Drop‐on‐demand inkjet printing is one of the most attractive techniques from a manufacturing perspective due to the possibility of fabrication from a digital layout at ambient conditions, thus leading to great opportunities for the realization of low‐cost and flexible thin‐film devices. Over the past decades, a variety of inkjet‐printed applications including thin‐film transistors (TFTs), radio‐frequency identification devices, sensors, and displays have been explored. In particular, many research groups have made great efforts to realize high‐performance TFTs, for application as potential driving components of ubiquitous wearable electronics. Although there are still challenges to enable the commercialization of printed TFTs beyond laboratory‐scale applications, the field of printed TFTs still attracts significant attention, with remarkable developments in soluble materials and printing methodology. Here, recent progress in printing‐based TFTs is presented from materials to applications. Significant efforts to improve the electrical performance and device‐yield of printed TFTs to match those of counterparts fabricated using conventional deposition or photolithography methods are highlighted. Moreover, emerging low‐dimension printable semiconductors, including carbon nanotubes and transition metal dichalcogenides as well as mature semiconductors, and new‐concept printed switching devices, are also discussed.

Designing softness into robots holds great potential for augmenting robotic compliance in dynamic, unstructured environments. However, despite the body's softness, existing models mostly carry inherent hardness in their driving parts, such as pressure-regulating components and rigid circuit boards. This compliance gap can frequently interfere with the robot motion and makes soft robotic design dependent on rigid assembly of each robot component. We present a skin-like electronic system that enables a class of wirelessly activated fully soft robots whose driving part can be softly, compactly, and reversibly assembled. The proposed system consists of two-part electronic skins (e-skins) that are designed to perform wireless communication of the robot control signal, namely, "wireless inter-skin communication," for untethered, reversible assembly of driving capability. The physical design of each e-skin features minimized inherent hardness in terms of thickness (<1 millimeter), weight (~0.8 gram), and fragmented circuit configuration. The developed e-skin pair can be softly integrated into separate soft body frames (robot and human), wirelessly interact with each other, and then activate and control the robot. The e-skin-integrated robotic design is highly compact and shows that the embedded e-skin can equally share the fine soft motions of the robot frame. Our results also highlight the effectiveness of the wireless interskin communication in providing universality for robotic actuation based on reversible assembly.

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Softening of thermoelectric generators facilitates conformal contact with arbitrary-shaped heat sources, which offers an opportunity to realize self-powered wearable applications. However, existing wearable thermoelectric devices inevitably exhibit reduced thermoelectric conversion efficiency due to the parasitic heat loss in high-thermal-impedance polymer substrates and poor thermal contact arising from rigid interconnects. Here, we propose compliant thermoelectric generators with intrinsically stretchable interconnects and soft heat conductors that achieve high thermoelectric performance and unprecedented conformability simultaneously. The silver-nanowire-based soft electrodes interconnect bismuth-telluride-based thermoelectric legs, effectively absorbing strain energy, which allows our thermoelectric generators to conform perfectly to curved surfaces. Metal particles magnetically self-assembled in elastomeric substrates form soft heat conductors that significantly enhance the heat transfer to the thermoelectric legs, thereby maximizing energy conversion efficiency on three-dimensional heat sources. Moreover, automated additive manufacturing paves the way for realizing self-powered wearable applications comprising hundreds of thermoelectric legs with high customizability under ambient conditions.

We report high performance and stable inkjet-printed stretchable silver electrodes on wave structured elastomeric substrates. Highly conductive silver electrodes were deposited directly on a ultraviolet ozone treated polydimethylsiloxane (PDMS) substrates having vertical wavy structures. Adhesion between printed silver lines and PDMS surface has been enhanced by intentionally roughened PDMS surface with wire-electro discharge machined aluminum mold. During slow (16.7 μm/s) stretching test, resistance of the printed silver electrode was increased only by three times at 30% tensile strain. Inkjet-printed silver electrodes also showed good mechanical stability during 1000-time fast (1 mm/s) cycling test with 10% tensile strain, showing maximum resistance change of less than three times.

Two-dimensional (2D) transition-metal dichalcogenides (TMDCs) have gained considerable attention as an emerging semiconductor due to their promising atomically thin film characteristics with good field-effect mobility and a tunable band gap energy. However, their electronic applications have been generally realized with conventional inorganic electrodes and dielectrics implemented using conventional photolithography or transferring processes that are not compatible with large-area and flexible device applications. To facilitate the advantages of 2D TMDCs in practical applications, strategies for realizing flexible and transparent 2D electronics using low-temperature, large-area, and low-cost processes should be developed. Motivated by this challenge, we report fully printed transparent chemical vapor deposition (CVD)-synthesized monolayer molybdenum disulfide (MoS) phototransistor arrays on flexible polymer substrates. All the electronic components, including dielectric and electrodes, were directly deposited with mechanically tolerable organic materials by inkjet-printing technology onto transferred monolayer MoS, and their annealing temperature of <180 °C allows the direct fabrication on commercial flexible substrates without additional assisted-structures. By integrating the soft organic components with ultrathin MoS, the fully printed MoS phototransistors exhibit excellent transparency and mechanically stable operation.

Gallium doped zinc oxide (GZO) thin films have been prepared by a simple sol–gel spin coating technique. XRD results showed the preferential c-axis orientation of the crystallites and the presence of the wurtzite phase of ZnO. A lowest resistivity of 3.3 × 10−3 Ω cm was obtained for the ZnO film doped with 2 at% of Ga after post-annealing at 500 °C for 45 min in a H2 atmosphere. All the films showed more than 80% of transparency in the entire visible region. Blue shifting of the optical band gap was observed with an increase in Ga doping, which can be explained on the basis of the Burstein–Moss effect. OLED devices were fabricated using 2 at% Ga-doped ZnO thin films as anodes. Preliminary results obtained demonstrated that spin-coated GZO films can be used as a promising TCO for optoelectronic device applications.

The interface engineering of two-dimensional (2D) transition-metal dichalcogenides (TMDs) has been regarded as a promising strategy to modulate their outstanding electrical and optoelectronic properties because of their inherent 2D nature and large surface-to-volume ratio. In particular, introducing organic molecules and polymers directly onto the surface of TMDs has been explored to passivate the surface defects or achieve better interfacial properties with neighboring surfaces efficiently, thus leading to great opportunities for the realization of high-performance TMD-based applications. This review provides recent progress in the interface engineering of TMDs with organic molecules and polymers corresponding to the modulation of their electrical and optoelectronic characteristics. Depending on the interfaces between the surface of TMDs and dielectric, conductive contacts or the ambient environment, we present various strategies to introduce an organic interlayer from materials to processing. In addition, the role of native defects on the surface of TMDs, such as adatoms or vacancies, in determining their electrical characteristics is also discussed in detail. Finally, the future challenges and opportunities associated with the interface engineering are highlighted.