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Inkjet printing of single-crystal filmsHiromi Minemawari1, Toshikazu Yamada1, Hiroyuki Matsui1, Jun’ya Tsutsumi1, Simon Haas1, Ryosuke Chiba1,2, Reiji Kumai1,3

& Tatsuo Hasegawa1

The use of single crystals has been fundamental to the developmentof semiconductor microelectronics and solid-state science1.Whether based on inorganic2–5 or organic6–8 materials, the devicesthat show the highest performance rely on single-crystal interfaces,with their nearly perfect translational symmetry and exceptionallyhigh chemical purity. Attention has recently been focused ondeveloping simple ways of producing electronic devices by meansof printing technologies. ‘Printed electronics’ is being explored forthe manufacture of large-area and flexible electronic devices by thepatterned application of functional inks containing soluble or dis-persed semiconducting materials9–11. However, because of thestrong self-organizing tendency of the deposited materials12,13,the production of semiconducting thin films of high crystallinity(indispensable for realizing high carrier mobility) may be incom-patible with conventional printing processes. Here we develop amethod that combines the technique of antisolvent crystalliza-tion14 with inkjet printing to produce organic semiconducting thinfilms of high crystallinity. Specifically, we show that mixing finedroplets of an antisolvent and a solution of an active semiconduct-ing component within a confined area on an amorphous substratecan trigger the controlled formation of exceptionally uniformsingle-crystal or polycrystalline thin films that grow at the liquid–air interfaces. Using this approach, we have printed single crystalsof the organic semiconductor 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) (ref. 15), yielding thin-film transistorswith average carrier mobilities as high as 16.4 cm2 V21 s21. Thisprinting technique constitutes a major step towards the use ofhigh-performance single-crystal semiconductor devices for large-area and flexible electronics applications.

Antisolvent crystallization is recognized as the best method ofachieving controlled and scalable solidification, which is useful inpharmaceutical science14, for example. To achieve this, an ‘antisolvent’(a liquid in which a substance is insoluble) is added to the solution ofthe substance in a solvent that is miscible with the antisolvent. Here wemake use of this concept in microliquid inkjet printing processes.

A solution of a semiconductor and an antisolvent for the semi-conductor are used as the two kinds of ink; the inks are individuallyprinted at arbitrary positions to form a microliquid intermixturebetween the inks on the top of substrates. We found that optimizedprinting conditions enable controlled formation of patterned single-crystal thin films having molecularly flat surfaces, in contrast to conven-tional inkjet printing processes that produce films with a non-uniformthickness distribution. This is a conceptual extension of the ‘double-shot’ inkjet printing process that was developed to produce films ofcharge-transfer compounds that are hardly soluble16,17. We used 1,2-dichlorobenzene (DCB) as the solvent and N,N-dimethylformamide(DMF) as the antisolvent for the semiconductor C8-BTBT. Theseorganic liquids show very different solubilities for C8-BTBT (the solu-bility at 20 uC is 400 times higher in DCB than in DMF), but havesimilar boiling points and are miscible with one another.

A schematic representation of this printing process is shown inFig. 1a. We used silicon wafers with 100-nm-thick silicon dioxide layers

as substrates. We produced the wetting/non-wetting surface patterningon the silicon dioxide layers by using a combination of ultraviolet/ozonetreatment, hexamethyldisilazane treatment, and photoresist pattern-ing18. We used a piezoelectric inkjet printing apparatus with doubleinkjet printing heads, from which a droplet of 60 picolitres is ejectedat a repetition frequency of 500 Hz. In the process, the antisolvent ink(pure anhydrous DMF) is printed first and then overprinted with thesolution ink (a 28 mM solution of C8-BTBT in DCB). In the formationof all the pieces of film shown in Fig. 1b, 42 shots of antisolvent ink wereprinted first and then 6 shots of solution ink were overprinted, all withina second. The deposited droplets are confined and intermixed in apredefined hydrophilic area on the upper surface of the substrate.

