Self-assembled monolayers of cyclohexyl-terminated phosphonic acids as a general dielectric surface for high-performance organic thin-film transistors.

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Self-Assembled Monolayers of Cyclohexyl-Terminated Phosphonic Acids as a General Dielectric Surface for High-Performance Organic Thin-Film Transistors Danqing Liu, Zikai He, Yaorong Su, Ying Diao, Stefan C. B. Mannsfeld, Zhenan Bao, Jianbin Xu, and Qian Miao* Self-assembled monolayers (SAMs) provide a versatile molecular platform that can dramatically change the surface properties[1] by varying the terminal groups. The cyclohexyl group is an unprecedented terminal group for all kinds of SAMs, to the best of our knowledge. Interestingly, cyclohexyl terminal groups are present in the phospholipid bilayers of acid- and heat-resistant bacteria (Bacillus acidocaldarius),[2] and make a major contribution to the acid- and heat-tolerance of the cell membrane.[3] Here, we report a novel cyclohexyl-terminated SAM from 12-cyclohexyldodecylphosphonic acid (CDPA in Figure 1a). The most important finding from this study is that molecules of CDPA self-assemble into a highly ordered molecular monolayer on AlOy/TiOx, a solution-processed high-k metal oxide,[4] providing a general dielectric surface for high-performance organic thin-film transistors (OTFTs).[5] The CDPA-modified AlOy/TiOx functions as an excellent dielectric enabling OTFTs with high field effect mobility of up to 5.7 cm2 V−1 s−1 for holes and 5.5 cm2 V−1 s−1 for electrons, good air stability with low operating voltage, and general applicability to solution-processed and vacuum-deposited p-type and n-type organic semiconductors. OTFTs are elemental units in organic integrated circuits, which, for example, are applied to drive individual pixels in active matrix displays and to operate radio-frequency identification (RFID) tags and sensors.[6,7] To fabricate OTFTs over a

D. Liu, Dr. Z. He, Prof. Q. Miao Department of Chemistry The Chinese University of Hong Kong Shatin, New Territories, Hong Kong, China E-mail: [email protected] Dr. Y. Su, Prof. J. Xu Department of Electronic Engineering The Chinese University of Hong Kong Shatin, New Territories, Hong Kong, China Dr. Y. Diao, Prof. Z. Bao Department of Chemical Engineering Stanford University Stanford, California 94305, USA Dr. S. C. B. Mannsfeld SLAC National Accelerator Laboratory Menlo Park, California 94025, USA Prof. Q. Miao Center of Novel Functional Molecules The Chinese University of Hong Kong Shatin, New Territories, Hong Kong, China

DOI: 10.1002/adma.201402822

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large area for these applications, organic semiconductors can be processed by vacuum deposition or more desirably by solutionbased methods, which are compatible with roll-to-roll and inkjet printing techniques of low cost. OTFTs are interface devices with their performance highly dependent on the interface between organic semiconductors and gate dielectrics. SAMs of organosilanes[8] and phosphonic acids[9] have been widely used as a powerful molecular platform to engineer the dielectric surface of OTFTs.[10,11] However, SAMs that are generally useful for both vacuum-deposited and solution-processed OTFTs are still

Figure 1. a) Structures of phosphonic acids. b) Structures of the p-type semiconductors tested in this study. c) Structures of the n-type semiconductors tested in this study.

© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 2. GATR-FTIR spectra taken from the SAMs of OPA, CDPA and CBPA.

