Strongly Photosensitive and Fluorescent F8T2 Electrospun Fibers

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Strongly Photosensitive and Fluorescent F8T2Electrospun Fibers

Isabel Ferreira,* Ana Catarina Baptista, Joaquim Pratas Leitao, Jorge Soares,Elvira Fortunato, Rodrigo Martins, Joao Paulo Borges*

Electrospun fibers of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-bithiophene] (F8T2) with excep-tional electro-optical performance are obtained. The I/T characteristics measured in fiberswith 7–15 mm diameter and 1 mm length show a semiconductor behavior; their thermalactivation energy is 0.5 eV and the dark conductivity at RT is 5� 10�9 (V cm)�1. Besidesexhibiting a photosensitivity of about 60 underwhite light illumination with a light power inten-sity of 25 mW � cm�2, the fibers also attain RTphotoluminescence in the cyan, yellow, and redwavelength range under ultraviolet, blue, andgreen light excitation, respectively. Optical micro-scope images of F8T2 reveal homogeneous elec-trospun fibers, which are in good agreement withthe uniformly radial fluorescence observed.

1. Introduction

Conductive organic fibers have attracted considerable

interest in the recent years due to their potential

application in micro to nanoscale electronic and optoelec-

tronic devices.[1–4] Electrospinning (ES) is a simple and

versatile method that utilizes electrical forces to produce

one-dimensional fibers from 2 nm to several micrometers

in diameter. Cylindrical, ribbons, or helices[5,6] are common

cross-sectional shapes produced by this technique. ES has

been successfully used to produce conductive nanostruc-

tures of polyaniline (PANI),[7] poly( p-phenylene vinylene)s

(PPVs),[8] polypyrrole (PPy),[9,10] and polythiophenes

Prof. I. Ferreira, A. C. Baptista, Prof. E. Fortunato, Prof. R. Martins,Prof. J. P. BorgesCENIMAT/I3N, Departamento de Ciencia dos Materiais, Faculdadede Ciencias e Tecnologia, FCT, Universidade Nova de Lisboa andCEMOP-UNINOVA, 2829-516 Caparica, PortugalE-mail: [email protected]; [email protected]. J. P. Leitao, Prof. J. SoaresDepartamento de Fısica and I3N, Universidade de Aveiro, CampusUniversitario de Santiago, 3810–193 Aveiro, Portugal

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(PThs).[1,11] For instance Chronakis et al.[10] reported

conductivities up to 10�5 (V cm)�1 in fibers with dimen-

sions of 300 nm obtained from solutions of 65% PPy

in poly(ethylene oxide) (PEO), while in polyethylene

terephthalate (PET) fibers covered with 10% PANI, Kim

et al.[12] achieved conductivities up to 2.5� 10�5 (V cm)�1

and Xin et al.[13] have produced PPV/PEO nanofibers with

conductivities up to 4� 10�4 (V cm)�1 using 80% of PPV in

PEO. Conductive electrospun fibers of PThs were also used

in the fabrication of aligned fibers with diameters around

300 nm composed by 75% of poly(3-hexylthiophene-2,5-

diyl) (P3HT) in PEO with conductivities of 0.3 (V cm)�1 after

iodine doping, as reported by Laforgue and Robitaille.[14]

All these studies used PEO to obtain homogeneous

fibers with controlled diameter. Generally, PEO is removed

after fiber production using appropriate solvents. Besides

time consumption, this procedure contributes for the

deterioration of fibers, namely conductivity, homogeneity,

and mechanical resistance. The production of conductive

fibers directly from conductive polymer solutions or melts,

that is, without a carrier polymer, and controlling their

diameter, homogeneity, and conductivity is still a great

challenge.

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Figure 1. Structural unit of F8T2.

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I. Ferreira et al.

