Article pubs.acs.org/Langmuir
Self-Aligned Growth of Organic Semiconductor Single Crystals by Electric Field Kenji Kotsuki,† Seiji Obata,‡ and Koichiro Saiki*,†,‡ †
Department of Chemistry and ‡Department of Complexity Science and Engineering, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan S Supporting Information *
ABSTRACT: We proposed a novel but facile method for growing organic semiconductor single-crystals via solvent vapor annealing (SVA) under electric field. In the conventional SVA growth process, nuclei of crystals appeared anywhere on the substrate and their crystallographic axes were randomly distributed. We applied electric field during the SVA growth of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) on the SiO2/ Si substrate on which a pair of electrodes had been deposited beforehand. Real-time observation of the SVA process revealed that rodlike single crystals grew with their long axes parallel to the electric field and bridged the prepatterned electrodes. As a result, C8BTBT crystals automatically formed a field effect transistor (FET) structure and the mobility reached 1.9 cm2/(V s). Electric-field-assisted SVA proved a promising method for constructing high-mobility single-crystal FETs at the desired position by a low-cost solution process.
rganic field effect transistors (OFETs) have been attracting much attention for next generation devices. To achieve a good performance of OFETs, improvements of crystallinity and control of the molecular orientation are essential. For this purpose, the use of the prepatterned templates for the vapor and solution crystal growth1−6 or the directional drying of liquid media7−9 have been widely performed. Application of external fields would be another potential way. So far, the applications of light,10,11 magnetic field,12,13 electric field,14−18 and mechanical vibration19 have been found effective for controlling the molecular orientation. The molecular orientation was affected by the interaction between the external field and the molecular dipole moment,10,12−14 or by wavelength-dependent light absorption of the crystal.11 In vapor growth, however, the external field worked on the surface-migrating molecules or on the growing crystallites on a substrate and thus the interaction with the underlying substrate would be often larger than that with the external field. As a result, the effect of external field was limited only within the grains smaller than 10 μm and aligning the molecular orientation beyond that size would be impossible. On the other hand, in the case of solution process molecules or growing crystallites could move more easily in solvent and thus external field might affect the crystal orientation in a macroscopic scale. Actually inorganic nanowires interacted with an external electric field in liquid and bridged the two electrodes separated at a distance of more than 10 μm.20,21 Such interaction, known as dielectrophoresis, was also effective for organic crystals’ alignment. We previously elucidated the effect of electric field during the drop-casting growth in which the appropriate electric field captured the drifting pentacene crystals with a size of several tens of micrometers in solution
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© 2015 American Chemical Society
and a possibility for automatic fabrication of the OFETs array was presented.22 Although this “drop-casting with electric field” was a simple fabrication process, pentacene had crystallized mostly before experiencing electric field. The wide distribution of the crystal shape or crystal size caused the difference in the degree of alignment under the electric field, which led to the variation of device performance such as carrier mobility. In the present work, we applied electric field to the growing crystals during a solvent vapor annealing (SVA) process of 2,7dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT, illustrated in Figure 1a). The C8-BTBT crystals formed via SVA had the FET mobility larger than 1 cm2/(V s).5,23 We performed the SVA process on a SiO2/Si substrate on which gold electrodes had been prepared beforehand. Using these electrodes electric field was applied during the SVA process. Real-time observation revealed displacement and rotation of the growing C8-BTBT crystals responding to the electric field and the rodlike crystals finally bridged the two electrodes. The as-grown crystal, forming automatically a FET structure, showed a mobility of 1.9 cm2/(V s). Application of external electric field proved useful for a facile top-down fabrication of single-crystal OFETs via electric-field-assisted SVA.
