**1. Introduction**

As the third-generation solar cells, dye-sensitized solar cell (DSSC) is one of the most promising alternatives to the silicon solar cells, due to their simple assembly procedure, good plasticity, transparency, mechanical robustness, ability to work at wider angles, and in low light and environmental friendliness [1, 2]. After a significant breakthrough in the photoelectric conversion efficiency (7.1–7.9%) of DSSCs in 1991, through introducing mesoporous film of

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

TiO2 nanocrystalline to adsorb dye instead of planar semiconductor electrode by O'Regan and Grätzel research group [3], DSSCs have stimulated a great research interest over the following 25 years and attained an efficiency of ca. 14% [4, 5].

explained in terms of current density *Jo*

efficiency, *η*, which is given by Eq. (2):

toward its theoretical maximum.

high catalytic activity, high stability in I−

causes a problem for its large-scale production.

*<sup>η</sup>* <sup>=</sup> *<sup>J</sup>*

*J*

(*Rct*) by Eq. (1):

where *J*

which is calculated from the charge transfer resistance

http://dx.doi.org/10.5772/intechopen.75398

redox shuttle, and better performance than the Pt

(1)

61

(2)

*<sup>o</sup>* <sup>=</sup> \_\_\_\_\_ *RT nFRct*

Porous Carbon Materials as Supreme Metal-Free Counter Electrode for Dye-Sensitized Solar Cells

where *R*, *T*, *n*, and *F* are the gas constant, the temperature, the number of electrons transferred

The overall performance of solar cell is evaluated by the solar-to-electrical energy conversion

*SC VOC FF* \_\_\_\_\_\_\_ *Pin*

As low-cost and environmentally friendly materials, porous carbon has high surface area,

CEs. The goal of this chapter is to discuss about the synthesis, characterization, and photovol-

As counter electrode (CE) is one of the most crucial components regulating the efficiency of DSSCs by catalyzing the reduction of the redox couples used as mediators to regenerate the sensitizer after electron injection, it is important to find a low-cost, high-efficiency, easy scalability, and corrosion-stable counter electrode. Platinum (Pt) has been widely employed as the standard CE in DSSCs due to its high catalytic reduction for redox shuttles, good chemical stability, and high conductivity. However, Pt is an expensive and scarce noble metal, which

Carbon materials are one of the promising substitutes of Pt CE due to their low cost, environmentally friendly, scale availability, high surface area, high catalytic activity, high electrical conductivity, high thermal stability, good corrosion resistance toward iodine, high reactivity for triiodide reduction, etc. In 1996, Kay and Grätzel first explored graphite-carbon black mixture as CE and achieved a power conversion efficiency of 6.7% [8]. Thereafter, intensive research efforts have been focused on carbonaceous materials, such as carbon black, mesoporous carbon, graphite [9], graphene [10, 11], carbon nanotubes [12–15], and carbon nanofibers

/I3 −

**2. Porous carbon materials as counter electrodes for DSSCs**

[16–18], and they have been successfully employed as counter electrodes.

taic performance of porous carbon materials as supreme counter electrode for DSSCs.

*sc* is the short-circuit current, *Voc* is the open-circuit voltage, FF is the fill factor, and *Pin* is the incident light intensity. FF depends on the charge transfer resistance, on series resistance, as well as on the overvoltage for diffusion and electron transfer. Low charge transfer resistance, series resistance, and overvoltage for diffusion and electron transfer lead to a higher FF value, thus resulting in greater efficiency and pushing the output power of the solar cell closer

in the elementary electrode reaction (*n* = 2), and the Faradays constant, respectively.

The working principle of DSSCs is not similar to conventional solar cell, but it is similar to natural photosynthesis process where light absorption and charge carrier transportation have different substances, as shown in **Figure 1**. DSSCs consist of three important components: dye-coated TiO2 film, counter electrode (CE), and electrolyte (or redox shuttle). In the DSSCs, after photoexcitation of sensitized dye, electrons from the LUMO level of dye molecule are injected into the conduction band of semiconductor metal oxide. Then, electrons transport to the anode and flow to the counter electrode (CE) via an external circuit. Finally, mediator electrolyte through reduction and oxidation carries electrons from counter electrode to the HOMO level of dye molecules, and dye is regenerated. This cycle is repeated again and again, and the device generates electric power continuously.

