Electrolytes for dye-sensitized solar cells based on interhalogen ionic salts and liquids

Article abstract of DOI:10.1021/ic062244b

In this paper, we report on the preparation of interhalogen ionic liquids of the general formula [K⁺]XY₂^-, where K ⁺ = 1,3-dialkylimidazolium, 1,2,3-trialkylimidazolium, or N-alkylpyridinium; XY₂^- = IBr₂^- or I₂Br^-. These compounds were characterized in solution and the solid state by NMR, IR, Raman, and mass spectroscopy. The crystal structure of the compound [Me₂Bulm]IBr₂ (7) shows that the IBr ₂^- anion has a linear Br-I-Br structure. Indications of an equilibrium between different forms of XY₂^- anions in solution are observed. Interhalogen ionic salts and liquids were used as electrolyte components for encapsulated monolithic dye-sensitized solar cells. Overall light-to-electricity conversion efficiencies up to 6.4%, 5.0%, and 2.4% at 1000 W/m² were achieved by using electrolytes based on interhalogen ionic salts and γ-butyrolactone, glutaronitrile, or native ionic liquids as solvents, respectively. Moreover, in terms of stability, the cell performance lost 9-14% of the initial performance after 1000 h illumination at 350 W/m².

Full text of DOI:10.1021/ic062244b

Inorg. Chem. 2007, 46, 3566−3575
Electrolytes for Dye-Sensitized Solar Cells Based on Interhalogen Ionic
Salts and Liquids
Mikhail Gorlov,† Henrik Pettersson,‡ Anders Hagfeldt,† and Lars Kloo*,†
Center of Molecular DeVices, Department of Chemistry, The Royal Institute of Technology,
Teknikringen 30, S-100 44 Stockholm, Sweden, and IVF Industrial Research and DeVelopment
Corporation, Argongatan 30, S-431 53 Mo¨lndal, Sweden
Received November 24, 2006
In this paper, we report on the preparation of interhalogen ionic liquids of the general formula [K+]XY2-, where K+
-
-
)
1,3-dialkylimidazolium, 1,2,3-trialkylimidazolium, or N-alkylpyridinium; XY2 )
IBr2 or I2Br-. These compounds
were characterized in solution and the solid state by NMR, IR, Raman, and mass spectroscopy. The crystal structure
-
of the compound [Me2BuIm]IBr2 (7) shows that the IBr2 anion has a linear Br
−I
−
Br structure. Indications of an
-
equilibrium between different forms of XY2 anions in solution are observed. Interhalogen ionic salts and liquids
were used as electrolyte components for encapsulated monolithic dye-sensitized solar cells. Overall light-to-electricity
conversion efficiencies up to 6.4%, 5.0%, and 2.4% at 1000 W/m2 were achieved by using electrolytes based on
interhalogen ionic salts and γ-butyrolactone, glutaronitrile, or native ionic liquids as solvents, respectively. Moreover,
in terms of stability, the cell performance lost 9
W/m2.
−14% of the initial performance after 1000 h illumination at 350
1. Introduction
Ionic liquids based on 1,3-dialkylimidazolium iodides have
been widely used as solvents for DSCs.2-4,6 However, the
Dye-sensitized nanostructured solar cells (DSCs), which
were presented in 1991 by Gra¨tzel, provide an alternative
concept to the present p-n junction photovoltaic devices.1
DSCs have been intensively investigated during the past
decade because of their potential low cost of production and
high-energy conversion efficiency.2-6 One concern for
practical applications of DSCs is the use of electrolytes based
on organic solvents. Attractive alternatives to organic solvents
are nonvolatile, room-temperature ionic liquids. In addition,
ionic liquids have favorable features such as chemical
stability, relative nonflammability, and a wide electrochemi-
cal window.
relatively high viscosity of this type of imidazolium ionic
liquids appears to limit their use because of transport
limitations. In order to decrease viscosity, mixtures of ionic
liquids containing different anions have successfully been
tested.2,3 Ionic liquids based on alkylimidazolium cations
were also used as key components for quasisolid (gel)
electrolytes.7-13 Different organic gelators7-11 or inorganic
additives12,13 have been used in order to solidify alkylimi-
dazolium ionic liquids; overall light-to-electricity conversion
efficiencies of about 6-7% have been achieved for certain
quasisolid electrolytes.13
* To whom correspondence should be addressed. E-mail: Larsa@kth.se.
† The Royal Institute of Technology.
(7) Kubo, W.; Kambe, S.; Nakade, S.; Kitamura, T.; Hanabusa, K.; Wada,
Y.; Yanagida, S. J. Phys. Chem. B 2003, 107, 4374.
(8) Kubo, W.; Kitamura, T.; Hanabusa, K.; Wada, Y.; Yanagida, S. Chem.
Commun. 2002, 374.
(9) Wang, P.; Zakeeruddin, S. M.; Exnar, I.; Gra¨tzel, M. Chem. Commun.
2002, 2972.
(10) Wang, P.; Zakeeruddin, S. M.; Moser, J. E.; Nazeeruddin, M. K.;
Sekiguchi, T.; Gra¨tzel, M. Nat. Mater. 2003, 2, 402.
(11) Kim, Y. J.; Kim, J. H.; Kang, M.-S.; Lee, M. J.; Won, J.; Lee, J. C.;
Kang, Y. S. AdV. Mater. 2004, 16, 1753.
(12) Scully, S. R.; Lloyd, M. T.; Herrera, R.; Gainnelis, E. P.; Malliaras,
G. G. Synth. Met. 2004, 144, 291.
‡ IVF Industrial Research and Development Corporation.
(1) Gra¨tzel, M. Nature 2001, 414, 338.
(2) Wang, P.; Zakeeruddin, S. M.; Moser, J.-E.; Gra¨tzel, M. J. Phys. Chem.
B 2003, 107, 13280.
(3) Wang, P.; Zakeeruddin, S. M.; Humphry-Baker, R.; Gra¨tzel, M. Chem.
Mater. 2004, 16, 2694.
(4) Otaka, H.; Kira, M.; Yano, K.; Ito, S.; Mitekura, H.; Kawata, T.;
Matsui, F. J. Photochem. Photobiol., A 2004, 164, 67.
(5) Xue, B.; Wang, H.; Hu, Y.; Li, H.; Wang, Z.; Meng, Q.; Huang, X.;
Sato, O.; Chen, L.; Fujishima, A. Photochem. Photobiol. Sci. 2004,
3, 918.
(6) Yamanaka, N.; Kawano, R.; Kubo, W.; Kitamura, T.; Wada, Y.;
Watanabe, M.; Yanagida, S. Chem. Commun. 2005, 740.
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Am. Chem. Soc. 2003, 125, 1166.
3566 Inorganic Chemistry, Vol. 46, No. 9, 2007
10.1021/ic062244b CCC: $37.00
© 2007 American Chemical Society
Published on Web 04/07/2007

Dye-Sensitized Solar Cells
Aside from 1,3-dialkylimidazolium ionic liquids, other
types of molten salts have been tested as solvents for DSCs.
Thus, it was found that trialkylsulfonium polyiodides exhibit
good electrical conductivity.14 We have recently reported that
DSCs with iodine-doped trialkylsulfonium iodides as elec-
trolyte achieved an overall light-to-electricity conversion
efficiency of 3.7% at a light intensity of 0.1 Sun (100
W/m2).15 Moreover, metal-iodide-doped trialkylsulfonium
iodides have been investigated as electrolytes in DSCs; an
overall light-to-electricity conversion efficiency of 3.1% (0.1
Sun) was achieved by using an electrolyte based on a (Bu2-
MeS)I:AgI:I2 mixture.16
It is well-known that strong interaction between cations
and anions in ionic liquids determines their physical proper-
ties, and in particular their viscosity. This fact was used for
the preparation of nonviscous ionic liquids and their use as
electrolytes for DSCs.2,3 The iodide/triiodide redox couple
is commonly used in DSCs. The diffusion coefficient of the
triiodide ion strongly depends on the viscosity of electrolyte.
Hence, chemical modification of the anionic and/or cationic
part of an ionic liquid can decrease its viscosity, with
improved performance of the solar cells as result.
attractive as solvents or components of electrolytes for DSCs.
