In the last years, many attempts using different modifications of the originally proposed cell have been made in order to improve its performance, most of them based on the use of different semiconductors (Tennakone et al., 1999), dyes (Wang et al., 2005) or ionic conductors (Wang et al., 2004). However, the extremely delicate sensibility of the charge transport and recombination dynamics to any alteration of the nature of the interfaces present in the cell should be considered (Haque et al., 2005). For instance, some of the most important routes of research have focused on the molecular engineering of suitable dyes having broader absorption spectra that show a better matching to the solar spectrum and higher molar extinction coefficients (Wang et al., 2005), thus yielding higher short circuit currents. However, further improvements in terms of cell stability and durability should be done. On this respect, the performance of cells using solid state hole conductor based DSSC (Bach et al., 1998) to increase the long-term stability still remains far from that achievable when liquid organic electrolytes are employed.
The quantification of the electrical kinetic parameters of the cell has attracted the attention of many research groups and a great effort has been made in this direction. However, less interest has been paid to the study and development of optical elements that could be introduced in the cell for boosting the optical path of light, thus increasing the probability for the photons to be absorbed. Although it was well-known that by optical means the output power of the cell can be enhanced through a higher photogenerated current, since it depends on the number of photons collected by the dye (Tachibana et al., 2002), it was also clear that the introduction of optical elements that can enhance light harvesting in DSSC was not straightforward. First, they are typically made of dense materials, which would block the flow of charged species in solution. Second, the standard fabrication and integration processes usually employed to make optical materials did not seem to be compatible with the colloidal chemistry approaches normally taken to prepare a DSSC. The first and most successful approach to light management in DSSC was based on the introduction of a diffuse scattering layer, as described below, which largely enhances the photon path length through the working electrode, thus increasing the probability of optical absorption to take place. More recently, approaches based on periodic structures are also being investigated with promising results. Apart from the large enhancements of efficiency these latter structures gives rise to, they present the added advantage of allowing for the precise selection of the spectral range at which optical absorption is amplified, leading to both control over the aspect and the semi-transparency of the cell.