Ladder-Type Silsesquioxane Copolymer Gate Dielectrics for High

Jan 25, 2016 - SKKU Advanced Institute of Nanotechnology (SAINT), School of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Republic of ...
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Ladder-Type Silsesquioxane Copolymer Gate Dielectrics for HighPerformance Organic Transistors and Inverters Woonggi Kang,† Gukil An,‡ Min Je Kim,† Wi Hyoung Lee,§ Dong Yun Lee,∥ Hyunjung Kim,‡ and Jeong Ho Cho*,† †

SKKU Advanced Institute of Nanotechnology (SAINT), School of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea ‡ Department of Physics, Sogang University, Seoul 121-742, Republic of Korea § Department of Organic and Nano System Engineering, Konkuk University, Seoul 143-701, Korea ∥ Department of Polymer Science and Engineering, Kyungpook National University, Daegu, 41566, Korea S Supporting Information *

ABSTRACT: A ladder-type poly(phenyl-co-methacryl silsesquioxane) (PPMSQ) copolymer was developed for use as a gate dielectric in high-performance organic field-effect transistors (OFETs). The ladder-type PPMSQ copolymer was synthesized via the hydrolysis of two types of monomers, methacryloxypropyltrimethoxysilane and phenyltrimethoxysilane, followed by a condensation polymerization. The phenyl groups in one monomer were introduced to enhance the structural ordering of the overlying organic semiconductors, whereas the methacryloxypropyl groups in the other monomer were introduced to cross-link the polymer chains via thermal- or photocuring. The curing process enhanced the electrical strength of the gate dielectric layer due to the formation of a network structure with a reduced free volume. Thermal curing reduced the surface energy of the gate dielectrics, which improved the structural order of the overlying organic semiconductors and promoted the formation of large grains. The ladder-type PPMSQ was used as a gate dielectric to produce benchmark p- and n-channel OFETs based on pentacene and N,N′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8), respectively. The resulting OFETs exhibited excellent electrical performances, including a high carrier mobility (0.53 cm2 V−1 s−1 for the p-type pentacene OFET and 0.17 cm2 V−1 s−1 for the n-type PTCDI-C8 OFET) and a high ON/OFF current ratio exceeding 104. The photocured patterned PPMSQ film was successfully used to fabricate complementary OFET-based inverters that yielded high gains. The use of the ladder-type PPMSQ gate dielectrics provides a novel approach to realizing next-generation organic electronics.

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14

INTRODUCTION

In addition to developing novel electrode and organic semiconductor materials, the development of gate dielectric materials is crucial to fabricating high-performance OFET devices. Thus far, the best device performances have been achieved using inorganic gate dielectrics, such as SiO2, Al2O3, or HfO 2 , prepared using high-cost and complex vacuum processes.15,16 Alternative dielectrics that can be formed using

Organic field-effect transistors (OFETs) have been a significant research focus because they are indispensable components in the development of large-area, flexible, and low-cost electronic devices, such as paper-like displays, smart cards, radio frequency identification (RF-ID) tags, and sensors.1-10 The performances of OFETs have improved considerably in the past decade and have already reached a level comparable or even higher than the performance levels obtained from amorphous silicon transistors. Flexible large-area OFETs with high performances may be achieved using solution-processable organic electronic components, including electrodes, semiconductors, and dielectrics.11© 2016 American Chemical Society

Received: October 20, 2015 Revised: January 20, 2016 Published: January 25, 2016 3501

DOI: 10.1021/acs.jpcc.5b10240 J. Phys. Chem. C 2016, 120, 3501−3508

Article

The Journal of Physical Chemistry C

Figure 1. (a) Schematic illustration of the top-contact OFETs prepared using the ladder-type PPMSQ gate dielectrics. The lower panel shows the chemical structure of the ladder-type PPMSQ used in this study. (b) FT-IR spectra of the ladder-type PPMSQ gate dielectric films cured at different temperatures (100, 150, and 200 °C) or cured by UV exposure. (c) Relative intensities of CO bonds connected to the C−C bonds compared with those connected to the CC bonds in the ladder-type PPMSQ films upon application of the thermal- and photocuring processes. (d) Surface energies and (e) dielectric constants of the ladder-type PPMSQ gate dielectrics as a function of the curing temperature. (f) Current density vs electric field plots of the ladder-type PPMSQ gate dielectrics cured at different temperatures.

