Scalable, Gravure-Printed Transparent Electronics: Materials and Process Design for Metal Oxide Thin-Film Transistors
- Author(s): Scheideler, William Joseph
- Advisor(s): Subramanian, Vivek
- et al.
Transparent metal oxides are one of the most promising material systems for thin film flexible electronics due to their unique balance of optical transparency and high electron mobility. Rapid progress in the field of solution-processed metal oxides in the last decade has established them as the leading material system for thin film transistors. Fully-integrating metal oxide materials with printing technologies could enable a wide range of applications such as flexible displays, imaging systems, resorbable bioelectronics, and low-power sensors that remain uneconomical or unmanufacturable with incumbent technologies. This thesis contributes materials and methods that can advance the scalable fabrication of transparent thin film transistors by high-speed gravure printing. Through these studies, an understanding of the interactions between ink chemistry, fluid parameters, printed feature formation, and electronic properties is presented.
In the past, gravure-printed electronics has been limited to printing organic inks and metal nanoparticles. Here we couple an understanding of the physics of gravure printing on glass with strategies for inorganic sol-gel ink design to extend the scope of high-speed printing to new high-performance inorganic materials including transparent conductive oxides (TCOs). We explore the impact that ink design has on material properties such as crystallinity, conductivity, transparency, and performance under mechanical bending stress, leading to effective co-optimization of patterning and TCO performance. Furthermore, these ink design principles are applied to silver nanowire hybrid sol-gel inks which allow gravure printing of high figure of merit mechanically robust, transparent conducting films using low processing temperatures.
Additionally, this thesis will discuss the design and construction of a custom gravure printer for high-speed patterning and accurate registration of printed layers. The novel design allows the decoupling of the individual subprocesses of gravure to allow mechanistic studies of major printing artifacts and improved areal uniformity. A new understanding of the role of gravure contact mechanics in ink transfer and doctoring is presented.
This work also develops high-performance materials for scaling down the process temperatures of printed metal oxide devices. Aqueous ink formulations are explored for inkjet printing transparent electrodes and channel materials in transparent thin film transistors. The unique printing physics, electrical contact properties to aqueous semiconductors, and low-temperature processability of these inks are studied to achieve high-resolution printed TCOs at plastic compatible temperatures below 220 °C. Additionally, low-temperature, UV-annealed high-k dielectrics are presented for processing printed metal oxide transistors. High-performance printed transistors are achieved with high operational stability and connections between bias-stress stability and low-temperature processing are studied.