Impact of Energetically Engineered Dielectrics on Charge Transport in

Article pubs.acs.org/JPCC

Impact of Energetically Engineered Dielectrics on Charge Transport in Vacuum-Deposited Bis(triisopropylsilylethynyl)pentacene Se Hyun Kim,† Junghwi Lee,‡ Namwoo Park,‡ Honggi Min,‡ Han Wool Park,§ Do Hwan Kim,*,§ and Hwa Sung Lee*,‡ †

Department of Nano, Medical and Polymer Materials, Yeungnam University, Gyeongsan 712-749, Republic of Korea Department of Chemical & Biological Engineering, Hanbat National University, Daejeon 305-719, Republic of Korea § Department of Organic Materials and Fiber Engineering, Soongsil University, Seoul 156-743, Republic of Korea ‡

S Supporting Information *

ABSTRACT: The surface functionality of the gate dielectrics is one of the important variables to have a huge impact on the electrical performance of organic field-effect transistors (OFETs). Here, we describe the impact of energetically engineered dielectrics on charge transport in vacuum-deposited 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) thin films for eventually realizing high-performance OFETs. A variety of self-assembled monolayers (SAMs) bearing amino, methyl, phenyl (PTS), or fluoro end groups were introduced onto the SiO2 dielectric surfaces to design energetically engineered surfaces that can be used to explore the impact of surface functionalities at a TIPS-pentacene/gate dielectric interface. The solvent-free vacuum deposition of TIPSpentacene was used to exclude solution-processing effects resulting from fluid flows and solvent drying processes. The TIPS-pentacene layer on the PTS-SAM yielded the best morphological and crystalline structures, which directly enhanced the electrical properties, exhibiting field-effect mobilities as high as 0.18 cm2/(V s). Furthermore, the hysteresis, turn-on voltage, and threshold voltage were correlated with the surface potentials of various SAM-dielectrics. We believe that systematic investigation of the energetically engineered dielectrics presented here can provide a meaningful step toward optimizing the organic semiconductor/dielectric interface, thereby implementing flexible and high-performance OFETs.

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INTRODUCTION Printable and flexible organic field-effect transistors (OFETs) have recently attracted considerable interest due to their unique large-area and low-cost applications in a variety of fields.1−4 A clear understanding of the electrical performances of OFETs and the molecular basis of these properties through organic semiconductors is critical to develop applications of the materials. In general, charge transport in OFETs takes place through a few nanometer thick channel near the organic semiconductor/gate dielectric interface between the two source and drain electrodes.5,6 Extensive research has shown that charge transport in OFET strongly depends on the interfacial characteristics, such as surface functionality,7−12 roughness,13,14 modulus,15,16 and dielectric constant17 of the gate dielectric. This is because that the properties of the gate dielectric strongly affect the crystalline microstructure (including the crystallinity, crystalline orientation, and degree of ordering) as well as the number of trap sites in the channel layer.7−21 Among the above factors, in particular, the surface functionality of the gate dielectrics is one of the interfacial factors that have been regarded to have a huge impact on the © 2015 American Chemical Society

electrical performance of OFETs and has been extensively explored. Several groups have demonstrated that the surface functionalities of a gate dielectric modulate the following interfacial properties near the channel: (i) the early stage nucleation and growth of organic semiconductor molecules,15−19 (ii) the formation of interface traps,22,23 and (iii) the gate dielectric surface potential which can tune the carrier density in the channel.24−26 Hydrophobic surface modification in oxide gate dielectrics can, for instance, improve the crystalline structures and the molecular orientations of organic semiconductors to favor charge transport along the direction of π−π overlap.15−26 Hydrophobic surfaces also prohibit a trap formation from polar functionalities at the interface (e.g., hydroxyl and carbonyl groups) and prevent the adsorption of H2O and O2 that can degrade the operational stability.22,23,27 Moreover, the charge carrier density in the channel can be tuned by the surface potential induced by molecular dipoles or Received: June 10, 2015 Revised: December 4, 2015 Published: December 4, 2015 28819

