Light which is transferred through a semiconductor material is reduced by a substantial amount when it passes through. The absorption of light rate is directly proportional to the intensity (photons flux) for a specific wavelength; explained differently, as light passes through the material the photons flux is reduced because on the way through some photons are absorbed. So, the amount of photons that would reach a specific point in a semiconductor depends on both the photon wavelength and the distance from the surface. The exponential decay of monochromatic (nearly single-wavelength) light as it passes through a semiconductor material is modeled by the equation below:
F(x) = F(X0) e-α (x- x0)
F(x): the intensity at point x underneath the surface of a semiconductor
F(x0): the intensity at point x0 on the surface
α: absorption coefficient
There are different absorption coefficients for different semiconductor materials. Materials which have higher absorption coefficients absorb photons more readily, thus exciting electrons into the conduction band.
Absorption Coefficient factors
The absorption coefficient defines how far light of a particular wavelength can penetrate into a material before being absorbed. If a material has a low absorption coefficient, light will be poorly absorbed, and a really thin material can appear transparent to that certain wavelength. The absorption coefficient depends on both the material and the wavelength of light being absorbed. Semiconductor materials have a clear advantage in their absorption coefficient, because light which has energy below the band gap doesn’t possess enough energy to excite an electron from the valence band into the conduction band. As a result, the light will not be absorbed.
Even for the photons that have energy above the band gap, the absorption coefficient will not be constant, but will depend strongly on the wavelength. The probability of absorbing a photon depends on the possibility of having an interaction between a photon and an electron as to move from one energy band to another. If the photon have an energy really close to that of the band gap, the absorption is rather low because only the electrons right at the edge of the valence band can cause absorption by interacting with the photon.
When the photon energy increases, not only the electrons that already have the energy close to the energy of the band gap can interact with the photon. Consequently, a bigger number of electrons have the ability to interact with the photon and the outcome is the absorption of the photon.
α which is the absorption coefficient, , is related to k which is the extinction coefficient, , by the formula below:
α = 4π k / λ
λ: the wavelength. (when the unit of λ is in nm, multiply by 107 to convert the absorption coefficient to the units of cm-1.
The absorption coefficients of materials helps in determining the material to be used in design of solar cells.