Thin-film materials have great potential in reducing costs: ①The thickness of cell thin-film materials ranges from micrometers to tens of micrometers, which is one-tenth of that of monocrystalline silicon and polycrystalline silicon solar cells, and the thin film is directly deposited without loss of slicing. , Can greatly save raw materials; ② can use integrated technology to form batteries sequentially, eliminating the component manufacturing process; ③ can use multi-layer technology, etc. Therefore, thin film solar cells have the potential to significantly reduce costs and achieve the goal of photovoltaic power generation competing with conventional power generation, thus becoming an alternative energy source. Because thin-film solar cells consume less silicon material, according to the results of the maximum efficiency of silicon solar cells calculated by MAGreen, the peak efficiency of silicon solar cells can reach 29% with a thickness of 80 m. Even if it is reduced to 1 μm, it can still reach 24%. In short, with Compared with crystalline silicon materials, although thin-film silicon batteries have low efficiency, a large area occupied by solar panels, and a less mature process, the main advantages are: low material consumption, which is one-tenth of that of monocrystalline silicon and polycrystalline silicon solar cells. low cost. Therefore, high-efficiency and low-cost thin-film solar cells have become one of the development directions of the solar cell industry.
The general requirements for thin-film solar cells are: high photoelectric conversion efficiency; the material itself does not cause pollution to the environment; it is convenient for industrial production and the material performance is stable. We analyze from the following aspects.
(1) Resource analysis of thin film materials
Among the constituent elements of various thin-film solar cells, gallium (Ga), indium (In) and tellurium (Te) are rare metals. The reason why this group of elements is called rare metals is due to the physical interaction between them. The chemical properties are similar, and they often exist in related minerals in the form of isomorphism, and it is difficult to form independent scattered metal deposits with separate mining value; second, they have a low average content in the crust and are associated in a rare and dispersed state. Among other minerals, they can only be comprehensively recovered and utilized in the process of beneficiation and smelting when the main metal deposit is mined. In other words, there is a problem of insufficient resources for these elements. For example, in the production of copper-steel-selenium thin-film batteries, if all batteries are made of copper-steel-selenium, the world’s proven steel reserves are not enough for a year in 2002. Therefore, the copper-steel-selenium thin-film cell will not achieve the development goal of large-scale industrialization. The cost of nickel arsenide batteries is also too high, about 10 times the cost of traditional batteries, and is mainly used in the aerospace field. The polysilicon film is rich in raw materials, which can be used for large-scale industrial applications and has resource advantages.
(2) Impact on the environment
In cadmium telluride batteries, cadmium is a heavy metal and highly toxic. Cadmium mostly exists in the form of sulfide cadmium ore in nature, and often coexists with zinc, lead, copper, manganese and other minerals. Although cadmium compounds are not toxic, toxic cadmium may be released during industrial production and use. Cadmium is very toxic. It can accumulate in the liver, kidney and other tissues of the human body, causing damage to various organs and tissues, especially the most obvious damage to the kidneys, and it can also lead to osteoporosis and softening. The main effects are: ①Dust containing cadmium causes harm to humans and other animals through the respiratory tract; @The pollution caused by the discharge of production waste water. As the raw material of gallium arsenide solar cells, arsenic has both metallic and non-metallic properties, and arsenic compounds are highly toxic. Arsenic mostly exists in trivalent (inorganic arsenic) and pentavalent (organic arsenic) forms, and trivalent arsenic compounds are more toxic than other arsenic compounds. Arsenic is easy to accumulate in the human body, causing acute or chronic poisoning. Chronic arsenic poisoning is fatigue and loss of vitality; more serious arsenic poisoning causes gastrointestinal mucositis, renal function decline, edema tendency, polyneuritis and so on. The oxide of arsenic, arsenic trioxide, commonly known as arsenic, is extremely toxic. Therefore, from a long-term environmental protection perspective, large-scale industrial applications of cadmium telluride batteries and gallium arsenide solar cells are not accepted by people, while polysilicon films are non-toxic and pollution-free, and have advantages in terms of environmental impact.
(3) Stability analysis
The industrialization of the cadmium telluride thin-film solar cell process still has several problems that need to be further resolved. First of all, the method of forming cadmium telluride film is not uniform, there are six or seven kinds, some of which have made solar cells with conversion efficiency greater than 12%. However, the efficiency of batteries prepared by different processes or the same process but by different personnel varies greatly. According to the requirements of industrialization, this film forming method is immature. Secondly, there are also problems with the stability of the components. The stability of batteries prepared by different researchers varies greatly, and some of them show obvious signs of deterioration after a period of aging. It is not yet clear whether the cause of the decline is the quality of the cadmium telluride material itself, the inter-diffusion of doped elements at the interface, or other problems that people have not yet realized. In short, the stability mechanism of cadmium telluride solar cells is not very clear, but it is certainly closely related to cell materials and manufacturing processes, which will become the biggest hidden danger of commercialization. Therefore, this cell is far away from industrial production.
The atomic ratio and lattice matching of copper-indium-selenium thin-film batteries often rely on precise control of the main semiconductor process parameters during the production process. Even at very low temperatures, the selenium content, metal diffusion, and impurity introduction are difficult to control. The repeatability of the process is poor and unstable. In addition, elements such as copper can react again, and the metastability of the film needs to be further explored.
