**8. Carbonaceous cathode for DSSC**

In this part, we show our effort on developing the counter electrode (CE). Generally, CE mainly functions as reducing the redox species from I3 to I2. A suitable redox charge mediator should effectively perform the function of shuttling the generated positive charge away from the light absorbing sensitizer residing on the semiconductor surface to the CE, thus completing the electrical circuit. For being an effective CE, it exhibit high electrical conductivity and electrocatalytic activity toward I3 reduction and corrosion resistant to iodide/triiodide electrolyte [164]. To date, a platinum (Pt) is the most common catalyst material for DSSC [165]. However, its cost remains a concern for the large-scale commercialization of the DSSC. Therefore, numerous researchers are trying to find new CE materials. Carbonaceous materials such as graphite [166, 167] carbon black [168, 169], activated carbon [170], hard carbon sphere [171], carbon nanotube [172], fullerene and graphene [173], conductive polymers [174], metal compounds [175] and composites have been developed and tested as promising counter electrodes. So far, carbonaceous materials are regarded as the most attractive option.

CPE. *β* indicated the capacitance of CPE and the deviation from the semicircle probably due to the porosity of electrode surface, respectively [41, 44, 178]. *R*<sup>s</sup> indicate the ohmic resistance of the electrolyte, the conductive glass and the carbon layer. In order to confirm the catalytic mechanism operating in an electrochemical

carbon electrode was used as the working electrode, a Pt coil as the counter electrode, and an Ag/Ag+ electrode as the reference electrode. **Figure 31(a).c** shows the CV curves obtained using CB and LPAH (versus Ag/AgCl). The negative current was

the carbon surface, respectively [179]. The LAPH sample showed a negative peak

voltammetry, the anode peak and the cathodic peak are related to the redox couple

*D*1/2*AC* where, *i*<sup>p</sup> is the peak current (A), *n* is number of electrons transferred in the redox event (usually 1), *ʋ* is scan rate in V/s, *A* is the area of working electrode, *C* is

In general, the equivalent circuit of the complete solar cell may be represented as indicated in **Figure 31(b)** [37, 176]. From right to left, it demonstrates impedance for charge transfer at electrolyte/catalytic materials-FTO interface, diffusion of I3

the Randles-Sevcik equation. The Randles-Sevcik equation: *ip* = (2.687 � 105

species in the electrolyte, electron transport and electron capture by the I3

TiO2/electrolyte interface and the electron transport at the FTO/TiO2 interface, respectively. These components can be simplified from proposed in DSSC model: *R*FTO/TiO2 is the resistance of the FTO/TiO2 contact and *CPE1* is the capacitance of this interface. TiO2 network consists of a diffusion element ZW1 that is in series connected with the charge-transfer element *R*TiO2, the two being in parallel with a capacitive (constant phase angle) element *CPE3*. *Z*W2 is the Warburg impedance

ance at the counter electrode, and *CPE2* is the double layer capacitance at the

The catalytic activity described the exchange current density (*J*0) using followed

*J*<sup>0</sup> ¼ *RT=ηFR*ct*:*

Here, *J*<sup>0</sup> is a kinetic parameter that depends on the reaction and on the electrode surface upon which the electrochemical reaction occurs, *R* is the gas constant,*T* is temperature (here T = 300 K), *n* is the gas constant and *F* is Faraday constant [44]. From the value of *J*0, we can know how easily the electrochemical reaction can occur on the electrode surface. The *R*ct of LPAH based CE shows approximately 35 times lower value at �3 μm thick film than that of a symmetric CB electrode at �8 μm thick film, leading a better energy efficiency at the DSSC (see **Figure 31(b)**). Furthermore, the relatively thin film of LPAH can be expected by reducing the internal series resistance of devices. The shifting the peaks of Bode phase at the high frequency region of the cells supported this expectation [181]. Consequently, LPAH would speed

� redox reaction, leading to improve the fill factor and cell efficiency. **Figure 31(c).b** described fitted Nyquist plot and *J-V* curves of the different CE (Pt, CB and LPAH) on DSSCs. The detailed parameters for internal resistance (*R*IR)

�) reactions and they can be used to estimate the diffusion coefficient (D) using

/s and 1.76 � <sup>10</sup>�<sup>4</sup> cm2

potential (�0.82 V) and a much higher current density (�0.02 mA/cm2

better reduction rate comparable to that CB (�1.4 V, �0.017 mA/cm2

/s and 6.3 � <sup>10</sup>�<sup>5</sup> cm2

� redox, cyclic voltammetry (CV) measurement was carried out. A

� to I� and positive current was the oxidation of I� at

. In the anodic and cathodic reaction, the

/s for CB. This result indicates that the

� redox reaction ascribed to a higher catalytic activity,

� in the electrolyte. *R*CE is the charge-transfer imped-

), indicating a

)*n*3/2*ʋ*1/2

�

� at the

). In cyclic

/s for LPAH were larger

system with I�/ I3

(I�/I3

assigned to the reduction of I3

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

the bulk concentration of analyte in mol/cm3

calculated *<sup>D</sup>* values of 1.05 � <sup>10</sup>�<sup>4</sup> cm2

LPAH would speed up the I�/I3

describing the diffusion of I3

equation,

up the I�/I3

**235**

electrolyte/catalytic materials-FTO interface [180].

