**1. Introduction**

In this century, the energy requisition and environment caring arrive at the highest point in history. The clean and economical renewable energy resource is urgently needed for us. Photovoltaics, named solar cells, tremendous progress has been achieved in efficiency (*η*), reproducibility, and stability [1–3]. It has been considered as one of the most promising renewable energy sources. Photovoltaics are classified to three generations, as shown in **Figure 1** [1, 4–7]. The first-generation solar cells, named silicon-based solar cells or the traditional solar cells, made up of crystalline silicon. These solar cells demonstrate high efficiency and significant demand in the market, but the production cost of crystalline silicon materials limited the large-scale industrial applications. The second-generation is cadmium telluride (CdTe)/cadmium indium gallium diselenide (CIGS) based solar cells. The solar cells could be produced with large-scale and well efficiency (14–22%). The first and second generations are the most widely solar cells at present. However, they are scarcity, the toxicity of materials, high-temperature, and high-vacuum processes

**350**

2010.

2017.

p. 154502, 2014.

*Solar Cells - Theory, Materials and Recent Advances*

[12] E. L. I. a. U. R. A. A. Kiselev, "Electron g factor in one- and zero-dimensional semiconductor nanostructures," *Phys. Rev. B ,* p. 6353 ,

[13] M. Dyakonov and V. Perel, "SPIN ORIENTATION OF ELECTRONS ASSOCIATED WITH THE INTERBAND," *Sov. Phys. JETP 33,*

[14] A. V. K. a. Y. V. Nazarov, "Spin-flip transitions between Zeeman sublevels in semiconductor quantum dots," *Phys. Rev.* 

1998.

p. 1053, 1971.

*B 64,* p. 125316, 2001.

[1] A. Kojima et al, "Organometalic halide perovskites as visible-light sensitize for photovoltaic," *J. Am. Chem.* 

[2] J. C. H. M. A. Loi, "Hybrid solar cells – perovskites under the Sun," *Nature* 

[3] P. M. H. J. Li, "Circular photovoltaic effect in organometal halide perovskite CH3NH3PbI3," *arXiv:,* p. 1607, 2016.

[4] M. Nakamura et al, "Spontaneous polarization and bulk photovoltaic effect driven by polar," *Phys. Rev. Lett,*

[5] C. M. e. al, "Revealing the role of organic cations in hybrid halide perovskite CH3NH3PbI3," *Nature Communications,* p. 6:7026, 2015.

[6] K. T. Y. S. a. T. M. Akihiro Kojima, "Organometal halide perovskites as visible-light sensitizers for photovoltaic cells," *J. Am. Chem. Soc.,* p. 131, 2009.

[7] T. D. H. S. K. e. a. Chiho Kim, "A hybrid organic-inorganic perovskite dataset," *Scient. Data 4,,* p. 170057 ,

[8] J. O.-M. a. W. R. Calderon-Munoz, "Thermal influence on charge carrier transport in solar cells based on GaAs PN junctions," *Journ. of Appl. Phys.,*

[9] C. C. S. M. G. K. e. a. Pierre-Adrien Mante, "Directional Negative Thermal Expansion and Large Poisson Ratio," *J. Phys. Chem. Lett,* p. 14398, 2017.

[10] M. C. B. J. M. L. M. L. R. J. E. a. J. C. J. A. J. Nozik, *Chem. Rev. 110,* p. 6873,

[11] G. Dresselhaus, "Spin-Orbit Coupling Effects in Zinc Blende Structures," *Phys.Rev. ,* p. 580, 1955.

*Soc,* p. 131, 2009.

**References**

p. 156801, 2016.

*Materials 12,* p. 1087, 2013.

#### **Figure 1.**

*The scheme of three generation photovoltaic solar cells.*

that restrict further applications. Dye-sensitized solar cells (DSSCs), classed thirdgeneration solar cells, have gained attention and be regarded as prospective solar cells for the photovoltaic technologies in recent years as potential cost-effective alternatives to the first and second generations solar cells [8–11]. Furthermore, the DSSCs have outstanding performance in an indoor, dim light environment [12–14].

