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Influence of Nylon 6 in I3-/I- Redox Electrolyte on the Photovoltaic Performance and Stability of Dye-Sensitized Solar Cells Seung Yong Lee,† Beomjin Yoo,† Min Ki Lim,‡ Tae-kyeong Lee,† A. R. Sathiya Priya,† and Kang-Jin Kim*,† †
Department of Chemistry, Korea University, Seoul, 136-713, Republic of Korea and ‡DongWoo Fine-Chem, Iksan-si, Jeonbuk, 802-8, Republic of Korea Received October 20, 2009. Revised Manuscript Received December 16, 2009
Nylon 6 fibers are used, for the first time, in dye-sensitized solar cells (DSSCs). The overall energy conversion efficiency obtained with 0.18 M nylon 6 reaches 6.2%, which is comparable to that (6.7%) obtained without adding nylon 6 on the day of cell fabrication. However, it is found that the long-term stability of the DSSCs with nylon 6 is superior to that of a reference electrolyte as a result of the complexation of nylon 6 with I3-. Furthermore, nylon 6 is found to be a corrosion inhibitor for silver metal in the electrolyte containing I3-.
1. Introduction Dye-sensitized solar cells (DSSCs) are regarded as low cost, next generation solar cells, and significant progress has been made to improve their performance and stability during the past decade.1 Typical DSSCs consist of two conductive transparent glasses, a ruthenium dye-sensitized TiO2 film, a platinum catalyst layer, and a liquid electrolyte containing the I3-/I- redox couple. In studies reported in the literature, attempts have been made to replace these components with new electrode materials,2-9 alternative sensitizers,10-14 and different types of electrolytes.15-21 In *To whom correspondence should be addressed. E-mail: kjkim@ korea.ac.kr. Tel.: 822-3290-3127; fax: 822-3290-3121. (1) O’Regan, B.; Gr€atzel, M. Nature 1991, 353, 737. (2) Saito, Y.; Kubo, W.; Kitamura, T.; Wada, Y.; Yanagida, S. J. Photochem. Photobiol. A: Chem. 2004, 164, 153. (3) Saito, Y.; Kitamura, T.; Wada, Y.; Yanagida, S. Chem. Lett. 2002, 31, 1060. (4) Imoto, K.; Takahashi, K.; Yamaguchi, T.; Komura, T.; Nakamura, J.; Murata, K. Sol. Energy Mater. Sol. Cells 2003, 79, 459. (5) Palomares, E.; Clifford, J. N.; Haque, S. A.; Lutz, T.; Durrant, J. R. J. Am. Chem. Soc. 2003, 125, 475. (6) Zaban, A.; Chen, S. G.; Chappel, S.; Gregg, B. A. Chem. Commun. 2000, 2231. (7) Chen, S. G.; Chappel, S.; Diamant, Y.; Zaban, A. Chem. Mater. 2001, 13, 4629. (8) Wang, Z.-S.; Huang, C.-H.; Huang, Y.-Y.; Hou, Y.-J.; Xie, P.-H.; Zhang, B.-W.; Cheng, H.-M. Chem. Mater. 2001, 13, 678. (9) Diamant, Y.; Chen, S. G.; Melamed, O.; Zaban, A. J. Phys. Chem. B 2003, 107, 1977. (10) Hara, K.; Dan-oh, Y.; Kasada, C.; Ohga, Y.; Shinpo, A.; Suga, S.; Sayama, K.; Arakawa, H. Langmuir 2004, 20, 4205. (11) Hara, K.; Sayama, K.; Ohga, Y.; Shinpo, A.; Suga, S.; Arakawa, H. Chem. Commun. 2001, 569. (12) Hara, K.; Kurashige, M.; Ito, S.; Shinpo, A.; Suga, S.; Sayama, K.; Arakawa, H. Chem. Commun. 2003, 252. (13) Kitamura, T.; Ikeda, M.; Shigaki, K.; Inoue, T.; Anderson, N. A.; Ai, X.; Lian, T.; Yanagida, S. Chem. Mater. 2004, 16, 1806. (14) Horiuchi, T.; Miura, H.; Uchida, S. Chem. Commun. 2003, 3036. (15) Kumara, G. R. A.; Kaneko, S.; Okuya, M.; Tennakone, K. Langmuir 2002, 18, 10493. (16) Kumara, G. R. A.; Konno, A.; Shiratsuchi, K.; Tsukahara, J.; Tennakone, K. Chem. Mater. 2002, 14, 954. (17) Stathatos, E.; Lianos, P.; Zakeeruddin, S. M.; Liska, P.; Gr€atzel, M. Chem. Mater. 2003, 15, 1825. (18) Usui, H.; Matsui, H.; Tanabe, N.; Yanagida, S. J. Photochem. Photobiol. A: Chem. 2004, 164, 97. (19) Cao, F.; Oskam, G.; Searson, P. C. J. Phys. Chem. 1995, 99, 17071. (20) Kubo, W.; Murakoshi, K.; Kitamura, T.; Wada, Y.; Hanabusa, K.; Shirai, H.; Yanagida, S. Chem. Lett. 1998, 27, 1241. (21) Wang, P.; Zakeeruddin, S. M.; Moser, J. E.; Nazeeruddin, M. K.; Sekiguchi, T.; Gr€atzel, M. Nat. Mater. 2003, 2, 402.
