Application of a New Cyclic Guanidinium Ionic Liquid on Dye

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Application of a New Cyclic Guanidinium Ionic Liquid on Dye-Sensitized Solar Cells (DSCs) Dongmei Li,† Meiyan Wang,‡,§ Junfeng Wu,† Quanxin Zhang,† Yanhong Luo,† Zhexun Yu,† Qingbo Meng,*,† and Zhijian Wu*,‡ †

Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China, ‡Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry,Chinese Academy of Sciences, Changchun 130022, China and §Graduate School, Chinese Academy of Sciences, Beijing 100049, China Received October 15, 2008. Revised Manuscript Received December 31, 2008

A new cyclic guanidinium ionic liquid OGI (1,3-dimethyl-2-N00 -methyl-N00 -octylimidazoguanidinium iodide) has been used as a quasi-solid-state electrolyte for dye-sensitized solar cells (DSCs), and 6.38% conversion efficiency was achieved at AM 1.5 simulated sunlight (9.81 mW cm-2). Further gelation with SiO2 nanoparticles afforded the solid-state electrolyte, which presented overall conversion efficiency of 5.85%. The diffusion properties of these OGI-based electrolytes were investigated. In the meantime, the optimal structure and ionpairing interaction in OGI have been proposed by density functional theoretical calculation (DFT) at the B3LYP/6-21G(d,p) level. In view of the experimental and theoretical calculation results, it is suggested that high asymmetry and good charge delocalization of the cyclic guanidinium cation can well restrain the recombination reaction between the injected electrons in the TiO2 conduction band and I3- ions through its flexible hydrocarbon group, thus giving relatively high efficiency.

Introduction The pursuit for cheap and renewable energy sources has stimulated the extensive development of the dye-sensitized solar cell (DSC) due to its easy fabrication, low cost, and relatively high efficiency.1 The DSC consists of three main components: a nanocrystalline porous film (mostly TiO2) coated with a monolayer of the dye, the electrolyte containing the charge carriers, and Pt sputtered on the conducting glass (FTO). So far, the I3-/I- redox couple is the most effective charge transporting carrier. The liquid electrolytes containing the I3-/I- redox couple can present higher conversion efficiency up to 11% at full sunlight (100 mW cm-2).2,3 However, some practical problems related with these organic solvent-based electrolytes have emerged to hinder DSCs’ large-scale application; for example, the leakage and evaporation will lead to technical complexity and inadequate durability for long-term operation.4 On the other hand, if the electrolyte can be supplied as a paste for covering on the photoanode film, the roll-to-roll technology will be introduced into the manufacture of flexible DSCs in the future. Thus, different kinds of electrolytes such as polymeric gels, ionic liquids, organic hole transport materials (HTM), etc., have facilitated the *Corresponding authors: Tel +86-10-82649242; Fax +86-1082649242; e-mail [email protected], [email protected]. ::

(1) O’Regan, B.; Gratzel, M. Nature (London) 1991, 353, 737–740. (2) Chiba, Y.; Islam, A.; Watanabe, Y.; Komiya, R.; Koide, N.; Han, L. Jpn. J. Appl. Phys. 2006, 45, L638–L640. (3) Nazeeruddin, M. K.; De Angelis, F.; Fantacci, S.; Selloni, A.; Viscardi, :: G.; Liska, P.; Ito, S.; Takeru, B.; Gratzel, M. J. Am. Chem. Soc. 2005, 127, 16835–16847. (4) Martinson, A. B. F.; Hamann, T. W.; Pellin, M. J.; Hupp, J. T. Chem.;Eur. J. 2008, 14, 4458–4467.

