Novel, Low-Cost, High-Capacitance Nanocomposite Dielectrics for Printed Electronics

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Abstract

Organic thin-film transistors (OTFTs) have been widely studied because of their promising potential for application in low-cost, large-area and flexible electronics. However, several challenges remain on the way towards practical OTFT devices, such as a high operating voltage (> 20 V) induced by the low charge carrier mobility of organic semiconductors and low capacitance of organic gate dielectrics. A low operating voltage is essential for various OTFTs applications, such as portable displays, radio frequency identification tags (RFIDs), smart textiles and sensors. The key to low voltage operation of OTFTs is reduction of the threshold voltage, inverse subthreshold slope which can be fulfilled by using a high-capacitance gate dielectric with superior interface properties. Since field-effect current is proportional to field-induced charge density, using a gate dielectric layer with high dielectric constant (high-k) enhances output current densities at much lower applied voltages. Very thin dielectric layers have reportedly suffered from poor dielectric properties, while very high-k gate dielectrics have led to inferior dielectric-semiconductor interface. As a result, unsatisfactory device performance, such as low charge carrier mobility and high gate leakage current, has been obtained. In addition, solution-processability on a variety of substrates and compatibility with most common semiconducting materials make high-k dielectric materials an unrivalled candidate for low-voltage, low-cost applications.Consequently, the aim of this project was to produce a high-quality, high-capacitance gate dielectric with excellent properties which is consistent with cheap, basic solution-processing manufacturing techniques. With great promise in hybrid materials, a novel, high-k dielectric material based on alternative organic-inorganic nanocomposites that combine very high dielectric constant values intrinsic to ferroelectric ceramic materials (nanoparticles) with mechanical flexibility, low-cost and easy processing of polymers was developed. Both low- and high-k polymer matrices have been used in formulating high-k nanocomposite dielectric suspensions. The uniformity of suspensions has been improved by surface modification of nanoparticles in the case of low-k polymers, while a combination of polymer choice, solvents and nanoparticle-to-polymer ratio led to homogenous suspensions based on high-k polymers. The nanocomposite preparation technique was also unique to this work and gave reproducibly stable nanocomposite suspensions. Finally, ultralow-voltage (~ 1) OTFTs have been successfully demonstrated by integrating nanocomposite bilayer dielectrics using a high-k fluorinated polymer. Bilayer dielectrics were formed by (partially) capping the surface of the nanocomposite films with an ultrathin capping layer. The capping layer was the key to the operation of low-voltage OTFTs as it allowed remarkable and advantageous use of the nanocomposite surface roughness while improving the dielectric-semiconductor surface roughness. Ultimately, such nanocomposite bilayers have a potential to pave the way towards low-cost fabrication and integration of low-voltage components and circuits on flexible substrates.

Keyword(s)

High- capacitance nanocomposite dielectric; Low-voltage OTFTs

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