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most cases, the studies were aimed at improving the properties and usability of the DSSCs, in order to meet the requirements of commercial applications and/or to reduce the production cost. The overall energy conversion efficiencies of DSSCs based on liquid electrolytes using organic compounds such as acetonitrile, propylene carbonate, and ethylene carbonate as solvents and the I3-/I- redox couple as an electrolyte have reached 10-11% under irradiation of AM1.5.22,23 Despite the high conversion efficiencies obtained for DSSCs with liquid electrolytes, their potential problems, such as the leakage and volatilization of the liquid, the possible desorption and photodegradation of the attached dyes, and the corrosion of the counter electrode, have brought about difficulties in cell fabrication and limited the long-term performance and practical use of these DSSCs.24 To prevent or reduce electrolyte leakage, several methods have been introduced. One strategy is to replace the volatile solvents with ionic liquids.25-28 Second, p-type semiconductors,29,30 inorganic hole transport materials,31 organic hole transport materials,32 and polymer/redox couple blends33 have been introduced as substitutes for the traditional liquid electrolyte in all-solidstate DSSC configurations. Furthermore, nanocomposites,34,35 (22) Gr€atzel, M. J. Photochem. Photobiol. A: Chem. 2004, 164, 3. (23) Chiba, Y.; Islam, A.; Watanabe, Y.; Komiya, R.; Koide, N.; Han, L. Jpn. J. Appl. Phys. 2006, 45, L638. (24) Wu, J. H.; Hao, S. C.; Lan, Z.; Lin, J. M.; Huang, M. L.; Huang, Y. F.; Fang, L. Q.; Yin, S.; Sato, T. Adv. Funct. Mater. 2007, 17, 2645. (25) Kawano, R.; Matsui, H.; Matsuyama, C.; Sato, A.; Hasan Susan, Md. A. B.; Tanabe, N.; Watanabe, M. J. Photochem. Photobiol. A: Chem. 2004, 164, 87. (26) Xue, B.; Wang, H.; Hu, Y.; Li, H.; Wang, Z.; Meng, Q.; Huang, X.; Chen, L.; Sato, O. C. R. Chim. 2006, 9, 627. (27) Reiter, J.; Vondr€ak, J.; Mich€alek, J.; Micka, Z. Electrochim. Acta 2006, 52, 1398. (28) Wang, P.; Zakeeruddin, S. M.; Moser, J. E.; Gr€atzel, M. J. Phys. Chem. B 2003, 107, 13280. (29) Murai, S.; Mikoshiba, S.; Sumino, H.; Kato, T.; Hayase, S. Chem. Commun. 2003, 13, 1534. (30) O’Regan, B.; Schwartz, D. T. Chem. Mater. 1998, 10, 1501. (31) O’Regan, B.; Lenzmann, F.; Muis, R.; Wienke, J. Chem. Mater. 2002, 14, 5023. (32) Ikeda, N.; Teshima, K.; Miyasaka, T. Chem. Commun. 2006, 16, 1733. (33) Han, H. W.; Liu, W.; Zhang, J.; Zhao, X. Z. Adv. Funct. Mater. 2005, 15, 1940. (34) Stergiopoulos, T.; Arabatzis, I. M.; Katsaros, G.; Falaras, P. Nano. Lett. 2002, 2, 1259. (35) Kaneko, M.; Hoshi, T. Chem. Lett. 2003, 32, 872.
