Flexible Electronic Circuits Research Articles

Anodization for Simplified Processing and Efficient Charge Transport in Vertical Organic Field‐Effect Transistors

AbstractVertical organic transistors are an attractive alternative to realize short channel transistors, which are required for powerful electronic devices and flexible electronic circuits operating at high frequencies. Unfortunately, the vertical device architecture comes along with an increased device fabrication complexity, limiting the potential of this technology for application. A new design of vertical organic field‐effect transistors (VOFETs) with superior electrical performance and simplified processing is reported. By using electrochemical oxidized aluminum oxide (AlOx) as a pseudo self‐aligned charge‐blocking structure in vertical organic transistors, direct leakage current between the source and drain can be effectively suppressed, enabling VOFETs with very low off‐current levels despite the short channel length. The anodization technique is easy to apply and can be surprisingly used on both n‐type and p‐type organic semiconductor thin films with significant signs of degradation. Hence, the anodization technique enables a simplified process of high‐performance p‐type and n‐type VOFETs, paving the road toward complementary circuits made of vertical transistors.

Flexible multimodal sensor inspired by human skin based on hair-type flow, temperature, and pressure

Flexible electronic circuits when integrated with different sensors to mimic the human skin can be referred to as E-skin. E-skin can be used for the monitoring of human health, human-machine interface and humanoids. To mimic skin, a variety of sensors have been studied, including those sensitive to different pressures, temperatures, flows, and strain and humidity levels. Herein, we demonstrate a multifunctional sensor encompassed in a single pixel which is inspired by human skin. The proposed device contains a pressure sensor, a temperature sensor and a piezoresistive hair-type flow sensor, the latter of which is fabricated using mixture of carbon nanotubes and polydimethylsiloxane piezoresistive materials. Additionally, the temperature sensor is printed by means of inkjet printing with MWCNT and Ag nanoparticle ink. Finally, all three sensors are vertically stacked to mimic the human skin, demonstrating interference-free characteristics. This study provides a new and cost-effective approach for multimodal sensor applications.

A Review of Vertical Organic Transistors

AbstractPowerful electronic devices require performant short‐channel transistors. For organic electronics, though, promising low‐cost and flexible electronic circuits, high processing costs for short channel devices are not acceptable. In this regard, vertical organic transistors (VOTs) are an attractive alternative, and in fact, today they reach the highest transition frequency (40 MHz) and the highest footprint current density (>1 MA cm−2) among all organic transistors. Here, all VOT concepts are reviewed, while discussing device physics, integration approaches, and highlighting the recent developments. The upcoming challenges for the VOT technology are also presented with a guideline for further developments.

Controlled reduction of graphene oxide laminate and its applications for ultra-wideband microwave absorption

In this work, we first screen print thin GO laminates on Kapton film and then chemically reduce to rGO with desired sheet resistance by controlling printing cycles and reduction time. The thin rGO laminates have a wide range of conductivity from 2.8 S m−1 up to 1.3×104 S m−1, exhibiting excellent consistency of electric performance even after 50000 times bending. Commonly used rGO reduction level characterization methods have been compared with the proposed sheet resistance method. It is revealed that the later has higher variation with reduction time, providing a new method for reduction monitoring of rGO. Benefiting from the tunability of its sheet resistance, the rGO laminate not only can be used in flexible electronic circuits as conductors or resistors, but also highly efficient to absorb microwave radiations with proper design. Therefore, a printed, three-layer, Jaumann structured microwave absorber has been designed, fabricated and characterised. The maximum microwave absorption rate is up to 99.9997% where the −10 dB fractional bandwidth is over 153%, which covers L, S and C bands. Moreover, the fabricated absorber can absorb efficiently even when the incident angle is as low as 20°. Full design procedure, simulation and measurement results are presented.

Environment-friendly aqueous processing of [60]fullerene semiconducting films for truly green organic electronics

Here we report an environmentally friendly and easy to use technique for fabrication of C60 semiconductor films suitable for different applications such as design of flexible electronic circuits or gas sensors.

