Basics of photovoltaic cells

Classification of photovoltaic cells according to photovoltaic materials

According to the materials used, photovoltaic cells can be divided into silicon photovoltaic cells, multi-compound photovoltaic cells and organic semiconductor photovoltaic cells, etc.

⑴Silicone photovoltaic cell
①Single crystal silicon photovoltaic cell
Monocrystalline silicon photovoltaic cells are manufactured from high-purity monocrystalline silicon wafers. The appearance of the cells and components is shown in Figure 1. The processing technology is the most mature. Because monocrystalline silicon is generally encapsulated with toughened glass and waterproof resin, it is sturdy and durable, and its service life is generally up to 15 years, up to 25 years. The efficiency of industrialized products is generally between 15% and 18%, and the peak power is about 120-140Wp/m2. Currently, the conversion efficiency of monocrystalline silicon cells produced by a laboratory at the University of New South Wales in Australia is the highest, reaching 24.7%. The market share of monocrystalline silicon photovoltaic cells is about 32%, ranking second. The reason is that the price of monocrystalline silicon photovoltaic cell materials and the corresponding cumbersome battery technology make the cost price of monocrystalline silicon remain high. It is very difficult to reduce its cost significantly. Although monocrystalline silicon photovoltaic cells can be used in many occasions, due to their price factors, they are mainly used in fields such as photovoltaic power plants and aerospace.

Figure 1 The appearance of monocrystalline silicon photovoltaic cells and modules

②Polycrystalline silicon photovoltaic cell
The raw materials for making polycrystalline silicon photovoltaic cells are square silicon ingots processed after melting, rather than being drawn into single crystals. The cut silicon wafers are aggregated from single crystal silicon particles. The appearance of the cells and components is shown in Figure 2. It is easy to distinguish from monocrystalline silicon photovoltaic cells. Although the photoelectric conversion mechanism of polycrystalline silicon cells is exactly the same as that of monocrystalline silicon cells, the conversion efficiency of polycrystalline silicon cells is lower due to the different sizes and orientations of the monocrystalline silicon particles in the silicon wafers. The theoretical efficiency can reach 20%, and the actual efficiency of the current product is between 12% and 14%, and the peak power is about 100 to 115Wp/m2. Due to the abundant raw materials of polysilicon and the simple production process, the output and market share of polysilicon photovoltaic cells are the largest, with a current market share of about 58%. Polycrystalline silicon photovoltaic cells are used in many other fields, except for photovoltaic power plants and photovoltaic building integration (BIPV).

Figure 2 The appearance of polycrystalline silicon photovoltaic cells and modules

③Amorphous silicon photovoltaic cell
Amorphous silicon photovoltaic cell is a new type of thin-film photovoltaic cell that appeared in 1976. Its atomic arrangement is in an irregular state, and its appearance is shown in Figure 3. It is completely different from the production methods of monocrystalline silicon and polycrystalline silicon photovoltaic cells. The process is greatly simplified, the silicon material consumption is small, and the power consumption is lower. The main advantage is that it can generate electricity under low light conditions. The photoelectric conversion efficiency of amorphous silicon photovoltaic cells is low. The theoretical efficiency is 18%, generally 4%-9%, and the peak power is about 50Wp/m2. The current international advanced level is 13%-14%. The main problem is that it is not stable enough. As time goes by, its conversion efficiency decays. The current market share is about 5%. It is mainly used in electronic consumer products such as watches, calculators, toys, etc., and can also be used in fields such as BIPV.

