Influence of Ionic Liquid on Recombination and Regeneration Kinetics

Article pubs.acs.org/JPCC

Influence of Ionic Liquid on Recombination and Regeneration Kinetics in Dye-Sensitized Solar Cells Feng Li,† James Robert Jennings,† Xingzhu Wang,† Li Fan,† Zhen Yu Koh,† Hao Yu,‡ Lei Yan,‡ and Qing Wang*,† †

Department of Materials Science and Engineering, Faculty of Engineering, NUSNNI-NanoCore, National University of Singapore, Singapore 117576 ‡ College of Chemistry, Key Laboratory of Low Dimensional Materials and Application Technology of Ministry of Education, Xiangtan University, Xiangtan 411105, Hunan Province, P. R. China S Supporting Information *

ABSTRACT: Nonvolatile electrolyte solutions are necessary for dye-sensitized solar cells (DSCs) with good long-term stability. Such electrolytes usually contain room-temperature ionic liquids (RTILs) and consequently possess higher viscosity and ionic strength than the volatile electrolytes used in current champion cells. In this study, we systematically investigated the effect of an RTIL additive on the performance of DSCs employing either a classical Ru-complex dye or a recently developed organic D-A-π-A dye, in combination with either I−/I3− or [Co(bpy)3]2+/3+ as redox mediator. Using impedance spectroscopy and transient absorption measurements under various background illumination intensities, recombination and regeneration kinetics were examined. Recombination is accelerated in the I−/I3− devices upon addition of RTIL, regardless of the dye used, but it is retarded in the [Co(bpy)3]2+/3+ devices. Addition of RTIL slowed regeneration in I−/I3− devices for both sensitizers, marginally accelerated it for [Co(bpy)3]2+/3+ with the Ru-complex dye, and did not significantly affect it for [Co(bpy)3]2+/3+ with the D-A-π-A dye. We show that these findings cannot be explained by diffusion limitations caused by increased solution viscosity or by a shift in the TiO2 conduction band relative to the electrolyte redox level. These findings should be useful for future optimization of RTIL-based DSCs.

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INTRODUCTION Dye-sensitized solar cells (DSCs) are a credible alternative to conventional silicon-based photovoltaics, having now achieved more than 12% power conversion efficiency for laboratory test cells.1,2 A typical DSC consists of a nanocrystalline mesoporous film of a wide band gap metal oxide, usually TiO2, supported by a transparent and conducting substrate where photogenerated electrons are eventually extracted into the external circuit. Onto the mesoporous film a monolayer of a transition metal complex3,4 or organic sensitizer5,6 is chemically adsorbed. The sensitizer absorbs photons and becomes oxidized after injecting electrons from its excited states into the semiconductor film. As a regenerative-type photoelectrochemical cell,7 the oxidized sensitizer is regenerated by reduced species in the electrolyte, while at the same time the oxidized species diffuse to the cathode and become reduced after accepting electrons that have flowed through the external circuit. Although the performance of champion DSCs is encouraging, to be applied commercially, DSCs must be reasonably stable, which will be problematic if volatile liquid solvents are used. With the impetus for long-term operation of DSCs, research into solvent-free ionic liquids, polymer electrolytes, and all-solid-state hole-transporting materials has recently surged.8−14 Specifically, studies of solvent-free room-temperature ionic liquids (RTILs) have increased dramatically thanks to their © 2014 American Chemical Society

electrochemical stability, nonvolatility, and high conductivity. DSCs with RTIL I−/I3− electrolytes have achieved over 9% power conversion efficiency (PCE) and good stability under long-term light-soaking and thermal stress tests.9 One major feature of RTILs is high viscosity, which may result in inefficient mass transport in DSCs operating under full sunlight, thus limiting the overall PCE. This is probably the main reason why all the high-efficiency RTIL DSCs adopt I−/I3− as redox mediator rather than [Co(bpy)3]2+/3+, as the transport of I−/I3− is more efficient because of not only its higher physical diffusion coefficient but also the acceleration from the Grotthuss exchange mechanism in highly concentrated electrolyte.8,15,16 Nevertheless, I−/I3− is far from perfect considering the possible corrosion of metal substrates in commercial modules, high free energy loss resulting from the large driving force for dye regeneration,17 and competitive light absorption in the visible spectrum. To further improve the performance of RTIL DSCs, it is essential to investigate charge-transfer processes and to identify Special Issue: Michael Grätzel Festschrift Received: March 7, 2014 Revised: April 8, 2014 Published: April 14, 2014 17153