There is also a possibility that lower optical absorption in
the visible region of the spectrum may allow higher halogen
concentrations, and thus, an extra enhancement of transport
-
efficiency may be achieved. Also, interhalogen anions XY2
consisting of different halide atoms X and Y in conjunction
with the corresponding halide ions X- or Y- can act as new
redox couples with more efficiently adapted energy levels
with respect to the sensitizing dye and counter electrode.
In the present work, we describe the preparation and
characterization of new interhalogen ionic liquids, and their
use as key components in electrolytes for dye-sensitized solar
cells. Three types of electrolytes, based on glutaronitrile,
γ-butyrolactone, or native ionic liquids as solvent, have been
studied.
2. Experimental Section
2.1. Materials and Instrumentation. Glutaronitrile and γ-bu-
tyrolactone were distilled under reduced pressure before use. All
other reagents and solvents were obtained from commercial sources
and used as received. C, H, and N elemental analyses were carried
out by Mikro Kemi AB (Uppsala, Sweden).
All quantum chemical calculations were performed using Gauss-
ian03 (rev B.04).29 Quasirelativistic effective core potentials were
used for both Br (28 electrons) and I (46 electrons).30 The valence
spaces of the halogens were uncontracted of (5s5p1d) and (7s6p1d)
quality for Br and I, respectively.30,31 Calculations were made using
the hybrid functionals B3LYP and B3PW91, as well as perturbation
theory using the second-order Møller-Plesset method. In order to
investigate the polarizing effect of a dielectric solvent, the
implemented version of the polarizable continuum model (PCM)
based on the integral equation formalism was used for water (ꢀ )
78.39), acetonitrile (ꢀ ) 36.64), and benzene (ꢀ ) 2.247). The united
atom model employing default radii was used to create the spherical
cavities around all atoms in the interhalogen molecules studied.
Positive-ion FAB mass spectra were obtained on a Trio 2000
instrument by bombarding 3-nitrobenzyl alcohol matrices of the
samples with 8 keV Xe atoms. Mass calibration for data system
acquisition was achieved using CsI.
The chemistry of trihalide anions in solution and in the
solid state has been widely studied for many years.17-20 The
nature of bonding in trihalide anions has been analyzed.21
The crystal structures and vibrational spectroscopic data are
obtained for salts containing organic as well as inorganic
cations.22-26 The synthesis and properties of 1,3-dialkylimi-
dazolium ionic liquids containing interhalogen anions such
-
as IBr2 , ICl2- , etc. have recently been reported.27,28 These
interhalogen ionic liquids (IILs) show much lower viscosity
in comparison with the corresponding iodides; in some cases,
the difference is a factor of 100-200.28 This fact makes IILs
(14) Stegemann, H.; Reiche, A.; Schnittke, A.; Fullbier, H. Electrochim.
Acta 1992, 37, 379.
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Sol. Energy Mater. Sol. Cells 2004, 82, 345.
(17) Popov, A. I. Halogen Chemistry; Gutmann, V., Ed.; Academic Press:
New York, 1967; Vol. I, p 225.
(18) Downs, A. J.; Adams, C. J. ComprehensiVe Inorganic Chemistry;
Bailar, J. C., Emeleus, H. J., Nyholm, R. S., Trotman-Dickenson, A.
F., Eds.; Pergamon: Oxford, 1973; Vol. II, p 1534.
(19) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. AdVanced
Inorganic Chemistry, 6th ed.; John Wiley & Sons: New York, 1999;
p 583.
(20) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, 2nd ed.;
Pergamon: Oxford; p 835.
(21) Landrum, G. A.; Goldberg, N.; Hoffmann, R. J. Chem. Soc., Dalton
Trans. 1997, 3605.
(22) Domercq, B.; Devic, T.; Fourmigue´, M.; Auban-Senzier, P.; Canadell,
E. J. Mater. Chem. 2001, 11, 1570.
(23) Murray, H. H., III; Porter, L. C.; Fackler, J. P., Jr.; Raptis, R. G. J.
Chem. Soc., Dalton Trans. 1988, 2669.
(24) Naito, T.; Tateno, A.; Udagawa, T.; Kobayashi, H.; Kato, R.;
Kobayashi, A.; Nogami, T. J. Chem. Soc., Faraday Trans. 1994, 90,
763.
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Venturini, E. L.; Azevedo, L. J.; Schirber, J. E. Inorg. Chem. 1985,
24, 1736.
Electrospray ionization mass spectra were recorded on a Bruker
esquire 3000 ion trap mass spectrometer, in both positive and
negative ion modes, equipped with an orthogonal electrospray
interface (Bruker Daltonics, Bremen, Germany). The Bruker
(29) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin,
K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G.
A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.;
Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R.
E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J.
W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.;
Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.;
Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari,
K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.;
Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A.
Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.
(30) Bergner, A.; Dolg, M.; Ku¨chle, W.; Stoll, H.; Preuss, H. Mol. Phys.
1993, 80, 1431.
(26) Svensson, P. H.; Kloo, L. J. Chem. Soc., Dalton Trans. 2000, 2449.
(27) Bortolini, O.; Bottai, M.; Chiappe, C.; Conte, V.; Pieraccini, D. Green
Chem. 2002, 4, 621.
(28) Bagno, A.; Butts, C.; Chiappe, C.; D’Amico, F.; Lord, J. C. D.;
Pieraccini, D.; Rastrelli, F. Org. Biomol. Chem. 2005, 3, 1624.
(31) Schwerdtfeger, P.; Dolg, M.; Schwarz, W. H. E.; Bowmaker, G. A.;
Boyd, P. D. W. J. Chem. Phys. 1989, 91, 1762.
Inorganic Chemistry, Vol. 46, No. 9, 2007 3567

Gorlov et al.
Daltonics esquire 5.2 (Build 382) software bundle consisting of
Bruker esquire Control 5.2 (Build 63.8) and Bruker Data Analysis
3.2 (Build 121) were used for acquiring spectra and data analysis,
respectively. The samples were delivered at a flow rate of 4 µL/
min using a syringe pump 74900 from Cole-Parmer Instrument
Company (Vernon Hills, IL), and N2 was used as drying and
nebulizing gas (flow rates 5 L/min).
Raman spectra were recorded with a Bio-Rad FTS 6000
spectrometer equipped with a low-power Nd:YAG laser (λ ) 1064
nm) and a liquid-nitrogen-cooled, solid-state Ge diode detector. A
resolution of 4 cm-1 was used. Typical laser power at the samples
was 500-800 mW.
IR spectra were recorded with a Bio-Rad FT-375C spectrometer
using NaCl plates for neat liquids or chloroform solutions.
1H and 13C NMR spectra were obtained on a Bruker DMX 500
spectrometer. The spectra were recorded at 298 K in CDCl3 solution
using TMS as external reference.
Diffraction data were collected on a Bruker-Nonius KappaCCD
diffractometer. Numerical absorption corrections were applied.32
The structure was solved using direct methods33 and refined on F2
with anisotropic thermal parameters for all non-H atoms.34 H atoms
were refined on calculated positions using a riding model. CCDC-
605792 contains the supplementary crystallographic data. These
data can be obtained free of charge from the Cambridge Crystal-
lographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Electrochemical experiments (cyclic voltammetry and conductiv-
ity measurements) were performed using a CHI-660 electrochemical
work station. Cyclic voltammetry and differential pulse voltammetry
were recorded in acetonitrile (concentration of IILs was 1.25 mM)
under nitrogen atmosphere. Scan rate was 0.5 V/s for 12 and 0.1
V/s for 7 and [HexPy]I3. Supporting electrolyte (n-C4H9)4NPF6 in
acetonitrile (50 mM), Pt foil as counter electrode, Ag/Ag+ as
reference electrode were all used.
was used as source of illumination. The lamp was calibrated versus
the solar spectrum, and the efficiencies correspond to the AM 1.5
spectrum.
