ARTICLE IN PRESS

Solar Energy Materials & Solar Cells 93 (2009) 1860–1864

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Solar Energy Materials & Solar Cells

0927-02

doi:10.1

� Corr

Taiwan

fax: +88

E-m

journal homepage: www.elsevier.com/locate/solmat

Enhancing the performance of dye-sensitized solar cells by incorporatingnanosilicate platelets in gel electrolyte

Yi-Hsuan Lai a, Chih-Wei Chiu b, Jian-Ging Chen a, Chun-Chieh Wang a,Jiang-Jen Lin c, King-Fu Lin c,d, Kuo-Chuan Ho a,c,�

a Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwanb Department of Chemical Engineering, National Chung Hsing University, Taichung 40227, Taiwanc Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwand Department of Materials Science and Engineering, National Taiwan University, Taipei 10617, Taiwan

a r t i c l e i n f o

Article history:

Received 11 September 2008

Received in revised form

1 March 2009

Accepted 29 June 2009Available online 23 July 2009

Keywords:

Dye-sensitized solar cells

Gel electrolytes

Nanosilicate platelets

48/$ - see front matter & 2009 Elsevier B.V. A

016/j.solmat.2009.06.027

esponding author at: Department of Che

University, Taipei 10617, Taiwan. Tel.: +886 2

6 2 2362 3040.

ail address: kcho@ntu.edu.tw (K.-C. Ho).

a b s t r a c t

Two kinds of gel-type dye-sensitized solar cells (DSSCs), composed of two types of electrolytes, were

constructed and the respective cell performance was evaluated in this study. One electrolyte, TEOS-

Triton X-100 gel, was based on a hybrid organic/inorganic gel electrolyte made by the sol–gel method

and the other was based on poly(vinyidene fluoride-co-hexafluoro propylene) (PVDF-HFP) copolymer.

TEOS-Triton X-100 gel was based on the reticulate structure of silica, formed by hydrolysis, and

condensation of tetraethoxysilane (TEOS), while its organic subphase was a mixture of surfactant

(Triton X-100) and ionic liquid electrolytes. Both DSSC gel-type electrolytes were composed of iodine,

1-propy-3-methyl-imidazolium iodide, and 3-methoxypropionitrile to create the redox couple of I3�/I�.

Based on the results obtained from the I–V characteristics, it was found that the optimal iodine

concentrations for the TEOS-Triton X-100 gel electrolyte and PVDF-HFP gel electrolyte are 0.05 M and

0.1 M, respectively. Although the increase in the iodine concentration could enhance the short-circuit

current density (JSC), a further increase in the iodine concentration would reduce the JSC due to increased

dark current. Therefore, the concentration of I2 is a significant factor in determining the performance of

DSSCs.

In order to enhance cell performance, the addition of nanosilicate platelets (NSPs) in the above-

mentioned gel electrolytes was investigated. By incorporating NSP-Triton X-100 into the electrolytes,

the JSC of the cells increased due to the decrease of diffusion resistance, while the open circuit voltage

(VOC) remained almost the same. As the loading of the NSP-Triton X-100 in the TEOS-Triton X-100 gel

electrolyte increased to 0.5 wt%, the JSC and the conversion efficiency increased from 8.5 to 12 mA/cm2

and from 3.6% to 4.7%, respectively. However, the JSC decreased as the loading of NSP-Triton X-100

exceeded 0.5 wt%. At higher NSP-Triton X-100 loading, NSPs acted as a barrier interface between the

electrolyte and the dye molecules, hindering electron transfer, hence, reducing the cell’s photocurrent

density. The same behavior was also observed in the PVDF-HFP gel electrolyte DSSC system.

& 2009 Elsevier B.V. All rights reserved.

1. Introduction

In the era with rapidly depleting energy resources, scientistsworldwide are searching for other substitute energies that areinexhaustible, clean, low cost and have high energy conversionefficiency. Among them solar energy attracts attention because itis renewable and abundant.