In this work we demonstrate the possibility to produce

F8T2 fibers by ES without using a carrier polymer. Poly[(9,9-

dioctylfluorene)-co-bithiophene), F8T2, is a rigid-rod poly-

mer with a nematic liquid crystalline phase at 265 8C, while

above 300 8C it melts into an isotropic liquid.[15] The F8T2

polymer backbone is relatively planar and elongated where

the octyl chains in the sp3-carbon atom of the chains bridge

come perpendicular to the conjugated planes, preventing

the p-p interchain stacking (Figure 1). Therefore, the charge

transport along the conjugated segments of the polymer

backbone can dominate the one related with interchain

hopping between adjacent chains.[16] Owing to its good

hole-transport properties and relative stability against

chemical doping by environmental oxygen and water

vapor, F8T2 has been investigated intensively for applica-

tion in p-type FETs[2,15] and in the development of solar

cells.[17]

For the best of our knowledge, this is the first report on the

production of F8T2 fibers by ES. Their electro-optical

properties show a semiconductor behavior with good

photosensitivity, around 60, and cyan, yellow, and red

fluorescence under the UV, blue, and green wavelength

range excitation, respectively.

2. Experimental Section

2.1. Fiber Production and Morphological

Characterization

A solution of F8T2 (Mw 20 000–60 000, purchased from Sigma-

Aldrich), 4% (w/v) in xylene, was electrospun. A syringe pump was

used to squeeze out the polymer solution at a speed between 0.1

and 1 mL �h�1 through a capillary. A voltage of 12.5 kV was applied

between the needle and the grounded collector at 15 cm of distance.

During this process, the F8T2 solution was heated up to 150 8C. The

produced fibers were observed by an optical microscope (Olympus

BX51).

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2.2. Film Production

The synthesized F8T2 polymer was dissolved in xylene, and a thin

film was prepared on a glass substrate using a conventional spin-

coating method with a rotation speed of 3000 rpm for 30 s. The

thickness of the films was �25 mm.

2.3. Conductivity Measurements

The F8T2 fibers were placed in a glass substrate coated with

aluminum electrodes separated 1 mm apart, which constitutes our

sample. The fibers were glued with silver paste to the aluminum in

order to keep constant the electric field along the fiber and to fix it,

as indicated in the sketch of Figure 3c. The probes were placed on

the metal electrodes being applied a voltage of 15 V. The current

passing through the fiber(s), about 1 mm long was measured in a

Keithley 618 ammeter. The sample was heated up from room

temperature (RT) to 360 K in a vacuum chamber (10�2 mbar) and

then cooled down using a computer controlled cryostat. The current

was measured during the cooling of the sample as a function of

temperature. The I–V curves were measured at atmospheric

pressure in dark and under white light with intensity of

25 mW � cm�2.

2.4. Photoluminescence Measurement

Steady-state PL measurements were carried at RT using an 85 BLS

DPSS laser (457 nm) and a Ventus laser (532 nm) as excitation

sources. The excitation power measured near the sample was

60 mW. The luminescence was dispersed by a Spex 1704

monochromator (1 m, 1200 mm�1) and detected by a cooled

Hamamatsu R928 photomultiplier. The fluorescence was observed

in a microscope Olympus BX51.

3. Results and Discussion

The fibers were produced by ES directly from a solution of

commercial F8T2 diluted in xylene and the viscosity of the

solution was controlled by heating it up to 150 8C. The

morphology of the obtained fibers is shown in Figure 2.

The optical transmission microscope image of a fiber with

a diameter around 15 mm shows two distinct regions

(Figure 2a): near the border and in the middle, attributed to

different molecular orientation. Indeed the optical micro-

graph obtained with crossed polarizers (Figure 2b) clearly

shows birefringence of fibers, which is not observed in the

F8T2 films (Figure 2c). These results agree with the recent

report of Werzer et al.[18] about the self-alignment of F8T2

polymer. They found that the polymer backbones gradually

align themselves (along the rubbing direction) with the

increasing temperature (up to 290 8C). Above that tem-

perature, the alignment of the polymer backbones is still

present but less significant.[18] The production of a fiber

by ES involves an extrusion process which is responsible

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Figure 2. Optical micrographs of an electrospun F8T2 single fiber with a diameter around15 mm: (a) transmission image obtained with parallel polarizers; (b) transmission imageobtained with crossed polarizers; and (c) optical micrograph of a F8T2 film, obtainedwith crossed polarizers, where no birefringence is observed.

