Ink-jet printing of organic semiconductor for fabricating organic thin-film transistors: Film uniformity control by ink composition

Ink-jet printing of organic semiconductor for fabricating organic thin-film transistors: Film uniformity control by ink composition

Synthetic Metals 159 (2009) 1381–1385 Contents lists available at ScienceDirect Synthetic Metals journal homepage: www.elsevier.com/locate/synmet S...

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Synthetic Metals 159 (2009) 1381–1385

Contents lists available at ScienceDirect

Synthetic Metals journal homepage: www.elsevier.com/locate/synmet

Short communication

Ink-jet printing of organic semiconductor for fabricating organic thin-film transistors: Film uniformity control by ink composition Dongjo Kim a , Sunho Jeong a , Seong Hui Lee a , Jooho Moon a,∗ , Jun Kwang Song b a b

Department of Materials Science and Engineering, Yonsei University, 134 Shinchon-dong Seodaemun-gu, Seoul 120-749, Republic of Korea Material Testing Team, Korea Testing Laboratory, 222-13 Guro-dong, Guro-gu, Seoul 152-718, Republic of Korea

a r t i c l e

i n f o

Article history: Received 31 July 2008 Received in revised form 6 February 2009 Accepted 16 February 2009 PACS: 61.25.he 68.55.am 72.80.Le Keywords: Ink-jet printing Organic thin-film transistor Ink Organic semiconductor

a b s t r a c t We have investigated the influence of solvent chemistry on ink-jet printed semiconductor patterns. Our research focuses on improving the uniformity of an ink-jet printed single dot semiconductor by adjusting the solvent mixture combination. Use of a solvent mixture with one solvent of a lower boiling point and a higher solubility with respect to the semiconductor molecules and another of a higher boiling point and a lower solubility allows us to produce a uniform single dot pattern by ink-jet printing. It was observed that the film uniformity of the semiconductor layer plays an important role in determining the electrical parameters of the transistor. In comparison with uncontrolled ink composition, the OTFT fabricated from the well-controlled ink exhibited better device performance, including a carrier mobility of 8.5 × 10−3 cm2 V−1 s−1 in the saturation regime, an on/off current ratio of 103 , and a threshold voltage of 0.39 V with subthreshold slopes of 0.95 V dec−1 . © 2009 Elsevier B.V. All rights reserved.

1. Introduction Organic thin-film transistors (OTFTs) have recently received considerable attention because of their potential applications in flexible, low-cost integrated circuits such as smart cards and radio frequency identification (RFID) tags, display backplanes such as liquid crystal displays and electronic paper, and organic electroluminescent displays [1]. Furthermore, the possibility of using low-cost solution or liquid fabrication techniques has fuelled the current surge in research interest in organic electronics [2–6]. When applied to OTFT fabrication, all or parts of the transistor’s constituents, i.e., electrode, gate dielectric, and semiconductor, can be solution processable. In particular, it is beneficial to produce the semiconductor layer by a direct-writing technique such as inkjet printing since it allows for selective deposition of expensive materials via precise quantities of ink. However, most recent works involving OTFTs still typically employ selective vapor deposition of the organic semiconductors through a shadow mask, an approach that is both time consuming and costly [7–10]. Organic semiconductors that can be printed by ink-jet printing must meet several requirements. Firstly they need to be to

∗ Corresponding author. Tel.: +82 2 2123 2855; fax: +82 2 365 5882. E-mail address: [email protected] (J. Moon). 0379-6779/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2009.02.025

