Jiaqing Zhao

Unencapsulated Air-stable Organic Field Effect Transistor by All Solution Processes for Low Power Vapor Sensing

With its excellent mechanical flexibility, low-cost and low-temperature processing, the solution processed organic field-effect transistor (OFET) is a promising platform technology for developing ubiquitous sensor applications in digital health, environment monitoring and Internet of Things. However, a contradiction between achieving low voltage operation and having stable performance severely hinder the technology to become commercially viable. This work shows that, by reducing the sub-gap density of states (DOS) at the channel for low operation voltage and using a proper low-k non-polar polymer dielectric layer, such an issue can be addressed. Stable electrical properties after either being placed for weeks or continuously prolonged bias stressing for hours in ambient air are achieved for all solution processed unencapsulated OFETs with the channel being exposed to the ambient air for analyte detection. The fabricated device presents a steep subthreshold swing less than 100 mV/decade, and an ON/OFF ratio of 106 at a voltage swing of 3 V. The low voltage and stable operation allows the sensor made of the OFET to be incorporated into a battery-powered electronic system for continuously reliable sensing of ammonia vapor in ambient air with very small power consumption of about 50 nW.

Current Status and Opportunities of Organic Thin-Film Transistor Technologies

Attributed to its advantages of super mechanical flexibility, very low-temperature processing, and compatibility with low cost and high throughput manufacturing, organic thin-film transistor (OTFT) technology is able to bring electrical, mechanical, and industrial benefits to a wide range of new applications by activating nonflat surfaces with flexible displays, sensors, and other electronic functions. Despite both strong application demand and these significant technological advances, there is still a gap to be filled for OTFT technology to be widely commercially adopted. This paper provides a comprehensive review of the current status of OTFT technologies ranging from material, device, process, and integration, to design and system applications, and clarifies the real challenges behind to be addressed.

Inkjet printed fine silver electrodes for all-solution-processed low-voltage organic thin film transistors

Inkjet printed silver (IJP Ag) source/drain (S/D) and gate electrodes were incorporated into a bottom-gate bottom-contact organic thin film transistor (OTFT) architecture to develop all-solution-processed low voltage OTFTs. With well controlled ink wetting on a cross-linked polyvinyl-alcohol surface, IJP Ag S/D electrode pairs were fabricated with controllable short channels down to 20 μm using a Dimatix 2831 inkjet printer with a 10 pL cartridge, and formed good contacts with the organic semiconductor layer. IJP gate electrodes with a low flat surface profile were also achieved to obtain OTFTs of high quality gate dielectric layer for low leakage current. The fabricated low voltage all-solution-processed OTFTs present good device performance with a low operation voltage below 2 V, a mobility of 0.3 cm2 V−1 s−1, and an ON/OFF current ratio larger than 104.

All-Solution-Processed Low-Voltage Organic Thin-Film Transistor Inverter on Plastic Substrate

In this paper, all-solution-processed low-voltage organic thin-film transistor inverters on polyethylene naphthalate plastic substrate were achieved in the bottom-gate bottom-contact device configuration. In the devices, 6,13-bis(triisopropylsilylethynyl)-pentacene blended with polystyrene was used as the channel layer, and ultraviolet cross-linked polyvinyl alcohol was used as the gate dielectric layer. With optimized inkjet jetting process parameters and a proper polymer dielectric substrate surface, fine silver electrodes were formed as the source, drain, and gate electrodes. The maximum processing temperature was 150°C. The devices show promising performance with a mobility of 0.8 cm2/(V·s), a subthreshold swing of 100 mV/decade and an ON/OFF ratio of about 104. The fabricated diode-load inverter has a high dc voltage gain up to 67.3 at a supply voltage of 3 V.

Controlling the surface wettability of the polymer dielectric for improved resolution of inkjet-printed electrodes and patterned channel regions in low-voltage solution-processed organic thin film transistors

A facile method for realizing both inkjet printed electrodes with improved resolution and patterned semiconductor islands was developed to fabricate all solution processed low-voltage organic thin film transistors (OTFTs). By reducing the surface wettability of the polymer gate dielectric layer through coating of a self-assembled monolayer (SAM), fine and narrow inkjet printed source/drain electrodes (a line width of about 35 μm) and short channels (about 15 μm) were formed with very good yield and uniformity using an inkjet printer with a 10 pL drop volume print head and limited registration accuracy. The coated SAM layer was then selectively removed by ultraviolet ozone treatment to create patterned wettable and less wettable regions to form self-assembled organic semiconductor islands. The fabricated low voltage OTFTs present a high quality semiconductor/dielectric interface and good device performance.