DOI: 10.1021/acs.jpcc.5b05533 J. Phys. Chem. C 2015, 119, 28819−28827

Article

The Journal of Physical Chemistry C

30 vol % H2O2) for 30 min at 100 °C and were washed with copious amounts of distilled water. The ultrathin SAMfunctionalized SiO2 dielectrics were fabricated by immersing the substrates in 10 mM solutions with anhydrous toluene (Sigma-Aldrich) as a solvent under an argon atmosphere for 2 h and then rinsing with toluene and ethanol several times. Finally, all samples were dried by nitrogen gas (99.9%) and then baked in the oven set at 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. TIPS-pentacene (organic semiconductor, Sigma-Aldrich Chemicals, no purification) was deposited from a quartz crucible onto the SAM-functionalized substrates at a rate of 0.2 Å/s using an organic molecular beam deposition (OMBD) system under a base pressure of approximately 10−7 Torr. The deposition rate (0.2 Å/s), film thickness, and substrate temperature were recorded by the monitor. Finally, the source/drain electrodes were defined on the pentacene film by thermally evaporating gold through a shadow mask, where the channel length (L) and width (W) were 100 and 1000 μm, respectively. Characterization. The surface energies (γs) of the dielectrics were obtained by measuring the contact angles of two probe liquids (water and diiodomethane) on the substrates using a contact angle analyzer (Phoenix 300A, SEO Co., Inc.). The γs values were calculated by fitting the following equation to the measured values of the contact angles:

weak charge transfer between the organic semiconductor and the gate dielectric.24−26 Although the impact of surface functionalities on a gate dielectric has been widely studied, conclusions based on previous results are often inconsistent or even contradictory.15−25 As a consequence, this reflects that surface response of organic semiconductors on the gate dielectric cannot be generalized in a specific manner to optimize the OFET performances, thereby providing a material-dependent mechanism and a corresponding accurate analysis according to organic semiconductors to design high-performance OFETs. Herein, we describe the first example of nucleation and growth of solvent-free, vacuum-deposited 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) molecules on energetically engineered dielectric surface to achieve efficient charge transport into the channel and realize high-performance OFETs. The TIPS-pentacene is one of pentacene derivatives with bulky side groups (−Si(CH2CH3)3) attached to 6- and 13carbon positions in pentacene backbone which are reactive points in the molecule, inducing the high oxidation stability to H2O and oxygen molecules. Chemically or energetically different dielectric surfaces were designed, and the impact of the surface functionalities at the interface between the TIPSpentacene and SiO2 gate dielectric was explored by introducing a variety of self-assembled monolayers (SAMs) bearing amino, methyl, phenyl, or fluoro end groups onto the SiO2 dielectric surfaces via a wet process. The presence of the SAMs provided a high degree of control over the dielectric surface properties, including the surface energy, polarity, and roughness. TIPS-pentacene, a representative organic semiconductor, has been used extensively by many groups because the introduction of bulky side groups into the pentacene backbone can significantly improve the π−π overlap and reduce the interplanar spacing relative to the unsubstituted polymer.28−30 These steps can increase the charge carrier mobility. The preparation of thin films through direct solution processes remains a challenge in part because the solution fluid flows are affected by the dielectric surface properties, such as the surface energy, roughness, and dipole moment, which induce chaotic nucleation and growth of TIPS-pentacene crystals.29,30 Wet approaches based on solution processing do not appear to be suitable for studying the surface responses of TIPS-pentacene molecules on dielectrics bearing versatile surface functional groups due to the strong crystalline packing properties of the molecules in the solution process. Therefore, this work employed a solvent-free dry approach such as vacuum deposition to remove the solvent effects and clearly examine the intermolecular coupling at the interface between the TIPSpentacene and gate dielectric. We also explored the relationship between the surface functionalities and charge transport in OFETs.

