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Transparent conducting electrodes (TCEs) have attracted considerable attention towards the development of flexible optoelectronic devices. In this study, mixed-dimensional TCEs are fabricated based on the two-dimensional graphene and one-dimensional electrospun metal fiber that can address the shortcomings of each electrode. In comparison with other TCEs, the Ag fiber/graphene hybrid electrodes exhibited a highly stable morphology (67% lower peak-to-valley ratio), low sheet resistance (approximately 11 Ω/sq), high transmittance (approximately 94%), high oxidation stability with excellent flexibility, and outstanding chemical stability. The multiple functionalities of the transparent and flexible hybrid structure highlight its potential for applications in emerging electronics and highly stable optoelectronics.

In this study, high-performance few-layered ReS2 field-effect transistors (FETs), fabricated with hexagonal boron nitride (h-BN) as top/bottom dual gate dielectrics, are presented. The performance of h-BN dual gated ReS2 FET having a trade-off of performance parameters is optimized using a compact model from analytical choice maps, which consists of three regions with different electrical characteristics. The bottom h-BN dielectric has almost no defects and provides a physical distance between the traps in the SiO2 and the carriers in the ReS2 channel. Using a compact analyzing model and structural advantages, an excellent and optimized performance is introduced consisting of h-BN dual-gated ReS2 with a high mobility of 46.1 cm2 V−1 s−1, a high current on/off ratio of ≈106, a subthreshold swing of 2.7 V dec−1, and a low effective interface trap density (Nt,eff) of 7.85 × 1010 cm−2 eV−1 at a small operating voltage (2 channel through Y-function method as a function of constant top gate bias.

To extract the confined waveguided light in organic light-emitting diodes (OLEDs), inserting a low refractive index (RI) periodic structure between the anode and organic layer has been widely investigated as a promising technology. However, the periodic-structure-based light extraction applied inside devices has been shown to severely distort spectrum and affect EL characteristics. In this study, a simple light extraction technology using periodic low-RI nanodot array (NDA) as internal light extraction layer has been demonstrated. The NDA was fabricated simply via laser interference lithography (LIL). The structural parameters of periodic pattern, distance, and height were easily controlled by the LIL process. From computational analysis using finite-difference time-domain (FDTD) method, the NDA with 300 nm pitch and 0.3 coverage ratio per unit cell with 60 nm height showed the highest enhancement with spectral-distortion-minimized characteristics. Through both computational and experimental systematic analysis on the structural parameters of low-RI NDA-embedded OLEDs, highly efficient OLEDs have been fabricated. Finally, as representative indicators, hexagonal and rectangular positioned NDA-embedded OLEDs showed highly improved external quantum efficiencies of 2.44 (+29.55%) and 2.77 (+57.38%), respectively. Furthermore, the disadvantage originating from the nanoscale surface roughness on the transparent conductive oxide was minimized.

Transition-metal dichalcogenide (TMD) materials with two-dimensional layered structures and stable surfaces are well suited for transparent and flexible device applications. In order to completely utilize the advantages of thickness control and fabrication of various heterostructure stacks, we proposed a transfer method of TMD field-effect transistors (FETs) and TMD complementary metal-oxide-semiconductor (CMOS) circuits from a Si/SiO2 substrate to a flexible substrate. We compared the characteristics of transferred MoS2 and WSe2 FETs with those of the corresponding devices transferred after channel passivation with an Al2O3 layer on a flexible substrate. Al2O3 passivation further stabilized the transfer of the entire device with electrodes. A CMOS circuit with MoS2 and WSe2 materials could be successfully transferred to a polyethylene terephthalate substrate after the channel passivation. This implies that TMD circuits can be easily fabricated on polymer substrates, which makes them suitable for use in semiconductor processes, for various applications.

Single-walled carbon nanotube (SWCNT) is generally used as a networked structure in the fabrication of a field-effect transistor (FET) since it is known that one-third of SWCNT is electrically metallic and the remains are semiconducting. In this case, the presence of metallic paths by metallic SWCNT (m-SWCNT) becomes a significant technical barrier which hinders the networks from achieving a semiconducting behavior, resulting in a low on/off ratio. Here, we report on an easy method of controlling the on/off ratio of a FET where semiconducting SWCNT (s-SWCNT) and m-SWCNT constitute networks between source and drain electrodes. A FET with SWCNT networks was simply sonicated under water to control the on/off ratio and network density. As a result, the FET having an almost metallic behavior due to the metallic paths by m-SWCNT exhibited a p-type semiconducting behavior. The on/off ratio ranged from 1 to 9.0 × 104 along sonication time. In addition, theoretical calculations based on Monte-Carlo method and circuit simulation were performed to understand and explain the phenomenon of a change in the on/off ratio and network density by sonication. On the basis of experimental and theoretical results, we found that metallic paths contributed to a high off-state current which leads to a low on/off ratio and that sonication formed sparse SWCNT networks where metallic paths of m-SWCNT were removed, resulting in a high on/off ratio. This method can open a chance to save the device which has been considered as a failed one due to a metallic behavior by a high network density leading to a low on/off ratio.

