A Review of Low‐Temperature Solution‐Processed Metal Oxide Thin‐Film Transistors for Flexible Electronics

Wide bandgap oxide semiconductors constitute a unique class of materials that combine properties of electrical conductivity and optical transparency. They are being widely used as key materials in optoelectronic device applications, including flat-panel displays, solar cells, OLED, and emerging flexible and transparent electronics. In this article, an up-to-date review on both the fundamental understanding of materials physics of oxide semiconductors, and recent research progress on design of new materials and high-performing thin film transistor (TFT) devices in the context of fundamental understanding is presented. In particular, an in depth overview is first provided on current understanding of the electronic structures, defect and doping chemistry, optical and transport properties of oxide semiconductors, which provide essential guiding principles for new material design and device optimization. With these principles, recent advances in design of p-type oxide semiconductors, new approaches for achieving cost-effective transparent (flexible) electrodes, and the creation of high mobility 2D electron gas (2DEG) at oxide surfaces and interfaces with a wealth of fascinating physical properties of great potential for novel device design are then reviewed. Finally, recent progress and perspective of oxide TFT based on new oxide semiconductors, 2DEG, and low-temperature solution processed oxide semiconductor for flexible electronics will be reviewed.

Wide Bandgap Oxide Semiconductors: from Materials Physics to Optoelectronic Devices

Ultrafast Laser Pulses Enable One-Step Graphene Patterning on Woods and Leaves for Green Electronics

Narrow-Band Green-Emitting Sr2MgAl22O36:Mn2+ Phosphors with Superior Thermal Stability and Wide Color Gamut for Backlighting Display Applications

‘Ideal’ transparent p-type semiconductors are required for the integration of high-performance thin-film transistors (TFTs) and circuits. Although CuI has recently attracted attention owing to its excellent opto-electrical properties, solution processability, and low-temperature synthesis, the uncontrolled copper vacancy generation and subsequent excessive hole doping hinder its use as a semiconductor material in TFT devices. In this study, we propose a doping approach through soft chemical solution process and transparent p-type Zn-doped CuI semiconductor for high-performance TFTs and circuits. The optimised TFTs annealed at 80 °C exhibit a high hole mobility of over 5 cm2 V−1 s−1 and high on/off current ratio of ~107 with good operational stability and reproducibility. The CuI:Zn semiconductors show intrinsic advantages for next-generation TFT applications and wider applications in optoelectronics and energy conversion/storage devices. This study paves the way for the realisation of transparent, flexible, and large-area integrated circuits combined with n-type metal-oxide semiconductor. Designing efficient thin-film transistors and circuits based on transparent p-type semiconductors remains a challenge. Here, the authors propose a solution-based doping approach to realize high performance transparent inorganic p-type semiconductors (Zn-doped CuI) by spin coating at 80 C with good operational stability.

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High-performance p-channel transistors with transparent Zn doped-CuI.

Neuromorphic computation, which emulates the signal process of the human brain, is considered to be a feasible way for future computation. Realization of dynamic modulation of synaptic plasticity and accelerated learning, which could improve the processing capacity and learning ability of artificial synaptic devices, is considered to further improve energy efficiency of neuromorphic computation. Nevertheless, realization of dynamic regulation of synaptic weight without an external regular terminal and the method that could endow artificial synaptic devices with the ability to modulate learning speed have rarely been reported. Furthermore, finding suitable materials to fully mimic the response of photoelectric stimulation is still challenging for photoelectric synapses. Here, a floating gate synaptic transistor based on an L-type ligand-modified all-inorganic CsPbBr3 perovskite quantum dots is demonstrated. This work provides first clear experimental evidence that the synaptic plasticity can be dynamically regulated by changing the waveforms of action potential and the environment temperature and both of these parameters originate from and are crucial in higher organisms. Moreover, benefiting from the excellent photoelectric properties and stability of quantum dots, a temperature-facilitated learning process is illustrated by the classical conditioning experiment with and without illumination, and the mechanism of synaptic plasticity is also demonstrated. This work offers a feasible way to realize dynamic modulation of synaptic weight and accelerating the learning process of artificial synapses, which showed great potential in the reduction of energy consumption and improvement of efficiency of future neuromorphic computing.

Synaptic Transistor Capable of Accelerated Learning Induced by Temperature-Facilitated Modulation of Synaptic Plasticity.