dx.doi.org/10.1021/jp502341a | J. Phys. Chem. C 2014, 118, 17153−17159

The Journal of Physical Chemistry C

Article

Figure 1. Chemical structures of (a) Z907 and (b) CBTC.

Table 1. Detailed Compositions of the Four Types of Electrolytes Useda iodide

PMII

GuSCN

tBP

solvent

label

regular ionic cobalt

0.6 M 0.6 M Co2+

0.05 M 0.05 M Co3+

0.1 M 0.1 M LiClO4

0.5 M 0.5 M tBP

ACN ACN + 40% vol. EMITFSI solvent

C-R-I, Z-R-I C-IL-I, Z-IL-I label

regular ionic

0.2 M 0.2 M

0.02 M 0.02 M

0.1 M 0.1 M

0.5 M 0.5 M

ACN ACN + 40% vol. EMITFSI

C-R-Co, Z-R-Co C-IL-Co, Z-IL-Co

I2

a

The last column shows the labels of the eight types of cells used in this work. PMII is the abbreviation for propylmethylimidazolium iodide, GuSCN for guanidinium thiocyanate, tBP for tert-butylpyridine, and ACN for acetonitrile.

3.1−3.2 μm thick were screen-printed on FTO (TEC-15) substrates using commercially available TiO2 paste (90-T, Dyesol). The sensitizing dye solution was either 0.15 mM Z907 in 1:1 acetonitrile/tert-butanol22 or 0.30 mM CBTC in a 1:4 mixture of ethanol and dichloromethane. The detailed electrolyte compositions are listed in Table 1. The cobalt complexes [Co(bpy)3](PF6)2 and [Co(bpy)3](PF6)3 (bpy =2,2′bipyridine) were synthesized following a previously reported procedure.23 For each dye−solvent−mediator combination, three cells were fabricated, and the two most similar cells (based on the j-V characteristics) out of the three were selected for full characterization after stabilization in the dark for 3 days. However, for clarity, and unless otherwise stated, data for only one cell in each group is shown in the subsequent sections. It must also be noted that only 40 vol % of acetonitrile was substituted by EMITFSI because of limited solubility of [Co(bpy)3]2+/3+ in pure EMITFSI. Optical and Photoelectrochemical Characterization. j-V characteristics under simulated AM 1.5G irradiance (both 0.95 sun and 0.15 sun) were determined using a Keithley model 2400 digital source meter and the PVIV software package (Newport). The simulated AM 1.5G light was provided by a Newport class A solar simulator with a 450 W xenon lamp, and its intensity was calibrated using a NIST-certified silicon reference solar cell. Light piping through the FTO from outside of the active cell area was precluded by a mask slightly smaller than the active area (0.196 cm2). Cells were preheated under illumination for 60 s before the j-V scan. IS spectra were measured under 627 nm illumination from a high-power LED using a potentiostat equipped with a frequency response analyzer (Ecochemie, AUTOLAB PGSTAT302N/FRA2) and the Nova 1.6 software package. Different light intensities were achieved by using neutral density filters mounted in an automated filter wheel system (Newport). TA and steadystate transmission were measured mainly as described elsewhere.24 Briefly, laser excitation was at 532 nm with a 5 ns pulse width and 5 Hz repetition rate. Probe light was provided by two near-infrared LEDs (850 and 970 nm, M850L2 and M970L2, Thorlabs). The overall bandwidth of the system was 35 MHz, limited by the rise time of the photodiode detector (Thorlabs FDS100). Various levels of background illumination