some extent soluble in relatively high boiling point solvents. The use of low boiling point solvents may give rise to nozzle clogging during printing. Secondly it is preferable that organic semiconductors exhibit stable electrical performance in air. Otherwise, the overall printing process must be performed in a controlled atmosphere. Among commercially available soluble organic semiconductors, oligomer-type ␣,␻-dihexylquaterthiophene (DH4T) is selected for use in the present study [11,12]. A major challenge in applying inkjet techniques for the deposition of semiconductor materials is the formulation of suitable inks. Ink chemistry and formulations determine not only the drop ejection characteristics and stability but also the quality of the printed patterns [13–16]. In particular, carrier charge transport occurs within a semiconductor layer. Therefore, the printed semiconductor layer should have good film uniformity and coherence with the dielectric layer, as this can play a critical role in determining the device performance, including carrier mobility and threshold voltage. Furthermore, the molecular ordering of organic semiconductor is influenced by the ink composition [17–20]. In this work, we have investigated the influence of solvent chemistry on ink-jet printed DH4T semiconductor patterns. By controlling solvent composition in the preparation of DH4T ink, coplanar-type OTFTs having a uniform DH4T layer have been fabricated by ink-jet printing. It should be noted that there have been other reports on uniformity improvement of ink-jet printed poly-

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Table 1 Selected solvent properties for various solvents used in this study. The contact angles were measured when HMDS-treated Si wafer was used as a substrate. Solvent type

Boiling point (◦ C)

Surface tension (mN/m)

Solubility (g/100 g)

Contact angle (◦ )

Tetrahydrofuran Chlorobenzene Xylene Dichlorobenzene Trichlorobenzene

65.9 131.6 138.3 180.4 213.5

26.4 33.6 30.1 26.8 24.8

1.34 0.94 1.17 1.15 0.79

10.9 11.1 14.5 11.6 33.4

mer or organic semiconductor, either by utilizing a bank structure that pre-defines the area of the semiconductor ink to be printed or by multi-printing to form a leveled pad [12]. The present approach focuses on enhancing the uniformity of ink-jet printed single dot DH4T by adjusting the solvent mixture combination.

drying at room temperature were observed by optical microscopy (Leica, DMLM) and confocal laser scanning microscopy (LEXT OLS3000, Olympus). I–V measurements were performed in air using an Agilent 5263A source-measure unit. 3. Results and discussion

2. Experimental DH4T (Syncom B.V.) was used as a soluble semiconductor material. Various solvents or their mixtures that can dissolve DH4T were tested, including tetrahydrofuran, chlorobenzene, xylene, dichlorobenzne, and trichlorobenzene (anhydrous, purity ∼ 99.9%, Sigma–Aldrich). In order to formulate a jettable ink, the ink should have a good solubility for DH4T. If the solvents of the ink have poor solubility, only dilute ink is usable; however, in this form, the ink cannot produce a uniform thin semiconductor layer. At a certain concentration, DH4T may be either precipitated or crystallized in the ink, potentially leading to nozzle clogging. Given these considerations, we measured the solubility of DH4T in various solvents. For the measurement of solubility, supersaturated DH4T solutions were dried in a vacuum oven for 24 h and the precipitates were separated by a 200-nm pore sized membrane filter. The physical properties of various solvents including the solubility are summarized in Table 1. Semiconductor inks were also prepared by dissolving DH4T in a mixture of two different solvents. The solvent combinations for DH4T inks are summarized in Table 2. The DH4T inks were printed onto a silicon wafer in air (relative humidity ∼35%) using an ink-jet printer. The wafer was sonicated in a piranha solution and cleaned by distilled water, followed by surface modification with hexamethyldisilazane (HMDS) selfassembling molecules. It is well known that HMDS assists molecular ordering of overlying DH4T, which in turn enhances TFT performance [7,9]. The printer set up consisted of a drop-on-demand (DOD) piezoelectric ink-jet nozzle manufactured by Microfab Technologies Inc. (Plano, TX) and the diameter of the nozzle was 50 ␮m. Uniform ejection of the droplets was performed by applying a ∼80 V impulse lasting ∼20 ␮s at a frequency of 1000 Hz. The volume and velocity of the ejected droplets were about 100 pl and 3 m/s, respectively. The DH4T ink was ink-jetted on the substrate after it was confirmed that each droplet had nearly identical volume and size. Once the semiconductor ink compositions were optimized, we fabricated a coplanar-type OTFT by ink-jet printing the DH4T ink between the source and drain electrodes. Prior to ink-jet printing, the SiO2 /Si surface was also treated with HMDS. The surface morphology and shape of the ink-jet printed DH4T single dot after