In early years, the development of solar cells was focused onthe single crystal silicon solar cell, whose efficiency is nearly

ll rights reserved.

mical Engineering, National

2366 0739;

reaching 25% [1]. However, the application has been limited dueto high production cost. The subsequent development of the thin-film solar cells also faces the same problem. However, since thedye-sensitized solar cells (DSSCs) were developed by the Gratzelgroup, their advantages, including low production cost andpotentially high conversion efficiency (11%) [2], stimulatedresearch in the DSSCs field.

Although classical DSSCs, composed of liquid electrolyte, haverather high energy conversion efficiencies, the vaporization andleakage of the liquid electrolyte result in low durability. In order toimprove the DSSCs’ stability, different types of electrolytes, suchas a gel-type and solid-state, have been proposed to replace theliquid-type electrolyte [3–10].

The TEOS-Triton X-100 hybrid gel and poly(vinyidene fluoride-co-hexafluoro propylene) (PVDF-HFP) copolymer have been

ARTICLE IN PRESS

Fig. 1. The structure of montmorillonite (MMT).

Y.-H. Lai et al. / Solar Energy Materials & Solar Cells 93 (2009) 1860–1864 1861

proposed as the gel electrolytes for DSSCs. The TEOS-Triton X-100hybrid gel is essentially a cross-linked polymer, which not onlycould be used for solidifying the electrolyte, but also suppressesthe vaporization, thus enhancing the stability of cells [11]. Onthe other hand, PVDF-HFP has been successfully used as thequasi-solid-state materials and shows relatively high ionicconductivities when it was in combination with ionic liquids,showing efficiency up to 6% [12]. Furthermore, because fluori-nated polymers are potentially stable even in the presence of TiO2

and Pt nanoparticles, this would improve the long-term stabilityof the DSSCs [13,14].

The nanosilicate platelets (NSPs) or montmorillonite are one ofthe most important materials for nanotechnology. Fig. 1 shows thestructure of montmorillonite. Their crystal structure consists oflayers made up of two tetrahedrally coordinated silicon atomsfused to an edge-shared octahedral sheet of either aluminum ormagnesium hydroxide. The layer thickness is around 1 nm, andthe lateral dimensions of these layers are around 100 nm. Stackingof the layers leads to a regular van der Waals gap between thelayers called the interlayer or gallery. Isomorphic substitutionwithin the layers (for example, Al3+ replaced by Mg2+) generatesnegative charges that are counterbalanced by alkali and alkalineearth cations situated inside the galleries. This type of layeredsilicate is characterized by a moderate surface charge known asthe cation exchange capacity (CEC), and generally expressed asmequiv/100 g. They could improve some material properties,including moduli, strength and heat resistance after incorpo-ration with NSPs [15]. The nanosilicate platelets also increase thesolvent retention of gels, due to physical and chemical interac-tions between the solvent and nanosilicate platelets [12].

In this work, two gel-type electrolytes, TEOS-Triton X-100 andpoly(vinyidene fluoride-co-hexafluoro propylene), were preparedto construct DSSCs. The TEOS-Triton, X-100, gel-type DSSCwas fabricated using a novel inorganic–organic hybrid gel fromTEOS and Triton X-100. The effect of iodine concentration on theperformance of the respective cell was studied. Also, in order tofurther improve the cell performance, the modified nanosilicateplatelets (NSPs), or modified montmorillonite (MMT), wereadded in gel-type electrolyte after which their effect on the cellefficiency was investigated.

2. Experimental

2.1. Materials

Anhydrous 1-propyl-3-methyl-imidazolium (PMII), iodine (I2),poly(ethylene glycol) (PEG) (molecular weight ¼ 20,000), and 4-tert-butylpyridine (TBP) were obtained from Merck. Titanium (IV)

isopropoxide (TTIP) (+98%) and guanidine thiocyanate (GUSCN)were bought from Acros. 3-methoxypropionitrile (MPN) waspurchased from Merck, and water molecules were removed byputting molecular sieves (4 A) into the solvent. The N3 dye wasthe commercial product obtained from Solaronix S.A., Aubonne,Switzerland. Tetraethyl silicate was obtained from Showa. TritonX-100 was purchased from Sigma. TiO2 (300 nm) was obtainedfrom Kronos. PVDF-HFP (molecular weight ¼ 400,000) wasobtained from Aldrich. NSP raw material was purchased fromNanocor Co.

