High-Resolution Ink-jet Printing of All Polymer Transistor Circuits

IntroductionImpressive advances in vapor-phase

deposition and photolithographic pattern-ing techniques have been fueling the sili-con microelectronics revolution over thelast 40 years. However, for many interest-ing classes of materials, including bio-logical materials or functional syntheticpolymers, vacuum deposition and photo-lithography are not the techniques ofchoice for producing ordered structuresand devices. Many of these materials self-assemble into well-ordered microstructureswhen deposited from solution,1 and pat-terning may be more readily achieved bysolution-based selective deposition anddirect-printing techniques. It is appealingto consider novel ways of manufacturingfunctional circuits and devices based ontechniques that are similar to printingvisual information onto paper.

Microfabrication by solution self-assemblyand direct printing is particularly attractivefor thin-film transistor (TFT) circuits basedentirely on organic polymers, in whichsemiconducting conjugated polymers areused for the active layers, conductingpolymers are used for electrodes and inter-connects, and conventional insulating poly-mers are used for the dielectric and isolationlayers. These are of interest for applicationssuch as active-matrix-display addressing2

or logic circuits in active identificationtags and labels.3 Over the last 3–4 years,rapid advances in performance have beenreported on the materials front since semi-conducting conjugated polymers with im-proved structural regularity and chemicalpurity have become available. By makinguse of supramolecular self-organizationmechanisms, it is possible to deposit or-dered thin polymer films from solution

in which charge-carrier mobilities of10�2–10�1 cm2/V s can be achieved.4–6

These values are approaching those ofthin-film amorphous silicon (0.1–1 cm2/V s)and small-molecule organic field-effecttransistors7–9 (0.1–5 cm2/V s).

All-polymer TFT circuits consisting of afew hundred transistors were first fabri-cated by a group at Philips Research Lab-oratories, The Netherlands, who used amore conventional photolithographic ap-proach for patterning with 1-�m resolu-tion.10 Circuits of similar complexity andimpressive performance have also beendemonstrated using small organic mole-cules deposited at the last stage of anotherwise conventional silicon process.11,12

Several nonconventional, direct-printingapproaches have been proposed. Screenprinting has been used to pattern source/drain and gate electrodes of conductinginks with channel lengths of 100 �m.13

Soft lithographic techniques based onpoly(dimethylsiloxane) (PDMS) stampshave been used to selectively deposit self-assembled monolayers onto thin films ofgold that can be used as etch masks for theetching of gold source/drain electrodes.14

Using PDMS stamps as phase-shift masks15

in a photolithographic process, gold source/drain electrodes have been defined withchannel lengths of only 0.1 �m. Selectivesolution-deposition of polymer electrodeshas been achieved by micromolding incapillaries (MIMIC).16,17

In this article, we discuss novel pattern-ing approaches that are based on ink-jetprinting.18 Ink-jet printing has emergedas an attractive patterning technique forconjugated polymers in light-emittingdiodes (LEDs),19,20 and it appears that it

will become the technique that will enablefull-color, high-resolution, polymer LEDdisplays.21 It has not been applied to or-ganic transistors yet, presumably becauseits resolution capability is considered to beinsufficient for the definition of practicalchannel lengths. The resolution of ink-jetprinting is limited to 20–50 �m by statisticalvariations of the flight direction of dropletsand their spreading on the substrate. Weshow that ink-jet printing allows the fabri-cation of complete TFT circuits, includingvia-hole interconnects, by successivesolution-deposition and printing steps.

