Po-Kang Yang

Triboelectric–Pyroelectric–Piezoelectric Hybrid Cell for High‐Efficiency Energy‐Harvesting and Self‐Powered Sensing

A triboelectric-pyroelectric-piezoelectric hybrid cell, consisting of a triboelectric nanogenerator and a pyroelectric-piezoelectric nanogenerator, is developed for highly efficient mechanical energy harvesting through multiple mechanisms. The excellent performance of the hybrid cell enhances the energy-harvesting efficiency significantly (by 26.2% at 1 kΩ load resistance), and enables self-powered sensing, which will lead to a variety of advanced applications.

Paper-based origami triboelectric nanogenerators and self-powered pressure sensors.

Discovering renewable and sustainable power sources is indispensable for the development of green electronics and sensor networks. In this paper, we present origami triboelectric nanogenerators (TENGs) using paper as the starting material, with a high degree of flexibility, light weight, low cost, and recyclability. Slinky- and doodlebug-shaped TENGs can be easily fabricated by properly folding printer papers. The as-fabricated TENGs are capable of harvesting ambient mechanical energy from various kinds of human motions, such as stretching, lifting, and twisting. The generated electric outputs have been used to directly light-up commercial LEDs. In addition, the as-fabricated TENGs can also serve as self-powered pressure sensors.

Robust Triboelectric Nanogenerator Based on Rolling Electrification and Electrostatic Induction at an Instantaneous Energy Conversion Efficiency of ∼55%

In comparison to in-pane sliding friction, rolling friction not only is likely to consume less mechanical energy but also presents high robustness with minimized wearing of materials. In this work, we introduce a highly efficient approach for harvesting mechanical energy based on rolling electrification and electrostatic induction, aiming at improving the energy conversion efficiency and device durability. The rolling triboelectric nanogenerator is composed of multiple steel rods sandwiched by two fluorinated ethylene propylene (FEP) thin films. The rolling motion of the steel rods between the FEP thin films introduces triboelectric charges on both surfaces and leads to the change of potential difference between each pair of electrodes on back of the FEP layer, which drives the electrons to flow in the external load. As power generators, each pair of output terminals works independently and delivers an open-circuit voltage of 425 V, and a short-circuit current density of 5 mA/m(2). The two output terminals can also be integrated to achieve an overall power density of up to 1.6 W/m(2). The impacts of variable structural factors were investigated for optimization of the output performance, and other prototypes based on rolling balls were developed to accommodate different types of mechanical energy sources. Owing to the low frictional coefficient of the rolling motion, an instantaneous energy conversion efficiency of up to 55% was demonstrated and the high durability of the device was confirmed. This work presents a substantial advancement of the triboelectric nanogenerators toward large-scope energy harvesting and self-powered systems.

Motion-driven electrochromic reactions for self-powered smart window system.

The self-powered system is a promising concept for wireless networks due to its independent and sustainable operations without an external power source. To realize this idea, the triboelectric nanogenerator (TENG) was recently invented, which can effectively convert ambient mechanical energy into electricity to power up portable electronics. In this work, a self-powered smart window system was realized through integrating an electrochromic device (ECD) with a transparent TENG driven by blowing wind and raindrops. Driven by the sustainable output of the TENG, the optical properties, especially the transmittance of the ECD, display reversible variations due to electrochemical redox reactions. The maximum transmittance change at 695 nm can be reached up to 32.4%, which is comparable to that operated by a conventional electrochemical potentiostat (32.6%). This research is a substantial advancement toward the practical application of nanogenerators and self-powered systems.

Fully Transparent Resistive Memory Employing Graphene Electrodes for Eliminating Undesired Surface Effects

A ZnO-based transparent resistance random access memory (TRRAM) employs atomic layered graphene exhibiting not only excellent transparency (less than 2% absorptance by graphene) but also reversible resistive switching characteristics. The statistical analysis including cycle-to-cycle and cell-to-cell tests for almost 100 cells shows that graphene plays a significant role to suppress the surface effect, giving rise to the notable increase in the switching yield and the insensitivity to the environmental atmosphere. The resistance variation of high-resistance state of ZnO is greatly suppressed by covering graphene as well. The device reliability investigation, such as the endurance more than 102 cycles and the retention time longer than 104 s, reveals the robust passivation of graphene for TRRAM applications. The obtained insights show guidelines not only for TRRAM device design and optimization against the undesired switching parameter variations but also for developing practically useful applications of graphene.

