Reverse Offset Printing of Semidried Metal Acetylacetonate Layers and Its Application to a Solution-Processed IGZO TFT Fabrication

High-Performance Flexible Electronics Fabricated Using a Surface Energy-Directed Assembly Process on Ultrathin Polyimide Substrates

Solution-based processes are emerging in the fabrication of flexible electronics owing to their cost-effectiveness, low-temperature processing capabilities, and vacuum-free operations. Currently, printing techniques, as the most widely used solution-based processes, suffer from low resolution and poor pattern fidelity. Although surface energy-directed assembly (SEDA) process enables unparalleled resolution and fidelity, multilayer fabrication and application on flexible substrates are rarely attempted. Here, a SEDA process is used for fabricating metal oxide thin film transistors (TFTs) on ultrathin and flexible polyimide (PI) substrates with a thickness of ≈35 µm. Comprehensive procedures to render PI substrates hydrophobic as well as to homogenize the hydrophobicity of the PI substrates and as-assembled layers are developed. All-solution-processed flexible TFTs are constructed by assembling indium oxide semiconducting channels, indium tin oxide source/drain electrodes, aluminum oxide gate dielectric layers, and indium tin oxide gate electrodes ordinally. The metal oxide TFTs exhibit excellent electrical properties with an average mobility of 22.01 cm2 V-1 s-1 and stable operation under mechanical strain. Flexible inverters with a high voltage gain of 78 and a high dynamic operation frequency of up to 1 kHz are also constructed, representing new opportunities of the SEDA process for the fabrication of next-generation flexible electronics.

All-Solution-Processed Electronics with Sub-Microscale Resolution and Nanoscale Fidelity Fabricated Via a Humidity-Controlled, Surface Energy-Directed Assembly Process

Solution-based processes have received considerable attention in the fabrication of electronics and sensors owing to their merits of being low-cost, vacuum-free, and simple in equipment. However, the current solution-based processes either lack patterning capability or have low resolution (tens of micrometers) and low pattern fidelity in terms of line edge roughness (LER, several micrometers). Here, we present a surface energy-directed assembly (SEDA) process to fabricate metal oxide patterns with up to 2 orders of magnitude improvement in resolution (800 nm) and LER (16 nm). Experiment results show that high pattern fidelity can be achieved only at low relative humidities of below 30%. The reason for this phenomenon lies in negligible water condensation on the solution droplet. Employing the SEDA process, all-solution-processed metal oxide thin film transistors (TFTs) are fabricated by using indium oxide as channel layers, indium tin oxide as source/drain electrodes and gate electrodes, and aluminum oxide as gate dielectrics. TFT-based logic gate circuits, including NOT, NOR, NAND, and AND are fabricated as well, demonstrating the applicability of the SEDA process in fabricating large area functional electronics.

Recent Progress in Solution-Based Metal Oxide Resistive Switching Devices

Metal oxide resistive switching memories have been a crucial component for the requirements of the Internet of Things, which demands ultra-low power and high-density devices with new computing principles, exploiting low cost green products and technologies. Most of the reported resistive switching devices use conventional methods (physical and chemical vapor deposition), which are quite expensive due to their up-scale production. Solution-processing methods have been improved, being now a reliable technology that offers many advantages for resistive random-access memory (RRAM) such as high versatility, large area uniformity, transparency, low-cost and a simple fabrication of two-terminal structures. Solution-based metal oxide RRAM devices are emergent and promising non-volatile memories for future electronics. In this review, a brief history of non-volatile memories is highlighted as well as the present status of solution-based metal oxide resistive random-access memory (S-RRAM). Then, a focus on describing the solution synthesis parameters of S-RRAMs which induce a massive influence in the overall performance of these devices is discussed. Next, a precise analysis is performed on the metal oxide thin film and electrode interface and the recent advances on S-RRAM that will allow their large-area manufacturing. Finally, the figures of merit and the main challenges in S-RRAMs are discussed and future trends are proposed.

Flexible electric-double-layer thin film transistors based on a vertical InGaZnO4 channel

Flexible electric-double-layer (EDL) thin film transistors (TFTs) based on a vertical InGaZnO₄ (IGZO) channel are fabricated at room temperature.

