The difference between monocrystalline silicon cells and polycrystalline silicon cells

  1. Monocrystalline silicon battery
    Since the monocrystalline silicon solar cell is cut from a cylindrical ingot, it is not a complete square, resulting in some waste of refined silicon material, so the manufacturing cost is high. Therefore, most of the monocrystalline silicon will have voids in the four corners, which are easy to distinguish in appearance. The structure and production process of monocrystalline silicon solar cells have been finalized, and the products have been widely used in space and on the ground. In order to reduce production costs, solar cells currently used on the ground use solar-grade monocrystalline silicon rods, and the material performance indicators have been relaxed. Some can also use the head and tail materials and waste monocrystalline silicon materials processed by semiconductor devices to be redrawn into monocrystalline silicon rods dedicated to solar cells.
    Monocrystalline silicon solar cells use high-purity monocrystalline silicon rods as raw materials, and the purity requirement is 99.999%. During production, the monocrystalline silicon rods are cut into slices, and the thickness of the slices is generally about 0.3mm. The silicon wafer is polished, cleaned and other processes to make the raw silicon wafer to be processed. The processing of solar cells requires doping and diffusion on the silicon wafer first. Generally, the dopants are trace amounts of boron, phosphorus, and brocade. Diffusion is carried out in a high temperature diffusion furnace made of quartz tubes, thus forming a PN junction on the silicon wafer. Then, the screen printing method is used to print the finely prepared silver paste on the silicon wafer to form grid lines. After sintering, the back electrodes are formed at the same time, and the surface with grid lines is coated with materials that reduce light reflection to prevent A large number of photons are reflected off the smooth silicon surface. After the single crystal silicon solar cell produced by the single-crystal silicon solar cell is checked and inspected by random, the solar cell module with a certain output voltage and current can be formed by the method of series and parallel according to the required specifications, and finally it is packaged with a frame and material. According to the system design, the user can form solar cell modules into various sizes of solar cell arrays (also called solar cell arrays).
    Monocrystalline silicon solar cells have the following characteristics.
    (1) The reserves of raw silicon are abundant. Due to the extremely low density of sunlight, large-area solar cells are practically required, so the supply of raw materials is very important, and the silicon material itself has a very low impact on the environment.
    (2) The density of silicon is low and the material is light.
    (3) Compared with polycrystalline silicon and amorphous silicon solar cells, its photoelectric conversion efficiency is higher.
    (4) The power generation characteristics are stable, and the durability is about 20 years.
    (5) In the main region of the solar spectrum, the light absorption coefficient is only about 103/cm, which is quite small. To enhance solar spectral absorption properties, 100 μm thick silicon wafers are required.
    At present, the development of monocrystalline silicon solar cells is moving towards reducing costs and improving efficiency. The photoelectric conversion efficiency of monocrystalline silicon solar cells is about 15%~17%, and the photoelectric conversion efficiency after moduleization is about 12%~15%. The definition of photoelectric conversion efficiency of a solar cell module is based on the lowest photoelectric conversion efficiency of solar cells in the module, rather than the average photoelectric conversion efficiency of solar cells.
    The most important problem in the practical application of solar cells is the development of solar cells with high cost performance. In fact, only a thin layer a few micrometers thick on the semiconductor surface is involved in photoelectric conversion in solar cells. At present, the most commonly used and most successful preparation technology is to use the vapor deposition method of thermally decomposing SiH4 gas to deposit a single crystal silicon film on sapphire.
  2. Polycrystalline silicon solar cells
    Although monocrystalline silicon solar cells have their advantages, their high price hinders the development of monocrystalline silicon solar cells in the low-cost market; while polycrystalline silicon solar cells are the first to reduce costs, and then to achieve efficiency. Although polycrystalline silicon solar cells and monocrystalline silicon solar cells have different crystalline structures, the photovoltaic principle is the same. There are three main ways to reduce the cost of polycrystalline silicon solar cells.
    (1) The purification process does not completely remove impurities.
    (2) Use a faster method to crystallize the silicon.
    (3) Avoid waste caused by slicing.
    These three reasons make polycrystalline silicon solar cells less expensive and time consuming to manufacture than monocrystalline silicon solar cells, but also make polycrystalline silicon solar cells poor in crystalline structure. The main reasons for the poor crystal structure of polycrystalline silicon solar cells are:
    (1) It contains impurities.
    (2) The speed of silicon crystallization is fast, and the silicon atoms do not have enough time to form a single crystal lattice and form many crystal particles. The larger the crystal particle, the closer the photoelectric conversion efficiency is to that of a single crystal silicon solar cell; the smaller the crystal particle, the worse the photoelectric conversion efficiency. Due to the poor bonding of silicon atoms on the crystal boundary, it is easily damaged by ultraviolet rays and produces more dangling bonds. With the increase of use time, the number of dangling bonds will also increase, and the photoelectric conversion efficiency will gradually decline. This is polysilicon. The main disadvantage of solar cells, and low cost is its main advantage.
    At present, polycrystalline silicon solar cells can achieve a unit photoelectric conversion efficiency of 15.8% per 100 cm2 (Sharp Company), and if in the laboratory, the unit photoelectric conversion efficiency per 4 cm2 area can reach 17.8% (UNSW). The general photoelectric conversion efficiency is about 10%~15%, and the photoelectric conversion efficiency of the component is about 9%~12%.
