**2.2 Conductive polymer materials**

Since 2000, the conductive polymer material has been discovered by Shirakawa, MacDiarmid, and Heeger [29]. Conductive polymers have attracted much attention as DSSC CEs owing to their excellent conductivity, good adhesion to the substrate, easy fabrication, light-weight, and good accessibility in terms of roll-to-roll processing. Common conductive polymers include poly(3,4-ethylenedioxythiophene) (PEDOT) [30, 35, 36, 40–43], poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) [30, 43, 48], poly(hydroxymethyl 3,4-ethylenedioxythiophene) (PEDOT-MeOH) [46], poly(3,4-propylenedioxythiophene) (PProDOT) [85, 86], poly(3,3-diethyl-3,4-dihydro-2H-thieno-[3,4-b][1,4]dioxepine) (PProDOT-Et2) [85–87], poly(2,2-dimethyl-3,4-propylenedioxythiophene) (PProDOT-Me2) [85], polythiophene (PT) [33], sulfonated poly(thiophene-3-[2- (2-methoxyethoxy) ethoxy]-2,5-diyl) (s-PT) [48], polyaniline (PANI) [34], and polypyrrole (PPy) [31, 38, 39, 44, 48], and their molecular structures are shown in **Figure 6**. However, conductive polymers often showed a flat or a mesoporous structure, meaning their lacks of the directional electron transfer pathways. Because of the synthetic difficulties, very few conductive polymers can form a 0D/1D/2D structure, as listed in **Table 2**.

The hierarchical nanosphere with PPy (denoted PPy-HNS) has the hierarchical nanospherical structure with an average diameter of 100–200 nm, as shown in **Figure 7(a)** [44]. The PPy-HNS has the following photovoltaic parameters: a VOC of 0.70 V, a JSC of 16.49 mA cm<sup>−</sup><sup>2</sup> , a FF of 0.58, and an *η* of 6.71%. The nanopatterning process is one of the methods to obtain the specific structure [42]. The nanopatterned PEDOT CE shows uniform hole patterns with ~100 nm diameter (**Figure 7(b)**) and exhibits an *η* of 6.71%. Regardless of the nanoparticle or nanosize hole, their corresponding *η* values are still lower than the *η* of Pt CE. Therefore, there are other structures, which were synthesized to overcome the challenge.

The flexible PPy membrane is composed of nanotubes that are about 50 nm in diameter, as shown in **Figure 7(c)** [38]. The paper-like PPy membranes exhibit

#### **Figure 6.**

*The molecular structures of (a) PEDOT, (b) PEDOT:PSS [30, 43, 48], (c) PEDOT-MeOH [46], (d) PProDOT [85, 86], (e) PProDOT-Et2 [85–87], (f) PProDOT-Me2 [85], (g) PT [33], (h) s-PT [48], (i) PANI [34], and (j) polypyrrole (PPy) [31, 38, 39, 44, 48].*

**85**

**Figure 7.**

*Structural Engineering on Pt-Free Electrocatalysts for Dye-Sensitized Solar Cells*

*ηs* of 5.27%, which is about 84% of the cell performance with a conventional Pt/ FTO CE (6.25%). Usually, the rod structure of PEDOT is obtained by a template method. The PEDOT-MeOH tube-coral array is synthesized by a template-free and bottom-up electropolymerization technique [46]. The PEDOT-MeOH TCA shows three advantages: (1) an enhanced conjugation of the PEDOT main chain due to the electron-donating MeOH group, (2) fast one-dimensional charge transfer routes, and (3) extended electroactive sites. The PEDOT-MeOH TCA has a highly porous surface with an average length of 5 mm and an average diameter of 500 nm, as shown in **Figure 7(d)**. Besides, PEDOT-MeOH TCA has vertically grown on their FTO substrates. The PEDOT-MeOH TCA CE, the co-existence of the 1D charge transfer pathways, and large active surface area on the PEDOT-MeOH TCA give its DSSC an *η* of 9.13%, which is higher than the Pt CE (8.94%). This performance is rarely found in pure conductive polymer materials. The patterned PEDOT with rod is obtained by the nanopatterning procedure. It has the height of 67 nm and width of 100 nm, as shown in **Figure 7(e)** [35]. The DSSCs of patterned PEDOT exhibits an *η* of 8.10%, which is close to Pt CE (8.26%). The prickly polyaniline nanorods (PPNR) display a prickly nanorod structure with the diameter of ~80 nm and the length of several micrometers, as shown in **Figure 7(f )**. The PPNR CE exhibits an *η* of 6.86%. The ultrathin polypyrrole nanosheets (UPNSs) have a nanoscale thickness of 50 nm and sheet morphologies by using a sodium decylsulfonate template,

*A partial list of literature on the DSSCs with conductive polymer material-based CEs.*

**Materials** *η* **(%)** *η* **of Pt (%) Structure Ref** PPy 6.71 7.47 Hierarchical nanosphere [44] PEDOT 7.10 7.60 Nanosized hole [42] PPy 5.27 6.25 Nanotubes [38] PEDOT-MeOH 9.13 8.94 Tube-coral array [46] PEDOT 8.10 8.26 Nanorod [35] PANI 6.86 7.21 Nanorod [34] PPy 6.80 7.80 Nanosheet [39]

as shown in **Figure 7(g)**. The DSSC using UPNS CE shows an *η* of 6.80%.

