Molten and Solid Trialkylsulfonium Iodides and Their Polyiodides as Electrolytes in Dye-Sensitized Nanocrystalline Solar Cells

Article abstract of DOI:10.1021/jp036859v

The overall solar-to-electric energy conversion efficiencies in solar simulators, and the incident monochromatic photon-to-current-efficiencies (IPCE) was measured for a selection of molten salt electrolytes to reveal their possible use as electrolytes in dye-sensitized nanocrystalline solar cells (DNSC). The IPCE results of (Et₂MeS)I and (Bu₂MeS)I were stable above 55% from 400 to 550 nm and then decreased on the red side of the spectra. The addition of iodine to (Bu₂MeS)I and (Bu₂EtS)I indicated increasing IPCE values at 520-540 nm; from 55 to 60% and 38 to 46%, respectively. (Te₂MeS)I, (Bu₂MeS)I, and (Bu₂EtS)I showed good conducting abilities at ambient conditions and provided good light-to-electricity conversion efficiencies while used as electrolytes in DNSC. The liquid electrolytes (Et₂MeS)I provided the highest conversion efficiencies at room temperature.

Full text of DOI:10.1021/jp036859v

J. Phys. Chem. B 2003, 107, 13665-13670
13665
Molten and Solid Trialkylsulfonium Iodides and Their Polyiodides as Electrolytes in
Dye-Sensitized Nanocrystalline Solar Cells
Hele´ne Paulsson,† Anders Hagfeldt,‡ and Lars Kloo*,†
Inorganic Chemistry, Department of Chemistry, Royal Institute of Technology, S-100 44 Stockholm, Sweden,
and Department of Physical Chemistry, Uppsala UniVersity, Box 532, S-751 21 Uppsala, Sweden
ReceiVed: September 25, 2003
Potential new electrolytes for dye-sensitized nanocrystalline solar cells (DNSCs) of Gra¨tzel type based on
trialkylsulfonium iodides have been investigated. Room temperature molten salts of (Et2MeS)I, (Bu2MeS)I,
and (Bu2EtS)I, with iodine in low concentrations, revealed good conducting abilities. DNSCs using iodine-
doped (Bu2MeS)I as electrolyte achieved an overall light-to-electricity conversion efficiency of 3.7% in
simulated AM 1.5 solar light at a light intensity of 0.1 Sun. The effects from varying the temperature during
the I/V measurements were studied, as well as the effects of 4-tert-butylpyridine treatment of the electrodes.
Introduction
containing conventional organic solvents may be sensitive to
stability effects due to evaporation and reactions with water and/
or oxygen. Alternative electrolytes of solid conducting materials
have been studied, such as polymer electrolytes (conducting
ions), and inorganic and also amorphous materials (conducting
holes).8-10
In photovoltaic systems, light energy is converted to electric-
ity, and common applications today are for satellites, light-
houses, and primary batteries. Because of the gigantic supply
of energy from the sun, covering 0.1% of the Earth’s surface
with solar cells (with an efficiency of approximately 10%) would
satisfy our present global needs.1
Enormous research activity is dedicated to finding cost-
effective solar cells. Photoelectrochemical (PEC) solar cells have
been studied since the 1960s. In 1991, Gra¨tzel and co-workers
presented a PEC with a highly porous, nanocrystalline TiO2
electrode sensitized with a ruthenium complex, by which high
power conversion efficiencies were achieved.2
In dye-sensitized nanocrystalline photoelectrochemical solar
cells (DNSCs), the function of light absorption is separated from
the charge-carrier transport.3 The active electrode film consists
of a monolayer of dye molecules, adsorbed on a nanostructured
semiconductor film with a highly porous structure. The working
electrode is connected to a counter electrode via an external
circuit. Between the electrodes an electrolyte penetrates the pores
in the nanostructured film and handles charge transport and dye
regeneration in this type of solar cell. The advantages of
nanocrystalline semiconductor films, as compared to single-
crystal and polycrystalline films, are related to the large surface
area and the high porosity, resulting in good electrical and
optical properties.2
Room temperature molten salts have some qualities that
would make them suitable as electrolytes, such as high
conductivity, ionic mobility, and negligible volatility. Dialkyl-
imidazolium salts are stable and good electrical conductors. Ionic
liquids based on 1,3-dialkylimidazolium cations and hydropho-
bic anions (RR′Im+X-) have shown to be promising solvents
for electrochemical and photovoltaic applications in studies
performed by Gra¨tzel and co-workers.11 They used mixtures of
hexyl-methyl-imidazolium iodide (HMImI) and organic solvents
or ionic liquids such as butyl-methyl-imidazolium triflate as
electrolytes in DNSCs.12 Quasi-solid-state electrolytes of HMImI
and iodine with addition of low-molecular-weight gelators have
been investigated by Kubo and co-workers.13,14 Solid-state
amorphous organic hole-transport materials have been success-
fully tested as mediators by Gra¨tzel and co-workers.15 Modifica-
tions of these compounds have provided improved results in
solar cell measurements.16-18
Trialkylsulfonium polyiodides have been found to exhibit
good electrical conductivity, 10-3 to 10-4 S/cm, which increases
with larger contents of iodine.19 This is explained by a
mechanism of electrical conduction in polyiodide chains via a
Grotthus mechanism, a relay mechanism, in which a net
transport of charge is achieved without any net transport of mass.
