Interface-Mediated Self-Assembly in Inkjet Printing of Single-Crystal

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Interface-Mediated Self-Assembly in Inkjet Printing of Single-Crystal Organic Semiconductor Films Makoto Yoneya, Hiromi Minemawari, Toshikazu Yamada, and Tatsuo Hasegawa J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b02143 • Publication Date (Web): 05 Apr 2017 Downloaded from http://pubs.acs.org on April 9, 2017

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The Journal of Physical Chemistry

Interface-Mediated Self-Assembly in Inkjet Printing of Single-Crystal Organic Semiconductor Films Makoto Yoneya ,∗,† Hiromi Minemawari ,† Toshikazu Yamada ,† and Tatsuo Hasegawa

‡

† National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba 305-8565, JAPAN ‡ Department of Applied Physics, School of Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 111-8656, JAPAN E-mail: [email protected]

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Abstract Recent reports have demonstrated that printing processes are more suitable for producing high-performance organic thin-film transistors (OTFTs) than vacuum processes, although the formation mechanism of solution-based films is not yet understood. Here, we use molecular dynamics simulations to show that prototypical solution-processable organic semiconductors (OSCs) form a temporal lyotropic liquid-crystalline (LLC) molecular layer at the air-liquid interface of the semiconductor solution, which subsequently serves as a versatile precursor to the growth of a single-crystalline film. The molecules exhibit spontaneous alignments of the molecular long axes to form a molecular layer that is parallel to the air-liquid interface, whereas alignment with the short axes is not observed and the molecules move diffusively within the layer. The onset of short-axis orientational and positional ordering was observed in the later stage of film growth. Our findings provide evidence that unique way of film growth occurs via stepwise order formation in solution-processed OSCs allowing the non-epitaxial growth of extremely large-area single-crystal films.

INTRODUCTION Organic semiconductors (OSCs) have a unique characteristic among semiconducting materials in that they can be processed in solution under ambient conditions. 1,2 This solution processability has created a great deal of interest in recent years because it may enable the low-cost production of flexible, lightweight, and large-area electronic products using print production (or ”printed electronics”) technologies. 3 In particular, several recent reports have demonstrated that some printing or solutionbased thin-film processing methods are more suitable for manufacturing high performance organic thin-film transistors (OTFTs) than vacuum-based thin-film processing methods. 4 These solution processes include spin-coating at high temperatures 5 and off-center spin coating 6 for the manufacture of self-aligned polycrystalline thin films as well as doubleshot inkjet printing (DS-IJP), 7 solution-shearing, 8 and edge-casting 9 techniques for the 2


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manufacture of large-area (order of several hundred microns square) single-crystalline thin films (a few tens of nm in thickness) on amorphous substrate surfaces. Note that singlecrystalline film growth on amorphous substrate surfaces is non-epitaxial and thus cannot be achieved with vacuum processes, although the formation mechanism of solution-based films is not yet understood. Film growth during vacuum processes proceeds in a rather conventional manner, wherein the nucleation and subsequent growth are controlled by the diffusive motion of organic molecules along the amorphous substrate surfaces, which naturally causes twodimensional (2-D) polycrystalline film growth. In contrast, film growth during solution processes should proceed in a more self-organizing manner with fully three-dimensional molecular diffusion within the fluid phases. 10 A common primary step in solution processes is to produce a thin OSC solution layer on top of the substrate surface. For example, spin coating is a process used to produce a thin solution layer via the centrifugal force resulting from spinning the substrates. Another example is the DS-IJP process, 7 in which antisolvent ink is printed first and then overprinted with the OSC solution ink, rapidly forming a thin OSC solution layer on top of the antisolvent droplet surfaces. DS-IJP was found capable of growing single crystalline thin-films over the entire area of the films with their crystalline layers parallel to the films. 7 This means that molecular diffusion and layered film growth parallel to the solution layer should be necessary for the formation of a single-crystalline film. Nonetheless, these molecular assembly dynamics within the solution layer have never been investigated because the experimental data we currently have on the DS-IJP process are limited to the observations with an optical microscope. In this study, we present a thorough investigation that applies molecular dynamics (MD) simulations to reveal the molecular diffusion and layer formation within an OSC solution. Because modeling the entire solution process by MD simulations is not possible due to the limitation of computational resources, we focused on an important and idealized cross

