Book excerpt: Semiconductor thin films and their junctions such as metal-semiconductor (Schottky barrier) and heterojunctions have received much attention due to their applications in various electronic and optoelectronic devices including solar cells. This book is based on the electrical and optical studies of thin film Schottky barriers and heterojunctions with an aim to use in optoelectronic devices. In this work, two wide band gap semiconductors, one Zinc Selenide (ZnSe) from group II-VI, and another transparent conducting oxide, Indium Tin Oxide are investigated in the forms of thin films and their junctions, for the possible uses as photovoltaic converter. The structural, morphological, optical and electrical (in dark and under illumination) properties of ZnSe and ITO thin films prepared at different growth conditions by thermal evaporation method have been studied by applying different experimental techniques. Schottky barriers and heterojunctions of ZnSe with different barrier metals and semiconductors have been fabricated and their current-voltage and photovoltaic characteristics are studied. The findings of it presented in this book will be helpful for the researchers of this field.