Wenjiang Liu

Light Trapping in Single Coaxial Nanowires for Photovoltaic Applications

We report a strong enhancement of the light absorption in single coaxial nanowires (NWs) of Si core/dielectric shells. We have calculated the light absorption coefficient within the framework of the Lorenz-Mie light scattering theory and found out that it is greatly increased by effective light trapping in Si cores owing to dielectric shells, as compared to that in Si NWs. We show that the strong absorption of light stems mainly from off-resonance enhancement and also from resonance contribution. By optimally tuning the core radius, the shell thickness, and the shell refractive index, we have obtained ~102% increase of the photocurrent.

A New Voltage Driving Scheme to Suppress Non-Idealities of Polycrystalline Thin-Film Transistors for AMOLED Displays

This paper presents a new voltage driving scheme with current feedback for polycrystalline silicon (poly-Si) thin-film transistors (TFTs) to drive active matrix organic light-emitting diode (AMOLED) displays. The pixel circuit is relatively simple, which is composed of 3 TFTs and 1 capacitor. Circuit simulation results show that the proposed driving scheme can effectively suppress the influence of nonidealities of typical poly-Si TFTs including kink effects, spatial nonuniformity of the electrical characteristics. This method also presents a settling time less than 3 μs with panel load of 100 pF/1.5 kΩ at all OLED current levels.

Universal Compact Model for Thin-Film Transistors and Circuit Simulation for Low-Cost Flexible Large Area Electronics

Thin-film transistors (TFT) in hydrogenated amorphoussilicon, amorphousmetal oxide, andsmallmolecule and polymer organic semiconductors would all hold promise as potential device candidates to large area flexible electronics applications. A universal compact dc model was developed with a proper balance between the physical and mathematical approaches for these thin-film transistors (TFTs). It can capture the common key parameters used for device performance benchmarking of the different TFTs while being applicable to a wide range of TFT technologies in different materials and device structures. Based on this model, a user-friendly tool was developed to provide an interactive way for convenient parameter extraction. The model is continuous from the off-state and subthreshold regimes to the above-threshold regime, avoiding the convergence problems when being used in SPICE circuit simulations. Finally, for verification, it was implemented into a SPICE circuit simulator using Verilog-A to simulate a TFT circuit examplewith the simulated results agreeing verywell with the experimental measurements.

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

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 Printable Organic Thin-Film Transistor Technology for Sensor Transducer

For many of future sensor applications, the sensors are required to be of low cost, easy production and to be able to work with multi-physics or bio/chemistry signals, and provide large area, flexible or comfortable surface coverage. All these bring challenges to the current silicon based manufacturing technology. This chapter introduces a hybrid integration concept combining the advantages of both the printed electronics and silicon technologies to address these issues. A fully printable low voltage organic thin-film transistor (OTFT) technology is developed for making the transducer in the hybrid sensor systems. A simple application example for pH sensor tag is demonstrated. This OTFT technology would provide a promising platform for developing general low cost and disposable multi-function integrated sensor systems.

Device/Circuit Mixed-Mode Simulations for Analysis and Design of Projected-Capacitive Touch Sensors

A device/circuit mixed-mode simulation method is proposed to effectively characterize the physical effects of the structure parameters and external noise signals on the sensing performance of projected-capacitive touch sensor devices for physical explanation and optimal design in touch screen applications. With this method, the electrostatic characteristics of the intrinsic capacitive sensor structure were obtained by numerical simulations, and then embedded into the circuit simulation environment to predict the resulted sensing performance. A single-layer mutual capacitance structure sensor device sample was fabricated to verify the simulation method. The related physical mechanisms during the touching procedure were analyzed with the simulation method, which was in agreement with the experimental measurement results.

Analytical models for delay and power analysis of zero-V load unipolar thin-film Transistor Logic Circuits

In thin-film transistor (TFT) logic circuit applications, propagation delay and power dissipation are two key constraints to be considered in optimal circuit design and synthesis. The unipolar zero-VGS-load logic design is widely used for implementation of TFT digital circuits, because of the simple structure, easy processing, and relatively high gain. In this paper, the analytical models for delay and power were developed for zero-VGS-load inverters, which clarify the relationships between device and design parameters and the two key design constraints. The proposed models were verified by circuit simulations, and could serve as a guideline for optimal design of unipolar zero-VGS-load logic circuits.

Noise Margin, Delay, and Power Model for Pseudo-CMOS TFT Logic Circuits

Flexibleelectronics based on thin-film transistors (TFTs) are promising in the area of Internet of Things and wearable devices, where the pseudo-CMOS logic is widely used in the unipolar TFT circuits. Though plenty of device models exist, analytical circuit-level models are still absent, preventing the further development of design and analysis of flexible TFT circuits. In this paper, we derive the noise margin (NM), delay, and power models for pseudo-CMOS logic circuits. Furthermore, we simplify thosemodels formanual analysis and design optimization. Allmodels are validated by SPICE simulations, where the device model and its parameters are extracted from the fabricated self-assembled monolayer organic TFTs. The average errors for NM, delay, and power models are 3%, 10%, and 3%, respectively. In addition, we exploit the delay models in a voltage-controlled oscillator design and its linearity of frequency characteristic is optimized with the proposed models, demonstrating their effectiveness.

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

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.

Transistor technologies and pixel circuit design for efficient active-matrix organic light-emitting diode displays

Following the advent of the digital multi-media era and the popularity of the internet, high-resolution flat-panel displays (FPDs) are becoming the central feature of many consumer products, from camcorders, mobile/smart phones, to notebook PCs. The FPD industry has been dominated by the liquid crystal displays (LCDs), which are advancing so rapidly that it seems difficult for other technologies to compete in the marketplace, despite all the enthusiasm and innovation emerging from research laboratories in both universities and industry for decades. On the other hand, with the continuous development of related materials and device architectures for efficient and stable organic light emitting diodes (OLEDs), the commercialization of active matrix organic light emitting diode (AMOLED) display products is accelerating [1]. AMOLED displays provide a set of attractive attributes to be used for high-quality video applications, such as excellent contrast ratio, ultra-fast response, vivid visual full-color appearance, and mechanically, a thinner and simpler structure compared to LCDs. AMOLEDs are rapidly expanding their market share for small-sized mobile applications since their mass production launch in 2007, and are also expected to enter the arena of larger-area displays for TVs soon [2].