N-Methyl-N-Allylpyrrolidinium Based Ionic Liquids for Solvent-Free

Feb 16, 2009 - Ning Cai, Jing Zhang, Difei Zhou, Zhihui Yi, Jin Guo and Peng Wang*. State Key Laboratory of Polymer Physics and Chemistry, Changchun ...
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J. Phys. Chem. C 2009, 113, 4215–4221

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N-Methyl-N-Allylpyrrolidinium Based Ionic Liquids for Solvent-Free Dye-Sensitized Solar Cells Ning Cai,†,‡ Jing Zhang,† Difei Zhou,†,‡ Zhihui Yi,† Jin Guo,† and Peng Wang*,† State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China, and Graduate School, Chinese Academy of Sciences, Beijing 100039, China ReceiVed: December 12, 2008; ReVised Manuscript ReceiVed: January 6, 2009

We prepared four new ionic liquids consisting of N-methyl-N-allylpyrrolidinium cation in conjunction with anions including iodide, nitrate, thiocyanate, and dicyanamide, respectively, and measured their physical properties of density, viscosity, and conductivity. Owing to the relatively lower melting point of electroactive N-methyl-N-allylpyrrolidinium iodide, in combination with three other nonelectroactive ionic liquids, we could construct solvent-free electrolytes possessing high iodide concentrations for dye-sensitized solar cells. We correlated temperature-dependent electrolyte viscosity with molar conductivity and triiodide mobility through applying an empirical Walden’s rule and a modified Stokes-Einstein equation, respectively. We have further found that these anions (nitrate, thiocyanate, and dicyanamide) have different influences on surface states and electron transport in the mesoporous titania film, resulting in different photovoltages and photocurrents of dye-sensitized solar cells. 1. Introduction Over the past two decades, the dye-sensitized solar cell (DSC) has inspired tremendous excitement for its feasibility to afford photovoltaic devices with an appreciable high performance/price ratio.1 The predominant attribute of DSCs consists in a mesoporous semiconducting film coated with a quasi-monolayer of light-harvesting dye molecules and immersed in a redox. This intriguing assembly with low-cost materials can perform efficiently the basic physical processes required by solar cells, such as light absorption, charge generation, and charge transport. On the basis of systematic device engineering benefiting from continuous efforts on material innovation and device physics, a state of the art DSC with a highly volatile electrolyte has achieved a validated efficiency of 11.1% measured under the air mass 1.5 global (AM1.5G) conditions.2 This efficiency could be further enhanced through the design of new sensitizers with a good spectral overlap with the AM1.5G solar irradiation, and exploration of multijunction tandem device architectures. Presently, a major drawback of the DSC technology is the usage of volatile electrolytes. This hurdle has precluded largescale outdoor application and integration into flexible cells even for its initial niche market. Among enormous efforts to develop solvent-free DSCs to address this issue, the use of room temperature ionic liquid electrolytes has proved to be the most successful strategy.3 This has been exemplified by a recent demonstration of a stable cell showing efficiencies of 8.5-9.1%.4 We note that so far, almost all DSCs showing efficiencies over 6% with solvent-free ionic liquid electrolytes have been made by taking imidazolium salts as the iodide source, except for binary tetrahydrothiophenium melts.5 Other ionic liquids with cations such as sulfonium,6 guanidinium,7 ammonium,8 or phosphonium9 have also been explored as * Corresponding author. E-mail: [email protected]. † Changchun Institute of Applied Chemistry. ‡ Graduate School.

solvent-free electrolytes but attained device efficiencies lower than 1.5% under full sunlight. Various N,N-dialkylpyrrolidinium ionic liquids and ionic plastic crystals10 have been intensively explored and used as electrolytes in lithium battery and fuel cells. Impressively, these pyrrolidinium salts exhibit a higher thermostability and a wider electrochemical window in contrast to their imidazolium counterparts. In this context, some solid pyrrolidinium iodides have been employed as iodide source for DSCs in conjunction with solid11 or liquid12 molecular solvents. However, these studies did not address the critical issue of the volatility of DSC electrolytes under thermal stress due to the simultaneous employment of molecular solvents with a considerable vapor pressure. The solvent-free ionic liquid electrolytes used in DSCs employ iodide melts as their major component, because a high concentration of iodide is required to intercept quantitatively the geminate charge recombination between the electrons injected by the photoexcited sensitizer and its oxidized form.3i During the exploration of pyrrolidinium ionic liquids for solventfree DSCs, we have found that solid iodide salts such as N,Ndimethylpyrrolidinium iodide,10f N-methyl-N-ethylpyrrolidinium iodide, and N-methyl-N-propylpyrrolidinium iodide have melting points over 150 °C and manifest limited solubilities in their corresponding low-viscosity pyrrolidinium ionic liquids, hampering the formulation of solvent-free electrolytes for efficient solar cells. In this paper, we report N-methyl-N-allylpyrrolidinium iodide exhibiting a much lower melting point as well as its corresponding room temperature ionic liquids with nitrate, thiocyanate, and dicyanamide anions. These pyrrolidinium melts can be used to make efficient DSCs, showing efficiencies up to 5.58% under the illumination of AM1.5G full sunlight. 2. Experimental Section 2.1. Materials. All solvents and reagents, unless otherwise stated, were of puriss quality and used as received. Allyl iodide, N-methylpyrrolidine, and guanidinium thiocyanate (GNCS) were purchased from Fluka. The Z907Na sensitizer, NaRu(4-carboxylic

10.1021/jp810973q CCC: $40.75  2009 American Chemical Society Published on Web 02/16/2009

