Inkjet-Printed Flexible Electronic Devices: from High-k Capacitors to h-BN/Graphene Thin Film Transistors

Abstract

The soaring global demand for flexible, wearable and transparent devices has been driven by the advent of the Internet-of-Things (IoT), where smart electronic objects are expected to be, at anytime and everywhere, aware of and interactive with the environment by means of a wireless network. During the last decade, printed electronics holds the promise of enabling low-cost, scalable solutions by exploiting the ability of innovative fabrication technologies that are both cost-competitive and eco-friendly, using disrupting materials to be employed as processed inks onto a large-area flexible substrate.

To date, different printing technologies have been employed to demonstrate appealing flexible devices such as organic light-emitting diode (OLED) displays, radio-frequency identification (RFID) antennas for intelligent packaging, solar cells or wearable healthcare sensors ( most of time based on organic thin-film transistors, OTFTs ). However, compared to the well-established silicon technology, all-printed devices must overcome several hurdles. Among them, the most critical issues concern parameters such as the electrical mobility (µ) and the relative permittivity (ε), whose low values in current available inks result in a low integration density.

The main objective of this Thesis was focused on demonstrating inkjet printing technology as a powerful tool for the oncoming flexible printed electronics development. Indeed, in this Thesis Project inkjet printing technology is validated as a versatile method for the manufacturing of different robust applications (in particular flexible sensors and hybrid circuitry), and especially as a complement and alternative to standard silicon technology for the development of high-quality flexible fully-inkjet printed passive and active devices such as capacitors, memristors and transistors based on novel outstanding functional nanomaterials. Basically, this Thesis addresses the exploration of new promising application areas, where inkjet-printing technology could contribute solving traditional issues by improving the manufacturing processes and the development of novel outstanding smart devices on flexible substrates. First of all, the dissertation deals with the demonstration of inkjet printing as promising technique in comparison to the other current printing methods for the fabrication of robust gas sensor circuitry platforms and as an alternative method to surface mount-technology (SMT), in order to assemble surface mount-devices (SMDs) onto previous inkjet-printed circuitry for flexible electronic applications.

The second main topic of this Thesis was focused on the implementation of new solutions to chase and follow the guidelines of the ITRS roadmap for the next generation of flexible electronic applications by the ink formulation of novel dielectric inks. The latter allowed the design, manufacturing and characterization (from both morphological and electrical points of view) of excellent fully-inkjet printed electronic devices.

Primarily, the study was centred on the high-k HfO2 dielectric ink and devoted to the investigation of the physical and chemical properties of the inkjet-printed insulator. Afterwards, the dielectric material was demonstrated as a suitable high-k material to achieve flexible fully-inkjet printed electronic devices such as metal-insulator-metal (MIM) capacitors (εHfO2~12.6 and a capacitance per unit area of ~1 nF mm2) and ReRAM memories (ION/IOFF ratio of around six orders of magnitude and more than 516 resistive switching cycles).

Bearing in mind the well-known outstanding properties of heterostructures based on two-dimensional (2D) atomic crystals, the aim of the Thesis moved to the formulation of an eco-friendly water-based ink made from 2D nanosheets of hexagonal boron nitride (h-BN), specially designed to be printed as dielectric material. The h-BN ink showed a high-k dielectric constant value εhBN ~ 6.9 and was demonstrated as suitable gate oxide for flexible fully-inkjet printed graphene/h-BN heterostructures for TFT devices. Distinctive electrical properties with state-of-the-art mobility up to 110 cm2V-1s-1 are reported, demonstrating the outstanding performance of the h-BN ink both in flat and under bending conditions, which have been achieved, to the best of our knowledge, for the first time in literature.

La creciente demanda de dispositivos flexibles, portátiles y transparentes ha sido impulsada por la irrupción del Internet de las cosas (IoT, de sus siglas en inglés), donde se pretende que los objetos electrónicos inteligentes estén constantemente sujetos a cambio e interactúen con el medio ambiente mediante una conexión inalámbrica (red 5G). Durante la última década, la electrónica impresa ha adquirido gran protagonismo con la promesa de permitir soluciones escalables y de bajo coste por medio de nuevas tecnologías de fabricación, rentables y respetuosas con el medio ambiente, empleando materiales novedosos en forma de tinta. En primer lugar, esta Tesis demuestra la viabilidad de la impresión por inyección de tinta como técnica prometedora, en comparación con otros métodos de impresión actuales, para la fabricación de circuitos sensores de gas y como método alternativo para el ensamblaje de chips sobre circuitos previamente impresos mediante inyección de tinta para aplicaciones electrónicas híbridas y flexibles. La segunda parte de esta Tesis se centró en el desarrollo de nuevas tintas dieléctricas como solución para seguir la hoja de ruta para la próxima generación de aplicaciones electrónicas flexibles. Primeramente, el estudio se centró en la caracterización de las propiedades físicas y químicas de la tinta de HfO2 de elevada constante dieléctrica (εHfO2 ~ 12.6) impresa mediante inyección de tinta. Acto seguido se demostró que el HfO2 es un dieléctrico adecuado para conseguir dispositivos electrónicos flexibles totalmente impresos por inyección de tinta, tales como condensadores MIM (Cox ~ 1 nF mm-2) y memorias ReRAM (más de 516 ciclos de conmutación resistiva). Finalmente, teniendo en cuenta las propiedades destacadas de las heteroestructuras bidimensionales (2D), la Tesis se focalizó en la formulación de una tinta a base de agua a partir de nano-láminas 2D de nitruro de boro (h-BN), especialmente desarrollada para ser impresa como material dieléctrico (permitividad εhBN ~ 6.9). La tinta de h-BN actúa correctamente como óxido de puerta para transistores flexibles con estructura semiconductor / aislante (grafeno / h-BN), respectivamente. Los dispositivos han sido totalmente fabricados por inyección de tinta sobre PET, lográndose una movilidad de huecos del canal semiconductor de grafeno de hasta 110 cm2V-1s-1.