**2. Dye-sensitized solar cells**

The structure and working principle of DSSCs are presented in **Figure 1**.

An electrode is a solid electrical conductor through which an electrical current enters or leaves an electrochemical cell or other mediums like metallic solids, liquids, gases, plasmas, or vacuums. Electrodes are usually made of good electrical conducting materials. In an electrochemical cell, an electrode is referred to as either an anode or a cathode. The anode is defined as the electrode at which electrons leave the cell and the oxidation occurs, while the cathode is defined as the electrode at

**295**

*Graphene-Based Material for Fabrication of Electrodes in Dye-Sensitized Solar Cells*

which electrons enter the cell and reduction occurs. For DSSCs, the anode is the light collector, thus it is also called photoanode. The anode had a deposited layer of a metal oxide semiconductor. The cathode is the electrode on which Pt and other conducting materials are deposited. The cathode in DSSCs is often called the counter electrode. In 1991, Grätzel first introduced the term "counter electrode" in

A photoanode is typically a layer of nanocrystalline titanium dioxide (TiO2), with a thickness of about 10 μm, coated on a transparent conductive oxide (TCO) glass substrate, such as an indium-doped tin oxide (ITO) or fluorine-doped tin oxide (FTO). Morphologies of TiO2 materials (rod, spherical, hierarchical, and tubular) significantly affect the light-harvesting, charge injection, and charge-

Dye-sensitizer plays a crucial role in improving light absorption within the visible region. Dye-sensitizer is described as an electron pump with inputting power from light photons. When the light irradiates the photoanode of DSSCs, electrons from the highest occupied molecular orbital (HOMO) of dye-sensitizer transfer to the lowest unoccupied molecular orbital (LUMO), then the electrons flow to the photoanode. Many types of dye-sensitizer have been used in DSSCs, such as ruthenium dyes and natural dyes. Dye N719 is a ruthenium-based dye that is used

After producing electrons, dye-sensitizer becomes oxidized states, and electrons need a medium to transfer back to dye-sensitizer and complete the external circuit of DSSCs. In DSSCs, electrolyte plays a role as a shuttle, which transfers electrons from cathode back to dye-sensitizer. A good electrolyte system used in DSSCs must have excellent electrical conductivity, low viscosity for easier and faster diffusion of electrons, and good interfacial contact with the nanocrystalline semiconductor of photoanode and cathode. The most popular electrolyte system used in DSSCs is

The last main component of a DSSC is the cathode. A typical cathode of DSSCs

is thin Pt layer coated on the TCO (typically FTO). At the interface between the Pt layer and the electrolyte, a reduction reaction of electrolyte occurs. This

his pioneering publication about dye-sensitized solar cells [10, 11].

collecting properties of DSSCs [12, 13].

/I3 −

) [10, 11].

widely in DSSCs [10, 11].

**Figure 1.** *Structure of DSSCs.*

iodide/triiodide (I−

*DOI: http://dx.doi.org/10.5772/intechopen.93637*

*Graphene-Based Material for Fabrication of Electrodes in Dye-Sensitized Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.93637*

**Figure 1.** *Structure of DSSCs.*

*Solar Cells - Theory, Materials and Recent Advances*

graphene oxide (ZnO/rGO) anodes.

and structure of these materials.

**2. Dye-sensitized solar cells**

DSSCs [3].