In general, evaporation of an ink-jet printed droplet leads to an uneven deposition of either the solutes or the particles dispersed in the ink. This phenomenon is known as the “coffee-ring effect”, since the deposited pattern is characterized by a ring shape that marks the original contact line of the ink droplet on the substrate [21]. Deegan et al. explained this effect in terms of pinning of the contact line of the droplet in combination with increased evaporation at the edges [22]. In order to pin the contact line, liquid evaporated at the edges must be replenished by liquid from the interior. The resulting outward flow can carry virtually all the dispersed materials to the edge. In efforts to overcome this effect and form a uniform film, mixtures of two different solvents have been utilized to form uniform polymer film, a silver conductive pattern, and photonic crystals [14,16,23]. The mixture should consist of two different solvents with a large difference in boiling point and/or surface tension in order to induce an inward flow in opposition to the outward convective flow. However, DH4T can be dissolved in organic solvents that have generally low surface tension while offering relatively large differences in solubility and boiling point, as shown in Table 1. In this regard, our approach to form a uniform DH4T film from a single ink droplet involves the use of a solvent mixture with differences in solubility and evaporation temperature. Fig. 1 shows optical microscopic images of the deposition pattern of DH4T single droplets ejected from an ink-jet nozzle as a function of the solvent composition. The solvent mixtures was comprised one solvent having a lower boiling point and a higher solubility and another solvent having a higher boiling point and lower solubility, as summarized in Tables 1 and 2. Both inks prepared from a mixture of tetrahydrofuran/chlorobenzene (Fig. 1a) and a mixture of chlorobenzene/trichlorobenzene (Fig. 1c) produced a relatively uniform deposition pattern, while some regions were lacking DH4T deposition. As the solvent of higher solubility and lower boiling point evaporates first when a droplet is placed on the SiO2 /Si substrate, the solvent dissolving potential toward DH4T gradually decreases during evaporation and the oligomeric DH4T molecules precipitate out before a significant convective flow develops to form a ring-shaped or non-uniform deposition pattern. To cover the whole region of a single dot, it is necessary to increase

Table 2 Compositions of solvent mixtures used for preparing DH4T semiconductor ink. The contact angles were measured when HMDS-treated Si wafer was used as a substrate. Ink ID

Solvent 1

Solvent 2

Composition by weight % (solvent 1:solvent 2)

Contact angle (◦ )

Fig. ID

1 2 3 4 5 6 7

Tetrahydrofuran Tetrahydrofuran Chlorobenzene Xylene Dichlorobenzene Dichlorobenzene Dichlorobenzene

Chlorobenzene Trichlorobenzene Trichlorobenzene Trichlorobenzene Trichlorobenzene Trichlorobenzene Trichlorobenzene

50:50 50:50 50:50 50:50 75:25 50:50 25:75

11.8 10.8 12.5 24.5 11.8 16.6 19.6

1a 1b 1c 1d 2a 2b 2c

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Fig. 1. Optical microscopic images of the deposition pattern of DH4T single droplets ejected from an ink-jet nozzle of 50 ␮m orifice size as a function of the solvent composition: (a) tetrahydrofuran:chlorobenzene = 50:50; (b) tetrahydrofuran:trichlorobenzene = 50:50; (c) chlorobenzene:trichlorobenzene = 50:50; (d) xylene:trichlorobenzene = 50:50. The solid loading of DH4T in the ink is 1 wt%. Scale bar denotes 50 ␮m.

