Dye-Sensitized Solar Cells with Solvent-Free Ionic Liquid Electrolytes

Aug 9, 2008 - Federal Institute of Technology, CH 1015, Lausanne, Switzerland ... increased compared to those for cells with the corresponding state o...
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J. Phys. Chem. C 2008, 112, 13775–13781

13775

Dye-Sensitized Solar Cells with Solvent-Free Ionic Liquid Electrolytes Yiming Cao,† Jing Zhang,† Yu Bai,† Renzhi Li,† Shaik M. Zakeeruddin,‡ Michael Gra¨tzel,‡ and Peng Wang*,† State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (CAS), Changchun 130022, China, and Laboratory for Photonics and Interfaces, Swiss Federal Institute of Technology, CH 1015, Lausanne, Switzerland ReceiVed: June 8, 2008; ReVised Manuscript ReceiVed: June 27, 2008

We systematically studied the temperature-dependent physicochemical properties, such as density, conductivity, and fluidity, of 1,3-dialkylimidazolium iodides. In combination with the amphiphilic Z907Na sensitizer, we have found that it is important to use low-viscosity iodide melts with small cations to achieve high-efficiency dye-sensitized solar cells. By employing high-fluidity eutectic-based melts the device efficiencies considerably increased compared to those for cells with the corresponding state of the art ionic liquid electrolytes. We propose a modified Stokes-Einstein equation by correlating ion mobility and fluidity to quantitatively depict the triiodide transport in ionic liquid electrolytes. These studies reveal that the viscosity-dependent transport of triiodide in ionic liquid electrolytes with high iodide concentration can be explained by two parallel processes. Apart from the normal physical diffusion, the coupling process of physical diffusion and bond exchange is responsible for the observed abnormally high diffusion coefficients. This work has provided useful insight for further improvement of solvent-free electrolytes based on rational design of their constituents, facilitating the large-scale practical application of lightweight, flexible dye-sensitized solar cells. 1. Introduction The presently unfolding planetary energy emergency warrants major efforts to develop renewable resources. Photovoltaic cells are becoming an increasing appealing option for energy production.1 Excitonic solar cells made from organic optoelectronic materials, which normally employ a donor-acceptor pair, offer a potentially cost-effective solution for the future photovoltaic market, as exemplified by dye-sensitized, polymer, and smallmolecule cells.2 By using a highly volatile electrolyte, the dyesensitized solar cell (DSC)3 has achieved a certificated 11.1% efficiency record4 among all types of organic photovoltaic cells, encouraging the surge to explore new organic materials for the conversion of solar to electric power. Striving to develop DSCs suitable for outdoor use, we recently have demonstrated a solar cell based on a low-volatility electrolyte, showing conversion efficiencies over 9% and a remarkable stability under long-term thermal and light-soaking dual stress.5 While the development of more efficient sensitizers and mesoporous semiconducting films cannot be overemphasized, we must remark on the pivotal role of electrolytes for further device performance enhancement of DSCs. Room temperature ionic liquids (RTILs) possessing good chemical and thermal stability, negligible vapor pressure, nonflammability, and high ionic conductivity have been intensively pursued as alternative electrolytes for DSCs6 and other electrochemical devices.7 Note that the above-mentioned “champion” DSC also employed ionic liquids in its electrolyte but in addition contained a large amount of volatile solvents.4 However, if the cells are not hermetically sealed, volatile solvents can leak out and evaporate under thermal stress, leaving only ionic liquids behind and deteriorating the device performance. * Corresponding author. E-mail: [email protected]. † Changchun Institute of Applied Chemistry, CAS. ‡ Swiss Federal Institute of Technology.

