Research ArticleAPPLIED SCIENCES AND ENGINEERING

Meniscus-controlled printing of single-crystal interfaces showing extremely sharp switching transistor operation

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Science Advances  07 Oct 2020:
Vol. 6, no. 41, eabc8847
DOI: 10.1126/sciadv.abc8847

Abstract

Meniscus, a curvature of droplet surface around solids, takes critical roles in solution-based thin-film processing. Extension of meniscus shape, and eventual uniform film growth, is strictly limited on highly lyophobic surfaces, although such surface should considerably improve switching characteristics. Here, we demonstrate a technique to control the solution meniscus, allowing to manufacture single-crystalline organic semiconductor (OSC) films on the highest lyophobic amorphous perfluoropolymer, Cytop. We used U-shaped metal film pattern produced on the Cytop surface, to initiate OSC film growth and to keep the meniscus extended on the Cytop surface. The growing edge of the OSC film helped maintain the meniscus extension, leading to a successive film growth. This technique facilitates extremely sharp switching transistors with a subthreshold swing of 63 mV dec−1 owing to the effective elimination of charge traps at the semiconductor/dielectric interface. The technique should expand the capability of print production of functional films and devices.

INTRODUCTION

Achieving sharp on/off switching at low voltages is a crucial subject for organic thin-film transistors (TFTs), the issue of which attracts considerable recent attentions (1, 2). For the sharp switching characteristics, it is fundamental to reduce carrier trap states at the semiconductor/gate dielectric interface (3). It was recently shown that excellent switching characteristics are achieved in devices produced through phase separation of small-molecule organic semiconductors (OSCs) and insulating polymers from the mixed solution (47). The technique is promising for the practical use in printed electronics technologies, although the obtained mobility is relatively lower than that expected for the inherent OSC characteristics, probably due to the polycrystalline nature of the polymer-blend devices. In contrast, it was also demonstrated that a model device, produced by a handmade lamination of defect-free OSC single crystal, exhibits both high mobility and excellent switching characteristics, showing small subthreshold swing (SS) as is close to the theoretical limit (810). The device uses a perfluorinated polymer gate dielectric of Cytop, which is known as the most lyophobic insulating amorphous material, and the low surface energy of Cytop should be effective to eliminate the charge traps (11, 12). Thus, it is ideal to produce uniform and highly crystalline small-molecule OSC layers on top of the highly lyophobic gate dielectric surfaces, although the issue is still a big challenge for the solution-based thin-film processing.

In solution processing for OSC films, a primary step is to produce a thin solution layer on top of the substrate surface. Frequently used technique is meniscus-guided coating (13). In this technique, semiconductor solution is kept (i.e., with a coating blade) and is moved unidirectionally to form a flat meniscus region with an extended thin solution layer on the substrate surface, followed by a conversion of solution layer into thin solid films at the receding edge (1315). It was pointed out that the OSC molecules are self-organized at the air-liquid interface (16, 17) and that some alkylated and rod-shaped OSC molecules are useful to obtain the highly crystalline film growth due to the high layered crystallinity (18, 19). From the features of the abovementioned process, it is implied that the formation of a small contact angle (θc) at the solution edge should be prerequisite, by which the films could be continuously grown. The requirement is partly associated to the fact that efficient solvent evaporation takes place around the air-liquid-solid contact line with a small θc. However, this condition is in contradictory to the large θc as is expected for sessile droplets on highly lyophobic surfaces, which makes it difficult to produce uniform OSC films by conventional solution processes (20). So, it is important and necessary to control the shape of solution meniscus on the lyophobic surfaces.

Here, we report the development of an extended meniscus-guided (EMG) coating technique that allows to manufacture OSC single-crystal films on top of highly lyophobic Cytop gate dielectric layer. The uniform film growth becomes possible by keeping extension of solution meniscus on the Cytop surface. The EMG coating allowed us to integrate single-crystal films of small molecule–based OSCs and Cytop gate dielectric in bottom-gate-bottom-contact (BGBC)–type organic TFTs, resulting in an extremely small SS value of 63 mV dec−1, which is close to the theoretical limit. We discuss the origin of the sharp on/off switching in terms of the interfacial quality of printed OSC single-crystal films on the Cytop gate dielectric.