Ink-Jet Printing Combined withSurface Free-Energy Patterning

Our approach to overcoming the resolu-tion limitations of ink-jet printing is toconfine spreading of water-based,conducting-polymer ink droplets on ahydrophilic substrate with a pattern of nar-row hydrophobic surface regions that definethe critical device dimension (Figure 1a).When water-based ink-jet droplets of theconducting polymer poly(3,4-ethylenedio-xythiophene) (PEDOT) doped with poly-styrene sulfonic acid (PEDOT/PSS) are deposited into the hydrophilic sur-face regions at a distance Rs from the hy-drophobic barrier, the droplets spread untilthey hit the repelling barrier. Using a piezo-electric ink-jet head, it is possible in this wayto print two parallel source/drain electrodelines, with a separation of only 5 �m, that are accurately aligned with the hydrophobic barrier.18 No PEDOT depo-sition occurs on top of the barrier regions, even if the width of the barrier ismuch smaller than the droplet diameter. Thesecond unconfined boundary of the printedlines is much more irregular, with a typicalstatistical roughness of the order of 10 �m (Fig-ure 1b). It is apparent that this would have ledto electrical shorts between the source anddrain electrodes had we attempted to definesuch a short channel without a confinementstructure. At present, the resolution of the inkconfinement process is limited only by ourphotolithography setup. Atomic forcemicroscopy (AFM) images such as Figure 1csuggest that even higher resolution may bepossible. Note that the boundaries ofprinted PEDOT lines are accurately pinnedat the edges of the hydrophobic barrier.

The high-resolution, surface free-energypattern is prefabricated on the substrate priorto any of the TFT deposition and printingsteps. A broad range of techniques may read-ily be used, since at this stage the substratedoes not yet contain any of the radiation-sensitive or chemically sensitive active lay-ers of the devices. In our firstdemonstration, the barrier was fabri-cated from a thin layer of hydrophobic

MRS BULLETIN/JULY 2001 539

High-ResolutionInk-Jet Printingof All-PolymerTransistor Circuits

H. Sirringhaus, T. Kawase, and R.H. Friend

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polyimide (PI) polymer patterned byphotolithography and etched in an O2plasma to expose regions of the bare, hy-drophilic glass substrate. The PI film isonly of the order of 500 Å thick. Since thespreading droplets in the liquid state havea much larger thickness of a few micro-

meters, the ink confinement must becaused mainly by the surface free-energycontrast and does not require a topo-graphic profile of the hydrophobic barrier.Indeed, we have recently achieved similarink confinement by soft lithography usingself-assembled monolayers of hydropho-bic alkyltrichlorosilanes selectively de-posited onto a hydrophilic substrate by aPDMS stamp.

After ink-jet printing of source/drainelectrodes, TFT devices are fabricated in atop-gate configuration by spin-coating acontinuous film of the active semiconduct-ing polymer (Figure 2a). We have fabricateddevices with a range of self-organizingconjugated polymers, including regio-regular poly(3-hexylthiophene) (P3HT)5

and liquid-crystalline polyfluorene-based

block copolymers, such as poly(9,9-dioctylfluorene-co-bithiophene) (F8T2).6Next, a dielectric polymer, such aspoly(vinylphenol), is spin-coated on topfrom a solvent that does not dissolve orswell the underlying semiconductingpolymer. Finally, a PEDOT/PSS gate elec-trode line overlapping the channel is ink-jet printed (Figure 2b). All ink-jet printingsteps are performed under atmosphericconditions.

