DOI: 10.1002/adfm.200600170

High-Resolution Characterization of Pentacene/PolyanilineInterfaces in Thin-Film Transistors**

By Kwang Seok Lee, Timothy J. Smith, Kimberly C. Dickey, Joung Eun Yoo, Keith J. Stevenson, andYueh-Lin Loo*

1. Introduction

Organic thin-film transistors (TFTs) are promising candi-dates for backplanes in electronic paper[1] and large-areadisplays,[2,3] as well as for electrically active components inradio-frequency identification tags (RFIDs)[4] and sensors.[5,6]

Recently, efforts in this field have focused on manipulating thethin-film morphology and structure of existing organic semi-conductors,[7] and on designing new materials,[8–11] to increasethe charge-carrier mobility and, accordingly, to enhance theperformance of organic TFTs. While the organic semiconduc-tor layer is the electrically active component within the organicTFT structure, the device performance is not solely governedby its inherent electrical characteristics. Rather, the perfor-mance of organic TFTs can frequently be plagued by a largecontact resistance originating from the interface between theorganic semiconductor and the metal electrodes, which in itself

is a manifestation of large energy barriers for charge transfer atthese interfaces.[12] At the device level, high contact resistanceresults in higher power consumption per unit current output,limited current-carrying ability, and increased resistor–capaci-tor (RC) charging times of circuit elements.[13] The elucidation,interpretation, and subsequent reduction of contact resistanceat the organic semiconductor/metal electrode interface aretherefore essential before organic TFTs can be fully incorpo-rated in practical applications.

To estimate the contact resistance in organic TFTs, severalresearch groups have analyzed the organic semiconductor/metal electrode interface within a device structure using thetransmission-line method.[14–18] In this method, the parasitic re-sistance is extracted by examining the output characteristics ofa series of TFTs with varying channel lengths. Generally, theextracted parasitic resistance is taken to be the sum total ofany resistance, other than the channel resistance, contributingto the TFT device performance.[18] More recently, the trans-mission-line method has been extended to organic TFTs withconducting-polymer electrodes.[19,20] These devices tend to ex-hibit a lower contact resistance than their counterparts withmetal electrodes; differences in energy alignment at the organ-ic semiconductor/electrode interfaces[21,22] have been specu-lated to be its origin.

In this paper, we present the first detailed report that directlycorrelates the reduced contact resistance in organic TFTs withfundamental structural characterization at the pentacene/poly-aniline (PANI) interface. For comparison, structural and elec-trical characterization was also carried out on reference TFTswith gold electrodes of similar dimensions and geometry. Con-sistent with previous work on conducting-polymer contacts byothers,[19,20,23] scanning surface potential microscopy (SSPM)[24]

experiments indicate that the contact resistance at the penta-cene/PANI interface is significantly smaller than that at the

Adv. Funct. Mater. 2006, 16, 2409–2414 © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2409

–[*] Dr. Y.-L. Loo, K. S. Lee, K. C. Dickey, J. E. Yoo

Department of Chemical EngineeringCenter for Nano- and Molecular Science and TechnologyUniversity of Texas at AustinAustin, TX 78712 (USA)E-mail: lloo@che.utexas.eduT. J. Smith, Dr. K. J. StevensonDepartment of Chemistry and BiochemistryCenter for Nano- and Molecular Science and TechnologyUniversity of Texas at AustinAustin, TX 78712 (USA)

[**] This work is supported by the NSF (DMR 0314707, CHE-0134884),the Dreyfus Foundation, the Keck Foundation, the Welch Foundation(Grant F-1529), the Beckman Foundation, and Ricoh Innovations. Wewould also like to thank the Il-Ju Academic Foundation in Korea foran Overseas Graduate Student Fellowship to K. S. L. and Dr. FranklinKalk at Toppan Photomasks for supplying the photomasks used inthis project.

We present the first detailed report that directly correlates the reduced contact resistance in organic thin-film transistors(TFTs) with fundamental structural and morphological characterization at the organic semiconductor/conducting polymer in-terface. Specifically, the pentacene grains are similar in size and continuous across the channel/electrode interface in bottom-contact TFTs with polyaniline (PANI) electrodes. On a molecular level, the fused rings of pentacene are oriented perpendicularto the surface both in the channel and on PANI. Accordingly, the contact resistance is small in such devices. In TFTs with goldelectrodes, however, the pentacene grains are different in size and are discontinuous across the channel/electrode interface.Further, the fused rings of pentacene are oriented perpendicular to the channel surface and parallel to the gold surface. Suchdifferences across the channel/electrode interface lead to structural and electronic disorder, which manifests itself as a signifi-cantly larger contact resistance in devices with gold electrodes.

FULL

PAPER

pentacene/gold interface. This phenomenon is directly corre-lated with the morphology and structure of the pentacene/elec-trode interface. Specifically, the pentacene grains are continu-ous across the channel/electrode interface in devices withPANI electrodes, while they are discontinuous across the anal-ogous interface in devices with gold electrodes. More impor-tantly, the pentacene ring orientation—perpendicular to thesurface—remains the same across the channel/PANI interface,whereas the pentacene ring orientation changes from beingperpendicular to the channel surface to being parallel to thegold electrode surface in reference devices.