RESULTS AND DISCUSSION

EMG coating of single-crystal films

A schematic of the EMG coating for obtaining OSC films on the highly lyophobic Cytop surface is presented in Fig. 1A. This technique is composed of two steps; first, a lyophilic U-shaped metal film is fabricated on the Cytop surface, and then the OSC solution is swept unidirectionally from the bottom of U shape with a glass blade (see movie S1). As a highly crystalline small molecule–based OSC material, we used phenyl/alkyl-substituted benzothieno[3,2-b]naphtho[2,3-b]thiophene (Ph-BTNT-Cn) (fig. S1) (21, 22) as a solution-processable and high-performance OSC that is known to form bilayer-type layered-herringbone (LHB) packing (18, 19, 2326). Using the EMG coating with Ph-BTNT-Cn, we successfully obtained uniform films covering a wide area of the lyophobic Cytop surface as well as the lyophilic metal film surface, as presented in Fig. 1B and fig. S2. We point out that the present EMG coating technique is based on the use of metal films that can act as source/drain (S/D) electrodes in BGBC-type TFTs, as shown in Fig. 1B. We also found that a mixture of Ph-BTNT-C10 and Ph-BTNT-C12 was effective in producing uniform films on top of the Cytop surface by the EMG coating. It is known from the previous report (26) that the mixture of unsymmetric substituted π-electron framework with different alkyl chain lengths allows the formation of single molecular bilayer films (fig. S1C).

Fig. 1 EMG coating of single-crystal films.

(A) Schematic of the EMG coating technique. (B) Optical micrograph of printed Ph-BTNT-Cn films fabricated on Cytop surfaces with gold films predefined for the EMG coating. Photo credits: Gyo Kitahara, University of Tokyo.

Here, we characterized the Ph-BTNT-Cn films on the Cytop surface fabricated by the EMG coating. The entire area of the film was monocolored in the nonpolarized micrographs (Fig. 1B), indicating that the films had a constant thickness and high uniformity (25). Atomic force microscopy (AFM) measurements also demonstrated that a single molecular bilayer, with a thickness of 5.2 nm, was formed over the entire area (fig. S3). We used a crossed-Nicols polarized microscope to identify the crystallinity of the OSC film, the results of which indicate that the film was composed of a few crystalline domains (fig. S2C). We also conducted in-plane x-ray diffraction (XRD) measurements for the single molecular bilayer films of Ph-BTNT-Cn, the results of which are shown in fig. S4. The OSC film fabricated on the Cytop surface exhibits a single sharp peak for the (020) diffraction, providing a clear evidence of the single crystalline nature. The estimated lattice constant along the b axis is 7.439 Å, which is very close to that on lyophilic silica surfaces (7.434 Å) and is roughly close to that of Ph-BTNT-C10 single crystal (7.406 Å) (21). The results indicate that the crystal lattice is not affected by surface energy of substrates.

In situ observation of OSC film growth and EMG coating mechanism

To reveal how the EMG coating technique allows the OSC film growth on highly lyophobic Cytop surfaces, we conducted in situ microscope observations for the blade coating on a U-shaped metal film pattern fabricated on the Cytop surface. The results are shown in Fig. 2 (A to C) and fig. S5. Before the receding edge of the solution meniscus reached the metal film area, the solution meniscus was strongly confined under the blade with a large θc, as seen from a relatively narrow width of dark vertical line area in Fig. 2A. The feature is due to the highly lyophobic nature of the Cytop surface, where no OSC film was obtained (see also fig. S6). When the receding edge of the solution meniscus reached the bottom of the U-shaped metal film area, the meniscus began to be extended along the receding direction, as seen from an increasing width of the dark area in Fig. 2B and fig. S5. After the fairly thin solution layer was formed at the receding edge on the metal film surface, uniform OSC film growth began. Here, we used Au films whose surface is treated with pentafluorobenzenethiol (PFBT) for their use as S/D electrodes. The treated Au surface is hydrophobic but wettable (lyophilic) for chlorobenzene that is used as solvent in the OSC solution in this study (see fig. S7). As a result, the film growth on the treated Au surface becomes possible as is similar to that on the lyophilic silica surface (see fig. S8).

Fig. 2 In situ observation and schematic view of the EMG coating with a U-shaped metal film.

