Inkjet Etching of Polymers and Its Applications in Organic Electronic Devices

polymers

Review Inkjet Etching of Polymers and Its Applications in Organic Electronic Devices

Wi Hyoung Lee 1,* ID and Yeong Don Park 2,* ID

1 Department of Organic and Nano System Engineering, Konkuk University, Seoul 05029, Korea 2 Department of Energy and Chemical Engineering, Incheon National University, Incheon 22012, Korea * Correspondence: [email protected] (W.H.L.); [email protected] (Y.D.P.); Tel.: +82-2-450-3468 (W.H.L.)

Received: 25 July 2017; Accepted: 7 September 2017; Published: 11 September 2017

Abstract: Inkjet printing techniques for the etching of polymers and their application to the fabrication of organic electronic devices are reviewed. A mechanism is proposed for the formation of via holes in polymer layers through inkjet printing with solvent, and recent achievements in the fabrication with inkjet etching of various three-dimensional microstructures (i.e., microwells, microgrooves, hexagonal holes, and concave structures) are discussed. In addition, organic electronic devices are presented that use inkjet-etched subtractive patterns as platforms for the selective depositions of an emissive material, a liquid crystal, an organic conductor, an organic insulator, and an organic semiconductor, and as an optical waveguide.

Keywords: inkjet; etching; organic electronic device; organic transistor; organic semiconductor

1. Introduction Inkjet printing has received considerable attention because of its potential use in future deposition and patterning technologies by the display and semiconductor industries [1–8]. This technique is a direct-write tool for the deposition of functional materials, so it can replace costly vacuum deposition methods such as sputtering and thermal evaporation. In addition, photolithography is not necessary in inkjet printing processes, thereby enabling maskless and photoresist-free patterning. Furthermore, the use of inkjet printing can reduce waste because droplets with picoliter volumes are printed at the desired position. The advantages of inkjet printing over conventional deposition and patterning tools mean that it can be used in the fabrication of components of organic electronic devices such as organic light emitting diodes (OLEDs) and organic thin-film transistors (OTFTs) [3,9–25]. Although inkjet printing is generally recognized as an additive tool for the deposition of functional materials on target substrates, it can also be used as a subtractive tool for the removal of very small areas of predeposited films [19,26,27]. As shown in Figure1, the inkjet printing of a solvent onto a polymer film can etch the polymer, which results in the formation of a ring-like deposit through the coffee-ring effect [28]. This inkjet-based, selective wet etching process has several advantages over conventional dry etching processes: it does not rely on conventional photolithography with masks and photoresists, and contamination of the sample can be greatly reduced. This one-shot removal process is also useful for the fabrication of components in organic electronic devices.

Polymers 2017, 9, 441; doi:10.3390/polym9090441 www.mdpi.com/journal/polymers Polymers 2017, 9, 441 2 of 16 Polymers 2017, 9, 441 2 of 16

Figure 1. TheThe applications applications of of the the inkjet inkjet etching etching of ofpolyme polymersrs to tovarious various organic organic electronic electronic devices: devices: an anoptical optical waveguide, waveguide, an anorganic organic light light emitting emitting diode, diode, a aliquid liquid crystal crystal microlen microlens,s, and and organic organic thin-film thin-ﬁlm transistors [[11,28–31].11,28–31].

In thisthis reviewreview paper,paper, we we present present various various implementations implementations of of inkjet inkjet etching etching as aas subtractive a subtractive tool tool for thefor fabricationthe fabrication of three-dimensional of three-dimensional microstructures. microstructures. Furthermore, Furthermore, the mechanism the mechanism for the structural for the developmentstructural development of subtractive of subtractive patterns is analyzedpatterns is by analyzed considering by considering the underlying the chemistryunderlying and chemistry physics ofand the physics etching of process. the etching In the process. following In section, the following the utilization section, of inkjet-etchedthe utilization microstructures of inkjet-etched in organicmicrostructures electronic in devicesorganic iselectronic reviewed. devices Inkjet-etched is reviewed. microstructures Inkjet-etched can microstructures be used directly can as be optical used waveguides,directly as optical and the waveguides, selective deposition and the selective of organic de materialsposition of into organic inkjet-etched materials microstructures into inkjet-etched can usedmicrostructures to fabricate can the componentsused to fabricate of organic the electroniccomponents devices. of organic Recent electronic achievements devices. and Recent future perspectivesachievements are and discussed. future perspectives are discussed.

2. Inkjet Etching of Polymers Inkjet etchingetching can can be be used used to fabricateto fabricate various variou typess types of subtractive of subtractive patterns. patterns. When aWhen single a solvent single dropletsolvent isdroplet ejected is fromejected an inkjetfrom an printer, inkjet it printer, collides it with collides the pre-depositedwith the pre-deposited polymer film. polymer Then, film. the polymerThen, the dissolves polymer anddissolves the solvent and the evaporates solvent evaporat [5,27]. Thees [5,27]. outward The convective outward convective flow in the flow dissolved in the polymerdissolved solution polymer leads solution to the redepositionleads to the of redeposi the polymertion atof the the pinned polymer contact at the line pinned [11,32]. Thiscontact process line results[11,32]. finallyThis process in the formationresults finally of a crater-likein the formation deposit, of as a showncrater-like in Figure deposit,1. The as shown dimensions in Figure of the 1. crater-likeThe dimensions deposit of the are crater-like determined deposit by several are determin factors suched by as several the properties factors such of theas the solvent properties and the of polymerthe solvent [33 ].and The the partial polymer drying [33]. of theThe first partial jetted dryi dropletng of can the hinder first thejetted consecutive droplet can printing hinder of inkthe whenconsecutive the boiling printing point of of ink solvent when is the too boiling low. If thepoint boiling of solvent point is of too the low. solvent If the is sufficiently boiling point high, of thisthe firstsolvent drop is problemsufficiently at thehigh, orifice this first can bedrop resolved problem and at cloggingthe orifice of can the be nozzle resolved does and not typicallyclogging of occur the duringnozzle thedoes printing not typically of solvents occur [34 ].during Nevertheless, the printi theng careful of solvents selection [34]. of theNevertheless, solvent is important the careful to ensureselection both of the the solvent dissolution is important of the polymer to ensure and both consecutive the dissolution droplet of ejection the polymer without and the consecutive formation ofdroplet satellites. ejection The without jettability the window formation of inkjetof satellites. printing The is typicallyjettability determined window of by inkjet considering printing the is rheologicaltypically determined properties by of considering an ink [35]. the The rheological Reynolds numberproperties (R ofe), an the ink Weber [35]. numberThe Reynolds (We), number and the Capillary(Re), the Weber number number (Ca) are (W regardede), and the as governingCapillary factorsnumber for (C defininga) are regarded printable as windows.governing The factors ratio for of thedefiningRe to printable the square windows. root of the TheW eratio (a quantity of the R callede to theZ number)square root was of first the identified We (a quantity as a parameter called Z fornumber) defining was the first jettability identified range as [a36 parameter]. However, for recent defining studies the revealedjettability that range the [36].We- CHowever,a space is recent better tostudies accurately revealed examine that the the W effectse-Ca space of the is viscous better andto accurately inertial forces examine [35, 37the]. effects For instance, of the viscous Nallan etand al. usedinertial mixed forces solvents [35,37]. with For different instance, surface Nallan tension et al. used and viscositymixed solvents to control with both different the We surface and the tensionCa and foundand viscosity the appropriate to control jettability both the window We and for the stable Ca and jetting found without the appropriate wavelike instability jettability [ 35window]. for stable jetting without wavelike instability [35].