2.2. Synthesis of Interhalogen Ionic Liquids. General Pro-
cedures for Preparation of 1-14. The starting materials, alky-
limidazolium or alkylpyridinium iodides or bromides, were prepared
by the reaction of 1-methylimidazole, 1,2-dimethylimidazole, or
pyridine with alkyliodides or alkylbromides in tetrahydrofuran
(boiling for 3 h with following washing with tetrahydrofuran and
drying under dynamic vacuum). Corresponding alkylimidazolium
or alkylpyridinium iodide (10 mmol) was dissolved in chloroform
(10 mL), and the solution was cooled to 5 °C (ice bath). An
equimolar amount of Br2 or IBr was dissolved in chloroform (5
mL), and the solution was added drop-by-drop to the solution of
alkylimidazolium or alkylpyridinium iodide under stirring. The
temperature of the mixture was kept in the interval 5-10 °C during
the whole reaction time. The color of the solution turned from
transparent to dark brown when the first drops of Br2 or IBr solution
were added (probably because of formation of iodine), but in the
end of the reaction it became orange-red, and two phases formed.
The upper layer was removed, and the remaining red-orange oil
was heated at 50 °C for 1 h under reduced pressure. The yield was
80-95%.
1-Methyl-3-ethylimidazolium Iododibromide, 1. Red liquid.
IR (liquid film): 3143 (vs), 3108 (vs), 2980 (m), 2949 (w), 1568
(vs), 1462 (s), 1446 (s), 1385 (w), 1336 (w), 1165 (vs), 1087 (w),
1026 (w), 958 (w), 829 (s), 743 (s), 700 (w), 642 (m), 619 (s)
cm-1. ESI-MS: m/z ) 111 [MeEtIm+], 287 [IBr2-], 334 [I2Br-].
1H NMR (CDCl3, 500 MHz): δ 1.62 (t, CH3), 4.05 (s, N-CH3),
4.35 (q, N-CH2), 7.14 (d, CH), 7.31 (d, CH), 9.08 (s, CH) ppm.
13C NMR (CDCl3, 500 MHz): δ 15.5 (s, CH3), 15.8 (s, CH3), 37.5
(s, CH2), 45.9 (s, CH3), 46.1 (s, CH3), 122.1 (s, CH), 122.4 (s,
CH), 123.8 (s, CH), 123.9 (s, CH), 136.3 (s, NCN), 136.4 (s, NCN)
ppm.
The diffusion constants D(A-) of I3-, IBr2-, and I2Br- were
determined by measuring the limiting current density Jlim, according
to the following equation35
1-Methyl-3-propylimidazolium Iododibromide, 2. Red liquid.
IR (liquid film): 3142 (vs), 3105 (vs), 2965 (vs), 2876 (m), 1568
(vs), 1456 (s), 1426 (m), 1340 (w), 1166 (vs), 1088 (w), 1018 (w),
902 (w), 829 (s), 799 (w), 748 (s), 647 (m), 620 (s) cm-1. ESI-
MS: m/z ) 125 [MePrIm+], 288 [IBr2-], 334 [I2Br-], 381 [I3-].
1H NMR (CDCl3, 500 MHz): δ 0.99 (t, CH3), 1.96 (sextet, CH2),
4.06 (s, N-CH3), 4.23 (t, N-CH2), 7.34 (d, CH), 9.30 (s, CH)
ppm. 13C NMR (CDCl3, 500 MHz): δ 11.0 (d, CH3), 23.7 (d, CH2),
37.4 (d, CH2), 52.2 (s, CH3), 122.4 (s, CH), 123.8 (s, CH), 136.7
(s, NCN) ppm.
Jlim ) 2nqD(A-)c(A-)NA/d
where n is the number of electrons, q is the elemental charge, c(A-)
is the concentration of I3-, IBr2-, or I2Br-, and NA is Avogadro’s
constant. Cyclic voltammetry was performed to measure Jlim using
a two-electrode cell consisting of two platinized conducting glass
electrodes separated at a fixed distance d (40 µm) and connected
to a potentiostat.
The overall light-to-electricity conversion efficiency (η), fill
factor (FF), open-circuit voltage (Voc), and short-circuit current
density (Jsc) were obtained through the current-voltage character-
istics of a solar cell at room temperature. These were monitored
and recorded using a computerized Keithley model 2400 source
unit. A microwave-powered sulfur plasma lamp, Light Drive 1000,
1-Methyl-3-hexylimidazolium Iododibromide, 3. Red liquid.
IR (liquid film): 3141 (s), 3107 (s), 2954 (vs), 2930 (vs), 2857
(s), 1566 (s), 1561 (s), 1464 (s), 1457 (s), 1377 (w), 1338 (w),
1164 (vs), 1104 (w), 829 (m), 742 (m), 648 (w), 619 (s) cm-1
.
Anal. Calcd for C10H19N2IBr2: C, 26.45; H, 4.19; N, 6.17. Found:
1
C, 26.8; H, 4.4; N, 6.3. FAB+-MS: m/z ) 167 [HexMeIm+]. H
NMR (CDCl3, 500 MHz): 0.83 (t, CH3), 1.28 (br, (CH2)2), 1.34
(br s, CH2), 1.90 (quintet, CH2), 4.04 (s, N-CH3), 4.23 (t, N-CH2),
7.33 (s, CH), 7.36 (s, CH), 8.89 (s, CH) ppm. 13C NMR (CDCl3,
500 MHz): δ 14.0 (s, CH3), 22.4 (s, CH2), 26.0 (s, CH2), 30.2 (s,
CH2), 31.0 (s, CH2), 37.6 (s, CH2), 50.9 (s, CH3), 122.6 (s, CH),
124.0 (s, CH), 136.1 (s, NCN) ppm.
(32) A red crystal of 7 was selected for X-ray structural analysis on a
Bruker-Nonius KappaCCD diffractometer at 299 K. The compound
crystallized in the space group P21/c, monoclinic, a ) 15.2999(10)
Å, b ) 7.7057(5) Å, c ) 13.4781(5) Å, â ) 111.123(4)°, V ) 1482.25-
(15) Å3, Z ) 4, Fcalcd ) 1.972 g‚cm-3, Mo KR radiation, λ ) 0.71073
Å. Final R values: R1 ) 0.0651, wR2 ) 0.1519; GOF ) 1.156 for
2572 unique reflections and 130 parameters. Herrendorf, W.; Ba¨r-
nighausen H.: HABITUS, a program for numerical absorption
correction; Universities of Giessen and Karlsruhe: Germany, 1997.
(33) Sheldrick, G. S. SHELXS97, a program for crystal structure solution;
University of Go¨ttingen: Go¨ttingen, Germany, 1997.
1-Methyl-3-hexylimidazolium Bromodiiodide, 4. Red liquid.
IR (liquid film): 3139 (s), 3103 (s), 2952 (vs), 2928 (vs), 2856
(s), 1568 (s), 1561 (s), 1464 (s), 1457 (s), 1376 (w), 1338 (w),
1163 (vs), 1104 (w), 1021 (w), 826 (m), 740 (m), 648 (w), 618 (s)
1
cm-1. ESI-MS: m/z ) 167 [MeHexIm+]. H NMR (CDCl3, 500
(34) Sheldrick, G. S. SHELXL97, a program for crystal structure refine-
ment; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
MHz): 0.83 (t, CH3), 1.28 (br, (CH2)2), 1.35 (br, CH2), 1.91 (quintet,
(35) Hauch, A.; Georg, A. Electrochim. Acta 2001, 46, 3457.
CH2), 4.05 (s, N-CH3), 4.25 (t, N-CH2), 7.33 (s, CH), 7.36 (s,
3568 Inorganic Chemistry, Vol. 46, No. 9, 2007

Dye-Sensitized Solar Cells
CH), 9.01 (s, CH) ppm. 13C NMR (CDCl3, 500 MHz): δ 14.0 (s,
CH3), 22.4 (s, CH2), 26.0 (s, CH2), 30.2 (s, CH2), 31.1 (s, CH2),
37.8 (s, CH2), 51.0 (s, CH3), 122.6 (s, CH), 124.0 (s, CH), 136.3
(s, NCN) ppm.
