Fc

ZNI

a

ARRAA

KPOECN

1

motsbawmsTuPaPimoew

e

0d

Synthetic Metals 160 (2010) 921–926

Contents lists available at ScienceDirect

Synthetic Metals

journa l homepage: www.e lsev ier .com/ locate /synmet

acile synthesis of dispersible spherical polythiophene nanoparticles byopper(II) catalyzed oxidative polymerization in aqueous medium

i Wang ∗, Yanyan Wang, Dong Xu, Eric Siu-Wai Kong ∗, Yafei Zhang ∗

ational Key Laboratory of Nano/Micro Fabrication Technology, Key Laboratory for Thin Film Microfabrication of the Ministry of Education,nstitute of Micro/Nano Science & Technology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China

r t i c l e i n f o

rticle history:eceived 17 October 2009eceived in revised form 15 January 2010ccepted 1 February 2010vailable online 23 February 2010

a b s t r a c t

Polythiophene (PT) nanoparticles with regular spherical morphology, which were not commonlyobserved in traditional iron(III) catalyzed oxidative polymerization of thiophene in organic solvents, havebeen synthesized by copper(II) catalyzed oxidative emulsion polymerization with high yields in aqueousmedium. Different copper(II) salts were shown to have significant influence on the rate of polymerization

eywords:olythiophenexidative polymerizationmulsion polymerization

as well as the properties and the morphology of PT. Cupric nitrate, cupric sulfate and cupric chloride arefound efficient to catalyze the polymerization, and spherical PT particles with diameters of 60–100 nmwere prepared with high yields (86–98%). While using cupric bromide only lead to irregular productswith relatively low yield (39%). The different effects of these copper salts on the surface resistivity andthermal stability of the product were also discussed. Additionally, sodium dodecyl sulfonate as anionic

ble o

opper(II) salt oxidantanoparticles

surfactant was found capasystem.

. Introduction

As one of the most important intrinsically conducting poly-ers, polythiophene (PT) has shown great potential in the field

f organic electronics [1–6], especially in the production of pho-ovoltaic devices [2,3,6]. The typical layered structures of polymerolar cell require that those conducting polymer including PT toe used in the form of thin films [7–9]. Those films were usu-lly cast from corresponding polymer solution. Unsubstituted PTas conventionally made by chemical and electrochemical poly-erization. However, it cannot be melted or dissolved in common

olvents, thus limiting its practical applications to a large extent.o date the most extensively used solution to address this insol-bility problem is to incorporate kinds of substituents onto theT backbone, such as alkyl [10], alkoxyl [11], perfluoroalkyl [12],mine [13], carboxyl [14] and zwitterionic groups [15], makingT soluble or dispersible in organic solvents, water or supercrit-cal fluids. These soluble PT derivatives are directly used as film

aterials. Great achievements have been made in the synthesisf soluble PT derivatives for polymer solar cells [16,17], how-ver, these synthetic procedures are quite complex and expensivehich generally require stringent conditions and several individ-

∗ Corresponding authors. Tel.: +86 021 34204322.E-mail addresses: ziwang@sjtu.edu.cn, joshua.w@163.com (Z. Wang),

swkong@sjtu.edu.cn (E.S.-W. Kong), yfzhang@sjtu.edu.cn (Y. Zhang).

379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2010.02.001

f increasing the polymer thermal stability for this aqueous polymerization

© 2010 Elsevier B.V. All rights reserved.

ual steps. Furthermore, some toxic solvents are usually involvedwhich are not environmentally favorable. In terms of organicphotovoltaic applications, the material stability under continuousillumination has to be taken into account [9]. The solubilizing sidechains may be passive in terms of light harvesting and chargetransport [18], and they may also make the materials soft andallow for diffusion of small molecules and constituents [19,20].Recently, Bjerring et al. [21] successfully prepared unsubstitutedPT films by solution processing thermocleaving method, in whichthe solubilizing side chains were removed after formation of thefilm.

