Room temperature solid-state synthesis of a conductive polymer for applications in stable I2-free dye-sensitized solar cells

Article abstract of DOI:10.1002/cssc.201200349

A solid-state polymerizable monomer, 2,5-dibromo-3,4- propylenedioxythiophene (DBProDOT), was synthesized at 25°C to produce a conducting polymer, poly(3,4-propylenedioxythiophene) (PProDOT). Crystallographic studies revealed a short interplane distance between DBProDOT molecules, which was responsible for polymerization at low temperature with a lower activation energy and higher exothermic reaction than 2,5-dibromo-3,4- ethylenedioxythiophene (DBEDOT) or its derivatives. Upon solid-state polymerization (SSP) of DBProDOT at 25°C, PProDOT was obtained in a self-doped state with tribromide ions and an electrical conductivity of 0.05 S cm^-1, which is considerably higher than that of chemically- polymerized PProDOT (2×10^-6 S cm^-1). Solid-state ¹³C NMR spectroscopy and DFT calculations revealed polarons in PProDOT and a strong perturbation of carbon nuclei in thiophenes as a result of paramagnetic broadening. DBProDOT molecules deeply penetrated and polymerized to fill nanocrystalline TiO₂ pores with PProDOT, which functioned as a hole-transporting material (HTM) for I₂-free solid-state dye-sensitized solar cells (ssDSSCs). With the introduction of an organized mesoporous TiO₂ (OM-TiO₂) layer, the energy conversion efficiency reached 3.5 % at 100 mW cm^-2, which was quite stable up to at least 1500 h. The cell performance and stability was attributed to the high stability of PProDOT, with the high conductivity and improved interfacial contact of the electrode/HTM resulting in reduced interfacial resistance and enhanced electron lifetime.

Full text of DOI:10.1002/cssc.201200349

DOI: 10.1002/cssc.201200349
Room Temperature Solid-State Synthesis of a Conductive
Polymer for Applications in Stable I2-Free Dye-Sensitized
Solar Cells
Byeonggwan Kim, Jong Kwan Koh, Jeonghun Kim, Won Seok Chi, Jong Hak Kim,* and
Eunkyoung Kim*[a]
A solid-state polymerizable monomer, 2,5-dibromo-3,4-propyle-
nedioxythiophene (DBProDOT), was synthesized at 258C to
produce a conducting polymer, poly(3,4-propylenedioxythio-
phene) (PProDOT). Crystallographic studies revealed a short in-
terplane distance between DBProDOT molecules, which was
responsible for polymerization at low temperature with
a lower activation energy and higher exothermic reaction than
2,5-dibromo-3,4-ethylenedioxythiophene (DBEDOT) or its deriv-
atives. Upon solid-state polymerization (SSP) of DBProDOT at
258C, PProDOT was obtained in a self-doped state with tribro-
mide ions and an electrical conductivity of 0.05 Scmꢀ1, which
is considerably higher than that of chemically-polymerized
PProDOT (2ꢀ10ꢀ6 Scmꢀ1). Solid-state 13C NMR spectroscopy
and DFT calculations revealed polarons in PProDOT and
a strong perturbation of carbon nuclei in thiophenes as
a result of paramagnetic broadening. DBProDOT molecules
deeply penetrated and polymerized to fill nanocrystalline TiO2
pores with PProDOT, which functioned as a hole-transporting
material (HTM) for I2-free solid-state dye-sensitized solar cells
(ssDSSCs). With the introduction of an organized mesoporous
TiO2 (OM-TiO2) layer, the energy conversion efficiency reached
3.5% at 100 mWcmꢀ2, which was quite stable up to at least
1500 h. The cell performance and stability was attributed to
the high stability of PProDOT, with the high conductivity and
improved interfacial contact of the electrode/HTM resulting in
reduced interfacial resistance and enhanced electron lifetime.
Introduction
Conducting polymers have received considerable attention for
various applications such as photovoltaic cells, organic field-
effect transistors (OFETs), organic light-emitting diodes
(OLEDs), and sensors because of their controllable conductive
and optical properties.[1–7] Among these, heterocyclic conduct-
ing polymers have become the most important materials
owing to their high conductivity, easy modification, and good
processability.[8–10] There are various polymerization methods
used for the synthesis of heterocyclic conducting polymers:
1) oxidative polymerization, including chemical polymerization
with an oxidant, electrochemical polymerization,[11–13] vapor-
phase polymerization,[14,15] and solution casting polymeri-
zation;[16,17] 2) chemical coupling polymerization with a cata-
lyst;[18] and 3) solid-state polymerization (SSP).[19] Although oxi-
dative and chemical coupling polymerizations have been ex-
tensively researched, SSP has rarely been reported.[20–25] SSP is
a unique synthetic method for well-ordered, highly conductive
polymers in films or bulk states without catalysts or solvent.
