The reliability on renewable energy sources will bring environmental benefits as well as overall energy security by diversifying energy supply. The major focus of this book is using polycrystalline Cu1.8Se1-xSx thin films as an alternative absorber layer to conventional Cu2-xSe for solar cell. This material will introduce an improvement in the performance of copper selenide- based solar cell which will be achieved by increasing band gap to match solar cell spectrum which in turn increasing the absorption and finally result in increasing efficiency. The other concentration of this book is using a new method, Inert gas condensation,as an original simply physical method to prepare a new material in the form of nanocrystalline thin films, which characterized by different properties other than polycrystalline films. By controlling the size of grains, we can obtain a different properties and consequently using nanocrystalline thin films in other applications such as field-effect transistors, quantum electronics, optoelectronics and sensors.