Inkjet Printed Organic Thin Film Transistors

Materials Science Forum Vol. 736 (2013) pp 250-274 © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.736.250

Inkjet Printed Organic Thin Film Transistors: Achievements and Challenges Saumen Mandal1,a, Gangadhar Purohit1,b, and Monica Katiyar1,c 1

Department of Materials Science & Engineering and Samtel Centre for Display Technology, Indian Institute of Technology Kanpur, Kanpur 208016, India a [email protected], [email protected], [email protected] Keywords: inkjet printing, organic thin film transistor, ink, applications

Abstract Inkjet printing of organic thin film transistors is an enabling technology for many applications requiring low cost electronics such as RFID tags, sensors, e-paper, and displays. This review summarizes the achievements and remaining challendges in the field. An all inkjet printed organic thin film transistor is feasible, but manufacturability needs to be improved. Often, a hybrid process in which only some layers are inkjet printed is used. Development of devices requires optimization of (1) ink chemistry, (2) inkjet process, (3) substrate ink interaction, and (4) new device structures. Several conducting, dielectric and semiconducting materials have been used to formulate ink. It appears that metal nanoparticle based conducting ink and PEDOT:PSS are widely used materials to fabricate source, drain and gate electrodes. PVPh is the most popular dielectric material for inkjet printing. To print semiconducting layer, both polymers and oligomers/small molecules are used. Many high performance organic semiconductors are p-type, but few n-type organic semiconductors show excellent performance. In addition to improved materials, challenges inherent in the inkjet process also need solutions. These are registration, alignment of the source,and drain with gate, resolution, reducing off-state current, and roll-to-roll processing. Introduction One of the important inventions of 20th century is solid state transistor – it revolutionized the microelectronics industry. It gave a new dimension to electronics, making possible consumer electronics like personal computers, video cameras, mobile phones, etc. A close relative to metaloxide-semiconductor transistor is amorphous silicon thin film transistor; it added the functionality of making electronics on large areas, e.g. information display devices. Organic thin film transistors provide an alternative to a-Si for flexible, transparent and low cost electronics. The fact that organic devices can be solution processed opens the door for utilization of techniques like printing to manufacture these devices and reduce the cost further. Many conventional printing technologies, gravure, offset, flexographic, screen and inkjet, are being experimented with to make OTFTs. In this review, inkjet is selected among all the printing techniques because it has several advantages: (1) non contact pattering method, (2) patterning and material deposition is integrated in one step, (3) the printing speed of inkjet is sufficiently high for mass production and (4) inkjet printing is very flexible and adaptable because computers generate printing patterns. Inkjet is all additive process (zero material wastage) and provides highest resolution among available printing techniques. The focus of this review is on inkjet printed organic thin film transistors. First organic thin film transistor (OTFT) was reported in 1983 [1]. Since then, research and development on OTFTs has resulted in its application in electronic paper, RFID tag and organic sensors (Samsung, SONG, NOKIA). Typically, standard microelectronics fabrication processes, thin film deposition, photolithography and etching, are used to make OTFTs. The first inkjet printed OTFT was reported in 2000 [2], where spin coating of semiconductor and insulator layers and inkjet printing of metal electrodes was used. Inkjet printed transistors can be classified in two categories: all-inkjet- and partial-inkjet-printed transistors. All inkjet printed transistor has all All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 115.248.114.51-05/11/12,06:17:22)

