Thermal Gradient During Vacuum-Deposition Dramatically Enhances

Feb 27, 2017 - A thermal gradient distribution was applied to a substrate during the growth of a vacuum-deposited n-type organic semiconductor (OSC) f...
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Thermal Gradient During Vacuum-Deposition Dramatically Enhances Charge Transport in Organic Semiconductors: Toward HighPerformance N‑Type Organic Field-Effect Transistors Joo-Hyun Kim,†,§ Singu Han,‡,§ Heejeong Jeong,‡ Hayeong Jang,‡ Seolhee Baek,‡ Junbeom Hu,‡ Myungkyun Lee,‡ Byungwoo Choi,‡ and Hwa Sung Lee*,‡ †

Department of Physics, North Carolina State University, Raleigh, North Carolina 27695, United States Department of Chemical and Biological Engineering, Hanbat National University, Daejeon 305-719, Korea

‡

S Supporting Information *

ABSTRACT: A thermal gradient distribution was applied to a substrate during the growth of a vacuum-deposited n-type organic semiconductor (OSC) film prepared from N,N′-bis(2-ethylhexyl)-1,7-dicyanoperylene3,4:9,10-bis(dicarboxyimide) (PDI-CN2), and the electrical performances of the films deployed in organic field-effect transistors (OFETs) were characterized. The temperature gradient at the surface was controlled by tilting the substrate, which varied the temperature onedimensionally between the heated bottom substrate and the cooled upper substrate. The vacuum-deposited OSC molecules diffused and rearranged on the surface according to the substrate temperature gradient, producing directional crystalline and grain structures in the PDI-CN2 film. The morphological and crystalline structures of the PDI-CN2 thin films grown under a vertical temperature gradient were dramatically enhanced, comparing with the structures obtained from either uniformly heated films or films prepared under a horizontally applied temperature gradient. The field effect mobilities of the PDICN2-FETs prepared using the vertically applied temperature gradient were as high as 0.59 cm2 V−1 s−1, more than a factor of 2 higher than the mobility of 0.25 cm2 V−1 s−1 submitted to conventional thermal annealing and the mobility of 0.29 cm2 V−1 s−1 from the horizontally applied temperature gradient. KEYWORDS: gradient thermal distribution, organic field-effect transistor, vacuum deposition, n-type organic semiconductor, thermal annealing

1. INTRODUCTION Organic field-effect transistors (OFETs) have received significant attention due to their potential utility in flexible electronic displays and highly efficient electronic devices.1−4 Recently, a variety of newly synthesized organic semiconductor (OSC) materials based on polymers or small molecules have been found to provide carrier mobilities exceeding 10 cm2 V−1 s−1.5−9 In addition to progress in chemical materials, researchers have concentrated on the development and optimization of processing parameters, because OSC crystals can assume a variety of shapes with different molecule arrays, depending on the growth direction, growth rate,10−14 and surface characteristics of the semiconductor and dielectric layers.15−19 Because an OFET’s performance is closely related to grain formation and the crystalline characteristics, the growth direction and molecular array must be meticulously controlled to achieve an optimal film structure.10−19 The morphological and crystalline structures of OSC thin films may be improved by guiding the self-organizing characteristics of OSC molecules using a variety of process methods, including thermal or solvent annealing and solvent soaking.20−23 Thermal annealing is widely used to control the © 2017 American Chemical Society

molecular rearrangement and crystallization of organic semiconductors, either by vacuum deposition or by solution casting, because thermal annealing efficiently improves the electrical properties of OSC films by tuning the process temperature and time.22,23 The application-level performances of OFETs, however, have been difficult to improve simply using thermal annealing processes because the thermal energy is applied randomly across the substrate. High-performance OFET devices may be obtained by combining well-controlled film morphologies and organized crystalline structures prepared through the directionally selective growth of OSC films.24 The zone annealing method is a representative method for achieving directional recrystallization via localized melting and cooling.25−32 The molecules in the OSC films became aligned under the thermal treatment, which was applied to a specific area of the predeposited film.25−35 The alignment and anisotropic growth of the molecular structure depended on the temperature gradient and annealing zone velocity. The Received: December 13, 2016 Accepted: February 27, 2017 Published: February 27, 2017 9910

DOI: 10.1021/acsami.6b15981 ACS Appl. Mater. Interfaces 2017, 9, 9910−9917

Research Article

ACS Applied Materials & Interfaces

thermally evaporating silver through a shadow mask. The channel length (L) and width (W) were 100 and 300 μm, respectively. 2.2. Characterization. The PDI-CN2 film morphologies were obtained using atomic force microscopy (AFM, Digital Instruments Multimode) measurements conducted over 5 × 5 μm2 scan areas via tapping-mode AFM using Si tips (42 N/m and 320 kHz, tip radius: 10 nm). Data analysis was performed using the Nanoscope 5.30 software. Two-dimensional grazing incidence X-ray diffraction (2D GIXD) studies were performed to investigate the crystalline structures of the PDI-CN2 films. These studies were carried out at the Pohang Accelerator Laboratory (PAL), Korea. The current−voltage characteristics of the PDI-CN2 OFETs were examined by operating the devices under a positive applied gate bias. The source electrode was grounded and the drain electrode was positively biased. The electrical parameters characterizing the PDI-CN2-OFETs were obtained at room temperature using an HP4156A instrument in a dark environment under vacuum conditions (10−4 Torr).