In this regard, new polymerization methods with new materials
are very important in materials science and fabrication because
they directly affect the electrical and optical properties of the
resultant polymers for desired applications. In SSP, polymeri-
zation occurs in crystalline states and thus short distances be-
tween the monomers are important to lower the activation
energy for polymerization. Thus, dihalogenated heterocyclic
monomers are a good choice of monomer for SSP owing to
their crystalline nature and the short distance between mole-
cules in their crystalline state, which ensures an exothermic re-
action with low activation energy for polymerization. The con-
ductivity, crystallinity of the resultant polymer, and polymeri-
zation rate are controlled by the polymerization temperature.
As dihalogenated heterocyclic monomers can easily penetrate
nanopores, they are important tools in the formation of hole
transporting channels in dye-sensitized solar cells (DSSCs).[26,27]
DSSCs represent a credible alternative to conventional crys-
talline silicon solar cells owing to their high energy conversion
efficiency (>10%) and low production costs.[28–33] There has
also been considerable interest in developing I2-free solid-state
DSSCs (ssDSSCs) using hole-transporting materials (HTMs)
owing to the potential to decrease the overall weight of the
cells and provide long-term durability and flexibility.[34–43] The
presence of I2 can decrease the durability and photocurrent of
the devices owing to the sublimation of I2 and incident light
absorption by I2. Furthermore, I2 can act as an oxidizing agent,
which corrodes the metals, such as grid metal Ag and Pt, of
the counter electrode. An I2-containing quasi-solid-state DSSC
[a] B. Kim, J. K. Koh, Dr. J. Kim, W. S. Chi, Prof. J. H. Kim, Prof. E. Kim
Active Polymer Center for Pattern Integration
Department of Chemical and Biomolecular Engineering
Yonsei University
50 Yonsei-ro, Seodaemun-gu, Seoul 120-749 (South Korea)
Fax: (+82)2-312-6401
E-mail: jonghak@yonsei.ac.kr
eunkim@yonsei.ac.kr
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201200349.
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J. H. Kim, E. Kim et al.
with the N719 dye showed poor stability (35% decrease of
energy conversion efficiency) after one week at 808C.[30]
Recently, we reported the preparation of a high efficiency (ca.
6.8% at 100 mWcmꢀ2), I2-free ssDSSCs using solid-state poly-
merizable conductive polymers as effective HTMs.[26,27] The so-
lution of dihalogenated heterocyclic monomers deeply pene-
trated the nanopores of the TiO2 layer to form a well-packed
nanostructure after solvent evaporation. Upon heating at
608C, SSP resulted in improved interfacial contact properties
between TiO2 and the conducting polymers.
energy, and higher exothermic reaction for polymerization
compared to DBEDOT. Herein, we report the synthesis of a mo-
nomer and its crystallographic analysis, polymerization at room
temperature inside nanopores of TiO2, and application to long-
term stable I2-free ssDSSCs for the formation of HTMs.
Results and Discussion
Solid-state polymerization (SSP)
In our previous work,[26,27] 2,5-dibromo-3,4-ethylenedioxy-
thiophene (DBEDOT) and 2,5-dibromo-3,4-ethylenedithiathio-
phene (DBEDTT) monomers reported by Meng et al.[19] were
used to produce conducting polymers as HTMs, poly(3,4-ethyl-
enedioxythiophene) (PEDOT) and poly(3,4-ethylenedithiathio-
phene) (PEDTT), respectively (Scheme 1a). However, these
Careful control of crystal formation is of pivotal importance in
solid-state polymerizable monomers because the crystal forms
can directly affect the electrical and optical properties of the
resultant conducting polymers. Here, DBProDOT was synthe-
sized by a facile synthetic method (Scheme 1b), which in-
volved the bromination of 3,4-propylenedioxythiophene
(ProDOT) and recrystallization from ethanol. The DBProDOT
crystals (0.1–0.5 g) were incubated at room temperature (258C)
for five days in a closed vial for the SSP synthesis of PProDOT.
In the PProDOT-X notation, X represents reaction temperature.
To investigate the crystalline properties, XRD studies were
performed on DBProDOT and PProDOT-25 (Figure 1a). The XRD
pattern of DBProDOT showed sharp and strong diffraction
peaks at 2q=12.8, 13.2, 20.1, 20.7, 21.6, 24.0, 24.4, 27.8, 30.4,
and 37.38, which indicate a well-ordered crystalline structure.
The diffraction peak at 24.08 corresponds to a d spacing of
3.70 ꢂ, which is assigned to the interplane distance between
DBProDOT monomers in the stack. Although this value is
slightly larger than that of DBEDOT (3.50 ꢂ), it is among the
shortest distances between molecules.[20] Furthermore, the in-
terplane distance between DBProDOT monomers is similar to
Scheme 1. (a) Solid-state polymerizable monomer structures of DBEDOT and
DBEDTT. (b) Synthesis of DBProDOT and polymerization of PProDOT. Mono-
mer synthesis: i) 1,3-propanediol, p-TSA, toluene, 1008C, 48 h; ii) N-bromo-
succinimide (NBS), chloroform/acetic acid=3:1, 08C to 258C, 1 h. Polymer
synthesis: iii) room temperature to
458C.