Published on Web 01/05/2010
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polymer gels,36,37 and solid-state or quasi-solid electrolytes have been prepared by the incorporation of various polymers38,39 and/ or nanoparticles40,41 into the liquid-type electrolytes. Among them, polymer electrolytes stand out, because of their excellent properties, such as their ease of fabrication, low cost, and good stability, so that they have attracted growing interest from the viewpoint of their practical applications. In the present study, using nylon 6, a semicrystalline polyamide, as the polymer host for the first time, 3-methoxypropionitrile as an organic solvent, and lithium iodide and iodine as the source of I3-/I-, an electrolyte was prepared. Surprisingly, nylon 6 was found to be very easily soluble in 3-methoxypropionitrile solutions containing I3-. Various properties of the electrolytes, such as their viscosity, charge transfer resistance, and photovoltaic characteristics, were studied. The corrosion of silver metal and long-term stability and performance of the DSSCs were investigated and compared with those of the cell containing a reference electrolyte, in order to examine the influence of nylon 6.
2. Experimental Section Nylon 6 was prepared from ε-caprolactam by hydrolytic reaction.42 When a mixture of 10 g of ε-caprolactam and 2 drops of water was heated at about 60 °C in an inert atmosphere of N2 for about 1 h, the ring structure of ε-caprolactam broke and it underwent polymerization. Then, the remaining water was removed by using a rotary evaporator to form fibers of nylon 6. Raman measurement of the solutions containing 1.0 M I2 and 2.0 M I- with and without 5.0 M nylon 6 in 3-methoxypropionitrile were made in backscattering geometry with a Jobin-Yvon LabRam HR fitted with a liquid nitrogen-cooled CCD detector under ambient conditions using the 514.532 nm line of an argon ion laser. A Hewlett-Packard Agilent 8453 diode array spectrometer was used for the measurements of the UV-vis absorption spectra of the electrolytic solutions containing 6 10-5 M I2, 1 10-4 M I-, and various concentrations of nylon 6 in 3-methoxypropionitrile. Viscosity measurements were conducted under an Ar atmosphere using an SV-10 viscometer. Chronoamperometric curves were also obtained from acetonitrile solutions containing 0.6 M 1-hexyl-2,3-dimethylimidazolium iodide, 0.05 M I2, 0.1 M LiI, and 0.5 M 4-tert-butylpyridine without and with different concentrations of nylon 6 using an EG&G PARC 263A potentiostat, with an electrochemical cell consisting of a Pt electrode (BAS, MF-2013, 1.6 mm in diameter), a Pt-wire auxiliary electrode, and a Ag/AgCl reference electrode. The potentials were applied at 1.00 and -1.00 V for the oxidation of I- and reduction of I3-, respectively. Dye-coated TiO2 films were prepared as working electrodes for the DSSCs as follows:43 a thin buffer layer of nonporous TiO2 was deposited from 5% titanium (IV) butoxide in ethanol on a cleaned FTO conducting glass, purchased from Libbey-Owens-Ford (TEC 8, 75% transmittance in the visible region), by spin coating at 3000 rpm. The thin layer coated FTO glass was cleaned and annealed at 450 °C. Dyesol titania paste (Dyesol Ltd.) was deposited on the above pretreated FTO glass by the doctor blade technique, using two pieces of transparent adhesive tape to delimit (36) Wang, P.; Zakeeruddin, S. M.; Exnar, I.; Gr€atzel, M. Chem. Commun. 2002, 2972. (37) Ileperuma, O. A.; Dissanayake, M. A. K. L.; Somasundaram, S. Electrochim. Acta 2002, 47, 2801. (38) Kim, D.-W.; Jeong, Y.-B.; Kim, S.-H.; Lee, D.-Y.; Song, J.-S. J. Power Sources 2005, 149, 112. (39) Biancardo, M.; West, K.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2006, 90, 2575. (40) Chen, Z.; Yang, H.; Li, X.; Li, F.; Yi, T.; Huang, C. J. Mater. Chem. 2007, 17, 1602. (41) Kim, J. Y.; Chung, I. J.; Kim, J. K.; Yu, J.-W. Curr. Appl. Phys. 2006, 6, 969. (42) Reimschuessel, H. K. J. Polym. Sci.: Macromol. Rev. 1997, 12, 65. (43) Kang, M. G.; Park, N.-G.; Chang, S. H.; Choi, S. H.; Kim, K.-J. Bull. Korean Chem. Soc. 2002, 23, 140.