the DH4T content within the ink. However, it was observed that the use of high concentration above 1 wt% prevents the formation of a stable jetting condition, and thus results in nozzle clogging. In the case of the ink based on a mixture of tetrahydrofuran/trichlorobenzene, DH4T was mainly deposited at the edge of the contact line, as shown in Fig. 1b. The boiling point of tetrahydrofuran (65.9 ◦ C) is much lower than that of trichlorobenzene (213.5 ◦ C). Tetrahydrofuran of higher solubility will evaporate so rapidly that most DH4T precipitates out at the edge of the droplet. This can pin the droplet at the contact line, causing coffee-ringshaped segregation, while a few DH4T particles settle in the central region of the ring. It is also observed that the use of a solvent having higher boiling point results in the deposition of large grain sized DH4T crystallites. Slow evaporation of trichlorobenzene of low solubility provides DH4T molecules with sufficient time to be crystallized (Fig. 1b–d). In contrast, the solvents of lower boiling point and higher solubility such as tetrahydrofuran and chlorobenzene cause rapid precipitation of DH4T, resulting in smaller grain sized DH4T (Fig. 1a). Based on the above results, we selected a combination of dichlorobenzene/trichlorobenzene. This mixture allows

the deposition of a uniform DH4T film without a significant difference in the solvent boiling points and at the same time promotes crystallization of DH4T. Fig. 2 shows the ink-jet printed single droplet of DH4T inks with a solvent mixture of dichlorobenzene and trichlorobenzene as a function of varying mixing ratios. The ink with dichlorobenzene:trichlorobenzene = 75 wt%:25 wt% showed a segregated deposition pattern (Fig. 2a). However, increasing the amount of trichlorobenzene led to the formation of a uniform film within a dot and the whole region was covered with DH4T, even when DH4T concentration was fixed to 1 wt% and each droplet had the same volume of about 100 pl. Furthermore, the surface energy of the solvent mixture with respect to the substrate plays a role in producing a uniform DH4T film. The contact angle of the ink with dichlorobenzene:trichlorobenzene = 25 wt%:75 wt% on the HMDS-treated SiO2 /Si substrate was 19.6◦ . This is larger than those of solvent mixtures based on dichlorobenzene/trichlorobenzene at other mixing ratios, as shown in Table 2. Due to a relatively large surface energy difference between the ink and substrate, the droplet placed on the substrate reaches an equilibrium shape without sig-

Fig. 2. Optical microscopic images of the deposition pattern of DH4T single droplets ejected from an ink-jet nozzle of 50 ␮m orifice size when the solvent mixture was composed of dichlorobenzene and trichlorobenzene. Relative solvent mixing ratio was varied as dichlorobenzene and trichlorobenzene: (a) 75:25; (b) 50:50; (c) 25:75. The solid loading of DH4T in ink is 1 wt%. Scale bar denotes 50 ␮m.

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Fig. 3. Confocal laser scanning microscopic 3-dimensional images of ink-jet printed single dots obtained from 1 wt% DH4T inks with various solvent compositions. Relative solvent mixing ratio was varied as dichlorobenzene and trichlorobenzene: (a) 75:25; (b) 50:50; (c) 25:75. The corresponding 2-dimensional profiles of DH4T as a function of the ink compositions are presented in (d). For comparison purpose, the deposition profile of DH4T obtained from the ink based on a solvent mixture of chlorobenzene:trichlorobenzene = 50:50, which represents an uncontrolled ink composition, was also shown.

nificant spreading prior to the solvent evaporation. This leads to the formation of a uniform film of smaller dot size compared to those made from the other inks. Fig. 3 shows 3-dimensional images and 2-dimensional surface profiles of the ink-jet printed DH4T single dots as a function of varying mixing ratios of dichlorobenzene/trichlorobenzene. When

using 75 wt% dichlorobenzene ink, DH4T is accumulated at the rim of the dot while the central region is almost empty, as observed in Fig. 3a. However, increasing the portion of trichlorobenzene drastically changes the deposition pattern within the dot. When 50 wt% dichlorobenzene ink is used (Fig. 3b), larger sized crystallites of DH4T are observed. Crystallites are spread over the whole region,

Fig. 4. Output and transfer characteristics of OTFT devices with ink-jet printed DH4T semiconductor layer with uncontrolled ink composition of (a, b) 50 wt% chlorobenzene and 50 wt% trichlorobenzene; with optimized ink composition of (c, d) 25 wt% dichlorobenzene and 75 wt% trichlorobenzene. Transfer characteristics were measured at a constant VD = −10 V. Insets show a top view of the fabricated OTFTs in which DH4T films were ink-jetted between the photolithographically defined Au source and drain electrodes. The substrate was heavily doped Si with a 200-nm thick SiO2 . Scale bar denotes 100 ␮m.