Direct ink writing of flexible electronic circuits and their characterization

The present work discusses the fabrication of low-cost, reliable and durable flexible electronic circuits from conductive carbon paste on flexible polyethylene terephthalate (PET) substrate using microdispensing direct ink write technique. Printing parameters such as pressure, substrate speed and gap size are optimized through Box–Behnken experimental design technique to achieve the desired quality of prints (patterns). For the nozzle with an inner diameter of 200 µm, the system is able to produce pattern line width ranging from 198 µm to 295 µm, respectively. Based on optimized parameters, circuits such as foil grid and rectangular spiral are printed on a flexible PET substrate. The deposited patterns of circuits are morphologically characterized by optical microscopy and scanning electronic microscopy. Current–Voltage (I–V) characteristic is performed to evaluate the conductive performance of circuits. The sensitivity of circuits to bending is assessed by studying resistance response under fully bend and unbend angle of 90° and 0°. Durability and reliability of circuits are analyzed by subjecting circuits to continuous 500 bend cycles and study its resistance response. The analysis revealed that these direct ink write circuits are reliable, durable and stable in performance and are applicable to the flexible electronic application.

Paper-like Foldable Nanowave Circuit with Ultralarge Curvature and Ultrahigh Stability.

Highly foldable conducting interconnects are fundamental elements for multipurpose flexible electronic circuits, including wearable electronics and biomedical devices. Traditional metalized thin-film interconnects demonstrate stable electronic performances in rigid devices but low deformation tolerance in flexibility. Recently, several remarkable research studies on flexible electronics have been carried out, as interconnect structures of serpentine, wavy, and nanowire networks. However, all of the reported flexible interconnects possess either mechanical instability or fabrication difficulty, which restrict their practical applications. Here, we report a new flexible circuit system, which consists of nanowave structure metal interconnects with highly foldable and large-scale manufactured features. This kind of nanowave interconnects presents both stable and prominent electrical performances under mechanical deformation (down to 0.2 mm bending radius with interconnecting resistance variation less than 10%). Further, a highly flexible paper-like wireless accelerometer based on the nanowave interconnects is fabricated and characterized under several extreme strain situations. Our approach affords a comprehensive direction for constitutional realization of new flexible designs and implements the assembly of next-generation foldable electronic equipment.

3D printed microstructures for flexible electronic devices

Flexible and stretchable electronics have attracted increasing attention and been widely used in wearable devices and electronic skins, where the circuits for flexible and stretchable electronics are typically in-plane-based 2D geometries. Here, we introduce a 3D microprinting technology that can expand one more dimension of the circuit in flexible electronics. We fabricated three-dimensional serpentine microstructures based on direct laser writing. These microstructures with a thin metal coated layer can be used as stretchable conducting meshes. Soft silicone serving as a substrate and encapsulations for these 3D microstructures enables great light transmittance (>90% in visible light range) and flexibility with 114° bending and 24° twisting. Further optimization of the mechanical design of the 3D microstructures can also enhance the stretchability up to 13.8%. These results indicate 3D flexible electronics can be realized by simple microprinting methods. Furthermore, 3D microprinting would also allow for the precise fabrication of other 3D structures, such as mechanically active 3D mesostructures, for the function of mechanical and electrical testing.

Fabrication of High-resolution Graphene-based Flexible Electronics via Polymer Casting

In this study, a novel method based on the transfer of graphene patterns from a rigid or flexible substrate onto a polymeric film surface via solvent casting was developed. The method involves the creation of predetermined graphene patterns on the substrate, casting a polymer solution, and directly transferring the graphene patterns from the substrate to the surface of the target polymer film via a peeling-off method. The feature sizes of the graphene patterns on the final film can vary from a few micrometers (as low as 5 µm) to few millimeters range. This process, applied at room temperature, eliminates the need for harsh post-processing techniques and enables creation of conductive graphene circuits (sheet resistance: ~0.2 kΩ/sq) with high stability (stable after 100 bending and 24 h washing cycles) on various polymeric flexible substrates. Moreover, this approach allows precise control of the substrate properties such as composition, biodegradability, 3D microstructure, pore size, porosity and mechanical properties using different film formation techniques. This approach can also be used to fabricate flexible biointerfaces to control stem cell behavior, such as differentiation and alignment. Overall, this promising approach provides a facile and low-cost method for the fabrication of flexible and stretchable electronic circuits.