aligned crystallinity reduced defect sites for carrier transport and enhanced the electrical properties of the OFET devices. Several research groups achieved high-performance OFET devices by applying a zone annealing process to OFETs.28−32 Despite its utility, the zone annealing process is inefficient, involving meticulous control over the process conditions to ensure optimal orientations, velocities, and delay times, as well as a high-cost mechanical installation process during device integration. The effect of zone annealing on vacuum-deposited small molecule OSCs such as pentacene was not outstanding, and zone annealing requires very high temperatures that approach the melting temperature.30,36 More useful and accessible post-thermal annealing approaches to enhance the morphological and crystalline properties of OSC films are required. Here, we demonstrated the efficient strategies for manipulating the thin film growth of thermally deposited OSCs by applying a thermal gradient to the substrate. The temperature gradient across the substrate was applied by vertically tilting the substrate on the heating plate, inducing high and low substrate temperatures at the bottom and upper sides, respectively. The tilted substrate was used during OSC molecule deposition, producing a film with a higher degree of crystallinity and good electrical properties along the vertical direction. The n-type N,N″-bis(2-ethylhexyl)-1,7-dicyanoperylene-3,4:9,10-bis(dicarboxyimide) (PDI-CN2) OSC was used as a model compound because the n-type electrical performance of this material is well characterized, particularly in the context of OFET applications.37,38 The field-effect mobilities of the PDICN2-based FETs were as high as 0.59 cm2 V−1 s−1 when fabricated on substrates subjected to a temperature gradient. This mobility value exceeded the value obtained from OFET devices prepared using conventional thermal annealing, 0.25 cm 2 V −1 s −1 . The crystalline structures and surface morphologies of the PDI-CN2 films were analyzed using two-dimensional grazing incidence X-ray diffraction (2D GIXD) and atomic force microscopy (AFM), respectively. The relationship between the crystalline morphology and the electrical performance of the resulting FET devices were investigated.

3. RESULTS AND DISCUSSION 3.1. OFET Device Fabrication. The top-contact bottomgate PDI-CN2 OFETs were fabricated on the ODTS-treated 300 nm thick gate dielectrics. Figure 1 shows the chemical

Figure 1. Schematic illustration of the film structure and crystalline alignment of PDI-CN2 molecules grown via vacuum deposition onto a substrate subjected to a temperature gradient.

structures of PDI-CN2 n-type organic semiconductor and a schematic representation of the system used to deposit the PDI-CN2 onto the substrate tilted through a 70° tilted angle. PDI-CN2 was vacuum-deposited at a rate of 0.2 Å/s onto the tilted substrate to a thickness of 50 nm. The 70° tilted substrate formed a temperature gradient between 68 °C at the bottom and 44 °C at the top regions, as shown in Figure S1. Considering the mechanism by which organic semiconductor films grow, including molecular absorption and surface diffusion, the thermal gradient across the surface was expected to affect the PDI-CN2 film morphology and crystallinity, inducing directional molecular rearrangement of the PDI-CN2 molecules and film growth along the vertical temperature gradient.25−28 Additionally, we confirmed the glass transition temperature (Tg) of the PDI-CN2 as shown in Figure S2. In the result, the Tg was found around 57 °C induced by the alkyl side chain, and melting temperature (Tm) was observed at 248 °C. Therefore, the molecules could be possible to arrange or rearrange on the heated substrates. The dependence of the PDI-CN2 film structure on the temperature gradient could be explained in terms of the morphological and crystalline nanostructures, which were investigated using AFM and synchrotron XRD techniques, respectively.

2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. A highly doped n-type silicon wafer with a 300 nm thick oxide layer was used as the substrate and the gate electrode. Prior to applying the silicon oxide layer surface treatment, the wafer was cleaned in piranha solution (70 vol % H2SO4 + 30 vol % H2O2) for 20 min at 120 °C and washed with copious amounts of distilled water. The as-purchased coupling agent octadecyltrichlorosilane (ODTS, Gelest) was used as an organic interlayer between the organic active material and the dielectric layer. The ODTS-treated SiO2 dielectrics were fabricated by dipping the substrates in 10 mM solutions of anhydrous toluene (Sigma-Aldrich) under an argon atmosphere for 2 h and then rinsing with toluene and ethanol several times. Finally, the samples were dried under nitrogen gas (99.9%) and then baked in an oven set to 120 °C for 20 min. After baking, the substrates were cleaned by ultrasonication in toluene and rinsed thoroughly with toluene and ethanol, followed by vacuum drying prior to use. PDI-CN2 (organic semiconductor, Polyera Activlnk, no purification) was deposited using an organic molecular beam deposition (OMBD) system onto the tilted or untilted substrates at a fixed rate of 0.2 Å/s under a base pressure of approximately 10−7 Torr. The angle of the tilted substrate was set to 70°. The deposition rates (0.2 Å/s), film thickness, and substrate temperature were recorded using a monitor. Finally, the source/drain electrodes were defined on the 50 nm thick PDI-CN2 film by 9911

DOI: 10.1021/acsami.6b15981 ACS Appl. Mater. Interfaces 2017, 9, 9910−9917

Research Article

ACS Applied Materials & Interfaces 3.2. AFM Morphology of PDI-CN2 Films. Figure 2 shows selected AFM topography images of 50 nm thick PDI-CN2

