Printed Ag Electrodes Research Articles

All-Solution-Processed OLEDs Using Printed Ag Electrodes and a PEI Bonding Layer

We demonstrated a bonding technology using polyethyleneimine (PEI) as a bonding layer between printed Ag electrodes and an emission material layer (EML) to fabricate all-solution-processed Organic LightEmitting Diodes (OLEDs). We manufactured Ag electrodes on a glass substrate by the screen printing method, which was bonded to another substrate with ITO anodes and an EML layer using a PEI layer. Since the bonding layer needs to have both good bonding characteristics and electrical conductivity, we investigated how both characteristics were affected by some experimental factors, namely, PEI concentration, the thickness of the layer and additives. The bonding strength and the electrical current density were investigated by tensile tests and electron only device (EOD) experiments, respectively. The results showed that at higher PEI concentration the bonding strength reached a higher value, but the electrical current through the PEI layer decreased rapidly with increased PEI layer thickness. When sorbitol was added into the PEI solution, both bonding strength and electrical conductivity were improved, and when 10 wt% of sorbitol was added into 0.1 wt% PEI solution, the device showed an electrical current density of 1,000 mA/cm<sup>2</sup> and a good bonding strength also. Finally, we manufactured a two-substrate OLED device using the bonding layer and measured the luminance, which revealed that the device turned on at 4 V and the maximum luminance was 7,000 cd/m<sup>2</sup>. These data, both electrical and optical, demonstrate the potential to fabricate all-solution-processed OLEDs using the two-substrate bonding technology.

Improved Electrical Properties of EHD Jet-Patterned MoS2 Thin-Film Transistors with Printed Ag Electrodes on a High-k Dielectric.

Electrohydrodynamic (EHD) jet printing is known as a versatile method to print a wide viscosity range of materials that are impossible to print by conventional inkjet printing. Hence, with the understanding of the benefits of EHD jet printing, solution-based MoS2 and a high-viscosity Ag paste were EHD jet-printed for electronic applications in this work. In particular, printed MoS2 TFTs with a patterned Ag source and drain were successfully fabricated with low-k silica (SiO2) and high-k alumina (Al2O3) gate dielectrics, respectively. Eventually, the devices based on Al2O3 exhibited much better electrical properties compared to the ones based on SiO2. Interestingly, an improvement of around one order of magnitude in hysteresis was achieved for devices after changing the gate insulator from SiO2 to Al2O3. In effect, the results of this work for the printed MoS2 and the printed Ag source and drains for TFTs demonstrate a new approach for jet printing in the fabrication of electronic devices.

Estimation of uniformity in Schottky contacts between printed Ni electrode and n-GaN by scanning internal photoemission microscopy

We report basic diode characteristics and uniformity of Schottky contacts between printed Ni electrode and n-GaN on GaN epitaxial wafers annealed at 400, 500, and 600 °C. The Schottky barrier height of the samples annealed at 400 °C and 500 °C were as high as 1.21 eV, which is comparable to a conventional evaporated Ni contact. When the annealing temperature was 600 °C, rectifying the characteristics that were lost. Scanning internal photoemission microscopy (SIPM) showed that contacts annealed at 500 °C exhibited better uniformity than those at 400 °C. In contrast, the photo yield signal in SIPM of the contact annealed at 600 °C became weak and noisy due to the interfacial reaction between Ni and GaN. SIPM revealed that the contacts printed Ni electrode on GaN was more uniform than those of the printed Ag electrode. These results also show that SIPM is useful to estimate the uniformity of electrode contacts on semiconductors by non-destructive visualization.

Stretchable inkjet-printed electronics on mechanically compliant island-bridge architectures covalently bonded to elastomeric substrates

Herein, we present an approach that allows versatile combination of inkjet-printed electronics and stretchable substrates. For this, we created a hybrid platform made out of stretchable Ecoflex covalently bonded via silane monolayers to flexible polyethylene terephthalate islands interconnected by bridges. The islands served as platforms where conductive lines, capacitive sensors and electrochromic devices (ECDs) were fabricated by inkjet printing. The robustness of the approach is highlighted by the minor influence of strain on the conductivity of printed Ag electrodes, which changed the resistance only by 1.3% at an applied strain of 50%. Furthermore, we demonstrated capacitor sensors capable of responding to strain changing their capacitance from 0.2 to 1.6 pF. To further show the applicability of the approach for multilayer/multimaterial optoelectronic elements, we processed ECDs capable of displaying information on the stretchable platform. Thus, we demonstrate how this digital and additive concept can be applied for the scalable integration of printed optoelectronic devices onto stretchable systems without relying on lithographic processes.