Hence it is pertinent to employ nonvolatile solvent-free ionic liquids or polymer electrolytes or all-solid-state hole-transporting materials in DSCs. Note that the solidification of volatile electrolytes by gel forming additives only affects their macroscopic fluidity. It cannot prevent solvent loss by leakage and evaporation under thermal stress as the presence of the gelating agent in the electrolyte affords only a small increase of its boiling point.8 Flexible and lightweight solar cells based on plastic matrix are attractive even if their solar conversion efficiency is moderate, i.e. in the 5% to 10% range. However, for these devices the use of organic solvents is not advisable, as they would permeate across the plastic cell walls. During the past years, solvent-free room temperature ionic liquid electrolytes of imidazolium melts have been actively pursued as an attractive solution to this dilemma. Recently, we demonstrated the concept of using a eutectic-based ionic liquid to reach over 8% efficiency measured under illumination by AM 1.5 G full sunlight.9 Here we scrutinize the extraodinary behavior of these melts and detail recent advances on dye-sensitized solar cells with such solventfree ionic liquid electrolytes. 2. Experimental Section 2.1. Materials. All solvents and reagents, unless otherwise stated, were of puriss quality and used as received. Alkyl iodides, guanidinium thiocyanate (GNCS), and 3-phenylpropionic acid (PPA) were purchased from Fluka. The 400-nm-sized TiO2 anatase particles were received as a gift from Catalysts and Chemical Ind. Co. (CCIC). The Z907Na sensitizer, NaRu(4carboxylic acid-4′-carboxylate)(4,4′-dinonyl-2,2′-bipyridine)(NCS)2, was prepared as previously reported.6i N-Butylbenzimidazole (NBB) was synthesized according to the literature method10 and distilled before use. 1,3-Dialkylimidazolium iodides were prepared by the direct reaction of 1-methylimidazole and alkyl iodides in the absence of any solvent at an

10.1021/jp805027v CCC: $40.75  2008 American Chemical Society Published on Web 08/09/2008

13776 J. Phys. Chem. C, Vol. 112, No. 35, 2008 almost unity yield. Fresh AgNCS was precipitated by mixing aqueous solutions of KNCS with AgNO3 at a molar ratio of 1/1 in the dark. Metathesis of 1,3-dialkylimidazolium iodides with AgNCS in deionized water according to the literature method11 produces the corresponding thiocyanates. The synthetic details of iodides and thiocyanates were described in the Supporting Information. 2.2. Electrolyte Characterization. The viscosity measurements were carried out with a Brookfield DV-II+Pro viscometer. Densities were determined with an Anton Paar DMA 35N density meter. A Radiometer CDM210 conductivity meter was used to measure conductivities. 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, consisting of a 5.0 µm radius Pt ultramicroelectrode as working electrode and a Pt foil as counter electrode, was used for the measurements of triiodide diffusion coefficients in combination with a CHI 660C electrochemical workstation. A heating-cooling cycle pump was employed for the control of sample temperatures. 2.3. Device Fabrication. A screen-printed double layer film of interconnected TiO2 particles was used as mesoporous 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 double-layer nanostructured TiO2 film have been reported in our previous paper.12 Unless otherwise stated with PPA as coadsorbent, a TiO2 electrode was stained by immersing it into a dye solution containing 300 µM Z907Na sensitizer in the mixture of acetonitrile and tert-butanol (volume ratio: 1/1) overnight. After being washed with acetonitrile and dried by air flow, the sensitized titania electrodes were assembled with thermally platinized conducting glass electrodes. The electrodes were separated by a 25 µm thick Surlyn hot-melt gasket and sealed by heating. The internal space was filled with a liquid electrolyte, using a vacuum back filling system. The electrolyteinjecting hole made with a sand-blasting drill on the counter electrode glass substrate was sealed with a Bynel sheet and a thin glass cover by heating. 2.4. J-V Measurements. A 450 W xenon light source (Oriel, USA) was used to give an irradiance of 100 mW cm-2 (the equivalent of one sun at air mass (AM) 1.5G) at the surface of solar cells. The spectral output of the lamp was matched in the region of 350-750 nm with the aid of a Schott K113 Tempax sunlight filter (Pra¨zisions Glas & Optik GmbH, Germany) so as to reduce the mismatch between the simulated and true solar spectra to less than 2%. The current-voltage characteristics were obtained by applying external potential bias to a cell and measuring the generated photocurrent with a Keithley model 2400 digital source meter (Keithley, USA). Photovoltaic performance was measured by using a metal mask with an aperture area of 0.158 cm2. All devices were measured within 3 h after fabrication. 2.5. Transient Photovoltage Kinetics. In the transient photovoltage experiment,13 steady-state lights were supplied with a homemade white light-emitting diode array tuned by varying the driving voltage. A red light-emitting diode array controlled with a fast solid-state switch was used to generate a perturbation pulse with a width of 200 ms. The pulse and steady-state white lights were both incident on the photoanode side of a testing cell. The pulse light intensity was controlled by the driving potential of red diodes to keep the modulated photovoltage

Cao et al.

