Printing technology has received considerable attention in various academic and industrial eco-friendly applications with respect to processability. In particular, the electronics field has been making significant progress in replacing photolithography to fabricate micro-/nano-patterned devices with...
Printing technology has received considerable attention in various academic and industrial eco-friendly applications with respect to processability. In particular, the electronics field has been making significant progress in replacing photolithography to fabricate micro-/nano-patterned devices with diverse printing techniques. Thus, printed and flexible electronic devices are expected to be commercialized in the near future. The thin-film transistor (TFT) is one of the unit components for electronic products such as displays, logic circuits, and sensors. In addition, TFT is considered a good test platform for evaluating the performance of printing technologies (e.g., ink formulation, electrical properties, pattern fidelity, etc.) because it requires the fabrication of multilayered structure using inks (semiconductor, conductor, and insulator) and fine patternability to reduce channel length to sub-micrometers. The performance of recent printed TFTs, including organic and/or inorganic materials (e.g. organic semiconductors, metal nanoparticles/nanowires, and carbon–based materials) patterned by a variety of printing techniques, is comparable or higher than that of the TFTs fabricated by vacuum processes.
Electrohydrodynamic (EHD) jet printing is frequently adapted due to positioning accuracy that enables thin-film deposition, direct writing, and spray forming. One printing parameter that affects the morphology and line width of the printed film is the “jetting mode”, which controls the ejection behavior of the ink droplet at the nozzle tip during the printing process. A unique feature of the EHD jet printing is the application of electrostatic fields between a metallic capillary nozzle and a substrate, by which various types of menisci can be formed at the end of the capillary tip depending on the surface tension of liquids. When the intensity of the electrostatic field is stronger than the surface tension force and/or viscous action of the liquid, the electrostatic stress elongates the meniscus in the direction of the field, resulting in the generation of a droplet or jet of liquid. The jetting modes of EHD jet printing are categorized as dripping, microdripping, cone-jet, and multi-jet mode. The details of each mode will be explained in the following section. However, the cone-jet mode is not always applicable for all inks. For example, highly viscous inks cannot be deformed to a conical meniscus in response to the applied electrostatic field, leading to the failure of the cone-jet mode. Moreover, the conductive ink including excess charges generate several unstable and irregular cone apexes over the meniscus, and an ink droplet hanging on the nozzle tip drops in current form rather than deforming the meniscus, or jet streams are multiply ejected on the meniscus. In these cases, optimal printing conditions for each ink (intensity of electrostatic field, flow rate, surface tension, viscosity, permittivity, and charge density) allow cone-jet mode printing.
In Chapter 1, Aligned silver nanowire (AgNWs) conductive electrodes are printed through an electrohydrodynamic (EHD) jet printing via cone-jet mode. We investigate effects of poly(ethylene oxide) (PEO) addition to AgNW inks in terms of surface energy, ink viscosity, and electrical conductivity. The optimized PEO content in the ink is determined to be ~10 wt%, at which well-defined AgNWs/PEO composite electrodes aligned along the printing direction can be prepared by cone-jet mode of EHD jet printing. We fabricate bottom contact organic thin film transistors (OTFTs) with TIPS-pentacene/PS blend as a semiconducting layer to take advantage of printed AgNWs/PEO electrodes. The resulting devices exhibit stable transfer characteristics for 12 h even in ambient air with average field-effect mobility (μFET), threshold voltage (Vth) and on/off current ratio (Ion/Ioff) of 0.51 cm2/Vs, –2.0 V, and ~106, respectively. To understand the characteristics of OTFTs based on AgNWs/PEO composite electrodes in detail, we examine crystalline morphology and molecular orientation of TIPS-pentacene/PS blend on various substrates through field emission scanning electron microscopy and two dimensional grazing incidence X-ray diffraction, respectively.
In Chapter 2, We demonstrate on the direct writing of multi-walled carbon nanotube (MWCNT) composite inks based on three different surfactants via the electrohydrodynamic (EHD) jet printing technique. All three surfactants, including two types of polymeric surfactants and an ionic surfactant, successfully dispersed the MWCNTs in the ink medium. Although the MWCNT composite with the ionic surfactant could not be printed by the EHD process, the MWCNT composites with polymeric surfactants could be successfully printed using this technique. Furthermore, the printed lines exhibited different electrical and electronic characteristics, depending on the type of surfactant. A large amount of the poly (4-styrenesulfonic acid) (PSS) surfactant was required to disperse the MWCNTs in ethanol, whereas a smaller amount of polymeric Triton X-100 (TX100) was required to obtain a MWCNT composite suspension in distilled water, and therefore, the printed lines of the latter provided higher conductivities. In addition, the surface potential and charge carrier injection properties of the EHD-printed MWCNT lines depended on the type of surfactant in the MWCNT composite. Finally, organic field-effect transistors (OFETs) employing source/drain electrodes based on MWCNT/surfactant composites exhibited opposing electrical characteristics depending on the type of surfactant. The MWCNT/PSS lines showed excellent electrical performance when used as electrodes in p-type OFETs, whereas the MWCNT/TX100 lines exhibited excellent performance when used as electrodes in n-type OFETs.
