high performance polymer field-effect transistors based on polythiophene derivative with conjugated...

High Performance Polymer Field-Effect Transistors Based onPolythiophene Derivative with Conjugated Side Chain

YOUJUN HE,1,2 WEIPING WU,1,2 YUNQI LIU,1 YONGFANG LI1

1Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry,Chinese Academy of Sciences, Beijing 100190, China

2Graduate University of Chinese Academy of Sciences, Beijing 100049, China

Received 6 April 2009; accepted 15 June 2009DOI: 10.1002/pola.23579Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Poly(3-[2-(5-hexyl-2-thienyl) ethenyl]-2,20-bithiophene) (P2, see Scheme 1)with conjugated thienylvinyl side chain was synthesized by copolymerization of the thio-phene units with and without conjugated side chain with Pd-catalyzed Stille couplingmethod. For comparison, P1 with the hexyl side chain instead of conjugated side chainwas also synthesized. P2 film shows broad absorption in the visible region with absorptionedge at about 700 nm. The solution-processed polymer field-effect transistors were fabri-cated and characterized with bottom gate/top contact geometry. The organic field-effecttransistors (OFET) based on P2 showed an average hole mobility of about 0.034 cm2/Vs(the highest value reached 0.061 cm2/Vs) upon annealing at about 180 �C for 30 min, witha threshold voltage of �1.15 V and an on/off ratio of 104 with n-octadecyltrichlorosilane(OTS) modified SiO2 substrate. In comparison, the OFET based on P1 displayed a holemobility of 8.9 � 10–4 cm2/Vs and an on/off ratio of 104 with OTS modified SiO2 substrate.The results indicate that the polythiophene derivative with conjugated thienylvinyl sidechain is a promising polymer for the application in polymer field-effect transistors. VVC 2009

Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5304–5312, 2009

Keywords: charge transport; conjugated polymers; conjugated side chain; holemobility; organic field effect transistors; polythiophene derivatives; UV–visspectroscopy

INTRODUCTION

In the past decades, organic conjugated polymersattracted great attentions for the applications inpolymer solar cells1–5 and organic field-effect tran-sistors (OFET).6,7 Molecular packing and chargetransport are key factors governing the use of con-jugated materials in electronic applications. Thecharge transport properties of the conjugatedpolymers relies on the control of the aggregation

and orientation of the polymer chains in the solidstate.8–13 Therefore, it is of crucial importance tostudy the effect of the molecular structure andsubstituents on the charge transport behavior ofthe conjugated polymers.

Solution-processable amorphous polymers areimportant semiconducting materials for the lowcost fabrication of OFETs. Veres et al. reportedthe fabrication of OFET based on polytriaryl-amines by spin-coating of the polymer solution,and achieved a OFET hole mobility close to10�2 cm2/Vs.14 Chung et al.15 synthesizedpoly[(1,2-bis-(20-thienyl)vinyl-50,500-diyl)-alt-(9,9-di-octyldecylfluorene-2,7-diyl] (PTVTF), a solution-processed OFET based on PTVTF with a bottom

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 47, 5304–5312 (2009)VVC 2009 Wiley Periodicals, Inc.

Correspondence to: Y. Li (E-mail: [email protected]) orY. Liu (E-mail: [email protected])

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gate/top contact geometry exhibited good deviceperformance with superior reproducibility. TheFET hole mobility reached 2 � 10�2 cm2/Vs with agood subthreshold swing of 1.7 V/decade. For theapplication of the OFETs, further improvement ofthe hole mobility is required.

Substitution of conjugated side chains on themain chains of polythiophene and poly(thienylenevinylene) was performed in our group to broadenthe absorption of the polymers for improving theirphotovoltaic properties in recent 3 years.16–21 Apolythiophene derivative with phenothiazine-vinylene conjugated side chains showed a highhole mobility of 6.8 � 10�3 cm2/Vs from OFETmeasurement.22 In order to further study theeffect of the conjugated side chains on the holemobility of the conjugated polymers, we synthe-sized a new polythiophene derivative with conju-gated thienylvinyl side chain (P2, as shown inScheme 1) by copolymerization of the thiopheneunits with and without the conjugated side chainwith Pd-catalyzed Stille coupling method. Forcomparison, P1 with the hexyl side chain insteadof conjugated side chain was also synthesized. Themaximum hole mobility of P2, measured from thesolution-processed OFET with bottom gate/topcontact geometry, reached 0.061 cm2/Vs (averagehole mobility of P2 is ca. 0.034 cm2/Vs), and theOFET showed an on/off ratio of 104 and thethreshold voltage (VT) of �1.15 V. The hole mobil-ity 0.061 cm2/Vs of P2 is one of the highest valuesamong the amorphous conjugated polymers.

