All-solution-processed foldable transparent electrodes of Ag nanowire mesh and metal matrix films for flexible electronics

In this study, we developed foldable transparent electrodes composed of Ag nanowire (AgNW) networks welded by Ag nanoparticles (AgNPs) reduced from commercial Ag ink. All the processes used were solution-based. Using the Meyer rod method, uniform AgNW networks were roll-to-roll coated on large-area polymer substrates, and the spin-coated AgNPs firmly welded the AgNWs together at junctions and to substrates. The hybrid films consisting of AgNWs and the Ag film matrix exhibited higher electrical conductivity (5.0–7.3 × 10⁵ S/m) than and equivalent transparency (90–95%) to the AgNW networks. Furthermore, the hybrid films showed significantly better bending stability than AgNW networks. During cyclic bending tests to 10,000 cycles at 5 mm bending radius and even when almost folded with r_b of 1 mm, the resistivity changes were negligible because AgNWs were tightly held and adhered to the substrate by Ag films covering wires, thereby hindering fracturing of AgNWs under tension. Because the films were fabricated at a low temperature, there was no oxidation on the surfaces of the films. Hence, flexible organic light-emitting diodes (f-OLEDs) were successfully fabricated on polyethylene terephthalates (PET) coated with the hybrid films. The f-OLED in the bent state was comparable to that in the flat state, validating the potential applications of these transparent hybrid films as electrodes in various flexible electronics.     

Neutral-pH PEDOT:PSS as over-coating layer for stable silver nanowire flexible transparent conductive films

The silver nanowire (AgNW) mesh film with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the over-coating layer is a promising flexible transparent conductive film technology. In this work, experimental studies show that the hygroscopic and acid properties of the common PEDOT:PSS lead to poor stabilities of the composite films, due to the conductivity degradation of PEDOT:PSS by the water absorption and the acid corrosion of AgNWs by PEDOT:PSS. By using the modified PEDOT:PSS of neutral pH as the over-coating layer, the long term shelf-life time, thermal and current stressing stabilities are all significantly improved without sacrifice of transparency, electrical conductivity and mechanical flexibility. Under both cases of thermal aging test at 210 °C for 20 min and 12 h continuous current stressing at a current density of 30 mA/cm², no obvious change of the conductivity is observed. The results clearly demonstrate that using the neutral-pH PEDOT:PSS as an over-coating layer can help to achieve flexible AgNW transparent conductive films with superior stability for flexible optoelectronic devices.     

Properties of transparent conductive ZnO:Al thin films prepared by magnetron sputtering

Middle-frequency alternative magnetron sputtering was used to deposit transparent conductive ZAO (ZnO:Al) thin films with ZAO (98 wt%ZnO+2 wt%Al₂O₃) ceramic target on glass and Si wafers. The influences of the various deposition parameters on the structural, optical and electrical performances of ZAO films have been studied. The structural characteristics of the films were investigated by the X-ray diffractometer and atomic force microscope, while the visible transmittance, carrier concentration and Hall mobility were studied by UV-VIS and the Hall tester, respectively. The lowest resistivity obtained in the work was 4.6×10⁻⁴ Ω cm for the film with average transmittance of 90.0% within the visible wavelength range and sheet resistance of 32 Ω, which was deposited at 250 °C and 0.8 Pa.     

Crystallization of sexiphenyl induced by polyurethane containing terphenyl groups affording high-mobility organic thin-film transistor

Novel polyurethane containing terphenyl groups were designed and synthesized as gate insulators to induce the crystallization of p-sexiphenyl(p-6P) for organic thin-film transistors (OTFTs). Different sizes and shapes of p-6P grains were measured by atomic force microscopy (AFM), and results showed that the large size of p-6P grain can improve the performance of OTFTs. About 900 nm thick films can be easily fabricated by spin-coating under ambient conditions, followed by curing at UV irradiation for 10 min. OTFTs with this film as gate insulator were found to have good processability, a high charge-carrier mobility of 1.1 cm²/V s, a threshold voltage of −25 V, and an on/off current ratio >10⁵. The result indicated that this material is a promising candidate for the exploration of devices using OTFTs.     

