Organic Electronics 10 (2009) 1102–1108

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Organic Electronics

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

High-resolution patterned nanoparticulate Ag electrodes towardall printed organic thin film transistors

Sung Kyu Park, Yong-Hoon Kim, Jeong-In Han *

Semiconductor and Information Display Research Division, Korea Electronics Technology Institutes, Gyunggi, Republic of Korea

a r t i c l e i n f o

Article history:Received 10 March 2009Received in revised form 17 May 2009Accepted 27 May 2009Available online 6 June 2009

PACS:73.40.�c73.61.Ph

Keywords:Photoresist-free processNon-relief-patterned surface patterningOrganic thin film transistor

1566-1199/$ - see front matter � 2009 Elsevier B.Vdoi:10.1016/j.orgel.2009.05.024

* Corresponding author.E-mail addresses: skpark@keti.re.kr (S.K. Park),

Han).

a b s t r a c t

High-resolution patterned nanoparticulate Ag electrode arrays and all printed organic thinfilm transistors (OTFTs) were demonstrated using a simple dip-casting and a photoresist-free, non-relief-pattern lithographic process. An octadecyltrichlorosilane self-assembledmonolayer was deposited to provide low surface energy and patterned by deep ultravioletlight, resulting in reproducible periodic arrays of patterned hydrophilic domains separatedfrom hydrophobic surroundings. Using a simple dip-casting with optimal withdrawalspeed, viscosity, and solvent polarity, dot size and electrode width of less than 1 lm and5 lm were obtained, respectively. All printed OTFTs were fabricated. Ink-jet printed6,13-bis(triisopropyl-silylethynyl) pentacene OTFTs including high-resolution patternednanoparticulate Ag source/drain electrodes (L < 10 lm) have shown similar performanceto the OTFTs with photolithographically patterned electrodes.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

Recently, flexible printing process has been attractivefor large area and low-cost electronics such as flexible dis-plays [1,2], radio frequency identification tags (RFIDs)[3,4], photovoltaic devices [5,6], etcetera. For the low-costmanufacturing of electronic devices, direct-printing tech-nologies for high-resolution electrodes and semiconduc-tors are one of challenging issues in roll-to-roll process[7–9]. To achieve the high-resolution and simple printingprocess, several kinds of technologies were demonstratedsuch as nano-scale imprinting [10], soft-lithography [11],self-formation [12], and barrier-like structure with photo-resist layer [7,13]. Recently, direct ink-jet printed Ag elec-trodes were widely researched for gate or data lines andsource/drain electrodes in flexible printed display or elec-tronics. To obtain fast integrated circuits and high resolu-

. All rights reserved.

hanji@keti.re.kr (J.-I.

tion display, generally high-resolution and fine-pitchelectrodes, and short channel devices are required.However, in direct-printing system such as ink-jet print-ing, due to the difficulty of controlling ink flow and spread,pitch and line width were typically limited around 50–100 lm. Therefore, patterning of substrate surface wassuggested for the control of ink flow and spread to obtainmorphological pathways of dewetting. Since Kagan et al.[14] have reported direct patterning technology usingdifferential surface energy on substrate, several groupshave researched on the direct patterning methods usingmolecular template barrier from stamping [15], e-beamlithography-patterned polymer barrier layer [7,13], oxy-gen-plasma treated differential surface state, etc. Althoughgood results have been demonstrated from the proposedtechniques, applying them to low-cost roll-to-roll processis still problematic due to process complexity and neces-sity of conventional relief-lithography related technolo-gies. Photoresist processing or relief-lithography relatedtechnologies may be a barrier for cheap mass-productiondue to the cost of resist, application, development, and

S.K. Park et al. / Organic Electronics 10 (2009) 1102–1108 1103

removal. Additionally, most of the recent researches typi-cally have used organic/polymer substance for the deposit-ing materials [16].

In this report, we demonstrate high-resolution pat-terned inorganic Ag nanoparticulate electrodes and organicsemiconductors using non-relief-lithography patteringtechnology for all printed organic thin film transistors(OTFTs). Using surface patterning of self-assembled mono-layer (SAM) by direct deep-ultraviolet (DUV) light expo-sure, photoresist processing can be avoided and barrier-like structure for defining electrodes were fabricated fromnon-relief-lithography process [8]. Additionally, for fastand cost effective processing, dip-casting process was car-ried out by controlling withdrawal speed of the surface-patterned substrates. In particular, our emphasis is onidentifying the physical conditions that ensure ideal pro-cesses for all printed OTFTs and electronics using the novelsurface patterning process and dip-casting.