Typically, DSSCs are consist of three sections, including photoanode, electrolyte, and counter electrode (CE), that respond to different functions, as shown in **Figure 2** [1, 4, 5, 8–10]. The photoanode converts the photon into the electron by the dye. The electrolyte keeps the function of the photoanode by iodine ion. The CE catalyzes the redox reduction in the electrolyte, which is an obvious influence on the photovoltaic performance, long-term stability, and cost of the device. In other words, the CE is a crucial component of DSSCs.

The CE is classified into three components, that are electrocatalyst, transparent conducting oxide, and substrate, as shown in **Figure 2**. Among them, the electrocatalyst is the key factor to promise the function of CE [1, 7–9, 15, 16]. As shown in **Figure 3**, between electrolyte and CE, the reaction of reduction iodide/ triiodide (I<sup>−</sup> /I3 − ) redox couple is that: The first stage, diffusion, triiodide diffuses from electrolyte bulk to near the CE for regenerating electrolyte. The second stage, decomposition, triiodide decomposes to iodide and iodine. The iodide is used to renew the dye and iodine will go to the next step. The third stage, adsorption, the CE adsorbs iodine near the CE. The fourth stage, electrocatalysis, electrocatalyst catalyzes reduction reaction, transferring iodine to iodide. The final stage, desorption, the CE desorbs iodide to complete regenerate the electrolyte. According to this mechanism, the electrocatalytic ability, it also represents the reaction rate in here, and the specific structure are the major affections for the reduction reaction.

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**Figure 4.**

*nanostructure.*

I3 −

**Figure 3.**

*The scheme of reduction iodide/triiodide (I−*

*Nanostructured Transition Metal Compounds as Highly Efficient Electrocatalysts…*

The traditional electrocatalyst of DSSCs is Platinum (Pt), which has an outstanding electrocatalytic ability [10, 15–20]. However, Pt, noble metal, is rare on earth that present expensive prices and difficult shapes the specific structure. Up to date, there are a few non-Pt nanomaterials that could have comparable electrocatalytic ability to that of Pt. There have two ways to raise the electrocatalytic reduction reaction. The intrinsic electrocatalytic ability of the electrocatalyst is directly related the electrocatalytic ability. In other words, the choice of material is very important. The other way is to design the nanostructure of the electrocatalyst for

*) redox couple in counter electrode.*

*/I3 −*

 reduction regarding with the charge transfer route and the surface area. Transition metal compounds (TMCs) possess d-electron filling in eg orbitals, which promote excellent electrocatalytic performance in partially filled condition [4, 19, 21–24]. So, they are interested to replace Pt. But most of TMCs still show poorer electrocatalytic ability than Pt. To overcome the challenge, TMCs are synthesized with various nanostructure, which is an important factor for increasing electrocatalytic ability [20–22, 25]. A nanostructure is defined if any dimension of the structure is lower than 100 nm, the structure is the nanostructure. Basically, nanostructure divides into four groups: zero-dimensional (0D, *e.g.* nanoparticle, nanocube, *etc.*), one-dimensional (1D, *e.g.* nanorod, nanotube, nanoneedle, *etc.*), two-dimensional (2D, *e.g.* nanosheet, nanopental *etc.*) and hierarchical nanostructures, as shown in **Figure 4**. In view of 1D, 2D, and hierarchical nanostructures have

*The scheme of zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), and hierarchical* 

*DOI: http://dx.doi.org/10.5772/intechopen.94021*

**Figure 2.** *The scheme of dye-sensitized solar cells and counter electrode (cathode).*

*Nanostructured Transition Metal Compounds as Highly Efficient Electrocatalysts… DOI: http://dx.doi.org/10.5772/intechopen.94021*

