**4.1 Dye-sensitized solar cells (DSSCs)**

360 Solar Radiation

Fig. 11. Schematic representation of the photocatalytic water splitting on a platinized

We have developed small closed circulation reactor for bench-scale tests. These reactors ensure the evaluation of the developed photocatalyst from lab scale to out-door scale, in a

The object of these studies is to improve hydrogen production and its storage under low

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,

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

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

semiconductor powder particle.

1991; Grätzel, 2003; Brennaman et al., 2011).

et al., 2009; Patrocinio et al., 2010; Brennaman et al., 2011).

batch mode.

**4. Solar cells** 

pressure.

The use of solar cells based on a combination of dyes and photosensitizers oxide semiconductor (DSCs) have attracted great attention since the pioneering work of Grätzel and collaborators (O'Regan & Gratzel, 1991**;** Grätzel, 2005). The most efficient sensitizers for wide band gap semiconductors are the well-known metallo-organic ruthenium complexes (Grätzel & O'Regan, 1991). Certain Ru(II) complexes have shown to be excellent photosensitizers for TiO2 in DSSCs, having gained the attention because of the high efficiencies achieved (11%) in converting sunlight into electricity (Nazeeruddin et al., 2005; Gao et al., 2008a, 2008b; Cao et al., 2009). In dye-sensitized solar cells, the conversion of visible light to electricity is achieved through the spectral sensitization of wide band gap semiconductors. Light is absorbed by the dye molecules, which are adsorbed on the surface of the semiconductor, thus inducing charge separation. Excitation of the dye molecules results in electron injection into the conduction band of the semiconductor. For electron injection to occur, the excited electrons must be at higher energy level than the semiconductor conduction band. An electrolyte of high ionic strength is also used in dyesensitized solar cells to facilitate charge transfer across the device.

DSSCs have emerged as one of the most promising devices for sustainable photovoltaics due to their usually reduced cost, low environmental impact, and fair efficiency of conversion of solar energy into electricity (Grätzel, 2003; Polo et al., 2004; Brennaman et al., 2011).

Research in this field has been intense, given the growing worldwide demand for new energy sources (Kamat, 2007; Jacobson, 2009), either with focus on new materials and components or on cell assemblies for development of more efficient and environmentally friendly devices (Garcia et al., 2003; Prochazka et al., 2009; Zakeeruddin & Grätzel, 2009; Snaith, 2010). It is increasingly urgent the need to diversify energy matrices in order to rely on truly renewable energy sources, cleaner and environmentally friendly, if the goal is to build an ecologically sustainable society (Kamat, 2007; Herrero et al., 2011).

However, the high cost of dyes based on Ru (II), due to the low abundance of this metal and use restrictions from the environmental point of view are aspects that restricts its application on a large scale, which has stimulated efforts to use photosensitizing dyes with good features, safe and low cost (Hamann et al., 2008; Mishra et al 2009; Imahori et al., 2009).

Several simple organic dyes, and especially xanthene dyes (Eosin Y, Rose Bengal, etc.), yield efficiencies comparable to those achieved with ruthenium complexes, especially when used to sensitize ZnO films (Guillén et al., 2008; Plank et al., 2009; Pradhan et al., 2007). Organic dyes such as these are inexpensive (Kroon et al., 2007), can be easily recycled (Lee et al., 2006) and do not rely on the availability of precious metals such as ruthenium. They also have high extinction coefficients and their molecular structures contain adequate anchoring groups to be adsorbed onto the oxide surface. However, solar cells sensitized with such dyes tend to have low stability. The development and optimization of solar cells is of great interest, both commercially and scientifically. However, dye sensitized devices are still not commercially available in large volumes. Disadvantages such as the low efficiency and stability of these cells pose a hindrance to their commercialization.

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

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 polyelectrolyte itself were still under investigation.

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 commercial application is considered.