In this manuscript, we demonstrated the synthesis of a ladder-type poly(phenyl-co-methacryl silsesquioxane) (PPMSQ) copolymer gate dielectric for use in high-performance OFETs. The ladder-type PPMSQ copolymer was simply synthesized through the hydrolysis and condensation polymerization of two types of monomers. The phenyl and methacryloxypropyl groups in each monomer were introduced to enhance the structural ordering of the overlying organic semiconductors and to form a network structure via thermal- or photocuring, respectively. These curing procedures enhanced the electrical strength of the gate dielectrics by forming a structured network among the polymer chains. The structural ordering and surface grain morphologies of the p-type pentacene and n-type N,N′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) films were systematically investigated using two-dimensional grazing incidence X-ray diffraction and atomic force microscopy. The reduced surface energy of the PPMSQ film by increasing the curing temperatures improved the structural order of the overlying organic semiconductors and increased the grain size. OFETs based on the 200 °C-cured PPMSQ gate dielectrics exhibited excellent electrical performances, including high carrier mobilities and ON/OFF current ratios. The use of ladder-type PPMSQ gate dielectrics provided a novel approach to realizing next-generation organic electronics.

solution processing methods, such as spin-coating or printing, are required. Polymeric gate dielectrics provide good candidates for OFETs due to their facile solution processability at low temperatures and their good film-forming characteristics, which are compatible with flexible substrates.17-19 Commercially available polymers, such as polystyrene, poly(methyl methacrylate), and polyimide, have been widely used as gate dielectrics; however, these polymers typically exhibit poor electrical strengths, which induces severe gate leakage currents and poor device stabilities. This, in turn, limits the applications of these polymers in logic circuits fabricated from OFETs. Alternatives to gate dielectrics include cross-linkable polymers, such as polyvinylphenol and poly(vinyl alcohol), prepared using cross-linking agents.20,21 The network structure formed by cross-linking dramatically reduced the free volume and thermal motions of the polymer chains, which reduced the leakage currents of the gate dielectrics. Residual hydroxyl groups in the film, however, formed detrimental charge trapping sites at the semiconductor−dielectric interface and at the gate dielectric, resulting in device instabilities. Cage-type polyhedral oligosilsesquioxanes (POSS) were synthesized as a single component cross-linking system, and a siloxane network film with minimal hydroxyl groups was fabricated by spin-coating and subsequent thermal (or photo) annealing.22-24 These cage-type POSS films were utilized as gate dielectrics to produce high-performance OFETs. Unfortunately, it proved to be difficult to control the thickness of the POSS films due to their oligomeric properties. The dramatic volume shrinkage induced by the large quantity of gases discharged during the curing process could not be prevented. Polymeric silsesquioxane films with a high dielectric strength and minimal hydroxyl groups would provide beneficial gate dielectric materials for use in high-performance OFETs.

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EXPERIMENTAL SECTION

Synthesis of Poly(phenyl-co-methacryl silsesquioxane). Ladder-type PPMSQ (Mw = 34,860, PDI = 3.6) was synthesized according to the following procedure. First, deionized water (2.4 g) and potassium carbonate (K2CO3, 0.02 g) were added to a 100 mL round-bottomed flask and 3502

DOI: 10.1021/acs.jpcc.5b10240 J. Phys. Chem. C 2016, 120, 3501−3508

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The Journal of Physical Chemistry C Table 1. Gate Dielectric Properties of the Ladder-Type PPMSQ thickness (nm) pristine 100 °C-cured 150 °C-cured 200 °C-cured UV-cured