In Chapter 3, In this study, 5-μm scale patterns of silver nanowires (AgNWs) flexible transparent electrode was successfully demonstrated by using a high resolution electrohydrodynamic (EHD) jet printing process. By optimizing the EHD jet printing parameters such as working distance and applied voltage, a mechanically flexible and robust acrylic polymer-silicate nanoparticle composite resin (iGloss®,in short IG) was printed on the AgNWs electrodes via EHD jet printing with a 5-μm-line-width, which, in turn, allowed us to fabricate the finely patterned AgNWs electrodes by removing the AgNWs under the un-covered region by IG. The EHD jet-printed AgNWs/IG electrodes showed the excellent optoelectronic properties, showing the optical transmittance of∼90% and electrical conductivity of∼45 ohms/sq. Furthermore, the IG-resin coated AgNWs electrode exhibited superior stabilities when placed under the severe bending, scratch and dipping tests in water and isopropyl alcohol due to the robustness of the IG.
In Chapter 4, AgNWs-based electrodes were directly patterned using an electrohydrodynamic(EHD) jet printing technique. We investigated EHD jet printing for AgNWs ink in detail, and established the optimum printing conditions in dragging mode for controlling the dimensions and conductivity of the AgNWs network, although the cone-jet printing mode has been the most conventionally used mode for EHD jet printing. The printed AgNWs were used as source/drain (S/D) electrodes of an organic thin film transistor (OTFT) with bottom-contact geometry, and yielded an average field-effect mobility (μFET), threshold voltage (Vth) and on/off current ratio (Ion/Ioff) of 0.48 /V, −11.2 V, and ∼, respectively. In addition, we investigate the interface morphologies between pentacene and AgNWs S/D electrode to figure out charge carrier injection property of AgNWs S/D electrode, by comparing vacuum-deposited Au ones.
Printing technology has received considerable attention in various academic and industrial eco-friendly applications with respect to processability. In particular, the electronics field has been making significant progress in replacing photolithography to fabricate micro-/nano-patterned devices with diverse printing techniques. Thus, printed and flexible electronic devices are expected to be commercialized in the near future. The thin-film transistor (TFT) is one of the unit components for electronic products such as displays, logic circuits, and sensors. In addition, TFT is considered a good test platform for evaluating the performance of printing technologies (e.g., ink formulation, electrical properties, pattern fidelity, etc.) because it requires the fabrication of multilayered structure using inks (semiconductor, conductor, and insulator) and fine patternability to reduce channel length to sub-micrometers. The performance of recent printed TFTs, including organic and/or inorganic materials (e.g. organic semiconductors, metal nanoparticles/nanowires, and carbon–based materials) patterned by a variety of printing techniques, is comparable or higher than that of the TFTs fabricated by vacuum processes.
Electrohydrodynamic (EHD) jet printing is frequently adapted due to positioning accuracy that enables thin-film deposition, direct writing, and spray forming. One printing parameter that affects the morphology and line width of the printed film is the “jetting mode”, which controls the ejection behavior of the ink droplet at the nozzle tip during the printing process. A unique feature of the EHD jet printing is the application of electrostatic fields between a metallic capillary nozzle and a substrate, by which various types of menisci can be formed at the end of the capillary tip depending on the surface tension of liquids. When the intensity of the electrostatic field is stronger than the surface tension force and/or viscous action of the liquid, the electrostatic stress elongates the meniscus in the direction of the field, resulting in the generation of a droplet or jet of liquid. The jetting modes of EHD jet printing are categorized as dripping, microdripping, cone-jet, and multi-jet mode. The details of each mode will be explained in the following section. However, the cone-jet mode is not always applicable for all inks. For example, highly viscous inks cannot be deformed to a conical meniscus in response to the applied electrostatic field, leading to the failure of the cone-jet mode. Moreover, the conductive ink including excess charges generate several unstable and irregular cone apexes over the meniscus, and an ink droplet hanging on the nozzle tip drops in current form rather than deforming the meniscus, or jet streams are multiply ejected on the meniscus. In these cases, optimal printing conditions for each ink (intensity of electrostatic field, flow rate, surface tension, viscosity, permittivity, and charge density) allow cone-jet mode printing.