Figure 1. Photographs of imidazolium melts and their mixtures at room temperature. The samples from left to right are HMII, BMII, PMII, EMII, DMII, DMII/EMII (1/1, molar ratio), AMII, DMII/EMII/AMII (1/1/1, molar ratio), respectively. All samples were dried at 80 °C under a vacuum of ∼3 Torr for 8 h.

below 10 mV. We used red diodes as a probe to generate a perturbation near the Voc of the cell under the bias white light and measured the voltage decay process thereafter. Normally, the decay follows closely a monoexponential form, thus the recombination rate, kr, can be extracted from the slope of the semilogarithmic plot. The electron density in the titania films under a given bias white light intensity can be measured by the charge extraction method. 3. Results and Discussion 3.1. Solvent-Free Iodide Ionic Liquid Electrolytes. The iodide/triiodide redox couple is the most efficient mediator in the mesoscopic DSC,4 although a 7.5% DSC measured under AM 1.5 G full sunlight was fabricated by using a solvent-free ionic liquid electrolyte with the SeCN-/(SeCN)3- couple.6h Our subsequent tests proved that it was hard to make a long-term stable DSC under the thermal and light-soaking dual stress with electrolytes containing the SeCN-/(SeCN)3- redox couple. Several alternative redox couples have been demonstrated to work for the DSC but the efficiencies were low.14 This has motivated us to enhance the DSC efficiency by using a highfluidity ionic liquid with a high-packing density of electroactive species. Hence, we revisite the iodide melts and explore their potential in DSCs as solvent-free electrolytes. The solvent-free ionic liquid electrolytes used in the DSC employ iodide melts as their major component, because a high concentration of iodide is required to intercept quantitatively the geminate recombination between the electrons injected by the photoexcited sensitizer in the nanocrystalline titania film and its oxidized form.6i The viscosity of the iodide melts should be low to avoid mass transport limitation of the photocurrent and loss of fill factor for a cell operated under full sunlight. Among iodide salts that are room temperature ionic liquids, 1-propyl-3-methylimidazolium iodide (PMII) has the lowest viscosity. Hence it was doped with iodine to make a ∼6.0% DSC,6e which is by far the most efficient ionic liquid system without containing nonelectroactive anions. In the family of 1,3-dialkylimidazolium iodides, 1-hexyl-3methylimidazoulium iodide (HMII), 1-butyl-3-methylimidazolium iodide (BMII), and 1-propyl-3-methylimidazolium iodide (PMII) are liquid at room temperature as shown in Figure 1. Upon further shortening of the alkyl chains, 1-ethyl-3-methylimidazolium iodide (EMII) and 1,3-dimethylimidazolium iodide (DMII) become solid with a melting point at 77.5 and 92 °C, respectively.9 This may be caused by the high lattice Gibbs energies stemming from the conformational rigidity of small and symmetric cations. The solid feature of EMII and DMII restricted their potential application as solvent-free

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Figure 2. Plots of fluidity versus temperature in the Arrhenius coordinate: (a) HMII; (b) BMII; (c) PMII; and (d) DMII/EMII/AMII (1/1/1, molar ratio).

Figure 3. Plots of density versus temperature in the Arrhenius coordinate: (a) HMII; (b) BMII; (c) PMII; and (d) DMII/EMII/AMII (1/1/1, molar ratio).

decrease of densities for all iodides, indicating lower ion packing densities due to the larger molecular volumes or enlarged voids within these melts. The ternary melt exhibits the highest densities owing to its highest percentage of the heavy atom iodide. From the temperature-dependent density measurements, we can derive the molar concentrations of these iodide melts in the order HMII < BMII < PMII < DMII/EMII/AMII (1/1/1, molar ratio) and molar volumes in the order HMII > BMII > PMII > DMII/ EMII/AMII (1/1/1, molar ratio) at a given temperature. The smaller the molar volume the higher the ion packing density, which should contribute to a higher conductivity, cooperating with the higher ion mobility influenced by the above-mentioned fluidity according to the following equation,