987.5 965.7 954.1 923.5 921.9

(±9.4) (±5.1) (±12.8) (±10.2) (±4.7)

roughness (nm) 0.4 0.5 0.4 0.3 0.3

(±0.1) (±0.2) (±0.1) (±0.1) (±0.1)

surface energy (mJ/m2) 50.0 47.6 44.4 44.8 44.5

(±3.1) (±2.1) (±2.7) (±3.5) (±1.9)

stirred for 10 min. Anhydrous tetrahydrofuran (THF) was then introduced and stirred for an additional 30 min. A 0.04 mol mixture of methacryloxypropyltrimethoxysilane and phenyltrimethoxysilane in a mole ratio of 1:1 was added dropwise using a syringe under nitrogen. The reaction proceeded at room temperature for 96 h, resulting in a crude solution with a colorless and a cloudy phase. The crude viscous products were obtained by decanting the colorless mixed solvents. The product was purified according to a previously reported method.25 The resulting synthesized ladder-type PPMSQ was analyzed by 1H-NMR, Si-NMR, and thermogravimetric analysis (TGA) (Supporting Information Figures S1 and S2), which is consistent with a previous report.25 OFET Fabrication. A Si wafer with a thermally grown 300 nm-thick oxide layer was used as a substrate. The substrate was cleaned with a Piranha solution (100 °C, 30 min) and then washed with deionized water. A 20 wt % PPMSQ solution in propylene glycol monomethyl ether acetate (PGMEA) was spin-coated onto the Si wafer at 3000 rpm for 60 s, and the film was stored in the vacuum chamber over 12 h to remove residual solvent. The thickness of the resulting film was 987 nm. Samples were annealed at various temperatures: 100, 150, or 200 °C, for 1 h under ambient conditions. A 50 nm-thick pentacene or N,N′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) film was deposited onto the prepared laddertype PPMSQ gate dielectrics at a rate of 0.2 Å/s using an organic molecular beam deposition (OMBD) system. Finally, gold source and drain electrodes were thermally deposited through a shadow mask. The channel length (L) and width (W) were 100 and 1000 μm, respectively. Characterization. Fourier transform infrared absorption (FT-IR) spectroscopy was used to investigate the thermal- and photoinduced chemical reactions in the ladder-type PPMSQ. The contact angles (θ) of the gate dielectrics were measured using distilled water and diiodomethane as probe liquids. The surface energies (γ) of the gate dielectrics were calculated using the geometric mean equation (1 + cos θ)γpl = 2(γsdγpld)1/2 + 2(γspγplp)1/2, where the superscripts d and p indicate the dispersion and polar terms of the surface energy, respectively. The subscripts of s and pl indicate the substrate and probe liquid, respectively. The surface morphologies of the pentacene and PTCDI-C8 films deposited onto the ladder-type PPMSQ gate dielectrics were measured using tapping-mode atomic force microscopy (AFM, Digital Instruments Multimode). The crystalline microstructures of the organic semiconductor films were investigated using two-dimensional grazing incidence Xray diffraction (2D GIXD) measurements collected at the 9A beamline of Pohang Light Source II at Pohang Accelerator Laboratory in Korea. The Keithley 2400 and 236 source/ measure units were used to measure the electrical properties of the devices. All electrical measurements were performed at room temperature under vacuum (∼10−5 Torr) in a dark environment.

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dielectric constant 3.1 3.6 3.5 3.6 3.6

(±0.3) (±0.2) (±0.3) (±0.2) (±0.1)

leakage current density @ 1 MV/cm (A/cm2) 1.5 4.2 4.0 6.2 4.8

(±0.3) (±0.6) (±0.4) (±0.3) (±0.3)