The effect is favored by a reduced distance between donors and
acceptors:19-20
A redox couple, most commonly iodide/triiodide, in the
electrolyte is the charge carrier between the electrodes. The
I-/I3 system is favorable due to its stability and reversibility,
-
low visible light absorption, high diffusion constants, ability to
provide fast regeneration of the oxidized dye, and very low dark
current.4,5 Recently, some cobalt complexes of substituted
polypyridine ligands were presented as possible alternative
-
I3- + I2 f I2 - I- ... I2 f I2 ... I- - I2 f I2 + I3 (1)
-
mediators to the I-/I3 redox couple.6 An ideal regenerative
To investigate and develop new electrolyte materials for the
DNSCs, we have synthesized and studied trialkylsulfonium
iodides, differing in chain-length of the alkyl groups. Cations
with long-chained alkyl groups are more likely to form room
temperature molten salts than their short-lengthened congeners
because of less favorable packing in a crystalline lattice. In this
work, the overall solar-to-electric energy conversion efficiencies
PEC solar cell does not change the composition of the electrolyte
during illumination.7 At any rate, the use of liquid electrolytes
* To whom correspondence should be addressed. E-mail: larsa@
inorg.kth.se (L. Kloo). Tel: +46-8-790 9343, Fax: +46-8-790 9349.
† Royal Institute of Technology.
‡ Uppsala University.
10.1021/jp036859v CCC: $25.00 © 2003 American Chemical Society
Published on Web 11/15/2003

13666 J. Phys. Chem. B, Vol. 107, No. 49, 2003
Paulsson et al.
DodMeS
TABLE 1: Synthesized Trialkylsulfonium Iodides, (R2R′S)I and Their Melting Points
Me2S
Et2S
Pr2S
Bu2S
MeI
EtI
1-PrI
1-BuI
1-PeI
1-HxI
1-DodI
white solid (187 °C)
white solid (30 °C)
white solid (172 °C)
white solid (160 °C)
white solid (87 °C)
white solid (162 °C)
white solid (68 °C)
yellow liquid
white solid (96 °C)
yellow solid (112 °C)
white solid (70 °C)
white solid (85 °C)
orange liquid
yellow liquid
white solid (80 °C)
yellow solid (77 °C)
yellow liquid
white solid (68 °C)
white solid (46 °C)
white solid (120 °C)
white solid (133 °C)
yellow liquid
yellow liquid
TABLE 2: Results from the Elemental Analyses
in solar simulators, and also the incident monochromatic photon-
to-current-efficiencies (IPCE), have been measured for a selec-
tion of molten salt electrolytes to reveal their possible use as
electrolytes in DNSCs.
(Me2DodS)I
(Et2MeS)I
H
(Bu2MeS)I
C
H
S
C
S
C
H
S
theor result (%): 46.9 8.7 8.9 25.9 5.6 13.8 37.5 7.3 11.1
exp result (%): 46.7 8.7 9.0 25.0 5.7 13.2 36.5 7.3 10.9
Experimental Section
Ramascope was only used for solid crystals). Addition of iodine
resulted in two new peaks in the Raman spectra, 113 and 152
Synthesis. Trialkylsulfonium iodides, (R2R′S)I, were syn-
thesized from dialkylsulfides (Me2S, Et2S, Pr2S, Bu2S, DodMeS)
and alkyl iodides (MeI, EtI, 1-PrI, 1-BuI, 1-PeI, 1-HxI, 1-DodI).
