This text deals with the advantages of rare earth activated phosphors for the development of solid state lighting technology and in enhancing the light conversion efficiency of Si solar cells. The book initiates with a short overview of the atomic and semiconductor theory followed by introduction to phosphor, its working mechanism, role of rare earth ions in the lighting and PV devices and host materials being used. Further, it introduces the applications of inorganic phosphor for the development of green energy and technology including advantages of UP/DC conversion phosphor layers in the enhancing the cell response of PV devices. Key Features: Focuses on discussion of phosphors for both solid state lighting and photovoltaics applications Provides introduction for practical applications including synthesis and characterization of phosphor materials Includes broad, in-depth introduction of semiconductors and related theory Enhances the basic understanding of optical properties for rare earth phosphors Covers up-conversion and down-conversion phosphor for energy harvesting applications