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layers of OTFT printed using inkjet printer. Whereas, at least one layer of the transistor is fabricated by inkjet printing in partial-inkjet-printed transistors. All-inkjet-printed transistor was first reported in 2004[3]. Considering current limitations of all-inkjet-printed devices, partialinkjet-printed organic transistors have become popular. In this chapter, partial-inkjet-printed OTFTs were categorized by the inkjet printed layer - electrode, dielectric and semiconductor. Finally, approaches to overcome the challenges of inkjet printing, inkjet encapsulation and applications of inkjet printed OTFTs are discussed. Operation of inkjet printer Inkjet printing is a five decades old technique used worldwide for conventional printing on paper, glass or ceramic. Basically, there are two parts to an inkjet printer: print-head and cartridge (ink reservoir). There are two types of print-heads - thermal and piezo, based on the mechanism used to create a droplet. Printing is done by software controlled print head that allows deposition of drops in a desired pattern. It is also amenable to making three-dimensional layered structures for making electronic devices. A big challenge and opportunity for material scientists is in discovering new functional ink and controlling interfaces between different layers. This has been the subject matter of most research papers recently. Also, a major challenge is low resolution of ink-jet printing. By using inkjet printing, making features smaller than 5µm is a big challenge, where as submicron feature can be done easily by photolithography technique. On the equipment side, there are many technological barriers/limitations to overcome. One current limitation is the lack of a simple robust inkjet printing system that could use a wide range of solutions and suspensions. In drop-on-demand peizo-inkjet printing, print head generates a piezo pulse according to applied voltage, it results in deformation of cartridge bag’s upper membrane; and a drop is generated at the nozzle and falls on the substrate. According to the printer head’s command, pattern will be generated on the substrate. Since organic materials are very sensitive to temperature, thermal drop on demand inkjet printer may degrade the ink. Hence, peizo-inkjet printing is more prevalent. Material parameters for printing Ink properties. In case of printing, viscosity and surface tension play important role. Surface tension decides satellite formation, which hampers resolution. Spreading of ink after printing depends on droplet volume and the wetability of the liquid on the surface. The spreading process of a solution is determined by both, the kinetic energy of the droplet and the interfacial free energy change between substrate and ink. [4]. Choice of solvent. Inkjet printing is a technique based on ink chemistry, so choice of solvent and its purity is crucial. Plotner et al used four different solvents, chloroform, trichloroethylene, chlorobenzene and xylene, to print poly-3-octylthiophene (P3OT) as active material in OTFTs [5]. Printing was not possible when chloroform was used as a solvent for P3OT, because of its fast evaporation it formed residue at the micropipette outlet, whereas trichloroethylene and chlorobenzene based solutions yielded reproducible single drops of size 200250 µm and 160-240 µm in diameter, respectively. Two problems were noticed, which are (1) huge bulge formation at the outer circular edge of the dot and (2) some annular waviness on the drop surface. The first one can be easily solved, if these edges are located outside the channel area, which is generally true for electrode spacing (10-50 µm). The second problem may be overcome by overlapping of single dots to form a printed semiconducting line. Lines were printed using trichlroethylene and chlorobenzene, and xylene as solvent. From trichloroethylene based semiconducting ink, printed line is having non-regular single dots as well as a flake like overlap of the dots, within the line, due to faster evaporation of trichloroethylene. Higher boiling point of chlorobenze and xylene is one of the reasons for better property of the transistors using P3OT films made from these solvents. The xylene based transistor showed negative threshold voltage due to its higher degree of purity compared to the chlorobenzene based one.

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Advances in Materials Development

Joung et al also observed that the performance of OTFT depends on optimization of inkjet printing parameters for P3HT, such as choice of solvent and use of hydrazine as a dedoping agent [6]. Three different solvent, chloroform, chlorobenzene and p-xylene were used, but higher mobilities (4.3×103 cm2/Vs) were observed in p-xylene than the other solvent, possibly because slow evaporation of the solvent enabled slower growth of the films and therefore allowed ordering. Effect of hydrazine as a dedoping agent was to enhance the mobility (1.72×10-5 cm2/Vs to 1.97×10-3 cm2/Vs), and improve saturation. Kim et al observed the effect of solvents on final transistor properties when triisopropylsilyl (TIPS) pentacene film was made by inkjet printing [7]. Before inkjet printing of TIPS pentacene, PVPh surface was treated with hexamethyldisilazane (HMDS). Finally, TIPS pentacene was ink jetted using three different solvents to formulate ink, and final transistor properties were compared. The OTFT fabricated from chloroform solution showed field effect mobility of 10-5cm2/Vs, whereas from chlorobenzene and anisole solutions the mobility was increased to 0.01 cm2/Vs and 0.04 cm2/Vs, respectively. TIPS pentacene films from high boiling point solvents had dendritic morphology, whereas chloroform based film was amorphous. Role of surfactants. Tailoring of viscosity and surface tension of PEDOT:PSS ink to make it printable was studied by Lopez et al [8]. Viscosity requirement for Dimatrix printer (DMP) is 112 cP, where as viscosity of as received PEDOT:PSS was 52 cP. Dilution using water (PEDOT:PSS:H2O) reduced viscosity linearly to 22.4cPs (3:1), 14 cP (3:2) and 9.8 cP (1:1). Although viscosity of 1:1 composition was in the acceptable range, nozzles in the printer cartridge were clogged during printing. So, use of surfactant was important. Three different surfactants were used dimethyl sulfoxide (DMSO), ethylene glycol (EG) and Triton X, in different combination and that allowed printing without clogging of the nozzles. Substrate. Substrate is an important part of printed electronics, it may be rigid or flexible, but it should have properties like impermeable, low thermal expansion coefficient, low roughness, chemically inert, mechanically robust, should withstand the processing temperatures etc. Silicon is widely used substrate in the field, because of its excellent semiconducting properties, tendency to form oxide easily, low surface roughness and comparatively lower cost compared to other inorganic substrates. But, main challenges with silicon substrates are its rigidity and opaque nature, as well as high cost for large area devices. Glass is an alternative rigid substrate having high transparency made of SiO2. Thermal expansion coefficient of corning glass used in display industry is almost similar to silicon [9]. Glass is also chemically inert and thermally very stable to withstand high temperature. Glass substrate is totally impermeable; water penetration level is around 10-12 g/m2 per day [10]. But, when flexibility is the concern, thin flexible glass, metal foil, plastic and paper are the options. Glass becomes flexible when its thickness is melting point of AgNO3, 212oC). The resulting molten AgNO3 flowed on the SiO2 surface and gave irregular shape printing. But, Xue et al introduced one poly (4vinylphenol) layer on top of SiO2 layer before AgNO3 printing, and surprisingly, curing temperature was lowered to 210oC and smooth continuous film was obtained. Barret et al observed that source/drain electrode and semiconductor interface play an important role in deciding the final performance of organic thin film transistor device [30]. In their work, contact resistance was compared for two semiconducting polymer: poly (3-hexylthiophene-2,5-diyl) (P3HT) and poly(3,3’-didodecyl-quarterthiophene) (PQT-12) in contact with different source and drain electrodes (evaporated gold, sputtered platinum, inkjetted silver nanoparticles and inkjetted PEDOT:PSS). The influences of source and drain work function, semiconductiong polymer ionization potential and nature of the interface (metal/polymer or polymer/polymer) on contact

