Application of Polymer Gel Electrolyte With GraphitePowder in Quasi-Solid-State Dye-Sensitized Solar Cells

Qinghua Li, Jihuai Wu, Qunwei Tang, Zhang Lan, Pinjiang Li, Tingting ZhangThe Key Laboratory for Functional Materials of Fujian Higher Education, Institute of MaterialsPhysical Chemistry, Huaqiao University, Quanzhou 362021, China

A polymer gel electrolyte with ionic conductivity of5.11 mS cm21 was prepared by using poly (acryloni-trile-co-styrene) as polymer matrix, acetonitrile and tet-rahydrofuran as binary organic mixture solvent, NaI + I2as electrolyte, graphite powder and 1-methylimidazoleas additives. The components ratio of the polymer gelelectrolyte was optimized, and the influence of thecomponents and temperature on the ionic conductivityof the polymer gel electrolyte and photoelectronicproperties of dye sensitized solar cell were investi-gated. On the basis of the polymer gel electrolyte withthe optimized conditions, a quasi-solid-state dye-sensitized solar cell was fabricated and its light-to-electricity energy conversion efficiency of 3.25%was achieved under irradiation of 100 mW cm22.POLYM. COMPOS., 30:1687–1692, 2009. ª 2008 Society ofPlastics Engineers

INTRODUCTION

Dye-sensitized solar cells (DSSCs) are one of the

promising candidates for the next generation of solar cells

because of their simple structure with relatively high con-

version efficiencies, inexpensive fabrication procedures in

contrast to amorphous silicon [1, 2]. On the basis of liq-

uid electrolytes, a photoelectric conversion efficiency of

11% for DSSC has been achieved [3, 4].

However, the potential problems caused by the liquid

electrolytes, such as the leakage and volatilization of liq-

uid, is considered as some of the critical factors limiting

the long-term performance and practical use of the

DSSCs. Thus, many efforts have been made to replace

the liquid electrolyte with solid or quasi-solid-type charge

transport materials such as polymer gel electrolytes [5, 6],

organic hole-transport materials [7], and solid polymer

electrolytes [8, 9], which not only offer hermetic sealing

and stability but also reduce design restrictions and endow

the cell with shape choice and flexibility.

Compared with other kinds of charge transport materi-

als, the polymer gel electrolytes have some advantages

including high ionic conductivities and stability. Up to

present, several types of gel electrolytes based on the dif-

ferent polymers have already been used in quasi-solid

state DSSCs [10–12]. At the same time, much effort has

been made to replace platinum as counter electrode. An

interesting low cost alternative for platinum is carbon ma-

terial such as graphite, carbon nanotube, or activated car-

bon. The carbon material combines good conductivity and

heat resistance, corrosion resistance toward I2, low cost,

and moreover has an large surface area because of their

porosity, which results in a higher catalytic efficiency for

electron exchange with the electrolyte [13–18].

In this article, a polymer gel electrolyte was prepared

by using poly (acrylonitrile-co-styrene) (AS) as polymer

matrix, acetonitrile (ACN) and tetrahydrofuran (THF) as

binary organic mixture solvent, NaI þ I2 as electrolyte,

graphite powder and 1-methylimidazole (MI) as additives,

based on the polymer gel electrolyte, a quasi-solid state

DSSC was fabricated by sandwiching the polymer gel

electrolyte between two electrodes. The properties of the

polymer gel electrolyte and its influence on the photovol-

taic performances of the DSSC were evaluated.

EXPERIMENTAL

Materials

Expanded graphite powder was purchased from Qing-

dao Tianhe Graphite, China. Poly (acrylonitrile-co-sty-rene) (AS) was commercially obtained from a chemical

company in China. Tetrabutyltitanate, titanium tetrachlor-

ide, sodium iodide, iodine, ACN, THF, alcohol, acetylace-

tone, and MI were all A R grade and purchased from

Shanghai Chemicals, China. All reagents were used with-

out further treatment before using.

