**3. Carbon nanotubes (CNTs)**

Carbon nanotubes (CNTs) are hollow cylinders consisting of single or multiple concentric layers of carbon atoms in a honeycomb lattice structure [26]. The CNT structure consists of enrolled graphite sheets, in a word, and can be classified as either or multi-walled (MWCNT) (**Figure 3(a)**) or single-walled CNT (SWNT) (**Figure 3(b)**) depending on its preparation method. In transmission electron microscopy (TEM) studies, MWCNTs were first observed by Iijima in 1991, while SWCNTs were independently developed by Iijima and Bethune in 1993 [26]. CNT has a spn hybridization (where n = 2) state of carbon material. However, because of the curved surface of CNT, it does not have a genuine sp2 hybridization. CNT has a sp2 + ? hybridization, which is in between n = 2 and 3. It is understood that CNT is a material lying between fullerenes and graphite as a new member of carbon allotropes [27]. Carbon nanotubes (CNTs) show very excellent adsorption characteristics because they have a high specific surface area and a nanoscale formation that constitutes many sites. It also has high electrical conductivity, mechanical strength, and a high aspect ratio [28].

#### **3.1 CNT based photoanode in DSSC**

CNT incorporated semiconductor material on the conducting electrode surface, offers efficient charge collection and transportation of charge carriers. The electrons

**Figure 3.** *(a) MWCNTs, (b) SWCNTs.*

through the semiconductor material toward the anode, and electrical energy is delivered to the external load (Eq. (3)). Then the electron further travels to complete the circuit and reaches the counter electrode (C.E.). The electron is transferred from the C.E. to the electrolyte. Dye regenerates when the dye accepts an

they receive ion from the C.E. (Eq. (5)) [25]. However, some unwanted reaction occurs, such as the recombination of dye (Eq. (6)), dye recombination to the ground state (Eq. (7)), and recombination of electrolyte (Eq. (8)) that reduces the

Oxidation of dye due to injection of electrons in TiO2 photoanode

e� ð Þþ TiO2 C*:*E*:* ! TiO2 <sup>þ</sup> e�

Restoration of electrolyte at the counter electrode 1 2 I3 � þ e�

D<sup>þ</sup> þ 3 2 I

� ! D þ

ð Þ <sup>C</sup>*:*E*:* ! <sup>3</sup> 2 I 1 2

� (Eq. (4)). I3

<sup>D</sup> <sup>þ</sup> <sup>h</sup><sup>ν</sup> ! <sup>D</sup><sup>∗</sup> (1)

<sup>D</sup><sup>∗</sup> ! <sup>D</sup><sup>þ</sup> <sup>þ</sup> <sup>e</sup>� ð Þ TiO2 (2)

ð Þ <sup>C</sup>*:*E*:* þ electrical energy

(3)

I3 (4)

� þ C*:*E*:* (5)

� ion float around, and

electron from the I� and I� gets oxidized to I3

*Solar Cells - Theory, Materials and Recent Advances*

overall cell's electron circulation performance. Excitation of dye upon illumination

Energy generation

**Figure 2.**

*Basic operating principles of DSSC.*

Regeneration of dye

**386**

injected from the excited dye into the semiconductor materials are then transferred through a CNT scaffold to generate photocurrent. Such 1-D nanostructures have been successfully exploited to improve the performance of DSSC [29]. **Figure 4** illustrates the role of CNT incorporated semiconductor material (i.e., TiO2) for efficient electron transportation in DSSC.

Studies by Brown et al. has shown that the presence of CNT (i.e., SWCNT) does not directly affect the primary charge injection process in the D\*/TiO2 system. SWCNT incorporated TiO2 (TiO2/SWCNT/D\*) films collect photoinduced electrons more effectively by charge separation than TiO2/D\* films [29]. Studies have also shown that SWCNT accepts and stores electrons when in contact with photoirradiated TiO2 semiconductor materials. The fermi equilibrium with photoirradiated TiO2 and SWCNT can store to 1 electron per 32 carbon atoms. When the dyes are linked to the TiO2-SWCNT suspension, the stored electrons are ready to discharge on demand [30]. SWCNT incorporated TiO2 showed 30% higher photoinduced current compared to without SWCNT incorporated TiO2. Though the SWCNT incorporated TiO2 based DSSC showed increases in the photoinduced current, the open-circuit voltage degrades in the SWCNT incorporated TiO2 based DSSC. This phenomenon can be explained by the electron capture properties of SWCNT. At equilibrium condition, a positive shift (20–30 mV) of the SWCNT causes loweropen-circuit voltage (which is directly related to the difference between fermi level of photoanode and redox electrolyte), as shown in **Figure 5**.