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resistance were discussed. Best transistor performance were observed in case of Au-PQT-12 and PEDOT:PSS- PQT-12 combination, because their work function and ionization potential are almost same. But, minimum contact resistance were observed in case of PEDOT:PSS-PQT-12 combination (0.36MΩcm at Vg= -80V). This is because of the improvement of interfacial doping of the semiconducting polymer close to the contact. It leads to decrease in the contact resistance and an additional dipole is created at the interface that shifts the HOMO level downwards. Whereas, Au and printed silver showed poor OTFT performance because of poor injection due to difference in work function and ionization potential. Noguchi et al fabricated top-contact-bottom-gate OTFT where source and drain electrodes was printed on organic semiconducting film. This device had TFT characteristics comparable to that of a device with evaporated source and drain electrodes [31]. Liu et al used a low cost ink for inkjet printing to fabricate high conductivity electrode [32]. A solution of 48 wt% silver nitrate (AgNO3), 25 wt% water and 27 wt% dimethyl sulfoxide (DMSO) filtered through a 0.4 µm syringe was used Table 2 Properties of the film made from different conducting inks. Material used

Substrate

Processing temperature (oC)

Conductivity (S/cm)

Ref. no.

Au-C4

glass

195

1.0×105

23

Au-C4, C6, C8

PET

155

1.51x103

24

5

25

Au-C6

Si

Laser sintering

1.85x10

AgNO3

Si

210

-

26 3

27 27

Ag-Cu nanoparticle

Si

200

7.3×10

Ag precursor

Si

250

1.7×105

Ag nanoparticle Ag nanoparticle

Si Si

200 Laser sintering

6.17x10

5

28

3.12x10

5

29

4

30

Ag nanoparticle

Si

130

5.88x10

Ag nanoparticle

PI

-

-

31

AgNO3

PI

300

6.67x104

32

Ag nanoparticle

glass

150

-

33

Silver metal organic precursor

glass

-

-

34

Cu nanoparticle

glass

325

5.81x104

35

1.42x10

5

36

5

37

Cu nanoparticle

-

-

Cu metal-organic

glass

320

2.27x10

Nano Au-organic composite

glass

300

105

38

Sb doped SnO2

glass

440, laser treatment

-

39

PEDOT:PSS

Glass, Si, quartz, plastic

-

-

2,4, 40-46

polypyrrole

Polyethylene sulfone

-

-

47

Au nanoparticle

-

Laser curing

7.14x106

48

as an ink. The work function of PEDOT:PSS, SPANI and Ag are 5.0-5.2, 5.1 and 4.3eV. However, in the inkjet printed Ag films, the chemisorption of oxygen on Ag surface usually increased the work function by 0.6-1.0 eV because the chemisorbed oxygen atoms capture electrons from the underlying Ag atoms. As a result, the printed Ag line work function was closer to that of SPANI or PEDOT:PSS.


