Versatile Solution-Processed Reductive Interface Layer for Contact Engineering of Staggered Organic Field-Effect Transistors.

Efficient charge injection/extraction from/to contact electrodes is essential to realize organic electronic and optoelectronic devices with optimum characteristics for many applications. Herein, we studied a versatile reductive interlayer based on sodium borohydride (NaBH4) to control the contact properties of the staggered organic field-effect transistors (OFETs) either by doping and/or by regulating the contribution of charge carriers. The versatile functionalities of the NaBH4 layer are mainly determined by the alignment of frontier molecular orbitals of donor-acceptor (D-A) type copolymer semiconductors and the work function of the contact electrode. After incorporating the NaBH4 layer, the work function of the bottom-contact gold electrode can be decreased significantly by 1.0 eV, which makes it favorable to efficient electron injection. An Ohmic contact is achieved by the spontaneous injection of electrons to the n-type organic semiconductors with high electron affinity while converting the OFET operation mode to n-type characteristics by blocking the counter-charge carriers for the other types of ambipolar and p-type semiconductors. The solution-processed reducing agent can be a valuable approach to develop high-performance printed and flexible electronic devices through careful engineering to obtain proper contributions of charge carriers either as electrons or holes in various D-A copolymer semiconductors.

Multimodal Capturing of Polysulfides by Phosphorus-Doped Carbon Composites for Flexible High-Energy-Density Lithium-Sulfur Batteries.

The widespread adoption of Li-ion batteries is currently limited by their unstable electrochemical performance and high flammability under mechanical deformation conditions and a relatively low energy density. Herein, high-energy-density lithium-sulfur (Li-S) batteries are developed for applications in next-generation flexible electronics and electric vehicles with long cruising distances. Freestanding high-S-loading carbon nanotubes cathodes are assembled with a phosphorus (P)-doped carbon interlayer coated on commercial separators. Strategies for the active materials and structural design of both the electrodes and separators are highly efficient for immobilizing the lithium polysulfides via multimodal capturing effects; they significantly improve the electrochemical performance in terms of the redox kinetics and cycling stability. The foldable Li-S cells show stable specific capacities of 850 mAh g-1 over 100 cycles, achieving high gravimetric and volumetric energy densities of 387 Wh kgcell -1 and 395 Wh Lcell -1 , respectively. The Li-S cells show highly durable mechanical flexibilities under severe deformation conditions without short circuit or failure. Finally, the Li-S battery is explored as a light-weight and flexible energy storage device aboard airplane drones to ensure at least fivefold longer flight times than traditional Li-ion batteries. Nanocarbon-based S cathodes and P-doped carbon interlayers offer a promising solution for commercializing rechargeable Li-S batteries.

A Novel Strategy to Overcome the Hurdle for Commercial All-Solid-State Batteries via Low-Cost Synthesis of Sulfide Solid Electrolytes.

Unlike commercial lithium-ion batteries, the high cost and low ionic conductivity of solid electrolytes (SEs) continues to be a big hurdle in commercially available all-solid-state batteries (ASSBs). Rather than the conventional dry-process and high-energy ball milling processes, the productive solution synthesis of bulk-type SEs is the most crucial issue in the successful application of high-energy-density ASSBs. In this study, the way is paved to overcome the hurdle for commercial lithium phosphorus sulfide chloride (LPSCl) SEs via a readily processable bulk-type solution-based synthesis without acquiring any high-energy ball-milling processes. By incorporating an elemental sulfur additive during the preparation process, Li2 S and S form a polysulfide, and P2 S5 is induced to react readily to provide LPSCl with excellent ion conductivity as high as 1.8 mS cm-1 . Surprisingly, the purity of bulk type precursors does not affect the final composition and ionic conductivity of sulfide electrolytes, which show the same electrochemical characteristics of ASSB cells with a high discharge capacity of 185.6 mA h g-1 . The study offers a promising strategy for saving the production cost of sulfide SEs, possibly up to 92%, and their commercial ASSBs are expected to be achieving a competitive cost per energy density of ≈0.46 $ W-1 .

Facile fabrication of solution-processed solid-electrolytes for high-energy-density all-solid-state-batteries by enhanced interfacial contact.

