Crystalline thin films of organic semiconductors are a good candidate for field effect transistor (FET) materials in printed electronics. However, there are currently two main problems, which are associated with inhomogeneity and poor thermal durability of these films. Here we report that liquid crystalline materials exhibiting a highly ordered liquid crystal phase of smectic E (SmE) can solve both these problems. We design a SmE liquid crystalline material, 2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10), for FETs and synthesize it. This material provides uniform and molecularly flat polycrystalline thin films reproducibly when SmE precursor thin films are crystallized, and also exhibits high durability of films up to 200 °C. In addition, the mobility of FETs is dramatically enhanced by about one order of magnitude (over 10 cm2 V−1 s−1) after thermal annealing at 120 °C in bottom-gate-bottom-contact FETs. We anticipate the use of SmE liquid crystals in solution-processed FETs may help overcome upcoming difficulties with novel technologies for printed electronics.

液晶物質の有機トランジスタへの展開 ; Liquid Crystals for Organic Thin Film Transistors

Attributed to its advantages of super mechanical flexibility, very low-temperature processing, and compatibility with low cost and high throughput manufacturing, organic thin-film transistor (OTFT) technology is able to bring electrical, mechanical, and industrial benefits to a wide range of new applications by activating nonflat surfaces with flexible displays, sensors, and other electronic functions. Despite both strong application demand and these significant technological advances, there is still a gap to be filled for OTFT technology to be widely commercially adopted. This paper provides a comprehensive review of the current status of OTFT technologies ranging from material, device, process, and integration, to design and system applications, and clarifies the real challenges behind to be addressed.

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Current Status and Opportunities of Organic Thin-Film Transistor Technologies

The contact resistance in organic thin-film transistors (TFTs) is the limiting factor in the development of high-frequency organic TFTs. In devices fabricated in the inverted (bottom-gate) device architecture, staggered (top-contact) organic TFTs have usually shown or are predicted to show lower contact resistance than coplanar (bottom-contact) organic TFTs. However, through comparison of organic TFTs with different gate-dielectric thicknesses based on the small-molecule organic semiconductor 2,9-diphenyl-dinaphtho[2,3-b:2’,3’-f]thieno[3,2-b]thiophene, we show the potential for bottom-contact TFTs to have lower contact resistance than top-contact TFTs, provided the gate dielectric is sufficiently thin and an interface layer such as pentafluorobenzenethiol is used to treat the surface of the source and drain contacts. We demonstrate bottom-contact TFTs fabricated on flexible plastic substrates with record-low contact resistance (29 Ωcm), record subthreshold swing (62 mV/decade), and signal-propagation delays in 11-stage unipolar ring oscillators as short as 138 ns per stage, all at operating voltages of about 3 V. The widespread adoption of organic thin-film transistors (TFTs) in low-voltage high-frequency device applications is impeded by the contact resistance in the TFTs. Here, the authors report record-low contact resistance in bottom-gate, bottom-contact organic TFTs with an ultrathin gate dielectric.

Small contact resistance and high-frequency operation of flexible low-voltage inverted coplanar organic transistors

Flexible sensors have the potential to be seamlessly applied to soft and irregularly shaped surfaces such as the human skin or textile fabrics. This benefits conformability dependant applications including smart tattoos, artificial skins and soft robotics. Consequently, materials and structures for innovative flexible sensors, as well as their integration into systems, continue to be in the spotlight of research. This review outlines the current state of flexible sensor technologies and the impact of material developments on this field. Special attention is given to strain, temperature, chemical, light and electropotential sensors, as well as their respective applications.

Flexible Sensors—From Materials to Applications

Sensors based on conductive polymers and their composites: a review

Flexible ammonia (NH3) sensors were fabricated on polyethylene terephthalate substrate using poly(3, 4-ethylene-dioxythiophene): poly(styrenesulfonate) (PEDOT:PSS)/silver nanowire (AgNW) composite film as the active layer. With the AgNWs of optimized concentration being incorporated into the PEDOT:PSS film, the sensitivity of the devices was significantly improved. Even with simple digitally dispensed parallel structure electrodes, the device achieved excellent sensing performance, and was shown to be able to detect very low NH3 concentration below 500 ppb. The mechanism for the sensing performance improvement was revealed. The sensor also showed considerable selectivity with respect to water and common organic vapors. Finally, the sensor was integrated with a self-designed portable data acquisition system to monitor the freshness of pork, demonstrating its feasibility for inspecting the meat quality in the early stage.