Correspondence to: Jihuai Wu; e-mail: jhwu@hqu.edu.cn

Contract grant sponsor: National Natural Science Foundation of China;

contract grant numbers: 50572030, 50372022; contract grant sponsor: the

Functional Nanomaterials Scientific Special Program of Fujian Province,

China; contract grant number: 2005HZ01-4.

DOI 10.1002/pc.20743

Published online in Wiley InterScience (www.interscience.wiley.com).

VVC 2008 Society of Plastics Engineers

POLYMER COMPOSITES—-2009

Conducting glass plates (FTO glass, Fluorine doped tin

oxide over-layer, sheet resistance 8 O cm22, Hartford

Glass, USA) were used as a substrate for precipitating

TiO2 porous film and were cut into 2 cm 3 1.5 cm

sheets. Sensitizing dye cis-dithiocyanate-N,N0-bis (4-car-

boxylate-4-tetrabutylammoniumcarboxylate-2,20-bipyridine)ruthenium (II) (N719) was purchased from Solaronix SA,

Switzerland.

Preparation of Graphite Powder

Expanded graphite powder of 1 g was mixed with

400 ml alcohol solution consisting of alcohol and distilled

water with a volume ratio of 7:3. The mixture was sub-

jected to ultrasonic irradiation with a power of 100 W for

3 h. Then it was filtered and dried at 808C to remove resi-

due solvents. The resulted mixture, called graphite powder

[19], was kept in a dry desiccator prior use.

Preparation of Polymer Gel Electrolyte

The appropriate amounts of NaI and I2 (10 mol% of

NaI), were dissolved in binary organic mixture solvent

consisting of 80 vol% ACN and 20 vol% THF. And the

graphite powder was ultrasonic dispersed in the above

mixture solution for 1 h. Then, 20 wt% of AS was added.

The resulting mixture was heated at 75–858C under vigor-

ous stirring until a viscous gel was formed, followed by

cooling down to room temperature, and the resultant poly-

mer gel electrolytes are shown in Fig. 1.

Assembling of the Quasi-Solid State DSSC

A fluorine-doped SnO2 conducting glass sheet (FTO)

was immersed in an isopropanol solution for 48 h to

remove any impurities. Then it was cleaned in Triton X-

100 aqueous solution, washed with ethanol. The FTO

sheet was precoated TiO2 underlayer by a spray pyrolysis

method to prevent direct contact between the SnO2 layer

and the electrolyte with graphite powder. This coating

was made by chemical vapor deposition of [(CH3)2CHO]4Ti (1 M in acetylacetone and ethanol (2:11) mix-

ture) on the SnO2 surface heated at 500–5508C [20]. A

thick nanoporous TiO2 layer was formed on FTO sheet

by a conventional sintering method at 4508C in air for

30 min [10]. The TiO2 layer was sensitized by 2.5 31024 M absolute ethanol solution of a ruthenium complex

dye for 24 h. Quasi-solid state DSSC was assembled by

injecting the polymer gel electrolyte into the aperture

between the TiO2 porous film electrode (anode electrode)

and a Pt plated conducting glass sheets (cathode elec-

trode, prepared by electrodepositing). The two electrodes

were clipped together and a cyanoacrylate adhesive was

used as sealant to prevent the electrolyte solution from

leaking. Epoxy resin was used for further sealing the cell

to measure the stability of the cell.

Measurements

The ionic conductivity of gel polymer electrolytes was

measured by using model DDB-6200 digitized conductiv-

ity meter (Shanghai Reici Instrument Factory, China).

The instrument was calibrated with 0.1 M KCl aqueous

solutions prior to experiments. The photovoltaic test of

quasi-solid state DSSCs was carried out by measuring the

J-V character curves under irradiation of white light from

a 100 W xenon arc lamp (XQ-500W, Shanghai Photoelec-

tricity Device Company, China) under ambient atmos-

phere. The incident light intensity and the active cell area

were 100 mW cm22 and 0.8 cm2, respectively. The pho-

toelectronic performances [i.e. fill factor (FF) and overall

energy conversion efficiency (g)] were calculated by the

following equations [21]:

FF ¼ Vmax 3 Jmax

Voc 3 Jscð1Þ

gð%Þ ¼ Vmax 3 Jmax

Pin

3 100%¼ Voc 3 Jsc 3 FF

Pin

3 100%

ð2Þwhere JSC is the short-circuit current density (mA cm22),

VOC is the open-circuit voltage (V), Pin is the incident

light power, and Jmax (mA cm22) and Vmax (V) are the

current density and voltage in the J-V curves, respec-

tively, at the point of maximum power output.

