**2.4 TiO2 interface for electron transport**

Unlike the typical the electron transport mechanism in bulk semiconductors, the electron transport mechanism of TiO2 in DSSC need to be extend from the properties of the individual nanoparticles to the particle connectivity or electronic coupling between the particles, and the geometrical configuration of the assembly. Interestingly, the mesoporous nanocrystalline TiO2 layer exhibit the highly efficient charge tranport through the nanocrystalline TiO2 layer, while the low inherent conductivity of the film ca. bulk mobility of TiO2 (1cm2 /Vs), ZnO (200cm2 /Vs), and SnO2 (250cm2 /Vs) as well as the presence of disorder from randomly arranged metal [16]. The main consequence of disorder in the electronic structure of the material is the appearance of localized states. This puzzling phenomenon has been explained by multiple-trapping model, in which the diffusion of conduction band electrons is affected by the trapping-detrapping events [17–20]. Electron transport in nanostructured oxide films impregnated with a highly concentrated electrolyte is believed to occur mainly by diffusion. It is generally accepted that this diffusional transport is influenced by the existence of electron localized states or traps in the semiconductor.

In the TiO2 nanoparticles, electrons undergo a number of processes. A main process is the transport of electrons in the extended states of the conduction band (CB), where the concentration of free electrons *nc* is defined as

$$n\_c = n\_0 e^{qV/k\_B T} \tag{10}$$

g Eð Þ¼ *NL*

we need to consider how the density of electrons in the CB.

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

conductivity *σ<sup>n</sup>* associated to free carrier transport is [21].

transport resistance of impedance spectroscopy [22–24].

**2.5 Electrolyte/counter electrode interface**

*kBT*<sup>0</sup>

exp

The carrier mobility is associated with random motion of free carriers, which is directed linked to the conductivity and resistivity. The expression of the electron

*<sup>σ</sup><sup>n</sup>* <sup>¼</sup> *<sup>q</sup>*2*D*<sup>0</sup>

Here *D0* is the free electron diffusion coefficient. The conductivity can be expressed by the temperature-dependent features using the framework of multiple trapping where electron displacement occure via extended states at the CB edge with the free electron diffusion coefficient. This result has been measured by the common techniques such as intensity modulated photocurrent spectroscopy (IMPS), transient photocurrent under a small perturbation of the illumination, or

In theory, the maximum photovoltage of the DSSC is determined by the energy

difference between the redox potential and the Fermi level of the metal oxide semiconductor. However, the output voltage under load is usually less than the open circuit voltage the theorical expected. This voltage loss is mainly attributed the the overall overpotential of electrolyt and counter electrode (CE) interface. The mass-transfer overpotential is largely affected by the ionic conductivity of electrolytes and the transportation of mediator species from the CE to the photoanode. The

kinetic overpotential or charge-transfer overpotential can be determined by electrocatalytic properties of the CE surface toward mediator reduction [25]. To be

electrocatalytic activity for reduction of the redox couple. The effect of electric field and transport by ionic migration can be negligible because of high ionic conductivity and ionic strength of liquid electrolyte. With increasing viscosity such as gel, quasi- solid and solid state electrolyte, an inadequate flux of redox components lead to sacrifice the photocurrent of the DSSC. In the case of the iodide/triiodide electrolyte, charge transport corresponds to formation and cleavage of chemical bonds:

� � �I2 � �I

counter electrode reduces the redox species. (triiodide !iodide) Platinum (Pt) materials is widely used as a suitable catalyst for reaction 1.14 to replenish the reduced species in the redox electrolyte. However, Pt is very expensive and rare so they have limited potential for commercial use. Therefore substantial researches are under way to develop inexpensive alternatives materials for larger commercialisation prospects. The charge transfer reaction between the counter electrode surface and the electrolyte occurs the charge-transfer resistance (RCT) in the DSSCs. The small RCT will give a higher fill factor (FF), which lead to a high conversion

The CE must exhibit catalytically fast reaction and low overpotential because the

� ! I

� þ I3

� (14)

effective CE, it should exhibit excellent conductivity and inhibit high

� ! I

I3 � þ I

efficiency.