EXPERIMENTAL

Materials

Tetrahydrofuran (THF) was dried over Na/benzo-phenone ketyl and freshly distilled prior to use.Compound 2, 2,5-bis (tributylstannyl) thiophene,

2,5-dibromo-3-hexylthiophene, P1 and compound 1were prepared by the procedure reported in the lit-eratures.17,18,23–25 All other reagents and solventswere purchased commercially as an analytical-grade quality and used without further purification.

Measurements

1H NMR spectra were measured on a BrukerDMX-400 spectrometer. Chemical shift of NMRwere reported in ppm relative to the singlet at7.26 ppm for 1H NMR of CDCl3. Splitting patternswere designated as s (singlet), d (doublet), t (tri-plet), m (multiplet), and br (broaden). Absorptionspectra were taken on a Hitachi U-3010 UV-visspectrophotometer. Photoluminescence spectrawere measured using a Hitachi F-4500 spectro-photometer. Absorption and Photoluminescencespectra measurements of the polymer solutionswere carried out in chloroform at 25 �C. Absorp-tion and photoluminescence spectra measure-ments of the polymer films were carried out onquartz plates spin-coated from the polymer solu-tions in chloroform at 25 �C. Molecular weight ofthe polymers was measured by GPC method withthe temperature of column at 30 �C and the flowspeed of the eluent THF at 1 mL/min, and poly-styrene was used as a standard. TGA measure-ment was performed on a Perkin-Elmer TGA-7.X-ray diffraction (XRD) measurements of the poly-mer thin films were carried out with a 2 kWRigaku x-ray diffraction system. The electrochemi-cal cyclic voltammetry was conducted on a ZahnerIM6e Electrochemical Workstation with Pt disc, Ptplate, and Ag/Agþ electrode as working electrode,counter electrode, and reference electrode respec-tively in a 0.1 mol/L tetrabutylammonium hexa-fluorophosphate (Bu4NPF6) acetonitrile solution.Ferrocene was employed as a reference redox sys-tem according to IUPAC’s recommendation.26

Fresh reference electrode was made before eachseries of measurements and calibrated against Fc/Fcþ. Polymer thin films were formed by drop-cast-ing 1.0 mm3 of the polymer solutions in THF (ana-lytical reagent, 1 mg/mL) onto the workingelectrode, and then dried in the air. AFM imageswere obtained using a Digital Instruments Nanop-robe Atomic Force Microscope in the tapping mode.

Fabrication of Polymer Field-EffectTransistor Devices

Polymer OFET devices were fabricated with a topcontact geometry (channel length L ¼ 50 lm and

Scheme 1. Molecular structures of P1 and P2.

HIGH PERFORMANCE POLYMER FET WITH POLYTHIOPHENE 5305

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

channel width W ¼ 3000 lm). Gold (50 nm thick)was used for the source and drain contacts, andsilicon dioxide (SiO2) with a thickness of 500 nmwas used as the dielectric layer. The SiO2 surfacewas cleaned and pretreated with n-octadecyltri-chlorosilane (OTS) to produce apolar and smoothsurfaces onto which the polymer films could beprepared by spin-coating. The polymer solutionsin chlorobenzene with a concentration of 0.5 wt %were filtered through a 0.45 lm pore size poly-vinylidene fluoride (PVDF) membrane syringefilter, and then spin-coated onto the substrates at3000 rpm. The films were dried on a hot platefor 30 min in a vacuum oven. Thermal annealingtreatment for the OFETs was performed at differ-ent temperatures for 30 min. The electrical char-acteristics of the OFET devices were measuredusing Keithley 4200SC semiconductor parameteranalyzer.

