Solar cell module

Generally, the solar cell module is composed of five layers from top to bottom, including photovoltaic glass, packaging adhesive film, cell chip, packaging adhesive film, and backplane:

（1） Photovoltaic glass

Due to the poor mechanical strength of the single solar photovoltaic cell, it is easy to break; The moisture and corrosive gas in the air will gradually oxidize and rust the electrode, and can not withstand the harsh conditions of outdoor work; At the same time, the working voltage of single photovoltaic cells is usually small, which is difficult to meet the needs of general electrical equipment. Therefore, the solar cells are usually sealed between a packaging panel and a backplane by EVA film to form an indivisible photovoltaic module with packaging and internal connection that can provide DC output independently. Several photovoltaic modules, inverters and other electrical accessories constitute the photovoltaic power generation system.

After the photovoltaic glass covering the photovoltaic module is coated, it can ensure a higher light transmittance, so that the solar cell can generate more electricity; At the same time, the toughened photovoltaic glass has higher strength, which can make the solar cells withstand greater wind pressure and greater diurnal temperature difference. Therefore, photovoltaic glass is one of the indispensable accessories of photovoltaic modules.

Photovoltaic cells are mainly divided into crystalline silicon cells and thin film cells. The photovoltaic glass used for crystalline silicon cells mainly adopts the calendering method, and the photovoltaic glass used for thin film cells mainly adopts the float method.

（2） Sealing adhesive film (EVA)

The solar cell packaging adhesive film is located in the middle of the solar cell module, which wraps the cell sheet and is bonded with the glass and the back plate. The main functions of the solar cell packaging adhesive film include: providing structural support for the solar cell line equipment, providing maximum optical coupling between the cell and solar radiation, physically isolating the cell and the line, and conducting the heat generated by the cell, etc. Therefore, packaging film products need to have high water vapor barrier, high visible light transmittance, high volume resistivity, weather resistance and anti PID performance.

At present, EVA adhesive film is the most widely used adhesive film material for solar cell packaging. As of 2018, its market share is about 90%. It has more than 20 years of application history, with balanced product performance and high cost performance. POE adhesive film is another widely used photovoltaic packaging adhesive film material. As of 2018, its market share is about 9% 5. This product is an ethylene octene copolymer, which can be used for packaging of solar single glass and double glass modules, especially in double glass modules. POE adhesive film has excellent characteristics such as high water vapor barrier rate, high visible light transmittance, high volume resistivity, excellent weather resistance and long-term anti PID performance. In addition, the unique high reflective performance of this product can improve the effective utilization of sunlight for the module, help to increase the power of the module, and can solve the problem of white adhesive film overflow after module lamination.

（3） Battery chip

Silicon solar cell is a typical two terminal device. The two terminals are respectively on the light receiving surface and the backlight surface of the silicon chip.

The principle of photovoltaic power generation: When a photon shines on a metal, its energy can be fully absorbed by an electron in the metal. The energy absorbed by the electron is large enough to overcome the Coulomb force inside the metal atom and do work, escape from the metal surface and become a photoelectron. Silicon atom has four outer electrons. If pure silicon is doped with atoms with five outer electrons, such as phosphorus atoms, it becomes an N-type semiconductor; If pure silicon is doped with atoms with three outer electrons, such as boron atoms, a P-type semiconductor is formed. When P type and N type are combined, the contact surface will form a potential difference and become a solar cell. When sunlight shines on the P-N junction, the current flows from the P-type side to the N-type side, forming a current.

According to the different materials used, solar cells can be divided into three categories: the first category is crystalline silicon solar cells, including monocrystalline silicon and polycrystalline silicon. Their research and development and market application are relatively in-depth, and their photoelectric conversion efficiency is high, occupying the main market share of the current battery chip; The second category is thin-film solar cells, including silicon based films, compounds and organic materials. However, due to the scarcity or toxicity of raw materials, low conversion efficiency, poor stability and other shortcomings, they are rarely used in the market; The third category is new solar cells, including laminated solar cells, which are currently in the research and development stage and the technology is not yet mature.

The main raw materials of solar cells are polysilicon (which can produce single crystal silicon rods, polysilicon ingots, etc.). The production process mainly includes: cleaning and flocking, diffusion, edge etching, dephosphorized silicon glass, PECVD, screen printing, sintering, testing, etc.

The difference and relationship between single crystal and polycrystalline photovoltaic panel are extended here

Single crystal and polycrystalline are two technical routes of crystalline silicon solar energy. If the single crystal is compared to a complete stone, the polycrystalline is a stone made of crushed stones. Due to different physical properties, the photoelectric conversion efficiency of single crystal is higher than that of polycrystal, but the cost of polycrystal is relatively low.

The photoelectric conversion efficiency of monocrystalline silicon solar cells is about 18%, and the highest is 24%. This is the highest photoelectric conversion efficiency of all kinds of solar cells, but the production cost is high. Because monocrystalline silicon is generally packaged with tempered glass and waterproof resin, it is durable and has a service life of 25 years.

The production process of polycrystalline silicon solar cells is similar to that of monocrystalline silicon solar cells, but the photoelectric conversion efficiency of polycrystalline silicon solar cells needs to be reduced a lot, and its photoelectric conversion efficiency is about 16%. In terms of production cost, it is cheaper than monocrystalline silicon solar cells. The materials are easy to manufacture, saving power consumption, and the total production cost is low.