    Conventional crystalline silicon solar cells are made on high-quality silicon wafers with a thickness of 350~450 um, which are sawn from pulled or cast silicon ingots, so more silicon material is actually consumed . In order to save material, people have been depositing polysilicon thin films on inexpensive substrates since the mid-1970s, but the grains of the grown silicon films were too small to make valuable solar cells. In order to obtain thin films with large-sized grains, research has not stopped and many methods have been proposed. At present, chemical vapor deposition methods are mostly used to prepare polysilicon thin film batteries, including low pressure chemical vapor deposition (LPCVD) and plasma enhanced chemical vapor deposition (PECVD) processes. In addition, liquid phase epitaxy (LPPE) and sputter deposition methods can also be used to fabricate polycrystalline silicon thin-film solar cells.
    Chemical vapor deposition mainly uses SiH2Cl2, SiHCl3, SiCl4 or SiH4 as the reactive gas, and reacts under a certain protective gas (atmosphere) to generate silicon atoms and deposit them on the heated substrate. The substrate materials are generally Si, SiO2, Si3N4, etc. . However, studies have found that it is difficult to form larger grains on non-silicon substrates, and it is easy to form voids between grains. The solution to this problem is to first use LPCVD to deposit a thin layer of amorphous silicon on the substrate, then anneal this layer of amorphous silicon to obtain larger grains, and then layer this layer of seed crystals. To deposit a thick polysilicon film, recrystallization technology is undoubtedly a very important link. The currently used technologies mainly include solid phase crystallization method and mid-zone melting recrystallization method. In addition to the recrystallization process, the polycrystalline silicon thin film battery adopts almost all the technologies for preparing monocrystalline silicon solar cells, and the photoelectric conversion efficiency of the solar cells obtained in this way is obviously improved.
    Typical characteristic parameters of industrially produced polycrystalline silicon solar cells are as follows:
    Isc=2950 mA; Voc=584 mV; fill factor FF=0.72; photoelectric conversion efficiency η=12.4% (test conditions: AM1.5, 1000 W/m2, 25℃).
    Other characteristics of polycrystalline silicon solar cells (such as temperature characteristics, changes in solar cell performance with incident light intensity, etc.) are similar to monocrystalline silicon solar cells. In terms of production cost, polycrystalline silicon solar cells are easier to manufacture than monocrystalline silicon solar cells, reduce power consumption, and have lower overall production costs, so they have been greatly developed. However, polycrystalline silicon solar cells have a shorter lifespan than monocrystalline silicon solar cells. In terms of cost performance, monocrystalline silicon solar cells are still better than polycrystalline silicon solar cells.
    In the utilization of solar photovoltaic, monocrystalline silicon solar cells and polycrystalline silicon solar cells play a huge role. At present, in order to make solar photovoltaic power generation have a large market and be accepted by the majority of consumers, it is necessary to improve the photoelectric conversion efficiency of solar cells and reduce production costs. From the current development process of international solar cells, it can be seen that its development trend is monocrystalline silicon, polycrystalline silicon, ribbon silicon, and thin film materials (including microcrystalline silicon-based thin films, compound-based thin films and dye thin films). From the perspective of industrialization development, the center of gravity has developed from single crystal to polycrystalline. The main reasons are:
    (1) There are fewer and fewer head and tail materials available for making monocrystalline silicon solar cells.
    (2) For solar cells, square substrates are more cost-effective, and square materials can be directly obtained from polysilicon obtained by casting method and direct solidification method.
    (3) The production process of polysilicon has made continuous progress. The fully automatic casting furnace can produce silicon ingots of more than 200 kg per production cycle (50h), and the size of the crystal grains reaches the centimeter level.
    (4) Since the polycrystalline silicon thin film battery uses much less silicon than the single crystal silicon battery, there is no problem such as efficiency decline, and it can be prepared on cheap substrate materials.
    (5) The cost of polycrystalline silicon thin film solar cells is much lower than that of monocrystalline silicon cells, and the photoelectric conversion efficiency is nearly 12.4%, which is higher than that of amorphous silicon thin film cells.
    Due to the rapid research and development of monocrystalline silicon technology in the past 10 years, its technology (such as selective etching of emitter junction, back surface field, etching texture, surface and bulk passivation, fine metal gate electrode, etc.) has also been applied to polycrystalline silicon cells. The use of screen printing technology can reduce the width of the shed electrode to 50μm and the height to more than 15μm. The rapid thermal annealing technology used in the production of polysilicon can greatly shorten the process time, and the single-chip thermal process can be completed within 1 minute. , the photoelectric conversion efficiency of solar cells made on 100cm2 polycrystalline silicon wafers by this process exceeds 14%. According to reports, the photoelectric conversion efficiency of solar cells fabricated on 50-60 μm polysilicon substrates exceeds 16%. The conversion efficiency of solar cells of 100cm2 polycrystalline silicon wafers using mechanical grooves and screen printing technology exceeds 17%, and the conversion efficiency of solar cells without mechanical grooves on the same area reaches 16%. Solar cell conversion efficiency reaches 15.8%