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but grains are separated by a region of empty space. It is observed, on other hand, that the use of DH4T ink at 75 wt% trichlorobenzene produces a fully covered film, as shown in Fig. 3c. The corresponding 2-dimensional profiles of the deposition patterns also clearly reveal improved uniformity of DH4T film (Fig. 3d). With an optimized solvent mixture composition, an OTFT device was fabricated by ink-jet printing of the DH4T ink. Insets in Fig. 4b and d show top views of the fabricated OTFT with two different ink compositions. Interdigitated electrodes of Au (49 nm)/Cr (1 nm) were photolithographically defined on top of a heavily doped n-type silicon wafer with a 200-nm thick thermal SiO2 layer. Channel length (L) and width (W) were 15 ␮m and 400 ␮m, respectively (W/L ratio ∼27). The optimized ink contains 1 wt% DH4T based on a solvent mixture of 25 wt% dichlorobenzene and 75 wt% trichlorobenzene, while a solvent mixture of 50 wt% chlorobenzene and 50 wt% trichlorobenzene was selected for the uncontrolled ink. Using both ink compositions, stable jetting without nozzle clogging was obtained, and thus the DH4T ink could be precisely deposited between the source and drain electrodes. The ink-jetted semiconductor from the optimized ink composition exhibited good uniformity and complete coverage between the channels, whereas a non-uniform channel region incompletely covered with DH4T was observed in the case of the uncontrolled ink composition. Fig. 4 shows the output and transfer characteristics of the devices fabricated by ink-jet printing two different DH4T compositions. The device with the uncontrolled solvent composition (chlorobenzene/trichlorobenzene) showed a mobility of a mobility of 7.3 × 10−5 cm2 V−1 s−1 in the saturation regime, and an on/off current ratio of 102 and a threshold voltage of about 2.66 V with subthreshold slopes of ∼2.24 V dec−1 (Fig. 4a and b). The inferior device performance can be attributed to the non-uniform discontinuous deposition of DH4T with partial empty regions obtained from the uncontrolled solvent composition as shown in the optical image (Fig. 1c) and the 2-dimensional profile (Fig. 3d). On the other hand, the OTFT device with optimized solvent composition (dichlorobenzene/trichlorobenzene) exhibited excellent field-effect transistor characteristics, which conform well to the conventional gradual channel model in both the linear and saturated regimes (Fig. 4c and d). This device showed a mobility of 8.5 × 10−3 cm2 V−1 s−1 in the saturation regime, and on/off current ratio of 103 and a threshold voltage of about 0.39 V with subthreshold slopes of ∼0.95 V dec−1 . These results clearly indicate that the coverage and uniformity of the semiconductor layer in the channel region significantly influence the device performance, which can be controlled by adjusting the semiconductor ink composition. 4. Conclusions We have demonstrated the fabrication of an organic thin-film transistor incorporating an ink-jet printed semiconductor layer. Ink compositions of DH4T semiconductor ink significantly influence the deposition pattern and film uniformity, which in turn determine the device performance. In order to produce uniform DH4T