This involves a nucleophilic attack by the sulfide on the
alkyliodide in a reaction such as:
cm-1, which indicate the presence of polyiodide ions.19-20 1
H
and 13C NMR spectroscopy was routinely used to check the
purity of the products. All NMR spectra were recorded on a
Bruker DMX 500 MHz spectrometer. Elemental analyses (C,
H, S) were performed for three selected products: (Bu2MeS)I,
(Et2MeS)I, and (Me2DodS)I. The experimental results cor-
respond very well to the theoretical values, see Table 2.
Conductivity measurements were performed at room tem-
perature in the frequency interval 1-10 000 Hz using a EG&G
PAR model 263 A potentiostate/galvanostate with impedance
capability and a conductivity cell with platinum electrodes. The
inner area of the cell was 2.75 × 5.9 mm2 with an electrode
R2S + R′I f (R2R′S)+ I-
(2)
About 5 or 10 g of the alkylsulfide was dissolved in 10-20
mL of acetone (dried) in a round bottle covered with aluminum
foil, since some of the reactants are light-sensitive. An equimolar
amount of the corresponding alkyl iodide was added. The
reaction atmosphere was protected from oxygen and water by
a continuous flow of dry nitrogen gas. The reaction mixture
was left for several days at ambient conditions. Addition of
diethyl ether resulted in two different kinds of product phases:
crystalline solid precipitates or liquid, molten products with
higher viscosity and density than the acetone/diethyl ether
solvent. The products were separated from the solvent followed
by thorough washing and recrystallizations in acetone and
diethyl ether. The final products were dried, and possible
remaining traces of solvent were slowly evaporated under
dynamic vacuum. The syntheses of the (R2R′S)I salts were
performed at room temperature. Higher reaction temperatures
were also tested, resulting in decreased yields of the products.
Most of the 22 compounds synthesized, all presented in Table
1, are white solids at room temperature. Some products from
syntheses with Et2S and Bu2S are yellow, viscous liquids
(molten salts). All pure products were stored in a freezer to
ensure that pure and noncontaminated electrolytes are studied.
By addition of iodine to the (R2R′S)I salts the viscosities were
decreased and the colors turned to different shades of purple-
red. Polyiodides were prepared from the trialkylsulfonium
iodides, without any additional solvents, by addition of iodine
in the following iodide-to-iodine ratios (100:1), (50:1), (10:1),
and (1:1). The reactants were heated to the melting point of the
appropriate trialkylsulfonium iodide. After slow cooling the
polyiodides obtained were stored in a freezer to ensure purity.
Structural Characterization and Physical Properties. The
melting points of the (R2R′S)I salts synthesized were determined
to confirm the degree of purity and also to verify the absence
of contaminants such as reactants. The (R2R′S)I salts were
structurally characterized by Raman and nuclear magnetic
resonance (NMR) spectroscopy. For Raman spectroscopy, two
kinds of equipments were used: Raman Bio Rad FTS 6000
equipped with an Nd:YAG laser (1064 nm), quartz beam splitter
and a liquid nitrogen Ge-detector, as well as a Renishaw
Ramascope with a diode laser (780 nm) and CCD detector (the
distance of 2 mm, which gave a cell constant of 1.23 cm-1
.
The electric conductivities of the molten salts (Et2MeS)I, (Bu2-
MeS)I, and (Bu2EtS)I were determined before and after the
addition of iodine. For the solid salt (Pr2MeS)I measurements
were feasible only after iodine addition. The results are given
in the Supporting Information, Table S1.
Current vs. Potential Characteristics. The photoelectric
conversion efficiency was experimentally obtained by measuring
the current-voltage characteristics of the solar cells at room
temperature. I/V diagrams were monitored and recorded using
a computerized Keithley model 2400 source measure unit. Two
different kinds of solar simulators were employed. A microwave-
powered sulfur plasma lamp, Light Drive 1000 provided an AM
1.5 spectrum with a light intensity of 100-1000 W/m2 (0.1-1
Sun). To investigate the possibility of applications for indoor
power supplies, the solar cells were also illuminated by
fluorescent light including UV wavelengths (170-200 Lux).