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the thickness of the film. The film was dried for 10 min at 70 °C, followed by annealing at 450 °C for 30 min. In this way, a porous TiO2 film with a thickness of about 10 μm was produced. The TiO2 film was cooled to room temperature, warmed again to 60 °C, and then sensitized by immersing it in an anhydrous ethanol solution of 0.3 mM N719 dye (Solaronix SA, Swiss) at room temperature for 24 h. To prepare the counter electrode, a drop of 5 mM hydrogen hexachloroplatinate (IV) hydrate (H2PtCl6 3 H2O) in 2-propanol was placed on the FTO glass substrate, followed by drying and annealing at 450 °C for 30 min. A DSSC was fabricated by placing the Pt counter electrode over the dye-coated TiO2 electrode, and the edges of the cell were sealed with 1-mm-wide strips of 60-μmthick Surlyn (Solaronix SA, SX1170 Hot Melt), in order to control the thickness of the electrolyte film. A hot press was used to press together the film electrode and the counter electrode. The electrolyte consisted of 0.05 M I2, 0.1 M LiI, 0.6 M 1-hexyl-2,3dimethylimidazolium iodide, and 0.5 M 4-tert-butylpyridine in 3-methoxypropionitrile with different concentrations of nylon 6. A drop of the electrolyte solution was introduced into the clamped electrodes through one of two small holes drilled in the counter electrode. After the electrolyte was soaked out through the other hole, the holes were then covered and sealed with microscope objective glass by soldering.43 Reference DSSCs were also prepared under identical conditions without nylon 6. The photoelectrochemical behavior of the DSSCs was characterized using a Keithley M236 source measure unit. An Oriel Xe lamp (300 W) with an AM 1.5 filter was used to illuminate the DSSCs at a light intensity of 1 sun (100 mW cm-2). The active area of the TiO2 electrode was 0.4 cm 0.4 cm. The thermally sealed cells fabricated above were used to test the long-term stability of the solar cells. The sealed cells were stored in a desiccator and subjected to electrochemical measurements every 24 h at 100 mW cm-2 at room temperature in order to study their long-term stability. To complement the photocurrent behavior of the DSSCs, their electrochemical impedance spectra were recorded using a Solartron 1287 potentiostat with a Solartron 1260 frequency-response detector. The measurements were performed under AM 1.5G, one sun, illumination at open-circuit condition over the frequency range of 10-2 Hz to 106 Hz with an ac amplitude of 10 mV. The data were analyzed using ZView (2.9b, Scribner Associate, Inc.) software. Tafel plots for the corrosion of silver were obtained with an EG&G PARC 273A potentiostat at a scan rate of 100 mV s-1, using an electrochemical cell consisting of a printed Ag film working electrode (5 mm 5 mm on glass), a Pt wire auxiliary electrode, and a Pt wire reference electrode.
3. Results and Discussion 3.1. Characterizations of the Electrolyte. Nylon 6 is insoluble in water, but newly found to be readily soluble in the aqueous solution containing I3-. This enhanced solubility of nylon 6 can be explained by the complexation between I3- and nylon 6, producing I3--nylon 6 complex,44 through the coordination of I3- to the hydrogen bonds in nylon 6, which is analogous to the enhanced solubility of molecular iodine in water by its complexation with iodide ion. The formation of I3--nylon 6 complex was confirmed by the measurements of Raman and UV-vis absorption spectra. Figure 1 shows the Raman spectra of the solutions containing 1.0 M I2 and 2.0 M I- with and without 5.0 M nylon 6 in 3-methoxypropionitrile, which agree well with those of the complex reported by Kawaguchi44 and Kang et al.45 Figure 2 reveals that the absorbance decreases and the absorption maximum shifts toward the long wavelength with increasing concentration of nylon 6 in 3-methoxypropionitrile solutions (44) Kawaguchi, A. Polymer 1994, 35, 2665. (45) Kang, Y. A.; Lee, Y. H. J. Appl. Polym. Sci. 2003, 88, 1138.
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Figure 1. Raman spectra of the solutions containing 1.0 M I2 and 2.0 M I- with (upper) and without (lower) 5.0 M nylon 6 in 3methoxypropionitrile.