× × × × ×

10−7 10−8 10−8 10−9 10−9

RESULTS AND DISCUSSION

The ladder-type poly(phenyl-co-methacryl silsesquioxane) (PPMSQ) copolymer was used as a gate dielectric layer in organic field-effect transistors (OFETs), as shown in Figure 1a. Two types of methoxysilane monomers with phenyl and methacryloxypropyl pendant groups were used to synthesize the copolymer. The phenyl groups in one monomer were introduced to improve both the molecular ordering and the surface morphology of the overlying organic semiconductors. Polystyrene bearing phenyl groups was inserted between the SiO2 and organic semiconductor layers to provide a highly crystalline organic semiconducting layer that yielded a high carrier mobility in the organic transistors based on this interlayer.25-27 The methacrylate groups in the other monomer were introduced to cross-link the polymeric chains via thermalor photocuring processes. Both the thermal- and photocuring steps applied to the ladder-type PPMSQ improved dramatically the electrical strength of the gate dielectrics by the formation of a network structure among the polymer chains. The surface energy and dielectric constant of the gate dielectric layer were simultaneously modulated by the thermal curing conditions. The dielectric surface characteristics affected the crystalline microstructures of the overlying organic semiconductors. The network structure formed among the PPMSQ chains was investigated by collecting the FT-IR spectra of the laddertype PPMSQ films subjected to curing at 100, 150, or 200 °C under ambient conditions, as shown in Figure 1b. Two absorption peaks at 1131 and 1040 cm−1, corresponding to Si− O−Si stretching vibrations, were observed in all samples. The bimodal absorption peaks indicated the formation of laddertype structures.25,28,29 The absorption spectra of the polyhedral oligomeric silsesquioxanes (POSS) with a cage structure exhibited one strong peak at 1100 cm−1. As the curing temperature was increased from 25 to 200 °C, the intensities of the peaks at 1636 and 1714 cm−1, which were ascribed to the stretching vibrations of the CC bonds and the CO bonds connected to CC bonds, respectively, decreased gradually. By contrast, the peak observed at 1731 cm−1, which corresponded to the stretching vibrations of the CO bonds connected to C−C bonds, increased.27 Figure 1c exhibited a gradual increase in the relative amounts of CO bonds connected to the C−C bonds compared with those connected to the CC bonds. The disappearance of the CC stretch upon thermal curing indicated the polymerization of the methacrylate groups and, as a consequence, the CO groups were no longer conjugated, consistent with the photocuring phenomena of cage-type POSS with methacryloxypropyl groups.29 The changes in the surface energy and dielectric constant of the ladder-type PPMSQ gate dielectrics were monitored as a function of the curing temperature. The surface energies decreased gradually as the curing temperature increased. The polar term of the surface energy (γp) decreased, whereas the dispersion term of the surface energy (γd) increased slightly, as 3503

DOI: 10.1021/acs.jpcc.5b10240 J. Phys. Chem. C 2016, 120, 3501−3508

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The Journal of Physical Chemistry C shown in Figure 1d. The decreased total surface energy (and the increase in the dispersion term) of the gate dielectrics was attributed to the lower surface energy of the C−C group formed by cross-linking as compared with the surface energy of the CC group.30 A lower surface energy typically benefits the molecular ordering in the overlying organic semiconductors (as discussed below). Figure 1e plots the dielectric constant as a function of the curing temperature. The dielectric constant increased with the thermal curing temperature, possibly due to a decrease in the free volume occupied by the polymer chains (after thermal cross-linking), as indicated by the reduced film thicknesses, measured by ellipsometry (Table 1). The electrical strength of the ladder-type PPMSQ gate dielectrics was evaluated by measuring the leakage current densities. A leakage current through the dielectric layer is undesirable in FETs mainly because it increases the static and dynamic power dissipation.31,32 Figure 1f shows plots of the current density (J)-electric field (E) obtained from the metal− insulator−metal (MIM) capacitors prepared using the laddertype PPMSQ dielectrics cured at various temperatures. The thermally cured dielectrics showed superior electrical strengths than the as-spun gate dielectrics. For example, the leakage current density of the as-spun gate dielectrics was 1.5 × 10−7 A/ cm2 at an electric field strength of 1 MV/cm, whereas the 200 °C-cured gate dielectrics exhibited a significantly lower current density of 6.2 × 10−9 A/cm2, despite its thinner thickness (Table 1). The enhanced electrical stability induced by the thermal curing step was originated from the reduced free volume and restrictions on the thermal dynamic motions of the polymer chains due to the cross-linked network structure in the PPMSQ chains. The gate dielectric properties of the laddertype PPMSQ are summarized in Table 1. The ladder-type PPMSQ gate dielectrics were assembled with a p-type benchmark semiconductor, pentacene, and an ntype benchmark semiconductor, PTCDI-C8. Pentacene was deposited onto the prepared ladder-type PPMSQ gate dielectrics cured at different temperatures (25, 100, and 200 °C). Figure 1a shows a schematic illustration of the top-contact OFETs employed in this study. More than five devices were fabricated and tested to estimate the device characteristics. Figure 2a shows the transfer characteristics (drain current (ID) vs gate voltage (VG)) of the pentacene OFETs based on the ladder-type PPMSQ gate dielectrics at a drain voltage (VD) of −60 V. All devices were found to be well-behaved p-type transistors. As the PPMSQ gate dielectric curing temperature increased, the current levels measured at the same gate voltages and the ON/OFF current ratios increased gradually. The carrier mobility (μ) was estimated in the saturation regime (VD = −60 V) according to the relationship ID = CSμW(VG − Vth)2/ (2L), where CS is the specific capacitance of the gate dielectric and Vth is the threshold voltage.33 The pentacene OFETs prepared with the as-spun ladder-type PPMSQ exhibited an average hole mobility of 0.17 (±0.04) cm2 V−1 s−1. Thermal curing of the gate dielectrics considerably increased the carrier mobilities. The hole mobility of the device prepared with the 200 °C-cured gate dielectrics was 0.53 (±0.10) cm2 V−1 s−1. Analogous to the results obtained from the pentacene OFETs, the thermally cured gate dielectrics improved the n-type PTCDI-C8 OFET device performance (provided a higher mobility and higher ON/OFF current ratio), compared with the devices prepared with the as-spun gate dielectrics: 0.03 (asspun) and 0.17 cm2 V−1 s−1 (200 °C-cured). Figure 2c and Table 2 summarize the electrical properties, including the