The photoinduced electrons are transferred to the SWCNT network, which minimizes the possibility of charge recombination at grain boundaries [29].

CNTs (i.e., SWCNTs) can be pristine (p-SWCNTs), metallic (m-SWCNTs) and semiconducting (s-SWCNTs) depending on their delocalized electrons occupying a 1-D density of states, chiral angles and diameters. The efficiency of SWCNT/TiO2 based DSSC depends on different parameters, such as eventual charge separation, charge transfer, charge transport, and recombination rates. Studies by Guai et al. showed that m-SWCNT incorporated TiO2 based photoanode does not significantly improve the recombination; however, it still enhances cell conversation efficiency of plain TiO2 based DSSC. This indicates that plain TiO2 has poor conductivity, which reduces the efficient charge transportation. On the other hand, s-SWCNT incorporated TiO2 based photoanode showed superior conductivity. s-SWCNT incorporated TiO2 based photoanode can also suppress the election recombination; thus, significant cell conversion efficiency is observed [31].

**Figure 6** illustrates the energy band diagrams of s-SWCNT and m-SWCNT incorporated TiO2 based DSSC. **Figure 6a** shows the photoinduced electron transfers from the photosensitizer's excited state to the TiO2 and quickly moves from s-SWCNT to FTO. Since there is less possibility of electrons transported back (or

*Energy-band diagrams of DSSCs with incorporated (a) s-SWCNTs and (b) m-SWCNTs [31].*

*Energy diagram illustrating D\*/TiO2 and transportation of photoinduced electrons without (a) and with*

*Improvement of Efficiency of Dye Sensitized Solar Cells by Incorporating Carbon Nanotubes*

*DOI: http://dx.doi.org/10.5772/intechopen.96630*

electrons are effectively transported and collected by the FTO. This results in enhanced photocurrent generation than p- and m-SWCNT based DSSCs. Also, the effective collections of the photoinduced electron at the anode results in a positive shift in Enf, increasing the open-circuit voltage of the SWCNT/TiO2 based DSSC. On the contrary, though m-SWCNT has better electron mobility than s-SWCNT, it has a higher disruption in charge carrier transportation, leading to increased back reaction, as shown in **Figure 6b**. As a result, fewer electrons are collected at the FTO, which leads to lower photocurrent and cell conversion efficiency [31]. Hence

TiO2 >p SWCNT*=*TiO2 > m SWCNT*=*TiO2 >s SWCNT*=*TiO2*:*

However, for the development of the high performance of SWCNT based DSSC, the combination of both m-SWCNT and s-SWCNT is used. Numerous researchers have been working on the combination of m-SWCNT and s-SWCNT. And the w/w

the relative cell conversion efficiencies can be estimated as follows:

% of s-SWCNT varies between 88 and 97% in the mix [32].

reduction, more photoinduced

recombination) to the liquid electrolyte to cause I3

**Figure 5.**

**Figure 6.**

**389**

*(b) SWCNT network.*

**Figure 4.** *CNT incorporated photoanode for DSSC.*

*Improvement of Efficiency of Dye Sensitized Solar Cells by Incorporating Carbon Nanotubes DOI: http://dx.doi.org/10.5772/intechopen.96630*

#### **Figure 5.**

injected from the excited dye into the semiconductor materials are then transferred through a CNT scaffold to generate photocurrent. Such 1-D nanostructures have been successfully exploited to improve the performance of DSSC [29]. **Figure 4** illustrates the role of CNT incorporated semiconductor material (i.e., TiO2) for

