4. Performance of DSSC and QDSSC with plasmonic materials incorporated

(Eq. 4). The I

226 Plasmonics

layer.

been replaced with S2�/S<sup>x</sup>

� ions diffuse back to the photoanode getting oxidized again into I

2D<sup>þ</sup> þ 3I� ! 2D þ I

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)

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

reduced (when it receives 2e) to S2� ions. The S2� ions will diffuse back to the photoanode to

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

<sup>2</sup>� ! Sx

<sup>2</sup>� <sup>þ</sup> 2e ! Sx�<sup>1</sup>

S þ Sx�<sup>1</sup>

S2� ions in the electrolyte. An example for a Cd chalcogenide is illustrated in Eq. (8):

As in DSSCs, the injected electrons will end up at the CE. S2�

Sx

complete the circuit as shown in Eqs. (9) and (10):

�

<sup>2</sup>� couple as the redox mediator in QDSSC. The working principle of

QD þ photons ! QD eð Þ � h (6)

CdX hð Þþ <sup>S</sup><sup>2</sup>� ! CdX <sup>þ</sup> <sup>S</sup> (8)

QD eð Þþ þ h TiO2 ! QD hð Þþ TiO2ð Þe (7)

�=I �

the circuit. The iodide ions neutralize the ionized dye molecules as shown in Eq. (5):

I � <sup>3</sup> þ 2e�

[6, 7] and cadmium selenide (CdSe) [8, 9]. Since the I

�

ð Þ <sup>C</sup>:E: ! 3I� (4)

<sup>3</sup> (5)

<sup>3</sup> couple is corrosive towards QD, it has

<sup>x</sup> ion in the electrolyte is then

<sup>2</sup>� <sup>þ</sup> S2� (9)

<sup>2</sup>� (10)

<sup>3</sup> to complete

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 Ag NPs were included.

Although efficiency can be enhanced by plasmonic effect, the long-term stability is a major concern, especially when I =I <sup>3</sup> redox mediator was used. This is because the iodide/triiodide 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 into TiO2 re-associated with the holes in the dye molecules or I <sup>3</sup> ions in the electrolyte. Several 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 surface of the NPs as a protective layer (Figure 3).

Sandwich structure (TiO2/Ag NPs/TiO2) has been developed by Lin et al. [38] to protect Ag NPs from I =I <sup>3</sup> redox couple. For this sandwich structure, although they have achieved 23% 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)


Au nanostar in active layer Au@SiO2 nanorod in active layer

Au@SiO2@Ag@SiO2

Au@Ag@SiO2 in active layer

Au NP on FTO Au NP on FTO Au NP on FTO Ag NP in active layer Ag NP in active layer Ag NP in active layer Ag NP in active layer Ag NP in active layer Ag NP in active layer Ag NP in active layer Ag NP in active layer

Ag NP in electrolyte

 TiO2 TiO2 TiO2 TiO2 TiO2 TiO2 TiO2 TiO2 TiO2 and ZnO nanorod

ZnO nanorod

TiO2 TiO2 nanorod

TiO2

N719 dye

 Liquid (I=I3 )

11.97\*–12.89

 0.73\* –

6.34\*–7.11

 12%

[35]

229

0.76

N719 dye N719 dye

—

 Liquid (I=I3 )

5.81\*–7.11

 0.72\* –

1.87\*–2.83

 51.3%

0.76

11.97\*–15.12 0.73 6.34\*–8.05

 27%

[35]

[36]

 CdS and CdSe

QDs

CdS QD

 Liquid

2.18\*–3.25

 0.59\* -

0.28\*–0.60

 > 100%

[34]

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

0.63

polysulphide

N719 dye

 Liquid (I=I3 )

Liquid

polysulphide

 12.4\*–16.20 13.27\*–15.67

 0.70\* –

4.80\*–5.92

 23.3%

0.74

 0.76

 7.10\*–8.90

 25%

[32]

[33]

nanofiber

 N719 dye

 Liquid (I=I3 )

 7.57\*–9.51

 0.8\* -

3.3\*–4.13

 25%

[31]

0.78

N3 Ruthenium

Liquid (I=I3 )

 7.52\*–8.28

 0.63\* –

2.83\*–3.62

 27.9%

0.71

N7 dye

GPE (I=I3 )

13.04\*–14.74

0.59 4.61\*–5.21

 13%

[29]

[30]

CdS QD

 Liquid

5.72\*–7.11

 0.47\* –

0.86\*–1.62

 88.4%

[28]

0.56

polysulphide

 in active layer TiO2

 TiO2

N719

N719

N719

N719 dye N719 dye

 Liquid (I=I3 )

12.83\*–12.34 0.82 6.5\*–6.3

 Liquid (I=I3 )

11.90\*–12.84 0.78 5.84\*–6.69

 Liquid (I=I3 )

13.89\*–16.67

 0.68

 6.55\*–7.72

 17.9%

 14.6%

 Efficiency decreases due to

high Schottky barrier

 Liquid (I=I3 )

 13.9\*–17.58 0.75 7.34\*–9.22

 25.6%

[24]

[25]

[26]

[27]

 Liquid (I=I3 )

13.15\*–15.88

 0.69\* –

5.86\*–7.21

 23%

[23]

0.73

 TiO2

N719

N749

 Liquid (I=I3 )

 8.66\*–10.69

 0.71\* –

3.53\*–5.13

 45.3%

0.73

 Liquid (I=I3 )

 15.1\*–17.2

 0.75\* –

7.00\*–8.45

 20.7%

[22]

0.78

Semiconductor

 Sensitizer

 Electrolyte

Jsc (mA/

Voc

Efficiency

Efficiency

enhancement

(V)

(%)

cm2)

Performance

Ref.


