Sunho Jeong

Role of Gallium Doping in Dramatically Lowering Amorphous‐Oxide Processing Temperatures for Solution‐Derived Indium Zinc Oxide Thin‐Film Transistors

[*] Prof. T. J. Marks, Dr. A. Facchetti, Dr. S. Jeong, Y.-G. Ha Department of Chemistry and the Materials Research Center Northwestern University 2145 Sheridan Road, Evanston, IL 60208 (USA) E-mail: t-marks@northwestern.edu; a-facchetti@northwestern.edu Prof. J. Moon Department of Materials Science and Engineering Yonsei University 134 Shinchon-dong, Seodaemun-gu, Seoul 120-749 (Korea) [+] Present address: Korea Research Institute of Chemical Technology, 19 Sinseongno, Yuseong, Daejeon 305-600 (Korea)

Low-temperature, solution-processed metal oxide thin film transistors

High-performance, solution-processable semiconductors have drawn significant attention for use in low-cost, functional electronic applications. Metal oxide semiconductors are the most promising building blocks for high performance electronic devices because of their electrical properties and solution-processability. However, the major impediment for metal oxide semiconductors is that the electrical properties applicable to electronic devices are activated by chemical/physical structural evolution at high temperatures, which critically limits the practical applications. This article reviews the recent progress in the development of high-performance oxide semiconductors processed at low temperatures which are compatible with plastic substrates and discusses the chemical/physical approaches to lower the annealing temperature.

Stable Aqueous Based Cu Nanoparticle Ink for Printing Well-Defined Highly Conductive Features on a Plastic Substrate

With the aim of inkjet printing highly conductive and well-defined Cu features on plastic substrates, aqueous based Cu ink is prepared for the first time using water-soluble Cu nanoparticles with a very thin surface oxide layer. Owing to the specific properties, high surface tension and low boiling point, of water, the aqueous based Cu ink endows a variety of advantages over conventional Cu inks based on organic solvents in printing narrow conductive patterns without irregular morphologies. It is demonstrated how the design of aqueous based ink affects the basic properties of printed conductive features such as surface morphology, microstructure, conductivity, and line width. The long-term stability of aqueous based Cu ink against oxidation is analyzed through an X-ray photoelectron spectroscopy (XPS) based investigation on the evolution of the surface oxide layer in the aqueous based ink.

Flexible low-voltage organic thin-film transistors enabled by low-temperature, ambient solution-processable inorganic/organic hybrid gate dielectrics

We report here on the design, synthesis, processing, and dielectric properties of novel cross-linked inorganic/organic hybrid blend (CHB) dielectric films which enable low-voltage organic thin-film transistor (OTFT) operation. CHB thin films (20-43 nm thick) are readily fabricated by spin-coating a zirconium chloride precursor plus an α,ω-disilylalkane cross-linker solution in ambient conditions, followed by curing at low temperatures (~150 °C). The very smooth CHB dielectrics exhibit excellent insulating properties (leakage current densities ~10(-7) A/cm(2)), tunable capacitance (95-365 nF/cm(2)), and high dielectric constants (5.0-10.2). OTFTs fabricated with pentacene as the organic semiconductor function well at low voltages (<-4.0 V). The morphologies and microstructures of representative semiconductor films grown on CHB dielectrics prepared with incrementally varied compositions and processing conditions are investigated and shown to correlate closely with the OTFT response.

An 8.2% efficient solution-processed CuInSe2 solar cell based on multiphase CuInSe2 nanoparticles

Multiphase CuInSe2 (CISe) nanoparticles including the CuSe phase are synthesized by the microwave-assisted solvothermal method. Without additional processing, multiphase CISe nanoparticles facilitate the solution-processed CISe absorber layer with a dense microstructure, large grains, high crystallinity, and composition controllability, which are essential for acceptable thin-film solar cell performance. The high performance, solution-processed CISe solar cell, with a conversion efficiency of 8.2%, is obtained through phase transformation, microstructural evolution, and composition adjustment by selenization (annealing under a Se atmosphere) at 530 °C.

