Silicon Nanoparticle Hybrid Layer as High-

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C: Energy Conversion and Storage; Energy and Charge Transport

Polymer/Silicon Nanoparticle Hybrid Layer as High-k Dielectrics in Organic Thin-FilmTransistors Xuesong Wang, He Wang, Yao Li, Zuosen Shi, Dong-Hang Yan, and Zhanchen Cui J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00866 • Publication Date (Web): 02 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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The Journal of Physical Chemistry

Polymer/Silicon Nanoparticle Hybrid Layer as High- Dielectrics in Organic Thin-FilmTransistors Xuesong Wang,a He Wang, b Yao Li, a Zuosen Shi,a Donghang Yan,b, * and Zhanchen Cuia, * a

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, P. R. China. b

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130012, P.R. China

ABSTRACT: In this work, a novel organic/inorganic hybrid dielectric material containing silicon nanoparticle had been designed and synthesized for using in organic thin-film transistors (OTFTs). And the inorganic silicon nanoparticle was modified by 2-hydroxyethyl methacrylate (HEMA) to completely disperse in organic polymers. The film made of these materials exhibited excellent smooth surface and high dielectric constants (5.2). The bottom-gate top-contact parahexaphenyl (p-6P)/vanadyl-phthalocyanine (VOPc) OTFTs with this dielectric thin-film as the dielectric layer exhibited more optimized threshold voltages (+4 V) and charge carrier mobility (0.7 cm2 V−1 s−1).

1. Introduction With the rapid development of electronic integrated industry and the breakthrough in the research of organic small molecules and semiconductor polymer materials, more and more

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attentions have been paid to OTFTs due to their unique qualities, including flexibility, low cost and large-area integration.1-4 In particular, para-hexaphenyl (p-6P)/vanadyl-phthalocyanine (VOPc) OTFTs, which possess high charge carrier mobility and excellent device performance. But high threshold voltage beyond −10 V makes them insufficient for practical applications.5– 7

Over the past two decades, a large number of studies concluded that the intrinsic properties of

the dielectric gate materials were very important for the performances of OTFTs devices. And using a dielectric layer with high dielectric constant is a convenient and effective approach to optimize the threshold voltage.8-12 However, how to choice a more prominent dielectric layer material is worth discussing for researchers. For electronic application, traditional inorganic dielectric materials with stable chemical properties and high dielectric constant are attractive, but inflexible processing, large leakage current limit the application of these materials in industry.13-16 Meanwhile, although most organic materials have the advantages of simple operations, low cost and large-area coverage, their dielectric constant is not big enough, which leads to larger threshold voltage of the device.17-23 Nano hybrid materials are that the inorganic nanoparticles are introduced into the polymers by chemical method, which combine the advantages of inorganic nanoparticles and organic polymers24,25 Hence, organic/inorganic hybrid materials have been widely studied and applied to the fabrication of OTFTs by merging their respective properties. Compared with the micro phase separation of sol-gel materials and tedious preparation process of layer by layer self-assembly, blending inorganic materials with organic polymers has attracted more attentions of researchers.26,27 Through the summary of our previous work,28-30 we knew that the dielectric constant can be effectively increased by adding inorganic silicon nanoparticles. But, the aggregation of inorganic nanoparticles and their poor compatibility with organic polymers can


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cause macroscopic phase separation and a larger leakage current, which reduce the device performances. Therefore, efficiently disperse silicon nanoparticles in organic polymer materials will be a focus in our research. In this paper, we designed and synthesized a novel organic/inorganic hybrid dielectric material that contained silicon nanoparticles. Silicon nanoparticles no longer existed in amorphous form, but formed dipoles in the insulating layer in the form of polysilicon, thereby increased the dielectric constant of the material. And we modified the surface of silicon nanoparticles by HEMA so that they could react with the organic functional groups to form chemical bonds and be uniformly fixed inside the polymer. By this method, the agglomeration and oxidation of nanoparticles could be prevented. Then we spin coated the hybrid material on silicon nitride to obtain a double-layer insulation layer. And compared with single silicon nitride dielectric devices31,32, the threshold voltage of p-6P/VOPc OTFTs with the novel dielectric layer was optimized significantly. Furthermore, the smooth surface of the dielectric layer and its good compatibility with the semiconductor layer facilitated higher charge carrier mobility in the device.