electrolytes in DSC unless they are combined with other room temperature ionic liquids or solvents.5 Interestingly, at a given temperature the conductivities of EMII and DMII melts are much higher than that of PMII. This finding encouraged us to mix DMII and EMII at different molar ratios. This preliminary experiment allows us to obtain a highly conductive eutectic salt with a melting point of 47.5 °C at the molar ratio of 1/1.9 The decreased melting point could be explained by the entropy increase.15 We further observed that 1-allyl-3-methylimidazolium iodide (AMII) is solid at room temperature as displayed in Figure 1 and also has a higher conductivity above melting point compared with that of PMII.9 To obtain a room temperature ionic liquid with a superior fluidity and a high ionic conductivity, DMII, EMII, and AMII were mixed at the molar ratio of 1/1/1 to form a room temperature liquid as shown in Figure 1. This ternary melt has a melting point below 0 °C and a strikingly high room temperature conductivity (σ) of 1.68 mS cm-1, exceeding that of PMII (0.58 mS cm-1) by almost a factor of 3.9 This higher conductivity can be rationalized by a lower viscosity, a higher ion concentration, as well as a smaller size of cations in the ternary melt.16 Figure 2 shows the temperature dependent fluidity (η-1) of imidazolium iodide melts in the Arrhenius coordinate. The plots can be well fitted by the Vogel-Fulcher-Tammann (VFT) viscosity equation,17

where F is the Faradic constant and z, c, and µ are the electronic charge, concentration, and mobility of the ith ion, respectively. Knowing the densities and conductivities of iodide melts at different temperatures, we can calculate the temperaturedependent molar conductivities (Λ). The behavior of ionic liquids can be further scrutinized by the empirical Walden rule (Λη ) constant). This means that the product of the molar conductivity and the viscosity for a particular electrolyte should be a constant. Recent research has shown that introducing an index parameter R into the empirical Walden rule can make the equation fit better to ionic liquids.19 The modified Walden rule can be expressed as,

η-1(T) ) AT-1⁄2 exp[-B/(T - T0)]

Ληa ) constant

(1)

where A and B are constants, T is the absolute temperature, and T0 is the thermodynamic Kauzmann temperature. The viscosity decreases in the order HMII > BMII > PMII > DMII/EMII/ AMII (1/1/1, molar ratio). At 25 °C, the viscosity (η) of this ternary melt is 403 cP, which is much lower than that of 1084 cP for PMII. The viscous behavior of ionic liquids can be understood by the interplay of Columbic and Van der Waals interactions as well as hydrogen bond formation.18 The electrostatic attraction between cations and iodide weakens along with lengthening the alkyl chain, but the Van der Waals interaction between the imidazolium cations becomes stronger and results in a lower fluidity. As shown in Figure 3, the temperature-dependent densities (F) can be well fitted by the following equation:

F ) A + BT

(2)

where A and B are constants and T is the absolute temperature. Along with the increase of temperature, there is a concomitant

σ ) F∑ziciµi

(3)

(4)

where R is the slop of the line in the Walden plot and reflects the decoupling degree of ions. The Walden plots of molar conductivity versus fluidity for four iodide melts are shown in Figure 4. The dotted “ideal line” represents the data of dilute aqueous KCl solution, in which ions are assumed to be fully dissociated. It is obvious that the experimental data of HMII, BMII, PMII and DMII/EMII/AMII (1/1/1, molar ratio) can be well fitted by the modified Walden rule. The slopes of the four fitted lines (HMII: 0.90; BMII: 0.90; PMII: 0.91; DMII/EMII/ AMII (1/1/1): 0.84) are slightly lower than unity predicted by the “ideal” Walden rule, indicating progressive augmentation in the population of less conductive ion pairs with increasing temperature. The molar conductivity decreases in the order DMII/EMII/AMII (1/1/1, molar ratio) > PMII > BMII > HMII at a given fluidity and this behavior, reflecting the size of cations in ionic liquid. It is noticeable that the four fitted lines approach or even cross over the “ideal line”, implying even faster transport of ions in view of the incomplete decoupling of ions in ionic

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Figure 4. Walden plots of molar conductivity versus fluidity: (a) HMII; (b) BMII; (c) PMII; and (d) DMII/EMII/AMII (1/1/1). The dotted “ideal” Walden line is also included.

