Quasi-single crystal-solar cell technology overview

1. Introduction to Quasi-Single Crystal Technology

1.1 Traditional monocrystalline silicon and polycrystalline silicon technology

We know that monocrystalline silicon is generally produced by the CZ method, which uses a single crystal seed crystal with a specific crystal orientation for seeding. After rotating and pulling, a single crystal silicon rod with the target crystal orientation is obtained. The resulting product contains only one grain and has the characteristics of low defects and high conversion efficiency. At present, the conversion efficiency of large-scale production of monocrystalline silicon cells has reached 18%, but this method has high requirements for raw materials and operations, and the single feed is small, the product cost is high, and the solar cell attenuation is large. Polycrystalline silicon is mainly produced by directional solidification method, with a large single feed amount, easy operation, low cost, etc. The cell attenuation is much smaller than that of monocrystalline silicon wafers, but under traditional ingot casting conditions, the ingot polycrystalline often contains a large number of grain boundaries and defects, making the conversion efficiency of polycrystalline silicon solar cells about 1.5%~2% lower than that of monocrystalline silicon cells.

1.2 Quasi-single crystal technology

The core of quasi-single crystal technology is single crystal ingot casting technology. The ingot casting process produces products similar to single crystals or even full single crystals, combining the advantages of single crystal silicon and polycrystalline silicon. Compared with polycrystalline silicon, quasi-single crystal silicon wafers have fewer grain boundaries and lower dislocation density; the conversion efficiency of solar cells is as high as 17.5% or more. Compared with single crystal silicon wafers, the light-induced attenuation of quasi-single crystal cells is about 1/4 to 1/2 lower; the furnace charge is large, the production efficiency is high, the slicing process is simple, and the cost is low.

2. Quasi-single crystal ingot casting technology

2.1 Implementation Method

There are two ways to achieve ingot single crystal, as follows:

(1) Seedless ingot casting. The seedless guided ingot casting process has high requirements for the initial growth control process of the crystal nucleus. One method is to use a crucible with a groove at the bottom. The key point of this method is to precisely control the temperature gradient and crystal growth rate during directional solidification to increase the size of the polycrystalline grains. The size of the groove and the cooling rate determine the size of the grains, and the groove helps to increase the size of the grains. Because there are too many parameters to control, the seedless ingot casting process is particularly difficult.

(2) Seeded ingot casting. Most of the quasi-single crystal technologies currently in mass production are seeded ingot casting. This technology first places the seed crystal and silicon material doping elements in a crucible, with the seed crystal generally located at the bottom of the crucible. The silicon material is then heated to melt, while the seed crystal is kept from being completely melted. Finally, the temperature is controlled to cool down and the temperature gradient of the solid-liquid phase is adjusted to ensure that the single crystal starts to grow from the seed crystal position.

2.2 Temperature control and process control

Quasi-single crystal ingot casting places high demands on temperature control and process control. In order to meet the requirements of quasi-single crystal ingot casting, the ingot furnace must have strict temperature gradient and solidification rate control, suitable interface shape, nucleation or single crystal control, and flow control. Currently, many manufacturers can provide quasi-single crystal ingot casting equipment, such as Shaoxing Jinggong JJL500/JJL660/JJL800 (G6), American GTDSS450HP/DSS650 (G5), Beijing Jingyuntong JZ460/JZ660 (G6), German ALD SCU450/SCU800, and French Cyberstar650/800. In addition, European, American and Japanese manufacturers such as REC (ALD improved type), SchottSolar (VGF), and Kyocera (VGF type) have specially designed furnaces with good results.

Among them, ALD developed equipment for BPsolar products in the early days. As early as 2006, BPsolar had done a lot of work on the topic of ingot single crystals and developed MONO2 products. Its patent US2007/0169684A1 reported a variety of methods. One of the methods is to place the seed crystal and silicon material separately, and pour the molten silicon liquid into a container with seed crystals for crystal growth. Later, because its parent company focused on fossil fuels, BPsolar terminated the research on ingot single crystals.

The single crystal area of ​​quasi-single crystal products can reach more than 90% of the entire silicon wafer, and the dislocation density is relatively low. About 95% or more of some silicon wafers have almost no dislocations, and there are "band-shaped" high dislocation areas on the edges. Some silicon wafers contain subcrystals. Take the Yuhui Virtuswafer product as an example: the area close to the crucible surface is polycrystalline, and the other areas are basically single crystals according to the crystal growth situation. If the crystal growth situation is good, as shown in Figure 1C.