2. Results and Discussion

Our ability to pattern PANI source and drain electrodes de-rives from a unique feature of the material. Unlike conven-tional PANI (not soluble in any common solvents), our PANIis water dispersible because it is protonated with a polymeracid, rather than a small-molecule acid,[25–28] at the onset ofsynthesis. The polymer acid of interest, poly(2-acrylamido-2-methyl-1-propane-sulfonic acid), or PAAMPSA, serves twocritical roles: the sulfonic acid pendant groups protonatesPANI to render it electrically conductive (0.1–1 S cm–1 depend-ing on the details of synthesis), and excess sulfonic acid groupspromote water dispersability of the final product.[25] We will re-fer to our water-dispersible, electrically conductive PANI asPANI–PAAMPSA from here on. To fabricate TFTs, wedefined PANI–PAAMPSA source and drain electrodes on sili-con substrates with 3000 Å silicon dioxide using a derivative of“stamp-and-spin-cast” that was previously developed in ourlaboratories.[25] The details associated with device fabricationare provided elsewhere[25] and are summarized in the Experi-mental Section. Separately, we also fabricated gold source anddrain electrodes of the same geometry and dimensions on anal-ogous silicon platforms by photolithography and lift-off. Penta-cene was simultaneously evaporated in the channel regions ofboth batches of electrodes through a shadow mask to completethe bottom-contact TFTs. The final TFTs had a channel width(W) of 1000 lm and a channel length (L) of 120 lm. To unam-biguously study how the different electrodes affect device per-formance, we took comprehensive measures to keep all otherparameters, including the geometry and dimensions of thechannels of the transistors, the chemical treatment of thedielectric surface, and the evaporation of pentacene, constant.

We fabricated and tested approximately fifty pentaceneTFTs with PANI–PAAMPSA electrodes and compared their de-vice characteristics with reference pentacene TFTs with goldsource and drain electrodes of analogous geometry and dimen-sions.[25] For all practical purposes, pentacene TFTs with PANI–PAAMPSA electrodes perform as effectively as TFTs with goldelectrodes. Specifically, when the TFTs are turned “on” com-pletely (known as the saturation regime), device characteristics,such as the saturation mobility and the on–off current ratios, ofboth batches of pentacene TFTs—either with PANI–PAAMPSAor with gold electrodes—are statistically comparable.[25] In thelow source–drain voltage (or linear) regime, however, TFTs with

PANI–PAAMPSA electrodes appear to have higher output cur-rents given the same input voltages compared with analogousTFTs with gold source and drain electrodes. Representative cur-rent–voltage (I–V) characteristics obtained in the linear regimeof two analogous TFTs, one with gold electrodes and the otherwith PANI–PAAMPSA electrodes, are shown in Figure 1a and b,respectively. Not only are the current outputs lower given thesame input voltages, but bottom-contact pentacene TFTs withgold electrodes also exhibit nonlinear, “hooking” I–V character-istics at the low drain–source voltage regime (Fig. 1a).[25] Thisphenomenon is commonly reported for bottom-contact organic

TFTs with metal electrodes, and is believed to originate fromlarge contact resistance at the pentacene/gold electrode interface.On the contrary, the bottom-contact pentacene TFT with PANI–PAAMPSA source and drain electrodes does not exhibit this“hooking” behavior in the linear regime (Fig. 1b); an observationwe attribute to reduced contact resistance at the pentacene/PANI–PAAMPSA interface.

To examine the details of the contact resistance, the TFTs inFigure 1 were subjected to SSPM measurements. During theseexperiments, both the surface topography and the surfacepotential distribution across the channels were measuredsimultaneously during device operation. To capture the surfacepotential difference in the linear regime, we applied a constantsource–drain voltage of –5 V and systematically changed thegate voltage (Vg) in increments of –10 V, ranging from –10 to–50 V. Figure 2a and b reveals the surface topography profiles,while Figure 2c and d contains the surface potential profilesacross the channels of the two TFTs, respectively. The channelregion for each device is highlighted in gray in Figure 2 forclarity. The topography profile in Figure 2a reveals gold sourceand drain electrodes with sharp edges that are characteristic offeatures generated using photolithography and lift-off. In con-trast, the PANI–PAAMPSA source and drain electrodes aretaller (≈ 500 nm, as opposed to 150 nm), and the edges aremore slanted due to the nature of the patterning process.[25] InFigure 2c, we observe how the –5 V bias that is appliedbetween the source and drain electrodes is distributed acrossthe channel region. Specifically, we observe a sharp drop-off atthe pentacene/gold source interface. This is followed by a grad-

2410 www.afm-journal.de © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2006, 16, 2409–2414

0 -1 -20.0

-0.1

-0.2

-0.3

0 -1 -20.0

-0.1

-0.2

-0.3

Source-Drain Voltage [V]So

urc

e-D

rain

Cu

rren

t [ µ

A]

a b

0.0

-0.1

-0.2

-0.3

0 -1 -20.0

-0.1

-0.2

-0.3

Figure 1. Linear-regime I–V characteristics of two pentacene TFTs with thesame channel dimensions (W = 1000 lm and L= 120 lm), with a) goldsource and drain electrodes and b) PANI–PAAMPSA electrodes. We in-creased the gate voltage from 0 to –50 V in increments of –10 V.

