Stable wire-shaped dye-sensitized solar cells based on eutectic melts

Article abstract of DOI:10.1039/c3ta13714g

Wire-shaped dye-sensitized solar cells (DSCs) have recently attracted increasing interest due to their unique and promising advantages, such as being lightweight, deformable and weavable compared to conventional planar structures. However, the use of liquid electrolytes based on organic solvents has largely limited their practical applications, e.g., low mechanical and thermal stabilities. Solid-state electrolytes are resistive to these problems in nature, and have been shown to have high stability. Herein, eutectic melts, for the first time, have been introduced as the electrolyte to develop novel wire-shaped DSCs. The resulting wire-shaped DSCs were flexible and highly stable thermally and their energy conversion efficiencies could recover entirely after heating up to 110 °C.

Full text of DOI:10.1039/c3ta13714g

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Materials Chemistry A
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Stable wire-shaped dye-sensitized solar cells based
on eutectic melts†
Cite this: J. Mater. Chem. A, 2014, 2,
3841
Houpu Li, Zhibin Yang, Longbin Qiu, Xin Fang, Hao Sun, Peining Chen, Shaowu Pan
*
and Huisheng Peng
Wire-shaped dye-sensitized solar cells (DSCs) have recently attracted increasing interest due to their unique
and promising advantages, such as being lightweight, deformable and weavable compared to conventional
planar structures. However, the use of liquid electrolytes based on organic solvents has largely limited their
practical applications, e.g., low mechanical and thermal stabilities. Solid-state electrolytes are resistive to
these problems in nature, and have been shown to have high stability. Herein, eutectic melts, for the first
time, have been introduced as the electrolyte to develop novel wire-shaped DSCs. The resulting wire-
shaped DSCs were flexible and highly stable thermally and their energy conversion efficiencies could
recover entirely after heating up to 110 ꢀC.
Received 16th September 2013
Accepted 12th December 2013
DOI: 10.1039/c3ta13714g
www.rsc.org/MaterialsA
Ionic liquids have been widely studied for various electronic
devices due to their high ionic conductivities and wide elec-
trochemical windows.11 In addition, they show a near-to-zero
Introduction
It is not an exaggeration to say that the world will endure a
global energy crisis in the near future. The foreseeable
exhaustion of fossil fuels has boosted the development of
alternative energy sources. As one of the promising candidates,
solar power has hitherto attracted massive amounts of attention
and thus inspired the exploitation of photovoltaic devices.1 The
traditional silicon-based technology has confronted serious
challenges as the heavy bulky packages are impractical for the
ever-growing requirements of portable, exible and low-cost
devices.
Emerging in the 1990s, dye-sensitized solar cells (DSCs) have
matured and ourished considerably over the last decade.2
Nevertheless, the conventional planar structure may still limit
their application in the upsurging elds of weavable and
stretchable photovoltaic devices.3 To this end, novel wire-sha-
ped DSCs were proposed and typically fabricated by intertwin-
ing two ber electrodes together to form a wire-shaped
structure.4–8 For a typical fabrication, the intertwined ber
electrodes are rst inserted into a transparent tube, followed by
injecting the liquid electrolytes with organic solvents and seal-
ing at two ends.9 The liquid electrolytes with organic solvents
suffer from leakage and low stability during use, which is
unfavourable for broad applications.10 The required bending or
other deformations in portable electronic devices make the
problem even more severe. Therefore, improvement of the
stability is critically important for the wire-shaped DSC.
vapor pressure, and are therefore immune to the problem of
vaporization. The combined remarkable electrochemical and
thermal properties make them suitable for promising applica-
tions in DSCs.12,13 Due to specic pendant groups such as ethyl
and regular structures, the ionic liquids that are solid at room
temperature are named ionic liquid crystals.14 Such ionic liquid
crystals are expected to replace the conventional liquid elec-
trolytes to obviate the longstanding leakage problem.10,15 These
ionic liquids, particularly ionic liquid crystals, may also repre-
sent a general and effective strategy to resolve the remaining
problems of wire-shaped DSCs, although no reports are avail-
able in this eld.