Au NP in active layer

Au NP in active layer

Au NP in active layer

Au NP in active layer Au NP in active layer

TiO2 TiO2 TiO2 TiO2 TiO2 TiO2 nanotube photonic

crystal

 TiO2

N719 dye Z907 dye

 Hole conductor (spiro-OMeTAD)

 Liquid (I=I3 )

 2.14\*–3.37

3–5\*

 0.72 1.2\*–2.2

 0.74 1.05\*–1.95

 86%

 83%

[15]

Au@SiO2 NP in active layer Au@TiO2 NP in active layer Au@TiO2 NP in active layer Au@TiO2 NP in active layer Au@SiO2 NP in active layer Au@SiO2 NP in active layer

Au nanorod in active layer Au nanorod in active layer

 TiO2

 TiO2

Y123 dye

 Liquid (I=I3 )

12.45\*–15.74

 0.71\* –

5.31\*–8.86

 66.9%

[21]

0.80

 TiO2

N3 dye N719 dye N749 dye

N3 dye

—

 Liquid (I=I3 )

 5.58\*–6.29 14.12\*–16.19

 0.63\* –

6.21\*–7.29

 17.4%

[20]

0.65

 0.65

 2.41\*–2.74

 13.7%

 Liquid (I=I3 )

 7.70\*–9.80

 0.70 3.52\*–4.81

 Liquid (I=I3 )

 8.25\*–10.31 0.70 3.88\*–4.63

 19.3%

 36.6%

[19]

 TiO2

N719 dye

 Liquid (I=I3 )

18.28\*–20.31

 0.73

 9.29\*–

9.9%

[17]

10.21

 TiO2 hollow sphere

 N719 dye

 Liquid (I=I3 )

 13.6\*–22.1

 0.72\* –

6.25\*–8.13

 30%

[18]

0.63

 TiO2

 TiO2

N719 dye N719 dye

 Liquid (I=I3 )

 18.28

 0.73\* –

9.29\*–9.78

 5.3%

0.79

 Liquid (I=I3 )

11.55\*–14.73 0.73 6.00\*–7.38

 23%

[16]

[17]

nanotube/

N719 dye

 Liquid (I=I3 )

 8.59\*–11.71 0.70 3.89\*–5.63

nanotube

 N719 dye

 Liquid (I=I3 )

 8.59\*–10.25

 0.70

 3.89\*–4.59

 18%

 44.7%

[14]

N719 dye

 Liquid (I=I3 )

18.67\*–20.11

 0.74\* –

9.59–10.8

 12.6%

[13]

0.79

CdS/ZnS QD Liquid polysulphide

N719 dye

N749 dye

 Liquid (I=I3 )

 3.89\*–6.42

 0.71–

2.09\*–3.12

 49.3%

0.75\*

9.85\*–9.48

 0.58\* –

2.63\*–2.96

 12.5%

[12]

0.61

—

Semiconductor

 Sensitizer

 Electrolyte

Jsc (mA/

Voc

Efficiency

Efficiency

enhancement

(V)

(%)

cm2)

5\*–6

 0.78 2.7\*–3.3

 22.2%

[10]

[11]

Performance

Ref.

228 Plasmonics


Ag NP on FTO

Ag NP at CE

Ag NP at CE

N719 =

tricarboxylato)

N3 =

ruthenium(II)

Cis-diisothiocyanato-bis(2,20-bipyridyl-4,40-dicarboxylic

[2,1-b,3,4-b']dithiphene-2-yl}-2-cyanoacrylic

(NCS)2; GPE = Gel polymer electrolyte;

\*is the value without metal NPs.

Table 1.

Plasmonic DSSC and QDSSC

performance.

 acid; C106 =

Short-circuit

 current density; Voc =

 Jsc =

tris(tetra-butylammonium);

Di-tetrabutylammonium

cis-bis(isothiocyanato)

 Z907 =

 acid)

ruthenium(II);

 Y123 =

NaRu(4,40-bis(5-(hexylthio)thiophen-2-yl)-2,20-bipyridine)(4-carboxylic

Open-circuit

 voltage.

bis(2,20-bipyridyl-4,40-dicarboxylato)

Cis-diisothiocyanato-(2,20-bipyridyl-4,40-dicarboxylic

ruthenium(II);

 N749 =

3-{6-{4-[bis(20,40-dihexyloxybiphenyl-4-yl)amino-]phenyl}-4,4-dihexyl-cyclopenta-

acid-40-carboxylate-2,20

 -bipyridine) Plasmonic Effect in Photoelectrochemical Cells http://dx.doi.org/10.5772/intechopen.79580 231

acid)-(2,20-bipyridyl-4,40-dinonyl)

ruthenium(II);

TiO2 TiO2 TiO2

N719 dye C106 dye

 Liquid (I�=I�3 )

 14.3\*–16.5

� 0.75 6.95\*–7.96

 Liquid (I�=I�3 )

 15.0\*–16.7

� 0.75 7.60\*–8.68

 14.2%

 14.5% Triisothiocyanato-(2,20,60,600-terpyridyl-4,40,400-

[52]

[53]

CdSe QD

 Liquid polysulphide

Semiconductor

 Sensitizer

 Electrolyte

Jsc (mA/

Voc

Efficiency

Efficiency

enhancement

(V)

(%)

cm2)

5.91\*–8.04

 0.53\* -

1.05\*–1.45

 38%

[51]

0.57

Performance

Ref.