1,2-Dimethyl-3-octylimidazolium Bromodiiodide, 10. Dark red
liquid. IR (liquid film): 3133 (s), 2952 (vs), 2925 (vs), 2854 (s),
1585 (s), 1559 (w), 1535 (s), 1464 (s), 1458(s), 1418 (m), 1378
(m), 1340 (w), 1278 (w), 1238 (s), 1163 (w), 1132 (m), 1106 (w),
1080 (w), 1036 (w), 746 (s), 662 (s), 626 (w) cm-1. ESI-MS: m/
z ) 209 [Me2OctIm+], 334.4 [I2Br-]. 1H NMR (CDCl3, 500
MHz): δ 0.82 (t, CH3), 1.21 ((CH2)2) 1.31 ((CH2)3), 1.83 (q, CH2),
2.73 (s, Cring-CH3), 3.89 (s, N-CH3), 4.09 (t, N-CH2), 7.22 (d,
CH), 7.27 (d, CH) ppm. 13C NMR (CDCl3, 500 MHz): δ 11.3 (s,
CH3), 14.1 (s, CH2), 22.6 (s, CH2), 26.5 (s, CH2), 28.9 (s, CH2),
29.0 (s, CH2), 29.8 (s, CH2), 31.7 (s, CH2), 36.8 (s, CH3), 49.7 (s,
CH3), 121.2 (s, CH), 123.0 (s, CH), 143.6 (s, NCN) ppm.
1-Methyl-3-dodecylimidazolium Iododibromide, 5. Yellow
solid. IR (liquid film, solution in CHCl3): 2993 (s), 2927 (vs), 2855
(s), 1587 (w), 1568 (m), 1462 (m), 1426 (w), 1378 (w), 1164 (s),
822 (w), 620 (m) cm-1. ESI-MS: m/z ) 251 [MeDodIm+], 286.7
[IBr2-], 334.2 [I2Br-]. 1H NMR (CDCl3, 500 MHz): 0.80 (t, CH3),
1.18 (br, (CH2)7), 1.31 (br, (CH2)2), 1.90 (quintet, CH2), 4.05 (s,
N-CH3), 4.22 (t, N-CH2), 7.32 (s, CH), 7.35 (s, CH), 8.85 (s,
CH) ppm. 13C NMR (CDCl3, 500 MHz): δ 14.1 (s, CH3), 22.6 (s,
CH2), 26.4 (s, CH2), 29.0 (s, CH2), 29.3 (s, CH2), 29.4 (s, CH2),
29.5 (s, CH2), 29.6 (s, CH2), 30.2 (s, CH2), 31.9 (s, CH2), 37.7 (s,
CH2), 51.0 (s, CH3), 122.6 (s, CH), 124.0 (s, CH), 136.2 (s, NCN)
ppm.
1-Methyl-3-propylimidazolium Tribromide, 6. Red liquid. IR
(liquid film): 3143 (vs), 3108 (vs), 2965 (vs), 2935 (w), 2876 (m),
1566 (vs), 1457 (s), 1387 (w), 1339 (w), 1165 (vs), 1104 (w), 1088
(w), 832 (s), 746 (s), 647 (m), 620 (s) cm-1. ESI-MS: m/z ) 125.2
[MePrIm+]. 1H NMR (CDCl3, 500 MHz): δ 0.70 (t, CH3), 0.99 (t,
CH3), 1.69 (sextet, CH2), 1.96 (sextet, CH2), 3.77 (s, N-CH3), 3.97
(t, N-CH2), 4.05 (s, N-CH3), 4.24 (t, N-CH2), 7.22 (d, CH), 7.32
(d, CH), 8.57 (s, CH), 9.05 (s, CH) ppm. 13C NMR (CDCl3, 500
MHz): δ 10.9 (d, CH3), 23.6 (s, CH2), 23.7 (s, CH2), 37.3 (s, CH2),
37.4 (s, CH2), 51.9 (s, CH3), 52.2 (s, CH3), 122.3 (s, CH), 122.6
(s, CH), 123.7 (s, CH), 123.9 (s, CH), 135.6 (s, NCN), 136.5 (s,
NCN) ppm.
1,2-Dimethyl-3-dodecylimidazolium Iododibromide, 11. Orange-
red solid. IR (liquid film, solution in CHCl3): 3134 (s), 2990 (m),
2925 (vs), 2854 (s), 1583 (m), 1558 (w), 1534 (m), 1465 (m), 1458
(m), 1419 (w), 1377 (w), 1339 (w), 1236 (w), 1133 (w), 626 (w)
cm-1. ESI-MS: m/z ) 265.0 [Me2DodIm+], 287.7 [IBr2-], 334.0
[I2Br-]. 1H NMR (CDCl3, 500 MHz): δ 0.80 (t, CH3), 1.19 ((CH2)7)
1.30 ((CH2)2), 1.81 (q, CH2), 2.70 (s, Cring-CH3), 3.88 (s, N-CH3),
4.07 (t, N-CH2), 7.20 (d, CH), 7.25 (d, CH) ppm. 13C NMR
(CDCl3, 500 MHz): δ 11.0 (s, CH3), 14.1 (s, CH2), 22.6 (s, CH2),
26.5 (s, CH2), 29.0 (s, CH2), 29.3 (s, CH2), 29.4 (s, CH2), 29.5 (s,
CH2), 29.6 (s, CH2), 29.7 (s, CH2), 31.7 (s, CH2), 36.5 (s, CH3),
49.5 (s, CH3), 121.2 (s, CH), 122.9 (s, CH), 143.7 (s, NCN) ppm.
N-Butylpyridinium Iododibromide, 12. Dark red liquid. IR
(liquid film): 3124 (w), 3055(s), 2960 (s), 2932 (s), 2868 (m), 1631
(vs), 1498 (s), 1485 (vs), 1464 (s), 1376 (w), 1317 (w), 1212 (w),
1171 (s), 1056 (w), 950 (w), 766 (s), 733 (w), 680 (vs) cm-1. Anal.
Calcd for C9H14N2IBr2: C, 25.55; H, 3.31; N, 3.31. Found: C,
1,2-Dimethyl-3-butylimidazolium Iododibromide, 7. Dark red
solid. IR (liquid film, solution in CHCl3): 3134 (s), 2989 (s), 2961
(vs), 2933 (s), 2872 (m), 1587 (s), 1535 (s), 1463 (s), 1459 (s),
1418 (m), 1381 (m), 1340 (w), 1232 (s), 1183 (w), 1132 (m), 1037
(w), 626 (w) cm-1. Anal. Calcd for C9H17N2IBr2: C, 24.56; H,
3.87; N, 6.37. Found: C, 25.5; H, 3.9; N, 6.5. FAB+-MS: m/z )
1
26.8; H, 3.6; N, 3.4. FAB+-MS: m/z ) 136 [BuPy]+. H NMR
(CDCl3, 500 MHz): δ 0.65 (t, CH3), 0.96 (t, CH3), 1.13 (sextet,
CH2), 1.44 (sextet, CH2), 1.76 (q, CH2), 2.04 (q, CH2), 4.43 (t,
N-CH2), 4.77 (t, N-CH2), 7.86 (t, CH), 8.13 (t, CH), 8.32 (t, CH),
8.52 (t, CH), 8.71 (d, CH), 9.01 (d, CH) ppm. 13C NMR (CDCl3,
500 MHz): δ 13.6 (s, CH3), 13.7 (s, CH3), 19.5 (s, CH2), 19.6 (s,
CH2), 33.5 (s, CH2), 33.7 (s, CH2), 62.4 (s, CH2), 62.9 (s, CH2),
128.9 (s, CH), 129.0 (s, CH), 144.3 (s, CH) 144.6 (s, CH), 145.6
(s, CH), 145.8 (s, CH). The deviation of experimental values in
elemental analysis data is due to negligible impurities which are
difficult to remove by distillation or recrystallization.
1
153 [Me2BuIm]+. H NMR (CDCl3, 500 MHz): δ 0.89 (t, CH3),
1.33 (sextet, CH2), 1.77 (q, CH2), 2.67 (s, Cring-CH3), 3.85 (s,
N-CH3), 4.06 (t, N-CH2), 7.25 (s, CH), 7.30 (s, CH) ppm. 13C
NMR (CDCl3, 500 MHz): δ 11.0 (s, CH3), 13.7 (s, CH2), 19.8 (s,
CH2), 31.7 (s, CH2), 36.5 (s, CH3), 49.2 (s, CH3), 121.2 (s, CH),
122.9 (s, CH), 143.6 (s, NCN) ppm.