2. Experimental section 2.1 Materials. The tetrachlorosilane, tetraoctylammonium bromide (TOAB), lithium aluminium hydride were purchased from Aldrich Chemical Co. 2-hydroxyethyl methacrylate (HEMA), glycidyl methacrylate (GMA), 2,2'-azobis(2-methylpropionitrile) (AIBN) were obtained from Alfa Chemical Co. And the tetrahydrofuran (THF) and the methanol were purified by refluxing and distilling.


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2.2 Synthesis of the organic polymer. Under a N2 atmosphere, HEMA (11.710 g, 90 mmol), GMA (1.420 g, 10 mmol), and anhydrous THF (200 ml) were mixed in a 500 ml three-neck boiling flask. After complete dissolution, AIBN (0.5 g) was added, and then was refluxed for 12 h in 80 ℃. After cooling to the room temperature, the solution was added dropwise to hexane for purifying the crude product, and then was dried in a vacuum oven to obtain the organic polymer. The preparation and structure of P-HEMA-&-GMA were illustrated in Supporting Information. 2.3 The modification of silicon nanoparticles (Si-HEMA) and the synthesis of hybrid dielectric materials. TOAB (20 g) and the anhydrous THF (30 ml) mixed in a conical flask. After complete dissolution, tetrachlorosilane (1 ml) was added quickly and stirred for 30 min, and then lithium aluminium hydride (3 ml) was cautiously added, dropwise, anhydrous methanol was putted in to remove the residue after the end of the reaction. The whole preparation process was carried out in the glove box to achieve anhydrous anaerobic environment. Twenty minutes later, HEMA (10 ml) and chloroplatinic acid (0.1 ml) were added to the above solution, so that we can modify the surface of the silicon nanoparticles with organic groups, and a yellow solid product was named Si-HEMA after purification. And then we mixed different ratios of SiHEMA with polymers and dispersed them into 2-ethoxyethanol. The result hybrid dielectric materials, Si-5 (Si-HEMA : polymer = 5 : 95, molar ratio) was obtained. Si-10 (Si-HEMA : polymer = 10 : 90, molar ratio), Si-15 (Si-HEMA : polymer = 15 : 85, molar ratio) and Si-20 (SiHEMA : polymer = 20 : 80, molar ratio) were prepared by a similar procedure, where the differences were the molar ratios of the materials. These materials were all used as dielectric layers in OTFT devices, and their preparation process was shown in Figure 1. 2.4 Device fabrication. To determine the capacitance and the dielectric constant of the novel materials, the devices with metal-insulator-metal (MIM, ITO/dielectric layer/Au) capacitor


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structure were fabricated on indium tin oxide (ITO) substrate. As shown in Figure 1, the dielectric materials were spin coated on top of ITO wafer. And then the films were heated at 80 ℃ for 1 h to remove all the solvent and were annealed at 160 ℃ for 2 h for cross-linking. Finally, the MIM devices were completed by evaporating the top Au electrode with a radius of 0.1 mm.

Figure 1. The synthesis of metal-insulator-metal (MIM) capacitor structure and the fabrication process of the organic /inorganic hybrid dielectric material. The para-hexaphenyl (p-6P)/vanadyl-phthalocyanie (VOPc) OTFTs had been fabricated on ITO substrate with SiNx and organic/inorganic hybrid materials as bi-layer dielectric layer that as shown in Figure 2. Firstly, a 300 nm SiNx layer was deposited by plasma enhanced chemical


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vapor deposition. And then the dielectric layer was completed after the hybrid materials were deposited on the SiNx film by spin-coating (15% cyclopentanone solution). A 3 nm p-6P layer was deposited on the surface of the dielectric thin-film, and then the 30 nm VOPc film was deposited on the p-6P layer. The deposition of semiconductor layers were deposited at 10−4–10−5 Pa at a rate of about 1 nm min−1 and the substrate temperature was maintained at 180 °C. Finally, the source and drain contacts were defined by thermally evaporating Au through a shadow mask in a vacuum of better than 5 × 10−6 mbar. The width and length of channel were 6000 and 200 µm.