Polyacrylate

Ag NP in active layer—

TiO2

sandwiched

TiO2

Ag-ion Ag@TiO2 NP in active layer Ag@TiO2 NP in active layer Ag@TiO2 NP in active layer

Ag@SiO2 NP

Ag NP in active layer

Biomass-coated

Ag nanowire in active layer Ag nanowire in active layer

Ag Ag nanoplate in active layer

Ag@TiO2

nanocube in active layer TiO2

nanowire@TiO2

 TiO2 TiO2

 TiO2

 TiO2

Beet root Metal orange

N719 dye N719 dye N719 dye N719 dye

 Liquid (I=I3 )

 6.51\*–9.54

 0.65\* –

2.85\*–4.26

 49.5%

0.70

 Liquid (I=I3 )

 8.22\*–12.47 0.7 2.81\*–3.84

 Liquid (I=I3 )

 9.85\*–12.01

 0.76

 4.68\*–5.31

 13.5%

 36.7%

 Liquid (I=I3 )

 9.69\*–11.83

 0.71

 5.45\*–6.26

 14.9%

[47]

[48]

[49]

[50]

 Liquid (I=I3 )

 4.21\*–6.77

 0.53 1.57\*–2.44

 Liquid (I=I3 )

 2.16\*–3.60

 0.56

 0.45\*–0.76

 68.9%

 55.4%

[46]

 Ag in active layer TiO2

TiO2 Nb-doped TiO2

N719 dye

 Liquid (I=I3 )

Solid (I=I3 )

> N719 dye

 Liquid (I=I3 )

 7.48\*–11.8

 0.76\* -

3.35\*–5.12

 52.8%

[45]

0.79

 14.1\*–16.1

 11.7\*–14.8

 0.85

 5.4\*–6.9

 27.8

 0.73

 6.8\*–7.6

 11.8%

[44]

N719 dye

 Liquid (I=I3 )

 10.2\*–13.85

 0.63\* -

4.30\*–6.16

 43%

[43]

0.66

 TiO2

 TiO2

N3 dye N719 dye

 Liquid (I=I3 )

 7.86\*–10.19

 0.65\* -

3.95\*–5.33

 34.9%

0.70

 TiO2

implantation

 in active layer TiO2 tri-layer

N719 dye N719 dye

 Liquid (I=I3 )

 5.12\*–6.67

 0.66\* -

1.42\*–1.83

 28.9%

0.69

6.07\*–8.31

 0.8 3.1\*–4.4

 42%

[41]

[42]

 Liquid (I=I3 )

 9.59\*–13.04 0.72 4.36\*–5.85

 34%

[39]

[40]

 structure TiO2/Ag/

 modified Ag NP

 TiO2

(Ru

Liquid (I=I3 )

 2.7\*–4.40

 0.78\* -

1.5\*–2.50

 66.6%

[37]

0.81

(bipy)2(SCN)2

dye

N3 dye

 Liquid (I=I3 )

2.7\*–6.18 0.83 1.43\*–3.01

 > 100%

[38]

Semiconductor

 Sensitizer

 Electrolyte

Jsc (mA/

Voc

Efficiency

Efficiency

enhancement

(V)

(%)

cm2)

Performance

Ref.

230 Plasmonics

Table 1. Plasmonic DSSC and QDSSC performance.

 Jsc =

(NCS)2; GPE = Gel polymer electrolyte;

\*is the value without metal NPs.

Short-circuit

 current density; Voc =

Open-circuit

 voltage.

TiO2/Ag NP electrode in 1.0 M titanium(IV) isopropoxide solution for 25 min. Two layers of TiO2 NPs with diameter of 18 and 400 nm, respectively, have been deposited by the doctorblade method. 5 mM AgNO3 in ethanol solution was used to deposit Ag NPs inside the porous TiO2 by drop-casting technique. From optical studies, the authors inferred that the Ag NPs are elliptical in shape. An improvement of 25% in efficiency was obtained when Ag NPs are

Plasmonic Effect in Photoelectrochemical Cells http://dx.doi.org/10.5772/intechopen.79580 233

