High-Performance Quasi-Solid-State Dye-Sensitized Solar Cell Based

Aug 2, 2008 - An electrospun membrane was prepared from a 16 wt % solution of poly(vinylidenefluoride-co-hexafluoropropylene) (PVdF−HFP) in a mixtur...
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Langmuir 2008, 24, 9816-9819

High-Performance Quasi-Solid-State Dye-Sensitized Solar Cell Based on an Electrospun PVdF-HFP Membrane Electrolyte A. R. Sathiya Priya,† A. Subramania,‡ Young-Sam Jung,† and Kang-Jin Kim*,† Department of Chemistry, Korea UniVersity, Seoul, 136-713, Korea, and Department of Industrial Chemistry, Alagappa UniVersity, Karaikudi-630 003, India ReceiVed May 3, 2008. ReVised Manuscript ReceiVed June 9, 2008 An electrospun membrane was prepared from a 16 wt % solution of poly(vinylidenefluoride-co-hexafluoropropylene) (PVdF-HFP) in a mixture of acetone/N,N-dimethylacetamide (7:3 wt %) at an applied voltage of 12 kV. It was then activated by immersing it in 0.6 M 1-hexyl-2,3-dimethylimidazolium iodide, 0.1 M LiI, 0.05 M I2, and 0.5 M 4-tertbutylpyridine in ethylene carbonate/propylene carbonate (1:1 wt %) to obtain the corresponding membrane electrolyte with an ionic conductivity of 10-5 S cm-1 at 25 °C. On the basis of this electrospun membrane electrolyte, quasisolid-state dye-sensitized solar cells were fabricated, which showed an open-circuit voltage (Voc) of 0.76 V, a fill factor of 0.62, and a short-circuit current density (Jsc) of 15.57 mA cm-2 at an incident light intensity of 100 mW cm-2. This yields a light-to-electricity conversion efficiency of 7.3%. Moreover, this cell possessed better long-term stability than that fabricated with conventional liquid electrolyte.

1. Introduction Dye-sensitized solar cells (DSSCs) are currently attracting extensive academic and industrial interest from researchers who envision this technology to be a powerful and promising way to generate electricity from the sun at low cost and with high efficiency. Nowadays, overall efficiencies of 11% have been reported for DSSCs using liquid electrolytes.1 Although DSSCs based on liquid electrolytes have already achieved high conversion efficiencies,2 substantial problems have been encountered when attempting to put DSSCs into practical use. For DSSCs, the electrolytes usually consist of a triiodide/iodide redox couple in organic solvents. However, the disadvantages of using liquid electrolytes are their lower long-term stability, difficulty in robust sealing, and evaporation and leakage of electrolyte in case of breaking of the glass substrates.3,4 To overcome these problems, many research groups have been searching for alternatives to replace the liquid electrolytes, such as inorganic or organic hole conductor,5,6 ionic liquid,7–9 polymer,10–12 and gel electrolytes.13–16 * To whom correspondence should be addressed. E-mail: kjkim@ korea.ac.kr. Tel.: 822-3290-3127. Fax: 822-3290-3121. † Korea University. ‡ Alagappa University.