Instead of commercial lithium-ion batteries (LIBs) using organic liquid electrolytes, all-solid-state lithium-ion batteries (ASSBs) employing solid electrolytes (SEs) are promising for applications in high-energy-density power applications and electric vehicles due to their potential for improving safety and achieving high capacity. Although remarkable progress in SEs has been achieved and has resulted in high ionic conductivity, which now reaches values comparable to those of liquid electrolytes, the typical use of a slurry process for the fabrication of conventional ASSBs inevitably causes harmful reactions between sulfide materials and polar solvents. Here, we studied the efficient infiltration process of SE slurry into conventional composite LIB electrodes (NCM622) for achieving high-energy-density ASSBs via a scalable solution-based fabrication process. Two methods are proposed to ensure that SE materials are evenly distributed and sufficiently infiltrated into the porous structures of LIB electrodes. The LPSCl SE solutions were effectively infiltrated into the electrodes at higher processing temperatures and the temperature was subsequently optimized at above the boiling point of the ethanol solvent due to the dynamic motion of SE molecules via a convective flow during solvent vaporization. Moreover, the porous LIB composite electrodes with a mixture of active materials of different particle sizes formed and filled capillary pores resulting in a high electrode density. The LPSCl SE-infiltrated NCM622 electrodes that used this strategy could remarkably improve the initial discharge capacity of ASSBs to as high as 177 mAh/g. These ASSBs also showed excellent performance even at high loading values (about 17 mg/cm2), making them competitive with LIBs using conventional liquid electrolytes.

Solution-Processed Nonvolatile Organic Transistor Memory Based on Semiconductor Blends.

Solution-processed nonvolatile organic transistor memory devices are fabricated by employing semiconductor blends of p-channel 6,13-bis(triisopropylsilylethynyl)pentacene and n-channel poly{[ N, N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]- alt-5,5'-(2,2'-bithiophene)} (P(NDI2OD-2T); N2200) on polystyrene-brush as a polymer electret. Electret-based memory characteristics are significantly changed depending on the frontier molecular orbitals of the active semiconductors because the charge-trapping efficiency is mainly determined by the energy barrier to transfer electrons and holes from the active channel to the electret layer. A semiconductor mixture with an optimized blending ratio results in an efficient programming and erasing process. Thus, we obtained a remarkably high ratio of ON/OFF current (memory ratio) about 107 and a large amount of shifts in the threshold voltage (memory window) between the programmed and erased states of 55 V, while single-component N2200 showed only writing-once-read-many (WORM)-type memory. Especially, the programmed data can be stably retained more than 10 years with a sufficient memory ratio of 103. Furthermore, our semiconductor blend system leads to preferable vertical phase separation, which affords good reliability under a sequential memory operation condition as well as stability in ambient air. It is expected that our memory devices can be applied for versatile data storage in printed and flexible electronic applications.

High Throughput Bar-Coating Processed Organic-Inorganic Hybrid Multi-Layers for Gas Barrier Thin-Films.

Herein, we demonstrate the preparation of a scalable bar-coated nanocomposite organic-inorganic hybrid film and developed robust barrier films for general purpose packaging. Using combinatory printing of polymers and nanocomposites by bar coating, a facile and effective barrier film fabrication method was developed. Based on a preliminary survey with several material combinations, a rationalized two-fold nanocomposite film was fabricated. The number of layers in the barrier film significantly modified oxygen barrier performance such that, for the 1 wt% ethylene vinyl alcohol (EVOH) intercalated film, the oxygen transmission rate (OTR) of the 5-layer sample was reduced to 31.69% of the OTR of the 3-layer sample (112.8 vs. 35.75 cc/(m² · day)). In addition, fine tuning the amount of EVOH polymer enabled further improvement of oxygen barrier performance. Intercalation of 2 wt% EVOH resulted an OTR improvement from 35.75 in the 1 wt% sample to 11.90 cc/(m² ·day), which is a 4.25-fold enhancement. Overall barrier characteristics proved that our approach could be used for large-area deposited, oxygen resistant, general purpose packaging applications.

Ultrafast Heating for Intrinsic Properties of Atomically Thin Two-Dimensional Materials on Plastic Substrates.

