**2. Fundamentals of dye-sensitized solar cells**

### **2.1 Operation principle**

The principle of operation of a DSSC is well documented in the literature [7]. The simplified principle of the thin layer DSSC is shown in **Figure 1**. A lightharvesting Ru complex potosensitizers adsorbed on the surface of a porous nanocrystalline film composed of a wide bandgap metal oxide such as TiO2, ZnO or SnO2 absorb incident photon flux. The photosensitizers are exited from the ground state (D) to the excited state (D\*) owing to the metal to ligand charge transfer (MLCT) transition (Eq. (1)). The exited electrons are injected into the conduction band of the TiO2 electrode, resulting in the oxidation of the photosensitizer (Eq. (2)).

$$\bullet \text{ D (adsorbed on TiO}\_2) + hv \rightarrow \text{D} \ast (\text{adsorbed on TiO}\_2) \tag{1}$$

$$\mathbf{D} \ast (\text{adsorbed on TiO}\_2) \to \mathbf{D}^+ \ (\text{adsorbed on TiO}\_2) + \mathbf{e}^- \ (\text{injected}) \tag{2}$$

Injected electrons in the conduction band of TiO2 are transported between TiO2 nanoparticles with diffusion toward the back contact (TCO) and consequently reach the counter electrode through the external load and wiring. The oxidized photosensitizer (D+ ) accepts electrons from the I� ion redox mediator, regenerationg the ground state (D), and I� is oxidized to the oxidized state, I3 � (Eq. (3)). The injected eletrons may recombine either with oxidized sensitier at the TiO2 (Eq. (4)).

$$\text{1D}^+ \text{ (adsorbed on TiO}\_2\text{)} + \text{3/2 I}^- \rightarrow \text{D (adsorbed on TiO}\_2\text{)} + \text{M}^- \text{I}^- \tag{3}$$

$$\text{D}^+ \text{ (adsorbed on TiO}\_2\text{)} + \text{e}^- \text{ (TiO}\_2\text{)} \rightarrow \text{D} \text{ (adsorbed on TiO}\_2\text{)}\tag{4}$$

The oxidized redox mediator, I3 �, diffuses toward the counter electrode and is rereduced to I� ions.

$$\text{I}^{-3} + 2\text{e}^- \left( \text{TiO}\_2 \right) \rightarrow \text{3I}^- \left( \text{anode} \right) \tag{5}$$

The primary energy conversion process in DSSCs is a photoinduced charge separation at the metal oxide/dye/electrolyte interface.

**2.2 Dye/metal oxide interface**

*A New Generation of Energy Harvesting Devices DOI: http://dx.doi.org/10.5772/intechopen.94291*

conduction band states [11].

**2.3 Dye/electrolyte interface**

follows:

(ε (I•

**189**

2.3 � <sup>10</sup><sup>10</sup> <sup>M</sup>�<sup>1</sup> <sup>s</sup>

to at the electron Fermi level of the electrode [12].

A monolayer dye molecules attached to the surace of the nanocrystalline film. The high surface area of nanocrystalline film ensured s a large concentration of the light absorber leading to good light haversting are critical to be efficient DSSCs. In addition, how and where the photosensitizer bound to the surface of TiO2 is another important issues since unexpected bonding between photosensitizer and the TiO2 surface lead to degrade electron injection performance. In the case of Ru complex, one carboxylic ligand provides good anchoring to metal oxide surface (Long-term stability against moisture condition can be improved by using phosphonic acid) as well as good electronic coupling of the excited states of photosensitizer with TiO2

To be the effective electron transfer, the lowest unoccupied molecular orbital (LUMO) level of excited dye molecules is appropriately lower than the highest unoccupied molecular orbital (HUMO) level of the metal oxide. Under illumination, the photoxcited sensitizers are injected into the metal oxide and diffused from the dye/metal oxide interface to substrate. In an efficient DSSC, a time scale of femtoseconds to picoseconds nanoseconds of injection process is witnessed on the strong electronic coupling between photoexicted sensitizer and metal oxide. The injected electrons experience fast recombination process and thermalisation down

After ultrafast electron injection from the photoexcited dye into the conduction band of TiO2, the dye is in its oxidized state and must be reduced by an electron donor in the electrolyte for regeneration. The standard electron donor is iodide. For many typed dye, high value of regeneration efficiency, which give the probability that an oxidized dye is regenerated by an electron donor in the electrolyte rather than by recombination with an electron in the TiO2, have been estimated in iodide.

) by iodide

�• is secondly

�• is by the composi-

� ! ð Þ D⋯I (6)

� ! ð Þ D⋯I2 � • (7)

� (9)

�• ð Þ! <sup>D</sup> <sup>þ</sup> I2 � • (8)

) complex.Finally, I2

The reaction mechanism from reduction of oxidized sensitizer (D<sup>+</sup>

ð Þþ D⋯I I

D⋯I2

lower potential of 0.1 V. Dissociation of ground-state dye D and I2

occurred from the formation of a (D���I2

�<sup>1</sup> in acetonitrile.

D<sup>þ</sup> þ I

I2 � • ! I3

Eq. (6) is most likely a one-electron transfer reaction between D<sup>+</sup> and I�.

/I�) = +1.33 V vs. NHE in aqueous solution.) The dye regeneration is firstly occured from the formation of a (D���I) complex since the redox potential (*ε*) of the iodine radical reacted to the sensitizer (ε (D���I) = +1.23 V vs. NHE in acetonitrile) shows a relatively lower potential of 0.1 V. The dye regeneration is firstly occured from the formation of a (D���I) complex since the redox potential (*ε*) of the iodine radical reacted to the sensitizer (ε (D���I) = +1.23 V vs. NHE in acetonitrile) shows a

�•

tion of triiodide and iodide. The second-order rate constant for this reaction is about

The oxidized sensitizer (specially, the most commonly reported Ru complex dye, named as N719) is regeneated by the iodide/iodine redox couple in the liquid

� þ I

#### **Figure 1.**

*Schematic diagram and principle of operation of proto-typed dye sensitized solar cells (inserted in the nanocrystalline TiO2 film).*

In order to elucidate DSSC working phenomenon, the physical kinetics and dynamics of charge transfer motion have been investigated in detail by many researchers, both experimentally and via computational modeling. Here, the electron transfer dynamics taking place at the oxide/dye/electrolyte interface for DSSCs are presented in **Figure 2** [7–9]. This figure also contains the information for several competing loss pathways, shown as red arrows. These loss pathways include decay of the dye excited state to ground, and charge recombination of injected electrons whith dye cations and with the redox couple.

#### **Figure 2.**

*State diagram of kinetics and energetics of a traditional DSSC. The positive charge transfer processes are indicated by blue color. The competing loss pathways of recombination with TiO2 trap sites and dye surface are shown in red. Reprinted from [8, 10].*