A printing technology combining screen-printing with a wet-etching process for the gate electrodes of organic thin film transistors on a plastic substrate

We have developed a practical printing technology for the gate electrode of organic thin film transistors (OTFTs) by combining screen-printing with a wet-etching process using nano-silver (Ag) ink as a conducting material. An Ag film was deposited onto a PVP (polyvinylphenol)-coated PC (polycarbonate) plastic substrate by screen-printing with nano-Ag ink, where Ag content of 20 wt.% was mixed using a terpineol solvent. Subsequently, the film was cured at 200 °C for 60 min, and then finally wet-etched through patterned positive photo-resist masks. The screen-printed Ag electrode exhibited a minimum line width of ∼5 μm, a thickness of ∼65 nm, and a resistivity of ∼10⁻⁶ Ω cm, producing good geometrical and electrical characteristics for a gate electrode. Additionally, it also provided good step coverage with the PVP dielectric layer, and consequently leakage current between the gate and source/drain electrodes was eliminated. Moreover, the electrical characteristic of the screen-printed Ag electrode was not significantly changed even after a bending test in which the Ag electrodes were bent with a bending radius of 6 mm and 2500 iterations of cyclic bending. OTFTs with the screen-printed Ag electrode produced a saturation mobility of 0.13 cm²/Vs and a current on/off ratio of 1.79 × 10⁶, being comparable to those of an OTFT with a thermally evaporated Al gate electrode.     

UV-directly patternable organic–inorganic hybrid composite dielectrics for organic thin-film transistors

A research on the design, synthesis, and characterization of novel cross-linked polymer organic–inorganic hybrid materials as gate insulators for organic thin-film transistors (OTFTs) with vanadyl-phthalocyanine as the organic semiconductor is presented. The hybrid films (0.5–1.2 μm thick) can be easily prepared by sol–gel technology and fabricated by spin-coating a mixture of zirconium n-butoxide sol with a side-chain triethoxysilane-capped polyurethane solution in ambient conditions, followed by curing at low temperatures (∼120 °C) and cross-linking under UV light. OTFTs with this film as gate insulator were achieved with good processability, high charge-carrier mobility of 0.56 cm²/Vs, surface roughness of around 0.49–0.59 nm, ultralow threshold of −6 V, and ultralow leakage of 0.24 mA. Hybrid films with various compositions were investigated, and the results showed that the field-effect mobility of the OTFTs was dominated by the high dielectric constant component ZrO₂. The result indicated that these hybrid materials are promising candidates for the exploration of devices using OTFTs.     

Preparation and properties of electrically conductive,flexible and transparent silver nanowire/poly (lactic acid) nanocomposites

As the everyday use of petroleum-based products has raised environmental concerns, there is an urgent need to replace them with green materials. In this work, an eco-friendly, highly conductive, flexible silver nanowire/poly (lactic acid) film has been fabricated through a simple casting method by embedding the silver nanowires (AgNWs) below the surface of the poly lactic acid (PLA) matrix. The fabricated film has a high optical transparency of 89.5% with a sheet resistance of 64.8 Ω/□ and a figure of merit (FoM) of 4.92 × 10⁻³ Ω⁻¹ which is comparable to that of indium tin oxide (ITO). These films demonstrate excellent flexibility, great adhesion, smooth surface with root mean square (RMS) roughness of 11.7 nm and high mechanical properties with tensile strength and Young's modulus of 39.8 (MPa) and 1.6 (GPa). The results obtained from different testing methods show that the AgNW/PLA nanocomposites are potential candidates in flexible electronics and optoelectronics.     

Interface optimization using diindenoperylene for C60 thin film transistors with high electron mobility and stability

C₆₀-based organic thin film transistors (OTFTs) with high electron mobility and high operational stability are achieved with (1 1 1) oriented C₆₀ films grown by using template effects of diindenoperylene (DIP) under layer on the SiO₂ gate insulator. The electron mobility of the C₆₀ transistor is significantly increased from 0.21 cm² V⁻¹ s⁻¹ to 2.92 cm² V⁻¹ s⁻¹ by inserting the template-DIP layer. Moreover much higher operational stability is also observed for the DIP-template C₆₀ OTFTs. A grazing incidence X-ray diffraction and ultrahigh-sensitivity photoelectron spectroscopy measurements indicate that the improved electron mobility and stability arise from the decreased density of trap states in the C₆₀ film due to increased (1 1 1) orientation of C₆₀-grains and their crystallinity on the DIP template.     