Figure 1. Transverse distribution of crystal size in a silicon ingot

3. Improvement of battery technology

准单晶产品也引发了各个企业对电池工艺的改进。硅片的晶向控制、位错密度、碳氧浓度和杂质分布，以及侧边问题会直接影响电池片效率。不同于普通多晶，准单晶产品更适合碱制绒工艺，形成倒金字塔型织构化表面，可显著提高成品电池片效率。晶澳太阳能针对准单晶电池片发明了先酸制绒后碱制绒的特殊工艺，目前maple系列电池片效率已经达到了18.2%。然而由于碱制绒的各项异性，准单晶中尤其边缘，非指定晶向处无法腐蚀，会在电池表面形成高亮区域，影响组件成品的外观。

Figure 2: ReneSola and JA Solar quasi-monocrystalline cells

4. Decisive factors of quasi-single crystal technology

4.1 Key points of technology research and development

(1) Improvement of temperature gradient. Research and development on thermal field to improve temperature gradient, while also paying attention to thermal field protection;

(2) Seed preparation. Research has found that the preparation of quasi-single crystal seeds will develop in the direction of ultra-large and ultra-thin;

(3) Precise melting control. This link is very difficult to control. It determines whether the quasi-single crystal can be stably produced, so a corresponding precise melting control device is required. It is understood that in order to obtain a stable control process, Phoenix Photovoltaic has developed a set of seed melting control equipment specifically for quasi-single crystals, which can enter the crystal growth stage at 0.5mm;

(4) Dislocation density. In many production processes, efficiency degradation is always inevitable. Therefore, controlling the dislocation density to a minimum is the key to this process.

(5) Corner polycrystalline control, that is, reasonable and effective control of the proportion of corner polycrystalline;

(6) Improvement of ingot casting yield. Currently, the yield is about 40% to 60%, which needs to be improved.

4.2 Determining Factors for Mass Production

(1) A feasible process route. If the developed quasi-single crystal does not have a feasible process route, the quasi-single crystal product will only remain in the laboratory stage;

(2) It is a stable control method;

(3) Precision melting control equipment;

(4) Low transformation and production costs, that is, to achieve transformation based on the original ingot casting furnace, thereby reducing costs. Phoenix Photovoltaic recently announced that the company successfully achieved the world's first mass production of quasi-single crystal by transforming the GTSolar DDS450 model furnace, and the cost was lower than that of crystalline silicon cells.

5. The significance of quasi-single crystal technology

Quasi-single crystal is not only a feasible way to produce high-efficiency silicon wafers, but also a way for ingot foundries to reduce costs. Regarding the issue of cost control, it is well known that in terms of the utilization rate of battery components, the silicon rods of CZ-monocrystalline silicon are cylindrical, and the four sides of the photovoltaic cell to be produced need to be cut off, and the yield rate of the battery components formed is about 50%. In comparison, the quasi-single crystal silicon ingot is a square ingot, and the slices used to make the cell are also right-angled squares, and the yield rate of the battery components formed is about 65%. In terms of process cost, CZ-monocrystalline silicon is 160 yuan/kg, while quasi-single crystal silicon is 60 yuan/kg. Considering the total cost of photovoltaic cells, under the premise of certain other costs such as silicon raw materials, slices, and components, the cost of the entire production chain can be reduced by 10% due to quasi-single crystal silicon ingot technology. However, to achieve the low cost of this technology, it is necessary not only to master the relevant processes and theoretical knowledge, but also to be proficient in practical operations. The last factor is high reliability, the core of which is whether the products produced can withstand such a large temperature difference.

Although quasi-single crystals have various advantages, judging from the above-mentioned technical difficulties, their development is still subject to many constraints, and more technological breakthroughs are needed to achieve long-term development.

6. Conclusion

Photovoltaic equipment manufacturers will face tremendous pressure in the future. Researching production processes in many aspects and using advanced equipment to meet the development needs of the photovoltaic industry are important ways out for photovoltaic equipment manufacturers. At present, China has surpassed Western countries in single crystal pulling, multi-crystalline ingot casting, and especially quasi-single crystal ingot casting technology. Several well-known domestic manufacturers have conducted varying degrees of research and practical application on quasi-single crystal production processes and technologies. Benefiting from quasi-single crystals, companies that started earlier will further consolidate their position in the ingot furnace market, adding chips to the increasingly competitive photovoltaic equipment market.

From a long-term perspective, with the continuous and large-scale production of quasi-single crystal products, more and more new technologies will be put into mass production, such as diamond wire cutting, all-single crystal ingots, direct thin silicon wafers, etc.