**4. Solar cells**

Photovoltaic cells are a good example of an alternative energy source, converting sunlight into electricity. Research in this field is quite intense given the importance of solar cells as sources of sustainable energy, as well as due to their reduced cost, low environmental impact, and fair efficiency for conversion of solar energy into electricity (O'Regan & Grätzel, 1991; Grätzel, 2003; Brennaman et al., 2011).

The efficiencies obtained for a silicon solar cell is about 24%, although at a very high manufacturing cost. Therefore, other materials have been studied in order to facilitate the conversion of solar energy into electrical energy (Zhao et al, 1998; Jayaweera et al., 2008; Cao et al., 2009; Patrocinio et al., 2010; Brennaman et al., 2011).

New developed devices such as dye solar cells, capable of converting solar energy into electrical (dye solar cells – DSCs), have been presented as alternatives for power generation (Hagfeldt & Gratzel, 1995; Gratzel & Hagfeld, 2000; Jayaweera et al., 2008). Despite its efficiency is still lower than that of silicon cells, the DSCs have been particularly interesting because they have a much lower production cost than devices based on silicon. In addition, the resulting material can be extremely thin as well as flexible and can be applied to almost any surface (Brabec et al., 2001; Kippelen & Bredas, 2009). Technically they are known as dye-sensitized solar cells, or DSSCs.
