[MUSIC] So we'll look now to the description of solar cells made from crystalline silicon. I remind you that they now represent more than 85% of the global market for photovoltaic solar cells in the world. I remind you that the solar cell is essentially a p-n junction exposed to sunlight. This is what is stated here in this figure. Solid p doped silicon, for example, is taken on the end zone, is heated near the surface by phosphorus diffusion, in order to obtain a p-n junction. The photons can be absorbed throughout the cell. So electron hold bare will be collected if the diffusion lengths of the carrier is such that it can reach the spacious region, shown here, where they will be separated by the electric field. I remind you that the interface area is very thin, in crystalline silicon of the order of a few microns, but less than the electron diffusion length that is of the order of magnitude of the total thickness of the cell, several hundred microns. So the carriers may easily diffuse into the spacious region. This is a very asymmetrical structure for the reasons I outlined earlier, and also the fact that the diffusion lengths of electrons is a minority carriers in the p area is much larger than the holes in the n area. However, for technological reasons, metallic impurities, n doped silicons tends to be increasingly used, in view of the reduction of the total thickness of the cells. In addition, what is shown in blue here, is passivated surfaces to limit the carrier recombinations. Indeed, the semiconductor surfaces areas with defects that can first act as recombination centers. We will say a word further about passivation processes. We'll also add it in the back, a so-called BSF, back surface field. Actually, it is a p+ layer. This p+ layer will create an electric field that will repel the electrons from the back. Also, this p+ layer will create an electric field that will repel the electrons from the back. Moreover, this layer, p+, facilitates ohmic contacts for holes. Likewise, your n+ layer on the front surface provide an anohmic contact for electrons. What are the principle of passivation processes? Si-O bond being very stable, so silicon is oxidized when it is exposed to air. It is covered with a very thin layer of SiO2, 1 or 2 nm thick. This thin oxide layer then protects the silicon wafer. The growth of this native oxide occurs naturally without any monitoring, so it can have many defects. The first thing to do is to remove the native oxide, to try to be as close as possible to the flat band condition. Hydrofluoric acid solution, HF, highly reactive on as our to under is generally used. HF will dissolve the native oxide, and further produce Si-H bonds on the surface will which passivate the surface, and prevent the further silicon oxidation for several hours, typically. Next, on on top of this passivity surface, a thin layer is deposited, generally an alloy of silicon, so as to get a good quality interface, free of defects. These thin layers are either silicon nitrate or silicon oxide which is deposited by plasma. Note that some of these materials, such as silicon oxinitride, can also be used as anti-reflection coatings. Intrinsic amorphous silicon can also be used as will be seen later. This will be the principle of heterojunctions. Note that the silicon passivation can be influenced by doping. Let's look now about metallic front contacts. Using a metallic grid, and it may have a n+ emitter over the entire surface, are located on the contact iron. The front surface can also be passivated before as mentioned above. The metallic gate needs to fit or compromise between transparency of the front surface under serious resistance. The shedding effect should be low as possible in order to convert the maximum photons. Moreover, all carriers will be collected with a good ohmic contacts between semiconductor and metal. The spacing of the metal in lines is therefore monitored by the diffusion lengths. In productions, silver printing pastes are generally used to make the green lines. The rear contact are presented here. The metal contact is obtained from an aluminium paste loaded with silver and then annealed. Passivation of this rear face is critical for the solar cell efficiency, as has been seen previously. A BSF layer, p+ layer, is hidden near this nearest face. Different feature are presented here, with localized contacts, if passivation is effective. We present here an example of a solar cell crystalline silicon p doped, optimized on small area. It is a PERL structure, passivated emitter rear locally diffused. The pyramid networks on the surface allows the trapping of the light to reduce reflection. The metallic grid on the front face is thick to decrease the series resistance effects. N+ layer on the emitter provides ohmic contacts for the electrons. In purple, one sees an oxide layer passivation of the front and in the rear faces. BSF structure that repels electrons is located so as to decrease interference of defects on the back contact. The performance of this cell, a few tens of square centimeters, are summarized here. Excellent performances are obtained with 25 conversion efficiency, with a Voc to the order of 700 milivolts on the short circuit current greater than 40 miliamps per square centimeter on the feed factor of 84%. At this stage, we can mention a way to significantly improve the performance of this type of silicon cells. It could be the deposition of a new thin film cells with a wider band gap than silicon, in order to reduce losses by thermalization. This new cell could be considered as an active layer. Such a structure could reach or exceed 30% efficiency. In practice, the deposition of this layer must always be carried out at very low cost. Perovskite are being considered in this perspective. Thank you. [MUSIC]
