This PhD dissertation focuses on the evaluation of the environmental impact of different photovoltaic technologies, with special focus on the benefits of replacing conventional fossil fuels with clean electricity production from photovoltaic systems. Emissions of CO2 are highlighted as the main reason of climate change. Human life style (transport, industry, etc.) has to be changed in order to follow the IEA directives. Sustainable energy has an important role to reduce the use of fossil fuels as energy source. Growing economies depend strongly on fossil fuels, and therefore, changing its energy policy are key to slow down global warming. The study follows a detailed methodology based on "Life Cycle Analysis" in combination with technical approaches to monitor working PV facilities and therefore provides quantitative results in term of avoided emissions and the potential for climate change mitigation of PV in different context, ranging from building integration of grid-connected systems to electrification of rural livelihoods.