**2. Dye sensitized solar cell**

improvements have been made in this regard, including energy technologies

Solar energy will effectively meet some of the energy needs of future generations. The 3x10<sup>24</sup> Joule/year supply of energy from the sun to the earth is 10 000 times more than the global energy requirement. This suggests that the use of 10% efficient photovoltaic cells will cover just 0.1% of the earth's surface area that could supply our current electricity needs [5]. In addition to photovoltaic device technologies, protect the global environment, and ensure economic growth with sustainable resources [6]. A solar cell or photovoltaic device is a solid-state device that is generally used for converting solar energy into useable electricity. The main advantage of the solar cells is that it does not require fossil fuel burning and does not produce any harmful emission [2]. The development of solar cells can be divided into three generations: First generation, second generation, and third generation solar cell. First generation or crystalline-silicon based solar cell has high cell efficiency(26.7% against theoretical limit 29%), which dominates the global solar cell market (90%) [7]. However, the rigid cell structure and high cost related to manufacturing silicon wafers limit the use of first generationsilicon-based solar cells. Second generation or thin-film solar cell is fabricated by depositing multiple layers of photovoltaic material on plastic, glass, or metal substrate. Thin-film solar cell thickness can vary from few nanometers to 10 microns. However, Indium and Tellurium's scarcity makes it somewhat difficult to commercializeand large-scale production of the thin-film solar cell. Also, cadmium is a highly toxic material, which poses both severe health and environmental hazards. Third-generation solar cells are targeted to achieve both high efficiency and low cost. The theoretical cell conversion efficiency of third-generation solar cells varies from 31 to 41%, and it is

focused on wind power, biofuels, solar panels, and fuel cells [3, 4].

*Solar Cells - Theory, Materials and Recent Advances*

expected that this limit can be easily overcome by using semiconductor

O'Regan in 1988 at UC Berkeley, USA and further developed by Michael GratzelÉcolePolytechniqueFédérale de Lausanne, Switzerland. The DSSCs are regarded as successful applicants for the substitution of high-cost conventional solar cells. Throughout the last twenty years, Gratzel and hisits colleagues have made considerable efforts to develop further. The different factors involved in the DSSCs system are still under development in several ways to make it ready for commercialization. To date, the maximum 14.7% power conversion efficiency (PCE) has been recorded for the DSSCs [9]. The theoretical PCE limit of the single junction DSSC under standard test conditions (STC) is 32%, according to Professor Michael Graetzel, and a two-level tandem DSSC structure could reach PCE of 46% [10]. Since there is a big difference between theoretical and experimental PCE of DSSC, there is aroom for further improvement. Different researchers have suggested and worked on adifferent methods to improve the PCE of DSSC, such as anode modification by doping material, anode modification by carbon nanotube, surface modi-

fication by TiCl4, dye modification, electrolyte modification, and cathode

**384**

modification by different carbon variant [11–13]. Advancements in DSSCs technology are occurring at an ever-increasing rate, as the development of novel carbonbased materials, such as carbon nanotubes (CNTs). The CNT structure consists of enrolled graphite sheets, in a word, and can be classified as either single-walled (SWCNT) ormulti-walled (MWCNT) depending on its preparation method [14]. Cell performance can be increased by adding SWCNTs/MWCNTs. The incorporation of CNTs into the semiconductor material (i.e., TiO2, ZnO, SnO2, ctc.) decreases

third-generation solar cells are still in the research stage.

nanoparticles [8]. Perovskite solar cell (PSC), copper zinc tin sulfide (CZTS) solar cell, quantum dot solar cell (QDSC), organic solar cell (OSC), and dye-sensitized solar cell (DSSC) are the example of the third-generation solar cells. Most of the

DSSC is a third-generation solar cells co-invented by Michael Gratzel and Brian

DSSC is different from other conventional solar cells in terms of both cell architecture and the physical process behind its operation. DSSC combines both solid and liquid phase in contrast to typical crystalline silicon or thin-film solar cell technology based on solid-state semiconductor materials. A typical DSSC consists of a transparent conducting oxide (TCO) glass substrate (i.e., ITO, FTO, AZO, etc.) as an anode, a wide band-gap semiconductor (a nanocrystalline semiconductor material, such as TiO2, ZnO, SnO2, SrTiO3, Zn2SnO4,and Nb2O5, etc. deposited on the TCO), dye sensitizer (anchored on to the surface of nanocrystalline semiconductor material), a volatile electrolyte (I/I3 , Br/Br3 , SCN/ (SCN)3 , and SeCN/ (SeCN)3 , etc. redox couple), and a platinum (or carbon) coated TCO glass substrate as a counter electrode [16–24]. A typical DSSC structure is shown in **Figure 1**.