Figure 2. Transfer characteristics of (a) pentacene and (b) PTCDI-C8 OFETs based on the ladder-type PPMSQ gate dielectrics cured at different temperatures. (c) Carrier mobilities and ON/OFF current ratios of the devices obtained from the transfer characteristics measured in panels a and b. (d) The output characteristics of the pentacene and PTCDI-C8 OFETs prepared based on the ladder-type PPMSQ gate dielectrics cured at 200 °C.

average carrier mobility, ON/OFF current ratio, and threshold voltage, of the devices. Figure 2d shows the output characteristics (ID vs VD) of both the pentacene and PTCDI-C8 OFETs based on the 200 °C-cured ladder-type PPMSQ gate dielectrics. Linear regimes, followed by saturation regimes at higher values of VD under a constant VG indicated the appropriate operation of the p-type and n-type transistors, respectively. The improved performances of the OFETs prepared with the thermally cured gate dielectrics resulted from the reduced surface energy of the gate dielectric layer upon thermal curing. The surface properties (i.e., surface energy) of the gate dielectrics strongly influenced the grain morphology and crystalline microstructure of the overlying organic semiconductor films.34,35 The interactions between the semiconducting molecules and the gate dielectric surface determined both nucleation and assembly of the molecules onto the surface, which played a critical role in the transistor performances. This was investigated by characterizing the pentacene and PTCDI-C8 films deposited onto the ladder-type PPMSQ gate dielectrics cured at different temperatures using AFM and 2D GIXD. Figure 3a shows AFM surface morphologies of 50 nm-thick pentacene films prepared on the ladder-type PPMSQ gate dielectrics cured at different temperatures. The pentacene films grown onto the as-spun PPMSQ surface exhibited globular structures with an average grain size of 231 nm. As the curing temperature increased, the grain size increased dramatically. The 200 °C-cured dielectrics displayed much larger grains with flat terraces and clear step edges separated by monomolecular steps. An AFM image of the PTCDI-C8 films (Figure 3b) also showed a pronounced increase in the grain size as the curing 3504

DOI: 10.1021/acs.jpcc.5b10240 J. Phys. Chem. C 2016, 120, 3501−3508

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The Journal of Physical Chemistry C Table 2. Electrical Properties of the OFETs Based on the Ladder-Type PPMSQ Gate Dielectrics pentacene mobility (cm2/(V s)) pristine 100 °C-annealed 200 °C-annealed UV-annealed

0.17 0.43 0.53 0.21

(±0.04) (±0.08) (±0.10) (±0.07)

