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Controlled Deposition of a High‐Performance Small‐Molecule Organic Single‐Crystal Transistor Array by Direct Ink‐Jet Printing

2011; Volume: 24; Issue: 4 Linguagem: Inglês

10.1002/adma.201103032

1521-4095

Yong‐Hoon Kim, Byungwook Yoo, John E. Anthony, Sung Kyu Park,

Advanced Memory and Neural Computing

Ink-jet printed small-molecule organic single-crystal transistors are realized by using selective surface energy modification, precise control of volume density of ink droplets on spatially patterned areas, and a co-solvent system to control solvent evaporation properties. The single-crystal formation in bottom-contact-structured transistors via direct printing is expected to permit high-density array fabrication in large-area electronics. Single-crystalline organic thin-film transistors (TFTs) have been investigated for studies of charge transport mechanisms and realizing commercial applications such as organic TFT-driven flexible active-matrix displays and sensor arrays, and generally show better performance and environmental stability compared to amorphous and polycrystalline organic TFTs.1 Although they have shown outstanding properties such as higher carrier mobility and on/off current ratio, the growth of single-crystals and controlling their crystal orientation and growth direction on active channel regions have posed significant technical challenges. Moreover, for low-cost fabrication, efficient organic single-crystal deposition and patterning processes are required. Several novel methods were reported to produce cost-effective patterning of organic semiconductors and organic single- crystals in both vapor- and solution-based processes.2 In these previous reports, spin casting (or drop casting) of solution, and submersion of surface-engineered substrates into a crystal suspension were proposed but both approaches can involve relatively significant material consumption and substrate contamination, respectively. Additionally, difficulties in controlling the spatial selectivity and film formation speed could limit the scalability of these methods. In order to overcome such limitations, the ink-jet printing technique has been considered as a candidate for cost-effective, volume controllable, and highly selective patterning method for organic single-crystal transistor arrays. However, achieving uniform and high-performance organic single-crystal arrays from ink-jet printing has confronted many difficulties due to complex drying mechanisms such as meniscus energy of the droplet ink, energy dissipation at the meniscus region, molecular diffusion in solvents, intra-molecular interaction, and solvent interaction with surfaces.3 Herein, we report high-performance ink-jet printed small molecule organic single-crystal transistors realized by simple selective surface energy modification, precise control of volume density of ink droplets on spatially patterned areas, and a co-solvent system to control solvent evaporation properties. The key differences and improvements of this research over the previously proposed techniques are easy control of single-crystal arrays growth and their spatial selectivity, versatility in large area application, and high-performing bottom-contact organic single-crystal transistor arrays using simple surface engineering and direct printing technology. It is also believed that previously reported dipping and drop casting methods can induce the single-crystal formation but one of the main advantages of ink-jet printing in comparison with the previously reported strategies is that it permits large-area application without the need of wetting all the substrate, leading to minimum material consumption. Additionally, utilization of bottom-contact source/drain structure rather than top-contact structure in realizing an organic single-crystal transistor array allows more usefulness in high-density array fabrication and manufacturing of large area applications. For spatial localization of organic single-crystals on channel regions using ink-jet printing, the surface state of the channel regions was selectively modified by using non-relief pattern lithography.4 An octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM) was selectively patterned to provide controllable hydrophilic domains separated from hydrophobic surroundings. The selective patterning of OTS-covered gate dielectric was achieved using deep ultraviolet (DUV) exposure (wavelengths of 254 nm (90%) and 185 nm (10%)) through a patterned quartz mask. DUV irradiation of silane SAMs typically induces a photochemical reaction which involves cleavage of Si–C bonds to form a reactive Si radical, which can react with