Figure 2. The metal-insulator-metal (MIM) capacitor structure (a) and the p-6P/VOPc OTFT device structure (b). 2.5 Measurements. Proton Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker AVANCE NMR spectrometer at a resonance frequency of 500 MHz for 1H NMR. And the chemical shifts were referenced to TMS at 0 ppm. The FT-IR analysis of the samples was acquired with an AVANAR 360 FT-IR infrared spectrophotometer. Energy Dispersive Spectrometer (EDS) was taken on a JEOL FESEM 6700F. For Transmission Electron Microscopy (TEM) observations, powder products were dispersed in ethanol by sonication for 10 min, and added on carbon-coated copper grids, they were observed using a JEOL JEM-2100F transmission electron microscope at an acceleration voltage of 150 kV. Differential scanning


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calorimeter (DSC) studies were performed with a NETZSCH DSC 204 analyzer at a heating rate of 10 ℃ min-1. And the thermogravimetric analysis (TGA) was carried out on a PerkinElmer TGA 7 thermogravimetric analyzer. The morphology of the thin-films were investigated by atomic force microscopy (AFM) using a SPA-300 HV microscope in tapping mode using a scan speed of 1 Hz, and all the root-mean-square (RMS) surface-roughness values were obtained in the area of 10 × 10 µm2 and 20 × 20 µm2. The capacitances of the dielectric polymers were measured with an Agilent E 4980A LCR meter. Current–voltage measurements were recorded under ambient conditions with two Keithley 236 source measurement units at room temperature.

3. Results and discussion 3.1 Structural characterization. We prepared P-HEMA-&-GMA, and then we grafted organic groups onto the surface by using HEMA to modify inorganic silicon nanoparticles that to make it well dispersed in polymers. The modified silicon nanoparticles were isolated as yellow solids and a contrast structures between Si-HEMA and HEMA were confirmed by 1H NMR as shown in S4. The signals at approximately 5.69 and 6.07 ppm attributed to the olefin double bond in the HEMA had disappeared due to modify the silicon nanoparticles. And the characteristic peak at 4.87 ppm was assigned to the proton on the hydroxyl group had occurred displacement. Similarly, the peak positions of methyl and methylene had changed accordingly, indicating the emergence of new substances. The FT-IR spectrum of Si-HEMA was shown in S5. The characteristic absorptions of hydroxyl groups at 3109-3514 cm-1, and the methyl and methylene appeared at 2923 and 2854 cm-1 respectively. By comparing the FT-IR spectrum of HEMA and Si-HEMA, we can see that the characteristic absorptions of -Si-CH- bonds appeared at 1268 and 726 cm-1.


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Through EDS analysis that was shown in S6, the presence of silicon and the disappearance of platinum and nitrogen elements showed that the silica nanoparticles had been thoroughly purified to avoid the influence on the properties of the dielectric material. By combining NMR, FT-IR and EDS analysis, it indicated that we successfully grafted HEMA onto silicon nanoparticles and achieved the purpose of modification. In order to verify the good dispersion of modified silicon nanoparticles, as shown in Figure 3, we could observe that no large area reunion phenomenon occurred and Si-HEMA distributed evenly. The dimension distribution of silicon nanoparticles was narrow, which was about 2.2 nm and the spacing of the lattice was 0.21 nm.

Figure 3. TEM images of Si-HEMA. The 1H NMR spectrum and organic structure of the Si-5 dielectric materials was shown in S7. The signals around 1.57 and 3.16 ppm were assigned to the hydrogen atoms in methylene, the


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characteristic peak around 0.88 ppm corresponded to the proton on the methyl that was belonged to the Si-HEMA, the signal at 3.49 ppm was attributed to the methylene groups of the HEMA. FT-IR spectrum of the Si-5 was shown in S8. The methyl group absorption was observed in 2920 cm-1. Furthermore, the characteristic absorption of the methylene group was appeared in 2850 cm-1, the carbonyl and epoxy at 1724 and 898 cm-1 respectively. These results indicated that the dielectric material, which mixed Si-HEMA and polymer, was successfully synthesized. And the 1

H NMR and FT-IR spectra of Si-10, Si-15 and Si-20 had been given in the in the Supporting

Information that had similar structures with Si-5.

Figure 4. The DSC cures of Si-5, Si-10, Si-15 and Si-20 with a heating rate of 10 ℃ min-1 in N2.