Strongly Photosensitive and Fluorescent . . .

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for the alignment of the polymer backbone and conse-

quently for the birefringence observed in the fibers. The

velocity profile inside the capillary (v¼ 0 near the wall of

the capillary and v¼ vmax at the center of the capillary) is

responsible for different degrees of molecular alignment

and therefore for the observed differences in the fiber’s

birefringence between the center and the periphery.

The production of F8T2 fibers by ES is controlled in terms

of fibers uniformity but the diameter distribution can be

improved by changing the rate at which the polymer

solution flows to the tip of the needle. For higher flow rates a

larger amount of polymer solution is available for fiber

elongation giving rise to larger fibers. Figure 3a and b shows,

respectively, a group of fibers and a single fiber placed

between two Al electrodes (dark regions). The groups of

fibers in Figure 3a evidence the difference in their

Figure 3. Optical images of (a) a group of electrospun F8T2 fibers and (b) a single fiber,both placed between two Al strips separated 1 mm apart as illustrated in the sketch at(c).

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diameters. A widespread distribution of

fibers diameter is a characteristic of the

ES process.[19] For conductivity measure-

ments the fibers were fixed to Al contacts

with conductive silver paste as sketched

in Figure 3c to guarantee a good electrical

ohmic contact between fibers and elec-

trodes. By applying an electric field j

(the ratio between the applied voltage V

and the length l of fiber) between the

two electrodes we measured the current I

along the fiber. The current was mea-

sured under vacuum conditions and the

temperature varied in the range 360–

298 K. The dark conductivity sd was

determined taking into account that

s¼ J/j [where J is the current density

given by the ratio between the current I

collected and the area S of the fiber (in this

case, a fiber with diameter of 15 mm): J¼ I/S]. Figure 4a

shows the corresponding Arrhenius plot (conductivity as

function of the inverse of the absolute temperature, T)

which gives information about the transport mechanism.

The results show a typical semiconductor conduction

mechanism with an activation energy DE of 0.5 eV and

dark conductivity at RT of 10�8 (V cm)�1. This behavior

reflects the mobility of polarons and bi-polarons along

the chain by the arrangement of the double and single

bonds in the presence of an applied electric field. Ordered

structures have typically an exponential variation of

conductivity with the temperature as s¼ s0 exp(�DE/kT),

where s0 is the pre-exponential factor and k Boltzmann’s

constant.[20] On the other hand, for disordered materials

or at very low temperatures in crystals, the conduction

mechanism is dominated by hopping or tunneling.

According to Mott and Davis[21] predic-

tion, the conductivity varies now with a

power of T as s¼ s0 exp(T0/T)�1/(1 þ d),

where d is the dimension of the hopping

path for a variable-rang hopping (VRH)

conduction mechanism. The experimen-

tal data depicted in Figure 4a, clearly

shows that in F8T2 the conduction

mechanism is dominated by the ordered

molecular chains. However, since charge

mobility in organic polymers is lower

than 10�2 cm2 �V�1 � s�1, the length of

fibers has a strong influence on the

conductivity.[22] Comparing the conduc-

tivity of fibers with the ones obtained in

films with the same electrodes config-

uration (co-planar), the former is about

three orders of magnitude higher than

the later ones. Other authors, for instance

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Figure 4. (a) Dark conductivity as a function of temperature; (b) modulus of current-voltage curves in dark and under white illuminationwith an intensity of 25 mW � cm�2, for a film with 25 mm of thickness and a fiber with a diameter of 15 mm (the lines are guides for eyes).

