Organic Electronics 11 (2010) 748–754

Contents lists available at ScienceDirect

Organic Electronics

journal homepage: www.elsevier .com/locate /orgel

Solution processable micron- to nanoscale conducting polymerpatterning utilizing selective surface energy engineering

Kwang-Ho Lee a, Byung-Yeon Choi a, Jeong-Woo Park a,b, Seok-Ju Kang a,b, Sang-Mook Kim a,Dong-Yu Kim a,b, Gun-Young Jung a,*

a Dept. of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, South Koreab Heeger Center for Advanced Materials, Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, South Korea

a r t i c l e i n f o a b s t r a c t

Article history:Received 17 December 2009Received in revised form 12 January 2010Accepted 15 January 2010Available online 21 January 2010

Keywords:Conducting polymerSurface energyOrganic field-effect transistorsNanoscale patterning

1566-1199/$ - see front matter � 2010 Elsevier B.Vdoi:10.1016/j.orgel.2010.01.014

* Corresponding author. Tel.: +82 62 970 2324; faE-mail address: gyjung@gist.ac.kr (G.-Y. Jung).

We developed a new conducting polymer microscale and nanoscale patterning method; abottom-up approach for applying a conducting polymer solution on a substrate withlocally varied surface energies. Selective surface energy engineering was achieved bycombining conventional photolithography and local hydrophobic treatments with aself-assembled monolayer (SAM) at the exposed surface. The regions under the photoresistpatterns remained hydrophilic after the photoresist removal. Here, a poly(3,4-ethylene-dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) conducting polymer solution wasdispensed with a razor blade on the substrate, wetting only the hydrophilic regions. Next,conducting source/drain polymer electrodes were fabricated (channel length: 5 lm) andutilized for poly(3-hexylthiophene) (P3HT) organic field-effect transistors. For nanoscaleconducting polymer line patterns, a photoresist template with nanoscale features wasfabricated using holographic lithography. Finally, multi-step spin coating method reliablyproduced polymer lines having a linewidth of 292 nm at 1 lm spacing.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

Conducting polymers have the potential to be widelyused as electrodes in the fabrication of organic devicessuch as sensors, actuators, capacitors, and display devices[1,2]. As a result, these polymers have received consider-able interest due to their intrinsic flexibility, thereby en-abling the construction of flexible devices, as well astheir ease of mass production, low cost, light weight, andsolution processing at low temperature. For this reason,there have been attempts to develop unconventional pat-terning techniques for active or passive organic devicesin the new era of plastic electronics [3].

However, conducting polymer patterning is the bottle-neck in the path to the development of these wider appli-cations. Conventional photolithography techniques require

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x: +82 62 970 2304.

conducting polymers that are photosensitive and cross-linkable during UV exposure, if they are to remain intactduring the subsequent development process; these restric-tions demand the development of new material designscompatible with the process [4]. Jung et al. demonstratedpolymer conducting nanolines by combining the conven-tional lift-off process with a nanoimprinted poly(methylmethacrylate) template, where a charged conducting poly-mer anchored to an oppositely charged self-assembledmonolayer (SAM) formed only at the trenches of the poly-mer template [5]. This process is not versatile, however,because the lift-off solvent could also dissolve the polymerfilm and/or degrade its conductance.

Alternatively, direct pattering techniques such as l-contact printing [6], ink-jet printed patterning [7], dip-pen lithography [8], and gravure printing [9] have beenintroduced. In attempt to resolve this issue, Rogers et al.[10] and Someya et al. [11] demonstrated submicrometerpatterns by injecting subfemtoliter ink through a nozzle

K.-H. Lee et al. / Organic Electronics 11 (2010) 748–754 749

sophisticatedly manufactured from a very fine capillaryglass tube having a tip diameter of <1 lm.

