Organic/Inorganic Nano-hybrids with High Dielectric Constant for Organic Thin Film Transistor Applications
© The Author(s). 2016
Received: 2 August 2016
Accepted: 1 November 2016
Published: 7 November 2016
The organic material soluble polyimide (PI) and organic–inorganic hybrid PI–barium titanate (BaTiO3) nanoparticle dielectric materials (IBX, where X is the concentration of BaTiO3 nanoparticles in a PI matrix) were successfully synthesized through a sol–gel process. The effects of various BaTiO3 contents on the hybrid film performance and performance optimization were investigated. Furthermore, pentacene-based organic thin film transistors (OTFTs) with PI-BaTiO3/polymethylmethacrylate or cyclic olefin copolymer (COC)-modified gate dielectrics were fabricated and examined. The hybrid materials showed effective dispersion of BaTiO3 nanoparticles in the PI matrix and favorable thermal properties. X-ray diffraction patterns revealed that the BaTiO3 nanoparticles had a perovskite structure. The hybrid films exhibited high formability and planarity. The IBX hybrid dielectric films exhibited tunable insulating properties such as the dielectric constant value and capacitance in ranges of 4.0–8.6 and 9.2–17.5 nF cm−2, respectively. Adding the modified layer caused the decrease of dielectric constant values and capacitances. The modified dielectric layer without cross-linking displayed a hydrophobic surface. The electrical characteristics of the pentacene-based OTFTs were enhanced after the surface modification. The optimal condition for the dielectric layer was 10 wt% hybrid film with the COC-modified layer; moreover, the device exhibited a threshold voltage of 0.12 V, field-effect mobility of 4.32 × 10−1 cm2 V−1 s−1, and on/off current of 8.4 × 107.
KeywordsPolyimide Barium titanate Hybrid films Sol–gel method Modified layer
Organic thin film transistors (OTFTs) have attracted considerable attention in recent years for their unique features, including low fabrication costs [1–5], flexibility [6, 7], and ease of processing in solution . Relevant research has sought to improve OTFT performance in organic and polymeric semiconductors by modifying their chemical structures [9–12]. Other approaches, such as controlling the deposition of crystalline organic films [13–15] and controlling the nature of the interfaces, have also been developed [16, 17].
In OTFTs, improving the semiconductor/dielectric interface has involved modifying the gate dielectrics with polymeric materials, organic–inorganic hybrids, and an organic–inorganic bilayer [18, 19]. Organic–inorganic hybrid material is a new type of material that demonstrates the desirable physical properties of both organic and inorganic components within a single composite. Moreover, inorganic material, such as a metal oxide, has a high-dielectric-constant material [20, 21]. These hybrid materials can enhance gate capacitance for accumulating more charge carriers in the channel. Additionally, the dielectric properties of mixtures of polymers and inorganic nanofillers [22, 23] including nanoparticles [24, 25], nanoclusters, and nanotubes [26, 27] can be tuned by varying the type and concentration of nanofiller materials.
In the literature, many polymeric dielectric materials have been applied as the dielectric materials in OTFTs, such as poly(styrene), poly(methyl methacrylate), poly(ethylene), poly(urethane), poly(vinyl alcohol), and poly(vinyl pyridine). However, among these polymeric dielectrics, polyimide (PI) is the best one and has been widely used as an insulating material for application in the field of electronic components due to its lower leakage current density, good thermal stability, mechanical toughness, and chemical resistance. In addition, the soluble polyimide can be applied to a low-temperature process and thus can prevent the need of high-temperature reaction for the dehydration and cyclization. On the other hand, barium titanate (BaTiO3) nanoparticles have a large dielectric constant and behave similarly to ferroelectric dielectric materials. Polyimide–BaTiO3 hybrid thin films providing high-quality dielectric nanocomposite materials were produced using simple solution techniques. Using the hybrid materials of high-dielectric-constant BaTiO3 nanoparticles in the PI matrix as gate dielectric materials can improve the performance of OTFT devices. In particular, to improve the electric characteristics and operational stability of OTFTs, the gate dielectric layers were modified with hydroxyl-free polymer insulators such as polymethylmethacrylate (PMMA) and cyclic olefin copolymer (COC).