In this device configuration, the ink con-finement barrier can be used to achieve adual result. It not only provides the high-resolution definition of the channel, butit also acts as an aligning template forthe deposition of the self-organizing semi-conducting layer, which is crucial forachieving high field-effect mobilities. Rigid-rod polymers such as F8T2 exhibit nematicliquid-crystalline phases at elevated tem-perature (TLC � 265�C for F8T2). They canbe aligned into monodomains on top ofa suitable alignment layer, such as analigned layer of PI, by annealing into theliquid-crystalline phase and subsequentquenching.6 By using mechanically rubbedPI ink confinement barriers, we have fab-ricated devices with uniaxial alignment ofpolymer chains in the channel of the TFT.The alignment is evident in optical mi-croscopy images observed under crossedpolarizers (inset in Figure 2b). The TFTchannel region appears bright, reflectingthe monodomain alignment of F8T2,which results in a change of polarizationof the incident light, enabling a fraction ofthe light to pass through the second, crossedpolarizer. The source/drain electrode re-gions where the F8T2 is in a multidomainconfiguration on top of PEDOT/PSS ap-pear dark. The degree of polymer alignmentcan be quantified by linearly polarizedoptical-absorption spectroscopy (Figure 3a).The dichroic ratio between light polarizedparallel and perpendicular to the rubbingdirection is 6–8, from which a lower-limitestimate for the structural order parame-ter P2 � 0.7 can be obtained.6 Printed TFTshave been fabricated with the direction ofcurrent transport parallel and perpendicu-lar to the alignment direction of polymerchains. Due to fast intrachain transportalong the polymer chain, enhanced field-effect mobility of 1 � 10�2 cm2/V s to2 � 10�2 cm2 V s are achieved for currentflow parallel to the polymer chains,whereas mobilities in the perpendiculardirection are lower by a factor of 5–7 (Figure 3b).

Figure 4 shows the output and transfercharacteristics of an ink-jet printed, all-polymer F8T2 TFT. The device exhibits ahigh on–off current ratio of the order of105, as measured between 0 V and �20 V,

540 MRS BULLETIN/JULY 2001

High-Resolution Ink-Jet Printing of All-Polymer Transistor Circuits

Figure 1. (a) Schematic diagram ofhigh-resolution ink-jet printing onto aprepatterned substrate. (b) Opticalmicroscopy image of ink-jet printedpoly(3,4-ethylenedioxythiophene)(PEDOT) source/drain electrodesconfined by a hydrophobic barrier ofpolyimide (PI) (L � 5 �m). (c) Tapping-mode atomic force micrograph showingaccurate pinning of ink-jet printedPEDOT/PSS (PSS is polystyrenesulfonic acid) source and drainelectrodes at the boundary of ahydrophobic PI barrier with L � 5 �m.

Figure 2. (a) Schematic diagramof top-gate ink-jet printed thin-filmtransistor (TFT) with a semiconductinglayer of poly(9,9-dioctylfluorene-co-bithiophene) (F8T2) (S � source,D � drain, G � gate). (b) Opticalmicrograph of an ink-jet printed TFT.The inset shows an enlargement of thechannel region seen under crossedpolarizers such that the TFT channel(L � 5 �m) appears bright due to theuniaxial, monodomain alignment of theF8T2 polymer on top of rubbed PI.

and good threshold voltage stability. Thehysteresis between characteristics measuredwith increasing and decreasing source/drain and gate voltage, respectively, isnegligible. F8T2 has good stability againstchemical doping by environmental oxy-gen or residual impurities such as mobilesulfonic acid in the PEDOT/PSS ink. Ingeneral, the performance of ink-jet printed,all-polymer F8T2 TFTs is by no means in-ferior to that of control devices fabricatedin a conventional way with photolitho-graphically defined gold electrodes. Thisshows that through careful choice of thesequence of solvents/polymers to avoiddissolution and swelling of underlyinglayers, our printing process maintains thecritical integrity and sharpness required ofthe different polymer–polymer interfacesin a multilayer TFT device. Note that dueto the low bulk conductivity of F8T2, no

patterning of the semiconducting layeris required.

In principle, even higher field-effectmobilities of up to 0.1 cm2/V s can beachieved with P3HT, which forms a mi-crocrystalline lamellar structure with effi-cient interchain transport in the plane ofthe film.4,5 However, P3HT has low stabil-ity against doping under atmospheric con-ditions. Since the ink-jet printing steps areperformed in air, printed P3HT devicesshow high bulk conductivity and lowon–off current ratios. Note that a reductivede-doping step, which would be neces-sary to restore P3HT to a low-conductivitystate, is not possible in the all-polymerTFT configuration, since the de-dopingwould adversely affect the conductivity ofthe PEDOT/PSS electrodes.