Figure 3. J-V curves of the DSSCs assembled with various concentrations of nylon 6 under illumination and in the dark. The light intensity was 100 mW cm-2. Table 2. Photovoltaic Parameters of the DSSCs with Electrolytes Containing Different Concentrations of Nylon 6a nylon 6 (M)
Jsc (mA cm-2)
0.00 18.20 0.18 18.02 0.44 15.77 0.88 14.47 1.8 12.63 4.4 10.66 a The same electrolyte as in Table 1.
Voc (V)
FF
η (%)
0.68 0.70 0.70 0.69 0.71 0.75
0.54 0.49 0.53 0.56 0.58 0.55
6.7 6.2 5.9 5.6 5.2 4.4
Figure 2. UV-vis absorption spectra of the solutions containing 6 10-5 M I2, 1 10-4 M I-, and various concentrations of nylon 6 in 3-methoxypropionitrile. Table 1. Viscosities of Electrolytes nylon 6 (M)
b
viscosity (mPa s)
0.00 1.24 0.44 1.32 0.88 1.45 a The electrolyte consisted of nylon 6, 0.1 M LiI, 0.05 M I2, 0.6 M 1hexyl-2,3-dimethylimidazolium iodide, and 0.5 M 4-tert-butylpyridine in 3-methoxypropionitrile. b Concentration of nylon 6 in the electrolytic solution.
containing 6 10-5 M I2 and 1.2 10-4 M I-, indicating formation of the complex. The recorded viscosities of the electrolytes containing nylon 6, 0.1 M LiI, 0.05 M I2, 0.6 M 1-hexyl-2,3dimethylimidazolium iodide, and 0.5 M 4-tert-butylpyridine in 3-methoxypropionitrile are given in Table 1. The viscosity of the electrolyte increased slightly in the presence of nylon 6. 3.2. Photovoltaic Performance. The effect of different concentrations of nylon 6 in the electrolyte on the DSSCs was investigated by measuring their photocurrent density-voltage (J-V) characteristics under an illumination of 100 mW cm-2. The results obtained on the day of cell fabrication are shown in Figure 3 and the corresponding parameters are listed in Table 2. With increasing concentration of nylon 6, the short-circuit photocurrent density (Jsc) decreased, whereas the open-circuit voltage (Voc) consistently showed a slight increase. As a result, the nylon 6-added DSSC showed a lower overall energy conversion efficiency 6640 DOI: 10.1021/la903951x
Figure 4. Chronoamperometric curves for (left) the oxidation of I- and (right) the reduction of I3- in the acetonitrile solutions containing various concentrations of nylon 6, 0.6 M 1-hexyl-2,3dimethylimidazolium iodide, 0.05 M I2, 0.1 M LiI, and 0.5 M 4-tert-butylpyridine.
(η) than that of the reference DSSC, i.e., 6.7%. The best performance was obtained when the electrolyte contained 0.18 M nylon 6, whereupon the Jsc, Voc, fill factor (FF), and η values of 18.02 mA cm-2, 0.70 V, 0.49, and 6.2% were obtained, respectively. One possible explanation for the decrease in the Jsc value is the slower diffusion of I3- and I- ions in the more viscous electrolyte in the presence of nylon 6 compared to the reference electrolyte. The difference in their rates of diffusion in the absence and presence of nylon 6 was estimated from the chronoamperometric plots. The results shown in Figure 4 reveal that the diffusionlimited currents (id), described by eq 1 below,46 for both I3(46) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; John Wiley & Sons: New York, 2001; p 163.
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Article Table 3. Electrochemical Parameters of the DSSCs Assembled with Nylon 6a nylon 6 (M)
Rs (Ω cm-2)
R1 (Ω cm-2)
R2 (Ω cm-2)
0.00 2.07 3.56 3.71 0.18 2.10 4.13 3.63 1.8 2.11 6.70 3.57 a The same electrolyte as in Table 1.
