Ultraflexible Devices Research Articles

Waterproof and ultraflexible organic photovoltaics with improved interface adhesion

Ultraflexible organic photovoltaics have emerged as a potential power source for wearable electronics owing to their stretchability and lightweight nature. However, waterproofing ultraflexible organic photovoltaics without compromising mechanical flexibility and conformability remains challenging. Here, we demonstrate waterproof and ultraflexible organic photovoltaics through the in-situ growth of a hole-transporting layer to strengthen interface adhesion between the active layer and anode. Specifically, a silver electrode is deposited directly on top of the active layers, followed by thermal annealing treatment. Compared with conventional sequentially-deposited hole-transporting layers, the in-situ grown hole-transporting layer exhibits higher thermodynamic adhesion between the active layers, resulting in better waterproofness. The fabricated 3 μm-thick organic photovoltaics retain 89% and 96% of their pristine performance after immersion in water for 4 h and 300 stretching/releasing cycles at 30% strain under water, respectively. Moreover, the ultraflexible devices withstand a machine-washing test with such a thin encapsulation layer, which has never been reported. Finally, we demonstrate the universality of the strategy for achieving waterproof solar cells.

Biomimetic Spun Silk Ionotronic Fibers for Intelligent Discrimination of Motions and Tactile Stimuli.

Innovation in the ionotronics field has significantly accelerated the development of ultraflexible devices and machines. However, it is still challenging to develop efficient ionotronic-based fibers with necessary stretchability, resilience, and conductivity due to inherent conflict in producing spinning dopes with both high polymer and ion concentrations and low viscosities. Inspired by the liquid crystalline spinning of animal silk, this study circumvents the inherent tradeoff in other spinning methods by dry spinning a nematic silk microfibril dope solution. The liquid crystalline texture allows the spinning dope to flow through the spinneret and form free-standing fibers under minimal external forces. The resultant silk-sourced ionotronic fibers (SSIFs) are highly stretchable, tough, resilient, and fatigue-resistant. These mechanical advantages ensure a rapid and recoverable electromechanical response of SSIFs to kinematic deformations. Further, the incorporation of SSIFs into core-shell triboelectric nanogenerator fibers provides outstanding stable and sensitive triboelectric response to precisely and sensitively perceive small pressures. Moreover, by implementing a combination of machine learning and Internet of Things techniques, the SSIFs can sort objects made of different materials. With these structural, processing, performance, and functional merits, the SSIFs prepared herein are expected to be applied in human-machine interfaces.

Dual Physically Crosslinked Silk Fibroin Ionoelastomer with Ultrahigh Stretchability and Low Hysteresis

Innovations in ionotronics have greatly facilitated the development of ultraflexible devices, displays, and machines. However, ionotronics often suffer from an inherent trade-off between stretchability and elasticity due to their viscous nature. Here, we overcome this limitation by incorporating dimension-and-content-confined nanocrystalline crosslinks into amorphous and molecularly entangled silk fibroin (SF) ionotronics. The welding-entanglement network exhibited a 34-fold increase in Young’s modulus, a 14-fold increase in tensile strength, and a 9-fold increase in toughness compared to the same SF material without the nanocrystal crosslinks. The welding-entanglement network also gained recoverable hysteretic energy dissipation with hysteresis as low as 22%. The stretchability and low hysteresis exhibited by these SF ionoelastomers far exceed those of previously reported ionotronics and bioelastomers. These mechanical merits, together with the inherent advantages of SF ionoelastomers, including their multiprocessability, sustainability, biocompatibility, and degradability, promote their applications in human–machine interfaces and bio-functional devices.

Simple Method for Creating Hydrophobic Ultraflexible Photovoltaics.