The effect of plasmonics in nanostructure oxide semiconductor on QDSSC performance has been studied by Zhao et al. [33]. They have constructed ZnO nanorod electrode doped with Ag and TiO2 NPs sensitized with CdS and CdSe as shown in Figure 4. ZnO nanorods were first grown on FTO glass followed by deposition of Ag and TiO2 NPs. The complete cell was constructed with polysulphide electrolyte and Cu2S as cathode. The higher absorption intensity due to LSPR was observed for ZnO/Ag/TiO2/CdS/CdSe electrode, whereas the absorption intensity for ZnO/TiO2/CdS/CdSe electrode was lower. An efficiency enhancement of 22% from 4.80 (without Ag NP) to 5.92% (with Ag NP) was obtained. Li et al. [31] have developed TiO2 nanofiber doped with Ag NPs by electrospinning. The thickness of nanofiber was 15 μm, controlled by the electrospinning time. The authors chose N719 dye to sensitize the TiO2 nanofiber. The DSSC efficiency increased from 3.3% for undoped DSSC to 4.13% for Ag-doped DSSC. The increased efficiency was attributed to increased Jsc. Optical studies revealed that the N719 absorption in Ag-doped semiconductor oxide layer was higher than that of the undoped layer. This is attributed to the strong localized electromagnetic field around the Ag NPs and resulting in higher Jsc and efficiency. The authors also found that the electron diffusion coefficient in photoanode increased with Ag-doped DSSC. Eskandari et al. [34] have varied the Ag NP concentration to study the effect on QDSSC performance. The electrolyte used was liquid polysulphide. Different concentrations of Ag NPs (1, 5 and 10%) have been doped into ZnO nanorod array and sensitized with CdS QD as photoanode. Chemical bath deposition (CBD) and successive ion layer absorption and reaction (SILAR) processes were employed for growing ZnO nanorods (NRs) and CdS QDs, respectively. The Ag NPs in their work have been coated with zinc sulphide (ZnS) shell by two-step dipping method. The first step is dipping in 0.05 M

included in the photoanode.

Figure 4. The arrangement of ZnO/Ag/TiO2/CdS/CdSe electrode [33].

Figure 3. Active layer arrangement to protect metal NPs: (a) TiO2/Ag sandwich structure and (b) metal core-shell.

layer, metal core-shell nanostructure, into the DSSC. SiO2 is corrosion resistant towards I =I 3 couple and stable during the sintering process. The authors have employed both I =I <sup>3</sup> redox couple containing liquid electrolyte and spiro-OMeTAD as a medium for charge transportation from cathode to photoanode. Incorporating bare Au NPs in DSSC resulted in lower fill factor, photovoltage and photocurrent due to the metal NPs acting as recombination centres. An increment of about 67% was obtained for photocurrent when the TiO2 mesoporous layer was incorporated with core-shell Au@SiO2 NPs and sensitized with Z907 dye. However, due to the insulating nature of SiO2, the excited dye molecules on SiO2 have a difficulty in injecting electrons into the SiO2 coating layer [41]. According to Choi et al. [17], the fact that DSSC with core-shell Au@SiO2 NPs exhibited higher Jsc is due to surface plasmon effect, while the higher Voc obtained in Au@TiO2 NPs is attributed to charging effect. In DSSC incorporating core-shell Au@TiO2, the electrons are more easily injected from dye molecules into TiO2 compared to SiO2 and stored in the Au core leading to upward shifting of the TiO2 Fermi level and increase Voc. The effectiveness of Au NP size on DSSC operation behaviour was studied by Wang et al. [13]. Three different Au NP sizes (5, 45 and 110 nm) were prepared and mixed into a TiO2 paste before deposition on the FTO or ITO substrate and consequently sensitized with N719 dye. The Au NPs were then protected by additional thin layer of TiO2. They have observed that smaller-size Au NPs exhibited higher efficiency attributed to the higher Voc which is due to the photocharging effect. Jeong et al. [32] have coated Ag NPs with TiO2 layer by refluxing TiO2/Ag NP electrode in 1.0 M titanium(IV) isopropoxide solution for 25 min. Two layers of TiO2 NPs with diameter of 18 and 400 nm, respectively, have been deposited by the doctorblade method. 5 mM AgNO3 in ethanol solution was used to deposit Ag NPs inside the porous TiO2 by drop-casting technique. From optical studies, the authors inferred that the Ag NPs are elliptical in shape. An improvement of 25% in efficiency was obtained when Ag NPs are included in the photoanode.

The effect of plasmonics in nanostructure oxide semiconductor on QDSSC performance has been studied by Zhao et al. [33]. They have constructed ZnO nanorod electrode doped with Ag and TiO2 NPs sensitized with CdS and CdSe as shown in Figure 4. ZnO nanorods were first grown on FTO glass followed by deposition of Ag and TiO2 NPs. The complete cell was constructed with polysulphide electrolyte and Cu2S as cathode. The higher absorption intensity due to LSPR was observed for ZnO/Ag/TiO2/CdS/CdSe electrode, whereas the absorption intensity for ZnO/TiO2/CdS/CdSe electrode was lower. An efficiency enhancement of 22% from 4.80 (without Ag NP) to 5.92% (with Ag NP) was obtained. Li et al. [31] have developed TiO2 nanofiber doped with Ag NPs by electrospinning. The thickness of nanofiber was 15 μm, controlled by the electrospinning time. The authors chose N719 dye to sensitize the TiO2 nanofiber. The DSSC efficiency increased from 3.3% for undoped DSSC to 4.13% for Ag-doped DSSC. The increased efficiency was attributed to increased Jsc. Optical studies revealed that the N719 absorption in Ag-doped semiconductor oxide layer was higher than that of the undoped layer. This is attributed to the strong localized electromagnetic field around the Ag NPs and resulting in higher Jsc and efficiency. The authors also found that the electron diffusion coefficient in photoanode increased with Ag-doped DSSC. Eskandari et al. [34] have varied the Ag NP concentration to study the effect on QDSSC performance. The electrolyte used was liquid polysulphide. Different concentrations of Ag NPs (1, 5 and 10%) have been doped into ZnO nanorod array and sensitized with CdS QD as photoanode. Chemical bath deposition (CBD) and successive ion layer absorption and reaction (SILAR) processes were employed for growing ZnO nanorods (NRs) and CdS QDs, respectively. The Ag NPs in their work have been coated with zinc sulphide (ZnS) shell by two-step dipping method. The first step is dipping in 0.05 M