2.2. Preparation of NSP-Triton X-100

Direct exfoliation of montmorillonite for preparing the randomNSP platelets with the Mannich polyamine salts has previouslybeen reported [16,17]. The exfoliated platelets were isolated usinga two-phase extraction process from toluene/aqueous NaOH(10 wt%), suspension in methanol at 60 1C, and extracted byagitating the slurry with water/toluene. NSP-Triton X-100 wasprepared using the ethylene oxide of Triton X-100 anchor on thesurface of NSP with the weight ratio of 30/70 to become anorganic platelet complex. An example of a preparative procedurefor preparing the Triton X-100/NSP hybrid is described below. Thewater slurry of 10 wt% NSP in water suspension (20 g) was addedwith Triton X-100 (4.7 g) in a round-bottomed flask, equippedwith a magnetic stirrer. The gelled products were collected andsubjected to freeze dryer for dryness.

2.3. Preparation of TiO2 thin films, electrolyte, and the cell assembly

Preparation of a TiO2 precursor and electrode fabrication werecarried out based on previous reports [14], except that after theautoclave treatment, the solution was concentrated to 13, and30 wt% poly(ethyleneglycol) (M.W. 20,000) was added to the TiO2

paste to prevent the film from cracking during drying. The TiO2

paste was coated on a fluorine-doped tin oxide (FTO) glass platewith the glass rod method. The TiO2 photoelectrode was preparedby subsequent coating of two types of TiO2 paste on a conductingfluorine-doped tin oxide glass. For the first two coatings, the TiO2

pastes incorporated with PEG having a molecular weight of20,000 was used, followed by a third coating (final coating) thatwas carried out with a mixture containing light-scatteringparticles of TiO2 (300 nm (30 wt%) and 20 nm (70 wt%)) forreducing the light loss by back scattering. The film thickness ofTiO2 by the above-mentioned three times coating was around12mm.

An active surface area of 0.16 cm2 of electrode was cut andimmersed overnight in a 2�10�4 M N3 dye solution consisting ofacetonitrile and tert-butanol with the volume ratio of 1:1.

A platinum-sputtered FTO (15O/&) conducting glass plate(Pt thickness of 25 nm) was used as the counter electrode and theelectrolyte was composed of 0.6 M PMII, 0.05–0.2 M I2, 0.5 M TBP,0.5 M GuSCN in MPN.

To prepare the TEOS-Triton X-100 gel polymer electrolyte,0.4 ml glacial acetic acid and 0.5 ml water were added to the 3 mlTEOS, and then the resulting solution was sonicated for 30 min, inorder to hydrolyze the TEOS. After the hydrolysis, 1.5 ml TritonX-100, the necessary concentration of I2/PMII redox couple of theelectrolyte in MPN, and a suitable amount of NSP-Triton X-100(0.5–5 wt%) were added into the hydrolyzed TEOS solution to formthe gel electrolyte.

The nanocomposite PVDF-HFP gel electrolyte was prepared byadding 5 wt% PVDF-HFP, NSP-Triton X-100 (1–10 wt%), and I2/PMIIredox couple of the electrolyte in MPN, and stirred at 100 1C for4 h. When the electrolyte was cooled down, it became gel.

ARTICLE IN PRESS

0

2

4

6

8

10

12

14

16

18 0.01 M I2 0.05 M I2 0.1 M I2 0.2 M I2

Cur

rent

den

sity

(mA

/cm

2 )

Voltage (V)

01234567

Effi

cien

cy (%

)

I2 concentration (M)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

0.00 0.05 0.10 0.15 0.20

Fig. 3. I–V curves for PVDF-HEP gel-type DSSCs at different iodine concentrations.

The inset shows the calculated cell efficiencies at different iodine concentrations.