monomers require a very long
polymerization time (several
months to two years) at low
temperatures such as 258C. For
the commercialization and mass
production of I2-free ssDSSCs,
device heating is to be avoided
to save energy, the fabrication
process needs to be simplified,
and the damage by heat to the
dye during HTM synthesis
should be reduced. To this end,
we designed a new solid-state,
polymerizable monomer, 2,5-di-
bromo-3,4-propylenedioxythio-
phene (DBProDOT), to synthesize
a conducting polymer, poly(3,4-
propylenedioxythiophene) (PPro-
DOT), at room temperature. The
DBProDOT crystals could be
Figure 1. (a) XRD patterns of the DBProDOT crystal and PProDOT-25. (b) Solid-state CP-MAS 13C NMR spectra of
DBProDOT and PProDOT-25. (c) UV-Vis-NIR spectra of PProDOT-25 coated on a glass slide and N719-dye-adsorbed
TiO2 on FTO glass. Inset: Magnified spectrum of PProDOT-25. (d) Isothermal DSC curves of DBProDOT at 25, 35,
polymerized at 258C because of
the short distance between the
monomers, lower activation and 458C. Inset: Arrhenius plot of the SSP reaction.
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the van der Waals distance (3.70 ꢂ). The difference (Dd) be-
tween the crystalline distance (dc) and the van der Waals dis-
tance (dV) is 0 and 0.2 ꢂ for DBProDOT and DBEDOT, respective-
ly. Therefore, a short distance between the bromine atoms in
the crystalline molecules facilitates polymerization at room
temperature. The broad diffraction peak at 21.88 was only ob-
served in PProDOT-25, which corresponds to a d spacing of
4.07 ꢂ and is assigned to the interplane distance between the
PProDOT molecules in the stack. The reduced crystalline order
and increased interplane distance in PProDOT may be because
mation]. The shortest intermolecular S···S, S···Br, and Br···Br dis-
tances are 3.65, 3.72, and 3.91 ꢂ, respectively, which are similar
to the van der Waals radii of S (1.80 ꢂ) and Br (1.85 ꢂ). The
short intermolecular Br···Br distance and short interplane dis-
tance (3.70 ꢂ) are responsible for the SSP of DBProDOT at low
temperature.
The structural changes from the monomer to polymer were
investigated using solid-state cross-polarization magic angle
spinning (CP-MAS) 13C NMR spectroscopy (Figure 1b). The
PProDOT polymer showed two strong peaks at 74 and 33 ppm
and very weak peaks at 148 and 126 ppm. In contrast, the
DBProDOT monomer showed clear and sharp peaks at 151,
126, 75, and 34 ppm, which are slightly different from those of
PProDOT. Through SSP, the DBProDOT monomers are polymer-
ꢀ
of the tribromide (Br3 ) dopants between the polymer crystal
lattice, which form spontaneously upon SSP.
The single crystals of DBProDOT were recrystallized from
ethanol to produce white, needle-like crystals (Figure S1, Sup-
porting Information). The single-crystal XRD data for DBProDOT
was collected at low temperature (200 K) because of its low
melting point (Table S1, the Supporting Information). The unit
cell of DBProDOT contained two independent molecules with
symmetric molecular structures (Figure 2a). DBProDOT com-
prises a planar thiophene ring and a protruding dioxypropy-
lene ring, which cause distances between the DBProDOT mole-
cules longer than those between the DBEDOT molecules. The
DBProDOT crystal revealed two packing types with p–p stack-
ing and a herringbone pattern (Figure 2b). The CꢀC bond
lengths of DBProDOT corroborated the calculated values of
DBProDOT [B3LYP/6-31G(d), Table S3 in the Supporting Infor-
ꢀ
ized to produce Br3 -doped PProDOT, which has polarons as
a result of the delocalization of electrons in PProDOT.[19] Thus,
the effect of the paramagnetic broadening strengthened the
carbon nuclei of the thiophenes, primarily because of the con-
jugation of conductive polymers and weak carbon nuclei of
the dioxypropylene bridge. The NMR spectrum of the chemi-
ꢀ
cally polymerized PProDOT (CP-PProDOT) doped with FeCl4
could not be measured owing to the strong paramagnetic
broadening.[20]
The ordered structure of the conductive polymer chains is
a key factor in the determination of their electrical conductivi-
ties. Compared to other polymerization methods, such as oxi-
dative polymerization and chemical coupling poly-
merization, SSP of heterocyclic conjugated rings
starts from monomers with a well-ordered, solid-state
crystal structure. The electrical conductivity of PPro-
DOT-25 and PProDOT-45 without any treatments or
additives reached 0.05 and 0.002 Scmꢀ1, respectively,
which is significantly higher than that of CP-PProDOT
(2ꢀ10ꢀ6 Scmꢀ1). Therefore, there is a strong depend-
ence of the ordered structure of the conductive poly-
mer on reaction temperature and polymerization
method.
According to the measurements, approximately
1.1 mg PProDOT per 1 cm2 of the TiO2 layer was ap-
propriate for deep infiltration into the TiO2 pores
without significant deposition on the TiO2 surface.
