This paper discusses the fundamental chemical and electrical properties, as well as the practical applications, of a digital skin device developed by the authors to provide tactile sensation for soft robots. Electronic artificial skin (e-skin) devices can electrically detect tactile sensations that humans typically perceive, such as pressure, slip, temperature, and humidity. To date, we have successfully demonstrated the detection of pressure, temperature, slip, humidity, hardness, and acceleration using our digital skin device. In this study, we specifically focus on the detection of pressure and slip as fundamental tactile modalities. Additionally, we present a wearable sensing system capable of accurately identifying object hardness by analyzing acquired tactile signals using artificial intelligence (machine learning).

The use of a top-gate/bottom-contact (TG/BC) configuration in organic thin-film transistors (OTFTs) is an effective strategy for achieving high electrical performance and facilitating device integration in organic electronic circuits. We found that TG/BC OTFTs incorporating spin-coated poly (3-hexylthiophene) (P3HT) films exhibited high field-effect mobilities, regardless of the substrate surface energy or the dielectric constant of the gate insulators. These results strongly suggest the formation of a highly ordered, edge-on-oriented structure at the surface of the P3HT films, which serves as the carrier transport region in TG/BC OTFTs. Solution-processed TG/BC OTFTs also demonstrated excellent electrical stability under gate bias stress, outperforming both bottom-gate OTFTs and inorganic TFTs. We further developed solution-processable nonvolatile memory devices by integrating polymer-based TG/BC OTFTs with the vertical phase separation observed in solution-processed organic blend films composed of a polymer insulator, such as polystyrene or poly(methyl methacrylate), and a soluble small-molecule semiconductor, TIPS-pentacene. The resulting solution-processed TG/BC P3HT OTFT memory exhibited a large threshold voltage shift of over 30 V when programmed under light illumination, due to the storage of photogenerated electrons in the organic floating-gate layer. Moreover, diketopyrrolopyrrole-based ambipolar polymer semiconductors enabled electrically programmable and erasable memory operation. By tuning the work functions of the gate electrodes, NAND-type memory operations were successfully demonstrated in OTFT memory devices connected in series.

Ultrathin perovskite solar cells (PSCs) fabricated on micron-thick plastic substrates have recently attracted significant attention due to their excellent flexibility and lightweight nature. By reducing the total substrate thickness to 3 μm, both mechanical flexibility and power-to-weight ratio were significantly improved compared to conventional thin-film photovoltaics. Our group has developed high-efficiency PSCs on such ultrathin substrates using a tin oxide (SnO₂)-based n-i-p structure. By employing thermally stable, transparent parylene-C/SU-8 substrates and flexible, amorphous ITO electrodes, our devices achieved an efficiency of 18.2%, comparable to that of PSCs on rigid glass substrates, while maintaining exceptional bending stability. These PSCs exhibit excellent mechanical durability, remaining stable even after bending deformations with a radius as small as 500 μm. Owing to their ultra-flexible and lightweight characteristics, ultrathin PSCs are being actively explored for use in wearable indoor energy sources, solar-powered drones, and space-based applications. Their flexibility, low weight, and high radiation tolerance against cosmic rays make ultrathin PSCs promising candidates for next-generation deployable solar paddles in space environments. This review outlines recent advances in ultrathin PSC technology, highlights our group’s work on high-efficiency SnO₂-based n-i-p structured ultrathin PSCs, and explores future pathways for their application in wearable electronics, aerospace, and space systems based on printed electronics techniques.

We have developed a sheet-type image sensor that enables high-resolution, high-speed reading. This sensor can capture high resolution images of fingerprints and veins for biometric authentication. In addition, it is capable of measuring pulse waves—a vital sign—and their spatial distribution. The sheet-type image sensor was fabricated by densely integrating a high-efficiency readout circuit, based on an active matrix of low-temperature polysilicon thin-film transistors, with a photodetector that employs a highly efficient organic semiconductor as its photosensitive layer. For fingerprint authentication, the sensor achieves a resolution of 508 dots per inch (dpi), meeting the standard requirement. The organic photodetector features a bulk heterostructured organic layer with high sensitivity to near-infrared light at a wavelength of 850 nm, and an external quantum efficiency exceeding 50%. Thanks to the 10 μm thickness of the polymer base material, the total thickness of the sensor is only 15 μm. This ultrathin form factor makes it easy to integrate into various devices and allows for attachment to curved surfaces, enhancing its versatility for applications in wearable and embedded systems

Stretchable displays are a promising technology for widening the application range from the current flat shapes to form-free 3D displays. Here, we introduce a highly stretchable display achieved through novel liquid metal wiring. This wiring enables three-dimensional dynamic deformation, such as concave, convex, and folding shapes. This unprecedented display has great potential to provide a completely novel media experience.