atmospheric oxygen and moisture to form silanol groups and produce a hydrophilic surface.5 Figure 1 shows the water contact angle of SiO2 surface with OTS SAM layer and after 10 min and 20 min of DUV exposure. When semiconductor ink is dropped on the channel region, surface energy differences direct the ink into the desired hydrophilic area to cover the OTS-patterned channel region. a) A schematic diagram of defining OTS patterned channel region using deep ultraviolet (DUV) exposure through quartz photo mask. b–d) Water contact angle on OTS treated SiO2 surface (109.1°) (b), after 10 min of DUV exposure (23.7°) (c), and after 20 min of DUV exposure (22.5°) (d). The solvent evaporation behavior at the meniscus during the drying process has been controlled to achieve homogeneous crystalline structure and to avoid the "coffee-ring" effect4, 6 during ink-jet printing. Using a co-solvent system including a solvent with higher boiling point and lower surface tension, evaporation-induced Marangoni flow can be obtained, which is a counter-balanced flow against convection flow in the droplet.3 For the co-solvent system, chlorobenzene/dodecane mixture with 3:1 volume ratio has been used, which was revealed as the optimum condition in our experiments. Figure 2 demonstrates schematic diagrams of the device structures and obtained microscopic images of ink-jet printed 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) films; A) on bare channel area (no OTS treatment) from 2 wt% chlorobenzene solution, B) on bare channel area (no OTS treatment) from 2 wt% co-solvent system, and C) surface patterned channel area (with OTS patterning) from 2 wt% co-solvent system, respectively. As shown in Figure 2A, even a small ink volume (∼20 pL) dropped on bare channel area spreads out to a diameter of around 300 μm, resulting in a circular shaped structure with small crystals growing at the edges due to the coffee-ring effect. Whereas, sample (B) typically demonstrated more crystal-like structures which were grown randomly on Au, SiO2, and the channel area. With selective OTS patterning by DUV exposure, creating a hydrophilic domain at the channel region in Figure 2C, the TIPS-pentacene crystals are more likely to form on the Au electrode and channel area. This is due to steering of the dropped TIPS-pentacene ink towards the Au electrode and channel area by the hydrophobic OTS regions as illustrated in Figure 2D. Figure 3A shows an array of single-crystal TIPS-pentacene TFTs from ink-jet printing, and the resulting crystalline structures were investigated by polarized optical microscope (POM) and out-of-plane mode X-ray diffraction (XRD) studies. As shown in the POM images, well-ordered single-crystals of TIPS-pentacene are formed on the channel and Au electrode regions when the OTS has been selectively removed (Figure 3B). On the other hand, only small crystallites of TIPS-pentacene were formed when the OTS layer has not been removed in the channel region (Figure 3C). The XRD data (Figure 3D) indicates the printed TIPS-pentacene single-crystals have preferential orientation of the (001)-axis normal to the surface which is similar to reference data obtained from drop-cast TIPS-pentacene single-crystals.7 Schematic in-plane and cross-section diagrams and microscopic images of ink-jet printed TIPS-pentacene films on bare channel area (no OTS treatment) from 2 wt% chlorobenzene solution (a), on bare channel area (no OTS treatment) from 2 wt% co-solvent system (b), and on surface patterned channel area (with OTS patterning) from 2 wt% co-solvent system (c). d) Illustrations of steering of dropped TIPS-pentacene ink into hydrophilic region and crystal growth. a) An optical microscopy image of ink-jet printed TIPS-pentacene TFT array, and polarized optical microscope (POM) images of ink-jet printed TIPS-pentacene TFTs with, surface patterned channel area (b) and hydrophobic channel area (without OTS removal by DUV) (scale bars: 200 μm) (c). d) X-ray diffraction pattern of TIPS-pentacene single-crystal array (out-of-plane mode). Recently, several studies were performed on the nucleation activity of anthracene molecules on Au surfaces functionalized with different SAMs as examples of site-selective nucleation.8 It was found that the nucleation density of anthracene depended on the surface energy of gate dielectric, and highly localized crystal growth onto higher surface energy regions was observed by submersion of substrates patterned with both the SAMs into a saturated anthracene solution and allowing solvent evaporation. Similar nucleation processes were observed in our case. Figure 4 shows time-scale evolution images of drying TIPS-pentacene inks on different surface structures. When