[MUSIC] We saw the homojunctions, we will now consider the multijunctions that combine several semiconductor materials. Thermalization losses would be truly reduced in a situation where solar photons are converted particularly as non-luminar energy. This will be closer to a thermodynamic limit. As shown in the figure, the blue photons are first converted with the semiconductor with a large bond gap. Lower energy photon are not converted by the first semiconductor. Then the second semiconductor converts the green photons, following the red and so on. In fact, this is not always possible with crystalline semiconductors. Right here we present the variation of the gap depending with the lattice parameter, limited to binary compounds. Tertiary or quaternary three fine compounds also exist. Crystal grows without defect, dislocation, stress, and so on implies the conservation of lattice perimeter epitaxic condition. Only materials lying in the same vertical in this figure can be combined. Thus, silicon can be only combined with wide bond gap materials, more than 2 eV, as GaP or AlP. However, germanium gap, 0.7 eV, has a situation much more favorable, since it can be combined with gallium arsenide, gap 1.3 eV, or other large band gap materials, binary, ternary, or quaternary. A rough estimation can be performed by limiting the ideal condition. The combination of three different bands achieves a conversion efficiency greater than 50% with concentration. Therefore, the use of multi-junction allows to significantly overcome the Shockley-Queisser limit. Finally, it can be pointed out that is the case of this order of semiconductor that are amorphous. The solid network can relax, allowing [INAUDIBLE] a semiconductor on top of another. This materials are well-treated for the fabrication of multi-junctions. In practice, there are two ways to proceed to achieve this multijunction. One can disperse specially light, as shown on the left figure. Or can realize the stack with decreasing gap value, as displayed on the right figure. The special dispersion is not used in practice because of the increase of the ground area it represents. With two-terminal stacks shown here, the currents must be kept constant in the multijunction, which requires a constraint on the thickness of the unit cell to tune optical absorption. Devices with three or four terminals involve inserting electrodes between the elementary cells. Which is inconsistent with the epitaxial conditions required for crystalline materials' conservation of the lattice perimeter, but possible with these other thin films. The four terminals' case, the two junctions are independent from each other, which is advantageous for the electrical point of view. The theoretical performance of the multiple junction with two or three unit cells are shown in this figure, without concentration. Convection efficiencies of about 50% can be obtained with three cells, even in the case where the real cell is crystalline silicon. The multi-junction therefore allows you to significantly overcome Shockley's limit. We considered up to now solar cells operating in theoretical condition. We will now discuss the values limitation in conversion efficiency. Let's return to the equivalent circuit of a solar cell. In the ideal case, the equivalent circuit was formed by a diode pn junction in parallel with the current source. The circuit shown here is a more realistic version with the presence of two new parameters that will affect the conversion efficiencies, a series on the parallel resistance. Series resistance may be related to the quality of the semiconductor contacts with metal, as we have seen, on any resistance in the device, such as your resistive depletion zone. The series resistance expresses all these parasitic effects. More generally, it is affected by carrier mobility. The parallel resistance represents electrical losses in the cells, for example, due to inhomogeneities. An electron whole pair has been created, but is not collected. It can be lost, for instance, in the grain boundaries of the polycrystalline material. It is generally assumed that this kind of loss is independent of the photocurrent. The right curve illustrates the effect of series on parallel resistances, which appear as the slopes at the origins. So the presence of series of parallel resistance decreases the field factor on the conversion efficiency. Now we can summarize the values' origins of limitation of the conversion efficiency. The short circuit current is directly dependent upon the photon flux, since it corresponds to the integral of the converted spectrum. Then the short circuit current depends on the conversion area. It is directly affected by the optical losses, including the reflection of the front surface exposed to the radiation. Isc is also affected by the collection of photogenerative pairs. Open circuit voltage, as we have seen, measures the difference between the quasi-Fermi levels of the n and p regions, so over the p increases VOC. More generally, the position of the quasi-Fermi level depends on the carrier density. For electrons' density varies exponentially with the distance between Ec, condition bond edge, on Ef, the Fermi level. Furthermore, VOC depends logarithmically on the light flux, thank you. [MUSIC]