leading to a more efficient DSSC.

than 6.70 � <sup>10</sup>�<sup>5</sup> cm2

In this book, we introduce large-effective-surface-area polyaromatic hydrocarbon (LPAH) for DSSC. A detailed description for generating LPAH species has been reported in an earlier publication [176]. The catalytic properties of LPAH can be calculated by EIS method. Device symmetric structure and the Randles-typed equivalent circuit model consisting of charge transfer resistance (*R*ct), a constant phase element (CPE) and a series resistance (*R*s) can be seen in **Figure 31**. Here, the *R*ct is a barrier for the charge transfer process at the LPAH/electrolyte interface [177]. The CPE is the interfacial capacitance, considering the roughness of the electrodes. In more detail, the impedance of CPE is described as *Z*CPE = *B*(i*ω*) *<sup>β</sup>* (0 ≤ *β* ≤ 1) where, *ω* is the angular frequency, *B* and *β* are frequency-independent parameters of the

#### **Figure 31.**

*(a) a. Equivalent circuit of a symmetrical cell used to fit the impedance spectra; b. Nyquist plots of different counter electrode catalytic materials (Platinum, carbon black and LPAH) prepared with identical electrodes; c. CVs of carbon black and LPAH films. (b) a. Nyquist plot of different counter electrodes. Equivalent circuit of the complete DSSC is given in the inset and b. J–V Characteristics of different counter electrode catalytic materials.*

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

**8. Carbonaceous cathode for DSSC**

*Solar Cells - Theory, Materials and Recent Advances*

activity toward I3

**Figure 31.**

*materials.*

**234**

In this part, we show our effort on developing the counter electrode (CE).

[164]. To date, a platinum (Pt) is the most common catalyst material for DSSC [165]. However, its cost remains a concern for the large-scale commercialization of the DSSC. Therefore, numerous researchers are trying to find new CE materials. Carbonaceous materials such as graphite [166, 167] carbon black [168, 169], activated carbon [170], hard carbon sphere [171], carbon nanotube [172], fullerene and graphene [173], conductive polymers [174], metal compounds [175] and composites have been developed and tested as promising counter electrodes. So far,

In this book, we introduce large-effective-surface-area polyaromatic hydrocarbon

(LPAH) for DSSC. A detailed description for generating LPAH species has been reported in an earlier publication [176]. The catalytic properties of LPAH can be calculated by EIS method. Device symmetric structure and the Randles-typed equivalent circuit model consisting of charge transfer resistance (*R*ct), a constant phase element (CPE) and a series resistance (*R*s) can be seen in **Figure 31**. Here, the *R*ct is a barrier for the charge transfer process at the LPAH/electrolyte interface [177]. The CPE is the interfacial capacitance, considering the roughness of the electrodes. In

*ω* is the angular frequency, *B* and *β* are frequency-independent parameters of the

*(a) a. Equivalent circuit of a symmetrical cell used to fit the impedance spectra; b. Nyquist plots of different counter electrode catalytic materials (Platinum, carbon black and LPAH) prepared with identical electrodes; c. CVs of carbon black and LPAH films. (b) a. Nyquist plot of different counter electrodes. Equivalent circuit of the complete DSSC is given in the inset and b. J–V Characteristics of different counter electrode catalytic*

suitable redox charge mediator should effectively perform the function of shuttling the generated positive charge away from the light absorbing sensitizer residing on the semiconductor surface to the CE, thus completing the electrical circuit. For being an effective CE, it exhibit high electrical conductivity and electrocatalytic

reduction and corrosion resistant to iodide/triiodide electrolyte

to I2. A

*<sup>β</sup>* (0 ≤ *β* ≤ 1) where,

Generally, CE mainly functions as reducing the redox species from I3

carbonaceous materials are regarded as the most attractive option.

more detail, the impedance of CPE is described as *Z*CPE = *B*(i*ω*)

CPE. *β* indicated the capacitance of CPE and the deviation from the semicircle probably due to the porosity of electrode surface, respectively [41, 44, 178]. *R*<sup>s</sup> indicate the ohmic resistance of the electrolyte, the conductive glass and the carbon layer. In order to confirm the catalytic mechanism operating in an electrochemical system with I�/ I3 � redox, cyclic voltammetry (CV) measurement was carried out. A carbon electrode was used as the working electrode, a Pt coil as the counter electrode, and an Ag/Ag+ electrode as the reference electrode. **Figure 31(a).c** shows the CV curves obtained using CB and LPAH (versus Ag/AgCl). The negative current was assigned to the reduction of I3 � to I� and positive current was the oxidation of I� at the carbon surface, respectively [179]. The LAPH sample showed a negative peak potential (�0.82 V) and a much higher current density (�0.02 mA/cm2 ), indicating a better reduction rate comparable to that CB (�1.4 V, �0.017 mA/cm2 ). In cyclic voltammetry, the anode peak and the cathodic peak are related to the redox couple (I�/I3 �) reactions and they can be used to estimate the diffusion coefficient (D) using the Randles-Sevcik equation. The Randles-Sevcik equation: *ip* = (2.687 � 105 )*n*3/2*ʋ*1/2 *D*1/2*AC* where, *i*<sup>p</sup> is the peak current (A), *n* is number of electrons transferred in the redox event (usually 1), *ʋ* is scan rate in V/s, *A* is the area of working electrode, *C* is the bulk concentration of analyte in mol/cm3 . In the anodic and cathodic reaction, the calculated *<sup>D</sup>* values of 1.05 � <sup>10</sup>�<sup>4</sup> cm2 /s and 1.76 � <sup>10</sup>�<sup>4</sup> cm2 /s for LPAH were larger than 6.70 � <sup>10</sup>�<sup>5</sup> cm2 /s and 6.3 � <sup>10</sup>�<sup>5</sup> cm2 /s for CB. This result indicates that the LPAH would speed up the I�/I3 � redox reaction ascribed to a higher catalytic activity, leading to a more efficient DSSC.

