High-Molecular-Weight Polyoxirane Copolymers and their Use in High-Performance Dye-Sensitized Solar...

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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

� 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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.



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

www.mme-journal.de 599

P. Petrov et al.

600

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



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


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.



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


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



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


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.



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


P. Petrov et al.

604

Keywords: copolymerization; crosslinking; dye-sensitized solarcells; high-molecular-weight PEO copolymers

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DOI: 10.1002/mame.200800008

High-Molecular-Weight Polyoxirane Copolymers and their Use in High-Performance Dye-Sensitized Solar...

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