Thus, a PProDOT-25 film with 1.1 mgcmꢀ2 was pre-
pared on a glass slide and its ultraviolet-visible-near
infrared (UV-Vis-NIR) spectrum was measured (Fig-
ure 1c). The PProDOT-25 film showed a broad peak
maximum at approximately 730 nm. The UV-Vis-NIR
spectrum of a N719-dye-adsorbed TiO2 film with
0.4 mgcmꢀ2 was also measured and showed strong
light absorption at 300, 400, and 530 nm. Thus, the
PProDOT synthesized by SSP hardly affected the inci-
dent light absorption of the N719 dye. Notably, the
solid-state polymerizable monomers at room temper-
ature reported by Patra et al.[25] overlapped with the
dye absorption, which implies reduced effectiveness
Figure 2. (a) Crystal structure of DBProDOT; Oak Ridge thermal ellipsoidal plot (ORTEP) di-
agram, the bond lengths between DBProDOT atoms, and two independent molecules in
the unit cell. (b) Crystal packing diagram of DBProDOT with short intermolecular contacts
between Br atoms. The thermal ellipsoids were set to 25% probability level.
as HTM for ssDSSCs.
The polymerization temperature is a key parameter
because it directly determines the reaction rate of
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SSP, which affects the conductivity of the resultant polymer.
The conductivity of PProDOT decreased with the increase in re-
action temperature because the ordered crystalline structure
collapsed at high temperatures owing to the low melting
point of the monomer (478C). Isothermal differential scanning
calorimetry (DSC) curves of DBProDOT were measured at 25,
35, and 458C (Figure 1d). The DSC curves showed a large full
width at half maximum (ca. 500 min) with exothermic heat
flow that involved part-to-part polymerization of micro-sized
DBProDOT crystals.[20,21,23] The heat of polymerization (DHp) was
ꢀ12.2ꢁ0.7 kcalmolꢀ1 (1 kcal=4.2 kJ). The activation energy
was calculated using the Arrhenius equation [Eq. (1)]:
production of a DBProDOT carbocation by bromine requires
a high activation energy of 115.2 kcalmolꢀ1, which makes it
the rate-determining step. Nonetheless, this value is smaller
than that for the DBEDOT reaction. The second step for the
production of a dimerized carbocation releases 11.0 kcalmolꢀ1,
and the last step for the release of bromine is exothermic with
116.1 kcalmolꢀ1. Thus, SSP for DBProDOT is an energetically fa-
vorable process and is autocatalyzed by the bromide ions Brꢀ
and Br3ꢀ. Despite the ordered structure of PProDOT and steric
bulkiness, the activation energy of the oligomerized carboca-
tions is less than that of the dimerized carbocations, mainly be-
cause of the formation of a stable delocalized structure in the
oligomer.[20]
ln ðkÞ ¼ ln ð1=t1=2Þ ¼ ln ðAÞꢀEa=RT
ð1Þ
Formation of HTMs for I2-free ssDSSCs at room temperature
where k is the rate constant, t1/2 is the time taken by SSP for
half conversion, A is the pre-exponential factor, Ea is the SSP
activation energy, R is the ideal gas constant, and T is the abso-
lute temperature for SSP. Using Equation (1), the Ea of DBPro-
DOT was determined to be 10.8ꢁ1.1 kcalmolꢀ1 (R2 =0.981).
The SSP process was fast owing to the high t value of DBPro-
DOT at room temperature, without the additional heat energy.
Thus, the full conversion time of DBProDOT (five days) was less
than that of DBEDOT (one to two months) at room tempera-
ture (22–258C).[20]
The I2-free ssDSSCs were constructed with fluorine-tin-oxide-
(FTO)-coated glass, an interfacial TiO2 layer, an N719-dye-ad-
sorbed nanocrystalline TiO2 layer, pore-filled PProDOT as an
HTM, and a Pt-coated FTO counter electrode (Figure 3a).
A conventional compacted TiO2 layer (CC-TiO2) was used as
a control sample, whereas an organized mesoporous TiO2 (OM-
TiO2) layer was introduced to reduce the interfacial resistance
and improve light transmittance. The 550 nm-thick, transparent
OM-TiO2 layer was synthesized through a sol–gel process using
an amphiphilic graft copolymer (see the Supporting Informa-
tion for details) as an interfacial layer.[27,45,46] After casting an
ethanol solution containing DBProDOT on the dye-adsorbed
TiO2 photoelectrode, SSP occurred to produce PProDOT at
room temperature. The cross-sectional scanning electron mi-
croscopy (SEM) images of the TiO2 photoelectrode (Figure 3b)
clearly show the deep penetration of PProDOT into the 11 mm-
thick nanocrystalline TiO2 layer to result in improved interfacial
contact of the electrode/HTM. The 550 nm-thick OM-TiO2 layer
also had good interfacial contact on the FTO glass without de-
fects. As shown in Figure 4a and b, the OM-TiO2 layer exhibit-
ed high porosity with good interconnectivity compared to the
uniformly dense CC-TiO2 layer. The average pore diameter was
approximately 40–70 nm. After in situ SSP of DBProDOT in the
TiO2 pores, the nanocrystalline TiO2 layer was completely filled
without defects (Figures 4c and d and S8).
