Absorption Depth


  1. The absorption depth is given by the inverse of the absorption coefficient, and describes how deeply light penetrates into a semiconductor before being absorbed.
  2. Higher energy light is of a shorter wavelength and has a shorter absorption depth than lower energy light, which is not as readily absorbed, and has a greater absorption depth.
  3. Absorption depth affects aspects of solar cell design, such as the thickness of the semiconductor material.

The relationship between absorption coefficient and wavelength makes it so that different wavelengths penetrate different distances into a semiconductor before most of the light is absorbed. The absorption depth is typically given in cm and is the inverse of the absorption coefficient on the previous page.

Absorption depth of materials as a function of wavelength.

The absorption depth is a useful parameter which gives the distance into the material at which the light drops to about 36% of its original intensity, or alternately has dropped by a factor of 1/e. Since high energy light (short wavelength), such as blue light, has a large absorption coefficient, it is absorbed in a short distance (for silicon solar cells within a few microns) of the surface, while red light (lower energy, longer wavelength) is absorbed less strongly. Even after a few hundred microns, not all red light is absorbed in silicon. The variation in the absorption depth for "blue" and "red" photons is shown below.

full_screen.png The blue photons are absorbed very close to the surface but most of the red photons are absorbed deep in the device.