**Figure 2** illustrates the basic operating principle of DSSC. Nanocrystalline semiconductor material (i.e., TiO2) is deposited on the TCO and provides the indispensable surface area for dye photosensitizer absorption. Photon energy from the sunlight is absorbed (or collected) by the dye photosensitizer layer and produce excited electron (D+) from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) (Eq. (1)). The excited dye injects an electron to the conduction band (C.B.) of the semiconductor material and the dye molecule oxidized and losses an electron (Eq. (2)). The injected electron travels

**Figure 1.** *Basic cell structure of DSSC.*

Recombination of dye

Dye recombination to ground state

*DOI: http://dx.doi.org/10.5772/intechopen.96630*

recombination losses within the solar cell.

**3. Carbon nanotubes (CNTs)**

and a high aspect ratio [28].

**3.1 CNT based photoanode in DSSC**

sp2 + ?

**Figure 3.**

**387**

*(a) MWCNTs, (b) SWCNTs.*

Recombination of electrolyte

TiO2∣D<sup>þ</sup> þ e�

*Improvement of Efficiency of Dye Sensitized Solar Cells by Incorporating Carbon Nanotubes*

Green arrows (path (1)–(5)) represent the electron transfer and movement of electrons within the solar cell, while red arrows (path (6)–(8)) represent potential

Carbon nanotubes (CNTs) are hollow cylinders consisting of single or multiple concentric layers of carbon atoms in a honeycomb lattice structure [26]. The CNT structure consists of enrolled graphite sheets, in a word, and can be classified as either or multi-walled (MWCNT) (**Figure 3(a)**) or single-walled CNT (SWNT) (**Figure 3(b)**) depending on its preparation method. In transmission electron microscopy (TEM) studies, MWCNTs were first observed by Iijima in 1991, while SWCNTs were independently developed by Iijima and Bethune in 1993 [26]. CNT has a spn hybridization (where n = 2) state of carbon material. However, because of the curved surface of CNT, it does not have a genuine sp2 hybridization. CNT has a

hybridization, which is in between n = 2 and 3. It is understood that CNT is a

CNT incorporated semiconductor material on the conducting electrode surface, offers efficient charge collection and transportation of charge carriers. The electrons

material lying between fullerenes and graphite as a new member of carbon allotropes [27]. Carbon nanotubes (CNTs) show very excellent adsorption characteristics because they have a high specific surface area and a nanoscale formation that constitutes many sites. It also has high electrical conductivity, mechanical strength,

I3

TiO2∣D<sup>∗</sup> ! TiO2∣<sup>D</sup> (6)

� þ 2e� ð Þ! TiO2 3I� (8)

ð Þ <sup>C</sup>*:*B*:* ! TiO2∣D (7)

**Figure 2.** *Basic operating principles of DSSC.*

through the semiconductor material toward the anode, and electrical energy is delivered to the external load (Eq. (3)). Then the electron further travels to complete the circuit and reaches the counter electrode (C.E.). The electron is transferred from the C.E. to the electrolyte. Dye regenerates when the dye accepts an electron from the I� and I� gets oxidized to I3 � (Eq. (4)). I3 � ion float around, and they receive ion from the C.E. (Eq. (5)) [25]. However, some unwanted reaction occurs, such as the recombination of dye (Eq. (6)), dye recombination to the ground state (Eq. (7)), and recombination of electrolyte (Eq. (8)) that reduces the overall cell's electron circulation performance.

Excitation of dye upon illumination

$$\mathbf{D} + \mathbf{h}\boldsymbol{\nu} \to \mathbf{D}^\* \tag{1}$$

Oxidation of dye due to injection of electrons in TiO2 photoanode

$$\mathbf{D}^\* \to \mathbf{D}^+ + \mathbf{e}^- \text{ (TiO}\_2\text{)}\tag{2}$$

Energy generation

$$\text{e}^-\left(\text{TiO}\_2\right) + \text{C.E.} \rightarrow \text{TiO}\_2 + \text{e}^-\left(\text{electric energy}\right) \tag{3}$$

Regeneration of dye

$$\rm D^{+} + \frac{3}{2}I^{-} \rightarrow D + \frac{1}{2}I\_{3} \tag{4}$$

Restoration of electrolyte at the counter electrode

$$\frac{1}{2}\mathbf{I}\_3^- + \mathbf{e}^-|\_{\text{(C.E.)}} \to \frac{3}{2}\mathbf{I}^- + \text{C.E.}\tag{5}$$

*Improvement of Efficiency of Dye Sensitized Solar Cells by Incorporating Carbon Nanotubes DOI: http://dx.doi.org/10.5772/intechopen.96630*

Recombination of dye

$$\text{TiO}\_2|\text{D}^\* \rightarrow \text{TiO}\_2|\text{D} \tag{6}$$

Dye recombination to ground state

$$\text{TiO}\_2\text{|D}^+ + \text{e}^-\text{|}\_{\text{(C.B.)}} \rightarrow \text{TiO}\_2\text{|D} \tag{7}$$

Recombination of electrolyte

$$\text{I}\_3\text{}^- + 2\text{e}^- \ (\text{TiO}\_2) \to 3\text{I}^- \tag{8}$$

Green arrows (path (1)–(5)) represent the electron transfer and movement of electrons within the solar cell, while red arrows (path (6)–(8)) represent potential recombination losses within the solar cell.