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for well-ordered alkyl chains in SAMs.[18–20] In comparison, the C–H stretches of the SAM of CBPA shift to larger wavenumbers (2854 and 2925 cm−1 for νs and νas, respectively). This shift is similar to that observed in the IR spectra of phosphonic acid SAMs with shorter straight alkyl chains, and is an indicator of disordered SAMs.[18,21] NEXAFS spectroscopy qualitatively assesses the structural order in SAMs with a high-intensity, monochromatic, and linearly-polarized X-ray from a synchrotron source to measure electronic transitions near the absorption edge of an atom, and analyses of the electronic transitions near the carbon K-edge are commonly used to characterize the ordering of long-chain alkyl groups in SAMs.[22,23] Therefore X-ray absorption spectra of the SAMs of CDPA, CBPA and OPA were collected at varied angles of incidence (θ). As found from the NEXAFS difference spectra at 90° and 25° X-ray incidence angles, the SAMs of CDPA and OPA exhibit an angle dependence (i.e., difference spectra intensity) at a peak related to σ*C–H (ca. 287.5 eV) as shown in the Supporting Information. In contrast, the SAM of CBPA does not exhibit angle dependence in the same experiment. These results are in agreement with the finding from GATR-FTIR and indicate that OPA and CDPA form well-ordered monolayers while CBPA forms a disordered monolayer,[22,23] since transitions to C–C and C–H antibonding orbitals (σ*) are known to depend on the alignment of the linearly polarized light with the antibonding orbital in relation to the orientation of alkyl chains in the SAMs.[24] In order to determine whether the ordered monolayer of CDPA is indeed crystalline, the SAM of CDPA was assessed using grazing incidence X-ray diffraction (GIXD), which is a powerful tool to investigate the structure of ordered monolayers by exposing thin films to high-intensity synchrotron X-rays at a very shallow incident angle (ca. 0.12°). This technique focuses the X-ray intensity into a region right above the substrate making it sensitive to both out-of-plane (perpendicular to the substrate) and in-plane periodic structures even in very thin films.[20] A crystalline monolayer can in principle exhibit a streak-like Bragg rod by satisfying the Bragg-condition in-plane. As shown in Figure 3, the GIXD patterns from the SAM of CDPA exhibit a Bragg rod at Qxy = 1.41 Å−1, which indicates the SAM is crystalline with a lattice constant of 4.45 Å. The appearance of only one Bragg rod in the detection range suggests that the SAM likely has a hexagonal lattice. The lattice constant is larger than that of the OPA SAM (4.2 Å)[16] by only 6%, suggesting a close packing of bulky cyclohexyl terminal groups. In relation to their application in OTFTs as a dielectric, CDPA and CBPA-modified AlOy/TiOx were studied in terms of surface roughness, surface energy, capacitance and leakage current (see Experimental Section). As found from atomic force microscopy (AFM), CDPA and CBPA generally formed very smooth surface with a root mean square (RMS) roughness of 0.2 nm over an area of 25 µm2. Such roughness is comparable to that of the ultrasmooth SAM of octadecyltrimethoxysilane (OTMS) on SiO2.[20] The surface energy of the SAM of CDPA measured 31.6 mN m−1 containing a polar component of 0.3 mN m−1 and a dispersion component of 31.3 mN m−1. The SAM of CDPA has a larger surface energy than that of OPA (26.6 mN m−1) by increasing the dispersion component while keeping the polar component almost unchanged. This is in agreement with the fact that CDPA contains a non-polar cyclohexyl terminal group,



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very rare. To demonstrate the general applicability of the CDPAmodified AlOy/TiOx, p- and n-channel OTFTs were fabricated from two p-type semiconductors and two n-type semiconductors as shown in Figure 1, respectively, by either vacuum deposition or solution-based methods. Here pentacene and C60 were chosen as representatives of vacuum-deposited semiconductors while 6,13-bis((triisopropylsilyl)ethynyl)-pentacene (TIPSPEN)[12,13] and 6,13-bis((triisopropylsilyl)ethynyl)-5,7,12,14tetraazapentacene (TIPS-TAP)[14,15] were chosen as representatives of solution-processed semiconductors because pentacene, C60 and TIPS-PEN are among the most extensively studied organic semiconductors and all of them possess very high fieldeffect mobilities. In order to better understand the structure–property relationship for CDPA, two structurally related phosphonic acids were also studied here in a comparison manner. Among them, 4-cyclohexylbutylphosphonic acid (CBPA) is a new phosphonic acid with the same cyclohexyl terminal group as CDPA but a shorter alkyl chain. Octadecylphosphonic acid (OPA), one of the most studied phosphonic acids for OTFTs, has a straight alkyl chain with the same number of carbon atoms as CDPA. CDPA and CBPA were conveniently synthesized in three steps as detailed in the Supporting Information. The SAMs of CDPA and CBPA were formed on the surface of AlOy/TiOx in a very similar way that was used to form SAMs of phosphonic acids on the same metal oxides (see Experimental Section).[15] The resulting SAM-modified AlOy/TiOx was characterized in two aspects. One is the molecular ordering in the SAMs, and the other is the surface and electrical properties related to its role as the dielectric layer in OTFTs. The cyclohexyl terminal group in CDPA and CBPA is larger than the methyl terminal group in well-studied long-alkylchain SAMs. Can such a bulky group be fitted into a highly ordered monolayer on the surface of AlOy/TiOx? To answer this question, the SAMs of CDPA and CBPA were studied in comparison with the crystalline SAM of OPA[16] using grazing angle attenuated total reflection Fourier transform infrared (GATR-FTIR) spectroscopy and Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. As shown in Figure 2, the GATR-FTIR spectra of the SAMs of CDPA and OPA exhibit two peaks at 2850 cm−1 and 2919 cm−1, which are attributed to the symmetric (νs) and asymmetric (νas) stretching modes of CH2 groups, respectively,[17] and can be used as a reference