One of the alternatives for the synthesis of soluble PT deriva-tives is to prepare dispersible PT nanostructures by heterogeneousoxidative polymerization. In addition to the intrinsic properties ofPT exhibited by these nanostructures, other bonus advantages mayalso be introduced, e.g. huge specific surface area especially usefulfor gas sensor application. In most cases, the oxidative polymeriza-tion of thiophene takes place in organic solvents such as chloroformand acetonitrile [22], being similar to precipitation polymerizationwith monomer soluble in continuous phase and polymer precipi-tating out during the polymerization. Tran et al. [23] pointed outthat with this classic method one can only obtain coagulated struc-ture with irregular morphology. Instead of direct polymerization

from thiophene monomer, they initiated the polymerization fromthiophene oligomer and 1-D PT nanowires were formed. Nonethe-less, how to prepare well-dispersed 0-D PT nanoparticles withregular morphology by templateless oxidative polymerization stillremains a challenge.

9 c Metals 160 (2010) 921–926

tpiosreooafsmcriy[tpdaeiicvl

bopmodops

2

2

us(p(aaR

2

aa3ai

rd

22 Z. Wang et al. / Syntheti

Recently, researchers started the effort of preparing PT nanos-ructures by using various surfactants in traditional oxidativeolymerization. Gök et al. [24] reported the oxidative polymer-

zation of thiophene in chloroform with ferric chloride (FeCl3) asxidant in the presence of surfactants. They found that anionicurfactant sodium dodecylbenzenesulfonate revealed interestingibbon structure and PT formed layered structure in the pres-nce of cationic and non-ionic surfactant, although the solubilityf synthesized PT was not mentioned. Li et al. [25] investigated thexidative polymerization of thiophene in acetonitrile with FeCl3s oxidant and cetyltrimethylammonium bromide as cationic sur-actant. PT nanoparticles were obtained by the optimization of theurfactant/FeCl3/monomer molar ratio, although with coagulatedorphology. As mentioned above many organic solvents such as

hloroform and acetonitrile are toxic. Although water is an envi-onmental friendly medium, oxidative polymerization of thiophenen aqueous medium has hardly been reported due to the very lowield caused by poor solubility of monomer in water [25]. Lee et al.26,27] first reported the oxidative polymerization of unsubstituedhiophene in water in two very recent papers. By using hydroxyleroxide as a “co-oxidant” to regenerate the Fe3+ from Fe2+ formeduring the polymerization, high yield could be achieved using onlysmall amount of FeCl3. The synthesized PT nanoparticles could beasily redispersed in various organic solvents although their shapes somewhat irregular according to the TEM micrographs. Furthernvestigations on how to perform facial synthesis PT nanoparti-les in aqueous medium are necessary. In addition, the effect ofarious oxidants on polymerization requires more study, since theiterature using the oxidants other than iron (III) salts are rare [28].

This paper reports the synthesis of spherical PT nanoparticlesy the oxidative polymerization of thiophene catalyzed by a seriesf copper(II) salts in aqueous medium. The effects of different cop-er(II) salts on the polymerization rate as well as the properties andorphology of PT were discussed. The results show that the rate

f polymerization strongly depends on the types of copper(II) saltsespite their very low concentration. Also these copper(II) oxidantsf small amount could lead to PT materials with quite different mor-hologies and properties such as surface resistivity and thermaltability.

. Experimental

.1. Materials

Thiophene (99% Lancaster) was stored in a refrigerator beforese. Sodium dodecyl sulfonate (SDS), hydroxyl peroxide (H2O2, 30%olution), cupric chloride dihydrate (CuCl2·2H2O), cupric bromideCuBr2), cupric nitrate trihydrate (Cu(NO3)2·3H2O), cupric sulfateentahydrate (CuSO4·5H2O), sodium chloride, dimethyl sulfoxideDMSO), N,N′-dimethylformamide (DMF), tetrahydrofuran (THF),cetone, methanol, iodine, 1,4-dioxane, ethyl ether and chloroformre all analytical grade reagents purchased from Shanghai Chemicaleagent Company, and were used as received.