1 + cos θ =

2(γsd)1/2 (γlvd)1/2 γlv

+

2(γsp)1/2 (γlvp)1/2 γlv

where γs and γlv are the surface energies of the dielectric surface and the probe liquid, respectively, and the superscripts d and p refer to the dispersive and polar (nondispersive) components of the surface energy, respectively. The thickness of the SAMs was determined using an ellipsometry (M-2000V, J.A. Woollam Co., Inc.). The surface morphologies and their root-meansquare (rms) roughness values were measured using atomic force microscopy (AFM, Digital Instruments Multimode) which is used via ex situ 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. The surface potentials of the SAM-functionalized dielectrics were characterized by measuring the secondary electron emission at the Pohang Accelerator Laboratory, Korea. The onset of the secondary electrons was determined from the intersection of the lines extrapolated from the background and the straight onset in each spectrum. All of the electrical parameters of the pentacene-FETs were obtained at room temperature using an HP4156A instrument in a dark environment. The field-effect mobility (μFET) and threshold voltage (Vth) were calculated in the saturation regime (drain voltage, VD = −60 V) using the equation ID = μFETCiWL−1(VG − Vth)VD, where ID, VG, and Ci are the drain current, gate voltage, and capacitance of the dielectrics, respectively. The Ci values of the SAM-functionalized SiO2 dielectrics, which were sandwiched between Au dots and highly doped n-type (100) Si substrates, were measured using an Agilent 4284 precision LCR meter.

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EXPERIMENTAL SECTION Materials and Sample Preparation. Highly doped n-type (100) Si wafers with a 300 nm thick thermally grown SiO2 layer were used as gate substrates. (3-Aminopropyl)trimethoxysilane (C6H17NO3Si, APS, Sigma-Aldrich), hexamethyldisilazane (C6H19NSi2, HMDS, Sigma-Aldrich), phenyltrichlorosilane (C6H5Cl3Si, PTS, Gelest), and (heptadecafluoro-1,1,2,2-tetrahydrodecyl)trichlorosilane (C10H4Cl3F17Si, FTS, Gelest) were used to form the various SAMs on SiO2 substrates without purification. Prior to surface treatment of SiO2 layer, the wafer substrates were cleaned in piranha solution (70 vol % H2SO4 +

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RESULTS AND DISCUSSION Before the interfacial properties in a TIPS-pentacene/gate dielectric system were investigated, the dielectric surface properties were explored using SAMs. The inherent phys28820


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introducing structural mismatch sites and defects that acted as trap states in the semiconductor film.34,35 A flat dielectric surface is required to achieve optimal OFET performances. The SAM-treated SiO2 dielectrics showed the same capacitance values (10 nF/cm2). These results suggest that the surface roughness and capacitance do not affect the semiconductor film growth or charge carrier transport under the conditions used in this study. The dielectric surface was also characterized by its surface energy, which provides a direct indication of the intermolecular forces and contributes to semiconductor film growth and interface trap formation.7−9,36,37 Competition between molecule−molecule and molecule−substrate interactions can determine the growth mode, thereby laying a foundation for the overall crystalline structure.15−19 In these reports, the surface energy relied on the sum of the polar components due to the presence of permanent dipoles (including hydrogen bonds) and dispersion components due to instantaneous dipole moments. Moreover, the surface energy was modulated by the chemical composition and packing density of the outmost surface functionalities.38 For example, nonpolar or weakly interactive surfaces generally provided lower surface energies. The surface energy in a fluorocarbon material decreased in the order of CH2 > CH3 > CF2 > CF3.39,40 The presence of more fluorines and a more close-packed structure at the surface reduced the surface energy. The inherent characteristics of fluorine, including its small atomic radius and low polarizability, also reduced the surface energy. The surface energies in the four SAM-treated and no-treated SiO2 dielectrics were calculated using the procedure described in the Experimental Section and are reported in Table 1. As expected, the surface energy of no-treated SiO2 dielectrics (bare) displayed the highest surface energy of 72.9 ± 3.5 mJ/m2 due to the presence of plentiful hydroxyl groups on the surface. The surface energy of FTS-SAM (18.6 ± 1.4 mJ/m2) was lowest among the surfaces tested here, whereas APS-SAM (51.4 ± 3.4 mJ/m2) displayed the highest surface energy among four SAM treatments due to a greater degree of polar character. PTSSAM (45.2 ± 1.7 mJ/m2) provided a higher surface energy than HMDS-SAM (31.8 ± 0.6 mJ/m2). However, the HMDS-SAM showed the higher polar term of the surface energy than the PTS-SAM one. Although methyl group (−CH3), a surface functional group of HMDS, is less polar than benzene of PTS,41,42 the thickness of HMDS is thinner than that of PTS (Table 1). In general, shorter chain length of SAM molecule (namely, thinner SAM thickness) hardly induces close packing between SAM molecules, thereby keeping the underlying oxide surface uncovered. From this reason, the polar term of the HMDS-SAM is higher than that of the PTS-SAM. The contribution of the polar component to the total surface energy (surface polarity) is a more meaningful indicator of the

