**1. Introduction**

Stretchable electronic components have attracted much research interest due to their considerable potential in biomedical instruments, smart skins, displays, and battery devices [1–3]. From 2010 to 2020, thin film transistors have been made predominantly from inorganic materials. The main reason for this is that the carrier mobility values of organic materials are too low compared with those of inorganic materials [4]. Because the performance of an organic thin film transistor (OTFT) [5,6] has not been able to reach the same performance of inorganic transistor, researchers have continued to study the use of various semiconductor materials [7–11] to improve their carrier mobility. In addition, the plastic soft board-based OTFTs can also be used on flexible substrates [12–15]. The rise of plastic substrates [16–18] has necessitated some flexural quality measurements and novel processing methods, such as stretching and coating, to increase the flexibility and mobility of components [19,20]. A roll-to-roll process that can be fabricated on a flexible substrate in a low-temperature environment could support future commercial development [21].

Hybrid materials [22–24] are organic-inorganic polymer blends that are molecularly mixed and blended through van der Waals forces, hydrogen bonds, ionic bonds, or covalent bonds, thus overcoming the phase separation that can usually be observed in traditional materials. These hybrid materials have the advantages of organic and inorganic materials, providing excellent material properties, including thermal, mechanical, optical, and electrical properties. To achieve a good nanoscale dispersion of organic-inorganic materials, the sol-gel method is the most commonly used method because it is flexible and materials prepared with the sol-gel method have high thermal stability and optical transparency.

This study used spin coating to replace the traditional vaporization for the fabrication of thin film transistors. Polyimide [25–27] was used for the preparation of OTFT due to its good thermal stability, chemical resistance, and mechanical properties. As practical applications continue to advance, the requirements for thermal and mechanical properties are becoming more and more demanding, so inorganic materials are often used to enhance the relevant properties. The most common inorganic materials are SiO<sup>2</sup> and TiO2, which can be prepared using tetraethoxysilane (TEOS) and titanium butoxide, respectively. The use of inorganic composite material TiO2-SiO<sup>2</sup> has also been featured in the literature [28,29]. Such an organic-inorganic hybrid film [30] was applied in OTFT as a dielectric film. The donor material, PffBT4T-2OD [31], was also used to replace the traditional pentacene [32] as a semiconductor layer in organic photovoltaic devices. However, PffBT4T-2OD has not been applied to OTFTs in other research.

Electronic products increasingly require the properties of flexibility and stretchability [33–37]. Therefore, some suitable polymers have been added to these advanced electronic products [38,39], such as Jeffamine D2000 [40] and polyurethane [41]. Another approach to enhance flexibility and stretchability is to connect the sidechain of the semiconductor layer material with an elastic polymer, such as poly(butyl acrylate)(PBA) or 2,6-pyridine dicarboxamide (PDCA). After this modification, the researchers expected that the device could retain its original performance after being subjected to stretching many cycles. The chemical structures of the polyimide-TiO2-SiO2, Jeffamine D2000, and polyurethane as well as the structural diagrams for the OTFT device and the experimental stretching directions are shown in Figure 1. Tensile properties depend on the ratio of TiO2-SiO<sup>2</sup> and whether Jeffamine D2000 or polyurethane is added. The addition ratio of TiO2-SiO<sup>2</sup> ranges from A0–A40 in the order of 0 wt.% to 40 wt.%, B0-B40 when Jeffamine D2000 is added, and C0–C40 when polyurethane is added. *Polymers* **2020**, *12*, x FOR PEER REVIEW 4 of 16 **3. Results and Discussion** Figure 1 shows the chemical structures of the polyimide-TiO2-SiO2 composite dielectric material, Jeffamine D2000, and polyurethane additives and the schematic for the OTFT device structure and the tensile direction. The OTFT devices exhibit tensile properties that depend on the addition ratio of TiO2-SiO2 inorganic nanoparticles and the presence or absence of Jeffamine D2000 or polyurethane additives. The addition ratio of TiO2-SiO2 inorganic nanoparticles ranges from 0 wt.% to 40 wt.%; the cases with those ratio values are denoted by A0–A40, B0–B40, and C0–C40, respectively, indicate the addition of Jeffamine D2000 and polyurethane additives in the order of 0 wt.% to 40 wt.%.

**Figure 1.** (**a**) Chemical structures of polyimide-TiO2-SiO2, Jeffamine D2000, and PU. (**b**) Device structure with illustration of each layer and strain direction in an organic thin film transistor. **Figure 1.** (**a**) Chemical structures of polyimide-TiO<sup>2</sup> -SiO<sup>2</sup> , Jeffamine D2000, and PU. (**b**) Device structurewith illustration of each layer and strain direction in an organic thin film transistor.

