**3.1 Carbonaceous materials**

*Solar Cells - Theory, Materials and Recent Advances*

[10, 11].

I3 −

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

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

The light illuminates the photoanode of the DSSCs then dye-sensitizer will

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).

After that, electrons move to external loads and get back to the DSSCs at the

as a result of reduction reactions of dye-sensitizers, as shown in Eq. (2).

−

between cathode and anode mainly relying on the diffusion process. The process is

The above processes take place when DSSCs are illuminated continuously. Then

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

is much higher than that of S<sup>+</sup>

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

. In excited states, the

after providing elec-

to the reduced state

in the electrolyte to restore to

+ → ∗+ SSe (1)

<sup>3</sup> 3I 2S I 2S −+ − + →+ (2)

receives electrons coming back to the DSSCs

. Redox mediators move back and forth

<sup>3</sup> I 2e 3I − − + → (3)

TiO2 e SS <sup>+</sup> + → (4)

TiO2 3 e I 3I − − + → (5)

SnO2 3 e I 3I − − + → (6)

. These recombination reactions

electrolyte, an electrolyte regenerator, and a conducting material [10, 11].

absorb appropriate wavelengths and turn into excited states S\*

cathode. Dye-sensitizer molecules turn into oxidized states S<sup>+</sup>

initial states and electrolyte changes from the oxidized state I−

tron. Subsequently, those molecules are reduced by I−

At the cathode, the reduced state I3

from the external load and restores to I<sup>−</sup>

reactions, as shown in Eqs. (4)–(6).

−

shown in Eq. (3).

*h*ν

**296**

concentration of I3

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

Carbon-based materials have shown great versatility due to the ability to chemically 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 several materials with different applications [17].

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 and charge transfer [18].

The three new materials, graphene, carbon nanotubes and fullerenes, can be called "nanocarbon" materials. Like graphite, the structures of these carbonaceous materials consist of sp2 orbitals. Fullerenes contain 12 pentagons and have some sp3 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 bending of sp2 hybridized carbon atoms, which produces angle strain. Fullerenes can 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].

Carbon nanotubes (CNTs) are formed by a single cylindrically shaped graphene sheet called single-wall carbon nanotubes (SWCNTs) or several graphene sheets arranged concentrically called multi-wall carbon nanotubes (MWCNTs). CNTs have been proposed as the prospective substitutes for the conventional Pt in DSSCs due to their outstanding advantages of large surface area, high electrical conductivity, and chemical stability [18, 19]. Additionally, CNTs could also be used for synthesis of composite materials of anodes in DSSCs, including the ZnO nanowires/ CNTs and TiO2/CNTs, in order to offer a potential platform to enhancement surface area and decrease of carrier recombination in DSSCs [20, 21].

As mentioned, carbonaceous materials are quite attractive for replacement of Pt in DSSCs due to the high electronic conductivity, corrosion resistance toward I2 electrolyte, high reactivity for I3 − reduction, and low cost. The lower catalytic activity of carbon compared to Pt can be compensated by increasing the active surface area of the electrode by using a porous electrode structure. For example, porous carbon electrodes are easily prepared from graphite powder, which consists of platelike crystals that, on deposition from a liquid dispersion and drying, will preferentially align in the plane of the counter electrodes, resulting in a high conductivity in this plane. Numerous carbonaceous materials were studied for the fabrication of electrodes in DSSCs, using carbon vulcan, carbon black, activated carbon, carbon nanofibers, carbon nanotubes, graphene or the combination of these materials to fabricate the high-performance electrodes of DSSCs, like graphite-activated carbon [22], carbon black-graphite [23]. Among these materials, graphene has attracted the most attention of researchers due to its outstanding properties [5, 24].
