Aim of this Ph. D. work was the study of the instabilities effects in Thin Film Solar Cells. The research was principally intended to a substantial experimental activity of electrical and optical measurements, to the physical modeling of solar cells behaviors and to the development of suitable electrical and optical conditioning techniques oriented to gain stability, reliability and an overall performance improvement for second and third generation solar cells. This project report consists of seven chapters.Chapter 1 represents an introduction to this research. A first part is dedicated to Renewable Energy Sources and their importance. It is here discussed about the challenges and objectives of Solar Energy by examining its properties, the different Photovoltaics Technologies and the various generations of solar cells available in the market, with a particular focus on Thin-Film solutions. The second part examines the hydrogenated amorphous Silicon (a-Si: H), pointing out on the main advantages and defects, listing its structural characteristics and focusing on the Staebler-Wronski Effect (SWE) occurring during light soaking of this type of cells. Here, in addition to the single junction analysis, it is discussed about how amorphous silicon is used in multi-junction cells, focusing attention on a-Si: H/microcrystalline Si tandem solar cells. Finally, the third part summarizes the reasons that led to the present thesis study and what are its goals.