Synthetic Metals 151 (2005) 275–278

Electrospun poly(3-hexylthiophene-2,5-diyl) fiber field effect transistor

Rosana Gonzalez, Nicholas J. Pinto∗

Department of Physics and Electronics, University of Puerto Rico, Humacao, PR 00791, USA

Received 22 April 2005; received in revised form 23 May 2005; accepted 25 May 2005

Abstract

One of the ways to reduce the size and increase component density in circuits is via the fabrication of devices based on semiconductornanofibers. We report on the fabrication of an electrospun regio-regular poly(3-hexylthiophene-2,5-diyl) fiber field effect transistor (FET).The hole mobility of the device was calculated to be 4× 10−4 cm2/V-s and the ON/OFF ratio was∼7. The results are compared to thoseobtained on a thin film FET. The large surface to volume ratio of the fiber makes it susceptible to doping, however, proper handling in an inertenvironment and pretreatment of the substrates should enhance device performance. Electrospinning is proposed as an easy one step processto fabricate one-dimensional polymer FET’s.©

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2005 Elsevier B.V. All rights reserved.

eywords:Poly(3-hexylthiophene); Transistor; Fiber; Electrospinning

. Introduction

Semiconducting �-conjugated organic polymers areidely used as active electronic components in the fabricationf flexible electronic displays and devices[1,2]. Techno-

ogically, the most important polymer based device fabri-ated and studied is the field effect transistor (FET), since itorms the basic building block in logic circuits and switchesor displays. Regio-regular poly(3-hexylthiophene-2,5-diyl)P3HT) is one such polymer that is used extensively as thective semiconducting layer in organic thin film FET’s as it

s soluble in common organic solvents and easily processedo from uniform thin films[3–5]. Relatively short, denselyacked whiskers and nanofibers can also be prepared via pre-ipitation from p-xylene and cyclohexane[6–8]. High valuesf the electric field induced charge mobility and on/off ratiosre important parameters that make such devices viable inractical applications. These characteristics are dependent on

he purity of the semiconductor and the fabrication processmong others. Most organic FET’s studied thus far have been

abricated in the form of thin films and are two-dimensional.

One of the ways to achieve device size reductionincrease component density in circuits is via the fabricationanofibers of these polymers making them one-dimensand more compact by design. Earlier, we demonstrateuse of a simple electrospinning technique to fabricate Fbased on conducting polymer nanofibers[9,10]. In this paperwe present results on the electrical characterization ofvidual, isolated submicron size fibers of P3HT preparedelectrospinning and compare the data with a thin film osame polymer. Careful control of the electrospinning paeters is expected to lead to true nanofibers of P3HT in astep process.

2. Experimental

Regio-regular P3HT and anhydrous chloroform (CHC3)were purchased from Aldrich and used as received. Iargon glove compartment, a 2 wt% solution of P3HTCHCl3 was prepared and stirred magnetically for 2 hsealed vial. The solution was then filtered through a 0.20�m

∗ Corresponding author. Tel.: +1 787 850 9381; fax: +1 787 850 9308.E-mail address:nj pinto@webmail.uprh.edu (N.J. Pinto).

PTFE syringe filter. A thin film FET was prepared (in air) byspin casting (5000 rpm/45 s) this solution on a prepatterneddoped Si/SiO2 wafer with gold electrodes. The thickness of

379-6779/$ – see front matter © 2005 Elsevier B.V. All rights reserved.

oi:10.1016/j.synthmet.2005.05.007

276 R. Gonz´alez, N.J. Pinto / Synthetic Metals 151 (2005) 275–278

Fig. 1. Schematic showing the main components of an electrospinning appa-ratus.