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section in the real solution process. The results revealed that the air-OSC solution interface provides OSC molecules with unique self-organizing fields for the spontaneous growth of aligned molecular layers parallel to the air-OSC solution interface. Furthermore, the initial stage of the OSC molecular assembly was found to gradually progress from the lessordered lyotropic liquid crystal (LLC) state, in which the molecules can move diffusively within the molecular layers, to the highly ordered state with short-axis orientational and positional ordering. We discuss how this stepwise order formation within the molecular layers enables the non-epitaxial growth of extremely large-area single-crystalline films by the solution processes. When we model the real solution process for MD simulations, we encounter serious difficulties in the treatment of highly non-equilibrium environments formed by processes such as solution flow or solvent evaporation. Here, we use the film growth process by the DS-IJP technique 7 as a specific and useful example because it is an ideal process whose film growth should not be directly affected by solution flow or solvent evaporation but rather is dominated by intrinsic intermolecular interactions. We particularly simulated a DS-IJP process in which 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (8BTBT8), 11 1,2-dichlorobenzene (DCB), and N,N-dimethylformamide (DMF) were used as the OSC material, solvent, and antisolvent for OSC, respectively. 7 In the DS-IJP process, the OSC solution is rapidly spread over the antisolvent surface within a few milliseconds after its deposition on the surface. 12 The thickness d of the OSC solution layer after complete spreading 13 is estimated to be d ∼ 0.8µm. 14 Assuming that a typical value of the diffusion constant is D ∼ 10−9 m2 s−1 , 12 the time scale of the vertical mixing of the solvent and antisolvent by molecular diffusion can be estimated as τmixing ∼ d2 /D ∼ 0.6ms. This solvent/antisolvent mixing readily forces the OSC solution layer to be supersaturated because of the rapid reduction in solubility. We define this stage of the DS-IJP technique (i.e., the period that the OSC solution layer undergoes supersaturation) as the key step for the film growth when the layer formation of the OSC molecules occur.

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We note that the OSC solution layer at this stage is still a liquid layer and that crystalline solid films have been observed at much later stages in experiments (i.e., approximately 5 s after deposition). 7 This means we will not investigate the crystallization stage, rather we focus on the diffusion and liquid layer formation stage of the DS-IJP process in this study.

METHODS We applied the flexible detailed-atom model with the exception that all of the bondstretching degrees of freedom were constrained to their equilibrium bond lengths. A general AMBER force field 15 in combination with the aliphatic CHn united atom parameters from a reoptimized united atom force field 16 was used for the inter- and intramolecular interactions. The restrained electrostatic potential (RESP) charges 17 were obtained using ab initio molecular orbital calculations at the B3LYP/6-31G(d) level using the Gaussian03 program, 18 and these values were used for the atomic charges. The trajectories were produced using the GROMACS program (version 4) 19 with leapfrog time integration and linear constraint solver (LINCS) bond constraints. 20 A steepest descent energy minimization was applied to the simulation system before the MD simulation. The time integration step was set to 4 fs because of the stability of the LINCS algorithm and the hydrogen mass repartitioning (i.e., to be heavier while preserving the total molecular mass). 21 Charge group-based twin-range 0.9 nm van der Waals and 1.8 nm electrostatic cut-off distances 19 were applied to the non-bonding interactions. The former value (0.9 nm) is the same as in the developments and evaluations of the force-field parameters that were applied in this study. 16 We did not apply Ewald-like methods, which usually require three-dimensional periodic boundaries, to the electrostatic interactions because we applied two-dimensional periodic boundary conditions in this study. 22 The simulation temperature was specified to be the same temperature, 30 ◦

C (303 K), as the IJP head and substrate temperature of the real DS-IJP process. The

simulation pressure was set to atmospheric pressure. For the temperature and pressure 5



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control, a robust Berendsen thermostat and barostat 23 with varying MD cell shapes were applied in the initial highly non-equilibrium part of the simulations. These were then switched to a velocity-rescaling thermostat 24 and a Parrinello-Rahman barostat 25 in the later parts. More detailed descriptions of the simulation methods are included in the supporting information. The initial structure was formed by placing a supersaturated thin OSC solution (8BTBT8s in DCB solvent) layer with a thickness of approximately 9 nm on the DMF antisolvent subphase and under the nitrogen gas phase as shown in Figure 1a-(i). [Figure 1 about here.] Although the solvent and antisolvent mixing should have started at this stage, we assume a pure DMF subphase as the initial condition.

RESULTS AND DISCUSSION The simulated temporal evolution showed unique dynamics especially at the air-OSC solution interface. Initially, the 8BTBT8 molecules were randomly oriented within the OSC solution layer, as shown in Figure 1a-(i). However, after 50 ns of the MD run, the 8BTBT8 molecules spontaneously formed well-aligned layers parallel to the air-OSC solution later interface (Figure 1a-(v)). The aligned-layer began to form at the air-OSC solution interface (Figure 1a-(iii)) and then proceeded to the inside of the solution towards the multilayer formation (Figure 1a-(vi)). Figure 1b shows the number density distributions of the component molecules (8BTBT8, DCB, and DMF molecules) as functions of the z coordinate (interface normal direction) after 4 ns and 50 ns of the MD run. The multilayer growth of 8BTBT8 from the air-OSC solution interface to the inside is clearly shown by the change of the 8BTBT8 density distributions from t = 4ns to t = 50ns. In the latter, three peaks are clearly observed in the density distribution of 8BTBT8, which correspond to the formation of three 8BTBT8 layers (Figure 1a-(v)). We found that the density distribution of the solvent DCB molecules 6