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Figure 1. Plots of specific conductivity of N-methyl-N-allylpyrrolidinium salts versus temperature with a heating procedure. Before measurements in a sealed tube, the samples were dried at 60 °C under a vacuum of ∼3 Torr for 6 h.

acid-4′-carboxylate)(4,4′-dinonyl-2,2′-bipyridine)(NCS)2, was prepared as reported previously.3i N-Butylbenzimidazole (NBB) was synthesized according to a literature method and distilled before use.13 Synthesis of N-Methyl-N-allylpyrrolidinium Iodide (P13′I). Allyl iodide (26.16 g, 0.12 mol) was added dropwise into N-methylpyrrolidine (8.51 g, 0.10 mol) under Ar. The temperature was maintained at 0 °C during the slow adding process. After that the mixture was stirred at 50 °C for 3 h. The crude compound was dissolved in 200 mL of water and extracted with dichloromethane at least three times. After removing water on a rotary evaporator, the resulting solid was dried at 100 °C under a vacuum of 3 Torr for 6 h, and 24.77 g of the product was obtained. Yield 98%. 1H NMR (400 MHz, d6-DMSO, δH) 6.04-6.11 (m, 1H), 5.59-5.66 (m, 2H), 3.99 (d, 2H), 3.42-3.49 (m, 4H), 2.98 (s, 3H), 2.10 (s, 4H). IR (νmax) 2006 (CsN), 1622 cm-1 (CdC). ESI-MS (m/z) 379.3 ([(P13′I)I]-), 885.3 ([(P13′I)3I]-), 1138.0 ([(P13′I)4I]-), 382.1 ([(P13′I)P13′]+), 886.7 1139.7 ([(P13′I)4P13′]+), 1645.2 ([(P13′I)3P13′]+), ([(P13′I)6P13′]+), 1898.3 ([(P13′I)7P13′]+). Synthesis of N-Methyl-N-allylpyrrolidinium Nitrate (P13′NO3). An aqueous solution of N-methyl-N-allylpyrrolidinium iodide (9.70 g, 0.04 mol) was added dropwise into an aqueous solution of silver nitrate (10.19 g, 0.06 mol), and the resulting slurry was stirred at room temperature for 30 min. Subsequent filtration of silver salts and removing water by a rotary evaporator afforded a colorless liquid. Yield: 93%. 1H NMR (400 MHz, d6-DMSO, δH) 6.04-6.10 (m, 1H), 5.59-5.65 (m, 2H), 3.99 (d, 2H), 3.40-3.51 (m, 4H), 2.98 (s, 3H), 2.10 (s, 4H). IR (νmax) 1644 (CdC), 1377 cm-1 (NO3). ESI-MS (m/ z) 816.3 ([(P13′NO3)4NO3]-), 1192.3 ([(P13′NO3)6NO3]-), 1380.0 ([(P13′NO3)7NO3]-), 1567.3 ([(P13′NO3)8NO3]-), 1755.5 ([(P13′NO3)9NO3]-), 1943.4 ([(P13′NO3)10NO3]-), 314.9 ([(P13′NO3)P13′]+), 691.1 ([(P13′NO3)3P13′]+), 879.8 ([(P13′NO3)4P13′]+), 1067.3 ([(P13′NO3)5P13′]+), 1255.4 ([(P13′NO3)6P13′]+), 1443.4 ([(P13′NO3)7P13′]+), 1631.3 ([(P13′NO3)8P13′]+), 1820.1 ([(P13′NO3)9P13’]+). The Ag+ and I- contents were confirmed with ICP-MS, being below 0.5% w/w. Synthesis of N-Methyl-N-allylpyrrolidinium Thiocyanate (P13′NCS). Silver thiocyanate was precipitated by mixing aqueous solutions of silver nitrate (10.98 g, 0.065 mol) and potassium thiocyanate (6.32 g, 0.065 mol), washed well with water to remove any unreacted reagents, and used immediately. The freshly prepared silver thiocyante was added to an aqueous solution of N-methyl-N-allylpyrrolidinium iodide (15.19 g, 0.06 mol) and the slurry was stirred at 40 °C for 1 h. Subsequent