Organic semiconductor thin film solar cells have the advantages of simple process, light weight, low price, and easy mass production. Although the conversion efficiency of the cell is low, and the degradation of organic matter affects the stability of the cell, it still has certain research value. Research institutions around the world have been actively working on research experiments to improve the conversion efficiency of organic thin-film solar cells. In July 2007, the University of California in the United States published an article in the scientific journal “Science” that “unit conversion efficiency is as high as 6.5% in the world”. Japan’s Sumitomo Chemical also announced in February 2009 that the conversion efficiency of the company’s organic thin-film solar cells reached 6.5%. The key to improving the conversion rate is that the donor material achieves a high open voltage of about 1V by introducing a structure that increases the energy gap between the donor material and the acceptor material in the polymer backbone. In addition, the introduction of substituents that can form the best power generation layer structure, taking into account the high level of short-circuit current and voltage, is expected to achieve 7% conversion efficiency around 2015; and their research has just started, and the current generation mechanism of organic semiconductor systems is still There are many points worth exploring. The stability is not very good, the conversion efficiency is still relatively low, and it is basically still in the exploratory stage.
Amorphous silicon thin-film solar cells are produced at low temperature, low cost, and convenient for large-scale production. However, the main problem of amorphous silicon cell as a ground power application is low efficiency and poor stability. The current laboratory efficiency is 15%, and the stable efficiency of the cell modules in production is 5.5% to 7.5%. The main reason for low efficiency and poor stability is light-induced decay. Studies have found that when amorphous silicon cells are irradiated with light for a long time, the cell efficiency will decrease significantly. This is the so-called SW effect, that is, light-induced decay. In addition, its optical band gap is 1.7eV, which makes the material itself insensitive to the long-wave region of the solar radiation spectrum, which limits its conversion efficiency. In order to solve these problems, people mainly study from the following aspects: ①Improve the doping efficiency, enhance the built-in electric field, and improve the stability of the cell; @Improve the stability of intrinsic amorphous silicon materials (including crystallization technology), and improve the amorphous The internal interface of the silicon cell reduces the grain boundary minority carrier recombination; ③The manufacture of double junction and multi-junction batteries improves the efficiency and cell stability. However, the effects of these measures are still far from people’s expectations. It can be seen from the above reasons that the possibility of large-scale production of amorphous silicon cells in recent years is relatively small.
From the above analysis of the resource analysis of various thin film cell thin film materials, the impact on the environment and the stability of the comparative analysis, it can be seen that the polycrystalline silicon thin film cell has both the high conversion efficiency and high stability of the single crystal silicon cell and the amorphous silicon thin film The advantages of the relatively simplified preparation process of cell materials have attracted people’s attention. Polycrystalline silicon thin film batteries not only have the characteristics of saving silicon raw material consumption and simplifying the silicon wafer manufacturing process, but also have the advantages of high conversion efficiency and stable performance of crystalline silicon batteries. Its efficiency is not only better than amorphous silicon thin film batteries, but also close to crystalline silicon batteries. In addition, even if the silicon layer of polysilicon thin-film solar cells is as thin as 10um, relatively high efficiency can still be achieved. Since polycrystalline silicon thin-film batteries combine the excellent photoelectric performance of crystalline silicon cells with the low-cost advantages of thin-film batteries, they are considered to be one of the most powerful candidates for second-generation solar cells. Although polycrystalline silicon thin film batteries have the above advantages, there are also the following issues to consider: ① Polycrystalline silicon thin film batteries are thicker than amorphous silicon thin film batteries, so it takes longer to deposit thin films, which requires an increase in the deposition speed; ② Compared with the crystalline silicon thin film cell, the annealing process is added, which requires energy consumption. Therefore, how to reduce the energy consumed during annealing as little as possible is a problem that needs to be studied carefully; ③The higher the annealing temperature, the high-temperature-resistant glass is required, and the higher the temperature is, the glass The higher the price is, so the annealing temperature is required to be as low as possible while forming a relatively high-quality polysilicon film during annealing.
From the above description of various solar cells, it can be seen that in addition to saving materials, thin-film batteries have many advantages and development potentials. Under the requirements of improving efficiency and reducing costs, solar cells are bound to become thinner. Silicon materials are used as the main material for the preparation of thin-film batteries because of their abundant resources, non-toxicity, suitable optical band gap, adequate research, and ease of mass industrial production. Polycrystalline silicon film combines the advantages of high mobility, high stability of crystalline silicon, raw material saving of amorphous silicon, simple process, convenient large-area modules, and flexible structure. It is considered to be the most promising solar cell material. In the process of industrialization of thin-film batteries, there are mainly problems such as mass production of equipment and high one-time investment in equipment.
In short, for crystalline silicon cells, its dominant position is still difficult to be replaced in a relatively short period of time, especially polycrystalline silicon cells that have lower production costs than monocrystalline silicon but still have good performance, and they are moving towards thin layering. develop. At the same time, the cost of raw materials has been continuously reduced with the development of new technologies and large-scale commercialization. The classification and performance comparison of solar cells are shown in Table 1.
|Solar cell type||Material||Material cost and process||Cell efficiency||Environmental protection||stability|
|Silicon solar cell||Monocrystalline silicon||High cost and cumbersome process||Highest||clean||Very high|
|Silicon solar cell||Polysilicon||Higher cost and simpler process than monocrystalline silicon||Higher||clean||high|
|Silicon solar cell||Polysilicon film||Low cost and complicated process||Higher||clean||Higher|
|Silicon solar cell||Amorphous silicon thin film||Low cost and complicated process||generally||clean||not tall|
|Multi-element compound thin film solar cell||Potassium arsenide||Low cost and complicated process||Highest||Arsenic is highly toxic||high|
|Multi-element compound thin film solar cell||Cadmium Telluride||Low cost, easy to scale production||Higher||Cadmium is highly toxic||Higher|
|Multi-element compound thin film solar cell||Copper Indium Tin||Raw material indium is scarce||Higher||Cleaner||Higher|
|Dye-sensitized solar cell||Low cost and complicated process||generally||clean||generally|
|Organic material thin film cell||Low cost, immature technology||Lower||clean||Poor|