Despite recent progress in producing flexible and stretchable electronics based on two-dimensional (2D) nanosheets, their intrinsic properties are often degraded by the presence of polymeric residues that remain attached to the 2D nanosheet surfaces following fabrication. Further breakthroughs are therefore keenly awaited to obtain clean surfaces compatible with flexible applications. Here, we report a method that allows the 2D nanosheets to be intrinsically integrated onto flexible substrates. The method involves thermal decomposition of polymeric residues by microwave-induced ultrafast heating of the surface without affecting the underlying flexible substrate. Mapping the C═O stretching mode by Fourier-transform infrared spectroscopy in combination with atomic force microscopy confirms elimination of the polymeric residues from the 2D nanosheet surface. Flexible devices prepared using microwave-cleaned 2D nanosheets show enhanced electrical, optical, and electrothermal performances. This simple technique is applicable to a wide range of 2D nanomaterials and represents an important advance in the field of flexible devices.

Organic nano-floating-gate transistor memory with metal nanoparticles.

Organic non-volatile memory is advanced topics for various soft electronics applications as lightweight, low-cost, flexible, and printable solid-state data storage media. As a key building block, organic field-effect transistors (OFETs) with a nano-floating gate are widely used and promising structures to store digital information stably in a memory cell. Different types of nano-floating-gates and their various synthesis methods have been developed and applied to fabricate nanoparticle-based non-volatile memory devices. In this review, recent advances in the classes of nano-floating-gate OFET memory devices using metal nanoparticles as charge-trapping sites are briefly reviewed. Details of device fabrication, characterization, and operation mechanisms are reported based on recent research activities reported in the literature.

Large Enhancement of Carrier Transport in Solution-Processed Field-Effect Transistors by Fluorinated Dielectric Engineering.

The universal role of high-k fluorinated dielectrics in assisting the carrier transport in transistors for a broad range of printable semiconductors is explored. These results present general rules for how to design dielectric materials and achieve devices with a high carrier concentration, low disorder, reliable operation, and robust properties.

Synergistic High Charge-Storage Capacity for Multi-level Flexible Organic Flash Memory.

Electret and organic floating-gate memories are next-generation flash storage mediums for printed organic complementary circuits. While each flash memory can be easily fabricated using solution processes on flexible plastic substrates, promising their potential for on-chip memory organization is limited by unreliable bit operation and high write loads. We here report that new architecture could improve the overall performance of organic memory, and especially meet high storage for multi-level operation. Our concept depends on synergistic effect of electrical characterization in combination with a polymer electret (poly(2-vinyl naphthalene) (PVN)) and metal nanoparticles (Copper). It is distinguished from mostly organic nano-floating-gate memories by using the electret dielectric instead of general tunneling dielectric for additional charge storage. The uniform stacking of organic layers including various dielectrics and poly(3-hexylthiophene) (P3HT) as an organic semiconductor, followed by thin-film coating using orthogonal solvents, greatly improve device precision despite easy and fast manufacture. Poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] as high-k blocking dielectric also allows reduction of programming voltage. The reported synergistic organic memory devices represent low power consumption, high cycle endurance, high thermal stability and suitable retention time, compared to electret and organic nano-floating-gate memory devices.

Enhanced response and sensitivity of self-corrugated graphene sensors with anisotropic charge distribution.

We introduce a high-performance molecular sensor using self-corrugated chemically modified graphene as a three dimensional (3D) structure that indicates anisotropic charge distribution. This is capable of room-temperature operation, and, in particular, exhibiting high sensitivity and reversible fast response with equilibrium region. The morphology consists of periodic, "cratered" arrays that can be formed by condensation and evaporation of graphene oxide (GO) solution on interdigitated electrodes. Subsequent hydrazine reduction, the corrugated edge area of the graphene layers have a high electric potential compared with flat graphene films. This local accumulation of electrons interacts with a large number of gas molecules. The sensitivity of 3D-graphene sensors significantly increases in the atmosphere of NO2 gas. The intriguing structures have several advantages for straightforward fabrication on patterned substrates, high-performance graphene sensors without post-annealing process.

Chemically doped three-dimensional porous graphene monoliths for high-performance flexible field emitters.

Despite the recent progress in the fabrication of field emitters based on graphene nanosheets, their morphological and electrical properties, which affect their degree of field enhancement as well as the electron tunnelling barrier height, should be controlled to allow for better field-emission properties. Here we report a method that allows the synthesis of graphene-based emitters with a high field-enhancement factor and a low work function. The method involves forming monolithic three-dimensional (3D) graphene structures by freeze-drying of a highly concentrated graphene paste and subsequent work-function engineering by chemical doping. Graphene structures with vertically aligned edges were successfully fabricated by the freeze-drying process. Furthermore, their number density could be controlled by varying the composition of the graphene paste. Al- and Au-doped 3D graphene emitters were fabricated by introducing the corresponding dopant solutions into the graphene sheets. The resulting field-emission characteristics of the resulting emitters are discussed. The synthesized 3D graphene emitters were highly flexible, maintaining their field-emission properties even when bent at large angles. This is attributed to the high crystallinity and emitter density and good chemical stability of the 3D graphene emitters, as well as to the strong interactions between the 3D graphene emitters and the substrate.

