Polycrystal is a kind of imperfect single crystal in essence.
In history, the earliest silicon wafer used in the production of solar cell is not like the mono-crystalline silicon wafer or polycrystalline silicon wafer produced by the specialized equipment but is the leftover bit and material using semiconductor wafer or the defective good. The price is quite expensive. As the qualified rate of semiconductor wafer is increasing gradually, the number of defective semiconductor wafer provided by the market is becoming less and less. When people hope to popularize silicon-based solar cells, there is no special solar mono-crystalline silicon production equipment. The cost of production of solar cell with semiconductor wafer is daunting, so people start to use the methods of casting, ingoting and directional solidification to produce crystalline silicon ingot, which is the so-called source of "Polycrystalline silicon wafers". Although the quality is poorer, the cost has reduced obviously. In the early 1980s, the technology used for producing solar monocrystalline silicon was becoming mature. In this period, a large number of solar power stations with more than 10 kw and above 1 mw emerged in Europe, America and China and the components produced with mono-crystalline silicon in these power stations are still used for generating electricity stably today. Accumulated attenuation in 30 years is less than 20%, greatly showing the stability of single crystal power.
The growth technics of polycrystal make itself not be able to generate large area of crystals with single crystal orientation (single crystal). The polycrystal is the aggregation of a large number of small single crystals in essence, as shown below:
The crystal boundary between small single crystal particles of polycrystalline ingot may reduce the generating capacity of cells. The simple and crude technologies of polycrystalline ingot make it expand in large scale easier but cannot control the dislocation defects and density of impurities at the lower level. These factors always affect the lifetime of minority carriers of polycrystal.
Power attenuation of components is divided into initial luminous decay and long-term attenuation. The comprehensive performance of single crystal is excellent.
Under the premise of reliable packaging materials for components, the key factor affecting the reliability difference of mono-crystalline and polycrystalline components is the power attenuation index. It is divided into initial luminous decay and long-term attenuation. Humans have studied on the problem of component attenuation before and after the 1970s. Through the exploration for decades, it is found that there are significant differences between single crystal and polycrystal in the two types of attenuation.
At present, mono-crystalline cell is mainly the Type P. This cell will have 2% ~3% fast power attenuation 2-3 weeks after the insolation. The reason is that boron is used as the dopant during the growth of crystal and many oxygen atoms are mixed in it at the same time. Also, substitutional boron and interstitial oxygen form deeper defects under the excitation of insolation so as to cause carrier recombination and cell performance degradation. However, this degradation can recover under the action of annealing. The power of solar cell can recover in 4 months or longer time (depending on the sunlight intensity and time). After one year, the accumulated attenuation is about 2.5%~3% and tends to be stable.
Polycrystalline cell does not have above problems, but due to the effect of high concentration of impurities and dislocation defects that cannot be overcome by itself, the cell performance will continue to decline to about 3% under the insolation, and will not recover.
On the current market, the guaranteed power of polycrystalline component is 97%-97.5% for the first year and 80% for 25 years. In other words, after the initial luminous decay is stable in the first year, it will attenuate 0.71%~0.73% per year later. Because mono-crystalline components use silicon materials with perfect crystal structure, the internal structure is more stable. The guaranteed power is 97% for the first year and 83.3% for 25 years. From the second year to the twenty-fifth year, the average annual attenuation is only 0.5%. These indexes are the guarantee values that can be written in the contract by the component manufacturer and also the indexes that the insurance company is willing to undertake. If the guaranteed power of polycrystalline component for 25 years is higher than that of single crystal, the insurance company is hard to promise.
The advantages of single crystal cannot be surpassed
In the past few decades, whether mono-crystalline cell or polycrystalline cell, its manufacturing technique is very rough. The performance of crystalline silicon material fails to reach the full potential. The difference of conversion efficiency is not obvious. With the continuous improvement of technology, the utilization of the electricity generation performance of crystalline silicon materials is refreshed constantly. Because of high quality characteristics of mono-crystalline material and high dislocation density and high impurity defect that cannot be overcome by polycrystalline material itself, the introduction of new technology must make the advantage of conversion efficiency of single crystal expand compared to the polycrystal. At present, the conversion efficiency advantage of Type P single crystal is 1.5 % compared to polycrystal. When the industrialization of PERC technology is realized, the efficiency of single crystal has increased 0.8-1%, but that of polycrystal can only increase 0.5-0.6%. In the future, IBC, HIT and other efficient technologies will introduced into the industrialization application and the advantage of single crystal will expand further.
Individual advantage and overall advantage
The advantage of single crystal compared to polycrystal is overall advantage. We don't exclude that the technology of some enterprises in the field of single crystal is poor. Comparing the single crystal technology in the past 3-5 years with current polycrystalline technology, in this individual case, the power stability and conversion efficiency of single crystal may reflect some disadvantages. In the same baseline, if the power difference between components is 15W, the price difference between single crystal and polycrystal will be covered by the cost difference of EPC. The investment cost of single crystal power station is lower than that of polycrystal power station. A very small amount of polycrystalline components with the power of 270W, 275W or more (packaging specifications for 60-piece cells, the same below) can be seen on the market, but the cost must be higher than that of 275W mono-crystalline component and this kind of component cannot be provided in bulk in 1-2 years. In short, photovoltaic generation is a kind of system engineering. The technical route that can realize industrialization and large-scale supply and has cost performance is the key in the market competition. The technology simply pursuing price advantage or conceptual advantage in some aspect can only be used as the transition in a particular period.