Full Paper
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High-Molecular-Weight PolyoxiraneCopolymers and their Use in High-PerformanceDye-Sensitized Solar Cells
Petar Petrov,* Iliyana Berlinova, Christo B. Tsvetanov, Silvia Rosselli,Andreas Schmid, Ameneh Bamedi Zilaei, Tzenka Miteva, Michael Durr,Akio Yasuda, Gabriele Nelles*
Low-crystalline random and gradient P(EO-co-PO) copolymers and amorphous PPO and PBO ofhigh molecular weight were synthesized by anionic coordination polymerization. Polymer gelelectrolytes based on these (co)polymers were prepared and tested for long-term performanceof DSSC. The DSSC based on P(EO-co-PO) copolymershave longer life time compared to the homo-PEO-and homo-PPO-based DSSC, respectively. The cellscontaining the chemically crosslinked copolymer gelexhibited a high efficiency of 6% after 25 d per-formance, whereas the solar cells based on physi-cally crosslinked copolymer gel showed fastdegradation.
Introduction
High-molecular-weight poly(ethylene oxide) (HMWPEO) is
one of the most investigated polyoxiranes in the field of
polymer electrolytes. The ethylene oxide (EO) repeating
unit presents a favorable arrangement for effective
interaction of the free electron pair on the oxygen with
cations in the electrolyte, in particular with the alkali
P. Petrov, I. Berlinova, C. B. TsvetanovInstitute of Polymers, Bulgarian Academy of Sciences,‘‘Akad. G. Bonchev’’ Str. 103A, 1113 Sofia, BulgariaFax: þ359 2 870 0309; E-mail: [email protected]. Rosselli, A. Schmid, A. B. Zilaei, T. Miteva, M. Durr, A. Yasuda,G. NellesMaterials Science Laboratory, Sony Deutschland GmbH, 70327Stuttgart, GermanyFax: þ49 711 585 8484; E-mail: [email protected]
Macromol. Mater. Eng. 2008, 293, 598–604
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metal cations. This occurs because the PEO chains are
capable of adopting a helical conformation with an
oxygen-lined cavity that presents ideal distances for
oxygen/cation interactions.[1] Due to its high crystallinity,
HMW PEO-based electrolytes show a reasonable ionic
conductivity only above its melting temperature. More-
over, the tendency to crystallize with time decreases
sensibly the long-term ionic conductivity. To obtain a
more amorphous polymer at ambient temperature, it is
necessary to introduce a certain degree of disorder in the
structure by, which could be done using plasticizers,
copolymers or crosslinked networks.[2] The copolymeriza-
tion of EO and other alkylene oxides can be a suitable
approach to obtain products of lower crystallinity, how-
ever, due to the much higher reactivity of EO compared
to the one of other alkylene oxides,[3] it is difficult to
control the regular incorporation of the comonomer units
throughout the PEO chain. Thus, one can obtain HMW
DOI: 10.1002/mame.200800008
High-Molecular-Weight Polyoxirane Copolymers . . .
copolymers of relatively high crystallinity.[4] Ikeda et al.
reported on HMWpoly[epichlorohydrin-co-(ethylene oxide)]
copolymers and comb-shaped poly(oxyethylene)s with
oxyethylene segments as side chains that exhibit very low
crystallinity.[5,6] The crystallinity of the resulting copoly-
mers is highly dependent on the copolymer architecture
and on the amount of the comonomer units in the main
chain.