Herein, to the best of our knowledge, we rst report a novel
wire-shaped DSC based on eutectic melts, a mixture of ionic
liquid crystal and ionic liquid, as electrolytes. The resulting
wire-shaped DSCs are exible, highly stable thermally and have
energy conversion efficiencies that can entirely recover aer
ꢀ
heating up to 110 C.
Experimental section
(1) Synthesis of 1-ethyl-3-methylimidazolium iodide (EMII)
The ionic liquid crystal, EMII, was synthesized according to the
following procedure.16 In a three-necked ask purged with
nitrogen, 0.1 mol (7.97 mL) 1-methylimidazole was dissolved in
15 mL of acetonitrile, and 0.1 mol (8.08 mL) ethyl iodide was
then added dropwise under vigorous stirring over 1 h. The
mixture was stirred for another 24 h at room temperature, fol-
lowed by adding 100 mL ethyl acetate under vigorous stirring.
White crystals were precipitated immediately. The resulting
State Key Laboratory of Molecular Engineering of Polymers, Department of
Macromolecular Science, Laboratory of Advanced Materials, Fudan University,
Shanghai 200438, China. E-mail: penghs@fudan.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ta13714g
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product was collected by ltering membranes, and then washed synthesized by chemical vapor deposition.19 The required elec-
with ethyl acetate and diethyl ether, for three times each. The trode was then prepared by twisting ve CNT yarns into a larger
yield was ꢁ70%. The product was characterized by H1-NMR ber with a diameter of 45 mm. Platinum nanoparticles were
(d/ppm): 1.56 (t, 3H), 4.10 (s, 3H), 4.46 (q, 2H), 7.85 (br d, 2H), deposited onto the CNT yarns by an electrochemical process.20
9.68 (s, 1H).
To achieve a high load of platinum nanoparticles on the CNT
substrate, twenty repeated deposition steps (double potential
step chronoamperometry) were generally used.
(2) Synthesis of 1-propyl-3-methylimidazolium iodide (PMII)
The synthesis of PMII was similar to that of EMII.16 In a three-
necked ask purged with nitrogen, 0.1 mol (7.97 mL) 1-meth-
(6) Characterization
ylimidazole was dissolved in 15 mL isopropanol, and 0.1 mol H1-NMR spectra were obtained from a Varian Mercury plus 400
(9.71 mL) propyl iodide was then added dropwise with vigorous MHz NMR spectrometer with acetone-D6 as the solvent. The
stirring over 1 h. The mixture was then stirred for another 24 h structures were characterized by scanning electron microscopy
at 80 ꢀC and cooled down to room temperature, followed (Hitachi FE-SEM S-4800 operated at 1 kV). The conductivities
by washing with ethyl acetate and diethyl ether, for three times were measured in a cell that had been made from two uorine-
each. The solvent was vaporized and the product was doped tin oxide glasses and a Surlyn lm with a thickness of
dried under vacuum for three days with a yield of ꢁ70%. The 60 mm (DHS-SN1760). A cuboid space with both a length and
resulting product was also characterized by H1-NMR (d/ppm): width of 5 mm and a thicknessꢀof 60 mm was formed in the cell.
0.94 (t, 3H), 1.97 (sx, 2H), 4.10 (s, 3H), 4.40 (q, 2H), 7.90 (br d, The melted electrolyte at 120 C was injected into the cuboid
2H), 9.69 (s, 1H).
cell through an ultrasound-drilled pinhole in one uorine-
doped tin oxide glass. The transmittances were measured based
on these cuboid cells by a SHIMADZU UV-vis spectrophotom-
eter (UV-2550). Thermal analysis was performed on a TA DSC
Q2000 differential scanning calorimeter at a heating rate of
(3) Preparation of eutectic melts
Different ratios of PMII and iodine in the electrolyte leads to
different ratios of Iꢂ and I3ꢂ, thus affecting the energy level of
the electrolyte, which, in the end, leads to a trade-off between
the voltage and current density for the solar cell.17 In this work,
we chose an optimized ratio of 24/1 according to the literature.18
Ionic liquid PMII (6.05 g, 24 mmol) and iodine (0.254 g, 1 mmol)
were mixed and dissolved in acetonitrile in a 10 mL volumetric
ask, obtaining a mixture with a PMII–iodine mole ratio of 24/1.
