The field of flexible electronics has rapidly expanded over the last decades, pioneering novel applications, such as wearable and textile integrated devices, seamless and embedded patch-like systems, soft electronic skins, as well as imperceptible and transient implants. The possibility to revolutionize our daily life with such disruptive appliances has fueled the quest for electronic devices which yield good electrical and mechanical performance and are at the same time light-weight, transparent, conformable, stretchable, and even biodegradable. Flexible metal oxide semiconductor thin-film transistors (TFTs) can fulfill all these requirements and are therefore considered the most promising technology for tomorrow's electronics. This review reflects the establishment of flexible metal oxide semiconductor TFTs, from the development of single devices, large-area circuits, up to entirely integrated systems. First, an introduction on metal oxide semiconductor TFTs is given, where the history of the field is revisited, the TFT configurations and operating principles are presented, and the main issues and technological challenges faced in the area are analyzed. Then, the recent advances achieved for flexible n-type metal oxide semiconductor TFTs manufactured by physical vapor deposition methods and solution-processing techniques are summarized. In particular, the ability of flexible metal oxide semiconductor TFTs to combine low temperature fabrication, high carrier mobility, large frequency operation, extreme mechanical bendability, together with transparency, conformability, stretchability, and water dissolubility is shown. Afterward, a detailed analysis of the most promising metal oxide semiconducting materials developed to realize the state-of-the-art flexible p-type TFTs is given. Next, the recent progresses obtained for flexible metal oxide semiconductor-based electronic circuits, realized with both unipolar and complementary technology, are reported. In particular, the realization of large-area digital circuitry like flexible near field communication tags and analog integrated circuits such as bendable operational amplifiers is presented. The last topic of this review is devoted for emerging flexible electronic systems, from foldable displays, power transmission elements to integrated systems for large-area sensing and data storage and transmission. Finally, the conclusions are drawn and an outlook over the field with a prediction for the future is provided.

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Metal oxide semiconductor thin-film transistors for flexible electronics

A water-based silver-nanowire (AgNW) ink is formulated for screen printing. Screen-printed AgNW patterns have uniform sharp edges, ≈50 μm resolution, and electrical conductivity as high as 4.67 × 10(4) S cm(-1) . The screen-printed AgNW patterns are used to fabricate a stretchable composite conductor, and a fully printed and intrinsically stretchable thin-film transistor array is also realized.

A Water-Based Silver-Nanowire Screen-Print Ink for the Fabrication of Stretchable Conductors and Wearable Thin-Film Transistors

The p-type inorganic semiconductor CuGaO2 as a hole-transporting layer (HTL) in perovskite solar cells (PSCs) provides higher carrier mobility, better-energy level matching, and superior stability, as well as low-temperature processing technique. Compared to organic HTL, a very competitive PCE of 18.51% with long-term stability is achieved. This indicates that CuGaO2 is a promising HTL for efficient and stable PSCs.

CuGaO2: A Promising Inorganic Hole-Transporting Material for Highly Efficient and Stable Perovskite Solar Cells

Gallium oxide (Ga2O3) is an emerging wide bandgap semiconductor that has attracted a large amount of interest due to its ultra-large bandgap of 4.8 eV, a high breakdown field of 8 MV/cm, and high thermal stability. These properties enable Ga2O3 a promising material for a large range of applications, such as high power electronic devices and solar-blind ultraviolet (UV) photodetectors. In the past few years, a significant process has been made for the growth of high-quality bulk crystals and thin films and device optimizations for power electronics and solar blind UV detection. However, many challenges remain, including the difficulty in p-type doping, a large density of unintentional electron carriers and defects/impurities, and issues with the device process (contact, dielectrics, and surface passivation), and so on. The purpose of this article is to provide a timely review on the fundamental understanding of the semiconductor physics and chemistry of Ga2O3 in terms of electronic band structures, optical properties, and chemistry of defects and impurity doping. Recent progress and perspectives on epitaxial thin film growth, chemical and physical properties of defects and impurities, p-type doping, and ternary alloys with In2O3 and Al2O3 will be discussed.

Recent progress on the electronic structure, defect, and doping properties of Ga2O3

The mechanical and electronic properties of two-dimensional materials make them promising for use in flexible electronics1–3. Their atomic thickness and large-scale synthesis capability could enable the development of ‘smart skin’1,3–5, which could transform ordinary objects into an intelligent distributed sensor network6. However, although many important components of such a distributed electronic system have already been demonstrated (for example, transistors, sensors and memory devices based on two-dimensional materials1,2,4,7), an efficient, flexible and always-on energy-harvesting solution, which is indispensable for self-powered systems, is still missing. Electromagnetic radiation from Wi-Fi systems operating at 2.4 and 5.9 gigahertz8 is becoming increasingly ubiquitous and would be ideal to harvest for powering future distributed electronics. However, the high frequencies used for Wi-Fi communications have remained elusive to radiofrequency harvesters (that is, rectennas) made of flexible semiconductors owing to their limited transport properties9–12. Here we demonstrate an atomically thin and flexible rectenna based on a MoS2 semiconducting–metallic-phase heterojunction with a cutoff frequency of 10 gigahertz, which represents an improvement in speed of roughly one order of magnitude compared with current state-of-the-art flexible rectifiers9–12. This flexible MoS2-based rectifier operates up to the X-band8 (8 to 12 gigahertz) and covers most of the unlicensed industrial, scientific and medical radio band, including the Wi-Fi channels. By integrating the ultrafast MoS2 rectifier with a flexible Wi-Fi-band antenna, we fabricate a fully flexible and integrated rectenna that achieves wireless energy harvesting of electromagnetic radiation in the Wi-Fi band with zero external bias (battery-free). Moreover, our MoS2 rectifier acts as a flexible mixer, realizing frequency conversion beyond 10 gigahertz. This work provides a universal energy-harvesting building block that can be integrated with various flexible electronic systems. Integration of an ultrafast flexible rectifier made from a two-dimensional material with a flexible antenna achieves wireless energy harvesting of Wi-Fi radiation, which could power future flexible electronic systems.