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RESULTS AND DISCUSSION Electric field was applied during the SVA process according to the process shown schematically in Figure 1b. The detailed procedure was described in Experimental Section. First the C8BTBT/poly(methyl methacrylate) (PMMA) double layer was Received: October 28, 2015 Revised: December 10, 2015 Published: December 22, 2015 644
DOI: 10.1021/acs.langmuir.5b03975 Langmuir 2016, 32, 644−649
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The histogram for 44 crystals revealed random orientation under the absence of electric field. Next, electric field was applied 3 min after the SVA process started. The left and right electrodes were set at 0 and +60 V so that the direct current (dc) electric field intensity was 1.2 × 106 V/m (VLeft = 0 V, VRight = +60 V, L = 50 μm). The total SVA period was 40 min just like the case of no electric field. Figure 1d shows the optical image taken after the completion of the SVA process. Although the crystal form was the same rodlike structure as that grown under the absence of electric field, the crystal orientation aligned approximately along the direction of electric field. The polarized microscope image in Figure 1e shows the same color among the crystals bridging the electrodes over the channel. The histogram in Figure 1g revealed that more than 70% crystals existed in the range of θ from 80−90°. This result indicated clearly that the electric field worked on the growing crystal to align its orientation in the channel. To elucidate the effect of electric field, we observed the growth process in real time by using optical microscopy. Figure 2 shows a sequence of images cut from the movie (Supporting
Figure 1. (a) Chemical structure of a C8-BTBT molecule. (b) Schematic illustration of the PMMA on Au configuration process. (c− e) Optical (c,d) and polarized optical (e) microscope images of C8BTBT single crystals after 40 min SVA under no (c) and 1.2 × 106 V/ m DC (e,f) electric field. (f,g) Distribution of crystal-alignment as a function of θ after 40 min SVA under no (f) and 1.2 × 106 V/m DC (g) electric field.
formed on the SiO2/Si substrate on which the gold electrodes had been deposited beforehand. This double layer was exposed to a saturated vapor of chloroform in a Petri dish. In the SVA process, the PMMA layer is known to adsorb chloroform vapor on its surface and chloroform vapor formed a thin liquid layer.24 Then, C8-BTBT was once dissolved into the liquid layer and recrystallized.25 PMMA enhanced the molecular mobility during the SVA process and C8-BTBT did not recrystallize without the PMMA layer.25 In our study, the nucleation of C8-BTBT crystals occurred in 1−2 min after the exposure to chloroform. Subsequently, Ostwald ripening process started to occur, during which larger crystals grew further and smaller crystals disappeared.25 In 40 min, many rodlike crystals with a length of around 50 μm were formed as shown in Figure 1c. The color of crystals in the reflection image differs from each other depending on the crystal thickness. These crystals had an angle of 106° in one end, meaning a single crystallinity of C8-BTBT with its long axis parallel to the [100] direction.5,26 All rodlike crystals had the same molecular orientation, which was confirmed by polarized optical microscopy. The crystals on the channel were longer than those formed on the electrodes. The growth on the electrodes decelerated probably because the large surface energy density of the electrode attracted much amount of chloroform and decreased the concentration of C8-BTBT in it. Figure 1f shows the angular distribution of crystal orientation as a function of θ, the angle between the C8-BTBT rod, and the electrode edge.
Figure 2. (a−f) In situ optical microscope images during the SVA process of C8-BTBT. For the initial 3 min, SVA was performed under no electric field (a). Then 1.2 × 106 V/m dc electric field was applied during SVA (b−f).