Thermally curable polymers consisting of alcohol-functionalized cyclotetrasiloxane and melamine derivatives for use as insulators in OTFTs

New thermally curable organic/inorganic hybrid polymers were designed and synthesized as insulators for organic thin film transistors (OTFTs). Cyclotetrasiloxane (CTS) was reacted with allyl alcohols through a hydrosilylation reaction in the presence of a catalytic amount of Pt(0) to give the alcohol-functionalized cyclotetrasiloxane (CTS-OH). The synthesized CTS-OH was then thermally cured with hexamethoxymethylmelamine (HMMM) at 80 °C in the presence of a catalytic amount of p-toluenesulfonic acid to form a hard and smooth thin film composed of a highly cross-linked network polymers (CTS-MMs). Devices with indium-tin-oxide/CTS-MM/Au configuration were fabricated to investigate electrical properties of the polymers such as capacitance, dielectric constant, and leakage current. The CTS-MM showed lower leakage current level than the well-known curable insulator consisting of poly(vinylphenol) (PVP) and a melamine derivative. Pentacene-based OTFTs were fabricated using the synthesized insulators as the gate dielectric layers, and their performances were compared to those of the device fabricated using PVP. The OTFTs fabricated using CTS-MM showed higher field-effect mobility than that of the PVP. The hole mobility of the pentacene based-OTFTs fabricated using CTS-MM as gate dielectric was 0.36 cm²/V s and the on/off current ratio was >10⁷.     

Effects of the Ag content on the geometrical and electrical characteristics of the screen-printed etched gate electrodes of OTFTs using Ag ink

During the fabrication of gate electrodes by Ag ink screen-printing combined with a wet-etching process, the effects of the Ag content on the geometrical and electrical characteristics such as the thickness and surface roughness of gate electrode, step coverage with the gate dielectric, leakage current associated with the step coverage, and the electrical performance of organic thin film transistors (OTFTs) were investigated. An increase of Ag content resulted in the thick and densely-packed Ag electrode, which had a stable and excellent conductivity. But, the large thickness of Ag electrode caused the worse step coverage of PVP (polyvinylphenol) dielectric layer on the edge of the Ag gate electrode, therefore, for Ag contents more than 40 wt.%, MIM (metal-insulator-metal) devices and OTFTs with the Ag gate electrodes had very large leakage current (>10⁻⁴ A/cm²) and off-state current (>∼19 pA/μm) due to the poor step coverage of PVP dielectric layers, respectively. Finally, we found that an Ag content of 20-30 wt.% was suitable for the screen-printed etched gate electrode of OTFTs using Ag ink. This range generated a mobility of 0.18 cm²/V s, an on/off current of 5 × 10⁶, and an off-state current of 0.002 pA/μm, which are suitable to drive e-paper.     

Properties of ZnO/Cu/ZnO multilayer films deposited by simultaneous RF and DC magnetron sputtering at different substrate temperatures

ZnO/Cu/ZnO transparent conductive multilayer films are prepared by simultaneous RF sputtering of ZnO and DC sputtering of Cu. The properties of the multilayer films are studied at different substrate temperatures. Sheet resistance of the multilayer film decreased initially with increase of substrate temperature and increased further with increase of substrate temperature beyond 100 °C. However, transmittance of the multilayer film increased with increase of substrate temperature. Good transparent conductive film of sheet resistance 9.3 Ω/sq and transmittance of 85% was found at a substrate temperature of 100 °C. The performance of the multilayer film was evaluated using a figure of merit. The observed property of the multilayer film is suitable for the application of transparent conductive electrodes.     

纳米银线-环氧树脂复合导电薄膜的制备及其在柔性触控上的应用

纳米银线透明导电薄膜具有优异的光电性能和机械性能,有望取代传统的氧化铟锡材料应用于柔性光电器件中。为解决纳米银线的黏附性、柔韧性和稳定性问题,结合转印法和烷基硫醇修饰法制备了纳米银线-环氧树脂复合导电薄膜。在此过程中,首先应用十八烷基三氯硅烷对转移衬底进行疏水化处理以提升透明电极转印的良率;接着,对转印后的纳米银线进行烷基硫醇分子的自组装修饰,以进一步提升纳米银线透明导电薄膜的稳定性;最后在此基础上制备了柔性投射式电容触控屏。所制备的透明导电薄膜具有优异的综合性能:品质因数约为300(Rs=29.7Ω/□,T=96.2%);薄膜经过100次胶带测试后,电阻变化小于25%;在1 000次弯曲(弯曲半径为1mm)测试后,电阻几乎不发生变化;高温高湿下老化一个月,电阻变化小于20%。本文结合转印工艺以及分子修饰技术开发的纳米银线-环氧树脂复合导电薄膜以及柔性触控工艺,有望推广应用至其他柔性电子元件中。     

The microwave properties of Li doped 0.7(Ba,Sr)TiO3-0.3MgO thick film interdigital capacitors on the alumina substrates