Figure 5. J-V characteristics of devices A-D measured under the illumination of AM 1.5 G full sunlight (100 mW cm-2).

TABLE 1: Detailed Photovoltaic Parameters of Devices A-D Measured under the Illumination of AM 1.5 G Full Sunlight (100 mW cm-2) device

Jsc/mA cm-2

Voc/mV

FF

η/%

A B C D

8.44 11.00 11.50 11.16

699 705 687 690

0.568 0.684 0.720 0.773

3.36 5.31 5.69 5.96

liquid. The origin of this observation is unclear and needs further investigation. To assess the photovoltaic performance of devices employing solvent-free iodide melt, we formulated four electrolytes by adding N-butylbenzoimidazole (NBB). The compositions of electrolytes tested were as follows: electrolyte in device A, HMII/I2/NBB (12/1/1); electrolyte in device B, BMII/I2/NBB (12/1/1); electrolyte in device C, PMII/I2/NBB (12/1/1); electrolyte in device D, DMII/EMII/AMII//I2/NBB (4/4/4/1/1). Figure 5 presents the J-V curves for devices A-D measured under the illumination of AM 1.5 G full sunlight and the detailed device parameters are listed in Table 1. While there is not too much variation on the open-circuit photovoltage, it is obvious that the device fill factor has a steady improvement along with the decrease of electrolyte viscosity. In addition, the benefit of increasing fluidity can also be perceived by the change of shortcircuit photocurrent densities for HMII, BMII, and PMII. In general, the device efficiency increases in the order DMII/EMII/ AMII (1/1/1, molar ratio) > PMII > BMII > HMII, mirroring their fluidity tendency. Our previous work has shown that the recombination of injected electrons in the titania film with triiodide is a critical channel for the photocurrent loss.5 The

Figure 6. J-V characteristics of devices E and F under the AM 1.5 G illumination (100 mW cm-2).

triiodide concentration was taken from the PMII-based electrolyte,6e which was optimized to avoid the photocurrent loss due to the mass transport limitation of triiodide. But in the ternary iodide system with a much higher fluidity, the diffusion flux of triiodide may be too high, causing a considerable photocurrent loss due to the charge recombination at the titania/ electrolyte interface. This may explain why we did not see the expected photocurrent advantage of low fluidity of electrolytes in device D. To prove the above assumption, we made two other electrolytes containing a low concentration of triiodide by almost a factor of 2. The compositions tested were as follows: electrolyte in device E, PMII/I2/NBB (12/0.5/1), and electrolyte in device F, DMII/EMII/AMII//I2/NBB (4/4/4/0.5/1). As shown in Figure 6, the short-circuit photocurrent density (Jsc), open-circuit voltage (Voc), fill factor (FF), and power conversion efficiency (η) of device E are 10.77 mA cm-2, 693 mV, 0.694, and 5.18%. Compared to device C, the decreased Jsc, FF, and the corresponding η of device E must be caused by the mass transport of triiodide. More interestingly, photovoltaic parameters (Jsc, Voc, FF, and η) of device F are 11.56 mA cm-2, 700 mV, 0.777 and 6.29%, respectively. This experiment has unambiguously confirmed that it is necessary to employ a high-fluidity iodide melt for efficiency enhancement. In using these low-viscosity iodide melts, we employed the strategy of molecular-scale interface engineering to reduce the charge recombination at the titania/electrolyte interface to enhance the performance. Guanidinium thiocyanate (GNCS) and 3-phenylpropionic acid (PPA) were used as additives in electrolyte and staining dye solution, respectively. The electrolyte composition tested for devices G and H is DMII/EMII/ AMII//I2/NBB/GNCS (4/4/4/0.5/1/0.2). Device H was fabricated by staining the titania film in the dye solution containing 300 µM PPA, and device G does not contain PPA. Photovoltaic parameters (Jsc, Voc, FF, and η) of device G are 12.07 mA cm-2, 719 mV, 0.778, and 6.75%. By comparing devices G and F (Figures 6 and 7), it is obvious that adding GNCS in the electrolyte without changing other compositions increased the Voc and Jsc, improving the device efficiency from 6.29% to 6.75%. This can be explained by the up-lifting quasi-Fermi level of electrons in the TiO2 film and retarding the recombination between triiodide and electrons in the mesoporous titania film.20 As shown in Figure 7, the cografting of PPA with Z907Na on the surface of titania nanocrystals can further enhance the device performance, which is consistent with our previous observation on the effect of PPA for a DSC with a binary ionic liquid electrolyte.6g Photovoltaic parameters (Jsc, Voc, and FF) of device H are 12.82 mA cm-2, 721 mV, and 0.768, respectively, yielding

