Takafumi Uemura

This paper presents a carbon-based organic field-effect transistor (OFET) developed using screen printing. Carbon materials are electrically conductive, chemically stable, and durable in outdoor settings, rendering carbon-based OFETs highly promising for environmental, Internet of Things, and biosensing applications. However, their temperature-dependent characteristics remain unexplored. We demonstrated carbon-based OFETs operating at 3 V and evaluated their electrical performance across a temperature range relevant to practical environments. By incorporating the observed temperature characteristics into a device simulation model, we achieved excellent reproducibility in circuit simulations. This study advances the understanding and future development of environmentally friendly, printed electronic devices.

Flexible imagers are currently under intensive development as versatile optical sensor arrays, designed to capture images of surfaces and internals, irrespective of their shape. A significant challenge in developing flexible imagers is extending their detection capabilities to encompass a broad spectrum of infrared light, particularly terahertz (THz) light at room temperature. This advancement is crucial for thermal and biochemical applications. In this study, a flexible infrared imager is designed using uncooled carbon nanotube (CNT) sensors and organic circuits. The CNT sensors, fabricated on ultrathin 2.4 µm substrates, demonstrate enhanced sensitivity across a wide infrared range, spanning from near‐infrared to THz wavelengths. Moreover, they retain their characteristics under bending and crumpling. The design incorporates light‐shielded organic transistors and circuits, functioning reliably under light irradiation, and amplifies THz detection signals by a factor of 10. The integration of both CNT sensors and shielded organic transistors into an 8 × 8 active‐sensor matrix within the imager enables sequential infrared imaging and nondestructive assessment for heat sources and in‐liquid chemicals through wireless communication systems. The proposed imager, offering unique functionality, shows promise for applications in biochemical analysis and soft robotics.

The use of piezoelectric film for pulse-wave measurement is advantageous in terms of power consumption, but it has limitations in terms of sensitivity and noise resistance. This study described a high-sensitivity pulse-wave sensor that used a piezoelectric film. Furthermore, we proposed a three-dimensional gel structure to improve the sensor's skin contact condition and measurement sensitivity through structural innovations. Our approach was to increase the strain of the piezoelectric film by utilizing a structure with a tapered rate, which increased the sensitivity of the sensor structure. The subject was a healthy adult male (24 years old). We also fabricated a prototype pulse-wave sensor with the proposed structure and continuously measured pulse waves at multiple locations on the radial artery of the wrist. Through signal processing, we were able to obtain measurement data that were effective for monitoring pulse wave propagation and showed that fluctuations in pulse wave velocity could be detected.

The monitoring of microvolt‐level biosignals such as electroencephalograms requires the application of low‐noise signal amplifier circuits, which, for single‐use cases, must be fabricated for disposability using low‐cost manufacturing techniques. One promising solution for the production of low‐cost amplifier circuits for digital biosensing is the emerging technology of low‐noise printed circuits. Here, a low‐noise‐electrode printing process for organic transistors that can carry out precision measurement of brain activity using a low‐noise organic amplifier is proposed. In the transistor fabrication process, Ag electrodes are processed via flat stamp parallel printing, and fine‐process optimization to minimize the charge‐trapping effect enables a three‐order reduction in transistor noise. The low‐noise transistors are used to produce an organic pseudo‐complementary metal‐oxide‐semiconductor amplifier with a small noise level of 2.2 µV_p‐p at 10 Hz, which enables precision brain wave monitoring in a close correlation with signals obtained using a commercialized measurement system.

Flexible electronics has paved the way toward the development of next-generation wearable and implantable healthcare devices, including multimodal sensors. Integrating flexible circuits with transducers on a single substrate is desirable for processing vital signals. However, the trade-off between low power consumption and high operating speed is a major bottleneck. Organic thin-film transistors (OTFTs) are suitable for developing flexible circuits owing to their intrinsic flexibility and compatibility with the printing process. We used a photoreactive insulating polymer poly((+/-)endo,exo-bicyclo[2.2.1]hept-ene-2,3-dicarboxylic acid, diphenylester) (PNDPE) to modulate the power consumption and operating speed of ultraflexible organic circuits fabricated on a single substrate. The turn-on voltage (Von) of the p and n-type OTFTs was controlled through a nanoscale interfacial photochemical reaction. The time-of-flight secondary ion mass spectrometry revealed the preferential occurrence of the PNDPE photochemical reaction in the vicinity of the semiconductor- dielectric interface. The power consumption and operating speed of the ultraflexible complementary inverters were tuned by a factor of 6 and 4, respectively. The minimum static power consumption was 30 +/- 9 pW at transient and 4 +/- 1 pW at standby. Furthermore, within the tuning range of the operating speed and at a supply voltage above 2.5 V, the minimum stage delay time was of the order of hundreds of microseconds. We demonstrated electromyogram measurements to emphasize the advantage of the nanoscale interfacial photochemical reaction. Our study suggests that a nanoscale interfacial photochemical reaction can be employed to develop imperceptible and wearable multimodal sensors with organic signal processing circuits that exhibit low power consumption.

A flexible organic thin-film transistor (OTFT) with gold nanowire (AuNW) networks as source/drain/gate electrodes (AuNW-OTFT) was fabricated and characterized. Its fabrication was enabled by the successful formation of AuNW network electrodes on a flexible thin-film substrate using the thermal pressing transfer method. The fabricated AuNW-OTFT exhibited typical p-type characteristics (mobility: 6.0 × 10⁻² cm² V⁻¹ s⁻¹, on/off ratio: 10⁵) and a lower contact resistance than that of an OTFT with vacuum-deposited Au electrodes in the same configuration. Furthermore, the AuNW-OTFT demonstrated high mechanical durability, as proven by the absence of significant characteristic changes before and after bending (bending radius:0.8 mm).

Flexible electronics have gained considerable attention for application in wearable devices. Organic transistors are potential candidates to develop flexible integrated circuits (ICs). A primary technique for maximizing their reliability, gain, and operation speed is the modulation of charge‐carrier behavior in the respective transistors fabricated on the same substrate. In this work, heterogeneous functional dielectric patterns (HFDP) of ultrathin polymer gate dielectrics of poly((±)endo,exo‐bicyclo[2.2.1]hept‐ene‐2,3‐dicarboxylic acid, diphenylester) (PNDPE) are introduced. The HFDP that are obtained via the photo‐Fries rearrangement by ultraviolet radiation in the homogeneous PNDPE provide a functional area for charge‐carrier modulation. This leads to programmable threshold voltage control over a wide range (−1.5 to +0.2 V) in the transistors with a high patterning resolution, at 2 V operational voltage. The transistors also exhibit high operational stability over 140 days and under the bias‐stress duration of 1800 s. With the HFDP, the performance metrics of ICs, for example, the noise margin and gain of the zero‐V_GS load inverters and the oscillation frequency of ring oscillators are improved to 80%, 1200, and 2.5 kHz, respectively, which are the highest among the previously reported zero‐V_GS‐based organic circuits. The HFDP can be applied to much complex and ultraflexible ICs.