Chapter IV - Light-Semiconductor Interaction

Abstract

This chapter highlights the interaction of light semiconductors including absorption, reflection and transmission. In the design of solar cells, the first consideration must be the interaction of light with the semiconductor. Study indicates that this interaction begins with the reflection of light from the front surface. There are many potential interactions of light with the interior of a semiconductor, other than the intra and interband interactions. Many of the interactions are used to probe the electronic structure of semiconductors, and others play major roles in solid state lasers and LEDs (light emitting devices). These phenomena play, at best, a secondary role in the operation of the solar cell. In absorbing a photon, exciting an electron across the forbidden gap of a semiconductor, and thus, creating a hole-electron pair, both the total energy and the momentum of the system (the particles involved may include electrons, holes, phonons, and atoms as well as photons) must be conserved. In addition, any applicable quantum mechanical “selection rules” for the energy shifts must be satisfied. In a direct gap semiconductor the conduction band minimum of energy possesses the same momentum as does the valence band maximum Therefore, no transfer of momentum is required when providing the valence band electrons with sufficient energy to transit the energy gap to the conduction band. However, many semiconductors are not direct gap materials, for example, silicon, which is the most widely used semiconductor.