Cai et al. filtration of silver salts and removing water by a rotary evaporator afforded the crude compound. To ensure a complete removal of silver salts from the product, dichloromethane was added to the crude compound and the solution was cooled in a freezer for at least 24 h. After filtering off a small quantity of silver salts and rotoevaporating dichloromethane, a colorless liquid was obtained. Yield 89%. 1H NMR (400 MHz, d6-DMSO, δH) 6.02-6.10 (m, 1H), 5.60-5.65 (m, 2H), 3.98 (d, 2H), 3.41-3.48 (m, 4H), 2.98 (s, 3H), 2.10(s, 4H). IR (νmax) 2053 (NCS), 1644 cm-1 (CdC). ESI-MS (m/z) 242.8 ([(P13′NCS)NCS]-), 797.5 ([(P13′NCS)4NCS]-), 1164.5 ([(P13′NCS)6NCS]-), 1347.4 ([(P13′NCS)7NCS]-), 1531.3 ([(P13′NCS)8NCS]-), 1716.3 ([(P13′NCS)9NCS]-), 1900.3 ([(P13′NCS)10NCS]-), 310.8 ([(P13′NCS)P13′]+), 678.5 ([(P13′NCS)3P13′]+), 863.4 ([(P13′NCS)4P13′]+), 1047.6 ([(P13′NCS)5P13′]+), 1232.1 ([(P13′NCS)6P13′]+), 1416.3 ([(P13′NCS)7P13′]+), 1601.1 ([(P13′NCS)8P13′]+), 1785.2 ([(P13′NCS)9P13′]+), 1968.2 ([(P13′NCS)10P13′]+). The Ag+ and I- contents were confirmed with ICP-MS, being below 0.5% w/w. Synthesis of N-Methyl-N-allylpyrrolidinium Dicyanamide (P13′DCA). Silver dicyanamide was precipitated by mixing aqueous solutions of silver nitrate (8.49 g, 0.05 mol) and sodium dicyanamide (4.45 g, 0.05 mol), well washed with water to remove any unreacted reagents, and used immediately. The freshly prepared silver dicyanamide was added to an aqueous solution of N-allyl-N-methylpyrrolidinium iodide (10.12 g, 0.04 mol), and the slurry was stirred at 40 °C for 1 h. Subsequent filtration of silver salts and the removal of water by a rotary evaporator afforded the crude compound. To ensure a complete removal of silver salts from the product, dichloromethane was added to the crude compound and the solution was cooled in a freezer for at least 24 h. After filtering off a small quantity of silver salts and rotoevaporating dichloromethane, a colorless liquid was obtained. Yield 86%. 1H NMR (400 MHz, d6-DMSO, δH) 6.02-6.11 (m, 1H), 5.60-5.66 (m, 2H), 4.00 (d, 2H), 3.42-3.49 (m, 4H), 2.98 (s, 3H), 2.10 (s, 4H). IR (νmax) 2235, 2196, and 2135 (N(CN)2), 1645 cm-1 (CdC). ESI-MS (m/z) 259 ([(P13′DCA)DCA]-), 644.8 ([(P13′DCA)3DCA]-), 836.9 ([(P13′DCA)4DCA]-), 1028.1 ([(P13′DCA)5DCA]-), 1220.3 ([(P13′DCA)6DCA]-), 1411.7 ([(P13′DCA)7DCA]-), 1604.6 ([(P13′DCA)8DCA]-), 1796.3 ([(P13′DCA)9DCA]-), 1988.4 ([(P13′DCA)10DCA]-), 318.4 ([(P13′DCA)P13′]+), 510.7 ([(P13′DCA)2P13′]+), 702.7 ([(P13′DCA)3P13′]+), 894.6 ([(P13′DCA)4P13′]+), 1086.7 ([(P13′DCA)5P13′]+), 1278.7 ([(P13′DCA)6P13′]+), 1470.6 ([(P13′DCA)7P13′]+), 1663.7 ([(P13′DCA)8P13′]+), 1854.6 ([(P13′DCA)9P13′]+). The Ag+ and I- contents were confirmed with ICP-MS, being below 0.5% w/w. 2.2. Viscosity, Conductivity, and Density Measurements. The viscosity (η) data were acquired with a Brookfield DVII+Pro viscometer. Density (F) was determined with an Anton Paar DMA 35N density meter. A Radiometer CDM210 conductivity meter was employed for conductivity (σ) measurement. The Radiometer CDC749 conductivity cell with a nominal cell constant of 1.70 cm-1 was calibrated with 0.1 M KCl aqueous solution prior to the experiments. A two-electrode electrochemical cell, with a 5.0-µm-radius Pt ultramicroelectrode as its working electrode and a Pt foil as the counter electrode, was used in combination with a CHI 660C electrochemical workstation to measure the triiodide diffusion coefficient (D). A heating-cooling cycle pump was used to control sample temperatures. 2.3. Device Fabrication. A screen-printed double layer film of interconnected TiO2 particles was used as a mesoporous

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J. Phys. Chem. C, Vol. 113, No. 10, 2009 4217

TABLE 1: Physical Properties of N-Methyl-N-allylpyrilidinium Melts at 25 °C Salt

Tm/°C

P13′I P13′NO3 P13′NCS P13′DCA

57.5 10.0

F/g cm-3

Vm/cm3 mol-1

σ/mS cm-1

Λm/S cm2 mol-1

η/cP

1.1511 1.0630 1.0472

163.5 173.4 183.6

3.89 11.15 19.27

0.64 1.93 3.54

284.1 75.5 25.6

negative electrode. A 7-µm-thick film of 20-nm-sized TiO2 particles was first printed on the fluorine-doped SnO2 conducting glass electrode and further coated by a 5-µm-thick second layer of 400-nm-sized light scattering anatase particles. The detailed preparation procedures of TiO2 nanocrystals, pastes for screenprinting, and nanostructured TiO2 film have been reported in a previous paper.14 A cycloidal TiO2 electrode (∼0.283 cm2 in area) was stained by immersing it into a dye solution containing 300 µM Z907Na sensitizer in the mixture of acetonitrile and tert-butanol (V/V, 1/1) overnight. After being washed with acetonitrile and dried in air flow, the sensitized titania electrodes were assembled with thermally platinized conducting glass electrodes. The electrodes were separated by a 30-µm-thick Bynel hot-melt gasket and sealed under heat and pressure. The internal space was filled with an ionic liquid electrolyte by using a vacuum back-filling system. The electrolyte-injecting hole drilled by sand-blasting on the counter electrode glass substrate was sealed with a Bynel sheet and a thin glass cover under heat. 2.4. Photovolatic Measurements. A LS100 solar simulator (Solar Light Com. Inc., USA) was utilized to produce an irradiance of 100 mW cm-2 (equal to 1 sun at AM1.5G) onto the surface of solar cells. The current-voltage characteristics of the cells under this condition were obtained by applying external potential bias to the cells and measuring the generated photocurrent with a Keithley model 2400 digital source meter (Keithley, USA). This process was fully automated with use of Labview8.0. A similar data acquisition system was used to control the incident photon-to-collected electron conversion efficiency (IPCE) measurement. Under full computer control, light from a 1000 W xenon lamp was focalized through a monochromator onto the photovoltaic cell under test. The IPCE (λ) is defined by IPCE (λ) ) hcJsc/eφλ, where h is the Planck constant, c is the light speed in vacuum, e is the electronic charge, λ is the wavelength (m), Jsc is the short-circuit photocurrent density (mA cm-2), and φ is the incident radiative flux (mW cm-2). Photovoltaic characteristics were determined with the cell covered by a metal mask with an aperture area of 0.166 cm2. 2.5. Transient Photoelectrical Measurements. In the transient photoelectric decay experiment, steady-state white light of different intensities was supplied with a homemade white light-emitting diode array by tuning the driving voltage, and a green light-emitting diode array controlled with a fast solid state switch was used to generate a perturbing pulse of a time width of 200 ms. The steady-state white light and pulsed green light were both incident on the photoanode side of the cell under test. The green pulse was carefully controlled by the driving potential of diodes to keep the modulated photovoltage below 5 mV. We used green light as a probe to generate a photovoltage perturbation near the open-circuit photovoltage (Voc) of the cell under the white light condition and measured the voltage decay process thereafter. Normally, the transient signals follow a monoexponential decay, therefore the recombination rate constant, kr, can be extracted from the slope of the semilogarithmic plot. The capacitance (Cµ) of the TiO2/electrolyte interface at Voc is calculated by Cµ ) ∆Q/∆V, where ∆V is the peak of the transient photovoltage, and ∆Q is the number of electrons