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Kim et al used inkjet printing to fabricate all three electrodes of the transistor [33]. Chung et al compared the contact resistance between Ag source/drain and pentacene layer for inkjettted Ag and evaporated Ag [49]. Contact resistance of inkjetted Ag and evaporated Ag are 1.79 Mohm.cm and 0.55 Mohm.cm, respectively. Osch et al optimized printing parameters of silver nanoparticle based ink [50]. Inkjet printing was performed on different substrates, like poly (tetrafluorethylene) (PTFE), polyarylate, PET and polyimide (PI). They concluded that the surface energy of substrate should not be too low, because it promotes bulge formation in the printed feature. This happened when printing was done on PTFE substrate having surface tension 17-22mN/m. Again, printing on PET and PI (surface tension of 38mN/m and 41mN/m) resulted in continuous and straight lines, but broad lines were obtained due to the wetting of the substrate by solvent. Polyarylate polymeric was best suited for printing by their ink. Copper ink. Park et al optimized inkjet printing of copper ink to make copper film of 17.2 µΩ cm resistivity[35]. The printed conductive films were annealed at various temperatures between 200°C and 350°C in vacuum (10−3 Torr). The resistivity decreased with increasing annealing temperature, there was a drastic change at around 250°C, indicating that conduction path between the particles was established by interparticle neck formation. The resistivity became almost constant after 325 °C. After heat treatment at 325°C for 1 h, the resistivity of 17.2 µΩ cm was obtained, this is about ten times higher than that of the bulk copper (1.7 µΩ cm). Kumashiro et al also optimized inkjet printing of nano copper ink which was formulated by laser irradiation of a copper complex in the liquid phase[36]. The generated copper nano-particles were covered by thin oxide layers. Therefore printed copper trace was treated with atomic hydrogen for metallization. The color of the traces changed from black to bronze, and the resistivity of the traces decreased to 7 µΩcm, where as resistivity of bulk copper is 1.6 µΩcm. Lee et al developed a copper metal-organic-based conductive ink which can be applied to printing and roll-to-roll processes [37]. The conductive copper ink was prepared by mixing the copper (II) neodecanoate and the nano-sized copper hydroxide powder using three-roll-milling in terpineol. Metal nanoparticle based composite ink. Dispersed Au nanoparticle in organic solvent is well established conducting ink, but ultra thin films of these materials have a tendency to crack during annealing because of the large intrinsic volume reduction that accompanies complete sintering of the clusters and the weak substrate adhesion and poor film cohesion in the nanometal. In addition, they require organic solvents such as toluene which can potentially harm underlying organic semiconductor layer. Finally, to deposit a 100 nm thick film by a single pass printing, for example the concentration of Au cluster should be >50mg/ml. Such a high concentration has been achieved only in few organic solvents. To overcome all these challenges, Sivaramakrishnan et al introduced controlled insulator to metal transformation in printable polymer composite with nanometal clusters [38]. He used modified Brust process, the monolayer protected Au clusters thus obtained were in the form of a salt with surface COO- charges compensated by -OC4N+ ions that were exceedingly soluble in the lower alcohols (for example, for a 2.2 nm Au cluster, 75mg/ml in methanol). They could be repeatedly isolated in dry waxy from and redispersed without ultrasonication. They found that those -OC4N+ surface ions could further be ion exchanged with Na+ to give a free flowing powder that is exceedingly soluble in water (70 mg/ml). These dispersions were mixed with corresponding solutions of poly (3,4-ethylenedioxythiophene) : poly (styrenesulphonate) (PEDT) or poly (4-hydroxystyrene) (PHOST) with a wide composition range of Au of up to 70 vol% without phase separation. This composite ink was stable for more than one year at room temperature. Film could be made of this ink by printing, drop casting and spin coating. Conductivity of the film made of nano Au ink and composite ink was measured with varying annealing temperature. Once percolative insulator to metal transformation temperature (Tp) was reached, conductivity increased drastically and finally it was in the range of 105 S/cm. Besides this, there was no crack formation in the films made of composite ink baked at 240oC, where as baked films made of Au cluster dispersion had crack formation at the same temperature.

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Inorganic oxide electrode. There is also extensive research going for developing inorganic oxide based inks, like antimony-doped tin oxide, nano sized SnO2 particles dispersed in acetylene black etc. Cranton et al introduced low temperature fabrication of transparent electrode element from thin films of antimony doped tin oxide (SnO2:Sb, ATO) by inkjet printing followed by excimer laser processing [439]. Conducting polymer. Poly(3,4-ethylenedioxythiophene) doped by poly (styrenesulfonate) (PEDOT:PSS) and polyaniline (PANI) are important conducting polymers used as conducting ink to fabricate electrodes. Kawase et al reported on inkjet-printed polymer electrodes and their applications [4]. Source and drain was printed first on glass substrate by inkjet printing of PEDOT:PSS ink. Minimum channel length was maintained 50 µm. F8T2 and PVPh were spin coated to make semiconducting and insulator layer. Finally, PEDOT:PSS was printed on overlapped portion of channel. Lim et al also inkjet printed PEDOT:PSS to fabricate source/drain electrode [40]. It was observed that changing the ink chemistry of PEDOT:PSS improves the thin film transistor property. Conductivity and film properties were significantly improved by the addition of polyalcohols or high dielectric solvents with PEDOT:PSS ink. Dimethyl sulfoxide (DMSO) was mixed into the PEDOT:PSS solution with a volume ratio of 1:5, significant improvement was noticed in the transistor properties. Conductivity was improved from 0.04 to 0.8 S/cm with the addition of DMSO. Contact resistance of semiconductor and PEDOT:PSS was also improved by this treatment. This improvement of contact resistance was explained by the scanning electron microscopy (SEM) images. The untreated PEDOT had a rough and irregular edge and dewetting occurred between the electrodes and P3HT film, where as DMSO treated PEDOT electrode had improved interfacial stability, resulting in a smooth edge and no dewetting in the contact region. Natori et al used PEDOT:PSS ink and glycerol modified PEDOT:PSS ink for inkjet printing on three different plastic substrates, polyester, polyacetylene and polyethylene terephthalate (PET) [51]. After printing and baking, sheet resistance of conducting polymers on different substrate was measured. On polyester substrate, inkjet printed PEDOT:PSS and glycerol modified PEDOT:PSS films’sheet resistance was 6.674×105Ω/□ and 1.985×105Ω/□, respectively. Whereas, sheet resistance on PET substrate was 1.832×106Ω/□ and 1.327 ×106Ω/□ for PEDOT:PSS and modified PEDOT:PSS film, respectively. PEDOT:PSS film on polyacethylene substrate was 1.022×106Ω/□. So, glycerol modification decreased sheet resistance and polyester substrate was best suited. Basiricò et al demonstrated inkjet printing of PEDOT:PSS to fabricate source/drain and gate in flexible and transparent organic thin film transistors and organic electrochemical transistors [46]. 6,13-bis (triisopropylsilylethynyl) pentacene for p-type and N1400 for n-type OTFTs were selected for their study. Shin et al demonstrated inkjet printing and vapour deposition polymerization (VDP) to fabricate inkjetted conducting polymer polypyrrole (PPy) source/drain for low contact resistance with pentacene compared to that of pentacene-Au combination [47]. Printing was performed using an initiator solution consisting of ammonium persulfate (20 wt.%) and poly (4-styrenesulfonate) as an additive in distilled water. After printing, devices with printed initiator were exposed to vapour of pyrrole monomers in a chamber with vaccum level of 10-1 Torr for 10 min for vapour deposition polymerization. Au was evaporated first to make gate on polyethylenesulfone substrate, then gate dielectric was made of spin coated crosslinked PVPh. PPy was printed on top of the evaporated pentacene active layer. A better OTFT performance was observed with PPy source/drain (fieldeffect mobility of 0.18 ± 0.03 cm2/V s, a threshold voltage of -1.99 V) compared to devices with Au source/drain (field-effect mobility of 0.15 ± 0.02 cm2/ Vs, a threshold voltage of -10.09 V). Barros et al introduced another route to prepare conducting polymer pattern on substrates, like plastic, transparent sheet, glossy paper [52]. Aniline and pyrrole were distilled twice under atmospheric pressure and stored in dark and at low temperature prior to synthesis. Solution of aniline and pyrrole, of desired concentration, in nitric acid and silver nitrate in aqueous solution was prepared and stored at low temperature before use. Then the patterns (lines and characters) were designed on a computer using a drawing software and printed on a substrate (paper, glossy paper or