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Figure 5. The TGA results of Si-5, Si-10, Si-15 and Si-20 with a heating rate of 10 min-1 in N2, from room temperature to 700 ℃. 3.2 Thermal analysis. The thermal analyses of the Si-5, Si-10, Si-15 and Si-20 were obtained by DSC and TGA measurements. Table 1 showed that the glass transition temperature (Tg) of the polymer films was in the range of 76-98 ℃. And from Figure 4, we could notice that the glass transition temperature of organic/inorganic hybrid materials decreased gradually when the proportion of silicon nanoparticles increased. Through thermo gravimetric analysis, the Figure 5 clearly indicated that the characteristic curve of 5% thermal decomposition of the hybrid


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materials occur at approximately 230 ℃ . Therefore, these results implied that the hybrid polymers were heat resistant properties.

Figure 6. The tapping mode AFM images (10 × 10 μm2 scan area) for evaluation of the film surface morphologies and the aqueous contact angles for the Si-5, Si-10, Si-15 and Si-20 thinfilms. 3.3 Properties of the thin polymer films. The surface morphology of the dielectric thin-films was generally recognized to play an important role in carrier transport by influencing the morphology of the overlying semiconductor, due to the carriers transferred within the semiconductor layer that closed to the insulating layer. And some chemical groups would affect the charge transport at the semiconductor–dielectric interface and further reflected in the performance of the equipment, such as hydroxyl group. Notably, the films had a relatively good hydrophobicity, which were confirmed by the aqueous contact angle (72°‒75°) on the film surfaces that as shown in Figure 6. The results also illustrated that the hydroxyl group in the structure of the hybrid material did not exist on the surface of the thin-film, which would not affect the device performance. The surface morphologies of the dielectric thin-films were smooth and without pinholes and were verified through the research of the atomic force microscopy (AFM) under ambient conditions in the tapping mode using a scan speed of 1 Hz. And all images


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were scanned on an area of 10 × 10 µm2, and the root-mean-square (RMS) surface roughness was around 0.3 nm. Moreover, the aggregation of inorganic silica nanoparticles did not be observed, and the thin-films surfaces were smooth and no corn appeared.

Figure 7. Capacitance–frequency curves for the hybrid dielectrics investigated in this study under 0 V bias voltages.


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Figure 8. Capacitance–voltage curves for Si-5, Si-10, Si-15 and Si-20,which were obtained using a dual-directional sweeping voltage from -20 V to +20 V in a radius of 0.1 mm round area. 3.4 Dielectric polymer film capacitances. The capacitance per unit area, which is one of the most pivotal parameters for dielectric materials, as given by formula (1): / 1 where is the vacuum dielectric constant, is the dielectric constant, and is the thickness of the dielectric thin-film. The capacitance characteristics of the dielectric thin-films were evaluated with metal-insulator-metal (MIM, ITO/polyurethane/Au) devices. The capacitance–frequency curves of the Si-5, Si-10, Si-15 and Si-20 thin-films were displayed in Figure 7. We could see


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that the dielectric thin- films exhibited greater capacitance (3.7–6.2 nF/cm2), and the calculated dielectric constants were 3.5-5.2 which were significantly higher than those of most of organic dielectric polymer materials, such as PS, PVP, etc. This result proved that the addition of silicon nanoparticles could increase the dielectric constants of the dielectric materials. Furthermore, we found that the electrostatic losses of the resulting layers were relatively low in the highfrequency alternating current, which shown that our materials were very stable. But, the abnormal behavior of the Si-20 was worth pondering. Therefore, we also measured the capacitance–voltage characteristics by using a dual-directional sweeping voltage from -20 V to +20 V in a radius of 0.1 mm round cure area that the result shown in Figure 8. It was noteworthy that the capacitances of Si-5, Si-10 and Si-15 were repeatable under the dual-directional sweeping voltage, but Si-20 indicated a strongly hysteresis in the thin-films. It was possible that the increase in the content of the silicon nanoparticles led to a reduction in the electrical stability of the Si-20 thin-film. The parameters of these materials were summarized in Table 1. Table 1． The parameters of the Si-5, Si-10, Si-15 and Si-20.