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are in antiphase with the 8BTBT8 peaks. In other words, the DCB molecules are enclosed by the 8BTBT8 layers in their inter-layer regions. Note that this nano-segregated structure of the layered 8BTBT8 and the DCB solvent near the air-OSC solution interface (Figure 1a-(v)) appears similar to the schematic picture of a LLC system at the air-liquid interface that has been reported in the literature (e.g., Figure 6 in Ref. 26 ). To confirm the liquid-crystalline nature of the simulated state, we investigated the diffusion motion of the 8BTBT8 molecules in terms of the mean square displacements (MSDs) of the molecules based on the trajectory analysis for the MD run. The MSD calculation was conducted only for the 8BTBT8 molecules that were involved within the multilayers, and molecules that intruded into the antisolvent subphase were excluded. The MSD plots for the last 2 ns of the MD run are shown in Supplementary Figure S1. The linear slopes of the plots and their anisotropy indicate molecular diffusion and provide clear evidence that the 8BTBT8 molecules within the multilayers were not in a crystalline state but rather were in a liquid-crystalline state. The diffusion constants obtained by the MSD plot slope are 1.5 ×10−10 and 0.3 ×10−10 m2 s−1 in the directions parallel and perpendicular to the layer, respectively. These diffusion constants are comparable to the values obtained for LLC systems by experiments. 27 The large anisotropy of the diffusion constants between the directions parallel and perpendicular to the layer is a common characteristic of the LLC systems. Next we discuss the molecular alignment dynamics in terms of the orientational order parameter of the 8BTBT8 molecules. Figure 1c shows the temporal evolution of the second-rank orientational order parameter, < P2 >; the values were obtained from the largest eigenvalue of the order parameter tensor Qαβ = (1/2N )

∑

i (3eiα eiβ

− δαβ ) 28

(written out explicitly in the Supplementary information section S2) , which is composed of molecular axis vectors ei that correspond to the long and short axes of the 8BTBT8 molecules. The long and short axes of the 8BTBT8 molecules were defined as the vector between the two outermost carbon atoms of the BTBT core and the vector between the sulphur atom and the center of the thiophene ring, respectively (see the inset in Fig.

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1c). < P2 >= 1 indicates a perfect alignment of the vectors, while < P2 >= 0 indicates random orientations. The figure clearly shows spontaneous alignment of the molecular long-axis and growth up to < P2 >∼ 0.7 during the simulation. In contrast, the shortaxis value of ca. 0.2 indicates nearly random orientations. The results clearly indicate the existence and absence of orientational order with the molecular long-axis and short-axis, respectively. These orientational order values correspond well to those of LLC systems 29 and differ distinctly from the corresponding values of the crystal phase of 8BTBT8, which are 0.99 and 0.68 for the long and short axes, respectively (the values obtained with the layered herringbone single crystal structure of 8BTBT8 30 ). We note that the low dip of the long-axis < P2 > at t ∼ 34 ns, as observed in Figure 1c, was caused by corrective re-orientation of the 8BTBT8 molecules within the top layer to a different orientation from the other 8BTBT8 molecules, which we will discuss in a later section. To further investigate the initial formation of LLC, we again consider the locations where the 8BTBT8 molecules start to form the aligned layers in the system. The simulation results in Figure 1a indicate that the aligned layer starts to form at the air-OSC solution interface. To confirm this, we studied a corresponding system without the airOSC solution interface by adding an extra DMF antisolvent layer on top of the OSC liquid layer, whose initial structure is shown in the supporting information Figure S2a-(i). In this simulation, formation of the layers parallel to the antisolvent-OSC solution interface was not observed, but rather irregular shaped one was formed as evident in the snapshots (Figure S2a). Lack of the layers parallel to the interface was also clear in the number density distributions, that is shown in Figure S3, as functions of the z coordinate (interface normal direction) after 50 ns of the MD run. We observed the molecular alignment of the 8BTBT8 long-axis as in temporal evolution of the orientational order parameter (Figure S2b). However, we found directions of the alignment were changed between different MD runs. We think this situation without the air-OSC solution interface is somewhat similar to the conventional antisolvent crystallization, which often leads to irregularly shaped product crystals. 31 Such irregularly shaped crystals are far different from the thin-film

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crystals with their crystalline layers parallel to the films by DS-IJP technique. These occur because the boundary at the antisolvent-OSC solution interface is not clear and irregularly shaped because the DCB solvent and the DMF antisolvent can mix with each other. In such a case, the LLC layer growth parallel to the interface should be lost. In contrast, clear material boundaries are prescribed at the air-OSC solution interfaces which can serve as an ideal field for the LLC layer growth parallel to the interface. We note that the molecular alignment in this case (Figure S2) should be mediated with rapid extraction of DCB solvents with the DMF antisolvent like as in the conventional antisolvent crystallization. In contrast, the aligned layer formation in the previous case (Figure 1) is mediated with the air-OSC solution interface. We also investigated the role of antisolvent, by simulating the temporal evolution of the dense OSC solution (8BTBT8 in DCB) without the antisolvent (DMF) subphase, the result of which is shown in the supporting information Figure S4. The results clearly show that the aligned-layer formation started at the air-liquid interfaces (free surfaces) and then proceeded to the inside but the growth of the inside layer was rather slow because of segregated DCB solvents filled up the inside region. This indicates that the LLC layer can be formed at enough high concentration of the solution, and that the antisolvent subphase works effectively as the solvent absorber to enable the multi-layer formation. We must also mention a fundamental role of the solvent molecules (i.e. lyotropic system) in the formation of the aligned 8BTBT8 layer. Because the melting temperature of 8BTBT8 molecules is 110◦ C, 32 they are in the crystal phase at the DS-IJP processing temperature of 30 ◦ C. 7 Thus, solvent molecules are necessary to maintain the fluidity that is required for the molecular diffusion and spontaneous alignment of the OSC molecules. To confirm this, we also studied a system without the DCB solvent molecules. The simulation results are shown in Figure S5 of the supporting information. In this simulation, the time for the molecular diffusion and spontaneous alignment was limited to only the initial stage. The system resulted in a nano-scale polydomain structure after 50 ns of the MD run and the orientational order of the 8BTBT8 molecules was limited to less than 0.5