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J. Phys. Chem. C, Vol. 112, No. 35, 2008 13779 TABLE 2: Detailed Photovoltaic Parameters of Devices I and J Measured under the Illumination of AM 1.5 G Full Sunlight (100 mW cm-2)

Figure 7. J-V characteristics of devices G and H under the AM 1.5 G illumination (100 mW cm-2).

Figure 8. Plots of recombination rate versus extracted charge density for devices G and H.

a power conversion efficiency of 7.0%,9 which is much higher than that of 6.0% previously reported6e for the corresponding device with a solvent-free, PMII-based ionic liquid electrolyte. Note that this ternary electrolyte composed of imidazolium iodides has shown a comparable efficiency to previously report solvent-free electrolytes containing relatively precious ionic liquids.6f-j The role of PPA in enhancing the Jsc can be further elaborated by measuring the transient photovoltage decays. Figure 8 displays the recombination rate (kr) at various extracted charge densities for devices G and H. At a given extracted charge density, device H always has a smaller kr than device G, indicating that the recombination between injected electrons and triiodide has been slowed down by cografting PPA with the Z907Na sensitizer. Further work is in progress to detail the role of PPA in device enhancement. 3.2. Ionic Liquid Electrolytes with Iodide and Nonelectroactive Anions. Due to the high polarity and localized charge density, until now an iodide ionic liquid with the viscosity lower than 200 cp at room temperature has not been reported to the best of our knowledge. However, some imidazolium salts with other anions6f,g,7,21 such as thiocyanate, dicyanoamide, tricyanomethide, tetracyanoborate, triflate, etc. show much lower viscosities at the ambient temperature. On the basis of the systematic studies on solvent-free ionic liquid electrolytes for DSCs, we have proved that compared to the pure iodide melts, some imidazolium melts containing at least one nonelectroactive anion apart from iodide can show higher power conversion efficiencies.6f-j,9 In our recent work, this strategy has also been successfully applied to make an efficient DSC with solventfree nonimidazolium ionic liquid electrolytes.22 We must note that mixing iodide melts with nonelectroactive ionic liquids to lower the electrolyte viscosity may not improve the iodide

