The mainstream of solar cells-the preparation of crystalline silicon solar cells

The mainstream of solar cells-the preparation of crystalline silicon solar cells

Monocrystalline silicon solar cells, polycrystalline silicon solar cells and banded silicon solar cells are collectively referred to as crystalline silicon solar cells, which are the mainstream of solar cells in the world. In 2005, crystalline silicon solar cells accounted for more than 93.5% of the total output of various solar cells. Crystalline silicon solar cells are made by diffusion bonding on single crystal or polycrystalline silicon chips. The preparation process of commercial crystalline silicon solar cells is roughly the same.

  1. Chemical cleaning of silicon wafer
Chemical cleaning of silicon wafer
Chemical cleaning of silicon wafer

(1) Impurities that may be contaminated on the surface of silicon wafer the impurities that may be contaminated on the surface of silicon wafer can be roughly divided into three categories: ① organic substances such as grease, rosin and wax; ② Metals, metal ions and various inorganic compounds; ③ Dust and other soluble substances.

(2) Decontamination effect of several common chemical cleaning agents

① Sulfuric acid hot concentrated sulfuric acid has a strong dehydration and carbonization effect on organic matter. In addition to dissolving many active metals and their oxides, hot concentrated sulfuric acid can also dissolve inactive copper and interact with silver to produce silver sulfate slightly soluble in water, but it cannot interact with gold.

② Aqua regia aqua regia has strong oxidation, corrosiveness and strong acidity. Its strong oxidation is mainly used in cleaning.

(3) Acidic and alkaline hydrogen peroxide solution alkaline hydrogen peroxide cleaning solution (also known as No. 1 cleaning solution) is composed of deionized water, 30% hydrogen peroxide and 25% concentrated ammonia. Their volume ratio is: water: hydrogen peroxide: ammonia = 5:1:1 to 5:2:1.

Acidic hydrogen peroxide cleaning solution (also known as No. I cleaning solution) is composed of deionized water, 30% hydrogen peroxide and 37% concentrated hydrochloric acid in proportion. Their volume ratio is water: hydrogen peroxide: hydrochloric acid = (6:1:1) ~ (8:2:1).

In fact, the above cleaning fluids are often used together to achieve better results. The general cleaning sequence of silicon wafer is: first remove the oil with organic solvent (such as toluene, etc.), and then remove the residual organic and inorganic impurities with hot concentrated sulfuric acid. After the surface of silicon wafer is corroded, it is thoroughly cleaned with hot aqua regia or no. I cleaning solution, and rinsed with deionized water after each cleaning solution.

  1. Remove the surface damage layer
Remove the surface damage layer
Remove the surface damage layer

After preliminary cleaning and decontamination, the silicon wafer is then subjected to surface corrosion, and about 10% of each surface is etched in the corrosion solution μ m. Its function is to remove the slicing mechanical damage on the surface and expose the silicon surface with complete lattice. Corrosive liquids are acidic and alkaline.

(1) The mixture of acid corrosion nitric acid and hydrochloric acid can play a good role in corrosion. The solution ratio is concentrated nitric acid: hydrochloric acid = 10:1 to 2:1. The function of nitric acid is to oxidize the elemental silicon into silicon dioxide. Hydrofluoric acid continuously dissolves the silica formed on the silicon surface and makes the reaction continue. The generated complex hexachlorosilicic acid is dissolved in water. The corrosion rate can be controlled by adjusting the ratio of nitric acid to hydrofluoric acid and the temperature of the solution. For example, adding acetic acid as buffer in the corrosion solution can make the surface of silicon wafer bright. The ratio of general acidic corrosive solution is nitric acid: hydrochloric acid: acetic acid = 5:3:3 or 5:1:1

(2) Alkaline corrosive silicon can act with alkali solutions such as sodium hydroxide and hydroxide clock to produce silicate and release hydrogen. For economic considerations, relatively cheap NaOH solution is usually used.