1 g of EMII and a certain amount of PMII–iodine solution (e.g.,
0.158 mL for 10%, 0.237 mL for 15%, etc.) were mixed and
ultrasonically treated for 30 min. The mixture was stirred for 8 h
and then heated to 80 ꢀC overnight, followed by drying in
vacuum to remove the acetonitrile. The eutectic melts appeared
dark purple in colour.
ꢂ1
ꢀ
ꢀ
10 C min in a temperature range of ꢂ50 to 150 C in a N2
atmosphere. Optical micrographs were taken by an Olympus
BX51 microscope with a DP72 image capturer. J–V curves were
produced by a Keithley 2400 Source Meter under illumination
(100 mW cmꢂ2) of simulated AM1.5 solar light generated from a
solar simulator (Oriel-Sol3A 94023A equipped with a 450 W Xe
lamp and an AM1.5 lter). The light intensity was calibrated
using a reference Si solar cell (Oriel-91150). The energy
conversion efficiencies were calculated by multiplying the
length and diameter of the working electrode to be comparable
with the available reports based on the same conditions.3–5,9,20,21
For the used mask that had the same length with a width that
was larger by 50% compared with the wire-shaped DSC, the
efficiencies may be overestimated by 33%. The effective areas of
the wire-shaped DSCs were calculated by multiplying the
(4) Preparation of working electrodes
The working electrode has been previously reported.7 To exposed length and diameter of the working electrodes, which
summarize, titanium wires (Alfa-Aesar, diameter of 0.005 inch is a generally accepted method.3–5,9,20,21
and purity of 99.9%) were anodized in an ethylene glycol solu-
tion containing 0.3 wt% NH4F and 8 wt% H2O at a bias voltage
Results and discussion
of 60 V. The anodized titanium wire was then annealed at 500 ꢀC
in air for 60 min. Aer that, the annealed titanium wire was
treated by 100 mM TiCl4 solution and annealed again at 450 ꢀC
in air for 30 min to increase the roughness of the TiO2 surface.
The eutectic melts were prepared by mixing ionic liquid crystals
of EMII with an amount of ionic liquid of PMII and iodine
(Scheme 1). Here EMII offers the ionic conductivity while PMII
ꢀ
Subsequently, the working electrode was heated to 80 C aer
the second annealing and then soaked in 0.3 mM N719 solution
in a solvent mixture of dehydrated acetonitrile and tert-butanol
(volume ratio of 1/1) for 12 h to absorb dye.
(5) Preparation of ber electrodes
Platinum (Pt) wires with a diameter of 25 mm (Alfa-Aesar, purity
99.95% Metals Basis) were used without treatment. Carbon
nanotube (CNT) yarns with an average diameter of 20 mm were
Scheme
1 Chemical structures of 1-ethyl-3-methyl imidazolium
iodide (EMII, left) and 1-propyl-3-methyl imidazolium iodide (PMII,
dry-drawn from spinnable CNT arrays which had been right).
3842 | J. Mater. Chem. A, 2014, 2, 3841–3846
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provides iodide anions (Iꢂ) for the redox couple, and iodine
generates triiodide ions (I3ꢂ) through reaction with iodide
anions. During the preparation of the eutectic melts, PMII and
iodine were previously mixed with a molar ratio of 24/1,18 and
the resulting PMII–iodine mixture was then added to the EMII
to form a uniform mixture. A series of eutectic melts could be
produced by varying the PMII–iodine weight percentage in the
nal mixture. Note that the molar ratio of PMII to iodine was
maintained as 24/1. The eutectic melts remained solid when the
PMII–iodine weight percentage was less than 40% and became
liquid at 40% or higher (Fig. 1).