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transparent sheet) previously soaked in an aqueous solution of the conducting polymer monomer. In the next step, a germicide lamp 20W was used to reveal the patterns. The reaction was instantaneous, as soon as the glossy film interacts with the light, the green or black (depends on the monomer aniline or pyrrole) polymer patterns was revealed as an indication that the conducting polymer is synthesized. The green color was a clear indication of synthesis of conducting polyaniline, which is confirmed by absorption spectra also. It is interesting to mention here, that the adherence of the conducting polymer to the substrate was very high, patterns remains even after washing with acid or base solution thoroughly. Annealing of films The annealing temperature of inkjet printed films is limited by the thermal budget of the substrate, especially for flexible polymer substrates. Therefore, there is need to find new ways for annealing conducting films, expecially based on metallic nanoparticles. Laser sintering. Ko et al used laser sintering technique to fabricate inkjet printed source and drain with lower channel length (down to 1-2µm) which was difficult to make by simple inkjet printing (~100µm) only [19]. First, inkjet printing (Microfab, 50µm nozzle diameter) of nano-gold having grain size 1-3 nm caped with thiol dissolved in alpha- terpineol (10wt%), was done on 25µm thick polymer substrate. The selectively laser sintered nanoparticle line produced a conductor line several micron wide while the original inkjet printed line had a width of ~100µm. Selective pulsed laser ablation by differential ablation threshold (SPLA-DAT) and laser sintering was introduced to fabricate organic thin film transistor [18]. Nano Au ink was printed first on polyimide film. Laser sintering was performed to sinter Au film. For transistor, after printing of nano Au on PVPh film, a 7 µm channel was made by SPLA-DAT. Most striking of this laser ablation is that underlying dielectric film was not affected at all. Bieri et al introduced laser curing of inkjet printed film to make conducting line [48]. Microwave sintering. Perelaer et al introduced microwave sintering to sinter inkjet printed silver film deposited on polymer foil [53] to overcome the large overall thermal energy impact together with the low writing speed of 0.2 mm/sec of translational stage of laser annealing. The microwave was selectively absorbed by the conductive particles, the polarization of dipoles in the thermoplastic polymer below the Tg was limited, which made the polymer foil’s skin depth almost infinite, hence transparent to microwave radiation. Therefore, only the conductive particles absorbed the microwaves and could be sintered selectively in a very short period (within 1-2 min). Inkjet printed dielectric material Dielectric inks can be classified into three groups which are organic, inorganic and inorganicorganic combined. Poly (methyl methacrylate) (PMMA), polyvinyl alcohol (PVA), poly-4vinylphenol (PVPh), polystyrene (PS) are the important dielectrics for inkjet printing used in transistors [54]. In addition to that varity of dielectrics, poly(perfluroroalkenylvinyl ether) (CYTOP, k = 2.1), poly(t-butylstyrene) (PTBS, k= 2.4), polystyrene(PS, k= 2.5), polyolefinpolyacrylate (ActivInk D2200, k = 3.2) have been used [55]. ZrO2, TiO2 and BaTiO3 based ceramic ink can be used in organic transistors in near future if the processing temperature can be reduced. In OTFT, inorganic nanoparticles are incorporated in organic dielectrics to enhance the dielectric constant. They are also classified as inorganic-organic hybrid materials. Cho et al introduced aerosol jet printing of ion-gel gate dielectric and finally fabrication of printed capacitor, transistor and inverter [56]. Ion gel films were formed by gelation of a triblock copolymer, poly(styrene-block-ethylene oxide-block-styrene) (PS-PEO-PS) (7wt%) in an ionic liquid, 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide ([EMIM][TFSI]. The PSPEO-PS triblock copolymers dissolved in [EMIM][TFSI] ionic liquid formed well defined physical gels through non covalent association of the PS block, but gelation occured at very low polymer weight fraction (as low as 4%) and that’s why high ionic conductivity was maintained. Films were formed readily by casting or printing an acetonitrile or ethyl acetate solution containing the [EMIM]

