[MUSIC] We'll now focus on the main material use in the Thin Film Solar Cells, which is Hydrogenated amorphous silicon. As we have seen previously, the thin film growth of amorphous silicon is obtained from a low pressure plasma which produces electrons. These electrons bike filing molecules that are introduced into the discharge. These electrons produce fragments containing silicon's which condense. The term glow discharge is used because the fragments are generally not produced is a ground state. So they relax by emitting light the method is called, PECVD, Plasma Enhanced Chemical Vapor Deposition. Because chemical vapor deposition is due to the fact that the films are deposited at relatively high temperature, 200 or 300 degrees. That is to say that it will therefore be possible to modify the molecular fragments on the surface on plasma enhance. Because the silent molecules are distributed before their positions. Radiofrequency plasma, 13.56 megahertz is generally used authorize legal vacancy. Capacitively coupling, this coupling is therefore compatible with planar electrodes which allows the extension in large areas. The deposition temperature is other of 200 or 300 degrees. And this is therefore compatible with conventional glass substrate. Therefore, at low cost which is an advantage of the PECVD method. On the other hand, your deposition rates are rather low of the order of a few microns per hour. In order to reduce the formation of defect in the semi-conductor. Note that in particular application, the thickness is of the layers are generally less than one micron. In general, the deposition condition as soft as possible for the same reason. Weakly dissociated plasma, typically 1%, on weak ion bombardment. The PECVD method make it possible to reduce the density of defects in the amorphous silicon to 10 to the 15, or 10 to the 16 per cubic centimeter. Which is therefore very low as compared to the total density of the material of the order of 10 to the 22. This PECVD method has been industrialized since many years notably in Asia. For the production of flat panel displays on large area typically greater than 10 square meters. Hydrogenated amorphous silicon is obtained from a pure silent plasma, but it is also possible to use gaseous mixture as shown in this figure. It is possible for instance to mix with the silent or gas guy of doping atoms phosphorus on born in order to obtain an N or P doping. It's just possible to prepare. Is the same reactor or structure PN, PIN as will be seen year in after. It is also possible to introduce methane or germane in order to obtain silicon carbon, or silicon germanium alloys. Which make it possible to vary the bone gap, as we will be seeing hearing after. Insulators in particular silicon nitrate, may also be depleted if silane is mixed with ammonia to dissociate the nitrogen. It is possible to proceed in the same way with oxygen and more generally to deposit SiO x and y alloys with optical properties that are advantageous for PV applications. Finally the hydrogen of fuel line base. Ancient plasmas make it possible to remove certain atoms by transferring them into the gaseous face. For example, from the reaction SI solid plus 4H give SIH 4 gaseous. In conclusion the PCV technique allows great flexibility of use on large fires. All the ozone plasma processes are very complex on difficult to optimize first. We have to consider the primary electrons silent reactions. For example, which produce fragments with electron energy dependent course sections as shown in appendix three. Next these fragments can react with the silent molecules the so-called secondary reaction. So the higher the pressure the more the presence of ternary on secondary reaction will increase, many channels exist. Let us mention an example, the conservation of the charge imposes conservation of the number of ions, but anions which reacts on silane produces another ions. But also produces an occult fragment consequently the relative proportion of ions, and those ionic bombardment decreases as a function of pressure. Because of the multiplication of the creation of neutral species, finally this fragments interact with growing surface. This interaction very with temperature on so on. What to remember great advantage of plasma techniques is to hello and how of equilibrium chemistry based on extremely reactive particles. Dissociation product, such as atomic hydrogen, for example. A particularly effective chemistry is thus obtained. Let's now return to the incorporation of hydrogen in amorphous silicon. Experimentally, the presence of hydrogen in the discharge leads to a great reduction in the defects of the thin film. But is the best version mechanism of thin film as shown on this figure. In fact, a typical defect density in a non hydrogenated amorphous silicon produce for example, by evaporation is obviously order of 10 to the -3. The amorphous silicon used in electronic applications, solar cells, or transistors for flat panel displays incorporates at least 10% of hydrogen in the material. It means 1,000 times more than the dangling bond density. It will be shown that the hydrogen incorporation, in fact, essentially contributes to preservation of weak bones of the disorder materials. The incorporation of hydrogen in hydrogenated amorphous silicon. An analogy with crystalline silicon will be made. The values defect of the crystalline silicon are very well known. One of them is a double vacancy, that is to say the lack of two neighbor crystalline silicon atoms. The lack of two atoms of crystalline silicone in the crystalline lattice should result in the appearance of six dangling bonds, which is not observed. On the contrary, a lattice reconstruction is observed, by means of weak bonds, a fraction of electron volt lattice. 10 times lower than the silicon, silicon bonds is in the crystal. We proceed by analogy with the crystalline silicon on consider the weak bones of the disorder of materials. The breakage of the weak bones resulting in the formation of two dangling bond. More precisely, we will assume that there is the thermodynamic equilibrium, this must action low between weak bonds and broken bonds. If the density of broken bonds corresponds to the incorporated hydrogen. It can be estimated at about 10%. The density of dangling bonds can be evaluated to 10 to the -3, as discussed above. We just obtained a Gibbs energy delta g of the order points heat and it converts approximately. The hydrogen passivates these weak bonds. And therefore creates two Si3-H bonds. And energy favored mechanism considering those kind of the SiH bonds. Since the material it is ordered. Zubin strange fluctuates within the amorphous silicon and hydrogen passivates weak bones, but as there is a thermodynamic equilibrium. It can move on this may explain the presence of possible phenomena of meta stability or instability. In fact such metastability phenomena are observed experimentally. For example, a variation in photoconductivity, which measures the increase of conductivity under illumination is given here. The photoconductivity of crystalline silicon remains constant during the exposure to the light. Other that of the amorphous silicon decreases over several other of magnitude as function of time of exposure to light as shown here. The ethic saturate after one day. This is a metastability effect since after one week if the film of amorphous silicon is annealed at 150 degrees the initial state particularly recovered. Similarly, the photovoltaic conversion efficiency of amorphous silicon solar cells decreases as a function of time. This is called the Staebler Wronski effect, the initial efficiency can also be recovered by annealing at 150 degrees. During this seconds, hydrogenated amorphous silicon, which is a principal thin film semiconductor has been studied. I refer you to appendix three for better understanding of the growth mechanisms of amorphous silicon. Thereafter, will be interesting in the thin film of nano crystalline silicon, thank you. [MUSIC]