In order to use ink-jet printed polymerTFTs in more complex integrated transis-

tor circuits, it is necessary to develop othercircuit components, which ideally shouldbe fabricated by ink-jet technology as well.Resistors can be printed by using differentcompositions of PEDOT/PSS ink becausethe resistance depends very sensitively onthe ratio of PEDOT to PSS. A particularlyimportant component for logic circuitsare via-hole interconnects, which provideelectrical connections between electrodesand/or interconnects in different layers.We have developed an efficient ink-jetprocess for via-hole fabrication. To fabri-cate a via hole through a dielectric layersuch as PVP, we ink-jet deposit a sequenceof droplets of a good solvent, which lo-cally dissolve the dielectric polymer. Re-markably, upon drying of the solvent, thepolymer redeposits on the side walls ofthe region defined by the printed droplet,but not in its center (Figure 5a). This isrepeated several times until the surfaceof the underlying layer is exposed, whichalso provides an automatic “etch stop.”Via holes are then filled with PEDOT/PSSduring the subsequent gate-electrodeprinting step. The mechanism for via-holeformation is believed to be similar to thatwhich results in the familiar “coffee-stain”effect, which occurs in drying dropletswhen the contact line is pinned. To com-pensate for an enhanced evaporation ratenear the contact line, a radial liquid flow isestablished that transports any dissolvedmaterial to the edges of the droplet. In ourvia-hole formation process, the dissolvedmaterial only redeposits at the side walls,such that the underlying layer is exposedin the center of the via hole. A more de-tailed description of this process is givenelsewhere.22

Combining the different components de-scribed, we have fabricated simple logictransistor circuits by ink-jet printing. In-verters are the basic building blocks ofa logic circuit. They can be implementedwith two p-type transistors in either anenhancement-load or a depletion-load con-figuration23 requiring via-hole interconnec-tions in both cases. The enhancement-loadconfiguration, in which the drain and gateof the load transistor are connected to-gether through a via hole, is appropriatefor normally off transistors such as ink-jetprinted F8T2 devices. Alternatively, in-verters with a printed resistor as the loadelement have been used. In both configu-rations, clean inverter action is observedfor switching of the output between �20 V(“high”) and �0 V (“low”) (Figure 5b).The hysteresis of the inverter characteris-tics is small, reflecting the stability of thetransistor threshold voltage. An importantrequirement for switching a large numberof subsequent stages in a more complex



Figure 3. (a) Optical-absorption spectra of a completed TFT device taken in the PEDOT-freesubstrate regions with incident light polarized linearly parallel (| |) and perpendicular (�) tothe direction of alignment of the F8T2 polymer. (b) Saturated transfer characteristics ofink-jet printed F8T2 TFTs fabricated on the same substrate on which the absorption spectrain (a) were taken.The channels were oriented such that current transport is parallel (| |) andperpendicular (�) to the chain alignment direction (L � 5 �m).

Figure 4. (a) Output and (b) transfer characteristics of an ink-jet printed, all-polymer F8T2TFT (L � 5 �m).The nonlinearities in the output characteristics at small source/drainvoltages are caused by source/drain contact resistance effects. Subsequent measurementsunder N2 atmosphere with increasing (�) and decreasing (�) source/drain and gatevoltage, respectively, demonstrate negligible hysterisis.

logic circuit is that the voltage gaindVout/dVin of the individual inverter stageis larger than 1. For the devices shown inFigure 5b, the voltage gain is �2. Highergains of 5–7 have been obtained in deviceswith larger load resistance at the expenseof a somewhat reduced switching speed.