Although the surface of alkali corroded silicon wafer is not bright and flat without acid corrosion, the performance of the battery made is exactly the same. At present, the application in the production of silicon solar cells outside China shows that alkali corrosion solution is an ideal silicon surface corrosion solution due to its low cost and less environmental pollution. In addition, alkali corrosion can also be used in the thinning technology of silicon wafer to manufacture thin silicon solar cells.

  1. Production of suede
Production of suede
Production of suede

The effective suede structure helps to improve the performance of solar cells. Due to the multiple reflection and refraction of incident light on the surface, the absorption of light is increased, and its reflectivity is very low, which is mainly reflected in the improvement of short-circuit current (ISC).

(1) Fabrication of monocrystalline silicon suede the fabrication of monocrystalline silicon suede is to use the anisotropic corrosion of monocrystalline silicon to form a nano tetrahedral pyramid structure (also known as pyramid structure) on the surface of silicon wafer. The pyramid structure on the surface of monocrystalline silicon was observed by electron microscope. Anisotropic corrosion, that is, the corrosion rate changes with different crystallization directions of single crystal. Generally speaking, the crystal surface

The higher the covalent bond density, the more difficult it is to corrode. For silicon, if the appropriate corrosion solution and temperature are selected, the corrosion rate of (100) surface can be dozens of times higher than that of (111) surface. Therefore, the anisotropic corrosion of (100) silicon wafer eventually leads to many dense tetrahedral pyramids with (111) surface on the surface. Due to the randomness of the corrosion process, the sizes of the pyramids are different to be controlled at 3 ~ 6 μ M is appropriate.

The anisotropic etching solution of monocrystalline silicon usually uses hot alkaline solution. The available bases include NaOH, Koh, LiOH, hydrazine and ethylenediamine. Most of them use cheap dilute NaOH solution (concentration is about 1%) to make suede silicon, and the corrosion temperature is 70 ~ 85 ℃. In order to obtain uniform suede, alcohol (the most commonly used are ethanol and isopropanol) should also be added to the solution as complexing agent to accelerate the corrosion of silicon, and general chemical cleaning should be carried out after the suede is corroded. The surface prepared silicon wafers should not be stored in water for a long time to prevent contamination. They should be diffused and bonded as soon as possible

(2) Fabrication of polycrystalline silicon suede the crystal surface structure of polycrystalline silicon is randomly distributed, which makes alkaline corrosion not very effective for polycrystalline silicon. The different reaction rates between different grains on the surface of polycrystalline silicon caused by alkali corrosion will also produce steps and cracks. In order to obtain uniform suede structure, people have done a lot of research on the preparation methods of polysilicon suede. At present, the promising methods suitable for polysilicon texturing are acid texturing (the mixture of HF, HNO3 and CH3COOH), reactive ion etching RIE (reactive ion etching), mechanical grooving and laser grooving. The surface texture of polycrystalline silicon is chemically etched by h3ch and h3ch; Reactive ion etching achieves the purpose of texture through stress-free dry etching. In the process of texture, uniform and fine pyramid like structure can be obtained on the surface of polycrystalline silicon by adjusting gas flow, RF power and reaction pressure. Large area polycrystalline silicon solar cells with laboratory efficiency of more than 17% use this method to texture. Mechanical grooving is to simultaneously carve V-shaped grooves on the surface of polycrystalline silicon with multiple blades to reduce the optical reflection on the surface of polycrystalline silicon. Laser Grooving uses laser to melt or vaporize silicon to form surface texture and achieve light trapping.

  1. Fabrication of p-n junction
Fabrication of p-n junction
Fabrication of p-n junction

At present, the methods of making p-n junction include thermal diffusion, ion implantation, epitaxy, laser and high-frequency electric implantation. Here we focus on the thermal diffusion method.