In contrast to top-down process, Tessler et al. reported abottom-up approach for conducting polymer patterningbased on a hydrophobic SAM formed on a 80 nm thickTiO2-covered substrate [12]. The SAM exposed to UV lightthrough the photomask became hydrophilic due to thephotocatalytic effect of TiO2 in the exposed regions and en-abled the solution-deposition of the conducting polymervia dewetting from the hydrophobic regions during subse-quent spin-casting. However, this technique requires a thinTiO2 layer for the photocatalytic reaction with SAM at ahumidity of ca. 40%. Other dewetting techniques hadhydrophobic regions defined by microcontact printing for�3 lm patterning [13] and electron-beam lithography fora 500 nm transistor channel length [14]. However, itshould be noted that electron-beam lithography is costlyand time-consuming process for large-area applications.

In this work, we propose a hybrid process that com-bines conventional photolithography and local surface en-ergy modification through hydrophobic treatment in orderto fabricate the conducting polymer source/drain elec-trodes of poly(3-hexylthiophene) (P3HT) organic field-ef-fect transistors (OFETs). This technique is then applied tothe fabrication of nanoscale conducting polymer patternsusing photoresist (PR) templates, where nanoscale resistfeatures are produced by holographic lithography.

2. Experimental section

2.1. Selective surface energy treatment for microscalepolymer patterning

The proposed fabrication process is shown in Fig. 1. Inbrief, a 50 nm thick silicon oxide (SiO2) surface on a heavilydoped n-type silicon substrate was successively cleanedwith acetone, ethyl alcohol, and deionized water in anultrasonic bath. For the initial hydrophilic surface modifi-cation, an oxygen plasma treatment was carried out at300 W for 60 s. A positive tone PR (AZ-6612, Clariant) solu-tion was spin-coated on the substrate at 2800 rpm for 30 s

Fig. 1. Schematic of conducting polymer patterning using selectivesurface modification. (a) Photoresist patterns from conventional photo-lithography and the subsequent development process. (b) OTS attach-ment to the exposed substrate surface by self-assembly. (c) After removalof the PR patterns. (d) Conducting polymer solution dispersion; onlyhydrophilic regions are wet.

and then baked at 100 �C for 1 min. A UV light (k = 365 nm)irradiated the PR film for 6.7 s through a clear-field photo-mask on which the desired patterns had been chromium-deposited. After development process, a gentle oxygenplasma treatment was performed at 100 W for 30 s in or-der to remove any residual PR layer under the trenchesand generate hydroxyl groups at the exposed surfaces,which anchor the hydrophobic molecules during the SAMtreatment.

The substrate with the PR patterns was immersed intoan octadecyltrichlorosilane solution (OTS; 1.5 wt.% in tolu-ene) for 5 min. At above 1.5 wt.%, aggregated OTS mole-cules were observed in all of the exposed regions. At lessthan 1.5 wt.%, the wettability contrast did not provide suf-ficient definition, especially for the 5 lm channel length,resulting in current leakage between the source and drainelectrodes. During the OTS SAM coating, the head groups(–SiCl3) reacted with the hydroxyl groups on the substratesurface in the trenches via siloxane linkage, and the alkylchains underwent van der Waals interaction between eachother with the hydrocarbon tail groups (–CH3) exposed tothe air, thereby rendering the surface hydrophobic nature.The substrate was then baked at 120 �C for 10 min to re-move any residual toluene solvent. After PR pattern re-moval by acetone soaking for 5 min in an ultrasonic bath,the protected regions underneath the PR patterns re-mained hydrophilic during the photolithography processand the subsequent OTS treatment. Thus, a substrate withlocally varied surface energies was achieved and subse-quently utilized for spontaneous polymer patterning.