4,4′-(Hexafluoroisopropylidene)dianiline (Lenexa, USA, 99 %), 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (Alfa Aesar, 98 %), and 4-aminobenzoic acid (ACROS, 99.5 %) were used to synthesize PI. Then, the prepared PI and barium titanate (Seechem Company PTYLTD, 99 %) was used as inorganic nanoparticles to prepare the hybrid dielectric films (PI-BaTiO3, IBX), and COC (Polyscience Inc.) and PMMA (Alfa Aesar, 98 %) were used as the polymer dielectric part. There are two kinds of solvents, tetrahydrofuran (ACROS, 99.9 %) and N,N-dimethylacetamide (ACROS, 99.8 %). Pentacene, an organic semiconductor material, was purchased from TCI Co. Ltd.
Synthesis of PI–BaTiO3 Hybrid Films
A solution–imidization technique was utilized to synthesize organo-soluble polyimide (6FDA–6FpDA–COOH) with carboxylic acid end groups . The molecular weight and end group functionality were controlled by the reactant stoichiometry. Firstly, 2.01 g (0.006 mol) of 4,4′-(hexafluoroisopropylidene)dianiline (6FpDA) was added into a 100-ml three-necked round-bottom flask, and 29.1 ml of NMP was used to dissolve the reactants. 5.331 g (0.012 mol) of 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) was then slowly added into the above solution with vigorous stirring under nitrogen purging. The mixture was allowed to react for 8 h at room temperature. Secondly, 1.6457 g (0.012 mol) of 4-aminobenzoic acid (4ABA) and 7.2 ml of 1,3-dichlorobenzene were added to the above solution. The 20 wt% of poly(amic acid) (PAA) solution was thus formed after constantly stirring the reactants for 16 h at room temperature. The PAA solution was then thermally imidized in a 1800 °C silicon oil bath for another 8 h and cooled to room temperature. The homogeneous 6FDA–6FpDA–4ABA–COOH solution was precipitated with 500 ml of methanol and re-dissolved in 30 ml of THF twice. A white-gray precipitate was recovered and dried in a vacuum oven at 1500 °C for 24 h to obtain 2.136 g of 6FDA–6FpDA–4ABA–COOH (yield, 23.8 %). The average acid value of 6FDA–6FpDA–4ABA–COOH was found to be 14 mg KOH/0.5 g polyimide using titration. The average molecular weight estimated by the acid value was around 4000. The weight average molecular weight estimated by GPC was 4276 with a polydispersity index of 1.31. It is noted that the yield of 6FDA–6FpDA–4ABA–COOH could be improved to nearly 50 % if monomers 6FDA, 6FpDA, and 4ABA were purified at 244–247, 195–198, and 187–189 °C, respectively, by sublimation/condensation procedure before the polyimide synthesis. The 6FDA–6FpDA–4ABA–COOH film was prepared using the following procedure: 0.5 g of 6FDA–6FpDA–4ABA–COOH was dissolved in 5 ml of DMAc while being stirred. The solution was filtered with a 0.45-m PTFE filter prior to use and spin-coated on to a silicon wafer at 1000 rpm for 20 s. The film was then baked at 60 °C on a hot plate for 10 min and at 150 °C for another 30 min to evaporate the solvent. The characteristic peaks of the FTIR spectrum for IB0 were observed as follows: 3434 cm−1 (COOH), 1788 cm−1 (CO), 1726 cm−1 (CO), 1610 cm−1 (C6H5), 1517 cm−1 (C6H5), 1438 cm−1 (C6H5), and 1370 cm−1 (CN) . Next, the IBX hybrid solutions were prepared. The PI was mixed with different weight ratios of BaTiO3 (0, 2, 5, 8, 10, and 12 wt%, i.e., IB0–IB12) in DMAc solvent and stirred uniformly to form the IBX hybrid solution. To prepare the IBX hybrid thin films, the precursor solution was spin-coated onto a silicon substrate. Finally, the hybrid dielectric thin films were baked at 60 °C for 30 min, 100 °C for 30 min, and 150 °C for 60 min [28–30].
Preparation of Modified Layer
Bilayer dielectrics consist of IBX hybrid films and a polymer layer. The dilute PMMA or COC was a mixture of monochlorobenzene and 1 % PMMA or COC. The volume ratio of monochlorobenzene and 1 % PMMA or COC was three to one. The bilayer dielectrics were baked at a temperature of 110 °C to remove moisture after being spin-coated onto the IBX thin films.
