Qiaofeng Li

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.

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.

Highly Sensitive and Transparent Strain Sensor Based on Thin Elastomer Film

Strain sensors of high sensitivity and excellent transparency were fabricated based on a thin (~20 μm) poly(dimethylsiloxane) (PDMS) elastomer film integrating a conductive composite of silver nanowires and neutral-pH poly (3,4-ethylenedioxythiophene): poly(styrenesulfonate). Such a thin PDMS film was peeled off from a glass substrate, which surface was modified with a self-assembled monolayer to reduce the required tensile stress in the elastomer film for the peeling-off process. The thin film sensor presented a sensitivity more than 20 times higher than that of strain sensors based on a thick (800 μm) PDMS film, and also a high optical transparency of nearly 80%, which are the desired features for developing unperceivable and invisible sensors in wearable applications. Moreover, the sensor showed excellent durability and fast response/recovery speed. An application for real-time monitoring of human neck posture was finally demonstrated.

Bias Stress Stability Improvement in Solution-Processed Low-Voltage Organic Field-Effect Transistors Using Relaxor Ferroelectric Polymer Gate Dielectric

Low-voltage organic-field effect transi-stors (OFETs) using relaxor ferroelectric polymer poly (vinylidene fluoridetrifluoroethylene-chlorofloroethylene) P (VDF-TrFE-CFE)) were fabricated. The measured hysteresis loop and the threshold voltage shift under negative bias stress (NBS) are opposite to that of the reference device using low-

Flexible Ammonia Sensor Based on PEDOT:PSS/Silver Nanowire Composite Film for Meat Freshness Monitoring

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Stable Thin-Film Reference Electrode on Plastic Substrate for All-Solid-State Ion-Sensitive Field-Effect Transistor Sensing System

CYTOP gate dielectric layer, in which the hysteresis and NBS-induced instabilities are explained by gate bias-induced charge trapping. The anomalous behaviors in the P (VDF-TrFE-CFE) OFETs are attributed to the stress-induced remnant polarization in P (VDF-TrFE-CFE), which induces additional mobile charges into the channel. By adding a thin CYTOP layer between the P (VDF-TrFE-CFE) layer and the semiconductor layer, the two effects of charge trapping and remnant polarization under gate bias are found to be neutralized with each other, resulting in low-voltage OFETs of negligible hysteresis and excellent NBS stability.

Inkjet-Printed Multi-Bit Low-Voltage Fuse-Type Write-Once-Read-Many Memory Cell

Flexible ammonia (NH3) sensors were fabricated on polyethylene terephthalate substrate using poly(3, 4-ethylene-dioxythiophene): poly(styrenesulfonate) (PEDOT:PSS)/silver nanowire (AgNW) composite film as the active layer. With the AgNWs of optimized concentration being incorporated into the PEDOT:PSS film, the sensitivity of the devices was significantly improved. Even with simple digitally dispensed parallel structure electrodes, the device achieved excellent sensing performance, and was shown to be able to detect very low NH3 concentration below 500 ppb. The mechanism for the sensing performance improvement was revealed. The sensor also showed considerable selectivity with respect to water and common organic vapors. Finally, the sensor was integrated with a self-designed portable data acquisition system to monitor the freshness of pork, demonstrating its feasibility for inspecting the meat quality in the early stage.

Fully Solution Processed Bottom-Gate Organic Field-Effect Transistor With Steep Subthreshold Swing Approaching the Theoretical Limit

Solid-state thin-film reference electrodes (REs) were fabricated on plastic substrate based on a titanium/gold/silver/silver chloride (Ti/Au/Ag/AgCl) multi-layer metal structure for ion-sensitive field-effect-transistor (ISFET)-based sensing systems. A porous structure polyvinyl butyral membrane was formed on top to maintain a constant concentration of chloride as well as acting as a bridge between the electrolyte inside the membrane and the test solution. Excellent measurement stability was achieved with the fabricated RE, with a small drift rate of the open circuit potential less than 1.7 mV per hour. An all-solid-state ISFET sensing system was thus able to be built using the RE for pH measurement, showing excellent reproducibility. The system was finally applied for measuring different beverages, and the measurement results agreed well with those obtained with a commercial pH meter.

Stable fully-printed polymer resistive read-only memory and its operation in mobile readout system

Low-voltage fuse-type write-once-read-many memories were fabricated using a common material inkjet printer to form both the contact pads and the resistor tracks with the same silver ink and process settings. Based on the dependence of the fusing voltage on the length of the printed resistor track, a new cell structure was proposed to achieve multi-bit memories. The fabricated 2-b memory cell presents excellent long-term storage and operation stabilities, and 16-b memories were achieved using this cell structure with greatly saved contact pad numbers for easier external electrical connections than the conventional design. A system demonstration of using the memories was finally provided to prove the potential of this technology for practical use.