pubs.acs.org/Langmuir © 2010 American Chemical Society
Influence of Electrolyte Composition on the Photovoltaic Performance and Stability of Dye-Sensitized Solar Cells with Multiwalled Carbon Nanotube Catalysts Seon Hee Seo,* Su Yeon Kim, Bo-Kun Koo, Seung-Il Cha, and Dong Yoon Lee Advanced Materials and Application Research Division, Korea Electrotechnology Research Institute, Changwon 641-120, Republic of Korea Received January 27, 2010. Revised Manuscript Received April 12, 2010 Efficient dye-sensitized solar cells (DSCs) were realized by using multiwalled carbon nanotubes (CNTs) as the counter-electrode catalyst. The catalytic layers were produced from an aqueous paste of mass-produced raw CNTs with carboxymethylcellulose polymer by low-temperature (70 °C) drying. We found that the highly disordered CNTs played the important role of increasing the fill factor of DSCs with electrolytes including large molecules and that the presence of Liþ as the counter charges for I3-/I- redox couples reduced the chemical stability when using the CNT catalyst. Our experiments showed that by replacing the conventional Pt catalyst and Liþ-based electrolyte with the proposed CNT catalyst and an electrolyte containing 1-butyl-3-methylimidazolium cations instead of Liþ, the energy conversion efficiency increased from 6.51% to 7.13%. This result suggests that highly defective CNT catalysts prepared by lowtemperature drying are viable cost-effective alternatives for DSCs, as long as the electrolytes composition is optimized.
1. Introduction Dye-sensitized solar cells (DSCs) are considered strong candidates for next-generation solar cells due to their potential for high energy conversion efficiencies and low manufacturing cost.1 However, current prototypes suffer from low efficiency2,3 and long-term stability problems,4 which are decisive issues. The requirements for a guarantee of ourdoor photovoltaic performances seem to be too tough for solar cells with organic components especially. Many researchers have, therefore, been concerned about lightweight and flexible solar cells based on plastic substrates because the applications can be disposable and require moderate stability. For flexible DSCs on plastic substrates, much more technological advancement will be required.5-7 One reason is that when using plastic substrates, the process temperature must be limited to ∼150 °C throughout fabrication. Thus, to realize the full potential of DSCs as a versatile energy source, it is first necessary to develop materials that can be handled at low temperature at low cost;a demanding task. A DSC is an electrochemical cell with a number of different interfaces;a fact that introduces complications not encountered with other types of solid-state solar cells.8,9 A DSC typically consists of conductive transparent oxides (TCO) deposited on two substrates, sensitizing dyes, a nanoporous TiO2 layer, an *To whom correspondence should be addressed. E-mail:
[email protected]. Telephone: 8255-280-1645. Fax: 8255-280-1590. (1) O’Regan, B.; Gr€atzel, M. Nature 1991, 353, 737. (2) Han, L.; Fukui, A.; Chiba, Y.; Islam, A.; Komiya, R.; Fuke, N.; Koide, N.; Yamanaka, R.; Shimizu, M. Appl. Phys. Lett. 2009, 94, 013305. (3) Wang, P.; Zakeeruddin, S. M.; Moser, J.-E.; Nazeeruddin, M. K.; Sekiguchi, T.; Gr€atzel, M. Nat. Mater. 2003, 2, 402. (4) Hinsch, A.; Kroon, J. M.; Kern, R.; Uhlendorf, I.; Holzbock, J.; Meyer, A.; Ferber, J. Prog. Photovoltaics 2001, 9, 425. (5) D€urr, M.; Schimid, A.; Obermaier, M.; Rosselli, S.; Yasuda, A.; Nelles, G. Nat. Mater. 2005, 4, 607. (6) Lindstr€om, H.; Holmberg, A.; Magnusson, E.; Lindquist, S.-E.; Malmqvist, L.; Hagfeldt, A. Nano Lett. 2001, 1, 97. (7) Pichot, F.; Pitts, J. R.; Gregg, B. A. Langmuir 2000, 16, 5626. (8) Wang, Q.; Ito, S.; Gr€atzel, M.; Fabregat-Santiago, F.; Mora-Sero, I.; Bisquert, J.; Bessho, T.; Imai, H. J. Phys. Chem. B 2006, 110, 25210. (9) Gr€atzel, M. Nature 2001, 414, 338.