Figure 4. The arrangement of ZnO/Ag/TiO2/CdS/CdSe electrode [33].

layer, metal core-shell nanostructure, into the DSSC. SiO2 is corrosion resistant towards I

Figure 3. Active layer arrangement to protect metal NPs: (a) TiO2/Ag sandwich structure and (b) metal core-shell.

couple containing liquid electrolyte and spiro-OMeTAD as a medium for charge transportation from cathode to photoanode. Incorporating bare Au NPs in DSSC resulted in lower fill factor, photovoltage and photocurrent due to the metal NPs acting as recombination centres. An increment of about 67% was obtained for photocurrent when the TiO2 mesoporous layer was incorporated with core-shell Au@SiO2 NPs and sensitized with Z907 dye. However, due to the insulating nature of SiO2, the excited dye molecules on SiO2 have a difficulty in injecting electrons into the SiO2 coating layer [41]. According to Choi et al. [17], the fact that DSSC with core-shell Au@SiO2 NPs exhibited higher Jsc is due to surface plasmon effect, while the higher Voc obtained in Au@TiO2 NPs is attributed to charging effect. In DSSC incorporating core-shell Au@TiO2, the electrons are more easily injected from dye molecules into TiO2 compared to SiO2 and stored in the Au core leading to upward shifting of the TiO2 Fermi level and increase Voc. The effectiveness of Au NP size on DSSC operation behaviour was studied by Wang et al. [13]. Three different Au NP sizes (5, 45 and 110 nm) were prepared and mixed into a TiO2 paste before deposition on the FTO or ITO substrate and consequently sensitized with N719 dye. The Au NPs were then protected by additional thin layer of TiO2. They have observed that smaller-size Au NPs exhibited higher efficiency attributed to the higher Voc which is due to the photocharging effect. Jeong et al. [32] have coated Ag NPs with TiO2 layer by refluxing

couple and stable during the sintering process. The authors have employed both I

232 Plasmonics

=I 3

=I <sup>3</sup> redox Zn (CH3COO)2.2H2O, and the second dipping is in 0.05 M Na2S solution. Improvements in Jsc and Voc were observed when 1 and 5% Ag NPs were embedded in the ZnO NR surface. This is due to LSPR and light scattering. The QDSSC performance dropped for 10% Ag NPs due to the NP corrosion. The authors attributed this to the imperfect coating of the ZnS shell. Guo et al. [42] have prepared several TiO2 electrodes doped with various quantities (0.05, 0.10, 0.15 and 0.20 wt.%) of Ag@TiO2 core-shells. The best Jsc and Voc combination was obtained when the DSSC was incorporated with 0.15 wt.% Ag@TiO2 core-shell NPs that subsequently lead to efficiency enhancement of 5.33% from 3.90% for DSSC without Ag@TiO2 core-shell NPs.

by 22.1% from 9.69 mA/cm<sup>2</sup> and increment of efficiency by 14.9% from 5.45% were achieved when Ag@SiO2 core-shell nanowire (NW) was applied to the DSSC with N719 dye [47]. Effectiveness of Ag nanowire on the DSSC operation behaviour that utilized natural dye has been investigated by Kazmi et al. [46]. The red shift in absorption spectra of TiO2-Ag nanowire was observed and attributed to SPR effect. The efficiency of DSSC using Beet root dye is 0.45%,

The interface between TiO2 and TCO can be modified by depositing metal NP in between TCO and TiO2. Doosthosseini et al. [51] have deposited Ag NPs on top of FTO glass which acts as interfacial layer followed by a layer of anatase TiO2 NPs (size = 20 nm) and a layer of rutile TiO2 NPs (size = 400 nm) sensitized with CdSe QD. The number of SILAR cycles affects the size of the deposited QDs. Since the CB of TiO2 is higher than the work function of Ag, electrons can be easily collected by the FTO substrate. The Jsc and efficiency of DSSC with Ag NPs as interfacial layer increased from 5.91 mA/cm2 to 8.04 mA/cm2 and from 1.05 to 1.45%, respectively. In a similar manner, Zhang et al. [26] have deposited FTO with different sizes of Au NP and by screen printing coating the assembly with TiO2 NP. The TiO2 was sensitized with N719 dye. The efficiency increased from 5.84% for without NP deposited FTO to 6.69% for FTO deposited with

Au NP. Efficiency enhancement is due to increased Jsc from 11.90 to 12.84 mA/cm2

factor increases by 4.9%. According to Ni et al. [54], the performance of DSSC by this arrangement (FTO/ NP/semiconductor) may be explained using the Schottky barrier height model. The Schottky barrier height, φb, is dependent on the metal–semiconductor contact. They have formu-

þ 1

ideality factor, D is electron diffusion coefficient and n0 = 1016 cm�<sup>3</sup> is the dark electron concentration. Further, the electron diffusion length is represented by L, film thickness by d

� kT q

ln 1 <sup>þ</sup> <sup>J</sup>

, <sup>T</sup> is temperature in kelvins (K), <sup>q</sup> is 1.602 � <sup>10</sup>�<sup>19</sup> C, <sup>m</sup> = 2 is

<sup>A</sup>∗T<sup>2</sup> exp ð�qφb=kT<sup>Þ</sup>

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

" #

, and the fill

(11)

235

and it was increased to 0.76% in the presence of Ag nanowire.

lated the J-V relationship as shown in the following expression:

ln <sup>ð</sup>Jsc � <sup>J</sup>ÞLcosh <sup>d</sup>ð Þ <sup>=</sup><sup>L</sup> qDn0sinh <sup>d</sup>ð Þ =<sup>L</sup>

� �

<sup>V</sup> <sup>¼</sup> kTm q

Figure 5. Work function of Au, Ag and TiO2.

