Xiaochen Ren

A Low‐Operating‐Power and Flexible Active‐Matrix Organic‐Transistor Temperature‐Sensor Array

An organic flexible temperature-sensor array exhibits great potential in health monitoring and other biomedical applications. The actively addressed 16 × 16 temperature sensor array reaches 100% yield rate and provides 2D temperature information of the objects placed in contact, even if the object has an irregular shape. The current device allows defect predictions of electronic devices, remote sensing of harsh environments, and e-skin applications.

High performance organic transistor active-matrix driver developed on paper substrate

The fabrication of electronic circuits on unconventional substrates largely broadens their application areas. For example, green electronics achieved through utilization of biodegradable or recyclable substrates, can mitigate the solid waste problems that arise at the end of their lifespan. Here, we combine screen-printing, high precision laser drilling and thermal evaporation, to fabricate organic field effect transistor (OFET) active-matrix (AM) arrays onto standard printer paper. The devices show a mobility and on/off ratio as high as 0.56 cm2V−1s−1 and 109 respectively. Small electrode overlap gives rise to a cut-off frequency of 39 kHz, which supports that our AM array is suitable for novel practical applications. We demonstrate an 8 × 8 AM light emitting diode (LED) driver with programmable scanning and information display functions. The AM array structure has excellent potential for scaling up.

High Dynamic Range Organic Temperature Sensor

IC A IO N Due to the compatibility with large-area fabrication techniques and low fabrication costs involved, organic transistors have been actively researched and various kinds of chemical or physical sensors based on organics transistors have been developed. These sensors can be used for the detection of moisture, [ 1 ] glucose, [ 2 ] pressure, [ 3 , 4 ] light intensity, [ 5 ] and temperature. [ 3b ] For pressure-sensing applications, Bao et al. have demonstrated micro-structured polydimethylsiloxane (PDMS) pressure sensors with sensitivity down to 3 Pa. Someya et al. have also prepared pressure-sensor arrays with memory properties by using pressure-sensitive rubber alongside pentacene organic thin-fi lm transistor (OTFT) circuits. [ 3a ] In optical-sensor applications, Forrest et al. have shown an integrated OTFT-photodetector device with a sensitivity dynamic range of 12 bits for monochromatic light sensing at 580 nm. [ 5 ] Apart from pressure and light, heat is another important physical parameter that is often measured and thermal sensors have a lot of application potentials. High-sensitivity temperature sensors can be used for electronic skins, electronic health monitoring, and detecting patients’ body temperatures. However, unlike the pressure and optical sensors, OTFT-based temperature sensing devices with high sensitivity are yet to be demonstrated. As the glass transition temperatures of organic semiconductors are relatively low, the operating temperature of such organic temperature sensors is usually limited to around 100 ° C, making it extremely suitable for electronic skin or medical applications. Unfortunately the conductivity variation of the organic thin-fi lms in the resistor or diode structures between room temperature and 100 ° C is usually less than 10. [ 3b ] This results in limited sensitivity of the organic temperature sensors, especially in comparison with the silicon-based devices. [ 6 ]

High Sensitivity, Wearable, Piezoresistive Pressure Sensors Based on Irregular Microhump Structures and Its Applications in Body Motion Sensing.

UNLABELLED A pressure sensor based on irregular microhump patterns has been proposed and developed. The devices show high sensitivity and broad operating pressure regime while comparing with regular micropattern devices. Finite element analysis (FEA) is utilized to confirm the sensing mechanism and predict the performance of the pressure sensor based on the microhump structures. Silicon carbide sandpaper is employed as the mold to develop polydimethylsiloxane (PDMS) microhump patterns with various sizes. The active layer of the piezoresistive pressure sensor is developed by spin coating PEDOT PSS on top of the patterned PDMS. The devices show an averaged sensitivity as high as 851 kPa(-1) , broad operating pressure range (20 kPa), low operating power (100 nW), and fast response speed (6.7 kHz). Owing to their flexible properties, the devices are applied to human body motion sensing and radial artery pulse. These flexible high sensitivity devices show great potential in the next generation of smart sensors for robotics, real-time health monitoring, and biomedical applications.

23 bits optical sensor based on nonvolatile organic memory transistor

Polymer electret transistor memory device has stable charge storage and memory properties. Here, we combine a large band gap organic semiconductor dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene with the polystyrene electret to form an optical sensor with memory effect. The blue light combined with programming bias leads to a positive threshold voltage shift for more than 100 V while the drain-source current shows a variation of seven orders of magnitude. The dynamic range of current device is up to 23 bits and the photo responsivity is 420 A W−1. The optically programmed transistor can be directly used for high-resolution optical sensor and multi-level data storage applications.

A UV-ozone treated amorphous barium–strontium titanate dielectric thin film for low driving voltage flexible organic transistors

Reducing the operating power and making them suitable for portable and wearable electronic applications are critical steps for the further development of organic field effect transistors (OFETs). One of the possible approaches to achieve this is by using a high dielectric constant (high-k) dielectric layer. Here, we propose a new kind of high-k amorphous Ba0.7Sr0.3TiO3 (BST) material derived by a layer-by-layer sol–gel method and solidified by UV-ozone treatments under atmospheric air and pressure. The dielectric constant of the amorphous BST thin film is around 11. Without employing the self-assembled monolayers (SAMs), the pentacene OFET on a polyethylene naphthalate (PEN) substrate shows saturation under −2.5 V, the saturation mobility of 0.252 cm2 V−1 s−1 with negligible hysteresis effects. The chemical composition of the dielectric film and the morphology structure are studied by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) respectively. We also performed compressive and tensile bending tests on the OFETs and the dielectric layer. Due to the amorphous nature of the BST thin film, it shows a constant C–f relationship in both compressive and tensile bending up to a bending radius of 3.3 mm. It demonstrates high application potential of the current BST thin film in flexible electronics and circuits. To confirm the application potentials, we develop an organic inverter based on the transistor and a resistor, and a gain as high as 20 can be achieved.

Fully transparent organic transistors with junction-free metallic network electrodes

We utilize highly transparent, junction-free metal network electrodes to fabricate fully transparent organic field effect transistors (OFETs). The patterned transparent Ag networks are developed by polymer crack template with adjustable line width and density. Sheet resistance of the network is 6.8 Ω/sq and optical transparency in the whole visible range is higher than 80%. The bottom contact OFETs with DNTT active layer and parylene-C dielectric insulator show a maximum field-effect mobility of 0.13 cm2/V s (average mobility is 0.12 cm2/V s) and on/off ratio is higher than 107. The current OFETs show great potential for applications in the next generation of transparent and flexible electronics.

Light programmable organic transistor memory device based on hybrid dielectric

We have fabricated the transistor memory devices based on SiO2 and polystyrene (PS) hybrid dielectric. The trap states densities with different semiconductors have been investigated and a maximum 160V memory window between programming and erasing is realized. For DNTT based transistor, the trapped electron density is limited by the number of mobile electrons in semiconductor. The charge transport mechanism is verified by light induced Vth shift effect. Furthermore, in order to meet the low operating power requirement of portable electronic devices, we fabricated the organic memory transistor based on AlOx/self-assembly monolayer (SAM)/PS hybrid dielectric, the effective capacitance of hybrid dielectric is 210 nF cm-2 and the transistor can reach saturation state at -3V gate bias. The memory window in transfer I-V curve is around 1V under +/-5V programming and erasing bias.