Organic Electronics 15 (2014) 1868–1875

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Organic Electronics

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

Laser-induced forward transfer of polythiophene-basedderivatives for fully polymeric thin film transistors

http://dx.doi.org/10.1016/j.orgel.2014.04.0291566-1199/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +33 0491829515.E-mail address: rapp@lp3.univ-mrs.fr (L. Rapp).

Ludovic Rapp ⇑, Catalin Constantinescu, Philippe Delaporte, Anne Patricia AlloncleAix Marseille University, CNRS, LP3 UMR 7341, 13288 Marseille, France

a r t i c l e i n f o a b s t r a c t

Article history:Received 16 September 2013Received in revised form 10 April 2014Accepted 18 April 2014Available online 11 May 2014

Keywords:Laser printingLaser-induced forward transferOrganic transistorPolymerPolythiopheneLow pressure

Polymeric thin-film transistors (pTFTs) have been fabricated by pulsed-laser printing ofsemiconductor and conductor polythiophene-based derivatives. Thin solid layers ofsemiconducting poly(3,30 00 didodecylquaterthiophene) (PQT-12) have been transferred by alaser-induced forward transfer (LIFT) technique on Si/SiO2 receiver substrates. Optimiza-tion of the transfer conditions and of the pixels morphologies has been realized. A markedimprovement in the quality of the pixels has been observed, in terms of morphologyand structure, by reducing the environmental pressure to 90 mbar during LIFT. Subse-quently, poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) has also beenlaser-printed and used as source/drain electrodes in the transistor configuration.Functional polymeric transistors have been obtained with high field-effect mobility up to2 � 10�2 cm2 V�1 s�1 together with current modulation of 104.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Since the discovery of conducting polymers [1], and thefirst reports on organic electronics in the 1970s [2,3], thedevelopment and synthesis of new materials has beenthe subject of intensive investigations. The production oforganic electronic components, including organic thin filmtransistors (OTFTs), is an alternative to inorganic devicesfor applications based on low-cost processes. The parame-ters of the OTFTs are continuously improved, since thesedevices will be one of the basic components of future plas-tic electronic devices [4]. Their advantages over conven-tional inorganic electronics essentially include thepossibility of being printed on different kinds of thin flex-ible supports, such as paper, polyimide, or polyethyleneterephthalate. They offer an accurate, cost efficient tech-nology for large area electronic applications, such asflexible displays, radio frequency identification (RFID) tags,

and sensors. The main mass printing processes/manufac-turing technologies used today are serigraphy-based tech-niques. Also, fully digital techniques are being investigated,e.g. inkjet printing. However, they require the use ofsoluble materials and an annealing step, which may pre-vent using low-cost flexible substrates. The study and theimplementation of rapid, yet simple techniques in manu-facturing electronic components on flexible supports arean important stage in developing plastic microelectronics.

Laser-based processes [5–7] offer versatile alternativesfor the deposition of thin films, to create organic electronicdevices operating on flexible supports. The laser-inducedforward transfer (LIFT) technique is an alternative wayfor the fabrication of organic electronic components, whenconventional techniques cannot be considered. Since it wasfirst investigated by Bohandy et al. in the mid ‘80s [8], thefeasibility of LIFT for the deposition of inorganic andorganic materials has been repeatedly demonstrated for alarge variety of materials [9–17]. LIFT is a proved techniquein fabricating various types of cost-efficient devices, e.g.organic light-emitting diodes (OLED) [18–20] and organic

L. Rapp et al. / Organic Electronics 15 (2014) 1868–1875 1869

thin film transistors (OTFT) [21]. The process itself hasbeen optimized and considerably refined, e.g. by the addi-tion of an intermediate sacrificial layer [17,18]. We havealready demonstrated the functional printing of oligomersemiconductors [22,23] leading to the fabrication of com-petitive OTFTs with thermally evaporated [23] or laser-printed [21,22] metallic electrodes.