Here, <sup>k</sup> is 1.38 � <sup>10</sup>�<sup>23</sup> J K�<sup>1</sup>

Liu et al. [16] demonstrated the effect of different Au@TiO2 core-shell NPs on DSSC performance. They found that thicker shell exhibited higher Voc, while higher Jsc was produced by thinner shells. Plasmonic DSSCs having Au@TiO2 with 5 nm shell exhibited efficiency of 7.38% with percentage increment of 23%. Combining Au and Ag NPs have been proposed in order to obtain a broader absorption region. The absorption region between 400 and 550 nm has been noted by Wang et al. [24] for Au coated with Ag and SiO2. The authors have constructed Au@SiO2@Ag@SiO2 NP structure with Au core-coated SiO2@Ag@SiO2 shell. The total NP size was about 100 nm. SiO2 acted as interfacial layer between Au and Ag. It is capable of reducing charge recombination which can be determined by impedance spectroscopy. DSSC fabricated with Au@SiO2@Ag@SiO2 electrode exhibited the highest electron lifetime and reported the efficiency as high as 9.22%. Researchers from the same group have also examined how the thickness of Au@Ag@SiO2 core-shell affects the DSSC operation [25]. The blue shift of absorption peak increased with increasing Ag shell thickness. This has been ascribed to the LSPR effect. The blue shift is due to the dielectric properties of Ag. Optimum efficiency of 7.72% was obtained for DSSC having Ag shell thickness of 15 nm.

The shape of nanoparticles influenced the optical properties. For example, Au with a spherical shape showed the absorption region between 400 and 500 nm, whereas Au nanostars exhibited a strong absorption up to near-infrared (NIR) region (500 nm to 1000 nm) [22]. Approximately 20% enhancement in efficiency from 7.1 (without Au) to 8.4% was obtained when the photoanode contained Au nanostars. Meen et al. [20] have studied the SPR effect of three different shapes of Au NPs (spherical, short and long nanorods) in the photoanode of DSSC. Spherical-shaped Au NPs with average diameter of 45 nm showed absorption peak at 540 nm, whereas the short Au NRs with length:width ratio of 2.5 displayed peaks at 510 and 670 nm. Peaks at 510 and 710 nm were observed for long Au NRs with aspect ratio of 4. The plasmonic bands of NRs are clearly dependent on the aspect ratio. The broader absorption wavelength for long Au nanorods resulted in higher efficiency of 7.29% followed by short Au nanorods and spherical Au with efficiencies of 7.08 and 6.77%, respectively. Without Au NPs, the efficiency of DSSC was only 6.21%. Similar results were obtained by Bai et al. [23] for Au nanorods with aspect ratio of 2.3. Two absorption peaks at 514 and 656 nm were observed. The absorption intensity of Au nanorods coated with SiO2 is higher compared to that without coating. The efficiency was increased from 5.86% for the DSSC without Au to 7.21% for DSSC with 2.0 wt.% Au. Introduction of Ag@TiO2 nanocube core-shells into reduced graphene oxide (RGO)-TiO2 nanotube (NT) has been studied by Chandrasekhar et al. [50]. RGO can increase adsorption surface area and reduce charge recombination rate. Its presence has resulted in a 43% efficiency enhancement from 2.85 to 4.26%. The efficiency was further increased by 21.8% when 0.2 wt.% Ag@TiO2 core-shell nanocubes were embodied in the photoanode. An increment in Jsc by 22.1% from 9.69 mA/cm<sup>2</sup> and increment of efficiency by 14.9% from 5.45% were achieved when Ag@SiO2 core-shell nanowire (NW) was applied to the DSSC with N719 dye [47]. Effectiveness of Ag nanowire on the DSSC operation behaviour that utilized natural dye has been investigated by Kazmi et al. [46]. The red shift in absorption spectra of TiO2-Ag nanowire was observed and attributed to SPR effect. The efficiency of DSSC using Beet root dye is 0.45%, and it was increased to 0.76% in the presence of Ag nanowire.

The interface between TiO2 and TCO can be modified by depositing metal NP in between TCO and TiO2. Doosthosseini et al. [51] have deposited Ag NPs on top of FTO glass which acts as interfacial layer followed by a layer of anatase TiO2 NPs (size = 20 nm) and a layer of rutile TiO2 NPs (size = 400 nm) sensitized with CdSe QD. The number of SILAR cycles affects the size of the deposited QDs. Since the CB of TiO2 is higher than the work function of Ag, electrons can be easily collected by the FTO substrate. The Jsc and efficiency of DSSC with Ag NPs as interfacial layer increased from 5.91 mA/cm2 to 8.04 mA/cm2 and from 1.05 to 1.45%, respectively. In a similar manner, Zhang et al. [26] have deposited FTO with different sizes of Au NP and by screen printing coating the assembly with TiO2 NP. The TiO2 was sensitized with N719 dye. The efficiency increased from 5.84% for without NP deposited FTO to 6.69% for FTO deposited with Au NP. Efficiency enhancement is due to increased Jsc from 11.90 to 12.84 mA/cm2 , and the fill factor increases by 4.9%. According to Ni et al. [54], the performance of DSSC by this arrangement (FTO/ NP/semiconductor) may be explained using the Schottky barrier height model. The Schottky barrier height, φb, is dependent on the metal–semiconductor contact. They have formulated the J-V relationship as shown in the following expression:

$$V = \frac{kTm}{q} \ln\left[\frac{(I\_{sc} - I)L\cosh(4\%)}{qDn\_0\sinh(4\%)} + 1\right] - \frac{kT}{q} \ln\left[1 + \frac{I}{A \ast T^2 \exp\left(-q\rho\_b/kT\right)}\right] \tag{11}$$