Polymers 2017, 9, 441 3 of 16

When a single droplet is insufficient to etch the polymer film, successive dropping of the solvent can remove the polymer film without the formation of residue on the substrate. Further, the consecutive printing of solvent droplets during the gradual movement of the inkjet printer head can lead to the formation of microgroove lines. The line width is typically determined by the volume of the droplets, the processing parameters, the substrate temperature, and the polymer-solvent interaction [32,38–40]. Several considerations are crucial to the fabrication of polymer-relief microstructures. First, the solubility of the polymer in the solvent must be appropriate: the inkjet-printed solvent should dissolve the pre-deposited polymer on the timescale of droplet evaporation. Evaporation that is too rapid can lead to the uncontrolled etching of the polymer. Second, the thickness of the pre-deposited polymer plays an important role in the design of the dimensions of the etched polymer microstructure. If the thickness of the polymer layer is not optimized, fingering instability can occur and the resulting patterns are not well defined [41,42]. On the other hand, if the polymer layer is too thick, the solvent does not completely etch the pre-deposited polymer. Third, control of the evaporation rate of the solvent enables the fabrication of well-defined microstructures. Note that the etching of the polymer is generally carried out under atmospheric conditions, so the temperature and humidity should be well-controlled [40]. Gans et al. examined the formation of various polymer-relief microstructures prepared by inkjet printing solvent onto PS films [31,32]. They used several kinds of solvent and found that the dimensions of the resulting holes or grooves are independent of the choice of solvent (ethyl acetate, isopropyl acetate, n-butyl acetate, toluene, anisole, and acetophenone) as long as it is a good solvent for PS [32]. Although the dimensions of the holes or grooves are governed by three factors—the spreading of solvent on the pre-deposited polymer film, the solvent evaporation rate, and the dissolution speed of the polymer film—the dissolution speed of each polymer/solvent pair usually exceeds the solvent evaporation rate. Thus, the solvent evaporation rate is not the governing parameter and the use of solvents with different vapor pressures does not affect the dimensions of the fabricated microstructures. On the other hand, they found that the molecular weight of the PS sample is critical to the formation of subtractive patterns. When they used PS with a high molecular weight (Mw of 282 kD), the dropping of acetophenone onto the polymer layer did not lead to the formation of such structures. However, the use of PS with low molecular weight (Mw of 137.4 kD) results in pattern formation, as shown in Figure2a, which shows an array of holes formed by inkjet printing isopropyl acetate onto pre-deposited PS with a thickness of 180 nm. The height of the ring formed by the coffee staining effect was measured to be approximately 1 µm. They also fabricated patterns of different types by varying the shape of the printing nozzle. When they used a rectangular nozzle, rectangular holes were fabricated (Figure2b); the use of a hexagonal nozzle resulted in the formation of hexagonal holes (Figure2c). Interestingly, they found that the distance between the solvent drops affects the shape of the patterns. When the distance between solvent drops was increased for a rectangular nozzle, the shape of the hole changed from rectangular to circular. These findings imply that in inkjet etching the control of the nozzle, i.e., its shape and speed, is important for the control of the shape and dimensions of the resulting pattern. Polymers 2017, 9, 441 4 of 16 Polymers 2017, 9, 441 4 of 16

FigureFigure 2.2. (a) Hole array in polystyrenepolystyrene (PS) etched by inkjetinkjet printingprinting withwith isopropylisopropyl acetateacetate [[33].33]. CopyrightCopyright 2007 2007 Royal Royal Society Society of Chemistry. of Chemistry. (b) Rectangular (b) Rectangular holes in PS holes inkjet-etched in PS withinkjet-etched acetophenone with droplets.acetophenone (c) Hexagonal droplets. holes (c) Hexagonal inkjet-etched holes with inkjet-etched isopropyl acetate with droplets.isopropyl The acetate surface droplets. proﬁles The are shownsurface beneath profiles the are images shown [beneath32]. Copyright the images 2006 [32]. Wiley. Copyright 2006 Wiley.

AlthoughAlthough GansGans etet al.al. diddid notnot findfind anyany relationshiprelationship betweenbetween thethe solventsolvent evaporationevaporation raterate andand thethe dimensionsdimensions ofof thethe fabricatedfabricated patterns,patterns, ZhangZhang etet al.al. found a clearclear relationshiprelationship betweenbetween them.them. TheyThey controlledcontrolled thethe solventsolvent evaporationevaporation raterate byby varyingvarying thethe substratesubstrate temperaturetemperature [[40].40]. FigureFigure3 a,b3a,b showshow comparativecomparative plots plots of of via via hole hole size size as a functionas a func oftion the of number the number of the drops. of the PVP, drops. with PVP, a thickness with a ofthickness 2.5 µm, andof 2.5 ethanol μm, were and usedethanol as the were polymer used film as andthe thepo etchinglymer solvent,film and respectively. the etching Increasing solvent, therespectively. substrate temperatureIncreasing the enhances substrate both temperature the solvent evaporationenhances both rate the and solvent the dissolution evaporation speed rate of and the polymerthe dissolution film. However, speed of the the effects polymer in this film. system However, of theincreased the effects solvent in this evaporation system of ratethe dominateincreased thesolvent effects evaporation of the increased rate dominate dissolution the speed.effects of Thus, the increased the etching dissolution speed of thespeed. PVP Thus, film the at 80 etching◦C is lowerspeed thanof the that PVP of film the PVPat 80 film °C is at lower 20 ◦C. than Accordingly, that of the the PVP inner film and at 20 outer °C. diametersAccordingly, of thethe holesinner areand reduced outer diameters for a substrate of the temperature holes are reduced of 80 ◦C. for Further, a substrate the height temperature of the rim of that 80 is°C. formed Further, at thethe height of the rim that is formed at the elevated temperature is lower because it depends on the elevated temperature is lower because it depends on the volume of the dissolved polymer. Note that volume of the dissolved polymer. Note that the outward convective flow at the pinned contact line the outward convective flow at the pinned contact line results in the movement of the polymer solution results in the movement of the polymer solution to the edge. Figure 3c shows a schematic to the edge. Figure3c shows a schematic representation of the formation of a hole through the inkjet representation of the formation of a hole through the inkjet printing of successive ethanol drops at a printing of successive ethanol drops at a high substrate temperature. This illustration suggests that the high substrate temperature. This illustration suggests that the reduced polymer transfer at a high reduced polymer transfer at a high substrate temperature is the reason for the reduced diameter of substrate temperature is the reason for the reduced diameter of the holes. They also examined the the holes. They also examined the effects of varying the polymer film thickness on the dimensions effects of varying the polymer film thickness on the dimensions of the holes fabricated by inkjet of the holes fabricated by inkjet etching: as the film thickness increases, the diameter of the holes etching: as the film thickness increases, the diameter of the holes increases [39]. Further, the number increases [39]. Further, the number of drops needed for complete etching also increases. Zhang et al. of drops needed for complete etching also increases. Zhang et al. showed that the substrate showed that the substrate temperature, film thickness, and polymer surface conditions determine the temperature, film thickness, and polymer surface conditions determine the dimensions of via holes dimensions of via holes fabricated with inkjet etching [39,40]. Grimaldi et al. examined the effects of fabricated with inkjet etching [39,40]. Grimaldi et al. examined the effects of varying the substrate varying the substrate temperature and etching solvent on the shape and dimensions of holes fabricated temperature and etching solvent on the shape and dimensions of holes fabricated with inkjet with inkjet etching [43]. They found that increasing the substrate temperature results in holes with etching [43]. They found that increasing the substrate temperature results in holes with a decreased a decreased diameter and an increased rim height. Furthermore, a mixed solvent can be adopted to diameter and an increased rim height. Furthermore, a mixed solvent can be adopted to induce a induce a change in the holes from concave to convex. This phenomenon arises because of the resulting change in the holes from concave to convex. This phenomenon arises because of the resulting change in the flow of the drying polymer solution during the inkjet etching process. change in the flow of the drying polymer solution during the inkjet etching process.

Polymers 2017, 9, 441 5 of 16

Figure 3. (a()a )Surface Surface profiles proﬁles obtained obtained after after dropping dropping 1 1 to to 23 23 droplets droplets of of ethanol onto ◦ b poly(4-vinylphenol) (PVP)(PVP) onon a a substrate substrate with with a temperaturea temperature of 20of 20C. °C. ( ) Surface(b) Surface proﬁles profiles of PVP of afterPVP dropping 1 to 23 droplets of ethanol onto PVP on a substrate at a temperature of 80 ◦C. (c) Schematic after dropping 1 to 23 droplets of ethanol onto PVP on a substrate at a temperature of 80 °C. (c) diagram of the formation of a hole resulting from the dropping of successive ethanol droplets at a high Schematic diagram of the formation of a hole resulting from the dropping of successive ethanol substrate temperature [40]. Copyright 2013 American Institute of Physics. droplets at a high substrate temperature [40]. Copyright 2013 American Institute of Physics.