RESULTS AND DISCUSSION

Scanning Electron Micrographof Polymer Gel Electrolytes

The scanning electron micrograph of the polymer gel

electrolytes with different graphite amount are shown in

FIG. 1. The appearance of polymer gel electrolytes: (a) the gel without

graphite powder, (b) the gel with graphite powder, and (c) the inversion

of (b). [Color figure can be viewed in the online issue, which is available

at www.interscience.wiley.com].

1688 POLYMER COMPOSITES—-2009 DOI 10.1002/pc

Fig. 2. Clearly, the morphologies of the samples with dif-

ferent concentration of graphite powder in the polymer

gel electrolyte are different. The conglomeration of graph-

ite powder gradually aggravate with increasing in the

amount of graphite powder added in the polymer gel elec-

trolyte. The micrograph of sample (b), contained 0.15%

graphite powder, shows a better graphite powder disper-

sion than the sample (c) contained 0.3% and the sample

(d) 0.4% graphite powder. The dispersion of graphite

powder is an important factor influencing the photoelec-

tronic properties of DSSC. Because of the conglomerating

of graphite powder, the irradiation may be partly blocked

and the catalytic activities of graphite powder declines.

Influence of the Amount of Graphite on the Conductivityof Polymer Gel Electrolyte

As shown in Fig. 3, the conductivity of polymer gel

electrolyte increases with increasing the percentage of

graphite powder. As well known that graphite powder is a

kind of fine electron conductors, with the increase of

graphite powder amount, more electrons conducting chan-

nels were constructed, and the conductivity of system is

enhanced. But the conductive mechanisms for polymer

gel electrolyte in DSSC is ionic conductivity of I32/

I2redox couple, higher concentration of graphite powder

will restrain the ionic conductivity of I32/I2 redox couple,

although the electron conductivity increase.

Influence of Temperature on theConductivity of Electrolytes

The lnr versus 1/T plots for the polymer gel electrolyte

with or without graphite powder shown in Fig. 4. Obvi-

ously, the conductivity increased with the increase in tem-

perature. To avoid the decomposition or aging, the elec-

trolyte generally works below 808C. It is said that the

polymer matrix is amorphous and has large amounts of

FIG. 2. Scanning electron micrograph of polymer gel electrolytes. (a) Without graphite powder, (b) graphite

powder 0.15%, (c) graphite powder 0.3%, (d) graphite powder 0.4%.

DOI 10.1002/pc POLYMER COMPOSITES—-2009 1689

free-volume cages. These free-volume cages increase with

an increase in temperature just as described in the free-

volume model [22]. The free volume increases with the

increase in temperature that enhances the mobility of

polymer chains and ions dissolved in polymer matrix.

As is apparent from Fig. 4, the lnr versus 1/T plots is

almost linear, which is accorded with the Arrhenius equa-

tion (3) shown as follow:

ln r ¼ � Ea

RTþ lnA ð3Þ

where Ea is the active energy, R the molar gas constant,

A is a constant, and T is the absolute temperature.

From the conductivity-temperature behaviors of the gel

polymer electrolytes in Fig. 4, the active energy (Ea) of

polymer gel electrolyte with graphite powder can be

obtained as 13.77 kJ mol21, which is lower than the

active energy of polymer gel electrolyte without graphite

powder, i.e. 15.37 kJ mol21. In other words, the polymer

gel electrolyte with graphite powder is more active and

the mobility of [I32] can be easier.