Two-electrode measurements were performed in air using an
open sandwich-type solar cell, which had not been sealed, with
a working electrode of dye-sensitized TiO2 and a counter
electrode of platinized conducting glass. The active electrode
area was typically about 0.8-1 cm2. The electrodes were
prepared following almost the same procedure first presented
by Nazeeruddin and co-workers in 1993.21-23 The nanostruc-
tured, porous film covering the working electrode was prepared
from a colloidal dispersion of TiO2 powder (Degussa P25 from
Solaronix S.A., Switzerland) deposited on a transparent, con-
ducting glass sheet (fluorine-doped SnO2). Treatment with a
newly developed press technique followed, not demanding any
thermal supply.23,24 Dye-sensitized electrodes were provided by
immersing the TiO2 electrodes, heated at 450 °C for 5-10 min
and then cooled to about 80 °C, into an 0.5 mM ethanolic dye
solution for at least 2 h. The dye used was N719, or Ru(dcbpy)2-
(CNS)2‚2TBA, with the full name cis-bis(isothiocyanato)-bis-
(2,2’-bipyridyl-4,4’-dicarboxylato)ruthenium(II)-bis-tetrabu-

Trialkylsulfonium Iodides in Solar Cells
TABLE 3: Best Results Obtained from I/V Measurements of DNSCs Using Different Light Sources as Solar Simulators
AM 1.5 (100 W m-2
fluorescent light (incl. UV) (170-200 Lux)
J. Phys. Chem. B, Vol. 107, No. 49, 2003 13667
)
electrolyte
prop
η (%)
FF (%)
V
OC (V)
JSC (mA cm-2
)
η (%)
FF (%)
VOC (V)
JSC (µA cm-2
)
(Et2MeS)I
(Et2MeS)I:I2
(Bu2MeS)I
2.0
2.8
1.5
3.7
5.0
5-6
58
60
59
62
66
60-70
0.57
0.49
0.63
0.60
0.64
0.6-0.7
0.6
1.0
0.4
1.0
11.8
1.0
0.5
0.1
1.7
0.6
47
31
44
55
0.38
20
10
54a
20
100:1
0.23
0.48
0.40
(Bu2MeS)I:I2
100:1
10:1
b
HMImI:I2
standard results for
50-60
0.45-0.55
15
a liquid electrolytec
a In irradiation of 176 Lux. b HMImI stands for 1-hexyl-3-methylimidazolium iodide. The irradiated area was 0.27 cm2 and the results can be
found in ref 14. c Results obtained from using an electrolyte of 0.5 M LiI, 50 mM I2 and 0.5 M 4-tert-butylpyridine in 3-metoxipropionitrile, at the
Dept. of Physical Chemistry, Uppsala University.
tyl- ammonium, provided by Solaronix S.A., Switzerland. 4-tert-
Butylpyridine was added to the electrodes and a silver conductive
paint applied on the part not covered with TiO2. The counter
electrode was composed of conducting glass, thermally plati-
nized and prepared from 5 mM H2PtCl6 in dry 2-propanol heated
at 380 °C for 10 min.23 The electrolyte was applied to the solar
cell assembly by using glass capillaries and then spread
thoughout the porous electrode structure. No complicated
injection techniques were required, since only open systems were
used.
Results and Discussion
Current vs Potential Characteristics. The performance of
the dye-sensitized solar cells is highly dependent on the structure
and the viscosity of the electrolytes used. The conducting
abilities of the (R2R′S)I compounds are also of importance. The
room-temperature molten salts (Et2MeS)I, (Bu2MeS)I, and (Bu2-
EtS)I showed good conductivities: about 6.8 mS/cm for (Et2-
MeS)I and 0.4 mS/cm for (Bu2MeS)I and (Bu2EtS)I, respectively
(Supporting Information, Table S1). They also provided the best
efficiency results obtained from the DNSC measurements, with
good short-circuit currents, photovoltages, and fill factors. By
using electrolytes of these ionic liquids in DNSCs, overall
energy conversion efficiencies of 2.0, 1.5, and 2.1% were
achieved for (Et2MeS)I, (Bu2MeS)I, and (Bu2EtS)I, respectively
(Table 3). It should be mentioned that all iodide liquids contain
a very small proportion of iodine, which guarantees the presence
of the iodide/triiodide redox couple in the electrolytes.
