In terms of solar cell manufacturing technology, due to the lack of silicon materials, the price has risen rapidly, which has greatly promoted the development of silicon solar cell wafer technology. At present, the thickness of silicon wafers has generally been reduced from 370 μm to 240 μm, and some manufacturers have reduced it to 220 μm. Still able to maintain a high yield. At the same time, it also promotes the improvement of solar cell efficiency. The general efficiency of polycrystalline silicon solar cells has reached 13%~14%, and some companies have been able to mass produce 15% products; while the efficiency of monocrystalline silicon cells has generally reached 14%~15%, Some companies have been able to mass produce 16% of the products, and now they have begun trial production of 200μm thin silicon wafers.
The vast majority of solar cells today are made from bulk crystals cut into silicon wafers. The typical thickness of these silicon wafers is 0.2~0.25mm, and most of the photoelectric conversion occurs in the outermost 0.02mm of the wafer. Silicon accounts for more than 50 percent of the cost of making solar panels, according to GE’s analysis. The trend is to maximize the use of silicon wafers, using thinner silicon wafers to generate the same power. To reduce the cost of silicon, the industry is cutting silicon wafers for photovoltaic cells thinner and thinner. Silicon wafers have an average thickness of about 300 μm, which has now been reduced to about 180 μm and is expected to be even thinner, according to DuPont.
Sanyo Electric announced in early April 2009 that the thickness of its HIT solar cells is only 85μm, which is reduced to less than the original I/2, and the conversion efficiency of about 103.3cm2 can still be as high as 21.4%. Cut the cost of silicon materials. For mass-produced products, if both high efficiency and low cost can be achieved, the competitiveness of HIT solar cells can be greatly improved. The open circuit voltage of the new HIT solar cell is higher, reaching 739mV. The short circuit current is 37.3mA/cm2 and the shape factor is 0.776. Sanyo Electric has previously developed a HIT solar cell with a conversion efficiency of 22.3%. However, the thickness of the solar cell is about 200 μm. Although the thickness has been greatly reduced this time, the conversion efficiency has hardly decreased, and the open circuit voltage has also increased from the original 725mA/cm2.
In addition, in September 2006, Ferro Electronic Materials Systems of California, USA, introduced a new material that enables solar cell manufacturers to greatly reduce the amount of silicon used. Aluminium- and silver-plated vehicle- and lead-free systems are said to improve the electrical properties of Si wafers, which are less than 180 μm thick, and “thin” wafers are now 240 μm thick.
Instead of making thin silicon wafers directly, GE has developed a technique for pouring silicon wafers from silicon powder. Although the silicon wafers produced by this casting are thicker and less energy efficient than conventional silicon wafers, they are produced faster and can reduce waste by 30% compared to conventional silicon wafer dicing processes.
Thinner silicon wafers also bring other changes. For example, thinner silicon wafers must be encapsulated with more advanced materials to be effectively incorporated into solar panels.
The director of the Fraunhofer Freiburg Institute for Solar Energy in Germany (former expert of the Berkeley Institute in the United States) proposed a new technology based on “dirty silicon”, whose production cost is one tenth of that of polysilicon, which can achieve 14%~16% % solar photovoltaic conversion (same conversion efficiency as polysilicon solar cells). Using this abundant and cheap “dirty silicon” containing metal impurities and defects can reduce the cost of solar cells, change the current situation where nearly 90% of solar cells are produced from refined pure silicon, and effectively solve the shortage of silicon and rising production costs. The main problem is to make solar photovoltaic power generation parity possible. If companies make full use of this “dirty silicon” as soon as possible, then an annual output of 8MW can be achieved. Of course, in order for “dirty silicon” to be fully utilized in the solar field, it must also be refined. Compared with the refining process of pure silicon, the refining of “dirty silicon” is relatively simple, and the refining degree can be as long as it can achieve the semiconductor effect and meet the needs of the semiconductor industry. The price of pure silicon for solar battery demand has doubled over the past few years, from $30 per kilogram to $400 in 2008. The price of initial metallurgical silicon, or “dirty silicon,” is $1. Even including the refining process and initial equipment investment that silicon producers are developing “dirty silicon”, the production cost will not exceed $15. Experts estimate that in the market, the price of “dirty silicon” after refining is about US$50 per kilogram, so if the cost of silicon is removed, manufacturers can also have enough room for profit. Such market prices will certainly promote the development of the photovoltaic solar energy industry. So experts predict that a lot of directly refined metallurgical silicon will be produced in the next two years, and high-performance solar batteries will account for 40% of the market by 2020.