5

624

0 wt %

Y.-H. Lai et al. / Solar Energy Materials & Solar Cells 93 (2009) 1860–18641862

To fabricate the solar cell, the electrolyte solution was spreadon both the dye-immobilized photoanode and Pt-sputtered FTOglass and immediately they were physically sandwiched togetherusing cell holders.

2.4. Instrumentation

The cells, composed of the TEOS-Triton X-100 gel-type electro-lyte, were illuminated by an Oriel solar simulator (#6266) equippedwith a water-based IR filter under AM 1.5G (Oriel, #81075). Thecells, composed of PVDF-HFP gel-type electrolytes, were illumi-nated by a Peccell solar simulator (PEC-L11) equipped with a water-based IR filter under AM 1.5G. Electrochemical impedance spectra(EIS) were obtained with a PGSTAT 30 (Autolab, Eco-Chemie, theNetherlands) potentiostat/galvanostat equipped with an FRA2module under constant light illumination of 100 mW/cm2. Thefrequency range explored was 10 mHz–65 kHz. The applied biasvoltage and ac amplitude were set at Voc of the DSSC and 10 mV,respectively, between the counter electrode and the workingelectrode, starting from the short-circuit condition in the illumina-tion state. The impedance spectra were analyzed by an equivalentcircuit model, interpreting the characteristics of the DSSCs [18]. Theimpedance parameters were determined by fitting of impedancespectra using Z-view software.

0

1

2

3

4

Effi

cien

cy (%

)

Content of NSP-Triton X-100 (wt % )

0

4

8

12

16

20 0.5 wt % 1 wt % 5 wt %

Cur

rent

den

sity

(mA

/cm

2 )

Voltage (V)0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

0 2 4 6

Fig. 4. Cell efficiencies for the TEOS-Triton X-100 gel-type DSSCs with different

loadings of the NSP-Triton X-100 in the electrolyte.

3. Results and discussion

Figs. 2 and 3 show the I–V curves for the gel-type DSSCs,composed of TEOS-Triton X-100 and PVDF-HFP electrolytes,respectively. It was found that the short-circuit current densities(JSC) of both types of DSSCs increased with the small increase iniodine concentration, which can be attributed to the existence ofI2, causing enhancement in the charge transfer in the microscopicmolecular networks of the polymers. However, the JSC of theDSSCs decreased as the iodine concentration was furtherincreased. This behavior can be explained by considering thefollowing redox reactions:

I2+I�2I3�

I3�+2e�-3I� (at Pt counter electrode)

0

2

4

6

8

10

12

14

16

18 0.01 M I2 0.03 M I2 0.05 M I2 0.1 M I2 0.2 M I2

Cur

rent

den

sity

(mA

/cm

2 )

Voltage (V)

0

2

4

Effi

cien

cy (%

)

I2 concentration (M)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

0.00 0.05 0.10 0.15 0.20

Fig. 2. I–V curves for TEOS-Triton X-100 gel-type DSSCs at different iodine

concentrations. The inset shows the calculated cell efficiencies at different iodine

concentrations.

3I�-I3�+2e� (at TiO2 photoanode)

I3�+2e�(CB)-3I� (side reaction at TiO2 photoanode)

According to reaction (1), the increase in concentration of I2

acts in favor of I3� formation when that of I� is much higher than

I2. Since a critical concentration level of I3� is necessary for the cell

functioning at the beginning, reactions (2) and (3) proceedeffectively and the JSC increases and saturates at once. However,further increase in the concentration of I2 results in favor ofreaction (4), and decreases the JSC of the DSSCs. Furthermore, I2

also absorbs some visible lights from the wavelength of400–500 nm, lowering the visible light absorption by thesensitizer [19].