From I/V diagrams the short-circuit current (JSC), open-circuit
voltage (VOC ), fill factor (FF), and overall energy conversion
efficiency (η) could be determined using the definitions:
(JV)MAX
FF )
(3)
(4)
JSCVOC
JSCVOCFF
IS
η )
Addition of iodine in small proportions (100:1) made a
negligible impact on the conducting abilities. However, further
addition resulted in considerably improved conductivities, about
20-40 mS/cm for iodine proportions of (1:3). This could
theoretically be explained by enhancing the so-called Grotthus
effect, describing the mechanism of electrical conductivity in
polyiodide chains. The charge-carrier transfer mechanism influ-
ences the overall transport with increasing iodine concentra-
tions.19 The connection between higher contents of iodine in
the electrolyte and enhanced conducting abilities was found to
be highly correlated with high conversion efficiencies in the
solar cell measurements.
where IS is the intensity of the incident light (W m-2).5 The
effects of pyridine treatment were studied for some selected
electrolytes. To investigate the influences of temperature, a
furnace was constructed to which the solar cells were attached.
The furnace contained heating elements (power resistors) and
a Pt-100 sensor, which were regulated by a PID thermostate.
The studies were performed from ambient conditions up to
80 °C, using electrodes not treated with 4-tert-butylpyridine.
IPCE Measurements. Photocurrent yields can be analyzed
by the expression of the incident monochromatic photon-to-
current conversion efficiency (IPCE) based on:25
From the I/V studies an addition of iodine to the salts, in
small proportions (100:1), resulted in enhanced conversion
efficiencies. For (Bu2MeS)I, the overall conversion efficiency
of the solar cell increased from 1.5% (pure salt) to 3.7% with
JSC 1.0 mA/cm2, VOC 0.60 V, and FF 62% (see Figure 1).
Regarding (Et2MeS)I an increase from 2.0% (pure salt) to 2.8%
with JSC 1.0 mA/cm2, VOC 0.49 V, and an FF of 61% was
obtained. The improvement from iodine-doping of (Bu2EtS)I
was less noticeable, since the efficiency stabilized just above
2%. The improved efficiencies were related to an increase in
the current, whereas the FFs and the photopotentials were almost
unchanged. The iodine-containing electrolytes of (Et2MeS)I,
(Bu2MeS)I, and (Bu2EtS)I showed high efficiencies until the
proportion (10:1) was exceeded. Higher concentrations, up to
equimolar amounts, resulted in decreasing efficiencies. This is
in contrast to the conductivity results, which are greatly
improved at higher contents of iodine. An explanation could
be that iodine lowers the photoresponse at all wavelengths, thus
disturbing the light-absorbing ability of the dye-sensitized film.
Also, recombination losses of photogenerated electrons to
hcJph 1240Jph
IPCE )
)
(5)
eλIS
λIS
where Jph is the photocurrent density (Am-2), λ is the
wavelength of the monochromatic light (nm), and IS is the light
power density (W m-2). 1240 is the product of the constants h,
c, and e, thus Planck’s constant, the speed of light in vacuum,
and the elementary charge, respectively. IPCE was measured
as a function of illumination wavelength for a two-electrode
solar cell using a computer-controlled potentiostat. The light
source was a 300 W xenon arc lamp, whose beam passed
through an 80 mm water filter (to avoid IR light) into a high-
intensity monochromator proceeded by a UV cutoff filter and
a focusing quartz lens. The two-electrode cell was illuminated
through the substrate (F:SnO2)/electrode interface (SE), since
this has provided higher efficiencies than through the electrolyte/
electrode interface (EE).25 The effects from 4-tert-butylpyridine
treatment were also studied.

13668 J. Phys. Chem. B, Vol. 107, No. 49, 2003
Paulsson et al.
Figure 2. Best conversion efficiency results obtained from temperature-
dependent I/V measurements of DNSCs using electrodes not treated
with 4-tert-butylpyridine (in irradiation AM 1.5, 100 W m-2).