prepared at a higher temperature, and the average grain deposited onto the high-temperature substrate was 179−256 nm long and 60−81 nm wide.11,40−44 The actual temperature of the untilted substrate heated at 80 °C was found to be 72 °C, higher than the temperature (68 °C) measured in the bottom region of the tilted substrate heated at 80 °C. Because the molecular packing and growth processes involved surface diffusion of the deposited PDI-CN2 molecules, that is, the surface thermal energy determined the deposition kinetics, it should be expected that the PDI-CN2 grains formed on the untilted substrate heated at 80 °C are larger than the grains formed in the bottom region of the tilted substrate because the actual temperature of untilted substrate is higher than the temperature of tilted bottom substrate. Our results differed from our expectations, although the differences in the grain sizes between on the untilted substrates and on the bottom region of the tilted substrate heated at 80 °C were not dominant. These results could be explained in terms of the effect of tilting on the molecular deposition and grain growth processes in the PDI-CN2 films. To understand these effects, it is important to understand how the grain structures formed in the PDI-CN2 films during molecular deposition and nucleation. The evaporated PDI-CN2 molecules first reach and then diffuse on the substrate. The PDI-CN2 molecules diffuse a longer distance across the surface at higher substrate temperatures and are more likely to be stabilized and captured by established PDI-CN2 islands, thereby increasing the island (grain) size. The PDI-CN2 films grown on the tilted system are assumed to show similar behavior with the one-sided feeding of the evaporated molecules. It is considered that the molecular diffusion was determined by the local surface temperature within the temperature gradient, and diffusion from the bottom to the upper regions occurred preferentially. As a result, the PDI-CN2 grain size was larger on the gradationally heated substrate by realizing a tilting system compared to the grain size formed on the untilted substrate, as shown in Figure 2 and Figure S3. The growth processes and morphological properties of the PDI-CN2 film were further explored by collecting AFM images of the films during the initial growth of 5 nm thick PDI-CN2 films deposited onto untilted or tilted substrates heated at 80 °C, as shown in Figure 3. Interestingly, the PDI-CN2 films formed two-dimensional (2D) layered grains during the initial

Figure 2. AFM morphologies of 50 nm thick PDI-CN2 films vacuumdeposited onto (a−c) tilted substrates at room temperature (25 °C) without application of a temperature gradient, or onto (d−f) tilted substrate heated at 80 °C and subjected to a temperature gradient (right). The actual temperatures of the tilted substrates heated at 80 °C are shown in Figure S1 of the Supporting Information.

films vacuum-deposited at 25 °C (left) or 80 °C (right) onto 70° tilted substrates. The vacuum-deposited PDI-CN2 films self-organized to form three-dimensional island structures with numerous small grains.39 The substrate deposited at 25 °C, shown in Figure 2a−c, revealed minimal differences in the PDICN2 film structure grain size and shape across the upper, middle, and bottom regions of the tilted substrate. The average length and width of the grains in the tilted case deposited at 25 °C were 82−127 nm and 42−54 nm, respectively. On the other hand, in the film deposited on a tilted substrate and deposited at 80 °C revealed variations in both the grain length and width at various substrate positions. The upper region of the tilted substrate featured smaller PDI-CN2 grain sizes compared to the bottom region. The grains in the bottom region were 183− 342 nm long and 59−132 nm wide, whereas the PDI-CN2 grains in the upper region were 164−243 nm long and 46−82 nm wide. As mentioned in Figure S1, the actual temperature of the substrate decreased from 68 °C in the bottom region to 44 °C in the upper region. These results suggested that the morphological variations in the PDI-CN2 films were closely related to the substrate temperature,11,40−44 which facilitated surface diffusion of the PDI-CN2 molecules and increased the grain size. The effect of the substrate temperature gradient on the PDICN2 morphological properties was examined by analyzing the morphologies of PDI-CN2 films deposited onto untilted substrates at 25 or 80 °C, as shown in Figure S3a and b, respectively, of the Supporting Information. As anticipated, larger PDI-CN2 grain sizes were observed on the substrate

Figure 3. AFM images of 5 nm thick PDI-CN2 thin film morphologies deposited at 80 °C, obtained from (a) untilted case, (b) tilted case at the bottom of the substrate, (c) tilted case at the middle of the substrate, and (d) tilted case at the upper substrate. 9912

DOI: 10.1021/acsami.6b15981 ACS Appl. Mater. Interfaces 2017, 9, 9910−9917

Research Article

ACS Applied Materials & Interfaces stages of film growth. The PDI-CN2−substrate interactions were strong, and the PDI-CN2 molecules tended to form 2D layered grain structures during the early stages of film growth. The PDI-CN2 molecules then formed island grains on the 2D layered grains because the interaction strength among the adsorbed molecules was comparable to the interaction strength between molecules and the underlying surface (see Figure 2). This growth mode is known as the Stranski−Krastanov (layerisland) mode.40,41,44 As shown in Figure 3, the PDI-CN2 film deposited onto the tilted substrate formed larger grain structures than the film deposited onto the untilted substrate, especially, across the bottom region of the tilted substrate. This is an interesting point: the thermal effects of the PDI-CN2 molecular deposition were more prominent on the tilted substrate, even though the actual temperature (68 °C) across the bottom region of the tilted substrate was lower than that (72 °C) of the untilted case, as mentioned above. We observed that the PDI-CN2 grain size decreased as the substrate temperature decreased from the bottom to the upper region, in agreement with the results shown in Figure 2. These results suggested that the PDI-CN2 films deposited onto the bottom region were connected across several layers, which improved the intergrain interconnectivity compared to the grain structure present in the upper region of the substrate. Some information about the grain growth direction in the PDI-CN2 films could be obtained from Figures 2 and 3. The evaporated PDI-CN2 molecules reached and then diffused across the surface prior to crystallizing at adjacent grains.40,41,44 The kinetics of surface diffusion is particularly sensitive to the ambient thermal energy. The PDI-CN2 molecules that diffused across a relative high-temperature substrate gradually stopped diffusing due to insufficient thermal energy (in lower temperature regions of the film) or due to stabilization of the molecules at a surface site, such as at a crystal grain. These effects induced directional crystalline growth in the PDI-CN2 films, with a crystallinity gradient that matched the temperature gradient. 3.3. Nanostructure Study of PDI-CN2 Films by GIXD. The vacuum-deposited PDI-CN2 films were characterized both structurally and in terms of their electrical properties. Twodimensional grazing-incidence X-ray diffraction (2D GIXD) measurements were performed on the 50 nm thick PDI-CN2 films prepared on the untilted or tilted substrates heated at 25 or 80 °C, to investigate the effects of the temperature gradient on the crystalline properties of the films, as shown in Figure 4. Only the (001) Bragg reflection along the qz (out of-plane) direction was observed in the 2D GIXD patterns collected from all PDI-CN2 films, indicating that the molecules were oriented with their long axis nearly normal to the surface.37,38,45−47 The position of the (001) reflection gave a d001 spacing value, i.e., the monolayer thickness, equal to 18.0 Å, more than 1.0 Å higher than the bulk monolayer thickness, d001(bulk) = 16.5 Å.37,38,45,46 The higher monolayer thickness was ascribed to the tilting of the average polyaromatic molecular plane toward c, the direction normal to the film surface (the ab plane) or the tilting and/or stretching of the alkyl chains.48−50 The 2D GIXD results revealed intense Bragg reflections along the qz direction at a given qxy (in-plane), suggesting that the PDI-CN2 molecules formed 3D crystals on the substrate.48−50 These 3D crystal structures represented an extraordinary degree of crystalline order along both the vertical and lateral directions. The 2D GIXD patterns obtained from the PDI-CN2 films prepared on the untilted substrates were compared, as shown in