All of the prepared hybrid films have optical transmittances greater than 90% with the film thickness about 200 nm. Figure 2a shows the UV-vis spectra of the optical transmittance of A0, A30, B30, and C30 hybrid thin films in the visible light region of 400–700 nm, the optical transmittance is

*3.1. Analysis of Optical and Thermal Properties*

morphology. When the particle size is less than 50 nm, light scattering can be negligible [42]. Moreover, Supplementary Figure S2 shows the optical transmittance of A0, A30, B30, and C30 films as the dielectric layer of OTFTs device in the visible light region of 400–700 nm. This indicates that the optical transmittances of all samples are greater than 75%. The thermal properties of the prepared polyimide-TiO2-SiO2 composite dielectric films were analyzed by thermogravimetric analysis (TGA). Figure 2b shows the TGA curves undertaken in a nitrogen atmosphere. It reveals that the decomposition temperature (Td) of A0, A30, B30, and C30 hybrid thin films are 418, 450, 461, and 443 °C, respectively. The relative parameters for thermal properties are listed in Table 1. This indicates that the thermal decomposition temperature increases with the content of TiO2-SiO2 nanoparticles due to the formation of chemical bonding between polyimide and TiO2-SiO2, which can restrict the polyimide chain reaction, and the Td and thermal stability for the hybrid films thus increases as TiO2- SiO2 content increases [22]. In addition, the addition of Jeffamine D2000 and polyurethane also increase the Td from 426 °C to 477 °C for B0–B40 and 405 °C to 454 °C for C0–C40. The increase in Td for B0–B40 is due to the hydrogen bonding between the N atom in the Jeffamine D2000 and the composite dielectric material. However, the polyurethane is a softer polymer, so the Td of B0–B40 is expected to be lower than that for the other two series of hybrid films. However, the Td for all of hybrid films nonetheless exceed 400 °C, indicating good thermal stability. In addition, none of the

#### **2. Experimental Section**

In this study, a stretchable OTFT was fabricated using Elastomer Tape 3M tape as the stretchable substrate and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS, Sigma Aldrich, Darmstadt, Germany) as the lower electrode. TiO2-SiO<sup>2</sup> inorganic nanoparticles and a soluble polyimide with COOH and a fluorine atom functional group were used to prepare the dielectric layer. The COOH on polyimide could be hydrolyzed and condensed with TiO2-SiO<sup>2</sup> to form a dense network structure, and the size of the CF group in PI molecule is quite big, which can cause an increase of free volume and a reduction of the interaction between the molecular chains, so as to increase the solubility and transparency for the prepared polyimide-TiO2-SiO<sup>2</sup> hybrid films. The film was used as an OTFT gate dielectric. In addition, soluble polyimide overcame the problem of the high temperature dehydration cyclization of thermal polymerization and was applicable to a stretchable OTFT device. In addition, Jeffamine D2000 and polyurethane could be used as additives to increase the tensile properties without the original electrical properties being affected.

#### *2.1. Preparation of Dielectric Gate Dielectric*

Briefly, the 4,4-oxydiphthalic anhydride (97%, Sigma Aldrich, Darmstadt, Germany) and the 2,2-*bis*(3-amino-4-hydroxyphenyl) hexafluoropropane (98%, Matrix Scientific, Columbia, SC, USA) in a three-necked flask were dissolved in the *n*-methyl-2-pyrrolidone (NMP, 99.9%, TEDIA, USA) with 1:1 molar ratio and mixed uniformly. After the further addition of isoquinoline (95%, Tokyo Chemical Industry) in a nitrogen atmosphere for 5 h, a yellow-brown solution was obtained, which was poly (amic acid) (PAA). The PAA was placed in an oil bath at 150 ◦C for 18 h. The polyimide solution obtained was placed in a water: methanol (98%, Mallinckrodt Baker, Phillipsburg, KS, USA) (1:3) mixed solvent to produce the precipitate. The filtrated precipitate was placed in a vacuum oven and dried at 60 ◦C for 2 days to obtain a soluble polyimide powder containing COOH and a fluorine functional group. Tetraethyl orthosilicate (TEOS, Sigma Aldrich, Darmstadt, Germany) was dissolved in ethanol (99.5%, Acros Organics, NJ, USA) added to an aqueous solution of nitric acid, and stirred for 30 min. Simultaneously, titanium(IV) butoxide (Ti(OBu)4, Sigma Aldrich, Darmstadt, Germany) was dissolved in 2-methyl-2,4-pentanediol (98%, Alfa Aesar, MA, USA) solvent, stirred for 30 min. The two aforementioned solutions were mixed and stirred for sol-gel reaction for 30 min, and the solvent was then removed with a rotary evaporator and finally placed in an oven to obtain the TiO2-SiO<sup>2</sup> inorganic nanoparticles. The polyimide dissolved in N,N-Dimethylacetamide (DMAc, 99.8%, TEDIA, USA) and the TiO2-SiO<sup>2</sup> nanoparticles dispersed in butanol solvent were mixed and stirred for 30 min to prepare three series of hybrid materials, namely polyimide-TiO2-SiO2, polyimide-TiO2-SiO2:D2000, and polyimide-TiO-SiO2:PU.To prepare the polyimide-TiO2-SiO<sup>2</sup> hybrid material, the different ratios of SiO2-TiO<sup>2</sup> (0, 10, 20, 30, and 40 wt.%) were mixed with polyimide and stirred for 1 h to obtain the PI-TiO2-SiO<sup>2</sup> precursor solution represented by AX (X = weight percentage of SiO2-TiO<sup>2</sup> in hybrid material). For preparation of the polyimide-TiO2-SiO2:D2000 and polyimide-TiO2-SiO2: PU hybrid material, the preparation procedure was the same as for polyimide-SiO2-TiO2. The only difference was that the polymer (Jeffamine D2000, Alfa Aesar, Massachusetts, USA) or polyurethane, (Sigma Aldrich, Darmstadt, Germany) was dropped gradually into the polyimide solution before the mixing with TiO2-SiO<sup>2</sup> inorganic nanoparticles. The polyimide-TiO2-SiO2:D2000 and polyimide-TiO2-SiO2: PU hybrid materials were represented by BX and CX, respectively, where X was the weight proportion of SiO2-TiO<sup>2</sup> in the hybrid material.