Rearrangement of 1D conducting nanomaterials towards highly electrically conducting nanocomposite fibres for electronic textiles.

Nanocarbon-based conducting fibres have been produced using solution- or dry-spinning techniques. Highly conductive polymer-composite fibres containing large amounts of conducting nanomaterials have not been produced without dispersants, however, because of the severe aggregation of conducting materials in high-concentration colloidal solutions. Here we show that highly conductive (electrical conductivity ~1.5 × 10(5) S m(-1)) polymer-composite fibres containing carbon nanotubes and silver nanowires can be fabricated via a conventional solution-spinning process without any other treatment. Spinning dopes were fabricated by a simple mixing of a polyvinyl alcohol solution in dimethylsulfoxide with a paste of long multi-walled carbon nanotubes dispersed in organic solvents, assisted by quadruple hydrogen-bonding networks and an aqueous silver nanowire dispersion. The high electrical conductivity of the fibre was achieved by rearrangement of silver nanowires towards the fibre skin during coagulation because of the selective favourable interaction between the silver nanowires and coagulation solvents. The prepared conducting fibres provide applications in electronic textiles such as a textile interconnector of light emitting diodes, flexible textile heaters, and touch gloves for capacitive touch sensors.

Monolithic Graphene Trees as Anode Material for Lithium Ion Batteries with High C-Rates.

Monolithically structured reduced graphene oxide (rGO), prepared from a highly concentrated and conductive rGO paste, is introduced as an anode material for lithium ion batteries with high rate capacities. This is achieved by a mixture of rGO paste and the water-soluble polymer sodium carboxymethylcellulose (SCMC) with freeze drying. Unlike previous 3D graphene porous structures, the monolithic graphene resembles densely branched pine trees and has high mechanical stability with strong adhesion to the metal electrodes. The structures contain numerous large surface area open pores that facilitate lithium ion diffusion, while the strong hydrogen bonding between the graphene layers and SCMC provides high conductivity and reduces the volume changes that occur during cycling. Ultrafast charge/discharge rates are obtained with outstanding cycling stability and the capacities are higher than those reported for other anode materials. The fabrication process is simple and straightforward to adjust and is therefore suitable for mass production of anode electrodes for commercial applications.

Suppressing spontaneous polarization of p-GaN by graphene oxide passivation: augmented light output of GaN UV-LED.

GaN-based ultraviolet (UV) LEDs are widely used in numerous applications, including white light pump sources and high-density optical data storage. However, one notorious issue is low hole injection rate in p-type transport layer due to poorly activated holes and spontaneous polarization, giving rise to insufficient light emission efficiency. Therefore, improving hole injection rate is a key step towards high performance UV-LEDs. Here, we report a new method of suppressing spontaneous polarization in p-type region to augment light output of UV-LEDs. This was achieved by simply passivating graphene oxide (GO) on top of the fully fabricated LED. The dipole layer formed by the passivated GO enhanced hole injection rate by suppressing spontaneous polarization in p-type region. The homogeneity of electroluminescence intensity in active layers was improved due to band filling effect. As a consequence, the light output was enhanced by 60% in linear current region. Our simple approach of suppressing spontaneous polarization of p-GaN using GO passivation disrupts the current state of the art technology and will be useful for high-efficiency UV-LED technology.

Charge-trap flash-memory oxide transistors enabled by copper-zirconia composites.

A solution-processed electrochemical charge-trap flash memory element is based on a solid solution of copper and zirconium oxides (Cu-ZrO2) as a charge-trapping layer. Because of the facile reduction of Cu(2+) to Cu(1+), Cu-ZrO2 thin films are especially effective in memory devices based on thin-film transistors when the devices are fabricated from combustion-processed metal-oxide semiconductors (In2O3 and an indium-gallium oxide).

Stable charge storing in two-dimensional MoS2 nanoflake floating gates for multilevel organic flash memory.