Although the the fabrication fabrication with with inkjet inkjet etching etching of ofvia via holes holes is useful is useful in several in several research research fields, fields, the featurethe feature resolution resolution is limited is limited to several to several tens tens or hu orndreds hundreds of micrometers. of micrometers. The The limited limited resolution resolution of theofthe inkjet inkjet etching etching process process is mainly is mainly due dueto the to massive the massive polymer polymer transport transport resulting resulting from the from coffee the stainingcoffee staining effect. effect.Bao et Baoal. fabricated et al. fabricated high-resol high-resolutionution concave concave microstructures microstructures by using by the using inkjet the imprintinginkjet imprinting technique technique [44]. [They44]. They used used pre-cured pre-cured polydimethylsiloxane polydimethylsiloxane (PDMS) (PDMS) as as a a viscoelastic polymer on which which the the viscoelastic viscoelastic dissipation dissipation of of so solventlvent occurs occurs [45]. [45 ].Figure Figure 4a4 ashows shows the the steps steps in thein the fabrication fabrication of ofconcave concave microstructures microstructures with with th thee inkjet inkjet imprinting imprinting technique. Microwells Microwells or microgrooves can be fabricated by controlling the drop distance: if the drop distance is larger than a critical value,value, microwells microwells are are fabricated; fabricated; on the on other the hand,other smallhand, drop small distances drop leaddistances to the formationlead to the of formationmicrogrooves. of microgrooves. The formation ofThe microwells formation or microgroovesof microwells is or dependent microgrooves on the PDMSis dependent pre-curing on time. the WhenPDMS thepre-curing pre-curing time. time When is less the than pre-curing 18 min, thetime ink is dropletless than sinks 18 min, into thethe PDMSink droplet film, whereassinks into when the PDMSthe pre-curing film, whereas time is when more the than pre-curing 30 min, the time ink is droplet more than remains 30 min, on thethe PDMSink droplet surface. remains The pattern on the PDMSmorphology surface. observations The pattern in morphology Figure4b indicate observations that the formationin Figure 4b of microwellsindicate that or the microgrooves formation isof microwellsdetermined or by microgrooves the PDMS pre-curing is determined time. In by addition, the PDMS the pre-curing line width time. is proportional In addition, to the pre-curingline width istime, proportional and the line to the height pre-curing is inversely time, proportionaland the line height to the is pre-curing inversely timepropor (Figuretional4 toc). the This pre-curing finding timeindicates (Figure that 4c). the This spreading finding ofindicates the PAA that solution the spreading on the PDMSof the PAA surface soluti is criticallyon on the affected PDMS surface by the isdegree critically of crosslinking affected byin the the degree PDMS of film. crosslinking Microwell in arrays the PDMS with film. various Microwell dimensions arrays were with utilized various in dimensionsthe trapping were of mammalian utilized in cells the (Figure trapping4d). Figureof mammalian4e shows acells fluorescence (Figure image4d). Figure of the microwells4e shows a withfluorescence trapped singleimage cells.of the Their microwells results demonstrate with trapped that concavesingle cells. microstructures Their results fabricated demonstrate with inkjet that concaveimprinting microstructures can be used to fabricated efficiently with trap inkjet single imprin cells forting fluorescence can be used imaging. to efficiently trap single cells for fluorescence imaging.

Polymers 2017, 9, 441 6 of 16 Polymers 2017, 9, 441 6 of 16

Figure 4. (a) The inkjet etching of concave microstructures; (b) Optical microscopy images of PDMS Figure 4. (a) The inkjet etching of concave microstructures; (b) Optical microscopy images of PDMS films with various degrees of pre-curing after etching with poly acrylic acid (PAA) solution; (c) Line films with various degrees of pre-curing after etching with poly acrylic acid (PAA) solution; (c) Line width and depth of microgrooves as functions of the polydimethylsiloxane (PDMS) film pre-curing width and depth of microgrooves as functions of the polydimethylsiloxane (PDMS) film time. The insets show images of the fabricated microgrooves (The scale bars in Figure4c are 5 µm); pre-curing time. The insets show images of the fabricated microgrooves (The scale bars in Figure 4c (d) The dimensions of the microwells used in the trapping of cells; (e) Fluorescence image of the are 5 μm); (d) The dimensions of the microwells used in the trapping of cells; (e) Fluorescence image microwells with trapped single cells [44]. Copyright 2015 Wiley. of the microwells with trapped single cells [44]. Copyright 2015 Wiley. 3. Applications in Organic Electronic Devices 3. Applications in Organic Electronic Devices Various three-dimensional polymer microstructures (i.e., microwells, microgrooves, hexagonal Various three-dimensional polymer microstructures (i.e., microwells, microgrooves, hexagonal holes, and concave structures) can easily be fabricated with inkjet etching, and can then be utilized holes, and concave structures) can easily be fabricated with inkjet etching, and can then be utilized as components in organic electronic devices [20]. In particular, nonlithographic patterning through as components in organic electronic devices [20]. In particular, nonlithographic patterning through inkjet etching has the advantage of avoiding the effects resulting from the use of photoresists and inkjet etching has the advantage of avoiding the effects resulting from the use of photoresists and developer. Further, the inkjet printing direct-write process enables simple fabrication steps that reduce developer. Further, the inkjet printing direct-write process enables simple fabrication steps that the quantities of used materials and costs. For instance, inkjet-etched polymer microstructures can be reduce the quantities of used materials and costs. For instance, inkjet-etched polymer directly utilized as optical waveguides [30]. In addition, they can be used as platforms for the selective microstructures can be directly utilized as optical waveguides [30]. In addition, they can be used as deposition of organic materials (i.e., emissive materials, liquid crystals, conductors, insulators, and platforms for the selective deposition of organic materials (i.e., emissive materials, liquid crystals, semiconductors), and thus in high-performance/high-throughput organic electronic devices. conductors, insulators, and semiconductors), and thus in high-performance/high-throughput The inkjet printing of a solvent onto a polymer film results in the formation of a microring; organic electronic devices. such a microstructure can be directly utilized to locally induce photon flows in an optical waveguide The inkjet printing of a solvent onto a polymer film results in the formation of a microring; (Figure5a). Zhang et al. fabricated a photonic circuit with a uniform microring pattern by using inkjet such a microstructure can be directly utilized to locally induce photon flows in an optical etching (Figure5b) [ 30]. A large-scale ring pattern with a uniform ring distance was fabricated on a waveguide (Figure 5a). Zhang et al. fabricated a photonic circuit with a uniform microring pattern by using inkjet etching (Figure 5b) [30]. A large-scale ring pattern with a uniform ring distance was fabricated on a flexible plastic substrate. This unique optical-resonator structure can efficiently

Polymers 2017, 9, 441 7 of 16 Polymers 2017, 9, 441 7 of 16

flexible plastic substrate. This unique optical-resonator structure can efficiently confine photon flow. confine photon flow. Figure 5c shows a coupled optical resonator consisting of two rings separated Figure5c shows a coupled optical resonator consisting of two rings separated by a distance of 500 nm. by a distance of 500 nm. The sub-micrometer scale resolution of the ring pattern was achieved by The sub-micrometer scale resolution of the ring pattern was achieved by controlling the inkjet etching controlling the inkjet etching conditions. The left ring is excited by incident light and the right ring isconditions. illuminated The asleft a result ring isofexcited resonator by incidentcoupling. light The andspectrum the right on ringthe right is illuminated hand side asof aFigure result 5c of demonstratesresonator coupling. the successful The spectrum enhancement on the rightof modulate hand sided resonance of Figure 5asc demonstratesa result of the theVernier successful effect. Thisenhancement inkjet-etching-based of modulated technique resonance is asa apromising result of themethodology Vernier effect. for the This fabrication inkjet-etching-based of optical waveguidestechnique is awith promising microring methodology patterns, and for theit fabricationis fully compatible of optical with waveguides plastic withsubstrates microring and patterns,high-resolution/larg and it is fullye-scale compatible production. with plastic substrates and high-resolution/large-scale production.