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film by ink-jet printing a single droplet onto the substrate, a wellcontrolled solvent mixture having proper physical properties is required. One solvent should have a lower boiling point and higher solubility and another should have a higher boiling point and lower solubility. Various deposition patterns of the DH4T single dot could be obtained depending on the relative differences in the solubility and boiling point between the two solvents as well as their mixing ratio. We have determined the optimal ink compositions to produce a uniform and well-defined ink-jet printed DH4T semiconductor layer, i.e., 1 wt% DH4T ink based on a solvent mixture of 25 wt% dichlorobenzene and 75 wt% trichlorobenzene. The OTFT fabricated from the optimized DH4T ink exhibited better device performance compared to those made from uncontrolled ink. Acknowledgments This work was supported by the Korea Science and Engineering Foundation (KOSEF) through the National Research Laboratory Program funded by the Ministry of Science and Technology (No. R0A-2005-000-10011-0). It was also partially supported by the Second Stage of the Brain Korea 21 Project in 2008. J.K. Song thanks the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea Government (MOST) (No. M10300000320-06J000032010). References [1] C.D. Dimitrakopoulos, P.R.L. Malenfant, Adv. Mater. 14 (2002) 99. [2] H. Sirringhaus, T. Kawase, R.H. Friend, T. Shimoda, M. Inbasekaran, W. Wu, E.P. Woo, Science 290 (2000) 2123. [3] Y. Wu, Y. Li, B.S. Ong, P. Liu, S. Gardner, B. Chiang, Adv. Mater. 17 (2005) 184. [4] S.P. Li, D.P. Chu, S.J. Newsome, D.M. Russell, T. Kugler, M. Ishida, T. Shimoda, Appl. Phys. Lett. 87 (2005) 232111. [5] Y. Liu, K. Varahramyan, T. Cui, Macromol. Rapid Commun. 26 (2005) 1955. [6] T.-W. Lee, Y. Byun, B.-W. Koo, I.-N. Kang, Y.-Y. Lyu, C.H. Lee, L. Pu, S.Y. Lee, Adv. Mater. 17 (2005) 2180. [7] T. Muck, J. Fritz, V. Wagner, Appl. Phys. Lett. 86 (2005) 232101. [8] K.R. Amundson, H.E. Katz, A.J. Lovinger, Thin Solid Films 426 (2003) 140. [9] T. Muck, V. Wagner, U. Bass, M. Leufgen, J. Geurts, L.W. Molenkamp, Synth. Met. 146 (2004) 317. [10] X. Liu, M. Knupfer, B.-H. Huisman, Surf. Sci. 595 (2005) 165. [11] H.E. Katz, A.J. Lovinger, J.G. Laquindanum, Chem. Mater. 10 (1998) 457. [12] D.H. Song, M.H. Choi, J.Y. Kim, J. Jang, S. Kirchmeyer, Appl. Phys. Lett. 90 (2007) 053504. [13] B.-J. de Gans, P.C. Duineveld, U.S. Schubert, Adv. Mater. 16 (2004) 203. [14] B.-J. de Gans, U.S. Schubert, Langmuir 20 (2004) 7789. [15] E. Tekin, B.-J. de Gans, U.S. Schubert, J. Mater. Chem. 14 (2004) 2627. [16] D. Kim, S. Jeong, B.K. Park, J. Moon, Appl. Phys. Lett. 89 (2006) 264101. [17] D.W. Breiby, E.J. Samuelsen, O. Konovalov, Synth. Met. 139 (2003) 361. [18] A.P. Kam, J. Seekamp, V. Solovyev, C. Clavijo Cedeno, A. Goldschmidt, C.M. Sotomayor Torres, Microelectron. Eng. 73/74 (2004) 809. [19] Y.H. Kim, Y.U. Lee, J.I. Han, S.M. Han, M.K. Han, J. Electrochem. Soc. 154 (2007) H995. [20] J.A. Lim, W.H. Lee, H.S. Lee, J.H. Lee, Y.D. Park, K. Cho, Adv. Mater. 18 (2008) 229. [21] H. Hu, R.G. Larson, J. Phys. Chem. B 106 (2002) 1334. [22] R.D. Deegan, O. Bakajin, T.F. Dupont, G. Huber, S.R. Nagel, T.A. Witten, Nature 389 (1997) 827. [23] J. Park, J. Moon, Langmuir 22 (2006) 3506.