Studies by Brown et al. has shown that the presence of CNT (i.e., SWCNT) does

CNTs (i.e., SWCNTs) can be pristine (p-SWCNTs), metallic (m-SWCNTs) and semiconducting (s-SWCNTs) depending on their delocalized electrons occupying a 1-D density of states, chiral angles and diameters. The efficiency of SWCNT/TiO2 based DSSC depends on different parameters, such as eventual charge separation, charge transfer, charge transport, and recombination rates. Studies by Guai et al. showed that m-SWCNT incorporated TiO2 based photoanode does not significantly improve the recombination; however, it still enhances cell conversation efficiency of plain TiO2 based DSSC. This indicates that plain TiO2 has poor conductivity, which reduces the efficient charge transportation. On the other hand, s-SWCNT incorporated TiO2 based photoanode showed superior conductivity. s-SWCNT incorporated TiO2 based photoanode can also suppress the election recombination;

thus, significant cell conversion efficiency is observed [31].

**Figure 4.**

**388**

*CNT incorporated photoanode for DSSC.*

not directly affect the primary charge injection process in the D\*/TiO2 system. SWCNT incorporated TiO2 (TiO2/SWCNT/D\*) films collect photoinduced electrons more effectively by charge separation than TiO2/D\* films [29]. Studies have also shown that SWCNT accepts and stores electrons when in contact with photoirradiated TiO2 semiconductor materials. The fermi equilibrium with photoirradiated TiO2 and SWCNT can store to 1 electron per 32 carbon atoms. When the dyes are linked to the TiO2-SWCNT suspension, the stored electrons are ready to discharge on demand [30]. SWCNT incorporated TiO2 showed 30% higher photoinduced current compared to without SWCNT incorporated TiO2. Though the SWCNT incorporated TiO2 based DSSC showed increases in the photoinduced current, the open-circuit voltage degrades in the SWCNT incorporated TiO2 based DSSC. This phenomenon can be explained by the electron capture properties of SWCNT. At equilibrium condition, a positive shift (20–30 mV) of the SWCNT causes loweropen-circuit voltage (which is directly related to the difference between fermi level of photoanode and redox electrolyte), as shown in **Figure 5**. The photoinduced electrons are transferred to the SWCNT network, which minimizes the possibility of charge recombination at grain boundaries [29].

efficient electron transportation in DSSC.

*Solar Cells - Theory, Materials and Recent Advances*

*Energy diagram illustrating D\*/TiO2 and transportation of photoinduced electrons without (a) and with (b) SWCNT network.*

#### **Figure 6.** *Energy-band diagrams of DSSCs with incorporated (a) s-SWCNTs and (b) m-SWCNTs [31].*

**Figure 6** illustrates the energy band diagrams of s-SWCNT and m-SWCNT incorporated TiO2 based DSSC. **Figure 6a** shows the photoinduced electron transfers from the photosensitizer's excited state to the TiO2 and quickly moves from s-SWCNT to FTO. Since there is less possibility of electrons transported back (or recombination) to the liquid electrolyte to cause I3 reduction, more photoinduced electrons are effectively transported and collected by the FTO. This results in enhanced photocurrent generation than p- and m-SWCNT based DSSCs. Also, the effective collections of the photoinduced electron at the anode results in a positive shift in Enf, increasing the open-circuit voltage of the SWCNT/TiO2 based DSSC. On the contrary, though m-SWCNT has better electron mobility than s-SWCNT, it has a higher disruption in charge carrier transportation, leading to increased back reaction, as shown in **Figure 6b**. As a result, fewer electrons are collected at the FTO, which leads to lower photocurrent and cell conversion efficiency [31]. Hence the relative cell conversion efficiencies can be estimated as follows:

TiO2 >p SWCNT*=*TiO2 > m SWCNT*=*TiO2 >s SWCNT*=*TiO2*:*

However, for the development of the high performance of SWCNT based DSSC, the combination of both m-SWCNT and s-SWCNT is used. Numerous researchers have been working on the combination of m-SWCNT and s-SWCNT. And the w/w % of s-SWCNT varies between 88 and 97% in the mix [32].