Figure 3 Appearance of amorphous silicon photovoltaic module

⑵Multi-element compound photovoltaic cell
Multi-compound photovoltaic cells refer to photovoltaic cells that are not made of a single element material. There are many types of research in various countries, and most of them are still for industrial production, mainly including the following types: cadmium sulfide (CdS) and cadmium telluride (CdTe) photovoltaic cells, gallium arsenide (GaAs) photovoltaic cells, copper steel selenium (CuInSe2) Photovoltaic cells, etc.
①Group Ⅱ-VI compound semiconductor photovoltaic cell
Cadmium sulfide photovoltaic cells were used in photovoltaic cell modules in 1988. Its theoretical conversion efficiency is 33.6%~44.4%. At present, the conversion efficiency of small-area cells is about 15%, and the conversion efficiency of large-area cells is 10%. above. Because of its high theoretical conversion efficiency and low cost, it may be useful in the future. Cadmium telluride and cadmium sulfide belong to the I-VI group of compound semiconductors, with a band gap of 1.5eV, which is very compatible with the solar spectrum, and is most suitable for photovoltaic energy conversion. It is a good PV material with stable performance. It is a technological development. A faster type of thin film photovoltaic cell. Its theoretical conversion efficiency is 28%. In fact, the conversion efficiency of small area battery cells is about 12.8%~16%, the conversion efficiency of large area battery cells is 7.7%~9.1%, and the average efficiency of commercial batteries is 8%~10 %. In the early 1990s, cadmium telluride photovoltaic cells had been produced on a large scale, but the market developed slowly, and the market share had been hovering around 1%. Although the efficiency of cadmium sulfide and cadmium telluride thin-film photovoltaic cells is higher than that of amorphous silicon thin-film cells, the cost is also lower than that of crystalline silicon cells, and they are easy to produce on a large scale, but cadmium is highly toxic and can cause serious pollution to the environment. Restricted the development of this series of photovoltaic cells.
②Ⅲ-VI group compound semiconductor photovoltaic cell
Gallium arsenide belongs to the II-VI group of compound semiconductors. The gallium arsenide compound material has a very ideal band gap and high light absorption efficiency, making it easy to manufacture high-efficiency batteries. At the same time, the multi-junction gallium arsenide-based battery can be made by stacking technology, which can further improve the conversion efficiency. Among them, the photovoltaic cell conversion efficiency of single-junction gallium arsenide-based cells ranges from 26% to 28%, and 2-junction and 3-junction gallium arsenide-based cells can reach 35% to 42%. However, due to the high price of gallium arsenide-based materials, gallium arsenide thin-film batteries are currently only used in the field of aerospace power generation.
③Copper Indium Selenide Photovoltaic Cell
Copper indium selenium (CulnSe2) is a light-absorbing material with excellent performance. It is a semiconductor material with a gradient band gap and can expand the solar energy absorption spectrum range and improve the photoelectric conversion efficiency. The conversion efficiency of copper indium selenium photovoltaic cells is significantly higher than that of silicon thin film photovoltaic cells, and can reach 18%. Moreover, so far in this series of thin-film photovoltaic cells, no light radiation has been found to cause performance degradation. However, selenium and steel are rare elements with few reserves, so large-scale development will inevitably be restricted by materials.

⑶Organic semiconductor photovoltaic cells
①Pigment-sensitized photovoltaic cell
The so-called dye-sensitized photovoltaic cell refers to a cell in which dye and electrolyte are added between two transparent electrode substrates on a glass substrate or a plastic substrate. This technology can produce transparent cells and cells of various colors. This type of battery is much cheaper than silicon-based batteries, requires abundant raw materials, is easy to process, and has little impact on the environment. The conversion efficiency of commercial batteries is about 7.2%. At the 89th Spring Meeting of the Chemical Society of Japan (March 2009), a research team from the University of Tokyo released a dye-sensitized photovoltaic cell using clay as an electrolyte medium. Confirm that its conversion efficiency reaches 10.3%.
②Organic thin film photovoltaic cells
Organic thin-film photovoltaic cells are composed of pigments or polymer materials. The energy conversion efficiency of organic thin-film photovoltaic cells in a small unit is about 5.7%, but the energy conversion efficiency of the module still stays at about 3%. A research team composed of Alan J. Heeger, a professor of physics at the University of California, Santa Barbara, has made the unit conversion efficiency of organic thin-film photovoltaic cells reach 6.5%, the highest in the world. Finland has developed a new type of solar panel, which can absorb solar energy efficiently with 3 layers of nano-coating, and can work normally at a high temperature of 300°C. Xinao Photovoltaic Energy Co., Ltd. successfully developed China’s first ultra-large 5.7m2 double-junction silicon-based thin-film photovoltaic cell module and light-transmitting BIPV module in 2009, as shown in Figure 4.

Figure 4 Translucent silicon-based thin-film photovoltaic cells

With the continuous advancement of technology and the rapid improvement of photoelectric conversion efficiency, organic thin-film photovoltaic cells have received more and more attention from the photovoltaic industry due to their low material consumption, simple process, low energy consumption, and certain advantages in cost. The development has been particularly rapid in the past three years. In 2008, the market share of thin film batteries including cadmium sulfide, copper indium selenium photovoltaic cells and organic thin film photovoltaic cells increased from 8.2% in 2006 to more than 15%. However, organic thin-film photovoltaic cells have the shortcoming of durability-generally less than 5 years, which restricts their development to a certain extent.