Optoelectronic devices, such as photodetectors and photovoltaics, are susceptible to surface contamination or water damage that can lead to reductions in performance or stability. Applying superhydrophobic coatings to these devices can introduce self-cleaning behavior and water resistance to extend their lifetime and improve their efficiency. However, existing methods for inducing superhydrophobicity have not been compatible with ultraflexible devices because of their thickness and complexity requirements. In this work, we introduce a procedure for inducing superhydrophobic and self-cleaning behavior on ultraflexible components using a combination of shrinkage-induced wrinkles and a low-surface-energy coating. We apply these techniques to an ultraflexible organic photovoltaics and demonstrate excellent hydrophobicity and self-cleaning behavior.

2D metal-organic frameworks for ultraflexible electrochemical transistors with high transconductance and fast response speeds.

Electrochemical transistors (ECTs) have shown broad applications in bioelectronics and neuromorphic devices due to their high transconductance, low working voltage, and versatile device design. To further improve the device performance, semiconductor materials with both high carrier mobilities and large capacitances in electrolytes are needed. Here, we demonstrate ECTs based on highly oriented two-dimensional conjugated metal-organic frameworks (2D c-MOFs). The ion-conductive vertical nanopores formed within the 2D c-MOFs films lead to the most convenient ion transfer in the bulk and high volumetric capacitance, endowing the devices with fast speeds and ultrahigh transconductance. Ultraflexible device arrays are successfully used for wearable on-skin recording of electrocardiogram (ECG) signals along different directions, which can provide various waveforms comparable with those of multilead ECG measurement systems for monitoring heart conditions. These results indicate that 2D c-MOFs are excellent semiconductor materials for high-performance ECTs with promising applications in flexible and wearable electronics.

(Invited) Stretchable Sensing Devices Combining Ionic Liquids and Soft Electrodes

In recent years, a variety of ultra-flexible devices have been proposed. Their applications include wearable devices and soft robots. Among ultra-flexible devices, the devices with stretchablity is attracting attention as next-generation sensing devices. In such devices, ionic liquids can be used as a sensing material. Ionic liquids are polymers in a liquid state and their structure can be easily altered, For example, ionic liquids can be developed reactive to temperature, humidity, light, gases, and many other factors. Furthermore, since they are in a liquid state, they are durable against device stretching. In this study, we report on a stretchable sensing device using ionic liquids. In particular, by using liquid metal and CNT(Carbon nanotube) as electrode materials, we propose a super-stretchable device and a super-flexible device with transparency and high breathability.<Temperature/humidity/oxygen/optical sensors using liquid metal electrodes> liquid metal was used as electrode, and ionic liquids as a sensing material. Equivalent circuit of the sensors were established based on Nyquist plot The sensor showed stable sensitivity to temperature without hysteresis as shown with a 0.039/°C increase in conductivity, which is quite high compared to other reports. We also show proof of concept for humidity, oxygen gas, and optical sensings using four kinds of ionic liquids, 1-Ethyl-3-methylimidazolium trifluoromethanesulfonate ([EMIM][Otf]), 1-Butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]), 1-Butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMPYR][NTf2]) and 1-butyl-3-(4-phenylazobenzyl)imidazolium bis(trifluoromethanesulfonyl)amide ([Azo][NTf2]) for optical sensing. The sensitivity of each ionic liquid to humidity and oxygen differed depending on the ionic liquid. For each type of stimuli, the sensing can be optimized by choosing the proper ionic liquid. In addition, using [Azo][NTf2], we demonstrate optical sensing and memory in this study.< Transparent and Breathable Ion Gel-based Sensors using CNT> highly transparent, ultra-flexible, and gas-permeable polymer thin-film sensors using ion gels as the sensing material, which demonstrated the capacity for selective detections, were proposed. Particularly, simultaneous and independent sensing of temperature and humidity was demonstrated in this study. The sensors were fabricated using a simple spray coating method on a thin silicone rubber film (around 25 µm thickness). Owing to their thin-film shape, they showed more than 80% visible light transmittance and a higher gas permeability of 58.7 g/m2 h than the human transepidermal water loss. Simultaneous and independent detection of temperature and humidity was achieved with a high sensitivity of 15.9%/°C and 2.5%/percentage of relative humidity, respectively, using two types of gels with ionic liquids.These results suggest that the easily modifiable nature of ionic liquids contribute to the development of stretchable electronics.