Rd (Ω cm-2)
D (cm2 s-1)
2.44 2.48 3.34
1.3 10-5 1.1 10-5 9.1 10-6
Figure 5. Nyquist plots of the DSSCs fabricated with the reference electrolyte and with the electrolyte containing 0.18 and 1.8 M nylon 6. The plots were recorded over the frequency range of 0.01-106 Hz, with an ac amplitude of 10 mV. The inset shows the equivalent circuit. Rs: solution resistance. R1 and R2: charge transfer resistance at the Pt/electrolyte and the TiO2/electrolyte, respectively. CPE1 and CPE2: capacitance element corresponding to R1 and R2, respectively. Ws: Warburg impedance.
reduction and I- oxidation decreased upon the addition of nylon 6 as compared to those obtained without nylon 6, confirming the decreased diffusion rates of both I3- and I- ions in the electrolyte containing nylon 6. id ¼ nFAD1=2 C=π1=2 t1=2
ð1Þ
where n is the electron transfer number per molecule, F is the Faraday constant, A is the surface area of the electrode, D is the diffusion coefficient, C is the concentration, and t is the time. This variation was also attributed to the structural change of the electrolyte associated with the formation of the I3--nylon 6 complexes. Nylon 6 contains an amide group which is connected to one another through hydrogen bonds, to which I3- ions coordinate to produce the complexes. However, this complexation does not form a conducting channel for the charge carriers, I3- and I-, which would facilitate the ionic transport through the electrolyte and, consequently, leads to a decrease in the photocurrent compared to that in the reference electrolyte. The electrochemical impedance spectra further supported the decreased Jsc as follows. Figure 5 compares the Nyquist plots of the DSSCs, measured at 100 mW cm-2, with and without nylon 6. The inset shows the equivalent circuit. The three semicircular shapes from left are assigned to impedances related to charge transport at the Pt electrode, at the TiO2/dye/electrolyte interface and in Nernstian diffusion within the electrolyte, respectively.47 The corresponding electrochemical parameters in Table 3 were obtained by fitting the plots with ZView software. It is noted that the first semicircle conspicuously increased with the addition of nylon 6 and the corresponding charge transfer resistance (R1) increased. R1 represents mainly the resistance at the Pt/electrolyte interface, and therefore, the increase in R1 may imply that the activation barrier for electron transfer from the Pt electrode to I3increases as a result of the formation of the I3--nylon 6 complex. Table 3 also reveals that the diffusion resistance (Rd) increased and, accordingly, the diffusion coefficient (D) of the I3- ions decreased. The diffusion coefficient was calculated using the equations given by Fabregat-Santiago et al. and Kang et al.48,49 The decrease in the diffusion coefficient of the I3- ions is (47) Han, L.; Koide, N.; Chiba, Y.; Mitate, T. Appl. Phys. Lett. 2004, 84, 2433. (48) Fabregat-Santiago, F.; Bisquert, J.; Palomares, E.; Otero, L.; Kuang, D.; Zakeeruddin, S. M.; Gr€atzel, M. J. Phys. Chem. C 2007, 111, 6550. (49) Kang, S. H.; Kim, J.-Y.; Kim, H. S.; Koh, H.-D.; Lee, J.-S.; Sung, Y.-E. J. Photochem. Photobiol. A: Chem. 2008, 200, 294.
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Figure 6. Variation of the photovoltaic parameters of the DSSCs assembled with the reference electrolyte (filled circles) and with the electrolyte containing 0.18 M nylon 6 (open circles) at room temperature over a period of 12 d.
consistent with the chronoamperometric data mentioned above, due to the increased viscosity of the electrolyte in the presence of nylon 6. The series resistance (Rs) and charge transfer resistance at the TiO2/dye/electrolyte interface (R2) were not influenced significantly by the addition of nylon 6. It is concluded therefore that the increased charge transfer resistance at the Pt/electrolyte interface and the slowed diffusion of the charge carriers explain the decrease in Jsc. To explain the slight, but consistent, increase of the Voc value with the addition of nylon 6, the J-V curves were measured in the dark. The dark currents, also shown in Figure 3, decreased with increasing concentration of nylon 6 in the electrolyte. This decrease in the dark current is thought to be related to the decrease in the effective concentration of I3- in the TiO2 film as a result of the formation of the I3--nylon 6 complexes. Thus, the decrease in the dark current should lead to an increase in Voc. 3.3. Long-Term Stability. Despite the overall energy conversion efficiency of the DSSCs with nylon 6 being slightly lower than that of the reference cell, its stability was increased enormously. For the stability test, the DSSCs with and without 0.44 M nylon 6 in the electrolyte were stored in a desiccator and their J-V curves were recorded at intervals of 24 h. The resulting variances of the photovoltaic parameters of the cells are compared in Figure 6. Relative to those in the absence of nylon 6, the Voc value remained almost constant (Figure 6a), whereas the FF value decreased slightly (Figure 6b) with time. The decrease in the FF can be ascribed to an increase of the ohmic drop over the series resistances in the cell due to the increases in the photocurrents with time as shown in Figure 5c.50 It was remarkable to observe (50) Kim, K.-J.; Benkstein, K. D.; van der Lagemaat, J.; Frank, A. J. Chem. Mater. 2002, 14, 1042.
