Crystalline silicon solar cells are divided into monocrystalline silicon solar cells and polycrystalline silicon solar cells.
Monocrystalline silicon solar cells are the solar cells with the highest conversion efficiency and the most mature technology among solar cells. This is because the monocrystalline silicon material and its related processing technology are mature and stable, the monocrystalline silicon structure is uniform, the content of impurities and defects is small, and the conversion efficiency of the battery is high. In order to produce low contact resistance, the surface area of the battery requires heavy doping, and high impurity concentration will enhance the recombination rate of minority carriers in this area, making the minority carrier life of this layer extremely low, so it is called the “dead layer” . And this area is the strongest light absorption area. Violet light and blue light are mainly absorbed here. Usually, the thickness of the solar cell layer is reduced to 0.1~0.2μm, that is, the “shallow junction” technology is used, and the surface phosphorous concentration is reduced. Controlled below the solid solubility limit value, the solar cell made in this way can overcome the influence of the “dead layer” and improve the blue-violet light response and conversion efficiency of the battery. This battery is called a “purple battery.” In addition, a P-P+ or N-N+ high-low junction is formed by establishing a concentration gradient of the same impurity between the battery matrix and the bottom electrode to form a back electric field, which can improve the effective collection of carriers, improve the long-wave response of solar cells, and increase the short-circuit current and open-circuit voltage. This kind of battery is called a “back-field battery.” In the 1980s, the Green team developed the “slotted battery” with the above technologies. The battery uses laser groove technology to carry out secondary heavy doping. Compared with the printing method, this method increases the battery efficiency by 10% to 15%. Since the 1980s, surface passivation technology has been developed. From the thin oxide layer (<10nm) of PESC battery to the thick oxide layer (about 110nm) of PERC and PERL battery, thermal oxidation surface passivation technology can reduce the surface state density. Below 1010/cm2, the surface recombination speed drops below 100cm/s. The use of various technologies has increased the conversion efficiency of monocrystalline silicon cells to 24.7%. According to expert predictions, the limit efficiency of monocrystalline silicon cells is 29%. In order to reduce the cost of the battery, while improving the conversion efficiency, people are currently exploring to reduce the thickness of the battery, that is, to achieve thin slices.
Polycrystalline silicon solar cells generally use polycrystalline silicon materials specially produced for solar cells. The most widely used polycrystalline silicon manufacturing method at present is the casting method, also known as the casting method. Polycrystalline silicon solar cells generally use low-grade semiconductor polycrystalline silicon, and most of the polycrystalline silicon wafers used are cut from controlled or cast crystalline silicon ingots. Polycrystalline silicon ingots are made by melting and casting defective silicon, waste single crystals and metallurgical grade silicon powder from the semiconductor industry as raw materials. At present, with the explosive development of solar cell production, the above-mentioned raw materials can no longer meet the needs of the solar cell industry, and a production industry specifically targeting polycrystalline silicon solar cells is now being formed, which will be described later.
In order to reduce the loss during the cutting of silicon wafers, the polycrystalline silicon wafers required for solar cells are prepared directly from molten silicon. The cells prepared by this method are generally called cells with silicon. There are two methods for preparing silicon with silicon: one is called EFG “Fixed Edge Feeding Film Method”. In industrial applications, a polysilicon tube is grown on eight sides, and each side is cut into silicon wafers; the other is called the “webbed crystallization method” and Evergreen Solar uses this method. The method is to use thin carbon rods to confine the molten silicon and pull it out of the molten pool, and the silicon liquid confined in the two thin rods is cooled and solidified to form a silicon ribbon. Compared with monocrystalline silicon solar cells, polycrystalline silicon solar cells have lower cost, and the conversion efficiency is closer to that of monocrystalline silicon solar cells. Therefore, in recent years, the development of polysilicon high-efficiency batteries has been rapid, among which the more representative ones are Geogia Tech batteries, UNSW batteries, Kyocera batteries, etc. Among the solar cells produced in recent years, polycrystalline silicon solar cells accounted for 52% more than monocrystalline silicon, and they are one of the main products of solar cells. However, compared with the existing energy prices, crystalline silicon solar cells cannot be widely commercialized because the cost of power generation is still too high.
To learn more about pn junctions, please read article Semiconductor and PN junction and PN junction and photovoltaic effect.