*The structures of conductive polymer materials include (a) hierarchical nanosphere of PPy [44], (b) nanosized hole of PEDOT [42], (c) nanotube of PPy [38], (d) hollow tubular structure of PEDOT-MeOH [46],*

*(e) nanorod of PEDOT [35], (f) nanorod of PANI [34], and (g) nanosheet of PPy [39].*

*DOI: http://dx.doi.org/10.5772/intechopen.85307*

**Table 2.**


*Structural Engineering on Pt-Free Electrocatalysts for Dye-Sensitized Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.85307*

**Table 2.**

*Nanostructures*

**2.2 Conductive polymer materials**

structure, as listed in **Table 2**.

of 0.70 V, a JSC of 16.49 mA cm<sup>−</sup><sup>2</sup>

the aggregations among the MWCNTs or among the rGOs. Thus, the MWCNT@ rGO rendered its DSSC an *η* of 6.91%, which is close to the Pt-based cell (7.26%).

Since 2000, the conductive polymer material has been discovered by Shirakawa, MacDiarmid, and Heeger [29]. Conductive polymers have attracted much attention as DSSC CEs owing to their excellent conductivity, good adhesion to the substrate, easy fabrication, light-weight, and good accessibility in terms of roll-to-roll processing. Common conductive polymers include poly(3,4-ethylenedioxythiophene) (PEDOT) [30, 35, 36, 40–43], poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) [30, 43, 48], poly(hydroxymethyl 3,4-ethylenedioxythiophene) (PEDOT-MeOH) [46], poly(3,4-propylenedioxythiophene) (PProDOT) [85, 86], poly(3,3-diethyl-3,4-dihydro-2H-thieno-[3,4-b][1,4]dioxepine) (PProDOT-Et2) [85–87], poly(2,2-dimethyl-3,4-propylenedioxythiophene) (PProDOT-Me2) [85], polythiophene (PT) [33], sulfonated poly(thiophene-3-[2- (2-methoxyethoxy) ethoxy]-2,5-diyl) (s-PT) [48], polyaniline (PANI) [34], and polypyrrole (PPy) [31, 38, 39, 44, 48], and their molecular structures are shown in **Figure 6**. However, conductive polymers often showed a flat or a mesoporous structure, meaning their lacks of the directional electron transfer pathways. Because of the synthetic difficulties, very few conductive polymers can form a 0D/1D/2D

The hierarchical nanosphere with PPy (denoted PPy-HNS) has the hierarchical nanospherical structure with an average diameter of 100–200 nm, as shown in **Figure 7(a)** [44]. The PPy-HNS has the following photovoltaic parameters: a VOC

terning process is one of the methods to obtain the specific structure [42]. The nanopatterned PEDOT CE shows uniform hole patterns with ~100 nm diameter (**Figure 7(b)**) and exhibits an *η* of 6.71%. Regardless of the nanoparticle or nanosize hole, their corresponding *η* values are still lower than the *η* of Pt CE. Therefore, there are other structures, which were synthesized to overcome the challenge. The flexible PPy membrane is composed of nanotubes that are about 50 nm in diameter, as shown in **Figure 7(c)** [38]. The paper-like PPy membranes exhibit

*The molecular structures of (a) PEDOT, (b) PEDOT:PSS [30, 43, 48], (c) PEDOT-MeOH [46], (d) PProDOT [85, 86], (e) PProDOT-Et2 [85–87], (f) PProDOT-Me2 [85], (g) PT [33], (h) s-PT [48],* 

*(i) PANI [34], and (j) polypyrrole (PPy) [31, 38, 39, 44, 48].*

, a FF of 0.58, and an *η* of 6.71%. The nanopat-

**84**

**Figure 6.**

*A partial list of literature on the DSSCs with conductive polymer material-based CEs.*

*ηs* of 5.27%, which is about 84% of the cell performance with a conventional Pt/ FTO CE (6.25%). Usually, the rod structure of PEDOT is obtained by a template method. The PEDOT-MeOH tube-coral array is synthesized by a template-free and bottom-up electropolymerization technique [46]. The PEDOT-MeOH TCA shows three advantages: (1) an enhanced conjugation of the PEDOT main chain due to the electron-donating MeOH group, (2) fast one-dimensional charge transfer routes, and (3) extended electroactive sites. The PEDOT-MeOH TCA has a highly porous surface with an average length of 5 mm and an average diameter of 500 nm, as shown in **Figure 7(d)**. Besides, PEDOT-MeOH TCA has vertically grown on their FTO substrates. The PEDOT-MeOH TCA CE, the co-existence of the 1D charge transfer pathways, and large active surface area on the PEDOT-MeOH TCA give its DSSC an *η* of 9.13%, which is higher than the Pt CE (8.94%). This performance is rarely found in pure conductive polymer materials. The patterned PEDOT with rod is obtained by the nanopatterning procedure. It has the height of 67 nm and width of 100 nm, as shown in **Figure 7(e)** [35]. The DSSCs of patterned PEDOT exhibits an *η* of 8.10%, which is close to Pt CE (8.26%). The prickly polyaniline nanorods (PPNR) display a prickly nanorod structure with the diameter of ~80 nm and the length of several micrometers, as shown in **Figure 7(f )**. The PPNR CE exhibits an *η* of 6.86%. The ultrathin polypyrrole nanosheets (UPNSs) have a nanoscale thickness of 50 nm and sheet morphologies by using a sodium decylsulfonate template, as shown in **Figure 7(g)**. The DSSC using UPNS CE shows an *η* of 6.80%.

#### **Figure 7.**

*The structures of conductive polymer materials include (a) hierarchical nanosphere of PPy [44], (b) nanosized hole of PEDOT [42], (c) nanotube of PPy [38], (d) hollow tubular structure of PEDOT-MeOH [46], (e) nanorod of PEDOT [35], (f) nanorod of PANI [34], and (g) nanosheet of PPy [39].*