Organic light-emitting diodes (OLEDs) with an enhanced outcoupling efficiency and a suppressed efficiency roll-off were fabricated by inserting a nanosize pixel-defining layer (nPDL) that defines the OLED emission region as an array of nanoholes. The outcoupling efficiency of the nano-arrayed OLEDs was increased through the reduced surface plasmon polariton loss caused by the wavy diffraction grating at the metal–organic interface, and their efficiency roll-off was suppressed through the diffusive exciton outside the exciton-formation zone. As a result, the nano-arrayed OLEDs exhibited enhancements of 148.7% in the power efficiency and 137.0% in the external quantum efficiency at 1000 cd m−2 compared with a reference device. Furthermore, the critical current density (J0) where the external quantum efficiency decreased to half of its initial value was improved by a factor of 2.5.

In this study, we designed a smooth, highly flexible, mechanically robust poly(vinyl-butyral) (PVB)/silver nanowire (AgNW) composite transparent conducting electrode (TCE) integrated with a random nanocone (RNC) to enhance the light extraction of flexible organic light-emitting diodes (OLEDs). The RNC was fabricated by reactive-ion etching (RIE) on AgNW embedded in PVB. As the etching time increased, the size of the RNC became larger. The sheet resistance and transmittance of PVB/AgNW with the RNC was 21.7 Ω/sq and ~87%, respectively. For the PVB/AgNW, the change in sheet resistance was only 2.6% when a 2,000-bend test was performed. The maximum external quantum efficiency was 28.3% when RNC 700 s was used as a green phosphorescent OLED. In addition, for current efficiency and power efficiency, RNC 700 s increased 1.4 times over RNC 0 s. RNC is free of viewing-angle-dependent color and brightness distortion. PVB/AgNW and RNC are practical ways to overcome the brittleness of conventional indium tin oxide and improve the efficiency of flexible OLEDs. Finally, this product is expected to be applied to various flexible optical devices.

Electrospun metal fiber is a promising flexible transparent electrode owing to its extremely long length and facile fabrication process. However, metal-fiber electrodes have problems with chemical and thermal stability and nonuniform emission in organic light-emitting diode (OLED) at low luminance. In this study, we proposed a Ag fiber/IZO composite electrode with high stability. Ag fiber/IZO composite electrodes exhibited chemical and thermal stability. In addition, it was demonstrated that the OLED with the Ag fiber/IZO composite electrode operated stably, and the uniform emission of the OLED with metal-fiber electrodes improved by using highly conductive IZO film.

This work was financially supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) (Grant No. 20143030011850), and funded by the Ministry of Trade, Industry and Energy (MOTIE). I. Kim acknowledges KUUC (KIST-UNIST-Ulsan Center for Convergent Materials) for partial financial support.

In this work, graphene field effect transistors (FETs) were fabricated on a trench structure made by carbonized poly(methylmethacrylate) to modify the graphene surface. The trench-structured devices showed different characteristics depending on the channel orientation and the pitch size of the trenches as well as channel area in the FETs. Periodic corrugations and barriers of suspended graphene on the trench structure were measured by atomic force microscopy and electrostatic force microscopy. Regular barriers of 160 mV were observed for the trench structure with graphene. To confirm the transfer mechanism in the FETs depending on the channel orientation, the ratio of experimental mobility (3.6-3.74) was extracted from the current-voltage characteristics using equivalent circuit simulation. It is shown that the number of barriers increases as the pitch size decreases because the number of corrugations increases from different trench pitches. The noise for the 140 nm pitch trench is 1 order of magnitude higher than that for the 200 nm pitch trench.

In this study, random nanoscale rods (RNRs) with a double refractive index were fabricated via spin coating, dry etching, and sputtering, which are processes that are extensively applied in industry. With regard to optical properties, the RNRs with a double refractive index (RNRsD) exhibited a total transmittance that was >90% in the visible range and an optical haze in the range of 42%-50% at a wavelength of 520 nm. Organic light-emitting diodes (OLEDs) with RNRsD, where SiO2 was deposited on the RNRs via radiofrequency sputtering, exhibited an enhancement of 34.5% in the external quantum efficiency compared with OLEDs with the bare substrate. Furthermore, the color variation of the OLEDs with the optimal RNRsD with respect to a change in the viewing angle was improved from color coordinates of Δ(x, y) = (0.032, 0.034) to Δ(x, y) = (0.014, 0.014). Therefore, the proposed film can be used as a scattering layer for enhancing the light extraction and viewing angle of OLEDs by reducing the substrate mode light loss and changing the direction of light. In addition to using a low-temperature fabrication process that does not employ a photomask and a lithographic template, the proposed method is applicable to flexible devices because it uses a polymer and a thin inorganic film.