Figure 1. I/V diagram from using iodine-doped (Bu2MeS)I as
electrolyte in DNSCs, providing an efficiency of 3.7% (in irradiation
AM 1.5, 100 W m-2).
absorption of the dye is therefore inhibited in the presence of
polyiodide ions and thus lower overall conversion efficiencies
achieved.
electron scavengers such as triiodide ions will become more
prominent the higher the content of iodine is in the electrolytes.22
Increased Light Intensities. On a regular basis, the photo-
electrochemical I/V measurements were performed in AM 1.5
illumination and at a light intensity of 100 W m-2. For
comparison, also higher light intensities within the interval of
100-1000 W m-2 were used; see Supporting Information,
Figure S1 and Table S3. The overall conversion efficiency of
the solar cells, while using the molten salt electrolytes, decreased
with increasing intensities, due to impaired FFs. It should be
noted that the decrease was less prominent in comparison with
standard electrolytes based on organic solvents. However, the
JSC and VOC values were both improved at higher light
intensities, but low efficiencies result because of overloading.
Temperature-Dependent Measurements. Since the working
temperature of DNSCs is expected to be considerably higher
than room temperature, also the performance at higher temper-
atures was investigated. The effects of temperature variation
were studied in DNSCs from ambient conditions up to 80 °C
using electrodes not treated with 4-tert-butylpyridine. The results
obtained are presented in Supporting Information, Table S4. The
changes noticed regarding the overall efficiency, JSC, VOC, and
FF with temperature were just moderate and different trends
probably depending on the structures and the viscosities of the
electrolytes measured. DNSCs with electrolytes of the liquid,
molten salts (Et2MeS)I, (Bu2MeS)I, and (Bu2EtS)I showed good
overall conversion efficiencies within the temperature interval
studied, but their individual trends were quite different (Figure
2). While using (Et2MeS)I, with and without iodine, the highest
values of efficiency, VOC and JSC were achieved at room
temperature, followed by decreasing values with increasing
temperatures. More viscous electrolytes based on (Bu2MeS)I
and (Bu2EtS)I showed different trends in the changes of potential
and current where increased temperatures resulted in enhanced
currents (JSC), though the potentials (VOC) were unchanged or
slightly decreased. The highest overall efficiencies were achieved
at different temperatures, for (Bu2MeS)I 1.8% at 50 °C and for
(Bu2EtS)I 1.3% at 80 °C. The iodine-containing electrolytes of
(Bu2MeS)I and (Bu2EtS)I, with lower viscosities than the pure
salts, achieved the highest conversion efficiencies at room
temperature. However, the iodine-doped (Bu2EtS)I provided
The cell performance of DNSCs with molten salt electrolytes
consisting of 1-alkyl-3-methylimidazolium iodides (C1-C9)
and 1/10 molarity of iodine has been reported by Kubo and
co-workers.14 The highest overall conversion efficiency, 5%,
was obtained by using 1-hexyl-3-methylimidazolium iodide
(HMImI). In Table 3, these results are presented together with
the best results recorded for the molten sulfonium iodide salts
in this work.
(R2R′S)I salts that are solids at room temperature were
difficult to apply, which complicated the measurements, thus
resulting in poor performance. By addition of iodine the melting
points were decreased, but the efficiencies at room temperature
were almost unchanged. DNSCs with electrolytes of the solid
salt (Me2EtS)I with iodine added in the proportion (100:1)
achieved an overall conversion efficiency of 1.5%. Using iodine-
containing electrolytes of (Et2BuS)I and (Pr2MeS)I provided
overall efficiencies of 0.5-0.8%, and for (Me2DodS)I, (Bu2-
PrS)I, and (DodEtMeS)I about 0.1%. I/V measurements of
DNSCs with electrolytes of the rest of the solid salts were
unfortunately impossible to obtain under the prevailing condi-
tions, mainly because of too high melting points. More complete
data from the I/V measurements can be found in Supporting
Information, Table S2.