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as shown in Figure S5b of the supporting information. These results demonstrate that the solvent molecules (i.e. lyotropic system) are indispensable to the formation of the aligned 8BTBT8 layer. [Figure 2 about here.] Although the orientational orders in Figure 1c are nearly equilibrated, we found that this is not the case if we focus on the single layer at the air-liquid interface that is specified with the dotted line box in Figure 1a-(v). The onset of additional short axis orientational ordering was observed at t ∼ 36ns for the single layer at the air-OSC solution interface, as shown in Figure 2. Figure 2 also shows corresponding snapshots of the selected layer before and after the onset of short-axis orientational ordering. These results clearly demonstrate that the ordering process was still progressing towards a crystalline state in the simulated LLC system. The corresponding temporal evolution of the diffusion constant within the layer is also shown in Figure 2 (red line). The diffusion constants after the shortaxis ordering decrease sharply to approximately one-quarter of that before the ordering. Nonetheless, the layer was still in an LLC state with a finite molecular diffusion constant (order of 10−10 m2 s−1 ). We also found that the density increases after the onset of shortaxis orientational ordering. The temporal evolution of the tilt angles of the molecular long-axis from the layer normal direction is shown in the supporting information Figure S6. The tilt angle increased slightly to more than 40 degrees after the short-axis ordering. The increase of the tilt angle corresponds a decrease of the corresponding molecular layer thickness and thus an increase of the effective density of the layer because the layer area and the number of molecules in the layer remain nearly constant. We believe that the increase of the tilt angle should not be the result of natural equilibration but should be temporally induced by the increase of the layer density. Considering these results, the time scale of the further ordering process is probably much longer than the accessible time scale of conventional MD simulations. To speed up the process, we performed an additional MD run with a model system composed of the selected single 8BTBT8 layer (shown in Figure 2 as snapshot after 50 ns) that is removed 10


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without the neighboring solvent molecules and placed on a model substrate surface (shown in Figure 3a). [Figure 3 about here.] The interaction between the molecules and the substrate surfaces was modeled by the 9-3 Steele potential 33 with interaction parameters that are weakened by half compared to those in the literature about silica surfaces. 22,34 The reason to utilize the Steele potential is minimizing external effects of dynamical fluctuation and structural disorder from the sublayer to the LC layer in order to speed up self-organizing process (e.g. herringbone-order formation) of the LC layer itself. Figure 3b shows snapshots of this MD run. The color of the molecule corresponds to values of the local herringbone order parameter Sherr (see the supporting information section S7 for the definition) of each molecule. The arrangement of the 8BTBT8 molecules initially shows parallel stacking structure (Figure 3b-(i) marked with the dotted box). However, the snapshot after 50 ns (Figure 3b-(v)) clearly demonstrates the subsequent evolution into the herringbone short-axis orientational ordering and positional (lattice) ordering of the 8BTBT8 molecules within the layer, which provide clear evidence of further ordering towards a crystalline state. However, herringbone angle value averaged over the molecules shown in Figure 3b-(ii) is 0.82 times smaller than the corresponding value obtained with the single-crystal structure of 8BTBT8 30 (with the same definition in the supporting information section S7). Also, the diffusion constant within the layer is two orders of magnitude smaller than the corresponding value at the end of Figure 2 (red line), but still has a value. We think that the simulated system at this stage is still in their course toward crystallization. 35 Based on the MD simulations, we suggest that the liquid crystallinity of the OSC material should allow the formation of the aligned layer at the air-OSC solution interface. 8BTBT8 is known to exhibit a layered (smectic) LC phase at high temperatures between its crystal phase and isotropic liquid phase (phase sequence: crystal 110◦ C smectic A 126◦ C isotropic). 32 The air-liquid interface is known to induce a vertical (interface normal) alignment of thermotropic and lyotropic LC systems. 26 The vertical alignment minimizes 11



