polarized emission from conjugated polymer chains aligned...

Research ArticlePolarized Emission from Conjugated Polymer Chains Aligned byEpitaxial Growth during Off-Center Spin-Coating

Takuya Anzai1 William Porzio2 and Varun Vohra1

1Department of Engineering Science University of Electro-Communications 1-5-1 Chofugaoka Chofu 182-8585 Japan2Istituto per lo Studio delle Macromolecole (ISMAC-CNR) Via Corti 12 20133 Milano Italy

Correspondence should be addressed to Varun Vohra varunvohrauecacjp

Received 25 July 2017 Accepted 13 September 2017 Published 17 October 2017

Academic Editor Yves Grohens

Copyright copy 2017 Takuya Anzai et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Due to theirmacromolecular nature conjugated polymers can be relatively easily aligned by applying a variety of processes resultingin either elongation or ordering of their conjugated backbones Processes that induce chain alignment include electrospinningmechanical rubbing epitaxial growth and nanoconfinement and unidirectional deposition techniques such as off-center spin-coating In this study we compare these deposition techniques by applying them to a green-emitting conjugated polymer materialthat exhibits liquid crystalline phase behavior Our study reveals that while methods such as electrospinning and mechanicalrubbing can be useful to locally generate polymer chain alignment the combination of epitaxial growth using 135-trichlorobenzeneas crystallizing agent with off-center spin-coating results in the formation of anisotropic nanofiber-like structures with enhancedcrystallinity degree and polarized light-emission properties The unidirectional epitaxial growth was also applied to a red-emittingpolymer that exhibits polarization ratios up to 41 Our results emphasize that this simple solution formulation and process canbe used for the fabrication of polarized thin films of a variety of conjugated polymers with potential applications in the advanceddisplay technologies or analytical equipment fields

1 Introduction

Conjugated light-emitting polymers have been increasinglystudied since the pioneering work from Burroughes et alin which they were used in light-emitting devices exhibitinghigh luminance and the potential for large-scale devicefabrication using simple solution processes [1] In particularthese materials have the potential to be relatively easilyoriented as the polymer chains can be mechanically alignedusing a variety of processes [2] Such processes includethe formation of nano- or microfibers by electrospinning[3 4] mechanical rubbing [5 6] unidirectional deposition[7] and epitaxial growth [8 9] The resulting films oftendisplay enhanced optical or charge transport properties andthin films with aligned conjugated polymer chains haveconsequently been used in a variety of optoelectronic devicessuch as organic field-effect transistors (OFET) [9] organicsolar cell-integrated liquid crystal displays [5] or bipolarizedlight-emitting diodes [10] In particular epitaxial growth

using crystallite templates formed by 135-trichlorobenzene(TCB) has shown remarkable results when applied to OFETwith hole mobilities values 275 to 466 times higher thanthose observed in conventionally deposited devices [9]However these values are only obtained from local measure-ments with interelectrode spacing (channel width) of 80 120583mDeposition processes based on TCB can also be appliedto the formation of films with polarized light-absorptionproperties [11] which could for example be used in analyticalequipment for light-polarization analysis (eg cosmic rays)Using a universal method that can be used with a wide rangeof conjugated polymers independently of their crystallinebehavior is consequently of major interest for the field ofmaterials science Furthermore to be able to practically usethin films containing aligned conjugated polymer chains thechain orientation should be achieved on a relatively largescale (areas of 1 cm2 or more) Here we demonstrate thatwhile techniques such as electrospinning and rubbing cangive extremely positive results locally they do not always

HindawiJournal of ChemistryVolume 2017 Article ID 8637108 9 pageshttpsdoiorg10115520178637108

2 Journal of Chemistry

F8BT

S

Cl

Cl

Cl Cl

NNn

n

n

MEH-PPV

PS CB TCB

（3

RO8（178（17

（3

（3

lowastR =

Figure 1 Chemical structures and properties of materials used in this study

match the requirements for large area fabrication Conse-quently we introduce a new method combining off-centerspin-coating and epitaxial growth with TCB which showslarge increases in polarized emission from films obtainedwith conjugated polymers exhibiting various crystallinebehaviors

2 Experimental Section

21 Materials Chlorobenzene (CB) and TCB poly(99-dioctylfluorene-alt-benzothiadiazole) (F8BT Mn 17000ndash23000) and polystyrene (PS Mwsim35000) were purchasedfromSigma-Aldrich and used as received Poly[2-methoxy-5-(2-ethylhexyloxy)-14-phenylenevinylene] (MEH-PPV) wasobtained from 1-Material The chemical structures of thematerials used in this study are summarized in Figure 1

22 Nanofiber andThin Film Formation The electrospinningprocess was performed using a F8BTPS blend containing10 to 25wt of F8BT with respect to the total polymerweight The solutions in CB were optimized at a concentra-tion of 200mgml to obtain the adequate viscosity for theelectrospinning process A potential of 15 kV was applied tothe metal spinneret placed 10 cm from the target electrodes(Figure 2(a)) A potential of minus5 kV was applied to the twoparallel target electrodes alternatively with a switching timeof 10 s to fabricate aligned nanofibers in the space betweenthe electrodes An additional elongationalignment step wasapplied to the fibers by increasing the space between the twoelectrodes from 2 to 3 cm after fiber deposition [3 4] Thefibers were then collected on a glass substrate cleaned by astandard cleaning procedure (ultrasonication in acetone andhot isopropanol)