2. Experimental

Fig. 1 shows the experimental procedure which wasused to obtain Ag nanoparticulate electrodes and allprinted TFTs. A hydrophobic octadecyltrichlorosilane(OTS) SAM was formed by immersing the substrate into a30 mM solution of OTS in hexane. Subsequently, ultrason-ication process was performed and another OTS formationprocess was carried out for building dense and stable SAM.Spatial patterning of OTS covered substrates was achievedusing DUV exposure through a quartz mask [17,18]. TheDUV radiation was obtained from a low pressure mercurylamp with primary output wavelengths of 254 nm (90%)and 185 nm (10%). The 185 nm exposure results in rapidremoval of the OTS SAM layer. Our preliminary measure-ment of contact angles from water droplet has shown100� and 20� before and after OTS removal, respectivelyon SiO2 surface. Deep ultraviolet light exposure was con-ducted by UV ozone cleaner (UV253H, Filgen, Japan).

Fig. 1. Process flow of non-relief-lithography process for pa

Ag nanoparticulate solution including an average nano-particle diameter of 10–20 nm was purchased from InkTec(www.inktec.com) Company. For controlling Ag ink viscos-ity, ethanol was added into the original solution and stirredat the temperature 60 �C for 2 h. The ink viscosity wasmeasured by SV-10 (Sine-Wave vibro viscometer, A&DCompany, Japan). Dip-casting process was performed tofabricate Ag electrodes using home-made dip-casting sys-tem with a stepping motor in air ambient at room temper-ature. Surface patterned substrates (patterned hydrophilicarea on SiO2/Si) were fully immersed into the Ag nanopar-ticulate solution and withdrawn with a controlled speed.After waiting for 30 min, the patterned Ag electrodes wereannealed with a temperature of 180 �C for 30 min on a hotplate in air ambient. The resistivity of Ag electrodes wereaveraged by measuring at least four point in a sample.

The bottom gate and bottom contact all printed OTFTswere fabricated on heavily doped p-type Si wafers with a200 nm thermally grown SiO2 layer. The Si wafer was usedas both substrate and common gate electrode, while theSiO2 layer acted as a gate dielectric layer. Using the non-re-lief patterned surface patterning, source/drain Ag elec-trodes for TFTs were fabricated on thermally oxidizedsilicon wafer by controlling various dip-casting parame-ters. For the organic and inorganic active layer deposition,two deposition methods such as an ink-jetting and a dip-casting were used, respectively. The solution-processed or-ganic semiconductors, 6,13-bis(triisopropyl-silylethynyl)pentacene (TIPS-pentacene) [19], was prepared from2 wt% chlorobenzene solution. After second surface pat-terning process on the pre-patterned Ag electrodes, activeorganic semiconductor layer deposition was performed byusing piezoelectric ink-jet printing system (UniJet UJ2100).The piezoelectric ink-jet nozzle had a diameter of 50 lm(orifice size of 50 lm). The frequency of the jetting was150 Hz and the diameter of the ink drop was approxi-mately 30–50 lm. All the electrical measurements wereperformed using Keithley 4200-SCS in a dark and air ambi-ent at room temperature.

tterning solution-processed organic semiconductors.

1104 S.K. Park et al. / Organic Electronics 10 (2009) 1102–1108

3. Results and discussion

3.1. Non-relief lithographic patterning of nanoparticulate Agelectrodes

To fabricate the barrier-like structure, a functional SAM(see experimental section) was patterned onto the sub-strate using deep ultra-violet (DUV) light exposure. Thedeposited nanoparticulate Ag solution can be steered intothe desired hydrophilic areas by low surface energy SAMsand the barrier-like structure. Because this approach doesnot require photoresist coating, development, and removalprocesses, the non-relief patterning of a SAM can easily re-sult in a fine-patterned barrier-like structure, making itcost effective. Direct DUV exposure of a SAM also offersgreat flexibility for two dimensional patterning. Patterningis not limited to surface energy effects and chemical reac-tivity or other surface properties can be used to meet theneeds of specific applications [18]. DUV irradiation of si-lane SAMs typically induces a photochemical reactionwhich involves cleavage of Si–O or O–C bonds to form areactive oxygen radical, which can react with atmosphericoxygen and moisture to form silanol or hydroxyl groupsand produce a hydrophilic surface [20]. Thus, for a hydro-phobic SAM, DUV irradiation can result in a hydrophobicto hydrophilic transformation.