injected during the green light pulse. The latter is obtained by integrating the transient curve of a short-circuit photocurrent generated by an identical green light pulse. This method may underestimate the actual injected electrons by the fraction that is lost due to recombination during the electron collection. The relative error, which is thought to be less than 30% in the worst case, does not affect the shape of the curve of calculated capacitance versus potential, but only alters the magnitude of the capacitance values. The electron density in the titania film under a given white light intensity was determined by charge extraction technique. 2.6. IMVS and IMPS Measurements. The intensitymodulated photovoltage spectroscopy (IMVS) and intensitymodulated photocurrent spectroscopy (IMPS) measurements were performed on a ZAHNER CIMPS system. A stationary DC voltage and a concurrent sinusoidal modulated AC voltage were imposed on a green LED, which gave out a green irradiance with a maximum wavelength at 546 nm. The LED was controlled by a potentiostatic feedback loop. The selected AC amplitude ranged from 5% to 15% of the stationary DC value. The transfer functions of IMPS and IMVS were determined by correlating the system response with the actual stimulation signal. The potential applied to the testing cell was controlled by a potentiostat unit. IMPS measurement was carried out under short-circuit condition while IMVS measurement was carried out under open-circuit condition. The measured shortcircuit photocurrent efficiency (Φext(ω)) of IMPS and the real and imaginary parts of modulated photovoltage ∆Voc of IMVS were fitted by using the Levenberg-Marquardt algorithm. 3. Results and Discussion 3.1. Physical Properties of Ionic Liquids and Binary Melts. By measuring the temperature-dependent specific conductivities with a gradual heating procedure to avoid the supercooling effect frequently met by organic molten salts, we estimated the melting points of N-methyl-N-allylpyrrolidinium salts as presented in Figure 1. The P13′I featuring an allyl substituent has a much lower melting point of 57.5 °C in contrast to other pyrrolidinium iodides with short saturated aliphatic chains such as N-methyl-N-propylpyrrolidinium iodide (mp >150 °C). We have noted that this scenario is distinctive from that of 1-methyl-3-propylimidazolium and 1-methyl-3-allylimidazolium iodides.3n The exact origin is not clear and needs to be clarified in further work. Moreover, its corresponding salts with nitrate, thiocyanate, and dicyanamide anions have higher conductivites and even lower melting points, becoming liquid at room temperature, due to the reduced electrostatic forces and/ or van der Waals interaction.15 These favorable features of allylsubstituted pyrrodinium melts have allowed us to formulate solvent-free ionic liquid electrolytes with high iodide concentrations discussed below. The physical properties of these salts at 25 °C are summarized in Table 1. We also measured their temperature-dependent densities, viscosities, and molar conductivities (Λ). As shown in Figure S1 (Supporting Information), the density of N-methylN-allylpyrrolidinium ionic liquids decreases linearly along with the increase of temperature, indicating a lower packing density

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Cai et al.

ΛηR ) C

Figure 2. Walden plots of molar conductivity versus fluidity of electrolytes A-C. The dashed “ideal” Walden line for 1 M KCl aqueous solution is also included.

of ions at a higher temperature owing to enlarged voids within the ionic liquid.15 From the density and molar weight data, we can derive the molar volume (Vm) of these ionic liquids in the order P13′DCA > P13′NCS > P13′NO3 and the molar concentration in the order P13′DCA < P13′NCS < P13′NO3 at a given temperature. Among these ionic liquids, P13′DCA has a lowest density at a given temperature, mirroring its smallest molar concentration. The fluidities (η-1) and molar conductivities depicted in Figures S2 and S3 (Supporting Information) can be well fitted by the corresponding VogelFulcher-Tammann (VFT) fluidity equations,16

η-1(T) ) AT-1/2 exp[-B/(T - T0)]

(1)

Λ(T) ) AT-1/2 exp[-B/(T - T0)]

