Now, we are going to discuss the materials that we used to put together the cells that are going to act as our devices that will transduce. Now, we'll change the energy from those solar radiation into electrical power that we can store in a battery or utilize directly if we need to. So the material that we're going to discuss is silicon. Silicon, it's a very common material in the crust of the Earth, about 20 percent of the crust of the earth is silicon, only exceeded by oxygen. So it's the second most common material. So as you can imagine, it's also very inexpensive because it's so common. We will like to have is crystalline silicon. Actually, we would love to have monocrystalline silicon that is just a single crystal. So silicon, of course, it's part of silica. It's part of an oxide on the Earth, and it has to be reduced chemically into silicon. Then it has to be purified because the silicon utilized in solar cell is extremely pure. Is as pure as or more than any of the pharmaceuticals that you used to remedy some illness. So what we would like to do is to describe how silicon is transformed from a wafer, that it's maybe eight inches in diameter into a series of cells that are going to be utilized, our solar cells. In order to do that, we have to fabricate in the surface or on the surface of the wafer, a diode. This diode, so you know what is a diode, a diode is a p-n junction. The p-n junction is fundamentally material that it's p, p meaning the charge that it releases into the material, it's a positive charge. So the phenomena that we utilize the process, it's called doping, and doping requires placing impurities on purpose in silicon in order to change the nature of the charges that are moving around. So there's only two types of charges, there are positive and negative. An absence of an electron in the lattice and the absence of a negative charge behaves like a positive charge. It's called a hole in this business. So we're going to have a junction between n material and p material. But in order to enhance the interaction of the solar rays with our device, we are going to put some material in the middle that it is neither n or p, it's called intrinsic. It's fundamentally a dielectric, is fundamentally an insulator. Our diode, it's going to be a p-i-n diode. Of course, the solar cell, it's black, it's very, very highly polished. What happen when you have a crystal that it's highly polished, it's also highly reflective. So we don't want the sunlight or those solar rays to come into your your cell and reflect back into the space. So we'll have to put something that avoid reflects and its call an anti-reflection coating. Most likely, the solar cells will have an anti-reflection coating. Remember, as the temperature decreases, the conductivity also decreases. Therefore, as the the temperature increases, the conductivity also increases. So the amount of current that you'll get from your solar cells, from the upper atmosphere you are trying to design, it's going to be in some ways sensitive to the ambient temperature. We have to remember that because it's counter-intuitive that in the winter, your solar cells are going to produce more current than in the summer, and say what? Sure, because how the resistance or the conductance, one is the reciprocal of the other how they behave us you increase and decrease temperature. In order to understand how the diode and how any electronic device, any semiconductor device, silicon is a semiconductor compound. Now, it's neither a very good electrical conductor, neither a very poor electrical conductor, neither a good insulator. It's somewhere in the middle, so it's a semi or semiconductor. Of course, all this materials obeyed certain very interesting laws in the physical theory called quantum mechanics. What happen is that the atoms when you look at it from the lens of quantum mechanics, actually, they have a series of energy levels. There are energy levels and it's interesting that the electrons are such a particle in quantum mechanics that it obeys a local, the Pauli Exclusion Principle. It's heavy, but actually, what it means is that you can have in an energy level just two electrons. One with a spin up and one with a spin down. What a spin is just a parameter. So it parameter is either up or down. So you can have two electrons or energy level, and the topmost electron energy level it's called the Fermi level. When you have many energy levels one under the other and they are all filled with electrons or they are empty of electrons, they are called energy bands. The bands structure of silicon is - of all semiconductors as a matter of fact - that you have two energy bands that are important. You have more energy bands, but two that are really important. One is the band where the energy of all the electrons participating in bonding, the ones that are tied to their neighbors. Okay. Their work in there is just hold tight with their neighbors to form this crystal structure. But once you break that bond-- and all those electrons that are holding on to their neighbors in order to provide bonding, there are called valence electrons, and the band it's called the valence band. If you break the bond and some of those electrons for some reason-- for example, one of the possible reasons is that there is a photon from the sun and the photon has more energy than the energy of the bond breaks the bond and that electron goes free, and if the electron is free and you can move it can carries current it forms a current. It is in the conduction band because it's participating in conductivity. It's actually taking charges from one end to another end of that semiconducting device. So there's two important band structures-- the valence band, where all the electrons participating in bonding, and the conduction bands where all the electrons participating in forming the current when you put a voltage difference. Of course, between the valence band - all the guys participating in bonding - and the conduction band there is a gap, and the gap means the following, and this is really important my friends. If this is the gap, and you provide this much energy, nothing happens. The electron cannot go to the center of the gap. The gap it's totally forbidden. Either you have this much energy or you don't have any energy. If you have less than this much energy -the gap - nothing will happen and you will not be able to put an electron into the conduction band no matter how much energy you get from the sun. Of course, if you'll get the exact energy of the gap, then you can go into the conduction band and participate in transport. So that band gap, it has a particular designation. Its E sub-g. It's the band gap energy, and the band gap energy for silicon it's about 1.12 electron volts. An electron volt is the energy an electron with the charge Q, will have if it drops through a gap having an energy of one volt. So one volt times the charge of electron, it's one electron volt. Is the unit of energy utilized in quantum mechanics. In our business of renewable energy, energy is designated by watt hours. The photovoltaic solar cells obtain all that energy. That is going to make the electrons break a bond, one electron coming up and participated in conduction. So I provided that 1.12 electron volt energy. The band gap energy the E sub g energy. I have provided, or the sun has provided. I have allowed the solar cells to receive it, and now the solar cell is producing a current. The current will go into charging your battery and that is where you're going to store the energy that you are going to use later on. When radiation from the sun, photons from the sun enters silicon, a PIN structure, and breaks a silicon bond or some of the intrinsic material-- Intrinsic means that it is not doped. Okay. So the PIN, P it's for positive, I for intrinsic, N for negative. So the I, the intrinsic, means that there is silicon in there. The only thing is that it is not doped. Remember what it's doped, when you put an impurity purposely in there just to change its conductivity. So when the radiation comes and break a bond it creates the absence of an electron. Remember what I told you about the absence of an electron. The electron needs a negative charge particle, but the absence of an electron behaves like a positively charged particle. Okay. So that positively charged particle that is created by the absence of an electron is called a hole, and when you absorbed light from the sun, you'll create an electron hole pairs. It is really important that you understand all this that we have been discussing as that relates to quantum mechanics and I tell you why. Because if you select the right material, you will be able to maximize the amount of energy that you'll get from the sun. You will maximize the possibility of taking very good care of your solar cells, but more than that, you maximize the power and you minimize the losses. But maximizing the power, it's really important because the power, as you remember from your elementary science, it's the product of the current times the voltage. We need both the current and the voltage to be amenable to us that we can in some ways maximize this. If we understand all the material science arguments in the selection of the best material for a solar cell, we will be able to maximize all those parameters that are important to us. We will be able to select the best electronics. We will be able to select where to place the best electronics and the best mechanics like those heliostats that follow the image of the sun. So we can maximize the amount of power that we can obtain from the sun.