1,2-Dimethyl-3-butylimidazolium Bromodiiodide, 8. Dark red
liquid. IR (liquid film): 3133 (s), 2957 (vs), 2931 (vs), 2868 (s),
1585 (vs), 1558 (w), 1535 (vs), 1458 (vs), 1418 (m), 1379 (m),
1340 (w), 1268 (w), 1244 (s), 1183 (w), 1132 (s), 1075 (w), 1036
(w), 741 (vs), 662 (s), 625 (w) cm-1. ESI-MS: m/z ) 153.1 [Me2-
BuIm+]. 1H NMR (CDCl3, 500 MHz): δ 0.94 (t, CH3), 1.39 (sextet,
CH2), 1.82 (q, CH2), 2.73 (s, Cring-CH3), 3.89 (s, N-CH3), 4.10
(t, N-CH2), 7.24 (d, CH), 7.29 (d, CH) ppm. 13C NMR (CDCl3,
500 MHz): δ 11.4 (s, CH3), 13.6 (s, CH2), 19.8 (s, CH2), 31.7 (s,
CH2), 36.8 (s, CH3), 49.4 (s, CH3), 121.3 (s, CH), 123.0 (s, CH),
143.7 (s, NCN) ppm.
N-Butylpyridinium Bromodiiodide, 13. Dark red liquid. IR
(liquid film): 3123 (w), 3055 (s), 2959 (s), 2931 (s), 2868 (m),
1632 (vs), 1498 (s), 1485 (vs), 1458 (s), 1376 (w), 1315 (w), 1211
(w), 1169 (s), 1055 (w), 949 (w), 765 (s), 733 (w), 679 (vs) cm-1
.
ESI-MS: m/z ) 136.1 [BuPy+], 334.6 [I2Br-], 380.3 [I2BuPy-].
1H NMR (CDCl3, 500 MHz): δ 0.98 (t, CH3), 1.44 (sextet, CH2),
2.07 (q, CH2), 4.75 (t, N-CH2), 8.13 (t, CH), 8.53 (t, CH), 8.93
(d, CH) ppm. 13C NMR (CDCl3, 500 MHz): δ 11.6 (s, CH3), 19.6
(s, CH2), 33.6 (s, CH2), 63.2 (s, CH2), 129.1 (s, CH), 144.6 (t, CH),
145.6 (s, CH).
1,2-Dimethyl-3-hexylimidazolium Iododibromide, 9. Dark red
liquid. IR (liquid film): 3133 (s), 2954 (vs), 2928 (vs), 2857 (s),
1585 (s), 1535 (s), 1464 (s), 1458(s), 1418 (m), 1378 (m), 1339
(w), 1268 (w), 1240 (s), 1171 (w), 1132 (m), 1105 (w), 1078 (w),
1036 (w), 743 (s), 662 (s), 626 (w) cm-1. ESI-MS: m/z ) 181
[Me2HexIm+], 288.2 [IBr2-], 334.6 [I2Br-]. 1H NMR (CDCl3, 500
MHz): δ 0.83 (t, CH3), 1.27 ((CH2)2), 1.32 (CH2), 1.81 (q, CH2),
2.70 (s, Cring-CH3), 3.87 (s, N-CH3), 4.07 (t, N-CH2), 7.21 (d,
CH), 7.25 (d, CH) ppm. 13C NMR (CDCl3, 500 MHz): δ 11.0 (s,
CH3), 14.0 (s, CH2), 22.4 (s, CH2), 26.3 (s, CH2), 29.7 (s, CH2),
31.1 (s, CH2), 36.5 (s, CH3), 49.5 (s, CH3), 121.2 (s, CH), 122.9
(s, CH), 143.7 (s, NCN) ppm.
N-Dodecylpyridinium Iododibromide, 14. Orange-red solid.
IR (liquid film, solution in CHCl3): 3126 (w), 3058 (m), 2990 (s),
2926 (vs), 2854 (s), 1631 (s), 1499 (m), 1486 (s), 1462 (m), 1373
(w), 1316 (w), 1171 (s), 677 (s) cm-1. ESI-MS: m/z ) 248.0
1
[DodPy+], 334.1 [I2Br-], 407.7 [Br2DodPy-]. H NMR (CDCl3,
500 MHz): δ 0.80 (t, CH3), 1.18 ((CH2)7), 1.34 ((CH2)2), 2.05 (q,
CH2), 4.69 (t, N-CH2), 8.13 (t, CH), 8.54 (t, CH), 8.90 (d, CH)
ppm. 13C NMR (CDCl3, 500 MHz): δ 14.1 (s, CH3), 22.6 (s, CH2),
26.2 (s, CH2), 29.0 (s, CH2), 29.3 (s, CH2), 29.5 (s, CH2), 29.6 (s,
CH2), 31.8 (s, CH2), 31.9 (s, CH2), 63.2 (s, CH2), 94.4 (t, CH2),
129.1 (s, CH), 144.5 (t, CH), 145.8 (s, CH).
Inorganic Chemistry, Vol. 46, No. 9, 2007 3569

Gorlov et al.
Table 1. Synthesized Interhalogen Ionic Salts and Liquids
BuIm]IBr2 (7) was characterized by single-crystal X-ray
diffraction analysis.32-34 The molecular and crystal structures
are shown in Figure 1. The compound consists of planar
imidazolium rings and linear IBr2- anions. It is important to
note that the iodine atom occupies the central position and
is surrounded by two bromine atoms. The I-Br distance is
2.7085(11) Å, which is slightly shorter than the calculated
value for the linear [Br-I-Br]- anion (2.766 Å; B3PW91).
Another essential characteristic of this compound is the
R
A-
R
A-
R
A-
-
-
-
1
2
3
4
5
6
C2H5
C3H7
C6H13
C6H13
C12H25 IBr2
C3H7
IBr2-
IBr2-
IBr2
7
8
9
C4H9
C4H9
C6H13
IBr2
12 C4H9
IBr2
I2Br- 13 C4H9
I2Br-
-
-
IBr2
14 C12H25 IBr2
I2Br- 10 C8H17
I2Br-
-
-
11 C12H25 IBr2
-
Br3
-
absence of significant interaction between the IBr2 anion
and the Me2BuIm+ cation in the crystal structure; the shortest
distance between Br or I atoms and H atoms is 3.1 Å.
Commonly, ionic liquids show a network of hydrogen bonds
(see, for example, refs 37 and 38). One can assume that the
absence of strong cation-anion interaction accounts for the
relatively low viscosity of interhalogen ionic liquids.
3. Results and Discussion
3.1 Synthesis and Characterization of Ionic Liquids.
In this work, three types of interhalogen ionic liquids were
studied: 1,3-dialkylimidazolium, 1,2,3-trialkylimidazolium,
and N-alkylpyridinium salts. These aromatic cations were
chosen because of their high electrochemical stability and
relatively low viscosity of the ionic liquids in comparison
with other types of ionic liquids, based, for example, on
Mass spectroscopy (fast atom bombardment and electro-
spray ionization spectroscopy) was also used for character-
ization of the compounds prepared. All spectra are shown
in the Supporting Information. Methanol solutions (concen-
tration 0.1 mg/mL) were used. Positive ion mode spectra of
all compounds contain single peaks corresponding to alky-
limidazolium or alkylpyridinium cations. It is important to
note that the negative ion mode spectra of all ionic liquids
-
tetraalkylphosphonium cations. The counterions were IBr2
and I2Br-. Compounds with different alkyl chains were
prepared; the length of alkyl chain varied from -C2H5 to
-C12H25.
The synthetic approach for the preparation of interhalogen
ionic liquids was based on the reaction between alkylimi-
dazolium or alkylpyridinium iodides and free bromine or
iodine monobromide in chloroform solution
-
-
with IBr2 anions contain peaks with m/z ) 286.7 [IBr2 ]
and m/z ) 334.7 [I2Br-]. This fact may be explained by the
existence of an equilibrium between different anions in
solution:
CHCl3
[K+]I- + Br-X 5-10 °C8 [K+][IBrX-]
2IBr2- a I2Br- + Br3
(1)
-
where K+ ) 1,3-dialkylimidazolium, 1,2,3-trialkylimidazo-
lium, or N-alkylpyridinium; X ) Br or I.
-
The ratio of intensities of peaks corresponding to [IBr2 ]
Earlier, this method was used for the preparation of a series
of IILs, but the reaction was carried out without any
solvent.27,28 We used mild reaction conditions (low temper-
ature and gradual addition of Br2 or IBr) in order to avoid
halogenation of the organic cations. A series of ionic liquids
was prepared on the basis of this reaction, see Table 1.