#### *2.2. OTFT Device Preparation*

First, the elastomer tape was attached to the glass and subjected to plasma treatment for 3 min to clean the tape surface. PEDOT:PSS was then spin coated on the elastomer tape and annealed at 100 ◦C for 30 min. The solution of polyimide-SiO2-TiO<sup>2</sup> (or polyimide-SiO2-TiO2:Jeffamine D2000 or polyimide-TiO2-SiO2:polyurethane) was spin coated onto PEDOT:PSS elastomer tape at 2000 rpm/20 s. The coated wafer was placed on a hot plate and thermally polymerized through stepwise heating. The baking process was performed at 60, 80, and 100 ◦C for 10 min and then, finally, at a temperature of 120 ◦C for 10 min. Three series of hybrid dielectric films, namely AX, BX, and CX, were obtained. The poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3"'-di(2-octyldodecyl)-2,2',5',2",5",2"' quaterthiophen-5,5"'-diyl) (PffBT4T-2OD, Sigma Aldrich) as the active layer was then spin coated onto the dielectric layer on a hot plate and heated at 90 ◦C for 5 min as an annealing process. The upper electrode (source and drain) EGaIn (99.99%, Alfa Aesar, MA, USA) was dropped onto the lower electrode and the PffBT4T-2OD surface, respectively, to fabricate the OTFT device. The device structure is shown in Figure 1.

### *2.3. Characterization*

The thermal properties of the prepared hybrids were assessed using a thermogravimetric analysis (TGA, TA Instruments, Q50) and differential scanning calorimeter analysis (DSC, TA Instruments, Q20/RSC90) at heating rates of 20 ◦C and 10 ◦C/min, respectively. The transmittances of the hybrid films coated on the quartz substrates were collected using an ultraviolet-visible spectrum (UV-Vis, Jasco, V-650). The morphologies of the thin films were observed with a high-resolution transmission electron microscope (HR-TEM, JEOL, JEM-2100), a scanning electron microscope (SEM, Hitachi, H-2400), and an atomic force microscope (AFM, Veeco, DI 3100). The thicknesses of the hybrid thin films were analyzed with a microfigure measuring instrument (Surface Profiler, α-*step*, ET-4000, Kosaka Laboratory Ltd.). For the metal-insulator-metal (MIM) structure analysis, 0.6-mm diameter Al electrodes were deposited directly onto the gate dielectric films through shadow masking. MIM direct current measurements and OTFT measurements were performed in ambient conditions using a probe station interface with an Agilent E4980A precision LCR meter (10 kHz to 1 MHz) and an Agilent B1500A semiconductor device parameter analyzer.

### **3. Results and Discussion**

Figure 1 shows the chemical structures of the polyimide-TiO2-SiO<sup>2</sup> composite dielectric material, Jeffamine D2000, and polyurethane additives and the schematic for the OTFT device structure and the tensile direction. The OTFT devices exhibit tensile properties that depend on the addition ratio of TiO2-SiO<sup>2</sup> inorganic nanoparticles and the presence or absence of Jeffamine D2000 or polyurethane additives. The addition ratio of TiO2-SiO<sup>2</sup> inorganic nanoparticles ranges from 0 wt.% to 40 wt.%; the cases with those ratio values are denoted by A0–A40, B0–B40, and C0–C40, respectively, indicate the addition of Jeffamine D2000 and polyurethane additives in the order of 0 wt.% to 40 wt.%.