During the initial stages of film formation, tiny floating bodies beginto form at the surface of the liquid and can be seen in microscopeimages (Supplementary Movie). Each floating body acts as a nucleusfor further crystallization and undergoes subsequent growth to form alarger floating body. These bodies eventually cover the entire surface ofthe droplet (step 3 in Fig. 1a). A few creases can be seen on the surfacesof the droplets during liquid evaporation, indicating the solid nature ofthe films (step 4 and Supplementary Fig. 1)19.

Although nuclei are generated randomly, mostly at the perimetersof the deposited droplet (solid–liquid–air interfaces), we found thatnucleation can be controlled through appropriate design of the dropletconfiguration, which is shaped by the predefined hydrophilic area aswell as by the ink volume. For example, a hydrophilic area containing aprotuberance, as presented in Fig. 1b, was quite effective in causinglocal seeding of floating bodies in the protrusive area. We propose thatlocal seeding is associated with the comparatively higher rate of solventevaporation in areas with a high surface area-to-volume ratio. Afterseeding, the growing front moves slowly to the other end of the dropletuntil the large single-domain floating body covers the entire liquid–airinterface (see Supplementary Movie).

The solvent then evaporates very slowly, taking about 10–50 timeslonger than is the case without the solute, most probably because thedroplet is completely covered by the solid film. During this slowevaporation, the creases in the films become smoothed out, and filmswith thickness of about 30–200 nm are eventually obtained on theamorphous substrate. The film adheres tightly to the substrate. Themorphology of the films as well as their single-domain nature dependson a variety of printing conditions, such as substrate temperature, theconcentration and volume of the solution, the solution–antisolventratio and the shape of the hydrophilic area on which the droplet isdeposited.

The thickness profile of the film differs markedly from that of con-ventional inkjet printing deposits. Conventional inkjet printing isknown to produce a characteristic thickness distribution in which bothends of the deposit are considerably thicker than its centre, known asthe ‘coffee-ring effect’ (see Supplementary Fig. 2)20. The uniform natureof the deposits produced by our process can be ascribed to temporaldiscrimination between solute crystallization and solvent evaporationwithin the deposited droplet (see Supplementary Fig. 1)16. The occur-rence of supersaturation in the intermixed microliquid droplet results

1National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 4, Tsukuba 305-8562, Japan. 2Department of Applied Physics, The University of Tokyo, Hongo 113-8656,Japan. 3CMRC, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801, Japan.

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in solute crystallization before solvent evaporation. In the microscopeimages of the films shown in Fig. 1d, we can see stripe-like features withintervals of several micrometres to several tens of micrometres.Atomic-force microscopy showed that the stripes are associated withthe height of the molecular step, which is estimated to be about 2.6–2.8 nm (Fig. 1e)21. This value is consistent with the thickness of onemolecular layer of C8-BTBT (csinb 5 2.92 nm, where c and b are theunit cell parameters) (ref. 22). We conclude that the stripe-like featuresare associated with the step-and-terrace structure of C8-BTBT.

In images recorded through crossed Nicol prisms, the colour ofalmost the entire film changes from bright to dark, simultaneously,on rotating the film about an axis perpendicular to the substrate(Fig. 1c). In addition, when we use hydrophilic areas with differentconfigurations such as a simple square, rectangle or circle, we obtainedpolycrystalline films composed of some crystal domains (see Sup-plementary Fig. 3). From these observations, we conclude that withappropriate design of both the droplet shape and printing conditions,single-domain crystal films that cover nearly the whole region of theprinted deposits could be produced with high probability (Supplemen-tary Fig. 4). We also noticed that the step-and-terrace structures inFig. 1d form concentric ellipses, and propose that this feature is formedby epitaxial growth on top of thinner single-domain crystal films at alater stage (see Supplementary Fig. 5).