The switching performance of the in-verter stages was tested by applying asquare wave input voltage and measuringthe output voltage with an active probewith a load capacitance of �20 pF. Theprobe capacitance is equivalent to a fan-outof 4–5, that is, the source/drain-to-gateoverlap capacitance of a discrete transis-tor, as used for the input transistor in theinverters, is 4–5 pF. The ink-jet printedinverters can be switched at frequenciesup to a few hundred hertz, as shown inFigure 5c for the resistor-load device ofFigure 5b. Further improvements are ex-pected from increasing polymer mobilityand PEDOT conductivity, and from reduc-ing channel length and overlap capacitance.

DiscussionRegarding the mechanism of surface

free-energy assisted, high-resolution ink-jet printing, it is interesting to considerwhich physical processes may be the reso-lution limiting factors. The spreading ofink-jet droplets on a partially wetting sub-strate is determined by the initial kineticenergy of the spherical droplets (radius R0)impinging on the substrate with velocityv0, and the free-energy lowering that theinitially spherical droplets can achieve bywetting the substrate and adopting aspherical cap shape with final radius Rg

and contact angle �g. The contact angles ofwater droplets on bare hydrophilic glassand hydrophobic PI are �g � 20–25� and�p � 70–80�, respectively. For typical ink-jet conditions used in our experiments, thefree-energy change is about a factor of 5larger than the initial kinetic energy, suchthat the viscous flow during spreading isgoverned by surface and interface tensionsrather than by the initial kinetic energy.24

Under these conditions, the spreadingdroplet may be assumed to have a spheri-cal cap shape with a base radius R continu-ously increasing toward Rg, and a dynamiccontact angle � decreasing toward �g. Ifthe fluid is incompressible, and evapora-tion can be neglected on the time scale ofspreading, there is a one-to-one relation-ship between � and R. In the later stagesof spreading, the velocity of the contactline v(R) can be shown to be related to �and R by25

, (1)v ��L

9ln�R

s ��1

��3 � �g3�

where �LV is the surface tension of the liq-uid, is the viscosity, and s is a character-istic length scale of molecular dimension(s R) in the vicinity of the contact line,over which slippage occurs at the bound-ary between the liquid droplet and thesolid substrate.26 When the droplets hit thebarrier with a dynamic contact angle of�(Rs) �p, they will at first spread a smalldistance into the hydrophobic region, butwill immediately be repelled by the un-compensated Young’s force per unit lengthof contact line

Fs � �SL � �LV cos �(Rs) � �SV

� �LV(cos �(Rs) � cos �p), (2)

where �SV and �SL are the interfacial ten-sions at the barrier–air and barrier–liquidinterfaces. The non-laminar fluid-dynamicsprocesses during repulsion of the dropletsare complex. However, for a qualitativediscussion, it is instructive to comparethe magnitude of the initial uncompen-sated Young’s force with the inertial mo-mentum of the moving contact line that isimpinging on the barrier per unit time,Fm � �hv2, where � is the density of theliquid, h � 4/3 R0

3/R2 is a characteristicthickness of the spreading droplet, andv is the contact line velocity. The Reynoldsnumber Re � �hv/ is estimated to beof the order of 50–100.

Figure 6 shows the calculated ratioFm/Fs as a function of the ratio Rs/Rg,where Rg is the maximum possible print-

542 MRS BULLETIN/JULY 2001


Figure 5. (a) Optical microscopyimage of two ink-jet printed via holesconnecting drain and gate electrodes ofthe load TFT in an enhancement-loadink-jet printed inverter. (b) Staticcharacteristics of an enhancement-loadinverter (L � 5 �m, width of the inputswitching transistor WI � 2150 �m, widthof the load transistor WL � 450 �m),and a resistance-load inverter(L � 5 �m, WI � 2150 �m,RL � 47 M ), measured withincreasing (solid line) and decreasing(dashed line) input voltage Vin

(VDD � 20 V). (c) Dynamic-switchingcharacteristics of the resistance-loadinverter in (b) (Vin � 250 Hz).Thespikes of the output voltage after abruptswitching of the input voltage areinduced by direct capacitive coupling onthe TFT substrate between input andoutput electrodes.The open circlesshow the square wave input signal, vin,while the filled circles show the invertedoutput signal.