Polymer gel electrolytes are largely used in lithium
batteries, fuel cells, smart windows and dye-sensitized
solar cells.[1,7–22] The electrolytes are prepared by incor-
porating a large amount of a liquid with the desired salts
and the redox active materials into a polymer matrix,
giving a stable gel. Owing to their unique hybrid structure,
polymer gels have both the cohesive properties of solids
and the diffusive transport properties of liquids. The
diffusion properties in the polymer gel electrolyte with
propylene carbonate/ethylene carbonate (PC/EC) as plasti-
cizerwere comparedwith ion diffusion in the polymer-free
electrolyte (pure PC/EC). It was reported that the ions can
diffuse as freely through the liquid enclosed in the polymer
network as they do in the bare liquid.[23,24]
Durr et al.[25] reported a two-compartment tandem
dye-sensitized solar cell (DSSC) with polymer gel electro-
lyte giving a power conversion efficiency of 10.5%, with
white light (100 mW � cm�2, AM 1.5). Long-term stability
studies were so far mainly reported for liquid electrolyte-
based DSSC[26] and only a few for polymer gel-based
DSSC.[21,27]
In this paper we report the synthesis of HMW amorphous
poly(propylene oxide) (PPO), poly(butylene oxide) (PBO) and
low crystalline random and gradient poly[(ethylene oxide)-
co-(propylene oxide)] P(EO-co-PO) copolymers and their use
in polymer gel electrolytes. All polymers were synthesized
using a calcium amide/alkoxide initiator which has been
Table 1. Experimental conditions for the synthesis of HMW polyoxir
Entry Comonomer
amount
Polymerization
temperature
g -C
PEO – 40
PPO 30 50
PBO 60 50
P(EO-co-PO21) 30 50
P(EO-co-PO27) 50 50
P(EO-co-PO17) 20 40
P(EO-co-PO38) 30 40
P(EO-co-PO44) 25 50
P(EO-co-PO68) 30 50
a)5 min, EO bubbling; 25(35) min, without bubbling.
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developed for the production of commercial HMW PEO.[28]
Furthermore, first experiment on the long-term perfor-
mance of DSSC, based on those copolymers gel electrolyte,
in comparison to homopolymer gel electrolyte is reported.
In addition comparison between solar cells prepared with
physical copolymer gel and chemically crosslinked gel
electrolyte is described.
Experimental Part
Materials
Ethylene oxide (Clariant), calcium (Neochim) and liquid ammonia
were used as received. PO and butylene oxide (Aldrich) were
distilled over CaH2. All solvents were purified by standard
procedures.
Synthesis
Preparation of the Initiator
In a 1-L four-necked flask equipped with a mechanical stirrer, an
argon inlet-tube, reflux condenser and thermometer, 75 mL of
liquid ammonia and 0.5 g of calcium metal were filled under
stirring forming a blue solution of calcium hexamine. 0.8 mL of
PO/acetonitrile mixture was added dropwise yielding a grey
slurry. The molar ratio of calcium to the modifiers was 1:1 and the
molar ratio of PO to acetonitrile was 3:2. Finally, the liquid
ammonia was evaporated by gentle warming to room tempera-
ture, 25 mL of heptane was added to the slurry and the mixture
was refluxed for 1 h to remove the traces of ammonia.
Synthesis of Homopolymers
The flask containing the initiatorwas immersed in awater bath at
a given temperature (Table 1) and 75mL of heptane was added. At
ane (co)polymers.
Polymerization
time
Feed
cyclea)PO
content
h min �minS1 mol-%
3.5 – 0
2 – 100
6 – –
2 – 21
1.5 – 27
4 5/25 17
4 5/25 38
3 5/35 44
1 5/25 68
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P. Petrov et al.
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zero time, an appropriate amount of monomer was added to the
initiating system and the polymerization was carried out for a
given period of time (Table 1). In the case of EO polymerization, the
monomer was bubbled through the reaction mixture. At the end
of the process, CO2 gas was bubbled through the mixture for
30 min. PPO and PBO obtained were isolated by evaporation of
both the solvent and unreacted monomer and dried. PEO was
collected by filtration, thoroughly washed with hexane and dried.
Synthesis of Random Copolymers
The synthetic procedure was similar to the one for homopolymers
as described above. At zero time, EO was bubbled and after 20 s a
given amount of PO was added. At the end of the process, CO2 gas
was bubbled through the reaction mixture for 30 min. The
obtained copolymer precipitate was separated by filtration,
thoroughly washed with hexane and dried.
Synthesis of Block-Like Gradient Copolymers
The synthetic procedure was as described above except that at
regular time intervals (Table 1) EO bubbling was stopped and the
polymerization was allowed to proceed without feeding EO for a
certain time.