Recently, cathodes of DSSCs are fabricated from platinum (Pt). Because Pt is a noble and expensive metal, the usage of Pt for fabrication of cathodes could lead to the increase in the production cost of DSSCs. Hence, many efforts had been made for the reduction of cathode fabrication costs to reduce the production costs of

Many research studies have been conducted for replacing Pt in cathodes by using other materials in combination with Pt. According to previous studies, copious low-cost materials were studied for fabrication of cathodes in DSSCs, such as carbonaceous materials, conductive polymers, alloys, metal oxide, transition metal-based materials including metal sulfides, metal carbides, metal nitrides, etc. [4]. Among these materials, carbon-based materials such as carbon vulcan, carbon black, activated carbon, carbon nanofibers, carbon nanotubes, and graphene have attracted more attention from researchers due to the relatively low cost, high stability, high chemical inertness, and high catalytic behavior [5]. In comparison with other carbon-based materials, graphene showed better properties, such as: having the highest electrical conductivity, fast charged carrier mobility, good chemical stability, and high surface area. These properties make graphene one of the most potential materials for fabrication of cathodes in DSSCs [6, 7]. Graphene can be synthesized from graphene oxide (GO) using the chemical reduction method, in which the synthesized graphene product is known as reduced graphene oxide (rGO). Numerous studies combined Pt and graphene for fabricating solar cells, fuel cells, and for other catalytic applications [8, 9]. In DSSCs, platinum/reduced graphene oxide (Pt/rGO) composite has been widely used for fabrication of cathode. For anodes, the electron recombination processes in anode material (ZnO and TiO2) were a phenomenon that decreased the efficiency of DSSCs. Due to the high electron mobility, lower recombination rate, electron lifetime is considerably higher in ZnO as compared to TiO2, good transparency to visible light, high photo activity and nanocrystalline ZnO of varying morphologies, ZnO is considered as a potential material for fabrication of anodes in DSSCs. In order to increase the efficiency of ZnO-based DSSCs, graphene was studied for combination with ZnO, which could reduce the electron recombination processes of anodes. Numerous efforts have been made to investigate the performance of DSSCs fabricated from zinc oxide/reduced

In order to emphasize the potential of graphene as a promising material for fabrication of cathodes and anodes in DSSCs, this chapter is aiming to provide an overview on the current issues of DSSCs that need to be improved and the recently studied materials for fabrication of electrodes in DSSCs, especially carbonaceous materials. Subsequently, the synthesis of Pt/rGO and ZnO/rGO composite materials and the effect of synthesized materials on the performance of fabricated DSSCs are discussed. Additionally, the characterization results of Pt/rGO and ZnO which were synthesized by our group were also presented to illustrate the morphologies

The structure and working principle of DSSCs are presented in **Figure 1**. An electrode is a solid electrical conductor through which an electrical current enters or leaves an electrochemical cell or other mediums like metallic solids, liquids, gases, plasmas, or vacuums. Electrodes are usually made of good electrical conducting materials. In an electrochemical cell, an electrode is referred to as either an anode or a cathode. The anode is defined as the electrode at which electrons leave the cell and the oxidation occurs, while the cathode is defined as the electrode at

**294**

which electrons enter the cell and reduction occurs. For DSSCs, the anode is the light collector, thus it is also called photoanode. The anode had a deposited layer of a metal oxide semiconductor. The cathode is the electrode on which Pt and other conducting materials are deposited. The cathode in DSSCs is often called the counter electrode. In 1991, Grätzel first introduced the term "counter electrode" in his pioneering publication about dye-sensitized solar cells [10, 11].

A photoanode is typically a layer of nanocrystalline titanium dioxide (TiO2), with a thickness of about 10 μm, coated on a transparent conductive oxide (TCO) glass substrate, such as an indium-doped tin oxide (ITO) or fluorine-doped tin oxide (FTO). Morphologies of TiO2 materials (rod, spherical, hierarchical, and tubular) significantly affect the light-harvesting, charge injection, and chargecollecting properties of DSSCs [12, 13].

Dye-sensitizer plays a crucial role in improving light absorption within the visible region. Dye-sensitizer is described as an electron pump with inputting power from light photons. When the light irradiates the photoanode of DSSCs, electrons from the highest occupied molecular orbital (HOMO) of dye-sensitizer transfer to the lowest unoccupied molecular orbital (LUMO), then the electrons flow to the photoanode. Many types of dye-sensitizer have been used in DSSCs, such as ruthenium dyes and natural dyes. Dye N719 is a ruthenium-based dye that is used widely in DSSCs [10, 11].