FULL

PAPER

K. S. Lee et al./ Pentacene/Polyaniline Interfaces in Thin-Film Transistors

ual drop in the surface potential across the channel, and finally,another sharp drop-off at the pentacene/gold drain interface.We have labeled the potential drops at the pentacene/goldsource interface and at the pentacene/gold drain interface DVs

and DVd, respectively. Potential drops at the organic semicon-ductor/electrode interfaces are in fact common in bottom-con-tact organic TFTs with gold source and drain electrodes, andare typically attributed to high energy barriers associated withcharge transfer at the interfaces in question.[29–33] Quantificationof the surface potential profiles also revealed that the potentialdrop at the pentacene/gold source interface (DVs) is larger thanthat at the pentacene/gold drain interface (DVd). A larger poten-tial drop translates to a larger contact resistance; our measure-ments therefore suggest that charge injection from the goldsource electrode into pentacene is more difficult than charge ex-traction from pentacene to the gold drain electrode.[32,33]

The surface potential profiles of the pentacene TFT withPANI–PAAMPSA source and drain electrodes, on the otherhand, look dramatically different from those of the referencepentacene TFTs with gold electrodes. While the dominant po-tential drop is still across the channel, we do not observe anysignificant potential drops across either of the pentacene/PANI–PAAMPSA interfaces in this device. Regardless, weestimated an upper bound of the potential drops, DVs and DVd

(see Fig. 2d), by examining the changes in the slopes of the sur-face potential profiles at the respective interfaces.

To quantify the individual resistances between the source anddrain electrodes—between which the –5 V bias was applied—weinvoked a simple model of five resistors in series as shown in Fig-ure 3a. In this model, the total resistance across the TFT consistsof the resistance along the drain electrode (R1), the contact resis-tance at the pentacene/drain interface (R2), the channel resis-tance (R3), the contact resistance at the pentacene/source inter-

face (R4), and the resistance along the source electrode (R5).Given the size of the scan window (≈ 100 lm), we scanned eachof the channel/electrode interfaces separately to span the entirechannel region. Our experimental setup allowed us to directlyprobe R2, R3, and R4; R1 and R5 fell outside the scan window.The dashed line in Figure 3a outlines the resistances that can bedirectly probed with measurements in the scan window. To quan-tify the individual resistances, we normalized each of the poten-tial drops by the source–drain current output acquired simulta-neously during SSPM. Given that DVs and DVd are negligible inthe TFT with PANI–PAAMPSA electrodes, R2 and R4 are clearoverestimates in this device. The channel resistance (R3) and thesum of the contact resistance (R2 + R4) are plotted as a functionof gate voltage for the TFTs with gold and PANI–PAAMPSAelectrodes in Figure 3b and c, respectively. We observe that boththe channel resistance and the sum of the contact resistance aregate-bias dependent. In fact, in the TFT with gold electrodes, thesum of the contact resistance (R2 + R4 580 kX cm at a gate volt-age of –50 V) becomes comparable with the channel resistance(R3 910 kX cm at the same gate voltage[34]) with increasinggate voltages. This device is therefore contact-limited, and thisphenomenon is entirely consistent with the electrical character-ization shown in Figure 1. This observation is not surprisingand has previously been reported for bottom-contact organicTFTs with metal electrodes.[14–16,19,20] On the other hand, weobserve that the contact resistance of the pentacene TFT withPANI–PAAMPSA electrodes is significantly lower than that ofthe channel resistance at all gate voltages. At an applied gatebias of –50 V, in particular, the sum total of the contact resis-tance at both the pentacene/PANI–PAAMPSA interfaces is26 kX cm, whereas the channel resistance is 290 kX cm.[34] Theperformance of the pentacene TFT with PANI–PAAMPSAelectrodes is therefore not contact limited.

In SSPM measurements, a –5 V bias was applied betweenthe source and drain electrodes that each spanned 7 mm fromthe edges of the scan window. Since gold is an excellent con-

Adv. Funct. Mater. 2006, 16, 2409–2414 © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.afm-journal.de 2411

0 40 80 120 160

0 40 80 120 160

-8

-6

-4

-2

0

0

100

200

300

400

500

Distance [µm]

Su

rfa

ce P

ote

nti

al [V

]H

eig

ht

[nm

] a b

∆Vd ∆Vs ∆Vd ∆Vs Vg=-50V

Vg=-10V

0 40 80 120 160

0 40 80 120 160

-8

-6

-4

-2

0

0

100

200

300

400

500

Distance [

Su

rfa

ce P

ote

nti

al [V

]H

eig

ht

[nm

] a

c d∆Vd ∆Vs ∆Vd ∆Vs Vg=-50V

Vg=-10V

Vg=-50V

Vg=-10V

Figure 2. Surface topography (top) and surface potential (bottom) profilesof the two TFTs whose I–V characteristics are shown in Figure 1. The pro-files on the left are acquired on the TFT with gold electrodes whereas theprofiles on the right are acquired during operation of the TFT with PANI–PAAMPSA electrodes. The surface potential profiles have been offset alongthe y-axis for clarity. The potential drops at the pentacene/gold-source andpentacene/gold-drain interfaces are labeled DVs and DVd, respectively.

