (DAs). Sub-GBs with misorientation of >0.7° always show strong recombination activity, which are the main cause for the efficiency degradation of solar cells. Moreover, the recombination activity of sub-GBs with misorientation of <0.7° and DAs is strongly dependent on dislocation density and metal contamination level. Phosphorus gettering is an effective way to reduce the effect of metal contamination at the sub-GBs and DAs in QSC silicon.

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.

Controlling dislocation gliding and propagation in quasi-single crystalline silicon by using -oriented seeds

Dislocations originating from the seed junctions are the most harmful defects in quasi-single crystalline silicon (QSC-Si). In traditional <100>-oriented QSC-Si ingots, dislocations easily develop into cascade shape, or even further form sub-grain boundaries as the ingot grows. In this paper, we have developed a novel <110>-oriented QSC-Si ingot to control the gliding and propagation of dislocations. It is found that the cascade shape is absent and dislocations only aggregate in the {100}/{100} boundary planes of <110>-oriented seeds, revealed by the minority carrier lifetime mapping and photoluminescence image. Accordingly, the internal quantum efficiency of <110>-oriented QSC-Si solar cells is obviously higher than that of the traditional <100>-oriented ones. Nevertheless, it should be noticed that the average photoelectric conversion efficiency of <110>-oriented QSC-Si solar cells could be 0.6% lower than that of traditional ones if the {100}/{100} boundary is included in the solar cell, due to a higher reverse leakage current. This problem can be involved by enlarging the size of seed crystals or adopting better seed arrangement. These results provide a new promising method for the development of high quality QSC-Si in photovoltaic industry.

Efficient nanostructured quasi-single crystalline silicon solar cells by metal-catalyzed chemical etching

Seed-assisted cast quasi-single crystalline silicon (Qsc-Si) technique allows the production of efficient, low-cost solar cells. However, most of the Qsc-Si wafers still consist of single- and multi-crystalline silicon grains, which lead to difficulties when attempting to achieve high efficiency by using conventional acid or alkali texture processes. This paper highlights the fact that nano-textured Qsc-Si solar cells can reach efficiencies ranging from 18.4% to 18.9% by using the same metal-catalyzed chemical etching technique, along with a depressed color difference. A parallel sub-cell model is proposed to explain how to enhance the performance of Qsc-Si cells.

Solar cell fabrication using two-step textured quasi-single crystalline silicon by alkaline and RIE texturing processes

After alkaline chemical texturing process, SF6/O2 reactive ion etching (RIE) was used for maskless textured Si surface independently from crystal orientation. Additional RIE texturing on alkaline textured Si surface increased the light receiving areas and decreased the surface reflectance by random pyramidal structures with a width of hundreds of nanometers. The weighted average reflectance (WAR) of the textured Si surface by two-step texturing processes was achieved as low as 9.6% in the 300–1200 nm wavelength range. Compared to the alkaline texturing, the alkaline and RIE texturing of front Si surfaces decreased the WAR by 6%. By means of two-step texturing processes for quasi-single crystalline silicon (QSC-Si) surfaces, performance parameters of the solar cell such as conversion efficiency, short circuit current density (Jsc), open circuit voltage (Voc) and fill factor (FF) were examined in 18.2%, 36.6 mA/cm2, 624 mV and 79.7%, respectively.

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.

Dislocation-induced variation of generation kinetics of boron–oxygen complexes in silicon

The behaviors of light-induced degradation (LID) using quasi-single-crystalline (QSC) silicon have been demonstrated. It is found that the dislocations, the main defect in QSC silicon, can significantly influence the generation kinetics of B–O complexes that are responsible for the LID effect. Compared to dislocation-poor samples, higher activation energy of 0.57±0.02eV for the B–O complex generation in dislocation-rich silicon has been obtained, and meanwhile, the pre-exponential factor is two orders of magnitude higher. It is believed that the dislocation-related electronic states charged with holes can cause an energy potential barrier for the capture of single-positive holes that is required for the transformation of B–O complexes from latent centers to immediate transient centers.