[MUSIC] We will now treat the operating principle of a solar cell. First is the ideal case, that is to say, assuming a perfect behavior. This is what is explained in the slide, I remind you the principle of converting solar energy to electricity with two condition to be met. First we must absorb a photon, that is to say induce a transition in electron from the valence bonds that is full to the conduction band, which is empty or almost empty. We will just excite an electron in the conduction band. As his band is empty it will tend to preferentially populate the lowest energy states part of the energy will thus be lost by thermalization. The useful energy corresponds to the band bap. It is the optical absorption phenomenon. Coming now to the device aspects of PN Junction diode, which is characterized by the presence of an electric field as the interface between the N and P regions. Is a photo generated carriers can diffuse to the space charge region there will be separated by the field. So separation of the carriers is a second condition to fulfill. Physically this means that the diffusion lengths of the carriers is of the order of magnitude of the total thickness of the diode. We'll see that this is the case of solar cells based on crystalline silicon. The ideal operation involve several assumptions. The first is perfect absorption of all photons that have an energy greater than the bond gap. Which implies the absence of reflection of the form surface of the semiconductor. For example, the circuit connect junction is a perfect collection of the photogenerated carriers. That is to say, lack of recombination in the p-n junction. And then perfect contact with the metallic electrode. The solar photon conversion mechanism is summarized in these figures. We have seen that from the electrical point of view, a cell behaved like a p-n junction out of equilibrium, in parallel with a current source that corresponds to the photogenerated carriers. The characteristic of a non-equilibrium diode follows the Shockley's law, I = Is(exponential minus qV on kT- 1). Which is in parallel with the source IL. The final characteristic IV is presented here. We also define two quantities, Voc which is open circuit voltage, which corresponds to I = 0, and the short-circuit current, Isc corresponds to V = 0. The solar cell acts as a generator in the third quadrant. P equal VI negative which is only illustrated here reversed. The current per unit area JS depends on the characteristic of the semiconductor. That is to say, bond gap diffusion constant of the carrier's lifetime of electrons on hold 2N on 2P, dumping densities and so on. So VOC is obtained from the expression of the characteristic at I = 0. We obtain an important consequent from the solar cell operation. VOC depends logarithmically on the photon flux IL. Thus, for example, VOC increases with the optical concentration. The maximum of the power, P equal VI, correspond to dP over dV equal 0. It can therefore be calculated analytically as shown here. So short-circuit current ISC corresponds to the photon conversion. It is obtained from the interior of the spectrum of solar photons integrated between Eg and infinity. These quantities are reported in this figure which again display the third quadrant of the grid IV. We define the Field Factor FF ratio JmVm. That is to say maximum power. Divided by the area of the rectangle ISC VOC. The most characteristic will be close to the rectangle the greater the fill factor will be. In practical application FF can reach 80% or even above. This figure shows a theoretical comparison of various semiconductors. The two top curves correspond roughly to the crystalline germanium on silicone. More the bond gap decreases, more solar photons are absorbed, leading to an increase of ISC. We observe a net positive behavior for the x Axis. Voc depends on the band gap still being slightly lower as we have seen previously. We can evaluate the theoretical maximum conversion efficiency for the various semiconductors. Remember that the solar photons that have a lower energy that the bond gap and that convert in. More of the gap is low, more of this loss is weak. In contrast losses by families a certain value in the opposite way. This opposite behaviors with EG lead to a compromise shown in this figure. A bond gap of 1.34 slightly higher than crystalline silicon corresponding to the better compromise between conversion on degradation. This corresponds to efficiency of 33%, under ideal operating conditions. It's called the Shockley–Queisser limit. It applies to the homojunction, that is to say, a P injunction based on single semiconductor material. This limit is well below the thermodynamic limit, more than 80%. The red dots correspond to the best yield Actually achieved. These values are found to be far below the theoretical limits particularly for the amorphous silicon thin film. Their actual recost are in the order of 25, 26% for crystalline silicone instead of 29% theoretically. The share of the different solar cell technologies is shown in this figure. The crystalline silicon base industries account for over 90% of the global market. We will come back to these technologies in the next chapter. The thin layer technology, CDT, amorphous silicone and so on represents the rest of the market, barely 10%. We have presented in this second the ideal operation of solar cells based on homojunction. That is to say, considering a single semiconductor material. We will see later how it's possible to overcome the short case limit 33% efficiency. Thank you. [MUSIC]