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the loss of attractive interactions with neighboring LC molecules. 36 These features of the smectic LC system are likely related to the ability for spontaneous LLC layer formation at the air-liquid interface that was observed in our simulation with the 8BTBT8 molecule. To confirm the generality of this scenario, we conducted additional MD simulations of other smectic LC materials, including 2,9-dioctyl-pentacene (8PEN8) 37 and non-LC compounds, including 6,13-bis(triisopropylsilyl-ethynyl)pentacene (TIPS-PEN). 38 The Structures of these materials are shown in Figure 4. [Figure 4 about here.] Because all of the materials in Figure 4 are in their crystal phases at the DS-IJP processing temperature (30 ◦ C), solvent molecules are necessary to maintain the fluidity as we discussed for the case of the 8BTBT8 molecule in the previous section. We also note that to the best of our knowledge, none of the three smectic LCs in Figure 4 have been reported to have semiconducting properties of the OTFTs via solution processing. The simulation results (see the supporting information section S8 for details) are summarized in Figure 4 along with the corresponding results for 8BTBT8 from the previous section. In this figure, L/S corresponds to the long- and short-axis length ratio of the molecules, which is known to be related to their liquid crystallinity. 29 < P2 >avr corresponds to the orientational order parameter of the molecular core long-axis averaged over the final 10 ns (t = 40 to 50 ns) of the MD runs for each molecule. The materials can be divided into two groups based on their L/S values: the smectic LC materials with high L/S and < P2 >avr values and the non-LC materials with low L/S and < P2 >avr values. The formation of the LLC layer was observed only in the former group in our simulations and did not occur in the latter group for at least 200 ns of the simulations (Fig. S11-S13). These results are consistent with a recent experiment that showed that it is not possible to produce single-crystalline thin films by DS-IJP with TIPS-PEN. 39 Actually, it was pointed out that additional solution-shearing flow is required to grow single-crystalline thin films for TIPS-PEN. 8 We note that the smectic LC forming ability of the OSC material might not be a nec12


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essary condition for obtaining single crystalline films by DS-IJP. Some materials without the ability to form smectic LCs themselves may (with adequate solvents) show spontaneous LLC layer formation at the air-liquid interface. For example, the ability to form LCs may be lost with shorter chain homologs, but they may (with adequate solvents) have the ability to spontaneously form LLC layers. Materials with the ability to form smectic LCs may be more likely to form LLC layers than materials without the ability to form smectic LCs.

CONCLUSIONS Our simulation results demonstrated that the molecular alignment and layer formation of 8BTBT8 OSC molecules in the DS-IJP process are preceded by the spontaneous formation of aligned LLC layers at the air-OSC solution interface. The LLC layers should cover the entire area of the air-liquid interface because the air-liquid interface acts as a vertical (interface normal) alignment layer. These molecularly connected LLC layers likely serve as a good precursor to single-crystal film growth due to the pre-organized long-range molecular orientational and layer ordering. These properties set up further orientational and translational ordering (Figure 2 and 3) that leads to the growth of not polycrystalline but rather extremely large-area single-crystalline films via controlled directional growth using an appropriately predefined hydrophilic area. 7 We actually observed that a much thiner film covering the whole surface of the printed deposit is formed at the initial stage of the DS-IJP process. 7 We’re thinking this observation may correspond to the LLC layer formation at the air-OSC solution interface in our simulations. Quite recently, the formation of a transient smectic LC phase of 8BTBT8 thin-films during hollow pen writing (8BTBT8 with toluene solvent as OSC ink) process was confirmed by in-situ microbeam grazing incidence X-ray scattering measurements. 40 We need the similar in-situ X-ray measurements during DS-IJP in future to further relate our simulation results to the real DS-IJP process. The utilization of LC layers as a precursor to high-quality polycrystalline thin films 13



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via spin-coating at high (LC phase) temperatures was proposed by Iino and Hanna. 5 Our results expand this concept to LLC layers as an indispensable precursor to obtaining single crystal films by means of printing processes at ambient temperatures. It is important to note that the aligned LLC layer formation that starts at the air-liquid interface and continues through stepwise order formation towards a crystalline state is markedly different from conventional crystal nucleation and subsequent epitaxial growth on crystalline substrate surfaces. Our simulation results imply that these unique features enable the non-epitaxial growth of large-area single-crystalline films by DS-IJP processes. Finally, we note that air-liquid interfaces, which we have demonstrated to be critical, are ubiquitous in various solution-based processes. We believe that this finding should trigger subsequent substantial development of next-generation printed electronics devices for real applications.

Acknowledgments We thank Prof. Yuka Tabe of Waseda University for fruitful discussions and Dr. Kenji Sakamoto of the National Institute for Materials Science (NIMS) for valuable comments. We also thank Dr. Satoru Inoue of Nippon Kayaku Co., Ltd. for helpful discussions.

Supporting Information Available: Results of the additional simulations. Definition of the local herringbone order parameter Sherr . Details of the the simulation models and methods. This material is available free of charge via the Internet at http://pubs.acs.org.

Notes and References (1) Anthony, J. E. Organic Electronics: Addressing Challenges. Nature Mater. 2014, 13, 773–775.