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[TFSI] and PS-PEO-PS (93:7 wt. ratio) directly onto flat substrate. The capacitance of the neat triblock copolymer film and ion gel film were ~ 0.007µF/cm2 and ~ 20µF/cm2 at 10Hz. Large capacitance was due to formation of nanometer thick electrical double layers at electrode-electrolyte interface with formation times of the order of 10µs. An intriguing potential advantage of ion gel dielectrics was that their high polarizability enables the gate electrodes to be physically offset from the channel. A printed OTFT, using P3HT as semiconductor, was fabricated where PEDOT:PSS gate electrode was offset by 60µm from the edge of the channel, but the transfer characteristics were remarkably similar to the current voltage trace for the aligned gate device. A potential disadvantage was high static off current (voltage fixed) of these devices. The probable reasons of high off-current are source to gate or drain to gate leakage and impurity in the ion gel. Inkjet printed semiconducting materials Organic semiconductors are basically two types, small molecules/oligomers and polymers. Pentacene and its derivative is well known p type small molecule/oligomer. But, most of the small molecules are not solution processed, so to make them solution processable functional groups are attached. Oligomers present many advantages over polymers, like a higher degree of purity in oligomers should allow for a better opportunity to control the molecular arrangement. Diel-elder pentacene and TIPS pentacene are good example of solution processable small molecule. Polymers are well known for solution processability. Wide ranges of polymer semiconductors, like poly (3hexylthiophene) (P3HT), poly (9,9’-dioctyl fluorine-co-bithiophene) (F8T2), regioregular poly (thiophene) (XPT) etc., are already used as ink. Polymer semiconductors p-type polymer. Paul et al selected two polymeric organic semiconductors for inkjet printing, poly (9,9’-dioctyl fluorine-co-bithiophene) (F8T2) and regioregular poly thiophene (XPT). Transistor properties were comparable to the spin coated OTFTs [57]. Poly (3-octylthiophene-2,5diyl) (P3OT), another p-type polymer, was printed on patterned Au on Si wafer with thermally oxidized 140 nm SiO2 [5]. Arias et al used inkjet printed OTFT in active matrix display backplane [58]. Poly[5,5'-bis(3-dodecyl-2-thienyl)-2,2'-bithiophene] (PQT-12) and F8T2 were chosen by them; because they can be processed in ambient condition and deposited from solution at room temperature. Joung et al observed the performance of 3-hexylthiophene (P3HT) based OTFT depends on optimization of printing process, like choice of solvent, annealing temperature and use of hydrazine as a dedoping agent [6]. Their results were discussed in an earlier section. Chen et al fabricated polymer based inverters and ring oscillator on PET substrate, where polythiophene was inkjet printed to fabricated OTFT on PET substrate [59]. This polymer OTFT operated in the enhanced mode and exhibited mobility, threshold voltage and Ion/Ioff ratio of 10-2 cm2/Vs, -4V and 80, respectively. Kim et al observed photocurrent decay of OTFT under gate bias [60]. α, ω-dihexylquarterthiophene (DH4T) was inkjetted on patterned-sputtered-AlNd gate and spin coated crosslinked PVPh dielectric surface. The mobility, Ion/Ioff ratio and threshold voltage of DH4T based OTFT are 0.035 cm2/V s, 106 and -6 V, respectively. Carboxylate functionalized polythiophene, another varaiant of polythiophene, had relatively high ionization potential (HOMO ~ -5.84eV, band gap ~ 2.35eV), which made them less prone to doping by an ambient oxygen but resulted in a considerable contact effect [11]. Schottky barrier for hole injection (Φb >0.3eV) was because of gold and carboxylate functionalized polythiophene contact. So, suitable choice of electrode material may improve OTFT performance further. Carbon nanoparticle (CNP) was incorporated into P3HT solution of different concentration, which was used as semiconducting ink by Lin et al [22]. Improvement in OTFT performance was observed with CNT incorporation.


