**2.2. Working principle of DSSCs**

**Figure 3** shows the energy levels in the working of a DSSC. The Fermi energy level of TiO2 will be aligned with the redox energy level when there is no light. Upon illumination, dye molecules (*D*) attached to the mesoporous TiO2 surface absorbs photons of energy, *hv*. Electrons in the highest occupied molecular orbital (HOMO) of the dye molecules will be excited into the lowest unoccupied molecular orbital (LUMO), see Eq. (1).

**Figure 3.** Schematic diagram showing the kinetic processes at the TiO2/dye/electrolyte interface.

$$D + h\mathbf{v} \to D^\* \quad \text{light absorption} \tag{1}$$

Here, *D\** is the excited dye molecule. Electrons in the LUMO of the dye will be transferred to the mesoporous TiO2 within femtoseconds, ~10−15 s. This process is called electron injection. The Fermi level of TiO2 will be increased towards the conduction band (CB). The dye molecule is now in an oxidized or ionized state (*D+* ), Eq. (2). The difference in the potential between the Fermi and the redox levels will be manifested as the voltage of the device.

$$D^\* \to e^-\_{(\text{no}\_2)} + D^\* \quad \text{electron injection} \tag{2}$$

The transferred electrons percolate through the interconnected nanocrystalline TiO2 network to the conducting substrate within milliseconds (10−3 s). For good performance of the DSSC, this process has to be completed with the recombination reaction displayed in Eqs. (3) and (4).

$$e^{-}\_{\left(^{\rm TO}{}\_{2}\right)} + D^{\*} \to D \tag{3}$$

$$\text{'} \ 2\text{e}^-\_{(\text{no}\_2)} + \text{I}^\cdot\_\text{\textbullet} \to \text{\textbulletI}^- \tag{4}$$

Eq. (3) describes electron recombination with the ionized dye molecule and Eq. (4) describes electron-triiodide ion recombination. Electrons exit the TCO substrate and travel towards the counter electrode through the external circuit and reduce a triiodide ion in the electrolyte to an iodide ion as shown in Eq. (5).

$$\frac{1}{2}\mathrm{I}^{-}\_{3} + e^{-}\_{\text{(CE)}} \rightarrow \frac{3}{2}\mathrm{I}^{-} \tag{5}$$

The iodide ion diffuses to the photoanode and is oxidized back to a triiodide ion regenerating the dye molecule in the process. This process occurs continuously as shown in Eq. (6).

$$D^{+} + \frac{3}{2}\mathcal{I}^{-} \to D + \frac{1}{2}\mathcal{I}\_{3}^{-} \tag{6}$$

#### **2.3. Dye sensitizer**

second layer can be prepared by grinding TiO2 of 21 nm size with nitric acid, a polymer of low molecular mass (e.g. polyethylene glycol of molecular mass 200 g/mol) with a little surfactant. This paste will be deposited over the blocking TiO2 layer and heated at ~450°C for 30 min. To ensure the dye adheres to the mesoporous TiO2 layer, the TiO2 films are soaked in the dye solution overnight. The larger surface area of the mesoporous TiO2 area allows a greater amount of dye to be adsorbed on its surface. An electrolyte usually with an iodide/ triiodide couple is needed for DSSC. The electrolyte can be in liquid or gel form. A catalytic active material (usually platinum) is required as the counter electrode to reduce the triio-

**Figure 3** shows the energy levels in the working of a DSSC. The Fermi energy level of TiO2 will be aligned with the redox energy level when there is no light. Upon illumination, dye molecules (*D*) attached to the mesoporous TiO2 surface absorbs photons of energy, *hv*. Electrons in the highest occupied molecular orbital (HOMO) of the dye molecules will be excited into the

dide ion (I

Here, *D\**

3

10 Nanostructured Solar Cells

<sup>−</sup>) to the iodide ion (<sup>I</sup>

**2.2. Working principle of DSSCs**

−).

lowest unoccupied molecular orbital (LUMO), see Eq. (1).

**Figure 3.** Schematic diagram showing the kinetic processes at the TiO2/dye/electrolyte interface.

\* *D hv D* + ® light absorption (1)

is the excited dye molecule. Electrons in the LUMO of the dye will be transferred to

the mesoporous TiO2 within femtoseconds, ~10−15 s. This process is called electron injection.

The dye sensitizer is one of the important components of the DSSC. It works as an absorber of light and produces electrons. For good light conversion into electricity, the dye or sensitizer must have the following:

