Dynamics of carrier injection in picene thin-film field-effect transistors with an ionic liquid sheet and ionic liquid gel

We fabricated picene thin-film field-effect transistors (FETs) with an ionic liquid gel and ionic liquid sheet as the gate electrolyte, and then used electron spin resonance (ESR) to investigate the carrier injection process in the organic electric double layer (EDL) FET. The ESR spectra strongly depended on the morphology of gate electrolytes. Three types of carrier injection processes in the EDL-FET were observed by examining the applied-bias time, organic-layer thickness, and gate-voltage dependencies of the electric-field-induced ESR spectrum: (1) interface injection due to electrostatic EDL formation, (2) bulk injection due to penetration of ions (electrochemical bulk doping), and (3) electrochemical reaction. These findings are significant for designing novel materials using the EDL-FET technique because three different carrier injection processes may lead to different physical properties, even in the same organic material.     

Controlling the crystal formation in solution-process for organic field-effect transistors with high-performance

We control the growth of high-quality organic semiconducting crystals in the aim of transistor application. By finely tuning the processing parameters, both isolated crystals showing characteristic facet angles and irregular-shaped, thin crystalline domains are obtained in large sizes (>400 μm). Structural investigations indicate that the various shapes of crystals are in the same crystal structure, and reveal that the irregular-shaped crystalline domains are composed by terrace of molecularly flat regions, which can be up to hundreds of microns in size. When applied in field-effect transistors, the thin crystalline domains exhibit the best performance showing μ_FET up to 4.4 cm²/V s. This is an order of magnitude higher than that of the transistors made from as-spun films and thick crystals. The approach well demonstrates the importance of fine control of crystal formation and can be generally used for getting organic crystal transistors.     

Electrical control of the distributed feedback organic semiconductor laser based on holographic polymer dispersed liquid crystal grating

In this paper, we demonstrate the electrical control of the distributed feedback (DFB) organic semiconductor laser based on a holographic polymer dispersed liquid crystal (HPDLC) grating for the first time. The grating is fabricated on the top of the organic semiconductor film to act as an external feedback structure. Experimental results show that the lasing intensity can be decreased by increasing the external electric field, and the lasing wavelength exhibits a slight blue-shift of 1.4 nm during the modulation process, indicating a good stability. The modulated performances are attributed to the decreases in the refractive index modulation and average refractive index of the HPDLC grating respectively as a result of the field-induced liquid crystal reorientation. This study provides some new ideas for the improvement of DFB organic semiconductor laser to enable envisioned applications in laser displays and integrated photonic circuits.     

Generalized enhancement of charge injection in bottom contact/top gate polymer field-effect transistors with single-walled carbon nanotubes

We investigate the influence of small amounts of dispersed single-walled carbon nanotubes (SWNTs) on the contact resistance and device characteristics of bottom contact/top gate polymer field-effect transistors (FETs). Five conjugated polymers representing different classes of polymer semiconductors with different HOMO/LUMO levels are employed, namely, polythiophenes (P3HT), polyphenylenevinylenes (MDMO-PPV), polyfluorenes (F8T2), naphthalene-bis(dicarboximide) bithiophene copolymers (P(NDI2OD-T2)), and diketopyrrolo-pyrrole-bithiophene copolymers (DPPT-TT). In all cases the presence of dispersed SWNTs reduces non-ohmic contact resistance and lowers threshold and onset voltages for charge transport. In some cases inherent ambipolar charge transport in conjugated polymers (F8T2 and P(NDI2OD-T2)) is revealed. The concentration of the SWNTs within the semiconducting layer remains below the percolation limit and thus the apparent mobilities and on/off ratios are still determined by the polymer and independent of the specific type of the carbon nanotubes (metallic or semiconducting). The degree of enhancement depends both on the energy level offset between the injecting gold electrode and the HOMO/LUMO level (i.e., Schottky barrier) and the charge carrier mobility of the respective polymer. The simplicity of this injection enhancement method and its broad applicability make it a step toward high performance polymer transistors without injection limitations.     

Improved ambipolar charge injection in organic field-effect transistors with low cost metal electrode using polymer sorted semiconducting carbon nanotubes

Solution-processed thin film transistors can be implemented using simple and low cost fabrication, and are the best candidates for commercialization due to their application to a range of wearable electronics. We report an ambipolar charge injection interlayer that can improve both hole and electron injection in organic field-effect transistors (OFETs) with inexpensive source-drain electrodes. The solution processed ambipolar injection layer is fabricated by selective dispersion of semiconducting single walled carbon nanotubes using poly(9,9-dioctylfluorene). OFETs with molybdenum (Mo) contacts and interlayer (Mo/interlayer OFETs) exhibit superior performance, including higher hole and electron mobilities, device yield, lower threshold voltages, and lower trap densities than those of bare transistors. While OFETs with Mo contacts show unipolar p-type behaviour, Mo/interlayer OFETs display ambipolar transport due to significant enhancement of electron injection. In the p-type region, transistor performance is comparable to devices with gold (Au). Hole mobility is increased approximately ten-fold over devices with only Mo contacts. The electron mobility of Mo/interlayer OFETs is 0.05 cm²V⁻¹s⁻¹, which is higher than devices with Au electrodes. The p-type contact resistances of Mo/interlayer OFETs are half those of OFETs with Mo contacts. Trap density in Mo/interlayer OFETs is one order magnitude lower than that of pristine devices. We also demonstrate that this approach is extendible to other metals (nickel) and n-type semiconductors with different energy levels. Injection by tunnelling is suggested as the mechanism of ambipolar injection.     