The introduction of CNTs into the semiconductor materials causes better dispersion of semiconductor materials particles and smaller crystalline size, structure with high porosity and coarse surface. These results in anincreasein the total surface area thus dye absorption increases and hence overall cell performance. However, the photosensitizer (i.e., metallic photosensitizer, organic photosensitizer, natural photosensitizer) used in DSSCusually anchors on the semiconductor materials (i.e., TiO2) surface, not the CNT surface. If the mass density of semiconductor materials decreases, the number of dyes loading or absorption will decrease. Thus, if the semiconductor materials are not uniformly distributed on the CNT's surface, total dye absorption will be poor, thereby decreasing the cell's conversion efficiency. To solve this problem, the bonding between semiconductor material and CNTs should be increased. Different research used various methods to solve the problem. CNTs treated with concentrated HNO3 and H2SO4 results in the introduction of -COOH anchoring group on the CNTs, and provides further improved bonding between semiconductor material and CNTs. The acid-treated CNT (i.e., SWCNT) can be used in two ways; either incorporated into the semiconductor material to improve charge transfer or introduced in semiconductor material/electrolyte interface as light scattering centers. In the first case, when acid-treated SWCNTs were incorporated into the semiconductor material (i.e., TiO2) films, the fabricated DSSC showed a 25% more enhancement in photocurrent than the untreated CNTs. In the second case, when acid-treated SWCNTs were introduced at the TiO2/electrolyte interface, the value of open-circuit voltage increases, whereas the value of photogenerated current remains constant. The improvement in open-circuit voltagegenerally impliesdecreased dark current and the negative shift of the conduction band of semiconductor material [33, 34]. **Figure 7** illustrates the TiO2 films with untreated SWCNTs and with acid treated SWCNT.

oxygen containing groups on the surface of the MWCNTs. The oxygen plasma treated MWCNT makes the surface more hydrophilic and improves the dispersion in TiO2, which leads to high surface area and enhanced dye absorption and hence the cell performance increased. According the Zhang et al. study, the plasma treated MWCNT can improve around 75% cell performance than untreated MWCNT/TiO2

*Improvement of Efficiency of Dye Sensitized Solar Cells by Incorporating Carbon Nanotubes*

**Figure 8** illustrates the surface morphology of the coated films of pure TiO2, MWCNT-TiO2, and plasma-treated MWCNT-TiO2 photoanodes using scanning electron microscopic (SEM). The introduction of MWCNT into the TiO2 semiconductor material causes immobilized uniformly. The FESEM images also indicate thatincorporating MWCNT into the TiO2 metal oxide material causes a highly porous and coarse surface (**Figure 8b**), which leads to a higher surface area for dye absorption. However, MWCNT-TiO2 photoanodes has irregular pore sizes and a non-uniform porous structure, which affect the total surface area improvement. On the other hand, plasma treated MWCNT has a higher dispersion in TiO2 metal oxide material leading to a more uniform porous surface structure (**Figure 8c**), which successfully increases the total surface area for better dye absorption [35].

Researchers have also explored other methods for employing CNTs for high surface area and hierarchical nanoporous structurefor higher cell conversion efficiency. Yun et al. employed TiO2 hollow sphere/CNT by direct mixing and showed 4.71% with 0.1 wt.% [36]. Muduli et al. sensitized TiO2/MWCNT composite by hydrothermal method and achieved the crystalline phase, which showed 50% more cell conversion efficiency than the one without the MW-CNTs [37]. Patrick et al. submerged TiO2 colloid in the optically transparent electrode and electrophoreticallydeposited SWCNTs for working photoanode. The modified photoanode showed better charge separation and prevented back reaction/recombination, showing 45% improvement in photocurrent [29]. Subha et al. fabricated singlecrystalline 1D rutile TiO2 nanorods/MWCNT composite template-free synthesis method and reported 60% improvement in cell performance. Due to the single

*FESEM and TEM images of the (a) pristine/pure TiO2, (b) MWCNTs/TiO2, and (c) plasma treated-*

photoanode based DSSC [34, 35].