The thermal diffusion p-n junction method is to use the heating method to make the group V a impurities doped into p-type silicon or the group III a impurities doped into n-type silicon. At high temperature, impurity elements enter the matrix due to thermal diffusion movement. Its distribution in the matrix varies according to the type of impurity elements, initial concentration and diffusion temperature. This distribution mode has a great impact on the electrical performance of the battery. At present, the most commonly used VA impurity element in silicon solar cells is phosphorus, and the Ⅲ a impurity element is boron.

The requirement of diffusion is to obtain the junction depth and diffusion layer block resistance suitable for the p-n junction of solar cell. The shallow junction has small dead layer and good short wave response, while the shallow junction causes the increase of block resistance. In order to keep the series resistance of the battery low, it is necessary to increase the number of grid lines of the upper electrode. The two are contradictory. In fact, both sides should be taken into account. The junction depth of conventional silicon solar cells is about 0.3 ~ 0.5 μ m. The block resistance is about 20 ~ 100 Ω / port.

The main thermal diffusion methods used in silicon solar cells include coating source diffusion, liquid source diffusion, solid source diffusion and so on. The diffusion process should be carried out in a very clean environment. Utensils and tools for diffusion shall be strictly cleaned, and attention shall be paid to ensuring the purity of deionized water and key chemical reagents.

After diffusion, diffusion layers are formed on both sides and around the silicon wafer. The p-n junction formed on the illumination surface of silicon wafer is called the front junction, which is necessary to realize photoelectric conversion. The p-n junction on the back and the surrounding diffusion layer must be removed in the subsequent process. After diffusion, the front junction must be properly protected. In practical work, it is often found that the reason for the low conversion efficiency and low yield of the battery is the careless operation after the junction, resulting in the damage of the p-n junction. For example, camera scratches and improper ultrasonic cleaning. The cone angles of tetrahedral cones on the surface of suede silicon are more vulnerable to damage, so you should be especially careful during operation.

  1. Edge cutting

In the diffusion process, a diffusion layer is also formed on the peripheral surface of the silicon wafer. The peripheral diffusion layer forms a short-circuit ring between the upper and lower electrodes of the battery, which must be removed. Any small local short circuit around the battery will reduce the parallel resistance of the battery and become waste.

The edge cutting method is corrosion method, that is, cover both sides of the silicon wafer and corrode it in the corrosion solution composed of nitric acid and hydrochloric acid for about 30s. The extrusion method is to use acid resistant rubber or plastic with the same size as the silicon wafer and slightly elastic, which is neatly separated from the silicon wafer. After a certain pressure is applied, it can prevent the corrosive liquid from penetrating into the gap and obtain masking. At present, plasma dry etching is used in industrial production. Under the condition of glow discharge, fluorine and oxygen alternately act on silicon to remove the periphery containing diffusion layer.

  1. Remove the back knot

The following three methods are commonly used to remove back knot: chemical corrosion method, grinding method and aluminum evaporation or screen printing aluminum paste sintering method.

(1) Chemical corrosion method chemical corrosion is a relatively early method. This method can remove the back knot and the surrounding diffusion layer at the same time, so the process of corroding the surrounding can be omitted. After corrosion, the back is flat and bright, which is suitable for making vacuum evaporation electrode. The masking of the front knot generally uses the method of coating black glue, which is made of vacuum sealing wax or high-quality asphalt dissolved in toluene, xylene or other solvents. After the silicon wafer is corroded and the back knot is removed, the vacuum sealing wax is dissolved with solvent, and then boiled and cleaned with concentrated sulfuric acid or cleaning solution.

(2) Grinding method grinding method is to grind the back knot with emery, or spray the sand particles carried by compressed air to the back of silicon wafer to remove it. A rough silicon surface is formed on the back of the grinding plate, so it is suitable for the back electrode made of electroless nickel plating.