icochemical characteristics of the SAMs, including the chain length, end-functionalities, and reactive groups and the reaction conditions, including the temperature, humidity, and solvent power, can affect the quality of the resulting molecular assembly such as the packing density or chain order.15,16,31−33 The electrical and optical properties as well as the interfacial wetting, adhesion, and friction properties of the substrate could be tuned by selecting an appropriate SAM. Figure 1 shows the

Figure 1. Schematic illustration of the top-contact TIPS-pentacene FETs and the chemical structures of the various SAMs used in this study. The background is an AFM image of APS dielectric.

schematic representation of an OFET device structure used in this study, composed of a SiO2 dielectric substrate treated with various SAMs. The chemical end groups of the SAMs are shown in Figure 1. The thickness of SAMs on SiO2 layer was summarized in Table 1. We could confirm that all SAM layers were well-defined on SiO2 dielectrics, showing their own functionalities on the basis of SAM thickness and surface energy as shown in Table 1. Note that AFM topographic images of amino- (APS), methyl- (HMDS), phenyl- (PTS), and fluoro-SAMs (FTS) are almost similar as seen in Figure 1. The AFM image in this figure is for APS-SAM surface as a representative one. All samples displayed amorphous smooth surface morphologies with no predominant features and similar root-mean-square (rms) roughness values of 0.34 ± 0.04 nm (scan size: 3 × 3 μm), which is an identical value with the rms roughness of 300 nm thick SiO2 surface (bare). The dielectric surface roughness effects (e.g., the peak-to-peak height of the undulated surface) significantly influenced the dynamic properties of the semiconductor film growth (nucleation and diffusion of molecules), thereby reducing the extent of π-conjugation and

Table 1. Summary of the Surface Energies of Various SAM-Modified Dielectrics and the TIPS-Pentacene FET Performances surface energya (mJ/m2) SAM thickness (Å) APS HMDS PTS FTS bare a

4.4 2.9 7.7 13.1

± ± ± ±

0.2 0.1 0.4 1.2

polar term 15.1 6.6 3.2 2.8 43.7

± ± ± ± ±

1.3 0.2 0.4 0.5 2.8

dispersion term 36.3 25.2 42.0 15.9 29.1

± ± ± ± ±

2.1 0.4 1.3 0.9 0.7

device performance field-effect mobility (cm V−1 s−1)

total 51.4 31.8 45.2 18.6 72.9

± ± ± ± ±

3.4 0.6 1.7 1.4 3.5

0.05 0.11 0.18 0.02 0.09

± ± ± ± ±

0.02 0.01 0.04 0.01 0.006

threshold voltage (V) −15.6 0.8 2.9 13.7 −0.9

± ± ± ± ±

2.2 0.4 1.3 2.7 0.3

turn-on voltage (V) −9.2 2.1 5.1 22.2 5.4

± ± ± ± ±

3.0 1.9 1.5 3.6 1.9

Surface energy calculation: geometric mean equation 1 + cos θ = 2(γds )1/2(γdlv)1/2/γlv + 2(γps )1/2(γplv)1/2/γlv (solvent: DI water and diiodomethane). 28821


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Figure 2. AFM images of the morphologies of the 10 nm (upper) and 50 nm thick (down) TIPS-pentacene films deposited onto (a, f) APS-, (b, g) HMDS-, (c, h) PTS-, (d, i) FTS-, and (e, j) bare-dielectrics.