Figure 5. (a(a) )Schematic Schematic representation representation of of the the fabrication fabrication of ofa photonic a photonic circuit circuit with with inkjet inkjet printing. printing. (b) (Photographb) Photograph of a of photonic a photonic circuit circuit with with a aunifor uniformm pattern. pattern. An An optical optical microscopy microscopy image image and and an atomic forceforce microscopy microscopy (AFM) (AFM) image image of of the the photonic photonic circuit circuit are are shown shown inin thethe middlemiddle andand right hand columns, respectively. (c) Optical microscopy image of coupled resonators with two rings at a hand columns, respectively. (c) Optical microscopy image of coupled resonators with two rings at a distance of 500 nm and the corresponding spectrum [30]. Copyright 2015 American Association for the distance of 500 nm and the corresponding spectrum [30]. Copyright 2015 American Association for Advancement of Science. the Advancement of Science.

Xia et et al. al. fabricated fabricated a regular a regular hole array hole arrayby inkjet by printing inkjet printing ethanol ethanolonto a PVP onto film; a PVPFigure film; 6a showsFigure 6aa photoluminescence shows a photoluminescence (PL) spectrum (PL) spectrum of the cr ofater-like the crater-like holes [19]. holes They [ 19 then]. They inkjet then printed inkjet printedpoly(9,9′ poly(9,9-dioctylfluorene-0-dioctylfluorene-co-benzothiadiazole)co-benzothiadiazole) (F8BT) emissive (F8BT) material emissive into material the regular into thehole regular array, resultinghole array, in resultingan integrated in an OLED integrated consisting OLED of consistinginkjet-patterned of inkjet-patterned PVP banks and PVP an banksinkjet-printed and an F8BTinkjet-printed emissive F8BTmaterial, emissive as shown material, in Figure as shown 6b. Figure in Figure 6c shows6b. a Figure PL image6c shows of the a fabricated PL image LED of array.the fabricated In their study, LED array.the inkjet-etched In their study, hole array the inkjet-etchedwas found to holeeffectively array confine was found the emissive to effectively layer locallyconfine during the emissive inkjet printing. layer locally Similarly, during Wang inkjet et al. printing. fabricated Similarly, microgrooves Wang inet a fluoropolymer, al. fabricated CYTOP,microgrooves by performing in a fluoropolymer, the successive CYTOP, inkjet by performingprinting of thesolvent successive [31]. The inkjet fabricated printing CYTOP of solvent banks [31]. Thewere fabricated utilized CYTOP for banksthe wereselective utilized fordeposition the selective with deposition inkjet with inkjetprinting printing of of a poly(dibenzothiophene-S,S-dioxide-coco-9,9-dioctyl-2,7-fluorene)-9,9-dioctyl-2,7-fluorene) (PF-FSO) (PF-FSO) film. film. The The CYTOP CYTOP banks enable thethe orthogonalorthogonal processing processing of of the the PF-FSO PF-FSO film fi becauselm because the solvent the solvent used inused the printingin the printing of PF-FSO of PF-FSOdoes not does dissolve not thedissolve CYTOP the banks CYTOP [46 ].banks Figure [46].6d showsFigure the 6d surface shows profilesthe surface of an profiles inkjet-etched of an inkjet-etchedCYTOP bank andCYTOP the inkjet-printed bank and PF-FSOthe inkjet-print film. Althoughed PF-FSO PF-FSO film. could Although be selectively PF-FSO deposited could intobe selectivelythe CYTOP deposited bank, it did into not the cover CYTOP the edge bank, region. it did This not explained cover the the edge reason region. for theThis difference explained in the PL reasonand electro for luminancethe difference (EL) in spectra, PL and as shownelectro inlumi Figurenance6e,f. (EL) Patterning spectra, of as the shown emissive in layerFigure in 6e,f. the PatterningOLED was of achieved the emissive through layer a nonlithographic in the OLED was process achieved with through inkjet etching. a nonlithographic process with inkjet etching.

PolymersPolymers 20172017, 9, ,9 441, 441 8 8of of 16 16

FigureFigure 6. 6. ((aa)) AA photoluminescencephotoluminescence (PL) (PL) image image of aof hole a hole by printing by printing ethanol ethanol on a PVP on film. a PVP (b) Schematicfilm. (b) showing structure of light emitting diode (LED) with poly(9,9’-dioctylfluorene-co-benzothiadiazole) Schematic showing structure of light emitting diode (LED) with (F8BT) emissive layer. (c) PL image of the fabricated LED array [19]. Copyright 2007 American poly(9,9’-dioctylfluorene-co-benzothiadiazole) (F8BT) emissive layer. (c) PL image of the fabricated Institute of Physics. (d) Surface profiles of inkjet etched CYTOP bank and inkjet printed LED array [19]. Copyright 2007 American Institute of Physics. (d) Surface profiles of inkjet etched poly(dibenzothiophene-S,S-dioxide-co-9,9-dioctyl-2,7-fluorene) (PF-FSO) film. (e) PL and (f) electro CYTOP bank and inkjet printed poly(dibenzothiophene-S,S-dioxide-co-9,9-dioctyl-2,7-fluorene) luminance (EL) images of organic light emitting diodes (OLED) based on PF-FSO emissive layer inkjet (PF-FSO) film. (e) PL and (f) electro luminance (EL) images of organic light emitting diodes (OLED) printed inside of CYTOP bank [31]. Copyright 2017 Royal Society of Chemistry. based on PF-FSO emissive layer inkjet printed inside of CYTOP bank [31]. Copyright 2017 Royal Society of Chemistry. Hwang et al. used inkjet-etched microwells to fabricate liquid crystal microlenses [29]. Figure7a showsHwang a schematic et al. used diagram inkjet-etched of the steps microwells in their method to fabricate for the liquid preparation crystal of microlenses a liquid crystal [29]. microlens. Figure 7aUV-curable shows a schematic monomer diagram (NOA61) of is the partially steps in cured thei withr method UVlight. for the Then, preparation deionized ofwater a liquid is dropped crystal microlens.onto the partially UV-curab curedle monomer NOA61 (NOA61) film and, subsequently,is partially cured high with intensity UV light. UV lightThen, is deionized utilized both water for isthe dropped complete onto curing the partially of the NOA61 cured filmNOA61 and thefilm drying and, subsequently, of the water droplet.high intensity After UVUV exposure, light is utilizeda crater-like both for microwell the complete forms. curing Here, of the the two-step NOA61 UV film curing and the process drying is theof the key water to the droplet. fabrication After of UVthe exposure, well-defined, a crater-like crater-like microwell microwell. forms. The Here, injection the oftwo-step liquid crystalUV curing into process the crater-like is the key microwell to the fabricationfinalizes the of fabrication the well-defined, of the liquid crater-like crystal microlens,microwell. andThe a injection rubbed polyimide of liquidfilm crystal on the into bottom the crater-likeplate is used microwell to align thefinalizes liquid crystal.the fabrication The optical of properties the liquid of thecrystal liquid microlens, crystal microlens and a atrubbed various polyimideapplied voltages film on were the bottom measured plate with is used polarized to align optical the microscopyliquid crystal. (Figure The 7opticalb). The properties color differences of the liquid crystal microlens at various applied voltages were measured with polarized optical microscopy (Figure 7b). The color differences indicate that the orientation of the liquid crystals in

Polymers 2017, 9, 441 9 of 16

Polymers 2017, 9, 441 9 of 16 indicate that the orientation of the liquid crystals in the microlens can be tuned with an applied bias. The simplicity of their reported fabrication method enables the high-volume production with inkjet the microlens can be tuned with an applied bias. The simplicity of their reported fabrication method etching of liquid crystal microlenses. enables the high-volume production with inkjet etching of liquid crystal microlenses.