the film was∼300 nm. The device with length (L= 20�m)and width (W= 800�m) was then electrically contacted viathe use of silver paint and placed in a temperature controlledvacuum chamber with a base pressure of 5× 10−4 Torr forfurther characterization. In order to prepare fibers, a 7 wt%of P3HT in CHCl3 was prepared in an argon glove compart-ment and stirred magnetically for 15 min in a sealed vial. Thesolution was heated for a few minutes to 50◦C to assist in thesolubility. About 0.5 ml of the solution was placed in a hypo-dermic syringe and the flow rate controlled using a precisionflow pump. A voltage of 20 kV was applied to the needlewith the cathode (Al foil) situated about 20 cm from the tipof the needle and grounded.Fig. 1shows the basic elementsof the electrospinning apparatus. Under these conditions, the

electric forces on the polymer droplet at the end of the nee-dle overcome the surface tension and a jet is issued forth. Asthe solvent evaporates, fibers and fine droplets of the poly-mer are seen to collect on the cathode due to its relativelylow molecular weight. By intercepting the jet momentarilywith a substrate (doped Si/SiO2 wafer), isolated fibers canbe captured for device fabrication. In order to make elec-trical contacts to individual fibers, a transmission electronmicroscope grid was carefully placed over a fiber and sil-ver metal thermally evaporated over it. Upon removal of thegrid, the fiber was seen to have metal contacts on eitherend (henceforth the source and drain terminals) as seen inthe scanning electron microscope image ofFig. 2where thepresence of beads along the fiber are also observed. Thesebeads could result from the simultaneous interplay betweensolution viscosity and rapid evaporation of the solvent dur-ing the electrospinning process. The average diameter of thefiber between the contacts was found to be∼670 nm andthe length was∼54�m. Electrical contacts were then madevia the use of silver paint and the device was placed in atemperature controlled vacuum chamber with a base pres-sure of 5× 10−4 Torr for further characterization. Electricalcharacterization of the devices were made with a Keith-ley Model 6517A electrometer at a temperature of 300 K.Each device was annealed at 340 K for 1 h prior to electricalcharacterization.

3

d

F fiber. T nds anu r of the

ig. 2. Scanning electron microscope image of an electrospun P3HTsed for external electrical contacts. The average length and diamete

. Results and discussion

Fig. 3 shows the drain–source current (IDS) versusrain–source voltage (VDS) of the device inFig. 2 for var-

he dark parallel bands indicate evaporated silver metal over the fiber ed werefiber was 54�m and 670 nm, respectively.

R. Gonz´alez, N.J. Pinto / Synthetic Metals 151 (2005) 275–278 277

Fig. 3. The drain–source current (IDS) versus drain–source voltage (VDS)of an electrospun P3HT fiber FET for different back gate voltages (VGS) at300 K. Inset: Variation ofIDS versusVGS with VDS kept fixed at−80 V andthe corresponding plot ofI1/2

DS versusVGS.

ious gate–source voltages,VGS. At VGS= 0 V, theI–V curveshowed a linear increase in the current with increasing neg-ative voltage indicating an ohmic conduction due to unin-tentional doping as the device was handled in air for sev-eral minutes prior to mounting it in the vacuum chamber.For other applied gates biases,IDS increased with increas-ing negativeVDS with evidence of some saturation in thedrain–source current at high drain source voltages. Theincrease in the drain–source current upon increasing the neg-ative gate bias demonstrates that the device operates as aFET and that the majority carriers are holes. The deviceON/OFF ratio was∼7. Since in the case of a fiber, the devicelength (L) � the width (W), a modified equation for obtain-

ing the charge mobility (�), IDS = µ(

CL2

)(VGS − VTH)2

was used, whereVTH represents the threshold necessaryto completely deplete the fiber,C andL correspond to thefibers capacitance and length respectively[10]. The fiberscapacitance per unit length with respect to the back gate isCL

= 2πεεo

ln(2h/r) , wherer represents the radius of the fiber,handε represent the thickness (200 nm) and average dielec-tric constant (∼2.5) of the device, respectively. The insetin Fig. 3 shows a plot ofIDS versusVGS with VDS heldconstant at−80 V together with the corresponding plot ofIDS

