The following takes polysilicon as an example to introduce the process flow of solar cells.
Polycrystalline silicon solar cells have developed rapidly due to their low cost of materials. By 2000, its output had accounted for 48.86% of the world’s total solar cell output, ranking first. The conversion efficiency of cast polycrystalline silicon solar cells has reached as high as 19.8% (with an area of 4cm2, under AM1.5 spectral conditions). Polycrystalline silicon solar cells are leading the current solar cell market. The production of polycrystalline silicon solar cells with the goal of reducing the cost of solar cell modules is divided into four stages, namely raw material technology, substrate technology, cell technology and module technology.
1. Selection of silicon wafers
The selection of silicon wafers is to select silicon wafers with the same performance. If silicon wafers with different performances are combined to form a single solar cell, the output power will be reduced. The main properties of silicon wafers include the conductivity type, resistivity, crystal orientation, dislocation, and life of the silicon material.
2. Surface treatment of silicon wafers
In the process of cutting silicon ingots into silicon wafers, the surface must be polluted to varying degrees, including oily metal and dust, etc., which are mixedly attached to the surface of silicon wafers, and at the same time, the surface of silicon wafers leaves mechanical damage made by cutting. The surface treatment of silicon wafers includes surface cleaning and surface polishing.
(1) Surface cleaning
Surface cleaning is the use of chemical cleaning agents to remove various impurities. Commonly used chemical cleaning agents include ionized water, organic solvents (such as toluene, xylene, acetone, trichloroethylene, carbon tetrachloride, etc.), concentrated acids, strong alkalis, and high-purity neutral detergents. When cleaning, place the silicon wafer in a special wafer basket and soak it in a washing solution heated to 100°C for overflow ultrasonic cleaning.
(2) Surface polishing
The surface of the cut silicon wafer is left with a highly distorted lattice layer and a deep elastic deformation layer, commonly known as the damage layer, with a thickness of 18-26 μm. There are infinitely many carrier recombination centers in these damaged layers, which must be completely removed before the surface of the silicon wafer is fabricated. In the production process of conventional monocrystalline silicon solar cells, chemical etching is often used to etch away 30~50μm from the rough cut surface to obtain a smooth and clean surface of the silicon wafer. After polishing the silicon wafer, use aqua regia (nitro hydrochloric acid) or acidic hydrogen peroxide (hydrogen dioxide) to remove ionic or atomic impurities on the surface of the remaining silicon wafer. In order to make high-efficiency silicon solar cells, chemical-mechanical polishing methods are generally not used, but chemical-mechanical polishing methods are used to process the silicon surface into a bright and flat mirror surface.
(3) Surface production
The surface of the pure silicon wafer has a high sunlight reflectivity. In order to reduce the surface reflectivity, the surface of the silicon wafer is structured to increase the absorption of solar radiation by the surface, that is, to reduce the reflection of the surface to solar radiation. In the solar cell production process, this structured silicon wafer surface is called the money surface.
3. Diffusion knot
The junction process is to generate diffusion layers of different conductivity types on a base material, and both it and the surface treatment before junction are key processes in the battery manufacturing process. The junction method includes thermal diffusion method, ion implantation method, epitaxy method, laser method and high frequency electric implantation method. The following mainly introduces the thermal diffusion method.
The main thermal diffusion methods used in silicon solar cells include coating source diffusion, liquid source diffusion, and solid boron nitride source diffusion.
(1) Coating source diffusion
Coating source diffusion is generally divided into simple coating source diffusion and silica latex source coating diffusion.
(2) Diffusion of liquid sources
Liquid source diffusion includes phosphorus oxychloride liquid source diffusion and boron liquid source diffusion. It is carried by the method of gas carrying impurities into the diffusion furnace to achieve diffusion. Its principle is shown in Figure 1.
(3) Solid-state boron nitride source diffusion
Solid-state boron nitride source diffusion usually uses flake boron nitride as the source and diffuses under nitrogen protection. There are two production methods for flake boron nitride: high-purity boron nitride rods can be cut into slices the same size as silicon wafers, or powdered boron nitride can be punched into slices. Before diffusion, the boron nitride sheet was preliminarily passed oxygen at the diffusion temperature for 30 minutes, so that the diboron trioxide on the surface of the boron nitride reacted with the silicon to form borosilicate glass deposited on the surface of the silicon, and the boron diffused into the silicon. The diffusion is more uniform at lower nitrogen flow rates.
4. Edge removal
The methods of edge removal mainly include corrosion method and extrusion method. The etching method is to cover both sides of the silicon wafer and etch it in an etching solution composed of nitric acid and hydrofluoric acid. The extrusion method is a method of using acid-resistant rubber or plastic with the same size as the silicon wafer and slightly elastic to separate the silicon wafer neatly, and applying a certain pressure to prevent the corrosive liquid from infiltrating the gap to obtain a masking method.
5. Remove the back knot
Common methods for removing back knots include chemical etching, grinding and sintering of aluminum by steam or screen printing.
(1) Chemical corrosion method
Chemical etching is an earlier method. This method can remove the back junction and the peripheral diffusion layer at the same time, so the process of etching the peripheral can be omitted.
(2) Grinding method
The grinding method is a method of grinding the back knot with emery, or spraying compressed air carrying sand grains to the back of the silicon wafer to remove the back knot.
(3) Steamed aluminum or screen printing aluminum paste sintering method
The first two methods of removing the back junction are applicable to both N+/P and P+/N type cells, and the steamed aluminum or screen-printed aluminum paste sintering method is only applicable to the N+/P type solar cell fabrication process.
The vaporized aluminum or screen-printed aluminum paste sintering method is to vacuum vapor-deposit or screen-print aluminum on the back of the diffused silicon wafer, and heat or sinter it to above the aluminum-silicon eutectic point (577°C) to sinter the alloy (Figure 2). After alloying, as the temperature drops, the silicon in the liquid phase will re-solidify to form a recrystallized layer containing a certain amount of aluminum. It is actually a process of doping silicon. It compensates for the donor impurities in the N+ layer on the backside, resulting in a P-type layer doped with Al. From the Si-Al binary phase diagram (Figure 3), it can be seen that the ratio of Al in the liquid phase increases as the alloy temperature increases.
6. Making the upper and lower electrodes
The electrode of the solar cell is a connecting conductor that forms close contact at both ends of the PN junction of the battery to connect the PN junction of the battery to form an electrical circuit that can supply power to the outside. Usually, the connecting conductor on the light-receiving surface of the battery is called the upper electrode, and the connecting conductor made on the back of the battery is called the lower electrode or the back electrode, also called the bottom electrode.
The methods of making solar cell electrodes mainly include vacuum evaporation method, electroless nickel plating method, screen printing and sintering method, etc. The metal materials used are mainly aluminum, titanium, silver, nickel and so on. Vacuum evaporation method and electroless nickel plating method are traditional process methods for making solar cell electrodes, which have disadvantages such as high production cost, high energy consumption and unsuitable for industrial production, and are not used in industrial production at present.
7. Make anti-reflection film
Although the silicon wafer made of textured surface can reduce the reflectivity of incident light to less than 10%, in order to reduce the reflection loss more, it is generally necessary to coat an anti-reflection film on its surface. Anti-reflection coating, also known as anti-reflection coating, the main function is to reduce or eliminate the reflected light on the surface, thereby increasing the amount of light transmission. The production process of making solar cells mostly adopts atmospheric pressure chemical vapor deposition of titanium dioxide anti-reflection film.
