Performance and Stability of Aerosol-Jet-Printed Electrolyte-Gated Transistors Based on Poly(3-hexylthiophene)

We report performance optimization and stability analysis of aerosol-jet-printed electrolyte-gated transistors (EGTs) based on the polymer semiconductor poly­(3-hexylthiophene) (P3HT). EGTs were optimized with respect to printed P3HT thickness and the completed device annealing temperature. EGTs with relatively thin P3HT films (∼50 nm) annealed at 120 °C have the best performance and display an unusual combination of metrics including sub-1-V operation, ON/OFF current ratios of 10⁶, OFF currents of <10^–10 A (<10^–6 A cm^–2), saturation hole mobilities of 1.3 cm² V^–1 s^–1, threshold voltages of −0.3 V, and subthreshold swings of 70 mV decade^–1. Furthermore, optimized EGTs printed on polyester substrates are extremely robust to bias stress and repeated mechanical bending strain. Collectively, the results suggest that optimized P3HT-based EGTs are promising devices for printed, flexible electronics

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

3D Hollow Framework Silver Nanowire Electrodes for High-Performance Bottom-Contact Organic Transistors

We successfully fabricated high performance bottom-contact organic field-effect transistors (OFETs) using silver nanowire (AgNW) network electrodes by spray deposition. The synthesized AgNWs have the dimensions of 40–80 nm in diameter and 30–80 μm in length and are randomly distributed and interconnected to form a 3D hollow framework. The AgNWs networks, deposited by spray coating, yield an average optical transmittance of up to 88% and a sheet resistance as low as 10 ohm/sq. For using AgNWs as source/drain electrodes of OFETs with a bottom-contact configuration, the large contact resistance at the AgNWs/organic channel remains a critical issue for charge injection. To enhance charge injection, we fabricate semiconductor crystals on the AgNW using an adsorbed residual poly­(<i>N</i>-vinyl­pyrrolidone) layer. The resulting bottom-contact OFETs exhibit high mobility up to 1.02 cm²/(V s) and are similar to that of the top-contact Au electrodes OFETs with low contact resistance. A morphological study shows that the pentacene crystals coalesced to form continuous morphology on the nanowires and are highly interconnected with those on the channel. These features contribute to efficient charge injection and encourage the improvement of the bottom-contact device performance. Furthermore, the large contact area of individual AgNWs spreading out to the channel at the edge of the electrode also improves device performance

Electrostatic-Force-Assisted Dispensing Printing of Electrochromic Gels for Low-Voltage Displays

In this study, low-voltage, printed, ion gel-based electrochromic devices (ECDs) were successfully fabricated. While conventional dispensing printing provides irregularly printed electrochromic (EC) gels, we improved the adhesion between the printed gel and the substrate by applying an external voltage. This is called electrostatic-force-assisted dispensing printing. As a result, we obtained well-defined, printed, EC gels on substrates such as indium tin oxide-coated glass. We fabricated a gel-based ECD by simply sandwiching the printed EC gel between two transparent electrodes. The resulting ECD, which required a low coloration voltage (∼0.6 V), exhibited a high coloration efficiency (η) of 161 cm²/C and a large transmittance contrast (∼82%) between the bleached and colored states at −0.7 V. In addition, electrostatic-force-assisted dispensing printing was utilized to fabricate directly patterned ECDs

Dense Assembly of Soluble Acene Crystal Ribbons and Its Application to Organic Transistors

The preparation of uniform large-area highly crystalline organic semiconductor single crystals remains a challenge in the field of organic field-effect transistors (OFETs). Crystal densities in the channel regions of OFETs have not yet reached sufficiently high values to provide efficient charge transport, and improving channel crystal densities remains an important research area. Herein we fabricated densely well-aligned single crystal arrays of the 6,13-bis­(triisopropylsilylethynyl)­pentacene (TIPS_PEN) semiconductor using a straightforward scooping-up (SU) methodology to quickly produce a large-area self-assembled semiconductor crystal layer. The resulting crystalline TIPS_PEN strip arrays obtained using the SU method revealed a packing density that was 2.76 times the value obtained from the dip-coated channel, and the mean interspatial distance between the crystal strips decreased from 21.5 to 7.8 μm. The higher crystal packing density provided efficient charge transport in the FET devices and directly yielded field-effect mobilities as high as 2.16 cm²/(V s). These field-effect mobilities were more than three times the values obtained from the OFETs prepared using dip-coated channels. Furthermore, the contact resistance between the source/drain electrodes and the TIPS_PEN crystals decreased by a factor of 2. These contributions represent a significant step forward in improving semiconductor crystal alignment for the fabrication of large-area high-performance organic electronics

Room-Temperature-Processable Wire-Templated Nanoelectrodes for Flexible and Transparent All-Wire Electronics

Sophisticated preparation of arbitrarily long conducting nanowire electrodes on a large area is a significant requirement for development of transparent nanoelectronics. We report a position-customizable and room-temperature-processable metallic nanowire (NW) electrode array using aligned NW templates and a demonstration of transparent all-NW-based electronic applications by simple direct-printing. Well-controlled electroless-plating chemistry on a polymer NW template provided a highly conducting Au NW array with a very low resistivity of 7.5 μΩ cm (only 3.4 times higher than that of bulk Au), high optical transmittance (>90%), and mechanical bending stability. This method enables fabrication of all-NW-based electronic devices on various nonplanar surfaces and flexible plastic substrates. Our approach facilitates realization of advanced future electronics