Figure 4. 2D GIXD patterns of 50 nm thick PDI-CN2 films vacuumdeposited onto an untilted substrate at (a) room temperature or (b) 80 °C. Panels (c) and (d) show the 2D GIXD patterns of 50 nm thick PDI-CN2 films deposited onto a tilted substrate heated at 80 °C, where the patterns were collected along the horizontal and vertical directions, respectively. The insets in each 2D GIXD pattern shown the in-plane X-ray intensity profiles collected from the 2D GIXD patterns along the qz direction at a qxy of 1.5 Å−1.

Figure 4a and b. The Bragg reflections were enhanced in the film deposited onto the substrate heated at 80 °C compared to the reflections obtained from the unheated film, as evidenced by the stronger peak intensities along the qz and qxy directions. These results suggested that the PDI-CN2 molecules readily rearranged themselves and self-organized under the thermal energy of the substrate heated at 80 °C, producing relatively well-oriented crystal structures in the PDI-CN2 films. It should be noted that the reflections along the qxy direction became more intense in the films prepared on the tilted substrate heated at 80 °C, as shown in Figure 4c,d, as compared to the untilted heated film, as shown in Figure 4b. The reflection intensities, scanned vertically from the bottom to the upper region, were stronger than those collected with horizontal scanning, indicating better crystalline properties in the PDICN2 films along the vertical directions. These results could be explained in terms of differences in the molecular packing mechanisms along the temperature gradient distribution of the tilted substrate. Better crystallinity in the PDI-CN2 films subjected to tilted at 80 °C agreed with the morphological structures observed in the films, as shown in Figure 3. 3.4. PDI-CN2 OFET Performance Study. Figure 5a−d shows the drain current−drain voltage (IDS−VD) output and drain current−gate voltage (IDS−VG) transfer characteristics collected from PDI-CN2 OFETs prepared using the tilted or untilted and heated or unheated systems. The reliability of the device characteristics was assessed by fabricating and testing more than 10 devices prepared under each set of conditions. The OFET device characteristics for the tilted case at 80 °C were horizontally measured at the bottom region of the substrate (refer to Figure S4). All devices were found to be well-behaved n-type transistors with a linear regime at small source−drain voltages (VDS) and a saturation regime at VDS 9913

DOI: 10.1021/acsami.6b15981 ACS Appl. Mater. Interfaces 2017, 9, 9910−9917

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Table 1. Summary of Filed-Effect Mobilities and Threshold Voltages Obtained in the Saturated Regime of PDI-CN2 Transistors

untilted case at 25 °C untilted case at 80 °C tilted case at 80 °C upper (horizontal) middle bottom tilted case at 80 °C (vertical)

mobility (cm2 V−1 s−1)

on/off current ratio

threshold voltage (V)