Synthesis of the Monomers

Synthetic route of the monomer is shown inScheme 2.

2,5-Dibromo-3-[2-(5-hexyl-2-thienyl)ethenyl]–thiophene, 3

Compound 2 (13.40 g, 0.040 mol) and phosphorousacid triethyl ester (6.64 g, 0.040 mol) were put ina flask and heated to 160 �C for 2 h. The productof (2,5-dibromothiophen-3-ylmethyl)-phosphoricacid diethyl ester was obtained and used directlywithout any purification. GC-MS: M/Z ¼ 376.

In a 250 mL one-necked flask, compound 1(6.47 g, 0.033 mol) and (2,5-Dibromothiophen-3-ylmethyl) phosphoric acid diethyl ester (15.04 g,0.040 mol) were dissolved in 200 mL DMF at 0 �C.A solution of NaOCH3 (2.10 g, 0.040 mol) in 2 mLDMF was later slowly added to the above mix-ture. The reaction mixture was stirred for 0.5 h at

0 �C, then slowly warmed to room temperatureand stirred for overnight. The reaction mixturewas added to 300 mL water. The organic phasewas separated, and the water phase wasextracted with chloroform (3 � 100 mL). The com-bined organics were washed with brine (3 � 100mL), water (3 � 100 mL), and dried over anhy-drous MgSO4. The solvent was removed in vac-uum. The residue was purified with silica columnchromatography eluted with light petroleum toyield compound 3 (8.46 g, 0.019 mol).

Yield: 59.1%. 1H NMR (CDCl3, 400 MHz) d(ppm) 7.17 (s, 1H), 7.02 (d, 1H), 6.92 (d, 1H),6.74 (d, 1H), 6.70 (d, 1H), 2.85 (t, 2H), 1.75 (m,2H), 1.37 (m, 6H), 0.90 (t, 3H). 13C NMR (CDCl3,400 MHz) d (ppm) 127.78, 125.57, 125.26, 111.3,29.7–27.6. M/z ¼ 434.

ELEM. ANAL. for C16H18S2Br2. Calculated: C,44.24%; H, 4.15%; S, 14.75%; Br, 36.87%. Found:C, 43.96%; H, 4.12%; S, 14.69%; Br, 36.79%. IR(KBr, cm�1): 3063.05, 3027.07, 2955.50, 2915.82,2854, 1621.64, 1463.91, 1354.29, 930.07, 784.37,719.68.

Preparation of the Polymers

The synthesis of two polymers was carried outusing palladium-catalyzed Stille-coupling bet-ween 2,5-bis (tributylstannyl) thiophene, and 2,5-dibromo-3-hexylthiophene or 3, as shown inScheme 3. Toluene and bromobenzene were driedafter refluxing 5 h with sodium and then was dis-tilled. All procedures were performed under anair-free environment.

Poly(3-hexyl-2,20-bithiophene) (P1)

GPC: Mw ¼ 74 K; Mn ¼ 38 K; Mw/Mn ¼ 1.95. 1HNMR (CDCl3, 400 MHz) d (ppm) 7.16–7.01 (br,3H), 2.80 (t, 2H), 1.71 (m, 2H), 1.35 (m, 6H), 0.91(t, 3H).

ELEM. ANAL. for (C14H16S2)n. Calculated: C,67.70%; H, 6.45%; S, 25.81%. Found: C, 66.80%;H, 6.32%; S, 24.93%.

Scheme 2. Synthetic route of monomer: (i) DMF,phosphorous acid triethyl ester, CH3ONa, 0 �C, 0.5 h,then room temperature 24 h.

Scheme 3. Synthetic routes of the polymers: (i) Tol-uene, Pd(PPh3)4, refluxed for 12 h.

