Ankit Mahajan

Optimization of Aerosol Jet Printing for High-Resolution, High-Aspect Ratio Silver Lines

Aerosol jet printing requires control of a number of process parameters, including the flow rate of the carrier gas that transports the aerosol mist to the substrate, the flow rate of the sheath gas that collimates the aerosol into a narrow beam, and the speed of the stage that transports the substrate beneath the beam. In this paper, the influence of process parameters on the geometry of aerosol-jet-printed silver lines is studied with the aim of creating high-resolution conductive lines of high current carrying capacity. A systematic study of process conditions revealed a key parameter: the ratio of the sheath gas flow rate to the carrier gas flow rate, defined here as the focusing ratio. Line width decreases with increasing the focusing ratio and stage speed. Simultaneously, the thickness increases with increasing the focusing ratio but decreases with increasing stage speed. Geometry control also influences the resistance per unit length and single pass printing of low-resistance silver lines is demonstrated. The results are used to develop an operability window and locate the regime for printing tall and narrow silver lines in a single pass. Under optimum conditions, lines as narrow as 20 μm with aspect ratios (thickness/width) greater than 0.1 are obtained.

Aerosol Jet Printed p- and n-type Electrolyte-Gated Transistors with a Variety of Electrode Materials: Exploring Practical Routes to Printed Electronics

Printing electrically functional liquid inks is a promising approach for achieving low-cost, large-area, additive manufacturing of flexible electronic circuits. To print thin-film transistors, a basic building block of thin-film electronics, it is important to have several options for printable electrode materials that exhibit high conductivity, high stability, and low-cost. Here we report completely aerosol jet printed (AJP) p- and n-type electrolyte-gated transistors (EGTs) using a variety of different electrode materials including highly conductive metal nanoparticles (Ag), conducting polymers (polystyrenesulfonate doped poly(3,4-ethylendedioxythiophene, PEDOT:PSS), transparent conducting oxides (indium tin oxide), and carbon-based materials (reduced graphene oxide). Using these source-drain electrode materials and a PEDOT:PSS/ion gel gate stack, we demonstrated all-printed p- and n-type EGTs in combination with poly(3-hexythiophene) and ZnO semiconductors. All transistor components (including electrodes, semiconductors, and gate insulators) were printed by AJP. Both kinds of devices showed typical p- and n-type transistor characteristics, and exhibited both low-threshold voltages (<2 V) and high hole and electron mobilities. Our assessment suggests Ag electrodes may be the best option in terms of overall performance for both types of EGTs.

Facile Method for Fabricating Flexible Substrates with Embedded, Printed Silver Lines

Insertion, curing and delamination is presented as a simple and scalable method for creating flexible substrates with embedded, printed silver lines. In a sequential process, aerosol-jet printed silver lines are transferred from a donor substrate to a thin reactive polymer that is directly adhered to a flexible substrate. Due to the unique ability of the aerosol jet to print continuous lines on a low energy surface, a 100% transfer of the printed electrodes is obtained, as confirmed by electrical measurements. Moreover, the root-mean-square roughness of the embedded electrodes is less than 10 nm, which is much lower than that for their as-printed form. The embedded electrodes are robust and do not show a significant degradation in electrical performance after thousands of bending cycles.

Formation of copper zinc tin sulfide thin films from colloidal nanocrystal dispersions via aerosol-jet printing and compaction

A three-step method to create dense polycrystalline semiconductor thin films from nanocrystal liquid dispersions is described. First, suitable substrates are coated with nanocrystals using aerosol-jet printing. Second, the porous nanocrystal coatings are compacted using a weighted roller or a hydraulic press to increase the coating density. Finally, the resulting coating is annealed for grain growth. The approach is demonstrated for making polycrystalline films of copper zinc tin sulfide (CZTS), a new solar absorber composed of earth-abundant elements. The range of coating morphologies accessible through aerosol-jet printing is examined and their formation mechanisms are revealed. Crack-free albeit porous films are obtained if most of the solvent in the aerosolized dispersion droplets containing the nanocrystals evaporates before they impinge on the substrate. In this case, nanocrystals agglomerate in flight and arrive at the substrate as solid spherical agglomerates. These porous coatings are mechanically compacted, and the density of the coating increases with compaction pressure. Dense coatings annealed in sulfur produce large-grain (>1 μm) polycrystalline CZTS films with microstructure suitable for thin-film solar cells.

High-Resolution, High-Aspect Ratio Conductive Wires Embedded in Plastic Substrates

A novel method is presented to fabricate high-resolution, high-aspect ratio metal wires embedded in a plastic substrate for flexible electronics applications. In a sequential process, high-resolution channels connected to low-resolution reservoirs are first created in a thermosetting polymer by imprint lithography. A reactive Ag ink is then inkjet-printed into the reservoirs and wicked into the channels by capillary forces. These features serve as a seed layer for copper deposition inside the channels via electroless plating. Highly conductive wires (>50% bulk metal) with minimum line width and spacing of 2 and 4 μm, respectively, and an aspect ratio of 0.6 are obtained. The embedded wires exhibit good mechanical flexibility, with minimal degradation in electrical performance after thousands of bending cycles.

Wettability Contrast Gravure Printing

Silicon gravure patterns are engineered to have cells that are wettable and lands that are not wettable by aqueous inks. This strategy allows excess ink on the lands to be removed without using a doctor blade. Using an aqueous silica ink, continuous lines as narrow as 1.2 μm with 1.5 μm space are gravure printed.

High-Resolution Transfer Printing of Graphene Lines for Fully Printed, Flexible Electronics

Pristine graphene inks show great promise for flexible printed electronics due to their high electrical conductivity and robust mechanical, chemical, and environmental stability. While traditional liquid-phase printing methods can produce graphene patterns with a resolution of ∼30 μm, more precise techniques are required for improved device performance and integration density. A high-resolution transfer printing method is developed here capable of printing conductive graphene patterns on plastic with line width and spacing as small as 3.2 and 1 μm, respectively. The core of this method lies in the design of a graphene ink and its integration with a thermally robust mold that enables annealing at up to ∼250 °C for precise, high-performance graphene patterns. These patterns exhibit excellent electrical and mechanical properties, enabling favorable operation as electrodes in fully printed electrolyte-gated transistors and inverters with stable performance even following cyclic bending to a strain of 1%. The high resolution coupled with excellent control over the line edge roughness to below 25 nm enables aggressive scaling of transistor dimensions, offering a compelling route for the scalable manufacturing of flexible nanoelectronic devices.

X-band printed phased array antennas using high-performance CNT/ion gel/Ag transistors

This paper reports a fully printed phased array antenna developed on a 125 micron thick flexible Kapton substrate. Switching for the phase delay lines is accomplished using printed carbon nanotube transistors with ion gel dielectric layers. Design of each element of the phased array antenna is reported, including a low loss constant impedance power divider, a phase shifter network, and patch antenna design. Steering of an X-band PAA operating at 10GHz from 0 degrees to 22.15 degrees is experimentally demonstrated. In order to completely package the array with electrical interconnects, a single substrate interconnect scheme is also investigated.