the TIPS-pentacene ink is dropped on hydrophobic OTS/SiO2 surface (water contact angle of 90° ∼ 110°), the diameter of dropped ink is relatively small due to the low surface energy. Also, the contact line is not pinned during the drying process and only the reduction of the droplet size is observed resulting in a non-crystallized structure (Figure 4A). On a less hydrophobic PFBT/Au surface (with water contact angle of 60° ∼ 70°), the contact line is pinned near the edge of the circular-shaped droplet and multiple crystals are grown from the edge to center regions (Figure 4B).9 If the TIPS-pentacene ink is dropped on the channel area with OTS selectively removed, the ink is confined to less hydrophobic surfaces and artificial contact lines are introduced. In this case, the nucleation and growth could start from one end or both ends of the PFBT/Au region (Figure 2D) into the OTS patterned hydrophilic area, and finally evolve into a large crystal on the hydrophilic domain at the channel region (Figure 4C). Time-scale evolution images of drying TIPS-pentacene inks on different surface structures, a) on OTS/SiO2 surface (more hydrophobic, water contact angle of 90° ∼ 110°), b) on PFBT/Au surface (less hydrophobic, water contact angle of 60° ∼ 70°), and c) on surface patterned channel region (scale bars: 200 μm). When thick and rough OTS films were deposited onto smooth SiO2 or a high surface energy area, it was typically observed that the selective nucleation of crystals is preserved in vapor-deposited organic single-crystal cases.10 It indicates that nucleation sites rely on a contrast in surface chemistry. The rough topography in surface domains was concluded to be the predominant factor in the selective nucleation, yielding crystal nucleation at the relatively stable defects and cleavage sites on the rough OTS surface possibly due to higher kinetic energy of thermally evaporated organic molecules. In contrast to vapor deposited organic single-crystal growth, inter-molecular interaction which fastens the adjacent molecules with molecular self-organization may dominate in solution-processed organic single-crystal growth possibly due to the weak interaction of the semiconductor molecules at the meniscus. The crystal growth can proceed with inter-molecular interaction resulting in lateral (directional) crystal growth. For this lateral crystal growth, one of the most important factors determining the inter-molecular interaction is film formation speed which depends on the rate of solvent evaporation at the meniscus.11 The transfer characteristic of ink-jet printed single-crystal TIPS-pentacene TFTs and statistics data of field-effect mobility from the TFT arrays are shown in Figure 5. The device had channel lengths of 10 ∼ 20 μm, channel widths of 50 ∼ 100 μm, and a 200 nm-thick SiO2 was used as gate dielectric. The single-crystal TIPS-pentacene TFT devices show extracted field-effect mobility as high as 0.6 cm2 V−1 s−1 (average of 0.20 cm2 V−1 s−1 from 42 devices) with on/off current ratio of 105 ∼ 106, and subthreshold slope of 0.4 ∼ 0.9 V decade−1. It is not clearly understood but we believe that the relatively low mobilities of the single-crystal TFTs are possibly due to contact issues similar to those of hand-picked laminated crystals on the source-drain electrodes.12 In case of using top-contact structure, the contact issue is less important due to formation of a conformal contact between source/drain electrodes and underlying organic semiconductor layer. Also, growth of organic single-crystal is not interfered by any physical barrier and more perfect crystals can be formed on the surface. Recently reported top-contact-structured organic single-crystal TFTs using 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene shows such advantages of using top-contact structure.13 On the other hand, in case of bottom-contact structure, the organic crystals must grow over the predefined Au source/drain electrodes having 50 ∼ 100 nm in thickness. In such case, the organic crystal growth can be interfered by the underlying electrodes and may lead to formation of a non-conformal contact especially at the edges of source and drain electrodes. To overcome the weak electrostatic bonding of a single-crystal onto predefined electrodes/gate dielectric and induce intimate contact, a solvent vapor annealing process was performed after the controlled single-crystal deposition. Figure 6 shows the evolution of transfer characteristics of a single-crystal TFT via toluene vapor annealing, resulting in increased mobility up to 1.7 cm2 V−1 s−1 which is comparable to the previously reported TIPS-pentacene single-crystal TFTs.14 In addition to the formation of an intimate