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Poly(3-[2-(5-hexyl-2-thienyl)ethenyl]-2,20-bithiophene) (P2)

The mixture of 3 (0.434 g, 1 mmol) and 2,5-bis(tributylstannyl) thiophene (0.66 g, 1 mmol) wasput into a three-necked flask. Then, 15 mL ofdegassed toluene was added under the protectionof argon. The solution was flushed with argon for10 min, then 15 mg of Pd (PPh3)4 was added. Thesolution was flushed with argon again for 20 min,and heated to reflux for 12 hours. The terminalbromobenzene and benzene boronic acid wereadded as end-cappers, with the bromobenzeneadded first and the benzene boronic acid added12 h later. After stirring for another 12 h, thereaction solution was cooled to room temperature.The polymer was precipitated by addition of50 mL methanol, and filtered through a Soxhletthimble, then subjected to Soxhlet extraction withmethanol, hexane, and chloroform. The polymerwas recovered as a solid sample from the chloro-form fraction by rotary evaporation. The solid wasdried under vacuum for 1 day to give the finalproduct P2 (370 mg).

GPC: Mw ¼ 41 K; Mn ¼ 29 K; Mw/Mn ¼ 1.44.1H NMR (CDCl3, 400 MHz) d (ppm) 7.16–6.64 (br,7H), 2.81 (t, 2H), 1.73 (m, 2H), 1.32 (m, 6H), 0.89(t, 3H).

ELEM. ANAL. for (C20H20S3)n. Calculated: C,67.42; H, 5.62; S, 26.97. Found: C, 66.83; H, 5.56;S, 25.81.

RESULTS AND DISCUSSION

Synthesis of the Monomers and Polymers

All the polymers were synthesized by Stille-cou-pling of 2,5-dibromo-3-hexylthiophene or mono-mer 3 with 2,5-bis (tributylstannyl) thiophenerespectively. The synthetic routes of the monomerand polymers are shown in Schemes 2 and 3,respectively.

The double bonds between thiophene rings in 3were formed by a Horner-Emmons reaction. Themolecular structure of 3 was determined by 1HNMR spectrum as shown in Figure 1. The 1HNMR spectrum indicates that there is no cis-iso-mer of the compound. The peak at 2.77 ppm iscorresponding to the a-hydrogen linking to thethiophene ring in the conjugated side chain of 3.Figure 2 shows the 1H NMR spectrum of P2.There is a peak at 2.81 ppm (peak 1) which isattributed to the hydrogen at position 1 on theside chain of P2 (see the molecular structure

Figure 1. The 1H NMR spectrum of Compound 3.

Figure 2. 1H NMR spectrum of P2.

Table 1. The Elemental Analysis, Weight-Average Molecular Weight and TGA Properties of the Polymers

Elemental Analysis (%) (Calculated/Found) Molecular Weight by GPC

Polymers C H S Mwa PDs T5d (�C)b

P1 67.70/66.80 6.45/6.32 25.81/24.93 38K 1.95 423P2 67.42/66.83 5.62/5.56 26.96/25.81 41K 1.44 398

aNumber-average molecular weight determined by GPC using polystyrene as the standard in THF solution.bDecomposition temperature determined by TGA in N2 gas based on 5% weight loss.



inserted in Fig. 2). All the polymers are randompolymers and readily soluble in common organicsolvents such as toluene, chloroform and THF.

The number-average molecular weights (Mn),thermal properties and elemental analysis of thepolymers were listed in Table 1.

Thermal Analysis

Thermal stability of the two polymers was investi-gated with thermo gravimetric analysis (TGA), asshown in Figure 3. The onset decomposition tem-peratures of P1 and P2 are at 423 �C and 398 �C,respectively. After introducing the conjugated sidechain on the polythiophene main chain, the ther-mal stability of the polymer decreased a little, butit is still good enough for the applications in opto-electronic devices.

Figure 3. TGA plots of P1 and P2 with a heatingrate of 10 �C/min under inert atmosphere.

Figure 4. Absorption spectra of (a) the polymer sol-utions in chloroform and (b) the polymer films onquartz plates.

Figure 5. Normalized PL spectra of the polymers inchloroform solutions.

Figure 6. Cyclic voltammograms of the polymerfilms on Pt electrode in 0.1 mol/L Bu4NPF6, CH3CNsolution with a scan rate of 100 mV/s.