(1) Gra¨tzel, M. J. Photochem. Photobiol., A 2004, 164, 3. (2) Nazeeruddin, M. K.; Kay, A.; Radices, I.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (3) Kubo, W.; Kitamura, T.; Hanabusa, K.; Wada, Y.; Yanagida, S. Chem. Commun. 2002, 374. (4) Wang, H. X.; Li, H.; Xue, B. F.; Wang, Z. X.; Meng, Q. B.; Chen, L. Q. J. Am. Chem. Soc. 2005, 127, 6394. (5) Saito, Y.; Fukuri, N.; Senadeera, R.; Kitamura, T.; Wada, Y.; Yanagida, S. Electrochem. Commun. 2004, 6, 71. (6) Meng, Q.-B.; Takahashi, K.; Zhang, X.-T.; Sutanto, I.; Rao, T. N.; Sato, O.; Fujishima, A.; Watanabe, H.; Nakamori, T.; Urigami, M. Langmuir 2003, 19, 3572. (7) Kubo, W.; Kitamura, T.; Hanabusa, K.; Wada, Y.; Yanagida, S. Chem. Commun. 2002, 374. (8) Wang, P.; Zakeeruddin, S. M.; Comte, P.; Exnar, I.; Gra¨tzel, M. J. Am. Chem. Soc. 2003, 125, 1166. (9) Wang, P.; Zakeeruddin, S. M.; Humphry-Baker, R.; Gra¨tzel, M. Chem. Mater. 2004, 16, 2694. (10) Nogueira, A. F.; Durrant, J. R.; De Paoli, M.-A. AdV. Mater. 2001, 13, 826. (11) Haque, S. A.; Palomares, E.; Upadhyaya, H. M.; Otley, L.; Potter, R. J.; Holmes, A. B.; Durrant, J. R. Chem. Commun. 2003, 24, 3008. (12) Nogueira, A. F.; Longo, C.; De Paoli, M.-A. Coord. Chem. ReV. 2004, 248, 1455.

Although solid-state electrolytes solve some of these problems, they show a lower solar-to-electricity conversion efficiency because of their lower electron injection efficiency. Recently, growing attention has been paid to quasi-solid-state DSSCs using polymer gel electrolytes, due to their unique hybrid network structure and favorable properties such as thermal stability, nonflammability, negligible vapor pressure, good contacting and filling properties with the nanocrystalline TiO2 electrode and counter electrode, and higher ionic conductivity, which is achieved by “trapping” a liquid electrolyte in polymer cages formed in a host matrix.13,17,18 Therefore, polymer gel electrolytes have been attracting intensive interest for diverse attentions. Up to the present, several types of polymer gel electrolytes have been used in quasi-solid-state DSSCs.19–21 Gel polymer electrolytes such as poly(acrylonitrile),22–24 poly(ethylene glycol),25,26 polymethylmethacrylate (PMMA),27–29 poly(2-hydroxyethyl methacrylate),30 poly(MMA-co(13) Wang, P.; Zakeeruddin, S. M.; Moser, J. E.; Nazeeruddin, M. K.; Sekiguchi, T.; Gra¨tzel, M. Nat. Mater. 2003, 2, 402. (14) Stathatos, E.; Lianos, P.; Zakeeruddin, S. M.; Liska, P.; Gra¨tzel, M. Chem. Mater. 2003, 15, 1825. (15) Mohmeyer, N.; Wang, P.; Schmidt, H. W.; Zakeeruddin, S. M.; Gra¨tzel, M. J. Mater. Chem. 2004, 14, 1905. (16) Komiya, R.; Han, L.; Yamanaka, R.; Islam, A.; Mitate, T. J. Photochem. Photobiol., A 2004, 164, 123. (17) Cao, F.; Oskam, G.; Searson, P. C. J. Phys. Chem. B 1995, 99, 17071. (18) Matsumoto, M.; Miyazaki, H.; Matsuhiro, K. Solid State Ionics 1996, 89, 263. (19) Dissanayake, M. A. K. L.; Bandara, L. R. A. K.; Bokalawala, R. S. P.; Jayathilaka, P. A. R. D.; Ileperuma, O. A.; Somasundaram, S. Mater. Res. Bull. 2002, 37, 867. (20) Wang, P. S.; Zakeeruddin, S. M.; Gra¨tzel, M. J. Fluorine Chem. 2004, 125, 1241. (21) Kubo, W.; Kambe, S.; Nakade, S. J. Phys. Chem. B 2003, 107, 4374. (22) Wang, G.; Zhou, X.; Li, M.; Zhang, J.; Kang, J.; Lin, Y.; Fang, S.; Xiao, X. Mater. Res. Bull. 2004, 39, 2113. (23) Illeperuma, O. A.; Dissanayake, M. A. K. L.; Somasundaram, S.; Bandara, L. R. A. K. Sol. Energy Mater. Sol. Cells 2004, 84, 117. (24) Kang, J.; Li, W.; Wang, X.; Lin, Y.; Li, X.; Xiao, X.; Fang, S. J. Appl. Electrochem. 2004, 34, 301. (25) Kim, Y. J.; Kim, J. H.; Kang, M.-S.; Lee, M. J.; Won, J.; Lee, J. C.; Kang, Y. S. AdV. Mater. 2004, 16, 1753. (26) Kim, J. Y.; Kim, T. H.; Kim, D. Y.; Park, N.-G.; Ahn, K.-D. J. Power Sources 2008, 175, 692. (27) Biancardo, M.; West, K.; Krebs, F. C. J. Photochem. Photobiol., A 2007, 187, 395. (28) Biancardo, M.; West, K.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2006, 90, 2575.