Thermal analysis was carried out to compare the solid
eutectic melts with different PMII–iodine weight percentages
(Fig. 2). As indicated, the melting point was 80 ꢀC for pure EMII.
When adding the PMII–iodine mixture, the melting points
Fig. 2 Thermal analysis of EMII before and after addition of the PMII–
iodine mixture with increasing the weight percentage. The number
above the line corresponds to the weight percentage.
gradually decreased. For example, 75 ꢀC for 10 wt% of PMII–
ꢀ
iodine and 60 C for a higher weight percentage of 35%. The PMII–iodine, the lowest transmittance was observed, indicating
peak sizes also lessened with the increasing PMII–iodine
mixture, meaning the added doping materials undermined the
EMII crystallization. As expected, no peak was observed for the
eutectic melts at the critical weight percentage of 40% when
they became liquids. Fig. 3 shows the optical micrographs of the
eutectic melts with different PMII–iodine weight percentages. It
is evident that the crystalline grains got smaller when more
PMII–iodine components were introduced, suggesting the
added PMII–iodine components as ion providers largely
reduced sizes of the crystalline grains, which was consistent
with the results in Fig. 2. The size changes of the crystalline
grains were not obvious at higher weight percentages beyond
25%.
the strongest scattering effect that was favorable for the light
harvest in DSCs.22 For the eutectic melts containing other
weight percentages of PMII–iodine, the resulting crystalline
grains were either too large or small, which diminished the
scattering effect.
Fig. S1† shows the optical transmittances of pure EMII and
the eutectic melts with different PMII–iodine weight percent-
ages. The pure EMII showed a relatively low transmittance
suggesting a very strong scattering effect ascribed to the crys-
tallization. The transmittance of 1 M EMII solution in acetoni-
trile was nearly 100%, while the solid EMII was only 40%. When
adding the PMII–iodine mixture, the transmittances decreased
in the rst place and then slightly increased, but the trans-
mittances were all lower than 40% in the wavelength range of
500 to 800 nm, where there should be no absorption for any
component of the electrolyte. The loss in the transmittances
should hence be attributed to the scattering effect of crystalline
grains in the electrolytes. For the eutectic melts with 20 wt%
Fig. 3 Optical micrographs of EMII before and after addition of a
Fig. 1 Photographs of pure EMII and eutectic melts with increasing PMII–iodine mixture (the same molar ratio of 24/1) with increasing the
PMII–iodine weight percentages from 10% to 40%. The labeled weight percentage from 10% to 40%. The numbers in the top left
number on the container corresponds to the weight percentage.
corners correspond to the weight percentage.
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The conductivity of pure EMII was unsatisfactory for the
restricted ion diffusion, but PMII exhibited a high conductivity.
The conductivity of eutectic melts increased with the addition of
the PMII–iodine mixtures. Fig. S2† shows that the conductivity
of eutectic melts was elevated by appropriately two times when
adding 35 wt% of the PMII–iodine.
To fabricate wire-shaped DSCs, titanium wires were rst
grown with a layer of aligned titania nanotubes on the surface
by an electrochemical anodization method. The titania nano-
tubes were grown perpendicularly to the Ti surface (Fig. S3†),
and their lengths can be controlled by varying the growth time.23
In this study, titanium wires were modied with 30 mm titania
nanotubes (grown in 6 hours). The titania nanotube-modied Ti
wire was immersed into N719 solution for dye absorption and
used as the working electrode in DSCs. The counter electrode
ber such as the Pt wire was twisted onto the working electrode
(Fig. 4a). The other bers, like bare CNT and the CNT/Pt
composite bers, were also employed as counter electrodes
Fig. 5 J–V curves of wire-shaped DSCs with increasing PMII–iodine
mixture weight percentages from 10% to 40% in eutectic melts. Pt wire
with a diameter of 25 mm was employed as the counter electrode to
perform these tests.