Research progress in skin-like ultraflexible organic field-effect transistors

<p indent="0mm">Field-effect transistor is one of the most important basic components in modern microelectronic industry. Organic field-effect transistors (OFETs) possess the outstanding advantages in flexibility, light weight, solution processability and good biocompatibility. They have presented great application potential in the field of skin-like electronics, such as software robots, implant devices and imperceptible wearable devices. Skin-like ultraflexible OFETs can be imperceptibly conformable to the three-dimensional static/dynamic surfaces such as skin and prosthesis, maintain the stable electrical performance under deformation, and give the wearer the minimized burden and maximized comfort. Decreasing the thickness and Young’s modulus will effectively reduce the resilience of the device due to deformation, which is the main strategy to obtain the skin-like ultraflexible devices with three-dimensional conformability. In this paper, two types of skin-like flexible OFETs, including ultra-thin transistors and stretchable transistors, are summarized. The research process of the skin-like ultraflexible OFETs is discussed.

Balancing efficiency and transparency in organic transparent photovoltaics

The challenges in transparent photovoltaic (TPV) fields are still that the device transparency and efficiency are difficult to be balanced to meet the requirements of practical applications. In this study, we systematically investigated the interrelationship between photovoltaic film properties, optical transmission, and photovoltaic performances in the near-infrared harvesting organic TPVs. The results indicate that the photovoltaic film thickness determines the TPV’s transparency and meanwhile affects the device efficiency; by contrast, the donor–acceptor ratio only affects device efficiency and has little effect on transparency. By controlling the film thickness and donor–acceptor ratio, the average visible transmission (AVT) of TPVs can be precisely managed in the range of 40% - 85%, and the device efficiency can achieve as high as 4.06% and 2.38% while the AVT exceeds 70% and 80%, respectively. Importantly, the large area (~10 cm2) TPV modules and ultra-flexible devices were then successfully prepared based on the systematical study.

P25. NEUROSTITCH: NANOFABRICATION OF A HIGH RESOLUTION, BIOMIMETIC PERIPHERAL NERVE INTERFACE

PURPOSE: Over a million people suffer limb loss annually. Existing prostheses are cumbersome with a limited capacity to recapitulate natural hand motion. A stable, high-fidelity neural interface is necessary for 2-way communication between robotic prostheses and the patient’s neural circuitry. Existing neural interfaces are limited by the trade-off between invasiveness, resolution and longevity. METHODS: Ultra-small, flexible electrodes of the same dimensions and compliance as human axons were fabricated. A ‘needle and thread’ approach was used to implant the ultra-flexible devices. Using histology, 2-photon imaging, and the Sciatic Functional Index (SFI), we assessed the neural injury and foreign body reaction (FBR) associated with the device. The Intan system was used to stimulate and record. RESULTS: SFI demonstrates resolution of the initial neuropraxia to baseline by 2 weeks post-implantation. Action potentials were recorded within 30 minutes of device insertion. The size of the electrodes and proximity to individual axons permitted local stimulation — down to a single toe. Immunohistochemistry demonstrated an absence of FBR at 6 months. CONCLUSION: We have fabricated and inserted biomimetic intraneural electrodes with minimal and recoverable nerve injury and absence of FBR. The Neurostitch is capable high selectivity stimulation. This device represents a step forward in the development of a high-resolution, stable man/machine interface for next-generation neuroprostheses.

Scalable batch fabrication of ultrathin flexible neural probes using a bioresorbable silk layer

Flexible intracerebral probes for neural recording and electrical stimulation have been the focus of many research works to achieve better compliance with the surrounding tissue while minimizing rejection. Strategies have been explored to find the best way to insert flexible probes into the brain while maintaining their flexibility once positioned. Here, we present a novel and versatile scalable batch fabrication approach to deliver ultrathin and flexible probes consisting of a silk-parylene bilayer. The biodegradable silk layer, whose degradation time is programmable, provides a temporary and programmable stiffener to allow the insertion of ultrathin parylene-based flexible devices. Our innovative and robust batch fabrication technology allows complete freedom over probe design in terms of materials, size, shape, and thickness. We demonstrate successful ex vivo insertion of the probe with acute high-fidelity recordings of epileptic seizures in field potentials as well as single-unit action potentials in mouse brain slices. Our novel technological solution for implanting ultraflexible devices in the brain while minimizing rejection risks shows high potential for use in both brain research and clinical therapies.

