Si Yun Park

Low‐Temperature, Solution‐Processed and Alkali Metal Doped ZnO for High‐Performance Thin‐Film Transistors

) and its dep-osition requires a high-cost vacuum process. More importantly, the poor transparency of silicon makes it unsuitable for trans-parent applications, and transparency is one of the key issues for future display technology. Consequently, in a search for alterna-tives for amorphous silicon, considerable interest has focused on metal oxide semiconductors, such as In, Ga, or Zn oxides, as these exhibit high optical transparencies, and have excel-lent electrical properties with high electron mobility, chemical stability, and solution processability. For example, ZnO-based semiconductors have been successfully incorporated into var-ious electronic devices, such as electron transfer layers for solar cells,

An effective energy harvesting method from a natural water motion active transducer

We demonstrated a new water motion active transducer (WMAT) without any external bias-voltage sources or additional processes, which critically limit the use of conventional passive capacitive transducers that convert mechanical motion into electric energy. From a simple structure, we successfully turned on an LED using various kinds of natural water motion. The WMAT, which has wide applicability, has good potential to be a candidate for generating sustainable electric energy.

UV–Visible Spectroscopic Analysis of Electrical Properties in Alkali Metal‐Doped Amorphous Zinc Tin Oxide Thin‐Film Transistors

Solution-processed and alkali metals, such as Li and Na, are introduced in doped amorphous zinc tin oxide (ZTO) semiconductor TFTs, which show better electrical performance, such as improved field effect mobility, than intrinsic amorphous ZTO semiconductor TFTs. Furthermore, by using spectroscopic UV-visible analysis we propose a comprehensive technique for monitoring the improved electrical performance induced by alkali metal doping in terms of the change in optical properties. The change in the optical bandgap supported by the Burstein-Moss theory could successfully show a mobility increase that is related to interstitial doping of alkali metal in ZTO semiconductors.

Novel Synthesis, Coating, and Networking of Curved Copper Nanowires for Flexible Transparent Conductive Electrodes

In this work, a whole manufacturing process of the curved copper nanowires (CCNs) based flexible transparent conductive electrode (FTCE) is reported with all solution processes, including synthesis, coating, and networking. The CCNs with high purity and good quality are designed and synthesized by a binary polyol coreduction method. In this reaction, volume ratio and reaction time are the significant factors for the successful synthesis. These nanowires have an average 50 nm in width and 25-40 μm range in length with curved structure and high softness. Furthermore, a meniscus-dragging deposition (MDD) method is used to uniformly coat the well-dispersed CCNs on the glass or polyethylene terephthalate substrate with a simple process. The optoelectrical property of the CCNs thin films is precisely controlled by applying the MDD method. The FTCE is fabricated by networking of CCNs using solvent-dipped annealing method with vacuum-free, transfer-free, and low-temperature conditions. To remove the natural oxide layer, the CCNs thin films are reduced by glycerol or NaBH4 solution at low temperature. As a highly robust FTCE, the CCNs thin film exhibits excellent optoelectrical performance (T = 86.62%, R(s) = 99.14 Ω ◻(-1)), flexibility, and durability (R/R(0) < 1.05 at 2000 bending, 5 mm of bending radius).

Solution-processed amorphous hafnium-lanthanum oxide gate insulator for oxide thin-film transistors

Solution-processed high-K dielectrics for oxide thin-film transistors (TFTs) have been widely studied with the objective of achieving high performance and low-cost TFTs for next-generation displays. In this study, we introduce an amorphous hafnium-lanthanum oxide (HfLaOx) gate insulator with high electrical permittivity which was fabricated by the simple spin-coating method. In particular, the solution-processed HfLaOx dielectric layer, which was achieved by a mixture of two Hf and La metal hydroxide precursors, showed amorphous properties, a low leakage current and a high dielectric constant. The solution-processed HfLaOx dielectric layers showed a breakdown voltage as high as 5 MV cm−1 in strength and a dielectric constant above 22. Based on their implementation as a gate insulator, the solution-processed ZnO/HfLaOx TFTs showed good and stable performances during operation at a low voltage. A mobility of μ = 1.6 cm2 V−1 s−1, an on/off current ratio of 106, and a threshold voltage of 0.0015 V were obtained under a 5 V gate bias. Our results show the possibility of the solution-processed amorphous HfLaOx dielectric layer as a gate insulator for oxide TFTs. We believe that this amorphous HfLaOx dielectric has good potential for next-generation high-performance TFT devices.