**191**

Here *NL* is the total DOS, and *T0* is a parameter with temperature units that determines the depth of the distribution, that is alternatively expressed as a coefficient α = T/T0. If we consider that electrons can only be transported via the CB, then

*E* � *Ec kBT*<sup>0</sup>

(12)

*kBT nc* (13)

here *n*<sup>0</sup> is an equilibrium concentration in the effective density of states of the CB and *kB* is the Boltzmann constant, q denotes elementary charge and T is the temperature unut. In a steady state, transport process is under the control of displacing electrons in the TiO2 CB with the trap in equilibrium occupation.

TiO2 nanoparticles exhibit numerous localized states in bandgap, which can capture and release electrons to the transport level. The position of Fermi level (*EFn*) plays an important role in calculating the probability of electron capture since trapping events below *EFn* are quiescent from almost fully occupied traps, while empty traps above the *EFn* happens in reverse.

A localized state at energy *Et* sets free electrons at a rate;

$$
\sigma\_t^{-1} = \beta\_n N\_c \sigma^{\frac{E\_t - E\_c}{k\_B T}} \tag{11}
$$

where *β<sup>n</sup>* is the time constant for electron capture (which is independent of trap depth). Since *τ<sup>t</sup>* increases exponentially with the depth of the state in the bandgap, the slowest trap is the deepest unoccupied trap. The effect of traps is dominant whenever a transient effect is induced, in which the position of the Fermi level is modified, with the correspondent need for traps release. Therefore, traps also become a dominant aspect of the recombination of electrons in a DSSC, that is a charge transfer to the electrolyte or hole conductor. The specific density of localized states (DOS) in the bandgap of TiO2, named as *g(E)*, can be also established by capacitance techniques that provide the chemical capacitance, that is defined as follows:

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

electrolyte in the range of 100 ns to 10 μs. The different regenation time can be explained by iodide concentration, the presence of additivies such as identity of caion salt (lithium ions) and tertbutylpyridine (tBP). To be the effective DSSC, the recombination process of electrons with either oxidized dye molecules or acceptors in the electrolyte must be minimized compared with regeneration process, which

Unlike the typical the electron transport mechanism in bulk semiconductors, the electron transport mechanism of TiO2 in DSSC need to be extend from the properties of the individual nanoparticles to the particle connectivity or electronic coupling between the particles, and the geometrical configuration of the assembly. Interestingly, the mesoporous nanocrystalline TiO2 layer exhibit the highly efficient charge tranport through the nanocrystalline TiO2 layer, while the low inherent conductivity

/Vs), ZnO (200cm2

/Vs) as well as the presence of disorder from randomly arranged metal [16].

The main consequence of disorder in the electronic structure of the material is the appearance of localized states. This puzzling phenomenon has been explained by multiple-trapping model, in which the diffusion of conduction band electrons is affected by the trapping-detrapping events [17–20]. Electron transport in nanostructured oxide films impregnated with a highly concentrated electrolyte is believed to occur mainly by diffusion. It is generally accepted that this diffusional transport is influenced by the existence of electron localized states or traps in the semiconductor. In the TiO2 nanoparticles, electrons undergo a number of processes. A main process is the transport of electrons in the extended states of the conduction band

*nc* ¼ *n*0*e*

here *n*<sup>0</sup> is an equilibrium concentration in the effective density of states of the CB and *kB* is the Boltzmann constant, q denotes elementary charge and T is the temperature unut. In a steady state, transport process is under the control of displacing electrons in the TiO2 CB with the trap in equilibrium occupation. TiO2 nanoparticles exhibit numerous localized states in bandgap, which can capture and release electrons to the transport level. The position of Fermi level (*EFn*) plays an important role in calculating the probability of electron capture since trapping events below *EFn* are quiescent from almost fully occupied traps, while

(CB), where the concentration of free electrons *nc* is defined as

A localized state at energy *Et* sets free electrons at a rate;