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(2) Sirringhaus, H. 25th Anniversary Article: Organic Field-Effect Transistors: The Path Beyond Amorphous Silicon. Adv. Mater. 2014, 26, 1319–1335. (3) Bauer, S. Flexible Electronics: Sophisticated Skin. Nature Mater. 2013, 12, 871–872. (4) Mei, J.; Diao, Y.; Appleton, A. L.; Fang, L.; Bao, Z. Integrated Materials Design of Organic Semiconductors for Field-Effect Transistors. J. of Am. Chem. Soc. 2013, 135, 6724–6746. (5) Iino, H.; Hanna, J.-i. Availability of Liquid Crystallinity in Solution Processing for Polycrystalline Thin Films. Adv. Mater. 2011, 23, 1748–1751. (6) Yuan, Y.; Giri, G.; Ayzner, A. L.; Zoombelt, A. P.; Mannsfeld, S. C.; Chen, J.; Nordlund, D.; Toney, M. F.; Huang, J.; Bao, Z. Ultra-High Mobility Transparent Organic Thin Film Transistors Grown by an Off-Centre Spin-Coating Method. Nature Commun. 2014, 5, 1–9. (7) Minemawari, H.; Yamada, T.; Matsui, H.; Tsutsumi, J.; Haas, S.; Chiba, R.; Kumai, R.; Hasegawa, T. Inkjet Printing of Single-Crystal Films. Nature 2011, 475, 364–367. (8) Diao, Y.; Tee, B. C.; Giri, G.; Xu, J.; Kim, D. H.; Becerril, H. A.; Stoltenberg, R. M.; Lee, T. H.; Xue, G.; Mannsfeld, S. C. et al. Solution Coating of Large-Area Organic Semiconductor Thin Films with Aligned Single-Crystalline Domains. Nature Mater. 2013, 12, 665–671. (9) Uemura, T.; Hirose, Y.; Uno, M.; Takimiya, K.; Takeya, J. Very High Mobility in Solution-Processed Organic Thin-Film Transistors of Highly Ordered [1] Benzothieno [3, 2-b] Benzothiophene Derivatives. APEX 2009, 2, 111501. (10) Whitesides, G. M.; Grzybowski, B. Self-Assembly at All Scales. Science 2002, 295, 2418–2421.

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(11) Ebata, H.; Izawa, T.; Miyazaki, E.; Takimiya, K.; Ikeda, M.; Kuwabara, H.; Yui, T. Highly Soluble [1] Benzothieno [3, 2-b] Benzothiophene (BTBT) Derivatives for High-Performance, Solution-Processed Organic Field-Effect Transistors. J. of Am. Chem. Soc. 2007, 129, 15732–15733. (12) Noda, Y.; Minemawari, H.; Matsui, H.; Yamada, T.; Arai, S.; Kajiya, T.; Doi, M.; Hasegawa, T. Underlying Mechanism of Inkjet Printing of Uniform Organic Semiconductor Films Through Antisolvent Crystallization. Adv. Funct. Mater. 2015, 25, 4022–4031. (13) When we treat the OSC solution as its solvent DCB, corresponding Young’s equation is, γDCB cosθ + γDM F −DCB = γDM F . Here, θ, γDCB , γDM F and γDM F −DCB are the contact angle of DCB droplet on DMF subphase, surface tension of DCB, DMF and interface tension of DMF-DCB interface, respectively. From this equation, cosθ = γDM F /γDCB − γDM F −DCB /γDCB . The first term in R.H.S., γDM F /γDCB ∼ 1. 12 The second term ∼ 0 because of γDM F −DCB ∼ 0 as solvent DCB and antisolvent DMF are miscible. 12 Then the contact angle θ ∼ 0◦ , i.e. complete wetting. (14) The thickness d ∼ 0.8µm is estimated from the size of the predefined patterned area S ∼ 800µm × 100µm and the droplet volume 60 pL. 7 (15) Wang, J.; Wolf, R. M.; Caldwell, J. W.; Kollman, P. A.; Case, D. A. Development and Testing of a General Amber Force Field. J. Comput. Chem. 2004, 25, 1157–1174. (16) Yang, L.; Tan, C.-h.; Hsieh, M.-J.; Wang, J.; Duan, Y.; Cieplak, P.; Caldwell, J.; Kollman, P. A.; Luo, R. New-Generation Amber United-Atom Force Field. J. of Phys. Chem. B 2006, 110, 13166–13176. (17) Bayly, C. I.; Cieplak, P.; Cornell, W.; Kollman, P. A. A Well-Behaved Electrostatic Potential Based Method Using Charge Restraints for Deriving Atomic Charges: the RESP Model. J. Chem. Phys. 1993, 97, 10269–10280.

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(18) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C. et al. Gaussian03, Revision C.02. Gaussian, Inc.: Wallingford CT, 2004. (19) Hess, B.; Kutzner, C.; Van Der Spoel, D.; Lindahl, E. GROMACS 4: Algorithms for Highly Efficient, Load-Balanced, and Scalable Molecular Simulation. J. Chem. Theory Comput. 2008, 4, 435–447. (20) Hess, B.; Bekker, H.; Berendsen, H. J. C.; Fraaije, J. G. E. M. LINCS: A Linear Constraint Solver for Molecular Simulations. J. Comput. Chem. 1997, 18, 1463– 1472. (21) Feenstra, K. A.; Hess, B.; Berendsen, H. J. Improving Effciency of Large Timescale Molecular Dynamics Simulations of Hydrogen-Rich Systems. J. Comput. Chem. 1999, 20, 786–798. (22) Yoneya, M.; Kawasaki, M.; Ando, M. Are Pentacene Monolayer and Thin-Film Polymorphs Really Substrate-Induced? A Molecular Dynamics Simulation Study. J. of Phys. Chem. C 2011, 116, 791–795. (23) Berendsen, H. J. C.; Postma, J. P. M.; van Gunsteren, W. F.; DiNola, A.; Haak, J. R. Molecular Dynamics with Coupling to an External Bath. J. Chem. Phys. 1984, 81, 3684–3690. (24) Bussi, G.; Donadio, D.; Parrinello, M. Canonical Sampling Through Velocity Rescaling. J. of Chem. Phys. 2007, 126, 014101. (25) Parrinello, M.; Rahman, A. Polymorphic Transitions in Single Crystals: A New Molecular Dynamics Method. J. of Appl. Phys. 1981, 52, 7182–7190. (26) Pershan, P. Liquid Crystal Surfaces. Le J. de Phys. Colloq. 1989, 50, C7–1. (27) Roeder, S. B.; Burnell, E. E.; Kuo, A.-L.; Wade, C. G. Determination of the Lateral