bottlenecks in the system, not only with the conventional I−/I3− redox couple, but also using the promising [Co(bpy)3]2+/3+ mediator. In this study, the effect of an ionic liquid on recombination and regeneration in DSCs was investigated, mainly using impedance spectroscopy (IS) and transient absorption (TA) measurements under various background illumination levels with cells biased to the open-circuit voltage. Both the Ru-complex sensitizer cis-disothiocyanato-(2,2′ -bipyridyl-4,4′dicarboxylic acid)-(2,2′-bipyridyl-4,4′-dinonyl) ruthenium(II) (Z907, Figure 1a) and a high-efficiency organic D-A-π-A dye (CBTC, Figure 1b, > 9% PCE under optimized conditions, details included in Part 1 of the Supporting Information) were used. 1-ethyl-3-methylimidazolium- bis(trifluoromethyl-sulfonyl) imide (EMITFSI) was adopted as RTIL additive because of its wide electrochemical window and large dissociation constant. It turns out that RTIL addition significantly influenced recombination and/or regeneration kinetics for most dye− mediator combinations studied. Capacitance−voltage measurements reveal that RTIL addition does not significantly shift the TiO2 conduction band relative to the electrolyte redox level in most cases, so this cannot be the reason for changes in recombination and regeneration kinetics. All measured rate constants are also far below the calculated diffusion limit, so it is unlikely that limited diffusion of reactants in the electrolyte is responsible for changes in kinetics. Addition of RTIL increases the ionic strength as well, which together with increased viscosity also influences electron-transfer kinetics by changing activity coefficients (i.e., a kinetic electrolyte effect), outersphere reorganization energies,18 and pre-exponential factors in the Arrhenius equations for the rate constants.19,20 Because of the complexity of these DSCs, it is difficult to distinguish between these possibilities, each of which warrants its own dedicated study utilizing simpler systems in the future.

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EXPERIMENTAL SECTION Fabrication of Dye-Sensitized Solar Cells. Eight types of DSCs were fabricated depending on the sensitizer−solvent− mediator combination used, and cells are labeled using the format dye-solvent-mediator (Table 1). Devices were fabricated based on a protocol described in previous work, and here only relevant details will be given.21 Mesoporous TiO2 films ca. 17154



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Table 2. Average j−V Characteristics of Eight Types of Cells Measured under 0.95 sun and 0.15 sun 0.95 sun

Voc (V)

jsc (mA cm−2)

FF (%)

PCE (%)

0.95 sun

Voc (V)

jsc (mA cm−2)

FF (%)

PCE(%)

C-R-I C-IL-I Z-R-I Z-IL-I 0.15 sun

0.788 0.767 0.815 0.785 Voc (V)

5.31 5.93 5.45 4.79 jsc (mA cm−2)

75 74 78 76 FF (%)

3.29 3.53 3.67 3.00 PCE (%)

C-R-Co C-IL-Co Z-R-Co Z-IL-Co 0.15 sun

0.745 0.758 0.785 0.819 Voc (V)

7.87 4.66 7.73 5.71 jsc (mA cm−2)

68 53 66 53 FF (%)

4.20 2.06 4.25 2.68 PCE(%)

C-R-I C-IL-I Z-R-I Z-IL-I

0.721 0.710 0.757 0.729

0.74 0.91 0.72 0.67

78 80 79 78

2.77 3.43 2.89 2.55

C-R-Co C-IL-Co Z-R-Co Z-IL-Co

0.679 0.691 0.717 0.738

1.16 1.11 1.03 1.02

79 74 78 75

4.14 3.81 3.81 3.75

Figure 2. Comparison of (a) chemical capacitance Cμ and (b) recombination resistance Rct derived from IS measurements made under various background light intensities at the open-circuit voltage. Voc is the open-circuit voltage and n is the electron density, obtained by integrating Cμ−Voc plots.