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rapid development of quasi-solid-state and solid-state DSCs.5-10 In our group, a new kind of solid-state composite electrolytes based on LiI addition compounds, such as [Li(HPN)4]I/SiO2 (HPN = 3-hydroxypropionitrile), [Li(C2H5OH)4]I/SiO2 systems, etc., have been employed on DSCs, and up to 6.1% of conversion efficiency was achieved.11-13 Room-temperature ionic liquids (RTILs) have been recognized as a kind of environmentally benign alternative in comparison with classical organic solvents due to their chemical and thermal stability, nonvolatility, and high ionic conductivity.14 They can be an appropriate component of quasi-solid-state and solid-state electrolyte systems for the DSCs. Among different kinds of ILs, imidazolium iodide electrolytes have been intensively studied; however, their conversion efficiencies are still dissatisfied (5) Stathatos, E.; Lianos, P.; Lavrencic-Stangar, U.; Orel, B. Adv. Mater. 2002, 14, 354–357. (6) Meng, Q.-B.; Takahashi, K.; Zhang, X.-T.; Sutanto, I.; Rao, T. N.; Sato, O.; Fujishima, A. Langmuir 2003, 19, 3572–3574. (7) Karim, M. A.; Cho, Y.-R.; Park, J. S.; Kim, S. C.; Kim, H. J.; Lee, J. W.; Gal, Y.-S.; Jin, S.-H. Chem. Commun. 2008, 1929–1931. (8) Kato, T.; Okazaki, A.; Hayase, S. Chem. Commun. 2005, 363–365. (9) Xia, J.; Masaki, N.; Lira-Cantu, M.; Kim, Y.; Jiang, K.; Yanagida, S. J. Am. Chem. Soc. 2008, 130, 1258–1263. :: (10) Schmidt-Mende, L.; Zakeeruddin, S. M.; Gratzel, M. Appl. Phys. Lett. 2005, 86, 013504. (11) Wang, H.; Li, H.; Xue, B.; Wang, Z.; Meng, Q.; Chen, L. J. Am. Chem. Soc. 2005, 127, 6394–6401. (12) Wang, H.; Liu, X.; Wang, Z.; Li, H.; Li, D.; Meng, Q.; Chen, L. J. Phys. Chem. B 2006, 110, 5970–5974. (13) An, H.; Xue, B.; Li, D.; Li, H.; Meng, Q.; Guo, L.; Chen, L. Electrochem. Commun. 2006, 8, 170–172. (14) Papageorgiou, N.; Athanassov, Y.; Armand, M.; Bonh^ ote, P.; :: Pettersson, H.; Azam, A.; Gratzel, M. J. Electrochem. Soc. 1996, 143, 3099–3108.

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comparable to the liquid electrolytes.15-17 High viscosity of the ILs is a serious drawback. It will dramatically limit the mass transfer of charge mediators, thus resulting in relatively low device performance. Attempts to reduce the viscosity by using other low-viscosity ILs, i.e., 1-ethyl-3-methylimidazolium tetracyanoborate, tricyanomethanide, dicyanamide, or thiocyanate, etc., were carried out. Over 7% efficiency has been achieved,18-21 but the thermal stability of DSCs based on these electrolytes was undesirable.21 In this respect, the search for new ILs systems is of great importance in order to meet the demand of highly efficient solid-state DSCs. The guanidinium ionic liquids are supposed to be a new kind of diverse ILs.22,23 Their physicochemical properties can be easily modified by introducing up to six different alkyl groups. Moreover, they also exhibit other desirable properties, i.e., good thermal stability and useful solubility. Therefore, specific RTILs may be obtained for different purposes. However, up to now, their application on DSCs remains rare, and the devices based on the known guanidinium iodide presented lower efficiencies and poor stability under higher light intensity.24 Inspiringly, guanidinium thiocyanate has been reported as an effective additive to the electrolytes, which can give up to 10.6% conversion efficiency.25-27 It is proposed that this guanidinium cation can remarkably screen the lateral Coulombic repulsion of the sensitizer and facilitate the self-assembly of a compact dye monolayer.25 So, it is necessary to further investigate guanidinium-based electrolytes on the DSCs. Herein, a new cyclic guanidinium ionic liquid with high asymmetry and good charge delocalization has been prepared and used as solidstate electrolytes for DSCs. Good conversion efficiency of 6.38% has been achieved at AM 1.5 simulated sunlight (9.81 mW cm-2).