-10 -20 -30 -40 -5010

1

102

103

104

R2+R

4

R3

R1+R

5

-10 -20 -30 -40 -5010

2

103

104

105

R3

R2+R

4

R4

sourcedrain

R1 R2R5

drain

-pentacene

contact

source

-pentacene

contact

R3

pentacene

RW

[k

Ω-c

m]

Gate Voltage [V]

101

102

103

104

2010

2

103

104

105

R4

sourcedrain

R1 R2R5

drain

-pentacene

contact

source

-pentacene

contact

R3

pentacene

R4

sourcedrain

R1 R2R5

drain

-pentacene

contact

source

-pentacene

contact

R3

pentacene

R4

sourcedrain

R1 R2R5

drain

-pentacene

contact

source

-pentacene

contact

R3

pentacene

a

b c

Figure 3. The organic TFTs are modeled as five resistors wired in series, asshown in (a). A dashed line defines the scan window. The contact resis-tance in the gold device (W 1000 lm, L= 120 lm) becomes comparablewith the channel resistance as shown in (b). Although the contact resis-tance in the TFT with PANI–PAAMPSA electrodes is negligible, the resis-tance of PANI–PAAMPSA electrodes is significant, as shown in (c).

FULL

PAPER

K. S. Lee et al./ Pentacene/Polyaniline Interfaces in Thin-Film Transistors

ductor, the resistances associated with the source and drainelectrodes outside the scan window, R1 and R5, are negligibleduring the operation of the TFT. The –5 V bias is thereforedropped entirely across the scan window. On the contrary,we see in Figure 2d that the potential drop across the scan win-dow is less than the –5 V bias applied between the PANI–PAAMPSA source and drain electrodes. This suggests thatsome of the applied bias is dropped outside the scan window(along the PANI–PAAMPSA electrodes). We attribute thepotential drop outside the scan window to the resistance alongthe PANI–PAAMPSA source and drain electrodes. This resis-tance, R1 + R5, can be calculated by normalizing the potentialdrop outside the scan window by the current output measuredduring the SSPM experiments. For completeness, we have alsoplotted this quantity in Figure 3c. This resistance is largelygate-bias independent (≈ 180 kXcm) as it originates from theinherent resistivity of PANI–PAAMPSA. While the TFT withPANI–PAAMPSA electrodes is not contact limited, its perfor-mance appears to be limited by the inherent conductivity ofthe polymer conductor. Efforts to improve the bulk conductiv-ity of PANI–PAAMPSA—both through materials design andmanipulation of its structure—are currently underway.[35]

Considering that the work functions of the two materials aresimilar (∼ 5 eV),[36,37] the large difference in the measured contactresistance between the pentacene TFTs with gold and PANI–PAAMPSA electrodes is surprising. This difference in contact re-sistance suggests that charge transfer at the pentacene/gold andthe pentacene/PANI–PAAMPSA interfaces are dramatically dif-ferent. To definitively understand the origin of the reduced con-tact resistance in TFTs with PANI–PAAMPSA electrodes, elec-tronic characterization of the interfaces in question must becarried out. In the meantime, we can draw parallels between ourmacroscopic electrical studies with the microscopic electronicstudies carried out by Koch and co-workers.[21,22] In examiningthe interfaces of pentacene/PEDOT–PSS—another pentacene/conductive-polymer interface—and pentacene/gold, Koch founda much smaller interfacial dipole barrier at the pentacene/PE-DOT–PSS interface (D = 0.1 eV) compared to that at the penta-cene/gold interface (D = 1.05 eV) despite the known similarity inwork functions of PEDOT–PSS and gold.[21,22] Given the similari-ties between the two systems, it is not unreasonable to surmisethat the interfacial dipole barrier is significantly smaller at thepentacene/PANI–PAAMPSA interface compared to the penta-cene/gold interface. We are currently carrying out UV photo-emission spectroscopy (UPS) experiments to quantify the interfa-cial dipole barrier in our system.