Purification of (Co)polymers
1 g of copolymer was dissolved in 100 mL of distilled water. CO2
was bubbled through the solution until it became transparent. The
solution was dialyzed against water for 14 d and the product was
isolated by freeze drying.
The homopolymers of PO and BOwere purified according to the
following procedure: 0.1 g of polymer was dissolved in 100 mL
hexane. The opalescent solutionwas passed trough Hyflo super-cel1
column (diatomaceous earth, Aldrich). The solventwas evaporated
and the polymer was dried.
Chemical Crosslinking of Copolymers
0.75 g of copolymerwasdissolved in an appropriate amount of EC/PC
(1:1 inweight) to obtain 5wt.-% solution at 65 8C to ensure complete
dissolution and homogeneity. A photoinitiator, benzophenone
(Sigma) and a crosslinking agent, N,N’-methylenebis(acrylamide)
(Fluka) (1:4 in weight and 5 wt.-% with respect to the polymer)
dissolved in 1 mL of PC/EC (1:1 in weight) was added under
stirring. The resulting homogeneous solution was poured into
Teflon dish forming a 2.5 mm thick layer and was irradiated with
full spectrum UV-Vis light at room temperature with a Dymax
5000-ECUV curing equipmentwith 400Wmetal halide flood lamp
for 2 min (93 mW � cm�2 input power).
Characterization
NMR Spectroscopy
The 1H NMR spectrum of polymer in CDC13 was recorded on a
Bruker WM250 spectrometer operating at 250 MHz. The
copolymers compositions were calculated comparing the relative
intensities of the proton signal characteristic for the methyl
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groups in the PO chain units at 1.1–1.2 ppm and the oxyethylene
and oxymethine signals at 3.4–3.6 ppm.
Viscometry
The viscosity measurements were carried out with an Ubbelohde
type viscometer equipped with a capillary of 0.45 mm diameter
and thermostated at 25 (toluene) or 30 8C (water). The constants
K¼1.25�10�4 dL � g�1 and a¼ 0.78 for PEO in water and K¼1.29� 10�4 dL � g�1 and a¼ 0.75 for PPO in toluene were used to
calculate the viscometric molecular weight.[29]
Gel Permeation Chromatography (GPC)
Molecular weight and molecular weight distribution were
determined by GPC (Waters) equipped with three m-styragel
columns (styragel linear, styragel 104 A and styragel 103 A) with
UV and RI detector. The measurements were carried out in
chloroform with 0.1 vol.-% triethylamine at 30 8C with a flow rate
of 1.0 mL �min�1. Polystyrene standards were used for calibration.
Static Light Scattering (SLS)
SLS measurements were carried out in methanol on a multi-angle
DAWN DSP Laser light scattering photometer (Wyatt Technology
Corp.) equipped with a He-Ne laser emitting at a wavelength of
632.8 nm. Analyzes were performed in a batch mode. The weight
average molar masses were calculated using Berry fit method
processed with Astra (Wyatt Corp.) software, v. 4.70. Specific
refractive index increments, dn/dc, were measured on a Wyatt
Optilab 903 interferometric refractometer operating at 633 nm.
The stock solutions for the light scattering measurements were
prepared at a concentration of 1�10�3 g �mL�1. They were
purified of dust using filter of 0.45 mm pore size and diluted with
filtered solvent.
