Carbon nanotubes as a material for functional inks.

Carbon nanotubes (CNTs) have been proposed as a material for use in printed electronics for a number of years. The potential to exploit the unique electrical and mechanical properties of these structures on the macro-scale is appealing; however there are a number of hurdles to overcome. Printing allows the deposition of CNT networks, the properties of which are governed by the CNT type and network density. The formulation of a suitable ink and deposition of a film with specific properties is challenging, and the work described in this thesis is concentrated on two specific areas, CNT ink development and CNT based device production. The CNT ink was developed by identifying key ink and dried film parameters for characterisation and assessing the effect of several major variables, namely the resin material, resin concentration, processing temperature, CNT concentration, CNT functionality and processing energy. A suitable research ink was developed and optimised using N-methyl-2-pyrrolidone as the solvent and polyvinyl alcohol as the resin at a concentration ratio of 1:1 with the CNT content. The effects of CNT concentration, CNT functionality and processing energy are shown to be interdependent. This is among the first reported studies to investigate the dependence of these factors upon a CNT ink for roll-to-roll processing. This ink system was then used in the production of CNT based thin film transistor (TFT) devices using flexography. Initially the concept was proven using MWCNTs. The design was then refined and devices were produced using SWCNTs at varying network densities. It was seen that the printing of CNT based devices using flexography is feasible, though careful control of the CNT network density is required to achieve suitable device performance. This is the first reported production of TFTs using flexography, and the first reported use of flexograi)hy to deposit CNTs

Effects of plasma modified carbon nanotube interlaminar coating on crack propagation in glass epoxy composites

AbstractFibre reinforced pre-preg systems have very good in plane properties, however they are weak in their through thickness (z) direction. This research aims to address this issue by adding plasma treated carbon nanotubes (CNTs) between the prepreg plies using a simple drawdown coating procedure. The significant test result shows by coating carbon nanotubes with a relatively low areal density (1.2g/m2) the propagation mode I toughness can be improved by up to 46%. Crack deviation leading to increased glass fibre bridging was observed for lower CNT coating concentrations explaining the improved performance. However at the highest areal coating density (2.0g/m2) fibre bridging disappeared and a stick–slip crack response was observed resulting in lower delamination resistance. This research demonstrates a simple method to incorporate a nanointerlayer that can manipulate crack propagation, leading to increased delamination resistance

Flexographic printing of polycarbazole-based inverted solar cells

The mass production of organic photovoltaics requires high throughput processes capable of producing a functional active layer which is homogenous, pinhole free and of a specified thickness. Common methods to achieve this are slot-die coating and gravure printing. We have demonstrated the fabrication of poly(N-9\u2032-heptadecanyl-2,7-carbazole-alt-5,5-(4\u2032,7\u2032-di-2-thienyl-2\u2032,1\u2032,3\u2032-benzothiadiazole (PCDTBT) based photovoltaic cells using the flexographic printing technique, which enables high throughput patterned active layers to be deposited on flexible substrates at lower cost. This was achieved by optimizing the flexographic plate pattern, print speed, the solvent and the drying process. By incorporating halftone patterning, a common process in the graphics printing industry, and optimizing the printing speed, the homogeneity of the active layer print was significantly improved. Further studies of suitable solvents and drying conditions led to reduced pinhole formation and improved uniformity. The functionality of the flexographically printed active layer was demonstrated by fabrication of 1 cm2 photovoltaic cells which showed an efficiency of up to 3.5%, which is comparable to alternative deposition techniques. These results demonstrate the suitability of flexography as a fabrication technique for bulk heterojunction organic photovoltaics.Peer reviewed: YesNRC publication: Ye

Flexo printed sol-gel derived vanadium oxide films as an interfacial hole-transporting layer for organic solar cells

In this paper we report on the synthesis and development of vanadium oxide precursor flexographic ink for the printing of hole-transporting layers in organic solar cells. For the synthesis of vanadium oxide inks, a sol-gel methodology was utilized. By modifying the vanadium alkoxide precursor with a right type of coordinating ligands a stable and flexoprintable ink has been successfully developed. Flexo-printing afforded smooth and uniform vanadium oxide sol-gel films on top of PCDTBT:PC<inf>70</inf>BM films. The conversion of the synthesized sol-gel film into a corresponding vanadium oxide layer was followed by DSC/TGA and XPS analyses. The inks were used for the fabrication of inverted organic solar cells by flexo-printing. Power conversion efficiencies ranging between 3.5 % and 4.5 % were achieved, which are slightly lower than the reference cells using vacuum-deposited MoO<inf>3</inf> as the hole-transporting layers.Peer reviewed: YesNRC publication: Ye

Inkjet printed thin and uniform dielectrics for capacitors and organic thin film transistors enabled by the coffee ring effect

The deposition of a thin and uniform dielectric layer is required for high performance printed capacitors and thin film transistors (TFTs), however this is difficult to achieve with printing methods. We have demonstrated inkjet-printed dielectrics with a uniform thickness from 70 nm to 200 nm by taking advantage of the coffee ring effect. A high capacitance per unit area of 230 pF/mm2 is achieved from capacitors with linear morphologies fully printed onto flexible substrates. We also demonstrate organic TFTs with an average mobility of 0.86 cm2/Vs and a source drain current of 57 \u3bcA obtained with a supply voltage of 15 V. This performance was shown to be consistent, with a standard deviation of 15% obtained from hundreds of printed organic TFTs on PET substrates. This consistency was further validated by the production of functional NAND, NOR, AND and OR logic gates. Our results demonstrate that the coffee ring effect, which is usually viewed as undesirable, can enable higher performance in printed electronic devices.Peer reviewed: YesNRC publication: Ye