By incorporating NSP-Triton X-100 into the above-mentionedelectrolytes, the JSC of the DSSCs increases obviously, as shown inFigs. 4 and 5, and the open circuit voltage (VOC) remains almostthe same. As shown in Fig. 4, when the loading of NSP-TritonX-100 in TEOS-Triton X-100 gel-type electrolyte was increased upto 0.5 wt%, the JSC and the conversion efficiency increased from 8.5

ARTICLE IN PRESS

0

4

8

12

16

20

24

0 wt % 1 wt % 5 wt % 10 wt %

Cur

rent

den

sity

(mA

/cm

2 )

Voltage (V)

0

1

2

3

4

5

6

Effi

cien

cy (%

)

Content of NSP-Triton X-100 (wt %)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

0 2 4 6 8 10

Fig. 5. Cell efficiencies for the PVDF-HFP gel-type DSSCs with different loadings of

the NSP-Triton X-100 in the electrolyte.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Nor

mal

ized

effi

cien

cy (%

)

Time (h)

0 wt % 5 wt %

0 100 200 300 400 500 600

Fig. 6. Stability for the TEOS-Triton X-100 gel-type DSSCs with different loadings

of the NSP-Triton X-100 in the PVDF-HFP gel electrolyte.

10

20

30

40

50

60

70

10

20

30

40

50

60

70

Rdi

ff(oh

m)

Rct

2 (o

hm)

Content of NSP-Triton X-100 (wt %)

0

10

20

30

40

50

60 0 wt % 0.5 wt % 1 wt % 5 wt %

-Z"(

ohm

)

Z'(ohm)20 40 60 80 100 120 140

0 1 2 3 4 5

Fig. 7. The Nyquist plot along with the equivalent electrical circuit model that is

used for fitting the impedances. The inset is a plot of the charge transfer

resistances (Rct2) and the diffusion resistances (Rdiff) for the TEOS-Triton X-100 gel-

type DSSCs with different loadings of the NSP-Triton X-100 in the electrolyte.

40 50 60 70 80 90 1000

4

8

12

16

20

24

280 wt % 1 wt % 5 wt % 10 wt %

-Z"(

ohm

)

Z'(ohm)

0 2 4 6 8 1010

20

30

40

50

60

10

20

30

40

50

60

Rdi

ff (o

hm)

Rct

2 (o

hm)

Content of NSP-Triton X-100 (wt %)

Fig. 8. The Nyquist plot along with the equivalent electrical circuit model used for

fitting the impedances. The inset is a plot of the charge transfer resistances (Rct2)

and the diffusion resistances (Rdiff) for the PVDF-HFP gel-type DSSCs with different

loadings of the NSP-Triton X-100 in the electrolyte.

Table 1The fitted impedance parameters for the TEOS-Triton X-100 gel-type DSSCs at

various loadings of NSP-Triton X-100 in the electrolyte.

Content of NSP-Triton X-100 (wt%) RS (O) Rct1 (O) Rct2 (O) Rdiff (O)

0 23.6 41.9 50.1 32.77

0.5 27.4 45.2 48.4 13.13

1 24.7 44.1 46.8 25.39

5 28.6 48.0 54.3 33.61

Y.-H. Lai et al. / Solar Energy Materials & Solar Cells 93 (2009) 1860–1864 1863

to 12 mA/cm2 and from 3.6 to 4.7%, respectively. On the otherhand, by loading 5 wt% NSP-Triton X-100, the conversion effi-ciency of DSSC, composed of the PVDF-HFP electrolyte, can beimproved from 4.0% to 5.2% (see Fig. 5).

Fig. 6 shows the stability of the cells with PVDF-HFP-based gelelectrolyte. The cell performance was evaluated at specificinterval, and after the evaluation, the cells were stored in thedesiccator at room temperature. In the first 120 h, the cellefficiency increased slightly as the gel electrolyte penetratedmore deeply into the porous TiO2 film. It can be found that the cellstability was improved by incorporation of NSP-Triton X-100 intothe electrolyte. After 600 h, the cell with the electrolyte containing5 wt% NSP-Triton X-100 only lost 5.2% of its initial efficiency;nevertheless, the other one without NSP-Triton X-100 lost about20% of its initial efficiency.

