10/29/2018
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`1. Absorption Coefficient and Penetration Depth - SolarWiki
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`1. ABSORPTION COEFFICIENT AND PENETRATION DEPTH
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`Light that is transmitted through the semiconductor material is attenuated by a significant amount as it passes through. The rate of
`absorption of light is proportional to the intensity (the flux of photons) for a given wavelength; in other words, as light passes through
`the material the flux of photons is diminished by the fact that some are absorbed on the way through. Therefore, the amount of photons
`that reach a certain point in the semiconductor depends on the wavelength of the photon and the distance from the surface. The
`following equation models the exponential decay of monochromatic (one-color or approximately single-wavelength) light as it travels
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`through a semiconductor :
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`where F(x) is the intensity at a point x below the surface of a semiconductor, F(x ) is the intensity at a surface point x , and α is the
`0
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`absorption coefficient, which determines the depth at which light of a certain wavelength penetrates the semiconductor. The
`wavelengths most important for solar application are in the infrared and visible parts of the electromagnetic spectrum. 
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`The absorption coefficient is related to the wavelength of light and another quantity called the extinction coefficient, which is also
`related to the wavelength of light (the electromagnetic waves propagated from the sun). This coefficient κ is an optical property of the
`semiconductor material and is related to the index of refraction n, which merely determines how much light is absorbed by the
`material. κ > 0 means absorption, while κ = 0 means the light travels straight through the material. The absorption and extinction
`coefficients are related by the following equation :
`1
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`where f is the frequency of the monochromatic light (related to the wavelength by λ=v/ƒ, where v is the velocity of the light wave), c is
`the speed of light, and π is a constant (≈ 3.14). The absorption coefficient is an important quantity that will show up in the following
`sections in the various models we have for semiconductor charge carrier generation, so it is good to keep in mind that it depends on
`both the incoming light and the intrinsic qualities of the material.
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`Above is an image of the ocean and the depth of absorbance by various wavelengths (energies) of light. As you can see, light in the
`higher and lower energies penetrate the least. In the different case of semiconductors, higher energy photons typically are absorbed
`more strongly, while low energy photons pass right through the semiconductor.
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`Source: <http://disc.sci.gsfc.nasa.gov/educat...ceanblue.shtml>
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`REFERENCES
`1. Goetzberger, Adolf et.al. Crystalline Silicon Solar Cells. Chichester: John Wiley & Sons Ltd., 1998.
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`https://photon.libretexts.org/The_Science_of_Solar/Solar_Basics/C._Semiconductors_and_Solar_Interactions/III._Absorption_of_Light_and_Generatio… 1/1
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`Petitioner Apple Inc. – Ex. 1025, p. 1
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