(2)

where A and B are constants, T is the absolute temperature, and T0 is the thermodynamic Kauzmann temperature. The molar conductivity trend (P13′DCA > P13′NCS > P13′NO3) at a given temperature follows closely the tendency of fluidity in the order P13′DCA > P13′NCS > P13′NO3. It is also noted that the ion size inversely related to ion mobility, which also has an influence on molar conductivity,17 does not make a dominant contribution here. To evaluate the potential of N-methyl-N-allylpyrrolidinium based melts, we have further formulated three solvent-free electrolytes with high iodide concentrations, to efficiently intercept the geminate charge recombination between oxidized sensitizer molecules and photoinduced electrons in titania. The compositions of these electrolytes are as follows: electrolyte A, N-methyl-N-allylpyrrolidinium iodide/N-methyl-N-allylpyrrolidinium nitrate/iodine/N-butylbenzimidazole/guanidinium thiocyanate (molar ratio, 24/16/1.67/3.33/0.67); electrolyte B, Nmethyl-N-allylpyrrolidinium iodide/N-methyl-N-allylpyrrolidinium thiocyanate/iodine/N-butylbenzimidazole/guanidinium thiocyanate (molar ratio, 24/16/1.67/3.33/0.67); and electrolyte C, N-methyl-N-allylpyrrolidinium iodide/N-methyl-N-allylpyrrolidinium dicyanamide/iodine/N-butylbenzimidazole/guanidinium thiocyanate (molar ratio, 24/16/1.67/3.33/0.67). Temperaturedependent conductivities and viscosities of these electrolytes can also be described by the Vogel-Fulcher-Tammann (VFT) equation. The trends of their densities, conductivities, and viscosities echo the difference in those of the only varied component, i.e. nonelectroactive pyrrolidinium salts (P13′NO3, P13′NCS, and P13′DCA). Generally, solvent-free ionic liquids can be well described by correlating the temperature-dependent fluidity with molar conductivity according to the modified Walden’s rule,17

(3)

where C is a constant and R is the slope of the line in the Walden plot, which reflects the decoupling of the ions. As shown in Figure 2, the fitted R values of our three electrolytes A-C are 0.834, 0.816, and 0.806, respectively. These values are smaller than unity as predicted by the ideal Walden rule, implying a progressive augmentation in the quantity of less conductive ion pairs along with the increase of temperature.18 The electrolyte molar conductivity grows in the order C > B > A at a given fluidity, reflecting the effect of anion size. It is noted that the three fitted lines all cross over the “ideal line”, implying even faster transport of ions in consideration of the incomplete decoupling of ions in ionic liquids.18 The origin of this observation is not fully clear and needs further investigation. We correlated the temperature-dependent fluidity with triiodide mobility by the modified Stokes-Einstein relation,3o

µconv )

zie0 6πrHηR

(4)

where µcon is the convention mobility, and zi, rH, and η are the electric charge of the ith ions, effective hydrodynamic radius, and viscosity, respectively, and R is the slope of the line in the Stokes-Einstein plot. As shown in Figure 3, the fitted slopes of electrolytes A, B, and C are 0.723, 0.724, and 0.743, respectively, which are all smaller than unity as is expected by the ideal Stokes-Einstein relation,17

µconv )

zie0 6πrHη

(5)

It is noticeable that at a given fluidity, triiodide shows an anomalous high mobility in the ionic liquid electrolytes. In our previous work, we have quantitatively described this eccentric phenomenon.3o Apart from the normal physical diffusion, the concurrent coupling process of physical diffusion with bond exchange is responsible for the observed abnormally high diffusion coefficient. This can be described by the following equation,3o

µconv )

zie0(1 + P) 6πrHηR

(6)

where P reflects the coupling transport containing the Grotthustype bond exchange20 relative to simple physical diffusion. To further prove this transport mechanism, a reference electrolyte D with the composition P13′I/EMITFSI/I2/NBB/GNCS (molar ratio, 24/1600/1.67/3.33/0.67) was constructed, where the bond exchange contribution to the triiodide transport is supposed to

Figure 3. Logarithmic plots of triiodide mobility versus fluidity. The orange line is calculated from the ideal Stokes-Einstein relation by using a hydrodynamic radius of 2.1 Å for triiodide.

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J. Phys. Chem. C, Vol. 113, No. 10, 2009 4219

Figure 4. (A) J-V characteristics of cells with solvent-free N-methylN-allylpyrrolidinium melts (a, c, and e) measured under an irradiation of AM1.5G full sunlight (100 mW cm-2) and (b, d, and f) in the dark: (a and b) cell I; (c and d) cell II; and (e and f) cell III. (B) Photocurrent action spectrum of cell III.

TABLE 2: Detailed Photovoltaic Parameters Measured Under an Irradiation of AM1.5G Full Sunlight (100 mW cm-2)a cell

Jsc/mA cm-2

Voc/mV

FF

η/%

I II III

7.78 10.38 10.94

718 659 719

0.740 0.712 0.709

4.14 4.87 5.58

The aperture area of the testing mask is 0.166 cm2. A UV-absorbing antireflection film is attached onto the photoanode side of a testing cell.

