**6. Conclusion**

362 Solar Radiation

commercially available in large volumes. Disadvantages such as the low efficiency and

A considerable increase in conversion efficiency of components of solar radiation into electrical energy by other photosensitizing dyes has been achieved in recent years. Macrocyclic systems such as porphyrins, phthalocyanines and derivatives have been shown

Special attention has been given to the electron recombination processes that limit the DSC efficiency (Wang et al., 2006; Peter, 2007a; Zhao et al., 2008). Experimental and theoretical studies have been carried out in order to better understand and control these processes (Kruger et al., 2003; Cameron & Peter, 2005; Peter, 2007b; Xia et al., 2007a), typical interface phenomena. Strategies have been proposed to prepare efficient blocking layers in DSCs by using different techniques, such as spray pyrolysis, sputtering or by immersion in oxide precursor solutions (Xia et al., 2007a; Xia et al., 2007b; Wang et al., 2003; Handa et al., 2007). For example, the application of a compact layer onto the FTO glass before the mesoporous oxide film can prevent electron recombination at the FTO/TiO2 interface. This blocking layer physically avoids the contact between the electrolyte and the FTO surface, decreasing the occurrence of triiodine reduction by photoinjected electrons (Patrocinio et al., 2010; Lei et al., 2010). Efficient layer-by-layer (LbL) TiO2 compact films is considered one of the most effective blocking layers to avoid recombination processes at FTO surface in DSCs (Patrocinio et al.,*.* 2009). Although not previously reported as blocking layers, LbL metal oxide films have been applied in several devices (Krogman et al., 2008; Srivastava & Kotov, 2008; Jia et al., 2008; Lu et al., 2009b), including DSCs (Tsuge et al., 2006; Agrios et al., 2006). Iha and coworkers have shown that an LbL film based on TiO2 nanoparticles and sodium sulphonated polystyrene, PSS, applied onto the FTO substrate before the mesoporous TiO2 layer improved the overall conversion efficiency of DSCs by 28% (Patrocinio et al., 2009). Other complementary effects of the compact LbL TiO2 layer in DSCs and the role of the

LbL films using polyelectrolytes with good thermal stability at the electrode sintering temperature (450o C), such as sodium sulphonated polystyrene and sulphonated lignin, SL, maintain the compact morphology, and act as effective contact and blocking layers in DSCs. TiO2 LbL films with poly(acrylic acid) as a polyanion presented similar morphology to that exhibited by TiO2/PSS and TiO2/SL films before sintering (Patrocinio et al., 2010). The best performance so far achieved is through the use of the TiO2/PSS compact layer that increases the overall efficiency of DSCs to 30%, from 5.6 to 7.3%. The LbL TiO2/PSS film imposes a longer time for a charge exchange at the electrode surface decreasing the electron recombination. The TiO2/SL films (23% improvement) can be a cost effective option if a

Despite the widespread use of titanium dioxide, modified or not, or even other semiconductors with photocatalytic activity in photodegradation and mineralization of organic matter (Agostiano et al., 2003; Mrowetz et al.,2004; Machado et al.,2008; Hoffmann et al.,2010; Gupta et al.,2011), and its other capabilities (Mrowetz et. al.,2004; Zaleska, 2008a, 2008b), these semiconductors have been little explored in the synthesis of compounds of interest, although it is recognized that the photocatalytic synthesis should enable the

**5. Organic synthesis mediated by heterogeneous photocatalysis** 

stability of these cells pose a hindrance to their commercialization.

to be capable of application in solar cells (Lu et al., 2009a).

polyelectrolyte itself were still under investigation.

commercial application is considered.

In this chapter we combined a fast literature review about the different applications of heterogeneous photocatalysis, involving environmental photocatalysis, Hydrogen production for power generation, solar energy conversion into electricity using dye/semiconductor oxide cells and organic synthesis, with some experimental results obtained in our research group.
