Wavy Architecture Thin‐Film Transistor for Ultrahigh Resolution Flexible Displays

A novel wavy‐shaped thin‐film‐transistor (TFT) architecture, capable of achieving 70% higher drive current per unit chip area when compared with planar conventional TFT architectures, is reported for flexible display application. The transistor, due to its atypical architecture, does not alter the turn‐on voltage or the OFF current values, leading to higher performance without compromising static power consumption. The concept behind this architecture is expanding the transistor's width vertically through grooved trenches in a structural layer deposited on a flexible substrate. Operation of zinc oxide (ZnO)‐based TFTs is shown down to a bending radius of 5 mm with no degradation in the electrical performance or cracks in the gate stack. Finally, flexible low‐power LEDs driven by the respective currents of the novel wavy, and conventional coplanar architectures are demonstrated, where the novel architecture is able to drive the LED at 2 × the output power, 3 versus 1.5 mW, which demonstrates the potential use for ultrahigh resolution displays in an area efficient manner.     

Thin‐Film Transistors

Thin‐film transistors (TFTs) matured later than silicon integrated circuits, but in the past 15 years the technology has grown into a huge industry based on display applications, with amorphous and polycrystalline silicon as the incumbent technology. Recently, an intense search has developed for new materials and new fabrication techniques that can improve the performance, lower manufacturing cost, and enable new functionality. There are now many new options – organic semiconductor (OSCs), metal oxides, nanowires, printing technology as well as thin‐film silicon materials with new properties. All of the new materials have something to offer but none is entirely without technical problems.     

Multifunctional Organic‐Semiconductor Interfacial Layers for Solution‐Processed Oxide‐Semiconductor Thin‐Film Transistor

The stabilization and control of the electrical properties in solution‐processed amorphous‐oxide semiconductors (AOSs) is crucial for the realization of cost‐effective, high‐performance, large‐area electronics. In particular, impurity diffusion, electrical instability, and the lack of a general substitutional doping strategy for the active layer hinder the industrial implementation of copper electrodes and the fine tuning of the electrical parameters of AOS‐based thin‐film transistors (TFTs). In this study, the authors employ a multifunctional organic‐semiconductor (OSC) interlayer as a solution‐processed thin‐film passivation layer and a charge‐transfer dopant. As an electrically active impurity blocking layer, the OSC interlayer enhances the electrical stability of AOS TFTs by suppressing the adsorption of environmental gas species and copper‐ion diffusion. Moreover, charge transfer between the organic interlayer and the AOS allows the fine tuning of the electrical properties and the passivation of the electrical defects in the AOS TFTs. The development of a multifunctional solution‐processed organic interlayer enables the production of low‐cost, high‐performance oxide semiconductor‐based circuits.     

Fabrication of Flexible MoS2 Thin‐Film Transistor Arrays for Practical Gas‐Sensing Applications

By combining two kinds of solution‐processable two‐dimensional materials, a flexible transistor array is fabricated in which MoS₂ thin film is used as the active channel and reduced graphene oxide (rGO) film is used as the drain and source electrodes. The simple device configuration and the 1.5 mm‐long MoS₂ channel ensure highly reproducible device fabrication and operation. This flexible transistor array can be used as a highly sensitive gas sensor with excellent reproducibility. Compared to using rGO thin film as the active channel, this new gas sensor exhibits much higher sensitivity. Moreover, functionalization of the MoS₂ thin film with Pt nanoparticles further increases the sensitivity by up to ～3 times. The successful incorporation of a MoS₂ thin‐film into the electronic sensor promises its potential application in various electronic devices.     

High‐Performance,Ultrathin, Ultraflexible Organic Thin‐Film Transistor Array Via Solution Process

Ultrathin organic thin‐film transistors (OTFTs) have received extensive attention due to their outstanding advantages, such as extreme flexibility, good conformability, ultralight weight, and compatibility with low‐cost and large‐area solution‐processed techniques. However, compared with the rigid substrates, it still remains a challenge to fabricate high‐performance ultrathin OTFTs. In this study, a high‐performance ultrathin 2,7‐dioctyl[1]benzothieno[3,2‐b][1]benzothiophene (C8‐BTBT) OTFT array is demonstrated via a simple spin‐coating method, with mobility as high as 11 cm² V⁻¹ s⁻¹ (average mobility: 7.22 cm² V⁻¹ s⁻¹), on/off current ratio of over 10⁶, switching current of >1 mA, and a good yield ratio as high as 100%. The ultrathin thickness at ≈380 nm and the ultralight weight at ≈0.89 g m⁻² enable the free‐standing OTFTs to imperceptibly adhere onto human skin, and even a damselfly wing without affecting its flying. More importantly, the OTFTs show good electrical characteristics and mechanical stability when conformed onto the curved surfaces and even folded in a book after 100 folding cycles. These results illustrate the broad application potential of this simply fabricated ultrathin OTFT in next‐generation electronics such as foldable displays and wearable devices.     