indicating efficient mass transport. This is consistent with the small mass transport resistance (RD) obtained from fitting IS data (only around 7 Ω for IL-I cells). Third, PCE of C-R-I is slightly lower than that of C-IL-I because of lower jsc, which will be later correlated to less efficient electron injection based on IS and TA results (differences in light harvesting efficiency between cells with or without EMITFSI were ruled out on the basis of UV−vis measurements). Last, the performance under 0.15 sun is worse than that under 0.95 sun. Impedance results (effective electron diffusion length) suggest that this may be caused by less efficient charge collection under weak light intensity (Supporting Information, Part 2). In contrast to I−/I3− cells, the story for cells with [Co(bpy)3]2+/3+ is simpler. First of all, small FF and jsc in combination with the fact that jsc is still increasing slightly in the reverse bias range on the j−V curve of IL cells (Supporting Information, Part 3) indicate inefficient mass transport. This is supported by the RD values obtained from fitting IS data (ca. 15 Ω for R cells and 120 Ω for IL cells). Second, the Voc of IL cells is higher than that of R cells, which is contrary to the situation in I−/I3− cells and will later be explained by retarded recombination. Last, the performance of IL cells becomes comparable to R cells under 0.15 sun. Impedance Spectroscopy: Conduction Band Position and Recombination Resistance. As stated in the Experimental Section, IS measurements were conducted under 627 nm illumination at the open circuit potential to ensure an approximately spatially uniform electron concentration profile throughout the film.25−27 The fitting was conducted using a nonlinear least-squares algorithm (ZView 3.1c, Scribner Associates). Figure 2a shows the dependence of the chemical

were provided by the same 627 nm LED used for IS measurements. A monochromator (Newport/Oriel) set to either 850 or 970 nm was used to prevent stray light from reaching the photodiode. Kinetic traces were averaged 384 to 512 times, depending on the optical density (OD) change. The probe light for steady-state transmission measurements at 970 nm was provided by the M970L2 LED and the background illumination was provided by the same 627 nm LED. The probe light was modulated at 9.96 kHz, and the signal was detected using a lock-in amplifier (SR830, Stanford Research Systems) to enhance the signal-to-noise ratio. Transmission spectra of solvent-filled, bare, or sensitized TiO2 electrodes and FTO substrates with compact TiO2 blocking layers were measured using a Shimadzu SolidSpec-3700 UV−vis−NIR spectrophotometer in direct transmission mode. The thickness of the TiO2 film was determined with a surface profiler (AlphaStep IQ, KLA-Tencor).

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RESULTS AND DISCUSSION j−V Characteristics. Table 2 shows average values of open circuit photovoltage (Voc), short circuit photocurrent (jsc), fill factor (FF), and PCE measured under 0.95 sun and 0.15 sun for all types of cell used in this study. Note that the overall PCEs of these cells were limited by the unusually thin TiO2 films (3.1−3.2 μm) which were essential for accurate TA measurements. For cells with I−/I3−, several points need to be noted. First, Voc of IL cells is always lower than that of R cells, which will later be correlated to faster recombination of electrons with acceptors in the electrolyte (electron−electrolyte recombination, EER) in IL cells based on the IS and TA results. Second, there was no significant degradation of FF for IL cells, 17155



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Figure 3. Typical 850 nm TA decays of (a) C-R-I and (b) C-IL-I cells measured under various background illumination levels. The laser pulse energy in each plot is identical for all background illumination levels. Dotted arrows indicate the increase of n in the color order of black → red → green → yellow → blue → pink → cyan → gray.