With a strongly interacting substrate where interlayer the TIPSpentacene/substrate interactions are stronger, the TIPSpentacene tends to form the 2D layered grains at the early stages of film growth. And then, TIPS-pentacene molecules formed island grains on the 2D layered grains because the interaction between adsorbed molecules is comparable to that between molecules and underlying surface. Such a growth mode has well-known as the Stranski−Krastanov (layer− island) mode.44,45 That is, this growth mode in the PTS-SAM and bare dielectrics are due to the strong π−π interaction of TIPS-pentacene/PTS-SAM intermolecules and the reactive substrate with high surface energy, respectively. By contrast, a typical growth mode of the Volmer−Weber (island) mechanism, whereby the adsorbed molecules have a higher attraction to each other than to the substrate, was observed in the other dielectrics (Figures 2a,b,d).45,47,48 FTS-SAM dielectric exhibited far smaller island size than other SAM dielectrics because of its lowest surface energy value, as described above. The morphological properties of the 50 nm thick TIPSpentacene films were examined, as shown in Figures 2f−j. The initial growth behaviors on each SAM-treated dielectric are far different (especially, interaction between TIPS-pentacene and substrates plays a critical role in determining initial crystal morphology: layer or island). However, all 50 nm thick films exhibited comparable surface crystal morphologies, even though the grain sizes of TIPS pentacene on the PTS-SAM and bare are much larger than others, which implies that the growth modes of all sample are similar after dielectric surfaces were covered with TIPS-pentacene molecules. Consequently, these observations suggested a possible mechanism to describe the interfacial effects of the functional dielectric surface. During the vacuum deposition process, TIPS-pentacene preferentially and easily covered the dielectric surface to decrease the surface energy (Figures 2a,c,e). On the other hand, the dielectric surfaces with relatively low surface energies repelled the TIPSpentacene molecules and formed deep grain boundaries and small grain sizes (Figures 2b,d). The crystalline structures of the 10 nm thick TIPS-pentacene films that had been vacuum-deposited onto the energetically engineered dielectric surfaces were examined using twodimensional grazing-incidence X-ray diffraction (2D GIXD) measurements. The interfacial crystalline features were extracted, as shown in Figure 3. The diffraction pattern

strength of attraction to polar molecules, and a high surface energy can enhance the adsorption of water and oxygen molecules, which creates trap states in a semiconductor/ dielectric interface. Additionally, the surface energy values in our system exhibited some differences (within 10%) from those reported (see Tables S1 and S2 in the Supporting Information).43 Although the surface energy characterized from contact angle measurement is a representative indicator to check the quality of SAMs, these exist some experimental error depending on the measurement conditions. Therefore, the accurate judgment of the SAM quality should be accompanied by other characterization for the SAM surfaces (e.g., surface roughness and thickness). As a result, the qualities of the APS-, HMDS-, PTS-, and FTS-SAMs are fully comparable to the previous result.43 The effects of the surface polarity of each SAM on charge carrier transport are discussed in detail in the following section. In addition to the electrochemical properties of the functionalities (such as the electronegativity or acidity), the SAM structure (including the molecular orientations and ordering) plays a critical role in determining the surface potential of the dielectric.15,16,31 SAM molecules possess permanent dipoles that depend on the end functionality. An ordered assembly of molecules can be described as the sum of the dipoles which build up a net electrical field at the surface. Most SAM thicknesses are on the molecular scale (1−2 nm). The effective strength of an electric field produced by a wellpacked SAM can be equivalent to the strength of an external bias (100 V) applied to a 300 nm thick SiO2 dielectric, which can reach values of a few MV/cm. The intrinsic potential created by a SAM at a semiconductor/SAM interface can be used to modulate the charge carrier density in the channel without application of an external bias. The mechanisms which energetically engineered dielectric surfaces affect the surface morphology were explored by preparing two TIPS-pentacene films (a 10 nm film formed during the early stages of film growth and a 50 nm bulk film) on a dielectric substrate. The AFM images shown in Figures 2a−e indicated that different TIPS-pentacene growth mechanisms were involved in the different SAM-dielectrics. Interestingly, the TIPS-pentacene film formed on the PTSSAM and bare dielectrics showed 2-dimensional (2D) layered grains during the early stages of film growth (Figures 2c,e). 28822