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electrolyte, and Pt catalyst;most of which form various interfaces. Thus, if one component is modified with a view to improving the energy conversion efficiency and/or stability, then other components would also require detailed analysis and readjustment.10-12 To increase device stability and limit the loss of electrolyte, many researchers have introduced the use of low volatile electrolytes such as ionic liquids, hole-transparent organic materials, and polymers.13-16 However, such electrolytes cannot easily access the interfaces causing increased resistances for moving charges;electrons and I3-/I-;at the TiO2/dye/electrolyte and catalyst/electrolyte interfaces, as well as within the electrolyte itself. The increased charge-transfer resistance reduces the current density (Jsc) and fill factor (FF), and thus the energy conversion efficiency. Thus, the composition of the electrolytes is very important factor determining the energy conversion efficiency because the electrolytes make various interfaces with every constituent part of DSCs. In DSCs, Liþ is widely used to enhance Jsc despite the fact that this reduces the open-circuit voltage (Voc) by causing a positive shift of the conduction band edge of TiO2.17-19 Other additives (10) Kuang, D.; Klein, C.; Ito, S.; Moser, J.-E.; Humphry-Baker, R.; Evans, N.; Duriaux, F.; Gr€atzel, C.; Zakeeruddin, S. M.; Gr€atzel, M. Adv. Mater. 2007, 19, 1133. (11) Martinson, A. B.; Elam, J. W.; Hupp, J. T.; Pellin, M. J. Nano Lett. 2007, 7, 2183. (12) Keis, K.; Lindgren, J.; Lindquist, S.-E.; Hagfeldt, A. Langmuir 2000, 16, 4688. (13) Cao, Y.; Zhang, J.; Bai, Y.; Li, R.; Zakeeruddin, S. M.; Gr€atzel, M.; Wang, P. J. Phys. Chem. C 2008, 112, 13775. (14) Bach, U.; Lupo, D.; Comte, P.; Moser, J. E.; Weiss€ortel, F.; Salbeck, J.; Spreitzer, H.; Gr€atzel, M. Nature 1998, 395, 583. (15) Stathatos, E.; Lianos, P.; Larrencic-Strangar, U.; Orel, B. Adv. Mater. 2002, 14, 354. (16) Lee, S. Y.; Yoo, B.; Lim, M. K.; Lee, T.-K.; Priya, A. R. S.; Kin, K.-J. Langmuir, 2010, in press, DOI: 10.1021/la903951x. (17) Nakade, S.; Kanzaki, T.; Kubo, W.; Kitamura, T.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 2005, 109, 3480. (18) Kambe, S.; Nakade, S.; Kitamura, T.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 2002, 106, 2967. (19) Kelly, C. A.; Farzad, F.; Thompson, D. W.; Meyer, G. J. Langmuir 1999, 15, 731.
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and imidazolium are also incorporated into electrolytes,20,21 basically to retard electron losses through alternative paths and realize high Jsc and reasonable Voc. However, there are few studies on improving the FF in laboratory-scale cells. Carbon nanotubes (CNTs), like other carbon materials,22 are usually resistant to chemical attack and also have large surface area, which would allow fast kinetics. Hence, we explored the possibility of using CNTs to improve the FF of DSCs. In this work, we studied the feasibility of using CTNs to improve the energy conversion efficiency and chemical stability of DSCs by selection of an appropriate electrolyte composition; an important factor, as mentioned above. A catalytic CNT layer was prepared from an aqueous paste by doctor blade printing and drying at a low temperature. The characteristics of the CNTs were evaluated by transmission electron microscopy (TEM), fieldemission scanning electron microscopy (SEM) and Raman spectroscopy. Then DSCs were assembled with prepared CNT catalytic layers and various electrolytes. The J-V characteristic curves and impedance spectra of these cells were measured to investigate the effect of electrolyte composition on the photovoltaic performance when using the CNT catalyst.