Compounds 1-4, 6, 8-10, 12, and 13 are red liquids,
immiscible with water and quite soluble in acetone. Salt 5
is a yellow solid, 7 is a dark red solid, and 11 and 14 are
orange-red solids. The yields of IILs were typically high
(80-95%). The melting points are 39 (5), 37 (7), 47 (11),
and 37 (14) °C.
-
and [BrI2 ] anions depends on the nature of the cation and
may reflect the interaction between cations and anions in
-
-
solution. For compounds 7 and 12, the ratio [IBr2 ]:[BrI2 ]
is about 2:1, which indicates that a significant amount of
-
[IBr2 ] anions is transformed into other forms.
Negative ion mode spectrum of the [Me2BuIm]I2Br (8)
-
ionic liquid contains three peaks: m/z ) 380.6 [I3 ], m/z )
-
-
334.7 [BrI2 ], and m/z ) 286.7 [IBr2 ], which can be
explained by the existence of an equilibrium in solution:
2I2Br- a IBr2- + I3
(2)
-
In order to confirm the structure of the cationic part of
1
the IILs, the compounds were characterized by H and 13C
It should be noted that similar behavior of potassium salts
containing interhalogen anions I2Cl- and I2Br- was observed
in aqueous and acetonitrile solutions.39 Thus, on the basis
of mass spectrometric results, the authors postulate the
existence of equilibria such as those in eqs 1 and 2.39 The
absence of some peaks in negative mode mass spectra, for
NMR spectroscopy in CDCl3 solution. In comparison with
the corresponding iodides, the IILs have almost identical 1H
NMR spectra (Supporting Information). It is interesting to
note that H and 13C NMR spectra of 1, 6, and 12 contain
double sets of the peaks. This probably may be explained
by the presence of free cations K+ and ion pair, such as
1
-
(K+)-(IBr2 ), in chloroform solution.
(36) Pettersson, H.; Gruszecki, T.; Bernhard, R.; Ha¨ggman, L.; Gorlov,
M.; Boschloo, G.; Edvinsson, T.; Kloo, L.; Hagfeldt, A. Prog.
PhotoVolt: Res. Appl., in press.
(37) Anderson, J. L.; Ding, R.; Ellern, A.; Armstrong, D. W. J. Am. Chem.
Soc. 2005, 127, 593.
(38) Vygodskii, Y. S.; Lozinskaya, E. I.; Shaplov, A. S.; Lyssenko, K. A.;
Antipin, M. Yu.; Urman, Y. G. Polymer 2004, 45, 5031.
(39) McIndoe, J. S.; Tuck, D. J. Dalton Trans. 2003, 244.
Several IILs prepared are solid at room temperature. This
gave us the possibility to use single-crystal X-ray diffraction
analysis in order to confirm the structure of the compounds
in crystalline form. It was particularly important to determine
the structure of the interhalogen anions. The compound [Me2-
3570 Inorganic Chemistry, Vol. 46, No. 9, 2007

Dye-Sensitized Solar Cells
Figure 1. Molecular (with thermal ellipsoids at 50% probability) and crystal structure of [Me2BuIm]IBr2 (7). Selected bond distances (Å) and angles (deg):
Br1-I1 ) 2.7085(11), N1-C5 ) 1.376(11), C6-C5 ) 1.316(13), N2-C6 ) 1.360(11), N2-C7 ) 1.317(10), N1-C7 ) 1.324(10), Br1-I1-Br1 )
180(0).
Table 2. Bands in Raman Spectra of Interhalogens in the Ionic Liquids
band, cm-1
calcd
compd
expt
B3PW91
B3LYP
MP2
ion
symm
[Me2BuIm]IBr2 (7)a
[Me2HexIm]IBr2 (9)a
[Me2C12H25Im]IBr2 (11)a
[HexMeIm]IBr2 (3)a
[PrMeIm]IBr2 (2)a
157.2 (s); 132.8 (m)
158.5 (s)
157.6 (s)
153.6 (s)
158.1 (s); 134.5 (m)
158.7 (s)
157.2 (s); 136.5 (m)
155.6 (s)
159.8 (s)
151
132; 179
144
125; 170
158
137; 198
[Br-I-Br]-
[Br-Br-I]-
D∞h
C∞V
-
IBr2
[EtMeIm]IBr2 (1)a
[BuPy]IBr2 (12)a
[C12H25Py]IBr2 (14)a
[MeC12H25Im]IBr2 (5)a
[Me2OctIm]I2Br (10)b
[Me2BuIm]I2Br (8)a
[BuPy]I2Br (13)a
157.8 (s); 133.4 (s); 111.8 (s)
153.9 (s); 132.3 (s); 110.9 (s)
157.2 (s); 132.4 (s); 110.8 (s)
157.6 (s); 132.4 (s); 111.0 (s)
125; 163
113
118; 154
107
130;170
116
[I-I-Br]-
[I-Br-I]-
C∞V
D∞h
I2Br-
[HexMeIm]I2Br (4)a
-
Br3
[PrMeIm]Br3 (6)b
159.5 (s)
158
109
151
103
166
113
[Br-Br-Br]-
[I-I-I]-
D∞h
D∞h
-
I3
a In CHCl3. b Without solvent.
example Br3- peaks, may be explained by the formation of
adducts between cations K+ and anions, such as [KBrn]-.
Indeed, negative ion mode spectrum of [Me2HexIm]IBr2 (9)
contains a peak at m/z ) 341.7 which corresponds to
[Me2HexImBr2]- anion.
in solution, Raman spectra for IILs were recorded. Also,
quantum chemical calculations were performed in order to
assign the low-frequency bands in the Raman spectra.29
Experimental and calculated frequency data for ionic liquids
-
-
formally containing IBr2 , I2Br-, and Br3 anions are
presented in Table 2. It should here be emphasized that
although the composition of IILs implies the presence of
Quantum chemical calculations were performed in order
to evaluate the thermodynamic stability of different anions
coexisting in solution of interhalogen ionic liquids.29 Cal-
culations of changes of Gibbs free energy for reactions 1
and 2 at the B3PW91 level give the following results (T )
298.150 K): ∆G˚ ) +33.2 kJ/mol for eq 1 and ∆G˚ ) +2.5
kJ/mol for eq 2. These correspond to equilibrium constants
of K1 ) 1.53‚× 10-6 and K2 ) 0.36. The inclusion of solvent
effects using the PCM formalism for water, acetonitrile, and
benzene, representing a wide scan in dielectric properties,
did not significantly shift the change in Gibbs free energy
of reactions 1 and 2 (<2 kJ mol-1). The main reason is most
likely that all molecular species in these two reactions with
respect to topology and charge distribution are very similar.
The experimental and calculational results allow us to
conclude that (1) the nature of, in particular, the organic
cation can play an important role with respect to shifts of
equilibria; (2) several forms of anions can coexist in solution.
In order to confirm the presence of IBr2- or I2Br- anions
-
XY2 anions, it is not necessary that these nominal ions
dominate in the liquid state (see mass spectrometry data).
One can state that predominance of XY2- anions is a starting
hypothesis in the interpretation of vibrational spectroscopic
data with the help of quantum chemical calculations.
Strong bands in region 153-159 cm-1 are observed in
-
Raman spectra of ionic liquids containing IBr2 anions in
chloroform solutions. This is in good agreement with
quantum chemical calculations showing one band at 151
cm-1 (B3PW91 level) for the Br-I-Br- anion (D∞h).
Asymmetry of these bands can be explained by interaction
between cations and anions in solution as well as the
-
existence of vibrational excitations. The presence of Br3
anion in solution due to equilibria 1 and 2 can also be
responsible for the asymmetry of these bands. According to
the calculations, the second possible isomer, [Br-Br-I]-,
has its strongest Raman band at 132 cm-1. One can assume
Inorganic Chemistry, Vol. 46, No. 9, 2007 3571

Gorlov et al.
Figure 2. A monolithic multicell with 24 individual cells (A) and schematic cross-section of the four-layer monolithic dye-sensitized photovoltaic cell (B).
that the presence of an additional, less intense band in the
region 132-136 cm-1 in the Raman spectra of certain IILs
(Table 2) may be attributed to the presence of a second
isomer in solution.
atmosphere. Compounds [Me2BuIm]IBr2 (7) and [BuPy]IBr2
(12) display three oxidation steps at 1.22, 0.75, and 0.56 V
versus NHE for 7, and at 1.10, 0.84, and 0.60 V for 12 versus
NHE (Supporting Information). It should be noted that IILs
decompose under electrochemical reduction: small drops
formed on the surface of acetonitrile solution at negative
potentials, which causes the quality of voltammograms.
