Balancing Hole and Electron Conduction in Ambipolar Split-Gate Thin-Film Transistors

Complementary organic electronics is a key enabling technology for the development of new applications including smart ubiquitous sensors, wearable electronics, and healthcare devices. High-performance, high-functionality and reliable complementary circuits require n- and p-type thin-film transistors with balanced characteristics. Recent advancements in ambipolar organic transistors in terms of semiconductor and device engineering demonstrate the great potential of this route but, unfortunately, the actual development of ambipolar organic complementary electronics is currently hampered by the uneven electron (n-type) and hole (p-type) conduction in ambipolar organic transistors. Here we show ambipolar organic thin-film transistors with balanced n-type and p-type operation. By manipulating air exposure and vacuum annealing conditions, we show that well-balanced electron and hole transport properties can be easily obtained. The method is used to control hole and electron conductions in split-gate transistors based on a solution-processed donor-acceptor semiconducting polymer. Complementary logic inverters with balanced charging and discharging characteristics are demonstrated. These findings may open up new opportunities for the rational design of complementary electronics based on ambipolar organic transistors. ? 2017 The Author(s).114Ysciescopu

Design and development of poly -(3 -hexylthiophene) field effect transistors

Organic field effect transistors (OFETs) with poly(3-hexylthiophene) (P3HT) as the active layer are developed and studied. The device characteristics are significantly affected by source/drain contact resistance, and P3HT-SiO 2 interface and the traps. These results are verified by the numerical device simulations. The temperature dependence of device mobility is studied, which indicates that the carrier transport is either heat-assisted or heat-limited at different temperature ranges. The on/off ratio and threshold voltage are found to be dependent on the temperature. Hysteresis effect due to gate electric stress is investigated. The silanol groups present at the SiO2 surface are thought to be the key factor, which could trap the gate-induced electrons forming immobile negative ions, and shift the device threshold voltage. Replacing gold with modified poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT-PSS) for the source/drain electrodes, reduces contact resistance and leads to an improved device performance. The SiO2 surface is also improved. Annealing the SiO2surface prior to the deposition of the P3HT layer is found to improve the performance of the device significantly. The device mobility is increased from 0.01 to 0.026 cm2/Vs, the on/off ratio increased from 2.3 × 103 to 8.2 × 103, and subthreshold slope decreased from 3.6 to 2 V/dec. The enhanced device performance is attributed to the possible reduction of physically adsorbed water molecules and hydroxyl groups at the SiO2 surface upon annealing. Polymer heterostructure OFETs are also developed for establishing a method to fabricate new devices and the possibility to increase the device performance. This idea stems from the conventional inorganic modulation doped field effect transistors (MODFETs) that have shown strikingly high carrier mobility. The operation of conventional MODFETs is based on the technique of modulation doping which provides a good means of introducing carriers into the conduction layer without the adverse effects of donors. A polymer heterojunction structure is made of P3HT and poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) and is integrated into a field effect transistor. The resulting device characteristic shows the modulation doping effect. To our knowledge, the modulation doping effect with a polymer heterojunction has not been reported so far. This finding opens a potential pathway to improve the OFETs\u27 device performance

Fabrication, characterization, and modeling of organic capacitors, Schottky diodes, and field effect transistors