Experimental Section The chemicals and solvents for preparing OGI (i.e., 1,3dimethyl-2-imidazolidinone, POCl3, n-octylamine, toluene, and CH3I) were predistilled before use.28 OGI (1,3-dimethyl-2-N00 methyl-N00 -octylimidazoguanidinium iodide) was prepared according to published procedures, except for using n-octylamine.28 :: (15) Stathatos, E.; Lianos, P.; Zakeeruddin, S. M.; Liska, P.; Gratzel, M. Chem. Mater. 2003, 15, 1825–1829. :: (16) Kawano, R.; Nazeeruddin, M. K.; Sato, A.; Gratzel, M.; Watanabe, M. Electrochem. Commun. 2007, 9, 1134–1138. (17) Hara, K.; Nishikawa, T.; Sayama, K.; Aika, K.; Arakawa, H. Chem. Lett. 2003, 32, 1014–1015. :: (18) Kuang, D.; Wang, P.; Ito, S.; Zakeeruddin, S. M.; Gratzel, M. J. Am. Chem. Soc. 2006, 128, 7732–7733. (19) Wang, P.; Wenger, B.; Humphry-Baker, R.; Moser, J.-E.; Teuscher, :: J.; Kantlehner, W.; Mezger, J.; Stoyanov, E. V.; Zakeeruddin, S. M.; Gratzel, M. J. Am. Chem. Soc. 2005, 127, 6850–6856. :: (20) Wang, P.; Zakeeruddin, S. M.; Moser, J.-E.; Gratzel, M. J. Phys. Chem. B 2003, 107, 13280–13285. :: (21) Wang, P.; Zakeeruddin, S. M.; Humphry-Baker, R.; Gratzel, M. Chem. Mater. 2004, 16, 2694–2696. (22) Mateus, N. M. M.; Branco, L. C.; Lourenc- o, N. M. T.; Afonso, C. A. M. Green Chem. 2003, 5, 347–352. (23) Duan, H.-F.; Guo, X.; Li, S.-H.; Lin, Y.-J.; Zhang, S.-B.; Xie, H.-B. Chin. J. Org. Chem. 2006, 26, 1335–1343. :: (24) Wang, P.; Zakeeruddin, S. M.; Gratzel, M.; Kantlehner, W.; Mezger, J.; Stoyanov, E. V.; Scherr, O. Appl. Phys. A: Mater. Sci. Process. 2004, 79, 73–77. :: (25) Gratzel, M. J. Photochem. Photobiol. A: Chem. 2004, 164, 3–14. (26) Murakami, T. N.; Ito, S.; Wang, Q.; Nazeeruddin, M. K.; Bessho, T.; :: Cesar, I.; Liska, P.; Humphry-Baker, R.; Comte, P.; Pechy, P.; Gratzel, M. J. Electrochem. Soc. 2006, 153, A2255–A2261. (27) Kuang, D.; Uchida, S.; Humphry-Baker, R.; Zakeeruddin, S. M.; :: Gratzel, M. Angew. Chem., Int. Ed. 2008, 47, 1–6. (28) Duan, H.-F.; Zhang, S.-B.; Lin, Y.-J.; Qiu, Z.-M.; Wang, Z.-M. Chem. J. Chin. Univ. 2003, 24, 2024-2026.

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Light yellow viscous OGI was dried in a vacuum for 24 h (yield: 80%). IR (KBr): 2952, 2925, 2858 (νC-H), 1626 (νs,C=N), 1465 (νas,C=N) cm-1. 1H NMR (ppm, CDCl3): 3.94 (s, 3H, CH3N), 3.47-3.35 (m, 4H, NCH2CH2N), 3.13 (s, 6H, 2CH3N), 1.67 (m, 2H, NCH2), 1.30-1.27 (m, 12H, 6CH2), 0.876 (m, 3H, CH3). 13 C NMR (ppm, CDCl3): 163.86 (s, CdN), 52.82 (s,=NCH3), 49.67 (d, 2C, NCH2CH2N), 37.05 (t, 2CH3N), 31.37 (s,=NCH2), 28.80 (d, 2CH2), 27.49 (t, 4CH2), 22.23 (s, CH2), 13.74 (s, CH3). MALDI-TOF m/z: 242 [OcMeN-C+ (CH3NCH2)2, 100%]. All reagents for DSCs (including the electrolytes and electrode materials) were used directly without further purification. Photoanodes were fabricated from a 10 μm nanoporous TiO2 film on a conducting glass substrate (F-doped SnO2, 10 Ω/sq) using a screen-printing technique.29 The films were sintered at 450 °C for 30 min. When the temperature was cooled down to 80 °C, they were immersed into a 0.3 mM ethanolic solution of N3 dye overnight (RuL2(SCN)2 3 2H2O, L = 2,20 -bipyridyl-4,40 -dicarboxylic acid, Solaronix). The TiO2 electrodes were rinsed with ethanol and dried in the air before measurement. A Pt-sputtered conducting glass was employed as a counter electrode. The electrolyte was sandwiched between the dye-anchored TiO2 film and the counter electrode to give the cell. A mask with a window of 0.15 cm2 was also clipped on the TiO2 side to define the active area of the cell. The thermal property of OGI was investigated using a DSC 2010 differential scanning calorimeter (TA Instruments). In the process of the measurement, the sample was heated from -100 to 100 °C with a rate of 10 °C min-1 under a N2 atmosphere. 1H and 13 C NMR spectra were obtained on BRUKER ARX 400. The MOLDI-TOF mass spectrum was carried out on a BIFLEX III. The conductivities were determined in a two platinum electrode conductivity cell (Kcell = 0.843) by an ac impedance technique using a HP 4192A impedance analyzer from 5 to 13 Hz under thermostated bath conditions. Viscosities were measured with a Brookfield DV-II+ viscometer on 0.50 mL samples while the temperature was maintained to 23 ( 1 °C. The steady-state voltammograms were conducted on a Zahner IM6e electrochemistry workstation using a two-electrode system equipped with a Pt ultramicroelectrode (radius 5 μm) as a working electrode and Pt wire as a counter electrode at the scan rate of 5 mV s-1. The cells were illuminated by an Oriel solar simulator 91192 under AM 1.5 irradiation (100 mW cm-2). The incident light intensity was measured by a radiant power/energy meter (Oriel 70260). The I-V characteristics of the cells were recorded on Princeton Applied Research, Model 263 A. All calculations here were carried out using the Gaussian 03 suite of programs.30 Molecular geometry of the compound was optimized by using the hybrid density functional method B3LYP.31-33 For the basis set, (29) Nazeeruddin, M. K.; Pechy, P.; Renouard, T.; Zakeeruddin, S. M.; Humphry-Baker, R.; Comte, P.; Liska, P.; Cevey, L.; Costa, E.; Shklover, V.; :: Spiccia, L.; Deacon, G. B.; Bignozzi, C. A.; Gratzel, M. J. Am. Chem. Soc. 2001, 123, 1613-1624. (30) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R., Jr.; Montgomery, J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, J.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara,Challacombe, A. M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. GAUSSIAN 03; Gaussian, Inc.: Pittsburgh, PA, 2003. (31) Becke, A. D. Phys. Rev. A 1988, 38, 3098–3100. (32) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789. (33) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652.