From a structural characterization standpoint, even less isknown about these interfaces. A recent study by Koch et. al.[38]

suggests that the electronic details of the organic/metal interfacecan depend on the subtle differences of the confirmation andstructure of the adsorbed molecules. To examine the morphologi-cal and structural details at the interfaces in question, we carriedout scanning electron microscopy (SEM) as well as near-edgeX-ray absorption fine structure spectroscopy (NEXAFS) experi-ments to complement our SSPM measurements. Figure 4a and breveals SEM images of thermally evaporated pentacene at thechannel/gold interface and the channel/PANI–PAAMPSA inter-

face, respectively. In the channel regions of both devices, penta-cene is directly deposited on hexamethyldisilazane (HMDS)-treated silicon dioxide. The surface morphology of pentacene inthese channels is therefore very similar. Specifically, we observe0.5–1 lm grains with discrete terraces that suggest a layer-by-layer growth of pentacene.[39] The surface morphology of penta-cene on gold, however, is quite different (Fig. 4a). On gold, pen-tacene appears to form small, round grains, on the order of sever-al hundred nanometers in diameter.[40,41] The pentacene grainsappear to be slightly larger on PANI–PAAMPSA (Fig. 4b), andpentacene appears to conform to the topography of the polymerconductor. More importantly, pentacene growth across the chan-nel/electrode interface is distinctly different in the two TFTs.Specifically, we observe distinctly disconnected grains across thechannel/gold interface. This discontinuity necessarily inducesstructural disorder, which has been speculated to be responsiblefor the large contact resistance reported in bottom-contact organ-ic TFTs with gold electrodes.[40,41] Such discontinuity within thepentacene thin film, however, is not observed at the channel/PANI–PAAMPSA interface where individual grains appear totraverse the interface.

Interestingly, the ensemble-averaged molecular orientation ofpentacene is also significantly different when pentacene is depos-ited on PANI–PAAMPSA, as opposed to on gold. To elucidatethe ensemble-averaged orientation of pentacene, we conductedNEXAFS experiments[42] at the carbon edge on pentacenedeposited on different substrates. Specifically, we examined thinfilms of thermally evaporated pentacene on a HMDS-treatedsilicon dioxide surface (analogous to the channel region), ongold, and on PANI–PAAMPSA. Figure 5a shows the pre- andpost-edge-normalized NEXAFS spectra of pentacene depositedon HMDS-treated silicon dioxide at different angles of incidence.We observe four dominant peaks: the two sharp peaks at 284 and285 eV are associated with carbon–carbon 1s → p* transitions ofthe fused rings within pentacene, while the other two broader

2412 www.afm-journal.de © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2006, 16, 2409–2414

Figure 4. SEM images of a) the pentacene/gold interface, and b) the pen-tacene/PANI–PAAMPSA interface.

FULL

PAPER

K. S. Lee et al./ Pentacene/Polyaniline Interfaces in Thin-Film Transistors

peaks at 294 and 301 eV are associated with carbon–carbon1s → r* transitions of pentacene.[43,44] As the incident angle is in-creased from 20° (glancing), to 50° and 75° (nearly perpendicularto the substrate), the p* intensities of the fused rings increase;this is accompanied by a decrease in the r* intensities. Figure 5dquantifies the integrated p* intensities as a function of the inci-dent angles from this particular specimen. The positive slope inFigure 5d indicates that the rings of pentacene are preferentiallyoriented upright with respect to the silicon substrate.[44] This pref-erential orientation is desirable, particularly in the channel re-gion, as it promotes p–p overlap, which can enhance chargetransport across the channel.[44] In contrast, when pentacene isdeposited on gold, the molecular orientation is dramatically dif-ferent. This is first seen in the pre- and post-edge-normalizedNEXAFS data in Figure 5b where the p* intensities decrease,and the r* intensities increase with increasing incident angles.The integrated p* intensities are quantified in Figure 5e; contraryto pentacene on HMDS-treated silicon dioxide, the negativeslope indicates that the rings of pentacene are preferentially ori-ented parallel to the substrate.[45] When pentacene is depositedon PANI–PAAMPSA, however, the integrated p* intensities in-creases with increasing incident angles (Fig. 5c and f) so penta-cene appears to adopt the same preferential orientation when de-posited on PANI–PAAMPSA as when it is deposited on HMDS-treated silicon dioxide. Given our NEXAFS results, the molecu-lar orientation of pentacene must change dramatically—fromrings perpendicular to the substrate to rings parallel—across thechannel/electrode interface in devices with gold electrodes. Incontrast, pentacene is organized in the same preferential orienta-tion across the channel/electrode interface in TFTs with PANI–PAAMPSA electrodes. It has been speculated that the details ofmolecular orientation of the organic semiconductor can drasti-cally affect charge injection and lateral charge transport withinthe thin film.[44,46,47] That the rings are organized in the samegeneral orientation serves to promote p–p overlap across the

pentacene/PANI–PAAMPSA interface,thereby facilitating charge transfer. At thepentacene/gold interface, however, theabrupt change in ring orientation is likelyto hamper charge injection into, andcharge extraction out of, pentacene inthese devices.

3. Conclusion

In summary, our surface potential mea-surements reveal large potential drops—which translate to large contact resis-tance—in TFTs with gold electrodes. Theuse of PANI–PAAMPSA, a conductivepolymer, to replace gold as source anddrain electrodes for organic TFTs effec-tively reduces the contact resistance at thepentacene/electrode interface. The differ-ence in the contact resistance can be di-rectly correlated with drastic structural

and morphological differences across the channel/electrode in-terface of these two types of devices.