.2. Synthesis of polythiophene nanoparticles

The reaction was performed in a 250 mL flask equipped withmechanical stirrer at 50 ◦C. 0.1 g SDS and 2 g thiophene was

dded into 60 mL deionized water and the mixture was stirred at00 rpm for 20 min. Then 9 g 30% H2O2 solution was introduced,

nd 2 × 10−4 mol copper(II) salts dissolved in 5 mL water was addedn one portion. The reaction was allowed to proceed for 7 h.

Yield calculation: A degassed syringe was used to extract 5 mLeacting mixture at given time intervals. The extracted mixture wasiluted with 5 mL deionized water, and the polymer was precip-

Fig. 1. Yield vs reaction time curves for oxidative polymerization of thiophene cat-alyzed by different copper(II) salts.

itated by adding sodium chloride followed by centrifugation. Theprecipitate was washed with deionized water for several times andthen dried under vacuum at 50 ◦C for 48 h.

Iodine vapor doping of PT: 0.1 g PT and 0.5 g iodine particles wereput in a sealed vessel. The iodine particles were not in direct contactwith PT samples. The vessel was put into an oven heated to 60 ◦Cfor 24 h.

2.3. Characterization

The morphology of PT samples was observed under a Zeiss Ultra55 field emission scanning electron microscope, with an accelerat-ing voltage of 5 kV and InLens observation mode. The samples werediluted with dionized water and coated on a silicone wafer for SEMcharacterization. The surface resistivity of PT samples coated onslides was measured by a RTS-8 four-probe resistivity instrument(Four Probe Tech.) at room temperature. UV–vis absorption spec-tra were recorded on a Shimadzu UV-2450 system and the samplewas prepared as dilute solution of DMSO or methanol in a quartzcell. Thermal stability test was performed on a PerkinElmer Pyris1 thermogravimetric analyzer with heating rate of 10 ◦C/min from50 to 800 ◦C.

PT dispersibility test: 30 mg dried PT samples were added into avial and then 10 mL of given organic solvent was introduced. Digitalphotos were taken after 24 h.

3. Results and discussion

3.1. Polymerization process

Fig. 1 shows the time-conversion curves for the oxidative poly-merization of thiophene by different copper(II) salts. The amountsof copper salt were kept the same (2 × 10−4 mol) for all the 4 poly-merization systems in order to investigate the effect of their types.The reaction rates for all the 4 systems were relatively high inthe beginning of polymerization, and they dropped gradually asthe reactions proceeded. Among all the 4 oxidants, Cu(NO3)2 andCuSO4 demonstrated the highest catalytic capability. The yieldswere above 70% at 45 min and exceeded 90% after 7 h. Although the

rate of the polymerization mediated by CuCl2 was slightly lowerthan the above two in the beginning of reaction (the yield was56% at 45 min), the yield was pushed up to 86% after 7 h. Whereasthe rate of polymerization in CuBr2 catalyzed system was signifi-cantly low, whose yield after 7 h was 39%. Since the concentrations

Z. Wang et al. / Synthetic Metals 160 (2010) 921–926 923

r(II) o

otattttpiHwmt

pCcrilC

ps

Fig. 2. SEM micrographs of PT samples synthesized with different coppe

f Cu2+ were the same, the deviation in the rates of polymeriza-ion should be attributed to the different oxidant anions involvedlthough their concentrations were low. Lee et al. [26] revealedhat in the presence of H2O2 a small amount of FeCl3 was enougho obtain high yield in thiophene polymerization. They believedhat the high yield was due to the continuous regeneration of Fe3+

hrough the oxidation of Fe2+ formed in reaction by H2O2. In theresent study H2O2 was also employed in order to regenerate Cu2+

on from Cu+, so only a small amount of Cu(II) salts were used.ence the concentrations of oxidant anions in the polymerizationere quite low. It can be inferred from above that the rate of poly-erization depends strongly on the types of oxidant anions despite

heir low concentrations.For further investigation on the effect of oxidant anions, an extra

olymerization run was performed. In addition to 2 × 10−4 moluBr2 as oxidant, 4 × 10−4 mol HNO3 was added to simulate thease of using Cu(NO3)2 as oxidant. However, the increase in theate of polymerization was not observed. This indicated that thentroduction of additional NO3

− in the presence of Br− could not

ead to the same reaction rate as that for the polymerization withu(NO3)2 as oxidant.