± ± ± ± ± ±

∼106 ∼106 ∼106 ∼106 ∼106 ∼106

9.0 11.3 10.6 9.8 11.7 5.9

0.078 0.224 0.170 0.264 0.291 0.588

0.021 0.089 0.073 0.066 0.079 0.100

substrates increased the average μFET values by more than a factor of 3 compared to the average μFET measured for the untilted unheated films, yielding an average mobility of 0.224 cm2 V−1 s−1 and a standard deviation of 0.089 cm2 V−1 s−1. These results were ascribed to an increase in the PDI-CN2 grain size due to the thermally activated diffusion of the molecules. Note that μFET measured along the vertical direction (the direction of the thermal gradient from high to low temperatures) was much higher compared to the value measured along the horizontal direction across the tilted substrate heated at 80 °C (see Figures 1 and 5e). The PDICN2 OFET properties measured along the vertical (thermal gradient) direction revealed an excellent average μFET of 0.588 cm2 V−1 s−1 with a standard deviation of 0.100 cm2 V−1 s−1, and maximum μFET of 0.690 cm2 V−1 s−1, much better than the value reported in the literature.8,38,52,53 The μFET characterizes charge transport and, thus, is intimately related to the degree of interconnectedness and ordered π−π stacking among the PDICN2 molecules.48,54 The charge carriers primarily flowed through the channel region within the PDI-CN2 film near the substrate surface. The crystalline and morphological structures of a few PDI-CN2 layers located at the dielectric surface were expected to determine the charge transport characteristics.10−19 As discussed previously, the PDI-CN2 layer deposited onto the tilted substrate heated at 80 °C produced directional film structures, as revealed by the morphological structures characterized during the early and late stages of film deposition, as well as the crystalline properties of the PDI-CN2 films. These directional PDI-CN2 film properties facilitated the formation of extensive π−π stacking interactions among the molecules. That is, the morphological and crystalline properties revealed why the PDI-CN2 OFETs based on the tilted heated films increased the electrical device performances, and the vertically oriented film displayed the best charge carrier transport among the OFETs studied here. The trend in Vth observed among the PDI-CN2 OFETs could not be fully understood in terms of the semiconductor morphological and crystalline structures, unlike the value of μFET. Vth shifted toward positive values, from 9.0 to 11.3 V, in the films deposited at 25 and 80 °C, respectively, prepared on the untilted substrates. Furthermore, the Vth values increased monotonically to 11.7 V along the horizontal direction of the film prepared on a tilted substrate heated at 80 °C, indicating rapid channel accumulation. On the other hand, Vth for the tilted heated film, measured along the vertical direction, showed a negative shift in Vth of 5.9 V. The multiple-trapping-andrelease (MTR) model, a model suitable for describing charge transport in organic electronics, predicted that both shallow and deep traps were dramatically decreased along the direction of the thermal gradient in the tilted heated film, due to the

Figure 5. Output (a,c) and transfer (b,d) characteristics of PDI-CN2 OFETs fabricated onto the untilted (a,b) and tilted substrates (c,d) at two substrate temperatures of 25 and 80 °C. Variations in the fieldeffect mobilities (e) and the threshold voltages (f) as a function of the deposition conditions for the PDI-CN2 films. The output and transfer characteristics of each FET were obtained with a stepped VGS of −10 V and at a fixed VD of −40 V, respectively.

values exceeding the gate voltage (VG), as shown in Figure 5a,c.11,14,38,51 The transfer curves shown in Figure 5b,d show that the on/off current ratios exceeded 106 for all PDI-CN2 devices, and the turn-on voltages of the devices approached 0 V.11,14,38,51 The field-effect mobility (μFET) and threshold voltage (Vth) were calculated from fits to the saturation regime (drain voltage VD = 40 V) of the IDS−VG transfer curves. The transfer characteristics were used to calculate the field-effect mobility in the saturation regime using the relationship IDS = CiμW(VG − Vth)2/2L, where W and L are the channel width and length, respectively, Ci is the specific capacitance of the gate dielectric, and μ is the field-effect mobility.11,14,38,51 The μFET and Vth of the PDI-CN2 OFETs as a function of the PDI-CN2 deposition condition are summarized in Figure 5e and f, respectively, and Table 1. The average μFET of the PDI-CN2 OFETs fabricated on the untilted substrate at 25 °C was 0.078 cm2 V−1 s−1, with a standard deviation of 0.021 cm2 V−1 s−1 in Figure 5e. Deposition of the PDI-CN2 films at 80 °C onto the untilted 9914

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providing the synchrotron radiation sources at 3C and 9A beamlines used in this study.

formation of well-ordered crystalline structures and good microscale grain evolution.55−57 Efficient processes that improve the electrical performances of a film are a prerequisite for OFET commercialization; therefore, the thermal gradient treatment using a tilted substrate, as described here, provides a versatile route to enhancing the electrical performances of organic semiconductor films, toward implementation of highperformance OFETs.