In Chapter 1, Aligned silver nanowire (AgNWs) conductive electrodes are printed through an electrohydrodynamic (EHD) jet printing via cone-jet mode. We investigate effects of poly(ethylene oxide) (PEO) addition to AgNW inks in terms of surface energy, ink viscosity, and electrical conductivity. The optimized PEO content in the ink is determined to be ~10 wt%, at which well-defined AgNWs/PEO composite electrodes aligned along the printing direction can be prepared by cone-jet mode of EHD jet printing. We fabricate bottom contact organic thin film transistors (OTFTs) with TIPS-pentacene/PS blend as a semiconducting layer to take advantage of printed AgNWs/PEO electrodes. The resulting devices exhibit stable transfer characteristics for 12 h even in ambient air with average field-effect mobility (μFET), threshold voltage (Vth) and on/off current ratio (Ion/Ioff) of 0.51 cm2/Vs, –2.0 V, and ~106, respectively. To understand the characteristics of OTFTs based on AgNWs/PEO composite electrodes in detail, we examine crystalline morphology and molecular orientation of TIPS-pentacene/PS blend on various substrates through field emission scanning electron microscopy and two dimensional grazing incidence X-ray diffraction, respectively.
In Chapter 2, We demonstrate on the direct writing of multi-walled carbon nanotube (MWCNT) composite inks based on three different surfactants via the electrohydrodynamic (EHD) jet printing technique. All three surfactants, including two types of polymeric surfactants and an ionic surfactant, successfully dispersed the MWCNTs in the ink medium. Although the MWCNT composite with the ionic surfactant could not be printed by the EHD process, the MWCNT composites with polymeric surfactants could be successfully printed using this technique. Furthermore, the printed lines exhibited different electrical and electronic characteristics, depending on the type of surfactant. A large amount of the poly (4-styrenesulfonic acid) (PSS) surfactant was required to disperse the MWCNTs in ethanol, whereas a smaller amount of polymeric Triton X-100 (TX100) was required to obtain a MWCNT composite suspension in distilled water, and therefore, the printed lines of the latter provided higher conductivities. In addition, the surface potential and charge carrier injection properties of the EHD-printed MWCNT lines depended on the type of surfactant in the MWCNT composite. Finally, organic field-effect transistors (OFETs) employing source/drain electrodes based on MWCNT/surfactant composites exhibited opposing electrical characteristics depending on the type of surfactant. The MWCNT/PSS lines showed excellent electrical performance when used as electrodes in p-type OFETs, whereas the MWCNT/TX100 lines exhibited excellent performance when used as electrodes in n-type OFETs.
In Chapter 3, In this study, 5-μm scale patterns of silver nanowires (AgNWs) flexible transparent electrode was successfully demonstrated by using a high resolution electrohydrodynamic (EHD) jet printing process. By optimizing the EHD jet printing parameters such as working distance and applied voltage, a mechanically flexible and robust acrylic polymer-silicate nanoparticle composite resin (iGloss®,in short IG) was printed on the AgNWs electrodes via EHD jet printing with a 5-μm-line-width, which, in turn, allowed us to fabricate the finely patterned AgNWs electrodes by removing the AgNWs under the un-covered region by IG. The EHD jet-printed AgNWs/IG electrodes showed the excellent optoelectronic properties, showing the optical transmittance of∼90% and electrical conductivity of∼45 ohms/sq. Furthermore, the IG-resin coated AgNWs electrode exhibited superior stabilities when placed under the severe bending, scratch and dipping tests in water and isopropyl alcohol due to the robustness of the IG.
In Chapter 4, AgNWs-based electrodes were directly patterned using an electrohydrodynamic(EHD) jet printing technique. We investigated EHD jet printing for AgNWs ink in detail, and established the optimum printing conditions in dragging mode for controlling the dimensions and conductivity of the AgNWs network, although the cone-jet printing mode has been the most conventionally used mode for EHD jet printing. The printed AgNWs were used as source/drain (S/D) electrodes of an organic thin film transistor (OTFT) with bottom-contact geometry, and yielded an average field-effect mobility (μFET), threshold voltage (Vth) and on/off current ratio (Ion/Ioff) of 0.48 /V, −11.2 V, and ∼, respectively. In addition, we investigate the interface morphologies between pentacene and AgNWs S/D electrode to figure out charge carrier injection property of AgNWs S/D electrode, by comparing vacuum-deposited Au ones.
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