In this study, a femtosecond (fs) laser is proposed to pattern and anneal a metal oxide active layer for the facile fabrication of metal oxide thin-film transistor (TFT) arrays. Compared with conventional photolithography and lift-off techniques, fs-laser ablation significantly simplified the patterning process due to its advantages such as no contact, high yield and high resolution. Uniform patterned arrays with sharp edges and small feature size (down to 32 μm) were achieved using fs-laser ablation. Moreover, fs-laser annealing was also applied for rapid conversion of the precursor to metal oxide lattices. The results showed that an increase in laser intensity lead to an improvement in charge carrier mobility and an improvement in the on–off current ratio of indium zinc oxide (IZO) thin film transistors (TFTs) along with a negative shift of the threshold voltage. X-ray photoelectron spectroscopy (XPS) analysis indicates that an increase in laser intensity enhanced the breakdown of weak or metastable chemical bonds within IZO films, leading to the removal of hydroxide-related (OH) species and enhancement of metal oxide composition and oxygen vacancies (Ov); it facilitated carrier transport and led to an average mobility up to 9.0 cm2 V−1 s−1. Furthermore, compared with thermal annealing (TA), fs-laser annealing could reduce the thermal budget and more effectively realize the dehydroxylation behavior for OH-related species, resulting in higher charge mobility. These results clearly demonstrated that fs-laser ablation and annealing could provide a promising tool to significantly simplify the fabrication of metal oxide TFT arrays due to their advantages such as high resolution, high yield and low cost. Hence, these techniques show great potential to be applicable in the fabrication of large-area metal oxide TFT arrays for use in display devices and circuits.

Solution-processed metal oxide arrays using femtosecond laser ablation and annealing for thin-film transistors

Luminescent materials such as Cd-based quantum dots, CuInS2 quantum dots, and perovskite-based materials play an important role as an exceptional class of optoelectronic materials due to their unique and novel characteristics for optoelectronic applications. However, a key challenge for luminescent materials and devices is their poor long-term stability, which is typically attributed to the irreversible changes of surface ligands or chemical structure against oxygen and moisture when exposed to ambient atmosphere. Thus, this review mainly focuses on recent important work on the stability-enhanced strategies of luminescent materials. Various strategies for their fabrication, including surface engineering and surface coating routes have been summarized, followed by their advantages and challenges. Furthermore, the corresponding optoelectronic applications of these promising stability-enhanced luminescent materials in light-emitting diodes, photodetectors, solid-state lighting chips, and solar cells were also reviewed. Afterward, the outlook and perspective are presented.

A review of stability-enhanced luminescent materials: fabrication and optoelectronic applications

Floating gate transistor photomemory (FGTPM) has been regarded as one of the most prospective nonvolatile photomemory devices because of its compatibility with transistor-based circuits, nondestruc...

Nonvolatile Multilevel Photomemory Based on Lead-Free Double Perovskite Cs2AgBiBr6 Nanocrystals Wrapped Within SiO2 as a Charge Trapping Layer.

Metal–oxide–semiconductors (MOS) have become an ideal candidate for the next-generation optoelectronic device applications. However, high processing temperature and complicate processes are still tremendous challenge in developing solution-based complementary MOS (CMOS) inverter. In this article, for the first time, femtosecond (fs) laser was used to realize controllable annealing for solution-based CMOS inverter. The achievement of nonstoichiometric p-type oxide thin films was ascribed to the photo-assisted conversion of precursor to metal–oxide (M–O) lattices along with the formation of atom vacancies in oxide lattice due to carrier excitation and relaxation using laser annealing (LA). The field effect mobility of the p- and n-type M–O thin-film transistors (TFTs) with inkjet printing (IJP) and LA was 0.91 and 7.07 cm2/ $\text{V}\cdot \text{s}$ , respectively. Moreover, location control capacity was exploited to separately anneal the p- and n-type oxide deposited with IJP for the fabrication of TFTs, which significantly simplified the fabrication process of the inverter. CMOS inverter with high noise margin and moderate voltage gain above 10 was also obtained. Our work significantly improved the ability to selectively manipulate the functionality and properties of the irradiated materials. The results demonstrated that logic gates based on all-oxide can be large area integrated using our strategy, exhibiting attractive properties and applications of the CMOS integrated circuits in oxide electronics.

Gengxu Chen

Papers

Synaptic Transistor Capable of Accelerated Learning Induced by Temperature-Facilitated Modulation of Synaptic Plasticity.

Solution-processed metal oxide arrays using femtosecond laser ablation and annealing for thin-film transistors

A review of stability-enhanced luminescent materials: fabrication and optoelectronic applications

Nonvolatile Multilevel Photomemory Based on Lead-Free Double Perovskite Cs2AgBiBr6 Nanocrystals Wrapped Within SiO2 as a Charge Trapping Layer.

Solution-Processed Oxide Complementary Inverter via Laser Annealing and Inkjet Printing