Calculation of energies required for the elementary reaction
To elucidate the reaction mechanism, the energy of each struc-
ture was calculated by using the Gaussian 03 program by
using DFT at the B3LYP/6-31G(d) level (Scheme 2 and
Table S2).[44] The dimerization of DBProDOT was exothermic at
11.9 kcalmolꢀ1, releasing more energy than the dimerization of
DBEDOT (11.2 kcalmolꢀ1) and dibromothiophene (8.3 kcal
molꢀ1, Figure 2a). In radical polymerization in the dark, the ho-
molytic cleavage energy of a CꢀBr bond in DBProDOT was
87.0 kcalmolꢀ1, which barely progresses at room temperature.
The electron donating dioxypropylene ring in DBProDOT stabi-
lized the cation of the heterolytic cleavage of the CꢀBr bond
in oxidative polymerization. In Figure 2b, the first step for the
Three types of devices were
fabricated using two interfacial
layers and PProDOT polymerized
at different temperatures; an
OM-TiO2 layer with PProDOT-25
(OM-PProDOT-25),
layer with PProDOT-25 (CC-PPro-
DOT-25), and CC-TiO2 layer
a
CC-TiO2
a
with PProDOT-45 (CC-PProDOT-
45). We controlled the electrical
conductivity of the HTM by con-
trol of the SSP temperature
using same monomer. The cur-
rent
density–voltage
(J–V)
curves, incident photon-to-cur-
rent efficiency (IPCE), and elec-
Scheme 2. (a) Calculated energy for dimerization of DBProDOT. (b) Proposed mechanism for the autocatalyzed ini-
tiation of the SSP of DBProDOT.
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Synthesis of a Conductive Polymer for Solar Cells
counter electrode/HTM. The Rb
value of OM-PProDOT-25 was
less than that of CC-PProDOT-25
and CC-PProDOT-45, which dem-
onstrates that the OM-TiO2 layer
results in reduced resistance at
the photoelectrode/HTM inter-
face.
Based on the EIS model, the
minimum angular frequency
(wmin) of the impedance semicir-
cles at the middle frequencies in
the Bode phase plots (Figure 5d)
can be used to estimate the
effective electron lifetime for
recombination (tr) according
to the following equation:
tr =1/wmin. Introduction of the
OM-TiO2 layer and PProDOT syn-
thesized at low temperature re-
sulted in the longest tr (39.8 ms),
which indicates enhanced elec-
tron transfer with reduced elec-
tron recombination.
Figure 3. (a) Facile fabrication process for I2-free ssDSSCs using conductive polymers as HTMs. i) Solid-state poly-
merizable monomer crystal formation with complete penetration by drop casting and drying of DBProDOT or
DBEDOT solutions. ii) Simple SSP of DBProDOT penetrated the cell at room temperature. iii) SSP of DBEDOT pene-
trated the cell at 608C. iv) Application of the counter electrode on the nanocrystalline TiO2 layer. (b) Cross-section-
al SEM images of the TiO2 photoelectrode with HTMs and the magnified interface between the OM film and nano-
crystalline TiO2 layer.
The long-term stability of the
I2-free ssDSSCs with PProDOT
was investigated as a function of
time without perfect encapsula-
tion under vacuum (Figure 6).
Epoxy resin was used for cell fix-
trochemical impedance spectroscopy (EIS) were measured at
100 mWcmꢀ2, as shown in Figure 5, and the results are sum-
marized in Table 1. The J–V curves of OM-PProDOT-25 showed
a short circuit current (Jsc) of 10.0 mAcmꢀ2, an open circuit
voltage (Voc) of 0.63 V, and an energy conversion efficiency (h)
of 3.5%, which were greater than those of CC-PProDOT-25 and
CC-PProDOT-45. The IPCE spectrum in Figure 5b also shows
the large quantum efficiency of OM-PProDOT-25 in the visible
wavelength region from 350–700 nm. These results illustrate
that the use of the OM-TiO2 layer and PProDOT synthesized at
low temperature effectively improved the cell efficiency.
To understand the cell performance, the internal resistance
and charge-transfer kinetics of the devices were investigated
by using EIS analysis (Figure 5c). Two semicircles were ob-
served in the Nyquist plots of the EIS spectra (0.05–105 Hz)
Figure 4. Surface SEM images of (a) the CC-TiO2 layer on the FTO glass,
(b) OM-TiO2 layer on the FTO glass, (c) nanocrystalline TiO2 layer, and
(d) nanocrystalline TiO2 layer with conducting polymers.
consisting of three components: ohmic resistance (Rs), charge-
transfer resistance at the interface of the counter electrode/
HTM (Ra), and charge-transfer re-
sistance at the interface of the
photoelectrode/HTM (Rb). The Ra
Table 1. Performances and EIS results of I2-free ssDSSCs fabricated with different interfacial layers and SSP
values of OM-PProDOT-25 and
CC-PProDOT-25 were not signifi-
cantly different, but that of CC-
PProDOT-45 was much greater.