Differential Scanning Calorimetry
The melting and crystallization behavior of the polymers was
studied with a PerkinElmer DSC-7 in a temperature range from
�100 to þ100 8C at a heating/cooling rate of 10 8C �min�1. The
crystallinity of PEO phase was calculated by the ratio of the
measured and equilibrium heats of fusion (DHf,0). DHf,0 was
196 J � g�1.
DSSC Preparation
The DSSC were assembled as follows: a 30-nm-thick bulk TiO2
blocking layer was formed on FTO (approx. 100 nm on glass). An
approx. 10-mm-thick porous layer of semiconductor particles was
screen printed on the blocking layer and sintered at 450 8C for half
an hour. Red-dye [cis-bis(isothiocyanato)bis(2,20-bipyridyl-4,40-
dicarboxylic acid) ruthenium(II)] molecules were adsorbed to
the particles via self-assembling out of a solution in ethanol
(3� 10�4M). The porous layer was filled with polymer gel
electrolyte containing I�/I�3 as redox couplewith c(I�3 )¼1.5� 10�3M
and 3 wt.-% (co)polymer dissolved in PC/EC (1:1 in weight)
mixture. A reflective platinum back electrode was attached with a
distance of 6 mm from the porous layer.
Photovoltaic Characterization
Current/voltage characteristics were taken under illumination
withwhite light from a sulfur lamp. Intensitywas 100mW � cm�2.
DOI: 10.1002/mame.200800008
High-Molecular-Weight Polyoxirane Copolymers . . .
Scheme 1. Schematic representation of one block of the block-likegradient p(EO-co-PO) copolymers.
Results and Discussion
Synthesis of the Polymers and CopolymersThe calcium amide/alkoxide initiating system was success-
fully employed for the synthesis of HMW PEO copoly-
mers.[5,30,31] In the present study, we used this initiating
system to prepare HMW PEO, PPO and PBO homopolymers
as well as HMW P(EO-co-PO) random and blocky-like
gradient copolymers. The anionic ring-opening polymer-
ization of EO, PO and butylene oxide was performed in
heptane under conditions enabling the preparation of
HMW polymers. While PEO was insoluble in heptane, PPO
and PBO formed highly swollen gels. Random P(EO-co-PO)
copolymers were synthesized in a similar way. The
copolymerization proceeded at a constant concentration
of EO, which was achieved by bubbling EO through the
reaction mixture during the entire process. The copolymer
composition depended on both the amount of PO in the
feed and the reaction temperature. At optimal experi-
mental conditions (Table 1), a copolymer with maximum
PO content of 27 mol-% was obtained [P(EO-co-PO27)
in Table 1].
To obtain polymers containing larger amount of PO, a
new synthesis method of HMW P(EO-co-PO) copolymers
with block-like gradient structure was developed. The
strategy is based on repeating short-time feeds of EO to the
reaction mixture in regular time intervals (feeding cycles)
during the polymerization [P(EO-co-PO17)/P(EO-co-PO68] in
Table 1). At the beginning of each cycle, the reaction
mixture was saturated with EO by bubbling for 5 min.
Then the polymerization was allowed to proceed for
a certain time. The ring-opening polymerization of
1,2-epoxides initiated by a calcium amide/alkoxide system
follows an anionic-coordination mechanism. The rate of
monomer incorporation into the growing chains depends
on both the monomer reactivity and monomer concentra-
tion. In our case, the reactivity of the monomers used
decreases with the increase in the bulkiness, i.e. EO is more
Table 2. Molecular characteristics of the HMW polyoxiranes (co)poly
Entry [h]a) Mv MGn
dL � gS1 kg �molS1 kg �m
PEO 12.7b) 2 600 –
PPO 3.7 878 –
PBO 1.9 – –
P(EO-co-PO21) – – 10
P(EO-co-PO27) – – 45
P(EO-co-PO17) 2.4 – 11
P(EO-co-PO44) 10.1 – 17
a)In toluene at 25 -C; b)In water at 30 -C.
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reactive than PO. Therefore, at the beginning of each cycle,
when the system is saturated with EO, the significant
difference in the reactivity of EO and PO will result in
preferred incorporation of EO into the chain. The
copolymer sequence will be richer of EO. At the end of
each cycle, EO should be entirely consumed and the
respective sequence should be richer of PO (Scheme 1).
The (co)polymers synthesized using the calcium amide/
alkoxide initiator contain Ca residues. Those have to be
removed to enable the preparation of a defined polymer
gel electrolyte. To purify the copolymer, CO2 was bubbled
through an aqueous copolymer solution until the milky
solution turned to transparent. This indicates that water-
insoluble Ca residues were converted to water-soluble
species whichwere subsequently removed by dialysis. The
isolated polymers did not contain Ca as confirmed by
elemental analysis. After purification, the copolymers
were characterized by 1H NMR and the monomer ratios
were calculated (Table 1). An increased PO amount in the
feed enabled the incorporation of larger amounts of PO.