Performance enhancement in mechanically stable flexible organic-field effect transistors with TIPS-pentacene:polymer blend

Flexible organic field-effect transistors (OFETs) with TIPS-pentacene: polystyrene (PS) blend are demonstrated to exhibit enhanced mobility and significantly improved electrical stability compared to neat TIPS-pentacene on poly(4-vinylphenol) (PVP) dielectric (bi-layer OFETs), along with high mechanical stability. Due to merit of high quality dielectric-semiconductor interface, pristine TIPS-pentacene: PS blend OFETs exhibited maximum mobility of 0.93 cm² V⁻¹ s⁻¹ with average of 0.44(±0.25) cm² V⁻¹ s⁻¹ compared to 0.14(±0.10) cm² V⁻¹ s⁻¹ for bi-layer OFETs with high current on-off ratios on the order 10⁵ for both. Both types of devices exhibited high electrical stability upon bending with increasing magnitude of strain or its duration up to 5 days. However, significant differences in electrical stability of devices were observed upon applying constant bias-stress for 40 min to 1 h. Pristine blend devices exhibited outstanding electrical stability with very low drain current decay of <5% compared to ∼30% for bi-layer devices. Even upon bias-stress after 5 days of bending, the drain current decay levels were only changed to <10% and ∼50% for blend and bi-layer devices respectively.     

Distributed collision control with the integration of packet size for congestion control in wireless sensor networks

Several great features offered by wireless sensor networks (WSN) result in its wide deployment in various remote and continuous monitoring applications. As such, managing huge collected readings in this domain posted many challenges due to its design limitations. In order to provide seamless data transmission, which is of utmost importance in those delay‐sensitive applications, minimum delay and packet loss occurrence should be considered. Specifically, this paper addresses the common issue of congested networks in WSN with the combination technique of variance‐based distributed contention control (DCC‐V) and packet size optimization. The proposed integration technique, which operates on medium access control layer, takes into consideration the packet size advantages as it plays a key role in determining successful data delivery, given the error‐prone nature of WSN. While ensuring fewer corrupted packets, the proposed contention window (CW) in DCC‐V minimizes the chances of packet collisions and so alleviates congestion. In this technique, CW is determined based on slot utilization and average collision values, which also involve standard deviation measurements. Simulation analysis using network simulator‐2 shows outstanding performance of the proposed solution compared with the existing IEEE 802.15.4 protocol. Copyright © 2014 John Wiley & Sons, Ltd.     

Numerical investigation of liquid cooling cold plate for power control unit in fuel cell vehicle

This paper proposed fifteen structure schemes of the liquid-cooled plate for thermal control of the power control unit (PCU) in fuel cell vehicle (FCV). At the given serpentine channel with inconstant width, pin fin arrays with various configurations were arranged to improve the performance of three heating zones with multiple heat sources. Based on the same setup and boundary conditions, numerical simulations were conducted for different schemes. The solutions were validated by grid independence check and comparison with previous researches. Effects of fin geometrical parameters (such as diameter, height, fin pitch and shape) on pressure drop and heat transfer characteristics were investigated. Furthermore, two dimensionless factors η_H and η_P were quantified to evaluate the heat transfer enhancement and pressure drop augmentation. The dimensionless performance evaluation factor P_EF was cited to assess overall performance of the cold plate. Based on three factors mentioned above, cooling performances of three heating zones and the whole plate were compared among all schemes. According to the performance comparison, scheme 12 employing circular fins with diameter of 4 mm was selected as the optimal solution for the cold plate.     

中继网络中不准确信道状态信息下抗多窃听者的物理层安全方案

该文研究存在多个相互勾结的单天线窃听者的多中继传输系统中,采用零空间人工噪声和放大转发的中继波束赋形的物理层安全传输方案。在中继窃听端的信道状态信息不准确的情况下,基于半定规划理论,对中继的波束赋形加权矩阵和人工噪声协方差矩阵进行联合优化,有效减少相互勾结的多个窃听者所获得的信息量,显著提高系统保密容量,是一种具有良好鲁棒性的物理层安全传输方案。仿真结果显示方案具有良好的性能。     

Passive arbitration in adaptive shared control of robots with variable force and stiffness scaling

In mixed-initiative shared control, the final control command to the robot is a weighted sum of the commands from two or more agents (human operators or automatic control systems). In force controlled robots, scaling of forces without power-consistent scaling of velocities leads to loss of passivity of the overall system. In this work, we first pose the problem statement related to position drift, while using a state-of-the-art, passivity ensuring method for scaling of forces. We then formulate adaptive mixed-initiative shared control as an adaptive stiffness control approach. We ensure passivity of the adaptive stiffness controller with a novel, model-independent method. The salient features, benefits and limitations of the approach are emphasized through analyses, simulations and hardware experiments. The proposed approach is finally validated with a practical shared control task.