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(Figure 3a). These features indicated that the TIPS-pentacene molecules were unfavorably oriented in the film such that the TIPS-pentacene backbones were stacked along a direction perpendicular to the gate dielectric modified with the APSSAM.49−51 The reflections from the TIPS-pentacene films deposited on the HMDS-SAM dielectric surface (Figure 3b) were enhanced compared to those obtained from the APS-SAM dielectric, as evidenced by the stronger peak intensity and the narrow diffraction patterns along the qz direction. These results indicated that the TIPS-pentacene nanocrystals were relatively large and well-oriented.49,50 The PTS-SAM dielectric shown in Figure 3c formed well-ordered and large crystalline domains, as observed in the AFM images shown in Figures 2c,h. These domains produced up to 3 orders of intense Bragg reflections along the qz (out-of-plane) direction at a given qxy (in-plane), suggesting that the TIPS-pentacene molecules formed 3D crystals on the PTS-engineered dielectric surface. These 3D crystal structures for the PTS case represented an extraordinary degree of crystalline order in both the vertical and lateral directions compared to APS, HMDS, FTS, and bare cases.49,52 The molecular packing structure of the TIPS-pentacene molecules changed dramatically due to the specific interactions with the benzene ring structure of the PTS-SAM during molecular deposition.53,54 Efficient charge transport in the lateral direction was achieved in the highly ordered film. Despite the different morphological features formed during the early stages of the FTS-SAM dielectric growth (Figure 2d), a circular diffraction pattern similar to those collected from the APS- and HMDS-SAM dielectrics was obtained, as shown in Figure 3d. The diffraction intensity of this pattern was relatively weak. This circular diffraction pattern corresponds to the general crystalline structure of the TIPS-pentacene film, except in the film that displayed extraordinary crystalline nature. In

Figure 3. 2D GIXD patterns of 10 nm thick TIPS-pentacene films deposited onto (a) APS-, (b) HMDS-, (c) PTS-, (d) FTS-, and (e) bare-dielectrics.

collected from the TIPS-pentacene film deposited on the APSSAM dielectric surface displayed a (001) reflection corresponding to a 16.8 Å in the TIPS-pentacene unit cell.49−51 An intense (011) peak was also observed along the qz (out-of-plane) axis

Figure 4. Output (upper) and transfer (down) characteristics of TIPS-pentacene FETs prepared using (a, f) APS-, (b, g) HMDS-, (c, h) PTS-, (d, i) FTS-, and (e, j) bare-dielectrics. Gate dielectrics were used with the SAM-modified 300 nm SiO2, and the output and transfer characteristics of each FET were obtained with a stepped VGS of −10 V and at a fixed VD of −60 V, respectively. 28823


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direction was as follows: the on-to-off swept transfer curve moved in the negative VG direction relative to the off-to-on curve. In general, hysteresis with this loop direction arises from interface traps at a semiconductor/dielectric interface, where hole or electron trapping and detrapping occurs.60 Traps in OFETs tend to originate from structural defects in the semiconductor layer. Surprisingly, the FTS-SAM, which displayed the smallest grain size and worst crystallinity exhibited negligible hysteresis, indicating that structure-induced traps did not affect the hysteresis in our system. Instead, the surface polarity of each SAM appeared to influence the hysteresis behavior. Water molecules adsorbed at the semiconductor/dielectric interface most likely created charge trap states that significantly degraded the device performance.61 The absorption of the water molecules depended strongly on the hydrophobicity of the dielectric surface. The nonpolar dielectric surface restricted the trap formation processes that produced hysteresis.62 The trend in Vturn‑on in our OFETs (FTS > bare > PTS > HMDS > APS), in which the subthreshold current increased exponentially, cannot be fully understood in terms of the semiconductor structure and morphology, unlike the case of μFET. The relationship between μFET/Vturn‑on and the surface potential was explored by collecting the secondary electron emission spectra of the SAMs (Figure 5a). The onset of