Investigation of Electrode Electrochemical Reactions in CH3 NH3 PbBr3 Perovskite Single-Crystal Field-Effect Transistors.

Optoelectronic devices based on metal halide perovskites, including solar cells and light-emitting diodes, have attracted tremendous research attention globally in the last decade. Due to their potential to achieve high carrier mobilities, organic-inorganic hybrid perovskite materials can enable high-performance, solution-processed field-effect transistors (FETs) for next-generation, low-cost, flexible electronic circuits and displays. However, the performance of perovskite FETs is hampered predominantly by device instabilities, whose origin remains poorly understood. Here, perovskite single-crystal FETs based on methylammonium lead bromide are studied and device instabilities due to electrochemical reactions at the interface between the perovskite and gold source-drain top contacts are investigated. Despite forming the contacts by a gentle, soft lamination method, evidence is found that even at such "ideal" interfaces, a defective, intermixed layer is formed at the interface upon biasing of the device. Using a bottom-contact, bottom-gate architecture, it is shown that it is possible to minimize such a reaction through a chemical modification of the electrodes, and this enables fabrication of perovskite single-crystal FETs with high mobility of up to ≈15 cm2 V-1 s-1 at 80 K. This work addresses one of the key challenges toward the realization of high-performance solution-processed perovskite FETs.

A Novel Strategy for Preparing Stretchable and Reliable Biphasic Liquid Metal

AbstractLow‐melting liquid metal is a hugely promising material for flexible conductive patterns due to its excellent conductivity and supercompliance, especially low‐cost and environmental liquid processing technology. However, the ever‐present fluidity characteristic greatly limits the stable shape and reliability of prepared liquid metal conductive electronics. Herein, a novel solidification strategy of liquid GaIn alloys by Ni doping and heat treatment is first reported, which can efficiently create a solid phase in the liquid metal and provide an effective solution for practical applications. Particularly, the liquid characteristic is preserved for conveniently fabricating different flexible electronic circuits, and then the solidification is carried out on prepared conductive patterns by heat treatment. The solidification mechanism is revealed by the interface chemical reaction between Ni and GaIn, creating the solid phase of intermetallic compound (Ga4Ni3 and InNi3) during heat treatment. Moreover, a biphasic GaInNi can be obtained by regulating the atomic ratio of gallium, indium, and nickel. As a result, the obtained GaInNi possesses extremely low sheet resistance (15 ± 4.5 to 135 ± 2.5 mΩ sq−1) and the variation of ΔR/R0 exhibits low level (0–2) when strained up to 100%, which offers a promising strategy to prepare stretchable and reliable liquid metal electronics.

Modified PDMS packaging of sensory e-textile circuit microsystems for improved robustness with washing

Electronic Textiles (e-textiles) should ideally be handled and cleaned like traditional textiles. Therefore, we can expect e-textiles to be machine washed or hand washed. As e-textiles enhance traditional fabrics with electronic functionality, any embedded microsystem i.e., flexible electronic circuits, will be expected to survive and show functionality after the e-textile has been washed multiple times to ensure the garment is practical. Therefore, the choice of encapsulation material for microsystems in a textile must be hydrophobic and offer minimal expansion when washed and ensure the electronics are undetectable when the textile is handled or cleaned. This paper evaluates five different base/curing agent mixing ratios—5:1, 7:1, 10:1, 15:1, and 20:1—of commercial polydimethylsiloxane (PDMS) as an electronic packaging encapsulation. Contact angle and aqueous permeability experiments were conducted to tailor the PDMS mixture specifically for washable e-textile applications. The experimental results show that 20:1 PDMS is the most suitable as it is sufficiently hydrophobic with minimal swelling in commercial washing machine trials. Following this, a 40.3 µm-thick 20:1 conformal encapsulation of PDMS upon an touch and proximity flexible circuit that can be integrated into textiles via knitting and/or weaving, was examined. Results show the washing spin speed is a crucial factor with washing cycle duration having minimal impact when determining circuit functionality survival. Overall, the e-textiles in this work survived between 10 and 15 washes with microscopic inspection of the circuits revealing failure of the external wires but not the PDMS encapsulation—suggesting its sufficient robustness and durability as a suitable encapsulation material for washable electronic textiles.

