After some generalities on the transport of semiconductors, we'll therefore be interested in the carrier injection into the semiconductor when exposed to the light in the context of photovoltaic applications. And we will deal with the interaction between a solar photon on the semiconductor material. In fact, two types of interaction can occur. The first, it was already introduced, the optical absorption. That is to say that the photon disappears into the semiconductor through the excitation of an electron in the conduction band. These are collection of electron/hole pair that is expected effect convert a photon in two just carriers. The overall process is a recombination. That is to say that an electron/hole pair that was created earlier recombines and restores the energy back. This recombination can potentially result in an emission of light, what is called radiative recombination. The existence of recombination can be easily demonstrated experimentally. Upon exposure to light of a semiconductor, the number of carriers increases then the conductivity goes up because it depends on the number for carriers. This phenomena versus conductivity increase with light exposure is called photoconductivity. Suppose now is that the semiconductor is exposed to the solar light continuously. The number of electron-hole pairs created will increase indefinitely. So normally, photoconductivity should also increase indefinitely. Obviously, this is not observed experimentally. So, is the photoconductivity does that increase indefinitely? During exposure to light, it proves that carriers recombines. And therefore, the recombination is a limiting factor. Light exposure of a semiconductor has been mentioned. However, Auger phenomena are possible. It may also be exposed to an electric field or ion beam and so on. A few words of direct recombination process as it is something very important in photovoltaic application since it is a limiting factor. We already talked about radiative emission, that is to say radiative recombination, we created an electron-hole pair for photon absorption from the light for example. This electron-hole pair recombines reverse process producing a photon. This is radiative emission (a). However, Auger phenomena are possible especially the so-called Auger recombination. In the Auger recombination process, an electron that has been excited as a condition band, falls back into valence band, but the corresponding energy instead of producing a photon, will transfer to another photon from the conduction band, increasing its energy, it's [inaudible] (b). Then, another recombination process very important in practical application can occur. It is called Schockley Read Hall mechanism. As we will see later, the presence of defect in a semi-conductor is correlated with the occasion of states is a band gap. Indeed in band gap normally, there is no possible state for perfect crystalline semi-conductor. In the case of an imperfect semi-conductor, that is to say in which there are some impurities, it may be possible states is a band gap. In this case, recombination can be performed in two stages. Initially, reconduction band will populate the state of energy in the band gap. And then secondly, the state falls back into the valence band. Recombination occurs in two times, that is to say, that is all is a band gap [inaudible] electron force in valence bands. This is exactly equivalent. That's the Schockley Read Hall mechanism. Why is it so important? The energy difference is smaller at each stage compared to the band gap versus the transition probability is larger according to Boltzmann laws. In the Boltzmann laws, exponential to the minus delta E of kT. So as delta E is smaller, the probability of the mechanism is higher. If we take the case of germanium for example the recombination time in a germanium crystal should be absorbed of one second. In fact, it is never observed experimentally. At best in the most pure crystalline germanium, recombination time is measure on the order of milliseconds, a thousand times smaller. In fact, there is no perfect crystalline germanium. There was always impurities even with the most sophisticated preparation methods. For example, it is shown that in the presence of copper in germanium which [inaudible] as low as ten to the minus seven, reduces the recombination time of several orders of magnitude. For direct proof of the existence of the possible energy states in the band gap, I invite you to refer to our next one which highlights the presence of the states optically. Since the crystalline silicon as band gap is unified on the human eye is not sensitive to unified. We take a case of carbon diamond which has a much higher band gap. In the case of diamond, these effects may be sensitive to the human eye. Another source of recombination which is very important in particular application is semiconductor surface. So there is no solid without free surface. Since the crystalline periodic array stops on the surface, it may be chemical bonds which may not be satisfied. For example, covalent bonds of silicon. This is called a dangling bond. However, the silicon network can reconstruct near the surface. By definition, the surface is a breaking of the periodic lattice which is therefore related to the presence of energy states is the band gap. They are specially localized as a surface. We'll see later that this surface states can be highly effective recombinant center lacks of impurities in the back material. This possible to hold back, explain the need of surface passivation process which is required in particular for photovoltaic applications. In the first part of the [inaudible] , we did a smaller review of the transport properties. If you want to see more details of presentation, the notion of diffusion length, life time, injection of carriers by light, I invite you to refer to Appendix 2 which details these notions. Thank you.