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Optical Properties of the Polymers

UV-vis absorption spectra could provide a good dealof information on the electronic structures of theconjugated polymers. Figure 4(a) shows the UV-visnormalized absorption spectra of the polymer solu-tions in chloroform. P2 display two absorptionpeaks: the one in visible region is attributed to thep-p* transition of the conjugated polymer mainchains; another in the UV region is attributed tothe conjugated side chains, which is a general phe-nomenon for the conjugated polymers with conju-gated side chains.16–22 In comparison with the visi-ble absorption peak of P1 at 466 nm, the visibleabsorption peaks of P2 is red-shifted by 16 nm.

Figure 4(b) shows the UV–vis absorption spec-tra of the polymer films on quartz plates. In com-parison with the absorption spectra of the poly-mer solutions, the visible absorption peak of P2film red-shifted and enhanced, which could beascribed to the aggregation and interchain inter-actions of conjugated polymer chains in the poly-mer film. The visible absorption peak of P2 film isat 532 nm with an absorption edge at 694 nm,which are red-shifted in comparison with those ofP1 by 26 nm and 56 nm respectively.

Figure 5 shows the photoluminescence (PL)spectra of the polymer solutions in chloroform. P1solution shows a PL peak at about 568 nm. P2 so-lution was excited at 320 nm and 420 nm respec-tively corresponding to the two absorption peaks,for the PL spectra measurement. Only one PLpeak at 598 nm was observed when the P2 solu-tion was excited at the two wavelengths, indicat-ing that there is a thorough intramolecular energytransfer of the excitons from the conjugated sidechains to the main chains when the polymer wasexcited at 320 nm. The same phenomenon wasalso observed in the PL spectra of other conjugatedside chain polythiophenes.16–22

Electrochemical Properties

Electrochemical property is one of the most impor-tant properties of the conjugated polymers and

many applications of the conjugated polymersdepend on the electrochemical properties. Westudied the electrochemical properties of P2 bycyclic voltammetry.

Figure 6 shows the cyclic voltammograms(CVs) of the polymer films on Pt electrode in 0.1mol/L Bu4NPF6, CH3CN solution with a scan rateof 100 mV/s. It was observed that the polymersexhibited reversible oxidation/re-reduction proc-esses at positive potential range and reduction/re-oxidation processes at negative potential range.From the onset oxidation potentials (uox) and theonset reduction potentials (ured) of the polymers,HOMO and LUMO energy levels as well as theenergy gap (Eec

g ) of the polymers were calculatedaccording to the Equations27

Table 2. Electrochemical Onset Potentials and Electronic Energy Levels of the Polymer Films

uox (V vs. Ag/Agþ)/EHOMO (eV) ured (V vs. Ag/Agþ)/ELUMO (eV) Eecg (eV) Eopt

g (eV)a

P1 0.35/�5.06 �1.94/�2.77 2.29 1.94P2 0.25/�4.96 �1.81/�2.90 2.06 1.79

aThe optical band gap was obtained from empirical formula, Eg ¼ 1240/kedge, in which the kedge is the onset value of theabsorption spectrum in the longer wavelength direction.

Figure 7. (a) Output and (b) transfer characteristicsof OFETs using P2 as active layer, IDS was obtainedat VDS ¼ �100 V for transfer characteristics.



HOMO ¼ �eðuox þ 4:71ÞðeVÞ;LUMO ¼ �eðured þ 4:71ÞðeVÞ;

Eecg ¼ eðuox � uredÞðeVÞ

where the units of uox and ured are V versus Ag/Agþ. The values obtained are listed in Table 2.ured and uox of P1 were obtained from our previ-ous work.24 The ured of P2 shifted positively by0.13 V compared to P1. The uox of P2 shifted neg-atively by 0.10 V compared to P1. And the electro-chemical energy gap of P2 is narrower than thatof P1 by 0.23 eV. The electrochemical propertiesof P2 indicate that the conjugated side chainsdecrease energy band gaps of the polymers.