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Kim et al.,[23] also found the conductivity of nanofibers

higher than that of films. This behavior is related to a

better alignment of the molecules in the fiber since the

extended conjugation length is longer, consequently

the interchain interaction and charge delocalization are

reduced when compared to the respective film material.[23]

In Table 1 we did an attempt to compare our data with

the one accessed in published data. Even considering that

the present ES fibers are much longer than the ones

found in literature (see Table 1), which are in the range

of few micrometers, it is interesting to notice that the

current response to the applied voltage still exhibits a

linear behavior with and without illumination (ohmic

Table 1. Summary of the main results obtained for the present work adark conductivity, sph/sd is the photosensitivity, PL the photolumine

Material Diameter [mm] Length [mm]

[Ru(bpy)3]2þ(PF�6 )2 0.8 0.5

5

F8T2 0.2 3 2.5

F8T2 15 1 000

F8T2 film 25� 10�4� 0.1� 0.4 cm3 a)

cellulose fibers

doped with iodine

1.0� 0.5� 0.2 cm3 b)

PA-6/Ppy 0.04� 1� 1 cm3 c)

a)Thin film; b)Piece of cotton; c)Bundle of fibers; the length was taken to

article.




behavior). The results are shown in Figure 4b where it is

plotted the absolute value of current density versus the

absolute value of applied voltage. As observed, the I–V

positive and negative values are almost coincident

showing a good linearity.

In the work performed by Moran-Mirabal et al.,[24] the

electrical conductivity of electrospun light-emitting nano-

fibers based on PEO in the dark and at RT is reduced by

a factor of 30, when the length of fibers increase one

decade (see Table 1). Based on that and assuming the same

proportionality for F8T2, we can expect conductivities in

the range of 10�5 (V cm)�1 for a 1 mm length fiber. Thus, our

results are comparable to the ones obtained in F8T2 fibers

nd comparison with other studies reported in the literature. sd is thescence peak, and DE is the activation energy.

sd (V cm)�1 sph/sd PL

[nm]

DE[eV]

Ref.

ffi5� 10�2 – – – [21]

ffi1.7� 10�3 – – –

� 10�5–1.4� 10�4 1.3 600 – [22]

4� 10�9 90 450 0.48 this work

2� 10�10 300 550 0.58

ffi5� 10�6 – – 0.68 [23]

ffi5� 10�5 – – – [24]

be 1 cm since no exact measure (‘a few millimeters’) is given in the

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produced by solution-assisted template wetting reported

by Garret O’Brien et al.[3]

On the other hand, the I–V characteristics measured in air

(Figure 4b) when compared to the conductivity calculated

for a voltage of 15 V taken under vacuum (see Figure 4a)

we found considerable differences (s is about two orders

of magnitude lower when measured in air than under

vacuum). This behavior can be related to the adsorption of

atmospheric agents (oxygen, water vapor, and others) at the

surface of ES fibers inducing surface defects that may trap

the carriers traveling along the backbone chains, which is

the predominant conduction mechanism in F8T2 fibers,

due to the molecular alignment induced by the ES process.

Although the fibers have a bulk to surface area ratio much

smaller than the films, the conductivity of fibers under

ambient conditions stills is two orders of magnitude

higher than that of films.

The photosensitivity (sph/sd) obtained under white light

illumination (25 mW � cm�2) is roughly 60 for fiber and 600

for film. To this difference contributes the effect of adsorbed

particles in the surface of fibers acting as trap carries as

stated before, or a non-uniform illumination due to their

cylindrical shape thus affecting the geometrical factor

considered to determine sph. Moreover, as the area of the

films under contacts is not fully isolated from the rest of

the film, we have a photolateral effect related to carriers

generated close to the boundaries of the area between

the contacts that lead to the overestimation of the photo-

sensitivity in the films, as also observed in other photo-

conductors.[25]

Figure 5 shows the fluorescence microscopy images of

fibers under UV (Figure 5a), blue (Figure 5b), and green

(Figure 5c) light excitation, respectively. These images,

taken at the same magnification and in the same fiber

region, clearly show cyan, green, and red luminescence,

respectively. There, we observe uniformly radiated

Figure 5. Optical microscopy fluorescence images of F8T2 fibers excitedof a fiber in the respective image.