Figure 7. (a) Schematic diagram of the steps in the preparation of a liquid crystal microlens. Figure 7. (a) Schematic diagram of the steps in the preparation of a liquid crystal microlens. (b) (b) Polarized optical microscopy images of the liquid crystal microlens under various applied voltages. Polarized optical microscopy images of the liquid crystal microlens under various applied voltages. R: Rubbing direction; A: Orientation of analyzer; P: Orientation of cross polarizer [29]. Copyright 2013 R: Rubbing direction; A: Orientation of analyzer; P: Orientation of cross polarizer [29]. Copyright Optical Society of America. 2013 Optical Society of America.

InkjetInkjet etchingetching isis alsoalso usefuluseful forfor applicationsapplications in OTFTs suchsuch asas interconnectioninterconnection and patterning. KawaseKawase etet al.al. fabricated inkjet-etched via holes to provide interconnectionsinterconnections betweenbetween toptop andand bottombottom electrodeselectrodes inin OTFTs OTFTs [11 ].[11]. Figure Figure8a shows 8a shows an atomic an forceatomic microscopy force microscopy (AFM) image (AFM) of an inkjet-etchedimage of an PVPinkjet-etched microhole. PVP As microhole. explained As in ourexplained introduction, in our theintroduction, formation the of theformation microhole of the is driven microhole by the is convectivedriven by ﬂowthe convective in the drying flow polymer in the solution, drying whichpolymer is itself solution, generated which by is the itself difference generated between by thethe solventdifference evaporation between the rates solvent of the middleevaporation and edgerates regions of the (Figuremiddle 8andb). Theedge PVP regions dielectric (Figure in an 8b). OTFT The wasPVP etcheddielectric with inkjetin an printing, OTFT andwas the etched deposition with of inkjet a poly(3,4-ethylenedioxythiophene) printing, and the deposition (PEDOT) of a electrodepoly(3,4-ethylenedioxythiophene) through the via hole resulted (PEDOT) in the electrode formation through of an interconnectionthe via hole resulted between in the the formation top and bottomof an interconnection PEDOT electrodes. between Kawase the top et al.and fabricated bottom PE threeDOT types electrodes. of inverters Kawase with et PEDOTal. fabricated electrodes three types of inverters with PEDOT electrodes connected by via holes in PVP films (Figure 8c), and the inkjet-printed, all-polymer circuits were all found to exhibit the required switching capability. Inkjet etching is particularly promising for such applications because via holes can easily be fabricated by inkjet printing the appropriate solvent onto the desired polymer region.

Polymers 2017, 9, 441 10 of 16 connected by via holes in PVP ﬁlms (Figure8c), and the inkjet-printed, all-polymer circuits were all found to exhibit the required switching capability. Inkjet etching is particularly promising for such applications because via holes can easily be fabricated by inkjet printing the appropriate solvent onto thePolymers desired 2017 polymer, 9, 441 region. 10 of 16

Figure 8. (a) AFM image of an inkjet-etched PVP microhole. (b) Schematic diagram of the flows in a Figure 8. (a) AFM image of an inkjet-etched PVP microhole. (b) Schematic diagram of the flows drying droplet. (c) Photographs of three types of inverters (depletion-type, enhancement-type, and in a drying droplet. (c) Photographs of three types of inverters (depletion-type, enhancement-type, resistor load) with poly(3,4-ethylenedioxythiophene) (PEDOT) electrodes connected by via holes and resistor load) with poly(3,4-ethylenedioxythiophene) (PEDOT) electrodes connected by via holes in PVP films. The insets on the right show the circuit diagrams of the three types of inverters [11]. in PVP films. The insets on the right show the circuit diagrams of the three types of inverters [11]. Copyright 2001 Wiley. Copyright 2001 Wiley. Inkjet etching can be utilized to etch unwanted dielectric materials in OTFTs. Khim et al. used anInkjet inkjet-printing-based, etching can be utilized soft-etching to etch unwantedtechnique dielectricto fabricat materialse high-speed, in OTFTs. ambipolar Khim etOTFTs. al. usedPoly([ an inkjet-printing-based,N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5 soft-etching technique to fabricate high-speed, ambipolar′-(2,2 OTFTs.′-bithiop 0 0 0 Poly([hene))N,N -bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5(P(NDI2OD-T2)) was used as the active layer and poly(vinylidenefluoride-trifluoroethylene)-(2,2 -bithiophene)) (P(NDI2OD-T2))(P(VDF-TrFE)), was and usedPS/P(VDF-TrFE) as the active were layer used andas the poly(vinylidenefluoride-trifluoroethylene) dielectric layers of p-channel and n-channel (P(VDF-TrFE)),transistors, respectively and PS/P(VDF-TrFE) [47]. Figure were 9a shows used as a theschematic dielectric diagram layers of of the p-channel fabrication and of n-channel an inverter transistors,with a dual respectively gate structure. [47]. Figure Inkjet9 etchinga shows was a schematic utilized to diagram selectively of the remove fabrication PS and of thus an inverter to prepare withp-channel/top-gate a dual gate structure. TFTs. Inkjet n-Butyl etching acetate was was utilized used to as selectively the etching remove solvent PS for and PS, thus which to prepare does not p-channel/top-gatedamage the P(NDI2OD-T2) TFTs. n-Butyl film. acetate Subsequent was used deposi as thetion etching of the solvent P(VDF-TrFE) for PS, whichdielectric does onto not the damageselectively the P(NDI2OD-T2) etched PS film film. resulted Subsequent in the deposition direct contact of the of P(VDF-TrFE) the P(VDF-TrFE) dielectric film onto with the the selectivelyP(NDI2OD-T2) etched PS film, film while resulted PS/P(VDF-TrFE) in the direct contact dual ofdiel theectric P(VDF-TrFE) was fabricated film with with the a P(NDI2OD-T2) view point from film,the while bottom-gate PS/P(VDF-TrFE) structure. dual The dielectric optical microscopy was fabricated images with in a viewFigure point 9b,c from show the that bottom-gate only the PS region was etched in the p-channel TFTs. The formation of a crater-like deposit in the soft-etched area is evident in Figure 9d. The integration of the p-channel TFT and the n-channel TFT resulted in the fabrication of the integrated circuit in Figure 9e. The use of the P(VDF-TrFE) dielectric increases the number of hole carriers and that of the PS dielectric increases the number of electron carriers. These features are illustrated schematically in Figure 9f,g. P(VDF-TrFE) and PS contain different functional groups, so their dipole moments take up opposing directions, which means that the band-bending of these materials occurs in the opposite directions. Finally, they succeeded in

Polymers 2017, 9, 441 11 of 16 structure. The optical microscopy images in Figure9b,c show that only the PS region was etched in the p-channel TFTs. The formation of a crater-like deposit in the soft-etched area is evident in Figure9d. The integration of the p-channel TFT and the n-channel TFT resulted in the fabrication of the integrated circuit in Figure9e. The use of the P(VDF-TrFE) dielectric increases the number of hole carriers and that of the PS dielectric increases the number of electron carriers. These features arePolymers illustrated 2017, 9, 441 schematically in Figure9f,g. P(VDF-TrFE) and PS contain different functional groups,11 of 16 so their dipole moments take up opposing directions, which means that the band-bending of these materialsfabricating occurs a ring in theoscillator opposite with directions. a frequency Finally, ofthey 16.7succeeded kHz by employing in fabricating an ainkjet-based ring oscillator etching with atechnique. frequency of 16.7 kHz by employing an inkjet-based etching technique.