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Figure 3. GIXD patterns of CDPA-modified AlOy/TiOx and bare AlOy/ TiOx (the arrows indicate the diffraction peak from the SAM).

which has a larger area of contact than the methyl terminal group in OPA. With an enhanced surface energy, the CDPAmodified AlOy/TiOx exhibits much better wettability than the OPA-modified AlOy/TiOx and is completely wettable by a variety of organic solvents, such as chloroform, isopropyl alcohol, ethyl acetate and toluene, with a static contact angle smaller than 5°. Such good wettability brought considerable freedom to our experiments to optimize solvents for drop casting. In comparison, the SAM of CBPA exhibited a larger surface energy (36.7 mN m−1) with a polar component of 1 mN m−1 and a dispersion component of 35.7 mN m−1. The larger surface energy of CBPA is likely due to the disordered and shorter alkyl chains of CBPA, which allow the probe liquids to have a better chance to interact with the metal oxides under the SAM. The capacitance per unit area (Ci) of CDPA- and CBPA-modified AlOy/ TiOx measured 210 ± 18 and 240 ± 16 nF cm−2, respectively. The capacitance slightly varied among different devices because the spin-coating process was not able to yield the AlOy/TiOx layer of uniformed thickness. The leakage current through the CDPA-modified AlOy/TiOx was about 1.6 × 10−6 A cm−2 as measured from a metal-insulator-metal structure with a voltage of 3 V. In comparison to the CDPA-modified AlOy/TiOx, the OPA-modified AlOy/TiOx has essentially the same leakage current, while the CBPA-modified AlOy/TiOx has a larger leakage current (3.4 × 10−6 A cm−2 with the same voltage). The larger