As a conducting polymer solution, we adopted theaqueous poly(3,4-ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) polymer solution (1:2.5 by weight,1 wt.% solid content, BAYTRON�, PH 500) due to its excel-lent processing ability, as well as its conducting, transpar-ent, and thin film characteristics. Recent studies haveshown that the conductivity of PEDOT:PSS can be dramat-ically increased with no loss in optical transparencythrough the addition of a small amount of diethylene gly-col dopant [15,16]. In this experiment, diethylene glycolwas mixed with PEDOT:PSS (1:10 by volume), and thesolution was then dropped and dragged on the speciallytreated substrate using the doctor-blade method with a ra-zor edge; there were no notable effects of blading directionrelative to the feature geometries. The solution was dew-etted by the hydrophobic regions and was thus spontane-ously confined in the hydrophilic regions duringspreading. Next, the sample was left in air for 2 min to al-low the solvent to evaporate; at this time the polymersolution contracted, reducing the surface energy, which re-sulted in contours with sharp edges at the corners. Finally,the conducting polymer patterns were heated at 110 �C for10 min to increase the conductivity for subsequent use asthe source and drain electrodes of OFETs.

2.2. Nanoscale polymer patterning

Laser interference lithography (i.e., holographic lithog-raphy) was used to fabricate nanoscale periodic PR pat-terns because it can produce nanoscale features withease. In brief, a diluted positive PR (AZ6612, Clariant) with

Table 1Contact angle measurement and corresponding surface energy calculated ateach step.

Experimental process Contactangle(�)

Surfaceenergy (mN/m)

Initial oxygen plasma treatment 2.2 89.8Exposed area After development 14.2 87.1

After OTS treatment 97.7 16.8Protected area After PR pattern

removal26.8 80.5

On PEDOT:PSS film 7.6 19.7

750 K.-H. Lee et al. / Organic Electronics 11 (2010) 748–754

a thinner (AZ1500, Clariant) (volume ratio = 1:2) was spin-coated to a 150-nm thickness on a clean substrate that hadpreviously been coated with hexamethyldisilazane(HMDS; above 98%, Fluka). Next, one-beam interferencelithography method was utilized at an incident angle (h)of 7.18 to fabricate repeated PR line patterns having a line-width of 290 nm at a 1.3 lm pitch size. A UV laser beamfrom the He–Cd ion laser (325 nm) was then passedthrough a spatial filter consisting of several lenses and apin hole (10 lm diameter) in the end, where diffraction-limited beam enhancement occurs; this enhanced beamwas projected onto the angle bracket where the sampleholder and Lloyd mirror were placed perpendicular to eachother. The PR was exposed to light for 50 s, equivalent to adose of 30 mJ/cm2, with a sinusoidal intensity owing to theconstructive and destructive interference between the twosplit beams, one directly from the laser and the other re-flected from the Lloyd mirror.

A specific pitch size was determined by K = k/2sinh,where h is the incident angle, K is the pitch size of the pat-tern, k is the laser wavelength, and the exposure time wasoptimized for the specific line width. After laser exposure,a developing process was performed by dipping the entiresubstrate into MIF 300 solution for 30 s and then rinsingwith deionized water.

However, the solution-based SAM coating method usedfor microscale features did not work well for the nanoscalePR features due to the incomplete wetting to such confinedareas caused by trapped air bubbles that prevented directcontact of the solution to the bottom of the exposed sub-strate surface between the PR features. Therefore, we ap-plied a hydrophobic treatment in vapor phase using atridecafluoro-1,1,2,2-tetrahydrooctyltrichlorosilane (Gel-est Inc., CF3–(CF2)5–(CH2)2–SiCl3), which is commonly usedas an anti-sticking material for the stamp surface treat-ment in nanoimprint lithography [17]. Perfluorocarbon-based trichlorosilanes have been reported to significantlydecrease the stiction and friction of surfaces compared tohydrocarbon-based surface coatings [18].

A substrate with nanoscale PR features was loaded intoa custom-built glass vacuum chamber and then pumpeddown to 10–3 torr. Then the pump was isolated from thechamber and the vapor of the hydrophobic material wasthen introduced into the chamber through a needle valve.The container of the hydrophobic agent solution was keptat 80 �C in order to maintain sufficient vapor pressure; thisprocess was kept for 10 min at a static pressure of 0.3 torr.Next, the valve supplying the vapor of the hydrophobicmaterial was closed and a second needle valve connectedto a container partially filled with DI water was openedto allow water vapor into the chamber. The chamber wasmaintained for another 10 min at a total pressure of0.8 torr in order to cross-link the neighboring attachedmolecules via a condensation reaction. Such a condensa-tion reaction can produce a dense network within themonolayer on the substrate surface [19]. Finally, the cham-ber was again pumped down to a pressure 10–3 torr andthe cycle was repeated twice more.