Raman spectra of IILs containing I2Br- anions contain
three strong bands centered at 155, 132, and 110 cm-1. The
presence of these bands may be explained by the coexistence
of the two isomers, [I-I-Br]- and [I-Br-I]-, ions in
solution. A strong band at 110 cm-1 may also originate from
-
Interhalogen anions IBr2 could in principle be oxidized to
-
give products with higher nuclearity. Compounds with I3Br4
-
I3 in the liquid. It should be noted that our results are in
anions are known, although only in the solid state.40,42 Taking
that into account, and the fact that, according to Raman and
mass spectra, IILs are present in solution in at least two
isomers, makes it difficult to draw conclusions about the
exact chemical composition of the oxidation products of
good agreement with experimental and theoretical data
recently obtained for series of organic salts with interhalogen
anions.40
In order to understand the electrochemical processes of
IBr2- anions, it is necessary to compare them with those of
IBr2 anions.
-
-
the well-known iodide/triiodide redox couple I3 /I-. Ac-
3.2. Solar Cell Performance. The solar cells used in this
work were encapsulated four-layer monolithic multicells. A
multicell means that several individual cells are manufactured
on the same substrate in parallel processes allowing a high
reproducibility of the cell performance. The multicell
substrate contains 24 individual monolithic dye-sensitized
solar cells with an active TiO2 surface of 0.48 cm2 (Figure
2A). Each monolithic cell consists of four layers: dye-
sensitized TiO2 layer, porous ZrO2 layer (spacer), adhesion
layer (carbon 1), and platinized carbon layer (carbon 2). As
can be seen in Figure 2B, the carbon 1 layer is placed next
to the spacer layer whereas the carbon 2 layer is placed on
top of the spacer layer and the carbon 1 layer. The
thicknesses of the layers were approximately 12 µm (TiO2),
-
cording to literature data, the I3 /I- couple is characterized
by two oxidation processes in solution: I- f I3- and I3- f
I2.41 These data are in agreement with electrochemical
behavior of alkylpyridinium and trialkylimidazolium triio-
dides. As an example, compound [HexPy]I3 displays two
oxidation processes in acetonitrile solution at +0.887 and
0.321 V versus NHE (cyclic voltammograms and differential
pulse voltammetry results in Supporting Information). The
electrochemical behavior of IILs was studied in acetonitrile
solution using Ag/Ag+ reference electrode under nitrogen
(40) Aragoni, M. C.; Arca, M.; Devillanova, F. A.; Hursthouse, M. B.;
Huth, S. L.; Isaia, F.; Lippolis, V.; Mancini, A.; Ogilvie, H. R.; Verani,
G. J. Organomet. Chem. 2005, 690, 1923.
(41) Oskam, G.; Bergeron, B. V.; Meyer, G. J.; Searson, P. C. J. Phys.
Chem. B 2001, 105, 6867.
(42) Minkwitz, R.; Berkei, M.; Ludwig, R. Inorg. Chem. 2001, 40, 25.
3572 Inorganic Chemistry, Vol. 46, No. 9, 2007

Dye-Sensitized Solar Cells
10 µm (spacer), 10 µm (carbon 1), and 50 µm (carbon 2).
Details of the multicell manufacturing have been described
in a separate work.36 In short, the manufacturing technique
is based on screen-printing the electrodes onto laser-
structured transparent conducting glass (TEC 15 Ω/square
from Hartford Glass). After sintering the electrodes, the glass
plates are immersed in the dye solution. For the results
presented in this paper, cis-di(thiocyanato)-bis(4,4′-dicar-
boxy-2,2′-bipyridine)Ru(II), in a concentration of 0.3 mM,
dissolved in acetonitrile/tert-butanol (50/50 by volume), was
used. The electrolyte solution is manually dispensed on the
monolithic electrodes. Finally, the cells are sealed tightly in
a vacuum-heat process using the encapsulation material
Surlyn.
The following IILs were chosen as components of the
electrolytes for monolithic DSCs: [PrMeIm]IBr2 (2), [HexMe-
Im]IBr2 (3), [HexMeIm]I2Br (4), [Me2BuIm]IBr2 (7), and
[BuPy]IBr2 (12).
Three types of electrolytes have been studied in this
work: (1) glutaronitrile as solvent, (2) γ-butyrolactone as
solvent, and (3) the native ionic liquids as solvent. The
composition of electrolytes, diffusion constants, and results
of IV-measurements are shown in Table 3. It is important to
note that, according to the Raman spectra of electrolytes,
interhalogen anions are present in the solutions even in the
presence of excess of iodide ions.
for DSCs.9,13,44 A comparison of efficiencies of these
electrolytes with the corresponding ones without addi-
tives (the E1-E2, E3-E4, E5-E6 pairs) shows that the
former ones are more efficient (the difference is up to
0.9% in efficiency at 1000 W/m2) due to an increase in open-
circuit photovoltage, as observed earlier.44 Guanidinium
thyocyanate may be adsorbed along with dye molecules at
the TiO2 surface, thus playing the role of a blocking layer
and/or facilitating self-assembly agent, as considered by
Gra¨tzel.45
4. The performance of the monolithic cells with electro-
lytes E9 and E10, containing imidazolium ionic liquids with
IBr2- and I2Br- anions, respectively, is very similar, which
can be explained by the presence of similar redox-active
species in solution due to the equilibria previously discussed
in this work.
5. The influence of alkyl chain length in imidazolium
cations on the performance of the monolithic cells was also
studied (electrolytes E6-E8). The efficiency of the cells
containing a short alkyl chain (C3H7) in imidazolium cations
is slightly higher than the one for the cells containing longer
alkyl chains (C6H13 and C12H25) at a light intensity of 1000
W/m2. However, at a light intensity of 100 W/m2, the
difference is insignificant.
6. A comparison of glutaronitrile electrolytes containing
-
the interhalogen anions IBr2 or I2Br- with glutaronitrile
electrolyte containing the I3- anion (E12) shows that the latter
one is more efficient (the difference is 0.5-1% in efficiency
at 1000 W/m2). It should be emphasized here that (1) the
concentration of additives and combination between materials
for electrodes were optimized for I-/I3- electrolytes; (2) the
On the basis of analysis of the results presented in Table
3, the following conclusions can be made.
1. Overall, light-to-electricity conversion efficiencies of
the cells with electrolytes based on ionic liquids as solvents
are lower than those for glutaronitrile- or γ-butyrolactone-
based electrolytes. For example, the difference between
efficiencies of the pairs E6-E9 and E7-E11 is 2-3% in
efficiency both at light intensities of 1000 and 100 W/m2.
This can be attributed to the higher viscosity of imidazolium
ionic liquids in comparison to glutaronitrile solutions, which
leads to slower ion transport in electrolyte (according to
literature,28 viscosity of [BuMeIm]I is 1110 cP and viscosity
of [BuMeIm]IBr2 is 57 cP). This is in agreement with lower
values of diffusion constants for the ionic liquid electrolytes.
It should be highlighted that the multicells have not yet been
optimized for low-viscous electrolytes. The performance of
cells using electrolytes E9 and E11 would benefit from the
use of a more porous TiO2 layer and a thinner spacer layer.
2. Overall light-to-electricity conversion efficiencies of the
cells with electrolytes based on γ-butyrolactone are higher
than those for glutaronitrile-based electrolytes mainly due
to higher short-circuit current density values. The difference
is 1.3-1.9% at light intensities of 1000 W/m2 (the E2-E13,
E4-E14, E7-E15 pairs). Probably, this may be attributed
to the lower viscosity of γ-butyrolactone compared with that
for glutaronitrile (1.63 and 5.67 cP at 28 °C, respectively).43
3. Glutaronitrile and γ-butyrolactone electrolytes contain
two additives, guanidinium thiocyanate and 1-N-methyl-
benzimidazole. N-donor organic additives based on benz-
imidazole are frequently used as components of electrolytes
-
performance of cells containing I-/I3 redox couple loses
10% of the initial performance after 1000 h illumination at
-
350 W/m2 (see ref 36); (3) in the case of I-/IBr2 or I-/I2-
Br- systems it is difficult to estimate which redox couple(s)
work in real cell due to equilibria between different anions
in solution.