The Ra value is largely depen-
dent on the conductivity of HTM
and the interfacial contact of the
temperatures.
Samples
Voc
[V]
Jsc
FF
h
Rs
[W]
Ra
[W]
Rb
[W]
wmin
tr
[mAcmꢀ2
]
[%]
[Hz]
[ms]
OM-PProDOT-25
CC-PProDOT-25
CC-PProDOT-45
0.63
0.61
0.61
10.0
8.2
5.2
0.56
0.56
0.56
3.5
2.8
1.8
16.1
16.7
15.3
38.6
38.0
54.3
57.1
61.3
85.2
25.1
33.7
45.4
39.8
29.7
22.0
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J. H. Kim, E. Kim et al.
nedioxythiophene)
(PProDOT),
were synthesized and character-
ized using XRD, solid-state cross-
polarization magic angle spin-
ning (CP-MAS) 13C NMR and UV-
Vis-NIR spectroscopy, and iso-
thermal DSC analysis. PProDOT-
25 (prepared at a reaction tem-
perature of 258C) doped with
tribromide ions showed an elec-
trical conductivity of 0.05 Scmꢀ1
without treatment, which was
higher than that of PProDOT-45
(0.002 Scmꢀ1
)
or CP-PProDOT
(2ꢀ10ꢀ6 Scmꢀ1) and indicates
the effectiveness of low-temper-
ature solid-state polymerization
(SSP).
The
solid-state
CP-
MAS13C NMR spectrum shows
the presence of polarons in
PProDOT and the strong effect
of paramagnetic broadening on
the carbon nuclei of the thio-
Figure 5. Performances of I2-free ssDSSCs with a conducting polymer at 100 mWcmꢀ2 (a) J–V curves, (b) IPCE
plots, (c) Nyquist plots, and (d) Bode phase plots of each device. Two different interfacial layers and SSP tempera-
tures are compared.
phenes. DBProDOT was also observed to have a low activation
energy, high exothermic reaction, and short interplane dis-
tance, which suggest the reaction mechanism of SSP at low
temperature. PProDOT deeply infiltrated the 11 mm-thick nano-
crystalline TiO2 layer and functioned as a HTM in I2-free
ssDSSCs, which was confirmed by SEM analysis. The ssDSSC
fabricated with a PProDOT-25 and 550 nm-thick OM-TiO2 layer
exhibited a high cell efficiency reaching 3.5% at 100 mWcmꢀ2
because of the high conductivity of HTM and the reduced in-
terfacial resistance of the electrode/HTM. Importantly, the ob-
tained efficiency was maintained within ꢁ15% for 1500 h
without perfect sealing, which indicates excellent long-term
stability of the device with solid-state polymerized HTMs.
Figure 6. Normalized cell efficiency of OM-PProDOT-25 maintained in room
light as a function of time.
Experimental Section
ation, and O2 remained in the cell. There have been few re-
ports on the long-term stability of ssDSSCs with conducting
polymers because conducting polymers synthesized by elec-
trochemical or chemical polymerization methods are highly
sensitive to the environment, such as O2. Interestingly, an h
value of 3.5% for OM-PProDOT-25 was maintained within
ꢁ15% for 1500 h, which is one of the highest long-term stabil-
ity values demonstrated in I2-free ssDSSCs fabricated with
HTM.[30] Furthermore, the long-term stability could not be ob-
tained from the liquid-state DSSC without a conducting poly-
mer because of imperfect encapsulation. The excellent long-
term stability might be a result of the lower sensitivity of PPro-
DOT synthesized by the SSP method.
Materials
3,4-Dimethoxythiophene, 1,3-propanediol, p-toluenesulfonic acid
(p-TSA), NBS, iron(III) chloride (FeCl3), poly(vinyl chloride) (PVC,
Mw =97000 gmolꢀ1), poly(oxyethylene methacrylate) (POEM), poly-
(ethyleneglycol) methyl ether methacrylate (Mn =475 gmolꢀ1), tita-
nium(IV) isopropoxide (TTIP), hydrogen chloride solution (HCl,
37 wt%), 1,1,4,7,10,10-hexamethyltriethylene tetramine (HMTETA),
copper(I) chloride (CuCl), anhydrous toluene, acetic acid, chloro-
form, ethanol, acetonitrile (ACN), isopropyl alcohol (IPA), 4-tert-bu-
tylpyridine (TBP), lithium bistrifluoromethanesulfonimide (LiTFSI),
lithium iodide (LiI), 1-methyl-3-propyl-imidazolium iodide (MPII),
chloroplatinic acid hexahydrate (H2PtCl6), and titanium bis(ethyl
acetoacetate) were purchased from Aldrich. Tetrahydrofuran (THF),
N-methyl-2-pyrrolidone (NMP), and methanol were purchased from
J. T. Baker. TiO2 paste (Solaronix, D20) and N719 dye were pur-
chased from Solaronix. The FTO glass (8 W per square) was pur-
chased from Pilkington Co. Ltd. Silica gel (60–230 mesh, Merck)
was used to purify the products. All solvents and chemicals were
reagent grade and used as received without further purification.