Consider the reaction process. In the very beginning of theolymerization, some of the thiophene monomers form micellestabilized by the anionic surfactant SDS, and the rest form monomer

xidants: (a and b) CuCl2; (c) CuBr2; (d and e) CuSO4; (f and g) Cu(NO3)2.

drops in aqueous phase. The copper salts which initiate the poly-merization are soluble in water. This initial status is similar tothat of traditional emulsion polymerization [29], except that thekinetics of reaction deviates from Smith–Ewart equations becausethe oxidative polymerization of thiophene does not seem to fol-low chain-wise mechanism [30]. Due to the overwhelming surfacearea of micelles compared to that of monomer drops, the reactiontaking place in those drops is actually not significant [29]. The pro-cedure of polymerization is generally recognized as follows [30]:(1) the metal cations diffuse into the micelles; (2) the oxidation ofthiophene monomer by the metal cations followed by the forma-tion of cationic radicals; (3) The coupling between the thiophenecationic radicals to form the dimers; (4) The dimers are further oxi-dized to form cationic radicals and continue the coupling reactionto gradually form high polymer chains. The functions of the oxi-dant anions are probably two folds: they may interfere with thediffusion process of metal cations from aqueous phase to micellesby electrostatic attraction; or they may interact with monomericand oligomeric cationic radicals thus affect their propagation reac-

tion. Although few papers directly investigated the effect of theinteraction between oxidant anions and metal cations on the oxida-tive polymerization of thiophene, the influence of the interactionbetween surfactant ions and metal cations was realized [26]. Theanionic surfactant was able to draw the metal cations into the

924 Z. Wang et al. / Synthetic Metals 160 (2010) 921–926

(NO3

mtwtctmdassittep

3

woiw(tpaCisbhdo

nosbtpdimamfnsmpcl

Fig. 3. Dispersions of PT samples (with Cu

icelles by electrostatic attraction to promote the initiation reac-ion therein. But the cationic surfactant may act in the oppositeay thus decrease the rate of polymerization. Since the same elec-

rostatic attraction exists between the oxidant anions and metalations in aqueous phase and micelles, it seems reasonable to takehe possibility of this electrostatic attraction affecting on the poly-

erization into account. Furthermore, there has been experimentalata showing that the chlorine anion in the oxidant (FeCl3) acts asdopant in the final PT materials synthesized [25]. Therefore it

eems reasonable to infer that, in this research the oxidant anionsuch as NO3

− might also interact with propagating cationic rad-cals thus interfere with the polymerization procedure. Althoughhis complicated effect of the oxidant anions on the polymeriza-ion of thiophene needs further investigation, it is quite clear thatven a low concentration of oxidant anions is enough to affect theolymerization rate significantly.

.2. Morphology of polythiophene

Fig. 2 showed the SEM micrographs of the PT samples. Fairlyell-dispersed PT nanoparticles with regular spherical morphol-

gy were synthesized using CuCl2 as oxidant, whose diameters falln the range of 60–100 nm (Fig. 2a and b). By contrast, the product

ith irregular bulk morphology was obtained with CuBr2 as oxidantFig. 2c). Using CuSO4 and Cu(NO3)2 could also lead to the forma-ion of spherical PT nanoparticles with similar diameters as thoserepared under CuCl2. Though the PT particles with CuSO4 (Fig. 2dnd e) agglomerated quite some with ill-defined edges, those byu(NO3)2 (Fig. 2f and g) seem to be dispersed better. The spher-

cal shape of these nanoparticles indicates that the stable colloidystem was formed as the polymerization proceeded, i.e. the com-ination of CuCl2 (or CuSO4, Cu(NO3)2)/water/SDS could offer quiteigh stabilization efficiency. These results demonstrate that usingifferent copper(II) salts as oxidants will influence the morphologyf the PT synthesized, in addition to the rate of polymerization.

