MoS2 thin films are directly synthesized over FTO/glass substrate in a one-step process and used as an efficient electron transport layer (ETL) for perovskite solar cells (PSCs).

Conductive and stretchable electrodes that can be printed directly on a stretchable substrate have drawn intensive attention for wearable electronics and electronic skins. Printable inks containing liquid metal (LM) are strong candidates for these applications, but the insulating oxide skin forming around LM particles limits their conductivity. This study reveals that hydrogen doping (H-doping) introduced by ultrasonication in the presence of aliphatic polymers makes the oxide skin highly conductive and deformable. X-ray photoelectron spectroscopy and atom probe tomography confirms hydrogen doping, and first-principles calculations are used to rationalize the obtained conductivity. Printed circuit lines show metallic conductivity (25,000 S/cm), excellent electromechanical decoupling at 500% uniaxial stretching, mechanical resistance to scratches, and long-term stability in wide ranges of temperature and humidity. The self-passivation of the printed lines allows direct printing of 3D circuit lines and double-layer planar coils that are used as stretchable inductive strain sensors.Stretchable electronic devices have received widespread attention for potential uses in healthcare monitoring 1-3 , electronic skins 4,5 , and wearable haptic devices 6,7 . One of the key technological issues in stretchable electronics is the fabrication of stretchable circuit lines, for which several characteristics are requested simultaneously; metallic conductivity, negligible resistance changes under deformations, electrical stability in harsh environments, printing of complicated circuit designs, passivation 8 , and good adhesion to elastomeric substrates 9 . Serpentine and buckled metal interconnections have achieved a few of the above requests such as metallic conductivity, small resistance changes, some degree of deformability, and environmental stability 10 . Other progress has been with conductive elastomer composites with respect to high

We present a new device structure for effective energy harvesting from human body movement using a ZnO nanogenerator. This is the first report on both piezoelectric and triboelectric effects in the same device. The fabricated device structure consists of a double-sided ZnO nanowire array sandwiched between gold coated ZnO nanowire arrays using polydimethylsiloxane. The peak open circuit voltage (V oc ) and short circuit current (I sc ) were recorded as 30 V and 300 nA respectively with power density of 0.390 mW cm 22 , when folding/bending the nanogenerator with the fingers. The ZnO nanogenerator was effectively utilized to harvest biomechanical energy from human body movements like stretching, folding, and pressing. The power density of fabricated nanogenerator was 9.49, 8.19 and 115 nW cm 22 by stretching, folding and pressing the fingers, respectively. The acquired output was used to drive a commercial light emitting diode. The generated output is combination of the piezoelectric effect from ZnO nanowire and the triboelectric effect from polydimethylsiloxane. This feasibility study substantiates human body movement as a source for energy harvesting. The energy generated by human activities (body movement) was sufficient to operate low power wearable electronic devices.

The fabrication of deformable devices has been explored by interconnecting nonstretchable unit devices with stretchable conductors or by developing stretchable unit devices consisting of all stretchable device components such as electrodes, active channels, and dielectric layers. Most researches have followed the first approach so far, and the researches based on the second approach are at the very beginning stage. This paper discusses the perspectives of the second approach, specifically focusing on the polymer semiconductor channel layers, that is expected to facilitate high density device integration in addition to large area devices including polymer solar cells and light-emitting diodes. Three different routes are suggested as separate sections according to the principles imparting stretchability to polymer semiconductor layers: structural configurations of rigid semiconductors, two-dimensional network structure of semiconductors on elastomer substrates, and ductility enhancement of semiconductor films. Each section includes two subsections divided by the methodological difference. This Perspective ends with discussion on the future works for the routes and the challenges related to other device components.

With the advent of foldable electronics, it is necessary to develop a technology ensuring foldability when the circuit lines are placed on the topmost substrate rather than in the neutral plane used in the present industry. Considering the potential technological impacts, conversion of the conventional printed circuit boards to foldable ones is most desirable to achieve the topmost circuitry. This study realizes this unconventional conversion concept by coating an ultrathin anisotropic conductive film (UACF) on a printed metal circuit board. This study presents rapid large-area synthesis of hydrogenated amorphous carbon (a-C:H) thin films and their use as the UACF. Since the synthesized a-C:H thin film has electrical transparency, the metal/a-C:H hybrid board reflects the complexity of the underlying metal circuit board. The a-C:H thin film electrically connects the cracked area of the metal line; thus, the hybrid circuit board is foldable without resistance change during repeated folding cycles. The metal/UACF hybrid circuit board can be applied to the fabrication of various foldable electronic devices.

Ultrathin transition metal dichalcogenides (TMDs) have exotic electronic properties. With success in easy synthesis of high quality TMD thin films, the potential applications will become more viable in electronics, optics, energy storage, and catalysis. Synthesis of TMD thin films has been mostly performed in vacuum or by thermolysis. So far, there is no solution phase synthesis to produce large-area thin films directly on target substrates. Here, this paper reports a one-step quick synthesis (within 45-90 s) of TMD thin films (MoS , WS , MoSe , WSe , etc.) on solid substrates by using microwave irradiation on a precursor-containing electrolyte solution. The numbers of the quintuple layers of the TMD thin films are precisely controllable by varying the precursor's concentration in the electrolyte solution. A photodetector made of MoS thin film comprising of small size grains shows near-IR absorption, supported by the first principle calculation, exhibits a high photoresponsivity (>300 mA W ) and a fast response (124 µs). This study paves a robust way for the synthesis of various TMD thin films in solution phases.

Although wafer‐scale single‐grain thin films of 2D metal chalcogenides (MCs) have been extensively sought after during the last decade, the grain size of the MC thin films is still limited in the sub‐millimeter scale. A general strategy of synthesizing wafer‐scale single‐grain MC thin films by using commercial wafers (Si, Ge, GaAs) both as metal source and epitaxial collimator is presented. A new mechanism of single‐grain thin‐film formation, surface diffusion, and epitaxial self‐planarization is proposed, where chalcogen elements migrate preferentially along substrate surface and the epitaxial crystal domains flow to form an atomically smooth thin film. Through synchrotron X‐ray diffraction and high‐resolution scanning transmission electron microscopy, the formation of single‐grain Si2Te3, GeTe, GeSe, and GaTe thin films on (111) Si, Ge, and (100) GaAs is verified. The Si2Te3 thin film is used to achieve transfer‐free fabrication of a high‐performance bipolar memristive electrical‐switching device.

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