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Stuttgart/Dresden relativistic effective core potential (SDD basis set) was used for I- ion.34-36 In addition, d polarization with an exponent of 0.267 for I- ion was used.37 For the remaining elements (i.e., C, H, and N atoms), the standard 6-21G(d,p) basis set was adopted. The optimization was carried out without imposing any symmetry constraints. Harmonic vibrational frequencies and zero-point vibrational energy were obtained at the same level of theory. The free energies were obtained by thermal corrections to free energies, which include entropy contributions by taking into account the vibrational, rotational, and translational motions of the species at 298.15 K.

Results and Discussion Guanidinium ILs with the conjugation of three nitrogen atoms and the central carbon atom exhibit better charge delocalization. The chainlike guanidinium iodide, N,Ndiethyl-N0 ,N0 -dipropyl-N00 -hexyl-N00 -methylguanidinium iodide (SGI, see Figure 1b), has already been applied on the :: DSC by Gratzel et al. 5.9% efficiency was obtained under weak intensity radiation of 9.47 mW cm-2. Its conversion efficiency was reported to dramatically decline at higher light intensities (>50 mW cm-2).24 Obviously, this is far away from the requirement of practical application. Here, a new cyclic octaalkylguanidinium iodide OGI was synthesized. The structure is shown in Figure 1a. It was found that in this kind of cylic guanidinium IL, N-substituted alkyl chain significantly influences the melting points. For example, OGI is a viscous liquid at room temperature whereas hexaalkylguanidinium iodide is a solid salt.28 Differential scanning calorimetry data told us that the liquid domain of OGI extends down to -54.6 °C, almost identical with 1-propyl-3-methylimidazolium iodide salt (PMII).38 In our experiment, OGI has the dual functions of the solvent and the mediator (I-) in the photoelectrochemical process. Thus, the mobility of the ions and ion-pairing interaction are very important to the electron transport within the TiO2 film as well as the reduction of triiodide ions at the electrolyte/Pt counter electrode interface. First, the ionic conductivity of neat OGI ionic liquid was tested. Figure 2 presents the plot of its ionic conductivities (σ) with dependence on the temperatures. A better fit to the Vogel-TammannFulcher (VTF) equation (1) is observed: σðTÞ ¼ AT -1=2 exp½ -B=ðT -T0 Þ

ð1Þ

where σ is conductivity, A and B are constants, T is the absolute temperature, and T0 is the glass-transition temperature. Thus, we can see that a relatively strong ion pairing still exists in OGI. However, OGI shows higher conductivity than SGI in the range from room temperature to ca. 55 °C.24 When OGI combines with I2 to give the electrolytes, the iodine will quantitatively transform into the I3- in the excess of I-, leading to the distinct change of their conductivity and viscosity. Figure 3 illustrates the effect of OGI/I2 molar ratios on the conductivity and viscosity of the electrolytes. With the (34) Schwerdtfeger, P.; Dolg, M.; Schwarz, W. H. E.; Bowmaker, G. A.; Boyd, P. D. W. J. Chem. Phys. 1989, 91, 1762-1774. :: (35) Andrae, D.; Haussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta 1990, 77, 123–141. :: (36) Bergner, A.; Dolg, M.; Kuchle, W.; Stoll, H.; Preuss, H. Mol. Phys. 1993, 80, 1431–1441. (37) Glukhovtsev, M. N.; Pross, A.; McGrath, M. P.; Radom, L. J. Chem. Phys. 1995, 103, 1878-1885. :: (38) Wang, P.; Zakeeruddin, S. M.; Comte, P.; Exnar, I.; Gratzel, M. J. Am. Chem. Soc. 2003, 125, 1166–1167.