(Invited) Progress in Carbon Nanotube Terahertz Photodetectors

Terahertz photodetectors based on carbon nanotubes have shown promising performance and could lead to new detection modalities due to the ability to fabricate the detectors on non-planar flexible substrates using inkjet printing. To realize this promise, a number of materials and device challenges must be overcome. In this presentation, I will discuss our recent work to understand the inner workings of these devices, and to improve their performance based on this knowledge. In particular, I will show how managing the materials and thermal properties of the system can lead to more sensitive and faster performance while enabling ultraflexible devices.

Giant Photoluminescence Enhancement in CsPbCl3 Perovskite Nanocrystals by Simultaneous Dual-Surface Passivation

The presence of localized trap states on the surface of CsPbCl3 perovskite nanocrystals (NCs) is one of the greatest challenges precluding the development of optoelectronic applications of these NCs. Passivation of these defect sites provides a promising pathway to remediating their electronic and optical properties, such as the photoluminescence quantum yield (PLQY). Herein, we demonstrate a postsynthetic dual-surface treatment using trivalent metal ion salts, i.e., YCl3, as a new passivation approach that enhances the PLQY up to 60% while preserving the NC size and crystal structure. Such remarkable enhancement of the PLQY along with prolongation of the average PL lifetimes of treated NCs samples indicates effective passivation of the surface defects and subsequent suppression of the formation of surface nonradiative recombination centers. As a segue toward optoelectronic applications, we probed the photoelectric performance of the NCs using ultraflexible devices; we found that YCl3-treated CsPbCl3 NC f...

(Invited) Ultra-Flexible Polymer Nanosheet for Bio and Electronics Applications

Integration of flexible devices into the living system is expected for advancing medical and healthcare devices, which will realize investigation of biological information at nano-biointerface. Such devices should be compatible to physical and mechanical properties of biological tissues. In this talk, we introduce free-standing polymeric ultra-thin films (referred to as “polymer nanosheets”) as a unique platform of ultra-flexible devices, which comprise tens- to hundreds-of-nanometer thickness close to the scale of biomembranes1. Various polymers (e.g., biodegradable polymers, conductive polymers, elastomers) are applicable as building blocks of polymer nanosheets; those are processed by spincoating, layer-by-layer and gravure coating techniques. The resulting nanosheets showed flexible and physically adhesive property owing to their ultra-thin structure. Polymer nanosheets are also functionalized by printing conductive polymers, nanomaterials or embedding thermo-sensitive dyes2-4. These nanosheet-based devices revealed several types of physiological properties in living organisms at different length scale from cell to tissue to body via functionalized interface, exemplified by temperature mapping of living muscle tissues and electromyogram of human bodies. The nanosheet technology will open a new avenue in the field of biotechnology and biomedicine. References T. Fujie, Polym. J., 2016, 48, 773.A. Zucca, K. Yamagishi, T. Fujie, S. Takeoka, V. Mattoli, F. Greco, J. Mater. Chem. C, 2015, 3, 6539.T. Miyagawa, T. Fujie, Ferdinandus, T. T. Vo Doan, H. Sato, S. Takeoka, ACS Appl. Mater. Interfaces, 2016, 8, 33377.M. Okamoto, M. Kurotobi, S. Takeoka, J. Sugano, E. Iwase, H. Iwata, T. Fujie, J. Mater. Chem. C, 2017, 5, 1321.

Reverse-Offset Printed Ultrathin Ag Mesh for Robust Conformal Transparent Electrodes for High-Performance Organic Photovoltaics.