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(1) Sekitani, T.; Zschieschang, U.; Klauk, H.; Someya, T. Flexible Organic Transistors and Circuits with Extreme Bending Stability. Nat. Mater. 2010, 9, 1015−1022. (2) Yi, H. T.; Payne, M. M.; Anthony, J. E.; Podzorov, V. UltraFlexible Solution-Processed Organic Field-Effect Transistors. Nat. Commun. 2012, 3, 1259. (3) Kim, S. H.; Hong, K.; Xie, W.; Lee, K. H.; Zhang, S.; Lodge, T. P.; Frisbie, C. D. Electrolyte-Gated Transistors for Organic and Printed Electronics. Adv. Mater. 2013, 25, 1822−1846. (4) Facchetti, A. Organic Semiconductors: Made to Order. Nat. Mater. 2013, 12, 598−600. (5) 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. (6) Smith, J.; Zhang, W.; Sougrat, R.; Zhao, K.; Li, R.; Cha, D.; Amassian, A.; Heeney, M.; McCulloch, I.; Anthopoulos, T. D. Solution-Processed Small Molecule-Polymer Blend Organic ThinFilm Transistors with Hole Mobility Greater than 5 cm2/Vs. Adv. Mater. 2012, 24, 2441−2446. (7) Li, J.; Zhao, Y.; Tan, H. S.; Guo, Y.; Di, C.-A.; Yu, G.; Liu, Y.; Lin, M.; Lim, S. H.; Zhou, Y.; Su, H.; Ong, B. S. A Stable SolutionProcessed Polymer Semiconductor with Record High-Mobility for Printed Transistors. Sci. Rep. 2012, 2, 754. (8) Gao, X.; Hu, Y. Development of n-type organic semiconductors for thin film transistors: a viewpoint of molecular design. J. Mater. Chem. C 2014, 2, 3099−3117. (9) Gao, X.; Zhao, Z. High Mobility Organic Semiconductors for Field-Effect Transistors. Sci. China: Chem. 2015, 58, 947−968. (10) Cheng, X.; Caironi, M.; Noh, Y.-Y.; Wang, J.; Newman, C.; Yan, H.; Facchetti, A.; Sirringhaus, H. Air Stable Cross-Linked Cytop Ultrathin Gate Dielectric for High Yield Low-Voltage Top-Gate Organic Field-Effect Transistors. Chem. Mater. 2010, 22, 1559−1566. (11) Lee, H. S.; Kim, D. H.; Cho, J. H.; Hwang, M.; Jang, Y.; Cho, K. Effect of the Phase States of Self-Assembled Monolayers on Pentacene Growth and Thin-Film Transistor Characteristics. J. Am. Chem. Soc. 2008, 130, 10556−10564. (12) Yang, H.; Shin, T. J.; Ling, M.-M.; Cho, K.; Ryu, C. Y.; Bao, Z. Conducting AFM and 2D GIXD Studies on Pentacene Thin Films. J. Am. Chem. Soc. 2005, 127, 11542−11543. (13) Kim, D. H.; Park, Y. D.; Jang, Y.; Yang, H.; Kim, Y. H.; Han, J. I.; Moon, D. G.; Park, S.; Chang, T.; Chang, C.; Joo, M.; Ryu, C. Y.; Cho, K. Enhancement of Field-Effect Mobility Due to SurfaceMediated Molecular Ordering in Regioregular Polythiophene Thin Film Transistors. Adv. Funct. Mater. 2005, 15, 77−82. (14) Lim, J. A.; Lee, W. H.; Lee, H. S.; Lee, J. H.; Park, Y. D.; Cho, K. Self-Organization of Ink-jet-Printed Triisopropylsilylethynyl Pentacene via Evaporation-Induced Flows in a Drying Droplet. Adv. Funct. Mater. 2008, 18, 229−234. (15) Facchetti, A.; Yoon, M.-H.; Marks, T. J. Gate Dielectrics for Organic Field-Effect Transistors: New Opportunities for Organic Electronics. Adv. Mater. 2005, 17, 1705−1725. (16) Dinelli, F.; Murgia, M.; Levy, P.; Cavallini, M.; Biscarini, F.; de Leeuw, D. M. Spatially Correlated Charge Transport in Organic Thin Film Transistors. Phys. Rev. Lett. 2004, 92, 116802. (17) Park, S. H.; Lee, H. S.; Kim, J.-D.; Breiby, D. W.; Kim, E.; Park, Y. D.; Ryu, D. Y.; Lee, D. R.; Cho, J. H. A Polymer Brush Organic Interlayer Improves the Overlying Pentacene Nanostructure and Organic Field-Effect Transistor Performance. J. Mater. Chem. 2011, 21, 15580−15586. (18) Yogev, S.; Matsubara, R.; Nakamura, M.; Zschieschang, U.; Klauk, H.; Rosenwaks, Y. Fermi Level Pinning by Gap States in Organic Semiconductors. Phys. Rev. Lett. 2013, 110, 036803. (19) Kim, D. H.; Lee, H. S.; Yang, H.; Yang, L.; Cho, K. Tunable Crystal Nanostructures of Pentacene Thin Films on Gate Dielectrics

4. CONCLUSIONS In summary, we prepared vacuum-deposition of PDI-CN2 organic semiconductor films on substrates subjected to a thermal gradient, and we evaluated the effect of this gradient on the morphological and crystalline properties of the resultant film. The tilted substrate provided a temperature gradient between the heated bottom and the cooled upper regions, revealing that the PDI-CN2 film might be grown by the onesided feeding effects of the deposited molecules. As a result, the crystalline ordering and molecular packing improved along the temperature gradient direction, inducing directional crystalline growth of the PDI-CN2 films and providing superior morphological and crystalline properties along the temperature gradient direction. The deposition of PDI-CN2 films across a thermal gradient, forming an organic semiconducting layer on the tilted substrate, produced a μFET in PDI-CN2 OFET devices that was as high as 0.59 cm2 V−1 s−1, measured along the vertical direction. This value was much higher than the μFET value of 0.25 cm2 V−1 s−1, measured in devices prepared using conventional thermal annealing, or 0.08 cm2 V−1 s−1, measured in devices prepared without annealing on a flat substrate. The application of a thermal gradient during the vacuum deposition process was shown to be an effective strategy for improving the morphological and crystalline qualities of an organic semiconductor film, thereby enabling the fabrication of highperformance organic electronics.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b15981. Variations in the actual substrate temperature across the tilted and untilted substrates; additional AFM images of PDI-CN2 films deposited onto untilted substrates. (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hwa Sung Lee: 0000-0001-9131-810X Author Contributions §

These authors (J.-H.K. and S.H.) contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF2014R1A1A4A01009458 and NRF-2016R1D1A1B03936094). The authors thank the Pohang Accelerator Laboratory for 9915