Firstly, we investigated the influence of withdrawalspeed of substrates which may be effective to control inkvolume deposited on hydrophilic area. In dip-casting pro-cess, a patterned OTS substrate is immersed into Ag nanop-articulate solution for a certain time (around 1 min), andthen withdrawn at a well-defined speed under controlledtemperature and atmospheric conditions. Ethanol wasmixed into original nanoparticulate Ag solution with a var-

Fig. 2. (a) Optical micrographs of patterned Ag nanoparticulate electrodes frompatterned electrodes as a function of withdrawal speed, and (c) an averaged lin

ious volume ratio to control the ink viscosity. Fig. 2a showsoptical micrographs of patterned Ag nanoparticulate elec-trode arrays from various withdrawal speeds. As shownin Fig. 2b, thinner thickness (thus smaller amount of inkin a hydrophilic area) was obtained from slower with-drawal speed because of stronger liquid–liquid interactionthan liquid–solid (substrate) interaction [7,21]. The elec-trodes from lower withdrawal speed from 1–10 mm/min.typically have shown some aggregation of Ag nanoparticu-lates with no fine-patterning structures (less than 40 lm),which is possibly due to fast solidification of Ag nanopar-ticulates from small amount of solvent in thinner film.Additionally, a significant aggregation of the ink was alsoobserved at faster withdrawal speed (Fig. 2a). Typicallysmaller volume of ink has led to thinner film and thushigher convective and Marangoni flow enhanced by the in-creased surface tension may induce faster evaporation ofsolvents in the films [22]. In our experiments, the thinnerfilms from withdrawal speed from 1–10 mm/min. resultin fast solvent evaporation before the splitting of ink,inducing incomplete dewetting or splitting. Fig. 2c indi-cates the averaged line width and pitch as a function ofwithdrawal speed from 10 times repeated experiments.The thicker films from the faster withdrawal speed are alsoproblematic for fine-patterning process.

The influence of Ag ink viscosity was also studied whilemaintaining withdrawal speed of 50 mm/min. Ink viscositywas controlled by the addition of ethanol into originalnanoparticulate Ag solution. High ink viscosity (>2 mPa s)typically has led to too thick film which is beyond the crit-ical thickness Hc for fine-pattern splitting. In contrast, low-er viscosity ink induced too thin film and thus fastevaporation of solvent occurred before the splittingof ink, resulting in incomplete dewetting. Similar to the

surface patterning and dip-casting process, (b) an averaged thickness ofe width and pitch of electrodes as a function of withdrawal speed.

S.K. Park et al. / Organic Electronics 10 (2009) 1102–1108 1105

influence of withdrawal speed, ink viscosity also appears tobe closely related to the ink volume or liquid thickness onthe pre-patterned area [23,24]. Fig. 3a and b demonstratesthe optical and confocal micrographs which show rela-tively uniform distribution of dip-cast Ag electrode fromthe surface patterning and dip-casting process. Theseimages may provide additional information regardingthickness uniformity of the self-formed Ag electrodes.The dip-casted Ag dots or line arrays from optimal condi-tions typically have shown an average thickness 120 nmwith ±15 nm variations and more than 80% uniformity.Fig. 3c shows an average limit of electrode resolution andpitch as a function of ink viscosity. Fig. 3d shows the resis-tivity of Ag electrodes as a function of electrode thicknessand viscosity. For the application of printed electronics,the conductivity of electrodes is also critical for large areaelectronics and device performance. Typically, the directprinted Ag electrodes including thickness >50 nm showsresistivity of low 10�5 X cm ranges through annealing at180 �C for 30 min, which is a little bit higher or similar tothat of vacuum deposited metal thin films.