PTCDI-C8 Vth (V)

ON/OFF current ratio 1.4 3.4 4.4 4.2

(±0.4) (±0.8) (±1.0) (±0.9)

× × × ×

4

10 104 104 104

−26.8 −27.2 −26.7 −29.1

(±4.8) (±3.5) (±5.1) (±7.2)

mobility (cm2/(V s)) 0.03 0.11 0.17 0.19

(±0.01) (±0.02) (±0.04) (±0.06)

ON/OFF current ratio 0.3 1.6 2.4 2.3

(±0.1) (±0.3) (±0.5) (±0.7)

× × × ×

4

10 104 104 104

Vth (V) −0.6 2.3 7.2 8.7

(±4.2) (±3.9) (±3.4) (±2.3)

Figure 3. AFM images of the 50 nm-thick (a) pentacene and (b) PTCDI-C8 films deposited onto the ladder-type PPMSQ gate dielectrics cured at different temperatures.

Figure 4. 2D GIXD patterns of (a) pentacene and (b) PTCDI-C8 films deposited onto the ladder-type PPMSQ gate dielectrics cured at different temperatures.

temperature increased. The morphological differences between the overlying organic semiconducting layers were strongly correlated with the surface nucleation and diffusion kinetics of the semiconductor molecules, which was mainly affected by both surface energy and roughness of the gate dielectrics.36 In this system, the surface roughness values of the gate dielectrics were similar, within the margin of error, whereas the surface energies decreased with the curing temperature (Table 1). Thus, the improvement in the grain morphologies of the hightemperature-cured dielectrics was attributed to the decrease in

the density of nucleation sites that formed grains, as a result of the decreased surface energy in the gate dielectric film. The structural characteristics of the pentacene and PTCDIC8 molecules deposited onto the ladder-type PPMSQ gate dielectrics cured at different temperatures were investigated by synchrotron 2D GIXD measurements (Figure 4). Two broad ring patterns at qz = 0.48 and 1.40 Å−1 were observed in all samples and originated from the ladder width and the spacing of the ladder superstructure in the PPMSQ films, respectively.37 As shown in Figure 4a, the pentacene films exhibited 3505

DOI: 10.1021/acs.jpcc.5b10240 J. Phys. Chem. C 2016, 120, 3501−3508

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The Journal of Physical Chemistry C

Figure 5. (a) SEM images of the ladder-type PPMSQ micropatterns prepared using the photocuring process. (b) Output and (c) transfer characteristics of the pentacene and PTCDI-C8 OFETs prepared using the photocured ladder-type PPMSQ gate dielectrics. (d) Schematic diagram and electrical connection setup of a complementary inverter prepared using the p-type pentacene and the n-type PTCDI-C8 OFETs. (e) Voltage transfer characteristics (left panel) of the complementary inverter under various VDD values and their corresponding inverter gain (right panel).

PPMSQ gate dielectric films could be patterned by light exposure. As shown in Figure 5a, lines with 5 μm spacings were clearly patterned without producing noticeable shrinkage or haziness. The changes in the other gate dielectric properties upon photocuring are summarized in Table 1. Figures 5b and 5c show typical output and transfer characteristics measured from the p-type and n-type OFETs prepared based on the photocured PPMSQ gate dielectrics, respectively, which exhibited device performances almost comparable to those of devices prepared with thermally-cured PPMSQ gate dielectrics (the pentacene OFETs displayed a hole mobility of 0.21 cm2 V−1 s−1 and the PTCDI-C8 OFETs displayed an electron mobility of 0.19 cm2 V−1 s−1). Finally, a complementary logic inverter was successfully fabricated by connecting a p-type pentacene OFET and an n-type PTCDI-C8 OFET with the photocured PPMSQ gate dielectrics, as shown in Figure 5d. The n-type OFET was connected to ground, whereas the ptype OFET was connected to the supply electrode. Both transistors shared the same input (VIN) and output terminals (VOUT). The corresponding equivalent circuit diagram is shown in the lower panel of Figure 5d. The resulting inverters exhibited reasonable voltage transfer characteristics, in which VOUT was switched between the applied VDD (40, 60, and 80 V) and 0 V as the VIN was swept (left panel of Figure 5e). This result indicated that the p- and n-type OFETs turned on and off, respectively, at low values of VIN and vice versa at high VIN. The right-hand panel in Figure 5e shows the signal inverter gain, defined as the absolute value of dVOUT/dVIN, as a function of the VIN at three different VDDs. A maximum inverter gain of 14.6 was obtained from the device at VDD = 80 V.