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ACS Applied Materials & Interfaces Possessing Surface-Order Control. Adv. Funct. Mater. 2008, 18, 1363− 1370. (20) Kim, D. H.; Park, Y. D.; Jang, Y.; Kim, S.; Cho, K. Solvent Vapor-Induced Nanowire Formation in Poly(3-hexylthiophene) Thin Films. Macromol. Rapid Commun. 2005, 26, 834−839. (21) Qiu, L.; Lee, W. H.; Wang, X.; Kim, J. S.; Lim, J. A.; Kwak, D.; Lee, S.; Cho, K. Organic Thin-film Transistors Based on Polythiophene Nanowires Embedded in Insulating Polymer. Adv. Mater. 2009, 21, 1349−1353. (22) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Two-Dimensional Charge Transport in Self-Organized, High-Mobility Conjugated Polymers. Nature 1999, 401, 685−688. (23) Jang, J.; Nam, S.; Chung, D. S.; Kim, S. H.; Yun, W. M.; Park, C. E. High Tg Cyclic Olefin Copolymer Gate Dielectrics for N,N′Ditridecyl Perylene Diimide Based Field-Effect Transistors: Improving Performance and Stability with Thermal Treatment. Adv. Funct. Mater. 2010, 20, 2611−2618. (24) Zhang, L.; Cao, Y.; Colella, N. S.; Liang, Y.; Bredas, J.-L.; Houk, K. N.; Briseno, A. L. Unconventional, Chemically Stable, and Soluble Two-Dimensional Angular Polycyclic Aromatic Hydrocarbons: From Molecular Design to Device Applications. Acc. Chem. Res. 2015, 48, 500−509. (25) Schweicher, G.; Paquay, N.; Amato, C.; Resel, R.; Koini, M.; Talvy, S.; Lemaur, V.; Cornil, J.; Geerts, Y.; Gbabode, G. Toward Single Crystal Thin Films of Terthiophene by Directional Crystallization Using a Thermal Gradient. Cryst. Growth Des. 2011, 11, 3663− 3672. (26) Ye, C.; Zhang, L.; Fu, G.; Karim, A.; Kyu, T.; Briseno, A. L.; Vogt, B. D. Controlled Directional Crystallization of Oligothiophenes Using Zone Annealing of Preseeded Thin Films. ACS Appl. Mater. Interfaces 2015, 7, 23008−23014. (27) Biniek, L.; Leclerc, N.; Heiser, T.; Bechara, R.; Brinkmann, M. Large Scale Alignment and Charge Transport Anisotropy of pBTTT Films Oriented by High Temperature Rubbing. Macromolecules 2013, 46, 4014−4023. (28) Ho, C.-C.; Tao, Y.-T. Crystallization of Rubrene on a Nanopillar-Templated Surface by the Melt-Recrystallization Process and Its Application in Field-Effect Transistors. Chem. Commun. 2015, 51, 603−606. (29) Pola, S.; Kuo, C.-H.; Peng, W.-T.; Islam, M. M.; Chao, I.; Tao, Y.-T. Contorted Tetrabenzocoronene Derivatives for Single Crystal Field Effect Transistors: Correlation between Packing and Mobility. Chem. Mater. 2012, 24, 2566−2571. (30) Duffy, C. M.; Andreasen, J. W.; Breiby, D. W.; Nielsen, M. M.; Ando, M.; Minakata, T.; Sirringhaus, H. High-Mobility Aligned Pentacene Films Grown by Zone-Casting. Chem. Mater. 2008, 20, 7252−7259. (31) Su, Y.; Gao, X.; Liu, J.; Xing, R.; Han, Y. Uniaxial Alignment of Triisopropylsilylethynyl Pentacene via Zone-Casting Technique. Phys. Chem. Chem. Phys. 2013, 15, 14396−14404. (32) Lin, Y.-J.; Tsao, H. Y. Ambient-Atmosphere Annealing Effect on the Carrier Conduction Behavior Based on the Linear-Regime Transfer Characteristics of Pentacene Thin Film Transistors. Microelectron. Eng. 2016, 149, 57−61. (33) Tripathi, A. K.; Heinrich, M.; Siegrist, T.; Pflaum, J. Growth and Electronic Transport in 9,10-Diphenylanthracene Single Crystals-An Organic Semiconductor of High Electron and Hole Mobility. Adv. Mater. 2007, 19, 2097−2101. (34) Liu, C.-Y.; Bard, A. J. Increased Photo- and Electroluminescence by Zone Annealing of Spin-Coated and Vacuum-Sublimed Amorphous Films Producing Crystalline Thin Films. Appl. Phys. Lett. 2003, 83, 5431−5433. (35) Liu, C.-Y.; Bard, A. J. In-Situ Regrowth and Purification by Zone Melting of Organic Single-Crystal Thin Films Yielding Significantly Enhanced Optoelectronic Properties. Chem. Mater. 2000, 12, 2353− 2362.