2. Experimental Section 2.1. Preparation of Multiwalled Carbon Nanotube Catalytic Layer. Multiwalled carbon nanotubes (CNTs) were
used electrolyte, AN50 (Solaronix), was used for a reference cell, which includes both lithium iodide and imidazolium iodide as iodide salts (www.solaronix.com). Liquid electrolytes with complex imidazolium iodide are denoted by BMI with 0.7 M 1-butyl-3-methylimidazolium iodide (BMImI), 0.03 M iodine, 0.1 M guanidine thiocyanate, 0.5 M 4-tert-butylpyridine dissolved in acetonitrile. The electrolytes were inserted into the cell by means of vacuum backfilling through a hole on the side of counter electrode. Finally, the hole was sealed up by additional thermoplastic sealant and a cover glass. 2.3. Measurements. Field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy were carried out on Hitachi S4800 and on Philips Tecnai F200 system. Powder samples were attached on slide glasses and then tested using 514 nm laser light and a mirco-Raman spectrometer at room temperature. Photocurrent-voltage (J-V) characteristics were obtained using a Keithley model 2400 source measure unit. The irradiation source was a 300 W xenon lamp on Oriel solar simulator with air mass 1.5G filter, whose power of solar simulator was calibrated by a NREL-calibrated Si solar cell equipped with an IR-cutoff KG-5 filter. To eliminate the diffuse scattering from the edge of glass substrates, black masks with the aperture area of 0.25 cm2 were attached on the surface of DSCs. Impedance spectra were measured with Princeton Applied Research Potentiostat 2273 at an open-circuit voltage (OCV) of the DSCs in a two-electrode configuration under 100 mW/cm2. Alternative signals were applied with the amplitude of 10 mV in the frequency range from 50 mHz to 100 kHz.
synthesized by thermal chemical vapor deposition process with Fe catalyst in a large scale (Applied Carbon Nano Technology, Korea). To prepare a viscous CNT paste, one gram of carboxymethylcellulose sodium (CMC) powder (viscosity 400-800 cP, Sigma-Aldrich) was dissolved in 100 g of distilled water and then stirred. Nine grams of raw CNT without any pretreatment were dispersed in the CMC aqueous solution. A small amount of CMC in the paste act as a binder to provide adhesion to FTO substrates as well as a viscosity adjust agent. To improve the dispersion of hydrophobic CNTs in aqueous solution, the mixed paste was ballmilled for 72 h. The viscosity of this paste was adjusted to about 75 000-80 000 cP. The prepared CNT paste was applied on fluorine-doped tin oxide (FTO) glass substrates (TEC8, sheet resistance 8-9 Ω/0, Pilkington) by doctor blade and the printed layers were located in a oven of 70 °C for 24 h in air. The final sheet resistance obtained was ∼160 Ω/0 on bare glass substrates. 2.2. Cell Fabrication. After the chemical cleaning of FTO glass substrates, a nanoporous TiO2 layer was prepared on the substrates by screen printing method with PST-18NR paste (Catalysts and Chemical Ind. Co.). The printed layers were dried at 110 °C for 1 h. The printing-drying processes were repeated four times and then the samples were annealed at 480 °C for 1 h, which makes a ∼12 μm thick transparent TiO2 layer. To enhance the Jsc of a DSC, we applied a scattering layer on the nanoporous layer with PST-400C paste (Catalysts and Chemical Ind. Co.). A ∼4 μm thick scattering