10.1021/la801375s CCC: $40.75  2008 American Chemical Society Published on Web 08/02/2008

Quasi-Solid-State Dye-Sensitized Solar Cell

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Figure 1. Chemical structure of PVdF-HFP.

MAA)/PEG,31 cyanoacrylate,32 poly(oligoethylene glycol methacrylate),33 poly(siloxane-co-ethylene oxide),34 poly(butylacry late),35 and poly(vinylidenefluoride-co-hexafluoropropylene) (PVdF-HFP)36–38 with different plasticizers have been employed in quasi-solid-state DSSCs. Among them, PVdF-HFP shows relatively high ionic conductivities at room temperature, and they are stable in the presence of TiO2 and Pt nanoparticles. In 2003, 6% solar energy conversion efficiency was reported in quasi-solid-state DSSCs based on a PVdF-HFP matrix, showing stable performance under both thermal stress and light soaking, matching the durability criteria applied to silicon solar cells for outdoor applications.13 Electrospinning was first studied by Zeleny39 in 1914 and patented by Formhals40 in 1934. Recently, this technique has regained a great deal of attention due to the surge of interest in nanotechnology, as continuous ultrafine fibers or fibrous structures of various polymers can be fabricated with diameters in the range from several micrometers down to tens of nanometers.41,42 Also, the electrospinning technique provides a simple, cost-effective approach for producing polymeric and inorganic nanofibers with structures that vary with the processing parameters. In this paper, we report for the first time on high-performance quasi-solid-state DSSCs fabricated using a novel electrospun membrane prepared from a 16 wt % PVdF-HFP solution in a mixture of acetone/N,N-dimethylacetamide (7:3 wt %) based on 0.6 M 1-hexyl-2,3-dimethylimidazolium iodide, 0.1 M LiI, 0.05 M I2, and 0.5 M 4-tert-butylpyridine in ethylene carbonate/ propylene carbonate (1:1 wt %) as the electrolyte. Thermally sealed cells were used to test the long-term stability of solar cells based on the electrospun membrane electrolyte and liquid electrolyte. The chemical structure of PVdF-HFP is shown in Figure 1.