(36) Ishii, H.; Kudo, K.; Nakayama, T.; Ueno, N. Electronic Processes in Organic Electronics; Springer: Berlin, 2015. (37) Cavallini, M.; D’Angelo, P.; Criado, V. V.; Gentili, D.; Shehu, A.; Leonardi, F.; Milita, S.; Liscio, F.; Biscarini, F. Ambipolar Multi-Stripe Organic Field-Effect Transistors. Adv. Mater. 2011, 23, 5091−5097. (38) Ferlauto, L.; Liscio, F.; Orgiu, E.; Masciocchi, N.; Guagliardi, A.; Biscarini, F.; Samori, P.; Milita, S. Enhancing the Charge Transport in Solution-Processed Perylene Di-imide Transistors via Thermal Annealing of Metastable Disordered Films. Adv. Funct. Mater. 2014, 24, 5503−5510. (39) Smith, D. Thin-Film Deposition: Principles and Practice; McGraw Hill Professional: New York, 1995. (40) Venables, J. A.; Spiller, G. D. T.; Hanbucken, M. Nucleation and Growth of Thin-Films. Rep. Prog. Phys. 1984, 47, 399−459. (41) Heringdorf, F.-J. M. Z.; Reuter, M. C.; Tromp, R. M. The Nucleation of Pentacene Thin Films. Appl. Phys. A: Mater. Sci. Process. 2004, 78, 787−791. (42) Stadlober, B.; Haas, U.; Maresch, H.; Haase, A. Growth Model of Pentacene on Inorganic and Organic Dielectrics Based on Scaling and Rate-Equation Theory. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 74, 165302. (43) Pratontep, S.; Nuesch, F.; Zuppiroli, L.; Brinkmann, M. Comparison between Nucleation of Pentacene Monolayer Islands on Polymeric and Inorganic Substrates. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 72, 085211. (44) Ruiz, R.; Choudhary, D.; Nickel, B.; Toccoli, T.; Chang, K.-C.; Mayer, A. C.; Clancy, P.; Blakely, J. M.; Headrick, R. L.; Iannotta, S.; Malliaras, G. G. Pentacene Thin Film Growth. Chem. Mater. 2004, 16, 4497−4508. (45) Jones, B. A.; Facchetti, A.; Wasielewski, M. R.; Marks, T. J. Effects of Arylene Diimide Thin Film Growth Conditions on nChannel OFET Performance. Adv. Funct. Mater. 2008, 18, 1329−1339. (46) Liscio, F.; Milita, S.; Albonetti, C.; D’Angelo, P.; Guagliardi, A.; Masciocchi, N.; Della Valle, R. G.; Venuti, E.; Brillante, A.; Biscarini, F. Structure and Morphology of PDI8-CN2 for n-Type Thin-Film Transistors. Adv. Funct. Mater. 2012, 22, 943−953. (47) Liscio, F.; Albonetti, C.; Broch, K.; Shehu, A.; Quiroga, S. D.; Ferlauto, L.; Frank, C.; Kowarik, S.; Nervo, R.; Gerlach, A.; Milita, S.; Schreiber, F.; Biscarini, F. Molecular Reorganization in Organic FieldEffect Transistors and Its Effect on Two-Dimensional Charge Transport Pathways. ACS Nano 2013, 7, 1257−1264. (48) Lee, W. H.; Min, H.; Park, N.; Lee, J.; Seo, E.; Kang, B.; Cho, K.; Lee, H. S. Microstructural Control over Soluble Pentacene Deposited by Capillary Pen Printing for Organic Electronics. ACS Appl. Mater. Interfaces 2013, 5, 7838−7844. (49) Briseno, A. L.; Aizenberg, J.; Han, Y.-J.; Penkala, R. A.; Moon, H.; Lovinger, A. J.; Kloc, C.; Bao, Z. A. Patterned Growth of Large Oriented Organic Semiconductor Single Crystals on Self-Assembled Monolayer Templates. J. Am. Chem. Soc. 2005, 127, 12164−12165. (50) Liu, S.; Wang, W. M.; Briseno, A. L.; Mannsfeld, S. C. B.; Bao, Z. Controlled Deposition of Crystalline Organic Semiconductors for Field-Effect-Transistor Applications. Adv. Mater. 2009, 21, 1217− 1232. (51) Usta, H.; Risko, C.; Wang, Z.; Huang, H.; Deliomeroglu, M. K.; Zhukhovitskiy, A.; Facchetti, A.; Marks, T. J. Design, Synthesis, and Characterization of Ladder-Type Molecules and Polymers. Air-Stable, Solution-Processable n-Channel and Ambipolar Semiconductors for Thin-Film Transistors via Experiment and Theory. J. Am. Chem. Soc. 2009, 131, 5586−5608. (52) Boudinet, D.; Benwadih, M.; Altazin, S.; Verilhac, J. M.; De Vito, E.; Serbutoviez, C.; Horowitz, G.; Facchetti, A. Influence of Substrate Surface Chemistry on the Performance of Top-Gate Organic ThinFilm Transistors. J. Am. Chem. Soc. 2011, 133, 9968−9971. (53) Castro-Carranza, A.; Nolasco, J. C.; Estrada, M.; Gwoziecki, R.; Benwadih, M.; Xu, Y.; Cerdeira, A.; Marsal, L. F.; Ghibaudo, G.; Iniguez, B.; Pallares, J. Effect of Density of States on Mobility in SmallMolecule n-Type Organic Thin-Film Transistors Based on a Perylene Diimide. IEEE Electron Device Lett. 2012, 33, 1201−1203. 9916

DOI: 10.1021/acsami.6b15981 ACS Appl. Mater. Interfaces 2017, 9, 9910−9917

Research Article

ACS Applied Materials & Interfaces (54) Duong, D. T.; Toney, M. F.; Salleo, A. Role of Confinement and Aggregation in Charge Transport in Semicrystalline Polythiophene Thin Films. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 86, 205205. (55) Mottaghi, M.; Horowitz, G. Field-Induced Mobility Degradation in Pentacene Thin-Film Transistors. Org. Electron. 2006, 7, 528−536. (56) Chesterfield, R. J.; McKeen, J. C.; Newman, C. R.; Ewbank, P. C.; da Silva Filho, D. A.; Bredas, J.-L.; Miller, L. L.; Mann, K. R.; Frisbie, C. D. Organic Thin Film Transistors Based on N-alkyl Perylene Diimides: Charge Transport Kinetics as a Function of Gate Voltage and Temperature. J. Phys. Chem. B 2004, 108, 19281−19292. (57) Letizia, J. A.; Rivnay, J.; Facchetti, A.; Ratner, M. A.; Marks, T. J. Variable Temperature Mobility Analysis of n-Channel, p-Channel, and Ambipolar Organic Field-Effect Transistors. Adv. Funct. Mater. 2010, 20, 50−58.

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DOI: 10.1021/acsami.6b15981 ACS Appl. Mater. Interfaces 2017, 9, 9910−9917