The basic structure of solar cells

Because the materials and processes used to produce solar cells are very different, the structures of solar cells are also diverse. The structure of a typical single-product silicon solar cell is shown in Figure 1 . The base material used is high-purity N-type and P-type single-product silicon rods. After cutting, grinding, polishing and other processes, the thickness is 0.25~ 0.5mm single product, its shape usually has two kinds of square and round shape, the base part of the wave-structured solar cell. Figure 1 shows an N+ /P-type solar cell made of P-type silicon material. The base material is the P layer, usually with a thickness of 0.2~0.5mm, and the base area composed of the base material can be referred to as the base area for short. The N layer region grown on the P layer is called the top layer region, also called the emission layer region, and can be referred to as the top layer for short. Usually the top layer is realized by high-temperature doping on the surface part of the base layer, and heavy doping is usually used, so it is marked as N+, and its thickness is 0.2~0.5μm. The doped top layer is located on the illuminated surface. A PN junction is formed at the junction of the P layer and the N layer. There is an upper electrode that forms an ohmic contact with it on the top layer. In order to ensure that the solar cell gets sufficient light, the electrode is made of bus bars and grid lines. The width of the grid lines is about 0.2 mm, and the width of the bus lines is about 0.5 mm. Connected by busbar. There is a lower electrode under the base layer that forms ohmic contact with it. The upper and lower electrodes can ensure that the solar cell can be connected to the load. In order to make the solar cell receive more light, it is usually necessary to add a transparent layer and an anti-reflection layer on the surface of the cell, and finally encapsulate it with a glass cover.

Figure 1 Single crystal silicon solar cell pin structure

The silicon solar cell can also use N-type silicon material as the base material to make a P+/N-type solar cell. Its structure is the same as that shown in Figure 1 , but it uses N-type base material and doped with P-type. Material to achieve.

In the context of the shortage of silicon raw materials and high prices, the mass production of crystalline silicon solar cells has been restricted, and their costs have continued to increase. Therefore, low-cost thin-film solar cells have a lot of room for development in the next few years. Compared with crystalline silicon solar cells, thin-film solar cells only need to use a very thin layer of optoelectronic materials, have a high light absorption coefficient, and use very few raw materials, so the cost is much lower than that of crystalline silicon solar cells. At the same time, the production process of thin-film solar cells can be continuous, so large-size cells can be produced, and large-scale production is easy. In addition, thin-film solar cells can also use flexible materials (such as stainless steel, plastics, etc.) as substrates, the cells are lighter in weight, can be rolled, and have a wide range of applications. At present, thin film solar cells mainly include amorphous silicon thin film solar cells, microcrystalline silicon thin film solar cells, polycrystalline silicon thin film solar cells, copper indium (gallium) selenium [CI(G)S] thin film solar cells, cadmium telluride thin film solar cells, Dye-sensitized thin-film solar cells, organic thin-film solar cells, etc.

Among various types of thin-film solar cells, silicon-based thin films, CIGS and CdTe (see Figure 2) are expected to achieve large-scale production. Among them, the rare metal selenium is used in the manufacturing process of CIGS thin-film solar cells, which makes large-scale production. The production cost of CIGS is relatively high, and the production process of CIGS is very complicated, which also causes certain difficulties for large-scale production, so the time is not yet fully ripe. As for CdTe thin-film solar cells, since the “cadmium” in its raw materials has been proven to be a carcinogen, there is a slight conflict with the green energy characteristics of solar cells. In addition, the “tellurium” in its raw materials is more expensive. So in comparison, silicon-based thin-film batteries are more suitable for large-scale production.

Figure 2 The structure of several main thin-film solar cells