*DOI: http://dx.doi.org/10.5772/intechopen.96630*

**Figure 8.**

**391**

*MWCNTs/TiO2photoanode [35].*

Similar to SWCNT, MWCNT also improves cell performance. However, the cell performance can be further improved by employing different treatment techniques. Numerous researchers are working on the topics and discovered different methods. Employing these methods can further improve the cell efficiency of DSSC. For example, Zhang et al. introduced RF induced oxygen plasma treatment for the

**Figure 7.**

*(a) TEM of untreated bundles of SWCNTs, (b) HRTEM of a single fragmented bundle of acid treaded SWCNTs (c) SEM of TiO2 films with untreated SWCNTs and (d) SEM of TiO2 films with acid treated SWCNTs [33].*

*Improvement of Efficiency of Dye Sensitized Solar Cells by Incorporating Carbon Nanotubes DOI: http://dx.doi.org/10.5772/intechopen.96630*

oxygen containing groups on the surface of the MWCNTs. The oxygen plasma treated MWCNT makes the surface more hydrophilic and improves the dispersion in TiO2, which leads to high surface area and enhanced dye absorption and hence the cell performance increased. According the Zhang et al. study, the plasma treated MWCNT can improve around 75% cell performance than untreated MWCNT/TiO2 photoanode based DSSC [34, 35].

**Figure 8** illustrates the surface morphology of the coated films of pure TiO2, MWCNT-TiO2, and plasma-treated MWCNT-TiO2 photoanodes using scanning electron microscopic (SEM). The introduction of MWCNT into the TiO2 semiconductor material causes immobilized uniformly. The FESEM images also indicate thatincorporating MWCNT into the TiO2 metal oxide material causes a highly porous and coarse surface (**Figure 8b**), which leads to a higher surface area for dye absorption. However, MWCNT-TiO2 photoanodes has irregular pore sizes and a non-uniform porous structure, which affect the total surface area improvement. On the other hand, plasma treated MWCNT has a higher dispersion in TiO2 metal oxide material leading to a more uniform porous surface structure (**Figure 8c**), which successfully increases the total surface area for better dye absorption [35].

Researchers have also explored other methods for employing CNTs for high surface area and hierarchical nanoporous structurefor higher cell conversion efficiency. Yun et al. employed TiO2 hollow sphere/CNT by direct mixing and showed 4.71% with 0.1 wt.% [36]. Muduli et al. sensitized TiO2/MWCNT composite by hydrothermal method and achieved the crystalline phase, which showed 50% more cell conversion efficiency than the one without the MW-CNTs [37]. Patrick et al. submerged TiO2 colloid in the optically transparent electrode and electrophoreticallydeposited SWCNTs for working photoanode. The modified photoanode showed better charge separation and prevented back reaction/recombination, showing 45% improvement in photocurrent [29]. Subha et al. fabricated singlecrystalline 1D rutile TiO2 nanorods/MWCNT composite template-free synthesis method and reported 60% improvement in cell performance. Due to the single

**Figure 8.**

*FESEM and TEM images of the (a) pristine/pure TiO2, (b) MWCNTs/TiO2, and (c) plasma treated-MWCNTs/TiO2photoanode [35].*

The introduction of CNTs into the semiconductor materials causes better dispersion of semiconductor materials particles and smaller crystalline size, structure with high porosity and coarse surface. These results in anincreasein the total surface area thus dye absorption increases and hence overall cell performance. However, the photosensitizer (i.e., metallic photosensitizer, organic photosensitizer, natural photosensitizer) used in DSSCusually anchors on the semiconductor materials (i.e., TiO2) surface, not the CNT surface. If the mass density of semiconductor materials decreases, the number of dyes loading or absorption will decrease. Thus, if the semiconductor materials are not uniformly distributed on the CNT's surface, total dye absorption will be poor, thereby decreasing the cell's conversion efficiency. To solve this problem, the bonding between semiconductor material and CNTs should be increased. Different research used various methods to solve the problem. CNTs treated with concentrated HNO3 and H2SO4 results in the introduction of -COOH anchoring group on the CNTs, and provides further improved bonding between semiconductor material and CNTs. The acid-treated CNT (i.e., SWCNT) can be used in two ways; either incorporated into the semiconductor material to improve charge transfer or introduced in semiconductor material/electrolyte interface as light scattering centers. In the first case, when acid-treated SWCNTs were incorporated into the semiconductor material (i.e., TiO2) films, the fabricated DSSC showed a 25% more enhancement in photocurrent than the untreated CNTs. In the second case, when acid-treated SWCNTs were introduced at the TiO2/electrolyte interface, the value of open-circuit voltage increases, whereas the value of photogenerated current remains constant. The improvement in open-circuit voltagegenerally impliesdecreased dark current and the negative shift of the conduction band of semiconductor material [33, 34]. **Figure 7** illustrates the TiO2 films