Conclusions
A new solid-state polymerizable monomer at room tempera-
ture, 2,5-dibromo-3,4-propylenedioxythiophene (DBProDOT),
and the corresponding conducting polymer, poly(3,4-propyle-
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Synthesis of a Conductive Polymer for Solar Cells
Synthesis of ProDOT
Fabrication of the OM-TiO2 Layer
3,4-Dimethoxythiophene (5.00 g, 34.7 mmol) and excess 1,3-pro-
panediol (13.2 g, 173 mmol) were dissolved in anhydrous toluene
(500 mL) and p-TSA (0.2 g) was added to the mixture at room tem-
perature under an argon atmosphere. The mixture was heated to
1008C for 12 h. The mixture was concentrated, washed with 5%
potassium hydroxide aqueous solution (500 mLꢀ1) and water
(500 mLꢀ2) to remove excess starting materials and extracted with
ethyl ether (200 mLꢀ3). The organic layer was dried with anhy-
drous magnesium sulfate, filtered, and evaporated. The crude
white oil was purified by column chromatography by using a mix-
ture of petroleum ether/ethyl ether=4:1 as an eluent to obtain
3.03 g (56%) of product as a white powder. 1H NMR (400 MHz,
CDCl3): d=6.53 (s, J=5 Hz, 2H; Th-H), 4.08 (t, J=5 Hz, 4H; ꢀOꢀ
CH2ꢀ), 2.20 ppm (m, 2H; ꢀCH2ꢀCH2ꢀCH2ꢀ); 13C NMR (100 MHz,
CDCl3): d=150.45, 106.37, 71.21, 33.86 ppm.
The OM-TiO2 film was prepared by using an optimized mixture of
synthesized amphiphilic PVC-g-POEM graft copolymer, TTIP, water
and HCl. HCl was slowly added to TTIP while stirring vigorously,
and distilled water was added to the mixture. The molar composi-
tion of the mixture was TTIP/H2O/HCl=2:1:1. Separately, PVC-g-
POEM graft copolymer (0.05 g) was dissolved in THF (1.5 mL) and
added to the mixture (0.6 mL). The final mixture was aged while
stirring at room temperature overnight and spin-coated on the
FTO glass at 2000 rpm for 20 s (SMSS Delta 80BM spin coater).
Upon calcination at 4508C for 30 min, the organic chemicals were
completely removed to produce the OM-TiO2 thin film.
Fabrication of I2-free ssDSSCs
The I2-free ssDSSCs were fabricated by drop-casting a monomer in
ethanol onto the photoelectrode and covering with a Pt-coated
counter electrode using a previously reported procedure.[26,27] For
the preparation of Pt counter electrodes, a H2PtCl6 solution (7mm
in IPA) was drop-cast on the conductive FTO glass; the glass was
heated to 4508C, which was maintained for 30 min, and then
cooled to 308C for 8 h. The TiO2 paste was cast onto the interfacial
layer of photoelectrodes coated with CC-TiO2 or OM-TiO2 film by
using a doctor-blade technique, dried at 508C for 30 min, which
was followed by successive sintering at 4508C for 30 min and cool-
ing to 308C within 8 h. Nanocrystalline TiO2 films were immersed
in the N719 solution (0.5mm in ethanol) at room temperature for
24 h. The monomer solution was prepared by dissolving DBProDOT
in ethanol. The 1 and 3 wt% solutions were directly cast onto the
photoelectrodes. After drying the solvent, the DBProDOT-filled TiO2
photoelectrodes were subjected to SSP to produce PProDOT at
258C for five days or 458C for two days. A drop of the mixture so-
lution [MPII (1.0m), TBP (0.2m), and LiI (0.2m) in ethanol] was cast
onto the dye-adsorbed TiO2 photoelectrodes with HTM. After evap-
oration of the solvent in a vacuum oven, sandwich-type ssDSSCs
were fabricated by clipping two electrodes and sealing with epoxy
resin. The active area was 0.25 cm2 (0.5 cmꢀ0.5 cm).
Synthesis of the DBProDOT crystal
ProDOT (0.7 g, 4.48 mmol) was dissolved in a mixture of chloro-
form (150 mL) and acetic acid (50 mL). NBS (1.75 g, 9.86 mmol,
2.2 equiv) was slowly added to the mixture at 08C under an Ar at-
mosphere, and the mixture was stirred at 258C for 1 h. The organic
layer was neutralized with 5% potassium hydroxide solution
(250 mLꢀ1) and washed with distilled water (250 mLꢀ3). The mix-
ture was dried with anhydrous magnesium sulfate, filtered, and
then evaporated. The light yellow powder was recrystallized from
ethanol to produce white needle-like crystals (1.36 g, 97%).