**Figure 3.** *The scheme of reduction iodide/triiodide (I− /I3 − ) redox couple in counter electrode.*

The traditional electrocatalyst of DSSCs is Platinum (Pt), which has an outstanding electrocatalytic ability [10, 15–20]. However, Pt, noble metal, is rare on earth that present expensive prices and difficult shapes the specific structure. Up to date, there are a few non-Pt nanomaterials that could have comparable electrocatalytic ability to that of Pt. There have two ways to raise the electrocatalytic reduction reaction. The intrinsic electrocatalytic ability of the electrocatalyst is directly related the electrocatalytic ability. In other words, the choice of material is very important. The other way is to design the nanostructure of the electrocatalyst for I3 − reduction regarding with the charge transfer route and the surface area.

Transition metal compounds (TMCs) possess d-electron filling in eg orbitals, which promote excellent electrocatalytic performance in partially filled condition [4, 19, 21–24]. So, they are interested to replace Pt. But most of TMCs still show poorer electrocatalytic ability than Pt. To overcome the challenge, TMCs are synthesized with various nanostructure, which is an important factor for increasing electrocatalytic ability [20–22, 25]. A nanostructure is defined if any dimension of the structure is lower than 100 nm, the structure is the nanostructure. Basically, nanostructure divides into four groups: zero-dimensional (0D, *e.g.* nanoparticle, nanocube, *etc.*), one-dimensional (1D, *e.g.* nanorod, nanotube, nanoneedle, *etc.*), two-dimensional (2D, *e.g.* nanosheet, nanopental *etc.*) and hierarchical nanostructures, as shown in **Figure 4**. In view of 1D, 2D, and hierarchical nanostructures have

#### **Figure 4.**

*The scheme of zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), and hierarchical nanostructure.*

*Solar Cells - Theory, Materials and Recent Advances*

*The scheme of three generation photovoltaic solar cells.*

words, the CE is a crucial component of DSSCs.

*The scheme of dye-sensitized solar cells and counter electrode (cathode).*

that restrict further applications. Dye-sensitized solar cells (DSSCs), classed thirdgeneration solar cells, have gained attention and be regarded as prospective solar cells for the photovoltaic technologies in recent years as potential cost-effective alternatives to the first and second generations solar cells [8–11]. Furthermore, the DSSCs have outstanding performance in an indoor, dim light environment [12–14]. Typically, DSSCs are consist of three sections, including photoanode, electrolyte, and counter electrode (CE), that respond to different functions, as shown in **Figure 2** [1, 4, 5, 8–10]. The photoanode converts the photon into the electron by the dye. The electrolyte keeps the function of the photoanode by iodine ion. The CE catalyzes the redox reduction in the electrolyte, which is an obvious influence on the photovoltaic performance, long-term stability, and cost of the device. In other

The CE is classified into three components, that are electrocatalyst, transparent conducting oxide, and substrate, as shown in **Figure 2**. Among them, the electrocatalyst is the key factor to promise the function of CE [1, 7–9, 15, 16]. As shown in **Figure 3**, between electrolyte and CE, the reaction of reduction iodide/

from electrolyte bulk to near the CE for regenerating electrolyte. The second stage, decomposition, triiodide decomposes to iodide and iodine. The iodide is used to renew the dye and iodine will go to the next step. The third stage, adsorption, the CE adsorbs iodine near the CE. The fourth stage, electrocatalysis, electrocatalyst catalyzes reduction reaction, transferring iodine to iodide. The final stage, desorption, the CE desorbs iodide to complete regenerate the electrolyte. According to this mechanism, the electrocatalytic ability, it also represents the reaction rate in here, and the specific structure are the major affections for the reduction reaction.

) redox couple is that: The first stage, diffusion, triiodide diffuses

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**Figure 2.**

triiodide (I<sup>−</sup>

**Figure 1.**

/I3 − complex structure. In this chapter, we will systematically discuss their plural strategies (including high electrochemical surface area, directional electron transferring pathways, decrease diffusion control, *etc.*) to promote the electrocatalytic ability for DSSCs performance.