(A to C) Shape of solution meniscus during the coating. (Left) Crossed-Nicols micrographs in the top view are shown. The coating was conducted continuously from (A) to (C). All scale bars are 500 μm. (Right) Schematics of cross-sectional meniscus shape during the coating process. (D) Schematic for film growth when both ends of air-liquid-solid contact line are anchored by metal films. (E) Schematic for the end of film growth due to the absence of metal films at both ends. (F) Magnified crossed-Nicols micrographs in the top view at the receding edge of solution meniscus during the blade coating. (Right) An enlarged image around the extended thin solution layer. Color, contrast, and brightness are adjusted for visibility. In (A) to (C) and (F), a part of the meniscus appears as dark colored area because of the absence of vertical reflection of incident light. Photo credits: Gyo Kitahara, University of Tokyo.

The most important observation is that the extended meniscus was maintained even after the coating blade was moved on the Cytop surface, as long as the air-liquid-solid contact line for the film growth continued to be anchored by both ends that placed on the outer metal film area (i.e., two parallel lines of the U shape), as depicted in Fig. 2D. The eventual extended meniscus on the Cytop surface was accompanied by a successive OSC film growth, as presented in Fig. 2C and fig. S5. Last, we observed that the meniscus got shrunk instantaneously, when the receding edge of the solution meniscus reached the end of the metal film area (i.e., top of the U shape), and that the OSC film growth was terminated, as shown in Fig. 2E and fig. S9B. These observations afford clear evidence that the extended meniscus is crucial for the OSC film growth on highly lyophobic Cytop surfaces in the EMG coating technique.

On the underlying mechanism of the EMG coating, we consider that the presence of interfacial OSC films, covering through over the air-liquid-solid contact line, should be responsible, as schematically shown in Fig. 2C (right). A microscope observation shown in Fig. 2F demonstrates that the OSC film grows around the edge of the thin solution layer, which continuously connects to that on the substrate surface. The growth and presence of OSC film at the air-liquid interface are consistent with the former reports based on molecular dynamics simulation (17) and in situ XRD measurements (16). The surface tension of solution edge on the highly lyophobic Cytop surface should not work effectively, when the interfacial OSC film covers through over the air-liquid-solid contact line. Furthermore, the outer metal film area at the both ends of the contact line is also necessary to prevent the shrinking of extended solution against the surface tension, as depicted in fig. S9A. If the outer metal film area does not exist, the surface tension of solution at the both ends becomes dominant, eventually leading to the disappearance of extended meniscus over the whole air-liquid-solid contact line (fig. S9B). Note that it is also possible to use discontinuous metal film patterns, as long as the above mechanism works in the EMG coating, as illustrated in Fig. 1B and fig. S10. In addition, the uniform OSC film growth is obtained over the whole area inside the outer metal film pattern, even if the area includes a variety of metal film patterns such as S/D electrodes, as seen in Fig. 1B and fig. S11.

Extremely sharp on/off switching in TFTs

To examine the electrical performance of printed OSC single-crystal films on the Cytop gate dielectric layer, we fabricated BGBC-type TFTs, as shown schematically in Fig. 3A. Using the EMG coating, we successfully obtained Ph-BTNT-Cn films composed of several single-crystal domains covering S/D gold electrodes and Cytop gate dielectric layer (Fig. 3B). Furthermore, we also successfully produced TFTs whose source electrodes were U-shaped (Fig. 3, C and D), and each TFT was isolated automatically by the EMG coating, the feature of which would be useful for integrated devices.

Fig. 3 Printed Ph-BTNT-Cn single-crystal TFTs produced by the EMG coating.

(A) Schematic cross-section of BGBC-type TFTs. (B) Crossed-Nicols polarized micrograph of TFTs. (C and D) Nonpolarized and crossed-Nicols micrographs of automatically isolated TFTs, respectively, where the source electrodes are used as U-shaped metal-film areas.

The electrical characteristics of the printed Ph-BTNT-Cn single-crystal TFTs (shown in Fig. 3B) are presented in Fig. 4. The carrier mobility reached up to 4.9 (linear regime) and 5.5 cm2 V−1 s−1 (saturation regime), showing good consistency to that of bottom-gate-top-contact (BGTC)–type TFTs composed of Ph-BTNT-Cn single-crystal films (21, 22). Low-voltage operation below 2 V was also achieved, along with highly sharp on/off switching, small threshold voltage (Vth) or turn-on voltage (Von) around 0 V, and negligible hysteresis.

Fig. 4 Typical electrical characteristics of printed Ph-BTNT-Cn single-crystal TFTs.

