3. Structure and mechanism of DSSC and QDSSC

be even cheaper. The third generation is based on thin films similar to second-generation solar cells. QDSSC, DSSC, perovskite, and organic PVs are third-generation solar cells. The drawback of thin-film cells is that if the thin layer photoactive semiconductor is impinged by photons of energy similar to that of the semiconductor energy gap, the photons are only partially absorbed. One of the promising methods to improve photon absorption is by employing plasmonic materials. Plasmonic materials are metals having negative dielectric constant. Plasmonic materials in solar cells can amplify electromagnetic field, trap, and scatter light strongly to the sensitizer. In this chapter, the effect of plasmonic materials on solar cell

A plasmon is a quasi-particle that can be described by oscillations of a collection of free charges (electrons). Plasmons at the boundary of a metal and a dielectric are called surface plasmons. When the oscillation frequency of conducting electrons equals that of the incoming light, surface plasmon resonance (SPR) occurs, and a strong electromagnetic field with energy greater than that of the incident photons is generated near the nanoparticles (NPs) and is referred to as the 'near electric field'. The non-propagating excited surface plasmon in plasmonic materials is referred to as the localized surface plasmon resonance (LSPR). Size and geometry of the plasmonic NPs influence LSPR. For example, the LSPR is red-shifted if the NP size is increased. This will lead to the increase in electric field wavelength. The dielectric materials surrounding plasmonic NPs also affect the frequency of LSPR (ωLSPR) [1] as can be described by Eq. (1):

> 1 1 þ 2ε<sup>m</sup>

The free electrons in the metallic NPs have a plasma frequency of ωp. τ is electron relaxation time and ε<sup>m</sup> is dielectric constant of the semiconducting materials. The LSPR frequency is red shifted when ε<sup>m</sup> increases. Surface plasmon polaritons (SPPs) are created when the electromagnetic field of incident light is combined or coupled with the plasmon. The SPPs propagate along the metal/dielectric interface. In addition, incident photons are efficiently scattered by plasmonic or metal NPs depending on their geometry and size. In general, there are three ways

i. By LSPR where the metal NPs act as subwavelength antennas through an oscillation of

ii. By SPP where the incoming light is trapped and promotes more light to be absorbed in

� <sup>1</sup> ω2 <sup>p</sup> τ<sup>2</sup>

(1)

!1=<sup>2</sup>

performance will be discussed.

224 Plasmonics

strong conducting electrons

the photoactive semiconductor

2. Enhancement mechanism of plasmonic materials

ωLSPR ¼ ω<sup>p</sup>

in which plasmonic materials can affect the performance of DSSCs and QDSSCs:

iii. By increasing light scattering and number of optical pathways

The main components of a DSSC are as shown in Figure 1. The photoanode which absorbs incident light and separates the charge is made up of a substrate (either glass or plastic). The substrate is coated on one side by a conducting oxide that is transparent. Hence, the substrate is referred to as transparent conducting oxide (TCO) substrate. The conducting oxide can be either fluorine doped tin oxide (FTO) or indium tin oxide (ITO). The TCO is coated with a semiconducting oxide layer (typically titanium dioxide (TiO2)) and a sensitizing dye. The counter electrode (CE) is where charges (electrons released by the sensitizer) are collected from the photoanode. CE comprises TCO coated with catalyst materials. The catalyst material can be platinum (Pt), carbon, conducting polymer, metal oxide or metal carbide. The electrolyte contains a redox couple. The iodide/triiodide I �=I � 3 couple is an example. For solid or gel electrolytes, the ion-conducting medium is placed between the photoanode and CE. Details of the DSSC working principle are as follows:

When the DSSC is illuminated, the sensitizing dye molecules D, on the TiO2 surface are excited D\* upon absorbing photons (Eq. 2) The excited molecules are immediately oxidized, D <sup>+</sup> . The released electrons are driven into the CB of TiO2 (Eq. 3). This can occur only if LUMO of the dye is at a higher position than the TiO2 Fermi level.

$$D + h\nu \to D^\* \tag{2}$$

$$D^\* \to e^-|\_{\text{(TiO}\_2)} + D^+ \tag{3}$$

The electrons that have entered the CB of TiO2 exit the cell via the TCO substrate, travel through the external circuit and reach the Pt CE. Electron transfer occurs at the Pt CE/electrolyte boundary when the I � <sup>3</sup> ions each receive two electrons from the CE and become I � ions

Figure 1. Schematic diagram of DSSC or QDSSC.

(Eq. 4). The I � ions diffuse back to the photoanode getting oxidized again into I � <sup>3</sup> to complete the circuit. The iodide ions neutralize the ionized dye molecules as shown in Eq. (5):

$$\rm I\_3^- + 2e^- \llcorner (\rm C.E.) \to \bf 3I^- \tag{4}$$

absorption of the sensitizer at the photoanode, the electrolyte must be very thin. Liquid