Since the reactivity of the 1,2-epoxides increases with
temperature, copolymers with higher PO content were
obtained at polymerization temperature 50 8C compared
to 40 8C. However, a further increase in temperature
resulted in a decrease in polymer yield due to poor control
of the polymerization. When the process was performed at
very high comonomer concentration a swelling of the
product in the reaction medium was observed, which
hampered the stirring of the mixture.
Viscosity measurements as well as GPC and SLS analysis
revealed the HMW of the (co)polymers (Table 2). GPC
mers.
PCM
GPCw Mw=M
GPCn M
SLSw
olS1 kg �molS1 kg �molS1
– – –
– – 300
– – –
0 270 2.7 200
0 1 090 2.4 460
8 387 3.2 220
6 405 2.3 270
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P. Petrov et al.
Table 3. DSC characterization data of the HMW polyoxiranes(co)polymers.
Entry Crystallinity Tm Tc Tg
% -C -C -C
PEO 77 65 43 S59
PPO 0 – – S67
PBO 0 – – S69
P(EO-co-PO21) 31 54 8 S68
P(EO-co-PO27) 16 51 17 S69
P(EO-co-PO17) 22 57 25 S68
P(EO-co-PO38) 11 52 16 S69
P(EO-co-PO44) 8 53 S1 S68
P(EO-co-PO68) 2 52 S24 S66Figure 1. I–V curve of DSSC employing different polyoxirane gelelectrolytes.
602
analyzes show monomodal distribution and apparent
molecular weight distributions in the range of 2.3–3.2
which are typical values for a heterogeneous suspension
polymerization.
Differential scanning calorimetry analysis of the
homopolymers showed that PPO and PBO are amorphous
polymers, while the degree of crystallinity of PEO is ca. 77%
(Table 3). The incorporation of a small amount of PO into
the PEO chain introduced a certain degree of disorder in the
polymer structure and reduced significantly the crystal-
linity of thematerials obtained. Generally, the crystallinity
and the melting temperature (Tm) of the PEO phase in the
copolymers decrease with the increased PO content. This
indicates that the PO units/segments inhibit the regular
crystallization of PEO chains.
Application in Dye-Sensitized Solar Cells (DSSC)
Dye-sensitized solar cells based on PEO, PPO and PBO
polymer gel electrolytes were studied (Table 4, Figure 1.).
To get a first insight into the influence of the reduced
crystallinity on the long-term stability, the efficiency of
the cells was recorded over several weeks. Hereby the
unsealed cells were kept in a desiccator in dark between
Table 4. DSSC characteristics employing different polyoxiranesgel electrolytes (best cell values).
Entry Current
density
Open circuit
voltage
Fill
factor
Efficiency
mA � cmS2 mV % %
PEO 18.7 735 56 7.70
PPO 20.3 695 49 6.97
PBO 18.2 765 53 7.41
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the measurements. The results indicate that the cells
containing HMW PEO have in average a better initial
performance than the other two homopolymers. The cells
containing PPO keep higher efficiency for longer time
(Figure 2).
This behavior can be correlated on one side with the
morphology of the polymer gel, in particular the increased
steric hindrance in the polymer chain from PEO to PBO that
reduces both, the void space for the solvent, which acts as a
plasticizer in the gel network, and the free bond rotation,
reducing the ion motion and with it the ion conductivity.
On the other hand, the amorphous structure of the PPO and
PBO plays a decisive role in the increase in the long-term
stability. In the case of PPO, those two parameters are
synergetic combined.
In order to achieve long-term stability with more effi-
cient solar cells, poly(EO-co-PO) copolymers were intro-
duced. We compared two copolymers having different
Figure 2. DSSC efficiency overtime for solar cells based on polymergel electrolyte of HMW PEO, PPO and PBO.