In this paper, we report on the laser-printing of twopolythiophene-based derivatives for fully polymeric OTFTdevices. One such material is used as the semiconductor/active layer, while the other acts as the conductive sourceand drain electrodes. The transfer of both materials hasbeen optimized, and the polymeric OTFTs were printed ina sequential laser process: first, the transfer of the activelayer, and second, the transfer of the contact electrodes.The conjugated polymer regioregular poly(3,30 0 0didodecyl-quarterthiophene) (PQT-12) is worth considering as a candi-date material for LIFT printing, for its high ionizationpotential and good stability against doping phenomena.PQT-12 is a solution-processable organic semiconductor,intensively studied by spin-coating [24–26] and inkjetprinting [27,28], due to its easy synthesis and relativelyhigh charge carrier mobility. Ong et al. reported on PQT-12 transistors with mobilities as high as 0.2 cm2 V�1 s�1

together with an On/Off ratio of 108 [25]. By using inkjettechniques, mobilities of 8 � 10�2 cm2 V�1 s�1 [27] andIOn/IOff of 105 were achieved with gold electrodes [29].Enhanced transistor performance can be further achievedby using the active layer together with well-adaptedsource/drain electrodes [26]. Poly(3,4-ethylenedioxythio-phene)/poly(styrenesulfonate) (PEDOT:PSS) is a solublepolymer, a mixture of two ionomers, such that the twocharged macromolecules form a macromolecular salt[30]: one component is made up of sodium polystyrenesulfonate (PSS) in which some of the sulfonyl groups aredeprotonated and carry a negative charge, while the sec-ond component is poly(3,4-ethylenedioxythiophene)(PEDOT), a conjugated polymer that carries positivecharges. Therefore, we used PEDOT:PSS as source/drainelectrodes for such laser-printed electronic devices [31].

2. Experimental details

The LIFT printing technique is used to transfer smallamounts of materials, under the action of a laser pulse,from a donor film to a receiver substrate [21]. The laserirradiates the donor sample from behind and pushes thematerial toward the receiver substrate. Only the irradiatedarea is transferred, which means that the laser defines thelateral shape of the deposition. With the goal of obtainingsquare pixels, we used a square mask (2 � 2 mm2) to selecta homogeneous part of the laser beam, and project itsimage on the donor thin film. With a converging lens, theresulting image size on the donor layer is 350 � 350 lm2.The irradiation and deposition areas were monitoredthrough a camera, for accurate alignment of the substrates.The substrates were maintained parallel and at a close con-tact distance from each other (the substrates are pressedtogether), with accurately controlled positioning by x, yand z motorized stages. Silicon (Si) wafers, covered by a

300 nm thick silicon dioxide (SiO2) (from Vegatec, France;Ci = 12 nF/cm2), were used as receiver substrates. Thesetwo materials also played the role of gate and dielectricmediums of the transistors, respectively. Both the donorand receiver substrates (suprasil and silicon wafers) werecleaned by successive immersions in an ultrasonic bath,in acetone, tetrahydrofuran, and then in ethanol, withdeionizer water rinsing at each step, and finally driedunder a stream of argon gas before use.

PQT-12 was supplied by American Dye Source Inc. inpowder form, dissolved at 25 �C in dichlorobenzene, andused without any further purification. Dichlorobenzenewas the solvent of choice, as it was demonstrated that highboiling point solvents lead to higher mobility in polythio-phene-based thin films [32,33]. Spincoated donor thinfilms were produced from solutions of 5 mg/ml, using70 ll of solution for each sample (80–200 rpm, 10 min,resulting in 80–300 nm film thickness). Optimal thin filmhomogeneity was obtained after two steps of subsequentannealing: 60 min at 80 �C, followed by 30 min at 110 �C.It was shown that faster annealing, at 120–140 �C for10 min, leads to inhomogeneous films, with cracks andvariable thickness [26].

PEDOT:PSS dispersion in water (typically at 1–3 wt.%solids) was supplied by Starck. Homogeneous thin films,with a thickness of 450 nm, are obtained using 300 ll ofthe emulsion for each sample (700–1000 rpm, 5 min),and then annealed at 80 �C for 30 min. Annealing for morethan 30 min leads to brittle donor films, while a shorterannealing time leads to printing of gelatinous pixels, andwith a large amount of splattered material around theiredges.

For the printing of PQT-12 and PEDOT:PSS pixels, weused a neodymium doped yttrium aluminium garnet(Nd:YAG) picosecond laser (Leopard S10/20 Continuum,355 nm wavelength, 50 ps pulse duration, 10 Hz repetitionrate). The quality of the laser-printed structures wasassessed directly by optical microscopy (Zeiss Axiotech100D and Leica DCM 3D), and atomic force microscopy(AFM) (PSIA XE-100).