Low temperature and solution-processed Na-doped zinc oxide transparent thin film transistors with reliable electrical performance using methanol developing and surface engineering

A transparent thin film transistor (TTFT), including zinc oxide (ZnO), has come into the spotlight as an innovative TFT that has the potential to drive the future of the information technology industry. Herein, we developed a new direct patterning method, drop-casting with a new developing method, through the combination of an aqueous ammonia–ZnO process with the doping of Na ions and surface engineering for high n-type semiconducting performance with good operational stability at low temperature. In particular, the effective decomposition and removal of the residual ammonia compounds using methanol have a successful effect on both intrinsic and Na doped ZnO precursor processes for TFTs and they showed the extensive possibility of ammonia based metal oxide precursor solutions. In this method, the Na doped ZnO TTFTs showed good operational stability even with the process of low temperature sintering. The mobility μ = 0.80 cm2 V−1 s−1 was obtained at 200 °C sintering and the mobility μ = 0.10 cm2 V−1 s−1 at 100 °C sintering. In addition, in ambient conditions, the patterned Na doped ZnO TTFT exhibited high electron mobility μ = 1.84 cm2 V−1 s−1 with excellent device operational stability and scant hysteresis with sintering at 300 °C. This method is not only simple as compared with photolithography and inkjet printing, but is also a sophisticated patterning process with good fidelity for solution-processed ZnO TFTs. Moreover, the proposed method can be extended to plastic substrates on a large scale because of the low temperature development process of the ammonia–ZnO precursor using methanol and continuous patterning at ambient conditions. We believe that this method can be adapted to the advanced process toward future printed transparent electronic devices.

The structural, optical and electrical characterization of high-performance, low-temperature and solution-processed alkali metal-doped ZnO TFTs

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.

Aqueous zinc ammine complex for solution-processed ZnO semiconductors in thin film transistors

We fabricated zinc oxide (ZnO) TFTs using a zinc ammine complex with various zinc oxide sources such as ZnO, intrinsic Zn(OH)2, and precipitated Zn(OH)2. From the analyses of the reaction mechanism, surface morphology, crystal structure, and oxygen vacancy in the ZnO films, we confirmed the same intermediate in ZnO semiconductor films irrespective of the type of zinc oxide source in the zinc ammine complex precursor. The results showed the analogous value of the average field effect mobility, on/off current ratio, and turn-on voltage in all solution-processed ZnO TFTs. In conclusion, we confirmed that directly dissolving pristine ZnO into ammonia water is the most efficient method for preparing the ZnO semiconductor precursor, the zinc ammine complex, for low-temperature, solution-processed, and high performance ZnO TFTs.

Enhanced electrochemical capabilities of lithium ion batteries by structurally ideal AAO separator

In this study, a novel inorganic separator, porous anodic aluminum oxide (AAO), is introduced for a rechargeable lithium ion battery system. The highly ordered AAO gives rise to an ideal structure for battery separators with appropriate porosity (67.4 %), extremely low tortuosity, and thermal durability. The prepared AAO separator has average pore sizes of 75 nm and thickness of 54 μm, which leads to enhanced ionic conductivity (2.196 mS cm−1), discharging capacity at high current rates (20.13 mA h g−1 at 10 C), and capacity retention (82.9%). Moreover, a computer simulation (COMSOL) model shows that the ideal AAO separator structure induces stable lithium ion battery operation in wide ranges of current rate, due to effective suppression of Li dendrite formation. The AAO separator has a strong potential in massive energy storage systems and electric vehicles.

Interface engineering for suppression of flat-band voltage shift in a solution-processed ZnO/polymer dielectric thin film transistor

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.