In this overall process of electron transfer, counter electrode plays a significant role on the photovoltaic parameters of DSSCs. The theoretical maximum photovoltage or open-circuit voltage of the DSSCs is higher than the output voltage after loading. This voltage loss is due to the mass transfer overpotential and the kinetic overpotential or charge transfer over potential. The former is mainly attributed to the ionic conductivity of electrolytes and the transportation of mediator species from the CE to the photoanode, whereas the latter is from the electrocatalytic activity of the CE surface toward mediator [6, 7]. The catalytic activity of the CEs can be

**Figure 1.** Schematic representation of the working principle of DSSCs.

explained in terms of current density *Jo* which is calculated from the charge transfer resistance (*Rct*) by Eq. (1):

TiO2

dye-coated TiO2

60 Emerging Solar Energy Materials

25 years and attained an efficiency of ca. 14% [4, 5].

and the device generates electric power continuously.

**Figure 1.** Schematic representation of the working principle of DSSCs.

 nanocrystalline to adsorb dye instead of planar semiconductor electrode by O'Regan and Grätzel research group [3], DSSCs have stimulated a great research interest over the following

film, counter electrode (CE), and electrolyte (or redox shuttle). In the DSSCs,

The working principle of DSSCs is not similar to conventional solar cell, but it is similar to natural photosynthesis process where light absorption and charge carrier transportation have different substances, as shown in **Figure 1**. DSSCs consist of three important components:

after photoexcitation of sensitized dye, electrons from the LUMO level of dye molecule are injected into the conduction band of semiconductor metal oxide. Then, electrons transport to the anode and flow to the counter electrode (CE) via an external circuit. Finally, mediator electrolyte through reduction and oxidation carries electrons from counter electrode to the HOMO level of dye molecules, and dye is regenerated. This cycle is repeated again and again,

In this overall process of electron transfer, counter electrode plays a significant role on the photovoltaic parameters of DSSCs. The theoretical maximum photovoltage or open-circuit voltage of the DSSCs is higher than the output voltage after loading. This voltage loss is due to the mass transfer overpotential and the kinetic overpotential or charge transfer over potential. The former is mainly attributed to the ionic conductivity of electrolytes and the transportation of mediator species from the CE to the photoanode, whereas the latter is from the electrocatalytic activity of the CE surface toward mediator [6, 7]. The catalytic activity of the CEs can be

$$J\_o = \frac{RT}{nFR\_{ct}}\tag{1}$$

where *R*, *T*, *n*, and *F* are the gas constant, the temperature, the number of electrons transferred in the elementary electrode reaction (*n* = 2), and the Faradays constant, respectively.

The overall performance of solar cell is evaluated by the solar-to-electrical energy conversion efficiency, *η*, which is given by Eq. (2):

$$\eta = \frac{J\_{\rm sc} V\_{\rm oc} \, FF}{P\_{\rm in}} \tag{2}$$

where *J sc* is the short-circuit current, *Voc* is the open-circuit voltage, FF is the fill factor, and *Pin* is the incident light intensity. FF depends on the charge transfer resistance, on series resistance, as well as on the overvoltage for diffusion and electron transfer. Low charge transfer resistance, series resistance, and overvoltage for diffusion and electron transfer lead to a higher FF value, thus resulting in greater efficiency and pushing the output power of the solar cell closer toward its theoretical maximum.

As low-cost and environmentally friendly materials, porous carbon has high surface area, high catalytic activity, high stability in I− /I3 − redox shuttle, and better performance than the Pt CEs. The goal of this chapter is to discuss about the synthesis, characterization, and photovoltaic performance of porous carbon materials as supreme counter electrode for DSSCs.