(A) Output characteristics. (B and D) Transfer characteristics in the linear regime (Vd = −0.2 V) and saturation regime (Vd = −2 V), respectively. (C and E) Gate voltage dependence of carrier mobility extracted from first derivative of transfer curve in the linear regime and saturation regime, respectively. Channel length L = 100 μm; channel width W = 800 μm; and gate capacitance Ci = 23 nF cm−2. Micrographs of the measured TFTs are shown in Fig. 3B.

Here, we focus on the switching characteristics for 56 TFTs produced on the same substrate (Fig. 5A). All the transfer curves exhibited extremely sharp on/off switching along with a small Von. The subthreshold characteristics provided the SS value as 63 mV dec−1, which is defined as the initial slope at Von, as shown in Fig. 5B. This value is remarkably small and is close to the theoretical limit of the SS value (59.6 mV dec−1 at 300 K). We also estimated the SS value by another method using the following relationSS=(d logIddVg)1(1)where Id and Vg are drain current and gate voltage, respectively. The values extracted from Eq. 1 are plotted as a function of Vg in Fig. 5C, whose minimum value corresponds to the SS value as is estimated from Fig. 5B. The distribution of the SS value (Fig. 5D) shows that the extremely sharp on/off switching presents excellent reproducibility (average value of 67 mV dec−1, with the smallest value being 63 mV dec−1) with a small SD of 3 mV dec−1. Furthermore, Von, which was extracted as the voltage at Id = 10−12 A (see Fig. 5B), is small around 0 V for all TFTs, as seen in Fig. 5E. These results demonstrate that the EMG coating facilitates the TFTs showing sharp on/off switching at low voltages, based on OSC single-crystal films on Cytop gate dielectric with an excellent interfacial quality. We also confirm that the sharp on/off switching (71 mV dec−1) is possible in the TFTs with automatically isolated OSC films as presented in fig. S12, where the source electrodes are U-shaped (shown in Fig. 3, C and D).

Fig. 5 Subthreshold characteristics of printed Ph-BTNT-Cn single-crystal TFTs.

(A) Transfer curves of 56 TFTs on the same substrate. (B) Transfer characteristics in the switching region. SS is extracted by the linear fit of current slope in the range from Id = 10−11.5 to 10−10 A. Turn-on voltage (Von) is extracted by the cross points of the linear fit for SS at Id = 10−12 A. (C) Gate voltage dependence of (dlog|Id|/dVg)−1 value extracted from the transfer curve. (D and E) Statistical distribution of the SS and Von, respectively, for 56 TFTs fabricated on the same substrate. Channel length L = 100 μm; channel width W = 800 μm; and gate capacitance Ci = 23 nF cm−2. Micrographs of a part of the measured TFTs are shown in Fig. 3B. All the characteristics are extracted from the forward sweep of transfer characteristics in the linear regime (Vd = −0.2 V).

The statistics of threshold voltage (Vth), hysteresis, mobility, and reliability factor (r factor) (27) are shown in fig. S13. The observed hysteresis was reproducibly small, indicating that the interfacial charge traps associated with the Vth shift, which are usually responsible for the hysteresis, were effectively eliminated due to the inert and stable nature of the Cytop gate dielectric (8, 11). On the other hand, the carrier mobility exhibited a relatively large distribution. From transmission line method analysis (fig. S14), both the contact resistance (RcW) and channel resistance (RchW) seem to vary from device to device, which causes a distribution of carrier mobility. In addition, some devices exhibit transfer curves showing “double slope” characteristics related to the Vg-dependent contacts (28, 29). The feature can be characterized by the r factor (fig. S13) as being larger than 1 (27). Nonetheless, extremely sharp SS values of approximately 67 mV dec−1 are reproduced for the devices, irrespective of showing the double-slope characteristics or not (fig. S15). The results imply that the Vg dependence of contacts has little effect on the on/off switching characteristics that are basically related to the diffusion-based carrier transport.