Self-Powered Triboelectric Nanosensor for Microfluidics and Cavity-Confined Solution Chemistry

Micro total analysis system (μTAS) is one of the important tools for modern analytical sciences. In this paper, we not only propose the concept of integrating the self-powered triboelectric microfluidic nanosensor (TMN) with μTAS, but also demonstrate that the developed system can be used as an in situ tool to quantify the flowing liquid for microfluidics and solution chemistry. The TMN automatically generates electric outputs when the fluid passing through it and the outputs are affected by the solution temperature, polarity, ionic concentration, and fluid flow velocity. The self-powered TMN can detect the flowing water velocity, position, reaction temperature, ethanol, and salt concentrations. We also integrate the TMNs in a μTAS platform to directly characterize the synthesis of Au nanoparticles by a chemical reduction method.

Si Hybrid Solar Cells with 13% Efficiency via Concurrent Improvement in Optical and Electrical Properties by Employing Graphene Quantum Dots.

UNLABELLED By employing graphene quantum dots (GQDs) in PEDOT PSS, we have achieved an efficiency of 13.22% in Si/ PEDOT PSS hybrid solar cells. The efficiency enhancement is based on concurrent improvement in optical and electrical properties by the photon downconversion process and the improved conductivity of PEDOT PSS via appropriate incorporation of GQDs. After introducing GQDs into PEDOT PSS, the short circuit current and the fill factor of rear-contact optimized hybrid cells are increased from 32.11 to 36.26 mA/cm(2) and 62.85% to 63.87%, respectively. The organic-inorganic hybrid solar cell obtained herein holds the promise for developing photon-managing, low-cost, and highly efficient photovoltaic devices.

Eliminating surface effects via employing nitrogen doping to significantly improve the stability and reliability of ZnO resistive memory

Metal oxides suffering from oxygen molecule chemisorption display environment-dependent metastability, leading to unstable resistive memory characteristics and performance degradation. To obtain ambient-independent characteristics, we introduced nitrogen into ZnO resistive memory devices, compensating for the native defects and suppressing oxygen chemisorption, giving rise to a significant improvement in switching behavior without undesired surface effects. Moreover, by thermal activation of the nitrogen doping via annealing, an increased yield ratio from 50% to 82%, a reduced current compliance from 15 mA to 5 mA, and more stable cycling endurance are obtained. Our findings give physical insight into designing resistive memory devices.

Resistive memory for harsh electronics: immunity to surface effect and high corrosion resistance via surface modification.

The tolerance/resistance of the electronic devices to extremely harsh environments is of supreme interest. Surface effects and chemical corrosion adversely affect stability and operation uniformity of metal oxide resistive memories. To achieve the surrounding-independent behavior, the surface modification is introduced into the ZnO memristors via incorporating fluorine to replace the oxygen sites. F-Zn bonds is formed to prevent oxygen chemisorption and ZnO dissolution upon corrosive atmospheric exposure, which effectively improves switching characteristics against harmful surroundings. In addition, the fluorine doping stabilizes the cycling endurance and narrows the distribution of switching parameters. The outcomes provide valuable insights for future nonvolatile memory developments in harsh electronics.

A Fully Transparent Resistive Memory for Harsh Environments.

A fully transparent resistive memory (TRRAM) based on Hafnium oxide (HfO2) with excellent transparency, resistive switching capability, and environmental stability is demonstrated. The retention time measured at 85 °C is over 3 × 104 sec, and no significant degradation is observed in 130 cycling test. Compared with ZnO TRRAM, HfO2 TRRAM shows reliable performance under harsh conditions, such as high oxygen partial pressure, high moisture (relative humidity = 90% at 85 °C), corrosive agent exposure, and proton irradiation. Moreover, HfO2 TRRAM fabricated in cross-bar array structures manifests the feasibility of future high density memory applications. These findings not only pave the way for future TRRAM design, but also demonstrate the promising applicability of HfO2 TRRAM for harsh environments.