2. Experimental Section 2.1. Materials. PVdF-HFP, acetone, N,N-dimethylacetamide, lithium iodide, iodine, 4-tert-butylpyridine, ethylene carbonate (EC), and propylene carbonate (PC) were purchased from Aldrich. 1-Hexyl2,3-dimethylimidazolium iodide was commercially obtained from the Kowoon Institute of Technology Innovation, Suwon University, Korea. All reagents were used without further purification. 2.2. Preparation of the PVdF-HFP Membrane. The electrospun membrane was prepared from a 16 wt % solution of (29) Yang, H.; Huang, M.; Wu, J.; Lan, Z.; Hao, S.; Lin, J. Mater. Chem. Phys. 2008, 110, 38. (30) Chen, D.; Zhang, Q.; Wang, G.; Zhang, H.; Li, J. Electrochem. Commun. 2007, 9, 2755. (31) Li, P. J.; Wu, J. H.; Huang, M. L.; Hao, S. C.; Lan, Z.; Li, Q.; Kang, S. Electrochim. Acta 2007, 53, 903. (32) Lu, S.; Koeppe, R.; Gunes, S.; Sariciftci, N. S. Sol. Energy Mater. Sol. Cells 2007, 91, 1081. (33) Masamitsu, M.; Hiromitsu, M.; Kikuo, M.; Yoshimasa, K.; Yoichi, T. Solid State Ionics 1996, 89, 263. (34) Liu, Y.; Lee, J. Y.; Hong, L. J. Power Sources 2004, 129, 303. (35) Kim, J. H.; Sung, M.-S.; Kim, Y. J.; Won, J.; Kang, Y. S. Solid State Ionics 2005, 176, 579. (36) Wang, P.; Zakeeruddin, S. M.; Exnar, I.; Gra¨tzel, M. Chem. Commun. 2002, 2972. (37) Scully, S. R.; Lloyd, M. T.; Herrera, R.; Giannelis, E. P.; Malliaras, G. G. Synth. Met. 2004, 144, 291. (38) Wang, P.; Zakeeruddin, S. M.; Gratzel, M. J. Fluorine Chem. 2004, 125, 1241. (39) Zeleny, J. Phys. ReV. 1914, 3, 69. (40) Formhals, A. U.S. Patent 1975504, 1934. (41) Dzenis, Y. Science 2004, 304, 1917. (42) Huang, Z.; Zhang, Y.; Kotaki, M.; Ramakrishna, S. Compos. Sci. Technol. 2003, 63, 2223.

Figure 2. Schematic representation of experimental technique for the preparation of the electrospun membrane.

PVdF-HFP in a mixture of acetone/N,N-dimethylacetamide (7:3 wt %) at 80 °C with constant stirring to form a transparent homogeneous polymer solution, which was then cooled to room temperature. This polymer solution was supplied to the stainless steel needle (24 G) using a syringe pump (KD Scientific, model 220), and a high voltage of 12 kV was applied to the end of the needle. The electrospun PVdF-HFP membrane was then deposited onto a grounded, polished stainless steel plate, where the tip-ground distance was fixed at 12 cm. A schematic representation of the experimental setup is shown in Figure 2. The thickness of the membrane was controlled as 30 µm using a micrometer. The membrane was vacuum-dried overnight at 80 °C to remove any remaining solvent and then hot pressed at 120 °C to reduce its thickness from 30 to 15 µm. 2.3. Characterization of the PVdF-HFP Membrane. The morphology of the electrospun PVdF-HFP membrane was observed using field-enhanced scanning electron microscopy (FE-SEM, Hitachi S-4100) under vacuum condition. The sample was gold-coated prior to the SEM measurement. 2.4. Conductivity Measurements. The electrospun PVdF-HFP membrane was soaked in 0.6 M 1-hexyl-2,3-dimethylimidazolium iodide, 0.1 M LiI, 0.05 M I2, and 0.5 M 4-tert-butylpyridine in EC/PC (1:1 wt %) to obtain the corresponding electrospun PVdF-HFP membrane electrolyte. The ionic conductivity of the electrospun PVdF-HFP membrane electrolyte was then measured using the complex impedance technique43 at 25 °C. The cells consisted of two identical Pt-sputtered FTO electrodes. The electrolyte resistance R was measured using a Solartron instrument (model 1287) with a 1260 frequency response analyzer controlled by a computer. The frequency limits were typically set between 10-2 and 106 Hz. The ac oscillation was 10 mV. The data were analyzed by Z-plot software. The ionic conductivity of the membrane σ was then calculated by the equation σ ) l/RA, where l is the thickness of the film, R is the electrolyte resistance, and A is the area of the film. 2.5. Fabrication of the Quasi-Solid-State DSSC. A thin layer of nonporous TiO2 film was deposited on cleaned FTO (LibbeyOwens-Ford TEC 8, 75% transmittance in the visible spectrum), using 5% titanium(IV) butoxide in ethanol by spin-coating at 3000 rpm, followed by annealing at 450 °C. The TiO2 electrode (TiO2 film thickness about 10 µm) was then obtained by spreading titania paste (Dyesol Ltd.) on the conducting glass substrate using a doctor blade technique. The film was dried at 70 °C for 10 min before removing the tape used for controlling the thickness of the film and then annealed at 450 °C for 30 min. The resulting TiO2 film was then sensitized by immersing it in a 0.3 mM anhydrous ethanolic solution of N719 dye ([(C4H9)4N]2[Ru(II)L2(NCS)2], where L ) 2,2′-bipyridyl-4,4′-dicarboxylic acid, ruthenium TBA 535, Solaronix SA) at room temperature for 24 h. To minimize the hydration of TiO2 from moisture in the ambient air, the films were immersed in the dye solution while they were at a temperature of around 100-120 °C after the annealing step. Afterward, the dye-sensitized TiO2 electrode was rinsed with anhydrous ethanol and dried in moisturefree air. (43) Singh, P. K.; Kim, K.-I.; Park, N.-G.; Rhee, H.-W. Macromol. Symp. 2000, 249-250, 162.

