[MUSIC] We'll now deal with the solar spectrum, which is shown in this figure. The yellow curve corresponds to the black body radiation at 59,000 degrees K, yellow color. So orange color matches the solar spectrum outside the atmosphere. So it corresponds to 1.3 kilowatts per square meter. The spectrum outside the atmosphere follow very well the Planck's law, so as the first order as the black body. Then the radiation is affected by interaction with the atmosphere. It is revealed by the third spectrum, sea level AM equal 1.5. Ultraviolet part is absorbed by the atmosphere. This is due to absorption by ozone. So ultraviolet contribution that correspond to the most energetic photons, more than 3 eV, represents only 5% of this spectrum. So visible ones, blue to red, from 0.4 to 0.75 microns, 40% of the radiation. Then the spectrum has a tail in the infrared, which represent more than half of the solar spectrum energy. The infrared portion, more than 0.75 micrometers, displays absorption bands relative to the absorption of photons by either the CO2 or the water vapor. Solar modules are characterized in normalized conditions, given here. This radiation absorption phenomenon by gaseous molecules can be explained quite simply. Consider the case of a CO2 molecule, which consists of two double CO bonds. These bonds' vibration modes is infrared. The infrared photons at this energy will thus excite these vibration modes CO, HO, and so on, and therefore will be absorbing. This is also the case for a SiO2, glass. That for the same reason, absorbs in infrared. This is the origin of the greenhouse effect. So gas of CO2 is transparent and invisible black body at 59,000 K, but absorbs infrared K, resulting in overheating. Another note in the course of semiconductors, we often mention energies, shown in electron volts. However, the solar spectrum is usually described in the form of wavelengths. So wavelength is inverse of energy. To easily switch from one to the other, we must remember that E, electron volt, is equal to 1.24 over lambda, the wavelength in micrometer. For example, the energy of a blue photon is 0.4 microns, corresponds to 3 eV. For red light, wavelength is around 0.6 microns, so energy is 2 eV. We present here the variation of the solar spectrum as a function of the air-mass ratio. Out of atmosphere, AM0, the integrating radiation is 1.3 kilowatts per square meter. At AM1, the sun at it's zenith, the power density is slightly less than 1 kilowatt per square meter. The standard solar module performances recorded at AM 1.5, graph 3. And graph 4 correspond to AM2. That is to say, at lower sun elevation. AM2 corresponds to less than 700 watts per square meter. So comparison of the values curves show that the high energy part of the spectrum is preferably absorbed by the atmosphere. This is why when the sun is at sunset high value AM appears, red or reddish. So far, we have considered only the direct solar radiation. But a sensor, such as a photovoltaic cell, is sensitive to all components of the radiation. Direct radiation is the one coming directly from the Sun. But part of the solar spectrum, that is scattered in the upper atmosphere, for example, by aerosol or water vapor droplets, the so-called Rayleigh scattering varies as lambda to minus four. So Rayleigh scattering tends to preferentially affect high-energy photons, which explain the blue color of the sky. Also sunlight can be scattered by other elements, such as clouds and so on, as shown here. This diffusion can be more or less anisotropic. Albedo corresponds to the ground reflection with an angular and spectral dependence strongly affected by the soil. For example, in the case of snow. The optical concentration, mirrors, lenses, concerns only direct radiation and only marginally affect the scattered radiation. All the components mentioned above vary considerably depending on the time of the day, on weather conditions. I represent data recorded on the campus of the Ecole polytechnique in summer. The blue curve corresponds to direct radiations, diffuse radiation in red, and the green one to the irradiance on horizontal plane, downwellingly. The first record, August 11, corresponds to a clear day with some clouds in the afternoon. Direct radiation almost reaching 1 kilowatt per square meter at noon, while the diffuse radiation is low, under 10%. So next day is a cloudy day and probably rainy, especially in the afternoon. Direct radiation, in blue, becomes very small. And diffuse radiation, in red, becomes dominant, close to 500 watts per square meter. Cloudy crossing greatly affects the impinging solar energy, its conversion into electricity, therefore affect significantly the power grid. Let's mention now that conversion of the solar spectrum by a semiconductor. As mentioned above, it is not simply a binary process, absorption of photon or not. Here we present the absorption coefficient for the main semiconductors. In particular, crystalline silicon which is the main technology used in solar cells, red triangles. Obviously, semiconductors do not absorb below the gap 1.1 eV for crystalline silicon. Absorption then increases very quickly as a function of the energy. In particular, all semiconductors strongly absorb the blue light wavelengths of penetration of around 10 nanometers. Meaning that the energetic photons are absorbed near the surface regardless of the type of semiconductor. I remind you that the crystalline silicon is indirect gap. In such a case, absorption close to the gap is lower since conservation of the vector k must involve vibration which reduce the probability of absorption. We treated, during these seconds, the solar spectrum. That is to say the available energy. We will look subsequently to the operating principle of solar cells. Thank you. [MUSIC]