contact, improvement in molecular ordering can be another possible reason for the increased mobility. Upon solvent vapor annealing process, the organic semiconductor molecules become more mobile by the evaporated solvent which can lead to reorganization of organic semiconductor molecules into a more energetically stable structure with improved molecular ordering, and also lead to phase change from amorphous to ordered structures.15 a) Transfer characteristic (log(ID)– VGS) (ID is the drain current, VGS is the gate-source voltage) of a single-crystal TIPS-pentacene TFT fabricated with surface patterned channel region. The channel length and width were 10 μm and 50 μm, respectively. The inset shows the printed single-crystal TIPS-pentacene TFTs (scale bar: 100 μm). b) Statistic data of single-crystal TIPS-pentacene TFT array with an average field-effect mobility of 0.20 cm2 V−1 s−1 from 42 TFT devices. Transfer characteristics of single-crystal TIPS-pentacene TFT before and after solvent annealing process using toluene as a solvent (solvent annealing time: 40 sec). The channel length and width of the TFT were 20 μm and 80 μm, respectively. The inset shows optical image of solvent-annealed TIPS-pentacene TFT. For the results described here, it is likely that the formation of solution-processed organic single-crystals is closely related to the optimization of solvent evaporation behavior, surface engineering, and volume density of the solute. Appropriate solvent evaporation at the meniscus region (liquid/vapor/solid interface) and optimum volume density of the semiconductor molecules on high surface energy substrates, encouraged by the controlled wetting area and co-solvent system, may facilitate directional lateral growth of organic single-crystal films. We believe that the large lateral single-crystals are a consequence of the liquid phase re-growth from the discontinuous and small solid seed-like points (nuclei) which were solidified rapidly in the meniscus. Hence, as the solvent begins to evaporate, crystal growth from these seeds can proceed on patterned hydrophilic surface, resulting in laterally-grown TIPS-pentacene single-crystals over the channel region. This qualitatively explains the observation of the large lateral organic single-crystal growth on the differential surface patterned region from direct ink-jet printing. Single-crystal organic transistors were fabricated using ink-jet printed TIPS-pentacene as the active material. A 200 nm thick layer of silicon dioxide and heavily doped (0.015 Ω-cm) n-type silicon wafers were used as the gate dielectric and gate electrode, respectively. Au/Ti source and drain electrodes were deposited by thermal evaporation and patterned using lift-off. Prior to SAM treatment and active layer deposition, the substrates were cleaned using deep ultraviolet (DUV) ozone. To improve the metal/organic contact, self-assembled monolayer (SAM) of pentafluorobenzenethiol (PFBT) was formed on the Au source/drain electrodes. The PFBT monolayer was formed by immersing the substrate into a toluene solution (0.1 mM) for 10 min. Then, a hydrophobic octadecyltrichlorosilane (OTS) SAM was formed by immersing the substrate into a solution of OTS in hexane (30 mM). The substrates were allowed to incubate with the self-assembly solution for 30 min. Subsequently, ultrasonication process was performed and another OTS formation process was carried out for building dense and stable SAM. After removal from the ultrasonication process, the OTS covered samples were washed with hexane and dried with a stream of dry nitrogen. Spatial patterning of OTS covered substrates was achieved using DUV exposure (wavelengths of 254 nm (90%) and 185 nm (10%)) through a quartz mask. The contact angles of water droplets were 109.1 degrees and 23.7 degrees before and after OTS removal, respectively. For growth of single-crystal active layer, 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) solution (2 wt%) from the mixture of chlorobenzene and dodecane with volume ratio of 3:1 were ink-jetted over the pre-patterned barrier-like SAM structures and dried. The deposition of TIPS-pentacene was performed using a piezoelectric ink-jet printing system (UniJet UJ2100). The piezoelectric ink-jet nozzle had a diameter of 50 μm (orifice size of 50 μm). The frequency of the jetting was 150 Hz and the diameter of the ink drop was approximately 30 ∼ 50 μm. The authors gratefully acknowledge financial support by a grant (F0004023-2010-33) from Information Display R&D Center, one of the 21st Century Frontier R&D Program funded by the Ministry of Knowledge Economy of Korean government, and the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (No. 2010-0002623).