DOI: 10.1002/mame.200800008
High-Molecular-Weight Polyoxirane Copolymers . . .
Figure 3. DSSC efficiency overtime based on polymer gel electro-lyte of HMW PEO and p(EO-co-PO) copolymers.
Figure 4. DSSC efficiency overtime based on polymer gel electro-lyte of HMW PEO and chemically crosslinked p(EO-co-PO) copo-lymer.
amounts of PO content, 17 and 44 mol-%, respectively,
yielding to middle [22%; P(EO-co-PO17)] and low crystal-
linity [8%; P(EO-co-PO44)], respectively. Solar cells based
on copolymer gel electrolyte similar to those based on
homopolymer gel electrolyte were prepared. The perfor-
mance as well as the long-term stability was investigated
as described above (Figure 3). The DSSC based on
P(EO-co-PO) copolymer containing a smaller amount of
PO comonomer (17 mol-%) show in average higher effi-
ciencies and have a longer lifetime compared to those
containing 44 mol-% and DSSC based on homo-PEO. This
teaches that a small content of comonomer in the
copolymer is enough to increase the long-term stability
without loosing the good ion diffusion properties of the
PEO-based polymer gel electrolyte.
So far we only reported about physical polymer gel
electrolytes, which can change its morphology with time.
Chemically crosslinked polymer gels show increased
mechanical stability due to the covalent bindings, and
thus should keep the plasticizer longer in the polymer
which should increase the long-term stability. To investi-
gate the influence of these features, chemically crosslinked
polymer gels were tested in DSSC. In the experiment, we
compared the long-term performance of solar cells based
on the standard physical PEO and the chemically cross-
linked P(EO-co-PO) copolymers gels with 21 mol-% PO
[P(EO-co-PO21)], under the same condition as before. The
results are summarized in Figure 4. Indeed an efficiency of
6% for cells containing the chemically crosslinked copoly-
mer gels could be maintained for more than 25 d, whereas
the solar cells based on physical copolymer show after
the some period an efficiency of 1% (Figure 3). Further
systematic investigation of DSSC containing cross-
linked polymer gels are ongoing and will be reported
elsewhere.
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Conclusion
The copolymerization of EO and PO initiated by the
calcium amide/alkoxide system resulted in HMW copoly-
mers of decreased crystallinity compared to the homo-PEO
polymer. Depending on the synthesis conditions various
copolymers with different contents of PO and different
crystallinities can be synthesized. The preparation of
polymer gel electrolytes based on HMW polyoxirane
(co)polymers and their long-term performance in DSSC
was investigated. It was found that the DSSC based on
P(EO-co-PO) copolymer containing small amounts of PO
(17–21 mol-%) show higher efficiencies and a longer
lifetime compared to both the P(EO-co-PO) copolymer with
high PO content (44 mol-%) as well as the homo-PEO and
homo-PPO-based DSSC. It seems that a small content of PO
in the copolymer is sufficient to introduce a certain degree
of disorder in the polymer structure which reduces
significantly the crystallinity and increases the long-term
stability. The good ion diffusion properties in a polymer gel
electrolyte of such copolymers are maintained. Impor-
tantly, the DSSC based on chemically cross-linked
P(EO-co-PO) gel electrolytes show much longer life-time
compared to physical P(EO-co-PO) gel electrolytes. This
result can be attributed to the fact that the chemically
crosslinked polymer gels show better mechanical stability
in terms of EC/PC leakage compared to the physical gel.
Acknowledgements: P. P. thanks Wyatt Technology Corporation(Santa Barbara, CA, USA) for the generous loan of the DAWN-DSP/Optilab light scattering system and Dr. Ch. Novakov for themeasurements.
Received: January 9, 2008; Revised: March 12, 2008; Accepted:March 17, 2008; DOI: 10.1002/mame.200800008
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P. Petrov et al.
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Keywords: copolymerization; crosslinking; dye-sensitized solarcells; high-molecular-weight PEO copolymers
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DOI: 10.1002/mame.200800008
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