Xinge Yu

Metal oxides for optoelectronic applications

Metal oxides (MOs) are the most abundant materials in the Earths crust and are ingredients in traditional ceramics. MO semiconductors are strikingly different from conventional inorganic semiconductors such as silicon and III-V compounds with respect to materials design concepts, electronic structure, charge transport mechanisms, defect states, thin-film processing and optoelectronic properties, thereby enabling both conventional and completely new functions. Recently, remarkable advances in MO semiconductors for electronics have been achieved, including the discovery and characterization of new transparent conducting oxides, realization of p-type along with traditional n-type MO semiconductors for transistors, p-n junctions and complementary circuits, formulations for printing MO electronics and, most importantly, commercialization of amorphous oxide semiconductors for flat panel displays. This Review surveys the uniqueness and universality of MOs versus other unconventional electronic materials in terms of materials chemistry and physics, electronic characteristics, thin-film fabrication strategies and selected applications in thin-film transistors, solar cells, diodes and memories.

Ultra‐Flexible, “Invisible” Thin‐Film Transistors Enabled by Amorphous Metal Oxide/Polymer Channel Layer Blends

Ultra-flexible and transparent metal oxide transistors are developed by doping In2 O3 films with poly(vinylphenole) (PVP). By adjusting the In2 O3 :PVP weight ratio, crystallization is frustrated, and conducting pathways for efficient charge transport are maintained. In2 O3 :5%PVP-based transistors exhibit mobilities approaching 11 cm(2) V(-1) s(-1) before, and retain up to ca. 90% performance after 100 bending/relaxing cycles at a radius of 10 mm.

Ultraflexible Polymer Solar Cells Using Amorphous Zinc−Indium−Tin Oxide Transparent Electrodes

Polymer solar cells are fabricated on highly conductive, transparent amorphous zinc indium tin oxide (a-ZITO) electrodes. For two representative active layer donor polymers, P3HT and PTB7, the power conversion efficiencies (PCEs) are comparable to reference devices using polycrystalline indium tin oxide (ITO) electrodes. Benefitting from the amorphous character of a-ZITO, the new devices are highly flexible and can be repeatedly bent to a radius of 5 mm without significant PCE reduction.

Synergistic Approach to High-Performance Oxide Thin Film Transistors Using a Bilayer Channel Architecture

We report here a bilayer metal oxide thin film transistor concept (bMO TFT) where the channel has the structure: dielectric/semiconducting indium oxide (In2O3) layer/semiconducting indium gallium oxide (IGO) layer. Both semiconducting layers are grown from solution via a low-temperature combustion process. The TFT mobilities of bottom-gate/top-contact bMO TFTs processed at T = 250 °C are ~5tmex larger (~2.6 cm(2)/(V s)) than those of single-layer IGO TFTs (~0.5 cm(2)/(V s)), reaching values comparable to single-layer combustion-processed In2O3 TFTs (~3.2 cm(2)/(V s)). More importantly, and unlike single-layer In2O3 TFTs, the threshold voltage of the bMO TFTs is ~0.0 V, and the current on/off ratio is significantly enhanced to ~1 × 10(8) (vs ~1 × 10(4) for In2O3). The microstructure and morphology of the In2O3/IGO bilayers are analyzed by X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy, revealing the polycrystalline nature of the In2O3 layer and the amorphous nature of the IGO layer. This work demonstrates that solution-processed metal oxides can be implemented in bilayer TFT architectures with significantly enhanced performance.

Hole mobility enhancement of pentacene organic field-effect transistors using 4,4′,4″-tris[3-methylphenyl(phenyl)amino] triphenylamine as a hole injection interlayer

Organic field-effect transistors(OFETs) were prepared and analyzed by inserting various thickness of 4,4′,4″-tris[3-methylphenyl(phenyl)amino] triphenylamine (m-MTDATA) between pentacene and goldelectrodes as a hole injection layer. These OFETs showed a significant enhancement of hole mobility comparing to the corresponding single layer device. The interfacial morphologies of pentacene and pentacene/m-MTDATA contact were characterized by atomic force microscopy. The hole mobility improvement of OFETs was attributed to an intermediate energy level formed between pentacene and gold heterojunction when inserting an ultrathin m-MTDATA layer, leading to a remarkable reduction of contact resistance at the metal-organic interface.