Here, <sup>k</sup> is 1.38 � <sup>10</sup>�<sup>23</sup> J K�<sup>1</sup> , <sup>T</sup> is temperature in kelvins (K), <sup>q</sup> is 1.602 � <sup>10</sup>�<sup>19</sup> C, <sup>m</sup> = 2 is ideality factor, D is electron diffusion coefficient and n0 = 1016 cm�<sup>3</sup> is the dark electron concentration. Further, the electron diffusion length is represented by L, film thickness by d

Figure 5. Work function of Au, Ag and TiO2.

Zn (CH3COO)2.2H2O, and the second dipping is in 0.05 M Na2S solution. Improvements in Jsc and Voc were observed when 1 and 5% Ag NPs were embedded in the ZnO NR surface. This is due to LSPR and light scattering. The QDSSC performance dropped for 10% Ag NPs due to the NP corrosion. The authors attributed this to the imperfect coating of the ZnS shell. Guo et al. [42] have prepared several TiO2 electrodes doped with various quantities (0.05, 0.10, 0.15 and 0.20 wt.%) of Ag@TiO2 core-shells. The best Jsc and Voc combination was obtained when the DSSC was incorporated with 0.15 wt.% Ag@TiO2 core-shell NPs that subsequently lead to efficiency enhancement of 5.33% from 3.90% for DSSC without Ag@TiO2 core-shell NPs.

Liu et al. [16] demonstrated the effect of different Au@TiO2 core-shell NPs on DSSC performance. They found that thicker shell exhibited higher Voc, while higher Jsc was produced by thinner shells. Plasmonic DSSCs having Au@TiO2 with 5 nm shell exhibited efficiency of 7.38% with percentage increment of 23%. Combining Au and Ag NPs have been proposed in order to obtain a broader absorption region. The absorption region between 400 and 550 nm has been noted by Wang et al. [24] for Au coated with Ag and SiO2. The authors have constructed Au@SiO2@Ag@SiO2 NP structure with Au core-coated SiO2@Ag@SiO2 shell. The total NP size was about 100 nm. SiO2 acted as interfacial layer between Au and Ag. It is capable of reducing charge recombination which can be determined by impedance spectroscopy. DSSC fabricated with Au@SiO2@Ag@SiO2 electrode exhibited the highest electron lifetime and reported the efficiency as high as 9.22%. Researchers from the same group have also examined how the thickness of Au@Ag@SiO2 core-shell affects the DSSC operation [25]. The blue shift of absorption peak increased with increasing Ag shell thickness. This has been ascribed to the LSPR effect. The blue shift is due to the dielectric properties of Ag. Optimum efficiency of 7.72% was

The shape of nanoparticles influenced the optical properties. For example, Au with a spherical shape showed the absorption region between 400 and 500 nm, whereas Au nanostars exhibited a strong absorption up to near-infrared (NIR) region (500 nm to 1000 nm) [22]. Approximately 20% enhancement in efficiency from 7.1 (without Au) to 8.4% was obtained when the photoanode contained Au nanostars. Meen et al. [20] have studied the SPR effect of three different shapes of Au NPs (spherical, short and long nanorods) in the photoanode of DSSC. Spherical-shaped Au NPs with average diameter of 45 nm showed absorption peak at 540 nm, whereas the short Au NRs with length:width ratio of 2.5 displayed peaks at 510 and 670 nm. Peaks at 510 and 710 nm were observed for long Au NRs with aspect ratio of 4. The plasmonic bands of NRs are clearly dependent on the aspect ratio. The broader absorption wavelength for long Au nanorods resulted in higher efficiency of 7.29% followed by short Au nanorods and spherical Au with efficiencies of 7.08 and 6.77%, respectively. Without Au NPs, the efficiency of DSSC was only 6.21%. Similar results were obtained by Bai et al. [23] for Au nanorods with aspect ratio of 2.3. Two absorption peaks at 514 and 656 nm were observed. The absorption intensity of Au nanorods coated with SiO2 is higher compared to that without coating. The efficiency was increased from 5.86% for the DSSC without Au to 7.21% for DSSC with 2.0 wt.% Au. Introduction of Ag@TiO2 nanocube core-shells into reduced graphene oxide (RGO)-TiO2 nanotube (NT) has been studied by Chandrasekhar et al. [50]. RGO can increase adsorption surface area and reduce charge recombination rate. Its presence has resulted in a 43% efficiency enhancement from 2.85 to 4.26%. The efficiency was further increased by 21.8% when 0.2 wt.% Ag@TiO2 core-shell nanocubes were embodied in the photoanode. An increment in Jsc

obtained for DSSC having Ag shell thickness of 15 nm.

