Low-Temperature Solution-Processed Gate Dielectrics for High-Performance Organic Thin Film Transistors

Highly Transparent and Surface-Plasmon-Enhanced Visible-Photodetector Based on Zinc Oxide Thin-Film Transistors with Heterojunction Structure

Highly transparent zinc oxide (ZnO)-based thin-film transistors (TFTs) with gold nanoparticles (AuNPs) capable of detecting visible light were fabricated through spray pyrolysis on a fluorine-doped tin oxide substrate. The spray-deposited channel layer of ZnO had a thickness of approximately 15 nm, and the thickness exhibited a linear increase with an increasing number of sprays. Furthermore, the ZnO thin-film exhibited a markedly smoother channel layer with a significantly lower surface roughness of 1.84 nm when the substrate was 20 cm from the spray nozzle compared with when it was 10 cm away. Finally, a ZnO and Au-NP heterojunction nanohybrid structure using plasmonic energy detection as an electrical signal, constitutes an ideal combination for a visible-light photodetector. The ZnO-based TFTs convert localized surface plasmon energy into an electrical signal, thereby extending the wide band-gap of materials used for photodetectors to achieve visible-light wavelength detection. The photo-transistors demonstrate an elevated on-current with an increase of the AuNP density in the concentration of 1.26, 12.6, and 126 pM and reach values of 3.75, 5.18, and 9.79 × 10⁻⁷ A with applied gate and drain voltages. Moreover, the threshold voltage (Vth) also drifts to negative values as the AuNP density increases.

Low-Voltage Organic Single-Crystal Field-Effect Transistor with Steep Subthreshold Slope

Anodization is a promising technique to form high- k dielectrics for low-power organic field-effect transistor (OFET) applications. However, the surface quality of the dielectric, which is mainly inherited from the metal electrode, can be improved further than other fabrication techniques, such as sol-gel. In this study, we applied the template stripping method to fabricate a low-power single-crystalline OFET based on the anodized AlO _x dielectric. We found that the template stripping method largely improves the surface roughness of the deposited Al and allows for the formation of a high-quality AlO _x high- k dielectric by anodization. The ultraflat AlO _x/SAM dielectric combined with a single-crystal 2,6-diphenylanthracene (DPA) semiconductor produced a nearly defect-free interface with a steep subthreshold swing (SS) of 66 mV/decade. The current device is a promising candidate for future ultralow-power applications. Other than metal deposition, template stripping could provide a general approach to improve thin-film quality for many other types of materials and processes.

High- k Gate Dielectrics for Emerging Flexible and Stretchable Electronics

Recent advances in flexible and stretchable electronics (FSE), a technology diverging from the conventional rigid silicon technology, have stimulated fundamental scientific and technological research efforts. FSE aims at enabling disruptive applications such as flexible displays, wearable sensors, printed RFID tags on packaging, electronics on skin/organs, and Internet-of-things as well as possibly reducing the cost of electronic device fabrication. Thus, the key materials components of electronics, the semiconductor, the dielectric, and the conductor as well as the passive (substrate, planarization, passivation, and encapsulation layers) must exhibit electrical performance and mechanical properties compatible with FSE components and products. In this review, we summarize and analyze recent advances in materials concepts as well as in thin-film fabrication techniques for high- k (or high-capacitance) gate dielectrics when integrated with FSE-compatible semiconductors such as organics, metal oxides, quantum dot arrays, carbon nanotubes, graphene, and other 2D semiconductors. Since thin-film transistors (TFTs) are the key enablers of FSE devices, we discuss TFT structures and operation mechanisms after a discussion on the needs and general requirements of gate dielectrics. Also, the advantages of high- k dielectrics over low- k ones in TFT applications were elaborated. Next, after presenting the design and properties of high- k polymers and inorganic, electrolyte, and hybrid dielectric families, we focus on the most important fabrication methodologies for their deposition as TFT gate dielectric thin films. Furthermore, we provide a detailed summary of recent progress in performance of FSE TFTs based on these high- k dielectrics, focusing primarily on emerging semiconductor types. Finally, we conclude with an outlook and challenges section.

Tailoring the Dielectric Layer Structure for Enhanced Performance of Organic Field-Effect Transistors: The Use of a Sandwiched Polar Dielectric Layer

To investigate the origins of hydroxyl groups in a polymeric dielectric and its applications in organic field-effect transistors (OFETs), a polar polymer layer was inserted between two polymethyl methacrylate (PMMA) dielectric layers, and its effect on the performance as an organic field-effect transistor (OFET) was studied. The OFETs with a sandwiched dielectric layer of poly(vinyl alcohol) (PVA) or poly(4-vinylphenol) (PVP) containing hydroxyl groups had shown enhanced characteristics compared to those with only PMMA layers. The field-effect mobility had been raised more than 10 times in n-type devices (three times in the p-type one), and the threshold voltage had been lowered almost eight times in p-type devices (two times in the n-type). The on-off ratio of two kinds of devices had been enhanced by almost two orders of magnitude. This was attributed to the orientation of hydroxyl groups from disordered to perpendicular to the substrate under gate-applied voltage bias, and additional charges would be induced by this polarization at the interface between the semiconductor and dielectrics, contributing to the accumulation of charge transfer.

Fabrication of Bendable Circuits on a Polydimethylsiloxane (PDMS) Surface by Inkjet Printing Semi-Wrapped Structures

In this work, an effective method was developed to fabricate bendable circuits on a polydimethylsiloxane (PDMS) surface by inkjet printing semi-wrapped structures. It is demonstrated that the precured PDMS liquid film could influence the depositing morphology of coalesced silver precursor inkjet droplets. Accordingly, continuous and uniform lines with a semi-wrapped structure were fabricated on the PDMS surface. When the printed silver precursor was reduced to Ag nanoparticles, the fabricated conductive film exhibited good transparency and high bendability. This work presented a facile way to fabricate flexible patterns on a PDMS surface without any complicated modification or special equipment. Meanwhile, an in situ hydrazine reduction of Ag has been reported using the vapor phase method in the fabricating process.