addition, the diffraction pattern in the bare case displayed an intense (011) peak along the qz-axis and a circular diffraction pattern similar to the FTS-SAM case in Figure 3e, despite the Stranski−Krastanov growth mode and large grain structures which is similar to the growth process of the PTS-SAM case. The results could be induced by the mismatched interaction between organic molecule and inorganic surface.18 Figure 4 shows the drain current−drain voltage (ID−VD) output and drain current−gate voltage (ID−VG) transfer characteristics of the TIPS-pentacene OFETs employing APS-, HMDS-, PTS-, and FTS-SAM-treated and non-treated (bare) SiO2 dielectrics. The field-effect mobility (μFET) and threshold voltage (Vth) were calculated from the fits to the saturation regime (drain voltage VD = −60 V), and the hysteresis and turn-on voltage (Vturn‑on) were extracted from the ID−VG transfer curves on a logarithmic scale. The electrical parameters extracted from these OFETs are listed in Table 1. The μFET of the PTS-SAM-based OFET was at least twice the values obtained from the other OFETs and was as high as 0.18 cm2/(V s). This value for the PTS-SAM case is much better than that of previous literature which is applied by vacuumdeposited TIPS-pentacene films.55 The μFET characterizes charge transport and thus, is intimately related to the degree of interconnectedness and ordered π−π stacking among TIPSpentacene molecules. The charge carriers primarily flowed through the channel that formed at the interface between the TIPS-pentacene layer and the SAMs in the OFETs.2−5 The crystalline morphology of the first few TIPS-pentacene layers located at the dielectric surface are expected to determine the charge transport characteristics. As discussed previously, the TIPS-pentacene layer grown on the PTS-SAM produced the largest grain size during the early stages of film growth, and the well-ordered crystalline structures favored extensive π−π stacking. This morphology explained why the phenyl-SAMbased OFET exhibited the best charge carrier transport among the OFETs studied here. Adjunctively, the μFETs obtained by vacuum-deposited TIPSpentacene in Figure 4 were lower compared to the controlled solution-processed methods, even though the maximum μFET of 0.23 cm2/(V s) for the PTS-treated device. For examples, Park’s group showed the TIPS-pentacene FETs with the μFET of 1 cm2/(V s) and current on/off ratio greater than 107.56 Bao’s group reported the highly aligned TIPS-pentacene crystals with metastable structures using lateral confinement, having the high μFET of 2.02 cm2/(V s).57 Cho’s group also reported the vertically segregated semiconductor-dielectric film with millimeter-sized spherulite-crystalline structures of the TIPSpentacene film on polymer gate dielectrics and the maximum μFET of 3.40 cm2/(V s),58 one of the highest mobility values for TIPS-pentacene OFETs fabricated by using a conventional solution process. However, something important to notice here is that they used the highly crystalline properties of the TIPSpentacene film by strong π−π packing properties between molecules in the controlled solution process, which differs from the heterogeneous responses of the intermolecular coupling between the TIPS-pentacene and the energetically engineered SAM-treated dielectrics. On the other hand, the μFET of TIPSpentacene FETs fabricated by the simple spin-coating method were usually shown around 0.1 cm2/(V s).59 Therefore, the device performances of our vacuum-deposited TIPS-pentacene FETs were reasonable and comparable. Interestingly, significant hysteresis was only observed in the APS-SAM OFET, as shown in Figure 4. The hysteresis loop

Figure 5. (a) Secondary electron emission spectra of the APS-, HMDS-, PTS-, and FTS-SAM-modified dielectric surfaces. (b) Variations in the Vturn‑on (closed symbols) and field-effect mobility (open symbols) of the TIPS-pentacene FETs as a function of the onset energy.