Flexible Ammonia Sensor Based on PEDOT:PSS/Silver Nanowire Composite Film for Meat Freshness Monitoring

We report a strong enhancement of the light absorption in single coaxial nanowires (NWs) of Si core/dielectric shells. We have calculated the light absorption coefficient within the framework of the Lorenz-Mie light scattering theory and found out that it is greatly increased by effective light trapping in Si cores owing to dielectric shells, as compared to that in Si NWs. We show that the strong absorption of light stems mainly from off-resonance enhancement and also from resonance contribution. By optimally tuning the core radius, the shell thickness, and the shell refractive index, we have obtained ~102% increase of the photocurrent.

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Light Trapping in Single Coaxial Nanowires for Photovoltaic Applications

This letter realizes both large gate dielectric capacitance and reduced sub-gap density of states at the channel in the same organic field effect transistor (OFET) structure by adopting optimized low-k/high-k bilayer gate dielectric. Subthreshold swing ( SS ) as small as 64 mV dec $^{-1}$ can be achieved with a thick (>360 nm) gate dielectric layer. This is the smallest SS value reported so far for all reported low voltage OFETs, and is even comparable with that of 22 nm technology node Si-FETs. The device can thus be switched within a very small voltage swing of 0.8 V, while having an ON/OFF ratio larger than 105. The device also shows excellent operational and storage stabilities.

Fully Solution Processed Bottom-Gate Organic Field-Effect Transistor With Steep Subthreshold Swing Approaching the Theoretical Limit

Thin-film transistors (TFT) in hydrogenated amorphoussilicon, amorphousmetal oxide, andsmallmolecule and polymer organic semiconductors would all hold promise as potential device candidates to large area flexible electronics applications. A universal compact dc model was developed with a proper balance between the physical and mathematical approaches for these thin-film transistors (TFTs). It can capture the common key parameters used for device performance benchmarking of the different TFTs while being applicable to a wide range of TFT technologies in different materials and device structures. Based on this model, a user-friendly tool was developed to provide an interactive way for convenient parameter extraction. The model is continuous from the off-state and subthreshold regimes to the above-threshold regime, avoiding the convergence problems when being used in SPICE circuit simulations. Finally, for verification, it was implemented into a SPICE circuit simulator using Verilog-A to simulate a TFT circuit examplewith the simulated results agreeing verywell with the experimental measurements.

Universal Compact Model for Thin-Film Transistors and Circuit Simulation for Low-Cost Flexible Large Area Electronics

This paper presents a new voltage driving scheme with current feedback for polycrystalline silicon (poly-Si) thin-film transistors (TFTs) to drive active matrix organic light-emitting diode (AMOLED) displays. The pixel circuit is relatively simple, which is composed of 3 TFTs and 1 capacitor. Circuit simulation results show that the proposed driving scheme can effectively suppress the influence of nonidealities of typical poly-Si TFTs including kink effects, spatial nonuniformity of the electrical characteristics. This method also presents a settling time less than 3 $\mu {\hbox{s}}$ with panel load of 100 pF/1.5 ${\hbox{k}}\Omega $ at all OLED current levels.

Wenjiang Liu

Papers

Flexible Ammonia Sensor Based on PEDOT:PSS/Silver Nanowire Composite Film for Meat Freshness Monitoring

Light Trapping in Single Coaxial Nanowires for Photovoltaic Applications

Fully Solution Processed Bottom-Gate Organic Field-Effect Transistor With Steep Subthreshold Swing Approaching the Theoretical Limit

Universal Compact Model for Thin-Film Transistors and Circuit Simulation for Low-Cost Flexible Large Area Electronics

A New Voltage Driving Scheme to Suppress Non-Idealities of Polycrystalline Thin-Film Transistors for AMOLED Displays