capacitance Cμ on Voc for the eight types of cell, which can be used to infer changes in the energy difference between the TiO2 conduction band edge (Ec) and the Fermi level of the redox electrolyte (EF,redox).28 According to the figure, there is no obvious shift of Ec relative to EF,redox upon addition of EMITFSI, with the exception of C-R-I and C-IL-I for which there appears to be up to a 20 meV shift of Ec toward EF,redox. Note, however, that the small difference between C-R-I and CIL-I cannot be simply attributed to a conduction band shift as the two plots meet at higher voltages, possibly suggesting variance in the distribution of bandgap states. Another interesting point lies in the bending of the plots at higher voltages. This may arise from charging of the Helmholtz capacitance (CH) that is in series with Cμ and the resultant conduction band unpinning.29 Among these cells, the bending for C-R-Co and C-IL-Co is the most obvious, possibly because the CBTC dye is more bulky, which would result in smaller CH if counterions are prevented from closely approaching the TiO2 surface. In sum, changes in Ec−EF,redox are unlikely to be the main cause for the Voc difference between cells in each pair (i.e., with or without IL) listed in Table 2. Figure 2b shows the recombination resistance Rct as a function of electron density n in the mesoporous film (chosen instead of Voc to exclude the influence of different Ec − EF,redox). Here n is obtained from n=

1 dAq

∫0

Voc

Cμ(V )dV

where R is the sum of the reactant radii, D the diffusion coefficient of reactant in the electrolyte solution, and C∞ is the bulk molar concentration of reactant. Assuming that this model is applicable to an electron or oxidized dye on the TiO2 particle surface,32 EER and regeneration rate constants can be estimated. Note that if the electrons or oxidized dye molecules are mobile, kd should be larger than predicted using eq 2. However, restriction of diffusion caused by pore structure, e.g., near-particle necking points, could feasibly reduce kd. The resultant diffusion-limited EER rate constant (kEER,d) in Co-IL cells estimated using eq 2 is 104∼105 times larger than the highest recombination rate constant determined from IS measurements with krec = 1/RctCμ, suggesting that the limited diffusion due to increased viscosity is not the cause of the retardation of EER rate in IL-Co cells. Although EER is probably not limited by slow diffusion, the RTIL could affect the recombination in other ways, as mentioned in the last part of the Introduction. Transient Absorption Decay: Regeneration Kinetics. To determine the dye regeneration rate constant and EDR rate constant, TA for each cell was measured under various background illumination levels, mainly following our previous protocol but with some improvements.24 Briefly, regeneration efficiency ηrg derived from the continuity equations of oxidized dye molecules D+ and n can be written as ηrg =

(1)

kobs,D+

=1−

kedrn χ kobs,D+

(3)

where [Red] is the concentration of reductive species, ϑ the reaction order in [Red] (equal to 1 for I− and [Co(bpy)3]2+), and kobs,D+ = krg[Red]ϑ + kedrnχ the observed pseudo-first-order rate constant for the decay of D+. kedr and krg are the rate constants for EDR and dye regeneration, respectively, and χ is the EDR reaction order in n. The laser-induced change in OD (ΔOD) was recorded at either 850 or 970 nm. Two stretched exponential functions were used to fit the 850 nm decay

where d is the film thickness and A is the cell area. Note that the main contribution to Rct is EER, apart from possibly at the highest few intensities where electron−dye recombination (EDR) becomes significant. It can be seen from the figure that at the same n, recombination becomes faster for I−/I3− cells with the addition of EMITFSI, regardless of sensitizer. Interestingly, obvious retardation of recombination is observed for [Co(bpy)3]2+/3+ cells with EMITFSI added, also regardless of the sensitizer. The retarded recombination is unlikely to be caused by slower diffusion of [Co(bpy)3]3+ in IL cells because of higher viscosity, which can be argued as follows. In general, the diffusion-limited rate constant for reaction with a hemisphere fixed on plane can be estimated by30 kd = C ∞2πRDNA103 s−1

k rg[Red]ϑ

β D+ ⎤ ⎡ ⎛ t ⎞ ⎥ ⎢ ⎟ ΔOD850 (t ) = ΔODO, D+ ,850 exp −⎜⎜ ⎢ ⎝ τWW,D ⎟⎠ ⎥ ⎣ ⎦

+ ΔODO,e

(2) 17156

βe−⎤ ⎡ ⎛ t ⎞ ⎥ ⎢ ⎜ ⎟ ⎟ ⎥ 850 exp⎢ − ⎜ − ⎣ ⎝ τWW,e ⎠ ⎦

−

(4)



