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. The authors also address the optical absorption features due to charge-transfer complexes at heterojunctions and some of their applications. In semiconductors where the VB maximum and CB minimum do not have the same crystal momentum, the direct transition excited by only a photon cannot occur, since the photon has a very small linear momentum. Considering a metal nanoparticle, the plasmon causes an enhancement of the local field intensity in the nearby space by redistribution of the optical field. A localized surface plasmon resonance induces sharp spectral absorption and scattering peaks so that chromophores situated in the region of enhanced field effectively absorb more light.