Rubbingwas performed using a nylon cloth (SavinaMXpurchased from KB Seiren) on F8BT films deposited froma 10mgml solution in CB by spin-coating on cleaned glassat 1000 rpm for 60 s (Figure 2(b)) The applied pressure andnumber of repetitions for the rubbing process were optimized

to 500 kPa and 10 times respectively Note that at lowerapplied pressure and rubbing times no polarized emissioncould be observed

All spin-coated films were deposited using 10mgmlconjugated polymer solutions in CB at 1000 rpm for 60 sAn additional amount of TCB (200mgml) was added tothe solutions for epitaxial growth using crystallite templatesOff-center spin-coating was performed by depositing thesolution 1 cm away from the center of the spin-coating stage(Figure 2(c)) Note that only small variations in polarizationratios (119875

119877) were observed when the solution was deposited

2 cm away from the center of the spin-coating stage

23 Nanofiber and Thin Film Characterizations The polar-ized photoluminescence (PL) spectra were collected by excit-ing the samples with a LED with an emission peak at 410 nmApolarizerwas placed between the samples and spectrometer(BLUE-Wave StellarNet) and the data was collected withan integration time of 1 s averaged over 5 measurementsfor each polarization angle (every 15 degrees from 0 to 180degrees) Note that no major differences could be observedbetween the 0 and 180 degrees measurements This con-firms that no photodegradation of the conjugated polymeroccurred during the PL characterization under these mildexcitation conditions 119875

119877was calculated by dividing the

integration of the highest intensity PL peak by the lowest one(polarizer placed at 90 degrees with respect to the highestintensity)

Optical and atomic force microscopy (AFM) images wereperformed using bright-field and contact mode respectivelyFluorescence and bright-field optical images were obtainedusing Nikon Eclipse and Olympus DSX510 microscopesrespectively X-ray diffraction (XRD) patterns carried outin Bragg-Brentano geometry were obtained at 25∘C usinga Siemens D500 diffractometer equipped with a sensibledetector (VORTEX) Soller slits (2∘) and narrow slits (03∘)and a Siemens FK 60-10 2000W tube (Cu K

120572radiation 120582 =

0154 nm)The operating voltage and current were 45 kV and

Journal of Chemistry 3

Switch

Solution feed

minus5 Ｅ６

15 Ｅ６

(a) Electrospinning setup

Nylon cloth

Rubbing direction

(b) Rubbing

Direction of centrifugal force

1000 ＬＪＧ

1 ＝Ｇ

Direction o

Ｇ

(c) Off-center spin-coating

Figure 2 Schematic representations of (a) the electrospinning (b) the rubbing and (c) the off-center spin-coating processes

40mA respectively Data were collected from 3∘ to 30∘ (2120579)at 005∘ intervals (9 s for each one)

3 Results and Discussion

31 Locally Polarized Emission in Electrospun Fibers andRubbed Films As mentioned in Introduction to generatealigned conjugated thin films for practical use involvingpolarized light the polarized emission properties shouldextend to large areas Electrospinning of conjugated polymershas been extensively studied to obtain conjugated polymeralignment andor to observe electroluminescence from thesefibers [12] Here we should emphasize the fact that asconjugated polymers have a rigid backbone electrospinningof pure conjugated polymers can hardly be done and theyare commonly blended with other stretchable polymer mate-rials to obtain the adequate viscoelastic properties for theelectrospinning process [3 4 12] Consequently we adapteda standard recipe and combined it with our electrospinningsetup for aligned electrospun fiber preparation The numberof collected fibers is directly proportional to the number ofswitching times which gave us the opportunity to observe thePL behaviors of single fiber (Figure 3(a)) and aligned fiberbundles (Figure 3(b))

While electrospinning on conventional collector screens(round shaped aluminum counter electrodes) gave relativelysmooth fibers using the alignment setup increased thedensity of bead formation during the depositionelongationprocesses Strong polarization could be observed from thestretched F8BTPS single fibers in the nonbeaded areas(Figure 3(a)) However Figure 3(a) also emphasizes that

unpolarized and strong fluorescence is emitted from thebeads Figure 3(b) clearly indicates that aligned fiber bundlescan be fabricated using our electrospinning setup Nonethe-less the large amount of unpolarized beads has a much largerPL intensity as compared to the thin smooth fibers andconsequently no polarization could be observed from thesesamples (119875

119877sim 1)

Similar issues arise in the case of rubbed samples Whilethe rubbed F8BT films display relatively high polarization inthe smooth parts the rubbing process itself generates somebead-like large aggregates which do not display any polarizedemission properties (Figure 3(c)) The images in Figure 3(c)contain unrubbed and rubbed parts of the same sampledeposited on a 1 cm2 substrate The image corresponds tothe part of the sample (approx 015 times 025 cm2) containingthe lowest amount of rubbing-induced beads and scratchesThe two images obtained using perpendicular polarizerorientations suggest that a strong119875

119877should be expected from

these samples The polarized PL measurements (Figure 3(d))contradict these assumptions due to the large number ofunpolarized beads in the measured areas (1 cm2)The highest119875119877value for rubbed films (119875