1/2 versusVGS. From this plot, we extract the value ofVTH (12 V) after extrapolation of the curve to theVGS axis.U y tob n-d tob hec d asp -b FET[

ac-t

Fig. 4. The drain–source current (IDS) versus drain–source voltage (VDS)of a P3HT thin film FET for different back gate voltages (VGS) at 300 K.Inset: Variation of IDS versusVGS with VDS kept fixed at−80 V and thecorresponding plot ofI1/2

DS versusVGS.

sus drain–source voltage (VDS) of this device for variousgate–source voltages,VGS. The characteristics show truesaturation behavior and larger measured currents when com-pared to the fiber with an ON/OFF ratio of∼1900. The largercontact area of the polymer with the gate dielectric betweenthe source and drain terminals compared to the fiber is respon-sible for the larger field effect induced currents. It is alsolikely that the superior contact with the gate dielectric andhence the number of defects and charge traps are fewer in thecase of the thin film FET. The conductivity of the film wasmeasured to be 2× 10−9 S/cm under no gate bias indicat-ing minimum doping. The charge mobility in this device wascalculated from the saturated section of theI–V curves usingthe standard FET equation[11]: IDS = µWCi

2L(VGS − VTH)2

whereCi is the capacitance per unit area of the 200 nm thickSiO2 gate dielectric. The inset toFig. 4 shows a plot ofIDS versusVGS with VDS held constant at−80 V togetherwith the corresponding plot ofI1/2

DS versusVGS. From thisplot, we extract the value ofVTH (−30 V) after extrapo-lation of the curve to theVGS axis and calculate the holemobility to be∼10−3 cm2/V-s in the accumulation mode andthe charge density to bep=GL/eµW∼ 6× 1011 cm−2, where

G =(

∂IDS∂VDS

)VGS

calculated atVGS=−50 V is the device con-

ductance. These values are similar to that reported in theliterature[3].

omF tot berF ndm beri ygena Theu entf ordert film

sing these equations, we calculate the hole mobilite∼4× 10−4 cm2/V-s in the accumulation mode. The couctivity of the fiber with no gate bias was calculatede∼6× 10−6 S/cm, due to unintentional doping, while tharge density along the length of the fiber is calculate=Q/eL=CVTH/eL≈ 6× 108 cm−1; this value is comparale to that obtained for a polyaniline nanofiber based

10].For comparison, a P3HT thin film FET was char

erized. Fig. 4 shows the drain–source current (IDS) ver-

A comparison of the device characteristics frigs. 3 and 4imply that the thin film FET is superior

he fiber FET. A large OFF current in the case of the fiET is partly responsible for the low ON/OFF ratio aobility. Due to the high surface to volume ratio, the fi

s more susceptible to being doped by atmospheric oxnd theI–V curves affected by adsorbed water vapor.se of purified starting materials and an inert environm

or fabricating and testing such devices is necessary ino maintain the superior characteristics seen in the thin

278 R. Gonz´alez, N.J. Pinto / Synthetic Metals 151 (2005) 275–278

FET. Pretreating the substrates to make the surface hydropho-bic will result in better fiber contact with the substrate therebyreducing the number of charge traps that inhibit mobility.Nevertheless, we show that electrospinning is an easy andquick technique to fabricate near one-dimensional polymerFET’s.

4. Conclusion

Submicron diameter fibers of regio-regular P3HT withlengths several tens of microns can be prepared within sec-onds under ambient conditions using a simple electrospinningtechnique. These fibers were captured on doped Si/SiO2wafers and electrically contacted to form a FET. TheI–Vchar-acteristic curves of this device shows evidence of near currentsaturation for high gate biases. A large OFF current due toexposure to air is seen to affect the device characteristicswhen compared to a thin film FET of the same semiconduc-tor. The use of purified starting materials, pretreatment ofthe substrates and fabrication in an inert atmosphere shouldimprove device characteristics. By careful control of theapplied electric field, solution viscosity and flow rate, fabri-cation of regio-regular P3HT nanofibers via electrospinningshould be achievable. Further work along these lines is cur-rently underway.

Acknowledgements

This work was supported in part by NSF under Grants0402766, 0412926 and by a Petroleum Research Fund Grant38880-B7.

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