capacitance and leakage current of the CBPA-modified AlOy/ TiOx are likely related to the shorter alkyl of chain of CBPA.[18] OTFTs of pentacene and C60 were fabricated by vacuumdeposition of the corresponding organic semiconductors on the CDPA- and OPA-modified AlOy/TiOx, and OTFTs of TIPS-PEN and TIPS-TAP were fabricated by drop-casting solutions of the corresponding organic semiconductors onto the CDPA-modified AlOy/TiOx. All of these OTFTs had top-contact gold as drain and source electrodes. Because the OPA-modified AlOy/TiOx is poorly wettable by most of the common organic solvents, the drop-casting technique cannot result in high-quality thin films on it.[15] Therefore solution-processed OTFTs of TIPS-PEN and TIPS-TAP were not fabricated on the OPA-modified AlOy/TiOx in this study. With a large capacitance per unit area (Ci), these OTFTs were able to operate at a gate voltage as low as 3 V. The OTFTs on CDPA- and OPA-modified AlOy/TiOx exhibited essentially the same threshold voltage. The field-effect mobilities of OTFTs on the CDPA- and OPA-modified AlOy/TiOx as measured in vacuum and in air are summarized in Table 1, where each average value was obtained from at least 20 channels on five different substrates. The typical output and transfer I–V curves of the best-performing OTFTs on the CDPA-modified AlOy/TiOx are shown in Figure 4.[25] When CDPA was replaced by CBPA, the vacuum-deposited OTFTs of pentacene and C60 exhibited lower field-effect mobility of 1.6 ± 0.2 cm2 V−1 s−1 (measured in air)[26] and 0.021 ± 0.005 cm2 V−1 s−1 (measured in vacuum), respectively, and the solution-processed OTFTs of TIPS-PEN and TIPS-TAP exhibited lower field effect mobility of 0.23 ± 0.07 cm2 V−1 s−1 (measured in air) and (7.3 ± 0.6) × 10−3 cm2 V−1 s−1 (measured in vacuum), respectively. To demonstrate the general applicability of the CDPAmodified AlOy/TiOx, other organic semiconductors, such as copper phthalocyanine, N,N′-dicyclohexyl-1,4,5,8-naphthalenetetracarboxydimide (C-NTCDI) and N,N′-dihexyl-1,4,5,8naphthalene-tetracarboxydimide (H-NTCDI),[27] were also tested. The vacuum-deposited OTFTs of C-NTCDI and solutionprocessed OTFTs of H-NTCDI on CDPA-modified AlOy/TiOx were air stable with high field effect mobilities of 1.50 ± 0.30 and 1.09 ± 0.26 cm2 V−1 s−1, respectively, as summarized in the Supporting Information. The field effect mobilities of pentacene and C60 as measured from their OTFTs on the SAM of CDPA are among the highest values for the two benchmark organic semiconductors. In comparison to the SAM of CDPA, the earlier reported crystalline SAM of octadecyltrimethoxysilane (OTMS) led to higher field effect mobility for vacuum-deposited OTFTs of C60 (4.7 ± 0.41 cm2 V−1 s−1 as measured in an atmosphere of N2) but lower field-effect mobility for pentacene (2.8 ± 0.2 cm2 V−1 s−1).[20] The maximum field-effect mobility

Table 1. Field-effect mobilities (cm2 V−1 s−1) of OTFTs fabricated on CDPA- and OPA-modified AlOy/TiOx as tested in vacuum (Vac.) or in air. pentacene

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TIPS-PEN

TIPS-TAP

CDPA

OPA

CDPA

OPA

CDPA

CDPA

Vac.

—

—

3.08 ± 0.93 highest: 5.5

0.93 ± 0.25 highest: 1.2

—

2.57 ± 0.89 highest: 5.0

Air

3.86 ± 0.47 highest: 5.7

2.21 ± 0.46 highest: 2.9

2.98 ± 0.83 highest: 5.1

0.66 ± 0.28 highest: 1.1

1.64 ± 0.55 highest: 2.7

0.78 ± 0.32 highest: 1.44


C60


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of the TIPS-PEN OTFTs on the SAM of CDPA (2.7 cm2 V−1 s−1) is higher than the reported maximum mobility from solutionprocessed unstrained films of TIPS-PEN (1.8 cm2 V−1 s−1),[13] but lower than that from strained single-crystalline films (11 cm2 V−1 s−1).[28] The electron mobility of TIPS-TAP OTFTs on the SAM of CDPA (5.0 cm2 V−1 s−1) is the highest among all the solution-processed polycrystalline n-channel OTFTs to the best of our knowledge. In comparison to the SAM of 12-methoxydodecylphosphonic acid (MODPA), which has a methoxyl terminal group to enhance surface energy for highmobility solution-processed n-channel OTFTs,[15] the SAM of CDPA not only enhanced the average field effect mobility of the two solution-processed semiconductors by about 50%, but also significantly improved the air stability of the n-channel OTFTs of TIPS-TAP. The improved air stability is a result of removal of polar oxygen atom from the SAM surface because the polar oxygen atoms can accumulate water molecules at the semiconductor-dielectric interface by hydrogen bonding. This is in agreement with the fact that the SAMs of MODPA and CDPA exhibit water contact angle of 75.2° and 99.8°, respectively. With the cyclohexyl group replacing the methoxyl group, the SAM of CDPA have a hydrophobic and organic-solventwettable surface, which is a key for fabricating air-stable solution-processed n-channel OTFTs.

