Click here to load reader
Upload
qinghua-li
View
218
Download
1
Embed Size (px)
Citation preview
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: [email protected]
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.
REFERENCES
1. B. O’Regan and M. Gratzel, Nature, 353, 737 (1991).
2. M.K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker,
E. Muller, P. Liska, N. Vlachopoulos, and M. Gratzel,
J. Am. Chem. Soc., 115, 6382 (1993).
3. M. Gratzel, J. Photochem. Photobiol. A: Chem., 164, 3
(2004).
4. M. Gratzel, Inorg. Chem., 44, 6841 (2005).
5. E. Stathtos, P. Lianos, U.L. Stangar, and B. Orel, Adv.Mater., 14, 354 (2002).
6. P. Wang, S.M. Zakeeruddin, J.E. Moser, M.K. Nazeeruddin,
T. Sekiguchi, and M. Gratzel, Nat. Mater., 2, 402 (2003).
7. U. Bach, D. Lupo, and M. Gratzel, Nature, 395, 583 (1998).
8. A.F. Nogueira, J.R. Durrant, and M.A. De Paoli, Adv.Mater., 13, 826 (2001).
9. T. Stergiopoulos, I.M. Arabatzis, G. Katsaros, and P. Fala-
ras, Nano. Lett., 2, 1259 (2002).
10. J. Wu, Z. Lan, and D. Wang, J. Photochem. Photobiol. A:Chem., 181, 333 (2006).
11. J. Wu, Z. Lan, and D. Wang, J. Electrochim. Acta, 51, 4243(2006).
12. Z. Lan, J. Wu, and D. Wang, Sol. Energy, 80, 1483 (2006).
13. A. Kay and M. Gratzel, Sol. Energy Mat. Sol. C, 44, 99
(1996).
14. J. Halme, M. Toivola, A. Tolvanen, and P. Lund, Sol.Energ. Mat. Sol. C, 90, 872 (2006).
15. K. Imoto, K. Takahashi, and T. Yamaguchi, Sol. Energ.Mat. Sol. C, 79, 459 (2003).
16. K. Suzuki, M. Yamaguchi, and M. Kumagai, Chem. Lett.,32, 28 (2003).
17. X. Fang, T. Ma, G. Guan, M. Akiyama, and E. Abe, J. Pho-tochem. Photobiol. A: Chem., 164, 179 (2004).
18. Z. Huang, X. Liu, K. Li, D. Li, Y. Luo, H. Li, W. Song,
L. Chen, and Q. Meng, Electrochem. Commun., 9, 596
(2007).
19. G. Chen, W. Weng, D. Wu, C. Wu, J. Lu, P. Wang, and X.
Chen, Carbon, 42, 753 (2004).
20. N. Ikeda, T. Miyasaka, Chem. Commun., 1886 (2005).
21. M. Gratzel, Prog. Photovoltaics, 8, 171 (2000).
22. T. Miyamoto and K. Shibayama, J. Appl. Phys., 44, 5372(1973).
23. H. Kusama and H. Arakawa, J. Photochem. Photobiol. A:Chem., 160, 171 (2003).
24. H. Kusama and H. Arakawa, J. Photochem. Photobiol. A:Chem., 162, 441 (2004).
25. T.S. Kang, K.H. Chun, and J.S. Hong, J. Electrochem. Soc.,147, 3049 (2000).
26. A. Zaban, S. Ferrere, and B.A. Gregg, J. Phys. Chem. B,102, 452 (1998).
1692 POLYMER COMPOSITES—-2009 DOI 10.1002/pc