As one of the most promising of the alternative energy sources, there are many advantages of solar energy: low carbon emissions; abundant and secure supplies; favoring developing countries; being cheap to run and maintain. Solar photovoltaic cells convert sunlight into electricity. This is achieved using semiconductors and some complicated physics. This paper explains how it works. The solar source of light energy is described and quantified, along with a review of the basic equations of photovoltaic device physics. Particular attention is given to efficiency limits for single-junction, tandem solar cells and triple junction solar cells. The efficiency of single-junction cells is presented as a function of the energy gap, and the efficiency of tandem cells is presented as a function of the energy gap of top and bottom cells.