From our experiments, the differential wettability fromsolution process seems to be mainly defined by contact an-gles and geometry of surface. Incomplete splitting of inkwas typically observed in samples which had too fast/slowwithdrawal speed and higher ink viscosity which are likelyto related with volume of the ink solution on hydrophilicarea. According fluid-dynamic model [25], in a homoge-neous solid substrate, the contact angle h satisfies theYoung equation [26].

cosðhÞ ¼ ðrVS � rLSÞ=rLV

where rVS, rLS, and rLV are the vapor–solid, liquid–solid,and liquid–vapor interfacial tensions, respectively. In ourpreliminary experiments, the contact angles of 100� (hd:

Fig. 3. (a) Optical micrographs (left) and confocal 3D micrographs (right) of patelectrodes as a function of ink viscosity, and (c) an averaged thickness and resis

contact angle on hydrophobic area) and 20� (hc: contactangle on hydrophobic area) were measured on hydropho-bic area (OTS SAM) and patterned hydrophilic area, respec-tively. During the dip-casting process, the hydrophilicstripes are completely covered by Ag ink, and the contactlines of this channel are located at the surface domainboundaries. As more Ag ink adds onto the hydrophilicstripes, the thickness of ink increases but they still havethe shape of cylindrical or spherical caps and their contactlines are still pinned at the domain boundaries, resulting inincreased contact angles, which no longer satisfies theYoung equation. With appropriate ink volume (from with-drawal speed/ink viscosity), the contact angle of Ag ink atthe boundaries are well consistent with differential wetta-bility condition [27] such as

hd < h < dc

If the ink volume on the hydrophilic stripes is more than acritical value (or thickness Hc), the contact angle exceedshd, developing a bulge as soon as the contact angle h > hd.If the two neighboring bulges are enough close to mergeor bridge, the patterned Ag electrodes typically haveshown instable patterning properties. This is well consis-tent with our observation which indicates narrow pro-cess-window in fine-pitch electrode arrays as well asinstable Ag ink patterns from too fast withdrawal speedand high ink viscosity.

As well as the effect of contact angle change, the pat-terning resolution during dip-casting process seems to bealso affected by polarity and vapor pressure of solvents.Typically, a polar solvent is affected more by a hydrophobicsurface, resulting in higher resolution patterning possible[14]. The flowing speed of the Ag nanoparticulate solutionon the heterogeneous surface should be faster than that ofsolvent evaporation speed, which can steer the liquid into

terned Ag nanoparticulate electrodes, (b) an averaged pitch of patternedtivity of patterned electrode as a function of ink viscosity.

1106 S.K. Park et al. / Organic Electronics 10 (2009) 1102–1108

desired area before solidification of the ink solution onhydrophobic area. In our experiments, ethanol is continu-ously evaporated during the dip-cast process, inducingchange of interface/surface energy of the liquid on thehydrophilic stripes. Therefore, if the solidification exceedsa critical value before splitting occurred, the interactionenergy for repelling between the liquid and hydrophobicsolid is critically diminished, resulting in poor dewettingproperties. Therefore, solvents including lower vapor pres-sure may limit the resolution of electrode arrays due totheir faster evaporation speed. For the results describedhere, it is likely that patterning geometry, withdrawalspeed, ink viscosity, solvent polarity, and vapor pressureare some of the key factors for the simple and high-resolu-tion Ag nanoparticulate patterning process.

Fig. 4 shows simple comparison between direct printedAg nanoparticulate electrodes by well-controlled ink-jetprinting including ink drop size of around 35 lm on non-surface treated substrate and dip-cast Ag nanoparticulateelectrodes on differentially patterned substrate using thenon-relief-lithography patterning technology. As shownin this figure, while it may be difficult to obtain high reso-lution and fine-pitch electrodes on untreated surface evenusing a well-controlled direct ink-jet printing due to thelarger ink-droplet and the difficulty of controlling inkspread, the dip-cast electrodes with non-relief patternedlithography patterning have shown high-resolution andfine-pitch, implying simple and high-resolution patterningpossible.

3.2. All printed organic thin film transistors

The bottom gate and bottom contact all printed OTFTswere fabricated on heavily doped p-type Si wafers with a200 nm thermally grown SiO2 layer. The Si wafer used asboth substrate and common gate electrode, while theSiO2 layer acted as a gate dielectric layer. Using the non-re-lief patterned surface patterning, source/drain Ag elec-trodes for TFTs were built on thermally oxidized siliconwafer. Fig. 5a shows the optical micrograph of fabricatedsource and drain electrodes on SiO2 layer. The channellength of 5–10 lm were easily achieved using the non-re-lief-patterned surface pattering and dip-casting process. Toachieve fine patterning of organic and inorganic semicon-ductors, second non-relief-patterned surface patterning

Fig. 4. Optical micrographs of (a) direct ink-jet printed Ag nanoparticulate electrusing the non-relief patterned lithography patterning and dip-casting.

was performed with the identical process of first surfacepatterning, resulting in a new barrier structure for confin-ing semiconductor solutions.