119877= 125) was obtained by

applying a rubbing pressure of 500 kPa and after 5 rubbingsNote that higher values can be obtained when using otherconjugated polymers with larger crystallinity such as poly(3-hexylthiophene) upon rubbing in similar conditions Addi-tionally polarized emission from F8BT has been demon-strated when using rubbed polyimide templates on whichF8BT is deposited but the formation of beads upon directrubbing of this small crystallinity polymermay explain why amultistep process (rubbing the polyimide template followed


Unpolarized

beads

8000

7000

6000

5000

4000

3000

2000

1000

0

PL in

tens

ity (a

u)

500 550 600 650 700 750Wavelength (nm)

ParallelPerpendicular

PR = 125

(a) (c)

(b) (d)

Figure 3 Fluorescence microscopy images of (a) a single and (b) a bundle of F8BTPS electrospun fibers (c) Fluorescence photograph and(d) PL spectra of rubbed F8BT films The two-side arrows represent the direction of the polarizer placed between the sample and camera

by deposition of F8BT) is necessary [13 14] Nonethelessthe scope of this study is to introduce a universal processto generate polarized conjugated polymer thin films andconsequently in the next section we propose a method thatprovides high119875

119877even for small crystallinitymaterials such as

F8BT

32 Ordering of Conjugated Polymer Films by EpitaxialGrowth during Spin-Coating Epitaxial growth during spin-coating using TCB has been previously studied using adiketopyrrolopyrrole-based conjugated polymer to increase

the charge transport properties of the polymer used in p-typeOFET [9]Thematerial used displayed strong crystallinepeaks without the addition of TCB which in this case onlyinduces radial growth and the formation of large spherulitesHere we discuss and demonstrate that similar formationof large spherulites composed of oriented nanofibers canbe obtained when using conjugated polymers with smallercrystallinity such as F8BT (Figure 4(a)) TCB crystallites areformed during spin-coating in a radial direction and are thenused as templates or nucleation centers that interact with theconjugated polymer molecules (Figure 4(b))


10

0010

(m)

0

(nm)100

(a)

F8BT + TCB solution

Centrifugal force

TCB crystallite template F8BT fibers

(1) Spin-coating(solution)

(2) TCB condensation(wet film)

(3) F8BT fiber formation(dried film)

1000 rpm 60 s

(b)

Figure 4 (a) Morphological characterization (photograph and AFM) and (b) schematic representation of the mechanism of F8BT radialnanofiber formation

While the formation of spherulites and nanofibers canbe clearly observed by eye in both F8BT and MEH-PPVsamples it is important to understand to which degreethese structures are ordered and whether the addition ofTCB to the spin-coated solution can force the crystallizationof the two polymer materials In as-spun films of F8BT orMEH-PPV the degree of order achieved is insufficient toassess a real detectable effect by XRD (Figure 5) [15ndash17]As previously demonstrated a higher degree of order maybe obtained by annealing these films at high temperature[17] Although the inclusion of TCB into the conjugatedpolymer solution leads to the formation of fiber-like struc-tures no interchain crystallinity peak could be measuredat the center of films spin-coated from solutions containingTCB In fact XRD patterns at the center of these films aresimilar to those from reference samples fabricated with-out TCB (data not shown) Interestingly when measuringareas approximately 05 cm away from the center of thesubstrate XRD peaks typically ascribed to interchain molec-ular packing can be observed in both F8BT and MEH-PPV samples prepared using TCB These peaks (markedwith arrows in Figure 5) correspond to 119889-spacing (interchainspacing) of 17 and 185 nm respectively for F8BT andMEH-PPV

33 Directional Epitaxial Growth Using Off-Center Spin-Coating Epitaxial growth resulting from the inclusion ofTCB in the spin-coated solution leads to the formation ofhighly ordered nanofiber structures that are relatively alignedwhen observing the samples away from the center of thespherulites Samples prepared from solutions containingTCBconcentrations of 250mgml lead to the formation of largespherulites that cover areas larger than 1 cm2 However asthe fibers grow in a radial manner away from the centerthe samples do not display any clear polarization and var-ious 119875

119877can be measured depending on the measurement

position relative to the center of the sample Here we explorea simple method for directional deposition namely off-center spin-coating and the morphology and polarizationproperties of films formed by off-center spin-coating ofpolymer solutions containing TCB XRD patterns from F8BTand MEH-PPV samples prepared using off-center epitax-ial growth are presented in Figure 5 Note that no majordifference in XRD patterns was observed for the edge ofcenter spin-coating samples (mentioned in Section 32) andoff-center samples containing TCB Additionally similar thinfilm properties could be found when the off-center spin-coating was performed at distances of 1 and 2 cm from thecenter of the spin-coating stage The results presented here


F8BT centerwithout TCB

F8BT off-centerwith TCB

MEH-PPV off-centerwith TCBMEH-PPV centerwithout TCB

Inte

nsity

(au

)

0

50

100

10 15 20 25 3052 (deg)