Figure 5. (A) Plots of chemical capacitance of cells II and III versus open-circuit photovoltage. (B) Plots of recombination rate constant of cells II and III versus open-circuit voltage. (C) Plots of recombination rate constant of cells II and III versus extracted charge density.

be negligible. On the basis of the calculated triiodide radius from electrolyte D, we derived the P values for electrolytes A, B, and C to be 1.78, 1.78, and 1.81, respectively. Notably, the P values are almost the same for the three electrolytes, indicating that these types of anions may have no effect on the triiodide transport behavior. 3.2. Photovoltaic Performance. We further investigated the influence of physical properties of these ionic liquid electrolytes on the photovoltaic performance of dye-sensitized solar cells. Electrolytes A, B, and C were utilized to make cells I, II, and III, respectively. Figure 4A presents their current density-voltage (J-V) characteristics at an irradiance of AM1.5G full sunlight and in the dark. Detailed cell parameters are listed in Table 2. The short-circuit photocurrent density (Jsc), open-circuit photovoltage (Voc), and fill factor (FF) of cell I are 7.78 mA cm-2, 718 mV, and 0.740, respectively, yielding an overall conversion efficiency (η) of 4.14%. The photovoltaic parameters of Jsc, Voc, FF, and η of cell II are 10.38 mA cm-2, 659 mV, 0.712, and 4.87%, respectively. Cell III with the highest fluidity electrolyte has a Jsc of 10.94 mA cm-2, a Voc of 719 mV, and a FF of 0.709, reaching an efficiency of 5.58%. Under various lower light intensities, the power conversion efficiencies are even higher up to 6.3% partly because of a higher fill factor. The photocurrent action spectra of cell III is shown in Figure 4B,

where the IPCE is plotted as a function of wavelength. From the overlap integral of this curve with the standard AM1.5G solar emission spectrum, a short-circuit photocurrent density (Jsc) of 11.8 mA cm-2 is calculated, which is even higher than the measured photocurrent density, owing to a slight nonlinear dependence of photocurrent on light intensity. We note that the calculated current is in close agreement with the measured photocurrent under a low light intensity of 20 mW cm-2. Therefore there is negligible spectral mismatch between our solar simulator and the standard AM1.5G sunlight. 3.3. Device Physics. We are curious to understand the physics of relatively different photovoltaic performance of these devices, shedding light on the further device improvement. Transient photoelectrical measurements21 were performed to scrutinize the origin of the remarkable difference in the opencircuit photovoltages of cells II and III. As presented in Figure 5A, the capacitances (Cµ) of cells II and III both increase exponentially along with the increase of Voc, which was generated by applying a gradually enhanced light intensity. At a given Voc, the apparently smaller Cµ of cell III is related to fewer surface states below the conduction band edge, indicating that more surface states have been passivated due to a stronger interaction of the mesoporous surface of TiO2 film with dicyanamide in contrast to thiocyanate. This is consistent with

a

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Cai et al. which will be analyzed by the IMVS/IMPS measurements.22 Our transient photoelectrical measurements have proved that cells I and III exhibit a similar profile of chemical capacitance versus voltage. It is obvious from Figure 6A that the electron lifetime (τn) of cell III measured by IMVS is much larger than that of cell I. As shown in Figure 6B, the electron diffusion coefficient (Dn) of cell III is also a little bit higher compared to that of cell I. This may be credited to an effective charge screening23 in cell III, stemming from a lower viscosity of its electrolyte and resulting fast moving of the pyrrolidinium cation. On the basis of electron diffusion coefficients and electron lifetime, we can estimate the electron diffusion length (Ln) with Ln ) (Dτn)1/2. Note that here we have roughly corrected the Fermi-level difference of titania measured under open- and short-circuit conditions from the results in ref 22g. The electron diffusion length shown in Figure 6C of cell III is remarkably larger than that of cell I, retaining a more efficient charge collection22g and thus a higher short-circuit photocurrent, discussed above. 4. Conclusions

Figure 6. Plots of (A) electron lifetime, (B) diffusion coefficient, and (C) diffusion length of cells I and II versus the Fermi level gap between titania film and electrolyte.

the higher Voc of cell III compared to cell II, because under the same light illumination the former cell will have a higher electron quasi-Fermi level in view of their indistinguishable electrolyte Fermi level. It is known that the charge recombination at the titania/ electrolyte interface depends on the thermodynamic driving force as well as charge densities.1c,d The pseudo-first-order recombination rate constants (kr) were determined from small perturbation photovoltage transient decays at different Voc, by adjusting output light intensities of white light-emitting diodes. As presented in Figure 5B, at the same Voc the charge recombination constant of cell II is considerably larger than that of cell III. However, on the other hand, the recombination kinetic processes are also affected by the electron concentration in the TiO2 film. Thus, the charge extraction technique was applied to detect the recombination kinetic constant at various electron concentrations in the titania film. As shown in Figure 5C, although cell II still displays a higher recombination rate at a given extracted charge density, the difference is considerably reduced in contrast to the comparison at the Voc axis shown in Figure 5B. These together may explain a relatively higher Jsc of cell III compared to cell II measured under the same light intensity. We remark that further work is needed to clarify the details. While cells I and III possess almost the same open-circuit photovoltage, their short-circuit photocurrents diverge obviously,