In general, the equivalent circuit of the complete solar cell may be represented as indicated in **Figure 31(b)** [37, 176]. From right to left, it demonstrates impedance for charge transfer at electrolyte/catalytic materials-FTO interface, diffusion of I3 � species in the electrolyte, electron transport and electron capture by the I3 � at the TiO2/electrolyte interface and the electron transport at the FTO/TiO2 interface, respectively. These components can be simplified from proposed in DSSC model: *R*FTO/TiO2 is the resistance of the FTO/TiO2 contact and *CPE1* is the capacitance of this interface. TiO2 network consists of a diffusion element ZW1 that is in series connected with the charge-transfer element *R*TiO2, the two being in parallel with a capacitive (constant phase angle) element *CPE3*. *Z*W2 is the Warburg impedance describing the diffusion of I3 � in the electrolyte. *R*CE is the charge-transfer impedance at the counter electrode, and *CPE2* is the double layer capacitance at the electrolyte/catalytic materials-FTO interface [180].

The catalytic activity described the exchange current density (*J*0) using followed equation,

$$J\_0 = RT/\eta \text{FR}\_{\text{ct}}.$$

Here, *J*<sup>0</sup> is a kinetic parameter that depends on the reaction and on the electrode surface upon which the electrochemical reaction occurs, *R* is the gas constant,*T* is temperature (here T = 300 K), *n* is the gas constant and *F* is Faraday constant [44]. From the value of *J*0, we can know how easily the electrochemical reaction can occur on the electrode surface. The *R*ct of LPAH based CE shows approximately 35 times lower value at �3 μm thick film than that of a symmetric CB electrode at �8 μm thick film, leading a better energy efficiency at the DSSC (see **Figure 31(b)**). Furthermore, the relatively thin film of LPAH can be expected by reducing the internal series resistance of devices. The shifting the peaks of Bode phase at the high frequency region of the cells supported this expectation [181]. Consequently, LPAH would speed up the I�/I3 � redox reaction, leading to improve the fill factor and cell efficiency.

**Figure 31(c).b** described fitted Nyquist plot and *J-V* curves of the different CE (Pt, CB and LPAH) on DSSCs. The detailed parameters for internal resistance (*R*IR)


### **Table 8.**

*Characteristic of different counter electrode materials (platinum, carbon black and LPAH). Reprinted from [176].* and photovoltaic properties are summarized in **Table 8**. The internal resistance

*R*IR = *R*<sup>0</sup> + *R*<sup>1</sup> + *R*<sup>2</sup> + *R*3. Although LPAH counter electrode has somewhat lower catalytic property compared with the Pt counter electrode, *V*oc, fill factor is close to Pt based DSSC electrode due to relatively similar *R*IR values. Finally, LPAH based DSSC show an overall energy conversion efficiency of 9.3% without mask, which is higher than the 7.5% achieved for CB counter electrode devices reported recently [169]. Therefore, we believe LPAH is a good candidate as a next catalytic material.

**9. Optimization of the DSSC performance for having maximum**

as the 3D cell [52]. In this design, it is very important how well the TiO2 nanoparticles have infiltrated among the ITO nanowire. In our earlier research, TiO2 solution including polymer binder is air-sprayed into ITO NWs. Although this method is easy and efficient enough to fill TiO2 NPs into ITO NW, it formed surface defect (such as crack) on the surface of TiO2 film during sintering process (see **Figure 32**). Through Electro-spraying process, crack free TiO2 film is deposited into

Based on these fundamental achievements, our efforts are headed for achieving an energy efficiency of over 12.3% by combining new materials and concept. For a photoanode, the ITO NR array with over 3 μm spacing and 10 μm thickness is used

*(a) SEM images of TiO2/ITO NRs on the different deposition technique; doctor-blade, air-sprayed from TiO2 solution including polymer binder and E-sprayed technique from EtOH and EtOH/terphineol (b) E-sprayed*

**performance**

**Figure 32.**

**237**

*TiO2/ITO NRs film inserted in the JV characteristic.*

(*R*IR,) can be calculated by the sum obtained from each resistance,

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