After removing the PR patterns with acetone, the poly-mer solution was cast on the substrate with locally differ-ent surface energies by using a multi-step spin-coating

method; 1000 rpm for 10 s, 3000 rpm for 20 s, and5000 rpm for 20 s.

3. Results and discussion

3.1. Surface energy measurement

To investigate the surface energy variation at each pro-cess step, the contact angle was measured with water atroom temperature and then converted into surface energyusing the Good–Girifalco–Fowkes–Young (G–G–F–Y) equa-tion [20]:

ð1þ cos hÞcLV ¼ 2ðcSVVLVÞ1=2

where cLV is the surface tension of the test liquor (water;72.8 dyne/cm), h is the contact angle, and cSV is the surfaceenergy of the medium of interest. Table 1 presents the re-sults of contact angle measurements and their correspond-ing calculated surface energies at each step.

The initial surface treatment with oxygen plasma gasproduced a very hydrophilic surface with a contact angleof 2.2�, equivalent to a high surface energy of 89.8 mN/m.The contact angle of the exposed substrate surface afterdevelopment increased to a value of 14.2�. The surface be-came hydrophobic after application of the OTS SAM coat-ing, attaining a contact angle of 97.7�, equivalent to asurface energy of 16.8 mN/m; this region was thermody-namically stable such that the surface is unlikely to reactto other chemical elements.

The initial hydrophilic nature of the region under thePR pattern remained after PR removal, though the valueincreased to 26.8� due to the instability of the surfaceSi–OH groups with time. During the sequential pro-cesses for PR patterning and removal, the increase incontact angle is associated with a decrease in hydroxylgroup coverage on the surface through the dehydrationreaction between two neighboring hydroxyl groups,leading to the formation of a siloxane (Si–O–Si) bond[21]. The contact angle was 15� immediately after PRremoval, and increased to 26.8� after 10 min in air. Assuch, polymer solution dispersion was completed within10 min after PR removal. This selective surface engi-neering drives the spontaneous patterning of the con-ducting polymer solution, covering energeticallyunstable surfaces of hydrophilic regions with a highsurface energy.

Fig. 2. (a) FE-SEM images of the conducting polymer source/drain electrodes with channel lengths from 5 to 100 lm. (b) AFM image of the surfacemorphology at the 20 lm gap. (c) Surface profile of a 500 lm wide conducting polymer electrode.

K.-H. Lee et al. / Organic Electronics 11 (2010) 748–754 751

The contact angle of the DEG-PEDOT:PSS film depositedon the hydrophilic region was also measured using water.Water was dropped through a syringe and wet the DEG-PEDOT:PSS film at a contact angle of 7.6�, correspondingto a surface energy of 89 mN/m. This illustrates that theDEG-PEDOT:PSS solution has a high affinity to water,which is driving force for spontaneous wetting in the local-ized hydrophilic regions.

3.2. Results of microscale polymer patterning

Fig. 2a presents SEM images of conducting polymer pat-terns (500 lm � 1000 lm) at gaps ranging from 5 to100 lm. The polymer patterns exactly reproduce the corre-sponding photomask patterns via the proposed patterningmethod. The solution wets the pre-determined hydrophilicareas. The significant outcomes are that the gap is clearlydefined along the 1000 lm length even at 5 lm gap andthat no cracks are observed within the polymer patterns.

Fig. 2b shows the AFM image of the surface morphol-ogy of the 20 lm gap, where it can be seen that the con-ducting polymer solution is well confined at thehydrophilic region at that resolution during dispersion.Fig. 2c represents the thickness profile of the polymerpattern measured by surface profiler (Alpha-Step IQ, KLA

Tencor); the polymer electrode has a maximum thicknessof 380 nm at the center. When a liquid interfaces with air,the molecules at the interface have a higher energy thanthat of the interior molecules; therefore, the eventual cur-vature of the thickness profile shown in the figure isformed to minimize the surface area and thereby reduceits surface energy.