Influence of the Amount of Graphite Powderon Photoelectronic Properties of DSSC

The influence of the amount of graphite powder in the

polymer gel electrolyte on photoelectronic properties of

DSSC is shown in Fig. 5. The JSC, VOC, FF, and g of

DSSCs increase with an increase of the graphite amount

in the polymer gel electrolyte from 0% to 0.15%. The

highest value overall energy conversion efficiency of

DSSC about 3.16% is attained at 0.15 wt% graphite

powder doped in the polymer gel electrolyte. Beyond the

0.15 wt% of graphite powder percentage, the JSC, VOC,

FF, and g of DSSCs decrease with the increase of the

graphite amount in the polymer gel electrolyte.

It is well known that graphite is also used as a catalyst for

I2þ I2 system in DSSC [15], as the reaction (4) and (5). The

more the concentration of graphite powder in polymer gel

electrolytes, the more I32/I2 redox couple could be catalyzed

and enlarge the surface areas of catalyst. So the overall

energy conversion efficiency of DSSC increases and attains

the maximum with the increase of graphite amount from 0 to

0.15 wt%. But the further increase in the concentration of

graphite powder, the overall energy conversion efficiency of

DSSC decreases. This is due to that Pt plate not only acts as

a catalyst for I2 þ I2 system, but also acts as a light reflector

for counter electrode, which has the added functionality to

reflect light that has not yet been absorbed by the photoelec-

trode back into the same [2]. With the added increase in the

concentration of graphite powder, it destroys the effect of the

light reflecting spacer. Moreover, the conglomeration of

graphite powder and light absorption of graphite powder

becomemore serious, which lead to the decline in catalytic ac-

tivity of the counter electrode. The catalytic activity generally

depends on the surface area of the catalyst. The effect of cata-

lytic activities of the graphite powder declines. These factors

result in the overall energy conversion efficiency of DSSC

decreases. Therefore, the highest overall energy conversion

efficiency of solar cell attains 3.15%, base on the polymer gel

electrolyte containing 0.15 wt % graphite powder.

I�3 þ 2e� ! 3I� ð4Þ3I2 þ 2e� ! 2I�3 ð5Þ

Influence of Additives MI on the PhotoelectronicProperties of DSSC

To investigate the influence of MI on the performance

of the quasi-solid state cells, a series of cells were fabri-

FIG. 3. Effect of the concentration of graphite powder on the ionic

conductivity (at 308C) of the polymer gel electrolyte. (Polymer gel elec-

trolyte contains 0.5 M NaI, 0.05 M I2, 20% AS and binary organic mix-

ture solvent consisting of 80 vol% ACN and 20 vol% THF).

FIG. 4. Temperature dependence of the conductivity of gel polymer

electrolytes. (Polymer gel electrolyte with 0.5 M NaI, 0.05 M I2, 20 wt%

AS and binary organic mixture solvent consisting of 80 vol% ACN and

20 vol% THF). [Color figure can be viewed in the online issue, which is

available at www.interscience.wiley.com].

1690 POLYMER COMPOSITES—-2009 DOI 10.1002/pc

cated by sandwiching the polymer gel electrolytes, which

contain 0.15 wt% graphite powder with or without MI

additives.

The photoelectronic properties of DSSC with the poly-

mer gel electrolyte contained different concentration of

MI are shown in Table 1. From Table 1, the r and JSCvalues decrease monotonously with increasing the concen-

tration of MI, and the VOC and FF values increase monot-

onously with increasing the concentration of MI. Accord-

ing to Eq. 2, higher VOC and FF are propitious to g, lowerJSC go against, therefore, when the concentration of MI is

at 0.30 M, an overall energy conversion efficiency (g) ofthe DSSC attains a maximum of 3.25%.

It is well known that the nitrogen-containing heterocy-

clic additives would enhance the VOC of cells and reduce

the JSC of cells [23–26]. The behavior that the ionic con-

ductivity of polymer gel electrolyte decreased with

increasing the concentration of MI can be explained as

the bad conductivity of MI, which results in the decrease

of JSC.

The enhancement of VOC and FF by adding MI is due

to due to the adsorption of MI on the bare TiO2 surface

and the suppression of back electron transfer from the

TiO2 electrode to I32. Adsorption of MI at the TiO2 sur-

face is caused by interaction between the Ti (IV) ion,

which has Lewis acidity and the lone electron pair of MI.