secondary electrons, which corresponded to the surface potential of the SAM surface,63 was determined by extrapolating two solid lines from the background and the straight onset from each spectrum. As shown in Figure 5a, the kinetic energies corresponding to the onset of secondary electrons followed the order FTS- > PTS- > HMDS- > APS-SAMs. A higher kinetic energy indicated that the surface produced a higher potential; that is, the surface was strongly electron-withdrawing.25,63 The high electronegativity of fluorine most likely contributed to the high potential of the FTS-SAM. By contrast, the surface potential of APS-SAM was lower than the surface potentials of the PTS- and HDMS-SAMs, although the amino group exhibited a higher dipole moment than the aliphatic or aromatic hydrocarbons. These results may have arisen from the electron-donating character induced by the unpaired electrons on the nitrogen atom. We did not compare the onset of secondary electrons on the bare gate dielectric because the organic (TIPS-pentacene)/organic (SAMs) and organic (TIPS-pentacene)/inorganic (SiO2) interactions are different, 28824



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which also induced the changes of the orientational degrees of freedom in the TIPS-pentacene films.64 Therefore, we considered that a comparison of the bare sample with SAMtreated dielectrics could be inappropriate. It should be noted that Vturn‑on was affected by the intrinsic electric field produced by the permanent dipole of the SAM layers and by the electrochemical reaction between the surface functional groups and the semiconductor molecules. Early studies about the dielectric surface chemistry demonstrated that surfaces with a strong electron affinity and/or acidic character should preferentially attract electrons at the semiconductor side, leading to the injection of extra holes that balance the charge and positively shift Vturn‑on and Vth.3,20 Therefore, the surface potential of a dielectric is critical to modulating the charge and electrical performance of the device. As observed from the secondary electron emission spectra of our SAMs, the trend in the surface potential agreed with the trend in Vturn‑on obtained from the corresponding OFETs, as shown in Figure 5b. The positive value of Vturn‑on in the FTS-SAM sample and the high off-current relative to the off-currents obtained from the other samples arose from the high electronegativity of the fluorine atoms; however, the poor morphology and crystalline structure of the pentacene film grown on the FTS-SAM surface produced more traps in the gap, which slowly increased ID in the subthreshold regime beyond Vturn‑on of the device. As a consequence, the FTS-SAM OFET exhibited a low μFET. These results indicated that charge transport was principally determined by competition between the electrochemical activity of the surface functionalities and the structural properties of the semiconductor layer.

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CONCLUSIONS

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (D.H.K.). *E-mail [email protected] (H.S.L.). Author Contributions

S.H.K. and J.L. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by Basic Science Research Program (NRF-2014R1A1A4A01009458) of the National Research Foundation of Korea (NRF) and the Center for Advanced Soft-Electronics under the Global Frontier Project (CASE2014M3A6A5060932) funded by the Ministry of Science, ICT and Future Planning.

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In summary, we have demonstrated the crystalline microstructure of solvent-free, vacuum-deposited TIPS-pentacene molecules on energetically engineered dielectric surface to optimize the electrical performances of OFETs. Chemically or energetically engineered dielectrics were designed by introducing a variety of functionalities, such as APS, HMDS, PTS, and FTS-SAMs, at the TIPS-pentacene/gate dielectric interface. The TIPS-pentacene film grown on the PTS-SAM provided the best morphological and crystalline structures, which directly enhanced the electrical properties, yielding μFETs as high as 0.18 cm2/(V s), due to the matching effects between TIPSpentacene and gate dielectric surface properties such as the surface energy, dipole moment, and functionalized surface molecules. Hysteresis and the Vturn‑on in the OFET device were clearly correlated with the surface potentials of the engineered SAM dielectrics. We believe that engineering gate dielectrics for achieving an optimal condition of crystalline microstructure of organic semiconductors will be an effective way to implement high-performance OFETs capable of showing high stability as well as high mobility.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b05533. Contact angles and surface energies of various SAMmodified surfaces; height profiles of 10 and 50 nm thick TIPS-pentacene films; full author list of references (PDF) 28825


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