Fluid characterisation and drop impact in inkjet printing for organic semiconductor devices

An inkjet printer can deposit a very small volume of liquid with high positional accuracy, high speed and low cost. As a maskless, non-contact additive patterning method, inkjet printing technology is increasingly being explored as an alternative to lithography, etching and vapour deposition processes to pattern electrical conductors and thin films with applications in printed electronic devices. The functional inks used in many of the applications involve non-linear viscoelasticity and their behaviours in the context of inkjet printing have not been fully understood. This thesis aims to characterise Newtonian and non-Newtonian properties of inkjet fluids and identify the key parameters affecting drop impact and spreading processes. Various fluid characterisation techniques such as the filament stretching rheometer and piezoelectric axial vibrator are explored. We propose an experimental method to assess the jettability of non-Newtonian inkjet fluids, without using an inkjet print head. The oblique collision of two continuous liquid jets leads to the formation of a thin oval liquid sheet bounded by a thicker rim which disintegrates into ligaments and droplets. Under certain conditions the flow structure exhibits a remarkably symmetrical “fishbone” pattern composed of a regular succession of longitudinal ligaments and droplets. Good correlation was found between the maximum included angle of the fishbone pattern and the maximum ligament length in the jetting experiments, which suggests that a test based on oblique impinging jets may be useful in the development of fluids for ink jet printing. High-speed imaging is used to analyse the impact and spreading of sub-30 μm drops of diethyl phthalate or polystyrene solutions in diethyl phthalate on to smooth glass surfaces with controlled wettability at speeds from 3 to 8 m s-1, under conditions representative of drop-on-demand inkjet printing. Data on drop height and spreading diameter are generated with high time and spatial resolution, over eight orders of magnitude in timescale. The effects of fluid viscosity and elasticity, which significantly affect jetting performance, are negligible throughout the whole deposition process, with no significant difference between spreading curves. The values of the fluid surface tension and the substrate wettability also have no effect on the kinematic, spreading or relaxation phases, but a marked influence on the wetting phase, in terms of the speed of expansion of the contact diameter and the final spreading factor

Influence of bank geometry on the electrical characteristics of printed organic field-effect transistors

The electrical characteristics of organic field-effect transistors (OFETs) based on small-molecule organic semiconductors (OSCs) have been significantly improved by employing various fabrication techniques in solution processes to enhance the OSC crystallinity. However, complicated fabrication and inhomogeneity of OFETs remain as challenges before commercialization. In this work, we have efficiently controlled the size and orientation of 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) crystalline domains by tuning the Cytop bank dimension, in which OSC inks are printed, to improve the device performance. The optimized bank pattern forms uniform thin film morphology and well-aligned TIPS-pentacene crystalline domains along the charge transport direction, resulting in four-fold increase in field-effect mobility and one third reduction in relative standard deviation.11Ysciescopu

Solution-Processed Vertically Stacked Complementary Organic Circuits with Inkjet-Printed Routing

The fabrication and measurements of solution-processed vertically stacked complementary organic field-effect transistors (FETs) with a high static noise margin (SNM) are reported. In the device structure, a bottom-gate p-type organic FET (PFET) is vertically integrated on a top-gate n-type organic FET (NFET) with the gate shared in-between. A new strategy has been proposed to maximize the SNM by matching the driving strengths of the PFET and the NFET by independently adjusting the dielectric capacitance of each type of transistor. Using ideally balanced inverters with the transistor-on-transistor structure, the first examples of universal logic gates by inkjet-printed routing are demonstrated. It is believed that this work can be extended to large-scale complementary integrated circuits with a high transistor density, simpler routing path, and high yield

Microfabrication of In Vitro Alveolar-Capillary Barrier Model by Inkjet-based Bioprinting

An in vitro alveolar-capillary barrier is one of the essential model systems for pulmonary drug and particle tests in disease studies, drug discovery and toxicology. An alveolar-capillary barrier in the gas exchanging region of the lung consists of epithelial and endothelial layers with a thickness of 2 μm. This thin structure is critical to sustain pulmonary function such as gas diffusion. There has been efforts to fabricate the biomimetic human alveolar-capillary barrier model, using microfluidic devices and bioprinting technology. However, none of the works has achieved to mimic this thin membrane, a key feature for the model. Here, we present a human alveolar-capillary model with a sub-10 mm-thick membrane, containing multi-type alveolar cells. We fabricated the alveolar-capillary barrier model with four types of human alveolar cell lines, including type 1 alveolar cell (NCI-H1703), type 2 alveolar cell (A549), lung fibroblast (MRC5), and lung microvascular endothelial cell (HULEC5a). High-resolution drop-on-demand inkjet printing enabled the fabrication of the thin alveolar-capillary barrier model under sub-10 mm thickness for the optimal structure by drop-on-demand deposition of multi-type alveolar cells as a thin layer. We evaluated the functions of the fabricated models by histology, barrier integrity test, and barrier permeability test to demonstrate the level of biomimicry. Inkjet-based bioprinting enabled the fabrication of reproducible in vitro alveolar-capillary models, which have biomimetic microstructures with customized and functionally designed micro-patterns. The inkjet-bioprinted alveolar-capillary models have a potential to replace animal testing as expecting to be applied in disease models for pathology, drug discovery, and toxicology1

Flexible and Printed organic integrated circuits and sensors

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Flexible and Printed Sensor Arrays and Integrated Circuits

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Inkjet-printing of organic electrochemical transistors for in vivo recordings of biological signals

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3D Monolithic Integration in Flexible Printed Organic Transistors

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7 Research Challenges for the Next-Generation Flexible Electronics

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Inkjet-based 3D Bioprinting Technology

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