device

Jsc/mA cm-2

Voc/mV

FF

η/%

I J

11.37 12.05

718 718

0.749 0.773

6.11 6.69

diffusion flux but solely increase the iodide diffusion coefficient.6i Actually, a high diffusion flux of iodide is desirable for the fast dye regeneration, to avoid the geminate charge recombination between oxidized sensitizers and injected electronsinthetitaniafilms.Weobservedtheenhancedphotocurrents6f-j,9 in DSCs with optimized binary ionic liquid electrolytes containing nonelectroactive anions, compared to pure iodide melts. Employing the transient absorption measurements, we have revealed that this may be caused by the suppression of another photocurrent loss channel, i.e. the reductive quenching of excited sensitizers in case of extraordinarily high packing densities of iodide.6i In addition, the high electrolyte fluidity should allow the use of a relatively low concentration of triiodide, without bringing a photocurrent loss due to the transport of triiodide. The use of less triiodide in the electrolyte not only decreases the dissipative optical absorption of triiodide but also prolongs the lifetime of electron in the titania film, which should benefit a higher charge collection yield. Hence we are curious to extend the above finding to make more efficient electrolytes containing nonelectroactive anions compared to state of the art binary systems.6f-j To explore the potential of using high-conductivity iodide eutectic for solventfree ionic liquid electrolytes containing nonelectroactive anions, we selected an abundantly available thiocyanate (NCS) for our preliminary understanding. We prepared ionic liquids of 1,3dimethylimidazoluim thiocyanate (DMINCS), 1-ethyl-3-methylimidazolium thiocyanate (EMINCS), 1-allyl-3-methylimidazolium thiocyanate (AMINCS), and 1-propyl-3-methylimidazolium thiocyanate (PMINCS), and measured their fundamental physicochemical properties. As depicted in Figures S2 and S3 (Supporting Information), their temperature-dependent specific conductivities and fluidities can be well fitted by the corresponding VFT equations. At a given temperature higher than 25 °C, DMINCS shows the highest conductivity, fluidity, and density compared to EMINCS, PMINCS, and AMINCS. This study helped to formulate new electrolytes using the eutectic melt of DMII and EMII in combination with EMINCS. Thus we prepared two electrolytes containing thiocyanate anion for photovoltaic evaluation: electrolyte in device I, PMII/ EMINCS/I2/NBB (12/8/1/1), and electrolyte in device J, DMII/ EMII/EMINCS/I2/NBB (6/6/8/1/1). The detailed photovoltaic parameters of devices I and J are listed in Table 2, clearly demonstrating the merit of using the eutectic melt of DMII and EMII to make a high conductivity, low viscosity electrolyte (Figures S5 and S6, Supporting Information). Taking advantage of these newly formulated ionic liquid electrolyte, we employed GNCS and PPA as additives in the above electrolyte and the dye solution, respectively. The composition in device K is DMII/ EMII/EMINCS//I2/NBB/GNCS (6/6/8/1/1/0.2). As shown in Figure 9, the Jsc, Voc, and FF of device K are 13.22 mA cm-2, 743 mV, and 0.774, respectively, yielding a power conversion efficiency of 7.6%, which has been considerably improved compared to that reported in our previous paper.6g 3.3. Transport of Triiodide in Ionic Liquid Electrolytes. We studied the triiodide transport in four related melts without GNCS and NBB as additives: melt I, PMII/I2 (12/1); melt II, DMII/EMII/AMII/I2 (4/4/4//1); melt III, PMII/EMINCS/I2 (12/

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D)

kBT 6πrHηR

(5)

where D, kB, T, rH, and η are the diffusion coefficient, Boltzmann constant, absolute temperature, effective hydrodynamic radius, and viscosity, respectively, and R is the slop of the line in the Stokes-Einstein plot, which reflects the decoupling degree of ions. Furthermore, the diffusion coefficient D can be described by the Einstein relation,

D)µ jabskBT

(6)

where µ j abs is the absolute mobility of ions. Hence the modified Stokes-Einstein equation can be expressed in another form, Figure 9. J-V characteristics of device K measured in the dark and under the AM 1.5 G illumination (100 mW cm-2).

µconv )

zie0 6πrHηR

(7)

where the conventional mobility is defined as µconv ) zie0µ j abs, zi is the electrical charge of the ith species, and e0 is element electron charge. The temperature-dependent triiodide mobility (µconv) versus fluidity (η-1) is plotted in Figure 10. It is noted that while log(µconv) increases linearly with log(η-1), the fitted slopes of melts I-IV are 0.694, 0.705, 0.758, and 0.726, respectively, which are all less than unity as expected by the ideal Stokes-Einstein equation,

µconv ) Figure 10. Stokes-Einstein plots of triiodide mobility (µconv) versus fluidity (η-1): (a-e) electrolytes I-V. The dashed line is calculated from the Stokes-Einstein relation, using a hydrodynamic radius of 2.1 Å for triiodide.

SCHEME 1: The Schematic Coupling Transport Mechanism of Triiodide in Ionic Liquid Electrolytes with a High Iodide Packing Densitya

8/1); melt IV, DMII/EMII/EMINCS/I2 (6/6/8/1). As shown in Figure S4 (Supporting Information), at a given temperature, the densities of these melts decrease in the order II > I > IV > III, reflecting the iodide/triiodide packing densities. Temperaturedependent specific conductivities, fluidities, and triiodide diffusion coefficients of these four melts can be well fitted by corresponding VFT equations as depicted in Figures S5, S6, and S7 (Supporting Information), respectively. In our recent papers,9,22 we described the triiodide transport in ionic liquid electrolytes by employing a modified Stokes-Einstein equation,23

(8)

The rH of triiodide derived from the fitted intercepts are unrealistically small (