All-printed TFTs with sharp on/off switching

Cytop is also important in printed electronics technologies, as it is useful for obtaining ultrafine conductive silver patterns (that can include electrodes for TFT arrays) using the surface photoreactive nanometal printing (SuPR-NaP) technique (30); Cytop is used as the base layer to fabricate photoactivated pattern that allows the exclusive chemisorption phenomenon for alkylamine/alkylacid encapsulated silver nanocolloids (AgNCs) (31). By using the SuPR-NaP technique and the EMG coating technique, we fabricated all-printed TFTs through low-temperature processes below 80°C, as schematically shown in Fig. 6 (A and B). Here, the SuPR-NaP technique was used for printing gate electrodes, S/D electrodes, and U-shaped metal films for the EMG coating. The all-printed TFTs typically exhibited low-voltage operation even with the use of relatively small gate capacitance (Ci = 2.9 nF cm−2) (Fig. 6, C to E): sufficiently sharp on/off switching with SS value at 170 mV dec−1, small turn-on voltage, negligible hysteresis, and carrier mobility up to 2.0 cm2 V−1 s−1. The results demonstrate that the excellent switching characteristics become possible by integrating the EMG coating technique and the SuPR-NaP technique.

Fig. 6 All-printed Ph-BTNT-Cn single-crystal TFTs produced by integrating EMG coating and SuPR-NaP techniques.

(A) Schematics of the all-printed fabrication processes. (1) Coating of Cytop on substrates, acted as a base layer for printing of gate electrodes. (2) VUV irradiation through a photo mask. (3) Printing of gate electrodes by the SuPR-NaP technique. (4) Coating of Cytop, acted as a base layer for printing of S/D electrodes and U-shaped metal films. Cytop also acts as a gate dielectric layer. (5) VUV irradiation through another photo mask. (6) Printing of S/D electrodes and U-shaped metal films by the SuPR-NaP technique. (7) Coating of OSC films by the EMG coating technique. (8) Resultant all-printed TFTs. (B) Schematic cross section of all-printed TFTs. (C to E) Typical electrical characteristics (output, transfer, and carrier mobility), with channel length L = 200 μm, channel width W = 800 μm, and gate capacitance Ci = 2.9 nF cm−2.

Application to other OSC material

To investigate whether the EMG coating technique is applicable to other OSC materials, we fabricated TFTs composed of another OSC material exhibiting bilayer-type LHB packing, phenyl/alkyl-substituted benzothieno[3,2-b]benzothiophene (Ph-BTBT-Cn) (18, 2326), as shown in Fig. 7A. Using the EMG coating technique, single-crystal films on Cytop gate dielectric were successfully obtained (Fig. 7B). With the use of a small Ci (2.5 nF cm−2), all the TFTs exhibited sufficiently sharp on/off switching (230 mV dec−1 on average), small Von, and small hysteresis with excellent reproducibility (Fig. 7, C to E). These results suggest that the EMG coating technique is applicable for Ph-BTBT-Cn as well. We also found that the mobility, in this case, remained at a relatively small value (average value of 0.29 cm2 V−1 s−1, with the highest value being 0.38 cm2 V−1 s−1), whereas Ph-BTBT-Cn single-crystal films have been reported to exhibit high carrier mobility over 1 to 10 cm2 V−1 s−1 in the BGTC geometry (25, 26). We conjecture that the small device mobility could be ascribed to the relatively large injection barrier originated from the insulating alkyl chain layers in the bilayer-type OSC single-crystal films (25), which might be more apparent in Ph-BTBT-Cn than in Ph-BTNT-Cn in the BGBC geometry.

Fig. 7 Electrical characteristics of printed Ph-BTBT-Cn single-crystal TFTs.

(A) Schematic cross section of TFTs. S/D electrodes are fabricated by the SuPR-NaP technique. (B) Crossed-Nicols polarized micrographs of Ph-BTBT-Cn crystal films. Two micrographs were taken at two polarization angles differing by 45°. Color, contrast, and brightness are adjusted for visibility. (C) Typical transfer characteristics in the linear regime (Vd = −5 V). (D and E) Transfer characteristics and statistical distribution of SS value for 15 TFTs produced on the same substrate, respectively. The SS value is extracted from the forward sweep of transfer characteristics in the linear regime (Vd = −5 V). Channel length L = 80 μm; channel width W = 800 μm; and gate capacitance Ci = 2.5 nF cm−2. Photo credits: Gyo Kitahara, University of Tokyo.