Direct ink write printing of resistive-type humidity sensors

To practice the development of printed electronics, researchers need to have the access to dedicated printer of electronic materials. However, commercial printing machines require large capital investment. Direct ink write (DIW) printing on the other hand operates with single printing head and less complex system therefore is promising for tight budget projects. Herein, a DIW printing system was home-built by making modifications to a 3D printer with ready to use components. Direct pattering and minimal printing line width of 230 ± 10 µm are demonstrated using an all solution-based Ag ink. Resistive-type humidity sensors were fabricated with DIW printed Ag electrodes and poly (diallyl dimethyl ammonium chloride) (PDDA) sensing layer. Humidity sensing performance of the sensors were examined and by taking advantage the flexibility of printing deposition, DIW printing was used to make quick modification toward a partially covered sensor structure for improved performance. Sensing response of more than 99% was obtained from 10% RH to 90% RH. Smaller hysteresis of ±4.4% RH and faster response of 7 s for adsorption and 213 s for desorption were obtained with the partially covered sensor. DIW printing is found to be a useful tool for device prototyping and ink development of printed electronics.

Thermal and radiation response of 4H–SiC Schottky diodes with direct-write electrical contacts

A high-sensitivity 4H–SiC temperature sensor and an alpha detector have been fabricated using additively printed metal contacts. The surface morphology and electrical conductivity of the printed electrodes were established prior to Schottky diode development. 4H–SiC Schottky diodes with direct-write printed silver contacts on the 5 μm-thick epilayer on 4H–SiC were characterized electrically in terms of the forward and reverse current–voltage and high-frequency capacitance–voltage characteristics. The turn-on voltage of the Schottky diodes, as established from the forward current–voltage characteristics measured up to a temperature of 400 °C, showed a linear temperature dependence. Schottky diodes with direct-write printed Ag electrodes were able to measure alpha particles emitted from Americium-241. The high temperature and radiation response of the Schottky diodes show their suitability for multi-modal sensor fusion on the 4H–SiC platform for harsh environment applications.

Silver film etching processing using the inkjet method for the organic thin film transistor

Abstract Thin-film transistors (TFT) are an important component of many printed electronics, as they greatly influence the performance of the device. Thus, optimum performance of a TFT is considered to be the most important factor in an electronic application field. This paper reports the enhanced performance of an organic TFT (OTFT) by a facile inkjet etching method for the first time. Thermally evaporated Ag with a thickness of 20–80 nm is used for the source–drain electrode. By employing temperature control and adopting a proper thickness of silver electrode, the source-drain patterning is improved and optimized for the fine pattern with accuracy of 50 μm-channel without residue. Accordingly, the field-effect mobility is enhanced to 0.454 cm2/V and the on/off current ratio is greater than 105. Furthermore, the fabricated OTFT with etched Ag electrode demonstrates more uniform electrical stability than printed Ag electrode.

Laser direct-writing electrode for rapid customization of a photodetector.

The interdigital Ag electrode is fabricated via a simple and fast approach called laser direct writing (LDW). The morphology and conductivity of electrode fingers are investigated systematically under different experimental parameters, including the laser spot size, laser power, and scanning speed. "Dose" describes the combined influence of these experimental parameters. It is found that overdose results in net-shape and dot-shape hollows in the middle of an Ag line due to the sintering degree and complex flow dynamics, which reduced the conductivity of the Ag lines. Based on the printed Ag electrodes with the best conductivity, a photodetector is customized further, which can detect the offset of the line-shape laser easily. Moreover, to the best of our knowledge, this is the first time the printed Ag electrodes are applied to photodetectors, which can be highly valuable for developing all-printed electronic devices by LDW in the future.

A Novel Photo Cross-Linkable Polymer Insulator for Printed OTFTs with Surface Selective Deposition By Vacuum-Ultraviolet Irradiation