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n-type polymer. Organic semiconductors are generally p-type, few n-type polymers exist. But compatibility of both p-channel (hole transporting) and n-channel (electron transporting) semiconductors with a single combination of gate dielectric and contact materials is highly desirable to enable powerful complementary circuit technologies, where p- and n-channel OTFTs operate in concert. Yan et al reported a new n-type polymer, poly{[N,N’-bis(2-octyldodecyl)naphthalene 1,4,5,8 bis(dicarboximide)-2,6-diyl] alt-5,5’-(2,2’-bithiophene)}[P(NDI2OD-T2)], having good solution processability as well as excellent final transistor property [55]. The best performing n-channel polymers are dithiophenealkylimide based and perylene based polymers, exhibiting promising electron mobilities of 0.001-0.01 cm2/Vs, (Ion/Ioff) ≈ 105 in vaccum) and the ladder type BBL material exhibiting a µe of 0.001-0.1 cm2/Vs (Ion/Ioff ≈ 101-104), where as [P(NDI2OD-T2)] based transistor had excellent OTFT characteristics (electron mobilities up to ~ 0.45-0.85 cm2/Vs) under ambient condition in combination with Au contacts and various polymeric dielectrics. The room temperature solubility of [P(NDI2OD-T2)] in conventional organic solvents such as xylene and dichlorobenzene (DCB) was as high as 60g/l. Despite the far larger band gap of the naphthalene (~3eV) versus the perylene-bis (dicarboximide) (~2eV) core, the optical absorption spectra of [P(NDI2OD-T2)] revealed an optical gap of only ~1.45 eV. Differential scanning calorimetry of [P(NDI2OD-T2)] indicated no thermal transition up to ~300oC. The mostly amorphous nature of [P(NDI2OD-T2)] is observed in wide angle X-ray diffraction. The spin coated OTFT devices using CYTOP, PS, PTBS, D2200 and PMMA dielectrics had good transistor characteristics with average values of µe ≈ 0.2-0.5 cm2/Vs, Ion/Ioff >106, Von ≈ 0-15V, Vth ≈ 5-20V, and S < 3V/dec, and devices yields approached 100%. Small molecules/oligomers as semiconductors. Generally, all small molecules are insoluble in organic solvent. That’s why, physical vapor deposition is applied to deposit small molecules as an active layer. Better transistor properties were observed when small molecules are used as an active material compared to polymers. Pentacene is well known and well established organic semiconductor, because of its highest mobility and good stability. But, it can not be used in inkjet printing. p-type small molecule/oligomer. Pentacene can be made soluble by Diel Alder reaction. Diel Alder penatcene contains side chain of N-S=O. After heating of Diel Alder Pentacene, it becomes pentacene by reverse Diel Alder reaction [61]. Volkman et al used first soluble pentacene to make inkjet printed transistor [61]. They took silicon substrate with 100 nm SiO2 layer as a substrate. Source drain was made by evaporated gold. Bottom contact OTFT was fabricated by inkjet printing of soluble pentacene on top of gold film. Extensive work was done to optimize inkjetted pentacene film. Temperature was varied from 120oC to 205oC and annealing time was varied from 1 min to 1 hour. Transistor properties were basically dependent on N-S=O elimination. Inkjet-printed pentacene based transistor’s mobility was higher than mobility of other printed-polymer based transistors. Finally, the conversion of the precursor into solvent resistant pentacene, is an extra advantage to printing multilayer, because generally solvent of the ink redissolves bottom organic layer. Lee et al optimized inkjet printing of 6,13-bis (triisopropylsilylethynyl) pentacene to fabricate active layer of organic thin film transistor (OTFT) [62]. Substitution with appropriate solubilizing ethynyl functions at the 6,13 carbon position of pentacene can lead to enhancement of solubility because it promotes extended π electron delocalization from the pentacene nucleus. Printing of TIPS pentacene was carried out at two different substrate temperatures, 26oC and 60oC. The W/L, ratio of channel width to channel length, of the OTFTs was fixed to 236 µm/6 µm. The OTFT printed at 26oC exhibited the field effect mobility (µfe) of 0.11 cm2/Vs in the saturation region, an Ion/Ioff ratio of 107,a threshold voltage (VT) of -1.8V and the gate voltage swing (S) of 0.8V/dec. On the other hand, the OTFT printed at 60oC exhibited the µfe of 0.24cm2/Vs, Ion /Ioff ratio of 107, VT of - 2V and S of 0.6V/dec. Kim et al observed the effect of solvents on final transistor properties when triisopropylsilyl (TIPS) pentacene film was made by inkjet printing [7]. To compare this result with spin coated TFTs, one OTFT was fabricated from chlorobenzene and anisole solution by spin coating. The field effect mobility and Ion/Ioff ratio of OTFT fabricated by inkjet printing and spin coating (when chlorobenzene was used as a solvent) were 0.015cm2/Vs, 105-106 and 0.021cm2/Vs, 104, respectively.