layer was formed by twice printingdrying and an annealing. Finally, the sintered TiO2 electrodes were immersed in a 0.3 mM RuIILL0 (NCS)2 (L = 2,20 -bipyridyl4,40 -dicarboxylate, L0 = 2,20 -bipyridyl-4,40 -ditetrabutylammonium carboxylate) (N719 dye, Solaronix) ethanol solution for 21-24 h at room temperature. For reference cells, Pt catalytic layers were thermally prepared on FTO substrates with Pt paste (Solaronix) at 400 °C for 30 min. The stained TiO2 working electrode and the catalytic counter electrode were assembled with 2 sheets of 60 μm thick thermoplastic Surlyn (SX1170-60, Solaronix). Commercially
3. Results and Discussion 3.1. Characteristics of CNT Catalytic Layer. We used highly defective multiwalled carbon nanotubes (CNTs) with discontinuous graphite layers on their outer walls, diameters of 5-20 nm, and lengths greater than 10 μm, as shown in Figure 1, parts a and b. A carbon electrode surface typically consists of egde and basal plane graphite that have noticeably different electrochemical properties.23 The representative TEM image in Figure 1a reveals uncapped graphite layers on the outer surface of CNTs, which is marked with white arrows. There are previous works that such edge-plane-like surface defect sites are responsible for catalytic properties of CNTs, which have greatly increased electro-reactivity in some systems.23 A charge transfer resistance of CNTs was as low as that of a thin Pt layer for I3-/Iredox couples.24 Raman shift, shown in Figure 1c, supports the highly disordered features, corresponding to the presence of the D-mode near 1355 cm-1 and the shoulder of the G-mode near 1580 cm-1, based on reference.25,26 This highly disordered CNTs were used for the reduction of I3- instead of Pt catalyst. Aqueous paste was a mixture of these CNTs and CMC polymeric binder, and the CNT layers were printed by doctor blade. The ratio of CMC was optimized in the perspective of the printability of aqueous CNT pastes on FTO substrates and the electrochemical properties of CNT layers. (see Supporting Information) The viscosity of the CNT paste was adjusted to ∼75 000 cP with CMC, which provided a 5-7 μm thick CNT layer on a FTO substrate after low-temperature drying. As shown in Figure 2, there were no macroscopic cracks or peelings over the whole area. A high thermal treatment to remove CMC polymer above the decomposition temperature, such as 350 °C, rather
(20) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; M€uller, E.; Kiska, P.; Vlachopoulos, N.; Gr€atzel, M. J. Am. Chem. Soc. 1993, 15, 6382. (21) Koops, S. E.; O’Regan, B. C.; Barnes, P. R. F.; Durrant, J. R. J. Am. Chem. Soc. 2009, 131, 4808. (22) Murakami, T. N.; Ito, S.; Wang, Q.; Nazeeruddin, Md. K.; Bessho, T.; Cesar, I.; Liska, P.; Humphry-Baker, R.; Comte, P.; Pechy, P.; Gr€atzel, M. J. Electrochem. Soc. 2006, 153, A 2255.
(23) (a) Banks, C. E.; Davies, T. J.; Wildgoose, G. G.; Compton, R. G. Chem. Commun. 2005, 7, 829. (b) Wang, J.; Musameh, M.; Lin, Y. J. Am. Chem. Soc. 2003, 125, 2408. (c) Moore, R. R.; Banks, C. E.; Compton, R. G. Analyst 2004, 129, 755. (d) Zhang, J.; Liu, X.; Blume, R.; Zhang, A.; Schlogl, R.; Su, D. S. Science 2008, 322, 73. (24) Trancik, J. E.; Barton, S. C.; Hone, J. Nano Lett. 2008, 8, 982. (25) Thomsen, C. Phys. Rev. B 2000, 61, 4542. (26) Nemanich, R. J.; Solin, S. A. Phys. Rev. B 1979, 20, 392.