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Diffusion Coefficient of Potassium Oleate in the Lamellar Phase. J. of Chem. Phys. 1976, 64, 1848–1849. (28) Allen, M. P.; Tildesley, D. J. Computer Simulation of Liquids; Oxford Science Publications: Oxford, 1987; Chapter 11.5. (29) de Gennes, P. G.; Prost, J. The Physics of Liquid Crystals, second edition ed.; Clarendon press: Oxford, 1993. (30) Izawa, T.; Miyazaki, E.; Takimiya, K. Molecular Ordering of High-Performance Soluble Molecular Semiconductors and Re-Evaluation of Their Field-Effect Transistor Characteristics. Adv. Mater. 2008, 20, 3388–3392. (31) O’Grady, D.; Barrett, M.; Casey, E.; Glennon, B. The Effect of Mixing on the Metastable Zone Width and Nucleation Kinetics in the Anti-Solvent Crystallization of Benzoic Acid. Chem. Eng. Res. and Design 2007, 85, 945–952. (32) Kosˇata, B.; Kozmik, V.; Svoboda, J.; Novotná, V.; Vanˇek, P.; Glogarová, M. Novel Liquid Crystals Based on [1] Benzothieno [3, 2-b][1] Benzothiophene. Liq. Cryst. 2003, 30, 603–610. (33) Steele, W. The Physical Interaction of Gases with Crystalline Solids:: I. Gas-Solid Energies and Properties of Isolated Adsorbed Atoms. Surf. Sci. 1973, 36, 317–352. (34) Pinilla, C.; Del Popolo, M.; Lynden-Bell, R.; Kohanoff, J. Structure and Dynamics of a Confined Ionic Liquid. Topics of Relevance to Dye-Sensitized Solar Cells. J. of Phys. Chem. B 2005, 109, 17922–17927. (35) We think that the layer in Figure 3c-(vi) may corresponds to a layer in the highlyorderd smectic (e.g., smectic E) phases with positonal and herringbone orders. 29 (36) Palermo, M. F.; Muccioli, L.; Zannoni, C. Molecular Organization in Freely Suspended Nano-Thick 8CB Smectic Films. An Atomistic Simulation. Phys. Chem. Chem. Phys. 2015, 17, 26149–26159. 18


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(37) Okamoto, K.; Kawamura, T.; Sone, M.; Ogino, K. Study on Liquid Crystallinity in 2, 9-Dialkylpentacenes. Liq. Cryst. 2007, 34, 1001–1007. (38) Anthony, J. E.; Brooks, J. S.; Eaton, D. L.; Parkin, S. R. Functionalized Pentacene: Improved Electronic Properties from Control of Solid-State Order. J. of Am. Chemi. Soc. 2001, 123, 9482–9483. (39) Minemawari, H.; Yamada, T.; Hasegawa, T. Crystalline Film Growth of TIPSPentacene by Double-Shot Inkjet Printing Technique. Jap. J. of Appl. Phys. 2014, 53, 05HC10. (40) Wan, J.; Li, Y.; Ulbrandt, J. G.; Smilgies, D.-M.; Hollin, J.; Whalley, A. C.; Headrick, R. L. Transient Phases During Fast Crystallization of Organic Thin Films from Solution. APL Mater. 2016, 4, 016103.

