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

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