ꢂ
the Iꢂ anions, oxidizing the Iꢂ anions into I3 anions, and the
dye is regenerated.
ꢀ
(Fig. S4†). The eutectic melts were heated to 120 C to form a
ꢂ
2dye+ + 3Iꢂ / 2dye + I3
(1)
melt and injected into a uorine-doped ethylene propylene
copolymer tube that was transparent and exible. The electro-
lyte was maintained at 80 C on a heating stage, and the inter-
ꢀ
ꢂ
twined two ber electrodes were inserted through the
transparent uorinated ethylene propylene (FEP) tube, followed
by sealing at two ends with an ultraviolet curing adhesive. The
eutectic melts were inltrated between the working and counter
electrodes (Fig. 4b–d).
The photoelectric performances of the wire-shaped DSCs
based on the eutectic melts with different weight percentages of
PMII–iodine as electrolytes are shown in Fig. 5. When a DSC is
illuminated, the dye (N719) molecule harvests photons and gets
excited. The excited dye molecules liberate electrons and inject
them to the conducting band of TiO2, then electrons are
transferred to the working electrode and go through the out
circuit succeedingly, leaving a hole in the dye (in other words,
the dye is oxidized). Then the dye cations capture electrons from
Aerwards, the I3 anions diffuse to the counter electrode
and obtain electrons from the counter electrode, reducing into
Iꢂ anions.
Pt or CNT
I3ꢂ þ 2e
3Iꢂ
(2)
ƒƒƒƒƒƒ!
Considering the above mentioned mechanism, the concen-
tration of Iꢂ anions will be a major determinant for the regen-
eration of N719 dye. Before the concentration of the redox
couple reaches a critical value for full regeneration of the dye
molecules, the JSC will increase with the increasing proportion
ꢂ
of Iꢂ/I3 anions. Above the critical value, the amount of Iꢂ
anions is adequate to regenerate the dye molecules. However,
the light absorption of iodine diminishes the illumination
intensity, leading to the decline of JSC, as shown in Fig. 5.
The energy conversion efficiencies were rst increased with
the increasing PMII–iodine weight percentage from 10% to
20%, since the conductivities were improved. With the further
increase of the weight percentage to 40%, the energy conversion
efficiencies were gradually decreased as a result of more light
absorption of the iodine. The maximal energy conversion effi-
ciency of 3.51% appeared at the PMII–iodine weight percentage
of 20% with the open-circuit voltage of 606 mV, short-circuit
current density of 9.51 mA cmꢂ2 and ll factor of 60.9%.
It should be noted that the cells based on eutectic melts
show a relatively lower VOC compared with the acetone-based
electrolyte duꢂe to their different energy levels caused by
different Iꢂ/I3 ratios. According to the Nernst equation, the
energy level of the Iꢂ/I3 redox couple may be changed by
ꢂ
varying the concentration ratio of the Iꢂ and I3 anions:
ꢂ
RT
nF
½Oxꢃ
E ¼ E0 þ
ln
(3)
½Redꢃ
Fig. 4 SEM images of the wire-shaped DSC. (a) Without electrolyte.
(b–d) After incorporation of the eutectic melts at different magnifi-
cations. Pt wire with a diameter of 25 mm was employed as the counter
electrode.
where E0, R, T, n, and F correspond to the standard half-cell
reduction potential of Iꢂ/I3ꢂ, ideal gas constant, temperature in
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the Kelvin scale, mole of electrons transferred in the cell reac- both liquid electrolytes and eutectic melts in the wire-shaped
tion or half-reaction and Faraday constant. In the case of DSCs, DSC. However, for the liquid electrolyte containing organic
there was a trade-off between JSC and VOC. A lower PMII–iodine solvent, like acetonitrile, that may be vaporized and leaked
ratio caused a positive shi in the redox potential of the elec- during heating, particularly above the boiling point, the energy
trolyte,24 leading to a lower VOC but higher JSC. The ratio of 24/1 conversion efficiencies cannot revert to the original values aer
was found to achieve the highest efficiency, similar to the other cooling down to low temperatures. In other words, the wire-
reports to maximize the efficiency through a trade-off between shaped DSCs based on the liquid electrolyte cannot be effectively
25,26
VOC and JSC
.