To realize printed manufacturing of electronic devices, including flexible displays, sensors, and integrated circuits that incorporate organic thin-film transistors (OTFT), developing and improving the materials and technologies used in this process is required. Solution processable organic semiconductors, insulating materials, and high-resolution patterning techniques of printed electrodes are all critical components of these electronic devices. The surface properties of insulators are particularly important because they contribute significantly to carrier transport at the interface of the organic semiconductors and affect the patterning of the printed electrode. Insulating materials that are solution-processable typically require high temperatures (∼150 °C) and long curing times during fabrication, which is undesirable because these conditions can cause significant distortion of common flexible substrates, such as polyethylene naphthalate. Thermal deformation of a flexible substrate prevents high-resolution patterning and the accurate alignment of the different layers within a device, and these problems become more significant as the size of the substrate increases. To overcome some of these challenges, we have recently developed a novel styrene-based polymer (PC200) for use as an insulating material that can be cured quickly via photo cross-linking. This material can be produced under atmospheric conditions, does not require heat treatment, is highly electrically insulating and has a low dielectric constant. Additionally, the surface of the PC200 insulating film can be transformed into a hydrophilic, wettable surface by selective irradiation using vacuum ultraviolet (VUV) radiation via a photomask. This allows the formation very fine printed electrodes using a metal nanoparticle ink. VUV exposure in air generates oxygen radicals and ozone molecules, which cause chemical bond dissociation on the polymer surface and render the region hydrophilic, resulting in patterned surfaces that are wettable. Compared with perfluoropolymers or parylene insulating films that have been reported recently, surface modification of the PC200 insulating film is relatively simple and requires less VUV irradiation. In this work, we have examined the electrical insulating properties of the PC200 insulator film and incorporated it into a short-channel OTFT with the recently developed soluble semiconductor (dithieno[2,3-d:2’,3’- d’]benzo[1,2-b:4,5-b’]dithiophene, DTBDT). The PC200 film was formed by spin-coating and subsequent UV exposure without heat treatment. The insulator film exhibited low surface roughness (RMS 0.5 nm), high electrical insulation (>3 MV/cm) and a low dielectric constant (εr=2.7). Additionally, we were able to form fine patterned electrodes on the polymer insulator with a minimum channel length of 5 μm using a VUV patterning technique. Importantly, these fine patterns (<10 μm) were formed without requiring a photoresist and wet-etching techniques, unlike conventional methods, such as photolithography. We fabricated short-channel OTFTs using the PC200 film as the patterning layer for printed Ag electrodes and DTBDT as the active layer. The OTFTs exhibited pronounced saturation curves with a steep onset and a sub-threshold swing of 0.3 V/dec, a low threshold voltage of −0.8 V, negligible hysteresis, a hole mobility of 0.5 cm2/Vs and a very large I on/I off ratio above 107. These results demonstrated that the PC200 insulator shows great promise for realizing a variety of flexible and printable electrical circuits for use in displays and sensors.

Development of printed and flexible dry ECG electrodes

Printed, flexible and wearable dry electrodes for monitoring electrocardiogram (ECG) signals, without any skin preparation and use of wet gel, has been developed. Silver (Ag) flake ink was screen printed on a flexible polyethylene terephthalate (PET) substrate to fabricate the dry ECG electrode. Multi-walled carbon nanotube (MWCNT)/polydimethylsiloxane (PDMS) composite, as a conductive polymer, was then deposited on the printed Ag electrode by using a bar coating technique. The performance of the printed electrodes was investigated by testing the MWCNT/PDMS composite conductivity and measuring the electrode-skin impedance for electrode radii varying from 8 mm to 16 mm. It was observed that the dry ECG electrode, with the largest area, demonstrated better performance, in terms of MWCNT/PDMS composite conductivity, ECG signal intensity and correlation when compared to a commercial wet silver/silver chloride (Ag/AgCl) electrode. In addition, the capability of the dry ECG electrodes for monitoring ECG signals in both the relaxed sitting position and while the subject is in motion, was also investigated and the results were compared with a wet Ag/AgCl ECG electrode (T716). While the subject is in motion, the printed dry electrodes were less noisy and were able to better identify the typical ECG characteristics in the signals due to its better conformal contact at the electrode-skin interface. The results obtained demonstrated the feasibility of employing conventional screen printing process for the development of flexible dry ECG electrodes for applications in the biomedical industry.

Morphological Behavior of Printed Silver Electrodes with Protective Self-Assembled Monolayers for Electrochemical Migration

We evaluated the electrochemical behaviors and reliability of printed silver (Ag) electrodes prepared from nanoparticle inks with the use of protective self-assembled monolayers (SAMs) under electronic bias conditions. The printed Ag electrodes were fabricated by inkjet printing on a hydrophobic substrate. The SAMs, which acted as barriers to moisture, were prepared by immersing the substrate in a pentafluorobenzenethiol solution at ambient temperature (25 °C). We investigated the electrochemical migration phenomenon using the water drop method, and the results showed that the formation of dendrites connecting the cathode and the anode, which can affect the electrochemical reliability of an electric device, was suppressed in the presence of the SAMs. The time before short circuit occurred was found to depend on the spacing between the electrodes, i.e., 130 s, when the distance between the electrodes was 200 μm in the presence of an SAM. We demonstrated that Ag electrodes treated using the procedure described in this work suppress the occurrence of electrical short circuits caused by Ag dendrite formation and thus their electrochemical properties are substantially improved.