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To better understand the effect of the cyclohexyl terminal group on the growth of organic semiconductors, particularly, in comparison with the methyl terminal group, vacuum-deposited films of the above organic semiconductors on the SAMs of CDPA and OPA were studied with X-ray diffraction (XRD) and atomic force microscopy (AFM). The X-ray diffractions from films of pentacene on the SAMs of CDPA and OPA (see Supporting Information) are essentially the same exhibiting four diffraction peaks in accordance with a thin-film phase of pentacene.[29] As found from the AFM images shown in Figure 5, the pentacene films deposited on the SAMs of CDPA and OPA are composed of grains with distinct terraces. The grains in the films on CDPA, with a size over 1 µm, are larger than those in the films on OPA presumably because layer-by-layer growth is favored by the SAM of CDPA, which has a higher surface energy.[30] Therefore, the difference in mobility of pentacene OTFTs on different SAMs can be attributed to the difference of film morphology. In agreement with the different film morphologies on the SAMs of CDPA and OPA, AFM images of very thin (3 nm thick) films reveal that the film of pentacene on CDPA has a higher coverage with less grain boundaries. As shown in the Supporting Information, the film of C60 on the SAM of CDPA exhibits two X-ray diffraction peaks at 2θ = 10.86° (d spacing = 8.19 Å) and 2θ = 21.78° (d spacing =



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Figure 5. AFM height images of thin films of pentacene, C60, and TIPSTAP deposited on different SAMs.

4.08 Å), which are in agreement with the (111) and (222) diffractions as derived from the single crystal structures.[31] This indicates a crystalline film, in which the (111) plane is parallel to the surface. In contrast, the film deposited on the SAM of OPA under the same condition exhibits only one small diffraction peak at 2θ = 10.86°, which indicates a film of apparently lower crystallinity. Unlike the C60 films grown on the SAM of CDPA, the polycrystalline films of C60 grown by molecularbeam deposition[32] or hot wall epitaxy growth[33] were reported to exhibit X-ray diffraction peaks at 2θ = 10.8°, 17.8°, and 20.9°, which corresponded to the (111), (220) and (311) planes respectively. These diffractions are indicative of films that contain crystalline domains aligned to different directions. From the above comparison, it can be concluded that the SAM of CDPA is able to initiate the growth of a highly crystalline film of C60 with unidirectional molecular alignment very possibly because

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of a quasi-expitaxy growth. In agreement with this, the vacuumdeposited films of C60 on the disordered SAM of CBPA are amorphous presumably due to lack of quasi-epitaxy growth and thus exhibit low mobility. As found from the AFM images of 40 nm-thick C60 films (Figure 5), the film on the SAM of OPA is composed of irregularly round grains with diameters of tens of nanometers, which are typical for vacuum-deposited films of C60[34] and likely associated with an island growth mode.[35] In contrast, the film on the SAM of CDPA contains flat crystallites of hundreds of nanometers long and wide. Each of the crystallites is presumably grown by a layer-by-layer mode[36] in agreement with the higher surface energy of the SAM of CDPA. Therefore, the high mobility of the C60 films on the SAM of CDPA should be attributed to both high crystallinity and good morphology, which result from the crystalline nature and higher surface energy of the SAM. The drop-cast films of TIPS-TAP on both the SAMs of CDPA and CBPA are crystalline as indicated by the similar XRD patterns from these films. The difference between the two films is revealed by the AFM images as shown in Figure 5. The dropcast film of TIPS-TAP on the SAM of CDPA contains highly ordered flat micro-stripes of ca. 1 µm wide, while the film on the SAM of CBPA exhibits a much rougher surface, which suggests lower ordering at the micrometer scale. In view that the CBPA SAM is disordered in nature, it can be concluded that the ordering of SAMs plays a role in determining the morphology and molecular ordering in solution-processed films. A crystalline SAM that is wettable by a variety of organic solvents is optimal for solution-processed OTFTs. In summary, the above study puts forth a new design of SAMs for interface engineering of high-performance OTFTs. The key of this design is CDPA, a phosphonic acid with an unprecedented cyclohexyl terminal group attached to a long alkyl chain. Self-assembly of CDPA on top of solution-processed AlOy/TiOx provides a dielectric surface that is generally useful for vacuum-deposited and solution-processed small-molecule organic semiconductors and results in p- and n-channel OTFTs with high field effect mobilities at low operating voltages and good air stability for n-channel transistors. Owning to its crystalline and wettable nature, the SAM of CDPA may also find other useful applications as a surface coating technique for a variety of metals and metal oxides. As CDPA is structurally closely related to ω-cyclohexyl undecanoic and tridecanoic acids, the major fatty acid components in the cell membrane of Bacillus acidocaldarius,[2] the crystalline SAM of CDPA might serve as a hint for understanding the structure of the acid- and heat-resistant cell membrane.