In conjunction with anions of iodide, nitrate, thiocyanate, and dicyanamide, four new low-melting pyrrolidinium melts have been prepared by introducing the allyl substitutent to cation. Furthermore, three solvent-free electrolytes exhibiting high iodide concentrations have been formulated for dye-sensitized solar cells on the basis of these new salts. We have correlated temperature-dependent electrolyte viscosity with triiodide mobility through applying a modified Stokes-Einstein equation to describe the triiodide transport in ionic liquids with a high iodide packing density. We have further found that different anions have different influences on surface states and electron transport in the mesoporous titania film, resulting in different photovoltages and photocurrents of dye-sensitized solar cells. These results will enlighten the future design of new ionic liquid electrolytes to fabricate high-performance dye-sensitized solar cells. Acknowledgment. The National Science Foundation of China (No. 50773078), the “CAS Knowledge Innovation Program” (No. KGCX2-YW-326), the National Key Scientific Program (No. 2007CB936700), and the “CAS 100-Talent Program” are acknowledged for financial support. We are grateful to Dyesol for supplying the 400-nm-sized scattering paste and DuPont Packaging and Industrial Polymers for supplying the Bynel film. Supporting Information Available: Plots of density, fluidity, and molar conductivity of N-methyl-N-allylpyrrolidinium ionic liquids vs temperature and plots of density, specific conductivity, molar conductivity, and fluidity of electrolytes A-C vs temperature. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (b) Gra¨tzel, M. Nature 2001, 414, 338. (c) Peter, L. M. J. Phys. Chem. C 2007, 111, 6601. (d) Bisquert, J. Phys. Chem. Chem. Phys. 2008, 10, 49. (2) Chiba, Y.; Islam, A.; Watanabe, Y.; Komiya, R.; Koide, N.; Han, L. Jpn. J. Appl. Phys., Part 2 2006, 45, L638. (3) (a) Papageorgiou, N.; Athanassov, Y.; Armand, M.; Bonhoˆte, P.; Pettersson, H.; Azam, A.; Gra¨tzel, M. J. Electrochem. Soc. 1996, 143, 3099. (b) Matsumoto, H.; Matsuda, T.; Tsuda, T.; Hagiwara, R.; Ito, Y.; Miyazaki, Y. Chem. Lett. 2001, 26. (c) Kubo, W.; Kitamura, T.; Hanabusa, K.; Wada, Y.; Yanagida, S. Chem. Commun. 2002, 374. (d) Wang, P.; Zakeeruddin, S. M.; Exnar, I.; Gra¨tzel, M. Chem. Commun. 2002, 2972. (e) Wang, P.; Zakeeruddin, S. M.; Comte, P.; Exnar, I.; Gra¨tzel, M. J. Am. Chem. Soc. 2003, 125, 1166. (f) Wang, P.;

N-Methyl-N-allylpyrrolidinium Based Ionic Liquids Zakeeruddin, S. M.; Moser, J.-E.; Gra¨tzel, M. J. Phys. Chem. B 2003, 107, 13280. (g) Wang, P.; Zakeeruddin, S. M.; Humphry-Baker, R.; Gra¨tzel, M. Chem. Mater. 2004, 16, 2694. (h) Wang, P.; Zakeeruddin, S. M.; Moser, J.-E.; Humphry-Baker, R.; Gra¨tzel, M. J. Am. Chem. Soc. 2004, 126, 7164. (i) Wang, P.; Wenger, B.; Humphry-Baker, R.; Moser, J.-E.; Teuscher, J.; Kantlehner, W.; Mezger, J.; Stoyanov, E. V.; Zakeeruddin, S. M.; Gra¨tzel, M. J. Am. Chem. Soc. 2005, 127, 6850. (j) Kuang, D.; Wang, P.; Ito, S.; Zakeeruddin, S. M.; Gra¨tzel, M. J. Am. Chem. Soc. 2006, 128, 7732. (k) Kato, T.; Okazaki, A.; Hayase, S. Chem. Commun. 2005, 363. (l) Zistler, M.; Wachter, P.; Schreiner, C.; Fleischmann, M.; Gerhard, D.; Wasserscheid, P.; Hinsch, A.; Goresa, H. J. J. Electrochem. Soc. 2007, 154, B925. (m) Wachter, P.; Zistler, M.; Schreiner, C.; Berginc, M.; Krasˇovec, U. O.; Gerhard, D.; Wasserscheid, P.; Hinsch, A.; Gores, H. J. J. Photochem. Photobiol. A 2008, 197, 25. (n) Bai, Y.; Cao, Y.; Zhang, J.; Wang, M.; Li, R.; Wang, P.; Zakeeruddin, S. M.; Gra¨tzel, M. Nat. Mater. 2008, 7, 626. (o) Cao, Y.; Zhang, J.; Bai, Y.; Li, R.; Zakeeruddin, S. M.; Gra¨tzel, M.; Wang, P. J. Phys. Chem. C 2008, 112, 13775. (p) Gorlov, M.; Kloo, L. Dalton Trans. 2008, 20, 2655, and references cited therein. (4) Shi, D.; Pootrakulchote, N.; Li, R.; Guo, J.; Wang, Y.; Zakeeruddin, S. M.; Gra¨tzel, M.; Wang, P. J. Phys. Chem. C 2008, 112, 17046. (5) Xi, C.; Cao, Y.; Cheng, Y.; Wang, M.; Jing, X.; Zakeeruddin, S. M.; Gra¨tzel, M. J. Phys. Chem. C 2008, 112, 11063. (6) (a) Paulsson, H.; Hagfeldt, A.; Kloo, L. J. Phys. Chem. B 2003, 107, 13665. (b) Paulsson, H.; Berggrund, M.; Svantesson, E.; Hagfeldt, A.; Kloo, L. Solar Energy Mater. Solar Cells 2004, 82, 345. (7) Wang, P.; Zakeeruddin, S. M.; Gra¨tzel, M.; Kantlehner, W.; Mezger, J.; Stoyanov, E. V.; Scherr, O. Appl. Phys. A: Mater. Sci. Process. 2004, 79, 73. (8) Santa-Nokki, H.; Busi, S.; Kallioinen, J.; Lahtinen, M.; KorppiTommola, J. J. Photochem. Photobiol. A 2007, 186, 29. (9) Kushihito, Y.; Hayakawa, H.; Tsunashima, K.; Sugiya, M. Bull. Chem. Soc. Jpn. 2007, 12, 2473. (10) (a) MacFarlane, D. R.; Meakin, P.; Sun, J.; Amini, N.; Forsyth, M. J. Phys. Chem. B 1999, 103, 4164. (b) MacFarlane, D. R.; Huang, J. H.; Forsyth, M. Nature 1999, 402, 792. (c) Golding, J.; Hamid, N.; MacFarlane, D. R.; Forsyth, M.; Forsyth, C.; Collins, C.; Huang, J. Chem. Mater. 2001, 13, 558. (d) MacFarlane, D. R.; Forsyth, M. AdV. Mater. 2001, 13, 957. (e) Forsyth, S. A.; Pringle, J. M.; MacFarlane, D. R. Aust. J. Chem. 2004, 57, 113. (f) Adebahr, J.; Seeber, A. J.; MacFarlane, D. R.; Forsyth, M. J. Phys. Chem. B 2005, 109, 20087. (g) Macfarlane, D. R.; Forsyth, M.; Howlett, P. C.; Pringle, J. M.; Sun, J.; Annat, G.; Neil, W.; Izgorodina, E. I. Acc. Chem. Res. 2007, 40, 1165. (h) Henderson, W. A.; Passerini, S. Chem. Mater. 2004, 16, 2881. (i) Nicotera, I.; Oliviero, C.; Henderson, W. A.; Appetecchi, G. B.; Passerini, S. J. Phys. Chem. B 2005, 109, 22814. (j) Henderson, W. A.; Young, V. G.; Passerini, S.; Trulove, P. C.; De Long,