3. Laser printing of PQT-12 pixels

The laser fluence is an important parameter, requiringcareful control in order to achieve adequate pixel transfer.The threshold transfer of such thin films is at 160 mJ/cm2,but the deposits have irregular shapes and/or they are onlypartially transferred. A series of optical microscope images,with pixels printed at several fluences, is presented inFig. 1a. The optimal printing conditions of the PQT-12 pix-els, meaning a square, repetitive shape and the transfer ofthe entire irradiated area, are between 220–280 mJ/cm2. InFig. 1a, we present a pixel printed at 260 mJ/cm2 that is lat-erally well resolved. However, some ejected particles arevisible all around and on it. This drawback is slightlyreduced when the laser fluence is decreased, but fluenceslower than 220 mJ/cm2 lead to the transfer of incompleteand/or irregular pixels. We printed longitudinal semicon-ducting lines by juxtaposing pixels with 20 lm overlap ofthe laser spot (at a laser fluence of 220 mJ/cm2), as shown

Fig. 1. (a) Optical microscopy images of PQT-12 semiconductor pixelslaser-printed at fluences of 200, 260, 300 and 350 mJ/cm2, and (b) laser-printed line with an overlap of 20 lm at 220 mJ/cm2; pixel thickness:80 nm.

Fig. 2. Schematic of the TFT structure used for the electrical measure-ments: laser-printed PQT-12 pixel on Si/SiO2 substrate, with the goldelectrodes thermally evaporated on the top of the PQT-12 pixel.

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in Fig. 1b. A pixel thickness of 80 nm was estimated byAFM, similar to the donor film. Using laser fluences of300 mJ/cm2 or higher leads to damage of pixel integrity:holes, irregular edges, large amounts of splattered mate-rial, etc. It was not possible to obtain better quality pixelsat this particular film thickness.

50 nm thick gold lines were thermally evaporated onthe semiconductor through a titanium mask (channelwidth: W = 300 lm, and length: L = 100 lm), at 1 � 10�6 -mbar and room temperature (Edwards AUTO306 evapora-tor), and further used as source-drain electrodes in TCconfiguration (Fig. 2). Current–voltage characteristics wereobtained with a Hewlett-Packard 4140B pico-ampereme-ter DC voltage source. The field effect mobility (l) wasextracted from the transfer characteristics in the saturationregime. The drain-source current (ID) in the saturationregime is governed by the following Eq. (1):

ðIDÞsat ¼W2L

lCiðVG � VTÞ2 ð1Þ

where Ci is the capacitance per unit area of the gate insu-lator layer, VG is the gate voltage, VT is the threshold volt-age, and l is the field-effect mobility. Typical output andtransfer characteristics are shown in Fig. 3a and b, respec-tively. Similar characteristics were obtained for all PQTpixels printed at fluences between 220 and 270 mJ/cm2.A mobility up to 5 � 10�3 cm2 V�1 s�1 was measured, witha threshold voltage VT = �9 V and IOn/IOff ratio of 103. Tran-sistors printed at higher fluences (above 280 mJ/cm2) werefunctional, but typically with very low electrical parame-ters: mobility in the order of 10�5 cm2 V�1 s�1 and On/Offratio of only 100, revealing a very narrow fluence rangefor reliable laser-printed pixels. These results prove thatthe laser irradiation and the mechanical transfer inducedamages in the semiconductor layers. No transistor effectwas measured for transistors made by the overlap of twopixels, with similar observations when the overlap zonewas slightly increased or decreased. AFM investigationsrevealed that the pixel edges present slight thinningeffects, discontinuities and higher roughness, with respectto the center area. However, we do not observe such effectstoward the center of the pixels. Moreover, the areas thatoverlap in the laser beam are irradiated twice. Thus, eitheran excessive laser irradiation in this area leads to thedestruction of the functionalities in the semiconductor[34] or, more probably, the contact between two pixelswas not good enough to allow charge carriers moving.Direct laser irradiation and ablation of such polymericmaterials may induce photochemical rearrangement reac-tions (referred to as photo-Fries rearrangement), leading tothe formation of various oxygenated species, conjugatedpolyenes, or other photo-chemical by-products due toring-opening reactions [35,36], but also by inducingmechanical degradation on impact.

