Jooho Moon

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)

Highly Transparent Low Resistance ZnO/Ag Nanowire/ZnO Composite Electrode for Thin Film Solar Cells

We present an indium-free transparent conducting composite electrode composed of silver nanowires (AgNWs) and ZnO bilayers. The AgNWs form a random percolating network embedded between the ZnO layers. The unique structural features of our ZnO/AgNW/ZnO multilayered composite allow for a novel transparent conducting electrode with unprecedented excellent thermal stability (∼375 °C), adhesiveness, and flexibility as well as high electrical conductivity (∼8.0 Ω/sq) and good optical transparency (>91% at 550 nm). Cu(In,Ga)(S,Se)₂ (CIGSSe) thin film solar cells incorporating this composite electrode exhibited a 20% increase of the power conversion efficiency compared to a conventional sputtered indium tin oxide-based CIGSSe solar cell. The ZnO/AgNW/ZnO composite structure enables effective light transmission and current collection as well as a reduced leakage current, all of which lead to better cell performance.

Influence of fluid physical properties on ink-jet printability.

Ink-jet printing is a method for directly patterning and fabricating patterns without the need for masks. To achieve this, the fluids used as inks must have the capability of being stably and accurately printed by ink-jetting. We have investigated the inter-relationship between ink-jet printability and physical fluid properties by monitoring droplet formation dynamics. The printability of the fluids was determined using the inverse (Z) of the Ohnesorge number (Oh) which relates to the viscosity, surface tension, and density of the fluid. We have experimentally defined the printable range as 4 < or = Z < or = 14 by considering characteristics such as single droplet formability, positional accuracy, and maximum allowable jetting frequency.

Direct writing of silver conductive patterns: Improvement of film morphology and conductance by controlling solvent compositions

The authors have investigated the influence of conductive ink compositions on the quality of ink-jet printed patterns. Controlling the solvent compositions plays an important role of reducing the so-called coffee ring effect. The use of a higher boiling point solvent with lower surface tension such as ethylene glycol enables the formation of a uniform deposit of silver nanoparticles due to surface tension gradient-induced inward Marangoni flow, which can compensate outward convective flow. It is demonstrated that the ink-jet printed film quality is directly related to the resistance difference in the conductive patterns.

A non-toxic, solution-processed, earth abundant absorbing layer for thin-film solar cells

Copper zinc tin sulfide (Cu2ZnSnS4, CZTS) has attracted significant attention in the past few years as a next generation absorber material for the production of thin film solar cells on large scales due to the high natural abundance of all constituents, tunable direct band gap energy ranging from 1.0 to 1.5 eV, and large absorption coefficient. In addition, to address the issue of expensive vacuum-based processes, non-vacuum solution-based approaches are being developed for CZTS absorber layer deposition. Here, we demonstrate the fabrication of a high quality CZTS absorber layer with a thickness of 2.8–3.0 μm and micrometre-scaled grains (1–2.5 μm) using air-stable non-toxic solvent-based inks. Our approach for the fabrication of CZTS absorber, reported here, will be the first step in achieving low-cost and large area solar cells with high efficiency.

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.

Fully Flexible Solution‐Deposited ZnO Thin‐Film Transistors

Electronic systems on fl exible substrates posses the advantage of mechanical fl exibility in actual use, but also provide more rugged rollable devices and may therefore result in lower manufacturing costs associated with continuous roll-to-roll fabrication. To realize these advantages of fl exible electronics, lowtemperature solution processing is strongly desirable. In this regard, organic semiconductor materials have been extensively researched. [ 1 ] Organic semiconductor polymers are soluble in a variety of solvents, and small molecules can be derivatized to soluble precursors. Organic transistors can also be fabricated by solution processing near room temperature, [ 2 ] compatible with temperature-sensitive plastic substrates. [ 3–5 ] Despite successful demonstrations of fl exible organic electronics, however, they are generally sensitive to operating conditions and are unstable during long-term operation. [ 6 ]

Preparation and characterization of the Sb-doped TiO2 photocatalysts

Doped TiO2 photocatalysts have been prepared by a coprecipitation method. Uniformly doped nanocrystalline TiO2 of 10–20 nm sizes was synthesized by calcinating the coprecipitated gels at 400–650°C. Photocatalytic characterization along with the microstructural investigation for each catalyst provides better understanding of the photocatalytic behavior. It was found that the photodegradation of methylene blue (MB) was a complex function of the doping type and its concentration and the microstructural characteristics of the catalysts. Antimony doing significantly improved photocatalytic performance as compared to the undoped TiO2. Post-treatment of the as-precipitated wet doped Ti gels in an organic solvent also increased the surface area, forming approximately 8 nm size doped TiO2 with surface area ∼149 m2/g. Superior catalytic activity was observed in the Sb-doped TiO2 samples at a doping concentration ranging from 1 to 5 at%. Using the 5 at% Sb-doped TiO2 catalyst treated in butanol, 100 ppm of MB could be decomposed completely within 1 h, which was better than the commercial Degussa P-25.

Ink-jet printing of cu-ag-based highly conductive tracks on a transparent substrate.

We have developed a Cu-Ag-based mixed metal conductive ink from which highly conductive tracks form on a flexible substrate after annealing at low temperature. Addition of small Ag particles significantly improves the particle packing density by filling the interstices formed between the larger Cu particles, which in turn facilitates better conductivity compared to pure Cu metal film. The particle size and volume ratio of the Ag particles added should be carefully controlled to achieve maximum packing density in the bimodal particle system, which is consistent with the theoretical considerations of the Furnas model. In addition, we demonstrate direct writing of complex patterns that exhibit high conductivity upon annealing at sufficiently low temperature (175-210 degrees C) to not damage the transparent plastic substrate such as polyethersulphone (PES).

Inkjet-Printed Zinc Tin Oxide Thin-Film Transistor

Recently, there has been considerable interest in adapting printing approaches that are typically used in the graphic arts to the printing of electronic circuits and circuit components. We report the fabrication of solution-processed oxide transistors using inkjet printing. A zinc tin oxide sol-gel precursor is utilized as the ink for directly printing a thin uniform semiconducting layer. The printed device performance is significantly influenced by printing conditions such as the surface wettability and substrate temperature. The inkjet-printed transistors exhibit reproducible electrical performance, demonstrating their potential application in low-cost manufacturing of large-area flat panel displays.