Light extraction in organic light-emitting diodes (OLEDs) was improved by applying SnO x nanocones grown via thermal annealing in a low-O2 atmosphere. SnO x was easily fabricated through thermal processing after Sn deposition. The diameter of the SnO x nanocones was controlled by changing the deposition thickness of Sn. The SnO x nanocones induced strong Mie scattering, which reduced the total internal reflection in the glass substrate. Consequently, the OLED with SnO x nanocones exhibited a 23% increase in the external quantum efficiency compared with a reference device.

We demonstrated light extraction improvement by applying a scattering layer of Ag nanoparticles physically synthesized through a low-temperature annealing process to flexible organic light-emitting diodes (OLEDs). In general, increasing the size of Ag nanoparticles is preferred to increase light scattering, but a high-temperature annealing process (∼400 °C) is required to produce them. However, flexible substrates generally cannot withstand high-temperature processes. In this study, we formed Ag nanoparticles at a low temperature of ∼200 °C by inserting a poly(3,4-ethylenedioxythiophene) polystyrene sulfonate buffer layer, thus promoting Ag dewetting. As a result, the scattering layer of enlarged Ag nanoparticles formed at low temperatures increased the external quantum efficiency by 24% in a flexible OLED compared to a reference device.

We investigated a low-temperature mask-free process for preparing random nanoscale rods (RNRs) as a scattering layer. The process involves spin coating and dry etching, which are already widely applied in industry. Our film exhibited 17–33% optical haze at 520 nm wavelength and 95% total transmittance in the visible range. Therefore, this film can be used as a scattering layer for improving viewing angle characteristics and decreasing substrate mode loss in organic light-emitting diodes (OLEDs). Specifically, we focussed on varying the height and density of the RNRs to control the optical characteristics. As a result, the OLEDs with RNRs revealed a variation in colour coordinates of Δ(x, y) = (0.007, 0.014) for a change in the viewing angle, which was superior to those without the RNRs that displayed a variation of Δ(x, y) = (0.020, 0.034) in CIE 1931. Moreover, the OLEDs with RNRs exhibited 31% enhanced external quantum efficiency compared to those of the OLEDs with the bare substrate. The flexibility of the polymer used for the RNRs and the plasma treatment suggests that the RNRs can be applied to flexible OLED displays and lighting systems.

We report on p-type and n-type thermoelectric (TE) materials made of single-walled carbon nanotube (SWCNT) networks incorporated into the cellulose fiber structure of a common packaging paper. This leads the paper to possess both mechanical flexibility from the cellulose fibers as a supporting matrix and the high electrical conductivity originating from the SWCNTs. Thermoelectric power of up to ±50 μV K−1 was successfully obtained as well, depending on their electronic type. Further, to demonstrate its thermoelectric voltage (VTEP) and generating power, a couple of thermoelectric modules composed of both p-type and n-type composite layers were assembled in series. The produced VTEP shows a quasi-linearity with respect to the number of p–n couples and the temperature difference ΔT. Our testing module enables the provision of VTEP and power generation as large as ≈16.8 mV and ≈75.5 nW upon inducing a 50 K temperature difference. The feasibility of commercial TE modules consisting of 10, 100 and 1000 p–n SWCNT couples was numerically calculated, taking into account our experimental results.

With the advances in the separation and purification of carbon nanotubes (CNTs), the use of highly pure metallic or semiconducting CNTs has practical merit in electronics applications. When highly pure CNTs are applied in various fields, CNT networks are preferred to individual CNTs. In such cases, the presence of an electrical path becomes crucial in the network. In this study, we report on the electrical percolation thresholds of semiconducting single-walled carbon nanotube (s-SWCNT) networks, and their electrical characteristics in field-effect transistors (FET). Using the Monte Carlo method, s-SWCNT networks were randomly generated in the channels defined by the source-drain electrodes of the FET. On the basis of percolation theory, the percolation thresholds of s-SWCNT networks were obtained at different channel lengths (2, 6, and 10 μm) by generating random s-SWCNT networks 100 times. The network density corresponding to the electrical percolation threshold was theoretically gained at each channel length. As a result, the network densities at the percolation thresholds for the channel lengths of 2, 6, and 10 μm were 6.8, 9.0, and 9.9 tube μm(-2), respectively. In addition, SPICE calculations were performed for each s-SWCNT network, constituting an electrical path between the source and the drain electrodes of the FET. In all channel lengths, the on/off ratio of the s-SWCNT networks was enhanced with increasing network density. Finally, we found a power law relationship between the on/off ratio of the s-SWCNT networks and the network density at the percolation threshold.