Bithiophenesulfonamide Building Block for π-Conjugated Donor–Acceptor Semiconductors

We report here π-conjugated small molecules and polymers based on the new π-acceptor building block, bithiophenesulfonamide (BTSA). Molecular orbital computations and optical, electrochemical, and crystal structure analyses illuminate the architecture and electronic structure of the BTSA unit versus other acceptor building blocks. Field-effect transistors and photovoltaic cells demonstrate that BTSA is a promising unit for the construction of π-conjugated semiconducting materials.

Performance enhancement of poly(3-hexylthiophene) organic field-effect transistor by inserting poly(methylmethacrylate) buffer layer

Electrode buffer layer has been extensively studied to improve the performance of organic field-effect transistor (OFET). Here, poly(methylmethacrylate) (PMMA) was employed as an electrode buffer layer between poly(3-hexylthiophene) (P3HT) layer and gold electrodes in OFETs. These OFETs exhibited nearly five-fold enhancement of hole mobility. Through atomic force microscope and grazing-incidence X-ray diffraction analyses, the performance enhancement was attributed to the uniformity and hydrophobicity of PMMA surface, which led to a remarkable reduction of contact resistance at P3HT/electrode interface. This study provides a facile strategy for the performance enhancement of OFET and insights into the essentiality of buffer layers.

Charge‐Trap Flash‐Memory Oxide Transistors Enabled by Copper–Zirconia Composites

A solution-processed electrochemical charge-trap flash memory element is based on a solid solution of copper and zirconium oxides (Cu-ZrO2) as a charge-trapping layer. Because of the facile reduction of Cu(2+) to Cu(1+), Cu-ZrO2 thin films are especially effective in memory devices based on thin-film transistors when the devices are fabricated from combustion-processed metal-oxide semiconductors (In2O3 and an indium-gallium oxide).

High performance unipolar inverters by utilizing organic field-effect transistors with ultraviolet/ozone treated polystyrene dielectric

High performance unipolar inverters based on a significant variation of threshold voltage (Vth) of organic field-effect transistors (OFETs), which was realized by introducing UV/ozone (UVO) treatment to polystyrene (PS) dielectric, were fabricated. A controllable Vth shift of more than 10 V was obtained in the OFETs by adjusting the UVO treating time, and the unipolar inverters exhibited inverting voltage near 1/2 driving voltage and a noise margin of more than 70% of ideal value. From the analysis of scanning electron microscopy, atom force microscopy, and X-ray photoelectron spectroscopy, the dramatic controllable Vth of OFETs, which played a key role in high performance unipolar inverters, was attributed to the newly generated oxygen functional groups in the PS dielectric induced by UVO treatment.

3D Tunable, Multiscale, and Multistable Vibrational Micro-Platforms Assembled by Compressive Buckling

Microelectromechanical systems remain an area of significant interest in fundamental and applied research due to their wide ranging applications. Most device designs, however, are largely two-dimensional and constrained to only a few simple geometries. Achieving tunable resonant frequencies or broad operational bandwidths requires complex components and/or fabrication processes. The work presented here reports unusual classes of three-dimensional (3D) micromechanical systems in the form of vibratory platforms assembled by controlled compressive buckling. Such 3D structures can be fabricated across a broad range of length scales and from various materials, including soft polymers, monocrystalline silicon, and their composites, resulting in a wide scope of achievable resonant frequencies and mechanical behaviors. Platforms designed with multistable mechanical responses and vibrationally de-coupled constituent elements offer improved bandwidth and frequency tunability. Furthermore, the resonant frequencies can be controlled through deformations of an underlying elastomeric substrate. Systematic experimental and computational studies include structures with diverse geometries, ranging from tables, cages, rings, ring-crosses, ring-disks, two-floor ribbons, flowers, umbrellas, triple-cantilever platforms, and asymmetric circular helices, to multilayer constructions. These ideas form the foundations for engineering designs that complement those supported by conventional, microelectromechanical systems, with capabilities that could be useful in systems for biosensing, energy harvesting and others.