Information, video S1) recorded during the SVA process under the electric field (60 V/50 μm: 1.2 × 106 V/m). One to two minutes after exposing the C8-BTBT/PMMA double layer to chloroform vapor, white dots appeared throughout on the substrate as shown in Figure 2a. These were nuclei of C8BTBT crystals. Three minutes after the SVA began, electric field was applied: the left and right electrodes were set at 0 and +60 V, respectively. In approximately 30 s, the crystals in the channel moved from the cathode (left) to the anode (right) as shown in Figure 2b. Then the larger crystals grew further and the smaller ones shrank via Ostwald ripening. During this ripening process, the growing crystals rotated their directions in the channel as shown in Figure 2c−f. Real-time observation revealed two kinds of electric field effects: movement from the cathode to the anode (in 30 s) and rotation along with applied electric field (during 30 min). A similar movement was observed for the applied voltages ranging from 30 to 90 V (electric field from 6 × 105 to 1.8 × 106 V/m; Figure 3). Lower applied voltage, however, needed longer time to complete the crystal alignment. Thus, the applied voltage was set at 60 V in the following experiment. In the case of alternating current (ac) 645
DOI: 10.1021/acs.langmuir.5b03975 Langmuir 2016, 32, 644−649
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the facts that the degree of orientation depended strongly on the aspect ratio and the dielectric polarization induced alignment has been also observed in isotropic silicon nanowires,21 we considered that the interaction between the electric field and the dielectric polarization dominantly contributed to the crystal rotation. The static electrophoresis and the electricfield-induced dipole moment thus contributed to the displacement and rotation of C8-BTBT crystals. Electric field was found to help the alignment of the growing C8-BTBT single crystals between the two Au electrodes. These electrodes for the generation of electric field could be used also for the FET measurement. We investigated the FET performance of the as-grown crystals by using the two electrodes as a source electrode and a drain electrode. The direction of the grown rods was parallel to the [100] axis of C8-BTBT crystal, the direction of which would be good for charge transport. Actually, the high mobility of more than 1 cm2/(V s) was already reported in the SVA-grown C8-BTBT crystals along the rod direction.23 The mobility of the present crystals as shown in Figures 1−3 was, however, as small as about 0.01 cm2/(V s). Nonohmic feature in a small drain voltage region in the output characteristics suggested a large contact resistance at the electrodes (Supporting Information Figure S1), probably arising from the remaining PMMA layer between the C8BTBT crystal and the Au electrodes. To improve the contact between the C8-BTBT crystals and the Au electrodes, we prepared another structure, “Au on PMMA” configuration as shown in Figure 4a. The detailed procedure was described in Experimental Section. In this process, the C8-BTBT and PMMA layers were fabricated separately. We first spin-coated PMMA solution in chlorobenzene on a SiO2/Si substrate. Then, the Au electrodes with 35 nm thickness and 50 μm channel length were deposited on the PMMA film and finally C8-BTBT solution was spin-coated.
Figure 3. In situ optical microscope images of C8-BTBT crystals grown via SVA with a dc electric field of 6 × 105 V/m (a) and 18 × 105 V/m (b).
electric field, C8-BTBT crystals swung between the two electrodes responding to the electric field during the SVA growth (Supporting Information, video S2). In this video, the longer crystals tended to keep their orientation parallel to the electric field, while the shorter crystals rotated randomly owing to the fluid resistance. On the basis of the above results, we discuss the mechanism of the crystal movements under electric field. The crystals seemed to feel two kinds of forces causing displacement (lateral movement) and rotation. With respect to the displacement, the growing crystals moved from the cathode to the anode (Figure 2a,b) like the negatively charged crystals, although the intrinsic C8-BTBT crystal itself was neutral. A similar electrification of organic crystals in solvent was observed in some kinds of organic substances such as pentacene, fullerene, and rubrene.17,22 Such electrification was contact electrification between substances and surrounding solvent. It is empirically known as Coehn’s rule that the intrinsically neutral substances are charged when they contact the materials (including solvents) with different dielectric constant.27 In most cases, the material with higher (lower) dielectric constant would be positively (negatively) charged. Although the dielectric constant of C8-BTBT was yet to be reported, those of organic semiconductors (4.0 of pentacene28 and 4.4 of C6029) lie between that of PMMA (2.330) and that of chloroform (4.831). This fact suggests the interaction between C8-BTBT and chloroform will be dominant for negative-charge electrification on the surface of C8-BTBT crystals. With respect to the rotation, the growing crystals gradually rotated toward the direction parallel to the electric field (Figure 2c−f). The orientation alignment by electric field has been observed in inorganic nanowires20,21 and organic crystals.17,22 This phenomenon is known to originate from the interaction between the electric field and the dielectric polarization induced in the inside of crystals.20,21 We previously observed that the degree of orientation of pentacene crystals has a strong dependence