These conducting polymer patterns were subsequentlyused as the source/drain electrodes in OFET devices. Usingthe four-point probe method, the conductivity of the con-ducting polymer pattern was measured to be ca.400 � 102 S/cm. The polymer electrodes for the bottomcontact OFETs (BC-OFET) were fabricated on top of a 50-nm thick gate insulating layer of silicon oxide that wasgrown on the heavily doped n-type silicon substrate. Aprior study investigating the surface-treatment effects onthe performance of OFETs indicated that OTS treatmentof the gate dielectric layer increased the crystallinity ofthe semiconducting active material and hence carriermobility [22]. In our case, as the exposed silicon oxide sur-face between the source/drain polymer electrodes was al-ready coated with OTS during the selective hydrophobictreatment, 1 wt.% of P3HT solution in chloroform wasspin-coated without further treatment and then baked at125 �C for 20 min inside a nitrogen glove box.

Gate voltage (V)-30 -20 -10 0 10

(Dra

in c

urre

nt)1

/2 (A

1/2 )

0.000

0.001

0.002

0.003

0.004

Dra

in c

urre

nt (A

)

10-10

10-9

10-8

10-7

10-6

10-5

10-4

Drain voltage (V)-10-8-6-4-20

Dra

in c

urre

nt (µ

A)

0.0

0.4

0.8

1.2

1.6

Fig. 3. (a) Output curves and (b) transfer characteristics, swept in bothdirections, of the P3HT OFFTs with the PEDOT:PSS source/drain electrodesfabricated using the selective surface engineering technique with channelwidth/length of 1000 and 5 lm, respectively.

752 K.-H. Lee et al. / Organic Electronics 11 (2010) 748–754

Fig. 3a shows the typical transistor output curve at gatevoltages from 0 to �10 V of BC-OFETs for a channel width/length of 1000 lm/5 lm. Fig. 3b shows the correspondingtransfer characteristics in both forward and reverse scanand the calculated field-effect mobility and the thresholdvoltage are obtained from plots of the square root of |ID|in the saturation region at VD = �30 V as a function of VG

(from +5 V to �30 V). All the measurements were per-formed in a nitrogen glove box. The output curves demon-strate a slight hysteresis phenomenon due to the hydroxylgroups at the surface of SiO2 gate dielectric layer inher-ently remained after the selective surface treatment. Thehydroxyl groups in the semiconducting materials or atthe interface are reported to act as charge trapping sites,being negatively charged, and cause the hysteresis depend-ing on the amount of hydroxyl groups [23]. Impurities suchas moisture, oxygen and mobile ions also bring about largehysteresis in the device characteristics [24]. However, asthe OFETs were fabricated and measured in dried nitrogenatmosphere in this experiment, the hysteresis was notnoticeable, especially in transfer characteristics at the scanfrom VG = 5 to �30 V and back to 5 V.

The charge carrier mobility in the saturation region isdetermined by the equation:

jIDj ¼W2L

lCi VG � VTð Þ2

where Ci is the capacitance per unit area of gate insulator(50-nm thick silicon oxide, 69 nF/cm2), W/L is the ratio ofchannel width to channel length, VT is the threshold volt-

age, and l is the charge carrier mobility of the OFET. Forour device, the charge carrier mobility was calculated tobe 1.4 � 10�3 cm2/V s at a threshold voltage of 0.2 V andan on/off current ratio of 1.2 � 104. With increasing thechannel length, the mobility was enhanced to 2.5 �10�2 cm2/V s (L = 100 lm) due to the reduced short chan-nel effect.