Typical IV-characteristics of DSCs based on the interh-
alogen ionic liquids are displayed in Figure 3 (glutaronitrile
or ionic liquid as solvent).
3.3. Stability of Monolithic Solar Cells with IILs
Electrolytes. The long-term stability of the dye-sensitized
solar cells is one of the most important characteristics for
their practical application. In order to evaluate the stability
under illumination of the monolithic solar cells containing
IILs, three cells with electrolytes E2, E4, and E6 were
illuminated for 1000 h (6 weeks) at 350 W/m2 using a sodium
lamp (open-circuit condition) protected by a UV cutoff filter
(cut off at 400 nm). Results from these experiments are
shown in Figure 4.
The efficiencies of all cells decrease slowly during
illumination. The relative decrease in efficiency after 6 weeks
of illumination is 9% for electrolytes E2 and E6 and 14.5%
for E4.
(44) Kusama, H.; Arakawa, H. J. Photochem. Photobiol. A: Chem. 2004,
162, 441.
(43) Phibbs, M. K. J. Phys. Chem. 1955, 59, 346.
(45) Gra¨tzel, M. J. Photochem. Photobiol. A: Chem. 2004, 164, 3.
Inorganic Chemistry, Vol. 46, No. 9, 2007 3573

Gorlov et al.
-
Table 3. Diffusion Constants D of IB2-, I2Br-, and I3 and Results Obtained from IV-Measurements of Monolithic Dye-Sensitized Multicells Based on
Interhalogen Ionic Liquidsa
1000 W/m2
100 W/m2
composition
D (cm2/s)
η (%)
FF (%)
59
Voc (V)
Jsc (mA/cm2)
η (%)
FF (%)
66
Voc (V)
Jsc (mA/cm2)
0.8 M [Me2BuIm]I
0.1 M [BuMe2Im]IBr2
in glutaronitrile
E1
E2
E3
E4
E5
E6
1.1 × 10-6
4.1
0.72
0.77
0.70
0.74
0.70
0.77
9.6
4.8
0.66
0.73
0.62
0.69
0.64
0.70
1.1
0.8 M [Me2BuIm]I
0.1 M [BuMe2Im]IBr2
0.1 M GuanSCNb
0.5 M NMBIc
8.6 × 10-7
1.2 × 10-6
1.1 × 10-6
4.9
4.1
4.5
3.6
4.5
61
59
58
57
60
10.2
9.8
6.0
4.2
6.0
4.6
5.8
72
69
71
70
70
1.1
1.0
1.2
1.0
1.2
in glutaronitrile
0.8 M [BuPy]I
0.1 M [BuPy]IBr2
in glutaronitrile
0.8 M [BuPy]I
0.1 M [BuPy]IBr2
0.1 M GuanSCN
0.5 M NMBI
10.3
8.9
in glutaronitrile
0.8 M HexMeImI
0.1 M [HexMeIm][IBr2 ]
in glutaronitrile
1.3 × 10-6
0.8 M [HexMeIm]I
0.1 M [HexMeIm]IBr2
0.1 M GuanSCN
0.5 M NMBI
d
-
9.9
in glutaronitrile
0.8 M [PrMeIm]I
0.1 M [PrMeIm]IBr2
0.1 M GuanSCN
0.5 M NMBI
E7
E8
1.9 × 10-6
1.4 × 10-6
5.0
4.5
59
54
0.74
0.71
11.5
11.5
6.0
6.1
71
73
0.68
0.66
1.2
1.3
in glutaronitrile
0.8 M [C12H25MeIm]I
0.1 M [C12H25 MeIm]IBr2
0.1 M GuanSCN
0.5 M NMBI
in glutaronitrile
0.1 M [HexMeIm]IBr2
0.1 M GuanSCN
0.5 M NMBI
E9
3.1 × 10-7
3.2 × 10-7
3.1 × 10-7
2.3
2.4
2.3
43
41
48
0.62
0.64
0.63
8.6
9.2
7.6
3.2
3.4
3.0
56
54
61
0.56
0.58
0.56
1.0
1.1
0.9
in [HexMeIm]I
0.1 M [HexMeIm]I2Br
0.1 M GuanSCN
0.5 M NMBI
E10
E11
in [HexMeIm]I
0.1 M [PrMeIm]IBr2
0.1 M GuanSCN
0.5 M NMBI
in [PrMeIm]I
0.8 M [BuMeIm]I
50 mM I2
0.1 M GuanSCN
0.2 M NMBI
in glutaronitrile
E12
E13
E14
E15
1.5 × 10-6
5.4
6.3
6.4
6.3
62
61
64
61
0.72
0.79
0.79
0.80
12.1
12.9
12.8
13.2
6.2
7.0
7.0
7.3
72
74
74
73
0.67
0.73
0.73
0.75
1.3
1.3
1.3
1.3
0.8 M [Me2BuIm]I
0.1 M [Me2BuIm]IBr2
0.1 M GuanSCN
0.5 M NMBI
d
-
in γ-butyrolactone
0.8 M [BuPy]I
0.1 M [BuPy]IBr2
0.1 M GuanSCN
0.5 M NMBI
d
d
-
-
in γ-butyrolactone
0.8 M [PrMeIm]I
0.1 M [PrMeIm]IBr2
0.1 M GuanSCN
0.5 M NMBI
in γ-butyrolactone
a η ) overall light-to-electricity conversion efficiency; FF ) fill factor; Voc ) open-circuit voltage; Jsc ) short-circuit current density. b GuanSCN )
guanidinium thiocyanate. c NMBI ) 1-N-methylbenzimidazole. d Limiting current density cannot be measured accurately because of the absence of well-
defined plateau in cyclic voltammograms.
3574 Inorganic Chemistry, Vol. 46, No. 9, 2007

Dye-Sensitized Solar Cells
Figure 3. IV-characteristics of monolithic dye-sensitized multicells based on glutaronitrile electrolyte E6 (left) and ionic liquid electrolyte E9 (right). Both
electrolytes E6 and E9 contain [HexMeIm]I and [HexMeIm]IBr2 (3).
lumination showed 9-14% relative efficiency degradation
after 1000 h of illumination at 350 W/m2. In conclusion, it
was shown that a new type of low-viscous interhalogen ionic
liquids containing a mixture of redox species provides
promising alternatives to viscous ionic liquids based on
alkylimidazolium iodides. In comparison with the classical
I-/I3- redox system, interhalogen I-/IBr2- or I-/I2Br- redox
couples are potentially more “flexible” redox species due to
equilibria between different anions in solution. In order to
achieve higher efficiency and improved long-term stability
of electrolytes based on interhalogen ionic liquids, optimiza-
tion of all components of DSCs (semiconductor materials,
dyes, sealing materials) will be required. The use of new
organic dyes and different sealing materials in conjunction
with interhalogen ionic liquid electrolytes is currently under
investigation in our group.
Figure 4. Efficiency changes with time for the cells with electrolytes E2,
E4, and E6 under illumination at 350 W/m2.
Acknowledgment. We thank Dr. A. Nazarov (Institut fu¨r
Anorganische Chemie, Universita¨t Wien, Austria) for record-
ing electrospray ionization mass spectra and Dr. G. Boschloo
for interesting discussions. The authors gratefully acknowl-
edge support from Swedish Energy Agency.
4. Conclusions
New interhalogen ionic liquids have been prepared and
characterized in solution and in the solid state. Ionic liquids
containing IBr2 and I2Br- anions were successfully used
as electrolyte components for monolithic dye-sensitized solar
multicells. Overall light-to-electricity conversion efficiencies
up to 6.4%, 5.0%, and 2.4% at 1000W/m2 were achieved
by using electrolytes based on interhalogen ionic salts and
γ-butyrolactone, glutaronitrile, or native ionic liquids as
solvents, respectively. The long-term stability under il-
-
Supporting Information Available: Cyclic voltammograms,
1
electrospray ionization mass spectra, and H, 13C NMR and IR
spectra of IILs. This material is available free of charge via the
Internet at http://pubs.acs.org.
IC062244B
Inorganic Chemistry, Vol. 46, No. 9, 2007 3575