The objectives of this project are to fabricate, characterize, and model organic microelectronic devices by traditional lithography techniques and Technology Computer Aided Design (TCAD). Organic microelectronics is becoming a promising field due to its number of advantages in low-cost fabrication for large area substrates. There have been growing studies in organic electronics and optoelectronics. In this project, several organic microelectronic devices are studied with the aid of experimentation and numerical modeling. Organic metal-insulator-metal (MIM) and metal-insulator-semiconductor (MIS) capacitors consisting of insulating polymer poly(4-vinylphenol) (PVP) have been fabricated by spin-coating, photo lithography, and reactive ion etching techniques. Based on the fabricated devices, the dielectric constant of the (PVP) is calculated to be about 5.6–5.94. The MIS capacitor consisting of organic semiconductor pentacene has been investigated. The hole concentration of pentacene is determined to be around 8 × 1016 cm −3. Schottky diodes consisting of aluminum and a layer of p-type semiconducting polymer poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) have been fabricated. Based on the current-voltage (I-V) and capacitance-voltage (C-V) measurements, the temperature dependence of hole mobility in MEH-PPV has been extracted by the space-charge limited conduction (SCLC) model, from 300 to 400 K. Moreover, the value of the effective hole density for MEH-PPV has been determined to be 2.24 × 1017 cm−3. Numerical simulations have been carried out to identify the parameters which affect the performance of devices significantly. Organic n- and p-channel field-effect transistors (FETs) have been designed and fabricated. By using Naphthalene-tetracarboxylic-dianhydride (NTCDA) as an organic semiconductor, n-channel FETs have been fabricated and characterized. At room temperature, the device characteristics have displayed electron mobility of 0.016 cm2/Vs, threshold voltage of −32 V, and on/off ratio of 2.25 × 102. Pentacene, an organic semiconductor offering high device performance, has been employed to fabricate the p-channel FETs. At room temperature, the device characteristics have displayed hole mobility of 0.26 cm2/Vs, threshold voltage of −3.5 V, subthreshold slope of 2.5 V/decade, and on/off ratio of 105. The temperature and field dependence of mobility has been studied based on the experimental results. Based on numerical simulations, the influence of bulk traps has also been identified, and the field-dependent mobility model has been used to obtain more accurate simulation results. Furthermore, electrostatically assembled monolayer (poly(dimethyldiallylammonium chloride) (PDDA)) is introduced at the organic/insulator interface to improve the performance of the FETs. The efforts carried out in this work appear to be the first reported attempt at the investigation of the temperature dependence of mobility for the given organic devices, and the surface modification of organic FETs by electrostatically assembled monolayer

Poly(ethylenedioxythiophene) based electronic devices for sensor applications

Organic electronic devices, based on Poly (3,4-ethylenedioxythiophene)-Poly (styrene sulfonic acid) (PEDOT-PSS) as the active layer for sensor applications, have been studied. Two sets of sensors have been developed. In one case, sensors consisting of PEDOT-PSS resistors have been realized and demonstrated for soil moisture monitoring. The resistor model for the soil moisture sensor enables the sensor device to be fabricated at low cost and easily tested with a simple structure. Unlike the large dimension device used in Time Domain Reflectometry (TDR), the sensors are small and are capable of capturing microscale behavior of moisture in soil which is useful for geological and geotechnical engineering applications. The Field Effect Transistors (FETs) based on PEDOT-PSS and GOx have been developed for a glucose sensing application. The sensitivity of the developed FET-based sensors is enhanced by selecting the channel as the active sensing region as compared with the previously reported devices which use the gate as the active sensing region. This also allows the devices to be designed by a simple and cost-effective means, unlike other complex platform designs for polymer-based sensor devices. PEDOT-PSS based sensors showed higher sensitivity and reversible electrical properties when compared to early versions of sensors fabricated using polymer electrolytes which showed irreversible change in the electrical properties when exposed to high moisture content. The output characteristics, which is the change in electrical sheet resistance of the PEDOT-PSS film versus the percentage change in relative humidity (%RH), show that the conductivity of the film decreases when it is exposed to increasing levels of moisture content. The change in the output resistance of the developed PEDOT-PSS based sensor device was observed to be from 2.5 MΩ to 4.0 MΩ when exposed to soil samples (e.g. Buckshot Clay, CH) with 15–35 % change in gravimetric water content. The FET-based glucose sensor using PEDOT-PSS and GOx as the channel materials, is designed and developed with the capability of precise, fast, and wide sensing range of measurement compared to that of traditional glucose sensors, which are costly and operate on a complex electrochemical based principle. The fabrication and characteristics testing steps of the present glucose sensor are also simpler in comparison to other glucose sensors, which use electrochemical cells for measurements. In the present device, GOx was immobilized on PEDOT-PSS conducting polymer film using a simple cost effective spin-coating technique. A linear increase in the FET drain current was observed, which was resulted from the increase in glucose concentration. The sensitivity of the glucose sensor was determined to be 0.3 Ampere per 1 mg/ml of glucose concentration. A linear range of response was found from 0.2 to 3 mg/ml of glucose, with a response time of 10–20 s. The results indicated that the reported FET-based glucose sensor retains the enzyme bioactivity and can be applied as a glucose biosensor. Moreover, the glucose sensor presented in this dissertation has displayed a reasonable level of sensitivity, repeatability, and stability. The evaluated range of glucose detection shows that the developed biosensor can be used to detect glucose concentration for normal and diabetic patients. This finding also opens a potential pathway for further development of novel biosensor devices

