mechanical flexibility, higher charge-carrier mobility than that of organic materials or amorphous silicon, and low processing temperature as compared to amorphous and polycrystalline silicon TFTs. [18][19][20][21] Parallel to the achievements of metaloxide TFTs, organic materials, carbon nanotubes (CNTs), and 2D materials have also been constantly developed. For recent reviews, see refs. [22,23]. The material approaches differ in their characteristics such as charge-carrier type (electrons or holes), charge-carrier mobility, environmental and electrical stability, mechanical flexibility, operation voltage, thermal budget of processing, technology readiness level for high-throughput fabrication, and sustainability (environmental footprint of materials and processes). There are also approaches that combine different material classes, for example, to implement complementary logic by combining the predominantly n-type oxide TFTs with p-type organic TFTs. [24] In this paper, we focus on the metal-oxide TFTs that are among the highest-performing alternatives for flexible thin-film devices that can be patterned by printing and that have already reached the maturity of being used in product fabrication (e.g., flat-panel displays). [2] The first metal-oxide-based TFT employed tin oxide (SnO 2 ) as the semiconductor and was invented by Klasens et al. in 1964, opening the door for research into transparent metal oxide TFTs. [25] Owing to the technology and material-processing limitations at that time, only a few studies focused on this area until 2003. During that year, zinc-oxide-based (ZnO) TFTs with mobility of µ = 2.5 cm 2 V −1 s −1 were successfully manufactured by Hoffman et al., Carcia et al., In the same year, Nomura et al. made a breakthrough with the first use of the quaternary-oxide semiconductor IGZO in a single-crystalline form to fabricate the TFTs, rather than some more conventional binary oxides such as ZnO, indium oxide (In 2 O 3 ), and SnO 2 . This kind of device that required annealing at 1400 °C displayed better electrical properties than its binary counterparts, with 80 cm 2 V −1 s −1 mobility and a 10 6 on/off ratio. [29] The following year in 2004, they reported flexible TFTs fabricated at room temperature on polymer film based on amorphous IGZO (a-IGZO) with saturation mobility of ≈6-9 cm 2 V −1 s −1 . [30] The discovery of a-IGZO raised substantial interest in research on amorphous metal-oxide TFTs and their low-temperature Metal-oxide-semiconductor-based thin-film transistors (TFTs) are exploited in display backplanes and X-ray detectors fabricated by vacuum deposition and lithographic patterning. However, there is growing interest to use scalable printing technologies to lower the environmental impact and cost of processing. There have been substantial research efforts on oxide dielectric and semiconductor materials and their interfaces. Materials for the source/ drain (S/D) contact electrodes and their interface to the semiconductor have received less attention, particularly concerning the usa...

Besides the metal oxide thin-film transistors (TFTs) in flat-panel displays that are fabricated using vacuum-processes, there is a growing interest in the fabrication of metal oxide TFTs by means of scalable, low-cost solution and printing processes for applications such as flexible displays and biosensors. Although devices with printed semiconductor and gate insulator can exhibit good electrical performance, source/drain-contacts (S/D) printed from silver (Ag) nanoparticles (NPs) typically suffer from deteriorated electrical characteristics and stability problems. On the other hand, metals providing good contacts, such as aluminum (Al), titanium (Ti) and molybdenum (Mo), cannot be formed as air-stable NPs. To overcome these issues, we have developed a patterning method based on high-resolution reverse-offset printing (ROP) of a sacrificial polymer resist layer. ROP delivers patterns with micrometer-level resolution and steep sidewalls, which are ideal for patterning vacuum-deposited metal contacts at high resolution via lift-off process. Solution-processed indium oxide (In2O3) TFTs were successfully fabricated by using ROP lift-off process for patterning of gate and S/D-electrodes using Al. The fabricated In2O3-based TFTs with Al S/D-contacts exhibit good uniformity, constant mobility (μsat) ~ 2 cm2/(Vs) over a wide range of width/length-ratios (W/L) and almost zero turn-on voltage (Von) ~ -0.2 V. TFTs down to 5 µm channel lengths were successfully patterned. Further development of the fabrication process could lead to flexible fully-print-patterned high-resolution TFT backplanes for flexible displays, biosensors, photosensors and X-ray detectors.