Compared with reported data on transistors obtainedby inkjet techniques and associated with thermallyevaporated gold, laser-printed transistors have slightlybetter mobility but a significantly lower On/Off ratio(l = 8 � 10�4 cm2 V�1 s�1, IOn/IOff = 105) [27,28]. The LIFTtechnique reveals itself as particularly effective, maintain-ing the electrical properties of the polymer up to a signifi-cant extent, but the transfer process needs to be optimizedin order to achieve competitive transistors.

4. Optimization of the printed pixel morphology

In order to improve the pixels’ quality and their electri-cal performance, the film thickness was increased, and an

Fig. 3. Output (a) and transfer (b) characteristics of PQT-12 based transistor equipped with 30 nm thick gold source/drain electrodes (W = 300 lm,L = 100 lm) printed at the laser fluence of 220 mJ/cm2.

L. Rapp et al. / Organic Electronics 15 (2014) 1868–1875 1871

optimization of the LIFT process by controlling the back-ground pressure was studied.

We use a picosecond-regime pulsed laser in our exper-iments, in order to minimize the thermal effects and thusreduce the damage to the molecular backbone/catenae ofthe polymer over the areas exposed to the laser [35,36].In spite of that, we observe some damage in the structureof the pixels (more significant at fluences above 280 mJ/cm2), as well as low transistor performance. We performedthermo-gravimetric analysis (TGA) and Fourier-infraredspectroscopy (FTIR) in order to investigate the decomposi-tion path of PQT-12 (Diamond TG/DSC Analyzer from Perk-inElmer Instruments), and observed that this polymercompletely decomposes in gaseous components. This isin support of our initial observations, i.e. thermally-induced degradation in laser-printed pixels.

It was reported by Zhao et al. that, in spincoated thinfilms, the PQT-12 tends to arrange by self-assembling in3D ‘‘packs’’, with ‘‘the size of the domains consisting of large(submicron level) anisotropic lamellar stacks, formed byarrays of p–p stowing of polymer chains’’, that are ‘‘highlydependent on the annealing temperature and covering therange from the crystal-to-liquid crystal to the liquid crystal-to-isotropic phase transitions’’ [26]. These extensive crystal-line domains are the reason for its higher air stability, up to300 �C [37], thus allowing a wider range of experimentalconditions for the optimal control of the transfer. To bemore evident, by comparison with the usual printing flu-ences that were used for LIFT (�80–350 mJ/cm2) [20,23],the PQT-12 integrity is significantly damaged only at veryhigh fluences (>500 mJ/cm2). Depending on the thicknessof the films, an excessive irradiation leads to the transferof pixels that are partially burned and/or with bubbles,holes and cracks in their structure. This is due to explosivegas formation [34] by photochemical decomposition and/or partial pyrolysis [38], and/or ruptures due to themechanical deformation. From our investigations, we con-cluded that the optimal thickness of the PQT-12 films is300 nm.

A natural development of the LIFT process is to controlthe background pressure during the laser transfer [39].Usually, LIFT is simply done in air and accurate transfershave been obtained in laboratory conditions. In the smallvolume delimited by the penetration depth of the beamand the irradiated surface, the radiation intensity is veryhigh and the absorbed energy is dissipated in a thermo-mechanical manner, including formation and propagationof a shockwave. The shockwave’s energy has been inten-sively studied [40,41]. Shadowgraphy experiments haveshown the generation of strong shockwaves at atmo-spheric pressure [41,42] and velocities higher than300 m/s have been reported. It has also been reported thatthe shockwave plays a significant role in affecting the qual-ity of the transfer [43]. Further investigations proved thatthe reflected shockwave may destroy the flyers at atmo-spheric pressure, but also showed that the flyers eitherfolded too much at low fluences or were destroyed onimpact at higher fluences [41–43]. Taking that intoaccount, we used a vacuum chamber and, after preliminaryresults, we set the background pressure at 90 mbar byusing a primary pump, a needle valve, and a standard pres-sure gauge, aiming to lessen the shockwave energy and tomore accurately control the quality of the pixels.

Optimal fluence conditions are between 320 and430 mJ/cm2, slightly increased due to the use of thickerPQT-12 donor thin films. This effect has already beenobserved for LIFT and it is not dependent on the back-ground pressure. No printing or partial printing occurredat fluences lower than 300 mJ/cm2, while the use of flu-ences higher than 500 mJ/cm2 led to the transfer of dam-aged pixels. For comparison, similar donor samples wereused in ambient atmosphere, but the resulting printed pix-els were highly inhomogeneous and irregular in shape.Fig. 4 shows a 300 nm thick PQT-12 pixel, printed at a flu-ence of 330 mJ/cm2. The transfer is clean, with almost nodebris even at close distance around the pixel. Such printedpixels are homogeneous, with a well-defined contour andsharp edges. Compared to the best-printed pixel at atmo-

Fig. 4. Optical microscope images at two different scales of laser-printedPQT-12 pixels at a background pressure of 90 mbar, and 330 mJ/cm2 laserfluence; pixel thickness: 300 nm.

