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Herein, we propose an ionic liquid–polymer dielectric layer for flexible electronics reinforced by a chemical interaction between the polymer matrix (PVP) and the ionic liquid. Due to the robust structures of the cross-linked PVP matrix and hydrogen bonding between the ionic liquid and PVP, the ionic liquid–PVP (IL–PVP) layer exhibited a good mechanical strength when bending up to 1000 times and a stable thermal behaviour up to 300 °C. Furthermore, the IL–PVP dielectric layer showed a high capacitance value of ∼2 μF cm−2 and was operated well as a gate insulator for flexible ZnO thin film transistors with a linear field-effect mobility of ∼3.3 cm2 V−1 s−1 at a gate bias of 3 V.

Flexible and transparent thin film transistors (FTTFTs) can lead to next generation displays that involve large area, future-oriented flexible and transparent displays. In order to achieve stable FTTFTs, solution processes of organic and inorganic compounds have received significant attention. Above all, transparent oxide semiconductors such as ZnO have been studied to enhance flexibility with high electrical performance by integration with organic dielectrics. However, interfacial traps between inorganic and organic compounds are derived by interface dipole, which induce a considerable flat band shift. Herein, we have developed a self-assembled inorganic layer (SAIL) via the photo-induced transformation of a mono-poly(dimethylsiloxane) (PDMS) layer as interface engineering. Especially, the shifting of flat band voltage (VFB) was effectively suppressed by the SAIL process, which was analyzed with a single-piece analytical model for ZnO TFTs. In addition, flexible ZnO/SAIL/polymer dielectric TFTs with low process temperature as high as 200 °C exhibited a good field-effect mobility μ = 0.28 cm2 V−1 s−1, more than 106 on–off current ratio and excellent device operational stability and flexibility.

A solution-processed ionic amorphous Al2O3 dielectric with a low temperature annealing process at 350 °C shows good compatibility and high performance in metal oxide semiconductor thin film transitors (TFTs) such as Li–ZnO TFTs and In–ZnO TFTs. The Li–ZnO/Al2O3 and In–ZnO/Al2O3 TFTs, with solution-processability and low temperature annealing at a maximum of 350 °C, exhibited field-effect mobilities of 46.9 cm2 V−1 s−1 in crystalline Li–ZnO/Al2O3 TFTs and 44.2 cm2 V−1 s−1 in amorphous In–ZnO/Al2O3 TFTs with an on/off current ratio of more than 105. The proton mobile ion, such as hydrogen ion (H+) from chemisorbed water, in the ionic Al2O3 dielectric remarkably induces a high performance capacitance by the formation of an electrical double layer. The chemisorbed water was monitored by FT-IR and ellipsometric porosimetry measurements. Furthermore, the addition of H2O2 to the ionic Al2O3 dielectric precursor successfully suppressed the oxygen vacancies in the dielectric layer, which caused the electrical trap and pass, and confirmed the stable operation. These ionic amorphous Al2O3 dielectrics show good potential as switching TFTs devices in advanced displays, because they can satisfy the various demands of next-generation high-performance TFTs, such as low-cost, solution-processability, and a relatively low-temperature process.

The structural, electrical and optical properties of high-performance, low-temperature and solution-processed alkali metal-doped ZnO TFTs were studied using various analytic instruments, including HR-TEM, AFM, XPS, EDS, electrical bias stability test and UV-vis spectroscopy. Furthermore, we successfully demonstrated that a change in the optical bandgap energy of Li-doped ZnO semiconductor films supported by Burstein–Moss theory can show a trade-off relationship between the field effect mobility of Li-ZnO TFTs and the Li doping concentrations. The relative broadening of the Eopt values, which are strongly related to the amount of excited electrons from the Fermi level in the valance band to the conduction band, was observed from the undoped ZnO film to the Li-doped ZnO film (10 mol%). The increase in the electron donor concentration was the dominant reason for the enhancement in the electron mobility of the alkali metal-doped ZnO TFTs.