(Invited) Solution-Processable Materials and Structuring Processes for Electrochemically Gated Thin Film Devices
Authors: Aghassi-Hagmann Jasmin 
Published: August 2024

Printed and flexible electronics enable interesting novel applications in the fields of sensors [1], bioelectronics [2], and security applications [3]. Most of them gain their functionality from a layered structure composed of different materials that interact with each other. For semiconducting layers, organic and inorganic materials can be used; in this regard, the respective functional materials need to be formulated into inks that are then printed on various substrates. The structuring process of printing is a complex process in which many chemical and physical process parameters need to be controlled at the same time, such as drop volume, the waveform of the electrical signal controlling droplet formation, or the drying time. Also, the surface of the substrate plays a decisive role in printing accuracy and layer morphology. After printing, post-treatments are often necessary to obtain the desired features, since the properties of bulk materials often differ from printed nanoparticulate or precursor-derived films. For instance, temperature sintering of printed layers can improve conductivities as well as UV-curing which also allows sintering at lower temperatures.
As functional materials for thin-film transistors and other devices, in particular inorganic materials such as metal oxide precursor and nanoparticulate inks are used. These can show advantages of high carrier mobilities, transparency, stability in air, non-toxicity, and can be prepared mostly by using water or alcohol-based solvents. We employ a large portfolio of various printing techniques, including inkjet, gravure, screen, and even direct laser printing. Nevertheless, for all of these techniques, printing thin pinhole-free dielectrics is a challenge and crucial for the whole device functionality, since pinholes can lead to short circuits or other defects of the device. Especially for transistors, this is a fundamental problem, since very thin dielectric layers are needed between the top gate and the semiconducting channel.To circumvent this problem, we have developed in the past years a composite solid polymer electrolyte (CSPE) consisting of two solutions, namely LiClO4+PC and PVA+DMSO. This approach allows employing a different geometry of the transistors and a thicker electrolyte layer while keeping the functionality. The electrolyte ink is inkjet-printable and dries at room temperature. The advantage of such electrolytes is their ability to cover well even rough film surfaces and create a large Helmholtz double layer capacitance, which causes low threshold voltages and hence allows for low-voltage operation of the printed devices. We have further expanded the electrolyte to chemically cross-linked ion gels that even show better device reliability concerning the humidity of the environment at similar electrical performance. Our recent works on device and component levels include ultra-low-power latches operating down to 0.6 V, pn- and Schottky-diodes, resistors, and logic gate standard cells such as inverter, NAND, and NOR-gates, as well as circuits such as ring oscillators, rectifiers, and security primitives [1].
This presentation starts by introducing various semiconducting inks based on either nanoparticles or precursor solutions and discusses thin film properties, microstructure, and finally the step-by-step device fabrication processes. We employ several structuring processes along the process chain, including laser ablation and lift-off, digital printing techniques such as inkjet, superinkjet, and direct laser printing. In addition, doping concepts to tune the threshold voltage and composite solid polymer electrolytes that are compliant with silver electrodes will be presented. The talk will further discuss a plethora of realized devices, including transistors, memristors, and even fully integrated circuits. In more detail, the fabrication and electrical performance of inkjet and laser-printed ZnO digital memristors as well as WOx analog memristors which show “learning” behavior will be discussed [4]. The conductance of analog-type devices can be gradually manipulated to emulate different behaviors of the biological synapse, such as short-time plasticity, long-term potentiation, and long-term depression.In addition, progress towards transistor architectures using nanoparticle-passed InO as the semiconductor and Pt or ITO as electrode materials is shown. Along these lines, transistors with InO-semiconducting nanoparticles printed into a 3D laser-written polymer reservoir with a 10 µm height are presented and their electrical characteristics discussed. Our devices show high ion currents in the range of ~mA for typical widths of a few hundreds of micrometers. These 3D hybrid devices have a huge potential for biohybrid systems, in which the reservoir structure can be used to accommodate biological material or tissue, or even single cells.
[1] Marques et. al., Advanced Materials 31, (2019), https://doi.org/10.1002/adma.201806483.
[2] Saghafi et. al., Adv. Funct. Mat., Early View, (2023), https://doi.org/10.1002/adfm.202308613
[3] Zimmermann et. al., IEEE Transactions on VLSI Systems 27 (2019), https://doi.org/10.1109/TVLSI.2019.2924081
[4] Hu et. al., Adv. Funct. Mat., EarlyView (2023), https://doi.org/10.1002/adfm.202302290