Self-Healable Conductive Hydrogels with High Stretchability and Ultralow Hysteresis for Soft Electronics.

Stretchable sensors based on conductive hydrogels have attracted considerable attention for wearable electronics. However, their practical applications have been limited by the low sensitivity, high hysteresis, and long response times of the hydrogels. In this study, we developed high-performance poly(vinyl alcohol) (PVA)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) based hydrogels post-treated with NaCl, which showed excellent mechanical properties, fast electrical response, and ultralow hysteresis properties. The hydrogels also demonstrated excellent self-healing properties with electrical and mechanical properties comparable to those of the original hydrogel and more than 150% elongation at break after the self-healing process. The high performance of the optimized hydrogels was attributed to the enhanced intermolecular forces between the PVA matrix and PEDOT:PSS, the favorable conformational change of the PEDOT chains, and an increase in localized charges in the hydrogel networks. The hydrogel sensors were capable of tracking large human motion and subtle muscle action in real time with high sensitivity, a fast response time (0.88 s), and low power consumption (<180 µW). Moreover, the sensor was able to monitor human respiration due to chemical changes in the hydrogel. These highly robust, stretchable, conductive, and self-healing PVA/PEDOT:PSS hydrogels, therefore, show great application potential as wearable sensors for monitoring human activity.

Universal Stretchable Conductive Cellulose/PEDOT:PSS Hybrid Films for Low Hysteresis Multifunctional Stretchable Electronics.

Skin-attachable conductive materials have attracted significant attention for use in wearable devices and physiological monitoring applications. Soft, skin-like conductive films must have excellent mechanical and electrical characteristics with on-skin conformability, stretchability, and robustness to detect body motion and biological signals. In this study, a conductive, stretchable, hydro-biodegradable, and highly robust cellulose/poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) hybrid film is fabricated. Through the synergetic interplay of a conductivity enhancer, nonionic fluorosurfactant, and surface modifier, the mechanical and electrical properties of the stretchable hybrid film are greatly improved. The stretchable cellulose/PEDOT:PSS hybrid film achieves a limited resistance change of only 1.21-fold after 100 stretch-release cycles (30% strain) with exceptionally low hysteresis, demonstrating its great potential as a stretchable electrode for stretchable electronics. In addition, the film shows excellent biodegradability, promising environmental friendliness, and safety benefits. High-performance stretchable cellulose/PEDOT:PSS hybrid films, which have high biocompatibility and sensitivity, are applied to human skin to serve as on-skin multifunctional sensors. The conformally mounted on-skin sensors are capable of continuously monitoring human physiological signals, such as body motions, drinking, respiration rates, vocalization, humidity, and temperature, with high sensitivity, fast responses, and low power consumption (21 µW). The highly conductive hybrid films developed in this study can be integrated as both stretchable electrodes and multifunctional healthcare monitoring sensors. We believe that the highly robust stretchable, conductive, biodegradable, skin-attachable cellulose/PEDOT:PSS hybrid films are worthy candidates as promising soft conductive materials for stretchable electronics.

Stretchable and Conductive Cellulose/Conductive Polymer Composite Films for On-Skin Strain Sensors.

Conductive composite materials have attracted considerable interest of researchers for application in stretchable sensors for wearable health monitoring. In this study, highly stretchable and conductive composite films based on carboxymethyl cellulose (CMC)-poly (3,4-ethylenedioxythiopehe):poly (styrenesulfonate) (PEDOT:PSS) (CMC-PEDOT:PSS) were fabricated. The composite films achieved excellent electrical and mechanical properties by optimizing the lab-synthesized PEDOT:PSS, dimethyl sulfoxide, and glycerol content in the CMC matrix. The optimized composite film exhibited a small increase of only 1.25-fold in relative resistance under 100% strain. The CMC-PEDOT:PSS composite film exhibited outstanding mechanical properties under cyclic tape attachment/detachment, bending, and stretching/releasing tests. The small changes in the relative resistance of the films under mechanical deformation indicated excellent electrical contacts between the conductive PEDOT:PSS in the CMC matrix, and strong bonding strength between CMC and PEDOT:PSS. We fabricated highly stretchable and conformable on-skin sensors based on conductive and stretchable CMC-PEDOT:PSS composite films, which can sensitively monitor subtle bio-signals and human motions such as respiratory humidity, drinking water, speaking, skin touching, skin wrinkling, and finger bending. Because of the outstanding electrical properties of the films, the on-skin sensors can operate with a low power consumption of only a few microwatts. Our approach paves the way for the realization of low-power-consumption stretchable electronics using highly stretchable CMC-PEDOT:PSS composite films.

Highly stretchable and robust transparent conductive polymer composites for multifunctional healthcare monitoring.