234 Plasmonics

and A\* is the Richardson constant for TiO2 (6.71 <sup>10</sup><sup>6</sup> A m<sup>2</sup> <sup>K</sup><sup>2</sup> ). Simulation results show that the fill factor and power output of DSSC are higher when ϕ<sup>b</sup> is smaller. The smaller value of ϕ<sup>b</sup> implies that the metal–semiconductor is ohmic. Hence, it will be easier for the electrons in CB of TiO2 to be collected by FTO. Au NP deposited on FTO glass was also studied by Dao and Choi [27]. The Jsc and efficiency slightly decreased from 12.82 mA/cm<sup>2</sup> and 6.54% for DSSC without Au NP to 12.34 mA/cm<sup>2</sup> and to 6.27% for FTO deposited with Au NP. However, an increment of Jsc and efficiency was observed when the Ag NP was deposited on FTO glass. The efficiency increased to 7.49%. This is due to the higher φ<sup>b</sup> in FTO/Au/TiO2 contact as compared to the FTO/Ag/TiO2 contact. The work function of Au is 5.3 eV, Ag is 4.12 eV and TiO2 is 4.0 eV as shown in Figure 5.

References

7608

2014;9:69

253107

Chemistry. 2017;41:1914-1917

Letters. 2014;14:6010-6015

quantum dots. ACS Nano. 2015;9:8157-8164

Applied Materials & Interfaces. 2016;8:34482-34489

tized solar cells. Electrochimica Acta. 2016;188:710-717

[1] Gangadharan DT, Xu Z, Liu Y, Izquierdo R, Ma D. Recent advancement in plasmon-

Plasmonic Effect in Photoelectrochemical Cells http://dx.doi.org/10.5772/intechopen.79580 237

[2] Lee HJ, Chen P, Moon S-J, Sauvage F, Sivula K, Bessho T, Gamelin DR, Comte P, Zakeeruddin SM, Il SS, Grätzel M, Nazeeruddin MK. Regenerative PbS and CdS quantum dot sensitized solar cells with a cobalt complex as hole mediator. Langmuir. 2009;25:7602-

[3] Heo JH, Jang MH, Lee MH, Shin DH, Kim DH, Moon SH, Kim SW, Park BJ, Im SH. Highperformance solid-state PbS quantum dot-sensitized solar cells prepared by introduction of hybrid Perovskite interlayer. ACS Applied Materials & Interfaces. 2017;9:41104-41110

[4] Lee Y-S, Gopi CVVM, Reddy AE, Nagaraju C, Kim H-J. High performance of TiO2/CdS quantum dot sensitized solar cells with a Cu–ZnS passivation layer. New Journal of

[5] Jun HK, Careem MA, Arof AK. Performances of some low-cost counter electrode materials in CdS and CdSe quantum dot-sensitized solar cells. Nanoscale Research Letters.

[6] Zhang J, Gao J, Church CP, Miller EM, Luther JM, Klimov VI, Beard MC. PbSe quantum dot solar cells with more than 6% efficiency fabricated in ambient atmosphere. Nano

[7] Kim S, Marshall AR, Kroupa DM, Miller EM, Luther JM, Jeong S, Beard MC. Air-stable and efficient PbSe quantum-dot solar cells based upon ZnSe to PbSe Cation-exchanged

[8] Huang F, Zhang L, Zhang Q, Hou J, Wang H, Wang H, Peng S, Liu J, Cao G. High efficiency CdS/CdSe quantum dot sensitized solar cells with two ZnSe layers. ACS

[9] Pandi DV, Muthukumarasamy N, Agilan S, Velauthapillai D. CdSe quantum dots sensitized ZnO nanorods for solar cell application. Materials Letters. 2018;223:227-230

[10] Nahm C, Choi H, Kim J, Jung D-R, Kim C, Moon J, Lee B, Park B. The effects of 100 nmdiameter Au nanoparticles on dye-sensitized solar cells. Applied Physics Letters. 2011;99:

[11] Jun HK, Careem MA, Arof AK. Plasmonic effects of quantum size gold nanoparticles on

[12] Zarazúa I, Esparza D, López-Luke T, Ceja-Fdez A, Reyes-Gomez J, Mora-Seró I, de la Rosa E. Effect of the electrophoretic deposition of Au NPs in the performance CdS QDs sensi-

dye-sensitized solar cell. Materials Today: Proceedings. 2016;3S:S73-S79

enhanced promising third-generation solar cells. Nano. 2017;6:153-175

The effect of plasmonic NP also can be observed when the NP is deposited at CE. The plasmonic effect of Ag NP with three different shapes (prism, sphere and rod) incorporated in Pt CE has been investigated by Ganeshan et al. [52]. They found that the Ag nanorod embedded at Pt CE shows stronger LSPR effect and better scattering property. Hence, 14% enhancement in efficiency was obtained when the Ag/Pt CE is used. About 15% efficiency enhancement was observed by Dong et al. [53] when Au NP was incorporated into CE. This was attributed to the high surface area at CE and the plasmonic effect.

### 5. Summary

One of the methods to upgrade the optical properties of DSSC and QDSSC components is by incorporating DSSC and QDSSC NPs which lead to better efficiency. These materials can be incorporated in the photoanode and counter electrode of the cells. Both methods improved light absorption efficiency. Different types, shapes, concentrations and sizes of NPs contributed to the plasmonic effect.