Characterization of Prepared Hybrid Composites
The structure of the prepared IBX hybrid thin films was determined using Fourier transform infrared spectroscopy (Perkin–Elmer Spectrum One), Raman spectroscopy (HORIBA iHR550), and an X-ray diffractometer (PANalytical X’Pert PRO MPD) by using CuKα radiation. The thermal properties of the IBX hybrid materials were measured using a TA Instruments Thermogravimetric Analyzer (Mettler Toledo TGA/SDTA851) and a differential scanning calorimeter (Perkin–Elmer Pyris 1) with heating rates of 20 and 10 °C min−1. The transmittances of the hybrid films coated onto quartz substrates were analyzed using ultraviolet–visible spectroscopy (Jasco V650). The surface morphologies of the thin films were examined using ultrahigh-resolution field emission scanning electron microscopy (FE-SEM, JEOL JSM-6500) and a student module of atomic force microscopy (AFM, Veeco DI3100).
OTFT Fabrication and Characterization
Results and Discussion
The IBX hybrid materials were fabricated as the gate dielectrics of the OTFT devices. PI is a highly thermal and environmentally stable material; therefore, it is well suited for use as the dielectric matrix. PI was synthesized in a two-step polymerization process that included PAA synthesis and chemical imidization. Hybrid thin films were spin-coated using a precursor solution, followed by thermal curing, as described in the experimental section. The dispersion and aggregation behaviors of the nanoparticles exert a crucial effect on the properties of IBX dielectric materials.
Structure Analysis of IBX Hybrid Dielectric Films
Optical Analysis of IBX Hybrid Dielectric Films
Surface Analysis of IBX Hybrid Dielectric Films
Summary of roughness of hybrid thin films
No modified layer
Ra roughness (nm)
Ra roughness (nm)
Ra roughness (nm)
Summary of contact angle and surface energies data of hybrid thin films
No modified layer
Water contact angle
DIM contact angle
Surfaces energy (mJ m−2)
Water contact angle
DIM contact angle
Surfaces energy (mJ m−2)
Water contact angle
DIM contact angle
Surfaces energy (mJ m−2)
OTFT Characteristics with IBX Hybrids as Dielectrics
Summary of electrical parameters for MIM device and pentacene OTFTs with different hybrid dielectrics
Capacitance (nF cm−2)
Dielectric constant (−)
μ (cm2 V−1 s−1)
V t (V)
I on/I off (−)
1.03 × 10−1
3.0 × 105
1.86 × 10−1
7.5 × 105
2.38 × 10−1
1.2 × 106
2.53 × 10−1
5.2 × 106
2.76 × 10−1
8.2 × 106
2.57 × 10−1
6.1 × 106
1.88 × 10−1
7.1 × 105
2.72 × 10−1
1.3 × 106
2.97 × 10−1
2.7 × 106
3.64 × 10−1
9.4 × 106
4.17 × 10−1
2.2 × 107
3.81 × 10−1
1.5 × 107
2.01 × 10−1
8.3 × 105
2.80 × 10−1
3.1 × 106
3.12 × 10−1
5.1 × 106
3.81 × 10−1
1.8 × 107
4.32 × 10−1
8.4 × 107
3.96 × 10−1
2.0 × 107
Pentacene-based OTFTs with a series of high-dielectric-constant IBX hybrid thin films, with different inorganic concentrations and polymer-modified layers used as a dielectric material, were successfully fabricated. The PI–inorganic materials provide a covalent-bonded surface, and the inorganic particles display a high degree of dispersion of BaTiO3 nanoparticles in the PI matrix. The dielectric constant of the composites is tunable by changing the concentration of BaTiO3 content incorporated with the PI matrix. The device performance and film properties reveal a favorable relationship with the weight percent of BaTiO3. Furthermore, the surface morphology and crystallinity of pentacene were significantly improved after the modification of hybrid gate dielectric layers with hydroxyl-free PMMA and COC polymer insulators. These PI hybrid materials demonstrate the development of transparent and environmentally safe gate dielectric materials for applications in transistors and related electronic devices.
The financial support provided by the National Science Council of Taiwan (Project No: MOST 104-2221-E-131-025-MY3) is greatly appreciated.
The authors declare that they have no competing interests.
YYY and WYL contributed ideas, conceived and designed the experiments, and wrote the manuscript. AHJ performed the experiments and material analyses. All authors read and approved the final manuscript.
YYY holds a professor position at Ming Chi University of Technology. AHJ is a master student at Ming Chi University of Technology. WYL holds an associate professor position at National Taipei University of Technology.
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