*Solar Cells - Theory, Materials and Recent Advances*

with untreated SWCNTs and with acid treated SWCNT.

**Figure 7.**

**390**

Similar to SWCNT, MWCNT also improves cell performance. However, the cell

*(a) TEM of untreated bundles of SWCNTs, (b) HRTEM of a single fragmented bundle of acid treaded SWCNTs (c) SEM of TiO2 films with untreated SWCNTs and (d) SEM of TiO2 films with acid treated SWCNTs [33].*

performance can be further improved by employing different treatment techniques. Numerous researchers are working on the topics and discovered different methods. Employing these methods can further improve the cell efficiency of DSSC. For example, Zhang et al. introduced RF induced oxygen plasma treatment for the

crystalline structure, there were no grain boundaries, which provides a smooth surface for electron transportation [38]. Zhu et al. sensitized rice grain-shaped TiO2/MWCNT composite by electrospinning process. Due to the single crystalline structure and high surface area of the rice grain-shaped TiO2/MWCNT composite, DSSC showed a 32% improvement in cell performance with 0.2 wt.% MWCNT [39].

electrodes for low-cost DSSC. Carbon materials are not only abundant but also highly resistant to corrosion. Carbon-based material, especially CNT, has attracted considerable interest because of its fast electron transfer kinetics and large surface area. **Figure 10** shows a basic schematic of a CNT based counter electrode for DSSC. Different methods have been explored for CNT based counter electrode. Nam et al. used paste printing and CVD growing methods for the counter electrode. The paste printing MWCNT based counter electrode based DSSC has lower cell efficiency (8.03%) than the Pt. counter electrode based DSSC's cell efficiency (8.80%). However, the CVD has grown MWCNT had higher cell conversion efficiency (10.04%) than the Pt. counter electrode based DSSC's cell efficiency (8.80%) [40]. Ramasamy et al. fabricated spray-coated MWCNT counter electrode and showed the effect of spray time/coating thickness [41]. Widodo also fabricated spray-coated CNT on FTO glass substrate for counter electrode for DSSC [42]. Prasetio et al. used different weight (0.01, 0.02 and 0.04 gram) of CNT and observed the cell performance of DSSC [43]. **Figure 11** illustrates the SEM of (a) the surface and (b) the

*Improvement of Efficiency of Dye Sensitized Solar Cells by Incorporating Carbon Nanotubes*

cross-section CNT based counter electrode [43].

*DOI: http://dx.doi.org/10.5772/intechopen.96630*

electrode.

**Figure 10.**

**393**

*CNT based counter electrode for DSSC.*

**4. Effect of CNT on the cell performance of DSSC**

natural dye sensitizer has been explored for DSSC operation.

**4.1 Effect of CNT based photoanode on the cell performance of DSSC**

As mentioned earlier (in Section 3.1.), adding CNT nanoparticles into mesoporous structure provides a strong light-harvesting capability and a large surface area for high-efficiency DSSC. Mesoporous semiconductor materials anchor on the long tubular CNT's outer surface and this assembly ensure efficient electron transport through CNTs. CNT improves the electron transport and increases the coating's thickness; thus, dye building on the anode material increase. CNT results in gains in the photocurrent without compromising the electron injection to the

From the previous discussion (Section 3.1.), CNT can be used either CNT/semiconductor material composite photoanode or counter electrode. For CNT-based photoanode for different types of dyes, such as metal complex dye sensitizer,