Figure 6. Plot of the calculated ratioof the inertial force Fm of the movingcontact line impinging on thehydrophobic barrier and the repellinguncompensated Young force Fs (leftaxis) and of the contact line velocity v(right axis) as a function of the printingdistance Rs from the barrier (normalizedto the equilibrium droplet radius on thehydrophilic surface Rg � 47.5 �m).�LV � 70.3 mN/m, � 3.4 mPa s,s � 1 �m, and R0 � 19.5 �m.

ing distance from the hydrophobic barriercorresponding to the equilibrium final ra-dius of the spherical cap on the hydro-philic surface (Rg � 47.5 �m). The simplemodel suggests that for a broad range ofprinting distances Rs (Figure 1a), the re-pelling Young’s force is orders of magni-tude larger than the inertial forces. Thismay explain why we have observed theconfinement process to be robust with re-spect to variations of the droplet volumeor the distance Rs, which are caused by ex-ternal perturbations of the droplet flightdirection or changing ejection conditionsat the nozzle. No evidence for accidentalshorts between source and drain electrodeswas found. The simple model also sug-gests that it may be possible to achieve sig-nificantly higher resolution by this methodof ink confinement. Experiments to inves-tigate resolution limits are under way.

ConclusionsWe have shown that solution self-

assembly and direct ink-jet printing tech-niques allow the controlled fabrication ofhigh-mobility, short-channel (5-�m) poly-mer transistors and complete transistorcircuits, including via-hole interconnects.The device performance of printed poly-mer thin-film transistors with mobilitiesof up to 2 � 10�2 cm2/V s and on–offcurrent-switching ratios of 105 is believedto be adequate for practical applications inactive-matrix-display addressing or logiccircuits in identification tags consisting ofa few hundred transistors. Ink-jet printinghas several advantageous attributes, someof which are particularly relevant if oneenvisions continuous, reel-to-reel manu-facturing of cheap integrated circuits onflexible substrates:� Ink-jet printing is a noncontact printingtechnique that allows low levels of particledefects and printing without abrasion andis capable of high-throughput, particu-larly if a large number of nozzles are usedin parallel. It is environmentally friendly,as it uses only a minimum amount ofpolymer materials and solvents. It alsoallows simultaneous printing of differentmaterials, which can be delivered frommultiple nozzles.

� Ink-jet printing provides accurate regis-tration over large areas because the ink-jethead can be aligned locally with respectto a previously deposited pattern. Thislocal registration capability, which can beautomated, is particularly important forflexible substrates that inevitably distortbetween subsequent patterning steps.� Application-specific or even end-user-specific circuits can be defined by simpleink-jet printing of a network of intercon-nections and via holes on a prefabricatedarray of transistor gates.� Compared with other liquid-phase pat-terning techniques, such as simple dip-coating of a substrate containing a surfacefree-energy pattern, ink-jet printing allowsprecise local control of deposited dropletvolume and drying time to form patternswith arbitrary shapes and thicknesses. Indip-coating, the patterns are defined byequilibrium configurations of the liquidand suffer from problems such as bulgeformation, capillary breakup, or a sensi-tive dependence of film thickness on theshape and size of the pattern.27

AcknowledgmentsWe acknowledge Dr. T. Shimoda and

Mr. S. Nebashi of Seiko-Epson Corp. forsupport of the ink-jet printing project;Dr. M. Inbasekaran, Dr. W. Wu, Dr. E.P.Woo, and Dr. J. O’Brien of Dow ChemicalCorp. for supplying the F8T2 polymer;Bayer Chemical Corp. for supplying thePEDOT/PSS; and Dr. C. Newsome andM. Banach for valuable contributions. Thework was supported by the Epson Cam-bridge Laboratory and the Royal Society.

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High-Resolution Ink-jet Printing of All Polymer Transistor Circuits