X-ray diffraction data for the films are shown in Fig. 2a and b. Theobserved out-of-plane diffraction spots are consistent with a molecularlayer structure that is parallel to the a and b axes. The observation ofBragg reflections up to 14th order indicates that the films have a highlycrystalline nature. At high incident angles of the X-rays, we observed 16diffraction spots that could be ascribed to Bragg reflections with indicesthat include an in-plane component (Fig. 2b), where the refined unitcell—monoclinic P21/a, a 5 5.91(15) A, b 5 7.88(1) A, c 5 29.12(19) A,b 5 91.0(8)u, V 5 1357(4) A3—is consistent with that of the bulk crys-tal22. These results provide unambiguous evidence that the films aresingle-crystalline with a long-range translational symmetry.

The data show that the growth direction is parallel to [1 21 0] inmany (about 60–70%) of the deposited films (Fig. 1c). On the other

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Figure 1 | Inkjet printing of organic single-crystal thin films. a, Schematic ofthe process. Antisolvent ink (A) is first inkjet-printed (step 1), and then solutionink (B) is overprinted sequentially to form intermixed droplets confined to apredefined area (step 2). Semiconducting thin films grow at liquid–airinterfaces of the droplet (step 3), before the solvent fully evaporates (step 4).

b, Micrographs of a 20 3 7 array of inkjet-printed C8-BTBT single-crystal thin-films. c, Crossed Nicols polarized micrographs of the film. d, Expandedmicrograph of the film, showing stripes caused by molecular-layer steps.e, Atomic-force microscopy image and the height profile (below) showing thestep-and-terrace structure on the film surfaces.

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hand, the bright-to-dark images observed through the crossed Nicolprisms originate from in-plane optical anisotropy of the single-crystalfilms. Figure 2c shows the polarized optical absorption spectra of oursingle-crystal films with the electric field of the light parallel to the a orthe b axis. The spectra show a clear anisotropy in their absorptionintensity and peak energy. The absorption intensity is much higheralong the a axis and peaks at 3.43 eV, whereas the absorption intensityalong the b axis is comparatively weak and peaks at a higher energy of3.47 eV. We note that the transition dipole for the lowest electronicexcitation between the highest occupied molecular orbital (HOMO)and the lowest unoccupied molecular orbital (LUMO) is polarizedparallel to the molecular plane (Fig. 2c). The difference in the absorp-tion intensity can be clearly ascribed to the orientation of the molecularplanes within the a–b plane. In contrast, the difference in peak energiesis due to Davydov splitting along the a and b axes; this is characteristicof herringbone-type molecular arrangements within single-crystalfilms, as observed in anthracene23 or pentacene24.

Field-effect devices were fabricated for the single-crystal films with atop-contact/top-gate geometry, composed of 30-nm Au films as thesource and drain electrodes, and films of parylene C (capacitance perunit area of C 5 4.2 nF cm22) as the gate dielectric layers. The typicalchannel width and length were 145mm and 100mm, respectively. Thedirect-current field-effect characteristics at room temperature (300 K)were measured in an argon-filled glove box. The transfer and outputcharacteristics of this device are shown in Fig. 3. The mobility in thesaturation regime reaches 16.4 cm2 V21 s21 on average, and themaximum value is as high as 31.3 cm2 V21 s21. The on/off currentratio is 105–107, and the subthreshold slope was about 2 V per decadewith a threshold voltage of about 210 V. Injection barriers at thesource/drain contacts may have remained, as manifested by theslightly nonlinear source/drain current–voltage (Isd2Vsd) dependenceat low voltages. Hardly any current hysteresis was observed in thetransfer and output characteristics, where the shift in the thresholdvoltage from forward to reverse sweeps was less than 0.1 V. This

feature is probably associated with the negligible charge-trappingeffects between the single-crystal surface of C8-BTBT and the parylenegate dielectric layer. The slope of the transfer curve (Fig. 3c) presentsa distinct kink feature, as reported in other organic single-crystal devices8, which clearly demonstrates the high quality of thesemiconductor–insulator interface. We also found that the character-istics were not influenced by the existence of a few domain boundariesand were not degraded by more than 10% after the films were kept inair for 8 months.