Soft, stretchable, conductive thin films have propelled to the forefront of applications in stretchable sensors for on-skin health monitoring. Stretchable conductive films require high conformability, stretchability, and mechanical/chemical stability when integrated into the skin. Here, we present a highly stretchable, conductive, and transparent natural rubber/silver nanowire (AgNW)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) composite film. Overcoating the PEDOT:PSS layer results in outstanding mechanical robustness and chemical stability by suppressing the mechanical and chemical degradation of the nanowire networks. Moreover, the introduction of the organic surface modifier enhances the bonding strength between the natural rubber substrate and AgNW at the interface. The highly conformable composite films are integrated into multifunctional on-skin sensors for monitoring various human motions and biological signals with low-power consumption. We believe that the highly stretchable, robust, and conformable natural rubber/AgNW/PEDOT:PSS composite film can offer new opportunities for next-generation wearable sensors for body motion and physiological monitoring.

Mitigating the Undesirable Chemical Reaction between Organic Molecules for Highly Efficient Flexible Organic Photovoltaics.

Organic photovoltaics (OPVs) with nonfullerene acceptors (NFAs) feature excellent device performance and device stability. However, they are facing problems when the amine-rich polyelectrolytes are used as cathode interfacial layers. In this work, a small molecule, ethanedithiol (EDT) at the polyethyleneimine ethoxylated (PEIE)/active layer interface is inserted for mitigating the undesirable reaction between amine-rich groups and electron-acceptor moieties in NFA. The main role of EDT is to passivate the PEIE surface and prevent electron flow to NFA and the unwanted reaction can be mitigated. It improves the performance of OPV devices by reducing the work function, decreasing trap-assisted recombination, and improving electron-mobility. As a result, the flexible device with the PEIE interfacial layer with a power conversion efficiency (PCE) of 7.20% can be improved to 10.11% after the inclusion of EDT. Moreover, EDT-modified device can retain 98.18% after it is bent for 200 cycles and can maintain 80.83% of its initial PCE under continuous light illuminated in ambient conditions without any encapsulation. Based on these findings, the proposed strategy constitutes a crucial step toward highly efficient flexible OPVs.

Indoor Organic Photovoltaics for Self-Sustaining IoT Devices: Progress, Challenges and Practicalization.

Indoor photovoltaics (IPVs) have great potential to provide a self-sustaining power source for Internet-of-Things (IoT) devices. The rapid growth in demand for low-power IoT devices for indoor application not only boosts the development of high-performance IPVs, but also promotes the electronics and semiconductor industry for the design and development of ultra-low-power IoT systems. In this Review, the recent progress in IPV technologies, design rules, market trends, and future prospects for highly efficient indoor photovoltaics are discussed. Special attention is given to the progress and development of organic photovoltaics (OPVs), which demonstrate great possibilities for IPVs, owing to their bandgap tunability, high absorbance coefficient, semitransparency, solution processability, and easy large-area manufacturing on flexible substrates. Highly efficient indoor organic photovoltaics (IOPVs) can be realized through designing efficient donor and acceptor absorber materials that have good spectral responses in the visible region and better energy-aligned interfacial layers, and through modulation of optical properties. Interfacial engineering, photovoltage losses, device stability, and large-area organic photovoltaic modules are surveyed to understand the mechanisms of efficient power conversion and challenges for IOPVs under indoor conditions as a self-sustaining power source for IoT devices. Finally, the prospects for further improve in IOPV device performance and practical aspects of integrating IOPVs in low-power IoT devices are discussed.

3D-printed Cu2O photoelectrodes for photoelectrochemical water splitting.

Photoelectrochemical (PEC) water splitting is an alternative to fossil fuel combustion involving the generation of renewable hydrogen without environmental pollution or greenhouse gas emissions. Cuprous oxide (Cu2O) is a promising semiconducting material for the simple reduction of hydrogen from water, in which the conduction band edge is slightly negative compared to the water reduction potential. However, the solar-to-hydrogen conversion efficiency of Cu2O is lower than the theoretical value due to a short carrier-diffusion length under the effective light absorption depth. Thus, increasing light absorption in the electrode-electrolyte interfacial layer of a Cu2O photoelectrode can enhance PEC performance. In this study, a Cu2O 3D photoelectrode comprised of pyramid arrays was fabricated using a two-step method involving direct-ink-writing of graphene structures. This was followed by the electrodeposition of a Cu current-collecting layer and a p-n homojunction Cu2O photocatalyst layer onto the printed structures. The performance for PEC water splitting was enhanced by increasing the total light absorption area (A a) of the photoelectrode via controlling the electrode topography. The 3D photoelectrode (A a = 3.2 cm2) printed on the substrate area of 1.0 cm2 exhibited a photocurrent (I ph) of -3.01 mA at 0.02 V (vs. RHE), which is approximately three times higher than that of a planar photoelectrode with an A a = 1.0 cm2 (I ph = -0.91 mA). Our 3D printing strategy provides a flexible approach for the design and the fabrication of highly efficient PEC photoelectrodes.