Our device performance is comparable to other transis-tors with PEDOT:PSS conductive electrodes fabricated bydifferent methods. For instance, Cho et al. reported transis-tors having ink-jet printed source/drain electrodes thatwere fabricated using a modified PEDOT:PSS solution(channel lengths from 25 to 150 lm) on P3HT film [25].The devices had an average carrier mobility of6.0 � 10�3 cm2/V s in the saturation region regime(Vd = �80 V), VT of 6 V, and on/off current ratio of 103. Inaddition, Azumi group demonstrated the microcontactprinting of PEDOT:PSS solution as the source/drain elec-trodes for a channel length of 12 lm on P3HT film, witha carrier mobility of 1.7 � 10�3 cm2/V s, and an on/off cur-rent ratio of 6.0 � 104 [26].

3.3. Nanoscale polymer line patterning

Fig. 4a shows the SEM image of separate PR line pat-terns after development with a linewidth of 290 nm, spac-ing of 1 lm, and a thickness of 120 nm, which wasfabricated using an interference lithography technique.Its corresponding AFM image (Fig. 4b) illustrates a PR tem-plate ready for the local surface energy treatment.

After hydrophobic treatment in vapor phase, several tri-als using the doctor-blade method for polymer solutiondispersion ended up with no polymer patterning at sucha tiny scale; the solution covered the whole substrate be-cause the generated solution film was too thick to be con-fined into the nanoscale hydrophilic line areas. Instead, amulti-step spin-coating method was utilized to producenanoscale polymer patterns. Preliminary experiments withsingle-step casting at a low spin speed generated polymercoverage over the entire substrate similar to the doctor-blading results; at a high speed, all the polymer solutionwas spinned off before the substrate surface was wetted.Therefore, the spin speed was stepwise increased in orderto provide a relaxation period to allow the polymer solu-tion to equilibrate to the local surface energy.

Fig. 4c shows that the polymer line patterns are wellmatched with the feature size of the PR template;292 nm wide lines at 1 lm spacing. However, some poly-mer residue can still be seen between the polymer linesin the AFM image (Fig. 4d). The cross-sectional surface pro-file (Fig. 4e) illustrates that 30 nm thick polymer nanolineswere generated by using the multi-step spin-coatingmethod.

4. Conclusions

In summary, a new process for conductive polymer pat-terning that combines conventional photolithography andself-assembly monolayer deposition of a hydrophobicmaterial was proposed. As the regions protected by the

Fig. 4. (a) FE-SEM image of the nanoscale PR features produced by holographic lithography and (b) its corresponding AFM image. (c) FE-SEM and (d) AFMimages of the nanoscale polymer line patterns with a width of 292 nm at 1 lm spacing produced by using the polymer template of (a). (e) Cross-sectionalsurface profile of the polymer line patterns.

K.-H. Lee et al. / Organic Electronics 11 (2010) 748–754 753

photoresist patterns remained hydrophilic following thephotoresist removal, a chemically patterned substrate withlocally different surface energies was generated. Based onthis process, we fabricated P3HT organic semiconductingfield-effect transistors using the PEDOT:PSS source/drainelectrodes formed by selective dewetting. The deviceshowed a comparable transistor performance to previousdevices: a field-effect mobility of 1.4 � 10�3 cm2/V s; athreshold voltage of 0.2 V; and on/off current ratio of 104

at a 5 lm channel length. In addition, the polymer solutioneffectively wetted the nanoscale hydrophilic regions, pre-determined by holographic lithography and subsequentselective hydrophobic treatment in vapor phase. As a re-sult, nanoscale polymer lines with a linewidth of 292 nmat 1 lm spacing were reliably fabricated by the multi-step

spin-coating method. In the near future, it is expected thatthis technique could be applied to solution-processibleplastic electronics.

Acknowledgements

This work was partially supported by the Ministry ofKnowledge Economy (MKE), Korea through a project ofthe Research Consortium for R2R printed RFID tags andthe System IC 2010 project from the Ministry of Com-merce, Industry and Energy (MCIE), Korea. This work wasalso supported by the Korea Science and Engineering Foun-dation (KOSEF) grant (No. R15-2008-006-03002-0, CELANCRC), and the Program for Integrated Molecular Systemat GIST.

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