Microstructured SiOx/COP Stamps for Patterning TiO2 on Polymer Substrates via Microcontact Printing

Microcontact printing (μCP) techniques have sparked a surge of interests in microfabrication since they help produce arrays on a wide range of target substrates in a facile and efficient manner. Polydimethylsiloxane (PDMS), as a well-established material for stamps, has constraints resulting from its hydrophobicity and softness, and the replication of PDMS stamps usually requires rigid masters or processes using a photoresist. Herein, a novel μCP stamp based on cyclo-olefin polymer (COP) is produced through vacuum ultraviolet (VUV) lithography. 2,4,6,8-Tetramethylcyclotetrasiloxane is selectively deposited at the affinity-patterns on the COP surface, and these patterned siloxane films are converted into SiO_x meanwhile protecting the COP beneath them from the VUV photoetching. By this means, a patterned relief is fabricated on the COP plates, resulting in a hydrophilic SiO_x/COP μCP stamp with punch heights of ∼180 nm. The novelty arises from the simplicity of the master- and photoresist-free microstructuring, and the higher stiffness of SiO_x/COP stamps prevents the deformation during pressing. Finally, an example μCP is given to transfer titania precursor gel and produce TiO₂ micropatterns on flexible polymer substrates. The SiO_x/COP stamps and the μCP of TiO₂ provide simple and cost-effective patterning techniques, which should contribute to the future design and creation of flexible devices.

Mechanisms of Adhesive Micropatterning of Functional Colloid Thin Layers

Thin-film layers of nanoparticles exhibit mechanical fragility that depends on their interactions. Balancing the cohesive force of particles with their interfacial adhesion to a substrate enables the selective transfer of micrometer-scale layer features. Here, the versatility of this adhesion-based transfer approach from poly(dimethylsiloxane) (PDMS) is presented by demonstrating micropatterns of various functional nanoparticulate materials, including Ag, Cu, indium tin oxide, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate, and dielectric silica. With the attachment of the Johnson-Kendall-Roberts interaction to a simple strain model of particle layers during the patterning process, the patterning criteria for successful printing at both macroscale and nanoscale levels are deduced. Discrete element modeling analysis was used to validate the scaling laws and to highlight the fracture modes of particle layers during the patterning process. In particular, the balance among cohesive forces in the tensile direction and in the shear direction and the adhesion force at the layer-PDMS interface mainly regulates the patterning quality of adhesion patterning.

High-Resolution and Large-Area Patterning of Highly Conductive Silver Nanowire Electrodes by Reverse Offset Printing and Intense Pulsed Light Irradiation

Conventional printing technologies such as inkjet, screen, and gravure printing have been used to fabricate patterns of silver nanowire (AgNW) transparent conducting electrodes (TCEs) for a variety of electronic devices. However, they have critical limitations in achieving micrometer-scale fine line width, uniform thickness, sharp line edge, and pattering of various shapes. Moreover, the optical and electrical properties of printed AgNW patterns do not satisfy the performance required by flexible integrated electronic devices. Here, we report a high-resolution and large-area patterning of highly conductive AgNW TCEs by reverse offset printing and intense pulsed light (IPL) irradiation for flexible integrated electronic devices. A conductive AgNW ink for reverse offset printing is prepared by carefully adjusting the composition of AgNW content, solvents, surface energy modifiers, and organic binders for the first time. High-quality and high-resolution AgNW micropatterns with various shapes and line widths are successfully achieved on a large-area plastic substrate (120 × 100 mm²) by optimizing the process parameters of reverse offset printing. The reverse offset printed AgNW micropatterns exhibit superior fine line widths (up to 6 μm) and excellent pattern quality such as sharp line edge, fine line spacing, effective wire junction connection, and smooth film roughness. They are post-processed with IPL irradiation, thereby realizing excellent optical, electrical, and mechanical properties. Furthermore, flexible OLEDs and heaters based on reverse offset printed AgNW micropatterns are successfully fabricated and characterized, demonstrating the potential use of the reverse offset printing for the conductive AgNW ink.