1H NMR (400 MHz, CDCl3): d=4.18 (t, J=5 Hz, 4H; ꢀOꢀCH2ꢀ),
2.26 ppm (m, J=5 Hz, 2H; ꢀCH2ꢀCH2ꢀCH2ꢀ); 13C NMR (100 MHz,
CDCl3): d=147.53, 92.29, 71.46, 33.32 ppm; CP-MAS 1H NMR
(500 MHz, solid state): d=5.34 (OꢀCH2ꢀ), 1.97 ppm (ꢀCH2ꢀCH2ꢀ
CH2ꢀ); CP-MAS 13C NMR (125 MHz, solid state): d=150.62, 130.73,
75.03, 34.06 ppm; EIMS: m/z (%): 316 (52) [M]+, 314 (100), 312 (51);
HRMS (EI): m/z: calc. for C7H6Br2O2S 311.8455 [M]+; found 311.8454;
elemental analysis calc. (%) for C7H6Br2O2S 313.9943: C 26.78, H
1.93, O 10.19, S 10.21; found: C 26.88, H 2.00, O 10.00, S 10.36.
Characterization
1
The H and 13C NMR spectra were obtained by using a Bruker Bio-
Synthesis of PProDOT Using SSP
spin Avance II at 400 and 100 MHz, respectively, using tetramethyl-
silane (TMS) dissolved in CDCl3 as a standard. Solid state 1H and
13C NMR spectra were acquired at 500 and 125 MHz, respectively,
by using a Bruker Avance II with a 4 mm magic angle spinning
probe with the spinning speed regulated at 7 kHz. The XRD data
was collected by using a Rigaku Ultima Idiffractometer using CuKa
(l=0.154 nm) radiation. UV-Vis-NIR spectra were obtained by
using a Perkin–Elmer Lambda 750 spectrometer. FTIR spectra were
collected by using a Bruker Tensor 37 FTIR Spectrometer with atte-
nuated total reflectance (ATR). DSC was performed by using
a Netzsch/DSC 200 F3 instrument under N2 gas flow at a heating
rate of 108Cminꢀ1, and the DBProDOT monomer was placed in
a tightly sealed Netzsch aluminum pan. The temperature was con-
trolled in the isothermal mode at 25, 35, and 458C to determine
the heat energy absorbed during SSP. Single-crystal XRD data was
collected by using a Bruker AXS Smart Apex CCD diffractometer,
MoKa (l=0.71073 ꢂ), T=200 K, installed at the KBSI (Jeonju,
Korea). PProDOT-25, PProDOT-45, and CP-PProDOT were pelletized
to a 3 mm thickness for conductivity measurements. The conduc-
tivity of the samples was measured by using an electrochemical
analyzer (CHI624B, CH Instruments Inc.) and a four-point probe
DBProDOT (0.2 g) was put in a vial and placed in an oven at room
temperature (22–258C) for five days, 358C for three days, or 458C
for two days. During the process, the color of the monomer crystal
changed from white to black, and brown bromine vapor formed
on the vial surface. For the film preparation, DBProDOT dissolved
in ethanol (1 wt% solution) was drop-cast on the slide glass by
evaporating the solvent at room temperature, and SSP was per-
formed in an oven at 258C. The drop-cast quantity was the same
amount used for ssDSSCs fabrication. CP-MAS 1H NMR (500 MHz,
solid state): d=5.44 (ꢀOꢀCH2ꢀ), 1.93 ppm (ꢀCH2ꢀCH2ꢀCH2ꢀ); CP-
MAS 13C NMR (125 MHz, solid state): d=147.55, 126.36, 74.06,
32.66 ppm.
Fabrication of the CC-TiO2 Layer
The 200 nm-thick CC-TiO2 layer was prepared by spin-coating a tita-
nium bis(ethyl acetoacetate) solution (2 wt% in n-butanol) on FTO
glass at 2000 rpm for 25 s, followed by calcination at 4508C for
30 min.
ChemSusChem 0000, 00, 1 – 9
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Received: May 19, 2012
Published online on && &&, 0000
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ChemSusChem 0000, 00, 1 – 9
ÝÝ
These are not the final page numbers!

FULL PAPERS
What a wait for solid state: 2,5-Dibro-
mo-3,4-propylenedioxythiophene
B. Kim, J. K. Koh, J. Kim, W. S. Chi,
J. H. Kim,* E. Kim*
(DBProDOT), a new monomer that is
a solid at room temperature, has been
synthesized to produce poly(3,4-propy-
lenedioxythiophene) (ssPProDOT). I2-free
solid-state dye-sensitized solar cells with
ssPProDOT exhibit exceptional long-
term stability (>1500 h) with an effi-
&& – &&
Room Temperature Solid-State
Synthesis of a Conductive Polymer for
Applications in Stable I2-Free Dye-
Sensitized Solar Cells
ciency of 3.5% at 100 mWcmꢀ2
.
ChemSusChem 0000, 00, 1 – 9
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org
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9
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These are not the final page numbers!