Effect of non-chlorinated solvents on the enhancement of field-effect mobility in dioctylbenzothienobenzothiophene-based top-gate organic transistors processed by spin coating

Abstract The use of non-chlorinated solvents for film deposition in organic field-effect transistors (OFETs) is highly desirable to fabricate flexible and printable electronic circuits by environmentally friendly processes. Here, we investigate the influence of the use of non-chlorinated solvents, such as toluene, p-xylene, cyclohexanone, and mesitylene, for the deposition of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) films by spin coating on the morphologies of these semiconductor films and the electrical characteristics of OFETs with a top-gate configuration. The results show that the size of polycrystalline domains in spin-coated C8-BTBT films tends to increase as the boiling point (b.p.) of the solvent increases, whereas an improvement in the field-effect mobility is restricted by the decrease in the extent to which the C8-BTBT film covers the substrate due to the long evaporation time of high b.p. solvents. The addition of the high b.p. solvent p-xylene to toluene allows a significant increase in polycrystalline domain sizes in spin-coated C8-BTBT films. Further, spin coating C8-BTBT using the high b.p. solvent mesitylene at a high spin rate is effective for the production of thinner C8-BTBT films with diminished surface roughness. Top-gate C8-BTBT FETs processed by spin coating using a mixed binary solvent and by high-speed spin coating achieved high average field-effect mobilities of 4.9 and 5.4 cm2 V−1 s−1, respectively.

Optimization of PET Ion-Track Membranes Parameters

Nowadays polymer ion-track membranes are used for a wide range of practical applications, which include various levels of filtration (micro-, ultra-, nanofiltration and osmosis), the creation of flexible electronic circuits and sensors based on polymer substrate, and using as templates for shape-controlled nanostructures synthesis. New applications demand clear understanding of the processes that occur during track membranes formation. For high-precision control of the end-product parameters, it is necessary to establish the correlation between etching conditions and track membranes characteristics (pores dimensions, porosity and membranes thicknesses). For this purpose, in the paper it is considered the technique of membranes formation with 10 nm – 10 μm cylindrical pores and correlation between their parameters and processing modes is studied

Impact of Substrate and Process on the Electrical Performance of Screen-Printed Nickel Electrodes: Fundamental Mechanism of Ink Film Roughness

In recent years, traditional printing methods have been integrated to print flexible electronic devices and circuits. Since process requirements for electronics differ from those for graphic printing, the fundamentals require rediscovery mainly to optimize manufacturing techniques and to find cost reduction methods without compromising functional performance. In addition, alternative inks need to be formulated to increase the variety of functional inks and to pioneer new product developments. In this report, we investigate a thermoplastic-based nickel ink prototype to print electrodes using a screen-printing process. Process fundamentals are explored, and cost reduction methods are addressed by studying the effect of substrate roughness, print direction, and number of ink layers on the electrical performance of printed nickel. Multilayered electrodes are printed on paper and heat stabilized engineered film. A novel fundamental mechanism is found that explains the effect of substrate roughness on ink film ...

Embedded Dipole Self‐Assembled Monolayers for Contact Resistance Tuning in p‐Type and n‐Type Organic Thin Film Transistors and Flexible Electronic Circuits

AbstractBased on the powerful concept of embedded dipole self‐assembled monolayers (SAMs), highly conductive interfacial layers are designed, which allow tuning the contact resistance of organic thin‐film transistors over three orders of magnitude with minimum values well below 1 kΩ cm. This not only permits the realization of highly competitive p‐type (pentacene‐based) devices on rigid as well as flexible substrates, but also enables the realization of n‐type (C60‐based) transistors with comparable characteristics utilizing the same electrode material (Au). As prototypical examples for the high potential of the presented SAMs in more complex device structures, flexible organic inverters with static gains of 220 V/V and a 5‐stage ring‐oscillator operated below 4 V with a stage frequency in the range of the theoretically achievable maximum are fabricated. Employing a variety of complementary experimental and modeling techniques, it is shown that contact resistances are reduced by i) eliminating the injection barrier through a suitable dipole orientation, and by ii) boosting the transmission of charge carriers through a deliberate reduction of the SAM thickness. Notably, the embedding of the dipolar group into the backbones of the SAM‐forming molecules allows exploiting their beneficial effects without modifying the growth of the active layer.