Understanding the Crystal Packing and Organic Thin‐Film Transistor Performance in Isomeric Guest–Host Systems

In order to understand how additives influence the structure and electrical properties of active layers in thin‐film devices, a compositionally identical but structurally different guest–host system based on the syn and anti isomers of triethylsilylethynyl anthradithiophene (TES ADT) is systematically explored. The mobility of organic thin‐film transistors (OTFTs) comprising anti TES ADT drops with the addition of only 0.01% of the syn isomer and is pinned at the mobility of OTFTs having pure syn isomer after the addition of only 10% of the isomer. As the syn isomer fraction increases, intermolecular repulsion increases, resulting in a decrease in the unit‐cell density and concomitant disordering of the charge‐transport pathway. This molecular disorder leads to an increase in charge trapping, causing the mobility of OTFTs to drop with increasing syn‐isomer concentration. Since charge transport is sensitive to even minute fractions of molecular disorder, this work emphasizes the importance of prioritizing structural compatibility when choosing material pairs for guest–host systems.     

High‐Performance Single‐Crystalline Perovskite Thin‐Film Photodetector

The best performing modern optoelectronic devices rely on single‐crystalline thin‐film (SC‐TF) semiconductors grown epitaxially. The emerging halide perovskites, which can be synthesized via low‐cost solution‐based methods, have achieved substantial success in various optoelectronic devices including solar cells, lasers, light‐emitting diodes, and photodetectors. However, to date, the performance of these perovskite devices based on polycrystalline thin‐film active layers lags behind the epitaxially grown semiconductor devices. Here, a photodetector based on SC‐TF perovskite active layer is reported with a record performance of a 50 million gain, 70 GHz gain‐bandwidth product, and a 100‐photon level detection limit at 180 Hz modulation bandwidth, which as far as we know are the highest values among all the reported perovskite photodetectors. The superior performance of the device originates from replacing polycrystalline thin film by a thickness‐optimized SC‐TF with much higher mobility and longer recombination time. The results indicate that high‐performance perovskite devices based on SC‐TF may become competitive in modern optoelectronics.     

Tailoring Thin‐Film Piezoelectrics for Crash Sensing

Crash sensing and its assessment play a pivotal role in autonomous vehicles for preventing fatal casualties. Existing crash sensors are severely bottlenecked by sluggish response time, rigid mechanical components, and space constraints. Miniaturized sensors embedded with custom‐tailored nanomaterials upholds potential to overcome these limitations. In this article, piezoelectric Zinc‐Oxide thin film as a crash sensing layer is integrated onto a flexible metal‐alloy cantilever. Material characterization studies are conducted to confirm piezoelectric property of sputtered ZnO film. The piezoelectric d ₃₁ coefficient value of ZnO film was 7.2 pm V^–1. The ZnO sensing element is firmly mounted on a scaled car model and used in a crash sensing experimental set‐up. A comprehensive theoretical analysis for two different real scenarios (nearly elastic and nearly inelastic collision) of crash events followed by experimental study is discussed. The crash sensor's output exhibits a linear relationship with magnitude of impact forces experienced at crash events. The response time of ZnO crash sensor is 18.2 ms, and it exhibits a sensitivity of 28.7 mV N^–1. The developed crash sensor has potential to replace bulk material sensors owing to its faster response time, high sensitivity, and compactness as the demand for crash sensors in next‐generation automobile industries is progressively growing.     

Exploring Two‐Dimensional Transport Phenomena in Metal Oxide Heterointerfaces for Next‐Generation,High‐Performance,Thin‐Film Transistor Technologies

In the last decade, metal oxides have emerged as a fascinating class of electronic material, exhibiting a wide range of unique and technologically relevant characteristics. For example, thin‐film transistors formed from amorphous or polycrystalline metal oxide semiconductors offer the promise of low‐cost, large‐area, and flexible electronics, exhibiting performances comparable to or in excess of incumbent silicon‐based technologies. Atomically flat interfaces between otherwise insulating or semiconducting complex oxides, are also found to be highly conducting, displaying 2‐dimensional (2D) charge transport properties, strong correlations, and even superconductivity. Field‐effect devices employing such carefully engineered interfaces are hoped to one day compete with traditional group IV or III–V semiconductors for use in the next‐generation of high‐performance electronics. In this Concept article we provide an overview of the different metal oxide transistor technologies and potential future research directions. In particular, we look at the recent reports of multilayer oxide thin‐film transistors and the possibility of 2D electron transport in these disordered/polycrystalline systems and discuss the potential of the technology for applications in large‐area electronics.