4. Experimental

Device Fabrication and Characterization: To define PANI–PAAMPSAfeatures on a highly doped silicon substrate with 3000 Å silicon dioxide,we first created hydrophilic and hydrophobic patterns on the substrate.Specifically, we started by irradiating the substrate with UV/ozone for20 min. A thin layer of HMDS was then spin-coated on the substrate.This was followed by the deposition of a thin layer of photoresist (AZ5214-E photoresist, Clariant Corporation). To define hydrophilic and hy-drophobic patterns, selective regions of HMDS were removed by photoli-thography. We found that the developer (AZ 400 K developer, ClariantCorporation) not only strips off the exposed AZ photoresist, it also read-ily strips off the underlying HMDS. The exposed regions thus became hy-drophilic after developing. To remove the photoresist in the unexposedregions, we rinsed the substrate with acetone. Since acetone does not re-move adsorbed HMDS, these regions remained hydrophobic. FollowingHMDS pattern definition on the substrate, a 5 wt % PANI–PAAMPSAaqueous solution was then spin-coated on the treated substrates. SincePANI–PAAMPSA selectively adsorbs in the hydrophilic regions [25],conductive PANI–PAAMPSA features in the predefined geometry ofsource and drain electrodes were created immediately after spin-coating.Gold source and drain electrodes were fabricated separately by photoli-thography and lift-off using the same photomask. To complete bottom-contact pentacene TFTs, 60–90 nm of pentacene was thermally evaporat-ed through a shadow mask onto the channel regions (0.15–0.3 nm s–1) ofboth batches of electrodes during the same evaporation run. The sub-strate temperature was held at 60 °C during pentacene evaporation. Tomeasure the device characteristics of these TFTs, we performed I–V mea-surements using a probe station equipped with an Agilent 4156C Preci-sion Semiconductor Parameter Analyzer. We used a Veeco InstrumentsBioscope atomic force microscope interfaced with a Nanoscope IV con-troller and operated in SSPM mode to directly measure the local surfacepotentials of pentacene TFTs in operation. All measurements were con-ducted with phosphorous-doped (n-type, doping level 1017 cm–3) Si can-tilevers (MikroMasch, ultrasharp tips, length 125 lm, tip radius < 35 nm,resonance frequency ca. 325 kHz, force constant ca. 40 N m–1). Electricalcontact to the tip was made by applying conductive silver paint (Ted Pel-la, No. 16034) to a fresh scratch in the oxide layer [48]. The gate bias was

Adv. Funct. Mater. 2006, 16, 2409–2414 © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.afm-journal.de 2413

0.0 0.2 0.4 0.6 0.8 1.0

1.0

2.0

3.0

4.0

0.0 0.2 0.4 0.6 0.8 1.0

1.4

1.6

1.8

2.0

2.2

0.0 0.2 0.4 0.6 0.8 1.0

1.0

2.0

3.0

4.0

270 285 300 315 3300.0

1.0

2.0

3.0

4.0

270 285 300 315 3300.0

1.0

2.0

3.0

270 285 300 315 3300.0

1.0

2.0

3.0

4.0

20º

50º

75º

a b c

d e f

sin2θ

Inte

gra

ted

*in

ten

sit

ies

[a.u

.]

Inte

nsit

y [

a.u

.]

Photon Energy [eV]

π*

*

0.0 0.2 0.4 0.6 0.8 1.0

1.0

2.0

3.0

4.0

0.0 0.2 0.4 0.6 0.8 1.0

1.4

1.6

1.8

2.0

2.2

0.0 0.2 0.4 0.6 0.8 1.0

1.0

2.0

3.0

4.0

270 285 300 315 3300.0

1.0

2.0

3.0

4.0

270 285 300 315 3300.0

1.0

2.0

3.0

270 285 300 315 3300.0

1.0

2.0

3.0

4.0

a

f

sin2

Inte

gra

ted

π*in

ten

sit

ies

[a.u

.]

Inte

nsit

y [

a.u

.]

Photon Energy [eV]

*

*

*

σ*

Figure 5. Pre- and post-edge normalized NEXAFS spectra of pentacene thin films on a) a HMDS-treated silicon dioxide–silicon substrate, b) gold, and c) PANI–PAAMPSA. The respective inte-grated p* intensities at 284 eV are plotted against the incident angles in (d–f).

FULL

PAPER

K. S. Lee et al./ Pentacene/Polyaniline Interfaces in Thin-Film Transistors

only applied to the TFT during surface potential scans, and not during to-pography scans, to minimize any electrostatic effects on the scan resolu-tion when obtaining surface topography [49,50]. Since the scanner usedfor the SSPM measurements had a range of 100 lm, each contact regionwas scanned sequentially to span the entire channel region. We have sinceperformed SSPM measurements on devices with smaller channel lengths( 30 lm); the smaller-channel devices exhibit qualitatively similar sur-face potential profiles.