on their aspect ratio (length ratio of long axis to short one): the torque effectively worked in the crystals with the aspect ratio larger than 1.5.22 In the case of present SVAprocessing, C8-BTBT single crystal grew along the [100] direction, increasing its aspect ratio up to 10. The increasing aspect ratio helped much the rotation of crystals toward the electric field direction, which was also suggested by the movement under ac electric field (Supporting Information, video S2). Anisotropy of dielectric constant might also align the crystal orientation. Although the dielectric anisotropy of C8BTBT crystal has not yet been known, it is expected not to be large considering the case of rubrene crystal with a similar herringbone crystal structure: dielectric constants of 2.55, 2.83, and 3.12 along the a, b, and c axes, respectively.32 Judging from
Figure 4. (a) Schematic illustration of Au on PMMA configuration process. (b−e) Optical microscope images (b,c) and angular distribution (d,e) after 60 min SVA under no (b,d) and 1.2 × 106 V/m DC (c,e) electric field. 646
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BTBT single crystals directly rode on the two Au electrodes and bridged them. The FET performance was measured for these as-grown crystals. Figure 5d,e shows the output and transfer characteristics of the crystal shown in Figure 5a. A good saturation behavior was observed in the output characteristics unlike the PMMA on Au configuration (Supporting Information Figure S1). The mobility and the threshold voltage of each device are described in Figure 5a−c. The mobility ranged from 0.85 to 1.9 cm2/(V s) and the threshold voltage ranged from −17 to −32 V. The mobility was smaller than that reported for the SVA-grown C8-BTBT (average 3.0 cm2/(V s)).23 In that study, the top-contact Au electrodes directly touched the C8BTBT crystal and a MoO3 layer between C8-BTBT and electrodes decreased the contact resistance. In the present study, the FET characteristics were measured for the as-grown crystals and the remaining impurities and/or PMMA were likely to deteriorate the performance. The device with smaller mobility tended to show the inferior output characteristics suggesting a larger contact resistance (Supporting Information Figure S2). Further study on the modification of the contact might improve the mobility. However, the key issue of the present work is that the SVA process with electric field automatically grew the C8-BTBT crystal between the FET electrodes and its mobility exceeded 1 cm2/(V s) without any treatment after the growth process.
This structure was exposed to a saturated vapor of chloroform to perform the SVA process. The optical image of C8-BTBT crystals by SVA on the Au on PMMA configuration under the absence of electric field was shown in Figure 4b. C8-BTBT recrystallized into rodlike crystals only in the channel region, the surface of which was covered with PMMA. On the Au electrodes, however, nuclei with a length of several micrometers that appeared once did not grow further because of the absence of the PMMA layer that helped self-assembly of C8-BTBT molecules. The rodlike crystals in the channel tended to grow parallel to the electrode edge and 45% of crystals aligned in the range of θ less than 10°, as shown in Figure 4d. This distribution contradicts the random orientation observed for the “PMMA on Au” configuration under the absence of electric field (Figure 1c,f). This might be ascribed to the difference in wettability of C8-BTBT to gold and PMMA.5 In addition, the Au electrodes deposited on the PMMA surface worked as a topographical barrier for the growing crystal to ride on. Therefore, the crystals longer than the channel length were forced to rotate in parallel to the electrode edge. When electric field (dc, 1.2 × 106 V/m) was applied, the growing crystals tended to align with the electric field (Figure 4c) like those observed for the PMMA on Au configuration. The growing crystals moved from the cathode to the anode and rotated toward the direction of electric field. Thirty-five percent of all crystals were aligned in the region of 80−90° (Figure 4e). The degree of alignment was smaller than that for the PMMA on Au configuration. Some of the crystals were prevented from rotating further when they touched the Au electrode, probably because the difference in wettability and the topographical step at electrode−channel interface impeded them to rotate further. Nevertheless, most of crystals successfully rode on the Au electrodes and formed a single-crystal FET structure. Under the ac electric field, however, the FET structure could not be formed. This is because the vibration of crystals between the two electrodes moved the crystal away from the electrode and prevented themselves from riding on the steps at the channel− electrode interface. Figure 5 shows the examples of a C8-BTBT FET structure that were fabricated automatically by the SVA process on the Au on PMMA configuration. Figure 5a−c shows that the C8-
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CONCLUSIONS We found the growing C8-BTBT crystals in SVA responded to the external electric field and behaved like a negatively charged substance in atmosphere. Adjusting the intensity of electric field could place the rodlike crystals just bridging the two electrodes. On the Au on PMMA configuration, the as-grown crystals showed a good FET performance without any additional process such as deposition of electrodes or chemical modification of electrodes. Application of electric field during SVA thus proved a promising way for fabricating facilely highmobility C8-BTBT FETs and its array.