FigureFigure 9. 9. ((aa)) SchematicSchematic diagramdiagram ofof thethe fabricationfabrication ofof anan inverterinverter withwith thethe inkjet-etchinginkjet-etching technique.technique. Poly([Poly([NN,N,N0-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,50-(2,20′-bithiophene))-(2,2′-bithiop (P(NDI2OD-T2))hene)) (P(NDI2OD-T2)) was used was used as the as the active active layer laye andr and poly(vinylidenefluoride-trifluoroethylene) poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE))(P(VDF-TrFE)) andand PS/P(VDF-TrFE)PS/P(VDF-TrFE) werewere usedused asas dielectricdielectric layerslayers forfor p-channelp-channel andand n-channeln-channel transistors,transistors, respectively.respectively. OpticalOptical microscopymicroscopy imagesimages ofof thethe PS-coatedPS-coated ((bb)) andand PS-etchedPS-etched ((cc)) regions.regions. (d(d)) AFMAFM imageimage of of a a inkjet-etched inkjet-etched PS PS film film on ona P( aNDI2OD-T2) P(NDI2OD-T2) layer, layer, showing showing the presence the presence of PS ofmicroholes. PS microholes. (e) Circuit (e) Circuit diagram diagram of ofthe the complementary complementary inverter. inverter. Band Band diagrams diagrams of thethe semiconductor-dielectricsemiconductor-dielectric contactcontact regionsregions ofof thethe n-channeln-channel ((ff)) andand p-channelp-channel ((gg)) transistors.transistors. Band-bendingBand-bending is is determined determined by by the the different different functional functional groups groups in the indielectrics the dielectrics [47]. Copyright[47]. Copyright 2013 American2013 American Chemical Chemical Society. Society.

An inkjet-etched microwell can be utilized as a platform for the crystallization of organic An inkjet-etched microwell can be utilized as a platform for the crystallization of organic semiconductors [15,28]. Kwak et al. fabricated two kinds of microwells with an inkjet-etching semiconductors [15,28]. Kwak et al. fabricated two kinds of microwells with an inkjet-etching technique: a PVP microwell was fabricated through the inkjet printing of ethanol, and a technique: a PVP microwell was fabricated through the inkjet printing of ethanol, and a poly(4-methyl poly(4-methyl styrene) (P4MS) microwell was fabricated through the inkjet printing of toluene [28]. styrene) (P4MS) microwell was fabricated through the inkjet printing of toluene [28]. They examined They examined the formation of these microwells by successive dropping of each solvent onto a the formation of these microwells by successive dropping of each solvent onto a polymer ﬁlm polymer film (Figure 10a). The diameter and depth of the hole increase with increases in the (Figure 10a). The diameter and depth of the hole increase with increases in the number of drops. number of drops. The height of the rim also increases due to the transfer of the polymer material The height of the rim also increases due to the transfer of the polymer material because of the because of the coffee staining effect (Figure 10b). Seven drops of solvent were necessary for the full coffee staining effect (Figure 10b). Seven drops of solvent were necessary for the full removal removal of the polymer layer. The fabricated microwells were crosslinked before the subsequent of the polymer layer. The fabricated microwells were crosslinked before the subsequent inkjet inkjet printing of 6,13-bis(triisopropylsilylethynyl)pentacene) (TIPS_PEN), which is a well-known printing of 6,13-bis(triisopropylsilylethynyl)pentacene) (TIPS_PEN), which is a well-known organic organic semiconductor for OTFTs [48–52]. The inkjet-etched microwell banks were then utilized to increase the patterning accuracy of the TIPS_PEN deposits. When the PVP bank was used, highly crystalline TIPS_PEN crystals were formed as a result of contact line pinning and subsequent crystallization at the bank edge (Figure 10c). The printing of the TIPS_PEN solution onto the P4MS bank resulted in small crystallites due to the receding of the contact line and crystallization at the bottom of the substrate (Figure 10d). The TIPS_PEN TFT containing a PVP bank was found to exhibit electrical properties that were superior to those containing a P4MS bank; surface treatment with hexamethyldisilazane led to a further increase in the mobility (Figure 10e,f). The hydrophilic PVP bank was found to induce better contact line pinning and crystal formation on the bank edge.

Polymers 2017, 9, 441 12 of 16 semiconductor for OTFTs [48–52]. The inkjet-etched microwell banks were then utilized to increase the patterning accuracy of the TIPS_PEN deposits. When the PVP bank was used, highly crystalline TIPS_PEN crystals were formed as a result of contact line pinning and subsequent crystallization at the bank edge (Figure 10c). The printing of the TIPS_PEN solution onto the P4MS bank resulted in small crystallites due to the receding of the contact line and crystallization at the bottom of the substrate (Figure 10d). The TIPS_PEN TFT containing a PVP bank was found to exhibit electrical properties that werePolymers superior 2017, 9 to, 441 those containing a P4MS bank; surface treatment with hexamethyldisilazane led12 to of a 16 further increase in the mobility (Figure 10e,f). The hydrophilic PVP bank was found to induce better contactThese line findings pinning indicate and crystal that the formation surface properties on the bank of edge.inkjet-etched These ﬁndings microwells indicate are important that the surface to their propertiesuse as platforms of inkjet-etched for the crystallization microwells are of important organic semiconductors. to their use as platforms for the crystallization of organic semiconductors.

Figure 10. (a) Optical microscopy images and (b) height profiles of inkjet-etched PVP films forFigure various 10. numbers(a) Optical of microscopy printed ethanol images droplets. and (b) height Polarized profiles optical of inkjet-etched microscopy PVP images films of for 6,13-bis(triisopropylsilylethynyl)pentacenevarious numbers of printed ethanol (TIPS_PEN) droplets. crystals Polarized that haveoptical been inkjetmicroscopy printed onimages a PVP of bank6,13-bis(triisopropylsilylethynyl)pentacene (c) and a poly(4-methyl styrene) (P4MS) bank (TIPS_PEN) (d). Electrical crystals properties that have of triisopropylsilylethynylbeen inkjet printed on a pentacenePVP bank (TIPS_PEN) (c) and thin-filma poly(4-methyl transistor (TFTs)styrene) on (P4MS) the PVP bank bank ( de)). andElectrical the P4MS properties bank (f ).of triisopropylsilylethynyl pentacene (TIPS_PEN) thin-film transistor (TFTs) on the PVP bank (e) and The term ‘bare’ indicates a SiO2 dielectric without surface treatment and ‘HMDS’ indicates a the P4MS bank (f). The term ‘bare’ indicates a SiO2 dielectric without surface treatment and ‘HMDS’ hexamethyldisilazane-treated SiO2 dielectric [28]. Copyright 2013 Wiley. indicates a hexamethyldisilazane-treated SiO2 dielectric [28]. Copyright 2013 Wiley.

Polymers 2017, 9, 441 13 of 16

4. Conclusions and Outlook We have reviewed recent reports of the fabrication of subtractive patterns in polymers through inkjet etching. Various types of polymer relief microstructures (i.e., microwells, microgrooves, hexagonal holes, and concave structures) have been fabricated with inkjet etching processes. The pattern shape and dimensions can be controlled by varying the volume of the droplet, the processing parameters, and the substrate temperature, and by modifying the polymer-solvent interaction. The preparation of via holes can be achieved by making use of the coffee staining effect, which is strongly related to the flows in drying polymer solutions that arise during inkjet etching processes. Thus, the in-situ observation of inkjet etching processes can be used to reveal the mechanism underlying the inkjet etching of polymers [53]. In addition, further study of the control of the flows in drying polymer solutions could enable the fabrication of new patterns [3,54]. In order to fully take benefit from inkjet printing technology, the polymer layer needs to be deposited by inkjet printing. In this case, the polymer layer is not flat mainly due to the coffee ring effect. In this regard, inkjet etching behaviors onto this nonflat polymer surface need to be studied for widening use of inkjet printing. The utilization of inkjet-etched microstructures in organic electronic devices was extensively reviewed. An inkjet-etched microring pattern has been directly utilized as an optical waveguide. The deposition of organic materials (i.e., emissive materials, liquid crystals, organic conductors, organic insulators, and organic semiconductors) onto pre-fabricated microstructures has been used to enhance the performance of organic electronic devices. Patterning of the emissive layer, liquid crystal, or organic semiconductor is feasible because inkjet-etched microstructures can be used as platforms for the selective deposition of organic materials. In addition, inkjet-etched via holes have been used as interconnections between electrodes, and inkjet etching has been utilized to etch unwanted dielectric materials in the preparation of OTFTs. Furthermore, inkjet-etched microwells have been utilized as platforms for the crystallization of organic semiconductors. Inkjet etching is an extremely simple process that does not require photolithography, and so can be used as a subtractive tool for the fabrication of various organic electronic devices. Reducing the feature size of inkjet etching products is vital to the commercialization of this technique in, for example, the display industry [55,56].

