The demand for flexible/wearable electronic devices that have aesthetic appeal and multi-functionality has stimulated the rapid development of flexible supercapacitors with enhanced electrochemical performance and mechanical flexibility. After a brief introduction to flexible supercapacitors, we summarize current progress made with graphene-based electrodes. Two recently proposed prototypes for flexible supercapacitors, known as micro-supercapacitors and fiber-type supercapacitors, are then discussed. We also present our perspective on the development of graphene-based electrodes for flexible supercapacitors.

Graphene-based materials for flexible supercapacitors

It is conventionally assumed that the growth of monodisperse colloidal nanocrystals requires a temporally discrete nucleation followed by monomer attachment onto the existing nuclei. However, recent studies have reported violations of this classical growth model, and have suggested that inter-particle interactions are also involved during the growth. Mechanisms of nanocrystal growth still remain controversial. Using in situ transmission electron microscopy, we show that platinum nanocrystals can grow either by monomer attachment from solution onto the existing particles or by coalescence between the particles. Surprisingly, an initially broad size distribution of the nanocrystals can spontaneously narrow. We suggest that nanocrystals take different pathways of growth based on their size- and morphology-dependent internal energies. These observations are expected to be highly relevant for other nanocrystal systems.

Observation of Single Colloidal Platinum Nanocrystal Growth Trajectories

To meet the rapid development of flexible, portable, and wearable electronic devices, extensive efforts have been devoted to develop matchable energy storage and conversion systems as power sources, such as flexible lithium-ion batteries (LIBs), supercapacitors (SCs), solar cells, fuel cells, etc. Particularly, during recent years, exciting works have been done to explore more suitable and effective electrode/electrolyte materials as well as more preferable cell configuration and structural designs to develop flexible power sources with better electrochemical performance for integration into flexible electronics. An overview is given for these remarkable contributions made by the leading scientists in this important and promising research area. Some perspectives for the future and impacts of flexible energy storage and conversion systems are also proposed.

Advances and challenges for flexible energy storage and conversion devices and systems

Plasmons in doped graphene exhibit relatively large confinement and long lifetime compared to noble-metal plasmons. Here, we study the propagation properties of plasmons guided along individual and interacting graphene nanoribbons. Besides their tunability via electrostatic gating, an additional handle to control these excitations is provided by the dielectric environment and the relative arrangement of the interacting waveguides. Plasmon interaction and hybridization in pairs of neighboring aligned ribbons are shown to be strong enough to produce dramatic modifications in the plasmon field profiles. We introduce a universal scaling law that considerably simplifies the analysis an understanding of these plasmons. Our work provides the building blocks to construct graphene plasmon circuits for future compact plasmon devices with potential application to optical signal processing, infrared sensing, and quantum information technology.

Graphene plasmon waveguiding and hybridization in individual and paired nanoribbons.

This Review describes the state-of-the-art of wearable electronics (smart textiles). The unique and promising advantages of smart electronic textiles are highlighted by comparing them with the conventional planar counterparts. The main kinds of smart electronic textiles based on different functionalities, namely the generation, storage, and utilization of electricity, are then discussed with an emphasis on the use of functional materials. The remaining challenges are summarized together with important new directions to provide some useful clues for the future development of smart electronic textiles.

Smart Electronic Textiles.

Flexible and transparent power sources are highly desirable in realizing next-generation all-in-one bendable, implantable, and wearable electronic systems. The developed power sources are either flexible but opaque or semitransparent but lack of flexibility. Therefore, there is increasing recognition of the need for a new concept of electrochemical device structure design that allows both high flexibility and transparency. In this paper, we present a new concept for electrochemical device design—a two-dimensional planar comb-teeth architecture on PET substrate—to achieve both high mechanical flexibility and light transparency. Two types of prototypes—dye-sensitized solar cells and supercapacitors—have been fabricated as planar devices and demonstrated excellent device performance, such as good light transparency, excellent flexibility, outstanding multiple large bending tolerance, light weight, effective prevention of short circuits during bending, and high device integration with up-date microelectronics...

Novel Planar-Structure Electrochemical Devices for Highly Flexible Semitransparent Power Generation/Storage Sources

Transparent, double-sided, flexible, ITO-free dye-sensitized solar cells (DSSCs) are fabricated in a simple, facile, and controllable way. Highly ordered, high-crystal-quality, high-density ZnO nanowire arrays are radially grown on stainless steel, Au, Ag, and Cu microwires, which serve as working electrodes. Pt wires serve as the counter electrodes. Two metal wires are encased in electrolyte between two poly(ethylene terephthalate) (PET) films (or polydimethylsiloxane (PDMS) films) to render the device both flexible and highly transparent. The effect of the dye thickness on the photovoltaic performance of the DSSCs as a function of dye-loading time is investigated systematically. Shorter dye-loading times lead to thinner dye layers and better device performance. A dye-loading time of 20 min results in the best device performance. An oxidation treatment of the metal wires is developed effectively to avoid the galvanic-battery effect found in the experiment, which is crucial for real applications of double-metal-wire DSSC configurations. The device shows very good transparency and can increase sunlight use efficiency through two-sided illumination. The double-wire DSSCs remain stable for a long period of time and can be bent at large angles, up to 107°, reversibly, without any loss of performance. The double-wire-PET, planar solar-cell configuration can be used as window stickers and can be readily realized for large-area-weave roll-to-roll processing.

Transparent, Double‐Sided, ITO‐Free, Flexible Dye‐Sensitized Solar Cells Based on Metal Wire/ZnO Nanowire Arrays

An “all-in-one” mesh-typed integrated energy unit for both photoelectric conversion and energy storage in uniform electrochemical system

Charge carrier dynamics in amorphous semiconductors has been a topic of intense research that has been propelled by modern applications in thin-film solar cells, transistors and optical sensors. Charge transport in these materials differs fundamentally from that in crystalline semiconductors owing to the lack of long-range order and high defect density. Despite the existence of well-established experimental techniques such as photoconductivity time-of-flight and ultrafast optical measurements, many aspects of the dynamics of photo-excited charge carriers in amorphous semiconductors remain poorly understood. Here, we demonstrate direct imaging of carrier dynamics in space and time after photo-excitation in hydrogenated amorphous silicon (a-Si:H) by scanning ultrafast electron microscopy (SUEM). We observe an unexpected regime of fast diffusion immediately after photoexcitation, together with spontaneous electron–hole separation and charge trapping induced by the atomic disorder. Our findings demonstrate the rich dynamics of hot carrier transport in amorphous semiconductors that can be revealed by direct imaging based on SUEM.

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Heng Li

Papers

Novel Planar-Structure Electrochemical Devices for Highly Flexible Semitransparent Power Generation/Storage Sources

Transparent, Double‐Sided, ITO‐Free, Flexible Dye‐Sensitized Solar Cells Based on Metal Wire/ZnO Nanowire Arrays

An “all-in-one” mesh-typed integrated energy unit for both photoelectric conversion and energy storage in uniform electrochemical system

Photo-excited hot carrier dynamics in hydrogenated amorphous silicon imaged by 4D electron microscopy

Low cost and flexible mesh-based supercapacitors for promising large-area flexible/wearable energy storage