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Figure 3. J-V characteristics of DSCs with CNT and Pt catalysts and different electrolytes under full sun illumination. AN50 is a commercial acetonitrile-base electrolyte with Liþ and imidazolium. BMI consists of 0.7 M 1-butyl-3-methylimidazolium iodide, 0.03 M iodine, 0.1 M guanidine thiocyanate, and 0.5 M 4-tert-butylpyridine in acetonitrile. Table 1. Photovoltaic Performances of DSCs with CNT and Pt Catalysts in Various Electrolytes under Full Sun Illumination electrolyte
catalyst
Jsc (mA/cm2)
Voc (V)
FF
η (%)
a
Figure 1. (a) Cross-sectional TEM and (b) SEM images of multiwalled carbon nanotubes (CNTs) synthesized by chemical vapor deposition. White arrows represent surface defects of discontinuous graphite layers. (c) Raman spectrum of the CNTs, showing highly disordered features.
Figure 2. SEM images of the CNT catalytic layers prepared by doctor blade method and dried at 70 °C in different magnifications of (a) 500 and (b) 40 000. The catalytic layers were continuous without macroscopic cracks over the whole area.
degraded the energy conversion efficiency of the assembled DSCs (see Supporting Information) and, moreover, the adhesion of Langmuir 2010, 26(12), 10341–10346
Pt 12.50 0.72 0.72 6.51 AN50 CNT 12.64 0.73 0.71 6.58 AN50a b Pt 11.98 0.80 0.68 6.52 BMI CNT 11.99 0.80 0.74 7.13 BMIb Pt 12.37 0.77 0.60 5.67 BMI-MPNc c CNT 12.27 0.79 0.67 6.46 BMI-MPN a AN50 is a commercial electrolyte from Solaronix (Swiss). Ingredient is noted at www.solaronix.com. b The electrolyte consisted of 0.7 M 1-butyl-3-methylimidazolium iodide, 0.03 M iodine (I2), 0.1 M guanidine thiocyanate, and 0.5 M 4-tert-butylpyridine dissolved in acetonitrile. c The electrolyte has the same composition as BMIb except MPN solvent.
CNT layers to FTO substrates became poor. The drying temperatures could be increased up to 250 °C without significant change in the catalytic properties of the CNT layer in the assembled DSCs so that the printed CNT layers were dried at 70 °C. Conventional DSCs use Pt nanoparticles or thin films as the catalyst made by high-temperature annealing of precursors or by sputtering in a expensive high-vacuum chamber. The former is inappropriate for plastic substrates and the latter requires rather expensive equipment. By contrast, our CNT catalytic layers required only a simple low-temperature fabrication process. 3.2. Influence of CNT Catalyst on Photovoltaic Performance. We found that CNTs are more efficient than conventional Pt catalysts in liquid electrolytes containing larger cations as counter charges for I3-/I- redox couples and larger solvent molecules. Figure 3 shows the J-V characteristics of DSCs with CNT catalysts and different electrolytes under full sun illumination. For comparison, we also fabricated a reference cell with thermal Pt catalyst and commercial Liþ-based AN50 electrolyte. The photovoltaic performance of the cell with the CNT catalysts was similar to that of the cell with the conventional Pt catalyst when using the AN50 electrolyte. This is in agreement with previous reports27,28 that the performance of CNT is comparable with those of conventional Pt cells when using Liþ-based electrolytes. However, in our experiment, there was a noticeable performance difference when using BMI electrolyte. The CNT cells showed improved performance when changing the iodide salt to [BMIm]I. As shown in Figure 3, with the BMI electrolyte, the CNT cell has a higher energy conversion efficiency of 7.13%. The (27) Lee, W. J.; Ramasamy, E.; Lee, D. Y.; Song, J. S. ACS Appl. Mater. Inter. 2009, 1, 1145. (28) Suzuki, K.; Yamaguchi, M.; Kumagai, M.; Yanagida, S. Chem. Lett. 2003, 32, 28.