Figure 5 F8BT and MEH-PPV XRD pattern observed either at the center of spin-coated samples without TCB or in off-center spin-coatedsamples with TCBThe dark blue and orange arrows correspond to the molecular packing peaks for F8BT (at 51∘ 17 nm) andMEH-PPV (at48∘ 185 nm) respectively

correspond tovspace5pt those obtained when the edge of thesubstrate was placed 1 cm away from the center of the spin-coating stage (Figure 6) The bright-field optical microscopyimages clearly emphasize that the quality of the fibers andtheir alignment increases with distance from the spin-coatingcenter In fact in the central part of the conventional spin-coated samples multiple growth directions (three in theimage presented in Figure 6) can be found Furthermoreboth the optical micrograph and the AFM image in Figure 4suggest that only small size fibers are formed close to thecenter which are mixed with conjugated polymer aggregatesOn the other hand the edge of the samples depositedusing conventional spin-coating with TCB already displaysstructures with a higher degree of orientation which isfurther improved when using off-center spin-coating Theoptical micrographs presented in Figure 6 correspond tosamples obtained using F8BT but a similar trend can beobserved for MEH-PPV samples The major morphologi-cal difference between F8BT and MEH-PPV fibers residesin their dimensions MEH-PPV fibers are thicker andhave a smaller height as compared to equivalent F8BTfibers formed at similar distances from the spin-coatingcenter Nonetheless these results correlate well with the XRDpatterns shown in Figure 5 as we can clearly observe that finefibers with enhanced crystallinity are formed in off-centerspin-coated samples prepared from solutions containingTCB

The fiber-like structure formation observed in our sam-ples (Figure 6) only occurs with the addition of TCBmolecules into the polymer solutions In fact the formation

of large spherulite structures during center spin-coatingstrongly suggests that the TCB molecule crystallization isformed along a preferential direction which correspondsto the solution spreadingdrying direction in which thecentrifugal forces are applied As these templates inducethe conjugated polymer fiber formation controlling thespreading direction of the solution is the key parameter toform aligned fiber-like structures This could be achieved byincreasing the strength of the applied centrifugal force usingoff-center spin-coating

The enhanced fiber alignment observed in the opticalmicrographs in Figure 6 can be easily understood whentaking into account the intensity of the centrifugal forcesapplied to the sample under each condition In fact theintensity of the centrifugal force (119865119888) applied to the solutioncan be expressed as119865

119888= 1198981205962119903 where119898120596 and 119903 correspond

to the mass angular speed and radius respectively As 120596 iskept constant at 1000 rpm as well as the volume of depositedsolution for each sample the only parameter that varies inthe above-mentioned equation is 119903 Consequently we canapproximate that 119865

119888is 5 and 10 to 20 times stronger at the

edge of the conventional spin-coated samples and in the off-center spin-coated samples relatively to the central part ofconventional spin-coated samples respectively Furthermorewe verified that orientation of the fibers and the conjugatedmolecules was actually obtained by measuring the polarizedPL spectra of all samples and calculating their 119875

119877(Figure 7)

Note that no polarized emission properties could be observedfor samples without TCB measured on different areas ofcenter and off-center spin-coated substrates


F8BT MEH-PPV

Central part Edge Off-center

5

00

(m)

(nm) (nm)

5

00

(m)

0 200 0 200

5 5

Edge (05ndash06 cm from center)

Central part (01 cm from center)

Off-center (1-2 cm from center)

Figure 6 Optical micrographs (200 times 200 120583m2) of conventional (central part edge) and off-center spin-coated (off-center) films producedfrom F8BT solutions containing TCB AFM images of nanofibers formed using off-center spin-coating from F8BT and MEH-PPV solutionscontaining TCB

The polarized PL spectra in Figure 7 clearly demonstratethat while no polarized emission can be observed in sampleswithout TCB the addition of TCB and the increasing distancefrom the spin-coating center result in large enhancementsof 119875119877 The F8BT samples prepared using TCB display 119875

119877

values of 135 and 224 for center (edge) and off-center spin-coating respectively The corresponding values for MEH-PPV are even higher and reach 284 and 409 for centerand off-center spin-coated samples respectively Further-more it may be worth remarking that the PL spectra ofsamples containing TCB display more structured spectralshapes with pronounced shoulders at 580 and 640 nmrespectively forF8BT and MEH-PPV Once again this phe-nomenon agrees well with the crystallinity enhancementobserved by XRD (Figure 5) as these enhanced shouldersare typically ascribed to an increase in interchain interac-tions of the conjugated polymer materials [18] The highestpolarized PL intensities for off-center spin-coated sampleswith TCB are measured at angles of 15 and 30 degreeswith respect to the fiber axis for F8BT and MEH-PPVrespectively This deviation with respect to the nanofiberaxis suggests that the conjugated polymer chains are notperfectly aligned inside the fiber However one should alsotake into account the fact that the optical transition dipole

moments of the conjugated polymers are not always parallelto their backbone axis [19] In general higher values of 119875

119877are

obtained for MEH-PPVThis is most probably resulting fromthe fact that unlike F8BT which is composed of two differentmonomers the transition dipole moment of MEH-PPV hasa higher probability of being parallel to its planar conjugatedbackbone

4 Conclusions

In summary we have demonstrated a universal methodto generate thin films of conjugated polymers composedof aligned nanofiber-like structures This was achieved bycombining two well-known methods for polymer chainalignment namely epitaxial growth using TCB and off-center spin-coating Our results further emphasize the factthat these two methods cannot generate large-scale andhigh polarization ratio films if used independently The thinfilms displaying the highest polarization reach 119875

