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Details of Absorption Coefficient

The factor by which photons are absorbed as they travel a unit distance
through a material.

The absorption coefficient determines how far into a material light of a
particular wavelength can penetrate before it is absorbed. In a material with a
low absorption coefficient, light is only poorly absorbed, and if the material
is thin enough, it will appear transparent to that wavelength. The absorption
coefficient depends on the material and also on the wavelength of light which is
being absorbed. Semiconductor materials have a sharp edge in their absorption
coefficient, since light which has energy below the band gap does not have
sufficient energy to raise an electron across the band gap. Consequently this
light is not absorbed. The absorption coefficient for several semiconductor
materials is shown below.

The above graph shows that even for those photons which have an energy abo

the band gap, the absorption coefficient is not constant, but still depends
strongly on wavelength. The probability of absorbing a photon depends on the
likelihood of having a photon and an electron interact in such a way as to move
from one energy band to another. For photons which have an energy very close to
that of the band gap, the absorption is relatively low since only those
electrons directly at the valence band edge can interact with the photon to
cause absorption. As the photon energy increases, there are a larger number of
electrons which can interact with the photon and result in the photon being
absorbed. However, for photovoltaic applications, the photon energy greater than
the band gap is wasted as electrons quickly thermalize back down to the band
edges.