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Figure 4. Impedance spectra of the DSCs in two electrode configuration are shown in (a) Nyquist plot and (b) the imaginary parts of the impedances as a function of the frequencies at OCV (Voc) under full sun illumination. To represent clearly the shrinkage of Z1 considered at high frequency region for CNT cells, the overlapped part of Figure 1a is magnified in the inset.
photovoltaic parameters are summarized in Table 1. This result suggests that the catalytic ability of CNTs is affected by the composition of the electrolyte. With the CNT catalysts and the BMIm-based electrolytes, Voc increases noticeably but Jsc decreases slightly irrespective of the kind of catalysts used a result of the absence of Liþ.17-19 Further, the FF is enhanced. Electrolytes with larger counter charges and larger solvent molecules would impede the charge transfer on the catalytic layer because they form an unfavorable electric double layer. However, the CNT catalytic layer has large surface area, as shown in Figure 2, which reduces the charge-transfer resistance on the counter electrode.24 To explain the charge transfer on the catalytic surface, the electrochemical impedance spectra were measured in the twoelectrode configuration at an OCV under full sun illumination (Figure 4). The Nyquist plots as shown in Figure 4a are very useful to distinguish the number of interfaces in a system, but do not provide information on the characteristic frequencies, which would provide a better understanding of the underlying interfacial reactions.29 Figure 4b shows the imaginary parts of the impedances as a function of the frequencies, allowing ready identification of the characteristic frequencies. The spectrum of a reference cell with Pt catalyst and AN50 electrolyte represents a typical feature of DSCs,30-32 three peaks (open circles in Figure 4) in frequency regions 105-103, 103-2, and 2-10-2 Hz, corresponding to the impedances at the catalyst/electrolyte interface (Z1), those at TiO2/dye/electrolyte interface (Z2), and the Nernstian diffusion (Z3) within the electrolyte, respectively. These frequency domains result from the different time constants for chargetransfer process at each interface, which is well established for DSCs elsewhere.30-32 Under full sun illumination at an OCV, Jsc varies with R2 associated with the electron recombination8,33 and (29) Orazem, M. E.; Tribollet, B. Electrochemical Impedance Spectroscopy; John Wiley & Sons, Inc.: Hoboken, NJ, 2008; Chapter 16. (30) Han, L.; Koide, N.; Chiba, Y.; Mitate, T. Appl. Phys. Lett. 2004, 84, 2433. (31) Fabregat-Santiago, F.; Bisquert, J.; Palomares, E.; Otero, L.; Kuang, D.; Zakeeruddin, S. M.; Gr€atzel, M. J. Phys. Chem. C 2007, 111, 6550. (32) Fabregat-Santiago, F.; Bisquert, J.; Garcia-Belmonte, G.; Boschloo, G.; Hagfeldt, A. Sol. Energy Mater. Sol. Cells 2005, 87, 117. (33) Hoshikawa, T.; Yamada, M.; Kikuchi, R.; Eguchi, K. J. Electrochem. Soc. 2005, 152, E68.
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Figure 5. Impedance spectra of sealed symmetric cells with two catalytic electrodes in AN50 electrolytes. The characteristic frequencies related with CNT catalysts are resolved clearly in the plot of part b. Midfrequency domain of the CNT/electrolyte reaction overlaps with that of the TiO2/dye/electrolyte reaction in Figure 4.