In recent years, it has been revealed that solvent-dispersibleanostructures of PT and its derivatives can be synthesized byxidative polymerization method with the addition of suitableurfactants. The morphology of PT varied much with different com-ination of solvents and surfactants. Nonetheless, in most caseshe obtained PT particles were in more or less agglomerated mor-hology, thus necessitating the improvement in the stability ofispersed polymer phase during polymerization. We believe that

f small molecular surfactants are used in the heterogeneous poly-erization of thiophene, water as the reaction medium may begood choice to form the stable colloid system during the poly-erization. In aqueous medium, the surfactant molecules will

orm spherical micelles spontaneously owning to their amphiphilicature, with the hydrophilic parts extending and the hydrophobic

egments aggregating inside. These micelles can accommodate theonomers and become the main loci for polymerization. As the

olymerization proceeds the hydrophobic segments of surfactantsan adsorb onto the polymer chains to form colloid particles, theatter will stabilize themselves through the electrostatic repulsion

)2 as oxidant) in various organic solvents.

(ionic surfactant) or steric effect (non-ionic surfactant). Throughthis way well-dispersed polymer micro- or nanoparticles with reg-ular spherical morphology can be obtained. On the other hand, itis difficult for these small molecular surfactants to aggregate intomicelles in organic solvents since the hydrophobic segment of sur-factant is soluble in continuous phase. When the molecular weightof the propagating oligomers exceeds the critical chain length forprecipitation, there is not enough driving force for the hydrophobicsegments of surfactant to adsorb onto the oligomer chains (namely“over soluble”). Then these oligomers coagulate and precipitateinstead of continuing the propagation reaction in the colloid par-ticles stabilized by surfactants. This might be the reason why itwas quite difficult to prepare well-dispersed spherical PT parti-cles without coagulation in organic solvents when small molecularsurfactants were employed. In order to achieve stable dispersedphase in organic solvents, amphiphilic macromolecular surfactantswith both solvent-soluble and solvent-insoluble segments may berequired. This proposed system resembles the classical dispersionpolymerization and the relevant research is underway in our labo-ratory.

After being dried, those PT nanoparticles can be redispersedin some commonly used organic solvents after being dried. Fig.3 showed the photographs of the dispersions of PT sample (withCu(NO3)2 as oxidant) in various solvents. Among all the 9 solventstested, PT nanoparticles showed the highest dispersibility in DMSOand DMF without any precipitation after 1 month. They could alsobe easily dispersed in THF, acetone, methanol and ethanol, althoughsome precipitation was observed after 2 weeks. By comparison, thePT sample could not be totally dissolved in dioxane under the testconditions. They could not be dispersed in ethyl ether or chloro-form since no color change was observed in the liquid phase of themixture.

Many practical applications demand the conducting polymerbeing used as films. One of the most important properties thatcharacterize the conducting film materials is the surface resistiv-ity. According to the data listed in Table 1, the surface resistivitiesof undoped PT are quite high. Among all the four samples the oneprepared with CuCl2 presents the lowest value of 25 k�/�, whilesamples prepared with CuSO4 and Cu(NO3)2 give slightly highervalues of 33 k�/� and 30 k�/� respectively. The surface resistiv-ity for the sample synthesized with CuBr2 is high as 97 k�/�. Afterbeing doped with iodine vapor, all the surface resistivities for PTsamples except that prepared with CuBr2 was lowered by about onemagnitude. The data for the two samples with CuSO4 and Cu(NO3)2become slightly lower than that with CuCl2. Although a significantdrop of surface resistivity was observed on doping for the samplewith CuBr2, it is still in the order of 104 �/�. Such a high surfaceresistivity for this sample may be attributed to its low yield andirregular morphology.

Fig. 4 showed the UV–vis absorption spectra for methanol andDMSO dispersions of PT samples. In comparison with the absorp-tion peak around 320 nm attributed to the �–�* transition forthiophene monomer, all the PT samples showed absorption around400 nm due to the �–�* transition for large �-conjugation struc-

Z. Wang et al. / Synthetic Metals 160 (2010) 921–926 925

Table 1Effect of different copper salts on polythiophene by oxidative polymerization.a.

