The aim of this chapter is to give an in-depth analysis of transition metal complexes that are useful in conversion of solar energy into electricity, and in organic light emitting diodes. In the first part we discuss the historical background of sensitization phenomenon, operating principles of dye-sensitized solar cells, tuning of photophysical and electrochemical properties of sensitizers, evolution of photovoltaic performance, present status and future prospects for dye-sensitized solar cells. In the second part, we elucidate the modulation of phosphorescent color and quantum yields in neutral, cationic, and anionic iridium complexes and their application in light emitting devices.