In this study, we investigated chemically exfoliated two-dimensional (2-D) nanoflakes of molybdenum disulfide (MoS2) as charge-storing elements for use in organic multilevel memory devices (of the printed/flexible non-volatile type) based on organic field-effect transistors (OFETs) containing poly(3-hexylthiophene) (P3HT). The metallic MoS2 nanoflakes were exfoliated in 2-methoxyethanol by the lithium intercalation method and were deposited as nano-floating gates between polystyrene and poly(methyl methacrylate), used as bilayered gate dielectrics, by a simple spin-coating and low temperature (<150 °C) process. In the developed OFET memory devices, electrons could be trapped/detrapped in the MoS2 nano-floating gates by modulating the charge carrier density in the active channel through gate bias control. Optimal memory characteristics were achieved by controlling the thickness and concentration of few-layered MoS2 nanoflakes, and the best device showed reliable non-volatile memory properties: a sufficient memory window of ∼23 V, programming-reading-erasing cycling endurance of >10(2) times, and most importantly, quasi-permanent charge-storing characteristics, i.e., a very long retention time (longer than the technological requirement of commercial memory devices (>10 years)). In addition, we successfully developed multilevel memory cells (2 bits per cell) by controlling the gate bias magnitude.

Regulating charge injection in ambipolar organic field-effect transistors by mixed self-assembled monolayers.

We report on a technique using mixed self-assembled monolayers (SAMs) to finely regulate ambipolar charge injection in polymer organic field-effect transistors. Differing from the other works that employ single SAM specifically for efficient charge injection in p-type and n-type transistors, we blend two different SAMs of alkyl- and perfluoroalkyl thiols at different ratios and apply them to ambipolar OFETs and inverter. Thanks to the utilization of ambipolar semiconductor and one SAM mixture, the device and circuit fabrications are facile with only one step for semiconductor deposition and another for SAM treatment. This is much simpler with respect to the conventional scheme for the unipolar-device-based complementary circuitry that demands separate deposition and processing for individual p-channel and n-channel transistors. Our results show that the mixed-SAM treatments not only improve ambipolar charge injection manifesting as higher hole- and electron-mobility and smaller threshold voltage but also gradually tune the device characteristics to reach a desired condition for circuit application. Therefore, this simple but useful approach is promising for ambipolar electronics.

Extremely efficient liquid exfoliation and dispersion of layered materials by unusual acoustic cavitation.

Layered materials must be exfoliated and dispersed in solvents for diverse applications. Usually, highly energetic probe sonication may be considered to be an unfavourable method for the less defective exfoliation and dispersion of layered materials. Here we show that judicious use of ultrasonic cavitation can produce exfoliated transition metal dichalcogenide nanosheets extraordinarily dispersed in non-toxic solvent by minimising the sonolysis of solvent molecules. Our method can also lead to produce less defective, large graphene oxide nanosheets from graphite oxide in a short time (within 10 min), which show high electrical conductivity (>20,000 S m(-1)) of the printed film. This was achieved by adjusting the ultrasonic probe depth to the liquid surface to generate less energetic cavitation (delivered power ~6 W), while maintaining sufficient acoustic shearing (0.73 m s(-1)) and generating additional microbubbling by aeration at the liquid surface.

Simultaneous enhancement of electron injection and air stability in N-type organic field-effect transistors by water-soluble polyfluorene interlayers.

Here, we report the simultaneous attainment of efficient electron injection and enhanced stability under ambient conditions for top-gate/bottom-contact (TG/BC), n-type, organic field-effect transistors (OFETs) using water-soluble polyfluorene derivatives (WPFs). When inserting the WPF interlayers between a semiconductor and the BC Au electrodes, initially the ambipolar (6,6)-phenyl-C61butyric acid methyl ester (PCBM) OFETs were fully converted to unipolar charge transport characteristics that were exclusively n-type with significantly increased electron mobilities as high as 0.12 cm(2)/(V s) and a decreased threshold voltage. These improvements were mostly attributed to the interfacial dipoles of WPF layers that aligned to form a favorable energy band structure for efficient electron injection and to effectively block counter charge carriers. These were confirmed when values for the reduced work function of metal electrodes with WPFs and their correlated contact resistance were measured via the ultraviolet photoemission spectroscopy and the transmission-line method, respectively. Moreover, the WPF interlayers played an important role in air stability of PCBM OFETs that exhibited higher and appreciably enhanced by increasing the ethylene-oxide side chain lengths of WPFs, which presumably was due to the water/oxygen/ion capturing effects in the hydrophilic interlayers.