Carrier trap density relating to SS value

Here, we discuss the extreme sharp on/off switching with a small SS value, achieved in printed OSC single-crystal films/Cytop gate dielectric interface. Theoretically, the SS value is determined by a combination of gate capacitance and carrier trap states in the channel layer. Under the assumption that the trap density is constant regardless of the energy level, the SS can be expressed asSS=kBTqln 10(1+q2DitCi)(2)where kB is the Boltzmann constant, T is the temperature, q is the elementary charge, Ci is the gate capacitance, and Dit is the maximum density of interface trap (3, 9). In principle, two strategies have been proposed for attaining a small SS value: by increasing the capacitance (Ci) or by reducing the trap density (Dit). The printed Ph-BTNT-Cn single-crystal TFTs presented in Fig. 5 show an extremely small SS (67 mV dec−1 on average), the result of which could be ascribed to the small interfacial trap density (Dit = 1.8 × 1010 eV−1 cm−2), as realized by the formation of OSC single-crystal films on top of the Cytop gate dielectric surface. Furthermore, all-printed Ph-BTNT-Cn single-crystal TFTs using the SuPR-NaP technique (Fig. 6) showed a small trap density (Dit = 3.5 × 1010 eV−1 cm−2), indicating that the interfacial quality is not critically degraded by the electrode printing procedure [i.e., vacuum ultraviolet (VUV) irradiation or blade coating of silver ink] (30). The printed Ph-BTBT-Cn single-crystal TFTs exhibited a small trap density at Dit = 4.5 × 1010 eV−1 cm−2 as well. These results demonstrate that the highly crystalline OSC films on the Cytop gate dielectric layer are promising for realizing sharp on/off switching TFTs, where carrier traps relating to SS values are effectively reduced.

Table 1 presents a comparison of the Dit values obtained in the present study, with the reported values (<1.0 × 1011 eV−1 cm−2) of TFTs showing sharp switching characteristics. These small Dit values have been achieved so far either by a simple handmade lamination of flake-like organic single crystals on top of the Cytop surface (9) or by using a mixed solution composed of small-molecule OSC and insulating polymer (47, 3234). The latter case is actually practical, although the achieved mobility remains relatively lower (<1 cm2 V−1 s−1) than the inherent characteristics of the small-molecule OSCs. Compared to these studies, here, the EMG coating technique enabled the production of Ph-BTNT-Cn single-crystal TFTs that showed both high mobility (up to 4.9 cm2 V−1 s−1) and sharp on/off switching with a small SS value (down to 63 mV dec−1), approximately reaching the theoretical limit.

Table. 1 A comparison of small SS value owing to reduced Dit (<1.0 × 1011eV−1cm−2) reported for organic TFTs.

Fig. 5* shows the smallest SS value and the highest carrier mobility achieved in this study. PVC, poly(vinyl cinnamate); C8-BTBT, 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene; PS, polystyrene; PVT, physical vapor transport; TIPS-pentacene, 6,13-bis(triisopropylsilylethynyl)-pentacene; PTS, trichlorophenylsilane; PVA, polyvinyl alcohol; C8-DTBDT, 2,7-dihexyldithieno[2,3-d:2’,3’-d’]benzo[1,2-b:4,5-b’]dithiophene.

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In summary, we successfully developed an EMG coating technique that enables the production of solution-processed OSC single-crystal films on the highly lyophobic Cytop gate dielectric surface. We used the lyophilic U-shaped metal film pattern both for initiating the OSC film growth and for assisting to keep the solution meniscus extended on the Cytop surface area that is surrounded by the outer metal film pattern areas. We found that the growing edge for the self-organized formation of OSC films is located at air-liquid interfaces and takes crucial roles in extending the solution meniscus on the Cytop surface. The continuous coverage of the molecular films across the air-liquid-solid contact line should suppress the surface tension of solution around the receding edge of the contact line and thus help maintain the extension of solution meniscus on the highly lyophobic Cytop surface. Using the EMG coating, we successfully produced BGBC-type printed single-crystal TFTs composed of Cytop gate dielectric layer. The obtained devices exhibited excellent characteristics, showing small SS values, which were close to the theoretical limit, small turn-on voltage, negligible hysteresis, and carrier mobility up to 4.9 cm2 V−1 s−1. The small SS value (67 mV dec−1 on average) could be ascribed to the small trap density (Dit = 1.8 × 1010 eV−1 cm−2), indicating that the excellent interfacial quality is realized in printed OSC single-crystal films on the Cytop gate dielectric. The EMG coating technique also facilitated the production of all-printed single-crystal TFTs with the help of the SuPR-NaP technique. Thus, the EMG coating technique paves the way to fully use the potential of OSC in realizing all-printed TFTs showing both sharp on/off switching and high carrier mobility.

