At present, along with the rapid development of artificial intelligence, biomedicine, flexible electronics, new energy and other emerging fields, making printed electronics has also achieved significant development results. The principle of printed electronics is to formulate electronic materials into electronic functional inks suitable for printing technology, and then complete the manufacturing of electronic products through the printing process. In printed electronics, inks are used to implement structures with a certain function. In order to print complex electronic structures, several kinds of inks are required, such as conducting, semiconducting, dielectric (or insulator inks). In some specific cases, light-emitting or photovoltaic inks are also used.
The conducting inks are synthesized as dispersed nanoparticles, dissolved organometallic compounds, dissolved or dispersed conductive polymers. Metals are the most commonly used conductive ink materials, which are in the form of nanoparticle suspensions or metal organic decomposition. Metal-nanoparticles are easy to disperse into inks for different printing methods due to their nanometer size; In addition to the metallic inks, carbon-based conductive inks including carbon nanotube and graphene have been also formulated, which are tailorable and used for specific applications. Conductive oxide ceramics have been also used as ink materials, which are usually produced by doping in order to alter the cation or the anion lattice. Examples of doped conductive oxide ceramics are antimony tin oxide (ATO), fluorin tin oxide (FTO), indium tin oxide (ITO), aluminum zinc oxide (AZO) and gallium zinc oxide (GZO).
The semiconducting layer of the printed electronics, serves as the active layer, where most of the electric activity occurs. Silicon and germanium have been the most common semiconductor materials due to their physical stability and high-performance. In addition to these, many ceramic oxides can be used as semiconductor materials, such as tin oxide (SnO2), zinc oxide (ZnO), indium oxide (In2O3) and gallium oxide (Ga2O3). In printed electronics, semiconducting inks are usually formulated from polymer blends and appropriate solvents. Semiconductors printed with such inks can be used to realize both p-type and n-type materials. Polymers mostly using holes as carriers are p-type polymer semiconductors, the most promising types of which are polythiophenes (PT) and polyfluorenes (PF). Examples of PTs are poly(3-alkylthiophene) (P3AT), P3HT and poly(3,3'-dialkyltetrathiophene) (PQT). N-type polymer conductors use electrons as carriers sucg as poly(9,9-dioctylfluorene-co-bithiophene) (F8T2).
Dielectric inks are used as insulator and capacitor layers in printed electronics. In comparison with conducting or semiconducting inks, it is challenging to make and print dielectric inks since the dielectric layer requires enough thickness in order to prevent electric leakage. Additionally, ceramic oxides can be used as dielectric materials; however, they have a tendency to form pinholes and cracks. There is an abundance of polymers suitable as dielectrics, which have low surface roughness, surface trap density, concentration of impurities, cost and sintering temperatures. Additionally, they are also compatible with organic semiconductors. The most commonly used polymer dielectric materials are polymethyl methacrylate (PMMA), polyimide (PI), polyvinylphenol (PVP), polystyrene (PS), polylactic acid (PLA), polydimethylsiloxane (PDMS), polyvinylalcohol (PVA) and benzocyclobutene (BCB).
Currently, many electronic components have been prepared commercially through printed electronics technology, such as solar cells, electronic tags (RFID), thin-film transistors, organic light-emitting diodes, electrode circuits, sensors, etc. At the same time, with the expansion of the electronics industry and the rising demand for smart electronics, the global printed electronics market has been further promoted, showing a huge scientific as well as commercial value.