Enhanced adhesion between liquid metal ink and the wetted printer paper for direct writing electronic circuits

Abstract Liquid metal (In75.4 Ga24.5, LM) with its high electrical conductivity and flexibility at room temperature, has been gradually introduced into printed electronics. However, the low adhesion effect of LM on the common printer paper may restrict its potential value in reducing cost and expanding use. In this study, the surface property of the printer paper has been modified by a wetting method with a coating of micro-size droplets on the paper surface to realize a smooth writing process using the liquid metal ink. The current strategy is expected to enable future flexible electronic circuits on the hard writing substrates.

Inkjet Printed Parallel Plate Capacitors Using PVP Polymer Dielectric Ink on Flexible Polyimide Substrates.

Inkjet printing (IJP) is an exciting new additive manufacturing technology that promises monolithic electronic circuit fabrication of $\mu \mathrm{m}$ thin low-cost large-area electronic low-cost body-worn flexible sensors. In this work, we demonstrate inkjet printed multilayer metal-dielectric-metal capacitors on flexible polyimide (PI) substrate by formulating a custom dielectric ink based on Poly 4-vinylphenol (PVP). Silver nanoparticle ink was used for printing theconductive parallel metal plates. We also demonstrate control over the capacitance values by varying the design parameters and succeeded in printing capacitors in the range 8.8 pF to 467 pF, with excellent repeatability on flexible PI substrates. A functional LC circuit using these printed capacitors has been designed and demonstrated to have a resonance frequency of 24.3 MHz. These results are a vital step forward towards monolithic printing of flexible electronic circuits using IJP technique formany applications such as body-worn sensors.

Inkjet printed flexible electronics on paper substrate with reduced graphene oxide/carbon black ink

A reduced graphene oxide (RGO) and carbon black (CB) ink was fabricated with a mixture of ethanol/ethanediol/propanetriol/deionized water as a solvent, and sodium carboxymethyl cellulose (CMC) as a binding and dispersant. The RGO was obtained by reducing graphene oxide using ascorbic acid as a green reductant at a mild temperature of 95 °C. The flexible paper-based electronic circuits were fabricated by inkjet printing the obtained ink on glossy photo paper substrate with an Epson piezoelectric printer. When the loads of RGO, CB, ethanol, ethylene glycol, glycerol, CMC and deionized water were 96 mg, 504 mg, 12 ml, 30 ml, 30 ml, 480 mg and 51 ml, the electrical conductivity, average particle size and viscosity of the ink were 122.4 µs/cm, 1.966 µm and 22.5 mPa s, respectively; and the ink exhibited good acid resistance. A continuous, dense and uniform conductive network was achieved when the printing pass number was 4 for a single circuit. The resistance at both ends of the aforementioned printed circuit (10 × 2 × 0.03338, length × width × thickness, mm) was 0.1 MΩ with a resistivity of 0.661 Ωm for the ink layer, and the circuits showed moderate uniformity, adhesion and mechanical flexibility. In the light-emitting diode operation, the three-dimensional conductive circuits also presented good electrical conductivity.

Spreading and orientation of silver nano-drops over a flat graphene substrate: An atomistic investigation

The spreading and orientation of silver nano-drop over the graphene substrate are important in various applications such as flexible electronic, bio-medical, electrochemical, and coating materials, etc and have been explored by molecular dynamics simulations. The tuning of interaction energy parameter between graphene and silver affects both spreading and phase transitions of silver nano-drop over the graphene substrate. At high interaction energy, silver nano-drops spread over the graphene substrate and solidified at a higher temperature compared to lower interaction energies. Silver depicts fcc with a layered hcp structure over the graphene substrate during cooling from 1500 K to 10 K. However, at the interface between graphene and silver nano-drop, silver shows the fcc{111} planner orientations. Spreading and orientation of silver nano-drop over the graphene substrate has been characterized by estimation of interaction energies, average number distributions, bond order parameters, the percentage of a crystalline atom during solidification. The results suggest that the silver nano-drop depicts the unique crystal structure with high thermal stability over the graphene substrate compared to the bulk. Therefore, it is extremely desirable material for the development of an efficient catalyst and flexible electronic circuits.