Mechanically durable transparent electrodes are needed in flexible optoelectronic devices to realize their long-term stable functioning, for applications in various fields such as energy, healthcare, and soft robotics. Several promising transparent electrodes based on nanomaterials have been previously reported to replace the conventional and fragile indium-tin oxide (ITO); however, obtaining feasible printed transparent electrodes for ultraflexible devices with a multistack structure is still a great challenge. Here, a printed ultrathin (uniform thickness of 100 nm) Ag mesh transparent electrode is demonstrated, simultaneously achieving high conductance, high transparency, and good mechanical properties. It shows a 17 Ω sq-1 sheet resistance (Rsh ) with 93.2% transmittance, which surpasses the performance of sputtered ITO electrodes and other ultrathin Ag mesh transparent electrodes. The conductance is stable after 500 cycles of 100% stretch/release deformation, with an insignificant increase (10.6%) in Rsh by adopting a buckling structure. Furthermore, organic photovoltaics (OPVs) using our Ag mesh transparent electrodes achieve a power conversion efficiency of 8.3%, which is comparable to the performance of ITO-based OPVs.

Thin-film organic semiconductor devices: from flexibility to ultraflexibility

Flexible thin-film organic semiconductor devices have received wide attention due to favorable properties such as light-weight, flexibility, reproducible semiconductor resources, easy tuning of functional properties via molecular tailoring, and low cost large-area solution-procession. Among them, ultraflexible electronics, usually with minimum bending radius of less than 1 mm, are essential for the development of epidermal and bio-implanted electronics, wearable electronics, collapsible and portable electronics, three dimensional (3D) surface compliable electronics, and bionics. This review firstly gives a brief introduction of development from flexible to ultraflexible organic semiconductor electronics, and design of ultraflexible devices, then summarizes the recent advances in ultraflexible thin-film organic semiconductor devices, focusing on organic field effect transistors, organic light-emitting diodes, organic solar cells and organic memory devices.

Ultraflexible Tactile Piezoelectric Sensor Based on Low-Temperature Polycrystalline Silicon Thin-Film Transistor Technology

In this paper, we present an ultraflexible tactile sensor, in a piezo-eletricoxide-semiconductor FET configuration, composed by a poly[vinylidenefluoride-co-trifluoroethylene] capacitor with an embedded readout circuitry, based on nMOS polysilicon electronics, integrated directly on polyimide. The ultraflexible device is designed according to an extended gate configuration. The sensor exhibits enhanced piezoelectric properties, thanks to the optimization of the poling procedure (with electric field up to 3 MV/cm), reaching a final piezoelectric coefficient $d_{33}$ of 47 pC/N. The device has been electromechanically tested by applying perpendicular forces with a minishaker. The tactile sensor, biased in a common-source arrangement, shows a linear response to increasing sinusoidal stimuli (up to 2 N) and increasing operating frequencies (up to 1200 Hz), obtaining a response up to 430 mV/N at 200 Hz for the sensor with the highest value of $d_{33}$ . The sensor performances were also tested after several cycles of controlled bending in different amount of humidity with the intent to investigate the device behavior in real conditions.

Fully-printed high-performance organic thin-film transistors and circuitry on one-micron-thick polymer films.

Thin, ultra-flexible devices that can be manufactured in a process that covers a large area will be essential to realizing low-cost, wearable electronic applications including foldable displays and medical sensors. The printing technology will be instrumental in fabricating these novel electronic devices and circuits; however, attaining fully printed devices on ultra-flexible films in large areas has typically been a challenge. Here we report on fully printed organic thin-film transistor devices and circuits fabricated on 1-μm-thick parylene-C films with high field-effect mobility (1.0 cm(2) V(-1) s(-1)) and fast operating speeds (about 1 ms) at low operating voltages. The devices were extremely light (2 g m(-2)) and exhibited excellent mechanical stability. The devices remained operational even under 50% compressive strain without significant changes in their performance. These results represent significant progress in the fabrication of fully printed organic thin-film transistor devices and circuits for use in unobtrusive electronic applications such as wearable sensors.