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Figure 3. Schematic diagram of a quasi-solid-state DSSC.

To prepare the counter electrode, a drop of 5 mM hydrogen hexachloroplatinate(IV) hydrate (H2PtCl6 · H2O) in 2-propanol was placed on the FTO glass substrate, followed by drying and annealing at 450 °C for 30 min. A quasi-solid-state DSSC was fabricated based on the electrospun membrane electrolyte by sandwiching a slice of the electrospun PVdF-HFP membrane between a dye-sensitized TiO2 electrode and a Pt counter electrode. For comparison, a reference DSSC based on liquid electrolyte was fabricated by placing the platinum electrode over the dye-coated TiO2 electrode; the edges of the cell were sealed with 1 mm wide strips of 60 µm thick Surlyn (Solaronix SA, SX1170 hot melt), in order to control the thickness of the electrolyte film and to avoid the short-circuiting of the cell. A hot press was used to press together the film electrode and the counter electrode. A drop of the electrolyte solution (0.6 M 1-hexyl-2,3-dimethylimidazolium iodide, 0.1 M LiI, 0.05 M I2, and 0.5 M 4-tert-butylpyridine in EC/PC (1:1 wt %)) was introduced into the clamped electrodes through one of two small holes drilled in the counter electrode. The holes were then covered and sealed with small squares of Surlyn strip. The resulting cells had an active area of 0.4 cm × 0.4 cm. A schematic diagram of the quasi-solid-state DSSC is shown in Figure 3. 2.6. Measurements. The photocurrent density-voltage (J-V) curves of the assembled DSSCs were measured with a computercontrolled digital source meter (Keithley, model M236). A 300 W Xe lamp with an AM 1.5 filter (Oriel) was used to illuminate the DSSCs at 100 mW cm-2. The incident light intensities were calibrated using a photodiode detector (Newport 818UV) and an optical power meter (Newport 1830-C). The J-V curves of the DSSCs with the electrospun membrane electrolyte were measured after an aging period of 24 h. Thermally sealed cells were used to test the longterm stability of the solar cells. The sealed cells were stored in a desiccator and applied to electrochemical measurement every 48 h in order to study their long-term stability. The photoelectrochemical parameters, i.e., the fill factor (FF) and light-to-electricity conversion efficiency (η), were calculated by the following equations:44

VmaxJmax FF ) VocJsc η(%) )

VmaxJmax VocJscFF × 100 ) × 100 Pin Pin

(1) (2)

where Jsc is the short-circuit current density (mA cm-2), Voc is the open-circuit voltage (V), Pin is the incident light power (mW cm-2), and Jmax (mA cm-2) and Vmax (V) are the current density and voltage in the J-V curves, respectively, at the point of maximum power output.

