Welcome in the new MOOC thin film silicon solar cell. This new MOOC proposed by Eric Johnson and myself, is a third MOOC of the photovoltaic series of Ecole Polytechnique. The first one, photovoltaic solar energy consists of a general short presentation of the solar electricity with a particular emphasis on socio-economic aspects. The MOOC entitled, physics of the silicon solar cells, address the physics background of the dominant PV technology based on crystalline silicon. In this third MOOC, we present another technology based on silicon thin films which are these other materials. The first chapter deals with the solar radiations on basics solar cell operation. This chapter is also included in MOOC A and B. As a consequence, in scat be skipped at your convenience. Likewise, the end of the third chapter dealing with heterojunctions is also treated in MOOC B. Let's first recall the principle of operation of a photovoltaic solar cell is ideal case. Photons are first absorbed by a solid. This energy is transferred to an electron that was initially bonded. It will then become free. In a further step, it will be possible to separate the positive and negative charges using an electrical field. Finally, the charges will be transferred to the metallic electrodes though a potential difference will appear. If the two electrodes are connected to a load circuit, power generation will take place and at the end of the cycle, the electron recombines. In reality, the process is more complex. As a first loss of energy, a part of the incident photon flux is reflected. Then all the photon energy is not transmitted to the electron. Some part is lost. This is called thermalization. This phenomenon will be addressed later in the course. Thereafter, the electrons are partial from positive charges is not perfect and some recombination takes place in the device. Finally, the contact between the solid semiconductor on the external circuit can depart from perfect behavior and you see new losses. The electron recombines at the end. Before discussing the physics of solar cell, first, let's look at solar radiation which is available source of energy. Then it will be seen as the principle of operation of the solar cell is an ideal case. The real case, we'll first focus on the limits of the conversion efficiency and then to the solar system cell optics. Solar radiation is defined from two quantities which are first irradiance in watts per square meter which is a power density delivered by the solar flux. This is an instantaneous quantity of power. The other is the irradiation which is therefore the time integral of irradiance energy. These quantities depend on the time of day, day of the season on latitude of the location considered. The sun is referred from two different angle, zenith and azimuth as shown in the figure. First, zenith which accounts for the height of the sun which is in fact, the complimentary angle of elevation. When the sun is vertical, the zenith is zero, and then, the azimuth varies from East to West during the day. We will now focus on seasonal variations in solar irradiance which is illustrated in this animation where we see the rotation of the Earth around the sun during the 12 months of the year. The Earth is tilted with respect to its plane of rotation. This is the origin of the various seasons. So, for six months of the year, the Northern Hemisphere is oriented towards the sun and during the other six months, it is the case of the Southern Hemisphere. We will look now at a few particular days which are the equinox and solstice as shown in the animation. Let's start with the autumn equinox. The autumn equinox has the particularity that the sun Earth's axis is located in the equatorial plane, so that the length of the day is 12 hours, the Northern and Southern Hemispheres being equivalent. These equinox, at noon, the zenith is equal to the latitude for the winter solstice Northern Hemisphere. So, this elevation is minimum. It correspond to the shortest day of the year. We take year as example of the latitude of the Ecole Polytechnique, then we pass through the spring equinox which is exactly equivalent to the autumnal equinox, then less 12 hours. Then we arrive at the summer solstice, the longest day of the year sunshine in the Christian world which is illustrated in this animation. I summarize the previous Theta year. Theta is zenith complementarity of elevation on Pi latitude. Again, taking the example of the location of Polytechnique. That is to say, latitude 48.7 degree at Polytechnique at summer solstice, the zenith is 23 degrees. That is to say, the sun is high, 667 degree at noon. In contrast to the winter solstice, the sun elevation is only 18 degree at noon which is very low. An important element to describe solar radiation is called air mass that measures the amount of atmosphere crossed by the sunlight. More specifically as illustrated in the figure, the air mass is the inverse of the cosine of the zenith. It is zero outside the atmosphere and then increases as a function of the inclination of the sun. If we consider a Ecole Polytechnique as summer solstice at noon which is the most favorable case, the air mass value is little more than one. At the winter solstice source end of December, it is more than three. At the equinox, the air mass is 1.5. This air mass ratio varies with latitude obviously. This figure shows the variation of the air mass in the Bruxelles region in Belgium our [inaudible] offers. Depending on the time of the day for all months of the year, the winter air mass is very large especially early or late in the day. It is larger than three, even at noon. However, the closer you are to the summer solstice, the more air mass is smaller. In June, July, even August, the air mass is smaller than two for most of the day. Let's consider us now changes in azimuth of the sun from East to West. Here, we present the polar coordinates. The red arrow indicates the zenith year. We always consider the case of the Paris region. Contrary to what we often think, the sun does not rise in the East and sets in the West. This behavior is strictly to two days a year that has the equinox. However, in winter the sun rises in the South East and sets in the South West. Summer on the contrary, the sun rises has the North East and sets in the North West. Such data are affected by the latitude. We investigated the Earth's trajectory around the sun. We will look thereafter to the solar spectrum on its variation depending on the wavelength of the light. Thank you.