Low temperature cross-linked, high performance polymer gate dielectrics for solution-processed organic field-effect transistors

Poly(methyl methacrylate) (PMMA) and polystyrene (PS) functionalized with both propargyl and azido groups have been synthesized by free radical copolymerization. The thin-films of the bi-functionalized PMMA or PS can be effectively cross-linked by TAAC reaction at a relatively low temperature (100 °C). This bi-functional approach significantly improved the efficiencies of the cross-linking reactions in the solid state and substantially enhanced solvent resistance of the cross-linked dielectric layers. The cross-linked thin-films displayed smooth surfaces and the cross-linked PMMA thin films showed low leakage current densities. The solution processed organic field-effect transistors of bottom-gate bottom-contact structures based on cross-linked PMMA gate dielectric layers exhibited high device performance with a field-effect mobility of 0.59 cm2 V−1 s−1, and an on/off current ratio of 105.

Low-Voltage pH Sensor Tag Based on All Solution Processed Organic Field-Effect Transistor

By reducing the sub-gap density of states at the channel, an all solution processed low-voltage organic field-effect transistor (OFET) was realized on a micrometer-thick SU8 gate dielectric layer. The device exhibits excellent operational stability under continuous bias stress. An integrated pH sensor tag was fabricated based on this low-voltage OFET, using an extended gate OFET for sensing and the other OFET as the load to convert the current signal to a voltage output for easy design of the subsequent readout circuit. With the low-voltage operation behavior and excellent operational stability of the OFET, the sensor tag was demonstrated to be operated in a 3.3 V battery powered electronic system for reliable pH sensing.

Top-Gate Dry-Etching Patterned Polymer Thin-Film Transistors With a Protective Layer on Top of the Channel

Photolithographic and dry-etching processes are developed to pattern the organic semiconductor (OSC) layer for top-gate organic thin-film transistors (OTFTs). A fluorine polymer layer is used to protect the OSC surface from the patterning processes so that the common photoresist can be used. The ON/OFF-current ratios of the patterned OTFTs are improved by about one order of magnitude compared with that of unpatterned devices. However, it is shown that removing the polymer protective layer can cause degraded subthreshold behavior due to increased interface trap density at the semiconductor/dielectric interface. A process without removing the polymer protective layer is thus developed to address this issue, and is shown to be able to provide a reliable route to achieve patterned top-gate OTFTs with high ON/OFF-current ratio, small subthreshold swing, and negligible hysteresis.

Inkjet printing narrow fine Ag lines on surface modified polymeric films

In the paper, we have demonstrated a simple but effective method to inkjet print fine Ag conductive lines with improved resolution on polymeric films. This is achieved by forming an ultra-flat polymeric film and modifying its surface energy. As a result, narrow Ag lines (<; 50 μm) and short gaps (~10 μm) were successfully achieved using optimized printing parameters. Meanwhile these formed Ag lines show fine morphology, smooth surface and high electrical conductivity. The improved resolution is attributed to the reduced spreading of ink droplets on the surface modified substrate. This method shows the potential of directly producing high-resolution electrodes for printed electronics applications.

Dual-

Threshold voltage (V_th) control using different metal gates (aluminum and gold) was applied to realize dual-V_th low-voltage solution processed organic thin film transistors (OTFTs). In the devices, the low-operation voltage was realized based on a channel engineering approach instead of using large gate dielectric capacitance, therefore, a relatively thick and low-dielectric constant polymer dielectric layer can be used. The devices present a mobility about 1.0 cm²/(V·s), ON/OFF ratio of 10⁵ and small subthreshold swing of 100 mV/decade with the maximum processing temperature not exceeding 100 °C. It was found that, even with a 400-nm thick polymer dielectric layer, since possible charge trapping effects was effectively suppressed by the crosslinking processes, well controlled V_th was achieved. Both unipolar single-V_th and dual-V_th inverters were fabricated, clearly showing the influence of V_th on the circuit operation and improved performance with the dual-V_th OTFT technology.