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increase of I3- concentration, the viscosity gradually decreases because I3- anions exhibit strong charge delocalization weakening the hydrogen bonding with the cations.39 On the other hand, addition of I2 into electrolytes will increase the conductivity, and this improvement is observed with increasing I3- concentration. However, the enhancement in conductivity is not completely linear. The same result was also seen in imidazolium ionic liquid systems.40,41 In DSCs, the conductivity of the electrolyte is generally expressed by the diffusion or migration of I- and I3- ions. If the conductivity is only determined by diffusion or migration of ionic species, it will decrease with the increasing of I2 concentration. This is obviously contrary to our experimental results. Thus, the OGI-based electrolytes are supposed to follow the Grotthuss-type charge transfer mechanism, where electron hopping and I-/I3- bond exchange are coupled to rationalize their high conductivities, shown as follows:14,40,41 I3- þ I - f I - - I2 :::I - f I - ::: I2 - I - f I - þ I3When the OGI/I2 electrolyte system was applied to DSCs, the effect of different OGI/I2 concentration ratios on the photovoltaic performance was discussed, such as short-circuit photocurrent density (Jsc), open-circuit voltage (Voc), fill factor (FF), and conversion efficiency (η). As shown in Table 1, the quantity of iodine incorporated in gel electrolytes plays a crucial role in the overall conversion efficiency when other ingredients are kept unchangeable. The Jsc changed a lot with the increasing of the OGI/I2 ratio, whereas the Voc changed in a relatively small range between 524.5 and 568.9 mV. The enhancement in Jsc may derive from faster dye regeneration rate with increasing the number of I- ions.11 When the OGI/I2 molar ratio was 9:1, the conversion efficiency reached the highest. However, if the ratio was kept higher, the electron transport from the counter electrode as well as the regeneration of dye molecules will be retarded due to too low I3- concentration. So, the conversion efficiency dropped to 3.90% when the OGI/I2 ratio was 10:1. It is clear that the I- diffusion into the inside of the nanoporous TiO2 film and the I3- ions from within the nanoporous TiO2 electrode into Pt counter electrode are extremely important for the respectable cell performance. On the basis of the above results, quasi-solid-state electrolyte A was defined as the mixture of 0.2 M I2, 0.5 M 4-tbp in OGI as the OGI/I2 molar ratio was equal to 9:1. It was further solidified to give solid-state electrolyte B by adding the “gelator”;SiO2 nanoparticles (5-7 wt %, Degussa, A150, average particle size 14 nm).13,38,42,43 The influence of silica nanoparticles on the photovoltaic performance has been investigated by the apparent diffusion coefficients (Dapp) of iodide (I-) and triiodide ions (I3-), which were determined by cyclic voltammetry measurement using Pt ultramicroelectrode (radius 5 μm) as a working electrode at a slow scan rate. Figure 4 presents the steady-state voltammograms of electrolytes A and B at the scan rate of 5 mV s-1. (39) Bonh^ ote, P.; Ana-Paula, D.; Papageorgiou, N.; Kalyanasundaram, :: K.; Gratzel, M. Inorg. Chem. 1996, 35, 1168–1178. (40) Kubo, W.; Murakoshi, K.; Kitamura, T.; Yoshida, S.; Haruki, M.; Hanabusa, K.; Shirai, H.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 2001, 105, 12809–12815. (41) Kawano, R.; Watanabe, M. Chem. Commun. 2003, 330–331. (42) Kimizuka, N.; Nakashima, T. Langmuir 2001, 17, 6759–6761. (43) Branco, L. C.; Crespo, J. G.; Afonso, C. A. M. Angew. Chem., Int. Ed. 2002, 41, 2771–2773.