preferentially oriented (00l) reflections along the qz direction, indicating that the pentacene crystals were oriented with their (00l) planes parallel to the gate dielectric surface.38 The pentacene films grown on an as-spun PPMSQ surface specifically displayed “bulk-phase” reflections (as indicated by the red arrows) and several highly-scattered patterns along the Debye rings at various qxy positions. Highly scattered intensities at each diffraction peak originated from crystal mismatches and dislocations along the vertical and lateral directions within the film. As the PPMSQ curing temperature increased, intense inplane reflections corresponding to the “thin-film” phase (as indicated by the white arrows) dominated the vertical direction at qxy = 1.33 and 1.64 Å−1 and were indexed to {1, ±1}T and {0, 2}T.37 These results indicated that the pentacene molecules were oriented toward the surface normal and were highly ordered along the in-plane direction. The PTCDI-C8 films deposited on the thermally cured PPMSQ surface exhibited pronounced Bragg rod reflections up to several orders at various qxy positions, indicating the extraordinary crystalline ordering of the PTCDI-C8. By contrast, the PTCDI-C8 films prepared on the as-spun gate dielectric surfaces displayed weak and highly scattered diffraction peaks (Figure 4b), consistent with the results obtained from pentacene films prepared on the same surface. The enhanced crystalline ordering in both organic semiconductors with increasing curing temperature primarily originated from good surface energy matching and the resulting enhanced molecular interactions between the semiconductor and gate dielectric surfaces, consistent with the AFM results. Importantly, the ladder-type PPMSQ could be cured through exposure to UV light (365 nm, 100 mW/cm2, 10 s). The FT-IR spectrum shown in Figure 1b revealed that the CC bonds almost disappeared, and the relative intensities of the CO bonds associated with C−C and the CO bonds associated with CC clearly increased (Figure 1c). The observed photocuring characteristics suggested that the ladder-type

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CONCLUSION In summary, we demonstrated the preparation of ladder-type PPMSQ gate dielectrics for general use in both p-type and ntype OFETs. The resulting ladder-type PPMSQ was cross3506

DOI: 10.1021/acs.jpcc.5b10240 J. Phys. Chem. C 2016, 120, 3501−3508

Article

The Journal of Physical Chemistry C

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linked via either thermal- or photocuring. Curing dramatically enhanced the electrical strength of the ladder-type PPMSQ film by forming a structured network among the polymer chains. The surface energy and dielectric constant were modulated by tuning the thermal curing temperatures. The optimized OFETs exhibited good electrical performances, including high carrier mobilities (0.53 cm2 V−1 s−1 for the p-type pentacene OFET and 0.17 cm2 V−1 s−1 for the n-type PTCDI-C8 OFET) and ON/OFF current ratios exceeding 104. The use of the laddertype PPMSQ gate dielectrics provides a novel approach to realizing next-generation organic electronics.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b10240. 1 H-NMR and Si-NMR spectra and thermogravimetric analysis of PPMSQ (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The copolymer was supported from Electronic Materials Division in DongJin Semichem, Korea. This work was financially supported by a grant from the Center for Advanced Soft Electronics (CASE) under the Global Frontier Research Program (2013M3A6A5073177) and Basic Science Research Program (2013R1A1A2011897, 2013R1A1A1008628, and 2009-0083540) of the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology, Korea. G.A. and H.K. acknowledge the support from NRF-2014R1A2A1A10052454.

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REFERENCES

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DOI: 10.1021/acs.jpcc.5b10240 J. Phys. Chem. C 2016, 120, 3501−3508

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DOI: 10.1021/acs.jpcc.5b10240 J. Phys. Chem. C 2016, 120, 3501−3508