Influence of pre-annealing of printed silver electrodes on ultrafast laser ablation of short thin-film transistor channels on flexible substrates

The cutoff frequency and current from an organic thin-film transistor (OTFT) are strongly dependent on the length and to some extent on the uniformity of the transistor channel. Reducing the channel length can improve the OTFT performance with the increase in the current and frequency. Picosecond laser ablation of the printed Ag electrodes, compatible with roll-to-roll fabrication, has been investigated. The ablation threshold was found to be similar for the laser wavelengths tested: 515 nm and 1030 nm. Short transistor channels could be opened both after light annealing at 70 °C and after annealing at 140 °C. The channels in the lightly cured films had a significantly less scale formation, which is critical for avoiding shunts in the device. By moving from bottom electrodes fully defined by printing to the bottom electrodes where the transistor channel is opened by the laser, the channel length could be reduced from 40 μm to less than 5 μm.

An inkjet printed Ag electrode fabricated on plastic substrate with a chemical sintering approach for the electrochemical sensing of hydrogen peroxide

The trend of developing electrochemical sensors toward cellular level detection put forward higher requirements of the electrodes in the detection performance. However, common disk electrodes or conventional screen printing electrodes meet up with some limitations in the electrocatalytic activity and electron transfer capability. In this work, we applied inkjet printing technology to fabricate electrodes to make some improvements. Highly conductive Ag nanoparticles based electrodes were obtained on plastic substrate by inkjet printing technology followed by a sintering process at room temperature. The resistivity of IPAgE is determined to be 64.0±5.3μΩcm. With better conductivity and the nanoparticle-based interface, superb electrochemical response of IPAgE for H2O2 was obtained, nearly 300-fold higher than the conventional screen printed Ag electrode. Moreover, high sensitivity of 287μAmM−1cm−2 with a LOD of 5.0μM was obtained under the optimized 20 printed layers. The inkjet printed Ag electrodes were also credibly applied in the detection of H2O2 release from living cells. This work demonstrates inkjet printing is a promising method for the high performance electrochemical sensors.

Direct writing of inkjet-printed short channel organic thin film transistors

Abstract A direct-writing fabrication process for fully inkjet-printed short-channel organic thin-film transistors (OTFTs) has been developed. Channels as narrow as 800 nm between two printed Ag electrodes were achieved by printing a special Ag ink on an SU-8 interlayer, which can be partially dissolved by the solvents used in the Ag ink. The ridge formed along the printed Ag line edges due to redistribution of the interlayer material during the drying process limits the ink spread, and separates neighboring printed lines, and is the key to defining an ultra-narrow channel for transistor fabrication. The short-channel OTFTs fabricated using this technique have demonstrated well-defined linear and saturation regimes. An extracted mobility of 0.27 cm 2 /Vs with an on/off ratio of 10 5 was obtained at a driving voltage of −12 V. The excellent performance of these devices demonstrates the potential of this technique in fabrication of short-channel devices using standard printing technologies.

Flexible carbon nanotube-enriched silver electrode films with high electrical conductivity and reliability prepared by facile screen printing

Flexible electrode films play critical and fundamental roles in the successful development of flexible electronic devices. In this study, carbon nanotubes (CNTs) were implanted into silver (Ag) ink to enhance the electrical conductivity and the reliability of the printed Ag electrode films. The fabricated carbon nanotubes-enriched silver (Ag-CNTs) electrode films were printed on the polyimide substrates by a facile screen printing method and sintered at a relatively low temperature. The resistivity of Ag-CNTs films was decreased by 62.27% compared with the pure Ag film. Additionally, the Ag-CNTs films exhibited excellent flexibility under a bending radius of 4mm (strain ε=2.09%) over 1000 cycles. Furthermore, the Ag-CNTs film displayed unchangeable electrical conductivity together with a strong adhesion after an accelerated aging test with 500 thermal shock cycles. These improvements were attributed to the Ag-CNTs interconnected network structure, which can provide electronic transmission channels and prevent cracks from initiating and propagating.