Fig. 5. Optical microscope image of a laser-printed PEDOT:PSS pixel atreduced background pressure (90 mbar) and �600 mJ/cm2 laser fluence;pixel thickness: �450 nm.

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spheric pressure, the overall quality is clearly improved:the pixel is more homogeneous and the quantity of ejectedfragments is greatly reduced, the edges are sharper andbetter defined. The transfer improvements obtained byreducing the pressure and increasing the thickness of thefilm are significant and allow the creation of higher qualitypixels.

5. LIFT of PEDOT:PSS

Wang et al. demonstrated that the PEDOT:PSS elec-trodes offer the advantage of forming a p-doped interfaciallayer with poly(9,9-dioctylfluorene-co-bithiophene) alternat-ing copolymer (F8T2) organic semiconductor, thus provid-ing a very efficient charge injection at the interface [44].It was also observed that PQT-12 transistor performancewas improved using PEDOT:PSS as source/drain electrodes[27]. Moreover, the use of a single technique to print boththe electrodes and the active layer with the same setupmake the process simpler and less costly. In a previouswork, we discussed the laser printing of functional linesof PEDOT:PSS, by using nanosecond and picosecond laser

sources [16], and concluded that an intermediate sacrificiallayer is necessary when printing in ambient atmosphere.The use of this layer considerably improved the transfer,but also had a huge drawback in contaminating the surfaceof the pixels. Thus, PEDOT:PSS was also printed at a back-ground pressure of 90 mbar. Best quality pixels are printedat laser fluences ranging from 550 to 750 mJ/cm2. Fig. 5shows a PEDOT:PSS pixel transferred at 600 mJ/cm2, witha thickness of �450 nm, measured by AFM. No debris orfragments are observed around the pixel. The quality ofthe PEDOT:PSS pixels is directly related to: (1) the anneal-ing time after spincoating, as the film becomes brittle if theannealing time is too long, or partially printed if this is tooshort (folded pixels or just fragments); (2) the laser flu-ence, as pixels can be photochemically damaged; (3) thebackground pressure during LIFT, since the existence ofair leads to the formation of a shockwave, hence to foldingand/or cracking of pixels, while the lack of it (pressure toolow) gives no cushion effect and/or no heat transfer possi-bilities during the impact with the receiver substrate(kinetic energy is transformed into heat leading to struc-tural and morphological degradation).

6. Laser printing and characterization of polymer TFTs

Polymeric bottom-gate top-contact transistors are fab-ricated by the following procedure. Firstly, 300 nm thickpixels of PQT-12 are laser-printed at a fluence of 330 mJ/cm2, on a Si/SiO2 substrate and under reduced backgroundpressure (90 mbar). Secondly, 450 nm thick PEDOT:PSSpixels are transferred on opposite sides of the PQT-12 pix-els at a laser fluence of 600 mJ/cm2, to act as the source andthe drain electrodes.

Fig. 6 is an image of such a fully polymeric thin-filmtransistor. Small deformations and/or folding of the PED-OT:PSS pixels are observed, together with a few minuscule,

Fig. 6. A functional polymeric TFT, starting from a laser-printed PQT-12pixel as active layer (brown color) at a fluence of �330 mJ/cm2 and twolaser-printed PEDOT:PSS pixels as source and drain electrodes (dark bluecolor), at a fluence of �600 mJ/cm2. Both prints were achieved in avacuum chamber at a background pressure of 90 mbar. (For interpreta-tion of the references to color in this figure legend, the reader is referredto the web version of this article.)

Fig. 7. Output (a) and transfer (b) characteristics of OTFT with laser-printed PQTL = 100 lm).