The crystalline and electrical properties of Li doped 0.7(Ba,Sr)TiO₃-0.3MgO thick film interdigital capacitors have been investigated. Screen printing method was employed to fabricate Li doped 0.7(Ba,Sr)TiO₃-0.3MgO thick films on the alumina substrates. (Ba,Sr)TiO₃ materials have high dielectric permittivity (>500 @ 1 MHz) and low loss tangent (0.01 @ 1 MHz) in the epitaxial thin film form. To improve dielectric properties and reduce sintering temperature, MgO and Li were added, respectively. 10 μm thick films were screen printed on the alumina substrates and then interdigital capacitors with seven fingers of 200 μm finger gap were patterned with Ag electrode. Current-voltage characteristics were analyzed with elevated temperature range. Up to 50 °C, the thick films showed positive temperature coefficient of resistivity (dρ/dT) of 6.11 × 10⁸ Ω cm/°C, then film showed negative temperature coefficient of resistivity (dρ/dT) of −1.74 × 10⁸ Ω cm/°C. From the microwave measurement, the relative dielectric permittivity of Li doped 0.7(Ba,Sr)TiO₃-0.3MgO thick films interdigital capacitors were between 313 at 1 GHz and 265 at 7 GHz.     

Performance improvement of organic thin film transistors with carbon nanotube/metal hybrid electrodes for S/D contacts

Electrode contact resistance is an important factor that seriously affects the performance of organic thin film transistors (OTFTs). In this paper, new low contact resistance carbon nanotube (CNT) based hybrid electrodes are introduced for the source and drain electrodes of OTFTs. The hybrid electrodes consist of solution-processed CNTs and a metal (Al; CNT/Al or Au; CNT/Au) layer evaporated on the CNTs. The contact resistance of the CNT/Al and CNT/Au hybrid electrodes was found to vary depending on the thickness of the Al and Au layer. The contact resistance of the CNT/Al hybrid electrodes exhibited a minimum of 2.9 kΩ cm at an Al thickness of 5 nm. It is notable that the minimum contact resistance of the CNT/Au was 0.9 kΩ cm at an Au thickness of 5 nm, and is the lowest value ever reported. It was lower than the 13 kΩ cm of the bare CNT electrodes, and tremendously less than the 4 MΩ cm of the pure Au electrode. The mobility of the OTFTs, which used pentacene as the semiconductor and polyvinylphenol as the gate dielectric, also followed the same dependence on metal thickness as the contact resistance. The maximum mobility of the OTFTs using CNT/Al and CNT/Au electrodes was 0.76 cm²/V sec and 1.0 cm²/V sec, respectively, at the same metal thickness of 5 nm, which was larger than 0.3 cm²/V sec of the bare CNT electrodes. The major origin of these enhancements was found to be the small energy difference between the work function of the CNT/metal hybrid electrodes and pentacene HOMO (5.1 eV), which was obtained at the metal thickness of 5 nm.     

Growth of rubrene crystalline thin films using thermal annealing on DPPC LB monolayer

High crystalline thin films of 5,6,11,12-tetraphenylnaphthacene (rubrene) can be obtained after in situ thermal post annealing using SiO₂ gate dielectric modified with a 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) monolayer obtained via Langmuir–Blodgett transfer. Such formed rubrene crystalline films are interconnected and highly ordered with defined molecular orientation. Organic thin film transistors (OTFTs) with high performance are reproducibly demonstrated with the mobility of 0.98 cm²/V s, the threshold voltage of −8 V and the on–off current ratio of higher than 10⁷. The results indicate that our approach is a promising one for preparing high quality rubrene crystalline films.     

Flexible transparent conductive film based on silver nanowires and reduced graphene oxide

Silver nanowires (AgNWs) with diameter of 90—150 nm and length of 20—50 μm were successfully synthesized by a polyol process. Graphene oxide (GO) was prepared by Hummers method, and was reduced with strong hydrazine hy-drate at room temperature. The flexible transparent conductive films (TCFs) were fabricated using the mixed cellulose eater (MCE) as matrix and AgNWs and reduced graphene oxide (rGO) as conductive fillers by the improved vacuum fil-tration process. Then, the optical, electrical and mechanical properties of the AgNWs-rGO films were investigated. The results show that for the AgNWs-rGO film produced with the deposition densities of AgNWs and rGO as 110 mg·m-2 and 55 mg·m-2, the optical transmission at 550 nm is 88.4% with Rs around 891 Ω·sq-1, whereas the optical transmission for the AgNWs-rGO film with deposition densities of AgNWs and rGO of 385 mg·m-2 and 55 mg·m-2 is 79.0% at 550 nm with Rs around 9.6 Ω·sq-1. There is little overt increase in Rs of the AgNWS-rGO film after tape tests for 200 times. The bending test results indicate that the change in Rs of AgNWs-MCE film is less than 2% even after 200 cycles of compressive or tensile bending. The excellent mechanical properties of the AgNWs-rGO film can be attributed to the burying of AgNWs and rGO at the surface of MCE     