*τ*�<sup>1</sup>

*<sup>t</sup>* ¼ *βnNce*

where *β<sup>n</sup>* is the time constant for electron capture (which is independent of trap depth). Since *τ<sup>t</sup>* increases exponentially with the depth of the state in the bandgap, the slowest trap is the deepest unoccupied trap. The effect of traps is dominant whenever a transient effect is induced, in which the position of the Fermi level is modified, with the correspondent need for traps release. Therefore, traps also become a dominant aspect of the recombination of electrons in a DSSC, that is a charge transfer to the electrolyte or hole conductor. The specific density of localized states (DOS) in the bandgap of TiO2, named as *g(E)*, can be also established by capacitance techniques that provide the chemical capacitance, that is defined as

*Et*�*Ec*

/Vs), and SnO2

*qV=kBT* (10)

<sup>k</sup>*B<sup>T</sup>* (11)

usually happens on a time scale of about 1 μs [7, 13–15].

**2.4 TiO2 interface for electron transport**

*Solar Cells - Theory, Materials and Recent Advances*

of the film ca. bulk mobility of TiO2 (1cm2

empty traps above the *EFn* happens in reverse.

(250cm2

follows:

**190**

$$\mathbf{g}(\mathbf{E}) = \frac{N\_L}{k\_B T\_0} \exp\left(\frac{E - E\_c}{k\_B T\_0}\right) \tag{12}$$

Here *NL* is the total DOS, and *T0* is a parameter with temperature units that determines the depth of the distribution, that is alternatively expressed as a coefficient α = T/T0. If we consider that electrons can only be transported via the CB, then we need to consider how the density of electrons in the CB.

The carrier mobility is associated with random motion of free carriers, which is directed linked to the conductivity and resistivity. The expression of the electron conductivity *σ<sup>n</sup>* associated to free carrier transport is [21].

$$
\sigma\_n = \frac{q^2 D\_0}{k\_B T} n\_c \tag{13}
$$

Here *D0* is the free electron diffusion coefficient. The conductivity can be expressed by the temperature-dependent features using the framework of multiple trapping where electron displacement occure via extended states at the CB edge with the free electron diffusion coefficient. This result has been measured by the common techniques such as intensity modulated photocurrent spectroscopy (IMPS), transient photocurrent under a small perturbation of the illumination, or transport resistance of impedance spectroscopy [22–24].

#### **2.5 Electrolyte/counter electrode interface**

In theory, the maximum photovoltage of the DSSC is determined by the energy difference between the redox potential and the Fermi level of the metal oxide semiconductor. However, the output voltage under load is usually less than the open circuit voltage the theorical expected. This voltage loss is mainly attributed the the overall overpotential of electrolyt and counter electrode (CE) interface. The mass-transfer overpotential is largely affected by the ionic conductivity of electrolytes and the transportation of mediator species from the CE to the photoanode. The kinetic overpotential or charge-transfer overpotential can be determined by electrocatalytic properties of the CE surface toward mediator reduction [25]. To be effective CE, it should exhibit excellent conductivity and inhibit high electrocatalytic activity for reduction of the redox couple. The effect of electric field and transport by ionic migration can be negligible because of high ionic conductivity and ionic strength of liquid electrolyte. With increasing viscosity such as gel, quasi- solid and solid state electrolyte, an inadequate flux of redox components lead to sacrifice the photocurrent of the DSSC. In the case of the iodide/triiodide electrolyte, charge transport corresponds to formation and cleavage of chemical bonds:

$$\text{I}\_3\text{}^- + \text{I}^- \rightarrow \text{I}^- \cdot \text{I}\_2 \cdot \text{I}^- \rightarrow \text{I}^- + \text{I}\_3\text{}^-\tag{14}$$

The CE must exhibit catalytically fast reaction and low overpotential because the counter electrode reduces the redox species. (triiodide !iodide) Platinum (Pt) materials is widely used as a suitable catalyst for reaction 1.14 to replenish the reduced species in the redox electrolyte. However, Pt is very expensive and rare so they have limited potential for commercial use. Therefore substantial researches are under way to develop inexpensive alternatives materials for larger commercialisation prospects. The charge transfer reaction between the counter electrode surface and the electrolyte occurs the charge-transfer resistance (RCT) in the DSSCs. The small RCT will give a higher fill factor (FF), which lead to a high conversion efficiency.