Table 1 summarizes the performance of DSSC and QDSSC incorporated with plasmonic materials. Gold (Au) and silver (Au) NPs are two most popular plasmonic materials that have been widely used for studying the plasmonic effect on the performance of DSSC and QDSSC. The most popular method to study the influence of NP on the PV cell characteristics is by introducing the NP in the semiconductor network. An improvement on the efficiency of DSSC from 2.7 to 3.3% was observed by Nahm et al. [10] when 100 nm Au NPs were incorporated into the TiO2 layer and sensitized with N719 dye. They found that the absorption was stronger in the cell with Au/TiO2 NP layer than in the cell without Au NPs. This showed that the Au NP plasmonic material helped to increase light absorption, which increased the number of electrons entering the TiO2 and increased the Jsc that led to efficiency improvement. Jun et al. [11] showed that with 5 nm Au NPs, the Jsc increased by 65% and efficiency increased from 2.09 to 3.12%. Saravanan et al. [30] studied the plasmonic effect Ag NPs produced from Ag+ ions treated with Peltophorum pterocarpum flower. An efficiency of 3.62% was noted when 2 wt.% of Ag NPs was doped into TiO2. Efficiency was only 2.83% for the undoped TiO2. Efficiency increased because of enhancement in light absorption via LSPR, SPP or increased optical pathways. Plasmonic effect on DSSC using phthaloyl chitosan and polyethylene oxide-based gel polymer electrolyte has been studied by Shah et al. [29]. Efficiency enhanced by 13% when

Although efficiency can be enhanced by plasmonic effect, the long-term stability is a major

couple can corrode the NPs. The presence of NPs in the semiconductor network can also increase recombination process that leads to shorter electron lifetime and lowering of Voc [31]. Due to the high electrical conductivity and the lower work function of the NPs than the CB of TiO2, the NPs can act as electron recombination centres where electrons that have been driven

efforts have been undertaken to prevent the metal NPs from being corroded. The efforts include utilizing sandwich structure and applying a coating or insulating layer or shell on the

Sandwich structure (TiO2/Ag NPs/TiO2) has been developed by Lin et al. [38] to protect Ag

enhancement in Jsc, the Ag NPs are still corroded during the illumination period. Hence, the authors have concluded that applying protective layer on Ag NPs is a necessity. The choice of materials (usually wide bandgap materials) and thickness of the protective layer also influence the performance of DSSC. Brown et al. [15] have incorporated Au NPs coated with silica (SiO2)

<sup>3</sup> redox couple. For this sandwich structure, although they have achieved 23%

<sup>3</sup> redox mediator was used. This is because the iodide/triiodide

<sup>3</sup> ions in the electrolyte. Several

Plasmonic Effect in Photoelectrochemical Cells http://dx.doi.org/10.5772/intechopen.79580 227

4. Performance of DSSC and QDSSC with plasmonic materials

electrolyte is usually used when the NP is deposited at the CE.

incorporated

Ag NPs were included.

NPs from I

concern, especially when I

=I  =I 

into TiO2 re-associated with the holes in the dye molecules or I

surface of the NPs as a protective layer (Figure 3).

$$2\text{ } 2\text{D}^+ + 3\text{I}^- \to 2\text{D} + \text{I}\_3^- \tag{5}$$

QDSSC has a similar structure with DSSC except that the sensitizers used are quantum dots (QDs) such as lead sulphide (PbS) [2, 3], cadmium sulphide (CdS) [4, 5], lead selenide (PbSe) [6, 7] and cadmium selenide (CdSe) [8, 9]. Since the I �=I � <sup>3</sup> couple is corrosive towards QD, it has been replaced with S2�/S<sup>x</sup> <sup>2</sup>� couple as the redox mediator in QDSSC. The working principle of QDSSC is the same as DSSC. Electron–hole (e-h) pairs are created upon photon absorption by QD (see Eq. 6). Electrons in the CB of the QD are driven into the TiO2 CB (Eq. 7), and the QD reverts back to its original state when the holes in the QD valence band receive electrons from S2� ions in the electrolyte. An example for a Cd chalcogenide is illustrated in Eq. (8):

$$\text{Q}D + \text{photons} \to \text{QD} \,(\varepsilon - \hbar) \tag{6}$$

$$\text{Q}D(\varepsilon+h) + \text{TiO}\_2 \rightarrow \text{QD}(h) + \text{TiO}\_2(\varepsilon) \tag{7}$$

$$\text{CdX}(h) + \text{S}^{2-} \rightarrow \text{CdX} + \text{S} \tag{8}$$

As in DSSCs, the injected electrons will end up at the CE. S2� <sup>x</sup> ion in the electrolyte is then reduced (when it receives 2e) to S2� ions. The S2� ions will diffuse back to the photoanode to complete the circuit as shown in Eqs. (9) and (10):

$$\rm S\_x \, ^{2-} + 2e \to \rm S\_{x-1} \, ^{2-} + S^{2-} \tag{9}$$

$$\text{S} + \text{S}\_{\text{x}-1}{}^{2-} \rightarrow \text{S}\_{\text{x}}{}^{2-} \tag{10}$$

The plasmonic NPs can be either deposited on the FTO or ITO surface of the TCO substrate or incorporated in the TiO2 semiconducting layer of the DSSC and QDSSC (Figure 2). However, the studies of plasmonic effect at the CE on the cell performance have also been investigated by some researchers. If the plasmonic NP is embedded in the photoanode, the electrolyte can be either in the solid, gel or liquid state. However, for SPR to occur at the CE and improve

Figure 2. Incorporation of plasmonic NP in photoanode (a) on the surface of TCO and (b) in the semiconductor active layer.

absorption of the sensitizer at the photoanode, the electrolyte must be very thin. Liquid electrolyte is usually used when the NP is deposited at the CE.
