ABSTRACT
This chapter provides a summary of the light absorption properties in a range of materials relevant to energy conversion including bulk inorganic semiconductors, semiconductor quantum dots, and organic molecules and materials. It addresses the optical absorption features due to charge transfer complexes at heterojunctions and some of the applications. The bonding character of a semiconductor largely determines the molecular orbitals that are involved in optical electronic transitions. For ionic solids, the valence band is usually formed from the highest occupied orbitals of the anions, with a mixture of the levels to a certain extent. The chapter discusses the several important effects: the plasma reflectivity, the Burstein shift, and the techniques based on the absorption of radiation by free carriers. The plasma absorption is due to a collective excitation of free carriers by the electrical field of the electromagnetic waves. The quantization leads to quasiparticles termed plasmons that obey the Bose statistics.