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Figure 1. Structures of (a) OGI (1,3-dimethyl-2-N00 -methyl-N00 -octylimidazoguanidinium iodide, all hydrogen atoms are omitted) and (b) SGI (N,N-diethyl-N0 ,N0 -dipropyl-N00 -hexyl-N00 -methylguanidinium iodide).24

Table 1. Photovoltaic Performance of DSCs Based on OGI/I2/4-tbpa Electrolytes with Different OGI/I2 Ratios under AM 1.5 Illumination with Light Intensity of 100 mW cm-2 OGI/I2

4

5

6

7

8

9

10

8.10 8.93 11.59 13.08 11.73 13.80 10.07 Jsc/mA cm-2 524.5 530.5 550.9 548.5 568.9 566.5 554.5 Voc/mV FF 0.59 0.64 0.66 0.67 0.68 0.69 0.70 η (%) 2.49 3.01 4.26 4.78 4.53 5.41 3.90 a 4-tbp = 4-tert-butylpyridine.

Figure 2. Plot of the conductivity of OGI versus temperature data in the Vogel-Tammann-Fulcher (VTF) coordinates. The T0 value labeled in the graph is taken from differential scanning calorimetry experimental data (T0 = Tg).

Figure 4. Steady-state voltammograms for electrolytes A (solid line) and B (solid dot) by using a Pt ultramicroelectrode as the working electrode. Scan rate: 5 mV s-1. Table 2. Photovoltaic Performances of DSCs with Different Electrolytes and Their Apparent Diffusion Coefficients Dapp compositions

Figure 3. Effect of I2 concentration on conductivity of the electrolytes (left axis; closed circle) and viscosity (right axis; closed triangle): the molar ratio of OGI/I2 is in the range from 4:1 to 10:1, measured at 23 °C. The Dapp values were obtained from the steady-state current Iss by the following equation: Iss ¼ 4nFDapp Ca

ð2Þ

where n is the electron number per molecule, F the Faraday constant, C the concentration of I3-, and a the radius of the Pt ultramicroelectrode. Because of the large excess of I- ions relative to I3- ions in the electrolyte, I3- ions are the current-limiting species. The diffusivities of electrolyte A and Langmuir 2009, 25(8), 4808–4814

A

B

HMIIa

light intensity/mW cm-2 100 9.81 100 9.81 100 13.80 1.68 11.01 1.47 9.59 Jsc/mA cm-2 566.5 510.3 589.8 531.2 550.8 Voc/mV FF 0.69 0.73 0.69 0.74 0.67 η (%) 5.41 6.38 4.47 5.85 3.54 0.40 0.54 0.950 Dapp(I3-)/10-7 cm2 s-1 a HMII = 1-hexyl-3-methylimidazolium iodide; the composition of HMII electrolyte was 0.5 M I2, 0.5 M 4-tbp in HMII.

its SiO2-solidified correspondence B are quite close, as revealed in Table 2. This is rationalized that silica nanoparticles act as physically cross-linked gelators, and the I3and I- ions can freely “flow” in this open silica nanoparticle networks.44,45 In fact, TiO2 (P25) is often used as (44) Stergiopoulos, T.; Arabatzis, I. M.; Katsaros, G.; Falaras, P. Nano Lett. 2002, 2, 1259–1261. (45) Kato, T.; Okazaki, A.; Hayase, S. J. Photochem. Photobiol. A: Chem. 2006, 179, 42–48.

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Figure 5. Schematic diagram of the role of silica nanoparticles in OGI/SiO2 solid-state electrolyte. Reproduced with permission from ref 46. Copyright 2006 Elsevier.

the electrolyte gelator for DSCs, too.8,46 Recently, Yanagida et al. have specialized the role of nanoparticles (especially TiO2) in gelating imidazolium-based ionic liquids and proposed that imidazolium cations can be adsorbed on the TiO2 nanoparticles, leading to the alignment of I3- and Iions by electrostatic force and thus facilitating the electron transport via the Grotthuss-type charge-transfer mechanism.46-48 To the nanosize silica particles, especially silica A150 (average size 14 nm) used in our experiment, its zeta potential was -15 mV, which means that the silica surface is basically electronegative.49 As a consequence, the electrostatic interaction between silica nanoparticles and the guanidinium cations may not be ignored, and this interaction also gives rise to I3- and I- ions exchange process, as depicted in Figure 5. Under illumination of 100 mW cm-2, the photovoltaic performances of DSCs based on electrolytes A and B are shown in Table 2, while their photocurrent-voltage characteristics are presented in Figure 6. For the DSC on quasi-solidstate electrolyte A, the photovoltaic performance parameters Jsc, Voc, FF, and η were 13.80 mA cm-2, 566.5 mV, 0.69, and 5.41%, respectively. After being solidified by SiO2 nanoparticles, it gave light-to-electricity conversion efficiency of 4.47% (see Table 2). For comparison, the device employing HMII-based electrolyte gave the conversion efficiency of 3.54%, though the diffusion coefficient of I3- ion of HMII electrolyte is larger than that of OGI, seen in Table 2. The reasons are not perfectly understood. It is suggested that the nature of their cations will remarkably influence the device performance; that is to say, better charge delocalization of guanidinium cations and its long n-octyl chain can well restrain the recombination reaction between the injected electrons in TiO2 conduction band and I3- ions, thus taking advantage over HMII. Under weak light intensity (around 10 mW cm-2), OGI-based electrolyte can give higher efficiency than SGI; for example, electrolyte A presented the conversion efficiency of 6.38% while its solidified counterpart gave 5.85% efficiency. (46) Yanagida, S. C. R. Chim. 2006, 9, 597–604. (47) Kang, M.-S.; Ahn, K.-S.; Lee, J.-W. J. Power Sources 2008, 180, 896–901. (48) Usui, H.; Matsui, H.; Tanabe, N.; Yanagida, S. J. Photochem. Photobiol. A: Chem. 2004, 164, 97–101. (49) Wang, H. X. PhD Thesis, Institute of Physics, Chinese Academy of Sciences, 2005.