Td(°C)

RMS Aqueous roughness contact angle (°) (nm)

Thickness (nm) (nF/cm2)

Dielectric constant ( )

99.1

235.2

0.345

75.8

358

6.2

3.5

Si-10

87.3

247.1

0.512

74.2

805

3.7

4.4

Si-15

79.3

240.9

0.423

73.1

747

4.2

5.2

Si-20

71.1

234.6

0.521

72.3

775

3.8

4.6

Hydric T (°C) materials g

Si-5


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Figure 9. The 3 nm surface morphology of p-6P andp-6P on the Si-5, Si-10, Si-15 and Si-20 as dielectric layer, respectively. 3.5 Properties of the p-6P/VOPc organic thin-film transistors. In order to study the effect of the organic/inorganic hybrid dielectric layer on the semiconductor morphology, a 3 nm p-6P was deposited on the dielectric surfaces. As shown in Figure 9, an independent p-6P was growing in the form of “island crystal”, but it was unfavorable for the crystallization of the VOPc molecule. However, after the silicon nanoparticles were putting in, the p-6P molecules could grow continuously on the surface of the dielectric layer in a schistose and large-size crystalline form. Moreover, it can be observed that the grain size of p-6P increased with the increase of the content of Si nanoparticles. This result may be attributed to that of a smooth surface of a dielectric layer and the excellent compatibility between dielectric thin-films and organic semiconductors. The smooth morphology of the p-6P crystal was beneficial to improve VOPc


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molecule crystalline appearance and size, and thus to obtain better device properties. And a suitable grain size of semiconductor was an important factor to fabricate p-6P/VOPc TFTs with high charge carrier mobility. For most circuit-based electronics, a typical bottom-gate top-contact OTFT, the electrons between the source and the drain contact were modulated by both the source-gate voltage (VGS) and the source-drain voltage (VSD). And when the device was in the source-gate voltage state (VGS > 0 V), an abrupt current increased must be achieved, but in the off state (VGS = 0 V), the channel current must be very low. The saturation current in organic thin-film transistors was generally calculated as follows:

,

− h 2

where µ was the charge carrier mobility, and were the width and length of the OTFTs channel and h was the threshold voltage, respectively. The Bottom-gate top-contact p-6P/VOPc TFTs were fabricated with SiNx film (300 nm) and organic/inorganic hybrid materials as the bilayer dielectric layer on indium tin oxide substrate. The output and transfer characteristic curves of p-6P/VOPc TFTs were shown in Figure 10. And VGS of −50 V was applied for the output characteristic curves, the transfer characteristic curves were measured by uni-directional sweeping voltage of -50 to +10 V. The p-6P/VOPc TFTs exhibited a more optimized thresholdvoltage (+ 4) and field-effect mobility (0.7 cm2 V−1 s−1). The data were summarized in Table 2. The results showed that the threshold voltage of the device was obviously adjusted with the addition of inorganic silicon nanoparticles. The introduction of the modified silicon nanoparticles


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could provide a large effective capacitance in devices, which was more positive than the traditional inorganic materials for adjusting threshold voltage.

Figure 10. Output characteristic curves and transfer characteristic curves p-6P/VOPc OTFT with Si-5, Si-10, Si-15 and Si-20 as dielectric layer, respectively. Table 2． The electric parameters of the p-6P/VOPc OTFTs with organic /inorganic hybrid materials as dielectric layers. Dielectric

μ

layer

(cm2/Vs)

Si-5

(V)

On/Off

0.1

-5

104

Si-10

0.7

-7

104

Si-15

0.6

+4

103

Si-20

0.5

-10

106

4. Conclusion Novel hybrid dielectric materials had been successfully designed and synthesized by mixing organic polymers with inorganic silicon nanoparticles that had been modified by HEMA. After


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modification, inorganic silicon nanoparticles could be well dispersed in organic polymers and uniformly distributed in the thin-films. These novel dielectric thin-films were stable, smooth and pinhole free. The dielectric constants of the materials were greatly improved by introducing silicon nanoparticles, so, the threshold voltage was obviously optimized. The results shown that with the addition of inorganic silicon nanoparticles, the overall performances of the device were enhanced. ASSOCIATED CONTENT Supporting Information The fabrication process, 1H NMR and FT-IR of the P-HEMA&GMA. The characterization of Si-polymer, Si-5, Si-10, Si-15, Si-20. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: +86-0431-85168217. *E-mail: [email protected]. Phone: +86-0431-85262165. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (grant no. 21374039, 51673081). REFERENCES (1) Tobjork, D.; Osterbacka, R. Paper Electronics. Adv. Mater. 2011, 23, 1935-1961.


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Silicon Nanoparticle Hybrid Layer as High-

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