# Overview on Different Types of Solar Cells: An Update
## 4 Dye-Sensitized Solar Cells
### Definition and Fundamental of Dye-Sensitized Solar Cells  
DSSC generally uses indium tin oxide (ITO) or fluorine-doped tin oxide (FTO) as a substrate. The oxide substrate layer functions as a current collector and the substrate material itself as a sealing layer between the cells in the DSSC and the outside air [155].  
Titanium dioxide (TiO\({}_{2}\)) is the material of choice used as a working electrode or photoelectrode in DSSC. The three modifications of TiO\({}_{2}\) include anatase, rutile, and brookite, but only anatase and rutile are stable [156]. Brookite is difficult to produce and therefore is not considered in DSSC applications [157]. The particle sizes of anatase and rutile increase[158] with increasing temperature. Figure 8, showing different growth rates, shows that rutile has a much higher growth rate than anatase. The growth rate of anatase is flat at 800 \({}^{\circ}\)C.  
The absorbance spectrum defines the possibility of a useful semiconductor for photocatalysts having a band gap (E\({}_{\rm g}\)) proportional to the photon energy of the visible or ultraviolet spectrum (Eg < 3.5 eV). Most authors have determined that the energy gap of rutile is 3.0 eV and that of anatase is 3.2 eV. A larger active surface area and a more effective photocatalyst [159] can be observed in the anatase phase.  
The photocatalytic process is based on the dual ability of a semiconductor material (TiO\({}_{2}\)) to absorb photons and carry out transformation reactions at the material junction simultaneously. Figure 9 shows several photoexcitation pathways [160] followed by electron and hole de-excitation. The enlarged portion of Figure 9 shows electrons excited from the valence band (E\({}_{\rm v}\)) to the conduction band (E\({}_{\rm c}\)) due to photons with energy (_hv_) equal to or greater than the band gap of the semiconductor.  
From Figure 9, it is assumed that the semiconductor remains intact, and the charge transfer is uniform to the adsorbed organic or inorganic molecules. The excited electron-hole pair can go through several paths as follows: the semiconductor can donate electrons on the surface to reduce the adsorbed acceptor (\(A/A^{-}\)), for example, oxygen (lane c). Holes can migrate to the surface and electrons from donors (D/D+) can join holes (c lanes). Competition for electron transfer to adsorbed molecules by the separate recombination of electrons and holes occurs in the volume of the semiconductor particles (band b) or on the semiconductor's surface (lane a).  
Electron paramagnetic resonance spectroscopy (EPR) showed that the detected electrons were either captured as Ti\({}^{3+}\) or as electrons in the conduction band, while the holes were trapped in the O\({}^{-}\) free-oxygen center resulting from the O\({}^{2-}\) lattice in the valence band. The reaction is written as follows [161]:  
\[\mathbf{e}^{-}+\mathbf{Ti}^{\mathbf{4}+}\rightarrow\mathbf{Ti}^{\mathbf{3}+}\ (\text{electrontrap}) \tag{7}\]  
\[\mathbf{h}^{+}+\mathbf{O}^{2-}\rightarrow\mathbf{O}^{-}\ (\text{holetrap}) \tag{8}\]  
Figure 8: Changes in the particle size of anatase and rutile as a function of annealing temperature [158].  
Figure 9: The schematic of photoexcitation followed by de-excitation on the semiconductor’s surface [160].