Two-dimensional MoS2-enabled flexible rectenna for Wi-Fi-band wireless energy harvesting.

Mechanically flexible mobile phones have been long anticipated due to the rapid development of thin-film electronics in the last couple of decades. However, to date, no such phone has been developed, largely due to a lack of flexible electronic components that are fast enough for the required wireless communications, in particular the speed-demanding front-end rectifiers. Here Schottky diodes based on amorphous indium-gallium-zinc-oxide (IGZO) are fabricated on flexible plastic substrates. Using suitable radio-frequency mesa structures, a range of IGZO thicknesses and diode sizes have been studied. The results have revealed an unexpected dependence of the diode speed on the IGZO thickness. The findings enable the best optimized flexible diodes to reach 6.3 GHz at zero bias, which is beyond the critical benchmark speed of 2.45 GHz to satisfy the principal frequency bands of smart phones such as those for cellular communication, Bluetooth, Wi-Fi and global satellite positioning.

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Flexible indium?gallium?zinc?oxide Schottky diode operating beyond 2.45 GHz

Indium–gallium–zinc-oxide thin-film transistors (TFTs) with solution-processed, high-capacitance Al x O y gate dielectrics have been fabricated at room temperature. The morphology and electrical properties of the anodized, ultra-thin Al x O y film have been studied. Several anodization voltages were used to create the gate dielectrics and the results showed that the TFTs gated with aluminum oxide anodized at 2.3 V (~3 nm) exhibited the best performance. The TFTs operate at an ultra-low voltage of 1 V with a high current on/off ratio >105 and a subthreshold swing ( SS ) as low as 68 mV/dec, which is very close to the theoretical limit of SS at 300 K. As a result, the presented devices possess a great potential for low-power electronics.

One-Volt IGZO Thin-Film Transistors With Ultra-Thin, Solution-Processed Al x O y Gate Dielectric

Lattice vibrations of highly a-axis oriented CoFe2O4 (CFO) films have been investigated by Raman scattering in the temperature range of 80-873 K. The five phonon modes T1 g (2), T1 g (3), E g , A1 g (1), A1 g (2), and their evolutions can be uniquely distinguished. It was found that an electron transfer between Co2+ and Fe3+ cations occurs in octahedral sites at about 173 K. The structure disorder in the CFO films appears with increasing the temperature, which indicates the cation migration between tetrahedral and octahedral sites. The phenomena suggest the structural transformation trend from inverse spinel to normal spinel at the elevated temperatures.

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Temperature dependent phonon Raman scattering of highly a-axis oriented CoFe2O4 inverse spinel ferromagnetic films grown by pulsed laser deposition

Oxide semiconductors are regarded as promising materials for large-area and/or flexible electronics. In this work, a ring oscillator based on n-type indium-gallium-zinc-oxide (IGZO) and p-type tin monoxide (SnO) is presented. The IGZO thin-film transistor (TFT) shows a linear mobility of 11.9 cm2/(V∙s) and a threshold voltage of 12.2 V. The SnO TFT exhibits a mobility of 0.51 cm2/(V∙s) and a threshold voltage of 20.1 V which is suitable for use with IGZO TFTs to form complementary circuits. At a supply voltage of 40 V, the complementary inverter shows a full output voltage swing and a gain of 24 with both TFTs having the same channel length/channel width ratio. The three-stage ring oscillator based on IGZO and SnO is able to operate at 2.63 kHz and the peak-to-peak oscillation amplitude reaches 36.1 V at a supply voltage of 40 V. The oxide-based complementary circuits, after further optimization of the operation voltage, may have wide applications in practical large-area flexible electronics.

https://www.mdpi.com/1996-1944/10/3/319/pdf

High Performance Complementary Circuits Based on p-SnO and n-IGZO Thin-Film Transistors.

An extremely sensitive dependence of the electronic properties of SnOx film on sputtering deposition power is discovered experimentally. The carrier transport sharply switches from n-type to p-type when the sputtering power increases by less than 2%. The best n-type carrier transport behavior is observed in thin-film transistors (TFTs) produced at a sputtering power just below a critical value (120 W). In contrast, at just above the critical sputtering power, the p-type behavior is found to be the best with the TFTs showing the highest on/off ratio of 1.79 × 104 and the best subthreshold swing among all the sputtering powers that we have tested. A further increase in the sputtering power by only a few percent results in a drastic drop in on/off ratio by more than one order of magnitude. Scanning electron micrographs, x-ray diffraction spectra, x-ray photoelectron spectroscopy, as well as TFT output and transfer characteristics are analyzed. Our studies suggest that the sputtering power critically affects the stoichiometry of the SnOx film.

Yunpeng Li

Papers

Flexible indium?gallium?zinc?oxide Schottky diode operating beyond 2.45 GHz

One-Volt IGZO Thin-Film Transistors With Ultra-Thin, Solution-Processed Al x O y Gate Dielectric

Temperature dependent phonon Raman scattering of highly a-axis oriented CoFe2O4 inverse spinel ferromagnetic films grown by pulsed laser deposition

High Performance Complementary Circuits Based on p-SnO and n-IGZO Thin-Film Transistors.

Extremely Sensitive Dependence of SnOx Film Properties on Sputtering Power.