CARBON NANOTUBE THIN FILM AS AN ELECTRONIC MATERIAL

Carbon nanotubes (CNT) are potential candidates for next-generation nanoelectronics devices. An individual CNT possesses excellent electrical properties, but it has been extremely challenging to integrate them on a large-scale. Alternatively, CNT thin films have shown great potential as electronic materials in low cost, large area transparent and flexible electronics. The primary focus of this dissertation is patterning, assembling, characterization and assessment of CNT thin films as electronic material. Since a CNT thin film contains both metallic and semiconducting CNTs, it can be used as an active layer as well as an electrode material by controlling the growth density and device geometry. The growth density is controlled by chemical vapor deposition and airbrushing methods. The device geometry is controlled by employing a transfer printing method to assemble CNT thin film transistors (TFT) on plastic substrates. Electrical transport properties of CNT TFTs are characterized by their conductance, transconductance and on/off ratio. Optimized device performance of CNT TFTs is realized by controlling percolation effects in a random network. Transport properties of CNTs are affected by the local environment. To study the intrinsic properties of CNTs, the environmental effects, such as those due to contact with the dielectric layer and processing chemicals, need to be eliminated. A facile fabrication method is used to mass produce as-grown suspended CNTs to study the transport properties of CNTs with minimal effects from the local environment. Transport and low-frequency noise measurements are conducted to probe the intrinsic properties of CNTs. Lastly, the unique contrast mechanism of the photoelectron emission microscopy (PEEM) is used to characterize the electric field effects in a CNT field effect transistor (FET). The voltage contrast mechanism in PEEM is first characterized by comparing measurements with simulations of a model system. Then the voltage contrast is used to probe the local field effects on a single CNT and a CNT thin film. This real-time imaging method is assessed for potential applications in testing of micron sized devices integrated in large scale

Charge transport in polymer semiconductor field-effect transistors

Ph.DDOCTOR OF PHILOSOPH

Passive micromixers and organic electrochemical transistors for biosensor applications

Fluid handling at the microscale has greatly affected different fields such as biomedical, pharmaceutical, biochemical engineering and environmental monitoring due to its reduced reagent consumption, portability, high throughput, lower hardware cost and shorter analysis time compared to large devices. The challenges associated with mixing of fluids in microscale enabled us in designing, simulating, fabricating and characterizing various micromixers on silicon and flexible polyester substrates. The mixing efficiency was evaluated by injecting the fluids through the two inlets and collecting the sample at outlet. The images collected from the microscope were analyzed, and the absorbance of the color product at the outlet was measured to quantify the mixing efficacy. A mixing efficiency of 96% was achieved using a flexible disposable micromixer. The potential for low-cost processing and the device response tuning using chemical doping or synthesis opened doorways to use organic semiconductor devices as transducers in chemical and biological sensor applications. A simple, inexpensive organic electrochemical transistor (OECT) based on conducting polymer poly(3,4- ethyelenedioxythiphene) poly(styrene sulfonate) (PEDOT:PSS) was fabricated using a novel one step fabrication method. The developed transistor was used as a biosensor to detect glucose and glutamate. The developed glucose sensor showed a linear response for the glucose levels ranging from 1 μM-10 mM and showed a decent response for the glucose levels similar to those found in human saliva and to detect glutamate released from brain tumor cells. The developed glutamate sensor was used to detect the glutamate released from astrocytes and glioma cells after stimulation, and the results are compared with fluorescent spectrophotometer. The developed sensors employ simple fabrication, operate at low potentials, utilize lower enzyme concentrations, do not employ enzyme immobilization techniques, require only 5 μL of both enzyme and sample to be tested and show a stable response for a wide pH ranging from 4 to 9