J. Phys. Chem. C, Vol. 113, No. 10, 2009 4221 H. C. Chem. Mater. 2006, 18, 934. (k) Zhou, Q.; Henderson, W. A.; Appetecchi, G. B.; Montanino, M.; Passerini, S. J. Phys. Chem. B 2008, 112, 13577. (11) Wang, P.; Dai, Q.; Zakeeruddin, S. M.; Forsyth, M.; MacFarlane, D. R.; Gra¨tzel, M. J. Am. Chem. Soc. 2004, 126, 13590. (12) Cerneaux, S. A.; Zakeeruddin, S. M.; Gra¨tzel, M.; Cheng, Y. B.; Spiccia, L. Chem. Commun. 2008, 3852. (13) Pilarski, B. Liebigs Ann. Chem. 1983, 1078. (14) Wang, P.; Zakeeruddin, S. M.; Comte, P.; Charvet, R.; HumphryBaker, R.; Gra¨tzel, M. J. Phys. Chem. B 2003, 107, 14336. (15) Wasserscheid, P.; Welton. T. Ionic Liquids in Synthesis; Wiley: Weinheim, Germany, 2002. (16) (a) Vogel, H. Phys. Z. 1921, 22, 645. (b) Fulcher, G. S. J. Am. Ceram. Soc. 1925, 8, 339. (c) Tammann, V. G.; Hesse, W. Z. Anorg. Allg. Chem. 1926, 156, 245. (17) Bockris, J. O.; Reddy, A. K. N. Modern Electrochemistry; Plenum: New York, 1998. (18) Ohno. H. Electrochemical Aspects of Ionic Liquids; Wiley: Hoboken, NJ, 2005. (19) Xu, W.; Cooper, E. I.; Angell, C. A. J. Phys. Chem. B 2003, 107, 6170. (20) (a) Dahms, H. J. Phys. Chem. 1968, 72, 362. (b) Ruff, I.; Friedrich, V. J. J. Phys. Chem. 1971, 75, 3297. (c) Ruff, I.; Friedrich, V. J.; Csillag, K. J. Phys. Chem. 1972, 76, 162. (21) (a) O’Regan, B. C.; Lenzmann, F. J. Phys. Chem. B 2004, 108, 4342. (b) Bailes, M.; Cameron, P. J.; Lobato, K.; Peter, L. M. J. Phys. Chem. B 2005, 109, 15429. (c) Kopidakis, N.; Neale, N. R.; Frank, A. J. J. Phys. Chem. B 2006, 110, 12485. (d) Walker, A. B.; Peter, L. M.; Lobato, K.; Cameron, P. J. J. Phys. Chem. B 2006, 110, 25504. (e) Quintana, M.; Edvinsson, T.; Hagfeldt, A.; Boschloo, G. J. Phys. Chem. C 2007, 111, 1035. (22) (a) Schlichtho¨rl, G.; Huang, S. Y.; Sprague, J.; Frank, A. J. J. Phys. Chem. B 1997, 101, 8141. (b) Dloczik, L.; Ileperuma, O.; Lauermann, I.; Peter, L. M.; Ponomarev, E. A.; Redmond, G.; Shaw, N. J.; Uhlendorf, I. J. Phys. Chem. B 1997, 101, 10281. (c) Schlichtho¨rl, G.; Park, N. G.; Frank, A. J. J. Phys. Chem. B 1999, 103, 782. (d) Park, N.-G.; Schlichtho¨rl, G.; van de Lagemaat, J.; Cheong, H. M.; Mascarenhas, A.; Frank, A. J. J. Phys. Chem. B 1999, 103, 3308. (e) Peter, L. M.; Wijayantha, K. G. U. Electrochim. Acta 2000, 45, 4543. (f) Kru¨ger, J.; Plass, R.; Gra¨tzel, M.; Cameron, P. J.; Peter, L. M. J. Phys. Chem. B 2003, 107, 7536. (g) Jennings, J. R.; Ghicov, A.; Peter, L. M.; Schmuki, P.; Walker, A. B. J. Am. Chem. Soc. 2008, 130, 13364. (23) (a) Kopidakis, N.; Schiff, E. A.; Park, N.-G.; van de Lagemaat, J.; Frank, A. J. J. Phys. Chem. B 2000, 104, 3930. (b) Lanning, O. J.; Madden, P. J. Phys. Chem. B 2004, 108, 11069.

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