119877values of

approximately 41 As this approach can be applied to virtuallyany conjugated polymer material the process we introducehere is very promising when it comes to fabricating newpolarized light-emitting-based technologies


With TCB off-center

PR = 224


0

5000

1 104

15 104

PL in

tens

ity (a

u)

600 700650550 750500

Wavelength (nm)

With TCB spin-coating (edge)

PR = 135


600 700650550 750500

Wavelength (nm)

0

5000

1 104

15 104

PL in

tens

ity (a

u)

No TCB spin-coating

F8BT

PR = 101


0

5000

1 104

15 104PL

inte

nsity

(au

)

600 700650550 750500

Wavelength (nm)

(a)

MEH

-PPV

PR = 104


0

1000

2000

3000

4000

5000

6000

PL in

tens

ity (a

u)

550 600 650 700 750500

Wavelength (nm)

PR = 284


0

1000

2000

3000

4000

5000

6000

PL in

tens

ity (a

u)

600 700650550 750500

Wavelength (nm)

PR = 409


0

1000

2000

3000

4000

5000

6000

PL in

tens

ity (a

u)

600 700650550 750500

Wavelength (nm)

With TCB off-centerWith TCB spin-coating (edge)No TCB spin-coating

(b)

Figure 7 Polarized PL spectra of (a) F8BT and (b) MEH-PPV samples prepared using center spin-coating with no TCB or with TCB at theedge of center spin-coated samples and from off-center samples

Conflicts of Interest

The authors declare that they have no conflicts of interest

Acknowledgments

This work was supported by the University of Electro-Communications Financial Support for Young FacultyMem-bers

References

[1] J H Burroughes D D C Bradley A R Brown et al ldquoLight-emitting diodes based on conjugated polymersrdquo Nature vol347 no 6299 pp 539ndash541 1990

[2] V Vohra and T Anzai ldquoMolecular orientation of conjugatedpolymer chains in nanostructures and thin films Review ofprocesses and application to optoelectronicsrdquo Journal of Nano-materials vol 2017 Article ID 3624750 18 pages 2017

[3] Y Ishii H Sakai and H Murata ldquoA new electrospinningmethod to control the number and a diameter of uniaxiallyaligned polymer fibersrdquo Materials Letters vol 62 no 19 pp3370ndash3372 2008

[4] M Campoy-Quiles Y Ishii H Sakai and H Murata ldquoHighlypolarized luminescence from aligned conjugated polymer elec-trospun nanofibersrdquo Applied Physics Letters vol 92 no 21Article ID 213305 2008

[5] R Zhu A Kumar and Y Yang ldquoPolarizing organic photo-voltaicsrdquo Advanced Materials vol 23 no 36 pp 4193ndash41982011

[6] V Vohra G Arrighetti L Barba K Higashimine W Porzioand H Murata ldquoEnhanced vertical concentration gradient inrubbed P3HTPCBM graded bilayer solar cellsrdquo The Journal ofPhysical Chemistry Letters vol 3 no 13 pp 1820ndash1823 2012

[7] Y Yuan G Giri A L Ayzner et al ldquoUltra-high mobilitytransparent organic thin film transistors grown by an off-centrespin-coating methodrdquo Nature Communications vol 5 article3005 2014

[8] C Muller M Aghamohammadi S Himmelberger et al ldquoOne-step macroscopic alignment of conjugated polymer systems byepitaxial crystallization during spin-coatingrdquo Advanced Func-tional Materials vol 23 no 19 pp 2368ndash2377 2013

[9] J Y Kim D S Yang J Shin et al ldquoHigh-performing thin-filmtransistors in large spherulites of conjugated polymer formedby epitaxial growth on removable organic crystalline templatesrdquoACS Applied Materials amp Interfaces vol 7 no 24 pp 13431ndash13439 2015


[10] A Bolognesi C Botta D Facchinetti et al ldquoPolarized elec-troluminescence in double-layer light-emitting diodes withperpendicularly oriented polymersrdquo Advanced Materials vol13 no 14 pp 1072ndash1075 2001

[11] B Dorling V Vohra T T Dao M Garriga H Murata andM Campoy-Quiles ldquoUniaxial macroscopic alignment of con-jugated polymer systems by directional crystallization duringblade coatingrdquo Journal of Materials Chemistry C vol 2 no 17pp 3303ndash3310 2014

[12] V Vohra U Giovanella R Tubino H Murata and CBotta ldquoElectroluminescence from conjugated polymer electro-spun nanofibers in solution processable organic light-emittingdiodesrdquo ACS Nano vol 5 no 7 pp 5572ndash5578 2011

[13] D-M Lee Y-J Lee J-H Kim and C-J Yu ldquoBirefringence-dependent linearly-polarized emission in a liquid crystallineorganic light emitting polymerrdquo Optics Express vol 25 no 4pp 3737ndash3742 2017

[14] S I Jo Y Kim J-H Baek C-J Yu and J-H Kim ldquoHighlypolarized emission of the liquid crystalline conjugated polymerby controlling the surface anchoring energyrdquo Japanese Journalof Applied Physics vol 53 no 3 Article ID 03CD04 2014