Relation between single crystal and polycrystal: polycrystal is a single crystal with defects.

With the rise of online bidding without subsidies and the increasing scarcity of installable land resources, the demand for efficient products in the global market is increasing. Investors’ attention has also shifted from the previous rush to the original source, that is, the power generation performance and long-term reliability of the project itself, which is the key to future power station revenue. At this stage, polycrystalline technology still has advantages in cost, but its efficiency is relatively low.

There are many reasons for the sluggish growth of polycrystalline technology: on the one hand, the research and development cost remains high, which leads to the high manufacturing cost of new processes. On the other hand, the price of equipment is extremely expensive. However, even though the power generation efficiency and performance of efficient single crystals are beyond the reach of polycrystals and ordinary single crystals, some price sensitive customers will still be “unable to compete” when choosing.

At present, efficient single crystal technology has achieved a good balance between performance and cost. The sales volume of single crystal has occupied a leading position in the market.

（4） Backplane

The solar backplane is a photovoltaic packaging material located on the back of the solar cell module. It is mainly used to protect the solar cell module in the outdoor environment, resist the corrosion of environmental factors such as light, humidity and heat on the packaging film, cell chips and other materials, and play a weather resistant insulation protection role. Since the backplane is located at the outermost layer on the back of the PV module and directly contacts with the external environment, it must have excellent high and low temperature resistance, ultraviolet radiation resistance, environmental aging resistance, water vapor barrier, electrical insulation and other properties to meet the 25 year service life of the solar cell module. With the continuous improvement of power generation efficiency requirements of the photovoltaic industry, some high-performance solar backplane products also have high light reflectivity to improve the photoelectric conversion efficiency of solar modules.

According to the classification of materials, the backplane is mainly divided into organic polymers and inorganic substances. The solar backplane usually refers to organic polymers, and the inorganic substances are mainly glass. According to the production process, there are mainly composite type, coating type and coextrusion type. At present, the composite backplane accounts for more than 78% of the backplane market. Due to the increasing application of double glass components, the market share of glass backplane exceeds 12%, and that of coated backplane and other structural backplanes is about 10%.

The raw materials of solar backplane mainly include PET base film, fluorine material and adhesive. PET base film mainly provides insulation and mechanical properties, but its weather resistance is relatively poor; Fluorine materials are mainly divided into two forms: fluorine film and fluorine containing resin, which provide insulation, weather resistance and barrier property; The adhesive is mainly composed of synthetic resin, curing agent, functional additives and other chemicals. It is used to bond PET base film and fluorine film in composite backplane. At present, the backplanes of high-quality solar cell modules basically use fluoride materials to protect the PET base film. The only difference is that the form and composition of the fluoride materials used are different. The fluorine material is compounded on the PET base film by adhesive in the form of fluorine film, which is a composite backplane; It is directly coated on PET base film in the form of fluorine containing resin through special process, which is called coated backplane.

Generally speaking, the composite backplane has superior comprehensive performance due to the integrity of its fluorine film; The coated backplane has a price advantage because of its low material cost.

Main types of composite backplane

The composite solar backplane can be divided into double-sided fluorine film backplane, single-sided fluorine film backplane, and fluorine free backplane according to the fluorine content. Because of their respective weather resistance and other characteristics, they are suitable for different environments. Generally speaking, the weather resistance to the environment is followed by double-sided fluorine film backplane, single-sided fluorine film backplane, and fluorine free backplane, and their prices generally decrease in turn.

Note: (1) PVF (monofluorinated resin) film is extruded from PVF copolymer. This formation process ensures that the PVF decorative layer is compact and free of defects such as pinholes and cracks that often occur during PVDF (difluorinated resin) coating spraying or roller coating. Therefore, the insulation of PVF film decorative layer is superior to PVDF coating. PVF film covering material can be used in places with worse corrosion environment;

(2) In the process of PVF film manufacturing, the extruding arrangement of molecular lattice along the longitudinal and transverse directions greatly strengthens its physical strength, so PVF film has greater toughness;

(3) PVF film has stronger wear resistance and longer service life;

(4) The surface of extruded PVF film is smooth and delicate, free of stripes, orange peel, micro wrinkle and other defects produced on the surface during roller coating or spraying.

Applicable scenarios

Because of its superior weather resistance, double-sided fluorine film composite backplane can withstand severe environments such as cold, high temperature, wind and sand, rain, etc., and is usually widely used in plateau, desert, Gobi and other regions; The single-sided fluorine film composite backplane is a cost reducing product of the double-sided fluorine film composite backplane. Compared with the double-sided fluorine film composite backplane, its inner layer has poor ultraviolet resistance and heat dissipation, which is mainly applicable to roofs and areas with moderate ultraviolet radiation.

6、 PV inverter

In the process of solar photovoltaic power generation, the power generated by photovoltaic arrays is DC power, but many loads need AC power. DC power supply system has great limitations, which is not convenient for voltage transformation, and the load application scope is also limited. Except for special electrical loads, inverters are required to convert DC power to AC power. The photovoltaic inverter is the heart of the solar photovoltaic power generation system. It converts the DC power generated by the photovoltaic power generation system into the AC power required by life through power electronic conversion technology, and is one of the most important core components of the photovoltaic power station.