Organic Field-Effect Transistor Propertiesof the Polymers

OFET were fabricated based on the two polymersby spin-coating with bottom gate/top contact ge-ometry. Figures 7(a,b) show the output and trans-fer characteristics of a representative device withP2 as the channel semiconductor. The outputbehavior follows the metal oxide semiconductorOFET gradual-channel model with very good sat-uration. The OFET properties of the two poly-mers, P1 and P2, are summarized in Table 3. ForP2, before annealing, the devices showed a satu-rated OFET mobility of about 4.2 � 10–4 cm2/Vsand a current on/off ratio of 102 only. The mobilityincreased significantly to 7.3 � 10�3 cm2/Vs withan on/off ratio of 103–104 upon annealing at about150 �C. After annealing at 180 �C, the hole mobil-ity of P2 reached 0.034 cm2/Vs on average, andthe highest value reached 0.061 cm2/Vs with theVT of �1.15 V and the on/off ratio of 104. While theOFET based P1 displayed a hole mobility of 8.9 �10–4 cm2/Vs and an on/off ratio of 104. These FETproperties, which were obtained from OFETs fab-ricated and characterized under ambient condi-tions, affirm that P2 is an excellent high-mobilityamorphous polymeric semiconductors for OFETs.

OFETs made from the two polymers all showedvery low threshold voltages (VT\ 10 V).

The structural characteristics of the polymerfilms before and after thermal annealing wereinvestigated by x-ray diffraction (XRD). Figure 8shows the XRD patterns of the polymer films. NoBragg refraction peaks were observed in the XRDpatterns of the polymer films [see Fig. 8(a)]. There

Table 3. Properties of the OFETs with Polymer Films Spin-Coated on OTS-Modified SiO2/Si Substrates

Polymers LUMO (eV) HOMO (eV)

UntreatedAfter Annealing

at 150 �CAfter Annealing

at 180 �C

l (cm2/Vs) Ion/off l (cm2 /Vs) Ion/off l (cm2/Vs) Ion/off

P1 �2.77 �5.05 1.0 � 10–4 103 8.4 � 10�4 104 8.9 � 10–4 104

P2 �2.90 �4.96 4.2 � 10–4 103 7.3 � 10�3 103�104 0.034 104

Figure 8. (a) XRD patterns of the P1 and P2 thinfilms. (b) XRD patterns of the P2 films annealed atdifferent temperatures for 30 min. [Color figure canbe viewed in the online issue, which is available atwww.interscience.wiley.com.]

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are still no Bragg refraction peaks after annealing[see Figure 8(b)]. The results indicate that thepolymer films are in amorphous state15 beforeand after annealing.

The hole mobility of 0.061 cm2/Vs for P2 isamong the highest values for the solution-proces-sible amorphous polymers. The results indicatethat conjugated side chain is helpful for improv-ing the hole mobility of the polymers.21,28

Atomic force microscopy was used to investi-gate surface morphologies in order to understandorigin of the different mobility before and afterannealing. As shown in Figure 9, the AFM imagesof the unannealed and annealed films based onP1 and P2 exhibit typical amorphous film stateswithout any crystalline domains. The AFM mor-phology results were consistent with the XRDanalysis results, indicating that the polymer thinfilms were amorphous.

CONCLUSIONS

Poly(3-[2-(5-hexyl-2-thienyl) ethenyl]-2,20-bithio-phene) (P2) with conjugated side chain was syn-thesized by Pd-catalyzed Stille coupling method,and P1 was synthesized for comparison. P2showed two absorption peaks corresponding totheir main chains and conjugated side chainsrespectively. OFETs based on the polymers byspin-coating were fabricated with bottom gate/topcontact geometry. The OFET hole mobility of P2reached 0.034 cm2/Vs on average, and the highestvalue reached 0.061 cm2/Vs with the on/off ratioof 104, after thermal annealing at 180 �C. TheXRD pattern and AFM images of the polymerfilms indicate that the polymer films are in amor-phous state before and after thermal annealing.The hole mobility of 0.061 cm2/Vs for P2 is amongthe highest values reported to date for the

Figure 9. AFM images of spin-coated films of (a) P1 before annealing, (b) P1 afterannealing, (c) P2 before annealing, and (d) P2 after annealing, (scale: 2 lm � 2 lm).



amorphous polymer films. The results indicatethat polythiophenes with conjugated side chainsare promising materials for the application in so-lution-processable OFET.

This work was supported by NSFC (Nos. 50633050,50673093, 20721061, 60736004).

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high performance polymer field-effect transistors based on polythiophene derivative with conjugated...

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