[15] V Vohra W Mroz S Inaba W Porzio U Giovanella and FGaleotti ldquoLow-cost and green fabrication of polymer electronicdevices by push-coating of the polymer active layersrdquo ACSApplied Materials amp Interfaces vol 9 no 30 pp 25434ndash254442017

[16] R J Kline M D McGehee and J Macromol ldquoMorphologyand charge transport in conjugated polymersrdquo Journal ofMacro-molecular Science Part C Polymer Reviews vol 46 no 1 pp27ndash45 2006

[17] S H Chen A C Su H L Chou K Y Peng and S A ChenldquoPhase behavior and molecular aggregation in bulk poly(2-methoxy-5-(21015840-ethylhexyloxy)-14-phenylenevinylene)rdquoMacromolecules vol 37 no 1 pp 167ndash173 2004

[18] O Y Posudievsky M S Papakin O P Boiko V G Koshechkoand V D Pokhodenko ldquoEnhanced and tunable photolumines-cence of polyphenylenevinylenes confined in nanocompositefilmsrdquo Nanoscale Research Letters vol 10 no 1 pp 1ndash7 2015

[19] T W Hagler ldquoNonparallel transition dipole moments and thepolarization dependence of electroabsorption in nonorientedconjugated polymer filmsrdquoChemical Physics Letters vol 218 no3 pp 195ndash199 1994

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CatalystsJournal of


F8BT

S

Cl

Cl

Cl Cl

NNn

n

n

MEH-PPV

PS CB TCB

（3

RO8（178（17

（3

（3

lowastR =

Figure 1 Chemical structures and properties of materials used in this study

match the requirements for large area fabrication Conse-quently we introduce a new method combining off-centerspin-coating and epitaxial growth with TCB which showslarge increases in polarized emission from films obtainedwith conjugated polymers exhibiting various crystallinebehaviors

2 Experimental Section

21 Materials Chlorobenzene (CB) and TCB poly(99-dioctylfluorene-alt-benzothiadiazole) (F8BT Mn 17000ndash23000) and polystyrene (PS Mwsim35000) were purchasedfromSigma-Aldrich and used as received Poly[2-methoxy-5-(2-ethylhexyloxy)-14-phenylenevinylene] (MEH-PPV) wasobtained from 1-Material The chemical structures of thematerials used in this study are summarized in Figure 1

22 Nanofiber andThin Film Formation The electrospinningprocess was performed using a F8BTPS blend containing10 to 25wt of F8BT with respect to the total polymerweight The solutions in CB were optimized at a concentra-tion of 200mgml to obtain the adequate viscosity for theelectrospinning process A potential of 15 kV was applied tothe metal spinneret placed 10 cm from the target electrodes(Figure 2(a)) A potential of minus5 kV was applied to the twoparallel target electrodes alternatively with a switching timeof 10 s to fabricate aligned nanofibers in the space betweenthe electrodes An additional elongationalignment step wasapplied to the fibers by increasing the space between the twoelectrodes from 2 to 3 cm after fiber deposition [3 4] Thefibers were then collected on a glass substrate cleaned by astandard cleaning procedure (ultrasonication in acetone andhot isopropanol)

Rubbingwas performed using a nylon cloth (SavinaMXpurchased from KB Seiren) on F8BT films deposited froma 10mgml solution in CB by spin-coating on cleaned glassat 1000 rpm for 60 s (Figure 2(b)) The applied pressure andnumber of repetitions for the rubbing process were optimized

to 500 kPa and 10 times respectively Note that at lowerapplied pressure and rubbing times no polarized emissioncould be observed

All spin-coated films were deposited using 10mgmlconjugated polymer solutions in CB at 1000 rpm for 60 sAn additional amount of TCB (200mgml) was added tothe solutions for epitaxial growth using crystallite templatesOff-center spin-coating was performed by depositing thesolution 1 cm away from the center of the spin-coating stage(Figure 2(c)) Note that only small variations in polarizationratios (119875

119877) were observed when the solution was deposited

2 cm away from the center of the spin-coating stage

23 Nanofiber and Thin Film Characterizations The polar-ized photoluminescence (PL) spectra were collected by excit-ing the samples with a LED with an emission peak at 410 nmApolarizerwas placed between the samples and spectrometer(BLUE-Wave StellarNet) and the data was collected withan integration time of 1 s averaged over 5 measurementsfor each polarization angle (every 15 degrees from 0 to 180degrees) Note that no major differences could be observedbetween the 0 and 180 degrees measurements This con-firms that no photodegradation of the conjugated polymeroccurred during the PL characterization under these mildexcitation conditions 119875

119877was calculated by dividing the

integration of the highest intensity PL peak by the lowest one(polarizer placed at 90 degrees with respect to the highestintensity)

Optical and atomic force microscopy (AFM) images wereperformed using bright-field and contact mode respectivelyFluorescence and bright-field optical images were obtainedusing Nikon Eclipse and Olympus DSX510 microscopesrespectively X-ray diffraction (XRD) patterns carried outin Bragg-Brentano geometry were obtained at 25∘C usinga Siemens D500 diffractometer equipped with a sensibledetector (VORTEX) Soller slits (2∘) and narrow slits (03∘)and a Siemens FK 60-10 2000W tube (Cu K