MATERIALS AND METHODS

Preparation and characterization of Ph-BTNT-Cn single-crystal films

As OSC materials, powdered Ph-BTNT-C10 and Ph-BTNT-C12 were synthesized according to the previously described procedure (21). After the synthesis, we prepared a 0.05 weight % (wt %) solution of Ph-BTNT-C10 and Ph-BTNT-C12 in chlorobenzene (Wako Special Grade; Fujifilm Wako Pure Chemical Corp., Japan), and these solutions were mixed at a volume ratio of 9:1. The OSC film was fabricated by blade coating or EMG coating, which uses a unidirectional sweep of a thin glass blade covered with a perfluoropolymer, Cytop (CTL-809 M; AGC Inc., Japan). The coating speed was controlled at 3.5 μm s−1 by a stepping motor (SHOT-302GS; Sigma Koki Co. Ltd., Japan) under ambient conditions. The thickness profile of the OSC film was measured by a tapping-mode AFM (VN-8010; Keyence Corp., Japan). The crystallinity of the OSC film was characterized by a crossed-Nicols polarized microscope (VHX-6000; Keyence Corp., Japan). The in-plane XRD measurements were conducted using a thin-film diffractometer (SmartLab; Rigaku Corp., Japan) with a monochromated CuKα radiation. Diffraction intensity is recorded with a scintillation counter under ambient conditions.

In situ observation of OSC coating

We observed solution meniscus from the top and sides during the coating process. Nonpolarized micrographs from the top view were taken by a zoom-type high-resolution lens (KCM-Z4D; Kenko Tokina Corp., Japan) equipped with a complementary metal-oxide semiconductor (CMOS) image sensor (dFK 72AUC02; The Imaging Source Asia Co. Ltd., Taiwan). For movie S1, a series of top view micrographs were taken at intervals of 2 s and connected at a frame rate of 50 frames per second. Crossed-Nicols micrographs from the top view and from the side view were taken with a digital microscope (VHX-6000; Keyence Corp., Japan).

Preparation and characterization of TFTs

P-type highly doped silicon wafers, with 100-nm-thick silica, were used as substrates. First, we conducted ultrasonic cleaning of the substrates using ultrapure water, acetone, isopropyl alcohol, and then again ultrapure water; they were then dried by a spin-coating process (MS-A100; Mikasa Co. Ltd., Japan). We prepared Cytop solution diluted using the solvent (CT-Solv.180; AGC Inc., Japan) at a volume ratio of 1:4 and formed a gate dielectric layer on the silica surface by spin coating at 2000 rpm for 60 s. Subsequently, the Cytop layer was cured at 50°C for 10 min at 0.08 MPa, 80°C for 15 min at 0.02 MPa, and 180°C for 60 min at 0.02 MPa. After drying, multilayer S/D electrodes and U-shaped metal films (0.5-nm-thick Cr and 25-nm-thick Au) were manufactured simultaneously via thermal evaporation on the Cytop surface. The electrode surfaces were then modified using chemical vapor treatment with PFBT (>95.0%; Tokyo Chemical Industry Co. Ltd., Japan) in a closed container. Thereafter, Ph-BTNT-Cn was coated on the substrates using the EMG coating, and solvent was dried at room temperature (25°C). All the TFT channels we used for the device characteristic measurements were isolated from floating U-shaped Au electrodes and unnecessary parts of OSC films to eliminate the current pathway outside channels and to evaluate the carrier motility correctly (35). We used a micromanipulator (Axis Pro; Micro Support Co. Ltd., Japan) for the isolation of TFT channels. The electrical characteristics were measured with a semiconductor parameter analyzer (E5270A; Agilent Technologies Inc., USA). In the devices, p-type highly doped silicon was used as a gate electrode, multilayer of 100-nm-thick silica and 25-nm-thick Cytop layer was used as a gate dielectric layer, multilayer of Au/Cr was used as an S/D electrode, and 5.2-nm-thick single bilayer of Ph-BTNT-Cn was used as a semiconductor channel layer. The gate capacitance was calculated as 23 nF cm−2, using series capacitance of Cytop and silica, based on relative permittivity (2.0 and 3.9) and thickness (25 nm and 100 nm), respectively.