Using fluorescent light as solar simulator resulted in overall
decreased values, regarding efficiencies, JSC, VOC, and FFs, in
comparison with the AM 1.5 results; see Table 3. However,
this negative effect excluded (Bu2MeS)I, which also provided
the best result with JSC 54 µA/cm2, VOC 0.48 V and FF 44% in
an illumination of 176 Lux. Electrolytes of (Me2EtS)I, (Et2-
MeS)I, (Pr2MeS)I, and (Bu2MeS)I achieved the best results
while applied as pure salts, whereas an addition of iodine
resulted in decreased current values, which was opposite to the
results obtained from the I/V measurements at AM 1.5 and also
the conductivity results. The explanation could be related to
the absorption spectrum of triiodide ions formed in the iodine-
doped electrolytes. From UV/vis spectroscopy investigations it
has been reported that the triiodide ion has absorption bands
both at 290 and 367 nm.20 In fluorescent light illumination the

Trialkylsulfonium Iodides in Solar Cells
J. Phys. Chem. B, Vol. 107, No. 49, 2003 13669
Figure 4. IPCE action spectra for DNSCs from using electrolytes of
(Bu2MeS)I with and without iodine.
Figure 3. IPCE action spectra for DNSCs from using (Et2MeS)I,
(Bu2MeS)I, and (Bu2EtS)I electrolytes.
wavelengths of 520-540 nm, which coincides with the peak at
530 nm present in the spectra of the dye.4
almost as high efficiencies also at 70-80 °C, because of the
increase in currents with higher temperatures.
Addition of iodine to (Bu2MeS)I and (Bu2EtS)I indicate
increasing IPCE values at 520-540 nm; from 55 to 60% and
38 to 46%, respectively; see Figure 4. This prevailed until the
proportion (50:1) with respect to iodine was exceeded. These
results correlate very well with the results from the I/V
measurements and also the conductivity determinations. The best
IPCE results obtained, just above 60% at 520-540 nm, are
compatible with results from earlier studies where solution
electrolytes have been used.4
No shifts of the highest peaks at 520-530 nm could be
noticed in the IPCE spectra following the effects of iodine-
doping of the electrolytes. However, a noticeable decrease
appeared on the blue side of the spectra, at wavelengths below
530 nm. This also correlates well with the results obtained from
the fluorescent light measurements, where iodine-doping of the
electrolytes caused a decrease in conversion efficiencies. Ad-
dition of iodine in the proportions (100:1) to the electrolytes of
(Et2MeS)I resulted in decreased IPCE values at 520-540 nm,
from 55 to 48%, which is an anomaly according to the I/V
results. This decrease was even enhanced with higher propor-
tions of iodine.
Among the compounds that are solids at room temperature
(Me2EtS)I provided the best results, as the iodine-doped (Me2-
EtS)I in the proportion (100:1) achieved an overall conversion
efficiency of 0.8% at about 40 °C. Higher temperatures gave
lower efficiency values. Some electrolytes provided neglible
currents; however, their open-circuit potentials were fairly
good: 0.5-0.6 V for (Pr2MeS)I, and 0.2-0.5 V for (Me2PrS)I,
(Me2DodS)I, (Pr2EtS)I, (Pr2BuS)I, (Bu2PrS)I, and (DodEtMeS)I.
I/V measurements of DNSCs with electrolytes of the remaining
room-temperature solids were unfortunately impossible to fulfill,
mainly due to their high melting points.
In summary, low viscosities are correlated to higher efficien-
cies. It is a well-known fact that doping causes melting-point
suppression. This is of particular importance for systems, like
the sulfonium iodides in this study, where the properties are
studied close to the melting point. This is also the reason for
the observed increase in efficiency with temperature for
electrolytes with high viscosities (melting points) at room
temperature.
IPCE Measurements. IPCE studies were performed for
DNSCs at wavelengths from 400 to 800 nm using the molten
electrolytes (Et2MeS)I, (Bu2MeS)I, and (Bu2EtS)I, with and
without addition of iodine. It should be mentioned that our
results have not been corrected for possible reflection and
absorption losses due to the conducting glass substrate.