Continuous 1D-Metallic Microfibers Web for Flexible Organic Solar Cells.

We report the use of a continuous 1D-metallic microfibers web (MFW) as transparent electrode for organic solar cells (OSCs). The MFW electrode can be produced with a process that involves simple electrospinning and wet etching of metal thin film. Au MFW exhibits a maximum optical transmittance of 90.8% (at 15 Ω/sq of the sheet resistance) and excellent mechanical flexibility. The MFW structure has an average width in the range from 4 to 6 µm and a junction-free structure, resulting in very smooth surface roughness. The OSCs with Au MFW electrode exhibited a higher power conversion efficiency (PCE) of 3.50% than the device with an indium tin oxide electrode (PCE = 3.20%). The optical modeling calculation showed that the Au MFW electrode induced light scattering and improved the light absorption in the active layer, resulting in an improved PCE in the OSCs.

Enhanced Photoelectrochemical Solar Water Splitting Using a Platinum-Decorated CIGS/CdS/ZnO Photocathode.

A Cu(InGa)Se2 film was modified with CdS/ZnO for application to solar water splitting. Platinum was electrodeposited on the ZnO layer as a hydrogen evolution catalyst. The effects of the electroplating time and acidity level of the electrolyte on the photocurrent density were studied. The highest photocurrent density of -32.5 mA/cm(2) under 1.5 AM illumination was achieved with an electroplating time of 30 min at a pH of 9. This photocurrent density is higher than those reported in previous studies. The markedly high performance of the CIGS/CdS/ZnO photocathode was rationalized in terms of its type II cascade structure that facilitated efficient charge separation at the interface junction.

Selective modulation of charge-carrier transport of a photoanode in a photoelectrochemical cell by a graphitized fullerene interfacial layer.

We show that a graphitic carbon interfacial layer, derived from C70 by annealing at 500 °C, results in a significant increase in the attainable photocurrent of a photoelectrochemical cell that contains a WO3 -functionalized fluorine-doped tin oxide (FTO) photoanode. Time-resolved photoluminescence spectroscopy, photoconductive atomic force microscopy, Hall measurements, and electrochemical impedance spectroscopy show that the increase in photocurrent is the result of fast and selective electron transport from optically excited WO3 through the graphitic carbon interfacial layer to the FTO-coated glass electrode. Thus the energy efficiency of perspective solar-to-fuel devices can be improved by modification of the interface of semiconductors and conducting substrate electrodes by using graphitized fullerene derivatives.

Role of additional PCBM layer between ZnO and photoactive layers in inverted bulk-heterojunction solar cells.

In order to induce greater light absorption, nano-patterning is often applied to the metal-oxide buffer layer in inverted bulk-heterojunction(BHJ) solar cells. However, current homogeneity was significantly disturbed at the interface, leading to an efficiency that was not fully optimized. In this work, an additional PC61BM layer was inserted between the ZnO ripple and the photoactive layer to enhance the electron extraction. The insertion of additional PC61BM layer provided substantial advantages in the operation of inverted BHJ solar cells; specifically, it enhanced current homogeneity and lowered accumulation and trapping of photogenerated charges at the ZnO interface. Inclusion of the additional PC61BM layer led to effective quenching of electron-hole recombination by a reduction in the number of accumulated charges at the surface of ZnO ripples. This resulted in a 16% increase in the efficiency of inverted BHJ solar cells to 7.7%, compared to solar cells without the additional PC61BM layer.

Surface modification of a ZnO electron-collecting layer using atomic layer deposition to fabricate high-performing inverted organic photovoltaics.

A ripple-structured ZnO film as the electron-collecting layer (ECL) of an inverted organic photovoltaic (OPV) was modified by atomic layer deposition (ALD) to add a ZnO thin layer. Depositing a thin ZnO layer by ALD on wet-chemically prepared ZnO significantly increased the short-circuit current (Jsc) of the OPV. The highest power conversion efficiency (PCE) of 7.96% with Jsc of 17.9 mA/cm2 was observed in the inverted OPV with a 2-nm-thick ALD-ZnO layer, which quenched electron-hole recombination at surface defects of ZnO ripples. Moreover, an ALD-ZnO layer thinner than 2 nm made the distribution of electrical conductivity on the ZnO surface more uniform, enhancing OPV performance. In contrast, a thicker ALD-ZnO layer (5 nm) made the two-dimensional distribution of electrical conductivity on the ZnO surface more heterogeneous, reducing the PCE. In addition, depositing an ALD-ZnO thin layer enhanced OPV stability and initial performance. We suggest that the ALD-ZnO layer thickness should be precisely controlled to fabricate high-performing OPVs.