L. Rapp et al. / Organic Electronics 15 (2014) 1868–1875 1873

rare splashes. The laser-printed transistors are functional;typical output and transfer characteristics are shown inFig. 7a and b, respectively. Mobilities up to 0.02 cm2 V�1 -s�1 were measured with a threshold voltage VT of �20 Vand an IOn/IOff ratio of 104. The average mobility of theprinted OTFTs was 0.012 ± 0.008 cm2 V�1 s�1. The highestmobility is obtained by using the PEDOT:PSS as electrodes,despite the higher resistivity of this material (compared tometallic electrodes), revealing better compatibilitybetween the PEDOT:PSS and PQT-12 pixels. We attributethese significantly higher electrical characteristics of thePEDOT:PSS/PQT-12 structure to the following thin filmparameters. Firstly, it is attributed to a better interfacialcontact between the two different polymer layers. Ade-quate contact between printed PQT-12 and PEDOT:PSS isof prime importance, as it is essential for carrier’s injection,which is the initial step to get operational OTFTs. The highkinetic energy of the transferred material induces a corre-spondingly impact pressure on the semiconductor layerand enhances physical contact between both layers. How-ever, the transfer is performed under low pressure and theimpact is not strong enough to destroy the receiving layer,as it has already been observed for the transfer at ambientatmosphere, and when using metallic electrodes [45]. Byregulating the ambient pressure during LIFT printing, onecan control the shockwave, but also the speed of the pixeland thus the impact pressure, to preserve the PQT-12 pix-els integrity and leading to optimized electrical contact/carrier’s injection. Although the direct laser irradiationmay induce surface damage to the printed pixel, it hasbeen shown that the PQT-12 pixels surface is homoge-neous. The PEDOT:PSS pixels are transferred on the top ofthe PQT-12 pixel, on a clean, homogeneous and relativelow surface roughness. It may be noted that PEDOT:PSSpixels are in contact with the PQT-12 pixels on the oppo-site side to the irradiated one, thus a clean, protected inter-face is assured. Furthermore, both materials are printed ina short time one after the other in solid-phase, thus avoid-ing the interaction of the solvents, a common problem of

-12 semiconductor and PEDOT:PSS source/drain electrodes (W = 300 lm,

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the wet techniques. Secondly, as it was discussed in theprevious section, the charge injection and transport inorganic electronic devices is influenced by the nature ofthe interface between the active layer and the source–drain electrodes. For instance, metallic electrodes onPQT-12 pixels may result in the formation of a rather largeinterface dipole. Therefore, the interface formation andelectronic structure have to be carefully considered inorder to maximize the efficiency and performance of theOTFTs. The output characteristic exhibits only very smallpositive current at low drain voltage and for all tested VG

polarizations. This parameter demonstrates that the leak-age currents between source–drain and gate electrodesthrough the insulating layer are very low. The chargetransport efficiency, as regarding the carrier mobility, isnot affected. The high work function of PEDOT:PSS pro-vides a better energy level matching with the PQT-12semiconductor. Moreover, the formation of an interfacialdoping zone between the polymer electrodes and semicon-ductor results in better electrical performance. Consideringthe higher mobility values together with similar on-statecurrent, the feasibility of an operating polymeric transistorprinted by LIFT is proved by using an optimized, singlelaser setup.

7. Conclusion

We demonstrate that laterally well-resolved, fully poly-meric square pixels have been printed. Their electricalcharacteristics prove the feasibility and reproducibility ofa laser direct-write technique on solid-state organic mate-rials: operational thin-film transistor devices can be fabri-cated by using the laser-induced forward transfer process.Reducing the background pressure to 90 mbar and increas-ing the thicknesses of both donor layers (300 nm for thePQT-12, and 450 nm for the PEDOT:PSS) result in clean,homogenous and well resolved printed pixels. PEDOT:PSSlaser-transferred pixels, printed on opposite sides of thePQT-12 pixels without the use of a sacrificial layer andby using a controlled background pressure, act as sourceand drain electrodes. High mobilities (up to 0.02 cm2 V�1 -s�1) are evidenced, revealing good contact between thepolymeric semiconductor–conductor interfacial zone ofthe pixels, compared to metallic electrodes, proving thatthe optimization of the printing process is fundamentalfor injection efficiency and high transistor performance.

Acknowledgements

This work was supported by a grant from the FrenchNational Research Agency (ANR), Programme Blanc – SIMI9 (Sciences de l’ingéniérie, matériaux, procédés, énergie2011), project number ANR-11-BS09-0011 – ‘‘ILTO’’(Impression laser de transistors organiques en films minces/Laser printing of organic thin film transistors).

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