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Figure 6. Photocurrent density-voltage characteristics of DSCs with OGI-based electrolytes under AM 1.5 illumination: curve (a) is for electrolyte A (100 mW cm-2, composition: OGI, I2, 4-tbp), curve (b) for solid electrolyte B (100 mW cm-2, composition: electrolyte A and SiO2 nanoparticles), and curves (c) and (d) for electrolytes A and B (9.81 mW cm-2), respectively. An active area of devices with mask: 0.15 cm2.

Inspiringly, under AM 1.5 illumination of full sunlight (100 mW cm-2), the DSCs with OGI-based electrolytes A and B exhibited quite stable conversion efficiency. This indicates that the cyclic structure of guanidinium cation is superior to the chainlike structure when both were used in DSCs. Here, one thing needs to be pointed out that our DSC with pure HMII presented relatively lower conversion efficiency (3.54%) than the known highest efficiency of the same photovoltaic cell.50 This is mainly caused by the properties of TiO2 films (including the porosity, thickness, roughness, etc.), which still need further optimization. Therefore, the potential improvement of the photovoltaic performances based on OGI electrolyte systems can be achieved. DFT Calculation on Geometry Optimization. According to our experimental result, the structure of guanidinium cation and its bonding to the I- ion will significantly affect the charge transport as well as the conversion efficiency of the (50) Kubo, W.; Kambe, S.; Nakade, S.; Kitamura, T.; Hanabusa, K.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 2003, 107, 4374–4381.

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Article

DSCs. The density functional method is one of the most widely used tools for investigating the geometric and electronic structures of molecules. Here, we chose DFT calculation to investigate the cation-anion interaction of OGI. Initially, the OGI cation was optimized. Then, reoptimization has been conducted after an iodide counterion was added to the cation. The proposed conformations of guanidinium cations at the B3LYP/6-21G(d,p) level are shown in Figure 7. OGI cation possesses high unsymmetry due to its stretching octyl group. This long alkyl tail will strongly influence the efficiently stacking ability of the OGI cations within the electrolyte. The net charges of OGI+ from natural bond orbital (NBO) analysis are presented in Table 3. As shown in Table 3, the positive charges of the OGI cation are localized on the central carbon atom and the H atoms (omitted in Table 3). The positive charge on the central carbon is ∼0.7, and the planar nature of the N18-C2-N8-N9 fragment demonstrates that the central carbon is a carbocation. Three neighboring nitrogen atoms bear slightly different negative charges, which further testify the OGI+ exhibits high asymmetry. Through extensive search (by optimization) on the potential energy surface, four favorable positions P1, P2, P3, and P4 for the iodide were obtained on the plane which is almost perpendicular to the guanidinium plane, as shown in Figure 8. OGI_P1 is the most stable configuration. In the four calculated structures, the C-H 3 3 3 I hydrogenbonding distances (av 3.083 A˚) are longer than that of imidazolium cation and iodide anion (av 2.93 A˚).51,52 The bond distance between the carbocation and iodide ion is av 3.604 A˚. The interaction energy ΔG is defined as the difference between the energy of the ion pairs system (GAX) and the sum of the energies of the purely cationic (GA+) and anionic (GX-) species: ΔG ðkJ mol -1 Þ ¼ 2625:5½GAX ðauÞ ðGAþ ðauÞ þ GX- ðauÞÞ To the above four OGI configurations, the calculated ΔG are given in Table 4. For OGI_P1, its interaction energy ΔG is -250.25 kJ mol-1, which absolute value is higher than other three configurations, indicating that it is the most stable configuration. This is mainly due to small steric hindrance around the carbocation and preference to the access of iodide ion in OGI_P1.52 The total dipole moment distributions of isolated guanidinium cations and OGI molecule are also calculated, as demonstrated in Table 4. The spherical iodide ion is supposed to carry zero dipole moment when it is isolated. However, when it is in the ionic liquid environment, its thermal fluctuation and polarization effect will result in nonzero instantaneous dipole moment. Here, the geometric center of the guanidinium cation OGI+ was chosen as its origin; as a result of high asymmetry, the instantaneous dipole moment of the isolated cation is 13.4 D. To OGI ionic liquid, the total dipole moment ranges from 11.9 to 13.0 D, less than the isolated guanidinium cation. The difference derives from the change of the electron (51) Turner, E. A.; Pye, C. C.; Singer, R. D. J. Phys. Chem. A 2003, 107, 2277–2288. (52) Wang, Y.; Li, H.; Han, S. J. Chem. Phys. 2005, 123, 174501.

