Paddy K. L. Chan

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.

Nonvolatile organic transistor-memory devices using various thicknesses of silver nanoparticle layers

We demonstrate the modification of the memory effect in organic memory devices by adjusting the thickness of silver nanoparticles (NPs) layer embedded into the organic semiconductor. The memory window widens with increasing Ag NPs layer thickness, a maximum window of 90 V is achieved for 5 nm Ag NPs and the on/off current ratio decreases from 105 to 10 when the Ag NPs layer thickness increases from 1 to 10 nm. We also compare the charge retention properties of the devices with different Ag NPs thicknesses. Our investigation presents a direct approach to optimize the performance of organic memory with the current structure.

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 ]

Physical mechanisms for hot-electron degradation in GaN light-emitting diodes

We report investigations on the degradation of GaN-based light-emitting diodes due to high dc current stress by examining two types of devices with the same fabrication procedures except for the growth conditions for the InGaN quantum wells (QWs). Higher trimethylindium and triethylgallium fluxes are used for type A devices resulting in a threefold increase in the InGaN QWs growth rate compared to type B devices. Detailed structural and optoelectronic properties of the devices are investigated by transmission electron microscopy, atomic force microscopy, thermal imaging, I-V measurements, and the low-frequency noise properties of the devices as a function of the stress time, tS. The experimental data show that the QWs in type B devices are dominated by spiral growth and they have substantially higher strain nonuniformity than type A devices. The highly strained GaN/InGaN interfaces in device B are also responsible for the faster increase in the defect density due to hot-electron injection. The defects enhance the trap-assisted tunneling in the multiple quantum wells (MQWs) resulting in the development of hot spots among type B devices after high current stressing of the MQWs. This in turn leads to an increase in the defect generation rate resulting in a thermal run-away condition that ultimately resulted in the failure of the device. The data show that an increase in the growth rate in the InGaN layer led to the domination by the step flow growth mode over the spiral growth mode in the MQWs. This is the main reason for the reduction in the dislocation density in type A devices and hence their increase in device reliability.We report investigations on the degradation of GaN-based light-emitting diodes due to high dc current stress by examining two types of devices with the same fabrication procedures except for the growth conditions for the InGaN quantum wells (QWs). Higher trimethylindium and triethylgallium fluxes are used for type A devices resulting in a threefold increase in the InGaN QWs growth rate compared to type B devices. Detailed structural and optoelectronic properties of the devices are investigated by transmission electron microscopy, atomic force microscopy, thermal imaging, I-V measurements, and the low-frequency noise properties of the devices as a function of the stress time, tS. The experimental data show that the QWs in type B devices are dominated by spiral growth and they have substantially higher strain nonuniformity than type A devices. The highly strained GaN/InGaN interfaces in device B are also responsible for the faster increase in the defect density due to hot-electron injection. The defects enh...

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.

Thermal relaxation time and heat distribution in pulsed InGaAs quantum dot lasers

Using a charge coupled device-based thermoreflectance technique, we achieve a high-resolution (∼700nm) cross-sectional temperature profile of a semiconductor laser. This two-dimensional profile allows us to identify separate heat sources due to contact heating and nonradiative recombination in the active region. By adapting the technique to pulsed operation and varying the laser’s duty cycle, we measure the thermal relaxation time constant. We also quantitatively determine the heat transfer from device-internal heat sources and demonstrate both the large effect of lateral heat spreading and the distinction between a laser’s top surface temperature and its active region temperature.

High performance self-organized InGaAs quantum dot lasers on silicon

We report the molecular beam epitaxial growth and characteristics of room temperature InGaAs quantum dot lasers grown directly on silicon utilizing thin (⩽2μm) GaAs buffer layers and quantum dot layers as dislocation filters. Cross-sectional transmission electron microscopy studies show that defect-free quantum dot active regions can be achieved. Room temperature photoluminescence emission from quantum dots grown on silicon is comparable, in intensity and linewidth, to that from similar dots grown on GaAs substrates. The best devices are characterized by relatively low threshold current (Jth∼1100A∕cm2), high output power (>150mW), large characteristic temperature (T0=244K), and constant output slope efficiency (⩾0.3W∕A) in the temperature range of 5–95°C.

Temperature mapping and thermal lensing in large-mode, high-power laser diodes

The authors use high-resolution charge-coupled device based thermoreflectance to derive two dimensional facet temperature maps of a λ=1.55μm InGaAsP∕InP watt-class laser that has a large (>5×5μm2) fundamental optical mode. Recognizing that temperature rise in the laser will lead to refractive index increase, they use the measured temperature profiles as an input to a finite-element mode solver, predicting bias-dependent spatial mode behavior that agrees well with experimental observations. These results demonstrate the general usefulness of high-resolution thermal imaging for studying spatial mode dynamics in photonic devices.

Short circuit current improvement in planar heterojunction organic solar cells by multijunction charge transfer

A multijunction structure was applied on an organic photovoltaic (OPV) device for broadening the absorption spectrum and enhancing the power conversion efficiency through charge transfer process. By inserting the tris[4-(2-thienyl)]amine (TTPA) into a boron subphthalocyanine chloride (SubPc)/C60 OPV device, the short circuit current density (Jsc) showed a 47.5% increases from 3.05 to 4.50 mA/cm2 in the bilayer planar heterojunction device, while the open circuit voltage (Voc) remained constant. Based on the single junction (TTPA/SubPc) device and photoluminescence absorption results, we confirmed both TTPA/SubPc and SubPc/C60 junctions are contributing to the exciton dissociation process hence the efficiency enhancement.