**4. The architecture of the dye-sensitized solar cell (DSSC)**

The architecture of a dye-sensitized solar cell (DSSC) is discussed in **Figure 8**. The DSSC consists of a dye-covered, nanoporous TiO<sup>2</sup> (titanium dioxide) layer and an electrolyte founded in between two glass substrates. Front and back electrodes are decorated with a transparent conducting oxide (TCO). Fluorine doped tin oxide (SnO<sup>2</sup> : F), FTO, is the most widely known. The FTO at the back electrode is covered with few nanometers of atomic layers of platinum (Pt), which enhance the formation of electrons through the redox reaction with the electrolyte. The front electrode is covered firstly with thin layer of TiO<sup>2</sup> as a blocking layer which is responsible to prevent the holes from reaching the anode and then coated with a nanocrystalline TiO<sup>2</sup> layer with an average particle size of 5–20 nm. Considering a layer thickness of 10 μm, the resulting effective surface is about 1000 times larger than the dense compact TiO<sup>2</sup> layer. Three modifications of TiO<sup>2</sup> exist: rutile, anatase, and brookite. In the DSSC preferably only the anatase modification is used [54].

Subsequently, a monolayer of dye molecules (scattering layer) is adsorbed on the surface of the TiO2 . The huge nanoporous surface allows for an adsorption of a sufficiently huge number of dye molecules for efficient light harvesting. The most widespread dye molecule employed in DSSC is usually a ruthenium (Ru) metal-organic complex, the so-called N719 [56]. The spectral absorption of the dye lies between 300 nm and 800 nm. Sufficient adsorption of the dye to the TiO<sup>2</sup> is critical and is obtained by the two carboxylic groups of the ligand (L = 2,2′-bipyridyl-4,4′-dicarboxylic acid) of the RuL<sup>2</sup> (NCS)2 . Finally, a liquid redox electrolyte is inserted between the two electrodes.

nanomaterials has imparted novel properties and applications in the photovoltaic field with enhanced performance. Aside from that, recently improved devices depend on a novel concept that have largely expanded the application range of titanium dioxide and also put forwarded new requirements for titanium dioxide properties. In this chapter, the major advances of applying titanium dioxide nanomaterials to photovoltaics have been discussed, including

Controlling the Microstructure and Properties of Titanium Dioxide for Efficient Solar Cells

http://dx.doi.org/ 10.5772/intechopen.72494

nano-

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the dye-sensitized solar cells. These steady progresses have demonstrated that TiO<sup>2</sup>

TiO2

a new PV energy source.

**Acknowledgements**

**Author details**

**References**

2017;**187**:228-242

materials play an important role in the search for efficient and low-cost photovoltaic technologies. The charge transfer process in these photovoltaic devices is nearly correlating with the features of titanium dioxide nanomaterials as well as the titanium dioxide interface. The unique physical and chemical properties of titanium dioxide nanomaterials can be controlled through modulation of nanocrystal structure, size, shape, and organization. Furthermore, the properties of titanium dioxide interface can be amended through the interaction between

 and the surrounding elements, including light harvesters, charge transport materials, additives, as well as interfacial modifiers. Moreover, many efforts have been done to develop large-scale preparation technique for high-quality, low-cost titanium dioxide nanomaterials and transformative technology. In order to realize the marketing economically of solar panels with wide application prospect, many studies have been considered. It is believed that future research efforts on new materials and key interfaces will make the titania-based solar cells as

The authors would like to extend their sincere appreciation to the Central Metallurgical Research and Development Institute, Egypt, for its financial support to pursue the work.

Ahmed Esmail Shalan\*, Ahmed Mourtada Elseman and Mohamed Mohamed Rashad

Electronic and Magnetic Materials Division, Advance Materials Department, Central Metallurgical Research and Development Institute (CMRDI), Helwan, Cairo, Egypt

[1] Iychettira KK, Hakvoort RA, Linares P, de Jeu R. Towards a comprehensive policy for electricity from renewable energy: Designing for social welfare. Applied Energy.

[2] de Souza JS, De Andrade LOM, Müller AV, Polo AS. Nanomaterials for solar energy conversion: dye-sensitized solar cells based on ruthenium (II) Tris-heteroleptic compounds or natural dyes. Nanoenergy. 2018:69-106. https://doi.org/10.1007/978-3-319-62800-4\_2

\*Address all correspondence to: a.shalan133@gmail.com

**Figure 8.** Schematic diagram of a solar cell sensitized by dye. The nanoporous semiconductor (TiO<sup>2</sup> ) and the redox electrolyte are founded between two substrates, covered with fluorinated tin oxide (FTO). The TiO<sup>2</sup> is filled with a dye, and the cathode is covered with a platinum layer. From Ref. [55]. Reprinted with permission from Springer Nature 2003.

#### **4.1. How does the DSSC work?**

When the solar light is irradiated, the dye sensitized it, and the electron excited from the valence band (VB) to the conduction band (CB) of the dye leaving a hole in the valence band. After that, the electron moves from the valence band of the dye to the valence band of the TiO<sup>2</sup> nanoparticles which facilitate the swimming of the electrons to the FTO anode substrate and then to the Pt cathode to generate electricity. On the other hand, the redox reaction occurs in the iodine electrolyte to facilitate the formation of the electron to fill the hole founded in the valence band of the dye to regenerate the electron in the cell, and the process occurs continuously.

#### **4.2. Uses of titanium dioxide in DSSC**

TiO2 is used for DSSCs for the following key properties: (i) is a suitable band that adjusts for electron injection from most commercial dyes, (ii) has a high surface area which is suitable for higher dye loading, and (iii) has high electronic mobility for photo-generated electron collection.

Nowadays, many groups and results mentioned that TiO<sup>2</sup> is the best available choice compared to other metal oxide semiconductors such as ZnO, SnO<sup>2</sup> , and so on due to the internal network structure which is important to achieve high charge collection efficiency and more electron transportation. These features enable a good electrode while fabricating a typical DSSC device.