Samples Copper salt Yield (%) Morphology Surface resistivityundoped (�/�)b

Surface resistivitydoped (�/�)c

PT-1 CuCl2·2H2O 86 Spherical 25 5.9PT-2 CuBr2 39 Irregular 97 26PT-3 CuSO4·5H2O 92 Spherical 33 3.5PT-4 Cu(NO ) ·3H O 98 Spherical 30 2.9

, t = 7 h.

ti4tIpsa

stoft

F

3 2 2

a Thiophene 2 g, SDS 0.1 g, H2O2 9 g, H2O 65 mL, copper salt 2 × 10−4 mol, T = 50 ◦Cb Surface resistivity of undoped polythiophene samples.c Surface resistivity of iodine doped polythiophene samples.

ure. These results are similar to those reported by Lee et al. [26]n FeCl3 catalyzed polymerization. However, the UV absorption at00 nm indicates somewhat limited effective conjugated length ofhe synthesized PT which needs to be improved in future study.t is also interesting to note that, the wavelength of absorptioneak for PT prepared with CuBr2 dispersed in DMSO (406 nm) waslightly higher than the other three (398 nm), despite its low yieldnd conductivity.

The data for thermal stability are described by TGA curveshowed in Fig. 5. The PT prepared with CuSO4 exhibited the best

hermal stability with the temperature for 10% weight loss (Td 10%)f 292 ◦C and the high residual weight of 25%. The thermal stabilityor the two samples synthesized with CuCl2 and Cu(NO3)2 is almosthe same, whose Td 10% are 283 ◦C and 282 ◦C respectively. The Td 10%

ig. 4. UV–vis absorption spectra of PT dispersed in (a) methanol and (b) DMSO.

Fig. 5. TGA curves of PT samples prepared in aqueous phase catalyzed by variouscopper(II) salts.

for the sample prepared with CuBr2 is the lowest, being 265 ◦C, andthe residual weight is only 10%. For the oxidative polymerizationof thiophene, the molecular weight rises with the monomer con-version. The molecular weight of the PT synthesized with CuBr2is probably low due to the low yield, thus decreasing its thermalstability. In addition, the TGA test for the PT synthesized withoutsurfactant (with Cu(NO3)2 as oxidant) was performed in order tocheck if the surfactant will affect the thermal stability. The resultshowed that Td 10% for this sample is 271 ◦C, lower than that usingSDS as surfactant. In the case here, it seems adding anionic sur-factant SDS can somewhat improve the thermal stability of the PTsynthesized.

Recently, high efficiency polymer solar cells based on unsubsti-tuted PT has been reported, showing the capability of unsubstitutedPT for photovoltaic applications [18]. The polymer ink plays animportant role in spin-coating as well as roll-to-roll process [31].Since these unsubstituted PT nanoparticles can be easily dispersedin some common organic solvents with quite high stability, theirpotential for making inks for solar cells and other electroactivedevices could be expected.

4. Conclusion

This paper reports the oxidative polymerization of thiophenein water with a series of copper(II) salts as oxidants in the pres-ence of anionic surfactant SDS and H2O2. The results indicate thatdifferent copper salts have quite obvious influence on the rate ofpolymerization as well as the morphology and the property of

PT synthesized. Using CuCl2, CuSO4 and Cu(NO3)2 as the oxidantscould achieve high yields in 7 h and PT nanoparticles with regularspherical morphology, relatively low surface resistivities and highthermal stability. By contrast, PT sample with CuBr2 as the oxidantwas obtained with low yield and irregular morphology, and it also

9 c Meta

stpwbat

A

oNN

R

[

[

[[

[[[[

[[[[

[[[[[

26 Z. Wang et al. / Syntheti

howed high surface resistivity and low thermal stability comparedo the other three samples. These PT samples could be easily dis-ersed in some common organic solvents such as DMF and DMSO,ith good stability and the surface conductivity of their films could

e improved by doping with iodine vapor. It was also found that thenionic surfactant SDS was able to increase the thermal stability ofhe product in aqueous oxidative polymerization of thiophene.

cknowledgements

This work is supported by National Natural Science Foundationf China No. 50730008, Shanghai Science and Technology Granto. 09JC1407400 and National Basic Research Program of Chinao. 2006CB300406.