Structural Characterization: SEM images were obtained at a lowoperating voltage (1–2 kV) to reduce damage to the conducting poly-mers and organic semiconductor thin films. A LEO 1530 scanning elec-tron microscope was used in our study. We performed NEXAFSexperiments at the NIST/Dow soft X-ray materials characterizationfacility located at beam line U7A at the National Synchrotron LightSource at Brookhaven National Laboratories (Upton, NY) [51]. Partialelectron yield was obtained at the carbon K-edge. Scan steps of 0.1 and0.2 eV were used to scan the regions of interest (280–310 eV) and thepre- and post-edge regions, respectively [43]. The data were collectedin a vacuum ( 1 × 10–8 Torr (1 Torr = 133.322 Pa)) at room tempera-ture. The obtained spectra were pre- and post-edge-normalized but notbackground-corrected. By examining the spectral changes as a functionof X-ray incident angles, we were able to qualitatively determine themolecular orientation of pentacene within the evaporate films [43].

Received: February 21, 2006Revised: April 24, 2006

Published online: November 3, 2006

–[1] J. A. Rogers, Z. Bao, K. Baldwin, A. Dodabalapur, B. Crone, V. R.

Raju, V. Kuck, H. Katz, K. Amundson, J. Ewing, P. Drzaic, Proc. Natl.Acad. Sci. USA 2001, 98, 4835.

[2] C. D. Sheraw, L. Zhou, J. R. Huang, D. J. Gundlach, T. N. Jackson,M. G. Kane, I. G. Hill, M. S. Hammond, J. Campi, B. K. Greening,J. Francl, J. West, Appl. Phys. Lett. 2002, 80, 1088.

[3] G. H. Gelinck, H. E. A. Huitema, E. van Veenendaal, E. Cantatore,L. Schrijnemakers, J. B. P. H. van der Putten, T. C. T. Geuns,M. Beenhakkers, J. B. Giesbers, B.-H. Huisman, E. J. Meijer, E. M.Benito, F. J. Touwslager, A. W. Marsman, B. J. E. van Rens, D. M.de Leeuw, Nat. Mater. 2004, 3, 106.

[4] P. F. Baude, D. A. Ender, M. A. Haase, T. W. Kelley, D. V. Muyres,S. D. Theiss, Appl. Phys. Lett. 2003, 82, 3964.

[5] B. Crone, A. Dodabalapur, A. Gelperin, L. Torsi, H. E. Katz, A. J.Lovinger, Z. Bao, Appl. Phys. Lett. 2001, 78, 2229.

[6] B. K. Crone, A. Dodabalapur, R. Sarpeshkar, A. Gelperin, H. E.Katz, Z. Bao, J. Appl. Phys. 2002, 91, 10 140.

[7] K. C. Dickey, J. E. Anthony, Y.-L. Loo, Adv. Mater. 2006, 18, 1721.[8] J. E. Anthony, J. S. Brooks, D. L. Eaton, S. R. Parkin, J. Am. Chem.

Soc. 2001, 123, 9482.[9] M. M. Payne, S. A. Odom, S. R. Parkin, J. E. Anthony, Org. Lett.

2004, 6, 3325.[10] M. M. Payne, S. R. Parkin, J. E. Anthony, C.-C. Kuo, T. N. Jackson, J.

Am. Chem. Soc. 2005, 127, 4986.[11] S. Aramaki, Y. Sakai, N. Ono, Appl. Phys. Lett. 2004, 84, 2085.[12] C. R. Kagan, P. Andry, Thin-Film Transistors, Marcel Dekker, New

York 2003.[13] S. M. Sze, Physics of Semiconductor Devices, Wiley, New York 1981.[14] P. V. Necliudov, M. S. Shur, D. J. Gundlach, T. N. Jackson, Solid-State

Electron. 2002, 47, 259.[15] H. Klauk, G. Schmid, W. Radlik, W. Weber, L. Zhou, C. D. Sheraw,

J. A. Nichols, T. N. Jackson, Solid-State Electron. 2002, 47, 297.[16] R. A. Street, A. Salleo, Appl. Phys. Lett. 2002, 81, 2887.[17] J. Zaumseil, K. W. Baldwin, J. A. Rogers, J. Appl. Phys. 2003, 93, 6117.[18] S. Luan, G. W. Neudeck, J. Appl. Phys. 1992, 72, 766.[19] M. Lefenfeld, G. Blanchet, J. A. Rogers, Adv. Mater. 2003, 15, 1188.

[20] G. B. Blanchet, C. R. Fincher, M. Lefenfeld, J. A. Rogers, Appl. Phys.Lett. 2004, 84, 296.

[21] N. Koch, A. Kahn, J. Ghijsen, J. J. Pireaux, J. Schwartz, R. L. Johnson,A. Elschner, Appl. Phys. Lett. 2003, 82, 70.