(3) The first two methods of removing back knot are applicable to N + / N and P + / N cells, while the aluminum steaming or screen printing aluminum paste sintering method is only applicable to the manufacturing process of N + / P solar cells. This method is to vacuum evaporate or screen print a layer of aluminum on the back of the diffused silicon wafer, heat or sinter the sintered alloy above the aluminum silicon eutectic point (577 ℃) (Fig. 3-32). After alloying, with the cooling, the silicon in the liquid phase will solidify again to form a recrystallized layer containing a certain amount of aluminum. In fact, it is a process of doping silicon. It compensates for the donor impurities in the back n + layer to obtain a p-type layer doped with aluminum. As can be seen from the silicon aluminum binary phase diagram (Fig. 3-33), the ratio of aluminum in the liquid phase increases with the increase of alloy temperature. Under sufficient aluminum content and alloy temperature, the back can even form an electric field in the same direction as the front junction, which is called the back field. At present, the process has been used in mass production process, which improves the open circuit voltage and short circuit current of the battery and reduces the contact resistance of the electrode.

Whether the back junction can be burned through is related to the following factors: the resistivity of the matrix material, the doping concentration and thickness of the back diffusion layer, the thickness of the back evaporation or printing aluminum layer, the sintering temperature, time and atmosphere.

  1. Preparation of optical antireflection film and surface passivation

Surface passivation of semiconductor devices can effectively reduce the surface density of States and improve the stability and reliability of devices. Passivation of the surface of the emitting region or base region of the silicon solar cell can effectively reduce the surface recombination of the emitting region and base region, improve the open circuit voltage of the solar cell, and improve the photoelectric conversion efficiency of the solar cell. The optical antireflection film on the light facing surface of the solar cell can effectively improve the light absorption of the solar cell, so as to increase the gain of the short-circuit current of the solar cell. Different from other semiconductor devices, solar cell is a kind of photoelectric converter, so in the specific process, the base surface passivation and optical antireflection must be considered comprehensively.

At present, SiO2, TiOx, SiNx: H and other thin film materials are suitable for optical antireflection films of crystalline silicon solar cells. Taking SiNx: h as an example, amorphous silicon hydride films are usually prepared by chemical vapor deposition or sputtering. Three basic chemical vapor deposition processes are: atmospheric pressure chemical vapor deposition (APCVD), the reaction pressure is one atmospheric pressure and the reaction temperature is 700 ~ 1000 ℃; Low pressure chemical vapor deposition (LPCVD), the reaction pressure is 0.1mbar (1mbar = 102 PA, the same below), and the reaction temperature is 750 ℃; Plasma enhanced chemical vapor deposition (PECVD), the reaction pressure was 1mbar and the reaction temperature was 500 ℃. Plasma enhanced chemical vapor deposition has been widely used in microelectronic industry because of its low deposition temperature. Sputtering method uses high-energy particles to bombard silicon targets, and carries out ion reaction in the presence of N + ions and H + ions to form silicon ammoniate molecules, which are deposited on the surface of solar cells.

Although SiNx: H films deposited by PECVD as optical antireflection and passivation films for crystalline silicon solar cells have entered commercial production in some foreign companies, there are still some disputes about the thermal stability and passivation mechanism of SiNx: H films. Internationally, some laboratories are constantly improving their performance to better meet the performance requirements of optical antireflection film and passivation film for crystalline silicon solar cells.

  1. Electrode fabrication – screen printing and sintering

The electrode of the solar cell with antireflection film is prepared by screen printing. The back electrode of silver paste or silver / aluminum paste is printed on the back of the cell, then the back electric field is printed on the aluminum paste after drying, and then the front electrode of silver paste is printed on the front of the cell, and then sintering is carried out. The sintering temperature is controlled at about 800 ~ 880 ℃, so as to complete the manufacturing of the positive and negative electrodes of crystalline silicon solar cell.

  1. Battery performance test and grading

The screen printed and sintered solar cells are classified according to the appearance color and electrical performance test results. The above is the international general silicon cell production process. In recent years, the efficiency of crystalline silicon solar cells has been continuously improved. There are many new structures and new processes, which have been used in large-scale industrial production.

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