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that the Jsc value was much higher for the DSSCs with nylon 6 compared with that for the reference cell (Figure 6c) from the day following cell fabrication. Apparently, on the day of cell fabrication, the increased viscosity of the electrolyte in the presence of nylon 6 led to the poor penetration of the electrolyte into the porous TiO2, resulting in a lower Jsc value. Primarily as a result of the slower decay of Jsc, the overall energy conversion efficiency of the DSSC with nylon 6 remained about 78%, whereas that of the DSSC with the reference electrolyte remained only 40% after being stored for 12 d (Figure 6d). This enhanced stability shown in Figure 6d suggests that the presence of nylon 6 in the electrolyte containing I3- suppresses the volatility of the electrolytic solutions. This enormously enhanced stability was attributed to the formation of the I3--nylon 6 complexes through the coordination of I3- to the hydrogen bonds in nylon 6.44 These complexes are well-dispersed in the electrolyte of 3-methoxypropionitrile and promote the coordination of the organic solvent molecules, enabling the electrolyte containing I3--nylon 6 complexes to hold the solvent molecules strongly and thus to suppress their volatilization. It is concluded that this solvent holding ability is responsible for the more stable performance of the DSSC with nylon 6 than that without nylon 6. 3.4. Corrosion of Silver Metal. The scaling up of the DSSC from the laboratory scale to practical utilizations requires the use of a silver grid as the current collector to reduce the sheet resistance of the conducting glass substrate. However, this silver grid should be protected to reduce its corrosion by I3- according to eq 2 2AgðsÞ þ I3 - / 2Agþ þ 3I -
ð2Þ
Figure 7 shows the potentiodynamic polarization curves for silver metal in the case of the electrolyte containing I3- in 3-methoxypropionitrile at room temperature. The corresponding inhibition efficiency (IE) was calculated using eq 3,51 IEð%Þ ¼ 100ði0 -iÞ=i0
ð3Þ
0
where i and i are the corrosion current measured in the absence and presence of nylon 6, respectively, determined by the extrapolation of the cathodic and anodic Tafel lines toward the corrosion potential. As shown in Table 4, the IE increased with increasing concentration of nylon 6, due to the decrease in the i. This increase in the IE is attributed to the decreased contact of silver metal with I3- as a result of the formation of I3--nylon 6 complexes. This result suggests that the corrosion of the silver (51) Ferriera, E. S.; Giacomelli, C.; Giacomelli, F. C.; Spinelli, A. Mater. Chem. Phys. 2004, 83, 129.
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Figure 7. Polarization curves for corroding silver metal in the reference electrolyte with (a) 0.00 M, (b) 0.18 M, (c) 0.44 M, (d) 0.88 M, and (e) 1.8 M nylon 6.
Table 4. Potentiodynamic Polarization Parameters of the DSSCs Fabricated with Different Concentrations of Nylon 6a nylon 6 (M)
current (mA)
0.00 1.55 0.18 1.00 0.44 0.29 0.88 0.28 1.8 0.10 a The same electrolyte as in Table 1.
IE (%) 35 81 82 94
grids by I3- in commercial DSSC modules can be prevented by the formation of complexes of I3- with suitable complex-forming agents.
4. Conclusions The performance of dye-sensitized solar cells, based on the addition of different concentrations of nylon 6 to the conventional liquid electrolyte, was evaluated under an illumination of 100 mW cm-2. The overall energy conversion efficiency of 6.2% obtained with 0.18 M nylon 6 in 3-methoxypropionitrile is comparable to that obtainable in the case of a liquid electrolyte. However, the addition of nylon 6 enormously improves the stability of the solar cells at room temperature, due to their high solvent holding ability of the I3--nylon 6 complexes. Silver metal was rendered resistant to corrosion in the electrolyte containing nylon 6 and I3-. Acknowledgment. This work was supported by the MKE new and renewable energy R&D project under contract 2006N-PV12-P-05.
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