The practical applicability of electronic devices is largely determined by the reliability of field effect transistors (FETs), necessitating constant searches for new and better-performing semiconductors. We investigated the stress-induced degradation of MoS2 multilayer FETs, revealing a steady decrease of drain current by 56% from the initial value after 30 min. The drain current recovers to the initial state when the transistor is completely turned off, indicating the roles of soft-traps in the apparent degradation. The noise current power spectrum follows the model of carrier number fluctuation–correlated mobility fluctuation (CNF–CMF) regardless of stress time. However, the reduction of the drain current was well fitted to the increase of the trap density based on the CNF–CMF model, attributing the presence of the soft-type traps of dielectric oxides to the degradation of the MoS2 FETs.

Fabrication of junction-free Ag fiber electrodes for flexible organic light-emitting diodes (OLEDs) is demonstrated. The junction-free Ag fiber electrodes are fabricated by electrospun polymer fibers used as an etch mask and wet etching of Ag thin film. This process facilitates surface roughness control, which is important in transparent electrodes based on metal wires to prevent electrical instability of the OLEDs. The transmittance and resistance of Ag fiber electrodes can be independently adjusted by controlling spinning time and Ag deposition thickness. The Ag fiber electrode shows a transmittance of 91.8% (at 550 nm) at a sheet resistance of 22.3 Ω □-1 , leading to the highest OLED efficiency. In addition, Ag fiber electrodes exhibit excellent mechanical durability, as shown by measuring the change in resistance under repeatable mechanical bending and various bending radii. The OLEDs with Ag fiber electrodes on a flexible substrate are successfully fabricated, and the OLEDs show an enhancement of EQE (≈19%) compared to commercial indium tin oxide electrodes.

We investigated the induction heating effect on device characteristics of flexible molybdenum disulfide (MoS2) field-effect transistors (FETs). A polyimide film was employed as a flexible substrate, and poly(3,4-ethylenedioxythiophene):polystyrene sulfonate was coated on the flexible substrate as a bottom gate. After the annealing process on the flexible MoS2 FETs by induction heating, the field effect mobility was enhanced from 2.13 to 5.58 cm2/V·s with the slight increase of the on-off ratio from 5.17 × 102 to 1.98 × 103. Moreover, the low field mobility was almost unchanged from 7.75 to 7.33 cm2/V·s, indicating that the induction heating mainly contributed to the enhancement of the device performances by contact improvement between electrodes and MoS2. With the simple model of the diode and resistor connected in series, it was confirmed that the Schottky diode disappeared with contact enhancement. Our findings can contribute to the contact improvement with minimum damage when low dimensional nanomateri...

To develop the advanced electronic devices, the surface/interface of each component must be carefully considered. Here, we investigate the electrical properties of metal-semiconductor nanoscale junction using conductive atomic force microscopy (C-AFM). Single-crystalline CdS, CdSe, and ZnO one-dimensional nanostructures are synthesized via chemical vapor transport, and individual nanobelts (or nanowires) are used to fabricate nanojunction electrodes. The current-voltage (I -V) curves are obtained by placing a C-AFM metal (PtIr) tip as a movable contact on the nanobelt (or nanowire), and often exhibit a resistive switching behavior that is rationalized by the Schottky (high resistance state) and ohmic (low resistance state) contacts between the metal and semiconductor. We obtain the Schottky barrier height and the ideality factor through fitting analysis of the I-V curves. The present nanojunction devices exhibit a lower Schottky barrier height and a higher ideality factor than those of the bulk materials, which is consistent with the findings of previous works on nanostructures. It is shown that C-AFM is a powerful tool for characterization of the Schottky contact of conducting channels between semiconductor nanostructures and metal electrodes.

We demonstrate the fabrication of shallow Si nanopillar structures at a submicron scale which provides broadband antireflection for crystalline Si (c-Si) solar cells in the wavelength range of 350 nm–1100 nm. The Si random nanopillars were made by reactive ion etch (RIE) processing with thermally dewetted Sn metals as an etch mask. The diameters and coverages of the Si nanopillars were adjusted in a wide range of the nanoscale to microscale by varying the nominal thickness of the Sn metals and subsequent annealing temperatures. The height of the nanopillars was controlled by the RIE process time. The optimal size of the nanopillars, which are 340 nm in diameter and 150 nm in height, leads to the lowest average reflectance of 3.6%. We showed that the power conversion efficiency of the c-Si solar cells could be enhanced with the incorporation of optimally designed Si random nanopillars from 13.3% to 14.0%. The fabrication scheme of the Si nanostructures we propose in this study would be a cost-effective and promising light trapping technique for efficient c-Si solar cells.