The IPCE results of (Et2MeS)I and (Bu2MeS)I are stable
above 55% from 400 to 550 nm and then decrease on the red
side of the spectra; see Figure 3. A threshold appeared at
4-tert-Butylpyridine Treatment. The effects from applying
4-tert-butylpyridine as an additive to the working electrode were
studied. From the I/V studies addition of 4-tert-butylpyridine
resulted in an increase in conversion efficiencies, thus indicating
enhanced currents, see Table 4. The greatest improvement was
noticed for iodine-containing (Bu2MeS)I in the proportion
(100:1), where the efficiency increased from 2.0 to 3.7% in the
presence of 4-tert-butylpyridine. The improvement, caused by
TABLE 4: Best Results Obtained from I/V Measurements of DNSCs Using Working Electrodes that Were Treated or Not with
4-tert-Butylpyridinea
addition of 4-tert-butylpyridine
no addition of 4-tert-butylpyridine
electrolyte
prop
η (%)
FF (%)
VOC (V)
JSC (mA cm-2
)
η (%)
FF (%)
VOC (V)
JSC (mA cm-2
)
(Et2MeS)I
2.0
2.8
1.5
3.7
2.1
2.2
58
60
59
62
56
62
0.57
0.49
0.63
0.60
0.61
0.52
0.6
1.0
0.4
1.0
0.6
0.7
1.7
1.8
1.4
2.0
0.6
1.2
65
54
67
42
55
55
0.69
0.42
0.59
0.53
0.74
0.68
0.4
0.8
0.3
0.9
0.2
0.3
(Et2MeS)I:I2
(Bu2MeS)I
(Bu2MeS)I:I2
(Bu2EtS)I
100:1
100:1
100:1
(Bu2EtS)I:I2
a In irradiation AM 1.5, 100 W m-2
.

13670 J. Phys. Chem. B, Vol. 107, No. 49, 2003
Paulsson et al.
ficiencies, photocurrents, and potentials, whereas the IPCE
results were moderately decreased. Concludingly, since the
molten salts provided promising capabilities as electrolytes in
DNSCs further investigations and optimizations of performance
will be pursued.
Acknowledgment. The Swedish National Research Council
is acknowledged for financial support. We thank Anna Holm-
stro¨m and Eva Magnusson at the Department of Physical
Chemistry, Uppsala University, for their kind help in preparing
the solar cells. Mårten Behm at the Department of Applied
Electrochemistry, Royal Institute of Technology, is acknowl-
edged for help with the conductivity measurements.
Supporting Information Available: Results for DNSCs
using (Bu2MeS)I electrolytes measured at different light intensi-
ties (Figure S1); results from conductivity measurements of
iodine-doped (Et2MeS)I, (Bu2MeS)I, and (Bu2EtS)I (Table S1);
results from IV measurements of DNSCs while using different
solar simulators (Table S2); results from IV measurements
performed at different light intensities (Table S3); results from
temperature-dependent I/V measurements of DNSCs (Table S4).
Figure 5. IPCE action spectra for DNSCs from using electrolytes of
(Et2MeS)I and electrodes that were treated or not with 4-tert-
butylpyridine.
References and Notes
iodine-doping the electrolytes of (Et2MeS)I and (Bu2MeS)I, was
more apparent while using electrodes treated with 4-tert-
butylpyridine. From literature, treatment of the TiO2 surface with
4-tert-butylpyridine may increase the voltage output of the solar
cell, whereas the current may decrease. Also, the rate constant
of the triiodide reduction is reduced, by preventing the loss of
conduction-band electrons to the electrolyte.4,22 From our I/V
studies, it was noticed that the JSC was consequently increased
in the presence of 4-tert-butylpyridine and thus caused improved
conversion efficiencies. The photovoltages were almost indif-
ferent to the addition of 4-tert-butylpyridine to the electrodes.
Good VOC’s of about 0.5-0.7 V were achieved with as well as
without 4-tert-butylpyridine present in the solar cells.
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Conclusions
The room temperature molten salts of (Et2MeS)I, (Bu2MeS)I,
and (Bu2EtS)I showed good conducting abilities at ambient
conditions. They also provided good light-to-electricity conver-
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provided overall conversion efficiencies of 2.8, 3.7, and 2.2%,
respectively. The liquid electrolytes of (Et2MeS)I provided the
highest conversion efficiencies at room temperature. By heating
treatment the viscosities of (Bu2MeS)I and (Bu2EtS)I were
decreased and thus higher overall efficiencies were obtained
above room temperature. The effects from 4-tert-butylpyridine
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