Optically enhanced absorption in thin silicon layers using photonic crystals

Yablonovitch derived a limit to enhancement in optical absorption in the weak absorption regime for a textured solar cell of 4n², where n is the refractive index of the medium. This limit assumes that the incident rays experience full randomization inside the medium so that all the photonic states are filled equally. We demonstrate that this statistical limit can be exceeded over a limited spectral and angular range using photonic structures. We theoretically examine the use of photonic crystals to improve the optical absorption. Photonic crystals have a periodic modulation of the refractive index that alters the photonic density of states. A simple model of a silicon photonic crystal is presented in order to illustrate the possible optical enhancement in thin silicon layers.