1 Introduction
After the solar module was produced, when the power test was performed, the power of the module was normal, but the customer received the module and found that the power was greatly attenuated when it was installed and operated. This phenomenon is mostly caused by the photo-induced attenuation of the cell sheet. This article will systematically briefly describe the phenomenon of light attenuation.
2 Light attenuation
The photo-induced attenuation of photovoltaic modules can be divided into two phases: initial photo-induced attenuation and aging decay.
2.1 Initial photo-attenuation
The initial photo-induced attenuation, that is, the output power of the photovoltaic module, dropped significantly in the first few days of initial use, but then stabilized. The main reason for this phenomenon is that the boron-oxygen complex in the P-type (boron-doped) crystalline silicon wafer reduces the lifetime of the minority carrier. By changing the P-type dopant, replacing the boron with crops can effectively reduce the photo-induced attenuation; or the pre-light treatment of the cell sheet is the initial photo-induced attenuation of the cell before the fabrication of the module and the initial photo-induced attenuation of the photovoltaic module. Can control in a very small range, but also improve the output stability of the component.
Photo-attenuation is more related to the cell chip manufacturers. The significance for component manufacturers is to select high-quality cells to reduce the effects of photo-induced attenuation.
2.2 Aging attenuation
Aging decay refers to the extremely slow power drop that occurs during long-term use. The main reason for this is related to the slow decay of the battery and to the deterioration of the performance of the packaging material. The main reason for the degradation of the performance of the main component of the module when irradiated with ultraviolet light. The long-term irradiation of ultraviolet rays causes aging and yellowing of the EVA and the backplane (TPE structure), which results in a decrease in the light transmittance of the module and a decrease in power.
This requires component manufacturers to strictly control the selection of EVA and backplane, and the selected materials must be very good in terms of aging resistance so as to reduce the power attenuation of components due to the aging of auxiliary materials.
3 Photo-induced attenuation mechanism
The early photo-induced attenuation of P-type (boron-doped) crystalline silicon solar cells was observed more than 30 years ago, after which a great deal of scientific research was conducted. Especially in recent years, scientific research has found that it is related to the concentration of boron and oxygen in silicon wafers. The basic consensus view is that light or current injection causes boron and oxygen in the silicon wafer to form a boron-oxygen complex, which reduces the lifetime of the minority carrier. However, after being annealed, the minority lifetime can be restored. The possible reactions are:
According to reports in the literature, the lifetime of sons and daughters of boron and oxygen-containing silicon wafers undergoes different degrees of attenuation after light irradiation. The greater the content of boron and oxygen in silicon wafers, the higher the boron oxygen produced in the body under light or current injection conditions. The more complexes, the greater the decrease in the life of their minority sons. In the low-oxygen, doped, and phosphorus-doped silicon wafers, the lifetime of the minority carriers increases with the increase of the illumination time, and the overall attenuation is minimal.
4 Solutions
4.1 Improve silicon single crystal quality
The early photo-induced attenuation of solar cell performance mainly occurs on monocrystalline silicon solar cells. For polycrystalline silicon solar cells, the early light-induced attenuation of conversion efficiency is small. This shows that the nature of the silicon wafer determines the early light-induced attenuation of solar cell performance. Therefore, it is necessary to solve the problem of early photo-induced attenuation of photovoltaic modules. It is necessary to solve the silicon problem manpower. Here are some scenarios for discussion.
A. Improve the quality of boron-doped P-type Czochralski silicon rods
The quality of some single crystal rods is indeed worrying, if not effectively changing this situation will seriously affect the healthy development of the photovoltaic industry in the boron-doped Czochralski single crystal products, the main problems and improvement measures:
1) Due to the shortage of raw high-purity polysilicon materials, some puller companies have introduced some high-quality silicon materials which should not be used in the base phosphorus and other harmful impurities. Solar cells produced using this type of material are not only inefficient, but also have very large early photo-induced attenuation. We strongly urge not to use low quality silicon.
2) In the high-purity polysilicon material, excessively low-resistivity N-type silicon germanium IC waste N-type silicon wafers, etc. are added. The boron-doped CZ silicon rod produced is a highly compensated P-type single crystal material. Although the resistivity is suitable, the concentration of boron and oxygen is very high, which results in a significant early photo-induced attenuation of the solar cell performance. We strongly urge not to use low resistivity N-type silicon.
3) Some companies have failed to pull the rods. The oxygen content in the crystalline silicon is too high, the internal stress is high, the density of dislocation defects is high, and the resistivity is not uniform, which directly affects the efficiency and stability of solar cells. We hope to improve the pulling process. Control oxygen content.
The solar cells made with the above-mentioned silicon wafers have a relatively large early photo-induced attenuation and will exceed the range acceptable to customers. In fact, the Czochralski process is very mature. As long as we use good material quality and produce according to the normal pull process, the quality of the silicon rod can be better controlled.
Bã€Using magnetron direct-draw silicon single crystal process (MCZ) to improve the quality of monocrystalline silicon rod products
This process can not only control the oxygen concentration in the single crystal, but also improve the uniformity of the resistivity in the longitudinal and radial directions of the silicon single crystal. This process has been started in some domestic pull rod companies.
C. Improve the quality of monocrystalline silicon rods by using zone melting single crystal silicon (FZ)
The zone melting single crystal silicon process avoids the intrinsic defects of a large amount of oxygen into the silicon crystal in the Czochralski process, thus completely solving the early photo-induced attenuation of the P-type (boron-doped) solar cell. Due to the high cost of the FZ process, it is mainly used for the manufacture of silicon wafers for ICs and other semiconductor devices, but at present, some companies have already carried out relevant transformations of the FZ process and reduced costs. It is suitable for the manufacture of solar cell silicon wafers. Some domestic pull rod companies have already started trial production in this area
D, change the dopant, use helium instead of boron
The battery made from the doped silicon wafer did not find the early photo-induced attenuation of the solar cell, and it was also one of the solutions to the early photo-induced attenuation of the solar cell.
E. Using phosphorus-doped N-type silicon wafers instead of boron-doped P-type silicon wafers
The use of germanium silicon wafers is also one of the methods to solve the problem of the initial test of photo-induced attenuation of the battery. However, from the perspective of the industrialized screen printing tantalum battery process, the tantalum battery has no advantage in conversion efficiency and manufacturing cost, and some key processes need to be solved.
4.2 Perform previous light attenuation on the cell
Since the early photo-induced attenuation of photovoltaic modules was caused by the early photo-induced attenuation of the cells, the solar cells were pre-attenuated so that the early photo-induced attenuation of the cells occurred prior to module fabrication. The early photo-induced attenuation of photovoltaic modules is very small and can be completely controlled within measurement errors. At the same time, the probability of hot spots in PV modules is also greatly reduced.
5 Summary
Improve the stability of the output of photovoltaic modules, to bring more benefits to our users.
Although the pre-attenuation by light is a method of remedying the damage of the dead sheep, it is an effective measure to solve the problem of early photo-induced attenuation of photovoltaic modules before the quality of the silicon wafer has been effectively improved.
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