Article

Figure 4. (a) kobs,D+ versus n determined from eq SI.1 in the Supporting Information. Lines are the corresponding fits. (b) Simulation of regeneration efficiency ηrg using the fits in panel a.

and one stretched exponential was used to fit the 970 nm decay βe−⎤ ⎡ ⎛ t ⎞ ⎥ ⎢ ⎟ ΔOD970 (t ) = ΔODO,e−,970 exp −⎜⎜ ⎢ ⎝ τWW,e− ⎟⎠ ⎥ ⎣ ⎦

Table 3. Average Dye Regeneration Rate Constants (krg) Determined from TA Measurements krg (M−1 s−1)

(5)

iodide

In the two expressions, ΔOD0 is the initial laser-induced OD change (at t = 0 s), β the stretch parameter, and τWW the characteristic stretched exponential lifetime. The subscripts e− and D+ denote parameters describing the decay of the corresponding species. For accurate comparison of decay lifetimes and rate constants, weighted average lifetimes (τobs) were adopted:31 τobs =

1 kobs

=

τWW ⎛ 1 ⎞ Γ⎜ ⎟ β ⎝β⎠

cobalt

C-R

C-IL

Z-R

Z-IL

9.39 × 104 ±8.2% 1.32 × 106 ±2.7%

2.56 × 104 ±12.6% 1.33 × 106 ±1.7%

8.15 × 104 ±1.2% 3.59 × 105 ±2.4%

3.78 × 104 ±1.5% 3.95 × 105 ±1.8%

measurements. Note that the local I− concentration in the pores is assumed to be equal to the bulk concentration here, as the depletion of I− in the pores is expected to lower [I−] by only less than 10%.24,32 Interestingly, for [Co(bpy)3]2+/3+ cells, krg is barely affected by adding EMITFSI when using the CBTC dye, and there is a marginal increase for Z907 cells (bordering on statistical significance). Note that krg,d estimated for [Co(bpy)3]2+/3+ cells by eq 2 is at least 103 times larger than the experimental value; thus, regeneration is unlikely to be limited by diffusion. It is quite interesting that addition of RTIL significantly influences regeneration in I−/I3− cells but not in [Co(bpy)3]2+/3+ cells, which cannot be explained by shifts in Ec and diffusion limitations. Determining the exact cause of this observation may be important for the further optimization of RTIL DSCs and deserves further study. To further demonstrate the influence of EMITFSI on regeneration and EDR kinetics, ηrg was calculated using the fits of Figure 4a with eq 3 over the range of n, leading to Figure 4b. Regeneration in IL cells is much less efficient than in R cells in the I−/I3− system at operationally relevant n (e.g., 1 sun electron density), which can be mainly attributed to much smaller krg in the former. It may explain why a very high concentration of I− (3 M or higher) is needed in RTIL DSCs to ensure efficient dye regeneration. Note that ηrg decreases with increasing n because of increasing EDR rate.21,24,33 In most cases χ is dependent on dye and electrolyte composition (Supporting Information, Part 5), which makes it impossible to comment on trends of kedr because of incompatible units; in addition, the physical origin of nonunity χ is unclear. Another approach is to examine kedrnχ (i.e., the pseudo-first-order EDR rate constant for a particular n) over the n range of interest (Supporting Information, Part 6). With the exception of Z-Co cells, adding EMITFSI did not induce a significant change in kedrnχ. This is not surprising as similar Ec implies a similar EDR driving force for a given dye. Meanwhile, diffusion limitation