eferences

[1] A.P.H.J. Schenning, E.W. Meijer, Chem. Commun. (2005) 3245.[2] S. Gunes, H. Neugebauer, N.S. Sacriftci, Chem. Rev. 107 (2007) 1324.[3] J.A. Mikroyannidis, M.M. Stylianakis, Q. Dong, Y. Zhou, W. Tian, Synth. Met. 159

(2009) 1471.[4] W. Liu, M. Pink, D. Lee, J. Am. Chem. Soc. 131 (2009) 8703.[5] B. Lim, K. Baeg, H. Jeong, J. Jo, H. Kim, J. Park, Y. Noh, D. Vak, J.-H. Park, J.-W.

Park, D.-Y. Kim, Adv. Mater. 21 (2009) 2808.[6] B.C. Thompson, J.M.J. Fréchet, Angew. Chem. Int. Ed. 47 (2008) 58.[7] F.C. Krebs, Sol. Energy Mater. Sol. Cells 93 (2009) 394.

[[[[[

ls 160 (2010) 921–926

[8] J. Brouclé, P. Ravirajan, J. Nelson, J. Mater. Chem. 17 (2007) 3141.[9] M. Jørgensen, K. Norrman, F.C. Krebs, Sol. Energy Mater. Sol. Cells 92 (2008)

686.10] R.D. McCullough, R.D. Lowe, M. Jayaraman, D.L. Anderson, J. Org. Chem. 58

(1993) 904.11] E.E. Sheina, S.M. Khersonsky, E.G. Jones, R.D. McCullough, Chem. Mater. 17

(2005) 3317.12] L. Li, K.E. Counts, S. Kurosawa, A.S. Teja, Adv. Mater. 16 (2004) 180.13] K. Ogawa, J.A. Stafford, S.D. Rothstein, D.E. Tallman, S.C. Rasmussen, Synth. Met.

152 (2005) 137.14] Z. Bao, A.J. Lovinger, Chem. Mater. 11 (1999) 2607.15] K.P.R. Nilsson, O. Inganas, Nat. Mater. 2 (2003) 419.16] E. Bundgaard, F.C. Krebs, Sol. Energy Mater. Sol. Cells 91 (2007) 954.17] R. Kroon, M. Lenes, J.C. Hummelen, P.W.M. Blom, B. De Bore, Polym. Rev. 48

(2008) 531.18] S.A. Gevorgyan, F.C. Krebs, Chem. Mater. 20 (2008) 4386.19] F.C. Krebs, K. Norrman, Prog. Photovolt. Res. Appl. 15 (2007) 697.20] K. Norrman, F.C. Krebs, Sol. Energy Mater. Sol. Cells 90 (2006) 213.21] M. Bjerring, J.S. Nielsen, N.C. Nielsen, F.C. Krebs, Macromolecules 40 (2007)

6012.22] K. Yoshino, S. Hayashi, R. Sugimoto, Jpn. J. Appl. Phys. 23 (1984) 899.23] H.D. Tran, Y. Wang, J.M. D’Arcy, R.B. Kaner, ACS Nano 2 (2008) (1841).24] A. Gok, M. Omastova, A.G. Yavuz, Synth. Met. 157 (2007) 23.25] X.-G. Li, J. Li, M.-R. Huang, Chem. Eur. J. 15 (2009) 6446.26] S.J. Lee, J.M. Lee, I.W. Cheong, H. Lee, J.H. Kim, J. Polym. Sci. Part A: Polym. Chem.

46 (2008) 2097.27] J.M. Lee, S.J. Lee, Y.J. Jung, J.H. Kim, Curr. Appl. Phys. 8 (2008) 659.28] H. Masuda, D.K. Asano, K. Kaeriyama, Synth. Met. 119 (2001) 167.29] G. Odian, Principles of Polymerization, 4th ed., John Wiley & Sons, 2004, p. 351.30] J. Roncali, Chem. Rev. 92 (1992) 711.31] F.C. Krebs, S.A. Gevorgyan, J. Alstrup, J. Mater. Chem. 19 (2009) 5442.