Acknowledgments: This paper was supported by Konkuk University in 2015. Author Contributions: Wi Hyoung Lee suggested this review and organized all sections. Wi Hyoung Lee and Yeong Don Park wrote the paper. Wi Hyoung Lee and Yeong Don Park made careful proofreading for the manuscript. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Calvert, P. Inkjet printing for materials and devices. Chem. Mater. 2001, 13, 3299–3305. [CrossRef] 2. De Gans, B.J.; Duineveld, P.C.; Schubert, U.S. Inkjet printing of polymers: State of the art and future developments. Adv. Mater. 2004, 16, 203–213. [CrossRef] 3. Singh, M.; Haverinen, H.M.; Dhagat, P.; Jabbour, G.E. Inkjet printing-process and its applications. Adv. Mater. 2010, 22, 673–685. [CrossRef][PubMed] 4. Belgardt, C.; Sowade, E.; Blaudeck, T.; Baumgartel, T.; Graaf, H.; von Borczyskowski, C.; Baumann, R.R. Inkjet printing as a tool for the patterned deposition of octadecylsiloxane monolayers on silicon oxide surfaces. Phys. Chem. Chem. Phys. 2013, 15, 7494–7504. [CrossRef][PubMed] 5. Tekin, E.; Smith, P.J.; Schubert, U.S. Inkjet printing as a deposition and patterning tool for polymers and inorganic particles. Soft Matter 2008, 4, 703–713. [CrossRef] 6. Yamada, K.; Henares, T.G.; Suzuki, K.; Citterio, D. Paper-based inkjet-printed microﬂuidic analytical devices. Angew. Chem. Int. Ed. 2015, 54, 5294–5310. [CrossRef][PubMed] 7. Tao, H.; Marelli, B.; Yang, M.M.; An, B.; Onses, M.S.; Rogers, J.A.; Kaplan, D.L.; Omenetto, F.G. Inkjet printing of regenerated silk ﬁbroin: From printable forms to printable functions. Adv. Mater. 2015, 27, 4273–4279. [CrossRef][PubMed] Polymers 2017, 9, 441 14 of 16

8. Teichler, A.; Perelaer, J.; Schubert, U.S. Inkjet printing of organic electronics-comparison of deposition techniques and state-of-the-art developments. J. Mater. Chem. C 2013, 1, 1910–1925. [CrossRef] 9. Kwon, J.; Takeda, Y.; Fukuda, K.; Cho, K.; Tokito, S.; Jung, S. Three-dimensional, inkjet-printed organic transistors and integrated circuits with 100% yield, high uniformity, and long-term stability. ACS Nano 2016, 10, 10324–10330. [CrossRef][PubMed] 10. Kwon, Y.J.; Park, Y.D.; Lee, W.H. Inkjet-printed organic transistors based on organic semiconductor/ insulating polymer blends. Materials 2016, 9, 650–665. [CrossRef][PubMed] 11. Kawase, T.; Sirringhaus, H.; Friend, R.H.; Shimoda, T. Inkjet printed via-hole interconnections and resistors for all-polymer transistor circuits. Adv. Mater. 2001, 13, 1601–1605. [CrossRef] 12. Cho, S.Y.; Ko, J.M.; Lim, J.; Lee, J.Y.; Lee, C. Inkjet-printed organic thin film transistors based on tips pentacene with insulating polymers. J. Mater. Chem. C 2013, 1, 914–923. [CrossRef] 13. James, D.T.; Kjellander, B.K.C.; Smaal, W.T.T.; Gelinck, G.H.; Combe, C.; McCulloch, I.; Wilson, R.; Burroughes, J.H.; Bradley, D.D.C.; Kim, J.S. Thin-film morphology of inkjet-printed single-droplet organic transistors using polarized raman spectroscopy: Effect of blending tips-pentacene with insulating polymer. Acs Nano 2011, 5, 9824–9835. [CrossRef][PubMed] 14. Kang, B.; Lee, W.H.; Cho, K. Recent advances in organic transistor printing processes. ACS Appl. Mater. Interfaces 2013, 5, 2302–2315. [CrossRef][PubMed] 15. Kjellander, B.K.C.; Smaal, W.T.T.; Anthony, J.E.; Gelinck, G.H. Inkjet printing of tips-pen on soluble polymer insulating films: A route to high-performance thin-film transistors. Adv. Mater. 2010, 22, 4612–4616. [CrossRef][PubMed] 16. Kwak, D.; Choi, H.H.; Kang, B.; Kim, D.H.; Lee, W.H.; Cho, K. Tailoring morphology and structure of inkjet-printed liquid-crystalline semiconductor/insulating polymer blends for high-stability organic transistors. Adv. Funct. Mater. 2016.[CrossRef] 17. Lim, J.A.; Kim, J.H.; Qiu, L.; Lee, W.H.; Lee, H.S.; Kwak, D.; Cho, K. Inkjet-printed single-droplet organic transistors based on semiconductor nanowires embedded in insulating polymers. Adv. Funct. Mater. 2010, 20, 3292–3297. [CrossRef] 18. Madec, M.B.; Smith, P.J.; Malandraki, A.; Wang, N.; Korvink, J.G.; Yeates, S.G. Enhanced reproducibility of inkjet printed organic thin film transistors based on solution processable polymer-small molecule blends. J. Mater. Chem. 2010, 20, 9155–9160. [CrossRef] 19. Xia, Y.J.; Friend, R.H. Nonlithographic patterning through inkjet printing via holes. Appl Phys. Lett. 2007, 90, 253513–253515. [CrossRef] 20. Wu, L.; Dong, Z.C.; Li, F.Y.; Zhou, H.H.; Song, Y.L. Emerging progress of inkjet technology in printing optical materials. Adv. Opt. Mater. 2016, 4, 1915–1932. [CrossRef] 21. Li, J.T.; Ye, F.; Vaziri, S.; Muhammed, M.; Lemme, M.C.; Ostling, M. Efficient inkjet printing of graphene. Adv. Mater. 2013, 25, 3985–3992. [CrossRef][PubMed] 22. Kim, D.; Lee, S.H.; Jeong, S.; Moon, J. All-ink-jet printed flexible organic thin-film transistors on plastic substrates. Electrochem. Solid State Lett. 2009, 12, 195–197. [CrossRef] 23. Alaman, J.; Alicante, R.; Pena, J.I.; Sanchez-Somolinos, C. Inkjet printing of functional materials for optical and photonic applications. Materials 2016, 9, 910–956. [CrossRef][PubMed] 24. Lee, W.H.; Park, Y.D. Organic semiconductor/insulator polymer blends for high-performance organic transistors. Polymers 2014, 6, 1057–1073. [CrossRef] 25. Lim, S.; Kang, B.; Kwak, D.; Lee, W.H.; Lim, J.A.; Cho, K. Inkjet-printed reduced graphene oxide/poly(vinyl alcohol) composite electrodes for flexible transparent organic field-effect transistors. J. Phys. Chem. C 2012, 116, 7520–7525. [CrossRef] 26. Jahn, S.F.; Engisch, L.; Baumann, R.R.; Ebert, S.; Goedel, W.A. Polymer microsieves manufactured by inkjet technology. Langmuir 2009, 25, 606–610. [CrossRef][PubMed] 27. Zhang, X.H.; Wei, X.X.; Ducker, W. Formation of nanodents by deposition of nanodroplets at the polymer-liquid interface. Langmuir 2010, 26, 4776–4781. [CrossRef][PubMed] 28. Kwak, D.; Lim, J.A.; Kang, B.; Lee, W.H.; Cho, K. Self-organization of inkjet-printed organic semiconductor films prepared in inkjet-etched microwells. Adv. Funct. Mater. 2013, 23, 5224–5231. [CrossRef] 29. Hwang, S.J.; Liu, Y.X.; Porter, G.A. Tunable liquid crystal microlenses with crater polymer prepared by droplet evaporation. Opt. Express 2013, 21, 30731–30738. [CrossRef][PubMed] Polymers 2017, 9, 441 15 of 16