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EXPERIMENTAL SECTION
Sample Preparation. Two kinds of specimens were prepared for the SVA process, PMMA on Au configuration and Au on PMMA configuration. The scheme of PMMA on Au configuration is illustrated in Figure 1b. First, a SiO2/Si substrate (the oxide layer of 300 nm thick) was cleaned by exposing it to ozone gas at 50 °C for 10 min using a UV/ozonizer (Samco, UV-1). Then, the 35 nm thick Au electrodes were deposited to form the channel with a length of 50 μm through a shadow mask. The double layer of C8-BTBT and PMMA was formed according to the process described in a previous paper.26 C8-BTBT (Nippon Kayaku) and PMMA (Tokyo Chemical Industry, Mw = 13 500−14 000) were dissolved in chlorobenzene with each concentration of 0.5 wt %. This solution was spin-coated at 2000 rpm onto the SiO2/Si substrate with the Au electrodes for 40 s. The difference in surface energy solidifies the PMMA faster than C8BTBT, forming a double-layer structure on the substrate until the completion of spin-coating.33 Then, this substrate was exposed to chloroform vapor in a Petri dish at room temperature for 40 min. During this SVA process, electric field was applied and crystallization of C8-BTBT was observed by an optical microscope. The scheme of Au on PMMA configuration is illustrated in Figure 4a. First, a SiO2/Si substrate was cleaned by a UV/ozonizer at 50 °C for 10 min. Then 1 wt % solution of PMMA in chlorobenzene was spin-coated at 2000 rpm for 40 s. Subsequently, Au electrodes (35 nm) were formed by evaporating Au in vacuum. The C8-BTBT layer was formed by spin-coating 1 wt % solution in toluene at 2000 rpm for 10 s. Instead of chlorobenzene, toluene was used as a solvent and the
Figure 5. (a−c) Optical microscope images of automatically formed C8-BTBT FETs on Au on PMMA configuration. (d,e) Output (d) and transfer (e) characteristics of the crystal shown in (a). 647
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Langmuir solution was drop-cast during rotation to prevent PMMA from dissolving. Then, this substrate was exposed to chloroform vapor in a Petri dish at room temperature for 60 min. The parameters of the concentration and the SVA time was optimized so as to provide the C8-BTBT crystal larger than 50 μm. FET Measurement. FET performance of all fabricated devices was measured in vacuum of less than 1 × 10−2 Pa at room temperature using a couple of KEITHLEY 6487 picoammeter/voltage sources. Source and drain terminals were connected to the Au electrodes, and a gate terminal was connected to silicon substrate. Gate insulator capacitance on the Au on PMMA configuration was evaluated to be 10.7 nF/cm2 by combining SiO2 capacitance (εSiO2 = 3.9, d = 300 nm) and PMMA capacitance (εPMMA = 2.3,30 d = 15 nm (AFM)). Fieldeffect mobility and threshold voltage were evaluated in the saturation region.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b03975. Figure S1 shows the output and the transfer characteristics of C8-BTBT single-crystal FET formed with PMMA on Au configuration. Figure S2 shows the FET characteristics of C8-BTBT single-crystal FETs formed with Au on PMMA configuration. (PDF) Video showing the motion of C8-BTBT crystals under dc electric field (VL = 0 V, VR = +60 V). (AVI) Video showing the motion of C8-BTBT crystals under ac electric field (VL = 60 V, VR = 0 V, f = 0.01 Hz). (AVI)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We would like to thank Nippon Kayaku Co. for providing us C8-BTBT. This work was partly supported by a Grant-in-Aid for Scientific Research from MEXT of Japan (No. 25107002).
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REFERENCES
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