3. Results and Discussion 3.1. Electrospun PVdF-HFP Membrane. Figure 4 shows the SEM image of the electrospun PVdF-HFP membrane. It consists of thin fibers with an average diameter of about 600 nm. The electrospun membrane has a three-dimensional network structure with high porosity, and the pores are fully interconnected. Hence, it should be able to uptake sufficient electrolyte solution in its pores and have enough mechanical strength due to its (44) Gra¨tzel, M. Prog. PhotoVoltaics 2000, 8, 1427.

Figure 4. SEM photograph of an electrospun PVdF-HFP membrane.

Figure 5. Photocurrent-voltage curves for DSSCs with liquid electrolyte and electrospun PVdF-HFP membrane electrolyte. Light intensity: 100 mW cm-2. Table 1. Photovoltaic Parameters of the Dye-Sensitized TiO2 Solar Cells Based on Liquid Electrolyte and Electrospun PVdF-HFP Membrane Electrolyte electrolyte

Jsc (mA cm-2)

Voc (V)

FF

η (%)

liquid PVdF-HFP membrane

16.86 15.57

0.73 0.76

0.63 0.62

7.8 7.3

three-dimensional network structure with numerous physical cross-linking points. The ionic conductivity of the electrospun PVdF-HFP membrane electrolyte was found to be of the order of 10-5 S cm-1 at 25 °C, which is almost comparable with that of a liquid electrolyte system. 3.2. Photovoltaic Performance. The J-V curves of the DSSCs with the conventional liquid electrolyte and quasi-solidstate electrolyte based on the electrospun PVdF-HFP membrane at a light intensity of 100 mW cm-2 are presented in Figure 5. The corresponding photovoltaic parameters are summarized in Table 1, which shows that a maximum η of 7.3%, Jsc of 15.57 mA cm-2, Voc of 0.76 V, and FF of 0.62 were obtained for the electrospun PVdF-HFP membrane electrolyte. It is noted that an enhanced Voc value (0.76 V) was observed, whereas the Jsc value (15.57 mA cm-2) decreased in the case of the electrospun membrane electrolyte when compared with the corresponding values of 0.73 V and 16.86 mA cm-2 for the liquid electrolyte, respectively. The FF remained the same, regardless of the presence of the PVdF-HFP membrane.

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The enhanced Voc value may be explained in terms of the reduction in the back electron transfer from the TiO2 conduction band to the I3- ions in the electrolyte. The Voc value kinetically manifests the degree of back electron transfer from the TiO2 conduction band to the I3- ions in the electrolyte (eq 3):45

Voc ) (kT/e) ln(Jsc/Jo)

(3)