Tuning the Work Function of Printed Polymer Electrodes by Introducing a Fluorinated Polymer To Enhance the Operational Stability in Bottom-Contact Organic Field-Effect Transistors

Poly­(3,4-ethylenedioxy­thiophene):poly­(4-styrene­sulfonate) (PEDOT:PSS) is a promising electrode material for organic electronic devices due to its high conductivity, good mechanical flexibility, and feasibility of easy patterning with various printing methods. The work function of PEDOT:PSS needs to be increased for efficient hole injection, and the addition of a fluorine-containing material has been reported to increase the work function of PEDOT:PSS. However, it remains a challenge to print PEDOT:PSS electrodes while simultaneously tuning their work functions. Here, we report work function tunable PEDOT:PSS/Nafion source/drain electrodes formed by electrohydrodynamic printing technique with PEDOT:PSS/Nafion mixture solutions for highly stable bottom-contact organic field-effect transistors (OFETs). The surface properties and work function of the printed electrode can be controlled by varying the Nafion ratio, due to the vertical phase separation of the PEDOT:PSS/Nafion. The PEDOT:PSS/Nafion electrodes exhibit a low hole injection barrier, which leads to efficient charge carrier injection from the electrode to the semiconductor. As a result, pentacene-based OFETs with PEDOT:PSS/Nafion electrodes show increased charge carrier mobilities of 0.39 cm²/(V·s) compared to those of devices with neat PEDOT:PSS electrodes (0.021 cm²/(V·s)). Moreover, the gate-bias stress stability of the OFETs is remarkably improved by employing PEDOT:PSS/Nafion electrodes, as demonstrated by a reduction of the threshold voltage shift from −1.84 V to −0.28 V

Direct Writing and Aligning of Small-Molecule Organic Semiconductor Crystals via “Dragging Mode” Electrohydrodynamic Jet Printing for Flexible Organic Field-Effect Transistor Arrays

Patterning and aligning of organic small-molecule semiconductor crystals over large areas is an important issue for their commercialization and practical device applications. This Letter reports “dragging mode” electrohydrodynamic jet printing that can simultaneously achieve direct writing and aligning of 6,13-bis­(triisopropylsilylethynyl) pentacene (TIPS-PEN) crystals. Dragging mode provides favorable conditions for crystal growth with efficient controls over supply voltages and nozzle-to-substrate distances. Optimal printing speed produces millimeter-long TIPS-PEN crystals with unidirectional alignment along the printing direction. These crystals are highly crystalline with a uniform packing structure that favors lateral charge transport. Organic field-effect transistors (OFETs) based on the optimally printed TIPS-PEN crystals exhibit high field-effect mobilities up to 1.65 cm²/(V·s). We also demonstrate the feasibility of controlling pattern shapes of the crystals as well as the fabrication of printed flexible OFET arrays

Direct Writing and Aligning of Small-Molecule Organic Semiconductor Crystals via “Dragging Mode” Electrohydrodynamic Jet Printing for Flexible Organic Field-Effect Transistor Arrays

Patterning and aligning of organic small-molecule semiconductor crystals over large areas is an important issue for their commercialization and practical device applications. This Letter reports “dragging mode” electrohydrodynamic jet printing that can simultaneously achieve direct writing and aligning of 6,13-bis­(triisopropylsilylethynyl) pentacene (TIPS-PEN) crystals. Dragging mode provides favorable conditions for crystal growth with efficient controls over supply voltages and nozzle-to-substrate distances. Optimal printing speed produces millimeter-long TIPS-PEN crystals with unidirectional alignment along the printing direction. These crystals are highly crystalline with a uniform packing structure that favors lateral charge transport. Organic field-effect transistors (OFETs) based on the optimally printed TIPS-PEN crystals exhibit high field-effect mobilities up to 1.65 cm²/(V·s). We also demonstrate the feasibility of controlling pattern shapes of the crystals as well as the fabrication of printed flexible OFET arrays

Impact of Energetically Engineered Dielectrics on Charge Transport in Vacuum-Deposited Bis(tri­isopropyl­silyl­ethynyl)pentacene

The surface functionality of the gate dielectrics is one of the important variables to have a huge impact on the electrical performance of organic field-effect transistors (OFETs). Here, we describe the impact of energetically engineered dielectrics on charge transport in vacuum-deposited 6,13-bis­(triisopropyl­silylethynyl)­pentacene (TIPS-pentacene) thin films for eventually realizing high-performance OFETs. A variety of self-assembled monolayers (SAMs) bearing amino, methyl, phenyl (PTS), or fluoro end groups were introduced onto the SiO₂ dielectric surfaces to design energetically engineered surfaces that can be used to explore the impact of surface functionalities at a TIPS-pentacene/gate dielectric interface. The solvent-free vacuum deposition of TIPS-pentacene was used to exclude solution-processing effects resulting from fluid flows and solvent drying processes. The TIPS-pentacene layer on the PTS-SAM yielded the best morphological and crystalline structures, which directly enhanced the electrical properties, exhibiting field-effect mobilities as high as 0.18 cm²/(V s). Furthermore, the hysteresis, turn-on voltage, and threshold voltage were correlated with the surface potentials of various SAM-dielectrics. We believe that systematic investigation of the energetically engineered dielectrics presented here can provide a meaningful step toward optimizing the organic semiconductor/dielectric interface, thereby implementing flexible and high-performance OFETs