All-printed TFTs

AgNCs used in the SuPR-NaP technique were synthesized as reported in the literature (31). We prepared glass substrates and cleaned them ultrasonically using the same procedure as used for silicon wafers (as stated above). Nondiluted Cytop solution was coated on the substrates by spin coating at 2000 rpm for 60 s and then cured at 80°C for 60 min at ambient atmospheric pressure. Silver gate electrodes were printed using the SuPR-NaP technique (30); the Cytop surface was irradiated and patterned by VUV light through a photomask, using a Xe2 excimer lamp processor (VUS-3150; ORC Manufacturing Co. Ltd., Japan). The synthesized AgNC ink was then swept with a glass blade at 2 mm s−1 using a bar coater (Automatic Film Applicator; TQC Sheen B.V., The Netherlands). Printed silver patterns were dried at room temperature for 10 min and then heated at 80°C for 10 min on a hot plate. The coating and annealing processes were conducted twice for obtaining low enough resistivity (as small as 10−5 ohm·cm). Subsequently, nondiluted Cytop solution was coated on the patterned gate electrodes/Cytop layer by spin coating at 2000 rpm for 60 s and cured at 80°C for 60 min at ambient atmospheric pressure. An approximately 600-nm-thick Cytop gate dielectric layer was obtained. S/D electrodes and U-shaped metal films were printed on the Cytop layer using the SuPR-NaP technique and following the same procedure as was used for obtaining gate electrodes. After PFBT modification of S/D printed electrodes, Ph-BTNT-Cn crystal films were obtained by EMG coating and then dried at room temperature (25°C). Last, the unnecessary part of OSC films was removed with a micromanipulator, and electrical measurements were done using a semiconductor parameter analyzer.

Ph-BTBT-Cn single-crystal TFTs

Powdered Ph-BTBT-C6 and Ph-BTBT-C10, used as OSC materials, were synthesized as described in the literature (18). After the synthesis, we prepared 0.05 wt % solutions of Ph-BTBT-C6 and Ph-BTBT-C10 dissolved in chlorobenzene, and these solutions were mixed at a volume ratio of 9:1. For device fabrication, p-type highly doped silicon wafers, with 100-nm-thick silica, were used as substrates and cleaned ultrasonically. Nondiluted Cytop solution was coated on the substrates by spin coating at 2000 rpm for 60 s, and the Cytop layer was cured for 10 min at 50°C and 0.08 MPa, for 15 min at 80°C and 0.02 MPa, and for 60 min at 180°C and 0.02 MPa. The S/D electrodes were then printed on the Cytop layer by the SuPR-NaP technique, which was followed by PFBT modification. Thereafter, Ph-BTBT-Cn crystal films were coated on the substrates by EMG coating, which was followed by drying at room temperature (25°C). Last, unnecessary parts of the OSC films were removed with a micromanipulator, and electrical measurements were done using a semiconductor parameter analyzer. Gate capacitance was calculated to be 2.5 nF cm−2 using series capacitance of Cytop and silica, based on relative permittivity (2.0 and 3.9) and thickness (650 and 100 nm), respectively.

SUPPLEMENTARY MATERIALS

Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/41/eabc8847/DC1

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REFERENCES AND NOTES

Acknowledgments: Funding: We thank Nippon Kayaku Co. Ltd. for providing Ph-BTNT-C10. This work was supported by JST CREST grant JPMJCR18J2, JST A-STEP grant JPMJTR1923, and JSPS KAKENHI grant JP18H03875, Japan. G.K. also thanks the financial support from the Leadership Development Program for Ph.D. (LDPP) in the University of Tokyo. The in-plane XRD measurements were conducted at the Advanced Characterization Nanotechnology Platform of the University of Tokyo, supported by “Nanotechnology Platform” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. Author contributions: G.K. conceptualized the work, performed the experiments, and analyzed all the data. S.I. and T.Hi. synthesized Ph-BTNT-Cn and Ph-BTBT-Cn. M.I. and T.Hay. improved the SuPR-NaP technique. S.A. helped perform XRD measurements. S.M., S.A., and T.Has. supervised the work. G.K. and T.Has. wrote the manuscript. All the authors checked the results and contributed to the development of the final manuscript. Competing interests: The authors declare that they have no competing interests. Data and materials availability: All data needed to evaluate the conclusion in the paper are present in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested form the authors.
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