Air-stable, surface-oxide free Cu nanoparticles for highly conductive Cu ink and their application to printed graphene transistors

Air-stable, surface-oxide free Cu nanoparticles are, for the first time, synthesized by surrounding completely the Cu surface with oleic acid incorporated as a capping molecule. XPS analysis, in conjunction with TEM analysis, revealed that the oleic acid is chemisorbed to the Cu surface via a chemical interaction wherein a monodentate bond is included, without leaving behind free (non-interacting) oleic acid, thereby providing complete surface protection against oxidation. By eliminating the surface oxide layer that critically degrades the electrical properties, the surface-oxide free Cu nanoparticle ink facilitates the realization of a solution-processed Cu electrode layer with resistivity as low as 4 μΩ cm, comparable to the resistivity of noble metal-based, solution-processed counterparts. In addition, high resolution Cu electrode patterns with 5 μm line-width are directly printed using an electrohydrodynamic inkjet technique, and graphene transistors with the printed Cu electrodes demonstrate potential applications in printed electronics.

Solution-deposited Zr-doped AlOx gate dielectrics enabling high-performance flexible transparent thin film transistors

Although high dielectric constant (k) oxide thin film has been considered as a key element for high performance and low-voltage driven thin-film transistors (TFTs), there are no solution processable high-k oxide dielectrics that satisfy the stringent requirements of low-temperature processability, mechanical flexibility, and good TFT performance. Here, we demonstrate that the incorporation of a zirconium component that has strong bonding to oxygen enables a significant reduction in the processing temperature for soluble alumina dielectrics to as low as 250 °C. Based on these Zr-AlOx films, high performance, low operational voltage, flexible TFTs are achieved. Flexible TFTs operate well under a gate bias of 5 V, exhibiting a high saturation mobility of 51 cm2 V−1 s−1, an on/off current ratio of 104, and a low threshold voltage of 1.2 V with good mechanical flexibility. This is the first study demonstrating the mechanical flexibility of all-oxide soluble high-k dielectric–semiconductor-based TFTs with an emphasis on the influence of annealing temperature on the solution-deposited high-k oxide dielectric characteristics.

Preparation of aqueous Ag Ink with long-term dispersion stability and its inkjet printing for fabricating conductive tracks on a polyimide film

We synthesized silver nanoparticles via chemical reduction method in aqueous medium and poly(acrylic acid) sodium salt (PAA), a kind of anionic polyelectrolyte, was used as a capping agent as well as a dispersant. Owing to the electrosteric repulsion characteristic of PAA, the prepared aqueous Ag ink exhibited a remarkable dispersion stability, which was confirmed by sedimentation test, zeta-potential measurement, and rheological analysis. We fabricated narrow conductive features onto the polyimide substrate using ink-jet printing method and demonstrated the influence of substrate temperature on the morphology of printed patterns. After annealing at 250 °C, the resistivity of the Ag pattern with a linewidth of 60 μm was 4 μΩ cm.

A solution-processed yttrium oxide gate insulator for high-performance all-solution-processed fully transparent thin film transistors

We studied high-k soluble yttrium oxide dielectrics for high performance oxide thin film transistors (TFTs). The electrical characteristics of yttrium oxide show leakage current density as less than 10−6 A cm−2 at 2 MV cm−1 regardless of annealing temperature and a high dielectric constant of nearly 16. For the first time, all solution-processed fully transparent ZnO-TFTs based on spin-coated YOx gate dielectric layers with a small interfacial trap density and high capacitance were demonstrated, exhibiting a field-effect mobility of 135 cm2 V−1 s−1, a threshold voltage of 1.73 V, and an on–off current ratio of 5.7 × 107 as well as low-voltage operation. In addition to microstructural and electrical analyses for solution-processed YOx dielectrics, we investigated the influences of dielectric–semiconductor interfacial quality on device parameters. Our results suggest that solution-processable fully transparent oxide TFTs have the potential for low-temperature and high-performance application in transparent, flexible devices.

All-Ink-Jet Printed Flexible Organic Thin-Film Transistors on Plastic Substrates

Organic thin-film transistors TFTs, composed of a conductor, a dielectric, and a semiconductor in a multilayered structure, have become a research focus for potential application in low-cost disposable electronic devices, such as smart cards and radio-frequency identification tags, as well as in flexible-display driver circuits, nonvolatile memories, and sensors. 1-7 In particular, the easy solution processability associated with organic semiconductors drives recent research interests, focusing on printed flexible electronic and display applications. Ink-jet printing and other solution-based deposition methods have been extensively adopted for the fabrication of printed TFTs. However, not all of the component layers in organic TFTs OTFTs have been solution-processed; instead, vacuum deposition or photolithography are utilized for the creation of at least one layer in the device structure. 8-13