120572radiation 120582 =

0154 nm)The operating voltage and current were 45 kV and


Switch

Solution feed

minus5 Ｅ６

15 Ｅ６


Nylon cloth

Rubbing direction

(b) Rubbing


1000 ＬＪＧ

1 ＝Ｇ

Direction o

Ｇ








119877sim 1)







Unpolarized

beads

8000

7000

6000

5000

4000

3000

2000

1000

0

PL in

tens

ity (a

u)

500 550 600 650 700 750Wavelength (nm)


PR = 125

(a) (c)

(b) (d)




F8BT




10

0010

(m)

0

(nm)100

(a)

F8BT + TCB solution

Centrifugal force





1000 rpm 60 s

(b)










Inte

nsity

(au

)

0

50

100

10 15 20 25 3052 (deg)










119877(Figure 7)



F8BT MEH-PPV


5

00

(m)

(nm) (nm)

5

00

(m)

0 200 0 200

5 5






119877



119877are


4 Conclusions


119877values of



With TCB off-center

PR = 224


0

5000

1 104

15 104

PL in

tens

ity (a

u)

600 700650550 750500

Wavelength (nm)


PR = 135


600 700650550 750500

Wavelength (nm)

0

5000

1 104

15 104

PL in

tens

ity (a

u)

No TCB spin-coating

F8BT

PR = 101


0

5000

1 104

15 104PL

inte

nsity

(au

)

600 700650550 750500

Wavelength (nm)

(a)

MEH

-PPV

PR = 104


0

1000

2000

3000

4000

5000

6000

PL in

tens

ity (a

u)

550 600 650 700 750500

Wavelength (nm)

PR = 284


0

1000

2000

3000

4000

5000

6000

PL in

tens

ity (a

u)

600 700650550 750500

Wavelength (nm)

PR = 409


0

1000

2000

3000

4000

5000

6000

PL in

tens

ity (a

u)

600 700650550 750500

Wavelength (nm)


(b)




Acknowledgments


References






























Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


Switch

Solution feed

minus5 Ｅ６

15 Ｅ６


Nylon cloth

Rubbing direction

(b) Rubbing


1000 ＬＪＧ

1 ＝Ｇ

Direction o

Ｇ








119877sim 1)







Unpolarized

beads

8000

7000

6000

5000

4000

3000

2000

1000

0

PL in

tens

ity (a

u)

500 550 600 650 700 750Wavelength (nm)


PR = 125

(a) (c)

(b) (d)




F8BT




10

0010

(m)

0

(nm)100

(a)

F8BT + TCB solution

Centrifugal force





1000 rpm 60 s

(b)










Inte

nsity

(au

)

0

50

100

10 15 20 25 3052 (deg)










119877(Figure 7)



F8BT MEH-PPV


5

00

(m)

(nm) (nm)

5

00

(m)

0 200 0 200

5 5






119877



119877are


4 Conclusions


119877values of



With TCB off-center

PR = 224


0

5000

1 104

15 104

PL in

tens

ity (a

u)

600 700650550 750500

Wavelength (nm)


PR = 135


600 700650550 750500

Wavelength (nm)

0

5000

1 104

15 104

PL in

tens

ity (a

u)

No TCB spin-coating

F8BT

PR = 101


0

5000

1 104

15 104PL

inte

nsity

(au

)

600 700650550 750500

Wavelength (nm)

(a)

MEH

-PPV

PR = 104


0

1000

2000

3000

4000

5000

6000

PL in

tens

ity (a

u)

550 600 650 700 750500

Wavelength (nm)

PR = 284


0

1000

2000

3000

4000

5000

6000

PL in

tens

ity (a

u)

600 700650550 750500

Wavelength (nm)

PR = 409


0

1000

2000

3000

4000

5000

6000

PL in

tens

ity (a

u)

600 700650550 750500

Wavelength (nm)


(b)




Acknowledgments


References






























Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


Unpolarized

beads

8000

7000

6000

5000

4000

3000

2000

1000

0

PL in

tens

ity (a

u)

500 550 600 650 700 750Wavelength (nm)


PR = 125

(a) (c)

(b) (d)




F8BT




10

0010

(m)

0

(nm)100

(a)

F8BT + TCB solution

Centrifugal force





1000 rpm 60 s

(b)










Inte

nsity

(au

)

0

50

100

10 15 20 25 3052 (deg)










119877(Figure 7)



F8BT MEH-PPV


5

00

(m)

(nm) (nm)

5

00

(m)

0 200 0 200

5 5






119877



119877are


4 Conclusions


119877values of



With TCB off-center

PR = 224


0

5000

1 104

15 104

PL in

tens

ity (a

u)

600 700650550 750500

Wavelength (nm)


PR = 135


600 700650550 750500

Wavelength (nm)

0

5000

1 104

15 104

PL in

tens

ity (a

u)

No TCB spin-coating

F8BT

PR = 101


0

5000

1 104

15 104PL

inte

nsity

(au

)

600 700650550 750500

Wavelength (nm)

(a)

MEH

-PPV

PR = 104


0

1000

2000

3000

4000

5000

6000

PL in

tens

ity (a

u)

550 600 650 700 750500

Wavelength (nm)