Full-solution processed, flexible, top-emitting polymer light-emitting diodes based on printed Ag electrodes

Full-solution processed, flexible, top-emitting polymer light-emitting diodes are fabricated by using low-cost and high-quality printed Ag electrodes.

Microfluidic Platform for Seawater Desalination by Coulometric Removal of Chloride Ions through Printed Ag Electrodes

This paper reports on the electrochemical characterization and comparison of printable silver inks for the fabrication of planar electrodes to integrate in a microfluidic platform for in situ desalination of seawater prior to detection of nutrients in marine environment. Screen printing and inkjet printing were investigated to overcome limitations of more conventional deposition techniques. The screen printed ink DuPont 5064H was chosen for the fabrication of the desalination platform as it displayed the best properties in terms of oxidation ability (15.5 mC/mm2) and absence of sample contamination. The platform was tested at different oxidation conditions; the optimum potential for desalination was found to be +0.9 V. The device desalination performance was then evaluated through conductivity measurements of the treated sample and a proof-of-concept was achieved: for a potential of +0.9 V, conductivity (hence concentration) could be lowered from an initial value of 42.13 mS (0.6 M) to 20.36 mS (0.206 M), 17.37 mS (0.157 M) and 6.38 mS (0.0516 M) for oxidation times of 120 s, 240 s and 360 s, respectively. The operating conditions allowed the regeneration of the silver electrode and the possibility to deploy it for six to ten cycles before depletion of the silver working electrode.

High Temperature Laser Based Drop on Demand Micro Joining of Thin Metallic Layers or Foils using Bronze Braze Preforms

In this paper a drop on demand based method of joining metallic layers of about 20 µm thick- ness with Cu-conductors of 100 µm diameter is presented. The process is based on melting spherical preforms of copper based braze (CuSn12) with a liquidus temperature of 990 °C (1). In the process, a braze preform is induced into a ceramic capillary, molten by a laser pulse and expelled out of the machining head via nitrogen overpressure. After a flight phase the braze droplet wets the surface of the electrode structure and the copper resulting in a firm joint after solidification. The shear strengths of the joints were measured to be 31 MPa for joints on Al2O3 substrates and 37 MPa for joints generated on LTCC substrates with screen printed Ag electrode structures. The shear strengths after temperature shock tests according to DIN EN 60068-2-14 indicate good performance of the joints in harsh environments. The average resistivity of a joint was tested by four wire testing and found to be 1.56 ± 0.33 mΩ. The introduced setup is capable to join thin metallic layers of 30 - 15 µm thickness with cu conductors of 100 µm diameter without the need of flux.

Printing high performance reflective electrodes for organic solar cells

In this paper we solve a long term material problem of thin film organic electronics, namely the solution processing of an opaque electrode. Solution processing of opaque metallic top electrodes typically leads to severe shunting problems. We solve this issue by reversing the electrode sequence and print a highly conductive but opaque bottom electrode from metallic precursors. Devices based on these printed bottom electrodes are compared to reference stacks based on evaporated silver. The transparent top electrode is solution processed from silver nanowire inks, which results in highly transparent electrodes with excellent conductivity. The optical, mainly reflective properties of the opaque silver electrode are investigated in comparison to screen-printed silver pastes. The outstanding smoothness of the printed Ag electrode results in high reflectivity and poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) based solar cells with η>2.5% and high fill factors performing on par with reference devices on evaporated silver electrode layers.

Direct metal micropatterning on needle-type structures towards bioimpedance and chemical sensing applications

Direct metal patterning methods, such as screen printing, inkjet printing and gravure/flexography printing, are widely used to form electrodes or interconnections for printed electronic devices due to their inexpensive, simple and rapid fabrication as compared to vacuum-based conventional metallization processes. Here, we present direct metal patterning by modified screen printing on the curved surface of needle-type rod structures (i.e. rods with radius of ρ < 1 mm). We achieved various microscale patterns such as straight lines, zigzag lines, wavy lines and alphabetic words with a minimum width of 70 µm on the surface of the rod. Also, four pairs of line patterns were printed on the single rod for electrical interconnection. Printed patterns on the surface of the rod were used as electrodes for the control of a light emission diode (LED) as well as the real-time electrochemical impedance spectroscopy of electrolyte solutions and solid objects by the rod insertion. Furthermore, needles with multiple pairs of microelectrodes were used to measure the electrical impedance of biological samples such as fat and muscle tissues of porcine meat. In addition, a needle-type probe sensor with gas sensing capability was demonstrated by using a needle with printed Ag electrodes and Pd thin films.