[MUSIC] We will look today to solar cells based on crystalline semiconductors, mainly crystalline silicone, which is largely dominant sector in the market currently. It will be first focused to silicon, the crystal growth, how to prepare the substrate which are used in solar cells. Then we will describe the main properties of solar cells based on crystalline silicon. At the end of the chapter, we'll focus on other crystalline cells based on sulfide semiconductors. Let’s deal with silicon, it has a huge advantage. The material resources are unlimited, since the silicon is more than a quarter of the Earth’s crust. The second most abundant element on the Earth, after oxygen. However, it is very abundant, but never pure in nature. In nature, it is always bound to oxygen, mainly in the form of silica, SiO2, or silicates. SiO chemical bonding is very strong bond, thus very stable. So we have to initially separate silicon from oxygen, which will require a lot of energy, because of the bonding strength of SiO. Historically, silicon is known for a very long time. From the Neolithic period have been found tools and flint weapons, which is a natural variety of quartz, so a form of silicon. 12,000 years BC appears the first glass from silicon. The large silicon boom occurs after 14,000 years, from the second half of the 19th century, from the appearance of the steel. Indeed, silicon is used in the form of alloy, iron silicon in steel production. The second great leap for us takes place in the second half of the 20th century. Indeed, silicon is a base material for the microelectronics industry. In particular, the most transistor are produced from monocrystalline silicon. So all the components you use everyday, mobile phone, computer, and so on, are based on microelectronics technology, and therefore use monocrystalline silicon. So now, as the monocrystalline silicon appears as a wafer, as you can see here, with a diameter of 200, this one, or 300 millimeters, which we've seen in the photo, for instance, with very high purity. The so called 8N, it means 8 times the number 9, the cost of the purification process is very high. It currently manufactures CMOS transistors whose critical length or gate length is smaller than 100 nanometers. So it is extremely miniaturized and parallelized, allowing to develop a large number of devices simultaneously. The 21st century marks the appearance of photovoltaics, which is now the first monocrystalline silicon users microelectronics. How do you get metallurgical silicon from silica, this is what is explained in this schema. The metalurgical process, in summary, consists in reducing silica to silicon for this chemical reaction. In practice, one uses coal on pure enough sand for silicon. At a very high temperature, that is to say, at more than 1500 degrees, one does obtain liquid silicon and CO. Then the CO oxidize as CO2, that is released into the atmosphere, this is what is seen in the upper left corner. In the top right of the figure are presented side reactions. Silicon immediately reacts with oxygen, the SiO bond will be very strong, producing SiO gas. The SiO gas can lead to solid silica, which is shown here. The solid silica is then recycled and re-reacts with carbon. The liquid silicon at high temperature then follows the refining step for purifying silicon on the solidify and crystalline form and then grinding to metalurgical silicone, sorted by size. What is important is that it is a high temperature process, more than 1,500 degrees, that corresponds to liquid silicon. This metalurgical process generates a large consumption of electric power. This gives us primary silicon, which contains about 1 to 3% impurities. We are therefore very far from the 8N silicon used in microelectronics. We must therefore refine the silicon so it can be used as a semiconductor, in particular, for subsecond doping. The second step is described here, that is to say, the transition from metallurgical silicon to the semiconductor silicon. Without going into too much detail, the purification of silicon is a chemical process. We first form trichlorosilane from the metalurgical silicon, at 350 degrees, which is explained in the drawing to the left. This trichlorosilane then undergoes a double purification by fractional distillation, it is then diluted in hydrogen. The various chemical reactions are summarized here. Triclocylane decomposes and gives SiH2Cl2, the SiH2Cl2 can then be decomposed into silicon and hydrochloric acid, HCl. This is one part of the process, the other part is related to the direction of trichlorosilane with hydrogen. Silicon is first obtained by a second channel, and similarly, purified silicon is obtained from chemical reaction at very high temperature. 1,100 degrees, so it is still a strong energy-consuming process. With these types of production of silicon metal, it may be implicit or doped, p or n. For doping, a gas carrier like boron oxide, B2O3, or phosphorous oxide should be added to the initial solute, thank you. [MUSIC]