(6)

where Γ() is the gamma function. Figure 3 shows typical 850 nm TA decays measured at open circuit under various background illumination levels for cells with C-R-I and C-IL-I. Decays of other cells are not included here for clarity as they are similar to these two plots. By fitting the TA decays to eq 4, kobs,D+ is extracted for each decay. One point that needs to be noted here is that ΔOD0,e−,850 is directly correlated to the number of electrons injected into the TiO2 film upon laser excitation. According to the fits, there is no significant difference in ΔOD0,e−,850 for all R−IL cell pairs except for the C-R-I and C-IL-I pair as shown in Figure 3, where over 20% more electrons are injected into the TiO2 film in C-IL-I cells. The same conclusion can be drawn by examining the initial ΔOD change at 850 nm, which is the sum of ΔOD0,e−,850 and ΔOD0,e+,850. To obtain krg[Red] and kedrnχ, the electron density n during TA measurements is needed. The detailed calibration linking the OD change at 970 nm to electron density n is included in the Supporting Information, Part 4. Oxidized dye decay rate constants kobs,D+ are plotted versus n as shown in Figure 4a, with corresponding fits to kobs,D+ = krg[Red] + kedrnχ. The resultant average krg values are listed in Table 3 using the bulk concentrations of I− or [Co(bpy)3]2+ for [Red]. According to the figure, for I−/I3− cells, krg of both dyes decreases greatly (ca. 50%). This is unlikely to be caused by limited diffusion of I− with the addition of EMITFSI because the estimated diffusion-limited regeneration rate constant krg,d (eq 2) is ca. 105 times larger than krg determined from TA 17157



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caused by an electrolyte viscosity increase plays no role here as the dye molecules are bonded to the TiO2 surface and the electrons are either inside or at the surface of the TiO2 particle, rather than in the electrolyte phase. The one exception is surprising and requires further study. A possible explanation is that ion pairing between [Co(bpy)3]2+/3+ and negatively charged Z907 is weakened upon the addition of RTIL because of charge screening (ion pairing is not expected for the organic dye),34 altering the EDR driving force.

CONCLUSION The effect of an RTIL additive on recombination and regeneration kinetics in DSCs employing Ru-complex or organic sensitizers in conjunction with I−/I3− or [Co(bpy)3]2+/3+ redox mediators was investigated. IS and TA measurements reveal that no simple relationship exists between recombination/ regeneration rate constants and electrolyte ionic strength or viscosity; detail depend on the particular dye/mediator combination. Specifically, EER is accelerated in the I−/I3− devices upon addition of RTIL, while regeneration in I−/I3− cells is slowed by the addition of RTIL, both of which lead to lower Voc. Recombination is slowed in [Co(bpy)3]2+/3+ cells, while regeneration is not affected or marginally (on the edge of statistical significance) increased, resulting in an increase in Voc. The changes in rate constants observed upon RTIL addition cannot be explained by diffusion-limited reaction rates because of increased solution viscosity in any of the studied cases, or by a shift in the energy of the TiO2 conduction band relative to the electrolyte redox level. Several other possible mechanisms exist which do not involve diffusion limitations and conduction band shifts (e.g., ionic strength-dependent activity coefficients and outer-sphere reorganization energies, viscosity-dependent preexponential factors in the Arrhenius equations), but because of the complexity of the system studied here, it is not possible to distinguish between them. An important objective of future studies will be to separate the effects of ionic strength and viscosity on charge-transfer rate constants in DSCs. Although only 40 vol % of the solvent was replaced by RTIL in this work (to ensure full dissolution of the [Co(bpy)3]2+/3+ electrolyte), the changes in rate constants brought about by RTIL addition should be applicable to full RTIL DSCs and the results may be helpful for future optimization. ASSOCIATED CONTENT

S Supporting Information *

Information on CBTC dye, electron diffusion length Ln, j−V characteristics of [Co(bpy)3]2+/3+ cells, calibration of electron density n, table of EDR rate constant and reaction order, comparison of pseudo-first-order rate constant. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: ++(65) 6516-7118. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Research Foundation of Singapore under its Competitive Research Program (CRP Award NRF-CRP4-2008-03) and SinBeRISE CREATE program. 17158



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Influence of Ionic Liquid on Recombination and Regeneration Kinetics

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