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Page 20 of 25

List of Figures 1

2

3

4

a i) Initial structure and snapshots after ii) 1 ns, iii) 4 ns, iv) 10 ns and v) 50 ns of the MD runs of the 8BTBT8 (red) and DCB (yellow) OSC solutions on the DMF (blue) subphase and under the nitrogen gas (light blue) phase. The green box corresponds to the simulation box with 2D periodic boundary conditions. b Number density distributions of the 8BTBT8 (red line), DCB (black line) and DMF (blue line) molecules as functions of the z coordinate (interface normal direction) after 4 ns (upper) and 50 ns (lower) of the MD run. c Time evolution of the orientational order parameter < P2 > of the 8BTBT8 core long-axis (black line) and short-axis (blue line) directions. . . . . . . . . . . . . . . . . . . . . . . Temporal evolution of the orientational order parameter < P2 > of the 8BTBT8 long-axis (black line) and short-axis (blue line) directions for the molecules within the top layer at the air-OSC solution interface. The corresponding in-layer diffusion constants are indicated by the red line. Snapshots before (t=30 ns) and after (t=50 ns) the onset of short-axis orientational ordering (t∼36 ns) viewed along the molecular long-axes are also shown. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a Side view of the simulated system with the selected 8BTBT8 single layer on the model substrate. b Enlarged views of (i) parallel stacking and (ii) herringbone packing shown in the dotted box in Figure 3c-(i) and (vi), respectively. In this figure, only the 8BTBT8 core parts are shown for clarity. c i) Initial structure and snapshots after ii) 1 ns, iii) 4 ns, iv) 10 ns, v) 20 ns and vi) 50 ns of the MD runs viewed along the molecular longaxes. The color of the molecule corresponds to the local herringbone order Sherr of each molecule. A selected molecule initially located approximately at the center is drawn thickly for reference. The green box corresponds to the substrate surface with 2-D periodic boundary conditions. . . . . . . Correlations between molecular long- and short-axis ratio L/S and simulated orientational order parameter < P2 >avr . . . . . . . . . . . . . . .

20


. 21

. 22

. 23 . 24

Page 21 of 25

a 8BTBT8

DCB

DMF

(i) t = 0 ns

(ii) t = 1 ns

(iii) t = 4 ns

c

(iv) t =10ns

(v) t = 50 ns

1.0

60.0

t = 4 ns 40.0

DMF

8BTBT8

long axis

0.8

DCB

20.0

0.6

0.0

b

number density (nm ) number density (nm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


t = 50 ns 60.0

S

0.4

C8H17

H17 C8 S

short axis

40.0

0.2 20.0

0.0 0.0

10.0

20.0

30.0

0.0

0.0

10.0

z (nm)

20.0

30.0

time (ns)

40.0

50.0

Figure 1: a i) Initial structure and snapshots after ii) 1 ns, iii) 4 ns, iv) 10 ns and v) 50 ns of the MD runs of the 8BTBT8 (red) and DCB (yellow) OSC solutions on the DMF (blue) subphase and under the nitrogen gas (light blue) phase. The green box corresponds to the simulation box with 2-D periodic boundary conditions. b Number density distributions of the 8BTBT8 (red line), DCB (black line) and DMF (blue line) molecules as functions of the z coordinate (interface normal direction) after 4 ns (upper) and 50 ns (lower) of the MD run. c Time evolution of the orientational order parameter < P2 > of the 8BTBT8 core long-axis (black line) and short-axis (blue line) directions.

21



-9

2.0

0.8

1.6

0.6

1.2

0.4

0.8

0.2

0.4

t = 30 ns

0.0 0.0

10.0

20.0

30.0

40.0

0.0 50.0

time (ns)

D (m2 s-1)

x10 1.0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 25

t = 50 ns

Figure 2: Temporal evolution of the orientational order parameter < P2 > of the 8BTBT8 long-axis (black line) and short-axis (blue line) directions for the molecules within the top layer at the air-OSC solution interface. The corresponding in-layer diffusion constants are indicated by the red line. Snapshots before (t=30 ns) and after (t=50 ns) the onset of short-axis orientational ordering (t∼36 ns) viewed along the molecular long-axes are also shown.

22


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a

b

(i)

(ii)

c S herr 1.0

0.8

0.6

0.4

0.2

0.0

(i) t = 0 ns

(iv) t = 10 ns

(ii) t = 1 ns

(v) t = 20 ns

(iii) t = 4 ns

(vi) t = 50 ns

Figure 3: a Side view of the simulated system with the selected 8BTBT8 single layer on the model substrate. b Enlarged views of (i) parallel stacking and (ii) herringbone packing shown in the dotted box in Figure 3c-(i) and (vi), respectively. In this figure, only the 8BTBT8 core parts are shown for clarity. c i) Initial structure and snapshots after ii) 1 ns, iii) 4 ns, iv) 10 ns, v) 20 ns and vi) 50 ns of the MD runs viewed along the molecular long-axes. The color of the molecule corresponds to the local herringbone order Sherr of each molecule. A selected molecule initially located approximately at the center is drawn thickly for reference. The green box corresponds to the substrate surface with 2-D periodic boundary conditions.

23



Si

1.0

S C8H17

8PhTTFPh8 7BNP7

H17 C8

smectic LC

0.8

S

8BTBT8 8PEN8 C8H17

Si

8BTBT8

0.6

TIPS-PEN S

avr

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 25

H17C8

8PEN8 0.4

H15C7

S

C7H15

non-LC

tBuBTBTtBu

7BNP7

0.2

tBuBTBTtBu

H25C12

S

TIPS-PEN S

0.0 C12H25

H17C8

1,6-12BTBT12

S

S

S S

0.0

2.0

4.0

6.0

L/S

C8H17

8.0

8PhTTFPh8

1,6-12BTBT12

Figure 4: Correlations between molecular long- and short-axis ratio L/S and simulated orientational order parameter < P2 >avr .

24


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85x47mm (200 x 200 DPI)


Interface-Mediated Self-Assembly in Inkjet Printing of Single-Crystal

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