After producing electrons, dye-sensitizer becomes oxidized states, and electrons need a medium to transfer back to dye-sensitizer and complete the external circuit of DSSCs. In DSSCs, electrolyte plays a role as a shuttle, which transfers electrons from cathode back to dye-sensitizer. A good electrolyte system used in DSSCs must have excellent electrical conductivity, low viscosity for easier and faster diffusion of electrons, and good interfacial contact with the nanocrystalline semiconductor of photoanode and cathode. The most popular electrolyte system used in DSSCs is iodide/triiodide (I− /I3 − ) [10, 11].

The last main component of a DSSC is the cathode. A typical cathode of DSSCs is thin Pt layer coated on the TCO (typically FTO). At the interface between the Pt layer and the electrolyte, a reduction reaction of electrolyte occurs. This

#### *Solar Cells - Theory, Materials and Recent Advances*

phenomenon happens when electrons move from external loads to the cathode of DSSCs. The Pt film has three main roles in a DSSC: a catalyst for redox reaction of electrolyte, an electrolyte regenerator, and a conducting material [10, 11].

The working principle of DSSCs is different from that of conventional siliconbased solar cells. In silicon-based solar cells, the semiconductor p-n junction carried out many processes, both absorbing light and sending out current. In DSSCs, those tasks are separate. The main role of the semiconductor is to transfer electron from dye-sensitizer to FTO glass, whereas electron suppliers are the dye-sensitizer [10, 11].

The light illuminates the photoanode of the DSSCs then dye-sensitizer will absorb appropriate wavelengths and turn into excited states S\* . In excited states, the dye releases electrons that will diffuse into the conduction band of semiconductor TiO2 and reach FTO glass. The exciting state of sensitizers is shown in Eq. (1).

$$h\nu + \mathbf{S} \to \mathbf{S}\* + \mathbf{e} \tag{1}$$

After that, electrons move to external loads and get back to the DSSCs at the cathode. Dye-sensitizer molecules turn into oxidized states S<sup>+</sup> after providing electron. Subsequently, those molecules are reduced by I− in the electrolyte to restore to initial states and electrolyte changes from the oxidized state I− to the reduced state I3 − as a result of reduction reactions of dye-sensitizers, as shown in Eq. (2).

$$\text{2gI}^{-} + \text{2S}^{\*} \rightarrow \text{I}\_{\text{g}}^{-} + \text{2S} \tag{2}$$

At the cathode, the reduced state I3 − receives electrons coming back to the DSSCs from the external load and restores to I<sup>−</sup> . Redox mediators move back and forth between cathode and anode mainly relying on the diffusion process. The process is shown in Eq. (3).

$$\rm I\_{\mathfrak{z}}^{-} + 2\mathbf{e} \rightarrow \mathfrak{z}\mathbf{I}^{\cdot} \tag{3}$$

The above processes take place when DSSCs are illuminated continuously. Then a current is generated and flow in the external load. However, electrons in the conduction band of TiO2 may follow three other routes to join in recombination reactions, as shown in Eqs. (4)–(6).

$$\mathbf{e}\_{\text{TOT}} + \mathbf{S}^\* \to \mathbf{S} \tag{4}$$

$$\mathbf{e}\_{\text{TOT2}} + \mathbf{I}\_{\text{g}}^{-} \to \mathbf{y} \mathbf{I}^{-} \tag{5}$$

$$\mathbf{e}\_{\text{SnOz}} + \mathbf{I}\_{\text{g}}^{-} \to \mathbf{y} \mathbf{I}^{-} \tag{6}$$

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*Graphene-Based Material for Fabrication of Electrodes in Dye-Sensitized Solar Cells*

make the external current deteriorate and cause declines in voltage of DSSCs, which