Electrochemical synthesis of CdTe/SWNT hybrid nanostructures and their tunable electrical and optoelectrical properties.

A facile electrodeposition technique was utilized to deposit single-walled carbon nanotubes (SWNTs) with cadmium telluride (CdTe) with well-controlled size, density, surface morphology, and composition. By controlling the applied charge, the morphology of these hybrid nanostructures was altered from CdTe nanoparticles on SWNTs to SWNT/CdTe core/shell nanostructures and the composition of the CdTe nanoparticles was altered from Te-rich (29 at% Cd) to Cd-rich (79 at% Cd) CdTe by adjusting the deposition potential. The electrical and optoelectrical properties of these hybrid nanostructures showed that photo-induced current can be tuned by tailoring the conductivity type (n-type or p-type), morphology, and size of the CdTe nanostructures, with a maximum photosensitivity (ΔI/I(0)) of about 30% for SWNT/Cd-rich CdTe (n-type) core/shell nanostructures. This work demonstrates a novel approach for synthesizing metal chalcogenide/SWNT hybrid nanostructures for various electrical and optoelectrical applications.

A high-performing nanostructured TiO2 filter for volatile organic compounds using atomic layer deposition.

Novel nanostructured gas filtering systems with TiO(2) thin films using atomic layer deposition (ALD) were developed for volatile organic compounds. A superior toluene adsorption efficiency was found for the nanostructured TiO(2) thin films.

ZnO nanosheets decorated with CdSe and TiO2 for the architecture of dye-sensitized solar cells.

Pure and TiO2- and CdSe-deposited ZnO nanosheets aligned vertically to the surface of ITO (Indium tin oxide) are prepared using electrodeposition, which is used for building blocks of dye sensitized solar cell. A significant improvement in the photovoltaic efficiency can be obtained by depositing TiO2 or CdSe on ZnO. Photoluminescence spectra show that the TiO2 and CdSe nanostructures suppress the recombination of the electron-hole pair of ZnO. We suggest that the interface charge transfer at TiO2-ZnO and CdSe-ZnO should be responsible for the suppression of the electron-hole pair recombination and enhanced solar cell efficiency by TiO2 and CdSe nanostructures.

Model catalysts of supported Au nanoparticles and mass-selected clusters.

In surface science, much effort has gone into obtaining a deeper understanding of the size-selectivity of nanocatalysts. In this article, electronic and chemical properties of various model catalysts consisting of Au are reported. Au supported by oxide surfaces becomes inert towards chemisorption and oxidation as the particle size became smaller than a critical size (2-3 nm). The inertness of these small Au nanoparticles is due to the electron-deficient nature of smaller Au nanoparticles, which is a result of metal-substrate charge transfer. Properties of Au clusters smaller than ∼20 atoms were shown to be non-scalable, i.e., every atom can drastically change the chemical properties of the clusters. Moreover, clusters with the same size can show dissimilar properties on various substrates. These recent endeavours show that the activity of a catalyst can be tuned by varying the substrate or by varying the cluster size on an atom-by-atom basis.

Electronic and geometric properties of Au nanoparticles on Highly Ordered Pyrolytic Graphite (HOPG) studied using X-ray Photoelectron Spectroscopy (XPS) and Scanning Tunneling Microscopy (STM).

Au nanoparticles grown on mildly sputtered Highly Ordered Pyrolytic Graphite (HOPG) surfaces were studied using Scanning Tunneling Microscopy (STM) and X-ray Photoelectron Spectroscopy (XPS). The results were compared with those of Ag nanoparticles on the same substrate. By varying the defect densities of HOPG and the Au coverages, one can create Au nanoparticles in various sizes. At high Au coverages, the structures of the Au films significantly deviate from the ideal truncated octahedral form: the existence of many steps between different Au atomic layers can be observed, most likely due to a high activation barrier of the diffusion of Au atoms across the step edges. This implies that the particle growth at room temperature is strongly limited by kinetic factors. Hexagonal shapes of Au structures could be identified, indicating preferential growth of Au nanostructures along the (111) direction normal to the surface. In the case of Au, XPS studies reveal a weaker core level shift with decreasing particle size compared to the 3d level in similarly sized Ag particles. Also taking into account the Auger analysis of the Ag particles, the core level shifts of the metal nanoparticles on HOPG can be understood in terms of the metal/substrate charge transfer. Ag is (partially) positively charged, whereas Au negatively charged on HOPG. It is demonstrated that XPS can be a useful tool to study metal-support interactions, which plays an important role for heterogeneous catalysis, for example.