Firstly, TIPS-pentacene from 2 wt% chlorobenzene solu-tion was ink-jetted over the pre-patterned barrier-likestructure. The schematic diagram of fabricated bottomcontact and bottom gate TIPS-pentacene OTFTs was dem-onstrated (bottom of Fig. 5b). As shown in Fig. 5b, droppedTIPS-pentacene ink was automatically confined within thepatterned OTS area. The diameter of dropped ink wasaround 30 lm with a volume of 30 pl. Fig. 5c shows

pID

and log(ID) versus VGS characteristics for VDS = �40 V, andID versus VDS characteristics for the TIPS-pentacene OTFTwith a surface energy patterned source–drain electrodeand active layer deposited from a 2 wt% chlorobenzenesolution. All electrical measurements were performed inan air ambient at room temperature. The device had a gatelength of 10 lm, a gate width of 100 lm, and a 200 nm-thick silicon dioxide gate dielectric. The electrical charac-teristics such as field-effect mobility of about 0.03–0.06 cm2/V s, on/off current ratios > 106, and subthresholdslope < 0.7 V/decade were obtained from more than 20 de-vices measurements. The typical performance of all printedOTFTs fabricated from the surface patterning and dip-cast-ing process was similar to that of same configurationOTFTs (0.02–0.06 cm2/V s) from ink-jet printed semicon-ductors and photolithographically patterned bottom con-tact electrodes [14,28,29].

Although we demonstrate non-relief-lithography-pat-terned Ag nanoparticulate source/drain electrodes basedOTFTs, adopting the novel patterning technology to poly-mer substrate or organic dielectric is still problematicdue to the difficulty of chemical reaction between OTSand polymer substance. Recently, a successful non-relief-lithography patterning of organic semiconductors onpolymer gate dielectric materials was reported using acomposite silane SAM layers [30]. The composite SAMlayer was used as a base layer to promote dense and uni-form formation of phenyl groups, which cannot beachieved by direct reaction between the SAMs and thepolymer surface. The composite layer also contains alkylgroups which lead a hydrophobic state. Additionally, inour recent experiments, differential surface states weresuccessfully obtained on polymer substrates and dielectricmaterials using the same non-relief-lithography patterning

odes on untreated surface and (b) patterned Ag nanoparticulate electrodes

Fig. 5. (a) Optical micrograph of patterned Ag nanoparticulate source/drain electrodes for unit TFT and simple circuit from surface pattering and dip-castingprocess, (b) optical micrographs of ink-jet printed TIPS-pentacene on the patterned electrodes, and (c) electrical characteristics of all printed TIPS-pentaceneOTFT (

pIDS � VGS and log(IDS) � VGS (VDS = �40 V), IDS � VDS) from surface patterning and dip-casting.

S.K. Park et al. / Organic Electronics 10 (2009) 1102–1108 1107

process and a specially developed surface surfactant. Basedon the recently developed technologies and materials, webelieve that forming multi-stacking layers will be possiblefrom the non-relief patterning technology and dip-castingfor all printed high-resolution flexible OTFTs andelectronics.

4. Conclusions

In summary, using a surface patterning, photoresist-free process and simple dip-casting, we have fabricatedreproducible all solution-processed transistors with pat-terned active layers and source/drain electrodes. The influ-ences of ink viscosity, deposition speed, solvent polarity,and boiling point were systematically investigated to ob-tain high-resolution and fine-pitch printed Ag nanopartic-ulate electrodes. Based on the patterned electrodes, wehave demonstrated all printed high-resolution and shortchannel OTFTs. This simple patterning technology for solu-tion-processed inorganic and organic materials can pro-vide not only high resolution but also reproducible andhigh performance flexible printed electronics. This workdemonstrates that surface energy patterning technologymay provide a path to low-cost and high performance allprinted electronics.

Acknowledgements

The authors gratefully acknowledge financial supportby a grant (F0004024-2008-31) from Information Display

R&D Center, one of the 21st Century Frontier R&D Programfunded by the Ministry of Knowledge Economy of Koreangovernment.

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