Solution processable bilayered gate dielectric towards flexible organic thin film transistors

In this study, we have successfully explored the potential of a new bilayer gate dielectric material, composed of Polystyrene (PS), Pluronic P123 Block Copolymer Surfactant (P123) composite thin film and Polyacrylonitrile (PAN) through fabrication of metal insulator metal (MIM) capacitor devices and organic thin film transistors (OTFTs). The conditions for fabrication of PAN and PS-P123 as a bilayer dielectric material are optimized before employing it further as a gate dielectric in OTFTs. Simple solution processable techniques are applied to deposit PAN and PS-P123 as a bilayer dielectric layer on Polyimide (PI) substrates. Contact angle study is further performed to explore the surface property of this bilayer polymer gate dielectric material. This new bilayer dielectric having a k value of 3.7 intermediate to that of PS-P123 composite thin film dielectric (k ∼ 2.8) and PAN dielectric (k ∼ 5.5) has successfully acted as a buffer layer by preventing the direct contact between the organic semiconducting layer and high k PAN dielectric. The OTFT devices based on α,ω-dihexylquaterthiophene (DH4T) incorporated with this bilayer dielectric, has demonstrated a hole mobility of 1.37 × 10⁻² and on/off current ratio of 10³ which is one of the good values as reported before. Several bending conditions are applied, to explore the charge carrier hopping mechanism involved in deterioration of electrical properties of these OTFTs. Additionally, the electrical performance of OTFTs, which are exposed to open atmosphere for five days, can be interestingly recovered by means of re-baking them respectively at 90 °C.     

Effects of post-thermal treatment on the properties of rf reactive sputtered ITO films

Indium tin oxide (ITO) thin film prepared by rf sputtering at various Ar-O₂ mixtures, were annealed at several temperatures. The electrical, optical and structural properties of the film were systematically investigated before and after post-thermal treatment. The influence of a reactive gas (O₂) on the sputtering rate of a metallic (indium/tin) alloy target was also investigated. The films were characterized by X-ray diffraction (XRD) measurement, scanning electron microscopy, and transmittance as a function of wavelength. The resistivity of 8.3×10⁻⁴ Ω cm has been achieved for the film thickness of 250 nm, deposited in pure Ar at room temperature (RT).     

High performance of pentacene organic thin film transistors by doping of iodine on source/drain regions

Contact doping was conducted by iodine in a top contact configuration in a pentacene organic thin film transistor (OTFT), to investigate its effects on contact resistance and the resulting electrical performance. Iodine doping in the pentacene film caused the change of pentacene structure, thus leading to an increase in electrical anisotropy, i.e. ratio of lateral to vertical resistivity. The two resistive components of doped pentacene film underneath the Au contacts were major contributors to the contact resistance, and a model to explain the dependence of contact resistance on iodine doping was presented. Finally, OTFTs fabricated on iodine doped source/drain contacts exhibited high mobility of 1.078 cm²/V s, two times that of OTFTs with undoped contacts, due to the low contact resistance.     

Study of contact resistance of high-mobility organic transistors through comparisons

Realization of high-frequency low-cost organic electronics requires high-mobility organic field-effect transistors (OFETs) with short channels, where influence of contact resistance becomes more serious than either lower mobility or longer channel devices. To reduce the contact resistance, we systematically and quantitatively investigate the influence of the lowest unoccupied molecular orbital (LUMO) level of an electron acceptor layer, the active layer thickness, and the side chain of active layer itself on contact resistance of top-contact high-mobility OFETs through a series of comparative analysis. We find that the acceptor of 1,3,4,5,7,8-hexafluoro tetracyano naphtha quinodimethane (F₆TNAP) with a deeper LUMO level is efficient for carrier injection and that the bulk resistance plays an important role in such devices. By optimizing the parameters, we get the lowest contact resistance of only 110 Ω cm, and thus recorded effective mobility of 8.0 cm²/V s is attained for polycrystalline thin film transistors and still kept as high as 6 cm²/V s at shorter channel lengths.