Macromol. Mater. Eng. 2012, DO



luminescence with constant intensity along the fibers,

revealing enhanced composition homogeneity, when

compared to F8T2 fibers produced with templates[3] or

even by other ES polymer, such as MEH-PPV/PEO.[26]

From the transmittance spectra of a 25 mm thickness

F8T2 film spin coated on a glass substrate (see Figure 6a) we

observe a high transmittance in the near infrared region

(above 60%) and a strong absorption to wavelengths

below 750 nm. Figure 6a also shows the square route of

absorption coefficient (a) and photons energy (Eph in eV)

versus Eph from which the optical transitions (due to energy

gap) can be calculated according to Tauc’s plot.[27] Two

distinct slopes were observed and the respective energy

gaps are about 1.75 and 3.15 eV that might be associated

to two different optical transitions. F8T2 has HOMO and

LUMO levels at, respectively, 5.5 and 3.1 eV. Thus an optical

transition gap of 2.4 eV was expected.[4] However, as

demonstrated by several authors, F8T2 has Raman and

optical dichroism[27] and the p-p conjugated interactions

in well aligned regions are responsible by low energy

emission, while disorder regions are responsible for

higher energy emission due to inter/chain interactions.[28]

So we conclude that the obtained energy gap of 1.75 might

be related with optical transitions of aligned regions

of the F8T2 chains and the 3.15 eV associated to the

disordered ones. Figure 6b shows the PL spectra of the F8T2

film excited with 532 nm and 457 nm wavelengths, with an

excitation power of 60 mW. A sharp PL peak is obtained at

630 nm for the former and a broad band for the later. These

results are in agreement with the fluorescence obtained

in the fibers.

Overall we have demonstrated the possibility to produce

F8T2 electropsun fibers without a carrier polymer, in

controlled conditions, being obtained highly photo-

sensitivity and photoluminescence fibers emitting in the

cyan, yellow, and red according to UV, blue, and green

with: (a) UV; (b) blue; and (c) green light. The inset is a magnification

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Figure 6. (a) Spectral variation of (aEph)1/2 and transmittance; (b) PL spectra of the F8T2 film excited with 532 nm and 457 nm laser light (inset:microscope image of the film with green light excitation).

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wavelengths excitation. The ES fibers exhibit conductivity

values similar to the ones obtained by template process

and therefore are suitable for applications in fiber field

effect transistors, as photossensible material in fiber solar

cells or even as phosphorescence fiber in optoelectronic

devices.

4. Conclusion

F8T2 fibers were successfully produced by means of ES

without a carrier polymer. The obtained fibers show

high photosensitivity and photoluminescence, in the

cyan, yellow, and red if excited with UV, blue, and green

wavelengths, respectively. The conductivity of fibers, is

thermally activated which is consistent with a semicon-

ductor material behavior and the values are similar to the

ones obtained by other methods. The great advantage of

ES fibers is the simplicity and facility of the production

process compatible with a good controllability of their

diameter, radial uniformity, and measurement length-

wise. The high photossensitivity 60 for 1 mm length fiber

is suitable for organic fiber photovoltaic and sensors

applications.

Acknowledgements: The authors acknowledge Dr. Paulo Lemosfor the microscopy images. This work was supported by FCT-MECthrough Strategic Project PEst-C/CTM/LA0025/2011. Ana CatarinaBaptista also acknowledges FCT-MEC for the doctoral grant SFRH/BD/69306/2010.

Received: January 12, 2012; Revised: February 13, 2012; Publishedonline: DOI: 10.1002/mame.201200009




Keywords: electrospun fibers; F8T2; fluorescence; photosensitivity;polythiophenes

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Strongly Photosensitive and Fluorescent F8T2 Electrospun Fibers

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