Before describing the hetero junction, amorphous silicon, crystalline silicon, let's come back on the performance of crystalline cells as a function of their thickness. So, back to crystalline silicon, we here shows a simulation that displays the evolution of the short-circuit current or maximum current density, depending on the thickness of the crystalline silicon cell treated with anti-reflective coating. This simulation shows that, a thickness of the order of 700 microns is needed to convert the entire solar spectrum, basically, saying silicon. But from an economic point of view, it is advantageous to reduce the thickness of silicon. This simulation reveals that reducing the thickness to 300 microns only induces a five percent loss, in terms of conversion efficiency. These estimations are calculated without optical optimization of the cell. We can significantly reduce the thickness of the crystalline silicon cell in order to reduce the cost of PV energy. Thus, the currently commercialized crystalline cells have a thickness of about 200 to 300 microns. We present here, the Voc estimation as function of the thickness of the crystalline silicon, whose front face is texture to improve the optical copying. The Voc increases as the thickness decreases. Dotted curve. The decrease of the total silicon into our thickness reduces carrier recombination in the cell. Then, Voc increases with higher densities. However, this increase in Voc at low thicknesses may be limited by surface recombination, whose relative effect can be dominant at low thickness as seen here on the solid curve. Compare the dotted and solid lines. The dotted curve represents recombination velocity, assumed to be equal on both sides, 10 times smaller than the solid line. In this last case, it is found that the Voc practically does not increase when the thickness decreases due to surface recombination. Therefore this simulation reveal, that a decrease in the thickness of the cell must be associated with a good surface passivation. The reduction of total thickness of the crystalline silicon is an economic issue. But what happens, experimentally, with reducing the thickness of crystalline silicon wafer? How 100 microns, or below, wafer are altered by high temperature processes, in particular, the diffusion of the dopant to create the p-n junction, often followed by annealing. In this case, as seen in this photo, silicon wafers become bended following the high temperature treatment. How to overcome this problem? Passivation of the surface is first required in order to reduce the total thickness as we have seen. But the p-n junction is also needed to separate carriers. Hydrogenated amorphous silicon is used to passivate the surface of crystalline silicon, intrinsic amorphous silicon, in order to reduce defects. Then the growth of a doped amorphous silicon layer allows to create the junction. These two layers are very thin, of the order of 10 to 20 nanometers in total. The transport of the photo generated charges mainly takes place in the crystalline silicon materials. The procedure is the same for the BSF on the back surface. HIT, heterojunction with intrinsic thin layer, is thus obtain whose preparation method is summarised here. The entire structure is shown here on crystalline silicon n- doped. The thin fields of amorphous silicon are deposited on both sides of the crystalline silicon diode on BSF. HIT structure is detailed here in application to crystalline silicon. N-doped, can be also p-doped, is initially texture in order to ensure optical copying. Tissue layers can be added especially to improve the optical performances. Let's look at the band diagram of the HIT structures starting with the front. To simplify, we don't mention here the intrinsic layer of amorphous silicon. Let's consider, for example, p-doped crystalline silicon. Therefore, the deposited amorphous silicon would be n-doped to clear the junction. At equilibrium, the formula there is constant in the system. But the bands on the surface are aligned relative to the vacuum level. There is, therefore, a band discontinuity here, delta EC, because the electron affinities of the amorphous silicon and crystalline silicon are different. What are the consequences? One of the advantages of the HIT structure is to increase Voc since amorphous silicon, which is combined with crystalline silicon, as a larger band gap. The distance between the two formula levels is increased as compared to the pure crystalline cells. We see that delta EV, the distance between the valence band edges, favours the collection of the holes since the barrier height is reduced. In contrast, delta Ec is a barrier for the electrons, but as this barrier is very thin, the electron will be able to cross it by tunnel effect. The band structure of Si:H (p) crystalline silicon and heterojunction is better suited. Consider now, the rear contact always in the case of crystalline silicon p-doped. Indeed, p plus doped amorphous silicon is deposited to create the BSF. Delta Ev creates a barrier for holes which can be crossed by tunneling. However, delta Ec is favorable for BSF since the electrons are repelled by this barrier. At this point, we can make a comparison between the different technologies of the cells based on silicon, crystalline or amorphous. The optical efficiency of crystalline silicon-based cells is excellent. In total, thin film cells are slightly affected by the two tissue layers which create extra interfaces and may be slightly absorbant. Finally, the cells based on crystalline silicon or HIT exhibit conversion efficiency well above the thin layers. The energy losses by lack of conduction of thermalization are the same order of magnitude as seen in this figure on the right. The recent changes, the best performances of crystalline silicon-based cells are presented here. The best cells are produced by Panasonic HIT and SunPower. Crystalline silicon with back contacts is a little case which just exceed the 25 threshold. The trend is towards very similar efficiencies for the cell and modules as discussed earlier. Thus, it can be predicted that the limit of crystalline silicon technology will be around 26 percent, that is to say, approximately only 10 percent in relative lower than the theoretical limit. We treated the thin film cells based on silicon and heterojunction. So transparent conductive oxide layers extensively mentioned are presented in the next one. Thank you.