Experimental Section Formation of AlOy/TiOx and SAMs: A thin layer of AlOy/TiOx was spin-coated onto a highly doped silicon substrate with an area of ca. 1 cm × 1 cm and resistivity smaller than 0.005 Ω cm following the reported solution-based procedure to form dielectrics.[4] To form SAMs of phosphonic acids, a AlOy/TiOx-coated Si wafer was treated with oxygen plasma for two minutes and then soaked in a solution of the corresponding phosphonic acid in isopropyl alcohol (1.5 mM) at room temperature for 12 hours, and then rinsed with isopropyl alcohol and dried with a flow of nitrogen.


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1

1

(1 + cosθ )γ l = (γ SDγ lD )2 + 2(γ SPγ lP )2

(1)

Here θ is the equilibrium contact angle made by each liquid on the solid surface, γ is the surface energy. The superscripts D and P refer to the dispersive and the polar components, respectively, and the subscripts l and s refer to the liquid and solid, respectively.[38] The dispersion and polar components of the surface tension are 21.8 mN m−1 and 50.9 mN m−1, respectively, for water, and 50.0 mN m−1 and 0 mN m−1, respectively, for CH2I2.[39] Measurement of Capacitance and Leakage Current: The capacitance of SAM-modified AlOY/TiOx was measured in a frequency range of 100 Hz to 100 kHz from a metal–insulator–metal structure, which had vacuum-deposited gold (0.2 mm × 1 mm) as the top electrode and a highly doped silicon substrate as the bottom electrode.[40] The average capacitance per unit area (Ci) of SAM-AlOY/TiOx was taken at the lowest frequency (100 Hz). The leakage current was measured from the same metal–insulator–metal structure with a voltage of −3 V to 3 V. Fabrication and Characterization of OTFTs: Thin films of pentacene and C60 were vacuum-deposited onto the SAM-modified AlOY/TiOx using an Edwards Auto 306 vacuum coating system at a pressure of 2.0 × 10−6 Torr or lower, with a deposition rate of ca. 1 Å s−1 to a thickness of 40 nm as measured by a quartz crystal sensor. During vacuum deposition the distance between source and substrate was 18.5 cm, and the substrate was kept at 60 °C for pentacene and 90 °C for C60 by heating with a radiant heater. Thin films of TIPS-PEN and TIPS-TAP were formed by dropping a 0.5 mg/mL solution in a mixed solvent of dichloromethane and acetone (a volume ratio of 1:1) onto the CDPA-modified AlOy/TiOx. The dropcast films were then placed in a vacuum oven overnight to completely remove solvent residues. A layer of gold was deposited through a shadow mask onto the organic films to form top-contact source and drain electrodes. The resulting devices had highly doped silicon as the gate electrode and the SAM-modified AlOY/TiOx as dielectrics. The fieldeffect mobility of these OTFTs in the saturation regime were extracted from transfer I–V curves using the equation: IDS = (µWCi/2L)(VG – VT)2, where IDS is the drain current, µ is field-effect mobility, Ci is the capacitance per unit area for the SAM-modified AlOY/TiOx, W is the channel width, L is the channel length, and VG and VT are the gate and threshold voltage, respectively. To obtain average values, at least 20 channels on five substrates were tested for each condition.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work was supported by the Research Grants Council of Hong Kong (project number: GRF402011 and CUHK7/CRF/012G). Received: June 25, 2014 Revised: August 4, 2014 Published online: September 10, 2014

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Adv. Mater. 2014, 26, 7190–7196

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