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Luzio studied top contact bottom gate and bottom contact bottom gate devices on Si/SiO2 substrate using 13,6-N-sulfinylacetamidopentacene as soluble pentacene precursor (SAP) that converts to pentacene at 150oC, and normal evaporated pentacene [45]. In one configuration, inkjetted PEDOT:PSS was used to fabricate source and drain on top of Si/SiO2 and SAP was coated on top it. This configuration offered field effect mobility, Ion/Ioff ratio, and threshold voltage of 0.27 cm2/Vs, 105, and -4.25 V, respectively. These values are comparable to the performance of standard device, where pentacene and gold was evaporated on top of Si/SiO2 substrate. Sansur et al successfully inkjet printed 9,9–didodecylfluorene-2,7-diboronic acid and nano Au to make active layer and source/drain electrode [63]. Choi et al reported the effect of surface treatment on OTFT performance and bias induced changes in the inkjet printed OTFT [64]. In their study, sputtered AlNd, spin coated photo definable photoacryl (PA) and thermally evaporated Ag were used as gate electrode, gate dielectric and source/drain, respectively. The dielectric and source/drain electrodes were treated by pentafluorothiophenol (PFBT) and phenethyltrichlorosilane (PTS) by dipping and spin coating on the substrate before inkjet printing. TIPS pentacene was inkjetted in the patterned photoacryl (PA) to fabricate active layer at 90oC substrate temperature. The OTFT with self assembled monolayer had good OTFT charcteristics, field effect mobility of around 0.18 cm2/Vs and excellent stability of threshhold voltage with gate bias stress. n-type small molecule/oligomer. Yuming Ai et al reported inkjet printing of a n-type semiconductor, 1,4,5,8-naphthalene-tetracarboxylic dianhydride (NTCDA), having conductivity of 10-4-10-5 S/cm [66]. Zhao et al reported a new n-type, stable, semiconducting materials, core expanded naphthalene diimide fused with 2 (1,3-dithiol-2-ylidene) malonitrile groups (NDI2ODDTYM2), exhibited highest field effect mobility of 1.2 cm2/Vs [67]. Its environmental stability and high temperature withstanding capability (without any degradation upto operating temperature ≤120oC) made it possible to do device processing in air. In this study, initially NDI2ODDTYM2 was deposited by spin coating, inkjet printing and brush painting on evaporated Au source-drain structure on Si/SiO2 substrate. To fabricate all-solution-processed OTFT, gate and source/drain were fabricated by inkjetted Ag, and dielectric by spin coated polyacrylonitrile (PAN) and polymethylsilsesquioxane (PMSQ). NDI2ODDTYM2 was inkjetted to fabricate active layer. This solution processed OTFT had field effect mobility, threshold voltage, Ion/Ioff ratio and subthreshold swing of 0.07–0.45 cm2/Vs, –0.6–10.5V, 104-106 and 1.1–2.8 V/dec, respectively. Grimaldi et al studied the effect of solvent mixture on the morphology of printed active layer and OTFT performance [68]. Perylene diimide (PDI-8CN2), an n-type organic semiconductor, was selected for their study. PDI-8CN2 dissolved in four different proportions of o-dichlorobenzene and chloroform was inkjetted on Si/SiO2 surface as an active layer of OTFT. Large crystallite interconnected with small crystal morphology, totally continuous and uniform film, was obtained with o-dichlorobenzene and chloroform ratio 3:2, it possessed good OTFT performance with field effect mobility 0.0035 cm2/Vs. Inkjet printed carbon nanotube. Beecher et al introduced inkjet printing of carbon nano tubes (CNTs) to make active material in organic thin film transistor (OTFT) [69]. Single wall carbon nano tubes can be dispersed in a variety of commonly used solvents, e.g., dimethylformamide, toluene, chloroform, acetone, anisole, methanol and N-methyl-2-pyrrolidone (NMP). CNTs tend to form dense clumps and it requires a significant amount of coercion in order to disperse them effectively and for this reason, a high power sonication and ultracentrifugation was employed. It was observed that NMP, a dipolar aprotic solvent, was very effective for producing CNT solution. The combination of the excellent solvency properties of NMP and intense dispersion techniques allowed for the preparation of quite stable solutions of CNTs without the need for a surfactant, which is usually necessary for long term stability of CNT dispersions. Other noticeable characteristics of NMP are its high boiling point (202oC) that helped to make smoother films after printing.


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Takenobu et al succeeded in fabricating high on/off ratio (~104) single walled carbon nanotube (SWCNTs) flexible TFTs by inkjet printing of water based dispersion without any additional processing steps to eliminate the effect of metallic SWCNTs [70]. Bottom gate TFTs were fabricated and SWCNT was inkjet printed.. The mobility and Ion/Ioff ratio of these transistors were 10-3-10-4cm2/Vs and 104, respectively. The observed water contact angles on Au and SiO2 surfaces were 51-57 o and 8-19o, respectively. This strong difference in wettability was attributed as the origin of poor contacts that prevented uniform SWCNT film fabrication at the Au/SiO2 interface. In order to improve electrical contacts, they shortened the drying process by heat treatment (50120oC). Through heat treatments (typically 50 and 70oC), the device performance was dramatically improved and the observed carrier mobility was 0.4cm2/Vs, although the on/off current ratio was quite small (