Figs. 7 and 8 show the equivalent electrical circuit and theelectrochemical impedance spectra under constant lightillumination of 100 mW/cm2, which was used for the fitting ofthe impedance results, and the analysis was mainly aimed at theNyquist plots for TEOS-Triton X-100 and PVDF-HFP gel-type DSSCswith a different content of NSP-Triton X-100, respectively.Moreover, the fitted parameters are summarized in Tables 1

and 2, respectively. In general, the EIS spectrum of the DSSC showsthree semicircles in the measured frequency range of10 mHz–65 kHz. The ohmic serial resistance, Rs, is associatedwith the series resistance of the electrolytes and electric contactsin the DSSCs. And Rct1, Rct2 and Rdiff correspond to the chargetransfer process occurring at the Pt counter electrode(corresponding to the first arc), the TiO2/dye/electrolyteinterface (corresponding to the second arc) and the Warburg

ARTICLE IN PRESS

Table 2The fitted impedance parameters for the PVDF-HFP gel-type DSSCs at various

loadings of NSP-Triton X-100 in the electrolyte.

Content of NSP-Triton X-100 (wt%) RS (O) Rct1 (O) Rct2 (O) Rdiff (O)

0 40.8 23.1 49.6 50.1

1 40.6 23.7 50.1 20.2

5 40.9 24.9 48.3 11.9

10 41.1 21.4 55.7 18.3

Y.-H. Lai et al. / Solar Energy Materials & Solar Cells 93 (2009) 1860–18641864

diffusion process of I�/I3� in the electrolyte (corresponding to the

third arc), respectively. Lower diffusion resistance of I_/I3_ (Rdiff)

resulting from smaller semi-circle is observed (frequency range:10 mHz–100 Hz) for the DSSC when loading suitable amounts ofNSP-Triton X-100. As shown in Figs. 7 and 8, the loadings of theNSP-Triton X-100 in TEOS-Triton X-100 and PVDF-HFP electrolyteswere increased from 0 to 0.5 and from 0 to 5 wt%, respectively; therespective cell diffusion resistance decreased from 32.8 to 13.1and from 50.1 to 11.9O. Decreasing diffusion resistance meansincreasing ion mobility, hence improving the cell efficiencies.Incorporating suitable amounts of NSP-Triton X-100 in the gelelectrolyte can enhance the ion mobility for both gel-typeelectrolytes, which can be interpreted as due to the increase ofthe ion exchange mechanism, such as the Grotthuss mechanism[20,21]. In the gel electrolyte, the negatively charged NSPs canattract imidazolium cations, which can align the anionic redoxcouple (I�/I3

�) by electrostatic force, facilitating electron transportby the ion exchange mechanism. When the amount of NSP-TritonX-100 in the TEOS-Triton X-100 gel-type electrolyte was increasedup to 5 wt%, a larger Rct2 at the TiO2/dye/electrolyte interface wasobserved. Since the nanosilicate would hinder the path of theelectron transfer, it would result in the decrease in JSC and cellefficiency. The same explanation can be applied for PVDF-HFP gel-type DSSCs that have similar trends.

4. Conclusions

TEOS-Triton X-100 and PVDF-HFP gel-type DSSCs were suc-cessfully fabricated in this study. The highest conversion efficien-cies for TEOS-Triton X-100 and PVDF-HFP gel-type DSSCs were3.6% and 4.0%, obtained as the iodine concentrations wereadjusted to 0.05 and 0.1 M, respectively. On the other hand, theconversion efficiencies of DSSCs based on TEOS-Triton X-100 andPVDF-HFP gel-type electrolytes were improved by adding suitableamounts of NSP-Triton X-100. Besides, the addition of NSP-TritonX-100 also improved the cell stability. Therefore, NSP-Triton X-100can be the promising material for improving the performance ofthe DSSCs.

Acknowledgements

This work was financially supported by the National ScienceCouncil (NSC) of Taiwan, the Republic of China, under Grant NSC97-2120-M-002-012. This work was partially supported by the

King Abdullah University of Science and Technology (KAUST)through the Global Research Partnership Centers-in-Developmentgrant (KAUST GRP-CID). Some instruments used in this study weresupported by the Academia Sinica, Taipei, Taiwan, the Republic ofChina, under Grant AS-97-TP-A08.

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