Many efforts have been made for commercialization of DSSCs, including the investigation to enhance the efficiency and reduce the production cost of DSSCs, especially the research on cathodes and anodes. Various materials have been studied for the fabrication of electrodes in DSSCs: carbonaceous materials, conductive polymers, alloys, metal oxide, transition metal-based materials, etc. Among these materials, graphene is one of the most prospective materials for the synthesis of

Carbon has long been an intriguing material because it had two allotropes which were widely known: diamond and graphite. Although these materials have the same elemental composition, the properties of graphite are very different from those of diamond. Although researchers working on carbon have long been aware of other forms, usually they were regarded as a nuisance and discarded if their formation could not be avoided. The importance of the recent "discoveries" of fullerenes and carbon nanotubes resides in the fact that their structures were elucidated for the first time. The most recent "discovery" is that of graphene, it is simply one of the many parallel sheets constituting graphite. The ingenuity resided in the preparation of a single isolated sheet, which opened the possibility of examining experimentally

Carbon-based materials have shown great versatility due to the ability to chemi-

Graphite is one of the most common allotropes of carbon, and the most stable form of carbon under standard conditions. Graphite is another promising electrode electrocatalyst and conducting layer material due to the abundant defect sites and good electronic properties. Particularly, the defect sites from edge planes are preferred to those of basal planes, as the former exhibits the faster electrons transport

The three new materials, graphene, carbon nanotubes and fullerenes, can be called "nanocarbon" materials. Like graphite, the structures of these carbonaceous

character. Fullerenes are spheroidal molecules and are made exclusively of carbon atoms (e.g. C60, C70). Their unique hollow cage-like shape and structural analogy with clathrin-coated vesicles in cells support the idea of the potential use of fullerenes as drug or gene delivery agents. Fullerenes display a diverse range of biological activity, which arises from their reactivity, due to the presence of double bonds and

act either as acceptors or donors of electrons. When irradiated with ultraviolet or visible light, fullerenes can convert molecular oxygen into highly reactive singlet oxygen. Thus, fullerenes have the potential to inflict photodynamic damage on biological systems, including damage to cellular membranes, inhibition of various enzymes [18].

orbitals. Fullerenes contain 12 pentagons and have some sp3

hybridized carbon atoms, which produces angle strain. Fullerenes can

cally combine with other carbon-based materials and with a range of different elements to form strong covalent bonds. As a result, these materials exhibit excellent characteristics such as high strength, high density, and high hardness. Their research, development and innovation are taking place in various fields, and studies employing the development of carbon-based materials have shown many positive results for a wide variety of structures, which has allowed the development of

*DOI: http://dx.doi.org/10.5772/intechopen.93637*

**3.1 Carbonaceous materials**

lead to low power conversion efficiency [10, 11].

composites for fabrication of electrodes in DSSCs [14, 15].

**3. Graphene for fabrication of electrodes in DSSCs**

what was already a well-studied material theoretically [16].

several materials with different applications [17].

and charge transfer [18].

materials consist of sp2

bending of sp2

Eq. (6) takes place on the interface between FTO and electrolyte, where TiO2 does not cover. The electron transfer from FTO to the external load is significantly faster than that of Eq. (6). Eq. (5) occurs more often than other ones because the concentration of I3 − is much higher than that of S<sup>+</sup> . These recombination reactions

### *Graphene-Based Material for Fabrication of Electrodes in Dye-Sensitized Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.93637*

make the external current deteriorate and cause declines in voltage of DSSCs, which lead to low power conversion efficiency [10, 11].

Many efforts have been made for commercialization of DSSCs, including the investigation to enhance the efficiency and reduce the production cost of DSSCs, especially the research on cathodes and anodes. Various materials have been studied for the fabrication of electrodes in DSSCs: carbonaceous materials, conductive polymers, alloys, metal oxide, transition metal-based materials, etc. Among these materials, graphene is one of the most prospective materials for the synthesis of composites for fabrication of electrodes in DSSCs [14, 15].