PR = 284


0

1000

2000

3000

4000

5000

6000

PL in

tens

ity (a

u)

600 700650550 750500

Wavelength (nm)

PR = 409


0

1000

2000

3000

4000

5000

6000

PL in

tens

ity (a

u)

600 700650550 750500

Wavelength (nm)


(b)




Acknowledgments


References






























Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


10

0010

(m)

0

(nm)100

(a)

F8BT + TCB solution

Centrifugal force





1000 rpm 60 s

(b)










Inte

nsity

(au

)

0

50

100

10 15 20 25 3052 (deg)










119877(Figure 7)



F8BT MEH-PPV


5

00

(m)

(nm) (nm)

5

00

(m)

0 200 0 200

5 5






119877



119877are


4 Conclusions


119877values of



With TCB off-center

PR = 224


0

5000

1 104

15 104

PL in

tens

ity (a

u)

600 700650550 750500

Wavelength (nm)


PR = 135


600 700650550 750500

Wavelength (nm)

0

5000

1 104

15 104

PL in

tens

ity (a

u)

No TCB spin-coating

F8BT

PR = 101


0

5000

1 104

15 104PL

inte

nsity

(au

)

600 700650550 750500

Wavelength (nm)

(a)

MEH

-PPV

PR = 104


0

1000

2000

3000

4000

5000

6000

PL in

tens

ity (a

u)

550 600 650 700 750500

Wavelength (nm)

PR = 284


0

1000

2000

3000

4000

5000

6000

PL in

tens

ity (a

u)

600 700650550 750500

Wavelength (nm)

PR = 409


0

1000

2000

3000

4000

5000

6000

PL in

tens

ity (a

u)

600 700650550 750500

Wavelength (nm)


(b)




Acknowledgments


References






























Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of





Inte

nsity

(au

)

0

50

100

10 15 20 25 3052 (deg)










119877(Figure 7)



F8BT MEH-PPV


5

00

(m)

(nm) (nm)

5

00

(m)

0 200 0 200

5 5






119877



119877are


4 Conclusions


119877values of



With TCB off-center

PR = 224


0

5000

1 104

15 104

PL in

tens

ity (a

u)

600 700650550 750500

Wavelength (nm)


PR = 135


600 700650550 750500

Wavelength (nm)

0

5000

1 104

15 104

PL in

tens

ity (a

u)

No TCB spin-coating

F8BT

PR = 101


0

5000

1 104

15 104PL

inte

nsity

(au

)

600 700650550 750500

Wavelength (nm)

(a)

MEH

-PPV

PR = 104


0

1000

2000

3000

4000

5000

6000

PL in

tens

ity (a

u)

550 600 650 700 750500

Wavelength (nm)

PR = 284


0

1000

2000

3000

4000

5000

6000

PL in

tens

ity (a

u)

600 700650550 750500

Wavelength (nm)

PR = 409


0

1000

2000

3000

4000

5000

6000

PL in

tens

ity (a

u)

600 700650550 750500

Wavelength (nm)


(b)




Acknowledgments


References






























Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


F8BT MEH-PPV


5

00

(m)

(nm) (nm)

5

00

(m)

0 200 0 200

5 5






119877



119877are


4 Conclusions


119877values of



With TCB off-center

PR = 224


0

5000

1 104

15 104

PL in

tens

ity (a

u)

600 700650550 750500

Wavelength (nm)


PR = 135


600 700650550 750500

Wavelength (nm)

0

5000

1 104

15 104

PL in

tens

ity (a

u)

No TCB spin-coating

F8BT

PR = 101


0

5000

1 104

15 104PL

inte

nsity

(au

)

600 700650550 750500

Wavelength (nm)

(a)

MEH

-PPV

PR = 104


0

1000

2000

3000

4000

5000

6000

PL in

tens

ity (a

u)

550 600 650 700 750500

Wavelength (nm)

PR = 284


0

1000

2000

3000

4000

5000

6000

PL in

tens

ity (a

u)

600 700650550 750500

Wavelength (nm)

PR = 409


0

1000

2000

3000

4000

5000

6000

PL in

tens

ity (a

u)

600 700650550 750500

Wavelength (nm)


(b)




Acknowledgments


References






























Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


With TCB off-center

PR = 224


0

5000

1 104

15 104

PL in

tens

ity (a

u)

600 700650550 750500

Wavelength (nm)


PR = 135


600 700650550 750500

Wavelength (nm)

0

5000

1 104

15 104

PL in

tens

ity (a

u)

No TCB spin-coating

F8BT

PR = 101


0

5000

1 104

15 104PL

inte

nsity

(au

)

600 700650550 750500

Wavelength (nm)

(a)

MEH

-PPV

PR = 104


0

1000

2000

3000

4000

5000

6000

PL in

tens

ity (a

u)

550 600 650 700 750500

Wavelength (nm)

PR = 284


0

1000

2000

3000

4000

5000

6000

PL in

tens

ity (a

u)

600 700650550 750500

Wavelength (nm)

PR = 409


0

1000

2000

3000

4000

5000

6000

PL in

tens

ity (a

u)

600 700650550 750500

Wavelength (nm)


(b)




Acknowledgments


References






























Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of





















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of

polarized emission from conjugated polymer chains aligned...

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