In addition, the energy conversion efficiencies used at high temperatures. In contrast, the efficiency derived
could also be improved by fabricating the wire-shaped DSC on from the eutectic melts was entirely recovered aer heating up to
different substrates, e.g., 4.22% and 4.67% on mirror and paper, 100 ꢀC and then cooling down to 30 ꢀC. Fig. 6d further shows that
respectively (Fig. S5†).
the wire-shaped DSC based on the eutectic melts can be stably
To elucidate the obtained short-circuit current density in the operated aer repeated heating and cooling for many cycles. In
wire-shaped DSC, the incident photon-to-electron conversion stark contrast, the wire-shaped DSC based on the conventional
efficiency (IPCE) of the device using electrolyte with 20 wt% liquid electrolyte reluctantly maintained the energy conversion
PMII–iodine was measured as a function of the wavelength efficiency by less than 10% aer only two cycles.
(Fig. S6†). A maximum of ꢁ75% was obtained at a wavelength of
As for the wire-shaped DSCs based on the eutectic melts and
530 nm. Note that the eutectic melts were also applicable to the liquid electrolytes, the efficiency evolution was traced respec-
other ber electrodes, apart from Pt wires, such as CNT bers tively over a period of 30 days under the same conditions
and CNT/Pt composite bers (Fig. S7 and S8†).
(Fig. 7). For the eutectic melts, the solar cell maintained an
Although the energy conversion efficiency is relatively lower energy conversion efficiency of 90%, while for the liquid elec-
than 6.58% for the case of a liquid electrolyte (Fig. S9†), Fig. 6 trolyte, only 8% remained. It should be noticed that these tests
implies that the stability of a DSC with eutectic melts is much were carried out using cells with makeshi encapsulation,
higher than that of a liquid electrolyte. Fermi levels of the similar to other published work.6 The electrolyte would corrode
photovoltaic materials were clearly changed under heating, the adhesives in the ends of the encapsulation tube, resulting in
correspondingly, the open-circuit voltages rapidly declined when pinholes where solvents of electrolyte may leak, which would
heated from 30 to 110 ꢀC (Fig. 6a). However, the diffusion of ions lead to a quick failure of conventional liquid electrolyte DSCs.
in electrolytes was enhanced at higher temperatures to increase In addition, the wire-shaped DSCs based on the eutectic melts
the short-circuit current densities (Fig. 6b). Overall, the energy were also exible. Fig. 8 shows that the energy conversion
conversion efficiencies typically decreased with the increasing efficiencies were well maintained at a bending curvature of
temperature (Fig. 6c). The above phenomena were consistent for 3.3 cmꢂ1
.
Fig. 6 A comparison of the thermal stability of the wire-shaped DSCs based on liquid electrolyte and the eutectic melts electrolyte with 20 wt%
PMII–iodine (molar ratio of PMII and iodine, 24/1). (a–c). Dependence of VOC, JSC, and h/h0 on the temperature, respectively. (d) h/h0 values at
30 ꢀC and 100 ꢀC. The wire-shaped DSCs were repeatedly heated to 100 ꢀC and then cooled down to 30 ꢀC. h and h0 correspond to the energy
conversion efficiencies of the as-fabricated wire-shaped DSC and the one after heating treatment. Pt wire with a diameter of 25 mm was
employed as the counter electrode to perform these tests.
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also want to thank B. Zhang and Z. Chen for their help with the
thermal analysis.
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3846 | J. Mater. Chem. A, 2014, 2, 3841–3846
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