30. Zhang, C.; Zou, C.L.; Zhao, Y.; Dong, C.H.; Wei, C.; Wang, H.; Liu, Y.; Guo, G.C.; Yao, J.; Zhao, Y.S. Organic printed photonics: From microring lasers to integrated circuits. Sci. Adv. 2015.[CrossRef][PubMed] 31. Wang, J.H.; Song, C.; Zhong, Z.M.; Hu, Z.H.; Han, S.H.; Xu, W.; Peng, J.B.A.; Ying, L.; Wang, J.; Cao, Y. In situ patterning of microgrooves via inkjet etching for a solution-processed oled display. J. Mater. Chem. C 2017, 5, 5005–5009. [CrossRef] 32. De Gans, B.J.; Hoeppener, S.; Schubert, U.S. Polymer-relief microstructures by inkjet etching. Adv. Mater. 2006, 18, 910–914. [CrossRef] 33. De Gans, B.J.; Hoeppener, S.; Schubert, U.S. Polymer relief microstructures by inkjet etching. J. Mater. Chem. 2007, 17, 3045–3050. [CrossRef] 34. Robin, M.; Kuai, W.L.; Amela-Cortes, M.; Cordier, S.; Molard, Y.; Mohammed-Brahim, T.; Jacques, E.; Harnois, M. Epoxy based ink as versatile material for inkjet-printed devices. ACS Appl. Mater. Interfaces 2015, 7, 21975–21984. [CrossRef][PubMed] 35. Nallan, H.C.; Sadie, J.A.; Kitsomboonloha, R.; Volkman, S.K.; Subramanian, V. Systematic design of jettable nanoparticle-based inkjet inks: Rheology, acoustics, and jettability. Langmuir 2014, 30, 13470–13477. [CrossRef][PubMed] 36. Jang, D.; Kim, D.; Moon, J. Influence of fluid physical properties on ink-jet printability. Langmuir 2009, 25, 2629–2635. [CrossRef][PubMed] 37. Kim, E.; Baek, J. Numerical study on the effects of non-dimensional parameters on drop-on-demand droplet formation dynamics and printability range in the up-scaled model. Phys. Fluids 2012.[CrossRef] 38. Pericet-Camara, R.; Bonaccurso, E.; Graf, K. Microstructuring of polystyrene surfaces with nonsolvent sessile droplets. Chem. Phys. Chem. 2008, 9, 1738–1746. [CrossRef][PubMed] 39. Zhang, Y.; Liu, C.; Whalley, D.C. The penetration limit of poly(4-vinyl phenol) thin films for etching via holes by inkjet printing. Appl. Phys. Lett. 2012, 101, 253302–253304. 40. Zhang, Y.; Liu, C.; Whalley, D.C. The impact of substrate temperature on the size and aspect ratio of inkjet-dissolved via holes in thin poly(4-vinyl phenol) dielectric layers. Appl. Phys. Lett. 2013, 102, 103303–103305. 41. Driscoll, M.; Delmotte, B.; Youssef, M.; Sacanna, S.; Donev, A.; Chaikin, P. Unstable fronts and motile structures formed by microrollers. Nat. Phys. 2017, 13, 375–379. [CrossRef] 42. Nagatsu, Y.; Ishii, Y.; Tada, Y.; De Wit, A. Hydrodynamic fingering instability induced by a precipitation reaction. Phys. Rev. Lett. 2014.[CrossRef][PubMed] 43. Grimaldi, I.A.; Del Mauro, A.D.; Nenna, G.; Loffredo, F.; Minarini, C.; Villani, F. Microstructuring of polymer films by inkjet etching. J. Appl. Polym. Sci. 2011, 122, 3637–3643. [CrossRef] 44. Bao, B.; Jiang, J.K.; Li, F.Y.; Zhang, P.C.; Chen, S.R.; Yang, Q.; Wang, S.T.; Su, B.; Jiang, L.; Song, Y.L. Fabrication of patterned concave microstructures by inkjet imprinting. Adv. Funct. Mater. 2015, 25, 3286–3294. [CrossRef] 45. Lee, J.N.; Park, C.; Whitesides, G.M. Solvent compatibility of poly(dimethylsiloxane)-based microfluidic devices. Anal. Chem. 2003, 75, 6544–6554. [CrossRef][PubMed] 46. Lee, W.H.; Suk, J.W.; Lee, J.; Hao, Y.F.; Park, J.; Yang, J.W.; Ha, H.W.; Murali, S.; Chou, H.; Akinwande, D.; et al. Simultaneous transfer and doping of cvd-grown graphene by fluoropolymer for transparent conductive films on plastic. ACS Nano 2012, 6, 1284–1290. [CrossRef][PubMed] 47. Khim, D.; Baeg, K.J.; Kang, M.; Lee, S.H.; Kim, N.K.; Kim, J.; Lee, G.W.; Liu, C.; Kim, D.Y.; Noh, Y.Y. Inkjet-printing-based soft-etching technique for high-speed polymer ambipolar integrated circuits. ACS Appl. Mater. Interfaces 2013, 5, 12579–12586. [CrossRef][PubMed] 48. Giri, G.; Verploegen, E.; Mannsfeld, S.C.B.; Atahan-Evrenk, S.; Kim, D.H.; Lee, S.Y.; Becerril, H.A.; Aspuru-Guzik, A.; Toney, M.F.; Bao, Z.A. Tuning charge transport in solution-sheared organic semiconductors using lattice strain. Nature 2011, 480, 504–508. [CrossRef][PubMed] 49. Anthony, J.E.; Brooks, J.S.; Eaton, D.L.; Parkin, S.R. Functionalized pentacene: Improved electronic properties from control of solid-state order. J. Am. Chem. Soc. 2001, 123, 9482–9483. [CrossRef][PubMed] 50. Lee, W.H.; Kim, D.H.; Jang, Y.; Cho, J.H.; Hwang, M.; Park, Y.D.; Kim, Y.H.; Han, J.I.; Cho, K. Solution-processable pentacene microcrystal arrays for high performance organic field-effect transistors. Appl. Phys. Lett. 2007, 90, 132106–132108. [CrossRef] 51. Lee, W.H.; Min, H.; Park, N.; Lee, J.; Seo, E.; Kang, B.; Cho, K.; Lee, H.S. Microstructural control over soluble pentacene deposited by capillary pen printing for organic electronics. ACS Appl. Mater. Interfaces 2013, 5, 7838–7844. [CrossRef][PubMed] Polymers 2017, 9, 441 16 of 16

52. Lim, J.A.; Lee, H.S.; Lee, W.H.; Cho, K. Control of the morphology and structural development of solution-processed functionalized acenes for high-performance organic transistors. Adv. Funct. Mater. 2009, 19, 1515–1525. [CrossRef] 53. Lim, J.A.; Lee, W.H.; Kwak, D.; Cho, K. Evaporation-induced self-organization of inkjet-printed organic semiconductors on surface-modiﬁed dielectrics for high-performance organic transistors. Langmuir 2009, 25, 5404–5410. [CrossRef][PubMed] 54. Lim, J.A.; Lee, W.H.; Lee, H.S.; Lee, J.H.; Park, Y.D.; Cho, K. Self-organization of ink-jet-printed triisopropylsilylethynyl pentacene via evaporation-induced ﬂows in a drying droplet. Adv. Funct. Mater. 2008, 18, 229–234. [CrossRef] 55. Sekitani, T.; Noguchi, Y.; Zschieschang, U.; Klauk, H.; Someya, T. Organic transistors manufactured using inkjet technology with subfemtoliter accuracy. Proc. Natl. Acad. Sci. USA 2008, 105, 4976–4980. [CrossRef] [PubMed] 56. Sele, C.W.; von Werne, T.; Friend, R.H.; Sirringhaus, H. Lithography-free, self-aligned inkjet printing with sub-hundred-nanometer resolution. Adv. Mater. 2005, 17, 997–1001. [CrossRef]

© 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).