where Jo is the exchange current density and is related to the rate of the back electron transfer. When the electrospun PVdF-HFP membrane is used in the DSSC, the polymer, which is hydrophobic in nature, reduces the ionic mobilities of the triiodide and iodide ions through the polymer membrane in the electrolyte solution. As a result, the triiodide ion concentration inside the TiO2 film decreases, leading to the suppression of the back electron transfer from the conduction band of the TiO2 electrode to the I3- ions. Consequently, the value of Voc is increased, according to eq 3 above. The adsorption of PVdF-HFP to the surface of TiO2 would shift the conduction band edge negatively like 4-tertbutylpyridine, which is known to shift the conduction band edge toward a negative potential due to its adsorption to TiO2.46 However, the contribution to the shift of the conduction band arising from the adsorption of PVdF-HFP, albeit possible, is considered insignificant, because the PVdF-HFP membrane is only in contact with the top surface of the TiO2 film. There are two possible explanations for the slightly lower Jsc value observed in the case of the electrospun PVdF-HFP membrane electrolyte relative to that observed in the case of the liquid electrolyte. First, the lower ionic mobilities of the triiodide and iodide ions in the presence of the polymer may be responsible for the decrease in the photocurrent in the presence of the PVdF-HFP membrane, as a result of the slower regeneration of the sensitizer molecules. Second, the number of excited states of the dye molecules, which are involved in the electron injection process, is reduced, due to the enhanced Voc value in the case of the DSSC with the PVdF-HFP membrane compared to that in the DSSC without the membrane. That is, the higher Voc in Table 1 indicates a shift of the conduction band edge of TiO2 toward a more negative value by approximately 30 mV in the presence of the PVdF-HFP membrane with respect to the position of the conduction band edge of TiO2 in the absence of the PVdF-HFP membrane. Assuming that the energy levels of the dye molecules either do not move at all with the band edge shift or move by less than 30 mV, one can understand the decrease in the photocurrent density in terms of the lower participation of the high-lying excited states of the dye molecules in the electron injection process.47 3.3. Long-Term Stability. The electrospun PVdF-HFP membrane based on the EC/PC organic electrolytes was employed to fabricate quasi-solid-state solar cells. The cells were sealed with thermal plastic in order to test the long-term durability of the cells. For comparison, solar cells with a liquid electrolyte were also assembled according to the method described in the Experimental Section. These two kinds of solar cells were stored in a desiccator, and their J-V curves were recorded in intervals of 48 h. The light-to-electricity conversion efficiency variances of these cells containing these two kinds of electrolytes are compared in Figure 6. The efficiency of the solar cell with the liquid electrolyte decreases gradually, due to the evaporation of the volatile liquid electrolyte, whereas that of the quasi-solid(45) Kaven, L.; Gra¨tzel, M. Electrochim. Acta 1989, 34, 1327. (46) Boschloo, G.; Lindstro¨m, H.; Magnusson, E.; Holmberg, A.; Hagfeldt, A. J. Photochem. Photobiol., A 2002, 148, 11. (47) Park, N.-G.; Chang, S.-H.; van de Lagemaat, J.; Kim, K.-J.; Frank, A. J. Bull. Korean Chem. Soc. 2000, 21, 985.

Figure 6. Normalized light-to-electricity conversion efficiency variation of the DSSCs with liquid electrolyte and electrospun PVdF-HFP membrane electrolyte.

state solar cell based on the electrospun PVdF-HFP membrane electrolyte remains at 96% of its initial value. In the DSSC assembled with the electrospun membrane electrolyte, the organic solvent containing the I3-/I- redox couple is well-encapsulated in the electrospun membrane electrolyte. Moreover, the electrolyte can promote strong interfacial contact between the dye-adsorbed TiO2 electrode and the Pt counter electrode. This gives more stable performance than the DSSC assembled with the liquid electrolyte. This phenomenon indicates that the liquid-conserving ability of the electrospun PVdF-HFP membrane electrolyte is extremely high. Finally, nanoparticles of larger size were not utilized for improving the performance of DSSCs, because the focus of this research was the use of the electrospun PVdF-HFP membrane in the cells. Having investigated the effect of the PVdF-HFP membrane as an electrolyte holder on the performance of DSSCs, we intend to incorporate larger TiO2 particles onto the electrospun PVdF-HFP membrane to improve the performance of the DSSCs by increasing the amount of scattered light.48

4. Conclusions An electrospun PVdF-HFP membrane was prepared from a 16 wt % solution of poly(vinylidenefluoride-co-hexafluoropropylene) in a mixture of acetone/N,N-dimethylacetamide (7:3 wt %) and employed for the first time to form quasi-solid-state DSSCs. The solar-to-electricity conversion efficiency of the quasisolid-state solar cells with the electrospun PVdF-HFP membrane electrolyte was 7.3%, which was slightly lower than the value of 7.8% observed for the solar cells with the conventional liquid electrolyte at an illumination intensity of 100 mW cm-2. The electrospun PVdF-HFP membrane encapsulated the electrolyte solution well without leakage and displayed better long-term stability than that with conventional liquid electrolyte. Thus, the quasi-solid-state DSSC fabricated with the electrospun PVdF-HFP membrane electrolyte is a promising candidate for practical solar cells with good durability. Acknowledgment. This work was supported by the new and renewable energy Research and Development project under contract 2006-N-PV12-P-05. LA801375S (48) Ferber, J.; Luther, J. Sol. Energy Mater. Sol. Cells 1998, 54, 265.