[MUSIC] We will be taking these seconds to the optical operation of the solar cells and in particular how to optimize the front surface of the semiconductor. This is a very important issue, because it will be shown that the semiconductor can reflect up to a third of the incident light. Let's start with some optical reminders, first reflective index that includes a clearly real path, only an imaginary path. The rate of being directly relative to the absorption coefficient alpha = 4 pi chi over lambda. The GAT function which is involved in this Maxwell equation is a square of reflective index, it is also complex quantity. Reflective indexes on the electric functions depend on the wavelength. The case of dielectric matter here, such quantity are purely real. Now, consider the reflection at the interface between a semiconductor on a dielectric. The reflection coefficient at normal incidence in given by this relationship. In the case of a metallic k is very large as compare to n, therefore R tends to one mirror behavior. The reflective indexes of silicone on other semiconductor, in general, acts such the reflection coefficient involves the order of at least 30% of the visible range. The radiation of this reflection is therefore an important issue for the optimization of the solar cells. For example, the presence of a dielectric on top of the surface of the semiconductor results in a reduction of R given by this relationship. Surface can also be textured to induce light trapping what we see here. The physical phenomenon depends on the ratio between the wavelengths of the light lambda and the typical size of the texturing d. If d is large compared to lambda, it corresponds to geometrical optics. It is the opposite situation, defraction phenomena will appear. In the value surface treatments, we must distinguish two different goals. The first objective is to reduce the reflection from the Sun's surface as previously explained. The second objective is the increase of optical path in the cell as shown here, it may require treatment on both sides. An increase is optical path allows reducing the physical thickness of the cell and thus its cost, because the semiconductor materials are often expensive. Let's deal now with Anti-Reflection Coatings ARC. Consider an absorbing semiconductor here in gray. Below a transparent electric layer in pink, with a refractive index L1, different from the air. For a given angle of incident at each interface, some of the light will be reflected and part transmitted. So an interference regime will be created inside the on between reflective waves in ambient, as this height appendix one. The interference will go from minima or maxima as a function of the wavelengths. On the thickness of the layer, more specifically depending on n d over lambda. This quantities will be chosen in order to cancel the reflection at the considered wavelength. Destructive interference by your quarter wave layer, the half wave layer would lead to the opposite behavior [INAUDIBLE] transmission. In both cases it is a combative effect. Dependent on lambda, one layer of thickness d will be both quarter wave and out wave for two different wavelengths. Further reflection is minimized is the reflective index of the anti-reflection layer is the square root of the product n0n2. [INAUDIBLE] n0 equal 1 [INAUDIBLE] in the case of silicon the ideal material as index slightly higher than 2. In practice, the geometric material are thin layer less than one micron thick. Hermeining is a case of silicon. The right curve shows the silicon reflections in the visible on the infrared more than 30%. To protect the silicon from the weather condition, it is encapsulated with glass dielectric index of about 1.5. The present of glass above silicone significantly reduces reflection, blue curve, around 20%. The green curve corresponds to the presence of an antireflection coating optimized in the red. The materials used for the antireflection layers are generally silicone alloys such as silicone nitride since it has a relatively high index 1.8 or 2. This layer cancels the reflection at the selected wavelengths on decreased reflection is vicinity of this wavelengths. Coming back to the surface texturing, this figure illustrates the effect of different silicon surface treatment. Wet across silicon or vacuum treatments. The appearance of order structure [INAUDIBLE] pyramidal facets is a consequence of the crystalline network preferentially operating under certain orientation of the. The refectivity is reduced to about 10% with this values surface. Notes that this type of treatment can be combined with the deposition of antireflection layers, further reducing the reflection of the cell. Returning to the other consequence of the surface texturing, the increase in the optical path. Take the case of the crystalline silicone cell in ideal operation. The typical thickness is of the order of 200 microns in currently commercialized cells. This allows the conversion of the visible range, quantum efficiency equal one, instead close to the gap, that is to say for wavelength greater than one micron, and increase optical path pass by an order of magnitude. It is needed for the complete collection of photogenetic carriers. This increase of optical path can be due to to different treatments on both surfaces. I show you here a solar cell based on multi-crystalline silicon material, that will be studied in the next chapter. Indeed, you see very clearly the presence of facets, therefore not mono crystalline silicon. You see very clearly the color of that cell. My question is, do you think that this multi-crystalline silicon cell includes an anti-reflection coating? Yes or not? [MUSIC] Obviously the answer is yes. Since normally photovoltaic cell absorbs solar radiation, in particular, visible radiation, therefore it appears black. The blue color reveals the presence of an optical surface treatment. Antireflection layer which modified its apparent color. I'll ask now a second question. This cell has an antireflective coating, this coating is optimized for what wavelength? In other words to what color? [MUSIC] The answer is red. Indeed if the cells appear blue by reflection, it means a lack of component in the spectrum of solar photons sensitive to the human eye. The blue appearance is relative to the lack of red low energy photons. The red corresponding to the maximum irradiance, this antireflective coating helps to optimize the cell optics. I present here finally the typical structure of a high performance crystalline silicone cell on small area. We will describe in detail this kind of cell in the next chapter. It is based on mono-crystalline p-type silicone. The surface is textured to reduce high friction. The typical size of pyramids fits with the domain of geometrical optics. Multiple reflection for light trapping, the back side is also textured to increase optical path. The presence of electrodes is necessary for the collection of the on the front, the size, and spacing of the metal grid are optimized according to their diffusion rates of the carriers in order to enable collection together with minimum shadowing. In general, the relative loss is of the order of 5 to 10%. However, the rear side is completely metalized. The presence of the overdot layer and plus and b plus at the semiconductor metal interfaces help to reduce the series resistance omit contacts. Thank you. [MUSIC]