Another reason is to raise the flatband potential (VFB) of

the TiO2 photoelectrode. Adsorbing additives in the elec-

trolytic solution onto the TiO2 surface may raise the flat-

band potential (VFB) of the TiO2 electrode. Under Fermi

level pinning, these two parameters are linked by

VOC ¼ jVFB � Vredj ð6Þ

FIG. 5. Influence of the percentage of graphite powder on the photovoltaic properties of quasi-solid state

DSSCs. (Polymer gel electrolyte contains 0.5 M NaI, 0.05 M I2, 20% AS and binary organic mixture solvent

consisting of 80 vol% ACN and 20 vol% THF). [Color figure can be viewed in the online issue, which is

available at www.interscience.wiley.com].

FIG. 6. Photovoltaic characterization of quasi-solid state DSSCs. [Color

figure can be viewed in the online issue, which is available at www.

interscience.wiley.com].

TABLE 1. Effects of the concentration of MI on the photovoltaic

properties of cells.a

Concentration

of MI (mol L21)

r(mS cm21)

JSC(mA cm22)

VOC

(mV) FF

g(%)

0.00 6.32 10.20 573 0.54 3.16

0.20 6.10 7.90 700 0.58 3.21

0.30 6.01 7.84 703 0.59 3.25

0.40 5.82 6.52 711 0.60 2.78

0.50 5.75 5.11 730 0.63 2.39

0.60 5.65 4.82 731 0.64 2.22

a Polymer gel electrolyte contains 0.5 M NaI, 0.05 M I2, 20 wt% AS

and binary organic mixture solvent consisting of 80 vol% ACN and

20 vol% THF.

DOI 10.1002/pc POLYMER COMPOSITES—-2009 1691

where Vred is the standard reduction potential of a redox

coupling. If Vred remains constant when MI is added, then

increasing the VFB by adsorbing MI onto the TiO2 surface

should increase the VOC. Raising the VFB would also

cause a negative shift in the conduction band edge of

TiO2, which would decrease the electron injection rate

from the exiting dye and explain the reduction in the JSCupon adding MI.

Photovoltaic Characterization of Quasi-SolidState DSSCs

Using three kinds of the polymer gel electrolytes: (a),

containing 0.5 M NaI, 0.05 M I2, 20 wt% AS and binary

organic mixture solvent consisting of 80 vol% ACN and

20 vol% THF; (b), (a) þ0.15 wt% graphite powder; (c),

(b) þ0.3 M MI as medium, DSSCs was assembled,

respectively. The photovoltaic characterization of the

quasi-solid state DSSCs were measured under irradiation

of 100 mW cm22 and shown in Fig. 6. It shows that the

overall energy conversion efficiency of the DSSC based

on the gel polymer electrolyte with 0.15 wt% graphite

powder was calculated as 3.16%, which improve 22.48%

compared to the DSSC based on the polymer gel electro-

lyte without graphite powder. The overall energy conver-

sion efficiency of the DSSC based on the gel polymer

electrolyte containing 0.15 wt% graphite powder and

0.3 M MI was approximately 3.25%, which improve

25.97% compared to the DSSC based on the gel without

graphite powder and additives.

CONCLUSION

A polymer gel electrolyte was prepared by using AS

20 wt% as polymer matrix, ACN 80 vol%, and THF

20 vol% as binary organic mixture solvent, 0.5 M NaI

þ 0.05 M I2 as electrolyte, graphite powder 0.15 wt%,

and 0.3 M MI as additives. The polymer gel electro-

lyte has an ionic conductivity of 5.11 mS cm21 at

308C. The influence of components and temperature on

the ionic conductivity of the polymer gel electrolyte

and photoelectronic properties of DSSC were investi-

gated. On the basis of the polymer gel electrolyte with

the optimized conditions, a quasi-solid-state DSSC

was fabricated by sandwiching the polymer gel elec-

trolyte between two electrodes. The overall energy

conversion efficiency of light-to-electricity of the

quasi-solid-state DSSC achieved 3.25% under irradia-

tion of 100 mW cm22.

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