Silver nanostructure enhancements for flexible conductive films and direct printed electrode applications
Date Issued
2015
Date
2015
Author(s)
Chen, Shih-Pin
Abstract
In the development of recent electrical industry, flexible printing electronic devices have become more attractive things for the future. It mainly utilized the printing technology such as off-set printing, screen printing, even like inkjet printing technology of these methods to fabricate the RFID system, O/PLED, OPVs, E-Paper and so on. Because of these printing technologies have many advantages like simple rapid process, material costs and wastes reduction, the lower waste pollution…etc. It makes the flexible printing electronic related researches with high anticipation and rapid development. For the further widespread applications, stretchable conductive thin films were developed for some related application along the innovation requirements such as E-skin sensor. Stretchable electronics differentiate with traditional flexible electronics and have become more popular in the future. These extended issues are still the aims of effort for related researchers. Here we applied a novel concept about low temperature chemically sintering nanowires for the reinforcement of network connection between each nanowires material. Recently, the related researches of flexible printing electronics are still under developing. Here we supply the thermal accelerative reaction to create the conductive lines of silver nanoparticles. Also used the silver nanowires to fabricate highly stretchable conductive films or compose the electronic devices with patterned nanowires films in direct printing method, that all needed to resolve in urgency. Therefore, the aim of this dissertation is resolving the encountered problems in the flexible printing electronics, to enhance the performance and find ways to make a breakthrough in flexible printing electronic researches. In general, there are no much difference between flexible printed electronics and conventional electronics. Both of these are designed to have a metallic conductor element or conductive tracks inside as a main component. But in practical details, it can be classified with a significant change of the development strategy or direction into three parts: substrate type, various conductive materials, processing improvement. In Chapter 1, we will give a description of flexible printing electronics, and further introduce the overall development process about pre-, in-, post-process categories, contents include of development of various conductive inks, commonly utilized printing technology, related theories in printing quality and post-treatment or test in flexible printing electronics. About the processing improvement section, whereas metallic nanoparticle inks often require high annealing temperatures (>150°C) to decompose stabilizing agents and other polymeric additives that inhibit electrical conductivity. Recent exploration into silver precursor inks has yielded promising results. Unfortunately, even these temperatures render the ink incompatible with many plastic and paper substrates used in flexible electronic and biomedical devices. In Chapter 2, we use a simple and effective silver ink formulation was developed to generate silver tracks with high electrical conductivity on flexible substrates at low sintering temperatures. Diethanolamine (DEA), a self-oxidizing compound at moderate temperatures, was mixed with a silver ammonia solution to form a clear and stable solution. After inkjet-printed or pen-written on plastic sheets, DEA in the silver ink decomposes at temperatures higher than 50 °C and generates formaldehyde, which reacts spontaneously with silver ammonia ions to form silver thin films. The electrical conductivity of the inkjet-printed silver films can be 26% of the bulk silver after heating at 75 °C for 20 min and show great adhesion on plastic sheets. About the substrate selection section, in stretching or elastic deformation processes with large strains, it is difficult for traditional conductors, such as metal tracks or indium tin oxide (ITO) thin films, to maintain electrical conductivity simultaneously with mechanical stability. Typical examples are using the film transfer methods, or buckled serpentine metal structures on stretchable substrates, or metal thin films on pre-strained substrates to preserve structural integrity of the laminated metal thin films at large strains, or using the percolating networks of m-NW thin films can more effectively accommodate strains and show good tolerance in the stretching process. One of the challenges to implement m-NWs as thin film conductors is that the insulating ligands used in the synthesis or solution dispersion must be removed after film deposition to ensure effective electrical contacts between m-NWs. Therefore, in Chapter 3, we demonstrate a new chemical soldering method to enhance silver nanowire (AgNW) network connections by a simple and fast solution-based process.Upon applying a reactive ink over AgNW thin films, silver nanoparticles are preferentially generated over the nanowire junctions and solder the nanomesh structures. The soldered nanostructure reinforces the conducting network and exhibits no obvious change in electrical conductivity in the stretching or rolling process with elongation strains up to 130%. About the various conductive materials section, one-dimensional nanostructure materials show more attractive advantages such as electrical conductivity, mechanical flexibility, excellent transparency and have received substantial attentions in the last few years. Like conductive silver nanowires, as a well promising candidate to replace the ITO conductor, but the use of silver nanowire networks still involves poor substrate adhesion issue. So there are rare papers have mentioned the patterning of silver nanowire thin films after materials deposition, such as lithography process post-treatment. In Chapter 4, we will demonstrate the controlled deposition of networks of various nanowires in well-defined patterns by inkjet printing for UV detection. Inks containing silver or titanium dioxide (TiO2) nanowires were first formulated adequately to form stable suspension for inkjet printing applications. The printed photodetector showed a high transparency and a fairly low dark current of 10-12–10-14 A with a high on/off ratio of 2000 to UV radiation. Under a bias voltage of 2 V, the detector showed fast responses to UV illumination with a rise time of 0.4 s and a recovery time of 0.1 s. This method shows the feasibility of applying inkjet printing technology to create nanowire thin films with specific patterns in printing electronics. Finally, in Chapter 5, we conclude and summarize the achievements of this thesis research. This study shows the related researches of printing applications that have opened up a whole new direction, and can be applied to business development of flexible printed electronics in the future.
Subjects
Silver nanowires
Silver nanoparticles
Conductive reaction inks
Highly stretchable conductive thin films
All-printed UV photodetector
Type
thesis
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