This device performance is much higher than the previous reportfor C8-BTBT21 and is comparable to the highest performance obtained(for a rubrene single-crystal device8). We consider that the followingcharacteristics of the film formation process are important to achievehigh-quality single-crystal film: (1) the liquid–air interfaces need to beideal locations for diffusion and self-organization of organic molecules(as for Langmuir–Blodgett films25) and (2) the gradual growth ofsingle-crystal films is only possible because of the fluidic nature ofthe microliquid droplet in which laminar flow dominates over tur-bulent flow26. The technique should be applicable to a broad class offunctional soluble materials.

The rather broad distribution of apparent mobility (Fig. 3b) indi-cates that further improvements of our technique should be possible,in areas such as ink composition, the optimization of equipment andthe environment, and also subsequent device processing. For example,there is plenty of scope for improving the source/drain contacts.Nonetheless, we believe that this drop-on-demand, non-vacuum androom-temperature printing process of patterned single-crystal semi-conductor films is in principle a useful new way of producing transistorarrays on top of plastic substrates, which is indispensable for realizinglarge-area, light-weight and high-speed electronic products.

Received 20 February; accepted 14 June 2011.

Published online 13 July 2011.

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Figure 3 | Transistor characteristics for the inkjet-printed C8-BTBT single-crystal thin films. a, Schematic of the device structure and micrograph of thethin-film transistors. b, Distribution of mobility and on/off ratio measured over54 transistors. Average mobility is 16.4 6 6.1 cm2 V21 s21. c, Transfercharacteristics at Vsd 5 260 V. d, Output characteristics at various gatevoltages Vg.

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Supplementary Information is linked to the online version of the paper atwww.nature.com/nature.

Acknowledgements We are grateful to Nippon Kayaku for providing C8-BTBT. Wethank K. Takimiya and S. Horiuchi for discussions, H. Okamoto and H. Matsuzaki forhelp with optical measurements, K. Kobayashi for help with the X-ray measurements,and T. Iwadate for help with atomic-force microscopy and measurements of device

characteristics. The synchrotron X-ray study was performed with the approval of thePhoton Factory Program Advisory Committee (no. 2009S2-003). This work wassupported by the New Energy and Industrial Technology Development Organization(NEDO) through a Grant for Industrial Technology Research and also by the JapanSociety for the Promotion of Science (JSPS) through its Funding Program forWorld-Leading Innovative R&D on Science and Technology (FIRST Program).

Author Contributions H.Minemawari was responsible for ink fabrication, inkjetprinting, microscopic observations, X-ray diffraction measurements, andmeasurements of the device characteristics of all the films. T.Y. prepared substrateswith thewet/non-wet surface patterning, assisted in inkjetprintingandX-ray diffractionmeasurements, and performed atomic-force microscopy and device characteristicsmeasurements. H.Matsui guided sample preparation and inkjet printing, andconducted DFT molecular orbital calculations. J.T. assisted with X-ray diffractionmeasurements and performed optical anisotropic absorption measurements. S.H.assisted with optical anisotropic absorption measurements. R.C. assisted in the inkfabrication. R.K. assisted with X-raydiffraction measurements. T.H. conceptualized anddirected the research project, and wrote most of the manuscript. All the authorsdiscussed the results and commented on the manuscript.

Author Information Reprints and permissions information is available atwww.nature.com/reprints. The authors declare no competing financial interests.Readers are welcome to comment on the online version of this article atwww.nature.com/nature. Correspondence and requests for materials should beaddressed to T.H. (t-hasegawa@aist.go.jp).

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