[22] A. Kahn, N. Koch, W. Gao, J. Polym. Sci. Part B 2003, 41, 2529.[23] H. Sirringhaus, T. Kawase, R. H. Friend, T. Shimoda, M. Inbasekaran,

W. Wu, E. P. Woo, Science 2000, 290, 2123.[24] M. Nonnenmacher, M. P. O’Boyle, H. K. Wickramasinghe, Appl.

Phys. Lett. 1991, 58, 2921.[25] K. S. Lee, G. B. Blanchet, F. Gao, Y.-L. Loo, Appl. Phys. Lett. 2005,

86, 074 102.[26] M. Angelopoulos, J. D. Gelorme, T. H. Newman, N. M. Patel, D. E.

Seeger, US Patent 53 070 825, 1994.[27] L. Sun, H. Liu, R. Clark, S. C. Yang, Synth. Met. 1997, 84, 67.[28] A. J. Heeger, Synth. Met. 2001, 125, 23.[29] L. Burgi, T. J. Richards, R. H. Friend, H. Sirringhaus, J. Appl. Phys.

2003, 94, 6129.[30] L. Burgi, H. Sirringhaus, R. H. Friend, Appl. Phys. Lett. 2002, 80, 2913.[31] T. Hassenkam, D. R. Greve, T. Bjornholm, Adv. Mater. 2001, 13, 631.[32] J. A. Nichols, D. J. Gundlach, T. N. Jackson, Appl. Phys. Lett. 2003,

83, 2366.[33] K. P. Puntambekar, P. V. Pesavento, C. D. Frisbie, Appl. Phys. Lett.

2003, 83, 5539.[34] While the linear mobility of the two devices at VDS = –5 V extracted

from the I–V characteristics are comparable, the channel resistance inthe device with gold electrodes appears significantly higher than that inthe device with PANI–PAAMPSA electrodes. We believe this is becausethe SSPM measurements were carried out approximately one weekafter the TFTs were made and tested electrically. The SSPM outputcharacteristics reveal a 58 % reduction in linear mobility for the TFTwith gold electrodes and insignificant reduction in the device withPANI–PAAMPSA electrodes. The drastic difference in electrical char-acteristics of the two devices on storage is currently under investigation.

[35] J. E. Yoo, J. L. Cross, K. S. Lee, T. L. Bucholz, M. P. Espe, Y.-L. Loo,unpublished.

[36] A. J. Heeger, I. D. Parker, Y. Yang, Synth. Met. 1994, 67, 23.[37] J. R. Posdorfer, B. Werner, B. Wessling, S. Heun, H. Becker, Proc.

SPIE-Int. Soc. Opt. Eng. 2004, 5214, 188.[38] N. Koch, G. Heimel, J. Wu, E. Zojer, R. L. Johnson, J.-L. Brédas,

K. Muellen, J. P. Rabe, Chem. Phys. Lett. 2005, 413, 390.[39] F. J. Meyer zu Heringdorf, M. C. Reuter, R. M. Tromp, Appl. Phys. A

2004, 78, 787.[40] I. Kymissis, C. D. Dimitrakopoulos, S. Purushothaman, IEEE Trans.

Electron Devices 2001, 48, 1060.[41] C. D. Dimitrakopoulos, P. R. L. Malenfant, Adv. Mater. 2002, 14, 99.[42] J. Stohr, NEXAFS Spectroscopy, Springer, Berlin 1992.[43] D. A. Krapchetov, H. Ma, A. K. Y. Jen, D. A. Fischer, Y.-L. Loo,

Langmuir 2005, 21, 5887.[44] D. M. DeLongchamp, E. K. Lin, D. A. Fischer, Proc. SPIE-Int. Soc.

Opt. Eng. 2005, 5940, 54.[45] W. S. Hu, Y. T. Tao, Y. J. Hsu, D. H. Wei, Y. S. Wu, Langmuir 2005,

21, 2260.[46] H. Sirringhaus, P. J. Brown, R. H. Friend, M. M. Nielsen, K. Bech-

gaard, B. M. W. Langeveld-Voss, A. J. H. Spiering, R. A. J. Janssen,E. W. Meijer, P. Herwig, D. M. De Leeuw, Nature 1999, 401, 685.

[47] D. M. DeLongchamp, S. Sambasivan, D. A. Fischer, E. K. Lin,P. Chang, A. R. Murphy, J. M. J. Frechet, V. Subramanian, Adv.Mater. 2005, 17, 2340.

[48] H. O. Jacobs, H. F. Knapp, A. Stemmer, Rev. Sci. Instrum. 1999, 70,1756.

[49] S. Sadewasser, M. C. Lux-Steiner, Phys. Rev. Lett. 2003, 91, 266 101.[50] T. J. Smith, K. J. Stevenson, unpublished.[51] D. A. Fischer, B. M. DeKoven, NSLS Newsletter 1996.

______________________

2414 www.afm-journal.de © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2006, 16, 2409–2414

FULL

PAPER

K. S. Lee et al./ Pentacene/Polyaniline Interfaces in Thin-Film Transistors