FF is inversely related with the generalized approximation of the series resistance Rs estimated from the impedance spectra: Rs ¼ R0 þ R1 þ R3 where R0 is the ohmic resistance of conducting substrate, R1 is the charge-transfer resistance between catalyst and electrolyte, and R3 is the Nernstian diffusion resistance within the electrolyte.34,35 We can expect the better photovoltaic performances of DSCs from larger R2 and smaller Rs of the impedance spectra. However, in our work, considering the DSCs with the AN50 electrolyte, although the J-V characteristics in Figure 3 are almost the same with both Pt and CNT catalysts, the impedance spectra are quite different;open circles and solid squares in Figure 4, respectively. The size of midfrequency domain with the peak frequency ω2 is enlarged at the CNT cell, which is not consistent with the J-V characteristics because such an enlarged midfrequency domain with the same ω2 is the result of a reduction in electron recombination on the TiO2/dye/electrolyte interface, which should lead to higher Jsc. In some cases, the enlarged R2 is not necessarily responsible for higher Jsc when the electron injection yield decreases, which would accompany a substantial shift of ω2 inversely related to electron lifetime and noticeably different J-V characteristics under full sun illumination.21,36 We emphasize that the investigated cells with different counter electrodes in the AN50 electrolyte represent very similar ω2, Jsc, and Voc except R2 as shown in Figures 3 and 4, and Table 1. In addition, for the CNT cell with the AN50 electrolyte, the shrinkage of Z1 considered at high frequency region shown in the inset of Figure 4a must have given better FF. To understand the interfacial reactions of CNT cells with AN50 electrolyte, we measured impedance spectra from symmetric cells with two catalytic layers37 at -100 mV, avoiding mass-transfer control region. As shown in Figure 5, for a Pt-Pt (34) Han, L.; Koide, N.; Chiba, Y.; Ialam, A.; Komiya, R.; Fuke, N.; Fukui, A.; Yamanaka, R. Appl. Phys. Lett. 2005, 86, 213501. (35) Seo, S. H.; Kim, H.-J.; Koo, B.-K.; Lee, D. Y. J. Electrochem. Soc. 2009, 156, F128. (36) Kern, R.; Sastrawan, R.; Ferber, J.; Luther, S. J. Electrochim. Acta 2002, 47, 4213. (37) Hauch, A.; Georg, A. Electrochim. Acta 2001, 46, 3457.
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Figure 7. Change of the photovoltaic parameters of the DSCs assembled with Pt and CNT catalyst and different electrolytes with acetonitrile solvent. LiAN electrolyte contains 0.7 M LiI, 0.03 M iodine, 0.1 M guanidine thiocyanate, and 0.5 M 4-tert-butylpyridine.
Figure 6. (a) J-V characteristics and (b) impedance spectra of DCSs with different catalysts and electrolytes containing larger iodide salt [BMIm]I and low volatile methoxypropionitrile solvent.
symmetric cell, two semicircles appear as a result of electrolyte diffusion and the Pt/AN50 interfacial reaction at 0.2 Hz and 20 kHz, respectively. However, a CNT-CNT symmetric cell shows a very different impedance spectrum. The Nernstian feature is buried in the tail of midfrequency domain. An impedance with a very fast time constant around 50 kHz is almost invariant with the applied voltage in the CNT symmetric cell, which is believed from the result of the charge transfer in solidstates CNTs. A unique feature of CNT/electrolyte interfaces is observed near 10-Hz region in the symmetric configuration, which seems to overlap exactly with the time constant from the TiO2/dye/electrolyte reactions in the two-electrode configurations in Figure 4b. In our experiment, it is difficult to separate the contribution of the CNT/electrolyte reactions on photovoltaic performances due to the overlapping midfrequency domains of Z2(TiO2/dye/EL) and Z1(CNT/EL). The CNT catalyst mainly improves the FF of DSCs with electrolytes containing large molecules in particular. As mentioned above, large molecules in electrolytes would alter ionic configurations within the electric double layer and then the reduction of I3- could be retarded on catalysts, which is closely associated with the increase of R1. The effect of large molecules in electrolytes was investigated on the basis of J-V characteristics and impedance spectra of DSCs with electrolytes containing larger iodide salt [BMIm]I instead of LiI, and with low volatile methoxypropionitrile (MPN) solvent instead of acetonitrile. As shown in Figures 4 and 6, with the Pt catalyst, the size and characteristic frequency of Z1(Pt/EL) is substantially changed with the size of iodide salt in the electrolyte, which is independent