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Figure 7. Optimized conformation of OGI cations at the B3LYP/ 6-21G(d,p) level. distribution. The optimal configuration OGI_P1 exhibits the smallest dipole moment. It can be rationalized due to the weak Coulombic forces felt by cations and iodides.53,54 And this relatively weak electrostatic attraction is supposed to be effective in the diffusion or migration of iodide ions in the photoelectrochemical process. In this regard, our calculation result is in agreement with the photoelectrochemical measurement. From the above calculation, high asymmetry of the OGI is further verified. Taking the experimental and DFT calculation results into account, this high asymmetry of OGI can effectively restrain charge recombination at the nanocrystalline TiO2 electrolyte interface; that is to say, the flexible hydrocarbon group from the low-symmetrical OGI cation may act as non-charge-carrying buffers between the charge centers and thus well suppress charge recombination in DSCs.

Conclusions A new cyclic guanidinium ionic liquid OGI has been prepared and applied in DSCs. When it was directly used as a quasi-solid-state electrolyte A, the DSC gave 6.38% efficiency at AM 1.5 illumination of 9.81 mW cm-2. When this quasi-solid-state electrolyte was further solidified by nanosized SiO2 particles to give electrolyte B, its light-to-electricity conversion efficiency also can reach 5.85%. Compared with the chainlike guanidinium SGI, the cyclic OGI-based electrolytes A and B could present stable conversion efficiency under AM 1.5 illumination of 100 mW cm-2. Density functional calculation revealed the optimal structure of OGI and interaction of the guanidinium cation and iodide anion. On the basis of the experimental and theoretical calculation results, it is suggested that high asymmetry of cyclic guanidinium (53) Bhargava, B. L.; Balasubramanian, S. J. Phys. Chem. B 2007, 111, 4477–4487. (54) Yan, T.; Burnham, C. J.; Del P opolo, M. G.; Voth, G. A. J. Phys. Chem. B 2004, 108, 11877–11881.

DOI: 10.1021/la8034209

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Li et al. Table 3. Selected Atom Charges for OGI+ from the Natural Bond Orbital (NBO) Analysis

atoms charges

C1 -0.267

C2 0.719

C3 -0.268

N8 -0.435

N9 -0.429

C10 -0.486

C14 -0.485

N18 -0.457

C19 -0.487

C23 -0.260

Figure 8. Optimized structures of OGI at the B3LYP/6-21G(d,p) level. All the distances are in angstroms. 1 stands for OGI systems, and P1, P2, P3, and P4 stand for different positions of I- ions. Table 4. Calculated Absolute Free Energies G (kJ mol-1), Interaction Energies ΔG (kJ mol-1), and Dipole Moment Distribution μ (D) between the I- Ion and the Corresponding Cation OGI+ μ (D)

cation and good charge delocalization could well restrain the recombination reaction of the injected electrons in TiO2 conduction band and I3- ions through its flexible hydrocarbon group.

I -30 287.13 -1 877 953.24 13.4 OGI+ -1 908 490.63 -250.25 11.9 1_P1a 1_P2 -1 908 487.14 -246.77 13.0 1_P3 -1 908 482.70 -242.34 12.0 1_P4 -1 908 478.03 -237.65 12.6 a 1 stands for OGI system, and P1, P2, P3, and P4 stand for different positions of I- ions around OGI+ cation.

Acknowledgment. This work was funded by the Natural Science Foundation of China (No. 20725311, 20673141, 20703063, 20873178, and 20721140647), the Ministry of Science and Technology of China (973 Project, No. 2006CB202606 and 863 Project, No. 2006AA03Z341), and the 100-Talents Project of Chinese Academy of Sciences.

G (kJ mol-1)

ΔG (kJ mol-1)

-

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