of the solvent. However, Z3(EL) depends on the solvent itself. So, both such larger R1 due to larger iodide salt and larger R3 due to large solvent molecule obviously decrease the FF of the DSCs as summarized in Table 1. Figure 6 shows the CNT catalyst has no effect on Jsc despite the enlarged midfrequency domain, because the midfrequency domain is not purely representative of the electron recombination resistance at the TiO2/dye/electrolyte as explained above. Meanwhile, the highly disordered CNTs supports a number of catalytic sites and then effectively reduce the charge-transfer resistance at the CNT/electrolyte interface surrounded by large molecules, which improves the FF without any other reductions in the photovoltaic parameters: from 0.68 to 0.74 for BMI in acetonitrile and from 0.60 to 0.67 for BMI in MPN. Langmuir 2010, 26(12), 10341–10346
3.3. Influence of Liþ on the Stability of DSCs with CNT Catalysts. The presence of Liþ in electrolytes is a crucial determiner of the chemical stability of highly disordered CNT catalyst. To test this influence, we compared the photovoltaic performances of the DSCs with CNTs and different electrolytes as a function of time. The values plotted in Figure 7 are the averages over four samples per each condition. All samples were stocked in a glovebox after the measurement of the J-V curves and impedance spectra. Because the composition of the commercial electrolyte of AN50 is not known precisely, we also prepared Liþbased electrolyte (LiAN) with the same additives as the BMI electrolyte, except LiI instead of [BMIm]I in order to exclude any effects from unknown additives in AN50. For a CNT cell without Liþ, CNT-BMI, a conspicuous Voc is seen as compared with the others, and an insignificant reduction in Jsc is seen the day after cell fabrication. After 16 days, the conversion efficiency of CNTBMI is maintained at about 100% to the initial one, just similar to that of 99% with reference to Pt-AN50. On the contrary, the overall efficiencies of the cells with Liþ, CNT-AN50 and CNTLiAN, deteriorated to about 90%. This decay can be ascribed to intercalation of the alkali metal into the CNTs through the defect sites.38,39 In our case, some of the Liþ could have been adsorbed and intercalated into the disordered CNTs, thus reducing the concentration of active Liþ near the TiO2 layer with time in a sealed DSC. This might alter the position of the TiO2 conduction band and thus decrease Jsc and slightly increase Voc with time, which is consistent with the stability trend shown in Figure 7. The similar Liþ intercalation problem occurs on the surface of TiO2 and then degrades stability, but this can be overcome by the extra addition of tert-butylpyridine.17 Since all electrolytes used in this work included pyridine derivatives even in AN50, Liþ intercalation into TiO2 would be negligible to the stability. In the view of longterm stability of the CNT catalyst in DSCs, we conclude that alkali metals should be prevented from intercalating into the CNTs.
4. Conclusion Highly disordered multiwalled CNTs were successfully used as catalysts for DSCs through a simple low-temperature fabrication process. The CNT catalysts play the important role of enhancing the FF in DSCs with electrolytes containing large molecules. In our experiment, the energy conversion efficiency was 7.13% with a CNT catalyst in an electrolyte containing 1-butyl-3-methylimidazolium instead of Liþ in acetonitrile, which is better than that achieved (6.51%) by a reference cell containing thermal Pt (38) Zhou, O.; Fleming, R. M.; Murphy, D. W.; Chen, C. H.; Haddon, R. C.; Ramirez, A. P.; Glarum, S. H. Science 1994, 263, 1744. (39) Kaskhedikar, N. A.; Maier, J. Adv. Mater. 2009, 21, 2664.
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catalyst and a commercial Liþ-based electrolyte. The chemical stability of CNT catalytic cells can be improved by eliminating Liþ from the electrolytes. Acknowledgment. This research was supported by the Converging Research Center Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (Grant Number: 2009-0093710).
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We also acknowledge the support from the Korea Electrotechnology Research Institute through a research program. Supporting Information Available: Figures showing further characterization of carbon nanotube film with respect to the CMC ratio to CNT weight and the post-treatment temperature after the printing. This material is available free of charge via the Internet at http://pubs.acs.org.
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